Internal Standards for Quantitative Analysis of Chemical Warfare Agents by the GC/MS Method: Nerve Agents

Tomas Capoun; Jana Krykorkova

doi:10.1155/2020/8857210

Internal Standards for Quantitative Analysis of Chemical Warfare Agents by the GC/MS Method: Nerve Agents

Capoun, Tomas;Krykorkova, Jana 2020-08-11 00:00:00 Hindawi Journal of Analytical Methods in Chemistry Volume 2020, Article ID 8857210, 11 pages https://doi.org/10.1155/2020/8857210 Research Article Internal Standards for Quantitative Analysis of Chemical Warfare Agents by the GC/MS Method: Nerve Agents Tomas Capoun and Jana Krykorkova Ministry of Interior—Directorate General of the Fire Rescue Service CR, Population Protection Institute, Na Luzci 204, Lazne Bohdanec 533 41, Czech Republic Correspondence should be addressed to Tomas Capoun; tomas.capoun@ioolb.izscr.cz Received 30 April 2020; Accepted 23 July 2020; Published 11 August 2020 Academic Editor: Ricardo Jorgensen Cassella Copyright © 2020 Tomas Capoun and Jana Krykorkova. %is is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. General conditions and requirements for an internal standard useful in the determination of chemical warfare agents (CWAs) by the method of gas chromatography coupled with mass detection (GC/MS) were deﬁned. %e determination is based on a GC/MS analysis of a mixture of a CWA with an internal standard, conversion of the TIC chromatogram to a chromatogram extracted at a particular m/z ratio, and calculation of the CWA concentration from the internal standard concentration, response factor, and chromatographic peak areas. Available internal standards were identiﬁed, and they were veriﬁed for seven organophosphorus nerve-paralysing agents. Corresponding response factors were determined as a ratio of gradients of the linear functions of the peak area and compound concentration. Linearity, repeatability, and accuracy of the measurements were evaluated. %e determination can be performed on all GC/MS systems of the Fire Rescue Service of the Czech Republic (FRS), where no CWA standards are available. substance diﬀerent from the analyte into the sample, to 1. Introduction determine these substances. %is method is based on the fact that within a certain concentration range, the ratio of According to the Czech Republic law, competences of the chromatographic peak areas and concentrations is constant. FRS include chemical countermeasures in case of CWA %is ratio is called the response or calibration factor [1]: spills or abuse. %e countermeasures include chemical re- connaissance, detection, identiﬁcation, and determination of A CWA /c CWA A CWA · c ISTD F � � , (1) CWAs. %is activity is ensured by specialized FRS chemical A ISTD/c ISTD A ISTD · c CWA laboratories. All laboratories are equipped by gas chroma- where F is the response factor, A is the CWA chro- tography with mass detection (GC/MS) as most frequent R CWA analytical systems. %ree types of systems are used in the matographic peak area, A is internal standard chro- ISTD matographic peak area, c is the CWA concentration in FRS: GC/MS 7890A/5975 (Agilent), GC/MS Intuvo 9000/ CWA 5977B of the same manufacturer, and EM 640 (Bruker the solution, and c is internal standard concentration in ISTD Daltonik). Usually, quantitative analysis is performed by the the solution. absolute calibration method on these machines, but only for %e internal standard method has several signiﬁcant those analytes where a standard of corresponding purity is advantages. Unlike the absolute calibration and standard available. addition methods, pure analyte standard is not required. %e %e fundamental issue is that pure and certiﬁed CWA absolute calibration and standard addition methods include standards are not available in the Czech Republic. %erefore, analysis of two separate samples which often introduces we had to turn our attention to a procedure based on an signiﬁcant errors into the results [1, 2]. %e same applies to the external standard method. internal standard, i.e., addition of a known amount of a 2 Journal of Analytical Methods in Chemistry When the internal standard diﬀers from the analyte, When using the internal standard method, the response factor value needs to be known in order to arrive at reliable compound losses during various phases of sample prepa- ration unavoidably diﬀer [1]. %ese diﬀerences can be eﬃ- analytical results. Additionally, the concentration range where the analyte and internal standard chromatographic ciently minimized by the use of double internal standards, peak area is a linear function must be known, as only then, when two neighbouring representatives from the homolo- the constant value of the response factor is ensured [1, 3]. gous series are used as internal standards for the determi- %is explains why many articles focusing on internal stan- nation of a speciﬁc agent [1, 19]. Further examples include dard applications begin by a detailed validation, especially determination of tocopherol in plasma, where pentam- testing linearity of the function and the limit of quantiﬁ- ethylchromanol was used as the internal standard [6], de- termination of 2,5-di-tert-butyl-3-methylphenol in chewing cation and repeatability of the analysis [4–16]. In case the sample is modiﬁed before the analysis (extraction, distilla- gums using 3,5-di-tert-butylphenol [26], determination of furaneol in tomatoes using maltol [27], or determination of tion, and solvent evaporation), maximum similarity of chemical and physical characteristics of the analyte and of carisoprodole in blood using benzyl carbamate [13]. Anisole was used for the determination of various air contaminants the internal standard should be ensured [1, 3]. Close re- tention times of the analyte and the internal standard are [28]. In the analysis of food industry raw materials and also required in order to eliminate peak area discrimination products, 5-nonanone was used for the determination of 34 at varying temperatures under temperature-programmed diﬀerent chemical compounds in honey by the dynamic conditions [1, 3]. On top of these published requirements for headspace method [5, 27], dinonyl phthalate for the de- the internal standard selection, one needs to consider the termination of policosanol components extracted from rice requirement that the internal standard must neither react or bran wax [29], octyl acetate for the determination of 35 volatile compounds in essential oils obtained by steam interact with the analyte nor constitute a decomposition product or other admixture of the analysed compounds. distillation of lemon tree leaves and bark [30], and crotonic acid for the determination of volatile fatty acids in cheeses For use in quantitative GC/MS analysis, the most eﬃ- cient and reliable solution is the use of such internal after steam distillation [31]. In water analysis, ﬂuorobenzene was used as the internal standard for the determination of standard which is identical or analogous to the analyte and labelled by a stable isotope [1, 17]. Examples include the trihalomethanes by the GC/MS method using SPME [32] determination of acrylamide in wheat samples, using and benzyl benzoate for the determination of phthalates [13C3]-acrylamide [18], determination of chrysene in a foil using the same method [33]. In the analysis of industrial using [2H12]-chrysene [19], determination of rotundone in products, diphenyl ether was used for the determination grapes and wine by the SPME method using [2H5]- of lactide monomer in polylactic materials [34], 1,2-di- rotundone [2], and determination of ethyl carbamate using chlorobenzene was used for the determination volatiles in water-oil emulsion by the SPME method [35], and tetra- [2H5]-ethyl carbamate, also in wine [9]. Additional exam- ples include the use of [2H8]-dibenzothiophene for the bromodiphenyl ether was used for the analysis of ﬁre re- tarders on the basis of polybrominated diphenyl ethers, determination of dibenzothiophene in crude oil, coal, and sediment extracts [20], [2H7]-meprobamate for the deter- organophosphates, and brominated aromatic hydrocarbons mination of meprobamate in blood [13], [13C12]-triclosane [15]. for the determination of triclosane in water [21], or [13C]- %is work focuses on the determination of CWAs by the dichlorodiphenyltrichloroethane for the determination of GC/MS method. Only few sources discussing internal dichlorodiphenyltrichloroethane (DDT) in the air [16]. standards for this purpose could be identiﬁed. Dipinacolyl However, internal standards labelled by a stable isotope are methyl phosphonate was described as an internal standard also used for samples where compounds other than ana- for the determination of tabun, cyclosarin, the VX agent, and logues of the standard are analysed. For the determination of nitrogen mustard [8]. %e TNO laboratory in Rijswijk, the Netherlands, focusing especially on the CWA analysis, uses a number of hydrocarbons and other volatile compounds in internal or external air, [2H6]-benzene [11], [2H8]-toluene deuterized sulphur mustard, [2H8]-bis-(2-chloroethyl)sul- phide, as internal standard for the determination of sulphur [22, 23] or [2H10]-ethylbenzene are used as internal stan- dards. Similarly, some deuterized components of gasoline mustard [36]. were used for the determination of aliphatic and aromatic When applying the internal standard, it is always nec- hydrocarbons in water by the SPME method [7]. Additional essary to evaluate the intensity of the analyte and standard examples of the internal standard use are [2H4]-1,2-di- chromatographic peaks. %e TIC area is usually considered chloroethane for the determination of 26 halogenated [1]. More precise results can be achieved when considering compounds in water [24], [2H5]-3,4-methylenediox- the peak area of a particular ion [2, 4], although the TIC area of the chromatographic peak can be sometimes more useful yamphetamine and [2H6]-hydromorphone for the deter- mination of narcotics and their metabolites in biological [5]. %is is discussed in detail in publication [4]. %is work focuses on a search for an available standard samples [4], and [2H14]-triﬂuarine, [2H6]-trans- permethrine, and [2H4]-nitrophenol for the determination which would fulﬁll the requirement of matching CWA and internal standard response ratios on all GC/MS systems in of 28 pesticides in the air [25]. %is makes labelled analyte analogues ideal as internal standards for the determination the FRS laboratories. %e aim of the work was to develop a of compounds by the GC/MS methods; however, their procedure for the FRS laboratories which would not only disadvantage is poor availability and high price. allow for the determination of CWAs in solutions, but could Journal of Analytical Methods in Chemistry 3 and only tailing peaks were integrated manually. For further be also used for a fast and simple determination of the active substance in the CWA preparates themselves. %ese pre- study, EIC chromatograms at a particular m/z ratio were extracted from the TIC chromatograms; peak areas corre- parates are further used for the calibration of the existing internal procedures for the determination of CWA, mostly sponding to the CWAs and the internal standards were by photometric and biochemical methods. obtained by an identical procedure. In order to assess the dependency of the chromato- graphic peak area on the concentration of the given com- 2. Materials and Methods pound in the solution, calibration curves were constructed. 2.1. Chemicals. %e quantitative analyses procedures by the %e linearity range was determined using statistical software GC/MS method were developed for the following CWAs: [37] based on the correlation coeﬃcient R and coeﬃcient QC O-ethyl-N,N-dimethylphosphoramidocyanidate (tabun, GA, values. Coeﬃcient values of R 0.99 and QC 5.00 CRIT CRIT 88%), O-isopropylmethylphosphonoﬂuoridate (sarin, GB, were considered as critical for the testing. %e gradient, y- 64%), O-(3,3-dimethyl-2-butyl)methylphosphonoﬂuoridate range, and standard deviation of the gradient and range were (soman, GD, 79%), O-cyclohexylmethylphosphonoﬂuoridate evaluated by software [37] in the identiﬁed linearity range. (cyclosarin GF, 58%), O-ethyl-S-(diisopropylaminoethyl) In order to assess the accuracy of CWA determination, a methylphosphonothioate (VX agent, 69%), O-ethyl-S- series of results from parallel determinations was compared (diethylaminoethyl)methylphosphonothioate (Edemo, 51%), to the known concentration. %e t-test was used for sta- and O-ethyl-S-(dimethylaminoethyl)methylphosphonothioate tistical evaluation [37], comparing the value of t criteria to (Medemo, 24%). All compounds were prepared in VOZ the critical value. Based on the results from the parallel Zemianske ´ Kostolany, Slovakia. Tabun assay was deter- measurement, the precision of the determination was tested. mined by potentiometric argentometric titration of cyanides %e method of concentration levels from parallel mea- by silver nitrate indicated by a silver electrode; assay of sarin, surements and calculation of relative standard deviation was soman, and cyclosarin was determined by potentiometric selected for the statistical evaluation [37]. lanthanometric titration of ﬂuorides by lanthanum (III) chloride indicated by a ﬂuoride ion selective electrode; assay 3. Results and Discussion of the VX agent, Edemo, and Medemo compounds was determined by potentiometric argentometric titration of 3.1. Study of Chromatographic Peak Area Dependence on thiols by silver nitrate indicated by a sulphide ion selective Concentration. %e primary aim of this work was to ﬁnd a electrode. suitable internal standard, applicable on all GC/MS systems Triethylphosphate (99.8+%, Sigma-Aldrich), heptan-1- across the FRS chemical laboratories in a universal proce- ol (>99%, Fluka), tri-n-butylphosphate (p.a., Merck), di-n- dure. %e procedure would be used especially for a quick and hexylamine (p.a., Merck), and di-n-amylether (p.a., Merck) simple determination of the active ingredient of own CWA were the internal standards used. Solutions of CWAs and preparates which are then used for the calibration of the internal standards were prepared in acetone or n-hexane existing determination procedures. %is means a binary (SupraSolv, for GC, Merck). mixture of the CWA and the internal standard is analysed, and hence, neither similarity of chemical properties of the analyte and the internal standard nor close physical char- 2.2. Measurement Conditions and Parameters. %e mea- acteristics are an issue here as these matter mostly in case of surements were performed on the following systems at sample preparation before the analysis itself. On the other conditions and parameters listed in Table 1. System A—GC/ hand, this requires highly reliable determination which is MS 7890A/5975C (Agilent Technologies, Inc., Wilmington, closely related to the linearity of the chromatographic peak USA); system B—GC/MS Intuvo 9000/5977B same manu- area as a function of the compound concentration. facturer; and system C—mobile GC/MS EM 640 (Bruker Assuming a linear function of the chromatographic peak Daltonik GmbH, Bremen, Germany). area and concentration, the relation can be described by the Solutions of CWAs and internal standard were mixed in following equation: a 1 : 1 (v/v) ratio, and the mixture was introduced into the injection inlet of the GC/MS system. %e linearity range of A � k · c + q, (2) the chromatographic peak area as a function of the com- pound concentration was studied in parallel both for the where A is the chromatographic peak area, k is the gradient, CWA and the corresponding internal standard. Hence, c is concentration, and q is the intercept on the peak area mixtures of CWAs and standards of the same concentration axis. %is equation can be combined with the response factor were injected. Triplicate measurements were performed for equation (1): each concentration of the compound and the standard. (k CWA · c CWA + q CWA) · c ISTD F � . (3) (k ISTD · c ISTD + q ISTD) · c CWA 2.3. Chromatogram Evaluation. Peaks corresponding to the CWA and the internal standard were identiﬁed in the TIC Assuming that the intercept on the peak area is negligible chromatogram recorded in the scan mode. %e peak area compared to the product of gradient and concentration, i.e., was obtained by integration using the evaluation software the linear function of CWA and internal standard peak area listed in Table 1. Generally, automatic integration was used, and concentration passes through the origin, the response 4 Journal of Analytical Methods in Chemistry Table 1: Measurement conditions and parameters. GC/MS 7890A/5975C Intuvo 9000/5977B EM 640 Column HP-5MS 30 m × 0.25 mm, 0.25 μm HP-5MS 30 m × 0.25 mm, 0.25 μm HP-5MS 25 m × 0.35 mm, 1 μm Carrier gas Helium, 147 kPa constant pressure Helium, 1.2 mL/min constant ﬂow Nitrogen, 500 hPa constant pressure Sampler Agilent GC 80 Agilent 7693A — Injection volume 1 μl 1 μl 1 μl ° ° ° 290 C, splitless mode, purge ﬂow 290 C, splitless mode, purge ﬂow 230 C, splitless mode, purge ﬂow Inlet 100 mL/min at 2 min 100 mL/min at 2 min 30 mL/min at 1 min ° ° ° ° ° ° ° ° ° 40 C (2 min), 10 C/min to 280 C 40 C (2 min), 10 C/min to 280 C 40 C (2 min), 10 C/min to 280 C Oven (10 min) (10 min) (10 min) Quadrupole MS, EI, scan mode, Quadrupole MS, EI, scan mode, Quadrupole MS, EI, scan mode, ° ° ° Detector transfer line 290 C, scan range transfer line 290 C, scan range transfer line 280 C, scan range 35–600 amu, solvent delay 6 min 35–600 amu, solvent delay 6 min 50–550 amu, solvent delay 6 min Agilent chemstation GC/MSD—data MassHunter Workstation software, Evaluation of Bruker Data Analysis, version 1.1., analysis, version E.02.02., Agilent version B.07.00, Agilent chromatograms Bruker Daltonik GmbH, 2003 Technologies, Inc., 2011 Technologies, Inc., 2014 factor equals the ratio of gradients of the two linear functions of In turn, this allows for a wider linearity range with the EIC CWA and the internal standard peak area and concentration: chromatograms than with the TIC chromatograms. More- over, the EIC chromatograms of the CWAs and the iden- k CWA F � . (4) tiﬁed standards meet the negligible intercept requirement k ISTD more easily. Reproducibility of the peak area readings is much higher for the EIC chromatograms than for the TIC Use of equation (4) for the determination of the re- chromatograms. Moreover, the EIC chromatogram peak sponse factor has two fundamental prerequisites. First, the area reading is more robust and resistant to interference with intercepts on the peak area axis must be negligible com- compounds with similar retention times than with the use of pared to the product of the gradient and lower limit of the TIC chromatograms. linearity range both for the CWA and for the internal standard. We have set the intercept must be lower than 10% of the product of gradient and the lowest useful concen- tration. Second, the response factor can only be applied in 3.2. Determination of Tabun. %e m/z values of 43, 70, 44, the concentration range where the function of CWA and the 133, and 162 dominate the mass spectrum. Methyllaurate internal standard peak area is linear. %e determination (m/z 43) and crotonaldehyde (m/z 70) were tested as the method was developed for the diﬀerent GC/MS systems available chemicals for the internal standard. However, the whose linearity ranges signiﬁcantly diﬀer. Moreover, the response factors highly varied across the GC/MS systems linearity range also depends on the analyte. Yet, linearity of (the F values of methyllaurate m/z 43 reached 0.75, 0.27, the response greatly inﬂuences reliability of the determina- and 1.02 for systems A, B, and C, respectively; for croto- tion. A typical example is illustrated in Figure 1 showing the naldehyde m/z 70, the values reached 0.64, 0.49, and 1.14 for chromatographic peak area of cyclosarin as a function of its systems A, B, and C, respectively). Although satisfactory concentration in the solution. results could be obtained when using dihexylamine at m/z 43 A number of potential compounds were tested during as the internal standard, we found out that as a weak base, the search for suitable internal standards. Both TIC chro- dihexylamine causes the decomposition of tabun after a matograms and EIC chromatograms extracted for a char- certain period of time, giving rise to O-ethyl-O-methyl N,N- acteristic ion m/z were evaluated. We have found that the dimethylamidophosphate. Hence, dihexylamine does not calculation of the response factor from the TIC chro- fulﬁll the requirement that the internal standard must not matograms recorded in scan mode is not suitable. Only few react with the analyte. Best results were achieved with standards with an identical response factor across the tested heptan-1-ol at m/z 70, as shown in Table 2. For simplicity, systems could be found; most standards exhibited high chromatographic peak area values are given in millions of diﬀerences between the systems. Moreover, a reproducible abundance units in all following tables. linear calibration curve could not be constructed at all for some of the compounds even in a narrow concentration range. In some cases, the requirement of a negligible in- 3.3. Determination of Sarin, Soman, and Cyclosarin. tercept on the peak area axis versus the product of gradient Other type G-type nerve-paralysing agents exhibit the m/z and concentration was not met. 99 as one of the most prominent mass spectrum peaks. %is %e use of EIC chromatogram, extracted both for the fact has led us to the selection of triethylphosphate as a CWA and the internal standard at a particular m/z value, is suitable internal standard. %e calculated response factor the optimal procedure for CWAs determination using the was identical across the three systems tested, as shown in the internal standard method. %is implies that an internal evaluation in Tables 3–5. %e molecule of soman (Table 4) standard exhibiting an intense peak corresponding to the contains two asymmetric atoms, namely, phosphorus and same ion as for the CWA mass spectrum has to be identiﬁed. carbon in the pinacolyl group, and forms four stereoisomers Journal of Analytical Methods in Chemistry 5 0 5 10 15 20 25 30 35 40 c (mg/L) Figure 1: TIC chromatographic peak area of cyclosarin as a function of its concentration in the solution, measured at the following GC/MS systems: 7890A/5975C (A), intuvo 9000/5977B (B), and EM 640 (C). Table 2: Evaluation of the chromatographic peak area of tabun and heptan-1-ol at m/z 70 as a linear function of their concentration (critical values of the correlation coeﬃcient R 0.99 and QC coeﬃcient QC 5.00). CRIT CRIT 7890A/5975C Intuvo 9000/5977B EM 640 GC/MS compound Tabun Heptanol Tabun Heptanol Tabun Heptanol Retention time (min) 13.3 10.4 10.3 7.8 12.6 9.8 Linearity range (mg/L) 2.5–40 2.5–40 0.5–10 0.5–10 10–50 10–50 R 0.9988 0.9994 0.9999 0.9968 0.9999 0.9996 QC 3.28 2.56 0.86 4.13 0.21 1.49 −6 Gradient (A × 10 × L/mg) 0.71 1.4 0.13 0.25 0.015 0.032 −6 Intercept (A × 10 ) 0.065 −0.34 −0.0030 −0.0090 0.00012 −0.015 Response factor F 0.49 0.52 0.48 Average value of F 0.50 [38] giving rise to two chromatographic peaks. Examples are and the internal standard must be performed in an apolar shown in Figure 2. %e total area is a sum of both peak areas. solvent. We found out that, in the methanolic environment, agent VX undergoes decomposition due to the presence of the basic amine, and the requirement that the internal 3.4. Determination of the VX Agent. %e VX agent exhibits a standard must neither react with the analyte nor interact in dominant peak at m/z ratio 114 and less pronounced peaks any way is, therefore, not fulﬁlled. %e whole amount of the at m/z 72 and 127. Tripropylamine (m/z 114 and 72) and 3- VX agent was completely decomposed already 6 hours after aminohexane (m/z 72) have much shorter retention time at mixing the methanolic solution of the VX agent and di-n- the separation conditions used, compared to the VX agent. hexylamine in a concentration range 5–50 mg/L. For the Further internal standards tested exhibited signiﬁcant var- analysis in methanolic solution, this implies that the ana- iances in the agent VX vs. internal standard response across lysed mixture would have to be mixed with the di-n-hex- the systems used. We have veriﬁed, e.g., m-chloroaniline ylamine solution immediately before the analysis, which is (F value for m/z 127 reached 0.12, 0.03, and 0.65 for systems not advantageous for larger sample sets. A, B, and C, respectively), 5-chloro-2-methoxypyrimidine (F value for m/z 114 reached 1.60, 0.30, and 1.21 for systems A, B, and C, respectively), 2-chloro-4-methoxypyrimidine 3.5. Determination of Edemo. %e Edemo agent exhibits two (F value for m/z 114 reached 1.80, 0.50, and 1.42 for systems dominant peaks corresponding to m/z ratios 86 and 99. %e available compounds tested as internal standards are sig- A, B, and C, respectively), and dimethyl adipate (F value for m/z 114 reached 3.00, 1.66, and 2.07 for systems A, B, and C, niﬁcantly more volatile than the analyte, and they have signiﬁcantly shorter retention times: triethylamine (m/z 86), respectively). Very close response factor values across dif- ferent GC/MS systems could be achieved with dipropyl- tripropylamine (m/z 86), 2-(diethylamino)ethan-1-ol (m/z tryptamine at m/z 114 (F value reached 0.71, 0.72, and 0.73 86), and triethylphosphate (m/z 99). Attention was also paid for systems A, B, and C, respectively). However, taking into to N-butylacetamide at m/z 86; however, although repro- account its problematic nature, dipropyltryptamine was not ducible results with close response factor values across the selected as the internal standard for the VX agent. GC/MS systems could be obtained (F value reached 0.97, Most reliable results could be achieved with di-n-hex- 0.93, and 0.94 for systems A, B, and C, respectively), the ylamine at m/z 114 as the internal standard. %e evaluation is compound was not selected as the internal standard for shown in Table 6. Analysis of the mixture of the VX agent Edemo determination due to its high price. Most reliable –6 Peak area, TIC.10 6 Journal of Analytical Methods in Chemistry Table 3: Evaluation of the chromatographic peak area of sarin and triethylphosphate at m/z 99 as a linear function of their concentration (critical values of the correlation coeﬃcient R 0.99 and QC coeﬃcient QC 5.00). CRIT CRIT 7890A/5975C Intuvo 9000/5977B EM 640 GC/MS compound Sarin Triethyl-phosphate Sarin Triethyl-phosphate Sarin Triethyl-phosphate Retention time (min) 7.4 13.2 5.1 11.7 7.1 13.3 Linearity range (mg/L) 1–40 2.5–40 0.5–10 0.5–15 5–40 5–50 R 0.9999 0.9986 0.9998 0.9996 0.9979 0.9988 QC 1.07 3.68 1.74 2.22 3.84 3.71 −6 Gradient (A × 10 × L/mg) 2.9 1.4 0.74 0.38 0.040 0.020 −6 Intercept (A × 10 ) 0.18 −0.24 −0.030 0.014 0.0088 0.0073 Response factor F 1.93 2.06 1.97 Average value of F 1.99 Table 4: Evaluation of the chromatographic peak area of soman and triethylphosphate at m/z 99 as a linear function of their concentration (critical values of the correlation coeﬃcient R 0.99 and QC coeﬃcient QC 5.00). CRIT CRIT 7890A/5975C Intuvo 9000/5977B EM 640 GC/MS compound Soman Triethyl-phosphate Soman Triethyl-phosphate Soman Triethyl-phosphate Retention time (min) 11.7 13.2 10.3 11.7 11.6 13.3 Linearity range (mg/L) 2.5–20 2.5–40 1–15 0.5–15 5–40 5–50 R 0.9971 0.9985 0.9991 0.9989 0.9988 0.9978 QC 4.32 3.63 3.07 3.76 3.16 4.79 −6 Gradient (A × 10 × L/mg) 1.6 1.4 0.45 0.39 0.024 0.021 −6 Intercept (A × 10 ) −0.65 −0.34 −0.030 −0.019 −0.0019 0.010 Response factor F 1.10 1.16 1.16 Average value of F 1.14 Table 5: Evaluation of the chromatographic peak area of cyclosarin and triethylphosphate at m/z 99 as a linear function of their con- centration (critical values of the correlation coeﬃcient R 0.99 and QC coeﬃcient QC 5.00). CRIT CRIT 7890A/5975C Intuvo 9000/5977B EM 640 GC/MS compound Cyclo-sarin Triethyl-phosphate Cyclo-sarin Triethyl-phosphate Cyclo-sarin Triethyl-phosphate Retention time (min) 14.6 13.2 12.5 11.7 14.7 13.3 Linearity range (mg/L) 5–30 2.5–40 1–12.5 0.5–15 5–50 5–50 R 0.9998 0.9992 0.9996 0.9998 0.9969 0.9996 QC 1.62 2.82 2.01 1.28 4.45 1.68 −6 Gradient (A × 10 × L/mg) 3.2 1.5 0.78 0.38 0.046 0.021 −6 Intercept (A × 10 ) −1.3 −0.29 0.0061 −0.017 0.022 0.0077 Response factor F 2.07 2.17 2.18 Average value of F 2.14 results were achieved with tri-n-butylphosphate as the in- 0.48, 0.18, and 0.40 for systems A, B, and C, respectively). ternal standard at m/z 99 (Table 7). Satisfactory results were achieved with di-n-amylether at m/ z 71 as the internal standard, as shown in Table 8. 3.6. Determination of Medemo. As regards to Medemo, as a dimethyl derivative of O-ethyl-S-(dialkylaminoethyl)meth- 3.7. Validation of the Determination. Accuracy and preci- ylphosphonothioate, the m/z values 58 and 71 dominate its sion of the determination were assessed by method vali- mass spectrum. Tri-n-octylamine (m/z 71) was tested as an dation. Accuracy was assessed by the statistical t-test. available compound. However, the linearity range of the Solutions to be analysed were prepared from diﬀerent chromatographic peak area as a function of concentration CWA batches than those used for the determination of was very narrow, rendering the compound practically response factors. Five parallel determinations were per- useless. Response factors of several other compounds tested formed for each solution. %e obtained set of concentration signiﬁcantly varied across the GC/MS systems used. We determinations was, then, evaluated by statistical software tested, for example, 3-aminohexane (F value for m/z 58 [37] to assess the t criterion which was compared to the reached 0.69, 0.34, and 0.93 for systems A, B, and C, re- critical value. %e procedure gives accurate results for spectively) or ethylbutyrate (F value for m/z 71 reached t< t . R CRIT Journal of Analytical Methods in Chemistry 7 Triethylphosphate Soman 10.00 12.00 14.00 16.00 18.00 20.00 22.00 24.00 26.00 28.00 30.00 32.00 34.00 (a) ×10 1.4 Soman Triethylphosphate 1.2 0.8 0.6 0.4 0.2 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 (b) ×10 Triethylphosphate 1.2 1.0 Soman 0.8 0.6 0.4 0.2 0.0 400 600 800 1000 1200 1400 1600 1800 2000 Time (s) (c) Figure 2: Examples of soman and triethylphosphate chromatograms extracted at m/z 99 measured on the following GC/MS systems: (a) 7890A/5975C, c � 10.7 mg/L, c � 15.3 mg/L, (b) intuvo 9000/5977B, c � 7.3 mg/L, c � 4.3 mg/L, and (c) EM 640, CWA ISTD CWA ISTD c � 39.0 mg/L, c � 31.9 mg/L. CWA ISTD Intens. 8 Journal of Analytical Methods in Chemistry Table 6: Evaluation of the chromatographic peak area of the VX agent and di-n-hexylamine at m/z 114 as a linear function of their concentration (critical values of the correlation coeﬃcient R 0.99 and QC coeﬃcient QC 5.00). CRIT CRIT 7890A/5975C Intuvo 9000/5977B EM 640 GC/MS compound VX Dihexyl-amine VX Dihexyl-amine VX Dihexyl-amine Retention time (min) 22.1 17.0 18.6 14.4 23.8 17.7 Linearity range (mg/L) 7.5–20 7.5–20 7.5–20 7.5–20 15–50 15–50 R 0.9934 0.9981 0.9987 0.9938 0.9999 0.9999 QC 4.38 4.81 4.68 3.89 0.43 0.23 −6 Gradient (A × 10 × L/mg) 0.38 0.67 0.24 0.45 0.020 0.036 −6 Intercept (A × 10 ) −0.21 −0.49 −0.031 0.033 0.0013 −0.0023 Response factor F 0.54 0.57 0.55 Average value of F 0.55 Table 7: Evaluation of the chromatographic peak area of edemo and tri-n-butylphosphate at m/z 99 as a linear function of their con- centration (critical values of the correlation coeﬃcient R 0.99 and QC coeﬃcient QC 5.00). CRIT CRIT 7890A/5975C Intuvo 9000/5977B EM 640 GC/MS compound Edemo Tributyl-phosphate Edemo Tributyl-phosphate Edemo Tributyl-phosphate Retention time (min) 20.5 21.2 16.4 17.1 18.9 19.6 Linearity range (mg/L) 5–30 5–30 2.5–15 2.5–12.5 10–50 10–50 R 0.9964 0.9996 0.9980 0.9967 0.9978 0.9981 QC 4.00 2.14 4.62 4.53 3.29 3.17 −6 Gradient (A × 10 × L/mg) 1.1 4.2 0.16 0.61 0.014 0.054 −6 Intercept (A × 10 ) −0.18 −2.1 −0.012 −0.11 0.0017 0.0045 Response factor F 0.27 0.25 0.26 Average value of F 0.26 Table 8: Evaluation of the chromatographic peak area of Medemo and di-n-amylether at m/z 71 as a linear function of their concentration (critical values of the correlation coeﬃcient R 0.99 and QC coeﬃcient QC 5.00). CRIT CRIT 7890A/5975C Intuvo 9000/5977B EM 640 GC/MS compound Medemo Diamyl-ether Medemo Diamyl-ether Medemo Diamyl-ether Retention time (min) 18.4 12.4 14.6 9.5 19.2 13.2 Linearity range (mg/L) 10–40 5–40 2.5–12 2.5–12 15–50 10–50 R 0.9988 0.9997 0.9997 0.9968 0.9995 0.9998 QC 2.58 1.77 0.81 4.01 1.68 0.79 −6 Gradient (A × 10 × L/mg) 0.53 2.5 0.11 0.49 0.017 0.084 −6 Intercept (A × 10 ) −0.10 −1.3 −0.014 −0.0034 −0.0089 −0.00095 Response factor F 0.22 0.21 0.20 Average value of F 0.21 Precision was statistically assessed by the method of 2017–2019 with six chemical laboratories equipped with three concentration levels from parallel measurements and cal- 7890A/5975C GC/MS systems, ﬁve Intuvo 9000/5977B sys- culation of relative standard deviations [37]. Precision was tems, and two mobile EM 640 systems. Accuracy of the results evaluated for the abovementioned sets of ﬁve results. was evaluated by statistical software [37]. %e value of the z- %e accuracy and precision assessment are summarized in score as a ratio of the diﬀerence of known and determined Table 9, indicating that the method yields accurate results and concentration and standard deviation was calculated. Results that the relative standard deviation does not exceed 15% which with an absolute value of z-score below or equal to 2.0 were generally corresponds to the precision of GC/MC-based de- considered as accurate. A relative standard deviation of 12.5% was chosen for the calculation of the z value. termination procedures. %e highest values of relative repeat- ability were achieved on the EM 640 system. On the 7890A/ Evaluation of the multilaboratory comparison is sum- 5975C and Intuvo 9000/5977B GC/MS systems, the maximum marized in Table 10. In total, 3.1% of inaccurate results were relative precision of CWAs determination reached 7%. obtained during the analysis of ﬁve CWAs. %e relative diﬀerence of known and determined concentration between 3.8. Multilaboratory Comparison. %e studied determina- the laboratories did not exceed 6%. Taking into account that tion method could also be veriﬁed in a multilaboratory participants of the multilaboratory comparison did not have comparison performed as part of the FRS chemical labo- standards of the nerve-paralysing agents available, these ratories proﬁciency testing. %e investigation was performed in results can be considered very good. Journal of Analytical Methods in Chemistry 9 Table 9: Assessment of CWA determination accuracy and precision testing (c —internal standard concentration, t—t criterion value, ISTD S —relative standard deviation, number of measurements n � 5, and t � 2.776). R CRIT c Known CWA concentration Determined concentration S ISTD R CWA/Internal standard GC/MS system t (mg/L) (mg/L) (mg/L) (%) 7890A/5975C 29.9 2.147 4.0 30.0 32.7 EM 640 31.6 1.152 11.1 Tabun/heptan-1-ol Intuvo 9000/ 4.0 3.4 3.6 1.263 2.2 5977B 7890A/5975C 14.5 0.049 3.8 20.0 14.5 EM 640 13.3 1.792 14.7 Sarin/triethyl-phosphate Intuvo 9000/ 5.0 9.1 9.7 2.046 6.0 5977B 20.0 19.5 7890A/5975C 18.3 0.311 5.6 EM 640 19.0 0.745 4.2 12.0 7.0 7890A/5975C 7.5 1.538 5.4 Soman/triethyl-phosphate Intuvo 9000/ 6.7 1.147 3.7 5977B EM 640 7.1 0.297 9.3 25.0 21.0 7890A/5975C 20.3 0.459 5.6 EM 640 20.7 0.612 4.2 12.0 8.8 7890A/5975C 9.5 2.005 5.4 Cyclosarin/triethyl- Intuvo 9000/ 7.9 1.916 3.7 phosphate 5977B EM 640 8.9 0.309 9.3 Intuvo 9000/ 5.0 3.7 4.0 1.714 6.0 5977B 15.0 17.7 7890A/5975C 17.0 0.699 5.6 Intuvo 9000/ 16.7 1.015 4.9 5977B VX agent/di-n- EM 640 15.3 1.973 8.4 hexylamine 10.0 19.3 7890A/5975C 17.6 1.514 6.8 Intuvo 9000/ 18.0 1.865 9.0 5977B 10.0 11.0 7890A/5975C 10.3 1.052 7.4 Intuvo 9000/ 11.2 0.328 3.9 5977B Edemo/tri-n- 27.0 22.2 7890A/5975C 23.6 0.874 5.8 butylphosphate EM 640 22.3 0.079 6.5 Intuvo 9000/ 8.0 6.1 5.9 1.232 6.6 5977B 40.0 35.0 7890A/5975C 35.8 1.051 4.8 EM 640 36.2 0.782 12.3 Medemo/di-n-amylether Intuvo 9000/ 10.0 6.5 6.9 0.429 5.1 5977B Table 10: Evaluation of multilaboratory comparison results, showing the determination of several CWAs using the GC/method with the internal standard procedure. Compound Tabun Sarin Soman VX agent Edemo Known concentration (mg/L) 7.5 9.9 15.0 18.7 23.5 Total number of results 16 24 18 20 20 Accurate/inaccurate results obtained on the system 7890A/5975C 6/0 9/0 8/0 6/0 8/0 Intuvo 9000/5977B 10/0 8/1 6/0 8/1 6/0 EM 640 0/0 6/0 4/0 4/1 6/0 Total number of accurate results 16 23 18 18 20 Average determined multilaboratory concentration (mg/L) 7.2 9.7 14.3 19.7 24.8 Relative diﬀerence of known and determined concentration (%) −4.0 −2.0 −4.7 +5.3 +5.5 Relative standard deviation between laboratories (%) 10.0 14.1 9.7 13.3 14.4 10 Journal of Analytical Methods in Chemistry 4. Conclusions Conflicts of Interest A procedure based on the use of an internal standard in GC/ %e authors declare that there are no conﬂicts of interest MS analysis was developed to determine the concentration regarding the publication of this paper. of nerve agents. We found that determination of response factors from TIC chromatograms recorded in the scan mode Acknowledgments is not suitable. A standard which would have an identical %is study was supported by the Ministry of Interior of the response factor across the systems tested could only be Czech Republic (Security Research and Development identiﬁed in exceptional cases; in fact, the response factor Project no. VI20152020009). values highly diﬀered in most cases. An optimal procedure was to use the EIC chromatogram extracted for a particular ion, extracting both CWA and internal standard peaks at the References same m/z ratio. Unlike TIC chromatograms, the EIC [1] I. G. Zenkevich, E. D. Makarov, and I. Y. 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Publisher: Hindawi Publishing Corporation
Copyright: Copyright © 2020 Tomas Capoun and Jana Krykorkova. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
ISSN: 2090-8865
eISSN: 2090-8873
DOI: 10.1155/2020/8857210
Publisher site: See Article on Publisher Site

Abstract

Hindawi Journal of Analytical Methods in Chemistry Volume 2020, Article ID 8857210, 11 pages https://doi.org/10.1155/2020/8857210 Research Article Internal Standards for Quantitative Analysis of Chemical Warfare Agents by the GC/MS Method: Nerve Agents Tomas Capoun and Jana Krykorkova Ministry of Interior—Directorate General of the Fire Rescue Service CR, Population Protection Institute, Na Luzci 204, Lazne Bohdanec 533 41, Czech Republic Correspondence should be addressed to Tomas Capoun; tomas.capoun@ioolb.izscr.cz Received 30 April 2020; Accepted 23 July 2020; Published 11 August 2020 Academic Editor: Ricardo Jorgensen Cassella Copyright © 2020 Tomas Capoun and Jana Krykorkova. %is is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. General conditions and requirements for an internal standard useful in the determination of chemical warfare agents (CWAs) by the method of gas chromatography coupled with mass detection (GC/MS) were deﬁned. %e determination is based on a GC/MS analysis of a mixture of a CWA with an internal standard, conversion of the TIC chromatogram to a chromatogram extracted at a particular m/z ratio, and calculation of the CWA concentration from the internal standard concentration, response factor, and chromatographic peak areas. Available internal standards were identiﬁed, and they were veriﬁed for seven organophosphorus nerve-paralysing agents. Corresponding response factors were determined as a ratio of gradients of the linear functions of the peak area and compound concentration. Linearity, repeatability, and accuracy of the measurements were evaluated. %e determination can be performed on all GC/MS systems of the Fire Rescue Service of the Czech Republic (FRS), where no CWA standards are available. substance diﬀerent from the analyte into the sample, to 1. Introduction determine these substances. %is method is based on the fact that within a certain concentration range, the ratio of According to the Czech Republic law, competences of the chromatographic peak areas and concentrations is constant. FRS include chemical countermeasures in case of CWA %is ratio is called the response or calibration factor [1]: spills or abuse. %e countermeasures include chemical re- connaissance, detection, identiﬁcation, and determination of A CWA /c CWA A CWA · c ISTD F � � , (1) CWAs. %is activity is ensured by specialized FRS chemical A ISTD/c ISTD A ISTD · c CWA laboratories. All laboratories are equipped by gas chroma- where F is the response factor, A is the CWA chro- tography with mass detection (GC/MS) as most frequent R CWA analytical systems. %ree types of systems are used in the matographic peak area, A is internal standard chro- ISTD matographic peak area, c is the CWA concentration in FRS: GC/MS 7890A/5975 (Agilent), GC/MS Intuvo 9000/ CWA 5977B of the same manufacturer, and EM 640 (Bruker the solution, and c is internal standard concentration in ISTD Daltonik). Usually, quantitative analysis is performed by the the solution. absolute calibration method on these machines, but only for %e internal standard method has several signiﬁcant those analytes where a standard of corresponding purity is advantages. Unlike the absolute calibration and standard available. addition methods, pure analyte standard is not required. %e %e fundamental issue is that pure and certiﬁed CWA absolute calibration and standard addition methods include standards are not available in the Czech Republic. %erefore, analysis of two separate samples which often introduces we had to turn our attention to a procedure based on an signiﬁcant errors into the results [1, 2]. %e same applies to the external standard method. internal standard, i.e., addition of a known amount of a 2 Journal of Analytical Methods in Chemistry When the internal standard diﬀers from the analyte, When using the internal standard method, the response factor value needs to be known in order to arrive at reliable compound losses during various phases of sample prepa- ration unavoidably diﬀer [1]. %ese diﬀerences can be eﬃ- analytical results. Additionally, the concentration range where the analyte and internal standard chromatographic ciently minimized by the use of double internal standards, peak area is a linear function must be known, as only then, when two neighbouring representatives from the homolo- the constant value of the response factor is ensured [1, 3]. gous series are used as internal standards for the determi- %is explains why many articles focusing on internal stan- nation of a speciﬁc agent [1, 19]. Further examples include dard applications begin by a detailed validation, especially determination of tocopherol in plasma, where pentam- testing linearity of the function and the limit of quantiﬁ- ethylchromanol was used as the internal standard [6], de- termination of 2,5-di-tert-butyl-3-methylphenol in chewing cation and repeatability of the analysis [4–16]. In case the sample is modiﬁed before the analysis (extraction, distilla- gums using 3,5-di-tert-butylphenol [26], determination of furaneol in tomatoes using maltol [27], or determination of tion, and solvent evaporation), maximum similarity of chemical and physical characteristics of the analyte and of carisoprodole in blood using benzyl carbamate [13]. Anisole was used for the determination of various air contaminants the internal standard should be ensured [1, 3]. Close re- tention times of the analyte and the internal standard are [28]. In the analysis of food industry raw materials and also required in order to eliminate peak area discrimination products, 5-nonanone was used for the determination of 34 at varying temperatures under temperature-programmed diﬀerent chemical compounds in honey by the dynamic conditions [1, 3]. On top of these published requirements for headspace method [5, 27], dinonyl phthalate for the de- the internal standard selection, one needs to consider the termination of policosanol components extracted from rice requirement that the internal standard must neither react or bran wax [29], octyl acetate for the determination of 35 volatile compounds in essential oils obtained by steam interact with the analyte nor constitute a decomposition product or other admixture of the analysed compounds. distillation of lemon tree leaves and bark [30], and crotonic acid for the determination of volatile fatty acids in cheeses For use in quantitative GC/MS analysis, the most eﬃ- cient and reliable solution is the use of such internal after steam distillation [31]. In water analysis, ﬂuorobenzene was used as the internal standard for the determination of standard which is identical or analogous to the analyte and labelled by a stable isotope [1, 17]. Examples include the trihalomethanes by the GC/MS method using SPME [32] determination of acrylamide in wheat samples, using and benzyl benzoate for the determination of phthalates [13C3]-acrylamide [18], determination of chrysene in a foil using the same method [33]. In the analysis of industrial using [2H12]-chrysene [19], determination of rotundone in products, diphenyl ether was used for the determination grapes and wine by the SPME method using [2H5]- of lactide monomer in polylactic materials [34], 1,2-di- rotundone [2], and determination of ethyl carbamate using chlorobenzene was used for the determination volatiles in water-oil emulsion by the SPME method [35], and tetra- [2H5]-ethyl carbamate, also in wine [9]. Additional exam- ples include the use of [2H8]-dibenzothiophene for the bromodiphenyl ether was used for the analysis of ﬁre re- tarders on the basis of polybrominated diphenyl ethers, determination of dibenzothiophene in crude oil, coal, and sediment extracts [20], [2H7]-meprobamate for the deter- organophosphates, and brominated aromatic hydrocarbons mination of meprobamate in blood [13], [13C12]-triclosane [15]. for the determination of triclosane in water [21], or [13C]- %is work focuses on the determination of CWAs by the dichlorodiphenyltrichloroethane for the determination of GC/MS method. Only few sources discussing internal dichlorodiphenyltrichloroethane (DDT) in the air [16]. standards for this purpose could be identiﬁed. Dipinacolyl However, internal standards labelled by a stable isotope are methyl phosphonate was described as an internal standard also used for samples where compounds other than ana- for the determination of tabun, cyclosarin, the VX agent, and logues of the standard are analysed. For the determination of nitrogen mustard [8]. %e TNO laboratory in Rijswijk, the Netherlands, focusing especially on the CWA analysis, uses a number of hydrocarbons and other volatile compounds in internal or external air, [2H6]-benzene [11], [2H8]-toluene deuterized sulphur mustard, [2H8]-bis-(2-chloroethyl)sul- phide, as internal standard for the determination of sulphur [22, 23] or [2H10]-ethylbenzene are used as internal stan- dards. Similarly, some deuterized components of gasoline mustard [36]. were used for the determination of aliphatic and aromatic When applying the internal standard, it is always nec- hydrocarbons in water by the SPME method [7]. Additional essary to evaluate the intensity of the analyte and standard examples of the internal standard use are [2H4]-1,2-di- chromatographic peaks. %e TIC area is usually considered chloroethane for the determination of 26 halogenated [1]. More precise results can be achieved when considering compounds in water [24], [2H5]-3,4-methylenediox- the peak area of a particular ion [2, 4], although the TIC area of the chromatographic peak can be sometimes more useful yamphetamine and [2H6]-hydromorphone for the deter- mination of narcotics and their metabolites in biological [5]. %is is discussed in detail in publication [4]. %is work focuses on a search for an available standard samples [4], and [2H14]-triﬂuarine, [2H6]-trans- permethrine, and [2H4]-nitrophenol for the determination which would fulﬁll the requirement of matching CWA and internal standard response ratios on all GC/MS systems in of 28 pesticides in the air [25]. %is makes labelled analyte analogues ideal as internal standards for the determination the FRS laboratories. %e aim of the work was to develop a of compounds by the GC/MS methods; however, their procedure for the FRS laboratories which would not only disadvantage is poor availability and high price. allow for the determination of CWAs in solutions, but could Journal of Analytical Methods in Chemistry 3 and only tailing peaks were integrated manually. For further be also used for a fast and simple determination of the active substance in the CWA preparates themselves. %ese pre- study, EIC chromatograms at a particular m/z ratio were extracted from the TIC chromatograms; peak areas corre- parates are further used for the calibration of the existing internal procedures for the determination of CWA, mostly sponding to the CWAs and the internal standards were by photometric and biochemical methods. obtained by an identical procedure. In order to assess the dependency of the chromato- graphic peak area on the concentration of the given com- 2. Materials and Methods pound in the solution, calibration curves were constructed. 2.1. Chemicals. %e quantitative analyses procedures by the %e linearity range was determined using statistical software GC/MS method were developed for the following CWAs: [37] based on the correlation coeﬃcient R and coeﬃcient QC O-ethyl-N,N-dimethylphosphoramidocyanidate (tabun, GA, values. Coeﬃcient values of R 0.99 and QC 5.00 CRIT CRIT 88%), O-isopropylmethylphosphonoﬂuoridate (sarin, GB, were considered as critical for the testing. %e gradient, y- 64%), O-(3,3-dimethyl-2-butyl)methylphosphonoﬂuoridate range, and standard deviation of the gradient and range were (soman, GD, 79%), O-cyclohexylmethylphosphonoﬂuoridate evaluated by software [37] in the identiﬁed linearity range. (cyclosarin GF, 58%), O-ethyl-S-(diisopropylaminoethyl) In order to assess the accuracy of CWA determination, a methylphosphonothioate (VX agent, 69%), O-ethyl-S- series of results from parallel determinations was compared (diethylaminoethyl)methylphosphonothioate (Edemo, 51%), to the known concentration. %e t-test was used for sta- and O-ethyl-S-(dimethylaminoethyl)methylphosphonothioate tistical evaluation [37], comparing the value of t criteria to (Medemo, 24%). All compounds were prepared in VOZ the critical value. Based on the results from the parallel Zemianske ´ Kostolany, Slovakia. Tabun assay was deter- measurement, the precision of the determination was tested. mined by potentiometric argentometric titration of cyanides %e method of concentration levels from parallel mea- by silver nitrate indicated by a silver electrode; assay of sarin, surements and calculation of relative standard deviation was soman, and cyclosarin was determined by potentiometric selected for the statistical evaluation [37]. lanthanometric titration of ﬂuorides by lanthanum (III) chloride indicated by a ﬂuoride ion selective electrode; assay 3. Results and Discussion of the VX agent, Edemo, and Medemo compounds was determined by potentiometric argentometric titration of 3.1. Study of Chromatographic Peak Area Dependence on thiols by silver nitrate indicated by a sulphide ion selective Concentration. %e primary aim of this work was to ﬁnd a electrode. suitable internal standard, applicable on all GC/MS systems Triethylphosphate (99.8+%, Sigma-Aldrich), heptan-1- across the FRS chemical laboratories in a universal proce- ol (>99%, Fluka), tri-n-butylphosphate (p.a., Merck), di-n- dure. %e procedure would be used especially for a quick and hexylamine (p.a., Merck), and di-n-amylether (p.a., Merck) simple determination of the active ingredient of own CWA were the internal standards used. Solutions of CWAs and preparates which are then used for the calibration of the internal standards were prepared in acetone or n-hexane existing determination procedures. %is means a binary (SupraSolv, for GC, Merck). mixture of the CWA and the internal standard is analysed, and hence, neither similarity of chemical properties of the analyte and the internal standard nor close physical char- 2.2. Measurement Conditions and Parameters. %e mea- acteristics are an issue here as these matter mostly in case of surements were performed on the following systems at sample preparation before the analysis itself. On the other conditions and parameters listed in Table 1. System A—GC/ hand, this requires highly reliable determination which is MS 7890A/5975C (Agilent Technologies, Inc., Wilmington, closely related to the linearity of the chromatographic peak USA); system B—GC/MS Intuvo 9000/5977B same manu- area as a function of the compound concentration. facturer; and system C—mobile GC/MS EM 640 (Bruker Assuming a linear function of the chromatographic peak Daltonik GmbH, Bremen, Germany). area and concentration, the relation can be described by the Solutions of CWAs and internal standard were mixed in following equation: a 1 : 1 (v/v) ratio, and the mixture was introduced into the injection inlet of the GC/MS system. %e linearity range of A � k · c + q, (2) the chromatographic peak area as a function of the com- pound concentration was studied in parallel both for the where A is the chromatographic peak area, k is the gradient, CWA and the corresponding internal standard. Hence, c is concentration, and q is the intercept on the peak area mixtures of CWAs and standards of the same concentration axis. %is equation can be combined with the response factor were injected. Triplicate measurements were performed for equation (1): each concentration of the compound and the standard. (k CWA · c CWA + q CWA) · c ISTD F � . (3) (k ISTD · c ISTD + q ISTD) · c CWA 2.3. Chromatogram Evaluation. Peaks corresponding to the CWA and the internal standard were identiﬁed in the TIC Assuming that the intercept on the peak area is negligible chromatogram recorded in the scan mode. %e peak area compared to the product of gradient and concentration, i.e., was obtained by integration using the evaluation software the linear function of CWA and internal standard peak area listed in Table 1. Generally, automatic integration was used, and concentration passes through the origin, the response 4 Journal of Analytical Methods in Chemistry Table 1: Measurement conditions and parameters. GC/MS 7890A/5975C Intuvo 9000/5977B EM 640 Column HP-5MS 30 m × 0.25 mm, 0.25 μm HP-5MS 30 m × 0.25 mm, 0.25 μm HP-5MS 25 m × 0.35 mm, 1 μm Carrier gas Helium, 147 kPa constant pressure Helium, 1.2 mL/min constant ﬂow Nitrogen, 500 hPa constant pressure Sampler Agilent GC 80 Agilent 7693A — Injection volume 1 μl 1 μl 1 μl ° ° ° 290 C, splitless mode, purge ﬂow 290 C, splitless mode, purge ﬂow 230 C, splitless mode, purge ﬂow Inlet 100 mL/min at 2 min 100 mL/min at 2 min 30 mL/min at 1 min ° ° ° ° ° ° ° ° ° 40 C (2 min), 10 C/min to 280 C 40 C (2 min), 10 C/min to 280 C 40 C (2 min), 10 C/min to 280 C Oven (10 min) (10 min) (10 min) Quadrupole MS, EI, scan mode, Quadrupole MS, EI, scan mode, Quadrupole MS, EI, scan mode, ° ° ° Detector transfer line 290 C, scan range transfer line 290 C, scan range transfer line 280 C, scan range 35–600 amu, solvent delay 6 min 35–600 amu, solvent delay 6 min 50–550 amu, solvent delay 6 min Agilent chemstation GC/MSD—data MassHunter Workstation software, Evaluation of Bruker Data Analysis, version 1.1., analysis, version E.02.02., Agilent version B.07.00, Agilent chromatograms Bruker Daltonik GmbH, 2003 Technologies, Inc., 2011 Technologies, Inc., 2014 factor equals the ratio of gradients of the two linear functions of In turn, this allows for a wider linearity range with the EIC CWA and the internal standard peak area and concentration: chromatograms than with the TIC chromatograms. More- over, the EIC chromatograms of the CWAs and the iden- k CWA F � . (4) tiﬁed standards meet the negligible intercept requirement k ISTD more easily. Reproducibility of the peak area readings is much higher for the EIC chromatograms than for the TIC Use of equation (4) for the determination of the re- chromatograms. Moreover, the EIC chromatogram peak sponse factor has two fundamental prerequisites. First, the area reading is more robust and resistant to interference with intercepts on the peak area axis must be negligible com- compounds with similar retention times than with the use of pared to the product of the gradient and lower limit of the TIC chromatograms. linearity range both for the CWA and for the internal standard. We have set the intercept must be lower than 10% of the product of gradient and the lowest useful concen- tration. Second, the response factor can only be applied in 3.2. Determination of Tabun. %e m/z values of 43, 70, 44, the concentration range where the function of CWA and the 133, and 162 dominate the mass spectrum. Methyllaurate internal standard peak area is linear. %e determination (m/z 43) and crotonaldehyde (m/z 70) were tested as the method was developed for the diﬀerent GC/MS systems available chemicals for the internal standard. However, the whose linearity ranges signiﬁcantly diﬀer. Moreover, the response factors highly varied across the GC/MS systems linearity range also depends on the analyte. Yet, linearity of (the F values of methyllaurate m/z 43 reached 0.75, 0.27, the response greatly inﬂuences reliability of the determina- and 1.02 for systems A, B, and C, respectively; for croto- tion. A typical example is illustrated in Figure 1 showing the naldehyde m/z 70, the values reached 0.64, 0.49, and 1.14 for chromatographic peak area of cyclosarin as a function of its systems A, B, and C, respectively). Although satisfactory concentration in the solution. results could be obtained when using dihexylamine at m/z 43 A number of potential compounds were tested during as the internal standard, we found out that as a weak base, the search for suitable internal standards. Both TIC chro- dihexylamine causes the decomposition of tabun after a matograms and EIC chromatograms extracted for a char- certain period of time, giving rise to O-ethyl-O-methyl N,N- acteristic ion m/z were evaluated. We have found that the dimethylamidophosphate. Hence, dihexylamine does not calculation of the response factor from the TIC chro- fulﬁll the requirement that the internal standard must not matograms recorded in scan mode is not suitable. Only few react with the analyte. Best results were achieved with standards with an identical response factor across the tested heptan-1-ol at m/z 70, as shown in Table 2. For simplicity, systems could be found; most standards exhibited high chromatographic peak area values are given in millions of diﬀerences between the systems. Moreover, a reproducible abundance units in all following tables. linear calibration curve could not be constructed at all for some of the compounds even in a narrow concentration range. In some cases, the requirement of a negligible in- 3.3. Determination of Sarin, Soman, and Cyclosarin. tercept on the peak area axis versus the product of gradient Other type G-type nerve-paralysing agents exhibit the m/z and concentration was not met. 99 as one of the most prominent mass spectrum peaks. %is %e use of EIC chromatogram, extracted both for the fact has led us to the selection of triethylphosphate as a CWA and the internal standard at a particular m/z value, is suitable internal standard. %e calculated response factor the optimal procedure for CWAs determination using the was identical across the three systems tested, as shown in the internal standard method. %is implies that an internal evaluation in Tables 3–5. %e molecule of soman (Table 4) standard exhibiting an intense peak corresponding to the contains two asymmetric atoms, namely, phosphorus and same ion as for the CWA mass spectrum has to be identiﬁed. carbon in the pinacolyl group, and forms four stereoisomers Journal of Analytical Methods in Chemistry 5 0 5 10 15 20 25 30 35 40 c (mg/L) Figure 1: TIC chromatographic peak area of cyclosarin as a function of its concentration in the solution, measured at the following GC/MS systems: 7890A/5975C (A), intuvo 9000/5977B (B), and EM 640 (C). Table 2: Evaluation of the chromatographic peak area of tabun and heptan-1-ol at m/z 70 as a linear function of their concentration (critical values of the correlation coeﬃcient R 0.99 and QC coeﬃcient QC 5.00). CRIT CRIT 7890A/5975C Intuvo 9000/5977B EM 640 GC/MS compound Tabun Heptanol Tabun Heptanol Tabun Heptanol Retention time (min) 13.3 10.4 10.3 7.8 12.6 9.8 Linearity range (mg/L) 2.5–40 2.5–40 0.5–10 0.5–10 10–50 10–50 R 0.9988 0.9994 0.9999 0.9968 0.9999 0.9996 QC 3.28 2.56 0.86 4.13 0.21 1.49 −6 Gradient (A × 10 × L/mg) 0.71 1.4 0.13 0.25 0.015 0.032 −6 Intercept (A × 10 ) 0.065 −0.34 −0.0030 −0.0090 0.00012 −0.015 Response factor F 0.49 0.52 0.48 Average value of F 0.50 [38] giving rise to two chromatographic peaks. Examples are and the internal standard must be performed in an apolar shown in Figure 2. %e total area is a sum of both peak areas. solvent. We found out that, in the methanolic environment, agent VX undergoes decomposition due to the presence of the basic amine, and the requirement that the internal 3.4. Determination of the VX Agent. %e VX agent exhibits a standard must neither react with the analyte nor interact in dominant peak at m/z ratio 114 and less pronounced peaks any way is, therefore, not fulﬁlled. %e whole amount of the at m/z 72 and 127. Tripropylamine (m/z 114 and 72) and 3- VX agent was completely decomposed already 6 hours after aminohexane (m/z 72) have much shorter retention time at mixing the methanolic solution of the VX agent and di-n- the separation conditions used, compared to the VX agent. hexylamine in a concentration range 5–50 mg/L. For the Further internal standards tested exhibited signiﬁcant var- analysis in methanolic solution, this implies that the ana- iances in the agent VX vs. internal standard response across lysed mixture would have to be mixed with the di-n-hex- the systems used. We have veriﬁed, e.g., m-chloroaniline ylamine solution immediately before the analysis, which is (F value for m/z 127 reached 0.12, 0.03, and 0.65 for systems not advantageous for larger sample sets. A, B, and C, respectively), 5-chloro-2-methoxypyrimidine (F value for m/z 114 reached 1.60, 0.30, and 1.21 for systems A, B, and C, respectively), 2-chloro-4-methoxypyrimidine 3.5. Determination of Edemo. %e Edemo agent exhibits two (F value for m/z 114 reached 1.80, 0.50, and 1.42 for systems dominant peaks corresponding to m/z ratios 86 and 99. %e available compounds tested as internal standards are sig- A, B, and C, respectively), and dimethyl adipate (F value for m/z 114 reached 3.00, 1.66, and 2.07 for systems A, B, and C, niﬁcantly more volatile than the analyte, and they have signiﬁcantly shorter retention times: triethylamine (m/z 86), respectively). Very close response factor values across dif- ferent GC/MS systems could be achieved with dipropyl- tripropylamine (m/z 86), 2-(diethylamino)ethan-1-ol (m/z tryptamine at m/z 114 (F value reached 0.71, 0.72, and 0.73 86), and triethylphosphate (m/z 99). Attention was also paid for systems A, B, and C, respectively). However, taking into to N-butylacetamide at m/z 86; however, although repro- account its problematic nature, dipropyltryptamine was not ducible results with close response factor values across the selected as the internal standard for the VX agent. GC/MS systems could be obtained (F value reached 0.97, Most reliable results could be achieved with di-n-hex- 0.93, and 0.94 for systems A, B, and C, respectively), the ylamine at m/z 114 as the internal standard. %e evaluation is compound was not selected as the internal standard for shown in Table 6. Analysis of the mixture of the VX agent Edemo determination due to its high price. Most reliable –6 Peak area, TIC.10 6 Journal of Analytical Methods in Chemistry Table 3: Evaluation of the chromatographic peak area of sarin and triethylphosphate at m/z 99 as a linear function of their concentration (critical values of the correlation coeﬃcient R 0.99 and QC coeﬃcient QC 5.00). CRIT CRIT 7890A/5975C Intuvo 9000/5977B EM 640 GC/MS compound Sarin Triethyl-phosphate Sarin Triethyl-phosphate Sarin Triethyl-phosphate Retention time (min) 7.4 13.2 5.1 11.7 7.1 13.3 Linearity range (mg/L) 1–40 2.5–40 0.5–10 0.5–15 5–40 5–50 R 0.9999 0.9986 0.9998 0.9996 0.9979 0.9988 QC 1.07 3.68 1.74 2.22 3.84 3.71 −6 Gradient (A × 10 × L/mg) 2.9 1.4 0.74 0.38 0.040 0.020 −6 Intercept (A × 10 ) 0.18 −0.24 −0.030 0.014 0.0088 0.0073 Response factor F 1.93 2.06 1.97 Average value of F 1.99 Table 4: Evaluation of the chromatographic peak area of soman and triethylphosphate at m/z 99 as a linear function of their concentration (critical values of the correlation coeﬃcient R 0.99 and QC coeﬃcient QC 5.00). CRIT CRIT 7890A/5975C Intuvo 9000/5977B EM 640 GC/MS compound Soman Triethyl-phosphate Soman Triethyl-phosphate Soman Triethyl-phosphate Retention time (min) 11.7 13.2 10.3 11.7 11.6 13.3 Linearity range (mg/L) 2.5–20 2.5–40 1–15 0.5–15 5–40 5–50 R 0.9971 0.9985 0.9991 0.9989 0.9988 0.9978 QC 4.32 3.63 3.07 3.76 3.16 4.79 −6 Gradient (A × 10 × L/mg) 1.6 1.4 0.45 0.39 0.024 0.021 −6 Intercept (A × 10 ) −0.65 −0.34 −0.030 −0.019 −0.0019 0.010 Response factor F 1.10 1.16 1.16 Average value of F 1.14 Table 5: Evaluation of the chromatographic peak area of cyclosarin and triethylphosphate at m/z 99 as a linear function of their con- centration (critical values of the correlation coeﬃcient R 0.99 and QC coeﬃcient QC 5.00). CRIT CRIT 7890A/5975C Intuvo 9000/5977B EM 640 GC/MS compound Cyclo-sarin Triethyl-phosphate Cyclo-sarin Triethyl-phosphate Cyclo-sarin Triethyl-phosphate Retention time (min) 14.6 13.2 12.5 11.7 14.7 13.3 Linearity range (mg/L) 5–30 2.5–40 1–12.5 0.5–15 5–50 5–50 R 0.9998 0.9992 0.9996 0.9998 0.9969 0.9996 QC 1.62 2.82 2.01 1.28 4.45 1.68 −6 Gradient (A × 10 × L/mg) 3.2 1.5 0.78 0.38 0.046 0.021 −6 Intercept (A × 10 ) −1.3 −0.29 0.0061 −0.017 0.022 0.0077 Response factor F 2.07 2.17 2.18 Average value of F 2.14 results were achieved with tri-n-butylphosphate as the in- 0.48, 0.18, and 0.40 for systems A, B, and C, respectively). ternal standard at m/z 99 (Table 7). Satisfactory results were achieved with di-n-amylether at m/ z 71 as the internal standard, as shown in Table 8. 3.6. Determination of Medemo. As regards to Medemo, as a dimethyl derivative of O-ethyl-S-(dialkylaminoethyl)meth- 3.7. Validation of the Determination. Accuracy and preci- ylphosphonothioate, the m/z values 58 and 71 dominate its sion of the determination were assessed by method vali- mass spectrum. Tri-n-octylamine (m/z 71) was tested as an dation. Accuracy was assessed by the statistical t-test. available compound. However, the linearity range of the Solutions to be analysed were prepared from diﬀerent chromatographic peak area as a function of concentration CWA batches than those used for the determination of was very narrow, rendering the compound practically response factors. Five parallel determinations were per- useless. Response factors of several other compounds tested formed for each solution. %e obtained set of concentration signiﬁcantly varied across the GC/MS systems used. We determinations was, then, evaluated by statistical software tested, for example, 3-aminohexane (F value for m/z 58 [37] to assess the t criterion which was compared to the reached 0.69, 0.34, and 0.93 for systems A, B, and C, re- critical value. %e procedure gives accurate results for spectively) or ethylbutyrate (F value for m/z 71 reached t< t . R CRIT Journal of Analytical Methods in Chemistry 7 Triethylphosphate Soman 10.00 12.00 14.00 16.00 18.00 20.00 22.00 24.00 26.00 28.00 30.00 32.00 34.00 (a) ×10 1.4 Soman Triethylphosphate 1.2 0.8 0.6 0.4 0.2 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 (b) ×10 Triethylphosphate 1.2 1.0 Soman 0.8 0.6 0.4 0.2 0.0 400 600 800 1000 1200 1400 1600 1800 2000 Time (s) (c) Figure 2: Examples of soman and triethylphosphate chromatograms extracted at m/z 99 measured on the following GC/MS systems: (a) 7890A/5975C, c � 10.7 mg/L, c � 15.3 mg/L, (b) intuvo 9000/5977B, c � 7.3 mg/L, c � 4.3 mg/L, and (c) EM 640, CWA ISTD CWA ISTD c � 39.0 mg/L, c � 31.9 mg/L. CWA ISTD Intens. 8 Journal of Analytical Methods in Chemistry Table 6: Evaluation of the chromatographic peak area of the VX agent and di-n-hexylamine at m/z 114 as a linear function of their concentration (critical values of the correlation coeﬃcient R 0.99 and QC coeﬃcient QC 5.00). CRIT CRIT 7890A/5975C Intuvo 9000/5977B EM 640 GC/MS compound VX Dihexyl-amine VX Dihexyl-amine VX Dihexyl-amine Retention time (min) 22.1 17.0 18.6 14.4 23.8 17.7 Linearity range (mg/L) 7.5–20 7.5–20 7.5–20 7.5–20 15–50 15–50 R 0.9934 0.9981 0.9987 0.9938 0.9999 0.9999 QC 4.38 4.81 4.68 3.89 0.43 0.23 −6 Gradient (A × 10 × L/mg) 0.38 0.67 0.24 0.45 0.020 0.036 −6 Intercept (A × 10 ) −0.21 −0.49 −0.031 0.033 0.0013 −0.0023 Response factor F 0.54 0.57 0.55 Average value of F 0.55 Table 7: Evaluation of the chromatographic peak area of edemo and tri-n-butylphosphate at m/z 99 as a linear function of their con- centration (critical values of the correlation coeﬃcient R 0.99 and QC coeﬃcient QC 5.00). CRIT CRIT 7890A/5975C Intuvo 9000/5977B EM 640 GC/MS compound Edemo Tributyl-phosphate Edemo Tributyl-phosphate Edemo Tributyl-phosphate Retention time (min) 20.5 21.2 16.4 17.1 18.9 19.6 Linearity range (mg/L) 5–30 5–30 2.5–15 2.5–12.5 10–50 10–50 R 0.9964 0.9996 0.9980 0.9967 0.9978 0.9981 QC 4.00 2.14 4.62 4.53 3.29 3.17 −6 Gradient (A × 10 × L/mg) 1.1 4.2 0.16 0.61 0.014 0.054 −6 Intercept (A × 10 ) −0.18 −2.1 −0.012 −0.11 0.0017 0.0045 Response factor F 0.27 0.25 0.26 Average value of F 0.26 Table 8: Evaluation of the chromatographic peak area of Medemo and di-n-amylether at m/z 71 as a linear function of their concentration (critical values of the correlation coeﬃcient R 0.99 and QC coeﬃcient QC 5.00). CRIT CRIT 7890A/5975C Intuvo 9000/5977B EM 640 GC/MS compound Medemo Diamyl-ether Medemo Diamyl-ether Medemo Diamyl-ether Retention time (min) 18.4 12.4 14.6 9.5 19.2 13.2 Linearity range (mg/L) 10–40 5–40 2.5–12 2.5–12 15–50 10–50 R 0.9988 0.9997 0.9997 0.9968 0.9995 0.9998 QC 2.58 1.77 0.81 4.01 1.68 0.79 −6 Gradient (A × 10 × L/mg) 0.53 2.5 0.11 0.49 0.017 0.084 −6 Intercept (A × 10 ) −0.10 −1.3 −0.014 −0.0034 −0.0089 −0.00095 Response factor F 0.22 0.21 0.20 Average value of F 0.21 Precision was statistically assessed by the method of 2017–2019 with six chemical laboratories equipped with three concentration levels from parallel measurements and cal- 7890A/5975C GC/MS systems, ﬁve Intuvo 9000/5977B sys- culation of relative standard deviations [37]. Precision was tems, and two mobile EM 640 systems. Accuracy of the results evaluated for the abovementioned sets of ﬁve results. was evaluated by statistical software [37]. %e value of the z- %e accuracy and precision assessment are summarized in score as a ratio of the diﬀerence of known and determined Table 9, indicating that the method yields accurate results and concentration and standard deviation was calculated. Results that the relative standard deviation does not exceed 15% which with an absolute value of z-score below or equal to 2.0 were generally corresponds to the precision of GC/MC-based de- considered as accurate. A relative standard deviation of 12.5% was chosen for the calculation of the z value. termination procedures. %e highest values of relative repeat- ability were achieved on the EM 640 system. On the 7890A/ Evaluation of the multilaboratory comparison is sum- 5975C and Intuvo 9000/5977B GC/MS systems, the maximum marized in Table 10. In total, 3.1% of inaccurate results were relative precision of CWAs determination reached 7%. obtained during the analysis of ﬁve CWAs. %e relative diﬀerence of known and determined concentration between 3.8. Multilaboratory Comparison. %e studied determina- the laboratories did not exceed 6%. Taking into account that tion method could also be veriﬁed in a multilaboratory participants of the multilaboratory comparison did not have comparison performed as part of the FRS chemical labo- standards of the nerve-paralysing agents available, these ratories proﬁciency testing. %e investigation was performed in results can be considered very good. Journal of Analytical Methods in Chemistry 9 Table 9: Assessment of CWA determination accuracy and precision testing (c —internal standard concentration, t—t criterion value, ISTD S —relative standard deviation, number of measurements n � 5, and t � 2.776). R CRIT c Known CWA concentration Determined concentration S ISTD R CWA/Internal standard GC/MS system t (mg/L) (mg/L) (mg/L) (%) 7890A/5975C 29.9 2.147 4.0 30.0 32.7 EM 640 31.6 1.152 11.1 Tabun/heptan-1-ol Intuvo 9000/ 4.0 3.4 3.6 1.263 2.2 5977B 7890A/5975C 14.5 0.049 3.8 20.0 14.5 EM 640 13.3 1.792 14.7 Sarin/triethyl-phosphate Intuvo 9000/ 5.0 9.1 9.7 2.046 6.0 5977B 20.0 19.5 7890A/5975C 18.3 0.311 5.6 EM 640 19.0 0.745 4.2 12.0 7.0 7890A/5975C 7.5 1.538 5.4 Soman/triethyl-phosphate Intuvo 9000/ 6.7 1.147 3.7 5977B EM 640 7.1 0.297 9.3 25.0 21.0 7890A/5975C 20.3 0.459 5.6 EM 640 20.7 0.612 4.2 12.0 8.8 7890A/5975C 9.5 2.005 5.4 Cyclosarin/triethyl- Intuvo 9000/ 7.9 1.916 3.7 phosphate 5977B EM 640 8.9 0.309 9.3 Intuvo 9000/ 5.0 3.7 4.0 1.714 6.0 5977B 15.0 17.7 7890A/5975C 17.0 0.699 5.6 Intuvo 9000/ 16.7 1.015 4.9 5977B VX agent/di-n- EM 640 15.3 1.973 8.4 hexylamine 10.0 19.3 7890A/5975C 17.6 1.514 6.8 Intuvo 9000/ 18.0 1.865 9.0 5977B 10.0 11.0 7890A/5975C 10.3 1.052 7.4 Intuvo 9000/ 11.2 0.328 3.9 5977B Edemo/tri-n- 27.0 22.2 7890A/5975C 23.6 0.874 5.8 butylphosphate EM 640 22.3 0.079 6.5 Intuvo 9000/ 8.0 6.1 5.9 1.232 6.6 5977B 40.0 35.0 7890A/5975C 35.8 1.051 4.8 EM 640 36.2 0.782 12.3 Medemo/di-n-amylether Intuvo 9000/ 10.0 6.5 6.9 0.429 5.1 5977B Table 10: Evaluation of multilaboratory comparison results, showing the determination of several CWAs using the GC/method with the internal standard procedure. Compound Tabun Sarin Soman VX agent Edemo Known concentration (mg/L) 7.5 9.9 15.0 18.7 23.5 Total number of results 16 24 18 20 20 Accurate/inaccurate results obtained on the system 7890A/5975C 6/0 9/0 8/0 6/0 8/0 Intuvo 9000/5977B 10/0 8/1 6/0 8/1 6/0 EM 640 0/0 6/0 4/0 4/1 6/0 Total number of accurate results 16 23 18 18 20 Average determined multilaboratory concentration (mg/L) 7.2 9.7 14.3 19.7 24.8 Relative diﬀerence of known and determined concentration (%) −4.0 −2.0 −4.7 +5.3 +5.5 Relative standard deviation between laboratories (%) 10.0 14.1 9.7 13.3 14.4 10 Journal of Analytical Methods in Chemistry 4. Conclusions Conflicts of Interest A procedure based on the use of an internal standard in GC/ %e authors declare that there are no conﬂicts of interest MS analysis was developed to determine the concentration regarding the publication of this paper. of nerve agents. We found that determination of response factors from TIC chromatograms recorded in the scan mode Acknowledgments is not suitable. A standard which would have an identical %is study was supported by the Ministry of Interior of the response factor across the systems tested could only be Czech Republic (Security Research and Development identiﬁed in exceptional cases; in fact, the response factor Project no. VI20152020009). values highly diﬀered in most cases. An optimal procedure was to use the EIC chromatogram extracted for a particular ion, extracting both CWA and internal standard peaks at the References same m/z ratio. Unlike TIC chromatograms, the EIC [1] I. G. Zenkevich, E. D. Makarov, and I. Y. 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Internal Standards for Quantitative Analysis of Chemical Warfare Agents by the GC/MS Method: Nerve Agents

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Internal Standards for Quantitative Analysis of Chemical Warfare Agents by the GC/MS Method: Nerve Agents

Internal Standards for Quantitative Analysis of Chemical Warfare Agents by the GC/MS Method: Nerve Agents

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