Anal. Chem. 2005, 77, 3406-3410
Mass Spectral Analysis of Chloropicrin under Negative Ion Chemical Ionization Conditions M. R. V. S. Murty, S. Prabhakar, V. V. S. Lakshmi, U. V. R. Vijaya Saradhi, T. Jagadeshwar Reddy, and M. Vairamani*
National Centre for Mass Spectrometry, Indian Institute of Chemical Technology, Hyderabad-500 007, India
Chloropicrin (CP) is a strong lachrymator and an irritant to all body surfaces of humans.1 Because of its high toxicity and irritating effects on the eyes, lungs, and mucus membranes, it was used as a chemical warfare agent during World War I.1 CP is covered under Chemical Weapons Convention (CWC) as a Schedule 3 chemical. It is also being used in industry as a grain fumigant and soil insecticide.2-4 CP may be released to the atmosphere as a result of its use as a fumigant or as a war gas. It is also found as a contaminant in drinking water, which may result from direct contamination of the water supply or from chlorination of other contaminants.5,6 Hence, its detection and unambiguous identification are important to control its production, usage, and stockpiling, and also in pollution control. CP is freely soluble in most of the common organic solvents such as hexane, chloroform,
carbon tetrachloride, dichloromethane, and ethanol, but it is sparingly soluble in water. CP is usually analyzed by gas chromatography (GC)7 with electron capture detector (ECD)4-6 or electron impact mass spectrometry (EIMS)5,6 because of its high volatility and high electrophilic nature due to the presence of chlorine and nitro groups. CP has been determined in drinking water along with other volatile organic compounds, grains, cereals, wine, and food using GC-ECD and GC/EIMS.7 Trace-level analysis of CP in water is being done by a purge-and-trap method followed by GC or GC/MS.8 It is highly volatile in nature, and it decomposes completely above 400 °C giving rise to phosgene and nitrosyl chloride, which are also toxic in nature. Hence, special desorption methods such as thermal desorption gas chromatography/mass spectrometry is reported for the analysis of this sample.9,10 When mass spectrometric methods are used for detection, the sensitivity of CP is higher in negative ion mode than in positive ion mode. Koida et al.11 reported the determination of CP in water by negative ion chemical ionization (NICI) and monitored the ion at m/z 46 (NO2-) for the quantitative estimation of CP, but the details regarding the analysis are not readily available. The positive ion EI mass spectrum is available in the literature and does not show the molecular ion.12 In such cases, chemical ionization (CI) using reagent gases such as methane, isobutane, etc., is a well-known technique for the determination of molecular weight and identification of the unknown compound unambiguously. The presence of molecular or quasimolecular ion [(M + H)+ or (M - H)-] of considerable relative abundance (>10%) under GC/CIMS is accepted as complementary evidence to GC/EIMS analysis in the official proficiency tests conducted by the Organization for the Prohibition of Chemical Weapons (OPCW). However, no such CI methods are available for the unambiguous identification of CP. Here, we report a systematic study on the behavior of CP with various reagent gases under positive and negative ion CI conditions. Reagent gases of different nature have been used to study the specific ion-molecule reactions for the structural elucidation of various organic com-
* Corresponding author. Telephone: +91-40-27193482. Fax: +91-40-27193156 or +91-40-27160757. E-mail:
[email protected]. (1) Franke, S. In Manual of Military Chemistry. Volume I. Chemistry of Chemical Warfare Agents; Office of the assistant chief of staff for intelligence, Department of the army, Washington, DC, 1967; pp 96-102. (2) Berck, B. J. Agric. Food Chem. 1965, 13 (4), 373-377. (3) Kallio, H.; Shibamoto, T. J. Chromatogr. 1988, 454, 392-397. (4) Daft, J. L. J. Agric. Food. Chem. 1985, 33, 563-566. (5) Kampioti, A. A.; Stephanou, E. G. J. Chromatogr., A 1999, 857, 217-229. (6) Nikolaou, A. D.; Lekkas, T. D.; Golfinopoulos, S. K.; Kostopoulou, M. N. Talanta 2002, 56, 717-726.
(7) Witkiewicz, Z.; Mazurek, M.; Szulc J. Chromatogr. 1990, 503, 293-357. (8) Ishizawa, F.; Ishiwata, T.; Miyata, K.; Yoshida, T. Chudoku Kenkyu, 2003, 16 (3), 339-343. (9) Muir, B.; Carrick, W. A.; Cooper, D. B. Analyst 2002, 127, 1198-1202. (10) Carrick, W. A.; Cooper, D. B.; Muir, B. J. Chromatogr., A 2001, 925, 241249. (11) Koida, K.; Saimatsu, J.; Mishima, K.; Kosa, K.; Tokumori, Y.; Kohno, Y.; Tsue, Y.; Oka, A. Hiroshima-shi Eisei Kenkyusho Nenpo (1987) 1988, 7, 31-33; Chem. Abstr. 1988, 111, 159839n. (12) NIST/EPA/NIH Mass Spectral Library, Version 2002.
A chemical ionization (CI) method is developed for the first time to obtain molecular weight information for chloropicrin (CP), which is used as a chemical warfare agent and as an insecticide. The study includes a detailed investigation on the behavior of CP under electron impact (EI) and CI. Reagent gases of different nature, i.e., methane, isobutane, ammonia, hydrogen, and carbon dioxide, were used for CI analysis. Negative ion mode is found more sensitive than positive ion mode for the EI/ CI mass spectrometric analysis of CP, but none of the methods provided molecular weight information, except negative ion CI using ammonia as the reagent gas (NICI (NH3)). The NICI (NH3) showed formation of the quasimolecular ion, [M + H]-, in addition to other adduct ions. The [M + H]- abundance critically depends on the source temperature, reagent gas pressure, and concentration of the analyte, and it can be 13% under optimized conditions by which CP can be confirmed unambiguously. This method meets the criteria used in official proficiency tests conducted by OPCW for confirming the molecular weight of the unknowns.
3406 Analytical Chemistry, Vol. 77, No. 10, May 15, 2005
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© 2005 American Chemical Society Published on Web 04/12/2005
pounds.13,14 In the present study, we have used methane, isobutane, hydrogen, carbon dioxide, and ammonia as reagent gases and the results are presented here. EXPERIMENTAL SECTION Chloropicrin was synthesized using a standard synthetic procedure,15 and the starting materials (nitromethane, sodium hypochlorite, calcium chloride) were purchased from Sd-fine Chemicals (Mumbai, India). The solvent hexane used for GC analysis was purchased from E-merck (Mumbai, India). The chemical structure of chloropicrin was characterized by mass spectral data. GC-ECD was used to monitor the purity of the compound. A stock solution of 1000 ppm (w/v) was prepared in hexane, and working standards (100, 75, 50, 25, and 10 ppm) were obtained by further dilution of stock solution with hexane. The reagent gases anhydrous ammonia (Matheson), deuterated ammonia (M/s. Stohler Isotope Chemicals), methane (M/s. ECM special gases), and isobutane (M/s. Hydro Gas India Pvt. Ltd., Mumbai, India) were commercially available. Pure helium and hydrogen were obtained from M/s. BOC India Ltd. (Hyderabad, India). The GC/MS analysis was carried out on an Agilent 6890N gas chromatograph (Agilent Technologies) equipped with a model 5973i mass selective detector. A CP-Sil 8 CB (Varian) capillary column (30-m length, 0.25-mm i.d., and 0.25-µm film thickness) was used for the analysis. The oven was programmed from an initial temperature of 50 °C (3 min) to the final temperature of 100 °C at the rate of 10 °C/min. The final temperature was held for 2 min. Helium at the rate of 1 mL/min was used as the carrier gas under constant flow mode. The inlet and interface temperatures were kept at 125 °C. The ion source (EI or CI) and the quadrupole temperatures were kept at 100 °C. A 1-µL 100 ppm sample in hexane was injected under splitless mode into GC, unless otherwise stated. The MS was scanned from 60 to 600 and 40 to 600 Da for PCI and NICI, respectively. Electron energy of 87 and 175 eV were used for PCI and NICI, respectively, for various reagent gases. The EIMS at low electronvolts (20 eV) and negative ion EIMS (70 eV) were recorded using an AutoSpec M mass spectrometer (Micromass, Manchester, U.K.) at a source temperature of 125 °C. The sample was introduced through the heated inlet reservoir at 125 °C. The reagent gas pressures were maintained at a regulator pressure of 200-250 kPa with a set flow of 20 and 40% of the total flow allowed into the source for PCI and NICI, respectively. RESULTS AND DISCUSSION The positive EI mass spectrum of the CP was recorded at 70 eV, and the spectrum matched with the reported one.12 The spectrum contained an ion at m/z 117 (+CCl3) as the base peak in addition to the ions at m/z 82 and 47. The ion at m/z 47 is formed from the ion at m/z 117 by the loss of chlorine molecule and the ion at m/z 82 is due to loss of ClNO2 from the molecular ion. It is difficult to identify CP by these ions as they can also be seen in the EI mass spectrum of carbon tetrachloride. Though (13) Harrison, A. G. In Chemical Ionization Mass Spectrometry; CRC Press: Boca Raton, FL, 1983. (14) John, B. W.; Main, M. A. Mass Spectrom. Rev. 1986, 5, 381-465. (15) Cheylan, E. Mem. Poudres. 1950, 32, 417-421; Chem. Abstr. 1950, 47, 8960h.
Figure 1. (a) PCI (CH4) spectrum and (b) NICI (CH4) spectrum of CP.
the spectrum of CP includes a small but characteristic ions at m/z 128, [M - Cl]+, and m/z 30, [NO]•+, there are more chances to miss these ions when the concentration of CP is too low. A similar spectrum is obtained even at 20 eV conditions, which contains ions at m/z 46 (10%), 63 (1%), 82 (8%), and 117 (100%). The negative ion EIMS of CP also did not yield molecular ion, but instead showed the ions at m/z 117 (12%), 46 (40%), and 35 (100%) due to CCl3-, NO2-, and Cl-, respectively. Absence of a molecular ion and the type of fragment ions formed suggests the C-N bond of CP is very weak and cleaves easily under EI. In fact, it was shown in the literature that the C-N bond in CP is longer than in nitromethane by ∼8%, due to which CP decomposes near its boiling point.16 To avoid thermal degradation of CP at or near its boiling point during the GC/MS analysis, the inlet (injection port), column, and interface are kept at low temperatures (125 °C) throughout the present study. Since, CP does not give a molecular ion under EI conditions, we decided to perform CI experiments using different reagent gases. Among the many reagent gases, methane, isobutane, and ammonia are primarily used for the formation of quasi-molecular ions; hence, we started experiments with these reagent gases. Positive Chemical Ionization (PCI). All the CI experiments were done at optimized conditions (see Experimental Section), as the CI spectra are known to depend on source temperature, reagent gas pressure, and energy of electron beam. The PCI (CH4) spectrum of CP did not show any quasi-molecular ion to reflect its molecular weight (Figure 1a). As expected, sensitivity is very poor in positive mode and showed the ion at m/z 117 as the base peak with other expected fragments. The PCI (iso-C4H10) spectrum showed weak signals due to CP, and background peaks dominated because of poor sensitivity (spectrum not shown). The PCI (NH3) behavior of CP is similar to that found with isobutane (spectrum not shown). Hence, we concluded that the PCI is not a suitable (16) Barss, W. M. J. Chem. Phys. 1957, 27 (6), 1260-1266.
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Table 1. Comparison of Sensitivities in PCI and NCI with Various Reagent Gases
a
reagent gas
rel peak areas in the total ion chromatogramsa PCI:NICI
CH4 iso-C4H10 NH3
1:7 1:5 1:25
Chloropicrin concentration,100 ppm.
Figure 2. NICI (CO2) spectrum of CP.
Figure 3. (a) NICI (NH3) spectrum and (b) NICI (ND3) spectrum of CP.
method for the analysis of CP. To overcome the above-mentioned problems, we decided to extend the study under negative ion mode, because CP is normally analyzed by GC with ECD. Negative Ion Chemical Ionization. Under NICI, we used CH4, iso-C4H10, H2, CO2, and NH3 as buffer gases for generating thermalized electrons suitable for the electron capture phenomenon.17 The CP signals (total ion currents) obtained in both positive and negative ion modes using reagent gases CH4, isoC4H10, and NH3 are presented in Table 1. It can be noticed that the sensitivity for the analysis of CP is increased under NICI conditions as expected, when compared with the PCI conditions. The major ions found in all NICI spectra are almost similar with the characteristic ions at m/z 46 (base peak), 117, and 128. Like PCI, the NICI (CH4) spectrum also did not yield molecular ion/ quasi-molecular ion (Figure 1b). The NICI (iso-C4H10) is similar to that obtained with CH4; however, the ion at m/z 128 is more abundant in the case of iso-C4H10 (spectrum not shown). The NICI (iso-C4H10) does show a very weak signal (
