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Determination of the concentration of explosives in air by isotope dilution analysis
Forensic Science, 6 (1975) 53-66 0 Elsevier Sequoia S.A., Lausanne
53 - Printed
in The Netherlands
DETERMINATION OF THE CONCENTRATION IN AIR BY ISOTOPE DILUTION ANALYSIS
G.A.St. JOHN, J.H. McREYNOLDS, A.C. SCOTT and M.‘ANBAR
W.G. BLUCHER,
Stanford Research Institute, Menlo Park, Calif. 94025 (Received
OF EXPLOSIVES
(U.S.A.)
April 24, 1975)
SUMMARY
The concentrations in air of dinitrotoluene (DNT), trinitrotoluene (TNT), nitroglycerin (NC;), ethylene glycol dinitrate (EGDN), and pentaerythritol tetranitrate (PETN) were measured at 25°C under equilibrium conditions, and that of cyclomethylene trinitramine (RDX) was measured at elevated temperature by means of an isotope dilution technique. Isotopically multilabeled compounds were synthesized and used as diluents. Field ionization mass spectrometry was used to measure the abundance ratios of the unlabeled materials. The concentrations in air at 25” C of DNT, TNT, NG, EDGN, PETN, and RDX are 184, 4, 31, 37,000, 7, and 0.8 ppb v/v, respectively. The data obtained may be used for the assessment of the required sensitivity of air-monitoring detection systems. INTRODUCTION
The detection of explosives by sensitive air monitors has been one of the suggested approaches for security systems. Highly specific monitors are expected to identify and detect the presence of molecules of the explosive material in the sampled air. The monitoring of other substances associated with certain explosives offers much less specificity and would result in less reliable security systems. To specify the required sensitivity of a detection system, it is necessary to know the concentrations in air of the explosive materials in question at ambient temperature. From the vapor pressures, or the concentration in air of the given material at ambient temperatures under equilibrium conditions, one can estimate the concentrations in air that would have to be detected by a practical monitoring system under dynamic conditions. When dealing with materials that have very low vapor pressures @TOIT or less) at room temperature, such as trinitrotoluene (TNT), pentaerythritol tetranitrate (PETN), or cyclomethylene trinitramine (RDX), it is practically impossible to use classic static vapor pressure measuring techniques. The possible errors made in measuring static pressures of about 10m9 atmospheres are
54
very large because the presence of small concentrations of more volatile impurities in the tested material may upset the measured vapor pressure by orders of magnitude. Rate of effusion techniques have been applied to such materials at elevated temperature, but these may also be affected by the presence of impurities. A far more reliable procedure, which is appropriate for the objective of specifying the sensitivity of an air monitoring system, is to determine the actual concentrations of the given material in air under equilibrium conditions. However, concentrations of 1 ppb v/v or about 10 pg/ml do not lend themselves readily to classic analytical techniques. We can tackle this problem in quantitative analysis by means of isotope dilution analysis, using nonradioactive multilabeled materials as carriers-diluents and nonfragmenting field ionization mass spectrometry as the measuring procedure. The principles of this methodology and the instrumentation used have been described elsewhere [ 11. In this paper we report on the dilution analysis determination of the concentration in air at room temperature of the explosive materials TNT, PETN, RDX, dinitrotoluene (DNT), nitroglycerin (NG), and ethylene glycol dinitrate (EGDN). Although the vapor pressure of each of these materials has been previously measured by the classic effusion technique, this is the first time all of them were determined at room temperature by the same absolute technique. The same technique can evidently be successfully applied to any other material of comparably low vapor pressure, and as su,ch it should be of interest to analytical chemists in general, in addition to those interested in explosives and their detection. EXPERIMENTAL
Materials Nitric acid-l80 (HN1*03) HN’ *03 was prepared by isotopic exchange of HN03 7N with Hz “0 98% according to the procedure of Anbar and Guttman [2]. HN1*03 (89% “O-labeled) was prepared by a two-stage exchange for use in the synthesis of NG, EGDN, and PFTN. After the exchange was complete, the acid solution was neutralized with ammonia and the water was removed by vacuum distillation. Freshly distilled methanesulfonic acid was added to a portion of this labeled ammonium nitrate and HN1’OJ was distilled off under vacuum. Nitroglycerin-‘*0 (NG- ‘*04) A nitrating mixture consisting of 14.25 g (95.0 mmol) trifluoromethane sulfonic acid and 2.01 g (23.7 mmol) anhydrous NH4N1’03 (89% ‘*O-labeled) was prepared in a 50 ml flask equipped with magnetic stirrer and thermometer. The solution was cooled to 2O”C, and 0.36 g (3.9 mmol) glycerol was added dropwise over 5 minutes. Stirring was continued for 2 hours at 20°C. The reaction mixture was quenched with 10 ml chilled water
55
and extracted with three 10 ml portions of CH2 Cl*. The combined CH2 Cl? extracts were washed with 10 ml saturated NaCl solution, 10 ml 1% Naz CO,, and two 10 ml portions of saturated NaCl solution, then dried over MgSO,. The solvent was removed by flushing with nitrogen, leaving 0.70 g (84%) of pale yellow liquid that was identified as NG by its i.r. spectrum and refractive index.
Ethyleneglycol
dinitrate-“0
(EGDN-‘804)
A nitrating mixture consisting of 9.45 g (63.0 mmol) trifluoromethane sulfonic acid and 1.35 g (15.8 mmol) anhydrous NH,,N1803 (89% 180-labeled) was prepared in a 50 ml flask equipped with magnetic stirrer and thermometer. The solution was cooled to 0°C and 0.3 g (4.84 mmol) ethylene glycol was added dropwise over 5 minutes. Stirring was continued for 30 minutes at O”C, and the reaction mixture was quenched with 10 ml chilled water. The resulting mixture was extracted with three 10 ml portions of CH2Cla. The combined extracts were washed with 10 ml saturated NaCl solution, 10 ml 1% NazCOa, and two 10 ml portions of saturated NaCl solution, then dried over MgS04. The solvent was removed, leaving 0.60 g (80.1%) of pale yellow liquid that was identified as EGDN by its i.r. spectrum and refractive index.
Pentaerythritol tetranitrate-‘“0 (PETN-“08) 1.86 g (27.3 mmol) of anhydrous HN1803 (89% “O-labeled)
in a 25 ml, two-necked, round bottom flask equipped with magnetic stirrer, drying tube, and thermometer was cooled to O”C, and 0.28 g (2.06 mmol) purified pentaerythritol was added in one portion. The reaction mixture was held below 5°C for 30 minutes. Then 15 ml of chilled water was added, and the white precipitate that formed was filtered off. The solid was digested for 1 hour on a steam bath with 5 ml of 0.5% NazCO,, filtered, washed with 2 X 15 ml H2 0, and recrystallized from acetone, giving 0.62 g (92% yield based on pentaerythritol) of a white solid that was identified as a PETN by its melting point and i.r. spectrum.
d6 -Dini tro toluene (d6 -DNT) A solution containing 3.63 g (24.2 mmol) of trifluoromethane sulfonic acid and 40 ml of CH2Clz was prepared in a 100 ml, three-necked, round bottom flask equipped with a mechanical stirrer, addition funnel, drying tube, and thermometer. 0.76 g (12.1 mmol) of anhydrous HN03 diluted with 5 ml CH2Cla was added dropwise over a 5 minute period. To this nitrating mixture was added 0.53 g (5.9 mmol) of toluene-d, diluted with 5 ml CH2 Clz dropwise over a 15 minute period. The reaction mixture was stirred at 20°C for 2 hours and quenched on 50 g crushed ice. The resulting mixture was extracted three times with 25 ml CHzClz . The combined CH2Clz extracts were washed until neutral with saturated sodium chloride solution and dried over MgS04; then the solvent was removed, leaving 1.10 g of a light yellow solid (99% yield based on toluened,) that was identified as a mixture of dinitro-
56
toluene isomers by its i.r. spectrum. This isomer mixture was used without further purification as isotopic carrier in the isotope dilution experiments because the isomers produce a single combined peak when.field ionized. The ionization efficiencies and vapor pressures of the two isomers are so close that no significant error is introduced during the isotope dilution procedure. d,-Trinitrotoluene (d,-TNT) A nitrating solution containing 1.94 g of 30% fuming sulfuric acid (7.28 mmol of S03) and 0.46 g (7.30 mmol) of anhydrous HN03 was prepared in an oven-dried, 50 ml, three-necked, round bottom flask equipped with a mechanical stirrer, addition funnel, drying tube, and thermometer. External cooling was used to keep the temperature between 20°C and 25” C during the addition of the nitric acid to the fuming sulfuric acid. 0.46 g (2.45 mmol) dgdinitrotoluene was added and the reaction mixture was heated to 70” C for 30 minutes; 80” C for 15 minutes and 90°C for 15 minutes, then quenched on 10 g crushed ice. Organics were extracted with 3 X 20 ml CH2Clz, and the combined extracts were washed with saturated sodium chloride solution (3 X 25 ml), dried over MgS04, and the solvent was removed, leaving 0.56 g (95% yield based on d6-dinitrotoluene) of a pale yellow solid that was identified as TNT by its i.r. spectrum. Cyclomethylenetrinitramine-d, (d6-RDX) A mixture containing 3.25 g acetic acid, 0.15 g acetic anhydride, and 0.08 g (1.0 mmol) NH4N03 was prepared in a 50 ml, four-necked, round bottom flask equipped with magnetic stirrer, thermometer, and three addition funnels. 0.15 g (1.60 mmol) anhydrous HN03 and 0.08 g (1.0 mmol) NH4 NOa were mixed and added to the above at 25°C. When the mixture was thoroughly mixed, the temperature was raised to 64”C, and three solutions were added simultaneously over 6 minutes: (i) 0.42 g (2.85 mmol) hexamine-dlz, 98% atom % D in 0.66 g acetic acid, (ii) 0.85 g (13.49 mmol) anhydrous HNOs and 0.66 g (8.25 mmol) NH,NOs, and (iii) 2.29 g acetic anhydride. The reaction mixture was aged at 64°C for 40 minutes and cooled; 10 ml chilled DsO was added and the mixture was refluxed for 30 minutes. After the mixture was cooled to room temperature, filtered, and washed with three 25 ml portions of DsO, the solid was dried over KOH at 25°C and 0.1 Torr. The crude RDX (0.76 g) was shaken with 325 ml 1,2-dichloroethane, filtered, and the 1,2-dichloroethane was evaporated, giving 0.64 g of solid that was identified as RDX (99+%) by its i.r. spectrum, melting point, and HPLC. Unlabeled explosives The corresponding unlabeled explosives conditions with unlabeled reagents.
were synthesized
under identical
Purity of reagents Unlike
classic methods
of determination
of vapor pressures
of materials,
our isotope dilution method does not require a high purity of the materials tested or of the isotopic diluents used. The mass spectrometric determination of the ratio of the unlabeled to labeled material is not affected by the incidental presence of impurities with molecular weights different from the materials being tested. On the other hand, the solvents used were of highest purity available, to avoid interference by impurities that may have molecular weights identical with either the labeled or the unlabeled explosives. The solvents used to wash the reagents were in excess by many orders of magnitude so that impurities in the solvents in the ppm range might cause significant interference. Control experiments were carried out on the pure solvents evaporated to dryness to ensure their mass spectrometric purity.
Instrumentation All mass spectrometric determinations were carried out in the field ionization mode [ 11. The instrumentation used has been described elsewhere [ 1,3] . It comprises a 45” sector magnet operated in the electrostatic scanning mode. Repeated multiscanning in the mass range of interest was carried out and the data were integrated on a 4096-channel multiscaler [ 1,4] . With all the explosive materials, the observation of a molecular ion required precise adjustment of the source temperature and the use of a high ionizing field. Typically, only a small latitude was found between the minimum temperature needed to maintain the sample in the gas phase and the maximum temperature consistent with the thermal stability of the molecule. The high fields required to remove an electron from materials with a high electron affinity impose strong polarization forces, which contribute to the natural tendency of these materials to be thermally unstable. Most other organic materials reach their optimum ionization efficiency well below the fields required to ionize these polynitro materials. Consequently, background interferences, normally minimal with field ionization, became significant unless scrupulous attention was given to the cleanliness of all systems and the purity of any solvents used. The isotope dilution experiments required the analysis of minimal amounts of material (multilabeled plus unlabeled). The transfer of submicrogram amounts of explosives (100-500 ng) imposed serious constraints on the inlet system. The gas inlet system consisted primarily of glass with Teflon and Viton components in the valves. Samples were taken in 50 ml glass bulbs fitted with glass valves having Teflon stems and Viton seals. For analysis, the sample bulb and valve assembly were attached by an O-ring joint directly to the gas transfer line. The inlet system could then be preevacuated and the transfer line pushed through a ball valve into the high vacuum region to form a tight seal with the inlet end of the ionization source. Even with an essentially all-glass system, certain precautions were required
58
to maintain a high sample transport efficiency. The uniform temperature required throughout the entire inlet system was achieved by placing the sample bulb, valve, and the exposed portion of the transfer line in a single surrounding oven. The optimum temperature of the inlet system was different for each of the materials. To moderate the flow rate into the source from our gas inlet system, we used a sintered tungsten substrate version of the multipoint ionizer [l] . With this source, the sample could be admitted directly from the gas sample bulb through the heated gas transfer line into the ionizing region, and the temperature of the oven used to control the inlet pressure. Known concentration ratios of unlabeled to labeled materials were analyzed to verify that the measured ion abundance ratios corresponded to the actual concentrations and that no background interferences were present. Isotopic assay of the multilabeled explosives
The field ionization spectra of the labeled explosives were recorded to determine their isotopic purity. Such spectra are exemplified by d6-DNT (Figure l), d5-TNT (Figure 2), d6-RDX (Figure 3), and NG(0”) (Figure 4). The critical parameter in each synthetic labeled material is the concentration of impurities having the molecular weight of the unlabeled material (including traces of the unlabeled material itself). In all the materials synthesized, this
de
182 M+O
rn”NTS
0.67K
Fig. 1. Molecular ion region of de-DNT.
59
r
t ITlie
COVNTS
I 227
232
?“I+0
h4+5 65.9K
0.54K
Fig. 2. Molecular
tV,e
ion region
222
of ds-TNT.
224
226
M+O
Fig. 3. Molecular
ion region
of dh-RDX.
60 I
I
I
I
I
I
I
I
I
I
I
I
I
I
m/e
227
229
231
233
235
237
239
MI0
!A+2
IA+4
IA+6
?.I+*
MtlO
IA+12
CO”NT5
12K
1.3K
1.x
3.m
6.8K
Fig. 4. Molecular
ion region of “O-labeled
&OK
56K
NG.
concentration was less than 0.176, and in some materials like TNT and DNT it was less than 0.01%. All materials with the exception of PETN were mass spectrometically assayed in molecular ion form. In the case of PETN we ultimately selected the NO2 ion for analysis because of the highly unstable nature of PETN as a molecule or as a molecular ion. This is not surprising in view of the use of this material as a primer and detonator, which requires a high degree of instability. Even though a fragment ion was used for quantitation in vapor pressure measurements, the presence of the molecular ion was verified in all experiments (Figure 5). Dilution analysis procedures In all the determinations of concentration of explosives in air, an amount of explosive was brought into equilibrium with a large volume of air (3 liters) at 25” C. A small volume (50 ml) of this equilibrated air was rapidly transferred (by suction) into an evacuated sampling bulb that contained a known amount of the multilabeled carrier. The change in pressure in the whole system was small (<2%). The sampling bulb was valved off without delay and separated from the large vessel. It was then cooled and air was removed by evacuation. Any losses of explosive at this stage did not affect the ratio of unlabeled to labeled material in the sampling bulb. The bulb was then attached
61
Fig. 5. Field ionization
spectrum
of unlabeled
PETN.
to the inlet system of the mass spectrometer for mass spectrometric analysis, and the ratio of unlabeled to labeled material was determined. From this ratio (I?), the known initial amount of the multilabeled carrier (a), and the volume of the sampling vessel (u), the concentration (C) of the unlabeled material could be readily calculated: C = aR/v The less volatile explosives were transferred to the mass spectrometer by dissolving the explosive in the bulb in an appropriate solvent, concentrating it, and transferring the concentrate into a glass melting point capillary [4]. The residual solvent was removed under a stream of dry nitrogen, and the explosive was introduced into the mass spectrometer by using a conventional temperature-controlled solid sample probe. The sample was then evaporated into the ionization source. This procedure again did not have to be quantitative, as any losses of the explosive do not affect the measured abundance ratio. Sampling techniques for individual materials Because each explosive was handled somewhat differently for optimal recovery and transfer, we will now describe the sampling techniques for each material measured.
62
EGDN
The sample bulb was loaded with 400 ng of 1*O-EGDN in -4 ~1 of acetone. The droplet was swirled around to coat the surface of the bulb until the acetone had evaporated. The bulb was cooled to 0°C in an ice bath and evacuated without exhaustive pumping. The evacuated sample bulb was then connected to a 3-liter glass vessel, which contained about 100 mg unlabeled EGDN in acetone solution. After complete equilibration of the unlabeled EGDN with the walls and the air of the 3 liter vessel, the valve of the sampling bulb was opened to take a sample. Following the sampling procedure described above, the sample bulb was attached to the heated glass inlet line of the mass spectrometer, cooled to O”C, and evacuated. When the bulb had pumped down sufficiently, the inlet tube was pushed through the ball valve to contact the back of the source [4] . The ice bath was removed, and an oven preheated to 100°C was placed over the sample bulb, which was then warmed to -85°C over 20 minutes. During this time, the molecular ion region was repeatedly scanned at a rate of one scan per second, and the multiplier output was integrated on a 4096channel multichannel analyzer. When the count rate had fallen to the background level, the ratio of the labeled to unlabeled ion counts was calculated from the integrated output of the multichannel analyzer. DNT
The sample bulb was loaded with 20 ng of d6 -DNT in benzene. The solvent was allowed to evaporate, and the bulb was evacuated at 0°C. The sampling of unlabeled DNT from the 3 liter vessel and the analyses were carried out as described above. NG
The internal standard, 400 ng ‘so-labeled NG in acetone, was added to the sample bulb, which was cooled to 0°C and evacuated by roughing pump only. This pumping was applied intermittently until no further pressure rise occurred when the sample valve was opened to vacuum. Following evacuation, a 50 ml sample was obtained from the 3 liter vessel as described above. This entire cycle of cooling, pumping, and sampling was repeated three more times with the same bulb until 200 ml of air had been sampled. The large vessel was brought up to atmospheric pressure, and the NG was allowed to return to equilibrium each time. This procedure allowed a larger volume containing the unlabeled material to be sampled. The material that was sampled is almost quantitatively adsorbed to the walls of the sampling vessel during the cooling and evacuation, so it was not lost during the repeated samplings. After the final sample was obtained, the bulb was cooled, evacuated, and analyzed as described above. RDX
The sample bulb was loaded with 400 ng of d6 -RDX dissolved in benzene.
63
The bulb was evacuated at 25°C and attached to the 3 liter vessel containing a few mg of unlabeled RDX. The equilibration vessel with the attached sampling bulb was placed in an oven at a preselected temperature and allowed to equilibrate for 1 hour. After that time, the bulb was opened to obtain the sample. The bulb was then cooled to room temperature, and the inner surface washed with 0.5 cc of spectrograde benzene. The benzene wash was removed and then concentrated by evaporation to -20 ~1 under dry nitrogen. The final volume was evaporated to dryness in a sample capillary tube [4] . The solid probe containing the sample capillary was placed against the ion
10
4
I
I
I
AH
= 13.6
To = 840
1/T
I
I
IoK x lo?
Fig. 6. Vapor pressure us temperature for RDX.
kcallmole K
64
source and slowly heated to 50-75”C, while the molecular ion region was being scanned. For measurements on this compound, samples were taken at 74”) 84”) and 96°C (Figure 6). The vapor pressure at room temperature was obtained from the Clausius-Clapeyron equation by plotting the log of the measured vapor pressures versus the reciprocal of the absolute temperature. TNT This material was analyzed as described for RDX, by the technique of washing the sample from the bulb and introducing it with the solid probe. The sampling was carried out at 25” C with 400 ng of d, -TNT used as isotopic diluent. PETN This material was sampled at 25°C with 400 ng of 180-PETN used as isotopic diluent. The sample was washed from the sample bulb and introduced with the solid probe. The instability of this molecule caused severe field-induced fragmentation, so it was necessary to monitor the NO2 ion region (m/e 46,48, 50). RESULTS
AND DISCUSSION
Table 1 summarizes the vapor pressure measurements obtained for all the explosives. For all the data, the number of counts in the unlabeled region was sufficient to give a statistical precision of better than +lO%. This variation is due mainly to the variation in the background level between individual samples. Repeated experiments and calibration runs showed variations that did not exceed 10% of the measured volume. A comparison of our findings with previous vapor pressure data, all of which were obtained by the Knudsen effusion method [5] or modification thereof [6,7], shows significant differences. We will distinguish here between results that, although different, still agree TABLE 1 Room temperature (25O It 2’C) vapor pressures of explosive compounds obtained by the isotope dilution method Compound
Concentration (g/ml)
P (Torr)
Mol/mol
EGDN DNT NG PETN TNT RDX
2.5 1.3 3.2 1.0 4.0 8.0
2.8 1.4 2.4 5.3 3.0 6.1
37 ppm 184 ppb 31 ppb 7 ppb 4 ppb 0.8 ppb
x x x x x x
lo-’ 10-g lo-lo 10-10 10-11 10-12
x x x x x x
10-2 10-4 1O-5 10-G 10-G lo-’
65
within the same order of magnitude and those cases where much larger differences were observed. In the first group we have EGDN, DNT, and TNT. An interpolated value of 6 X low2 mm at 25°C can be calculated for EGDN from literature data [8] , and even the extrapolated value of 1 X 10-l calculated from older data [9] is just three times higher than the value we obtained. The literature values for the vapor pressure of DNT obtained in the temto 25” C, yield 3 X lo-” mm, perature range 59” to 69” C [lo] , extrapolated which is lower by a factor of 4 from our findings. A vapor pressure of 1.2 X 10m5 mm at 25°C can be extrapolated from values obtained for TNT in the temperature range 55”-76°C [lo]. A value of 1 X 10s5 mm can be extrapolated from Edwards data [ 111 obtained in the range 50”-143°C. These values are higher by a factor of 3-4 than that found by us at 25°C. The significantly large differences, which may be expected from two utterly different techniques, do not indicate any systematic error in our procedure. A comparison of data for other explosives shows even larger differences. Our value for NG is significantly lower (by a factor of about 20) than the interpolated value obtained from literature data [12]. On the other hand, the concentration in air of RDX found by us (by extrapolation form 74”-96”) is higher by a factor of 200 than that obtained by extrapolation of effusion data in a comparable temperaturerange (56”-98°C) [ 131. The largest difference was observed for PETN, where we were able to measure at 25°C a concentration of 7 ppb (corresponding to 5.3 X 10e6 mm), which is 3 orders of magnitude higher than the value of 8 X lo-’ mm obtained by extrapolation from data measured in the temperature range 97”-139°C. We have no reasonable explanation for the large differences in the latter cases. From the practical standpoint, however, our data are much more pertinent for the purpose of assessing the required sensitivity of air monitoring because our measurements have been of actual concentrations in air. Moreover, they indicate that an air monitoring system with a sensitivity of 0.1 ppb could detect any of the explosives in air under a fairly wide dynamic range of conditions. This is in contrast to estimates from older vapor pressure data, which indicate that the detection of RDX or PETN in air at room temperature would require lOO- to lOOO-fold higher sensitivities. It should be noted that the detection of concentrations of air contaminants of about 0.1 ppb is well within the state of the art at present, but attainment of much higher sensitivities is problematic, if possible at all.
ACKNOWLEDGEMENTS
This research has been carried out under the sponsorship of the U.S. Postal Service under Contract No. 74-00810. The authors wish to acknowledge Mr. R.G. Cumings of the U.S. Postal Service with whom we had many helpful discussions throughout the progress of this project.
66 REFERENCES 1 M. Anbar and W. Aberth, Field ionization mass spectrometry: a new tool for the analytical chemist, Anal. Chem., 46 (1974) 59A-64A. 2 M. Anbar and S. Guttmann, The catalytic effect of chloride ions on the isotopic oxygen exchange of nitric and bromic acids with water, J. Amer. Chem. SOC., 83 (1961) 4741-4745. 3 M. Anbar, J. McReynolds, W. Aberth and G.A. St. John, Nonradioactive multilabeled molecular tracers in biomedical research, Proc. First Intern. Conf. Stable Isotopes in Chemistry, Biology and Medicine, held at Argonne National Laboratory, III. May 9-11, 1974, AEC Publication No. CONF-730525, pp. 274-234. 4 M. Scolnick, W. Aberth and M. Anbar, An integrating multiscanning field ionization mass spectrometer, Int. J. Mass Spectrom. Ion Phys., 17 (1975) 139-146. 5 M. Knudsen, Experimental determination of the vapor pressure of mercury at 0’ and at higher temperatures, Ann. Phys., 29 (1909) 179-193. 6 T.H. Swan and E. Mack, Vapor pressures of organic crystals by an effusion method, J. Amer. Chem. Sot., 47 (1925) 2112-2116. 7 M. Davies and B. Kybett, Sublimation and vaporization heat of long chain alcohols, Trans. Faraday Sot., 61 (1965) 1608-1617. 8 Engineering Design Handbook, Explosive Series - Properties of Explosives of Military Interest. Army Materiel Command Pamphlet 706-177. 1971, p. 144. 9 W.H. Rickenbach, The properties of glycol dinitrate, Ind. Eng. Chem., 18 (1926) 1195-1197. 10 C. Lenchitz and R.W. Velicky, Vapor pressure and heat sublimation of three nitrotoluenes, J. Chem. Eng. Data, 15 (1970) 401-403. Trans. Faraday Sot., 46 11 G. Edwards, The vapour pressure of 2 : 4 : 6_trinitrotoluene, (1950) 423-427. 12 Engineering Design Handbook, Explosive Series - Properties of Explosives of Military Interest, Army Materiel Command Pamphlet 706-177, 1971, p. 233. 13 J.M. Rosen and C. Dickinson, Vapor pressure and heats of sublimation of some high melting organic explosives, J. Chem. Eng. Data, 14 (1969) 120-124. 14 G. Edwards, The vapour pressure of cycle-trimethylene-trinitramine (cyclonite) and pentaerythritol-tetranitrate, Trans. Faraday Sot., 49 (1953) 152-154.
53 - Printed
in The Netherlands
DETERMINATION OF THE CONCENTRATION IN AIR BY ISOTOPE DILUTION ANALYSIS
G.A.St. JOHN, J.H. McREYNOLDS, A.C. SCOTT and M.‘ANBAR
W.G. BLUCHER,
Stanford Research Institute, Menlo Park, Calif. 94025 (Received
OF EXPLOSIVES
(U.S.A.)
April 24, 1975)
SUMMARY
The concentrations in air of dinitrotoluene (DNT), trinitrotoluene (TNT), nitroglycerin (NC;), ethylene glycol dinitrate (EGDN), and pentaerythritol tetranitrate (PETN) were measured at 25°C under equilibrium conditions, and that of cyclomethylene trinitramine (RDX) was measured at elevated temperature by means of an isotope dilution technique. Isotopically multilabeled compounds were synthesized and used as diluents. Field ionization mass spectrometry was used to measure the abundance ratios of the unlabeled materials. The concentrations in air at 25” C of DNT, TNT, NG, EDGN, PETN, and RDX are 184, 4, 31, 37,000, 7, and 0.8 ppb v/v, respectively. The data obtained may be used for the assessment of the required sensitivity of air-monitoring detection systems. INTRODUCTION
The detection of explosives by sensitive air monitors has been one of the suggested approaches for security systems. Highly specific monitors are expected to identify and detect the presence of molecules of the explosive material in the sampled air. The monitoring of other substances associated with certain explosives offers much less specificity and would result in less reliable security systems. To specify the required sensitivity of a detection system, it is necessary to know the concentrations in air of the explosive materials in question at ambient temperature. From the vapor pressures, or the concentration in air of the given material at ambient temperatures under equilibrium conditions, one can estimate the concentrations in air that would have to be detected by a practical monitoring system under dynamic conditions. When dealing with materials that have very low vapor pressures @TOIT or less) at room temperature, such as trinitrotoluene (TNT), pentaerythritol tetranitrate (PETN), or cyclomethylene trinitramine (RDX), it is practically impossible to use classic static vapor pressure measuring techniques. The possible errors made in measuring static pressures of about 10m9 atmospheres are
54
very large because the presence of small concentrations of more volatile impurities in the tested material may upset the measured vapor pressure by orders of magnitude. Rate of effusion techniques have been applied to such materials at elevated temperature, but these may also be affected by the presence of impurities. A far more reliable procedure, which is appropriate for the objective of specifying the sensitivity of an air monitoring system, is to determine the actual concentrations of the given material in air under equilibrium conditions. However, concentrations of 1 ppb v/v or about 10 pg/ml do not lend themselves readily to classic analytical techniques. We can tackle this problem in quantitative analysis by means of isotope dilution analysis, using nonradioactive multilabeled materials as carriers-diluents and nonfragmenting field ionization mass spectrometry as the measuring procedure. The principles of this methodology and the instrumentation used have been described elsewhere [ 11. In this paper we report on the dilution analysis determination of the concentration in air at room temperature of the explosive materials TNT, PETN, RDX, dinitrotoluene (DNT), nitroglycerin (NG), and ethylene glycol dinitrate (EGDN). Although the vapor pressure of each of these materials has been previously measured by the classic effusion technique, this is the first time all of them were determined at room temperature by the same absolute technique. The same technique can evidently be successfully applied to any other material of comparably low vapor pressure, and as su,ch it should be of interest to analytical chemists in general, in addition to those interested in explosives and their detection. EXPERIMENTAL
Materials Nitric acid-l80 (HN1*03) HN’ *03 was prepared by isotopic exchange of HN03 7N with Hz “0 98% according to the procedure of Anbar and Guttman [2]. HN1*03 (89% “O-labeled) was prepared by a two-stage exchange for use in the synthesis of NG, EGDN, and PFTN. After the exchange was complete, the acid solution was neutralized with ammonia and the water was removed by vacuum distillation. Freshly distilled methanesulfonic acid was added to a portion of this labeled ammonium nitrate and HN1’OJ was distilled off under vacuum. Nitroglycerin-‘*0 (NG- ‘*04) A nitrating mixture consisting of 14.25 g (95.0 mmol) trifluoromethane sulfonic acid and 2.01 g (23.7 mmol) anhydrous NH4N1’03 (89% ‘*O-labeled) was prepared in a 50 ml flask equipped with magnetic stirrer and thermometer. The solution was cooled to 2O”C, and 0.36 g (3.9 mmol) glycerol was added dropwise over 5 minutes. Stirring was continued for 2 hours at 20°C. The reaction mixture was quenched with 10 ml chilled water
55
and extracted with three 10 ml portions of CH2 Cl*. The combined CH2 Cl? extracts were washed with 10 ml saturated NaCl solution, 10 ml 1% Naz CO,, and two 10 ml portions of saturated NaCl solution, then dried over MgSO,. The solvent was removed by flushing with nitrogen, leaving 0.70 g (84%) of pale yellow liquid that was identified as NG by its i.r. spectrum and refractive index.
Ethyleneglycol
dinitrate-“0
(EGDN-‘804)
A nitrating mixture consisting of 9.45 g (63.0 mmol) trifluoromethane sulfonic acid and 1.35 g (15.8 mmol) anhydrous NH,,N1803 (89% 180-labeled) was prepared in a 50 ml flask equipped with magnetic stirrer and thermometer. The solution was cooled to 0°C and 0.3 g (4.84 mmol) ethylene glycol was added dropwise over 5 minutes. Stirring was continued for 30 minutes at O”C, and the reaction mixture was quenched with 10 ml chilled water. The resulting mixture was extracted with three 10 ml portions of CH2Cla. The combined extracts were washed with 10 ml saturated NaCl solution, 10 ml 1% NazCOa, and two 10 ml portions of saturated NaCl solution, then dried over MgS04. The solvent was removed, leaving 0.60 g (80.1%) of pale yellow liquid that was identified as EGDN by its i.r. spectrum and refractive index.
Pentaerythritol tetranitrate-‘“0 (PETN-“08) 1.86 g (27.3 mmol) of anhydrous HN1803 (89% “O-labeled)
in a 25 ml, two-necked, round bottom flask equipped with magnetic stirrer, drying tube, and thermometer was cooled to O”C, and 0.28 g (2.06 mmol) purified pentaerythritol was added in one portion. The reaction mixture was held below 5°C for 30 minutes. Then 15 ml of chilled water was added, and the white precipitate that formed was filtered off. The solid was digested for 1 hour on a steam bath with 5 ml of 0.5% NazCO,, filtered, washed with 2 X 15 ml H2 0, and recrystallized from acetone, giving 0.62 g (92% yield based on pentaerythritol) of a white solid that was identified as a PETN by its melting point and i.r. spectrum.
d6 -Dini tro toluene (d6 -DNT) A solution containing 3.63 g (24.2 mmol) of trifluoromethane sulfonic acid and 40 ml of CH2Clz was prepared in a 100 ml, three-necked, round bottom flask equipped with a mechanical stirrer, addition funnel, drying tube, and thermometer. 0.76 g (12.1 mmol) of anhydrous HN03 diluted with 5 ml CH2Cla was added dropwise over a 5 minute period. To this nitrating mixture was added 0.53 g (5.9 mmol) of toluene-d, diluted with 5 ml CH2 Clz dropwise over a 15 minute period. The reaction mixture was stirred at 20°C for 2 hours and quenched on 50 g crushed ice. The resulting mixture was extracted three times with 25 ml CHzClz . The combined CH2Clz extracts were washed until neutral with saturated sodium chloride solution and dried over MgS04; then the solvent was removed, leaving 1.10 g of a light yellow solid (99% yield based on toluened,) that was identified as a mixture of dinitro-
56
toluene isomers by its i.r. spectrum. This isomer mixture was used without further purification as isotopic carrier in the isotope dilution experiments because the isomers produce a single combined peak when.field ionized. The ionization efficiencies and vapor pressures of the two isomers are so close that no significant error is introduced during the isotope dilution procedure. d,-Trinitrotoluene (d,-TNT) A nitrating solution containing 1.94 g of 30% fuming sulfuric acid (7.28 mmol of S03) and 0.46 g (7.30 mmol) of anhydrous HN03 was prepared in an oven-dried, 50 ml, three-necked, round bottom flask equipped with a mechanical stirrer, addition funnel, drying tube, and thermometer. External cooling was used to keep the temperature between 20°C and 25” C during the addition of the nitric acid to the fuming sulfuric acid. 0.46 g (2.45 mmol) dgdinitrotoluene was added and the reaction mixture was heated to 70” C for 30 minutes; 80” C for 15 minutes and 90°C for 15 minutes, then quenched on 10 g crushed ice. Organics were extracted with 3 X 20 ml CH2Clz, and the combined extracts were washed with saturated sodium chloride solution (3 X 25 ml), dried over MgS04, and the solvent was removed, leaving 0.56 g (95% yield based on d6-dinitrotoluene) of a pale yellow solid that was identified as TNT by its i.r. spectrum. Cyclomethylenetrinitramine-d, (d6-RDX) A mixture containing 3.25 g acetic acid, 0.15 g acetic anhydride, and 0.08 g (1.0 mmol) NH4N03 was prepared in a 50 ml, four-necked, round bottom flask equipped with magnetic stirrer, thermometer, and three addition funnels. 0.15 g (1.60 mmol) anhydrous HN03 and 0.08 g (1.0 mmol) NH4 NOa were mixed and added to the above at 25°C. When the mixture was thoroughly mixed, the temperature was raised to 64”C, and three solutions were added simultaneously over 6 minutes: (i) 0.42 g (2.85 mmol) hexamine-dlz, 98% atom % D in 0.66 g acetic acid, (ii) 0.85 g (13.49 mmol) anhydrous HNOs and 0.66 g (8.25 mmol) NH,NOs, and (iii) 2.29 g acetic anhydride. The reaction mixture was aged at 64°C for 40 minutes and cooled; 10 ml chilled DsO was added and the mixture was refluxed for 30 minutes. After the mixture was cooled to room temperature, filtered, and washed with three 25 ml portions of DsO, the solid was dried over KOH at 25°C and 0.1 Torr. The crude RDX (0.76 g) was shaken with 325 ml 1,2-dichloroethane, filtered, and the 1,2-dichloroethane was evaporated, giving 0.64 g of solid that was identified as RDX (99+%) by its i.r. spectrum, melting point, and HPLC. Unlabeled explosives The corresponding unlabeled explosives conditions with unlabeled reagents.
were synthesized
under identical
Purity of reagents Unlike
classic methods
of determination
of vapor pressures
of materials,
our isotope dilution method does not require a high purity of the materials tested or of the isotopic diluents used. The mass spectrometric determination of the ratio of the unlabeled to labeled material is not affected by the incidental presence of impurities with molecular weights different from the materials being tested. On the other hand, the solvents used were of highest purity available, to avoid interference by impurities that may have molecular weights identical with either the labeled or the unlabeled explosives. The solvents used to wash the reagents were in excess by many orders of magnitude so that impurities in the solvents in the ppm range might cause significant interference. Control experiments were carried out on the pure solvents evaporated to dryness to ensure their mass spectrometric purity.
Instrumentation All mass spectrometric determinations were carried out in the field ionization mode [ 11. The instrumentation used has been described elsewhere [ 1,3] . It comprises a 45” sector magnet operated in the electrostatic scanning mode. Repeated multiscanning in the mass range of interest was carried out and the data were integrated on a 4096-channel multiscaler [ 1,4] . With all the explosive materials, the observation of a molecular ion required precise adjustment of the source temperature and the use of a high ionizing field. Typically, only a small latitude was found between the minimum temperature needed to maintain the sample in the gas phase and the maximum temperature consistent with the thermal stability of the molecule. The high fields required to remove an electron from materials with a high electron affinity impose strong polarization forces, which contribute to the natural tendency of these materials to be thermally unstable. Most other organic materials reach their optimum ionization efficiency well below the fields required to ionize these polynitro materials. Consequently, background interferences, normally minimal with field ionization, became significant unless scrupulous attention was given to the cleanliness of all systems and the purity of any solvents used. The isotope dilution experiments required the analysis of minimal amounts of material (multilabeled plus unlabeled). The transfer of submicrogram amounts of explosives (100-500 ng) imposed serious constraints on the inlet system. The gas inlet system consisted primarily of glass with Teflon and Viton components in the valves. Samples were taken in 50 ml glass bulbs fitted with glass valves having Teflon stems and Viton seals. For analysis, the sample bulb and valve assembly were attached by an O-ring joint directly to the gas transfer line. The inlet system could then be preevacuated and the transfer line pushed through a ball valve into the high vacuum region to form a tight seal with the inlet end of the ionization source. Even with an essentially all-glass system, certain precautions were required
58
to maintain a high sample transport efficiency. The uniform temperature required throughout the entire inlet system was achieved by placing the sample bulb, valve, and the exposed portion of the transfer line in a single surrounding oven. The optimum temperature of the inlet system was different for each of the materials. To moderate the flow rate into the source from our gas inlet system, we used a sintered tungsten substrate version of the multipoint ionizer [l] . With this source, the sample could be admitted directly from the gas sample bulb through the heated gas transfer line into the ionizing region, and the temperature of the oven used to control the inlet pressure. Known concentration ratios of unlabeled to labeled materials were analyzed to verify that the measured ion abundance ratios corresponded to the actual concentrations and that no background interferences were present. Isotopic assay of the multilabeled explosives
The field ionization spectra of the labeled explosives were recorded to determine their isotopic purity. Such spectra are exemplified by d6-DNT (Figure l), d5-TNT (Figure 2), d6-RDX (Figure 3), and NG(0”) (Figure 4). The critical parameter in each synthetic labeled material is the concentration of impurities having the molecular weight of the unlabeled material (including traces of the unlabeled material itself). In all the materials synthesized, this
de
182 M+O
rn”NTS
0.67K
Fig. 1. Molecular ion region of de-DNT.
59
r
t ITlie
COVNTS
I 227
232
?“I+0
h4+5 65.9K
0.54K
Fig. 2. Molecular
tV,e
ion region
222
of ds-TNT.
224
226
M+O
Fig. 3. Molecular
ion region
of dh-RDX.
60 I
I
I
I
I
I
I
I
I
I
I
I
I
I
m/e
227
229
231
233
235
237
239
MI0
!A+2
IA+4
IA+6
?.I+*
MtlO
IA+12
CO”NT5
12K
1.3K
1.x
3.m
6.8K
Fig. 4. Molecular
ion region of “O-labeled
&OK
56K
NG.
concentration was less than 0.176, and in some materials like TNT and DNT it was less than 0.01%. All materials with the exception of PETN were mass spectrometically assayed in molecular ion form. In the case of PETN we ultimately selected the NO2 ion for analysis because of the highly unstable nature of PETN as a molecule or as a molecular ion. This is not surprising in view of the use of this material as a primer and detonator, which requires a high degree of instability. Even though a fragment ion was used for quantitation in vapor pressure measurements, the presence of the molecular ion was verified in all experiments (Figure 5). Dilution analysis procedures In all the determinations of concentration of explosives in air, an amount of explosive was brought into equilibrium with a large volume of air (3 liters) at 25” C. A small volume (50 ml) of this equilibrated air was rapidly transferred (by suction) into an evacuated sampling bulb that contained a known amount of the multilabeled carrier. The change in pressure in the whole system was small (<2%). The sampling bulb was valved off without delay and separated from the large vessel. It was then cooled and air was removed by evacuation. Any losses of explosive at this stage did not affect the ratio of unlabeled to labeled material in the sampling bulb. The bulb was then attached
61
Fig. 5. Field ionization
spectrum
of unlabeled
PETN.
to the inlet system of the mass spectrometer for mass spectrometric analysis, and the ratio of unlabeled to labeled material was determined. From this ratio (I?), the known initial amount of the multilabeled carrier (a), and the volume of the sampling vessel (u), the concentration (C) of the unlabeled material could be readily calculated: C = aR/v The less volatile explosives were transferred to the mass spectrometer by dissolving the explosive in the bulb in an appropriate solvent, concentrating it, and transferring the concentrate into a glass melting point capillary [4]. The residual solvent was removed under a stream of dry nitrogen, and the explosive was introduced into the mass spectrometer by using a conventional temperature-controlled solid sample probe. The sample was then evaporated into the ionization source. This procedure again did not have to be quantitative, as any losses of the explosive do not affect the measured abundance ratio. Sampling techniques for individual materials Because each explosive was handled somewhat differently for optimal recovery and transfer, we will now describe the sampling techniques for each material measured.
62
EGDN
The sample bulb was loaded with 400 ng of 1*O-EGDN in -4 ~1 of acetone. The droplet was swirled around to coat the surface of the bulb until the acetone had evaporated. The bulb was cooled to 0°C in an ice bath and evacuated without exhaustive pumping. The evacuated sample bulb was then connected to a 3-liter glass vessel, which contained about 100 mg unlabeled EGDN in acetone solution. After complete equilibration of the unlabeled EGDN with the walls and the air of the 3 liter vessel, the valve of the sampling bulb was opened to take a sample. Following the sampling procedure described above, the sample bulb was attached to the heated glass inlet line of the mass spectrometer, cooled to O”C, and evacuated. When the bulb had pumped down sufficiently, the inlet tube was pushed through the ball valve to contact the back of the source [4] . The ice bath was removed, and an oven preheated to 100°C was placed over the sample bulb, which was then warmed to -85°C over 20 minutes. During this time, the molecular ion region was repeatedly scanned at a rate of one scan per second, and the multiplier output was integrated on a 4096channel multichannel analyzer. When the count rate had fallen to the background level, the ratio of the labeled to unlabeled ion counts was calculated from the integrated output of the multichannel analyzer. DNT
The sample bulb was loaded with 20 ng of d6 -DNT in benzene. The solvent was allowed to evaporate, and the bulb was evacuated at 0°C. The sampling of unlabeled DNT from the 3 liter vessel and the analyses were carried out as described above. NG
The internal standard, 400 ng ‘so-labeled NG in acetone, was added to the sample bulb, which was cooled to 0°C and evacuated by roughing pump only. This pumping was applied intermittently until no further pressure rise occurred when the sample valve was opened to vacuum. Following evacuation, a 50 ml sample was obtained from the 3 liter vessel as described above. This entire cycle of cooling, pumping, and sampling was repeated three more times with the same bulb until 200 ml of air had been sampled. The large vessel was brought up to atmospheric pressure, and the NG was allowed to return to equilibrium each time. This procedure allowed a larger volume containing the unlabeled material to be sampled. The material that was sampled is almost quantitatively adsorbed to the walls of the sampling vessel during the cooling and evacuation, so it was not lost during the repeated samplings. After the final sample was obtained, the bulb was cooled, evacuated, and analyzed as described above. RDX
The sample bulb was loaded with 400 ng of d6 -RDX dissolved in benzene.
63
The bulb was evacuated at 25°C and attached to the 3 liter vessel containing a few mg of unlabeled RDX. The equilibration vessel with the attached sampling bulb was placed in an oven at a preselected temperature and allowed to equilibrate for 1 hour. After that time, the bulb was opened to obtain the sample. The bulb was then cooled to room temperature, and the inner surface washed with 0.5 cc of spectrograde benzene. The benzene wash was removed and then concentrated by evaporation to -20 ~1 under dry nitrogen. The final volume was evaporated to dryness in a sample capillary tube [4] . The solid probe containing the sample capillary was placed against the ion
10
4
I
I
I
AH
= 13.6
To = 840
1/T
I
I
IoK x lo?
Fig. 6. Vapor pressure us temperature for RDX.
kcallmole K
64
source and slowly heated to 50-75”C, while the molecular ion region was being scanned. For measurements on this compound, samples were taken at 74”) 84”) and 96°C (Figure 6). The vapor pressure at room temperature was obtained from the Clausius-Clapeyron equation by plotting the log of the measured vapor pressures versus the reciprocal of the absolute temperature. TNT This material was analyzed as described for RDX, by the technique of washing the sample from the bulb and introducing it with the solid probe. The sampling was carried out at 25” C with 400 ng of d, -TNT used as isotopic diluent. PETN This material was sampled at 25°C with 400 ng of 180-PETN used as isotopic diluent. The sample was washed from the sample bulb and introduced with the solid probe. The instability of this molecule caused severe field-induced fragmentation, so it was necessary to monitor the NO2 ion region (m/e 46,48, 50). RESULTS
AND DISCUSSION
Table 1 summarizes the vapor pressure measurements obtained for all the explosives. For all the data, the number of counts in the unlabeled region was sufficient to give a statistical precision of better than +lO%. This variation is due mainly to the variation in the background level between individual samples. Repeated experiments and calibration runs showed variations that did not exceed 10% of the measured volume. A comparison of our findings with previous vapor pressure data, all of which were obtained by the Knudsen effusion method [5] or modification thereof [6,7], shows significant differences. We will distinguish here between results that, although different, still agree TABLE 1 Room temperature (25O It 2’C) vapor pressures of explosive compounds obtained by the isotope dilution method Compound
Concentration (g/ml)
P (Torr)
Mol/mol
EGDN DNT NG PETN TNT RDX
2.5 1.3 3.2 1.0 4.0 8.0
2.8 1.4 2.4 5.3 3.0 6.1
37 ppm 184 ppb 31 ppb 7 ppb 4 ppb 0.8 ppb
x x x x x x
lo-’ 10-g lo-lo 10-10 10-11 10-12
x x x x x x
10-2 10-4 1O-5 10-G 10-G lo-’
65
within the same order of magnitude and those cases where much larger differences were observed. In the first group we have EGDN, DNT, and TNT. An interpolated value of 6 X low2 mm at 25°C can be calculated for EGDN from literature data [8] , and even the extrapolated value of 1 X 10-l calculated from older data [9] is just three times higher than the value we obtained. The literature values for the vapor pressure of DNT obtained in the temto 25” C, yield 3 X lo-” mm, perature range 59” to 69” C [lo] , extrapolated which is lower by a factor of 4 from our findings. A vapor pressure of 1.2 X 10m5 mm at 25°C can be extrapolated from values obtained for TNT in the temperature range 55”-76°C [lo]. A value of 1 X 10s5 mm can be extrapolated from Edwards data [ 111 obtained in the range 50”-143°C. These values are higher by a factor of 3-4 than that found by us at 25°C. The significantly large differences, which may be expected from two utterly different techniques, do not indicate any systematic error in our procedure. A comparison of data for other explosives shows even larger differences. Our value for NG is significantly lower (by a factor of about 20) than the interpolated value obtained from literature data [12]. On the other hand, the concentration in air of RDX found by us (by extrapolation form 74”-96”) is higher by a factor of 200 than that obtained by extrapolation of effusion data in a comparable temperaturerange (56”-98°C) [ 131. The largest difference was observed for PETN, where we were able to measure at 25°C a concentration of 7 ppb (corresponding to 5.3 X 10e6 mm), which is 3 orders of magnitude higher than the value of 8 X lo-’ mm obtained by extrapolation from data measured in the temperature range 97”-139°C. We have no reasonable explanation for the large differences in the latter cases. From the practical standpoint, however, our data are much more pertinent for the purpose of assessing the required sensitivity of air monitoring because our measurements have been of actual concentrations in air. Moreover, they indicate that an air monitoring system with a sensitivity of 0.1 ppb could detect any of the explosives in air under a fairly wide dynamic range of conditions. This is in contrast to estimates from older vapor pressure data, which indicate that the detection of RDX or PETN in air at room temperature would require lOO- to lOOO-fold higher sensitivities. It should be noted that the detection of concentrations of air contaminants of about 0.1 ppb is well within the state of the art at present, but attainment of much higher sensitivities is problematic, if possible at all.
ACKNOWLEDGEMENTS
This research has been carried out under the sponsorship of the U.S. Postal Service under Contract No. 74-00810. The authors wish to acknowledge Mr. R.G. Cumings of the U.S. Postal Service with whom we had many helpful discussions throughout the progress of this project.
66 REFERENCES 1 M. Anbar and W. Aberth, Field ionization mass spectrometry: a new tool for the analytical chemist, Anal. Chem., 46 (1974) 59A-64A. 2 M. Anbar and S. Guttmann, The catalytic effect of chloride ions on the isotopic oxygen exchange of nitric and bromic acids with water, J. Amer. Chem. SOC., 83 (1961) 4741-4745. 3 M. Anbar, J. McReynolds, W. Aberth and G.A. St. John, Nonradioactive multilabeled molecular tracers in biomedical research, Proc. First Intern. Conf. Stable Isotopes in Chemistry, Biology and Medicine, held at Argonne National Laboratory, III. May 9-11, 1974, AEC Publication No. CONF-730525, pp. 274-234. 4 M. Scolnick, W. Aberth and M. Anbar, An integrating multiscanning field ionization mass spectrometer, Int. J. Mass Spectrom. Ion Phys., 17 (1975) 139-146. 5 M. Knudsen, Experimental determination of the vapor pressure of mercury at 0’ and at higher temperatures, Ann. Phys., 29 (1909) 179-193. 6 T.H. Swan and E. Mack, Vapor pressures of organic crystals by an effusion method, J. Amer. Chem. Sot., 47 (1925) 2112-2116. 7 M. Davies and B. Kybett, Sublimation and vaporization heat of long chain alcohols, Trans. Faraday Sot., 61 (1965) 1608-1617. 8 Engineering Design Handbook, Explosive Series - Properties of Explosives of Military Interest. Army Materiel Command Pamphlet 706-177. 1971, p. 144. 9 W.H. Rickenbach, The properties of glycol dinitrate, Ind. Eng. Chem., 18 (1926) 1195-1197. 10 C. Lenchitz and R.W. Velicky, Vapor pressure and heat sublimation of three nitrotoluenes, J. Chem. Eng. Data, 15 (1970) 401-403. Trans. Faraday Sot., 46 11 G. Edwards, The vapour pressure of 2 : 4 : 6_trinitrotoluene, (1950) 423-427. 12 Engineering Design Handbook, Explosive Series - Properties of Explosives of Military Interest, Army Materiel Command Pamphlet 706-177, 1971, p. 233. 13 J.M. Rosen and C. Dickinson, Vapor pressure and heats of sublimation of some high melting organic explosives, J. Chem. Eng. Data, 14 (1969) 120-124. 14 G. Edwards, The vapour pressure of cycle-trimethylene-trinitramine (cyclonite) and pentaerythritol-tetranitrate, Trans. Faraday Sot., 49 (1953) 152-154.