(5) J. March, "Advanced Organic Chemistry: Reactions, Mechanisms, and Structure", McGraw-Hill, New York, 1968, p 758. (6) J. W. Loveland and G. R . Dimeler, Anal. Chem., 33, 1196 (1961). (7) R. F. Dapo and C. K . Mann, Anal. Chem., 35, 677 (1963). (8) M. Michlmayr and D. T. Sawyer, Electroanal. Chem. lnferfacial Electrochem., 23, 375 (1969). (9) P. J. Smith and C. K. Mann, J. Ora. Chem.. 34, 1821 (1969). (10) A. D. Goolsby and D. T. Sawyer, h a / . Chem., 3g, 41 1 (1967). (11) H. 0. House, E. Feug, and N. P. Peet, Jr., J. Org. Chem., 36, 2371 11971) \ .I.
(12) L. C. Gruen and P. T. McTigue, Aust. J. Chem., 17, 953 (1964).
(13) L. C. Portis, V. V. Bhat, and C. K. Mann, J. Org. Chem., 35, 2175 (1970). (14) L. C. Portis, J. C. Roberson, and C. K. Mann, Anal. Chem., 44, 294 (1972).
RECEIVEDfor review May 6,1976. Accepted June 11,1976. This work was supported by the National Science Foundation under Grant No. CHE73-05204.
Generator for Producing Trace Vapor Concentrations of 2,4,6Trinitrotoluene, 2,4-Dinitrotoluene, and Ethylene Glycol Dinitrate for Calibrating Explosives Vapor Detectors Peter A. Pella Analytical Chemistry Division, National Bureau of Standards, Washington, D.C. 20234
A vapor generator was constructed to produce known vapor concentrations of explosives such as 2,4,6-trinitrotoluene, 2,4-dinitrotoluene, 2,6-dinitrotoluene, and ethylene glycol dlnitrate below 1 ppb by volume for calibratingtrace explosives vapor detectors. The system Is temperature controlled which permits a wide range of equilibrium vapor concentrations to be generated. These vapor concentrationsare diluted by slngle-stage, dynamic, gas blending to obtaln concentrationsas low as 0.05 ppb. A quantitative gas chromatographic procedure was developed to evaluate this system by measuring the output vapor concentrations. The systematic error was usually within 15 to 20% of the values expected for TNT, and within 30% for EGDN. The applicability of the system for calibration purposes is demonstrated by performance data obtained with three commercial trace explosives vapor detectors.
Vapors of explosives such as 2,4,6-trinitrotoluene (TNT) and ethylene glycol dinitrate (EGDN) at concentration levels below 1 part per billion (ppb) by volume are required to establish the limits of detection of explosives vapor detectors. These detectors are used in a variety of law enforcement applications and are primarily designed for the detection of vapors of T N T or dynamite in air at trace concentrations. The physicochemical techniques employed for detection are discussed elsewhere ( 1 ) and include electron-capture ( Z ) , electron-capture gas chromatography ( 3 ) ,bioluminescence ( 2 , 4 ) , mass spectrometry (5),and ion-mobility (6). It is important that the generator produce known vapor concentrations from pure explosive materials for testing purposes, because some of these detectors are also sensitive to vapors from other compounds often present in an explosive material, such as dinitrotoluene (DNT) in TNT. Trace vapor generators employing dynamic dilution of vapors of T N T and EGDN have been developed by several workers for testing detector performance. Wall et al. (7) have generated vapors with a system based on the weight loss of the explosive using a thermogravimetric analyzer and measured flow rates. Liebel and Roberts (8) constructed a device for T N T vapor generation, and Dravnieks (9) has used a double-stage system for dilution of EGDN vapors at room temperature by a factor of 106. In this work, a dynamic gasblending system employing a single dilution stage was developed for producing known vapor concentrations of 2,4,61632
TNT, 2,4-DNT, 2,6-DNT, and EGDN. Similar devices have been constructed and evaluated a t the National Bureau of Standards for a number of applications (IO).An equilibrium vapor concentration of either T N T , DNT, or EGDN is generated a t a known temperature by passing nitrogen through a column containing the explosive dispersed on an inert support. As the nitrogen leaves the column, it is saturated with the explosive vapor. The equilibrium vapor is diluted with air by gas blending to provide the vapor of the desired final concentration. This vapor generator was evaluated by measuring the diluted vapor concentrations of TNT, DNT, and EGDN by a gas chromatographic method. Measurements of the equilibrium vapor concentrations of each of these materials were also made as a function of temperature. Performance data were obtained with three explosives vapor detectors of different manufacture to determine if the range of vapor concentrations generated by this system was suitable for the purpose of calibration.
EXPERIMENTAL Description of Generator. A diagram of the system is presented in Figure 1. Dry nitrogen passes through a charcoal filter and a molecular sieve (13x-40/60 mesh) trap before entering the column containing the explosive material. The column is immersed in a temperature regulated bath and was controlled to f0.05 OC. The accuracy of the controller was checked with an NBS platinum resistance thermometer and was found to be accurate to better than h0.2 "C. The equilibrium vapor from the column enters an electronic metering valve (AMs Co., Box 873, Lake Elmo, Minn. 55042) which precisely dispenses a predetermined flow of the vapor to the mixing manifold. The explosives vapor is diluted with air in the mixing manifold, and then passes to the sampling manifold where it exits the system through two ports. The flow rates of the equilibrium explosives vapor and diluent air are regulated by means of up- and downstream differential flow controllers (UFC and DFC) (Moore Products CO., Spring House, Pa.). The flow rates are measured by the calibrated flowmeters R1 and Rz. To minimize the adsorption and condensation of explosives vapors on the glass surfaces, the glass tubing a t the column exit is heated to 80-90 "C. The glass surfaces were also pretreated with dimethyl dichlorosilane to minimize adsorption of the vapors. The metering valve, mixing manifold, and sampling manifold are maintained a t 40-50 "C. Electronic Metering Valve, The flow rate of the equilibrium vapor ranged from 30 to 50 ml/min. The diluent air flow rate ranged from 1000 to 7000 ml/min, permitting a dilution of up to about 200fold. However, dilutions greater than 1000-fold are necessary to produce output EGDN concentrations of less than 0.5 ppb. This was
ANALYTICAL CHEMISTRY, VOL. 48, NO. 11, SEPTEMBER 1976
~~~
Table I. Melting Points of Explosive Materials Used Suppliers Material
Eastman Kodak
K&K Lab.
Military Grade
Literature
2,4-DNT 2,6-DNT 2,4,6-TNT
69.6-70.1 " C
69.8-70.9 "C 57.3-57.7 "C 80.2-81.2 "C
...
70-71 "C (11) 56.6 " C (12) 82 " C ( 1 1 )
... ...
8o.a-Gi.o Q C
accomplished by reducing the flow of the equilibrium vapor to the mixing manifold by means of an electronic metering valve. The valve is operated so that a preselected fraction of the equilibrium vapor from the gas flow is vented through the bleed outlet. The gas flow F1 delivered to the mixing manifold is
NF where N can be varied from 50 to 999, and F is the equilibrium vapor flow rate from the column. The expected diluted vapor concentration from the generator, C, in ppb, is given by =
C,-
Fi
Fi
hn , M O L SIEVE TRAP
F1=lOOO
c,
MOL. SIEVE TRAP
~
IM UI X
Ob
H
I
EPOATS
w s
IMANIFOLO MANIFOLOJ L- - - - -- - - GASBLENOING SYSTEM
W
Figure 1. Schematic diagram of vapor generator
+ F2
where C, is the equilibrium vapor concentration, in ppb, from the column at a constant temperature; and F2 is the flow rate of the diluent air entering the mixing manifold. The ratio F I / ( F 1 + F2) is the dilution factor. Materials. Table I lists the melting points of the T N T and DNT samples measured in this work after recrystallizing from benzene. The corresponding literature values are included for comparison. EGDN (lot No. E-B-28) was synthesized at the Trojan U.S. Powder Corporation, Allentown, Pa., on special order using reagent grade ethylene glycol and nitric acid. The material as received contained 60% EGDN and 40% reagent acetone by weight. Analysis of this material by electron-capture GC indicated that no other compounds were present. The solvents used in this work were benzene (Mallinckrodt Nanograde; or Matheson, Coleman and Bell pesticide quality), ACS reagent acetone, and spectrograde quality cyclohexane. Preparation of Columns for Explosive Materials. Three spiral-shaped glass columns (0.64-cm 0.d. by 190 cm long), the glass tubing connections, and the glass mixing and sampling manifolds were first silanized by treatment for 20 min with a 1C%solution of dimethyl dichlorosilane followed by thorough rinsing with toluene, and then drying at 110 "C. Two grams of either TNT, 2,4-DNT, or 2,6-DNT were mixed with 18 g of Chromosorb G (60/80 mesh) and 25 ml benzene. The slurry was stirred thoroughly until the explosive material dissolved, and then placed in an oven and heated to about 50 "C until nearly all the benzene was removed. The slurry was again vigorously stirred to ensure good mixing of the>materialwith the Chromosorb particles, and then returned to the oven and heated at 60-70 "C until dry. Each of the three glass columns was then filled with approximately 20 g of one of these mixtures and plugged with silanized glass wool at both ends. Because of the hazardous nature of pure EGDN, a different procedure was used to prepare the EGDN column. A spiral-shaped glass column (0.64-cm 0.d. by 110 cm long) was filled with about 3 g of Chromosorb P (hexamethyl disilazane treated, lOO/l20 mesh) and plugged with silanized glass wool a t both ends. A 4.0-ml aliquot of EGDN-acetone with an EGDN concentration of 77.2 mg/ml was added to the column. Sufficient air flow was applied to the end of the column containing the solution until the entire length of the column was wetted with the solution. Dry nitrogen was passed through the column a t room temperature for about 5 h to completely remove the acetone. It was estimated from the EGDN equilibrium concentration reported in the literature (13)and the flow rate, that the amount of EGDN removed in this process did not exceed 15 mg. Dry nitrogen was flowed through each column containing the explosive materials for several hours before any measurements of the explosives vapor concentrations were made. Analytical Procedures. Known volumes of T N T and EGDN equilibrium vapors were collected directly a t the exit of the column as shown in Figure 2 in a collection tube which contained a glass wool-charcoal adsorbent. Vapor Collection Tubes. Vapor collection tubes were made from disposable transfer pipettes. The adsorbent consisted of a glass wool plug about 3 cm long followed by 1mg of finely divided charcoal and
'I
,COLLECTION TUBE
1 mm H O L E ~ R U B B E A SEPTUM METAL UNION
COLUMN
Figure 2. Collection of equilibrium vapor concentrations (shown outside temperature bath)
a 1-cm glass wool plug as shown in Figure 3. The efficiency of the tubes was determined by collecting known volumes of equilibrium vapors of the explosives in a tube 13 cm long containing double sections of the glass wool-charcoal adsorbent. Each adsorbent section was removed and analyzed separately. Less than 2% of the total amount of the material collected was found on the second section. Additional measurements showed that the collection efficiency was essentially 100% for flow rates from 10 to 100 ml/min. The collection efficiency was also checked when these tubes were used to collect output vapor concentrations from the sampling manifold. They were found to be 99-100% efficient at flow rates up to 410 ml/min. Device for Collecting Diluted Vapors. In order to measure diluted vapors at concentrations below 1 ppb, it was necessary to sample relatively large volumes of vapor (e.g., 10-50 1.) to obtain adequate analytical sensitivity. A device for vacuum sampling large volumes of vapor, as described by Cadoff (141, was constructed for this purpose. With this system, the flow rate is adjustable and, once selected, pressure drops as high as 500 mm Hg (67 kPa) can be tolerated without any measurable change in the flow rate through the tube. To ensure reliable operation, a tight packing of the glass wool in the collection tube was avoided. A flow rate of 300 ml/min can be maintained constant at a pressure drop of 500 mm Hg or less. Variation in the flow rate was checked with a collection tube in place by monitoring the flow rate at periodic intervals for 48 h. The maximum observed deviation from 300 ml/min did not exceed +6 ml/min. Collection flow rates were calibrated with a wet-test meter positioned upstream from the collection tube. When this variable flow device was used to sample diluted vapors exiting the system, the flow rate of the diluted vapor was at least four times the collection rate.
ANALYTICAL CHEMISTRY, VOL. 48,
NO. 11,
SEPTEMBER 1976
* 1633
-,
1 cm 1.0.
Table 111. Measured Equilibrium Vapor Concentrations of EGDN with Temperature
2.6 cm
Temperature,
CHARCOAL GRANULES
OC
Concn, ppb
-33.0 -33.0 -33.0 -33.0 -30.0 -30.0 -30.0 -30.0 -18.5 -18.5 -18.5 -5.0 -5.0
148 143 156 148 234 225 228 252 1180 1040 1110 5130 5420 4930 5760 9.66 x 104 10.70 X l o 4 8.98 x 104 10.27 X l o 4 9.95 x 104
-5.0 -5.0 FLOW
25.0 25.0 25.0 25.0 25.0
Figure 3. Vapor collection tube
Table 11. Measured Equilibrium Vapor Concentrations of TNT with Temperature "C
14.0 14.0 14.0 21.0 21.0 25.0 25.0 25.0 25.0 25.3 25.3 26.5 35.0 35.0 35.0 40.0 40.0 45.0
Concn, ppb
O C
11.8
45.0 45.0 45.0 45.0 45.0 45.0 45.0
10.8
50.0
11.0
50.0 55.0 55.0 55.0 55.0 56.5 56.5 56.5 56.5 56.5
2.8 2.9 3.0 3.8 4.3 8.9
12.4 12.9 16.9 74.0 76.0 59.0 91.0 93.0 130.0
Concn, ppb
Table IV. Equilibrium Vapor Concentrations of 2,4-DNT with Temperature
152 132 142 173 182 151 155 251 269 457
Temperature O C
4.0 4.0 10.0 10.0
506 521 584 614 577 610 577
Gas Chromatographic Analysis. T N T , 2,4-DNT, and 2,6-DNT were eluted from the collection tubes with benzene. EGDN was eluted with 10-25 p1 of acetone followed by cyclohexane. These solutions were then diluted to a known volume. The acetone concentration of the EGDN-cyclohexane solutions did not exceed 2%. The concentration of explosives material in these solutions was measured by comparing the chromatographic peak areas obtained with those from . explostandard solutions using sample volumes of 1.0 to 5.0 ~ 1The sives vapor concentration, C (ppb), was calculated from the equation:
(3) where Cs = the concentration of the standard solution in nanograms (ng) per ml of solution. A E = chromatogram area of eluted sample. As = chromatogram area of standard sample. VE = volume of eluted sample, ml. V = volume of vapor collected, 1. G = molar gas volume in l./mol at 25 "C. M = molecular weight of the explosive. Standard solutions for calibration were prepared by dissolving known amounts of purified T N T and DNT in benzene. Working standards were made by serial dilution. An EGDN standard solution was prepared by dilution of a 60% EGDN, 40% acetone solution with acetone. Working EGDN standards were prepared by serial dilution with cyclohexane so that the acetone concentration in these solutions did not exceed 2%. Higher acetone concentrations were avoided because of severe tailing which interfered in the EGDN analysis. The gas chromatograph was operated isothermally, and the electron capture detector contained either a titanium tritide foil operated a t 200 "C or a scandium tritide foil a t 230-270 "C. The standing current during operation was a t least 1.3 X 10-8 A. A 1.8-m glass column (2-mm id.) packed with a 4% UCW-98 on Chromosorb WHP (100/120 1634
20.0 20.0 30.0 30.0 40.0 40.0 40.0 40.0 40.0 40.0 50.0 50.0 60.0 60.0
460
71.0
71.0
Dilution factor
Measured concn, ppb
Equilibrium concn, ppb
0.025 09 0.024 53 0.023 68 0.023 98 0.022 87 0.022 87 0.023.01 0.023 0 1 0.024 01 0.024 01 0.022 33 0.022 33 0.022 33 0.020 96 0.022 74 0.022 74 0.009 049 0.009 049 0.009 326 0.009 326
0.397 0.405 0.877 0.897 3.92 4.03 9.8 10.8 43.5 38.4 35.3 44.0 34.8 33.9 154.2 157.6 146.6 149.9 457.0 519.0
15.8 16.5 37.0 37.4 171.4 176.2 425.0 471.0 1810.0 1600.0 1580.0 1970.0 1560.0 1620.0 6780.0 6930.0 1.62 x 104 1.66 x 104 4.89 x 104 5.57 x 104
mesh)-AW-DMCS was used for measurement of T N T and DNT. The column temperature was adjusted between 140-160 OC to give retention times of 83 s for T N T , 70 s for 2,4-DNT, and 110 s for 2,6DNT. The injection port temperature was 165 "C. High purity nitrogen containing less than 0.5 ppm oxygen was used as the carrier gas a t a flow rate of 60 ml/min. For EGDN measurements, a 0.2-cm i.d. X 70-cm glass column was packed with 12%DEGS on Chromosorb WHP (100/120 mesh) and operated at 125 "C to give retention times of about 70 s.
RESULTS AND DISCUSSION An important factor to be considered in the generation of equilibrium vapor concentrations is that the nitrogen flow rate through the column must be maintained at a sufficiently low level to permit saturation with the vapor of the explosive. The effect of flow rate on reaching equilibrium conditions was determined by varying the nitrogen flow from 10 to 100 ml/ min through a column containing 2,4-DNT at 25.0 'C. The measured 2,4-DNT concentration was found to be independent of the nitrogen flow rate over this range. Measurements made with EGDN showed no differences in vapor concentrations when the nitrogen flow rate was varied from 30 t o 40
ANALYTICAL CHEMISTRY, VOL. 48, NO. 11, SEPTEMBER 1976
Table VII. Comparisons of Experimental and Calculated TNT Vapor Concentrationsa as a Function of Dilution
Table V. Equilibrium Vapor Conc.entrationsof 2,6-DNT with Temperature Temgerature C
4.0 4.0 10.0 10.0
20.0 20.0 20.0 20.0 20.0 30.0 30.0 30.0 30.0 30.0 40.0 40.0 40.0 50.0 50.0
Dilution factor
Measured concn, ppb
Equilibrium concn, ppb
0.024 24 0.024 24 0.023 1 9 0.023 1 9 0.023 87 0.023 87 0.023 00 0.023 00 0.023 00 0.009 233 0.009 233 0.009 350 0.009 350 0.023 32 0.023 48 0.023 48 0.023 48 0,009 167 0.009 167
0.825 0.812 2.45 2.44 9.43 9.89 8.6 7.8 8.5 13.2 13.5 13.8 14.4 31.1 114.0 113.0 108.0 153.0 157.0
34.0 33.5 105.6 105.3 395.0 414.0 373.0 338.0 371.0 1427.0 1 464.0 1475.0 1 542.0 1332.0 4 838.0 4 825.0 4 612.0 16 700.0 17 100.0
Table VI. Constants in the Equation log Cc = (-u,/T) and Standard Deviations
+ a,, No.
Material
a1
TNT 2,4-DNT 2,6-DNT EGDN
5175 4992 5139 3476
a0
Std dev of a ,
Std dev Std dev data of a , of residuals pts
18.43 19.20 20.11 16.67
105.0 59.4 52.3 21.5
0.336 0.193 0.175 0.083
0.091 0.056 0.035 0.030
36 20 19 20
ml/min, but a 15%decrease in the vapor concentration was noted at a nitrogen flow rate of 70 ml/min. For this reason, the nitrogen flow rates used for generating EGDN equilibrium vapor concentrations did not exceed 37 ml/min over the temperature range from -33 to +25 "C. Measured values of the equilibrium vapor concentrations of T N T generated a t temperatures from 14.0 to 56.5 "C, and EGDN generated from -33.0 to +25.0 OC are presented in Tables I1 and 111, respectively. These measurements were made directly at the column output as shown in Figure 2. A different procedure was used to obtain equilibrium vapor concentrations of 2,4-DNT and 2,6-DNT. This consisted of generating equilibrium vapor concentrations from 4.0 to 71.0 "C for 2,4-DNT, and from 4.0 to 50.0 "C for 2,6-DNT. These concentrations were then diluted in the gas-blending system and the resulting diluted concentrations were measured. The equilibrium vapor concentration was calculated from Equation 2. The results obtained are summarized in Tables IV and V. Each data set in Tables I1 through V was plotted as log C, vs. 1/T and treated by the least-squares method. The equation is -a
+
log c, = 1
a0 (4) T where C, is the equilibrium vapor concentration in ppb, a1 and a0 are the slope and intercept in this equation, respectively, and T is the absolute temperature. The values for the slopes and intercepts are presented in Table VI and can be used to compute the expected equilibrium concentrations with temperature. The equilibrium vapor concentrafions calculated by least-squares are generally in agreement with the measured values to better than f15%. The vapor pressures and the heats of sublimation of these materials were calculated by means of the Clausius-Clapeyron equation and are compared to
Dilution factor
Measured concn, ppb
Predicted concn, ppb
Relative error, %
0.043 25 0.97 0.988 -2 0.042 49 0.82 0.971 -1 6 0.023 69 0.50 0.541 -8 0.023 65 0.43 0.540 -20 0.023 07 0.50 0.527 -5 0.023 07 0.52 0.527 -1 0,009 758 0.22 0.223 -1 0.009 493 0.16 0.217 -26 0,009 341 0.18 0.213 -1 6 0.009 195 0.19 0.210 -1 0 0,009 136 0.21 0.209 0 0.004 974 0.12 0.114 +5 0.004 854 0.12 0.111 +8 0.004 845 0.084 0.111 -24 @Equilibriumvapor concentration = 22.85 ppb, calculated from Equation 4 and Table VI. Column temperature was 30 "C.
Table VIII. Comparison of Experimental and Calculated EGDN Vapor Concentrations as a Function of Dilution Dilution factor
Equilibrium Measured vapor concn, ppba :oncn, ppb
Predicted concn, ppb
21.5 21.33 0.0246 9 864 20.2 20.19 0.0233 7 864 0.0093 1 3 864 7.8 8.05 0.0284 8 235 6.1 6.69 0.0236 3 23 5 4.6 5.55 4.38 0.0050 68 864 4.2 0.0293 8 156 3.6 4.58 0.0293 8 3.6 4.58 156 0.0113 8 23 5 2.5 2.67 0.0105 9 156 1.3 1.65 0.0094 24 156 1.1 1.47 0.0090 1 0 1.2 1.41 156 0.0049 90 23 5 0.90 1.17 0.0070 80 156 0.85 1.10 0.0046 25 156 0.57 0.72 0.0023 73 156 0.24 0.37 0.0011 5 235 0.23 0.27 0.0011 5 235 0.23 0.27 0.0011 5 235 0.23 0.27 0.0011 6 156 0.12 0.18 0.0005 80 156 0.055 0.090 5 a Calculated from Equation 4 and Table VI.
Relative error, %
1 0
-3 -9 -1 7
-4 -2 1 -21 -6 -2 1 -25 -1 5 -2 3 -23 -21 -3 5 -1 5 -1 5 -1 5 -33 -39
values reported in the literature by several workers, and discussed in a separate publication (15). The concentration of explosives vapor from the generator is preselected by adjustment of the column temperature and dilution factors for each explosive material. T o demonstrate the accuracy of the system for producing a wide range of vapor concentrations, output concentrations were measured and compared to values predicted from Equation 2. An equilibrium vapor concentration of T N T generated at 30.0 "C was diluted 20- to 200-fold. The results are presented in Table VII. The measured values averaged about 10% lower than the predicted values. Similar results were obtained when EGDN concentrations were diluted 40- to 1000-fold as shown in Table VIII. However, in this case, the negative bias was about 20% on the average. These results indicate that the loss of material may be due to adsorption on the glass surfaces in the system. Applicability to T r a c e Vapor Detectors. In order to demonstrate the applicability of the generator to the calibration of commerciallyavailable detectors, performance data
ANALYTICAL CHEMISTRY, VOL. 48, NO. 11, SEPTEMBER 1976
1635
h
X
0.64 ppb
room airblank
a
0
15
€0
30
Time,
0.48 p p b
(5)
Figure 4. Response of instrument A to various concentrations of TNT
0.32 p p b
(a) blank; (b) 0.21 ppb; (c) 0.40 ppb; (d)1.0 ppb; (e) 1.3 ppb. Symbols (.)and (x) represent replicate measurements
0.16 p p b 8.40 p p b
h k L i
0.08 p p b 0.04 ppb
2.60 p p b
EGON signal Figure 6. Response of instrument C to various concentrations of EGDN
0 68 p p b
0.34 p p b
0.19 ppb
0.08 p p b blank
2.4-DNT signal Figure 5. Response of instrument B to various concentrations of 2,4DNT
were obtained with three explosives vapor detectors of different manufacture. Each instrument was used to measure one explosive material, that is, instrument A was tested with TNT, instrument B with 2,4-DNT, and instrument C with EGDN. These instruments all employ electron-capture detection, but vary in the manner in which the explosives vapor is preconcentrated. Generally speaking, an alarm is triggered at some threshold concentration and/or a recorder trace in the form of a chromatogram is provided indicating the presence of detected explosive. The results obtained are presented in 1636
ANALYTICAL CHEMISTRY, VOL. 48,
NO. 11,
Figures 4 to 6. The measurement procedure consisted of placing the sampling probe of the detector at the bottom port of the sampling manifold while the top port was vented into a fume hood. The vapor sampling rates of these instruments varied considerably from one another. This difference can be important if the detection sensitivity depends on the volume of vapor sampled by the instrument. The time-dependent response of instrument A to various T N T concentrations from 0.2 to 1.3 ppb is plotted in Figure 4. At each concentration level, a maximum reading was obtained between 40 and 60 s. If the T N T vapor sampling was continued for 2 min, the scale reading actually decreased. The alarm threshold for this instrument as received in our laboratory was preset so that the alarm was triggered when a reading slightly above 20 was reached. For instrument B, the sampling flow rate is high (e.g., 5-6 l./min) at the start of the vapor sampling cycle, but then decreases rapidly to a few ml/min at the end of the cycle. Unlike instruments A and C, the sampling cycle time was fixed in this instrument. Measurements of 2,4-DNT from 0.08 to 8.4 ppb were made using instrument B, and are shown in Figure 5. An alarm can be triggered by this instrument by setting the appropriate controls, but this feature was not used in these experiments. Data obtained with instrument C are shown in Figure 6 for EGDN concentrations from 0.04 to 0.64 ppb. This instrument operates at a constant sampling flow rate and allows the operator to select the sampling time. The data shown were taken at a sampling rate of about 1l./min for 20 s. For proper sampling, the flow rate of vapor exiting the sampling manifold must be greater than the sampling rate of the explosives detector. In these experiments, the output flow rate of the explosives vapors from the generator was 6 l./min,
SEPTEMBER 1976
which is many times greater than the vapor sampling rate of instruments A and C. For instrument B, the vapor sampling rate was approximately equal to the flow rate from the generator only for a few seconds at the start of the sampling cycle. This, however, was not considered to present a problem because of the relatively long sampling time of this instrument. It was concluded that this trace vapor generator can be used effectively to determine the explosives vapor concentration for which a detector alarm is triggered, or to determine the explosives vapor concentration for which the detector response is just recognizable. ACKNOWLEDGMENT The author thanks Edwin C. Kuehner and Stephen Cheder for their assistance, and Lorne Elias of the National Research Council, Ottawa, Canada, for his helpful suggestions. LITERATURE C I T E D (1) J. W. Harrison, "Comparative Evaluation of Trace Gas Technology", Vol. Il-Analysis and Evaluation of instrumental Methods, Contract DAAKO273-C-0128, Research Triangle Park, N.C. 27709 (1973). (2) W. A. Wall and H. M. Gage, Technical Memorandum 74-14, U.S. Army Land Warfare Laboratory, Aberdeen Proving Ground, Md. 21005 (AD-921744). (3) G. E. Spangler, Report 2089, USAMERDC, Attn: STSFB-XR, Fort Belvoir, Va. 22060. (4) W. A. Wall and H. M. Gage, Technical Memorandum 73-02, U.S. Army Land Warfare Laboratory Aberdeen Proving Ground, Md. 21005 (AD-921867).
(5)G. E. Spangler, Report 2083, USAMERDC, Fort Belvoir, Va. 22060. (6) W. A. Wall and H. M. Gage, Technical Report No. 74-13, U S . Army Land Warfare Laboratory, Aberdeen Proving Ground, Md. 21005 (AD-921743). (7) W. A. Wall, H. M. Gage, and H. T. Reiiiy, "Calibratlon of Effluvia Detectors, Techniques and Results," Technlcal Report 74-85, USALWL AD-922270L, Aberdeen Proving Ground, Md. 21005. (8) B. W. Liebel and R. M. Roberts, "Final Report of Trace Gas Acquisition System (TGAS)," Analytical Research Laboratories. Inc., Monrovia, Calif. 91016, Contra:t No. DAAK02-70-C-0644. (9) A. Dravnieks, Bomb Detection System Study," IlTRl Technical Report No. ADS-81 on FAA Contract No. FA 6 s WA-1200 (1966). (10) E. E. Hughes, W. D. Dorko, E. P. Scheide, L. C. Hall, A. L. Bellby, andJ. K. Taylor, Gas Generation Systems for the Evaluation of Gas Detecting Devices, National Bureau of Standards internal Report No. 73-292 (1973). (11) R. C. Weast and S. M. Selby, "Handbook of Chemistry and Physics", 48th ed., Chemical Rubber Co., Cleveland, Ohio, 1967-68. (12) W. C. McCrone and J. H. Andreen, Anal. Chem., 28, 1997 (1954). (13) J. D. Brandner, J. Ind. Eng. Chem. 30, 681 (1938). (14) B. Greifer, B. C. Cadoff. J. Wing, and J. K. Taylor, Development of Solid State Samplers for Work Atmospheres, National Bureau of Standards Internal Report No. 74-527 (1974). (15) To be published in J. Chem. Thermodynam.
RECEIVEDfor review May 6,1976. Accepted June 16,1976. Work supported by The National Institute of Law Enforcement, Department of Justice. Certain commercial equipment, instruments, or materials are identified in this paper in order to adequately specify the experimental procedure. In no case does such identification imply recommendation or endorsement by the National Bureau of Standards, nor does it imply that the material or equipment identified is necessarily the best available for the purpose.
Comparison of Various Structural Analyses for Pitch Fractions Yoshio Yamada," Takeshi Furuta, and Yuzo Sanada' National Research Institute for Pollution and Resources, Ka waguchi, Saitama, Japan 332
Comparison of structural parameters calculated by denslmetric, NMR, x-ray dlffraction, and computer methods has been made for solvent fractions from pitches. New structural parameters have been proposed by the combination of Brown-Ladner's NMR method and that of Diamond's x-ray diffraction. The results obtained by the combination method were in good agreement wlth those obtained by the computer method. The fact implies that the both structural analyses are available for the structural analysis of such carbonaceous materials as pitch and coal extracts. It was found, moreover, that the calculated parameters provide useful Information not only on the chemical structure but also on the solubillty of pitch.
The structural analysis of coal was first studied by Van , the basis of additivity of specific volume. Krevelen et al. ( l )on Thereafter, a number of analyses have been carried out for coal extracts, petroleum fractions, and so on, by means of many experimental techniques (2-5). Recently, Hirsch and Altgelt ( 6 ) developed a method for estimation of many structural factors of petroleum heavy ends by using a computer. This procedure was modified by Katayama et al. (7) and successfullyapplied to tar-pitches with highly condensed aromatic components and to others containing naphthene rings or aromatic components alone. The computer method for obtaining structural parameters uses four kinds of experimental values, viz., elemental analysis, molecular weight, Present address, F a c u l t y o f Engineering, H o k k a i d o University, H o k k a i d o , Japan 060.
density, and NMR data. Such parameters as aromaticity can be calculated also by Van Krevelen's and Brown-Ladner's method ( I , 2 ) , but it is impossible to evaluate the number of structural units or aromatic rings per molecule without assumptions relating to the compactness of fused rings (810). The purpose of this investigation is to calculate the structural parameters by the computer method and to compare them with the results obtained by a combination of the NMR method and x-ray analysis termed Diamond's method (11, 12). EXPERIMENTAL F o u r k i n d s o f coal t a r - a n d petroleum-pitches were used in t h i s study. T h e samples were extracted w i t h benzene in a Soxhlet apparatus, a n d t h e n t h e extracts were separated i n t o t w o fractions o w i n g t o solubility in e t h y l ether; t h e soluble fraction is referred t o as A, a n d t h e insoluble fraction as B. T h e results o f elemental analysis for these fractions are indicated in Table I, a n d molecular weight, density, a n d hydrogen d i s t r i b u t i o n from NMR d a t a are summarized in T a b l e 11. T h e experimental procedure for these d a t a i s described in d e t a i l elsewhere ( 1 3 ) .
RESULTS AND DISCUSSION Definition of Terms. The following terms are used in the subsequent calculations. FC = fraction of aromatic carbon in fused rings per molecule. 4 = ring compactness factor described by Hirsch et al. (6). FB = ratio of attachments of aliphatic chains to peripheral ring carbons.
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