4;
usual electrochemical significance. Values of X ( a t ) for various potentials have been published by Nicholson and Shain for the first-order case (21), and by Shuman for the second-order case (15). Cyclic voltammograms were recorded for various concentrations of Hg(II), an example being shown in Figure 2. Assuming that the reaction is secondorder, the diffusion coefficient of Hg(I1) was calculated to be 0.13 ==I 0.01 X lo+ cm2 sec-l, which is in good agreement with results obtained at the RPDE. The shape of the linear scan voltammogram (curve a) is also characteristic of the second-order process. Values for - E p ) of 0.079 V agree well with the value of 6.080 V predicted by Shuman (after correction for temperature). Finally, a plot of log C US. Ep yielded a slope of 0.048 V in agreement with the value of 0.052 V predicted for the second-order reaction. The peak potential was independent of sweep rate over the range studied (0.025-0.30 V sec-I), and the peak height was directly proportional to concentration. The shape and position of the curve obtained on reversal (curve b) supports the above contention. These observations confirm the reduction of Hg(I1) to dimeric Hg(1) to be reversible under these experimental conditions on a platinum surface.
Table 11. Verification of i - u ~Dependence / ~ for Reduction of Hg(I1) to Hg(0) at RPDE. [ A = 8.8 X 10-3 cm*, C = 1.0 mM Hg(I1)I il, PA, (measured at -0.05 V DS. Ag/Ag+) a, (rad sec-1) h/a1/2, (A rad-”%secl’t) 3.35 3.77 4.77 5.83 6.80 7.77 8.28 9.63 10.70
26.4 33.4 53.5 81.7 111 146 165 224 275
to Hg(I1). Under these conditions, Hg(1) is found to be stable in this melt as Hg22+. Using the standard potentials calculated above, we can calculate the equilibrium constant for the disproportionation
VOLTAMMETRY OF Hg(1)
Samples of Hg2(N03)2.2H20 added to the melt were rapidly and quantitatively converted to Hg(I1) under the normal experimental conditions. The presence of metallic mercury, condensed on the cooler surfaces of the cell, indicated that, as expected, the other product was Hg(0). When the experiment was performed in a sealed cell, in the presence of large amounts of metallic mercury, the composite currentvoltage curve (Figure 3) was recorded at the RPDE. The portion of the composite wave above the zero current axis corresponds to the reduction of Hg(I1) to Hg(1) and the portion below this axis corresponds to the oxidation of Hg(1)
6 . 5 2 x 10-7 6.51 6.51 6.46 6.48 6.47 6.47 6.42 6.44 Av 6.48 i.0 . 0 3 X lo-’
K=
[Hgz‘+I [Hg2+l[Hgol
The value calculated is 1.5 X lo2. This compares to K = 3.6 X lo3 obtained in the A1Cl3-KC1-NaCl eutectic (3), and the value of 1 . 1 X l o 2 calculated using aqueous standard potentials (22). RECEIVED for review April 26, 1972. Accepted July 14, 1972. (22) W. M. Latirner, “Oxidation Potentials,” 2nd ed., PrenticeHall, Englewood Cliffs, N.J., 1952.
Negative Ion Mass SpectrometryA New Analytical Method for Detection of Trinitrotoluene J. Yinonl and H. G . Boettger Jet Propulsion Laboratory, California Institute of Technology, Pa,radena, Calif. 91 I03
W. P. Weber Department of Chemistry, University of Southern California, Los Angeles, Calif. 90006 THEDETECTION of poly-nitro aromatic compounds concealed in airline baggage, such as trinitrotoluene (TNT), is an extremely relevant analytical problem, which requires a highly sensitive as well as specific method for its solution. One of the most sensitive analytical devices available for the detection of trace quantities of material is the mass spectrometer. The use of positive ion mass spectrometry is difficult because it is approximately equally sensitive to all types of volatile organic compounds, so that TNT, which is in the presence of all sorts of other organic molecules-such as those derived from perfume-must first be separated by gas chromatography and subsequently identified by its complete mass spectrum.
Negative ion mass spectrometry, on the other hand, is extremely selective in its sensitivity. With this thought in mind, we have examined the negative ion mass spectrum of TNT.
Reagent grade TNT was purchased from Eastman Kodak Co., Rochester, N.Y. The negative ion mass spectra were determined with an AEIiGEC MS9 high resolution mass spectrometer, modified for negative ionization ( I ) . The sample was admitted into the source using the direct insertion probe. Negative ion mass spectra were recorded at various
NRC Resident Research Associate, on leave of absence from the Weizrnann Institute of Science, Rehovot, Israel.
(1) J. Yinon and H. G. Boetiger, Znt. J. Muss Spectrum. Zun Phys., in press.
EXPERIMENTAL
ANALYTICAL CHEMISTRY, VOL. 44, NO. 13, NOVEMBER 1972
2235
m/e NO;
E
0
l 0
I ,
,
(
20
,
,
I
40
,
I.
60
80
100
120
140
I
1 6 0 ’ 180
200
220
240
mle Figure 1. Negative-ion mass spectra of trinitrotoluene (TNT) at 2 and 6 eV
electron energies, while the total electron emission current was regulated. An integrating experiment was carried out by introducing a known amount of T N T via the probe, as the mass spectrometer was focused at m/e 46 and the electron energy was set a t 6 eV. An oscillograph was used to record the total ion current a t m/e 46. The sensitivity in coulomb/pg is obtained by multiplying the ion current by the recording time and dividing by the amount of TNT. RESULTS AND DISCUSSION
The negative ion mass spectra were recorded as a function of electron energy. Figure 1 shows the mass spectrum of TNT a t 2 and 6 eV with the suggested ion structures. The dominant peak in the mass spectrum is m/e 46 (NO?-). At 6 eV the ion current due to the anion NO9- is 57 of the total negative ion current of TNT, while a t 2 eV it is only 18 %. These large differences in the spectra are a result of the different physical chemical processes which play a role in negative ion formation (2). The bulk of the ion current at lower energies is carried by a number of structurally significant ions which are formed by relatively simple processes ( I ) . Most of the ions are the same as those found in the positive ion spectrum of T N T and other nitro-aromatics. Thus we find the ion at mje 210 which is due to the loss of OH through interaction with the methyl group in ortho position to two of the nitro groups. N o loss of OH is observed in nitrobenzenes (3). Rearrangement reactions involving multiple loss of NO, similar to the ones reported for positive ion spectra, can account for the ions at m/e 197, 167, and 137. A metastable peak a t 170.96 supports at least the first step ( 3 , 4 ) . Simple cleavage leads to the formation of the ion at mass 181 through loss of NO?. This ion in turn loses N O to form the second most intense ion of the 2 eV spectrum a t mje 151. Similarly, the formation of the (2) Charles E. Melton, “Principles of Mass Spectrometry and Nega-
tive Ions,” Marcel Dekker, New York, N.Y., 1970. (3) C. L. Brown and W. P. Weber, J. Amer. Cliem. SOC.,92, 5775 (1970). (4) J. H. Bowie, Org. Mass Specirom., 5,945 (1971). 2236
remaining ions can be accounted for. While the 2-eV spectrum provides a considerable amount of structural information, the 6 eV spectrum with its intense peak a t m/e 46 is much more suitable for use in detection scheme for traces of TNT and other nitrated explosives which have been shown to possess an intense peak a t m/e 46 ( I , 3). Using the m/e 46 peak as a characteristic peak of TNT, a n integrating experiment was carried out in order to define the absolute sensitivity of detection of TNT by negative-ion mass spectrometry. The resulting sensitivity is 6.5 X lo-* coulomb/ pg. This figure gives the sensitivity of our MS9 mass spectrometer for TNT. It includes the amplification factor of the electron multiplier and of the amplifier, the transmission efficiency of the mass spectrometer, and the number of ionizing electrons used in the ion source. A figure of sensitivity of detection of TNT by negative ion mass spectrometry, which is independent of a specific mass spectrometer, would be the number of negative ions a t mJe 46 per pg TNT per pA electron ionizing current-at 6 eVformed in the ion source. The number obtained is lo6 ions/ (pg-PA). The available amount of TNT can be calculated from the vapor pressure of T N T at room temperature by using the Antoine equation B log P = A - _ _ _ ~ C T - 273
+
where P is the vapor pressure in mm Hg and Tis the absolute temperature. The values of the constants for T N T are ( 5 ) : A = 3.8673, B = 1259.406, C = 160. At 20 “C the vapor pressure of TNT is 7.5 X lop4 Torr. In case of equilibrium with the surrounding atmosphere, the TNT vapor would be about 1 part in lo6 of the atmospheric pressure, which is well within the range of detection of specific compounds by focusing on a unique, intense peak in the mass spectrum. This is demonstrated by the mass spectrometer leak detector. More recently, it has been reported that (5) “Handbook of Chemistry,” N. A. Lange, Ed., Tenth Edition, McGraw-Hill, New York, N.Y., 1967, p 1450.
ANALYTICAL CHEMISTRY, VOL. 44, NO. 13, NOVEMBER 1972
picogram quantities of organic substances were detected in a mass spectrometer by focusing on one characteristic peak (6, 7). Furthermore, it has been found that in the negative ionization mode, at an electron energy of 6 eV at m / e 46, there are no measurable peaks derived from other organic compounds. This is particularly true for substances which could be expected to be found in travelers’ baggage-i.e., ethyl alcohol. There is also no measurable contribution at m / e 46 due to the isotope peaks of COz or the NOz peak from air because negative ionization of C 0 2and NO? results in dissociative electron capture forming an 0-anion (8,9). A proposed TNT detector could be a small mass analyzer of high ion transmission, focused on mje 46, and adjusted for maximum sensitivity at an electron energy of 6 eV. In many (6) V. L. Talrose, G. D. Tantsyrev, and V. I. Gorshkov, J. A n d . Khim., 20,103 (1965). (7) R. Reimendal and J. Sjovall, ANAL.CHEM., 44,21 (1972). (8) R. K. Asundi, J. D. Craggs, and M. V. Kurepa, Proc. Phys. SOC.,82,967 (1963). (9) R. E. Fox, J. Chem. Phys., 32,285 (1960).
ways, it would be similar to a conventional mass spectrometer type leak detector, which is usually focused on a tracer gas such as He. The proposed TNT detector would have an atmospheric sniffing probe (viscous leak of adjustable rate) which would be brought close to or into the luggage. Thus the TNT vapor coming from a concealed explosive device would be at equilibrium with the surrounding atmosphere in the vicinity of the source of TNT. If the explosives to be detected would eventually be contained in air-tight plastic bags, it would certainly make detection more difficult. However, there still would probably be sufficient TNT vapor available for detection due to the traces left on the plastic bag from the handling of the explosives. RECEIVED for review February 2, 1972. Accepted July 28, 1972. This paper presents the results of one phase of research carried out at the Jet Propulsion Laboratory, California Institute of Technology, under Contract No. NAS 7-100, sponsored by the National Aeronautics and Space Administration.
Programmed Temperature Gas Chromatography of Boron Hydrides E. J. Sowinski Western Electric Company, Allentown, P a . 18103
I. H. Suffet’ Department of Chemistry and Encironmental Engineering and Science Program, Drexel Unicersitj,, Philadelphia, P a . 19104
BORONHYDRIDES represent an important class of boron containing compounds of significance in air pollution and industrial hygiene ( I , 2). The environmental significance of these compounds relates to their industrial use as semiconductor dopants, as parent compounds for the synthesis of of carborane polymers, and as propellants. A need exists for improved analytical methods for mixtures of boron hydrides which may occur as by-products from industrial operations. Complex mixtures of hydrides can occur particularly from applications involving elevated temperature (394). Table 1 shows the gas chromatographic techniques which have been reported for the analysis of boron hydrides; isothermal column conditions have been employed for separating boron hydride mixtures in the range BBHB-BSH~l To whom reprint requests should be sent.
EXPERIMENTAL
( 1 ) U. S. Department of Health, Education and Welfare; Public
Health Service; National Air Pollution Control Administration, “Preliminary Air Pollution Survey of Boron and its Compounds,” Raleigh, N.C., October 1969. (2) American Conference of Governmental Industrial Hygienists, “Threshold Limit Values of Airborne Contaminants,” Cincinnati, Ohio, 1971. ( 3 ) R. Baylis, H. Pressley, and C. Stafford, J . Amer. Chem. Soc.. 88, 2428 (1966).
(4) D. J. Dumin, J . Electrocheni. Soc., 116, 133 (1969).
(5-8). One of the major drawbacks of isothermal column operation is the necessity for using either two different columns or two different temperatures for the analysis of mixtures of boron hydrides in the range of B2H6 to B10H14. Each of the previously reported techniques was at a relatively high gas chromatographic detection limit of gram; whereas, present needs are for low gas chromatographic detection limits of 10-9 gram (nanogram) for air pollution and industrial hygiene studies. This paper shows the development of a programmed temperature chromatographic separation of four boron hydrides in the range B2H6-B10H14with the possible application of separating and tentatively identifying other intermediate boron hydride compounds or higher molecular weight compounds. Gas chromatographic detection limits are described at the nanogram level. Apparatus. The gas chromatographic unit was a Tracor 220 equipped with a temperature programmer, a Beckman 10( 5 ) K. Boerer, A. B. Littlewood, and C. S. Phillips, J. Inorg. N~rcl. Cliem., 15, 316 (1960). (6) J. J. Kaufman, J. E. Todd, and W. S. Koski, ANAL.CHEM., 29, 1032 (1957). (7) L. J. Kuhns, R. S. Braman, and J. E. Graham, ibid., 34, 1700 (1962). (8) L. H Hall and W. S . Koski, J. Amer. Chem. Soc., 84, 4205 (1 962).
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