2610
Anal. Chem. 1984, 56,2610-2611
can then be used to quantify and correct for the interference.
ACKNOWLEDGMENT We thank Nitram, Inc., for allowing us to collect samples at their plant. We especially thank Ed Epps of Nitram for his assistance during the testing. We also thank Jack Suggs of EPA for statistical advice. Registry No. NO,, 11104-93-1; NH3, 7664-41-7; “OB, 7697-37-2. LITERATURE CITED (1) Code of Federal Regulations, Title 40, Part 60, July 1, 1980, Appendix A, pp 347-352. (2) Reference 1, Appendlx 6,p 462. (3) “A Review of Standards of Performance for New Stationary Sources Nitric Acid Plants”; EPA Report 450/3-79-013, March 1979, pp 4-9. (4) Fed. Regist. 1983, 48, 46472-46478. (5) Code of Federal Regulations, Tltie 40, Part 60, July 1, 1982, Appendix A, pp 404-409. (6) “Methods for Chemical Anaiysls of Water and Wastes”: EPA Report 600/4-79-020, March 1979, Method 350.2. (7) “Transportable Continuous Emission Monitoring System Operational Protocol”; EPA Report 340/1-83-016, January 1983. (8) Margeson, John H.; Mltcheli, William J.; Suggs, Jack C.; Midgett, M. Rdnev J. As Pollut. Control Assoc. 1082. 32. 1210-1215. (9) “Encyclopedia of Chemical Technology”, 3rd ed.; Wiley: New York, 1978; Voi. 2, p 473.
(10) Hamii, H. F.: Thomas, R. E. “Collaborative Study of Method for the Determination of Nitrogen Oxlde Emissions from Stationary Sources”: EPA Report 65014-74-028, May 1974. (11) Margeson, John H., NH, interference in Methods 7C and 7D, March 30, 1983, unpublished information. Available from the author.
John H. Margeson* Joseph E. Knoll M. Rodney Midgett Environmental Monitoring Systems Laboratory U.S. Environmental Protection Agency Research Triangle Park, North Carolina 27711 Guy B. Oldaker, I11 Kenneth R. Loder Entropy Environmentalists, Inc. P. 0. Box 12291 Research Triangle Park, North Carolina 27709 Peter M. Grohse W. F. Gutknecht Center for Environmental Measurements Research Triangle Institute Research Triangle Park, North Carolina 27709 RECEIVED for review April 11, 1984. Accepted July 19, 1984.
AIDS FOR ANALYTICAL CHEMISTS Effect of Electron Capture Agents in Chemical Ionization Mass Spectrometry Patrick Rudewicz, Ta-min Feng, Karl Blom, and Burnaby Muneon* Department of Chemistry, University of Delaware, Newark, Delaware 19716 In chemical ionization mass spectrometry (CIMS) the extent of conversion of sample molecules to sample ions and, therefore, the sensitivity of the technique depend upon the reaction time and the rate constants for the reactions between the reactant ions and the sample molecules. We wish to report a method whereby the extent of conversion of ion/molecule reactions is increased in positive ion CIMS by the addition of small amounts of electron capture agents into the ion source. Because of its high proton affinity and the consequent weak acidity of the ammonium ion, ammonia has been widely used as a selective reagent gas in chemical ionization mass spectrometry, CIMS (1-4). Under electron bombardment at high pressures, ammonia gives the set of reactant ions, NH4(NH3),+, for which n = 0 corresponds to the ammonium ion a t m / z 18 and n = 1,2,3, etc. are the solvated ammonium ions at m / z 35,52,69, etc. The relative abundances of these reactant ions are very sensitive to the experimental parameters of the CIMS system: temperature, pressure, and path length of the ion source, as well as the field strength (or repeller voltage) within the ion source. During some of our work with ammonia as a reagent gas in GC/CIMS experiments ( 5 ) , it was observed that when certain compounds with large electron capture cross sections passed through the source of the mass spectrometer, the relative abundances of the more highly solvated reactant ions increased. The total ion current also increased when these electron capture agents passed through the source. In additional experiments with constant amounts of polar samples introduced from the probe or oven, an increase in the sample ion current, (M + NH4)+,was noted as these electron capture species were eluted from the gas chromatograph. Figure 1
shows the effects of certain compounds on the ion currents (and therefore, the relative distributions) of the reactant ions at a pressure of 0.5 torr of NH3 a t 100 OC. When CC14, iodobenzene, and nitrobenzene pass through the source of the mass spectrometer, one sees significant decreases in the ion current of NH4+and significant increases in the ion currents of the solvated ions, NH4(NH3)+and NH4(NH3),+. No effects are observed when equal amounts of toluene and bromobenzene enter the source. No (M + H)+, (M + NH4)+,or any other sample-containing ions are observed for these compounds, except for nitrobenzene, which gives (M + NH4)+at m / z 141. The change in pressure as the samples pass through the source of the mass spectrometer is less than 0.002 torr a t a total pressure 0.500 torr. The electron capture coefficients (relative to chlorobenzene = 1) for carbon tetrachloride, iodobenzene, and nitrobenzene are 7000,370, and 390 and for bromobenzene and toluene 6 and 0.003 (6, 7). We attribute these changes in the distributions of the reactant ions to changes in the ionic residence times as these electron capture agents elute through the mass spectrometer. Under the conditions of high pressure and high electron current in CIMS, the positive ions and the negative particles are not independent of each other. At high concentrations of charged species, the positive ions and the negative particles diffuse together at the ambipolar diffusion rate which is the composition average of the diffusion rates of all of the charged species (8). When the electron capture compounds enter the source of the mass spectrometer, the rapidly moving electrons are converted into much more slowly moving negative ions. Consequently, the ambipolar diffusion rate decreases, and there is an increase in the residence time of ions within the
0003-2700/84/0356-2610$01.50/0@ 1984 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 56, NO. 13, NOVEMBER 1984
2611
w CClq
/
/@/
w/o
cc14
A/
@/A
A 1 2 pg o f Testosterone
Figure 2. Effect of CCI, on NH, C I sensitivity for testosterone. Pressure: 0.5 torr, 155 "C.
TIME e
Flgure 1. Effect of equimolar quantities of CCI,, toluene, bromobenzene, iodobenzene, and nitrobenzene eluting from the GC on the relative abundances of the reactant ions In ammonia at 0.5 torr, 100 OC.
source of the mass spectrometer. This increase in ionic residence time causes an increase in the extent of conversion of the ammonium ions to the more highly solvated species and also an increase in the extent of conversion of reactant ions to sample ions. If the major loss process for positive ions is ambipolar diffusion to the source walls, then a decrease in this diffusion could account for the observed increase in total ion current. An alternative mechanism may be proposed if ion/ion or ion/electron recombination, rather than lateral diffusion, is the major ionic loss process. If the neutralization rate constants for positive ions with negative ions are significantly smaller than those for positive ions with thermal electrons, replacing electrons by negative ions from the electron capture reagents will decrease the loss of positive ions and increase the total ion current. In addition, if the rate constants for neutralizationn of the solvated ammonium ions are larger than the rate constant for neutralization of NH4+,then this reduction in the loss of positive ions will give a larger increase in the ion current for the solvated species than for NH4+but will not cause a decrease in ion current for NH4+. The interpretation of these observations is not clearcut, and additional experiments are under way in our laboratory to investigate the mechanism of this electron capture effect in more detail. The analytical utility of this phenomenon is illustrated in Figure 2. Two sets of experiments were done: one with a small amount of CC14 in the source of the mass spectrometer at 0.5 torr of NH3 and 155 "C and the other without this electron capture agent. Different amounts of testosterone, 178-hydroxy-4-androsten-3-one, were completely vaporized from a probe into the mass spectrometer. The total sample ion current of the (M+ NH4)+ ion was measured for each
experiment. As indicated in Figure 2, the integrated area for the adduct ion is greater by at least a factor of 2 when CCld was present in the source than for pure NH3. Similar experiments have been reported recently which show an even larger enhancement of sensitivity for other electron capture reagents (9). Preliminary experiments indicate that this enhancement by electron capture reagents is general, since we observed increases in sample ion current upon the addition of electron capture reagents to CHI, CH4/NH3,and i-C4HI0as CI reagent gases.
EXPERIMENTAL SECTION These experiments were done by using a Du Pont 21-492B mass spectrometer and a Varian 2740 gas chromatograph. The source pressure was measured by using a MKS Baratron capacitance manometer (MKS Instruments, Burlington, MA) connected to the source by a glass line through the probe inlet. The reagent gas pressure for these experiments was 0.5 torr (0.42 torr of NH, and 0.08 torr of He). The electron energy was 75 eV, and the emission current was 250 wA. The repeller voltage was set to zero, and the accelerating voltage was approximately 1750 V. Registry No. CC14, 56-23-5; nitrobenzene, 98-95-3; iodobenzene, 591-50-4; toluene, 108-88-3; bromobenzene, 108-86-1; testosterone, 58-22-0.
LITERATURE CITED (1) Hunt, D. F.; McEwen, C. N.; Upham, R. A. Tetrahedron Lett. 1971, 4 7 , 4539-4542. (2) Hunt, D. F. Prog. Anal. Chem. 1973, 6 , 359-376. (3) DJerassl,C.; Tecon, P.; Hlrano, Y. Org. Mass Spectrom. 1982, 77, 277-285. (4) Keough, T.; Destefano, A. J. Org. Mass Spectrom. 1981, 76, 527-533. ( 5 ) Feng, T.; Rudewicz, P.; Munson, B., paper presented at the 30th Annual Conference on Mass Spectrometry and Allied Topics, Honolulu, HI, June 6-11, 1982. (6) Lovelock, J. E. Nature (London) 1961, 789, 729-732. (7) Deveaux, P.; Gulochon, G. J . Gas Chromatogr. 1967, 5 , 341-352. (8) Siegel, M. W. Pract. Spectrosc. 1980, 3 , 297-306. (9) Foltz, R. L., paper presented at the 31st Annual Conference on Mass Spectrometry and Allled Topics, Boston, MA, May 8-13, 1983.
RECEIVED for review April 20,1984. Accepted June 18,1984. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this research. Supported in part by the National Science Foundation, CHE-8312954.
