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Anal. Chem. 1980, 60, 2218-2220
pected by varying the window function and by computer synthesis and matching to the experimental spectrum. Other frequency analysis methods, which give very narrow lines, such as the maximum entropy method (27-29), may also be a suitable alternate data treatment method for cases such as these.
LITERATURE CITED (1) Gross, Michael L. Hlgh ferfwmance Mass Spectrometry; ACS SympoSlUm Series 7 0 American Chemical Society: Washington, DC, 1078; p 124. (2) Rayleigh, Lord fhlbs. Mag. 1879. 8 , 261-274. (3) Fowler Transform Inkared Spectroscopy; Ferraro, J. R., Basiie, L. J., Eds.; Academic: New York. 1978. (4) Ernst, R. R.; An&rson, W. A. Rev. Scl. Instrum. 1086, 3 7 , 93-102. (5) Combrow, M. B. Anal. Chlm. Acta 1085, 178, 1-15. (6) Laude, D. A., Jr.; Johlman, D. L.; Brown, R. S.; Weil, D. A,; Wilkins, C. L. Mass Spectrom. Rev. 1086, 5 . 107-166. (7) Russell, D. H. Mass Spectrom. Rev. 1088, 5 , 167-189. (8) Marshall, A. G. Acc. Chem. Res. lB85, 18, 316-322. (9) Frelser, B. S. Talanta 1985, 32, 697-708. (10) Gross, M. L.; Rempei, D. L. Science (Washington,D . C . ) 1084, 226, 261-268. (11) Wanczek, K. P. Int. J . Mass Spectrom. Ion Processes 1984, 6 0 , 11-60. (12) Ekkers, J.; Flygare. W. H. Rev. Sci. Instrum. 1978, 47, 448-454.
(13) Lee, J. P.; Comlsarow, M. B. Appl. Spectrosc. 1087, 4 1 , 93-98. (14) Aarstol, M.; Comlsarow, M. B. I n t . J . Mass Spectrom. Ion fhys. 1987. 76. 287-297. (15) Llndon, J.' C.; Ferrlge., A. G. frog. NMR Spechosc. 1980, 1 4 , 27. (16) Comisarow, M. €3.; Lee, J. Anal. Chem. 1985, 5 7 , 464-468. (17) Marpie, S. L., Jr. Rec. I€€€ I n t . Conf. Acoust., Speech and Signal Process 1077, 74-44. (18) Comisarow, M. B. Adv. Mass Spectrom. 1080, 8 , 1698-1706. (19) Lee, J. P.; Comisarow, M. B. J . Magn. Reson. 1087, 72, 139-142. (20) Chow, K. H.; Comlsarow, M. B., unpublished work. (21) Harris, F. J. froc. I€€€ 1978, 66, 51-63. (22) Bax, A. Two Dlmenshnal Nuclear Magnetic Resonance in LiquMs; Reidel: Boston, 1982. (23) Williamson, M. P. J . Magn. Reson. 1983. 55, 471-471. (24) Ke-eier, J.; Neuhaus. D. J . Magn. Reson. 1085, 63. 454-472. (25) Comlsarow, M. B.; Melka, J. Anal. Chem. 1979, 5 1 , 2198-2203. (26) Rempei, D.L. 35th ASMS Conference on Mass Spectrometry and Allied Topics, Denver, CO, May 24-29, 1987; paper FPA 34. (27) Burg, J. P. Ph.D. Thesis, Stanford Unlverslly 1975. (28) Rahbee, A. Int. J . Mass Spectrom. Ion Processes 1986, 72, 3-13. (29) Sibisi, S. Nature (London) 1983, 301, 134-136.
RECEIVED for review October 26, 1987. Resubmitted June 8, 1988. Accepted June 28, 1988. This research was supported by the Natural Sciences and Engineering Research Council of Canada.
Discrimination between Coprecipitated and Adsorbed Lead on Individual Calcite Particles Using Laser Microprobe Mass Analysis L. C. Wouters and R. E. Van Grieken* Department of Chemistry, University of Antwerp ( U I A ) ,B-2610 Antwerp- Wilrijk, Belgium
R. W. Linton Department of Chemistry, University of North Carolina, Chapel Hill, North Carolina 27514
C. F. Bauer Department of Chemistry, University of New Hampshire, Durham, New Hampshire 03824
Laser mlcroprobe mass analysis was applled to calcite particles contalnlng elther copreclpltated or adsorbed lead, at bulk concentratlons of either ca. 100 or 1000 ppm. For this system it appeared that a dlstlnctlon could be made between the two partlde types; the lead mas peaks were significantly more promlnent for the particles with adsorbed lead, partlcularly when low laser lrradlances were used.
Laser microprobe mass analysis (LAMMA) offers definite advantages for environmental research. Its single particle analysis capability provides information on the heterogeneity of a particle population that cannot be obtained directly by bulk analysis techniques. The detection limits of LAMMA, although element-dependent, are generally much lower than those of electron-probe X-ray microanalysis. Spectra recorded in the so-called laser desorption mode (i.e. recorded at low laser irradiances without visible damage of the particle) can give indications about the surface coating on individual particles. Since particle surfaces are sites of various important phenomena, this kind of information can be valuable. However, artifacts such as preferential evaporation can produce 0003-2700/88/0360-2218$01.50/0
resulb that do not reflect the actual composition of the surface layer. Several authors previously have reported the detection by LAMMA of surface-enriched species for various model systems, e.g. polynuclear aromatic hydrocarbons coated on NaCl particles ( I ) , asbestos fibers coated with organic compounds such as benzo[a]pyrew and phthalates ( 2 , 3 )and NaCl particles with a nitrate coating ( 4 , 5). In this paper, we report on the results of an investigation on the potential of LAMMA to discriminate between lead adsorbed on the surface of carbonate particles or coprecipitated homogeneously within them.
EXPERIMENTAL SECTION Instrumentation. In the LAMMA-500 instrument (Leybold-Heraeus, Cologne, F.R.G.) a frequency quadrupled Qswitched Nd:YAG laser (1O'O W cm-2; T = 15 ns) is focused onto the sample with the aid of a collinear He-Ne laser. The laser intensity can be varied by a set of optical filters. Laser-generated ions are accelerated and collimated into the drift tube of a time-of-flight (TOF) mass spectrometer. The instrument is coupled to a 32-kbyte memory transient recorder (Lecroy TR 8818B-MM 103/8),controlled by an IBM PC-AT,which provides the data acquisition, display, and analysis. Further information 0 1988 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 60, NO. 20, OCTOBER 15, 1988
Table I. Description of the Calcite Samples ( 8 ) sample no.
sample type
bulk Pb concn, ppmw
1 2 3 4
coprecipitated Pb coprecipitated Pb adsorbed Pb adsorbed Pb
134 1210 73 1120
about the LAMMA-500 instrument, the transient recorder, and the software package used can be found in the literature (6, 7). Samples. Two seta of calcite samples with adsorbed and coprecipitated lead at similar total concentration levels were studied and compared to calcite blanks. Details about samples and sample preparation can be found elsewhere (8). Essentially, homogeneous coprecipitation was carried out by adding Ca(N0J2 and Pb(N03)2tda solution of Na2C03. Surface coating of CaC03 particles was achieved by stirring them in a Pb(NO& solution. The size of the analyzed particles was around 2 Mm. The bulk lead concentrations as determined by atomic absorption spectroscopy (AAS) are listed in Table I. Measurements (8) by secondary ion mass spectrometry and X-ray photoelectron spectroscopy have confirmed the surface adsorbed form of lead in samples 3 and 4 and its homogeneous distribution in the particles of samples 1and 2. Lead could not be detected in any of the samples by electron-probe X-ray microanalysis in the energy-dispersive detection mode. X-ray diffraction showed the crystalline form to be calcite for all samples (8). Because of the transmission geometry of the LAMMA-500 instrument, particles had to be mounted on Formvar foil coated electron microscope grids. This was done by simply bringing the grids in contact with the powder.
RESULTS AND DISCUSSION Positive ion LAMMA spectra of calcite are dominated by the Ca+ mass peaks and the typical (CaO),', (CaO),H+, and (CaO),Ca+ cluster ion peaks (9). Within the mass resolution of the LAMMA-500 instrument, the mass peak of 208Pb+,the most abundant lead isotope, coincides with that of ('Ca1e0)3MCa+. At low laser irradiances, the contribution of this cluster to the m / z 208 peak was always negligible. At high laser irradiances, the ml2 206 and 207 lead isotope peaks (their relative abundances being, respectively, 23.6% and 22.6%)can provide information about the contribution of lead to the m / z 208 peak. Analyses in the Desorption Mode. When particles with high adsorbed lead concentrations (sample 4)were analyzed in the desorption mode (Le. for laser shots with a power density of approximately 10% of the value needed for total evaporation and with a slightly defocused beam aimed at the particle surface facing the mass spectrometer),this invariably yielded spectra that were dominated entirely by mass peaks at m / z 206,207, and 208 as seen in Figure la. In some spectra, one or more of the most abundant ions typical for calcite were also seen. For the particles of sample 3, the low concentration of surface-enriched lead was still clearly detected in most spectra, as seen in Figure lb. Ion intensities in these spectra show large variabilities from shot to shot, with relative standard deviations on the absolute 208Pbsignals of 118%. The lack of precision is partially due to some differences in size and composition among individual particles but probably more to the unavoidable variations in laser intensity and focusing conditions. The latter factor will be most important since achieving reproducible focusing from shot-bshot on successive particles is not straightforward with the LAMMA-500 instrument. The two samples containing coprecipitated lead (samples 1and 2), analyzed by using the same instrumental parameters, never yielded lead peaks in the desorption mode. Spectra were identical with those of the calcite blank, shown in Figure IC. Analyses at High Laser Irradiance. Figure 2 shows average LAMMA spectra (n = 15) of samples 2 and 4, recorded
2210
I
I
45
63
84
108
135
165
198
234 ml;
t
1 45 63 84 H)B 135 165 198 234mlz Flgure 1. Average of 15 positive-ion LAMMA spectra recorded with similar instrumental parameters (input range = 500 mV), in the desorption mode for (a) sample 4 (1 120 ppmw Pb, adsorbed), (b) sample 3 (73 ppmw Pb, adsorbed), and (c) blank calcite particles.
r 1
45
63
84
108
135
165
198
-
234 mlz
I
-
45 63 84 108 135 165 198 234 mlz Flgure 2. Average of 15 positive-ion LAMMA spectra, recorded with
similar instrumental parameters (input range = 1000 mV), at &#I hser Irradiance, for (a) sample 2 (1210 ppmw Pb, coprecipltated)and (b) sample 4 (1120 ppmw Pb, adsorbed).
with the same instrumental parameters as above, except a t high laser intensity. The individual particles were completely evaporated with a single laser shot. As in the desorption mode, it was possible to distinguish between a particle population with coprecipitated (sample 2) or with adsorbed lead (sample 4) a t the same concentration level. This appears from the average V b + / % a + ratios, based on peak areas. Rather than a cluster ion, an elemental ion was used for normalization as the intensities of the latter are more reproducible. %a+ was chosen because its signal was the least disturbed by the considerable tailing of larger calcium isotope peaks. For sample 4 vs 2 with ca.1OOO ppmw (parta per million by weight) adsorbed vs coprecipitated lead, the ratios were 0.29 (with a standard deviation of 0.08) and always below 0.15, respectively. In other words, a higher useful ion yield for lead is obtained in the adsorbed sample. For sample 3 (73 ppmw of adsorbed lead) the ratio was CO.10, while for sample 1 (134 ppmw of
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Anal. Chem. 1988, 60,2220-2227
coprecipitated lead), the typical lead isotope pattern could not be detected systematically. It is thus obvious that, at least for this system, the distribution of a minor element within a particle has a significant influence on the relative intensity of its mass peaks observed at high laser irradiance. This so-called geometry effect on useful ion yield was already noticed by Bruynseels and Van Grieken (10).Studying carbon bilayers, consisting of a layer of natural carbon and a second one enriched in 13C, they found ion extraction from the side of the sample facing the mass spectrometer to be promoted. The effect seemed to be the most pronounced at laser intensities near the threshold value for sample perforation. For the lead-coated calcite particles, it is rather difficult to verify this decrease of the geometry effect with increasing laser intensity. This is due to the fact that changes in laser power density at the sample arise from variations in both laser beam intensity and focusing conditions, and, as mentioned earlier, particles are unfavorable samples for reproducible focusing.
CONCLUSION In the laser desorption mode, the signal from lead adsorbed on calcite particulates is so enhanced relative to the one of coprecipitated lead that LAMMA allows unambiguous differentiation between surface enriched and homogeneously distributed lead down to 100 ppmw or lower. Knowledge about the elemental distribution within particles is of interest, e.g. to assess their eventual environmental impact. Even when the laser intensity increases, such that the entire particle is ablated, differences between lead signals of corresponding adsorbed and coprecipitated lead particles persist.
This implies that, whenever semiquantitative analysis is considered, the distribution of the element of interest within the particle should be taken into account. Moreover, one also has to be aware of this geometry effect when performing standardization procedures using vacuum deposition of a thin film onto the sample, as applied by e.g. Schroder et al. (11).
ACKNOWLEDGMENT We thank Luc Van Vaeck for valuable discussions. LITERATURE CITED Nmssner, R.; Klockow, D.; Bruynseels, F. J.; Van Weken, R. E. Int. J . Environ. Anal. Chem. 1985. 22, 281-295. De Waele, J. K.; Vansant, E. F.; Van Espen, P. J.; Adams, F. C. Anal. Chem. 1983, 55, 671-677. De Waele, J. K.; Gijbels, J. J.; Vansant, E. F.; Adams. F. C. Anal. Chem. 1983, 55, 2255-2260. Otten, Ph. M.; Bruynseels, F. J.; Van Grieken, R. E. Bull. SOC.Chim. Be@. 1986, 95, 447-453. Bruynseels, F. J.; Van Grieken, R. E. Atmos. Envkon. 1985, 79, 1969-1970. Denoyer, E. E.; Van Grieken, R. E.; Adams, F. C.; Natusch, D. F. S. Anal. Chem. 1982. 54, 26A-41A. Van Espen, P. J.; Van Vaeck, L.; Adams, F. C. I n Thkd International Laser Microprobe Mass Spectrometry Workshop, 1986, Antwerp, Belgium, Aug 26-27, 1986. Fulghum, J. E.; Bryan, S. R.; Llnton, R. W.; Bauer, C. F.; @MIS,D. P. Environ. Sci. Technol. 1988. 22, 463-467. Bruynseels, F. J.; Van Grleken, R. E. SpeclrocMm. Acta, Parts 1983, 388,053-050. Bruynseels, F. J.; Van Grieken. R. E. Int. J . Mass Spectrom. Ion Processes 1986, 74, 161-177. Schriider, W.; Frings, D.; Stmve, H. Scannlng Electron Mlcrosc. 1980. I I , 647-654.
RECEIVED for review November 2,1987. Accepted May 16, 1988. This work was partially supported by the Belgian Ministry of Science Policy under Contract 84-89/69.
Atmospheric Sampling Glow Discharge Ionization Source for the Determination of Trace Organic Compounds in Ambient Air Scott A. McLuckey,* Gary L. Glish, Keiji G. Asano, and Barry C. Grant Analytical Chemistry Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831
A new atmospheric sampling ion source, based on the establlshment of a glow dlscharge In amblent alr drawn into a region of reduced pressure, Is described. The source is shnpie, rugged, and relatlvely maintenancefree, exhlbits a very short memory, and Is extreme& wnsltive for compounds wlth hlgh proton affinities, hlgh electron afflnitles, hlgh gas-phase acidities, and/or low Ionization potentlals. The effects of dlscharge voltage and Ion source pressure on the nature of the mass spectra observed are descrlbed. These operating parameters affect the absolute number of Ions observed and, particularly for positive ions, affect the distribution of the reagent Ions and the degree of fragmentetlon. For Illustrative purposes, the limit of detectlon and dynamlc range of the ion source coupled wlth a mass spectrometer are discussed for 2,4,6-trlnltrotoluene. For the present system, a detection hit of 1-2 parts per trMion and a Unear dynamk range of at least 6 orders of magnitude are observed.
The detection of trace quantities of organic compounds in ambient air is a commonly encountered problem often re-
quiring both high specificity and low detection limits. For example, the detection of organic explosives in ambient air requires detection limits in the parts per trillion range (or lower) along with high specificity to minimize false positives (I). The method of choice for many trace analyses of constituents present in ambient air is the combination of a sensitive ionization method with mass spectrometry. Several approaches have been taken in ionizing compounds present in ambient air. One approach is to leak air into the mass spectrometer and ionize by electron bombardment (2, 3). Air may be leaked into the system directly or through a selective membrane that passes certain organic compounds more readily than the normal constituents of air. A widely used approach is to ionize at atmospheric pressure by using either a @ emitter such as 63Ni( 4 , 5 ) or a corona discharge (6-9)as a source of ionizing electrons. Both methods are commonly referred to as atmospheric pressure ionization (API). A number of reviews are available that describe API as a sensitive means for detecting trace quantities of certain organics in air (6, 10,11). Several characteristics are usually desirable for an ion source intended for monitoring trace organics in ambient air. These include the following: low detection limits for the compounds
0003-2700/88/0360-2220$01.50/00 1988 American Chemical Society
