Anal. Chem. 1987, 59, 1911-1914
rg, which is a t the measuring limit. Values obtained for the molybdenum trioxide analyzed were near the detection limit of the method but still gave a distinct result (Table IV). The results in Table I indicate that both hydrogen as well as citrate ion concentrations decrease the value of the distribution coefficient appreciably but that the effect of hydrogen ion concentrations is more pronounced. Both citrate and especially hydrogen ion concentration therefore have to be controlled when an effective adsorption on a small column is planned. Nitric acid, when necessary for dissolution of the metal, and as used by Spano and Green (3),has to be removed as described above. Neutralization is of no avail because the neutralizing cation may compete for exchange sites even more strongly than the hydrogen ion it replaces. Table I1 shows that the presence of phosphoric acid also depresses the distribution coefficient of vanadium(1V) quite significantly. Though the effect is less pronounced than in the case of titanium ( 1 ) the presence of relatively small amounts of phosphoric acid still will mobilize the vanadium on the column, as has been shown above. By use of a considerably larger column (Figure 2) it becomes possible to accommodate moderate amounts of phosphoric acid (up to about 0.5 M).
1911
The dissolution of molybdenum trioxide in citric acid is quite slow. It takes a long time but little actual work. Considerably faster dissolution can be obtained by using phosphoric acid-hydrogen peroxide mixtures (1,3),especially when the amount of phosphoric acid is increased to 10 g. But this means that the hydrogen peroxide has to be removed completely afterward because it prevents the reduction of vanadium(V) by SOP The amount of actual working time therefore is considerably larger, and, in addition, larger ion exchange columns have to be used, as has been indicated.
Registry No. V, 7440-62-2; Mo, 7439-98-7; H,MoO,, MOOS, 1313-27-5.
7782-91-4;
LITERATURE CITED (1) (2) (3) (4) (5) (6)
Streiow, Franz W. E. Anal. Chem. 1986, 5 8 , 2408. Strelow, Franz W. E . S. Afr. J . Chem. t987, 4 0 , 1. Spano, Ernest F.; Green, Thomas E. Anal. Chem. 1966, 38, 1341. Klement, Robert ffesenius’ Z . Anal. Chem. 1952, 736, 17. Budevsky, 0.;Johnova, L. Talanta 1965, 72, 291. Streiow, Franz W. E.; Victor, AndrQ H. S . A h . J . Chem. 1975, 28, 272.
RECEIVED for review October 31,1986. Accepted March 27, 1986.
Analysis of Aerosols Using Total Reflection X-ray Spectrometry D. J. Leland, D. B. Bilbrey, and D. E. Leyden* Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523
Peter Wobrauschek and Hannes Aiginger Atominstitut der Osterreichischen Universitaten, Schuttlestrasse 115, A-1020 Vienna, Austria
Hans Puxbaum Institut fur Analytische Chemie, Technische Universitat Wien, A-1060 Vienna, Austria
Total reflectlon X-ray fluorescence (TXRF) was used to determine sulfur, chlorlne, potasslum, and calclum In atmospherlc aerosol samples. Aerosols collected In a slze fractlonatlon cascade Impact collector were dissolved In water, and the solution was placed on a quartz X-ray reflector plate. X-ray fluorescence spectra were acquired by using a prototype TXRF spectrometer. Cobalt was used as an Internal standard added to the solution after dlssolutlon of the aerosol. Detection limlts as low as 325 pg for sulfur and 60 pg for calcium were obtained. Aerosol samples between 38 and 387 pg were used to prepare the specimens. Good agreement was obtalned between sulfur calculated as sulfate obtained by TXRF and sulfate obtalned by Ion chromatographic determlnatlon. These results show that TXRF may hold significant potential as an analytkai method for the determlnatlon of major and mlnor elements in aerosol samples by using no more than a few hundred micrograms of sample.
The mass and chemical composition of urban aerosols as a function of particle size are important in the evaluation of potential health hazards and the determination of contributing sources in atmospheric pollution. Particulates in the atmo0003-2700/87/0359-1911$01.50/0
sphere generally follow a trimodal size distribution (1). The collection and size fractionation of aerosols are often done with a cascade impactor to separate particles in the range of 0.01-25-llm aerodynamic diameters. However, multielement determinations on such samples are often complicated by the small amount of mass collected during a given sampling interval. As little as a few tens or hundreds of micrograms may be available. Analytical methods such as inductively coupled plasma optical emission or atomic absorption spectrometry usually require 0.5-2.0 mL of solution, which makes the determination of minute masses difficult (2). X-ray spectrometric methods are attractive for the analysis of aerosols because, in principle, the determinations may be performed by using the specimen on the filter. However, conventional X-ray spectrometers normally lack the lower limit of detection required for these determinations. Particle induced X-ray emission (PIXE) spectrometry is a method which is used for multielement determinations of small quantities of material (3). Direct analysis of aerosols using PIXE usually requires correction for sample thickness because of practical difficulties in the assumption of an infinitely thin specimen. Sample inhomogeniety may also be a problem when PIXE is used ( 4 , 5). The purpose of this investigation was to determine the potential of total reflection X-ray fluorescence (TXRF) for 0 1987 American Chemical Society
1912
ANALYTICAL CHEMISTRY, VOL. 59, NO. 15, AUGUST 1, 1987
the characterization of the elemental content of aerosols collected with a cascade impactor. TXRF is a relatively new X-ray spectroscopic technique, which is particularly well suited for multielement determinations of limited sample quantities. The principles and instrumentation of TXRF (6, 7) allow for quantitative determinations by using microliter volumes of sample applied as a thin film to a substrate. Previous applications of TXRF have shown the method capable of application to a wide variety of environmental samples (8, 9).
400
1
I
I
I
320
Ln
240
E 0 W
: %
EXPERIMENTAL SECTION
I
160
0 _I
Instrumentation. The spectrometer used is a prototype TXRF spectrometer previously described (10). The system used a Siemens 100-kV stabilized X-ray generator to supply power to a fine-focus 2700-W Siemens X-ray tube equipped with a copper anode operated at 40 kV and 50 mA. After collimation, the X radiation from this tube irradiates an area of 0.2 X 12 mm. X radiation is detected with a Princeton Gamma Tech (Princeton, NJ) Si(Li) detector with an active area 3f 12 mm2 and a fwhm of 165 eV at 5.9 keV. A Canberra Model 2020 amplifier is used along with a Nuclear Data ND66 multichannel analyzer. A quartz reflector is located between the collimator and the specimen. The angle of the incident X-ray beam on this reflector is adjusted to cutoff energy above approximately 10 keV to minimize the intensity of the continuum above the characteristiclines of the X-ray tube. The incidence angle of the radiation on the quartz sample reflector is adjusted to 1.5 mrad to be below the angle required for total reflection. Data for lower limits of detection and all determinations were obtained by using lo00 s live time. Standards used to determine sensitivity factors were counted for 250 s live time. K a intensities were extracted from the spectra by using a modified version of SEEK (Ortec, Oak Ridge, TN). Standard solutions were prepared by using commercial standards (Merck) or from reagent grade salts. Sample Collection and Preparation. The aerosol samples used in this study were collected in Linz, Austria, during Jan 1985. Linz is a city of approximately 200000 inhabitants and contains two large industrial complexes near residential areas. Air quality in the region is regularly monitored by local authorities for SOz, NO,, CO, and total suspended particulate matter. Sampling of respirable atmospheric aerosols was conducted by using a six-stage cascade impactor, Type TU 80 ( I I ) , for periods of 5-12 h over 2 consecutive days. The impactors are low pressure types with round orifices, which are arranged on one circle concentric to the axis of each stage. The aerosol was deposited on aluminum foils in five logarithmically divided fractions in the size range of 0.04-25-pm aerodynamic diameter (a.d.). The a.d. of a particle is the diameter of a sphere with unit density that has the same terminal velocity as the given particle. The nominal air flow rate was. 5 m3 h-l. The aerosol mass was determined by weighing the aluminum foil before and after loading. The aerosol deposits were locally concentrated in a narrow ring-shaped array in the form of discrete spots. The spots (11-252 depending on the size fraction) were transferred to solution by cutting a portion of the foil and placing it into a small test tube. Two milliliters of water was added to each sample and then spiked with 20 pL of a 120 pg/L cobalt standard. The samples were then agitated in an ultrasonic bath for 30 min. A 20-pL aliquot of the solution was pipetted onto the surface of a reflector plate and the solvent removed under vacuum. The samples could not be dissolved in acid because the high concentration of dissolved aluminum salts would cause a substantial reduction in the signal to background ratio. Blank reflectors were measured to assure that the sample support was free of contamination. The determination of sulfate by ion chromatography was performed by extracting a portion of the foil with 1 mL of water by using ultrasonic agitation. A Dionex 10 ion chromatograph w a ~ used with a 250 X 4 mm Dionex anion separator column, a 50 X 4 mm, guard column filled with Chelex 100, and a 9 X 100 mm Dionex anion suppressor column. The eluent was 3 mM NaHC03/2.4 mM Na,CO, and the injection volume was 100 pL. Standardization. The sensitivity factors used for quantitative determinations are defined as the ratio of the counts per second per microgram of an element to that for cobalt, the arbitrarily selected reference element. Sensitivity factors were obtained under
80
0
I
12
15
I
I
I
I
18
21
24
27
PTOMIC NUMBER Figure 1. Plot of lower limit of detection vs. atomic number for S,K, Ca, Cr, and Co.
the s0me excitation conditions and geometry used for the analysis of th, aerosol samples. The multielement standard mixtures were prepared to avoid overlapping peaks. Three separate specimens were analyzed for each mixture.
RESULTS AND DISCUSSION TXRF has been shown to be especially useful in applications in which multielement determinations of limited sample quantity is required. Improvements in instrument design have resulted in low limit of detection (LLD) on the order of picograms for ideal specimens (12). Because the depth of penetration of the incident beam into the reflector surface is only a few nanometers, the contribution to scatter from the reflector is essentially negligible. However, the contribution to scatter from the specimen depends upon the matrix. Therefore, it is important to keep excess salts to a minimum to obtain the best lower limits of detection (12). Several definitions of LLD are used for X-ray results (13). Perhaps the most common definition is that quantity of analyte element which provides a net signal 3 times the square root of the background signal under the peak. That definition is used here. Figure 1 is a plot of LLD values obtained for 1000 s live time by using a Cu anode X-ray tube. These range from 16 pg for cobalt to 325 pg for sulfur. Because only 20 pL of solution was used, these convert to a few micrograms per liter in concentration units. Used with separation and enrichment techniques, TXRF should provide practical lower limits of detection in the range of a few parts per billion. Application of the specimen to the reflector plate as a thin film containing an internal standard and the use of sensitivity factors for the elements of interest result in a simple method for the quantitation of these elements. Because the specimen is infinitely thin by X-ray absorption criteria, no matrix or absorption/enhancement effects are encountered. Figure 2 shows a plot of the sensitivity factors obtained experimentally vs. atomic number. With cobalt used as the internal standard element and the normalization element for the sensitivity factors, the mass of each element on the reflector plate is calculated by
Mi =
Ui/ICo)
x (Mc~/K~,c~)
where M , and Mco are the mass of the analyte element and cobalt in the specimen, respectively, and I , and Ico are the intensities of the analyte element and cobalt, respectively. Ki,&is the sensitivity factor of the analyte element normalized to that of cobalt. Figure 3 shows an example of a spectrum of an aerosol sample obtained by using the TXRF spectrometer and sample
ANALYTICAL CHEMISTRY, VOL. 59, NO. 15, AUGUST 1, 1987
1913
Table I. TXRF Results for the Determination of SO4'-, C1, K,and Ca in an Aerosol Sample Collected January 17, 1985, in Linz, Austria size fraction,
stage
pm
1
a.d.
0.04-0.1 0.1-0.4 0.4-1.6 1.6-6.5 6.5-25
2 3 4 5
ambient aerosol mass, pg/m3
so42-
concentration, pg/m3 C1K+
4 20 48
0.51 2.96
0.01 0.06
7.82
0.25
15
1.60 0.17
0.48 0.95
7
mass used for extraction, kg
ea2+
0.02 0.14 0.21
0.03 0.10
38 217 387
0.04
0.48
163
0.02
0.53
72
0.01
SO4
BY TXRF [ U t M-31
Flgure 4. Plot of sulfate determined by I C vs. that calculated from the TXRF results. Symbols indicate size fraction (Fm a.d.): 0, 0.04-0.1; 0,0.1-0.4; +, 0.4-1.6; X 1.3-6.4; A *6.4-25.
CI
n I)
i W
3
i*?*ML
I
I
lm6 1.2
/
1
1
Spectrum of an aerosol sample obtained by using the TXRF spectrometer. Figure 3.
I
preparation methods described above. When the acquisition parameters established for this study were used, S, C1, K, and Ca were typically found to be present. The relative concentrations of these elements were found to be dependent upon the particulate fraction of the aerosol. The silicon peak arises from the quartz reflector plate and interferes with the determination of silicon in the aerosol samples. Table I shows the results of the analysis of aerosol samples collected on January 17,1985, during a 10-hsampling period. The amount of aerosol extracted into solution for the TXRF measurement ranged from 38 to 387 pg. The concentrations of Ca and C1 were greatest in the coarse mode particles (1.6-26 pm a.d.), whereas the concentrations of K and SO:- were greatest in the nucleation mode (0.044.1 pm a.d.) and accumulation mode (0.1-1.6 pm a.d.1 particles. The formation of coarse mode particulates is due to abrasive processes such as grinding, whereas the formation of nucleation and accumulation mode particulates is primarily a result of gas-phase condensation and aggregation of small
.B
.4 I;
I
I
1.2
1.6
2
BY TXRF (UG M-31
Figure 5. Plot of potassium determined by IC vs. TXRF results. Symbols indicate size fraction (Fm a.d.); 0, 0.04-0.1; 0,0.1-0.4; +,
0.4- 1.6.
particles. Energy dispersive X-ray fluorescence cannot distinguish the chemical form of an element such as sulfur. However, previous studies have shown the predominate form of sulfur in aerosols to be sulfate, and sulfur determined by TXRF was calculated as sulfate for comparison with data from ion chromatography (IC) (1). Figure 4 is a plot of sulfate determined by IC vs. that calculated from the TXRF results for various samples collected over a 48-h period. The data show good agreement; a linear regression fit yields a slope of 1.09 f 0.031, an intercept of -0.078 0.156, and an R = 0.989. Fewer data points were available for a comparison of potassium values obtained by IC and TXRF. The highest concentrations of potassium were found to be present in the
*
Anal. Chem. 1987, 59, 1914-1917
1914
of 0.822 f 0.041, an intercept of 0.050 f 0.036, and an R = 0.968. Initial results have shown that TXRF is a potentially valuable method for the determination of elements in aerosol particulates, especially when limited sample is available as in the case of size fractionation. The principal advantages of TXRF are the small sample size requirements, the low limits of detection, simple standardization, lack of matrix effects, and the relatively simple sample preparation. Modification of the instrumentation to optimize the excitation could lead to a method for the determination of elements of lower concentration.
4
3.2 ?
2.4
W
2 U I
+ 1.6 m
-I U
.a
ACKNOWLEDGMENT The assistance of C. Streli and V. Casensky is gratefully acknowledged. Registry No. S, 7704-34-9; C1, 7782-50-5; K, 7440-09-7;Ca, 7440-70-2.
(1
CL BY TXRF
(UG M - 3 1
LITERATURE CITED
Figure 6. Plot of chloride determined by IC vs. TXRF results. Symbols i n d i t e size fraction &m a.d.): 0, 0.04-0.1; 0,0.1-0.4; 0.4-1.6; X 1.6-6.4; A, 6.4-25.
+.
size range of 0.1-1.6 pm a.d. IC data were not available for stages 4 and 5 covering the size range of 1.6-25 pm a.d. TXRF data for stage 4 and 5 (not shown) indicate that potassium concentrations were below 0.10 pg m-3 over the sampling period. Figure 5 is a plot of potassium values obtained by IC vs. TXRF for stages 1-3. The slope of the regression line is 0.982 f 0.082 with an intercept of -0.026 f 0.046 and an R = 0.951. Figure 6 is a plot of chlorine data obtained by TXRF and IC. The data show generally higher values for TXRF compared to IC, especially prevalent are the results from stage 5 for particles in the 6.4-25 pm size range. The discrepancy between IC and TXRF was much larger for stage 5 and may be due to contamination during the storage or preparation of the specimen. Data shown for stage 5 were not used in calculating the regression line in Figure 6. The line has a slope
(1) Whitby, K. T. Atmos. Envlron. 1978, 12, 135-159. (2) Broekaert, J. A. C.; Wopenka, B. W.; Puxbaum, H. Anal Chem. 1982, 5 4 , 2174-2179. (3) Nelson, J. W. I n X-ray Fluorescence Analysis of Environmental Samples; Dzubay, T. G . , Ed.; Ann Arbor Publishers: Ann Arbor, MI, 1977; pp 19-34. (4) Johansson, T. B.; Van Greiken, R. E.; Nelson, J. W.; Winchester, J. W. Anal. Chem. 1975, 4 7 , 855-860. (5) Lannefors, H.; Carisson, L. E. X-ray Spectrom. 1983, 72, 138-147. (6) Yoneda, Y.; Horiuchi, T. Rev. Sci. Instrum. 1971, 4 2 , 1069-1070. (7) Wobrauschek, P.; Aiginger, H. Anal. Chem. 1975, 4 7 , 852-855. (8) Michaelis, W.; Knoth, J.; Prange, A.; Schwenke, H. Adv. X-Ray Anal. 1985, 28, 75-83. (9) Stossei, R.; Prange, A. Anal. Chem. 1985, 5 7 , 2880-2885. (IO) Alglnger, H.; Wobrauschek, P. Adv. X-Ray Anal. 1985, 28, 1-10, (11) Preining, 0.; Berner, A. EPA-600/2-79-105, 1979; Environmental Protection Agency, Research Triangle Park, NC. (12) Knoth, J.; Schwenke, H. Fresenlus’ 2.Anal. Chem 1980, 307, 7-9. (13) Currie, L. A. Anal. Chem. 1968, 4 0 , 568-593.
RECEIVED for review February 20, 1987. Accepted April 20, 1987. This work was supported in part by a grant from Tracor X-ray Inc. D.J.L. and D.B.B. thank the Fulbright Commission of the U S . Information Agency for funds to conduct this research.
Selective Reduction of Infrared Data Robert J. Anderegge and Dong-jin Pyo Department of Chemistry, University of Maine, Orono, Maine 04469
As gas chromatography/lnfrared spectrometry (GC/IR) becomesroutkrely available, method^ mu81 be developed to deal wlth the large amwnt of data produced. We demonstrate computer methods that quickly search through a large data file, locating those spectra that clrspfay a spectral feature of interest. Based on a modiffed library search routlne, these selective data reduction methods retrieve all or nearly all of the compounds of interest, whHe rejecting the vast majority of unrelated compounds. A greater degree of selectlvlty Is observed than wlth chemigram-type routines.
The coupling of chromatographs to various types of spectrometers has led to the development of a number of extremely powerful instrument systems for the analysis of complex mixtures. Gas chromatography/mass spectrometry (GC/MS) and, more recently, gas chromatography/infrared spectrom-
etry (GC/IR) are probably the two most widely used examples. These computerized systems are prodigious data producers, capable of generating hundreds of spectra per hour. One of the great challenges of modern analytical chemistry is to try to make sense of data as fast as instruments can spew it forth. To that end, we have been interested for some time in developing computerized methods of selective data reduction. The goal of these methods is the rapid sorting of hundreds of spectra to find the relative few which may be important enough in a given analysis to warrant further attention. For example, if a complex mixture of unknown compounds is to be analyzed, the sample might be chromatographed and the GC effluent be directed into a continuously scanning spectrometer. After 30 min of analysis, 900 spectra might have been collected and stored in a data file. Let us assume, in this particular case, that the analyst is interested in finding and identifying all of the compounds in the mixture that are chlorinated. If the GC effluent had been detected with a
0003-2700/87/0359-1914$01.50/00 1987 American Chemical Society
