X-Ray Spectrometry for Particulate Air PollutionA Quantitative Comparison of Techniques J. V. Gilfrich, P. G . Burkhalter, and L. S. Birks Naval Research Laboratory, Washington. D. C. 20375
X-Ray fluorescence spectrometry is well suited to the determination of elemental composition of air pollution particulate samples because no sample preparation is required for filter collections, 1 00-second detection limits are 1 to 100 ng/cmz for most elements of interest, the technique is nondestructive, and 10 or 2 0 elements can be measured simultaneously using presently available commercial equipment. The best detection limits are achieved by reducing the background primary radiation scattered into the measuring system by the sample and its filter substrate. In this work, comparisons were made among various excitation sources, X-ray tubes, fluorescers, radioisotopes and 5-MeV protons and alpha particles, and between wavelength dispersion using a crystal spectrometer and energy dispersion using a Si(Li) solid state detector. Incinerator and other actual samples are analyzed to show application of the method.
The elemental analysis of air pollution particulate samples is a unique problem. The total amount of material is ordinarily small but the sample may contain a large number of elements over a broad atomic number range and a t widely different concentrations. An effective analytical scheme must measure most of the elements of interest; it must have good detectability in order to provide reliable results for those elements present a t low concentrations even in the presence of large amounts of other elements. The objective of the work reported here is to compare all of the X-ray spectrochemical techniques except those using electron excitation, under as nearly the same conditions as possible. The results should provide a suitable basis for choosing the most appropriate X-ray method for analyzing many samples in a routine manner. There are three instrumental techniques other than X-ray fluorescence spectrometry which have been used to varying degrees for the analysis of particles filtered from the air. A) Optical emission spectroscopy has been used for about fifteen years in federal monitoring activities and has been applied t o the measurement of seventeen elements ( 1 ) . This technique is still only semiquantitative and has rather poor detection limits for some elements (2). B) Atomic absorption is a much more quantitative technique and is very sensitive for some elements. I t is. however, limited to those elements for which hollow-cathode lamps are available (although the tunable laser may show some promise for other elements). More severe limitations are the measurement of only one element at a time and the requirement that the sample be in solution. Because much air pollution is the result of high temperature combustion, experience shows that the particulate matter may be quite refractory and difficult to dissolve for (1) G. B . Morgan. G Ozolius, and E. C. Tabor. Science. 170, 289 ( 1970). (2) A. P. Altshuller, "Analytical Problems in Air Pollution Control," in Analytical Chemistry: Key to Progress In National Problems, Nat. Bur. Stand. f U . S . )Spec. P u b / . . 351, 266 (1972).
2002
atomic absorption analysis. C) The use of neutron activation analysis has also received considerable attention recently. It has the advantage of being nondestructive but is slow and suffers from widely different detection limits among the elements of interest. In samples containing many elements, it is necessary to perform radiochemical separations (no longer nondestructive) or to count a t intervals over an extended period up to a month ( 3 ) in order to avoid interferences. X-Ray fluorescence spectrometry appears most attractive for the analysis of air pollution particulate matter for several reasons. 1) No specimen preparation is required for filter collections; the material on the filter is analyzed directly. 2 ) Detectability is fairly uniform across the periodic table and all elements from atomic number 11 (Na) upward can be analyzed. 3) The X-ray technique is nondestructive and the sample can be retained for further analysis or as legal evidence. 4) Ten or twenty elements can be analyzed simultaneously with presently available commercial equipment for a cost of about a dollar and a half per sample, as estimated in Table I. X-Ray measurements of air pollution samples have been reported in the literature but those references were limited in scope to the use of either wavelength dispersion or energy dispersion and further limited in the choice of excitation sources evaluated. Goulding and Jaklevic ( 4 ) have examined the capabilities of energy dispersion using their most improved guard-ring Si (Li) detector with radioisotopes or low powered X-ray tube and fluorescer excitation. Bowman et al ( 5 ) used a radioisotope-fluorescer assembly with a solid state detector to examine the lead and bromine concentration in Berkeley, Calif., over a sevenyear period. Dittrich and Cothern (6) also used radioisotopes and a solid state detector to analyze pollution particulate samples. Johansson et al (7) made energy dispersion measurements using proton excitation in the MeV range. as did Watson e t al (8) using 50-MeV alpha particles. Cooper (9) has compared energy dispersive particle and photon excited sensitivities using protons at 2 and 4 MeV, alpha particles a t 30 and 80 MeV. a Mo transmission target X-ray tube and radioisotopes (55Fe and losCd). Leroux and Mahmud (10) and Greenfelt et al ( 2 1 ) used
R. Dams, J. A. Robbins, A. K . Rahn. and J. W. Winchester, Anal. Chem.. 42, 861 (1970). F . S. Goulding and J . M. Jaklevic, Lawrence Berkeley Laboratory. UCRL-20625, May 1971 H . R. Bowman. J . G Conway, and F . Asaro, Environ. Scr. Techno/.. 6 . 558 (1972). T. R . Dittrich and C. R . Cothern, J . Air Pollut. Confr. Ass.. 21, 716 (1971). T. 6.Johansson, R. Akselsson, and S. A. E. Johansson, Lund Institute of Technology, LUNP 7109. August 1971 R . L. Watson. J . R. Sjurseth. and R . W. Howard, N u c i . instrum Methods. 93. 69 (1971) J. A. Cooper, Nucl. Instrum. Methods. 106, 525 (1973). J . Leroux and M. Mahmud. J . Air Poilut Contr. Ass.. 20, 402 (1970) P. Greenfelt, A. Akerstrom, and C. Grosset. Atmos. Environ.. 5 , 1 (1971).
ANALYTICAL CHEMISTRY, VOL. 45, NO. 12, OCTOBER 1973
Table I . Estimate of Cost for Large Scale Analysis Using Multiple Crystal Spectrometer I nstrumenta TECTOR
Eq uipment 1. Original Instrument $100,000, amortized over 5 years 2. Replacement X-ray tube 3. Service and maintenance
$20,000 2,000 5,000 Subtotal
Labor (including overhead) 1. Technician 2. Supervisor
LUCITE SAMPLE HOLDER CRYSTAL SPECTROMETER
$27,000
Figure 1. Schematic of crystal spectrometer
$12,000 25,000 Subtotal
Space Equipment and Office
$37,000 $ 2,000
Total
$66,00O/year
$66,00O/year
= $1.47 per sample. 45,000 samples/year a Assume analysis time of 6 hours per day, 250 days per year, measuring one sample every 2 minutes (45000 samples per year).
W
Table II. Instrumental Parameters for Wavelength Dispersion Collimation Crystals
Detector Pulse height analyzer X-Ray tubes
X-Ray tube windows X-Ray tube power
0.125pm X 10 c m (0.072" divergence) LiF (200) Graphite KAP (Potassium Acid Phthalate) Gas flow proportional counter (90% Ar-10% CH4) FWHM of pulse amplitude distribution for X-ray line being measured W target Machlett SEG-SOH Rh target Machlett SEG-50H Cr target Philips FAQ 60/1 W and Rh targets, 0.1 25 m m Be Cr target, 0.3 mm Be 900 Watts [45 kV (c.P.), 20 mA]
Table III. Experimental Conditions for Energy Dispersion Radioisotopes
'
Fluorescers Fluorescer excitation X-Ray tubes used directly
X-Ray tube power
7 mCi 55Fe 70 mCi 09Cd Mn, Cu, Ag, and Cr-Zr W target Machlett OEG-50 X-Ray tube (45 kV, 20 mA) Mo target Machlett OEG-50 W target Machlet OEG-50 (with and without Ni primary filter) 150 Watts [50 kV (c.P.),3 m A ] (1'/2-mm diameter aperture between tube and sample, 3 c m from focal spot)
c r y s t a l spectrometers w i t h X - r a y t u b e e x c i t a t i o n t o m e a sure p o l l u t i o n p a r t i c u l a t e samples, as did L u k e e t al. (12).
EXPERIMENTAL Wavelength Dispersion. T h e equipment used for the wavelength dispersion measurements consisted of a standard Philips Universal Vacuum Spectrometer coupled t o Hamner counting electronics. T h e spectrometer geometry is shown schematically in Figure 1 which also illustrates t h e special Lucite sample holders having a n internal diameter large enough so t h a t , with proper alignment of the X-ray tube, the primary X-ray beam did not strike t h e sample holder in a n area which could be viewed by the collimator. Thus, the only mass which can scatter primary radia( 1 2 ) C. L. Luke, T. Y . Kometani. J. E. Kessler, T . C . Loomis, J. L. Bove, and B. Nathanson, Environ. Sci. Techno/., 6, 1105 (1972).
Figure 2. Schematic of energy dispersion chamber, shown for fluorescer excitation. A radioisotope or X-ray tube can be placed at the fluorescer position
-
A IONS
u \2MpLE BEAM --c
DUMP
COLLIMATOR
DETECTOR
Figure 3. Schematic of Van de Graaff chamber
tion directly into the spectrometer is the sample and its substrate. T h e particulate matter on its filter was sandwiched between two layers of 4-pm Mylar and held in place on the sample holders by a n "O-ring." T h e substrates used, W h a t m a n filter paper a n d Millipore filter material, have mass thicknesses of 10 and 5 mg/cm2, respectively. The two layers of Mylar added only about 1 mg/cm2. T h e spectrometer was always evacuated to eliminate the air scattering contribution t o background. Other instrumental parameters are listed in Table 11. E n e r g y Dispersion. T h e energy dispersion measurements used a Nuclear Diodes (now Edax International) solid state Si(Li) detector having energy resolution of about 280 eV a t 5.9 keV and 1000 Hz. A special vacuum chamber, as shown in Figure 2 . was constructed to eliminate air scattering and yet permit close coupling between t h e source. sample, and detector. The offset geometry prevented t h e detector from viewing any part of the chamber wall which was illuminated by the source. T h e chamber was a long cylinder which permitted any of several standards or s a m ples t o be translated into t h e analysis position. Various types of excitation sources (radioisotopes, fluorescers, or X-ray tubes) could be placed outside the window. Table I11 lists t h e experimental conditions for these measurements. Ion excitation using 5 MeV protons and alpha particles was carried out with the cooperation of t h e Van de Graaff Branch a t the S a v a l Research Laboratory ( S R L ) . Figure 3 is a schematic diagram of the Van de Graaff scattering chamber as set up for X-ray analysis. T h e same solid state detector was used as for the other energy dispersion measurements. T h e ion beam was collimated so t h a t t h e sample holder was not illuminated a n d the detector was shielded from radiation originating a t t h e beam d u m p . S t a n d a r d s . T h e air pollution particulates as collected on filter paper constitute a sample which is thin t o X-rays. I t is t o be expected t h a t such samples will exhibit no matrix effect. Standards were prepared by depositing a small measured volume of a known
ANALYTICAL CHEMISTRY, VOL. 45, NO. 12, OCTOBER 1973
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concentration solution of a salt of the element onto disks of Whatman filter paper. The disks were large enough so t h a t the solution did not spread t o t h e edges; the area t o which the solution spread was measured and the filter paper allowed to dry slowly. Uniformity of deposition was evaluated by cutting one such sample into ten equal size pieces and measuring the X-ray intensity from each. T h e relative standard deviation for these ten measurements was 4% indicating adequate uniformity. This technique produced usable standards for elements whose characteristic X-rays had wavelengths shorter t h a n about 2 [Kcu lines for elements above atomic number 25 ( M n ) and La lines for elements above atomic number 64 (Gd)]. Calibration curves were straight lines up to a few hundred gg/cm2, t h e maximum concentration normally expected in the pollution samples. For the longer wavelength radiation, the X-ray absorption in the filter paper becomes significant for elements deposited throughout the thickness of the filter. T h e air pollution particulate material is deposited mostly on the surface of the filter a n d therefore long wavelength radiation is less absorbed than for soluble salt standards on filter paper as described above. A correction was made for the filter absorption (13) and tested by comparing X-ray intensities from the soluble salt standards with some a n a lyzed. light-element insoluble salt standards prepared by vacuum filtering a suspension of insoluble salts onto Millipore filters. Table IV shows this comparison for four light elements. T h e filter materials used to collect pollution samples or prepare standards must be as pure as possible, of course. Historically, air pollution particulate matter has been collected on glass fiber filters because these are very strong and are capable of withstanding rather severe collection conditions such as might be encountered in sampling the high temperature effluent from a n incinerator or power plant. Unfortunately, t h e high impurity level in these glass fibers makes them undesirable. I t has been demonstrated (11)that Whatman filter paper has fewer impurities t h a n any other material readily available and t h a t certain types of Millipore filter material are also very good. These two filters were used for preparation of standards and sample collection for the work reported here.
pending on the impurity level in the substrate, the magnitude of the primary radiation scattered by the sample and its substrate, and stray radiation originating as scattering or fluorescence from various parts of the equipment. The detection limits are shown in Table VI and discussed below. Air Pollution Samples. During the course of this work, numerous samples were analyzed for as many as 17 elements per sample. These samples were collected by EPA in their usual monitoring activities but mostly on Millipore filters in order to minimize the substrate impurity problem. Among these specimens were included automotive emissions and samples from the stacks of incinerators, power plants, and a cement plant. Because of the high temperature of automotive exhaust, glass fiber filters were required for the particulates from a vehicle operated through a standard driving cycle. A preliminary experiment on such a sample showed 1.2 kg Fe/cm2 and 73 pg Pb/cm2. As examples of some of the samples analyzed for a large number of elements, Table VI1 lists the analytical results by three different types of X-ray analysis for the particulate emission from several types of fixed sources. The X-ray techniques used were wavelength dispersion with a crystal spectrometer (Column A), and energy dispersion with a solid-state detector and both lo9Cd radioisotope (Column B ) and X-ray tube (Column C) excitation. Some of these samples were also analyzed by EPA for Fe, Ni, Cu, Zn, As, Se. Cd, and P b using atomic absorption spectrometry; those results are also shown in the table (Column D).
RESULTS
DISCUSSION Sensitivity. The data shown in Table V are presented in two ways: 1) as measured in the laboratory with available equipment, and 2) as extrapolated from the measurements to values which should be attainable with state-ofthe-art commercial equipment assuming identical geometry. The extrapolation is considerably different for the various types of X-ray analysis. For the wavelength dispersion measurements, the data are extrapolated from the 900 watts X-ray tube power used to the 2500 watts available with present-day high powered X-ray spectrographic tubes. The sensitivity thus improves by the factor 2500,’ 900. Energy dispersion measurements were also extrapolated to the maximum tube power of 2500 watts for the fluorescers or to the maximum counting rate of 10000 c/s, a limitation imposed by detector resolution and dead-time considerations. For radioisotope excitation, the limiting criterion is the maximum activity practical in a compact sealed source, in practice about 150 mCi. The ion-excited sensitivities tabulated have been extrapolated to 50 p C using the rationale that a beam current ten times greater than used in these measurements could be tolerated. In order to compare sensitivities for the various techniques most realistically, the extrapolated values should be used because they reflect present-day capability. As shown in Table V, the sensitivities for the various X-ray techniques generally vary smoothly with atomic number except where we change from K to L lines. For X-ray tube excitation. there is enhanced sensitivity for elements which are efficiently excited by the characteristic radiation from the tube target. This can be seen in Table V for the crvstal mectrometer measurements of K and Ca with the Cr”tube, S with the Rh tube, and Fe with the w tube. For the energy dispersion measurements, there is increased sensitivity for Se with the Mo tube and
The results reported herein are divided into two general categories: 1) Measurement of sensitivities and detection limits for the various X-ray techniques; and 2) quantitative analysis of elements in real samples provided by the Environmental Protection Agency (EPA). The most important goal was to compare the various X-ray techniques in terms of sensitivity and limit of detection for elements of interest. By sensitivity we mean the slope of the X-ray intensity us. concentration curve (count per microgram per square centimeter); it depends upon excitation-source strength, spectrometer and/or detector geometry: and detector efficiency. For the detection limit, we use the International Union of Pure and Applied Chemistry (IUPAC) definition “signal above background equal to three times the standard deviation of the background for a given unit of time” ( 2 5 ) . Sensitivity, Sensitivity was determined from the calibration standards. The X-ray intensities were measured for three concentrations of each element which allowed construction of a calibration curve. Table V lists the sensitivities for a selection of elements of interest measured by the various X-ray techniques. Detection Limits. The detection limit usually improves with sensitivity as might be expected. However, the additional dependence of the detection limit on the background intensity makes the detectability fluctuate somewhat. The background varies from element t o element de(13) J w. Criss. Naval Research Laboratory, private communication. 1972. 1141 R . Dams, K . A . Rahn, and J W Winchester. Environ Sci Tech, , n o / . . 6, 441 (1972). (15) IUPAC Commission V . 4 on Spectrochemical and other Optical Procedures of Analysis, Appendices on Tentative Nomenclature, Symbols. Units and Standards, No. 26, November 1972. p 17.
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ANALYTICAL CHEMISTRY, VOL. 45, NO. 12, OCTOBER 1973
again for Fe with the W tube. If the characteristic lines are filtered from the X-ray tube spectrum, the sensitivity becomes more or less uniform over a fairly wide atomic number range but is somewhat degraded for the lower atomic numbers because the long wavelength continuum is also filtered out of the spectrum. This is illustrated by the data for the energy dispersions measurements using the W tube with a Ni filter. Radioisotopes and fluorescers are essentially monoenergetic sources; they are efficient in exciting neighboring elements but become less and less efficient for elements of lower atomic number. The sensitivity for the fluorescer excitation is about 100 times as high as for the radioisotopes (comparing the Ag fluorescer with lo9Cd and the Mn fluorescer with 55Fe) illustrating the effect of the higher intensity available from the fluorescers excited by X-ray tubes. Excitation with 5 MeV protons or alpha particles is particularly effective for low atomic numbers but less efficient as the absorption edge energy increases; the loss in efficiency is more pronounced with alpha particles than with protons. Detection Limits. The detection limit data in Table VI, like the sensitivities in Table V, are shown both as measured and as extrapolated to higher power, higher activity, or higher count rate where appropriate. In addition, the energy dispersion measurements have had their detection limits improved for the effect of the better energy resolution of a state-of-the-art solid-state detector. For this additional extrapolation, we have assumed a detector capable of maintaining 165 eV resolution a t 5.9 keV up to 10000 c/s. The background is therefore multiplied by the factor (ideal resolution)/(measured resolution); a t 5.9 keV photon energy and 10000 c j s this would be 1651500. The detection limit is improved by the square root of this factor. It must be emphasized that these are single element detection limits determined on standards containing no interfering elements. If the background is increased because of other elements present in the sample, the detection limit will degrade by the square root of the increase in the background. Since the magnitude of neighboring element interference is a function of the resolution of their characteristic lines, the problem is more serious for energy dispersion than for wavelength dispersion. With the exception of ion excitation, the detection limits by energy dispersion are comparable with wavelength dispersion only for direct X-ray tube excitation. Ion excitation constitutes a special case because different standards were used. The filter paper standards described previously could not be used in the Van de Graaff because they would not withstand the ion beam current of about 50 nA on 1 to 2 mm2. Special standards were prepared by evaporating KBr, Cu, and Au onto thin (10 to 20 ,ug/cm2) carbon or nitrocellulose films and Mylar simultaneously and calibrating in the X-ray spectrometer by comparison with the filter paper standards. Air Pollution Samples. The results on the real samples shown in Table VI1 point out some important facets in the comparison of wavelength dispersion with energy dispersion. The most obvious observation is the fact that the major elements can be measured by either of the techniques using any of the sources. Intermediate concentrations can be analyzed by either energy dispersion or wavelength dispersion providing X-ray tube excitation is used. Elements present a t the lowest detectable concentrations can only be measured using wavelength dispersion. This last observation might seem to be a t odds with the comparable single element detection limits demonstrated for the two methods of data acquisition. However. the results
Table I V . Comparison of Sensitivities for Soluble-Salt Standards, after Correction for filter Paper Absorption with Sensitivities for Insoluble-Salt Standards Sensitivities [(c/100 s e c ) / ( @ g / c m * ) ]
Calculated correction factor
Element
31 20
Mg AI Ca V
2.2
1.4
Insolublesalt standards
Soluble-salt standards Measured
Corrected
1.5 8.0 730 1300
46 160 1600 1800
Measured
50 120
1400 1600
Particle
Si
0.2
si'o, in 5pm
\
1
\
0 0.01 0.I I.o PARTICLE SIZE X LINEAR ABSORPTION COEFFICIENT
10
Figure 4. Effect of particle size on X-ray intensity
illustrate the effect on the detection limit caused by the presence of interfering elements a t widely different concentrations. Admittedly, computer stripping of the spectra might have succeeded in identifying one or more of the missing elements but the stripping technique estimates low concentrations from the small difference between two large numbers and is a t best semiquantitative analysis and suspect even for that purpose when operating near the detection limit. Atomic absorption results for some of the samples in Table VI show significantly lower concentrations than the X-ray measurements. This is consistent with our earlier argument that the pollution particles may be quite refractory and difficult (or impossible) to dissolve for atomic absorption analysis. I t should be mentioned that although measurements were made for As and Se, the volatility of the oxides of these elements (SeOl sublimes a t 315 "C and As203 sublimes a t 193 "C) makes it unlikely that much will be collected on the filter. Particle Size Effect. The total amount of material collected for X-ray analysis (up to -1 mg/cm2) does not show the usual interelement effects encountered in bulk specimens. However, there will be a significant size effect for particles larger than about five micrometers. Figure 4 shows a calculated (16) general particle size curve. For the 5-pm particles indicated, the intensity would be reduced to 60 to 80% of its value compared with the same mass concentration of smaller particles. Although it is possible to calculate the particle size effect on X-ray intensity as shown in this figure, the application of this technique to real samples requires that the particle size or its distribution be known. For size fractionated collections, this is straightforward; but for general air pollution particulate samples, the particle size distributions of the various con(16) J W Criss, Naval Research Laboratory unpublished data
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Table V. Sensitivities for Various X-Ray Techniques, (c/lOO sec)/(pg/cm') V Fe Cu K Ca AI S
Zn
Se
Br
Zr
Au
Pb
WAVELENGTH DISPERSION
X-Ray tube excitation As measured (900 watts) Cr tube Rh tube W tube Extrapolated to 2500 watts Cr tube Rh tube W tube
78 400
687 3970 290
217 1110
1910 11000 806
5620 1430
1840 1610 1540
440 2100 2420
328 1580 1700
361 1430
48 450 600
109 460 400
58 300
156000 3970
5110 4470 4280
1220 5830 6720
911 4390 4720
1000 3970
133 1250 1670
303 1280 1110
161 833
24 1.8
7P 3.9
6.0
14
29
54
15
26 3.9
75 8.4
13
30
62
116
32
1210 660 187 700
2940 1270 332
930 630
2000 1700
3300
770
3360 1830 520 1940
81 70 3530 895
2580 1750
5560 4720
91 70
2140
570
460
380 4600 740
1300 6900 1200
2200 9800 1600
2900 6500 1500
5800 4400 2100
2600 2000 1100
1300
760 4600 2100
2600 6900 3400
4400 9800 4600
5800 6500 4300
11600 4400 6000
5200 31 00 31 00
8920 1960
24800 5440
ENERGY DISPERSION
Isotope excitation As measured 7 MCi 55Fea 70 mCi lo9Cde Extrapolated to 150 mCi 55Fe lo9Cd Fluorescer excitation As measured (900 watts Mn
cu Ag Cr-Zr Extrapolated to 2500 watts Mn cu Ag Cr-Zr X-Ray tube excitation As measured (1 50 wattsb) Mo tube W tube W tube-Ni foil Extrapolated to 10,000, c/sc Mo tube W tube W tube-Ni foil 5-MeV ion excitation As measured (5 F C d ) Protons Alpha particles Extrapolated to 50 p C Protons Alpha particles
21 0
600
2200 209
6110
45000 36000
12500 160
4300 360
3400 200
450000 360000
125000 1600
43000 3600
34000 2000
a Measurement with 55Fe made for 2000 sec because of low activity. 0.002 steradian aperture between X-ray tube and sample. X-Ray tubes operated at 150 watts with aperture gave count rates of: Mo, 5000 c ' s ; W , 10,000 c/s; W with filter, 3500 c I s . Ion measurements made for time necessary to accumulate 5 pC at beam dump, approx. 100 to 200 sec. e Note added in Proof: Expert opinion has indicated that the geometry of this source is not ideal for X-ray excitation; a smaller source, perhaps 20 mCi with the proper geometry, might give equivalent sensitivity and detection limits.
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ANALYTICAL CHEMISTRY, VOL. 45, NO. 1 2 , OCTOBER 1973
Table VI. 1 00-Second Detection Limits for Various X-Ray Techniques, ng/cmi Br
Zr
Au
Pb
Ai
S
K
360 85
52 13 52
3 18
10 29
53 33 29
150 30 36
160 49 40
180 51
820 150 100
390 21 0 160
1000 260
220 50
31 8 31
2 11
6 17
32 20 17
90 18 22
96 29 24
110 31
490 90 60
230 130 100
600 160
110 6200
180 21 00
2200
1400
420
230
84 3200
130 1100
1200
740
220
120
370
58 62 31 0 84
51 68 360
180 210
69 44
50
190
27 28 150 38
24 31 170
84 100
31 20
23
84
160
350 33 140
160 34 90
120 39 120
100 160 110
48 110 81
110 190 110
160 19
71 20 34
53 22 46
44 92 42
21 63 31
49 110 42
Ca
V
Fe
cu
Zn
Se
WAVELENGTH DISPERSION
X-Ray tube excitation As measured (900 watts) Cr tube Rh tube W tube Extrapolated to 2500 watts Cr tube Rh tube W tube ENERGY DISPERSION
Isotope excitation As measured 7 MCi 55Fea 70 mCi l o 9 C d Extrapolated to 150 mCi 55Fe lo9Cd Fluorescer excitation As measured (900 watts) Mn cu Ag Cr-Zr Extrapolated to 2500 watts Mn cu Ag Cr-Zr X-Ray tube excitation As measured (150 watts*) Mo tube W tube W tube-Ni foil Extrapolated to 10,000, c/sc Mo tube W tube W tube-Ni foil 5-MeV ion excitation As measured (5G d ) Protons Alpha particles Extrapolated to 50 p C Protons Alpha particles
570
220
220
84
53
700
44
350
20
1 1
2
0.2 0.2
0.5
10
5 20
0.2 2
5
1
1
*
Measurement with 55Fe made for 2000 sec because of low activity. 0.002 steradian aperture between X-ray tube and sample. 150 Watts tube power with aperture produced count-rates of: Mo. 5000 c/s: W, 10,000 c / s ; W with filter, 3500 c/s. At these count rates, detector resolution varied from 400 to 500 eV. Ion measurements made for time necessary to accumulate 5 gC at beam dump, approx. 100 to 200 sec.
ANALYTICAL CHEMISTRY, VOL. 45, NO. 12, OCTOBER 1973
2007
stituents may be difficult, if not impossible, to obtain. If a significant fraction of the particles is larger than a few micrometers, an error will be introduced into the X-ray analytical result which might approach a factor of two. There is no obvious solution to this problem except to suggest increasing use of fractionating impactors.
CONCLUSION
0
-
z
0
51
II
a z
0
s 0
m 0 0 0 0 0 0
UI??"?
0 0 0 0 0
0 7
0
222 0 0 0
0
2
(51
._ L
V
i
' t
0
cu 0
0
w
r
;-E
z x I
E E
On the basis of the results of this investigation, it can be concluded that X-ray fluorescence spectrometry can measure air pollution particulate samples for the elements of interest a t the concentrations encountered in practical situations with a measuring time of 100 seconds. Most of the early literature references to X-ray analysis of pollution particulate samples (4-8) used energy dispersion because the specimens contained many elements which would make scanning with a crystal spectrometer prohibitively time consuming. Close examination of the problem points out two difficulties with energy dispersion which are not likely to be overcome in the near future: 1) The best solid state detector ( - 150 eV resolution) will not separate the Kcu line of one element from the KP line of the next lower atomic number in the region of sulfur to nickel. Thus, all these elements will require mathematical unfolding to determine the X-ray intensities. Although unfolding is an acceptable process for intensities of similar magnitude (17), it is not adequate for the range of 2 or 3 orders of magnitude concentration which exist in pollution samples. 2 ) The solid state detectors are limited to about lo4 c/s a t their best resolution. In the energy dispersive mode of operation, the detector must accept all the radiation a t the same time, including the charactertistic lines of all the elements of the sample as well as the scattered primary radiation (which may contribute 50% or more of the total). For those elements present a t low concentration in the sample, counting times must be long to achieve reasonable statistics; e . g . , 50 ng/cm2 in a sample plus substrate weighing 5 mg/cm2 represents 10 ppm and would require processing of 2 x 105 other photons for each one photon of interest. At a counting rate of 104 per second, it would take 2000 seconds to accumulate the 100 counts for 10% statistics. Based on these two limitations for energy dispersion, we would have to conclude that large scale quantitative analysis of typical air pollution particulate samples requires the better resolution obtainable with crystal spectrometers in order to separate the lines of interest from possible interferences and to retain reasonable counting rates for low concentrations. For routine analysis, the only practical approach is the use of multichannel wavelength spectrometer instruments. One advantage which is gained with these commercial instruments is that each channel can be optimized (most efficient resolution, best crystal and detector, etc.) for the particular element being measured. Thus, it might be expected that detection limits could be obtained which would be better than those reported here, even when extrapolated to the same X-ray tube power used in the commercial equipment (2500 watts). In order to confirm this supposition, measurements were made on the soluble salt standards using four modern multichannel instruments with the cooperation of the X-ray equipment manufacturers who produce such analyzers. Although the details of the data cannot be (17) R M Dolby X-Ray Optics and Microanalysis H H Pattee V E Cosslett A Engstrom Ed Academic Press New York N Y 1963 p 483
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ANALYTICAL CHEMISTRY, VOL. 45, NO. 12, OCTOBER 1973
given here, the results did confirm that, with only simple modifications to the normal bulk specimen holders (in order to minimize the scattered background), all four of these instruments demonstrated detection limits better than the extrapolated values shown in Table VI for most of the elements examined. One hundred second detection limits in the range of 1 to 10 ng/cm2 were observed for a significant number of elements. There is, however, a limitation to the use of these simultaneous multichannel X-ray analyzers. Their flexibilit y is limited to the analysis of a fixed set of elements because of the use of fixed spectrometers. True, a particular spectrometer can be replaced with a different one or, in some instruments, set for a different element. If the instrument is equipped with a scanning spectrometer, it can be programmed to measure a series of elements sequentially. Any of these operations takes time however, somewhat negating the advantage of simultaneous analysis. Where the elements of interest can be defined beforehand as should eventually be the case for air pollution samples, the multiple spectrometer instruments should be ideal for large scale analysis. I t may be convenient for such an instrument to be equipped with an energy dispersive channel for a qualitative examination of the sample. There are two situations where wavelength dispersion is not required:
1) If only one or a few major elements of interest are present, for example, Fe and P b near a major highway, energy dispersion analysis with a low power X-ray tube or a high activity radioisotope might be adequate. If one element is present a t a concentration much above any others, then a simple proportional counter could make the measurement. For more than one major element, a solidstate detector would be desirable. 2) Fqr one or two elements near the detection limit and in the presence of strong interferences, a strong monochromatic source may allow selective excitation. The best resolution solid-state detector would be required for the measurements; counting times might have to be quite long.
ACKNOWLEDGMENT The technical assistance of K. L. Dunning and A. R. Knudson of the Van de Graaff Branch, Naval Research Laboratory, is gratefully acknowledged for the ion excitation effort. B. M. Klein assisted with the analysis of the pollution samples and D. J . Nagel participated in the preparation of standards. Received for review February 9, 1973. Accepted May 14, 1973. This work was supported by the Environmental Protection Agency under Interagency Agreement 690114.
Novel Method of Raman Data Acquisition J a m e s E. M o o r e and Lewis M. F r a a s lnstituto de Fisica "Gleb Wataghin. Universidade Estadual de Campinas. Campinas. S. P . , Brasii "
Signal-to-noise information is presented that supports the feasibility of a novel approach to the acquisition of Raman data. I n this approach. the fluorescence background interference common in many Raman experiments, is minimized through selective phonon population enhancement.
Until the advent of the laser, the Raman spectrometric technique, when used for the elucidation of structure of molecules of biological interest, had been severely restricted for the following experimental reasons: small sample size, sample photosensitivity, absorption heating, and fluorescence interference. However, with the advent of the laser, most of the aforementioned problems have been solved. For example, Beattie ( I ) has reported Raman data from a 0.1-mg sample of octasulfur. Without a laser, a sample size of a t least an order of magnitude greater would be required. The problems of photosensitivity and optical absorption can be minimized by the proper selection of excitation frequency. As an example, some compounds may be highly absorptive or photosensitive to radiation a t shorter wavelengths but rather unperturbed by red radiation. (1) I R Beattie Chem B n r , 3 . 347 (1967)
The use of the laser has aided in the reduction of interference from fluorescence as well. Still, fluorescence is presently the greatest problem in the successful recording of Raman data for large molecules. The present paper is mainly concerned with an experimental method of obtaining Raman data in the presence of fluorescence interference. Some successful Raman experiments have been reported where a large fluorescence background exists (2, 3) and Tobin ( 4 ) has discussed the intrinsic signal-to-noise ratio for a spontaneous Raman signal superimposed on a large fluorescence background. We begin this paper in section I with a summary of Tobin's method of Raman data acquisition. In section 11. we propose a radical departure from the Tobin approach. The experimental method of section I1 is based on the fact that little fluorescence is found on the anti-Stokes side of the exciting line. In the spontaneous Raman effect, thermal equilibrium exists, and Raman signals in the anti-Stokes region are small. If, however, phonon populations can be selectively enhanced, then thermal equilibrium will no longer prevail and Raman signals will be found without significant fluores(2) T G Spiro and T C Strekas Proc Nat Acad Sci U S A 69. 2622 (1972) (3) P S Hendra H A Willisand H Zichi Polymer in press ( 4 ) M C Tobin J Opt SOC Amer 58. 1057 (1968)
A N A L Y T I C A L C H E M I S T R Y , VOL. 45, NO. 12, O C T O B E R 1973
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