821
Anal. Chem. 1980, 52,821-824
Inaccuracies Encountered in Sulfur Determination by Particle Induced X-ray Emission L. D. Hansen,*’ J. F. Ryder, N.
F. Mangeison, M. W.
Hill, K. J. Faucette, and D. J. Eatough
Departments of Chemistry and Physics, Brigham Young University, Provo, Utah 84602
Negative errors as large as 50 % may occur in particle induced X-ray emission (PIXE) determinationsof sulfur concentration in environmental samples. The low values obtained by PIXE In this study are due to chemical reactions induced by irradiatlon and i o loss of sulfur specles whlch are volatile at the temperatures produced by the particle beam under some conditions. The use of small beam currents can eliminate the losses due to simple heating but not those due to radiationinduced reactions.
We have recently found a discrepancy in proton PIXE determination of sulfur in several environmental samples. The problem encountered is readily seen by inspection of Table I where proton PIXE analysis results are compared with those of direct injection enthalpimetry (DIE) and ion chromatography (IC). The purpose of this paper is to present the results of experiments which were designed to evaluate the sources of the errors in sulfur determination by PIXE as currently being done by us, and to assess whether the problems are unique to our procedures or have general applicability to X-ray emission methods of analysis. X-ray emission techniques are being used extensively for determining the elemental composition of environmental samples. Since sulfur is considered to be one of the primary indicators of man-made air pollution, it is very important that any systematic errors present in the analytical methods be identified. Several intercomparison studies among laboratories using X-ray emission methods and between the X-ray methods and other methods of sulfur determination have been done (1-3). The laboratory intercomparisons have largely been made using synthetic standards or small numbers of filters on which airborne particulate matter has been collected. In neither case is the intercomparison likely to detect those environmental samples which exhibit the problems discussed in this paper. Another potential source of systematic error in the interlaboratory studies using airborne particulate matter is the long length of time between collection and analysis. Any highly reactive or volatile compounds initially present could be lost or altered during this time period. Furthermore, “outlying” results in such studies are usually deleted from consideration when, in fact, these samples actually may be behaving differently in different procedures. In those studies in which large numbers of environmental samples have been analyzed by two or more methods, the X-ray method has usually been applied to the sample first, since it has been considered to be nondestructive. However, it is our hypothesis in this paper that some X-ray methods can cause significant changes in sample chemistry. Even when methods have been tested in parallel, “outliers” have traditionally been deleted from the data for statistical reasons. The process of X-ray emission depends on the creation of a vacancy in an inner electron shell of an atom in the sample. Current (temporary) address: Laboratory for Energy-Related Health Research, University of California, Davis, Calif. 95616 0003-2700/80/0352-0821$01 .OO/O
Table I. Determinations of Sulfur in Selected Environmental Samples’” S0,Xbv
sample coal fly ash: oil fired power plant flue dust Plant 1: Plant 2: Plant 3: particulates from a Cu smelter plume:
extractant
S b y DIEor P1XE.b omoi/ pmbl/ g
g
62 56
103 135
0.1 M HCl, 2 . 5 mM FeCl, 0.1 M HC1, 2.5 mM FeCl, 0.1 M HC1, 2 . 5 mM FeC1,
2500
4700
750
1400
4200
7000
none, direct analysis on Mylar film 0.1 M HCl, 2 . 5 mM FeC1,
350
d
1710
2650
H 20
0.1 M HCl, 2 . 5 mM FeCl,
1480 3010 a The overall accuracy of all the determination is estimated to be f 10% unless systematic errors exist. PIXE indicates proton induced X-ray emission analysis of air dried aliquots of solutions. DIE indicates direct injection enthalpimetry with BaC1, after oxidation with Cr20,*- in 0.1 M HCI, 2.5 mM FeCl, and IC indicates automated ian exchange chromatography of H,O extracts. Methods not atmlicable.
H*0
The creation of this electron vacancy can be accomplished by using X-ray photons or charged particles as the primary exciting radiation. Particle-induced X-ray emission (PIXE) techniques usually employ protons ( 4 ) or a-particles ( 5 ) ,although heavier ions have been used. Systems using X-ray photon excitation (XRF) commonly employ as many as three X-ray tubes or secondary fluorescing targets to carry out a sample analysis ( 6 ) . Both photon and charged particle excited systems share the characteristic that not all of the energy of the primary exciting radiation is used in the simple creation of inner shell electron vacancies. A significant amount of the irradiation energy may be absorbed by the sample and cause a temperature rise. Certain compounds may be volatilized and therefore lost because of sample heating. This previously has been recognized as a serious problem in thick target methods where most of the energy of the exciting beam is absorbed by the target (7). However, only a few reports have been published of problems with sample heating when using targets such as those used in this study. The thin targets absorb only a small fraction of the beam energy, and consequently the problem is expected to be much less. Shaw and Willis (8)have studied the effects of sample heating by photon and charged particle beams. They concluded that the use of small currents of charged atomic particles causes a rapid rise in the temperature 0 1980 American Chemical Society
822
ANALYTICAL CHEMISTRY, VOL. 52, NO. 6 , MAY 1980
Table 11. Analyses of Standard Na,SO, Solutions by Proton PIXE Staken,
Sfound',
ng
ng
compound Na,SO,
a
i
101 0.98
+
6
0.98 1.05 1.08
8.3
5
42.0
43 91 390 721
i
408 773
foun d
0.2 1
8. Q
89 .o
Stakenl
*
7
27
Uncertainties are given as the standard deviation of
three measurements. __ ~
___-___---
of small, solid particles (1 gm) and leads to loss by sample decomposition and volatilization. Verbecke, Van Espen, and Adams have reported large losses of C1 from atmospheric particulate matter during analysis by XRF (9). King has reported the loss of sulfur from XRF standards prepared with elemental sulfur and dibenzyldisulfide (IO). Methods other than PIXE, e.g., scanning electron microscopy (SEM), which employ an electron beam apparently are subject to the same type of sample heating problems ( I I , 1 2 ) . However, because the sample heating is a poorly understood function of beam current, type of particle, particle energy, and target composition, it is not possible at the present time to extrapolate with any certainty from one experimental design to another. There are reports of procedures in which a sample is analyzed by an X-ray fluorescence method and then the same sample is analyzed for specific chemical species by other, less general methods ( 3 , 13). If the irradiation causes any decomposition or chemical alteration of the sample, the subssquent determination of specific species is obviously invalid.
were coated with a thin layer of Blair clear plastic spray coating. The calorimetric equipment and procedure (DIE) for determining sulfate has been described previously (16). The ion chromatograph was a Dionex Model 10. The eluent used was 3.5 mM Na2C03-2.6 mM NaOH. The eluent pump was run at 69 mL/hr and a 8 X 250 mm separator column was used. Under these conditions, the retention times of sulfite and sulfate ions are, respectively, 8 and 11 min ( 2 7 ) . A Phillips Model PW 1410 vacuum path, wavelength dispersive X-ray spectrometer was used to obtain K a and KP emission spectral lines for sulfur. Pellets were pressed as described in reference 18 using Whatman CF-11 cellulose powder. Reagents. All reagents used were analytical reagent grade chemicals except VOS04-2H20,which was reagent grade, and HOCH,SO,Na, which was 9 8 % . The solutions were prepared by dissolving a weighed amount of the compound in distilled, deionized water and diluting to a known volume. The H,S04 solution was prepared by diluting 100 pL of stock 18 hl H 3 O 4 (Baker Analyzed, suitable for Hg analysis) to 500 mL with distilled-deionized water. All glassware was previously cleaned with Alconox and rinsed thoroughly with distilled-deionized water. Standard solutions of Na2S04were prepared by weight in 0.1 M HCI, 2.5 mM FeCl, solution.
RESULTS AND DISCUSSION SLandard solutions of sodium sulfate were analyzed as a calibration check of the proton PIXE analysis for total sulfur. The results of these analyses are given in Table 11. The ratio, S+&,,/Sfowd, in the last column indicates that the proton PIXE data are accurate to better than 10% on sodium sulfate. Solutions of other sulfur-containing compounds which were selected 9s representing the more reactive or volatile sulfur compounds that exist in environmental samples were analyzed by proton PIXE and IC. The results of these experiments are shown in Table 111. An inspection of the fourth column, EXPERIMENTAL where the ratio of the total sulfur found by proton P I X E to Analytical Equipment and Procedures. The proton PIXE the total sulfur found as sulfite and sulfate ion is given, shows equipment and procedures have been described previously (14). agreement between proton PIXE and IC for the VOS04 and Briefly, 5.1 p L of solution was pipetted onto a hydrophilic Mylar M n S 0 4 solutions. The results on various iron sulfate comfilm (3.2 pm thick, 0.46 mg/cm2). The film was air dried and then pounds indicate that proton P I X E sulfur analysis is about used as the target in the proton beam. The sample, contained 30% low for these compounds. The proton P I X E analysis within the 0.43 cm2 beam irradiation area, was irradiated with of NaHS03 solutions indicates a 59% loss for this compound. 2-MeV protons for approximately 10 min at 150 nA. The X-rays However, when only the sulfate ion concentration determined emitted were detected with an energy dispersive Si(Li) detector by IC is compared with the total sulfur concentration found which does not discriminate between the K a and K/3 X-rays from by proton PIXE, good agreement is seen between the two sulfur. A few PIXE runs using 18 MeV a particles were also made. methods. This indicates a quantitative loss of sulfite ion in The equipment and procedures used in these runs are described the proton PIXE analysis. Sulfur is also lost from HOCH2in reference 5. In these experiments, the targets were irradiated S03Na, the adduct of NaHSO, with formaldehyde, although for 30 s at 8 nA. [These conditions are equivalent to 16 nA of not to the same extent as from NaHS03 itself. The proton 4.5-MeV protons ( 1 5 ) ] . The area of the beam was 0.15 cm2. P I X E analysis of H,SO, solutions showed the greatest deHydrophobic Mylar target material (3.2 pm thick, 0.53 mg/cm2) viation when compared with the IC sulfate ion analysis. was used in the cy irradiation experiments. On this material, the The results of analyses of standard solutions of some of the 5.0 pL of the solutions dried to produce a single spot of material same or similar sulfur compounds by 0-PIXE are given in 0.3 to 1 mm in diameter. Therefore, absorption correction;7 were Table IV. In this table the values of Staken/Sfound are sigapplied to the X-ray data based on the calculated thickness of nificantly low only for the ferrous sulfate targets and the the spot ( - 5 pm). One half of the targets for each material studied ________._____ ________ Table 111. Analyses of Aqueous Solutions of Pure Compounds by Proton PIXEa mole ratio mole ratio proton PIXEPC proton proton PIXE,C IC, SO,'-/ compound IC,b mM SO,'^ mM S PIXE/IC S/metal PIXE metal VOS0,.2H, 0 2.43 2.24 i 0.03 0.92 1.07 k 0.01 1.16 2.56 2.72 i 0.03 1.06 0.87 0.01 0.82 MnSO;H,O 2.56 1.81 i 0.17 0.71 0.67 t 0.03 0.95 FeS0;7 H, 0 3.10 2 . 2 0 * 0.03 0.71 1 . 4 1 * 0.02 1.99 Fe(",), ( S O4 )2,6H,0 Fe( NH,)(SO,),.lBH,O 3.67 2.37 t 0.28 0.65 1.25 i 0.13 1.93 ___ _._ 3.60 0.77 r 0.10 0.21 %SO, -._._ (",)'SO, 2.89 2 . 2 4 i 0.17 0.78 1.13 i 0.19 0.85 (SO,'-), 0.41(Total) --1 . 4 6 (SO,,'.) -t 1.33 (SO,") NaHSO __~ --_ 1.23 z 0.04 0.75 1.65 ( S O , ' - ) HOCH,SO,Na l'cilal estimated uncertainty is 5%. Error limits given All solutions were prepared in deionized or distilled water. as the standard deviatioii of three determinations. I
ANALYTICAL CHEMISTRY, VOL. 52, NO. 6, MAY 1980
Table IV. Analyses of Aqueous Solutions of Pure Compounds by a-PIXE mole ratio,' Sfound/
compound K304
FeS0;7H20 H*SO, (",),SO4
Sfound/Staken' 0.88 f 0.07' (coated) 0.59 f O.Ogb (uncoated) 0.74 0.94 0.88
f
* *
0.03'
0.05' 0.04c
metal found 0.43 f 0.01 0.43 i- 0.02 0.75 f 0.04
___
_--
' Sfound and metalfound were corrected for absorption effects assuming a sample thickness of 5 pm (Veigele, W. J., Atomic Data 1 9 7 3 , 5 , 51.) This introduces an Error estimated uncertainty of - 5 % in these results. limits given as the standard deviation of the mean of four No significant difference was seen determinations. between coated and uncoated targets. Therefore all eight determinations were combined.
uncoated K2S04targets. Except for K2S04,no differences were seen between the coated and uncoated targets. The mole ratio of S to metal is accurate within the limit of error for K2S0, and is 25% low for FeS04. Based on these results five possible error sources were identified. These error sources are: (1)Mechanical loss from PIXE targets. (2) Evaporative loss during target preparation. (3) Decomposition of the sample during irradiation by the proton beam due to sample heating. (4)Differences in X-ray intensity due to S oxidation state. ( 5 ) Chemical reactions induced by the particle beam which result in the formation of volatile products. Each of these error sources was investigated as described below. Mechanical Loss from PIXE Target. Flaking off of the dried sample from the Mylar backing material is possible either during transport to the accelerator laboratory or during loading in the irradiation chamber. The accuracy of the proton PIXE data for the metals as indicated in the last column in Table I11 indicates that this is not a problem with the thin target samples. If the sample were flaking off the backing material, a large negative error would be expected in the data for the metals. The uncoated K2S04 targets used in the a-PIXE runs apparently did show some mechanical loss of material from these much thicker targets. The mole ratio of S to K, however, shows that they are lost equally from the target. The mole ratio of sulfur to metal, which is independent of the total amount of compound on the target, shows that S, but not the metal, is lost from the sample from all of the ironsulfur compounds tested, irrespective of the particle beam used or whether or not the target was coated with plastic glue. It is significant to note that although a large percentage of H2S04was lost from the proton-PIXE targets, the amount detected was quite reproducible. Evaporative Loss during Target Preparation. To investigate loss during target preparation, aliquots of solutions of NaHS03, FeS04.7H20,and H2S04were treated by procedures similar to PIXE target preparation procedures and were then redissolved and analyzed by IC. In one procedure, 2 mL of stock solution were pipetted into a clean polystyrene vial and allowed to evaporate in a clean air hood. After complete evaporation, 2 mL of distilled-deionized water were added to reconstitute the sample. The same procedure was followed for a second set except that the samples were stored under vacuum (0.22 Torr Hg) for 18 h before reconstitution in order to assess the effect of the accelerator analysis system vacuum on the targets. The evaporative procedure and vacuum had little or no effect on the samples of FeS04 and H2S04solutions. The NaHS03 solution showed quantitative loss of sulfite ion in both sets of samples.
823
Thermal Decomposition during Irradiation with 2MeV Protons or 18-MeV a Particles. While passing through the sample and backing material, the particles lose energy. This causes a temperature rise and can lead to loss of elements or compounds which are volatile or decompose at elevated temperatures. An upper limit to the temperature which the sample reaches when irradiated has been determined by irradiation of substances with known melting points. The substances (Temprobe temperature test kit; Omega Engineering, Inc.) were finely ground and suspended in distilled-deionized water. An aliquot of the solution was pipetted onto a Mylar backing and allowed to dry. The targets were irradiated under normal operating conditions (150 nA of protons for 10 min or 8 nA of a particles for 30 s), and then examined under a microscope to determine which samples had melted. Using this technique we have established the maximum sample temperature to be 225 h 7 "C for the proton beam and 72 f 6 "C for the a-particle beam. This is an upper limit on temperature, since the particles present on the temperature-study targets were considerably larger and consequently absorb a greater amount of the beam energy than the particles normally found on a target prepared by evaporation of a solution or collection of aerosols. Simple thermal volatilization is the mechanism for loss of H2S04and (NH4&304 from the proton PIXE targets (Table 111)since these compounds are retained on the a-PIXE targets (Table IV). The difference in the temperatures of the targets in the proton and a-particle beams is a result of the difference in the beam current. Consequently, to avoid the loss of these compounds, the beam current should be kept as low as possible and it should be verified that no loss occurs under the specific conditions of the analysis. Other workers have reported the loss of NH4C1, NH4N03, H2S04,(NH4I2SO4,and NH,HS04 from samples in an ESCA spectrometer at temperatures well below 225 "C (19). Also, flash volatilization studies have shown these compounds to be volatile below 250 "C at 1 atm (20). All of the above compounds are important components of airborne aerosols. X-ray Intensity Differences due to S Oxidation State. The shape and intensity of the S K@peak is known to change with the oxidation state of the S atom (21-24). To investigate the possibility of errors due to this effect of the S oxidation state, pellets of ZnS, ZnS04, and a 1:1 mole mixture of ZnS and ZnS04 were pressed (It?), and a wavelength-dispersive X-ray spectrometer was used to obtain the X-ray emission spectrum. The measured peak areas for both S K a and S KP peaks were observed to decrease with a change in oxidation state. However, the differences in peak areas were caused by the matrix effect of the pellets. When the peak areas were normalized to the same ZnS matrix, no differences (
