Anal. Chem. 1983, 55, 1911-1914
19‘11
Multielement Analysis of Unweighed Oil Samples by X-ray Fluorescence Spectrometry with Two Excitation Sources Ronald W. Sanders,* Khris B. Olsen, and Walter C. Weimer Pacific Northwest Laboratory, Richland, Washington 99352
Kirk K. Nielson Rogers & Associates Engineering Corporation, P.O. Box 330, Salt Lake City, Utah 84110
A method has been developed to compute matrlx correctlons for unwelghed oil samplos from the comblned results of two separate energy-dlspersllve X-ray fluorescence spectra from dlfferent (TI Ka, Zr K a ) excitation sources. The method utilizes the Incoherent and coherent scatter intensities from the Zr-excited spectrum as a basis for multlelement backscatter/fundamental parameter matrlx correctlons for both spectra. The TI-excltetl spectrum Is collected In a helium atmosphere and provldes more sensitlve determination of elements In the AI-Ca range. The method utlllres thln film multlelement calibratlons of the spectrometer and permits the direct analysls of samples of varying unknown mass and composltlon with hlgh accuracy and precision.
An important feature of energy-dispersive X-ray fluorescence (EDXRF) analysis is its ability to determine simultaneously numerous elemental concentrations in a single sample. However, it is often necesisary to perform two separate analyses of the sample using different excitation X-ray energies to achieve adequate sensitivities for widely separated suites of elements. Although the resulting spectra are usually interpreted independently by using separate empirical calibrations, the data can be more efficiently utilized in fundamental parameter calculations which take advantage of the features which are common to the two analyses. These include identical excitation and detection geometry, sample thickness, and sample matrix composition, and, therefore, many identical X-ray absorption characteristics. The equality of these parameters reduces the number of unknowns which must be determined empirically. In addition, the combined determination of a greater number of elements provides a more reliable basis for accurate mathematical matrix corrections. This paper demonstrates a new data analysis method which utilizes the common parameters from dual EDXRF analyses of oil samples to perform accurate matrix corrections for quantitative midtielement analysis. The new method is designed for direct, multielement analysis of samples without similar standards and is an extension of the previously reported backscatter/fundamental parameter (BFP) method (1, 2) for performing matrix corrections. The BFP method utilizes separate incoherent and coherent backscatter intensities to help define the complete matrix composition of each sample. Heavy element masses (per unit area) are first estimated from their fluorescent intensities, from which their contributions to the two scatter intensities are calculated. These contributions are then subtracted from the two observed scatter intensities, and the differences are used to characterize the unobserved, lightelement component of the sample (H, C, N, 0, etc.). T w o representative light elements are chosen from the light-element scatter intensity ratio, and their masses are estimated by relating their intensities to their scatter cross sections. Matrix
corrections for self-absorption and enhancement are then computed from both light- and heavy-element masses and are applied to all fluorescent and scatter intensities. The lightelement selection and matrix correction process is iteratively repeated with newly corrected intensities until no further changes are obtained. Element concentrations and total sample mass are then computed from the corrected heavyand light-element masses. In the new BFP method, the two backscatter peaks from one analysis are used in combined matrix corrections for balth analyses. The fluorescent peaks in the second spectrum are used to define additional heavy-element masses, and their contributions to the scatter in the first spectrum are included with the contributions from the other heavy elements observed in the first spectrum. The main difference in the dualspectrum approach i13 the use of a different element sensitivity curve and a different excitation energy in computing the matrix corrections for peaks in the second spectrum. It is, thus, particularly useful in extending the BFP tipproach to low-energy secondary excitation sources such as Ti (4.5 keV), whose incoherent and coherent peaks cannot be resolved and are, therefore, not suitable for BFP analysis. The new method is otherwise similar to the previous BFP approach and features (a) calculation of the effective sample thickness, (b) definition of twlo “representative light element” masjaes which approximate the absorption properties of the oxygen, carbon, or other nonobserved light elements, and (c) calculation of matrix ciorrections for self-absorption and enhancement. It is also similar in some respects to a previously reported method for oil analysis which used scatter intensities in matrix correctioiis (3). This paper demonstrates an application of the dual-spectrum BFP matrix correction method with the analysis of five NBS fuel oil standards and a liquified coal sample. Zirconium and Ti secondary excitation sources were chosen for ithe present application because the elements determined with the Ti source could also be potentially determined with Zr excitation. This allowed a comparison of results from the dual-spectrum approach with those from the conventional BFP methods. In routine use, Ti and Ag secondary sources or other more widely separated sources are often used and have greater utility for analysis of separate suites of elements with less overlap. The present analyses utilized thin-film spectrometer calibrations and did not rely on similar oil standards. The same calibrations for the Zr source previously gave accurate results for rock, soil, coal, orchard leaves, rind oil shale (4). Sample thicknesses were computed by the maitrix correction program (1)and did not require weighing of liquid samples or preparation to a known thickness. Since absorption corrections were applied to the scatter peaks used for matrix corrections, sample thicknesses were not restricted to any particular range. By avoiding sample-specific calibrations and minimizing sample preparation, the dual-spectrum BFP approach greatly reduces analysis time and costs, sample han-
0003-2700/83/0355-1911$01.50/00 1983 American Cliemicni Society
ANALYTICAL CHEMISTRY, VOL. 55, NO. 12, OCTOBER 1983
1912
SAMPLE: SRM 1634a NBS FUEL OIL
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Flgure 1. Multielement thin-fllm calibration for Ti Ka,p and for Zr Ka,p X-ray excitation.
dling and contamination, and possible biases from poorly chosen "similar" standards.
EXPERIMENTAL SECTION The feasibility of dual-spectrum BFP matrix corrections was evaluated in terms of precision and accuracy by analyses of five different aliquots of each of five NBS fuel oil standards, SRM 1621b, 1622b, 1623a, 1624a, and 1634a. A typical application of the method to fossil fuel analyses is also presented, featuring replicate analyses of a coal liquid (an SRC-I1 fuel oil blend). The results for this sample illustrate the precisions and improved detection limits which are obtained for low sulfur concentrations using Ti K a excitation without sample treatment or the use of similar standards. Samples were prepared by injecting the liquids directly into 33 mm diameter polyethylene sample cells fitted with 0.006 mm thick polypropyleneX-ray windows. The samples were transferred with disposable pipets and in some cases required slight warming to facilitate handling. No control was maintained on sample mass or volume except to ensure that the entire bottom surface of the sample cell was covered by the sample. The cells were then placed on the sample changer of the excitation system (Model 0810a, Kevex Inc., Foster City, CA), and the sample chamber was purged with helium for 10 to 15 min until an argon fluorescent peak could no longer be detected. The helium flow was then reduced and maintained at 1.5 L/min during the EDXRF analyses to minimize X-ray attenuation and backscatter from the air in the X-ray path. A Vacion pump was used after the analyses to maintain the vacuum in the detector cryostat. Each sample was analyzed first with a Ti secondary source and then with a Zr secondary source and filter. A W X-ray tube was operated at 20 kV and 10 mA for the Ti source, and at 40 kV and 30 mA for the Zr source. Analysis livetimes were 10 min with the Ti source (2 to 3% deadtime) and 25 min with the Zr source (25 to 35% deadtime). The detector was a 30 mm2Si(Li) diode with a resolution of 180 eV fwhm at 6.4 keV. Calibration of the EDXRF spectrometer was based on thin-film, single element standards evaporated onto thin (0.003mm) Mylar substrates (Micromatter Inc., Eastsound, WA). Since the thin-fii calibration factors had been previously determined for the Ti and Zr secondary excitation sources, it was necessary only to normalize
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Flgure 2. Illustration of the formation of the composite spectrum.
these factors to the excitation intensities used for the present analyses. The resulting element sensitivity factors are plotted for both secondary sources in Figure 1. A single thin-film standard was analyzed with each set of samples to determine the exact excitation intensity for normalizing the calibration factors for the sample set. Details of the calibration procedure have been reported previously (1, 2). In order to simplify data analysis for the BFP calculation, the SAP3 computer code ( I ) previously used for single-spectrum BFP calculations was modified to accommodate dual excitation sources. For input to the modified SAP3 program, a composite spectrum was generated from the dual spectra with a splicing program. The splicing program read each of the EDXRF spectra from disk, erased the >4.09 keV region from the Ti spectrum and the C4.09 keV region from the Zr spectrum, and then added the two together. Figure 2 illustrates the formation of the composite spectrum. Since SAP3 uses a background-independent method (5)for integrating net peak intensities, the resulting complicated background shape in the composite spectrum in Figure 2 did not interfere with the spectrum analysis. The modifications to SAP3 consisted of altering the mass absorption coefficient calculations to account correctly for the lower excitation energy for the peaks below 4.09 keV. The calculation of corrections both for X-ray absorption and enhancement were affected by the modifications. Calibration factors from both of the curves in Figure 1were entered into a new calibration library for SAP3. Scatter cross sections in the library were retained for the Zr excitation energy only, however, since the Zr scatter intensities were used in computing the BFP matrix corrections. The use of 17.7 keV (Zr K P ) scatter cross sections in computing ab-
ANALYTICAL CHEMISTRY, VOL. 55, NO. 12, OCTOBER 1983
19'13
Table I. Comparison of XRFA Results for NBS SRM 1634a, Trace Elements in Fuel Oil (Mean of Five Replicates and Standard Deviation), p g / g element Si P S c1 K Ca Ti V Cr Mn Fe
c Ni c 11 )1:
Zn Ga Hg Se Pb As Br Rb Sr a
XRFA-Zr spectrum
XRFA-Zr/Ti spectrum
< 3000 < 1500
NBS
