1782
Anal. Chem. 1082, 5 4 , 1782-1786
(8) Wampler, J. E. Amerlcan Society for Photobiology, Program and Abstracts, 1977, p 75. (9) Tamura, T.; Tenabe, K.; Hlrashl, J.; Saeki, S. Bunsekl Kagaku 1979, 28, 591-595. (IO) Wampler, J. E.; Mulkerrin, M. G.; Rlch, E. S. Clln. Chem. (WinstonSalem, N . C . ) 1979, 25, 1628-1834. (1 1) Wampler, J. E. "Modern Fluorescent SPectroscopy"; . . Plenum: New York,. 1976; Chapter 1. (12) Wampler, J. E. "Blolumlnescence in Action"; Academlc Press: London, 1978; Chapter 1. (13) Wilson, P. E.; Edwards, T. H. Appl. Spectrosc. Rev. 1976, 72, 1-81.
(14) (15) (18) (17)
Wampler, J. E.; DeSa, R. J. Appl. Spectrosc. 1971, 25, 823-827. Chesler, S. N.; Cram, S. P. Anal. Chem. 1971, 43, 1922-1933. Petitclerc, T.; Guiochon, G. Chromatographla 1975, 8, 185-192. Demas, J. N.; Crosby, G. A. J. Phys. Chem. 1971, 75, 991-1024.
RECEIVED for review January 28,1982. Accepted June 4,1982. This work was supported in part by the National Science Foundation (Grant PCM 8012433).
Analysis of Steels by Energy Dispersive X-ray Fluorescence with Fundamental Parameters Kirk K. Nielson" Rogers & Associates Engineering Corporation, P.O. Box 330, Salt Lake City, Utah 841 10
Ronald W. Sanders and John C. Evans Pacific Northwest Laboratory, Richland, Washington 99352
A simple fundamental parameters method was developed for energy dlsperslve X-ray fluorescence analysls of nonradloactlve and radloactlve steels. The method utilizes a thinfilm multlelement Calibration of the spectrometer, wlth mathematlcal matrix correctlons for self-absorptlon and enhancement. The method allows direct analysls of steels of varylng physical conflguratlon and composltlon with hlgh accuracy and preclslon and does not requlre specialized standards or sample preparation. Preclslons are dominated by countlng statistics, and slgnlflcant relative errors due to sample form averaged 3.8%. Analyses of radloactlve steels were accomplished by subtractlng the radlonucllde spectrum from the energy dlsperslve X-ray fluorescence spectrum before spectral analysls.
A fundamental parameter method has been developed for energy dispersive X-ray fluorescence (EDXRF) analysis of steels without reference to standards of similar thickness, composition, or physical form. The method is based on a spectrometer calibration from thin-film standards, and on mathematical corrections for matrix self-absorption and enhancement effects. The excitation and detection conditions of the EDXRF spectrometer are suitable to detect nearly all significant constituents of the steels simultaneously, and thereby provide the basis for accurate matrix corrections. The method is validated by analyses of three National Bureau of Standards steels in various physical configurations to demonstrate the insensitivity of the method to sample configuration and to illustrate its precision and accuracy. One important application of the method is in the analysis of radioactive steel from the internal structures of a nuclear power reactor. The determination of niobium in the steel is of particular importance for the eventual decommissioning of the reactor. This is due to the neutron activation of niobium to "b (20000 year half-life), an important source of long-term radioactivity. Niobium is difficult to determine by many analytical techniques; however, EDXRF provides an inexpensive and reliable means for simultaneous determination of Nb along with the other constituents. The method reported
here is particularly applicable to the analysis of radioactive metals because it minimizes sample handling and preparation and permits subtraction of the radionuclide contributionsprior to spectral analysis. The application of X-ray fluorescence for steel analysis is well established and has traditionally and competently relied on empirical matrix corrections such as those of Rasberry and Heinrich ( I ) , Lucas-Tooth and Pyne (2),Beattie and Brissey (3), LaChance and Trail1 (4), or Claisse and Quintin (5). Although these methods provide accurate results for steel analyses, the calibrations and requirements for standards for many of them are time-consuming and costly. Fundamental parameter methods avoid elaborate multielement empirical calibrations for steels and are suitable over virtually any range of concentrations of any of the observed elements. Equations for these methods have been developed over the past few decades (6-B), and comprehensive computer programs such as that of Criss and Birks (9) have been developed to implement them for both wavelength and energy dispersive X-ray fluorescence. Particularly simple applications of fundamental parameter methods have been reported (IO) for EDXRF spectrometers using monochromatic or secondary source excitation. The steel analysis method presented here is included as an option of a more general fundamental parameters program, SAP3 ( I I ) , which was designed for monochromatic and secondary source excitation. SAPB normally estimates light element concentrations (carbon, oxygen, etc.) from backscatter peaks (12)and also allows the user to enter known concentrations of analyte elements not observed in the spectrum. However, in the analysis of steels, the incident radiation provided by Ag Ka,P X-rays (22.104, 24.987 keV) provides simultaneous excitation and measurements of virtually all contributing constituents. Therefore, only observed elements were utilized for matrix corrections with the exception of the known silicon concentrations in two of the NBS steels. If the silicon were in greater concentration or of greater interest, it could be determined by using a lower atomic number fluorescer such as titanium, and could utilize matrix corrections from the Ag-excited elements with appropriate modifications of the present procedure. The general SAPB program also has
0003-2700/82/0354-1782$01.25/00 1982 American Chemlcal Society
ANALYTICAL CHEMISTRY, VOL. 54, NO. 11, SEPTEMBER 1982 PEAK ANALYSIS
4
CALCULATION OF THIN.SAMPLE ELEMENT MASSES I
, I
CORRECTIONS FOR
PEAK OVERLAPS
TO M l N l M U N "INFINITE' THICI+
e> L A S ITERATION ~
I""
-CALCULATION OF
Figure 1. Calculation seqluence for matrix corrections based on
thin-film calibrations.
the ability to estimate sample thickness based on backscatter; however, the high density of the steels allowed simplifications of matrix calculations by assuming infinite sample thickness. The backscatter intensities were therefore not used under the program options chosen for the present analyses.
EXPERIMENTAL SECTION Mathematical Procedure. The present mathematical procedure differs from most previous fundamental parameter steel analyses in that it uses thin-film calibrations instead of thick calibration standards. Although thick-sample calibrations also could have been used to accomplish these analyses, the present thin-film approach offers several notable features due to its generality. The same thin-film calibration curve used in the present work has also beien successfully used in multielement analyses of biological and geological samples of varying intermediate thickness under other options of the SAPBprogram. The thin-film multielement calibration is a fundamental description of the spectrometer respoinse to various elements, and once it is determined, it can be conveniently incorporated into calculations for nearly all sample matrices in thin, intermediate, and thick sample configurations. The thin-film spectra used for the multielement calibration cuwe are also used to defiie peak overlap factors on an absorption-free, mass ratio basis (13,14). These factors facilitate accurate peak overlap corrections for all sample configurations by application to absorption-corrected element masses rather than to spesctral peak intensities. Element concentrations were determined from the EDXRF spectra by the iterative calculation procedure illustrated in Figure 1. Net peak areas were used to estimate initial elemental quantities from linear, thin-film calibrations of the EDXRF spectrometer by simply dividing the net peak intensities by the appropriate calibration factors (counts/min per rg/cm2). The elemental quantities were then corrected iteratively for self-absorption and enhancement effects due to the thickness of the sample and its variable composition. Peak overlap corrections were also performed in the literation loop because the peak overlap coefficients were expressed on an absorption-free, mass ratio basis (13,141. They therefore required correction for matrix effects before being applied.
where oj = fluorescent yield for element j; Jj = absorption function jump ratio for element j; rj(i) = 0, KsJ energy < abs. edgei energy; rj(i) = R / ( R + l),Ks,ienergy > abs. edgei energy > KaJenergy; rj(i) = 1,KaJenergy > abs. edgei energy;R = Ks/Ka intensity ratio for element j. Additional enhancement by the backscattered exciting radiation was included by two additional terms in Equation 2, in which the subscript j referred to the incoherent and coherent intensities, respectively, and the ojrj(i)(l- l/Jj)yj(e) factors were deleted. In their place, the respective summations X k Q k q k and CkQkuCk were substituted for the incoherent and coherent scattering terms, where aIkand uck were the incoherent and coherent scatter cross sections for the excitation energy for element k. The matrix corrections were then applied to the thin film mass estimates as Iifi Qi
= Ki(l
+ Ei)
where Zi = net peak intensity for element i (count/s) and Ki = thin-fii calibration fador (counts/min per rg/cm2), Peak overlap corrections were applied to the matrix-corrected element quantities as (4)
where cij was the peak overlap coefficient as a mass ratio (12,13), expressed as the apparent mass of element i per unit mass of element j. The corrected element quantities determined from eq 4 were next used in a repeated sequence of matrix corrections, as illustrated in Figure l. This procedure was repeated until successive iterations did not alter the resulting corrections by more than about 0.05%. The element quantities were then normalized to unity and printed as fractional concentrations in the steel sample. In order to prevent the thick-sample corrections from continually increasing the sample mass in successive iterations for thick samples, the total mass was normalized to a constant value sufficient to absorb the excitation radiation (Fex= 5). For samples of intermediate thickness, the normalization could be based on a user-furnished thickness,if known, or left at the value calculated from backscatter using other options of the SAPBprogram. In these cases, the self-absorption corrections were modified in eq 3 to include the additional factor 1/[1 - exp(-fi)]. Procedure. Calibration of the EDXRF spectrometer was based on analyses of single-element thin-film standards (50-100 pg/cm2) evaporated onto thin Mylar backings (Micromatter Inc., Seattle, WA). Sensitivities determined from these standards are plotted as a function of X-ray energy in Figure 2 and formed a smooth
1784
ANMYTICM CHEMISTRY. VOL. 54. NO. 11. SEPTEMBER 1982
I
I
I
I
5
10
16
mr .,
.IC.
L
X - W " RlEnG" In.",
ne*. 2. -M
mkcfhn calibration cuve forAg Ku,B X-ray
excRation.
,
._II
curve. The excitation system (Kevex Subsystem 081OA, Kevex Inr.Foster City, CA) was operated at 50 kV and 10 mA to excite a dver seoondary fluoreseer for sample excitation. The excitation intensity for a given set of steel analyses was normalized to that uaed for the thin f h ealihrations hy analysis of a single thin film standard with each set of steel samples and normalizing the analysis livetime accordingly. In order to evaluate the precision and accuracy of the preaent analysis method for steels in various geometric configurations, we analyzed three National Bureau of Standards (NBS) steel standards in various forms. The standarda D-849 and 1155 were obtained in the form of solid disks, some of which were used to prepare samples in the form of lathe or drill turnings. Both sampleswere analyzed diredly from the solid disks, from machine turnings, and from pellets prepared from the turnings. The steel standard 123-C was obtained in the form of small chips and was analyzed directly and also as a compressed pellet. Figure 3 illustrates the actual samples of these three steels which were analyzed. The chips and turnings samples were supported in the sample chamber by a thin plastic tape. Because of the variable geometry of the samples, they were not weighed, hut were arranged to approximately fill the sensitive sample area viewed by the detector. Five replicate analyses were conducted on each of the forms of the samples to evaluate analytical precision separately from sample geometry and preparation effects. Several radioactive steel samples were analyzed hy the same method and are discussed here to illustrate ita application in the preaence of radioactivity. The aamplea were type 304L stainless steel and type 110 Inconel taken from the fuel support structure of a Westinghouse pressurized water reactor. The samples had been irradiated for a full fuel cycle (2 years) and had been out of the reactor for about 10 years. Bemw of their former p i t i o n in the high flux region of the reactor, the samples contained high levels of radioactivity, primarily MFe,T o , "Ni, and 69Ni. The reactor samples were obtained in the form of small chips and were also analyzed between pieces of plastic tape. In order to avoid the effects of the radioactivity, we collected a spectrum resulting only from the internal radioactivity of the aample by the !%DW system with the X-ray tube h e d off. A subsequent EDXRF analysis was then conducted for an equivalent livetime, and the spectrum from the radioactivity was subtracted prior to analysis by the s o 3 program. Data analpie with the s ~ pprogram j utilized a library wnthining X-ray pea!+ energies. sensitivities, and other fundamental wnatanta required for the matrix corrections Mass absorption mfficienta
, l*'l,
.
I
s
Flpun 9. photogaphs ofm6 sold W s . pressed palets. and tunhss samples of the NBS steels as itmy were analyzed.
and jump ratios were taken from McMaster et al. (17) and fluorescent yields were taken from Bambynek e t al. (28).
RESULTS A N D DISCUSSION The results of the replicate NBS standard steel analysea 81e presented in Table I. The arithmetic means of all analyses of all forms of a given standard are presented. followed by the standard deviations among the replicate analyses of each sample form, the standard deviation estimated from peak counting statistics, and the standard deviation among the results for the different forms of a given steel. Finally, the absolute errors between the mean of all analyses and the certified or provisional values given by NBS are reported. As indicated in Table I, the values of S(REP) and S(STAT) are generally wmparable, suggesting that peak counting statistics are the dominant source of variation in reolicate analvses of a given sample. The variation among different sample forms of a given steel was noted in Table I to be significant at the probability
