Anal. Chem. 1991, 63, 2237-2244
portant analytical technique. Meanwhile, AIPA, in conjunction with statistical analysis program8 for data handling and interpretation, is proving to be a powerful technique for studying atmospheric particle chemistry.
ACKNOWLEDGMENT We thank Michael Palma for his help with sample preparation. Registry No. Na, 7440-23-5; Mg, 7439-95-4;Al, 7429-90-5; Si, 7440-21-3;P, 7723-14-0; K,7440-09-7;Ca, 7440-70-2;Ti,7440-32-6; Fe, 7439-89-6. LITERATURE CITED Lee, R. J.; Fisher, R. M. NBS Spec. Publ. 1960, No. 533, 63-83. Kelly, J. F.: Lee, R. J.; Lentz, S. Scennlng Elecbw, Mkmsc. 1980, No. 1.. 911-922. . Cassucio, 0. S.; Janocko, P. B.; Lee, R. J.; Kelly, J. F.; Dattner, S. L.; Mgebroff, J. S. JAPCA 1983. 3 3 , 937-943. Bernard. p. c.: Van Qrieken, R. E.; Eisma, D. E"?. Sci. Technol. 1986, 2 0 , 467-473. Anberson. J. R.; Agptt, F. J.; Bueeck. p. R.; brmani, M. s,;s h t tuck, T. W. Envlron. Sci. Technol. 1968. 2 2 , 611-818.
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Saucy, D. A.; Anderson, J. R.; Buseck, P. R. Atmos. Envkon. 1987, 2 1 , 1649-1657. Leysen, L. A.; Roekens, E. J.; Storms, H.; Van (Lleken, R. E. Atmos. €nv&on. 1987, 2 1 , 2425-2433. Hopke, P. K. Receptor Modelng in Envhonmental Chemktry; Wiley: New York, 1985; pp 67-91. Johnson, D. L.; McIntyre, 6.; Fortmann, R.; Stevens, R. K.; Hanna, R. B. Scannlng Ekctron Microsc. 1981, No. 1, 469-476. Germani. M. S. J. Forensic Sci. 1991. 331-342. Shattuck. T. W.; Germani, M. S.; Buseck. P. R. Anal. Chem., in press. Fritz, (3. S.; McCerthy. J. J.; Lee, R. J. Microbeam Anal. 1901, 57-60. Small, J. A.; Heinrich. K. F. J.; Newbury, D. E.; Myklebust, R. L.; Fiorl, C. E. NBS Spec. Publ. 1900, No. 533, 29-38. Armstrong, J. T.; Buseck. P. R. Anal. Chem. 1985, 47, 2178-2192. Chayes, F. oeochm. Cosmochim. Acta 1967, 31, 463-464. Flanagan, F. J. Geochim. Cosmochim. Acta 1987, 3 1 , 289-308. (17) Weigand, P. W.; Thoresen. K.; Griffin, W. L.; Heler, K. S. U . S . -1. Swv. Prof. Paper 1976, No. 840, 79-81.
RECEIVED for review January 17,1991.Accepted July 11,1991, This work was supported by Grant Nos. ATM-8022849 and ATM-8404022 from the Atmospheric Chemistry Division of the National Science Foundation.
General Approach for Quantitative Energy Dispersive X-ray Fluorescence Analysis Based on Fundamental Parameters Fei He and Pierre J. Van Espen* Department of Chemistry, University of Antwerp (UZA),B-2610 Wilrijk, Belgium
A general procedure for quantltatlve energy dkperdve X-ray fluorescence analyrk Is presented. The method Is based on t h fund"!hl par8meters approach wlth options1 use of the coherently and Incoherently scattered radlatlon to estimate the comporltlon and mam of the low atomlc number matrlx. Because of a rlgorous lmplementatlon of the fundamental parameter oquatlons for the most general case of Intermedate thlck samples and polychromatk excltatlon, the method Is sppllcable to a large varlety of samples uslng different excltatlon sources. I n prlnclple, callbratlon wlth only one standard Is pomlble. For hlgher accuracy, callbratlon wlth more standards Is recommended. The procedure has been Implemented as part of a larger software package AXIL-OXAS, whlch Includes spectrum acqulsttlon and spectrum analysts and runs on a personal computer.
INTRODUCTION Being a truly multielement and nondestructive technique, X-ray fluorescence is one of the most versatile and cost effective techniques in analytical chemistry. Wavelength-dispersive X-ray fluorescence (WD-XRF) is most valuable for the analysis of large series of similar samples as, e.g., encountered in industrial laboratories. Excellent accuracy can be obtained by using empirical, semiempirical, and fundamental parameter quantization procedures, providing the system is well calibrated with a large set of suitable standards. For energy-dispersive X-ray fluorescence (ED-XRF), it is difficult to compete in accuracy with WD-XRF, mainly because of the relatively poor detector resolution. Nevertheless
* Corresponding author. 0003-270019110383-2237$02.50/0
ED-XRF has many interesting features: the analysis of truly unknown samples is easier and faster due to the quasi simultaneous character of the detection process; there are few restrictions to the sample form and shape, and different excitation sources such as low-power X-ray tubes, secondary targets, and radioisotopes can be used. This makes ED-XRF ideally suitable when in a laboratory a large variety of different samples need to be analyzed and each sample or small sample batch must be considered as a unique problem. This type of analysis requires a quantitative procedure that can accommodate a wide variety of matrix compositions while not relying on a large set of standards, because this might not be cost effective or because standards might not be available. The aim of this work was to develop such a quantitative procedure for ED-XRF and to implement it on a personal computer. The procedure can handle nearly any sample form and matrix and provide acceptable results even if only a limited number of standards is used. Because of the good understanding of X-ray fluorescence physics, the availability of reliable physical quantities, and computer power, the fundamental parameter method is an obvious choice to implement such a procedure. Compared to empirical and semiempirical methods, this method can deal with more complex samples, especially when the available standards are different from the samples to be analyzed. The derivation of primary, secondary, and even tertiary fluorescence intensities for infinitely thick samples was already given in the early work of Sherman, Shiraiwa, Fujino, and others (1-5). Van Dyck (6) derived theoretical equations for secondary fluorescence in intermediately thick samples and applied them to secondary target XRF analysis. Quantification is, however, hampered by the presence of low-Z elements (Z < 11, e.g. C and 0 in geological, biological, and environmental samples), which do not produce measurable 0 199 1 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 63, NO, 20, OCTOBER 15, 1991
Table I. Definition of Symbols Used in the Fundamental Parameter Equations def
polychromatic X-rays, the relation between the weight fraction wiof an element i and the net count rate Nil of a particular line 1 can be written as wi
weight fraction of element i
sample density ( ~ c m - ~ ) linear sample thickness (cm) absorption edge energy of subshell X of element i (keV) characteristic radiation energy of line 1 of element i (keV) primary X-ray intensity at energy E, of radiation source net count rate of count 1 of element i instrument constant relative detection efficiency for the characteristic radiation Eil incident and emergent angles relative to the normal of the sample surface incident and emergent solid angles of X-ray beams production cross section of the characteristic line 1 of element i for the excitation by photons E, fluorescence yield of X shell (K or one of L subshells) of element i transmission probability of the characteristic line 1 of element i Coster-Kronig transition probability for the radiationless transition between the subshells m and n of L shell absorption edge jump ratio of shell X (K shell or one of L subshells) photoelectric mass absorption cross section of element i at energy E (cm2.g-’) incoherent scattering cross section of element i at energy E (cm*.g-’) coherent scattering cross section of element i at energy E (cm*.g-l) mass attenuation cross section of element i at energy __ E (cm2.g-’) absomtion factor for the line 1 of element i excited at E” enhancement factor for the line 1 of element i excited at E. fluorescence lines but are nevertheless essential to calculate the absorption correction. To extend the applicability of the method to a wider range of matrices, the scattered characteristic excitation radiation can be used to estimate the composition of the l o w 4 matrix and/or the sample mass. Nielson introduced the use of scattered radiation into a fundamental parameter approach (7,8). Since then, the method has been refined and widely used in quantitative analysis (9-14). All these methods rely on the use of monochromatic excitation radiation. For the case of continuum excitation, a semiempirical method has been developed (15). Considering performance characteristics, computer requirements, and availability, Bilbrey (16) has made a comparison of the most widely distributed programs, implementing either fundamental parameter or semiempirical methods: NRLXRF (17,18),CORSET (19),DATAPLUS (20),XRFll (211,SAPS (81,and NBSCSC (22). Most of the existing software has been developed primarily for WD-XRF and has some restrictions for use with certain matrices and experimental conditions. A more general and flexible approach designed for ED-XRF is therefore desirable.
THEORY In this work, the fundamental parameter approach is applied to the most general case of polychromatic excitation and intermediately thick samples. Other conditions (i.e., thin or thick sample, monochromatic excitation) are considered as special cases of this general case. The nomenclature used in the following equations is defined in Table I. Relation between Fluorescence Intensity and Concentration. For a flat, homogeneous sample excited with
= Nil/Gt(Ei1)CI(En)[Qil(En)AiI(E+n)H i ~ ( E n ) l n
(1)
where I(&)is the intensity distribution (in arbitrary unita) of the primary X-rays from the excitation source as they enter the sample. If the KLYline is chosen as the analytical line, the production cross section is
Eip > If the La line is chosen, this term becomes QidEn)
= ~ i , ~ f l i , ~ o [ 7 i , ~ 3 (+ E n?,L2(En)f2,3 )
En
+ + f i , d 2 , 3 ) I (3)
Ti,~l(En)(fi,3
in which, the L subshell photoelectric absorption c r m sections are respectively given by
The absorption correction factor accounts for the absorption of the primary X-rays and the characteristic X-ray lines in the sample and is given by &(En)
=
1- ~ X P [ - X ( E ~ & ) P ~
x (EirBn)
(7)
(8) The absorption correction factor as defined by eq 7 only accounts for the absorption at each energy En. To appreciate the magnitude of the absorption correction in the case of polychromatic excitation, a total (or apparent) absorption correction factor can be defined for each element AiI,T = NO,iI/Np,il (9) where No$ is the fluorescence intensity calculated by eq 1with the absorption correction term set to 1and the enhancement correction term set to 0 and Npj,is the fluorescence intensity accounting for X-ray absorption only. With this definition, the absorption correction factor varies linearly with the sample mass for a thin sample and becomes independent of the sample mass for an infinitely thick sample. The measured characteristic line intensity of element i can be enhanced by secondary excitation by element j in the sample when the energy of the characteristic photons of element j is above the absorption edge energy of element i. This results in an addition to the primary characteristic line in-
ANALYTICAL CHEMISTRY, VOL. 63, NO. 20, OCTOBER 15, 1991
tensity of element i, which needs to be taken into account for quantitative analysis. After rearrangement of the equations given by Van Dyck, the enhancement fador can be calculated
as
where j denotes the element whose characteristic line m can excite the characteristic line 1 of element i. The term Yil is rather complex and given by
F, =
R=
2239
For a Si(Li) detector, the detector efficiency appearing in eq 1 is calculated as €(E)= exp[-pBe(E)tB,l exp[-pAu(E)tAt~Iexp[-pdl(E)tdll (1 - e x ~ [ - ~ ~ i ( E ) t s i(20) l) where t b , tAu, t d , and tsi are respectively the mass thicknesses (gem-') of the Be window, the Au contact layer, the Si dead layer, and the Si cryatal. The parameters required to calculate the detector efficiency can be obtained from the manufacturer or determined experimentally (24). The instrument constant G is related to the excitation and detection geometry and is theoretically equal to G = dQld&/4T COS 81 (21) In practice, however, the value of G is obtained by measuring standards. This factor also takes the absolute intensity of excitation radiation into account. Relation between Scattered Intensity and Sample Composition. A characteristic line of energy E in the excitation spectrum (e.g., the Rh K a line from a Rh-anode X-ray tube) will be coherently and incoherently scattered by the sample. The count rate of these two peaks is respectively given by N m h = GcohC(E)Acoh(E)Z(E)CWia,h,i (22) i
Nine = G i n c c ( E i n c ) A i n c ( E ) z ( E ) CWiQinc,i i
(23)
where a d and uincare respectively the coherent and incoherent differential cross sections for scattering at an angle 0 = el + e2. When the scatter contribution of the high-2 elements is separated from that due to low-2 elements, the scattering cross section of the l o w 4 elements can be calculated as
(25) where the weight fractions of the high-2 elements can be calculated from their fluorescence intensities. The low-2 matrix can be represented by two elements 2 and 2 + 1,which are selected in such a way that
Elis an exponential integral function calculated via a series expansion (23)
...
in which y = 0.57721 is Euler's constant. In practice, the function El is calculated via several fitted polynomial functions instead of a series expansion. For the infinitely thick sample the logarithmic terms F3and F4dominate in eq 11, so that in this case
(26) and the weight fractions of these two representative elements, w z and w ~ +are ~ ,obtained by solving the system of equations %h = wZawh,Z + WZ+lacohZ+l (27) acoh,Z+l/uinc,Z+l
sgh/sf,c
acoh,Z/ainc,Z
= WZainc,Z + WZ+lainc$+l (28) The second representative element can be specified as 2 + 2 instead of 2 1 if the major elements of the l o w 2 matrix are rather different. The scatter peak intensities can further be used to estimate the effective sample mass. Equations 22 and 23 can be rewritten as -- 1 - exp[-x(E&dI Ncoh Acoh(E) = SZc
+
G c o h c ( E ) I ( E ) zwi'Ji,coh I
x(EB)
(29) Similar to the absorption correction factor, a total enhancement correction factor can be defined for each element as the ratio of the total fluorescence intensity (Nail+ Np,i!) to the fluorescence intensity Npjl induced by primary excitation only
1- exp[-~(E8i&dl (30) X(E,Einc) from which the sample mass pd can be determined. The
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ANALYTICAL CHEMISTRY, VOL.
63,NO. 20, OCTOBER
15, 1991
LT-J Main Rogrom
CONCENTRATIONS C A
L
Y
I
No
I k i n Progrom
Flguro 1. Flow chart of the calculations involved in the quantitative
analysis procedure. incoherently scattered intensity rather than the coherently scattered intensity is used to estimate the sample mass since uincvaries less sharply with the atomic number 2 compared to ucoh* The instrument constants for the coherent and incoherent scattering Ghcand G& can be determined experimentally by using standards or estimated from the instrument constant for the fluorescence G on the basis of the relation
(31) The instrument constant Cineis more reliable when low-2 standards are used, while Gcoh is more reliable with high-2 standards. This is because the incoherent scatter dominates for low-2 elements whereas coherent scatter dominates for the high-2 elements. As an alternative, a procedure based on experimentally determined scattering cross section (14,15) has also been implemented. Primary Excitation Spectrum. In order to make eq 1 generally applicable to any excitation source (direct tube, secondary target, radioactive source, or synchrotron radiation), the intensity distribution I(E,) of the excitation source is required. Experimentally determined and calculated X-ray tube spectral distributions have been reported in the literature (25-33). The procedure described here either accepts a user supplied intensity distribution or calculates the intensity distribution of the X-ray tube on the basis of the algorithm developed by Pella (32).For secondary target excitation, the tube spectrum is employed to calculate the intensity distribution of the characteristic lines of the secondary target. To obtain a more accurate estimate of the excitation spectrum, the continuum component of the secondary target spectrum resulting from the scatter of the tube spectrum can be taken into account. For a radioisotope excitation, the intensity distribution can be estimated from theoretical transition probabilities (34). Calculation Procedure. In principle, one element in a sample of known mass and composition can be used to estimate the instrument constant G. With more elements measured in one or more standards, instrument constants for each element can be obtained from which an average instrumental constant can be calculated. Alternatively, the individual instrument constants can be used in the quantization procedure. The latter will compensate to a large extent for any systematic errors in the fundamental parameters, in the primary X-ray distribution and in the detector efficiency. The procedure to calculate the instrument constant is relatively straightforward compared to the procedure to calculate the composition of unknown samples as no iterative procedure is required. Figure 1shows a simplified flow chart of the procedure for quantitative analysis. On the basis of the instrument constants, the sensitivities for all the analytes are calculated. If the sample thickness and/or matrix composition are unknown,
the ratio of the coherently to the incoherently scattered intensity is used for estimating the composition of the sample matrix and the intensities are used for estimating the sample thickness. An estimated mass per unit area for each analyzed element is obtained by comparing its fluorescence intensity with its corresponding sensitivity without considering absorption corrections. Subtracting the sum of these masses, the total mass of the low-2 elements is estimated if it is assumed that the sample contains low-2 elements. In this way an initial estimate of the sample composition is obtained. For an infinitely thick sample containing low-2 elements, the critical thickness is used as sample thickness. From this point on the program enters into an iterative process. When scatter peaks are used, an iterative procedure is executed for the sample matrix and/or sample thickness in addition to the iteration for the weight fraction of the analyzed elements. Using the initial composition and thickness, the weight fraction for each analyzed element is iteratively calculated until the calculated composition converges. The absorption and enhancement corrections are carried out by using the composition and thickness estimated from the latest iteration. If scatter peaks are used, the procedure returns to the iteration for the estimation of the low4 matrix composition and sample thickness and the newly estimated thickness and/or percentage of the low-2 matrix are compared with the previous ones. When two subsequent iterations of both iterative procedures yield no significantly different results, the iteration loops are terminated and the analysis results are reported. The accuracy of obtained results depends on the accuracy of the fundamental parameters. Compilation and comparison of the available data for fluorescence yields, mass attenuation coefficients, absorption edge energies, etc., have been made by many researchers (35,43). In this work the fluorescence yield for the K and L shells and the Coster-Kronig yields for L subshells are adopted from the data given by Krause (37). The mass absorption and photoelectric cross sections are calculated from McMaster’s compilation (38). Differential scattering cross sections are calculated from the tabulated data of Hubbell et al. (39). The absorption edge jump ratio is calculated from a least-squares-fitted polynomial function based on McMaster’s tables (40). The transition probabilities, in terms of line ratios, are based on Scofield’s work (41-43). EXPERIMENTAL SECTION Equipment. A direct tube excitation and a secondary target ED-XRF system were used to test the proposed quantitative analysis procedure. Because the tests are most demanding in the case of polychromatic excitation, emphasis is given to the direct tube excitation system. The direct tube excitation system makes use of a 17.5-W, air-cooled, side-window Rh X-ray tube. The Be window of the tube has a thickness of 0.127 mm, and the take-off angle of the X-rays from the Rh target is 20°. The tube operating voltage can be varied between 1 and 50 keV. Five filters of different composition can optionally be placed between the X-ray tube and the sample to optimize the excitation for a specific group of elements. The average incident and take-off angles are 4 5 O . The resolution of the detector is about 160 eV for the Mn K a line. The spectrometer is interfaced and controlled by a personal computer that acquires, displays, and stores the spectral data and other relevant information. The secondary target system consists of a high-power, Mo X-ray tube, and a Zr secondary target. The tube has a take-off angle of 6’. A Zr filter is placed between the secondary target and sample to increase the monochromatic character of the excitation. The average incident and take-off angles for the secondary target and for the sample are 45O. All measurements were done in air as the sample chamber cannot be evacuated. Sample Preparation. Different types of sample were used for calibration and for analysis. Commercially available thin films consisting of single element or simple compounds (1+170 wcm-2) evaporated onto a 4 pm thick Mylar foil were used. Thick samples
ANALYTICAL CHEMISTRY, VOL. 63,NO. 20, OCTOBER 15, 1991
2241
AI 0.001
i LL
C
*
,p0.0001
0 n VI
-
20s
40@
6W
8W
I
, ID00 12W Channel )*ub.s
Typical Xqay spectrum obtained wlth Rh-tube fit of the region including the scatted peaks.
Figure 2.
IZI
Ed excitation and
of pure elements and simple compounds were also made by pelletizing fine powders at a pressure of 10 tons.cm-2. Alloy samples were prepared by metallographic polishing until a scratch-free surface was obtained. Geological samples were prepared by wet grinding of 0.5 g of material during 2 min in a mechanical mill fitted with corundum grinding elements. A 3-mL portion of the resulting suspension was pipetted onto a Mylar foil glued to a Teflon ring. The sample was dried for 8 h at 60 OC to form a more or less homogeneous layer of approximately 5 mgcm-2 on the Mylar foil. The particle size was found to be less than 10 pm after 2 min grinding, which is sufficient to minimize particle size effects (44). Biological samples were measured as pressed pellets with a mass per unit area in a range of 50-200 mg.cm-*. Measurements. The Rh tube excitation system was operated at 35 kV with a 0.05-mm Rh filter. This excitation condition provides an adequate excitation efficiency for the elements above C1 and produces sufficient scattered Rh K a intensity to estimate the sample mass and low-2 matrix composition. For the determination of the elements A1 to C1, the samples were measured at 15 kV without a filter. All measurements were performed with the sample chamber under vacuum. Spectra were acquired for lo00 s with a dead time of less than 25%. For the secondary target system, the Mo tube was operated at 30 kV with a thin Zr filter placed between the Zr secondary target and the sample. All measurements were done in air and the spectra were acquired for 3000 s. Spectrum Evaluation. Net peak areas were obtained by the nonlinear least-squares-fitting procedure implemented in the AXJL-QXASsoftware package (45).The spectral data are described by series of Gaussian functions and an exponential polynomial background. Spectral artifacts such as peak tailing, escape, and sum peaks are taken into account. Nevertheleas, small inaccuracies in the model can introduce systematic errors in the peak area determination of very small peaks in the neighborhood of large peaks. The incoherently scattered peak, which is asymmetric and much broader due to the fact that the scattering occurs over a range of angles, is modeled by a series of closely spaced Gaussians. Figure 2 shows a typical tube-excited spectrum and the fit in the region of the scatter peaks.
RESULTS AND DISCUSSION The importance of the absorption correction in relation to the sample thickness is illustrated in Figure 3. The absorption correction factor (eq 9) is calculated for the IAEA soil-5 standard by assuming excitation with a Rh tube at 35 kV and using a 0.05-mm Rh filter. The sample composition is based on the certified element concentrations, assuming all elements are in the oxide form. From the figure, it is obvious that the curves start deviating from linearity at different sample thicknesses for different elements. For low energetic X-ray lines (A1 Ka,Si Ka) the absorption correction becomes significant even for a sample mass as low as 1 mg-cm-2. Therefore, in most cases, the absorption correction must be included in the calculations, especially for elements whose
-A-
1E-05
1E 4 6 0.0 1
0. I
~
1000
100
10
Sample Mass (mg/cmZ)
Figure 3.
Absorption correction factors (eq 9) as a function of the
sample mass for the IAEA soil-5 standard using Rh X-ray tube excl-
tation.
c 1.1 Y
-
0 c
I-
1.05
1 'LO 1
m,,,
0.1
1
,
,
1
.
1
1
10
1
1
1
1
I
100
I
,
,
,
,
1000
Sample Mass (mg/cm2)
Figure 4. Enhancement correction factors (eq 19) as a function of the sample mass for the IAEA soli-5 standard using Rh X-ray tube
excitation. 1.4 1.2
6
8
10 12 Line Energy (rev)
14
16
I
18
Instrument constants for the Rh X-ray tube system obtained from various standards. Figure 5.
characteristic lines are in the low-energy region. For intermediately thick samples, the enhancement correction is often neglected. Figure 4 shows how the enhancement correction factor (eq 19) varies with the sample mas8 for an IAEA soil-5 sample. Similar to the absorption correction factor, the enhancement correction factors are important for the elements that emit low-energy characteristic lines even a t low sample masses. The instrument constant plays a vital role in the quantification procedure. In principle, it is constant for all the elements. However, it varies in practice from element to element because of uncertainties in the fundamental parameters, the calculated excitation spectrum, and the detector efficiency. Figure 5 shows the instrument constants of various elements measured with a Rh X-ray tube system. The constants were obtained from thin-film standards, pressed pellets, and pure metals. The calculated values are nearly constant, except for Nb and Mo because these elements have absorption
2242
ANALYTICAL CHEMISTRY, VOL. 63, NO. 20, OCTOBER 15, 1991
Table 11. Results of the Analysis of Stainless Steel Samples Compared with Certified Values and the Results Obtained with the NBSGSC Propam' element Ti V Cr Mn
Fe
co Ni cu
SRM 1155 stainless steel certified this work mGSC 0.047 18.45 1.63 64.46b 0.101 12.18 0.169
0.065 18.50 1.64 64.42 0.147 12.28 0.159
0.032 18.37 1.63 64.40 0.127 12.11 0.169
SRM 1171 stainless steel certified this work NBSGSC 0.34
0.30
0.38
17.4 1.80 68.4b 0.10 11.2 0.121
17.12 1.75 66.74 0.12 10.94 0.137
17.46 1.77 66.01 0.12 10.79 0.120
0.158
0.151
Nb Mo 2.38 2.35 2.36 0.165 Values obtained by difference. a Concentrations are given in w %.
SRM 1291 Stainless steel certified this work NBSGSC
