Fluorescent X-Ray Spectral Analysis of Powdered Solids by Matrix Dilution E. 1. GUNN Humble Oil and Refining Co., Bayfown, Tex.
A matrix dilution technique has been employed to minimize the effects of elemental interaction in a generalized x-ray fluorescence analysis in which the matrix is composed of a powder sample blended with lithium carbonatestarch carrier. A selected group of 1 1 elements of interest, ranging from 20 through 42 in atomic number, were examined in the oxide form; the developed calibration curves are linear over most or all the range in concentration, which indicates that the effects of elemental interaction are minor. The introduction of a large amount of a substance of lower or much higher atomic weight into the sample produces detectable interaction effects and the judicious use of reference standards is recommended for such component types. The coefficient of variation of a for measurement is approximately replicates a t a designated concentration level. In the measurement of synthetic samples containing from 4 to 1670 of the elements, the average analysis value varies from synthesis by about 1%.
370
T
sensitivity, and precision of x-ray fluorescent analyses on substances of varied composition and physical structure place this technique in the forefront of present-day methods of instrumental analysis. Although, in principle, x-ray fluorescence is similar to optical emission spectroscopy, in that each is derived from changes in energy states of the atom, the unique advantages and limitations of the methods result in their being complementary rather than competitive for generalized application. To illustrate specifically, the optiral emission of substances containing cobalt and molybdenum is so complex that interpretation is practically impossible, whereas the simple x-ray spectrum presents no problem. On the other hand, optical emission is regularly applied to elements of lorn atomic number-e.g., sodium and magnesium-for which fluorescent measurement is very difficult, if not impossible. with currently available x-ray equipment. I n the petroleum industry such diverse applications as measurement of lead and bromine in ethyl gasoline (2, I O ) , metal additives in lubricating oils HE SPEED,
184
ANALYTICAL CHEMISTRY
(6), and contaminant metals in fluid cracking catalyst ( 7 ) have been reported. Although each application reported is satisfactory for measuring an element in a particular substrate or liasic substance, the methods have one notable characteristic in commonthe procedure described is highly specific to the substance analyzed. This constitutes a limitation of the technique to applications of a generalized nature. I n the petroleum laboratory powdered solids are frequently encountered, in which the identity and concentrations of the component elements vary widely. It is for this type material that a more generalized x-ray fluorescent method of analysis is needed. The aim of the present study has been to investigate the possibilities of applying a generalized technique to powdered solids of interest in petroleum refining. The x-ray fluorescent spectrum is practically unequivocal in providing identification of the elements which give rise to it. I n turn, measurement of the spectral x-radiation provides a means for the quantitative estimation of the amounts of the elements in a substance. Generally, however, this measurement is not straightforward for major components (16). I n some instances this could be due to interference through spectral coincidence. But more significantly, it is the result of inherent interaction effects between the elements themselves in a substance which prevents a straightforward measurement of major concentrations. This interaction, rt hich results in nonlinearity of fluorescent intensity with concentration in binary and other mixtures, is well known, and has been demonstrated experimentally for systems nhich contained some of the ele-
Table I.
ments in the present study (3, 8, 13, 14). If the radiation emitted by each and every element present is measuredfor example, in a series of four-component alloys containing the same elements but in varying ratios-the interelemental effects of absorption and enhancement can be evaluated (1). Then the introduction of empirical interaction coefficients and experimental values into simultaneous equations makes it possible t o determine the amount of each element in an unknown alloy. If, instead of a n alloy, the substance is a powdered solid which contains both metals and nonmetals, some of which are measured and some not measured, the same type of solution obviously would be very difficult, if not practically impossible to apply. A remaining recourse is to minimize the interaction effects of sample absorption and excitation, by diluting with a high ratio of noninterfering carrier or matrix substance. This idea has been applied in x-ray fluorescent analysis of ores (4, 11), etc., and it serves both in reducing the emitted x-ray intensity to a satisfactory level for measurement and in minimizing the effects of elemental interaction. The preqent problem employs this approach. MATRIX A N D SPECIMEN PREPARATION
The selection of a suitable matrix diluent substance for the analysis of powder samples by x-ray fluorescence depends upon certain physical properties. The substance should be composed of elements of lon- atomic number to minimize absorption effects, be free of impurities in the elements sought, blend readily and intimately with the sample, and lie amenable to rapid and
Properties of Substances Suitable for Matrix Dilution Absorption Coefficients at Other Properties Selected K a v e Lengths" Pellet fabrica0 71 1 54 1 934 2 50 A. A. A. A. Purity Blending tion 0 974 7 85 15 33 31 56 High Good Good
-
CornJtarch Spectroscopic 0 605 4 52 8 77 18 0 graphite Lithium carbonate 0 940 8 17 16 08 33 19
aObtained or calculated from literature values.
High High
Poor Good Excellent Good
Density, g./ml. 0 92 0 84 0 72
Figure 1.
reproducible preparation of the specimen for analysis. Some of the substances considered to meet these requirements are listedin Table I. A 1 t o 1 weight blend of cornstarch and reagent grade lithium carbonate powder was selected as complying best with requirements. A preliminary comparison of the reproducibility of specimen preparation as between a loose-packed powder a.nd a briquetted I-inch-diameter pellet of the powder was made. The briquet or pellet mas found to be more reproducible, probably because of greater constancy in bulk density and surface from specimen to specimen; therefore it was selected as the form for measurement. In this discussion the “sample” refers to the original material before blending, and the results of a n analysis are reported on this basis; the “matrix” refers t o the sample after blending with lithium carbonatestarch; the “specimen” refers to the matrix portion or briquetted pellet examined in the instrument.
Cell holder for briquetted pellets
PROCEDURE
Table II.
Ele
men!
No.
Ca
20
V
23
Calibration Conditions and Straight Line Equations for Measuring Elements b y X-Ray Fluorescence
K Peak a
Background CI
Background Baokground
Cr
24
Background Background
Mn
25
26
Co
27
28
Background
29
Zn
30
Background A8
M”
33 42
63.0
51.?
B
B a
B a
B a
Background Background
71.0 69.4 66.5 62.4 60.0
B
a
Background
69.1
56.7
Background
Background
113.0 10.90 76.9 74.0
B
CI
Background
eter 28
60.0
Background
Background Cu
B u_
Background Ni
a
Background Background
Fe
B
Goniom.
a
60.0
54.0 52.a 55.2 47.55 55.2 48.7 55.2 43.7 41.0 45.05 47.6 40.55 39.5 41.85
39.5 37.6 39.5 34.05 32.5 20.9
16.5
Source Tube
Calibration Equation
E”. Ma. 35
20 % Ca
= 0.06481 (c.p.8. - 60)
25
15 % V
=
35
20 % V
=
20
10 yo Cr - 0.03289 (0.p.s. - 0)
25
15 70Cr
20
10 % Mn = 0.02427(c.p.s. - 10)
25
15 % M n
25
15
70Fe
20
10
% Co = 0.01657 (c.P.s. - 15)
25
15 yo Co
=
14
10 % Ni
= 0.06345 (c.P.s.
25
15 YoNi
20
10
25
15 % Cu
- 110) = 0.09090(c.p.s. - 0)
20
10 % Zn
=
20
10 YoZn
= 0.1319 (c.p.8.
20
10 % As
=
30
15 % Ma
= 0.05344 (c.p.~. 100)
Cu
=
- 20) 0.04242 (0.p.s. - 20)
0.02128 (c.p.8.
- 0)
0.05195 (c.P.s.
= 0.04706(c.p.8. - 10) =
0.02258 (c.p.8. - 50)
0.03529 [c.P.s. - 75)
- 160) = 0.02712 (c.p.s. - 165) = 0.05714(c.p.s.
0.01875(c.p.s.
0.06048 (c.p.8.
- 0) - 62) - 40) -
The range of elements that can be measured is practically determined by the optical system of the instrument. I n the procedure rvhich follows, in which lithium fluoride resolution is used, the elements of atomic numbers 20 t o 42 can be measured by their characteristic IC emission. The range of concentration as metal oxides is 1 to 100% in the sample.
Sample Preparation. A portion of t h e powdered sample, sufficiently divided t o pass a 200-mesh sieve, is added t o 19 times its weight of lithium carbonate-starch by careful analytical weighings. T h e lithium carbonatestarch blend is composed of 1 t o 1 weights of the chemically pure reagent compounds. T h e sample is homogeneously mixed in the matrix by use of mortar and pestle. Lithium carbonate-starch has excellent blending properties with powdered substances. A waferlike briquet specimen 1inch i n diameter and 3/a t o inch thick is prepared by t h e use of a n Applied Research Laboratories briquetting press with a pressure of about 80,000 pounds per square inch. Apparatus. T h e North American Phillips (Norelco) x-ray fluorescence spectrometer is used for t h e analysis. A lithium fluoride crystal (d = 2.0138 A.) is employed i n t h e goniometer for diffracting the fluorescent spectrum. A leaf collimator with 0.02-inch spacings is used before the Geiger tube. A plastic bag is used t o surround the optical path during all measurements, so t h a t helium may be passed through t h e svstem: a rate of 1 liter ner minute Gas bden found adequite. Air absorption of longer wave lengths is thus reduced, so t h a t satisfactory VOL. 29, NO. 2, FEBRUARY 1957
185
Table 111.
Typical Calibration Data for Measuring Elements in Synthetic Powders by X-Ray Fluorescence
Rlolybdenum~ Concn., Intensity, % C.P.S. 0 100 6.7
,.
218
Concn.,
%
Nickels Intensity, c.p.s.
0
162
8.0
277
Calciumc Concn., Intensity,
%
c.p.9.
0
6.5
Moos, Fe*Oa, CaCOs in synthetic standards.
* NiO, MnOe, CaO in synthetic standards.
CaO (or CaC03),Fe203, MOOJin synthetic standards.
sensitivity is obtained. A tungsten target source tube, Machlett OEG-50, is used. A modified sample holder for the brip e t t e d specimen is shown in Figure 1. Conditions of Measurements. Peak measurements of the elements are corrected for background near the peak to provide net peak intensity values. Suggested conditions for measurement of the elements are shown in Tables I1 and 111. These conditions, especially the primary source tube settings, are flexible and may be selected for the specific concentration range of interest. For a peak measurement, 102,400 Geiger counts normally are taken, and for background 12,800 counts are taken. Because the statistical precision factor of counting can be expressed as the reciprocal square root of the number of counts, the variance of a measurement imposed by the random counting error above can be predetermined. Duplicate measurements, either on opposite sides of a briquet or on a side of two briquets, are taken and averaged for the analysis. The measurement of both the CY and p peaks of an element often proves advantageous, both from the standpoint of choice where spectral interference may be experienced with one peak, and in providing for a more satisfactory counting rate. The position of each peak is approximately located by scanning the spectrum of the element and more exactly located by point set readings with manual control. Standard for Calibration. The standards are prepared for calibration by the use of high purity osides of the metals preignited a t 1000° F. to cons t a n t weight. Portions of the oxides are weighed out to prepare the sample containing a designated concentration of the metal. The briquetted specimen is prepared for analysis exactly as described above. Preliminary Qualitative Inspection. A preliminary qualitative inspection
186
ANALYTICAL CHEMISTRY
of the composition will usually save time in analysis by x-ray fluorescence. The inspection may be made by a strip chart fluorescence scan or by a n emission spectrum on the sample. I n case the composition of the sample is very different from that of the standards-e.g., has a large component amount of low or very high atomic number elements-new standards should be prepared to simulate the composition of the unknown as closely as possible. Whatever the composition, standard reference samples should be interspersed in measurements of the unknown samples. RESULTS
Quality of Excitation Spectrum. The quality of the excitation spectrum from a Machlett OEG-50 tungsten xray tube is reflected in the strip chart tracing of Figure 2. A lithium carbonate-starch blank specimen was irradiated to obtain the spectrum. Both first- and second-order reflections are indicated on the tracing. S h o m also in Figure 2 are the angular positions for measuring the metals selected for study. The tungsten L peaks are characteristic and inherent to the target material. The copper, nickel, and iron peaks, which are significant in magnitude, especially copper and nickel, are due to the presence of traces of these
Table IV.
Element Ca
v
CC Mn Fe Co Ni
CU Zn AS
Mo
Wave Lengths of
impurity elements within the x-ray tube itself. The intensity of nickel increases slowly with tube use and thus constitutes interference of a particularly troublesome nature for lorn concentrations of this element. Other than detection a t a high intensity level, the measurement of zinc Kp superimposed upon tungsten Lp is not a serious problem from the standpoint of time-to-time reproducibility. On the other hand, nickel in the sample is superimposed upon the variable peak contributed by the tube. As a consequence, frequent recalibration for drift still may be slightly uncertain in providing highly reliable values for nickel in very low concentrations. Characteristic Emission and Absorption Constants. An examination of the x-ray emission and absorption constants of the group of elements selected for this study is useful in understanding the problem of interaction between them. Table IV presents the wave length values for the K emission lines and the critical absorption wave lengths of the measured elements. Table V lists the absorption coefficients a t selected wave lengths for these measured elements as well as for the eIements of the matrix. To iilustrate the relationship between wave length and absorption in the spectral range of interest, one may consider a system in which copper and iron coesist. Copper ( K a , 1.54 A,) is of a wave length t o the left of the iron absorption edge and is notorious in producing iron fluorescence: Copper x-radiation is strongly absorbed by iron, which further enhances the emission of the iron spectrum. By contrast, manganese (Ka,2.098 A.) is t o the right of the iron absorption edge and hence does not exhibit this effect. To illustrate further, iron in a sample is highly absorptive for calcium ( K a , 3.35 A), but this wave length, of course, does not cause the iron to fluoresce as do ware lengths to the left of its K absorption edge. A similar qualitative prediction of behavior for each element of the systems measured in
K Series Lines (A.) of Elements Measured (5) Emission
SO.
20 23 24 25 26 27 28 29 30 33 43
(Y
3.352 2.498 2.285 2.098 1.932 1.788 1.655 1.537 1.432 1.173 0.708
P
Critical Absorption
3.083 2,280 2.081 1.906 1.753 1.617 1.479 1.389 1.293 1.055 0.631
3.064 2.263 2.066 1.892 1.739 1.604 1.484 1.377 1.281 1.043 0.618
the present study can thus be derived from the data of Tables IV and V. Correction for Coincidence Loss. The Geiger counter fails to respond linearly a t high counting rates, because of inability of the counter t o resolve discrete impulses a t high reception rates. This finite time of resolution is known as the “dead time” of the counter, which is of the order of 200 to 300 microseconds for the type of Geiger tube used. Geiger tubes may be calibrated for nonlinearity by a multiple foil method (9). Aluminum foil n-as used in the present work. All measurements a t rates greater than 700 counts per second were corrected, using the expression A: = A-o/ (1 - NOT)where SOis the observed value, T the resolving time, and the N the true count. Calibration Curves. The conditions suggested for establishing calibrations for the elements are delineated in Table 111, and typical straight-line calibrations are expressed as the equation for each element K peak intensity measured us. concentration. I n most instances the curves in Table I11 do not pass through the origin. This, of course, is dependent upon the choice of positions for background measurement, as is demonstrated by the profile of Figure 2-e.g., the intensity a t the element position for zero concentration may be greater than the background reference position. Typical calibration data for measuring the highest (molybdenum), an intermediate (nickel), and the lowest (calcium), atomic number elements considered in this paper are shown in Table 111. Khere the limitation in counting rate due to the dead time of the Geiger tube is not exceeded, the curves are essentially linear over the total concentration range in the sample. To obtain linearity for all ranges of an element, the excitation power of the source tube will, for certain elements, have to be reduced below that shown in Table 11. This observed linearity indicates that interelemental effects are minor or negligible for elemental systems containing atomic numbers 20 to 42 in the matrix. The slope of a given curve is determined by the ratio of matrix dilution of the sample and the operating power of the exciting tube. If these are selected and remain fixed, the calibration is reproducible within the precision of measurements. The scatter of calibration points in various blends generally was greater for calcium than for the other elements measured. Probable explanations for this are the high susceptibility of calcium x-radiation to absorption, its lower fluorescent yield as compared with other higher atomic number elements, and the less favorable optics of measurement employed (2 e = 113”). VOL. 29, NO. 2 , FEBRUARY 1957
187
Table V.
Element H Li
So.
1 3
c
6
8
C Na
11
c1
17 20 23 24
Ca
v
Cr
lfn Fe
25
CO Si
cu
26 27 28 29
Zn
30
33 42 82
AS
3x0
Pb
-
X-Ray Absorption Coefficients of Elements in Matrix (5, 72) V'ave Length, A.
0 710 1.389 0 47 0.43 0 86 0.22 3 35 0 70 8 10 1 50 23 4 3.36 76 7 11 6 120 19 8 26 5 30.4 (203) .. 33 5 250 38 3 267 41.6 286 47 4 37 49.7 54.8 .. 69 5 .. 20 2 141 I .
1.436 0'54 4.43 11.4 25.6 85 142 186 213 234 270 292 325 42 49 3 63.5 136 202
1.542 0.48 0 68 5 50 12.7 30 9 103 1i2 227 259 284 324 354 49.3 52.7 59 76 5 164 241
1.659 0'87 6 76 16.2 37.9 126 210 275 316 348 397 54.4 61 65 72 1 43.8
197 294
1.790 1'13
8 50
20 2 46.2 158 257 339 392 43 1
59 9 65.9
75 1 79 8 88 5
115 242 354
1.937 0.53 1.48 10.7 25.2 56.9 199 317 422 490 63.6 72.8 80.6 93.1 98.8 109 142 299 429
Precision of Measurements for Three Elements a t Two Concentration Levels
Table VI.
yo,of hIeasurementE
Av. Intensity, C p . ~ . ~
Std. Dev. Peak C.P.S. Equiv. 70 2 83 0 066 363 7 5 Fe A B 6 2416c 86 7 0 38 Ca .4 6 326.4 5.28 0.12 C 6 394OC 105 1.3 ?rro B 6 424 7 5 40 0 09 6 2574 29 5 1 1 a A. 10% CaC03, 10% Fe20s,80% MOOS. B. 20% CaC03, 70% Fe203, 10% 11003. C. 100% CaCOs. * Tube pover. Iron, 25 kv., 15 ma.; calcium, 50 kv., 45 ma.; molybdenum, 35 kv., 20 ma. c Departure from linearity exhibited at this counting level after correction for Geiger coincidence. Element
Blenda
Background 88 0 135.1 404.7 306.0 192 2
2 103
2 201
1 76 13 8
2 50 0.55
2 I1 17.0 40 1 02 5
4.1) 18.0
32 2 i2 3
245 100 530
308 ,508 -I i
4
1.; 5 124
1 (:3-4,5)
300
1020
600 .. ..
89 9
70 5 79 6 00 9
99 4
115 126 145
102 116 123 135 175 360 499
169 218 439 585
of one of these both vanadium iron produced u-hatsoever in molybdenum.
. .
147
375
180 197 228
450 495 575
..
1.54
3 57 (1.0) (10.7) 55.2
..
..
..
..
..
..
..
samples with 1.0% of and nickel and l . i % no detectalile effects measuring cobalt and
Table VII. Precision and Measurement for Cobalt and Molybdenum in Alumina-Base Catalyst"
Sample A
B
KO 6 594 5 97 0 27
Kcu 6 610 6 28 0 31
Peak
Ka
KR
S o . rpplicates
6
6
Molyl,denum Peak S o . replicates Av c ire,
V.P.
c:
0,
5
Cobalt
PRECISION AND ACCURACY
The precision of measurements for iron, calcium, and molybdenum a t a low and high concentration level of each is shoivn in Table VI. These data were obtained by measuring each face of three briquetted specimens prepared from a matrix powder. I n each set of measurements the standard deviation, by reference to a calibration curve, is no greater than an equivalent of 1.3% of the element in the synthetic. Somewhat high counting rates were recorded for the greater concentrations of iron, calcium, and molybdenum. Corrections for Geiger coincidence loss n-ere made, but the detection response is still not linear a t these high counting levels, which indicates that the correction factors were applied beyond their limits in these measurements. Reduction of the tube poiyer t o 35 kv. and 20 ma. for calcium and 30 kv. and 20 ma. for molybdenum, for example, provided linear curves up to and including these concentrations. Very little deviation from linearity was observed for iron under the suggested conditions of op188
0
ANALYTICAL CHEMISTRY
eration. Regardless of counting rate, these replicate results on precision indicate satisfactory homogeneity in briquet preparation as well as instrumental stability during the period of measurement. It may be concluded that if standards similar to the unknown are available for concurrent measurement with the unknon-n, a n analysis of high reliability can be obtained. The convenience of briquets for easy handling simplifies accumulating reference standards which simulate or match the composition of many unknowns. Iron, calcium, and molybdenum measurements n eie chosen to illustrate precision because these represent the intermediate, highest, and lon-est fluorescent .r\-avelengths, respectively, of the group of eleven elements selected for this study. Further illustrations of precision for the x-ray fluorescent-matrix method are sho15 n in Table VII. One measurement for each element was made on each of six briquet faces. The coefficient of variation is about 37, for cobalt and 5% for molybdenum on this type of sample. The deliberate contamination
5
2 84 2.47 0 05 a. 7 0 07 a S o m i n a l roncentration. 6.3% 310, 2 . 7 5 Co.
co,
Table VIII. Molybdenum in Molybdenum-Alumina Catalyst
7 hloOp Chemicala S-ra>-fluorescence
7.9 7 88
a Gravimetric, lead molybdate precipitation.
Data in Table VI11 compare chemical and x-ray fluorescence values on a catalyst substance containing molybdenum. Evcellent agreement is shown between methods, A single analysis of the catalyst was performed chemically. Four faces of briquets n-ere measured to obtain the u-ray fluorescence value in this and folloIving examples. An analysis of a sample containing most all the elements involved in this study is of interest from the standpoint of spectral interference. A blend con-
Table IX.
Element
Analysis of Synthetic Blends
(Per cent) .ldded Found
Uift‘.
Blend A . 10% of Each LIetal O d e Ca 4 0 4 6 +O6 7 0 0 0 Fe 7 0 1 5 -2 2 \lo 6 7 C
