Anal. Chem. 1981, 53, 1788-1792
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the detector signal. Second, the optical efficiency of the system must be improved to allow an increase in the number of surfaces on the cold station while still allowing overnight data collection with a reasonable S/N ratio in the spectra. Plans for up to 50 separate deposition surfaces are under consideration in order to analyze the hundreds of compounds found in many environmental samples, and the necessary variations in the optics are currently under evaluation. The present studies augment those of Reedy, Bourne, and Cunningham (10, 11) in demonstrating the practicality of matrix isolation spectroscopy in GC detection and point to the utility of the technique in characterization of complex real samples. Of course, there is no fundamental limitation of the technique to IR spectrometry; the use of other forms of matrix isolation spectroscopy (including molecular fluorescence) in chromatographic detection is presently being scrutinized in this laboratory.
(5) Hinton, E. R., Jr.; Mamantov, G.; Wehry, E. L. Anal. Lett. 1979, 12, 1347. (6) Wehry, E. L.; Mamantov, G.; Hembree, D. M.; Maple, J. R. In "Polynuclear Aromatic Hydrocarbons: Chemistry and Biological Effects”; BJorseth, A., Dennis, A. J., Eds.; Battelle Press: Columbus, OH, 1980; p 1005. (7) Hinton, E. R„ Jr. Ph.D. dissertation, University of Tennessee, 1979. (8) Erickson, M. D. Appl. Spectrosc. Rev. 1979, 15, 261. (9) Griffiths, P. R. In “Fourier Transform Infrared Spectroscopy, Applications to Chemical Systems”; Ferraro, J. R., Baslle, L. J., Eds.; Academic Press: New York, 1978; Vol. 1, p 143. (10) Reedy, G. T.; Bourne, S.; Cunningham, P. T. Anal. Chem. 1979, 51, 1535. (11) Bourne, S.; Reedy, G. T.; Cunningham, P. T. J. Chromatogr. Sol. 1979, 17, 460. (12) Schiller, J. E.; Mathiason, D. R. Anal. Chem. 1977, 49, 1225. (13) Schiller, J. E. Hydrocarbon Process. 1977, 56(1), 147. (14) Anderson, R. J.; Griffiths, P. R. Anal. Chem. 1978, 50, 1804. (15) “Chromatography and Mass Spectrometry Products”, Scientific Glass Engineering, Inc.: Austin, TX, 1979; p 17. (16) Hertz, H. S.; Brown, J. M.; Chester, S. N.; Guenther, F. R.; Hllpert, L. R.; May, W. E.; Parris, R. M.; Wise, S. A. Anal. Chem. 1980, 52, 1650. (17) Driscoll, J. N. J. Chromatogr. 1977, 134, 49. (18) Hester, N. E.; Meyer, R. A. Environ. Sol. Technol. 1979, 13, 107.
ACKNOWLEDGMENT We are grateful to W. E. May (National Bureau of Standards) for the shale oil sample, to W. L. Griest (Oak Ridge National Laboratory) for the coal-derived crude oil sample, and to P. W. Jones (Electric Power Research Institute) for the sample of SRC-1.
Received for review February 27, 1981. Accepted July 22, 1981. This work was supported by Contract RP-1307-1 with the Electric Power Research Institute. Purchase of the FTIR spectrometer was assisted by National Science Foundation Research Instrument Grant GP-41711. D.M.H. thanks the Tennessee Eastman Corp. for a Graduate Fellowship. Portions of this research were described at the Fourth International Symposium on Polynuclear Aromatic Hydrocarbons, Oct 1979, the Second Chemical Congress of the North American Continent, Las Vegas, NV, Aug 1980, and the 181st National Meeting of the American Chemical Society, Atlanta, GA, March 1981.
LITERATURE CITED G .Anal. Chem. 1979, 51, 643A. (1) Wehry, (2) Mamantov, G.; Wehry, E. L; Kemmerer, R. R.; Hinton, E. R. Anal. Chem. 1977, 49, 86. (3) Tokousbalktes, P.; Hinton, E. R., Jr.; Dickinson, R. B., Jr.; Bilotta, P. V.; Wehry, E. L; Mamantov, Q. Anal. Chem. 1978, 50, 1189. (4) Hembree, D. M.; Hinton, E. R., Jr.; Kemmerer, R. R.; Mamantov, G.; Wehry, E. L. Appl. Spectrosc. 1979, 33, 477. E.
L; Mamantov,
Determination of Sulfur and Heavy Metals in Crude Oil and Petroleum Products by Energy-Dispersive X-ray Fluorescence Spectrometry and Fundamental Parameter Approach Leif Hojslet Christensen*1 and Allan Agerbo Department of Chemistry, Aarhus University, DK-8000 Aarhus, Denmark
major, minor, and trace elements in crude oil and petroleum products. In 1967 X-ray fluorescence was approved as an ASTM standard method for the determination of sulfur. The current edition of this method, designated D 2622-77, was approved in 1977, and the 1979 Annual Book of ASTM standards (1) describes the analytical procedure in detail. Gamage and Topham (2) published a paper on a nondispersive on-line analyzer dedicated to the determination of sulfur. The primary excitation beam may be generated either in an X-ray tube or by a radioisotope. Recently, the Institute of Petroleum proposed and approved an X-ray fluorescence method for sulfur (IP 336/78 T) (3) based on a 65Fe radioisotope excitation source and a nondispersive analyzer. These standard methods both use a linear calibration procedure and they are prone to the same general problem inherent in X-ray fluorescence, i.e., the problem of matrix effects. For liquid samples the enhancement effect is negligible in most cases and only a matrix absorption correction is
The combination of energy-dispersive X-ray fluorescence based on the secondary target excitation principle and of a quantification model based on the fundamental parameter approach provides a method for the determination of sulfur and heavy metals in crude oil and petroleum products. Samples are analyzed directly without any kind of sample preparation. For sulfur the analysis Is performed under vacuum in an open cell arrangement. Further, a calibration procedure has been developed Independent of both the sample matrix and instrument setting. This procedure and the quantification model have been applied to various certified reference materials and the results obtained proved to be accurate to within 2-5%. The precision (1 ) of the method Is, however, better than 1.5% as long as the counting statistics are not the
limiting factor.
necessary. Several papers (4-7) have dealt with different experimental procedures to compensate for matrix effects in the sample. In a very general review by Williams (8) methods based on internal standards, solid internal standards, and incoherently
Wavelength-dispersive X-ray fluorescence spectrometry has often been the method of choice for the determination of 1 Present address: The Isotope Division, Rise National Laboratory, Postbox 49, DK-4000 Roskilde, Denmark.
0003-2700/8170353-1788$01.25/0
©
1981 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 53, NO. 12, OCTOBER 1981
Table I. Basic Equations for the Fundamental Parameter Approach
Ii= K^-K^-Ai-Wi K
Kn,i
Ai µ
=
(2)
KA,s (jU0' CSC i//L m
+
µ{
=µ·
7=1 WH + Wc
Sc
CSC
1)"1
(3)
(4) =
Sg Ay-
Iq
(1)
a i
1
-
XlVj
(5) (6)
Ac
scattered tube lines are described. Unfortunately, none of these provides a general approach to the problem and, furthermore, except for the latter, these methods complicate the sample preparation. Another more general approach has been suggested by Bird and Toft (9). They calculated the matrix absorption correction by using tabulated mass attenuation coefficients. In this more fundamental approach the concentrations of carbon and hydrogen are required. The concentration of carbon was estimated from experimentally determined intensity ratios of the coherent-to-incoherent scattered Cr Ka tube line. The concentration of hydrogen was determined by difference. Although the contribution to the backscatter peak intensities from the analyzed elements can be estimated on a fundamental basis, Bird and Toft introduced a set of experimentally determined correction factors into the linear relationship between the carbon weight percentage and the coherent-to-incoherent scattering ratio. Recently the use of energy-dispersive X-ray fluorescence spectrometers has been described in the literature (10-14). Again the calibration is performed by use of standard addition or internal standard methods. In the work by Giauque et al. (15) transmission measurements are used for estimating the matrix absorption correction for V, Fe, and Ni in fuel oil. The coherently scattered Mo Ka excitation radiation is used as an internal standard. In our work we have combined an energy-dispersive X-ray fluorescence spectrometer based on the secondary target excitation principle and the fundamental parameter approach to provide a universal, simple, and rapid method for determining part-per-million levels of sulfur and heavy metals in crude oil and petroleum products. Two methods for estimating the weight fractions of the nonanalyzed light elements are discussed and compared.
THEORY The Fundamental Parameter Approach. The basic equations in the fundamental parameter approach have been derived and discussed in various papers for the case of monochromatic excitation (16,17). The excitation radiation emitted by a secondary target is considered monochromatic. This is accomplished by a suitable weighing of the Ka and ß energies. For the purpose of this work, the working equation relating the observed fluorescence intensity, from a sample of infinite thickness and the elemental weight fraction, tF¡¡, is given by eq 1 in Table I. KA¡i CKa,b) is the absolute calibration constant for element i (s). For a fixed irradiation geometry, KAi depends on the kV/mA setting of the high-voltage generator, the secondary target used for the analysis, and various fundamental physical parameters, e.g., the photoelectric cross section, the absorption jump ratio, the fluorescence yield, etc. In order to make the calibration of the spectrometer independent of the high-voltage generator setting, we introduced a relative calibration constant, Kr,,, in eq 1. As shown in eq
·
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2 the relative calibration constant can be calculated from experimentally determined absolute calibration constants. The absolute calibration constant for one of the elements analyzed within a secondary target group (Ti target: P, S, Cl, K, Ca) is used as a normalization factor. Thereby, the tedious relative calibration can be done once and for all. In fact, the same relative calibration can be used for all sample types. In eq 1 and 3, A¡ is the matrix absorption correction taking into account both the attenuation of the primary radiation, and 1/2 are the angles µ0', and fluorescence radiation, µ{. of incidence and emergence of the primary radiation and of the fluorescence radiation, respectively. In our system they are each approximately 45°. A¡ is calculated by using tabulated mass attenuation coefficients, #tj, for the pure elements (18). The concentration dependency of µ', eq 4, and, thereby, of A¡ implements an iterative solution of eq 1. The Estimation of the Carbon and Hydrogen Concentrations. For samples containing light elements not amenable to X-ray fluorescence analysis the concentration of these elements has to be determined by other analytical techniques or, as we will outline below, by means of either an a priori choice of the carbon-to-hydrogen ratio or by using the information given in the backscattered peaks of the spectrum. However, the latter two methods may be termed pseudomethods because the concentration values obtained are not
real.
Method 1 makes use of an a priori choice of the carbonto-hydrogen weight ratio and this ratio together with eq 5, Table I, is used for determining the concentrations of hydrogen and carbon. Method 2, which is somewhat comparable to the method used by Bird and Toft (9), uses the information given in the backscattered Ka and/or ß lines from the secondary target. The coherent-to-incoherent ratio, Ir//C eq 6, is proportioned to the ratio of the coherent, St, and incoherent, Sc, scatter factors and to the ratio of the absorption correction factors for the coherent, A„ and incoherent, Ac, scattered radiation. The geometry-dependent proportional constant or calibration factor is termed G in eq 6. The contributions to the backscatter peak intensities from all the elements in the sample matrix are included in ST and Sc. Sr, Sc, AT, and Ac are calculated by using tabulated scattering cross sections and mass attenuation coefficients (18). The light element fraction is represented by carbon and hydrogen. The weight fraction of these two elements can now be estimated by solving eq 5 and 6. This is done for each iteration.
EXPERIMENTAL SECTION Spectrometer. Our energy-dispersive X-ray fluorescence spectrometer has been described in detail in two recent papers
(19, 20). The spectrometer is based on a conventional highpowered X-ray tube and a set of easily changeable secondary targets (Ti, Se, Mo, Ag, Te, and Sm).
Spectrum Analysis and Fundamental Parameter Approach Calculations. The net peak intensities in the acquired,
spectra are determined by using either a nonlinear least-squares fitting method or a simple summation method, the latter being adequate for uncomplicated spectra usually obtained in the analysis of petroleum. Calculations based on the fundamental parameter approach have been performed by using either a general fundamental parameter approach program, matrix, running on a medium-size time-shared computer or using a TI-59 programmable pocket calculator. Except for the rather complicated and memory-demanding calculations involved in the solution of eq 6, all can easily be performed on the TI-59 calculator. A library of programs dedicated to the determination of S, Ca, V, Fe, Ni, Zn, Sr, and Ba in petroleum products has been developed for the TI-59
calculator (21).
Calibration Standards and Sample Preparation. For all
elements except sulfur we have used 1-2% w/w standard solutions
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·
ANALYTICAL CHEMISTRY, VOL. 53, NO. 12, OCTOBER 1981
Table II. Absolute Calibration Constants for Elements Analyzed with Ti and Se Secondary Targets (Ka Lines)
Ai( element
Ti
S
35 kV 35 mA Se
35 kV 10 mA
Zn Cu Ni Co Fe Mn Cr Ca
K Fe
KA.i (iltf), (counts/s)·
W, %
g/cm2 X 10"2
1.11 4.99 10.01
0.35 0.32 0.28
3.02 (0.03) 3.04 (0.02) 2.97 (0.01)
1.91 1.91 1.93 1.25 1.89 1.96 1.95 1.90 1.46
4.32 3.90 3.05 2.94 2.67 1.91 1.54 0.64 0.54
4.89 4.20 3.79 3.01 2.48 1.83
1.89 1.70 1.52 1.34 1.16 0.93
2.67
2.70 2.74 2.77 2.80 2.84
cm2/g
X 10e
(0.05) (0.01) (0.01) (0.03) (0.006) (0.01) 1.47 (0.01) 0.26(0.008) 0.15 (0.03)
2.50(0.01) 2.50(0.009) 2.52 (0.004)
roughly is equivalent to the diameter of the beam spot. The view diameter of the detector defined by a collimator is 8 mm. The sample cups are always filled to the rim resulting in a sample thickness of approximately 2 cm. The low Z elements S, Cl, K, and Ca are determined under vacuum by means of the Ti secondary target. The counting time per sample is usually 100 for elemental concentrations above 1% and below 100-500 s. V, Fe, and Ni are determined by use of the Se secondary target and a counting time of 500-1000 s.
RESULTS AND DISCUSSION Calibration of the Spectrometer. Equation 1 applies only to samples of infinite thickness, and this requirement may not be fulfilled for all fluorescence energies of interest. For an oil matrix containing, say, 12% H and 88% C, infinite thickness approximately equals 0.01 cm, 0.15 cm, 0.4 cm, 0.6 cm, and 2 cm for S, V, Fe, Ni, and Se Ka energies, respectively. Thus, in order to reproduce the irradiation and counting geometry as well as possible, calibration is performed by means of samples having nearly the same mass thickness, m (g/cm2), as the unknown samples. For samples of finite thickness the absorption correction factor, A;, eq 1, should be replaced by Aí-tttMI exp(-m/A¡)). m is determined gravimetrically. In order to verify the reproducibility of our calibration procedure, we have experimentally determined the absolute calibration constants for elements analyzed by means of the Ti and Se secondary targets. For each element five samples were prepared from the same stock solution and measured. Table II gives the mean value of the calculated KA[i values, eq 1, and the associated standard deviation. This deviation reflects the reproducibility of the sample preparation, the sample changer mechanics, the applied vacuum, and the intensity determination. In general, the relative standard deviation is better than 1%. Within a secondary target group, e.g., Se, the absolute calibration constants decrease with decreasing Z value. This decrease is caused partly by a decrease in the photoelectric
C/H ratio
G
5.95 6.00 6.97 7.81 8.79 10.43 10.50
1.31 1.34 1.23 1.25
Se
ß Backscattered
Peaks
1.20 1.23 1.26
Table IV. Effect of the Carbon-to-Hydrogen Ratio on the Determination of Sulfur in NBS SRM 1621a and SRM 1622a Residual Fuel Oil Standards 0 W, %
C/H sample
2.48 (0.006) 2.48 (0.002) 2.48 (0.01)
of water-soluble compounds for the absolute and relative calibration of the spectrometer. For sulfur we used di-ierf-butyl sulfide diluted with white oil. The calibration of the backscattered peaks was performed by measurements of cyclohexane and toluene and various mixtures of these two compounds. Samples are analyzed directly without any kind of sample preparation. An open cell arrangement consisting of a spectro cup and a very thin Mylar window (
