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ANALYTICAL CHEMISTRY, VOL. 50, NO. 7, JUNE 1978
included both fibrous and bulk crystalline material. Thus, although the spectrum appears to be characteristic of the serpentine minerals, no incontrovertible evidence can be advanced for the equivalence of the Raman spectra of chrysotile and antigorite. The results summarized above illustrate certain capabilities of the Raman microprobe in application to microanalysis and micromineralogy. For the analytical chemist, this new microprobe offers the possibility of mineralogically identifying a microcrystallite in a single measurement, and, at very least, provides unique data which complement those obtained by other microanalytical techniques. The technique is no panacea for microanalysis in t h a t limitations can arise from optical absorption and sample heating. For the mineralogist, this probe can extend the tools of vibrational spectroscopy for chemical and structural studies to the microscopic domain. Thus, it is now possible to obtain vibrational spectroscopic data not previously available for most minerals simply because of the nature of the (bulk) samples. These data are obtained from single crystals; they are relatively free of artifacts associated with crystal size and shape; and they cover the complete spectrum of lattice and molecular vibrations. Systematic studies of important mineral classes are needed in order to fully exploit the potential of the Raman microprobe for both analytical and mineralogical applications.
ACKNOWLEDGMENT The authors thank C. C. Gravatt and K. F. J. Heinrich for support and helpful suggestions during the course of these investigations; B. Mason for providing samples; J. Small and P. Johnson for assistance in SEM-micrography and electron
probe analysis; P. Pella for x-ray fluorescence data; C. McDaniel and B. Velde for aid in determining and interpreting x-ray powder diffraction patterns; B. Velde for helpful discussions of the molecular spectrometry of sheet silicates; and K. F. J. Heinrich and M. Ross for commenting on the manuscript.
LITERATURE CITED (1) A. N. Lazarev, "Vibrational Spectra and Structure in Silicates", ConsuRants Bureau, New York, N.Y., 1972. (2) "The Infrared Spectra of Minerals", V. C. Farmer, Ed.. Mineralogical Society, London, 1974. (3) R. Ruppin and R. Englman, Rep. Prog. Phys., 33, 149 (1970). (4) G. J. Rosasco and E. S. Etz. Res./Dev..28, 20 (1977). (5) R. Ruppin, J . Phys. C., 8, 1969 (1975). (6) E. Salje, J , Appl. Crysta//ogr..6, 442 (1973). (7) J. F. Scott and T. C. Damen, Opt. Comrnun., 5, 410 (1972). (8) W. A. Deer, R. A. Howie, and J. Zussman, "Rock Forming Minerals", Voi. I-V, Longmans, Green, and Co., Ltd., London, 1963. (9) E. Loh, J . Phys. C, 6, 1091 (1973). (10) W. B. White in "Infrared and Raman Spectroscopy of Lunar and Terrestrial Minerals", C. Karr, Jr., Ed., Academic Press, New York, N.Y., 1975, p 325. (1 1) T. R. Gilson and P. J. Hendra, "Laser Raman Spectroscopy", WileyInterscience, London, 1970. (12) R. G. Burns and R. G. J. Strens, Science, 153, 890 (1966).
RECEIVED for review December 1,1977. Accepted February 27, 1978. J. J. Blaha was temporarily assigned to the National Bureau of Standards. Certain commercial equipment, instruments, or materials are identified in this paper in order to adequately specify the experimental procedure. In no case does such identification imply recommendation or endorsement by the National Bureau of Standards, nor does it imply that the material or equipment is necessarily the best available for the purpose.
Use of Dibenzyldithiocarbaminate as Coprecipitant in the Routine Determination of 12 Heavy Metals in Pharmaceuticals by X-ray Fluorescence Spectroscopy H. R. Linder, H. D. Seltner, and B. Schreiber" Analytical Research and Development, Pharmaceutical Division, Sandoz Lfd., 4002-Basle, Switzerland
A sample preparation and enrichment procedure for 12 heavy metals (Mn, Fe, Co, Ni, Cu, Zn, As, Se, Cd, Sb, Hg, and Pb) is described. Samples are decomposed with sulfuric acid and hydrogen peroxide in a temperature programmed wet digestion apparatus. After the decomposition, buffer solution is added and the metal traces are coprecipitated with the dibenzylaminosalt of dibenzyldithiocarbamic acid. The precipitate is collected by pressure filtration on a millipore filter for direct lrradlatlon in the x-ray spectrometer. The procedure allows the preparation and measurement of up to 12 elements slmultaneously in eight 1-g samples. The recovery of the procedure was checked with radloactlve isotopes, conflrmlng that 90 to 100% of all elements are found in the precipitate.
Coprecipitation is successfully used as sample preparation technique for x-ray fluorescence (XRF) if the concentration of the elements to be determined (heavy metals) is too low for a direct pelletization of the sample. 0003-2700/78/0350-0896$0 1.OO/O
This is usually the case in the concentration range below 10 ppm. A gain in sensitivity of 2 orders of magnitude for solid samples and three orders of magnitude for aqueous liquid samples can be achieved by this method. Thanks to the small thickness and the low atomic number of the precipitate, nearly all the atoms of the enriched trace elements can be excited by XRF. In addition, the characteristic x-rays of the sample atoms are not lost by absorption from the sample matrix. With direct pelletization on the other hand, only a small part of the sample contributes to the signal. Most coprecipitation procedures are based on an organic reagent, forming insoluble compounds with the trace metals and a carrier element, that has to be added to the sample to get an adequate amount of precipitate for trapping the trace elements. Oxine, cupferrone, thionalide and pyrrolidine dithiocarbaminate are used in conjunction with a heavy metal carrier (1-4). There are some drawbacks with the use of a metal carrier: The absorption coefficient of the precipitate is increased resulting in lower x-ray intensities of the trace elements. In C 1978 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 50, NO. 7, JUNE 1978
addition, the element used as carrier cannot be determined in t h e sample. Pueschel ( 5 ) was t h e first t o introduce PAN for the coprecipitation of heavy metals without the use of a carrier element. T h e reagent itself is precipitated in the sample solution a n d collects t h e trace metals by co-crystallization. This technique has the following disadvantages. Precipitation has to be performed in a n alkaline medium where trace elements tend to be adsorbed by the container walls and alkaline earth elements are also precipitated. In addition, the range of elements that can be determined is quite narrow. A better group of compounds, as far as general applicability is concerned, would be represented by t h e dithiocarbamates. Of these, the dibenzyldithiocarbamate showed most promise as coprecipitation agent because of t h e low water solubility of t h e metal complexes. Dibenzyldithiocarbaminate has been used as a spectrophotometric reagent (6, 7); it can also be seen as a potential coprecipitant for use in acidic solutions without a carrier element to collect nano- and microgram amounts of heavy metals from aqueous samples containing considerable amounts of alkali and alkaline earth salts. T h e possibility of using such a technique for sample preparation in conjunction with routine XRF work is explored in this paper. EXPERIMENTAL Apparatus and Operating Conditions. Measurements were performed with a Siemens Model SRS 1 vacuum x-ray spectrometer with a sample changer for 10 samples. Instrument parameters were chosen as follows: Chromium tube, operated at 50 kV, 40 mA, LiF 100 crystal, pulse height analyzer, collimator 0.15 or 0.40°, 8-mm diameter beam constriction, vacuum system held a t 0.3 Torr throughout all measurements. The lines were measured with argon methane flow proportional and NaJscintillation counters. Pressure filtration of the precipitates was performed with a Schleicher and Schuell pneumatic pressure syringe, Model Antlia Pro Aqua, equipped with Millipore filterholders, 13-mm diameter and filters with 8-pm pore size, Type SCWP 1300. Teflon cylinders (50 mm diameter X 10 mm high) served to hold the filters in position during XRF measurements in graphite sample holders. Reagents. The sodium salt of N,N-dibenzyldithiocarbamic acid was obtained from Kodak (No. 10089). The dibenzylaminosalt of dibenzylaminodithiocarbamic acid was prepared as follows (8): 0.05 mol carbon disulfide, 0.11 mol dibenzylamine and 10 mL benzene are mixed at room temperature, heated to 70 "C for an hour, cooled and evaporated to dryness on a Rotavac. The residue is crystallized twice from diethylether. Recovery was 12.9 g; mp 80-82 "C. Sulfuric acid (96% w/w), nitric acid (65% w/w), hydrogen peroxide (30% w / w ) , acetic acid (99% w / w ) , methanol (99% w/w), ammonia (25% w/w), sodium acetate, sodium iodide, ascorbic acid, analytical grade Merck were used. The indicator solution was from Merck (No. 9177, pH 0-5). Metal solutions of certified concentration, Fixanal, Riedel de Haen, were used for standard preparation. Standard solutions containing 1 ppm of each of the elements studied were prepared by dilution of Fixanal standards with 10% (v/v) nitric acid. Indicator buffer solution was prepared from 37 g sodium acetate, 143 mL acetic acid, and 12 mL indicator solution, filled up to 1 L with doubly distilled water. The coprecipitation reagent consisted of 1% (w/v) of sodium dibenzyldithiocarbamate or dibenzylaminodibenzyldithiocarbamate in methanol. The solution is stable for two weeks, if stored in a refrigerator at 4 "C. The reduction solution was made from 15 g sodium iodide and 2.5 g ascorbic acid, filled up to 100 mL with doubly distilled water. Procedures. S a m p l e Decomposition. Samples (1 g) were weighed into 50-mL quartz Kjeldahl flasks and a mixture of 2 mL sulfuric acid and 8 mL nitric acid was added. The flasks were heated to the boiling point of nitric acid for an hour. The temperature was raised to 250 "C for 10 min and then lowered to 150 "C. Then hydrogen peroxide was added in 50-pL aliquots until a clear, colorless solution was obtained. Finally, the
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temperature was again raised to 250 "C for 10 min and samples were cooled to room temperature. Coprecipitation. To 1C-200 mL of the strongly acidic solution, approximately 10 mL of distilled water and 1 mL of the reducing reagent were added. After 15 min, another 1 mL of the reducing agent and 20 mL of indicator-buffer solution were added and the solution was adjusted to pH 4 i 1 with ammonia. The total volume of the sample should be between 40 and 250 mL. Now, 2 mL of the coprecipitatant were pipetted into a Teflon-beaker, the sample solution was added and thoroughly mixed. The suspension was transferred to a pressure syringe, equipped with a 13-mm diameter filter. After filtration, the precipitate was washed with 20 mL of doubly distilled water and dried at room temperature. The filter was centered on a Teflon block and fitted into the sample holder of the x-ray spectrometer for measurement.
RESULTS A N D D I S C U S S I O N Recovery Tests. Trace element recoveries were tested by (a) comparison of the x-ray intensities of coprecipitates of metal spiked samples and of metal solutions spotted on filter paper (1 to 10 pg/element) and (b) addition of radioactive tracers to the samples prior to decomposition and measurement of the activity of the dissolved precipitate. Recoveries of more than 90% were found for Cu, Fe, Zn, Pb, Ni, As, Se, Cd. Sb, Mn, Co, and Hg. Radioactivity measurements showed that even nanogram amounts of Pb, Fe, and Zn are collected quantitatively within the precipitate. Recoveries did not vary between 5 ng and 10 pg of these elements. For t h e experiments the following conditions had to be considered: (a) Filtration had to be performed within 10 min after precipitation. The Antlia pressure filtration device permitted filtration of only 1 to 2 min per sample. (b) T h e total amount of all metals precipitated was never to exceed 100 pg. (c) As and S b were not precipitated in the +5, Se and Te in the +6 oxidation state. Prior reduction with KJ/ascorbic acid, however, resulted in a quantitative recovery of these elements. (d) Mn and Zn formed rather weak complexes, and thus exhibited reduced recoveries (60 to 80%) if another element was present in an amount higher than 20 l g . pH P r o f i l e f o r the Coprecipitation Step. T h e p H can influence recoveries via two mechanisms. One is dissociation of the complex resulting in free metal ions which are not included in the precipitate; and secondly hydrolysis of the reagent resulting in amine and CS2formation. p H adjustment is awkward if performed with glass electrodes on a large number of samples. The procedure should therefore work in a pH-independent plateau of maximum recovery. p H profiles were measured for all elements mentioned. Figure 1 gives a summary of the profiles of P b , Ni, Cu, Zn, Fe, and Se indicating that there is no dependence in the p H range from 2 to 5 . E f f e c t of T e m p e r a t u r e . The solubility and crystalline form of the precipitate are usually temperature-dependent. Precipitates formed a t 10 "C or lower have a very small grain size; hence, filters become clogged and filtration is markedly more time-consuming. Between 20 and 40 "C there is no temperature effect, whereas a t 50 "C recovery of zinc is reduced to 80%. Higher temperatures can also lead to the melting of t h e precipitate and should be avoided. Aging of the P r e c i p i t a t e . Solutions containing coprecipitates of 10 pg of each of the elements Fe, Sb, Cd, Se, P b , Zn, Cu, Ni, and Mn were filtered 10, 30, 60, and 180 min after precipitation. Only Zn and Mn showed an effect. The recovery of Zn began to decrease after 1 h and was only 81% after 3 h. Recoveries for manganese decreased to 83% after 30 min and further to 65% after 3 h. Filtration should therefore be performed immediately after precipitation. Calibration Curves f o r 12 Elements. Linear calibration functions were recorded for all elements in the concentration
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ANALYTICAL CHEMISTRY, VOL. 50, NO. 7, JUNE 1978
Table 11. Standard Samples, Analyzed by Atomic Absorption and the Proposed XRF Method" Gelatine standard
cu Fe Mn
AAS 38 51 42 41 49
Ni
Zn
sb
XRF
1.2
0.5
42 53 44 42 46
0.7 2.1 1.0 4.2
10 23 229 80
1.5 2.0 2.6
sb
2.8 3.0
3.2 0.4 1.9
NBS Orchard Leaves
cu Zn Fe Mn
* Cu.Pb 0 ~
0
Zn Se
Ni Fe
11
3
2
PH
4
5
Figure 1. pH profile for the coprecipitation of trace metals with di-
benzyldithiocarbaminate Table, I. Characteristic Data of the Calibration Functions for Coprecipitated Elements Measured by XRF Line measured/collimator/- Slope, Correlation Detection Element counter" cps/pg coefficient limits, p g Se Pb As Zn CU Ni CO Fe Mn Hg Cd Sb
Ka/O.lS/SZ LOl/O.l5/SZ Ka/O.l5/SZ Ka/0.4/SZ Ka/0.4/DZ Ka/0.4/DZ Ka/0.4/DZ Ka/0.4/DZ Ko/O.4/DZ La/O.l5/SZ Lp5/0.4/DZ Lp,/O.4/DZ
20 12 16 61 46
53 64 63 5 10 7
19
0.9999 0.9999 0.999 0.995 0.998 0.9999 0.9999 0.9999 0.9999 0.9996 0.997 0.999
0.2 0.2 0.3 0.8 0.4 0.2 0.2 0.2 0.2 0.2 0. I 0.5
" SZ = NaJ scintillation counter. DZ = flow proportional counter. range of up to 10 pg. Calibration functions are characterized by the slope (counts per second and microgram), the coefficient of correlation between 0 and 10 pg and the realistic detection limit, calculated by a formula which takes into account the standard deviation of the background and of the standard values (9). Table I gives a summary of the recorded calibration functions. As a matter of convenience, a chromium tube was used for all the measurements. Use of a molybdenum tube improves detection limits for Se, As, P b , and Hg by a factor of 2-3, whereas Fe, Cu, Cd, and S b show an opposite effect because of the high bqckground of Fe and Cu and the poor fluorescence yield for S b and Cd. Peak intensities were measured for all the indicated lines, collecting a minimum of 20000 counts (usually between 20000 and 200000). The measurement time was between 20 and 100 s. One single background intensity was measured in the continuum a t 33' with the LiF 100 crystal and was used to correct peak intensities. The quotient of peak over background intensity was used to calculate the linear regression function. This method was found to be better than the subtraction of the background intensity. Parallel Analyses of S t a n d a r d Samples w i t h XRF a n d Atomic Absorption Spectroscopy. NBS Orchard leaves (NBS Standard No. 1571) and gelatine standard material, spiked with trace elements (IO)were analyzed for Cu, Fe, Mn,
10
26 256 82
1.0 2.1
22.0 3.0
" Values in ppm. Mean values of 6 determinations of each sample are given. s = absolute standard deviation in ppm. The very high deviation of the iron values i n the NBS sample measured by XRF is caused by the large volume of the precipitate. Iron should be measured from a separate sample of smaller weight. Ni, and Zn by the proposed technique and by an atomic absorption method (11). Six samples of each material were decomposed and measured by both methods. Data are presented in Table 11. CONCLUSION The use of dibenzyldithiocarbaminate as coprecipitant for traces of heavy metals has significant advantages compared to other dithiocarbamate derivatives and to other coprecipitation techniques. The low solubility of the reagent in water eliminates the need to add a carrier metal to the sample. The reagent itself acts as carrier when it is precipitated from methanolic solution by the aqueous sample. This leads to very uniform precipitates, since thickness is not a function of the amount of element precipitated but is given only by the concentration of reagent, added to the sample solution. Both absence of a heavy metal as collector and uniformity of the collected precipitate improve the linearity and slope of the calibration line and result in lower detection limits. The low water solubility of these complexes also has a positive effect on the recovery of trace elements from very dilute solutions (1 to 10 ppb). The p H of the sample solution is not critical and can be somewhere between 2 and 5 . This is attributed to the high stability of the reagent toward attack by acids (decomposition to carbon disulfide and amine). From the trace analytical point of view, precipitation from acidic solutions is advantageous since many elements form insoluble hydroxides in the neutral and alkaline range. The uncritical pH also makes the procedure easier because time-consuming pH adjustment can be omitted. In general, the method is straightforward and easy to handle. The enrichment step including collection of the precipitate with the pneumatic pressure filtration device can be performed within 10 min. The proposed method is in routine use in our laboratories for purity control of organic materials and compounds containing alkali or alkaline earth metals. Ten samples (2 standards and 8 samples) can be measured for 10 elements within 3 h. Sample preparation is matched to the throughput and is performed with an automated wet ashing apparatus with a maximum capacity of sixteen 2-g samples per 3 h. The method successfully replaces atomic absorption and classical methods used until now. LITERATURE CITED (1) G. D. Thorn and R . A. Ludwig, "The Dithiocarbamates and Related Compounds", Elsevier, Amsterdam, 1962. (2) 0. G. Koch and G. A . Koch-Dedic, "Handbuch der Spurenanalyse", Springer-Verlag, Berlin, 1974. (3) A. Hulanicki, Talanta, 14, 1371 (1967).
ANALYTICAL CHEMISTRY, VOL. 50, NO. 7, JUNE 1978 (4) C.L. Luke, Anal. Chim. Acta, 41, 237 (1968). (5) R. Pueschel, Talanta, I S , 351 (1969). (6) T. Fukusawa and T. Yamane. Anal. Chim. Acta, 84. 195 (1976). (7) R. I. Martens and R. E. Githens, Sr., Anal. Chem., 24, 991 (1952). (8) F. Haase and R. Wolfenstein, Ber. Msch. Chem. &s., 37, 3228 (1904). (9) R. Plesch. GIT fachz. Lab., 17, 677 (1973).
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(10) D. H. Anderson, Anal. Chem., 48, 117 (1976). (1 1) Perkin-Elmer, Norwalk, Conn., "Analytical Methods for Atomic Absorption Spectroscopy", 1971.
RECEIVEDfor review March 28,1977. Accepted March 6,1978.
Energy Dispersive X-ray Fluorescence Spectrometric Determination of Trace Elements in Oil Samples Hideo Kubo' and Robert Bernthal Nuclear Physics Laboratory, University of Colorado, Boulder, Colorado 80309
Thomas R. Wildeman" Department of Chemistry and Geochemistry, Colorado School of Mines, Golden, Colorado 8040 1
A method is described for the determination of trace elements in petroleum by energy dispersive x-ray fluorescence spectrometry. Minimum sample preparation is required. This is achieved by making small targets and spiking the sample with a solution of Cr and Rh in H2S04or organo-Rh in mineral oil. Use of two spiking elements with different x-ray energies facilitates the determination of x-ray absorption corrections. I n the NBS fuel oil (SRM 1634), V, Fe, Ni, and Mo were detected and the results of the analyses correspond well with the NBS certified values. I n the shale oil, Fe, Ni, Zn, As, and Se were detected. The one-element splklng method works well for samples of low viscosity where the sample can be spread on the supporting foil thin enough so that the absorption of measured x-rays can be ignored, whereas the two-element spike is needed for high viscosity samples (NBS fuel oil) where samples cannot be made thin enough to ignore sample absorption.
Recently, there have been a number of studies on analysis of petroleum and petroleum products for trace elements (1-4). This interest is because elements such as As, Se, and P b may poison the catalysts used in refineries and automobiles. Also elements such as Cd, As, Se, Hg, a n d P b can cause accumulative detriment to the environment when they are present in fossil fuels. T h e presence of P b in gasoline is a n example of both situations. T h e question of trace elements in petroleum is of more concern to t h e development of synthetic fuels made from coal or oil shale. In these processes, the solids must be heated (usually to temperatures of around 500 "C) and this allows the possibility of elements bound in the rock t o be transferred t o the synthetic crude oils (5-7). The trace element analysis of petroleum products is heavily dependent on atomic absorption analyses (8). However, recent studies by cooperating laboratories have shown that digestion procedures are needed for the atomic absorption analysis of heavy crude oils and vacuum residues (2-4). Typically, these techniques require a good deal of experience before they become routine (2-4, 7). Multielement analyses of wear metals in oils have been done by plasma-source emission spectrometry and apparently interelement and matrix effects are minimal (8). Also, neutron activation has been used for multielement analyses of petroleum (9). However, this technique is not *Onleave from the Department of Radiology, Kitasato School of Medicine, Sagamihara, Kanagawa, Japan 228. 0003-2700/78/0350-0899$0 1.OO/O
readily available and reactor operators are especially worrisome about sample leakages or explosions when these types of samples are subjected to intense neutron bombardment. Consequently, t h e study of other multielement analysis procedures for petroleum is attractive. X-ray fluorescence spectrometric (XRF) analysis of oils would be useful, especially if methods could be found so digestions would not be necessary. This paper is a report of the results of the development of an analytical procedure for trace elements in petroleum by energy dispersive X R F t h a t requires minimum sample preparation. It has been demonstrated that if proper attention is paid to instrument parameters, a n energy dispersive X R F system can detect trace elements in the part-per-million range in typical samples (1&12). I t has also been shown that with good matrix correction programs, multielement analyses on such systems are accurate (10,13). Energy dispersive instruments have been designed to analyze either thin targets (12)or thick targets (10, 11). The system used here was the thin target type primarily designed for the analysis of trace elements in biological samples (12). In this instrument the samples are circular targets 3-4 mm in diameter weighing 300 pg. Weighing and uniformly depositing such a small quantity of petroleum was found to be too difficult. Instead, a n internal spike of an appropriate metallic element was used to eliminate uncertainty over the target thickness. Just how t h e internal spike was used in the determination of sample thickness is shown later in t h e paper.
EXPERIMENTAL Apparatus. The small sample XRF system at the Nuclear Physics Laboratory of the University of Colorado has been previously described (12). In this study, the direct x-ray beam was provided by a tungsten anode tube. Three sets of primary filters (between the x-ray tube and sample) and tube voltages were chosen to change the minimum detection limit (MDL) over various element regions. At the 95Y0 confidence level, the MDL is equal to 3 &/sensitivity (with units of ppm) where B is background counts under the x-ray peak. For adequate MDL over a broad energy range, a 0.12-mm thick Lu filter and 55-kV tube voltage were used. To improve MDL for lighter elements (K through Zn), a 0.51-mm A1 filter and a tube voltage of 40 kV were used. To improve MDL for heavy elements (As through Sr), a 0.10-mm Sn filter and 55-kV tube voltage were used. Table I gives sensitivity in counts per nanogram per coulomb of integrated anode current for various vacuum deposited thin metal films for the three system parameters. The unit of coulombs is used rather than time or power because in this system the x-ray tube current changes from sample to sample. The value for V is obtained by inter1978 American Chemical Society
