1240
Anal. Chem. 1982, 5 4 , 1240-1243 COMMERCIAL
PEANUT
OIL
AT
ester are commonly acquired (10). By the use of the T P F inlet system, the mass spectra of fatty acids in oil-bearing nuts can be obtained directly, at least for fingerprinting identification purposes.
5OO~C
' I
ACKNOWLEDGMENT
i
The authors thank S. Lai, the univeristy glass-blower, for fabricating the fractionator, C. P. Luk for optimizing the linear temperature programmer, and L. P. Ng for typing the manuscript. They also thank United Oversea Enterprises, Ltd., Phillips Petroleum International, Inc., and Dow Chemical (Hong Kong), Ltd., Tsing Yi Plant, for providing the commerical polymer samples.
LITERATURE CITED -
-
c
a A
Y
E 5 -
0
IIh , , I 40
I Bo
80
100
120
140
l l 0
180
200
MIE
Flgure 5. Mass spectra of (i) a sample of commercial peanut oil at 500 OC and (ii) a bit of untreated peanut at 500 O C , obtained at a temperature programming rate of 50 'C/min.
(1) Hu, J. C. Anal. Chem. 1981, 53, 942. (2) Schulten, H. R.; Dilssel, H. J. J. Anal. Appl. Pyro/ysis 1980/81, 2 , 293. (3) Futrell, J. H.; Wells, G.; Voorhees, K. J. Rev. Scl. Instrum. 1981, 52, 735. (4) Shlmizu, Y.; Munson, E. J. Po/ym. Sci., Polym. Chem. Ed. 1979, 17, 1991. (5) Lattlmer, R. P.; Harmon, D. J.; Welch, K. R . Anal. Chem. 1979, 51, 1293. (6) Udseth, H. R.; Friedman, L. Anal. Chem. 1981, 53,29. (7) Jane, I., 28th Annual Conference on Mass Spectrometry and Allied Topics, 1980;paper RPMOA6, p 540. (8) Yuen, H. K.; Mappes, G. W.; Grote, W. A. 1l t h North Amerlcan Thermal Analysis Society Conference, 1981; paper 54; Thermochlm.Acta 1982, 52, 143. (9) Heller, S. R.; Mllne, G. W. A. "EPA/NIH Mass Spectral Data Base"; U.S. Government Printing Office: Washington, DC, 1978; Natl. Stand. Ref. Data Ser. (U.S., Natl. Bur. Stand.), NSRDS-NBS 63. (10) Ryhage, R.; Stenhagen, E. J. Lpld Res. 1960, I , 361.
RECEIVED for review January 18,1982. Accepted February 18, 1982.
Determination of Trace Elements in 13 Organic Solvents by Instrumental Neutron Activation Analysis F. S. Jacobs, V. Ekambaram, and R. H. Filby* Nuclear Radiation Center and Department of Chemistry, Washington State University, Pullman, Washington 99 164
The reagent blank in many trace element analysis methods is an important source of error and is often the limiting factor in reducing elemental detection limits. Trace element contents have been measured for high-purity water (1-3), nitric (1,4), sulfuric and hydrofluoric (1,5) acids, and for other inorganic reagents (1, 4 , 5 ) . Methods have been developed for the preparation of ultra-pure reagents using such techniques as subboiling distillation (6). The study of trace element distributions in fossil fuels, for example, petroleum, oil sand bitumen, and coal-derived liquids, requires solvent extraction, size exclusion chromatography, and/or liquid chromatography (analytical and preparative HPLC) in which relatively large volumes of organic solvents are used in relation to sample size. Examples of such separations in trace element studies are the extraction and separation of oil sand bitumen (7), liquid chromatographic separation of solvent refined coal ( 8 , 9 ) ,size exclusion chromatography of coal liquids (10,11),and solvent extraction of solvent refined coal (12). Some studies of this type have been carried out with little regard for the trace element blank from solvents used in separation or extraction procedures. The assumption often made is that organic 0003-2700/82/0354-1240$0 1.25/0
solvents have extremely low trace element contents relative to those of the sample. Determinations of trace elements in organic solvents have been reported by Russian authors. Ovrutskii et al. (13)determined 18 trace metals in organic solvents by emission spectroscopy following evaporation of solvents on a carbon matrix. Detection limits obtained ranged between 0.3 ng 8-l and 10 ng g-l. A similar procedure was used by Kuz'min et al. (14) to determine 14 trace elements in isopropyl alcohol, rn-xylene, dioxane, isoamyl alcohol, and chloroform with detection limits between 0.1 ng 8-l and 10 ng g-l. In later work Kucherova et al. (15)measured 10 trace elements in carbon tetrachloride, chloroform, and methylene chloride and obtained results comparable to those of Ovrutskii et al. (13) and Kuz'min et al. (14). Recently Manoliu et al. (16)used atomic absorption spectrometry to analyze solvent evaporation residues and reported concentration values for Cu, Ni, Fe, Zn, Mn, Mg, Na, K, Ca, and Ag in methanol and isopropyl alcohol to be within the range 3-78 ng 8-l. Most organic solvents manufactured and used in the United States are not routinely analyzed for trace metals, except for 0 1982 Amerlcan Chemical Society
ANALYTICAL CHEMISTRY, VOL. 54, NO. 7, JUNE 1982
certain elements $suchas P b and Cu. In an effort to evaluate the solvent blank contribution to trace element data from chromatographic fractions of coal, petroleum, and oil sands it was found necessary to determine the trace element composition of some common organic solvents used in this laboratory. In this ,study, a total of 26 trace elements were measured by instrumental neutron activation analysis in toluene, heptane, hexane, pentane, tetrahydrofuran, pyridine, 2-propanol, ethyl acetate, diethyl ether, ethanol, methanol, chloroform, and acetone.
EXPERIMENTAL SECTION Fifty milliliters (100 mL for some solvents) of solvent was evaporated in a high purity quartz vial (8 mm X 30 mm). The quartz vials were cleaned prior t o use by washing with double distilled nitric acid and water. The solvent was allowed to drip at a precisely timed rate from a separating funnel into the quartz vial placed on a hot plate set to maintain the solvent at a subboiling temperature and constant level. When the solvent had completely evaporated, the vial was sealed in a 2-dram polyethylene irradiation vial. A blank quartz vial was prepared in the same manner as the sample except that no solvent was applied. National Bureau of Standards, Standard Reference Materials SRM 1632 Coal, SRM 1633 Fly Ash and SRM 1571 Orchard Leaves, as well as U.S. Geological Survey, Standard Rocks GSP-1, PCC-1, and BCR-1 were used as standards by sealing 15-100-mg aliquots in 2/6-dranlpolyethylene vials which were then packaged in 2-dram polyethylene irradiation vials. Samples and standards were irradiated in the Washington State University TRIGA-fueled Mark-I11 research reactor at a neutron flux of 6 X 1OI2 n cm-’s-’ for a period which depended on the half-life of the induced radionuclides (see Table I). Decay and counting times are also shown in Table I. At the end of the irradiation for the measurement of short half-life (n,y) nuclides, the quartz vials were counted directly on a Ge(Li) y-ray spectrometer. For the intermediate and long half-life nuclides, each quartz vial was leached with 8 M HNO, and then washed with CSz. The leachates and washings were transferred to a pietri dish and mixed with quartz powder to maintain constant counting geometry. Determination of radionuclide activity was made by Ge(Li) y-ray spectrometry using a Nuclear Data ND 6620 spectrometer. Conversion of y-ray spectra to elemental concentrations was carried out by using either SPANAL or FOURIER programs in the Washington State University Amdahl 470 computer (17, 18). Corrections for decay during counting (for TI < 10 min), overlapping y-ray peaks (e.g., zo3Hg/76Se at 280 ked), and neutron fluxvariations betwtaen irradiation levels were made in the FOURIER and SPANAL programs. Full details of counting procedures and y-ray spectra reduction have been presented elsewhere (17,18).
RESULTS AND DISCUSSION Table I1 presents the trace element concentrations in ng L-l measured in 13 organic solvents. For heptane, hexane, and tetrahydrofuran, different solvent grades were analyzed. The associated error terms resulting from counting statistics are not reported due to space limitation. In general they are found to be less than 15% (relative). It should be noted that the trace element concentration ranges of these solvents are comparable to those previously reported for Russian and Rumanian solvents (13-,(6). A measure of the overall trace element content of each solvent, X I , was computed by summation of elemental concentrations, including “less than” values. The value for X I thus represents an upper “impurity” limit and is an approximate measure of overall trace element purity for the solvents considered in this study. It is significant that toluene contains the highest X I value and highest concentrations of Na, K, Co, Cr, and Fe of any solvent because toluene is commonly used in the extraction of fossil fuel components. It is evident that the Mallinckrodt
1241
Table I. Irradiation and Counting Procedures for Instrumental Neutron Activation Analysis irradiation time
dements determined V, Cl,Mn
decay time
Short Half-Life Nuclides 3-5 min 1-30 min
Intermediate-Lived Nuclides As, Br, Cu, Ga, K, 8h 36 h La, Mo, Na, Srn Long-Lived Nuclides Ba, Ce, Co, Cr,Ch, 8h 21 d Eu, Fe, Hf, Hg, Ni, Rb, Sb, Sc, Se, Sr, Ta, Rb, Th, Zri, Zr
count time 150-1000 s 4000 s
80000 s
-
analytical-reagent grade heptane contains lower concentrations of As, Br, Cr, Fe, K, Mn, and Zr than the Baker reagent and HPLC grades. The Waters and Baker HPLC grades of hexane are similar in trace element contents. The tetrahydrofuran data show few differences in trace element content. It may be concluded that HPLC grades are not necessarily lower in overall trace element contents than reagent grades and this is not particularly surprising because HPLC-grade solvents are produced to have lower contents of organic rather than inorganic impurities. As would be expected from its ability to complex metals and to dissolve some inorganic compounds, pyridine contains several trace elements, e.g., Fe, Mn, Zn, and Sb a t higher concentrations than in most other solvents. In general, the n-alkane solvents contain very low concentrations of most trace elements. Trace element concentrations of components separated using large solvent-to-sample ratios may require significant correction for the reagent blank. An example of the importance of the solvent correction is the extraction of bitumen from Athabasca oil sand by toluene. Ten grams of bitumen was extracted from Athabasca oil sand with 1000 mL of toluene (Baker reagent grade) using a Soxhlet extractor. The solvent was removed with a rotary evaporator and the bitumen was then analyzed for trace elements by neutron activation analysis. The measured and solvent-corrected concentrations of nine elements are shown in Table 111. At the 95% confidence level the solvent-corrected concentrations of Br, Co, Fe, Mn, and Hg are not significantly different from the observed values. However, for K and Zn the solvent correction represents 56% and 61 %, respectively, of the observed values. For Na the corrected value is not significantly different from zero. For the other 17 elements measured in the oil sand bitumen, the solvent contributions to the observed values were negligible. In this study it was assumed that trace elements in the solvent are present in nonvolatile forms and would be incorporated in the solute during solvent extraction and subsequent evaporation. This assumption does not preclude the existence of organic halides (RX), organoarsenic (R,As), organoselenium (R2Se),cdrganoantimony (R,Sb) and volatile Hg species, in which case the measured values represent only nonvolatile componentti. It was on the basis of this assumption that the experimental design was chosen.
CONCLUSION The data reported here should assist the analyst in evaluating the solvent blank when organic solvents are used in extraction or separation of materials subjected to trace element analysis. Care and judgement need to be exercised using these data; it should be emphasized that the values apply only to the individual bottles analyzed. It is likely that different lots of a given solvent would show different trace element contents
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ANALYTICAL CHEMISTRY, VOL. 54, NO. 7, JUNE 1982
Table 11. Trace Element Concentrations for Organic Solvents (in ng L-I)
element As Br Ce
c1 co cr cs Eu Fe Ga Hf Hg K La Mn Mo Na Sb sc Se Sm Tb Th V Zn Zr
(LaL-')
element As Br ce
a co cr cs
Eu Fe Ga Hf Hg K La Mn Mo Na Sb sc Se Sm Tb Th V Zn Zr X I ( r g L-I) element As Br Ce
c1 co Cr cs Eu Fe Ga
pentane Baker Reagent 4.3
