Anal. Chem. 1984, 56, 1758-1760
1758
Table 11. Tentative Assignment of Gas Chromatographic Peaks from TG/GC/MS Experiment on Coal Sample, 1.6 to 3.0% Weight Loss carbon dioxide sulfur dioxide methylcyclohexane alkane alkane and dimethylnaphthalene dimethylnaphthalene alkane and dimethylnaphthalene alkane alkane trimethylnaphthalene
1 2 3 4
5 6 7 8
9 10 11 12 13
alkane ZCI7 alkane alkane
14
alkane I I
TG/MS data, one can then plot important ions in the mass spectra of these compounds, such as 64 for sulfur dioxide and 156 for dimethylnaphthalene, as a function of temperature. Figure 9 contains these ion vs. temperature plots along with a plot of the total ion current observed during the TG/MS experiment. The results indicate that dimethylnaphthalenes are evolved much earlier in the run than is sulfur dioxide. This approach assumes that there are no interferences from other compounds a t m / z 64 and 156. The experimental results from the TG/GC/MS analysis of coal have shown that the on-column cryogenic trapping technique employed in this work is a relatively simple and highly efficient approach to evolved gas analysis. It is capable of trapping, separating, and identifying compounds ranging from gases like sulfur dioxide to high-boiling alkanes and aromatics. Although the present TG/GC/MS system has only been used to date in qualitative analysis of evolved species, research is planned for evaluating methods for quantitative analysis.
LITERATURE CITED (1) Langer, H. G. I n "Treatise on Analytical Chemistry"; Kolthoff, I.M., Elvlng, P. J., Eds.; Wlley-Intersclence: New York, 1982; Vol. 12,
f
l
I
Y)
1m
,
I
1%
XC TernpB'slurc
250
,
3W
1
350
I'CI
Figure 9. Ion current vs. temperature plots from TG/MS experiment,
0-3% weight loss.
bining the information gathered in both the TG/MS and TG/GC/MS modes. For example, from the TG/GC/MS experiment one observed the presence of compounds such as sulfur dioxide and dimethylnaphthalene. By using the
Chapter 6. Chiu, J. Thermochlm. Acta 1970, 1 , 231-238. Chang, T.; Mead, T. E. Anal. Chem. 1971, 43, 534. Barnes, P. A.; Stephenson, G. Anal. R o c . 1981. Dec, 538. Jennlngs, W. "Gas Chromatography with Glass Capillary Columns", 2nd ed.; Academic Press: New York, 1980; p xi. (6) Moncur, J. G.; Sharp, T. E.; Byrd, E. R. HRC CC, J. High Resolut. Chromatogr. Chromatogr. Commun. 1981, 4 , 603. (2) (3) (4) (5)
RECEIVED for review September 1,1983. Resubmitted March 30, 1984. Accepted April 2, 1984.
Low-Contamination Digestion Bomb Method Uslng a Teflon Double Vessel for Biologkal Materials Kensaku Okamoto* and Keiichiro Fuwa
National Institute for Environmental Studies, Enuironmental Agency of Japan, Yatabe-machi, Tsukuba, Ibaraki 305,Japan The National Institute for Environmental Studies (NIES) recently initiated a certified reference material (CRM) program, the objective being the preparation and certification of biological and environmental standards, to serve the needs of environmental scientists and laboratories. The certified reference materials issued by NIES are Pepperbush (I),Pond Sediment ( 2 ) ,and Chlorella, the elemental compositions of which have been certified. The certification process requires analytical data for the elements to be obtained from independent and established analytical techniques. At NIES atomic absorption spectrometry (AAS), flame emission spectrometry (FES), and inductively coupled plasma atomic emission spectrometry (ICP-AES) have been extensively used for the determination of trace elements in NIES reference materials. These analytical techniques invariably require sample to be in liquid form so that for solid sample types complete dissolution of the sample is required. There have been a number of reports concerning sample dissolution, and recently wet digestion utilizing a Teflon decomposition bomb (3-5)has been in-
creasingly used due to the following advantages over conventional wet digestion: (1)faster digestion, (2) high efficiency of destruction under pressure, (3) virtually complete elimination of losses of volatile elements, (4) relatively small quantity of mineral acid required for digestion which reduces the contamination from the acid. Through the NIES CRM program, it has been required to investigate and develop a contamination-free digestion system for biological materials, since trace element levels in these matrices are so low that contamination during sample dissolution procedure often became a limiting factor for trace analysis. A t an earlier stage, a Teflon bomb developed by Iida et al. (6) was used for the decomposition of NIES Chlorella and Freeze-Dried Serum reference materials. Recently, we developed an improved Teflon bomb method that uses a small screw cap vial for sample decomposition inside the Teflon digestion vessel. This digestion system can reduce the risk of sample leakage and contamination with extraneous materials and can be applied to biological materials up to approximately 300 mg of dry weight.
0003-2700/84/0358-1758$01.50/00 1984 American Chemical Society
ANALYTICAL CHEMISTRY. VOL. 56, NO. 9. AUGUST 1984
1759
Table I. Analytioal Results for NBS Oyster and NIES Mussel Reference Materials'
element
Sr Cd Ni Ag Cr
Ph co Hg
analyt tech
GFAAS ICP GFAAS GFAAS GFAAS GFAAS GFAAS GFAAS CVAAS
f *
NBS ovster found, p g / g S RSD, %
12.6 0.4 9.96 f 0.20 3.6 f 0.1 1.15 f 0.02 0.89 0.02 0.68 0.02 0.48 0.02 0.37 i 0.01 0.056 f 0.01
*
2.0 2.8 1.7 2.2 2.9 4.2 2.7 1.8
certified value
13.4 f 1.9 10.36 0.56 3.5 + 0.4 1.03 f 0.19 0.89 f 0.09 0.69 f 0.27 0.48 0.04 (0.4) 0.057 i 0.015
*
NIES mussel found (pg/g) i*s RSD, %
9.1 f 0.3
16.5 f 0.4 0.83 i 0.03 0.97 0.05 0.030 + 0.002 0.65 f 0.02 0.92 f 0.03 0.42 f 0.01 0.061 f 0.001
1.9 3.7 4.0 1.9 3.3 2.4 3.6 5.2 6.7 3.1 3.3 2.4 1.6
'Key: AAS, air-acetylene flame atomic absorption spectrometry; GFAAS, graphite furnace AAS; CVAAS, cold vapor AAS ICP, inductively coupled plasma atomic emission spectrometry; i, mean value; s, standard deviation; RSD, relative standard deviation, n = 3 (NBS ovster) n = 6 (NIES mussel).
-C .B
A
0 cm 5 Flgure 1. Douole vessel digestion bomb: (A) PFA Tul-Tainer Screw cap vial (7 mi, (E) PTFE vessel (23 mL) w8h PTFE lid. (C)stainless
steel jacket
EXPERIMENTAL SECTION Figure I illustrates the construction of ihe digestion homh, whirh MnPisw of a Teflon PFA (perfluoroalkoxy)'l'uf-'lkiner vial (Pierce Chemical Co.) of 7 ml. capacity IA), a 'l'dlon PTFE (p,,lyttpimfl~iaroethylene))vessel of 23 ml. caparity (R),and a stainless SIPPI jacket ICJ. At NIES, this dimtion system has heen successfully used for sample dissolution of SIES Hair. Mussel. and Tea Leaves reference materiala and NBS rNational Bureau of Standards) Oyster. Cirrua I.eaws. and Hovine 1.iver Standard IMerenrt Materials. where similar procedures were employed varying ihe cumbination and quantities of mineral arid added. The digestion of Mussel and Oyster reference materials is drwrihed helow BO an exnmple. Weigh the sample tup to 3W mg dry weight) in a PFA tial and add 3 ml. uf subboiling distilled nitric arid Close the cap of the vial tightly by hand and then put it into the PTFE ves.iel. Add I mL uf distilled water, cover uiih the PTI.'E lid and insert it into the stainless steel jacket. Fasten the atainleia steel cap tightly with a wrench and place it in an air-o%,en.Heat the homh at 90 'c' for 2 h and then increase the temperature t n 140 "C and hold it for 4 h at temperature. After cooling the bomb overnight. open the iorinlrw* i t e l cap carefully and expel the gas inside the PTFE vessel. Take out the PFA $,ialand wash it with distilled water. After wiping the vial with tissue paper, loosen the CHI, slwdy and carefully until the huhhling of dissolved gas starts. Allnw it to siand for 20-30 min to release CO?. SO2gas until the wlnr of the digest changes irum dark green w yelluw.green. Remove the cap and wash the inside of the cap with small vnlume of distilled water and add tn the digest. For ordinary biological samples, the dige*t w l u t i u n can he applied IO AAS analysis after appropriate dilution with distilled u'ater. For the determinatron of Hg. add
1 mL of 2% KMnO, (AAS grade) solution to the whole digest solution and then subject the mixture to cold-vapor AAS. since ICP-AES requires complete destruction of organic matter, which may cause matrix interferences, it is preferahle to further treat the digest with perchloric acid on a hot plate. The following open digestion was continued for the decomposition of Mussel and Oyster reference materials. Place the vial on a hot plate and evaporate the content to approximately 0.3 mL at 130-140 "C. Add 0.5 mL of hydrofluoric acid tn dissolve siliceous materials contained in Mussel and Oyster samples. (In case that the sample does not contain siliceousmaterial, this procedure can he omitted.) Heat to approximately 0.3 mL and, after cooling, add 0.5 mL of perchloric acid and heat to white fumes of perchloric acid. Add 0.5 mL of nitric acid and evaporate the content to approximately 0.3 mL. Add a small volume of distilled water and transfer the content quantitatively to a 25-mL volumetric flask and dilute to volume with distilled water. The sample preparation and evaporation procedures were done in a class 100 clean hood. This digestion procedure could he performed on a 2 day cycle The solutions were subjected to ICP-AES and AAS analysis. A Jarrell-Ash 975 Atom Comp atomic emission spectrometer was used for the determination of major, minor, and trace elements in Mussel and Oyster samples. A two-point calibration procedure was employed for the standardization of the instrument, using the high standard (10 pg/mL of the required elements in 0.1 M nitric acid) and the low standard (0.1 M nitric acid). A Hitachi 180-80Zeeman atomic absorption spectrometer was used in the airacetylene flame and graphite furnace atomization modes. For the determination of trace elements by GFAAS, a standard addition method was employed to compensate for matrix interferences due to major constituents, particularly C1. In the case of Ph determination by GFAAS, ammonium nitrate (AAS grade) was added as a matrix modifier to give a concentration of 0.5% in the sample solutions. Mercury was determined by cold-vapor AAS with a Tokyo Koden ANA-K80 mercury analyzer.
RESULTS AND DISCUSSION Decompition methods of biological materials for elemental analysis should he evaluated on the basis of losses of the desired elements, contamination from acids, and, with decomposition vessels, speed, safety, and cost. With respect to the losses, the cap of the PFA vial has an inner secondary seal so that excellent holding of the content can he maintained. As shown in Table I, the analytical values for NBS Oyster SRM were in g o d agreement with the NBS certified values, indicating that no significant losses of the elements occurred during the sample decomposition by this system. Even for volatile elements such as Hg and As, excellent data were obtained. Small relative standard deviations obtained for each element in Oyster and Mussel reference materials (Table I) show good reproducibility of this digestion technique.
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ANALYTICAL CHEMISTRY, VOL. 56, NO. 9, AUGUST 1984
Contamination with extraneous materials is a serious problem in trace element determination in biological materials. In this system sample decomposition is performed in the PFA vial inside the PTFE vessel; therefore, there is no possibility of contact with the outer stainless steel jacket. For the elements given in Table I, no significant peak was detected for the blank solutions. High-purity inorganic acids are required to reduce contamination. Nitric acid purified by subboiling distillation with a synthetic quartz still was used in this experiment. Perchloric acid and hydrofluoric acid used were of analytical grade and purification of the acids by subboiling distillation is under way. The amount of a sample applicable to the PFA vial is an important point. It is preferable to use as much sample as possible for trace element determination, but the quantity of the sample is limited for safety against gas pressure which results from decomposition of organic material. Uchida et al. (7) used a PTFE inner vessel (3.8 mL) for the digestion of 1-2 mg of biological materials, followed by AAS and FES determinations with discrete nebulization in a microliter sample volume. In this digestion system up to approximately 300 mg of the dry biological materials, a relatively large amount for closed digestion system, could be digested in the vial. Concerning the safety of the bomb, there was no burst problem for the digestion of a number of biological materials
under the conditions mentioned above. For safe operation, however, careful attention must be given to the size and nature of the sample, the amount of acid, and operating temperature. The disadvantage of this system is, however, that the PFA vial should be replaced after four to five uses under pressure. This replacement is a little costly. ACKNOWLEDGMENT The authors wish to thank Yukihiro Nojiri and Takashi Uehiro for their valuable advice on the construction of the bomb. LITERATURE CITED (1) Okamoto, K., Ed. "Preparation, Analysis and Certification of Pepperbush Standard Reference Material"; National Institute for Environmental Studles: Ibaraki, Japan, 1980; Research Report No. 18. (2) Okamoto, K., Ed. "Preparation, Analysis and Certification of Pond Sedlment Certified Reference Material"; National Institute for Environmental Studies: Ibaraki, Japan, 1982; Research Report No. 38. (3) Bernas, B. Anal. Chem. 1968, 4 0 , 1682-1686. (4) Van Eenbergen, A,; Bruninx, E. Anal. Chlm. Acta 1978, 98, 405-406. (5) Uhrberg, R. Anal. Chem. 1982, 5 4 , 1906-1908. (6) Iida, C.; Uchida, T.; Kojima, I . Anal. Chim. Acta 1980, 113, 365-368. (7) Uchida, T.; Kojima, I.; Iida, C. Anal. Chlm. Acta 1980, 116, 205-2 IO.
RECEIVED for review February 27,1984. 1984.
CORRECTION Simultaneous Determination of Inorganic Anions a n d Cations by Ion Chromatography w i t h Ethylenediaminetetraacetic Acid as Eluent Manabu Yamamoto, Hirofumi Yamamoto, Yuroku Yamamoto, Susumu Matsushita, Nobuyuki Baba, and Tetsuo Ikushige (Anal. Chem. 1984,56, 832-834). There are unfortunate errors in Table 11, appearing on p 833: The concentration of Cl- of 4.6 in tap water should read 14.6. The concentration of Sod2of 0.38 in rainwater and that of 0.43 in tap water should read 3.8 and 4.3, respectively.
Accepted April 18,
