2060
Anal. Chem. 1900, 6 0 , 2060-2062
Wet Microwave Digestion of Diet and Fecal Samples for Inductively Coupled Plasma Analysis Gwen M. Schelkoph and David B. Milne*
USDA, ARS, Grand Forks Human Nutrition Research Center, P.O. Box 7166, University Station, Grand Forks, North Dakota 58202
A method for the mlcrowave dlgestlon of diet and fecal samples from metabollc balance studles Is described. Samples, enclosed In vessels (CEM Corp.) made of Teflon PFA brand fluorocarbon resin with a nltrlc-hydrochlorlc acld mixture, were dlgested In a mlcrowave oven for a period of up to 30 mln. Inductlvely coupled plasma emlsslon spectroscoplc (ICP-ES) analysis of the dlgestates for elgM dlfferent mineral elements lndlcated excellent agreement with certlfled values In several NBS Standard Reference Materials. Recoverles of 92-104% and precision between 0.1 and 9.3% relative standard deviation lndlcated that this procedure was well suited for provldlng dlgested samples for elemental determination by ICP-ES.
It is necessary t o destroy and remove the organic matrix of samples prior to trace element analysis by atomic absorption spectroscopy or by inductively coupled plasma emission spectroscopy (ICP-ES). Gorsuch (1)has discussed many of the problems related t o different dry-ashing and wet-ashing procedures. Wet ashing with perchloric and nitric acids is a widely used technique for digesting biological samples. However, it is relatively time consuming, presents a potential explosion hazard, requires constant operator attention, and often requires large amounts of acid to completely digest the sample. Open vessel work requires special hoods, can lead to corrosion of equipment, is open to potential contamination of the sample, and risks mechanical and volatile loss of the analyte. Recently, several laboratories have experimented with .the use of microwave energy to digest a variety of materials for trace mineral analysis (2-7). Studies with biological materials either have utilized open vessels in specially vented systems (2, 7) or have been limited to the analysis of a few specialized types of samples and had limited recovery data ( 3 , 4 , 6, 8). During the course of human metabolic balance studies, we routinely analyze many foods, diet composites, and fecal samples for several different essential trace elements. Here we describe a method for using microwave energy for rapid digestion of diet and fecal samples in closed Teflon PFA vessels for ICP-ES analysis of several different trace elements.
EXPERIMENTAL SECTION Reagents and Standards. Specific NBS Standard Reference Materials (SRMs) used were citrus leaves (SRM 1572), wheat flour (SRM 15671, and mixed diet (RM-8431). Recovery standards were prepared from 10 000 ppm reference standards obtained from VHG Labs, Inc., Manchester, NJ. Vycor double distilled acids (GFS Chemicals, Columbus, OH) were used for the digestions. Equipment. Digestions were accomplished by use of a microwave digestion system (CEM Corp., Indian Trail, NC, MDS81D), equipped with a 600-W magnetron that is adjustable in 1% increments. Included in the system were a rotating turntable and a variable-speed exhaust fan to direct any corrosive fumes to a hood. Time and power settings were controlled by a programmable microprocessor. Digestions were carried out in 120-mL This article not subject to U.S.
Copyright.
Table I. Analysis of NBS Reference Materials (Hg/g) after Microwave Digestion
calcium certified determined re1 std dev copper certified determined re1 std dev iron certified determined re1 std dev potassiumn certified determined re1 std dev magnesium certified determined re1 std dev manganese certified determined re1 std dev phosphorus" certified determined re1 std dev
wheat flour
mixed diet
citrus leaves
(1567)
(8431)
(1572)
190.0 f 10 185.8 1.7%
1.94 f 0.14" 1.89 0.8%
31.5 f 1.0' 30.9 1.1%
2.0 f 0.3 2.2 6.8%
3.36 f 0.33 3.1 2.2%
16.5 i 1.0 16.1 3.1%
18.3 i 1.0 16.5 2.5%
37.0 f 2.6 36.0 0.8%
90.0 f 10 80.6 1.6%
1.36 f 0.4 1.2 1.4%
7.9 i 4.2 7.9 1.0%
18.2 f 0.6 17.4 0.7%
b 346.6 2.1%
650 f 40.0 698.5 0.8%
5.8 i 0.3' 5.7 1.1%
8.5 f 0.5 8.0 2.1%
8.12 f 0.31 8.2 1.0%
23.0 f 2.0 21.9 2.2%
b 1.2 1.7%
3.3 i 0.3 3.3 1.0%
1.3 i 0.2 1.3 1.2%
10.6 i 1.0 9.94 0.7%
17.0 f 0.6 16.3 1.0%
29.0 f 2.0 28.3 3.4%
zinc
certified determined re1 std dev
'mg/g. No certified values. Determined values are the mean of three replicates. Teflon PFA vessels (CEM Corp., Indian Trail, NC) that were equipped with safety relief valves and vented into a beaker of dry ice to condense any fumes that may escape. The Teflon PFA vessels are sealed, using a capping station, to 12 f t lb torque for proper valve operation. Trace element analyses were accomplished with a Jarrell-Ash Model 1140 ICP-ES system, capable of analyzing up to 20 elements simultaneously. This consisted of an ICP excitation source, a direct reading spectrometer, and a PDP 11/04 dedicated minicomputer for data acquisition and readout. An adjustable cross flow pneumatic nebulizer was employed to aspirate liquid samples through the torch assembly. Digestion Procedure. Between 0.5 and 1.0 g of sample was weighed directly into the digestion vessels to which 5 mL of concentrated nitric acid and 2 mL of 6 M hydrochloric acid were added. The digestion vessels were sealed to 12 ft lb torque using an automated capping station. Twelve vessels were digested at a time using the following two procedures. The first, used for wheat flour and diet samples, consisted of 5 min, 50% power; 5 min, 0 power; 5 min, 50% power; 5 min, 75% power; 5 min, 0 power; and 5 min, 75% power. This method of alternating power fluxes was chosen to minimize possible venting of fumes from the
Published 1988 by the American Chemical
Society
ANALYTICAL CHEMISTRY, VOL. 60, NO. 19, OCTOBER 1, 1988
RESULTS AND DISCUSSION
Table 11. Recovery of Added Standards to Diet and Fecal Pools diet
added covereda Cu
Fe Mg Mn Zn
1.5 6.5 20 30 450 600 10 20 20 30
1.38 6.24 18.8 29.0 426.2 520.7 10.5 20.8 20.4 30.6
analyzed and certified values for all elements when the various SRMs were digested and analyzed by our method. Recoveries of the various ions added to diet composites or freeze-dried fecal samples were between 87 and 110% (Table 11). Within run precision of triplicate samples was between 0.1 and 9.3%. Run to run precision for four runs on different days averaged 5.3% for diet pools and 6.5% for fecal pools. There was also reasonable agreement between the microwave digestion and a traditional nitric-perchloric acid digestion procedure (9) (Table 111). Microwave digestions, however, yielded consistently higher values than did nitric-perchloric digestions for the elements Ca, Fe, and P when diets or feces were compared. Conversely, slightly higher values for Cu were seen with the nitric-perchloric digested SRMs. A small amount of residue remained on the sides of some of the vessels after microwave digestion. We were unable to find detectable amounts of mineral elements in the residue upon redigestion and analysis. Infrared spectroscopic analysis of the residue from a fecal digestion indicated that it was comprised mainly of undigested amino acid residues. This is consistent with other studies that have shown that many organic materials are not completely decomposed by a nitric-hydrochloric acid combination and incomplete digestion products may remain in suspension ( 4 , 10). Several different combinations of acids were attempted with variable success. The addition of HF, HC104, or HzOZto the microwave digestion mixture, as suggested by others ( 2 , 3 , 5 ,
amount, pg rerecovery, added covereda %
recovery,
re-
As shown in Table I, there was excellent agreement between
feces
amount, r g '70 92 96 94 97 95 87 105 104 102 102
5 10 25 40 700 1000 15 25 30 40
4.8 9.9 23.0 41.7 648.7 988.7 14.9 26.1 29.8 39.8
96 99 92 104 93 99 99 104 99 99
Amount recovered is the average of tridicate determinations. vessels at pressures above 120 psi. The second procedure consisting of 12.5 min, 50% power; 5 min, 0 power; and 10 min, 75% power, was used for the dissolution of more difficult samples such as citrus leaves and freeze-dried feces that were not completely digested by the first program. Spontaneous venting did not normally occur under these conditions. After the digestion programs were completed, the samples were cooled, vented, and, using volumetric flasks, diluted 1:50 with deionized water prior to ICP-ES analysis. The microwave digestion procedure was compared with a traditional nitric-perchloric acid digestion method (9).
Table 111. Comparisons between Microwave and Open Hot-Plate Nitric-Perchloric Acid Digestions diet pool microwavea
HN03-HC104"
fecal pool microwave"
225.4 f 1.5 0.37 f 0.02 5.17 f 2.30 0.91 f 0.01 122.9 f 0.6 2.24 f 0.06 430.2 f 2.5 4.14 f 0.04
37.7 76.4 343.4 22.2 10.2 208.3 33.2 412.1
256.1 f 5.3 0.34 f 0.02 5.11 f 0.30 1.54 f 0.04 143.4 f 2.7 2.65 f 0.08 460.0 f 8.3 4.89 f 0.15
microwave" Ca
30.9 16.0 80.6 17.4 5.7 21.9 1.2 28.3 59.2
cu
Fe K Mg Mn P Zn B a Average
citrus leaves (1572) HN03-HC104"
of triplicate determinations.
30.1 17.0 87.6 16.7 5.3
certified 31.6 16.5 90.0 18.2 5.8 23.0 1.3 29.0
...
1.3 28.9
microwave"
f 1.0 f 1.0 f 10.0
f 1.3 f 1.9 f 3.1 f 0.2 f 3.0 f 3.4 f 0.7 f 4.4
26.6 f 66.5 f 323.5 f 18.8 f 8.2 f 206.0 f 26.6 f 395.5 f
wheat flour (1567) HN03-HC104"
0.14 0.5 2.8 0.1 2.8 1.4 0.6 4.2
certified
...
185.8 2.2 16.5 1.2 346.6 8.1 1.22 9.9
f 0.6 f 0.3 f 2.0 f 0.2 f 2.0
HN03-HC104"
190 f 10.0 2.0 f 0.3 18.3 f 1.0 1.36 f 0.04
2.8 16.5 1.54 390.0 8.0 1.40 10.1
b 8.5 f 0.5
b 10.6 f 1.0
b
No certified values.
Table IV. Comparison of Different Acid Mixtures Used for Microwave Digestion of Citrus Leaves (1572)
Ca
cu
Fe K Mg Mn P Zn
" Described method.
HNOa-HCl"
HNO3-HC1 HClOI
HNO3-HCl HF
HNOS-HCl HzOz
30.9 f 0.3b 16.1 f 0.5 80.6 f 1.3 17.4 f 0.1 5.7 f 0.06 21.9 f 0.5 1.3 f 0.02 28.3 f 1.0
27.6 f 0.01 14.4 f 0.04 76.7 f 4.9 11.0 f 0.66 4.4 f 0.01 19.0 f 0.09 1.1f 0.005 25.0 f 0.32
23.9 15.3 53.2 19.2 4.0 22.0 1.3 28.3
30.8 15.0 78.0 21.9 5.2 22.1 1.4 28.0
Mean triplicate determinations.
2061
f 3.4
f 0.3
f 12.9 f 1.7 f 0.5 f 0.5 f 0.04 f 1.2
f 0.4 f 0.5 f 3.9
f 1.3 f 0.07
f 0.3 f 0.02 f 0.4
NBS certified value 31.5 16.5 90.0 18.2 5.8 23.0 1.3 29.0
f 1.0 f 1.0 f 10.0 f 0.6 f 0.3 f 2.0 f 0.2 f 2.0
2062
Anal. Chem. 1988, 60, 2062-2066
6, I I ) , led to variable recoveries and low values for most elements (Table IV). Hydrofluoric acid has been used for the dissolution of geologic samples (6). There has also been a report of H F use with some biological samples, particularly when silicon analyses are needed (3). However, when H F was added to our system, unacceptable amounts of particulate matter remained after digestion of the fecal samples; there were poor recoveries of most elements except for phosphorous and zinc. Digestions seemed to be more complete with nitric acid-hydrogen peroxide or nitric-perchloric acid combinations. However, some elements, including iron and copper, were poorly recovered when these combinations were used. We have successfully used our method to digest a wide range of diet, food, and fecal samples with differing fat contents. Traditional open flask digestion procedures (9) generally require up to 20 mL of a 1:5 perchloric-nitric mixture and 8-13 h to digest a 0.5-g sample of either freeze-dried feces or diet in an open 50-mL Erlynmeyer flask on a hot plate. Compared with this open-flask, hot-plate procedure, the microwave procedure offers considerable advantages in speed and safety. Blank values are lower because less acid is required and the sample is not exposed to a hood environment for long periods of time. The closed system maintains sample integrity well and thus permits the determination of more volatile elements. In a preliminary study, we obtained 97-105% recoveries of added boron by using the closed microwave digestion method, whereas boron recoveries were nil when conventional nitric-perchloric digestion techniques (9) were
used. Thus, the microwave digestion method is relatively safe, simple, and rapid with good precision and accuracy. This procedure seems to be particularly suited for providing digested samples for elemental determinations by ICP-ES. ACKNOWLEDGMENT We wish to express our appreciation to Kathy Huot and Elaine Westerlund for their competent technical assistance. LITERATURE C I T E D (1) Gorsuch, T. T. The Destruction of Organic Matter: Pergamon: New York, 1970. (2) Abu-Samra, A.; Morris, J. S.;Kolrtyohann, S. R. Anal. Chem. 1975. 4 7 , 1475-1477. (3) Nadkarni, R. A. Anal. Chem. 1984. 5 8 , 2534-2541. (4) Kingston, H. M.: Jassie, L. B. Anal. Chem. 1988, 5 8 , 2534-2541. (5) Mahan, K. I., et al. Anal. Chem. W87, 5 9 , 938-945. (6) Papp, C. S. E.: Flscher, L. B. Analyst (London) 1987, 772, 337-338. (7) Barrett, P. B.; Davldowski, L. J., Jr.; Penaro, K. W.; Copland, T. R. Anal. Chem. 1978, 5 0 , 1021-1023. (8) Aysola, P.; Anderson, P.; Langford, C. H. Anal. Chem. 1987, 5 9 , 1582-1583. (9) Analytical Methods Committee Analyst (London) 1980, 8 5 , 643. ( 10) Bock, R. A Handbodc of DecMposiMw, Methods in Ana&thl Chemisfry;translated and revlewed by Man, I. L.; Wiley: New York, 1979. (11) White, R. T., Jr.: DouthiR, G. E. J . Assoc. Off. Anal. Chem. 1985, 68, 766-769.
RECEIVED for review March 4,1988. Accepted June 27, 1988. Mention of a trademark of proprietary product does not constitute a guarantee or warranty of the product by the U.S. Department of Agriculture and does not imply its approval to the exclusion of other products that may also be suitable.
Laser-Enhanced Ionization as a Selective Detector for the Liquid Chromatographic Determination of Alkyltins in Sediment K. S. Epler a n d T. C. O’Haver Department of Chemistry and Biochemistry, University of Maryland, College Park, Maryland 20742
G. C. T u r k * a n d W. A. MacCrehan Center for Analytical Chemistry, National Bureau of Standards, Gaithersburg, Maryland 20899
Laser-enhanced ionlzatlon, used as an element-speclflc detector for llquld chromatography (LC), Is appiled to trlalkyltln determinations In sediment. The LC separates the organotlns and resolves spectral interferences from the analytes. A rapid method for extractlng trlbutyttln (TBT) from sediment Into 1-butanol Is described. The detection limit is 3 ng/mL tin as TBT or 0.06 ng of tin.
Many approaches to metal speciation involve the combination of chromatographic separation with element-specific atomic spectroscopic detection (1-11). Two factors determine the success of such combinations. First, the effluent from the chromatographic system must be compatible with the method of atomization employed by the atomic spectroscopic detector. Second, the spectroscopic detector must be sufficiently sensitive to compensate for the dilution of analyte resulting from chromatographic dispersion. For many environmental or clinical applications, this will require sub-parts-per-billion detection capability. The combination of liquid chromatography (LC) with laser-enhanced ionization (LEI) spectroscopy, using :I flame as an atom reservoir, meets these criteria. The
suitability of flame spectroscopy as a detector for LC is seen in a number of successful applications of atomic absorption spectroscopy (AAS)(2,6,10, 11). A flame is easily interfaced with an LC column and is compatible with a wide variety of LC mobile phases a t normal LC flow rates. In the area of sensitivity, LEI is generally at least 100 times more sensitive than flame AAS. Berglind et al. have demonstrated the feasibility of LC-LEI with a separation of organometallic forms of iron and chromium (12). We present here a further demonstration of LC-LEI as applied to the determination of trialkyltin compounds in water and sediment. The environmental significance of alkyltin compounds stems from the widespread use of tributyltin (TBT) as the primary toxicant in marine antifouling paints used for pleasure and commercial water craft. Recently, such use of T B T coatings has been implicated in contributing to the loss of economically important nontarget organisms such as fish and mollusks. T B T concentrations in water in the sub-partper-billion range have been shown to cause mutations in juvenile oysters (13). Because of its organophilicity, TBT tends to bioaccumulate in marine organisms and has also been found concentrated in sediments, particularly in harbors. Sediment TBT originates from both the controlled-release paint on the
0003-2700/88/0360-2062$01.50/0 1988 American Chemical Society
