Anal. Chem. 1886, 58,2340-2342
2340
plication of indirect LCEC are continuing in this laboratory. Registry No. Isopropyl alcohol, 67-63-0; 1-propyl alcohol, 71-23-8; 1-butyl alcohol, 71-36-3; isobutyl alcohol, 78-83-1;propionic acid, 79-09-4; 1-butyric acid, 107-92-6; isovaleric acid, 503-74-2; 1-hexanoic acid, 142-62-1;p-hydroquinone, 123-31-9.
(7) Snyder, L. R.;Kirkland. J. J. Introduction to Modern Liquid C b r m t o g raphy, 2nd ed.; Why: New York, 1979; pp 140-145.
J i a n n o n g Ye Richard P. Baldwin* Department of Chemistry University of Louisville Louisville, Kentucky 40292
LITERATURE C I T E D (1) Small, H.; Miller, T. E., Jr. Anal. Chem. 1982, 5 4 , 462. (2) Bobbltt, D. R.; Yeung, E. S.Anal. Chem. 1964, 56, 1577. (3) Bobbltf, D. R.; Yeung, E. S. Anal. Cbem. 1985, 57, 271. (4) Mho, S . 4 . ; Yeung, E. S.Anal. Chem. 1965, 5 7 , 2253. (5) Banerjee, S.Anal. Chem. 1965, 57, 2590. (6) Mills, A.; GMdings, S. L. Anal. Chem. 1986, 58, 153.
K. Ravichandran Department of Chemsitry University of Georgia Athens, Georgia 30602
RECE~VED for review January 27,1986. Accepted May 1,1986.
AIDS FOR ANALYTICAL CHEMISTS Digestion of Bldogkal Materials for Mineral Analyses Uslng a Combination of Wet and Dry Ashlng A. D. Hill,* K. Y. Patterson, C. Veillon, a n d E. R. Morris
US.Dewartment of Agriculture.. Aaricultural Research Service, Beltsuille Human Nutrition Research Center, Beltsuilie, Maryland 20705 Periodically in our laboratory, it is necessary to analyze large numbers of diet samples and other biological materials from human studies for a wide variety of essential minerals. Atomic absorption spectrometry is accepted as the preferred method (1). Atomic absorption analysis requires the sample be in an aqueous form and this usually requires the destruction of the organic material. Some common procedures for digestion are low-temperature dry ashing in an oxygen plasma (2-6))high-temperature dry ashing (6-13))and low-temperature wet ashing in an acid solution (13-19). None of these methods is without problems. The equipment for low-temperature ashing in an oxygen plasma is expensive. Most ashers can handle only a few small samples a t one time. Low-temperature ashing can be very time-consuming when high concentrations of salts are present. Some minerals have been reported to be lost during hightemperature ashing through either volatilization or adsorption onto the walls of the container (20-22). Contamination from the ashing vessel has also been reported (23). Wet ashing with perchloric acid is widely used but potentially explosive conditions are needed to totally digest the lipid portion of the sample. Large volumes of acids are sometimes needed and constant operator attention is required. Sulfuric acid, which is used extensively in wet ashing procedures, has been shown to interfere in atomic absorption analysis (24) particularly with calcium. The procedure described in this paper utilizes dry ashing a t 375 "C in a muffle furnace for 24-48 h. This temperature will char the sample and burn off the major portion of the organic matrix including the lipid material without causing loss of the seven biologically important minerals investigated in this study. The ash is suspended in small amounts of deionized water and nitric acid. The samples in borosilicate tubes are then placed in a heating block and hydrogen peroxide is added to complete the digestion. Very little operator time is required. Only small amounts of reagents are used lowering the chance
Table I. Recovery of Added Metals
amt recovered," amt added, Kg
cu
1.0
Fe
6.5 5.4
4.0 2.0 4.0
4.01
2.8
2.07 4.07
7.2
8.0
8.52
10.0 20.0
9.80 19.5 40.3 4.96
40.0
5.0
Zn
10.0 20.0
35
Ca
70
75
M€!
re1 std dev, %
0.99 2.03
2.0
Mn
bg
150
10.2
19.9 36.7 70.8 81 155
2.0 2.8 9.4 4.4 2.1
6.5 2.8
% recovery
99
102 100 104 102
107 98
98 101
99 102 99
1.6 12.7 6.9
105 101
8.4
108
9.3
103
Amount recovered is the averaze of tridicate determinations. ~
of contamination and resulting in low blanks and allowing for more accurate analysis a t low concentrations in samples. EXPERIMENTAL SECTION Procedure. Homogenized, dried samples of about 1 g are weighed into 20 X 150 mm borosilicate test tubes. The tubes are placed in 1000-mLglass beakers, covered with a watch glass, and placed in a muffle furnace. Furnace temperature is increased 50 deg/h to 375 "C and held at that temperature for 48 h. After cooling, samples are removed from the furnace and 0.20 mL of deionized water and 0.20 mL of Ultrex nitric acid (Baker, Phillipsburg, NJ) are added to each. Tubes are placed in heating blocks (Thermolyne Dri-Bath, Dubuque, IA) and the temperature is raised to 90 "C.Hydrogen peroxide (50%) is added in 0.1-mL aliquots at 10-15 min intervals until all black carbon particles are digested. Samples are allowed to evaporate to dryness and
This article not subJect to US. Copyright. Published 1966 by the Amerlcan Chemlcal Society
ANALYTICAL CHEMISTRY, VOL. 58, NO. 11, SEPTEMBER 1986
Table 11. Percent Recoveries of uZn, 14Mn, 19Fe,and IICr in Rat Tissues after Endogenous Labeling
liver heart muscle kidney serum retained in tube wall a
@‘Zn
64Mn
6sFe
SICr
102 97 96 100 99
98 100 101 102 96 1
94 103 107 97 108 2
95 102 104 103 101 1
0
Values given are the mean of three redicates.
cooled and 2 mL of deionized water and 1 mL of Ultrex HC1 are added to each (presumably one could use HNOBin situations where chloride ion is undesirable). Samples are reheated at 90 OC for 15 min to dissolve the residue, cooled, transferred quantitatively to 10-mLvolumetric flasks,and adjusted to volume with deionized water. After mixing, samples are transferred to polypropylene storage tubes. Standards (1000 ppm, Baker, Phillipsburg, NJ) are diluted with 10% HC1 to cover the linear range for each element. Sample solutions can be aspirated directly and diluted when necessary with 10% HC1 solution. Standards and samples to be analyzed for calcium should be diluted with lanthanum solution (0.5% La). Recovery of Added Minerals. One-gram samples from a homogenized, freeze-dried diet composite were spiked with three levels of a solution containing an inorganic form of copper, manganese, iron, and zinc made from the 1000 ppm standards. The solution contained copper at 2 pg/mL, manganese at 4 pglmL, iron at 20 pg/mL, and zinc at 10 pg/mL. Calcium and magnesium recoveries were from individual additions to samples of U.S. National Bureau of Standards materials Rice Flour (SRM 1568) and Bovine Liver (SRM 1577a), respectively. The samples were digested by use of the procedures described in this paper and analyzed by flame atomic absorption spectrometry (Model 5000, Perkin-Elmer Corp., Norwalk, CT).
RESULTS AND DISCUSSION The problems with this type of recovery experiment have been discussed by Gorsuch (21). Specifically, the additions are made with an inorganic form of the element which could differ from that found natively in the organic matrix. Losses by volatilization or adsorption can be offset with contamination from reagents or equipment because the oxidation step cannot be isolated from other steps necessary for the final analysis. These problems can be avoided by using radioisotopes. Rat tissues were endogenously labeled by intraperitoneal injections with zinc, iron, manganese, and chromium radiotracer solutions and the tissues were removed and digested as described. The radiotracers were “carrier-free”, so the total dosage for each is well below physiological levels. The results are given in Table 11. Geometry differences between ashed and unashed samples contribute to the variation in the results. Volumes of ashed sample solutions were adjusted to approximately match the unashed tissue sample volume. The U S . National Bureau of Standards (NBS) certifies plant and animal materials for a large number of elements. The results of analysis using the described procedure and the certified values are given in Table III. The NBS certified value is the mean determined by using a t least two independent methods of analysis f the estimated uncertainty. The results given in Tables I and I1 show no significant losses of minerals during the digestion procedure. The recovery of minerals added to a diet composite averaged 100% for copper, 104% for manganese, 99% for iron, 100% for zinc, 102% for calcium, and 106% for magnesium. Recovery for rat tissues endogenously labeled with radioisotopes averaged 99% for zinc, 99% for manganese, 102% for iron, and 101% for chromium. Table I11 shows the results of analysis of NBS reference materials. The results are within the certified range for both
a
2341
Table 111. Analysis of NBS Reference Materials” (pglg)
copper certified determined re1 std dev manganese certified determined re1 std dev iron certified determined re1 std dev zinc certified determined re1 std dev magnesium certified determined re1 std dev calcium certified determined re1 std dev
Wheat Flour
Rice Flour
Bovine Liver
Citrus Leaves
(1567)
(1568)
(1577a)
(1572)
2.0 f 0.3 2.05 4.2%
2.2 f 0.3 2.12 2.3%
158 f 7 156 1.3%
16.5 f 1.0 16.7 3.4%
8.5 f 0.5 8.38 1.3%
20.1 f 0.4 20.0 1.3%
9.9 f 0.8 10.2 0.7%
23.0 f 2.0 23.5 1.7%
18.3 f 1.0 8.7 f 0.6 17.7 8.64 3.3% 2.4%
194 f 20 187 1.4%
90 f 10 89.7 5.7%
10.6 f 1.0 19.4 f 1.0 10.4 20.2 1.5% 2.8%
123 f 8 121 0.9%
29 f 2 31.0 3.1%
n.c.b
n.c.
379 4.1%
419 1.3%
600 f 15 585 1.2%
5800 f 300 5540 2.5%
190 f 10 198 3.8%
140 f 20 140 2.1%
120 f 7 118 1.9%
31500 f 1000 31100 2.5%
nglg
nglg
88 f 12c 85 14 %
800 f 200 817 6.9%
chromium certified n.c. determined re1 std dev
n.c.
Analyzed values are the mean of four replicates. Not certified (no value given). SRM 1577: SRM 1577a not certified for Cr. plant material and animal tissue. The relative standard deviations show acceptable reproducibility for all analyses. This procedure may be applicable to other elements; however, quartz test tubes may be necessary in some cases where contamination from borosilicate tubes is a problem. Registry No. Cu, 7440-50-8; Mn, 7439-96-5; Fe, 7439-89-6; Zn, 7440-66-6; Cr, 7440-47-3; Mg, 7439-95-4; Ca, 7440-70-2.
LITERATURE CITED Faulkner. W. R. Lab. Management 1081, July, 21. Gleit, C. E.; Holland, W. D. Anal. Chem. 1062, 3 4 , 1454-1457. Lockwood, T. H.; Lirnitiaco, L. P. Am. Ind. Hyg. Assoc. J . 1075, 3 6 , 57-62. ..
Guthrie, 6. E.; Wolf, W. R.: Velllon, C. Anal. Chem. 1078, 5 0 , 1900-1902.
Raptis, S. E.; Knapp. G.; Schaik, A. P. Z . Anal. Chem. 1083, 316, 482-407.
Locke, J. Anal. Chim. Acta 1070, 104, 225-231. Heanes, D. L. Analyst (London) 1081, 106, 182-187. Donaldson, J.; St. Pierre, T.: Minnichi. J. L.; Barbeau, A. Can. J . Bochem. 1073, 5 1 , 87-92. Baker, A. D. J . Assoc. Off. Anal. Chem. 1071, 5 4 , 951-952. Adrian, W. J.; Stevens, M. L. Analyst (London) 1077, 102, 446-452. &line, D. R.; Schrenk, W. 0. J . Assoc. Off. Anal. Chem. 1077, 60, 1170-1 174. Isaac, R. A.; Johnson, W. C. J . Assoc. Off. Anal. Chem. 1075, 5 8 , 436-440. Smith, D. L.; Schrenk. W. G. J . Assoc. Off. Anal. Chem. 1072, 5 5 , 669-675. Marks, G. E.: Moore, C. E.; Kanabrocki, E. L.: Oester, Y. T.; Kaplan, E. Appl. Spectrosc. 1071. 26, 523-527. Evans, W. H.; Dellar, D.: Lucas, 6. E.; Jackson, F. J.; Read, J. I. Analyst (London) 1080, 105, 529-543. Johnson, C. A. Anal. Chlm. Acta 1078, 81, 69-74. Arafat, N. M.; Glooschenko, W. A. Analyst (London) 1081, 106, 1 174-1 178. Clegg, M. S.; Keen, C. L.: Lonnerdal, 6.; Hurley, L. S. 6/01. Trace Elem. Res. 1081, 3 , 107-115. Ladefoged, K. Clln. Chim. Acta 1080, 100, 149-153. Clegg. M. S.; Keen, C. L.; Lonnerdal, 6.; Hurley, L. S. 6iOl. Trace Elem. Res. 1001. 3 , 237-244. Gorsuch, T. T. Analyst (London) 1050, 8 4 , 135-173. Gorsuch, T. T. Analyst(London) 1062, 8 7 , 112-115.
2342
Anal. Chem. 1986, 58,2342-2343
(23) wurts, L.; Smeyers-Verbeke; Massary, D. L. Clin. Chim. Acta 1976, 72, 405-407. (24) Basson, W. D.:Bohmer, R. G. Analyst (London) 1972, 97, 482-489.
RECEIVED for review February 24,1986. Accepted April 14,
1986. Trade names are included for the benefit of the reader and do not any endorsement Or preferential treatment of the product listed by the US.Department of Agriculture. Other brands of acids of sufficient purity to produce acceptable blanks have also been used.
Generation of a Helium Inductlvely Coupled Plasma In a Low-Gas-Flow Torch Shi-Kit Chan, Raymond L. Van Hoven,and Akbar Montaser*
Department of Chemistry, The George Washington University, Washington, D.C. 20052 In an earlier communication ( I ) , we reported the generation of three types of helium inductively coupled plasmas (He ICP) a t atmospheric pressure by using modified conventional torches: the hollow He ICP, the filament-type He ICP, and the annular He ICP. Preliminary results indicated that the annular He ICP was capable of exciting elements such as C1 and Br, which possess high excitation energies. Atomic emission detection limits measured for aqueous chloride and bromide solutions were improved by more than 1 order of magnitude, as compared to the results obtained from the Ar ICP. Two impediments to the investigation of the annular He ICP as a new vaporization-atomization-ionization-excitation source for atomic spectrometry were the high gas flow requirement (57 L/min He) and the necessity to follow a relatively complicated procedure for the generation of plasma ( I ) . T o circumvent such limitations at atmospheric pressure, a number of investigators have recently reported the use of reduced-pressure He ICPs (2-4), especially for the determinations of non-metals. The applications of these reducedpressure He ICPs, however, are limited to gas analysis. Relatedly, introduction of aqueous samples into filament-type He ICPs (1,5,6),operated at atmospheric pressure, has been extremely difficult. In the present study, we report on the design and evaluation of a new demountable, low-gas-flow torch for generating an annular He ICP at atmospheric pressure. For the new design, the total He gas flow has been reduced by a factor of 7 as compared to our previous work ( I ) , and the procedure for the formation of the plasma has been simplified significantly to a one-step plasma generation. Detection limits of aqueous bromide obtained at Br 1827.24 nm are also reported, using a photodiode array detector.
EXPERIMENTAL SECTION Except for the He ICP torch and the detection system, the instrumentation for the inductively coupled plasma atomic emission spectrometry (ICP-AES) system has been discussed elsewhere (1, 7). Modifications on the induction load coil and the impedance matching network were similar to those previously described ( I ) . The schematic diagram of the demountable, lowgas-flow torch is shown in Figure 1. The torch consisted of three components a torch base, a threaded insert, and a high precision quartz tube (13 mm i.d. and 15 mm o.d., Wilmad, Buena, NJ), which were closely fitted together by means of nitrile O-rings (Catalog no. 2-113 and 2-015, Parker, Lexington, KY) placed in the square-shaped grooves of the base and the insert. The torch base and the threaded insert were made of Macor machinable glass ceramics (Corning Glass Works, Corning, NY). High-purity helium gas (99.997%, MG Industries, Valley Forge, PA) was introduced tangentially as the plasma gas via the torch base while the injector gas was directed through an 0.5 mm orifice at the center of the insert. Aerosol of liquid sample, produced by an ultrasonic nebulizer ( I ) , or gaseous sample was transported by the injector gas into the He ICP. 0003-2700/86/0358-2342$01.50/0
Table I. Generating and Operating Conditions for the He ICP
forward power, W reflected power, W plasma gas flow, L/min injector gas flow, L/min sample uptake rate, mL/min observation height above the load coil, mm
1500 5 7 1 2 25
A 1024-elementintensified (700 active elements) linear photodiode array detector (Model 1420R, EG&G Princeton Applied Research, Princeton, NJ) with a detector module and a system processor (Models 1463 and 1460) was used to monitor atomic emission of Br I 827.24 nm. The diode array detector was cooled to -5 "C and was scanned repetitively 100 times at a rate of 100 milscan for each signal integration. A sharp-cut-off, red filter (Catalog no. CS2-63,2424, Corning Glass Works) was used to eliminate any possible interference from second- or third-order spectra. The entrance slit of the monochromator was set at 50
m. RESULTS AND DISCUSSION A. Design Considerations. Because of its excellent electrical resistivity, thermal shock resistance, zero porosity, chemical resistance, and machinability, Macor machinable glass ceramics was chosen for constructing the torch base and the threaded insert. Other materials such as nylon and Teflon for the torch base and brass and boron nitride for the threaded insert were also tested but were proven to be less satisfactory. For example, when a brass insert was used, an annular He ICP could be generated a t about 3 L/min, but the plasma could not be operated at forward power levels greater than 1 kW because of thermal expansion of brass and the subsequent breakage of the outer tube. Since the Macor ceramics could be machined to a very high precision, the demountable torch could be assembled or dissembled within a minute, and no alignment was necessary after the assembly. The most critical parameters in the design were the groove's geometry and dimension of the threaded insert, which determined the flow pattern and total gas flow for sustaining the He ICP. The insert was quadra-threaded a t 1.54 pitch per cm and the dimensions of the V-shaped groove were 1.17 and 0.45 mm for the width and the depth, respectively. T o ensure proper introduction of the plasma gas into the grooves, the tangential plasma gas inlet was machined to form a 1.5 mm X 9 mm rectangular slot. The conditions for generation and operation of the annular He ICP formed in the new torch are listed in Table I. At 1500 W forward power, the plasma gas flow and the injector gas flow were 7 and 1L/min, respectively. The plasma was usually self-ignited to form a very stable annular He ICP, Figure 2A,B. No external cooling was necessary to prevent overheating of the plasma torch. To the best of our knowledge, this is the f i t annular He ICP generated directly from helium 0 1888 American Chemical Society
