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Anal. Chem. 1980, 52, 120-124
(5) Stary, J. Anal. Chim. Acta 1963, 28, 132-149. (6) Riley, J. P.; Robertson, D. E.;Dutton. J. W. R.; Mitchell, N. T.; Williams, P. J. "Chemical Oceanography", Riley, J. P.,Skirrow, G., Eds., Academic Press: London, 1975; Vol. 111, Chapter 19. (7) Milner, G. W. C.; Wilson, J. D.; Barnett, G. A,; Smales, A. A. J . Elecfroanal. Chem. 1961, 2, 25-38. (8) Brooks, R. R. Talanta 1965, 12, 511-516. (9) Armitage, B.; Zeitin, H. Anal. Chim. Acta 1971, 53, 47-53. (IO) Bender, M. L.; Klinkharnmer, G. P.; Spencer, D. W. Deep-sea Res. 1977, 2 4 , 799-812. (11) Sholkovitz, E. R. Geochim. Cosmochim. Acta 1976, 4 0 , 831-845.
(12) Segar, D. A.; Cantillo, A. Y. "AnaMicai Methods in Oceanography", Gibb, T. R. P., Jr., Ed., ACS: Washington, D. C., 1975; Chapter 7. (13) Kiinkhamrner, G. P.; Bender, M. L., submitted for publication in Esfuar. Coast. Mar. Sci. (14) Klinkhammer, G. P., submitted for publication in Chem. Geol. (15) Klinkhammer, G. P., submitted for publication in Earth Phnet. Sci. Lett.
RECEIVED for review July 30,1979. Accepted October 10,1979. Work supported by the NSF under contract OCE 77-05184.
Determination of Silicon and Aluminum in Biological Matrices by Inductively Coupled Plasma Emission Spectrometry F. E. Lichte' and S. Hopper Environmental Trace Substances Research Center, University of Missouri, Columbia, Missouri 6520 1
T. W . Osborn" The Procter & Gamble Company, Miami Valley Laboratories, P.O. Box 39175, Cincinnati, Ohio 45247
An analytical method for the determination of silicon and aluminum in blood, urine, feces, and animal feed is described. The samples were ashed, fluxed wlth sodium carbonate, and dissolved in hydrochloric acid. The solutions were analyzed by Induction Coupled Plasma-Atomic Emission Spectrometry (ICP-AES). The 251.6mm emission line was used to measure silicon, and the 394.4-emission line was used to measure aluminum. The method was tested by analyzing samples to which known quantities of silicon and aluminum had been added. The method was further tested by analyzing pure water samples to which known quantities of silicon and aluminum were added. The precision of the method was limited by the instability and heterogeneity of the biological samples.
Considerable interest has developed concerning the behavior of trace elements in biological systems (1-3). Two elements of current interest are silicon and aluminum, ubiquitous elements that represent major fractions of the lithosphere. Silicon is an essential trace metal ( 4 ) ,although in some specific forms it may produce toxic reactions (5,6). Aluminum may be a n essential trace metal, although some of its compounds can cause toxic reactions ( 7 ) . In pharmacokinetic or toxicology studies, the preferred way t o find out how animals absorb, distribute, and excrete a n element is to use a radioisotope of the element. Unfortunately, the radioisotopes of silicon and aluminum have such short half-lives or low specific activities that they are not suitable for biological studies. Therefore, we sought a method of analysis t h a t could be used for tracing the absorption, distribution, and excretion of silicon and aluminum without using radioisotopes. Techniques that measure single elements, such as nitrous oxide flame emission (8) dc arc or spark emission spectrometry (9),graphite furnace atomic absorption ( I O ) , and colorimetry (I1,12) have been used for the determination of silicon and aluminum. However, we needed a method that would measure both elements a t once, because we anticipated analyzing a Present address, U.S.Geological Survey, Branch of Analytical Laboratories, Mail Stop Lakewood, Colo. 80225. 0003-2700/80/0352-0120$01.00/0
great many samples, some of which are not large enough to carry out two separate determinations. Inductively coupled radio-frequency plasma-atomic emission spectrometry (ICPAES) was chosen as the analytical technique most likely to meet the analysis requirements (13). ICP-AES is a multielement technique that is relatively free of chemical interferences. It possesses high sensitivity and is particularly useful for the determination of refractory elements such as silicon and aluminum. This paper describes the application of ICP-AES for the determination of silicon and aluminum in urine, feces, blood, and animal feed.
EXPERIMENTAL Apparatus. Figure 1 is a block diagram of the po;ychromator and associated equipment. The plasma was viewed axially (14) with the center of the plasma focused onto the entrance slit. Other features of the system are given in Table I. The analytical methods and optimization of the operating parameters will be discussed below. Reagents. All reagents were Analytical Reagent grade or better and included concentrated HCl, Na2C03,NaOH, Na202,NaCl, and deionized/distilled/deionized water. Silicon reference solutions were prepared from both pure quartz and from Fisher certified solution (Lot 2755125-12). The aluminum reference solutions were prepared from Fisher certified solution (Lot 0753439). All dilutions were made on a weight basis in polyethylene to eliminate contamination from glassware. Samples. Samples were obtained from a variety of animal species. The excreta samples were collected from animals housed in stainless steel metabolism cages. Plastic fecal cups were used to prevent contamination of urine samples by feces. The cages were kept in a room with air filters to keep dust from them. The cages and sample collection apparatus were washed with triple distilled water. Animal personnel wore talc-free plastic gloves, hair nets, and protective clothing to reduce the possibility of contaminating the samples. Blood samples were collected using a stainless steel needle and a polyethylene syringe. The samples, after collection,were stored in acid washed, screw-cap polyethylene bottles until they were analyzed. Sample Preparation. The tissues and excreta were prepared for analysis in a Class 100 horizontal laminar flow work station (Contamination Control, Inc.). Samples were weighed into nickel or platinum crucibles. Liquid samples were evaporated to dryness on a hot plate. The crucibles were then covered with the lid and ashed in a muffle furnace for 1 h at 200 "C, 1 h at 400 "C, and 4 h at 600 "C. The platinum crucibles were found to be superior 1979 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 52, NO. 1, JANUARY 1980
1
Group U 3
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Analog to
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Digital 4
Converter Program Timer 8
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121
Automatic Altair 8800 + Micro + Printer Processor (optional)
*
r
1
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Control Group
PlasmaPower
Plasma Excitation Stand
-
4-
-
*-IL
-+-+-+=
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Diffraction Grating
Figure 1. Simplified block diagram of instrumentation
Table I. Instrumental Facilities and Parameters component operating conditions plasma generator 1100 W forward power, 27.1 MHz Model 27-40 International Plasma Corp, Hayward, Calif 94544 15 L/min coolant Ar flow, 0.58 L/min aerosol carrier gas flow with a plasma torch 1.5-mm i.d. aerosol injection orifice all quartz Environmental Trace Substances Research Center construction (15 ) pneumatic nebulizer a two-channel Technicon AutoAnalyzer-pumped sample solution and buffer cross flow type solution to the nebulizer at a total flow rate of 2 . 5 mL/min; sample Ames Laboratory construction solution pumped a t 0.2 m l l m i n . ; buffer solution pumped at 2.3 mL/min spray chamber (17) polychromator system grating: concave halographic, 1666.7 linesimm, reciprocal dispersion 0 . 6 Model FAS-2 nm/mm, entrance slit 40 pm, exit slit 75 pm Baird Atomic Bedford, Mass. 01730 data acquisition 28K, 8-bit memory Altair 8800 Microprocessor MITS, Inc. Albuquerque, N.M. 87108 analysis lines Si 251.6 nm, 1st order; A1 394.4 nm, 1st order to nickel crucibles, particularly for urine which tended to corrode the nickel crucibles. The ash was then mixed with Na2C03as indicated in Table I1 and fluxed for 5 min over a Meeker burner to render the silicates soluble. The fusion cake was dissolved with a solution containing 20 wt% hydrochloric acid, then diluted with deionized/distilled/deionized water as shown in Table 11. Analysis. The solutions were nebulized into the plasma and integrated intensities a t 251.6 nm (Si) and 394.4 nm (Al) were compared to analytical calibration curves. The reference solutions
were prepared to match the total salt and acid contents of the sample solution by the addition of NaCl and hydrochloric acid. Reagent and total analysis blanks were determined with each series of samples and were subtracted from the sample concentrations. All samples and reference solutions were prepared in polyethylene labware that had been leached for 24 h with approximately 3 M HN03,and rinsed three times with deionized/distilled/deionized water immediately before use. The samples were analyzed by making at least three consecutive 12-s integration measurements
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Table 11. Fluxing and Dilution Conditions
weight weight weight of of analyzed, Na,CO,, solution, material urine and blood feces and feed
g
2 0.5
g 0.5 2
g 60 120 I
per determination. The average value was reported. A reference solution was run with every sixth sample. Working curve drift was corrected by interpolation of the sensitivity at the time of each determination. The sensitivity drift was generally less than 3% between sensitivity checks. If a drift greater than 6 % was observed, the determinations were repeated.
RESULTS AND DISCUSSION Sample Preparation. A dry ashing technique was chosen as the method for destroying the organic material because it required the fewest reagents and could be conducted within loosely covered crucibles. Our studies demonstrated that complete destruction of the organic material was achieved a t 600 "C without loss of silicon or aluminum. The temperature in the muffle furnace had to be raised slowly to avoid rapid combustion and mechanical loss of the analyte. The resulting residue was fluxed with sodium carbonate to render the silicates soluble. Other fluxing agents studied included sodium peroxide and sodium hydroxide. Sodium peroxide was a more powerful oxidizing agent, but it had high levels of silicon and had to be used with more expensive zirconium crucibles. Sodium hydroxide was not used because silicon and aluminum concentrations varied considerably from batch to batch. Sample Introduction. Several nebulizer systems were tested. The final choice was a cross-flow pneumatic nebulizer (16) with a double Pyrex spray chamber (17). The efficiency and the uptake rate of solution by the nebulizer are dependent upon the salt and acid content of the solution (18). Therefore, reference solutions were made to closely match total salt and acid content of the sample solutions. One channel of a Technicon multichannel AutoAnalyzer pump was used to pump the sample or reference solution to the nebulizer a t a rate of 0.2 mL/s. A second channel of the peristaltic pump was used to add a buffer solution containing 2000 kg/mL of sodium at 2.3 mL/s. The flow from the second channel diluted the samples and further reduced differences in acid and salt content between the samples and the reference solutions. Measurement. Selection of emission lines for elemental analysis from a radio-frequency plasma has been investigated (19). Our choice of lines was limited to those pre-cut in the polychromator. The silicon 251.6-nm line was used because it had a higher signal-to-noise ratio than the 288.1-nm line. Aluminum was measured at the 394.4-nm line because it was more intense than either the 308.2- or 256.7-nm line, and because the S/N ratio was most favorable. A known limitation of the 394.4-nm line is that it is subject to Rowland ghost interference from nearby high intensity calcium lines (20). The calcium intensity a t the aluminum line is shown in Figure 2. Information for this scan was obtained with a Hilger-Engis spectrometer, but it demonstrates the effect present in the polychromator. This interference with aluminum was proportional to the net intensity of the Ca emission line a t 393.4 nm. Thus, a linear Ca interference correction was performed on each sample. The degree of the interference was calibrated for each analysis run. Several related parameters of the induction-coupled plasma were optimized concurrently for the measurements. They were the area of the plasma observed, input power, and the argon gas flow. The external optics of the system were adjusted to allow light only from the central region of the axially viewed
.-+x VI
0 C c C
-
I' Figure 2. Effect of calcium intensity on aluminum background
0.2
0.4
0.6
0.8
1.0
Flow Rate (l/rnin.\
Figure 3. Optimization of carrier gas flow rate for silicon and aluminum
plasma to enter the spectrometer. A 3-mm high mask a t the entrance slit was used to eliminate the argon emission in the outer region of the plasma. Input power of 1100 W produced the most stable plasma. Higher power input required higher coolant gas flow rates to minimize Si erosion in the quartz torch. Higher gas flow rates increased the noise due to turbulence in the plasma. At lower power input (
