A N A L Y T I C A L CHEMISTRY, VOL. 50, NO. 1, JANUARY 1978
5 . T h e reported detection limit for the NAA procedure ( 3 ) is 50 ppb, limited by the aluminum content of the resin and reagents and presence of radiocontaminants in the activated resin. On t h e other hand, because of lower aluminum contamination in the AA procedure, its detection limit is 20 ppb. A comparison of relative standard deviations in routine analyses by the two methods shows that the AA procedure results in data with lower RSD values.
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15) G. C. Goode, C. M. Howard, A. R. Wilson, and V. Parsons, Anal. Chim. Acfa, 58, 363 (1972). ( 6 ) H. Thurston, G. R. Gilmore, and J. D. Swales, Lancet, 881 (1972). ( 7 ) C. Fuchs, M. Brasche, K. Paschen. H. Norbeck, and E Quellhorst, Clin. Chim Acfa. 52. 71 (1974). (8) D. A. Lord,J. W: McLaren:and R. C. Wheeler, Anal. Chem., 49, 257 (1977). (9) A. C. Alfrey. G. R. LeGendre, and W. D. Kaehny, N . Engl. J . Med., 184, Jan. 22, 1976. (10) J. R. McDermott and I. Whitehill, Anal. Chim. Acta, 85, 195 (1976). ( 1 1) Report No. 1, Intercomparison of Trace and m e r Elements in IAEA Animal Muscle H-4 (1976), Internatlonal Atomic Energy Agency, Vienna, Austria.
LITERATURE CITED RECEIVED for review August 23, 1977. Accepted October 25, 1977. The authors acknowledge support from NIH Biomedical Research Support Grant RR-07055 (UN-L Research Council), and by the Omaha V.A. Hospital (MRIS 7319). This is ERDA document number C00/1617-55.
(1) J. R. J. Sorenson, I. R. Campbell, L. B. Tepper, and R. D. Lingg, Environ. Health Persp., 8, 3 (1970). (2) J. R. J. Sorenson, "Biochemistry", Vol. 2, J. 0.Nriagu, Ed., Ann Arbor Science Publishers, Ann Arbor, Mich., 1976, p 427. (3) A. J. Blotcky, D. Hobson, J. A. Leffler. E. P. Rack, and R. R . Recker, Anal. Chem., 48, 1084 (1976). (4) K. Fritze and R. Robertson, J . Radioanal. Chem., 7, 213 (1971).
Determination of Lead in Rocks and Glasses by Temperature Controlled Graphite Cup Atomic Absorption Spectrometry Darryl D. Siemer" and Horng-Yih Wei Department of Chemistry, Marquette University, Milwaukee, Wisconsin 53233
A Varian Techtron Model 63CRA used with standard atomizer cups (not tubes) and rods was utilized for the work. A gas shield box fabricated from 2-mm thick alumi.num sheet was used to reduce the air entrainment and consequent fairly rapid atomizer cup and rod deterioration noted in our preliminary experiments. The two halves of the box are mounted with screws fitting the holes already tapped in the atomizer. Additional holes in the shield box allow insertion of the temperature sensing probe and permit a variety of different optical axes to be used (Figure 1). Graphite powder was obtained by grinding up scrap pieces of National AGKS graphite rod in the Wi;:-L-Bug. The graphite used for sample dilution was 325 mesh or finer. For solution analyses, an Instrumentation Laboratory Model 455 graphite furnace was used. Solutions were pipetted into "microboats" which were dried under a heat lamp before insertion into the atomizer. The manufacturer's recommended operational conditions were used. Both of the nonflame atomizers were used with an Instrumentation Laboratory Model 251 atomic absorption spectrometer. 11, hollow cathode and hydrogen continuum lamps were used. The 28X3-nm lead resonance line was utilized for all of the work described in this paper. The constant temperature power supply used with the cup atomizer is depicted schematically in Figure 2. A triac switches the mains current through an autotransformer if, and only if, three requirements are satisfied. Two of the three requirements are that both inputs to the first NAND gate be at logic "1". This will be true only if the atomization timer chip (IC 2) output is "high" and the photon emission (a measure of temperature) of the carhon atomizer cup itself is lower than some value determined 11y the voltage at the noninverting input of the operational dniplifier. A forward biased diode is used at the emitter of the phiitotransistor as a sensing resistor because it has a logarithmic response to the various photocurrents. This allows a much wider range of temperatures 1900 to 1800 "C) to be set with Rio than would be permitted with a simple resistor. The other requirement for turning on the triac is that the voltage across the triac itself is lower than about 5 V. This is sensed t)y the SCR trigger IC itself. Four 6-\' lantern batteries in series 5erve for the power supply for the logic elements and the triac trigger IC.
Lead (Pb) in rocks, glasses, and fly ash is determined by graphite cup atomic absorption spectrometry. Finely ground samples of silicate based materials are mixed with graphite powder, loaded into graphite atomizer cups, and the lead is atomized by heating the cup to a temperature which volatilizes that metal but leaves the bulk of the matrix behind. The integrated atomic absorption signals of the samples were compared to signals obtained from matrix-free aqueous lead standards to obtain the lead concentrations. Excellent agreement of results with published values in a variety of standard reference materials was obtained.
There has been considerable interest recently in applying direct graphite furnace atomic absorption (GFAA) spectrometry t o the determination of trace metals in a variety of solids (1-9). Potential advantages of this type of approach over conventional arc emission spectroscopic techniques are greater sensitivity and, more importantly. a much greater independence of the analytical signal from the nature of the sample matrix. L'vov's and Langmyhr's review articles are t h e most comprehensive descriptions of what has been achieved to date (2, 9). LVe undertook to investigate the use of a commercially available carbon cup atomizer for the determination of lead in a variety of rocks and glasses.
EXPERIMENTAL Samples were reduced to a powder with a "LVig-L-Bug"grinder which shakes the sample along with a ball bearing in a small hardened steel vial. Weighed portions of powdered rock (325 mesh or finer) were mixed with graphite powder by shaking them together in polystyrene vials with the same machine. One or two minutes of grinding suffices to reduce rock or glass samples to the required state of subdivision. A set of sieves was used to classify particle sizes. The grinder, vials, and sieves are obtainable from Spex Industries, Inc.. Metuchen, N.J. 0003-2700/78/0350-0147S01 O O / O
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1977 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 50, NO. 1, JANUARY 1978
in type from brown beer bottles to a Kimax test tube were also examined. A homemade analog peak height and peak area measuring and recording circuit was used to supplement the spectrometer's electronics in order to enable simultaneous measurements of both parameters t o be made. The circuit may be obtained by writing the authors. Nitrogen gas at various flow rates (usually 8 Ljminute) was used to flush air from around the atomizer cup during heating cycles. Stainless steel tweezers were used t o handle the carbon atomizer cups. A microbalance was used to weigh the atomizer cups before and after the sample was added to them with a stainless steel spatula. Alternate values of the lead content of various specimens were obtained by first dissolving powdered samples in hydrofluoric acid. A suitable quantity of the material was boiled in 20 times its weight of a 1:1mixture of concentrated HF and HNOBand taken just to dryness in a Teflon beaker on a hot plate. More HF was then added and the solution reduced almost to dryness with further heating. This was repeated three or four more times. Then half a milliliter of "OB was added and the solution boiled down to about 0.2 mL. Following this, the solution was transferred to a volumetric flask, made up to volume, and then analyzed by conventional GFAA techniques using the 455 graphite furnace with the square cross section atomizer tubes and "microboats".
Figure 1. Atomizer with gas shield box, phototransistor mount, and
RESULTS AND DISCUSSION
light pipe
T h e object of our research was to determine conditions under which individual lead atoms would each contribute equally to an integrated atomic absorbance signal regardless of when during the atomization cycle they come out of the cup. Under these conditions, a time integrated atomic absorption signal should be an excellent measure of the amount of lead present in a sample regardless of the matrix. We chose a carbon cup atomizer (Varian Model 63) for this project because it allows easy replacement of the atomizer CUPS,a choice of optical axes (either through or above the CUP), and the additional flexibility of possibly using a hydrogen diffusion flame around the CUP. We deemed it desirable t o use a commercially available atomizer so that successful results would be of immediate relevance to practicing analysts. It has been often observed t h a t different chemical forms of lead vaporize from graphite surfaces at different times during conventional GFAA analysis (10) done with short tubular furnaces. This results in several overlapping atomic absorption peaks instead Of a sing1e The lead atoms in each peak come off a t different temperatures and diffuse from the Portion of the optical axis sampled by the spectrometer a t different rates and therefore spend unequal times in the optical path. This renders time integration of the total
The output of the autotransformer was fed into the high voltage side of a 4 KVA 10/1stepdown transformer whose low voltage winding powered the carbon cup atomizer. The autotransformer served to allow variations in the rate at which the atomizer cup approached its preset working temperature and allowed control of low temperature drying stages during which the atomizer CUP did not radiate visible light. The triac was cooled by mounting its stud on a 4-mm thick aluminum plate to which was glued a flattened Of Tygon tubing through which the cooling water first circulated through the atomizer was passed. ~ i g h emitted t by the atomizer cup is led to the phototransistor with a silver plated (except the ends) light pipe made of 3-mm diameter Pyrex rod, The phototransistor is mounted in a plastic grommet fitted through an aluminum plate screwed to the base of the atomizer. The free end of the light pipe is inserted through a hole in the gas box and is aimed at the atomizer cup (Figure 1). The instantaneous atomizer temperature was monitored by means of a storage oscilloscope probe at the emitter of the phototransistor. An optical pyrometer (Leeds & Northrup Co.) was used to calibrate the temperature setting potentiometer (Rlo)and to relate the phototransistor emitter voltage to a definite temperature, Reference materials analyzed included NBS-614 (glass), NBS 1633 (fly &-,), and Canadian Centre for Mineral Technology SY-2, Sy-3, and MRG-1 rock samples. A variety of other glasses ranging +6
+6
" h o t " , 120 V A C l i n e ~
7-5 ?3 34
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-% 7 T
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Figure 2. Power supply schematic. R,, R,, 330R; 5-kQ pot; R, R,, 3.3 kQ; R6, 10 k!2; R,. 100 12; R,, 1 MQ pot; R,, 33 kQ; C1, 100 wF, 25 V; C2, 0.33 WF, 25 V; Cs, 0.1 gF, 25 V; C4, 0.47 pF, 25 V ; C5. 0.05 pF, 200 V; Q,, GE L1461 phototransistor; Q2, GE SC260 25-A 400 PRV Triac; D1,10-mA silicon switching diode; IC1,741 op. amp.; ICz,555 monostable; IC3, 7400 quad l T L NAND gate; IC4, Fairchild WA 742 zero-crossing triac switch; T,, Powerstat type 242 4.2KVA autotransformer, Superior Elec. Co., Bristol, Conn.; Tz. 4KVA 10: 1 stepdown transformer, Badger Transformer Co., Milwaukee, Wis. All resistors 0.5 W, 10% carbon composition
ANALYTICAL CHEMISTRY, VOL. 50, NO. 1, JANUARY 1978 3ower
“Off’
Figure 3. Nonatomic background absorbance signals as a function of atomizer cup temperature. 1:l Sy-B/graphite; fast ramp; 8 L/min N2
atomic absorbance signals to be of questionable utility for correction of the multipeak problem with these furnaces. I t is partially to deal with these difficulties that instrument manufacturers recommend sample pretreatment with oxy acid to convert analyte atoms to a single chemical form prior to GFAA analysis. We decided to investigate the use of graphite powder mixed with finely ground sample to effect a similar conversion of the lead to a single form, hopefully, elemental metal atoms. In addition to assuring a reducing environment, the graphite powder serves to control the diffusion of lead from the cup by presenting a short column of fine particles through which the volatilized material must pass to escape the cup. This could be expected to slow the actual loss of lead atoms from the cup until a somewhat higher and more uniform gas temperature above the cup is achieved. We also felt that it was reasonable to expect that control of maximum atomization temperature by means of suitable power supply circuitry would allow distillation of lead from relatively involatile matrices without much concomitant nonatomic absorbance or scattering signal interference. Temperature control reduces gross perturbation of atom residence times above the cup caused by convection due to volatilized gaseous matrix material. There were certain experimental limitations or “boundary values” to variations in experimental parameters inherent in our system. First, the spectrometer used had a maximum integration time capability of 16 s. Therefore, it was necessary to use a sufficiently high atomization temperature to cause essentially all of the lead to be volatilized in this time. Second, atomic absorption analytical curves typically deviate from linearity a t absorbances greater than unity. Therefore, atomization temperatures and/or sample quantities should be limited so that atomic absorbances greater than this value do not occur a t any time during the atomization. Another limitation inherent in state of the art AA spectroscopy is that effective background correction is only possible up to a limited value of “smoke” absorbance. However, atomization temperature control effectively keeps these signals well within the background correction capability of most instruments. Figure 3 depicts the background absorbance signals seen when 3 mg of SY-3 syenite rock mixed 1:l by weight with graphite powder is atomized a t different temperatures. In each case an autotransformer setting of 80% was used resulting in times for the atomizer cup to reach the indicated temperatures of 1.1, 1.4, 1.6, 1.8, 2.0, 2.2 and 2.4 s for the seven temperatures listed in increasing order. Table I gives both peak height and integrated background corrected atomic absorbance signals seen for the same sample under identical atomization conditions. I t is apparent that a t temperatures of 1200 t o 1500 “C it is possible to quickly volatilize the lead and leave the bulk of interfering matrix concomitants in the atomizer cup. A similar conclusion can be reached with all of the samples that we investigated regardless of significant differences in gross matrix composition.
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Table I. Peak Height and Area Data for SY-3 Rock/Graphite Atomized with Identical Heating Rates to Different Final Temperatures Final temperature, C
Time, s
Peak areaa (absis)
1100
1.2
0.4 1
1200
1.4
1350
1.6
1500
1.9
1700
2.2
1800
2.5
0.41 0.4 1 0.43 0.41 0.41 0.35 0.38 0.38 0.4 3 0.41 0.38
Peak height ( abs ) 0.14 0.16 0.20 0.28 0.37 0.41
0.33 0.30 0.31 0.34 0.41 0.38
a Conditions: autotransformer setting 80%; N, flow, 8 Limin; optical axis 2 mm above grazing incidence; 3 mg of the rock sample.
Table 11. Peak Height and Area Data for SY-3/Graphite Atomized at Different Heating Rates to a Common Final Temperature (1350 C) Peak Peak Autotrans- Time t o areaa height, former reach setting 1350 “C, s (abs i s ) (abs) 0.4 1 0.37 80% 1.6 0.35 0.28 0.37 0.34 0.41 0.41 0.34 0.23 70% 3.5 0.41 0.33 0.38 0.30 0.4 1 0.30 0.35 0.28 60% 5.9 0.38 0.22 0.34 0.20 0.4 6 0.34 0.4 1 0.18 50% 15 0.35 0.11 0.33 0.11 0.33 0.10 a Conditions: N, flow, 8 Limin; optical axis 2 mm above grazing incidence; 3 mg of the rock.
Table I1 gives background corrected lead atomic absorbance peak height and peak area (integrated) data seen when 3-mg samples of SY-3 mixed 1:1 with graphite are subjected to 1350 “C maximum atomization temperature atomization cycles with different rates of attainment of that temperature. T o effect this, the autotransformer setting was varied. Maximum peak heights increase markedly when the rate of heating (or “ramp” slope) is increased. This indicates that much of the lead is volatilized before the equilibrium atomization temperature is reached if slow “ramps” are used. Therefore true isothermal conditions within the optical volume sampled by the AA spectrometer during the time that lead atoms are within it are not achieved under these conditions. However, it is equally apparent that the integrated lead signals are not greatly affected by the change in heating rates. This indicates that atom residence times are not much affected by these temperature variations. A reason for this might be that the atoms are observed in a region considerably removed from the surface of the graphite powder matrix-a region in which the actual gas temperature is probably much lower than that of the cup. The “flushing” or convection effect of the nitrogen gas in which the cup is suffused probably is the dominant mechanism of atom removal from this zone, not simple diffusion. Figure 4 is a tracing of the stripchart re-
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