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Anal. Chem. 1986, 58,1462-1467
Direct Analysis of Solids by Graphite Furnace Atomic Absorption Spectrometry Using a Second Surface Atomizer Thomas M. Rettberg and J a m e s A. Holcombe*
Department of Chemistry, University of Texas at Austin, Austin, Texas 78712
The dlrect graphlte furnace atomic absorptlon spectrometrlc analysis of sollds using the second surface atomlzer has been lnvestlgated. The atomlzer features a gas-cooled Ta Insert wlthln the graphlte furnace onto whlch the analyte can be condensed, after which atomlzatlon Is performed by ralslng the furnace to a higher temperature and shutting off the coolant gas. The analyses were conducted on standard reference materlal fly ash, rlver sedlment, and cltrus leaves, In addltlon to fllter paper samples. All analyses were conducted wlthout sample pretreatment or use of matrtx modifiers. Quantltatlon was done by uslng simple aqueous standards. By use of peak helghts, the recoveries varied from 81 % to 127%, although several determinations were wlthln the certified concentration range. The procedures typlcally gave low background absorbances and peak shapes that were relatively Independent of the original sample matrlx.
Advances in graphite furnace atomic absorption (GFAA) instrumentation and methods in recent years have made trace-metal determination achievable for an increasing number of difficult sample types. Nonetheless, the direct analysis of solids or aqueous samples with high concentrations of dissolved salts is still considered difficult partially because of the inherent problems associated with sample handling, weighing, and the assurance of homogeneity. Although overall acceptance of one preferred design has not yet occurred, most major instrument manufacturers as well as numerous research laboratories have special atomizers (1-9) available for the direct analysis of solid samples. Many of the proposed GFAA methods for direct analysis of solids represent compromised atomization conditions compared to those used for aqueous samples. An erroneous conclusion might be drawn that furnace atomic absorption techniques are fundamentally ill-suited for the direct microanalysis of solids. However, there is no inherent reason why the sample must be a liquid, and GFAA has other characteristics that actually suggest strong potential for the direct analysis of solids. Among these are the use of discrete samples and the ability to integrate the absorbance signal to minimize the effect of varying vaporization rates (10). Since the direct determination of impurities in metal chips was first investigated by Nicolaev in 1965 ( I I ) , the determination of trace components in solid metal samples has probably received the most attention. The analyses of steels, non-ferrous metals, and alloys have focused primarily on the determination of trace quantities of the more volatile elements, including Ag, Bi, In, Pb, Sb, Se, Sn, Te, and T1 (e.g., ref 12 and 13), since these metal impurities can be detrimental to the physical properties of the metal. These elements generally are more amenable to direct GFAA analysis since they can be more readily volatilized from the bulk. Several elements, including Pb, Bi, Se, Te, T1, and Sn, were successfully determined in alloys by Marks et al. (12), although calibration had to be made against solid, alloy standards. The relative standard deviation (RSD) varied from 7% for Bi to 25% for Sn. They also reported that the buildup of residual material in an HGA-2100 furnace tube had no effect on subsequent 0003-2700/66/0358-1462$01.50/0
analyses or tube lifetimes, although this was not corroborated by Price et al. (4) who found that the buildup of metal residues severely degraded their graphite furnace tubes. Takada and Hirokawa (5) found relatively slow release of Ag, Cu, Mn, and P b from submilligram amounts of steel and suggested that diffusion of the analyte in the metal substrate as well as the volume and surface area of the sample was responsible. In order to completely vaporize Cu and Mn from steel, this group, as well as Sommer and Ohls (141, utilized repetitive atomization cycles for a single sample and added the collected absorbances until the analyte signal was no longer considered significant. The possibility of calibrating solids against aqueous standards using peak areas was investigated by Lundberg and Frech (10). Their results supported the use of peak areas when isothermal atomization conditions were employed. Peak areas were found to be superior to peak heights for accurate determinations of Pb, Bi, Zn, Ag, and Sb in steel and Ni alloys using a microboat sample introduction system (3). Solid biological samples, including orchard leaves, pine needles, and bovine liver, have been atomized from capped sample cups inserted into a constant-temperature furnace (9). The enclosed cups lowered the large background signal that would have otherwise existed for these samples, and recoveries of 90-110% for Pb, Cd, Cu, and Zn were achieved. Using open sample cups, Atsuya and Itoh (7) concluded that Zeeman background correction was necessary to correct for the large background signal generated by various plant, as well as bovine liver powders. A T a “scoop” was successfully used by Langmyhr et al. (15) to introduce pulp and paper material into a graphite furnace for quantitation of Cd, Pb, Mn, and Cu using peak heights. A graphite cup intended to behave similar to the platform has been utilized for direct trace-metal determination in plant, soil, PVC, and textile materials (1,2). Matrix modifiers were used in some cases. Although the analyte peak shapes from these samples varied appreciably from those obtained with aqueous standards, the use of peak areas resulted in reasonable accuracy. This overview of just a few of the studies on direct analysis of solids by GFAA reveals some of the furnace-related problems repeatedly encountered: high background levels, buildup of residual material, sample-dependent peak shapes, choice of suitable standards, and difficulties with less volatile elements in many matrices. The second surface atomizer offers an approach that is particularly suitable for solids analysis, since more latitude was shown to exist in the ashing procedure to separate the analyte from the problematic matrix (16, 17). Furthermore, the shape of the analyte absorbance peak for a given matrix was characteristically independent of the original sample composition (17). The second surface atomizer for solids analysis features, in addition to the cooled Ta “plug”, a reusable sampling cup for containing the sample. The sampling cup is present in the furnace only during the preatomization heating steps. Since it is removed along with matrix residues from the furnace prior to the atomization cycle, problems associated 0 1986 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 58, NO. 7, JUNE 1986
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*
Table I. Parameters Used for Analysis Flgure 1. Second surface adapted to a Varian GTA-95 workhead: (a)
pneumatic arm, (b) coolant gas delivery tube, (c) coolant exhaust port, (d) Ta plug in holding unit, (e) graphite cup. with residues in the furnace can be avoided. The only source of background originates from components of the sample that co-condense with the analyte onto the T a second surface during the transfer step. Since the and@ is “distilled” from the sample to the cooled Ta insert and the remaining solid residue is physically removed from the furnace, the background levels during atomization are significantly lower than would be obtained by complete volatilization of the entire sample. The thin, uniform layer of condensate on the plug is devoid of most of the original matrix, and the analyte supply function is less dependent on the original sample. Thus, peak heights have been used successfully.
EXPERIMENTAL SECTION Apparatus. All absorbance data were recorded as peak heights on a Varian AA-315 spectrometer with deuterium lamp background correction. Absorbance, time, and temperature data were displayed on the GTA-95 video display, and traces of these data were made for permanent recording. Hollow cathode lamps were used for all determinations using recommended currents. Less sensitive atomic lines were employed when necessary to provide the sensitivity appropriate for the analyte being determined. A Varian GTA-95 graphite furnace atomization system was used to provide power and control of the appropriate furnace heating. This unit also was used to electronically control the coolant gas flow. The READ function on the GTA-95 keyboard was programmed to initiate coolant gas flow during the appropriate heating cycles. For one study, Cu in standard reference material (SRM) 1645, a trigger for peak area integration was used that tapped into the GTA-95 sheath gas control circuit. The GTA-95provided up to 3.1 L/min of an Ar sheath gas flow through the furnace ends. Typically, high flows (>2 L/min) were used during ash, low flows (2. Citrus Leaves. Citrus leaves (SRM 1572) were analyzed for Cd, Pb, and Cu using the second surface atomizer. This material can be ashed at relatively low temperatures (e500 "C). A known mass of sample (ca. 2.5 mg) was weighed in the graphite sample cup. Following the ash and transfer cycles, the cup and residue were removed prior to the atomization step. Citrus leaves generate copious amounts of smoke during ashing or combustion. If ashing was performed with the plug inserted into the furnace, the carbonaceous material evolved at temperatures below 330 "C would transfer to the plug and create high background levels during atomization. However, transfer of this carbonaceous material to the plug was easily avoided by ashing with the plug retracted from the furnace, after which time the plug was reinserted and the temperature raised for transfer of the analyte. Some difficulties were encountered in ashing the sample completely without loss of Cd. For example, a t 450 "C complete combustion of the leaves could be obtained, yet loss of Cd was significant with either pure Ar or 1% 0 2 in Ar. However, with 20% O2in Ar, the loss was less severe indicating a shift in the Cd release to higher temperatures brought about by O2 (21). A t the other extreme, matrix volatilization was not achievable at 310 "C with a 60-9 hold cycle. It was determined that ashing had to be conducted only in the range 330-400 "C in 20% 02.To fully combust the matrix in this temperature range, a 50-s ash cycle was necessary. By use of the parameters shown in Table I for Cd determination, the background during atomization was below 0.1 absorbance units. The determined concentration of Cd was within the certified range (Table 11),although the relative standard deviation was about 25%. P b or Cu are less volatile than Cd, and no problems were encountered in ashing the leaf material without loss of the analyte. When aqueous standards were used for calibration, the determined concentrations fell within the certified range of values (Table 11). The relative standard deviation values for P b and Cu were 3% and 16%, respectively. Fly Ash. Coal fly ash (SRM 1633) provides an example of a material having an inorganic oxide base with analytical and environmental interest. This material contains or adsorbs chemical species evolved in coal combustion, including a variety of transition metals at the microgram per gram level (22). Silicon makes up over 20% (w/w) of the sample mass and A1 another 1370, accounting for most of the mass as aluminosilicates (23). Other elements present in excess of 1% include C, Fe, Ca, K, and Mg. When heated in the furnace, this material underwent few obvious changes in its structure until about 1500 "C, above which conversion into a "glassy" phase occurred. This phase dominated until about 2000 O C at which point the material vaporized and generated large amounts of persistent background, likely due to particulate scatter. Coal fly ash would be extremely difficult to analyze by direct vaporization in the furnace (e.g., 2300 "C atomization) because of background absorbance levels in excess of 2.0. Since the backgroundgenerating matrix was transferable to the plug also, an upper
1465
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i
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-
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---
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00
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Flgure 4. Second surface atomization absorbance and temperature vs. time traces for Cu in fly ash (SRM 1633). Shown are (a) the background-corrected Cu signal (solid line) and (b) the background (dashed line) during atomization.
limit on the transfer temperature of about 1950 "C was dictated by the vaporization of the matrix. When transfer was conducted below 1950 "C, atomization temperatures above 2000 "C could be used without risk of producing copious background absorption because the bulk of the matrix was removed with the sampling cup. Transfer of P b was observed at wall temperatures between 1300 and 1750 "C in the fly ash samples. For the analysis, a transfer temperature of 1650 OC was chosen with a IO-s hold at this temperature. Background absorption during the atomization step was below 0.2 absorbance unit. The concentration value determined for P b (Table 111) represents a recovery of about 80%. The P b peak shapes from the solid samples were virtually identical with those obtained for aqueous standards; thus the use of peak areas probably would not improve the recovery. The background level was dependent upon the transfer temperature chosen and the subsequent amount of matrix condensed onto the plug. For example, the background absorbance signals at 244 nm during a 2400 "C atomization step was about 0.25,1.0, and >1.5 for 1900 "C, 2000 "C, and 2100 "C transfer steps, respectively. At a transfer temperature of 2000 "C or above, the peak appeared to have distortion due to background correction system overcorrection. In the determination of Cu, as with Pb, it was necessary to transfer the analyte onto the plug below that temperature at which the matrix appreciably volatilized, viz., 1950 "C. The measured Cu value was 27% greater than the certified value. Sample sizes averaged 1.1mg with the majority of the sample matrix removed with the sampling cup prior to atomization. The background absorbance was about 0.45 during atomization. Figure 4 shows an absorbance vs. time trace for both Cu signal and background. For comparison, a fly ash sample of similar mass was atomized directly from a standard, commercial furnace wall (Figure 5). When the second surface atomizer was used, background was greatly reduced. River Sediment. This standard reference material (SRM 1645) originates from a canal in a steel manufacturing city; hence it has high concentrations of heavy metals with Al, Cr, Fe, and Zn accounting for over one-sixth of the mass. When
ANALYTICAL CHEMISTRY, VOL. 58, NO. 7, JUNE 1986
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r
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0
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Figure 5. Absorbance and temperature vs. time trace for Cu in fly ash (SRM 1633)using wall atomization of a 1-mg sample: (a) "corrected" Cu signal (solid line) and (b) the background (dashed line) during atomization.
noted previously that Sommer and Ohls (13)and Takada and Hirokawa (5) added the absorbances of repetitive atomization cycles for a single metal chip with the objective of attaining the "total" absorbance signal. Analogously, the use of repetitive transfer cycles, or more conveniently, one long transfer cycle to "extract" the Cu from the sample bulk could be used with the second surface atomizer to avoid repetition of the atomization cycles. To ascertain whether extraction of Cu could be achieved by longer transfer times, a number of 1750 "C transfer and 2500 "C atomization steps were performed on a single sample. In typical fashion, the sample cup was removed after each transfer cycle. A Cu absorbance signal was detected for several subsequent cycles, reflecting the amount of Cu transferred per transfer cycle. The net weight loss of the total sample was about 5% per cycle, using a 10-9 transfer temperature hold at 1750 "C. As expected, the absorbance peak area decreased for each subsequent firing. The data suggested an exponential decay, and a plot was constructed according to the following:
Table IV. Certified and Determined Concentrations for Cd and T1 in SRM 1645
In A , = -kt element
certified
determined
Cd
10.2 k 1.5 1.44 f 0.07
9.1 f 0.4 1.26 k 0.07
T1
heated in a furnace, the organic fraction volatilized between 400 and 600 "C. At higher temperatures the sample behaved similar to a metal, consolidating into a round "bead" of metallic-appearing material at about 1600 "C, and wetting the graphite surface a t higher temperatures. Vaporization of the residue from the cup required two or more clean cycles (2900 "C). In some cases a residue remained, probably as metal carbide species, even after multiple clean cycles. This residue in the cups did not interfere with subsequent analyses using these cups. Thallium was determined by use of a 1300 "C transfer cycle, which was well below the temperature a t which the bulk of the sample volatilized. Background absorbance levels during atomization were C0.3 and probably originated from co-condensed organic and salt species on the second surface. The T1 peak shapes were nearly identical for aqueous standards and solid river sediment samples. Table IV indicates a determined concentration within 13% of the certified value. For comparative purposes, a 2-mg sample of river sediment was placed directly onto the wall of a conventional furance. Quantitation was not possible with this instrument due to the background signal in excess of 1.0 absorbance unit produced during atomization a t 2300 "C. These data suggest that the second surface atomizer successfully kept the nonspecific background within a magnitude easily correctable by most background correction systems. Cadmium determination was straightforward in this SRM using the less sensitive 326-nm Cd line. The concentration determined with the second surface atomizer was within the certified range (Table IV) and showed a relative standard deviation of 4.4%. There is a maximum furnace wall temperature beyond which an element can no longer be held on the plug. This is a function of coolant gas choice and flow rate. Under optimal conditions, this is approximately 2000 "C for Cu. However, in this particular sample, the Cu was volatilized slowly below 2000 "C. Slow evolution of Cu was not unexpected, since the release of a metal contained in another metal(s) is based upon mutual solubilities and their respective diffusion coefficients (5). Transfer parameters sufficient for complete transfer of Cu deposited as aqueous samples provided only partial transfer of Cu to the plug from this sediment sample. It was
+ In (ATC)
(2)
where A, is the peak area absorbance after the nth transfer cycle, AT represents the total Cu peak area absorbance that would be obtained by completely extracting the Cu from the bead, t is transfer time, and C and 12 are constants. Although not quantified in this study, the variables k and C relate to the rate of release of Cu from the bead and would be expected to be dependent upon many physical factors, including the bead surface area, bead mass,Cu solubility and diffusion rate, temperature, and any energy barriers associated with vaporization from the surface. Similar first-order kinetic expressions have previously been used to describe diffusion of interstitial metals within metals (e.g., ref 24). The logarithms of six absorbance measurements taken a t 10-s increments of transfer time (60-s total transfer time) were plotted against time. The furnace temperature during these transfer intervals was 1750 O C . The plot of In A, against time was linear with a correlation coefficient of 0.996, suggesting that the amount of Cu transferred under these conditions followed this first-order relationship. The energetics for release of Cu from this metal may be calculable by use of this approach. Additionally, quantification of Cu may also be achievable utilizing this relationship; however, it was not attempted with this sample due to the significant mass loss and the changing chemical composition of the bead, which would likely affect the slope of the curve. If applicable to solid systems generally, the use of this equation may open up unique analytical possibilities. These studies are presently being pursued. The relationship shown by eq 2 has been followed in another case examined. Data provided in a paper by Takada and Hirokawa (5)for Cu atomization signals from ingot iron using repetitive atomization cycles were plotted according to eq 2. A linear plot was observed with these data also. The routine utilization of long, ca. 50 s, transfer times for a "single shot" determination of Cu in SRM 1645 was precluded because of overheating of the power supply transformer when used for the long duration of high-temperature heating. In addition, the lifetime of the T a plug was reduced to approximately 25 total shots under these conditions. Filter Paper. The determination of Ag and Mn from filter paper samples was investigated to pursue the possibility of gathering samples in the field using on-site pipetting onto filter paper disks for later lab analysis. By use of this sampling method, losses due to adsorption onto container surfaces could be avoided and sample transport could be simplified. Airborne particulate matter could also be analyzed by direct analysis of filter paper material. Besides these potential uses, the direct
ANALYTICAL CHEMISTRY, VOL. 58, NO. 7, JUNE 1986
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100
150
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Figure 6. Callbration curves for Ag deposited as aqueous samples directly onto the furnace wall (0)and as aqueous samples deposited onto filter paper disks (X).
Figure 7. Calibration curve for aqueous Mn standards deposited directly onto the furnace wall (0)and standard additions curve for aqueous Mn standards deposited onto disks of filter paper (X).
trace-metal analysis of paper associated with chromatographical supports, archival studies, or pulp material is of interest. These analyses were performed prior to the development of the solid sampling cup. With the plug retracted, the samples were placed directly into the furnace through the opening for the T a plug. Because a sampling cup was not used, no attempt was made to remove the ash residue after the transfer step, and atomization was conducted immediately after transfer. There may be some utility in using the solid sampling cups for these analyses also, since the occasional problem with the paper ash residue blowing about inside the furnace would likely be avoided. Background levels are below 0.2 absorbance unit during atomization for these analyses. Sampling was done while holding the paper disks with tweezers and wetting them with an aqueous standard before placement in the furnace. The paper was dried and ashed in 1%oxygen leaving only a small ash residue. The plug was then inserted, and the transfer and atomize cycles were initiated. Figure 6 shows the calibration curves using absorbance peak heights obtained for 2-pL samples of aqueous Ag deposited on the furnace wall and on the filter paper. The peak shapes were virtually identical for the two sample types, and the superimposable curves suggest no interference by the paper on the Ag signal. Manganese was also determined from filter paper samples, but unlike Ag, there was Mn contamination in the paper itself. Thus, Figure 7 shows a calibration curve for aqueous standards pipetted directly and a standard additions curve for Mn solution standards placed on the filter paper prior to insertion. The similarities in the slopes between the two curves suggest little effect of the filter paper on the determination. Background levels at 279.5 nm were below 0.2 absorbance unit during atomization for these analyses.
of the analyte from the matrix. In rare cases where analyte and matrix volatilities directly overlap, condensation of the matrix on the plug can increase the background levels and interferences. Sample pretreatment may be of utility in these instances. In general, the technique provides a convenient, quantitative method for direct trace analysis of solids when high accuracy is not a requisite.
CONCLUSION The reasonable accuracy and precision for most determinations using peak heights confirm that the second surface atomizer can successfully minimize background and chemical interferences for direct solids analysis. The in situ separation of analyte and matrix resulted in peak shapes nearly identical for aqueous and solid samples. The time required for an analysis is an important criterion in GFAA. For routine work using the second surface atomizer, the prior characterization of the sample is minimal if reasonable estimates or measurements are made concerning the volatility of the matrix. The transfer temperature is chosen accordingly from a range of usable temperatures. This range of transfer temperatures spans several hundred degrees and, in most cases, offers sufficient latitude to facilitate separation
ACKNOWLEDGMENT We thank Schunk and Ebe Carbon Corp. (Menomonee Falls, WI) for their cooperation. Registry No. Cd, 7440-43-9; Pb, 7439-92-1; Cu, 7440-50-8; T1, 7440-28-0; Ag, 7440-22-4; Mn, 7439-96-5; Ta, 7440-25-7; graphite, 7782-42-5.
LITERATURE CITED Vollkopf, V.; Grobenski, 2.; Tamm, R.; Welz, B. Analyst (London) 1979, 110, 573-577. Carnrick, G. R.; Lumas, B. K.; Barnett, W. B. 1985 Pittsburgh Conference, New Orleans, LA, Paper 159. Backman, S.; Karlsson, R. Analyst (London) 1979, 104, 1017-1029. Price, W. J.; Dymott, T. C.; Whiteside, P. J. Spectrochim.Acta, Part8 1980, 3 5 8 , 3-10. Takada, K.; Hirokawa, H. Fresenius' 2. Anal. Chem. 1982, 3 1 2 , 109-113. Kurfurst, U.; Rues, B.; Wachter, K. H. Fresenius' Z . Anal. Chem. 1983, 3 7 4 , 1-5. Atsuya, I.; Itoh, K. Spectrochim. Acta, Part 8 1983, 3 5 8 , 1259-1264. Frech, W.; Lundberg, E.; Barbooti, M. M. Anal. Chim. Acta 1981, 131, 45-52. Nlchols, J. A.; Jones, R. D.; Woodrlff, R. Anal. Chem. 1978, 5 0 , 2071-2076. Lundberg, E.; Frech, W. Anal. Chim. Acta 1979, 104, 75-84. Nikolaev, G. I. 2.Anal. Khim. 1965, 2 0 , 445-447. Marks, J. Y.; Welcher, G. G.; Spellman, R. J. Appl. Spectrosc. 1977, 3 1 , 9-11. Headridge, J. B. Spectrochim. Acta, Part 8 1980, 3 5 8 , 785-793. Sommer, D.; Ohls, K. Fresenius' 2. Anal. Chem. 1979, 2 9 8 , 123-127. Langmyhr, F. J.; Thomassen, Y.; Massoumi, A. Anal. Chim. Acta 1974, 6 8 , 305-309. Rettberg, T. M.; Hoicombe, J. A. Spectrochim. Acta, Part 8 1984, 3 9 8 , 249-260. Rettberg, T. M.; Holcombe, J. A. Spectrochim. Acta, in press. Chureila, D. J.; Copeland, T. R. Anal. Chem. 1978, 5 0 , 308-314. Paveri-Fontana, S. L.; Tessari, G.; Torsi, G. Anal. Chem. 1974, 4 6 , 1032-1 038. L'vov, B. V. Atomic Absorption Specfrochemlcal Analysls ; Elsevier: New York, 1970; pp 117-119, 220. Salmon, S. G.; Holcombe, J. A. Anal. Chem. 1982, 5 4 , 630-634. Holcombe, L. J.; Eynon, B. P.; Switzer, P. Environ. Sci. Technol. 1985, 19, 615-620. Kautherr, N.; Shenasa, M.; Llchtman, D. Environ. Sci. Technol. 1985, 19, 609-614. Wheeler, J. A.; Wlnslow, F. R. Diffusion in Body Centered Cubic Metals; American Society for Metals: Metals Pk., OH, 1965; p 4.
RECEIVED for review October 11, 1985. Accepted February 6, 1986. This research was supported, in part, by National Science Foundation Grant CHE 8409819.
