Anal. Chem. 1984, 56,163-168
163
complex can be observed (less than 1% of H,+is complexed by the crown ether). In order to avoid an eventual competition between H,+ (or H,O+) and the studied metal (Na+, UOz2+, etc...), the concentration of the studied metal has to be chosen as high as possible. Registry No. Na, 7440-23-5;propylene carbonate, 108-32-7.
(11) Norberg, K. Talanta 1966, 13, 745-752. (12) Izutzu, K.; Kolthoff, I . M.; Fujlnaga, T.; Hattori, M.; Chantooni, M. K. Anal. Chem. 1977, 4 9 , 503-508. (13) L’Her, M.; Courtot-Coupez, J. J . Nectroanal. Chem. 1973, 4 8 , 265-275. (14) Talarmin, J.; L’Her, M.; Laquenan, A.; Courtot-Coupez, J. J . Nectroanal. Chem. 1980, 106, 347-358.
LITERATURE CITED
(16) Gosse, B.; Denat, A. J . flectroanal. Chem. Interfacial Nectrochem. 1974, 5 6 , 129-147. (17) Spiess. B. Thesis, Strasbourg, France, 1981. (18) Brown, A. S. J . Am. Chem. SOC. 1934, 5 6 , 846-647. (19) Talarmln, J.; L’Her, M.; Courtot-Coupez, J. J . Chem. Res. 1977, 5 ,
. .
Ser. C 1978, 2 8 7 , 105-108. (2) McClure, J. E.; Reddy, T. B. Anal. Chem. 1988, 4 0 , 2064-2066. (3) Lakshminarayanalah, N. ”Membrane Electrodes”; Academic Press: New York, 1976: pp 50-94. (4) Rechnitz, G. A. Chem. f n g . News 1967, 45 (25),146-158. (5) Eisenman, G. I n “The Glass Electrode”; Wlley: New York, 1965;pp 213-369. (6) Buck, R. P. Anal. Chlm. Acta 1974, 7 3 , 321-328. (7) Buck, R. P.; Boles, J. H.; Porter, R. D.;Margolls, J. A. Anal. Chem. 1074, 4 6 , 255-261. (8) Romberg, E.; Cruse, K. 2.Nektrochem. 1959, 6 3 , 404-418. (9) Coetzee, J. F.; Padmanabhan, G. R. J . Phys. Chem. 1962. 66, 1708- 1713. (IO) Teze, M.; Schaal, R. Bull. Soc. Chlm. Fr. 1982, 7 , 1372-1379.
(15) Kolthoff, I. M.; Harris, W. E. J . Am. Chem. Soc. 1945, 6 7 ,
1484-1491.
28-29. (20) Sabatlni, A.; Vacca. A.; Gans, P. Talanta 1974, 2 1 , 53-77. (21) Talarmln, J.; L’Her, M.; Laquenan, A.; Courtot-Coupez, J. J . flectroanal. Chem. 1979, 103, 203-216. (22) Ingri, N.; Kakolowlcz, W.; Sillen, L. G.; Warnqvist, 8. Talanta 1967. 14, 1261-1286. (23) Fux, P.; Lagrange, J.; Lagrange, P., unpublished results. (24) Kolthoff, I.M.; Wang, W. J.; Chantoonl, M. K., Jr. Anal. Chem. 1063, 5 5 , 1202-1204.
RECEIV~, for review July 11,1983. Accepted October 11,1983.
Chloride Interferences in Graphite Furnace Atomic Absorption Spectrometry Walter Slavin,* G. R. Carnrick, and D. C. Manning Perkin-Elmer Corporation, Main Avenue, Norwalk, Connecticut 06856
The premature loss of analyte as a volatile halide Is shown to be an important interference In m e sttuations. When this char step interference is avolded, an interference In the atomization step can arise when large amounts of halide are present during that step. This atomizatlon interference results from the binding of the anaiyte as a vapor phase metal halide, thereby preventing some portlon of the analyte from absorbing atomic radiatlon. This vapor phase interference can be circumvented in several ways. The most convenient Is the use of high concentrations of HNO, or H2S0, which drlves off more of the chloride as HCI durlng the dry and char steps. As much CaCi, In solution as 1 mg reduced the Mn slgnal In the presence of HNO, and Mg(N03), by only 15%. Analogous experience was found wlth TI. We believe the experience is qulte general. The halide which interferes in the vapor phase is retained from the char step by adsorption or intercalation on the graphite tube. Thus we can expect stili further improvements in the handling of large amounts of hailde matrix when glassy carbon or solid pyrolytic tubes become available.
L’vov’s original publication on electrothermal atomic absorption spectrometry (1) included a theoretical and experimental justification of the “absolute” nature of the technique. All of the analyte was converted to an atomic vapor and, if the experimental conditions were correctly chosen, the expected signal could be calculated. This implied independence of the signal from the nature and amount of the matrix. The instrumental design changes which deviated from those proposed by L’vov led to the extensive literature of interferences over the 25 years since his first publication. However, modern work with the various systems that are often described as “constant temperature furnaces” (2-7) and which use the L’vov
conditions have shown that, within limits, determinations by furnace AAS are indeed independent of the matrix, as L’vov predicted. Our version of these modern furnace systems, called the stabilized temperature platform furnace, STPF, has been described (7) and we have shown furnace analyses within 10 to 20% without using standards (8). This paper explores some of the limits to the situations to which L’vov’s theory can be applied using the STPF technology. Metal halide interferences are, beyond doubt, the most widely reported interferences, and the nature and control of these interferences are gradually becoming clearer. While we use chloride for the experimental work in this paper, the other halogens provide similar results. Alkali and alkaline-earth chlorides have provided the most serious interferences. These interferences apply to most analyte metals. The interferences depend more upon the chloride that is present than upon the metal that is associated with the chloride, but there is influence from both. Important goals of the constant temperature furnace of Woodriff (2)and of the improvements in furnace technology by many other workers (3-7) have been directed toward understanding and controlling these alkali and alkaline-earth chloride interferences. By now we have controlled this metal halide interference on a large variety of analytes and an equally large variety of metal halide matrices. However, when the amount of metal halide matrix is large enough, we and others with improved furnace systems find that an interference arises. Typically, this amount of chloride is on the order of 200 pg of salt on the platform which, in the 2 0 - ~ Lsample, is about 1% salt in the solution. This 200 pg of metal chloride is not a sharp cutoff, larger amounts can be handled for some analytes and for some metal chlorides. In this paper we have addressed some of the sources of these problems and their control. While we have a better under-
0003-2700/84/0356-0163$01.50/00 1984 American Chemical Society
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standing of the source of the problems, our principal progress has been in the control of the interferences. The theory is still speculative and requires additional work.
THEORY A variety of theories have been suggested to explain the halide interference. Copeland (9) and others have suggested that analyte is trapped, occluded, within the crystal of dried matrix in the furnace and some of the crystals are ejected thermally without volatilization. This is a sort of mechanical carrier effect. Matousek (10) argues that occlusion is an unlikely explanation for halide interference. L'vov (11)suggested that part of the analyte is bound in halide in the vapor phase. He showed that the metals displaying strong halide interference had large dissociation constants for the gaseous metal halides. Elements with larger halide dissociation constants added to a sample reduced the halide interference by scavenging the halide in the vapor phase. Thus, additions of Li reduced chloride interference on Cu and Pb. This theory explained the observation that H, added to the argon gas reduced the chloride interference because HCl has a relatively high dissociation constant. The theory was further supported by the observation (12) that the strong MgCl, interference on P b was unaltered when the P b and MgCl, were deposited in separate parts of the furnace tube. The theory was also consistent with our observation (13) that the degree of MgClz interference on Mn depended upon the char temperature. Charring a t higher temperature left less MgC1, to interfere in the vapor phase during the atomization step. We also observed that a higher vapor phase temperature reduced the MgC12 interference on T1 (14). Thus, by now it seems secure to believe that the primary cause of the halide interference is a vapor phase removal of free analyte atoms as gaseous halides. Different quantitative levels of halide interference will depend somewhat upon many variables including the temperature of the gas phase, how much halide is present during the time the analyte atoms are in the vapor phase, and the amount and binding strength of other atoms present in the vapor phase when the analyte absorbance is being measured. The latter effects explain why different metal halide matrices have different interference effects upon a particular analyte, even though the amount of halogen in the sample was the same. Although the primary cause of halide interference appears to be established, some secondary effects are less well understood and are important in determining what to do to control the interference. While developing a method for 13 elements in natural waters (15),we recognized that an independent source of halide interference involved the char loss of volatile lead chloride. We will discuss this effect below. EXPERIMENTAL SECTION
All analyses were performed on a Zeeman/5000 using the L'vov platform, Perkin-Elmer Part No. B0109-324, and pyrolytically coated graphite tubes, Perkin-Elmer Part No. B0109-322. The AS-40 autosampler was used to introduce samples and the matrix modifiers. The Perkin-Elmer Data System 10 and a HewlettPackard 7225.4 graphics plotter were used for plotting the atomization profiles necessary for evaluating performance. After the preliminary work with Pb below, we chose a metal of medium volatility, Mn, and of high volatility, T1, for our experimental protocol. We studied the effect of MgCl, and CaCl, on Mn but we have used only the CaCl, data below because the results were similar. Experiments with Fe in NaCl have not been included because the resuIts were very similar to Mn in CaC12. The analytical parameters for Mn and T1 are included in Tables I and 11. The parameters for Pb were reported earlier (15). All analytical data utilized integrated absorbance, A-s, signals. The temperatures were set with an Ircon optical pyrometer, 2000 series. The arrangements and precautions for setting temperatures were established in a paper dealing with Se (16). We have not ade-
Table I. Analytical Parameters for Mn dry char atom. clean cool temp, "C 280 1400 2200 ramp, s 1 1 0 hold, s 60 45 6 300 0 int flow, mL/min 300 recorder, s -5 read, s -1 wavelength, 279.5 nm Mn HCL, 20 mA 0.7 nm slit width
2600
20
0
1
6
20
0
300
atom. clean
cool
Table 11. Analytical Parameters for T1 dry
char
temp, "C 280 600 1500 ramp, s 1 1 0 hold, s 60 45 6 int flow, mL/min 300 300 0 recorder, s -5 read, s -1 wavelength, 276.8 nm T1 EDL, 6 W 0.7 nm slit width
2600
20
0
1
6
20 300
0
Flgure 1. The curtain tube.
quately emphasized in our previous papers the necessity to use the base-line-offsetcompensator, which is called the BOC in the instruction manuals (17 ) ,in conjunction with the measurement of integrated absorbance signals. For some experiments in this paper we have modified the standard pyrolytically coated tubes to limit the length of the tube which is in contact with the atomic vapor. We have added four holes equally spaced on each of two circles to permit the argon to flow out of the tube without flowing through the hot center of the tube. We call this a "curtain tube" since the argon flowing through these holes provides a limiting curtain for the diffusing analyte vapor. Even when argon is not flowing, these holes permit the gaseous sample matrix and analyte to leave the furnace tube before reaching the cooler ends of the tube. The arrangement is illustrated in Figure 1. In a previous publication (18)we showed that an absorption path length of about 1.5 cm is within 100 "C of the temperature of the center of the tube at the highest furnace temperatures. The eight holes are each 1.3 mm in diameter. Thus, on each circle, the open area for gas to flow out is 0.055 cm'. In contrast, the diameter of the fill hole is 1.6 mm, an area of 0.020 cm2. The pneumatic impedance thus greatly favors the gases leaving through the ring of holes.
RESULTS Pb in MgC1,. Char curves are shown in Figure 2 for P b with no matrix, compared to a matrix containing MgC1, and to one containing ammonium phosphate also. P b on the platform, when no matrix is present, begins to volatilize a t about 600 OC, although small amounts are lost at even lower temperatures. Phosphate will stabilize the P b to higher temperatures. We have shown (15) that still higher char
ANALYTICAL CHEMISTRY, VOL. 56, NO. 2, FEBRUARY 1984 O7
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"
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600"
'
" 800"
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1200"
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PLATFORM
20
I
ob
10
1400"
CHAR TEMP
Figure 2. Char temperature study for 1.5 ng of Pb in the presence
of 20 pg of MgCI, and the combination of MgCI, and 80 pg of NH,H,P04.
temperatures are possible if Mg(N03)zis also added with the phosphate. P b is stable to about lo00 "C in this mixture. The fact that Mg is present as the chloride is not significant, as we have shown (15) that it is the Mg that acts as the matrix modifier, not the compound. Note however that when MgClz alone is added to Pb, the P b is volatile at a lower temperature. This is specific to the chloride and is not observed if Mg is introduced as the nitrate. Thus the many workers who showed that MgClz interfered with P b (12) were suffering to some extent from char losses, not from vapor phase interferences. Presumably some P b is lost as the chloride at low char temperatures. The presence of additional anions, phosphate in this case, prevents the loss of P b as the chloride. In this particular situation, the MgC1, is probably hydrolyzed to the oxide a t a temperature between 500 and 1000 "C, which is consistent with physical chemistry experiments on the properties of MgC1, at high temperatures. Not only will phosphate delay the P b signal, so will sulfate (19,20) and probably many other anions. Other metal halides will also cause these char losses for P b unless an anion such as phosphate or sulfate is present. Having understood this phenomenon, we observed that the same effect can be seen for many other analytes and matrices. For instance, char curves prepared for Cd in the presence of phosphate and Mg(NO& permitted charring at several hundred degrees higher temperature than Cd in the same matrix modifier but with large amounts of seawater present (21). Similar effects were found with Se (16). Manganese. In a summary of our STPF experience (7), we listed from the literature the amount of various matrices which did not interfere for several analytes. That list, and some subsequent publications, showed that for the volatile analytes like Pb, Cd, T1, and Sn, about 200 pg of various metal chlorides just began to interfere. For the more refractory analytes, like Mn, Cu, Cr, and Ni, about 500 pg of various metal chlorides began to interfere. Since these large amounts of matrix produced background signals that were beginning to rise to troublesome levels for the non-Zeeman background correction systems that had been employed, it was difficult to determine the source of the interferences and their magnitude. Thus, we repeated some of these chloride interference studies to see if Zeeman background correction altered the interference. Mn, 200 pg, in the presence of CaCl,, was studied in Figure 3. Comparison is made with atomization studies from a tungsten wire (13),without matrix modifier, and from a similar platform arrangement with Mg(N0J2, using Dz background correction (22,23). The Zeeman-corrected data from these studies, also utilizing Mg(NO&, were similar to our previous experience. For 800 pg of CaClZ,the background absorbance was about 0.7 A . A similar experiment for Mn in MgC1, produced similar interference curves with and
100
1003
CaC12 ( 8 9 )
"
Flgure 3. The interference of CaCi, on Mn using normalized A - s signals. I n the platform 4- Mg(NO,), data (23), 400 pg of Mn and 15 pg of Mg(N03), produced a signal of 0.53 A .s at 0 CaCI,. The wire data are from ref 13. I n the platform, Zeeman, Mg(NO,), data (this work), 200 pg of Mn and 250 pg of Mg(NO,), produced a signal of 0.48 A ' s at 0 CaCI,. I n the "OB, Zeeman curve, 100 pg of Mn in 50 pug of Mg(NO,), and 5 % HNO, produced a signal of 0.28 A -sat 0 CaCI,, using the cool-down step after charring. I n the curtain tube curve, 290 pg of Mn in 50 p g of Mg(NO,), produced a signal of 0.35 A .s at 0 CaCi,.
"
TIME
(SEC)
Figure 4. The comparison of absorbance profiles for 290 pg of Mn
in a standard pyrolyticaliy coated tube and a curtain tube in the presence of 50 pg of Mg(NO,),. The char temperature was 1400 OC and the atomization temperature was 2200 O C . No argon was flowing in either tube.
without Zeeman background correction. Since we had shown that the interference of large amounts of CaClz on Mn absorbance was the same whether conventional Dzbackground correction or Zeeman correction was used, and the interference wm not a char loss, the interference must result from vapor phase binding of Mn when the concentration of chloride was large enough. One source of the chloride available during the atomization cycle might be material volatilized during the char cycle, condensed on the cooler ends of the furnace tube (18), and then revolatilized during the atomization cycle. We thus conducted some experiments with what we call a curtain tube, which is designed to sweep away the material volatilized during the char step so that it will not be available during the subsequent atomization step. The design of the tube is discussed in the Experimental Section. We used Mg(NO& as a matrix modifier, permitting a char temperature of 1400 OC. Figure 4 shows that, even with no internal Ar flow, there was 2-fold reduction in sensitivity for Mn using the curtain tube. When an internal flow was used with the curtain tubes, the sensitivity was decreased further. By use of a curtain tube with 100 mL/min internal argon flow and a char temperature of 1400 OC, the curve marked "curtain tube" on Figure 3 showed that there was some reduction of interference as compared to our previous work, although there were still interferences at large concentrations.
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'1 0
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BACKGROUND -NO M n ADDED
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