2567
Anal. Chem. 1988, 60, 2567-2572
researchers have considered that molybdenum oxide contributes to a formation of uniform film with less defects @), to a formation of amorphous film (6),etc. Nevertheless, it still is left unclear whether or not molybdenum influences the film characteristics to the response of hydrogen ion and the attack of chloride ion in an acid solution at present. In the case of the molybdenum oxide film electrode (3), it had resistance against the chloride ion present in a solution of a pH range between 3 and 11but showed a Nernstian response in the narrower pH range and a worse selectivity versus pH than that of oxide film formed on stainless steel. Selective coefficient of potassium ion was in level of for a molybdenum oxide electrode but below the measurable level for an oxidecoated SUS316 electrode. These results suggest that a composite metal oxide film is superior to a monometal oxide fiim as a pH-sensing electrode. The initial drift after the immersion in solutions was about -60 mV/2 h at the pH value of 1.68, and about 20 mV/2 h at a pH value of 4.0. The response became stable after about 3 h, and a slight drii at the rate of about 1mV/h was observed in each solution. Even when pH measurement was carried out just after dipping a dry electrode, a Nernstian slope could also be obtained at the moment though the response drifted to some extent. The output voltage kept constant within the deviation of at most 30 mV with the lapse of time. In a solution of pH 2.2 containing 0.5 M NaC1, the gate voltage decreased suddenly by about 10 or 20 mV once every 2-4 hours but recovered in about 10-20 min. After 2 days the gradient decreased to about 30 mV/pH. Such a phenomenon was not observed in 0.5 M NaCl solutions with pH values higher than 3.5. Although pit incubation or nucleation (5) might be originated by the attack of chloride ion, the pit growth was prevented in the oxide film on stainless steel in the high pH solutions. If the hydrogen ion concentration were
high, the active sites could produce a number of positive sites by the adsorption of H30+. The change of the surface charge would permit the approach of chloride ions to get into the oxide film. Concerning the long-term stability of the response, the electrode will work well for more than a month if the electrode is not dipped in a solution of NaCl at a pH value less than 3.5.
CONCLUSION The oxide films on SUS316 were found to be superior to the oxide films on SUS304 from the view point of suppression of the interference caused by chloride ion in solutions. Calibration of pH needs to be carried out at every measurement as slow drift cannot be controlled at present. The calibration can be easily carried out by adjusting the applied voltage or current between the source and the drain of FET by using only one standard solution. As any shape of electrode can be fabricated easily depending on the purpose of the pH sensor, the oxide-coated stainless steel electrode will be useful for various types of pH measurements. Because the sensing film is separated from the FET and can be fabricated very easily, metal oxide coated stainless steel sheets are disposable. LITERATURE CITED (1) Fog, A.; Buck, R. P. Sens. Actuators 1984, 5 , 137. (2) Akiyama, T.; Ujihira. Y . ; Okabe, Y . ; Sugano, T.; Niki, E. I€€€ Trans. Electron Devices 1982, ED-29, 1936. (3) Nomura, K.; Ujihira, Y . Anal. S d . 1987, 3 , 125. (4) Boshin, R.; Benzhen, Y . Nihon Kagaku Kaishi 1987, 1054. (5) Nishimura, R.; Kudo, K. Corrosion-NACE 1988, 4 4 , 29. (6) Hoar, T. P. J . €/ectrochem. SOC. 1970, 117, 17C.
RECEIVED for review February 8,1988. Accepted September 1, 1988.
Investigation and Elimination of Chloride Interference on Thallium in Graphite Furnace Atomic Absorption Spectrometry Bernhard Welz,* Gerhard Schlemmer, and Jayateerth R. Mudakavi' Department of Applied Research, Bodenseewerk Perkin-Elmer & Co GmbH, 0-7770 Ueberlingen, Federal Republic of Germany
Chiorlde Interferences on thallium are In part caused by voiatllizatlon of thallium chiorlde In the pyrolysis stage and in part by formation of TIC1 In the gas phase during the atomlration stage. The palladium modifier Is not as effective for thalllum as it is for other elements. Its stabilizing power can be Improved substantially when the modifler Is pyrolyzed at 1000 OC before the sample Is pipetted. Use of hydrogen purge gas has a similar effect on the stabilizing power of palladium. I t was found necessary to apply both these measures, pyrolysis of the modmer prior to the additlon of the sample and use of hydrogen, for interference-free determlnatlon of thallium in samples with high chloride content such as seawater and urlne. A detection llmil(3a) of 2 pg L-' was obtained for thalilum In both sample types with 10-pL sample volumes, and the characterlstlc mass was 19 pg.
Present address: Department of Chemical Engineering, Indian Institute of Science, Bangalore, India. 0003-2700/88/0360-2567$01.50/0
Thallium is an element of substantial toxicity and is therefore of considerable analytical interest particularly in biological and environmental samples. Graphite furnace atomic absorption spectrometry (GF-AAS)provides the required sensitivity and selectivity and is an important tool for determining thallium at low concentrations. However, there appear to be tenacious interferences in the determination of thallium, particularly in matrices containing high chloride concentration. Many analysts therefore prefer complexation and extraction of the analyte prior to its determination, as is summarized in a recent review article on this element (1). In this work we wanted to investigate to what extent interferences in the determination of thallium, particularly due to chloride, could be overcome when up-to-date equipment and techniques were used. This included for example the use of the stabilized temperature platform furnace (STPF) concept (2) and Zeeman-effect background correction ( 3 , 4 ) . Slavin and co-workers, have shown in several publications that atomization from a L'vov platform substantially reduced the 0 1988 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 60, NO. 23, DECEMBER 1, 1988
Table I. Typical Graphite Furnace Temperature Program for the Determination of Thallium" 1
2
3
4
5
6
temperature, "C 90 120 1000 1650 2650 20 ramp time, s 1 1 0 1 0 0 1 1 hold time, s 20 20 20 6 5 5 read on internal gas flow rate, 300 300 300 0 300 300 mL min-'
" In some experiments a lower pyrolysis temperature (step 3) or a higher atomization temperature (step 4) was used. In the presence of high salt concentrations the ramp and hold times in step 3 were increased to 40 s each. interference from sodium chloride on thallium ( 5 ) . The addition of sulfuric acid as a chemical modifier further increased the range of interference-free determination (6, 7). However, atomization from a platform and the use of sulfuric acid could not completely eliminate chloride interferences on thallium. Lelow et al. (8)found it necessary to use a standard addition method for the determination of thallium in biological material to correct for matrix effects, and even when the work of Slavin et al. (6, 7) was consulted, it became apparent that smaller amounts of the interferent had a more pronounced influence on the thallium signal than did larger amounts. This found expression in a "dip" in the curve when the thallium signal was plotted against the interferent concentration, and this dip was found around 10 pg of NaCl and 0.1 M HC104, respectively. L'vov (9) stated that formation of gaseous monohalides is the most frequent case of chemical interferences in GF-AAS, and the influence of chloride on thallium is an example of this interference. For eliminating or diminishing this effect, he proposed to increase the atomization temperature by vaporizing the samples from a graphite platform and to bind the free chlorine into molecules by the addition of lithium. Recently we showed that, in agreement with L'vov's data, addition of lithium reduced the chloride interference on thallium but did not remove it completely (10). Shan et al. (11) hgve developed a method for the determination of thallium in wastewater by using palladium as a modifier which allowed the pyrolysis temperature to be raised to lo00"C. We investigated this modifier and its applicability to a number of elements and found that a mixture of palladium nitrate and magnesium nitrate resulted in a higher ruggedness and better tolerance for interferents (12, 13) compared to palladium alone. Voth-Beach and Shrader (14) also found that the performance of palladium alone as a modifier was strongly dfected by the sample matrix and that the addition of a reducing agent provided for more consistent performance. Their resulb were seriously impaired, however, because these authors used atomization from the tube wall and not from a platform, which made the analysis more susceptible to interferences and less comparable to the results of others. In recent work Manning and Slavin (15) confirmed that palladium nitrate is an effective modifier for thallium de-
termination. They found it advantageous, however, to work without the addition of a modifier while abandoning the pyrolysis step completely, which reduced total analysis time to about 1 min. The authors diluted their samples with nitric acid and used Zeeman-effect background correction to cope with the high background absorption that resulted from these conditions. Thallium was fully recovered in the presence of about 30 Fg of NaCl or KC1, which is, however, an order of magnitude lower interferent mass compared to what others have found when a modifier and a pyrolysis step were used. Beyond the analytical aspect of thallium determination in chloride-containing matrices we also carried out some investigations on the mechanism of chloride interferences. For this purpose we used the previously described dual-cavity platform (16, 171, which allowed spatially separated pipetting and volatilization of analyte and interferent in the same graphite tube. This arrangement made it possible, to a certain extent, to distinguish between condensed-phase and gas-phase interferences.
EXPERIMENTAL SECTION Instrumentation. A Perkin-Elmer Zeeman/BO30 atomic absorption spectrometer with HGA-600 graphite furnace and AS-60 furnace autosampler was used throughout this work. S i evaluation was performed exclusively by means of integrated absorbance values (peak area) computed by the AA instrument. Pyrolyticgraphite coated tubes (Perkin-Elmerpart no. B010-9322) with pyrolytic graphite platforms (Perkin-Elmer part no. B0109324) were used for most of the experiments. Special "dual cavity platforms" with two cavities instead of the usual one were custom-made from solid pyrolytic graphite (Ringsdorff-Werke, GmbH, Bonn-Bad Godesberg, FRG) and were used for some of the experiments. They were inserted into conventional PerkinElmer pyrolytic graphite coated tubes in which the dosing hole was enlarged to form a slot in order to facilitate pipetting into the two separate cavities, as was dwribed in detail elsewhere (16). An electrodeless discharge lamp for thallium, operated at 7 W from an external power supply, was used for all determinations. The wavelength was set to 276.8 nm and the slit width to 0.7 nm (low). The graphite furnace temperature program given in Table I was used unlesa stated otherwise. For some determinations the modifier solution was pipetted and pyrolyzed before the sample solution was pipetted on top. A typical temperature program for this sequential treatment is given in Table 11. Reagents and Samples. All reagents used were of the highest purity available and at least of analytical-reagent grade. Nitric acid was purified by distillation in a quartz sub-boiling still (Kuemer Analysentechnik, Rosenheim, FRG). Thallium standard stock solution, loo0 mg L-l, was prepared from Titxisol concentrate (Merck, Darmstadt, FRG). Reference solutions were prepared daily by further dilution with 0.2% (m/v) nitric acid. Palladium nitrate solution, 3000 mg L-' Pd, was prepared by dissolving 300 mg of palladium metal powder (-22 mesh, m4N8 = 99.998%, order no. 400030, Alfa Ventron Products) in 1 mL of concentrated nitric acid with the addition of 10 pL of concentrated hydrochloric acid under gentle heating and diluting to 100 mL with deionized water. Magnesium nitrate solution, 2000 mg L-' Mg(NO& was prepared by dissolving 350 mg of magnesium nitrate hexahydrate (Suprapure, order no. 5855, Merck, Darmstadt, FRG)in deionized water and diluting to 100 A.The palladium nitratemagnesium nitrate mixed modifier solution was
Table 11. Graphite Furnace Temperature Program for Sequential Pipetting and Pyrolysis of the Modifier Prior to the Sample Solution 1
2
30
temperature, "C 90 120 1000 ramp time, s 1 20 10 hold time, s 15 5 10 read 300 300 300 internal gas flow rate, mL min-' a Program interruDted for cool-down and samDle deDosition.
5
6
7
8
9
90 1 15
120 20 5
1000 10 20
1800 0 5
2650 1 5
20
300
300
300
300
300
4
1 5
on 0
ANALYTICAL CHEMISTRY, VOL. 60, NO. 23,DECEMBER 1, 1988
2569
\
b
\
\
Pylolysis Temperature,
Flgure 1. Influence of pyrolysis temperature on thallium signaldual cavlty platform: (a) TI In aqueous solutlon; (b) TI with 10 pg of NaCl in the other cavity: (c)TI mixed with 10 pg of NaCI.
O C
Figure 2. Influence of pyrolysis temperatwe on thallium signal In 0.2% (m/v) HN0,dual davity platform: (a) TI in nitric acid solution; (b) TI with 10 pg of NaCl in nitric acM solution in the other cavity; (c)TI mixed with 10 pg of NaCl in nitric acid solution.
prepared by mixing equal volumes of the above-mentioned solutions. Ten microliters of this solution was added to the measurement solution for each determination, correspondingt~ masses of 15 pg of Pd (32.5 pg of Pd(N03)J and 10 pg of Mg(N03)Z. NASS-1 Open Ocean Seawater Reference Material was obtained from the Marine Analytical Chemistry Standards Program, Division of Chemistry, National Research Council, Ottawa, Ontario, Canada K1A OR9. SERONORM Trace Element Urine, batch no. 108, was obtained from Nycomed AS Diagnostics, Oslo, Norway. .d
-
+.
RESULTS AND DISCUSSION Dual-Cavity Platform Experiments. This set of experiments was carried out in order to obtain more information about the mechanisms involved in the chloride interference on thallium. For the same reason some of the experiments were carried out without the addition of a chemical modifier. The dual-cavity platform (DCP), under ideal circumstances, allows differentiation between interferences in the gas phase and those originating from condensed-phase interactions. Interferences of the former type should persist even when analyte and interferent are in different cavities because they are expected to mix readily when they are released into the gas phase. Interferences of the latter type, however, should disappear because analyte and interferent do not come into contact in the condensed phase. But, in addition to these "ideal" cases, we have observed in earlier work interactions of compounds volatilized from one cavity with the condensed phase in the other cavity (16,17). Figure 1 shows the influence of the pyrolysis temperature on the integrated absorbance signal obtained for thallium without the addition of a modifier. In pure aqueous solution the thallium signal decreases gradually with increasing pyrolysis temperature. The decrease is much more pronounced in the presence of sodium chloride, particularly for pyrolysis temperatures above 300 OC, and when thallium and sodium chloride are mixed in one cavity of the DCP. This is in accordance with the assumption that the chloride interference on thallium is caused by volatilization losses of TlC1, which has a melting point of 430 OC and a boiling point of 720 "C. However, there is also a substantial effect of sodium chloride on the thallium signal when analyte element and interferent are separated on the DCP. This cannot be explained by volatilization losses of TlCl prior to the atomization stage. It must rather be due to the formation of gaseous TlCl molecules in the atomization stage which are carried out of the graphite tube without being atomized. When the same experiment is carried out in 0.2% (m/v) nitric acid solution (Figure 2), the individual pyrolysis curves at first glance appear to be not much different from those in Figure 1. It is nevertheless striking that for low pyrolysis temperatures the effect of sodium chloride is more pronounced when analyte and interferent are separated on the DCP than when they are mixed. In the presence of nitric acid sodium
!
200 300
LOO 500 600 700 Pyrolysis Temperature T
Figure 3. Influence of pyrolysis temperature on thallium signal in 0.2% (m/v) HNO,-dual cavity platform: (a)TI pyrolyzed first, 10 pg of NaCl added later in other cavity; (b) TI pyrolyzed first, 10 pg of NaCl added later into the same cavity: (c) 10 pg of NaCl pyrolyzed first, TI added later in other cavity; (d) 10 pg of NaCl pyrolyzed first, TI added later
into the same cavity.
chloride is converted into sodium nitrate and hydrochloric acid according to NaCl
+ HNOB= NaN03 + HC1
(1)
The HC1 is volatilized in the drying stage and can according to previous experience (17)react with the analyte in the other cavity. The 2530% lower sensitivity (compared to matrix-free solutions), for pyrolysis temperatures up to about 500-600 OC is then predominantly due to losses of the undissociated monochloride in the atomization stage, and the more severe influence at 700 "C includes incremental losses of TlCl in the pyrolysis stage. The relatively small influence of sodium chloride on thallium in nitric acid solution when the solutions are mixed, and pyrolysis temperatures are low, is most probably due to the fact that chloride has been volatilized effectively in the form of HC1 in the drying stage according to eq 1,and thallium is imbedded in a large excess of sodium nitrate which prevents losses. The more pronounced drop of the curve for mixed solutions and pyrolysis temperatures higher than 400 OC in Figure 2 compared to Figure 1can be explained by the higher volatility of thallium nitrate (mp 206 OC;bp 430 "C) compared to the chloride. The volatilization of thallium may well be enhanced by the decomposition of sodium nitrate, which begins around 380 "C. This is in agreement with previously made observations that a high percentage of the analyte may be carried out of the furnace tube when the matrix decomposes in a vigorous reaction (16,17). In an additional set of experiments, depicted in Figure 3, either the analyte element or the interferent was pipetted onto the platform, dried, and pyrolyzed but not atomized. After cooling, the other component, interferent or analyte element, was pipetted into the other cavity or on top of the pyrolyzed solution, and the complete temperature program executed.
ANALYTICAL CHEMISTRY, VOL. 60,NO. 23. DECEMBER 1, 1988
2570
E
-
231,
d o
,
,
,
,
,
,
,
600 800 1000 Pyrolysis Temperature O C Figure 4. Influence of pyrolysis temperature on TI using palladium nitrate-magnesbmnitrate modifief-dual cavity platform: (a) TI in nitric acld solution with modlfier; (b) TI with 10 pg of NaCl in the other cavity; (c) TI mixed with 10 pg of NaCI. 200
LOO
The first observation is that there is very little influence from sodium chloride (in nitric acid solution) on the thallium signal when pyrolysis temperatures of 200-400 "C are used, irrespective of the experimental protocol. This is easy to explain when sodium chloride is injected and pyrolyzed first as the chloride is expected to be removed from the furnace under these conditions according to eq 1. The pyrolysis curve should essentially reflect volatilization of thallium nitrate, supported by the decomposition of sodium nitrate, which is expressed by the pronounced drop in sensitivity when pyrolysis temperatures higher than 400 "C are used. When thallium is dried and pyrolyzed first and sodium chloride added later into the other cavity of the DCP, the pyrolysis curve is very close to that for an aqueous thallium solution. The HCl gas generated according to eq 1 on decomposition of sodium chloride in nitric acid solution in the one cavity apparently cannot under these conditions react with thallium in the other cavity and cause the losses which became apparent in Figure 2. A possible explanation for this striking difference is that thallium, during the pyrolysis, has undergone a change that makes it less reactive to HCl gas. It may be sufficient for the thallium that the solvent is removed completely to avoid reaction with the HCl gas, which is otherwise readily absorbed by the wet sample. There may also be an additional stabilization of thallium by the graphite surface which makes it less susceptible to an attack by HC1. When palladium nitrate (which is not shown here) or a mixture of palladium nitrate and magnesium nitrate is used as a chemical modifier, thallium is stabilized to pyrolysis temperatures of 1000 "C as is depicted in Figure 4. When thallium is determined in the presence of 10 pg of NaC1, the stabilizing power of both modifiers is not quite satisfactory. The integrated absorbance for thallium is under all conditions lower in the presence of sodium chloride and decreases further with increasing pyrolysis temperature. When thallium and the modifier are in one cavity of the DCP and the nitric acid solution of sodium chloride is in the other, there is very little influence up to a pyrolysis temperature of 400 "C. Only when higher pyrolysis temperatures are applied is the thallium signal some 10% lower than in the absence of sodium chloride. This is generally in agreement with earlier findings that the chloride interference in nitric acid solution is due to volatilization losses of TlCl and not due to a gas-phase interaction. Neither palladium alone nor a mixture of palladium nitrate and magnesium nitrate can apparently eliminate this interference completely, which is in agreement with earlier findings (10, 14, 15).
A possible explanation would be that the modifier must be in a particular chemical form, such as the reduced, metallic form, in order to prevent losses of thallium and that it cannot develop its full stabilizing power at the relatively low temperatures at which TlCl is volatilized. This could also be an
01
1 10 lnterferent Moss , pg NoCl
loo 200
Figwe 5. Influence of NaCl on thallium signal using different modifiers
(pyrolysls 800 "C; atomization 1650 "C; purge gas argon): (e)Pd(NO3)?modifier; (0)Pd(N03)? Mg(NO& modifler; (0) Pd(N03), iascorbic acid modlfier.
+
interpretation for the puzzling shape of the pyrolysis curves in Figure 4, which all show a "dip" around 800-900 "C. We have observed similar curves earlier for selenium (18) and other elements and have interpreted them as resulting from a rearrangement of the analyte in a thermally more stable compound at the higher temperature. Thallium may therefore form a more stable compound with the palladium modifier at 1000 "C than it does at 800 "C or 900 "C. In an additional set of experiments thallium and the palladium nitrate-magnesium nitrate modifier were injected together, dried, and pyrolyzed at 1000 "C, and the program was interrupted after the pyrolysis stage (see Table 11). After cooling, sodium chloride in nitric acid solution was pipetted on top of the pyrolyzed analyte and, in the other cavity of the DCP,respectively, and treated as usual. Quantitative recovery of thallium was obtained in any case, which means that there is no chloride interference under these conditions, and confirms that thallium and the modifier at lo00 "C form a compound that is no longer affected by the presence of high amounts of chloride. Chloride Interference and Modifier Action. ln aqueous solution, and when no modifier is used, sodium chloride even at very low concentration depresses the thallium signal significantly. In 0.2% (m/v) nitric acid solution the effect is diminished but still very pronounced and depends, as expected for a volatilization interference, very much on the pyrolysis temperature. Addition of palladium nitrate, alone or mixed with magnesium nitrate (12)or with 60 pg of ascorbic acid (14) improved the stability of thallium but could not remove the interference of sodium chloride in masses greater than 0.5-1 pg as depicted in Figure 5. It is interesting to note that all interference curves exhibit a "dip", typically around a mass of 10 pg of NaCl. These curves showed only minor variation when the pyrolysis temperature was varied between 500 and lo00 "C and are not depicted here. They also did not change noticeably when a cool-down step was introduced between pyrolysis and atomization as proposed by Slavin et al. (19), and there was essentially no difference between NaCl and KC1 as the interferent, which is in contrast to the reports of Manning and Slavin (15). The dip in the curve of Figure 5 first of all means that palladium, alone or mixed with magnesium nitrate or ascorbic acid, cannot prevent losses of thallium in the presence of high chloride concentrations. It also means, however, that the interference of about 10 pg of NaCl on thallium is more pronounced than that of a 10 times higher mass of sodium chloride. This dip is also described in the work of Manning and Slavin (15) and it is shown but not discussed in earlier publications of Slavin et al. (6, 7). This behavior may be explained by the formation of a thermally more stable, i.e. less volatile, compound of thallium in the presence of a greater excess of chloride. It is not possible, however, to draw con-
ANALYTICAL CHEMISTRY, VOL. 60, NO. 23, DECEMBER 1, 1988
I
I
01
10
1
Kx)
I
0001
300
lnterferent Mass, pg NaCl
Figure 6. Influence of NaCl on thallium signal (pyrolysis 800 "C; atomization 1650 "C): (0)Pd(N03), Mg(N03), modifier, pyrolyzed Pd(NO,), at 1000 OC prior to sample deposition, purge gas Ar; (0) Mg(NO,), modifier, purge gas 5 % H,/Ar; (0)Pd(NO,), modifier, purge gas 5 % H,/Ar.
+
+
clusions from our experiments on stabilization mechanisms or compounds formed. In essence, the performance of the palladium nitrate modifier in its various combinations was analytically not satisfactory for thallium determination because the chloride interference could not be eliminated. In an earlier experiment we have observed that the chloride interference disappeared when thallium and the modifier were pyrolyzed at 1000 "C on the platform before the sodium chloride solution was added. Grobenski et al. (20) found for the determination of mercury that the palladium modifier was much more effective when it was injected onto the platform first and pyrolyzed at 1000 "C before the sample was injected. For pure thallium solutions there was no difference in sensitivity or stabilizing power when analyte and modifier were injected together or the modifier pyrolyzed first and the analyte added later. There were no losses of thallium at pyrolysis temperatures up to 1100 "C in any case. There was a dramatic difference, however, with respect to the chloride tolerance as becomes apparent from Figure 6. The dip disappeared completely when the mixture of palladium nitrate and magnesium nitrate was injected onto the platform and pyrolyzed at 1000 "C before analyte and interferent were added, and up to 100 pg of NaCl had no influence on the analyte signal. With a slight modification of the temperature program that was extending the ramp and hold times in the pyrolysis step (step no. 6 in Table 11)to 40 s each, the interference-freerange could even be extended up to 300 pg of NaCl. Voth-Beach and Shrader (14) have shown that palladium was reduced to the metal at 1000 "C in the course of the pyrolysis stage. They suspected that palladium as the metal acts as a modifier for thallium and proposed that addition of a reducing agent would guarantee that the palladium is present as the metal early in the temperature program. The results depicted in Figure 5 show, however, that at least ascorbic acid, which was among the proposed reducing agents, did not help at all. Another reducing agent proposed by these authors was hydrogen (5% H2in Ar) used as a purge gas. The results depicted in Figure 6 show that this combination actually works as well as does the pretreatment at 1000 "C. There is also no difference between pure palladium nitrate and palladium mixed with magnesium nitrate as the modifier. The argonhydrogen mixture was used as the purge gas (through the tube) while pure argon was retained as the sheath gas around the tube. There was no difference in the performance when the argon-hydrogen mixture was used during the drying and pyrolysis steps only or during the entire program cycle. It was considered more economical, however, to use the expensive gas mixture only when required, i.e. in program steps 1to 3. From these experiments it appears that there are at least two equally effective ways to overcome the chloride inter-
001 01 1 lnterferent Concentration, '/o ( m i v l HCI
Figure 7. Influence of HCI on thallium signal using Pd(NO,), (NO,), modifier: (a) purge gas Ar; (b) purge gas 5 % H,/Ar.
2571
I
3
+ Mg-
.#
;
-
+-
201
d o
0001
0 01 01 1 3 lnterferent Concentration,% Imivl HCIOL
Figure 8. Influence of HCIO, on thallium signal using Pd(N03), Mg(N03), modifier: (a) purge gas Ar; (b) purge gas 5 % H,/Ar.
+
ference on thallium: the pyrolysis of the modifier at lo00 "C and the use of hydrogen as the purge gas during the drying and pyrolysis stages. Hydrochloric acid and perchloric acid are also among the chloride-containing compounds reported to have a strong interference effect on thallium. We have briefly investigated these two acids by using argon and hydrogen in argon, respectively, as the purge gas with all the other conditions unchanged. The addition of hydrogen to the purge gas increases the range of interference-free determination of thallium quite substantially in the presence of both acids as is depicted in Figures 7 and 8. The mechanism may be that, in the presence of hydrogen, palladium is reduced to the metal earlier in the temperature program and is present in a different physical form as was proposed by Voth-Beach and Shrader (14). However, it may also be that hydrogen reacts with the chlorine and helps to remove the interferent from the graphite tube as suggested by L'vov (9) and Frech and Cedergren (21, 22) for the chloride interference on lead determination, and as it is well known that palladium can adsorb large amounts of hydrogen, it is possible that a combined effect of palladium and hydrogen adsorbed to palladium stabilizes the analyte element and helps to volatilize the interferent. Analysis of Seawater and Urine. Seawater and urine are typically considered to be among the most troublesome samples to analyze because of the high chloride content. Unfortunately there are no such standard reference materials available with a certified value for thallium. We therefore analyzed NASS-1 Seawater and SERONORM Trace Elements Urine and spiked these materials with thallium in order to test the proposed procedures. Both types of samples exhibited a very similar analytical behavior. When 10 pL of undiluted seawater or urine was injected into the furnace for analysis, none of the previously established procedures, neither the sequential pyrolysis of the modifier prior to sample deposition nor the use of hydrogen, gave satisfactory results. The recovery of added thallium was typically found between 60% and 80%. Only a combination of both, use of hydrogen in the purge gas and the separate
2572
ANALYTICAL CHEMISTRY, VOL. 60, NO. 23, DECEMBER 1, 1988
Table 111. Integrated Absorbance Readings for Thallium (in Duplicate) in Matrix-Free Solution (Diluent) and in NASS-1 Seawater Using Separate Pyrolysis of the Modifier Prior to Sample Deposition and 5% Hydrogen in Argon Purge Gas
T1 added, ng 0 0.2 0.5 1.0
005
integrated absorbance reading diluent NASS-1 0.003, 0.049, 0.104, 0.189,
0
095
0.001 0.049 0.103 0.184
010
0.002, 0.049, 0.102, 0.189,
015
0.001 0.048 0.098 0.192
0 20
Added Moss of Thallium ( n g i
as long as chloride concentrations are low. In the presence of up to about 10 pg of NaCl or KC1, an alternative appears to be to abandon the pyrolysis step and to work without the addition of a modifier. This also reduces the total analysis time to about 1 min. For samples with high chloride concentrations such as undiluted urine and seawater, however, the only way to an interference-free determination appears to be a separate pyrolysis of the palladium modifier prior to sample deposition and the addition of hydrogen to the purge gas. Under these conditions we have seen no difference in performance between palladium nitrate alone and the palladium nitrate-magnesium nitrate mixed modifier, and it may be simpler not to add magnesium nitrate in this case. The fact that both measures, pyrolysis of the modifier prior to sample deposition and use of hydrogen as the purge gas, have to be taken suggests that the action of hydrogen is not, or at least not only, a reduction of palladium to the element. This reduction can be expected to be quantitative after pyrolysis of the modifier on the graphite surface. It may be, however, that not all of the analyte element comes into intimate contact with the modifier when high sodium chloride concentrations have to be analyzed, and stabilization of the analfie element is therefore not optimal. Reactions that are likely to take place in the presence of hydrogen gas are an adsorption of hydrogen on the reduced palladium and/or removal of chloride in the form of HC1 gas.
Flgure Q. Thallium in matrix-free solution and in urine using separate pyrolysis of the modifier prior to sample deposition. Pur* gas is argon (broken line) and 5 % H, In argon (solid line), respectively.
pyrolysis of modifier prior to sample deposition finally gave around 100% recovery. In addition it was found advantageous in the case of seawater to increase the ramp and hold times in the pyrolysis stage (step no. 6 in Table 11) to 20 and 50 s, respectively. Under these conditions the integrated absorbance readings were the same, within experimental error, for thallium in matrix-free solution and added to seawater (Table 111), which indicates an interference-free determination. Similarly, the slopes of the analytical curve for matrix-free solutions and for thallium added to the urine are the same as is depicted in Figure 9. There is a substantial difference in the slopes when argon is used as the purge gas, and essentially the same difference in the slopes was obtained when hydrogen was used as the purge gas but sample and modifier were injected and pyrolyzed together in the graphite tube. Ten repetitive determinations of 10-pL portions of seawater resulted in a mean integrated absorbance of 0.0017 f 0.0016. This translates into a detection limit (3 times the standard deviation of the blank) of 2 pg L-l thallium in undiluted seawater. The characteristic mass of 19 pg of T1 is in good agreement with published data (23). Ten repetitive determinations of 10-pL portions of undiluted, unspiked urine gave a mean integrated absorbance of 0.0088 f 0.0015, which corresponds to a thallium content in Seronorm Trace Elements Urine of about 4 pg L-l, and results in the same detection limit of 2 pg L-' as in the seawater.
CONCLUSION For practical analytical work, the previously recommended palladium nitrate-magnesium nitrate modifier can be used
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RECEIVED for review April 20,1988. Accepted September 6, 1988.
