2064
Anal. Chem. 1984, 56,2064-2066
Determination of Inorganic Tellurium Species in Natural Waters M. 0. Andreae
Department of Oceanography, Florida State University, Tallahassee, Florida 32306
Te(1V) and Te(V1) are copreclpltated from seawater and other natural waters with Mg(OH)2. Followlng redlssolutlon of the preclpltate wlth HCI, Te( I V ) Is reduced In 3 M HCI to tellurium hydride. The hydrlde is trapped lnslde the graphlte tube of a graphlte furnace atomic absorptlon spectrometer heated to 300 'C, and then determlned by atomlratlon. Te(V1) Is reduced to Te(1V) by bolllng wlth HCI and then determined as Te( IVj. The llmlt of detectlon Is 0.5 pmol L-' and the precision Is 10-20%.
There are no published data on the concentration and speciation of tellurium in natural waters. This is due to the lack of an analytical method to determine this element a t the picomolar concentrations at which it exists in nature. The analytical chemistry of T e has been reviewed by Nazarenko and Ermakov (1). The determination of Te using borohydride reduction and atomic absorption detection has been described by several authors (241,but detection limits obtained by these workers were orders of magnitude above the levels in natural waters. No attempt was made by them to separate Te(1V) and Te(VI), both of which could be present even though on the basis of thermodynamic equilibrium calculations Te(1V) should be predominant. This paper presents for the first time a combination of preconcentration and analytical procedures which allow the determination of Te(1V) and Te(V1) at the concentrations typical of natural waters. EXPERIMENTAL SECTION Apparatus. The analyte solution (25 mL) is contained in a
reaction vessel with an injection port for the borohydride solution, a fritted gas inlet tube, and a gas outlet. The injection port is attached 2.5 cm above the bottom of the vessel. From the reaction vessel, the gas stream containing the tellurium hydride (TeH,) is introduced by a transfer tube into the internal argon purge inlet of a HGA 400 graphite furnace(Perkin-Elmer Corp., Norwalk, CT) using an arrangement similar to the one described for the determination of Ge species by Hambrick et al. @),but omitting the switching valves and cold traps. Argon is added to the carrier gas stream by means of a T-joint in the transfer tube. The internal glass surfaces are passivated with a commercial silanizing reagent (Sylon CT; Supelco, Bellefonte, PA). The gas stream enters the graphite furnace through the left internal purge inlet; the right inlet is left open to vent since we use graphite tubes without the central injection hole (Ultra-Carbon, Bay City, MI). The gas flow rates are 80 mL m i d He and 100 mL min-' Ar. The graphite furnace is mounted in a Perkin-Elmer 5000 atomic absorption spectrophotometer. A Te electrodeless discharge lamp is used; the wavelength is set at 214.3 nm and the slit at 0.2 nm. Under these conditions, the absorption response is linear to ca. 1.2 absorbance units. The deuterium background corrector is used but probably not required for routine analysis. A chart recorder is used as readout and peak height is used for quantitation. Standards and Reagents. Standards are prepared from tellurium oxide (TeOz, ultrapure; Alfa Products, Danvers, MA) for Te(1V) and from telluric acid (H2Te04.2H20,99.5%;Alfa Products) for Te(V1). To prepare 1 L of a 0.01 M stock solution of Te(IV), 1.596 g of Te02 is dissolved in about 100 mL of concentrated HCl and diluted to 1 L with deionized (DI) water. The 0.01 M stock solution for Te(V1) is prepared by dissolving 2.297 g of telluric acid in 1 L of DI water. These stock solutions are stable for several months. They are diluted to the required concentrations daily.
Sodium borohydride (Fisher Scientific Co., Orlando, FL) is dissolved in DI water to make a 4% solution. One milliliter of 2 M NaOH is added to 100 mL of this solution to improve the stability of the borohydride solution. It is then stable for several days. All other chemicals are reagent grade. Methods. The tellurium in the sample is preconcentrated by coprecipitating with Mg(OH)2. For seawater, 12 mL of a 2 M NaOH solution is added to 4.0 L of sample in a clear polycarbonate container under vigorous stirring with a magnetic stirrer. The solution is stirred for another 2 h and then allowed to settle overnight. For water samples containing little magnesium, 2 mL of a 1 M MgCl2 solution is added prior to the addition of NaOH. After the Mg(0H)z has settled, the clear solution is syphoned off and the remaining suspension is centrifuged. The Mg(OH)zpellet is redissolved in 10 mL of 6 M HC1, and the solution transferred to a 50-mL volumetric flask. The precipitation vessel and the centrifuge tubes are then washed successively with 15 mL of 6 M HC1 and 15 mL of DI water and the wash solutions transferred to the volumetric flask. The solution is then made up to 50 mL with DI water, resulting in a final HC1 concentration of 3 M. The determination of Te in the preconcentrated solution is based on the reduction of Te(1V) with borohydride and the trapping of the generated TeH, in the graphite furnace by thermolysis at 300 "C, followed by atomization. The following furnace program is used: step 1, 300 "C, hold time 120 s; step 2, stop flow (purge gas flow off), 2750 "C, hold time 12 s. The sample (25 mL) is pipetted into the reaction vessel and sparged with He for 4 min. The furnace cycle is then initiated and 2 mL of the borohydride solution is injected during the first 20 s by using a plastic syringe with a stainless steel needle. The tip of the syringe needle should not touch the analyte solution during the injection. At 75 s, the He flow is stopped to prevent the presence of He or hydrogen in the graphite furnace during atomization. At 115 s, the chart recorder is started and the atomization signal is recorded. Only Te(1V) reacts under these conditions. To determine Te(VI), an additional aliquot of the preconcentrated solution is boiled at 3 M HCl for 10-20 min. This reduces Te(V1) to Te(1V) and a determination of total Te is made. Te(V1) is then obtained by difference. RESULTS A N D DISCUSSION Hydride Generation. All previous authors agree that
relatively high HC1 concentrations are necessary to reduce Te(1V) to the hydride (2-4). For the chemically similar Se, Cutter (6) found a HCl concentration of 4 M to be required. Figure la shows the dependence of the peak size from 0.2 nmol of Te(1V) on the HCl concentration. The response increases sharply to 2 M HCl and then remains constant to 6 M. I am using a HCl concentration of 3 M for routine work. The response increases with the amount of borohydride solution up to about 1.5 mL, and then remains constant; for practical work, 2 mL is used. When the borohydride is injected slowly over a period of 40-60 s, the response is lower than with a rapid injection (20 s). This is in contrast to our observations with As and Sb, where a slow injection was found to be optimal (7). It is likely that at the very high acid Concentrations used here, borohydride undergoes such rapid hydrolysis that the necessary BH4- concentration cannot be reached if the borohydride is introduced too slowly. I found that both the response and precision were improved when a short syringe needle was used which did not allow contact between the solution and the tip of the needle. This is probably due to interference by the metals dissolved from the needle tip when
0003-2700/84/0356-2064$01.50/00 1984 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 56, NO. 12, OCTOBER 1984
b
a
Table I. Te(1V) Concentrations in Seawater and Rainwater (Results of Duplicate Determinations)
r
[Te(IV)],pmol L-I Seawater, Florida Straits (24O12'N,
f ,/ 5:
n
2065
82O49'W)
0.41
surface 10 m depth 20 m depth 30 m depth 70 m depth 110 m depth 250 m depth 400 m depth
L -
20002200 2400 2600 2000 M HCI
O C
Figure 1. Dependence of the absorption response from 0.2 nmol Of Te(IV)on (a)the HCI concentration in the reaction solution and (b) the graphite furnace atomization temperature. Each point represents at least three replicate determinations. the needle is immersed into the acidic solution. Reaction and Stripping Time. Experiments using the direct introduction of the TeH, generated during the reaction into a quartz burner cuvette like the one used in the determination of Sb (7)showed that all the hydride generated was sparged from the solution during the injection of the borohydride solution and within a few seconds following the injection. The sparging period is therefore limited to 75 s by switching off the He flow at that time. No improvements in response resulted from extending this period. Graphite Furnace Operation. The technique for the trapping of hydrides within the graphite furnace was first proposed by Lee (8) for bismuth. He found that collection efficiency did not change with graphite furnace temperature between 25 "C and 350 "C. I observed an increase in the efficiency for the trapping of the tellurium hydride from room temperature, where little Te was collected, up to 150 "C. The efficiency then remained constant to 500 "C. The atomization signal is dependent on the furnace temperature during atomization and on the type of graphite tube used. I found unpyrolyzed tubes to produce a response about four times higher than pyrolyzed ones. The dependence of the atomization response on furnace temperature is shown in Figure Ib. For routine work, a temperature of 2750 "C is used to extend the life of the graphite tubes (2-4 days under normal conditions). Reduction of Te(V1). Reduction with boiling HC1 has been used to reduce Se(V1) to Se(1V) (6). The reduction of Te(V1) to Te(1V) can be performed in a similar way by boiling the sarhple solution for a period of 10-20 min after adjusting to a HC1 concentration of 2-6 M. This reduction does not occur at a rapid rate at room temperature; Te(V1) solutions in 6 M HCl were only reduced to about 20% after 2 weeks. Preconcentration of Te from Natural Waters. The levels of Te in natural waters are too slow to permit direct determination without preconcentration. Coprecipitationwith Mg(OH)2 has been used to preconcentrate various elements from seawater. This approach is particularly suitable for seawater due to the high Mg concentration present (53 mmol kg-'). The efficiency of the coprecipitation was evaluated by adding 0.1 nmol ?f Te to a 2-L seawater sample. The Mg(OH)2 was then precipitated and worked up as described above. The collection efficiency was found to be between 90% and 100% for both Te(1V) and Te(V1). Interferences. Using standard addition techniqdes to seawater and rainwater matrices, we have not observed any interferences with these matrices when compared to deionized water. High concentrations of transition metals have been shown to produce significant interference for the borohydride reduction for a number of elements (9). We have not observed problems with any of the samples analyzed to date, but interferences should be expected in polluted waters. Addition
4.1 f 0.8 3.4 0.7 6.8 0.7 3.1 0.6 1.9 0.7 2.4 zk 0.6 5.6 zk 0.8 6.2 0.8
** **
*
Seawater, N. Gulf of Mexico (29'49'N, 8 4 O 31'W) 3.0 zk 0.5
Rainwater, Tallahassee, Florida 26-28 Feb 1984 6-7 Mar 1984
*
26 3 4.0 zk 0.6
of EDTA (5) or cleanup using a cation exchanger column (IO) may eliminate these interferences. Since the sample solution after preconcentration is about 0.25 M in MgC12,I investigated the possibility of an interference by Mg ion. A t Mg concentrations up to 0.5 M, no reduction in the signal was observed. When Mg(OH), is precipitated from surface seawater with a high content of organic matter, a large fraction of the dissolved organics is coprecipitated. This results in a highly colored solution which foams strongly when N&H4 is injected. The foaming can be eliminated by adding a drop of octanol or decanol to the solution. The presence of organics did not cause any interference when checked by a standard addition of Te(1V). Precision, Accuracy, and Detection Limits. The precision as determined by repeated injections of a sample solution is about 10% at 0.05 nmol of Te. It shows some dayto-day variation, probably due to changes in the operation of the graphite furnace. Due to the lack of certified standards for aqueous tellurium, the accuracy of the method cannot be assessed rigorously. Standard additions of known amounts of Te species to natural waters show quantitative recovery. The procedural blank was determined by stripping seawater of Te by the Mg(OH)2method and then adding additional MgC12 to the seawater and restripping. The blank in this sample was the same as that obtained by reacting only the HC1 with NaBH,: 3 pmol. The detection limit is based on the variability of the blank determination. The standard deviation of the blank is 15%, or 0.45 pmol. Using two times the standard deviation of the blank results in an absolute detection limit of 0.9 pmol. Since the effective sample volume is about 2 L (volume before preconcentration), this corresponds to a concentration detection limit in the sample of ca. 0.5 pmol L-l. Results from Environmental Samples. Table I contains the results of the determination of Te(1V) on several samples of seawater and rainwater. There was no evidence for the presence of Te(V1) in these samples at the level of the precision obtainable for the difference between Te(1V VI) and Te(IV), ca. 20% of the value reported for Te(1V). Registry No. Mg(OH)*, 1309-42-8; Te, 13494-80-9; water,
+
7732-18-5.
LITERATURE CITED (1) Nazarenko, I I . ; Ermakov, A. N. "Analytical Chemistry of Selenium and Tellurium"; Halstead Press: New York, 1972. (2) Fernandez, F. J. A t . Abs. News/. 1973, 72, 93. (3) Thompson, K. C. Anawst (London) 1975, 700, 307. (4) Fleming, H. D.; Ide, R. G. Anal. Chim. Acta 1976, 83, 67
2066
Anal. Chem. 1984, 56,2066-2069
(5) Hambrick, G. A.; Froelich. P. N.; Andreae, M. 0.; Lewis, B. L. Anal. Chem. 1984, 56, 421. (6) Cutter, G. A. Anal. Chim. Acta 1978, 9 8 , 59. (7) Andreae, M. 0.; AsmodB, J.-F.; Foster, P.; Van? dack, L. Anal. Chem. 1981, 53, 1776. (8) Lee, D. S. Anal. Chem. 1982, 5 4 , 1682. (9) Thompson, M.; Pahlavanpour, B. Anal. Chim. Acta 1979, 109, 251. (IO) Hashimoto, Y.; Kobayashi, R.; Chiou, K. Y.; Winchester, J. W.
"Proceedings of VIth World Congress on Air Quality"; Sepic: France, 1983; Vol. 1, p 329.
RECEIVED for review March 26,1984. Accepted May 21,1984. This work was supported in part by the Science Foundation, Grant No. OCE-8200931.
Ion Chromatographic Determination of Nitrate and Sulfate in Natural Waters Containing Humic Substances Gyorgy Marko-Varga,* Istvan Csiky, and Jan Ake Jonsson University of Lund, Department of Analytical Chemistry, P.O. Box 740, S-220 07 Lund, Sweden
Determinations of anions In natural waters by various methods, lncludlng Ion chromatography, are disturbed by the lnevltable presence of humlc substances. A cleanup column, packed with a chemically bonded amine material (Nucleosll 5 NH,) was found to effectively remove lnterferlng humic substances. This materlal Is superior to other types of chemlcally bonded materials for the Intended purpose. No Influence was found from humlc substances In concentrations up to 45 mg/L on Ion chromatographic analysls of nitrate and sulfate (10-100 mg/L) after passage through the cleanup column. The method was applled to the determination of nitrate and sulfate in sol1 lysometer waters.
In the study of the pollution of natural waters, the concentrations of nitrate and sulfate are important factors. The levels of these ions are related to such problems as acidification and eutrophication of the environment. A review of the commonly used methods for nitrate and sulfate determination in soil and natural waters can be found in a recent book (1). Nitrate is determined with a nitrate-selective electrode, spectrophotometrically, or from the total nitrogen content of the sample. Sulfate can be determined by turbidimetry, nephelometry, colorimetry, or titrimetry or from the total sulfur content. All these analytical methods are to varying extents influenced by the presence of humic substances in natural waters. In recent years, the technique of ion chromatography (IC) has developed into a convenient, sensitive and efficient means for anion analysis. The determination of small amounts of nitrate and sulfate in various matrices is already routine (2). However, the application of IC to the analysis of anions in natural waters is seriously impeded by the presence of humic substances, as those substances are strongly adsorbed on the IC column, eventually destroying it irreversibly. Humic substances (humic acid) are the major organic constituents of soil and they occur in almost all terrestrial and aquatic environments. The chemical structure is not known in detail, but it is known to involve aromatic polymers of high molecular weight, to which an unusally high number of functionalities are attached, such as carbonyl, carboxyl, hydroxyl, amine, phenolic, and quinone groups (3). Most of these groups contribute to the ion-exchange properties of the humic substances. The acidic character is generally accepted, and it is mainly due to carboxyic and phenolic groups (4). The humic substances form colloidal complexes with silica par-
Table I. Materials for Adsorption Columns material
type0
a
5 SA 5 Cia 100-5 5 SR 5 NH2
b C
d e
functional group
-so3-CldH3-
-SOH -N(CH3)3+
-"*
adsorption capacity'
0.005 0.01 0.01 1.51
13'
a All materials are of the Nucleosil series (Macherey-Nagel and Co., Duren, FRG). 'Milligrams of humic aid per gram adsorbent. 'This refers to "fraction 2" (see the text).
ticles. The ions in the water thus take part in a complicated equilibrium system, the details of which are largely unknown. Several studies of the removal of organic matter from water, as well as the separation of the organic matter into different fractions, have been presented. Techniques such as ultrafiltration (5, 6), steric exclusion chromatography (7), adsorption on Amberlite XAD-2 (8),adsorption on a bipolar ion exchanger (9),and adsorption or DEAE-cellulose (10) have been employed. Some of these methods can probably (10) also be used to remove the interference from humic substances prior to determination of ions. To selectively adsorb the interfering humic substances we suggest the use of a short cleanup column, packed with a bonded-phase material for liquid chromatography. After passage through this column, the anions in the water sample can be subjected to conventional ion chromatographic analysis. EXPERIMENTAL SECTION Chemicals. Stock solutions of nitrate and sulfate, as well as the phthalate solution used as mobile phase, were prepared from their corresponding potassium salts. The pH of the phthalate solution was adjusted with NaOH. The water used was purified with a Milli-Q/RO-4unit (Millipore, Bedford, MA). Humic acid (Fluka AG, Buchs, Switzerland) with a molecular weight of 600-1000 and an ash content of 10-1570 was used to prepare reference solutions of humic substances. The solution was stirred for 20 h at 60 OC and filtered through a 0.65-pm membrane filter (Millipore). Elemental analysis of an aliquot of the reference humic solution, evaporated to dryness, gave the following results: C, 43.5%; H, 3.0%; N, 0.95%. The content of dry matter was 45 mg/L. Thus, the content of dissolved organic carbon (DOC) was 20 mg carbon per L. Chromatographic Apparatus. The chromatographic system consisted of a pump (Constametric111, LDC, Riviera Beach, FL), a loop (100or 500 pL), an injector (Model 7000, Rheodyne, Cotati, CA), a column (see below), two detectors in series (a conducto-
0 1984 American Chemical Society 0003-2700/84/0356-2066$01.50/0
