Anal. Chem. 1998, 70, 2038-2043
Development of an ICP-IDMS Method for Dissolved Organic Carbon Determinations and Its Application to Chromatographic Fractions of Heavy Metal Complexes with Humic Substances Jochen Vogl and Klaus G. Heumann*
Institute of Inorganic Chemistry and Analytical Chemistry, Johannes Gutenberg-University, Becherweg 24, D-55099 Mainz, Germany
A sensitive and fast method for direct determinations of dissolved organic carbon (DOC) in bulk samples and in chromatographically separated fractions by inductively coupled plasma isotope dilution mass spectrometry (ICPIDMS) is developed. A 13C-enriched spike solution of benzoic acid is used for the isotope dilution step. Equal ionization efficiencies are obtained for carbon, independent of the type and molecular weight of the dissolved organic substance, which is the most important precondition for DOC determinations by ICP-IDMS. The detection limits achieved are 0.3 mg L-1 for bulk analyses and 7 × 10-4 µg s-1 for transient signals of chromatographic peaks. The results for different water samples, analyzed by ICP-IDMS and a conventional DOC method, agree well within the limits of error. By using a 13C spike solution, which also contains enriched isotopes of heavy metals, simultaneous determinations of DOC and heavy metal concentrations in separated fractions of complexes with humic substances are possible by coupling ICPIDMS with HPLC and using size exclusion chromatography. Dissolved organic carbon (DOC) is one of the most important quality parameters in water analysis today. In this connection, humic substances (HS) play an important role because the major portion of DOC in natural aquatic systems originates from HS. HS show polyfunctional structures, which enables these compounds to interact either with inorganic (e.g., heavy metals) or organic substances (e.g., pesticides).1 The formation of heavy metal complexes with HS strongly influences the mobility and bioavailability of these metals in the environment. It is, therefore, of great importance that reliable analytical methods are available for DOC determinations in bulk samples but also for quantification in chromatographic fractions of compounds which interact with HS. The conventional method for DOC determination is an IR detection after catalytic conversion of the organic substance into CO2 at high temperatures. In this case, the conversion process can be a problem, and the whole systems must be externally calibrated. The development of other DOC determination meth(1) Ziechmann, W. Huminstoffe; Verlag Chemie: Weinheim, 1980; pp 13-57.
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ods, which result in accurate data, is therefore, an interesting alternative. Today, inductively coupled plasma mass spectrometry (ICP-MS) is a wide-spread method, especially for water analyses. Isotope dilution mass spectrometry (IDMS) is considered to be one of the most accurate methods in trace element and elemental species analysis.2-4 This is based on the fact that isotope ratios instead of absolute intensities are determined, which avoids, for example, matrix effects which normally affect conventional calibration strategies. Therefore, an ICP-IDMS method was developed for DOC determinations. ICP-IDMS can also be coupled with chromatographic methods.5,6 Indications of organic substances in separated fractions are often obtained by using UV detection, which cannot be used for quantification of DOC because of the strong dependence of the UV absorbance on the type of substance. Fluorescence detection or thin-film reactors were, therefore, also used for the determination of HS in chromatographic fractions.7,8 However, other substances, such as heavy metals, cannot be detected simultaneously with these methods. If the C+ signal obtained by ICP-MS is independent of the compound, quantification of DOC is possible by using the isotope dilution technique. In addition, the multielement capability of an ICP-MS allows simultaneous determination of heavy metals in chromatographic fractions if a spike solution, containing enriched isotopes of the metal of interest and a 13C-enriched compound, is applied. EXPERIMENTAL SECTION Instrumentation. An ICP-MS, type ELAN 5000 (Perkin-Elmer SCIEX), connected with a cross-flow nebulizer and a Scott spray chamber made of quartz (AHF, Tu¨bingen, Germany), was used. The ICP-MS operating conditions are listed in Table 1. The separation of heavy metal/HS complexes was carried out with a metal-free high-performance liquid chromatographic system (Sykam, (2) Heumann, K. G. Mass Spectrom. Rev. 1992, 11, 41-67. (3) Heumann, K. G. Int. J. Mass Spectrom. Ion Processes 1992, 118/119, 575592. (4) Fassett, J. P.; Paulsen, P. J. Anal. Chem. 1989, 61, 643A-649A. (5) Rottmann, L.; Heumann, K. G. Fresenius J. Anal. Chem. 1994, 350, 221227. (6) Heumann, K. G.; Rottmann, L.; Vogl, J. J. Anal. At. Spectrom. 1994, 9, 13511355. (7) Susic, M.; Boto, K. G. J. Chromatogr. 1989, 482, 175-187. (8) Huber, S. A.; Frimmel, F. H. Anal. Chem. 1991, 63, 2122-2130. S0003-2700(97)01283-3 CCC: $15.00
© 1998 American Chemical Society Published on Web 04/02/1998
Table 1. ICP-MS and HPLC Instrumental and Operational Parameters ICP-MS (ELAN 5000) rf generator 40.68 MHz free running rf power 1300 W argon flow rates plasma 14.4 L min-1 auxiliary 1.10 L min-1 nebulizer 1.15 L min-1 sampler cone platinum (AHF) skimmer cone platinum (AHF) measurement parameters points across peak 1 dwell time 15 ms sweeps per replicate 10 HPLC mobile phase flow rate wavelength of UV absorption
Milli-Q water 0.55 mL min-1 254 nm
Gilching, Germany). This HPLC system consists of a pump (S111 PEEK), a sample injection valve (Rheodyne, model 9125), fitted with a 500 µL sample loop made of PEEK, and a UV detector (Linear UVIS 204) with an 8 µL KEL-F cell. Size exclusion chromatography (SEC) was applied for separation, where the separation column consisted of PEEK filled with HEMA SEC BIO 300 material (Alltech, Munich, Germany). HPLC control and data handling was performed by using the Axxi-Chrom 727 software (Axxiom Chromatography Inc.), except for calculation of the UV chromatograms, which was carried out by using an in-housewritten computer program. Additional operational parameters for the HPLC system are listed in Table 1. For calibration of the mass flow of the spike solution, a standard injection valve (model 5020, ERC, Alteglofsheim, Germany) with a 1500 µL sample loop made of PTFE was placed behind the UV detector. The spike solution was continuously added into the system by a peristaltic pump (Gilson Miniplus 3, Abimed, Langenfeld, Germany). Total mixture of the spike solution with the eluent, which comes from the separation column, or mixture with the standard solution for calibration of the spike mass flow, was obtained by a v-formed connecting piece, where the eluent is introduced at one side of the arrow and the spike solution at the other one. The mixture of the eluent and spike solution was directly introduced into the ICP-MS. The schematic figure of the HPLC/ICP-IDMS system used for simultaneous DOC and heavy metal determinations in chromatographic fractions is presented in Figure 1. More general details of a similar system can be obtained from the literature.5 Chemicals and Spike Solutions. Potassium hydrogen phthalate (KHP, p.a.), potassium carbonate (p.a), poly(ethylene glycol)-20 000 (PEG 20000), and β-cyclodextrin were from Merck (Darmstadt, Germany). Nitric acid of p.a. grade (Merck) was purified under subboiling conditions in a quartz distillation apparatus. A 13C-enriched benzoic acid solution, with isotope enrichment in the phenyl group (Promochem, Wesel, Germany), was dissolved in water for preparation of the 13C spike solution used for DOC bulk analyses. The spike solution used for simultaneous determination of DOC and heavy metals in chromatographic fractions was prepared by mixing the 13C-enriched benzoic acid with a multielement spike solution already applied in previous
Figure 1. Schematic diagram of the HPLC/ICP-IDMS system for simultaneous DOC and heavy metal determinations in chromatographic fractions.
investigations and enriched in 65Cu, 62Ni, 97Mo, 206Pb, and 68Zn, respectively.9 This mixed spike solution was acidified with nitric acid (total HNO3 concentration in the spike solution was 1.3 wt %) to avoid adsorption of metals. Characteristic data of the two spike solutions applied in this work are listed in Table 2. The element concentrations in the spike solutions were determined by an inverse IDMS technique, using KHP and heavy metal standard solutions of natural isotopic composition for the isotope dilution step. Dissolution of chemicals and dilution of solutions were always carried out by using Milli-Q water (18 MΩ) from a Millipore system equipped with a special filter for absorption of organic material. The same water was also used as an eluent during chromatographic separations with SEC. Samples. One sewage sample, groundwater, brown water, and seepage water sample from soil as well as different solutions prepared from isolated fulvic acids of these original samples were analyzed. After sampling, the original water samples were always filtered using 0.45 µm pore size filters of poly(vinylidene difluoride) (Millipore). The fulvic acids were isolated by the Engler-BunteInstitute of the University of Karlsruhe within the research project “Refractory Organic Acids in Waters”, applying the XAD-8 separation method described in the literature.10-12 RESULTS AND DISCUSSION Optimization of ICP-MS Sensitivity and Mass Resolution. DOC concentrations of natural waters are normally in the milligram per liter range. Therefore, a KHP solution of 10 mg L-1 was used for the optimization of instrumental parameters. This solution results in a counting rate for the 12C+ ions from 4 × 105 to 106 counts s-1, which is usually too high for detection. In the case of the ELAN 5000 ICP-MS instrument, ion counting rates can be reduced by increasing the offset voltage at the quadrupole (called OmniRange). Figure 2 represents the ion counting rate of a KHP solution dependent on the quadrupole offset voltage. (9) Rottmann, L.; Heumann, K. G. Anal. Chem. 1994, 66, 3709-3715. (10) Aiken, G. R.; Thurman, E. M.; Malcolm, R. L. Anal. Chem. 1979, 51, 17991803. (11) Malcolm, R. L.; MacCarthy, P. Environ. Int. 1992, 18, 597-607. (12) Abbt-Braun, G.; Frimmel, F. H.; Lipp, P. Z. Wasser-Abwasser-Forsch. 1991, 24, 285-292.
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Table 2. Characteristic Data of the Spike Solutions Used for DOC Analyses and for Simultaneous Heavy Metal Determinations in Chromatographic Fractions element C
isotope ratio 13C/12C
(2)
C Cu Mo Ni Pb Zn
enriched isotope
enrichment (%)
reference isotope (%)
element concn (ng/g)
(1) 13C Spike Solution for DOC Bulk Analysis (ICP-IDMS) 13C 67.50 32.50
21.65 × 103
13C
and Heavy Metal Spike Solutions for Simultaneous DOC and Heavy Metal Determinations in Chromatographic Fractions (HPLC/ICP-IDMS) 13C 79.50 20.50 63Cu/65Cu 65Cu 92.49 7.51 97Mo/98Mo 97Mo 92.21 3.85 60Ni/62Ni 62Ni 87.30 3.58 206Pb/208Pb 206Pb 90.47 3.81 66Zn/68Zn 68Zn 91.00 3.30 13C/12C
Figure 2. Reduction of the 12C+ and 13C+ ion counting rates and corresponding isotope ratios by increasing the offset voltage at the quadrupole ICP-MS instrument ELAN 5000 (KHP solution of 10 mg L-1).
From the corresponding 13C/12C isotope ratio, it can be seen that this value is not affected by an offset voltage of e4 V. Using an offset voltage of 3 V, the reduction of the 12C+ intensity is normally large enough to determine most of the natural water samples. Necessitated by variations in the ICP-MS system, the adjustment of the offset voltage should be carried out, at least, once a day. For IDMS, two different isotopes must be analyzed. Under low-resolution conditions, high intensities of 14N+ can contribute to the intensity of 13C+. The ELAN 5000 has the option of two different adjustments for mass resolution. With the higher resolution of about 23 (20% valley definition) at mass number 14, separation between the 14N+ peak of nitrogen and the 13C+ peak of carbon is sufficient, as can be seen from Figure 3, where a KHP solution of 10 mg of DOC L-1 with natural isotopic composition was analyzed. Carbon Ionization Efficiency for Different Substances. Equal ionization efficiencies of carbon from different organic substances are an essential precondition for applying ICP-IDMS in DOC determinations. To also check possible ionization effects on the molecular weight, substances with very different molecular weights were analyzed. Solutions of similar DOC concentrations of the different substances were obtained by exactly weighing the corresponding compounds, dried at 107 °C for 36 h, and by dissolving them in Milli-Q water. The results listed in Table 3 for the 12C+ and 13C+ counting rates (normalized to the DOC content) demonstrate that there is no significant dependence of the C+ sensitivity and, therefore, also of the ionization efficiency within 3 times the standard deviation (n ) 3) of 0.7% on the type of organic compound. 2040 Analytical Chemistry, Vol. 70, No. 10, May 15, 1998
13.80 × 103 25.06 6.25 11.17 26.30 28.04
Figure 3. Mass spectrum in the mass range of mass numbers 1114.5 determined with the ICP-MS ELAN 5000 at a mass resolution of 23 using a KHP solution of natural isotopic composition (DOC concentration 10 mg L-1). Table 3. ICP-MS Sensitivities of C+ Detection, Normalized to the DOC Content, of Solutions Containing Organic Substances with Different Molecular Weight but Similar DOC Content
substance KHP β-cyclodextrin PEG 20000
DOC molecular content weight (mg L-1) 198 1 135 20 000
67.13 51.34 56.70
sensitivity [counts s-1 (mg L-1)-1] 12C+ 13C+ (8.43 ( 0.02) × 103 152 ( 1 (8.51 ( 0.03) × 103 156 ( 1 (8.47 ( 0.02) × 103 156 ( 1
Removel of Inorganic Carbon and Blank Contributions. For the direct determination of DOC in water samples by ICPIDMS, it is necessary to remove all inorganic carbon from the sample. Usually, dissolved CO2 and carbonates are the dominant inorganic species in water samples, which can be removed by acidifying the sample and by purging it with an inert gas. Under these conditions, volatile organic compounds are also removed. However, in most natural water samples, volatile organic substances can be neglected compared with the nonvolatile ones. To check the efficiency of the removal of inorganic carbon by the acidification and purging process, two solutions, containing 10 and 50 mg L-1 carbonate as a potassium salt, were acidified with nitric acid to a pH of 4 and then purged with helium gas for 30 min. The acidified and purged samples showed only about 1-4% of the 12C+ intensity compared with the original solutions. This remaining carbon must be attributed to organic impurities in the carbonate chemical used, because background correction
Table 4. DOC Bulk Determination in Water by ICP-IDMS and Comparison with the Results of a Conventional Method DOC content (mg L-1) sample
ICP-IDMS
conventional method
groundwater sewage water brown water solution of fulvic acids isolated from groundwater solution of fulvic acids isolated from brown water
4.2 ( 2.1 26.7 ( 2.0 19.3 ( 0.6 47.4 ( 1.6
5(2 21 ( 2 20 ( 7 47 ( 1
55.3 ( 3.4
49 ( 1
has always taken place. The background was determined by measuring the corresponding counts of a pure water sample (MilliQ) in which the carbonate was dissolved. The background mainly results from organic impurities in the plasma argon gas but also from those in the pure water. The importance of purging the sample for the removal of CO2 was demonstrated by treating pure water samples differently. There was no distinct difference in the measured 12C+ intensity for bidistilled water samples (distilled in a quartz apparatus) and those which were additionally acidified by nitric acid. Also, the storage of these water samples in glass or PE bottles shows no significant effect. On the other hand, a reduction of the carbon concentration by a factor of about 2 was obtained for an acidified and helium-purged water sample. Additional reductions of the carbon blank by a factor of about 1.5 could be achieved by using water purified with a Millipore system equipped with a special filter for the absorption of organic material. The use of such a largely carbon-free water as an eluent in SEC separation is essential for DOC determinations in chromatographic fractions. DOC Determinations in Natural Water Samples by ICPIDMS. The total DOC content of different natural water samples, containing various concentrations of humic substances, and of two solutions of isolated fulvic acids was analyzed by ICP-IDMS. After filtration (0.45-µm filter), an exactly weighed amount of the 13C enriched spike solution was added, and the samples were then acidified and purged as described before. The results (mean with standard deviation of three independent determinations) are summarized in Table 4. The same samples were also analyzed with a conventional DOC analyzer (Carbon Analyzer 555, Ionics Inc., Watertown, MA) for comparison using IR detection of CO2. In this case, the DOC content was not directly determined but was analyzed as the difference of the total carbon and the inorganic carbon. As can be seen from the results in Table 4, good agreement between the ICP-IDMS data and those obtained by the conventional method were obtained within the limits of error. The standard deviations obtained are mainly influenced by instrumental instabilities and uncertainties during sample pretreatment, respectively. In principle, the ICP-IDMS method has the advantage that it is not affected by matrix effects and is independent of the dissolved organic substance, which cannot be assumed in all cases for the conventional method. In addition, the isotope dilution technique is an internal calibration method, whereas the conventional DOC techniques must be calibrated by external standard solutions. Recently, Oweczkin et al. published two papers, identical in
Figure 4. 13C/12C isotope ratio chromatogram of an isotope diluted sample (solution of fulvic acids isolated from brown water) determined with HPLC-SEC/ICP-MS.
content, where the DOC concentration was determined by inductively coupled plasma atomic emission spectrometry (ICPAES).13,14 The calculated detection limits (2σ) were 0.03 and 1.09 mg L-1, respectively, for two different carbon emission lines. However, this ICP-AES method was only applied for DOC determinations in real samples at levels above 100 mg L-1. Further experiments must, therefore, show the reliability of this AES spectroscopic method for the low DOC concentration range where the normal level of natural water systems lies. The detection limit of ICP-IDMS was found to be 0.3 mg L-1 (3σ), which is in the same range as for sensitive conventional DOC analyzers. The detection limit for ICP-IDMS is mainly limited by the precision of the 13C/12C spike isotope ratio measurement with the applied quadrupole ICP-MS. For this measurement, the spike solution was diluted in Milli-Q water to a DOC concentration normally present in real samples, and then it was acidified and purged as described before. However, by applying more precise ICP-MS instruments, further improvement of the detection limit should be possible. Determination of DOC in Chromatographic Fractions. For DOC determinations in chromatographic fractions, the HPLC/ ICP-IDMS system, schematically shown in Figure 1, with a size exclusion column is used. All analyzed samples are poor in carbonate. However, for carbonate-rich samples, a suppressor column must be connected in series with the separation column to avoid a breakthrough of carbonate. After optimization of the ICP-MS sensitivity and after calibration of the spike mass flow, the sample is injected, and the time-resolved signals of the UV absorption and of the 13C/12C isotope ratio are followed. A representative isotope ratio chromatogram, obtained with a solution of fulvic acids isolated from a brown water sample, is shown in Figure 4. In the beginning, when no DOC-containing fraction is eluted from the column, the isotope ratio of the spike solution is measured. The isotope ratio shifts in the direction of the natural isotopic composition of carbon when DOC-containing fractions leave the separation column because the spike flow is totally mixed with these fractions before entering the ICP-MS. By using the data of the calibrated spike mass flow and an in-house-written computer program, which is, in principle, de(13) Oweczkin, I. J.; Kerven, G. L.; Ostatek-Boczynski, Z. Commun. Soil Sci. Plant. Anal. 1995, 26, 2739-2747. (14) Oweczkin, I. J.; Kerven, G. L.; Ostatek-Boczynski, Z. Commun. Soil Sci. Plant Anal. 1996, 27, 47-55.
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Figure 5. HPLC-SEC/ICP-IDMS chromatogram of DOC and UV absorption curve from a solution of fulvic acids isolated from a brown water sample.
scribed in more detail elsewhere,5 the isotope ratio chromatogram can be converted into a mass flow chromatogram of DOC. This is represented in Figure 5 for the same solution of fulvic acids as was used for the experiment shown in Figure 4 (this sample is not identical with the fulvic acid sample from brown water in Table 4). In addition, the UV absorption curve is also represented in Figure 5. Because the mass flow of DOC is directly correlated with the concentration in the original sample and in the eluted fraction, HPLC/ICP-MS in connection with the isotope dilution technique is the only analytical method, up to now, resulting in real-time concentrations in chromatographic fractions. The given amounts for the two separated DOC fractions of Figure 5 (15.2 and 5.5 µg) are the mean of three independent analyses of the same sample. In this case, the DOC amount of the first fraction was determined, with a relative standard deviation of 0.3% (1σ), and the amount of the second fraction was determined, with 8% standard deviation, which covers the total range of precision for all the determinations carried out with the HPLC-SEC/ICPIDMS system. The detection limit, calculated as 3 times the standard deviation of the spike isotope ratio under HPLC/ICPIDMS conditions, is 7 × 10-4 µg of DOC s-1. Another great advantage of DOC determinations in chromatographic fractions by ICP-IDMS compared with the usually used UV absorption can be seen from Figure 5. All fractions containing any dissolved organic compound could be quantified, whereas UV qualitatively indicates only those organic substances which are active for UV absorption at the wavelength used (for absorption at a wavelength of 254 nm, conjugated π-electron systems are necessary). If quantification of DOC in chromatographic fractions is not necessary, DOC concentration ratios of different fractions can easily be determined simply by running the 12C+ or 13C+ intensity chromatogram (without any isotope dilution step), as was recently shown by Vogl and Heumann.15 In addition, the approximate range of molecular weights of the different fractions can be determined by using standard substances, even if exact molecular weight calibrations for substances separated by size exclusion chromatography are a general problem. Polysaccharide standards are acceptable calibration compounds for humic substances.16 From such a standard solution, containing pullulans with molecular weights from 12 000 to 853 000 (Shodex P-82, Macherey & Nagel, Germany), the two (15) Vogl, J.; Heumann, K. G. Fresenius J. Anal. Chem. 1997, 359, 438-441.
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Figure 6. HPLC-SEC/ICP-IDMS chromatograms of DOC and molybdenum from a solution of fulvic acids isolated from a brown water sample (notice that the mass flow scales for DOC and molybdenum differ by 3 orders of magnitude).
separated fractions in Figure 5 correspond to >100 000 at 10-min retention time and to 20 000-30 000 at about 13-min retention time. Simultaneous Determination of DOC and Heavy Metals in Chromatographic Fractions of Humic Substances. Using the second spike solution listed in Table 2, containing also enriched heavy metal isotopes besides 13C, simultaneous quantification of DOC and heavy metals in separated fractions of humic substances is possible. This enables more detailed information to be obtained on heavy metal/HS complexes, which are not available at the moment by any other analytical method. For example, the chromatograms of DOC and molybdenum are represented in Figure 6 for a solution of fulvic acids isolated from brown water (sample identical with that listed in Table 4). Molybdenum was not added to the sample but is the natural contribution from the original brown water after isolation of fulvic acids. Two separated DOC fractions are obtained, where the second fraction could not be identified by UV absorption at 254 nm. The first fraction of humic substances (10-min retention time; molecular weight > 100 000), which containssin contrast to the second fractionssignificant amounts of π-electron groups, forms complexes with about 70% of the total molybdenum. The remaining 30% of the molybdenum interacts with the second fraction of these fulvic acids, whereas no free molybdate ions appear at higher retention times. However, it is interesting to note that only molybdenum of all detected heavy metals (Cu, Mo, Ni, Pb, and Zn) also interacts with the second low-molecular-weight fraction of this fulvic acid sample (nonaromatic; molecular weight range about 20 00030 000). In addition, exact comparison of the DOC and molybdenum chromatograms indicates that it is not the total amount of HS in the second DOC fraction that interacts with molybdenum but, preferably, organic compounds at the lower molecular side of this fraction (right shoulder of second DOC peak; 12.3-min retention time). Copper, nickel, and zinc are exclusively found in the first fraction (not shown in Figure 6), and lead is below the detection limit. From the total DOC concentration of this fulvic acid solution (see Table 4) and the injection volume of 500 µL, it can be concluded that 80% of the total organic substance is (16) Swift, R. S. In Humic Substances in Soil, Sediment and Water; Aiken, G. R., McKnight, D. M., Wershaw, R. L., MacCarthy, P., Eds.; Wiley: New York, 1985; pp 387-408.
this result could only be received by addition of zinc to the sample. On the other hand, HPLC/ICP-MS can usually detect HS/heavy metal complexes much better at the natural concentration level due to its high sensitivity.5,6,9,15 In addition, HPLC/ICP-AES must be externally calibrated, whereas the HPLC/ICP-IDMS method allows the simultaneous determination of real-time concentrations of heavy metals and DOC.
Figure 7. HPLC-SEC/ICP-IDMS chromatograms of DOC, copper and zinc from a sample of seepage water from soil (for better representation, the mass flow scale of Zn is shifted upward the y-axis; mass flow scale for the metals is 3 orders of magnitude lower than for DOC).
distributed among the two fractions shown in Figure 6. This portion of HS, separated by the SEC column, agrees with those reported in the literature, where recoveries of 73-96% are reported for humic substances separated by size exclusion chromatography.17 Figure 7 represents the DOC, copper, and zinc chromatograms of a seepage water sample from soil, which shows that only the high-molecular-weight HS fraction of this sample forms complexes with these two heavy metals. The detected heavy metals again represent the natural concentration in this sample. Molybdenum and nickel could not be detected in this sample, and lead was near the detection limit but was identified only in the first fraction. That zinc preferably forms complexes with high-molecularweight humic acids was also found by Itoh and Haraguchi using an HPLC-SEC/ICP-AES system and a commercially available HS reagent.18 Because of the distinctly higher detection limit of ICP-AES for most metal determinations compared with ICP-MS, (17) Knuutinen, J.; Virkki, L.; Mannila, P.; Mikkelson, P.; Paasivirta, J. J. Water Res. 1988, 22, 985-990.
CONCLUSION A reliable and sensitive method for DOC determinations by ICP-IDMS was developed which is of special interest as an alternative technique to conventional DOC methods because ICPIDMS is matrix independent and also independent of the type of the dissolved organic compounds. Another aspect as an alternative method is the increasing broad distribution of ICP-MS instruments all over the world, especially in water-analyzing laboratories. However, an exceptionally great advantage of this DOC determination method is its application to chromatographically separated fractions by on-line coupling of HPLC with ICPIDMS. Such a coupling system also allows simultaneous determinations of DOC and other elements, which enables the quantitative characterization of metal complexes with humic substances or, in general, with other important organic compounds such as anthropogenic complexing agents in the environment or proteins in medical samples. ACKNOWLEDGMENT We thank the Deutsche Forschungsgemeinschaft (DFG) for financial support, which was granted within the research project “Refractory Organic Acids in Waters (ROSIG)”. We also thank all members of this DFG project for excellent cooperation, especially Prof. F. H. Frimmel and Dr. G. Abbt-Braun, University of Karlsruhe, for providing the samples investigated in this work. Received for review November 24, 1997. February 19, 1998.
Accepted
AC971283P (18) Itoh, A.; Haraguchi, H. Chem. Lett. 1994, 1627-1630.
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