Anal. Chem. 1994,66, 3709-3715
Determination of Heavy Metal Interactions with Dissolved Organic Materials in Natural Aquatic Systems by Coupling a High-Performance Liquid Chromatography System with an Inductively Coupled Plasma Mass Spectrometer Lothar Rottmann and Klaus G. Heumann' Institut fur Anorganische Chemie der Universitat Regensburg, Universitatsstrasse 3 1, 0-93040 Regensburg, Germany
A high-performanceliquid chromatographic (HPLC) system with a size exclusion column is coupled with an inductively coupled plasma mass spectrometer (ICP-MS) for the determination of interactions between heavy metals and different fractions of dissolved organic matter in natural aquatic systems. Direct coupling is applied to get specific distribution patterns of the heavy metal complexes, and on-line isotope dilutionmass spectrometryis performed to quantify heavy metals accurately in different organic fractions. With respect to the separation properties of a size exclusion column by molecular size, different distribution patterns could be found for the heavy metals depending on the type of aquatic system. Differentdistribution patterns in the various fractions of dissolved organic material (preferably of humic substances) could also be observed for the metals in the same natural water sample. In addition, a high-resolution ICP-MS is applied for the first time as an element-specific detector in connection with a HPLC system which also allow interference-free detection of iron species.
Dissolved organic matter (DOM) is very important in natural waters because it engages natural metabolisms in aquatic ecosystems by acting as a substrate in heterotrophic processes or as enzyme, vitamin, or toxin in organisms.' In natural waters, humic substances (HS) represent the major part of the dissolved organic material. Depending on the type ofwater, theconcentration of humic substancesvaries between 0.1 and 115 mg L-lS2 The elemental composition of HS depends on the origin, whereby the major constituents are carbon (40-60%), oxygen (30-40%), and hydrogen (4-6%~).~ H S have no homogeneous chemical structure and their molecular weights range from a few hundred to some 10 OO0.4 Considering the complex and polydisperse nature of HS, a full structural elucidation had not been achieved until now, although many structural analytical techniques (NMR,5,6 IR,798pyrolysis GC/MS9) have been applied to this problem. It is generally assumed that H S contain aromatic backbones (1) Steinberg, C.; Stabel, H.-H. Vom Wasser 1978, 51, 11-32. (2) Boggs, S.;Livermore, D. G.;Seitz, M. G. J. Macromol. Sci. 1985, C25.599-
657.
(3) Abbt-Braun, G.;Schmiedel, U.; Frimmel, F. H. Vom Wasser 1990,75,59-73. (4) Rashid, M. A.; King, L. H. Geochim. Cosmmhim. Acta 1969, 33, 147-151. (5) Wilson, M. A.; Collin, P. J.; Malcolm, R. L.;Perdue, E. M.; Cresswell, P. Org. Geochem. 1988, 12, 7-12 (6) Wershaw, R. L. In Humic Subslances in Soil, Sediment, and Water; Aiken, G . R., McKnight, D. M., Wershaw, R. L., MacCarthy, P., Eds.; Wiley: New York, 1985; Chapter 22. (7) Abbt-Braun, G.; Frimmel, F. H. Vom Wasser 1990, 74, 307-324. 0003-2700/94/0366-3709$04.5010
0 1994 American Chemical Society
which, depending on their origin, carry a diversity of functional groups as, for example, carboxylic or phenolic groups. Due to the fact that H S have hydrophobic as well as polyfunctional groups, they can interact with many organic (e.g., PCB, PAH, and pesticides) and inorganic (e.g., heavy metals) pollutants in aquatic systems. The physicochemical properties of the heavy metals are altered by interaction with DOM and are often dominated by the properties of the organic substance. For that reason a more detailed knowledgeof these interactions is an essential precondition for the toxicity, accumulation, and bioavailability of heavy metals in the analyzed system. Complexation of metal ions also affects geochemical transport mechanism of metals in the environment or the efficiency of removal of heavy metals in drinking water preparation steps. Due to the importance of those elemental species in natural waters, sensitiveand reliable analytical methods are necessary to obtain accurate analytical results for the various species. During the past few years, different analytical methods have been applied to investigate heavy metal interactions with organic substances in aquatic systems, including anodic stripping voltammetry (ASV),l0J streaming current detection (SCD),I2fluorescence techniques,'3J4 ion selective electrodes (ISE),'S electron paramagnetic resonance spectroscopy (EPR),16and solid state spectr~photometry.'~ Furthermore, different separation techniques (ion chromatography, reversedphase chromatography, size exclusion chromatography, diafiltration) in combination with various detection methods (e.g., atomic absorption spectrometry (AAS)1G23or radioanalytical (8) MacCarthy, P.; Rice, J. A. In HumicSubstances inSoil,Sediment, and Water; Aiken, G. R., McKnight, D. M., Wershaw. R. L., MacCarthy,P., Eds.; Wilcy: New York, 1985; Chapter 21. (9) Abbt-Braun, G.;Frimmel, F. H.; Schulten, H.-R. Warer Res. 1989,23,1579-
1591.
(10) Powell, H. K. J.; Town, R. M. AMI. Chim. Acta 1991, 248, 95-102. (1 1) van den Hoop, M. A. G. T.; van Lceuwen, H. P. AM^. Chim. Acta 1993,273, 275-287. (12) Weis, M.; Frimmel, F. H. 2.Naturforsch. 1990, 456, 887-891. (13) Susetyo, W.; Carreira, L. A.; Azarraga, L. V.; Grimm, M. D. Fresenius J . Anal. Chem. 1991, 339, 624-635. (14) Dobbs, J. C.; Susetyo, W.; Knight, F. E.; Castles, M. A.; Carreira, L. A.; Azarraga, L. V. Int. J . Environ. Anal. Chem. 1989, 37, 1-17. (15) Buffle, J.; Greter, F.-L.; Haerdi, W. AMI. Chem. 1977, 49, 216-222. (16) Chesire, M. V.; Berrow, M. L.; Goodman, B. A,; Mundie, C. M. Geochim. Cosmochim. Acta 1977, 41, 1131-1138. (17) Ohzeki, K.; Tatehana, M.; Nukatsuka, I.; Ishida, R. A M l p t 1991,116, 199205. (18) Douglas, G. S.; Quinn, J . G. In Aquafic Humic Substances; Suffet, I. H., MacCarthy, P.; Eds.; Advances in Chemistry Series 219; American Chemical Society: Washington, DC, 1989; Chapter 20. (19) Matsunaga, K.; Negushi, M.; Fukase, S.Gemhim. Cosmochim. Acra 1980, 44, 1615-1619.
Analytical Chemistty, Vol. 66, No. 21, November
I, 1994 3709
technique^^^) have been applied. In the majority of these investigations commercial HS have been used, which are significantly different from DOM in natural system^.^^-^^ In other cases, sample enrichment steps or other preparation steps,under conditionsnot comparable with the natural system, were applied prior to speciation. Here, changes of the chemical form during the sample treatment are very probable, so that conclusionson the original situation of the species in the sample are difficult to draw from these results. By the use of an inductivelycoupled plasma atomic emission spectrometer (ICP-AES) as an elemental detector for a highperformance liquid chromatographic system (HPLC) with a gel filtration column,the interaction of magnesium and calcium with organic matter in natural waters was investigated without further sample preparation steps (except filtration through a 0.45 pm filter).27 The sensitivity of ICP-AES is sufficient for the determination of these alkaline earth elements but unsatisfactory for many of the heavy metal species in natural water systems. Size exclusion chromatography (SEC) is an excellent method to get information about the molecular size distribution of DOM and HS.28-34 Therefore, in this study, a HPLC system, equipped with a size exclusion column, is coupled with an inductively coupled plasma mass spectrometer (ICP-MS) to get specificmolecular size distribution patterns of DOM and their corresponding heavy metal interactions in different types of natural waters. This analytical system implies three important features: (1) HPLC as a rapid and powerful separation technique, which carries out separation of DOM in the water sample without any further preparation step; (2) ICP-MS as a sensitive detector with multielement capabilities which determine most of the elements within one run without any enrichment step prior to the analysis; (3) ICP-MS for determination of isotope ratios, which makes it an ideal tool for the definitive method of isotope dilution mass spectrometry (IDMS). For the determination of the size distribution patterns, UV absorbance is used for detection of the organic substances and the ICP-MS technique is applied for the detection of the heavy metals. On the one hand, direct coupling and time-resolved ICP-MS detection of isotope intensities of the elements of interest without quantification is applied for determining the interaction of heavy metals with different DOM fractions, and on the other hand, an on-line isotope dilution technique with a species-unspecificspike is used for reliable quantification ~
~
~~
(20) Taga, M.; Tanaka, S.; Fukushima, M. Anal. Sci. 1990,6,611-615. (21) Burba, P.; Willmer, P. G. Fresenius J . Anal. Cfiem. 1992, 342, 167-171. (22) Truitt, R. E.; Weber, J. H. Anal. Cfiem. 1981, 53, 337-342. (23) Hiraide, M.; Arima, Y.; Mizuike, A. Anal. Cfiim Acta 1987, 200, 171-179. (24) Warwick, P.; Hall, T. Analyst 1992, 117, 151-156. (25) MacCarthy, P. In Aquutic Humic Substances; Suffet, I. H.,MacCarthy, P., Eds.; Advances in Chemistry Series 219; American Chemical Society, Washington, DC, 1989; Chapter 4. (26) Kile, D. E.; Chion, C. T. In Aquatic Humic Substances; Suffet, I. H., MacCarthy, P.. Fds.; Advancesin Chemistry Series 219; American Chemical Society: Washington, DC, 1989; Chapter 10. (27) Gardner, W. S.; Landrum, P. F.; Yates, D. A. Anal. Cfiem.1982,54, 1 1 9 6 1198.
(28) Vartiainen, T.; Liimatainen, A.; Kauranen, P. Sci. Total Enuiron. 1987, 62, 75-84.
(29) Saito, Y.; Hayano, S.J. Cfiromatogr. 1979, 177, 390-392. (30) Miles, C. J.; Brezonik, P. L. J. Cfiromcltop. 1983, 259, 499-503. (31) Paolini, J.; Chitty, W. Sci. Total Enuiron. 1990, 91, 107-114. (32) Bcchcr, G.; Carlberg, G. E.; Gjessing, E. T.; Hongsio, J. K.; Monarca, S. Enuiron. Sci. Tecfinol. 1985, 19, 422-426. (33) Fuchs, F.; Raue, B. Vom Wasser 1981, 57, 95-106. (34) Huber, S.; Gremm, T.; Frimmel, F. H. Vom. Wasser 1990, 75, 331-342.
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AnalytIcalChemistry, Vol. 66, No. 21, November 1, 1994
of the different elemental species. A detailed description of the method of on-line isotope dilution with species-unspecific spikes is given by Rottmann and H e ~ m a n n .Recently, ~~ a new double-focusing high-resolution ICP-MS (HR-ICP-MS) was constructed by Finnigan MAT GmbH, Bremen, Germany. A detailed description of this instrument is given by Giessmann and Greb.36 With such a HR-ICP-MS instrument, a HPLC system is coupled for the first time to investigate interactions of elements, like iron, with DOM as well, which normally cannot be determined by a quadrupole ICP-MS because of spectral interferences.
EXPERIMENTAL SECTION Instrumentation. The separation of DOM is performed by commercial high-performance liquid chromatographic equipment (Sykam GmbH, Gilching, Germany). The system consists of a HPLC pump (SlOOOPEEK), a sample injection valve (Rheodyne, Model 9125), fitted with a 500 pL sample loop made of PEEK, and an UV monitor Linear UVIS 204 with an 8 pL KEL-F cell. The employed separation system consists of a glass guard column filled with Top Off Gel 3PW and a size exclusion column (TSK 3000 PW glass, TosoHaas GmbH, Stuttgart, Germany). The HPLC system control and the data handling are performed by a computer using the Axxi-Chrom 727 Chromatography Data Station Software (Axxiom Chromatography Inc.). When the on-lineisotopedilution technique is being applied, a flow injection (FI) valve (ERC 5020, ERC GmbH, Alteglofsheim, Germany) fitted with a 1.5 mL sample loop is installed behind the UV monitor for calibration of the spike flow.35 The outlet of the FI valve is connected to an inverse Y-junction, where the species-unspecificspike is continuously added by a peristaltic pump (Gilson Minipuls 3, Abimed, Langenfeld, Germany) to the eluent. The tubes of the peristaltic pump are made of Isoversinic (Abimed, Langenfeld, Germany). All constituents that come into contact with the eluent are made of plastic materials such as PEEK, KEL-F, or PTFE, so that metal contaminations from the separation system can be avoided. The eluent is delivered directly to the cross-flow nebulizer (Perkin-Elmer), where the aerosol generation takes place. A low-resolution quadrupole ICP-MS (Elan 5000, Perkin Elmer) is used for the determination of the time-resolved ion intensities and isotope ratios, respectively, of all heavy metals except iron. For the determination of iron, a new double-focusing HR-ICP-MS (Element, Finnigan MAT GmbH, Bremen, Germany) is used. In this case, the magnet scanning mode is chosen for data collection and the resolution of the instrument is set to 3000. A schematic diagram of the HPLC/ICP-MS system used for the investigations of this work is shown in Figure 1. The instrumental operating parameters for the quadrupole ICP-MS and the HPLC system are summarized in Table 1. Samples. Different water samples are collected from various aquatic systems (bog, lake, river) in Bavaria, filtered directly after sampling through a 0.45 pm PTFE filter (Minisart SRP 25, Sartorius AG, Gottingen, Germany), and stored in precleaned PE bottles under exclusion of light. The (35) Rottmann, L.; Heumann, K. G. Fresenius J. Anal. Chem., in press. (36) Giessmann, U.; Greb, U. Fresenius J. Anal. Cfiem., in press.
Sample injection valve
IiPLC pump
Inverse Y-junction for mixing up eluent and spike
Pump for
Standard injection for calihration of spike flow
S i x exclusion column
.'.--..'.---i ;
ICP-MS
e-
-.--. .--. .
Components only necessaryfor icotopc dilution technique
Figure 1. Schematic diagram of the HPLC/ICP-MS system including the on-line isotope dilution technique with a species-unspecificspike. Table 1. HPLC and ICP-MS 1nst"taI Parameters
rf generator rf power
and Operatlonal
ICP-MS (ELAN 5O00) 40.68 MHz free running
nebulizer spray chamber argon flow rates plasma auxiliary nebulizer sampler cone skimmer cone measurement parameters points across peak dwell time sweeps per replicate mobile phase flow rate wavelength of UV absorption mncn of the spike solution flow rate of the spike solution
1200 w
cross-flow nebulizer ( PE-Sciex) Scott-Type (PE-Sciex) 15.0 L m i d 0.80 L min-I 0.9-1.1 L m i d
platinum platinum direct coupling
on-line IDMS
1 200 ms 1
30 ms 10
1
HPLC purified water 0.55 mL m i d 254 nm On-Line IDA -5 pg L-' per element -0.5 mL min-'
sampleorigin, DOC content, and total concentration of heavy metals in the water samples are summarized in Table 2. Chemicals and Spike Solution. Prior to the coupling of the HPLC system with the ICP-MS, the sensitivity of the ICPMS is optimized using an ICP multielement standard solution. This solution contains 10 ng g1of 23 different elements and is prepared by dilution of a commercially available 1000 pg g-' solution (ICP Multielement Standard IV, Merck, Darmstadt, Germany). The multielement standard for calibration of the spike flow is prepared by addition of an ICP standard solution of molybdenum (Alfa Products, Johnson Matthey GmbH, Karlsruhe, Germany) to thediluted ICP Multielement Standard IV. The species-unspecificspike solution,containing the isotopicallyenriched elements of interest in concentrations of - 5 pg L-l, is prepared by dilution of corresponding stock solutions, which are received by dissolution of the commercially available isotopically enriched metals or metal oxides (Medgenix Group GmbH, Ratingen, Germany; Euriso-Top Groupe CEA, Saint-Aubin, France; Cambridge Isotope Laboratories, Cambridge, MA; Promochem, Wesel, Germany) with suitable acidic or alkaline solutions. The isotope abundances of the investigated heavy metals measured by
ICP-MS in standard solutionsof natural isotopic composition and used for all samples as well as those of the spike solution are summarized in Table 3. By using these measured isotope abundancesinstead of corrected values for IDMS calculations, a possible mass bias by the ICP-MS instrument is widely eliminated and therefore complicated assumptions for corrections are not necessary. Dilution of all standard and spike solutions is carried out with purified water, which is made by a 2-fold distillation in quartz apparatus. This water is also used as mobile phase for the chromatographic separation instead of buffer solutions,as potential changes of the original chemical form of the species of interest during separation are minimized, and ion exchangeprocesses between heavy metals interacting with DOM and the cations of the buffer solution can be avoided. Procedure for Heavy Metal Species Determination. After optimization of the ICP-MS sensitivity, the HPLC system is coupled with the ICP-MS. The sample loop is filled by a suction loading technique, which avoids contact with a metal syringe needle and, therefore, possible heavy metal contamination. Simultaneously with the injection of the sample onto the column, the detection of the UV absorption is initiated and the ICP-MS intensities of the isotopes of interest are measured. When direct coupling without isotope dilution is employed, the different isotopes of interest are detected "simultaneously" using the GRAPHICS software of the ELAN 5000 during separation. At the end of the analysis, the time-resolved ICP-MS data are transferred by a selfwritten program into a data interchange format (DIF) and combined with the UV data. When theon-line isotopedilution technique is applied the spike solution is continuously pumped into the eluent flow after the eluent passes the UV monitor. A complete mixing of the eluent and spike solution is carried out by an inverse Y-junction. Before starting the analysis, the spike flow has to be calibrated. This calibration is carried out by injection of a multielement standard solution via the FI valve, which is positioned between the UV monitor and the inverse Y-junction (see Figure 1). After calibration of the spike flow, monitoring of the time-resolved isotope ratios in the GRAPHICS application of the ELAN software is started simultaneouslywith the UV detection and the injection of the sample. The calculation of the mass flow of the different elements by the determined isotope ratios and the spike flow is performed by a self-made computer program as described elsewhere.35 RESULTS AND DISCUSSION Heavy Metal Pattem. In SEC the molecular sizes of the separated molecules normally can be related to the retention time and appropriate calibration curves. However, in case of polydisperse samples, like DOM in natural aquatic systems, SEC can only be used to determine molecular weight distribution profiles.37 For DOM, the macromolecular structure is not exactly known, therefore causing a lack of authentic substances to calibrate the separation system. In SEC, elution of polyelectrolytes is also influenced by pH and ionic strength. Changes in these parameters lead to wrong (37) Barth, H.G. J. Chromarogr. Sci. 1980, 18,409-429.
Analytical Chemistry, Vol. 66, No. 21, November 1, 1994
3711
Table 2. Orlgln, DOC Content, and Concentratlon of Heavy Metak of the Investigated Sampler
sample origin
abbrev
'%Zf
Danube (Regensburg) Naab (Pielenhofen) Regen (Zeitlarn) Schwarze Laaber (Laaber) Latschensee (Bayer. Wald) Hochmoor-See (Steinwald) Schwetzendorfer Weiher (Pettendorf)
DO NA RE SL LS MS
river river river river bog bog lake
a
sw
DOC (mg L-I) 5.2 9.o 7.5 6.0 7.7
Ni
1.40 f 0.22 1.75 f 0.15 2.33 0.10 0.82 f 0.17 1.14 0.21 1.21 f 0.28 4.78 f 0.06
2.60 f 0.10 2.71 f 0.14 1.44 f 0.03 2.75 f 0.31 0.42 f 0.02 0.82 i 0.05 5.64 f 0.15
*
18.1 7.5
mncna (rg L-1) Mo
cu
Pb
1.54 f 0.03 2.1 1 f 0.14 0.12 0.01 0.06 f 0.01
-
