Seawater atomic spectroscopy

Determination of trace metals in seawater represents one of the most challenging tasks in chemical analysis because the parts per billion (ppb) or sub-ppb levels of analyte are very susceptible to matrix interference from alkali or alkaline-earth metals and their associated counterions. For instance, the alkali metals tend to affect the atomisation and the ionisation equilibrium process in atomic spectroscopy, and the associated counterions such as the chloride ions might be preferentially adsorbed onto the electrode surface to give some undesirable electrochemical side reactions in voltammetric analysis. Thus, most current methods for seawater analysis employ some kind of analyte preconcentration along with matrix rejection techniques. These preconcentration techniques include coprecipitation, solvent extraction, column adsorption, electrodeposition, and Donnan dialysis. 

Grobenski Z, Lehmann R, Radzuck B, Voellkopf U (1984) The determination of trace metals in seawater using Zeeman graphite furnace AAS. In Atomic Spectroscopy Application Study No. 686 (1984) Papers presented at Pittsburgh Conference, Atlantic City, NJ, USA... 

Recently, flameless atomization techniques have been developed for atomic spectroscopy, particularly for atomic absorption. Absolute sensitivities of atomic absorption using these flameless atomizers are, for most elements, comparable with or better than those attainable by any other technique. Additionally, unlike most other techniques the atomic absorption method is relatively free from interferences by other components of the sample matrix. Therefore, flameless atomic absorption holds great promise for direct analysis of trace metals in seawater and other environmental samples. This paper reports the successful apphcation of a new design of commercial atomizer to direct analysis of several metals in seawater. 

FIGURE 17.7 Analyte and blank spectral scans of (a) Co, (b) Cu, (c) Cd, and (d) Pb in NASS-4 open-ocean seawater certified reference material, using flow injection conpled to ICP-MS. (From S. N. Willie, Y. lida and J. W. McLaren, Atomic Spectroscopy, 19[3], 67, 1998.)... 

Many of the published methods for the determination of metals in seawater are concerned with the determination of a single element. Single-element methods are discussed firstly in Sects. 5.2-5.73. However, much of the published work is concerned not only with the determination of a single element but with the determination of groups of elements (Sect. 5.74). This is particularly so in the case of techniques such as graphite furnace atomic absorption spectrometry, Zeeman background-corrected atomic absorption spectrometry, and inductively coupled plasma spectrometry. This also applies to other techniques, such as voltammetry, polarography, neutron activation analysis, X-ray fluroescence spectroscopy, and isotope dilution techniques. 

Methods for determining metals in seawater have been published by the Standing Committee of Analysts (i.e., the blue book series, HMSO, London) they are not reproduced in this book, as they are available elsewhere. These methods are based on chelation of the metals with an organic reagent, followed by atomic absorption spectroscopy. 

Mullins [ 189] has described a procedure for determining the concentrations of dissolved chromium species in seawater. Chromium (III) and chromium (VI) separated by coprecipitation with hydrated iron (III) oxide and total dissolved chromium are determined separately by conversion to chromium (VI), extraction with ammonium pyrrolidine diethyl dithiocarbamate into methyl isobutyl ketone, and determination by atomic absorption spectroscopy. The detection limit is 40 ng/1 Cr. The dissolved chromium not amenable to separation and direct extraction is calculated by difference. In the waters investigated, total concentrations were relatively high, (1-5 pg/1), with chromium (VI) the predominant species in all areas sampled with one exception, where organically bound chromium was the major species. 

Table 5.4. Analysis of poor quality seawater by potentiometric stripping analysis and atomic absorption spectroscopy and good quality seawater by potentiometric stripping analysis... 

Wrembel and Pajak [486] evaporated mercury from natural water samples with argon and amalgamated the mercury with a gold foil. The mercury was excited in a ring-discharge plasma and determined by atomic emission spectroscopy. The method was applied to the determination of mercury in seawater in the range 0.01-1.0 xg/l. 

Monien et al. [515] have compared results obtained in the determination of molybdenum in seawater by three methods based on inverse voltammetry, atomic absorption spectrometry, and X-ray fluorescence spectroscopy. Only the inverse voltammetric method can be applied without prior concentration of molybdenum in the sample, and a sample volume of only 10 ml is adequate. Results of determinations by all three methods on water samples from the Baltic Sea are reported, indicating their relative advantages with respect to reliability. 

Petit [563] has described a method for the determination of tellurium in seawater at picomolar concentrations. Tellurium (VI) was reduced to tellurium (IV) by boiling in 3 M hydrochloric acid. After preconcentration by coprecipitation with magnesium hydroxide, tellurium was reduced to the hydride by sodium borohydrate at 300 °C for 120 seconds, then 257 °C for 12 seconds. The hydride was then measured by atomic absorption spectroscopy. Recovery was 90 - 95% and the detection limit was 0.5 pmol/1. 

The major anions and cations in seawater have a significant influence on most analytical protocols used to determine trace metals at low concentrations, so production of reference materials in seawater is absolutely essential. The major ions interfere strongly with metal analysis using graphite furnace atomic absorption spectroscopy (GFAAS) and inductively coupled plasma mass spectroscopy (ICP-MS) and must be eliminated. Consequently, preconcentration techniques used to lower detection limits must also exclude these elements. Techniques based on solvent extraction of hydrophobic chelates and column preconcentration using Chelex 100 achieve these objectives and have been widely used with GFAAS. 

Spectroscopic analysis can also benefit from a preceding electrochemical preconcentration. In particular, such coupling has been widely used for minimizing matrix interferences in atomic absorption spectroscopy (AAS). For example, lead, nickel, and cobalt have been determined in seawater with no interferences from the high sodium chloride content [80]. By adjusting the deposition potential and the pH, it is possible to obtain information on the oxidation and com-plexation states of the metal ions present [81]. 

Batley, G.E. and Matousek, J.R (1977) Determination of heavy metals in seawater by atomic absorption spectroscopy after electrodeposition on pyrolytic graphite coated tubes. Anal. Chem., 49, 2031-2035. 

Riley, J.P. and Taylor, D. (1968) Chelating resins for the concentration of trace elements from seawater and their analytical use in conjunction with atomic absorption spectroscopy. Anal. Chim. Acta, 40, 479-485. 

Various spectroscopic techniques such as flame photometry, emission spectroscopy, atomic absorption spectrometry, spectrophotometry, flu-orimetry, X-ray fluorescence spectrometry, neutron activation analysis and isotope dilution mass spectrometry have been used for marine analysis of elemental and inorganic components [2]. Polarography, anodic stripping voltammetry and other electrochemical techniques are also useful for the determination of Cd, Cu, Mn, Pb, Zn, etc. in seawater. Electrochemical techniques sometimes provide information on the chemical species in solution. 

Olsen et al. (48, 20) have described an interesting method for the determination of lead in polluted seawater using FIA and flame atomic absorption spectroscopy. The system incorporates a Chelex-100 column for on-line preconcentration of the sample. The preconcentration and elution step improves the detection limit for lead by a factor of four (50 nM). Further increases in sensitivity are easily possible. The combination of this preconcentration step with a more sensitive detector, such as anodic stripping voltammetry, may make possible the determination of trace metals in seawater on a routine basis. 

Analytical methods have been developed which are sensitive enough to measure the low concentration levels of trace metals in seawater. Well defined methods, like emission spectroscopy, neutron activation analysis, anodic stripping voltammetry, atomic absorption spectroscopy, and mass spectroscopy, can be used individually or collectively to obtain the necessary data on trace metal concentrations. So why, even with these well developed methods, are we not getting reliable results from the analysis of trace metals in natural water ... 

Naturally-occurring humic-metal complexes have been isolated from estuarine systems and seawater using solid phase extraction (SPE) onto a Cig HPLC column to preconcentrate the sample (JO-12). Samples were subsequently eluted from the SPE colunm at a much higher concentration and injected onto another HPLC column and detected by UV absorbance and a metal-sensitive detector, such as atomic fluorescence spectroscopy. The concentration of metal-humic complexes in natural aquatic environments was then calculated. However, there was some evidence of competitive binding of the metal ion between the organic matter and free silanol groups in the stationary phase resulting in a loss of metal in the column and erroneously low metal values (10). 

Potentiometric stripping analysis, as stated in one review,92 "is not as general an analytical technique for the determination of metal traces as is graphite-furnace atomic absorption spectroscopy." It is used as a complementary technique for assay of some toxic metals in water (zinc, cadmium, lead, and copper in potable water and wastewater,93 94 and lead and thallium in seawater.95 The advantage of anodic stripping voltammetry (ASV) is summarized in two steps, which include electrolytic preconcentration and the stripping process. There are a number of interfering ions that can affect the... 

Several other hydroxides have been used both for general coprecipitation and for specific separations. Lanthanum hydroxide has been used (47, 48) as a convenient collector and subsequent release agent for atomic absorption spectroscopy. Titanium hydroxide coprecipitates copper and zinc from seawater (49), and chromium(III) hydroxide precipitates cadmium(II) (50) and zinc (57). Other authors have used magnesium hydroxide (52, 53), manganese dioxide (54, 55), and zirconium hydroxide (56, 57) to collect trace metals from natural waters. Thallium(III) was coprecipitated with zirconium hydroxide at pH 4.3-6.7 and thallium(I) at pH 7.0-13.0 (57). Beryllium (58) and antimony (59) can be quantitatively coprecipitated with a variety of hydroxides including titanium and zirconium, depending on the pH used. 

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