Analysis of seawater

The buffer solution is prepared according to Section 12.4.2.4-Reagents. A solution is prepared of 400 mg of sodium dibenzyldithiocarbamate in 10 g of methanol. The solution can be stored, if cooled, for 1 or 2 days. A 2+1 (v v) mixture is prepared of CHQ3 and CH3OH. A dilution of a metal standard solution (e.g., 10mg/L Co) is prepared. 

A tuft of quartz wool is placed in a glass beaker and is leached for several hours in warm 10 % HCl. One layer of leached and rinsed quartz wool, approximately 5 mm, is placed in the undermost position of the augmented part of the capillary (Fig. 12-26). Above this layer, 80 % of the remaining space is filled with dry silica gel (approximately 1.5 mL of Chromo-sorb W/AW-DMCS). The column is closed with another 5 mm plug of quartz wool. The quartz column is coimected to a vacuum pump. The column is rinsed slowly with approximately 300 mL of 3 % HO and then with ultra-pure water until the obtained water passed is of neutral ph. The column is cleaned by performing repeated separation procedures as described in the following section until the blank values are constant. 

The sample reservoir and the column are cleaned by rinsing without applying vacuum with approximately 4 mL of CHCI3 subsequently they are conditioned with at least 2 mL of CH3OH. The column is now ready for the next sample or blank. 

An automated system for this separation and concentration procedure has been recently described by Gerwinski and Schmidt (1998). 

A 100-300/rL aliquot of of the organic eluate is transferred into the highly polished quartz sample carrier and evaporated to form a dry residue by means of a special device (see Fig. 12-27) designed to adjust the sample spot to meet the excitation beam. 

Salomon et al. [123] investigated in great detail the potential errors that could be encountered in the determination of nanomolar concentrations of aluminum in seawater, using conventional LS GF AAS with Zeeman-effect BG. In addition, HR-CS AAS was used as a diagnostic tool in order to identify the source of interference. 

They concluded that the presence of the calcium line at 309.528 nm in both spectra could obviously result in a small error. The authors also point out, considering the Zeeman-splitting in the range of 10-30 pm that the emission line of nickel partially coincides with the magnesium absorption line at 309.690 nm, and some overlap between the aluminum and magnesium absorption lines at 309.284 nm and 309.299 nm, respectively, occurred as well, resulting in the observed deviations. Although the authors did not use HR-CS AAS for the determination of aluminum in seawater, it is needless to say that the interferences detected for LS AAS with Zeeman-effect BC are absent in the former technique, so that an interference-free determination could be expected with an improved overall performance. 

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Sea Water. The analysis of seawater (Parravano, C., State University of New York at Purchase Steams, C., unpublished) is a three-part experiment employing ion exchange techniques titrations to study the salt content of this familiar... 

This book covers all aspects of the analysis of seawater using both classical and the most advanced recently introduced physical techniques. 

Until fairly recently, the analysis of seawater was limited to a number of major constituents such as chloride and alkalinity. 

In the analysis of seawater, the only significant interference arises from turbidity caused by particles in the sample. Prior filtration of the sample is therefore necessary. For anoxic waters, however, sulfide concentrations over 2 pm were found to decrease the absorbance. This was overcome by adding an excess of either Cd2+ or Hg2+ to the sample [171,172],... 

The work carried out by Dimmock and Midgley [14-16] at the Central Electricity Board, UK is concerned with the analysis of cooling waters, saline and non-saline. Although the analysis of seawater is not specifically discussed, their work may be of some relevance. 

Prior to the introduction of ion-selective electrode techniques, in situ monitoring of free copper (II) in seawater was not possible due to the practical limitations of existing techniques (e.g., ligand competition and bacterial reactions). Ex situ analysis of free copper (II) is prone to experimental error, as the removal of seawater from the ocean can lead to speciation of copper (II). Potentially, a copper (II) ion electrode is capable of rapid in situ monitoring of environmental free copper (II). Unfortunately, copper (II) has not been used widely for the analysis of seawater due to chloride interference that is alleged to render the copper nonfunctional in this matrix [288]. 

Atienza et al. [657] reviewed the applications of flow injection analysis coupled to spectrophotometry in the analysis of seawater. The method is based on the differing reaction rates of the metal complexes with 1,2-diaminocycl-ohexane-N, N, N, A/Metra-acetate at 25 °C. A slight excess of EDTA is added to the sample solution, the pH is adjusted to ensure complete formation of the complexes, and a large excess of 0.3 mM to 6 mM-Pb2+ in 0.5 M sodium acetate is then added. The rate of appearance of the Pbn-EDTA complex is followed spectrophotometrically, 3 to 6 stopped-flow reactions being run in succession. Because each of the alkaline-earth-metal complexes reacts at a different rate, variations of the time-scan indicates which ions are present. 

Fang et al. [661] have described a flow injection system with online ion exchange preconcentration on dual columns for the determination of trace amounts of heavy metal at pg/1 and sub-pg/1 levels by flame atomic absorption spectrometry (Fig. 5.17). The degree of preconcentration ranges from a factor of 50 to 105 for different elements, at a sampling frequency of 60 samples per hour. The detection limits for copper, zinc, lead, and cadmium are 0.07, 0.03, 0.5, and 0.05 pg/1, respectively. Relative standard deviations are 1.2-3.2% at pg/1 levels. The behaviour of the various chelating exchangers used was studied with respect to their preconcentration characteristics, with special emphasis on interferences encountered in the analysis of seawater. 

The extension of inductively coupled plasma (ICP) atomic emission spectrometry to seawater analysis has been slow for two major reasons. The first is that the concentrations of almost all trace metals of interest are 1 xg/l or less, below detection limits attainable with conventional pneumatic nebulisation. The second is that the seawater matrix, with some 3.5% dissolved solids, is not compatible with most of the sample introduction systems used with ICP. Thus direct multielemental trace analysis of seawater by ICP-AES is impractical, at least with pneumatic nebulisation. In view of this, a number of alternative strategies can be considered ... 

In the analysis of seawater, isotope dilution mass spectrometry offers a more accurate and precise determination than is potentially available with other conventional techniques such as flameless AAS or ASV. Instead of using external standards measured in separate experiments, an internal standard, which is an isotopically enriched form of the same element, is added to the sample. Hence, only a ratio of the spike to the common element need be measured. The quantitative recovery necessary for the flameless atomic absorption and ASV techniques is not critical to the isotope dilution approach. This factor can become quite variable in the extraction of trace metals from the salt-laden matrix of seawater. Yield may be isotopically determined by the same experiment or by the addition of a second isotopic spike after the extraction has been completed. 

One of the advantages of the isotope dilution technique is that the quantitative recovery of the analytes is not required. Since it is only their isotope ratios that are being measured, it is necessary only to recover sufficient analyte to make an adequate measurement. Therefore, when this technique is used in conjunction with graphite furnace atomic absorption spectrometry, it is possible to determine the efficiency of the preconcentration step. This is particularly important in the analysis of seawater, where the recovery is very difficult to determine by other techniques, since the concentration of the unrecovered analyte is so low. In using this technique, one must assume that isotopic equilibrium has been achieved with the analyte, regardless of the species in which it may exist. 

The benefits imparted by preconcentration to improved sensitivity are illustrated in the example of lead preconcentration on Chelex 100 resin [871,872], followed by analysis by ICP-AES. Without preconcentration the best detection bmit achievable is 60 ng/1, via direct nebubsation. When the Chelex 100 preconcentration step is included, the detection limit improves to 0.6 ng/1, i.e., 100 times better, which is a very important improvement achieved in the analysis of seawaters. Examinaton of Table 5.12 reveals that the following metals can be determined with detection limits in the 1 -10 ng/1 range beryllium (0.6 ng/1),... 

Chapter 3 discusses the analysis of seawater constituents and reiterates the scientific problems that necessitate a number of seawater-based reference materials. 

A wide variety of solvents, reagents, and structural materials encountered normally in the trace-element analysis of seawater have been analyzed for trace-element impurities by neutron activation analysis and gamma (y)-ray spectrometry.17 Some of the results obtained for 10 trace elements are shown in Table 6.4 and indicate that many substances contain high impurity levels of various elements. Particular note should be made of the high concentrations of zinc in... 

Analysis of seawater by ICP-MS is complicated by low concentrations of many elements of interest, sensitivity reductions due to the high salt concentrations, and Cl containing molecular ion spectral overlaps [314]. Standard reference seawaters are available from the National Research Council (Canada). 

The concentrations of trace elements in seawater varies geographically, spatially, and in depth [7j. Such variations are generally caused by the biological activities and physicochemical processes in the oceans. Thus, readers should note that the concentrations of trace elements given in Table 1 are not representative of those for all oceans. However, the extremely low levels of elements in concentrated salt matrix make accurate and precise analysis of seawater difficult. In recent interlaboratory investigations of seawater analysis by skillful marine or analytical chemists, a wide range of... 

EXPERIMENTAL CONDITIONS AND DETECTION LIMITS IN FLAMELESS ATOMIC ABSORPTION SPECTROMETRIC ANALYSIS OF SEAWATER EXTRACTS ... 

TRACE METALS ANALYSIS OF SEAWATER USING MIBK SINGLE EXTRACTION AND MIBK-HNOj SUCCESSIVE EXTRACTION METHODS [37]... 

Selco, J. I. Roberts, J. L., Jr. Wacks, D. B. The Analysis of Seawater A Laboratory-Centered Learning Project in General Chemistry, /. Chem. Educ. 2003, 80, 54-57. 

Coale, K. H., and Mart, L. (1985) Analysis of Seawater for Dissolved Cadmium, Copper and Lead An Inter-comparison of Voltammetric and Atomic Absorption Methods. Mar. Chem. 17, 285-300. 

TTHE BENEFITS OF AUTOMATING NUTRIENT ANALYSES IN SEAWATER have been recognized and utilized for several decades. Automated analyses allow samples to be processed faster and generally with better precision and accuracy than is possible with most manual methods. The only technique that was available at a reasonable cost for the automated analysis of seawater nutrients, until recently, was segmented continuous-flow analysis (CFA). Segmented CFA is characterized by the use of air bubbles to segment the liquid in the reaction tube so that dispersion of the sample is limited. 

Anderson (11) was the first to report on the use of FIA for the analysis of seawater micronutrients. He developed a method for the simultaneous determination of nitrate and nitrite. The chemical reactions for the analysis of nitrate were based on the reduction of nitrate to nitrite by a copper-ized cadmium column placed in the flow path. The nitrite was then analyzed as an azo dye (11). This reaction sequence is conventionally used in both segmented CFA and manual analyses of nitrate and nitrite in seawater (2, 6, 7). The detection limits are 0.1 fxM for nitrate and 0.05 juM for nitrite. 

The application of the ASV technique to the analysis of seawater has been reviewed (6-9). Its advantages over other trace-metal methods are several (1) sea salts do not interfere (2) a high degree of sensitivity and selectivity can be achieved through electrolytic preconcentration, thus the analysis can be carried out directly in seawater, and few or no reagents need to be added (3) the measurement is easily automated (4) relatively inexpensive, rugged, and portable instrumentation can be built for field use (5) more than one metal may be measured at one time with a sensitivity approaching parts per trillion. 

Extractive pre-concentration techniques have been used for some time in trace metal analysis of seawater. In particular, the ammonium pyrrolidinecarbodithioate-methyl isobutyl ketone (APCD-MIBK) system has found moderate use (1-7). Because of the potential for multielement analysis of a single extract and the apphcabihty of the technique for shipboard use, this method was evaluated for the analysis of Cu, Cd, Pb, Ni, and Zn at natural levels in coastal waters. The results presented for those five metals show that this method is a reliable and valuable analytical tool for trace metal analysis in seawater if the limitations of the method are realized and the proper precautions are taken. 

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