Metal analysis preconcentration

Examples of applications involving preconcentration may be found in references (79-85). Modified electrodes capable of preconcentrating the analyte onto the electrode surface have been employed to enhance the analytical sensitivity and selectivity. The most common applications appear to be in the area of metal analysis. Preconcentration has been achieved on the basis of electrostatic attraction for analysis of chromium as Cr207" (78). For more selective preconcentration, however, complexation has been employed (79-81). Dimethylglyoxime has been employed for nickel determinations (81), and dithiocarbamates (79, 80, 82, 83) for copper determination. Dithiocarba-mate modified electrodes have been used for mercury analysis (84, 85). 

In order to one of the most effective separation and preconcentration procedure in trace metal analysis is solid phase extraction (SPE) of analyte. 

Shipping analysis is an extremely sensitive electrochemical technique for measuring trace metals (19,20). Its remarkable sensitivity is attributed to the combination of an effective preconcentration step with advanced measurement procedures that generate an extremely favorable signal-to-background ratio. Since the metals are preconcentrated into the electrode by factors of 100 to 1000, detection limits are lowered by 2 to 3 orders of magnitude compared to solution-phase voltammetric measurements. Hence, four to six metals can be measured simultaneously in various matrices at concentration levels down to 10 10 i. utilizing relatively inexpensive... 

A logical approach which serves to minimise such uncertainties is the use of a number of distinctly different analytical methods for the determination of each analyte wherein none of the methods would be expected to suffer identical interferences. In this manner, any correspondence observed between the results of different methods implies that a reliable estimate of the true value for the analyte concentration in the sample has been obtained. To this end Sturgeon et al. [21] carried out the analysis of coastal seawater for the above elements using isotope dilution spark source mass spectrometry. GFA-AS, and ICP-ES following trace metal separation-preconcentration (using ion exchange and chelation-solvent extraction), and direct analysis by GFA-AS. These workers discuss analytical advantages inherent in such an approach. 

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. 

The advantage of using HPLC-ICP-OES for metal analysis is when direct nebulisation of solutions of samples can cause matrix interference on ICP-OES. An important use of this technique would be the detection of variable oxidation states of elements and it can also preconcentrate trace elements on a column which can be eluted from the column and nebulised using ICP-OES friendly solvents. See schematic diagram 7.16 showing an anion and cation HPLC coupled with ICP-OES. 

The first requirement can be easily fulfilled by the preconcentration of the analyte before the analysis. Preconcentration has been applied to sample preparation for flame atomic absorption (25) and, more recently, for ICP (79,80) spectroscopy. However, preconcentration is not completely satisfactory, because of the increased analysis time (which may be critical in clinical analysis) and the increased chance of contamination or sample loss. Most important, however, a larger initial sample size is necessary. The apparent solution is a more sensitive technique. Table 2 lists concentrations of various metals in whole blood or serum (81,82) in comparison to limits of detection for the various atomic spectroscopy techniques. In many cases, especially for the toxic heavy metals, only flameless atomic absorption using a graphite furnace can provide the necessary sensitivity and accommodate a sample of only a few microliters (Table 1). The determination of therapeutic gold in urine and serum (83,84), chromium in serum (85), skin (86) and liver (87), copper in semen (88), arsenic in urine (89), manganese in animal tissues (90), and lead in blood (91) are but a few examples in analyses which have utilized the flameless atomic absorption technique. 

C.D. Stalikas, Micelle-mediated extraction as a tool for separation and preconcentration in metal analysis, Trends Anal. Chem. 21 (2002) 343. 

Nevertheless, modem methods of electroanalytical chemistry can, in certain instances, either successfully complement atomic absorption spectroscopy or provide information not otherwise obtainable. A synergistic effect can, in fact, be obtained by a combination of the two techniques, and such work has been described by Lund and Larsen (39) and by Fairless and Bard (40). Controlled potential electrolysis is first used to preconcentrate the element or elements of interest on an electrode. This step also separates the elements from the matrix and possibly from interfering elements. The electroplated elements are then removed and measured by atomic absorption. The combination of the two techniques seems ideal for problems in petroleum trace metal analysis. Much research remains to be done, however, before its applicability can be defined. 

M. Bengtsson, F. Malamas, A. Torstensson, O. Regnell, and G. Johansson, Trace Metal Ion Preconcentration for Flame Atomic Absorption by an Immobilized N,N,N -Tri-(2-Pyridylmethyl)ethylene Diamine (TriPEN) Chelate Ion Exchanger in a Flow Injection Analysis. Mikrochim. Acta, III (1985) 209. 

Probably the most widely used procedure for trace metal analysis of seawater over the last 20 years has been preconcentration followed by ETAAS because of the wide availability, good sensitivity, and large range of elements that can be measured by this method. Preconcentration procedures have in general been one of three types. First, co-precipitation, with iron(III) hydroxide or cobalt pyrrolidinedithio-carbamate, being the most widely used co-precipit-ants. Second, cornplexation followed by solvent extraction with a number of different complexants, of which dithiocarbamates, 8-quinolinol, and dithi-zone are particularly popular, and with extraction into a range of solvents. The third approach is extraction on to a chelating column (usually Chelex-100 but recently also other complexants such... 

The determination of metals in water samples by neutron activation analysis (NAA) shows different sensitivities for different samples, including several cases where NAA sensitivity is better than all the analytical techniques. Several of the factors which can affect the sensitivity of the method are sample composition, neutron flux, irradiation time, decay time, coxmting time, and detector efficiency [328,329]. Different preconcentration methods have also been applied to NAA protocols for metal analysis. For instance, the use of coprecipitation method [330,331], chelating adsorbents [332], etc. One of the additional advantages of this methodology is that both the irradiation and neutral activation can be directly performed on the resin, without eluting the metals from the column. 

Wang, Y., M. L. Chen, and J. H. Wang. 2007 New developments in flow injection/ sequential injection on-line separation and preconcentration coupled with electrothermal atomic absorption spectrometry for trace metal analysis. Appl. Spectrosc. Rev. 42 103-118. 

Nicola, M., Rosin, C.,Tousset, N., and Nicolai,Y. (1999).Trace metals analysis in estuarine and seawater by ICP-MS using on line preconcentration and matrix elimination with chelating resin. Talanta 50(2), 433. 

Benkhedda, K., Infante, H. G., Ivanova, E., and Adams, F. C. (2000) Trace metal analysis of natural waters and biological samples by axial inductively coupled plasma time-of-flight mass spectrometry (ICP-TOFMS) with flow injection on-line adsorption preconcentration using a knotted reactor. J. Anal. At Spectrom., 15,1349. 

Martins, AO Silva, EL Carasek, E. Sulphoxine immobilized onto chitosan microspheres by spray drying application for metal ions preconcentration by flow injection analysis. Talanta, 2004, 63, 397-403. 

The term direct TXRF refers to surface impurity analysis with no surface preparation, as described above, achieving detection Umits of 10 °—10 cm for heavy-metal atoms on the silicon surface. The increasit complexity of integrated circuits fabricated from silicon wafers will demand even greater surfrce purity in the future, with accordingly better detection limits in analytical techniques. Detection limits of less than 10 cm can be achieved, for example, for Fe, using a preconcentration technique known as Vapor Phase Decomposition (VPD). 

Essentially, stripping analysis is a two-step technique. The first, or deposition, step involves die electrolytic deposition of a small portion of the metal ions hi solution into die mercury electrode to preconcentrate the metals. This is followed by die shipping step (the measurement step), which involves die dissolution (shipping) of die deposit. Different versions of stripping analysis can be employed, depending upon die nature of the deposition and measurement steps. 

Potentiometric stripping analysis (PSA), known also as stripping potenhometry, differs from ASV in the method used for stripping the amalgamated metals (22). hi this case, the potentiostatic control is disconnected following the preconcentration, and the concentrated metals are reoxidized by an oxidizing agent [such as O2 or Hg(II)] that is present in the solution ... 

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. 

Adsorptive cathodic stripping voltammetry has an advantage over graphite furnace atomic absorption spectrometry in that the metal preconcentration is performed in situ, hence reducing analysis time and risk of contamination. Additional advantages are low cost of instrumentation and maintenance, and the possibility to use adapted instrumentation for online and shipboard monitoring. 

In many applications, such as the analysis of mercury in open ocean seawater, where the mercury concentrations can be as small as 10 ng/1 [468,472-476], a preconcentration stage is generally necessary. A preliminary concentration step may separate mercury from interfering substances, and the lowered detection limits attained are most desirable when sample quantity is limited. Concentration of mercury prior to measurement has been commonly achieved either by amalgamation on a noble-metal metal [460,467, 469,472], or by dithizone extraction [462,472,475] or extraction with sodium diethyldithiocarbamate [475]. Preconcentration and separation of mercury has also been accomplished using a cold trap at the temperature of liquid nitrogen. 

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. 

Preconcentration and removal of the metals of interest from the seawater matrix prior to ICP analysis... 

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