Adsorption trace analysis

Adsorptive stripping analysis involves pre-concentration of the analyte, or a derivative of it, by adsorption onto the working electrode, followed by voltanmietric iiieasurement of the surface species. Many species with surface-active properties are measurable at Hg electrodes down to nanoniolar levels and below, with detection limits comparable to those for trace metal detemiination with ASV. 

The first (direct reading) method is fairly simple and results are available immediately. However, the instruments have limited sensitivity and must be recalibrated periodically. The second (absorption in a liquid or adsorption on a medium) and third (gas container) methods are generally considered more sensitive and more accurate method for trace analysis by gas chromatographs, infrared... 

Adsorptive stripping analysis greatly enhances the scope of stripping measurements toward numerous trace elements (23,24). This relatively new strategy involves the... 

Trace analysis has its special hazards for the unwary. The most important of these are loss of material in the analytical process and contamination by outside sources. Everyone realizes that trace constituents can be lost from samples, but few are aware of the many ways in which this can occur. For example, phosphate has been observed to disappear mysteriously from water samples in polyethylene bottles (10). Nitric acid, used to clean plastic vials, has been observed to convert these surfaces to ion exchangers, which readily take up as much as 10 12 moles per sq. cm. of trace metals (16). Lead nitrate solutions unless made distinctly acidic, plate out much of the lead on the walls of glass bottles. While everyone realizes that formation of a precipitate is liable to carry out trace constituents either by adsorption or occlusion, it is not as well-known that vanishingly small amounts of precipitates—amounts likely to be overlooked on casual observation—may also do this. The fly-ash and soot, which seem to be inescapable components of city air,... 

Adsorptive stripping analysis greatly expands the scope of stripping measurements towards numerous trace metals (e.g., Cr, Al, Fe, Y, Mo... 

Far from the metal trace analysis, our initial studies with BCFMEs were focused on the determination of folic acid [122], In this case, the main goal was the optimisation of the electrode pretreatment for this analyte. An acidic medium (0.1M perchloric acid) was considered optimum for folic acid determination by differential pulse voltammetry. A linear range between 2.0 x HT8 and 1.0 x 10 6M with a detection limit of 1.0 x 10 8M was obtained. Nevertheless, in this work, the adsorptive properties of the folic acid on mercury were noted and the employment of mercury-coated carbon fibre UMEs for folic acid determination has been targeted as a future goal. 

J. B. F. Lloyd, Adsorption Characteristics of Organic Explosives Compounds Typically Used in Clean-up and Related Trace Analysis Techniques, Journal of Chromatography 328 (1985) 145. 

Carbon-mineral adsorbents are of interest because of their potential applicability in practice, and as model adsorbents in research. Such composite materials can be applied for the adsorption of both polar and nonpolar compounds.1,2 These adsorbents have been utilized in chromatography, trace analysis, environmental protection, technology of water and sewage purification, and in other processes.1... 

Lloyd JBF. 1985a. Adsorption characteristics of organic explosives compounds on adsorbents typically used in clean-up and related trace analysis techniques. J Chromatogr 328 145-154. 

Zimmermann, S., Menzel, C.M., Berner, Z., Eckhardt, J.D., Stiiben, D., Alt, F., Messerschmidt, J., Taraschewski, H., Sures, B. Trace analysis of platinum in biological samples a comparison between sector field ICP-MS and adsorptive cathodic stripping voltammetry following different digestion procedures. Anal. Chim. Acta 439, 203-209 (2001)... 

Column efficiency can affect S/N measurements therefore analysts should account for both the type and the age of the column when determining the LOD. The maintenance status of chromatographic components (e.g., detectors and injectors) will also affect the ability to measure limits.In trace analysis, the LOD is greatly influenced by the recovery of the compound adsorption to glassware, instability or volatility, incomplete reaction (during derivatization), and poor laboratory technique are some of the causes of sample loss during analysis. 

Besides the requirement for adequate analytical reliability, crucial for routine use are robustness and the possibility of substantial automation. For these reasons, we used a HS 40 headspace sampler (Perkin-Elmer) directly coupled to an HP 5890 series II gas chromatograph (Hewlett-Packard) This combination avoids adsorptive and reactive surfaces in the sample path and allows the temperature to be controlled from cleavage to detection. This makes even trace analysis very reliable and reproducible. 

The belief that the isolation of macrocomponents from solution as sparingly soluble compounds is inadmissible in trace analysis, because of the considerable losses of traces caused by adsorption, is not necessarily true if the precipitation is done in acid medium. This has been confirmed in the following examples. 

Solid-phase microextraction (SPME) is a technique that was first reported by Louch et al. in 1991 (35). This is a sample preparation technique that has been applied to trace analysis methods such as the analysis of flavor components, residual solvents, pesticides, leaching packaging components, or any other volatile organic compounds. It is limited to gas chromatography methods because the sample must be desorbed by thermal means. A fused silica fiber that was previously coated with a liquid polymer film is exposed to an aqueous sample. After adsorption of the analyte onto the coated fiber is allowed to come to equilibrium, the fiber is withdrawn from the sample and placed directly into the heated injection port of a gas chromatograph. The heat causes desorption of the analyte and other components from the fiber and the mixture is quantitatively or qualitatively analyzed by GC. This preparation technique allows for selective and solventless GC injections. Selectivity and time to equilibration can be altered by changing the characteristics of the film coat. 

Adsorptive stripping voltammetry (ASV) is another specialised technique where the SMDE electrode is used for reducible species and carbon paste electrodes for oxidisable ones. This allows enrichment (by factors of 100-1000) of ions at the working electrode before stripping them off for measurement this improves the detection limits. This technique is rapid, sensitive (10 "M), economical and simple for trace analysis. The basic instrumentation for stripping analysis is apotentiostat (with voltammetric analyser), electrode and recorder. While voltammetry is generally very useful for compounds that do not have a chromophore or fluorophore, stripping analysis is the best analytical tool for direct, simultaneous determination of metals of environmental concern, e.g. lead, cadmium, zinc and copper in sea water. 

ScHLEiCH C and Henze G (1990a) Trace analysis of germanium. Part 2. Polarographic behaviour and determination by adsorptive stripping voltammetry. Fresenius J Anal Chem 338 145-148. 

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