Modifiers, organic trace analysis

When trace analysis is performed, well-established protocols have to be followed to obtain reliable results. The production of free atoms in the flame or furnace can be modified according to the composition of the matrix in which the sample is found. Therefore, salts and mineral or organic reagents are often introduced in the solutions to be nebulised. 

The chromatographic procedure intially investigated was reverse-phase liquid chromatography (RPLC) using Cig-bonded silica gel. This technique has been used previously for the preconcentration of trace metals from seawater prior to analysis by atomic absorption spectrophotometry (23, 24) and for the isolation of organically bound copper from seawater (4). These methods were modified and adapted for automation. 

This chapter has identified the main extraction techniques used for the extraction of organic analytes from solid matrices, e.g. soil. The main purpose of each technique has been to remove the analyte from the matrix as effectively as possible. In some instances, e.g. supercritical fluid extraction, some attempt is made to achieve selectivity of extraction by altering the operating conditions, such as temperature and pressure, or by the addition of an organic modifier. In other situations, the sole purpose of the extraction technique is to remove the analyte from the matrix under the strongest possible conditions without any concern for selectivity. The selectivity in these circumstances results exclusively from the method of analysis which follows. It should also be borne in mind, depending on the level of contamination of the sample, that further extract pre-concentration and/or clean-up may be required prior to analysis in order to achieve trace level analytical results. The methods used for effective pre-concentration are described later in Chapter 10. 

In some instances the method for carbon determination has to be modified, e.g., the determination of trace amounts of what is referred to as dissolved organic carbon in water after inorganic carbon has been removed. This type of carbon determination involves wet oxidation activated by silver ions in a solution of potassium persulphate in sulphuric acid. The oxidation of organic compounds gives carbon dioxide, which is adsorbed by molecular sieves. The molecular sieves are then heated in a flow of helium to desorb the carbon dioxide, the amount of which is measured by a TCD. The lowest concentration of organic carbon that can be measured in water is 0.2—2ppm [55]. The application of chromatographic elemental analysis to the determination of the total carbon content in water has been described [56]. 

For trace determinations of cesium cations the procedure is executed in two steps First, 3 /il of a standard solution containing a known amount of cesium chloride is applied to the emitter by means of the modified syringe technique s and a signal at m/z 133 is recorded. S nd, between 0.2 pA and 1 1 of sample are applied to the same emitter and desorbed under identical conditions. From the peak areas of the evaporation profiles obtained in both measurements, the unknown amount of the alkali element present in the sample is calculated. Usually, one analysis (calibration + sample analysis) can be performed within 30 min. Thus, cesium in sample sizes of 0.2 to 1 jul, which contain 0.3 to 1000 pg of the element, can be determined. The accuracy of repeated measurements of a standard solution is 10% and that of the technique for the determination of unknown concentrations 20 %. A sensitivity between 1.4 and 2.5 X 10 C per g is obtained for cesium. Since a good sensitivity value for an organic FD ion, namely the molecular ion of cyclophosphamide, has been reported to be 1 to 2X 10 C per it is clear that FDMS is about a factor of 100 more sensitive for the [Cs] ion. 

The sampler itself should be constructed from noncontaminating and, equally important, nonadsorptive material. The sampler should also be capable of being lowered through the sea surface closed so as to avoid contamination from the organic-rich sea surface microlayer. Unfortunately, there is no ideal sampler. The modified General Oceanics Go-Flo bottles widely used by workers in the field of oceanic trace inorganic analysis are often the most suitable systems that are commonly available. In some cases (e.g., for halocarbons), special devices have been constructed to allow uncontaminated withdrawal for the sample... 

Insofar as the derivatization can be considered one of the stages of sample preparation for chromatographic analysis, it can be combined with other procedures, for instance, the preconcentration of traces of analytes. For example, the yield of solid-phase extraction or microextraction of organic compounds from aqueous solutions with modified silica gels is better for more hydrophobic substances the preliminary conversion of acidic compounds into suitable derivatives is recommended. ... 

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