Eluent choice

Limitations on eluent choice (minimisation of solvent interferences)... 

Table 7.87 shows the main features of on-line micro LC-GC (see also Table 7.86). The technique allows the high sample capacity and wide flexibility of LC to be coupled with the high separation efficiency and the many selective detection techniques available in GC. Detection by MS somewhat improves the reliability of the analysis, but FID is certainly preferable for routine analysis whenever applicable. Some restrictions concern the type of GC columns and eluent choice, especially using LC columns of conventional dimensions. Most LC-GC methods are normal-phase methods. This is partly because organic solvents used as eluents in NPLC are compatible with GC, making coupling simpler. RPLC-GC coupling is demanding water is not a suitable solvent for GC, because it hydrolyses the siloxane bonds in GC columns. On-line RPLC-GC has not yet become routine. LC-GC technology is only applicable to compounds that can be analysed by GC, i.e. volatile, thermally stable solutes. LC-GC is appropriate for complex samples which are difficult or even impossible to analyse by a single chromatographic technique. Present LC-GC methods almost exclusively apply on-column, loop-type or vaporiser interfaces (PTV).

SPE consists of analyte immobilization on the sorbent in the first stage, followed by selective elution of the components of interest by an appropriate solvent. Solid, adsorptive phases are selected according to their high affinity for the analytes (significantly higher than between analyte and donor phase). Proper eluent choice is the second factor influencing SPE efficiency. 

The classification of eluents into the two categories mentioned above makes sense and is necessary only in the framework of conductivity detection with its different application requirements. Eluent choice is much easier for applications using spectrophotometric or amperometric detectors. In photometric detection, both the photometric properties of the eluent ions and their chemical properties have to be taken into account nevertheless, a large number of eluents are available. The alkali salts of phosphoric acid, sulfuric acid, and perchloric acid have proved to be successful, because they all feature good UV transmittance. In the field of amperometric detection, the choice of eluents is even much higher. The electrolyte concentration in the mobile phase must be about 50-100 times higher than the analyte ion concentration. The mobile phase acts as a support electrolyte that, by the reduction of the mobile-phase resistance, Rt, ensures that the voltage drop, i Rt, is kept low. Chlorides, chlorates, and perchlorates of alkali metals and alkali hydroxides and carbonates are suited for use as support electrolytes. 

Similarly, for the analysis of polymers using high-temperature solvents, the important concern is column stability and durability. Eor this reason, 10-/am particles are the best column choice. Waters Styragel HT columns are designed for this kind of application. Similarly, these columns are also more tolerant to eluent changes. Therefore, these columns are also recommended... 

While partially concurrent eluent evaporation is easier to use, and is preferred for the transfer of normal phase solvents, concurrent eluent evaporation with co-solvent trapping is the technique of choice for transfer of water-containing solvents, because wettability is not required. 

The optimization of preparative and even micropreparative chromatography depends on the choice of an appropriate chromatographic system (adsorbent and eluent), sample application and development mode to ensure high purity, and yield of desirable compounds isolated from the layer. For the so-called difficult separations, it is necessary to perform rechromatography by using a system with a different selectivity. But it should be taken into account that achievement of satisfactory results frequently depends on a compromise between yield and the purity of the mixture component that is being isolated. 

IEC continues to have numerous applications to the detection and quantification of various inorganic ions.1 1 This is particularly true in water analysis.5-14 Inorganic ions in a variety of other sample types, such as food and beverages,1518 rocks,19-23 biological fluids, (blood, urine, etc.),24-31 pharmaceutical substances,32 33 concentrated acids,34 alcohols,35 and cleanroom air36 have also been analyzed by IEC. IEC has also been employed in isotopic separation of ions,37 including the production of radioisotopes for therapeutic purposes.3839 Typical IEC sample matrices are complex, and may contain substances that interfere with measurement of the ion(s) of interest. The low detection limits required for many IEC separations demand simple extraction procedures and small volumes to avoid over-dilution. Careful choice and manipulation of the eluent(s) may be needed to achieve the desired specificity, especially when multiple ions are to be determined in a single sample. 

In environmental analytical applications where analyte concentrations, e.g. surfactants or their metabolites, are quite low, extraction and concentration steps become essential. Solid phase extraction (SPE) with cartridges, disks or SPME fibres (solid phase micro extraction) because of its good variety of SP materials available has become the method of choice for the analysis of surfactants in water samples in combination with FIA as well as LC—MS analysis. SPE followed by sequential selective elution provides far-reaching pre-separations if eluents with different polarities and their mixtures are applied. The compounds under these conditions are separated in the MS spectrometer by their m/z ratios providing an overview of the ionisable compounds contained in a sample. Identification in the sense it has been mentioned before, however, requires the generation of fragments. 

The adduct formation can be largely controlled and directed into the formation of a single selected species by adequate choice of the ionisation mode (possibly at the expense of sensitivity), the eluent composition (buffer addition, pH adjustment, type of organic modifier) and by optimisation of the ion source parameters influencing the stability of individual (adduct) ions. In contrast to the variations in adduct or cluster formation, which principally can be diagnosed by recording more than one (adduct) ion in SIM mode, the occurrence of ion suppression requires more careful diagnosis. 

---