Hyphenated and Specialized Systems

The possibility of combining separation and detection techniques gives origin to the so-called hyphenated-techniques , analytical systems able to answer these questions. Well described approaches on the subject are in the special issue of Spectrochimica Acta devoted to Speciation [111]. 

A chromatographic system with autosampler and fraction collector under the control of software specially adapted for hyphenated experiments. 

Body fluids, such as serum, contain several different carotenoids in low amounts. The crucial point in the isolation and analysis of these samples is the enrichment factor. Serum samples can be directly analysed with hyphenated extraction-sample enrichment-separation systems, such as on-line SPE-HPLC employing tailored stationary phases [29]. By using special restricted access materials (RAMs) for sample enrichment, the carotenoids are retarded on the pre-column while the protein binding is broken and the macromolecules are eluted. The preparation of artifacts is hindered, as the whole analysis steps take place under conditions of light- and oxygen-exclusion. The scheme of on-line SPE-HPLC is presented in Figure 5.2.2. 

Conventional packed column injection ports are used for the sample introduction on packed and megabore columns. In case of capillary columns, unless a special injection technique is used, the low maximum allowable sample volume which may be introduced on the column negatively affects the experimental detection limits as only a tiny fraction of the derivatized extract is finally processed in the hyphenated system. Online preconcentration and injection of derivatized organolead species can solve that problem. It consists of three consecutive processes taking place in the injection hner sample injection, solvent venting and release of the analytes on to the column. Up to 25 fil can be processed at a time and larger amounts can be handled by successive injections of 20 to 25 /a1 volumes at 1 min intervals to remove the solvent. 

Although originally FIA was conceived as a special technique for delivery of a sample segment into the instrument, the combination of flow injection as a sample pretreatment tool with atomic spectrometry has been shown to be of great potential for enhancing the selectivity and sensitivity of the measurements. Moreover, contamination problems are reduced due to the closed system used, making this interface suitable for ultratrace determination of metal species. Hyphenated techniques such as FIA/ SIA with flame atomic absorption spectrometry, inductively coupled plasma (ICP)-optical emission spectrometry, and ICP-mass spectrometry (MS) have been exploited extensively in recent years. The major attraction of FIA-ICP-MS is its exceptional multi-elemental sensitivity combined with high speed of analysis. In addition, the possibility of... 

The selective detectors discussed in the previous sections often do not provide enough information to elucidate with 100% probability the nature of the eluting solutes. For this reason, data with selective detectors can be erratic. The future in this respect definitely belongs to the spectroscopic detectors that allow. selective recognition of the separated compounds. Today, the hyphenated techniques CGC-mass spectroscopy (CGC-MS), CGC-Fourier transform infrared spectroscopy (CGC-FTIR), and CGC-atomic emission detection (CGC - AED) are the most powerful analytical techniques available. They provide sensitive and selective quantitation of target compounds and structural elucidation or identification of unknowns. The applicability and ease of use of the hyphenated techniques were greatly increased by the introduction of fused silica wall coaled open tubular columns. The main reason for this is that because of the low flows of capillary columns, no special interfaces are required and columns are connected directly to the different spectrometers. The introduction of relatively inexpensive benchtop hyphenated systems has enabled many laboratories to acquire such instrumentation, which in turn has expanded their applicability ever further. 

Mass spectrometers hyphenated to gas chromatographs have the same design as those in LC-MS except that there is no need of a special interface for removal of high amounts of liquid vapors. A constant flow of carrier gas, together with the chromatographically separated compounds, is, via transfer Une, directly brought into an ionization chamber. There the compounds are ionized by two different systems, and the ions are analyzed and detected by the use of different analyzer systems and electron- or photomultipliers. Turbo pumps provide a high vacuum of down to 10 mbar. Molecules can be ionized by either electron impact (El) ionization or chemical ionization (Cl). 

Note A detailed treatment of ion mobihty spectrometry is beyond the scope of this book, nonetheless, a brief description of the method is required. While IM-MS systems can in principle be regarded as hyphenated instrumentation (treated in more detail in Chap. 14), the pecuharities of ion mobility demand for an integrated instrumental approach rather than gas or hquid chromatography couplings that can rapidly be attached and removed from a mass spectrometer as required [235,241], Therefore, IM-MS is dealt with here as a special case under the heading of instrumentation. 

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