Multi-column technique

Fig. 57. Multi-column technique for product analysis. From ref. 280.

On-line multicolumn preffactionation is an efficient but complex method of isolating fractions for HPLC analysis [I]. Switching devices can implement the transfer of fractions to other extraction columns or to an analytical column. Each solvent used for extraction must be compatible with the subsequent step. Such online procedures minimize sample loss from manual manipulations. On-line multi-column techniques are most effective for high-volume repetitive procedures where the time involved in method development is inconsequential due to throughput. If cross-contamination must be avoided, columns or cartridges must be discarded after each use In other cases, regeneration is possible. 

Pfrepper et al. [37] verified the equivalence of Eqs. 2 and 3. They compared values for Hf isotopes obtained, first, in batch experiments and by conventional elution chromatography in the anion-exchange system HS36 (a resin with the quaternary triethyl-ammonium group) -0.27 M HF/x M HNO3 (x variable) with, second, values obtained in the same system with the multi-column technique (MCT). They used the short-lived Hf isotopes and detected the daughter... 

Multi-column switching can be an effective approach for the determination of high and low concentrations of sample components in complex mixtures. This is a very powerful technique for the analytical and preparative separation of components... 

This multi-column swithching (GC-GC) technique has also been shown to be a powerful method for the separation of benzene and 1-methyl-cyclopentane in gasoline, as well as for the analysis of m-andp-xylenes in ethylbenzene. 

Constant overhead composition, vaiying reflux. If it is desired to maintain a constant overhead composition in the case of a binaiy, the amount of reflux returned to the column must be constantly increased throughout the run. As time proceeds, the pot is gradually depleted of the hghter component. Finally, a point is reached at which the reflux ratio has attained a veiy high value. The receivers are then changed, the reflux is reduced, and an intermediate cut is taken as before. This technique can also be extended to a multi-component mixture. 

An on-line supercritical fluid chromatography-capillary gas chromatography (SFC-GC) technique has been demonstrated for the direct transfer of SFC fractions from a packed column SFC system to a GC system. This technique has been applied in the analysis of industrial samples such as aviation fuel (24). This type of coupled technique is sometimes more advantageous than the traditional LC-GC coupled technique since SFC is compatible with GC, because most supercritical fluids decompress into gases at GC conditions and are not detected by flame-ionization detection. The use of solvent evaporation techniques are not necessary. SFC, in the same way as LC, can be used to preseparate a sample into classes of compounds where the individual components can then be analyzed and quantified by GC. The supercritical fluid sample effluent is decompressed through a restrictor directly into a capillary GC injection port. In addition, this technique allows selective or multi-step heart-cutting of various sample peaks as they elute from the supercritical fluid... 

Another application of SFC-GC was for the isolation of chrysene, a poly aromatic hydrocarbon, from a complex liquid hydrocarbon industrial sample (24). A 5 p.m octadecyl column (200 cm X 4.6 mm i.d.) was used for the preseparation, followed by GC analysis on an SE-54 column (25 m X 0.2 mm i.d., 0.33 p.m film thickness). The direct analysis of whole samples transferred from the supercritical fluid chromatograph and selective and multi-heart-cutting of a particular region as it elutes from the SFC system was demonstrated. The heart-cutting technique allows the possibility of separating a trace component from a complex mixture (Figure 12.21). 

Specifically for triazines in water, multi-residue methods incorporating SPE and LC/MS/MS will soon be available that are capable of measuring numerous parent compounds and all their relevant degradates (including the hydroxytriazines) in one analysis. Continued increases in liquid chromatography/atmospheric pressure ionization tandem mass spectrometry (LC/API-MS/MS) sensitivity will lead to methods requiring no aqueous sample preparation at all, and portions of water samples will be injected directly into the LC column. The use of SPE and GC or LC coupled with MS and MS/MS systems will also be applied routinely to the analysis of more complex sample matrices such as soil and crop and animal tissues. However, the analyte(s) must first be removed from the sample matrix, and additional research is needed to develop more efficient extraction procedures. Increased selectivity during extraction also simplifies the sample purification requirements prior to injection. Certainly, miniaturization of all aspects of the analysis (sample extraction, purification, and instrumentation) will continue, and some of this may involve SEE, subcritical and microwave extraction, sonication, others or even combinations of these techniques for the initial isolation of the analyte(s) from the bulk of the sample matrix. 

The significant intrinsic limitation of SEC is the dependence of retention volumes of polymer species on their molecular sizes in solution and thus only indirectly on their molar masses. As known (Sections 16.2.2 and 16.3.2), the size of macromolecnles dissolved in certain solvent depends not only on their molar masses but also on their chemical structure and physical architecture. Consequently, the Vr values of polymer species directly reflect their molar masses only for linear homopolymers and this holds only in absence of side effects within SEC column (Sections 16.4.1 and 16.4.2). In other words, macromolecnles of different molar masses, compositions and architectures may co-elute and in that case the molar mass values directly calculated from the SEC chromatograms would be wrong. This is schematically depicted in Figure 16.10. The problem of simultaneous effects of two or more molecular characteristics on the retention volumes of complex polymer systems is further amplifled by the detection problems (Section 16.9.1) the detector response may not reflect the actual sample concentration. This is the reason why the molar masses of complex polymers directly determined by SEC are only semi-quantitative, reflecting the tendencies rather than the absolute values. To obtain the quantitative molar mass data of complex polymer systems, the coupled (Section 16.5) and two (or multi-) dimensional (Section 16.7) polymer HPLC techniques must be engaged. 

Unstable radionuclei result on subjecting the nuclei of some elements to neutron bombardment. During the decay process, in which the radionuclei return to more stable forms, characteristic radiation is emitted. The energy of the radiation is characteristic of the element, and its intensity forms the basis for quantitative elemental analysis. The advantages of NAA for trace analysis include low detection limits, good sensitivity, multi-element capability and relative freedom from matrix effects. However, for successful application of this technique skilled personel are required and because of the low sample throughput the amount of work involved in the analysis of column fractions, for example, is prohibitively high. In addition, it may take up to several weeks before the results are available. Further, only few laboratories have easy access to a neutron source. 

The idea of chromatographic CF is the use of timely chromatographic apparatus and technique for this purpose. Basically, it is the application towards copolymer investigation of what is known among chromatographers as multi-dimensional or coupled column chromatography. 

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