Solvent elimination interface

Consequently, the major experimental options for the analyst are packed or capillary SFC, mobile phase with/without modifier and off-line or on-line mode, namely direct deposition (DD-SFC-FUR) vs. flowcell. Both small-bore packed columns and narrow-bore open-tubular columns have been used for SFC-FTIR analysis using a pressure-stable, thermostated, flow-cell or solvent elimination interfaces. 

The solvent-elimination interface offers various advantages over the flow-cell approach ... 

LC-FHR has been reviewed [204,507]. Various mobile-phase elimination designs were discussed by White [167]. Resolution of complex LC-FTIR spectroscopy data was described [508]. A general overview of flow-cell based IR detection and of early solvent-elimination interfaces for LC-FHR has recently appeared solvent-elimination RPLC-FHR interfaces have also been described [500]. 

Since the major part of LC involves reversed-phase separations, more recent work in the field of LC-FTIR has concentrated on the development of interfaces suitable for the elimination of aqueous eluents. Solvent-elimination interfaces with which the eluent is eliminated prior to IR detection have shown to be much more versatile and to yield interference-free spectral information for considerably smaller amounts of analytes, which is the primary objective of LC-FTIR. 

Attributes of commercially available reversed-phase LC-FTIR systems in which the column effluent is either sent directly to the solvent elimination interface or is mixed online with a reagent, prior to solvent evaporation, to facilitate... 

Fujimoto et al [24] used a packed capillary column with 10% methanol in hexane as the mobile phase for analysing silicones. In this case, the column was coupled to a solvent elimination interface enabling the effluent from the column to be deposited on to crystals of potassium bromide for infrared analysis. With this method, several peaks were distinguished as cyclic rather than linear polysiloxanes. 

Figure 5 Schematic representation of a solvent-elimination interface. (A) Side-view during solvent-elimination and analyte deposition (B) Top-view from analyte deposits on substrate. (Reprinted with permission from Gagel JJ and Biemann K (1986) Analytical Chemistry 58(11) 2184-2189 86 American Chemical Society.

Table 3 Typical characteristics of flow cell and solvent-elimination Interfaces for HPLC...

The coupling of LC techniques with FTIR is an important complementary technique. Because of the robustness, the simplicity and the cost effectiveness, LC-FTIR is the method of choice in most applications. The hyphenation of LC and FTIR can be realized in two ways (1) on-line mode via a flow cell [98-101] and (2) off-line mode via a solvent elimination interface. The typical characteristics of the two approaches are summarized in Table 3 [102]. 

Both LC-ETIR and LC-NMR can be applied in combination with solvent gradients. In both cases there are some complicating factors. In the case of FTIR, only solvent elimination interfaces can realistically be used. This implies that the effluent from the LC is sprayed into a (heated) evaporation chamber and that the nonvolatile analyte polymers are deposited on a suitable substrate (for example, a germanium disc). By moving the spray or the substrate, the entire chromatogram can be recorded. Some authors have programmed the deposition conditions to obtain optimal results for gradient elution LC-FTIR. However, as was mentioned ear-... 

Table 4 Typical characteristics of SEC-FIR online flow cell and off-line solvent elimination interfaces...

Condition Fiow ceii interface Solvent elimination interface... 

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