Solvent elimination LC-FTIR

In solvent-elimination LC-FTIR, basically three types of substrates and corresponding IR modes can be discerned, namely, powder substrates for diffuse reflectance (DRIFT) detection, metallic mirrors for reflection-absorption (R-A) spectrometry, and IR-transparent windows for transmission measurements [500]. The most favourable solvent-elimination LC-FTIR results have been obtained with IR-transparent deposition substrates that allow straightforward transmission measurements. Analyte morphology and/or transformation should always be taken into consideration during the interpretation of spectra obtained by solvent-elimination LC-FTIR. Dependent on the type of substrate and/or size of the deposited spots, often special optics such as a (diffuse) reflectance unit, a beam condenser or an FITR microscope are used to scan the deposited substances (typical diameter of the FITR beam, 20 pm). 

Deposition of analytes in solvent-elimination LC-FTIR is performed on powdered substrates, mirrors, or IR-transparent windows. Correspondingly, diffuse reflection Fourier-transform infrared (DRIFT) detection, reflection-absorption (R-A) spectroscopy, or transmission measurements are applied to investigate the analyte deposits. 

Hgure 7 FTIR spectra of isomeric chloropyrenes recorded during solvent-elimination LC-FTIR of a chlorinated pyrene sample. Based on the spectral data (from the top downward) the isomers could be identified as 1,6-dichloropyrene, 1,8-di-chloropyrene, and 1,3-dichloropyrene, respectively. 

Next to pneumatic nebulization, ultrasonic ne-bulization can be applied for solvent-elimination LC-FTIR. The eluent spray is now formed by disrupting the liquid surface at ultrasonic frequencies. Carrier gas can be used to enhance eluent evaporation and to focus the spray toward the deposition substrate. A further increase in the evaporation capacity is accomplished by placing the ultrasonic nebulizer and substrate in a vacuum chamber. Such a system is suitable for the evaporation of high-boiling eluents at relatively high flow rates (100-200 pi min ). Various manufacturers have commercialized ultrasonic nebulizers for LC-FTIR Lab Connections offers the LC Transform 300 Series... 

The choice for the type of LC-FTIR coupling (flow cell or solvent-elimination) depends on the particular application of the user, where aspects such as type of spectral information needed, required sensitivity, and ease of use are main criteria. Flow-cell LC-FTIR is relatively simple and straightforward, and has developed into a special-purpose technique that can be used in a routine fashion for the monitoring of major mixture constituents with specific functional groups. Solvent-elimination LC-FTIR is somewhat more complicated requiring (sometimes complex) evaporation interfaces, but allows characterization of minor sample components with a high level of confidence. At present, the practical application of FTIR detection in LC is still quite limited. 

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]. 

Temperature-programmed packed capillary liquid chromatography (TP-CLC), coupled off-line to solvent elimination FTIR (LC Transform ) has recently been used for gradient separations of Irganox 1010/1076/3114 dissolved in DMF with LOD of about 40 ng [511]. Low... 

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... 

The solvent elimination approach is better than the one above as full spectral information from the analytes can be obtained and the chromatographic conditions do not have to be modified as much. In practice, the elnting compounds are deposited onto potassium bromide pellets, the mobile phase is evaporated and the pellets are then transferred to the FTIR for spectral analysis by diffuse reflectance. Hence the solvents nsed in the mobile phase must be more volatile than the analytes being smdied. A microdispenser for interfacing LC to IR or Raman which uses the solvent elimination approach has been reported . 

Additives in polymers were identified by solvent-elimination based coupling of reversed-phase column liquid chromatography(LC) and FTIR spectrometry. A spray-jet interface was used to deposit the effluent from a narrow-bore LC column on a zinc selenide window. The deposited additives were analysed by FTIR transmission microscopy, yielding identification limits in the low-nanogram range. High-quality IR spectra were obtained for components present in PVC and PP samples. 

The strong IR absorption of most eluents reduces the attainable detection Emits in flow-cell FTIR and has directed LC-FTIR research toward a solvent-elimination approach in which the eluent is removed prior to detection. To accomplish this, generally the eluent is directed to a nebulizer, often aided with (heated) nebulizer gas, to achieve solvent evaporation. Almost simultaneously, the separated analytes are deposited (immobilized) on a moving substrate to collect the analytes individually and retain the chromatographic integrity. After deposition, IR spectra from the immobilized chromatogram are acquired. Dependent on the type of substrate used (see below) and/or size of the deposited spots, for detection often special optics, such as a (diffuse) reflection unit, a beam condense or an IR microscope, are used. 

In order to permit higher (aqueous) flow rates (> 5 pi min ) in LC-FTIR, use of interfaces with an enhanced evaporation capacity are essential. As effective solvent elimination is an important issue as... 

With pneumatic nebulization for LC—FTIR, optimum mass sensitivity is achieved when microbore-LC (typical flow rate, 20-50 pi min ) is used in combination with a ZnSe deposition substrate and IR microscopy for detection. With such systems it is possible to acquire full spectra from 1 to 10 ng of analyte. In order to achieve complete evaporation of 100% aqueous eluents, enhanced solvent elimination power is required. One solution to this problem is the placement of the nebulizer inside a vacuum chamber to facilitate the evaporation of water. Another option is the online liquid-liquid extraction of the LC eluent... 

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-... 

---