Liquid Chromatography-Infrared Spectroscopy Techniques

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. 

Solvent-elimination approaches include evaporative spray deposition onto infrared-transparent surfaces (141) or reflective surfaces and powders (142, 143). Other approaches include partial evaporation of the mobile phase before spray deposition (144, 145), and continuous liquid-liquid extraction systems that transfer solutes from LC mobile phases to solvents possessing an infrared window (146). Spray systems include both pneumatic and ultrasonic nozzles (147). 

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 

Despite the distinct advantages of pneumatic nebulizers, ultrasonic nebulizers may alternatively be used, in some instances, with success. In a recent application, a variation of ultrasonic nebulizer called spray nozzle-rotating disk FTIR interface was successfully applied to confirm the presence of methyltestosterone, testosterone, fluoxymesterone, epitestosterone, and estradiol and testosterone cyp-ionate in urine, after solid-phase extraction and reversed-phase LC separation (151). Using a commercial infrared microscopy spectrometer, usable spectra from 5 ng steroid deposits could be readily obtained. To achieve success with this interface, phosphate buffers in the mobile phase were not used because these nonvolatile salts accumulate on the collection disk and their spectra tend to swamp out small mass deposits. Another limitation of the method was that only nonvolatile analytes could be analyzed because volatile compounds simply evaporated off the collection-disk surface prior to scanning. 

Interface type LC flow-rate ( l/min) Substrate Infrared mode Identification limit Ref. 

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High performance liquid chromatography, infrared spectroscopy, UV and visible spectrophotometry, and polarography are some of the other major analytical techniques used to determine many diverse classes of compounds. 

In addition, before the routine use of instrumentation such as gas-liquid chromatography, infrared spectroscopy, nuclear magnetic resonance spectrometry, etc., the identity of components and compositions were determined by isolation. The latter involved the problems of separating compounds of closely related properties. This often explains conflicting and inaccurate data published in the literature. The modern techniques of determining compositions have resolved some of these problems. 

Other combinations of chromatography techniques with MS which may be useful in environmental studies are the coupling of high performance liquid chromatography (LC) with MS [84,384,504,506,530,585-593],LC with MS-MS [181, 594 - 599], LC with atmospheric pressure chemical ionization MS (LC-APCI-MS) [600], and Fourier transform infrared spectroscopy-fast atom bombardment coupled to LC-MS (FTIR-FAB-LC-MS) [514]. 

Modern analytical techniques have been developed for complete characterization and evaluation of a wide variety of sulfonic acids and sulfonates. Titration is the most straightforward method of evaluating sulfonic acids. Spectroscopic methods for sulfonic acid analysis include ultraviolet spectroscopy, infrared spectroscopy, and lH and l3C nmr spectroscopy. Modem separation techniques of sulfonates include liquid chromatography and ion chromatography. See also Chromatography. 

The main spectrometric identification techniques employed are gas chromatography/mass spectrometry (GC/MS) (13), liquid chromatography/tandem mass spectrometry (LC/MS(/MS)) (14), nuclear magnetic resonance (NMR) (11), and/or gas chromatography/Fourier transform infrared spectroscopy (GC/FL1R) (15). Each of these spectrometric techniques provides a spectrum that is characteristic of a chemical. MS and NMR spectra provide (detailed) structural information (like a fingerprint ), whereas an FUR spectrum provides information on functional groups. 

Mass spectrometry (MS), infrared (IR) spectroscopy, and nuclear magnetic resonance (NMR) spectroscopy with their numerous applications are the main instrumental techniques for the detection and identification of CWC-related chemicals. During the last few years, however, less laborious techniques such as liquid chromatography (LC) and capillary electrophoresis (CE) have become attractive for the analysis of water samples and extracts where sample preparation is either not required or is relatively simple. 

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