Direct stop-flow sample presentation

Frequently industrial hygiene analyses require the identification of unknown sample components. One of the most widely employed methods for this purpose is coupled gas chromatography/ mass spectrometry (GC/MS). With respect to interface with mass spectrometry, HPLC presently suffers a disadvantage in comparison to GC because instrumentation for routine application of HPLC/MS techniques is not available in many analytical chemistry laboratories (3). It is, however, anticipated that HPLC/MS systems will be more readily available in the future ( 5, 6, 1, 8). HPLC will then become an even more powerful analytical tool for use in occupational health chemistry. It is also important to note that conventional HPLC is presently adaptable to effective compound identification procedures other than direct mass spectrometry interface. These include relatively simple procedures for the recovery of sample components from column eluate as well as stop-flow techniques. Following recovery, a separated sample component may be subjected to, for example, direct probe mass spectrometry infra-red (IR), ultraviolet (UV), and visible spectrophotometry and fluorescence spectroscopy. The stopped flow technique may be used to obtain a fluorescence or a UV absorbance spectrum of a particular component as it elutes from the column. Such spectra can frequently be used to determine specific properties of the component for assistance in compound identification (9). 

Recently, NMR spectrometers directly coupled with LC systems have become commercially available. Spectra can be acquired in either of two modes, continuous or stopped flow. In continuous flow mode the spectrum is acquired as the analyte flows through the cell. This method suffers from low sensitivity since the analyte may be present in the cell for only a brief period of time, but it has the advantage of continuous monitoring of the LC peaks without interruption. Fig. 12A shows a contour plot of the continuous flow NMR analysis of a mixture of vitamin A acetate isomers.Fig. 12B shows the spectra taken from slices through the contour plot. These plots highlight the olefinic region of the spectra which provided ample information for the identification of each of the isomers. With very limited sample quantities, the more common method of LC-NMR analysis is stopped flow. Here the analyte peak is parked in the flow cell so any of the standard NMR experiments can be run. 

More recently, another multicommutated flow method was developed for the simultaneous determination of tartaric acid and potassium as a tool for evaluating the tartrate stability of wines (Oliveira et al., 2010). This system also resorted to inline dialysis of the samples in order to minimize matrix interferences for tartaric acid determination. A detailed study with different configurations of the dialysis unit was presented and various membrane materials were compared. The continuous-flow dialysis process was optimized for acceptor and donor channel flow rates, flow directions, and stop flow periods. 

The end point of the reaction is conveniently determined electrometrically using the dead-stop end point procedure. If a small e.m.f. is applied across two platinum electrodes immersed in the reaction mixture a current will flow as long as free iodine is present, to remove hydrogen and depolarise the cathode. When the last trace of iodine has reacted the current will decrease to zero or very close to zero. Conversely, the technique may be combined with a direct titration of the sample with the Karl Fischer reagent here the current in the electrode circuit suddenly increases at the first appearance of unused iodine in the solution. 

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