Chromatography optimizing speed

Others have examined the necessary parameters that should be optimized to make the two-dimensional separation operate within the context of the columns that are chosen for the unique separation applications that are being developed. This is true for most of the applications shown in this book. However, one of the common themes here is that it is often necessary to slow down the first-dimension separation system in a 2DLC system. If one does not slow down the first dimension, another approach is to speed up the second dimension so that the whole analysis is not gated by the time of the second dimension. Recently, this has been the motivation behind the very fast second-dimension systems, such as Carr and coworker s fast gradient reversed-phase liquid chromatography (RPLC) second dimension systems, which operate at elevated temperatures (Stoll et al., 2006, 2007). Having a fast second dimension makes CE an attractive technique, especially with fast gating methods, which are discussed in Chapter 5. However, these are specialized for specific applications and may require method development techniques specific to CE. 

To identify a compound, five data points per peak may be sufficient. Quantitation may require at least 10 data points across a peak. Many of today s laboratories still house standard detectors (UV, ELSD, fluorescence, etc.) with maximum data acquisition rates at or below 20 Hz. Many conventional LC/MS methods acquire data at rates of 5 Hz or less. As shown in Figure 3.8, this is not sufficient for modem speed optimized chromatography. Obviously, selecting the wrong data acquisition rate will nullify all attempts to optimize chromatography. 

Solvents, UV cut-olf values, 70 Solvents, miscibility, 75 Solvophobic effect, 201,203 Solvophobic inleHlclidHk, IS2, 20i Solvophobic ion chromatography, 242 Solvophobic theory, 141,148,152,155, 158, 202, 203, 226, 228, 246 8omatostedn, 263,290 Sorbents, polymeric, 127 Sorption isottom, 159 Soiption kineties, efbet on column effi-cieney in RPC, 227 Speed of aepantion, optimization [Pg.172]

J. D. Thompson and P. W. Carr, High-Speed Liquid Chromatography by Simultaneous Optimization of Temperature and Eluent Composition, AnaL Chem. 2002, 74, 4150. 

In addition to HPLC, microchips have also been used in other modalities of liquid chromatography, including capillary electrochromatography and mi cel -lar electrokinetic chromatography. Many workers have attempted to achieve nano separations at high speed of different molecules with high efficiency, reproducibility, and low detection limits. The state of the art of separation in these modalities is discussed in this chapter, with special emphasis on their applications, optimization, and mechanisms of separation. 

Insofar as liquid solvents are concerned, the most important factor governing / is viscosity 77, as the Stokes equation clearly demonstrates. Therefore any systematic effort to increase separation speed requires close attention to viscosity, with an emphasis on finding solvents and conditions for which viscosities are minimal. The reduction of viscosity can be pursued systematically in place of a hit or miss search for low viscosity solvents. The approach below was developed by the author and his colleagues [32] for use in optimizing size-exclusion chromatography, but the conclusions are generally applicable to separations. 

The aim of any chromatographic separation may be defined as the achievement of an optimal combination of speed of elution, sample size, and resolution of the solutes. Good resolution can only be obtained if there is adequate control over the differential migration rates of a group of solutes as they move down a column (column selectivity) and over the extent of zone dispersion for each of the solutes (column efiiciency). Historically, the various modes of liquid chromatography have been considered as separate and independent phenomena. It is now clear that they all have a common theoretical basis. Column selectivity in HPLC, irrespective of the mode, arises due to differences in the distribution equilibria... 

K. Matsuda, S. Matsuda, and Y. Ito, Toroidal coil counter-current chromatography. Achievement of high resolution by optimizing flow-rate, rotation speed, sample volume and tube length, /. Chromatogr. A 808 95-104 (1998). 

The scan speed, the pulse amplitude in DPV was optimized and an inhibition calibration curve was obtained using carbofuran as reference pesticide. The linear range was observed to be 1 nM to 1 pM. A lowest detection limit of 9 nM has been achieved using this AChE immobilized tetracyanoquinodimethane (TCNQ)/NF modified SPE. In the case of carbofuran contaminated water samples, the detection limit was observed to be 20 nM and the developed sensor has a good analytical performance in comparison with the results obtained by standard methods using gas chromatography coupled to a Finningan Mat 800 ion trap detector mass spectrometer (GC-ITDMS). 

For both GC and LC, the efficiency of a chromatographic system is optimal at intermediate flow velocities. Optimal performance is usually not obtained in practice because of the emphasis on separation speed, which requires the use of greater than optimal flow rates. Theoretical considerations of the thermodynamic and kinetic aspects of chromatography led to the development of HPLC and capfllary GC, both of which possess the speed necessary for clinical analyses. 

In terms of theory, the types of stationary and mobile phases, and applications, thin-layer and liquid chromatography are remarkably similar. In fact, thin-layer plates can be profitably used to develop optimal conditions for separations by column liquid chromatography. The advantages of following this procedure are the speed and low cost of the exploratory thin-layer experiments. Some chromatogra-phers have taken the position that thin-layer experiments should always precede column experiments. 

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