Liquid Mobile Phases

SERS has also been applied as a sensitive, molecule-specific detection method in chromatography, e.g. thin layer, liquid, and gas chromatography. SERS-active colloids were deposited on the thin layer plates or mixed continuously with the liquid mobile phases. After adsorption of the analytes, characteristic spectra of the fractions were obtained and enabled unambiguous identification of very small amounts of substance. 

Consider the heat balance of the cell. However, as opposed to the GC column, the heat capacity of the liquid mobile phase in an in LC column is relatively large. Consequently, heat convected from the cell by the mobile phase must also be taken into account. It follows that... 

A liquid mobile phase is far denser than a gas and, therefore, carries more momentum. Thus, in its progress through the interstices of the packing, violent eddies are formed in the inter-particular spaces which provides rapid solute transfer and, in effect, greatly increases the effective diffusivity. Thus, the resistance to mass transfer in that mobile phase which is situated in the interstices of the column is virtually zero. However, assuming the particles of packing are porous (i.e., silica based) the particles of packing will be filled with the mobile phase and so there will... 

The main product, benzene, is represented by solute (B), and the high boiling aromatics are represented by solute (C) (toluene and xylenes). The analysis of the products they obtained are shown in Figure 12. The material stripped form the top section (section (1)) is seen to contain the alkanes, alkenes and naphthenes and very little benzene. The product stripped from the center section appears to be virtually pure benzene. The product from section (3) contained toluene, the xylenes and thiophen which elutes close to benzene. The thiophen, however, was only eliminated at the expense of some loss of benzene to the lower stripping section. Although the system works well it proved experimentally difficult to set up and maintain under constant operating conditions. The problems arose largely from the need to adjust the pressures that must prevent cross-flow. The system as described would be virtually impossible to operate with a liquid mobile phase. 

The statement made by Martin in 1941 contains all the necessary conditions to realize both the high efficiencies and high resolution achieved by modern LC columns. Despite his recommendations, however, it has taken nearly fifty years to bring his concepts to fruition. In the same paper Martin and Synge suggested that it would be advantageous to replace the liquid mobile phase by a gas to improve the rate of transfer between the phases and thus, enhance the separation. The recommendation was not heeded and it was left to James and Martin (5) to bring the concept to practical reality in the... 

Before considering these in detail, it is necessary to revisit the inherent incompatibilities between mass spectrometry and liqnid chromatography. These are, as discussed previously, that HPLC utilizes a liquid mobile phase, often containing significant amounts of water, flowing typically at 1 mlmin while the mass spectrometer must be maintained under conditions of high vacuum, i.e. around 10- torr (1.333 22 x IQ- Pa). 

Detection in SFC can be achieved in the condensed phase using optical detectors similar to those used in liquid chromatography or in the gas phase using detectors similar to those used in gas chromatography. Spectroscopic detectors, such as mass spectrometry and Fourier transform infrared spectroscopy, are relatively easily interfaced to SFC compared to the problems observed with liquid mobile phases (see Chapter 9). The range of available detectors for SFC is considered one of its strengths. 

Lee, S. T. and Olesik, S. V., Normal-phase high-performance liquid chromatography using enhanced fluidity liquid mobile phases, /. Chromatogr. A, 707, 217, 1995. 

Competition between liquid mobile phase and solid adsorbent... 

In lc there are other sorption mechanisms that can cause separation, depending on whether we choose to use a liquid or a solid as the stationary phase, or what kind of solid we use. Liquid-liquid chromatography (11c) uses a liquid stationary phase coated onto a finely divided inert solid support. Separation here is due to differences in the partition coefficients of solutes between the stationary liquid and the liquid mobile phase. In normal phase 11c the stationary phase is relatively polar and the mobile phase relatively non-polar, whilst... 

Some, uPLC systems are equipped with UV absorbance detection, and other systems allow for both UV absorbance and fluorescence detection. Fluorescence detection increases the sensitivity and selectivity of certain applications and is the method of choice in many separation-based assays. The liquid (mobile phase + sample) leaving the individual flow cells designated for UV detection is transferred through capillaries to a bank of 24 flow cells designated for fluorescence detection. 

Equipment for HPLC is slightly more complicated (see Figure 2.18), due to the need to handle a liquid mobile phase. This requires the presence of one or... 

Preparative chromatographic processes are of increasing importance particularly in the production of fine chemicals. A mixture of compounds is introduced into the liquid mobile phase, and this then flows through a packed column containing the stationary solid phase. The contacting scheme is thus differential, but since the adsorption characteristics of the compounds in the mixture are similar, many equivalent theoretical stages are required for their separation. Chromatographic processes are mostly ran under transient conditions, such that... 

In liquid-solid and liquid-liquid systems, manipulation of mobile phase properties is the dominant approach. Mixtures of poor and good solvents for the types of molecules being separated are prepared and retention observed. There are fewer stationary phases for liquid mobile phase systems for this reason and at least one other. 

It may be difficult to imagine a liquid mobile phase used with a liquid stationary phase. What experimental setup allows one liquid to move through another liquid (immiscible in the first) and how can one expect partitioning of the mixture components to occur The stationary phase actually consists of a thin liquid film chemically bonded to the surface of finely divided solid particles, as shown in Figure 11.8. It is often referred to as bonded phase chromatography (BPC). Such a stationary phase cannot be removed from the solid substrate by heat, reaction, or dissolving in the mobile phase. 

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