Definitions, chromatography-related

Equipment used in GLP studies must be validated for appropriateness. Each piece of equipment must have SOPs for operation, calibration, and routine maintenance. All routine and nonroutine maintenance must be documented. What is the definition of a piece of equipment Any item that can have an impact on the results of an anal5dical procedure. In the typical non-GLP laboratory, records are kept on analytical equipment such as spectrophotometers or gas chromatography units. Under GLP, however, the definition expands to include items such as pipets, thermometers, incubators, refrigerators, and mixing devices, as long as it is possible that the use of the item can affect the outcome of the test. For the non-GLP lab, implementation of this standard will dramatically increase the number of equipment-related SOPs. 

The term A is related to the flow profile of the mobile phase as it traverses the stationary phase. The size of the stationary phase particles, their dimensional distribution, and the uniformity of the packing are responsible for a preferential path and add mainly to the improper exchange of solute between the two phases. This phenomenon is the result of Eddy diffusion or turbulent diffusion, considered to be non-important in liquid chromatography or absent by definition in capillary columns, and WCOT (wall coated open tubular) in gas phase chromatography (Golay s equation without term A, cf. 2.5). 

ODOR. An important property of many substances, manifested by a physiological sensation caused by contact of their molecules with the olfactory nervous system. Odor and flavor are closely related, and both are profoundly affected by submicrogram amounts of volatile compounds. Attempts to correlate odor with chemical structure have produced no definitive results, Objective measurement techniques involving chromatography are under development. Even potent odors must be present in a concentration of 1,7 x I07 molecules/cc to be detected. It has been authentically stated that the nose is 100 times as sensitive in detection of threshold odor values as the best analytical apparatus. 

The most popular method for measuring the polarity of a solute entails determination of the distribution constant between water and a water-immiscible solvent, e.g., octanol. However, because there is difficulty in dissolving proteins in the solvent, a two-phase aqueous system was developed (Shanbhag and Axelson, 1975). Albertson (1986) reported the construction of various aqueous phase systems for partitioning proteins, other macromolecules, and even cells. Recently, simpler aqueous biphase systems were selected for hydrophobic partitioning of proteins (Hachem et al., 1996). However, because of restrictions similar to those for HIC, as discussed above, it may be premature to replace the method used in Basic Protocol 5. The definition of hydrophobicity is based on the polarity of chemical compounds, which is closely related to the distribution between solvents of different polarities. This theory is similar to the elution mechanism of phase distribution chromatography as well as phase partition. However, complexity in the partition system and procedure hampers the broad use of the phase partition approaches. 

Detector sensitivity is best explained in terms of signal to noise ratio, which is the minimum detectable quantity with a signal to noise ratio of two (Willard, 1988). Detector sensitivity is linked to the method detection limit, a concept that we routinely use in environmental project work. (The definitions of detection limits in environmental pollutant analysis are discussed in Chapter 4.5.1.) The MDLs, however, while being related to detector sensitivity, greatly depend on the analytical method, sample matrix, and the analyte itself. In this chapter, we will address detector sensitivity in relative terms by comparing sensitivities of various chromatography detectors. 

The methods of quantitative analysis presented in this chapter are not unique to chromatography, but their principles will be discussed as they relate to chromatographic measurements. They will be preceded by a short section dealing with conventional statistical definitions and terms. 

Unfortunately, there has been considerable confusion and disagreement over the respective units that should be used to define certain detector specifications and in some cases there has been dissension over the exact definition of the specifications themselves. This confusion has arisen largely from the adoption of criteria used in other instrumental devices that have been borrowed for use in detector technology and are sometimes not precisely applicable. In addition, specifications have also been adopted by some manufacturers in order to present their products in the best possible light and which may not be easily translatable into specifications that are pertinent to the chromatographer. In the following discussions on detector specifications, the units that are used have been chosen as those most relevant to chromatography and that are directly related to the needs of the analyst, albeit for use in GC or LC. 

Solvent interaction model for normal-phase liquid chromatography. The solvent-interaction model of Scott and co-workers (Scott and Kucera, 1979) assumes that the analyte partitions between the bulk mobile phase and a layer of solvent absorbed onto the stationary phase. The quantitative description of the relationship between retention and the composition of the mobile phase in the solvent-interaction model requires the definition of the void volume corrected retention volume (V), which is related to the retention volume (F ) and the void volume (Fq) by... 

However, the definitions related to specific processes have to be kept in mind. In chromatography the plate height is a measure that lumps together the contribution of the fluid dynamic non-idealities (axial dispersion) and the mass transfer resistance... 

The confusion that a combination of these different definitions can generate, together with the difficulties encoimtered in the determination of some of the column characteristics involved (particularly the internal and the external porosities) makes useful a careful consideration of these issues. Given the stage of sophistication that the modeling of chromatography has now reached, it is not possible to tolerate errors, confusions, or approximations in the definitions nor in the estimations of the critical parameters related to the porosities, the velocities, and the equilibrium constants, nor to accept that more errors be made in the estimation of these parameters than those that are always involved in any measurement process. 

Solvent strength and selectivity are the properties commonly used to classify liquid stationary phases as selection tools for method development in gas chromatography [29,102-104]. Solvent strength and polarity are often used interchangeably and can cause confusion. Polarity is sometimes considered to be the capacity of a stationary phase for dipole-type interactions alone, while more generally solvent strength is defined as the capacity of a stationary phase for all possible intermolecular interactions. The latter definition is quite sensible but unworkable because there is no substance that is uniquely polar that might be used to probe the polarity of other substances. Indirect measurements of polarity, such as those scales related one way or another to the... 

L4.3.1 Chromatography Although the final detection is based on the mjz ratios, optimized chromatographic separation is crucial to obtain high-quality data since it is directly related to peak definition and subsequent data... 

In Chapter 1, definitions and terms were presented to facilitate the description of the chromatographic system. In this chapter, additional terms are introduced and related to the basic theory of chromatography. Please refer to Table 1.1 in Chapter 1 for a listing of some of the symbols. Make special note of those that are recommended by the lUPAC they are the ones used in this book. 

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