Chromatographic system capacity factor

The curves show that the peak capacity increases with the column efficiency, which is much as one would expect, however the major factor that influences peak capacity is clearly the capacity ratio of the last eluted peak. It follows that any aspect of the chromatographic system that might limit the value of (k ) for the last peak will also limit the peak capacity. Davis and Giddings [15] have pointed out that the theoretical peak capacity is an exaggerated value of the true peak capacity. They claim that the individual (k ) values for each solute in a realistic multi-component mixture will have a statistically irregular distribution. As they very adroitly point out, the solutes in a real sample do not array themselves conveniently along the chromatogram four standard deviations apart to provide the maximum peak capacity. 

A general approach to the problem of identification, should more definitive detectors not be available, is to change the chromatographic system , which in the case of HPLC is usually the mobile phase, and redetermine the retention parameter. The change obtained is often more characteristic of a single analyte than is the capacity factor with either of the mobile phases. 

Many chromatographic systems show linear relationships between the logarithm of the capacity factor and the reciprocal of the column temperature (van t Hoff plots) [255,258-261]. In thermodynamic terms the interaction of the solute with the stationary phase can be described by... 

Standards and blanks are the usual controls used in analytical HPLC. Standards are usually interspersed with samples to demonstrate system performance over the course of a batch run. The successful run of standards before beginning analysis demonstrates that the system is suitable to use. In this way, no samples are run until the system is working well. Typically, standards are used to calculate column plate heights, capacity factors, and relative response factors. If day-to-day variability has been established by validation, the chromatographic system can be demonstrated to be within established control limits. One characteristic of good science is that samples... 

The retention time tj is a non-linear function of the ring size n and thus allows the identification of new species of the homologous series S by interpolation. Furthermore, the logarithm of the capacity factor k = (t — (with = death time of the chromatographic system) is a linear function of the ring size, n, although two such functions are obtained see Fig. 7... 

To compare different chromatographic systems, however, it is more useful to use the relative retention time (also called the capacity factor, k-). This parameter is defined as the retention of the compound relative to a nonretained chemical species, such as a very polar organic compound or an inorganic species such as nitrate ... 

The resolution achievable in any chromatographic system is proportional to the capacity, efficiency, and selectivity of the system. Each of these factors must be considered and controlled to achieve success. The theoretical expression for resolution is Equation B4.2.2, where k is the average capacity factor for the two peaks, N is the efficiency factor for the system, and a is the selectivity factor of the medium. 

The separation of safflower oil (SFO)-linseed oil (LSO) methyl esters is shown in Fig. 16. Free fatty acid methyl ester elution reproducibility, resolution, and baseline stability were maintained at sample sizes of 17-170 /zg, although capacity factors (k) decreased approximately 25% between the 17- and 170-/zg sample sizes. The trend of longer retention times with smaller sample sizes was consistent throughout their studies. Peak distortion, such as observed when gas chromatographic columns are overloaded, was not observed in their system. Perhaps larger FAME samples compete for silver ion sites the same way the ACN cosolvent competes for those sites. Excellent peak shapes were obtained, even with sample elution times of 1.5-2.0 h. 

Chromatographic system (see Chromatography, in the general procedure (621)) The liquid chromatography is equipped with a 280-nm detector and a 4.6 mm x 15-cm column that contains 5-/im packing L7. The flow-rate is about 0.8 ml/min. Chromatograph the System suitability solution, and record the peak responses as directed for Procedure the capacity factor, k , is not less than 6 the column efficiency is not less than 3000 theoretical plates the tailing factor is not more than 1.5 and the relative standard deviation (RSD) for replicate injections is not more than 1%. 

After a particular chromatographic technique has been selected a chromatogram should be recorded. This is not always straightforward, since recording a chromatogram requires that the components in the sample show reasonable retention times (capacity factors). Often, therefore, we have to adjust the chromatographic system in order to get all sample components to appear as a peak (but not necessarily a well-resolved one) in the... 

In a chromatographic system, the capacity factor will again depend on the distribution coefficient for the solute X, which reads for the example in which the aqueous phase is the mobile one ... 

The selectivity a of all chromatographic modes can be defined as the relative separation achieved between adjacent solute peaks and thus reflects the overall performance in relative selectivity of a chromatographic system. In particular, selectivity a is given by the ratio of capacity factors for adjacent peaks Pj and , i.e.,... 

Equations (105) and (106) provide an important linkage between the three essential parameters that dictate the overall quality of the chromatographic resolution, namely, the relative retention, expressed in terms of the capacity factor k the relative selectivity a, and the extent of peak dispersion Nt or he i. Higher system performances and thus larger values of Rs, per unit time... 

In this context, the capacity factor k = (/r — /o)//o is important to is the dead time of the chromatographic system, roughly the time the pure eluent needs to travel from the injector to the detector. It may be determined by injection of a methanoEwater mixture since H2O does not show any significant retention in a C18 column. For cyclic and acycUc organic polysulfanes, it has been found that the logarithm of the capacity factor is a linear function of the number of sulfur atoms (equation 118). 

The retention times and capacity factors depend, of course, on the chromatographic system and its operating... 

Hammers, W.E., Meurs, G.J., DeLigny, C.L. (1982) Correlations between hquid chromatographic capacity factors ratio data on Lichrosorb RP-18 and partition coefficients in the octanol-water system. J. Chromatogr. 247, 1-13. 

The most important detector specification is probably detector sensitivity as it not only defines the minimum concentration of solute that can be detected but also allows the overall mass sensitivity of the chromatographic system to be calculated. The detector sensitivity also places a limit on the maximum (k ) (capacity factor) at which a solute can be eluted from a chromatographic column. In order to calculate the mass sensitivity or the maximum (k ) value, the detector sensitivity must be available in concentration units, e.g. g/ml. Moreover, if all detector sensitivities were given in units of g/ml, then all detecting devices, functioning on quite different principles, could then be rationally compared. 

Thus (m ), the mass sensitivity of the chromatographic system depends on the detector sensitivity, column dimensions, column efficiency and the capacity factor of the eluted solute. However, irrespective of the column properties, the mass sensitivity is still directly related to the detector sensitivity. It will also be seen that the column radius will depend on the extracolumn dispersion, much of which arises from the detector connecting tubes and sensor. It follows that the design of the detector and its sensitivity has a major influence on the mass sensitivity of the overall chromatographic system. 

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