Resolution, influencing factors

Table 17-4 summarizes the qualitative influence of changing the gradient parameters on resolution, retention factor, and run time. The bold rows of Table 17-4 offer the best approach to reducing analysis time and will be discussed in more detail. The nonbold rows represent parameters that should be optimized during the initial stages of method development to optimize resolution. 

Image quality in CT is determined both by spatial resolution and contrast. It is the tube current (mA) that primarily affects spatial resolution, whilst the peak tube potential (kVp) affects both spatial and contrast resolution. Other factors influencing spatial resolution - the ability to observe small details - include in the imaging plane the collimation (in SDCT) - or section collimation (in MDCT) and the focal spot size of the tube. The slice (section) thickness and the choice of pitch influence spatial resolution along the Z-(long) axis of the patient - thinner slices and a lower pitch leading to an improvement in spatial resolution (Huda et al. 2002). 

Equations 12.21 and 12.22 contain terms corresponding to column efficiency, column selectivity, and capacity factor. These terms can be varied, more or less independently, to obtain the desired resolution and analysis time for a pair of solutes. The first term, which is a function of the number of theoretical plates or the height of a theoretical plate, accounts for the effect of column efficiency. The second term is a function of a and accounts for the influence of column selectivity. Finally, the third term in both equations is a function of b, and accounts for the effect of solute B s capacity factor. Manipulating these parameters to improve resolution is the subject of the remainder of this section. 

Depth resolution in NRA is influenced by a number of factors. These include energy loss per unit depth in the material, straggling effects as the ions travel through the sample, and the energy resolution of the detection system. 

As described above, resolution can be improved by variations in plate number, selectivity or capacity factor. However, when considering the separation of a mixture which contains several components of different retention rates, the adjustment of the capacity factors has a limited influence on resolution. The retention times for the last eluted peaks can be excessive, and in some cases strongly retained sample components would not be eluted at all. 

A particular problem with GRAFA and RBL is the reproducibility of the retention data. The retention time axes should be perfectly synchronized. Small shifts of one time interval (thus the ith spectrum in X, corresponds with the i+lth spectrum in X ) already introduce major errors (> 5%) when the chromatographic resolution is less than 0.6. The results of an extensive study on the influence of these factors on the accuracy of the results obtained by GRAFA and RBL have been reported in Ref. [37]. Although some practical applications have been reported [38,39], the lack of robustness of RBL and GRAFA due to artifacts mentioned above has limited their widespread application in chromatography. 

To a first approximation the three terms in equation (1.46) and (1.47) can be treated as independent variables. For a fixed value of n Figure 1.8 Indicates the influence of the separation factor and capacity factor on the observed resolution, when the separation factor equals 1.0 there is no possibility of any separation. The separation factor is a function of the distribution coefficients of the solutes, that is the thermodynamic properties of the system, and without some... 

Figure 1.8 Influence of varying the separation factor and capacity factor on the observed resolution for two closely spaced peaXs.

The object of a chromatographic separation is to achieve satisfactory resolution of solutes in the minimum time. Resolution is influenced by the capacity factor of the solutes and the selectivity and plate number of the column. 

Table 5 illustrates the effects of emulsion thickness on resolution as indicated by HD (distance from the line within which 50% of all the silver grains from it lie). The data indicate that resolution is improved as emulsion thickness is decreased. If the emulsion thickness exceeds 2 pm, an insignificant quantity of (3-particles (see radioisotopes) will reach the emulsion from most biological specimens. Some factors which influence emulsion thickness include dilution of the emulsion, temperature of the emulsion, temperature and wetness of the slide as well as the temperature and humidity during drying. The reader is referred to Rogers (7) for thorough discussions of each. 

Example The influence of relative slit width on peak shape and resolution is demonstrated on the second isotopic peak of toluene molecular ion, m/z 94 (Fig. 4.25). With the entrance slit at 50 pm and the exit slit at 500 pm the peak is flat-topped (left), because a narrow beam from the entrance sweeps over the wide open detector slit keeping the intensity constant as the scan proceeds until the beam passes over the other edge of the slit. Closing the exit slit to 100 pm increases resolution to 2000 without affecting the peak height (middle), but reduces the peak area by a factor of 4 in accordance with an increase in resolution by the same factor. Further reduction of the exit slit width to 30 pm improves... 

Several CD derivatives (charged and uncharged) are available which should allow the separation of most chiral molecules with at least one of them. However, due to the complexity of chiral recognition mechanisms, the determination of the best selector based on the analyte structure is challenging. Eurthermore, separations using CDs are influenced by numerous factors, so that no general rule can be applied for the successful resolution of enantiomers. ... 

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