nbsp;   The Peak Capacity of a Chromatographic Column The peak capacity of a column has been defined as the number of peaks that can be fitted into a chromatogram between the dead point and the 'last peak', each peak being separated from its neighbor by 4s. The 'last peak' of  chromatogram is vague term because it depends somewhat on a number of unrelated factors such as the detector sensitivity and the column efficiency. As a result, the 'last peak' can be either arbitrarily specified or defined by the properties of the column and/or the chromatograph with which it is used. Limited peak capacity can be a serious problem in the analysis of multi-component mixtures if the capacity of the chromatogram is insufficient to contain all the peaks discretely. Isocratic development results in the early peaks being adequately separated and the late peaks very broad and eluted at concentrations is so low that they can hardly be detected.

nbsp; The curves show that the peak capacity increases with the column efficiency, as one would expect, but the major factor that influences peak capacity is the capacity ratio of the last peak. Thus, 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 (28) have pointed out that the theoretical peak capacity is an exaggerated value of the true peak capacity. The individual (k') values for each solute in a realistic multi-component mixture will have a statistically irregular distribution. They very adroitly point out, that the solutes in real samples do not array themselves conveniently along the chromatogram four standard deviations apart to provide the maximum peak capacity. Nevertheless, the theoretical peak capacity values given by equation (77) can be used as a reasonable practical

peak height (h) is the distance between the peak maximum and the base line geometrically produced beneath the peak. The peak width (w) is the distance between each side of a peak measure at 0.6065 of the peak height (ca 0.607h). The peak width measured at this height is equivalent to two standard deviations (2s) of the Gaussian curve and thus has significance when dealing with chromatography theory. The peak width at half height (w0.5) is the distance between each side of a peak measured at half the peak height. The peak width measured at half height has no significance with respect to chromatography theory. The peak width at the base (wB) is the distance between the intersections of the tangents drawn to the sides of the peak and the peak base geometrically produced. The peak width at the base is equivalent to four standard deviations (4s) of the Gaussian curve and thus also has significance when dealing with chromatography theory. Factors

phase was ethanol and the flow rate 100 ml/minute. The sample load was 400 mg dissolved in 5 ml of ethanol. It is seen in figure 21 that after the first cycle, there is very little resolution of the enantiomers, but an impurity is separated on the front of the composite peak. This peak is diverted to waste a procedure that is termed peak shaving (from the main peak). During the second cycle, the separation of the enantiomers is beginning, although the isomers are insufficiently resolved for peak collection to be initiated. During the third cycle, the first major peak is 'shaved' from the composite peak. It is important to note that as the overloaded peak is asymmetrical and tails, the first peak will be collected virtually pure. The second peak will remain contaminated with a small amount of the first peak. After the fourth cycle is complete, the trace of the first isomer is shaved from the major peak and passed to waste. The remainder of the peak is then collected with little or

for the production of high purity fractions by peak cutting. This advantage is illustrated in the last example given. The first peak can be collected as a fraction up to a point just before the sharp front of the second peak starts. This will produce a very pure fraction of the first peak The second peak is re-run in an identical manner and as the first peak is now an impurity (and therefore present at a low concentration) it is not overloaded and, therefore, will be eluted as a symmetrical peak in front of the second main peak. If required, the second peak can be collected just after the trace of the first peak is eluted and will also be extremely pure. The first fraction of the second separation, still containing a mixture of both peaks, although containing a very small percentage of the total mixture can be recycled if considered appropriate. 3. If large sample loops are employed the injection must be cut so that tail of sample left in the loop does not cause serious peak

Detectors . Peak Dispersion from the Overall Detector Time Constant Peak dispersion resulting from the time constant of the sensor and its associated electronics can be significant in both GC and LC, particularly when filter circuits are introduced to remove inherent detector noise. The effect of the detector time constant can be theoretically examined (see Extra Column Dispersion ) and calculated and the results from such calculations are shown in figure 6. The undistorted peak, that would be monitored by a detector with a zero time constant, is about 4 seconds wide. Thus, for a GC packed column operating at 20 ml/min. this would represent a peak having a volume of about 1.3 ml. It is important to note that the dispersion is only apparent. The term apparent is used as the solute concentration profile, itself, is not actually changed, only the profile as presented on the recorder or printer.    Figure 6. Peak Profiles Demonstrating Distortion Resulting