Plate number resolution enhancement

Owing to the EK bias in favor of faster migrating species in stack injection, the signal enhancement ranged from 31 to 8. However, stack injection produced less resolution, i.e., fewer plate numbers (N), as compared to those obtained in pinched injection. In addition, RSD (n = 6) in peak areas for stack, non-stacked, and pinched injection are 2.1%, 1.4%, and 0.75%, respectively [346]. 

When packed into chromatography columns, TRIM beads imprinted with Boc-L-Phe were shown to have column efficiencies and separation abilities superior to ground and sieved bulk material [5]. The theoretical plate number was approximately double that obtained with conventional crushed polymer under the same conditions and the resolution of a racemate was also slightly enhanced. The difference, however, was not that great considering the additional preparation time and effort involved. 

Anything that increases the column efficiency N, the column selectivity a. or the retention factor k will enhance the separation power of the column. Packed columns are characterized by low plate numbers and PCGC is therefore a low-resolution technique. The lower efficiency is compensated by the high. selectivity a of the stationary phase, and this is the main reason why so many different stationary pha.ses have been developed for PCGC. Capillary columns on the other hand have very high plate numbers and, therefore, the number of stationary phases can be restricted because the selectivity is less important. In fact, most separation problems can be handled with four basic. stationary pha.ses and half a dozen tailor-made stationary phases. Other important features of capillary columns are their inertness and compatibility with spectroscopic detectors. In the framework of this discussion, emphasis is, therefore, on capillary columns. 

One would obtain a quite small improvement in the resolution as a result of a reasonable extension of the separahon time (k value of 6). Therefore, the enhancement of the interaction (e.g., the increase of the water content of the eluent) is a common, easily implemented, but mostly quite inefficient procedure for routinely improving resolution. If, however, the plate number could be improved to 15,000, for example (smaller particles, injection tricks, etc.), in the case of a k value of 4, then the resolution would improve to 1.17. 

Optimization of the primary parameters of the gradient program may only lead to sufficient separation if the selectivity is sufficiently large. If the a values between one or more pairs of solutes are low, resolution may be enhanced by a reduction of the programming rate and by increasing the number of plates. However, this will be at the expense of increased analysis times and the resolution will never be better than under constant elution conditions. 

The number of theoretical plates and, thus, the separation efficiency, is determined by the length of the separator column. If two separator columns are used in series, the resulting enhancement of separation efficiency leads to a better resolution between ions with similar retention characteristics, with a corresponding increase in retention times. The separator column length also determines the exchange capacity. An increase of the exchange capacity via elongation of the separator column is recommended in all cases where the ion to be analyzed is present in an excess of another component. 

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