Column evaluation

It is important to appreciate that, in all aspects of column evaluation and column design in GC, the compressibility of the mobile phase must be taken into account or serious enors will be incurred. Either equation (13) or (15) can be employed but, as already stated, equation (13) is recommended as the more simple to use. 

TABLE 21.3 Column Evaluation Data for an Acceptable High Temperature GPC Column... 

The final step in the process of standardizing our columns was to try and maintain the high quality of columns from batch to batch of gel from the manufacturer. This was done by following the basic procedures outlined earlier for the initial column evaluation with two exceptions. First, we did not continue to use the valley-to-peak ratios or the peak separation parameters. We decided that the D20 values told us enough information. The second modification that we made was to address the issue of discontinuities in the gel pore sizes (18,19). To do this, we selected six different polyethylenes made via five different production processes. These samples are run every time we do an evaluation to look for breaks or discontinuities that might indicate the presence of a gel mismatch. Because the resins were made by several different processes, the presence of a discontinuity in several of these samples would be a strong indication of a problem. Table 21.5 shows the results for several column evaluations that have been performed on different batches of gel over a 10-year period. Table 21.5 shows how the columns made by Polymer Laboratories have improved continuously over this time period. Figure 21.2 shows an example of a discontinuity that was identified in one particular evaluation. These were not accepted and the manufacturer quickly fixed the problem. 

The performance of columns is determined by using a simple procedure for both new and used columns. The test method and the results are usually printed on a sheet provided by the column manufacturer. Each manufacturer uses the most suitable method for their column to obtain the best theoretical plate number. The column evaluation provides important information for all users of liquid chromatography. 

In an attempt to avoid interactions with residual silanol groups, Abidi and Mounts investigated the separation of the molecular species of PC, PE, and SPH on polymeric C18 columns by RP-HPLC (103). Of the three polymer columns evaluated, the best HPLC results were obtained with an octadecanoyl polyvinyl alcohol (ODPVA) stationary phase. High-performance LC on ODPVA with an A/M/W mobile phase provided significantly faster analysis and greater detection sensitivity than assays with C18 silica columns. 

Isocratic or gradient Column Evaluate method with different... 

After several resins have been tested in this manner, the resin is selected for column evaluation which has a high loading per ml of resin or a low residual with larger resin quantities. 

For multicomponent systems try to apply component-specific detectors Favor measurement methods with packed columns Evaluate concentration range of the application process Check applicability of measurement method Determine single-component parameters first... 

General uncertainty of measurement is said to be dispersed between the measured values and parameter values that are interrelated. In other words, in the course of measurement of physical quantities, such as personnel, equipment and measurement of environmental impact, the degree of uncertainty for measurement result. And evaluation of uncertainty in measurement is by means of a quantitative determine the extent of such uncertainty, the purpose of which is to determine the distribution of measurement range and measurement result reliability level. Observation column evaluation results of the statistical analysis of standard uncertainty is called type A standard uncertainty. Are different from those used for observing statistical analysis to evaluate the uncertainty of standard uncertainty is known as type B standard uncertainty . 

A discussion of column evaluation may seem inappropriate to biochemists who have traditionally packed gel SEC columns themselves and run them without testing. Such columns would eventually be unpacked when the performance degraded, and the packing material would be cleaned or discarded. The quality of the column was not significant in relationship to the time necessary to evaluate it. HPSEC columns are usually more expensive than their classical counterparts, and consequently, users are concerned about quality assurance, column longevity, sample recovery, and extension of column lifetime. Because there are major differences in columns from different manufacturers, users must have a qualitative basis for column comparison. Columns should also be evaluated periodically during their use so that loss of efficiency can be monitored and columns cleaned, when necessary, to restore resolution. 

Three types of column evaluations should be implemented ... 

The manipulations involved in introducing a sample for analysis or column evaluation are relatively simple. With a conventional valving system, helium flow is directed through the bypass valve, and a gas sample from the steel container or a vacuum-line storage volume is admitted to the previously evacuated sample loop. After the gas pressure is recorded ( 0,lmm of Hg) and the sample isolated. 

As an example, we will consider only the column evaluation obtained in mobile phase 1, as that obtained in mobile phase 2 is very similar (data not shown). 

First of all, the number of test compounds to be employed should be as low as possible. In our ongoing example, we show how a set of 14 test compounds can be successfully reduced to seven, while still maintaining the same quality of column evaluation. The 14 previously selected test compounds possessed different physicochemical properties and could be classified on the basis of size and polarity. 

If we keep two molecules per physicochemical group (the ones more easily commercially available), the physicochemical domain is still well covered and consequently a valuable column evaluation should be maintained. 

---