Unretained peak

In practice, the retention volume of an unretained peak eluted at the dead volume (Vo), will be made up of the volume of mobile phase in the column (Vm) and extracolumn volumes, from sample valves, connecting tubes, unions, etc. (Ve)-... 

Figure 3. Capillary Column Curves Relating H against u for an Uncoated Column (Unretained Peak)...

Having chosen the test mixture and mobile diase composition, the chromatogram is run, usually at a fairly fast chart speed to reduce errors associated with the measurement of peak widths, etc.. Figure 4.10. The parameters calculated from the chromatogram are the retention volume and capacity factor of each component, the plate count for the unretained peak and at least one of the retained peaks, the peak asymmetry factor for each component, and the separation factor for at least one pair of solutes. The pressure drop for the column at the optimum test flow rate should also be noted. This data is then used to determine two types of performance criteria. These are kinetic parameters, which indicate how well the column is physically packed, and thermodynamic parameters, which indicate whether the column packing material meets the manufacturer s specifications. Examples of such thermodynamic parameters are whether the percentage oi bonded... 

TYPICAL RANGES FOR VALUES OF COLUMN VARIANCE AND PEAK VOLUMES OF AN UNRETAINED PEAK... 

Another advantage of small bore columns concerns the small peak widths that are produced. In Section 2.3.3 we calculated the retention volume and the peak width of an unretained peak from a 25 cm x 4.6 mm column. 

When the column is ready to be used, the chromatogram of a suitable test mixture should be obtained. The plate number and retention times of the test solutes should be noted, and the peaks should have a satisfactory shape (minimal tailing). For measurement of the plate number, the recorder should be used at a high chart speed. Fig. 5.1b(i) and (ii) show test chromatograms for a C-18 column prepared by the above method, and Fig. 5.1c and 5.Id show the data that you should report with the chromatogram. The retention for an unretained peak is taken as the small baseline disturbance just before the first peak. 

Possible difficulties in obtaining accurate retention data for physicochemical measurements should be recognized, however. The evaluation of to, the elution time of an "unretained peak (274) is often connected with systematic error and the measurement of the retention time of asymmetrical peaks may not be accurate. Moreover, no satisfactory methods are available for the precise evaluation of the phase ratio in the column. Consequently, the measurement of the equflibrium constant proper is beset with difficulties as discussed in Section VII.B. 

In practice,for an unretained peak eluted at the dead volume,(Vo),... 

Furthermore, the effect of sample volume on peak width will be greatest for the early peaks (the most narrow peaks) and will progressively decrease for all subsequent peaks. As the resolution of both late and early peaks must be given equal priority, the sample volume must be chosen such that the increase in the width of the first unretained peak is restricted to 5%. 

The stochastic model applies to processes involving the stationary phase. To analyze the chromatogram, we need to subtract contributions to peak broadening from dispersion in the mobile phase and extra-column effects such as finite injection width and finite detector volume. These effects account for the width of the unretained peak. To subtract the unwanted effects, we write... 

For Component A, stationary phase observed unretained peak... 

Adjusted retention volume. V The solute retention volume minus the retention volume for an unretained peak. 

Carbon tetrachloride is an unretained peak. An example chromatogram is shown in Figure 9-1. 

C. On a separate sheet of paper construct Tables 9-1 and 9-2 and record the appropriate performance parameters. Is carbon tetrachloride an unretained peak How do you support this conclusion ... 

Fig. 1 Diagram depicting the retention volume, corrected retention volume, dead point, dead volume, and dead time of a chromatogram. Fq total volume passed through the column between the point of injection and the peak maximum of a completely unretained peak F total volume of mobile phase in the column F (a) retention volume of solute A F (a) corrected retention volume of solute A F extra column volume of mobile phase volume of mobile phase, per theoretical plate vy. volume of stationary phase per theoretical plate distribution coefficient of the solute between the two phases n number of theoretical plates in the column Q column flow rate measured at the exit.

An unretained ideal peak with k = 0 and a very small injection volume is likely to have a peak width of 0.032 min at the base. An indirect injection of a much larger volume requires that the loop contents be swept onto the column over an extended time. If the total flow rate is 70 mL/min and the initial concentration is 5% modifier (modifier flow = 3.5 mL/min), a 2-mL loop is swept in 0.285 min. An unretained peak would have a peak width >0.285 min. Injecting into the modifier line thus artificially broadens a peak compared to the ideal with a small injection volume. 

Figure 4.8 Chromatograms displaying injection peak. The injection peak is the unretained peak that allows access to retention factors. This is normally the first peak on the chromatogram. It can interfere with other early-eluting anions such as fluoride (Chromatograms from AUtech).

From the raw retention values, retention factors can be calculated, which are dimensionless numbers correcting for variations in instrumental instabilities such as variation in flowrate. The retention factor is defined as the ratio of the net retention time (tT - tu) and the retention time of an unretained peak (tu), where tr is the retention time of the solute. This is standard practice in liquid chromatography [Christian O Reilly 1986]. Moreover, taking the logarithms of these retention factors ideally linearizes the data, e.g., with respect to fractions of organic modifier in the mobile phase composition [Schoenmakers 1986],... 

The number of theoretical plates, N, is not the best measure of the column efficiency since in measuring tr the retention time (tm) of an unretained peak has... 

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