Peak time correction factors

Outside the optimum range this is no longer true. If ten peaks are equally resolved (r = 1) with S values of 0.001, then according to eqn.(4.47), four million plates are required for adequate resolution. Moreover, we can see from figure 4.11 that the required analysis time is a factor of about 600 larger (under constant flow and diameter conditions) than it would be if S equalled 0.1. If S was 0.5, the analysis time would be a factor of about 200 larger than in the optimum. Hence, we may conclude that for optimization processes during which the capacity factors may be expected to vary dramatically, a time correction factor is required even when r is used as the optimization criterion. 

If the mobile phase is a liquid, and can be considered incompressible, then the volume of the mobile phase eluted from the column, between the injection and the peak maximum, can be easily obtained from the product of the flow rate and the retention time. For more precise measurements, the volume of eluent can be directly measured volumetrically by means of a burette or other suitable volume measuring vessel that is placed at the end of the column. If the mobile phase is compressible, however, the volume of mobile phase that passes through the column, measured at the exit, will no longer represent the true retention volume, as the volume flow will increase continuously along the column as the pressure falls. This problem was solved by James and Martin [3], who derived a correction factor that allowed the actual retention volume to be calculated from the retention volume measured at the column outlet at atmospheric pressure, and a function of the inlet/outlet pressure ratio. This correction factor can be derived as follows. 

Thermal conductivity detectors used in gas chromatographs do not respond equally to all FAMlis. To correct for varying detector sensitivity, peak area for each FAME should be multiplied by the proper response correction factor (RCF) (Table E6.2). If extra time is available, you may want to calculate your own response correction factors for the fatty acid methyl esters. The factors are experimentally determined on a gas chromatograph by comparing the area under a GC peak due to a known amount of compound to the area under a GC peak represented by a reference compound. 

If it suspected that the enantiomers of interest are coeluting, a correction to the areas of the affected peaks may be applied. The correction can be determined from a knowledge of the peak areas on a nonpolar and polar achiral column. If the shape of the peak and the areas are the same on both columns, then no coelution is occurring. If the areas are different, then a correction factor can be applied by comparing retention times of the separated enantiomers, and subtracting the areas of those peaks that are coeluting, which were determined from the chromatograms obtained from the achiral columns. It should be emphasized that this is not an appropriate correction to make if accurate quantitative information is required. 

Calibration standards can be of two types external standards and internal standards. With external standards, multiple concentrations of the standards are injected, areas are measured, and a calibration curve is platted. Unknown samples are then injected, chromatograms run, and areas are calculated and compared with the calibration curves to determine amounts of each compound present. With internal standards, known amounts of an internal standard are added to each known concentration of standard compound and areas or peak height response factors relative to those of the internal standard are calculated. When unknowns are run, a known amount of internal standard is added to the unknown sample, response factors are calculated relative to the internal standards, and amounts of each unknown present are calculated from the standards calibration factors. Internal standards are usually used to correct for variations in injection size due to different operators and injection techniques. Internal standards can also be used to correct for extraction variation in GC/MS target compound quantitation, this standard is referred to as a surrogate standard. Generally, an internal standard is used for one purpose or the other, not both at the same time. 

Impurity Relative retention time [5-7] Correction factor [6, 7] Limit (%) [5-7] Limitation of peak area Relative response factor (F) [5]... 

Calculation of the levels of each impurity is performed by multiplying the peak areas of impurities A, F, G, and H with the corresponding correction factor of those impurities, as provided in Table 6.1. In addition, the requirement that the peak area of each impurity in the test solution corresponds with the principal peak of rocuronium bromide obtained from the RS is also detailed in Table 6.1. In addition, all compendias [5-7] suggest that any peak eluted prior to the elution of impurity A should be ignored due to the blank and to bromide ion that elutes just before impurity A. Furthermore, any peak with an area less than 0.5 times of the principal peak of rocuronium bromide should be disregarded. 

It is also possible to improve the accuracy of the mass calibration by making use of a peak of known composition to correct the electric field term for changes in the number of ions [16], In other words, if the identity of some component of the mixture is known by its retention time and/or mass spectrum, a peak of known mass in the mass spectrum may be used to apply a correction factor, x, to the electric field term in the calibration equation, which may be rewritten as ... 

One method for the quantitative determination of the concentration of constituents in a sample analyzed by gas chromatography is area normalization. Here, complete elution of all the sample constituents is necessary. The area of each peak is then measured and corrected for differences in detector response to the different eluates. This correction involves dividing the area by an empirically determined correction factor. The concentration of the analyte is found from the ratio of its corrected area to the total corrected area of all peaks. For a chromatogram containing three peaks, the relative areas were found to be 16.4, 45.2, and 30.2, in order of increasing retention time. Calculate the percentage of each compound if the relative detector responses were 0.60, 0.78, and 0.88, respectively. 

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