Retention times factors affecting

The definition of polymer thermal stabiUty is not simple owing to the number of measurement techniques, desired properties, and factors that affect each (time, heating rate, atmosphere, etc). The easiest evaluation of thermal stabiUty is by the temperature at which a certain weight loss occurs as observed by thermogravimetric analysis (tga). Early work assigned a 7% loss as the point of stabiUty more recentiy a 10% value or the extrapolated break in the tga curve has been used. A more reaUstic view is to compare weight loss vs time at constant temperature, and better yet is to evaluate property retention time at temperature one set of criteria has been 177°C for 30,000 h, or 240°C for 1000 h, or 538°C for 1 h, or 816°C for 5 min (1). 

Internal standardization circumvents the effects of time-variant instrument response, but does not compensate for different ionization efficiencies of analyte and standard. For internal standardization, a compound exhibiting close similarity in terms of ionization efficiency and retention time is added to the sample at a known level of concentration, e.g., an isomer eluting closely to the analyte or a homologue may serve for that purpose. It is important to add the standard before any clean-up procedure in order not to alter the concentration of the analyte without affecting that of the standard. For reliable results, the relative concentration of analyte and standard should not differ by more than a factor of about ten. 

In robustness tests, peak measurementfanalysis parameters can also be considered. Such parameters are related to the measurement of the detector signal and they affect responses, such as peak areas, peak heights, retention time, and resolution. They allow improving the quality of these responses. These factors can be found in the data treatment software of an instrument, where often only the default settings are used by the analyst. 

Other results obtained from the ruggedness test are the definition of optimized method conditions for the factors and of system suitability criteria for a number of responses. System suitability parameters [6,17] are defined as an interval in which a response can vary for a rugged method. The system suitability criteria are the range of values between which a response (e.g. retention time, capacity factor, number of theoretical plates, resolution) can vary without affecting the quantitative results of the analysis. For instance, a design is performed and the retention time of the main substance varies between 200 s and 320 s without affecting the quantitative determination of the substances. The system suitability criteria for the retention time is then defined as the interval 200 s - 320 s. 

Liquid chromatography can be operated under mild conditions in terms of pH, ionic strength, polarity of liquid, and temperature. The apparatus used is simple in construction and easily scaled up. Moreover, many types of interaction between the adsorbent (the stationary phase) and solutes to be separated can be utilized, as shown in Table 11.1. Liquid chromatography can be operated isocratically, stepwise, and with gradient changes in the mobile phase composition. Since the performance of chromatography columns was discussed, with use of several models and on the basis of retention time and the width of elution curves, in Chapter 11, we will at this point discuss some of the factors that affect the performance of chromatography columns. 

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. 

The quality of chromatographic separation depends on the composition of the stationary phase of the column and the instrument settings. The temperature regime of the oven the flow rate of the mobile phase gas through the column the temperature of the injection port—all of these factors influence compound retention time and peak resolution. The reduction in the chromatographic analysis time may adversely affect compound resolution. Because commercial laboratories always balance a need for a sufficient resolution with a need to perform analysis within the shortest possible period of time, the quality of chromatographic resolution is often traded for the speed of analysis. 

The capacity parameters do not affect the selectivity (a), but they do have an effect on the capacity factor (fc) and hence on the resolution (Rs see eqn.1.22). The physical parameters only affect the resolution through the efficiency (N). They also have an effect on the retention time through the hold-up time t0 (see eqn.1.6). 

As shown by Eq. (1), the resolution of components in a liquid chromatographic separation is dependent on (1) their relative retention on a particular chromatographic system and (2) their peak widths. To optimize these parameters for maximum resolution, a clear understanding of their nature and the factors that affect them is necessary. Although the retention time of a component adequately describes the amount of time a particular solute takes to elute from a chromatographic system, a more useful parameter describ-... 

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