Program rates, resolution

Figure 7. Graphs of Resolution of the Isomers of 4-Benzyl-2-oxazol-idinone against Program Rate for Three Different Initial Temperatures...

It is seen that the resolution rapidly falls with increased program rate. However, the manner in which the resolution changes with temperature is complicated by the fact that the standard free enthalpies of the two isomers differ and, thus, the effect of... 

However, the IC/5 offers all comfort functions available today measurement with up to eight sensors with AutoZero and AutoTune as virell as capability of simultaneous control of two evaporator sources. Moreover, it offers 24 material programs, with which 250 coatings in 50 processes can be programmed. To simplify operation and avoid errors, the unit also has a diskette drive. All types of crystal holders can be connected here. The thickness resolution is around 1 A, the rate resolution for rates between 0 and 99.9 A/s around 0.1 A/s and for rates between 100 and 999 A/s around 1 A/s. A particularly attractive option offered by the IC/5 is a microbalance... 

Resolution in programmed temperature GC is enhanced if the programming rate (rr/F) is decreased and if the initial temperature (T) is decreased. Giddings [606] suggested that the first peak in a programmed analysis should not appear within about five times the hold-up volume of the column. Since the temperature has little effect on the selectivity in GC (see section 3.1.1), the optimization of temperature programs is a process that may be seen as resolution optimization rather than as selectivity optimization. 

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. 

This procedure is a general method for calculating the effect of the program rate on retention time. Basically, the corrected retention volume must be measured for each solute of interest at two different temperatures to provide the thermodynamic constants. The above equations will then allow the effect of different linear temperature programs on the corrected retention volume to be calculated for each solute. The effect of program rate on resolution can also be observed if some solutes elute close together. In fact, the... 

At first the chromatographic conditions of the chiral main column must be optimized carefully. Capillary columns coated with chiral stationary phases of suitable enantio-selectivities are used as main columns. Chiral resolutions are commonly achieved isothermally or by low temperature programming rates, starting at least 20°C below precolumn temperature. Precolumns are chosen with respect to (i) the versatility of application, to (ii) the direct injection of high sample volumes and (iii) with respect to the requisite time of analysis. 

Lower temperatures (and lower program rates) will lead to Increased values of k (and of a), and permit the system to deliver Improved resolution, but another result will be longer analysis times, and (because the KDs are larger as the solutes are delivered from the end of the column to the detector,) lower sensitivities (1). 

Flinshaw provides a useful comparison of experimental results arising from experiments where different isothermal temperatures are used, the temperature programming rate is altered, column length is altered, carrier gas flow rate is altered, and where column ID is altered to achieve equivalent resolution performance using a combination of a narrower and shorter column. 

The traditional GC separation with a reduced run time, i.e., fast GC, is now the cutting edge and can be achieved by the use of shorter capillary columns, columns with reduced internal diameter, thinner stationary-phase films, H2 as the carrier gas, higher carrier gas velocities, faster oven temperature programming rates, and combinations of these parameters. However, one must be careful to understand the impact of these changes on the chromatographic resolution. Despite the unavoidable decrease in efficiency, the overall analytical results are obtained with 95% reduction in analytical time, typically less than 2... 

The ramp rate governs the tradeoff between analysis time and resolution. The compromise between resolution and time of analysis is contrasted in Figure 3.64 for parallel capillary column separations of lemon oil generated at three different programming rates. Relying on experience and intuition in establishing optimum column temperature conditions can be time-consuming and inefficient. An alternative route is the use of computer simulation for method development this approach is discussed in much further detail in the next chapter (98-102). 

FIGURE 4.13 Peak resolution as a function of program rate for peak pairs in the pesticide test sample. For conditions, see Figure 4.11. [Reprinted from LC/GC Magazine with permission of Advanstar Publications (14).]... 

Retention time is also strongly influenced by the choice of stationary phase. Several factors must be taken into account when selecting a column for separating androgens. Retention time and resolution will be affected by the choice of column length, stationary film thickness and polarity, and the temperature program rate. The programmed temperature optimization of a mixture of anabolic... 

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