Temperature programming optimization

Choice of column and oven temperature program optimization for optimal GC separation, paying particular attention to analytes with similar fragmentation patterns. 

In this step, theoretical optimum conditions for the entire catalyst bed involving a number of pertinent parameters, such as temperature, pressure, and composition, are determined using mathematical methods of optimization [7,8]. The optimum conditions are found by attainment of a maximum or minimum of some desired objective. The best quality to be formed may be conversion, product distribution, temperature, or temperature program. 

Other thermal zones which should be thermostated separately from the column oven include the Injector and detector ovens. These are generally insulted metal blocks heated by cartridge heaters controlled by sensors located in a feedback loop with the power supply. Detector blocks are usually maintained at a temperature selected to minimize detector contamination from condensation of column bleed or sample components and to optimize the response of the detector to the sample. The requirements for i injectors may be different depending on the injector design and may include provision for temperature program operation. 

Sllylation, preparation of bonded phases (LC) 324 Simplex optimization selectivity (LC) 478 temperature program (GC) 56 Single column ion chroBatography 434, 438 Sintered glass layer (TLC) 671 Size-exclusion chroBatography 439... 

Semperatuie proqcammlnq (computer simulation 55 optimization 53 preparative separations 211 programming rate 53 retention indices 178 theoretical models 54 Temperature programming LC 83 SFC 630... 

In order to answer these questions, the kinetic and network structure models were used in conjunction with a nonlinear least squares optimization program (SIMPLEX) to determine cure response in "optimized ovens ". Ovens were optimized in two different ways. In the first the bake time was fixed and oven air temperatures were adjusted so that the crosslink densities were as close as possible to the optimum value. In the second, oven air temperatures were varied to minimize the bake time subject to the constraint that all parts of the car be acceptably cured. Air temperatures were optimized for each of the different paints as a function of different sets of minimum and maximum heating rate constants. 

Splitless Trace analysis (ppb) possible Cold trapping and solvent effects provide sharp peaks More complex than split Limited to temperature programming Several parameters to optimize Loss of low-volatility, labile analytes... 

T.A. Kunt, T.J. McAvoy, R.E. Cavicchi, and S. Semancik. Optimization of temperature programmed sensing for gas identification using micro-hotplate sensors . Sensors and Actuators B53 (1998), 24-43. 

If we allow the lateral interactions to vary over a too large range during the optimization, then we occasionally get adlayer structures in the kMC simulations that differ from those found experimentally. This does not mean necessarily that very different temperature-programmed desorption spectra are... 

Figure 2.2—Optimum linear velocity and viscosity of carrier gas. The optimal mean linear velocities of the various carrier gases are dependent on the diameter of the column. The use of hydrogen as a carrier gas allows a faster separation than the use of helium while giving some flexibility in terms of the flow rate (which can be calculated or measured). This is why the temperature program mode is used. The significant increase in viscosity with temperature can be seen for gases. In addition, the sensitivity of detection depends on the type of carrier gas used.

Helium is the most common carrier gas and is compatible with most detectors. For a flame ionization detector, N2 gives a lower detection limit than He. Figure 24-11 shows that H2, He, and N2 give essentially the same optimal plate height (0.3 mm) at significantly different flow rates. Optimal flow rate increases in the order N2 < He < H2. Fastest separations can be achieved with H, as carrier gas, and H2 can be run much faster than its optimal velocity with little penalty in resolution.11 Figure 24-12 shows the effect of carrier gas on the separation of two compounds on the same column with the same temperature program. 

Instead of setting the reference flow equal to the flow in the analytical column, it can be optimized for minimum drift during temperature-programming (a) Set the flows equal. (b) Make a temperature program run, observing the drift on a recorder,. 

The configuration of the GC system also affects the result. Many factors, such as column type, gas flow control, and temperature programming, if not set up correctly, will affect the performance of the GC. In the authors opinion, the first thing to do in a GC analysis is to choose the right column. One can then elucidate the optimal conditions for other factors (see Critical Parameters). 

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. 

If the program is optimized so that all sample components are eluted under optimal conditions, then other (secondary) parameters may be used for the optimization of the selectivity. However, changes in the secondary parameters may imply that the parameters of the program need to be re-optimized. For example, if the selectivity in a temperature programmed GC analysis is insufficient, then another stationary phase may be used to enhance the separation. However, the optimum program parameters obtained with one stationary phase cannot be transferred to another column that contains another stationary phase. The re-optimization of the temperature program for the other column will require at least one additional experiment to be performed. 

In this optimization procedure a segment usually refers to a part of the temperature program at which heating takes place. Such segments may be separated by isothermal periods, during which the temperature is kept constant. In the present discussion we will refer to the two kinds of segments as non-isothermal and isothermal, respectively. 

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