Flow programming

Flow Programming with a Non-Compressible Mobile Phase 

If the mobile phase is not compressible, then the contribution (AV) to the retention volume (Vr) after time (t) for a period (At) will be given by 

if (tr) is the elution time (in seconds) of a solute having a retention volume (Vr), (ocq) is expressed as the change in flow rate per second, (At) is taken as 1 second and (nt) the number of seconds. 

Marcel Dekker, 270 Madison Avenue, New York, New York 10 

Using equation (1) with a simple computer program that identifies (n) when 

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Figure 1. Effect of Flow Program Rate on Elution Time...

Flow Programming with a Compressible Mobile Phase... 

Flow programming with a compressible mobile phase is a far more complicated process to examine theoretically. We shall assume that, under flow programming conditions, the mass flow rate will be increased linearly with time, i.e. (Qo(t)) = (Q o + otQt) where (Q o) is the initial exit flow rate, (Qo(t)) is the exit flow rate after time (t) and (aq) is the flow program rate. 

These conditions are usual for modern gas flow programming devices that utilize mass flow controllers which are often computer operated. Now, if (AV(o)) is an... 

Figure 3. Curves Relating Elution Time to Flow Program Rate for Solutes having Different Retention Volumes...

Temperature programming was introduced in the early days of GC and is now a commonly practiced elution technique. It follows that the temperature programmer is an essential accessory to all contemporary gas chromatographs and also to many liquid chromatographs. The technique is used for the same reasons as flow programming, that is, to accelerate the elution rate of the late peaks that would otherwise take an inordinately long time to elute. The distribution coefficient of a solute is exponentially related to the reciprocal of the absolute temperature, and as the retention volume is directly related to the distribution coefficient, temperature will govern the elution rate of the solute. 

Flow programming Varying the HPLC flow rate during the course of a separation. 

Figure 1.12 Comparison of isothermal (A), flow programmed (B), and temperature programmed (C) separation of C g to n-alkanes. (Reproduced with permission from ref. 154. Copyri t Dr. Alfred Huethig Publishers).

Solute property detectors, such as spectroscopic andj electrochemical detectors, respond to a physical or chemical] property characteristic of the solute which, ideally, is] independent of the mobile phase. Althou this criterion is rarely met in practice, the signal discrimination is usually sufficient to permit operation with solvent changes (e.g., flow programming, gradient elution, etc.) and to provide high sensitivity with aj wide linear response range. Table 5.4. Solute-specific detectors complement ulk property detectors as they provide high ... 

The principal advantages of flow programming are that it shortens separation time for mixtures of wide volatility, while ... 

Temperature and Flow Programming in Liquid Chroma tography... 

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