Mobile phase flow rate

An eluted solute was originally identified from its corrected retention volume which was calculated from its corrected retention time. It follows that the accuracy of the measurement depended on the measurement and constancy of the mobile phase flow rate. To eliminate the errors involved in flow rate measurement, particularly for mobile phases that were compressible, the capacity ratio of a solute (k ) was introduced. The capacity ratio of a solute is defined as the ratio of its distribution coefficient to the phase ratio (a) of the column, where... 

It is clear that the separation ratio is simply the ratio of the distribution coefficients of the two solutes, which only depend on the operating temperature and the nature of the two phases. More importantly, they are independent of the mobile phase flow rate and the phase ratio of the column. This means, for example, that the same separation ratios will be obtained for two solutes chromatographed on either a packed column or a capillary column, providing the temperature is the same and the same phase system is employed. This does, however, assume that there are no exclusion effects from the support or stationary phase. If the support or stationary phase is porous, as, for example, silica gel or silica gel based materials, and a pair of solutes differ in size, then the stationary phase available to one solute may not be available to the other. In which case, unless both stationary phases have exactly the same pore distribution, if separated on another column, the separation ratios may not be the same, even if the same phase system and temperature are employed. This will become more evident when the measurement of dead volume is discussed and the importance of pore distribution is considered. 

The column used in the upper chromatogram was 24 cm long, 4.6 mm I.D. the solvent was tetrahydrofuran, the solute benzene and the flow rate 1 ml/min. The column used in the lower chromatogram was 1 m long, 1 mm I.D. using the same solvent and solute but at a mobile phase flow rate of 40 ml/min. It is seen that the reduction in cell volume has a dramatic effect on peak shape. The large 25 pi cell... 

It follows from equation (2) that the sample load will increase as the square of the column radius and thus the column radius is the major factor that controls productivity. Unfortunately, increasing the column radius will also increase the volume flow rate and thus the consumption of solvent. However, both the sample load and the mobile phase flow rate increases as the square of the radius, and so the solvent consumption per unit mass of product will remain the same. 

Column Column Length Column Diameter Column Packing Mobile Phase Flow Rate Detector Sample Volume... 

Mobile Phase Flow Rate Detector Sample Volume... 

Unfortunately, neither the computer nor the potentiometric recorder measures the primary variable, volume of mobile phase, but does measure the secondary variable, time. This places stringent demands on the LC pump as the necessary accurate and proportional relationship between time and volume flow depends on a constant flow rate. Thus, peak area measurements should never be made unless a good quality pump is used to control the mobile phase flow rate. Furthermore, the pump must be a constant flow pump and not a constant pressure pump. 

The time taken for an analyte to elute from a chromatographic column with a particular mobile phase is termed its retention time, fan- Since this will vary with column length and mobile phase flow rate, it is more useful to use the capacity factor, k. This relates the retention time of an analyte to the time taken by an unretained compound, i.e. one which passes through the column without interacting with the stationary phase, to elute from the column under identical conditions (to). This is represented mathematically by the following equation ... 

Figure 4.9 Schematics of electrospray LC-MS interfaces with (a) a heated capillary and (b) a heated block to allow high mobile-phase flow rates. From applications literature published by (a) Thermofinnigan, Kernel Hempstead, UK, and (b) Micromass UK Ltd, Manchester, UK, and reproduced with permission.

Mobile phase flow rates from nlmin to in excess of 1 mlmin can be used with appropriate hardware, thus allowing conventional and microbore columns to be employed. 

In this study, the effect of mobile-phase flow rate, or more accurately, the rate of flow of liquid into the LC-MS interface, was not considered but as has been pointed out earlier in Sections 4.7 and 4.8, this is of great importance. In particular, it determines whether electrospray ionization functions as a concentration-or mass-flow-sensitive detector and may have a significant effect on the overall sensitivity obtained. Both of these are of great importance when considering the development of a quantitative analytical method. 

One mobile phase contains polyethylene oxide = SOOfiOO) (For details see Table II). Mobile phase flow rate 4.2 mL min water mobile phase containing poly-ethylene oxide (M - 300,000) (A) 0.0I67M MgSO (pH - 2.28) ... 

Detector Mobile phase Flow rates Retention time... 

Mobile phase flow rate Mobile phase... 

Coupling HPLC to a mass spectrometer is far more complicated than in a GC system because of the large amount of mobile phase solvent expanding into the system (see Table 1 for expansion volumes). Typical mobile phase flow rates for HPLC are 0.5-2 mL min which translates into gas flow rates of 100-3000 mL min . ... 

Stationary phase Mobile phase Flow rate... 

Figure 1.15 Fast analysis of a test mixture on a 10 cm x 4.6 mm I.D. column packed with 3 micrometer octa< ecylsilanized silica with a mobile phase flow rate of 3.4 mi. i.n (acetonitrile-water 7 3) and operating pressure of ca. 340 atmospheres. Peaks 1 uracil, 2 phenol, 3 - nitrobenzene, 4 - toluene, 5 -ethylbenzene, 6 - isopropylbenzene, and 7 - tert.-butylbenzene. (Reproduced with permission from ref. 222. Copyright Friedr. Vieweg 6 Sohn).

Ghodbane, S. and Guiochon, G., Effect of mobile phase flow-rate on the recoveries and production rates in overloadde elution chromatography a theoretical study, /. Chromatogr., 452, 209, 1988. 

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