Column optimum flow rate

The flow rote Is the most effective porometer for improving the resolution. For 6 to 8 mm l.d. columns, optimum flow rates are from 50 to 100 / l min. A slight improvement in the bandwidth of peaks Is often achieved by an Increose in column temperature. 

Theoretical plates per metre of column Optimum flow rate at 5300 3300 2700 1600... 

Thus, for significant values of (k") (unity or greater) the optimum mobile phase velocity is controlled primarily by the ratio of the solute diffusivity to the column radius and, secondly, by the thermodynamic properties of the distribution system. However, the minimum value of (H) (and, thus, the maximum column efficiency) is determined primarily by the column radius, secondly by the thermodynamic properties of the distribution system and is independent of solute diffusivity. It follows that for all types of columns, increasing the temperature increases the diffusivity of the solute in both phases and, thus, increases the optimum flow rate and reduces the analysis time. Temperature, however, will only affect (Hmin) insomuch as it affects the magnitude of (k"). 

The maximum and minimum flow rate available from the solvent pump may also, under certain circumstances, determine the minimum or maximum column diameter that can be employed. As a consequence, limits will be placed on the mass sensitivity of the chromatographic system as well as the solvent consumption. Almost all commercially available LC solvent pumps, however, have a flow rate range that will include all optimum flow rates that are likely to be required in analytical chromatography... 

There remains the need to obtain expressions for the optimum column radius (r(opt)), the optimum flow rate (Q(opt)), the maximum solvent consumption (S(sol)) and the maximum sample volume (v(sam))-... 

The solvent consumption appears to be in conflict with the corresponding optimum flow rates. Substances with high (a) values have a very high optimum flow rate (over 11 per min. for (a=1.2) and the column diameter is over 6 mm which would indicate a very large solvent consumption. However, because the separation is simple, a very rapid separation is achieved with analysis times of less than a second. As a consequence, only a few ml of solvent is necessary to complete the analysis. The apparatus, however, must be designed with an exceedingly fast response and very special sample valves would be necessary. In contrast, a very... 

The optimum flow rate for most SEC separations using conventional PLgel column dimensions (internal diameter 7.5 mm) is 1.0 ml/min. It may be of some benefit to work with lower flow rates, particularly for the analysis of higher molecular weight polymers where the reduced flow rate improves resolution through enhanced mass transfer and further reduces the risk of shear... 

You can see that these dispersion mechanisms are affected in different ways by the flow rate of mobile phase. To reduce dispersion due to longitudinal diffusion we need a high flow rate, whereas a low flow rate is needed to reduce dispersion due to the other two. This suggests that there will be an optimum flow rate where the combination of the three effects produces minimum dispersion, and this can be observed in practice if N or H (which measure dispersion) are plotted against the velocity or flow rate of the mobile phase in the column. The shape of the graph is shown in Fig. 2.3f. 

Simple and comprehensive 2D HPLC was reported in a reversed-phase mode using monolithic silica columns for the 2nd-D separation (Tanaka et al., 2004). Every fraction from the lst-D column, 15cm long (4.6 mm i.d.), packed with fluoroalkylsilyl-bonded (FR) silica particles (5 pm), was subjected to the separation in the 2nd-D using one or two octadecylsilylated (Cig) monolithic silica columns (4.6 mm i.d., 3 cm). Monolithic silica columns in the 2nd-D were eluted at a flow rate of up to lOmL/min with separation time of 30 s that provides fractionation every 15-30s for the lst-D, which is operated near the optimum flow rate of 0.4-0.8 mL/min. The 2D-HPLC systems were assembled, as shown in Fig. 7.6, so that the sample loops of the 2nd-D injectors were back flushed to minimize band broadening. 

Explain how you would use a van Deemter plot, as shown in Fig. 14.2 to determine the optimum flow rate for a separation. What are the key variables When using theoretical plate measures for comparing columns, what experimental conditions must be controlled ... 

Figure 3.11 illustrates the effect of varying the flow rate of the mobile phase on the efficiency of the separation process and provides a standard method of determining the optimum flow rate for a specific column and mobile phase system. 

In liquid chromatography, the diffusion rates are slower than that in gas chromatography, and the values of DM and D are very small therefore, the minimum H value is obtained at a low flow rate, as shown by curve E in Figure 5.6. The value of H increases slowly at higher flow rates in liquid chromatography. An experimental result is shown in Figure 5.7. The HETP was minimal at a certain flow rate, and the measured optimum value was less than 10 pm for this column. The optimum flow rate was about 0.9 ml min - corresponding to a linear flow velocity of about 55 mm min -. ... 

HPLC systems have recently become commercially available, which allow the use of pressures up to 1000 bar. Columns of particle size around 1.7 p,m and up to approximately 10-cm long can be operated at their optimum flow rate within the pressure capability of the system. The main advantage of these columns is that they can generate the same plate count as longer columns of larger particles but in a shorter analysis... 

Although HPLC column technology is considered to be a mature field now, improvements and new developments are being made continuously in the stationary phases. One of the improvements has been the reduction in particle sizes. Smaller particles help to improve mass transfer and provide better efficiency. Manufacturers are producing particles down to 1.5 J,m in diameter, although 3- and 5- J,m particles are still the most popular. Because of the smaller particle sizes, the backpressure increases proportionally to the inverse of the square of the particle size. Most commercially available HPLC systems cannot accommodate the pressures required to operate these columns at optimum flow rates. This has led to the introduction of systems that run at high pressures. 

The optimum mobile phase velocity will also be determined in the above calculations and, as the minimum radius will also be calculated in order to achieve minimum solvent consumption and maximum mass sensitivity, the optimum flow rate can also be identified. The column specifications and operating conditions can be summarized as follows. 

It Is seen from equation (30) that the optimum flow-rate Is also proportional to the extra column dispersion and, as a consequence, the total volume of mobile phase employed in an analysis will also depend on the extra column dispersion. It follows that the economy of the analysis lies in the hands of the designer of the chromatograph, a responsibility for which, many instrument makers are not aware. Steps taken in the design of the chromatographic system that would reduce the extra column dispersion by a factor of two would also halve the volume and cost of solvent used in the... 

The optimum flow rate is also proportional to the applied pressure, the fourth power of (or l) and the inverse of the viscosity. It follows, somewhat surprisingly that by selecting a solvent system of low viscosity would also permit a higher flow rate. Whether this will also increase the solvent consumption will be seen in due course. Employing equation (30) curves were constructed relating optimum column flow-rate to the separation ratio of the critical pair and these are shown in figure (5). 

It is seen that the optimum flow-rate increases rapidly as the separation becomes less difficult and, in fact, there is a flow-rate change that extends over three orders of magnitude. Again the trend in column design becomes more apparent, simple separations are carried out on short, wide columns, packed with relatively large particle and operated at at high flow-rates. 

Having determined the optimum column radius, the optimum velocity being known then the optimum flow-rate volume is given by,... 

It is seen that equation (15) is very similar to that for the optimum flow-rate for an analytical column except that (oe) Is replaced by the expression (IIOM/w) as the extra column dispersion no longer controls the column radius. 

The results of the breakthrough studies indicated that the optimum flow rate would be approximately 50-100 bed volumes/h. These results were used to scale-up the bench-scale columns for pilot plant studies. 

The van Deemter plots in Figure 25-3 show that small particles reduce plate height and that plate height is not very sensitive to increased flow rate when the particles are small. At the optimum flow rate (the minimum in Figure 25-3). the number of theoretical plates in a column of length L (cm) is approximately3... 

One reason why small particles give better resolution is that they provide more uniform flow through the column, thereby reducing the multiple path term, A, in the van Deemter equation (23-33). A second reason is that the distance through which solute must diffuse in the mobile and stationary phases is on the order of the particle size. The smaller the particles, the less distance solute must diffuse. This effect decreases the C term in the van Deemter equation for finite equilibration time. The optimum flow rate for small particles is faster than for large particles because solutes diffuse through smaller distances. 

A reversed-phase HPLC Cl8 or C30 narrow-bore column is typically used for LC/MS with APCI. Details about chromatography columns used for carotenoids are contained in unit F2.3. For most APCI systems, the optimum flow rate into a mass spectrometer or tandem mass spectrometer equipped with APCI, as controlled by a syringe pump or HPLC pump, is usually between 100 and 300 pl/min, which is ideal for narrow-bore HPLC columns. Larger diameter columns should be used with a flow splitter postcolumn to reduce the solvent flow into the mass spectrometer. For example, if a 4.6 mm i.d. column was used at a flow rate of 1.0 ml/min, then the flow must be split postcolumn 5 1 so that only 200 pl/min enters the mass spectrometer. 

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