Reduced linear velocity

Assumed reduced parameters h = 3, v = 4.5. These are optimum values fromagraphofreducedplateheightversus reduced linear velocity of the mobile phase. 

To allow the comparison of chromatographic performance with different phases, reduced (dimensionless) plate height, h, and reduced linear velocity, v, are typically... 

Fig. 3.30 Reduced plate height as a function of reduced linear velocity for four columns (Marme,...

Figure 33 Van Deemter curves fa- various retention factors, (a) Apparent reduced plate height vs. apparent reduced linear velocity, (b) Reduced plate height vs. reduced linear velocity, corrected with the extra-column volume. Condititms Premixed 50/50 (v/v) water to acetonitrile as the moMe phase ...

Vi is the injection volume, dc is the column diameter, and L is the column length. For a successful method transfer, the reduced linear velocity of the mobile phase (v) must be kept constant, because this value is independent of the column geometry. Hence, for a geometrical transfer, the new flow rate (F) can be calculated with the following equation ... 

Assumed reduced parameters h -linear velocity of the mobile phase. 

Using Poiseuille s formula, the calculation shows that for concentric-tube nebulizers, with dimension.s similar to those in use for ICP/MS, the reduced pressure arising from the relative linear velocity of gas and liquid causes the sample solution to be pulled from the end of the inner capillary tube. It can be estimated that the rate at which a sample passes through the inner capillary will be about 0.7 ml/min. For cross-flow nebulizers, the flows are similar once the gas and liquid stream intersection has been optimized. 

Reduce linear flow velocities to eliminate static charge buildup during feed... 

If the linear velocity is high and u Dm/r, then equation (1) reduces to. 

The primary section of the chamber is characterized by its cross-sectional area (W X H) and by its length (L). The cross-sectional area is designed to be larger than the inlet and exit ducts in order to reduce substantially the gas stream s inlet linear velocity. The length of the chamber determines the amount of time the particles remain at the redueed rate. This starving of the gas s forward motion allows the partieles sufficient time to settle out into the hoppers. 

Generally, size exclusion chromatography is carried out using columns with an internal diameter of 7.8 mm. However, some SEC applications require the use of expensive solvents. For this purpose, size exclusion columns with a smaller internal diameter (4.6 mm) have been developed. Of course one should use proportionally lower flow rates with narrow-bore columns. If the standard column size uses a flow rate of 1 ml/min, then the smaller 4.6-mm columns should be used at a flow rate of 0.35 ml/min. This provides the same linear velocity as 1 ml/min on 7.8-mm columns. The decreased flow rate reduces solvent consumption and solvent disposal cost. The performance of the smaller diameter columns is not compromised if properly optimized instrumentation is used. 

Conventionally, analytical SEC columns have been produced with an internal diameter of 7.5 mm and column lengths of 300 and 600 mm. In recent years environmental and safety issues have led to concerns over the reduction of organic solvent consumption, which has resulted in the development of columns for organic SEC that are more solvent efficient (13). By reducing the internal diameter of the column, the volumetric flow rate must be reduced in order to maintain the same linear velocity through the column. This reduction is carried out in the ratio of the cross sectional areas (or internal diameters) of the two columns. Eor example, if a 7.5-mm i.d. column operates at 1.0 ml/min, then in order to maintain the same linear velocity through a 4.6-mm i.d. column the flow rate would be... 

When a pump is taking its suction from a tank, it should be located as close to the tank as possible in order to reduce the effect of friction losses on the NPSH available. Yet the pump must be far enough away from the tank to ensure that correct piping practice can be followed. Using a larger diameter line to limit the linear velocity to a level appropriate to the particular liquid being pumped can usually reduce pipe friction. Many industries work with a maximum velocity of about 5 feet per second, but this is not always acceptable. 

High linear velocity (reduced extraction time)... 

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