Injector and Detector Requirements

In packed column SFC, injection is normally carried out by switching the contents of a sample loop into the mobile phase. Flow and column diameter here do not generally affect separation efficiency. For capillary SFC, much smaller injected volumes are required, and these are introduced by splitting the injected sample, or more usually by time-split injection, in which the injection valve is opened and closed very rapidly by pneumatic operation. A retention gap, located before the column, may permit focusing of larger amounts of sample. Injection volumes are determined from a general equation which relates ti and tc, the standard deviations in the peak widths due to the injector (or detector) and the column. N and Ni are respectively the number of theoretical plates provided by the column and the number of those filled by injector or detector or detector volume:

4KndF

Table 1

ficients

Sample loadability ratios for different distribution coef-

From eqn [21], an expression has been derived linking fractional loss of resolution, ARs, due to the injector or detector volume to column and retention characteristics:

Table 1

ficients

Sample loadability ratios for different distribution coef-

Kd

Lp

Lc

1

31

10

101

100

373

Packed column: ¿, = 20 cm; dp = 5 |im; dpc = 1 mm; £¡ = 0.5; £e = 0.4; £T = 0.7. Open tubular column: Lc10m; dc = 50 |im; dF = 0.2 |im.

where vi is the injection volume and L is column length. Since H is a function of dc, vi is proportional to d5J2 and for a 10 m x 50 |im column operated under practical conditions, a 1% loss of resolution requires an injection volume of only 50 nL.

For equal distribution coefficients, KD, an equation relating the maximum loadabilities, Lp and Lc, for packed and open tubular columns has been derived:

where dpc is the diameter of the packed column; g;, ge and gT are intraparticle, interstitial and total porosities, respectively; and Lp and Lc are the lengths of the packed and open tubular columns, respectively.

Table 1 lists the maximum sample loadability ratios for different KD values for typical packed and open tubular columns; as might be expected, sample loadabilities are much greater on packed columns.

Packed column: ¿, = 20 cm; dp = 5 |im; dpc = 1 mm; £¡ = 0.5; £e = 0.4; £T = 0.7. Open tubular column: Lc10m; dc = 50 |im; dF = 0.2 |im.

Increasing the column diameter is the simplest approach to increasing the sample capacity of packed columns, since the surface area of stationary-phase support particles is generally maximized. However, for capillary columns, sample capacity can be increased by increasing the stationary phase film thickness, dF. This should also lead, in theory, to a reduction in column efficiency but in practice such a reduction is only small, with film thickness up to 1 |im.

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