138 1220 Infinity II LC System User Manual
8
Detector Description
Match the Flow Cell to the Column
Therefore, flow cells with longer path lengths yield higher signals. Although
noise usually increases a little with increasing path length, there is a gain in
signal-to-noise ratio. For example, the noise increases by less than 10 %, but a
70 % increase in signal intensity is achieved by increasing the path length from
6 mm to 10 mm.
When increasing the path length, the cell volume usually increases — in our
example, from 5 μL to 13 μL. Typically, this causes more peak dispersion. As
Figure 50 on page 138 demonstrates, this does not affect the resolution in the
gradient separation in our example.
As a rule-of-thumb, the flow cell volume should be about 1/3 of the peak
volume at half height. To determine the volume of your peaks, take the peak
width as reported in the integration results, multiply it by the flow rate and
divide it by 3.
Figure 50 Influence of cell path length on signal height
Traditionally, LC analysis with UV detectors is based on comparing
measurements with internal or external standards. To check photometric
accuracy of the detector, it is necessary to have more precise information on
path lengths of the flow cells.
The correct response is:
expected response * correction factor
Details of the flow cells are shown in Table 24 on page 139 and Table 25 on
page 140.
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