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76 Agilent 1260 Infinity VWD User Manual
5
How to optimize the detector
Match the Flow Cell to the Column
Flow Cell Path Length
Lambert- Beer’s law shows a linear relationship between the flow cell path
length and absorbance.
where
Therefore, flow cells with longer path lengths yield higher signals.
Although noise usually increases little with increasing path length, there is
a gain in signal- to- noise ratio. For example, in
Figure 31 on page 77 the
noise increased by less than 10 % but a 70 % increase in signal intensity
was achieved by increasing the path length from 6 mm to 10 mm.
When increasing the path length, the cell volume usually increases — in
the example from 5 – 14 µL. Typically, this causes more peak dispersion.
As demonstrated, this did not affect the resolution in the gradient
separation in the example shown below.
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).
T is the transmission, defined as the quotient of the intensity of the transmitted light I
divided by the intensity of the incident light, I
0
,
e is the extinction coefficient, which is a characteristic of a given substance under a
precisely-defined set of conditions of wavelength, solvent, temperature and other
parameters,
C
[mol/L] is the concentration of the absorbing species,
d [m] is the path length of the cell used for the measurement.