35
Optimizing the detector
AZURA® Detector DAD 6.1L, DAD 2.1L, MWD 2.1L Instructions, V6700
small, the noise may be higher and the signal may be too small due to
less light reaching the photodiodes.
Ideal flow cell volume therefore is a compromise between peak broade-
ning and sensitivity (see Fig. 25).
Fig. 25:
Volume comparison
A good rule of thumb is that the flow cell volume should not be more
than 1/3 of the peak volume of your separated sample. 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.
Cartridge flow cells with volumes of 2 µl, 6 µl and 10 µl are available for
the detectors. Narrow-bore columns (~ 2,1 mm ID) are suitable for flow
cells with smaller volumes. Columns with with a larger inner diameter
(³ 3,0 mm ID) are less affected by the volume of the flow cell.
The flow rate should also be taken into consideration. A lower flow rate in-
creases the axial and longitudinal diffusion and adds to a broadened flow
profile which may lead to a peak broadening.
Path length
As described by the Beer-Lambert law, the path length of a flow cell af-
fects the light intensity that is detected.
A:
measured absorption at a given wavelength
T:
transmittance, defined as the quotient of the light intensity (
I)
after passing through the sample and the initial light intensity (
I
0
)
before passing through the sample
ε
:
molar absorptivity coefficient (wavelength and temperature
dependent)
d:
path length
c:
analyte concentration (temperature-dependent)
