DAD 6.1L/DAD 2.1L/MWD 2.1L user manual V6700, version 3.2
Optimizing the Detector
29
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 (
≥
3,0 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
increases 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 affects
the light intensity that is detected.
For the same concentration, the peak height will be higher if the path
length is longer. Path lengths of 3 mm, 10 mm, and 50 mm are available for
the detectors. A longer path length, therefore, increases the sensitivity of a
method (see figure 25). The limit of detection is inversely proportional to
the path length.
Wetted Parts
The flow cell materials must be chemically compatible with the solvents and
sample you are working with. Titanium and metal-free flow cells are avail-
able. All variants are bio-inert.
Pressure Stability
The different flow cells can withstand various maximum pressures. The
upper pressure limits of the flow cells are 30 bar, 50 bar, or 300 bar. The
flow cell should not be subjected to the maximum pressure for a long
period of time.
Fig. 24
Beer-Lambert law
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 depen-
dent)
b:
path length
c:
analyte concentration (temperature-dependent)
Fig. 25
path length comparison
A
T
log
–
I
I
o
----
log
ε
d
c
×
×
=
=
=
