6
•
Operation
Diode Array/Multiple-Wavelength Detectors (VC-D11, VC-D12, VF-D11)
Operating Manual
Page 103
Bandwidth
The bandwidth specifies the spectral bandwidth at which a
chromatogram (UV_VIS channel) is recorded. You can accept the
standard bandwidth in Chromeleon, or set the bandwidth to a higher or
lower value. Setting a wider bandwidth will average more photodiode
signals. This process is referred to
photodiode bunching
.
Note the following:
•
The averaging always takes place symmetrically to the selected
wavelength.
•
When performing a detection near the edge of the spectral range
If the bandwidth setting results in an averaging asymmetrically to
the selected wavelength, the device will correct the setting.
Example:
If you perform a detection at a wavelength of 195 nm with
a bandwidth of 20 nm, the photodiode bunching is set to 190 to
200 nm, i.e. the largest possible symmetrical range around the
detection wavelength. As a result, the bandwidth is changed to
10 nm. This is done to avoid a wavelength shift as would be the case
with an asymmetrical averaging.
The following table serves as guidance for selecting the bandwidth
based on the spectral features of the analyte to be detected:
Spectral Features
Bandwidth
Effect
Samples with extremely fine
spectral features, such as
benzene, and very high
absorptions.
< 4 nm
May increase baseline noise.
Samples for "normal" analysis
with fine spectral features,
such as caffeine.
4 nm – 8 nm
Reasonable compromise between
low baseline noise, good linearity
and low cross-sensitivity
Samples with broad spectral
features.
> 8 nm
Most suitable for low noise. A
reduced linearity and increased
cross-sensitivity may occur.