Agilent Technologies 1290 Infinity LC System System Manual And Quick Reference page 63

3
Optimization of the Agilent 1290 Infinity LC System
How to Achieve Higher Sensitivity
Slit Width
Light transmission into the spectrograph and the optical bandwidth are
controlled by the variable aperture entrance slit. The default setting for the slit
width is 4 nm which is appropriate for most applications as it gives good
all-round performance. The performance characteristics affected are
sensitivity, spectral resolution and linearity. Considering a particular
wavelength entering the spectrograph, its light will effectively fall onto a small
band of diodes, the width of which is proportional to the width of the entrance
slit. The description of the slit as 4 nm describes this behavior – the light falls
on the number of diodes that detect a bandwidth of 4 nm. It follows that the
minimum optical resolution will be 4 nm and therefore the diode-array (or
digital) bandwidth should be set to 4 nm or greater. For optimum sensitivity
the 8 nm setting will allow most light in and will minimize noise but spectral
resolution is at its lowest. This is not usually a problem with UV spectra as
their natural bandwidths are usually greater than 25 nm without any fine
structure. The optical bandwidth at 8 nm reduces the linearity range
compared to 4 nm slit so it is important that a validated method always uses
the slit width that was used for validation. For optimum spectral resolution
the 1nm setting is best. This will enable fine structure such as in the benzene
spectrum to be resolved (see Figure 24 on page 63). Very few compounds
display such fine detail in solution spectra. The light level will be lower so the
signal will have more noise - the noise level depends on the wavelength and
mobile phase solvents used.
Figure 24 Benzene at 1 and 4 nm Slit Width (Principle)
Agilent 1290 Infinity LC System Manual and Quick Reference 63