The 1290 Infinity III Diode Array Detector (G7117B) has a variable slit at the entrance of the spectrograph. This is an effective tool to adapt the detector to changing demand of different analytical problems.
A narrow slit provides spectral resolution for analytes with very fine structures in the absorbance spectrum. An example of such a spectrum is benzene. The five main absorbance bands (fingers) are only 2.5 nm wide and just 6 nm apart from each other.
A wide slit uses more of the light shining through the flow cell. This gives lower baseline noise as shown in.Influence of the slit width on baseline noise
However, with a wider slit, the spectrograph’s optical resolution (its ability to distinguish between different wavelengths) diminishes. Any photodiode receives light within a range of wavelength determined by the slit width. This explains why the fine spectral structure of benzene disappears when using a 8 nm wide slit.
Furthermore, the absorbance is no longer strictly linear with concentration for wavelengths at a steep slope of a compound’s spectrum.
Substances with fine structures and steep slopes like benzene are very rare.
In most cases the width of absorbance bands in the spectrum is more like 30 nm as with anisic acid ( Optimization of wavelength setting).
In most situations, a slit width of 4 nm will give the best results.
Use a narrow slit (1 or 2 nm) if you want to identify compounds with fine spectral structures or if you need to quantify at high concentrations (> 1000 mAU) with a wavelength at the slope of the spectrum. Signals with a wide bandwidth can be used to reduce baseline noise. Because (digital) bandwidth is computed as average of absorbance, there is no impact on linearity.
Use a wide (8 nm) slit when your sample contains very small concentrations. Always use signals with bandwidth at least as wide as the slit width.
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