How does resolution (slit width) influence spectral peak height and shape?

Besides spectral resolution, the slit also controls the amount of light energy incident on the sample. Therefore, the spectral noise level will be significantly affected by changes in the slit setting. The larger the slit setting, the lower the noise in the spectra. The smaller the slit setting, the higher the noise in the spectra.

Above are spectra that show the results of a lack of resolution. The black spectrum is fully resolved with a slit width five times lower than the FWHM. The red spectrum has a slit width equal to the FWHM. Note how the unresolved spectrum has photometric values lower than their “real values”. In addition, the unresolved spectrum has broader peaks with less separation between overlapping bands. Having an instrument with a large fixed slit width could cause inaccurate absorbance measurements on materials where narrow bands are a possibility.