Waters 996 - Nonidealities; Detector Noise; Absorbance Spectra of a Compound at Two Concentrations

60 Spectral Contrast Theory
5
Figure 5-5 Absorbance Spectra of a Compound at Two Concentrations
5.4 Nonidealities
Shape differences between absorbance spectra can be caused by one or more of the
following nonideal phenomena:
• Detector noise
• Photometric error caused by high sample concentration
• Variation in solvent composition
These sources of spectral variation can cause chemically pure, baseline-resolved peaks
to exhibit a small level of spectral inhomogeneity. You can assess the significance of
spectral inhomogeneity by comparing a Spectral Contrast angle to a
Threshold angle (see
Section 5.4.4).
5.4.1 Detector Noise
Statistical and thermal variations add electrical noise to the absorbance measurements
made by the 996 detector. The noise manifests itself as fluctuations in the baseline, known
as
baseline noise . The magnitude of any absorbance differences caused by statistical and
thermal variations can be predicted from the instrument noise in the baseline region of a
chromatogram.
Normalized Absorbance
Wavelength (nm)
Spectral Contrast Angle: 3.4
°
Normalized Spectra of a Compound at Different
Concentrations