Waters 2996, Operator'S Manual

Page: 64 / 84

42 Spectral Contrast Theory
5
Figure 5-1 Comparing Spectra of Two Compounds
5.2 Representing Spectra as Vectors
The Spectral Contrast technique uses vectors to quantify differences in the shapes of
spectra. Spectral Contrast converts baseline-corrected spectra to vectors and then
compares the vectors. Spectral vectors have two properties:
• Length – Proportional to analyte concentration.
• Direction – Determined by the relative absorbance of the analyte at all wavelengths
(its absorbance spectrum). Direction is independent of concentration for peaks that
are less than 1.0 AU across the collected wavelength range.
Vector direction contributes to the identification of a compound, since the direction is a
function of the absorbance spectrum of the compound. The ability of spectral vectors to
differentiate compounds depends on the resolution of spectral features. As both
wavelength range and spectral resolution increase, the precision of a spectral vector for the
resultant spectrum increases. A vector derived from the 2996 PDA Detector can include
absorbances in any range from 190 to 800 nm. To enhance spectral sensitivity, set the
bench resolution to 1.2 nm.
Compound B
Compound A
Ab
245
Ab
257
--------------- 2.2 =
Ab
245
Ab
257
--------------- 0.7 =
Compound A:
Compound B:
245 nm
257 nm