Waters 2998, Обзор и руководство по обслуживанию

Страница: 61 / 94

December 5, 2017, 715004753 Rev. B
Page 61
5
Spectral Contrast Theory
The spectral contrast algorithm compares the UV/Vis absorbance spectra of samples that
the detector collects. This chapter describes the theory on which the algorithm is based by
explaining how it exploits differences in the shapes of the absorbance spectra. It also
explains how spectral contrast represents those spectra as vectors, determining whether
differences among them arise from the presence of multiple compounds in the same peaks
(coelution) or from nonideal conditions like noise, photometric error, or solvent effects.
5.1 Comparing absorbance spectra
When measured at specific solvent and pH conditions, the shape of a compound’s absorbance
spectrum characterizes the compound. The varying extent of UV/Vis absorbance occurring at
different wavelengths produces a unique spectral shape.
The following figure shows the absorbance spectra for two compounds, A and B. The ratio of the
absorbance at 245 nm to that at 257 nm is about 2.2 for compound A and 0.7 for compound B.
Note that this comparison of a single wavelength pair’s absorbance ratios yields little information
about a compound. For more information, you must compare the ratios of multiple wavelength
pairs.
Figure 5–1: Comparing spectra of two compounds
TP02838
Ab
245
Ab
257
--------------- 2.2 =
Ab
245
Ab
257
--------------- 0.7 =
Compound A:
Compound B:
nm
AU
220.00 240.00 260.00 280.00 300.00 320.00 340.00
0.40
0.20
0.00
Compound A Compound B
245 nm
257 nm