56
Tip:
To prevent detector noise, don’t include wavelengths where there is little
or no analyte absorption.
Vectors derived from two wavelengths
The spectral contrast algorithm uses vectors to characterize spectra. To
understand the vector principle, consider the two vectors in the figure below,
which are based on the spectra depicted in the previous figure.
Plotting vectors for two spectra:
In this figure, the axes reflect the absorbance units of the two wavelengths
used to calculate the absorbance ratio of the previous figure. The head of the
vector for Compound A lies at the intersection of the absorbance values (for
Compound A), at the two wavelengths represented by each axis. The
remaining vector is similarly derived from the spectrum of Compound B.
Compound B’s vector points in a different direction from Compound A’s.
Expressed by the spectral contrast angle (
), this difference reflects the
difference between the two compounds’ absorbance ratios at wavelengths
245 nm and 257 nm. A spectral contrast angle greater than zero indicates a
shape difference between spectra (see
“Spectral contrast angles” on page 57
).
Finally, note that the length of the vectors is proportional to concentration.
0
0.1
0.2
0.3
0.4
0.1
0.2
0.3
0.4
AU at 245 nm
AU at 257 nm
Compound B
Compound A
Summary of Contents for ACQUITY UPC2
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