Gilson 155 User Manual page 61

Operation
Ratio threshold
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In dual wavelength mode, output channel 3 shows
a ratio of the absorbance at wavelength 1 to the
absorbance at wavelength 2. The ratio can help you
determine peak purity.
The absorbance ratio is displayed only when the
absorbance at both wavelengths exceeds a specified
threshold.
Positive ratios indicate that the peak at wavelength
1 is larger than the peak at wavelength 2. Negative
ratios indicate that the peak at wavelength 2 is
larger than the peak at wavelength 1.
Consider two points when setting the ratio
threshold:
• What is the height, in full scale absorbance
units, of the trace at each wavelength?
• How high is the noise floor, in absorbance
units, on each trace.
Set the ratio threshold (in AU) higher than the
highest baseline noise level. This prevents the
detector from interpreting a noisy baseline as a
series of peaks.
In general, the higher the baseline noise, the larger
the ratio threshold setting.
Two useful references that describe the use of ratio
plots are:
• "Real-Time Absorbance Ration and
Absorbance Difference Measurements Using
Dual-Wavelength HPLC Detectors." J. Chrom.
Sci., 21:447, 1983.
• "Absorbance Ratio Plots in High Performance
Liquid Chromatography: Some Software
Problems and Remedies." J. Chrom. Sci.,
23:227, 1985.
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