130 1220 Infinity II LC System User Manual
8
Detector Description
Agilent 1220 Infinity II LC Diode Array Detector (DAD)
step of 4 nm would be sufficient. However a step of 2 nm gives a more optimal
display of the spectrum.
Threshold
Sets the peak detector. Only spectra from peaks higher than threshold will be
stored when a peak-controlled storage mode is selected.
Margin for Negative Absorbance
The detector adjusts its gain during balance such that the baseline may drift
slightly negative (about -100 mAU). In some special case, for example, when
gradient with absorbing solvents are used, the baseline may drift to more
negative values.
Only for such cases, increase the margin for negative absorbance to avoid
overflow of the analog-to-digital converter.
Optimizing Selectivity
Quantifying Coeluting Peaks by Peak Suppression
In chromatography, two compounds may often elute together. A conventional
dual-signal detector can only detect and quantify both compounds
independently from each other if their spectra do not overlap. However, in
most cases this is highly unlikely.
With a dual-channel detector based on diode-array technology, quantifying
two compounds is possible even when both compounds absorb over the whole
wavelength range. The procedure is called peak suppression or signal
subtraction. As an example, the analysis of hydrochlorothiazide in the
presence of caffeine is described. If hydrochlorothiazide is analyzed in
biological samples, there is always a risk that caffeine is present which might
interfere chromatographically with hydrochlorothiazide. As the spectra in
Figure 43 on page 131 shows, hydrochlorothiazide is best detected at 222 nm,
where caffeine also shows significant absorbance. It would therefore be
impossible, with a conventional variable wavelength detector, to detect
hydrochlorothiazide quantitatively when caffeine is present.
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