Negative Absorption

Negative absorbance can be the result of a decreasing absorption of the eluent after Autozero (which usually occurs at the beginning of a run). At the default setting, the detector will become saturated when the actual transmission is 20% above the autozero level. The extent of negative absorbance depends on:

• The measured wavelength,

• The spectrum of the lamps (deuterium and tungsten),

• The spectrum and concentration of the different eluent components (present in the cell), and

• The spectrum and concentration of other substances (present in the cell).

NegativeAbsorptionLevel

Usually, the PDA, PDA-100, and PDA-3000 can detect signal values down to about -0.1 AU (default value 1) at a wavelength of 240 nm. Use the NegativeAbsorptionLevel command to change the default and to enter the level up to which negative values can be detected. If the baseline becomes flat and truncated and shows no noise, increase the NegativeAbsorptionLevel setting. This increases the available signal range to lower values. This is especially recommended when running a gradient application in which the absorbance of the eluent decreases by several hundred mAU.

The following table is intended as a guideline and shows which approximate values in mAU can be achieved for certain wavelengths when the negative absorption level setting is changed:

 Tip:

Baseline noise may increase as the negative absorbance level increases.