I am using a cellulose based chiral column to separate the enantiomers of organophosphates on an HP 1100 system that is equipped with a diode array detector. I observe an increase in peak areas when the flow rate is reduced. One example, when the flow rate (100% hexane) was decreased from 0.8 ml/min to 0.3 ml/min the areas of the separated enantiomers increased by a factor of three (monitored at 205 nm). My sources tell me that UV response is relatively insensitive to flow rate, but to date I have observed the same phenomenon for four organophosphates. The injected amounts are in the lower range of the linear calibration lines that were determined for each organophosphate.

UV response expressed as peak height is insensitive to flow rate. This is not necessarily so for UV response expressed as peak area.

Think of the peak as a volume of liquid emerging from the column and entering the detector. The detector cell contains a finite volume (much) less than that of the peak. If the peak flows slower through the detector, it is measured over a longer time. Hence, the peak area (which is usually expressed in units like volt-seconds) is greater when the flow rate is lower.

Assuming no change in column efficiency, the relationship should be direct. Hence in your case one would expect peak area to increase by a factor of 0.8/0.3 = 2.7, which is approximately what you report.

If you were to change the units of peak area from volt-seconds to volt-milliliters (by multiplying by the flow rate) you would observe little or no change in peak area due to change in the flow.