Q: At our company, we recover all of the eluate ejected from the detector in the reservoir.
The baseline probably rises because the amount of sample component in the mobile phase originating from the eluate increases, and consequently the absorbance of the mobile phase itself increases.
The negative peak probably represents a decrease in target component concentration in the elution band at the position of the target component. When sample solvent is injected in the mobile phase, which contains the target component, an elution band with a relatively small amount of target component is created. This band travels through the column at the same speed as the target component and so it gives rise to a negative peak.
If a negative peak appears at the elution position of the target component, the influence of a reduction in area is particularly large when there is a low concentration of target component. If there is a large amount of mobile phase available, rather than remake the mobile phase, re-inject the standard sample, and redo the calibration curve.