determination of mercury by cold vapor with sodium borohydride

good evening. I am analyzing a sample in the vp100 (thermo scientific hydride generator - cold vapor) but when introducing the sample the controls go down. For example, the control before the sample gives me 40 ug/L and after the sample it drops to 25 ug/L. I must let water or HCl 1.5% flow for up to 20 min so that the control reaches 40 ug/L again. the sample is digested as follows: 0.5 g of solid plus 5 ml of concentrated HNO3. dissected by microwave oven and subsequently filtered to 50 mL. the sample apparently has no solids and is not cloudy. I would appreciate your comments