Negative DV values

Dear NM users: I have a dataset where some of the concentrations are reported as negative values. I believe that the concentrations were calculated using a standard curve. My instinct is to impute all the negative values to zero, but worry that it will introduce bias. A 2nd thought is using the absolute value of the lowest (negative) concentration as LLOQ. All the concentrations below LLOQ will be treated as zero. By doing this, some positive and negative values both will be zero out which will help to cancel some of the unevenness that the 1st method may introduce. I believe that the 2nd method is better but wonder if there is any other better way to do it. Any comments will be greatly appreciated. Thank you in advance. Siwei