Because there are many contaminants in our hands, like cream, lotion, or toxins that may make the substance of our sample has been adulterated by these substances and makes the result invalid.
Because we are in the hands of contaminants such as creams, lotions, or toxins, which may cause our sample had been contaminated by these substances. And the result was not valid.
Because we have in hand many contaminants such as cream, lotion or toxins, which may make the sample has been contaminated by these substances. The result was incorrect.