Which of the following is a cause of contamination during the sample preparation?

• A. Carryover of previous sample due to uncleaned sample preparation apparatus.
• B. Sample contained in a required container following the precautions specified in the sampling method.
• C. Sample preparation apparatus is cleaned and washed according to requirements of sampling method.
• D. None of the above.

Answer: (A)

Carryover of residues from a previous sample is a common source of contamination in sample preparation. If the apparatus and tools aren’t cleaned between runs, materials from the earlier sample can be transferred to the next one, altering the composition and skewing results. This is why uncleaned, previously used equipment is identified as a contamination risk. Using properly specified containers and following the sampling method’s precautions helps keep samples isolated, and cleaning the apparatus according to the method minimizes contamination. So the carryover from uncleaned equipment best explains the contamination.