Should samples be frozen?

• A. Yes
• B. No
• C. Sometimes
• D. Only after analysis

Answer: (B)

Preserving samples to maintain their original composition is essential. Freezing is not used for most environmental samples because ice formation can introduce artifacts that compromise the data. When a sample is frozen, volatile components can be lost during storage and thawing, and chemical forms can change as the matrix rearranges. Ice crystals can rupture cells in biological or microbial samples, releasing intracellular material and altering measured concentrations. Precipitation or adsorption to container surfaces can also occur during freezing and thawing, changing the apparent amount of analyte. Refrigeration on ice (2–6°C) slows biological activity and chemical reactions without causing those freeze-related artifacts, helping keep the sample representative until analysis within its holding time. Only analyze after freezing if the method has specifically validated freezing as acceptable; otherwise, avoid freezing.