Cryogenic freezing: A reliable preservation method of samples for seawater nutrient analysis

Abstract

Freezing is a strongly recommended preservation method of samples for seawater nutrient analysis when onboard analysis is not available. Previous studies on the effects of freezing methods have explored issues including freezing period, prefiltration, and freezing speed. In this study, the effects of various freezing and storage temperatures on seawater nutrient analysis were examined using a domestic freezer (-18 degrees C), scientific freezer (-23 degrees C), and cryogenic freezer (-80 degrees C). The results showed no difference between the fresh and frozen samples stored in the cryogenic freezer. The observed differences between samples from the domestic freezer and those from the scientific freezers can be attributed to the freezer auto-defrost function. Transferring samples between the scientific and cryogenic freezers maintained their condition during freezing and storage when a scientific freezer without an auto-defrost function was used. Silicate concentrations in frozen samples in the cryogenic freezer were similar to those of the fresh samples, even when the frozen samples were analyzed several hours after thawing. Therefore, a cryogenic freezer is highly recommended for freezing and storing seawater nutrient samples because of its stability and independence from silicate polymer formation.