Salting Out: A Simple and Reliable Method to Distinguish Between Common Fluid Preservatives and Estimate Alcohol Concentration

Abstract

This paper details the salting-out method, which uses the salts potassium carbonate and sodium chloride to distinguish between the three most commonly used fluid preservatives: ethanol, isopropanol, and formalin. A summary of other methods to identify fluid preservative type and a review of the salting-out method published by Mayfield (2013, Distinguishing between ethanol and isopropanol in natural history collection fluid storage, Society for the Preservation of Natural History Collections, https://spnhc.org/wp-content/uploads/2018/11/Mayfieldfinalwithtablechanges.pdf) are provided. A new salting-out method is presented, which requires a small fluid sample (2–4 ml). It is simple, quick, and relatively inexpensive to implement, making it a viable method to distinguish between common fluid preservatives. The materials and equipment for the salting-out test cost just over $100 US, and tests take approximately 3 minutes per container. Results of testing on known concentrations and combinations of ethanol, isopropanol, and formalin (a solution of formaldehyde in water) and on samples of fluid preservatives from specimen containers in the Smithsonian National Museum of Natural History and Bernice Pauahi Bishop Museum collections are presented. The results of salting-out tests have been verified by direct analysis in real time mass spectrometry (DART-MS) (Cody et al., 2005, Versatile new ion source for the analysis of materials in open air under ambient conditions, Analytical Chemistry 77(8):2297–302), which confirmed the results of salting-out tests but also highlighted some limitations, particularly when combinations of fluid preservative are encountered.