If you are performing oil & grease analyses according to EPA Method 1664, you are familiar with the requirement to dry your extract prior to evaporation. There are those who might perform this step for reasons such as “this is the way we’ve always performed our extractions” or “the government-regulated method told me so” or “we have a giant container of sodium sulfate in the lab, so we might as well use it”; however, there is sound logic in removing water from your organic solvent prior to evaporating it.
If you read the first blog post (the basics of solvent drying) in this 2-part series, you know that it covered a brief summary of solvent drying – and how it differs from solvent evaporation. If you feel like you need a quick refresher, feel free to skim through it before reading on. Looking for a more detailed comparison between solvent drying and solvent evaporation? This infographic (solvent drying vs solvent evaporation) will help with that.
Once your hexane extractable materials have been extracted, it is important to dry the extract to remove any residual water. If water is present during the solvent evaporation step, analyte recoveries will suffer from both poor accuracy and reproducibility. Most organic solvents have boiling points that are well below that of water. So when you set your evaporation system to run at 40˚C or 50˚C (or whatever temperature is sufficient, given the boiling point of the organic solvent you are working with), the water will remain behind, relatively unaffected by the gentle heating. If you were to make the gravimetric measurement of the extract, the weight of the water would contribute to the weight of the sample and produce an inaccurate result. The reproducibility of your results will not fare any better. Each extraction will result in slight differences in the amount of water that ends up in the extract, so if you are not thoroughly drying your extracts during the evaporation step, the water weight that contributes to your results will vary from extraction to extraction.
There are a few different approaches you can take when drying your extracts. One of the more commonly used approaches involves the use of sodium sulfate. The reasons for using it seem obvious –
- you can buy it in bulk
- it’s pretty safe to use (assuming you follow standard safe laboratory practices)
- the procedure for using it is easy to learn
- it’s easy to dispose of when you’re done with it
On the downside, sodium sulfate has to be added post-extraction (which means one more step in your process and one more opportunity to contaminate your sample), it has to be carefully stored to keep it dry prior to use, inconsistencies in technique can affect analyte recoveries, and chemical drying creates the chance for unwanted by-products.
A better alternative is to use physical separation to remove water from the extracts. The WaterTrap™ membrane is perfectly suited for this.
In addition to being economical and super easy to use, the WaterTrap provides in-line drying – no post-extraction drying required! The WaterTrap uses physical separation to remove water, eliminating the possibility for unwanted side reactions. Plus, the materials in the WaterTrap are non-hazardous so when the extraction is done, you can just throw it in the trash and install a new trap for the next sample!
Too good to be true? It isn’t! Check out this product note (The WaterTrap for Water Removal) to make your own assessment.