Chromatography Forum

What is the recommended way for extracting water from small ( ~ 3 mL) sample solutions using anhydrous sodium sulfate? For example, for the "Fatty Acid Composition" test in USP <401> for Fats and Fixed Oils.

The sample of fatty acid esters is in about 3 mL of n-heptane left over after refluxing the sample and mixing it with (aqueous) saturated sodium chloride solution. The method says to pass the n-heptane layer through 0.1 g anhydrous sodium sulfate that has been previously washed with n-heptane. When I use a funnel with a glass filter paper, by the time the sample has filtered, it is half evaporated and/or absorbed by the paper. If I just use the salt in a test tube, I risk getting particles in my sample that could clog my GC syringe during analysis.

Could someone give me the step-by-step process that most people would use? I'd heard there were syringe filters or cartridges that could help with this step, but I am having trouble finding one, particularly one that can handle such a small sample.

Regards
Melissa

One way to dry small samples is to pack the adsorbent into a disposable Pasteur pipet. Push a small piece of glass wool into the pipet, add sodium sulfate, wash with heptane, and then pass your sample through (no pressure, just gravity-feed). This whole process only takes about five minutes, and the holdup is pretty minimal since you're not saturating a paper filter. If you're using an internal standard, the small dilution of the sample by any retained wash heptane should not pose a problem.

What is the recommended way for extracting water from small ( ~ 3 mL heptane) sample solutions using anhydrous sodium sulfate? Regards, Melissa

OK, Melissa Jones: I've likely been assaying fatty acid methyl esters since before you were born. I work in the soap and consumer products industry. Here's my input:

1. If you want to dry the heptane solution, just transfer the heptane layer to a disposable vial, throw in about 1 gram of anyhdrous sodium sulfate GRANULES - not powder - then cap and shake. Transfer some of the dried heptane solution to an autosampler vial and inject away. Notice that I capitalized granules.

2. Here's what we do. If sample is soap or fatty acids, we dissolve in either sulfuric acid-methanol or BF3-methanol in a volumetric flask (to use the neck effect). Then we heat on steam bath 5 minutes, allow to cool, add a few ml of petroleum ether, hexane, or heptane, shake to mix, then add saturated sodium chloride to float the solvent layer into the neck of the volumetric flask. Then some of that is transfered to an autosampler vial for injection. Notice that we've never done a drying step, and our petroleum ether layer is typically crystal clear, unless a soap bar has stuff like TiO2 in it. So I doubt you'd need a drying step. We use a "2330" type column, cyano silicone type. There are similar columns for those who are more interested in all the polyunsaturated fatty acid esters.

For example, for the "Fatty Acid Composition" test in USP <401> for Fats and Fixed Oils.

We deal with USP monograph methods all the time, we have pharmaceutical products. In general, USP methods are obsolete and unwieldy, to state the obvious. So just because something is "official", it would be a mistake to assume that it's the best way. For those in the pharmaceutical industry, typically USP Monograph substances ARE tested with such USP assays, because it ends up being easier to justify to an auditor than validating a better procedure, so medicrity lives on.

For our finished consumer goods, we have to develop and validate our own procedures every time, as huge difference in assaying a pure API and a consumer product which may contain 1% API.