I have aqueous samples heavily loaded (to saturation) with CaCO3 and CaOH2. I analyse my analytes (sugars) by derivatising them as alditol acetates followed by GCMS analysis. In theory, the last step of the derivatisation, which is a liquid-liquid extraction, should leave behind any ions (like calcium) as I'm only taking the dichloromethane (DCM) phase which contains my derivatised sugars. In reality, the GCMS instrument seems to be under strain due to my samples.
How could I make sure I remove all calcium from my samples? I was thinking of adding an extra step at the begging with addition of 0.5mL pure methanol to my samples (0.1mL), followed by mixing and centrifugation to precipitate calcium, followed by complete freeze drying and posterior resuspension of the residue with 0.1mL of water (as my samples were before). The main problem I have is that my freeze-drier only goes to -55C so the samples in methanol never freeze; all liquid evaporates in 1.5-2h but I can't help but feeling that this might be too long.
Any thoughts? I can't buy another freeze drier. Would any other solvent (that precipitates calcium) freeze at -55C and?
You can use a solution of Oxalic acid in your extraction. That will give you a preciptiate of Calcium Oxalate.
Either of the above methods would work. If you are freeze drying your samples post-derivitization, then I would recommend using acetone to dissolve the organics. In my experience this is a good way to remove salts from a solid sample.
Oxalic acid is very good. Its solubility in water is 9,52 g in 100 g water (20 °C). However oxalic acid is very toxic and there might be interactions with your solutions. Therefore I would recommend to use the potassium salt of oxalic acid (C2O4K2). Its solubility in water is 39,5 g/ in 100 g water (20 °C). The solubility of other salts of oxalic acid is always distinctly lower. The solubility data are from Chemiker Kalender, Springer 1984.
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