Does anyone have any good ideas on how to desalt small volumes of seawater (1mL)? My aim is to remove inorganic salts prior to injection on LC-MS for analysis of LMW organic acids and GC-MS for sugars and amino acids. I have considered a few desalting columns, but many appear to be geared at analysing proteins and not LMW organic compounds. I have tried passing some sample through a resin with both cation and anion exchange, but there still seemed to be high salt at the end.
Dear Belinda, if you are using LC-MS, I figure with a RP colums, you don't need to desalt your samples, just use a split valve to discharge the first part of your chromatography. for the GC-MS it depends on your system. You can use a SPE cartridge to absorb your analites, filters the salt, then ricover your analites.
Dear Belinda, a possible way to remove inorganic salts from your samples is to use a SPE cartridge, that can also be useful to concentrate your analites. If you are using a HPLC-MS with a RP column you can simply dischcarge the initial part of your eluate by using the swithch valve of the MS.
If you're looking at organic acids, they would come off your column quite early, so directing the first minute of your eluent to waste might mean you could loose the first one or two peaks. My tip: try it anyway. Do you know the RT of your smallest org. acid? If your LC column can go 100% aqueous, try that for your starting conditions. Normally I would also recommend to add ~0.1% Formic acid to your mobile phase to make sure the acids stay protonated, but I'm guessing you're detecting in (-)ve mode?
Massimo is right. RP-SPE might be a good starting point (numerous SPE protocols are available from vendor websites)(Review Agilent, Thermo and Waters website). However, sugars are polar. Are you aliquoting them separately for LC-MS and GC-MS ? In some LC-MS set-up, on-line desalting could be performed using the right trap column (RP C8 would perhaps work).
Thanks for all the feedback. I think I have to do some research to choose the correct resin for SPE to make sure I don't loose the smaller molecules. I'm very limited by my sample volume so can only afford limited trials. I've achieved a good tip for the future: get out of marine science 😊
Hi Luke,
From what I can tell from dialysis tubing it separates based on molecular weight. As the metabolites I am interested in are also low molecular weight I am unsure if this would adequately separate them from the salts, unless there is a specific membrane you are referring to?
Thanks,
Belinda
Thanks Gaetan,
I am worried that the precense of salt may supress the signal on the GCMS as well as potentially muck up the column etc...I am using the derivisiating agents TMCS and BSTFA. Perhaps there are ones better suited for complex matrices?
As alredy suggested, SPE will probably work. Using mixed-mode RP -anion exchange cartridges would probably be a good approach. Alternatively you can use RP cartridges with loading and washing solutions that maintain your acids uncharged, and/or including a volatile ion pairing reagent. If the probles is very frequent in your analytical work, implementing an online SPE (even using a guard column as SPE colum) would be faster and cheaper.
for sure. salts will destroy you GC/MS
I worked with synthetic wastewater analysis,
any way you can first filter your sample and then add 1 ml of n-hexane to the 5 ml of seawater, shakewell for several minutes, centrifuge it and use the upper layer for GC.
you may also read this discussion I hope it will be help' you :
https://www.researchgate.net/post/How_to_purify_sea_water_before_injecting_it_into_the_GC_MS
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