Pyridine substitute
Chromatography Forum: LC Message Board: Pyridine substitute
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By Anonymous on Monday, July 5, 2004 - 08:27 pm:

I am wanting to use a Supelcosil LC-SCX cation-exchange column to seperate arsenic compounds, namely arsenobetaine, arsenocholine, trimethylarsine oxide and tetramethylarsonium. Methods suggest aqueous 20mM pyridine mobile phase adjusted to pH 2.4 with formic acid. Can anyone suggest a "safer"alternative to pyridine?

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By Anonymous on Tuesday, July 6, 2004 - 05:01 am:

You could try nitric acid on its own. (adjust pH to 2.4 with nitric = approximately 4mM).
John

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By Alexander on Wednesday, July 7, 2004 - 06:23 pm:

Try non-toxic, UV-transparent and readily available hexamethylenetetramine (urotropine, pKa 5.13) in place of pyridine (pKa 5.17). It may work to replace pyridine as a base for the buffer. Alternatively, try hydroxylamine sulfate (pKa 5.96). pH adjustment may be necessary with the latter.

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By Chris Pohl on Saturday, July 10, 2004 - 02:52 pm:

Actually, it's not a good idea to use simple acid eluent on silica based cation exchange materials. When the stationary phase is totally in the hydronium form, the localized pH is sufficient to cause stationary phase attack. You would be better off using a UV transparent cation such as tetramethylammonium ion, tetraethylammonium ion or a cesium salt.

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By Jim Reeves on Monday, July 12, 2004 - 07:28 am:

Is it OK to use phosphate buffers pH 2.4 on a silica based SCX column? I have been doing that for some time and now appear to be getting voids in the column,shown as split peaks. Reversal of teh column gets rid of the splitting.
Jim

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By Mark on Monday, July 12, 2004 - 11:24 am:

Jim,
If you had a void at the column head and reversed the column flow wouldn't you still have some poor peak shape due to mixing after the peak eluted from the column? I have seen the effect you describe when there is a partial blockage of the inlet frit on the column and when you reverse the flow either the particles are purged off the frit or the partial plug at the outlet side seems to make very little difference in the peak shape.
Possibly some precip of the mobile phase buffer or particles not filtered out during mobile phase prep? Just some random thoughts,

Regards,
Mark

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By Jim reeves on Tuesday, July 13, 2004 - 02:02 am:

Thanks very much for that Mark. What you say is consistent with what I've seen.
Jim