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Methanesulfonic Acid in Mobile Phase
Posted: Tue Jul 19, 2005 12:13 am
by syx
I have found a method using methanesulfonic acid in the mobile phase:
Methanesulfonic acid solution – Prepare a solution of methanesulfonic acid in water (3:1000)
Mobile phase – Prepare a filtered and degassed mixture of acetonitrile, isopropyl alcohol, and Methanesulfonic acid solution (20:15:65); while mixing adjust with triethylamine to a pH of 4.0 +/- 0.05 …
Questions:
Is there any purpose of methanesulfonic acid other than as pH controller in the mobile phase?
Could methanesulfonic acid be substituted by other kind of acids?
Posted: Tue Jul 19, 2005 10:28 am
by javaslinger
I have seen methanesulfonic acid used as an ion-pairing agent for charged compounds to slow the elution as they often come out with the solvent front otherwise....
Posted: Wed Jul 20, 2005 1:57 am
by Uwe Neue
Does anybody know what the pKa of methanesulfonic acid is?
Posted: Wed Jul 20, 2005 7:30 am
by HW Mueller
Since I also wondered about this: Checked my usual sources, there was nothing, tried the net and found
http://www.cem.msu.edu/~reusch/VirtualText/acidity2.htm
MSA was given there as pKa = ~-2. Therefore, it seems to me that if there is an increase of rt due to this acid it would have to be via its adsorption on the stat phase and ion exchange with the analyte?
Posted: Wed Jul 20, 2005 7:43 am
by syx
It is corrosive to iron, steel, brass, copper, and lead (Merck Index). Its pKa= 1.92.
Posted: Wed Jul 20, 2005 9:46 pm
by Uwe Neue
If its pKa is indeed 1.92, what is this method doing at a pH of 4.0? If both pieces of information are correct, the method is junk.
Posted: Thu Jul 21, 2005 7:16 am
by syx
The method is taken from USP General Monographs for Cyproheptadine Hydrochloride Tablets...
Posted: Thu Jul 21, 2005 7:37 am
by HW Mueller
syx,
you either forgot the minus sign(-) or got the second diss. constant for H2SO4 (CRC tables: pKa2 = 1.92; Ka2 = 1.2x10^-2). Methanesulfonic has a Ka ~ 10^2 (pKa ~ -!!2), so is ~ totally dissociated.
Posted: Thu Jul 21, 2005 6:37 pm
by Mark Tracy
Methanesulfonic acid has good UV properties and can be a substitute for trifluoroacetic acid. Dionex uses MSA in eluents for many of our cation exchange methods.