Methanesulfonic Acid in Mobile Phase

[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?

[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....

[Uwe Neue]

Does anybody know what the pKa of methanesulfonic acid is?

[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?

[syx]

It is corrosive to iron, steel, brass, copper, and lead (Merck Index). Its pKa= 1.92.

[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.

[syx]

The method is taken from USP General Monographs for Cyproheptadine Hydrochloride Tablets...

[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.

[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.