I was wondering if anyone has any experience in quantitating levels of Na2S2O5 in drug products by HPLC (reversed/normal phase or ion chromatography).

In aqueous solution, metabisulfite is unstable (except in very concentrated solutions) and decomposes back to 2 equivalents of sulfite anion. So, it's not possible to directly measure this species via chromatography. The sulfite, of course, can be measured readily via ion chromatography. Most any column will work for this purpose but in order to get quantitative results it is necessary to either exclude oxygen from your eluent or to stabilize the sulfite by addition of formaldehyde to the sample.

We add 1ml of a 1% solution of metabisulfite to our analyte stds as a "preservative". A typical standard is prepared in 250mls (5mls ylene chloride, the rest 1-propanol). I have often questioned why or how this is needed. Would the unstable metabisulfite (as sulfite anion) be a "scavenger" to prevent oxidation of the analyte?

John,

Yes, undoubtedly in the case you mention above the metabisulfite is functioning as an oxygen scavenger to prevent oxidation of the analyte. Sulfite (the breakdown product of metabisulfite) is quite effective as an oxygen scavenger. I've used it on occasion to remove oxygen contamination from a column in order to reduce interconversion of oxidizable species.