I now know what the problem was, it was a bad case of eye-brain
miscoordination. Yes, I am now blushing heartily as I admit that, just
upstream of my lovely new IE column, I still had the RP guard column
plumbed in. Arragghhhh! My only excuse is that I thought the thing just
had filters in, which is a bit of a crap excuse, I admit. My only
consolations are that nobody else in the lab spotted it either, reducing
their rights to laugh mercilessly at me, and that I guess the IE column is
fine after all (haven't had a chance to check it out yet). The worst thing
(but also the way I worked it out) was that the person using the system
after me had to spend all of yesterday cleaning my junk out of it - ooops.
Luckily he was too busy laughing at me to hit me.
Sorry for being such a dunce, but thanks for all your many replies - I
now know a million times more about such things than I used to, anyway. I
will now crawl back into the woodwork and write out one thousand times
"Guard columns bind peptide".
Yours embarrassedly,
Jenny