Good column for inert trace analysis in helium

Hello. I am looking for advice on choosing a column to run inert trace analysis (o2, n2, ar) in helium. I have a 7890B with PDHID running thru a Carboxen 1006 column currently. I am not getting great sensitivity even using a 5cc sample loop!

I am trying to see below 1 ppm, hopefully down to 250 ppb level. Any suggestions?