Chromatography Forum

I'm trying to figure out the answer to this question. Our Agilent instruments use make-up gas with their FIDs, but our Perkin Elmer Autosystem XL does not. Why? I can't seem to find this answer anywhere... We use a hydrogen flow of 45 mL/min and air at 450 mL/min, versus 40 mL/min and 400 mL/min on the Agilents. Carrier pressure (He) 9.5 psig on the PE versus 35 psi on the Agilents. Does it have something to do with the differences in gas flow? Sorry, I'm a newbie...

Welcome to the forum.

Make-up gas was originally used to flush the eluent from capillary columns through the large volumes of plumbing in FIDs the were made for packed columns. As companies redesigned their FIDs to work with the lower carrier gas flows of capillary columns some of them dropped the make-up gas - the fuel hydrogen is enough to flush the eluent quickly through the flame.

However, nitrogen as make-up gas also increases signal:noise by slowing diffusion at the base of the flame where analyte is hydrogenated by hydrogen radicals diffusing upstream form the combustion zone. How big the improvement is varies from a few percent to 30% in different studies - some companies consider this worth the extra plumbing and cost, others not.

Peter

Thank you, Peter.