Chromatography Forum

I'm having a problem pressure balancing a Deans card for a dual FPD system. I've trimmed the restrictor column to what I think is the right length (when the Deans is in off position then all of analyte goes to vent detector, and nothing shows up in the analytical detector). However, when I have the Deans on, there is analyte showing up in both detectors, when it should only be showing up on the analytical column. I did swap out Deans cards with the same result. Any suggestions?

Deans switch is tuned by calculating pressure/flows in branches using special calculator (or piece of paper and pencil).

Switching is done turning on/off or changing flow through control control channel.
E.g. applying same pressure at both ends of analytic column completely stop separation and blow out detector ends.

In my hands, the calculator has gotten me close but I had to develop my own sort of balancing refinement.

Mostly, I'm looking for low concentration components. I analyze alcoholic beverages so if a little ethanol bleeds through the switch, I don't care. My systems are 30 m x 0.32 mm DB-5-type precolumn going into the switch. I have a 30 m x 0.32 mm Wax-column coming out of the switch and going to a MS detector. The transfer line to the monitor detector (FID) is as I recall 2 m of 0.18 mm DB-5 type fused silica. Maybe it's only 0.10 mm tubing. I'm remembering this so bear with me here.

I choose a sample that has some analytes that are big enough that I still want to get 100% transfer to the second column. I then use the calculator settings as a starting point. I run that sample in heartcut mode (100% diversion of the chromatogram to the second column) and see what happens. I then change the head pressure on the precolumn (leaving the midpoint pressure alone) until I get the desired recovery of the peak of interest and only minimal bleed of the really big components (like ethanol) to the precolumn detector.

If I can get 100% of the peak of interest, it's been my experience that I get 100% recovery of everything smaller than that. It's a bit of a balancing act between optimum flows and good peak shapes - especially for the early eluters.

Here's an example of the kind of chromatography I get with my systems (I have several). It's about the 3rd post down with the graphics:

https://www.chromforum.org/viewtopic.ph ... or#p380164

Separate coeluting (on the DB-5) menthol from camphor.

I believe the system I describe above has 9.5 psig as the headpressure of the first column and 4.3 psig as the midpoint pressure (feeds the Wax going to the mass spec). I run in constant pressure mode. I've never tried constant flow with the switch. I get what I need with constant pressure.