Deans card balancing problem

[The Dude]

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?

[antonk]

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.

[rb6banjo]

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.