stripping column / purge

[sq]

llo,

I am using a commercial GC/PID for ambient air analysis.
(Syntech Spectras GC955 serie 811)

the samples are pre-concentrated on a Tenax trap, then they go to injection/stripping stage before going to the column.
all lines are connected through a ten ways valve. (zero-air gas, inlet, trap, outlet, stripper column, analysis column)

I am not familiar with the use of a stripping column. I understand it is the same as the analysis column but shorter, and it used to make analysis time shorter by pre-separating the compounds, right?

I changed the original (default from the company) temperature program to have a better separation.
I decreased the starting temperature and then the heaviest compounds (last) don't appear anymore. For my purpose it is OK because I am not analyzing them.
But, I want to make sure of what happens to them and that they don't appear in the next chromatogram under other peaks (it's continuous analysis).

During the time of the analysis the stripper column is back flushed, can you confirm that this step is removing these undesirable compounds?

Thanks.

[chromatographer1]

When you don't get those peaks in later chromatograms you might assume they were removed, but you will probably always have a background presence of them unless you return to the original temperatures the company provided to do the analysis. They apparently felt it necessary to select a temperature they determined would work.

Change it at your own risk. Of course a single bakeout run may always fix your concern.

best wishes,

Rod

[Bricevan]

The purpose of a stripper column is to prevent your heavier components from making it onto the main column. You are correct that it is just a short section of your main column.

Exactly how it works will depend on how your instrument and the method is configured. But one way it might work, is your components of interest pass through the stripper column and onto the main analytical column. Immediately after your last component leaves the stripper column, the valve actuates and backflushes the heavier components still trapped on the stripper column to vent.

What it sounds like you're saying is that you are no longer seeing your last few heavier components at the end of your run.

By lowering your temperature, all of your components of interest might not be making it off of the stripper column and onto the main column before the valve actuates, and they get sent to vent instead of the detector. One way to correct this is to adjust the timing on the valve actuation. Make it happen a little later in the method, to allow all components to elute to the main analytical column before the stripper column is backflushed.

This is just a guess, and I'd have to see your plumbing diagram and a normal chromatogram to know how it works for sure.

-Brice

[sq]

Thanks for your comments.

The default program from the company is set for 30minutes analysis (which is perfect) but for a range of compounds larger than the one I am looking at .
Therefore the light compounds I want to analyze are badly separated at the beginning and after 12 minutes there are only peaks I have no interest in.

I now have a good temperature program where all the compounds of interest all appear in 25min which is good. (I did not change any valve actuation parameters)
But still I want to make sure where all the undesirable compounds are?

With an oven temperature of 70°C during the stripping/injection step the heavy undesirable compounds go to the analysis column.
with the new temperature program during the stripping/injection step the oven is at 50°C and at the end of the analysis goes to 90°C for 4mins. I expect these heavy undesirable compounds to be removed from the stripper column during that heat up stage.
Please tell me if this correct?

the measurements are supposed to go 24h/7days, so I only plan to stop for calibration. Would bake out also be necessary?

sorry I didn't manage to attach the pictures of plumbing diagram and chromatograms

thanks again!

[Bricevan]

If the stripper column is getting backflushed to vent, you should be OK. You can look at the instrument diagram to confirm this or fax me a copy and I will take a look.

Columns have the same properties forwards as backwards so if the heavy components are being foreflushed through the stripper column for 5 minutes, they will require backflushing for 5 minutes in order to remove them (at isothermal temperature). Since you are not running isothermal, the backflush period could be shorter than the foreflush time and the components will still make it off the stripper column in time for the end of the run.

Without seeing a diagram and a chromatogram I can only guess. But I would say you are probably fine, and all of your heavier components are being backflushed from the stripper column to vent and you won't require any sort of baking out.

If you want to do some experimenting, you can play around with the valve timing and see how your chromatogram changes. For example, delay the valve switch by 15 seconds and see if you then get your heavy items back in your chromatogram. You will also have to extend your runtime to do this since you're running at a lower temperature than the original program.

-Brice

fax: (970) 221-9364