Trouble reaching 1:1 split on Agilent 6890

[mazeroth]

We have a test specification for testing residual solvents that require a 1:1 split on an Agilent 6890. I rarely have to run them but some colleagues of mine do more often and I hear them complain sometimes that the instrument can not achieve the 1:1 split and they will move to a different instrument. I can get the column info tomorrow but I believe it's an DB-624 75m x 0.53mm x 3um. My questions are:

1. Why would anyone develop a method with a 1:1 split? If you are only trying to divert 50% of the sample from reaching the column why even split? Run splitless with a timed purge.

2. What makes it difficult for an instrument to achieve a 1:1 split? I think I remember, from a LONG time ago, an Agilent rep giving a presentation saying something about a low split being 5:1 and you should really aim for a 10 or 20:1 split. I would think it would be easier for the GC to run 1mL/min through the column and 1mL/min through the split, rather than the Agilent spec when they calibrate our GC, which is a 918:1 split (no joke). 0.2mL/min to the column and around 200mL/min through the split vent. Any insight on why higher splits are easier?

Thanks in advance!

[chromatographer1]

The restriction of the column is fixed and very reproducible from column to column.

The variable hardware restriction must control the vent flow very accurately. It is easier to control an orifice to get a flow of 200mL/min. than to constrict the orifice with accuracy to get a flow of 3(?) mL/min.

Try to adjust a needle valve manually to get a low flow while refraining to break off the internal needle in the orifice of the valve body and you will experience the pain.

best wishes,

Rod

[R13]

1. Most likely because inexperienced chemist not aware of the inctrument construction/limitations was told to do so by the manager who newer worked with GC at all.

2. Slightly close Your lips and exhale trying to maintain constant air flow. Now close lips almost completely, exhale and try to blow very tiny flow with constant speed....

[mazeroth]

Thanks for the input. It really helps me understand.

I should have mentioned this is a headspace run. Is it not advisable to run splitless with a headspace? I ask this because all of our headspace runs are split. If it doesn't matter than I don't understand why he would have developed the method with a 1:1 split when he could have ran splitless. Most of the peaks are of decent size, except we sometimes have trouble with dimethylformamide.

Thanks again.

[AICMM]

Two reasons to run split instead of splitless. First, the flow path for a split injection is down the injector and not across the top (on the old 5890's especially.) This means that you will get better transfer of the analyte to the column with a split rather than splitless (been there, done that.) The second is that the sweep may be better served by 15 ml/min rather than just 7.5. It is more obvious with something like a 4:1 where you might have 35 ml/min across a trap (for example) rather than just 7.5 but the concept is still the same.

So, if it is an old method (z.b. originally developed on a 5890) then you may be stuck with the 1:1.

To answer the second 1/2 of your question, just for fun, try to generate 1 or even 3 mL a minute of a gas flow without using a column, that is to say with a pressure gauge and a restrictor. Very, very hard to do. It can be done with a fixed restrictor but it is a very challenging scenario.

Best regards,

AICMM

[Karen01]

Thanks for the input. It really helps me understand.

I should have mentioned this is a headspace run. Is it not advisable to run splitless with a headspace?

No IMO if you have loop headspace sampler and you have the instrument setup correctly for headspace.

First you should have the septum purge turned off and the instrument plumed (or flows set) so that all (or most depending on how plumbed) of the carrier comes into the inlet through the headspace transfer line. I've had 5890s,6890, and 7820s (well can't turn off septum purge there) set up that way for headspace. That minimizes sample dilution.

With that setup empirically I found that higher splits give shaper peaks (so better chromatography/separations) and sometimes even more sensitivity.

I think the reason has to do with band width.

With a loop headspace sampler if your column flow is 1 ml/min and you have 1 mL loop, as 1st approximation it will take 1 min to flush the sample from the loop and so the the analyte will be deposited on the column over a 1 minute. If it migrates at all at the initial column temperature you wind up with broader peaks, lower peaks with potentially less resolution between peaks.

The higher the split the the less material (which helps separations also), but the narrower the band on the column.

Does that makes sense?

I know you are likely running a validated method you can't change... but if you have the freedom to play a bit and your instrument is setup correctly for headspace, try injecting your low standard (or dilute it to the bottom of your quantitation range) at 5:1 and 10:1 splits and see what the injections look like.

HTH
- Karen

[aldehyde]

I've had to explain this to many customers, maybe it was me who told you about it . Everyone has pretty much covered it, but running lower than 5:1 is not recommended because the pressure measurements take place after the valve that controls split. If you want to achieve the desired head pressure it will take forever with the slow flow of gas through the valve. You can set a higher split ratio, achieve head pressure, then set it to 1:1 and you might have a little more success (although you would need to manually do this or time program it into your method or something).

The best work-around is to validate the method at 5:1 split and turn the septum purge off.

The best solution is to buy a volatiles interface and run it in direct mode https://www.ecs.umass.edu/eve/facilitie ... erface.pdf

[mkigls]

Gas phase injections need to be split, period. You don't get solvent refocusing effects in gas phase injections. The only way to make a splitless or low split method work is to cold trap the analytes, which I assume you are not doing. I'd start with a 20:1 split - typically we run 30:1 or 40:1 splits. As Karen correctly points out, the peaks are much sharper (and the DLs lower), since you're allowing the column to do its job.

As to the ability of the 5890s to split at 1:1, it ain't happenin'. The split flow controller is only good down to 5 mL/min, and even there it is questionable. It will move all over the place if you try to actually control at that flow. Your best bet is to change it out for a low-flow controller (like the SGE, for example).

[thohry]

I think in gas analysis, it's no good to run splitless
The total flow of an inlet (split flow, column flow) must not too small, say 2-3 ml/min.
So you can have 1:1 split stably if the flow of the column is about 3ml/min, and it's ok for 0.53 column and it's also good for gas analysis.

[BZK]

In head space I suppose that you inject some 1-2 ml of air sample in fast way. With lower ratio of split (less than 5-10) the head pressure increase to 1 or 2 more bar than the carrier only and decrease slowly. This cause at least two serious drawbacks : cold trapping for heavy compounds back to the carrier line, very wide initial band for non cold trapped molecole. So, at the end, inject with 1:2 of split ratio it isn't a good idea anyway, even for liquid.

Roby