7890 + CombiPAL Headspace Injections

[Schmitty]

Well, I have been working with 5890s for a while now, and we have started replacing them with 7890s. I bought an install kit for one of my CombiPALs and moved it from a 5890 to a 7890.

The issue I am having appears to be over-loading of the injection port. If I use the same conditions from the 5890 on the 7890 (flows, temperatures, splitless mode, etc.) the peaks are split on the 7890 output.

There are only 2 things that I have been able to do in order to improve peak shape; Turn on the split or inject less. Both of these solutions reduce the sensitivity of this analysis to the point that I can no longer use my usual low standard (1ppm MeOH, IPA and EtOH in water).

The problem persists if I change the flow type from constant fow to constant flow (like the 5890 would be).

I see there are many more options on the 7890 front panel, than the set of three knobs on the 5890.

General instrument conditions:
DB-WAXetr 30mx0.53 2u film
35°C (yes, this is below the recommended operating temperature) for 10 minutes, ramp to 200+
injection port 220
liner is a single taper split/splitless with quartz wool (part#5062-3587)
detector 250 (FID)
flow (have tried from 2-8 mL/min)

Samples are 10mL in 20mL vial, heated at 80°C for 20min. Syringe at 85°C. Have tried slow and fast injection speeds for the plunger.

I guess my question is: What is the 7890 doing differently to create this problem? I'm guessing the injection port's gas control is to blame, but what can I do?

One last thing I plan on trying soon: change the injection port liner to a smaller diameter splitless liner.

[Peter Apps]

Hi Schmitty

What is the injection volume and injection speed ?, if these are too high the injected gas plug can go in directions that are highly undesirable. Contrary to expectation a faster injection will not necessarily give you a narrower peak. With a Gerstel clone of the CombiPal on a 6890 I found that injection speeds above about half the total flow through the inlet gave distorted peaks for ethanol in water.

Why are you starting the column below its minimum temperature ?, no good can come of this. Why have quartz wool in the inlet ? - headspace samples are clean and in the vapor phase already.

Peter

[Schmitty]

Peter,

The volumes I typically use are 1-2mL. I generally use a large enough injection volume to generate a S/N ratio of 1:10 for my lowest level standard.

The injection speed of the plunger on the Combi with the 7890 was varied from 100 to 1000 µL/sec, with no appreciable difference seen.

The goal of using the lower temperature is to separate the alcohols (IPA and ethanol, specifically). I would rather use a different column, but we developed this method a few years ago, with the intent of separating methanol and ethanol. But now we have the added issue of samples with IPA, which interferes with ethanol on this column.

I have the wool in the liner because of direct injections. The Combi can be easily switched to DI, so I figured I'd just leave it in. This wasn't an issue with the 5890.

I can bump the temperature up to 40 or 45, to see if any of the WAX properties become more apparent. However, I think the little separation I do have will be lost.

Thanks

[Peter Apps]

Hi Schmitty

Even the slowest injection speed that you tried; 100ul/s is 6 ml/min, which is nearly the same as your fastest gas flow rate, and causes a pressure pulse in the inlet. The reason that this is a problem on the 7890 and not the 5890 is that from the 6890 onwards the inlet pneumatics changed.

This is one of those situations where you can start going around in circles - because the peaks are distorted the detectability is poor, so you increase the sample volume, which causes a bigger pressure pulse .................. You might actually get better detectability with a smaller injection volume injected more slowly.

If having the column that cool really does improve the separation then the phase must still be liquid, but I would still give it a shot starting at its minimum temp.

Peter

[Schmitty]

Thanks, Peter.

I guess I was right in assuming that the inlet it too smart for my own good. I will consider your suggestions and continue my plight.

Schmitty

[chromatographer1]

Schmitty,

If you add some OV-1301 column to your Wax column you will separate Ethanol and IPA, but that will require a change in method, but then your analysis did change with the addition of IPA.

You can increase the pressure required for the analysis which will compress your sample plug before it reaches the column (this reduces the split caused by the pressure pulse or you can simply reduce the size of your sample as Peter suggested.

There are different possibilities to increase the pressure. We can discuss these if you decide that is the direction you wish to investigate.

(sorry for the short reply but I am away on vacation. Back next week.)

best wishes,

Rod