Headspace Artifact Peak with DMA

[Alchemist5]

I am analyzing a sample by headspace using the diluent DMA. I have an Agilent G1888 using the settings:
Flow: 20ml/min
Oven: 80°C
Loop/Line: 95/95°C
Vial Eq/Pressuriz time: 45/0.1 min
Loop Fill/Eq time: 0.2/0.1 min
Inject time: 1.0 min
The run is 15 min long going from 50 to 230°C. This is a capillary column (G43) 30x.0.53x3µm with a flow of 5ml/min and a split of 15:1. DMA comes off at 7 min, but I get a growing artifact peak around 2.4min right after methanol (analyte). It tails good too compared to the sharp analyte peak. I have removed this peak once before, but on another system (developing the current configuration. Now its back.
Any thoughts on this? And how I can remove it?

[chromatographer1]

it is probably an amine which develops from the DMA, water, heat, and time.

You could acidify your matrix if that does not cause issues.

best wishes,

Rod

[GaryR]

Hi Alchemist5

What happens if you increase (or add) a hold time at 230C at the end of your run?
If this does not change the retention of your artifact peak it is coming from your headspace (carry over/contamination or analyte reaction as noted by chromatographer1).
If it does, you can put this into your temerature ramp so that it is eluting prior to your next injection.

Regards
Gary

[Alchemist5]

Acidifying the matrix is not an option, unfortunately. I do believe it is some amine byproduct from the diluent. And as far as an extended bake out time, I will try to increase the 6 minutes already employed to around 10 (the artifact eluting off between 2 and 3 minutes of the run) and hopefully this will resolve it. Thanks Chromatographer1 and GaryR!

I will report back the results for information to others.

[chromatographer1]

No news is good news, I guess.

[Alchemist5]

Sorry about not getting back sooner. It still is unknown why or where that peak (sometimes peaks) originates from. The headspace vial prep did allow me to try and acidify the matrix, since it is comprised of 5 mL DMA and 1 mL water. Changing the water to 0.001N HCl eliminated to main obtrusive peak. A smaller, yet not interferring peak near the DMA response did not disappear. This peak acted in the same manner as the previously described one (also an amine, I assume).

The strange thing is that we performed intermediate precision on this method using a second instrument and column (same manufacturer, but different lot) and neither interferring peak appeared. We still used the 0.001N HCl. Now the problem either was the column itself (would a column contribute to the degradation/decomposition of DMA? Remember, it's a G43), or there was some 'hot/reactive' spot in the inlet port of the instrument originally used.

The problem was solved by using the 0.001N HCl, but the question remains: Why was that peak there? I've never eperienced anything like that before and I even had the Agilent guy that came in for other reasons stumped on this one.

Thanks Rod for the solution input!

Burt

[chromatographer1]

Alchemist5,

When dimethylacetamide is heated with water over a length of time usually taken by most published methods a reaction takes place and methylamine(?) and dimethylamine(?) are formed and possibly other artifacts or impurities in the DMA are seen to elute.

Other reactions can take place depending upon the matrix and other factors.

This methylamine peak elutes early, often near methanol on many columns.

The peak near DMA is DMF or MMA and should elute just before DMA, this MMA is a normal small impurity found in most lots of DMA.

What can happen? If your flow path is reactive rather than inert (this is all relatively speaking now) the MeAmine or DiMeAmine can be absorbed rather than elute as a distinct peak. A well used system will often not elute the peak although other alcohols, ketones, amides, etc will elute without a noticeable problem.

The HCl you added makes an amine salt of the MeAmine or diMeAmine and inhibits its volatility so it never get to the flow path or at least is suppressed in concentration.

BTW, I found that if one uses smaller samples the LOD for most analytes does DOWN, not up, which is counterintuitive, but true. I never use more than 100µL of dissolved sample to do headspace analysis and am able to achieve a reproducible equilibrium for most residual solvents in 6 minutes of heating. Alcohols like i-BuOH or IPA will take 8 to 10 minutes to equilibrate to a near maximum level. This greatly reduces the artifacts that can be formed.

If you read my short two page HS article published in the Journal of Analytical Chemistry in 1997 you will see that I stated there:

"The dimethylacetamide produces artifactual peaks which coelute with methanol and dichloromethane and are adjacent to hexane, isobutanol, and dioxane. "

I only heated my samples for 6 minutes and was able to see and separate 1ppm of 18 residual solvents (including dioxane) using only 1mg of sample dissolved into 25µL of water (with a small amount of DMA).

I used the Tekmar 7000 modified with fused silica coated components, of which I was a beta tester.

Oh, for the good old days.....................

best wishes,

Rod