Chromatography Forum

Hello, all. I'm trying to set up a headspace method to replace a liquid extraction technique that we use to determine the concentration of xylenes in a wash solution in our plant. I've previously used dynamic headspace but the equipment I have is only capable of static headspace.

I've tried using very small sample sizes (10-15 mg) for TVT/FET but reproducibility in results is non-existant. I bumped up sample size to ~ 100 mg and have done multiple extractions on the same sample. Where I'd expect to see a regression curve, I see a horizontal line or even an increase. My GC run time is 30 minutes (I have no backflush capabilities), so I have the vial taken out of the incubation chamber. When the vial comes out, there is no visible liquid so, even with this somewhat large sample size, I'm postulating that I'm still volatilizing all of the material. If you're investigating a hodge podge of materials, what would you expect the results of MHE would be?

Here's what I'm using:
Combi-Pal with Static Headspace (2 mL syringe) and incubation chamber
Incubation: 100 °C for 15 minutes
Needle: 105 °C
Inlet: 250 °C
Column: 40 °C initial for 4 min, ramp 10°/min
Constant flow

Sample: Ethanol, toluene, xylenes, naphtha, random petroleum distillates
Prep: 20 mL Headspace vial, 50 mg ethylene glycol diethyl ether (ISTD), 50 mg sample

I don't have the mhe attachment, so I come over and vent the vial manually after every injection.

Eventually, I want to be able to perform analysis on paint samples as a modification of ASTM D6886. I'm tired of ruining columns and liners from extracted materials into methanol and it makes sense to test volatile compounds via headspace, but this thing is giving me a real headache. Any help would be greatly appreciated.

Thanks!

The first question has to be; why are you doing multiple extractions ? That you get a horizontal line with multiple injections from the same vial could be due to there being plenty of analyte in the sample, so that the depletion at each cycle is too small to measure against the large signal.

There are dozens of possible causes of poor repeatability with syringe headspace - try a search of the forum archives.

Peter

The first question has to be; why are you doing multiple extractions ? That you get a horizontal line with multiple injections from the same vial could be due to there being plenty of analyte in the sample, so that the depletion at each cycle is too small to measure against the large signal.

There are dozens of possible causes of poor repeatability with syringe headspace - try a search of the forum archives.

Peter

I'm dealing with paint and I expect that I'll have severe matrix effects to overcome. To do that, my options are to either use ultra-small sample sizes to get complete evaporation of the volatiles (TVT/FET) or to do multiple headspace extraction (MHE)and plot the regression. Getting the logarithmic slope, I can then solve for total analytes in the system.

Yes, I could dilute the sample but, because I'm looking for ALL volatile materials present in my sample, I don't want to see any bleedover from the diluent which might be hiding small concentrations of other volatile compounds, including the diluent itself. For example, I could dilute my paint in methanol and perform headspace analysis but the bleedover would mask the presence of ethanol, acetone and other materials and make it impossible to determine if methanol was present in the sample, initially.

Regarding the horizontal line: in MHE technique, one vents the vial between samplings to force the headspace to re-equilibrate. I would think that, with venting, the concentration of analyte would decrease sufficiently to be detected. It may be that I need to pressurize the vessel to fully remove the headspace but I'm not experienced enough in this technique to say for sure.

In the first post you wanted to analyse wash solutions, now it's paint.

What kind of paint is it, or what kind of wash solution ?, and what ballpark range of concentrations of analytes are you expecting ? If it is paint is it still liquid, or are you looking at residual solvents in films ?

What sample volume do you take from the vial into the syringe ?

Your thinking on matrix effects and multiple extractions makes sense, but it also makes the analysis more tricky. How accurate and precise do you need the results to be ?

Peter

In the first post you wanted to analyse wash solutions, now it's paint.

What kind of paint is it, or what kind of wash solution ?, and what ballpark range of concentrations of analytes are you expecting ? If it is paint is it still liquid, or are you looking at residual solvents in films ?

What sample volume do you take from the vial into the syringe ?

Your thinking on matrix effects and multiple extractions makes sense, but it also makes the analysis more tricky. How accurate and precise do you need the results to be ?

Peter

I'm currently analyzing a tank wash solution, which contains trace amounts of resin, but I want to adapt the technique to analyze paint. I'm always going to be doing analysis on liquid materials. In the case of paints, it's so I know concentrations of various things "in the can" for reporting VOC level the of the material.

In the wash solution, I'm looking at the concentration of xylenes (o-, m-, & ethyl benzene) which totals ~ 40%. For paints, it's going to be any volatile material up to 10%, roughly.

I've done 2 mL injections but recently switched to 1 mL because I believe I'm overloading.

I'd rather do one analysis using FET but I wanted to confirm results with a MHE attempt. I'd like my results to be as precise as possible but I am only accurate to +/- 0.5% when doing liquid extraction analysis on the same machine. I'd like to at least be comparable to that with headspace; using sample sizes (with ITSD) of 20-30 mg, though, I'm seeing +/- 5%. When I bumped up sample size (with ISTD) to 100-150 mg, though, I saw repeatability of +/- 0.75% between two samples. I'm about to run a third to see if results are still comparable.

I'll also be running an actual paint sample later today to see what I get with FET, an attempt at MHE and then an extraction and liquid injection to see how results compare.

A difference of 0.75% between replicates (I'm assuming that this is relative to the mean of the two ?) is actually pretty good for headspace.

Are you really using microgram sample quantities - if so how on earth do you weight them accurately, bearing in mind that they have lots of volatiles in them ?.

Even if it is micrograms - say 50 ug to be in the middle of what you have tried. With complete evaporation into 20 ml, that gives you 5 ug in the syringe with a 2 ml sample. With an FID just 1% of that (50 ng) will give you a nice peak to integrate. So I would decrease the injection volume to 100 or 200 ul, and set the split ratio on the GC at 10:1 to see what it looked like.

What inlet and column do you have ?

Peter

A difference of 0.75% between replicates (I'm assuming that this is relative to the mean of the two ?) is actually pretty good for headspace.

Are you really using microgram sample quantities - if so how on earth do you weight them accurately, bearing in mind that they have lots of volatiles in them ?.

Even if it is micrograms - say 50 ug to be in the middle of what you have tried. With complete evaporation into 20 ml, that gives you 5 ug in the syringe with a 2 ml sample. With an FID just 1% of that (50 ng) will give you a nice peak to integrate. So I would decrease the injection volume to 100 or 200 ul, and set the split ratio on the GC at 10:1 to see what it looked like.

What inlet and column do you have ?

Peter

30% +/- 0.75% were the results I obtained in duplicate performed over two days.

My morning started early and I had a brain fart. 50 milligrams is what I meant (corrected the post). Apologies for the confusion.

I'm injecting 1 mL at 40:1 split ratio. I could never decide if it was better to get a large sample and a higher split ratio or a smaller sample and a lower split ratio. How does one decide? I should also mention that I've got the column split to two detectors (FID & Mass Spec), so there is a reduced amount going to the FID. I need a moderately high volume so that I can determine the components of interest via fragmentation pattern.

I'll get back to you on inlet/column later.

Thanks for the discussion

Harlan