Dynamic Headspace, purge and Trap and SPME?

[Karen01]

I need to to be able to quantitate some high boiling (obviously hydrophobic) hydrocarbons from aqueous media and want to avoid doing liquid-liquid extraction and want the least equipment hassles. I don't think I could get enough sensitivity with static headspace.

What comes to mind is Dynamic Headspace/Purge and Trap and SPME. I have not use any of those techniques myself. (lots of static headspace work)

Logically it seems to me that Dynamic Headspace and Purge and Trap are the same the, but i came across things on line that seem to imply they are two separate techniques. Is there a difference?

Both SPME and And Dynamic headspace might give me the sensitivity i need. Any reason to prefer one over the other? What's vendors auto-samplers for these techniques should I look at?

Thanks,
- Karen

[Peter Apps]

Hi Karen

It depends first on how low you want to go, and second on what instruments you already have.

SPME would need only a special syringe add on to some autosamplers, and a way of controlling sample temperature. For a quick and dirty test you can even just buy a fiber and work it by hand. The ultimate LOD/Q is determined by the small volume of the absorbent, but for heavy hydrocarbons a methyl silicone fiber will have very favourable partitioning and very low potential lower limits.

Purge and trap needs much more complicated (and expensive) hardware. Dynamic headspace as far as I know is purge and trap. The lower limts can be much lower (and are often fixed by interfering compounds in the samples rather than by the detection limits of the instrument and method). I would guess that there are more standard methods based on purge and trap than on SPME, especially in the environmental contaminants area.

You can get an "in-needle" trap as an add-on to a combi pal type autosampler, which is a sort of hybrid of purge and trap with SPME hardware.

Peter

[Karen01]

Thanks,

Some of the compounds boil up to about 300C, and I need to go down to at least 0.1 PPM from aqueous media.

I would be buying a new instrument for this project so I could go either way.

- Karen

[Bigbear]

Tekmar HT3 is a static headspace unit that can also do dynamic ( P&T) if configured for it.
Both SPME and P&T should work for you. As Peter eluded to the dynamic range for SPME is limited due to the small amount of adsorbent.
We do both P&T and SPME( using a Gerstel injection unit ) here.
My personal bias is to recomment you go with purge and trap. As I said earlier the HT3 can do both static and dynamic headspace ( no experience with HT3). We are now looking to replace an old Tekmar 7000 with an HT#.

[chromatographer1]

A simple sample prep procedure may be all you need.

A reversed phase cartridge will trap the alkanes and then you could elute them with a non-polar solvent. With a large volume injection or by carefully reducing the volume of the purge solvent (pentane?) you could then concentrate the extract and reach the limit you seek.

Very inexpensive and multiple samples could be processed in parallel.

good luck,

Rod

[Peter Apps]

Rod makes a good point, I thnk that there may even be off the shelf hardware that can do automated SPE -desorption-large volume injection - probably fairly pricey though.

Peter

[Karen01]

A simple sample prep procedure may be all you need.

A reversed phase cartridge will trap the alkanes and then you could elute them with a non-polar solvent. With a large volume injection or by carefully reducing the volume of the purge solvent (pentane?) you could then concentrate the extract and reach the limit you seek.

Very inexpensive and multiple samples could be processed in parallel.

I actually had thought of that and figured it would work. An encapped C4 or C8 SPE cartridge eluted with Hexane would work OK (if not C18 for sure).... I've used SPE to concentrate hydrophobic molecules from aqueous solution and developed an SPE prep method for a phospholipid assay (using a manifold) both for HPLC use in the past (many years ago now), but those were for relatively low numbers of samples.

Before too long, for this application, we are talking 50- 100 samples a day. SPE sounds like a lot of labor and messing with significant volumes of flammable solvents, even if using using an extraction manifold.
Purge and trap would help keep the inlet cleaner as well.

Also hood space to prep samples is at a premium here. In the long run, considering everything, I am not sure if going the Sep-Pak route would be cheaper, but I did mention it to my boss the other day.

But maybe I worry too much about such things ... Given the eventual need for that type of throughput, what do you think?

Thanks,
- Karen

[Karen01]

I've found out the difference between purge and trap and dynamic headspace.

in Purge and trap you bubble the carrier through the liquid to volatilize the compounds and in dynamic headspace you just sweep the headspace. In my mind both do a purge but they are significantly different practically.

I need to go up to about C16... from what I've found purge and trap is only used up to about c10 so dynamic headspace likely won;t work above that.

I have not found any SPME applications that go above c10 either, though in principle if the fiber is in the liquid it should be able to work with the "right" non polar fiber. Anybody have any experience with this?

For extractions the standard environmental method from water they first acidify the samples, then extract with hexane or pentane (sep funnel), run that down a small silica or other polar column with some Mag Sulfate to remove water and hydrocarbons with polar functional groups, then concentrate, then inject on GC.

Maybe SPE with different washes would work, but extracting hydrocarbons from the cartridges could be an issue... if that worked well i would have expected environmental labs to be doing that.

Can anybody here that does TPH environmental work comment? From what I can find , from aqueous media they don't use SPME or Reversed SPE to extract > C10 hydrocarbons from water.

Thanks,
- karen

[rbardsley]

Karen
I am an application chemist with Teledyne Tekmar. We have a Gasoline Range Organics in Water headspace appliaciton note on our web site. See http://www.teledynetekmar.com/ . It is under the HT3 page. Also I have looked at DRO in the past. No application notes on this one.

Thanks
Roger Bardsley

[AICMM]

Karen,

An alternative to consider if you have the funds. What about a micro-extraction and a large volume injection? The extraction may not be the most exhaustive but with a large volume injection you could certainly get down to the detection limits you are interested in pretty quickly.

Something like 30 mL of water in a VOA vial, 5 mL of hexane or DCM, shake a bunch of them for a couple of minutes, vial up and go. This works as long as your lightest compounds are not too light. You can do a lot of samples this way....

I would be worried about SPME lifetime for the number of samples you are talking about. (Don't get me wrong, for the right application I love SPME, but it might not be best for what you describe.)

Just a thought.

AICMM

[chromatographer1]

This is a good suggestion but I would not suggest DCM as too much water carryover is possible/probable?

Hexane or maybe pentane, again the issue is water carryover.

Good suggestion, AICMM

Rod

[Karen01]

That actually is what i was thinking about, I have a Agilent GC with a Mutli-Mode inlet that can do large volume injections.

I think I would need to run through a sodium sulfate Sep-pak to dry it, which would mean more volume to elute it. The question is will there be enough sensitivity after that.

- Karen

[chromatographer1]

Karen,

I once did an analysis looking for ppm levels of hydrocarbons (pump oil actually) from C16 to C37 in a water solution (the matrix acted like a soap so the extraction was difficult).

I was able to get down to total amounts of 0.5 ppm total for the C16-C37 range from 100 grams of matrix dissolved into 300mL of water. (the limit sought was 5 ppm so the bosses were quite pleased)

I used a 3 meter 0.53mm ID OV-17 1 micro film column for this analysis, running from 40 to 275C at 30C/min. FID One 30m commercial column became 10 test columns.

I extracted the aqueous solution with hexane and CAREFULLY with a gentle flow of nitrogen blew down the several mL of hexane to 0.25mL or less. Internal std is recommended unless you can measure small amounts of the final volume accurately.

I did not dry the solution as the water collected nicely on the inside of the culture tube I used to hold the last few mL of solvent.

I think I injected 2 microliters SLOWLY into a injection liner DIRECTLY attached to the column (NO SPLIT)

If done carefully it was very reproducible and could be done in bulk concurrently with a proper hood setup. (the hexane gets quite cold after a few minutes of evaporation and the process slows but that is ok, you don't want the hydrocarbons to be lost.

Just a FYI, it can be done, and I did no drying of the samples. It took time, but doing samples concurrently, and with the short run time on the GC we got the project completed in a timely manner. There were a lot of samples and spiked samples to demonstrate recovery. I worked 20 hour days for 4 days to get the emergency testing done by the deadline, but I got a couple of days off to catch up on my sleep.

Good luck,

Rod

[Karen01]

I once did an analysis looking for ppm levels of hydrocarbons (pump oil actually) from C16 to C37 in a water solution (the matrix acted like a soap so the extraction was difficult).

Yes, I am concerned about emulsions... That is why i was originally thinking of starting with a RF SPE cartridge and wash with a low MeOH water solution, elute off with hexane and run that through a drying cartridge ... But not sure if that would work and is a lot of manipulation ..and even if it did it would probably require some evaporation.

I used a 3 meter 0.53mm ID OV-17 1 micro film column for this analysis, running from 40 to 275C at 30C/min. FID One 30m commercial column became 10 test columns.

I will look into that column. Almost all my GC experience up until now was for residual solvents. This is a whole different kettle of fish!

I extracted the aqueous solution with hexane and CAREFULLY with a gentle flow of nitrogen blew down the several mL of hexane to 0.25mL or less. Internal std is recommended unless you can measure small amounts of the final volume accurately.

How did you deal with the emulsion issue in the extraction?

I did not dry the solution as the water collected nicely on the inside of the culture tube I used to hold the last few mL of solvent.

Purging with dry nitrogen through a needle in a vial with a septum (preferably a "v" vial) that was vented with a needle could prevent that. That could be done on a manifold so 6- 8 could be done at time. But as hood space is at real premium i would like to avoid having to remove solvent that way.

I think I injected 2 microliters SLOWLY into a injection liner DIRECTLY attached to the column (NO SPLIT)

In theory the Muti-mode inlet should let me inject up to about 50 uL and as I will be able to program l control the Inlet temperature, i should be able to selectively evaporate the hexane first there. I've never used this inlet so I'll have to see what it can really do.

Just a FYI, it can be done, and I did no drying of the samples.

Being able to avoid the drying step would help for sure! All the environmental assays I've seen do it as then then separate the aromatics from from the aliphatic hydrocarbons on a florisil column before injecting on GC.

Thanks,
- karen

[chromatographer1]

The carryover of water was resolved by letting the glass surface of the vial collect the water. I had to continually rinse the sides of the evaporation tube with the hexane so the hydrocarbons did not 'stick' to the glass. This is why a static evaporation requires rotation of the tube. I did use a long needle to flow the nitrogen.

The active surface of the glass tube acts like a silica or florisil trap to retain the polar carryover components.

This procedure requires technique and practice. Not every tech in the lab could do this procedure. They were too impatient and thought they could just blast it down to near dryness and it would work. Gentle and careful evaporation with continual washing of the vial sides is required or loss of hydrocarbons resulted.

Good luck,

Rod