Chromatography Forum

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If you're even considering doing headspace with manual injections, I'd stick to internal standard procedures only. It's practically impossible to get repeatable injections doing this manually, even with valved syringes. I assume that you know that the 5890 (although an excellent GC, and we still use several) has been out of factory support for several years.

yes but fortunately due to its simplicity and robustness it has outstanding aftermarket support on places like this and vendors like restek, is working well for us.


so whats the big pitfall in doing the method with internal standards? poor accuracy? too time consuming? seems simple enough with an oven, septa vial, and the right syringe.

If you're even considering doing headspace with manual injections, I'd stick to internal standard procedures only. It's practically impossible to get repeatable injections doing this manually, even with valved syringes. I assume that you know that the 5890 (although an excellent GC, and we still use several) has been out of factory support for several years.

Hi

What "budget" do you have?

A used and "refurbished" 7694 Headspace sampler is around 10000 $ or so.
The B version is also still supported by Agilent if I recall right. Also I think you even can hook up an older Perkin Elmer HS 40 model on the 5890.

Manual Headspace sampling is a pain especially if the incubation temperature is high enough to force you to wear gloves. For good headspace analysis it is essential that you always incubate for the same time period at the same temperature every time. The sample should be taken through the septa using a gas tight syringe (not like the video ). Forget any thoughts of precision of rsds in fractions of % , between 3 and 10% are more common and I have seen 20-50% rsds for manual

If you are going to have to buy a GC , consider getting a GC /Headspace combination. A less popular PE GC with headspace may be the same price as a 5890 be prepared to compromise on GC if it allows you to get a headspace autosampler. Tekmar and CTC samplers work with most GCs
No shortage on labx

Try the manual method on an existing GC, it might convince you an autosampler is essential

yeah id say the 10-12K ive seen for 7694 is definitely out of our budget. wed like to get an old 5890 for a few grand instead.

+/ 10% might be doable, we only need resolution to .01% which seems way above the resolution of that method. base on this varian article, they are only getting 1.96% RSD with a headspacesampler, but that corresponds to an SD of 0.0007, so i think 10% RSD wouldnt be out of the question

http://www.varianinc.com/image/vimage/d ... i-0773.pdf


is there an generic astm method for manual headspace sampling?

If automated equilibrium headspace is outside the budget, manual SPME might be the way to go. The repeatability of manual equilibrium headsdpace is so poor (unless you can do it at room temperature and your room temeprature is controlled) that it is really only a qualitative method.

Peter

Look at this
http://www.griffinanalytical.com/headspace.html
Is a manual headspace with a calefactor block for the vials, and the sirynge is coverede with a furnace.

Maximum equilibrium time of 99 seconds ?

Peter

whats manual SPME? is there a link describing this method?

thanks

These links could be useful for clarification of your question:

https://www.sigmaaldrich.com/analytical ... video.html

http://www.science.uwaterloo.ca/chemistry/pawliszyn/

Disclaimer: I don't work for Supelco

Best regards

Well, I was able to get quite a decent results by using combination of home-made sand bath (i.e. beaker with sand, hot plate with fine adjustment and a thermometer), 2 mL crimp-cap vials and classical autoinjector (not even a gas-tight syringe!). Used it for determination of residual hexane in oils and pressed seeds. Each sample was equilibrated in sand bath for 1 h, and afterwards I had to run with a hot vial to another room, but it did the job. The most problematic step was actually preparation of spiked samples (it's a bit hard to spike 1 mL/0.5 g of sample with e.g. 0.1 or 0.3 uL of hexane), but after some practice I was able to get excellent linearity and very good precision (sorry, I don't remember the exact RStDev).

Headspace is finicky enough with an actual sampler, the thought of rigging up a sand bath and running through the halls with a hot vial makes me ill .

I know you said its out of your budget but heed the warnings in this thread. You will likely have a lot of trouble trying to do this.

JF,

Given the fact that you are not able to use a headspace analyzer or to trust yourself to use SPME, then I have another solution which may meet your fiscal requirements. My assumption is that your analytical requirements, the number of analytical measurements ie samples is relatively low.

Buy a simple GC capable of performing packed column analysis.

Buy a porous polymer column, almost any will do.

Dilute your biodiesel samples with methylene chloride for high concentration methanol samples. Use undiluted samples for extremely low concentration methanol samples.

Given good sampling and standard addition techniques (if you don't know what I mean then take a class or read a book) you should have no problems analyzing for methanol with excellent accuracy.

Historically, I have analyzed both solid and liquid samples for methanol at levels far below 1000 ppm accurately and with minimal sample dilution you should be able to reach 50 ppm without difficulty.

Now accuracy of analysis with a large sample load requires a headspace analyzer, but if you are only testing several samples a day, a packed column solution will suffice. What do you think people did for such analyses in the 60s, 70s. and the 80s, before the common use of capillary columns ?

You should rotate the column on a daily basis and bake out the end of the column overnight at a temperature well below the max temp of the packing.

But tell your bean counters, if you have a large number of tests, and will be performing the tests over a long period of time, it will be poor economics to NOT use a headspace analyzer, unless of course, you work at less than minimum wage and don't object to working 80 hours a week.

If you are not permitted to do the tests properly, then send out the samples and see what they cost then. Maybe your bean counters will talk a different story at that point.

wishing you the best with your financial, not an analytical, problem,

Rodney George
consultant

Maximum equilibrium time of 99 seconds ?

Peter

Hi
It's "seconds" is a mistake. I worked with it 3 years ago, and is "minutes"
I don't sell it