Dear Dalma Sas,

you should check the technical notes of the MEGA-624, respectively ask the manufacturer to provide information about temperature and solvent stability. Indeed, most GC columns do not tolerate water. But, injecting headspace of aqueous samples is usually unproblematic. Just use an adequate equilibration temperature (about 80 °C) and check routinely the performance of your column, since its performance will deteriorate over time. If you recognize worsens peak shape, you can cut the column at the injection port (about 10 to 30 cm) to restore column performance.

When using manual injection of the gas phase, be sure that your syringe is tempered and that you use an adequate internal standard to take into account the variance of injection volume.

So for your analysis, headspace GC coupled with FID or MS should be fine.

Kind regards

Michael

Take HS-GC. Works reliably...

Dear Dalma,

if you don't mind changing your column, you might change to a new developed stationary phase, that is intended for aqueous injections: Watercol 1910.

As an example please find the separation of acetone, 2-propanol, methanol and ethanol out of an aqueous injection below:

http://www.sigmaaldrich.com/catalog/product/supelco/g1006712

Best regards,

Frank

DB-624 works good with HS for water matrix, but even direct injection of water with alcohols is an acceptable solution. Start the separation at relatively low temperatury like 40 or 50 deg.C and allow the water to be eluted from the column (at this stationary phase at 60 deg. C water is eluted slightly after dead time).

FID is a good detector for this purpose you don't need a GC-MS system. Only problem (if you inject the water directly) is the flame stability during the elution of water. Split injection, small volumes of injected sample allow to overcome this problem.

best regards,

G

Thank you for your help. The ideas was very useful for my work.

This week I start to analyse alcohols from water solution by PDMS SPME. I keep the SPME fiber into the water and alcohols solution 10 minutes before I inject to the GC-FID. I try to analyse 10ppm solution, but up to now I can't analyse repeatability. I don't no what is causing my problem.

FID: 250°C, Air: 300ml/min, Hydrogen: 30ml/min, Split 1:10

Temperature program: 37°C 2 min, 20°C/min 70°C 3min, 20°C/min 80°C 2min, 20°C/min 200°C

I used the same method to this two chromatogram. (the bigger peaks 10ppm, the less 50 ppm)

I don't understand, why I colud not repeat the measurement at the same condition.

Thank you for your help!

Dear Dalma,

Why you put the fiber into water? Try to use a HS-SPME. Close a 5 mL solution in 22 mL vial for headspace, close it with a cup with membrane. Puncture the membrane by SPME holder and take out the fiber – all fiber should be in the free gas phase of the vial (the headspace). Put it into a thermostated chamber for 30 or 45 min (for each sample precisely control the time). After that time take out the SPME and put it to GC injector – what is the temperature of the injector – it should be above 250 deg. C.

 Remember that proper liner should be used for SPME analysis. The first chromatogram reveals that each peak is doubled (small “hill” at the front of the peak) this is an injection/desorption phenomena problem.

If you have a gastight syringe – try to forget about SPME for few hours. Thermostate the sample take 0,2 mL of the gas phase and immediately inject to GC. In my opinion you don’t need SPME for this purpose.

For better reproducibility the syringe can be also heated prior to taking a sample.

best regards,

G

Dear Grzegorz,

Thank for your help.

The last week I tried to analyse with gastight syringe, but the alcohols like the water phase, so can't go into the gas phase. Before the inject, the sample thermostated in 90°C water bath 25min. I can't  heat my sample over 90°C.

I think manual inject with gastight inject include some problem.

Yes, I should change my injector temperature 250°C, so I can escape "tailing peak".

So this is why I use SPME.

This alcohols are very volatile, at 80-90 degrees the equilibrium is strongly moved to gas phase. The thermostating in water bath can be problematic due to the fact that not whole vial is in the water right? If you have another GC instrument you can use it as an oven ;)

Unfortunately I can't use another GC instrument, because somebody always use it. I can use drying cabinet, but in the cabinat the samples can not be mixed.

When I use SPME I dont't need to heat my sample. Another problem when I used gastight inject, I analyzed lot of another component from laboratory air and with GC-FID I can not filter out this peak.

So try the SPME from the gas phase, and heat the sample during the extraction to 40-60 deg.C. I think PDMS is not the best choice of the fiber for alcohols.

Thank you for your help.

Unfortunately, I can not solve the problem about heating my sample and I would like to use SPME fiber.

What do you think which fiber is the best choice for me?

What is the highest concentration that I can examine? (Last week I tried the analyses with 10ppm solution)

Between 2 measurements method I conditione the fiber at 250°C at 10 min. When I take the fiber from the GC, I don't wait as long as my fiber get cool at room temperature before I put the fiber into the aqueus phase. This may cause problem.

From your answers I think your still immersing the fibre in the solution.  I don't think its a good idea in this case you may have 2 separate phases not a single liquid phase.  Thus I think Headspace is the way to go as mentioned earlier.  I would increase your absorption/exposure time and try a few different times.  Run blanks spme runs between samples this will give you an idea if you have any carry over or not.  I assume your not using a split injection.  You can always incorporate a split later if you require it.  Also ensure that that your using a SPME inlet liner.

Thank You for your advice.

Unfortunately, I can not solve the problem about heating my sample so I can not use headspace inject.

I use this liner: http://www.sigmaaldrich.com/catalog/product/supelco/2051305?lang=hu®ion=HU

Water will not damage any modern GC stationary phase especially at low starting temperature. It will just go through in the void volume and cause no damage. Headspace is the way to go. I don't believe methanol binds at all and ethanol weakly to most SPME phases.  

What kind of conditions should I use to make ethanol SPME in FID with DVB / Carboxen / PDMS fiber and HP5 Agilent column in concentrated juice?

Since you can't do headspace GC, another simpler alternative is direct injection of the liquid phase of the alcohol mixture into the GC, but for this you would need to use either an old-fashioned polar packed column or a polar capillary column with a large split in the injector port (to avoid overloading the capillary column). Non-polar columns (even modern cross-linked ones) will be degraded over time if you inject strong polar solvents like water.

https://www.gerstel.com/pdf/p-gc-an-2005-01.pdf

I use this column https://www.restek.com/catalog/view/1433

60 meter. is perfect to analyse alcohols and aldehydes from aqueous solutions.

You can concentrate the samples with SPME, however, analysis of the sample with a such non-polar column is not expected to get good results as the alcohols are not well separated in the column given the low affinity to the stationary phase.

Hi Dalma... HSS is the best system for alcohol analysis... But, you´ll be need a special cap. Mega col (water resistent) Id: 530 maybe SPB- 5 MEGABORE ID: 0.53 mm SUPELCO 2-5351 carrier He. I hope help you