Sample preparation! Help plz

[Chem-Eng]

Hi there,


I am working on cardon dioxide reduction and need to identify and quantify the products using GC. The products I am expecting are tiny amounts of methanol, formic acid,and formaldehyde in aqueous phase, which initialy contained KCl and pyridine at PH=5.2. Which sample preparation is appropreate here?and how to quantify the products accuratly?

Before performing the experiment I first want to understand how to develop a good sample preparation and accurate and efficient method of analysis : So I tried to dilute methanol in dionised water and introduced in the GC, I only could see water and no trace of methanol in the spectrum! I also tried the SPME with the same solution and gave the same spectrum! I increased the concentration and only when the methanol concentration was high 1:1 methanol: water the spectrum shows the methanol peak.But , this is not benefical to me as my products are with very low concentration.

Can anyone help me with this please?

Thank you in advance.

[chromatographer1]

what is your detector using GC? I hope you are not using MSD.

Using GC headspace and an FID I have measured 1ng of methanol from 25 microliters (mg) of water, that is:

1ng from 25,000,000 ng of water.

formic acid should be done by titration or ion chromatography.

formaldehyde should be done by wet chemistry and or spectroscopy.

Use the best technique for each analyte for the most accurate determination at low levels.

best wishes,

Rod

[Chem-Eng]

Thanks a lot Rod

[quote="chromatographer1"]what is your detector using GC? I hope you are not using MSD.

Yes, I am using MSD, is it not appropriate for this analysis?? Why FID is better in this case???

My products will be in a mixture, do I need to separate them first then analyse each using the techniques you specified!!??

Thank you again

Dalia

[chromatographer1]

Most likely you can analyze your sample for each of the components without any preliminary processing.

tiny amounts of formic acid and formaldehyde should not interfere with the methanol GC-FID analysis.

Likewise the acid should separate from alcohol and aldehyde in IC or with a titration.

And performing an aldehyde determination should be straightforward as long as there are no other aldehydes present in your sample.

why GC-FID instead of GC-MSD?

I suspect you may not be using the MSD properly or there are problems with the conditions you are using.

best wishes,

Rod

[Chem-Eng]

Thanks Rod,

I would bother you again with an other question about the HSGC and FID. I have never done this before, I am reading about it.

I would appreciate if you give me details how you analysed methanol in water, things like vial type and size you used: is it possible to use an Agilent 1.5 ml vial or do I need to purchase a specific one !? How much of volume of the sample in relation to the vial size I need to introduce? I read about the addition of a matrix modifer, is it needed here?

Also I would like to know how is it heated and I guess below the boiling point of methanol, right!!! Do I need to do for long (time), is there any other paramters I need to take into account to maximaze the amount of the analyte in the headspace . Is the injection done using std GC manual syringe for example?

Best wishes,
Dalia

[chromatographer1]

My paper on headspace was published in Analytical Chemistry

Rapid Analysis of USP Organic Volatile Impurities and Thirteen Other Common Residual
Solvents by Static Headspace Analysis, Analytical Chemistry Vol. 69, No.11, p2221-2223

A comparison of vial sizes, 20 to 3.5mL is included. 25 microliters samples were used to measure solvent contents of 0 to 1000ng in each of the 25mg solutions (water). No matrix modifier was needed. I found that a gain of 0 to 20% of sensitivity was not worth the opportunities for error and sample contamination when using a modifier. 80C equilibration temperature and 6 minute times were chosen. Sample volumes over 250 microliters and equilibration times greater than 15 minutes were counter productive. 100 microliters and 10 minutes and 6mL vials are a nice compromise for sample size and analysis time. High concentrations (2 to 100 micrograms per 100 microliters) may require 12 or 20mL vials using the sample volume and equilibration time.

While you may not have a headspace analyzer, headspace using a SPME fiber syringe might be an excellent low cost solution for your needs. Reproducibility is not as good as automated HS but SPME is able to measure levels more than 3 orders of magnitude lower then HS. Ambient temperatures can be used for methanol and times can be researched easily. Call or email Supelco technical support for suggestions for the proper fiber and conditions. This is an excellent choice for 1 ppb to 1 ppm levels of solvent.

best wishes,

Rod

[dpr]

We have a formaldhyde method that uses a cyanodrin derivation prior to analysis on a ppq packed column. But is you only what to use one GC.......

Hach Lange have a nice analytical cuvette test solution that will goes down to around 0.5ppm.

[Chem-Eng]

Thanks a lot Rod, I'll try your method

[Chem-Eng]

Hi dpr

Could you give me more details about the cyanodrin derivatisation, please? (a paper that talks about that for example?).

I did not get what you mean by only GC ? what detector you are using?

Thanks

[dpr]

To look for formadehyde we take the following sets.
In a 100ml vol.
90mls sample and add 5mls 1% KCN
mix and stand a few mins
Then add 5mls of 10% H3PO4.
Again mix and stand few mins

All at room temperature.

This is then analysed on a ppq packed column.

Oven conditions - isothermal at 160deg on FID - External standard

This has been shown to be good down to around 10ppm wt/wt in aq samples.


Hope this helps. My comment on the number of GCs' was that I do not know if you have another GC with packed injectors available.

Dave