help please

[renu]

Hi,
I am a newbie in this field of GC. I am doing GC/MS analysis to detect oxidation rate in my sample (mixture of tomato paste+corn oil) by detecting oxidation compounds.I have decided on using one of the two internal standard C19-Nonadecanoic acid and C17-Heptadecanoicacid. The problem is they are in powder form, I want to know how should I make the desired solution for standard curve.Please help me, If possible please give me the procedure to make standard curve. Thank you.

[Don_Hilton]

There are many ways to handle the addition of an internal standard. The simplest would be to weigh a quantity of each into a largge volume of your extracting solution and then let the internal standard follow with the analytes through the entire sample preparation procedure. The solution would be made at a level where the quantity of internal standard would be similar to or slightly greather than the quantity of the analytes you are looking for in any give sample. If you will extract samples with a fixed volume of sample and inject the extraction mixture directly into the GC, this type of technique works well.

But this is the easy one and, unfortunatly has limited applicability.

Creation of a standard curve for this technique would involve spiking portions of the extration solution with known quantities of each analyte at varying levels.

But, a bit more detail about what you need to do for sample preparation would help before anyone could give good advice on specifics for making the internal standard solution and building a calibration curve.

[renu]

Thank you Don for your reply. I am actually doing a semi-quantitative analysis my basic goal is to just identify the rate of oxidation. I will be also performing other test like TBA and PV and relate it to GC result. My professor has advised me to do headspace gas analysis so I will not be doing any extraction. I was thinking to load my sample in vial and spike it with the internal standard. This will be done to assure stable performance of the instrument throughout the test. During my analysis I may detect new compounds in my sample due to oxidation.These compounds may increase or decrease during storage period for which I was planning to do an standard curve.I know my method is different but my whole purpose is just to analyse rate of formation of oxidation compounds.

can you please tell me if it will be appropriate to mix the internal stanard which is in powder form? can I dissolve it in some solution?

[Don_Hilton]

If your are doing headspace analysis, these organic acids are going to be tricky to deal with. They do not go into water well. And, if you put them in a volatile organic solvent and spike the solvent into the sample, the solvent can become the dominant vapor in the headspace and saturate the fiber.

You may be able to put the fatty acid into a solvent such as chloroform and add a few microliters of solvent to gram quantities of sample. But, you have to add sufficient quantities of the fatty acid to see it - and you must keep the solvent quantity to a minimum.

Another possibility is to take all of the tomato paste/ corn oil mix that will be used in your study and a portion of the fatty acid and blend them together. The problem of water soluability remains - and the acids may be only partly disolved. But, you need to be sure these acids are not subject to degredation.

And I will point out that these acids are not particularly volatile, so you may not find them transfering well to the fiber. I don't believe that I've noticed fatty acids of this size transferring significantly to a SPME fiber in the work that I have done. (but that may be a matter of how hot I've wanted to go with the sample or how long I've equilibrated the fiber in the headspace.) So you may want to try a couple of experiments to see if you can even see your internal standards. And that is as simle as adding a few flakes of each acid to a headspace vial with some corn oil and sampling it. I suspect the fatty acid may partition to the corn oil.

If you are looking for aldehydes and ketones as oxidation products, I would suggest that you look for an aldehyde or ketone as an internal standard. It is likely to be more volatile.

[renu]

Mr.Hilton I found a paper that says solubilty of nonadecanoic acid in water as 2.37e-06 mg/mL ..I am not able to understand this...can you please clarify if possible. Also, I am planning for serial dilution for which I am not sure how to start with my bad chemistry basic...would you please give me an example... like If I need to use 1ul of IS how do I start up with the powdered nonadecanoic acid standard. Please let me know I really do not have anyone to guide me. Thank you

[Don_Hilton]

The paper you have found tells you that nonadecanoic acid does not disolve in water to any extent that will be of interest to you. Not only is nonadecanoic acit not particularly soluable in water, but it not particularly volatile, so measurement by headspace has a very low chance of working. If you expect the oxidation products in your mixture to be organic acids, I would strongly advise youy find a different analytical technique.