Problems with the derivatization of a perfluorinated acid

Discussions about sample preparation: extraction, cleanup, derivatization, etc.

17 posts Page 1 of 2
Hi all,

I have some doubts regarding GC analysis of some perfluorinated compounds. I tried derivatizing a perfluorinated acid with acetonitrile, pyridine, isobutyl alcohol and isobutyl chloroformate mixture and extracting the ester formed with hexane. But when I run the GC, I am getting so many peaks and the actual peak for the ester is a real small one.I am really new to this field and so I dont really know how to correct these errors. I tried with BSTFA also;but of no use. How can I make the small peak a significant one?


Do you have any suggestions?

The first question is what is the concentration of the acid (or derivative) at the time of injection? Low concentrations give small peaks.

And what kind of detector are you using?

Assuming that you are using a reasonable detector and have sufficient compund present, then the conditions under which you deritize the acids and the chromatographic conditons might help someone to see where things have gone wrong for you.

Also, are you following a literature prodecure?

Thanks for your reply

Concentration of the acid is 10mg/l.I tried increasing the concentration ;but somehow the peaks are still small.

I am using a GC-EI/MS.

The temperature program that I use is :-
The oven temperature held at 40 ◦C for 3min, ramped at 10 ◦C/min to 170 ◦C and then held for 3min.


Also, how important the transfer line temperature is in getting these results?The temperature should be 300deg celsius but I am using 280deg cel. Is it going to make a huge difference?


Yes,I am following a literature method. I dont understand why so many peaks are coming.


Also the paper suggests the peak area to be around 417,000±30,800 a.u.
Isn't this value a bit small? For my small peak, I am getting area somewhat near this value.

I was just wondering whether I can use this method to detect that eventhough it is giving a small peak.

If your compound is failry well behaved in the ion source (ionizes well and does not breakd down into many fragments) you should be able to see if with a mass spec.

Withoug knowing the retention characterists of your compund, I can not say anything about the transfer line temperature. If it elutes at an oven temperature below 280, the transfer line should be OK there.

If you are getting a signal close to what was published in the literature, you are proably doing about as well as can be expected without seriouly revising the method. (People tend to show the good results in a paper.)

Why you are getting so many peaks? It all depends on what went into the instrument. From the number of component in the mixture, I would expect several right away. Depending on what you are using for reagents and solvents, some peaks may be impurities in them. Use the mass spec to look at the spectra and see if you can identify the peaks. If so, you may have your answer.

Without details on the method and analytes, I can not make any guesses at how to optimize it.

>How can I make the small peak a significant one?

I would recommend running in SIM (Single Ion Monitoring) mode instead of scan mode. With SIM you can get much higher sensitivity because you're looking at just the most abundant fragments - usually your base peak and maybe a couple of other main fragments.

The other mode would be Scan mode where you're scanning an entire mass spectrum. In SIM mode you can have a much higher dwell time on just a few masses which gives you an enormous increase in sensitivity for the compound of interest.

You'll need to be able to see a mass spectrum for your compound to choose which ions to monitor. The NIST has a very large number of compounds with mass spectra http://webbook.nist.gov/chemistry/

Hi "chromatographer"and "Don_Hilton",
Thanks for your replies..

I tried using SIM mode also..But the peak is not getting bigger...I have found that 2 of the unnecessary peaks are coming from Hexane and Isobutyl chloroformate.Hexane that I use is HPLC grade,95%.

I tried using other solvents instead of acetonitrile,like methanol,acetone and all.But no use.I used ether for extraction also.But all these were of no use.

Also, the peak I am getting is at a temperature around 90 degrees. The boiling point of the compound formed after derivatization,methyl ester of the acid, is 158degree celsius. The temperature program that I use is 40 ◦C for 3min, ramped at 10 ◦C/min to 170 ◦C and then held for 3min.Do you think that changing the temperature ramp will help detecting it?


Also in the mass spectra, the library suggests some weird compound names. I dont understand from where it came. I tried running GC with acetone alone many times..But still I could see such compounds.Is it the impurities present or is the column contaminated?

I dont know much about GC.So I cant think of any measures to correct these errors.

Any thoughts will be helpful...

Thanks!

The only way that a change in temperature ramp would increase sensitivity would be if it were to compress the peak, increasing the height. A fast ramp through a peak can do this, but from what you describe, you need more that the increase this would give to you.

You can post some pictures and we can comment on the quality of the chromatography.

Strange library names are not uncommon when searching a spectrum that is not in the library - sometimes even for a spectrum that is in the library. A typical library search matching technique is to make a multidimenional vector out of the spectrum and to comptue the match as the cosine of the angle created by the library spectum and the unknown. And, the technique works pretty well. But it misses many finer points that would be observed by a knowledgable chemist - which is nice because if the computer could work unaided, then GC/MS would go over to the information services deparment and we would have even more unemployed scientists.

Blank sample
Image


After derivatization
Image


Overlayed chromatographs
Image


Peak at 8minutes
Image[/b]

Does SIM mode give you a clean trace for your peak? And, where is this concentration relative to your anticpated analytical range?

juniorchem,

Do you have access to a GC with an ECD? Polyfluorinated might respond quite well and somewhat selectively on an ECD?

Best regards.

Hi AICMM,
We dont have a GC-ECD in our lab.Just an EIMS.


juniorchem

Hi,

The results that I produce are for the SIM mode. Scan is even worse! The concentration that I used is 10 mg/l and the range as per the method is 0.08 to 10 mg/l.


The compound that I am trying to analyze is PFOA (Perfluorooctanoic acid).

I had been using 99%pure acetonitrile. I dont think that was an HPLC grade. But when I started using HPLC grade acetonitrile, the peaks were a little less. But the thing is now the unnecessary series of peaks are coming from hexane(HPLC 95%) and isobutyl chloroformate. The peaks from hexane are coming at around 3minutes(the pictures dont show that,since these peaks were absent that day) and those for isobutyl chloroformate that you can see at time = 8. 5 to 10minutes in the pics.

Day by day, the results are getting worse...


Also, does anyone know the significance of temperature in an ultrasonic bath? As part of the derivatization procedure, I have to use the ultrasonic for 20s. But the paper doesn't say anything about the conditions of the ultrasonic bath. Is there a standard condition or something?

Also does anyone know whether we can use glass GC vial for PFOA samples?I use polypropylene vials for stock solution and sample preparation;but glass vials for GC.



Any thoughts will be helpful...

Thanks

juniorchem

Scan mode will be worse - it includes the noise from all masses which do not have signal. While it may theoretically average out - in reality it does not.

You say that results are getting worse day by day. What is the peak shape doing (possible indication of column problems) and how often is the inlet liner changed. And what kind of liner are you using? (open, with glass wool, a frit?) Given the chemistry of the derivatization, I would be careful of dirt in the inlet.

Are you reusing the polypropylene tubes? (If they are disposable - dispose of them cleaning could be an issue.)

Given that there is not a standard ultrasonic bath, it is hard to guess conditions for yours. I would suggest try a couple of tubes each at varying times and see if you get an improvement. The method of addition may make a difference. I normally do low volume work with microliter syringes - and a quick expulsion of a solution into a vial will get thngs well started on being mixed. Addition from a plastic pipette tip will be different. Be sure that gross mixing of the sample occurs. If the ultrasonic bath just makes the liquid jiggle around, cause some gross mixing before going to the ultrasonic bath.

I don't note an inlet temperture in the conditions you listed - I would suggest that you check to be sure you are using the same as in the paper.

You also don't indicate your carrier and carrier flow. Most people doing MS work use helium. And the flow should be set for optimum linear velocity or juat a bit faster.

I have not tried this reaction, but from my reading these compounds are difficult to derivatize. The ester derivatives are not always stable.

Also, fluorinated interferences can be coming from many different sources - every Teflon component could contribute. Even the septa and liners in the GC vials can contribute. From what I have heard, low level analysis of these types of compounds is very difficult.

Can you you negative ion CI? That should produce better sensitivity.
Merlin K. L. Bicking, Ph.D.
ACCTA, Inc.

I have not tried this reaction, but from my reading these compounds are difficult to derivatize. The ester derivatives are not always stable.

Also, fluorinated interferences can be coming from many different sources - every Teflon component could contribute. Even the septa and liners in the GC vials can contribute. From what I have heard, low level analysis of these types of compounds is very difficult.

Can you you negative ion CI? That should produce better sensitivity.
Merlin K. L. Bicking, Ph.D.

ACCTA, Inc.
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