Peak shape Problem with amphetamine

[jetlag]

I have an issue i can´t explain, hope some of you could give me some explanation.

I have a method for the analysis of amphetamine by GC-FID. I have changes only in the shape of the amphetamine peak (no in the internal satandard) but not in the area so the quantification is good as i´ve injected several times a standard and the results are ok.

In the first injection the amphetamine peak is symmetrical, narrow and sharp and in the subsequence injection the peak becomes less narrow, and broader but the area is the same.

These are the parameters:

Injector Temp: 250ºC
Split 10:1

Constant Flow N2: 2,9ml/min

Oven: 58ºC 2min then 15Cº/min to 280ºC 2min

I´ve also tried to change the split ratio to 20:1 and the Flow to 1.1ml/min

any suggestions?

Thanks

[chromatographer1]

Replace your injection liner. Trim the inlet side of the column by some suitable length.

A Lewis acid is present somewhere in the sample path, either in the liner or on the column.

This broadens the amine peak as it makes a 'sticky' spot.

best wishes,

Rod

[swheasler]

At risk of being obvious - does your method use the free base form of amphetamine, or is it a salt form (amphetamine hydrochloride, amphetamine sulfate, etc.)?

That (a salt form) would be the simplest reason to explain your chromatography.

[chromatographer1]

The salt form would not give any peak. However, if it came into contact with a Lewis base in the matrix or basic sites in the injector a small about could be converted to the free base. Salts of bases are NOT volatile when it refers to molecules the size we are discussing.

best wishes,

Rod

[swheasler]

In my experience, dissolving amphetamine salts in methanol and injection on a GC will not only get you a peak, but a hideous mess of peaks. Restek has an article about this issue. Figure 1 is an example of what amphetamine salts can look like directly injected. Sometimes I get almost none of "peak 1" and almost all of "peak 2" from figure 1. Running it as its base will fix it well enough for most applications (figure 2).


http://www.restek.com/pdfs/adv_2008_01_11.pdf

[chromatographer1]

The splitting of the peak with the second peak 'blob' is an artifact of injection. The cation of the base has very little volatility. It can react with the methanol (which contains water) and form a free base and a methoxide salt temporarily. And of course, there are sites in the injector, carbon dioxide in the air injected with the sample in the syringe....... lots of chemistry going on when we don't think about it.

Same thing happens with a ammonium chloride. As one old chromatographer told me forty years ago: Ions don't exist in the vapor phase (at lab temperatures), thus no volatility, not unless your injector is VERY VERY HOT. However, there is something called ionic equilibrium.

have a wonderful weekend,

Rod

[swheasler]

I didn't mean to suggest or imply that there isn't complicated injection port chemistry that ultimately introduces the base form onto the column.

I'm just saying that my first thought with the subject and original post was "with methamphetamine and amphetamine - if you inject a solution of an amphetamine salt your chromatography is a mess". I knew there was an obvious sample preparation explanation that would be easy to troubleshoot. Wouldn't want the poor chap to be trying all sorts of instrument maintenance to fix a problem in his or her volumetrtic flask.

All is well.

SWW

[chromatographer1]

Another issue can be producing artifacts is DCM as a solvent for amines. A little water in a Lewis acid can cause a variety of interesting effects with a hot injector port.

If I remember correctly the use of hexane to extract the free base is highly recommended.

best wishes,

Rod