After the first run (injection of internal standard: benzyl benzoate) and second run (mixture of phthalate derivatives DEP+ DBP+ BBZP+ DEHP+Benzyl Benzoate), the peak of internal standard disappears in the third injection. Any idea why is this happening?
Hello Sarra,
just for a better understanding, for the first run you just injected the IS.
The second run was the IS plus the phthalate mixture.
Both times, for the first and second run you got a peak signal for the IS and made sure via the MS, that it is indeed the standard?
Was the peak the same in height/area (assuming you used the same amount), shape and retention time for the second run?
What did you inject for the third run? The mixture again?
The disappearance of a peak can have a large variety of reasons so I will only list the most prominent ones that come into my mind:
So, first of all make sure, that this peak we are talking about is indeed caused by the internal standard and not by some impurities or contaminations in your system.
If this is set be sure to use the same GC and MS parameters to analyze your samples. This can happen if you accidentally used a slightly different method.
How long was the time between the injections, and how did you store your mixture? Make sure to either check UV stability of your standard or use brown glass. How is the stability with regard to the temperature? Make sure to know the condition to avoid disintegration of your standard. You could easily check this by preparing a fresh solution and measure it directly.
Does your standard react with your analytes? This should be avoided at all costs. I am not really familiar with phthalates so I can answer that question for you, but you should check if you have chosen the right standard for your analytical problem. For that you can measure your IS without the phthalates and check if you get the peak again.
If it is none of the above check your chromatogram again. How are the other peaks besides the missing standard? Are they exactly where they are supposed to be? Do they elute at the same retention time as before? Do they show tailing, fronting, or are also considerably smaller than you would expect? If so, then you should check your GC system for leaks.
These are all troubleshooting steps valid for every GC-MS Analysis not just the phthalates.
I hope this was a bit helpful.
Kind regards,
Kristina
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