hp 5971 and 72 stability

[vestelshirley]

I recall a protocol for spaced internal standards when doing ms quant because of the instability of the ms detector as compared with a fid which can use en external standard. I have two mass specs - a hp5971 and a hp5972. How often should I put in an internal standard? Years ago, I used 6 minutes or so.

[gpronger]

I assume that by saying to have an internal standard every 6 minutes is that you are adding to the sample, compounds (often deuterated) at known concentrations so that their retention times through-out the run are spread out by one of the internal standards eluting about every 6 minutes. you are then using the response from the internal standard to quantitate the analytes in the samples.

If I am interpreting you correctly, That would typically be sufficient. If you pay attention to the analytes whose retention times are the greatest from the internal standard you've assigned it, if they are erratic, then you may need to add an internal standard more closely eluting to the compound that's erratic. In this case, you could always utilize the deuterated analog of the problem compound as an internal standard and ensure that its "misbehavior" is handled.

[vestelshirley]

I have someone here who does nt want to use internal standards for the 5971 and 72. I fould never from my past excperience, but wanted to be fair to the person by requesting an outside opinion.

Would you run withut internal standards for quantifying compounds?

[Peter Apps]

"Would you run withut internal standards for quantifying compounds?"

The answer is an uneqivocal "That depends", on what your analytes are in what matrix at what concentration, the sample preparation, is there any empty space on the chromatogram, how much do the standards cost, what accuracy and precision do you need, etc etc

There is no one-size-fits-all answer.

Peter

[gpronger]

Peter: since this is GC-MS versus GC-FID, I don't think there being room on the chromatogram will be much of an issue so long as any chosen internal standards do not have ions that overlap with the analytes of interest.

Peter is correct in that it is pretty much your call. Internal standards are more work, and cost. If you are getting sufficient stability and quantitation (where you're defining "sufficient") than quite simply why do more work.

The lab I'm associated with (Suburban Labs near Chicago) performs predominately EPA work; for the EPA is is more uncommon to not be utilizing internals standards (in other words we almost always do). But we have little control over matrix so we need everything we can get to help offset matrix effects on a sample-to-sample basis. Hence the need for internal standards.

Internal standards should improve reproducibility and quantitation. If you have a widely varying sample matrix, where you have concerns on quantitation and reproducibility, than internal standards typically improve the situation or at least let you know what is going on. If you do have a widely varying matrix, you could start adding internal standards and discover that you have issues going on that were not previously apparent.

So, it does come down to your call after considering your requirements for precision and accuracy as well as the samples themselves.

Greg

[Don_Hilton]

I have ocasionally encountered folks who learn that they have a very stable instrument and "discover" the idea of improved efficienty by omitting internal standards. They are delighted to know that there is even a name for the technique: external standard calibration. And then, the delight goes away when the question comes up: "What happens to your results if the autosapler draws up an air bubble?" Some will get as far with the answer as: "You will know it's a bad result because..." (they stop here.)

There are instances when it becomes difficult to introduce an internal standard because of the nature of the sample and the sampling technique and there is no gain in adding internal standard and having the sample change on you.

If you want to move that other person in the direction of using internal standards, my suggestion is to pose the question of how to detect some sample pereparation and/or injection problems that woud only be caught by eratic performance of the internal standard.

Also RSD's are just about always worse with external standard calibration methods. If limits of detection are important, that answers the question.

[Peter Apps]

Greg is right that with an MS running SIM you can clean up the chromatogram to make space for an internal standard, but you can only use SIM if both your target analyte and the standard have a distinctive ion as a strong peak in their spectra (it does not have to be the same ion). For pesticides and a lot of environmental contaminants this works very well, but for natural products e.g. flavours you have to be very lucky indeed to find an analyte whose spectrum is distinctively different from the chemical background.

Putting in internal standards that elute at intervals during a run implies to me that the MS is drifting during the run. How large is this drift ?

Peter

[vestelshirley]

So, even if the question of stability is restricted to these two instruments, it is still in question and the operator must know his/her own instrument.

Gentlemen, I want to thank you for my friend. He was trying to minimize time and cost. I told him I would help him find the space for the internal standards since he is shorthanded, but I won't buy him anything except an occasional lunch.

Again, I thank you.

[gpronger]

An occasional lunch sounds like a fair deal to me.

Hope all goes well.

sincerely,
Greg Pronger