I am using a multicomponent pesticides analysis (SPE extraction) in surface water with UPLC and Q-TOF (Bruker Maxis impact) in ESI+ and 5ul injection. I use a 5 point external calibration. I want to identify a component with 2 exact masses (check ratio in calibration versus samples). For the identification I would like to follow: NTA 8379 (en), Water quality - Guidelines for the identification of target compounds by gas and liquid chromatography and mass spectrometry
I have two options:
1. Parent + isotope
In several cases the isotope mass has a relatively low intensity (8-15%) If there is Cl/Br in the ion, there is a second ion with a higher response thus no problem. I also use the ion ratio for identification. At low concentrations the ion ratio varies a lot if the second mass has a low intensity. For example carbamazepine 237,1022 and 238,1054 (17%).
2. parent + fragment
With the instrument we can also alternating measure total ion and fragmentation. I then select the parent and a suitable fragment. For carbamazepine the fragment is 194,0956. It has a very good response compared to the mass 238,1054.
Because the lower mass of the fragment it is less specific for that component (the higher mass the better?)because it can also be a fragment of a co-eluting component. I will check this later in the validation of the method. I am calculating the parent/fragment ratio in the calibration and samples and want to use this in the identification criteria.
What is, according to you, the best option to identify a component, option 1 or 2? I prefer option two when second m/z has a low intensity, because I can identify a component on a lower concentration. In the software it is difficult to select and handle 2 masses plus a fragment. But if that was possible it was the best option.
Thank you for your time.
Dear Gerrit, we have more or less the same questions....after several trials, we have noticed that your option two , as the most "sensitive" (available for lower concentrtaion) is the most robust. But isotopic pattern is a complementary way to confirm the identification.
We use a WAters system and we can keep the two options simultaneously
I have very interested to exchange with youi and other team involved in this new technology to compare our practices!!
Regards
ANne TOGOLA
For sure, option 2 is the best. It is always preferable to use fragments ( clearly established) than isotopes becasue they have to be consider more selective and with the ion ratio interval defined in experimental conditions.
Obviously it is a general rule, in some especific cases Cl or Br isotopes are very helpful and even the first choice but ths selection is case by case.
Additionaly to say that; at very low concentration high variations in intensity can be related with the resolution provided by the instrument
Your choice for option 2 is the way to go due to its relatively higher sensitivity that you'll need for your analysis. I agree very much with Amadeo that variations at lower concentrations can be related to the instrument's resolution.
While I certainly agree with the other respondents, there are a few things to keep in mind. When discussing anything about approach/procedures to use, one should always be discussing signal/noise. There is none of that explicit anywhere in this discussion, but it should be everywhere. Also you are using far too many significant digits for the Bruker Impact's mass accuracy, given its resolving power, which can misguide your options in some cases. Your "exact mass" is not very exact mass for any of the QToFs, that is manufacturer's sales pitch (everyone does it). Given the Impact's capabilities, Option 2 is the best as the others have stated. However, if you had 10-50x the resolving power such as the SolariX XR or Thermo Fusion, the choices become far more complex (but better). The ion source is a critical thing to discuss, and here you are referring to the Apollo II ESI, but a modest addition such as the Captive Spray (for Bruker) can increase your s/n 2-10x. Lastly, the discussion so far completely ignores the UPLC part, which is critical in s/n discussion - we need far more info regarding the peak characteristics, especially in terms of peak width, with regards to how much time the QToF has to work with, to define enough points during elution. Can't discuss the MS part without the LC part. In some cases, the UPLC can work against your s/n as it takes away time for MS/MS & signal averaging for the MS to do its thing. This is 2-3 book chapters-full of discussion possible here, so the specifics of your setup is critical to narrow it down.
Thank you all for your time to write comment and tips for my question. I am aware that the resolution of our TOF is not capable to see the difference in masses at the 4 digits I mentioned. But I just wrote down the exact mass in 4 digits.
I am convinced the second option is the best. I will run some more tests to see if the "fragment"ratio is as constant as the isotope ratio in both calibration samples and surface water extracts. Also I will spend some more time checking the noise levels, peak shape and scan times.
Dear all!
You might want to get in contact with Nikos Thomaidis from Athens University (Greece). His group did some really interesting work on this.
On the other hand, eawag has a very interesting concept, even if it is on an Orbi ;) Heinz (Singer) and Matze (Ruff) do a terrific job there.
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