Dear all,
What is the criteria for selecting the positive mode or negative mode in LC-MS/MS analysis?
In brief, when is positive and negative mode of ionizations are used inn LC-MS analysis?
Hi,
the best way to select the polarity in LC-MS analysis is to try to inject your compound of interest in both polarities and observe which of the two ones gives the most intense signal and the highest S/N.
There are general rules to select the polarity:
NEGATIVE POLARITY --> molecules already charged negatively, molecules containing phosphates, acidic moieties, molecules with many Oxygen atoms, molecules contaning phenolic groups ecc.
POSITIVE POLARITY --> proteins, amines, molecules already charged positively, e.g. anthocyanins...
BUT the best way to select polarity still remains to try both polarities and select the most suitable to you.
BTW, if you are interest in a specific compound, look at the literature, there will be surely information about the experimental conditions, including polarity
At first need the analyte to scan in positive and negative ionization mode.
In broad sense the analyte have the capacity to accept H+ ion in acidic condition can run in positive ionization mode and the compound can release H+ in neutral mode can run in negative ionization mode.
In general :
- for basic compounds M, we use the positive mode. To assist the ionization, 0.1 % formic acid is generally added in the mobile phase. In this case, the most probable ion is (M+H)+
- for acidic compounds M, we use the negative mode. To assist ionization, 0.1 % of ammonium hydroxide is generally added in the mobile phase. In this case, the most probable ion is (M-H)+
Regards.
I think you will find the answers to your questions in this free training article.
- "HPLC Mobile Phase Composition and LC-MS Electrospray Voltage"; https://hplctips.blogspot.com/2018/11/hplc-mobile-phase-composition-and-lc-ms.html
In our training classes we teach to try BOTH modes (pos/neg) with your samples to determine experimentally, for your method and conditions, then review the data generated.
Thanks to all the experts for your kind responses. William Letter Elias Bou-Maroun Luca Narduzzi Md Humayun Kabir
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