difference between LC/MS and LC/MS/MS
Posted: Wed Jan 16, 2013 12:42 pm
by rrm1987
1)can anyone tell what is the difference between LC/MS and LC/MS/MS and its applications?
2)in MS two modes were there i.e., positive and negative ionisation mode?what exactly it implies?
pl do clarify
Re: difference between LC/MS and LC/MS/MS
Posted: Fri Jan 18, 2013 9:16 pm
by JMcK
Since no one else has responded, I'll take a stab at this.
1) In LC/MS, the effluent from the LC is ionized, and the ions are directed into a mass spectrometer. The spectrometer generally records all ions within the set range. You can collect all ions within the working range and plot the total ion chromatograph (TIC). If the analyte that you are observing has a unique mass, you can get better resolution of that analyte by performing SIM, single ion monitoring, where you only monitor the specific ion.
You would use MS/MS if you need to confirm that the mass you are monitoring belongs to the analyte of interest. You could for instance monitor anisole at m/z 108. You would detect all ions at this m/z ratio, including other ions of other molecules such as benzyl alcohol, or fragments formed from degradation of larger molecules. You can perform MS/MS however. With the first mass separation, collect only m/z 108 ions. Then you would allow this ion to fragment and perform mass separation and look for a fragment with m/z 65 (C5H5+). That will give you further confirmation of the analyte's structure.
2)Some molecules form anions easier than cations, and vice versa. Sometimes you get more structural information using positive mode than negative mode, etc. Use the mode that is better suited for your analyte and your separation conditions.
Re: difference between LC/MS and LC/MS/MS
Posted: Sat Jan 19, 2013 5:18 am
by rrm1987
Thank you
Re: difference between LC/MS and LC/MS/MS
Posted: Sun Jan 20, 2013 4:28 am
by vdnsrinivas
LC MS vs LC MS/MS
LC-MS @ See all masses in Scan (Poor specificity)
LC-MS/MS@ More Specificity due to fragmentation in Q2 (Great sensitivity)
Modes:
If the sample has functional groups that readily accept H+ (such as amide and amino groups found in peptides and proteins) then positive ion detection is used
If a sample has functional groups that readily lose a proton (such as carboxylic acids and hydroxyls as found in nucleic acids and sugars) then negative ion detection is used
