"MSMS" as well as "Daughter", "Parent" and "Neutral-Loss" scans are used for special applications. This is a short tutorial to show how this works and how it can be utilised.
I worked for some time with a VG-Micromass Tribrid GC-MS system so the examples are from this instrument. Basically this is a instrument with
Ion source (I)
two electrostatic sectors (E),
a magnetic sector (B)
a Quadrupole (Q)
three collision cells (C), that can be filled with e.g. Argon
and two multipliers (M)
Often the E and B sector are operated together as EBE sector.
Triple quadrupole instruments basically work the same, though they normally work only on one collision cell (equivalent to C1 and C2).
Collision results are dependent on the kinetic energy of the respective ions and the pressure of the collision gas as well as the molar mass of the collision gas (normally only argon is used).
Daughter scans are used to get more information unknown fragments.
Basic question: what is?
First analysator operates in Quasi SIM at mass x to isolate the fragment is question. In the collision chamber it is further fragmented, which is analysed in the second analyser which scan from x to 0. During data evaluation it should be considered that generally no isotopic pattern will be detected, except the fragment ion has a mixed isotopic contents (such as 35Cl237Cl2).
Parent scans can be used to find the origins of some fragments e.g. the molecular ions.
Basic question: What is the origin?
The second analyser operates in quasi SIM at mass x, while the first analyser scans from x+1 to high masses.
Neutral loss scans are used to find compounds with special properties in the chromatogram such as nitroaromatic compounds. It is essential that the respective compounds do a significant cleavage in the collision cell (nitro groups are cleaved: loss of 46, CO2 may be cleaved: loss of 44 or CO (28)).
During this experiment both analysers scan in parallel but with the respective difference. The multiplier can only be hit by ions that cleave the respective mass in the collision cell.
With sectorfield instruments this is generally hard to perform as the magnets show a hysteresis that can hardly be taken into account in this experiment. Do that with a Triple Quad!
In MRM specific cleavages are used to quantify compounds in complex matrices.
Basic question: were is my quantification signal in this jungle of peaks.
Both analysers work on (co-ordinated) SIM programs. Instead of the mass (as in SIM a specific cleavage (x->y) is analysed.
I used it for the analysis of hopanes and steranes in crude oil. In SIM these give one chromatogram each with hundreds of compounds and lot of peaks from even different compounds. The MRM experiment gave about 16 chromatograms (in 1 GC run) with about 2-5 peaks in each.
In MSMS always a loss of physical signal has to be faced. Even with a lot of optimisation a loss of signal of about 20 has to be faced. So this is not feasible if already the physical limits of detection of the instrument are reached. If detection is limit to the high background or structural information is needed from compounds. MSMS methods can give good solutions or information that cannot be gained any other way.
For quantification in HPLC-MS the second MS is highly recommended, as the ESI or APCI ion source gives significantly less information than an EI spectrum. Also HPLC separation is poorer than one obtained in gas chromatography.
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