Geplaatst op 26 januari 2005 - 18:22
Geplaatst op 26 januari 2005 - 18:30
What is tandem or multi stage mass spectrometry (MS/MS)?
Sometimes further structural information than that generated by the ionisation method used is required. For example, an ion produced by FAB will indicate the molecular weight of the sample but often may not fragment much, if at all, thus giving little structural information.
By coupling together two analysers, separated by a collision cell, further information can be obtained. The first analyser is used to select the ion of interest. This ion is then passed into the collision cell which usually will have been pressurised with an inert gas such as argon. Collision of the ion with the atoms in the cell can induce dissociation of the ion. This is known as collision induced dissociation (CID) - the original ion is referred to as the "precursor" ion and the dissociated ions are known as "product" ions. These product ions are then analysed in the second mass spectrometer thus generating a product ion mass spectrum of the original precursor ion. Figure 8, shows the full scan conventional mass spectrum of 4-OH Tamoxifen and the MS/MS spectrum of the molecule ion.
Tandem mass spectrometry can be used for quantification. This is achieved by, in effect, doing two stages of selected ion recording. The first mass spectrometer is set to transmit the first ion of interest in to the collision cell, and then one of its product ions is monitored after the second mass spectrometer. Again, by careful selection of an internal standard, both stages of the mass spectrometer can be rapidly switched between the four ions (two precursor ions and the two associated product ions) and the corresponding product ion signal intensities monitored. This technique is known as multiple reaction monitoring (MRM) and is capable of very high levels of specificity.
Tandem mass spectrometry can also be used for screening purposes and two principal techniques are available. The second mass spectrometer is set statically to transmit product ions of only one selected mass to charge ratio. This mass is monitored continuously whilst the first mass spectrometer is scanned. A signal will be detected only when a precursor ion fragments to the product ion that is monitored. This technique is known as precursor ion scanning and is often used, for example, to screen a complex matrix (e.g. urine or blood extracts) in order to locate compounds of related structure, such as the metabolites of a known drug.
The other tandem screening method is known as constant neutral loss scanning. Here both mass spectrometers are scanned simultaneously but are offset by a set amount that corresponds to the difference between precursor and product ion masses. A signal only appears when a precursor ion yields a product ion with the mass-difference selected. This technique can be used to screen for compounds that contain a specific structural feature that yields a common fragmentation process.
( http://www.bmss.org...._is/whatis.html )
Geplaatst op 26 januari 2005 - 18:44
De MS/MS kun je in allerlei modes laten opereren.
Product-ion Select Scan
Precursor-ion Scan Select
Selected Reaction monitoring Select Select
Neutral loss Scan Scan (MS2 scant achter MS1 aan)
RF daughter Transmit Scan (screening bij method developm)
Meest gangbare apparaten die een MS/MS spectrum op kunnen nemen:
Geplaatst op 26 januari 2005 - 18:45
en mijn opmaak boven van me tabelletje is verschoven, naja so be it. Het staat boven nog mooier beschreven
Veranderd door Napoleon1981, 26 januari 2005 - 18:51
Geplaatst op 26 januari 2005 - 18:53
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