GC MS GC/MS
GC separates components Mass spectrum of eluting
component
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Mass Spectrometers
Tandem Mass Spectrometry (MS/MS)
Mass spectrometers are commonly combined with separation devices such as gas chromatographs (GC) and liquid chromatographs (LC). The GC or LC separates the components in a mixture, and the components are introduced, one by one, into
Simple Schematic of a Mass Spectrometer Interfaced to a GC
the mass spectrometer . MS/MS is an analogous technique where the first-stage separation device is another mass spectrometer .Suppose that we analyze a mixture of components by a "soft" ionization method (such as chemical ionization, fast atom bombardment, or electrospray ionization). Each component produces characteristic ionic species such as [M+H]+. To keep the discussion simple, let's assume that each component of the mixture has a unique molecular weight. The mass spectrum of the mixture contains peaks for each compound present in the mixture. Now, suppose that we would like to identify one of the mixture components. All the mass spectrum tells us is the
m 1+→m 2++N
Set MS1to select this mass,m 1MS1MS2CID:Gas-filled collision chamber.
m 1breaks apart to produce fragments
Fragments of m 1
MS/MS
molecular weight, but we would really like to see fragment ions that provide structural information for the component of interest.The simplest form of tandem mass spectrometry combines two mass spectr
ometers. The first mass spectrometer is used to
Simple Schematic of a Two Mass Spectrometer Tandem MS/MS
select a single (precursor ) mass that is characteristic of a given analyte in a mixture. The mass-selected ions pass through a region where they are activated in some way that causes them to fall apart to produce fragment (product ) ions. This is usually done by colliding the ions with a neutral gas in a process called collisional activation  (CA) or  collision-induced dissociation  (CID). The second mass spectrometer is used to separate the fragment ions according to mass. The resulting "MS/MS" spectrum consists only of product ions from the selected precursor . Chemical background and other mixture components are absent.
Kinds of MS/MS experiments
Consider a precursor  ion, m1+ that decomposes to produce a product  ion, m2+, and a neutral loss , N :
MS/MS experiments can be classified according to which of these species (product, precursor, or neutral loss) is a constant. Note that there is no requirement that the product and precursor ions be positively charged; the terminology is identical for negative ions.
Product-ion scan (m1+ specified)
If we do an MS/MS experiment to find out all of the product ions, m2+ that result from the decomposition of a specified parent ion m1+, then this is called a product-ion scan. This is the most common and well-known MS/MS experiment. It is used to determine structurally significant fragment ions for a selected precursor ion.
Precursor-ion scan (m2+ specified)
If we perform an MS/MS experiment that tells us all of the possible precursor ions m1+ that decompose to produce a specified product ion m2+, then this is called a precursor-ion scan. This is useful when you know that a particular product (fragment) ion mass is characteristic of a class of compounds, and you would like to identify the mixture components that belong to that compound class.
Constant neutral-loss scan (N specified)
An MS/MS experiment that looks for all pairs of precursor ions and product ions that differ by a constant neutral loss, N, then this is called a constant neutral loss scan. This is useful when you know t
hat a particular neutral loss mass is characteristic of a class of compounds, and you would like to identify the mixture components that belong to that compound class. Note that the peaks in a constant neutral loss scan can be labeled with either the product ion mass or the precursor ion mass. Both naming conventions are valid, but it is more common to label the peaks
with the precursor mass because it is assumed that you are interested in the identity of the intact analyte molecules.
Selected reaction monitoring
Selected reaction monitoring (SRM) is an MS/MS experiment that is analogous to selected ion monitoring (SIM) for target compound identification. In SIM, one only measures a set of preselected analyte masses. Other masses and the baseline noise between peaks are not detected. Compare to scanning experiments, SIM experiments provide improved selectivity and sensitivity for target compound identification and quantitative analysis. In SRM, one specifies sets of product and precursor masses that are known to be characteristic of certain target compounds.
Ion activation methods
There are many different ways to increase the internal energy of ions so that chemical bonds will break and fragments will be formed. The various ion activation methods differ in how the energy is imparted to the ions, how much energy is imparted, and how the energy is distributed in the activated ions. These factors affect the selectivity, efficiency and reproducibility of the mass spectra, and they can have dramatic effects on which product ions are formed from an activated ion. The best way to characterize an ion activation method is to look at a plot of the internal energy distribution of the activated ion (Wysocki, V. H.; Kenttamaa, H. I. ; Cooks, R. G. Int. J. Mass Spectrom. Ion Proc.
75 (1987), 181-208). The internal energy distribution can be estimated from the relative abundances of fragment ions from well-characterized test compounds such as tungsten carbonyl. A broad internal energy distribution will result in many different kinds of fragmentations and an information-rich MS/MS spectrum.
A narrow internal energy distribution will result in efficient conversion of the precursor ions to only a few specific product ions. Note that a low, narrow internal energy distribution willreaction mass
Wysocki, Kettämaa, & Cooks, IJMSIP, 75 (1987), 181-208
MS/MS High-energy vs. Low-energy Conversion
favor bond rearrangements while a high, narrow internal energy distribution will favor simple bond clea
vage. The experimental time scale is important. It takes a certain amount of time for an activated ion to decompose. Mass analyzers such as time of flight and magnetic sector mass spectrometers may detect activated ions before they have time to decompose. In that case, the precursor ions may require additional internal energy to cause them to decompose fast enough for the product ions to be observed. This is called a kinetic shift.
Metastable ions
The majority of ionization methods form some ions that have enough excess energy to decompose spontaneously. Some ions are stable and long-lived, such as molecular ions that are observed in electron ionization (EI) mass spectra, or [M+H]+ ionsobserved in chemical ionization (CI) mass spectra. Some ions are unstable and decompose rapidly in the ion source, such as

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