GC MS GC/MS
GC parates components Mass spectrum of eluting
component
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Mass Spectrometers
Tandem Mass Spectrometry (MS/MS)
Mass spectrometers are commonly combined with paration devices such as gas chromatographs (GC) and liquid chromatographs工作单位意见
(LC). The GC or LC parates 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 paration device is another mass spectrometer .Suppo 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 prent in the mixture. Now, suppo 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 lect 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 e fragment ions that provide structural information for the component of interest.The simplest form of tandem mass spectrometry combines two mass spec
trometers. The first mass spectrometer is ud to
Simple Schematic of a Two Mass Spectrometer Tandem MS/MS
lect a single (precursor ) mass that is characteristic of a given analy运动口号
te in a mixture. The mass-lected ions pass through a region where they are activated in some way that caus 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 cond mass spectro如何发群公告
meter is ud to parate the fragment ions according to mass. The resulting "MS/MS" spectrum consists only of product ions from the lected precursor . Chemical background and other mixture components are abnt.
Kinds of MS/MS experiments
Consider a precursor ion, m1+ that decompos to produce a product ion, m2+, and a neutral loss , N :
MS/MS experiments can be classified according to which of the species (product, precursor, or neutral loss) is a constant. Note that the睡前故事短篇
re is no requirement that the product and precursor ions be positively charged; the terminology is交通安全教育主题班会
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 ud to determine structurally significant fragment ions for a lected 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 decompo to produce a specified product ion m2+, then this is called a precursor-ion scan. This is uful when you know that a particular product 珍爱生命的名言
(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, th结膜炎会传染吗
en this is called a constant neutral loss scan. This is uful when you kno
w that 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 becau 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 lected ion monitoring (SIM) for target compound identification. In SIM, one only measures a t of prelected analyte mass. Other mass and the baline noi between peaks are not detected. Compare to scanning experiments, SIM experiments provide improved lectivity and nsitivity for target compound identification and quantitative analysis. In SRM, one specifies ts of product and precursor mass that are known to be characteristic of certain target compounds.
Ion activation methods
There are many different ways to increa 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. The factors affect the lectivity, efficiency and reproducibility of the mass spectra, and they can have dramatic effects on which product ions are formed from an activated ppt加背景音乐
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 will
Wysocki,入体原则
Kettmaa, & 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 cl
eavage. The experimental time scale is important. It takes a certain amount of time for an activated ion to decompo. Mass analyzers such as time of flight and magnetic ctor mass spectrometers may detect activated ions before they have time to decompo. In that ca, the precursor ions may require additional internal energy to cau them to decompo fast enough for the product ions to be obrved. This is called a kinetic shift.
Metastable ions
The majority of ionization methods form some ions that have enough excess energy to decompo spontaneously. Some ions are stable and long-lived, such as molecular ions that are obrved in electron ionization (EI) mass spectra, or [M+H]+ ionsobrved in chemical ionization (CI) mass spectra. Some ions are unstable and decompo rapidly in the ion source, such as
