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Mass Spectrometry and Related Techniques 2
Lecture Date: February 27th, 2008
Ion and Particle Spectrometry 2 - Outline
Atomic and Molecular Mass Spectrometry – Skoog et al. Ch 11 and 20.
Please read this additionalreference:
– R.Aebersold and D. R. Goodlett, “Mass Spectrometry in Proteomics”,Chem. Rev., 2001, 101, 269-295.
Ion Mobility Spectrometry – If interested, see:
G. W.Eiceman, Critical Rev. Anal. Chem., 1991, 22, 471-489.
D. C. Collins and M. L. Lee, “Developments in ion mobility spectrometry – mass spectrometry”,Anal. Bioanal. Chem., 2002, 372, 66-73.
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Homework Problems If you read March’s paper on ion traps:
– Whatis resonant excitation? Summarizehow resonant excitation
is used in typical ion trap MS experiments.
If you read the Russell and Edmondson paper on MALDI-TOF and accurate mass:
– Summarizethe advantages and disadvantages of MALDI-TOF (with DE and reflection) versus FTICR (including ESI-FTICR), especially in biochemical applications.
If you read the Aeberold and Goodlett proteomics paper:
– Why is MS used so heavily in the study of post-translational modifications? Briefly describe an application to phosphopeptide sequence determinations.
If you read the Sleno and Volmer ion activation methods paper:
– Pick any two of the ion activationprocesses described in the paper (e.g. in Table 1), describe how it works and the approximateenergies involved,and list one advantage
Applications of Mass Spectrometry
Interpretationof mass spectra is the key to most applications of the technique
Information contained in a mass spectrum:
– Molecular weight (viaexact or mono-isotopic mass). Usually obtained though a suitably accurate measurement of:
M+• (the molecularion, an odd-electronspecies)
[M+H]+ and [M-H]- (the protonated/de-protonatedmolecule, an even-electron species)
In sometechniques, can be confirmedby [M+Na]+, [M+K]+, [M+NH4]+,dimers, trimers,and other adducts, etc…
– Molecular formula
– Ionizationenergies
– Isotopic incorporation (ex. 13C, 14C, 2H, 3H…) – Fragmentationand ion stability
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Quasi-equilibrium Theory Once we make an ion, what happens to it?
In EI, and similar techniques: the ionizingelectron has littlemass and high KE, so it barely movesthe molecule that it hits but leaves it in a higher rotational/vibrational state.
Ionizationenergies can sometimesbe determinedfromion intensities.
Diagram from Strobel and Heineman, Chemical Instrumentation,A SystematicApproach,Wiley, 1989.
Molecular Structural Analysis: Fragmentation
Fragmentationcan also be used to determine structure – commonfragmentationpathways and rearrangementscan be predicted in many cases
Cl General rules:
- More stable carbocationsare more stablefragments (ex. tertiary carbocations are morestablethan
primary)
O
p-chloro-benzophenone
- Resonance can stabilize fragments,ex. allylic carbocationsand benzyl/tropyliumions
- Loss of small,neutral, stablemolecules is favored
Figure from R. M. Silverstein, Spectrometric Identification of Organic Compounds,6th Ed., Wiley, 1998.
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Molecular Structural Analysis:
Isotope patterns can be used to determine molecular structure
Isotope Patterns
M (100%)
– Example: the well-known methods of calculating(M+1) and (M+2) intensities
– Especially useful for detecting chlorine, bromine,sulfur, silicon and many other elementswith characteristic profiles
Isotope patterns can also be used to extract out “isotope incorporation profiles”for labeled compounds
– Examples:13C, 14C, 2H, 3H-labeled moleculesfor metabolism studies
– Applicationsin isotope chemistry include the detection of stable and radioactive isotopes in synthetic products and in nuclear chemistry.
M+1 (19.28%)
M+2 (33.99%)
M+3 (6.21%)
215 220 m/z
O
Cl
p-chloro-benzophenone
Molecular Structural Analysis: Accurate Mass
Nuclide masses are not integers. Example: Four things that weigh “28” amu:
– CO, 27.9949 – 14N2,28.0062
– CH2N, 28.0187 – C2H4, 28.0312
m/z measurements to four decimal places or higher are needed
Accurate mass analysis is often used as a final confirmation of structure, or for unravellingcomplex fragmentation
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Molecular Structural Analysis: Mass Defects
Mass Defect (Da)
0.02 2H 0.01 1H
0
13C 14N 15N
Atomic Mass Defects (All Different)
-0.01 12C 16O Mass Defect =
-0.02 Atom Mass – Nearest Integer
-0.03 Every C H N O S mass is unique!
31P 32S
-0.04
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34
Mass (Dalton)
Picturecourtesy Prof. Alan Marshall, FSU/NHMFL
Molecular Structural Analysis: MS-MS, and MSn
Step 1 – mass selection of an ion formed in the source Step 2 – dissociation of the parent ion via collisions
Step 3 – mass analysis of the dissociated “daughter” ions Step 4 – repeat…
+ + + + +
Mass Analyzer 1
Collisions
Mass Analyzer 2
+ + + + +
MassAnalyzer and Collision Chamber
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