different components of mass spectroscopy
Answers
The main components of a mass spectrometer are:
- Inlet system (LC, GC, Direct probe etc...)
- Ion source (EI, CI, ESI, APCI, MALDI, etc...)
- Mass analyzer (Quadrupole, TOF, Ion Trap, Magnetic Sector)
- Detector (Electron Multiplier, Micro Channel Plates MCPs)
Explanation:
What is Mass Spectrometry?
Mass spectrometry is an analytical technique that involves the study in the gas phase of ionized molecules with the aim of one or more of the following:
Molecular weight determination
Structural characterization
Gas phase reactivity study
Qualitative and quantitative analysis of components in a mixture.
Mass spectrometry consists basically of weighing ions in the gas phase. The instrument used could be considered as a sophisticated balance which determines with high precision the masses of individual atoms and molecules. Depending on the samples chemical and physical properties, different ionization techniques can be used. One of the main factor in choosing which ionization technique to be used is thermolability. For samples that are not themolabile and relatively volatile, ionization such as Electron Impact and/or Chemical Ionization can be effectively used. For samples that are thermolabile such as peptides, proteins and other samples of biological interest, soft ionization techniques are to be considered. Among the most used soft ionization techniques are Electrospray (ESI) and Matrix Assisted Laser Desorption (MALDI). The name given to a particular mass spec technique is usually pointing to the ionization method being used.
Atomic and molecular masses are assigned relative to the mass of the carbon isotope, 12C, whose atomic weight is defined as exactly 12. The actual mass of 12C is 12 daltons, with one dalton is equal to 1.661 10-24 g. The mass of a molecule or an ion can be presented in daltons (Da) or kilodaltons (kDa).
The Mass Spectrometer
Mass spectrometry uses an instrument called a mass spectrometer. The main components of a mass spectrometer are:
Inlet system (LC, GC, Direct probe etc...)
Ion source (EI, CI, ESI, APCI, MALDI, etc...)
Mass analyzer (Quadrupole, TOF, Ion Trap, Magnetic Sector)
Detector (Electron Multiplier, Micro Channel Plates MCPs)Samples can be introduced to the mass spectrometer directly via solids probe, or in the case of mixtures, by the intermediary of chromatography device (e.g. Gas chromatography, Liquid chromatography, Capillary electrophoresis, etc...). Once in the source, sample molecules are subjected to ionization. Ions formed in the source (molecular and fragment ions) acquire some kinetic energy and leave the source. A calibrated analyzer then analyzes the passing ions as a function of their mass to charge ratios. Different kind of analyzer(s) can be used, Magnetic, Quadrulpole, Ion trap, Fourier Transform, Time of Flight, etc...The ion beam exiting the analyzer assembly is then detected and the signal is registered. Common ionization method acronyms include:
EI=Electron Impact;
CI=Chemical Ionization;
SIMS=Secondary Ions Mass Spec;
FAB=Fast Atom Bombardment;
LDMS=Laser Desorption Mass Spec;
PDMS=Plasma Desorption Mass Spec;
TS=Thermospray;
AS=Aerospray;
ESMS=Electrospray Mass Spec.
Common mass analyzer acronyms include:
EB=Electrostatic-Magnetic;
IT=ion trap;
Q=Quadrupole;
TOF=Time of Flight.
Ionization Methods
Selection of the proper ionization method for the analysis of your sample is extremely important. Although we can offer suggestions, it is your responsibility to understand and select the method(s) appropriate for your research compounds.
Electron Impact EI Ionization
Chemical Ionization CI
Negative Ion Chemical Ionization
Electrospray Ionization Techniques
Matrix Assisted Lazer Desorption (not offered in our facility, but available elswhere on campus)
Atmospheric Pressure Chemical Ionization APCI
Electron Impact Ionization
M + e-(70eV) -----> M+. + 2e-
EI ionization method is suitable for non thermolabile compounds. The volatility of the sample is required. Sample molecules in vapor state are bombarded by fast moving electrons, conventionally 70 eV energy. This results in ion formation. One electron from the highest orbital energy is dislodged, and as a consequence molecular ions are formed. Some of this molecular ions decompose and fragment ions are formed. The fragmentation of a given ion is due to the excess of energy that it requires within the ionization. Fragment ions can be odd electron or even electron. Molecular ions formed in electron impact ionization are odd electron ions. Odd electron fragment ions are formed by direct cleavage(e.g. direct cleavage of a C-C bond). Even electron fragment ions are often formed by rearrangement(e.g. proton transfer). Sample can be introduced to the EI source via a gas chromatography device, for example in the case of mixtures, or directly via a solids probe device. The quantities needed for an experiment is usually less than a microgram of material.
EI mass spectra, in most of cases, contain intense fragment ion peaks and much less intense molecular ion peak. When the molecular ion peak is not observed in the mass spectrum, chemical ionization can be used in order to get molecular ion information. One helpful rule for determining whether an ion is a molecular ion is the Nitrogen Rule.