Mass Spectrometry


Glossary of Terms - Content

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  1. Mass Spectrometry
  2. Sources of Ions
  3. Analysers
  4. Linear Analysers
  5. Trap Analysers
  6. Multi-Analyser Configurations
  7. Detectors
  8. Spectra
  9. Sample Introduction
  10. Additional Resources

Mass Spectrometry This is the technique in which an instrument is employed to produce ions from molecules (the source) which are then separated according to their charge-to-mass-ratios (m/z) and detected (the analyser). IRMS, FTMS, TOF, Tandem MS, GC-MS, LC-MS ,
Ion Trap-MS
Type of Instrument IRMS Isotope Ratio MS is capable of very precise determination of 13C/12C ratios. It is exploited principally in examining trace enrichment of 13C in small molecular-weight analytes (e.g. protein-derived amino acids) after biosynthetic incorporation of a 13C-labeled precursor. Applications of IRMS include the study of substrate disposition in humans after infusion of 13C-labeled precursors. IRMS can be combined with GC-MS. Other elemental ratios can also be measured; for example 2H/1H, and 18O/16O, but 13C/12C and 15N/14N are the principal emphasis of this resource.
  FTMS The basis for FTMS is an ion trap (Penning cell) that allows ions formed by EI, CI, MALDI, and ESI to be accumulated and stored for time periods as long as minutes. During this time, reactions of the ions with neutral molecules can be followed. The method has the highest resolving power in mass spectrometry, a high upper mass limit, high sensitivity, nondestructive detection, and high accuracy for mass measurement. Because it uses Fourier transform detection, signal averaging and simultaneous wide-mass detection are possible. The capabilities of this instrumentation are evolving, and its potential for high-performance measurements is the highest.
  TOF  
  Quadrupole MS GC-MS and LC-MS systems are available.
  Tandem MS Tandem MS or MS/MS is used for structure determination of molecular ions or fragments. In Tandem MS, the ion of interest is selected with the first analyzer (MS-1), collided with inert gas atoms in a collision cell, and the fragments generated by the collision are separated by a second analyzer (MS-2). In Ion Trap and Fourier transform experiments, the experiments are carried out in one analyzer, and the various events are separated in time, not in space. The information can be used to sequence peptides and small DNA/RNA oligomers, to determine structure and connectivity of polysaccharides, to determine the position and structure of fatty acids in complex lipids, and to carry out other structure determinations.
  Ion Trap MS GC-MS and LC-MS systems are available.
 
Sources of Ions    
APCI
(Atmospheric Pressure Chemical Ionisation)
Ionisation by reaction with reagents formed with a plasma discharge in air.
 
AMS
(Accelerator Mass Spectrometry)
A specialist technique in which atomic ions are formed by charge stripping in a very high voltage source, fed from a Van de Graff accelerator. The chosen technique for trace level analysis of the 14C isotope in radiocarbon dating and biological tracer studies.
 
Californium Fission Ionisation Ionisation as a result of impact with fission fragments from 252Cf.  
CI (Chemical Ionisation) Ionisation by reaction with gaseous reagents. CI is applied to similar samples; it is used to enhance the abundance of the molecular ion. For both ionization methods, the molecular weight range is 50 to 800 Da. Accuracy of the mass measurement at low  resolving power is 0.1 Dalton and in the high resolution mode, 5 ppm.
DI (Desorption Ionisation) This is a general term encompassing all forms of direct ionisation from a solid or liquid sample (see FAB, FI, Californium Fission Ionisation).  
ESI
(Electrospray Ionisation)
Ionisation produced by spraying a sample solution through a conducting capillary tube at a high potential. ESI allows production of molecular ions directly from samples in solution. It can be used for small and large molecular-weight biopolymers (peptides, proteins, carbohydrates, and DNA fragments), lipids. It is a continuous ionization method that is suitable for using as an interface with HPLC or capillary electrophoresis. Multiply charged ions are usually produced. ESI should be considered a complement to MALDI. The sample must be soluble, stable in solution, polar, and relatively clean (free of nonvolatile buffers, detergents, salts, etc.). Electrospray ionization is installed on the BRUKER ESQUIRE~LC and ESQUIRE 3000.
EI
(Electron Ionisation)
Ionisation in the gas phase by interaction with a beam of electrons. EI is widely used in mass spectrometry for relatively volatile samples that are insensitive to heat and have relatively low molecular weight. The spectra, usually containing many fragment-ion peaks, are useful for structural characterization and identification. Small impurities in the sample are easy to detect.
Most of MS-libraries based on EI (defined conditions) like NIST, Wiley
FAB
(Fast Atom Bombardment) (see LSIMS)
Ionisation by impact of a beam of energetic atoms and/or ions from an ion gun onto a liquid matrix and the subsequent transfer of energy from the matrix to the analyte.  
FD/FI
(Field Desorption /
Field Ionisation)
The formation of ions by removal of electrons in a high field.  
GDI
(Glow Discharge Ionisation)
Ionisation from a solid surface by application of a glow discharge.  
ICP-MS
(Inductively Coupled Plasma Mass Spectrometry)
Ionisation of elements in a high temperature plasma.  
LSIMS (Liquid Surface Ionisation MS) ( see FAB).  
MALDI
(Matrix Assisted Laser Ionisation)
Ionisation by effect of illumination with a beam of laser generated light onto a matrix containing a small proportion of analyte. MALDI is used to determine the molecular weight of peptides, proteins, oligonucleotides, and other compounds of biological origin as well as of small synthetic polymers. The amount of sample needed is very low (fmoles or less). The analysis can be performed in the linear mode (high mass, low resolution) up to a molecular weight of m/z 300,000 (in rare cases) or reflectron mode (lower mass, higher resolution) up to a molecular weight of 10,000. The analysis is relatively insensitive to contaminants. Mass accuracy (0.1 to 0.01%) is not as high as for other mass spectrometry methods. Recent development in Delayed Extraction TOF allow higher resolving power and mass accuracy. Structural information for molecules can be obtained in a "Post-Source Decay" mode, or by collisional activation.
NICI
(Negative Ion Chemical Ionisation)
Ionisation as a consequence of capture of an electron in the gaseous phase.  
TI
(Thermal Ionisation)
Ionisation induced at high temperatures in a gaseous sample with microwave or an inductively coupled plasma.  
TSP
(Thermospray Ionisation)
Ionisation from a solution of analyte by passing through a heated tube.  
Soft Ionisation Methods leading to the formation of ions with low internal energies.
(see CI, FAB, MALDI, ESI, APCI)
 

 

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