Mass Spectrometry

Mass SpectrometryByK . Rakesh Gupta

CONTENTSIntroductionPrincipleWorking of the mass spectrometerInstrumentationTheory of mass spectrometryApplications

INTRODUCTIONJ. J. Thomson (1913) separated the isotopes 20Ne and 22NeAtlantic Refining Company (1942), first commercial useThis technique resolves ionic species by their m/e ratio

Francis William Aston won the 1922 Nobel Prize in Chemistry for his work in mass spectrometry

Replica of an early mass spectrometer

Some of the modern techniques of mass spectrometry were devised by Arthur Jeffrey Dempster and F.W. Aston in 1918 and 1919 respectively. In 1989, half of the Nobel Prize in Physics was awarded to Hans Dehmelt and Wolfgang Paul for the development of the ion trap technique in the 1950s and 1960s. In 2002, the Nobel Prize in Chemistry was awarded to John Bennett Fenn for the development of electrospray ionization (ESI) and Koichi Tanaka for the development of soft laser desorption (SLD) and their application to the ionization of biological macromolecules, especially proteins.The earlier development of matrix-assisted laser desorption/ionization (MALDI) by Franz Hillenkamp and Michael Karas has not been so recognized despite the comparable (arguably greater) practical impact of this technique, particularly in the field of protein analysis. This is due to the fact that although MALDI was first reported in 1985, it was not applied to the ionization of proteins until 1988,after Tanaka's report.

WHAT IS MASS SPECTROMETRYIt is an analytical technique for the determination of the elemental composition of a sample or molecule

MS is a powerful analytical technique Identify Unknown Compounds quantify known materials Elucidation of structural and chemical properties Requires minute Quantities (6000 C)Only a small amount of analyte is utilized (< 1%)

Electron ionization Electron ionization (EI, formerly known as electron impact) is an ionization method in which energetic electrons interact with gas phase atoms or molecules to produce ions. This technique is widely used in mass spectrometry, particularly for gases and volatile organic molecules

The following gas phase reaction describes the electron ionization process :

where M is the analyte molecule being ionized, e- is the electron and M+ is the resulting ion.

Diagram representing an electron ionization ion source

Chemical Ionization (CI) Ion SourceA modified form of EIHigher gas pressure in ioniation cavity (1 torr)Reagent gas (1000 to 10000-fold excess) added; usual choice is methane, CH4Reagent gas is directly ionized instead of analyteGentle; little fragmentation; even-electron ions produced more stable than odd-electron ions produced in EIExcess energy of excited ions removed by many ion-reagent gas collisions

Chemical Ionization ReactionsReagent gas ionization:CH4 CH4+ +e (also CH3+, CH2+)

Secondary reactions:CH4+ + CH4 CH5+ + CH3CH3+ + CH4 H2 + C2H5+ (M+29)

Tertiary reactionsCH5+ + MH CH4 + MH2+ (M+1) proton exchangeCH3+ + MH CH4 + M+ (M1) hydride exchangeCH4+ + MH CH4 + MH+ (M) charge exchange

Fast Atom BombardmentIon source forbiological moleculesAr ions passedthrough low pressureAr gas to producebeam of neutral ionsAtom-sample collisionsproduce ions as large as 25,000 Daltons

glow discharge

Sputtering of the cathode material (the sample) by an argon plasma.Ionisation of the elements of the sample in the plasma.Extraction and acceleration of ions.Ions separation with a magnetic sector (Mattauch Herzog configuration).Ions detection by a Faraday cup or an electron multiplier

Matrix-Assisted Laser Desorption/Ionization (MALDI)Analyte mixed with radiation-absorbing material and driedSample ablated with pulsed laserOften coupled to time-of-flight (TOF) detectorExcellent for larger molecules, e.g. peptides, polymers

MASS ANALYZERSQuadrupole AnalyzerIons forced to wiggle between four rods whose polarity is rapidly switchedSmall masses pass through at high frequency or low voltage; large masses at low frequency or high voltageVery compact, rapid (10 ms)R = 700-800

TOF Time of Flight Mass AnalyzerSeparates ions based on flight time in drift tubePositive ions are produced in pulses and accelerated in an electric field (at the same frequency)All particles have the same kinetic energyLighter ions reach the detector firstTypical flight times are 1-30 sec

Time of Flight Mass AnalyzerSeparation PrincipleAll particles have the same kinetic energyIn terms of mass separation principles:Vparticle = Her/mHold H,e, and r constantVparticle = 1/m (constant)Vparticle is inversely proportional to mass

Quadrupole Ion TrapIons follow complex trajectories between two pairs of electrodes that switch polarity rapidlyIons can be ejected from trap by m/z value by varying the frequency of end cap electrodes

Detectorselectron multiplierFaraday cupsMicrochannel plate detectors

Electron multiplier

Continuous dynode electron multiplierAn electron multiplier (continuous dynode electron multiplier) is a vacuum-tube structure that multiplies incident charges. In a process called secondary emission, a single electron can, when bombarded on secondary emissive material, induce emission of roughly 1 to 3 electrons. If an electric potential is applied between this metal plate and yet another, the emitted electrons will accelerate to the next metal plate and induce secondary emission of still more electrons. This can be repeated a number of times, resulting in a large shower of electrons all collected by a metal anode, all having been triggered by just one.

Faraday cupA Faraday cup is a metal (conductive) cup designed to catch charged particles in vacuum. The resulting current can be measured and used to determine the number of ions or electrons hitting the cup.The Faraday cup is named after Michael Faraday who first theorized ions around 1830.

Schematic of a Faraday cup

Faraday cup cont..When a beam or packet of Ions hits the metal it gains a small net charge while the ions are neutralized. The metal can then be discharged to measure a small current equivalent to the number of impinging ions. Essentially the faraday cup is part of a circuit where ions are the charge carriers in vacuum and the faraday cup is the interface to the solid metal where electrons act as the charge carriers (as in most circuits).

Faraday cup with an electron-suppressor plate in front By measuring the electrical current (the number of electrons flowing through the circuit per second) in the metal part of the circuit the number of charges being carried by the ions in the vacuum part of the circuit can be determined.

Micro-channel plate (MCP)It is a planar component used for detection of particles (electrons or ions) and impinging radiation (ultraviolet radiation and X-rays).

It is closely related to an electron multiplier, as both intensify single particles or photons by the multiplication of electrons via secondary emission.

However, because a micro channel plate detector has many separate channels, it can additionally provide spatial resolution.

A micro-channel plate is a slab made from highly resistive material of typically 2 mm thickness with a regular array of tiny tubes or slots (microchannels) leading from one face to the opposite, densely distributed over the whole surface.

The microchannels are typically approximately 10 micrometers in diameter (6 micrometer in high resolution MCPs) and spaced apart by approximately 15 micrometers; they are parallel to each other and often enter the plate at a small angle to the surface (~8 from normal).

ReferencesSkoog, Instrumental analysis, cengage learning , Indian edition.www.youtube.comwww.google.comwww.wikipedia.com