Mass spectrometry

Mass spectrometery:

Mass spectrometery

Definition of mass spectrometery:

Definition of mass spectrometery Mass spectrometry is the study of systems causing the formation of gaseous ions,with or without fragmentation,which are then characterised by their mass to charge ratios (m/z) and relative abundance.

Sequence of process:

Sequence of process Stage 1: Ionisation The atom is ionised by knocking one or more electrons off to give a positive ion ( mass spectrometer always works with positive ions even with chlorine or argon) Ion source

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Stage 2: Acceleration The ions are accelerated so that they all have the same kinetic energy. Stage 3: Deflection The ions are then deflected by a magnetic field according to their masses and their charges

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The lighter they are, the more they are deflected. The more the ion is charged, the more it gets deflected. Mass analyzer Stage 4: Detection The beam of ions passing through the machine is detected electrically. Detector

Basic principle::

Basic principle: charged particle passing through a magnetic field is deflected along a circular path on a radius that is proportional to the mass to charge ratio, m/e. In an electron impact mass spectrometer, a high energy beam of electrons is used to displace an electron from the organic molecule to form a radical cation known as the molecular ion . If the molecular ion is too unstable then it can fragment to give other smaller ions. The collection of ions is then focused into a beam and accelerated into the magnetic field and deflected along circular paths according to the masses of the ions. By adjusting the magnetic field, the ions can be focused on the detector and recorded.

Terminology:

Terminology Molecular ion The ion obtained by the loss of an electron from the molecule Base peak The most intense peak in the MS, assigned 100% intensity M+ Symbol often given to the molecular ion Radical cation +ve charged species with an odd number of electrons Fragment ions Lighter cations formed by the decomposition of the molecular ion. These often correspond to stable carbocations.

Ion sources:

Ion sources The choice of ionization depends upon the sample Three types of ionization extensively used Electron impact Thermal ionization Plasma ionization (microwave and ICPMS)

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Electron impact Principal method for gas analysis Electrons are generated from incandescent filament The production of ions Ions are accelearated through the slit system Ions enter into the magnetic field

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2. Thermal ionization (TIMS): 1 st way: Material that is available only in solid form and in small amounts is usually dissolved in acid and concentrated into a small volume of solution, then evaporated 2 nd way: material deposited onto a metal filament.This filament is then incorporated into a source where it is used both for evaporation and ionization (a single filament source) or for evaporation only (other filaments are used to ionize the vapor). .

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The slit system that accelerates the ions create is similar to the gas source

Plasma ionization (microwave and ICPMS): :

Plasma ionization (microwave and ICPMS) : Plasma is an extremely hot charged gas confined to a small region Fluid droplets of sample are injected directly into a plasma where the droplets are dissociated and constituent atoms or molecules are ionized by charge exchange with the plasma gas.

HOW DOES THE ANALYZER WORK ?:

HOW DOES THE ANALYZER WORK ? Use of filtering to sort ions according to their mass to charge ratio Various types of analyzers magnetic sectors, quadrupole mass filters, quadrupole ion traps, Fourier transform ion cyclotron resonance spectrometers, and time-of-flight mass analyzers.

Magnetic sectors:

Magnetic sectors Bend the trajectories of ions into circular paths of radii that depend on the momentum of mass-to-charge ratios of the ions. Ions of larger m/z follow larger radius paths than ions of smaller m/z values so ions of differing m/z values are dispersed in space. By changing the ion trajectories through variations of the magnetic field strength, ions of different nominal mass-to-charge ratios can be focused on a detector.

Double focusing mass spectrometers:

Double focusing mass spectrometers Double focusing mass spectrometers use a combination of magnetic and electrical fields to focus and sort ions The electric sector focuses the ions with respect to differences in kinetic energy that they may have as they exit the source region.

Quadrupole mass filter :

Quadrupole mass filter consists of four parallel poles or rod mass sorting depends on ion motion resulting from simultaneously applied constant (dc) and radiofrequency electric (rf) electric fields. Scanning is accomplished by systematically changing the field strengths, thereby changing the m/z value that is transmitted through the analyzer. Quadrupole mass spectrometers provide lower resolution than double focusing instruments but tend to be more easily interfaced to various inlet systems and to be less costly.

The quadrupole ion trap mass spectrometer :

The quadrupole ion trap mass spectrometer operates on a principle similar to a quadrupole mass filter But it does not operate as a filter. the ion trap stores ions for subsequent experiments and analysis. a ring electrode in the middle with cap electrodes on each end. Within a selected range of mass-to-charge ratios determined by the applied voltages, the device traps ions in the space bounded by the electrodes. Typically, a mass spectrum is produced by scanning the applied rf voltages to eject ions sequentially of increasing mass-to-charge ratio through an end cap opening for detection.

Fourier transform ion cyclotron resonance (FT-ICR):

Fourier transform ion cyclotron resonance (FT-ICR) ions are trapped electrostatically within a cubic cell in a constant magnetic field. A covalent orbital ("cyclotron") motion is induced by the application of a radio-frequency pulse between the excite plates. The orbiting ions generate a faint signal in the detect plates of the cell. The frequency of the signal from each ion is equal to its orbital frequency, which in turn is inversely related to its m/z value.

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The signal is amplified and all the frequency components are determined, yielding the mass spectrum. The FT-ICR instrument is used to generate very high resolution spectra.

Time-of-flight mass analyzers :

Time-of-flight mass analyzers seperate ions by virtue of their different flight times over a known distance. A brief burst of ions is emitted from a source. These ions are accelerated so that ions of like charge have equal kinetic energy and then are directed into a flight tube. Since kinetic energy is equal to 1/2 mv2, where m is the mass of the ion and v is the ion velocity, the lower the ion's mass, the greater the velocity and shorter its flight time.

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The travel time from the ion source through the flight tube to the detector, measured in microseconds, can be transformed to the m/z value through the relationships described above. Because all ion masses are measured for each ion burst, TOF mass spectrometers offer high sensitivity as well as rapid scanning. They can provide mass data for very high-mass biomolecules.

Ion detection :

Ion detection Faraday Cup Electron and Photon Multipliers Array Detectors

Faraday cup:

Faraday cup The ion beam is allowed to collide with the interior walls of an open-ended metal cup and all secondary electron emission is suppressed. The cup is maintained at virtually at ground potential. Minute current is produced Amplification of that current Most commonly used detector

Application of mass spectrometery:

Application of mass spectrometery For molecular weight determination For molecular formula For structural determination

How can mass spectrometric data be used for structure analysis? :

How can mass spectrometric data be used for structure analysis?

Exact mass measurement:

Exact mass measurement

What other techniques are used to produce ions? :

What other techniques are used to produce ions?

Other techniques to produce ions:

Other techniques to produce ions Chemical ionization (CI) The sample molecules are exposed to a large excess of ionized reagent gas. Transfer of a proton to a sample molecule M, from an ionized reagent gas such as methane in the form of CH5+, yields the [M+H]+ positive ion.

Use of MS various separation techniques:

Use of MS various separation techniques gas chromatography (GC), liquid chromatography (LC), capillary electrophoresis and supercritical fluid chromatography

How is mass spectrometry used for quantitative analysis? :

How is mass spectrometry used for quantitative analysis? commonly done in environmental pollutant work pharmacokinetic studies where the goal is quantitation at very low concentrations in complex mixtures. Only a single type of ion is produced i.e molecular ion

Selected ion monitoring :

Selected ion monitoring

What is mass spectrometry/mass spectrometry (MS/MS)?? :

What is mass spectrometry/mass spectrometry (MS/MS)?? Used in identifying compounds in complex mixtures and in determining structures of unknown substances "parent" or "precursor" ions are fragmented and then the product ions resulting from the fragmentation are analyzed in a second stage of mass analysis

What is elemental mass spectrometry? :

What is elemental mass spectrometry? a technique used mostly for inorganic materials Elemental mass spectrometry provides quantitative information about the concentrations of those elements

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The ion source is ordinarily an atmospheric-pressure discharge such as the inductively coupled plasma (ICP) the decomposition of the sample into its constituent atoms and ionization of those atoms occurs in a specially designed source. The resulting atomic-ion beam is then separated or sorted by a mass spectrometer and the signal as a function of m/z used to determine the sample composition.

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