mass spect

Billones Lecture NotesThe Health Sciences Center

UNIVERSITY OF THE PHILIPPINES MANILA

Mass SpectrometryIn mass spectrometry, a substance is converted into rapidly moving gaseous ions which are then separated on the basis of their mass-to-charge ratio.

Mass spectrometry is the most widely applicable of all the analytical tools.

It provides information about:

1) the qualitative and quantitative composition of both inorganic and organic analytes in complex mixtures.

2) the structures of a wide variety of complex molecular systems.

3) isotopic ratios of atoms in samples

4) the structure and composition of solid surfaces



Atomic and molecular weights are generally expressed in terms of atomic mass unit (amu).

The atomic mass unit is based upon a relative scale in which the reference is the carbon isotope 126C, which assigned a mass of exactly 12 amu.

Mass spectroscopists also call the amu the dalton.

1 amu = 1 dalton

€

=112

12g12C /mol12C6.0221x1023atoms12C /mol12C

= 1.66054 x 10-24 g/atom 12C

The atomic weight of an isotope, such as 3517Cl is then related to that of the reference 126C atom.

Chlorine-35 isotope is 2.91407 times greater than the mass of carbon-12 isotope.



Atomic mass 3517Cl = 12.0000 dalton x 2.19407 = 34.9688 dalton

Because 1 mol of 126C weighs 12.0000 g, the atomic weight of 3517Cl is 34.9688 g/mol.

In mass spectrometry, the exact mass of particular isotopes of an element or the exact mass of compounds is determined.

The exact mass, m, of particular isotopes of an element or the exact mass of compounds containing a particular set of isotopes is measured.

For example a CH4 ion gives the following peaks in the mass spectrum:

12C1H4 m = (12.000 x 1) + (1.007825 x 4) = 16.031 amu

13C1H4 m = (13.00335 x 1) + (1.007825 x 4) = 17.035 amu



12C1H32H1 m = (12.000 x 1) + (1.007825 x 3) + (2.0140 x 1) = 17.037 amu

In mass spectrometry, exact masses are quoted to three or four decimal places because high-resolution mass spectrometers have this precision.

Nominal mass is used to imply a whole-number precision in a mass measurement.

12C1H4 m = 16.031 amu 16 amuExamples: Nominal mass

13C1H4 m = 17.035 amu 17 amu

Chemical atomic weight (A) of an element is given by the equation

A = A1p1 + A2p2 + . . . + Anpn



where A1, A2, ... , An are atomic masses in amu of the n isotopes of A p1, p2, ... , pn are the fractional abundances the isotopes

The mass-to-charge ratio, m/z, of an atomic or molecular ion is obtained by dividing the atomic or molecular mass of an ion m by the number of charges z that the ion bears.

Examples:

12C1H4+ m/z = 16.031/1 = 16.031 13C1H4

2+ m/z = 17.035/2 = 8.518

Chemical molecular weight is the sum of the chemical atomic weights of the atoms in the compound.

Because most ions in mass spectrometry are singly charged, the term mass-to-charge ratio, is often shortened to mass.



The evolution of Mass Spectrometry is shown in the following table.

Development Approximate Date Application

Behavior of ions in magnetic field described 1920 Determination of isotopic

abundances of elements

Double focusing 1935 High mass resolution achieved

First commercial mass spectrometer 1950 Quantitative analysis of petroleum

products

Spark source 1955 Quantitative elemental analysis

Theory describing fragmentation of molecular species 1960 Identification and structural

analysis of complex molecules

Interfacing mass spectrometers with chromatographs 1965 Qualitative and quantitative

analysis of complex mixtures

Tandem mass spectrometers 1970 High speed analysis of complex mixtures

New ionization techniques 1970 Enhanced capacity for structure elucidation

Fourier Transform applied to mass spectrometry 1980 Improved mass resolution and

signal-to-noise ratios

Improved sources for nonvolatile species 1980 Analysis of polymeric molecules

and surfaces

Billones Lecture Notes


The Health Sciences Center

Inlet system

Ion source

Mass analyzer

Detector

Vacuum system

Signal processor

Readout

10-5 to 10-8 torr

sample

Components of a Mass Spectrometer



The inlet system introduces a very small amount of sample (micromole) into the mass spectrometer. It contains a means for volatilizing solid and liquid samples.

The ion source converts the components of a sample into ions by bombardment with electrons, molecules, or photons or by thermal or electrical energy.

The mass analyzer is analogous to that of the grating in an optical spectrometer. The dispersion is based upon the mass-to-charge ratios.

Mass spectrometers fall into several categories, depending upon the nature of the mass analyzer.

The detector converts the beam of ions into an electrical signal.




Inlet system

External sample introduction system

A sample probe for inserting a sample directly into the ion source

The sample is volatilized externally and then allowed to leak into the ionization region.

Proximity to ion source makes it possible to obtain spectra of thermally unstable compounds.

Batch Inlet System

Direct Probe Inlet

This is applicable to gaseous and liquid samples having boiling points up to 500 oC.

Solids and nonvolatile liquids can be introduced by means of a probe.

This is used when the quantity of the sample is limited.




Detectors

Discrete dynode electron multiplier

Continuous dynode electron multiplier

This detector is like the PMT for UV-Vis radiation, with each dynode being held at a successively higher voltage.

Cathode and dynodes have Cu/Be surfaces.

20 dynodes provide a current gain of 107.

This trumpet-shaped device made of glass is doped with Pb.

A potential of 2 kV is impressed across the length of the detector.

Ions striking the surface near the entrance eject electrons, then skip along the surface, ejecting more electrons with each impact.

Current gain of 105 is achieved with this type of detector.

Electron Multipliers




Magnetic Sector Analyzer

Mass spectrometers are classified based on the nature of the mass analyzer.

Mass Analyzer

Magnetic sector analyzers employ a permanent magnet or an electromagnet to cause the beam from the ion source to travel in a circular path of 180, 90, or 60 deg.

The next figure shows a 90-deg sector instrument.

The ions formed by electron impact are accelerated through slit B into the metal analyzer tube maintained at 10-7 torr.

Ions of different mass can be scanned across the exit slit by varying the field strength of the magnet or the accelerating potential.




Schematic of a Magnetic Sector Spectrometer

The ions passing through the exit slit fall on a collector electrode, resulting in an ion current that is amplified and recorded.




The capability of a mass spectrometer to differentiate between masses is expressed in terms of its resolution, R

Two peaks are considered to be separated if the height of the valley between them is no more than 10% of their height.

Thus, a spectrometer with a resolution of 4000 would resolve peaks occurring at 400.0 and 400.1 (or 40.00 and 40.01).

R = m/Δm

where Δm is the mass difference between two adjacent peaks resolved, and m is the nominal mass of the first peak (sometimes mean mass)

What resolution is needed to separate C2H4+ (m = 28.0313) and CH2N+ (m = 28.0187) ions.

Δm = 28.0313 - 28.0187 = 0.0126R = m/Δm = 28.025/0.0126 = 2.22 x 103

Example




It can be shown that the mass-to-charge ratio is related to the voltage in the ionization chamber (V), magnetic field strength (B), radius of the curvature (r), and the charge of the ion (e = 1.60 x 10-19C) according to the equation

€

mz

=B2r2e2V

Most modern sector mass spectrometers contain an electromagnet in which ions are sorted by holding V and r constant while varying the current in the magnet and thus B.

In sector spectrometers that use photographic recording, B and V are constant and r is the variable.




What accelerating potential will be required to direct a singly charged water molecule through the exit slit if the magnet has a field strength of 0.240 tesla and the radius of curvature of the ion through the magnetic field is 12.7 cm?

First convert variables into SI units.

Example

charge per ion, ez = 1.60 x 10-19 C

radius, r = 0.127 m

mass, m = 18.02 g H2O+/mol x 10-3 kg/g

6.02 x 1023 H2O+/mol

= 2.99 x 10-26 kg H2O+

magnetic field, B = 0.240 T = 0.240 W/m2 (W= Weber)

V = B2r2ez/2m = [0.240 W/m2]2[0.127 m]2[1.60 x 10-19 C]

2 x 2.99 x 10-26 kg

V = 2.49 x 103 W2C/m2kg (or V)




Double-Focusing SpectrometersThe magnetic sector instruments are sometimes called single-focusing spectrometers.

Magnetic field is used to act on ions with diverging distribution in order to produce converging distribution of ions leaving the field.

The ion beam is first passed through an electrostatic analyzer (ESA) which limits the kinetic energy of the ions reaching a magnetic sector.

The term double focusing is a p p l i e d t o m a s s spectrometers in which the directional and energy aberrations are minimized.

This is achieved by the use of combinations of electrostatic and magnetic fields.




Quadrupole Mass Filters

Quadrupole mass spectrometers are more compact, less expensive, more rugged than their magnetic sector counterparts.

They offer advantage of low scan times (<100 ms). They are the most common mass analyzers used today.

Quadrupole is analogous to variable, narrow-band filter because it transmits only ions with small range of m/z ratios.

Quadrupoles function by selective removal of ions. Thus they are called mass filters.




A Quadrupole Mass Spectrometer

The heart of the instrument is the set of four cylindrical metal rods that serve as the electrodes of the mass filter.

Ions having energies larger than average strike the upper side of the ESA slit and are lost to ground.

Ions having energies less than average strike the lower side of the ESA slit and are thus removed.




Ion Trap Analyzers

Ion trap analyzers are more compact, less expensive, more rugged than sector or quadrupole instruments.

An ion trap is a device in which gaseous anions or cations can be formed and confined for extended periods by electric or magnetic field.

In TOF instruments, cations are produced periodically by bombardment with brief pulses of electrons, secondary ions, or laser generated photons.

The particles are accelerated and allowed to pass to a field-free drift tube.

Time-of-flight Analyzers

Because the ions have the same KE, their velocities must vary inversely with their masses, the lighter particles arriving at the detector earlier than the heavier ones.




Molecular Spectra from Various Ion Sources

The appearance of mass spectra




Molecular Spectra from Various Ion Sources

Name Abbreviation Type Ionizing Agent Date of

Use

Electron Ionization EI Gas phase Energetic electrons 1920

Chemical ionization CI Gas phase Reagent ions 1965

Field Ionization FI Gas phase High-potential electrode 1970

Field Desorption FD Desorption High-potential electrode 1969

Fast Atom Bombardment FAB Desorption Energetic atoms 1981

Secondary ion mass SIMS Desorption Energetic atoms 1977

Laser desorption LD Desorption Laser beam 1978

Plasma desorption PD Desorption High-E fission fragmentsfrom 252Cf

1974

Thermal desorption Desorption Heat 1979

Electrohydrodynamic ionization EHMS Desorption High field 1978

Thermospray ionization ES Positive charges imparted to fine

droplets of sample soln1985




Molecular Spectra

mass spect

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