Mass Spect

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Mass Spectrometry In 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 The Health Sciences Center

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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

 1 12g12C / mol12C =  23 12 12  12  6.0221x10 atoms C / mol C  = 1.66054 x 10-24 g/atom 12C The atomic weight of an isotope, such as 3517Cl is then related to 12 C atom. that of the reference 6 €

Chlorine-35 isotope is 2.91407 times greater than the mass of carbon-12 isotope. The Health Sciences Center

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UNIVERSITY OF THE PHILIPPINES MANILA 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 35 Cl is 34.9688 g/mol. 17 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: 12C1H 4

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

13C1H 4

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

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UNIVERSITY OF THE PHILIPPINES MANILA 12C1H 2H 3 1

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. Examples: 12C1H 4 13C1H 4

m = 16.031 amu

Nominal mass 16 amu

m = 17.035 amu

17 amu

Chemical atomic weight (A) of an element is given by the equation A = A1p1 + A2p2 + . . . + Anpn The Health Sciences Center

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

Chemical molecular weight is the sum of the chemical atomic weights of the atoms in the compound. 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: 12C1H + 4

m/z = 16.031/1 = 16.031

13C1H 2+ 4

m/z = 17.035/2 = 8.518

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

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

Development

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Components of a Mass Spectrometer sample 10-5 to 10-8 torr

Inlet system

Ion source

Mass analyzer

Vacuum system

Detector

Signal processor

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UNIVERSITY OF THE PHILIPPINES MANILA 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-tocharge 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. The Health Sciences Center

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UNIVERSITY OF THE PHILIPPINES MANILA Inlet system Batch Inlet System The sample is volatilized externally and then allowed to leak into the ionization region. This is applicable to gaseous and liquid samples having boiling points up to 500 oC. External sample introduction system

Direct Probe Inlet Solids and nonvolatile liquids can be introduced by means of a probe. This is used when the quantity of the sample is limited.

A sample probe for inserting a sample directly into the ion source The Health Sciences Center

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

UNIVERSITY OF THE PHILIPPINES MANILA Detectors This detector is like the PMT Electron Multipliers

for UV-Vis radiation, with each dynode being held at a successively higher voltage. Cathode and dynodes have Cu/Be surfaces.

Discrete dynode electron multiplier

20 dynodes provide current gain of 107.

a

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.

Continuous dynode electron multiplier The Health Sciences Center

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

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Mass Analyzer

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

Magnetic Sector 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. The Health Sciences Center

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THE The ions UNIVERSITY passing throughOF the exitPHILIPPINES slit fall on a MANILA collector electrode, resulting in an ion current that is amplified and recorded. Schematic of a Magnetic Sector Spectrometer

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

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). Example What resolution is needed to separate C2H4+ (m = 28.0313) and CH2N+ (m = 28.0187) ions. Δm = 28.0313 - 28.0187 = 0.0126 R = m/Δm = 28.025/0.0126 = 2.22 x 103 The Health Sciences Center

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UNIVERSITY OF THE PHILIPPINES MANILA 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 2 2

m Bre = z 2V 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.

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

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Example UNIVERSITY OF THE PHILIPPINES MANILA 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. 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 = 2.99 x 10-26 kg H2O+

6.02 x 1023 H2O+/mol 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) The Health Sciences Center

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UNIVERSITY OF THE PHILIPPINES MANILA Double-Focusing Spectrometers The magnetic sector instruments are sometimes called singlefocusing spectrometers. Magnetic field is used to act on ions with diverging distribution in order to produce converging distribution of ions leaving the field. The term double focusing is applied to mass spectrometers in which the directional and energy aberrations are minimized. This is achieved by the use of combinations of electrostatic and magnetic fields.

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

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UNIVERSITY OF THE PHILIPPINES MANILA 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.

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UNIVERSITY OF THE PHILIPPINES MANILA 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. The Health Sciences Center

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Ion Trap Analyzers UNIVERSITY OF THE PHILIPPINES MANILA 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.

Time-of-flight Analyzers 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. 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. The Health Sciences Center

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Molecular Spectra from Various Ion Sources The appearance of mass spectra

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UNIVERSITY OF THE PHILIPPINES MANILA Molecular Spectra from Various Ion Sources

Name

Abbreviatio n

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 1978

Laser desorption

LD

Desorption Laser beam

Plasma desorption

PD

Desorption

Thermal desorption Electrohydrodynamic ionization

Thermospray ionization The Health Sciences Center

High-E fission fragments from 252Cf

Desorption Heat

EHMS Desorption High field ES

Positive charges imparted to fine droplets of sample soln

1974 1979 1978 1985 Billones Lecture Notes

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Molecular Spectra

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