Chapter 20: Molecular Mass Spectroscopy

CHE. 331

Chapter 20: Molecular Mass Spectroscopy.

Introduction:

In Mass Spectroscopy (MS), atomic and molecular weights are generally expressed in terms of atomic mass units (amu). The atomic mass unit is based on upon a relative scale in which the reference is the carbon isotope 126C, which is assigned a mass of exactly 12 amu. Thus the amu is defined as 1/12 of the mass of one neutral carbon atom.

Mass spectroscopy is perhaps one of the most widely applicable of all the analytical tools available to the analytical chemist in the sense that this technique is capable of providing information about

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

(1b) this instrument measures compounds with molecular masses up to 200, 000 Daltons.

(2) the structures of a wide variety of complex molecular species

(3) isotopic ratios of atoms in samples and

(4)the structure and composition of solid surfaces.

The four main components of a molecular mass spectrometer

In MS, in contrast to most types of chemistry, we are often interested in the exact mass m of particular isotopes of an element or the exact mass of compounds containing a particular set of isotopes. Thus, we may need to distinguish between the masses of compounds such as

12C1H4m = 12.000  1 + 1.007825  4 = 16.031 Dalton

13C1H4m = 13.00335  1 + 1.007825  4 = 17.035 Dalton

12C1H32H1m = 12.000  1 + 1.007825  3 + 2.0140  1 = 17.037 Dalton

Exact masses are quoted to three or four figures to the right of the decimal point as shown, because high-resolution mass spectrometers have this precision.

Nominal mass : This simply the term used to imply a whole-number precision in a mass measurement. Thus the nominal, mass of the three isomers just cited are 16, 17, and 17 Daltons, respectively.

The chemical atomic weight or the average atomic weight (A) of an element in nature is given by the equation

A = A1p1 + A2p2 + ...... + Anpn

where A1, A2, ...... An are the atomic masses in Daltons of the n isotopes of the element and p1, p2 ...... pn are the fractional abundance of these isotopes.

Mass-to-charge ratio is the term obtained by dividing the atomic or molecular mass of an ion m by the charges z that the ions bears.

The Mass Spectrometer:

The block diagram below shows a simplified draft of the major components of a mass spectrometer. A detailed description of each component follows.

The Sample Inlet System:

The sample inlet system has as its main objective to permit the introduction of a representative sample into the ion source with minimal loss of vacuum. Two versions are outlined below.

Batch Inlet Systems:

These systems are the simplest and simply involve the volatilization of the sample externally and then the gradual leakage of the volatilized sample into the evacuated ionization chamber. For gases, the sample is introduced into the metering volume container and then expanded into the reservoir flask where it is then leaked into the ionization chamber. For liquids, a small quantity of sample is introduced into the reservoir and the pressure of the system is reduced to about 10-5 torr. The inlet system is lined with glass to avoid losses of polar analytes by adsorption.

The Direct Probe Inlet:

These systems are used for solids and non-volatile liquids and in these systems the sample is introduced into the ionization region by means of a sample holder, or probe, which is inserted through a vacuum lock. Probes are also used when the amount of the sample to be analyzed is small. With a probe, the sample is generally held on the surface of a glass or aluminum capillary tube and positioned within a few meters of the ionization source.

Detectors:

There are several types of detectors available for mass spectrometers but the electron multiplier is the detector of choice for most routine instruments.

Electron Multipliers: A discrete-dynode electron multiplier is designed for detection of positive ions. Each dynode is held at successively higher voltage and there is a burst of electrons that is emitted when struck by energetic electrons or ions. A continuous-dynode electrons electron multiplier is a trumpet-shaped device made of glass that is heavily doped with lead.

In general, electron multipliers are rugged and reliable and are capable of providing high current gains and nanosecond response times. These detectors can be placed directly behind the exit slit of a magnetic selector because they usually posses enough energy to eject electrons from the first stage of the device.

Electron multipliers can also be used with mass analyzers that utilize low-kinetic ion beams but in these devices the ion beam exciting the analyzer is accelerated to several thousand electron volts prior to striking the first stage.

The Faraday Cup detector: This detector functions as follows. When positive ions strike the surface of the cathode, electrons move flow from the ground through the resistor to neutralize the charge. The resulting potential drop across the resistor is amplified via a high-impedance amplifier.

Mass Analyzers:

There are several methods available for separating ions with different mass-to-charge ratios. Ideally, the mass analyzer should be capable of distinguishing between minute mass differences.

Resolution of Mass Spectrometers: Resolution, in MS, refers to the ability of a mass spectrometer to differentiate between masses and is quantitatively defined as

R = m / m

where m is the mass difference between two adjacent peaks that are just resolved and m is the nominal mass of the first peak (the mean mass of the two peaks is sometimes used instead).

Magnetic Sector Analyzers:

Magnetic sector analyzers employ a permanent magnet or electromagnet to cause the beam from the ion source to travel in a circular path of 180, 90, or 60 degrees. Here, ions are formed by electron impact.

The translational energy of an ion of mass m and charge z upon exciting slit B is given by

K. = Zev = ½ mv2 Equation 1

where V is the voltage between A and B, v is the velocity of the ion after acceleration, and e is the charge of the ion.

The path in the sector described by the ions of a given mass and charge represents a balance between two forces acting upon them. These two forces are the centripetal force and the magnetic force and equating these two forces yields:

Bzev = mv2/rwhich rearranges tov = Bzer/m

Substituting the above equation into (1) givesm/z = B2r2e/2V

The last equation shows how mass spectra can be obtained by varying one of the three variables B, V, or r.

Double Focusing Instruments:

These type of instruments, unlike single-focusing which simply minimize directional errors, are designed to limit both the errors introduced because ions are initially moving in different directions and also the errors introduced due to the fact that ions of the same mass-to-charge ratio may have different translational energies. A schematic of a double-focusing instrument is shown below. The Electrostatic analyzer (ESA) in the figure consists of two curved plates at across which a dc potential is applied. Ions with energies larger than the average strike the upper side of the ESA slit and are lost to the ground. Ions having energies that are less than average strike the lower side of the ESA and are thus removed.

Quadrupole Mass Filters:

Quadrupole mass spectrometers are usually more compact, less expensive, more rugged than their magnetic sector counterparts. A quadrupole is analogous to a narrow-band filter in that it , set at any operating conditions, it transmits only ions within a small range of m/z values. All other ions are neutralized and carried away as uncharged molecules. By varying the electrical signals to a quadrupole it is possible to vary the range of m/z values transmitted, thus making spectral scanning possible.

Time-of-Flight Analyzers:

In time-of-flight instruments, positive ions are produced periodically by bombardment of the sample with brief pulses of electrons, secondary ions or laser generated photons. The ions produced are then accelerated by an electric field and then made to pass into a field-free drift tube about a meter long. Because all ions entering the tube ideally have the same kinetic energies, their velocities in the tube must vary inversely with their masses, with the lighter particles arriving at the detector earlier than the heavier ones.

From the standpoint of resolution and reproducibility, instruments employing TOF separators are not as satisfactory as those based on magnetic or quadrupole separators.

Computerized Mass Spectrometers:

Minicomputers and microprocessors are integral part of modern mass spectrometers. The figure below is a block diagram of the computerized control and data acquisition system of a triple quadruple mass spectrometer. Ion separation is carried out by placing the atom in the beam, via ionization with high temperature. Due to the fact that most ions are short lived and is contained under a pressure of approximately 10^-5 and 10^-8 torr most procedures are performed under a vacuum. The ions are then analyzed and detected by the instrument that is also attached to a computer which serves as a visual aid.

This figure shows two features a detailed and also a simplified sketch that will be encountered on any modern instrument; the presence of a computer as the main instrument controller and a set of microprocessor.

Ion Sources:

The appearance of mass spectra for a given molecular species is highly dependant upon the method used for ion formation. The table below lists many of the new sources available and they are arranged into two categories; gas phase sources and desorption sources.

Ion sources are usually categorized as hard or soft. The principal hard source is the electron impact source while the chemical ionization and desorption sources are categorized as soft.

Isotopes:

By definition isotopes are atoms having the same atomic number but different mass numbers this technique is advantageous because in mass spectroscopy isotopes are easily differentiated. As depicted below: a mixture which contains isotopes are differentiated and shown in a spectrum.

bromine

Bromine consists of a mixture of isotopes one with a mass of 79amu and the other of 81amu also to the right two additional one 158 and 162 respectively.

Vinyl chloride also by mass spectroscopy is shown on the spectrum to also have a mixture of two pairs of isotopes. This shows how beneficial mass spectroscopy is because these atoms differ by as little as one amu. As shown below vinyl chloride has one isotope 35amu and 37amu and lastly to the right 62 and 64 amu.

vinyl chloride

Gas-Phase Sources:

Gas-phase sources require volatilization of the sample before ionization and thus are limited to thermally stable compounds that have boiling points less than about 500C.

Electron Impact Source : In the sources, electrons emitted from a filament are accelerated by a potential of about 70 V and made to collide with gaseous atoms or molecules of the sample causing ionization. Electron-impact ionization is not very efficient and only about one molecule in a million undergoes the primary reaction

M + e-  M.+ + 2e-

Electron Impact spectra are very complex due to the high energies possessed by the accelerated electrons which collide with the sample and lead to fragmentation. These complex spectra are very useful for compound identification.

Advantages of Electron Impact sources

(1)They are convenient and produce high ion currents.

(2)Extensive fragmentation can lead to unambiguous identification of analytes.

Disadvantages of Electron Impact sources

(1)The need to volatilize the sample limits this method since it excludes analysis of thermally unstable compounds.

(2)Excessive fragmentation can lead to the disappearance of the molecular ion peak therefore preventing the molecular mass of the analyte to be determined.

Chemical Ionization Sources:

These sources employ the use of a reagent to impart energy to the sample. The reagent is bombarded with highly accelerated electrons and then made to collide with the sample in its gaseous phase. The following figure contrasts the spectra from chemical ionization and electron impact sources.

The electron impact spectra not only is more complex but also does show the molecular ion peak. In fact, it shows know detectable peaks above 112.

Desorption Sources:

In desorption methods, energy is introduced in various forms to the liquid or solid sample in such a way as to cause direct formation of gaseous ions. As a consequence, spectra are greatly simplified and often consist of only the molecular ion or the protonated molecular ion.

An example of a desorption source are Fast Atom Bombardment (FAB) sources in which samples, in their condensed state are bombarded by energetic Xenon or Argon atoms.

Identification of Pure Compounds by Mass Spectroscopy:

Mass spectroscopy can be used to determine the molecular weight of a compound but this involves an identification of a molecular peak and a comprehensive study of a spectrum.

TANDEM MASS SPECTROSCOPY: This type of spectroscopy simply involves the coupling of one mass spectrometer to another and this hyphenated technique has resulted in dramatic progress in the analysis of complex mixtures.

SECONDARY ION MASS SPECTROSCOPY: This is one of the most highly developed of the mass spectrometric surface methods, with several manufacturers offering instruments for this technique. It involves the bombarding of a surface with a beam of ions formed in an ion gun. The ions generated from the surface layer are then drawn into a spectrometer for mass analysis.

Useful Websites Dealing With Molecular Mass Spectroscopy

TSRI Mass Spectrometry Home Page:

American Chemical Society:

Chemical Abstracts Service:

Chemical Center Home Page:

Science Magazine:

Journal of Chemistry and Spectroscopy:

More Useful Internet Websites Dealing with Molecular Mass Spectroscopy.