Molecular Mass Spectroscopy Chem. 331

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 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

Technique is capable of providing information about the qualitative and quantitative composition of both organic and inorganic analytes in complex mixtures this instrument measures compounds with molecular masses up to 200, 000 Daltons. the structures of a wide variety of complex molecular species isotopic ratios of atoms in samples and the structure and composition of solid surfaces.

The four main components of a molecular mass spectrometer

The Sample Inlet System 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.

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 to v = Bzer/m Substituting the above equation into (1) gives m/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

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.

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.

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 They are convenient and produce high ion currents. Extensive fragmentation can lead to unambiguous identification of analytes.

Disadvantages of Electron Impact sources The need to volatilize the sample limits this method since it excludes analysis of thermally unstable compounds. Excessive fragmentation can lead to the disappearance of the molecular ion peak therefore preventing the molecular mass of the analyte to be determined.

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.

References TSRI Mass Spectrometry Home Page: http://masspec.scripps.edu/hist.html American Chemical Society: http://www.acs.org Chemical Abstracts Service: http://www.cas.org Chemical Center Home Page: http://www.chemcenter/org Science Magazine: http://www.sciencemag.org Journal of Chemistry and Spectroscopy: http://www.kerouac.pharm.uky.edu/asrg/wave/wavehp.html http://www.uis.edu/trammell/che425/ms-2 http://www.usf.edu/~sl/mass­_spec/MS_instrumentation.html http://minyos.its.rmit.edu.au/~rcmfa/mstheory.html http://www.cem.msu.edu/~reusch/VirtualText/Spectrpy/MassSpec/masspec1.htm http://www.abrf.org/ABRFNews/1996/September1996/sep96iontrap.html