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Mass Spectrometry

Chem 546 Fall 2010 Introduction to SpectroscopyProf. Rob Ronald. High resolution mass spectrometry (HRMS), while more difficult to obtain than LRMS, provides not only the molecular weight, but also the molecular formula of the ions. In HRMS the fractional mass units reflect the nuclear p

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Mass Spectrometry

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    1. Chem 546 Fall 2010 Introduction to Spectroscopy Prof. Rob Ronald Mass Spectrometry Mass spectrometry is an analytical method in which molecules in a sample are ionized and the mass/charge ratio of the ions is measured. There are two major modes for MS: Low resolution MS, which has a molecular weight resolution of 1 amu; and High Resolution MS, HRMS, which gives fractional mass resolution. Low resolution MS will usually provide the molecular weight, and give a characteristic fragmentation pattern that can be compared to spectra stored in data banks - for many compounds this is sufficient to provide identification.

    2. Chem 546 Fall 2010 Introduction to Spectroscopy Prof. Rob Ronald High resolution mass spectrometry (HRMS), while more difficult to obtain than LRMS, provides not only the molecular weight, but also the molecular formula of the ions. In HRMS the fractional mass units reflect the nuclear packing fractions of the various elements that make up the molecule. That is, while a 12C1H2 group and a 14N atom both have nominal masses of 14, the exact mass of 12C1H2 is 14.01565 amu, but the exact mass of 14N is 14.00307 amu. The difference, 0.01258 amu represents the mass that was consumed in the nuclear synthesis of 14N. These small, but significant, fractional mass differences can be used to determine the molecular compositions of ions. HRMS is now routinely used as part of the characterization of new organic compounds. HRMS has largely replaced combustion analysis for the purpose of confirming composition.

    3. Chem 546 Fall 2010 Introduction to Spectroscopy Prof. Rob Ronald A mass spectrometer consists of three main parts: 1) an ionizer, where the sample is converted to ions. There are many forms of this part of the spectrometer. The ionizer is crucial as it determines the types of ions that are to be generated. 2) a mass analyzer, where the ions are sorted by mass based upon their mass to charge ratio, m/z; there are also many forms of mass analyzers. 3) a detector, where the ion current is measured. Typically this is usually some form of Wheatstone bridge that measures the ion current.

    4. Chem 546 Fall 2010 Introduction to Spectroscopy Prof. Rob Ronald There are three principal types of mass analyzers: the magnetic (and also electric) sector analyzer; the time of flight (TOF) mass analyzer; and the quadrupole mass filter. The magnetic sector analyzers have a curved tube held in a variable magnetic (or electric) field. These are typically rather large instruments as they have big magnets. The TOF analyzers have a straight tube connecting the ionizer and the detector. A TOF is the size that can sit on a desktop or table. The quadrupole mass filters are the smallest and most compact of all the mass analyzers. Quadrupole analyzers have metal rods (typically about 10-20cm long) that have DC voltages and Rf frequencies applied to them that vary in a regular way. Because they are small they can sit on a lab bench, and they can even made to be portable. LRMS is usually done with quadrupole mass analyzers. HRMS is usually done with magnetic sector mass analyzers; but it can also be done by TOF.

    5. Chem 546 Fall 2010 Introduction to Spectroscopy Prof. Rob Ronald In a Magnetic Sector Analyzer the ions are constrained to pass through a curved tube under the influence of an external magnetic (or electric) field As the strength of the magnetic (or electric) field is increased, ions of increasing m/z can pass through the tube without hitting the walls of the tube. At any given magnetic field strength only ions in register can pass through the tube without hitting the walls. Ions that are too heavy are not deflected sufficiently; and ions that are too light are deflected too much. The magnetic sector analyzers used in HRMS can determine the mass of ions to four or five decimal places; this is sufficient resolution to determine molecular formulas.

    6. Chem 546 Fall 2010 Introduction to Spectroscopy Prof. Rob Ronald Ions produced in the source are focused into a beam and introduced into the ion channel Ions too heavy hit the outer wall Ions too light hit the inner wall Ions in register are detected

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