H3 Mass Spect Notes 2026 (student copy)

H3 Chemistry 9813 Mass Spectrometry © 2026/JC2/Chem Dept 1 Anglo-Chinese Junior College Department of Chemistry Mass Spectrometry Prepared by: ACJC Chemistry Department Section Content 1 2 3 4 4.1 4.2 4.3 4.4 4.5 5 Introduction Basic Features of Mass Spectrometer Relative Atomic Mass of an Element Relative Molecular Mass of Molecules Mass Spectra of Simple Molecules Molecular Formulae from Accurate Masses (High Resolution Mass Spectrometry) Fragmentation Patterns Determination of Number of Carbon Atoms in Molecules using the 12C:13C Ratio Identification of Halogen Compounds using the (M), (M+2) and (M+3) peaks Rearrangement Accompanying Fragmentation No. References 1 2 K. B. Yeong; Mass Spectrometry Lecture Notes; ACJC; 2008 J. McMurry; Organic Chemistry; Brooks/Cole Publishing Company; 3rd Edition; 1992 Learning Outcomes Section Candidates should be able to: (a) outline the basic pinciples of the mass spectrometry, with reference to (i) ionisation and fragementation (ii) mass/charge ratio, m/z [Detailed knowledge of instrumentation is not required] (b) understand the following features and use them in the interpretation and prediction of mass spectra: (i) molecular ion peak (ii) isotopic abundances including the use of (M+1) peak caused by 13C and (M+2) and (M+4) peaks for the identification of halogen compounds (iii) major fragement ions [fragment ions obtained from rearrangements are not included] 2 3 4.1, 4.3 5 4.3

H3 Chemistry 9813 Mass Spectrometry © 2026/JC2/Chem Dept 2 (b) outline the basic principles of mass spectrometry, with reference to (i) ionisation and fragmentation (ii) mass/charge ratio, m/z 1. Introduction Mass spectrometry is not strictly a branch of spectroscopy as it does not involve the absorption of electromagnetic radiation. Mass spectrometry is one of the most useful and valuable analytical techniques used in Chemistry and Biochemistry to determine the chemical structure and formula of compounds, particularly organic compounds. The principle of mass spectrometry is very simple, although modern mass spectrometers are very sophisticated, precision -made instruments, capable of determining molecular masses to an accuracy of 1 part in 100,000. In addition, it is often possible to gain information about an unknown structure by measuring and identifying the masses of fragments produced when high -energy molecules fly apart. Structures of large complex unknown organic molecules can thus be elucidated. 2. Basic Features of Mass Spectrometer Figure 1: Double-focusing Mass Spectrometer Six processes occur in a double-focussing mass spectrometer (see figure 1): 1. The compound is first dissolved in a solvent and injected into the mass spectrometer. Once in the mass spectrometer, the compound is vaporised in an oven. Since the interior of the instrum ent is kept under a high vacuum, o nly a small vapour pressure is required. Thus, mass spectrometry can be used even when only a small amount of the compound is available.

H3 Chemistry 9813 Mass Spectrometry © 2026/JC2/Chem Dept 3 2. Low energy e lectrons (70 eV) from the e lectron gun are then fired at the gaseous molecules and knock off other electrons from some of the molecules producing radical cation . When a molecule loses one electron, it then has a positive charge and one unpaired electron. This ion is called a radical cation: M + e- → M+· + 2 e- In addition to ionising a molecule, the impact of an energetic electron may cause the molecule to break apart. This fragmentation process gives a characteristic mixture of ions (molecular ion and fragment ions). 3. The gaseous ions are accelerated by passing through an electric field (at a voltage of 5-10 kV). 4. The fast-moving ions now pass through the poles of an electromagnet, where they are deflected. The deflections are proportional to the ions' charge to mass ratios. If all ions have a +1 charge (which is usually the case) the extents of deflection will be inversely proportional to their masses. 5. The deflected ions pass through a narrow slit and are collected on a metallic plate connected to an amplifier. For a given strength of magnetic field, only ions of a certain mass pass through the slit and hit the collector plate. As the (positive) ions hit the plate, they cause a current to flow through the amplifier. The more ions there are, the larger the current. 6. A mass spectrum is produced, which plots relative (or percentage) abundance against mass/charge (m/e) ratio. Important Features of Mass Spectrum Mass spectrum of a compound is usually presented as a vertical bar graph. (i) Vertical axis Signal intensity or peak height, measures the relative abundance of the ion which gives rise to the peak. (ii) Horizontal axis (x –axis) mass/charge or m/e ratio. Since e (charge of the ion = +1) the m/e values are, therefore, the masses of ions responsible for the peak. (iii) Base peak Tallest peak. It is arbitrarily assigned an intensity of 100%, and all other peaks are reported relative to its intensity . The base peak usually corresponds to a particularly stable fragment of the molecules under investigation. (iv) Parent peak (or molecular ion peak) The peak that corresponds to the unfragmented cation radical is called the parent peak or the molecular ion peak.

H3 Chemistry 9813 Mass Spectrometry © 2026/JC2/Chem Dept 4 3. Relative Atomic Mass of an Element The following information can be obtained from the mass spectrum of a monoatomic element: Information Evidence from mass spectrum • number of isotopes present in the element • number of peaks or lines • relative isotopic mass of each isotope • m/e value of each peak • relative abundance of each isotope • height of each peak Ar of an element = sum of (relative isotopic mass x relative abundance) = sum of (m/e value x relative abundance) An analysis of the mass spectrum of an element allows us to calculate its relative atomic mass, Ar. For elements with only one isotope, only one peak is obtained, e.g. Na. Figure 2: Mass spectrum of Na For elements with more than one isotope, several peaks corresponding to ions of different isotopes are obtained, e.g. Fe. Figure 3: Mass spectrum of Fe The m/e value of each peak corresponds to the relative isotopic mass of each isotope. The relative intensity (height) of each peak corresponds to the natural relative abundance of that isotope. % abundance (100%) 23 m/e % abundance (5.84%) 54 56 58 m/e 100 57 (91.68%) (2.17%) (0.31%) abundance total abundance) relative mass isotopic( A r   =

H3 Chemistry 9813 Mass Spectrometry © 2026/JC2/Chem Dept 5 Example 3.1 The mass spectrum of chlorine is as shown: i) How many isotopes of chlorine are there? 2 ii) Write down their relative isotopic masses. 35, 37 iii) Which is the more abundant isotope? 35Cl iv) Calculate the relative atomic mass of chlorine.      +      374 1354 3 = 35.5 Quick Check 1 The figure shows the mass spectrum of elemental rubidium. i) State the isotopes present in rubidium. ii) Calculate the respective percentage abundance of each isotope iii) Calculate the relative atomic mass of rubidium.

H3 Chemistry 9813 Mass Spectrometry © 2026/JC2/Chem Dept 6 (c) Understand the following features and use them in the interpretation and prediction of mass spectra: (ii) Molecular ion peak Example 3.2 Given Isotope % abundance 50Cr 4.31 52Cr 83.76 53Cr 9.55 54Cr 2.38 i) Sketch the mass spectrum that would be obtaine d