Instrumentation & Spectroscopy (HL)

Mass spectrometry is an analytical technique used to determine the relative molecular mass of a compound and to identify structural fragments. A sample is first vaporised and then ionised, typically by electron impact, to form positively charged ions (molecular ions, M⁺). These ions are then accelerated through an electric field and deflected by a magnetic field. The extent of deflection depends on the mass-to-charge ratio (m/z) of the ion; lighter ions and more highly charged ions are deflected more. A detector records the m/z values and their relative abundances, producing a mass spectrum. The molecular ion peak (M⁺) corresponds to the relative molecular mass of the compound. Fragmentation of the molecular ion can occur, leading to peaks at lower m/z values, which provide valuable information about the compound's structure. The most intense peak in the spectrum is called the base peak.

Infrared spectroscopy is a technique used to identify the functional groups present in a molecule. Molecules absorb infrared radiation at specific frequencies, causing their bonds to vibrate (stretch or bend) at characteristic energy levels. Different functional groups have unique absorption frequencies, which are typically measured in wavenumbers (cm⁻¹). An IR spectrum plots the percentage transmittance of IR radiation against wavenumber. A strong absorption at a particular wavenumber indicates the presence of a specific functional group. For example, a strong absorption around 1700 cm⁻¹ indicates a carbonyl (C=O) group, while a broad absorption between 3200-3600 cm⁻¹ suggests an alcohol O-H group, and a very broad absorption between 2500-3300 cm⁻¹ indicates a carboxylic acid O-H group. The region below 1500 cm⁻¹ is known as the 'fingerprint region' and is unique to each compound, but it is often too complex for detailed analysis at Leaving Cert level.

Nuclear Magnetic Resonance (NMR) spectroscopy provides detailed information about the carbon-hydrogen framework of a molecule, specifically focusing on the environment of hydrogen nuclei (protons, ¹H NMR). Certain atomic nuclei, such as ¹H and ¹³C, possess nuclear spin. When placed in a strong external magnetic field, these nuclei can align either with or against the field. Applying radiofrequency radiation causes these nuclei to 'flip' their spin state, absorbing energy at specific frequencies. The exact frequency of absorption, known as the chemical shift (δ), is measured in parts per million (ppm) relative to a standard reference (tetramethylsilane, TMS, at δ = 0 ppm). The chemical shift depends on the electronic environment of the nucleus, providing clues about the functional groups nearby. A ¹H NMR spectrum provides four key pieces of information: 1. Number of signals: Indicates the number of different types of chemically equivalent protons in the molecule. 2. Chemical shift (δ): Provides information about the electronic environment of the protons (e.g., protons near electronegative atoms are 'deshielded' and appear at higher δ values). 3. Integration (area under peak): The relative area under each signal is proportional to the number of protons giving rise to that signal. 4. Splitting pattern (multiplicity): Caused by spin-spin coupling with neighbouring non-equivalent protons. The 'n+1 rule' states that a signal for a proton with 'n' equivalent neighbouring protons will be split into 'n+1' peaks (e.g., a singlet, doublet, triplet, quartet).