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Glossary#

adsorption - the adhesion of molecules to a surface. The molecules that attach are called adsorbate and the surface is the adsorbent. Adsorption is vital to chromatography.

chemical shift – a measure of the chemical environment of a particular nucleus within a molecule. Chemical shift values are quoted relative to a standard compound tetramethylsilane (TMS) and are measured in parts per million (ppm) of the strength of the magnetic field strength required for resonance in TMS.

chromatogram - the visible record of the separation of substances in mixtures by chromatography.

chromatography - a laboratory technique used to separate substances in a mixture. The mixture is dissolved in a mobile phase, which is then passed through a stationary phase. Substances are separated on the basis of their affinity with the stationary phase.

column chromatography – a form of chromatography in which the mixture to be separated is dissolved in a solvent. This is the eluent and is passed through a column containing a stationary phase (often aluminium oxide). The components of the mixture move at different rates through the column and can be collected as they run out of the bottom.

eluent – the mobile phase in column chromatography. Consists of the mixture to be separated dissolved in a solvent.

fingerprint region – the region below 1500 cm-1 in an infrared spectrum, where there are many peaks which are difficult to assign. An exact match of the fingerprint region with a reference spectrum gives the identity of a compound.

fragmentation – a phenomenon whereby some ions disintegrate into smaller fragments during mass spectrometry, leading to multiple peaks in the resulting spectrum.

gas–liquid chromatography – a form of chromatography in which a mixture is injected into a coiled capillary tube inside an oven, where it vaporizes. An unreactive carrier gas (the mobile phase) transports the mixture through the coil, where the components interact with a powder lining the coil, which acts as the stationary phase. The components exit the coil at different times after injection and may pass straight into a mass spectrometer.

high performance liquid chromatography - a form of column chromatography in which the eluent moves through the column under high pressure. This results in a faster and more effective separation.

infrared spectroscopy – a technique that measures the absorption of different frequencies of infrared energy by a sample of a substance. This gives information about the types of functional group present in a compound.

infrared spectrum – a plot of transmission of infrared radiation against wavenumber (1 / wavelength).

integration – a mathematical technique for working out the area under the peak on a line graph. This is useful in proton NMR as the area under a peak in a 1H NMR spectrum is proportional to the number of hydrogen atoms in that chemical environment. The integration trace 20911. Analytical techniques is sometimes shown as a line of the spectrum whose height is proportional to the number of hydrogen atoms of that type.

mass spectrometry – an accurate instrumental technique that can be used to determine the relative isotopic mass and the relative abundance for each isotope of an element in a sample. This can be used to identify the sample.

mass spectrum – a plot of the mass-to-charge ratio (m/z) of detected fragments against their relative abundance.

mass-to-charge ratio – the mass divided by the charge of an ion. Often called the m/z value.

mobile phase – the part of chromatography consisting of the mixture to be separated dissolved in a solvent and passed over/through the stationary phase. The mobile phase can be a liquid or gas.

molecular ion – the ion formed when a molecule loses an electron during mass spectrometry but does not fragment. Represented by the symbol M+

molecular ion peak – the peak on the mass spectrum of a compound at the highest mass-to-charge ratio, which corresponds to the relative atomic mass of the molecular ion.

NMR spectroscopy – a technique used to investigate the chemical environment of isotopes in a molecule which has an odd mass number (and so exhibit spin). NMR spectroscopy gives us information about the local environment of specific atoms in a molecule, and so can be used to deduce information about the molecular structure of a sample.

NMR spectrum – a plot of the different chemical shift values (in parts per million or ppm) of peaks found in NMR analysis of a compound. NMR spectra usually give information about the local chemical environments of 13C or 1H atoms in a molecule. The integration of the area under the peaks in a 1H NMR spectrum gives the relative number of protons in each type of chemical environment.
Splitting of peaks in a 1H NMR spectrum gives information about the number of protons attached to adjacent carbon atoms.

retention time - the time it takes for a component of a mixture to pass through the coil during gas–liquid chromatography.

spin coupling - a phenomenon that occurs as the spins on atoms which are close together interact with each other.

splitting is the name given to the observation that peaks in 1H NMR spectra often appear as a number of clustered peaks rather than one single peak. This is due to spin coupling interactions between adjacent atoms, and therefore gives information about the number of nearby hydrogen atoms.

stationary phase - the part of chromatography that remains stationary and separates the components of a mixture by interacting with them to different degrees. The stationary phase is usually a solid, often in powder form.

thin layer chromatography (TLC) - a form of chromatography in which the stationary phase is a thin layer of adsorbent material attached to a plate of glass or plastic (TLC plate). A small drop of mixture is placed on the plate, which is placed vertically inside a jar containing solvent. This is the mobile phase and separates the components of the mixture as it rises up the TLC plate.

TMS - Tetramethylsilane. A substance used as a reference compound for NMR as its methyl groups are particularly well shielded and so it produces a strong, single peak at the far right of an NMR spectrum.

wavenumber - the number of cycles of waves in a unit length; the reciprocal of wavelength (1 / wavelength). Usually measured in m-1 or cm-1