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M (g) + e - M + (g) + 2e -. Mass spectrometry. Mass spectrometry is an analytical technique that can be used to deduce the molecular formula of an unknown compound. Gaseous molecules of the compound are bombarded with high-speed electrons from an electron gun.
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M(g) + e- M+(g) + 2e- Mass spectrometry Mass spectrometry is an analytical technique that can be used to deduce the molecular formula of an unknown compound. Gaseous molecules of the compound are bombarded with high-speed electrons from an electron gun. These knock out an electron from some of the molecules, creating molecular ions (M+), which travel to the detector plates: The relative abundances of the detected ions form a mass spectrum: a kind of molecular fingerprint that can be identified by computer using a spectral database.
The molecular ion peak The peak with the highest mass-to-charge ratio (m/z) is formed by the heaviest ion that passes through the spectrometer. This value of m/z is equal to the relative molecular mass of the compound. High resolution mass spectrometry can be used to determine the molecular formula of a compound from the accurate mass of the molecular ion. molecular ion peak 100 80 60 abundance (%) 40 20 0 m/z 0 160 80 120 40 mass spectrum of paracetamol
What is fragmentation? A molecular ion is a positively-charged ion, which is also a radical as it contains a single unpaired electron. It is therefore sometimes represented as M+•. During mass spectroscopy, the molecular ion can fragmentinto a positive ion and a radical: M+• → X+ + Y• NB: Only the ions are detected by the mass spectrometer. For example, in the case of propane: CH3CH2CH3+• → CH3CH2+ + CH3• or CH3CH2CH3+• → CH3CH2• + CH3+ This fragmentation process gives rise to characteristic peaks on a mass spectrum that can give information about the structure of the molecule.
Interpreting mass spectra Origins of some common peaks in mass spectra: loss of a methyl ion methyl positive ion (15) phenyl positive ion (77) Mass differences between peaks indicate the loss of groups of atoms (fragments). For example, loss of a methyl group leads to a mass difference of 15 between peaks.
Uses of mass spectrometry Some uses of mass spectrometry: • Identifying elements in new or foreign substances, for example analysing samples from the Mars space probe. • Monitoring levels of environmental pollution, for example amounts of lead or pesticide in a sample. • In biochemical research, for example determining the composition of a protein by comparing it against a database of known compounds.
Infrared spectroscopy Infrared spectroscopy is an analytical technique that provides information about the functional groups present in a compound. Certain groups of atoms absorb characteristic frequencies of infrared radiation as the bonds between them undergo transitions between different vibrational energy levels. Infrared energy is only transferred to a bond if the bond contains a dipole that changes as it vibrates. Symmetrical molecules such as O2 or H2, are therefore IR inactive. The particular wavelengths absorbed are specific to that particular configuration of bonds and atoms (functional group). bonds absorb IR energy
Infrared spectra An infrared spectrum is a plot of transmission of infrared radiation against wavenumber (1 / wavelength). Any wavelength that is absorbed by the sample will transmit less than the others, forming a dip in the graph. The pattern in the fingerprint region (1500-400 cm-1) is unique to each molecule, and so can be used for identification purposes. transmission (%) wavenumber (cm-1) IR absorption spectrum for chloroethane
Uses of IR spectroscopy Some uses of infrared spectroscopy: • Use of IR spectra to follow the progress of a reaction involving change of functional groups (e.g. in the chemical industry to determine the extent of the reaction). • Use of IR spectra to assess the purity of a compound. • Breathalyzers: modern breathalyzers calculate the percentage of ethanol in the breath by looking at the size of the absorption caused by the C–H bond stretch in the alcohol.
What does an NMR spectrum tell us? Different chemical environments (bonds and atoms surrounding a nucleus) affect the strength of magnetic field that must be applied to a nucleus in order for it to enter the resonance state. By measuring the strength of magnetic field that must be applied, NMR spectroscopy gives us information about the local environment of specific atoms in a molecule. This can be used to deduce information about molecular structure. The environments of 13C and 1H atoms are most commonly studied in NMR spectroscopy.
Interpreting 13C NMR spectra The 13C NMR spectrum of ethylamine contains two peaks. This is because ethylamine has two unique 13C environments, each requiring the application of a different magnetic field strength for that carbon nucleus to enter the resonance state. One peak is due to the carbon atom with three hydrogen atoms attached to it, and the second to the carbon atom with two hydrogen atoms and an amine group attached to it.
Chemical shift and TMS The horizontal scale on an NMR spectrum represents chemical shift (δ). Chemical shift is measured in parts per million (ppm) of the magnetic field strength needed for resonance in a reference chemical called TMS. TMS (tetramethylsilane)is universally used as the 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. The signal from the carbon atoms in TMS is defined as having a chemical shift of 0.
13C NMR chemical shift assignment The chemical shift values of peaks on the 13C NMR spectrum can help us identify the types of carbon atom in a compound. The likely source of spectrum peaks can be identified using a data table of typical chemical shift values. Type of carbon δ/ppm 5–40 20–50 190–220 chemical shift
Integration and the number of hydrogens The height of the peaks in an NMR spectrum does not give us any useful information. However, the area under the peaks on a 1H NMR spectrum is proportional to the number of hydrogen atoms causing the signal. The ratio of the areas under the peaks tells you the ratio of 1H atoms in each environment. 3 2 1 The spectrum can be integratedto find this information.
1H NMR chemical shift assignment The chemical shift values of peaks on an 1H NMR spectrum give information about the likely types of proton environment in a compound. Type of proton δ/ppm 0.7–1.2 2.1–2.6 9.0–10.0
Uses of NMR spectroscopy NMR spectroscopy uses the same technology as magnetic resonance imaging (MRI). This is an important non-invasive method of gaining information about internal structures in the body used in diagnostic medicine and scientific research. NMR spectroscopy is also used in the pharmaceutical industry to check the purity of compounds. Often, a combination of mass spectrometry, infrared spectroscopy and NMR spectroscopy is used in modern analysis to elucidate the structure of organic molecules.
What is chromatography? Chromatography is a series of analytical techniques that can be used to separate mixtures of compounds for further use or for analysis. In all forms of chromatography, a mobile phase moves through or across a stationary phase. • stationaryphase – this phase does not move. Compounds in the mixture are attracted to it (adsorbed) and slowed down. Either a solid or a liquid. • mobile phase – this phase moves. The more soluble compounds in the mixture are carried faster as the mobile phase moves. Either a liquid or a gas.
pump injector eluent reservoir data analysis column detector High performance liquid chromatography High performance liquid chromatography (HPLC) is a development of column chromatography in which the eluent is pumped through the column at high pressure. This results in better and faster separation than can be achieved in standard column chromatography. components collected
Gas chromatography–mass spectroscopy In both GL chromatography and HPLC, the output from the chromatography column can be passed through a mass spectrometer. The spectra obtained can be compared to spectra of known compounds. Gas chromatography–mass spectroscopy (GC–MS) is used extensively in forensics, environmental monitoring and in airport security systems. It is sensitive enough to detect minute quantities of substances
