H15 Big Instruments Destructive analysis The original sample is not recoverable Non-destructive testing The original sample is recoverable. Needed for Identification of artworks Establishing the authenticity of historical artefacts To prove that the metal in jewellery is precious Examples of instrumental analysis that can be used for small sample sizes and non-destructive testing include: Mass spectroscopy Spectroscopy involving electromagnetic radiation Mass spectroscopy Form of analysis that separates and identifies substances on the basis of the masses of the positive ion formed by the substances when they are bombarded by high-energy particles (usually electrons) in a high vacuum. Has been used to: Show the existence of isotopes and to measure their relative masses. Identify the presence of a known substance in a sample. Deduce considerable information about the structure of unknown compounds. Mass spectrum The record of the ion current (through the detector) as a function of the mass-to-charge ratio. Since all the ions have a charge of +1 the mass spectrum records the current produced by ions of different mass. Electromagnetic radiation Type of wave motion that consists of oscillating electric and magnetic fields. The properties of electromagnetic radiation include: Transfer energy without transferring mass Transfer energy through a vacuum Travels with the speed of light The forms of electromagnetic radiation differ in their wavelength (or frequency) and include: Radio waves Microwaves Infrared radiation Visible light Ultraviolet light X-rays -rays Spectroscope Instrument in which the observations of spectra are made by eye. Spectrograph Instrument in which the observations of the spectra are recorded on photographic plates. Spectrometer Instrument in which the observations of the spectra are produced as a recorder or computer graph. Dispersion Separating white light into its spectrum. Monochromator An instrument that allows particular wavelengths of light to be selected. Emission of light Light is emitted from an atom when electrons that have been promoted to a higher energy level jump back to a lower energy level. Emission spectra The series of bright colour light emitted by an atom as its electrons return from higher energy levels to lower energy levels as the atom cools. Atoms of different elements produce unique emission spectra because the energy of the orbitals occupied by their electrons, is different. Emission spectra are used to determine the elements present in the sample but are unable to give any information about the nature of their compounds. Ultraviolet-Visible spectroscopy A compound will absorb radiation of the wavelength that will excite one of its valence electrons into a higher energy level. A comparison of the intensity of the light passing through the sample with that of a reference beam gives a measure of the absorption by the sample at that wavelength. The u.v.-light spectrum gives useful information about the structure of the sample without necessarily identifying it. Infrared spectroscopy An i.r. spectrum is obtained by scanning through the whole wavelength range and plotting percentage absorption against wavelength. Infrared radiation is absorbed at particular wavelengths that correspond to the vibrational energy associated with the bending or stretching of particular bonds in molecules. The i.r. spectrum gives information about the type of bonds in a molecule and this information can be used to help identify the compound. Analysis of surfaces The surfaces of objects can be of great interest and there are a number of techniques that can be used to determine the nature of stains on surfaces or the composition of the first few nanometres of an object. These techniques include Electron spectroscopy for chemical analysis (ESCA) X-ray photoelectron spectroscopy (XPS) Scanning tunnelling microscopy Atomic-force microscopy Advantages of spectroscopic methods of analysis Fast and easy to perform Can be used on very small samples Disadvantage Spectrometers can be very expensive.