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SPECTROMETRY 1. General 2. High Energy 3. Medium Energy 4. Low Energy 5. Mass Spectrometry 6. References |
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| SPECTROMETRY 3. Medium energy - the optical range Most chemical analysis is done in this range, using emission, absorption, and fluorescence. The range generally covers ultraviolet, visible, and infrared, though materials and techniques are different for the infrared. As a rough rule of thumb, ultraviolet and visible radiations involve changes of electronic energy levels of molecules, while infrared affects vibrations between atoms and groups and far infrared involves rotations of molecules. Monochromators, which separate light beams into individual wavelengths, are usually based on either prisms or gratings. Early instruments used prisms, but later production techniques have put gratings in the lead. In either type, incoming light is focused on an entrance slit, dispersed by the prism or grating, and imaged on an exit slit through which the light goes on to the detector. The slit width establishes the width of the wavelength band being observed. A different system uses a rotating ‘circular interference filter’ which changes the pass wavelength with rotation. Much care is taken to assure that only light of the desired wavelength gets through the exit slit. Other light is ‘stray light’. Usually it is desirable to use very narrow slits to produce as nearly as possible ‘monochromatic’ light, but narrowing the slits reduces the energy available for the detector. In special cases, tunable laser sources offer extremely narrow bands but high energy over limited spectral ranges. 3.1 Emission spectrometry (for metals) Arc/spark emission: energy is put into the sample by an electrical arc or spark. Elements in the sample emit light in many narrow ‘lines’ which are separated in a monochromator. The light may be measured by one or more detectors positioned along the spectrum, or by photographic film. Standards of known composition are used to generate spectra for calibration. Flame emission: solutions of standards or samples are sprayed into a flame. Some elements, especially lithium, sodium, and potassium, are ‘excited’ by the flame, and then emit their characteristic lines. With relatively few such lines in this case, the monochromator is often replaced by a color filter. ICP (Inductively Coupled Plasma) spectrometry: like flame emission, but with a much hotter flame. Solutions are sprayed into a flow of argon gas, which is excited to become a plasma as it passes through a high-frequency induction coil. In a ‘sequential’ ICP spectrometer, a single detector is moved from one position to another along the spectrum to measure certain lines. A ‘simultaneous’ ICP spectrometer has numerous detectors (often 30) preset to determine numerous elements. 3.2 Absorption spectrometry Light from a source (a tungsten lamp, or a hydrogen or deuterium arc) passes through a monochromator. The selective wavelength passes through the sample to a detector. The proportion of light absorbed is proportional to the concentration of the atom or molecule being determined. The sample is often dissolved in a solvent, held in a ‘cuvet’. Typical cuvets are 1 cm x 1 cm x 5 cm high, of glass or quartz, but there are many other sizes and shapes. The light path through the sample can vary from less than 1 mm up to 100 mm or more. In ‘atomic absorption’, ‘AA’, photometry, a liquid sample is sprayed into a relatively long cuvet and individual atoms absorb specific wavelengths, which are typically produced by ‘hollow cathode’ lamps. Such lamps are specific for one or a few elements. In ‘graphite furnace’ AA, a small sample is placed in a carbon (graphite, etc.) tube through which a heavy current can be passed for heating. The sample is evaporated to dryness at low current/temperature, and then vaporized by very high temperature. A ‘diode array’ is an assembly of a few hundred photodiodes packed closely together. It can record a UV/Visible spectrum at one time without having to ‘scan’ monochromator wavelengths. It is particularly useful in a spectrophotometric detector for high-pressure liquid chromatography, where it can generate absorption spectra of individual components as they elute from the chromatograph column. Infrared waves are often considered ‘heat’ waves, and sources are hot, but usually not hot enough to produce much visible light, which would be wasted in an infrared system. Two common sources are the ‘Globar’ and the ‘Nernst glower’. Large salt prisms were used in early spectrometers, but gratings have taken over now. The infrared waves do not have enough energy for the detectors used in the visible and UV range, and thermal detectors (thermocouples, etc.) are used. Plastic, glass and quartz all absorb too strongly for use in IR, and the sample is often a thin film between a pair of polished salt (NaCl) plates. Thicker cells (~ 1 mm) with salt windows are also used. Samples may be ground (‘mulled’) with mineral oil or a fluorocarbon oil; ground with potassium bromide (KBr) and pressed into flat disks (‘pellets’) or dissolved in solvents with relatively little IR absorbance (such as carbon tetrachloride and carbon disulfide). Aqueous solutions are bad for IR. Water absorbs IR strongly. Worse yet, it dissolves salt sample holders. The solubility problem can be avoided with more expensive materials such as sapphire, or by ‘total internal reflection, ‘multiple internal reflection’ or ‘attenuated total reflectance (ATR)’ plates of material transparent to IR. The IR beam enters at an angle and is internally reflected one or more times at the surface. Some of the IR beam (‘evanescent wave’) effectively passes outside the plate and back inside at each reflection. When there is a sample in contact with the reflecting surface an IR spectrum is produced. The worst problem for IR spectrometry has been lack of sensitive detectors, combined with the low energy in a narrow band of wavelengths from a monochromator. That is over come with Fourier-Transform systems (FTIR). Those systems use interferometers rather than monochromators, and effectively measure all wavelengths at once. The detector sees a relatively strong signal which, after passing through a computer program, produces the sample IR spectrum. Raman spectrometry uses an intense monochromatic laser beam. Some of the light is absorbed in the sample, and some of that light is re-emitted at a different wavelength. The energy difference is the same as the energy of certain absorbed IR bands. Raman spectrometry usually uses visible light, so problems with water and materials are avoided. However, some bands are different in the two kinds of spectra. Absorption measurements: If the intensity of light going into a sample is I0, and that coming out is I, then:
The last relation, A = abc, is the Bouguer-Lambert-Beer law in which:
Beer stated the relation for c, and ‘Beers’ law, stating
that absorbance is proportional to concentration, is particularly important
for chemical analysis. |