Transcript Slide 1
Chapter 25 Instruments for Optical Spectrometry 25 A Instrument components Most spectroscopic instruments in the UV/visible and IR regions are made up of five components: (1) a stable source of radiant energy; (2) a wavelength selector to isolate a limited region of the spectrum for measurement; (3) one or more sample containers; (4) a radiation detector, to convert radiant energy to a measurable electrical signal; and (5) a signal-processing and readout unit consisting of electronic hardware and in modern instruments a computer. Spectroscopic sources are of two types: 1. Continuum sources emit radiation that changes in intensity only slowly as a function of wavelength. A continuum source provides a broad distribution of wavelengths within a particular spectral range. 2. Line sources, which emit a limited number of spectral lines, each of which spans a very narrow wavelength range. Sources can also be classified as continuous sources, which refer to the fact that they emit radiation continuously with time, or pulsed sources, which emit radiation in bursts. The continuum sources for IR radiation are normally heated inert solids. A Globar source consists of a silicon carbide rod. Infrared radiation is emitted when the Globar is heated to about 1500°C by passing electricity through it. A Nernst glower is a cylinder of zirconium and yttrium oxides that emits IR radiation when heated to a high temperature by an electric current. Monochromators generally have a diffraction grating to disperse the radiation into its component wavelengths. The output wavelength of a monochromator is thus continuously variable over a considerable spectral range. The wavelength range passed by a monochromator, called the spectral bandpass or effective bandwidth, can be less than 1 nm for moderately expensive instruments to greater than 20 nm for inexpensive systems. Other instruments used for emission spectroscopy contain a device called a polychromator, which contains multiple exit slits and multiple detectors. This arrangement allows many discrete wavelengths to be measured simultaneously. Angular dispersion results from diffraction, which occurs at the reflective surface. The radiation entering the monochromator is shown as consisting of just two wavelengths, 1 and 2, where 1 is longer than 2. The two wavelengths are focused by another concave mirror onto the focal plane of the monochromator. A high-quality monochromator will exhibit an effective bandwidth of a few tenths of a nanometer or less in the ultraviolet/visible region. Most gratings in modern monochromators are replica gratings. A grating for the ultraviolet and visible region typically has from 50 to 6000 grooves/mm, with 1200 to 2400 being most common. One of the most common types of reflection gratings is the echellette grating. A parallel beam of monochromatic radiation approaches the grating surface at an angle i relative to the grating normal. The incident beam is depicted as consisting of three parallel beams that make up a wave front labeled 1, 2, 3. The diffracted beam is reflected at the angle r, which depends on the wavelength of the radiation. The angle of reflection r is related to the wavelength of the incoming radiation by the equation n = d(sin i + sin r) Filters operate by blocking or absorbing all but a restricted band of radiation. There are two types of filters used in spectroscopy: interference filters and absorption filters. Interference filters are typically used for absorption measurements. These filters generally transmit a much greater fraction of radiation at their nominal wavelengths than do absorption filters. Interference filters are used with ultraviolet and visible radiation, as well as with wavelengths as long as about 14 mm in the infrared region. An interference filter relies on optical interference to provide a relatively narrow band of radiation, typically 5 to 20 nm in width. The nominal wavelength for an interference filter lmax is given by the equation max = 2t/n where t is the thickness of the central fluoride layer, h is its refractive index, and n is an integer called the interference order. Absorption filters, which are generally less expensive and more rugged than interference filters, are limited in use to the visible region. Absorption filters have effective bandwidths that range from perhaps 30 to 250 nm. Filters have the advantages of simplicity, ruggedness, and low cost. Since one filter can only isolate a single band of wavelengths, a new filter must be used for a different wavelength band. In the IR region of the spectrum, most modern instruments do not disperse the spectrum at all. Instead an interferometer is used, and the constructive and destructive interference of electromagnetic waves are used to obtain spectral information through a technique called Fourier transformation. To obtain spectroscopic information, the radiant power transmitted, fluoresced or emitted, must be detected in some manner and converted into a measurable quantity. A detector is a device that identifies, records, or indicates a change in one of the variables in its environment such as pressure, temperature, or electromagnetic radiation. In modern instruments, the information of interest is encoded and processed as an electrical signal. A transducer converts nonelectrical quantities, such as light intensity, pH, mass, and temperature, into electrical signals that can be subsequently amplified, manipulated, and finally converted into numbers proportional to the magnitude of the original quantity. The electrical signal produced by the transducer should be linearly related to the radiant power P of the beam. G = KP + K’ where G is the electrical response of the detector in units of current, voltage, or charge. The proportionality constant K measures the sensitivity of the detector in terms of electrical response per unit of radiant power input. K ’, is a small constant response known as a dark current, even when no radiation strikes the surfaces. Under ordinary circumstances, G = KP There are two general types of transducers: one type responds to photons, the other to heat. All photon detectors are based on the interaction of radiation with a reactive surface either to produce electrons (photoemission) or to promote electrons to energy states in which they can conduct electricity (photoconduction). Photoelectrons are electrons that are ejected from a photosensitive surface by electromagnetic radiation. A photocurrent is the current in an external circuit that is limited by the rate of ejection of photoelectrons. Figure 25-13 Diagram of a photomultiplier tube. (a) photograph; (b) cross-sectional view; and (c) electrical diagram illustrating dynode polarization and photocurrent measurement. Photoconductive transducers consist of a thin film of a semiconductor material deposited on a nonconducting glass surface and sealed in an evacuated envelope. Absorption of radiation by these materials promotes nonconducting valence electrons to a higher energy state, which decreases the electrical resistance of the semiconductor. A semiconductor is a substance having a conductivity that lies between that of a metal and that of a dielectric (an insulator). Crystalline silicon is a semiconductor, a material whose electrical conductivity is less than that of a metal but greater than that of an electrical insulator. Silicon has four valence electrons, each of these is combined with electrons from four other silicon atoms to form four covalent bonds. The conductivity of silicon can be greatly enhanced by doping. A pn junction or a pn diode, which is conductive in one direction and not in the other. In its conduction mode, the positive terminal of a dc source is connected to the p region and the negative terminal to the n region. The diode is said to be forward biased under these conditions. Photodiodes are semiconductor pn-junction devices that respond to incident light by forming electron–hole pairs. When a voltage is applied to the pn diode such that the p-type semiconductor is negative with respect to the n-type semiconductor, the diode is said to be reversed biased. The majority carriers are drawn away from the junction, leaving a nonconductive depletion layer. Silicon photodiode detectors respond extremely rapidly, usually in nanoseconds. Diode arrays can also be obtained commercially with front-end devices called image intensifiers to provide gain and allow the detection of low light levels. In charge-transfer detectors individual detector elements are arranged in rows and columns. In a charge-injection device (CID) detector, the voltage change arising from movement of the charge from the region under one electrode to the region under the other is measured. In a charge-coupled device (CCD) detector, the charge is moved to a charge-sensing amplifier for measurement. Four types of thermal transducers are used for infrared spectroscopy. The most widely used is a tiny thermocouple or a group of thermocouples called a thermopile. The bolometer consists of a conducting element whose electrical resistance changes as a function of temperature. A pneumatic detector consists of a small cylindrical chamber that is filled with xenon and contains a blackened membrane to absorb infrared radiation and heat the gas. Pyroelectric detectors are manufactured from crystals of a pyroelectric material, such as barium titanate or deuterated triglycine sulfate. A signal processor is an electronic device that may amplify the electrical signal from the detector. The signal processor may convert the signal from dc to ac (or the reverse), change the phase of the signal, and filter it to remove unwanted components. The signal processor may also perform such mathematical operations on the signal as differentiation, integration, or conversion to logarithms. Sample containers, which are usually called cells or cuvettes, must have windows that are transparent in the spectral region of interest. 25 B Ultraviolet/visible photometers and spectrophotometers A spectrometer is a spectroscopic instrument that uses a monochromator or polychromator in conjunction with a transducer to convert the radiant intensities into electrical signals. Spectrophotometers are spectrometers that allow measurement of the ratio of the radiant powers of two beams, a requirement to measure absorbance. Photometers use a filter for wavelength selection in conjunction with a suitable radiation transducer. Most spectrophotometers cover the UV/visible and occasionally the near-infrared region, while photometers are most often used for the visible region. Photometers find considerable use as detectors for chromatography, electrophoresis, immunoassays, or continuous flow analysis. Many modern photometers and spectrophotometers are based on a doublebeam Design. A double-beam-in-space instrument is one in which two beams are formed by a V-shaped mirror called a beam-splitter. One beam passes through the reference solution to a photodetector, and the second simultaneously passes through the sample to a second, matched photodetector. In a double-beam-in-time spectrophotometer, the beams are separated in time by a rotating sector mirror that directs the entire beam through the reference cell and then through the sample cell. Figure 25-20 Instrumental designs for UV/visible photometers or spectrophotometers. (a), a single-beam instrument and (b), a double-beam-inspace instrument. In the double-beam-in-time instrument (c), the beam is alternately sent through reference and sample cells before striking a single photodetector. With multichannel systems, the dispersive system is a grating spectrograph placed after the sample or reference cell. The photodiode array or CCD array is placed in the focal plane of the spectrograph. These detectors allow the measurement of an entire spectrum in less than 1 s. 25 C Infrared spectrophotometers Two types of spectrometers are used in IR spectroscopy: the dispersive type and the Fourier transform variety. In most UV/visible instruments the cell is located between the monochromator and the detector in order to avoid photodecomposition of the sample, which may occur if samples are exposed to the full power of the source. Fourier transform IR instruments contain no dispersing element, and all wavelengths are detected and measured simultaneously. Instead of a monochromator, an interferometer is used to produce interference patterns that contain the infrared spectral information. Fourier transform spectrometers detect all the IR wavelengths all the time. They have greater light-gathering power than dispersive instruments and consequently better precision.