25 Instruments for Optical Spectrometry

Transcription

1 25 Instruments for Optical Spectrometry 25A INSTRUMENT COMPONENTS (1) source of radiant energy (2) wavelength selector (3) sample container (4) detector (5) signal processor and readout (a) (b) (c) Fig components of various types of instruments for optical spectroscopy. (a) The arrangement for absorption measurements is shown. (b) The configuration for fluorescence measurements is shown. Two wavelength selectors are needed to select the excitation and the emission wavelengths. The selected source radiation is incident on the sample and the radiation emitted is measured, usually at right angles to avoid scattering. (c) The configuration for emission spectroscopy is shown. A source of thermal energy, such as a flame, produces an analyte vapor that emits radiation that is isolated by the wavelengths selector and converted to an electrical signal by the detector. 25A-1 Optical Materials cells, windows, lenses and wavelength dispersing element (Fig. 25-2) silicate glass: for visible region fused silica or quartz: < 380 nm Fig Transmittance ranges for various optical materials. 158

2 25A-2 Spectroscopic Sources generate a beam of radiation with sufficient and stable power (1) continuous sources: emit radiation that changes in intensity only slowly as a function of wavelength (2) line sources: emit a limited number of bands of radiation, each of which spans a very limited range of wavelength. Fig Spectral source types. The spectrum of a continuum source (a) is much broader than that of a line source (b). Table 25-1 Continuous Sources for Optical Spectroscopy Source Wavelength Region, nm Type of Spectroscopy Xenon arc lamp Molecular fluorescence H 2 and D 2 lamps UV molecular absorption Tungsten/halogen lamp UV/vis/near-IR molecular absorption Tungsten lamp Vis/near-IR molecular absorption Nernst glower ,000 IR molecular absorption Nichrome wire ,000 IR molecular absorption Globar ,000 IR molecular absorption Continuous Sources in the UV/Visible Region provides radiation of all wavelength within a particular spectral region. (a) (b) Deuterium (hydrogen) lamps: nm (Fig. 25-4) 159 Fig (a) A tungsten lamp of the type used in spectroscopy and (b) its spectrum. Intensity of the tungsten source is usually quite low at wavelengths shorter than about 350 nm. Note that the intensity reaches a maximum in the near-ir region of the spectrum (~1200 nm in this case). A cylindrical tube (contains deuterium at a low pressure) with a quartz window (the radiation exits) Fig (a) A deuterium lamp of the type used in spectrophotometers and (b) its spectrum. Note that the maximum intensity occurs at ~ 225 nm. Typically instruments switch from deuterium to tungsten at ~350 nm. (a) (b)

3 Continuous Sources in the IR Region Globar source: 1-40 μm (Globar heated to about 1500 ) 5- by 50-mm silicon carbide rod. Nernst glower: a cylinder of zirconium and yttrium oxides. Nichrome wire 25A-3 Wavelength Selectors enhance both the selectivity and the sensitivity 1. Monochromators and Polychromators advantage: the output wavelength can be varied continuously over a considerable spectral range. (the more common type) 2. Grating disperse radiation into its component wavelengths qualitative analysis: narrow slits and minimum effective bandwidths quantitative analysis: wider slits permit operation at lower amplification (greater reproducibility). (a) Fig Types of monochromators: (a) grating monochromator and (b) prism monochromator. In both cases, λ 1 > λ 2. Fig Output of an exit slit as the monochromator is scanned from λ 1 - δλ to λ 1 + δλ. Fig Mechanism of diffraction from an echellette-type grating. 160

4 The Echellette Grating Concave Grating Holographic Grating Fig Dispersion of radiation alomg the focal plane AB of a typical prism (a) and echellette grating (b). The position of A and B in the scale in (c) are shown in Fig Radiation Filters advantage: simplicity, ruggedness and cheapness interference filter: effective bandwidths of 5 to 20 nm Dielectric material: CaF 2 of MgF 2 absorption filter: effective bandwidths of 50 to 250 nm Fig Bandwidths for two types of filter. Fig (a) Schematic cross section of an interference filter. Note that the drawing is not to scale, and the three central bands are much narrower than shown. (b) Schematic to show the conditions for constructive interference. 25A-4 Detecting and Measuring Radiant Energy detector: indicates the existence of some physical phenomenon. ex: photographic film pointer of a balance mercury level in a thermometer human eye 161

5 transducer: converts signals, such as light intensity, ph, mass and temp. into electrical signals that can be subsequently amplified, manipulated and finally converted into numbers proportional to the magnitude of the original signal. Properties of Radiation Transducers 1. responds rapidly to low levels of radiant energy over a broad wavelength range. 2. produces an electrical signal that is easily amplified and has a relatively low noise level. 3. electrical signal produced by the transducer be directly proportional to the power of the beam P: G = KP + K' G = KP Types of Transducers G : electrical response of the detector in units of current, resistance or potential. K : proportionality constant (sensitivity of the detector) K': dark current (produced by a photoelectric detector in the absence of light). Table 25-2Common Detectors for Absorption Spectroscopy Type Wavelength Range, nm Type of Spectroscopy Photon Detectors Phototubes UV/visible and near-ir absorption Photomultiplier tubes UV/visible and near-ir absorption, molecular fluorescence Silicon photodiodes Visible and near-ir absorption Photoconductive cells ,000 IR absorption Heat Detectors Thermocouples ,000 IR absorption Bolometers ,000 IR absorption Pneumatic cells ,000 IR absorption Pyroelectric cells ,000 IR absorption Photon Detectors (1) Phototubes *a semicylindrical photocathode: supports a layer of photoemissive material, such as alkali metal or metal oxide; emitted photoelectrons, producing a current (photocurrent) *a wire anode 162 Fig A phototube and accompanying circuit.

6 (2) Photomultiplier Tubes (PMT) (Fig 25-13) more sensitive cathode: emitted electrons are accelerated toward a dynode dynode: at 90 V more positive than cathode. Fig Diagram of a photomultiplier tube: (a) cross-sectional view, (b) electrical diagram illustrating dynode polarization and photocurrent measurement. (3) Photoconductive Cells. A thin film of a semiconductor: PbS, mercury cadmium telluride or indium antimonide (4) Silicon Photodiodes and Photodiode Arrays crystalline silicon: semiconductor conduction a semiconductor involves the movement of electrons and holes in opposite directions. conductivity of silicone : enhanced by doping, a process whereby a tiny, controlled amount ( 1 ppm) of a Group V or Group III element is distributed homogenously throughout a silicon crystal. Ex:1. a crystal is doped with a group V element, such as As, four out of five of the valence electrons of the dopant form covalent bonds with four silicone atoms leaving one electron free to contribute to the conductivity of the crystal containing unbonded electrons (negative charges): n-type majority carrier: electrons Extra electron n-type Fig Two-dimensional representation of n-type silicon showing impurity atom. 163

7 Ex:2. the silicon is doped with a group III element, such as Ga, which has but three valence electrons, an excess of holes develops, which also enhances conductivity (Fig ) containing an excess of holes (positive charges): p-type majority carrier: holes Fig Two-dimensional representation of p-type silicon showing impurity atom. p-type Vacancy (or hole) pn junction or pn diode Fig (a) Schematic of a silicon diode. (b) Flow of electricity under forward bias. (c) Formation of depleton layer, which prevents flow of electricity under reverse bias. (5) Diode-Array Detectors (6) Charge Transfer Devices (CTD) Charge Injection Devices (CID) Charge Coupled Devices (CCD) Fig Cross section of one of the pixels of a charge transfer device. The positive hole produced by the photon hν is collected under the negative electrode. Heat Detectors, thermal detector 25A-5 Sample Containers cells or cuvettes: 0.1 to 1-cm path length Fig Typical examples of commercially available cells for the UV/visible region. 164

8 25B UV/Visible Photometers and Spectrophotometers spctrophotometers: employ a grating or a prism monochromator to provide a narrow band of radiation for measurements that the wavelength used can be varied continuously, thus making it possible to record entire absorption spectra. photometers: use an absorption filter or an interference filter. advantages: simplicity, ruggedness and low cost. 25B-1 Single-Beam Instruments Spectronic 20: spectral range nm (an accessory phototube extends the range to 950 nm). effective bandwidth of 20 nm wavelength accuracy of ±2.5 nm 0 % T calibration or adjustment: 100 % T calibration or adjustment: blank (a) (b) Fig The Spectronic 20 spectrophotometer. A photography of the instrument is shown in (a), while the optical diagram is seen in (b). 25B-2 Double-Beam Instruments (Fig b,c) Fig (a) A single-beam instrument (a) A aingle-beam instrument, radiation from the filter or monochromator passes through either the reference or the sample cells before striking the photodetector. 165

9 Fig (b) A double-beam-in-space instrument (b) A double-beam-in-space instrument, radiation from the filter or monochromator is split into two beams that simultaneously pass through the reference and sample cells before striking two matched photodetectors. Fig (c) A double-beam-in-time instrument (c) A double-beam-in-time instrument, the beam is alternately sent through reference and sample cells before striking a single photodetector. Only a matter of milliseconds separates the beams as they pass through the two cells. 25B-3 Multichannel Instruments possible to record an entire ultraviolet or visible spectrum Chips length: 1-6 cm individual diodes widths: mm Fig Diagram of a multichannel spectrometer based on a grating spectrograph with a photodiode array detector. 166

10 25C Infrared Spectrophotometers 25C-1 Dispersive IR Instruments cell compartment is located between the source and the monochromator any scattered radiation generated in the cell compartment is largely removed by the monochromator. IR sources: heated solids IR gratings: much coarser than those required for UV/visible IR detectors: respond to heat rather than photons optical components of IR: polished salts such as NaCl or KBr. 25C-2 Fourier Transform Instruments great speed, high resolution, high sensitivity and unparalleled wavelength precision and accuracy no dispersing element and all wavelengths are detected and measured simultaneously. 167