CHAPTER 7. Components of Optical Instruments

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1 CHAPTER 7 Components of Optical Instruments From: Principles of Instrumental Analysis, 6 th Edition, Holler, Skoog and Crouch. CMY 383 Dr Tim Laurens NB Optical in this case refers not only to the visible region of the electromagnetic spectrum, but also includes the UV and IR regions. 7A: General designs of optical instruments 1. Optical spectroscopic measurements are based on six phenomena: 1. Absorption 2. Fluorescence 3. Phosphorescence 4. Scattering 5. Emission 6. Chemiluminescence 2. Instruments utilizing the above phenomena differ in configuration but most of their components are similar. 3. Typically spectroscopic instruments require: 1. A stable source of radiant energy 2. A transparent container of holding the sample 3. A device that isolates a restricted region of the spectrum for measurement 4. A radiation detector for conversion of radiant/light energy into an electric signal 5. Signal processor and readout 1

2 2

3 Optical characteristics of the individual components 1. Note that the instrument components differ in detail, depending on the wavelength region within which they are going to be used. 2. Qualitative vs quantitative design 3. Molecular vs atomic spectroscopy 3

7B SOURCES OF RADIATION 1. Typically the source must generate a beam with sufficient radiant power for easy detection and measurement 2. The output power should be stable for reasonable periods 3.

4 7B SOURCES OF RADIATION 1. Typically the source must generate a beam with sufficient radiant power for easy detection and measurement 2. The output power should be stable for reasonable periods 3. Typically, the radiant power of a source varies exponentially with the voltage of its power supply: i.e. a well regulated power supply to the source is needed to provide the required stability. 4. Alternatively the problem of source stability can sometimes be circumvented with double beam designs in which the ratio P/P o serve as analytical variable. a. In double beam designs the intensities of the two beams are measured simultaneously so that the fluctuation in source intensity is largely cancelled 5. Two types of sources: a. Continuum sources: emit a continuum of radiation, changing slowly over a wavelength range b. Line sources: emit a limited number of lines (bands of radiation) each spanning a limited range of wavelengths. 6. Sections 7B-1 and 7B-2 4

7C Wavelength selectors a. Most spectroscopic analyses require radiation that consists of a limited, narrow, continuous group of wavelength called a band b. A narrow band width: a.

5 7C Wavelength selectors a. Most spectroscopic analyses require radiation that consists of a limited, narrow, continuous group of wavelength called a band b. A narrow band width: a. Enhances the sensitivity of the absorbance measurement. b. Provide selectivity to emission/absorption measurements c. Is required to obtain a linear relationship between the detected signal and concentration, i.e. linear calibration graph c. Ideally, the output from a wavelength selector would be radiation of a single wavelength or frequency But there are no wavelength selectors that can select only one wavelength, therefore wavelength selection usually results in a band of wavelengths (figure 7-11) d. The effective bandwidth is an inverse measure of the quality of the wavelength selector The narrower the bandwidth the better the performance of the wavelength selector. e. There are two types of wavelength selectors: a. Filters b. Monochromators 7C-1: Filters 1. Two types of filters are employed for wavelength selection: a. Interference filters (also called Fabry-Parot filters) Suitable for the UV, Vis and IR regions Rely on optical interference to provide narrow bands of radiation 5

6 Consist of two metal films with a dielectric layer in between. The metal films are sandwiched between two glass plates in turn. The thickness of the dielectric layer is carefully controlled and determines the wavelength of the transmitted radiation. The particular wavelength that is reflected in the reinforced due to constructive interference. Deduce eq 7-5 (p177) Not Fabry-Parot Etalon, leave out up to end of holographic filters b. Absorption filters restricted to the Vis region only Function by selective absorption of portions of the spectrum The most common type consists of coloured glass or a dye suspended in gelatine between glass plates. Coloured glass greater thermal stability. Cut-off filters: have transmittances nearly 100% over a portion of the visible region and then rapidly decrease to zero transmittance over the remainder :Figure 7-17 (p180) 6

7C-2: Monochromators Components of a monochromator 1. An entrance slit that provides a rectangular optical image 2. Collimating lens or mirror that produces a parallel beam 3.

7 7C-2: Monochromators Components of a monochromator 1. An entrance slit that provides a rectangular optical image 2. Collimating lens or mirror that produces a parallel beam 3. A prism/grating that disperses the radiation into its component wavelengths. By rotating the dispersing element, one band or the other can be focuses on the exit slit. Grating dispersion varies linearly with wavelength as opposed to the non-linear dispersion of a prism. 4. A re-focussing element that reforms the image of the entrance slit and focuses it on a planar surface called a focal plane 5. An exit slit in the focal plane that isolates the desired spectral band. 7

8 Lease rad up to end of Prism monochromators and Grating monochromators on p183. Leave Echellette Grating and up to end of Section 7C p190 8

9 7D Sample containers Sample holders, cuvettes, or the cells that will contain the sample during a measurement must be made of material that is transparent to radiation in the spectral region of interest. Quartz or fused silica required in for work in the UV region (below 350nm). Both Quartz and fused silica also suitable for the Visible and IR regions as well Silicate glasses can be used in the region between nm. Plastic containers can also be used in the visible region Crystalline NaCl most common material used for IR work 7E Radiation Transducers 7E-1: Introduction Early detection devices: human eye, photographic plat/film Transducers that convert radiant energy into an electrical signal have replaced the older detection devices Properties of the ideal Transducer: o Should have a sensitivity o High Signal-to-Noise ratio o A constant response over a wide range of wavelengths o It should exhibit a fast response o It should have a zero output signal in the absence of illumination (k d small/zero) o The electrical signal should be directly proportional to the radiant power, P. S=kP + k d (k d = zero) Types of radiation transducers o Two general types : Respond to photons Respond to heat 7E-2: Photon Transducers Several types: 1. Photovoltaic cells 2. Vacuum photo tubes 3. Photomultiplier tubes 4. Photoconductivity transducers 5. Silicon photo diodes 6. Charge-transfer transducers 9

10 7. These lectures covers only o Vacuum photo tubes (p193) and Photomultiplier tubes (p194) Vacuum photo tubes 10

11 Photomultiplier tubes Rest of Chapter 7 for reference reading 11

Basic Components of Spectroscopic. Instrumentation

Basic Components of Spectroscopic Ahmad Aqel Ifseisi Assistant Professor of Analytical Chemistry College of Science, Department of Chemistry King Saud University P.O. Box 2455 Riyadh 11451 Saudi Arabia