= m
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UV Light
Beer's Law
Diode Array
Deuterium Discharge Lamp
Low Pressure Zinc Discharge Lamp
High Pressure Mercury Discharge Lamp
Low Pressure Cadmium Lamp
Tungsten Halogen Lamp
Lens and Window Material in Spectrometers
Fluorescence Reagents
Diffraction Grating
Fourier Transform IR Spectrometer
Halide Disks
Mull Samples
Film Samples for IR Spectroscopy
Light Pipes
Attenuated Total Reflectance Spectroscopy
Multiple Internal Reflectance
External Reflectance
Specular Reflectance
Diffuse Reflectance
Photoacoustic Spectroscopy
Beam Splitter
Raman Scattering
Rayleigh Scattering
Raman Spectroscopy
Atomic Spectroscopy
Atomic Emission Spectroscopy
Atomic Absorption Spectroscopy
The Inductively Coupled Plasma Torch
The Helium Plasma Torch
Emission Spectrometer
Atomic Absorption Spectrometry
Flame Atomic Absorption Spectrometer
Flame AA
Hollow Cathode Lamp
Electrothermal Atomization
Graphite Furnace
L’vov Platform
Electron Paramagnetic Resonance
Zeeman Effect
Continuous Wave
Electron Paramagnetic Resonance
Pulsed EPR
Electron Spin Echo
Multple Resonance Spectroscopy
Magnetic Resonance Spectroscopy
Nucleus Spin Decoupling in NMR
Superconducting Magnets
NMR Microcells
Electron Impact Ionisation
Chemical Ionization
Inductively Coupled Plasma Ionization
Secondary Ion Mass Spectrometry
Fast Atom Bombardment
Plasma Desorption Mass Spectrometry
Laser Desorption Mass Spectrometry
Matrix Assisted Desorption mass Spectrometry
Field Desorption Ionization
Thermospray Ionization
Electrospray Ionization
Atmospheric Pressure Ionization
Particle Beam Interface
Permeable Membrane Interface
Sector Mass Spectrometer
Quadrupole Mass Spectrometer
Ion Trap Mass Spectrometer
Time of Flight Mass Spectrometer
Optical RotationCircular Dichroism
Circularly Polarized Light
Verdet Constant
Faraday Effect

UV and Visible spectrometer Light Sources

The Low Pressure UV Discharge lamps
Today there are many types of UV, visible, near infrared and infrared lamps available but only a selected few pertinent to UV and Visible light will be mentioned here. One of the early lamps used for the production of UV light were the low-pressure mercury vapour lamps, which generate the majority of its light, at a high intensity at 254 nm. There are a number of other lamps of similar type that can be used to provide UV light of a specific wavelength and they are the low-pressure cadmium lamp which generates the majority of its light at 225 nm and the low pressure zinc lamp that emits largely at 214 nm. None of the lamps emits strictly monochromatic light and light of other wavelengths is always present but usually at a significantly lower intensity. The emission spectra of the mercury , cadmium and zinc lamps are shown in figure 9
It is seen that if a completely monochromatic source of light is required, then an appropriate filter would be needed. The low-pressure mercury light source (major emission at wavelength 253.7 nm) is the lamp that provides the closest to true monochromatic light of all three lamps. However, it does provide light of significant intensity below 200 nm, but light of such wavelengths is often absorbed and eliminated by walls of the discharge tube unless very pure silica is used.
The zinc lamp has a major emission line at 213.9 but the emission line at 307.6 is also of comparable intensity and would probably need to be removed by a suitable filter if light of the lower wavelength was exclusively required. The cadmium lamp has a major emission line at 228.8 but light is emitted at both lower and higher wavelengths and so an appropriate filter would again be desirable. Suitable interference filters can be quite expensive to construct, which may account for the unpopularity of these two lamps.

Figure 9. emission Spectra for Three Discharge Lamps

They do, however, emit light at wavelengths, which would provide an increased sensitivity to substances such as proteins and peptide s, which might make their use worthwhile in the biotechnology field.
The deuterium Lamp
The deuterium lamp, in one of its many forms, is probably the most popular broadband UV emission lamps. A photograph of a deuterium light source and a deuterium lamp is shown in figure 10.

Deuterium Light Source

Deuteriun lamp

Courtesy of “Newport Resource and Spectra-Physics

Figure 10. deuterium Light Source and deuterium Lamp

Stray light can be a difficult problem in UV spectroscopy as many UV Light sources have a black body Spectrum with low UV and high Visible output. As a result, the intensity of the Visible light often exceeds the intensity of the real signal of the UV. However, with deuterium lamps the intense continuum up to 400 nm and the low Visible and infrared emission gives a high signal to noise ratio for the UV measurements. As a result, deuterium lamps are the preferred source for UV spectroscopy. mercury lamps can give an improved performance at specific wavelengths but for obvious reasons, they are not suitable for continuous Spectrum work. deuterium lamps have been constructed with a range of modifications to meet different specifications. For example the case of the lamp can be made of impure silica , which restricts the emission of the lower wavelengths (lower limit about 200 nm) and, thus, eliminates the production of ozone . Other lamps are designed to provide maximum emission others maximum stability. An example of the emission curves for a range of different deuterium lamps is shown in figure 11.

Courtesy of “Newport Resource and Spectra-Physics

Figure 11. emission Characteristics for a Range of Different deuterium Lamps.

Xenon Flash Lamps
Xenon lamps are exceedingly strong light sources and are widely employed in many photometric instruments. Flash lamps can have certain advantages over constant current DC operated lamps as they exhibit a relatively higher UV emission when operated in small pulses. Power supplies for such lamps must provide both high voltages and high currents and also a carefully controlled voltage pulse to strike the arc and, thus, initiate the discharge. The lamp is made of pure silica containing xenon or krypton or a mixture of both and contains three hermetically sealed electrodes. The lamp may be straight, U shaped, circular or spiral.
The anode is usually made of tungsten and the cathode of porous tungsten filled with a barium compound to reduce the work function of the electrode and, thus, increase its electron emission. Whereas DC operated lamps have pointed electrodes to keep heat away from the quartz envelope, flash tubes usually have cathodes with flat discharge surfaces Two electrodes carry the arc current and the other initiates the arc. The emission characteristics of Xenon lamps operated in the continuous and pulsed mode are shown in figure 12. It is seen that the pulsed lamp provides far more light at the low UV wavelengths than does the continuously operated lamp.
The pulsed Xenon lamp is frequently used as the excitation light source for the Fluorescence spectrometer. The discharge tubes used for the Xenon flash lamps often have carefully arranged guiding electrodes that ensure that the electric discharge has repeatable arc paths and, thus, ensures that a stable light output is achieved. Light stability can be maintained to within 2%.

Courtesy of “Newport Resource and Spectra-Physics

Figure 12. The emission Characteristics of Xenon Lamps Operated in the Continuous and Pulsed Mode

The Tungsten Halogen Lamp
The tungsten halogen lamps are the most popular Visible and near infra red light sources due to their smooth spectral emission curve and that they do not exhibit peak outputs at specific wavelengths as do other light sources. A diagram of a tungsten halogen emission bulb is shown in figure 13.

Courtesy of “Newport Resource and Spectra-Physics

Figure 13. A Tungsten Halogen Light Bulb

The filaments of most tungsten lamps are doped and the whole is contained in a quartz envelope. The envelope is filled with a rare gas containing a trace of a suitable halogen gas. The tungsten filament operates at about 3000K. Without doubt the Tungsten Halogen Lamp is the ‘work-horse’ of those light emitters used for Visible and near infrared spectroscopy.
The relative existence of the tungsten filament, a grey body radiator and a full radiator is shown in figure 14. It is seen that the tungsten surface is about equivalent to the grey body radiator and about 40% of the ideal full radiator. Tungsten is not the only metal that can be used for Visible and near infra red light emission but it appears to be the metal of choice for most spectrometer manufacturers. From the curve in figure 14 it is seen that the wavelength of the light emitted from the Tungsten Halogen Lamp comprehensively covers not only the Visible range of wavelengths but also the near infrared range as well.

Courtesy of “Newport Resource and Spectra-Physics

Figure 14. The Spectral Exitance of a Full Radiator, (1), A Tungsten Surface (2) and a Grey Body with an Emissivity of 0.425 at 3100 K.

The Lamda UV/Vis/NIR Spectrometers (950, 850 and 650)
The Lamda series of instruments are typical of those available for UV/vis/NIR spectroscopy applications. A diagram of the general layout of a Lamda instrument is shown in figure 15. This particular instrument was designed to accommodate a wide variety of sample types, including a range of sampling modules that simply clip into place as required. The sample and detector compartments can also be rapidly changed to suit the nature of the sample. The instrument can be fitted with deuterium and tungsten/halogen light sources and, to ensure low stray light, a double holographic grating monochromator is used. A common beam mask allows a precise adjustment of beam height to match samples of different dimensions and a common beam depolarizer permits the accurate measurement of bi-refringent samples. A chopper switches between sample and reference beam, and to accommodate highly absorbing samples, sample and reference beam attenuators are provided. A high sensitive photomultiplier and Peltier-controlled lead sulphide sensors are available for detection. A special universal reflectance accessory is also available that allows the angle of measurement to be changed with no adjustment to the sample or optics.

Courtesy of the Perkin Elmer Corporation
1.Deuterium an Tungsten/Halogen Light Source. 2. Double Holographic Grating3. Common Beam mask. 4.Common Beam Deploarizer. 5.Chopper.6. Sample and Reference Beam attenuators. 7. Sample compartment.8. Photomultiplier. 9. Second sampling area.

Figure 15. The General Layout of a Lamda UV/Vis/NIR spectrometer

The Perkin Elmer Lamda 25/35/45 series of UV/vis spectrometers are simpler and accurate alternatives but with no near infrared (NIR) facilities. The double beam system, that allows references to be measured in real time, provides high stability, and the sealed quartz coated optics ensures consistent lifetime performance. The spectrometers employ deuterium and tungsten lamps and fast scanning facilities are provided.
The results of some standard tests carried out on the instrument are shown in figures 16A and 16B.
The test utilizes a 0.02% w/v toluene in n-hexane with a 1nm slit width. The ratio of the peak and trough near 269 nm and 266 nm respectively was found to be greater than 1.9 the pass criterion being at least 1.5. In the wave length accuracy test using a 4mg/ml solution of holmium perchlorate a summary of the results are as follows.
Specified Value (nm) Measured Value (nm) Maximum Tolerance (nm) 241.15 241.10 +/-1.0 361.50 361.10 +/-1.0 536.30 536.40 +//3.0
It is seen the results are well within the required criteria.



Courtesy of the Perkin Elmer Corporation

Figures 16A and 16B. Results from the Pharmacopeia Resolution Tests and Wavelength Accuracy

Matrix Spectrometer
The Orial matrix spectrometer incorporates an interesting device that allows very weak signals in the 190-500 nm range to be measured with a resolution of 0.6 nm.

Courtesy of Newport Resource and Spectra-Physics

Figure 17. The Oriel Matrix spectrometer

This device is ideally suited for Fluorescence applications, thin film reflectance and atomic emission. A photograph of the instrument is shown in figure 17 and the optical layout in figure in figure 18. The capability of an optical system to transmit radiation is measured by the optical throughput or the Etendu. For any given optical system the etendue (E) is constant and is defined as the product of the entrance aperture (or slit area) and the solid angle through which the light is accepted.
Consider an input aperture that consists of an optical matrix containing N x N apertures.

Now if (Ω) is the solid input angle,
(f) is a factor that takes into account that only a fraction of the aperture elements are transparent,
(g) is a factor that takes into account that a number of pixels are discarded for one row of the mask

Then the etendue is given by E = f x g x N2 x Ω
For the systems being considered, f = 0.5, g = 3 and n = 24

Thus, for a slit the same height as the mask, E = 3 x 24 x 1 x Ω

For a fibre input (24 μm in diameter) E = 1 x 1 x Ω

For the Matrix Mask E = 0.5 x 3 x N x N x Ω = 864 Ω

It is seen that the etendue of the mask matrix is 12 time greater than a slit and 864 times greater than a single fibre.


Courtesy of Newport Resource and Spectra-Physics

Figure 18. The Optical Layout of the Oriel Matrix Spectrometer

The device has been given the (perhaps a little vainglorious) name Multimodal Multiplex Spectroscopy (MMS). Essentially, it consists of a 2.3mm x 0.58 mm encoded aperture mask, a concave grating, a cooled two-dimensional Diode Array and a compute data processing system.
The cooled aperture consists of 2304(28 x 32) lithographically etched apertures having dimensions 24 mm x 24 mm. The total perforated mask dimensions are 2.3 mm x 0.58 mm. Light enters the optical system through a mask providing a total of 2304 virtually point light sources. The etched mask must be fully illuminated by the source. A fixed concave grating is employed having a groove density of 200 lines per mm. The peak efficiency of the grating is at 300 nm. The sensor can be cooled to below 20oC to provide a good signal to noise ratio and takes the form of a back thinned 512 x 512 CCD (charged couple device) array (a sensor similar to that employed in a digital camera). Each pixel accepts light from 2304 point light sources. The dark current output is automatically taken before the measurements are started, the integration period is then set and the data collected. The software can process emission transmission, reflectance and adsorption data. The advantages of the etendue differences calculated above are demonstrated in figure 19. A slit/fibre spectrometer maps each spectral channel separately onto a pixel column in the two dimensional detector. However in the Multimodal Multiplex system each pixel in the detector measures several spectral channels simultaneously which results in a significant improvement in the signal-to-noise ratio. The improvement in signal to noise is clearly demonstrated by the results shown in figure 19.

Courtesy of ìNewport Resource and Spectra-Physics

Figure 19. Spectra Demonstrating the Advantages of the Encoded mask input Relative to the Optical Fibre and the Simple Slit.

The emission of a mercury lamp was measured employing three different configurations each with an integration time of 4 seconds. The top Spectrum represents the results from a 48 μm fibre input, the middle Spectrum from a 24 μm slit and the lower Spectrum from the MMS system. The results demonstrate the large improvement in the signal-to-noise ratio that is achieved by the Multimodal Multiplex system. The quantum efficiency of the system is shown in figure 20.

Courtesy of “Newport Resource and Spectra-Physics

Figure 20. Graph Showing the Quantum Efficiency of the Multimodal Multiplex System.


About the Author
RAYMOND PETER WILLIAM SCOTT was born on June 20 1924 in Erith, Kent, UK. He studied at the University of London, obtaining his B.Sc. degree in 1946 and his D.Sc. degree in 1960. After spending more than a decade at Benzole Producers, Ltd. Where he became head of the Physical Chemistry Laboratory, he moved to Unilever Research Laboratories as Manager of their Physical Chemistry department. In 1969 he became Director of Physical Chemistry at Hoffmann-La Roche, Nutley, NJ, U.S.A. and subsequently accepted the position of Director of the Applied Research Department at the Perkin-Elmer Corporation, Norwalk, CT, U.S.A.
In 1986 he became an independent consultant and was appointed Visiting Professor at Georgetown
University, Washington, DC, U.S.A. and at Berkbeck College of the University of London; in 1986 he retired but continues to write technical books dealing with various aspects of physical chemistry and physical chemical techniques. Dr. Scott has authored or co-authored over 200 peer reviewed scientific papers and authored, co-authored or edited over thirty books on various aspects of physical and analytical chemistry. Dr. Scott was a founding member of the British chromatography Society and received the American Chemical society Award in chromatography (1977), the M. S. Tswett chromatography Medal (1978), the Tswett chromatography Medal U.S.S.R., (1979), the A. J. P. Martin chromatography Award (1982) and the Royal Society of Chemistry Award in Analysis and Instrumentation (1988).
Dr. Scott’s activities in gas chromatography started at the inception of the technique, inventing the Heat of Combustion Detector (the precursor of the Flame Ionization Detector), pioneered work on high sensitivity detectors, high efficiency columns and presented fundamental treatments of the relationship between the theory and practice of the technique. He established the viability of the moving bed continuous preparative gas chromatography, examined both theoretically and experimentally those factors that controlled dispersion in packed beds and helped establish the gas chromatograph as a process monitoring instrument. Dr. Scott took and active part in the renaissance of liquid chromatography, was involved in the development of high performance liquid chromatography and invented the wire transport detector. He invented the liquid chromatography mass spectrometry transport interface, introduced micro-bore liquid chromatography columns and used them to provide columns of 750,000 theoretical plates and liquid chromatography separations in less than a second. Dr. Scott has always been a “hands-on” scientist with a remarkable record of accomplishments in chromatography ranging from hardware design to the development of fundamental theory. He has never shied away from questioning “conventional wisdom” and his original approach to problems has often produced significant breakthroughs.

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