Photomultiplier tubes spectrophotometers

Photomultiplier tubes or photodiodes (light sensors) are used as detectors in UV-VIS spectrophotometers, while thermcouples (heat sensors) are used as detectors for infrared (IR) spectrometry. This is the reason UV-VIS instruments are called spectrophotometers while IR instrument are called spectrometers. 

Except for the movable photomultiplier tube, a light scattering photometer is very nearly identical to an ordinary spectrophotometer, which measures the ratio of the intensity of transmitted light to the intensity of incident light /,//0. The absorbance per unit optical path cabs is defined in terms of this quantity as... 

The photomultiplier tube a very sensitive device that has a linear response over seven decades - has for a long time been the most widely used detector in spectrophotometers. Its efficiency depends on the yield of the photocathode, which varies with wavelength (e.g. 0.1 e /photon at 750 nm). and with the signal gain provided by the dynode cascade (e.g. gain of 6 x 105). With such values, the impact of 10000 photons per second produces a current of 0.1 nA. 

In routine spectrophotometers, photomultiplier tubes are replaced by photodiodes (Fig. 11.11), which have excellent sensitivity, linearity and dynamic range. The photoelectric threshold, in the order of 1 eV, allows detection up to wavelengths of 1.1 pm. In diode array systems, each rectangular rectangular diode (15 pm x 2.5 mm) is associated with a capacitor. The electronic circuit sequentially samples the charge of each capacitor. While a photomultiplier tube measures the instant intensity in watts, a diode measures the emitted energy in joules over a time interval. 

As with any spectrophotometer instrument, the light intensity is determined at each wavelength using a photomultiplier tube. Emission measurements can be made over a wide dynamic range, which is an interesting feature because elements with widely different concentrations or sensitivity can be measured in a single sample solution. 

Phosphors for cathode-ray tubes, television screens, monitor screens, radar screens, and oscilloscopes are tested under electron excitation. Electron energy and density should be similar to the conditions of the tube in which the screen will be used. The phosphors are sedimented or brushed onto light-permeable screens and coated with an evaporated aluminum coating to dissipate charge. The luminescence brightness and color of the emitted light are measured with optical instruments such as photomultipliers or spectrophotometers. 

A Perkin-Elmer MPF-2A Fluorescence Spectrophotometer was used to determine the excitation and emission wavelengths required for achieving maximum fluorescence intensity for the pesticides studied. The MPF-2A contained a 150 watt xenon arc and an excitation monochromator with a grating blazed at 300 nm as the excitation unit a Hamamatsu R 777 photomultiplier tube (sensitivity range 185 - 850 nm) and an emission monochromator grating blazed at 300 nm as the emission detection unit. A DuPont Model 848 Liquid Chromatograph was used for HPLC (Figure 2). The accessory injection device included a Rheodyne Model 70-10 six-port sample injection valve fitted with a 20 y liter sample loop. A Whatman HPLC column 4.6 mm x 25 cm that contained Partisil PXS 1025 PAC (a bonded cyano-amino polar phase unspecified by the manufacturer) was used with various mobile phases at ambient temperature and a flowrate of 1.25 ml/minute. 

The detection system of a spectrophotometer is a light sensitive photomultiplier tube. Operation of this device is described in detail in Chapter 3 and is not, therefore, discussed here. In instruments with a broad spectral range it is usually necessary to use two different photomultipliers, one most sensitive between 200 and 600 nm and a second sensitive to... 

For absorption measurements, the authors used a Cary recording spectrophotometer in which the absorbances were displayed directly on a recorder chart as the different spectral regions were scanned. In this instrument, the absorption cell is located between the exit slit and photomultiplier tube detector, thus minimizing any photochemical effect during the measurement. Corrections were applied to the recorded absorbances for any ozone decomposition during the time of measurement. 

Once the electrical signal leaves the photomultiplier tube, it is fed to a recorder if a printout is required, or, more usually, to a screen where the absorption spectrum can be displayed. Most modern spectrophotometers are now interfaced to a personal computer to allow storage of large amounts of data, or to allow access to a library of stored spectra on the hard drive of the machine. This allows comparison of stored spectra with the experimentally derived results from the laboratory and aids in the identification of unknown compounds. 

Spectra and intensity measurements were made with a 0.5 meter Jarrell-Ash Ebert spectrophotometer using an RCA 1P21 photomultiplier tube. A pair of large condensing lenses served to focus the image of a cross section of the flame on the entrance slit. The spectrophotometer was mounted on a table which could be moved at constant speed parallel to the axis of the reaction cell. In this way the intensity of the flame was recorded as a function of distance along the tube. 

Figure 14.10 Optical scheme for a spectrophotometer with an echelle grating. For clarity only the central section of the beam issuing from source 1 is represented (this beam should cover the whole mirror 2). The echelle grating 5, separates the radiations arriving from the source in the horizontal plane (in x). The prism then deviates the radiation in the vertical plane (in y). The path of three different spectral lines is shown. The images of the entrance aperture 2 are in the focal plane 8. In the past, to detect these radiations, photomultiplier tubes (PMT) of reduced size were installed in specific places, but now charge transfer devices (charge coupled or charge injection devices, CCD/CID) are used, as an electronic equivalent of photographic plates. This allows a continuous spectral cover from 190 to 800 nm with excellent resolution. Sensors of 500 X 2000 pixels (each 12 x 12p,m) are now used.

Classical transducers such as the photomultiplier tube and the photodiode have been used in most the flow systems [96,97], Since the 1980s, the tendency to use diode array spectrophotometers has increased, mainly for simultaneous determinations [98] and/or for implementing dual wavelength spectrophotometry [99], the latter being more exploited in segmented flow analysis. 

Spectrometers that use phototubes or photomultiplier tubes (or diode arrays) as detectors are generally called spectrophotometers, and the corresponding measurement is called spectrophotometry. More strictly speaking, the journal Analytical Chemistry defines a spectrophotometer as a spectrometer that measures the ratio of the radiant power of two beams, that is, PIPq, and so it can record absorbance. The two beams may be measured simultaneously or separately, as in a double-beam or a single-beam instrument—see below. Phototube and photomultiplier instruments in practice are almost always used in this maimer. An exception is when the radiation source is replaced by a radiating sample whose spectrum and intensity are to be measured, as in fluorescence spectrometry—see below. If the prism or grating monochromator in a spectrophotometer is replaced by an optical filter that passes a narrow band of wavelengths, the instrument may be called a photometer. 

So, using this approach, for a UV-Vis spectrophotometer, the source is a deuterium/ tungsten lamp, the sample is the cuvette or flowcell, the discriminator is the monochromator, the detector is a photomultiplier tube and the output device is a computer with an analogue-to-digital converter (Figure 1.2). 

Optical polished Nd Lu20j transparent samples were used for the spectroscopic measurement. Linear optical transmittance of 3at.%Nd Lu203 transparent ceramic was measured in region of 190-1100 nm on a UV. VIS/NIR spectrophotometer (Lambda 2, Perkin Elmer, U.S.A.). The fluorescence spectrum of the specimen was recorded by a spectrofluorometer (Fluorolog-3, Jobin Yvon, Edision, U.S.A.) equipped with a Hamamatsu R928 photomultiplier tube. A 808nm continuous wave diode laser was used as the excitation source. 

Electronic Absorption and Emission Spectroscopy. UV and visible spectra were recorded on Cary 14, Cary 171, or Perkin-Elmer 576 ST spectrophotometers. Luminescence excitation and emission spectra. were recorded on an Hitachi-Perkin-Elmer MPF-2A spectrofluorimeter equipped with a red-sensitive Hamamatsu R-446 photomultiplier tube. Conventional flash photolysis experiments were performed as described previously (41). The samples were degassed by several cycles of freeze-pump-thaw and sealed under vacuum. 

The spectra were not corrected for the spectral variation in the excitation source and photomultiplier tube sensitivity. Background scattered light was zeroed electronically before the acquisition of the fluorescence of 4-methylcoumaro-[222]cryptand. Absorption spectra were obtained using Perkin Elmer Lambda 7 UV/VIS Spectrophotometer. 

R. sphaeroides wild-type strain NCIB 8253 was grown photoheterotrophically at 30°C under various light intensities as described by Holmes et ai (14). Near-IR absorption spectra were obtained at 295 K on a Johnson Research Foundation DBS-3 spectrophotometer equipped with a Hamamatsu R-406 photomultiplier tube. Total Bacteriochlorophyll (BChl) was determined in acetone methanol water (7 2 1, vol/vol) using the extinction coefficient of Clayton (15). 

A Raman spectrophotometer analyzes the radiation scattered by molecules when they are illuminated with monochromatic exciting radiation. Currently, most Raman spectrometers are either FT instruments equipped with cooled germanium transducers or multichannel instruments based upon CCDs. These transducers, in contrast to photomultiplier tubes, are sensitive to radiation at 785 nm produced by diode lasers, which provide Raman excitation of many compounds without significant fluorescence. CCDs are not sensitive to the 1064-nm radiation from an Nd/YAG laser. 

Figure 2 Properties of ideal turbidimeter and spectrophotometers. For a clear sample, the position of the detector (PMT, photomultiplier tube) is unimportant (A). If a turbid sample is examined in a spectrophotometer where the PMT is far removed from the cuvette (B), the instrument will behave as a sensitive turbidimeter because it will detect little of the scattered light. If a turbid sample is examined in a spectrophotometer where the PMT is close to the cuvette (C). the instrument will detect absorbance even though much of the light is scattered.

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