Detectors light source intensity

Fig. 11. Schematic design of a fluorescence sensor. A strong light source creates radiation with low wavelengths. Optics like lenses and filters extract and focus the desired excitation light which is sent through the window into the measuring solution. Only a small fraction of the fluorescent light arrives at the window, passes this, and is collected by appropriate optics and fed to a sensitive detector (usually a photomultiplier). Variations in the light source intensity can be compensated by a comparative measurement. When optical fibers are used inside the instrument, the dichroitic mirror shown is obsolete...

Visible and UV spectrophotometers are by far the most frequently used type of detectors in FI systems. This is also true for FI separation systems. Provided the light source intensity is strong enough, a conventional batch spectrophotometer can easily be converted into a flow-through spectrophotometer by substituting the conventional cuvette with a flow-through cell. 

We can evaluate the impact of indeterminate error due to instrumental noise on the information obtained from transmittance measurements. The following discussion applies to UVWIS spectrometers operated in regions where the light source intensity is low or the detector sensitivity is low and to IR spectrometers where noise in the thermal detector is significant. 

Ideally, using the reference beam would make measuring the baseline unnecessary. In practice, due to differences in the optical elements efficiencies and alignment between the two paths the baseline correction is usually still necessary for accurate measurements. The main advantage of the dual beam spectrophotometer is to compensate for short term (seconds to minutes) variations on the light sources intensities and the efficiency of the optical comptments common to both beams. Most variations in this time scale are due to light source intensities and detector efficiency changes due to temperature variatirai. 

Direct photography of drops in done with the use of fiber optic probes using either direct or reflected light. StiU or video pictures can be obtained for detailed analysis. The light transmittance method uses three components a light source to provide a uniform collimated beam, a sensitive light detector, and an electronic circuit to measure the amplified output of the detector. The ratio of incident light intensity to transmitted intensity is related to interfacial area per unit volume. 

Light from an appropriate light source (a xenon arc or a halogen or tun ten lamp) passes through a monochromator (probe monochromator). The exit intensity at wavelength "k, IqCK), is focused onto the sample by means of a lens (or mirror). Tbe reflected light is collected by a second lens (mirror) and focused onto an appropriate detector (photomultiplier, photodiode, etc.). For simplicity, the two lenses (mirrors) are not shown in Figure 2. For modulated transmission the detector is placed behind the sample. 

Because the Raman cross-section of molecules is usually low, intense light sources and low-noise detectors must be used, and high sensitivities - as required for surface analysis - are difficult to achieve. Different approaches, singly and in combination, enable the detection of Raman spectroscopy bands from surfaces. 

The simplest fluorescence measurement is that of intensity of emission, and most on-line detectors are restricted to this capability. Fluorescence, however, has been used to measure a number of molecular properties. Shifts in the fluorescence spectrum may indicate changes in the hydrophobicity of the fluorophore environment. The lifetime of a fluorescent state is often related to the mobility of the fluorophore. If a polarized light source is used, the emitted light may retain some degree of polarization. If the molecular rotation is far faster than the lifetime of the excited state, all polarization will be lost. If rotation is slow, however, some polarization may be retained. The polarization can be related to the rate of macromolecular tumbling, which, in turn, is related to the molecular size. Time-resolved and polarized fluorescence detectors require special excitation systems and highly sensitive detection systems and have not been commonly adapted for on-line use. 

Ratio imaging nicely cancels out some of the main complications in the interpretation of wide-field images in that it normalizes fluorescence intensity differences caused by for example, cell height (Fig. 7.T1) as well as possible slow drift in excitation intensity. Light sources invariably are much less stable than detectors. Incidentally, for these reasons emission ratio imaging has been applied for over 3 decades by the Ca2+ imaging community. 

Nonetheless, near-IR is the most widely used IR technique. Less intense water absorptions permit to increase the sampling volume to compensate, to some extent, for the lower near-IR absorption coefficients and the inferior specificity of the absorption bands can for many applications be overcome by application of advanced chemometric methods. Miniaturised light sources, various sensor probes, in particular based on transmission or transflectance layouts, and detectors for this spectral range are available at competitive prices, as are (telecommunications) glass or quartz fibres. 

Optical methods are especially useful for the selective detection of CO and C02 concentrations. In low-priced sensors, a simple miniature light bulb is used as IR-source. The radiation emitted enters an absorption chamber, through which the flue gas is pumped. An added interference filter lets only the absorption spectra of the target gas pass. The IR detector determines the reduction of the light intensity, which is then transformed into an electrical signal. The correlation between the source intensity and the received intensity is given in the Lambert-Beer equation. 

A problem encountered with atomic absorption is that emission from the flame may fall on the detector and be registered as negative absorption. This can be eliminated by modulating the light source, either mechanically or electronically, and using an a.c. detector tuned to the frequency of modulation of the source. D. C. radiation, such as emission from the flame, will then not be detected. A high intensity of emission, however, may overload the detector, causing noise fluctuations. 

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