Fiber-optic photometer

Experimental arrangement for continous monitoring of inhibitors of acethyl-cholinesterase (AChE) in drinking water. The enzyme converts the yellow substrate into a blue product. If the AChE is inhibited, the blue dye no longer is formed and this is measured via the fiber-optic photometer. 

Apparatus. The fiber optic photometer is similar to the instrument used in earlier work (2). It includes a tungsten-halogen source, a photomultiplier detector, interference filters for wavelength selection and a bifurcated fiber optic bundle 3 mm in diameter at the common end. In this study, excitation filters had peak transmittances at 420 and 480 nm and the emission filter had peak transmittance at 520 nm. The bandwiths at half-maximum transmittance were 9.4, 8.2 and 8.6 nm for the 420, 480 and 520 nm filters, respectively. An automated filter wheel allowed for rapid switching from one filter to another. 

Instrnmentation for UV-vis process analysis falls into fonr categories scanning instruments, diode-array instrnments, photometers, and fiber-optic spectrophotometers with photodiode-array (PDA) and charge-conpled device (CCD) detectors. The former two are more typically enconntered in at-line or near-line applications, whereas the latter two are better snited to actnal on-line analyses. 

The measurements by ultraviolet analyzers can utilize (1) single-beam (2) split-beam (3) dual-beam, single-detector (4) dual-beam, dual-detector (5) flicker photometer (6) photodiode and (7) retroreflector designs. The standard errors of these measurements are 2% FS, whereas it is 1% FS for the fiber-optic diode-array designs. These analyzers can handle process pressures up to 50 barg (750 psig) and temperatures up to 450°C (800°F). 

Figure 5.19. Progress in miniaturization of spectrophotometric flowthrough cells, (a) Hellma cuvette, which fits into most commercial photometers, b) Z-cell, where A are the transparent windows B, Teflon made body C, cell house, and CH, the inlet channel, (c) A fiber optic reflectance cell for optosensing at active surfaces [848], where CH is the channel of a microconduit, R is the chemically active reflecting material, OF is the optical fiber.

Figure I3-I6b is a schematic representation of a double-beam photometer used to measure the absorbance of a sample in a flowing stream. Here, the light beam is split by a iwo-branched (bifurcated) fiber optic, which transmits about 50% of the radiation striking it in the upper arm and about. 50% imho lower arm. One beam passes through the sample, and the other passes through the reference cell. Filters are placed after the cells before the photodiode transducers. Note that this is the double-bcam-in-spacc design, which requires photodiodes with nearly identical response. The electrical outputs from the two photodiodes arc converted...

FIQURE 13-16 Single-beam photometer (a) and double-beam photometer for flow analysis (b). In the single-beam system, the reference cell is first placed in the light path and later replaced by the sample cell. In the double-beam system (b). a fiber optic splits the beam into two branches. One passes through the sample cell and the other through the reference cell. 

NIR is widely used for rapid and nondestructive analysis in industries such as agriculture, food, pharmaceuticals, textiles, cosmetics, and polymer production. NIR has several advantages. In addition to nondestructive and rapid analysis, several components can be assayed simultaneously. The sample is supplied to the photometer without pretreatment. Both dry and wet materials can be accepted as a sample. Therefore, both aqueous and solid fermented samples can be accepted. NIR is an assay technique with high precision, and on-line measurement is available with fiber optics. These characteristics of NIR are an excellent match for the requirements of monitoring systems for the fermentation process. NIR will be increasingly apt for fermentation process monitoring. 

The use of ultraviolet (UV) spectroscopy for on-line analysis is a relatively recent development. Previously, on-line analysis in the UV-visible (UV-vis) region of the electromagnetic spectrum was limited to visible light applications such as color measurement, or chemical concentration measurements made with filter photometers. Three advances of the past two decades have propelled UV spectroscopy into the realm of on-line measurement and opened up a variety of new applications for both on-line UV and visible spectroscopy. These advances are high-quality UV-grade optical fiber, sensitive and affordable array detectors, and chemometrics. 

In 1986, Foret et al.41 described an on-line UV absorbance detector that employed a commercial photometer and optical fibers in direct contact with the outer walls of the separation capillary. The optical fibers (200 (im I.D. fused silica core) conducted the light beam perpendicularly across the migrating zones one fiber was connected to a mercury lamp to serve as the illumination source, and the other directed light to a photomultiplier tube for detection. The detector was found to be linear in the range of 10"5 to 10 3 M (r = 0.994 for 10 measurements), with detection limits of 1 X 10 5 M for picric acid (S/N = 2). 

Figure 2 Optical configuration for antibody-based FOGS (A) light source (laser In this work), (B) shutter, (C) beam splitter, (D) focusing lenses, (E) Incident end of fiber In a positioner, (F) sensing end of fiber, (G) monochromator/PMT, (H) photometer, and (l) recorder.

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