Ultraviolet/visible photometer

All compounds with UV or visible (VIS) range absorbance Ultraviolet/visible photometer PAHs (EPA 8310) Nitroaromatics, nitramines (EPA 8330) Nitroglycerine (EPA 8332) —Detection based on a single wavelength is prone to interferences. —Does not have the selectivity for positive compound identification. 

Table 16-1 gives the rough limits of each of the three regions. Measurements in the near-lR region are often made with photometers and spectrophotometers similar in design and components to the instruments described in earlier chapters for ultraviolet-visible spectrometry. The most important applications of this spectral region have been to the quantitative analvsis of industrial and agricultural materials and for process control. Applications of near-IR spectrometry are discussed in Section 17D. 

We use the term colorimeter to designate an instrument for absorption measurements in which the human eye serves as the detector using one or more color-comparison standards. A photometer consists of a source, a filler, and a photoelectric transducer as well as a signal processor and readout. Note that some scientists and instrument manufacturers refer to photometers as colorimeters or photoelectric colorimeters. Filter photometers are commercially available for absorption measurements in the ultraviolet, visible, and infrared regions, as well as emission and fluorescence in the first two wavelength regions. Photometers designed for fluorescence measurements arc also called fluorometers. 

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. 

Ultraviolet I visible UVjVIS) photometer—a non-selective detector that responds to a majority of organic compounds... 

UV/Visibie detector. A photometer used in LC. This detector measures changes in the absorbance of ultraviolet (UV) or visible (VIS) light resulting as the components pass through the detector. This is a sensitive and specific detection technique for compounds that contain chromaphoric groups. The wavelength of absorbance can be chosen to enhance specificity and/or sensitivity for a specific compound. 

By definition, photometers do not respond to radiation in the infrared or the ultraviolet (Fig. 4-4a). They are light meters in the sense that they mimic human vision that is, they respond to photons in the visible region, similar to the light meter on a camera. A candle is a unit of luminous intensity, originally based on a standard candle or lamp. The current international unit is called a candela (sometimes still referred to as a candle ), which was previously defined as the total light intensity of 1.67 mm2 of a blackbody radiator (one that radiates maximally) at the melting temperature of pure platinum (2042 K). In 1979 the candela was redefined as the luminous intensity of a monochromatic source with a frequency of 5.40 x 1014 cycles s-1 (A, of 555 nm) emitting 0.01840 Js-1 or 0.01840 W (1.464 mW steradian-1, where W is the abbreviation for watt and steradian... 

Over the past decade, there has been considerable development in imaging type detectors for the measurement of ultraviolet (UV) and visible light. These new detectors have attracted the interest of a number of analytical spectroscopists. For absorption spectroscopy, analytical chemists have traditionally used such instruments as the photometer, which uses a narrow-band light source (for example the 254 nm emission line from a low pressure Hg lamp or a continuous source with a filter), a sample cell and a photomultiplier tube (FMT) as the detector. While useful for many specific applications, the single-wavelength photometer cannot determine multiple sample components simultaneously or provide a general absorbance characterization of the system. When information at multiple wavelengths is desired,... 

Case I uncertainties often appear in less expensive ultraviolet and visible specirophoiomeiers or photometers equipped with meters or digital readouts with limited resolution. For example, some digital instruments have 3 /i-digit displays. These can display the result to 0.1%r. Here, the readout resolution can limit the measurement precision such that the absolute uncertainty in T s the same from 1)%T to 100% T. A similar limitation occurs with older analog instruments with limited meter resolution. 

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