Photometric detectors spectrophotometers

Photometric detectors are widely used recent versions (T2) require only small volumes (less than 10 nl) of eluate in a flow cell, and the sample may be recovered. A spectrophotometer can be used to produce... 

Flow techniques use a wide range of photometric detectors from simple colorimeters to CCD and diode array spectrophotometers. While photometers typically available in the laboratory are often adapted to flow systems by using an appropriate cell, a number of specially designed detectors are also currently available. 

Unlike the filter photometric detector, which is limited to one or a few wavelengths, the spectrophotometers can choose the wavelength that is closest to an absorption maximum of the analyte(s) in order to obtain the highest sensitivity. 

A photometric flow-through sensor for the determination of carbamate pesticides (carbofuran, propoxur and carbaryl) based on similar principles as regards the detector and sensor used (a diode array spectrophotometer and a flow-cell packed with C,g resin, respectively) was employed to monitor the formation of the products resulting from hydrolysis of the analytes and online coupling of the respective phenols with diazotized sulphanilic acid. This... 

Photometric titrations are usually performed with a spectrophotometer or a photometer that has been modified so that the titration vessel is held stationary in the light path. After the instrument is set to a suitable wavelength or an appropriate filter is inserted, the 0%7 adjustment is made in the usual way. With radiation passing through the analyte solution to the detector, the instrument is then adjusted to a... 

Tests based on visual comparison of a standard and a sample were named colorimetric analyses, whereas photometric analysis make used of a photometer. Spectrophotometric analysis is based on spectrophotometers with incorporated monochromoters and detector. 

Since this is a book concerned primarily with applications, no further details are given concerning instrumentation. The reader is referred to Alpert et al. (1970), in which are discussed an optical diagram of a double-beam spectrophotometer operating variables (resolution, photometric accuracy) components of infrared spectrophotometers (sources, types of photometers, dispersing elements, detectors, amplifiers, and recorders) special operating features, such as optimization of scan time and available instruments and their specifications. The books by Martin (1966), Conn and Avery (1960), and Potts (1963), and the chapter by Herscher (1966) are also recommended for details on some of these topics. 

Estimation of true vitamin E in foods requires quantitative determination of all its components since they vary in their biological potency. This vitamin consists of four tocopherols (a, jS, y, and 6) and four tocotrienols (a, jS, y, and d), but the three major constituents responsible for vitamin E activity are the a-, jS-, and y-tocopherols. While these compounds are fluorescent, their esters must be reduced to free alcohols for total tocopherol assays. Total vitamin E can be directly obtained through fluorimetry, but the determination of individual components is carried out using LC with fluorimetric detection. This procedure has been used to determine the composition of vitamin E in seed oils from maize, olives, soya beans, sesame, safflower, and sunflower by measuring the content of all the four tocopherols plus a-tocotrienol. The simultaneous determination of tocopherols, carotenes, and retinol in cheese has been carried out using LC with two programmable detectors coimected in series, a spectrophotometer and a fluorimeter. Carotenes have been determined photometrically, and fluorimetric measurements have been obtained for tocopherol and retinol. 

The first detector to be used for SFA was a photometer, and photometric determinations still form the vast majority of current methods. Other detectors in common use are UV spectrophotometers, used primarily for pharmaceutical compovmds and for bitterness in beer flame photometers, for potassium and sodium determination fluorimeters, used primarily for measuring low levels of determinants in the presence of interferences, such as the determination of histamine in blood, and vitamins in food extracts and ion-selective electrode and pH detectors. In principle, almost any detector with flow-through capability can be used with SFA systems, and determinations based on densitometry, thermometry, and luminescence have been published, among others. 

For routine HPLC analysis, the detection of flavins is carried out either spectro-photometrically, using variable- or fixed-wavelength HPLC detectors in the ultraviolet (e.g., 254 nm) or visible (e.g., 405 nm) region, or fluorimetrically. For riboflavin, the excitation wavelength for fluorimetric detection is usually 440 to 450 nm, and the emission wavelength 530 nm. The detection limit for fluorescence detectors is >1 pmol (0.38 ng) riboflavin, whereas <30 pmol (11 ng) can be detected spectrophotometrically at 254 nm. Photodiode array detectors are significantly less sensitive than normal HPLC spectrophotometers (38). 

---