Photometric analysis automated

Small amounts of fructose on the surface of anhydrous glucose crystals prepared from sucrose have been measured by selective dissolution, reaction with a modified diphenylamine reagent that is 147 times more sensitive to fructose than glucose, and spectro-photometric analysis at 640 An automated Elson-Morgan... 

Various approaches to the analysis of dissolved silicon have been tried. Most of them are based on the formation of /J-molybdosilic acid [ 199-203 ]. Dissolved silicon exists in seawater almost entirely as undissociated orthosilicic acid. This form and its dimer, termed reactive silicate , combine with molybdosilicic acid to form a- and /I-molybdosilicic acid [180]. The molybdosilicic acid can be reduced to molybdenum blue, which is determined photometrically [206]. The photometric determination of silicate as molybdenum blue is sufficiently sensitive for most seawater samples. It is amenable to automated analysis by segmented continuous flow analysers [206-208]. Most recent analyses of silicate in seawater have, therefore, used this chemistry. Furthermore, reactive silicate is probably the only silicon species in seawater that can be used by siliceous organisms [204]. 

In general, the apparatus for titrimetric analysis is simple in construction and operation. A typical analysis procedure would involve measurement of the amount of sample either by mass or volume, and then addition of the titrant from a burette or micro-syringe. Apart from visual indication, the course of a titration may be followed by electrochemical or photometric means in neither is the equipment required complex. A simple valve voltmeter or conductivity bridge will suffice on the one hand, and a simple spectrophotometer or filter photometer with minor modifications on the other. Varying degrees of automation may be incorporated. 

The determination of ascorbic acid in foods is based, in part, on its ability to be oxidized or to act as a reducing agent. The most common method for determination of vitamin C in foods is the visual titration of the reduced form with 2,6-dichloroindophenol (DCIP) (4-7). Variations in this procedure include the use of a potentiometric titration (6), or a photometric adaptation (S) to reduce the diflSculty of visually determining the endpoint in a colored extract. The major criticisms of this technique are that only the reduced vitamin, and not the total vitamin C content of the food, is measured, and that there can be interference from other reducing agents, such as sulfhydryl compounds, reductones, and reduced metals (Fe, Sn, Cu), often present in foods. The DCIP assay can be modified to minimize the effects of the interfering basic substances, but the measurement is still only of the reduced form. Egberg et al. (9) adapted the photometric DCIP assay to an automated procedure for continuous analysis of vitamin C in food extracts. 

In Section 8B-6 we described various automated sample handling techniques including discrete and continuous flow methods. In this section, we explore the instrumentation and two applications of flow-injection analysis with photometric detection. 

Figure 26-18 is a diagram of a dialysis module in which analyte ions or small molecules diffuse from the sample solution through a membrane into a reagent stream, which often contains a species that reacts with the analyte to form a colored product, which can then be determined photometrically. Large molecules, which interfere in the determination, remain in the original stream and are carried to waste. The membrane is supported between two Teflon plates in which complementary channels have been cut to accommodate the two stream flows on opposite sides of the membrane. The transfer of smaller species through the membrane is usually incomplete (often less than 50%). Thus, successful quantitative analysis requires close control of temperature and flow rates for both samples and standards. Such control is easily accomplished in automated flow-injection systems. 

GC can achieve the highest resolution of the essential oils, but there are some significant limitations with regards to preparative scale separations. Typically, as the sample capacity is increased, the resolution of the chromatographic separation is reduced. On a lab scale, equipment is available that permits 24-hour automated and unattended separations, however, the recovery yield and sample resolution are still problematic [57]. Capillary column GC has become so routine for essential oil analysis that one rarely finds a lab without that capability. A multitude of detectors exist for GC thermal conductivity (TCD), flame ionization (FID), flame photometric (FPD), thermionic specific (TSD), photoionization (PID), electron capture (ECD), atomic emission (AED), mass spectrometry (MS), and infrared spectroscopy (FTIR) [58,59]. The TCD is used primarily with preparative-GC (packed column) because it is... 

Automated wet analytical reactions can be performed in autoanalyzer systems to monitor intracellular compounds [167]. Ahlmann et al. reported the development of an on-line system for analysis of intracellular penicillin G amidase produced by genetically modified bacteria. The time delay between sampling and photometric detection was 30 min. The whole system included automated sampling, cell disruption, and enzyme activity determination [167]. 

The analysis of phosphorus in waters has historically been based on the photometric measurement of 12-phosphomolybdate or the phosphomolybdenum blue species, which are produced when phosphomolybdate is reduced. The majority of manual and automated methods of phosphate determination are based on the spectrophotometric determination of phosphorus as phosphomolybdenum blue, i.e.. 

Flame photometric detection allows a fast and fully automated means for sulfur analysis. Typical limits of detection obtainable are 1.2 ng for solids and 0.2 ng for liquids. 

---