Continuous-flow analyzers

Accoe, E, Berglund, M., Geypens, B., Taylor, P. (2008) Methods to reduce interference effects in thermal conversion elemental analyzer/continuous flow isotope ratio mass spectrometry 5 180 measurements of nitrogen-containing compounds. Rapid Communications in Mass Spectrometry, 22(14), 2280-2286. 

Flow injection analysis (FIA) was developed in the mid-1970s as a highly efficient technique for the automated analyses of samples. °> Unlike the centrifugal analyzer described earlier in this chapter, in which samples are simultaneously analyzed in batches of limited size, FIA allows for the rapid, sequential analysis of an unlimited number of samples. FIA is one member of a class of techniques called continuous-flow analyzers, in which samples are introduced sequentially at regular intervals into a liquid carrier stream that transports the samples to the detector. ... 

By passing a continuous flow of solvent (admixed with a matrix material) from an LC column to a target area on the end of a probe tip and then bombarding the target with fast atoms or ions, secondary positive or negative ions are ejected from the surface of the liquid. These ions are then extracted into the analyzer of a mass spectrometer for measurement of a mass spectrum. As mixture components emerge from the LC column, their mass spectra are obtained. 

Assay of Enzymes In body fluids, enzyme levels aie measured to help in diagnosis and for monitoiing treatment of disease. Some enzymes or isoenzymes are predominant only in a particular tissue. When such tissues are damaged because of a disease, these enzymes or isoenzymes are Hberated and there is an increase in the level of the enzyme in the semm. Enzyme levels are deterrnined by the kinetic methods described, ie, the assays are set up so that the enzyme concentration is rate-limiting. The continuous flow analyzers, introduced in the early 1960s, solved the problem of the high workload of clinical laboratories. In this method, reaction velocity is measured rapidly the change in absorbance may be very small, but within the capabiUty of advanced kinetic analyzers. 

Dimensional Analysis. Dimensional analysis can be helpful in analyzing reactor performance and developing scale-up criteria. Seven dimensionless groups used in generalized rate equations for continuous flow reaction systems are Hsted in Table 4. Other dimensionless groups apply in specific situations (58—61). Compromising assumptions are often necessary, and their vaHdation must be estabHshed experimentally or by analogy to previously studied systems. 

Differential and Integral Balances. Two types of material balances, differential and integral, are applied in analyzing chemical processes. The differential mass balance is valid at any instant in time, with each term representing a rate (i.e., mass per unit time). A general differential material balance may be written on any material involved in any transient process, including semibatch and unsteady-state continuous flow processes ... 

The Pt film, with a surface area corresponding to NG=4.2-10 9 mol Pt, measured via surface titration of oxygen with C2H4,1,4 is exposed to po2 — 4.6 kPa, PC2H4 = 0.36 kPa at 370°C in a continuous flow gradientless (CSTR) reactor of volume 30 cm3. The rate of C02 formation is monitored via an infrared analyzer.1,4... 

The reaction test was carried out at atmospheric pressure using a vertical continuous flow fixed bed reactor. The content of effluent gas was analyzed by a gas chromatograph (HP 5890). 

Catalytic activity for the selective oxidation of H2S was tested by a continuous flow reaction in a fixed-bed quartz tube reactor with 0.5 inch inside diameter. Gaseous H2S, O2, H2, CO, CO2 and N2 were used without further purification. Water vapor (H2O) was introduced by passing N2 through a saturator. Reaction test was conducted at a pressure of 101 kPa and in the temperature range of 150 to 300 °C on a 0.6 gram catalyst sample. Gas flow rates were controlled by a mass flow controller (Brooks, 5850 TR) and the gas compositions were analyzed by an on-line gas chromotograph equipped with a chromosil 310 coliunn and a thermal conductivity detector. 

CK catalyzes the reversible phosphorylation of creatine in the presence of ATP and magnesium. When creatine phosphate is the substrate, the resulting creatine can be measured as the ninhydrin fluorescent compound, as in the continuous flow Auto Analyzer method. Kinetic methods based on coupled enzymatic reactions are also popular. Tanzer and Gilvarg (40) developed a kinetic method using the two exogenous enzymes pyruvate kinase and lactate dehydrogenase to measure the CK rate by following the oxidation of NADH. In this procedure the main reaction is run in a less favorable direction. 

The cracking of diphenylmethane (DPM) was carried out in a continuous-flow tubular reactor. The liquid feed contained 29.5 wt.% of DPM (Fluka, >99%), 70% of n-dodecane (Aldrich, >99% solvent) and 0.5% of benzothiophene (Aldrich, 95% source of H2S, to keep the catalyst sulfided during the reaction). The temperature was 673 K and the total pressure 50 bar. The liquid feed flow rate was 16.5 ml.h and the H2 flow rate 24 l.h (STP). The catalytic bed consisted of 1.0 g of catalyst diluted with enough carborundum (Prolabo, 0.34 mm) to reach a final volume of 4 cm. The effluent of the reactor was condensed at high pressure. Liquid samples were taken at regular intervals and analyzed by gas chromatography, using an Intersmat IGC 120 FL, equipped with a flame ionization detector and a capillary column (Alltech CP-Sil-SCB). 

One of the most far reaching analyzes along these lines of thought was given by Commenge [114] in the context of gas-phase reactions in continuous-flow processes. Specifically, he analyzed four different aspects of micro reaction devices, namely the expenditure in mechanical energy, the residence-time distribution, safety in operation, and the potential for size reduction when the efficiency is kept fixed. 

Catalytic tests were performed in a gas-phase continuous-flow reactor. The outlet flow of the reactor was either sampled for the analysis of the gaseous components, or condensed in a dry frozen trap, for the analysis of the solid and liquid products. Two liquid layers formed an organic layer containing the unconverted n-hexane, and an aqueous layer, some products dissolved preferentially in the organic layer, others in the aqueous one. Both layers were analyzed by gas chromatography. 

The simplex approach to the optimum is also an experimental method and has been applied more widely to pharmaceutical systems. Originally proposed by Spendley et al. [9], the technique has even wider appeal in areas other than formulation and processing. A particularly good example to illustrate the principle is the application to the development of an analytical method (a continuous flow analyzer) by Deming and King [6]. 

Semibatch or semiflow processes are among the most difficult to analyze from the viewpoint of reactor design because one must deal with an open system under nonsteady-state conditions. Hence the differential equations governing energy and mass conservation are more complex than they would be for the same reaction carried out batchwise or in a continuous flow reactor operating at steady state. 

The ideal continuous stirred tank reactor is the easiest type of continuous flow reactor to analyze in design calculations because the temperature and composition of the reactor contents are homogeneous throughout the reactor volume. Consequently, material and energy balances can be written over the entire reactor and the outlet composition and temperature can be taken as representative of the reactor contents. In general the temperatures of the feed and effluent streams will not be equal, and it will be necessary to use both material and energy balances and the temperature-dependent form of the reaction rate expression to determine the conditions at which the reactor operates. 

There are now a number of options open to scientists who need to analyse stable isotopes as part of their research. The choice is dependent on many factors, such as the type of sample, the precision of measurements required, the amount of sample available, and the number of samples to be analysed. Dual inlet IRMS is still the technique of choice those scientists who need to measure a few samples to very great precision. More often, scientists want to determine a trend in conditions over time, or a flux integrated over an area. In these cases, CF-IRMS will be preferred. Continuous-flow IRMS is also preferred when a limited amount of sample material is available, as it is able to analyze far smaller samples (between 10 and several hundred times smaller) than DI-IRMS. 

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