Absorption measurements flow cells

Radiation from a xenon or deuterium source is focussed on the flow cell. An interchangeable filter allows different excitation wavelengths to be used. The fluorescent radiation is emitted by the sample in all directions, but is usually measured at 90° to the incident beam. In some types, to increase sensitivity, the fluorescent radiation is reflected and focussed by a parabolic mirror. The second filter isolates a suitable wavelength from the fluorescence spectrum and prevents any scattered light from the source from reaching the photomultiplier detector. The 90° optics allow monitoring of the incident beam as well, so that dual uv absorption and fluorescence... 

The instrumentation for sensors based on absorption measurements can be designed on the traditional spectrophotometers by using a flowthrough cell for automatic sampling with the sensors mounted inside the flow-through cell shown in Fig. 20a.3. For remote optical sensing using optical fibers, the chromophores can be immobilized in reflective... 

The ultraviolet (UV) absorption HPLC detector is basically a UV spectrophotometer that measures a flowing solution rather than a static solution. It has a light source, a wavelength selector, and a phototube like an ordinary spectrophotometer. The cuvette is a flow cell, through which the column effluent flows. As the mobile phase elutes, the chromatogram traces a line at zero absorbance, but when a mixture... 

For ultraviolet and visible spectroscopic detectors, a standard solution of a compound whose molar absorption constant is known must be prepared, and placed in the flow cell. The absorbance obtained is then compared with the value measured by a standard spectrophotometer. 

The photodiode array detector (PDAD) measures absorption of light waves by a sample. This is considered the most powerful of the ultraviolet spectrophotometric detectors. The optical system focuses light from a deuterium source through the sample flow cell onto several photodiodes. These act as capacitators by holding a fixed amount of charge. When light strikes the photodiodes, they discharge a certain amount of current. 

In the oxidation of ascorbate by Oj catalysed by ascorbate oxidase, the formation of the monodehydroascorbate free radical was demonstrated by EPR spectroscopy in a flow cell. A steady state was usually reached within 50 ms. The production of the free radical was also followed by the reduction of Fe(in)-cytochrome c. Thus the oxidation of ascorbate occurs in a one-electron step The formation of the monodehydroascorbate free radical was also measured directly by spectrophotometry at 360 nm, where the free radical shows an absorption maximum... 

Monochromatic detection. A schematic of a monochromatic absorbance detector is given in Fig. 3.12. It is composed of a mercury or deuterium light source, a monochromator used to isolate a narrow bandwidth (10 nm) or spectral line (i.e. 254 nm for Hg), a flow cell with a volume of a few pi (optical path 0.1 to 1 cm) and a means of optical detection. This system is an example of a selective detector the intensity of absorption depends on the analyte molar absorption coefficient (see Fig. 3.13). It is thus possible to calculate the concentration of the analytes by measuring directly the peak areas without taking into account the specific absorption coefficients. For compounds that do not possess a significant absorption spectrum, it is possible to perform derivatisation of the analytes prior to detection. 

We performed transient absorption measurements on BP(OH>2 with a spectrometer based on two noncollinearly phase matched optical parametric amplifiers (NOPAs) pumped by an homebuilt regenerative Ti Sapphire laser system or a CPA 2001 (Clark-MXR) [1,7]. The tunable UV pump pulses are generated by frequency doubling the output of one of the NOPAs. The other NOPA provides the visible probe pulses. The cross correlation between pump and probe pulses has a typical width (FWHM) of 40 fs. The sample is a cyclohexane solution of BP(OH)2 pumped through a flow cell with a 120 pm thick channel. 

TDLAS measures the absorption of monochromatic light by CO in a multipass flow cell (77). The time response is controlled by the flow of air through the cell. In theory, the instrument is fast enough for application to aircraft measurement of CO flux by eddy correlation. A reported precision of 1 ppbv or 1% is superior to other techniques. The instrument is not commercially available. 

The liquid (tap water) was pumped through the tracer injection fitting, where a determined tracer volume was injected. Before entering the spray nozzle, the liquid flow was monitored for the dye concentration using an optical flow cell. The liquid passed the channels of the monolith and was collected with a small stirred vessel (mixing-cup). From this mixing cup the liquid was led on to a second probe, which measured the dye concentration by spectroscopy. The absorption of light is related to the concentration of the tracer by the Beer-Lambert law. 

The components separated by FFF are washed out of the channel and into a detector and/or fraction collection device. The detectors are mainly those used in liquid chromatography where light absorption, refractive index changes, and so on, are measured as components flush through a small flow cell following their elution. 

Aeration Apparatus The apparatus, shown in Fig. 16, consists of a flowmeter (a), capable of measuring flow rates from 500 to 1000 mL/min, connected via a three-way stopcock (b), with a Teflon plug, to 125-mL gas washing bottles (c and d), followed by a drying tube (e), and finally a suitable quartz liquid absorption cell if), terminating with a vent (g) to a fume hood. 

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