Photodiode array spectrophotometers

Membrane separation coupled on-line to a flow-injection system was employed for the monitoring of propazine and terbutryn in natural water. A microporous hydro-phobic membrane was utilized in which the analytes were extracted from the aqueous medium into an organic solvent that was carried to the flow cell of a photodiode-array spectrophotometer. The LCDs were 4-5 qg so the technique could potentially be used for screening purposes. Samples with positive detection would require further analysis. 

Stopped flow mixing of organic and aqueous phases is an excellent way to produce dispersion within a few milliseconds. The specific interfacial area of the dispersion can become as high as 700 cm and the interfacial reaction in the dispersed system can be measured by a photodiode array spectrophotometer. A drawback of this method is the limitation of a measurable time, although it depends on the viscosity. After 200 ms, the dispersion system starts to separate, even in a rather viscous solvent like a dodecane. Therefore, rather fast interfacial reactions such as diffusion-rate-limiting reactions are preferable systems to be measured. 

In a dispersive spectrometer (Figure 20-1), only one narrow band of wavelengths A photodiode array spectrophotometer reaches the detector at any time. In a photodiode array spectrophotometer (Figure 20-14), all measures all wavelengths at once, giving... 

Figure 20-14 Schematic design of photodiode array spectrophotometer.

Additionally, the measurements are carried out on a steep slope of the absorption curve. It is therefore of importance that the wavelength be set accurately (e.g., by use of a photodiode array spectrophotometer). 

Elisabeth, I. Rene, S. Dieter, J. Screening for drugs in clinical toxicology hy high-performance liquid chromatography Identification of barbiturates by post-column ionization and detection by a multiplex photodiode array spectrophotometer. J. Chromatogr. 1988, 428, 369. 

Fourier transform infrared spectrophotometers for use with HPLC are commercially available and as with ultraviolet photodiode array spectrophotometers enable the absorbance data to be displayed as a function of wavelength and time. Further details are given in Chapter 7. 

Most multiparametric procedures are restricted to two active components, and the main challenge is in using different reagents and detection wavelengths. The use of photodiode array spectrophotometers and, more recently, multichannel fiber optic spectrophotometers facilitates this approach. Solenoid valves, to control the flow system, have proved to be very useful in the development of techniques for simultaneous determination of Zn, Fe, Cu, Ca, and Mg in pharmaceutical drugs. Derivative... 

Dispersive spectrophotometers described so far scan through a spectrum one wavelength at a time. A diode array spectrophotometer records the entire spectrum at once. The entire spectrum of a compound emerging from a chromatography column can be recorded in a fraction of a second by a photodiode array spectrophotometer. At the heart of rapid spectroscopy is a photodiode array such as the one in Figure 19-11, which contains 1 024 individual semiconductor detector elements (diodes) in a row. 

What characteristic makes a photodiode array spectrophotometer suitable for measuring the spectrum of a compound as it emerges from a chromatography column and a dispersive spectrophotometer not suitable What is the disadvantage of the photodiode array spectrophotometer ... 

Absolute and differential absorption spectra were obtained on a Hewlett-Packard photodiode array spectrophotometer. Difference spectra were generated by subtracting the absorbance spectra of the test solution from the initial effluent and then subtracting the absorbance spectra of the solution surrounding the cells containing BA from its counterpart which did not contain BA. Difference spectra generated for (1) the dye solution with and without BA and (2) the initial effluent over time, show no absorbance differences. In addition, spectra were run of dye solution with known [Ca ] to generate a standard curve. 

Several months are required to acid clean the scale from the evaporators thus leaving the evaporators unavailable to perform an important process fimction. To help predict the rate of scale formation, salt solutions that feed the evaporators are analyzed for silica. The molybdosilicate colorimetric method for silica was developed for remote determination by using fiber optics to couple the light source and photodiode array spectrophotometer outside the shielded cell to the flow cell inside the cell. This remote method has considerable advantages in terms of speed and minimizing radiation dose to personnel versus analyzing the high radiation samples outside the shielded cell. 

A diode array spectrophotometer is one that utilizes a series of photodiodes to detect the fight intensity of all wavelengths after the fight has passed through the sample. See Figure 8.9. The advantage is that an absorption spectrum can be measured in a matter of seconds. 

An HPLC detector is often a modified spectrophotometer equipped with a small flow cell, which monitors the concentration (or mass) of eluting analytes.Common detectors in the pharmaceutical laboratory are listed in Table 2 with their respective attributes and sensitivity levels. A recent survey found that 85% of pharmaceutical applications use absorbance detectors such as UVA/ is or photodiode array detectors (PDA). These two detectors are covered in more detail in this section. 

The emergence of sensitive and affordable array detectors in 1980s and 1990s has also improved measurement capability in the UV-vis. Commercially available UV-vis instrumentation with photodiode-array (PDA) detectors appeared in the mid-1980s. This made it possible to produce a UV-vis spectrophotometer... 

Instrnmentation for UV-vis process analysis falls into fonr categories scanning instruments, diode-array instrnments, photometers, and fiber-optic spectrophotometers with photodiode-array (PDA) and charge-conpled device (CCD) detectors. The former two are more typically enconntered in at-line or near-line applications, whereas the latter two are better snited to actnal on-line analyses. 

Reverse-phase columns with a gradient elution in combination with UV-Vis spectrophotometers using photodiode-array (PDA) (Fig. 1.6) and spectrofiuorimeters are common devices employed in this technique. In a lesser extent, MS, tandem mass spectrometry (MS-MS), and nano liquid chromatography-electrospray ionization-quadrupole time-of-flight tandem mass spectrometry (nanoLC-nanoESI-Q-qTOF-MS-MS) has been used as detection system. This instrumentation has been mainly used in the analysis of dyes and proteinaceous media, and in some extent, in the analysis of drying oils and terpenoid varnishes [47,48],... 

Figure 11.13 —Schematic am optical path showing the principle and simplified view of a diode array spectrophotometer. The shutter is the only mobile piece in the assembly, allowing subtraction of the background signal (dark current) without any light intensity striking the photodiodes. This inverted optical design allows the sample to be exposed to the exterior light. These instruments are widely used as detectors in liquid chromatography (cf. 3.7).

FigurB 25-26 Application of the method development triangle to the separation of seven aromatic compounds by HPLC. Column 0.46 x 25 cm Hypersil ODS (C)e on 5-(j.m silica) at ambient temperature ( 22°C). Elution rate was 1.0 mL/min with the following solvents (A) 30 vol% acetonitrile/70 vol% buffer (B) 40% methanol/60% buffer (C) 32% tetrahydrofuran/68% buffer. The aqueous buffer contained 25 mM KH2P04 plus 0.1 g/L NaN3 adjusted to pH 3.5 with HCI. Points D, E, and F are midway between the vertices (D) 15% acetonitrile/20% methanol/65% buffer (E) 15% acetonitrile/16% tetrahydrofuran/69% buffer (F) 20% methanol/16% tetrahydrofuran/64% buffer. Point G at the center of the triangle is an equal blend of A, B, and C with the composition 10% acetonitrile/13% methanol/11% tetrahydro-furan/66% buffer. The negative dip in C between peaks 3 and 1 is associated with the solvent front. Peak identities were tracked with a photodiode array ultraviolet spectrophotometer (1) benzyl alcohol (2) phenol (3) 3, 4 -dimethoxyacetophenone (4) m-dinitrobenzene (5) p-dinitrobenzene ...

Explain the differences between a spectrophotometer that uses a phototube for a detector and one that uses a photodiode array detector. 

Spectrophotometers were improved somewhat by dual-wavelength detectors, but it was not until the development of the photodiode array (PDA) that spectrophotometric techniques were revolutionized. The photodiode array detector, which can acquire data in both the time and spectral domains, has led to considerable improvements in HPLC food analysis for the purpose of identifi-... 

Rapid-scanning Spectrophotometers. These en loy multi-channel detectors. The most commonly encountered detector of diis t e is tlie linear photodiode array. The reversed-optics mode is employed, so that radiation is passed throu tiie sample or reference cell, tiien dispersed by a dif action grating polychiomator integrated intensity of radiation incident on it which is determined by tiie spectial dispersion photo ode ratio. If, for example, a 200-nm txmdwidtii of radiation were dispersed across 256 photodiodes, tiie nominal resolution per photodiode woitid be 0.78 nm. 

Semiconductor silicon diodes, called photodiodes, with special conductance properties are generally used in diode array spectrophotometers. Photodiodes are very useful because scan times of the order of a few milliseconds and thus many scans may be obtained in a very short time. In addition, the data obtained, being digital, can be further processed to generate spectra with a very high degree of resolution and accuracy. 

Reverse Optic Spectrophotometers-Photodiode Array Detectors... 

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