Ultraviolet photodiode array

Herrera MC and Luque de Castro MD. 2005. Ultrasound-assisted extraction of phenolic compounds from strawberries prior to liquid chromatographic separation and photodiode array ultraviolet detection. J Chromatogr A 1100(1) 1 —7. 

Figure 25-20 Photodiode array ultraviolet detector for HPLC. (a) Dual-beam optical system uses grating polychromator, one diode array for the sample spectrum, and another diode array for the reference spectrum. Photodiode arrays are described in Section 20-3. (fc>) Reversed-phase chromatography (using C18-silica) of sample containing 0.2 ng of anthracene, with detection at 250 nm. Full-scale absorbance is 0.001. (c) Spectrum of anthracene recorded as it emerged from the column. [Courtesy Perkln-Elmer Corp.. Norwalk. Cl]...

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 ...

Alvarez JC, De Mazancourt P. Rapid and sensitive high-performance liquid chromatographic method for the simultaneous determination of retinol, a-tocopheroi, 25-hydroxyvitamin Dj and 25-hydroxyvitamin D2 in human plasma with photodiode-array ultraviolet detection. J Chromatogr 2001 755 129-35. 

Riond, J.L. Hedeen, K.M. Tyczkowska, K. Riviere, J.E. Determination of doxycycline in bovine tissues and body fluids by high-performance liquid chromatography using photodiode array ultraviolet-visible detection. J.Pharm.Sci., 1989, 78, 44-47... 

Detectors. A wide variety of detection methods for CE have been reported many of which are directly comparable to those employed in HPLC. Optical detectors including ultraviolet, photodiode array, ultraviolet-visible and fluorescence have proved to be the most popular and all are commercially available. With these detectors the optically opaque polyimide layer must be removed and the somewhat limited sensitivity has been significantly improved by modifying the internal geometry of the capillary to give an increased pathlength. 

E. Blanco, M.C. Casais, M.C. Mejuto and R. Cela, Analysis of tetrabromobisphenol A and other phenolic compounds in water samples by non-aqueous capillary electrophoresis coupled to photodiode array ultraviolet detection, J. Chromatogr. A, 1071, 205-211, 2005. 

Hplc techniques are used to routinely separate and quantify less volatile compounds. The hplc columns used to affect this separation are selected based on the constituents of interest. They are typically reverse phase or anion exchange in nature. The constituents routinely assayed in this type of analysis are those high in molecular weight or low in volatility. Specific compounds of interest include wood sugars, vanillin, and tannin complexes. The most common types of hplc detectors employed in the analysis of distilled spirits are the refractive index detector and the ultraviolet detector. Additionally, the recent introduction of the photodiode array detector is making a significant impact in the analysis of distilled spirits. 

FL, Fluorescence UV, ultraviolet PAD, photodiode-array detection. ACN, acetonitrile HCOOH formic acid MeOH methanol. 

HPLC and LC/MS. HPLC methodology coupled with ultraviolet (UV), fluorescence (FL), photodiode-array (PDA) and/or a mass spectrometry (MS) detection has been developed. In general, neonicotinoids can be determined by HPLC/UV. Typical HPLC operating conditions are given in Table 2. 

Molecular weights of the copolymers were determined by gel permeation chromatography (GPC) with four p-styragel (Haters) columns calibrated using polystyrene standards. Chloroform was used as the eluate at a flow rate 1.5 ml/min. An LKB-2140 Ultraviolet Photodiode Array detector was used to detect the polymer with a scan range from 190 to 370 nm. 

Cuyckens F and Claeys M. 2002. Optimization of a liquid chromatography method based on simultaneous electrospray ionization mass spectrometric and ultraviolet photodiode array detection for analysis of flavonoid glycosides. Rapid Commun Mass Spectrom 16(24) 2341—2348. 

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. 

Since amphenicols exhibit strong ultraviolet absorption, they are ideal for direct determination by liquid chromatography, without any need for derivatization (Fig. 29.2.1). Their detection wavelengths have been set at 224 nm for thiamphenicol, 220 or 225 nm for florfenicol, and 270-290 nm for chloramphenicol (Table 29.2). Use of photodiode array detectors has been suggested for tentative confirmation of the identity of chloramphenicol residues analyzed by liquid chromatography (26, 37, 38, 40, 53, 58, 59, 61, 63, 66, 67). 

The system relies upon preliminary fractionation of the microbial crude extract by dualmode countercurrent chromatography coupled with photodiode array detection (PDA). The ultraviolet-visible (UV-Vis) spectra and liquid chromatography-mass spectrometry (LC-MS) of biologically active peaks are used for identification. Confirmation of compound identity is accomplished by nuclear magnetic resonance (NMR). Use of an integrated system countercurrent chromatography (CCC) separation, PDA detection, and LC-MS rapidly provided profiles and structural information extremely useful for metabolite identification (dereplication, Figure 14.1). 

A photomultiplier tube is a sensitive detector of visible and ultraviolet radiation photons cause electrons to be ejected from a metallic cathode. The signal is amplified at each successive dynode on which the photoelectrons impinge. Photodiode arrays and charge coupled devices are solid-state detectors in which photons create electrons and holes in semiconductor materials. Coupled to a polychromator, these devices can record all wavelengths of a spectrum simultaneously, with resolution limited by the number and spacing of detector elements. Common infrared detectors include thermocouples, ferroelectric materials, and photoconductive and photovoltaic devices. 

A photodiode array was used to record the ultraviolet spectrum of each peak in Figure 25-12 as it was eluted. By this means, it was possible to determine which compounds were in each peak. 

The way we know which peak is which is to record the entire ultraviolet spectrum of each peak as it is eluted, using a photodiode array spectrometer. 

A short guard column containing the same stationary phase as the analytical column is placed before the analytical column to protect it from contamination with particles or irreversibly adsorbed solutes. A high-quality pump provides smooth solvent flow. The injection valve allows rapid, precise sample introduction. The column is best housed in an oven to maintain a reproducible temperature. Column efficiency increases at elevated temperature because the rate of mass transfer between phases is increased. Mass spectro-metric detection provides quantitative and qualitative information for each substance eluted from the column. Ultraviolet detection is most common and it can provide qualitative information if a photodiode array is used to record a full spectrum of each analyte. Refractive index detection has universal response but is not very sensitive. Evaporative light scattering responds to the mass of each... 

Post-column detection of chlorophyll derivatives is often accomplished by ultraviolet and visible spectroscopic techniques, which take advantage of the strong electronic absorption spectra of these pigments (units F4.3 F4.4). While these methods have enjoyed wide application (Schwartz et al., 1981 Khachik et al., 1986), a major advance was made with the introduction of photodiode array (PDA) detection. Multichannel photodiode array detection allows for simultaneous monitoring of multiple wavelengths, resulting in the generation of online electronic absorption spectra of a compound as it elutes from the HPLC column. Because of the uniqueness of electronic absorption spectra of individual chlorophyll derivatives, these techniques have enjoyed extensive application for tentative identification of components from complex mixtures and extracts... 

Crude extract was also separated and collected on another Waters system, which consisted of a 600 pump, a 2996 Photodiode Array Detector, and a 2767 fraction collector. The detection wavelength was set in the ultraviolet (UV) between 190 and 400 nm. The column used was a 150 x 21 mm long ACE AQ with 10-mm particles (Advanced Chromatography Technologies, Aberdeen, UK). The system was operated at room temperature. The injection volume was 1500 pL. The mobile phase consisted of 1 3 acetonitrile water with 0.01% trifluoroacetic acid, which was flowing at a rate of 10 mL/min. The system was operated in the isocratic mode. Fractions of 1.25 mL were collected every 7.5 s. 

Also, to facilitate the use of this detector an extensive computer software system has been written which allows the user almost unlimited freedom in the way that the data taken over the bandwidth of the detector may be plotted or displayed for visual examination. The linear photodiode arrays used as detectors have been shown to have a significant response in the ultraviolet region of the spectrum. 

The scope of ultraviolet and visible spectrophotometry can be further extended when combined with a chromatographic separation step such as HPLC. The development of rapid-scanning detectors based on the linear photodiode array permits spectra to be acquired during the elution of peaks. Computer-aided manipulation of these spectra has led to new strategies for the examination of chromatographic peak homogeneity, based on classical techniques in spectroscopy. The use of microcomputers enables the development of archive retrieval methods for spectral characterisation (A. F. Fell etal, J. Chromat., 1984, 316, 423-440). 

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