Photodiode array detectors sensitivity

Electrochemical detection is sensitive and selective, and it gives useful information about polyphenolic compounds in addition to spectra obtained by photodiode array detectors. Differences in electrochemically active substituents on analogous structures can lead to characteristic differences in their voltammetric behavior. Because the response profile across several cell potentials is representative of the voltammetric properties of a compound, useful qualitative information can be obtained using electrochemical detection (Aaby and others 2004). 

Light detection can also be achieved by semiconductor photodiodes or by photodiode array detectors. Their sensitivity, so far, is lower than that of PMTs but they possess the great advantages of much smaller dimensions and lower demand on power supply. These features make them attractive, especially for the construction of portable chemiluminometers. The sensitivity of these detectors... 

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. 

Confirmation of the identity of the -lactam residues detected by liquid chromatography has been attempted through use of photodiode array detectors (73, 75,11-19. This procedure is relatively simple, but does not offer the specificity and the sensitivity required to determine or identify trace levels of residual -lactam antibiotics in edible animal products. Better residue confirmation can be more readily attained by treatment of the suspected samples with -lactamase or penicillinase and their reanalysis (71, 80, 86-89, 105, 106-111). In this instance, absence of a chromatographic peak with the proper retention time provides unequivocal evidence that a given residue is not present above the detection limit of the method. Thus, use of -lactamase provides a simple, inexpensive and... 

Confirmatory analysis of suspected liquid chromatographic peaks is usually accomplished by a photodiode array detector that continuously collects spectral data during the chromatographic separation and further compares the spectrum (200-550 nm) of the eluted suspected compound with that of a standard (37, 38, 66, 161, 163, 166-168, 178, 180, 181). Online absorbance ratio techniques combined witlr off-line thin-layer chromatography have been also reported (171). Although tliese confirmation techniques are relatively simple, their sensitivity is not generally adequate to identify trace levels of residual nitrofurans in edible animal products. 

Although confirmation with a photodiode array detector is simple, specificity and sensitivity are not sufficient to determine or identify trace levels of residual anthelminthics in edible animal products. 

To confirm anticoccidials in liquid chromatography-based methodologies, the photodiode array detector, which collects continuous spectral data during the analysis to check for interfering substances by comparing the spectrum of the sample witli that of tlie standard, has been used (66,379,385). Although confirmation with a photodiode array detector is simple, specificity and sensitivity are not sufficient to determine or identify trace levels of residual anticoccidials in edible animal products. 

Selected methods for determining vitamin K in a variety of foods are summarized in Table 13. The photodiode array detector lacks the required sensitivity for identifying phylloquinone in foods other than green leafy vegetables. Careri et al. (248) reported on the use of particle beam mass spectrometry for the determination and unequivocal identification of phylloquinone in some vegetable samples. The proposed LC-MS method permitted phylloquinone assay at levels down to 0.1 yug/g with high specificity. 

Analogous to the principal concept of multiplex CARS microspectroscopy (cf. Sect. 6.3.5), in multiplex SRS detection a pair of a broad-bandwidth pulse, eg., white-light femtosecond pulse, and a narrow-bandwidth picosecond pulse that determine the spectral width of the SRS spectrum and its inherent spectral resolution, respectively, is used to simultaneously excite multiple Raman resonances in the sample. Due to SRS, modulations appear in the spectrum of the transmitted broad-bandwidth pulse, which are read out using a photodiode array detector. Unlike SRS imaging, it is difficult to integrate phase-sensitive lock-in detection with a multiplex detector in order to directly retrieve the Raman spectrum from these modulations. Instead, two consecutive spectra, i.e., one with the narrow-bandwidth picosecond beam present and one with that beam blocked, are recorded. Their ratio allows the computation of the linear Raman spectrum that can readily be interpreted in a quantitative manner [49]. Unlike the spectral analysis of a multiplex CARS spectrum, no retrieval of hidden phase information is required to obtain the spontaneous Raman response in multiplex SRS microspectroscopy. 

This is considered the method of choice for the analysis of flavones and flavonols due the high resolution of the chromatographic separations and the sensitivity of the detection methods that include UV, fluorescence, electrochemical, and MS detectors. The best combination is the detection system that links on-line UV detection with a photodiode array detector (PDA) that allows the registration of the UV spectra of the eluting compounds, with an MS/MS detector as an ESI ion trap that allows the isolation and fractionation of specific ions, even if they coelute under the same chromatographic peak. 

During LC-MS analyses, the simultaneous monitoring of the absorption spectrum of the eluate is recommended. The combination of LC-MS coupled with a photodiode array detector can provide additional information regarding the chemical structure of unknown degradation products. The ability of the photodiode array detector to monitor all wavelengths simultaneously assures that any analate eluting within the selected scan range will be monitored at its maximum sensitivity. 

The photodiode-array detector is a powerful analytical instrument that has provided enhanced detection capabilities with the addition of detailed spectral information via its multisignal detection technology. Its applications are HPLC based and can be found in basic research, automated analysis, pharmaceutical product development, and the clinical laboratory environment. Through spectral acquisition and analysis, a wealth of information can be obtained about the identity and purity of a compound. Combined with high selectivity and sensitivity, this mode of de-... 

A photodiode array detector (PDA), also known as a diodearray detector (DAD), provides UV spectra of eluting peaks while functioning as a multiwavelength IJV/Vis absorbance detector. It facilitates peak identification and is the prefered detector for method development. Detector sensitivity was lower in earlier models but has improved significantly in recent years. 

RP HPLC has proved to be the method of choice for the separation of a variety of flavonoids in different samples. The phenolic nature of these compounds requires the use of acidic mobile phases for satisfactory separation and peak shapes, whereas the detection is usually carried out with photodiode array detectors which are also very helpful for their identification of the characteristic absorption spectra of the flavonoids. In the last decade, mass spectrometers connected to HPLC systems introduced a greater selectivity and sensitivity in flavonoid analysis. Improving the characteristics of the stationary phases and developing more sophisticated instruments as well as devices for more efficient and faster sample preparation are the challenges for all modem analysts. Discovering... 

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