Photodiode array specifications

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. 

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. 

Figure 3.5 Three-dimensional display of the photodiode array absorbance data obtained by HPLC/PDA/MS for a M. truncatula extract. The first dimension is HPLC retention time, second is wavelength, and third is absorbance. The data can be rapidly previewed for specific absorbance regions characteristic of functional groups.

Identification (ID) tests in Category IV require only specificity for their validation. Identification by HPLC usually involves comparison of the retention time (%) or relative retention time (RRT) of a sample and standard injection. The increasing use of photodiode array (PDA) detectors in HPLC methods also allows identification by comparison of UV spectra for standards and samples, in addition to retention characteristics. The information required for either ID test by HPLC can be gathered while performing any other HPLC method for a given sample. Identification tests are often incorporated into the assay method and the satisfactory completion of specificity for the assay will meet the requirements for ID as well. 

Most of the detectors permit peak recognition but provide no structural information, which can be particularly important for identification of unknown compounds. From this point of view, the spectro-metric detectors, specifically mass spectrometer and photodiode array detectors, add a third dimension to the multidimensional system and give additional information useful in components identification. 

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 (Fig. 29.8.1) and further compares the spectrum (200-450 nm) of the eluted suspected compound with that of a standard (281, 289, 296, 311). Although confirmation with a photodiode array detector is simple, specificity and sensitivity are not sufficient to determine or identify trace levels of residual tetracyclines 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. 

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. 

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. 

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