Diode array detection spectra

Figure 3 Pattern of lycopene isomers in a human blood sample. (Insert) Photo-diode array detection spectra of all-E-lycopene and 5-Z-lycopene. (From Ref. 8.)...

For the characterisation of the biodegradation intermediates of C12-LAS, metabolised in pure culture by an a-proteobacterium, Cook and co-workers [23] used matrix-assisted laser desorption/ionisation (MALDI)-time of flight (TOF)-MS as a complementary tool to HPLC with diode array detection and 1H-nuclear magnetic resonance. The dominating signal in the spectrum at m/z 271 and 293 were assigned to the ions [M - H] and [M - 2H + Na]- of C6-SPC. Of minor intensity were the ions with m/z 285 and 299, interpreted to be the deprotonated molecular ions of C7- and C8-SPC, respectively. 

A general approach to the analysis of multicomponent analytes bearing chromophores was demonstrated with a mixture of nitrophenylhydrazines (250). In a FLA system the mixture was preconcentrated by SPE on Q x bonded silica, followed by desorption with a buffer and detection by UW on a diode array. The spectrum, resolved for three components, had RSD 1.43% for 11 samples containing 2 x 10 5 M of 250c. The method allowed up to 40 samplings per hour527. 

The first measurement we make when starting a fluorescence study is not usually a fluorescence measurement at all but the determination of the sample s absorption spectrum. Dual-beam differential spectrophotometers which can record up to 3 absorbance units with a spectral range of 200-1100 nm are now readily available at low cost in comparison to fluorimeters. The wide spectral response of silicon photodiode detectors has made them preeminent over photomultipliers in this area with scan speeds of a few tens of seconds over the whole spectral range being achieved, even without the use of diode array detection. 

Here, the outlet of the chromatographic system is connected to an NMR detection cell. The NMR spectra are acquired continuously while the sample is flowing through the detection cell. The result is a set of one-dimensional (ID) NMR spectra which cover the whole chromatogram and are typically displayed as a two-dimensional (2D) matrix showing NMR spectrum against retention time, similar to an LC-diode array detection (DAD) plot. 

Fig. 3 Investigation of chromatographic peak purity by diode array detection (A) spectra of drug substance and impurity (B) coelution of a mixture containing about 10% impurity (C) coelution of a mixture containing about 0.5% impurity. The spectra were obtained in the peak maximum, at about 5% and 50% of each side of the peak. The normalization was performed with respect to the first spectrum at the peak front (a matchfactor of 1000 means identical spectra) using commercial software.

Diode-Array detection systems become more and more standard detectors in HPLC-analysis. Based on the principle of UVA/lS-spectrophotometers, DAD facilitates the simultaneous detection and registration of UV-chromatograms at different wavelength and the spectra of the single substances. This additional spectrum information is often required for the definite identification of the single substance peaks in UV/ VIS-chromatograms [15]. 

Diode array detection of PAHs after LC allows acquisition and storage of each UV spectrum of the eluting peaks. The chromatogram can then be reconstructed at any defined wavelength after the analysis. The spectral output of a diode array is used not only for qualitative and quantitative analysis but also to augment the resolution of chromatographic separation, since poorly resolved components can be identified by deconvolution of the eluent spectra. 

Among the methods for analyzing OA, HPLC combined with UV, fluorescence, or diode array detection is the most used. The applied HPLC conditions are summarized in Table 3, and a typical chromatogram and UV-spectrum of OA is presented in Fig. 2. 

Modem UVWIS spectrophotometers at moderate prices are available with diode array detection that offer rapid sampling and readout (>1 Hz) of the entire 190-800 nm spectrum using flow equipped cells. 

UV detection, diode-array detector (DAD) and fluorescence have been the detection techniques used, coupled to HPLC for the analysis of OTC. UV detection is set at 355 nm [49-51], 350 nm [40], or at 353 nm [52], Using the diode array detector [49] offers advantages that the target peak can be identified by its retention time and absorption spectrum. Compared to UV detection, fluorescence detection is generally more specific and is less interfered by other compounds in the sample matrix [51]. A HPLC method with electrochemical detection has also been suggested recently. Zhao et al. [53] described HPLC with a coulometric electrode array system for the analysis of OTC, TC, CTC, DC, and methacycline (MC) in ovine milk. An amper-ometric detection coupled with HPLC was developed by Kazemifard and Moore [54] for the determination of tetracyclines in pharmaceutical formulations. 

In order to observe a short-lived species it may be necessary to employ a rapid-scanning spectrometer, such as a diode-array instrument (Sms for a 240nm-800nm spectrum). In addition, the absorbances of electrogenerated species can be very small and signal-averaging or phase-sensitive detection may be necessary to achieve the required signal-to-noise ratio (cf. EMIRS and FTIR). 

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. 

The use of a fixed wavelength UV detector for liquid chromatographic separations was first described by Horvath and Lipsky in 1966 [1], and is possibly the most popular HPLC detector in general use today. Although other detection techniques are more sensitive, the UV detector provides a simple and universal answer to the majority of HPLC applications [2]. Developed in 1982, the diode array UV detector measures the full absorption spectrum of each analyte peak, and was a... 

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