Diode array spectrometry

Each of the detectors measures the radiation intensity on a spectral width resulting from a division of the linearly dispersed wavelength interval by the number of photosensitive receivers disposed side to side. Alternatively, diode array spectrometry can be employed, which supposes a transformation of the continuum into discrete values, generally followed by interpolation and smoothing procedures. 

It is thus obvious that diode array spectrometry can be differentiated from the other spectrometric techniques by the following considerations ... 

Figure 1 Basic principle of diode array spectrometry.

The spectral resolution for diode array spectrometers is dependent upon the number of photosensitive diodes distributed in the focal plane of the concave diffraction grating. The distance, in terms of wavelength, between two adjacent diode centers is called sampling interval. Higher resolution means not only more diodes composing the array but also smaller diodes. Spectrometers for the UV-Vis domain (180-820 nm) equipped with a 1024 photodiode array will consequently ensure a resolution of 0.6nm. Howevei a 2nm resolution can be considered as the usual value in diode array spectrometry, which means significantly lower capabilities compared to resolutions of 0.5, 0.2, and even 0.1 nm, which is quite usual for scanning instruments. The basic requirement in spectrometry is that the... 

Noise can be controlled in diode array spectrometry by spectral averaging. The Felgett advantage (noise reduction equals the square root of acquired data points) leads to a significant improvement of the signal-to-noise ratio. Large bandwidth acquisition not only reduces noise but also lowers sensitivity. 

The simultaneous sampling of all spectral channels by means of diode array spectrometry leads to realtime spectral data acquisition (usually 0.1s for the whole spectrum). Data processing and storage may add a supplementary 0.5 s delay. Consequently, diode array spectrometry is a first choice for dynamic system measurements (e.g., flow injection analysis and liquid chromatography detection, process control, fast kinetic measurements). Spectral averaging capabilities represent a corollary of the increased acquisition speed. 

Diode array spectrometry covers a broad range of wavelengths, from soft X-rays to the near-infrared (NIR) domain. A typical diode array spectrometer is designated to make spectral measurements from 200 to 1200 nm, with a resolution of 1 nm. A complete spectrum can be obtained in less than 100 ms. Due to these characteristics, this technique has been considered as a useful technique in different branches of analytical sciences (quantitative and qualitative determination) as well as in fundamental research. 

Some other major applications of NIR diode array spectrometry are summarized in Table 1. 

Table 1 Major application fields for NIR diode array spectrometry ...

Online applications are by far the most important utilization of diode array spectrometry. High-performance liquid chromatography, supercritical fluid chromatography, capillary electrophoresis, and flow-injection techniques produce enhanced sensitivity and structure-related information due to coupling with diode-array-based detectors. Emission of the microwave-induced plasma generated in atomic emission detectors for capillary gas chromatography is also analyzed by means of UV-Vis diode array instruments. 

C. Aguilar, I. Feirer, R Bonnll, R. M. Marce and D. Barcelo, Monitoring of pesticides in river water based on samples previously stored in polymeric cartridges followed by on-line solid-phase extraction-liquid cliromatography-diode array detection and confirmation by atmospheric pressure chemical ionization mass spectrometry . Anal. Chim. Acta 386 237-248 (1999). 

R. M. Marce, H. Prosen, C. Crespo, M. Calull, R Boirull and U. A. Th Brinkman, Online ti ace enrichment of polar pesticides in environmental waters by reversed-phase liquid cliromatography-diode array detection-particle beam mass spectrometry , J. Chromatogr. 696 63-74 (1995). 

C. Aguilar, R Bomtll and R. M. Marce, Determination of pesticides by on-line trace enrichment-reversed-phase liquid cliromatogr aphy-diode-array detection and confirmation by particle-beam mass spectrometry , Chromatographia 43 592-598 (1996). 

I. Eeirer, V. Pichon, M. C. Hennion and D. Barcelo, Automated sample preparation with exti action columns by means of anti-isoproturon immunosorbents for the determination of phenylurea herbicides in water followed by liquid chi omatography diode array detection and liquid clrromatogi aphy-atmospheric pressure chemical ionization mass spectrometry , 7. Chromatogr. Ill 91-98 (1997). 

THF = tetrahydrofuran. ACN = acetonitrile, p.s. = particle size. i.d. = internal diameter, o.d. = outer diameter. MeOH = methanol. MS = mass spectrometry. DAD = diode array detector, n.a. = not available. THC = tetrahydrocurcumin. = exdtation wavelength. X = emission wavelength. 

Hvattum, E., Determination of phenolic compounds in rose hip (Rosa canina) using liquid chromatography coupled to electrospray ionisation tandem mass spectrometry and diode-array detection, Rapid Commun. Mass Spectrom., 16, 655, 2002. 

In conclusion, synthetic dyes can be determined in solid foods and in nonalcoholic beverages and from their concentrated formulas by spectrometric methods or by several separation techniques such as TEC, HPLC, HPLC coupled with diode array or UV-Vis spectrometry, MECK, MEECK, voltammetry, and CE. ° Many analytical approaches have been used for simultaneous determinations of synthetic food additives thin layer chromatography, " " derivative spectrophotometry, adsorptive voltammetry, differential pulse polarography, and flow-through sensors for the specific determination of Sunset Yellow and its Sudan 1 subsidiary in food, " but they are generally suitable only for analyzing few-component mixtures. 

When measuring a signal, one records the magnitude of the output or the response of a measurement device as a function of an independent variable. For instance, in chromatography the signal of a Flame Ionization Detector (FID) is measured as a function of time. In spectrometry the signal of a photomultiplier or diode array is measured as a function of the wavelength. In a potentiometric titration the current of an electrode is measured as a function of the added volume of titrant. 

S. Hakkinen and S. Auriola, High performance liquid chromatography with electrospray ioniza tion mass spectrometry and diode array ultraviolet detection in the identification of flavonol aglycones and glycosides in berries, J. Chromatogr. A, 829, 91 100 (1998). 

Vallejo F, Tomas-Barberan FA and Ferreres F. 2004. Characterisation of flavonols in broccoli (Brassica oleracea L. var. italica) by liquid chromatography-UV diode-array detection-electrospray ionisation mass spectrometry. J Chromatogr A 1054(1—2) 181—193. 

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