The Diode Array Detector

The main disadvantage of the system lies in the fact that li t of all wavelengths is present in the cell simultaneously and consequently, there is a real possibility that there will also be fluorescent light present at the wavelength being monitored. 

In comparison with other detection techniques fluorescence affords greater sensitivity to sample concentration but less sensitivity to instrument instability and such macroscopic properties as temperature and pressure. In part this may be 

Iq is the intensity of the incident light c is the concentration of the solute k is the molar absorbance 1 is the path length of the cell. 

Light from a broad emission source is focussed by means of a ellipsoidal mirror onto a toroidal mirror and hence onto a grating. Monochromatic light, selected by the angular position of the grating, is then focussed by a spherical mirror and then by an ellipsoidal mirror onto the sample contained in a sample cell. 

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The diode array detector can be used in a number of unique ways and an example of the use of a diode array detector to verify the purity of a given solute is shown in figure 9. 

Another interesting example of the use of the diode array detector to confirm the integrity of an eluted peak is afforded by an application published by the Perkin Elmer Corporation showing the separation of a mixture of aromatic hydrocarbons. The separation they examined is shown in figure 10. 

The versatility and advantages of the diode array detector are obvious but it is basically a research instrument or, from the point of view of the analyst, would be extremely useful in method development. Its use in routine analysis, however, might be considered vernacularly as "overkill". In any routine analysis, its versatility would be hardly used and its expense might be difficult to justify. 

This an excellent example of the value of the diode array detector. If the chromatogram shown in figure 3 was monitored at two different wavelengths, then a peak ratio curve would immediately disclose the presence of the second peak (see page 175) and it would no longer be necessary to resort to changes in mobile phase composition to establish the presence of the impurity. 

It is seen that however sophisticated the software might be, it would be virtually impossible to de-convolute the peak into the three components. The peaks shown in the diagram are discernible because the peaks themselves were assumed and the composite envelope calculated. The envelope, however, would provide no basic data there is no hint of an approximate position for any peak maximum and absolutely no indication of the peak width of any of the components. The use of the diode array detector, monitoring at different wavelengths, might help by identifying uniquely one or more of the... 

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. 

FIGURE 3.7 Optimizing arrangement of LC modules for very complex systems. Four-detector (DAD, ELSD, CLND, and SQ-MS) LCMS system. The capillary connections from the diode array detector have been highlighted for better visibility. (Courtesy of Kenneth Lewis, OpAns Pic.)... 

Figure 3.7 Diode array detector. Light from the lamp passes through the flow cell and to a holographic reflectance grating and the resulting spectrum is focused on the diode array. Detectors frequently have a spectral range of 190-800 nm and can offer bandwidths as low as 1.0 nm.

Fig. 2. 245 -nm excited resonance Raman spectrum of (A) 0.225 mM Craq002 + and (B) 50 pM Craq1801802 + in 0.02 M HC104 at 0°C. Negative peaks arise from subtraction of the intense 4 bands at 934 and 629 cm-1. The asterisk indicates a burned spot on the intensifier of the diode-array detector. Reproduced with permission from J. Am. Chem. Soc. 1995, 117, 6483-6488. Copyright 1995 American Chemical Society. 

The wavelength accuracy of the diode array detector is commonly determined by comparing a measured absorbance with the absorbance maxima of a reference... 

Figure 3.14—Principle of the diode array detector. The flow cell is irradiated with a polychromatic UV/Vis light source. The light transmitted by the sample is dispersed by reflection on a grating and the reflected intensities are monitored by an array of photodiodes. Several hundred photodiodes can be used, each one monitoring the mean absorption of a narrow band of wavelengths (i.e. 1 nm).

Figure 11.29—A PC-plug-in spectrophotometer. This instrument is a fully integrated system for colorimetric analysis. The source is linked by an optical fiber to the diode array detector fixed onto the card (Model CHF.M2000 reproduced by permission of Ocean Optics Europe.)...

Analytes are identified on the basis of retention time compared to standards and with the addition of the suspected compound to the sample (55). The diode array detector has been used recently as an additional aid in the identification of sweeteners and the determination of peak purity (56). Quantification is performed by the internal or external standard method on the basis of peak height or area. 

In a polychromator PDA spectrometer it was previously mentioned that there is no exit slit - all diffracted wavelengths (after order sorting) fall onto the diode-array detector. In this case what defines the analyser spectral bandwidth Each individual detector element integrates the signal falling onto it, and allocates that signal to a specific wavelength. In... 

Spectroscopic detectors measure partial or complete energy absorption, energy emission, or mass spectra in real-time as analytes are separated on a chromatography column. Spectroscopic data provide the strongest evidence to support the identifications of analytes. However, depending on the spectroscopic technique, other method attributes such as sensitivity and peak area measurement accuracy may be reduced compared to some nonselective and selective detectors. The mass spectrometer and Fourier transform infrared spectrometer are examples of spectroscopic detectors used online with GC and HPLC. The diode array detector, which can measure the UV-VIS spectra of eluting analytes is a... 

The coulometric array detection mimics the diode array detector if two peaks are eluted together, they can be electrochemically and spectrophotometrically resolved and quantified. While a diode array detector relies on the different UV-Vis spectra of various compounds and characterizes analytes on the basis of their retention times and spectroscopic features, the coulometric array detector takes advantage of the variability of the voltammograms for diverse analytes and typifies them based on retention time and reaction potential. 

There are two basic types of multi-wavelength detector, the dispersion detector and the diode array detector, the former being the more popular. In fact, today very few dispersion instruments are sold but there are many still used in the field and so their characteristics will be discussed. Both types require a broad emission light source such as deuterium or the xenon lamp, the use of the deuterium lamp being the most widespread. 

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