Diode array detector . See

Diode array detectors (see Section 6.10) can measure the eluate at several different wavelengths simultaneously in addition to the ratio between two extinctions. This extracts more information from the chromatogram and provides important data for the qualitative analysis of unknown samples. This is a good solution to use for the problem demonstrated in Figure 6.7. Electrochemical detectors that monitor the eluate simultaneously at two different potentials are also marketed. 

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. 

The aim of all the foregoing methods of factor analysis is to decompose a data-set into physically meaningful factors, for instance pure spectra from a HPLC-DAD data-set. After those factors have been obtained, quantitation should be possible by calculating the contribution of each factor in the rows of the data matrix. By ITTFA (see Section 34.2.6) for example, one estimates the elution profiles of each individual compound. However, for quantitation the peak areas have to be correlated to the concentration by a calibration step. This is particularly important when using a diode array detector because the response factors (absorptivity) may considerably vary with the compound considered. Some methods of factor analysis require the presence of a pure variable for each factor. In that case quantitation becomes straightforward and does not need a multivariate approach because full selectivity is available. 

HPLC methods can be ntilized for the pre-concentration of aromatic amines from polluted waters on silica gel or octadecyl silica (ODS) colnmns [55], The determination is then performed by RP HPLC using ODS packings as the stationary phases and a mixture of methanol, isopropanol, and water as the mobile phase [55], RP HPLC with diode array detector (DAD) methods coupled on-line with a continnons seqnential anaerobic/aerobic reactor system have been employed in wastewaters treatments [56], A continnons monitoring of the possible presence of aromatic amines in azo dyes wastes is based on indncing in the waste, the reaction of a reduction of the dye, followed by HPLC/ UV or HPLC/MS analysis [57-59], The redncing agent solutions are sodium dithionite or tin(II) chloride in an aqneons acidic medinm at 70°C, followed by SPE [58,59], LLE [60,61], or SEE [60-62],... 

The spectral characteristics of a standard can be monitored during HPLC using a diode-array detector (unitfu). A directory of standard spectra can be stored, enabling additional identification of sample peaks. The actual absorption maxima and fine structure will be dependent on the composition of the mobile phase (see Fig. F2.2.4). Peak I may only occur as a shoulder with civ-carotenoids. while an additional peak is observed at around 340 nm (see Fig. 2.2.1). 

Schematically, two main systems can be used to collect 3D fluorescence data (time, wavelength, number of photons, see fig. 1). In a first type of system, light is directed into a monochromator connected to a photomultiplier tube and then to a fast oscilloscope (PM detection). The experimentalist thus collects luminescence decays at various wavelengths. This system is known to be very efficient for luminescence decay acquisition but is very time-consuming for the acquisition of emission spectra. In the second type of system, light is directed to a diode array detector (or CCD camera) and a subsequent electronic detection device (diode detection). The experimentalist collects emission spectra at various delay times (time zero for the pulse entering in the sample). This system is very efficient for emission data acquisition but, on the other hand, time-consuming for luminescence decay acquisitions. From this very schematic description, it appears that a system combining the two types of detections would be the optimum.

Photodiode array A linear array of photodiodes that can detect multiple wavelengths simultaneously see diode array detector. Photoelectric colorimeter A photometer that responds to visible radiation. 

FIGURE 8.7 From top to bottom the elctronic absorption spectra of polyynes with w = 4, 5, 6, 7, and 8. The assignment is unequivocal (see Table 8.1). These spectra were recorded by the diode-array detector of the HPLC system on each peak shown in Figure 8.6(A). 

Figure 18.10 Two-dimensional separation of a mixture of phenolic and flavone antioxidants [after P. Jandera, University of Pardubice, Czech Republic see also F. Cacciola et a ., J. Chromatogr. A, 1149, 73 (2007)]. Conditions in the first dimension column, 15 cm x 4.6 mm i.d. stationary phase, PEG silica 5 pm mobile phase, 0.3 ml min" water-acetonitrile, gradient 1-55% acetonitrile in 200 min. Interface ten-port valve with two storage columns X-Terra Cl8 2.5 pm, 3 cm x 4.6 mm i.d. which concentrate the eluate. Cycle time, 5 min. Conditions in the second dimension column, 10 cm x 4.6 mm i.d. stationary phase, SpeedROD RP-18e (monolith) mobile phase, 2 ml min" water-acetonitrile, gradient 1-40% acetonitrile in 5min. Diode array detector with 254 + 260 + 280 + 320 nm. Of the numerous identified analytes only the most important ones are specified in the figure.

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