Diode array detectors, variable

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. 

Series 10 chromatography LC-95 variable wavelength UV/visible detector, LC-90 variable wavelength UV detector, LC-135 and L-235 diode array detectors, LC 1-100 computing integrator, 1SS-100 intelligent amjj in s stjm, Series 410 LC pump... 

High-Performance Liquid Chromatography. A Varian 5060 delivery system was used for this work with detection by UV absorption. Either a Varian UV-50 variable wavelength detector or a Hewlett Packard 1040A scanning diode array detector was used. All HPLC columns were packed in our laboratory (10) with 5-/um particle size Spherisorb-ODS, Spherisorb-CN (Phase Separations), or 8-pm particle size Zorbax-CN (Dupont Ltd). HPLC columns (20 or 25 cm X 4.6 mm i.d.) were coupled via short lengths of stainless steel capillary tubing (5 cm X 0.25 mm i.d.). Separation conditions were as follows ... 

NOTE While a variable wavelength UV-Vis detector is sufficient for the protocols in this unit, a UV-Vis diode array detector, which collects spectra for individual peaks, greatly enhances the methods. 

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. 

Turning initially to multidetection, and here first to simultaneous usage, an obvious application is to combine the gradient FIA techniques with the use of detectors that provide several signals at several values of the instrumental variables, which indeed gives FIA a doubly dynamic character. In these techniques, which have already been mentioned in Section 2.4, advantage can be taken by multidetectors, such as the fast-scan vol-tammetric detectors [288] or by inductively coupled plasma atomic emission spectroscopy [808] or by diode array detectors [1017, 1075, 1382]. Combined with the advantages offered by chemometrics, these multidetection procedures may in fact be extended to multideterminations. 

Figure 5.10. Schematic diagram of a variable wavelength dual beam absorption detector (A) and a diode array detector employing reverse optics (B).

In addition, the absorbance ratio has been shown to be a satisfactory substitute for the diode array detector because it monitors the intrinsic variability of methods, allowing for the development of a new protocol for distinct matrices. It should be borne in mind that minor variations required in the A obviously depend on grooves originating from a prism or comb, which are the principal determining factors... 

Diode array detector. The diode array detector records in about 10 msec the complete spectrum from 190 to 600 nm using an array of over 200 photodiodes. The advantage of this is that the complete spectrum at any point on a peak can be obtained and compared with that at any other point. This gives confidence (or not) as to peak homogeneity and also as to peak identity. However, it is less sensitive than the variable wavelength detector. 

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