Gradient scanning

If only mixtures of a given eluotropic strength are considered as the result of a gradient scan, then a further optimization of the primary parameter (solvent eluotropic strength) is not contemplated and the number of parameters involved in the optimization process is effectively reduced by one. In the optimization of a ternary mobile phase composition one of the three volume fractions is defined by the two others, as their sum must equal one. 

Thus, a FIA readout can be obtained in several ways, of which peak-height measurement has been the most popular so far. The reason for this choice is its simplicity, since peak height obviously is the parameter most convenient to locate. Other readouts have to be identified by means of a time scale allocated to the dispersed sample zone, with the origin at the time of injection (gradient dilution, gradient scanning, or gradient kinetics), or via a selected level of a detector for peak-width measurement. 

Gradient Dilution and Gradient Scanning Gradient Dilution... 

Gradient scanning is an extension of the gradient dilution in that it uses a dynamic detector, which measures continuously a physical parameter by repeatedly scanning it within a certain range. Originally demonstrated by means of a voltametric detector (current/potential scan [288, 464]), the technique was later extended to spectroscopy [817, 1080] as shown on the emission/wavelength scan in Fig. 2.19. 

Figure 4.26. Flow system and setup for simultaneous multicomponent gradient scanning by flame photometric FI A embodying standard addition. The sample (5) is initially aspirated into loop L, which upon turning of the valve (V) is propelled forward by pump P through the FI A system and toward the detector (F), the sample being sandwiched between the inert carrier solution (water) and an infinitely long zone of standard carrier solution (SC) F, flame nebulizer-burner T, timer A/, scanning monochromator O, storage oscilloscope and R, X-Y recorder.

Z. Fang, J. M. Harris, J. RdiiCka, and E. H. Hansen, Simultaneous Flame Photometric Determination of Lithium, Sodium, Potassium and Calcium by Flow Injection Analysis with Gradient Scanning Standard Addition. Anal. Chem., 57 (1985) 1457. 

Fig.3. The principle of the FIA Gradient scanning technique, allowing a series of spectra to be recorded on a dispersed sample zone (left) S, point of sample injection, and t, time. The three-dimensional recording (right) shows an electrochemical spectrum, that is, a voltammogram of a 0.01 M solution of copper(Il)amine complex being reduced on a mercury electrode. The peak (forming a ridge) is located at a potential (U) at which maximum current intensity (i) is observed due to reduction of Cu(II). Fifty scans (each lasting 2 sec.) were recorded on the tailing section of the dispersed sample zone. ...

Vacha M, Yokoyama N, Tokizaki T, Furuki M and Tani T 1999 Laser scanning microscope for low temperature single molecule and microscale spectroscopy based on gradient index optics Rev. Sc/. Instrum. 70 2041-5... 

Figure 7 DMTA scan for (a) normal, and (b) shear gradient processed polymer.

Thin-layer chromatography (TLC) is used both for characterization of alcohol sulfates and alcohol ether sulfates and for their analysis in mixtures. This technique, combined with the use of scanning densitometers, is a quantitative analytical method. TLC is preferred to HPLC in this case as anionic surfactants do not contain strong chromophores and the refractive index detector is of low sensitivity and not suitable for gradient elution. A recent development in HPLC detector technology, the evaporative light-scattering detector, will probably overcome these sensitivity problems. 

The HPLC system comprised a 75 ftm x 15 cm PepMap column with a linear gradient of acetonitrile/0.1% aqueous formic acid (5 to 50% acetonitrile over 45 min) at a flow rate of 250 nlmin . Positive-ion electrospray ionization was employed using a nanospray interface. MS-MS specna were acquired over the range m/z 40 to 2000 at a rate of 1 s per scan. 

FIGURE 6.17 Densitograms of Azulan extract scanned at 410 nm divided into fractions a to i (a) isocratic development, ethyl acetate in chloroform (1 5), (b) stepwise gradient development, 10 to 40% v/v of ethyl acetate in chloroform. (From Matysik. G., Soczewinski, E., and Polak, B., Chromatographia, 39, 497-504, 1994. With permission.)... 

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