SCOFT detection

Other methods for flow measurement include the use of fluorescence correlation spectroscopy to trace the hydrodynamic flow of a fluorophore in a Si microchannel [412]. Velocity of flowing particles was measured using SCOFT detection as described in Chapter 7, Section 7.1.3 [413]. 

FIGURE 7.10 Micromachined glass electrophoresis chip for SCOFT detection (a) schematic showing the solution reservoirs, one electrophoresis channel (4.5 cm long, 15 pm deep, 50 im wide at the top, 20 pm wide at the bottom), and patterned Cr layer with micromachined slits (300 pm-wide, 700 pm-spaced center to center) (b) scanned image of the actual 75-mm x 75-mm chip. The six different channel systems can be seen [698]. Reprinted with permission from the American Chemical Society. 

Among the mathematical transformation techniques, CC and HT substantially improve the S/N ratio in CE separations. It should be noted that Shah convolution Fourier transform (SCOFT) detection yielded results that differ from those obtained with either CC or HT. Although fundamental studies have demonstrated the potential of SCOFT (6,11), both the advantages and disadvantages remain to be determined. Nevertheless, SCOFT apparently has the potential to improve the resolution and/or sensitivity of microchip electrophoresis (ME). In this chapter, we discuss the principles, instrumentation, and performance of CE and ME methods using CC, HT, and FT techniques. 

The hrst successful use of an electrophoretic technique employing the FT is the SCOFT detection proposed by Crabtree et al. (6). In SCOFT, a slit positioned at a hxed distance is inserted into the separation channel of the microchip. The analyte ions are simultaneously separated and detected as they pass through the separation channel with the slit, resulting in an electropherogram with equally spaced peaks for each analyte. A schematic illustration of SCOFT detection is shown in Figure 17.5. 

In FT electrophoresis, well-designed microchips are employed to obtain an electropherogram with periodic peaks. Crabtree et al. (6) achieved SCOFT detection on a microchip with a Cr layer patterned on top of the separation channel. In their first report on SCOFT detection, fifty-five 300 Lim-wide slits, spaced such that each slit is separated by 700 qm measured from its center, that is, 400 im-wide detection windows, were aligned at 300 pm intervals. A... 

While the use of FT in the capillary format remains to be used for electrophoresis, FT has been employed in microchip electrophoretic separations. Figure 17.7 shows the results obtained using a microchip with detection windows consisting of 55 slits. In Figure 17.7, the resolution obtained in the frequency domain (Fig. 17.7c) was less than that seen in the time domain with singlepoint detection (Fig. 17.7a). Kwok and Manz reported that the S/N ratio was enhanced -9-fold in the SCOFT detection (11). However, the analyte concentrations used in the SCOFT detection were much greater than those used in conventional LIF detection. 

Multiple-point fluorescent deteclion has been proposed to enhance detection sensitivity. This method is based on the use of a detector function, such as the Shah function. The time-domain signals were first detected, and they were converted into a frequency-domain plot by Fourier transformation. Therefore, this technique was dubbed Shah convolution Fourier transform detection (SCOFT). As a comparison, the single-detection point time-domain response is commonly known as the electropherogram [698,699,701]. 

FIGURE 7.11 One-component injection detected by SCOFT. (a) Time-domain data produced when a fluorescein sample is injected down the separation channel 55 peaks, spaced at 0.58 s, superimposed on the Gaussian distribution of the expanded laser beam, (b) Fourier transformation of part (a). Peaks for 1.74 Hz (fundamental) and 3.43 Hz (second harmonic) can be seen. The frequency of 1.74 Hz was correlated to the migration time of 0.58 s [698]. Reprinted with permission from the American Chemical Society. 

In SCOFT for multipoint fluorescent detection, a Shah function is used (Figure 7.11). What are the two different types of methods in implementing the shah function (4 marks)... 

FIGURE 17.5. Schematic illustrations of multipoint detection for generation of periodic signals and the expected results in SCOFT. For the details, refer to Referenee 6. 

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