Streaking procedures

The streak camera gives a time resolution of about 5 ps. It requires a rather complex calibration procedure since the incident light intensity appears as the thickness of trace on the screen. It is used mostly for luminescence kinetics measurements, one of its advantages being that it can record single events. 

Dry in a hot air oven — if any white streaks are found on the sides of the vessels the rinsing procedure is inadequate. This may be a result of failure to rinse sufficiently or it may indicate that the deionised water supply is faulty. 

Herpfer and Jeng [146] used streak particle imaging velocimetry for planar measurement of droplet velocities and sizes. They encountered problems due to out-of-focus effects of the PIV imaging procedure. 

This test is done to rule out any TLC artifacts that can occur from the sample applied to the origin not moving at the same rate as the drug substance upon the first pass in the mobile phase. It is often utilized to rule out TLC procedure-related bands, as described later in this chapter (Impurity Isolation and Characterization by TLC). A plate is spotted, not streaked, with an appropriate amount of sample at a nominal concentration (typically 10-15 pL). This is done in several applications of about 3-5 units each to prevent diffusion. It is spotted on the lower left corner, at the origin, and this lane is labeled as first pass. The plate is developed in the normal fashion. Once dried, the plate is rotated 90° counterclockwise, another spot is applied in the lower left corner, labeled as second pass, and developed in the same manner. The result shows any TLC artifacts present in the second pass. 

Some specialized cellulases are used to minimize dye streaking on the fabric, to cause more abrasion near seams, or any of a myriad of other desired effects. Cellulases operating at high pH (> 9) have not been successful, as alkaline pH is used to terminate the enzyme reaction to prevent unwanted damage, and this procedure is ineffective with alkaline cellulase. 

Typically, in measurements of time-resolved luminescence in the time regime of tens of picoseconds, data obtained from 10 to 20 laser shots are averaged to improve the signal-to-noise ratio and to minimize the effects of shot-to-shot variations in the laser pulse energy and shape. Once the reliability of the data has been ensured by application of the corrections described above and made necessary by detector-induced distortions, the time-resolved fluorescence data is analyzed in terms of a kinetic model which assumes that the emitting state is formed with a risetime, xR, and a decay time, Tp. Deconvolution of the excitation pulse from the observed molecular fluorescence is performed numerically. The shape of the excitation pulse to be removed from the streak camera data is assumed to be the same as the prepulse shape, and therefore the prepulse is generally used for the deconvolution procedure. Figure 6 illustrates the quality of the fit of the time-dependent fluorescence data which can be achieved. 

Another type of image noise arises from nonrandom or systematic addition of counts due to imaging devices or procedural artifacts. For example, bladder uptake of 18F-FDG may obscure the lesions in the pelvic area. Various streak type artifacts introduced during reconstruction may be present as noise in the image. 

If the system works with a potential gradient less than 20 V/cm the procedure is called low voltage electrophoresis. There are no additional needs to control evaporation of liquid and warming up the filter paper strips. The filter paper strip is wetted with the electrophoresis buffer and positioned into the electrophoresis equipment. The sample is applied either in the form of drops or streaks. Commercially available applicators can be used for this purpose. Streaks are used to improve resolution of two closely moving zones while dots are preferred if repeated application is needed because of a large volume of diluted sample. After electrophoresis the paper strip is removed from the apparatus and dried in a ventilated oven at 100°C. 

TLC can be scaled-up and used for the isolation of large (10-100 mg) quantities of pure component. The practice of the technique is similar to that for analytical, qualitative scale work. The main difference lies in the plates used. Almost all preparative scale work is carried out, in the adsorption mode, principally on silica gel plates of varying thickness, 1-5 mm, and of 20 x 20 cm dimensions. The sample is applied as a streak, either by a pasteur pipette, syringe or a motorised streak applicator . Advantage can be taken of multiple development techniques, which allow efficient separation of components of markedly different polarities. Bands incompletely resolved can be applied to a fresh plate and rechromatographed with a suitable solvent and development procedure. Once development is complete the bands of component can be scraped off with a razor blade or spatula and the component washed off the adsorbent with a suitable solvent. Plates for preparative chromatography are available with added fluorescent indicator which facilitates non-destructive location of the components. The fluorescent indicator is irreversibly bound to the silica. 

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