Excitation wavelength limitations

Overcoming the Excitation Wavelength Limitation by Multiphoton Excitation... 

Region Wavelength limits Erequency or photon energy Transitions observed or excited... 

If the fluorescence was excited at a wavelength of = 313 nm, then the detection limits were 200 ng (caffeine, theophylline) to 400 ng (theobromine) substance per chromatogram zone. At an excitation wavelength X = 365 nm the detection limits were appreciably lower at 100 ng substance per chromatogram zone for theobromine and theophylline. The sensitivity remained unchanged in the case of caffeine. 

These spherical nano-particles about 55 nm in diameter have a fluorescent material of ruthenium pyridine inside, and the shell of silicon dioxide, as shown in Fig. 36. The excitation wavelength of the ruthenium pyridine is 480 nm and the emission wavelength is 592 nm [81]. In order to get a clear image of nano-particles, the mass concentration of the fluorescent particles should be limited to a very low level. 

In most work on electrochemical systems, use is made of two effects that greatly enhance the Raman signals. One is resonance Raman spectroscopy (RRS), wherein the excitation wavelength corresponds to an electronic transition in an adsorbed molecule on an electrode surface. The other effect is surface-enhanced Raman spectroscopy (SERS), which occurs on certain surfaces, such as electrochemically roughened silver and gold. This effect, discovered by Fleischmann et al. (1974), yields enhancements of 10 to 10 . The vast majority of publications on Raman studies of electrochemical systems use SERS. The limitations of SERS are that it occurs on only a few metals and the mechanism of the enhancement is not understood. There is speculation that only a small part of the surface is involved in the effect. There is a very good review of SERS (Pemberton, 1991). 

A specific and sensitive fluorimetric method was proposed by Al-Majed for the determination of (7))-penicillamine in its pure state and in its dosage forms [24], The method is based on the coupling between (/))-penicillamine and 4-fluoro-7-nitroben-zo-2-oxa-1,3-diazole, and analysis of the fluorescent product was measured at an excitation wavelength of 465 nm and an emission wavelength of 530 nm. The fluorescence intensity was found to be a linear function of the drug concentration over the range of 0.6-3 pg/mL, and the detection limit was 2 ng/mL (13 nM). 

Historically, this has been the most constrained parameter, particularly for confocal laser scanning microscopes that require spatially coherent sources and so have been typically limited to a few discrete excitation wavelengths, traditionally obtained from gas lasers. Convenient tunable continuous wave (c.w.) excitation for wide-held microscopy was widely available from filtered lamp sources but, for time domain FLIM, the only ultrafast light sources covering the visible spectrum were c.w. mode-locked dye lasers before the advent of ultrafast Ti Sapphire lasers. 

Dobutamine hydrochloride may be determined in plasma levels, after extraction, on a C18 reversed-phase column eluted with 22% aceto-nitrile-78% 0.1 M phosphate buffer (pH 2.0) at 2 ml/minute. The drug and its metabolite are detected by a fluorescent detector with an excitation wavelength of 195 nm and a 330 nm emission cut off filter. The retention times of dobutamine and the 3-methoxy metabolite are 5.2 and 7.9 min., respectively. The lower limit of sensitivity is 10 ng/ml. Reproducibility is 5% over a 25-300 ng/ml range. Nylidrin is used as an internal standard (6). 

Weiss and Worsham 259 indicated that the most important factor governing mean droplet size in a spray is the relative velocity between air and liquid, and droplet size distribution depends on the range of excitable wavelengths on the surface of a liquid sheet. The shorter wavelength limit is due to viscous damping, whereas the longer wavelengths are limited by inertia effects. 

The suitability of organic dyes for multicolor signaling at single wavelength excitation is limited due to their optical properties (Fig. Id, f and Table 1). With... 

Fluorescence measurements are useful in limit tests where the trace impurity is fluorescent or can be rendered fluorescent by chemical modification. An example is the determination of aluminium in water for use in haemodialysis solutions by formation of its salt with 8-hydroxyquinolone (Fig. 7.5) followed by quantifieation of the complex using fluorescenee speetrophotometry. The excitation wavelength is set at 392 nm and the emission is measured at 518 nm. This type of fluorescent complex can be used to determine low levels of a number of metal ions. 

---