Vapor-phase fluorescence

Detection reagents can also be applied to the layer as a vapor, as mentioned above for iodine. Other reagents delivered to the layer by vapor exposure include t-butyl hypochlorite and HCl, both of which form fluorescent derivatives with a variety of compounds. The Analtech vapor-phase fluorescence (VPF) visualization chamber provides detection of compounds such as sugars, lipids, steroids, flavonoids, and antibiotics by induced fluorescence after heating the sealed chamber, containing the plate and ammonium bicarbonate crystals, on a hotplate to a temperature that decomposes the salt to ammonia. 

MaxweU, R.J. An efficient heating-detection chamber for vapor phase fluorescence TLC. J. Planar Chromatogr. Mod. TLC 1988,1, 345-346. 

Figure 8.6 Vapor phase fluorescence visualization chamber. (Photograph courtesy of Analtech.)...

The design of a leak-proof heating-detection chamber from a 2-L resin kettle was described by Maxwell (1988), and this apparatus was used for the comparison of the induced vapor-phase fluorescence detection of four polycyclic ether antibiotics and lipids on Cig and silica gel layers (Maxwell and Unruh, 1992). 

Maxwell, R. J., and Unruh, J. (1992). Comparison of induced vapor phase fluorescent responses of four polycyclic ether antibiotics and several lipid classes on RP-18 and silica gel HPTLC plates. J. Planar Chromatogr.—Mod. TLC 5 35-40. 

Fig. 1 Fluorescence scan of 30 ng thiamine after vapor-phase derivatization (A) and an accompanying blank track (B).

DeWild JF, Olson ML, Olund SD. 2002. Determination of methyl mercury by aqueous phase ethylation, followed by gas chromatographic separation with cold vapor atomic fluorescence detection, U.S. Geological Survey Open File Report 01 45, 23 p. 

Designing a conjugated polymer sensor based on FQ, however, is not only a matter of making a fluorescent polymer for which the photoinduced electron transfer reaction is energetically favorable. There are other important factors that must be considered and requirements that must be met to rehably detect any analyte of interest, including TNT, from the vapor phase. In the broadest sense, these considerations distill to the two primary considerations for any sensing system, sensitivity and selectivity. 

Fisher, M. and C. Cumming. Detection of trace concentrations of vapor phase nitroaro-matic explosives by fluorescence quenching of novel polymer materials, in Proceedings of 7th International Symposium on the Analysis and Detection of Explosives, Defense Evaluation and Research Agency, Edinburgh, Scotland, UK, June, 2001. 

Fisher, M. and C. Cumming. Utilization of novel fluorescent polymer materials for trace level vapor-phase detection of nitroaromatic explosives, in Proceedings of the U.S. Federal Aviation Administration s Third International Aviation Security Technology Symposium, Atlantic City, NJ, November 27-30, 2001c. 

Fluorescence was observed for the TMB family such as 3,5-dimethoxyphenol, l,3-dihydroxy-5-methoxybenzene (5-methoxyresorcinol), l-acetoxy-3,5-dimethoxyben-zene, and l,3,5-trimethoxy-2-methylbenzene. These results indicated that complete symmetry of the substitution on O atoms is not necessary to observe fluorescence from the TMB family, and that the variation of parent molecules of fluorescent radical cation is possibly performed [153]. Fluorescence was also detected from hexamethxybenzene as an example of pseudo-Dgh molecules. The discussion of the symmetry has been described here on the fluorescence from fluorobenzenes in the vapor-phase or noble gas matrices. 

Just as photoassociation may be inferred from the observation of fluorescence self-quenching, so may the negative temperature coefficient of KM be adopted as a criterion of excimer dissociation in the absence of excimer fluorescence. The application of Eq. (31) to data obtained for the selfquenching of anthracene in the vapor phase leads to a value of — AHa 7.6... 

Benzo[c]furan (4) exhibits a long-wave absorption band of medium intensity in the region of 340 nm. Lack of solvent dependence together with mirror relationship to the fluorescence spectrum signifies a tt-ti band a rotational analysis of the vapor phase spectrum led to an assignment as 82 <- Ap 1,3-Diaryl-substituted benzo[c]furans show a strong absorption band in the region of 415 nm in sterically hindered compounds, this... 

In the vapor phase, the emission from excited acetone has been shown to be a mixture of fluorescence and phosphorescence [254]. The estimated lifetime of the excited singlet state is 10 ns, a figure commonly accepted as a reasonable approximation to the excited singlet to triplet (St - TJ transition time in aliphatic ketones. The overlap of the fluorescence and phosphorescence spectra reflects the fact that the energetic separation between the lowest n, n single and triplet states is small, at least in comparison with the S2 — Tt splitting between lowest excited n, n singlet and triplet states. 

Several formal and informal intercomparisons of nitric acid measurement techniques have been carried out (43-46) these intercomparisons involve a multitude of techniques. The in situ measurement of this species has proven difficult because it very rapidly absorbs on any inlet surfaces and because it is involved in reversible solid-vapor equilibria with aerosol nitrate species. These equilibria can be disturbed by the sampling process these disturbances lead to negative or positive errors in the determination of the ambient vapor-phase concentration. The intercomparisons found differences of the order of a factor of 2 generally, and up to at least a factor of 5 at levels below 0.2 ppbv. These studies clearly indicate that the intercompared techniques do not allow the unequivocal determination of nitric acid in the atmosphere. A laser-photolysis, fragment-fluorescence method (47) and an active chemical ionization, mass spectrometric technique (48) were recently reported for this species. These approaches may provide more definite specificity for HN03. Challenges clearly remain in the measurement of this species. 

The fluorescent yield of hexafluorobenzene at all wavelengths is very small60 and, with the exception of 2800 A, increases linearly with pressure of hexafluorobenzene or of inert gas. Thus it is clear that there is some process competing with collisional loss of vibrational energy. There were indications that the normal techniques used to estimate triplet-state yields were not successful in this instance. The hexafluorobenzene triplet state probably has a very short mean life, but it seems unlikely that the competing process is an intersystem crossing. This process could be an isomerization, and indeed, Haller has identified Dewar hexafluorobenzene as a product in the vapor-phase photolysis of hexafluorobenzene.69 The yield was very small. Since the isomers are both formed and destroyed photochemically, the steady-state concentration of isomers is usually low. 

As has been previously reported, when pyrene is adsorbed on silica gel there is evidence for ground state association which is not present In solution or the vapor phase.9-13 but which has been described as being present when pyrene is dissolved in a plastic medium. This is also a manifestation of surface inhomogeneity -some sites enhance the tendency to form a ground state bimolecular complex, whereas other sites contain isolated pyrene molecules. The interaction differences are sufficient to yield significant spectral shifts in absorption and the ground state complex emits with the characteristic pyrene excimer fluorescence. Fig. 5 shows a typical set of spectra Illustrating this association and Fig. 6 presents evidence that this observation Is not due to microcrystal formation. 

Another example is found with benzene vapor at room temperature. The emission yield (fluorescence only) is about 0.20 0.04 and essentially independent of pressure100,101. There are products formed, although there is some disagreement on this subject and the yields in the gas phase have not been determined. Also there is a crossover from the singlet to the triplet state so that all or nearly all of the primarily excited singlet state molecules either fluoresce or cross over to the triplet state. Nevertheless, in the vapor phase at room temperature, about 80 per cent of the molecules which absorb neither react chemically nor emit radiation100, 101. 

Figure 15-27. Normalized fluorescence excitation spectra of ABN, DMABN, and DIABN in a free jet. The excess energies are with respect to their origin bands. Inset plots represent the fluorescence lifetimes (in ns), measured as a function of the S excess vibrational energy. Vapor-phase absorption spectra of ABN and DIABN are also shown (dashed curves). (Reprinted with permission from Ref. [60].)...

There are experimental evidence for the assignment of Sj to the ttct state for both HFB and PFB. Figure 15-33(a) presents the fluorescence excitation and dispersed fluorescence spectra of HFB in supersonic free jet [74], The fluorescence excitation spectra very closely mimic the vapor-phase irir S0 absorption spectra of the compound. It is evident that there is no spectral overlap between the fluorescence and the tht <- S0 absorption spectra of HFB. The energy difference between the absorption and emission maxima is greater than 11 000 cm-1. Moreover, the full width at half maximum (FWHM) of the absorption is about 3000 cm-1, whereas that of the dispersed emission is about 5500 cm-1. For fluorinated benzenes with four or less F atoms, the absorption and emission bands overlap with the Stokes shift of about 4000 cm-1, and the FWHM of both bands is about 3000 cm-1. The FWHM absorption bandwidth of 3000 cm-1 is characteristic of tht (Lft) S0... 

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