Applications of LIF

Laser-induced fluorescence (LIF) depends on the absorption of a photon to a real molecular state, and is therefore a much more sensitive technique, capable of detection of sub-part-per-billion concentrations. Thus, this is the most suitable for measurement of those minor species which are the transient intermediates in the reaction network. Here a tunable laser is required, as well as an electronic absorption system falling in an appropriate wavelength region serendipitously, many of the important transient species have band systems which are suitably located for application of LIF probing. The ability to sensitively detect transitions originating from electronically as well as vibrationally excited levels of a number of molecules offers the possibility of inquiring into the participation of non-equilibrium chemistry in combustion processes. 

An interesting application of LIF is the measurement of relative population densities N v", 7") and their distribution over the different vibrational-rotational levels (v", J ) under situations that differ from thermal equilibrium. Examples are chemical reactions of the type AB - - C AC -I- B, where a reaction product AC with... 

Figure 1 Scope of application of LIF spectroscopy in clinical analysis.

An interesting application of LIF is the measurement of relative population densities // ) and their distribution over the different vibrational-... 

Since 1990, laser-induced fluorescence (UF) detection has become increasingly important. A good review of the principles and applications of LIF detection is given in [25]. For more detailed information on fluorescence detection, see [26] [27]. 

A very interesting application of LIF is the measurement of population distributions N(v., J. ) in rovibronic molecular levels under situations which differ from thermal equilibrium. Examples are chemical reactions of the type AB + C - AC + B, where a reaction product AC with internal energy is formed in a reactive collision between the partners AB and C. The measurement of the internal state distribution N q(v,J) can often provide useful information on the reaction paths and on the potential surfaces of the collision complex (AB)C. The fact that for some of these reactions -population invevsion has been observed allowing the operation of chemical lasers [8.71] may elucidate the importance of such studies. A better knowledge of the reaction mechanisms can help to optimize the conditions for maximum inversion. 

In peptide syntheses, where partial racemization of the chiral a-carbon centers is a serious problem, the application of 1-hydroxy-1 H-benzotriazole ( HBT") and DCC has been very successful in increasing yields and decreasing racemization (W. Kdnig, 1970 G.C. Windridge, 1971 H.R. Bosshard, 1973), l-(Acyloxy)-lif-benzotriazoles or l-acyl-17f-benzo-triazole 3-oxides are formed as reactive intermediates. If carboxylic or phosphoric esters are to be formed from the acids and alcohols using DCC, 4-(pyrrolidin-l -yl)pyridine ( PPY A. Hassner, 1978 K.M. Patel, 1979) and HBT are efficient catalysts even with tert-alkyl, choles-teryl, aryl, and other unreactive alcohols as well as with highly bulky or labile acids. 

From shock compression of LiF to 13 GPa [68] these results demonstrate that X-ray diffraction can be applied to the study of shock-compressed solids, since diffraction effects can be observed. The fact that diffraction takes place at all implies that crystalline order can exist behind the shock front and the required readjustment to the shocked lattice configuration takes place on a time scale less than 20 ns. Another important experimental result is that the location of (200) reflection implies that the compression is isotropic i.e., shock compression moves atoms closer together in all directions, not just in the direction of shock propoagation. Similar conclusions are reached for shock-compressed single crystals of LiF, aluminum, and graphite [70]. Application of these experimental techniques to pyrolytic BN [71] result in a diffraction pattern (during compression) like that of wurtzite. 

Another application of the electrolysis of tantalum and niobium in fluoride melts is in the preparation of intermetalic compounds as a result of the interaction between the electrochemically precipitating metal and the cathode material. Based on an investigation of the electrochemical reduction of K2TaF7 or K2NbF7 in a LiF - NaF melt on nickel cathodes, Taxil and Qiao [565] determined the appropriate conditions for the formation of TaNi3 or NbNi3 in the form of stable phases in the bulk of the obtained layer. 

Temporal sequence of OH-LIF measurements captures a localized extinction event in a turbulent nonpremixed CH4/H2/N2 jet flame (Re 20,000) as a vortex perturbs the reaction zone. The time between frames is 125 ps. The velocity field from PIV measurements is superimposed on the second frame and has the mean vertical velocity of 9m/s subtracted. (From Hult, J. et al.. Paper No. 26-2, in 10th International Symposium on Applications of Laser Techniques to Fluid Mechanics, Lisbon, 2000. With permission.)... 

However, using alkaline earth metal fluorides gives a less pronounced improvement. The use of CsF and LiF as CIM layer has the same effect. However, unlike LiF, CsF reacts directly with A1 and releases Cs metal, whereas the dissociation of LiF in the presence of A1 is thermodynamically disallowed and proceeds only in the presence of suitable reducible organic materials such as Alq3. Thus CsF is more generally applicable to many organic materials. 

LS Hung, CW Tang, MG Mason, P Raychaudhuri, and J Madathil, Application of an ultrathin LiF/Al bilayer in organic surface-emitting diodes, Appl. Phys. Lett., 78 544—546, 2001. 

While LIF monitoring is established in other manufacturing industries, it is relatively new within the pharmaceutical industry. The application of real-time intrinsic LIF for the manufacture of pharmaceuticals in general can be broken in to three areas (i) classic (small molecule) final drug product manufacturing (ii) biopharmaceutical prodnct manufacturing and (iii) factory operational applications. 

NIR methods are not the only on-line applications for blend monitoring FT-Raman " and laser induced fluorescence (LIF) have been utilized. Refer to Chapter 11 for a comprehensive review of LIF. As stated herein, NIRS is well established as an effective and advantageous means to deem blend homogeneity and blending end point, however there are circumstances in which NIR is insufficient. For example, LIF can be more suitable for blends with low drug load. Lai and Cooney illustrated in a lab-scale experiment that LIE yielded a limit of detection below 0.02% w/w for a given API. ... 

FTIR, TDLAS, and LIF are in situ techniques, whereas DOAS is a long-path method that gives only a path-integrated result. NO, N02, NO,3, and HONO have been successfully measured with DOAS even in rural and remote regions. PAN, HN03, and NH3 have been measured with FTIR in urban areas, but its sensitivity at present is not adequate for levels below a few parts per billion by volume. NO, N02, PAN, HN03, and NH3 have been measured with TDLAS down to sub-ppbv levels. A review with references for applications of these three methods is available (13). The LIF method has been more recently developed, has been applied to the measurement of NO, N02, NH3, and HONO (see reference 14 for an example), and offers sensitivity down to the parts per trillion by volume level. 

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