Optical detection systems laser-induced fluorescence

Using the same PAbs an optical biosensor system has been developed for 2,4,6-TCP [224]. The principle is the detection of laser-induced fluorescence (LIF) in single microdroplets by a homogeneous quenching fluorescence immunoassay (QFIA). The competitive immunoassay occurs in microdroplets (d=58.4 mm) produced by a piezoelectric generator system. A continuous Ar ion laser (488 nm) excites the fluorescent tracer and its fluorescence is detected by a spectrometer attached to a cooled, charge-coupled device (CCD) camera... 

The most widely employed optical method for the study of chemical reaction dynamics has been laser-induced fluorescence. This detection scheme is schematically illustrated in the left-hand side of figure B2.3.8. A tunable laser is scanned tlnough an electronic band system of the molecule, while the fluorescence emission is detected. This maps out an action spectrum that can be used to detemiine the relative concentrations of the various vibration-rotation levels of the molecule. 

Samples are introduced into the capillary by either electrokinetic or hydrodynamic or hydrostatic means. Electrokinetic injection is preferentially employed with packed or monolithic capillaries whereas hydrostatic injection systems are limited to open capillary columns and are primarily used in homemade instruments. Optical detection directly through the capillary at the opposite end of sample injection is the most employed detection mode, using either a photodiode array or fluorescence or a laser-induced fluorescence (LIF) detector. Less common detection modes include conductivity [1], amperometric [2], chemiluminescence [3], and mass spectrometric [4] detection. 

Sensitive Optical Detectors. More sensitive optical techniques that have been used with CE include fluorescence, refractive index, chemiluminescence, Raman spectrophotometry, and circular dichroism. The most sensitive optical detection method used in CE is laser-induced fluorescence (LIE), which is capable of detection limits in the 10 to 10" mol (or better) range. This detection mode is easily accomplished with analytes that are either easily labeled with a fluorescent substrate (e.g., intercalators for double-stranded DNA) or are naturally fluorescent (e.g., proteins or peptides containing tryptophan). CE systems have also been interfaced with mass spectrometers, and electrochemical detection methods have been developed, although such detectors must be isolated electrically from the electrophoretic voltages. 

The trapped ions can be monitored either by laser-induced fluorescence [1222, 1223] or by the RF voltage that is induced in an outer RF circuit by the motion of the ions [1217]. The LIF detection is very sensitive for a true two-level system, where the fluorescence photon rate /f of a single ion with a spontaneous upper-level lifetime may reach R = (2xk) [s ] (Sect. 9.1.4). Even for a three-level system, this can be achieved if a second laser is used that refills the ground-state level, depleted by optical pumping (Fig. 9.5). For t = 10 s, this implies that for sufficiently large laser intensities a single ion emits up to 5 x 10 fluorescence photons per second, which allows the detection of a single stored ion [1225,1226]. 

The detection system is the other cornerstone to improve the concentration limits of detection. Although CE is more easily interfaced with optical detection methods based on UV—visible absorption and laser induced fluorescence (LIE), a variety of detection systems are suitable for the identification and quantification of inorganic compovmds, organic contaminants, and NOM in the enviromnent. Table 5 lists the most common detectors used in CE enviromnental applications. 

Laser-induced fluorescence (LIF) is particularly suited for detection on the micro chip. It is easy to focus the light bundle on the optical path of the system and it is little susceptible to background noise due to its specificity. In addition to this, combining LIF with sensitive photomultiplier tubes (PMT) makes it very sensitive. The LIF... 

---