Wavelength, emission

Compound Solvent pH Excitation wavelength, nm Emission wavelength, nm... 

An instrument for measuring fluorescence that uses filters to select the excitation and emission wavelengths. 

The analysis of cigarette smoke for 16 different polyaromatic hydrocarbons is described in this experiment. Separations are carried out using a polymeric bonded silica column with a mobile phase of 50% v/v water, 40% v/v acetonitrile, and 10% v/v tetrahydrofuran. A notable feature of this experiment is the evaluation of two means of detection. The ability to improve sensitivity by selecting the optimum excitation and emission wavelengths when using a fluorescence detector is demonstrated. A comparison of fluorescence detection with absorbance detection shows that better detection limits are obtained when using fluorescence. 

Excitation and emission wavelengths are for the unconjugated fluorophore. Wavelengths for conjugates may vary according to the composition of the conjugate. 

Although the LED is one of the most basic optoelectronic devices, there exists a variety of complex and interacting material and stmctural considerations in designing these devices. These include the choice of materials for emission wavelength of the LED as well as the geometry and fabrication methods of the device. The principal stmctural properties of commercially available LEDs are summarized in Table 1. 

Fig. 9. Luminous efficiency vs peak emission wavelength for ( ) conventional commercial LED technologies. Also shown are data for the emerging...

Pig. 11. Luminous performance vs peak emission wavelength for the best reported high brightness LEDs. Some high performance lighting sources are indicated. The numbers in parentheses correspond to the source wattage. Also shown is the (—) eye response curve (as defined by the Commission... 

Laser sources that emit in the mid-ir region of the spectmm (2—5 -lm) are useful for detection of trace gases because many molecules have strong absorption bands in that region. Other appHcations include remote sensing and laser radar. Semiconductor lead—salt (IV—VI) lasers that operate CW at a temperature of 200 K and emission wavelength of 4 p.m are commercially available however, they have relatively low output powers (<1 mW) (120). 

Direct-reading polychromators (Figure 3b) have a number of exit slits and photomultiplier tube detectors, which allows one to view emission from many lines simultaneously. More than 40 elements can be determined in less than one minute. The choice of emission lines in the polychromator must be made before the instrument is purchased. The polychromator can be used to monitor transient signals (if the appropriate electronics and software are available) because unlike slew-scan systems it can be set stably to the peak emission wavelength. Background emission cannot be measured simultaneously at a wavelength close to the line for each element of interest. For maximum speed and flexibility both a direct-reading polychromator and a slew-scan monochromator can be used to view emission from the plasma simultaneously. 

The composition must be controlled to give the required emission wavelength. Techniques utilized include molecular-beam epitaxy (MBE) and liquid-phase epitaxy (LPE). 

Detection is carried out using a fluorescence detector, with an extinction wavelength of 340 nm and an emission wavelength of 445 nm. With this method it is possible to detect amino acid at concentrations of 5 pmol ml in the sample, which corresponds to 450 fmol per amino acid injected. The method may be applied to samples containing between 5 and 400 pmol mU per amino acid. 

More precise control over the emission color may be achieved by employing equimolar quantities of a fc/.v-phosphonium salt, e.g., 67 and a dialdehyde containing a flexible unit sueh as 66. This approach has been exploited by a number ol researchers 98-102[. The emission wavelength of these materials is in the blue te blue-green region of the spectrum (470-495 nm). 

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