FLUORESCENCE
FLASH AND CONTINUOUS LIGHT SOURCES IN FLUORESCENCE
For fluorescence measurements from the ultraviolet through the visible and into the near
infrared region, the typical light source is the Xenon arc lamp. This particular source has been
widely used due to its favorable characteristics, primarily the ability to deliver high energy
output over a wide wavelength range. Xenon lamps in fluorescence instruments can be
operated in either the continuous mode or as flash lamps. Both types of lamps consist of two
electrodes, i.e. an anode and a cathode, housed within a bulb containing highly purified Xenon
gas at several atmospheres pressure. In continuous operation a trigger voltage on the order of
10 to 20 kV is delivered to the electrodes creating an arc that then ignites the Xenon gas
contained in the lamp bulb. Once ignited the arc discharge is made continuous by maintaining
constant DC current, on the order of 10A for a 150W lamp. More recently, Xenon flash lamps
have been introduced as sources in fluorometers and fluorescence detectors for HPLC. In
such lamps a trigger voltage is supplied to the electrodes in a bulb containing Xenon gas
under pressure, creating an arc across the electrodes in a manner similar to continuous lamps.
The difference is that in the case of the flash lamps, the arc is not maintained. Rather,
successive arcs are triggered repetitively at a maximum rate approaching 100Hz. The
essential features of continuous operation xenon arc lamps are illustrated in Figure 1. (1)
Figure 1
Schematic of a typical 150W Xe lamp in continuous operation After ignition, several minutes are required for the lamp to reach temperature equilibrium and stable energy output. During operation, the Xe gas pressure increases approximately three‐fold over the pressure at ambient temperature. In order to maintain constant lamp energy, it is essential to have a well‐regulated power supply. As indicated in Figure 1, it is typical to have a dedicated power supply to trigger lamp ignition and a separate power supply for maintaining constant lamp current. As indicated in the center section of Figure 1, the energy distribution and hence the light intensity is not evenly distributed between the lamp electrodes but is more intense closer to the cathode. Continuous lamps distribute the energy over quite a large area, as indicated in the polar coordinate plot in the right section of Figure 1. In fluorescence instruments, this allows for efficient collection of energy by focusing mirrors and more energy distributed over wavelength selection elements such as diffraction gratings, which results in improved instrument performance. The spectral distribution of a typical 150W Xe lamp operated in the continuous mode is shown in Figure 2. (1) Spectral Distribution of a Continuous Operation Xe Lamp The energy distribution curve shown in Figure 2 indicates that during continuous operation, the Xe lamp has relatively low energy output below 300nm, more or less constant energy output between 350 and 750nm and several intense lines in the near infrared between 800 and 1,000nm. It is important to note that the ordinate scale represents lamp power per square cm measured at a distance of 50cm from the source. The curve will shift up or down depending on the wattage of the lamp, i.e. it would be higher for a 300W lamp and lower for a 75W lamp, but the shape of the curve and its characteristics will not change. For example, the ratio of energy at 500 and 200nm for a 300W continuous lamp is approximately 20 as is the same ratio for a 150W lamp. Therefore, one cannot increase the relative lamp efficiency at lower UV wavelengths simply by using a more powerful lamp. On the other hand, nearly constant energy from 350 through 750nm is advantageous in terms of signal stability and measurement repeatability as well as in the fact that the excitation and emission wavelengths of many fluorescent compounds lie within this wavelength range. A schematic of a typical Xe flash lamp is shown in Figure 3. In this lamp a voltage is applied across the anode and cathode and arc formation is triggered through the probes. (2) Figure 3 Flash Lamp Construction The spectral distribution of a Xe flash lamp is illustrated in Figure 4. (2) Compared with a continuously operated lamp, the relative energy output from the flash lamp is significantly higher in the ultraviolet region and significantly lower in the visible region. For example, in the continuously operated lamp, the energy ratio at 450/250 nm is approximately 4/1 while for the flash lamp, the energy ratio at the same wavelengths is 0.4/1. This difference lies in the fact that the output of the flash lamp is used at its ignition point which is at a significantly higher temperature(color temperature of 15000K) than the operating temperature(color temperature of 6000K)of a continuously operated lamp. The absolute power of most continuously operated Xe lamps (typically 150W) used in fluorescence instruments is significantly greater than that of flash lamps (35W max) but many factors determine how efficiently that energy is used. Figure 4 Spectral Distribution of a Xe Flash Lamp Flash lamps are obviously an advantage for time‐resolved measurements where light pulses with well defined pulse‐width, duration, and intensity are required. These types of lamps require significantly smaller power supplies and associated electronics than continuously operated lamps. In steady state measurements of fluorescence spectra, quantitative measurements, or reaction rates there is arguably little benefit, if any, in a flash as opposed to a continuous source. There is, however, a significant difference between the two types of sources in terms of useable lifetime. A typical specification for the maximum number of flashes per lamp for a flash lamp is 109, and the maximum flash rate is typically on the order of 100 Hz (2). Thus, the theoretical lifetime for a flash lamp operated under these conditions would be roughly 2,700 hours. On the other hand, for a 150W, continuously operated Xe lamp, it is not uncommon for the manufacturer to guarantee a lifetime of 1,000 to 1,200 hours with average lifetimes approaching 1,800 to 2, 500 hours (1). It must be remembered, however, that the working lifetime of a lamp in a particular instrument is almost always shorter than the lamp lifetime guaranteed by the lamp manufacturer. When a lamp manufacturer does performance characterizations on their products, they rightly select optimum standardized conditions for the tests, i.e. optimum operating voltage, exquisitely controlled power supplies and optimum sampling conditions. Optimum sampling conditions for a continuous lamp would obviously be under continuous operation with a single ignition followed by measurement until the lamp performance falls below specifications. This is far removed from normal instrument operation where the lamp is ignited on numerous occasions during its lifetime and per use operating times can range from an hour or less to an eight‐hour shift, to more extended use. The same effect of operating conditions on working lifetime can be said for flash lamps. Signal stability is another performance characteristic for both types of lamps, perhaps more so in the case of flash lamps where repeated firings on the order of 80 to 100 Hz are required. The differences in stability between continuously operated Xe lamps and flash lamps are illustrated in Figure 5 (1,2). Figure 5 Fluctuation Data for Continuous and Flash Lamps The data illustrated in Figure 5 is representative of best performance under ideal test conditions. Actual signal stability in fluorescence instruments depends on numerous factors. Nevertheless, the data shown in Figure 5 does illustrate that a 150W Xe lamp in continuous operation for 500 hours is still well within the specification of ± 1% fluctuation. The same specification for a 10 to 15W ‘Super‐Quiet’ flash lamp at 500 hours operation is ± 2.5%, and the fluctuation specification for a 10W “High‐Power” flash lamp is ± 5%. An additional consideration is the need for increased shielding of electronics and detectors during lamp ignition. Since there is extremely high current flow during the firing of a Xe lamp, there is associated electromagnetic noise. In continuously operated lamps such noise is quickly dissipated after a stable arc has formed but is obviously constant in flash lamps since these lamps fire at a high frequency. Such considerations as lamp stability and noise are perhaps more significant when flash lamps are used as sources in chromatographic detectors. Both types of lamps have characteristics that render them suitable for particular applications. There is no ‘one and only choice’ for sources for fluorescence instrumentation. Rather, the application itself will determine which source, which measurement method, and which instrument is best suited for the task(s) at hand. References 1. “Super Quiet Xenon Lamps”, Technical Literature TLSX1002E06, Hamamatsu Photonics K.K, March, 2000. 2. “Xenon Flash Lamps”, Technical Literature TLSX1008E04, Hamamatsu Photonics K.K. April, 1998. Shimadzu Scientific Instruments
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