Principles of Fluorescence Spectroscopy

PRINCIPLES OF FLUORESCENCE SPECTROSCOPY 33
Figure 2.8. Compact xenon arc lamps. Reprinted from Cermax lamp
engineering guide.
2.2.3. High-Pressure Mercury (Hg) Lamps
In general Hg lamps have higher intensities than Xe lamps,
but the intensity is concentrated in lines. It is usually better
to choose the excitation wavelengths to suit the fluorophore,
rather than vice versa. These lamps are only useful
if the Hg lines are at suitable wavelengths for excitation.
2.2.4. Xe–Hg Arc Lamps
High-pressure mercury–xenon lamps are also available.
These have higher intensities in the ultraviolet than Xe
lamps, and the presence of Xe tends to broaden the spectral
Figure 2.9. Spectral output of a low pressure mercury–argon lamp.
Revised from [7]. Courtesy of Ocean Optics Inc.
output. The output of these lamps is dominated by the Hg
lines. The Hg–Xe lamp has slightly more output between
the Hg lines. When first started the Hg–Xe lamp output is
due mostly to Xe. As the lamp reaches operating temperature
all the Hg becomes vaporized, and the Hg output
increases.
2.2.5. Quartz–Tungsten Halogen (QTH) Lamps
These lamps provide continuous output in the visible and
IR regions of the spectrum. Previously such lamps were not
useful for fluorescence because they have low output below
400 nm, and are thus not useful for excitation of UV
absorbing fluorophores. However, there is presently
increasing interest in fluorophores absorbing in the red and
near infrared (NIR), where the output of a QTH lamp is significant.
2.2.6. Low-Pressure Hg and Hg–Ar Lamps
These lamps yield very sharp line spectra that are useful
primarily for calibration purposes (Figure 2.9). Previously
the lamps contained only mercury. Some lamps now contain
both mercury and argon. The mercury lines are below
600 nm and the argon lines are above 600 nm (Table 2.1).
The use of these lamps for wavelength calibration is
described in Section 2.3.5.
2.2.7. LED Light Sources
LEDs are just beginning to be used as light sources in spectrofluorometers.
8–9 A wide range of wavelengths are available
with LEDs (Figure 2.10, see also Figure 4.13). In order