Geoscience Reference
In-Depth Information
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In-plane scattered light
is light that is not wanted in the dispersion plane of the mono-
chromator and is due mainly to variations in groove spacing or depth of the grating. The
intensity of diffuse scattered light exiting a monochromator is proportional to the slit area
and also to 1/
λ
2
.
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Ghost light
is primarily a scattering effected in mechanically ruled gratings and is caused
by periodic errors in the groove spacing at the time of ruling. It manifests as systematic
periodic spikes on the background signal. Holographic gratings do not, in general, suffer
from this potential scattering problem.
Stray light that is not caused by the scattered light from the grating is called instrumen-
tal stray light. Every monochromator will reflect light in the zero order and this must be
trapped to minimize its contribution to the overall instrumental stray light. Similarly light
from the other diffraction orders may also find its way to the exit slit and therefore con-
tributes to the stray light. Careful instrument design, particularly with respect to baffles,
along with correct illumination of the optics, minimization of sharp edges, and “nonopti-
cal” reflective surfaces will all contribute to minimizing stray light problems.
The key performance indicator for any instrument is the overall SNR, which evaluates
the ratio of diffracted light to unwanted light. Because this is an instrument function there
is no clear rule-of-thumb that indicates what grating type, ruled or holographic, might pro-
vide the higher SNR. It is the SNR of an instrument that will determine the system linearity
and dynamic range.
5.4.14 Cuvettes, Cleaning and Handling
There is a large range of optical cells available for spectroscopy measurements and they
vary in materials, size, shape, and spectral transmission characteristics. The most commonly
used sample holder in fluorescence spectroscopy is a 10 mm × 10 mm × 45 mm volume
cuvette made from fused silica (for UV to near-IR operation), glass (visible), or a plastic
that is often polycarbonate and disposable. Of course, it is essential that the cuvette cho-
sen is suitable for the application and experiment and that it is clean, and handled with
maximum care.
Checking the transmission of the cuvette and solvent in the wavelength range of exci-
tation and emission is critical. This should be done using a quality absorption spectropho-
tometer. After all, if there is no transmission of light to the sample or the emission signal
is blocked to the analyzing channel of the fluorimeter then there will be no fluorescence
measurement. Plastic cuvettes are useful because they are inexpensive, disposable, do not
require cleaning and are robust. Even so, such cuvettes are unsuitable for some applications
such as:
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They demonstrate strong polarization effects and are therefore not suitable for any mea-
surement involving polarizers.
They can be dissolved by many organic solvents commonly used for fluorescence
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measurements.
