Geoscience Reference
In-Depth Information
1
0.8
0.6
0.4
0.2
0
200
300
400
500
Wevelength (nm)
600
700
800
Figure 5.20. Relative intensity of the excitation channel of a fluorimeter as a function of wavelength.
A wide range of quantum counter-designs have been proposed but none overcome the prob-
lems associated with polarization, geometry, concentration, photo-bleaching, and limited
spectral range. Finally, these approaches fundamentally measure photon flux as opposed
to optical power. Nearly all fluorimeter systems are equipped with a reference detector,
usually a UV-sensitive photodiode, to correct the fluorescence signal both spectrally and
temporally. The photodiode is usually calibrated against a known irradiance standard under
the exact same conditions of illumination and spectral range as used in the instrument. This
provides a known and traceable responsivity of the photodiode, R pd (λ . The measurement
of the excitation light level can then be made by monitoring the signal output from the pho-
todiode S pd (λ , and this can be used to correct resulting spectra.
5.4.18 Correcting Excitation Signal Channels
Correction can be divided into two types:
To use a previously measured correction file, or
To actively monitor the light intensity during the measurement and make the correction
live
In the first method it is important to know the spectral response of the reference detector.
This can be measured separately using a calibrated tungsten light source and the mono-
chromator under test in the system if the spectral response of the reference detector is not
already known. Normally, the correction file, , Ex Co ( λ , is normalized to give a range
from 0 to 1, where the peak intensity is 1 and the minimum fluorescence intensity is 0.
If the measured spectrum is S m ( ) then the corrected sample file Sc Ex
( ) = ( ) /
λ
S m
λ
Ex Co ( λ .
It is imperative that the Ex Co ( λ file should be determined under the same bandpass
parameters as the sample measurement. More important is to ensure that the step size for
each file is the same and that the Ex Cor
( λ file covers at least the same spectral region
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