Geoscience Reference
In-Depth Information
Table 5.5.
Measurement and analysis of signal-to-noise using the water Raman test
Measurement conditions
Excitation:
Emission scan:
Integration time:
λ
exc
= 350 nm, with
Δλ
exc
= 5 nm
370 <
λ
em
< 460 nm, with
Δλ
em
=5nm, and scan
in
δλ
=1 nm steps
1 s per step
Analysis method
Signal = (peak signal at 397 nm) - (Average back ground signal)
SNR: Signal/background noise in range 450-460 nm
370 380 390 400 410
Wavelength (nm)
420 430 430
Figure 5.17. Typical spectra showing the water Raman measurement using an excitation wavelength
of 350 nm.
In reality, the long-term stability of an instrument is one of the key concerns of both the
manufacturer and the user. Thus, day-to-day and even instrument-to-instrument validation is
needed. One of the best known methods to achieve this is to undertake the so-called “water
Raman test.” Assuming the water sample is of very high purity and is contained in a fully
sealed cuvette to prevent ingress of contaminants, this test can be reliable at wavelengths
below 400 nm. The reason for this is that the intensity of the Raman signal changes by 1/
λ
4
,
thus at longer wavelengths of excitation the signal levels are considerably reduced. For
many manufacturers the water Raman test and the resulting SNR become a measure of their
instrument performance guarantee. Not all manufacturers record this signal nor calculate the
resulting SNR in the same manner, so considerable care should be applied in the exact defi-
nitions and comparison. Presented in
Table 5.5
are some suggested measurement conditions
and analysis methods for routine assessment of the instrument performance in terms of total
signal and water Raman SNR. A water Raman spectra is shown in
Figure 5.17
.
