Horiba Scientific FluoroMax-4 Operation Manual page 49

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FluoroMax -4 & FluoroMax -4P with USB rev. D (30 Jul 2012)
Calculation of water-Raman signal-to-noise ratio
Introduction
The water-Raman test is a good measure of relative sensitivity between different in-
struments, if the experimental conditions used to compare the systems are the same.
Unfortunately, there are different ways of handling the data, all of which are valid but
which will give quite different values. Therefore, it is important not only to know how
the water-Raman S/N values are measured, but also how the data were treated. The wa-
ter-Raman S/N test method combines a value for system sensitivity (a signal) with a
value for system noise (no signal) to show the overall performance of the instrument.
Definitions
At HORIBA Scientific, we define the S/N ratio as the difference of peak and back-
ground signal, divided by the square root of the background signal.
S
S
peak
N
The peak signal is measured at the water-Raman peak (397 nm for 350 nm excitation)
and the noise in a region (450 nm) where no Raman signal is present. An "ideal" sys-
tem would give a signal value of zero at 450 nm. Thus the equation becomes
S
S
397
N
Another commonly used method is to divide the difference (peak signal minus back-
ground signal) by the rms value of the noise on the background signal. This second
method is used by a few other manufacturers:
S
S
peak
N N
Example
Some actual data from a typical FluoroMax
tween the two methods. The experimental conditions were:
Excitation
Emission
Data-
handling
Note:
All HORIBA Scientific systems' parameters are specified with a
R928P photomultiplier tube at room temperature.
S
background
Our method
S
background
S
nm 450 nm
S
450 nm
S
background
rms, background
350 nm, 5 nm bandpass
365–450 nm, 5 nm
bandpass
No smoothing
Make sure the test is carried out with the actual detector used.
Other manufacturers' method
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system serve to show the difference be-
Increment 0.5 nm
Integration 0.5 s
time
Detector Standard room-temperature, red-
sensitive
3-13
System Operation