Geoscience Reference
In-Depth Information
5.4.9 Linearity, Signal to Noise, and Dynamic Range
The dynamic range of a spectroscopic instrument is of considerable importance because if
allows the measurement of both weak and strong signals that might be closely spaced in
wavelengths. There are several ways to consider the dynamic range and it is often quoted
in a simple way that might not always reflect reality. For any instrument, the key figures of
merit are the maximum signal that can be measured without distortion and what the noise
level is. This gives a SNR maximum level, or the dynamic range of the instrument.
For a fluorimeter instrument the noise level is determined by several factors:
•
The noise level from the photomultiplier detector
•
The stability of the xenon lamp, that is, light fluctuation or noise in the excitation light
level
The stray light performance of the instrument at the wavelengths of interest
•
Single-photon counting fluorimeters exhibit exceptional dynamic ranges compared to their
analog counterparts. Typical photon counters in modern fluorimeters are capable of up to
100 Mega counts per second (Mcps) if the signal is repetitive. Ideally, two photon events
are identified by appropriate discriminators and counted as two events. However, in a prac-
tical situation two problems exist. First, the incoming photon rate is random and as such
the available counting rate is reduced, as we need to be able to distinguish between photon
pulses; and second, the pulse width of each photon pulse is of finite value because of the
detector time responses. Therefore, it is necessary to distinguish between two pulses, that
is, the pulse-pair resolution. If the time between photon pulses is less than or equal to the
time to resolve the two photon pulses then they look like a “single” event and the signal is
lost. This is the time after a first pulse within which the system cannot distinguish the next
photon pulse, that is, a pile up phenomenon and is referred to as the dead-time of the sys-
tem. In this case, the available count rate for a random signal is given by:
I
p
=
(5.11)
where
I
m
is the maximum possible count rate and
D
i
is the effective dead-time of the
system.
Therefore if the maximum count rate is 100 Mcps, and the dead time is about 26-27 ns,
this would give a maximum practical signal count rate of approximately 22 Mcps.
For photon counting, the photomultiplier dark noise level defines the noise level of
the instrument, and this is determined by the operating conditions of the detector itself:
type of detector photocathode, applied high voltage, voltage divider arrangement, and
discriminator threshold value. Typically, for blue-sensitive photomultipliers the noise
level is under 100 counts per second (cps), and for red-sensitive detectors it may be 10
times higher at room temperature owing to the lower work function photocathode and
higher intrinsic dark noise level from such a detector. However, routine cooling can reduce
dark count levels to a photon per second. Therefore the performance of any fluorescence
