Geoscience Reference
In-Depth Information
Table 5.6.
Typical signal, noise and dynamic range characteristics of photon counting
devices
Method
Peak signal
Noise
Dynamic range
Typical photon counting systems
4-22 Mcps
100 cps
40,000-200,000: 1
detection system is usually defined in terms of its signal-to-noise performance over a
dynamic range (
Table 5.6
).
5.4.10 Speed and Sensitivity
The speed of wavelength scanning is often considered a prime specification in a fluores-
cence spectrometer system. Many users equate this to the ability to perform a large range of
measurements in a short time. Thus high scan speed is considered advantageous. However,
high scan speed is of no importance if the instrument is not able to achieve a reasonable
SNR level during sample measurement at that scan speed. This is the well known speed
versus accuracy or speed versus sensitivity issue. The terms “scan rate” and “slew rate”
are often interchanged although they have distinctly different meanings (see
Table 5.7
).
Manufacturers may quote slew rates to demonstrate high rates of change in wavelength, but
from a practical point of view it is the scan rate that is the important parameter.
The scan rate is determined by the
•
Mechanical rotation speed of the monochromator grating
•
Focal length of the monochromator
•
Groove density of the grating
The focal length and groove density determine the reciprocal linear dispersion of the
monochromator, or how much of the spectrum is spread across a specific distance in the
output plane of the monochromator. Thus, short focal length monochromators with coarsely
ruled diffraction gratings can produce relatively high scan rates.
Short focal length monochromators can have poor resolution compared to longer focal
length units. Also, coarsely ruled gratings produce low spectral resolution outputs. So to
determine the equivalent scan rate of instruments an understanding and comparison of
focal lengths and grating parameters is necessary. Ultimately, there must be a trade-off
between scan speed, spectral resolution, and system sensitivity. Many fluorimeters spec-
ify their scan rates as very slow, slow, medium, fast, and very fast and at the same time
they claim 30,000 nm min
-1
or higher slew rates. As a result, the scan and/or the slew rates
have little practical meaning and it is almost impossible to determine the real instrument
performance in this regard. In practice, most users will select a scan speed and slit width
commensurate with their measurement requirements from an SNR point of view, although
the choices they make are often qualitative assessments of spectral quality rather than con-
sidered quantitative requirements.
