Geoscience Reference
In-Depth Information
Table 5.7.
Definitions of slew rate and scan rate
Scan rate
The
scan rate
represents the continuous speed of scanning a spectrum between
two wavelengths.
Slew rate
The
slew rate
represents the maximum rate of change of wavelength.
The single-photon counting fluorimeter has many performance advantages over con-
ventional fluorimeter instruments. One of the most important is that it uses single-photon
counting as the measurement technique. This delivers unparalleled sensitivity. It means
you can collect data faster, measure more samples, or work at lower concentrations with
more accuracy. At the same time, the scanning capability of the instrument means more
data can be measured and exposure time to sample is limited, which reduces the possibil-
ity of photo-bleaching or sample degrading with time and hence damaging the integrity
of the sample and results. Single-photon sensitivity allows the user to analyze samples
at low concentrations that is simply not possible with non-photon-counting instruments.
If the emission signal from the samples is strong, then short integration times permit fast
scanning, while maintaining the same level of accuracy and saving measurement time.
The stronger the measured signal, the lower the statistical noise, the better the SNR and
hence the greater the accuracy. Many fluorescence detection systems employ photodiode
array detectors (e.g., high-performance liquid chromatography). These offer single-photon
sensitivity, albeit at lower resolution, as the exit slit width is effectively a single element in
the actual array, e.g., 25 μm. Unfortunately, an in-depth discussion concerning photodiode
arrays detection systems is beyond the scope of this chapter.
5.4.11 Wavelength Accuracy
Wavelength accuracy is a fundamental requirement in any spectrometer system and one
that should be checked on a periodic basis. The normal method to check wavelength accu-
racy is to undertake measurements using a suitable line source lamp such as a low-pressure
mercury lamp. This lamp provides a range of discrete lines that can be used to demonstrate
the wavelength calibration of the system as well as wavelength linearity. At the same time
it can be used to demonstrate the correct and repeatable operation of the grating turret.
The emission lines for a low-pressure mercury lamp are demonstrated in
Table 5.8
, with
the common wavelengths for calibration highlighted in bold.
Most monochromator systems utilize drive mechanisms that use stepping motors. These
stepping motors operate in discrete angular increments called steps. Thus a wavelength
calibration can be made by recording the position of the calibration lines from the lamp as
a function of the number of steps driven from the zero order of the grating and hence the
angular position of the grating. Often this mapping of spectral position with step position
is linear or close to linear in form. A suitable fitting with either a straight line or some form
of polynomial will yield a calibration curve of wavelength versus step (angle) position
