568
R. W. Kessler and W. Kessler
23.3.4.2 Absolute and Relative Measurements: Detection Limits (DL)
The different spectroscopic methods can be divided into two categories: absolute
and relative measurements [33, 43]. In absolute measurements, the signal grows
from zero or the background noise upwards. This means that a single photon can be
detected on a dark background. Absolute technologies include NMR, fluorescence,
and Raman spectroscopy, where the signal is proportional to the concentration of the
analyte. For relative measurements, the measured parameter refers to a reference,
which is defined as a 100% signal at the defined wavelength, as in MIR, NIR, and
UV–Vis spectroscopy.
The detection limit for absolute measurements is defined as the standard deviation
SD of the background noise with an SNR equals to 1:1. For absolute measurements,
the SNR depends on the value of the background noise. This means that reducing
this noise increases the SNR, which can be achieved simply by cooling the detector.
The detection limit of relative measurements is limited by the normalized signal
magnitude and thus also depends on the values that are output, e.g., as counts or as
voltage. This means that all parameters such as light source (intensity, fluctuations),
beam guidance, polarization of the photons, characterization of the spectrometer used
(e.g., scattered light, losses), the detector used (background, sensitivity, dynamic
range), and also the sample or sample chamber must be controlled and kept constant
in order to compare the individual measurements [43].
23.4 Inline Spectroscopy of Liquids: Interfacing
with Probes
23.4.1 Synopsis and Taxonomy of Probes
Generally, probes may be divided into two types: immersion probes which are directly
integrated into the system (inline), and flow through probes (online), which are
coupled to the system via a bypass. In both these categories, there is a variety of
different configurations.
With the transmission probe, the liquid is illuminated directly and the wavelengthdependent absorption is measured. Derived from a transmission probe is the transflectance probe. In this case, after light has passed the medium it is reflected back
by a mirror. Thus, the path length is twice the path length of the transmission setup.
Apart from this, the construction of the light source and detector is identical to the
transmission probe. Reflection probes belong to the type of backscattering probes and
detect light which is reflected or scattered back from particles or bodies. Fluorescence
probes and Raman probes are special probes for the measurement of fluorescence and
Raman signals, respectively, and similar to scattering probes and will not be shown
here. A particular type of probe is the attenuated total reflectance (ATR) probe. With
this type of probe, the light undergoes total reflection in the probe surface. The
R. W. Kessler and W. Kessler
23.3.4.2 Absolute and Relative Measurements: Detection Limits (DL)
The different spectroscopic methods can be divided into two categories: absolute
and relative measurements [33, 43]. In absolute measurements, the signal grows
from zero or the background noise upwards. This means that a single photon can be
detected on a dark background. Absolute technologies include NMR, fluorescence,
and Raman spectroscopy, where the signal is proportional to the concentration of the
analyte. For relative measurements, the measured parameter refers to a reference,
which is defined as a 100% signal at the defined wavelength, as in MIR, NIR, and
UV–Vis spectroscopy.
The detection limit for absolute measurements is defined as the standard deviation
SD of the background noise with an SNR equals to 1:1. For absolute measurements,
the SNR depends on the value of the background noise. This means that reducing
this noise increases the SNR, which can be achieved simply by cooling the detector.
The detection limit of relative measurements is limited by the normalized signal
magnitude and thus also depends on the values that are output, e.g., as counts or as
voltage. This means that all parameters such as light source (intensity, fluctuations),
beam guidance, polarization of the photons, characterization of the spectrometer used
(e.g., scattered light, losses), the detector used (background, sensitivity, dynamic
range), and also the sample or sample chamber must be controlled and kept constant
in order to compare the individual measurements [43].
23.4 Inline Spectroscopy of Liquids: Interfacing
with Probes
23.4.1 Synopsis and Taxonomy of Probes
Generally, probes may be divided into two types: immersion probes which are directly
integrated into the system (inline), and flow through probes (online), which are
coupled to the system via a bypass. In both these categories, there is a variety of
different configurations.
With the transmission probe, the liquid is illuminated directly and the wavelengthdependent absorption is measured. Derived from a transmission probe is the transflectance probe. In this case, after light has passed the medium it is reflected back
by a mirror. Thus, the path length is twice the path length of the transmission setup.
Apart from this, the construction of the light source and detector is identical to the
transmission probe. Reflection probes belong to the type of backscattering probes and
detect light which is reflected or scattered back from particles or bodies. Fluorescence
probes and Raman probes are special probes for the measurement of fluorescence and
Raman signals, respectively, and similar to scattering probes and will not be shown
here. A particular type of probe is the attenuated total reflectance (ATR) probe. With
this type of probe, the light undergoes total reflection in the probe surface. The