Experimental Design and Quality Assurance
199
a concern. One example is the portable SMF2 spectrophotometer (Safe Training Systems
Ltd., Wokingham, UK), which utilizes a xenon flash lamp, bandpass and interference filters for peak excitation wavelength of 280 nm. The fluorescence signal of tryptophan is
measured between 350 and 360 nm. Measurements from this sensor can be calibrated using
diluted river or waste water samples to detect effluent in natural systems (Baker et al.,
2004).
A great deal of effort has gone into transitioning fluorescence spectroscopy from the
laboratory benchtop to in situ field measurements. In Section 6.3, we will focus on the
most common submersible design types that are commercially available at the time of this
book’s publication that can offer analysts an appreciation of the options for their particular
applications.
6.3 Instrument Design Types
6.3.1 Sensor Configurations
There exists a wide range of sensor designs used by researchers to collect in situ OM
fluorescence measurements. The challenges of in situ measurements are the same – how
to collect high-quality fluorescence data that is inexpensive and limited by low-power UV
capabilities? Now we will focus on the most commonly designed, commercially available field sensor designs. Quite simply, an in situ fluorometer consists of five main optical
components: a UV light source, optical hardware to bring excitation light to the sample
volume, optical hardware to collect the emitted fluorescence from the sample, optical filters to separate the excitation and emission wavelengths of interest, and a photodetector
(Figure 6.7). These components are housed within pressure housings of two types of sensor geometries: (1) the open-faced design, which can either be right-angle (Figure 6.7a)
or flat-faced (optical backscatter type; Figure 6.7b) with a fixed cell geometry and volume
and (2) the flow-through design, which may require a pump to pass the sample through a
quartz cuvette flow tube (Figure 6.7c). With either geometry, sensors can be configured to
collect single or multispectral measurements. Collection over multiple wavelengths greatly
improves the ability to characterize FOM as well as provide a measurement of the bulk
intensity or amount of FOM.
The flow-through geometry type tends to have greater efficiency in signal output compared to the open-faced design, in part due to the absorption of light by the plastic (or epoxy
resin) facing of open-faced meters. Flow-through designs are an ideal choice for waters
with low OM concentration and few particles as the sensor geometry (sample volume) is
well defined as the width and length of the detection area is based primarily on the cuvette
and the geometric area of the excitation and photo diode detector assembly. This geometry
is also well suited for waters with high OM concentration as they have small pathlengths
and are therefore less susceptible to alterations in the sample volume cone that can erroneously reduce fluorescence signals. Limitations of flow-through designs exist for deployment in waters that are not optically clear due to difficulty in maintaining cleanliness of the
199
a concern. One example is the portable SMF2 spectrophotometer (Safe Training Systems
Ltd., Wokingham, UK), which utilizes a xenon flash lamp, bandpass and interference filters for peak excitation wavelength of 280 nm. The fluorescence signal of tryptophan is
measured between 350 and 360 nm. Measurements from this sensor can be calibrated using
diluted river or waste water samples to detect effluent in natural systems (Baker et al.,
2004).
A great deal of effort has gone into transitioning fluorescence spectroscopy from the
laboratory benchtop to in situ field measurements. In Section 6.3, we will focus on the
most common submersible design types that are commercially available at the time of this
book’s publication that can offer analysts an appreciation of the options for their particular
applications.
6.3 Instrument Design Types
6.3.1 Sensor Configurations
There exists a wide range of sensor designs used by researchers to collect in situ OM
fluorescence measurements. The challenges of in situ measurements are the same – how
to collect high-quality fluorescence data that is inexpensive and limited by low-power UV
capabilities? Now we will focus on the most commonly designed, commercially available field sensor designs. Quite simply, an in situ fluorometer consists of five main optical
components: a UV light source, optical hardware to bring excitation light to the sample
volume, optical hardware to collect the emitted fluorescence from the sample, optical filters to separate the excitation and emission wavelengths of interest, and a photodetector
(Figure 6.7). These components are housed within pressure housings of two types of sensor geometries: (1) the open-faced design, which can either be right-angle (Figure 6.7a)
or flat-faced (optical backscatter type; Figure 6.7b) with a fixed cell geometry and volume
and (2) the flow-through design, which may require a pump to pass the sample through a
quartz cuvette flow tube (Figure 6.7c). With either geometry, sensors can be configured to
collect single or multispectral measurements. Collection over multiple wavelengths greatly
improves the ability to characterize FOM as well as provide a measurement of the bulk
intensity or amount of FOM.
The flow-through geometry type tends to have greater efficiency in signal output compared to the open-faced design, in part due to the absorption of light by the plastic (or epoxy
resin) facing of open-faced meters. Flow-through designs are an ideal choice for waters
with low OM concentration and few particles as the sensor geometry (sample volume) is
well defined as the width and length of the detection area is based primarily on the cuvette
and the geometric area of the excitation and photo diode detector assembly. This geometry
is also well suited for waters with high OM concentration as they have small pathlengths
and are therefore less susceptible to alterations in the sample volume cone that can erroneously reduce fluorescence signals. Limitations of flow-through designs exist for deployment in waters that are not optically clear due to difficulty in maintaining cleanliness of the