322
A. L. Chakraborty and A. Roy
on recent calibration-free WMS techniques that have proved to be a very important
development in this area.
1.1 Applications of TDLS
Industrial applications of TDLS are one of the large-scale and demanding applications that test the robustness of these systems. TDLS is used extensively for noninvasive, high-sensitivity detection of trace gases as well as to extract gas parameters
such as mole fraction, pressure and temperature [1–6]. Careful measurement and
monitoring of key process parameters is critical to ensure the health of large installations, reliable functioning of equipment, optimization of critical processes and
preempting potentially hazardous situations. Industrial applications often present
harsh operating conditions such as high pressure and temperature, high humidity
levels, vibrations that cause misalignment of the laser-detector pair, dirt that accumulates on optical surfaces, and corrosive effect of gases [7, 8]. The challenge is
to engineer rugged hardware and robust signal extraction algorithms that can withstand long-term exposure to such harsh environments. Environmental applications
also use TDLS heavily for long-term and continuous atmospheric trace gas measurements over large areas [9–13]. Accurate in situ measurements of greenhouse gases
(GHGs) is a key to managing the burden of emissions. Reliable measurements are
also required to build accurate climate models that help to understand the sink and
source patterns of GHGs and also to assess the effectiveness of international agreements that aim to reduce GHG emissions. Medical applications [14–18] have also
emerged as a very interesting field of application with clinical breath analysis [19]
being an attractive diagnostic technique for point-of-care use. The human breath has
several bio-markers such as acetone, ammonia, oxides of nitrogen that are linked to
abnormal physiological conditions. The primary attraction here is that sample collection is easy but the challenge is that the breath matrix is extremely complicated.
Microbiological applications of TDLS have received very little attention to date [20]
although bacteria are known to thrive in an astonishing variety of extreme physical
and chemical conditions [21–27], and also because the threat of antimicrobial resistance looms large [28, 29]. Recently a non-invasive and non-destructive WMS-based
technique was used to monitor growth by measuring the mole fraction of CO 2 emitted
by Escherichia coli and Staphylococcus aureus over several hours of their life cycles
[30]. WMS has clear advantages over traditional methods based on cell number density such as haemocytometry, plate counting method, ring mounted slide method
and electronic particle counter, that are often erroneous (besides being destructive)
because of improper staining, small cells becoming indistinguishable or other particles being mistaken for a microbe. Turbidity-based measurements become inaccurate
at high cell concentrations [31]. WMS requires no sample extraction or preparation,
which eliminates the associated risk of contamination. Interesting microbial traits
such as bioluminescence, virulence, and formation of bio-films that are expressed
at specific stages of growth [32–34] depending on the conditions and the strain’s
A. L. Chakraborty and A. Roy
on recent calibration-free WMS techniques that have proved to be a very important
development in this area.
1.1 Applications of TDLS
Industrial applications of TDLS are one of the large-scale and demanding applications that test the robustness of these systems. TDLS is used extensively for noninvasive, high-sensitivity detection of trace gases as well as to extract gas parameters
such as mole fraction, pressure and temperature [1–6]. Careful measurement and
monitoring of key process parameters is critical to ensure the health of large installations, reliable functioning of equipment, optimization of critical processes and
preempting potentially hazardous situations. Industrial applications often present
harsh operating conditions such as high pressure and temperature, high humidity
levels, vibrations that cause misalignment of the laser-detector pair, dirt that accumulates on optical surfaces, and corrosive effect of gases [7, 8]. The challenge is
to engineer rugged hardware and robust signal extraction algorithms that can withstand long-term exposure to such harsh environments. Environmental applications
also use TDLS heavily for long-term and continuous atmospheric trace gas measurements over large areas [9–13]. Accurate in situ measurements of greenhouse gases
(GHGs) is a key to managing the burden of emissions. Reliable measurements are
also required to build accurate climate models that help to understand the sink and
source patterns of GHGs and also to assess the effectiveness of international agreements that aim to reduce GHG emissions. Medical applications [14–18] have also
emerged as a very interesting field of application with clinical breath analysis [19]
being an attractive diagnostic technique for point-of-care use. The human breath has
several bio-markers such as acetone, ammonia, oxides of nitrogen that are linked to
abnormal physiological conditions. The primary attraction here is that sample collection is easy but the challenge is that the breath matrix is extremely complicated.
Microbiological applications of TDLS have received very little attention to date [20]
although bacteria are known to thrive in an astonishing variety of extreme physical
and chemical conditions [21–27], and also because the threat of antimicrobial resistance looms large [28, 29]. Recently a non-invasive and non-destructive WMS-based
technique was used to monitor growth by measuring the mole fraction of CO 2 emitted
by Escherichia coli and Staphylococcus aureus over several hours of their life cycles
[30]. WMS has clear advantages over traditional methods based on cell number density such as haemocytometry, plate counting method, ring mounted slide method
and electronic particle counter, that are often erroneous (besides being destructive)
because of improper staining, small cells becoming indistinguishable or other particles being mistaken for a microbe. Turbidity-based measurements become inaccurate
at high cell concentrations [31]. WMS requires no sample extraction or preparation,
which eliminates the associated risk of contamination. Interesting microbial traits
such as bioluminescence, virulence, and formation of bio-films that are expressed
at specific stages of growth [32–34] depending on the conditions and the strain’s