Chemistry Reference
In-Depth Information
Traditionally the OP-FTIR technique employs non-coherent light sources based on a resistive arrangement
heated from 1000 to 1800°C. On the other hand, mid-IR coherent light sources such as lead-salt tunable diode
lasers (TDL) have been successful used for the detection of NO, NO
2
, CO and CO
2
for distances of several
meters [10] and detection of CO at distances of 610 m [11]. Moreover, open path ozone, water vapor and CO
2
detection over distances of 5800 m, employing two quantum cascade lasers have been also reported [12].
6.1.2.2
Stand-off Raman spectroscopy
Raman spectroscopy is also a useful technique for interrogating the sample from a certain distance by using
a telescope. This technique is usually called remote or stand-off Raman spectroscopy and it was explored
theoretically by T. Hirschfeld [13]. As can be easily assumed, the main advantage of those stand-off systems
is that the operator is distanced from a potential danger [14] avoiding the physical contact with the sample
and enhancing in situ measurements possibilities [15]. Stand-off measurements have been employed to
investigate the mineralogy of meteorites [16] and mineral characterization on planetary surfaces [17].
Raman-based stand-off systems have the potential to characterize materials through glass and plastic
containers, allowing the analysis of the content without opening the container. This property has been used to
discriminate between sodas, juices and liquors and hazardous liquids such as liquid explosives or chemical
warfare agent simulants from a distance of 7 m approximately [18,19].
Two different geometries can be adopted in stand off Raman systems named coaxial and oblique which can
also be referred as bi-axial (see Figure 6.3). Coaxial illumination maximizes the collection efficiency of
Raman scattered photons and allows Raman measurements at different distances without system realignment
[20]. In the oblique mode the laser is focused directly to the target sample, ensuring that all the laser power
reaches the sample, and the telescope collects the scattered radiation at an oblique angle, determined by the
distance from the sample to the Raman instrument. This geometry has provided increased sensitivity as
compared to the coaxial mode [21].
The distances that separate sample and operator in stand-off Raman spectroscopy using the oblique
geometry vary from 10 to 50 m. On the other hand, coaxial systems have provided useful Raman spectra of
minerals organic and inorganic material at 9 to 120 m distance [22].
6.1.2.3
UV Differential Optical Absorption Spectroscopy (DOAS)
The DOAS technique is based on the fact that all chemical compounds absorb light at specific wavelengths.
The spectroscopic absorption from various molecular species over a path length is proportional to the
absorptivity and concentration of a compound and to the length of the light path. The technique has many
advantages, including real-time measurements and the ability to simultaneously measure several species [23].
DOAS can be also considered as a clean technique, involving operation in the open air.
The DOAS equipment consists of an emitter, a receiver and an analyzer. The distance from the emitter to
the spectrometer can vary from several meters to kilometers. DOAS can be grouped in active, in both
monostatic and bistatic modes (similar to OP-FTIR), and passive mode, depending on the emitter used. In
passive DOAS sunlight is employed as light source and it has been successfully used to measure stratospheric
and tropospheric trace gases [24] and halogen compounds at various sites [25]. In the active DOAS, the
ultraviolet light source can be a xenon arc lamp, a tungsten halogen or a deuterium lamp. Technological
progress during recent decade has made LEDs as an interesting alternative to conventional broadband light
sources [26].
A recently developed imaging DOAS (IDOAS) technique [27] has been used to obtain trace gas information
resolved in two dimensions, allowing for a better understanding of atmospheric emissions, their transport,
and relevant chemical reactions.
