Laser absorption spectroscopy

Optical detection techniques in the MetAMCII project are based on laser spectroscopy, which studies the interaction between light and matter. Figure 1 shows the fundamental principle of a common laser spectrometer. The wavelength of the laser source is tuned over a transition of the molecular species of interest, which causes attenuation in the detected signal. The partial pressure can be deduced from the observed absorbance, which is proportional to the amount of the target molecules.

Figure 1. Schematics of laser absoption spectroscopy.

Typically, the AMCs have very low partial pressures in the air corresponding to the substance fraction in the range of few nmol/mol (ppb) or below. The sensitivity of a simple single-pass laser spectrometer is often not sensitive enough to detect such low concentrations. In the MetAMCII project several more sophisticated measurement techniques are developed to improve the sensitivity to a trace level.


Cavity enhanced methods

Figure 2. A commercial CRDS instrument is used for detecting NH3 and a home-built mid-IR CRDS is used for detecting HCl.

Cavity enhanced methods use an optical cavity to increase the effective absorption path length. By using a pair of highly reflective (R ~ 99.999%) cavity mirrors, it is possible to reach effective path lengths up to several km. In cavity ring-down spectroscopy (CRDS), the decay time of light circulating inside the cavity is related to the absorbance of the gas species. Changes in this decay time are used to determine the concentration of specific AMCs. Figure 2 shows the basic principle of a cavity ring-down spectrometer which can be tuned over several transitions enabling multicomponent analysis using a single instrument.

A different cavity-enhanced technique being developed by NPL uses “Noise Immune Cavity Enhanced Optical Heterodyne Molecular Spectroscopy (“NICE-OHMS”). The experimental arrangement is shown in Figure 3. The laser output frequency is modulated, typically at both ~15 MHz and ~1.5 GHz. The lower frequency is used to provide an error signal so that the laser can be locked to the cavity and the higher modulation frequency is equal to the cavity free spectral range. This allows the carrier frequency and ~1.5 GHz sidebands to resonate inside the cavity. A signal from the light transmitted through the cavity can be used to generate a signal whose size is proportional to the AMC concentration in the cavity. This technique combines the advantages of a long cavity effective path length with the low noise detection limit from heterodyne detection. A key difference between NICE-OHMS and CRDS is that the laser is locked to the cavity in NICE-OHMS leading to lower detection limits and shorter measurement times.

Figure 3. Simplified schematic for a NICE-OHMS spectrometer for trace AMC measurement. The laser wavelength is determined by the wavelength of strong absorption lines of the target AMC; for ammonia this is 1531 nm, and for HCl this is 1742 nm.


Time division multiplexed direct TDLAS and wavelength modulation spectroscopy

The time division multiplexed spectroscopic technique combines the benefits of absolute concentration measurements using calibration-free direct tunable diode laser absorption spectroscopy (dTDLAS) with the enhanced noise rejection of wavelength modulation spectroscopy (WMS) together with a multipass gas cell. The laser can be modulated by alternating the modulation between a triangle-ramp (dTDLAS) and an additional high frequency sinusoidal modulation (WMS). PTB has developed an optical gas standard wherein the amount fraction measured using dTDLAS is traceable to the SI, through an unbroken chain of measurements. The method used in MetAMCII project combines the metrological traceability of dTDLAS with the enhanced sensitivity offered by WMS.  More details can be found in doi:10.3390/s141121497.

Figure 4. A typical time division multiplexed dTDLAS/WMS measurement configuration.


Photoacoustic spectroscopy

Photoacoustic spectroscopy is another often used ultra-sensitive spectroscopic method for trace gas analysis. It is based on detecting not the light, but local pressure variation caused by local heating due to non-radiative relaxation of excited molecules. A sensitive microphone is used to detect the pressure variation. Laser sources can be tuned in similar fashion as in standard direct absorption experiments, making it also suitable for multicomponent detection. In MetAMCII project the microphone used in the photoacoustic detectors is a patented cantilever sensor, which is much more sensitive than common electrical microphones traditionally used in photoacoustic spectroscopy. The movement of the cantilever, caused by photoacoustic effect, is detected by optical readout interferometer.

Figure 5. The MetAMCII project is studying, for example, the use of a novel cantilever based photoacoustic spectrometer for detecting HCl in the low nmol/mol (ppb) range.