Spectroscopic mole fraction analyzer designs
Greenhouse gas mole fraction measurement techniques for gas mixtures based upon IR spectroscopy use either absoiption bands or individual transitions within bands of the GHG of interest. Absorption bands of individual molecules are well separated in wavelength. This separation lends itself to instrument designs aimed at a particular molecular species. However, where bands of different molecules overlap in wavelength, either these regions are avoided, or a strategy must be used to separate contributions from different molecular species. For example, water vapor is ubiquitous in atmospheric measurement situations as it has bands at several IR wavelengths and can be a confounding element in instrumental designs. Some field installations avoid or minimize water vapor interferences by drying the sample air prior to introduction to an instrument’s measuring volume, while others do not, often depending on instrument application. An instrument’s molecular specificity is realized by confining its detector response to radiation of wavelength absoiption by the species to be measured. For the generalized NDIR instrument design shown in Figure 8, this is achieved by the band pass filter. For example, CO, has a very strong absoiption band in the 4.2 pm to 4.3 pm region with no other molecule having a significantly interfering band. This band, the so-called “fundamental band” with the strongest absorption, is the basis for many instrument designs. These use a filter that transmits this range of radiation wavelengths to the detector while rejecting all others. Typically, such a filter would have a passband with center wavelength of 4.26 pm and width of 100 nm to 200 mn.
In all instrumental designs however, source photon removal via molecular absoiption along the instrument’s sensitive path is a fundamental objective. Since absoiption band strengths of molecules differ substantially, instrument designs are adjusted to meet application requirements. Although maximizing photon absorption by the pertinent GHG molecule is a main instrumental goal, several other processes may be simultaneously at work, resulting in additional intensity reductions at the detector. Such interference effects may be minimized with a blend of instrumental design and calibration procedures using the appropriate gas mixture standards discussed above.
Figure 8 shows a form of the Beer-Lambert law and a general instrument design incorporating an IR source, absoiption path, bandpass filter, and detector. This general design uses tungsten filament electrical lamp sources that emit broadly across the infrared, but mostly outside the wavelength band of interest. (Recent advances in infrared light emitting diode technologies are an emerging alternative appearing in some research grade and commercial devices.) Band pass filter technologies for GHG molecules are well developed and widely available. The majority of instrumental designs utilize a single detector and bandpass filter, although in the last few years multi-detector assemblies with integrated electronic amplification and wavelength selectable filters have become available. These support strategies of integrated absoiption signal/reference signal detection using a single gas sample path. The reference filter’s pass band is selected to be in a wavelength region where molecular absoiption does not occur. In this way, the reference channel signal compensates for parasitic effects that impact both detectors.
The form of the Beer-Lambert relation shown in Figure 8 assumes multiple absorbing species. Source radiation transmitted to the detector, Td, is the ratio of the radiant flux transmitted through the gas, Ir, and measured at the detector, to the radiant flux incident on the measurement path, Io. Td depends
Figure 8. IR absorbance-based mole fraction measurement, Non-Dispersive IR (NDIR).
on the attenuation parameter, a, for the i* gas species observed, the attenuation pathlength, l, and the number density, n , of the attenuating species. The number density is the value desired for mole fraction determination. In most instrument designs, absorbing pathlength is fixed and the attenuation parameter, cr, can be determined from spectral reference data, i.e., absorbance information, obtainable from references such as HITRAN.
The Beer-Lambert relation is a widely-used instrument design foundation for mole fraction measurement, however, complications may arise for some applications. Although Г., f. and the pathlength may be measured directly, they are more often inferred from the ratio of the signal to reference channels, or in single channel instruments by turning the lamp off periodically. Other instrumental effects that increase the uncertainty in NDIR mole fraction measuring instrument designs must be compensated. These include drift characteristics in IR detector response and source emission. Compensation for various scattering processes due to elements comprising the optical path may also be needed. All these effects can limit NDIR measurement accuracy, although perhaps not its sensitivity, in most cases. Such deficiencies can be compensated considerably or eliminated by instrument calibration procedures based on mole fraction standards for the gas or gases of interest. This has been the practice in many applications where field measurement requirements approach those of standard lab capabilities.
6.2.1 Long absorption pathlength instrument designs
Typical NDIR instrument designs have pathlengths ranging from a few to a 30 centimeters and usually employ an analyte molecule’s strongest absorption bands for maximum signal amplitude. Spectroscopic investigation of weakly absorbing molecular bands in the early to mid-20th century resulted in the development of several optical cavity designs aimed at substantially increasing the absorption pathlength with multi-pass designs. These are often based on reflections between two or more minors to fold the absoiption path within tractable cavity lengths. An early design still in current use is the White cell (White, 1942), a basic version is shown in Figure 9. By adjusting the orientation of the two smaller concave minors appropriately, multiple passes of the input beam transit the cavity, thereby increasing pathlength. The configuration shovra supports eight transits with precise adjustment resulting in more passes. Succeeding cell designs feature more passes due to greater mechanical stability (Heniott, 1965).
With the advent of high-reflectance mirror technology, reflectance > 0.9995, and lasers, resonant cavities, such as those illustrated in Figure 10, came in to widespread use. Technological innovations driven significantly by the telecommunications revolution resulted in frequency-stable, tunable infrared lasers, optical fiber components, and IR detector technologies that have advanced molecular spectroscopy research and measurement instrument designs. Due to effective pathlength increase and other strategies, these typically have performance superior to NDIR instrument designs. The optical cavities used lengthen absoiption paths from fractions of meters to multiple meters and kilometers at bench scale or smaller physical sizes through the application of a number of strategies (Romanini, 2014).
High reflectivity, resonant and non-resonant optical cavities have shifted measurement strategies from use of all or most of an absoiption band to that of a single or a few adjacent individual transitions within a band. This wavelength, or frequency, selectivity also provides means to compensate for overlap of absoiption features of multiple molecules in the same wavelength region by direct observation and compensating analysis.
Analytical instrument designs based on these technological innovations and applied to concentration measurement instrument designs for CO„ CH4, N,0, CO, and other small molecular species at high
Figure 9. White cell optical paths
Figure 10. CRDS-based mole fraction measuring instrument design. Image courtesy of J. Hodges and D. Long, NIST.
sensitivities also featuring good temporal stability began appearing commercially in the late 1990s and early 2000s. Resonant optical cavities were combined with the exquisitely fine control of emission frequency of tunable IR lasers. This resulted in designs with long absorption pathlengths and much greater control of the frequency of the light used to probe molecular absorption features. These designs have effective path lengths of 100s to 1,000s of meters. One approach, and variations on it, based on the use of resonant cavity configurations, is used in several commercial instruments as well as in research applications and is known as cavity ringdown spectroscopy (CRDS). Figure 10 shows the main components of a CRDS design along with a conceptual diagram of the spectral properties of the resonant cavity using a Fabry-Perot interferometer cavity (Busch, 1999). The high reflectivity optical resonator (mirror reflectance of 0.999 and higher) not only provides the means to substantially lengthen the absorption path, but it also replaces the bandpass filter of NDIR designs. A cavity’s filtering properties result in a comb of frequencies, illustrated as the series of transmission peaks shown in blue in Figure 10. These equally-spaced, comb frequencies allow the cavity to accept laser light only at those exact frequencies or wavelengths. Frequency spacing of cavity resonances, termed the free spectral range, Af, of the cavity is dependent upon mirror spacing and the speed of light.
where: c = speed of light and
I = mirror spacing.
The 400-megahertz (MHz) resonant frequency separation shown in Figure 10 corresponds to a mirror separation of ~ 37 centimeters (a 12 inches). Cavity length tuning via the piezoelectric actuator shown in the figure mainly changes the frequency position of comb teeth.
The combination of laser frequency timing and cavity length control gives exceedingly fine timing of light either entering the cavity at a comb frequency, or strongly rejecting if the laser is timed off a comb frequency. Measurement of absorption featur e properties, both amplitude and frequency, is accomplished by successively tuning the excitation laser to comb frequencies. As an example, an idealized molecular absorption feature (the red line labelled ‘absorption feature’ in Figure 7) is shown superimposed over the cavity’s resonant frequency comb. The black dots indicate cavity resonance overlap with the absorption line. Adjustment of laser frequency and cavity resonance conditions are the means of probing absorption featur e shapes and amplitudes, a widely-used method for accurate measurement of absorption line shapes/ amplitudes. Cavities designed for high-sensitivity measurement of spectral reference data often have ~ 100 MHz cavity resonance separations and widths in the tens of kilohertz range, giving them the ability to measure absorption features with significantly higher accuracy than previously (Yi, 2018).
Long-pathlength instrument designs transform signal acquisition from one that relies on long-term temporal stability of the detector, as in NDIR designs, to one that relies on short-term temporal stability where drift effects are negligible. In the case of CRDS-based designs, incident radiation from the laser is tuned to the center frequency of a comb tooth, allowing the cavity’s optical field to increase. The detector monitors interior cavity field strength and when it reaches a preset level, the laser frequency or the comb tooth frequency is changed. Without further laser input, the cavity optical field intensity decays exponentially due predominantly to molecular absoiption, although other cavity loss mechanisms are present but tend not to har e the temporal nature of the molecular absoiption process. The detector monitors the exponentially decreasing signal (the ‘ring-down’ time depicted at the right in Figure 10). The cavity’s optical field is allowed to decay far below its original value, after that time, the procedure can be repeated. Detector voltage is digitized throughout, and this data is fit over the ringdown tune to an exponential in order to obtain a ring-down time value. Since ring-down tunes typically have durations of a few to tens of microseconds, drift in detector response is effectively eliminated. Ring-down times with the cavity evacuated and filled with the gas mixture to be measured allow calculation of mole fraction values based upon line strength and frequency properties intrinsic to the analyte molecule. Reference information for the molecular absoiption feature being used by the instrument design is the basis for this analysis. Auxiliary instrument parameters values, e.g., gas mixture pressure and temperature, must also be measured in order to compute mole fraction values.
Commercially available CRDS instruments used for atmospheric monitoring of greenhouse gases har e demonstrated excellent stabilities over several weeks or longer, based upon repeated observations of GHG mole fraction standards (Kwok, 2015; Karion, 2013). Uncertainty in instrument response of ~ 0.1 pmol/mol at nominal CO, values ranging from ~ 400 pmol/mol to 800 pmol/mol while supported by periodic observations of standard gas mixtures are routine.
6.2.2 High-accuracy spectra1 reference data
Spectral reference data of line shape and amplitude are determined using research-grade CRDS instruments measuring mole fraction of primary gas concentration standards whose mole fraction value has been assigned using the gravimetric methods discussed previously. CRDS determinations of molecular absoiption line and band spectral reference data for the greenhouse gas molecules and other species of interest, such as oxygen, carbon monoxide, and water vapor provide spectral reference data (Polyansky, 2015) are used in applications such as remote sensing. The combination of these measurement methods has been shown to provide line strength and shape data at previously unattained accuracy levels. These data har e been incorporated in widely available spectral reference datasets, such as the HITRAN database, as a primary means of disseminating them to both the instrument design community and to the atmospheric remote sensing community. Such information is widely used in remote sensing applications.