As shown in the figure, the complete instrument includes an inlet valve, pump, and pressure sensor to automatically and continuously capture the gas sample. The instrument automatically maintains the cavity at a constant and sub-atmospheric pressure. The small sample volume of the cavity (about 33cc) allows very fast gas exchange rates, enabling high measurement speed. Because of the reduced sample pressure, the near-IR spectrum of a trace gas molecule appears as a series of narrow but well-resolved lines - much more so than it does at atmospheric pressure. The task of WS-CRDS is to measure the strength (height) of one of these lines - this peak height is linearly proportional to absorbance and hence concentration.
Schematic of Picarro WS-CRDS analyzer showing optical cavity and sample gas flow.
Sample spectrum showing the incredibly high wavelength resolution (0.0001cm-1) of the Picarro analyzer - this is over 1000 times better than FTIR.
Meticulously measuring and controlling temperature and pressure is crucial to producing a spectrophotometer suitable for quantitative analysis. In competitive, first-generation cavity-based instruments, the absorbance is measured by taking data at just two points: at the wavelength corresponding to the absorption line apex and baseline wavelength. The laser is alternately stepped between these two points. There are several sources of noise and errors with this approach. First, simple statistics indicate that such as a single-point measurement of peak height is limited by system shot noise (inherent on all two-mirror cavities). Second, this approach assumes the laser is always on line-center, which is frequently not true. But even if the laser wavelength does not drift off line-center, both the line position and line shape may drift unless the temperature and pressure of the instrument are very precisely stabilized. And third, whenever another absorption line from an unexpected rogue species overlaps the target line, or sits anywhere near the chosen baseline wavelength, the competitive instruments can not recognize that fact. In any of these scenarios, the intensity data and hence concentration will be corrupted.
Temperature and pressure control of the sample and measurement cavity are critical in achieving high-stability measurements over long periods of time. Shown here is a subset of a 45-day NOAA field trial comparing atmospheric CO2 data taken by a Picarro G1200 analyzer with that from NOAA's NDIR analyzer. The Picarro analyzer was calibrated only once over the 45-day period, whereas the NDIR analyzer required calibration every 4 hours. The average difference is < 180 ppbv/day. The Picarro analyzer was able to sample unconditioned gas and drifted < 0.8 ppbv/day.
The typical drift of a Picarro analyzer measuring methane is ~0.5ppb (0.026% @2ppmv) over >30 days without calibration and the guaranteed specification is < 3ppb (0.16%) over that time period.
All these potential problems are eliminated by using WS-CRDS (Wavelength-Scanned Cavity Ring Down Spectroscopy). During every WS-CRDS measurement, the laser is tuned to multiple known points across the target absorption line. The instrument then uses a proprietary multi-order fitting routine to get the most accurate fit for the line shape and hence its true peak height.
WS-CDRS achieves very accurate concentration values by taking data across the entire absorption line, rather than just a single "line-center" data point. The shortest ring down times occur at the wavelength corresponding to the peak of the optical absorption of the molecule of interest.
Similarly, several points along the baseline are used to fit that value with very high precision. Of course, this is only possible if the laser wavelength is accurately known at all times. In Picarro instruments, we use a proprietary and patented wavelength monitoring technique that actually exceeds the precision of any commercially available wavelength monitor. The end result is measurement precision that is one to two orders of magnitude higher than that from first-generation, single-wavelength CRDS instruments.
By incrementing the wavelength of the laser, measuring the ring down time at each wavelength, and analyzing the resulting absorption spectra, the analyzer can determine the concentration of the molecules of interest with extreme sensitivity. Further, the patented wavelength meter allows the laser to be immediately tuned to any arbitrary point on the curve without slowly scanning the full lineshape - this increases speed and results in lower noise and higher sensitivities than competing techniques.
Wavelength scanning also is key to the amazing specificity of WS-CRDS. A complex gas mixture such as human breath or vehicle exhaust can contain many different and sometimes unexpected trace gas species. Each of these gives rise to absorption lines in the near-IR, which can therefore be quite crowded with lines. Wavelength scanning over an extended range ensures that the baseline measurement is true a zero-point. And more important, it can fit a line shape, regardless of whether or not there is some overlap with a nearby line from another species. With WS-CRDS the concentration of one trace gas species is therefore measured with high absolute accuracy and is also immune to changes in other gas concentrations. The graph below shows how this works in the case of H2S, a species that is notoriously difficult to measure at trace levels by any other optical technique, because of crosstalk from other species. In this test, the concentration of CO2 and H2O are varied at the percent level with no effect on the H2S detection channel at the ppbv level!
Data showing how WS-CRDS is immune to interferences from changing concentrations of other molecular species in the background gas.
Picarro WS-CRDS analyzers have been developed for a large number of single trace gases. In addition, we use a modular design in which each analyzer can accommodate multiple laser modules. And often a single laser can be tuned to measure absorption lines for two or more separate gas molecules. As a result, we can offer analyzers that can simultaneously measure the concentration of an arbitrary number of species: one, two or as many as six (for a recent application). And because of the high linear dynamic range of our instrument, the same instrument can measure parts per trillion concentrations of a trace gas, while measuring ambient levels of other species. The high dynamic range also enables isotope measurements even when there is a large difference in the natural abundance of the isotopes.
The table below summarizes the precision and lower detection limit for just some of the gas species that can be measured by Picarro's WS-CRDS analyzers. The analyzers are able to achieve these sensitivities with a range of five to six orders of magnitude, enabling ppt to % level dynamic range.
