Frequently Asked Questions
- What differentiates the spectroscopy of CRDS from similar techniques?
- How does CRDS compare to ICOS?
- How does CRDS compare to FTIR, NDIR, and TDLAS?
- How is the wavelength chosen for measuring a specific gas?
- How many gases can be detected simultaneously?
- Can the instrument be re-configured in the field for a different target gas?
- Can you measure liquids?
- Is CRDS really calibration-free?
Fundamentally, both of these applications measure very small changes in concentration of various species, and therefore require very high precision and accuracy with exceptional stability. All Picarro analyzers utilize:
1. Highly stable temperature control - to 20 milli degrees C (0.006% of room temperature) - of the sample, the measurement cavity and surrounding electronics, ensuring confidence in your data even in changing ambient temperatures (see link here).
2. Highly stable sample pressure control - to 0.024 Torr (0.003% of an atmosphere) - again ensuring the highest confidence in your data even as ambient pressure changes.
3. A patented wavelength monitor that provides highly accurate measurement and control of the laser's wavelength - to 2MHz (0.000001% of its center wavelength) - used to precisely interrogate the spectral feature of interest.
This unprecedented level of control provides uncompromised precision and accuracy over time and ambient conditions - since the spectral feature of interest will be unchanging during the course of the measurement.
CRDS is a completely time-based technique whereas ICOS is inherently an absorption (intensity-based) measurement. To determine the path length, ICOS must use a time-based ring-down measurement at the end of each spectral absorption scan, an additional step that is not necessary in CRDS. Intensity-based measurements like ICOS are limited by laser noise and drift. In contrast, the time-based CRDS measurement is actually taken while the laser is off and is therefore not subject to any laser noise or drift. One additional difference unique to Picarro's CRDS technology is the use of an ultra-precise (~2MHz) wavelength monitor with an ability to actively target the laser to specifically-known wavelengths. In contrast, ICOS does not employ this and in fact, uses laser current to sweep the wavelength of the laser, which is inherently nonlinear. These nonlinearities are translated directly into the absorption spectrum measurement and can cause errors in the concentration calculations.
CRDS differs from these other techniques in that it uses a time-based measurement to interrogate the absorption spectrum of the gas, rather than a traditional intensity-based absorption method. This approach has the advantage that it is independent of laser noise. Since absorbance-based instruments such as FT-IR and NDIR measure the ratio of the absorbed-to-incident light, sensitivity is limited by noise from the light source, mirrors and detector. CRDS also has a significantly longer path-length than the other techniques, resulting in higher sensitivity and precision, as well as lower detection limits. The "optical path length" of the measurement cell is the primary theoretical determinant of sensitivity for all these techniques.
CRDS uses a coherent laser light source while FTIR and NDIR instruments use an incoherent light source to generate photons. The properties of incoherent light limit the optical path length, and therefore sensitivity, of these instruments. Although TDLAS uses a coherent light source, CRDS achieves a dramatically longer optical path length than TDLAS due to the use of a resonant optical cavity instead of a multi-pass cell. Typically multi-pass cells, such as Herriott cells, are limited by practical issues of mirror manufacture and alignment that do not enable them to achieve the path lengths and hence sensitivity/precision of CRDS.
CRDS is able to acquire relatively narrow spectra due to the narrow tuning range of currently available diode lasers.
Our focus is on producing a stable, reliable instrument that can be operated and maintained with minimal effort. With the laser being a key component, we select individual spectral features that enable us to use specific NIR DFB lasers that have years of reliability and design validation in the telecom industry. These lasers have lifetimes exceeding 20 years. There is a range of NIR DFB lasers available, so we optimize the spectroscopy by choosing among several available spectral lines for each molecule. The spectral line is chosen on the basis of its strength and absence of interference from other molecular species in the sample to be analyzed.
It depends on the target gases, background gases, and performance requirements. If there are multiple absorption peaks of interest within the tuning range of our laser, our analysis algorithms can be adapted to measure each peak of interest. Our typical users prefer the measurement of only a few target gases at the highest precision and sensitivity.
Measuring a different gas generally involves switching the laser and wavelength monitor. These items are not currently field-replaceable.
Yes, we can measure isotopes of liquid water (as well as water vapor). We have not developed methods for other liquids.
Contrary to some claims, CRDS does need some type of calibration. However, these are generally very straightforward compared to other optical techniques...
The molar absorptivity, or extinction coefficient, must be measured and calibrated for each target gas. This is done by Picarro during the development of the spectroscopy for each instrument. These are factored into the spectroscopic fitting algorithm programmed at the factory and never require customer calibration or adjustment.
Large changes in background concentration and operating pressure changes can cause pressure broadening effects that must be calibrated. We calibrate these effects, and our spectroscopy routines account for these effects. These are factored into the spectroscopic fitting algorithm programmed at the factory and never require customer calibration or adjustment.
Depending on the application, periodic measurement of a known standard may be required for verification purposes.