R&D METHODS FOR PAMS/VOCS OZONE PRECURSORS
4. Tunable Diode Laser Absorption Spectroscopy - TDLAS
[Basis | Range | Min. Detection Level | Operating Temp. | Interferences | Notes of Interests | References]
1. Basis: "Absorption spectroscopy" is a chemical analysis technique made possible by the phenomenon that a given molecule absorbs light at selected wavelengths. The wavelengths absorbed are characteristic of each molecule’s atomic features (Southwest Sciences, 1999). The amount of light radiation absorbed by a substance depends on two factors: the number of molecules in the path of the light, and the characteristics of the molecule (e.g., absorption cross-section). Measurement of changes in the light intensity as it passes through the molecules, and the use of calibration and reference data, enable the determination of the number of molecules encountered.
Tunable Diode Laser Absorption Spectroscopy (TDLAS) uses a diode laser as the excitation light source for the absorption spectroscopic measurement. "Diode lasers" contain a small "diode" which is constructed of bits of various elements in crystalline form. When electrical current or heat, or both, are applied to the diode, it emits the laser light beam. The wavelength of the light beam is highly specific with respect to the material of the diode. This wavelength can be tuned over a small spectral window by varying the electrical and heat forces applied. Adjusting the wavelength is called "tuning" the diode, giving the laser the name "tunable diode laser" (Schiff, 1996; Werle, 1998).
TDLAS has been used typically in the measurement of trace gases in the lower and upper atmosphere. This technique has been applied to greenhouse gases such as CF4 and C2F6, vehicular exhaust gas such as CO and CO2, in-stack HF emission (e.g., Schiff, 1996).
2. Range: The measurement range is the typical span of species concentration that is measurable by a TDLAS device.
3. Minimum
Detection Level: Typically, if the molecular absorption cross
sections or absorption coefficients are known, the detection sensitivity
for a gas species can be calculated. Smaller detection limits correspond
to more sensitive detection. The following data were available at
http://www.swsciences.com/sensors.html
using
an 1 meter optical path, lead-salt diode except HF, assuming 1E-5 absorbance,
and 1 Hz bandwidth.
| Species | Wavelength (nm) | Detection Limit (ppbv) |
| Water | 5940 | 2.0 |
| Nitric oxide | 5250 | 5.8 |
| Carbon dioxide | 4230 | 0.13 |
| Carbon monoxide | 4600 | 0.75 |
| Nitrogen dioxide | 6140 | 3.0 |
| Formaldehyde | 3550 | 8.4 |
| Ozone | 9500 | 11 |
| Ammonia | 10300 | 0.80 |
| Sulfur dioxide | 7280 | 14 |
| Hydrogen fluoride (HF) | 1310 (Near-IR) | 10 |
The sensitivity of TDLAS can be improved significantly by using a multipass cell (see Notes of Interests).
4. Operating Temperature: Tunable diode laser spectrometers have been used both in-situ and remote-sensing modes. For in-situ applications, the operating temperatures would be those of the laboratory, vehicle, or aircraft where the equipment is installed and operated. Lead-salt tunable diode lasers are the most widely used for atmospheric trace gas monitoring. A major disadvantage of the lasers is that they operate at temperatures generally below 100K and thus require cryogenic cooling (Kolb et al., 1995).
5. Known Interferences: Optical filtering, beam shaping, and collimating adjustment is generally required to ensure the TEMOO beam quality during the operation (Kolb et al., 1995). In spite of the tunable feature of the diodes, their actual tunable wavelength range is generally limited. The property of the semiconductor diode laser thus represents a major limiting factor for a wide application of this technique.
a. There are many reports on the use of TDLAS. Traditional single-path laser absorption techniques are not capable of 1.0 ppbv level of detection. Detection improvements can be achieved by using multi-pass cells (Pustogov, 1994); however, the cost of such devices is significant (See MayComm Research Inc. and New Focus, Inc. at the References Section).
b. Although at least five types of diodes are available on the market these days (Southwest Sciences, 1999), InGaAsP, AlGaAs, AlGaInP, Lead-salt lasers, and Antimonide lasers, the most commonly used diode laser in TDLAS is the lead-salt diode. It has a wide tunable range, over wavelengths from 3.3 mm to 30mm (Kolb et al., 1995), compared to other diode lasers.
c. TDLAS is a fast detection technique, and is specific to the analyte of interest. Multiple lasers can be multiplexed to design of an instrument capable of measuring multiple gas species in one box. The complexity and cost of TDLAS instrument are the major disadvantages of the technique.
1. Kolb, C. E., et al., "Recent Advances in Spectroscopic Instrumentation for Measuring Stable Gases in the Natural Environment". Chap. 8 in: Biogenic Trace Gases:Measuring Emissions from Soil and Water, ed. P. A. Matson and R. C. Harris, Blackwell Sci. Ltd., London, 1995.
2. MayComm Research Company, "Herriott Cells and Tutorial". http://www.spectrasensors.com/tutorial.htm. [Recently bought by SpectraSensors, Inc.]
3. Pustogov, V. V., et al., "Pressure Broadening of NO2 by NO2 , N2 , He, Ar, and Kr Studied with TDLAS". J. Mole. Spectrosc. 167: 288-299, 1994.
4. New Focus Company, Inc., "6300 Velocity Lasers", and " Multipass Cells": http://www.newfocus.com/Online_Catalog/1/160/1120/body.html, Santa Clara, California, 1999
5. Schiff, H. I., "1995 Fisher Scientific Award Lecture: Reflections of an Atmospheric Chemist Wondering Why He Won an Analytical Chemistry Award". Canadian J. Chem. 74:1765-1773, 1996.
6. Southwest Sciences, Inc., "Diode Laser Gas Sensing". http://www.swsciences.com/sensors.html Santa Fe, New Mexico, June, 1999.
7. Werle, P., "A Review of Recent Advances in Semiconductor Laser Based Gas Monitors". Spectrochimica Acta Part A, 54:197-236, 1998.
