NARSTO COMPENDIUM

Laser-Induced Fluorescence (LIF)

A. Basic principle:

Fluorescence spectroscopy is widely used these days in environmental monitoring, clinical chemistry, DNA sequencing, genetic analysis, and many other applications.  In excited singlet states, the electron in the excited orbital is paired (of opposite spin) to the second electron in the ground-state orbital.  Consequently, return to the ground state is spin-allowed and accompany by emission of a photon.  This process is called the "fluorescence emission" (Lakowicz, 1999; Garden et al., 1992; West, 1992).  The emission light can be detected by using a device such as a photomutiplier tube, a diode array, and/or a charge-coupled device.  The amount of analyte present in the sample is proportional to the intensity of the emission light (e.g., as in photon counts) of a specific wavelength and quantification is possible by using calibration data.

Figure 1 shows a conceptual schematic of the laser-induced fluorescence process.  Fluorescence measurements can be broadly divided into two types of measurements, steady-state and time-resolved.  Steady-state measurements are those performed with constant illumination and observation.  This is the most common type of measurement.  The sample is illuminated with a continuous beam of light, and the intensity of emission spectrum is recorded.  Because of the nanosecond timescale of fluorescence, most measurements are steady-state measurements.  Time-resolved measurement is used for measuring intensity decays.  For these measurements, the sample is exposed to a pulse of light, where the pulse width is typically shorter than the decay time of the sample.  This intensity decay is recorded with a high-speed detection system that permits the intensity to be measured on the nanosecond timescale.  A time-resolved system is generally more complex than the steady-state system, but the former offers the possibility of measuring much of the molecular information available from fluorescence that the steady-state time-averaging process does not.  In atmospheric research, LIF method has been used mostly in the measurement of hydroxyl and hydroperoxyl radicals.

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An example is that Holland et al. (1995), Brune et al. (1992), and Bailey et al. (1997) inferred ~ 10⁵ OH molecules cm^-3 was the detection limit at the signal-to-noise ration greater than 2 for 1-minute data integration time.

LIF instruments are typically operated inside a climate-controlled housing (e.g., at normal room conditions) such that operating temperature, humidity, and so on remain at a stable level.

(a) Laser power modulation - laser excitation could generate OH radicals biasing the measurement (Wennberg et al., 1994)

(b) Chemical modulation - collision quenching of OH by ambient air molecules (Bailey et al., 1997)

Use of LIF on measurement of ozone precursors such as OH and HO₂, primary oxidants in photochemical ozone production processes, has been done extensively.  Use of LIF in the measurement of volatile organic compounds of interest to PAMS is under development.

(a) Hydroxyl radical: OH, an oxidant which plays a role in the conversion of trace gases to CO₂ or to water soluble components. It is typically shorter-lived in the troposphere than ozone, and its overall role in tropospheric chemistry is still under investigation.

(b) PAMS : Photochemical Assessment Monitoring Stations, defined in 40CFR, Part 58.1 (1998 edition). Part 58.41 describes the PAMS network, consisting of stations to collect data in ozone non-attainment areas. PAMS stations are required to report data on volatile organic carbons, as defined by EPA, and meteorological variables.

• Gardner, C. S., Lidar, McGraw Hill Encyclopedia of Science and Technology. 7^th ed., 1992, v.10, pp.40-41.
• West, W., Absorption of Electromagnetic Radiation, McGraw Hill Encyclopedia of Science and Technology. 7^th ed., 1992, 1: 18-24.
• Brune, W. H., et al., Measuring OH and HO₂ in the Troposphere by Laser-Induced Fluorescence at Low Pressure, J Atmos. Sci. 52(19): 3328-3336.
• Holland, F., et al., In Situ Measurement of Tropospheric OH Radicals by Laser-Induced Fluorescence -- A Description of the KFA Instrument, J. Atmos. Sci. 52 (19): 3393-3401.
• Bailey, A. E., et al., Collisional Quenching of OH ( A^2S+ , n’ = 0) by N₂ , O₂ and CO₂ between 204 and 294 K. Implications for Atmospheric Measurements of OH by Laser -induced Fluorescence, J. Chem. Soc., Faraday Transactions. 93(16): 2915-2920, 1997.
• Creasey, D. J. et al., Instrumentation and Initial Deployment of a Field Instrument for Measurement of OH and HO₂ in the Troposphere by Laser-induced Fluorescence, J. Chem. Soc., Faraday Transactions. 93(16): 2907-2913, 1997.
• Lakowicz, J. R., Principles of Fluorescence Spectroscopy, 2^nd Ed., Kluwer Academic, NY, 1999.
• Wennberg, P. O. et al., Aircraft-borne, laser-induced fluorescence instrument for the in-situ detection of hydroxyl and hyroperoxyl radicals, Rev. Sci. Instrum. 65(6): 1858-1876.