SPECTROSCOPY: Photoacoustic laser-absorption technique identifies sports doping substances

In sports, doping has been practiced since the original Olympics. Today, the World Anti-Doping Agency (Montreal, Que., Canada) and national agencies try to eliminate this practice through testing. The analysis of dopant samples is performed by authorized laboratories, which mostly use gas chromatography/mass spectrometry. These are time-consuming and costly methods that should be paralleled by simpler approaches, especially for quick screening. Laser spectroscopy could surely play a significant role because the optical spectra of dopants, even those of large organic molecules, are unique.

A team at ETH Zurich (Zurich, Switzerland) is exploring the potential of laser-absorption spectroscopy using a laser in the spectral range from 3 to 4 μm, where many organic molecules show vibrational-rotational bands.^1-4 In a photoacoustic cell containing the trace gas to be analyzed, spectral absorption produces acoustic waves that can be detected by sensitive microphones, especially if resonant acoustic modes of the cell have been excited. The microphone signal scales directly with the absorbed power at low background. However, at low laser powers of, say, below 1 mW, and elevated temperatures above approximately 60°C, the photoacoustic scheme is less suitable and other techniques like direct absorption spectroscopy appear preferable.

Pulsed OPG/OPA system

Because the IR spectral bands of large molecules in a buffer gas under pressure are continuous rather than consisting of individual lines, tunable lasers of broad bandwidth can often be sufficient for first studies. For that reason, the Swiss group set up a pulsed Nd:YAG-laser-pumped optical-parametric-generation and -amplification (OPG/OPA) system, realized by passing the Nd:YAG-laser beam through a temperature-controlled periodically poled lithium niobate (PPLN) crystal. Laser pulses of 6 ns duration and 5 kW peak power were repeated at 5.72 kHz, which was the first longitudinal-resonance frequency of the photoacoustic cell.

The PPLN crystal creates two longer-wavelength beams (“signal” and “idler”). Depending on the poling period, the crystal works for a certain wavelength range that can be shifted by varying the temperature. The idler wave reached an average power of several mW and showed a linewidth of 8 cm^-1, corresponding to 8.7 nm at a 3.3 μm wavelength, which could be tuned over 10⁶ cm^-1 (120 nm) by changing the crystal temperature by 100°C. Using a single crystal with adjacent gratings of different periodicity, other spectral sections were accessible. The microphone signals were detected using a lock-in amplifier.

The vapor of a chemically pure liquid or solid doping substance under test was transferred from the sample holder into the photoacoustic cell, following a repetitive evacuating and filling cycle using synthetic air (cell temperature was 60°C and air pressure 500 mbar). Hence, the absolute concentration of the doping substance in the cell remained unknown. Spectra of doping substances belonging to the classes of stimulants, anabolica, diuretica, and beta blockers were recorded and compared to known spectra. An example is that of the stimulant nikethamide (see figure), which has a melting point of 25°C (liquid at room temperature). The amount of vapor obtained at the 60°C test temperature yields a spectrum orders of magnitude higher than the noise level.

Recordings of numerous doping agents from different classes have been made. In most cases, no comparable gas-phase spectra exist. Despite the fact that currently no absolute sensitivity of the method can be given, the results demonstrate the considerable potential of laser photoacoustic spectroscopy for doping-substance detection.

Spectral analyses of substances in liquid solution would be interesting, as obtained in urine or blood samples; however, IR absorption bands in liquids are broad and strong, yielding a background that can impede identification and analysis of diluted substances. “Vapor-phase spectra with better spectral resolution can enable specific detection of doping agents without elaborate sample preparation even when diluted in urine,” says Markus Sigrist, head of the research group. “A narrowband laser source is realized by difference-frequency generation between the beams of the Nd:YAG laser and of a tunable near-IR diode laser in the PPLN crystal. A novel heatable multipass cell has been developed that permits measurements at considerably higher temperatures, and hence at higher vapor pressures, than is possible with the photoacoustic cell. The multipass configuration still allows sensitive absorption measurements.”

REFERENCES

1. C. Fischer et al., Appl. Phys. B (2006) DOI 10.1007/s00340-006-2367-y.
2. C. Fischer et al., Proc. SPIE 5697, 56 (2005)
3. R. Bartlome et al., Proc. SPIE-OSA, SPIE 5864, 58640N-1 (2005).
4. R. Bartlome and M. W. Sigrist, Techn. Dig. CLEO/QELS 2006, Long Beach, CA, May 21-26, 2006, paper CFL6.