Continuous wave external-cavity quantum cascade laser-based high-resolution cavity ring-down spectrometer for...

Area of Science:

• Spectroscopy
• Laser Technology
• Analytical Chemistry

Background:

• Nitric oxide (NO) is a critical biomarker in various physiological and pathological processes.
• Ultrasensitive detection of NO is essential for medical diagnostics and environmental monitoring.
• Existing methods for NO detection often lack the required sensitivity or specificity.

Purpose of the Study:

• To develop and characterize a high-resolution cavity ring-down spectroscopic (CRDS) system for ultrasensitive nitric oxide (NO) detection.
• To demonstrate the system's capability for measuring low concentrations of NO in various matrices.
• To explore the potential of the developed CRDS system for broader applications in trace gas analysis.

Main Methods:

• Utilized a continuous wave (cw) mode-hop-free (MHF) external-cavity quantum cascade laser (EC-QCL) operating at λ∼5.2 μm.
• Employed high-resolution CRDS to measure rotationally resolved Λ-doublet components of NO.
• Achieved a noise-equivalent absorption coefficient of 1.01×10⁻⁹ cm⁻¹ Hz⁻¹/² with an empty cavity ring-down time of 5.6 μs.

Main Results:

• Successfully measured the rotationally resolved e and f components of the P(7.5) line in the fundamental band of NO.
• Demonstrated ultrahigh sensitivity and specificity for detecting parts per billion (ppb) concentrations of NO in N₂ and human breath samples.
• Achieved a standard deviation of 0.11% with averaging of six ring-down time determinations.

Conclusions:

• The developed EC-QCL based cw-CRDS instrument provides ultrasensitive and specific detection of NO.
• The CRDS sensor is advantageous for measuring ppb NO concentrations in complex samples like human breath.
• The CRDS system's broad tuning range and high resolution offer potential for simultaneous multi-species trace gas analysis and hyperfine structure studies.