DEVICES AND METHODS FOR QUARTZ ENHANCED PHOTOACOUSTIC SPECTROSCOPY
In quartz-enhanced photoacoustic spectroscopy (QEPAS), an analyte (typically in gas phase) generates a pressure wave in response to incident laser light. A quartz tuning fork (QTF) resonant at the frequency of the pressure wave transduces the pressure wave into an electrical signal. Pulsing the laser briefly reduces the amount of thermal chirp and increases the fraction of time that the laser emits at the wavelength(s) of interest. This increases the measurement efficiency. Pulsing the incident laser light with bursts of short pulses at the QTF resonant frequency increases signal strength. Exciting the sample with a two pulses at different laser wavelengths, separated by a half QTF period yields signal and background acoustic waves that partially cancel when integrated by the QTF, producing a differential measurement. Pulsing the incident laser light at a frequency faster than the gas response cut off frequency can improve the noise performance of a QEPAS measurement.
1 . A method of making a spectroscopic measurement of a sample, the method comprising:
emitting a sequence of pulses from a single-mode laser;
illuminating the sample with the sequence of pulses; and
detecting radiation transmitted, reflected, and/or scattered by the sample in response to the sequence of pulses with a detection system having a low-pass cutoff frequency less than a pulse repetition frequency of the sequence of pulses.
2 . A method of making a spectroscopic measurement of a sample, the method comprising:
modulating a single-mode laser with a repetitive pulse sequence so as to cause the laser to emit a periodically pulsed laser beam, each period of the repetitive pulse sequence comprising a plurality of pulses;
illuminating the sample with the periodically pulsed laser beam;
generating an electrical signal representing radiation reflected, scattered, transmitted, and/or emitted by the sample in response to the periodically pulsed laser beam; and
bandpass filtering the electrical signal at a band centered on the pulse repetition frequency.