Sensing of gaseous HF at low part-per-trillion levels using a tunable 2.5-µm diode laser spectrometer operating at ambient pressure

Abstract

13]. Methods to measure ambient concentrations of gaseous HF are important for regulatory enforcement and monitoring, worker safety and health, and atmospheric research applications. For hazard identification, methods need to detect HF far below dangerous levels and work in real time to allow source identification and to provide evacuation warnings, if necessary [7]. Worker exposure to industrial HF can lead to respiratory and musculoskeletal problems [8,14], and in the USA, the National Institute for Occupational Safety and Health (NIOSH) limits exposure to 6 parts-per-million by volume (ppm) for short-term exposure and 3 ppm averaged over an 8-h workday [15]. Aside from the human toxicity, HF is a highly corrosive gas that can attack metals, glasses, and plastics with deleterious effect.Atmospheric researchers, using the sun as a back-light, have measured total HF concentrations in the upper atmosphere using Fourier transform infrared (FTIR) techniques [1]. These measurements necessarily measure the total integrated concentration from the upper atmosphere to the detector, using models to estimate the distribution of HF as a function of height [16]. It is highly desirable to supplement these path-integrated measurements with sensitive point measurements using tunable diode laser absorption spectroscopy (TDLAS) or FTIR spectrometers to validate the models [10,17]. For atmospheric research, these sensors must also be compact and lightweight and have sufficiently low power consumption suitable for weather balloons [17,18] or aircraft [19] instrument packages. Ideally, they should operate at sufficiently high acquisition speeds to allow detection of small changes in background concentrations on time scales of a few seconds. In the stratosphere, HF is long-lived and measured concentrations are 0.5-0.6 parts-per-billion by volume (ppb) [20], depending on altitude, latitude, and season. Closer to the ground, rain Abstract We demonstrate a sensor based on tunable diode laser absorption spectroscopy for the detection of hydrogen fluoride (HF) gas at ambient pressure. Absorption from the HF R(1) ro-vibrational peak at ν̃ = 4038.962 cm −1 (2.476 µm) in the fundamental (Δν = 1) band is measured. A quantitative spectral fit based on HITRAN data is used to account for overlapping spectral peaks of HF and water vapor, with an rms residual noise of 5 × 10 −4 absorbance units. The sensor is optimized for the detection of transient variations in HF concentration. We measure noise-equivalent concentrations for HF of 38 parts-per-trillion by volume (ppt) for 1-s integration times and 2.3 ppt for 10-min integration times.

Sub-parts-per-trillion level sensitivity in trace gas detection by cantilever-enhanced photo-acoustic spectroscopy

“…So far, the highest HF sensitivity using laser absorption spectroscopy has been achieved in the work by Craig et al . 11 where they report a NEC of 38 ppt in 1 s, whereas in our experiment a NEC of 5 ppt is achieved in 1 s.…”

“…More specifically, we combine a sensitive CEPAS detector, a particularly stable high-power narrow-linewidth optical parametric oscillator (OPO), and a strong absorption cross-section of hydrogen fluoride (HF), to reach a noise-equivalent concentration (NEC) of 650 parts-per-quadrillion (ppq) by averaging for 32 minutes. The result, to our knowledge, is the lowest concentration ever reported for CEPAS, the highest HF sensitivity ever reported for a laser-based technique 11 , 12 , and one of the few laser-based gas analyzers capable of sub-ppt level sensitivity in trace gas detection 8 , 13 , 14 . Detection of HF at ultra low concentrations is needed in, for example, registering of hazards at work sites 15 .…”

“…In general to reduce such effects and increase the response time of the system, one can resort to an open cell design and high flow rates as in Ref. 11 . To implement such a design using photo-acoustic detection, one could make use of a differential photo-acoustic detection scheme 32 .…”