Novel optical method for background reduction in resonant photoacoustics

Abstract

We report a new way of reducing the background originated by window absorption in resonant photoacoustics. The technique employs a secondary light source that is absorbed by the window but not by the gas sample. This auxiliary source is modulated 180 degrees apart from the one used to probe the gas. This way the window is heated almost uniformly during each cycle, thus lessening the associated background signal. We tested the scheme on a photoacoustic setup, conceived to measure NO(2), which is excited by an array of blue light-emitting diodes. Another array of near-infrared, light-emitting diodes served as the secondary light source. With the addition of this cancelling method, the detection limit was decreased to 4% of the previous reached without it.

Photoacoustic-Based Gas Sensing: A Review

“…Nitrogen Dioxide (NO 2 ) LD [108][109][110][111][112] Multimode LD [113] LED [114][115][116][117][118] Nitric Oxide (N 2 O) QCL [112,119] OPO [120] Nitrogen oxide (NO) QCL [121] LD [122] Ammonia (NH 3 ) QCL [123][124][125] CO 2 laser [126,127] LD [128] Carbon Dioxide (CO 2 ) QCL [129] CO 2 laser [130] LD [131][132][133][134] OPO [135] Table 2. Cont.…”

Differential, LED-excited, resonant NO2 photoacoustic system

“…1), that combines the benefits of previous setups. In one of them, we employed two types of LEDs, blue (465 nm), that excite NO 2 , and infrared (780 nm), that compensate the optical absorption of Pyrex glass [2]. Another, resorted to blue and violet LEDs (405 nm); this enabled us to decide whether the sample contains an interfering species.…”

“…In previous articles [1,2], we reported on the development and further improvement of a blue-LED-excited, transversally illuminated, resonant photoacoustic system, devoted to NO 2 measurements. The interest in monitoring this substance has not decreased for it continues to be a harmful urban pollutant produced in combustion.…”

Low-cost, portable photoacoustic setup for solid samples

“…Still, the lock-in is not a low-cost device. On the other hand, the simple computer sound card can be used successfully as a lock-in [1,7,8].…”