High Sensitivity Monitoring of VOCs in Air through FTIR Spectroscopy Using a Multipass Gas Cell Setup

Abstract

Human exposure to Volatile Organic Compounds (VOCs) and their presence in indoor and working environments is recognized as a serious health risk, causing impairments of varying severities. Different detecting systems able to monitor VOCs are available in the market; however, they have significant limitations for both sensitivity and chemical discrimination capability. During the last years we studied systematically the use of Fourier Transform Infrared (FTIR) spectroscopy as an alternative, powerful tool for quantifying VOCs in air. We calibrated the method for a set of compounds (styrene, acetone, ethanol and isopropanol) by using both laboratory and portable infrared spectrometers. The aim was to develop a new, and highly sensitive sensor system for VOCs monitoring. In this paper, we improved the setup performance, testing the feasibility of using a multipass cell with the aim of extending the sensitivity of our system down to the part per million (ppm) level. Considering that multipass cells are now also available for portable instruments, this study opens the road for the design of new high-resolution devices for environmental monitoring.

Keywords: FTIR; VOCs; accuracy; acetone; ethanol; isopropanol; ppmv; sensor; styrene.

Figure 1

Schematic view of the setup used for the calibration experiments. The multipass gas cell, located in the sample compartment of a Vertex 70v Michelson interferometer, was connected to a sealed evaporation chamber. The PID system was installed on top of the evaporation chamber and was connected to a computer for real-time monitoring of evaporated VOCs.

Figure 2

FTIR spectra of the studied VOCs in the MIR spectral region ranging between 750 and 1300 cm⁻¹. Colored dashed lines highlight the absorbance peaks considered for the evaluation of the calibration curves of each VOC: styrene (red), acetone (blue), ethanol (pink) and isopropanol (green).

Figure 3

Calibration curves for styrene (a), acetone (b), ethanol (c) and isopropanol (d), referred to as the integrated absorbances A_i vs. ppm_v. The experimental data are indicated by points and the fit curves by the dashed lines. In panel (a), we reported the A_i related to sub-ppm_v concentrations in yellow, indirectly estimated. The root-mean-square error (RMSE) was used for the estimation of the differences between the experimental values and the adopted linear model. (The integrated area errors are 0.1 × 10⁻⁴ cm⁻²).

Figure 4

Enlargement view of the calibration of the integrated absorbance, normalized at the optical path, vs. ppm_v for styrene (a), acetone (b), ethanol (c) and isopropanol (d) obtained with the multipass (light blue line), benchtop (red line) and the portable device (green line), respectively. The whole calibration curves of the integrated absorbance vs. ppm_v obtained with the three setups are reported in Figure S3 of the Supporting Information. The integrated area errors were 0.1 × 10⁻⁴ cm⁻² for multipass data (blue), 4∙10⁻⁴ cm⁻² for benchtop data (red) and 0.001 cm⁻² for portable data (green).

Figure 5

(a) Spectra of mixtures. Colored areas were calculated with the integration method of the OPUS™ 8.2 software. (b) Fit of styrene and ethanol absorbance peaks in the range 830–1160 cm⁻¹ for a mixture of styrene+ethanol (orange) and for a ternary mixture (blue).