Analysis of VOCs in Liquids through Vaporization in a Tubular Oven Monitored by Chemical Ionization Mass Spectrometry

Abstract

The analysis of chemical compounds present at trace levels in liquids is important not only for environmental measurements but also, for example, in the health sector. The reference technique for the analysis of Volatile Organic Compounds (VOCs) in liquids is GC, which is difficult to use with an aqueous matrix. In this work, we present an alternative technique to GC to analyze VOCs in water. A tubular oven is used to completely vaporize the liquid sample deposited on a gauze. The oven is heated in the presence of a dinitrogen flow, and the gas is analyzed at the exit of the oven by a chemical ionization mass spectrometer developed in our laboratory. It is a low magnetic field Fourier Transform Ion Cyclotron Resonance (FT-ICR) optimized for real-time analysis. The Proton Transfer Reaction (PTR) used during the Chemical Ionization event results in the selective ionization of the VOCs present in the gas phase. The optimization of the desorption conditions is described for the main operating parameters: temperature ramp, liquid quantity, and nitrogen flow. Their influence is studied using a 100 ppmv aqueous toluene solution. The analytical method is then tested on a mixture of seven VOCs.

Keywords: Fourier Transform Ion Cyclotron Resonance; mass spectrometry; proton transfer reaction; real-time analysis; tubular oven; volatile organic compounds.

Figure 1

The ICR cell. The direction of the magnetic field $\overrightarrow{\mathbf{B}}$, shown in blue, is perpendicular to the image.

Figure 2

(a) Scheme of the piezoelectric valve. The sample gas flows through the valve from inlet (A) to outlet (B). (b) Zoom showing the situation when no voltage is applied to the piezoelectric ring stack (valve closed) and (c) when voltage is applied to the piezoelectric ring stack (valve open).

Figure 3

Typical timing of the FT-ICR analytical sequence. The numbers refer to the experimental events described in the text.

Figure 4

Scheme of the experimental setup.

Figure 5

Toluene mixing ratio as a function of time in the gas flow when a gauze soaked with 0.5 mL of a 100 ppmv solution of toluene in water is heated using different temperature ramps at 20 mL/min N₂ flow rate.

Figure 6

Influence of the carrier gas (N₂) flow rate with a 0.5 mL of 100 ppmv toluene solution and a ramp of 27 °C/min.

Figure 7

Influence of the liquid volume deposited on the non-woven gauze on the temporal profile observed for the mixing ratio of toluene in gas phase. Operating conditions: 0.5 mL of 100 ppmv toluene solution, 27 °C/min, 20 mL/min of N₂ gas flow.

Figure 8

Evolution of toluene mixing ratios as a function of time. The area corresponding to the integration of the curve between 100 and 1600 ms is shown in hatching. Experimental conditions: 50 ppm toluene solution, 20 mL/min of N₂, 27 °C/min.

Figure 9

Mean integrated mixing ratio for toluene as a function of the toluene concentration in the solution.

Figure 10

Intensity evolutions of a mixture of VOCs as a function of time.

Figure 11

Mass spectrum obtained after the analysis of the standard mixture of VOCs. The concentrations of compounds were 300 µg/mL.

Figure 12

Response of MS/tubular oven to increasing concentrations of VOCs.