Development of microfluidic analytical method for on-line gaseous Formaldehyde detection

Highlights

• Novel analytical micro-device for quantification of gaseous formaldehyde.

• Gaseous Formaldehyde uptake through the establishment of a microfluidic annular flow.

• Formaldehyde detection using colorimetric cell using a liquid-core-waveguide.

Abstract

This paper reports on the development of a novel colorimetric analytical method based on microfluidic technologies for the detection of low airborne Formaldehyde concentrations, representative of those found in indoor air, i.e. 10–100 μg m⁻³. The new analytical technique operates according to 4 distinct steps: 1) gas sampling, 2) gaseous Formaldehyde uptake into the aqueous solution using an annular gas/liquid flow at room temperature, 3) derivatization reaction with acetylacetone solution at 65 °C producing 3,5-Diacetyl-1,4-dihydrolutidine (DDL) and 4) colorimetric DDL detection with a liquid-core-waveguide.

Laboratory experiments were performed to determine the experimental conditions permitting to obtain a stable annular flow, i.e. a liquid flow rate above 5 μL min⁻¹ and gas to liquid flow rate ratios greater than 1000. Effects of liquid-core-waveguide of internal diameter and length were also investigated.

Using liquid and gas flow rates of 35 μL min⁻¹ and 35 mL min⁻¹ respectively, the resulting uptake yield of gaseous Formaldehyde in aqueous solution was around 90% and the detection limit of gaseous Formaldehyde was 0.7 μg m⁻³, which is consistent with the guideline values for indoor air. In addition, the low reagent consumption increases significantly the device autonomy up to 20 days with 1L of acetylacetone solution.

Introduction

Formaldehyde is a major pollutant of indoor air due to its multiple sources (materials, combustion, painting, etc.) [1]. Several studies [2], [3], [4] have shown that indoor Formaldehyde concentrations are 2 to 15 times higher than those measured outdoor, and may vary typically between 10 and 100 μg m⁻³. Casset et al. [5] have shown that Formaldehyde is implicated in allergic diseases, particularly for asthmatic people. As of 2004 Formaldehyde is considered as a carcinogenic compound for humans by the International Agency for Research on Cancer (IARC) [6]. French recommendations aim at limiting Formaldehyde concentrations in public buildings to 30 μg m⁻³ by 2015 and to only 10 μg m⁻³ by 2023. Therefore, sensitive methods and potentially transportable instruments performing Formaldehyde measurements are highly required.

The conventional method to measure airborne Formaldehyde concentration [7], [8], [9], [10] uses a DNPH-cartridge sample followed by an off-line laboratory HPLC/UV analysis which is time-consuming and involves a laboratory treatment. In addition, such technique gives average concentrations over intervals of a few tens of minutes to one week, for active and passive samplings, respectively.

Many alternative methods oriented towards real-time and in-field detection, have been developed such as Hantzsch monitor, PTR-MS, infrared diode laser spectroscopy [11], [12], [13], [14]. However most of these methods rely on bulky apparatus and then are not adapted for indoor air monitoring where portability is a key point. Among the developed methods, some of them used reagents which react with Formaldehyde to produce color change which is conveniently used for the detection. Miniature and portable systems have been developed [15], [16]. Nevertheless, autonomic and continuous monitoring over several weeks was often not possible since the reaction on the solid detection element was not reversible. In addition, the resolution of such sensors was usually not as good as needed to detect the very low Formaldehyde concentrations recommended by the state legislature.

As argued by Salthammer et al. in a review on Formaldehyde published in 2010 [1], the method with derivatization to 3,5-Diacetyl-1,4-dihydrolutidine (DDL) in aqueous solution followed by fluorimetric determination is probably the most reliable method of measuring Formaldehyde in indoor air. A similar analytical technique, based on the uptake and concentration of Formaldehyde into an aqueous solution, followed by its specific and complete reaction with acetylacetone in presence of ammonium acetate to form DDL and then the quantification of DDL by fluorescence, has also been developed in our laboratory [17]. Such analyser is very sensitive (LOD < 1 μg m⁻³) and is considered to be portable and real-time due to its low weight (8 kg), its response time and temporal resolution (10 min). However, its major drawback is the high liquid reagent consumption since it operates at a liquid flow rate of 1.1 mL min⁻¹, which is a real limitation since the autonomy is then estimated to be only 15.2 h with 1L of acetylacetone solution. In addition, more miniaturization is always needed for in-field monitoring.

This work aims at developing a new Formaldehyde analytical method which preserves precision, selectivity, sensitivity and fast analysis in a single miniaturized instrument which will be suitable for field measurement with reagent autonomy sufficient for a one month monitoring. The contribution of microfluidics in analytical system can allow advantageously achieving all of these goals.