30 Air Monitoring


PORTABLE MICROFLUIDIC ANALYSER FOR CONTINUOUS MONITORING OF FORMALDEHYDE IN INDOOR AIR


At a time when pollution has become a major public health concern, reliable monitoring techniques for air quality must be available. Formaldehyde is a pollutant found mainly in indoor air. It is present in concentrations 2 to 15 times higher than those in outdoor air, with values typically ranging from 10 to 100 µg m-3 [1]. A wide variety of formaldehyde sources are present indoors: building and furnishing products, household products, domestic combustion, etc. Formaldehyde is also present in many products due to its disinfectant and conservation properties.


Formaldehyde has irritating effects on the respiratory system and eye mucosa, and its involvement in allergic asthma, is well known [2]. The International Agency for Research on Cancer classifi ed it as carcinogen in 2004. Health agencies recommended thus to reduce exposure to lowest feasible concentration: values of 19.7 µg m−3


(16 ppb) (NIOSHa ) and 30 µg m-3 (24.4 ppb) (ANSESb (300 ppb) (over 8 h) and 487 µg m-3 ) for


long term exposure while occupational exposure limits are set at 243 µg m-3


(600 ppb) (over 15


min) under EU Directive 2019/983. Rapid, sensitive, easy-to-use, and robust continuous analysis techniques need to be developed to democratise the formaldehyde monitoring.


1. Materials and methods


In this section, we will describe the formaldehyde monitoring analytical system (microF), the gas generators and the set-up for the evaluation of the microF performance regarding humidity exposition and its use in a fi eld campaign.


1.1. MicroF


The microF (Figure 1) is a portable microfl uidic formaldehyde analyser commercialised by Chromatotec. It was fi rst developed and patented by a research team at ICPEESc


(CNRSd and University of


Strasbourg). As represented in Figure 2, this device is based on the continuous detection of 3,5-diacetyl-1,4-dihydrolutidine (DDL), a product of the reaction of formaldehyde with fl uoral-P (or acetylacetone solution). The air sample and liquid reagent solution fl ow at a constant rate through the micro-F. Both gas and liquid phases are co-injected into a 530 µm ID capillary column. Due to the high solubility of formaldehyde, the aqueous solution traps all the gaseous formaldehyde. After degassing, Fluoral-P reacts specifi cally with formaldehyde in an oven set at 65°C for about 3.5 min, ensuring the full conversion of formaldehyde into DDL, which is a fl uorescent molecule. The DDL is excited by a LED at 415 nm and the fl uorescent light is collected at 530 nm by a photomultiplier and then amplifi ed. The fl uorescence is therefore proportional to the formaldehyde concentration [3].


1.2. Humidity and gaseous formaldehyde generation


Whereas humidity is omnipresent in air, this parameter is often overlooked by analysers’ manufacturers. To assess the humidity impact on the analysis of formaldehyde by the microF, a mixture of controlled humidity and formaldehyde concentration was required. Humidity was generated from distilled water contained in a bottle in which a microporous tube was immersed. Nitrogen fl owed in the microporous tube and water was evaporated at the interface into the fl ow, resulting in a mixture of air at controlled humidity. This humidity generator was described and used by Becker et al. [4] Gaseous formaldehyde was generated using a paraformaldehyde permeation tube. The permeation tube was placed in an oven and heated at 60°C. The system was supplied by a constant fl ow of nitrogen (60 mL min-1


) and


generated formaldehyde at a fi xed concentration of 339 µg m-3


. It was further diluted with humid or dry nitrogen, producing concentrations between 50 and 152 µg m-3


humidity (RH) [5]. Both formaldehyde and water gaseous fl ows combined in a mixing chamber to which the microF sampling inlet was connected. In these studies the standard method ISO 16000-3 was used as a reference to monitor formaldehyde concentration [6].


Figure 1: MicroF: Microfl uidic formaldehyde analyser aNIOSH: National Institute for Occupational Safety and Health (American federal agency) bANSES: French Agency for Food, Environmental and Occupational Health & Safety (French government agency) IET NOVEMBER / DECEMBER 2023


cICPEES: Institute of Chemistry and Processes for Energy, Environment and Health dCNRS: French National Centre for Scientifi c Research


at 8% or 50% relative


Figure 2: Scheme of the microF showing the sampling, the uptake, the reaction, and the detection steps (adapted from [3]).


1.3. Measurements of formaldehyde in a French public school


An indoor study was carried out in two elementary schools in the city of La Rochelle (France) both constructed between 1965 and 1974. The classroom selected in the fi rst school was rectangular, with a volume of 187 m3


and located on the fi rst fl oor. It was


occupied by 25 students of 9-11 years old. Ventilation was done with manual opening of the windows only. A CO2


was used to monitor CO2 detector (LUM’Air) in the room as a confi nement indicator.


The objective of the campaign was to quantify formaldehyde in schoolroom, identify its sources and assess the effect of different ventilation scenarios, i.e., usual school aeration procedure, aeration according to the CO2


level and aeration instructions given by the French Indoor Air Quality Observatory (OQAI).


2. Results and discussion 2.1. The microF: a micro-analyser not aff ected by humidity.


Formaldehyde is highly soluble, so that many instruments dedicated to formaldehyde can be potentially affected by the presence of humidity. Various formaldehyde concentrations in the range 50–152 µg m–3


was generated either in dry conditions


or at a relative humidity of 8 % (green triangle shapes) 50% (blue circle shapes). MicroF response (i.e., fl uorescence intensity) was plotted as function of formaldehyde concentration determined by the DNPH reference method (Figure 3).


Whatever the humidity, the calibration curves in both conditions were identical, showing that water in the air does not have an impact on the measurements of HCHO by microF. It has also been


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