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Methods of Detection and Analysis
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- Atmospheric monitoring methods
- In the environment
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Methods of Detection and Analysis
Formaldehyde can be characterised by a number of methods including spectrophotometry, high performance liquid chromatography (HPLC), colorimetry, fluorimetry, polarography, gas chromatography (GC) using flame ionisation detection (FID), and infrared detection. Methods based on spectrophotometry are the most widely used, and have sensitivities of 8 to 25 ppb (10 to 30 g/m3). HPLC is another method commonly used and has a detection limit of 1.7 ppb (2 g/m3). The most sensitive method of detection is flow injection, with a detection limit of 9 ppt (0.011 g/m3). Information on methods of detection and analysis for formaldehyde in various media is abundant and has been summarised in a number of reviews (IPCS, 2002; ATSDR, 1999; IARC, 1995; IPCS, 1989). For all methods, organic and inorganic chemicals, such as sulphur dioxide, other aldehydes and amines, can cause interference. Therefore, the method of sampling and the treatment of the sample before analysis are important factors in the accuracy of the determination. This section focuses on the methods commonly used in Australia for detecting formaldehyde in the atmosphere of workplaces, ambient air, indoor air and emissions from products releasing formaldehyde, such as wood and textiles. Methods of detection in other media, such as water and in biological samples, are also briefly discussed.
For personal monitoring during full shifts or tasks, workers are equipped with a sampler (tube or badge) placed in the breathing zone. For area monitoring, the tube or badge is placed at a fixed location in the workplace environment. Tubes are connected to a portable metering pump, whereas badges sample the air by diffusion. At the end of the sampling period, the tube or badge is sealed and transferred to a laboratory, where the chemical is liberated from the absorbent and quantified using different analytical methods. The result is expressed as ppm or mg/m3 over the duration of the sampling period. The analytical detection limit depends on the airflow across the absorbent and the duration of the sampling period. The US National Institute of Occupational Safety and Health (NIOSH) methods (NIOSH, 1994) are commonly used in Australia. They are summarised in Table 6.1. Table 6.1: NIOSH methods of detection for formaldehyde (NIOSH, 1994)
TWA, time weighted average; STEL, short-term exposure limit. Several other atmospheric monitoring methods for detecting formaldehyde in the workplace are summarised in the CICAD (IPCS, 2002). These include some methods that have been used in Australia, such as use of a formaldehyde passive sampler/monitor followed by chromotropic acid test (detection limit of 0.083 ppm) and gas tube detectors with infrared analysers (detection limit of 0.33-0.42 ppm). Instantaneous measurement of the concentration of airborne formaldehyde, such as by direct read, hand-held electronic formaldehyde devices, is also used in Australia. For example, formaldehyde meters and Interscan machines provide instantaneous readings. The sensor of formaldehyde meters is an electro-chemical cell which contains electrodes that are used for temperature compensation and to improve the selectivity. The sensor response is linear with the concentration of formaldehyde in air. Two filters are used to eliminate interferences. Measurements are first made with a filter that permits determination of the background or baseline. Insertion of a second filter then permits the measurement of formaldehyde. The limit of detection is 0.01 ppm. The Interscan machine is an electrochemical gas detector operating under diffusion-controlled conditions. Gas molecules from the sample are adsorbed on an electrocatalytic-sensing electrode, after passing through a diffusion medium, and are electrochemically reacted at an appropriate sensing electrode potential. This reaction generates an electric current directly proportional to the gas concentration. This current is converted to a voltage for meter or recorder readout. The limit of detection is 0.01 ppm.
The methods commonly used for measuring the concentration of formaldehyde in ambient air fall into the following two categories (EA, 2001):
The most widely used method for discrete air sampling involves the collection of air into a stainless steel canister over a predetermined period of time, such as 24 hours, followed by GC or GC-MS analysis. Discrete sampling methods determine average pollutant levels over the sample collection time. A commonly used continuous in-field analysis method uses an optical remote sensing system to determine the concentration of the chemical by means of the differential absorption of transmitted light by gaseous compounds along the light path. The system consists of a light transmitter and sensor placed at a given distance apart at the monitoring site. Alternatively, the concentration in air can be analysed by semi-continuous gas chromatography. Samples are collected directly onto solid absorbents, desorbed thermally onto the GC column and analysed while the next sample is collected. Compared with discrete sampling method, continuous or semi-continuous methods enable more detailed information about concentration variations. The analytical limit of detection of the above methods typically ranges from
In addition, several other methods of detection for measuring ambient air formaldehyde levels are available including:
A recent NEPM document (NEPC, 2004) recommended use of two other US EPA testing methods:
Formaldehyde concentrations in indoor air can be measured by either active or passive sampling using a sampler to collect the formaldehyde followed by analysis using a number of methods. The use of passive sampling techniques should be fully verified by active means. Currently, there is an Australian Standard for testing formaldehyde in indoor air, AS 2365.6-1995, Methods for the Sampling and Analysis of Indoor Air. Method 6: Determination of Formaldehyde –Impinger Sampling- Chromotropic Acid Method (Standards Australia, 1995). However, there are problems with use of chromotropic acid due to interferences and quality-related issues. There are more suitable methods including active collection onto DNPH, which are analysed via HPLC or GC/MS or equivalent analytical methods. The US EPA methods discussed above (TO5, TO11, TO-11A, and TO-15) are also suitable for measuring indoor air formaldehyde. There are a number of International Organization for Standardization (ISO) documents on indoor air formaldehyde testing. They are:
(ISO, 2004a)
The Standards Australia Indoor Air Committee advised that the Committee would be considering these ISO methods along with other methods such as the US EPA methods when determining suitable testing methods for indoor air formaldehyde in the future. Methodology for the simultaneous sampling of a number of indoor airborne aldehydes including formaldehyde is also available. A recent paper investigated detecting indoor air formaldehyde using a direct reading device (Suzuki, 2003). However, this method has certain limitations and serves mainly for screening purposes.
Four methods have been developed to measure formaldehyde emissions from wood products and details have been summarised in recent reviews (IPCS, 2002; IARC, 1995). The Standards Australia has published a number of methods for the measurement of formaldehyde emission from particleboard, fibreboard and medium density fibreboard (MDF). A summary of these standards is provided in Table 6.2. Standard testing methods for formaldehyde emissions from plywood (AS/NZS 2098.11:2004) and laminated veneer limber (AS/NZS 4357.4:2004) products are currently being considered by the Standards Australia. Table 6.2: Standards Australia methods for the measurement of formaldehyde emissions from wood-based products
The concentration of formaldehyde in biological samples, such as blood and breath, has been used in attempts to monitor workers’ exposure (ATSDR, 1999). Formic acid or formate, a metabolite of formaldehyde, has been measured in workers’ urine and blood. However, it has been suggested exposure to formaldehyde cannot be adequately assessed by these methods because formaldehyde is rapidly metabolised and is highly reactive. Therefore, it is unlikely to be present in samples. Urinary formate levels are also an unreliable biomarker as formate is a metabolite of many other substances.
Methods for the collection and determination of formaldehyde in atmospheric water, drinking water and fog water have been summarised by ATSDR (1999). These methods are similar to those for ambient air described above. The methods for formaldehyde in drinking water and fog water rely on the formation of the DNPH derivative followed by HPLC. The method for measuring formaldehyde in atmospheric water relies on the reaction of formaldehyde in atmospheric water with diketone (2,4-pentanedione) and ammonium acetate to form a fluorescent derivative that is measured spectrophotometrically in a flow injection analysis system.
One method for measuring formaldehyde in soil has been reported (Klasco, 2003). The soil is dried by addition of magnesium sulfate. Freon 113 is then used to extract the formaldehyde and the sample is scanned with a spectrophotometer. The concentration is determined from a calibration curve.
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