Formaldehyde (FA) has been widely used in wood, paper and textile industries as well as in the production of a number of chemicals and for the preservation of biological material. It also present in almost all common foods and it’s estimated that adult dietary intake is 11 mg/day. Occasionally, it is used as a disinfectant to disinfect water filters. (ADWG, 2004)
FA can be toxic allergenic and carcinogenic to human beings (Lyon, 2006). Several epidemiological studies of occupational exposure to formaldehyde have indicated an increased risk of nasopharyngeal cancers, leukemia and eye irritations (OSHA, 2008). The International Agency for Research on Cancer has concluded that FA is probably carcinogenic to humans (IARC 1987).
FA may be present in water through industrial effluents, ozonation of naturally occurring humic materials, contamination by accidental spills and overflows as well as deposition from the atmosphere (ADWG, 2004). A study showed that the FA concentrations in rainwater are expected to be up to three orders of magnitude higher than in surface water, which indicated that atmospheric deposition is a significant source of FA in aquatic systems (Kieber et al., 1999).
Generally, the concentration of formaldehyde in water is very low which has a low environmental risk to human and organisms. However, when accidental spills or overflows happened, chemical analyses and monitoring programs are needed.
1.1 Formaldehyde in drinking water
FA enters in drinking-water mainly from the oxidation of natural organic matters such as humic materials during ozonation (Glaze et al., 1989) and chlorination (Becher et al., 1992). Leaching from polyacetal plastic fittings in which the protective coating has been broken can sometimes be one of the resources of FA in drinking-water (IPCS, 2002).
According to Australian guideline value, the concentration of formaldehyde in drinking water should not exceed 0.5 mg/L (ADWG, 2004).
1.2 Formaldehyde in wastewater
FA has been used in many industrial activities as a key chemical. In organic synthesis industry, the synthesis of special chemicals such as pentaerythritol and ethylene glycol used FA as one of the agents. In addition, FA is essential in production of resins, textiles, paper products, medicinal products and drugs (Khiaria et al., 2002). Therefore, effluents arising from these industrial applications may contain significant amounts of FA which is needed to be determined and treated.

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1.3 Chemical analysis of formaldehyde in water and wastewater
Since the concentration of formaldehyde in water can be occasionally high which may be potential risk to human health, we should conduct some methods to measure the accurate concentration of it. The chemical analysis of formaldehyde can provide meaningful information on the quality of water therefore actions can be taken immediately to ensure that water suppliers provide consumers with water that is safe to use and meet the public recreational and aesthetic requirements if changes occurred.
Advice on sample collection
In sample collection, the sampling site, time and weather conditions are needed to be considered to obtain a volume of water which can be the representative of the water body. Before it is analyzed in laboratory, we should try to keep it in such a manner during store and transport processes, sometimes preservatives can be added in order to minimize any changes that may occur (Private Water Supplies website).
The essential steps in sampling program are shown below (From unit 5, lecture notes).
Problem Definition
Formaldehyde Sampling Program Design
Sample Preparation
Chemical Analyses
Field Sampling
Reporting
Data Analysis
2.1 Sample containers
Formaldehyde belongs to volatile organic compound, therefore, it’s recommended to use 40mL brown glass vial or transparent glass vial with aluminum foil covered outside as the sample container to prevent it from releasing to air or deteriorating after exposing to light. The cap must have teflon-lined septum. The polypropylene screw caps should be used instead of typical phenolic resin caps due to the possibility of sample contamination from FA (US EPA 1998). In addition, when taking samples, we should use pre-cleaned bottles that are free from volatile organics (Standard operating procedures for water sampling – methods and analysis, WA, 2009).
2.2 Sample collection
Sample collection is very important in determining the safety of water, so it’s essential to ensure that the samples are representative, reliable and full validated. For complicated and unstable water quality such us wastewater effluent, sample collection should also cover the random and regular variations in water quality as well as the fixed conditions.
2.2.1 Types of Sampling
The types of sampling include grab sampling, composite sampling, flow-related sampling, automatic sampling and continuous monitoring. Each method has its own characteristics and suitable for different water body and sampling purpose.
For drinking water, we use grab sampling method. For grab sampling, all of the test material is collected at one time. So the grab sample can only reflect the water quality state at a particular site and time, and then only the sample was properly collected can it represent the water body we concerned (Norwalk Wastewater Equipment Company website). Grab sampling has some advantages. For example, some specific type of unstable parameters such as VOCs, chlorine residual and nitrites in water treatment plant can be effectively analyzed. Sometimes, grab sampling can also be conducted for pH, temperature and DO monitoring (NWEC website). For drinking water, the water was well mixed, stable and generally free of contamination. Therefore, grab samples can already be good representations of the water quality. In addition, this method is very common, easy and low capital cost.
For wastewater, we use composite sampling method. Composite sampling is another sample collection technique which consists of many individual discrete samples that have been taken at regular intervals over a period of time. Therefore, the collected samples can reflect the average performance of water quality during the collection period (NWEC website). Wastewater treatment plants receive unfixed and variable amounts of sudden increased waste flows from industries and households during a day followed by intermittent periods of no flow (NWEC website). Analyzing a single grab sample of effluent at a fixed time and site can introduce some bias and cannot reflect the real varying flow patterns in effluent outlets. Therefore, composite sampling method is more plausible for evaluating the holistic performance and state of wastewater quality.
2.2.2 Sampling sites
Drinking water sampling
For drinking water sampling, we can either take a sample from a customer’s tap, or storage tank or some representative places.
From a tap
Choose a tap which is most frequently used.
Any external fittings such as filters and contaminants such as grease and sediment build-up around the spout should be removed prior to testing.
Since tap outlets are suspected to be contaminated, disinfection should be conducted by swabbing both outside and inside of the tap several minutes before sample collection. The disinfection reagent can be 0.1% sodium hypochlorite solution (Forensic and Scientific Services, Queensland Government 2008).
To get a representative fresh water sample, the tap should be run for a while (about 2-3 minutes) to remove the stagnant water in the tube.
From a storage tank
For shallow depth (Small water supply tank),
To get a representative sample of the source of supply, the sampling depth is recommended to be 0.5m.
The bottle inside and the cap inside should not be touched.
The neck should be plunged downwards into the water and then turned upwards until the water is overfilled and mouth is towards upside (FSS, QLD Government 2008)
For deep depth (Large water supply dam),
To get a representative sample, the water sample should be collected by using a suitable depth sampling device such as hosepipe, sampling rod or pump etc.
Be careful not to disturb bottom sediment.
Wastewater sampling
For wastewater sampling, we should take the samples from outlets of wastewater treatment plant. Since we use composite sampling method for wastewater analysis, we should pour equal portions of freshly collected samples into the appropriate container.
2.2.3 Collection instructions
According to surface water sampling methods and analysis – technical appendices in Western Australia in 2009, the recommended collection techniques are listed as follows:
The containers for holding samples should not be pre-rinsed.
It is recommend that the bottles should be used to collect sample directly rather than decanting. However, in some cases, decanting samples from big collection vessels into sample vials are acceptable provided that all the containers are free of contamination. For example, sometimes a clean bucket with about 10L capacity or a large 1L breaker can be used to collect the surface sample and then transfer to the laboratory sample container.
To minimize the exposure to air and light, samples should be overfilled containers and then the cap should be tightly sealed free of air bubbles and faced down to help prevent leakage.
2.2.4 Complete lab form and sample label
After sample collection, we should complete the lab form which contains the sampling information such as water volume, sampling sites, etc and stick label on each sample container which recorded sampling location and time.
2.3 Preservation
Filtration
For wastewater samples, the filtration treatment should be conducted since some suspended particulates may block the testing instruments.
Preservation
Some experiments indicated that aldehydes are susceptible to microbiological decay.
To inhibit microbial decomposition of organic compounds, it is recommended to add 0.1 ml of CHCl3 (Economou et al., 2002) or alternatively, 15 mg of copper sulfate pentahydrate in water samples (US EPA 1998).
2.4 Sample transportation and storage
Transportation
During transportation process, we should minimize the contamination and disturbance to water samples, conserve them in the dark and maintain in cool condition with a chilled insulated container and then return to the lab as quick as possible (Environmental health guide, WA, 2006).
Storage
Before the lab analysis, the samples are recommended to be refrigerated but not freezed at 1-4°C in the dark.
Available techniques for sample extraction
3.1 Available techniques
Sample extraction is used to concentrate the analyte for its successful analysis by instruments. There are various methods for FA extraction. Each method has its own characteristics. The object is to choose an optimal technique to avoid excessive loss of the analyte and achieve desired performance.
Soxhlet extraction and solvent extraction are traditionally common extraction techniques, particularly for organic compounds. However, since their limitations such as the need of a large volume of solvent, lack of thermal stability and volatility of some analytes and interference from contaminants in the extraction thimbles (Grob et al., 2004), they may not desirable for FA extraction. According to recent studies and researches, some extraction techniques have already achieved good results. They are listed as follows.
Solid-phase extraction (SPE)
SPE is a technique including two extraction steps. The first step is the non-equilibrium removal of the analytes from the liquid sample by retention on a sorbent. The second step is the solvent elution or thermally desorption of the selected analytes (Grob et al., 2004).
One successful approach for determining formaldehyde in drinking water has been to use colorimetric-solid phase extraction with EmporeTM Anion Exchange-SR 47-mm extraction membranes as extraction cartridges and elution from the SPE cartridge by sodium hydroxide solvent (Hill et al., 2009). Another successful approach for formaldehyde analysis in water was by using poly (allylamine) beads for solid-phase extraction and eluting from the C18 cartridge by hydrochloric acid (HCl) solvent (Kiba et al., 1999).
SPE is one of the most widely used techniques in FA analysis. Due to its high sensitivity and efficiency, it can determine the low FA concentrations down to 80 ppb by several minutes (Hill et al., 2009). However, one of the drawbacks of it was its e high packing and sorbent selection requirements which might be costly and time-consuming in stuff preparation. Another problem is SPE may have analyte loss during elution when analyte passing though tube.
Ultrasonic extraction (USE)
USE is a fast technique using ultrasound assisted method to assure good contact between sample and solvent (Grob et al., 2004). One of the researches has stated the successful use of USE in FA extraction. Formaldehyde was first extracted with water by ultrasound assisted, and directly introduced into a derivatization column which was packed with a moderately sulfonated cation-exchange resin. The resin was charged with 2, 4-dinitrophenylhydrazine (DNPH) previously and used as solid support. The formaldehyde DNPH derivative was eluted by sodium dihydrogen phosphate in 50% ACN solvent (Chen et al., 2008).
Compared with traditional techniques, this method was proved to be fast, accurate, sensitive and labour-saving. In addition, only small quantities of solvent and sample were required. Therefore, it’s a promising extraction method (Chen et al., 2008). However, the drawback of this method was its low recovery efficiency. For low concentrations of analytes in samples, multiple extractions are often required (Grob et al., 2004).
Supercritical fluid extraction (SFE)
SFE is a fast and efficient technique. Analytes are more soluble in supercritical fluids (SFs), which are dense gases above their critical temperature and pressure, when they are in their liquid state. Therefore, the important properties such as the melting point and solubility of analytes in the SF are needed to be considered (Grob et al., 2004).
A study has stated the SFE in FA analysis using CO2 as the extraction fluid, and the experiment was carried out at 13.8 MPa, 120°C with 15 min of static extraction time, 15 min of dynamic extraction time and 80 μl of modifier (methanol). The DADHL derivative which was the product of the condensed FA with ammonia and acetylacetone can be detected by UV spectrometer (Reche et al., 2000).
However, one of the drawbacks of SFE in FA analysis was the use of supercritical CO2 fluid. Since the low polarity of CO2 but the polarity of FA, the extraction was difficult and recoveries are poor.
Solid-phase microextraction (SPME)
SPME is a good extraction method which can incorporate with GC or HPLC to get the high performance in sample analysis. It used a fiber coated with an extracting phase which can concentrate the analytes and then the fiber is transferred to the injection port of separating instruments and analytes are desorbed from the fiber and rapidly delivered to the column (Pawliszyn, 2009).
One of the researches has stated the SPME experiment for FA analysis. Prior to use, the 75 μm Carboxen-Polydimethylsiloxane fiber was conditioned in the injection port of GC at 300°C under helium flow for 1.5 h. Then the extraction was carried out at 80°C for 30 min using the fiber with a medium stirring of sample. Next, the thermal desorption was reacted in a splitless mode at a temperature of 310°C for 3 min (Bianchi et al., 2007).
SPME technique has some advantages in FA. SPME is a simple, easily-conducted and solvent-free technique. The detection limits can reach parts per trillion which is really useful in FA analysis since the concentration of FA is water is very low. In addition, SPME is fast and low cost which can minimize sample holding times, reduce analyte loss and sample contamination (Trenholm et al., 2008). However, one of the problems is SPME may have analytes loss during extraction that nearly 1% of analytes goes on fiber (Leap technologies website).
Stir bar sorptive extraction (SBSE)
The theory of SBSE is similar to SPME which used a spinning glass-covered magnetic bar coated with a thick layer of polydimethylsiloxane to extract analytes, then thermal desorption can be carried out in the GC injection port (Grob et al., 2004). SBSE has been applied successfully to trace analysis especially VOCs and semi-volatile compounds in environmental, biomedical and food applications. The detection limits can be extremely low which are suitable for FA analysis in water (David et al., 2003).
There’s limited information of FA analysis relating to SBSE technique, however, it’s still a promising method in the future.
Newer techniques
Newer techniques such as pressurized liquid extraction (PLE), subcritical water extraction (SWE) and microwave-assisted solvent extraction (MWE) are enhanced liquid extraction techniques. Compared with traditional soxhlet extraction and solvent extraction, these methods are less time-consuming, less solvent consumption and more efficient and can be exerted to low concentrations of analytes in samples.
3.2 Water and wastewater sample extraction
For drinking water formaldehyde analysis, we can use solid-phase extraction (SPE) which is commonly used, easy-operated and available in laboratory.
For wastewater formaldehyde analysis, we should remove particulates by filtration prior to extraction because particulate matter in the sample can interfere with the analysis such as absorbing some analytes of interest and causing low analytical recoveries. And then we can use SPE, USE, SPME or other advanced techniques for sample extraction.
Current techniques for sample analysis
Spectrophotometric methods
The theory of spectrophotometer is to measure the intensity and amount of light which have been absorbed or reflected by the analytes as function of colour or wavelength (Skoog et al., 2007).
4.1.1 Reflectance spectrophotometer
One of the studies has developed a method that successfully monitored the FA concentrations in water samples using purpald as the colorimetric reagent (Hill et al., 2009). Firstly, a colourless intermediate was formed by purpald reacting with FA in alkaline solution. And then an intensely purple tetrazine was formed due to the oxidization of intermediate. The purple tetrazine was served as the colorimetric product (Dickinson et al., 1974).
After completing the colour reaction in the syringe, the 1mL sample is passed through an extraction disk. The amount of extracted analyte is then measured on-disk using a BYK-Gardner diffuse reflectance spectroscopy. The reflectance data can be collected at 20nm intervals over the visible spectral range. After that, the BYK-Gardner QC-Link software in PC is used to calculate the Kubelka-Munk function F(R). Then the analyte was compared to a calibration plot of F(R) at 700nm which was the most effective analytical wavelength to determine the FA concentration (Hill et al., 2009).
This method successfully analyzed the FA concentrations at the range of 0.08 to 20ppm using only 1mL samples and just costing several minutes.
4.1.2 UV/Visible spectrophotometer
UV/Visible spectrophotometer can be used to measure the absorbance which is the difference of intensity of light before and after passing through a sample by an object as function of wavelength or color (Skoog et al., 2007).
One of the studies for FA determination used Hantzsch Reaction for derivatives. The colourless solution became yellow colour gradually owing to the synthesis of DADHL which formed from the condensed formaldehyde and acetylacetone in the condition of excess of ammonium salt. Then UV/Vis detection was carried out with a UV-1603 Spectrophotometer (Reche et al., 2000). The maximum absorbance was approximately at 415nm which was used in analyzing FA concentration in sample and the standard solution (Shimadzu Application News).
4.1.3 Advantages and disadvantages
The spectrophotometers are widely used in many laboratories and institutes. This method has advantages such as the lower instrument capital and operational cost and easy operation. However, the sensitivity and selectivity are lower than GC and HPLC method. For extremely low FA concentrations in water sample, this method is limited in application.
4.2 Chromatography methods
For chromatography, since FA concentrations in water and wastewater are very low, it must be derivatized prior to analysis to ensure quantitative and qualitative detection. Nash reagent, dinitrophenylhydrazine and PFBHA reagent are typical agents which can have color reactions with FA. Then their derivatives can provide better sensitivity for UV, fluorescence or MS detection (Michels et al., 2001).
4.2.1 GC
GC is widely used in FA analysis. According to many researches using GC for FA determination, the mobile phase is usually helium and the different stationary phases were covered on column. By measuring the different retention time of the analytes, FA concentrations can be calculated out.
1. GC/MS
In FA analysis, different molecules in solution can be separated during the sample travel by GC and then the mass to charge ratio of ionized fragments of FA can be detected by MS (Robert et al., 2007).
One of the researches used the pentafluorobenzyl hydroxylamine (PFBHA) reagent to form derivatives and then used a Varian CP-3800 GC system connected with a Varian 4000 ion trap MS system for detection. The injector was operated at 250°C in split mode and separation was conducted on a 0.25μm DB5-MS capillary column. Electron impact ionization (EI) in full scan from 150 to 275m/z was used in MS analysis (Trenholm et al., 2008). Then the FA concentrations can be measured by recording mass to charge ratio. Another similar study also used PFBHA derivatization reagent and GC/ MS method (Bianchi et al., 2007).
Advantages and disadvantages of GC/MS:
Combining GC with MS can have better identification and separation of molecules than single GC since molecules behave different in GC and MS. For FA analysis, GC/MS is widely used due to its rapid operation, high precision and selectivity. Having considered its good performance and cost effectiveness, it is proposed to be an alternative of traditional methods (US EPA 1998).
One of the drawbacks is GC/MS is less sensitive than HPLC in identification of the FA derivatives. Another problem is GC/MS is susceptible to interference. For wastewater sample, the compounds are often complex, therefore the interferences may lead to imprecise analysis.
2. GC/ECD
US EPA offers another alternative method to measure FA. The oxime derivatives were formed by adding pentafluorobenzyl hydroxylamine (PFBHA) reagent to FA solution at pH of 4. Then they are extracted from the water with 4mL hexane. After processing through an acidic wash step, the extracts are analyzed by GC with electron capture detection (GC/ECD). After comparing with the calibration standard, the analytes can be identified. Two chromatographic peaks have been observed for FA that both (E) and (Z) isomers are formed for FA carbonyl compounds (US EPA 1998).
Comparison of GC/MS and GC/ECD:
ECD offers better detection limits (<1 μg/L) than MS (10 μg/L) while GCMS has better specificity. In addition, GC/ECD method may require larger sample volumes and additional solvents and chemicals compared with GC/MS method. Moreover, GC/ECD may have less selectivity than GC/MS since MS can also be used to separate molecules.
4.2.2 HPLC
1. Nash reagent derivatization
One approach has been to use HPLC with post-column reaction (PCR) derivatization method for the analysis of free FA in water. The flow chart of HPLC instrumentation for this experiment is shown below. (Michels et al., 2001)
HPLC Column
Injector
Pump #1
Mobile Phase
pe
UV
Detector
70°C
Data
System
Post-
Column Reactor
Pump #2
Nash Reagent
FL
Detector
The Pump #1 delivered chromatographic mobile phase to injector and the Pump #2 supplied Nash reagent to the post-column reactor. After separating the mixed sample in analytical column of HPLC, the elution of the components can be firstly monitored by a UV detector. Next, FA derivatives formed by FA and Nash reagent was eluted in the void volume of the system and can be detected by FL detector as a single peak. The FL detector has a function of increasing the selectivity and sensitivity (Michels et al., 2001).
For this approach, the FA incorporating components can be retained in analytical column and all FA derivatives were eluted in PCR HPLC system. Therefore, there’s only one peak occurred in FL detector. 　
2. DNPH derivatization
Another method reported by ADWG for FA determination was by formation of the DNPH derivative followed by analysis with HPLC and UV detection (Whittle et al., 1988).
The method is based on the detection of DNPH formed by FA and 2,4-dinitrophenylhydrazine under acidic conditions. The derivatives were extracted with dichloromethane and taken up in acetonitrile. Then the solution should be transferred into a sealed 3.5 ml vial immediately. Reversed-phase HPLC with UV detection at 254 nm and a 10-μm LiChrosorb RP-18 column are used to separate and detect the DNPH. After samples injecting via a 20μl sample loop, instruments worked and graphs were presented on PC. Then the total FA concentration can be calculated by integrating the peak areas with reference to the peak areas obtained by blank and standard solutions which have the same procedures (Whittle et al., 1988).
US EPA has published methods using DNPH derivatization followed by LC. However, it also stated that this method can get pleased performance in clean waters such as water from water treatment plant, but in highly contaminated or alkaline solutions, recoveries are often low (US EPA 1998).
3. Advantages and disadvantages of HPLC:
HPLC method is sensitive with a detection limit in water to approximately 6 ppb. This method is also relatively specific. However, interferences cannot be eliminated entirely. For example, the FA peak can be interfered with other unrelated colorful compounds. In addition, the separation of reversed-phase HPLC is based on hydrophobic interaction of the analyte and the stationary phase. Therefore, polar FA solutions are often not very well analyzed.
4.2.3 Advantages and disadvantages
Chromatography methods generally achieved high level of sensitivity and selectivity and are useful in FA analysis of water and wastewater samples since FA concentrations are often very low in water. They also have some advantages such as convenience and time saving.
However, the instruments are costly that are only equipped in some laboratories and universities. They are not widely used in many factories or industrial sectors. In addition, the instruments are susceptible that need specialized staff to operate them.
4.3 Other methods
Other methods such as capillary zone electrophoresis (CZE) can be applied for FA determination as well (Michels et al., 2001).
Accurate quantitation of the analyte
5.1 How to undertake the quantitation
The accurate quantitation of the FA in water and wastewater mainly referred to the determination of absolute or relative abundance of FA which can help in analyzing specific properties. Calibration is an essential way for accurate quantitation. And we can use either internal standard or external standard method to establish a calibration curve or standard addition curve.
For example, if we use the external standard method, we should prepare a series of standard solutions of known FA concentrations to get a standard curve. In Luo’s study, six calibration standards with FA concentration of 1.65, 3.90, 6.15, 8.85, 10.6 and 16.0 μg/ml were prepared. The regression equation of the standard curve was Y=103X+11.8 with a linear regression coefficient of 0.997, where Y is the peak area count (mV) of derivatized FA and X (μg/ml) is the FA concentration in unknown sample (Luo et al., 2001). The concentration range of standard solutions should cover the concentrations of unknown samples to reduce the error.
5.2 Calibration
5.2.1 A calibration curve
As reported in some materials, a calibration curve is often used in FA accurate quantitation which is a common method by setting a series of standard solutions of known concentrations for the determination of unknown sample concentration. Most of the time, the calibration plot of instrument response vs. concentration of analyte exhibited a linear profile. For example, in Hill and Lipert’s FA study in drinking water, they used a reflectance spectrophotometer to get the diffuse reflectance R and then used PC software to calculate KM function F(R) vs. analyte concentration at 700nm for calibration curve. In KM function, F(R) = (1-R)2/2R. In this experiment, they prepared the FA concentrations in standard solutions from 0.08 to 20 ppm. Thus the FA concentrations in unknown samples can be determined by interpolating from calibration curve (Hill et al., 2009).
For our drinking water samples, using a calibration curve is an effective way for accurate quantitation of the FA.
But this method is not suitable for quantitation of FA in wastewater samples which contain many impurities. The calibration curve is obtained from pure standard samples. However, the impurities can interference with the analyte in unknown samples then the instrumental response might be changed. Thus, interpolating from calibration curve, we are likely to get biased concentrations.
5.2.2 Standard addition
The theory of standard addition method is similar to a calibration curve. Moreover, this method has widely applications in GC and can be used to solve matrix effect problem which occurs when there are many impurities in the unknown sample.
In a FA study, a set of FA standard solutions of different concentrations were added to unknown samples, and the readings changed before and after adding the standard solutions were measured by HPLC. Therefore, in the calibration, the impurities were also accounted for. Since the same reagents were added to standard and unknown solutions, researchers can get FA concentrations in unknown samples by extrapolation without bias (Luo et al., 2001).
For our wastewater samples, using standard addition is an effective way for accurate quantitation of the FA.
5.3 Internal standard and external standard
5.3.1 Internal standard
The internal standard is also performed in FA accurate quantitation. One of the researches used Acetone-d6 as the internal standard of FA solutions. Acetone-d6 has the similar molecule weight and retention time with FA. It is added in water samples, standard soluti