Fluorescent whitening agents

The presence of FWAs indicates human effluent.

Washing powder manufacturers regularly add organic compounds called fluorescent whitening agents (FWAs) that adhere to woven fabrics, and which adsorb the short (ultraviolet) wavelengths from sunlight and emit most of the absorbed energy as higher wavelength blue light. This fluorescence, which is more noticeable with white fabrics, makes clothing appear brighter and cleaner.

Most household plumbing mixes the effluent from toilets with "grey water" from washing machines and, consequently, FWAs are usually associated with human faecal contamination from both individual septic tank systems and community wastewater schemes.

Although a range of FWAs is available, only one (4,4'-bis[(4-anilino-6-morpholino-1,3,5-triazin-2-yl)-amino]stilbene-2,2'-disulfonate) is used in New Zealand (FWA 1), which simplifies their analysis.

FWA 1

FWA 1

 

The key features of FWAs include:
  • They are not known to occur in nature.
  • They are highly polar adsorbing strongly to the polysaccharides of paper and clothing.
  • Irradiation by sunlight causes them to bind irreversibly to cellulose of protein, enabling binding to cotton and nylon fabrics.
  • They are highly water soluble.
  • They undergo photochemical degradation; the half-life is several hours under summer noon sun.
  • There are no known health effects of FWAs at levels seen in effluent or water.
Synchronous scanning spectrofluorophotometry

A Synchronous Scanning Spectrofluorophotometer (SSS) is programmed to scan through a defined wavelength range for both the excitation and emission wavelengths of a fluorescent compound. This analysis produces a plot of fluorescence intensity versus emission wavelength and is very useful for identifying a mixture of fluorescent molecules in an unknown sample. Fluorescence sensitivity is up to a thousand times better than absorbance methods and allows analysis to nanogram and picogram levels. SSS is a rapid and inexpensive method, which is able to provide preliminary and discriminatory information on the types of contaminants present in a sample, e.g. the presence of FWAs, fuels, polyaromatic hydrocarbons and pesticides in an aquatic environment.

Samples positive by SSS are further analysed by the quantitative method of High Pressure Liquid Chromatograpy (HPLC). FWAs are extracted from 50 mL samples and analysed by HPLC as previously described (Gregor et al., 2002 (external link) ). Results are expressed in parts per billion (ppb) equivalent to μ/litre. Levels of FWA greater than 0.1 μ/L suggest human sewage, with levels >0.2 μ/L a strong indication of human sewage. Here [PDF, 88 KB] is an example of results generated by fluorescent whitening agents analysis. The comparative costs of FWA analyses can be found under Costs for faecal discrimination testing.

A further advantage of FWA analysis is the ability to track human contamination along a watercourse to identify the entry point of a faecal source e.g. a beach side community where a series of septic tanks line a waterway flowing into the beach. SSS can be performed on water samples taken along the course of the waterway and HPLC on presumptive positive samples can indicate the septic tank system which is the likely source of contamination.

Sample collection and storage

If PCR analysis is requested alongside FWA analysis then the 1 litre of water is sufficient for all PCR and FWA analyses.

Additional literature on FWAs can be found here.

 

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