GETTING THE RIGHT RESULT
The importance of sample preparation
M Drikas, C Kapralos, E Kozlik, R Fabris
Publication Date (Web): 26 June 2017
Analytical water quality is determined by measurement of a range of parameters; many of those parameters need to be determined following filtration to remove suspended matter and obtain a result for the “dissolved” component. Filtration through 0.45μm membrane filters is the internationally accepted standard for analysis of dissolved parameters. Whilst this appears a simple and straightforward aspect of sample preparation, we have shown it can have a significant impact on parameters such as colour, UV absorbance and dissolved organic carbon (DOC) measurement which are critical for optimisation of coagulation in drinking water treatment and removal of DOC to improve disinfectant stability and reduce disinfection by-product formation.
Historically filtration has been undertaken using membrane filters and applying a vacuum. Recently the use of syringe filters has also been adopted particularly when filtering small volumes as it is more convenient, particularly when in the field. Differences obtained using vacuum versus syringe filtration were observed when undertaking analyses for UV absorbance at 254 nm (UV254) and DOC when analysing turbid raw water. Previous investigations undertaken when developing a suitable method to analyse true colour showed that differences in colour could be attributed to ineffective filtration caused by colloidal particles passing through the membranes. This resulted in the recommendation that the residual turbidity after membrane filtration should be less than 2% of the colour in Hazen units. In ensuing years, measurement of UV254 and DOC to assess the organic character of water, both alone and in combination, has increased. The impact of filtration on these parameters has been assessed by other researchers and resulted in standard methodology to highlight the impact of filtration when measuring UV254. A range of filter types, including glass fibre, polycarbonate, polyethersulphone and hydrophilic polypropylene, are recommended. However, the method also states that other filter papers that neither sorb UV-absorbing organics nor leach interfering substances may be used and that filter pore size will influence test results, particularly in raw waters. The method also states that polypropylene syringes may be used instead of vacuum filtration.
This study measured colour, UV254 and DOC on a range of water sources following filtration with both vacuum and syringe filtration using 0.45 and 0.22μm mixed cellulose esters (MCE) and polyethersulphone (PES) membranes. This showed that the filtration step impacted the resulting water quality results due to a range of causes including fouling of the membrane, ineffective removal of turbidity, leaching of organics and /or “over-pressuring” (during syringe filtration). Review of this data has proved that effective filtration is critical when preparing samples for analysis to ensure accurate and consistent results. Whilst the use of 0.22μm PES membranes provided similar results, the use of 0.45μm membranes is consistent with international standards. Therefore, it is recommended that filtration for preparation of samples to be analysed for true colour, filtered UV absorbance and DOC be undertaken with 0.45μm PES membranes using vacuum filtration.
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