Analysis of drinking waters

Drinking water standards in the United Kingdom are some of the toughest in the world.

The European Union has recently issued revised quality standards relating to drinking water.

These and some stricter national standards have been adopted and written into national law in The Water Supply (Water Quality)(England) Regulations 2000 that come fully into force at the end of 2003, although some of the amendments to the previous 1989 regulations have already been adopted.

The primary responsibility for assessing compliance against standards and regulations lies with the Drinking Water Inspectorate (DWI).

It independently assesses the performance of every water company, and reports annually. Local authorities also have some responsibility for checking drinking water quality.

Origins of disinfection by-products: DBPs are chemical compounds that have toxic potential and have been shown to be mutagenic and carcinogenic in low doses on those studies conducted on animals.

They are formed in low concentrations during the disinfection of water supplies and include chloroform and chloroacetic acid from chlorination, and bromate (a by-product of ozone treatment) on raw waters containing bromide.

Disinfection of waters is generally achieved with treatment using chlorine dioxide or ozone.

Bromide in water causes a catalytic disintegration of ozone and forms hypobromite as an intermediate that then reacts further with an overdose of ozone to form bromate.

The intermediate product hypobromite is predominantly present at higher pH values, at lower values hypobromous acid is formed.

Hypobromous acid does not react further with ozone, and thus at low pH no bromate is formed, but it does lead to the formation of brominated organic compounds in the presence of organic matter.

Concentrations of DBPs in drinking waters are extremely low but it has been suggested that prolonged exposure to low doses may have the same effect as short-term exposure to high doses.

Some contaminants in low concentrations have been found to pose special health risks to particular groups of people.

These include infants, young children, and the elderly as well as those people with compromised immune systems due to certain illnesses.

Other symptoms of possible bromate poisoning include abdominal pains, hearing impairment, and kidney failure.

It is not realistic to stop disinfecting water supplies because the theoretical risk of toxicity is weighted against the certainty that non-disinfected water can cause illness and death.

Disinfection was first adopted in the United Kingdom back in 1936 after an outbreak of typhoid in Croydon.

In the intervening years, the occurrence of drinking water-related illnesses has been reduced, and death from cholera and typhoid has been eliminated Determination of bromate in drinking waters: The Environmental Protection Agency (EPA) has developed EPA Method 317.0 for the determination of inorganic oxyhalide DBPs in drinking waters using ion chromatography with the addition of a post column reaction (PCR) for trace bromate analysis.

The method was developed as a multiple component procedure employing both suppressed conductivity detection and post column UV-Vis detection in series.

The method employs the same hardware (prior to the addition of PCR) as EPA Method 300.1 that determined inorganic anions in drinking waters and can be performed with the post column hardware off-line.

The mandatory standard for bromate in drinking waters as governed by the 1998 European Drinking Water Directive, in a paper published by the DWI, is 25µg/l by the end of 2003 with a lower mandatory standard of 10µg/l by the end of 2008.

The directive does however affirm that member states should strive for lower values wherever possible without compromising disinfection.

Traditionally bromate analysis has been performed by ion chromatography with conductivity detection (EPA Method 300.1), whereby it was possible to get down low µg/l levels with no real difficulty. However as water treatment processes continue to improve and evolve over time it is likely that the concentrations of bromate actually contained in supplies of drinking water will be well below the mandatory standard of 10µg/l and the use of conductivity detection may well prove problematic.

With this in mind it is preferable that one can accurately quantify bromate well below this limit.

By use of a post column reaction, one is able to convert the eluting bromate from the ion chromatographic column into a highly sensitive chromophore that can be detected using UV-Vis detection, which literature states is a more sensitive detection technique than conventional conductivity.

The use of PCR for bromate allows selective detection by UV-Vis with no interference from chloride, nitrate and other common anions. Ion chromatography - an overview: Chromatography is a method for separating mixtures of substances using two phases, one of which is stationary and the other mobile moving in a particular direction.

Chromatography techniques are divided up according to the physical states of the two participating phases.

The term ion exchange chromatography or ion chromatography (IC) is a subdivision of high performance liquid chromatography (HPLC).

A general definition of ion chromatography can be applied as follows; "ion chromatography includes all rapid liquid chromatography separations of ions in columns coupled online with detection and quantification in a flow-through detector".

A stoichiometric chemical reaction occurs between ions in a solution and a solid substance carrying functional groups that can fix ions as a result of electrostatic forces.

In anion chromatography these are quaternary ammonium groups.

In theory, ions with the same charge can be exchanged completely reversibly between the two phases.

The process of ion exchange leads to a condition of equilibrium; the side to which the equilibrium lies depends on the affinity of the participating ions to the functional groups of the stationary phases.

Collection and treatment of drinking water samples: The water samples should be treated in accordance with the guidelines of EPA Method 317.0.

In the event of a field sample being collected from a plant employing chlorine dioxide or ozone treatment, then the field sample must be first purged with an inert gas prior to the addition of ethylenediamine (EDA) preservative equivalent to 50mg/l per volume of sample.

The drinking water samples for bromate analysis should be collected in plastic or glass bottles thoroughly cleaned and rinsed with reagent water.

Samples collected in this manner and stored at less than 6C can then be held for up to 28 days.

EDA preserves the integrity of bromate concentrations by binding with the intermediate hypobromous acid/hypobromite ion to prevent further formation of bromate ions.

The presence of chlorite can interfere with the quantification of low concentrations (0.5-15µg/l) of bromate using PCR with UV-Vis detection.

Treatment plants that use chlorine dioxide treatment as part of their treatment process can produce high levels of chlorite in their samples that must be removed prior to analysis.

The EDA stabilised sample is acidified to pH 5-6 before ferrous iron solution is added and allowed to react producing an oxidation-reduction reaction with chlorite. The sample is filtered, first through a 0.45µm filter to remove the precipitated ferric hydroxide, and then a cation exchange cartridge to remove the excess soluble iron, prior to injection. Method for analysis of bromate in drinking waters: The water samples were treated as appropriate.

200µl of sample was injected directly into the ion chromatograph and the response for the peaks recorded using a mobile phase eluent of sodium carbonate/sodium bicarbonate with the Metrohm A SUPP 5 analytical column.

The calculation was carried out automatically using integration software IC Net 2.1 against previously prepared calibration plots.

The modular system used comprised the Metrohm 709 IC pump, 732 IC detector, 733 IC separation centre, two 752 pump units, 762 IC interface and Post Column Reagent kit, Bischoff Lambda 1010 UV-Vis Detector, and Jones 7971 column heater.

The 709 IC pump comprises a dual piston pump with two valves to guarantee low residual pulsation and excellent flow stability.

The pressure is measured extremely precisely by piezo-resistive means, which allows the facility to set pressure upper and lower limits to safeguard pressure sensitive columns. The 732 IC detector comprises a detector block with a built-in measuring cell that is thermally and electrically shielded from outside influences by use of a faraday cage.

The measuring cell is thermostatted and has a thermal stability of <0.01C. Together with the electronic pre-amplification in the detector block, one is able to obtain very high sensitivity coupled with an optimal signal-to-noise ratio.

The liquid contacting parts and the electronic parts are separated from each other.

The 733 IC separation centre provides thermal stability and the metal network built into the casing provides an effective shield against electromagnetic radiation.

The separation centre is the home of all the components associated with the wet chemistry part of ion chromatography analysis; guard columns, analytical columns, detector cell, pulsation dampner, Metrohm Suppressor Module (MSM) and valves all work under steady and constant ambient conditions.

If it is required to determine bromate, or other species not detected by UV-Vis, by conductivity detection then the use of the MSM is required.

This is built into the IC separation centre in certain instrument configurations and consists of three micro-bed packed suppressor channels located inside a rotor.

One channel is used in-line for the analysis with the other two operating off-line.

A fresh suppressor channel is used each time for analysis, ensuring that the cation exchanger is operating at full capacity.

The advantages of this type of suppressor are that there are no sensitive membranes, no contamination by sulphuric acid, no risk of destruction by pressure or heavy metals, and no hydrogen gas production in the laboratory.

The 752 IC pump unit is used with the MSM supplying the two off-line channels; one is sulphuric acid for regeneration and the other deionised water for regeneration.

A second pump unit is configured within the instrument set-up to continuously deliver PCR solution to the PCR kit for the UV-Vis detection of bromate.

With the 762 IC interface and Metrohm IC Net software, complete control of the Metrohm hardware is guaranteed under a Windows operating system.

The IC interface is responsible for the data acquisition and analogue to digital conversion of the outputted signal from the detector.

It is possible to control up to 16 IC or HPLC peripheral devices through the interface with only a single connection to the PC.

The IC Net software allows full control of all the individual instruments in any given system through the use of pictographic icons as well performing the all important integration functions.

The Bischoff Lambda 1010 is a programmable UV-Vis detector for HPLC that allows continuous variation in wavelength over the range 190-800nm (with the different lamp options).

The excellent signal-to-noise ratio allows for effective detection even with low sample concentrations.

The 7971 Jones column heater is used to provide an elevated heated medium for the Metrohm PCR kit.

EPA Method 317.0 recommends a temperature of between 60-80C for the reaction that takes place between PCR (o-dianisidine dihydrochloride solution) and the bromate species.

It was stated in EPA Method 317.0 that operating at a temperature of 80C dramatically increased the method's sensitivity.

When a large sample throughput is required, it is recommended to automate the system with one of three Metrohm autosamplers; the 766 IC sample processor, the 788 IC filtration sample processor, or the 813 compact autosampler.

The calibration plots obtained are typical ranges described by EPA Method 317.0 over which the instrument should be calibrated and demonstrate the linearity of bromate using both forms of detection.

In-house application work at Metrohm has shown it is possible to get down to 0.5µg/l bromate using both conductivity and UV-Vis detection operating together in series.

However EPA Method 317.0 states that for bromate concentrations between 5.0 and 15.0µg/l, then far better precision and accuracy is attained when PCR with UV-Vis detection is used.

The method EPA 317.0 recommends the use of conductivity detection for samples containing greater than 15.0µg/l bromate. An additional benefit of using PCR with UV-Vis detection is that chloride interference is minimised.

No response would be observed for a large chloride peak eluting after bromate when using UV-Vis detection.

The use of PCR with UV-Vis detection allows the quantification of bromate down to low µg/l in the presence of large excesses of chloride without the need for any sample pre-treatment of the chloride.

A surrogate solution added to all solutions was employed in EPA Method 317.0.

This was used to monitor method performance using conductivity detection.

No response is seen for the surrogate using UV-Vis detection and if only absorbance detection is employed then the surrogate solution can be omitted.

Conclusion of bromate analysis in drinking waters: All water treatment strategies will result in the formation of disinfection by-products.

The use of chlorine dioxide for disinfection of drinking water generates the inorganic DBPs chlorite and chlorate, while the presence of chlorate has been reported in waters treated with hypochlorite. Ozonation produces bromate as a DBP anion if the source water contains naturally occurring bromide.

The monitoring of bromate (and other DBPs) in drinking water is of critical importance bearing in mind the known toxicological risks that they possess. In addition, there is the issue of water quality that must be regulated and controlled in order to supply wholesome drinking water to the consumer.

Ion chromatography offers a reliable, precise analytical technique that can attain outstanding limits of detection; a factor recognised in its inclusion in many of the EPA methods.

If a mandatory standard is exceeded then the cause is immediately investigated by the water company concerned and remedial action taken.

In the majority of cases, any exceedances are minor and temporary in nature.

All such instances must be reported to the DWI which can investigate further should it deem necessary.

Using the hardware described in this article it is possible to switch between EPA Methods 317.0 and 300.1 by simply running with the PCR hardware off-line allowing flexibility of the methodology employed. References: The following internet sites were used extensively as references and can be used to obtain further information:- www.croydon.gov.uk www.dwi.gov.uk www.epa.gov www.metrohm.ch www.stwater.co.uk