Drinking water matrix effects

The results described here demonstrate the importance of appropriate treatment and monitoring in actual drinking water processing plants, with attention to the specific requirements of the raw water matrix in use. In particular, the adverse effect of certain processes, namely pre-chlorination, which has been implicated in the inhibition of biodegradation in subsequent steps, and in the formation of alternative metabolites, is highlighted. Furthermore, the variable efficiency of GAC filtration in practice, emphasises the need for regular monitoring and quality control. The duration of specific process steps has also been shown to influence the efficacy of the technique, and should be addressed in application. 

Figure 5-3 shows a strong matrix effect in the analysis of perchlorate (CIO4 ) by mass spectrometry. Perchlorate at a level above 18 p,g/L in drinking water is of concern because it can reduce thyroid hormone production. Standard solutions of C104 in pure water gave the upper calibration curve in Figure 5-3. The response to standard solutions with the same concentrations of CIO4 in groundwater was 15 times less, as shown in the lower curve. Reduction of the ClOj" signal is a matrix effect attributed to other anions present in the groundwater.

A common way to assess the matrix bias is to analyse a drinking water (therefore containing a matrix) in the proficiency testing. For this analysis, laboratories calibrate their instruments using standard, commercial or in-house solutions. If a CRM is available, a possible matrix effect can be corrected by adjusting operational instrument parameters to match the certified value. 

A modified reduction-aeration method to analyse mercury in environmental water samples is described. After aeration of the sample, the mercury is pre-concentrated on gold-coated sand which is then analysed by thermal desorption and cold vapour atomic absorption spectrometry (CVAAS). When applying reduction-cells of volumes up to 1 1, limits of detection as low as 1 ng.r are obtained. The method avoids matrix-effects caused by complexing agents such as Cl and 1, which interfere in the direct measurement of mercury by common CVAAS-techniques. The method was applied to dty drinking-water. Mercury levels ranging from less than 1 ng.l" to 10 ng.T were found. 

When electrolyte concentrations are not too great, it is often useful to swamp both samples and standards with a measured excess of an inert electrolyte. The added effect of the electrolyte from the sample matrix becomes negligible under these circumstances, and the empirical calibration curve yields results in terms of concentration. This approach has been used, for example, in the potentiometric determination of fluoride ion in drinking water. Both samples and standards are diluted with a solution that contains sodium chloride, an acetate buffer, and a citrate buffer the diluent is sufficiently concentrated so that the samples and standaids have essentially identical ionic strengths. This method provides a rapid means of measuring fluoride concentrations in the part-per-million range with an accuracy of about 5% relative. 

Friant and Suffet chose four model compounds that are polar and representative of the intramolecular forces of dispersion, dipole orientation, proton-donor capabihty, and proton-acceptor capability (31). These were methyl ethyl ketone, nitroethane, n-butanol and p-dioxane. The pH of the aqueous sample matrix had no effect on all except for nitroethane. An optimum salt concentration was 3.35M in sodium sulfate and an optimum HS sampling temperature of 50°C. For example, the partition coefficient of methyl ethyl ketone increased from 3.90 to 260 when the temperature of the aqueous sample that contained the dissolved ketone was increased from 30°C to 50°C and at the same time, the concentration of salt increased from zero to 3.35M. Otson et al. compared static HS, P T, and LLE techniques for the determination of THMs in drinking water and found that static HS showed the poorest precision and sensitivity. However, a manual HS sampling technique was used back in this era and might possibly have contributed to this loss of precision. LLE was found to be quite comparable to P T in this study (32). 

Other potential sources of interference are the sample matrix and the solvent used for making the sample solution. The sample matrix is anything in the sample other than the analyte. In some samples, the matrix is quite complex. Milk, for example, has a matrix that consists of an aqueous phase with suspended fat droplets and suspended micelles of milk protein, minerals, and other components of milk. The determination of calcium in milk presents matrix effects that are not found when determining calcium in drinking water. Sample solutions with high concentrations of salts other than the analyte may physically trap the analyte in particles that are slow to decompose, interfering in the vaporization step and causing interference. Differences in viscosity or surface tension between the standard solutions and the samples, or between different samples, will result in interference. Interference due to viscosity or surface tension occurs in the nebulization process for FAAS... 

The resnlts obtained in the UHPLC-MS/MS analysis of different food matrices show that matrix effect depends on the kind of matrix. For instance, signal suppression for most of the PFCs considered was generally lower in drinking water, higher in fish, and the highest in liver. In agreement, method detection limit (MDL) values are much lower in drinking water than in solid matrices. 

Consequently, a value higher than 100% means that signal enhancement occurs in the mass spectrometer because of the coextracted and coeluted matrix. A value lower than 100% implies that signal snppression occnrs in the mass spectrometer. This matrix effect was measured twofold and calcnlated for ultrapure water, drinking water, and surface water at a concentration level of 0.2 mg/L for each compound. In order to investigate the effect of the matrix on the response of the mass spectrometer, the signal suppression was calculated for 15 deuterated standards in sample extracts of snrface water and wastewater compared to a standard in drinking water. The results were corrected for the blank values. 

Table 5.10. Matrix effects are, of course, influenced by the sample intake, which varied depending on the type of matrix. The data in Table 5.10 show that the LODs range from 1 to 22 ng/L for drinking water and from <1 to 10 ng/L in surface waters. In wastewaters, LODs can be much higher because of stronger matrix suppression of the signal. In effluents, LODs varied from 1 to 28 ng/L and, in influents, from 2 to 152 ng/L.

Atomic absorption spectrometry is one of the most widely used techniques for the determination of metals at trace levels in solution. Its popularity as compared with that of flame emission is due to its relative freedom from interferences by inter-element effects and its relative insensitivity to variations in flame temperature. Only for the routine determination of alkali and alkaline earth metals, is flame photometry usually preferred. Over sixty elements can be determined in almost any matrix by atomic absorption. Examples include heavy metals in body fluids, polluted waters, foodstuffs, soft drinks and beer, the analysis of metallurgical and geochemical samples and the determination of many metals in soils, crude oils, petroleum products and plastics. Detection limits generally lie in the range 100-0.1 ppb (Table 8.4) but these can be improved by chemical pre-concentration procedures involving solvent extraction or ion exchange. 

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