Interlaboratory Proficiency Testing on the Analysis of Alkylphenols, Alkylphenol Ethoxylates and Bisphenol A in Surface and Waste Waters According to ISO/CD 18857-2 Using New 13C-Marked Internal Standards

By: Olaf P. Heemken, Ernst Stottmeister, AnalytiX Volume 8 Article 5

Member of Working Group DIN NA 119-01-03-02-02 “Phenols with GC”
Convener of Working Groups ISO TC 147/SC 2/ WG 17 “Phenols” and DIN NA 119-01-03-02-02 “Phenols with GC” 

As reported previously1, alkylphenols and their shortchain ethoxylates have considerable environmental relevance because of their endocrine-disrupting properties. For this reason, they have been introduced into several international measurement programmes. The most important monitoring programmes for surface water in Europe include the “Water Framework Directive” 2000/60/EC and the “OSPAR Convention for the Protection of the Marine Environment” for waters in the northeast Atlantic.

Among the alkylphenols, nonylphenol and its ethoxylated derivates present a particular analytical challenge because they are technical mixtures of isomers. Figure 1 shows the chromatogram of technical nonylphenol and the mono- and diethoxylate. Their identification is based on the peak pattern (fingerprint) of the chromatogram, although the relation between the individual peaks of the pattern may differ in samples and standards. Substances co-eluting with nonylphenol and its mono- and diethoxylates can interfere in the determination. This may have a large influence on the result, since these three analytes are quantified from the sum of all peaks belonging to the chromatographic pattern. The analyst should use caution to include only those peaks from the sample that are attributable to the multicomponent analyte. In complex environmental samples, such interferences can occur even if GC-MS is used. The difficulty of analysing these compounds is illustrated by the fact that different interlaboratory trials resulted in widely varied results2,3. Another disadvantage in current analytical procedures is the lack of appropriate isotopemarked internal standards.

Figure 1 Chromatogram of target analyte mix and internal standards mix

Figure 1 Chromatogram of target analyte mix and internal standards mix

For these reasons, the German Institute for Standardisation (referred to as DIN) submitted in April 2006 a new work-item proposal to the International Organisation for Standardisation (ISO) to develop an ISO standard method for the determination of alkylphenols, alkylphenol ethoxylates, and Bisphenol A in non-filtered water samples using solid phase extraction and gas chromatography with mass selective detection.

For a final evaluation, the method referred to as ISO/CD 18857-24 had to be validated by an interlaboratory trial in order to assess the performance of the method, to give information on the comparability of the analytical results attained at different laboratories, and to evaluate critical points such as the suitability of the proposed internal standard for quantifying multi-component analytes (e.g. the technical mixture of nonylphenol isomers) that was specially developed for this analytical approach. The interlaboratory study was evaluated according to ISO 57255 and included two duplicate non-filtered water samples: surface water (2 x 1 L) containing the target compounds in an analyte-concentration range from 0.05 to 0.4 µg/L, and waste water (2 x 100 mL) containing the target compounds in a concentration range from 0.1 to 5 µg/L. All laboratories were asked to follow the procedure exactly as prescribed in the draft standard. Briefly, the method applied consists of the following steps:

  1. Acidification of the water sample to pH 2 with sulphuric or hydrochloric acid.
  2. Addition of 13C-marked internal standards, specified in tab. 2.
  3. Solid phase extraction and drying of the cartridge.
  4. Elution, concentration, and reconstitution of the extract.
  5. Derivatization with MSTFA.
  6. Measurement and quantification of the trimethylsilane derivatives.

 

Figure 2 Molecular structures of 13C-labelled octyl- and nonylphenol and corresponding ethoxylates.

Figure 2 Molecular structures of 13C-labelled octyl- and nonylphenol and corresponding ethoxylates.

Details of the method are given elsewhere4. The interlaboratory trial was organized by the Federal Environment Agency (Bad Elster, Germany) and started in November 2007. Fourteen laboratories from four different countries (Austria, Canada, Italy, and Germany) participated. Thirteen laboratories from four countries reported results; one laboratory withdrew.

Table 1 shows a summary of the results from this trial. The accuracy of the method is expressed by the recovery, and the precision of the method is determined by the variance coefficients of the reproducibility CVR as a measure of the comparability between the different laboratories, and the repeatability CVR as a measure of internal laboratory precision. The data show that the method developed under ISO/CD 18857-2 represents a robust procedure for the quantitative analysis of octyland nonylphenol and their mono- and diethoxlyates in water samples. Bisphenol A can also be determined by this method. Results obtained were of good accuracy and reproducibility. The isotope-marked standard compounds developed in this context have been proven to be reliable internal standards that allow a precise and accurate quantitation of all compounds specified in ISO/CD 18857-2.

Table 1 Performance data from the interlaboratory validation of ISO/CD 18857-2 of octyl- and nonylphenol (OP, NP), the corresponding ethoxylates (1,2EO), and Bisphenol A (BPA) in surface waters

Table 1 Performance data from the interlaboratory validation of ISO/CD 18857-2 of octyl- and nonylphenol (OP, NP), the corresponding ethoxylates (1,2EO), and Bisphenol A (BPA) in surface waters

Sigma-Aldrich now offers the entire range of isotopemarked compounds in ISO/CD 18857-2 for the quantitativeanalysisofoctyl- andnonylphenolandtheirmono-and diethoxylates. Table 2 lists the available concentrations with the corresponding product numbers. In addition to the 13C-marked compounds for analysis with GC-MS, there are also twice-deuterated compounds that are ideal for analysis with LC-MS/MS.

Compound Package size Cat. no. Package size Cat. no.
4-tert-octylphenol (Ring 13C6) 1 ml / 10 µg/ml 33565 10 ml / 1 µg/mL 33566
4-tert-octylphenol monoethoxylate (Ring 13C6) 1 ml / 10 µg/ml 33563 10 ml / 1 µg/mL 33564
4-tert-octylphenol diethoxylate (Ring 13C6) 1 ml / 10 µg/ml 33229 10 ml / 1 µg/mL 33244
4-(3,6-dimethyl-3-heptyl)-phenol (Ring 13C6) 1 ml / 10 µg/ml 33574 10 ml / 1 µg/mL 33575
4-(3,6-dimethyl-3-heptyl)-phenol monoethoxylate (Ring 13C6) 1 ml / 10 µg/ml 33572 10 ml / 1 µg/mL 33573
4-(3,6-dimethyl-3-heptyl)-phenol diethoxylate (Ring 13C6) 1 ml / 10 µg/ml 33207 10 ml / 1 µg/mL 33222
 
4-tert-octylphenol (Ring D2) 1 ml / 10 µg/ml 33557 10 ml / 1 µg/mL 33559
4-tert-octylphenol monoethoxylate (Ring D2) 1 ml / 10 µg/ml 33523 10 ml / 1 µg/mL 33525
4-tert-octylphenol diethoxylate (Ring D2) 1 ml / 10 µg/ml 33254 10 ml / 1 µg/mL 33257
4-(3,6-dimethyl-3-heptyl)-phenol (Ring D2) 1 ml / 10 µg/ml 33569 10 ml / 1 µg/mL 33571
4-(3,6-dimethyl-3-heptyl)-phenol monoethoxylate (Ring D2) 1 ml / 10 µg/ml 33567 10 ml / 1 µg/mL 33568
4-(3,6-dimethyl-3-heptyl)-phenol diethoxylate (Ring D2) 1 ml / 10 µg/ml 33249 10 ml / 1 µg/mL 33252
 
Target analyte mix according to ISO/CD 18857-2     10 ml / 1 µg/mL 33623
Internal standard mix according to ISO/CD 18857-2     10 ml / 1 µg/mL 33627

Table 2 Labelled octyl- and nonylphenol and corresponding ethoxylates from Sigma-Aldrich

Materials

     

References

  1. Heemken, O. P.; Amann, N., Analytix 2007, 4, pp 6–7.
  2. Loos, R.;Wollgast, J.; Castro-Jimenez, J.; et al., TrAC Trends in Analytical Chemistry 2008, 27 (1), 89–95.
  3. Sobiecka, E.; Van der Sloot, H.; Hansen, N.; Gawlik, B. M., Project HORIZONTAL Validation Report, Office for Official Publications of the European Communities, Luxembourg 2007, ISBN 978-92-79-07123-2.
  4. International Standardisation Organisation, ISO/CD 18857-2, 2008.
  5. International Standardisation Organisation, ISO/CD 5725-6, 1994.

 

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