aldehydes in water
953.5. Method validation
3.6. Determination of aldehydes in water
In order to verify the effectiveness of the MLLE–PTV–GC–MS method proposed for the application in question, ten water samples were analysed, including drinking, well and swimming pool waters. Tables 4 and 5 show the aliphatic and aromatic LMMA concentrations, respectively, found in the different waters. The results were validated against those provided by EPA Method 556.1, which were also included in these tables. As can be seen the two methods provided similar results; however, EPA Method 556.1 failed to detect the majority of aromatic aldehydes (except for BA and 3-HBA) in all waters owing to its lower sensitivity.
Aliphatic aldehydes (Table 4) were present in all waters including well water at concentrations from 0.01 to 38 µg L−1, C1 and C2 being the most prevalent in all instances, and dicarbonyl aldehydes were those found at lower concentrations. The high concentrations of aliphatic aldehydes found in swimming pools is noteworthy since the concentration of organic matter and especially residual chlorine were higher in these water samples: 1–3 mg L−1 for outdoor–indoor pools versus 0.3–0.5 mg L−1 for tap waters; so the concentrations of these DBPs increased (between 2 and 9 times from tap to swimming pool waters). With regard to aromatic aldehydes (Table 5) only BA, 3-MBA and 3-HBA were found in tap water but the six aromatic compounds studied were present in swimming pool waters at concentrations from 0.2 to 11 µg L−1, BA being the most prevalent. Finally, Fig. 3 shows the chromatogram (SIM mode) obtained from the indoor swimming pool water 1. As can be seen, (E) and (Z) isomers for aliphatic and aromatic aldehydes can be resolved. Four isomers can be formed for compounds with two carbonyl groups, which co-eluted in the case of G (see peak 11 in Fig. 3) and can be resolved for MG (see peak 12). The “R” peak was identified as the excess from PFBHA but it does not interfere with the analysis of the aldehydes studied.
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Table 4 Results (± standard deviation) for the determination of aliphatic aldehydes in water samples by the proposed (MLLE) and the reference (EPA 556.1) methods (n =5). Water Content ± SD (µg L-1) MLLEEPA 556.1 C1C2C3C4C5GMGC1C2C3C4 C5GMG Tap 1 2.3 ± 0.2 1.3 ± 0.1 1.4 ± 0.1 2.2 ± 0.2 0.7 ± 0.1 0.1 ± 0.1 0.3 ± 0.1 2.1 ± 0.2 1.2 ± 0.1 1.6 ± 0.1 2.5 ± 0.2 0.6 ± 0.1 0.1 ± 0.1 0.2 ± 0.1 Tap 2 3.8 ± 0.3 1.8 ± 0.1 2.2 ± 0.2 3.2 ± 0.3 0.5 ± 0.1 0.2 ± 0.1 0.2 ± 0.1 4.2 ± 0.3 2.3 ± 0.2 1.7 ± 0.1 2.8 ± 0.2 0.6 ± 0.1 0.4 ± 0.1 0.1 ± 0.1 Tap 3 4.3 ± 0.3 1.7 ± 0.1 1.1 ± 0.1 1.9 ± 0.2 0.4 ± 0.1 0.08 ± 0.010.1 ± 0.1 4.5 ± 0.4 2.0 ± 0.2 1.4 ± 0.1 2.0 ± 0.2 0.5 ± 0.1 0.06 ± 0.01 0.09 ± 0.01 Tap 4 3.2 ± 0.3 2.7 ± 0.2 1.6 ± 0.1 2.0 ± 0.2 0.9 ± 0.1 0.5 ± 0.1 0.3 ± 0.1 3.0 ± 0.3 2.2 ± 0.2 1.9 ± 0.2 1.9 ± 0.2 0.8 ± 0.1 0.7 ± 0.1 0.5 ± 0.1 Well 11.5 ± 0.1 0.8 ± 0.1 1.0 ± 0.1 1.1 ± 0.1 0.2 ± 0.1 0.01 ± 0.010.03 ± 0.011.1 ± 0.1 1.1 ± 0.1 1.2 ± 0.1 1.5 ± 0.1 <0.3 an.d.b <0.05 Well 20.9 ± 0.1 1.2 ± 0.1 1.0 ± 0.1 1.0 ± 0.1 0.3 ± 0.1 0.02 ± 0.010.04 ± 0.011.2 ± 0.1 0.8 ± 0.1 0.9 ± 0.1 1.3 ± 0.1 <0.3 <0.03 <0.05 Outdoor swimming pool 1 14 ± 17.2 ± 0.6 2.6 ± 0.2 3.1 ± 0.3 1.2 ± 0.1 0.6 ± 0.1 0.8 ± 0.1 17 ± 17.6 ± 0.7 2.3 ± 0.2 2.9 ± 0.2 0.8 ± 0.1 0.5 ± 0.1 1.2 ± 0.1 Outdoor swimming pool 2 20 ± 2 11 ± 14.1 ± 0.3 3.9 ± 0.3 3.0 ± 0.3 1.4 ± 0.1 1.1 ± 0.1 18 ± 18.4 ± 0.7 4.5 ± 0.4 3.7 ± 0.3 3.4 ± 0.3 1.2 ± 0.1 1.3 ± 0.1 Indoor swimming pool 1 38 ± 3 13 ± 16.5 ± 0.6 5.8 ± 0.6 4.5 ± 0.4 2.7 ± 0.2 1.5 ± 0.1 36 ± 3 12 ± 16.3 ± 0.5 6.3 ± 0.6 5.0 ± 0.4 2.3 ± 0.1 1.2 ± 0.1 Indoor swimming pool 2 30 ± 3 19 ± 28.1 ± 0.7 7.7 ± 0.6 6.5 ± 0.6 2.3 ± 0.2 2.9 ± 0.2 33 ± 3 24 ± 28.6 ± 0.7 7.4 ± 0.7 6.1 ± 0.5 2.6 ± 0.2 3.2 ± 0.2 a < LOQ. b n.d., not detected.
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Table 5 Results (± standard deviation) for the determination of aromatic aldehydes in water samples by the proposed (MLLE) and the reference (EPA 556.1) methods (n = 5). Water Content ± SD (µg L-1) MLLEEPA 556.1 BA3-MBA2-EBA2,5-DMBA3-HBA2,5-DHBABA3-MBA2-EBA2,5-DMBA3-HBA2,5-DHBA Tap 1 1.1 ± 0.1 0.06 ± 0.01<0.15an.d.b5.6 ± 0.6 n.d.1.3 ± 0.1 n.d.n.d.n.d.5.9 ± 0.7 n.d. Tap 2 0.6 ± 0.1 0.09 ± 0.01n.d.n.d.1.7 ± 0.2 n.d.0.5 ± 0.1 n.d.n.d.n.d.1.5 ± 0.2 n.d. Tap 3 0.4 ± 0.1 0.4 ± 0.1 <0.15n.d.2.5 ± 0.3 n.d.<0.5 <0.7 n.d.n.d.2.3 ± 0.2 n.d. Tap 4 0.2 ± 0.1 <0.05n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d. Well 10.07 ± 0.01n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d. Well 2 0.4 ± 0.1 n.d.n.d.n.d.n.d.n.d.<0.5 n.d.n.d.n.d.n.d.n.d. Outdoor swimming pool 1 3.4 ± 0.3 0.2 ± 0.1 0.2 ± 0.1 0.4 ± 0.1 1.2 ± 0.1 0.5 ± 0.1 3.1 ± 0.3 n.d.n.d.n.d.1.5 ± 0.2 n.d. Outdoor swimming pool 2 2.1 ± 0.2 0.5 ± 0.1 0.5 ± 0.1 0.3 ± 0.1 0.9 ± 0.1 <0.3 1.8 ± 0.2 <0.7 n.d.n.d.<1n.d. Indoor swimming pool 1 11 ± 1 0.9 ± 0.1 0.4 ± 0.1 0.9 ± 0.1 7.9 ± 0.8 0.7 ± 0.1 10 ± 10.8 ± 0.1 n.d.<38.3 ± 0.8 n.d. Indoor swimming pool 2 4.7 ± 0.5 0.6 ± 0.1 0.9 ± 0.1 0.3 ± 0.1 3.1 ± 0.3 0.5 ± 0.1 5.2 ± 0.5 <0.7 <2n.d.2.8 ± 0.3 n.d. a < LOQ. b n.d., not detected.
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Fig. 3. PTV–GC–MS chromatogram (SIM mode) obtained from the analysis of the indoor swimming pool water 1 (see Tables 3 and 4) by the proposed MLLE method. Peak identification: 1, C1; 2, C2; 3, C3; 4, C4; 5, C5; 6, BA; 7, MBA; 8, 2-EBA; 9, 2,5- DMBA; 10, 3-HBA; 11, G; 12, MG; 13, 2,5-D3-HBA; IS, internal standard and R, PFBHA excess.
4. Conclusions
LLE is one of the most commonly extraction techniques used in water analysis, but it requires large volumes of solvent and, therefore, alternative extraction techniques allowing minimum or no solvent consumption are advocated.
Nevertheless if we compare LLE with micro scale alternatives, it can be observed
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that the classical solvents in conventional LLE (e.g. n-hexane, ethyl acetate and methyl tert-butyl-ether), which provide excellent extraction efficiency and compatibility with GC–MS, have been replaced by other solvents which give lower extraction efficiency (e.g. octanol, hexanol, undecanol). These solvents, with higher boiling points, are less volatile and can be retained either inside the column or in the mass spectrometer ion source, with the corresponding well-known problems. The proposed MLLE method demonstrated that it is possible to use conventional solvents in miniaturised techniques with all the advantages that this means. For the first time a fast, simple, sensitive and robust method has been developed to derivatise and extract thirteen LMMAs in one step for their determination by GC–
MS at trace levels in water. The MLLE method-based on EPA Method 556.1 – is characterised as being an environment-friendly microextraction choice that uses a low volume of solvent and presents the following innovations: (i) the derivatisation reaction with PFBHA can be performed in a strong acidic medium (pH 1.1) at 60 ºC for 1 min versus the weak acidic medium (pH 4.0) at 35 ºC for the 2 h required by EPA Method 556.1; (ii) the addition of magnesium sulphate to the aqueous phase – it heated this layer up to ~60 ºC – allows the simultaneous derivatisation/
microextraction of aldehydes in only 1 min; and (iii) the use of an aqueous/organic phase ratio of 45 versus 5 (EPA Method 556.1) together with a LVI (50 µL) coupled to PTV–GC–MS improved the sensitivity with LODs at ng L−1 levels. Finally, it should also be emphasised that the combination of this MLLE method with the direct LVI of the extract in the PTV–GC–MS system minimises the generation of hazardous residues in accordance with the principles of “Green Chemistry”.
Acknowledgements
This work was funded in the framework of Projects CTQ2010- 17008 (Spanish Ministry of Education) and P09-FQM-4732 (Andalusian Regional Government). FEDER provided additional funding. María Serrano is grateful for the award of a pre-doctoral grant from Project CTQ2010-17008.
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