The Transition and the politics of memory

and full scan data to be collected in a single run. A full scan (40–400 amu) was used for identification and SIM for quantification, selecting the characteristic ions in each case. The characteristic ions for the fourteen HKs studied are shown in Table 1.

2.3. Sample preparation and stability

Samples were collected in 125 mL amber glass bottles with polytetrafluoroethylene (PTFE) screw caps and adjusted at pH ~1.5 by adding 100 µL of concentrated H₂SO₄. Bottles were completely filled in order to avoid evaporation of volatile compounds. In these acidic conditions, the concentration of the fourteen target analytes in water remained constant for 1 week at 4 ºC and, as result, no dechlorinating agent for residual chlorine was necessary (see Section 3.7).

When the time between sample col-lection and analysis exceeded 1 week, samples could be stored at −20 ºC.

2.4. MLLE procedure

A volume of 12 mL of treated water (prepared as stated in the previous section) or mineral water (adjusted at pH ~1.5 by adding10 µL of concentrated sulphuric acid), containing from 0.02–0.4 to 50–200 µg/L of each HK and 2 µg/L of 1,2-dibromopropane, was placed in a 15 mL glass vial. Then, 200 µL of MTBE and 4 g of Na₂SO₄ were added and the vial was immediately closed with a PTFE screw cap, vortexed for 1 min and left undisturbed for 2 min. After that, ~70 µL of the upper MTBE layer was transferred to a 0.1 mL conical glass placed inside a 2 mL amber glass GC vial containing ~10 mg of Na₂SO₄ to dry the extract. Finally, a volume of 50 µL of the dried extract was aspirated with a 100 µL GC microsyringe and injected into the PTV–GC–MS instrument for analysis.

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Table 1 Main analytical parameters of the proposed MLLE and EPA 551.1 (with LVI–PTV–GC–MS technique) methods. Compoundm/zaMLLEEPA 551.1 LODLinear Range RSD (%) LODLinear Range RSD (%) (ng/L) (μg/L)Intra-day Inter-day (ng/L) (μg/L)Intra-day Inter-day CA43, 92, 94 100.03–505.36.2 300.1–2005.76.3 1,1-DCA43, 83, 85 100.03–505.46.4 300.1–2005.76.3 1,1,1-TCA43, 125, 127 6 0.02–505.15.8 200.07–2004.85.4 1,3-DCA49, 77, 79 120.04–505.66.5 300.1–2005.26.2 1,1-DBA43, 173, 175 6 0.02–505.06.1 200.07–2005.36.4 1,1,3-TCA77, 79, 83 100.03–505.56.3 300.1–2005.86.7 1,1,3,3-TeCA83, 85, 196 6 0.02–505.26.3 200.07–2005.05.9 1,1,1,3-TeCA77, 79, 117 120.04–505.76.8 300.1–2006.17.1 1,1,1-TBA43, 251, 253 180.06–506.17.0 600.2–2005.16.1 1,1-DB-3-CA77, 79, 173 180.06–506.27.3 600.2–2006.57.3 1,1-DB-3,3-DCA173, 201, 203 600.2–2006.77.61800.6–7006.07.1 1,1,3-TBA121, 123, 296 600.2–2006.87.91800.6–7006.47.2 1,3-DB-1,3-DCA127, 129, 157 600.2–2006.67.91800.6–7005.86.7 1,1,3-TB-3-CA127, 129, 201 1200.4–2007.58.43501.2–7007.88.8 a Base peaks used for quantification are boldfaced.

177 2.5. EPA Method 551.1 procedure

The LLE protocol for the determination of HKs in water was that reported by EPA Method 551.1 [23] proposed for the determination of halogenated VOCs.

Samples were collected in 62 mL amber bottles with a PTFE screw cap containing 6 mg of ammonium chloride, without headspace to avoid evaporation of VOCs. A 12 mL aliquot was withdrawn from the sample bottle and dis-carded and the pH of the remaining sample (50 mL) was adjusted to 4.5–5.5 with diluted H₂SO₄. Then, fifty microlitres of a 10 mg/L standard solution of 1,2-dibromopropane (IS), 3 mL of extracting solvent (MTBE), 20 g of Na₂SO₄ and 1 g of copper sulphate were added and the mixture was stirred for 4 min; once the HKs were extracted, the vial was left to stand for 2 min for phase separation. Then, 1 mL of the upper MTBE layer was transferred to a 2 mL glass vial and 0.1 g of Na₂SO₄ was added to dry the extract. Finally, 50 µL of the extract were injected into the PTV–GC–MS instrument for analysis.

3. Results and discussion

3.1. Selection of the organic solvent

There are some conditions that need to be met when selecting an organic solvent (extractant), such as high extraction capability for the target analytes and low solubility in an aqueous solution. Also, there are two requests related to the development of the analytical method: in MLLE the extractant should have lower density than water in order to simplify the aspiration of the extract into the microsyringe, and in the LVI–PTV technique, the boiling point of the extractant influences performance in the solvent vent injection mode. Three solvents that satisfy these requirements: MTBE, n-hexane and ethyl acetate (boiling point ~55,

~69 and ~77 ºC, respectively), were used as extractants to evaluate their performance. In addition, 1,2-dibromopropane was used as the internal standard (IS) to correct errors associated with the partial dissolution of each organic solvent in the aqueous phase and make the extraction efficiency in the different solvents comparable. In this study, each extraction variable (sample pH, the amount of salt

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and the aqueous/organic volume ratio) was initially set according to EPA Method 551.1. Accordingly, preliminary tests were accomplished by adding 12 mL of mineral water (spiked with 20 µg/L of each of the fourteen HKs and 2 µg/L of IS) to 15 mL vials, adjusted at pH 4.5–5.5 with diluted H₂SO₄, 0.7 mL of each solvent and 5 g of Na₂SO₄. The vial was recapped and shaken by hand for 4 min, after which it was allowed to stand for another 2 min in order to facilitate the separation of the two phases. Then, about 300 µL of the upper organic layer were transferred into a 2 mL glass vial containing ~10 mg of Na₂SO₄ and finally, 20 µL of the extract were injected into the LVI–PTV–GC–MS instrument for analysis. A vent flow rate of 20 mL/min was used for 0.01 min. Each extraction test is based on the average of quintuplicate measurements. As can be seen from Fig. 1, all assayed organic solvents extracted the fourteen HKs (average extraction efficiency ranged between 60% and 80%), however lower values were achieved for the most polar analytes (CA, 1,1-DCA and 1,3-DCA) when n-hexane was used. Although MTBE and ethyl acetate provided similar extraction efficiency for chlorinated acetones, MTBE was the most effective organic solvent, especially for the brominated ones. This could be due to their moderate polarity and because the difference between the boiling points of the target analytes and the MTBE was the greatest. This different volatility is very important in the PTV technique since it allows the evaporation of the solvent without sweeping out any volatile HK. Consequently, MTBE was selected as the optimum organic solvent and used in subsequent studies.

The volume of the organic solvent, a key parameter to determine the enrichment factor, was studied by performing the extraction with a series of MTBE volumes: from 100 to 700 µL. The ratio peak areas decreased when the volume of MTBE increased due to the dilution effect of the analytes at a higher volume of extractant, although it was very difficult to collect the upper layer when the initial volume of organic solvent was less than 200 µL. In any case, and with the primary focus being to achieve high enrichment factors, a volume of 200 µL was used for further experiments even though it was only possible to collect ~70 µL of the supernatant due to the significant dissolution of the MTBE in the aqueous phase.

This extract was transferred to a 0.1 mL conical glass insert that contained ~10 mg of Na₂SO₄ to dry it. Practically the whole extract, 50 µL, was injected into the PTV–

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