Rapid determination of methyl tert-butyl ether using dynamic headspace/ion mobility spectrometry
Introduction
Methyl tert-butyl ether (MTBE) has been used as an octane-enhancing replacement for lead in gasoline since the 1970s. Since the 1990s, MTBE production has increased because of its use as an oxygenated additive in fuel. In general, partially oxidized additives, such as MTBE, promote the complete combustion of gasoline, reduce CO, NOx levels and cut down the emissions of volatile organic compounds (VOCs). “Cleaner” burning of so-called oxygenated and reformulated gasoline has significantly improved air quality. Despite a continuous rise in automobile traffic, smog-causing pollutants, including VOCs, decreased 17% (the equivalent of 16 million cars less) in the 4-year period (1995–1999) in the United States [1], [2].
Chief concerns about MTBE are detecting it in surface and groundwater sources (the consequence of leaking gasoline tanks and pipes), and its high solubility in water (approx. 4%, w/w). To date, limited data are available on the effects of MTBE on health. Notwithstanding this, US Environmental Protection Agency (EPA) has concluded that at high doses, MTBE is a potential human carcinogen and recommended that MTBE levels in drinking water be kept below a range of 20–40 ppb [1]. California State has proposed that primary and secondary maximum contaminant levels (MCLs) for MTBE in drinking water [1] be kept as low as 13 and 5 ppb, respectively.
There is no official EPA method for the determination of MTBE in water. One of the most reliable methods of determining MTBE in water is method 8260 which is based on gas chromatography/mass spectrometry (GC/MS) [3] after extraction from the water sample (use of purge and trap is recommended). Efficient extraction and concentration of volatile compounds from the sample using P&T, together with the sensitivity and specificity of GC/MS, result in advantages such as detection limits at levels below 1 ppb and no risk of false identification due to interfering substances. Nevertheless, other instrumental approaches should be taken into consideration when other factors are critical. Examples of other critical factors are the stability of the original concentrations of volatile MTBE during sample transport and storage, the need for rapid, frequent and numerous data generation, and the requirements of in-field and portable tools of analysis.
Ion mobility spectrometry (IMS) is a tried and tested, portable, and relatively inexpensive technique that uses a hand-held device for monitoring volatile organic compounds. Relevant fields and sectors are pharmaceuticals, explosives, chemical war agents, industrial hygiene, and the environment [4]. In IMS, vapor samples are ionized first. Next, product ion velocities (v) are measured in a drift tube’s weak electric field (E) so as to obtain ion mobilities (K); these provide a means of identifying and quantifying vapor substances (Eq. (1)) [4]:Since K depends on temperature and pressure, for practical purposes, normalized ion mobility (k0) is used, as defined in Eq. (2):Stach et al. [5] recently presented a well-designed method for the rapid analysis of water-borne MTBE using IMS. This produced good reproducibility and detection limits in the 30 ppb range. Briefly, small water samples were added to excess water-adsorbing polymer in a tube. The sample headspace was then withdrawn with a syringe and injected into the IMS via an inlet port, that had been modified and equipped with a septum that could be pierced.
The aim of our project is to demonstrate a simpler, more sensitive, reproducible, and totally on-line method for determining MTBE in water-borne samples named as dynamic headspace/IMS (DHS/IMS). MTBE is removed from the sample and swept directly into the IMS by means of a built-in IMS sampling pump acting as an air stream generator. There is no need for instrument modifications and off-line syringe withdrawals. An adsorbing-cryogenic trap further improves detection limits. Experimental set-ups, as well as detection limits, calibration curves, and inter- and intra-day reproducibility tests are presented and discussed.
Section snippets
Instrumental set-up
An IMS (Raid-1, Bruker, Leipzig, Germany) inlet was connected with Teflon tubes to Drechsel bottles which contained both sample and activated carbon (Fig. 1). A valve along the line allowed switching from the sample bottle to the carbon bottle. IMS was operated in positive ion mode, controlled by IMS Bruker software (version 4.12.). Ions were formed at atmospheric pressure by using a Ni63 β source. The ionization chamber was set at 80 °C. All experiments were performed at room temperature. A
Results and discussion
The instrumental set-up shown in Fig. 1 permitted a rapid switch from measure mode to stand-by mode; at the same time, thanks to the activated carbon traps, the constant withdrawal of organic-free air was made possible.
Fig. 2 shows a typical spectrum for MTBE reference solutions at 200 (Fig. 2a) and 20 ppb (Fig. 2b), the latter being the detection limit. Identification and quantification of low MTBE concentrations was best carried out on the de-convolved spectrum line automatically provided by
Acknowledgements
We thank Bruker Daltonics (Leipzig, Germany; Milan, Italy) for their kind hospitality, precious training, and invaluable technical support.
References (5)
- United States Environmental Protection Agency, Office of Research and Development, Oxygenates in Water: Critical...
- B.Z. Shakhashiri, Chemical of the Week—Chem. 104, University of Wisconsin-Madison, 28 January 2002...
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