## 4 Conclusions  
In this study, two typical and reliable preparation technologies (P&T and HS-SPME) were optimized for simultaneous detection of fuel oxygenates. Both of the two preparation methods provided excellent chromatographic resolution in subsequently GC-QqQ-MS/MS detection system. The precursor ions and product ions of these detected fuel oxygenates in this study were selected and identified with the optimized GC-QqQ-MS/MS operation parameters. Optimizations of P&T and HS-SPME process were conducted comprehensively. The LODs and LOQs of these selected oxygenate compounds were discovered to be 0.52-32 ng/L in P&T-GC-QqQ-MS/MS detection system method and 0.11-151 ng/L in HS-SPME-GC-QqQ-MS/MS detection system. Meanwhile, the intra-day and inter-day precision in this study for these selected oxygenate compounds were found in the range of 1.99-8.87% and 1.18-12.7% with the P&T-GC-QqQ-MS/MS detection system, 1.19-9.65% and 1.09-9.78% with the HS-SPME-GC-QqQ-MS/MS detection system, respectively. For BTEX, the HS-SPME-GC-QqQ-MS/MS system provided outstanding detection performance because of the lower LODs obtained. However, for other selected compounds, the P&T-GC-QqQ-MS/MS system offered lower LODs. Detection results indicated that both P&T and HS-SPME pretreatment methods can be used to conduct simple and rapid preparation of water samples for fuel oxygenates detection, with good linearity, repeatability, precision, low LODs in the GC-QqQ-MS/MS detection system.  
The following are available online at [[http://www.mdpi.com/2073-4441/12/8/2266/s1](http://www.mdpi.com/2073-4441/12/8/2266/s1)]([http://www.mdpi.com/2073-4441/12/8/2266/s1](http://www.mdpi.com/2073-4441/12/8/2266/s1)), Figure S1: the mass spectra of the twelve analytes. (A) Full scan spectra in EI mode; (B) daughter scan spectra of quantitative ions; (C) daughter scan spectra of qualitative ions, Figure S2: chromatogram of quantitative identification ions of the twelve analytes (5 ug/L) in MRM: (A) PT-GC-MS/MS chromatogram; (B) HS-SPME-GC-MS/MS chromatogram, Table S1: name, segments, retention time, dwell time, precursors and product ions of quantitative transition and confirmation transition, and collision energy for the twelve analytes, Table S2: linearity of standard calibration curves, LODs, LODs, method precision and validity for the twelve analytes using the P&T-GC-QqQ-MS/MS methodology, Table S3: linearity of standard calibration curves, LODs, LOQs, method precision and validity for the twelve analytes using the HS-SPME-GC-MS/MS methodology.  
Methodology, K.Z.; software, Y.L.; formal analysis Q.S. and L.H.; data curation, M.Z.; writing--original draft preparation, K.Z.; writing--review and editing, Y.L. and K.Z.; supervision, K.Z.; project administration, Y.L. and K.Z.; funding acquisition, K.Z. and Y.L. All authors have read and agreed to the published version of the manuscript.  
This work was financially supported by the Project of Introducing and Cultivating Young Talent in the Universities of Shandong Province and the Natural Science Foundation of Shandong Province (No. ZR2017 BEE016).  
We are grateful for assistance with Shandong Provincial Key Laboratory of Water and Soil Conservation and Environmental Protection, College of Resources and Environment, Linyi University, Linyi, 276000, China.  
The authors declare no competing financial interests.