Title:

Pre-concentration and a special scan function for more sensitive and stable ambient ionization mass spectrometry

Introduction Ambient mass spectrometry (AMS) is a group of novel ionization techniques operated at atmospheric pressure without tedious sample pretreatment. Albeit quick and simple, the sensitivity of AMS is often similar to or lower than conventional LC-MS methods. It is challenging to direct analyze parts-per-trillion level of compounds in environmental samples. In practical environmental analysis, various contaminants, such as perfluorinated compounds, are well below the limit of detection (LOD). Therefore, the development of a simple, universal, and robust pre-concentration technique will greatly facilitate the quantitative application of AMS. Methods 10 µL sample solution of very low concentration (<1 ppb) was loaded by micropipette into pulled glass emitters. The glass emitter was placed on a home-made sample holder horizontally at room temperature. Solvent was evaporating and drying for 20 minutes, and the non-volatile analytes concentrated at the edges of the glass emitters. A piezo inkjet printer head was modified to infuse pL-nL pure solvents for extraction and ionization of the dried (pre-concentrated) sample. A constant plasma source was used to trigger the relay spray ionization. Meanwhile, a special MS/MS scan function, in which the analyte and the internal standard were isolated together but fragmented separately in two consecutive scans, was used to mitigate the signal fluctuations in conventional continuous scans. Preliminary Data 10 µL dye solution was almost colorless when being loaded into the glass emitter. As the solvent evaporated in glass emitter, deep purple stains were observed at the edge of the emitter. This phenomenon could be explained by Coffee Ring Effect. To reconstitute the dried sample, a small volume of pure solvent should be infused into the emitter. Piezoelectric inkjet was utilized to infuse spray solvent of well controlled volume, in the form of tiny droplets, on the pre-concentrated sample by applying pressure pulse through a nozzle. 50:50 methanol: water was used as the solvent. The frequency of the pressure pulse was 2 kHz. The pulse width was 20 µs, and the maximum voltage applied was 18 volts. Under these conditions, it took 1 s to eject 2000 droplets and the volume of each droplet was 10 pico-liters. Consequently, 10 nL spray solvent was loaded into the glass emitters, extracting the analyte(s) and migrated to the tip end by capillary action. Discharge plasma was driven by piezoelectric transformer (PZT) in ambient air. Spray could be pushed continuously, stably and focusedly by the PZT after adding a direct current (DC) high voltage behind both the capillary and discharge plasma. The high intense quantifier product ion m/z80 (SO3-) of PFOS (499/80) and 13C8-PFOS (507/80) overlaps in MS/MS scan. The special MS/MS scan function isolated parent ion m/z 499and m/z 507 in one ion injection but fragmented in two consecutive scans. Compared with the conventional ion trap MS/MS scan, not only the overlapped fragments could now be used for quantitation, but also the RSD decreased by 50%, and the LOD was lowered by a factor of 4 Novel Aspect Plasma Source trigger ESI, Piezoelectric inkjet, Novel MS Scan, Pre-concentraion

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