Gas modifier assisted re-circulation mode high-field asymmetric waveform ion mobility spectrometry (FAIMS)

- The gas-saving detection with a closed-loop arrangement allows a FAIMS to be used in applications such as on-site detection.
- LOD, gas flow rate stability, gas and sample reserve time of this FAIMS system were investigated respectively.
- With helium modifiers applied, the separation capacity of the FAIMS system was remarkably improved.

In a regular FAIMS system, a carrier gas is used to drive samples through the drift tube. Thus, high consumption of sample and carrier gas can be expected, especially when multi-cycle detection is needed for high precision detection. In this study, a novel re-circulation mode FAIMS was developed. Ion separation capacity and limit of detection (LOD) of this FAIMS system were investigated. The stability of this FAIMS system was also studied by changing the flow rate of the carrier gas. Using this newly designed re-circulation mode FAIMS system, the detection results of o-, p- m- xylenes in pure nitrogen and helium/nitrogen mixtures were compared. The sample and the carrier gas reserve time which ensure the success of multi-cycle detection was examined. The characteristic parameters (α2,α4) and their deviation were also precisely calculated through 18 CV − DV spectra. Results show that 0.6 ng o-xylene can be detected in a 25 ml closed loop re-circulation arrangement. The effective detection time is 6.2 min in this detection process. In fact, the sample and the carrier gas used in this experiment are 70 and 826 times less than a regular open loop FAIMS system respectively. The peak position and the full width at half-maximum (FWHM) of the obtained CV-DV spectra stay almost unchanged during th.entire experiment process. The ion signal peaks can be fitted with a Gauss curve, which means that the stability of the carrier gas flow rate fully meet the requirement for on-site detection. Furthermore, the use of a helium modifier is studied and proved to be an effective method to improve the detection capacity of the FAIMS for VOCs. In conclusion, the gas-saving re-circulation design along with the gas modifiers provide new opportunities for FAIMS systems to be applied on-site high precision chemical detection.

Schematic of the closed loop for FAIMS running in re-circulation mode. The entire closed loop design consists of a micro rotary vane pump, a rotameter, a buffer/cooler vessel, a sample injector, a filter, and of course a FAIMS chip. A rotary vane pump (Fürgut GmbH) was chosen in this study because of the low impurity interference compared to a membrane pump. Pulsation problem caused by pump electromotor can be solved by adding a simple vessel on the vacuum side of the pump which acts both as a buffer to absorb the pressure shocks and a cooler to reduce heat generated by the pump. For the purpose of long duration monitoring of the flow rates, we connected a rotameter into the loop. It also acts as a buffer for the pulsation. A quartz sample injector was designed for sample introduction. Samples were introduced by a micro-syringe through a capillary connected to the injector.111