Multiplexing Techniques Applied to Ion Mobility Measurements

Austen Davis

Ion mobility spectrometry (IMS) is an analytical technique that provides rapid separation of gas-phase ions. IMS is capable of measuring the gas-phase mobility coefficient (K) of ions under specific operational conditions. IMS devices are increasingly coupled to the front-end of mass spectrometers (MS) which can simultaneously measure drift time and m/z to provide ion density measurements. The combined chemical information provided by IMS-MS instruments enables rapid separation of isomers and isobars which can be useful for the identification of unknown compounds. The greater peak capacity and ion manipulation afforded by modern high resolution mass spectrometers places a premium on maximizing ion transfer to the mass detector without sacrificing the separation and chemical information afforded by IMS. The common method of gating ions into the drift cell of IMS devices at atmospheric pressure suffers from low duty cycles that limits the ions ultimately transmitted to the mass detector. To increase ion transmission without physical modifications to the IMS device it is necessary to increase the duty cycle of the IMS gate achieved by increasing the gate pulse width or incorporating multiplexing strategies. The gate pulse width is a crucial parameter in IMS operation that influences the resulting sensitivity and resolution. Multiplexing techniques for IMS instruments traditionally incorporate either the Fourier or Hadamard transform which have not yielded the signal-to-noise ratio improvement predicted by theory at the higher gating duty cycles. The application of the proposed signal processing techniques are tailored to IMS-MS measurements and demonstrated to realize enhanced signal-to-noise ratio without sacrificing resolution or duty cycle of the ion gate. The signal processing technique known as cross-correlation IMS is demonstrated with IMS resolution that approaches the theoretical limit. This signal processing technique is applied for the purpose of identifying the surrounding chemical environment of the uranyl for attribution purposes. Additionally, the use of a signal processing technique known as basis pursuit denoising is demonstrated and applied for improved resolution at extended gating duty cycle with applications of mobility measurements acquired using slow acquisition mass detectors and combining individual IMS measurements across multiple electric fields for accurate mobility measurements.

Multiplexing Techniques Applied to Ion Mobility Measurements

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