An ambient detection system for visualization of charged particles generated with ionization methods at atmospheric pressure.

RATIONALE: With the current state-of-the-art detection of ions only taking place under vacuum conditions, active pixel detectors that operate under ambient conditions are of particular interest. These detectors are ideally suited to study and characterize the charge distributions generated by ambient ionization sources. METHODS: The direct imaging capabilities of the active pixel detector are used to investigate the spatial distributions of charged droplets generated by three ionization sources, named electrospray ionization (ESI), paper spray ionization (PSI) and surface acoustic wave nebulization (SAWN). The ionization spray (ESI/PSI) and ionization plume (SAWN) originating from each source are directly imaged. The effect of source parameters such as spray voltage for ESI and PSI, and the angle of the paper spray tip on the charge distributions, is investigated. Two types of SAWN liquid interface, progressive wave (PW) and standing wave (SW), are studied. RESULTS: Direct charge detection under ambient conditions is demonstrated using an active pixel detector. Direct charge distributions are obtained of weak, homogeneous/focused and dispersed spray plumes by applying low, intermediate and high spray potentials, respectively, for ESI. Spray plume footprints obtained for various angles of PSI shows the possibility to focus the ion beam as a function of the paper angle. Differences between two designs of the SAWN interface are determined. Droplet charge flux changes are illustrated in a way similar to a total ion chromatogram. CONCLUSIONS: The use of this active pixel detector allows the rapid characterization and optimization of different ambient ionization sources without the actual use of a mass spectrometer. Valuable illustrations are obtained of changes in spatial distribution and number of charges detected for ESI, PSI and SAWN ion plumes. Copyright © 2015 John Wiley & Sons, Ltd.

Direct ion imaging approach for investigation of ion dynamics in multipole ion guides.

A key requirement of electrospray ionization (ESI) and other techniques facilitating ionization at elevated pressures is the efficient transport of free gas-phase ions into the high vacuum region of the mass spectrometer. Radio frequency (RF) multipole ion guides that allow for collisional cooling are one of the most popular means of achieving this. However, their performance is highly dependent on several experimental factors, including pressure and various electrode potentials along the ion path. To experimentally visualize these effects, we have employed a position-sensitive detector at the exit of a quadrupole mass spectrometer (QMS) instrument operated in RF only mode that employs an RF only octopole as a collisional cooling ion guide. This allows the spatial distribution of the ions, and its dependence on experimentally determined conditions, to be directly visualized at the exit of the quadrupole. This investigation provides a detailed insight into the ion dynamics occurring inside multipole ion guides. This knowledge can directly be applied to instrument development and to improve the ion transmission efficiency and, thus, sensitivity. Numerical simulations using custom-developed trajectory simulation software are compared and contrasted with the experimental observations.

A micropixelated ion-imaging detector for mass resolution enhancement of a QMS instrument.

An in-vacuum position-sensitive micropixelated detector (Timepix) is used to investigate the time-dependent spatial distribution of different charge state (and hence different mass-to-charge (m/z)) ions exiting an electrospray ionization (ESI)-based quadrupole mass spectrometer (QMS) instrument. Ion images obtained from the Timepix detector provide a detailed insight into the positions of stable and unstable ions of the mass peak as they exit the QMS. With the help of image processing algorithms and by selecting areas on the ion images where more stable ions impact the detector, an improvement in mass resolution by a factor of 5 was obtained for certain operating conditions. Moreover, our experimental approach of mass resolution enhancement was confirmed by in-house-developed novel QMS instrument simulation software. Utilizing the imaging-based mass resolution enhancement approach, the software predicts instrument mass resolution of ∼1,0000 for a single-filter QMS instrument with a 210-mm long mass filter and a low operating frequency (880 kHz) of the radio frequency (RF) voltage.

Experimental investigation of the 2D ion beam profile generated by an ESI octopole-QMS system.

In this paper, we have employed an ion imaging approach to investigate the behavior of ions exiting from a quadrupole mass spectrometer (QMS) system that employs a radio frequency octopole ion guide before the QMS. An in-vacuum active pixel detector (Timepix) is employed at the exit of the QMS to image the ion patterns. The detector assembly simultaneously records the ion impact position and number of ions per pixel in every measurement frame. The transmission characteristics of the ion beam exiting the QMS are studied using this imaging detector under different operating conditions. Experimental results confirm that the ion spatial distribution exiting the QMS is heavily influenced by ion injection conditions. Furthermore, ion images from Timepix measurements of protein standards demonstrate the capability to enhance the quality of the mass spectral information and provide a detailed insight in the spatial distribution of different charge states (and hence different m/z) ions exiting the QMS.

Quadrupole mass filter: design and performance for operation in stability zone 3.

The predicted performance of a quadrupole mass filter (QMF) operating in Mathieu stability zone 3 is described in detail using computer simulations. The investigation considers the factors that limit the ultimate maximum resolution (Rmax) and percentage transmission (%Tx), which can be obtained for a given QMF for a particular scan line of operation. The performance curve (i.e., the resolution (R) versus number (N) of radio frequency (rf) cycles experienced by the ions in the mass filter) has been modeled for the upper and lower tip of stability zone 3. The saturation behavior of the performance curve observed in practice for zone 3 is explained. Furthermore, new design equations are presented by examining the intersection of the scan line with stability zone 3. Resolution versus transmission characteristics of stability zones 1 and 3 are compared and the dependence of performance for zones 1 and 3 is related to particular instrument operating parameters.

Factors influencing the QMF resolution for operation in stability zones 1 and 3.

This study uses a computer model to simulate a quadrupole mass filter (QMF) instrument under different operating conditions for Mathieu stability zones 1 and 3. The investigation considers the factors that limit the maximum resolution (R(max)), which can be obtained for a given QMF for a particular value of scan line. Previously, QMF resolution (R) has been found to be dependent on number (N) of radio frequency (rf) cycles experienced by the ions in the mass filter, according to R = N(n)/K, where n and K are the constants. However, this expression does not predict the limit to QMF resolution observed in practice and is true only for the linear regions of the performance curve for QMF operation in zone 1 and zone 3 of the stability diagram. Here we model the saturated regions of the performance curve for QMF operation in zone 1 according to R = q(1 - 2c(N))/∆q, where c is a constant and ∆q is the width of the intersection of the operating scan line with the stability zone 1, measured at q-axis of the Mathieu stability diagram. Also by careful calculations of the detail of the stability tip of zone 1, the following relationship was established between R(max) and percentage U/V ratio: R(max) = q/(0.9330-0.00933U/V). For QMF operation in zone 3 the expression R = a - bc(N) simulates well the linear and saturated regions of the performance curve for a range of operational conditions, where a, b, and c are constants.

QMS in the third stability zone with a transverse magnetic field applied.

We report here a study using a quadrupole mass spectrometer (QMS) in which a static magnetic field is applied transversely to the body of the mass filter operating in stability zone 3. Significant improvement in QMS performance was obtained under certain magnetic field conditions, and these have been explained in terms of our theoretical model. The theoretical approach assumed in the model is that the QMS contains hyperbolic rods as electrodes and that the magnetic field acts over the full length of the mass filter assembly. Our latest analysis also predicts for what values of operating parameters an enhancement of the quadrupole resolution is achieved when a transverse magnetic field is applied. The model predicts instrument resolution R > 5000 for Ar with a 100 mm long mass filter and R > 3500 for a HT and D(2) mixture with a 200 mm long mass filter via application of a transverse magnetic field.