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Time-of-flight mass spectrometry (TOFMS) is one of the simplest and most powerful approaches for mass spectrometry. Realization of the advantages inherent in TOFMS requires innovation in the theory and practice of the technique. Instrumental developments, in turn, create new capabilities that enable applications in chemical measurement. This review focuses on the recent advances in TOFMS instrumentation. New strategies for ion acceleration, multiplexed detection, miniaturized TOFMS instruments, approaches to extend the length of ion flight, and novel ion detection technologies are reviewed. Techniques that change the basic paradigm of TOFMS by measuring m/z based on ion flight distance are considered, as are applications at the frontiers of instrumental performance. © 2020 John Wiley & Sons Ltd. Mass Spec Rev.

A modified halo-shaped Flowing Atmospheric Pressure Afterglow (h-FAPA) source is coupled to an Atmospheric Pressure Interface Time of Flight Mass Spectrometer (API-TOFMS) for the analysis of gas samples. The h-FAPA is an ionization source easy to manufacture, operate and handle, and with low consumption in terms of gas flow rates and electric power. The analytical performance of this plasma-based ambient ionization source is evaluated for the direct analysis of VOCs, using benzene as a model compound. The influence of the different operating parameters, such as sampling distance, discharge current, and gas flow rates, is evaluated to optimize the detection of the benzene molecular ion. Matrix effects are evaluated using mixtures of different VOCs, and spiked synthetic air and human breath. Quantitative recoveries of benzene concentrations are around 100% in different VOC mixtures, showing low matrix effects. Moreover, calibration curves with similar sensitivities for benzene are obtained using spiked synthetic air and human breath. Limits of detection are in the order of 1 ng L−1 in both cases.

The development of new Ambient Desorption/Ionization (ADI) sources has been an important research topic in recent years, arising from the need for reducing cost, analysis time and equipment complexity using mass spectrometry detection. This work presents a miniaturized plasma device with ADI capabilities based on a concentric FAPA source. The discharge device, the “needle-Flowing Atmospheric Pressure Afterglow” (n-FAPA), has been developed with easily replaceable commercial parts. In particular, the discharge is generated in flowing He using two concentric electrodes: a stainless steel capillary tube acting as the anode (outer electrode), and a hypodermic needle with a bevel-cut edge (inner electrode) acting as the cathode. Electrical and optical studies of n-FAPA have been carried out and the existence of a plasma regime transition (e.g. between normal glow and glow-to-arc transition), depending on the gas flow and current intensity conditions, has been shown. Rotational temperature and spatial distribution of temperatures have been calculated for both plasma regimes. Finally, the n-FAPA desorption/ionization source, coupled to a quadrupole mass spectrometer, has been tested in both plasma regimes for direct analysis of caffeine. Additionally, commercial drinks, carboxylic acids in water, and olive oil have been tested as model analytes.

Atmospheric pressure glow discharges have been widely used in the last decade as ion sources in ambient mass spectrometry analyses. Here, an in-house flowing atmospheric pressure afterglow (FAPA) has been developed as an alternative ion source for differential mobility analysis (DMA). The discharge source parameters (inter-electrode distance, current and helium flow rate) determining the atmospheric plasma characteristics have been optimized in terms of DMA spectral simplicity with the highest achievable sensitivity while keeping an adequate plasma stability and so the FAPA working conditions finally selected were: 35 mA, 1 L min(-1) of He and an inter-electrode distance of 8 mm. Room temperature in the DMA proved to be adequate for the coupling and chemical analysis with the FAPA source. Positive and negative ions for different volatile organic compounds were tested and analysed by FAPA-DMA using a Faraday cup as a detector and proper operation in both modes was possible (without changes in FAPA operational parameters). The FAPA ionization source showed simpler ion mobility spectra with narrower peaks and a better, or similar, sensitivity than conventional UV-photoionization for DMA analysis in positive mode. Particularly, the negative mode proved to be a promising field of further research for the FAPA ion source coupled to ion mobility, clearly competitive with other more conventional plasmas such as corona discharge.

An in-house atmospheric pressure glow discharge source, designed to be used as ionization/desorption source for ambient mass spectrometry, has been electrically characterized, and its optical emission spectra evaluated in detail. Electrical characterization showed that the plasma regime can vary from glow discharge to arc discharge depending on operating conditions (i.e. He flow rate and inter electrode distance). Furthermore, bidimensional images of the optical emission of some plasma species using filters as wavelength selectors, were registered from inside and outside the discharge chamber (inner region and afterglow region respectively), showing the spatial distribution of excited species (i.e. He*, N 2 + and O*). These distribution patterns are useful to study the chemistry of the discharge plasma, since different production pathways and different excitation energies affect the presence of these species in the plasma regions.

In recent years particular effort is being devoted towards the development of pulsed GDs because this powering operation mode could offer important analytical advantages. However, the capabilities of radiofrequency (rf) powered glow discharge (GD) in pulsed mode coupled to optical emission spectrometry (OES) for real depth profile quantification has not been demonstrated yet. Therefore, the first part of this work is focussed on assessing the expected advantages of the pulsed GD mode, in comparison with its continuous mode counterpart, in terms of analytical emission intensities and emission yield parameters. Then, the capability of pulsed rf-GD-OES for determination of thickness and compositional depth profiles is demonstrated by resorting to a simple multi-matrix calibration procedure. A rf forward power of 50 W, a pressure of 600 Pa, 1000 Hz pulse frequency and 50% duty cycle were selected. The quantification procedure used was validated by analysing conductive layers of thicknesses ranging from a few tens of nanometer up to about 20 μm and varied compositions (hot-dipped zinc, galvanneal, back contact of thin film photovoltaic solar cells and tinplates).

Ambient desorption/ionization (ADI) sources coupled to mass spectrometry (MS) offer outstanding analytical features: direct analysis of real samples without sample pretreatment, combined with the selectivity and sensitivity of MS. Since ADI sources typically work in the open atmosphere, ambient conditions can affect the desorption and ionization processes. Here, the effects of internal source parameters and ambient humidity on the ionization processes of the flowing atmospheric pressure afterglow (FAPA) source are investigated. The interaction of reagent ions with a range of analytes is studied in terms of sensitivity and based upon the processes that occur in the ionization reactions. The results show that internal parameters which lead to higher gas temperatures afforded higher sensitivities, although fragmentation is also affected. In the case of humidity, only extremely dry conditions led to higher sensitivities, while fragmentation remained unaffected.