ocus on ass Spectrometry & Ambi Andy Ray and Jackie Mosely, ww g.uk


Ambient ionisation mass spectrometry allows the rapid analysis of samples or objects in their native state in the open environment with no prior preparation. The ability of these techniques to provide selective analyte desorption and ionisation in combination with mass spectrometry (MS), has provided a growing number of powerful analytical alternatives across broad application areas, both quantitative and qualitative in nature, including pharmaceutical analysis, process chemistry, biological imaging, in vivo analysis, proteomics, metabolomics, forensics, and explosives detection. The commercial availability of DART, DESI and ASAP have enabled these techniques to be widely adapted within many industries. However with the emergence of new ambient ionisation methods, and the complementary nature of existing desorption and/or ionisation techniques, additional hyphenated methods have been devised, which pushes the total number of documented methods to almost thirty, these often combine separate desorption and ionisation processes in a single method.


The commercial availability of atmospheric pressure ionisation (API) sources, such as electrospray ionisation (ESI), has given greater flexibility within mass spectrometric analysis, and has overcome a number of issues in bridging the gap between the prepared sample and its introduction into the mass spectrometer vacuum system. The distinction between atmospheric pressure ionisation and ambient ionisation is vital in gaining clearer understanding of the impact of these new methods. Whilst ambient ionisation methods often have operational commonality with atmospheric pressure ionisation, techniques that fall strictly under the ambient ionisation banner display a number of basic characteristics; namely the direct analysis of untreated samples or objects in the open environment, whilst largely maintaining the native condition and spatial integrity of the sample. Here, analyte molecules derived from the sample, but not the whole sample itself, are transferred into the mass spectrometer. These rapid techniques offer unprecedented flexibility in sample analysis in the open environment, often remote from the mass spectrometer, and have made a significant impact on the world of analytical science. The term Direct Analysis has been used in relation to these techniques to distinguish from API techniques but the British Mass Spectrometry Society’s (BMSS) Special Interest Group (SIG) (http://www.bmss.org.uk/SIG_ambient-ion.shtml) feels that Ambient Ionisation is the correct term to be used.


Boundaries between ambient ionisation techniques can be somewhat blurred, however to remain consistent with the initial disclosure of each technique, similar methods are classed as separate techniques, but can be discussed together under the same general mechanism. A broad classification based upon the technique(s) having the most influence on the resultant mass spectra (e.g. ESI type spectra for DESI) allows effective comparison of mechanisms.


ESI or Spray-Related Techniques (DESI, EESI, ND-EESI)


A simple assessment of the number of papers published since the invention of DESI by the Cooks group demonstrates the broad utilisation of this technique across a number of diverse application and analyte types, for both small (e.g. drugs) and large (e.g. proteins) molecule applications. This belies an inherent ease of incorporation, where often little modification of existing mass spectrometer hardware is required. Indeed, many early applications of DESI relied upon only slight modification to vendor-standard ESI sources. Commercial DESI sources are now available which helps make the technique amenable to incorporation in an industrial setting.


In the DESI process, a spray of charged micro-droplets from an pneumatically-assisted electrospray needle is directed towards the object or analyte of interest in the ambient environment and allowed to impact the surface, giving desorption of the analyte into the gas phase and subsequent ionisation, prior to being sampled by the mass spectrometer or ion collection device. The incident angle of the spray plume (relative to the sample) has been investigated primarily to maximise analyte response and selectivity, but also to help elucidate contributing or competing mechanisms for sample ionisation, often being optimised around 45 – 55° with shallow ion-collection angles (collector relative to the sample) of around 10°. Capillary-tip-to-sample and sample-to-collector distances, whilst largely dependent upon instrumental configuration, sample type and desired outcome (e.g. reduction of source fouling vs. maximised sensitivity), play a crucial part in devising a successful DESI experiment.


Variations on ESI spray-based techniques, have afforded a number of alternative methods, where subtle experimental changes have given shifts in analytical performance. For example, neutral desorption extractive electrospray (ND-EESI), a variant upon the extractive electrospray ionisation (EESI) method. ND-EESI relies upon initial desorption of sample molecules into a neutral gas stream, rather than a charged solvent stream used in EESI, which coincides with an ESI plume to give ESI-like sample spectra. The major benefits of ND-EESI are non-proximate


sample introduction, the removal of voltages from the ambient sampling process, and increased selectivity over conventional solution-based ESI, particularly for low molecular weight analytes. Similarly, the removal of spray voltages used in the DESI experiment have given rise to desorption extractive electrospray ionisation (DeESI), which is now called easy ambient sonic- spray ionisation (EASI). The use of a micro-junction interfaced to electrospray (LESA) has shown promise as a hyphenated method where the sampling and ionisation are separate processes and may overcome some of the issues associated with MALDI imaging.


Paperspray ionisation has recently been shown where ions are generated by applying a high voltage to a paper triangle wetted with a small volume of solution. Samples can be preloaded onto the paper, added with the wetting solution or transferred from surfaces using the paper as a wipe.


Spray-Based Photon/Energy Techniques (ELDI, MALDESI)


The disclosure of spray-based ionisation techniques that are combined with laser ablation sample desorption, have given rise to methods such as electrospray laser desoprtion ionisation (ELDI) and matrix-assisted laser desorption electrospray ionisation (MALDESI). Recent ELDI experiments have shown the use of femtosecond laser pulses combined with ESI post-sample desorption. These methods provide additional selectivity and scope, compared with conventional LDI experiments. Limitations in spatial resolution due to addition of matrix can be an issue for MALDESI and, technically, one could argue that addition of matrix (i.e. prior sample preparation) might preclude MALDESI from being classified as an ambient ionisation method. Recently, the advent of desorption atmospheric pressure photo-ionisation (DAPPI), has offered selectivity and soft ionisation, particularly for neutral, non-polar or highly-conjugated compounds by using dopants.


Spectroscopy on in Mass Spectrometry


Figure 1. DART Ion Source (Direct Analysis in Real Time - Courtesy Peter Ryan, KR Analytical)


INTERNATIONAL LABMATE - JANUARY/FEBRUARY 2013


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