Desorption electrospray ionisation

Two new independently developed techniques called Dart ° (direct analysis in real time) and Desi (desorption electrospray ionisation) are making a huge impact on mass spectrometry. Together they remove the need for sample preparation and vacuum, speed up analysis time and can work in the open air. The sample is held in a gas or liquid stream at room temperature and the impact induces the surface desorption of ions. The ions then continue into the vacuum interface of the MS for analysis. Samples can be hard, soft or even liquid in nature. Ifa et al. have used Desi to image biological samples in two dimensions, recording images of tissue sections and the relative concentrations of molecules therein. Jeol have launched a commercial Dart ion source for non-contact analysis of materials in open air under ambient conditions. 

Jackson, A.T., Williams, J.P., and Scrivens, J.H. (2006) Desorption electrospray ionisation mass spectrometry and tandem mass spectrometry of low molecular weight synthetic polymers. Rapid Commun. 

Bodzon-Kulakowska, A., Drabik, A., Ner, J., KotUnska, J.H., and Suder, P. 2014, Desorption electrospray ionisation (DESI) for beginners— How to adjust settings for tissue imaging, Rapid Commun. Mass Spectrom., 15 1-9. 

Seng, J.A., Ellis, S.R., Hughes, J.R., Maccarone, A.T., Truscott, R.J., Blanksby, S.J., and Mitchell, T.W. 2014. Characterisation of sphingolipids in the human lens by thin layer chromatography-desorption electrospray ionisation mass spectrometry, Biochim. Biophys. Acta, 1841 1285-1291. 

Harry, E.L., Reynolds, J.C., Bristow, A.W., Wilson, I.D., and Creaser, C.S. 2009. Direct analysis of pharmaceutical formulations from non-bonded reversed-phase thin-layer chromatography plates by desorption electrospray ionisation ion mobility mass spectrometry, Rapid Commun. Mass Spectrom., 23 2597-2604. 

Desmarestiales, 2893-2894 0-Desmethyl-angolensin, 2398, 2446 Desorption electrospray ionisation (DESI), 181 Desorption electrospray ionization (DESI), 1038... 

Williams, J. R Scrivens, J. H., Coupling desorption electrospray ionisation and neutral desorption/extractive electrospray ionisation with a travelling-wave based ion mobility mass spectrometer for the analysis of drugs . Rapid Communications in Mass Spectrometry 2008, 22, 187-196. 

Fast atom bombardment (FAB) Plasma desorption (PD) Liquid secondary-ion mass spectrometry (LSIMS) Thermospray (TSP)/plasmaspray (PSP) Electrohydrodynamic ionisation (EHI) Multiphoton ionisation (MPI) Atmospheric pressure chemical ionisation (APCI) Electrospray ionisation (ESI) Ion spray (ISP) Matrix-assisted laser desorption/ionisation (MALDI) Atmospheric pressure photoionisation (APPI) Triple quadrupole (QQQ) Four sector (EBEB) Hybrid (EBQQ) Hybrid (EB-ToF, Q-ToF) Tandem ToF-ToF Photomultiplier... 

In off-line coupling of LC and MS for the analysis of surfactants in water samples, the suitability of desorption techniques such as Fast Atom Bombardment (FAB) and Desorption Chemical Ionisation was well established early on. In rapid succession, new interfaces like Atmospheric Pressure Chemical Ionisation (APCI) and Electrospray Ionisation (ESI) were applied successfully to solve a large number of analytical problems with these substance classes. In order to perform structure analysis on the metabolites and to improve sensitivity for the detection of the various surfactants and their metabolites in the environment, the use of various MS-MS techniques has also proven very useful, if not necessary, and in some cases even high-resolution MS is required. 

Several years later, the next step in the application of MS-MS for mixture analysis was developed by Hunt et al. [3-5] who described a master scheme for the direct analysis of organic compounds in environmental samples using soft chemical ionisation (Cl) to perform product, parent and neutral loss MS-MS experiments for identification [6,7]. The breakthrough in LC-MS was the development of soft ionisation techniques, e.g. desorption ionisation (continuous flow-fast atom bombardment (CF-FAB), secondary ion mass spectrometry (SIMS) or laser desorption (LD)), and nebulisation ionisation techniques such as thermospray ionisation (TSI), and atmospheric pressure ionisation (API) techniques such as atmospheric pressure chemical ionisation (APCI), and electrospray ionisation (ESI). 

S. G. Penn, M. T. Cancilla, M. K. Green, and C. B. Lebrilla, Direct comparison of matrix-assisted laser desorption/ionisation and electrospray ionisation in the analysis of gangliosides by Fourier transform mass spectrometry, Euro. J. Mass Spectrom., 3 (1997) 67-79. 

Two techniques that have become preffered for ionisation of proteins/peptides is electrospray ionisation (ESI) and matrix-assisted laser desorption/ionisation (MALDI). Although different combinations of ionisation techniques and mass analyser exist, MALDI usually is coupled with a time-of-flight (TOF) (Figure 7) tube as a mass analyser while ESI is tradionally combined with quadrupole mass analysers. Instruments capable of MS/MS have the ability to select ions of particular m/z ratio from a mixture, to fragment selected ions and to record the precise masses of the resulting fragment ions. If this process is applied to the analysis of peptide ions, in principle the amino acid sequence of the peptide can be deduced. 

Mass spectrometry has assumed great importance in determinations of the molar masses of biological macromolecules, even quite large ones. This is due to developments such as electrospray ionisation (ESI) and matrix assisted laser desorption/ ionisation (MALDI), which have made it possible to determine the molar masses of biopolymers up to several 100 kDa (Pitt 1996 Kellner et al. 1999 Snyder 2000). The combination of MALDI techniques with time-of-flight mass spectrometers (MALDI-TOF) is of particular significance for determination of the molar masses of proteins with high sensitivity (typically pmol quantities, although exceptionally fmol) and precision (proteins up to 100 kDa with precision of about 0.01 %). Mass spectrometry can provide very accurate measurements of protein molar mass that can yield information about even minor structural modifications not readily accessible by other means. 

The classical area of application of mass spectrometry has been with small volatile compounds, although non-volatile samples could be analysed if they were suitably derivatised. The application of mass spectrometry to large complex molecules like proteins has been made possible by the development of novel ionisation techniques which enable large molecules (> 200 kDa) to be introduced into the mass spectrometer in an intact form suitable for analysis (Siuzdak 1996 Dass 2000). Of the various techniques that have been developed, electrospray ionisation (ESI) and matrix-assisted laser desorption ionisation (MALDI) are the ones best suited for use with macromolecules and macromolecular complexes. 

Although FAB has been used in polymer analysis, problems with fragmentation and the relatively low mass limit has made this less popular as new techniques have emerged. Plasma desorption has been used successfully but this too has waned in popularity with commercial spectrometers not really readily available. To a large extent polymer mass spectrometry equates to MALDI time-of-flight and the remainder of this article will bear this in mind. However, the use of electrospray ionisation (ESI) will be considered in conjunction with either quadrupole detectors or ion cyclotron resonance (ICR) N. B. ICR detectors can also be used with MALDI, as this is important and probably not as widely used as it could be. 

Mass spectroscopy [electron ionisation (El), chemical ionisation (Cl), electrospray ionisation (ESI), fast atom bombardment (FAB), matrix-associated laser desorption ionisation (MALDI), inductively coupled plasma-mass spectrmetry (ICP-MS, cf and ), etc]... 

Benomar, S. H., Clench, M. R., and Allen, D. W., The analysis of alkylphenol ethoxysulfonate surfactants hy high-performance liquid chromatography, liquid chromatography-electrospray ionisation-mass spectrometry and matrix-assisted laser desorption ionisation-mass spectrometry. Anal. Chim. Acta, 445, 255-267, 2001. 

The mass spectrometry of oligonucleotides is a rapidly expanding research area dominated by the techniques of electrospray ionisation and matrix assisted laser desorption-ionisation, time of flight (MALDI-TOF) mass spectrometry. A number of reviews of this area have been published this year. Several... 

Since the 1980s a revolution in the use of mass spectrometry for biological analyses has occurred and continues today. A major reason for this development was the introduction of new ionisation techniques such as fast atom bombardment (FAB), plasma desorption (PD) and thermospray (TSP) permitting the production of gas phase ions from charged and polar biopolymers [7—10). It has reached a first culmination with the recent award of the 2002 Nobel prizes in chemistry to two scientists pioneering the development of electrospray-ionisation and laser desorption mass spectrometry, John Fenn and Kuichi Tanaka [11, 12]. 

The choice of an ionisation method depends on the analyte characteristics and the required type of analytical information. Classically, hard ionisation methods such as electron ionisation (El) or chemical ionisation (Cl) make use of their fragmentation capabilities to gain stractural information, typically of small organic molecules. In contrast, soft ionisation techniques such as electrospray ionisation or laser desorption are used to obtain mass spectra of intact molecules with little or no fragmentation, being capable of analysing complex multi-component mixtures. 

Focal plane (array) detectors can detect a range of masses simultaneously. This provides a multichannel advantage that can improve the sensitivity for magnetic sectors, and detection limits can be improved if the analysis is limited by the analyte ion current instead of the chemical background level. This is the case for experiments such as MS/MS, electrospray ionisation, and field desorption. Array detectors can be used with pulsed ionisation methods, but the array detectors for commercial magnetic sector mass spectrometers can only detect a portion of the entire mass range at any given instant. 

Hunt, S. M., Sheil, M. M., and Derrick, P. J., Comparison of Electrospray Ionisation Mass Spectrometry with Matrix-assisted Laser Desorption Ionisation Mass Spectrometry and Size Exclusion Chromatography for the Characterisation of Polyester Resins, Eur. Mass Spectrom., 4, 475, 1998. 

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