Size weight and position ion mobility spectromet

Authors: András Kiss Ron M A Heeren

Publish Date: 2011/01/13

Volume: 399, Issue: 8, Pages: 2623-2634

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Abstract

Size weight and position are three of the most important parameters that describe a molecule in a biological system Ion mobility spectrometry is capable of separating molecules on the basis of their size or shape whereas imaging mass spectrometry is an effective tool to measure the molecular weight and spatial distribution of molecules Recent developments in both fields enabled the combination of the two technologies As a result ionmobilitybased imaging mass spectrometry is gaining more and more popularity as a bioanalytical tool enabling the determination of the size weight and position of several molecules simultaneously on biological surfaces This paper reviews the evolution of ionmobilitybased imaging mass spectrometry and provides examples of its application in analytical studies of biological surfacesKey questions in analytical biology and biochemistry evolve around the determination of three basic molecular properties size weight and position The complex interplay between molecules in living systems results in a continuous alteration of these properties at the molecular level The need for analytical approaches that provide better insight into these processes is growing as understanding of these systems is increasing A cell a tissue section a tissue extract or a body fluid contains a huge amount of chemical and biological information A single analytical method is usually insufficient when a better understanding of the function of these different biological systems is targeted This is caused by the complexity of these systems For that reason a combination of different instruments is extensively used in biochemical research Ion mobility spectrometry IMS coupled with imaging mass spectrometry MS is one of these combined approaches that have the potential to provide direct insight into the shape structure and position of biomolecules The combination of MS with IMS has already proven to be an extremely successful technique for determining the structures of ions in the gas phase as it allows the separation of different structural isomers The addition of a molecular imaging component enhances the molecular detail providedThe basics of IMS have been known for a long time Rutherford 1 was one of the first researchers who performed measurements based on the mobility of ions when he measured the velocity of different gas ions Still the first ion mobility spectrometers that enabled the separation of ions on the basis of their physical collision crosssection were built only in the 1960s Ions were simply forced to drift in a gas cell The first ion mobility mass spectrometers coupled with magnetic sector instruments were also built in the 1960s 2 almost immediately after the development of the first ion mobility spectrometers Since then the combination of ion mobility cells with both continuous and pulsed ion sources has been solved and ion mobility spectrometers have been coupled with many different kinds of mass analysers including timeofflight TOF analysers 3 4 magnetic sector instruments 2 quadrupoles 5 6 ion traps 7 8 9 and Fourier transform ion cyclotron resonance instruments 10 11 12 The first ion mobility instruments were pure drifttime instruments Two other types of ion mobility spectrometers evolved differential mobility analysers also called highfield asymmetric ion mobility spectrometers and recently travellingwave instrumentsInitially ion mobility spectrometers were used only in the field of physical chemistry and for plasma physics research The development of the matrixassisted laser desorption and ionization MALDI 13 14 15 and electrospray ionization ESI 16 17 methods in the late 1980s enabled the measurement of large biomolecules by MS This also resulted in a major breakthrough in the field of ion mobility MS IMMS in the 1990s The first work where the mobility of biomolecules was measured was undertaken by Clemmer et al 18 In this publication the possibility of measuring the conformation of proteins with IMMS was demonstrated The results led to the maturation of the technique and since then IMMS has been used for structural studies of several different biomolecules such as proteins 18 19 20 lipids 21 22 23 24 25 26 carbohydrates 27 28 29 30 and even intact viruses 31 32 33 It was also shown in several works that the separation of optical isomers is possible using chiral modifiers in the collision gas 34 35 36 The potential of using ion mobility mass spectrometers in biological and biochemical research accelerated the development of the instrumentation and moved the field from custombuilt instruments to commercially available instruments Nowadays several vendors sell ion mobility mass spectrometers and these instruments are becoming more and more popularIn parallel to the development of IMMS another bioanalytical technology has emerged imaging MS 37 38 Imaging MS is an MS technique where the spatial distribution of different classes of molecules can be determined on a surface It is perceived as one of the emerging tools in proteomics lipidomics and metabolomics Recent developments in imaging MS allow the direct detection of biomolecules as proteins lipids and drugs on tissue Labelfree imaging with MS uses an intrinsic molecular parameter the molecular mass to identify and visualize the distribution of a wide variety of biomolecules at surfaces This allows the combined determination of molecular weight and localization An imaging MS experiment requires surface molecules to be desorbed and ionized Two general approaches can be distinguished in imaging MS The first and oldest approach uses energetic charged particles to generate surface ions for mass analysis This approach is common to secondary ion MS SIMS The second approach uses photons to generate surface ions for mass analysis This is the domain of laserbased MS technologies such as MALDI There is a continuous stream of new desorption and ionization technologies that extend the capabilities of imaging MS Desorption ESI and laser spray are examples of recently conceived ambient desorption and ionization methods that further the application areas of imaging MS The interested reader is referred to recent imaging MS reviews 37 38 for an overview of all relevant innovation in desorption and ionization for imaging MS Here we will briefly introduce the key concepts of SIMS and MALDI for imaging MSSIMS involves the analysis of secondary ions generated from a surface when this surface is bombarded with primary particles These primary particles are typically elemental ions generated from a liquid metal ion source The ‘sputtered’ secondary particles will be electrons neutrals and ions These ions are amenable to analysis with a mass spectrometer and provide insight into the chemical composition of the surface As a rule of thumb with a primary ion dose below 1013 ions/cm2 each individual primary ion samples a fresh spot on the surface This is referred to as the ‘static limit’ An ioninduced modification of the surface is sampled with a primary ion dose above this static limit Under these conditions the surface changes rapidly and is sputtered with a fixed sputter rate The regime where the primary ion dose exceeds the static limit is hence referred to as ‘dynamic SIMS’ Traditionally SIMS has been applied in the domain of surface physics and solidstate physics Over recent decades SIMS has developed more and more as an analytical tool for biological materials such as cells and tissue Abundant lipids and small peptides can be studied from these surfaces through the use of sample modification strategies 39 and new elemental cluster ion sources Au n m+ and Bi n m+ 40 SIMS is traditionally implemented on a TOF mass spectrometer that ensures a sensitive and highthroughput analysisMALDI is a versatile and effective method to desorb and ionize polymers small organic molecules lipids and amino acids but also especially biomolecules such as peptides proteins and oligonucleotides 14 The basic experiment involves irradiating the sample with a pulsed laser to generate analyte ions from the surface A matrix solution is applied to the tissue sample to facilitate effective MALDI analysis These solutions are used for the extraction of the analyte molecules from the tissue and to create analytedoped matrix crystals During the analysis the surface is irradiated by a pulsed laser thus inducing ablation of these analyterich crystals which results in cooperative motion of the analyte and matrix ions into the vacuum MALDI can be applied directly for the analysis of biological surfaces and tissue sections This makes it a key technology for imaging MS

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