- Email: [email protected]
‘Next generation’ laser-based biological mass spectrometry
Methods 104 (2016) 1–2
Contents lists available at ScienceDirect
Methods journal homepage: www.elsevier.com/locate/ymeth
Guest Editor’s Introduction
‘Next generation’ laser-based biological mass spectrometry In the last few years laser-based biological Mass Spectrometry (MS) has made substantial advances in countless and very diverse directions. Having been dominated by classical Matrix-Assisted Laser Desorption/Ionization (MALDI) and its application in proteomics, it has now overcome many limitations of early MALDI MS and is further exploiting the unique possibilities given by the use of lasers as precise and rapid energy delivery tools. As there is a new wave of methods and applications using lasers in biological MS it is timely to have a closer look at some of these new developments, such as new ionization modes and combinations, and provide an update on the more established techniques, such as (MA)LDI MS imaging as well as new applications of classical MALDI MS in proteomics. In 2014, Franz Hillenkamp, one of the co-inventors of MALDI MS, passed away and it can be said that without his and his group’s early efforts in laser-based biological MS this field would not be as advanced as it is now. It really feels like that there is now a ‘next generation’ in laser-based biological MS emerging, demonstrating the fantastic opportunities that lasers can offer in an analytical field that is progressing beyond classical MALDI due to new concepts and ideas in laser-based desorption and ionization and their applications, often driven by the needs of miniaturization, higher throughput and/or speed in analysis. Although there have been some recent books on the latest developments of MALDI and laser-induced soft ionization MS [1,2], there has been much more activity in the field, which this issue tries to capture. It is therefore not surprising that the response to contribute to this issue was overwhelming, which unfortunately also meant that its publication experienced a slight delay. For this, I would like to apologize to all contributors, in particular the ones who sent in their contributions, revisions and proofs on time. I hope that the final product will compensate for this. The first article in this issue is provided by Bierstedt and Riedel [3] and is a good example of how a laser-based desorption method can be combined with another ionization method, in this case dielectric barrier discharge ionization, and how this combination can provide a decisive advantage for the detection of a specific analyte class. This article also provides the cover image of this issue. The next six articles continue to demonstrate innovation in the field, from underpinning optimization work to new hyphenations and applications of novel methods, mainly based on MALDI-TOF MS. In the article by Ryumin et al. [4] important information is provided on improving the ion yield of multiply charged analyte ions in liquid MALDI MS. Similarly, Moskovets et al. [5] present their results for the production of singly charged MALDI ions, comparing two different ion source pressure regimes. The articles by Steinhoff et al. [6] and Schröter et al. [7] describe the use of MALDI-TOF MS in monitoring cell growth/productivity and the kinetics of http://dx.doi.org/10.1016/j.ymeth.2016.06.012 1046-2023/Ó 2016 Elsevier Inc. All rights reserved.
phospholipase activity, respectively. These articles show the strength of MALDI for high-throughput (HTP) microarray screening and the analysis of a wide range of bioanalytes, in particular lipids. Evaluating cell growth by MALDI-TOF MS analysis has also been described in the article by Sparbier et al. [8] and exploited for antibiotic resistance/susceptibility testing of cell cultures, promising fast HTP assays for the clinical microbiology laboratory. Marchetti-Deschmann et al. [9] employed MALDI-TOF MS for assaying bioconjugates using the example of a mycotoxin conjugated to the carrier protein conalbumin, a bioconjugate of agricultural importance. Performance evaluation for some methods is provided by Trimpin et al. [10] and Skraskova et al. [11] with studies comparing laser-based MS techniques with other (non-laser) MS techniques. Skraskova et al. undertook their comparative studies with respect to mass spectrometry imaging (MSI) of gangliosides in murine brain, comparing MALDI with desorption electrospray ionization (DESI) and showing their complementarity. The following seven articles describe further work on MALDI MS with two articles introducing novel concepts in laser-based MSI. The article by Shi et al. [12] presents non-resonant (matrixfree) femtosecond laser vaporization coupled to electrospray ionization for biological tissue (leaf) imaging. Shariatgorji et al. [13] applied straightforward laser tissue ablation to imaging elements and compared their results to the more established laser ablation inductively coupled plasma (LA-ICP) method. The article by Mitchell et al. [14] reports the application of standard MALDI MSI for the imaging of the distribution and effect of emollient treatments on sections of reconstructed living skin equivalents while the two articles by Steven et al. [15,16] further investigate critical MALDI MSI parameters of the laser (energy) delivery. The next three articles are reviews, of which the first two focus on MSI, i.e. high resolution laser MS bioimaging [17] and small molecule MALDI MSI [18]. The third review focuses on the analysis of low-molecular weight compounds in biological matrices [19]. Following the reviews, three articles describe some specific applications of lasers in MS-based proteomics. The first article by Longuespée et al. [20] investigates various sample preparations of laser-microdissected formalin-fixed paraffin-embedded (FFPE) tissue samples for MS-based proteomic analysis, which is also of relevance to MALDI MSI. The other two articles employ electrophoretic separation of proteins prior to MS-based proteomic analysis. In the work by Lohnes et al. [21], MALDI MS analysis is applied to intact proteins after separation by isoelectric focusing and alternatively combined with the results of a (subsequent) ‘bottom up’ analysis of gel-separated protein digests by MALDI MS while Komatsu et al. [22] present their method development for
2
Guest Editor’s Introduction / Methods 104 (2016) 1–2
the characterization of immunoglobulins through the analysis of glycopeptides by MALDI MS. The final two articles of this issue present new efforts in improving the sample targets in laser-based desorption/ionization for MS analysis, introducing a new low-cost disposable metalcoated polymeric sample target plate [23] and a highly porous silver foil as sample substrate for the efficient analysis of lipids [24]. Last but not least, I would like to thank all contributors to this issue. Without their continuing work in this field, the analytical landscape and the sciences supported by it would be much poorer. Given the great variety and analytical power of laser-based biological MS, this field will undoubtedly continue to support great research and other areas where fast, sensitive and accurate analysis at the biomolecular level is needed. For my part, I am looking forward to finding laser-based mass spectrometry more and more in important application areas such as clinical diagnostics, drug discovery, environmental screening, food authentication, forensics and industrial QC/QA, to name but a few. References [1] F. Hillenkamp, J. Peter-Katalinic (Eds.), MALDI MS: A Practical Guide to Instrumentation, Methods, and Applications, Wiley-VCH, Weinheim, Germany, 2014, ISBN 978-3-527-33331-8. [2] R. Cramer (Ed.), Advances in MALDI and Laser-Induced Soft Ionization Mass Spectrometry, Springer, Cham, Switzerland, 2016, ISBN 978-3-319-04818-5. [3] Andreas Bierstedt, Jens Riedel, High-repetition rate laser ablation coupled to dielectric barrier discharge postionization for ambient mass spectrometry, Methods 104 (2016) 3–10. [4] Pavel Ryumin, Jeffery Brown, Michael Morris, Rainer Cramer, Investigation and optimization of parameters affecting the multiply charged ion yield in APMALDI MS, Methods 104 (2016) 11–20. [5] Eugene Moskovets, Alexander Misharin, Viktor Laiko, Vladimir Doroshenko, A comparative study on the analytical utility of atmospheric and low-pressure MALDI sources for the mass spectrometric characterization of peptides, Methods 104 (2016) 21–32. [6] Robert F. Steinhoff, Daniel J. Karst, Fabian Steinebach, Marie R.G. Kopp, Gregor W. Schmidt, Alexander Stettler, Jasmin Krismer, Miroslav Soos, Martin Pabst, Andreas Hierlemann, Massimo Morbidelli, Renato Zenobi, Microarray-based MALDI-TOF mass spectrometry enables monitoring of monoclonal antibody production in batch and perfusion cell cultures, Methods 104 (2016) 33–40. [7] Jenny Schröter, Rosmarie Süß, Jürgen Schiller, MALDI-TOF MS to monitor the kinetics of phospholipase A2-digestion of oxidized phospholipids, Methods 104 (2016) 41–47. [8] Katrin Sparbier, Sören Schubert, Markus Kostrzewa, MBT-ASTRA: a suitable tool for fast antibiotic susceptibility testing?, Methods 104 (2016) 48–54 [9] Martina Marchetti-Deschmann, Christopher Stephan, Georg Häubl, Günter Allmaier, Rudolf Krska, Barbara Cvak, Determining and characterizing hapten loads for carrier proteins by MALDI-TOF MS and MALDI-TOF/RTOF MS, Methods 104 (2016) 55–62. [10] Sarah Trimpin, Shameemah Thawoos, Casey D. Foley, Daniel W. Woodall, Jing Li, Ellen D. Inutan, Paul M. Stemmer, Rapid high mass resolution mass spectrometry using matrix-assisted ionization, Methods 104 (2016) 63–68.
[11] Karolina Škrášková, Emmanuelle Claude, Emrys A. Jones, Mark Towers, Shane R. Ellis, Ron M.A. Heeren, Enhanced capabilities for imaging gangliosides in murine brain with matrix-assisted laser desorption/ionization and desorption electrospray ionization mass spectrometry coupled to ion mobility separation, Methods 104 (2016) 69–78. [12] Fengjian Shi, Jieutonne J. Archer, Robert J. Levis, Nonresonant, femtosecond laser vaporization and electrospray post-ionization mass spectrometry as a tool for biological tissue imaging, Methods 104 (2016) 79–85. [13] Mohammadreza Shariatgorji, Anna Nilsson, Maximilian Bonta, Jinrui Gan, Niklas Marklund, Fredrik Clausen, Patrik Källback, Henrik Loden, Andreas Limbeck, Per E. Andren, Direct imaging of elemental distributions in tissue sections by laser ablation mass spectrometry, Methods 104 (2016) 86–92. [14] Christopher A. Mitchell, Michael Donaldson, Simona Francese, Malcolm R. Clench, MALDI MSI analysis of lipid changes in living skin equivalents in response to emollient creams containing palmitoylethanolamide, Methods 104 (2016) 93–100. [15] Rory T. Steven, Alex Dexter, Josephine Bunch, Investigating MALDI MSI parameters (Part 1) – A systematic survey of the effects of repetition rates up to 20 kHz in continuous raster mode, Methods 104 (2016) 101–110. [16] Rory T. Steven, Alex Dexter, Josephine Bunch, Investigating MALDI MSI parameters (Part 2) – On the use of a mechanically shuttered trigger system for improved laser energy stability, Methods 104 (2016) 111–117. [17] Kermit K. Murray, Chinthaka A. Seneviratne, Suman Ghorai, High resolution laser mass spectrometry bioimaging, Methods 104 (2016) 118–126. [18] Paul J. Trim, Marten F. Snel, Small molecule MALDI MS imaging: current technologies and future challenges, Methods 104 (2016) 127–141. [19] András Kiss, Gérard Hopfgartner, Laser-based methods for the analysis of low molecular weight compounds in biological matrices, Methods 104 (2016) 142–153. [20] Rémi Longuespée, Deborah Alberts, Charles Pottier, Nicolas Smargiasso, Gabriel Mazzucchelli, Dominique Baiwir, Mark Kriegsmann, Michael Herfs, Jörg Kriegsmann, Philippe Delvenne, Edwin De Pauw, A laser microdissectionbased workflow for FFPE tissue microproteomics: important considerations for small sample processing, Methods 104 (2016) 154–162. [21] Karen Lohnes, Neil R. Quebbemann, Kate Liu, Fred Kobzeff, Joseph A. Loo, Rachel R. Ogorzalek Loo, Combining high-throughput MALDI-TOF mass spectrometry and isoelectric focusing gel electrophoresis for virtual 2D gelbased proteomics, Methods 104 (2016) 163–169. [22] Emy Komatsu, Marjorie Buist, Rini Roy, Andrey Giovanni Gomes, Edward de Oliveira, Apolline Salama Bodnar, Jean-Paul Soulillou, Hélène Perreault, Characterization of immunoglobulins through analysis of N-glycopeptides by MALDI-TOF MS, Methods 104 (2016) 170–181. [23] Stefan Bugovsky, Wolfgang Winkler, Werner Balika, Manfred Koranda, Günter Allmaier, Polymer-based metal nano-coated disposable target for matrixassisted and matrix-free laser desorption/ionization mass spectrometry, Methods 104 (2016) 182–193. [24] Andreas Schnapp, Ann-Christin Niehoff, Annika Koch, Klaus Dreisewerd, Laser desorption/ionization mass spectrometry of lipids using etched silver substrates, Methods 104 (2016) 194–203.
Rainer Cramer Department of Chemistry, University of Reading, Whiteknights, Reading RG6 6AD, UK E-mail address: [email protected] Available online 17 June 2016
Contents lists available at ScienceDirect
Methods journal homepage: www.elsevier.com/locate/ymeth
Guest Editor’s Introduction
‘Next generation’ laser-based biological mass spectrometry In the last few years laser-based biological Mass Spectrometry (MS) has made substantial advances in countless and very diverse directions. Having been dominated by classical Matrix-Assisted Laser Desorption/Ionization (MALDI) and its application in proteomics, it has now overcome many limitations of early MALDI MS and is further exploiting the unique possibilities given by the use of lasers as precise and rapid energy delivery tools. As there is a new wave of methods and applications using lasers in biological MS it is timely to have a closer look at some of these new developments, such as new ionization modes and combinations, and provide an update on the more established techniques, such as (MA)LDI MS imaging as well as new applications of classical MALDI MS in proteomics. In 2014, Franz Hillenkamp, one of the co-inventors of MALDI MS, passed away and it can be said that without his and his group’s early efforts in laser-based biological MS this field would not be as advanced as it is now. It really feels like that there is now a ‘next generation’ in laser-based biological MS emerging, demonstrating the fantastic opportunities that lasers can offer in an analytical field that is progressing beyond classical MALDI due to new concepts and ideas in laser-based desorption and ionization and their applications, often driven by the needs of miniaturization, higher throughput and/or speed in analysis. Although there have been some recent books on the latest developments of MALDI and laser-induced soft ionization MS [1,2], there has been much more activity in the field, which this issue tries to capture. It is therefore not surprising that the response to contribute to this issue was overwhelming, which unfortunately also meant that its publication experienced a slight delay. For this, I would like to apologize to all contributors, in particular the ones who sent in their contributions, revisions and proofs on time. I hope that the final product will compensate for this. The first article in this issue is provided by Bierstedt and Riedel [3] and is a good example of how a laser-based desorption method can be combined with another ionization method, in this case dielectric barrier discharge ionization, and how this combination can provide a decisive advantage for the detection of a specific analyte class. This article also provides the cover image of this issue. The next six articles continue to demonstrate innovation in the field, from underpinning optimization work to new hyphenations and applications of novel methods, mainly based on MALDI-TOF MS. In the article by Ryumin et al. [4] important information is provided on improving the ion yield of multiply charged analyte ions in liquid MALDI MS. Similarly, Moskovets et al. [5] present their results for the production of singly charged MALDI ions, comparing two different ion source pressure regimes. The articles by Steinhoff et al. [6] and Schröter et al. [7] describe the use of MALDI-TOF MS in monitoring cell growth/productivity and the kinetics of http://dx.doi.org/10.1016/j.ymeth.2016.06.012 1046-2023/Ó 2016 Elsevier Inc. All rights reserved.
phospholipase activity, respectively. These articles show the strength of MALDI for high-throughput (HTP) microarray screening and the analysis of a wide range of bioanalytes, in particular lipids. Evaluating cell growth by MALDI-TOF MS analysis has also been described in the article by Sparbier et al. [8] and exploited for antibiotic resistance/susceptibility testing of cell cultures, promising fast HTP assays for the clinical microbiology laboratory. Marchetti-Deschmann et al. [9] employed MALDI-TOF MS for assaying bioconjugates using the example of a mycotoxin conjugated to the carrier protein conalbumin, a bioconjugate of agricultural importance. Performance evaluation for some methods is provided by Trimpin et al. [10] and Skraskova et al. [11] with studies comparing laser-based MS techniques with other (non-laser) MS techniques. Skraskova et al. undertook their comparative studies with respect to mass spectrometry imaging (MSI) of gangliosides in murine brain, comparing MALDI with desorption electrospray ionization (DESI) and showing their complementarity. The following seven articles describe further work on MALDI MS with two articles introducing novel concepts in laser-based MSI. The article by Shi et al. [12] presents non-resonant (matrixfree) femtosecond laser vaporization coupled to electrospray ionization for biological tissue (leaf) imaging. Shariatgorji et al. [13] applied straightforward laser tissue ablation to imaging elements and compared their results to the more established laser ablation inductively coupled plasma (LA-ICP) method. The article by Mitchell et al. [14] reports the application of standard MALDI MSI for the imaging of the distribution and effect of emollient treatments on sections of reconstructed living skin equivalents while the two articles by Steven et al. [15,16] further investigate critical MALDI MSI parameters of the laser (energy) delivery. The next three articles are reviews, of which the first two focus on MSI, i.e. high resolution laser MS bioimaging [17] and small molecule MALDI MSI [18]. The third review focuses on the analysis of low-molecular weight compounds in biological matrices [19]. Following the reviews, three articles describe some specific applications of lasers in MS-based proteomics. The first article by Longuespée et al. [20] investigates various sample preparations of laser-microdissected formalin-fixed paraffin-embedded (FFPE) tissue samples for MS-based proteomic analysis, which is also of relevance to MALDI MSI. The other two articles employ electrophoretic separation of proteins prior to MS-based proteomic analysis. In the work by Lohnes et al. [21], MALDI MS analysis is applied to intact proteins after separation by isoelectric focusing and alternatively combined with the results of a (subsequent) ‘bottom up’ analysis of gel-separated protein digests by MALDI MS while Komatsu et al. [22] present their method development for
2
Guest Editor’s Introduction / Methods 104 (2016) 1–2
the characterization of immunoglobulins through the analysis of glycopeptides by MALDI MS. The final two articles of this issue present new efforts in improving the sample targets in laser-based desorption/ionization for MS analysis, introducing a new low-cost disposable metalcoated polymeric sample target plate [23] and a highly porous silver foil as sample substrate for the efficient analysis of lipids [24]. Last but not least, I would like to thank all contributors to this issue. Without their continuing work in this field, the analytical landscape and the sciences supported by it would be much poorer. Given the great variety and analytical power of laser-based biological MS, this field will undoubtedly continue to support great research and other areas where fast, sensitive and accurate analysis at the biomolecular level is needed. For my part, I am looking forward to finding laser-based mass spectrometry more and more in important application areas such as clinical diagnostics, drug discovery, environmental screening, food authentication, forensics and industrial QC/QA, to name but a few. References [1] F. Hillenkamp, J. Peter-Katalinic (Eds.), MALDI MS: A Practical Guide to Instrumentation, Methods, and Applications, Wiley-VCH, Weinheim, Germany, 2014, ISBN 978-3-527-33331-8. [2] R. Cramer (Ed.), Advances in MALDI and Laser-Induced Soft Ionization Mass Spectrometry, Springer, Cham, Switzerland, 2016, ISBN 978-3-319-04818-5. [3] Andreas Bierstedt, Jens Riedel, High-repetition rate laser ablation coupled to dielectric barrier discharge postionization for ambient mass spectrometry, Methods 104 (2016) 3–10. [4] Pavel Ryumin, Jeffery Brown, Michael Morris, Rainer Cramer, Investigation and optimization of parameters affecting the multiply charged ion yield in APMALDI MS, Methods 104 (2016) 11–20. [5] Eugene Moskovets, Alexander Misharin, Viktor Laiko, Vladimir Doroshenko, A comparative study on the analytical utility of atmospheric and low-pressure MALDI sources for the mass spectrometric characterization of peptides, Methods 104 (2016) 21–32. [6] Robert F. Steinhoff, Daniel J. Karst, Fabian Steinebach, Marie R.G. Kopp, Gregor W. Schmidt, Alexander Stettler, Jasmin Krismer, Miroslav Soos, Martin Pabst, Andreas Hierlemann, Massimo Morbidelli, Renato Zenobi, Microarray-based MALDI-TOF mass spectrometry enables monitoring of monoclonal antibody production in batch and perfusion cell cultures, Methods 104 (2016) 33–40. [7] Jenny Schröter, Rosmarie Süß, Jürgen Schiller, MALDI-TOF MS to monitor the kinetics of phospholipase A2-digestion of oxidized phospholipids, Methods 104 (2016) 41–47. [8] Katrin Sparbier, Sören Schubert, Markus Kostrzewa, MBT-ASTRA: a suitable tool for fast antibiotic susceptibility testing?, Methods 104 (2016) 48–54 [9] Martina Marchetti-Deschmann, Christopher Stephan, Georg Häubl, Günter Allmaier, Rudolf Krska, Barbara Cvak, Determining and characterizing hapten loads for carrier proteins by MALDI-TOF MS and MALDI-TOF/RTOF MS, Methods 104 (2016) 55–62. [10] Sarah Trimpin, Shameemah Thawoos, Casey D. Foley, Daniel W. Woodall, Jing Li, Ellen D. Inutan, Paul M. Stemmer, Rapid high mass resolution mass spectrometry using matrix-assisted ionization, Methods 104 (2016) 63–68.
[11] Karolina Škrášková, Emmanuelle Claude, Emrys A. Jones, Mark Towers, Shane R. Ellis, Ron M.A. Heeren, Enhanced capabilities for imaging gangliosides in murine brain with matrix-assisted laser desorption/ionization and desorption electrospray ionization mass spectrometry coupled to ion mobility separation, Methods 104 (2016) 69–78. [12] Fengjian Shi, Jieutonne J. Archer, Robert J. Levis, Nonresonant, femtosecond laser vaporization and electrospray post-ionization mass spectrometry as a tool for biological tissue imaging, Methods 104 (2016) 79–85. [13] Mohammadreza Shariatgorji, Anna Nilsson, Maximilian Bonta, Jinrui Gan, Niklas Marklund, Fredrik Clausen, Patrik Källback, Henrik Loden, Andreas Limbeck, Per E. Andren, Direct imaging of elemental distributions in tissue sections by laser ablation mass spectrometry, Methods 104 (2016) 86–92. [14] Christopher A. Mitchell, Michael Donaldson, Simona Francese, Malcolm R. Clench, MALDI MSI analysis of lipid changes in living skin equivalents in response to emollient creams containing palmitoylethanolamide, Methods 104 (2016) 93–100. [15] Rory T. Steven, Alex Dexter, Josephine Bunch, Investigating MALDI MSI parameters (Part 1) – A systematic survey of the effects of repetition rates up to 20 kHz in continuous raster mode, Methods 104 (2016) 101–110. [16] Rory T. Steven, Alex Dexter, Josephine Bunch, Investigating MALDI MSI parameters (Part 2) – On the use of a mechanically shuttered trigger system for improved laser energy stability, Methods 104 (2016) 111–117. [17] Kermit K. Murray, Chinthaka A. Seneviratne, Suman Ghorai, High resolution laser mass spectrometry bioimaging, Methods 104 (2016) 118–126. [18] Paul J. Trim, Marten F. Snel, Small molecule MALDI MS imaging: current technologies and future challenges, Methods 104 (2016) 127–141. [19] András Kiss, Gérard Hopfgartner, Laser-based methods for the analysis of low molecular weight compounds in biological matrices, Methods 104 (2016) 142–153. [20] Rémi Longuespée, Deborah Alberts, Charles Pottier, Nicolas Smargiasso, Gabriel Mazzucchelli, Dominique Baiwir, Mark Kriegsmann, Michael Herfs, Jörg Kriegsmann, Philippe Delvenne, Edwin De Pauw, A laser microdissectionbased workflow for FFPE tissue microproteomics: important considerations for small sample processing, Methods 104 (2016) 154–162. [21] Karen Lohnes, Neil R. Quebbemann, Kate Liu, Fred Kobzeff, Joseph A. Loo, Rachel R. Ogorzalek Loo, Combining high-throughput MALDI-TOF mass spectrometry and isoelectric focusing gel electrophoresis for virtual 2D gelbased proteomics, Methods 104 (2016) 163–169. [22] Emy Komatsu, Marjorie Buist, Rini Roy, Andrey Giovanni Gomes, Edward de Oliveira, Apolline Salama Bodnar, Jean-Paul Soulillou, Hélène Perreault, Characterization of immunoglobulins through analysis of N-glycopeptides by MALDI-TOF MS, Methods 104 (2016) 170–181. [23] Stefan Bugovsky, Wolfgang Winkler, Werner Balika, Manfred Koranda, Günter Allmaier, Polymer-based metal nano-coated disposable target for matrixassisted and matrix-free laser desorption/ionization mass spectrometry, Methods 104 (2016) 182–193. [24] Andreas Schnapp, Ann-Christin Niehoff, Annika Koch, Klaus Dreisewerd, Laser desorption/ionization mass spectrometry of lipids using etched silver substrates, Methods 104 (2016) 194–203.
Rainer Cramer Department of Chemistry, University of Reading, Whiteknights, Reading RG6 6AD, UK E-mail address: [email protected] Available online 17 June 2016