Application Note # MT-94 Direct Read-out of Thin Layer Chromatography (TLC) using MALDI-TOF

Transcription

1 Bruker Daltonics Application Note # MT-94 Direct Read-out of Thin Layer Chromatography (TLC) using MALDI-TOF Thin Layer Chromatography (TLC) is broadly established to separate, characterize and quantify food and biological ingredients/components. Recently interest in mass spectrometric analysis of TLC-separated compounds has increased significantly. Here we will introduce the direct coupling of TLC with MALDI-TOF mass spectrometry: TLC-MALDI; an adapter target for TLC plates and an extension of the standard flexsoftware that turns the MALDI into a molecular TLC readout platform. Also included are the first results from lipid analysis. Introduction TLC separations remain indispensable for a number of applications such as forensics, herbals, food, cosmetics and clinical/industrial applications. Particular analytes which prove difficult for HPLC separations, such as lipids, are widely characterized by TLC. In addition, TLC remains a quick and simple method to monitor the progress of chemical reactions in the organic synthesis laboratory. For many of these applications, the assignment or confirmation of molecular structures to TLC-separated spots is important, as RF ( ratio of fronts ) values alone are often not highly reproducible and do not allow the unequivocal assignment of a certain compound. However, currently established TLC-MALDI protocols involve scratching the chromatographic phase off the carrier and elution of the analyte prior to spectroscopic or mass spectrometric analysis. This process is not only time consuming and carries the risk of loosing some compounds, but it also relies on previously detected and manually marked spots. Therefore, the structural analysis depends on the ability to visualize the particular analyte by specific staining agents or by unspecific fluorescence quenching. As staining methods do not resolve individual compounds with similar RF values, the achievable analytical resolution is typically limited by the detection method. Here we introduce matrixassisted laser desorption/ionisation time-of-flight mass spectrometry (MALDI-TOF MS) to directly read out TLC traces reproducibly whilst maintaining the chromatographic resolving power. Workflow The hyphenated TLC-MALDI approach requires the uniform coverage of the TLC lanes with MALDI matrix as a preparative step comparable with the effort of staining. The mass spectrometric information is subsequently read out automatically. MALDI plate movement along the chromatographic lanes is driven by the standard movement mechanics of the MALDI ion source without additional robotic requirements. A chromatographic run is generated for each lane. It can then be analyzed using TLC-MS software, providing the extracted ion chromatograms of individual analytes independent of dye staining and at a selectable resolution which largely depends on the chromatography itself.

2 Fig. 1: MTP sized TLC-MALDI Adapter Target (Order No #255595) for 50x75 mm TLC Silica gel (aluminum backed). After TLC separation and matrix coating they are ready for MALDI analysis. Experimental TLC All TLC-MALDI experiments were performed using either aluminum backed mm TLC Silica gel 60 F 254 plates, 200 µm layer thickness (Merck, # ) or mm TLC Silica gel 60 plates on aluminum backs, 200 μm layer thickness) (Merck, # ) and developed in horizontal developing TLC chambers using CHCl 3, ethanol, water, triethylamine (35:35:7:35, v/v/v/v) as the solvent system for lipids and phospholipids, whereas glycolipids were separated under acidic conditions (CHCl 3, CH 3 OH, acetic acid (65:25:10, v/v/v)). Lipids were obtained by extraction of cellular suspensions or biological tissues, prepared and visualized by spraying with a solution of primuline as described [1-3]. ImagePrep allows fully automated matrix preparation with highest spatial resolution and reproducibility. A 100 mg/ ml solution of DHB in acetonitrile/water (1:1, v/v) was used for manual matrix application. 50 mg/ml DHB was used for ImagePrep preparations. Generally, in contrast to conventional MALDI, a larger excess of the matrix over the analyte is needed if spectra are to be recorded directly from the TLC plate. TLC-MALDI software Coupling of TLC with MALDI-TOF MS The mm TLC plates were cut to mm to fit the TLC-MALDI Adapter target (Bruker, Order Nr ); mm TLC plates were unaltered (Fig. 1). MALDI spectra were acquired from the matrix coated TLC plates as described in [1]. Matrix was added either manually using conventional dried droplet preparation on discrete primuline stained spots (marked by pencil) or the entire TLC plate was homogeneously coated with matrix (1-5 mg/cm 2 ) for MALDIimaging or TLC-scanning using the ImagePrep preparation device (Bruker Daltonics) [4]. Fig 2: TLC-MALDI setup dialog. The wizard driven software interface allows for simple analysis setup with multiple chromatographic lanes and defined step rasters along with or orthogonal to the chromatographic axis.

3 MALDI-TOF MS Selected Spots: All MALDI spectra from primuline spots were acquired on an Autoflex MALDI-TOF-MS with 50 Hz nitrogen laser. The extraction voltage was 20 kv and gated matrix suppression was applied to prevent the saturation of the detector by matrix ions. All spectra were acquired in reflector mode using delayed extraction as described in [5]. Spectra were calibrated using a standard lipid mixture desorbed from a standard DHB preparation next to the spots of interest (positive ion mode). For negative ion mode, the characteristic signals of the DHB matrix were used for calibration [6]. MALDI: Imaging and scanning of lanes on TLC plates was performed on an ultraflex III MALDI-TOF/TOF with 200 Hz smartbeam laser in reflector mode. The extraction voltage was set to 25 kv and matrix suppression to m/z <200. Data were acquired under Compass 1.2 (FC 3.0, FA 3.0). TLC-Imaging: A pixel raster of 400 μm 400 μm spots was defined with fleximaging 2.0 on the entire TLC lane area. 200 laser shots were accumulated for each pixel. False colors were assigned to each mass of interest and their spatial distributions across the TLC plate were displayed as a heat map. TLC-Scanning: The position of chromatographic lanes on the TLC plate can be read directly from xy-scales on the adapter target and entered into a new dedicated software tool for TLC-MALDI (Fig. 2), which is available as a TLC- MALDI software patch compatible with Compass 1.2 for chromatographic readout. The Compass 1.3+ software will incorporate TLC-MALDI functionality. The location and dimensions of multiple TLC lanes can be defined, as can raster dimensions (here 400 µm). The intuitive, easy-tohandle TLC-MALDI software controls not only automatic data acquisition but also provides a dedicated TLC data viewer and post-processing tools. Results Erythrocyte membrane lipids cells were extracted using CHCl 3 / CH 3 OH/ H 2 O 2:1:1 (v/v/v) as described in more detail in [2]. The TLC plate was analyzed using three different approaches as shown in Fig. 3. The traditional analysis used specific lipid staining by primuline (Fig. 3A), which forms a non-covalent complex with lipids, therefore, not interfering with subsequent MALDI analysis. Three major spots consisting of PE, PC and SM (see Tab. 1) were detectable using primuline and a minor spot represented LPC. Detailed evaluation of the TLC-MALDI imaging dataset revealed 3 more groups of compounds (LPI, PI, PS) and permitted the differentiation of lipids that could not be distinguished chromatographically Abbr. Lipid name MH + PE Phosphatidylethanolamine 16:0/18: G Ganglioside PS Phosphatidylserine 18:0/20: PC Phosphatidylcholine 16:0/20: SM Sphingomyelin 16: LPC Lyso-Phosphatidylcholine 16: PI Phosphatidylinositol 16:0/18: PA Phosphatidic acid 16:0/18: Tab. 1: Detected and characterized lipids from erythrocyte membrane, stem cells and brain: abbreviation, name, structure and typical protonated molecular ion (for several lipids, e.g., MNa + are actually detected). with the staining-scraping procedure: PE 18:0/20:4 vs. PE 16:0/18:1; LPI 22:5 vs. PI 18:0/18:2 (Figs. 3b, 4). Detection of major lipids Although not shown, all major lipids (phospholipids, apolar lipids such as cholesterol, cholesteryl esters and triacylglycerols) present in biological systems are easily detectable using this approach. Our studies have also provided evidence that even low abundance lipids (< 1% of the total lipids) such as PI are easily detectable within a single dataset providing a dynamic range of ~ 3 orders of magnitude. Even mixtures of polyphosphoinositides (PI, PIP, PIP 2 ) can be easily separated and analyzed (PI peak labeled blue in Fig. 3 and present in Fig. 4 b). Glycolipids require a more acidic solvent system in order to separate them from the common and more abundant phospholipids [7]. Although compounds such as gangliosides are characterized by a much higher polarity resulting in reduced detectability, they are easily detectable using this approach. However, glycolipids are often negatively charged (due to the presence of phosphate or sulphate groups), and require negative mode analysis.

4 Classical Staining vs. Imaging and Chromatographic Readout of TLC-MALDI Data x Mass Spectrum A B C TLC Lane MALDI Image MALDI Chromatogram Extracted Ion Cromatogram m/z Fig 3: A) Classical primuline staining of separated lipids on a TLC plate; B) TLC-MALDI imaging analysis of identical TLC plate with matrix coating; C) chromatographic TLC readout as provided by the TLC-MALDI software permitting direct access to all molecular species (m/z values on the x-axis) along the chromatographic separation on the y-axis. The colored circles correspond to the compounds that are visualized in the TLC- MALDI image. Full readout in minutes Although imaging readouts such as Fig. 3B appear attractive as an analysis format comparing directly and favorably with classical stains (Fig. 3A), they require time consuming, manual data interrogation. Therefore, they do not appear well suited for routine analysis purposes. However, chromatographic treatment of the TLC data from acquisition to visualization and analysis appears to be more efficient and powerful, providing a full readout of the information contained in the TLC-MALDI dataset (Fig. 3C). Acquisition times are reduced from hours to ~ 5 min/lane and the large data file size for an image is reduced to a fraction of the size for the chromatogram. Therefore the TLC-MALDI software has been designed to support the quick and simple definition of lanes in the visual interface for the acquisition of chromatographic traces. Up to 4 lanes can be defined per TLC plate. After acquisition, the entire dataset is visualized using a dedicated DataViewer (Fig. 3C). It provides direct access to all separated compounds through a heat map of all MS peaks along the chromatographic separation (y-axis). For a cursor-selected position in the RF vs. m/z plane, the extracted ion chromatogram is displayed on the right side and the corresponding MS spectrum is shown on the top. This representation of the data allows direct access to all chromatographic mass peaks and a direct distinction can be made between the peaks and background ions which are represented by vertical streaks in the DataView. In addition, distinguishing largely overlapping lipids is straight- forward as their mass separation adds another dimension not available with the mass image. Structure confirmation After the analysis of the separated compounds, structure confirmation typically requires the acquisition of MS/MS spectra. The TLC-MALDI DataViewer facilitates manual pick a peak, which permits the direct manual acquisition of MS/MS spectra, in a few seconds, without the need to find the proper spots (not shown).

5 TLC-MALDI Analysis of Erythrocyte Membrane Lipids 8 PE 18:0/20:4 7 PE 16:0/18:1 8 7 PE PE 18:0/20:4 ( m/z = 812.5) PE 16:0/18:1 ( m/z = 762.5) 6 PC 16:0/20:4 Fragments of LPI 22:5 ( m/z = and 551.4) 5 PC 16:0/18:1 PI 18:0/18:2 ( m/z = 885.6) 16:0 LPC 18: SM 16:0 SM 24: m/z (a) PC SM LPC (b) Fragment of PS 16:0/20:4 ( m/z = 618.5) Unknown (Impurity?) ( m/z = ) PC 16:0/20:4 ( m/z = 804.6) PC 16:0/18:1 ( m/z = 760.6) SM 24:0 ( m/z = 837.7) SM 16:0 ( m/z = 725.6) LPC 18:0 ( m/z = 524.3) LPC 16:0 ( m/z = 496.3) Fig. 4: Spectrum overview of various lipids detected in positive ion mode from erythrocyte membrane. Spectra are numbered according to their elution order. TLC-MALDI imaging data (b) proved to be much more sensitive than common staining techniques (a: primuline stain). MS spectra obtained from (b) provided detailed differences in fatty acyl compositions (1-8). Conclusion TLC-MALDI runs on any Bruker Flex mass spectrometer equipped with a scoutmtp ion source under Compass 1.2 and higher. The ImagePrep is equipped with TLC methods to produce a uniform matrix coating with TLC separated lipids. Previously, TLC-MALDI coupling was expected to be problematic [8] because pieces of the stationary TLC phase might disintegrate and damage the MS. However, no such events were observed even under routine laboratory conditions. It is also remarkable that under TLC-MALDI conditions, fragmentation of analytes was not significantly exaggerated in comparison with standard MALDI. The new technology was applied to the analysis of lipids from various sources such as mesenchymal stem cells [2], human erythrocytes [2] or chicken egg [1] extracts providing evidence that MALDI is a great readout platform for TLC separations of lipids [9]. These analyses demonstrated significantly improved detection limits compared to the classical primuline staining approach. Imaging analysis allowed the visualization of many hitherto undetected lipids and to distinguish several compounds in broad primuline spots. Although imaging mass spectrometry compares favourably with primuline staining analytically, the relatively long acquisition time (~1-2 h) and data evaluation process associated with imaging (~1-2 h), calls for different approaches in routine analysis. Therefore, we developed software to automate the linear scanning of TLC traces with scan times in the 5 min range and use DataViewer to work with TLC-MALDI data in chromatographic format, which drastically reduces manual evaluation times of the analysis to few minutes. All, hardware and software methods are available now for the simple application of TLC-MALDI to routine analysis. Use of optimized matrix protocols with new matrices (such as 9-aminoacridine) are expected to further extend the application range of this new and exciting method. TLC- MALDI is interesting in a great number of applications such as oligosaccharides [10], food, cosmetics, herbals or for quick analytical support in the organic chemical synthesis lab.

6 Bruker Daltonics is continually improving its products and reserves the right to change specifications without notice. Bruker Daltonics , # Acknowledgements We thank Michael Schulz, Merck KGaA, Darmstadt, Germany, for providing various phases and plate formats for protocol optimization. References [1] B. Fuchs, J. Schiller, R. Süß, M. Schürenberg and D. Suckau 2007 A direct and simple method of coupling matrix-assisted laser desorption and ionization time-of-flight mass spectrometry (MALDI-TOF MS) to thin-layer chromatography (TLC) for the analysis of phospholipids from egg yolk Anal. Bioanal. Chem. 389: [2] J. Schiller, B. Fuchs, R. Süß, M. Zscharnack, A. Bader, P. Müller, M. Schürenberg, M. Becker and D. Suckau 2008 Analysis of stem cell lipids by offline TLC-MALDI-TOF MS Anal. Bioanal. Chem. 392: [3] B. Fuchs, J. Schiller, R. Süß, A. Nimptsch, M. Schürenberg and D. Suckau 2008 Capabilities and drawbacks of combined matrix-assisted laser desorption and ionization time-of-flight mass spectrometry (MALDI-TOF MS) and high-performance thin-layer chromatography (TLC): Analysis of egg yolk lipids JPC J. Planar Chromatogr. (in press). [4] Bruker Daltonics Technical Note 18, A New Matrix Application Device for MALDI Tissue Imaging [5] J. Schiller, R. Süß, J. Arnhold, B. Fuchs, J. Leßig, M. Müller, M. Petkovic, H. Spalteholz, O. Zschörnig and K. Arnold 2004 Matrix-assisted laser desorption and ionization time-of-flight (MALDI-TOF) mass spectrometry in lipid and phospholipid research Prog. Lipid Res. 43, [6] J. Schiller, R. Süß, B. Fuchs, M. Müller, M. Petkovic, O. Zschörnig and H. Waschipky 2007 The suitability of different DHB isomers as matrices for the MALDI-TOF MS analysis of phospholipids: which isomer for what purpose? Eur. Biophys. J. 36, [7] Fuchs, B., Nimptsch, A., Süß, R., Schiller, J Analysis of Brain Lipids by Directly Coupled Matrix-Assisted Laser Desorption Ionization Time-of-Flight Mass Spectrometry and High-Performance Thin-Layer Chromatography J. AOAC Int. 91: [8] Fuchs, B., Süß, R., Nimptsch, A., Schiller, J. 200 Matrix-assisted laser desorption and ionization time-of-flight mass spectrometry (MALDI-TOF MS) directly combined with thin-layer chromatography (TLC) - A review of the current state. CHROMATOGRAPHIA. [9] Franz-Josef Mayer-Posner, Jochen Franzen 2002 United States Patent TLC/MALDI carrier plate and method for using the same. [10] Z. Zhang, J. Xie, F. Zhang, and R.J. Linhardt R.J Thin-layer chromatography for the analysis of glycosaminoglycan oligosaccharides. Anal. Biochem. 371, Authors Martin Schürenberg 1, Detlev Suckau 1, Beate Fuchs 2 and Jürgen Schiller 2 1 Bruker Daltonics, Bremen, Germany 2 Inst. Med. Physics & Biophysics, Univ. Leipzig, Germany For research use only. Not for use in diagnostic procedures. Keywords TLC-MALDI lipids Instrumentation & Software autoflex TOF/TOF ultraflex TOF/TOF ImagePrep flexsoftware 1.2 TLC-MALDI Adapter Target # Bruker Daltonik GmbH Bremen Germany Phone +49 (421) Fax +49 (421) sales@bdal.de Bruker Daltonics Inc. Billerica, MA USA Phone +1 (978) Fax +1 (978) ms-sales@bdal.com