NOVEL MATRIX SUBLIMATION DEVICE FOR MALDI MASS

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NOVEL MATRIX SUBLIMATION DEVICE FOR MALDI MASS SPECTROMETRY IMAGING Roberto Fernandez, Jone Garate, Lucía Martín-Saiz, Igor Galetich, and Jose A. Fernandez Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.8b04765 • Publication Date (Web): 05 Dec 2018 Downloaded from http://pubs.acs.org on December 6, 2018

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Analytical Chemistry

NOVEL MATRIX SUBLIMATION DEVICE FOR MALDI MASS SPECTROMETRY IMAGING Roberto Fernández, Jone Garate, Lucia Martín-Saiz, Igor Galetich and José A. Fernández * Department of Physical Chemistry, Faculty of Science and Technology, University of the Basque Country (UPV/EHU), Barrio Sarriena S/N, Leioa 48940, Spain. ABSTRACT: Sublimation is a widely used method for matrix deposition in imaging mass spectrometry experiments. Still, most of the time standard glass sublimators are used for this purpose, which do not enable optimal matrix deposition reproducibility, compromising inter-experiment comparison of the results. Here we present an in-house designed stainless steel sublimator in which the parameters that have the strongest influence over matrix deposition reproducibility can be easily monitored. Using sections of human colon biopsies we demonstrate the capabilities of this new prototype.

One of the key aspects of MALDI-IMS (matrix assisted laser desorption/ionization - imaging mass spectrometry) is matrix deposition.1, 2 Aside from using the appropriate substance for the sample to be analyzed, it is necessary to create a uniform film, with the optimal amount of matrix per surface area. The two most popular methods in the literature to achieve a good deposition are matrix spraying and matrix sublimation,3-5 although several other alternatives also exist.6-8 The former has the advantage of inducing a more efficient analyte extraction, increasing signal intensity at the expenses of spatial resolution. This is the method of choice for proteins, where signal intensity is always an issue.9, 10 However, in the case of lipids, where signal intensity is usually stronger, matrix sublimation is very popular, as it avoids using solvents that could favor lipid migration. Nevertheless, recent studies have demonstrated that even with sublimation, the matrix induces lipid migration and extraction. 11 With this method, spatial resolutions of a few micron have been reported.3, 12-15 Still, this method may be used for proteins, if it is followed by a re-crystallization stage, although, once more, it implies a limitation in the highest spatial resolution achievable.6, 16 Sample preparation and matrix deposition should be considered as key steps to obtain good quality images with high spatial resolution.17 Regarding matrix deposition, reproducibility is important to obtain comparable results from different samples. Usually, matrix deposition by sublimation is performed using the standard glass sublimators available from different vendors, sometimes with home-made modifications. Some of them are very sophisticated and lead to obtaining images that accurately reproduce the histology of the tissue with excellent spatial resolution and S/N ratio.13, 18 However, in some occasions such devices do not enable an optimal reproducibility. For example, the surface of the cold finger is usually irregular and therefore, the temperature of the sample varies between experiments, as the contact between the sample holder and the cold finger also changes. Matrix heating is also problematic, as the contact between the glass recipient and the heater is far from being perfect, although this may be in part solved by using sand baths.3 Vacuum control is also a problem,

due to the rudimentary rubber o-rings used to seal the sublimator and the vacuum connections, which are usually made with rubber tubing. Such tubing requires of very careful handling, otherwise the glass connector may break. Inserting a proper vacuum meter to monitor the pressure inside the sublimator is not simple, and it is usually positioned close to the mechanical pump, instead of directly in the sublimator container.13 To increase matrix deposition reproducibility we present here an in-house designed, fully controlled stainless steel (SS) sublimator. To demonstrate the capabilities of this new device, we will present the optimization curves showing that a reproducibility  5% was achieved over glass slides. We will also present some images of human colon recorded at 25 m pixel size, obtained using sections prepared alternatively with the glass and SS sublimators, demonstrating that the latter enables achieving at least similar spatial resolution, with a better reproducibility in the number of species detected and in the amount of matrix deposited, at a significantly reduced sample preparation time. EXPERIMENTAL SECTION Samples Ethics Research Committee of the Balearic Islands specifically approved the sample collection for this study (#IB 2118/13 PI). Patients undergoing colonoscopy according to the clinical criteria were invited to participate in the study. Informed consent was obtained in written form from each patient before performing the colonoscopy. Endoscopic biopsies were resected using a Radial Jaw Standard Capacity Biopsy Forceps (Radial Jaw™ 4, Boston Scientific, USA) and immediately snapped frozen in liquid nitrogen and saved at −80 °C until sample preparation. Sections of ~10 μm thickness were prepared using no cryoprotective substances or embedding material in cryostat (Leica CM3050S) at −20 °C and placed on plain glass microscope slides. A consecutive section was stained with hematoxylin and eosin (Sigma–Aldrich, Madrid, Spain) for structure identification. Sample preparation for MALDI-IMS

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Matrix deposition for MALDI-IMS was performed using the glass sublimator and the new SS sublimator and two matrices: 2-mercaptobezonthiazole (MBT)19 for positive-ion mode and 1,5-diaminonaphtalene (DAN)3 for negative-ion mode (both 97%, Sigma-Aldrich Chemie, Steinheim, Germany). Detailed description of the protocol for matrix deposition using the glass sublimator may be found elsewhere.12 Briefly, an Ace Glass 8023 standard sublimator (Vineland, NJ, USA) was filled with the appropriate matrix substance and the glass slide containing the tissue section was attached to the bottom of the cold finger. Then, the sublimator was evacuated with the aid of a mechanical pump (Pfeiffer Vacuum, model DUO 2.5, Germany) and placed on top of the hot plate that was already set at a temperature of 120 ºC. The sublimation process was carried out with the vacuum on during 8 min both for MBT and DAN matrices. Under such conditions, a uniform film of matrix of  0.1-0.2 mg/cm2 was obtained, which is the optimal amount of matrix for MALDI-IMS experiments, according to our experience.12 Similar matrix densities have also been reported by other authors.11 The final amount of matrix was determined using an analytical balance (Mettler Toledo XP105DR, Switzerland).

Figure 1. (A) Picture of the prototype. (B, C) Schematics of the chamber of the stainless steel sublimator. Additional detailed drawings and pictures may be found in Supporting Information Figs. S1 and S2.

Matrix deposition with the stainless steel sublimator followed a slightly different protocol. Sample temperature was set at 5 ºC and controlled during the whole process (see supplemental material). Vacuum was also monitored precisely, allowing us to start the sublimation when the vacuum pressure reached 0.4 mbar. Although vacuum pump was operating during the whole procedure, vacuum pressure changed from 0.6 mbar when sublimation commenced down to 0.2 mbar by the end of the process. Optimal deposition time was 10 min for MBT and either 8 min or 12 min for DAN, depending on the heating temperature (see below). The times reported are from starting the heater until it was turned off. Optical images of the matrix layers obtained were recorded using a scanning electron microscope (JEOL JSM-6400,

Japan). Instrument parameters were set as follows: U = 20 kV, I = 10-11 - 10-10 A. As the sample were non-conductive they were covered with a 20 nm Au conducting layer to remove the charge and shield the incident beam from the charge accumulated in the bulk material. MALDI Imaging Mass Spectrometry MALDI imaging of tissue samples was performed on an LTQ-Orbitrap XL (Thermo Fisher Scientific, Waltham, MA, USA) instrument, equipped with an N2 laser (LTB, Berlin, model MNL 100, 100 μJ max power, elliptical spot, 60 Hz repetition rate).12 Acquisitions was made with a pixel size of 25 μm in oversampling mode. Mass resolution of 60000 at m/z = 400 was used to record the data on a 550-1200 Da mass window, in negative-ion mode. Laser energy was set in the 2030 µJ interval for the experiments to obtain an optimal signalto-noise ratio (S/N). Instrument calibration was performed once a week using the standard calibration mixture (ProteoMass™ MALDI Calibration Kit for LTQ XL™ and LTQ Hybride, Sigma-Aldrich, MA, USA). Data were acquired using Xcalibur software, version 2.0.7 (Thermo Fisher Scientific, Waltham, MA, USA). Image data were visualized and analyzed using MSIAnalyst, version 2.0 (NorayBioinformatics S.L, Spain) and in-house software built in Mathlab version R2017a (MathWorks, Natick, MA, USA). Mass accuracy obtained in the experiments was typically  10 ppm. Further details may be found in the Supporting Information. RESULTS AND DISCUSSION Stainless steel sublimation prototype Figure 1 shows the picture and the schematics of the new sublimation apparatus. The main advantages of this sublimator is that it offers a better control over the main variables that influence the deposition of the matrix: vacuum inside the sublimation chamber, temperature of the sample, sublimation temperature and deposition time, thanks to several sensors incorporated in the device. It is also significantly faster than the glass sublimator we used before (see below). To control the sample cooling, the sublimator was equipped with a Peltier cell that maintains the sample at 5 ºC during the process. In principle, it could be possible to reach lower temperatures, but then heat dissipation becomes a major issue. The Peltier cell is attached to a copper block that helps dissipating the heat produced during the cooling (see picture in the supplemental material). This block is refrigerated with the aid of a heat dissipater cooled by a fan. The temperature of the sample is monitored in real time using a controller and a thermocouple that reeds the temperature on the copper holder. Vacuum is also monitored in real time with the aid of a digital vacuum gauge controller. This allows one to turn the matrix heater on only once certain vacuum has been reached, reducing the variability between samples. The best results were obtained when vacuum reached 0.4 mbar, both in the case of DAN and MBT, although this optimal pressure may depend on the pump capacity and the temperature applied to the matrix holder. Matrix warming showed to be a critical parameter. A fine control over the temperature was achieved using a digital controller and a thermocouple attached to the matrix compartment. In this way, ±1 ºC control was achieved. Another parameter whose impact in the sublimation process may not be


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Analytical Chemistry evident is the geometry of the receptacle for the matrix (Figure S2). How the matrix is deposited in this receptacle is also important. For example, if the matrix is spread covering a large surface, sublimation proceeds faster than if the matrix is confined to a smaller area. Also, a different contact with the metal holder has some influence on the sublimation speed. Several holder geometries were tested and the best results were achieved with a cylindrical matrix tray with a receptacle of 1 cm in the center. The matrix substance was deposited on this receptacle as powder (see supplemental material). Size of matrix crystals may also have some influence on the sublimation process, although that variable was not explored.

Figure 2. Influence of the temperature (a, b) and deposition time (c, d) on the amount of MBT and DAN deposited by sublimation on a glass slide. See also Figure S4 of the supporting information.

Optimization of the sublimation parameters To determine the influence of all the parameters identified as important for the reproducibility of the amount of matrix deposited, 434 sublimation experiments were carried out, although only the results for 200 of such experiments are reported here, as the rest correspond to the stage at which other type of variables were explored. According to our previous studies, the optimal amount of matrix is  0.1-0.2 mg/cm2. Similar densities have been also reported by other authors.11 The exact amount depends on the type of tissue (fatty tissues require of more matrix) and the acquisition time, when a nonatmospheric pressure source is used: longer acquisition times require of thicker matrix layers as the matrix sublimates under vacuum conditions.14 Plain glass slides (75 mm  25 mm) were used for the test. Caution must be taken to position the glass slides always in the same position relative to the matrix holder, as according to our experience, this is another parameter that may compromise matrix deposition reproducibility. Heating temperature vs Deposition time There is a compromise between heating temperature and deposition time to deposit an optimal amount of matrix, although not all the combinations lead to similar reproducibility. Figure 2 shows the variation of the amount of matrix deposited as a function of the temperature for a deposition time of 8 minutes (MBT) and 12 minutes (DAN), which are the deposition times that resulted in better reproducibility. Such reproducibility changes for MBT along the interval of temperatures explored. Apparently, there is a threshold at 120 ºC: above such temperature, sublimation proceeds faster. Conversely, DAN deposition is more reproducible and the amount of matrix is approximately

proportional to the heating temperature in the whole interval explored. The amount of matrix deposited with time for a given temperature increases smoothly, both for MBT and DAN, although, once again, deposition of DAN is more reproducible, being close to 5 % for some conditions. Similar experiments were carried out with the glass sublimator, for two sets of conditions (Figure S4): 115º and 8 and 12 min, obtaining matrix densities of 0.313 ± 0.035 mg/cm2 and 0.122 ± 0.037 mg/cm2 respectively. Comparison with the values obtained with the new device, 0,083 ± 0.0087 and 0.09 ± 0.004 mg/cm2 demonstrate the advantage of the new device. The advantage is even clearer is the best conditions for the SS sublimator are used (120º and 8/12 min): 0.125 ± 0.007 and 0.115 ± 0.009 mg/cm2, The deposition times reported in this work for the SS sublimator were taken from the instant were the heating system was turned on, until it was turned off, and therefore it includes the warming stage, although due to the speed of the heating system, reaching the nominal temperature takes less than 1 min. In this respect, the SS sublimator is significantly faster than our old glass sublimator. The total sample preparation for deposition of DAN using a sublimation time of 12 min takes 110 min with the glass sublimator and only 18 min with the SS sublimator. This may also be a source of variability in the case of the glass sublimator: while with the SS prototype start and stop sublimation times are well defined, they are not that clear in the case of the glass sublimator, as it takes some time to accommodate the system over the heating device, introducing variations in the final amount of matrix deposited. Size of the crystals formed during deposition While reproducibility of the amount of matrix deposited is an important factor for multi-experiment studies, the size of the crystals formed over the tissue is a relevant parameter to achieve high spatial resolution.20 We reported in previous works small crystal size ( 1 µm) formation both for DAN and MBT using a glass sublimator.12, 21 We tested also for the new stainless steel design if the same crystal size was achieved,

using scanning electron microscopy. Figure 3. SEM pictures of the MBT (a) and DAN (b) matrices deposited over plain glass slides and over mouse heart tissue sections, using the stainless steel (SS) and the standard glass sublimators. See also Figures S5 and S6 of the supporting information.

Fig. 3 shows examples of matrix deposition over plain glass slides and over a section of heart tissue, both for DAN and MBT. Although deposition over plain glass is substantially more homogeneous, a small crystal size was achieved with both sublimators. Apparently, differences in the experimental conditions between the two devices led to differences in the



morphology of the crystals in the case of MBT: they are flat crystals in the case of the glass sublimator and elongated prismatic crystals when deposition was carried out with the new prototype. In any case, slightly smaller crystal were formed for both matrices when the new prototype was used. Inspection of the crystals in the microgarphs show an average crystal size for DAN of 1.42 ± 0.11 µm when the glass sublimator was used and of 0.76 ± 0.04 µm with the stainless steel sublimator. Comparison in the case of MBT is not easy, due to the change on crystal morphology and the larger size dispersion. Measuring the width of the biggest and smallest crystals in the micrograph, we obtain a size interval of 0.6 – 1.3 µm for the stainless steel sublimator and of 0.7 – 1.6 µm for the glass sublimator. Comparison between images obtained with both sublimators. As a final test of the performance of the developed stainless steel sublimator, sections of biopsies of human colon were coated with DAN using both sublimators and images of lipid distribution at 25 m/pixel were recorded, in negative-ion mode (Figure 4 and S7). The images of lipid distribution show similar resolution. The pixel size used allowed us to easily discriminate the colonic crypts (blue segments in Figure 4) from the lamina propria (white segments), in good agreement with previous publications.21

of segments to 3 (the segment containing the pixels outside the tissue was omitted). The tissue architecture is easily recognize in both experiments; Images obtained in negative-ion mode using a pixel size of 25 m. Additional experiments may be found in the supporting information (Figure S7). Cryp: colonic crypts.

To estimate the relative signal intensity and reproducibility of the results with both sublimators we run several tests. Figure 5 shows the distribution of the TIC along all the spectra recorded over each section. The spectra with lower TIC correspond to those pixels outside the tissue and therefore, they only contain matrix peaks. The number of the spectra recorded outside the tissue changes between experiments and in the case of the middle panel, they account for a large number of spectra.

Figure 5. Comparison between the TIC of the spectra recorded directly from the samples covered using the glass sublimator (blue) and the SS sublimator (orange). Bimodal distributions were obtained with a maximum at lower intensities corresponding to the spectra recorded outside the tissue and that only contain matrix peaks. The insets are zooms over part of the graphics. In all cases, the spectra recorded over the sections covered with the SS sublimator present higher intensities. All the experiments carried out in negative-ion mode.

Figure 4. (A, B) Optical images of two sections of biopsies of human colon, consecutive to those used to record lipid distribution using MALDI-IMS. The section in A was covered with DAN using the glass sublimator and the section in B was covered using the SS sublimator. (C, D) Images of the distribution of selected lipid species over section A; (E, F) distribution of the same species over section B; (G, H) segmentation images obtained setting the number

Regarding the intensity of the spectra recorded over tissue, in all three pairs of images, those sections covered using the SS sublimator (orange), rendered stronger signals than those covered with the glass sublimator (blue). The stronger signal resulted in a better reproducibility of the spectra. Figure S9 (ESI) shows the number of species whose standard deviation < 50% of its intensity. This parameter gives an estimation of the reproducibility of the species detected, as it only takes into account those mass channels with an inter-spectrum variation below the given threshold. Clearly, the results obtained with the SS sublimator were superior, although the reason behind such observation is not clear: in principle, the type of sublimator used should only affect to the reproducibility of the amount of matrix deposited and to the crystal size of the matrix, but one would expect to obtain similar signal intensity with both sublimators for a given amount of matrix. Perhaps the more uniform cooling of the sample during the sublimation process enabled a more uniform matrix distribution, improving the final performance of the experiment. It may also be related with the crytsal size and the migration of lipids from the tissue towards the matrix layer. In a recent publication,11 this has been shown to be a key factor in the number of species detected in MALDI-IMS experiments. CONCLUSIONS


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Analytical Chemistry We presented here an in-house designed stainless steel sublimator. Compared with our previous standard glass device, the new prototype enables to finely control the main parameters that influence reproducibility of matrix deposition. Thanks to the better control over sample temperature during sublimation, the vacuum inside the chamber and the sublimation temperature, more reproducible conditions for matrix deposition were achieved, also enabling a better reproducibility of the species detected in MALDI-IMS experiments over sections of human colon biopsies. The blue prints of the device are freely available under request.

ASSOCIATED CONTENT Supporting Information Detailed description of the experimental procedure, additional figures and drawings of the prototype, figures with the variation of the matrix deposition with the different variables identified as critical. Images of all six sections, average spectra over all six sections. The Supporting Information is available free of charge on the ACS Publications website.

AUTHOR INFORMATION Corresponding Author * Dr. José A. Fernández, Dep. Of Physical Chemistry, Faculty of Science and Technology, University of the Basque Country (UPV/EHU), Barrio Sarriena, S/N, Leioa 48940, Spain. Phone: +34946015387; Fax: +34946013500; email: [email protected].

ACKNOWLEDGMENT We would like to thank Dr. Barceló-Coblijn for the donation of the samples of tissue sections. This study was supported by the Spanish Ministry of Economy and Competitiveness, grant RTC-2015-36931 (RETOS program), cofinanced by Fondo Europeo de Desarrollo regional (FEDER) EU, and the Basque Government (IT971-16). Technical and human support provided by the Servicio de Lipidómica of the SGIKER (UPV/EHU, MINECO, GV/EJ, ESF) is gratefully acknowledge. JG thanks the UPV/EHU for a predoctoral fellowship. LM thanks the MINECO for a predoctoral fellowship.

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6 Baker, T. C.; Han, J.; Borchers, C. H. Recent advancements in matrix-assisted laser desorption/ionization mass spectrometry imaging, Current Opinion in Biotechnology. 2017, 43, 62-69. 7 Wu, Q.; Comi, T. J.; Li, B.; Rubakhin, S. S.; Sweedler, J. V. On-Tissue Derivatization via Electrospray Deposition for MatrixAssisted Laser Desorption/Ionization Mass Spectrometry Imaging of Endogenous Fatty Acids in Rat Brain Tissues, Anal. Chem. 2016, 88, 5988-5995. 8 Muller, L.; Kailas, A.; Jackson, S. N.; Roux, A.; Barbacci, D. C.; Albert Schultz, J.; Balaban, C. D.; Woods, A. S. Lipid imaging within the normal rat kidney using silver nanoparticles by matrix-assisted laser desorption/ionization mass spectrometry, Kidney Int. 2015, 88, 186-192. 9 Angel, P. M.; Baldwin, H. S.; Gottlieb Sen, D.; Su, Y. R.; Mayer, J. E.; Bichell, D.; Drake, R. R. Advances in MALDI imaging mass spectrometry of proteins in cardiac tissue, including the heart valve, Biochimica et Biophysica Acta (BBA) - Proteins and Proteomics. 2017, 1865, 927-935. 10 Schwamborn, K.; Kriegsmann, M.; Weichert, W. MALDI imaging mass spectrometry — From bench to bedside, Biochimica et Biophysica Acta (BBA) - Proteins and Proteomics. 2017, 1865, 776-783. 11 Van Nuffel, S.; Elie, N.; Yang, E.; Nouet, J.; Touboul, D.; Chaurand, P.; Brunelle, A. Insights into the MALDI Process after Matrix Deposition by Sublimation Using 3D ToF-SIMS Imaging, Anal. Chem. 2018, 90, 1907-1914. 12 Garate, J.; Fernandez, R.; Lage, S.; Bestard-Escalas, J.; Lopez, D. H.; Reigada, R.; Khorrami, S.; Ginard, D.; Reyes, J.; Amengual, I.; Barcelo-Coblijn, G.; Fernandez, J. A. Imaging mass spectrometry increased resolution using 2-mercaptobenzothiazole and 2,5-diaminonaphtalene matrices: application to lipid distribution in human colon, Anal. Bioanal Chem. 2015, 13 Chaurand, P.; Cornett, D. S.; Angel, P. M.; Caprioli, R. M. From whole-body sections down to cellular level, multiscale imaging of phospholipids by MALDI mass spectrometry, Molecular & Cellular Proteomics. 2011, 10 14 Kompauer, M.; Heiles, S.; Spengler, B. Autofocusing MALDI mass spectrometry imaging of tissue sections and 3D chemical topography of nonflat surfaces, Nat Meth. 2017, 14, 9096. 15 Soltwisch, J.; Kettling, H.; Vens-Cappell, S.; Wiegelmann, M.; Müthing, J.; Dreisewerd, K. Mass spectrometry imaging with laser-induced postionization, Science. 2015, 348, 211. 16 Yang, J. H.; Caprioli, R. M. Matrix Sublimation/Recrystallization for Imaging Proteins by Mass Spectrometry at High Spatial Resolution, Anal. Chem. 2011, 83, 5728-5734. 17 Buchberger, A. R.; DeLaney, K.; Johnson, J.; Li, L. Mass Spectrometry Imaging: A Review of Emerging Advancements and Future Insights, Anal. Chem. 2018, 90, 240-265. 18 Caughlin, S.; Park, D.,Hee; Yeung, K.,K.-C.; Cechetto, D.,F.; Whitehead, S.,N. Sublimation of DAN Matrix for the Detection and Visualization of Gangliosides in Rat Brain Tissue for MALDI Imaging Mass Spectrometry, JoVE. 2017, e55254. 19 Astigarraga, E.; Barreda-Gomez, G.; Lombardero, L.; Fresnedo, O.; Castano, F.; Giralt, M. T.; Ochoa, B.; RodriguezPuertas, R.; Fernández, J. A. Profiling and Imaging of Lipids on Brain and Liver Tissue by Matrix-Assisted Laser Desorption/Ionization Mass Spectrometry Using 2Mercaptobenzothiazole as a Matrix, Anal. Chem. 2008, 80, 9105– 9114.. 20 Jurchen, J. C.; Rubakhin, S. S.; Sweedler, J. V. MALDI-MS Imaging of Features Smaller than the Size of the Laser Beam, J. Am. Soc. Mass Spectrom. 2005, 16, 1654-1659. 21 Bestard-Escalas, J.; Garate, J.; Maimó-Barceló, A.; Fernández, R.; Lopez, D. H.; Lage, S.; Reigada, R.; Khorrami, S.; Ginard, D.; Reyes, J.; Amengual, I.; Fernández, J. A.; Barceló-



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Analytical Chemistry

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