Positive Ion Images Research Articles

The Seven S Criteria for Evaluating the Performance of a MALDI Mass Spectrometer for MSI

Our goal in this work is to evaluate a new combination linear/reflector MALDI-TOF instrument toward satisfying all "7S criteria" for the ideal MSI mass spectrometer. The linear analyzer satisfies all of the 7 criteria except for Specificity. The new instrument described here adds a reflector to provide up to 50,000 mass resolving power with ppm mass accuracy and with no sacrifice in speed, spatial resolution, and sensitivity demonstrated earlier for the linear MALDI-TOF. This instrument employs new laser optics that produces a 5 kHz laser beam with 2.5-25 μm diameter under computer control. The most important advance is the patented combination of laser and ion optics that provides very high efficiency for production and detection of ions generated by laser desorption using small diameter laser beams. This provides spectra with a wide dynamic range summing a relatively small number of laser shots/pixels. Rat and mouse brain tissues have been used for these initial studies. Examples of negative ion images of lipids and positive ion images from tryptic digestion of proteins are presented. These results demonstrate a very high speed for MSI. This speed is derived from a combination of high laser rate (5 kHz), fast motion of sample relative to the laser beam (20 mm/s), very high ionization efficiency (up to 50%), and the ability to acquire, process, and save spectra at a very high rate (1000/s). As a result, the speed that is possible is imposed by other limits, including the mass range, concentration of samples on the surface, and the spatial resolution required.

Imaging of lipids in rat heart by MALDI-MS with silver nanoparticles

Lipids are a major component of heart tissue and perform several important functions such as energy storage, signaling, and as building blocks of biological membranes. The heart lipidome is quite diverse consisting of glycerophospholipids such as phosphatidylcholines (PCs), phosphatidylethanolamines (PEs), phosphatidylinositols (PIs), phosphatidylglycerols (PGs), cardiolipins (CLs), and glycerolipids, mainly triacylglycerols (TAGs). In this study, mass spectrometry imaging (MSI) enabled by matrix implantation of ionized silver nanoparticles (AgNP) was used to map several classes of lipids in heart tissue. The use of AgNP matrix implantation was motivated by our previous work showing that implantation doses of only 10(14)/cm(2) of 2 nm gold nanoparticulates into the first 10 nm of the near surface of the tissue enabled detection of most brain lipids (including neutral lipid species such as cerebrosides) more efficiently than traditional organic MALDI matrices. Herein, a similar implantation of 500 eV AgNP(-) across the entire heart tissue section results in a quick, reproducible, solvent-free, uniform matrix concentration of 6 nm AgNP residing near the tissue surface. MALDI-MSI analysis of either positive or negative ions produce high-quality images of several heart lipid species. In negative ion mode, 24 lipid species [16 PEs, 4 PIs, 1 PG, 1 CL, 2 sphingomyelins (SMs)] were imaged. Positive ion images were also obtained from 29 lipid species (10 PCs, 5 PEs, 5 SMs, 9 TAGs) with the TAG species being heavily concentrated in vascular regions of the heart.

The adsorption and configuration of octyl hydroxamic acid on pyrochlore and calcite

XPS, adsorption density and ToF-SIMS measurements were performed on pyrochlore and calcite treated with octyl hydroxamic acid (OHA). XPS results indicated a slight shift of the binding energies of the Ca 2p electrons (0.19eV) on calcite after OHA treatment but the N1s binding energy spectrum of OHA did not change, showing a physical adsorption mechanism. On OHA-treated pyrochlore, the binding energies of both Nb 3d and Ca 2p electrons shifted (0.24 and 0.35eV, respectively), and the N1s electron binding energy peak of OHA split from 400.3eV to 400.7 and 398.5eV, representing the neutral RNHOH and the ionized RNHO− groups. The ratio of the integrated areas of the 400.7eV peak to the 398.5eV peak was 3.8 to 1. Therefore OHA possibly adsorbed on pyrochlore by both physisorption and chemisorption, with the number of OHA molecules involved in the adsorption at 3.8 to 1. The adsorption isotherm of OHA on pyrochlore showed two plateaus, with the first plateau corresponding to a vertically oriented monolayer of OHA. The second plateau was 4 times higher than the first plateau. Therefore, the adsorption isotherm was consistent with XPS results, in that the vertically oriented monolayer (first plateau) formed the chemisorbed layer, and the second plateau contained mostly physisorbed OHA. The adsorption isotherm of OHA on calcite showed a lower plateau which corresponded to horizontally oriented OHA. ToF-SIMS positive ion images on OHA treated mixtures of pyrochlore and calcite confirmed that OHA preferentially adsorbed on pyrochlore.

Relationship between MALDI IMS Intensity and Measured Quantity of Selected Phospholipids in Rat Brain Sections

MALDI IMS positive ion images of rat brain show a regional distribution of phosphocholine species that is striking in the apparent distinctiveness and reproducibility of such depictions. The interpretation of these images, specifically the relationship between MALDI IMS ion intensity and the amount of the phosphocholine (PC) species in the tissue is complicated by numerous factors, such as ion suppression, ion molecule chemistry, and effects of tissue structure. This study was designed to test the hypothesis that the intensity of PC molecular species does relate to the quantity of molecules in a tissue sample. A set of comparison studies for a limited but representative selection of cell-derived PC molecular species was carried out using LC/MS/MS to measure the amounts of these species in brain tissue extracts. There was good correlation between the MALDI IMS ion abundance of PC molecular species and the relative abundance of corresponding PC molecular species in microdissected regions analyzed by LC/MS/MS.

Time-of-flight secondary ion mass spectrometry of fatty acids in rat retina

The retina consists of many kinds of central nervous cells, and some cells contain fatty acids such as palmitic acid, stearic acid and oleic acid. Time-of-flight secondary ion mass spectrometry (TOF-SIMS) has a possibility to detect kinds and quantity of materials in relation to the cell or tissue. We applied TOF-SIMS to detect the palmitic acid, stearic acid and oleic acid in the visual cell of the rat retina. We used 4- and 18-month-old normal Wistar Kyoto rats. After pentobarbital anesthesia, the eyes were enucleated, and immediately put into liquid nitrogen without any fixation and then cut into semithin sections (10 μm) with a cryo-ultramicrotome, and laid it on a silicon wafer plate and air-dried. Ion images were detected with TOF-SIMS. Positive ion images were examined with a Ga + source at an acceleration voltage of 15 keV. The secondary ion acceleration voltage was 4.5 keV. In the 4-month-old rat, palmitic and stearic acid were detected in the photoreceptor outer segment and nuclear parts, but not in the inner segment. In the 18-month-old rat, the oleic acid is significantly decreased compared to that in the 4-month-old rat. TOF-SIMS is a useful tool to detect the changes of fatty acids corresponding to changes of physiological conditions in relation to the histological features.

Comparison of NFIM and FIM of polymer layers with tetracyanoethylene and benzene as image gases

Field ion microscopy (FIM) with positive ions and negative ions (NFIM) is compared for polymer layers of two structures grown by field induced anionic polymerization of tetracyanoethylene (TCNE) on a metal tip. The positive and negative ion images of the polymer layer mainly consisting of linear polymer chains are complementary, i.e. positive ion emission is preferentially observed in dark areas of the corresponding negative ion images. For the layer with a crosslinked polymer structure the bright rings of negative ion images are not observed in positive ion images. The dissimilarities between NFIM with TCNE and FIM with benzene and TCNE are attributed to differences in the excess electron and electron hole conductivities of the polymer layers exposed to the image gases.