Biological cluster mass spectrometry

Abstract

This article reviews the new physics and new applications of secondary ion mass spectrometry using cluster ion probes. These probes, particularly C(60), exhibit enhanced molecular desorption with improved sensitivity owing to the unique nature of the energy-deposition process. In addition, these projectiles are capable of eroding molecular solids while retaining the molecular specificity of mass spectrometry. When the beams are microfocused to a spot on the sample, bioimaging experiments in two and three dimensions are feasible. We describe emerging theoretical models that allow the energy-deposition process to be understood on an atomic and molecular basis. Moreover, experiments on model systems are described that allow protocols for imaging on biological materials to be implemented. Finally, we present recent applications of imaging to biological tissue and single cells to illustrate the future directions of this methodology.

Figure 1

Cross section of an Au target 8.8 ps after Au₄ bombardment at 16 keV. The color code represents temperature relative to the melting temperature of Au. Green is the melting temperature, and red is twice the melting temperature. Figure taken from Reference , used by permission from the American Physical Society.

Figure 2

Summary of the erosion model parameters (a) Y_tot, (b) σ_D, (c) d, and (d) ε as a function of 40-keV C₆₀⁺ incident angle. Figure adapted from Reference , used by permission from the American Chemical Society.

Figure 3

Snapshots of the atom positions for Au₃ and C₆₀ bombardment of a 2.5-nm film of ice (red) on Ag (blue). The incident particle impinges from the left with 15 keV at an angle of 40° with respect to the surface normal. The time snapshots are at 1, 3, and 5 ps for the frames from top to bottom, respectively. Figure adapted from Reference , used by permission from the American Physical Society.

Figure 4

Molecular ion signal (M-H)⁻ at m/z 146 representing a glutamate film on Si as a function of sputtering time using 3-keV C_n⁻ projectile ions. Figure adapted from Reference , used by permission from the American Vacuum Society.

Figure 5

Molecular depth profile of (a) a 300-nm trehalose film doped with GGYR peptide, (b) a 300-nm cholesterol film, (c) a multilayer stack of alternating Langmuir-Blodgett films of approximately 50-nm width, and (d) a sequence of Irganox 3114 delta layers embedded into an Irganox 1010 matrix on Si measured using C₆₀⁺ projectile ions for sputter erosion and data acquisition.

Figure 6

3D depth profiling of a peptide-doped 300-nm thin film of trehalose on Si, patterned using a Ga⁺ ion probe. (Left panel) Atomic force microscopy (AFM) picture of the patterned letters whose troughs are 9.6 μm in width and 220 nm in depth. (Middle panel) A 3D secondary ion mass spectrometry (SIMS) image acquired by directly stacking 100 2D images acquired during erosion with C₆₀. The Ga signal at m/z 69 is red, the peptide signal at m/z 452 is blue, and the Si signal at m/z is green. The information becomes intertwined as the film is removed. (Right panel) The same data with the depth scale corrected using information obtained largely from the AFM measurements. The depth resolution in this case approaches 3 nm. Figure taken from Reference , used by permission from the American Chemical Society.

Figure 7

Secondary ion mass spectrometry (SIMS) images of steatotic vesicles within a liver tissue slice. (a) Vitamin E signal at m/z 429. (b) The cholesterol signal at m/z 369. (c) The sum of all the triacylglycerides near m/z 862. (d) The sum of C₁₆ fatty acid carboxylate ions. The relative signal intensity is represented by the color bar to the right of the images, with yellow representing the most intense and cyan blue representing the least intense. Figure adapted from Reference , used by permission from the American Chemical Society.

Figure 8

Secondary ion mass spectrometry (SIMS) images of (a) a patterned cholesterol film on Si created using physical vapor deposition, (b) film cooled in vacuum (−196°C) with H₂O redeposition, and (c) after etching with a dose of 10¹³ C₆₀⁺ ions cm⁻². The m/z 369 signal is shown in green (cholesterol), the m/z 18 signal in blue (H₂O), and the m/z 28 signal in red (Si). (d) Line scans across the film features reveal that the distribution of cholesterol on the surface is maintained when C₆₀⁺ is used to remove the water overlayer versus when the surface is not cleaned with the C₆₀⁺ nanotome. Figure adapted from Reference , used by permission from the American Society of Mass Spectrometry.

Figure 9

Isosurface rendering of thyroid tumor cells acquired using a Bi₃⁺ source for spectral acquisition and a C₆₀⁺ source for erosion. The m/z 23 signal is shown in blue (Na), the m/z 39 signal in green (K), the m/z 184 signal in red (phosphocholine head group), and the m/z 86 signal in yellow (phosphocholine head-group fragment). (a) A single cell with a forward image tilt of 45°. (b) A cell cross section with a forward image tilt of 15°. Figure taken from Reference , used by permission from Wiley Interscience.

Figure 10

3D images of freeze-dried oocyte, showing changes in (a) signal summed over the m/z range 815–960, representing mainly membrane lipid distributions, and (b) the cholesterol peak at m/z 369. Color scale is normalized for total counts per pixel for each variable (m/z range). The imaged species are localized differently, not only along the lateral dimensions, but also along the depth. Figure adapted from Reference , used by permission from the American Chemical Society.