Evidence of near-field laser ablation inductively coupled plasma mass spectrometry (NF-LA-ICP-MS) at nanometre scale for elemental and isotopic analysis on gels and biological samples

Abstract

Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) has been established as a powerful surface analytical method for local elemental analysis on metallic, ceramic, geological or biological sample surfaces. Here we show a new way of nanometre scale analysis of elements on sample surfaces by near-field LA-ICP-MS (NF-LA-ICP-MS). This technique uses the near-field enhancement effect on the tip of a thin silver needle in a laser beam (Nd:YAG laser, wavelength 532 nm) on the sample surface. The thin silver needle was etched electrolytically in an electrochemical cell using a droplet of citric acid as electrolyte. For nanolocal analysis by NF-LA-ICP-MS on soft matter (e.g., on 2-D gels and biological samples) a small volume transparent laser ablation chamber was constructed and coupled to a double-focusing sector field inductively coupled plasma mass spectrometer (ICP-MS). A small amount of soft sample material is ablated at atmospheric pressure by a single laser shot in the near-field of the silver tip in the defocused Nd:YAG laser beam. The ablated material is transported with argon as carrier gas into the inductively coupled plasma (ICP) ion source of the sensitive double-focusing sector field mass spectrometer with reverse Nier-Johnson geometry. By single-shot analysis on 2-D gels and biological surfaces doped with uranium in the mg g À1 range using NF-LA-ICP-MS an enhancement of ion intensities of transient signals in comparison with the background signal of up to factor 60 was observed. In gels doped with isotopically enriched 65 Cu and 67 Zn spikes by NF-LA-ICP-MS (single shot analysis) ion intensities up to the n  10 5 cps range and isotope ratios (235 U/ 238 U, 65 Cu/ 63 Cu and 67 Zn/ 64 Zn) were measured at a lateral resolution in the nanometre scale. Using the near-field effect in LA-ICP-MS, it was demonstrated that nanolocal analysis is possible in single-shot measurements of elements on biological samples and on a gel surface with spatial resolution at the hundreds of nanometres range. This first experiment on near-field LA-ICP-MS opens up a new, challenging path for future applications in nanoimaging of elements in life science, biology and medicine, e.g., for analyses of single cells, cell organelles or biological structures at nanometre range in order to detect neurodegenerative diseases, but also in material science, nanotechnologies and nanoelectronics.

Established and emerging atmospheric pressure surface sampling/ionization techniques for mass spectrometry

“…59 The concentration of copper present was calibrated using synthetic matrix-matched standards and found to be on average in the range of 3-7 µg/g. Another exciting possibility in LA-ICP/MS was illustrated by Becker's group, 60 namely, the use of the 'near-field effect' 61 to beat the wavelength limitation of the laser light to achieve laser desorption (ablation) spot sizes on the order of hundreds of nanometers. This spatial sampling resolution is sufficient for spot sampling within, and imaging, of individual cells or organelles.…”

Metallomics: Guidelines for terminology and critical evaluation of analytical chemistry approaches (IUPAC Technical Report)

“…Inhomogeneous (often layered) site-specific metal distribution in tissue sections (as demonstrated in human brain different regions) can be obtained. The resolution can be decreased to the 100-nm level using nano-secondary-ion mass spectrometry (SIMS), nanoprobe synchrotron XRF, or near-field LA-ICP-MS [32], paving the way to metal distribution imaging in single cells and cell organelles.…”

Inorganic mass spectrometry as a tool for characterisation at the nanoscale

“…the tip of a thin Ag needle) immersed in the radiation field. Becker et al [35,36] have investigated the use of NF-LA-ICP-sector field MS (SFMS) for elemental analysis at the nanometre scale in twodimensional gels, biological samples and thin Au films. The proposed NF-LA-ICP-SFMS arrangement employed thin Ag needles (d~180-250 nm) electrolytically etched, and the distance between the needle and the sample surface was controlled by measuring the tunnel current.…”