Laser-based mass spectrometry techniques allow one to spatially resolve and analyze the molecular, elemental, or isotopic signatures in a solid at a lateral spatial resolution dictated by the laser’s spot size. Typically, UV/Vis/IR wavelength lasers are used with mass spectrometers to map signatures in a solid, but their lateral spatial resolution is limited to ≥1 µm. Short-wavelength lasers in the EUV regime bring new opportunities to laser-based mass spectrometry methods by realizing nanoscale (≤100 nm) ablation due to their high absorptivity in materials (i.e., 10’s of nanometers) as well as their ability to efficiently ionize the removed material in their laser-created plasmas. In this talk, we will discuss how we are using an EUV laser, operating at a wavelength of 46.9 nm, for material ablation and ionization with a time-of-flight mass spectrometer to map isotopic information down to the nanoscale in nuclear and geologic materials. We will also discuss how we are working towards expanding the use of the EUV laser by connecting it to a more sensitive mass spectrometer so that nanoscale analyses can be realized with increased precision and accuracy.
We describe the methods for automating the workflow for rapidly measuring and producing elemental maps of large-area samples using the Submicron Resolution X-ray Spectroscopy Beamline (SRX) at the National Synchrotron Light Source II, Brookhaven National Laboratory, through a novel combination of supervised (support vector machine) and unsupervised (cluster analysis) machine learning algorithms. SRX has the capability to create centimeter area full spectrum x-ray fluorescence (XRF) maps non-destructively with special detector and beam configurations. To facilitate the automation of this process, we discuss the development of the Synchrotron Network Automation Program in Python (SnapPy) software package that automates measurements such that SnapPy will control everything from beamline machine control to data acquisition and analysis. The only intervention that will need to be performed by beamline staff will be to physically install and remove samples. This will allow us to run measurements overnight or during times when beamline staff would not otherwise be available.
Extreme ultraviolet (EUV) lasers possess unique properties for ablation and ionization at the nanoscale (≤100 nm) due to their short wavelength, high absorptivity in most materials (i.e., 10’s of nanometers), and efficient photoionization in the laser-created plasmas. When coupled with a mass spectrometer, an EUV laser can be used to analyze and map chemical information in three dimensions with nanoscale spatial resolution. We have previously built an EUV time-of-flight mass spectrometer (EUV TOF) that achieved ~80 nm lateral and ~20 nm depth resolution when mapping the chemical content in organic and inorganic solids. Here, we present results from a recent study that extends EUV TOF’s high resolution capabilities to the analysis of an isotopically heterogenous uranium fuel pellet that was made by blending two isotopically distinct starting materials. We show that EUV TOF can map 235U/238U heterogeneity at the 100 nm scale, revealing micron to submicron heterogeneity. For comparison, nanoscale secondary ionization mass spectrometry (NanoSIMS) maps a similar distribution of U heterogeneity on a similar subsample at the same spatial scale. We also show that EUV TOF can measure the isotope ratio in a silver sample using single shot spectra. These results position EUV TOF as a promising technique for performing isotopic analyses at the nanoscale, finding applications in nuclear forensics, geology, and biology as well as in the semiconductor industry.
In EUV TOF MS, bright laser pulses from a compact 46.9-nm-wavelength laser [1] are focused into nanometer size spots to ablate craters a few nanometers deep on selected regions of the sample. Elemental and molecular ions in the laser-created plasma are extracted and identified by their mass-to-charge ratio (m/z) using a time-of-flight (TOF) mass spectrometer. Analysis of the spatially resolved mass spectra obtained as the sample is displaced with respect to the focused laser beam enables one to construct 3-D composition images with nanoscale resolution [2]. In this talk I will describe recent advances of extreme ultraviolet MSI that show its unique capabilities to identify low concentration of high Z elements into glass matrices, and to map isotopic ratios [3].
[1] S. Heinbuch et al, "Demonstration of a desk-top size high repetition rate soft x-ray laser," Opt. Express 13, 4050-4055 (2005).
[2] I. Kuznetsov et al, "Three dimensional nanoscale molecular imaging by extreme ultraviolet laser ablation mass spectrometry, " Nature Communications, Vol. 6, Article No. 6944(2015).
[3] T. Green, et al, “Characterization of extreme ultraviolet laser ablation mass spectrometry for actinide trace analysis and nanoscale isotopic imaging,” J. Analy. At. Spectrom. Vol. 32, 1092 (2017).
We have previously shown soft x-ray laser ablation time-of-flight mass spectrometry has the ability to detect singly ionized alanine molecules arising from the single shot ablation of a ∼50 zeptoliter volume. This superior sensitivity results from the ability to focus the 46.9 nm wavelength (26.4 eV energy per photon) laser beam to the diffraction limit, the strong absorption, and the efficient photoionization of the soft x-ray photons. In this paper we describe results on the application of soft x-ray laser mass spectrometry to elemental trace analysis in inorganic materials. Two dimensional imaging with spatial resolution of 80 nm in inorganic samples is also demonstrated.
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