Imaging of Organic Samples with Megaelectron Volt Time-of-Flight Secondary Ion Mass Spectrometry Capillary Microprobe

Abstract

Time-of-flight Secondary Ion Mass Spectrometry (TOF SIMS) with MeV primary ions offers a fine balance between secondary ion yield for molecules in the mass range from 100 to 1000 Da and beam spot size, both of which are critical for imaging applications of organic samples. Using conically shaped glass capillaries with an exit diameter of a few micrometers, a high energy heavy primary beam can be collimated to less than 10 μm. In this work, imaging capabilities of such a setup are presented for some organic samples (leucine-evaporated mesh, fly wing section, ink deposited on paper). Lateral resolution measurement and molecular distributions of selected mass peaks are shown. The negative influence of the beam halo, an unavoidable characteristic of primary beam collimation with a conical capillary, is also discussed. A new start trigger for TOF measurements based on the detection of secondary electrons released by the primary ion is presented. This method is applicable for a continuous primary ion beam, and for thick targets that are not transparent to the primary ion beam. The solution preserves the good mass resolution of the thin target setup, where the detection of primary ions with a PIN diode is used for a start trigger, reduces the background, and enables a wide range of samples to be analyzed.

Chemical Imaging of Organic Materials by MeV SIMS Using a Continuous Collimated Ion Beam

“…As can be seen in Figure , there is virtually no beam halo after collimation (up to film radiation contrast and the optical microscope sensitivity), which is a significant improvement compared to the beam profile after collimation through a glass capillary . Only the central ion beam spot is visible.…”

“…The same sample was imaged by ions collimated through a glass capillary (results are presented in ref ). Comparing these two measurements, it is immediately apparent that the image produced with the ion beam collimated by a simple aperture is clearer with better contrast (∼20 times higher signal-to-noise ratio in the image).…”

“…The desorbed molecules are accelerated toward a spectrometer by a voltage difference of +3 kV applied to a sample holder (the TOF extractor is always grounded). The triggering system for the electron START (for TOF measurement) is similar to the system presented in ref . Initially, the extraction voltage is at zero potential to ensure that the generated secondary electrons are not attracted toward the sample holder.…”

“…Moreover, no active elements are used for ion focusing, so the size of the beam spot should be virtually independent of the type and energy of the ions. Regardless of the good results published in ref , two main problems have been identified for MeV SIMS imaging based on the collimated beam with the glass capillary: Despite a good lateral resolution (between 3 and 20 μm), almost 50% of the beam ends in a beam halo blurring 2D chemical images and raising a minimum detection limit. The START signal for TOF measurement is generated by the secondary electrons emitted from the sample surface, which can be problematic for samples with low secondary electron emission (consequently low detection efficiency). Even more, variation in the secondary electron emission efficiency over the sample surface leads to unrealistic variations in the 2D chemical images since each image pixel is normalized to the incoming number of primary ions. …”

“…At the same time, we have developed the MeV SIMS technique in the so-called “transmission” mode, which allowed us to perform two-dimensional (2D) mapping of lipids within a single cell with a spatial resolution of 400 nm . In addition to applications in biology, MeV SIMS has also been used to identify synthetic organic pigments in modern paint materials − and in forensics to determine the deposition order of different inks at the crossing lines. − Recently, a new MeV SIMS system for chemical–biological mapping has been developed based on the collimation of the beam with a glass capillary. , The great advantage of the MeV SIMS setup presented in ref is the ability to accept a wide range of energies and masses of primary ions, especially high-energy heavy ions that have high electronic stopping power. According to the work of Brajković et al, these ions should provide the largest yield of intact molecules, which is crucial for SIMS imaging.…”