In secondary ion mass spectrometry, the molecular environment from which a sample is analyzed can influence ion formation, affecting the resulting data. With the recent surge in studies involving examination of biological specimens, a better understanding of constituents commonly found in biological matrixes is necessary. In this article we discuss results from an investigation directed at understanding the role of salts doped as alkali chlorides in a model biological environment, arginine. The data show that addition of salt to the model system causes ion suppression of all the major mass spectral peaks attributed to arginine, with KCl having the largest suppression effect. Potential causes for the suppression effects are briefly discussed in relation to collected data. These theories include sample degradation, formation of salt adduct peaks, and anion neutralization. Investigation of the arginine salt data in comparison with data collected from pure salt systems indicates that suppression of the positive secondary ions is likely caused by a neutralization process involving the salt counteranion, chloride. To address the suppression issue, various procedures were performed on the arginine films such as sample washing with a cleaning solution (ammonium formate, ethanol, water) and analysis of films in a frozen-hydrated state. We present data from the analysis of the frozen-hydrated samples that shows both an ion yield enhancement and a significant amelioration of the salt suppression effects when compared to the samples run under standard conditions, demonstrating that it is a helpful approach to dealing with salt suppression.

Although the benefits of decreased sample temperature for the molecular profiling of organic materials with time-of-flight secondary ion mass spectrometry (TOF-SIMS) have been established, the mechanism behind spectral changes observed at low temperature, particularly increased protonated molecular ion (M + H)(+) yields, have not been examined in detail. We have developed a procedure to investigate these effects by monitoring secondary ion yields under sustained primary ion bombardment as the sample temperature is cooled from room temperature down to 80 K. Examination of biomaterials such as an amino acid (arginine), a polypeptide (Gly-Gly-Tyr-Arg), a lipid (1,2 dipalmitoyl-sn-glycero-3 phosphatidylcholine), and a drug molecule (cyclosporine A) each provide evidence of ion yield enhancement at 80 K under either 20 keV C(60)(+) or 20 keV Au(3)(+) bombardment. For example, arginine shows a 2-fold increase in the steady-state intensity for the (M + H)(+) ion at 80 K compared to the steady state at 300 K. It is shown that there is a correlation between the yield enhancement and a reduction in the damage cross section, which for arginine under 20 keV Au(3)(+) bombardment decreases from 5.0 ± 0.4 × 10(-14) cm(2) at 300 K to 2.0 ± 0.3 × 10(-14) cm(2) at 80 K. The role of water as the facilitator for this reduction is explored through the use of H(2)O and D(2)O dosing experiments at 80 K.

Sample preparation continues to be a major challenge for secondary ion mass spectrometry studies of biological materials. Maintaining the native hydrated state of the material is important for preserving both chemical and spatial information. Here, we discuss a method which combines a sample wash and dry protocol discussed by Berman et al1 (1) followed by plunge freezing in liquid ethane for a frozen-hydrated analysis of mammalian cells (HeLa). This method allows for the removal of the growth media and maintains the hydrated state of the cells so that they can be prepared frozen-hydrated without the need for a freeze-fracture device. The cells, which were grown on silicon, have been successfully re-grown after the cleaning procedure, confirming that a significant portion of the cells remain undamaged during the wash and dry. Results from preliminary SIMS measurements show that is it possible to detect a large variety of bio-molecular signals, including intact lipids from the plasma membrane in the mass range of 700–900 Da from single cells, with little external water interference at the surface.