Salt Effects on Ion Formation in Desorption Mass Spectrometry: An Investigation into the Role of Alkali Chlorides on Peak Suppression in Time-of-Flight-Secondary Ion Mass Spectrometry

Abstract

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.

Dynamic Reactive Ionization with Cluster Secondary Ion Mass Spectrometry

“…[12] were obtained at the beginning of a depth profile (i.e., with an ion fluence of 5 × 10 11 ions/cm 2 , whereas the data presented here (Figure 5) refer to the steady state region of the depth profile, where the protonated molecule signal has already been reduced because of the buildup of some surface damage (see Figure 2). Also, it is possible that the presence of Cl − ions somewhat limits the benefit arising from the enhanced protonation efficiency, since these ions might react with a protonated molecule and, hence, lead to ion suppression effects as demonstrated previously by the Vickerman group [13]. However, Cl − might react with other fragment ions from trehalose, leaving a high chance of formation of protonated molecules, which we have observed in the experiment.…”

“…For example, matrices employed in MALDI experiments have been particularly useful in boosting the intensity of higher mass secondary ions, presumably through proton donation to form the protonated molecule, [M + H] + . Salt addition can, under some conditions, improve ionization through adduct formation [13]. Similarly, Cs flooding or Cs + bombardment enhances ionization through [M + Cs] + formation [14, 15].…”

Developments in molecular SIMS depth profiling and 3D imaging of biological systems using polyatomic primary ions

“…It is clear that salt in biological material is a source of signal suppression and is responsible for the generation of new ionic species. A subsequent study has sought to understand the source of the organic ion suppression (Piwowar, Lockyer, & Vickerman, 2009). Arginine films evaporated from water solution onto a silicon shard formed the basis of the model system.…”

“…Thus carrying out the analysis in a frozen state may well be the ideal. As outlined in Section IIC the frozen hydrated state may also help to reduce or overcome the effect of salt in the matrix (Piwowar, Lockyer, & Vickerman, 2009). However, the fact that in the study by Dubois et al the sputtering C 60 beam was only operated at 10 keV may also have been an issue.…”

Relative ion yields in mammalian cell components using C60 SIMS

“…[12,13] The organic ion suppression effect was also evident from the study on salt-doped arginine films. [14] The loss of negative chloride ion intensity in the negative ion spectrum during depth profiling suggests that the chloride anion ion paired with the positive organic secondary ions, which leads to suppression of the organic signals.…”