Abstract
MeV-SIMS is an emerging mass spectrometry imaging method that employs fast, heavy ions to desorb secondary molecules from the analyzed sample. High yields and low fragmentation rates of large molecules, associated with the dominating electronic sputtering process, make it particularly useful in biomedical research, where insight into the distribution of organic molecules is vital. Both yield and fragmentation of desorbed molecules in MeV-SIMS rely on characteristics of the primary ion but may also be impaired by poor instrumental settings. After utilizing secondary ion optics in the linear mass spectrometer at the micro-analytical center of the Jožef Stefan Institute, we demonstrate very efficient detection of secondary ions. As a result, the secondary ion yield, using such settings, solely depends on the species and the characteristics of the primary ion. In order to analyze the yield dependence on the primary ion energy, and the corresponding stopping power within the electronic excitation regime, we used a continuous electron multiplier detector to measure the primary ion current during each measurement of the mass spectra. Secondary ion yield as a function of the primary ion energy and charge is presented as well as fragmentation rates of organic molecules arginine and leu-enkephalin. Other influential instrumental drawbacks are also studied, and their effect on the results is discussed.
Highlights
Used mass spectroscopy imaging (MSI) methods, such as secondary ion mass spectroscopy (SIMS)[1,2] or matrix asisted laser desorption ionization (MALDI),[3,4] rely intensely on the efficiency of sputtering secondary ions
Journal of the American Society for Mass Spectrometry stopping power is approximately linear as a function of energy within the employed range, we present our results in relation to the primary ion energy
Such numbers are below values, which would allow employing hydrogen or some other light charged particle as a start signal for time-of-flight measurements
Summary
Used mass spectroscopy imaging (MSI) methods, such as secondary ion mass spectroscopy (SIMS)[1,2] or matrix asisted laser desorption ionization (MALDI),[3,4] rely intensely on the efficiency of sputtering secondary ions. In the case of SIMS, the secondary ion yield is strongly correlated to the stopping power of the primary ion within the target. Secondary ion yield heavily relates to the fragmentation rate of sputtered molecules, as well as to the ionization probability of emitted particles, which typically has very low values. In the case of keV primary ion clusters, which are commonly used to desorb heavier molecules with conventional SIMS, the ionization probability is of the order of 10−3 or lower.[5] Such low values hint the potential need of postionization protocols. Some postionization techniques, e.g., laser postionization, interfere with the mass analysis of secondary ions and, are often avoided. Other techniques, such as dynamic reactive ionization,[6] are being used to enhance the ionization of protonated molecular ions
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