Ion-Ion Interactions in Charge Detection Mass Spectrometry
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J. Am. Soc. Mass Spectrom. (2019) DOI: 10.1007/s13361-019-02343-y
RESEARCH ARTICLE
Ion-Ion Interactions in Charge Detection Mass Spectrometry Daniel Y. Botamanenko, Martin F. Jarrold Chemistry Department, Indiana University, 800 E Kirkwood Ave, Bloomington, IN 47405, USA
Abstract. Charge detection mass spectrometry (CDMS) is a single-particle technique where the masses of individual ions are determined by simultaneously measuring their mass-to-charge ratio (m/z) and charge. Ions are usually trapped inside an electrostatic linear ion trap (ELIT) where they oscillate back and forth through a detection cylinder, generating a periodic signal that is analyzed by fast Fourier transforms. The oscillation frequency is related to the ion’s m/z, and the magnitude is related to the ion’s charge. In early work, multiple ion trapping events were discarded because there was a question about whether ion-ion interactions affected the results. Here, we report trajectory calculations performed to assess the influence of ion-ion interactions when multiple highly charged ions are simultaneously trapped in an ELIT. Ion-ion interactions cause trajectory and energy fluctuations that lead to variations in the oscillation frequencies that in turn degrade the precision and accuracy of the m/z measurements. The peak shapes acquire substantial high and low m/z tails, and the average m/z shifts to a higher value as the number of trapped ions increases. The effects of the ion-ion interactions are proportional to the product of the charges and the square root of the number of trapped ions and depend on the ions’ m/z distribution. For the ELIT design examined here, ion-ion interactions limit the m/z resolving power to several hundred for a typical homogeneous ion population. Keywords: Ion-ion interactions, Charge detection mass spectrometry, CDMS, Electrostatic linear ion trap, ELIT Received: 3 July 2019/Revised: 16 September 2019/Accepted: 24 September 2019
Introduction
E
lectrospray has made it possible to place very large ions into the gas phase [1]. However, their analysis by mass spectrometry has been hindered by heterogeneity which makes it increasingly difficult to resolve charge states in the m/z spectrum as the mass increases [2, 3]. This limitation restricts the upper mass limit to well below a megadalton, except for a few cases where special care was taken to reduce heterogeneity [4, 5]. This shortcoming inspired the development of novel instrumentation capable of directly measuring the mass of individual ions [6]. One such technique is charge detection mass spectrometry (CDMS) [7–10]. Here, ions are directed through a conducting cylinder; each ion induces a charge on the cylinder as it passes through, and the induced charge is detected by a charge-sensitive amplifier. The flight time through the cylinder yields the m/z, and the amplitude of the
Correspondence to: Martin Jarrold; e-mail: [email protected]
signal provides the charge. The product of these two quantities is the mass. The main drawback with single-pass CDMS measurements i
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