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Title 

Ion yields for some salts in MALDI: Mechanism for the gas-phase ion formation from preformed ions

Authors 

Jeong Hee MoonY S ShinY J BaeM S Kim

Publisher 

Elsevier

Issue Date 

2012

Citation 

Journal of American Society for Mass Spectrometry, vol. 23, no. 1, pp. 162-170

Keywords 

Ion formation mechanismIon pairIon yieldMALDIPreformed ion

Abstract 

Preformed ion emission is the main assumption in one of the prevailing theories for peptide and protein ion formation in matrix-assisted laser desorption ionization (MALDI). Since salts are in preformed ion forms in the matrix-analyte mixture, they are ideal systems to study the characteristics of preformed ion emission. In this work, a reliable method to measure the ion yield (IY) in MALDI was developed and used for a solid salt benzyltriphenylphosphonium chloride and two room-temperature ionic liquids 1-butyl-3-methylimidazolium hexafluorophosphate and trihexyltetradecylphosphonium bis(2,4,4-trimethylpentyl)phosphinate. IY for the matrix (α-cyano-4-hydroxycinnamic acid, CHCA) was also measured. Taking 1 pmol salts in 25 nmol CHCA as examples, IYs for three salts were similar, (4-8)∈×∈10 -4, and those for CHCA were (0.8-1.2)∈×∈10 -7. Even though IYs for the salts and CHCA remained virtually constant at low analyte concentration, they decreased as the salt concentrations increased. Two models, Model 1 and Model 2, were proposed to explain low IYs for the salts and the concentration dependences. Both models are based on the fact that the ion-pair formation equilibrium is highly shifted toward the neutral ion pair. In Model 1, the gas-phase analyte cations were proposed to originate from the same cations in the solid that were dielectrically screened from counter anions by matrix neutrals. In Model 2, preformed ions were assumed to be released from the solid sample in the form of neutral ion pairs and the anions in the ion pairs were assumed to be eliminated via reactions with matrix-derived cations.

ISSN 

1044-0305

Link 

http://dx.doi.org/10.1007/s13361-011-0278-6

Appears in Collections

1. Journal Articles > Journal Articles

Registered Date

2017-04-19


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