Dual polarity accurate mass calibration for electrospray ionization and matrix-assisted laser desorption/ionization mass spectrometry using maltooligosaccharides

Abstract

In view of the fact that memory effects associated with instrument calibration hinder the use of many mass-to-charge (m/z) ratios and tuning standards, identification of robust, comprehensive, inexpensive, and memory-free calibration standards is of particular interest to the mass spectrometry community. Glucose and its isomers are known to have a residue mass of 162.05282Da; therefore, both linear and branched forms of polyhexose oligosaccharides possess well-defined masses, making them ideal candidates for mass calibration. Using a wide range of maltooligosaccharides (MOSs) derived from commercially available beers, ions with m/z ratios from approximately 500 to 2500Da or more have been observed using Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) and time-of-flight mass spectrometry (TOF-MS). The MOS mixtures were further characterized using infrared multiphoton dissociation (IRMPD) and nano-liquid chromatography/mass spectrometry (nano-LC/MS). In addition to providing well-defined series of positive and negative calibrant ions using either electrospray ionization (ESI) or matrix-assisted laser desorption/ionization (MALDI), the MOSs are not encumbered by memory effects and, thus, are well-suited mass calibration and instrument tuning standards for carbohydrate analysis.

Figure 1

Comparison of the commonly observed positively and negatively charged MOS found in beer generated using ESI and MALDI with mass analysis by FT-ICR-MS. While the positive mode ESI was dominated by sodium (circles) and potassium adducts (most abundant series), the carbohydrates observed in the negative mode were primarily observed as deprotonated [M-H]⁻ species and to a very small extent a hydrated series (squares). For MALDI analysis, [M+Na]⁺ ions were the most abundant series observed, whereas in the negative mode the major series corresponded to products of two sequential cross-ring cleavages (the two most abundant MOS series).

Figure 2

Influence of ESI cone voltage on the relative abundance and observed MOS ions in both the positive and negative modes with mass analysis by FT-ICR-MS. In the positive mode the type of adduct observed was relatively unchanged with cone potential; however, total ion abundances were highly dependent upon this value. Higher cone potentials in the negative mode induced fragmentation (labeled in smaller font), whereas lower cone potentials yielded the deprotonated species. For the MOS sample shown, the positive mode was dominated by potassiated adduct, with a smaller series corresponding to the sodiated adducts (circles). Additional series were observed; however, the relative abundance of these series limits their utility as calibrants.

Figure 3

ESI-FT-ICR MS/MS with IRMPD of selected beer MOS. The vertical dotted lines provide a visual guide to the parent ion masses. The remaining peaks resulted from the IRMPD of the precursor ions and represented either the loss of water or cross-ring cleavage (see legend). The m/z region of each spectrum below the parent ion has been magnified for clarity.

Figure 4

Annotated nano-LC/FT-ICR-MS separation of MOS derived from beer. Using PGC as the stationary phase, singular m/z values within multiple nano-LC elution peaks were often observed indicating the presence of isomeric carbohydrates.

Figure 5

Positive ion mode ESI-TOF-MS analysis of PGC purified beer MOS with an effective dilution factor of 100. Above m/z 1600, the intensity scale has been expanded by a factor of 25 for clarity. The major MOS series corresponds to [M+Na]⁺ ions.

Figure 6

Infusion of beer MOS using ESI-FT-ICR-MS. The plot of total ion intensity observed over time illustrates the reproducible nature of five 1 µL beer maltooligosaccharide injections and the lack of carryover between samples.