SpheriCal^®‐ESI: A dendrimer‐based nine‐point calibration solution ranging from m/z 273 to 1716 for electrospray ionization mass spectrometry peptide analysis

Abstract

Rationale A calibration solution for mass spectrometry needs to cover the range of interest with intense and sufficiently narrowly spaced peaks. Limited options fulfilling this may lead to compromises between performance and ease of use. SpheriCal®‐ESI was designed to combine high calibration performance for electrospray ionization (ESI) mass spectrometric analysis of peptides in positive mode with quick and easy use. Methods The developed calibration solution was tested using three mass spectrometers: two ion traps and one tandem quadrupole. The m/z errors of SpheriCal®‐ESI itself and of a tryptic digest of cytochrome C were measured after calibration. The results were compared with those achieved with ESI Tuning Mix. The memory effects of the dendrimers, and contamination from Na+ in the calibration solution, were evaluated. Results SpheriCal®‐ESI showed good shelf life as powder and was quickly reconstituted for use. Achieving intense and stable signals was straightforward. The accuracies and precisions were as expected for the instruments. SpheriCal®‐ESI was more precise and at least as accurate as ESI Tuning Mix. The memory effects and Na+ contamination were found to be negligible in typical peptide solvents. In addition, the dendrimers showed predictable dissociations with product ions common to collision‐induced dissociation in both ion trap and tandem quadrupole mass spectrometers. Conclusions SpheriCal®‐ESI provided easily accessible calibration by showing intense signals at low infusion rates and at source settings equal or similar to those used in peptide analysis. Nine calibration points in the range of interest gave precise and accurate results. Memory effects and contamination were negligible even without rinsing.

Chemical mass shifts of cluster ions and adduct ions in quadrupolar ion traps revisited and extended

“…Protonated, deprotonated, and Na + ‐adducted analytes showed no obvious mass shifts relative to the calibration solution used (see supporting information ‐ Supplementary mass shifts Figures 5–20), except in the extended mass range where two of the high‐mass dendrimers showed error toward lower m/z relative to protonated ESI tuning mix (Figure 2). In line with these results, previous studies on lower mass Na + ‐adducted dendrimers of SpheriCal®‐ESI showed no mass shifts relative to peptides, and no peak distortion, on the amaZon 16 . The Cs + ‐adducted high‐mass dendrimers showed mass shifts compared to the Na + ‐adducted dendrimers, but not as severe as the shifts of some of the cluster ions (Figure 2).…”

“…In line with these results, previous studies on lower mass Na + ‐adducted dendrimers of SpheriCal®‐ESI showed no mass shifts relative to peptides, and no peak distortion, on the amaZon. 16 The Cs + ‐adducted high‐mass dendrimers showed mass shifts compared to the Na + ‐adducted dendrimers, but not as severe as the shifts of some of the cluster ions (Figure 2 ). As it has been shown that, for example, protonated and Na + ‐adducted analytes had different stabilities, 4 , 6 , 8 we assume that the difference found here between Na + adducts and Cs + adducts has a similar origin.…”

“…The peptide mixture consisted of Leucine‐enkephalin, Angiotensin II, Angiotensin I, Bombesin, and adrenocorticotropic hormone (ACTH) amino acids18–39 at 0.8 μM each, in 3:2 MeCN:H 2 O, with 0.1 vol% HCOOH. Low mass Na + ‐adducted dendrimers (SpheriCal®‐ESI) were prepared according to the literature 16 . The high‐mass dendrimer mixture consisted of four dendrimers, C 128 H 240 O 18 (“D2089”), C 52 H 34 O 13 I 12 (“D2412”), C 100 H 82 O 30 I 10 (“D3055”), and C 124 H 106 O 37 I 12 (“D3732”), 100 μg/ml each, with 5 μg/ml NaHCOO, dissolved in THF:DMSO 1:1 and injected as such.…”

“…Low mass Na + ‐adducted dendrimers (SpheriCal®‐ESI) were prepared according to the literature. 16 The high‐mass dendrimer mixture consisted of four dendrimers, C 128 H 240 O 18 (“D2089”), C 52 H 34 O 13 I 12 (“D2412”), C 100 H 82 O 30 I 10 (“D3055”), and C 124 H 106 O 37 I 12 (“D3732”), 100 μg/ml each, with 5 μg/ml NaHCOO, dissolved in THF:DMSO 1:1 and injected as such. Cs + adducts of the dendrimers were obtained by adding 2 vol% 5 mM CsI.…”