Mass Calibration Options for Accurate Electrospray Ionization Mass Spectrometry

Abstract Calibration is vital for accurate m/z measurements. This review informs on calibration and possible calibrants for ESI-MS, highlighting analytical considerations important for high accuracy and precision. External calibration is often easier to perform, but the best accuracies are obtained with internal calibration, especially with calibrants close to and bracketing the analyte(s). Clusters of CsI allow the highest m/z calibration points, but other clusters give more closely spaced signals (clusters are unsuitable for ion trap calibration). Polymers such as PEG and PPG give closely spaced signals over a limited mass range, but suffer from memory effects. Commercial calibration solutions are easy to use and good for general calibration, but may be expensive and hard to modify. Proteins, peptides, small organic molecules, and some other calibrants offer high customization and may be preferred for more specific analyses, often for analysis of similar molecules. Because both accuracy and precision need to be known before measuring unknowns, the instrument should be evaluated. To avoid interferences, such as overlapping signals, it is important to keep the instrument and solutions free from contamination. Overlapping signals that cause shifts can, but may not, result in peak asymmetry, and smoothing can further obscure partially resolved interferences. Although increased resolution may resolve interfering signals, increased resolution can lead to lower sensitivity, potentially leading to loss of precision and accuracy due to poor ion statistics. Low intensity can to some extent be compensated by averaging more spectra. Trapping instruments suffer from space charge effects, so for the best results the number of ions needs to be limited but still high enough for sufficient ion statistics, and matching between calibration and analysis.