Effect of mass spectrometric parameters on peptide and protein identification rates for shotgun proteomic experiments on an LTQ-orbitrap mass analyzer

The success of a shotgun proteomic experiment relies heavily on the performance and optimization of both the LC and the MS systems. Despite this, little consideration has, so far, been given to the importance of evaluating and optimizing the MS instrument settings during data-dependent acquisition mode. Moreover, during data-dependent acquisition, the users have to decide and choose among various MS parameters and settings, making a successful analysis even more challenging. We have systematically investigated and evaluated the effect of enabling and disabling the preview mode for FTMS scan, the number of microscans per MS/MS scan, the number of MS/MS events, the maximum ion injection time for MS/MS, and the automatic gain control target value for MS and MS/MS events on protein and peptide identification rates on an LTQ-Orbitrap using the Saccharomyces cerevisiae proteome. Our investigations aimed to assess the significance of each MS parameter to improve proteome analysis and coverage. We observed that higher identification rates were obtained at lower ion injection times i.e. 50150 ms, by performing one microscan and 1215 MS/MS events. In terms of ion population, optimal automatic gain control target values were at 5 x 1051 x 106 ions for MS and 3 x 1031 x 104 ions for MS/MS. The preview mode scan had a minimal effect on identification rates. Using optimized MS settings, we identified 1038 (+/- 2.3%) protein groups with a minimum of two peptide identifications and an estimated false discovery rate of similar to 1% at both peptide and protein level in a 160-min LC-MS/MS analysis.