The LTQ-Orbitrap is a hybrid mass spectrometer combining the strengths of two fundamentally different mass analysers. The front part is a robust very sensitive linear ion trap (dynamic electric field ion trap) mass spectrometer with high scan rates for high throughput analyses. The second mass analyser, the Orbitrap (static electric field ion trap), measures the m/z dependent frequency of a harmonic ion oscillation along a central electrode rode using non-destructive ion detection. With the Orbitrap analyser a mass resolution of 100,000 and mass accuracies of <3ppm (<1ppm after calibration) are achievable in routine analyses.
The LTQ-Orbitrap instrument is equipped with a nanospray ionisation source that is in-line coupled to nano-flow liquid chromatography. This setup allows high throughput and high accuracy mass spectrometric analyses of tryptic digests from very complex protein mixtures. Typically tryptic peptides are fractionated by nano-flow reverse phase liquid chromatography and directly injected through an emitter tip into the ion source of the mass spectrometer. While the Orbitrap determines the accurate mass of the tryptic peptides that elute from the LC column during a 1.5 sec scan the linear ion trap performs collision induced dissociation tandem mass spectrometry on the strongest 4-6 precursor signals to gain sequence information. The combination of high accuracy mass determination of tryptic peptides with sequence information from a very efficient CID MS/MS gives high confidence in protein identification.