Mass Analyzers - Advanced Analytical Chemistry - Lecture Slides, Slides of Analytical Chemistry

Download Mass Analyzers - Advanced Analytical Chemistry - Lecture Slides and more Slides Analytical Chemistry in PDF only on Docsity! Mass Analyzers - Outline  Sector Mass Analyzers (Magnetic and Electrostatic)  Quadrupole Analyzers  Ion Traps  Ion Cyclotron Resonance  Time-of-Flight  and many more…. Ionization Source Mass Analyzer Detector docsity.com Properties of Mass Analyzers Resolution (R): R = m/m m = mass difference of two adjacent resolved peaks m = mass of first peak or average Example: R = 500 (“low” resolution) resolves m/z=50 and 50.1, and m/z=500 and 501 Example: R = 150000 (“high” resolution) resolves m/z=50 and 50.0003, and m/z=500 and 500.0033 docsity.com Electrostatic Sector Mass Analyzers 2 v reV z m  Therefore: Ion kinetic energy: 2 2 1 mvzeVT  eVFm  r mvFc 2  Mc FF  Forces: Only ions with equal forces will pass: V can be varied to bring ions of different KE (and different m/z ratio to the exit) Diagram from Strobel and Heineman, Chemical Instrumentation, A Systematic Approach, Wiley, 1989. docsity.com Double-Focusing Sector Mass Analyzers  If a batch of ions of equal m/z but with different kinetic energies enters a magnetic sector instrument, this will result in a spread-out beam  Soution: minimize directional and energy differences between ions of the same m/z.  Example of a double- focusing MS: the Nier- Johnson geometry Diagram from Strobel and Heineman, Chemical Instrumentation, A Systematic Approach, Wiley, 1989. docsity.com Double-Focusing Sector Mass Analyzers  Another design, the Mattauch-Herzog geometry  This geometry is analogous to CCD-based optical electronic spectroscopy systems, while Nier-Johnson instruments are similar in nature to traditional scanning monochromator spectrometers. Diagram from Strobel and Heineman, Chemical Instrumentation, A Systematic Approach, Wiley, 1989. docsity.com Time-Of-Flight (TOF) Mass Analyzers  The reflectron – a method of compensating for different ion KE’s Figure from http://www.abrf.org/ABRFNews/1997/June1997/jun97lennon.html docsity.com Time-Of-Flight (TOF) Mass Analyzers  The reflectron – a method of compensating for different ion KE’s Figure from http://www.abrf.org/ABRFNews/1997/June1997/jun97lennon.html docsity.com Quadrupole Mass Analyzers  The quadrupole (named for its “electrical structure”) is one of the simplest and most effective mass spectrometers. Diagrams from Skoog et al. docsity.com Ion Trap Mass Analyzers  Ion trap: a device for trapping ions and confining them for extended periods using EM fields  Used as mass analyzers because they can trap ions and eject them to a detector based on their mass.  Theory is based on Mattieu’s work on 2nd order linear differential equations (in the 1860’s), and on Wolfgang Paul’s Nobel Prize winning implementations R. E. March and R. J. Hughes, Quadrupole Storage Mass Spectrometers, Wiley, 1989. See also Chem. Eng. News 1991; 69(12):26-30, 33-41 Figure from W. Paul Nobel Lecture, December 8, 1989. docsity.com Ion Trap Mass Analyzers  The stability region of an ion trap – based on differential equations 22 0 8    mr eUaz 22 0 4   mr eVqz )cos(0 tVU   Most ITMS systems don’t use DC (U), i.e. only qz is controlled R. E. March and R. J. Hughes, Quadrupole Storage Mass Spectrometers, Wiley, 1989. docsity.com Ion Trap Mass Analyzers  Layout of a cylindrical ion trap mass analyzer: Diagram courtesy of M. Olsen, GlaxoSmithKline + Main RF Ring Endcap Lenses Octopole Optimized Asymptote Angle End Cap Shutter Focus Electron Multiplier Conversion Dinode Low Amplitude Dipole Field (1/3 frequency of main RF) + + ++ + + docsity.com Ion Cyclotron Resonance  How ICR works: – The ions are circulated in a field – An RF field is applied to match the cyclotron frequency of the ions – this field brings them into phase coherence (forming ion “packets”)! – The image current is produced as these little packets of ions get near the plates. The frequency of the image current is characteristic of the ion packet’s m/z ratio. http://www-methods.ch.cam.ac.uk/meth/ms/theory/fticr.html docsity.com Ion Cyclotron Resonance and Magnetic Field  Parallels between NMR/EPR and ICR: B   B =  ze B m =  B Picture courtesy Prof. Alan Marshall, FSU/NHMFL docsity.com The Orbitrap: A “Hybrid” Trap – Between IT and ICR  The Orbitrap is an electrostatic ion trap with FT/MS read-out of image current, coupled with MS/MS  Advantages – Ease of use – Resolving power (superior to TOF) – Precision and accuracy – Versatility, dynamic range  A lower-resolution, more economical ICR docsity.com The axial oscillation frequency follows the formula Where  = oscillation frequency k = instrumental constant m/z = mass-to-charge ratio zm k /  Frequencies and Masses Ions in the Orbitrap generate a complex signal whose frequencies are determined using a Fourier Transformation Image/animation from Thermo Electron Inc. See A. Makarov et al., Anal. Chem. 2006, 78, 2113-2120. docsity.com Multiple-Stage MS: MS-MS, and MSn  Also known as Tandem MS or MSn Mass Analyzer Mass Analyzer  Multiple quadrupoles are very common (e.g. triple-quad or QQQ systems, EB for double-focusing, Q-TOF for quad time-of-flight…)  Why tandem MS? Because of the possibility of doing CID – collisionally induced dissociation. Ions are allowed to collide with a background gas (He) for several millliseconds, prior to analysis. Allows for MSn experiments in an ion trap. … docsity.com Comparison of Mass Analyzers  A comparison of the properties of some common mass analyzers Analyzer Cost Scan speed Resolution Double-focusing High Slow High Quadrupole Low Medium Low-medium Trap Low Medium Medium TOF Medium Medium Medium-high ICR High Fast High docsity.com Detectors: Others  Super-conducting tunner junction – high mass range, used with MALDI – Can detect fmol of 150 kDa proteins – Can measure both energy and arrival time (2D MS – plots of m/z vs. kinetic energy)  Focal-plane array detectors/CCD – Like in electronic spectroscopy, much more challenging to design for ion detection – Would combine well with “mini-traps” or other small MS systems docsity.com MS-Chromatography Interfaces  GC-MS: gas eluent from a column is piped directly to the MS source  LC-MS: the ionization methods themselves serve as interfaces – techniques like ESI, APCI and APPI work on liquid phase samples. The methods are generally tolerant to RP LC solvents and some NP solvents. Some buffers can quench ionization of analytes though: – Bad: Phosphate – leaves a solid upon evaporation. Also ionizes preferentially – Bad: any other non-volatile additives are also bad – Good: TFA, ammonium acetate, formic acid – Good: lower concentrations, <50 mM docsity.com