Mass Analyzers Used in Mass Spec (6) Flashcards
What are mass separation methods based on?
the movement if ions with different KE, momentum, or a combination; inherent diffusion rates; motion of charged particles
What are the main performance characteristics of a mass analyzer?
separate ionized masses based on m/z to be sent to detector
single focusing
accelerates ions with an electric field and separates them with a magnetic field; can use magnetic sector or time of flight
double focusing
ions separated twice, once with electric field once with magnetic. high resolution
quadrupole focusing
mass filters
ion trap
ions confined by electric/magnetic fields for extended periods of time
ICR analyzers
aka Fourier transform, uses principles of ion cyclotron resonance
What are the physical principles behind magnetic sector MS?
permanent magnet to steer ionized beam from MS source in circular path. ions of different mass scanned by varying field strength of magnet
How are ions sorted in a magnetic sector mass analyzer?
heavier ions travel through sector at lower velocities
What is the typical resolution for a magnetic sector MS?
2000, m/z of 10-10000
What are the advantages of a magnetic sector MS?
widely used, relatively simple
What are the physical principles behind a time of flight MS?
accelerate ions in an electrostatic field to a constant KE
How are ions sorted in a time of flight MS?
ions of different masses have different velocities but the same KE; sorted temporally (in time) with lighter mass reaching detector first and heavier later
What is the typical resolution for a time of flight MS?
< 1000
What are the advantages of a time of flight MS?
can avg many mass spectra for sensitivity; useful for detection of transient species with short lifetimes; simple; inexpensive
What is a reflectron time of flight MS?
variation on a time of flight
What are the advantages of the reflectron design?
improve resolution;
What is a double focusing analyzer?
the use of two types of mass analyzers in series
What are the physical principles behind double focusing MS?
ion beam passes through electrostatic analyzer, which limits KE of ions reaching magnetic sector
What is the function of the electrostatic analyzer?
elect narrower range of KE than conventionally feasible
What is the function of the magnetic sector?
provide directional focusing
What are some typical double focusing designs?
What is the resolution for a double focusing analyzer?
10e5
What are the advantages of a double focusing analyzer?
much higher resolution
What are the physical principles behind quadrupole mass analyzers?
transmits only ions within a narrow range of m/z. all others neutralized and carried away
How are quadrupoles different from single and double focusing analyzers?
What is the layout of the quadrupole electrodes?
4 parallel metal rods serving as electrodes held at a DC voltage and modulated with RF frequencies
How do the DC and AC voltages interact with ions of different masses?
if m/z is correct, ion will travel all the way to the detector; if m/z is not correct, ion will collide and neutralize to not be detected;
What determines resolution in a quadrupole?
ratio of AC/DC potential
What is the typical resolution for a quadrupole?
2000 - 4000
What are the advantages of a quadrupole?
no slits or ion dispersion needed; higher sensitivity; compact; rugged; less expensive; low scan time; can be used as chromatography detectors
What is a tandem MS?
quadrupole mass analyzers placed in a series
Function of first quadrupole in tandem?
output largely molecular ions (parents); serves to separate parent ions by mass selection
What is the resolution of a typical tandem MS instrument?
What is an ion trap?
device in which ions can be formed and confined for extended periods by electric and/or magnetic fields
How does an ion trap work?
analyte ions admitted to cell through grid in upper end cap; RF voltage applied to ring electrode confining ions in trap; variable RF voltages applied to ring electrode destabilizes lighter ions that are then swept from cell, passing through lower end cap to detector; mass spectrum obtained by increasing RF amplitude, destabilizing ions of increasing mass
What are the advantages of an ion trap?
rugged, less costly, used as GC detectors
What is the typical resolution of an ion trap MS?
500 - 1000
Why use fourier transforms in mass spectrometry?
improved speed, S/N, sensitivity, and resolution
What is the physical principle behind fourier transform mass spectrometry?
ion cyclotron resonance: motion of gas ion in magnetic field becomes circular in a plane perpendicular to the field direction
What is an image current?
result of coherent motion of resonant ions, experimentally observed to decrease with time; induced by circular motion of charged ions interacting with detector plates in ICR cell
Function of second quadrupole in tandem?
introduce collision gas to fragment parent ions selected by first quadrupole
Function of third quadrupole in tandem?
allows mass selective detection of daughter ion fragments
