Mass Analyzers Used in Mass Spec (6)

Question 1

What are mass separation methods based on?

Answer 1

the movement if ions with different KE, momentum, or a combination; inherent diffusion rates; motion of charged particles

Question 2

What are the main performance characteristics of a mass analyzer?

Answer 2

separate ionized masses based on m/z to be sent to detector

Question 3

single focusing

Answer 3

accelerates ions with an electric field and separates them with a magnetic field; can use magnetic sector or time of flight

Question 4

double focusing

Answer 4

ions separated twice, once with electric field once with magnetic. high resolution

Question 5

quadrupole focusing

Answer 5

mass filters

Question 6

ion trap

Answer 6

ions confined by electric/magnetic fields for extended periods of time

Question 7

ICR analyzers

Answer 7

aka Fourier transform, uses principles of ion cyclotron resonance

Question 8

What are the physical principles behind magnetic sector MS?

Answer 8

permanent magnet to steer ionized beam from MS source in circular path. ions of different mass scanned by varying field strength of magnet

Question 9

How are ions sorted in a magnetic sector mass analyzer?

Answer 9

heavier ions travel through sector at lower velocities

Question 10

What is the typical resolution for a magnetic sector MS?

Answer 10

2000, m/z of 10-10000

Question 11

What are the advantages of a magnetic sector MS?

Answer 11

widely used, relatively simple

Question 12

What are the physical principles behind a time of flight MS?

Answer 12

accelerate ions in an electrostatic field to a constant KE

Question 13

How are ions sorted in a time of flight MS?

Answer 13

ions of different masses have different velocities but the same KE; sorted temporally (in time) with lighter mass reaching detector first and heavier later

Question 14

What is the typical resolution for a time of flight MS?

Answer 14

< 1000

Question 15

What are the advantages of a time of flight MS?

Answer 15

can avg many mass spectra for sensitivity; useful for detection of transient species with short lifetimes; simple; inexpensive

Question 16

What is a reflectron time of flight MS?

Answer 16

variation on a time of flight

Question 17

What are the advantages of the reflectron design?

Answer 17

improve resolution;

Question 18

What is a double focusing analyzer?

Answer 18

the use of two types of mass analyzers in series

Question 19

What are the physical principles behind double focusing MS?

Answer 19

ion beam passes through electrostatic analyzer, which limits KE of ions reaching magnetic sector

Question 20

What is the function of the electrostatic analyzer?

Answer 20

elect narrower range of KE than conventionally feasible

Question 21

What is the function of the magnetic sector?

Answer 21

provide directional focusing

Question 22

What are some typical double focusing designs?

Question 23

What is the resolution for a double focusing analyzer?

Answer 23

10e5

Question 24

What are the advantages of a double focusing analyzer?

Answer 24

much higher resolution

Question 25

What are the physical principles behind quadrupole mass analyzers?

Answer 25

transmits only ions within a narrow range of m/z. all others neutralized and carried away

Question 26

How are quadrupoles different from single and double focusing analyzers?

Question 27

What is the layout of the quadrupole electrodes?

Answer 27

4 parallel metal rods serving as electrodes held at a DC voltage and modulated with RF frequencies

Question 28

How do the DC and AC voltages interact with ions of different masses?

Answer 28

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;

Question 29

What determines resolution in a quadrupole?

Answer 29

ratio of AC/DC potential

Question 30

What is the typical resolution for a quadrupole?

Answer 30

2000 - 4000

Question 31

What are the advantages of a quadrupole?

Answer 31

no slits or ion dispersion needed; higher sensitivity; compact; rugged; less expensive; low scan time; can be used as chromatography detectors

Question 32

What is a tandem MS?

Answer 32

quadrupole mass analyzers placed in a series

Question 33

Function of first quadrupole in tandem?

Answer 33

output largely molecular ions (parents); serves to separate parent ions by mass selection

Question 34

What is the resolution of a typical tandem MS instrument?

Question 35

What is an ion trap?

Answer 35

device in which ions can be formed and confined for extended periods by electric and/or magnetic fields

Question 36

How does an ion trap work?

Answer 36

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

Question 37

What are the advantages of an ion trap?

Answer 37

rugged, less costly, used as GC detectors

Question 38

What is the typical resolution of an ion trap MS?

Answer 38

500 - 1000

Question 39

Why use fourier transforms in mass spectrometry?

Answer 39

improved speed, S/N, sensitivity, and resolution

Question 40

What is the physical principle behind fourier transform mass spectrometry?

Answer 40

ion cyclotron resonance: motion of gas ion in magnetic field becomes circular in a plane perpendicular to the field direction

Question 41

What is an image current?

Answer 41

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

Question 42

Function of second quadrupole in tandem?

Answer 42

introduce collision gas to fragment parent ions selected by first quadrupole

Question 43

Function of third quadrupole in tandem?

Answer 43

allows mass selective detection of daughter ion fragments