MASS SPECTROMETRY

Question 1

have powerful analytic capabilities with widespread clinical applications

Answer 1

coupled techniques, GC/MS and LC/MS

Question 2

first volatilized and then ionized to form charged molecular ions and fragments that are separated according to their mass-to charge (m/z) ratio; the sample is then measured by a detector, which gives the intensity of the ion current for each species.

Answer 2

sample in an MS

Question 3

Molecule identification is based on formation of characteristics fragments

Answer 3

MASS SPECTROMETRY

Question 4

These steps take place in the four basic components that are standard in all MSs:

Answer 4

sample inlet, ionization source, mass analyzer, and ion detector

Question 5

kinds of Sample Introduction and Ionization

Answer 5

• Electron Ionization
• Atmospheric Pressure Ionization
• Electrospray Ionization
• Atmospheric Pressure Chemical Ionization

Question 6

most common form of ionization used in GC/MS is electron ionization (EI). This method requires a source of electrons in the form of a filament to which an electric potential is applied, typically at 70 eV

Answer 6

Electron Ionization

Question 7

o The molecules in the source are bombarded with high-energy electrons, resulting in the formation of charged molecular ions and fragments

o Since most instruments use the same 70 eV potential, the fragmentation of molecules on different days and different instruments is remarkably similar, allowing the comparison of unknown spectra to spectra

Answer 7

Electron Ionization

Question 8

ion source of this type of instrument is not included in the high-vacuum region of the instrument.

Answer 8

Atmospheric Pressure Ionization

Question 9

ESI and APCI also differ from EI in that they are “soft” ionization techniques that leave the molecular ion largely intact in the source

Answer 9

Atmospheric Pressure Ionization

Question 10

ionization techniques used in LC/MS produce fragments and therefore mass spectra that are somewhat less reproducible between instruments than EI used in GC/MS

Answer 10

Atmospheric Pressure Ionization

Question 11

ESI can be applied to a wide range of biological macromolecules in addition to small molecules and has become the most common ionization source for LC/ MS

Answer 11

true

Question 12

Coulombic repulsion of like charges leads to the ejection of ions from the droplet

Answer 12

Electrospray Ionization

Question 13

adept at forming singly charged small molecules, but larger molecules can also be ionized using this method

Answer 13

Electrospray Ionization

Question 14

important ionization source is APCI, which is similar to ESI in that the liquid from LC is introduced directly into the ionization source. However, the droplets are not charged and the source contains a heated vaporizer to allow rapid desolvation of the drop

Answer 14

Atmospheric Pressure Chemical Ionization

Question 15

high voltage is applied to a corona discharge needle, which emits a cloud of electrons to ionize compounds after they are converted to the gas phase

Answer 15

Atmospheric Pressure Chemical Ionization

Question 16

actual measuring of the m/z occurs when the gas phase ions pass into the mass analyzer. It generates electric fields that can manipulate the charged molecules to sort them according to their m/z

Answer 16

Mass Analyzer

Question 17

kinds of Mass Analyzer

Answer 17

• Quadrupole
• Ion trap
• Tandem Mass spectrometry
• High-Resolution MS

Question 18

o Most common mass analyzer in use today
o This technique will generate a full scan mass spectrum

Answer 18

Quadrupole

Question 19

specific masses can be selected to monitor a few target analytes and it allows a longer dwell time and therefore, higher sensitivity

Answer 19

Selected ion monitoring (SIM)

Question 20

Full scan would be preferable for general unknown screening while SIM analysis is more suitable for target compound analysis

Answer 20

Quadrupole

Question 21

o A modified quadrupole
o unique feature of ion trap MSs is that they trap and store ions generated over time, effectively concentrating the ions of interest and yielding a greater sensitivity

Answer 21

Ion trap

Question 22

A linear ion trap employs a stopping potential on the end electrodes to confine ions along the ___ axis of the quadrupoles

Answer 22

2D

Question 23

o Can be used for greater selectivity and lower detection limits
o A common form of MS/MS is to link three quadrupoles in series; such an instrument is referred to as a triple quad

Answer 23

Tandem Mass spectrometry

Question 24

used to scan across a preset m/z range and select an ion of interest

Answer 24

First quadrupole (Q1)

Question 25

functions as a collision cell
o In a process called collision-induced dissociation, the ions are accelerated to high kinetic energy and allowed to collide with neutral gas molecules (usually nitrogen, helium, or argon) to fragment the ions.

Answer 25

Second quadrupole (Q2)

Question 26

serves to analyze the product ions generated in Q2

Answer 26

The third quadrupole (Q3)

Question 27

o Newer technologies utilizing high-resolution mass spectrometers based on time-of-flight (TOF) or Orbitrap (Thermo Fisher)
o These instruments can measure large numbers of analytes simultaneously in complex biological matrices and have been particularly useful for drug screening application

Answer 27

High-Resolution MS

Question 28

TOF mass spectrometers achieve resolutions of ________________ FWHM utilizing the principle that given the same kinetic energy, lighter ions travel faster than heavier ions.

Answer 28

10,000 to 50,000

Question 29

• The most common means of detecting ions employs an electron multiplier
• In this detector, a series of dynodes with increasing potentials are linked. When ions strike the first dynode surface, electrons are emitted.

Answer 29

Detector