Topic 6: Mass Spectroscopy

Question 1

What is mass spectroscopy?

Answer 1

A technique used to measure the mass-to-charge ratio of molecules and atoms. Often characteristic ions produced by an induced unimolecular dissociation of a molecule are measured. These characteristic ions can be used to “finger print” a molecule for identification or identity confirmed.

Question 2

What are the four essential components of mass spectroscopy

Answer 2

• Sources of ions-in vacuum or at atmosphere
• Mass analyzer – must be under vacuum
• Ion detection device
• Data acquision system

Question 3

Describe the basic physics fundamental of mass spectroscopy

Answer 3

Magnetic fields exert a force on a moving electrical charge that is perpendicular to the direction of motion

Question 4

What are the advantage and disadvantages for using the magnetic sector of mass spectroscopy

Answer 4

• Advantage:
  
  • high resolution possible (with electric)
  • Simultaneous monitor isotopes
  • Versatile ionization techniques
• Disadvantages:
  
  • Very expensive and massive
  • High voltage/vacuum-operation challenge
  • Upper mass range limited by magnet

Question 5

Explain how “time of flight” works in Mass spectrometry

Answer 5

High voltage is applied to the backplate to accelerate ions towards the right and expel them from the ion source into the drift region
If ions have the same kinetic energy, but different masses, the lighter ones travel faster than the heavier ones.

In the simplest form of time-of-flight mass spectrometer, it’s just a long, straight, evaculated tube with the source at one end and the detector a the other end. Ions expelled from the source drift to the detector in order of increasing mass, because the lighter ones travel faster.

Question 6

What are the advantages and disadvantages of Time of Flight MS?

Answer 6

• Advantages
  
  • Good resolution
  • No upper mass limit
  • Biomolecule application
• Disadvantages
  
  • Pulsed source required
  • Fast electronic required (expensive)
  • Sensitivity limitation
  • High mass resolution difficult
  • High voltage/vacuum

Question 7

What are the advantages and disadvantages of using quadrupole?

Answer 7

• Advantages:
  
  • Relatively inexpensive, reliable
  • Simple to use
  • No high voltages (1000 V), small size
  • Mass scale is linear
• Disadvantages
  
  • Limited resolution
  • Limited mass range.

Question 8

Label the component on the ion trap and its function.

Answer 8

Question 9

What are the advantages and disadvantages for quadrupole ion trap

Answer 9

• Advantages
  
  • Small size, relatively inexpensive
  • Applicable for MS/MS, LC/MS
  • Inherent sensitivity (no transmission losses)
• Disadvantages
  
  • Limited mass range
  • Complicated operation (pulsed ionization)

Question 10

Describe Fourier Transform Mass Spectroscopy.

Answer 10

• Gaseous ions move in a circular motion in a strong magnetic field. The angular frequency of this motion, the cyclotron frequency depends only upon the inverse of the m/z value, in a fixed field.
• Ions trapped in a magnetic field can absorb energy from an ac electric field if the applied frequency matches the cyclotron frequency. The absorbed energy increases the velocity of a specific m/z ion. The movement of the ions induces a capacitor current that is dependent on the velocity of ion movement. This induced current can be monitored as an image current. Since the entire process is link to the m/z of the ion, m/z specific information can be obtained
• But very expensive, with very high resolution

Question 11

How to calculate mass resolution (R)?

Answer 11

Average is m

Question 12

Name the different types of techniques for ionization in mass spec?

Answer 12

• Under vacuum
  
  • Electron impact
  • Chemical ionization (+ve and ECNI)
  • MALDI
  • FAB
• Atmospheric pressure
  
  • MALDI
  • Electrospray (ESI)
  • Chemical ionization (APCI)
  • Photoionzation (APPI)
  • “new” real time techniques (DART, DESI, ASAP)
  • ICP

Question 13

How is a molecule ionized?

Answer 13

A gaseous vapor of analyte (M) in traduced into a small chamber, held under vacuum. Electrons are generated by emission from a heated filament and then accelerated by a potential difference, typically 70 eV. There electrons collide with the neutral analyte molecles and transfer energy to the analyte, inducing ionization and fragmentation.

Question 14

Explain the method of chemical ionization.

Answer 14

Chemical ionization produces less fragmentation than electron ionization. For chemical ionization, the ionization source is filled with a reagent gas such as methane, isobutene, or ammonia, at reduced atmospheric pressure. Energetic electrons convert CH4 in to a variety of reactive products.

Question 15

Explain what is electron capture negative chemical ionization (ECNI)

Answer 15

An analyte molecule (M) will normally only form a significant population of M- if the electron affinity (EA) is positive, or in other words the anion is more stable than the neutral species. Polyhalogenated species generally have high electron affinities, whereas, many sources of chemical noise, ie., unsubstituted hydrogencarbons generally do not. The rate and efficiency of electron capture reactions can approach the collision limit, therefore, much more efficient than EI.
Biologically important analytes, steroids, neurotransmitters, amino acids, etc. can be determined after derivation to add “high EA” functional group ie. –C₆F₅

Question 16

Explain how Fast Atom bombardment (FAB) works

Answer 16

A beam of energetic atoms are focused on a sample dissolved in a non volatile solvent (glycerol) and the molecular ions produced are accelerated into the mass spectrometer

Question 17

Explain how Matrix Assisted Laser Desportion Ionization (MALDI) works.

Answer 17

An intense laser beam is focused on a sample. The matrix is selected to have a very strong absorption of the laser wavelength. Get desorption, desolvation and charge transfer to the analyte. Proteins up to mass 300 000 Daltons have been ionized in this manner.

Question 18

Explain how Electrospray ionization (ESI) works.

Answer 18

The sample is mixed with a solvent and introduced to a vessel (capillary). High voltage is applied to the tip, which charges the molecules in the solvent. Once the solution is injected into the evaporation chamer, the like charges repel, forming Taylor cone, then burst into fine spray. Solvent gradually evaporates and nitrogen gas forces ions closer together to repel (Coulomb force) until all droplets split up into single charged molecule.

Question 19

Explain how DART works (Direct Analysis in Real Time)

Answer 19

DART source produces electronically excited He or vibrationally excited N2, which are directed at the surface of an object to be sampled in open air

Question 20

Explain how does low-temperature plasma work

Answer 20

• Low-temperature plasma is created by passing He, Ar, N₂ or ambient air through a glass tube with a ground wire at the center
• Excited-state species in the plasma ionize and dislodge molecules from a surface such as human skin into the source of a mass spectrometer.