Mass Analyzers

Question 1

Calculate “z”

Answer 1

z = 1/delta(m/z)

Question 2

Mass Analyzer Function and Characteristics

Answer 2

• separate ions of different (m/z)
• important characteristics
  
  • mass resolution
  • mass accuracy
  • speed

Question 3

Mass Resolution

Answer 3

• the observed m/z value divided by the smallest difference (delta m/z) for two ions that can be separated
• two common definitions
  
  • FWHM- full width at half max peak (single peak)
  • 10% valley - the separation between two adjascent equal magnitude peaks such that the valley between them is a specified fraction of the peak height

Question 4

Mass Accuracy

Answer 4

• the measured error in m/z divided by the theoretical value

((experimental(m/z)-theoretical(m/z))/theoretical (m/z))*10⁶ yields mass accuracy in ppm

Question 5

Mass Range

Answer 5

• maximum allowable m/z ratio amenable to analysis

Question 6

Linear Dynamic Range

Answer 6

• range over which an ion signal is linear with the analyte concentration

Question 7

Speed

Answer 7

• number of scans per unit time

Question 8

Filtering Mass Analyzer

Answer 8

• mass analyzer which functions as a filter allowing only ions of a singel specific m/z to exit while all others are discarded
• low fraction of generated ions are detected
• filtering analyzers need to be scanned to generate the full spectrum
• select ion monitoring (SIM) of a selected m/z value allows for more precise measurement of ion abundance

Question 9

Quadrapole Mass Analyzer

Answer 9

• assembly of four parrallel equidistant spaced rods
• masses are separated by their vibratory motion in a high-frequency oscillating field which is created by applying direct current and radio frequency potentials to the electrodes
• mass spectrum is obtained by changing both the direct current and the radio frequency while keeping their ratio constant
• ions which are unstable under these conditions will crash into the rods
• fast scan speeds but has low mass resolution R=200 at m/z 200
• can be operated as a mass analyzer or mass filter

Question 10

Three Dimensional Quadrapole Ion Trap

Answer 10

• mass separation is achieved by storing ions in the trapping space and manipulating their motion in time rather than space
• traps all ions in a radio frequency field consisting of two end cap electrodes and a ring electrode
• by scanning different radio frequency voltages ions with different m/z are ejected (slow ejection allows for better mass resolution)
• low levels of detection than quadrapole
• capable of MS/MS
• is only able to trap a limited number of ions and has a low mass resolution delta m/z = 1

Question 11

Quadrapole Ion Trap Operational Principles

Answer 11

• instrument creates stable trajectories for ions of certain m/z or m/z range while removing unwanted ions by letting them collide with the walls or removing them through axial ejection due to unstable trajectories

Question 12

Linear Quadrapole Ion Trap (LIT)

Answer 12

• shoebox composed of quadrapoles plus trapping potentials at both ends of the box
• mass is analyzed by trapping and ejection of desired ions depending upon their m/z
• can trap more ions (increased sensitivity) and has a high scan rate approx 20Hz
• still has low resolution delta m/z = 1

Question 13

FT-ICR

Answer 13

• Fourier-Transform Ion Cyclotron
• traps ions using a static magnetic field
• ions circle with radius r (unchanged kinetic energy) which is effected by their m/z ratio
• by applying a broadband pulse certain m/z are synchronized and become excited thereby increasing their radius while maintaining the same frequency of rotation
• as the orbit of the ions nears the walls of the device the ions pass by electrodes inducing an image current

Question 14

Cyclotron Detection

Answer 14

• image current induced by ions passing electrodes on the walls of the instrument creates a potential which can be amplified and measured
• Fourier transform is performed on the time dependent current to yeild frequencies for various ions which can be related to their m/z
• allows for high mass resolution in excess of 1,000,000 and accuracy <1ppm of error despite long aquision times
• increasing the scan speed lowers the resolution to approximately 600,000