Chapter 7 - Mass spectrometry (part 2)

Question 1

The construction of a typical QIT mass spectrometer consists of

Answer 1

a ring electrode with a hyperbolic inner surface and two electrically common hyperbolic end-cap electrodes.

Question 2

The trajectory of ions inside the QIT resembles

Answer 2

a figure 8 with rotating and oscillating simultaneously.

Question 3

As the ions repel each other in the QIT, their trajectories

Answer 3

expand as a function of time.

Question 4

To avoid QIT expansion, the trap is maintained with a pressure of ________ gas around _______ Torr.

Answer 4

Helium

10^-3

Question 5

A single high-vacuum pump with a flow of about ________ is sufficient to maintain QIT vacuum compared with _______ for other mass spectrometers.

Answer 5

40 L/s

250 L/s

Question 6

Ion motion in QIT can be analyzed by______ equations.

Answer 6

Mathieu

Question 7

When solving equations of motion for ions withing QIT analyzer, three fundamental parameters of fundamental importance emerge

Answer 7

a, q, and beta

Question 8

Most commercial QITs operate by applying a

Answer 8

fundamental RF voltage to the ring at constant frequency and varied amplitude.

Question 9

The quadrupole ion trap will be operated along the ______ axis.

Answer 9

q_u (x)

Question 10

At a fixed value of the RF potential V applied to the ring electrode, heavier ions will have lower ______ values.

Answer 10

q_z

Question 11

If the value of V is equal to 0.908, beta_z is equal to one, and the ion has reached its _________.

Answer 11

stability limit.

Question 12

At the stability limit, a slight increase in V will cause

Answer 12

the ion to have an unstable trajectory and will be expelled from the trap in the z direction.

Question 13

In QIT, only _____% of expelled ions will reach the detector.

Answer 13

50

Question 14

The maximum internal mass can be increased by

Answer 14

increasing V at higher values without arcing, reducing the size of the trap, or using a lower RF frequency.

Question 15

In an ideal QIT, the motions of the ions in the radial and axial directions are

Answer 15

mutually independent.

Question 16

If a supplementary RF voltage to the end-cap electrodes is applied to the end caps,

Answer 16

resonant ejection of ions occurs at q_z < 0.908

Question 17

Principle of resonant ejection

Answer 17

Ions are stored in the 3D trap at a voltage V_A of the fundamental RF.

Question 18

MS scan based on resonant ejection may either be carried out in a

Answer 18

forward (low to high mass) or reverse manner.

Question 19

Resonant ejection can serve for precursor ion selection in

Answer 19

tandem MS experiments.

Question 20

_______ rather than _______ MS occurs in QIT.

Answer 20

Time-dependent, space-dependent

Question 21

QIT can perform not only MS², but also

Answer 21

MS³ and MSⁿ

Question 22

QIT instruments do not support what kind of scans?

Answer 22

constant neutral loss and precursor ion

Question 23

General sequence for MS in QIT.

Answer 23

1. Select ions of one m/z ratio
2. Let those ions fragment
3. Analyze the ions by one of the scanning methods
   
   1. Alternatively, select a fragment in the QIP, and let it fragment further

Question 24

To fragment ions by resonance excitation,

Answer 24

an RF voltage must be applied to adjust the q_z value for the selected m/z ratio to the frequency of the RF generator used.

Question 25

In general, fragments with m/z values lower than about _____ of the precursor ions m/z will be lost in QIT.

Answer 25

20%

Question 26

Space charge effect

Answer 26

QIT has the capacity for a limited number of ions at any one time before repulsive charges cause the excess ions to be ejected.

Question 27

Concentrations of ions above the space charge limit lead to

Answer 27

poor performance with grossly deteriorated resolution and shifts in the mass axis calibration.

Question 28

The linear ion trap (LIT) adds sections

Answer 28

at each end of the quadrupole to create a potential well.

Question 29

Once in the LIT, ions are cooled by

Answer 29

collision with an inert gas.

Question 30

Placing electrodes of higher potential near the front and back ends of RF-only quadrupole creates

Answer 30

a trapping potential within the quadrupole.

Question 31

Two modes for the mass selective ejection of ions used in LIT

Answer 31

1. ions are expelled axially using fringe field effects
   
   1. slots are hollowed out in two opposite rods and mass selective radial expulsion of ions is obtained

Question 32

One great advantage of LITs in comparison with QITs is a

Answer 32

more than 30-fold higher ion trapping capacity before space charge effects, and a more than 10 times higher trapping efficiency.

Question 33

LIT advantages increase

Answer 33

sensitivity and the dynamic range.

Question 34

The performance of LITs is highly dependent upon

Answer 34

mechanical construction.

Question 35

2D and 3D ion traps are commonly operated at _____ resolution with scan rates on the order of _____ m/z per second.

Answer 35

unit

11,000

Question 36

2D and 3D ion traps can trade spectral acquisition rate for

Answer 36

resolution.

Question 37

The orbitrap mass analyzer consists of

Answer 37

an outer barrel like external electrode and a central spindle like internal electrode along the axial axis.

Question 38

In orbitrap, both electrodes are connected to

Answer 38

independent voltage supplies.

Question 39

In orbitrap, the outer electrode is split into

Answer 39

an ion excitation part and an ion detection part.

Question 40

In orbitrap, the space between the internal and external electrodes forms

Answer 40

the measurement chamber under high vacuum conditions (~10^-9).

Question 41

In orbitrap, ion introduction is achieved by using a

Answer 41

field compressor.

Question 42

Orbitrap uses a ___________ field, which is the sum of a quadrupole field of the ion trap and a logarithmic field of a cylindrical capacitor.

Answer 42

quadro-logarithmic

Question 43

Curved linear traps function in the same way as their linear counterparts with the added advantage that

Answer 43

ions can be focused in narrow areas to be ejected as compact bursts of ions.

Question 44

In curved linear traps, ions are focused by

Answer 44

altering the trapping fields of the electrodes at the front and rear of the instrument.

Question 45

The curved linear ion trap is one of the most _______ ways of injection ions to the orbitrap.

Answer 45

efficient

Question 46

Slow injection

Answer 46

When ions are injected equatorially.

Question 47

In order to eliminate gas carryover from the ion trap (linear or curved),

Answer 47

the ion beam is usually deflected by a set of transfer lenses.

Question 48

Fast injection

Answer 48

When ions are injected into the orbitrap at a position offset from its equator.

Question 49

The _______ mode allows for the highest mass resolution in orbitrap.

Answer 49

Fourier Transform

Question 50

Two modes of orbitrap mass detection

Answer 50

1. Fourier Transform Mode
2. Mass Selective Instability Mode

Question 51

MSI can be achieved by

Answer 51

resonance excitation.

Question 52

A convenient means of detection in MSI mode is

Answer 52

collision with a conversion dynode in the outer electrode.

Question 53

Instances in which MSI mode should be used:

Answer 53

• Ions with certain m/z ratio can be stored for MS/MS analysis
  
  • High intensity signals from unwanted ions can be ejected to improve dynamic range

Question 54

The mass resolving power is half the

Answer 54

frequency resolving power.

Question 55

The orbitrap provides a resolving power up to 140000 at m/z

Answer 55

200