Mass spectrum_L2

Question 1

list four analysers

Answer 1

QUADRUPOLE

TIME-OF-FLIGHT (TOF)

ION TRAP

ORBITRAP

Question 2

what are the three key parameters of assessing the analyser?

Answer 2

(1) upper mass limit: what is the smallest mass to charge ratio that the analyser can successfully separate
(2) ion transmission: the capacity of ions that the analyser can separate to be detected by the detector
(3) resolution: the ability of separating ions with similar mass to charge ratio

Question 3

what are the features of quadrupoles?

Answer 3

(1) Uses a quadrupolar electrical field (comprising RF and DC components) to separate ions
(2) The quadrupole consists of four parallel rods
(3) Each diagonal pair of rods is connected electrically
(4) The electrical field is obtained by the application of a voltage made up of a DC component (to one pair) and a RF potential (to the other pair)
(5) electrons that are able to be detected are right at the centre of the four poles, they are in harmony with the electromagnatic field, those undetectable are in disharmony
(6) the mass to charge ratio can be calibrated aganist the known quadrupole electric field

Question 4

pros and cons of quadropoles

Answer 4

pros:

(1) small
(2) robust
(3) low cost
(4) Rapid scanning
(5) Ideal for GC-MS (EI ionization)
(6) Widely used for ES-MS
cons:
(1) lowest upper limit anong all the analysers
(2) lowest resolution

Question 5

What principles the ion trap analyser are based on and what procedures are involved?

Answer 5

(1) These analyzers operate on a similar principle to quadrupole analyzers but they do not operate as a filter
(2) The ion trap stores ions using fields generated by RF (and sometimes DC) voltages applied to electrodes arranged in a sandwich geometry
(3) The sandwich is composed of a ring electrode in the middle with cap electrodes on each end
(4) Ions within a selected m/z range are trapped within the electrodes, ions enter from the entrance endcap electrode and are trapped in the centered small space which is termed as ring electrode for circulation, higher the energy they possesses, greater the radius they circulates, eventually, the electrons are ejected from the exit endcap electrode and go to the detector
(5) in recent years higher performance ion traps have been introduced which are called “linear ion traps”; the ions are trapped over a larger (linear) volume which helps to overcome problems of ion interference and increases storage capacity, more ions being trapped can improve the data quality efficiently

Question 6

pros and cons of ion trap analyser

Answer 6

pros:
(1) better resolution
(2) better separation
(3) capable of handling ions of higher mass to charge ratio

cons:
(1)if not all ions are trapped in the ring electrode, i.e. the ion transmission is crucial, the data is still unreadable

Question 7

What principles the orbitrap are based on and what procedures are involved?

Answer 7

(1) The central spinal electrode is covered by the barral
(2) Ions are trapped by a static electrostatic fields
(3) Ions orbit around central electrode and oscillate in axial direction in the electrode
(4) m/z ratio relates to the frequency of ion oscillation along axial direction
(5) Fourier transform (FT) algorithm converts time-domain signal to m/z

Question 8

pros and cons of orbital

Answer 8

pros:

(1) High resolution: able to separate similar mass to charge ratio
(2) High mass accuracy
(3) Good upper mass range and ion transmission

cons:
(1)high cost

Question 9

What principles the TOF(time of flight) analysers are based on and what procedures are involved?

Answer 9

(1) Ions generated from the source travels towards the detector in the vaccum which is set to ensure the migratory efficiency by reducing the conflict with air
(2) Ions of higher mass to charge ratio migrate in slower velocity and vice versa
(3) Ions are separated by differences in velocities as they move in a straight path towards the detector - “unlimited” mass range
(4) The calibration curve can be plotted, the mass to charge ratio can be translated from the time of flight

Question 10

pros and cons of TOF analysers

Answer 10

pros:
(1) upper limit can be guareenteed

cons:
(1) Early analysers had poor resolution and mass accuracy

Question 11

what are the principles of reflection?

Answer 11

(1) Newer analyzers incorporate reflectrons which correct for the effects of kinetic energy distribution of the ions and give a longer flight path
(2) Initially produced ions travel in different velocities and are separated due to difference in speed. Series of electrodes select the ions of same charge.
(3) The reflectron is an ion mirror that reverses the direction of travel of the ions - ions of greater kinetic energy penetrate further and therefore have a longer flight path, thereby compensating for the faster travelling speed, theoretically, all the ions need to be in the same energy level, however, the delayed extraction can still happen, therefore, even ions with same mass to charge ratio can have slightly different energies, the reversal in the direction is known as ionmerisation, the process greatly improves the analyser resolution ability
(4) Linear analyzers have low resolution (<1000); reflectrons give much higher resolution (>3,000)

Question 12

What changes will the molecular resolution have in the absence or presence of reflection?

Answer 12

With reflection, the resolving power is increased and peaks are narrowed down from the wide, broad peak which will present when the reflection is not applied

Question 13

How does PM-photomultiplier work?

Answer 13

PM-photomultiplier detects photons (ions strike a dynode which results in electron emission; electrons then strike a phosphorous screen which in turn releases a burst of photons; the photons then pass into the multiplier where amplification occurs in a cascade fashion), the peak occurs on the spectrum.

Question 14

How does EM-electron multiplier work?

Answer 14

EM-electron multiplier: detects electrons (ions strike dynode; emission of electrons; amplification when they strike next dynode etc), cascade pattern passes down the electrons to amplify the secondary electrons, sharp peaks appear on the spectrum, the amplification power is up to 6-10 times

Question 15

How does MCP- micro-channel plate array detectors work?

Answer 15

MCP- micro-channel plate array detectors: for simultaneous detection of multiple m/z values (many separate channels – spatial resolution), multiple electro-detectors spread all over the place to increase the sensitivity and accuracy

Question 16

What are the two common features in all the detectors?

Answer 16

(1) detect separated ions

(2) distinguish which ions are present in high concentration and which are in low concentration

Question 17

what instruments are usually combined with EI and why? What pros can the combination result in?

Answer 17

(1) EI sources are most commonly found on instruments with quadrupole analysers (cheap but low resolution)
(2) GC-MS is widely used
(3) A reminder that EI requires molecules to be volatile: low MWt and non-polar
(4) EI has low resolving power, so it does not matter to EI of the analysers have ultra high performance or not
(5) small, robust, easy to use

Question 18

what instruments are usually combined with MALDI and why? What pros can the combination result in?

Answer 18

(1) MALDI sources are most commonly found on instruments with TOF analysers, TOF does not have mass limit
(2) MALDI-TOF is the method of choice for “mass fingerprinting” complex mixtures of polymers – both chemical and biological complexes
(3) MALDI-flight tube-detector
(4) improved resolving nature and the combination is slightly larger

Question 19

what instruments are usually combined with MALDI and why? What pros can the combination result in?

Answer 19

(1) ES sources are most commonly found on triple quadrupole, ion trap, orbitrap or Q-TOF instruments
(2) ES is especially useful for multiply charged molecules e.g. peptides and proteins
(3) high resolution

Question 20

What are the hybrid instruments and what advantages they potentially have?

Answer 20

(1) more than 1 mass analyser are associated to maximise the beneficial properties
(2) allow 2-dimensional mass spectrometry
(3) The main instrument manufacturers have their favoured instrument geometries and they compete on performance (resolution, sensitivity etc), price, robustness etc

Question 21

Give some examples of hybrid instruments and in what senarios they can be applied?

Answer 21

(1)Q-TOF, TOF-TOF (usually combined with a MALDI source) and linear ion trap-orbitrap
(2)MALDI TOF-TOF is sensitive and has specific analytic ability, MALDI ion-trap-TOF is a high-performance mass spectrometer
(3)Not only two analysers’ combination is possible, but triple-instrument combination even gives rise to highest performance, such as ES-quadrupole-ion-trap-ion-trap instrument:
High resolution capability, 1,000,000 (FWHM) at m/z 200
･< 1 ppm RMS using internal calibration
･Sub fmol sensitivity
(4)Hybrid instruments are used for MS/MS experiments

Question 22

What kinds of molecular ions can the three ion sources produce and what molecular mass they have respectively and why?(if the electron mass is neglected)

Answer 22

ELECTRON IMPACT: RADICAL CATIONS M+(M), the outer orbital electron is ejected

MALDI: SINGLY CHARGED CATIONS and ANIONS i.e. [M+H]+(M+1), [M+Na]+(M+23) etc and [M-H]-(M-1)

ES: MULTIPLY CHARGED CATIONS and ANIONS i.e. [M+nH]n+(M+n), [M+nNa]n+(M+23n) etc and [M-nH]n-(M-n)

Question 23

Which equation is used to calculate the mass to charge ratio(m/z)?

Answer 23

the molecular mass divided by the number of charges