Mass spec Flashcards Preview

The Medicine (Y2) > Mass spec > Flashcards

Flashcards in Mass spec Deck (40):
1

MS requires generation of analyte ions in order to measure the relative mass of molecules. True or false?

True

2

What is on the x and y axis?

Y is abundance of each ion
X is m/z ratio

3

What are the four major stages of MS?

sample vaporisation
ion generation
ion separation
ion detection

4

What are the three ways in which sample volatilisation can be achieved?

Heat
Placing sample in a vacuum
Using fast atom bombardment

5

How can sample ionisation be achieved?

Bombarding the volatilised molecules with electrons from an electron gun. This will knock an electron out of the molecule, leaving it with a positive charge. Charged plates then accelerate the ionised molecules into the deflection chamber

6

In electron impact ionisation, the sample is vaporised by heat. True or false?

True

7

In electron impact ionisation, how is ionisation achieved?

By bombarding the volatilised molecules with an electron beam

8

When an analyte molecule loses an electron, does its mass change?

No

9

In electron impact ionisation, what are the fragments analysed based on?

mass to charge ratio

10

How does chemical ionisation work?

Uses a stream of electrons to ionise a reagent gas (ammonia or methane). The ionisation of the reagent gas produces a strong acid. Volatilised analyte molecules are ionised by strong acid via protonation

11

Electron ionisation and chemical ionisation are gentle methods and produce very little fragmentation. True or false?

False - not gentle methods and produce a lot of fragments

12

Electron ionisation and chemical ionisation are suitable for small volatile molecules with molecular weight more than 1000 Da. True or false?

False - less than 1000 Da

13

What atom does fast atom bombardment involve?

Xe

14

What is the role of the matrix in fast atom bombardment?

Protect the analyte from fragmentation

15

In FAB, do analyte molecules need to be evaporated beforehand?

No - bombardment by Xe atoms has dual action:
desorption of analyte molecules into gas phase and
ionisation

16

FAB only generates positively charged ions. True or false?

False - generates both positive and negative ions

17

Does the molecular weight of molecule change in FAB?

Yes - loss/gain of proton will change mass as protons have a weight

18

Is FAB a gentle technique? Does it produce fragments?

Yes, a gentle technique with very little fragmentation

19

FAB is suitable for the analysis of molecules up to 6000 Da. True or false?

True

20

In FAB, energy transfer takes place from the matrix to Xe atoms. True or false?

False - from Xe to matrix

21

What is the main difference between FAB and MALDI?

In MALDI, energy transferred from laser beam to matrix, whereas in FAB, energy is transferred from Xe to matrci

22

FAB is a more expensive technique than MALDI, True or false?

False

23

MALDI is suitable for biomolecules of up 500,000 Da. True or false?

True

24

What is the role of the pH modifier in electrospray ionisation?

to produce ions - this will determine whether ionisation proceeds via protonation or deprotonation

25

In which type of ionisation is the analyte dissolved in a mixture of organic solvent and water?

Electrospray ionisation

26

In electrospray ionisation, a high voltage is applied to the capillary. Why?

To disperse the droplets into the air

27

The droplets formed in electrospray ionisation are big, how are they diminished in size and why is this done?

a warm gas is applied (nitrogen) - creates smaller ions which are easier to ionise

28

Electrospray ionisation is used for non-polar analytes. True or false?

False - used for polar analytes

29

Atmospheric pressure chemical ionisation is similar to electrospray ionisation in that an organic solvent and a pH modifier is used. True or false?

True

30

APCI is more effective at analysing less polar species than electrospray ionisation. True or false?

True

31

The advantage of APCI is that the molecular mass range is typically less than 2000 Da. True or false?

False - this is a disadvantage

32

Out of all the ionisation techniques, which one creates the most fragmentation?

Electron impact ionisation, followed by chemical ionisation

33

Why does fragmentation take place?

Because the molecular ion produced by ionisation is in an excited state. This corresponds to excess vibrational energy leading to fragmentation

34

Neutral species can be seen in MS. True or false?

False - only ionised species can be seen

35

What does the process of ion separation involve?

once ionised, the analyte ions are separated by their interaction with an electrical or magnetic field in high vacuum

36

How can the mass-to-charge ratio of fragments be changed?

By varying the strength of the magnetic field

37

How are ions detected?

the positive ions and molecular fragments produced in the ionisation chamber are accelerated into an analysing tube. The path of the charged molecules is bent by an applied magnetic field

38

In ion detection, high momentum ions are deflected the most and collide with the walls of the analyser. True or false?

False - deflected the least and collide with the walls. Low momentum ions are deflected the most, but too collide with the walls.

39

Ions have to have the proper mass-to-charge ratio to collide with the collector and generate an electrical current. True or false?

True

40

Analysis of MS involves re-assembling of fragments, after identifying their molecular structure, and working backwards to generate the original molecule. True or false?

True