IRMS

Question 1

MS vs IRMS

Describe the purpose of MS

Answer 1

Compound identification by mass fragments (scan magnet across range)

Question 2

MS vs IRMS

Describe the purpose of IRMS

Answer 2

Compound known, different isotopic compositions of a single compound quantified (fix magnet to quantify specific masses of interest)

Question 3

What is Fractionation?

Answer 3

Partitioning of stable isotopes of an element between coexisting phases

Question 4

When does fractionation occur?

Answer 4

During phase transitions

Question 5

What is fractionation controlled by?

Answer 5

Reaction conditions (eg temperature)

Question 6

IRMS Ion Sources

Gas source - Electron ionisation

Answer 6

Sample ionised by electrons emitted by a heated filament

Question 7

IRMS Ion Sources

ICP-MS - Plasma Ionisation

Answer 7

Sample ionised by introduction into Ar plasma produced by RF induction - either ‘wet’ or ‘dry’ plasma options

Question 8

IRMS Ion Sources

AMS source - Sputtering

Answer 8

Sample ions released via energy imparted after Cs+ ions impact sample surface.

Question 9

Otzi

What does oxygen isotope ratio in humans can tell you?

Answer 9

Where the water came from the person was drinking

Question 10

Otzi

What does strontium isotope ration in human scan can tell you?

Answer 10

Where the food came from the person was drinking

Question 11

What is a quadrupole?

Answer 11

Four magnets, hyperbolic in cross section are arranged in a diamond formation. One pair has an applied direct current, the other an alternating current

Question 12

What is the purpose of a quadrupole mass filter?

Answer 12

Only a particular mass ion can ‘resonate’ properly and reach he detector

Question 13

Detectors

What are discrete dynodes?

Answer 13

Dynodes emit multiple electrons in response to being hit by ions/electrons.

Question 14

Detectors

Application of dynodes

Answer 14

Can be used in analogue (large signals) or pulse counting (small signals) mode

Question 15

Detectors

What is the operating principle of Faraday Cup Detectors?

3 points

Answer 15

1. Ion enters the cup and transfer their charge to it
2. Charge usually transferred to electronics outside the vacuum system
3. Type of electronics determined whetheer measured as charge, current or voltage

Question 16

Detectors

What is the advantage of Faraday Cup Detectors?

3 points

Answer 16

• High precision abundance measurements of specific ions
• Offer very stable peak shape and linear response to high ion currents
• Modern multi-collector instruments eliminate ions beam instability and gain differences between Faraday detectors

Question 17

What are the 3 differences in behaviour between isotopes?

Answer 17

1. The lighter isotope reacts faster and requires less energy of activation (e.g light isotopes of water are more likely to evaporate than heavy ones)
2. The heavier isotopes make the strongest bonds. (e.g. Bacteria are more likely to break the light isotope bonds of molecules).
3. The heavier isotope is concentrated in the higher oxidation state

Question 18

What is the point of using a standard?

2 points

Answer 18

1. Picking up problems during measurement (mass spec has an air leak?)
2. Negate effects of drift

Question 19

2 types of gas sources for IRMS

Answer 19

1. Dual inlet
2. Continous flow

Question 20

Gas source for IRMS

Describe how dual inlet works

Answer 20

Samples prepared off-line and the gas is inserted into the mass spectrometer alternating with a reference gas. Gases expanded into bellows under vacuum, expanded to a volume(based on pressure) which gives an equal signal from sample and reference

Question 21

Gas source for IRMS

What are the 2 advantages for dual inlet

Answer 21

1. Better precision
2. Longer measurements possible

Question 22

Gas source for IRMS

Describe principle of work of continious flow

Answer 22

Samples prepared on-line and transferred into the mass spectrometer in a flow of helium with small amounts of reference gas injected into the He stream before and after the sample gas has passed through.

Question 23

What are isobars

Answer 23

ions or molecules with the same m/e as the isotope of interest for measurement

Question 24

What at the two ways of measurin IRMS of radioisotopes?

Answer 24

1. Measure sample activity
2. Measure isotope ratios