IRMS Flashcards
MS vs IRMS
Describe the purpose of MS
Compound identification by mass fragments (scan magnet across range)
MS vs IRMS
Describe the purpose of IRMS
Compound known, different isotopic compositions of a single compound quantified (fix magnet to quantify specific masses of interest)
What is Fractionation?
Partitioning of stable isotopes of an element between coexisting phases
When does fractionation occur?
During phase transitions
What is fractionation controlled by?
Reaction conditions (eg temperature)
IRMS Ion Sources
Gas source - Electron ionisation
Sample ionised by electrons emitted by a heated filament
IRMS Ion Sources
ICP-MS - Plasma Ionisation
Sample ionised by introduction into Ar plasma produced by RF induction - either ‘wet’ or ‘dry’ plasma options
IRMS Ion Sources
AMS source - Sputtering
Sample ions released via energy imparted after Cs+ ions impact sample surface.
Otzi
What does oxygen isotope ratio in humans can tell you?
Where the water came from the person was drinking
Otzi
What does strontium isotope ration in human scan can tell you?
Where the food came from the person was drinking
What is a quadrupole?
Four magnets, hyperbolic in cross section are arranged in a diamond formation. One pair has an applied direct current, the other an alternating current
What is the purpose of a quadrupole mass filter?
Only a particular mass ion can ‘resonate’ properly and reach he detector
Detectors
What are discrete dynodes?
Dynodes emit multiple electrons in response to being hit by ions/electrons.
Detectors
Application of dynodes
Can be used in analogue (large signals) or pulse counting (small signals) mode
Detectors
What is the operating principle of Faraday Cup Detectors?
3 points
- Ion enters the cup and transfer their charge to it
- Charge usually transferred to electronics outside the vacuum system
- Type of electronics determined whetheer measured as charge, current or voltage
Detectors
What is the advantage of Faraday Cup Detectors?
3 points
- 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
What are the 3 differences in behaviour between isotopes?
- 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)
- The heavier isotopes make the strongest bonds. (e.g. Bacteria are more likely to break the light isotope bonds of molecules).
- The heavier isotope is concentrated in the higher oxidation state
What is the point of using a standard?
2 points
- Picking up problems during measurement (mass spec has an air leak?)
- Negate effects of drift
2 types of gas sources for IRMS
- Dual inlet
- Continous flow
Gas source for IRMS
Describe how dual inlet works
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
Gas source for IRMS
What are the 2 advantages for dual inlet
- Better precision
- Longer measurements possible
Gas source for IRMS
Describe principle of work of continious flow
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.
What are isobars
ions or molecules with the same m/e as the isotope of interest for measurement
What at the two ways of measurin IRMS of radioisotopes?
- Measure sample activity
- Measure isotope ratios
