AMS Flashcards
steps in atomic mass spectrometic analysis
- atomization
2 ionization - separation of the ions
- Counting the number of ions
Atomization
conversion of sample to atoms
Ionization
conversion of atoms to a stream of ions (usually positive)
Separation of ions
separate based on their mass-to-charges (m/s)
Counting the number of ions
- counting each types
- or measuring ion current produced when the ions formed from sample strike a suitable transducer
plot for mass spec
intensity v. m/s
atomic masses of isotopes are ____
discriminated
what is nominal mass
implies a while number precision in a mass measurement… if you get 17.3, your nominal mass is 17
what is the detector response
mass spectrum
two types of mass spectra
- Atomic mass spectra
2. Molecular mass spectra
components of mass spectrometer
- inlet
- ion source
- mass analyzer
- Vacuum pump
- ion transducer
- signal processor and readout
inet
introduction of sample to ion source
ion source
produces stream of ions which are accelerated to mass analyzer
mass analzer
separates ions base don m/z ratios
ion transducer
converts ion beam to electrical signal
signal processor
processes signals from transducer
readout
plot of signal v. m/z
Types of inlets
- nebulizers
- electrothermal vaporization
- Hydride generation
- chromatography (GC, LC, SFC)
vacuum pump
- maintains low pressure in ion source, analyzer and ion transducer
- no air should be present, rather O2 and N2
ion sources in AMS
- ICP
- DCP
- MIP
- Spark source (SS)
- Thermal ionization (TI)
- Glow discharge (GD)
- Laser Microbe (LM)
- Secondary ionization (SI)
Samples must be ___, ___, and ___ in AMS
vaporized
atomized
ionized
what dont we need in AMS
excitation
Types of mass analyzers
- magnetic sector analyzers
- double focusing analyzers
- QUADRUPOLE ANALYZERS
- Time of flight analyzers
Magnetic sector analyzer
ions are accelerated through a magnetic sector , where they are separated based on mass ratios
in magnetic sector analyzer, which is more positive slit A or B
A is more positive than B
Small molecules are (more or less) affected by magnetic field in magnetic sector
more
Large molecules are (more or less) affected by magnetic field in magnetic sector
less
disadvantage of magnetic sector analyzer
not a high enough resolution
what forces are ions subject to in magnetic sector analyzer
magnetic field (Fm) centripital force (Fc)
how can mass sprectra be obtained in magnetic sector analysis
by varying one of the following and keeping the other two constant:
- Magnetic field strength (B)
- Voltage difference between slits A and B (V)
- circulat path radius (r)
modern magnetic sector MS vary which variable
Magnetic field strength (B)
what does double focus analyzer employ
electrostatic and magnetic sector analysis
what does the electrostatic analyzer do in double focus
separates ions based on kinetic energies
do ions get separated by mass or kinetic energies first in double focus analysis
- kinetic energies
2. then mass
what is quadrupole mass analyzer composed of
4 parallel cylinder rods that act as electrodes
how are the cylindrical rods set up in quadrupole
+,-,+,-
- rods connected directly to a variable dc power source such that one pair is positive and one is negative
with dc, what is applied to each quadrupole rod
- variable radio frequency
- ac voltages
what influence is int he xz plane of quadrupole
positive rods
in xz plane of quadrupole, if no ac current is applied, what is the result
- big and small ions go through
- no fltering
in xz plane of quadrupole, if ac current is applied, what is the result
-big ions will make it through, small ions will not make it through
why do big ions make it and small ions not make it through xz plane of quadrupole, if ac current is applied
- big ions take a lot of time to respond to directional changes so they pass the filter
- small ions respond strongly to direction change so they end up hitting the rod and are neutralized and do not pass the filter
what influence is int he yz plane of quadrupole
negative rods
in yz plane of quadrupole, if no ac current is applied, what is the result
No ions will pass
in yz plane of quadrupole, if ac current is applied, what is the result
-small ions make it through, big ions will not make it through
why do small ions make it and big ions not make it through in yz plane of quadrupole, if ac current is applied
the ac voltage offsets the dc effect so the small ions make it, while the big ions get attracted to the negative rods
in the both planes, why is there a narrow window of m/z that pass
bc ions must be heavy enough to not be eliminated by high mass filter and light enough to not be removed by low mass filter
for a mass spectrum to be obtained on this device, what must be
- ions accelerated into space between rods by potential difference of 5-10 V
- AC and DC voltages are increased simultaneously while maintaining a constant ratio
- All ions except those in a limited m/z range strike rod and converted to neutral atoms
- only ions int he narrow m/z range reach the transducer
what else is quadrupole called
mass filter
how can selectivity be increased using quadrupoles
by placing multiple quadrupoles in series, the selectivity can be increased
how are ions formed during time of flight mass analyzers
formed during pulse laser impact
what happens in drift tube of time of flight mass analyzer
ions spread due to their different velocities
what happens at ion detector of time of flight mass analyzer
ions reach detector one by one
order of ion arrival in time of flight mass analyzers
smaller ions move quicker so they arrive first
larger ions move slower so their arrive last
downfall of time of flight mass analyzer
it must have a fast respinse
what are ions accelerated by in time of flight mass analysis
electric field pulse
what are typical flight times
1-50 µs
what gives you a better resolution in time of flight mass analysis
a longer drift tube
advantages of time of flight mass analysis
- simple
- ruggedness
- ease of accessibility of ion source
- virtually unlimited mass range
disadvantages of time of flight mass analysis
- limited resolution
- limited sensitivity
- requires fast electronics due to ion arrival at transducer are only fractions of seconds appart
AMS transudcers
- electron multiplies
- Faraday cup
- Array transducer
In discrete electron multiplier, what does ion beam strike
Cu/Be oxide cathode with enough energy to create a burst of electrons
After the Cu/Be oxide electrode, what happens to the electrons in discrete electron multiplier
they bounce between multiple dynode surfaces to create several more electrons
In 20 discrete electron multiplier dynode, how many electrons are produced for 1 electron
10,000,000 electrons
What is the most sensitive and common transducer in AMS
continuous- dynode electron multiplier
what is the continuous- dynode electron multiplier composed of and what’s its shape
Horn shaped glass doped with Pb
What electron gain can be seen in continuous- dynode electron multiplier
100000-100000000 electrons
what does the ion beam hit to produce electrons then continue to hit
restive conductive surface
In Faraday cup what do ions strike
the collector electrode,surrounded by a cage that prevents the escape of reflected ions and ejected secondary electrons
In Faraday cup, what are the collector electrode and cage attached to
ground through large resistor
In faraday cup, how are positive ions neutralized
by electrons flowing from ground through the resistor
In faraday cup, how is the voltage drop amplified
by a high-impedance amplifier
In Faraday cup, what is the transducer response independent of
- Energy of ion
- mass of ion
- chemical nature of ion
advantages of Faraday cup
- inexpensive
- mechanically and electronically simple
disadvantages of faradays cup
- need high impedance amplifier bc that limits speed of spectral scanning
- provides no internal amplification
Types of array transducers
- Electrooptical ion detector(EOID)
2. microchannel plate
what does the ion beam hit in EIOD and what results
microchannel plate to produce cascade of electrons
what to electrons hit in EIOD and what results
phosphorescent screen
radiation results in photons
what do the photons hit in EIOD
the transducer to convert optical signal to electrical signal
How is amplification carried out in microchannel plate
-particles strike inner surface of tiny channels taht act as individual electron multipliers to produce about 1000 electrons for every incoming particle
What is resolving power
ability for mass spectrometer to separate two peaks of similar mass
most common mass spec technique
ICPMS
other kinds of mass spec techniques
- DCPMS
- MIPMS
- SSMS
- TIMS
- GDMS
- LMMS
- SIMS
characteristics of ICPMS
- low LOD (0.1-10 ppb)
- High selectivity
- good precision (2-4%)
- good accuracy
- 90% of periodic table can be analyzed bc large mass range
- 1 m/z resolution
- Dynamic range of 10^6
- approx. 10s analysis time
- various sample introduction method
why would one use laser ablasion for ICPMS
so your sample doesnt have to be dissolved
Spectroscopic interferences
- isobaric interference
- poly-atomic ion interference
- Oxide and hydroxide species interference
- matrix effects
Isobaric interference
two elements with isotopes differing by < 1 mass unit
Isobaric interference correction
another element isotope is used
Why are isobaric interferences not that bad
bc theyre predictable and correction can be done with appropriate software
Poly-atomic interference
occurs when poly-atomic species form from interactions between species in the plasma and the species in the matrix or atmosphere
poly-atomic interference corection
- blank measurement
- using different analyte isotope
Oxide and hydroxide species interference
MO+ or MOH+ formed from the analyte, matrix components, solvent, and plasma gasses
Oxide and hydroxide species interference correction
adjusting the following variables may reduce the interference:
- injection flow rate
- radio-frequency power
- sample skimmer spacing
- sample orifice size
- plasma gas composition
- oxygen elimination
- solvent removal
Matrix effects
important when concentration of species in the matrix is too large which can cause the the reduction of analyte signal
matrix effect corrections
- use more dilute solutions
- altering sample introduction procedure (such as LC)
- LLE or SPE to separate out offending species
- Use appropriate internal standard’
what can icpms be used for
- qualitative
- semi quantitative
- quantitative
- iosotope ratio measurements
qualitative and semi-qualitative analysis
Perform analysis to determine what’s in sample with rough concentration estimate
most widely used quantitative method of ICPMS anaysis
multiple standard calibration
what plots are generally used for ICPMS and why
log-log plots bc dynamic range is so large
Isotope ratio measurement
the measurement of isotope ratios is important in science and medicin
archaeology application of icpms
carbon dating to determine the age of artifacts and various deposits
medicinal application of icpms
using an isotope to see how long it takes a drug to circulate the body and where it ends up
advantages of ICP-MS over ICP-OES
- detection limits are better
- simple spectra, easy to interpret
- ability to measure isotope ratios
disadvantages of ICP-MS over ICP-OES
- much more expensive
- instrument drift
- isotopic interferences
ICP-MS involves atomization and ___
ionization
ICP-OES involves atomization and ____
excitiation
order of detection limits
ICPMS
