Download Mass spectrometry How Electric and Magnetic Fields work in Mass

Mass spectrometry
Reading: White #15, F&M 4.4
Also Required: Visit and study Pudue AMS lab web site: http://www.physics.purdue.edu/primelab/
Guide Questions:
What is the means used to separate isotopes in most isotope ratio mass specs?
What means is used to steer and focus all the masses together into a single ion beam prior to their
separation according to mass?
What are the 4 key ways of generating ions in isotope ratio mass specs? What are the strengths and
weaknesses of each one?
What is the fundamental difference between Faraday buckets and ion counters and what are the normal
uses and limitations of each?
What equation can be used to estimate the uncertainty related to random fluctuations in ion arrival rate
at the detectors?
What are the three methods used to determine the magnitude of the mass bias in mass spectrometers
and how do they work?
Need to quantify different isotopes: Options:
1) Either: Measure some radiation given off that is isotope-dependent- separate absoption/emission
energies
Nuclear magnetic resonance (no, few nuclei are NMR-active)
IR radiation- recently developed (!)- precision not so great yet
2) Or: Separate and count isotopes:
How can we manipulate isotopes- move them around?
We can push them around with electric and magnetic fields if we ionize them first
This is how mass spectrometry works.
How Electric and Magnetic Fields work in Mass Specs
Electric fields and magnetics field act differently on particles:
Assume we will shoot ions at high speed through a field and separate them:
All will have same energy, e.g., 10keV
But this means different velocities because E = 1/2mv2
1. Magnetic Fields Sort Ions according to mass:
Magnetic force = B e v, where B = field, e = charge, v = veloc.
So force α veloc.
Compare Hydrogen and Deuterium:
Veloc. of H is 2 times greater (kinetic E = 1/2mv2)
 Force on H is 2 times greater
 Acceleration (a=F/m), 2 2 times greater (it’s 1/2 the mass)
But, greater v means less time in the field (factor of 2 )
And Displacement α t2, so this means 1/2 the displacement
 Net effect: Displacement is 2 greater for H+ than for D+.
Conclusion: Ions in a mag. field have circular paths:
m
B 2 r2
(B = mag field (Gauss), V = accel. voltage, e is charge (in integer
= 4.825 ×10−5
e
V
units of e- charges))
r2 α kinetic energy α veloc.2
r2 α mass
**Note that an ion with 2 times the mass and a double charge will follow the
same path.
2. Electric fields sort ions according to kinetic energy alone:
Force depends on charge alone: Same force for H and D
H accel. = 2 D accel.
Time in field: 2 times greater for D than for H
But displacement = at2
Therefore: the greater time and slower accel. for D cancel out.
Conclusion: In an electric field, all ions with same velocity/energy take the same
path. Electric fields can be used as “energy filters”
2E
r=
(r = path radius, E = energy of ions, V = field voltage)
Vsector
Utility: Collisions in imperfect vacuum slowed ions (need to filter out)
Components of a Mass Spec
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Vacuum for ions to travel in
Ion source to make the ions and shoot them through the vacuum
Magnetic field that can be adjusted for various masses
Optional Electrostatic field to filter out ions with wrong energy
Ion collection system that can measure number of ions.
1. Ion source to make the ions and shoot them through the vacuum
Ions created by one of the following:
1. Electron bombardment of a gas, if you can make one easily.
2. Thermal effects- TIMS
3. Bombardment by ions (Cs+, O-): Ion probe, AMS
4. Plasma: ICP-MS
Accelerate and focus ions: Use Electric fields only- want to keep all ions on same path
Plates with slits in them to accelerate ions through
Plates/donuts to act as lenses
Split plates to steer beam for max intensity
Defining slit, grounded (0Volts) at the end- like the hole in a pinhole camera
Total potential for a benchtop instrument- 10kV
AMS (accelerator mass spec): use 106 eV- level Van de Graaf accelerator- fills a large
room- advantage: particles with this much energy can be manipulated and detected
differently from the smaller mass specs (see Purdue AMS lab web site- required)
2. Mass filter- magnetic field
a) Magnetic sectors: Straightforward uniform, static magnetic field
Must be quite strong- several Amperes through a series of windings
Heat generated- causes drift, monitor the field with a “Hall probe”
Must be able to vary field quickly for rapid measurements
ALSO possible to adjust the Accel. Voltage
b) Quadrupole mass filters: Another mass filter option:
• Four metal rods
• Radio frequency variation of potentials (sinusoidal; AC current):
• Causes selected ions to spiral down the axis; heavier and lighter ions crash
• More compact and cheaper than a magnetic sector, but less precise
• Impossible to get entire ion beam into the detector at once- smeared out.
• ±1% precision, maybe 0.1% if you’re very careful
• Good enough for concentration measurement, not most isotope ratios
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+
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Ion Traps: Another type of “mass analyzer”
• Less precise than quadrupoles, but even cheaper
• Used for relative measurement of organic compounds, for example
3. Ion beam measurement:
Typical ion beams are roughly 10-11 Amperes: 1A = 1C/sec. = 6.2*1018 /sec.
10-11 A= 6.2*107 ions/sec.
Some isotopes (e.g.,230Th) are rare and beams are much smaller
a) Faraday bucket/cup: Catch all ions, measure current as ions flow to ground.
1011 Ohm resistor- largest stable resistor, I think
Measure voltage as ions flow across this resistance to ground
1V = 10-11A beam
- What is lower limit?
Noise: Circuit has noise at around the 0.1 mV level.
Depending on your required precision, you need at least 10mV beam, probably more
like 100mV (=10-12A) to get good data.
Cannot get ion beams this large for rare isotopes (e.g., 230Th)
Multiple collection:
No beam is perfectly steady- beam switching is slow and introduces error
An array of multiple collectors is better
b) Ion Counting: For beams < 10-13 A, we resort to individual counting of ions:
Ions coming in about every 10-8 seconds
Detection circuits: Ordinary ones need 10-8 seconds to reset or more
Thus, need to factor in “dead time”: some ions missed
Need a “multiplier” device- Single charge arrival is not measureable
Pulse of current generated from each ion registered
a) Electron multiplier- ion impact starts electron cascade
Pulse of many electrons  counting device
“Channeltrons” are one type of electron multiplier
b) Photomultiplier- Daly detector
Ion beam diverted to Al knob, Knocks some electrons off
Daly knob is negatively charged- repels e’s
Electrons shoot across to scintillator
Light pulse- photomultiplier tube sends pulse to counting device
c) AMS instruments can use a variety of fancy detectors because the ions are at such high
energy- ions MeV energies and can be detected by semiconductor materials or charged gas
detectors.
Practical Mass Spec Guide
What can each type of mass spec measure? Where can they be found? Also see some info at:
http://www.geochemistry.syr.edu/cheatham/timsins.html
1) Gas Source Mass Spec: H, C, N, O, S, Cl
• Very common: Most major universities
• Dual Inlet variety: Older method, highest precision
• Continuous flow variety: Newer, allows one to measure out put from a gas
chromatograph or a combustion analyzer. Quicker, easier way to get C, H, O, N
and S isotope ratios from 1) individual organic compounds or 2) total C and N
derived by combusting soils, etc. Smaller sample requirement than dual inlet.
2) TIMS: Sr, Nd, Pb, Os, Cr, Se, Fe, U, Ra, Pa, Li, Th (sometimes)
• Very common: Many major universities
3) TIMS with high abundance sensitivity filter (=energy filter): Th
• Most newer TIMS instruments (1/3 of them?)
4) Negative ion TIMS: Capable of measuring – ions: Os, Se, B
• Most newer TIMS instruments (1/3 of them?)
5) PIMMS: (Plasma Ionization Multicollector Mass Spec, also called multicollectorICP-MS): All elements except H, C, N, O, Cl, Ar and maybe a few others
• Recent development, about 20 exist nationwide.
• Can use a laser sampling system
6) ICP-MS: Usually this means an instrument with one detector, no designed for
isotope ratio work. Use for measuring concentrations and a few isotope ratios (e.g,
Th) to relatively low precision
• Very common: Most major universities
7) AMS (accelerator mass spec): Use for cosmogenic nuclides, because ratios like
14
C/12C and 36Cl/35Cl are VERY small. Smaller mass specs cannot distinguish the
tiny ion beams from background noise.
• Multi-million dollar instruments, very different from all above, which are
commercially available and fit in a small lab
• North American facilities: U. Arizona, Lawrence Livermore National Lab,
Purdue, MIT/Woods Hole, Toronto
8) Noble gas mass specs: Very similar to gas source mass specs, except: Can close
off vacuum pumps, leak gas in and ionize without pumping it away- very efficient,
can analyze very tiny amounts of He or Ar. Systems for Ar age dating are used only
for that, I think.
Counting statistics:
In some cases, the precision is limited by the number of ions that arrive at the detector
• Isotope ratios can be measured to precision approaching 1 part in 106. At such a high
level of precision, the fact that ions arrive randomly at the detectors becomes a factor
• When measuring rare isotopes (e.g., 226Ra and other U-series nuclides), the rate of ion
detection can be as low as, say, 30 counts per second. Again, randomness becomes a
factor because the total number of ions measured may not be great enough to get a good
average intensity.
Example: Ions arrive randomly at the detector: For a 104 ions/sec. beam:
• If you made repeated measurements with a 10-6 second time window, would see mostly 0 arrivals
with an occasional arrival and very rare 2 arrivals in the same interval
• With a 10-4 sec. window, would see some zero’s many ones, some twos
• With a 10-3 sec. window, would see some 8’s, lots of 9’s and 10’s, some 11’s, etc.
• With a 10-2 sec. window, you would see a range of values, centering on 100 counts
• With a 1 sec. W=window, you would see 10,000 counts ± a little variation
So…How well do you know the beam intensity for each window size? (for one observation)
Conclusion: The more ions you count, the better you know the rate.
Counting statistics is the name given to the relationship between precision and the number of ions
measured. As long as the incoming ions are randomly distributed in time:
1
where n is number of ions measured
n
Example: If you have measured 104 ions, the precision is 10-2, or ±1%
Example: If you want to attain 1 part in 106 precision, you must measure 1012 ions
Typical ion beam, 10-11 Amperes, or 6.2*107 ions/sec.- measure for 4.5 hours!
σ=
Mass Spec Bias or Discrimination:
The raw ratios measured by mass specs are affset from the true sample ratios
e.g., with TIMS, light isotopes ionize faster- faster kinetics
e.g., with ICP-MS, heavy isotopes are favored- transmitted through interface better
Approaches to correct for discrimination:
1. Sample-standard bracketing: Run standards between samples to monitor discrim. and drift
Only successful when samples and standards have exactly the same conditions
Gas source mass specs (for H, C, N, O, S, Cl): always used
TIMS- can’t use; each filament is unique and evolves over time during the run
ICP-MS: often used
2. To measure radiogenic isotopes, use two non-radiogenic isotopes to monitor discrimination
e.g., 87Sr/86Sr measurement- assume 86Sr/88Sr = 0.1194.
• Deviations from 86Sr/88Sr = 0.1194 are assumed to be caused by either mass spec
discrimination or natural isotope fractionation
• Measure the deviation and correct 87Sr/86Sr accordingly
This used for Sr and Nd, Os, Hf, Cr isotopes.
• Doesn’t work for Pb, only 204 is stable. Use double spike (below)
3. Double isotope spike technique:
• Need four or more stable isotopes
• Add two pure isotopes, in a known ratio, to the sample
• Other two isotopes are the target ratio
• Monitor the ratio of these two isotopes during measurement
• Calculate the discrimination and correct the measurement