Measurements I: Mass spectrometry– Faraday cups & electron multipliers • Some types of MS –...

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Measurements I: Mass spectrometry

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• Introduction– Uses, criteria, commonalities

• Ion Sources– Electron impact, TI, ICP, SI

• Analyzers– E & M sectors, QMS, TOF, Ion Traps

• Detectors• Detectors– Faraday cups & electron multipliers

• Some types of MS– Gas source, TIMS, SIMS, ICPMS,AMS







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Uses for Mass Spectrometers

• Trace element determination sensiti it & isolation– sensitivity & isolation

• Quantitative analysis– reproducibility & separation

• Molecular structure determination– deconstruction of molecules

• Material identification and separation• Isotope abundance determination

Commonalities• An ion source

Atoms need to be charged to be– Atoms need to be charged to be manipulated

– Accelerates, focuses, collimates a beam• A mass analyzer

– Selects momentum and energyR f b– Refocus beam

• A detector– Measure and discriminate beam

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In addition• Sample preparation and introduction system

– Often integrated into machine– Depends on sample type and machine type

• Data gathering and processing system– Digitization of analogue signals – Counting of ions

• Instrument control– Control of all parts– Automation and book-keeping– More reliable, cost effective, humane

Decisions to make

• Sample character: gas liquid or solid purity?• Sample character: gas, liquid or solid, purity? • How abundant is the material? • Isotopic abundance: similar or drastically

different?• Range: do isotope ratios vary a lot or a little?• Are there interferences? • Are there “matrix effects”?• Do we need to know the “absolute” ratio?

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Figures of Merit• Resolution: smallest difference in mass• Sensitivity: how small a signal can be

discerned? • Blank: what do you get with no sample at all?• Abundance sensitivity: the smallest isotopic

ratio • Stability and reproducibilityStability and reproducibility• Discrimination and accuracy• How much does a measurement depend on the

presence or absence of other species?• Memory effect?

Resolution• Are there interfering

isobars?– How small a mass difference

can you distinguish?• Caveat emptor: watch the

definitions!• You can separate• You can separate

entangled peaks if you know the peak shape very well– Not trivial to do…

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Types of Ion Sources• Electron impact

– gases– gases• Thermal ionization

– solids• Inductively coupled plasma source

– liquids• SIMS and Laser Ablation

– solid surface analysis• There are others

– But not important for isotope work

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Electron Impact Source

• Generally has more plates than thi

Electron impact ionization• Depends on

atomic structure• Can produce

multiply charged ions

• Sensitivity generallygenerally around n x 10-4

amps/torr1 amp = 6.24 x 1018 e s-1

1 torr = 1/760 atmospheres

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Thermal Ionization Source• Solid sample dissolved and concentrated into

a few drops• Deposited on a refractory filamentDeposited on a refractory filament

– Often with 2ndary coatings• Filament heated in an ion source

– Diffusion of material to surface

– Volatilization– Ionization– Usually multiple filaments

Thermal Ionization Source• Solid sample dissolved and concentrated into a few

drops• Deposited on a refractory filament

– Often with 2ndary coatings

• Filament heated in an ion source– Diffusion of material

to surface– Volatilization– Ionization– Usually multiple filaments

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Microwave Ion SourcePlasma maintained by microwave discharge,

Supported by inflowing gas through capillary

Contained by axial magnetic fields

Ions extracted and focused by electric fields

Inductively Coupled Plasma Source

• For “direct” injection of solutions– Amenable to automation

Simpler chemistr (?)– Simpler chemistry (?)• Ionization ~ 100%• Process:

– Ar from liquid Ar source– Torch made of quartz (high temp!)– electrostatic ignition of Ar– Maintenance of plasma by RF (1-2 KW)– Nebulization of solution into Ar flow– Coaxial flows of Ar (for cooling too)– Ion extraction by “skimmers” (cooled)– Skimmers interface vacuum to Ar pressure

• Issues: matrix effects• Recent developments with laser ablation

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Secondary Ion Mass Spectrometry• For surface analysis of solids

– E.g., thin sections

• Spatial resolution ~ 10-6m• Spatial resolution 10 m– Laterally and in depth

• Process– Create ion beam (Ar, Cs, etc)– Extract & Ionize sputtered atoms

typically < 10% ionized– Accelerate collimate & focusAccelerate, collimate & focus– Big energy spread in ions

• Need to energy filter

• Issues: surface & ionization biases– Standard surfaces/matrices?







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Mass Analyzers

• Magnetic and Electric SectorsMost common for isotope st dies– Most common for isotope studies

• Time of Flight– Simple designs

• Quadrupolar Mass Analyzers– Robust rapid scanners

• Ion Traps– Compact and emerging tools

Magnetic and Electric Sectors• Electric sectors:

– Energy filtersBalance between:– Balance between:

• Centripetal force mv2/R• Electrical force qV

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or2

Analyzer

K

mvqVR

ER

=

=

• Resolution related to R and defining slit dimensions

Analyzer

RqV

=

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Magnetic and Electric Sectors• Magnetic sectors:

– Momentum filters– Balance between:

2

CmvF qvBR

= =

• Centripetal force mv2/R• Magnetic force qvB

– Since energy = qV• We have

R l ti l t d t R d

ormvRqB

=

212

E mv qV= =

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2mVRqB

=

• Resolution related to R and defining slit dimensions

Beam SlitR RMM R

Δ + ΔΔ≅

Time of Flight Instruments• Conceptually simple• Not widely used for isotope studiesNot widely used for isotope studies• Operated in pulse mode

– “duty cycle” low• Momentum filters:

ΔT = L / v

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Quadrupolar Mass Filters• Reliant on orbital stabilty in RF electric fields:

– Only stable orbits passed– Only stable orbits passed• Complicated optical solutions

– Integrated Mathieu equations• Compact and robust• Low voltage• Fast scannersFast scanners• Wide range• Constant resolution vs Mass

• Reliant on orbital stabilty in RF electric fields:– Only stable orbits passed

• Complicated optical solutions

Quadrupolar Mass Filters

– Integrated Mathieu equations• Compact and robust• Continuous mode operation• Low voltage• Fast scanners• Wide rangeg• Constant resolution vs Mass

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Ion Traps

• Invented by the same• Invented by the same guy (Paul)

• Opposite to QMS– All ions in stable orbits– Desired ions ejected – Operated in pulse mode– Operated in pulse mode

• Compact & easily made

• Emerging technology







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Faraday Cups• For intense ion beams

– From 10-15 – 10-9 ampere range– From 10 – 10 ampere range• Very simple: collect charge

– Bleed through electrometer– Easily created arrays of cups

• Watch for secondary electrons– From beam in cup– From beam outside of cup– Secondary electron suppression plates

Electron Multipliers• For ion beams < 10-12

amps ( < 107 ions /s)• Relies on secondary

electron cascade • Discrete or continuous• Current or pulse mode• Run at 102 > Gain < 108

• Ion feedback issuesGain depends on high voltage applied to device

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Electron Multipliers• For ion beams < 10-12

amps ( < 107 ions /s) Current vs. pulse counting modes:

( ) i d d !• Relies on secondary electron cascade

• Discrete or continuous• Current or pulse mode• Run at 102 > Gain < 108

(a) gain dependency!

(b)dark current discrimination

• Ion feedback issues

Gain and NOISE depends on high voltage applied to device







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Gas Source Mass Spectrometers

• Dynamic vs. static mode of operation– Dynamic (mass spec pumped continuously)Dynamic (mass spec pumped continuously)

• Matched/calibrated dual viscous leaks*• Larger sample requirement• Rapid cycling between standard & unknown• Very precise: e.g., stable oxygen, carbon, nitrogen, &

sulfur isotopes

– Static• Introduction into vacuum envelope (arrested pumping)• Smaller samples, more sensitivity

– For ion counting limitations– Matrix and pressure effects

*non isotopically fractionating

Accelerator Mass SpectrometryWhere isotopic ratios are extraordinarily small,

high voltage and many “filters” are required to separate isotopes. E.g., 14C, 10Be, 129I

Negative C ions created off graphite t t f 8 KVtargets from 8 KV Cs beam (no –N)

Most non-C ions eliminated in 40 KV MS (recombinator)-C accelerated to 2 5 MV2.5 MV

E-stripping in Ar produces +3C

Accelerated through 2.5 MV to ground

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Accelerator Mass Spectrometry

12C and 13C separated off into f d i

Where isotopic ratios are extraordinarily small, high voltage and many “filters” are required to separate isotopes. E.g., 14C, 10Be, 129I

faraday cups using a magnet

Additional electric and magnetic sectors to select against scattered ions

Detected in gasDetected in gas ionization detector (additional discrimination)

Accelerator Mass SpectrometryWhere isotopic ratios are extraordinarily small,

high voltage and many “filters” are required to separate isotopes. E.g., radiocarbonThe future:

Using a gas source g g(microwave plasma) for continuous flow analysis & interfacing to GCs, etc for single compound analysis

NEC 1.5SDH-1 withextended LE Tube

HE MagnetR=75 cmG=75 mm

LE MagnetR=26.8 cmG=70 mm

LE ESAR=30 cmG=75 mm

NEC 134 SampleMC SNICS

HE ESAR=75 cmG=50 mm

Detector

Gas IonSource

MC-SNICS

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Why CFAMS?Why CFAMS?

ChargePrepar- AMSSystem

ChargeExchangeCanal

Gas IonSource

Combu-stion

Prepar-atory

Device

Direct interfacing or injection attractive:Direct interfacing or injection attractive:“real time” feedback“real time” feedbackf (l bl k?)f (l bl k?)fewer steps (lower blank?)fewer steps (lower blank?)Quicker turnaroundQuicker turnaroundlower cost per analysislower cost per analysisless intrinsic memory than “hybrid” sourcesless intrinsic memory than “hybrid” sources

Gas (or Liquid) Chromatography:Polycyclic aromatic hydrocarbons (PAH’s) in soot

• Environmental Forensics: – E.g., Identification of fossil fuel vs

“natural” sources• Environmental Processes:

– Bio-degradation of oil spills• Extreme dilution tracer

experiments

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Programmed pyrolysis, oxidation, or combustion

Gas supplied to sample, ≈ 30 mL/mineither He (pyrolysis),He + 1% O2 (oxidative release),or He + 25% O2 (in situ combustion).

Additional gas supplied to secondg ppreactor, ≈ 5 mL/min, He + O2 tomaintain full oxidation of CuO catalyst.

5 C°/min

T, °C100 200 300 400 500 600

CO

2, m

ol/Δ

T

30 C°/min

20 C°/min

10 C°/min

Laser Ablation for spatial information- e.g., Spleothems, otoliths, Corals, Tree Rings

1 mm holes

Laser

O2+He CO2+He

SampleFrit

O2+He CO2+He

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AMSSystem

ChargeExchangeCanal

Gas IonSource

Combu-stion

Prepar-atory

Device

LaserSample Laser

O2+He CO2+He

SampleFrit

Ion Source Performance• Ion Source:• Ion Source:

300300 μμl/min = 1 4 · 10l/min = 1 4 · 101717 C atoms/sC atoms/s Effi i 5 4%Effi i 5 4%

• Mg Charge Exchange Canal:

• Total:

• Mg Charge Exchange Canal:

• Total:

300 300 μμl/min 1.4 10l/min 1.4 10 C atoms/s C atoms/s 1200 1200 μμA CA C++ = 7.6 · 10= 7.6 · 101515 C ions/sC ions/s

Efficiency= 5.4%Efficiency= 5.4%

400 400 μμA CA C++ = 2.5 · 10= 2.5 · 101515 C ions/s C ions/s 6363 μμA CA C-- = 3.8 · 10= 3.8 · 101414 C ions/sC ions/s

Efficiency= 15%Efficiency= 15%

Efficiency= 0.3Efficiency= 0.3--0.8%0.8%• Emittance, and Energy Spread• Emittance, and Energy Spread

~70 ~70 ππ·· mm·mrad at 30 kV Extraction (Cmm·mrad at 30 kV Extraction (C++ ions)ions)~80~80--140 140 ππ·· mm·mrad at 30 kV Extraction (Cmm·mrad at 30 kV Extraction (C-- ions)ions)±± 320 eV beam energy spread at 30 kV Extraction320 eV beam energy spread at 30 kV Extraction

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C- ion beam from the Prototype MIS+CXC ion source unit⎯ Test stand results

Data from the test stand:

MIS:0.2 mL/min CO2 flow rate, 30 kV extraction voltage;

CXC:490 oC magnesium vapor

• Intensity: ~ 60-80 μA C-

• Beam size at CXC exit: r = 5 mm• Beam size at CXC exit: r = ~ 5 mm• Divergence at CXC exit: r’ = ~16 mrad• Emittance: ~ 80 π mm·mrad• Beam energy spread: ± ~320 eV (±1%)

(SRIM calculation result)

Memory-CO2 from OXI compared to CO2 from C-1

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The low-energy end with an energy achromat and a second port for the NEC-SNICS source

LE MagnetR=26.8 cmG=70 mm

NEC 1.5SDH-1 withextended LE Tube HE Magnet

R=75 cmG=75 mm

HE ESA

LE ESAR=30 cmG=75 mm

Gas IonSource

NEC 134 SampleMC-SNICS

R=75 cmG=50 mm

Detector

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Measurements I: Mass spectrometry– Faraday cups & electron multipliers • Some types of MS –...

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Transcript of Measurements I: Mass spectrometry– Faraday cups & electron multipliers • Some types of MS –...