Mass spectrometry 8/23/12

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# Mass spectrometry 8/23/12 - PowerPoint PPT Presentation

Mass spectrometry 8/23/12. What are the principles behind MS? What do all MS instruments have in common? What are the different types of MS?. Lecture outline: Introduction to mass spectrometry sample introduction systems, mass analyzers popular combinations in geosciences.

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## Mass spectrometry 8/23/12

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Presentation Transcript

Mass spectrometry 8/23/12

What are the principles behind MS?

What do all MS instruments have in common?

What are the different types of MS?

• Lecture outline:
• Introduction to mass spectrometry
• sample introduction systems, mass
• analyzers
• popular combinations in geosciences

JJ Thomson’s cathode ray tube, 1897

Introduction to Mass Spectrometry

Sample

introduction

Count ions

Separate

masses

Collect results

Ionization

Minimize collisions, interferences

Nier-type

mass spec

Basic equations of mass spectrometry

Ion’s kinetic E function of accelerating voltage (V) and charge (z).

Centrifugal force

Applied magnetic field

balance as ion goes through flight tube

Combine equations to obtain:

Fundamental equation of mass spectrometry

Change ‘mass-to-charge’ (m/z) ratio by

changing V or changing B.

NOTE: if B, V, z constant, then:

If:

B in gauss

r in centimeters

m in amu

V in volts

z in electronic charge

then….

What magnetic field strength would be required to focus a beam of CO2+ ions on

a collector of a mass spectrometer whose analyzer tube as a radius of 31.45cm,

assuming a voltage of 1000V?

Change your magnetic field strength by -10%, what voltage puts the CO2 ions

into the collector?

Examples of mass spec data output

Ex: B

You can scan in B or V to sweep masses

across a single detector.

OR

You can put different masses into

multiple cups without changing B or V.

Sample Introduction Systems (aka “front ends”)

1) Gas source (lighter elements)

dual inlet - sample purified and measured with standard gas at identical conditions

precisions ~ ±0.005%

continous flow - sample volatized and purified (by EA or GC) and injected into

mass spec in He carrier gas, standards measured before and after,

precisions ~ 0.005-0.01%

2) Solid source (heavier elements)

TIMS - sample loaded onto Re filament, heated to ~1500°C, precisions ~0.001%

laser ablation - sample surface sealed under vacuum, then sputtered with laser

precisions ~0.01%?

3) Inductively coupled plasma (all elements, Li to U)

ICPMS - sample converted to liquid form, converted to fine aerosol in nebulizer,

injected into ~5000K plasma torch

Ionization occurs in the ‘source’

Electron Ionization

Gas stream passes through beam of e-,

positive ions generated.

Thermal Ionization

Plasma: Gas stream passes through plasma

maintained by RF current and Ar.

Themal: Filament heated to ~1500C

Mass Analyzers - the quadrupole vs. magnetic sector

Changes DC and RF

voltages to isolate

a given m/z ion.

PRO: cheap, fast, easy

Magnetic Sector:

Changes B and V to focus

a given m/z into detector.

PRO: turn in geometry means

less ‘dark noise’,

higher precision

Two types of ion detectors

A) Faraday collector - long life, stable, for signals > 2-3e6 cps

B) Electron multiplier - limited life, linearity issues, high-precision, signals < 2e6 cps

Popular combinations

Gas source

1) Dual inlet isotope ratio mass spec (at GT, Lynch-Steiglitz and Cobb)

- O, C, H ratio analyses

2) Elemental analyzer IRMS (at GT, Montoya)

- N, C, S ratio analyses

3) Gas chromatograph IRMS (at GT, Chemistry)

- compound-specific ratio analyses

Solid source

1) Thermal Ionization mass spec (multi-collector)

- heavy metals, REE

ICP

1) ICP quadrupole mass spec (at GT, Taillefert)

- trace metal analysis

2) Single collector magnetic sector ICPMS

- higher-precision trace metal

analysis

2) Multi-collector ICPMS (nearest at USC)

- U/Th dating, TIMS

replacement

Micromass IsoProbe - MC-ICPMS

Inductively Coupled Plasma Mass Spectrometry

detector

Shared components

of all ICPMS machines

mass/charge

discriminator

high vacuum

10-7 bar

sample cone

skimmer cone

“fore” vacuum

10-4 bar

instrument housing

atmospheric

pressure

• - measure concentrations
• as low as several ppt
• - no fuss sample preparation
• (dissolve in 5% HNO3)
• - get beam intensity
• vs. mass/charge ratio

or magnetic

sector

and ion counter (electron multiplier)

The torch box of an

Agilent 7500 ICPMS

spray chamber

Ar feed

torch

RF coil

The sample cone isolates the

torch from the interior.

High-resolution ICPMS

2. High resolution ICPMS

aka double-focusing ICPMS

aka magnetic sector ICPMS

- same front end as Q-ICPMS

- combines magnet w

electrostatic analyzer

electrostatic

analyzer

separates

ions by charge

magnet

separates

ion by mass

and EM

Multi-collector ICPMS

3. MC-ICPMS

- same front end as other ICPMS

- same magnet and ES as

HR-ICPMS

- multiple detectors spaced 1amu

apart enable simultaneous

measurement of many (~7) isotopes

-good for what kinds of systems?

Low vs. High – resolution ICPMS and Interferences

56Fe

very low concentrations

in environmental samples,

but high interest (why?)

Unfortunately, 56Fe has the

same atomic wt as ArO

(40Ar+16O)

INTERFERENCE!

HR-ICPMS measurement =

can distinguish 56Fe from ArO

NOTE: most elements can be

distinguished with a low

The importance of standards in mass spectrometry

ICPMS: Can determine concentration to ~1% R.E. using calibration curve (below)

Can monitor Sensitivity (signal response for given

solution concentration) over time

unknown sample =

8.2e7 cps,

conc ~ 10.5ppb

REMEMBER: all mass spectrometers are “black boxes” we really have no idea what goes on from sample container to detector signal

• Ex: you measure a count-rate of 10,000 cps for a given element, but you need to know how many atoms of that element, or its concentration, were in your sample
• measuring isotope ratios is a powerful approach because we can measure
• samples against standards with known isotopic ratios (it’s much more difficult to change a material’s isotopic ratios than it is to change its elemental concentration!)
• isotope dilution takes advantage of ability to precisely measure ratios
• ALL measurements need to include blanks and standards (either concentration or ratio standards)

Isotope dilution principle

Isotope dilution is an analytical technique used in combination with mass spectrometry

to determine the concentration of element x in unknown samples.

ex: Rb

A known amount of “spike” with

known elemental concentration

and isotopic abundances

(what’s the diff?)

is added to sample with unknown

elemental concentration but

known isotopic abundances.

• Requirements:
• The sample has natural (or known) isotopic abundance (usually true).
• The spike and sample isotopic ratios are different.

More Commonly used ICPMS terms

Nebulization efficiency – the amount of solution that reaches the plasma (~1%)

- varies with sample matrix

- surface tension, viscosity, and density of solution will affect neb. eff.

- usually all standards, spikes, and samples are introduced as 2-5% HNO3

- an acid solution reduces complexation, surface adsorption

Matrix effects – the changes in ICP characteristics with variable matrices

- largely black box (we see these effects, cannot wholly explain/predict them)

- you must carefully match the matrices of your standards/samples to

obtain quantitative results

Ionization efficiency – the amount of ions produced per atoms introduced

- depends on matrix, focusing of beam through cones, lenses

- usually no better than 1/1000

ICP detection limits for a variety of elements

ICP-OES

ICP-MS

Detection limit – defined as 3 x the S.D. of the signal as the concentration of the analyte approaches 0 (measure stability at a variety of conc’s, extrapolate to 0; or measure

5% HNO3 blank solution)

Ion microprobe

(or

Secondary

Ion

Mass

Spectrometry

 SIMS)

• use an ion beam (usually Cs+1) to “sputter” a sample surface; secondary ions fed into mass spec

20μm

Accelerator Mass Spectrometry

The AMS at University of Arizona (3MV)

• prior to AMS samples were 14C-dated by counting the number of decays
• - required large samples and long analysis times
• -1977: Nelson et al. and Bennett et al. publish papers in Science demonstrating
• the utility of attaching an accelerator to a conventional mass spectrometer
• Principle:
• You cannot quantitatively remove interferring
• ions to look for one 14C atom among several
• destroy molecular ions (foil or gas)
• filter by the energy of the ions (detector)
• to separate the needle in the haystack.

The AMS at LLNL (10MV)

c) ACCELERATOR

generates 2.5 million volts,

accelerates C- ions

b) INJECTOR MAGNET

separates ions by mass,

masses 12, 13, and 14 injected

d) TERMINAL

C- ions interact with

‘stripper’ gas Ar,

become C+ ions,

molecular species CH

destroyed

e) ELECTROSTATIC DEFLECTOR

specific charge of ions selected (3+)

a) ION SOURCE

generates negative

carbon ions

by Cs sputtering

f) MAGNETIC SEPARATION

13C steered into cup, 14C

passes through to solid detector

g) Si BARRIER DETECTOR

pulse produced is proportional to the energy of ion, can differentiate b/t 14C and other ions count rate for modern sample = 100cps

http://www.physics.arizona.edu/ams/education/ams_principle.htm

Hurdles in mass spectrometry

1) Abundance sensitivity - ratio of signal at mass

m to signal at m+1

- better with better vacuum

- acceptable values: 1-3ppm at 1amu

2) Mass discrimination

- heavier atoms not ionized as

efficiently as light atoms

- can contribute 1% errors to

isotope values

- can correct with known (natural)

isotope ratios within run, or with

known standards between runs

Hurdles in mass spectrometry (cont.)

3) Dark Noise - detector will register signal even without an ion beam

- no vacuum is perfect

and

- no detector is perfect

- must measure prior to run to get “instrument blank” if needed

4) Detector “gain” - what is the relationship between the electronic signal recorded

by the detector and the number of ions that it has counted?

- usually close to 1 after factory calibration

- changes as detector “ages”

- must quantify with standards

Cardinal rule of mass spectrometry: