Mass spectrometry 8/31/10

What are the principles behind MS?

What do all MS instruments have in common?

What are the different types of MS? Lecture outline:1) Introduction to mass spectrometry

2) sample introduction systems, mass analyzers

3) popular combinations in geosciences

JJ Thomson’s cathode ray tube, 1897

Introduction to Mass Spectrometry

Sample introduction

IonizationMinimize collisions, interferences

Separatemasses

Count ionsCollect results

Nier-type mass spec

Basic equations of mass spectrometry

21

2mv zV

2 /F mv R

F Bzv2 /mv R Bzv

2 2/ / 2m z B R V

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

Centrifugal force

Applied magnetic field

balance as ion goes through flight tube

Fundamental equation of mass spectrometry

Combine equations to obtain:

Change ‘mass-to-charge’ (m/z) ratio bychanging V or changing B.

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

r m

If: B in gaussr in centimetersm in amuV in voltsz in electronic chargethen….

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 ionsinto the collector?

V

rBx

z

m 22510825.4

Examples of mass spec data output

You can scan in B or V to sweep massesacross a single detector.

OR

You can put different masses intomultiple cups without changing B or V.

Ex: B

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 IonizationPlasma: Gas stream passes through plasmamaintained by RF current and Ar.

Themal: Filament heated to ~1500C

Mass Analyzers - the quadrupole vs. magnetic sector

Quadrupole:Changes DC and RFvoltages to isolatea given m/z ion.PRO: cheap, fast, easy

Magnetic Sector:Changes B and V to focusa 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 combinationsGas source1) 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, ????)- compound-specific ratio analyses

Solid source1) Thermal Ionization mass spec (multi-collector)

- heavy metals, REE

ICP1) 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- U/Th dating, TIMS replacement

Micromass IsoProbe - MC-ICPMS

Inductively Coupled Plasma Mass Spectrometry

sample cone

skimmer cone

instrument housing

mass/chargediscriminator

detector

atmosphericpressure

“fore” vacuum10-4 bar

high vacuum10-7 bar

Shared componentsof all ICPMS machines

or magneticsector

Faraday cupand ion counter (electron multiplier)

1. Quadrupole ICPMS- measure concentrations

as low as several ppt

- no fuss sample preparation(dissolve in 5% HNO3)

- get beam intensityvs. mass/charge ratio

2. High resolution ICPMSaka double-focusing ICPMSaka magnetic sector ICPMS

- same front end as Q-ICPMS

- combines magnet welectrostatic analyzer

electrostaticanalyzer

separatesions by charge

magnetseparates

ion by mass

Faraday cupand EM

High-resolution ICPMS

Multi-collector ICPMS

3. MC-ICPMS

- same front end as other ICPMS

- same magnet and ES as HR-ICPMS

- multiple detectors spaced 1amuapart enable simultaneous

measurement of many (~7) isotopes

-good for what kinds of systems?

56Fevery low concentrations

in environmental samples,but high interest (why?)

Unfortunately, 56Fe has thesame atomic wt as ArO

(40Ar+16O)

Quadrupole measurement =INTERFERENCE!

Low vs. High – resolution ICPMS and Interferences

HR-ICPMS measurement =can distinguish 56Fe from ArO

NOTE: most elements can bedistinguished with a lowresolution quadrupole

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 givensolution 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 measuresamples 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 spectrometryto 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 unknownelemental concentration butknown isotopic abundances.

Requirements: 1) The sample has natural (or known) isotopic abundance (usually true).2) 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-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)

ICP detection limits for a variety of elements

Ion microprobe(orSecondaryIonMassSpectrometry 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 demonstratingthe utility of attaching an accelerator to a conventional mass spectrometer

The AMS at LLNL (10MV)

Principle:You cannot quantitatively remove interferring ions to look for one 14C atom among several

quadrillion C atoms.Instead, you

a) destroy molecular ions (foil or gas)b) filter by the energy of the ions (detector)

to separate the needle in the haystack.

a) ION SOURCEgenerates negative

carbon ionsby Cs sputtering

b) INJECTOR MAGNETseparates ions by mass,

masses 12, 13, and 14 injected

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

c) ACCELERATORgenerates 2.5 million volts,

accelerates C- ions

d) TERMINALC- ions interact with

‘stripper’ gas Ar,become C+ ions,

molecular species CHdestroyed

e) ELECTROSTATIC DEFLECTORspecific charge of ions selected (3+)

f) MAGNETIC SEPARATION13C steered into cup, 14C

passes through to solid detector

g) Si BARRIER DETECTORpulse produced is proportional to the energy of ion, can

differentiate b/t 14C and other ions count rate for modern sample = 100cps

Hurdles in mass spectrometry

1) Abundance sensitivity - ratio of signal at massm 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

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:Your measurements are only as good as your STANDARDS!

Standards (both concentration and isotopic) can be purchased from NIST

Hurdles in mass spectrometry (cont.)

Ex: NBS-19, O, C carbonate isotopic standard