An Introduction to ICP MS

7/27/2019 An Introduction to ICP MS

1/47

1

The world leader in serving science

From first principles:

An introduction to the

ICP-MS technique

This presentation describes the history of how ICP-MS was developed,

gives the characteristics of modern ICP-MS instruments and explains the

theory of how ICP-MS works.


2/472

2

Design requirements for new analytical massspectrometer March 1971

Courtesy of Kym Jarvis, National Environmental Research Centre (NERC) Facility, UK

The story of ICP-MS begins back in 1971 at the University of Surrey, UK.

Environmental and geological scientists were looking for a new analytical

mass spectrometer to help characterize their samples. Their initial designrequirements were quite modest by todays standards. They wanted to be

able to analyze between 4 and 6 samples per hour (10-15 minutes per

sample) and detect all metallic elements from lithium to uranium mass

range from 6 to 238 amu. They were searching for detection limits of

around 100 ppb - considerably below the possible detection level of the

early optical ICP instruments around in those days - and they would be

happy with precision of around 25% RSD. They were interested in

elemental concentrations, but also isotopic information (mostly given asratios), which was another reason for looking at mass spectrometry.


3/473

3

Some development landmarks

Alan Gray, Applied Research Laboratories

(1974, 1975)DC plasma

3000-6000K

Sufficient to ionise elements


4/474

4

First DCP - 1971

Alan Gray, Applied Research Laboratories

This slide shows a photograph of the Alan Grays first DCP-MS instrument

in around 1971. The DCP-MS proved that a plasma source mass

spectrometer could be a useful and successful tool for inorganic analysisand detection limits looked reasonably promising. The source did limit the

techniques element coverage and there was a need for a higher energyionisation method.


5/475

5

Development landmarks

Decision to replace DC plasma with Inductively Coupled Plasma

(ICP) Parallel developments in UK (Uni of Surrey and VG now

Thermo), USA (Iowa State Uni) and Canada (Sciex)

First spectra obtained by Alan Gray & Alan Date at Uni of Surrey

in 1981

First commercial instruments ordered in 1983 (VG/Thermo and

PE/Sciex)

This lead to a decision to replace the DC plasma with an inductively

coupled plasma (ICP), since this excitation source was known to be able

to produce more energetic plasmas, capable of ionizing more elements toa higher degree. At about this time (~1980), parallel developments for an

ICP-MS began in the UK with Alan Grays group at the University of

Surrey, in the USA with Sam Houks group at Iowa State University, and in

Canada with Don Douglass group at the University of Toronto.

Associations were formed between Grays group and a company called

VG (now part of Thermo), and Douglass group and a company called

Sciex (now a joint-venture with PerkinElmer Instruments). The race wason to produce a commercial product!

The first commercial instruments shipped at around the same time in 1983from both VG (Thermo) and Sciex.


6/476

6

Early development: Experimental instruments

Vertical Plasma instrument

Uni of Surrey

c.a. 1980

Angel Plasma instrument

Uni of Surrey

c.a. 1981

These two photos show some early experimental ICP-MS instruments

from Grays group. The left photo shows a vertical plasma instrument from

around 1980 and the right hand photo shows the so-called Angel plasmainstrument. These were experiments in ion sampling from the plasma.


7/477

7

Towards commercialization

NERC research instrument

Uni of Surrey/VG (Thermo)

early 1980s

Prototype PQ instrument atBGS Grays Inn Road

VG (Thermo), early 1980s

These photos show some prototype instruments slightly closer to the

finished commercial product from VG (Thermo). The instrument on the left

is still functioning in Kym Jarviss lab at the NERC facility in the UK. Theinstrument on the right is a prototype supplied to the British Geological

Survey. These prototypes paved the way towards the first VG commercialinstrument.


8/478

8

Finally a product! And another

Thermo (VG) commercialize the

first PlasmaQuad ICP-MS1983

1990 PQ 2 ICP-MS Launched

Sensitivity increased, stability

improved, ease-of-use improved

Finally in 1983 VG (Thermo) launched the PlasmaQuad instrument. This

was a successful product that was installed in many research facilities. It

had limited application to the routine analysis market since it requiredsome expertise to use.

In 1990 VG launched the PlasmaQuad 2 which was a refinement of the

original PlasmaQuad that was repackaged in an easier to use format. In

addition, some of the performance characteristics, such as sensitivity and

stability were improved. This platform was incredibly successful and reallybegan to find its way into routine labs.


9/479

9

And anotherand another

PQ 3 ICP-MS Launched

Huge ease-of-use improvements,

sensitivity improvements, compact

design, all computer-controlled,cool plasma capability

1994

1997

VG PQ ExCell ICP-MS Launched

1st PQ with Collision CellTechnology, new software platform

Collision/Reaction Cell Technology for

ICP-MS invented

1999

By 1994, the demand for routine usage of the technique had grown. The

VG response was the PQ 3, a compact ICP-MS that was under complete

PC control. This allowed much greater automation and made it mucheasier to use. Some performance enhancements were also added,

including cool plasma that allowed the measurement of several elements

that were previously impossible due to high backgrounds or gas-based

interferences. PQ 3 instruments also shipped under brand names such as

Fisons Instruments and TJA Solutions, as VG was acquired by first

Fisons and then Thermo Electron Corporation as part of the ThermoJarrell Ash (TJA) group.

By 1997, there had been a new technological development in ICP-MS:

the introduction of collision/reaction cells. This new technology provided

an instrumental method of removing polyatomic interferences one of the

longstanding problems for the technique. In 1997, collision reaction cell

technology for ICP-MS was invented by GVI (previously Micromass, now

Thermo) and in 1999 the VG PQ ExCell was launched. This was the first

collision cell PQ and was based on the PQ 3 platform. The collision cell

opened up new applications and enabled the analysis of previously

challenging or impossible analytes and sample types. A new softwareplatform also simplified the use of the system.


10/4710

10

And another 25 years of improvements!

X Series ICP-MS Launched

Super-compact design for ease of use

and low maintenance, 2nd Generation

Collision Cell with KED, newsoftware, fastest ever analysis

20

00

20

05

2001

Xi / Environmental Interface Launched

XSeriesII ICP-MS Launched

Xt and Xs interfaces ultimate matrix

tolerance,-Extraction, 3rd Generation

Collision Cell lowest ever DLs and

freedom from interference

In 2000, we launched the environmental interface (later known as the Xi interface); a

cone set with special properties to aid routine environmental analysis. This enabled the

measurement analytes typically found at high concentrations in environmental samples,

while still allowing ppt level measurements of toxic analytes. It also improved theinstruments tolerance to highly concentrated matrices by reducing drift and polyatomicinterferences.

In 2001, the X Series ICP-MS was launched. This heralded the modern age of ICP-MS. It

was a super-compact bench-top system that redefined the ease-of-use and maintenance

of the technique and massively improved the speed of analysis by incorporating

productivity enhancing design features and software functionality. Performance was also

improved with the release of our improved 2nd generation collision cell technologydesigned for kinetic energy discrimination.

In 2005 Thermo launched the new XSeriesII. This builds on the ideas in the X Series and

gives the customer more productivity, more practicality and more performance. The key

innovations include developments in interface technology (the Xt and Xs interfaces),

designed for even better tolerance to high matrices; a new optics design (PI extraction),

producing the lowest ever detection limits and; our 3rd generation of collision cell

technology, providing the simplest, most effective interference removal technologyavailable.

More than ever before, this latest generation of ICP-MS provides overlap with Optical ICP

characteristics, but also produces detection limits at the ppt-level and below withexcellent freedom from interference.

So where has the modern technique of ICP-MS got to? We are now able to analyze

samples for up to 75 elements for many sample types in timescales of between 2 and 6

minutes per sample (10-30 samples per hour), with detection limits in the ppq to ppt

range for most elements and RSDs of around 1-5%. CRC technology also removes many

of the old problems with interference. Modern ICP-MS has more than surpassed theexpectations of Alan Grays group in the early 1970s.


11/4711

11

Key features of modern ICP-MS

A quantitative elementalanalysis technique

Accommodates a broadrange of samples andmatrices

Nearly all elements inperiodic table

Excellent sensitivity

Wide linear dynamic range

Enough history. Now onto some characteristics.

ICP-MS has many unique features which make it attractive as a techniquefor quantitative trace element analysis.

ICP-MS is capable of accommodating a broad range of sample types.

Although in its basic form, ICP-MS analyses materials as a solution, it can

be successfully coupled to laser (enabling direct analysis of solids) or

gases (for example by hydride generation). More recently, ICP-MS has

been coupled with techniques such as HPLC and GC to providespeciation analysis of certain elements.

The technique is applicable to nearly all elements in the periodic table, so

that all commonly occurring elements can typically be measured in oneanalysis.

ICP-MS is a highly sensitive technique, providing the analyst with

determinations down to sub ppt levels, lower than most other analytical

techniques. This sensitivity is very well complemented by its very widedynamic range.

As environmental regulations and process control require ever-increasing

sensitivity, ICP-MS continues to become the analytical technique of choicein a wide variety of application areas.


12/4712

12

ICPIon optics and mass

spectrometerIon detection

What is ICP-MS?

Instrument comprises five basic steps:

Generating an aerosol of the sample

Ionizing sample in the ICP source

Extracting ions in the sampling interface

Separating ions by mass

Detecting ions, calculating the concentrations

Sampling

interface.

Sample intro

We will now move onto the theory of how ICP-MS works.

The technique can be simplified in five main stages or processes.

Although ICP-OES and ICP-MS use the same kind of plasma source, the

steps towards the analytical outcome are quite different. The sample

introduction stages are essentially the same in both ICP-OES and ICP-

MS. The sample is introduced into the instrument by nebulization into afine aerosol which is then carried into the plasma to generate excitation.

In ICP-OES the excitation and relaxation processes occurring in theplasma produce light emitted from the elements of interest. In ICP-MS the

goal of the plasma is to remove electrons from the analyte atoms and

generate ions. The ICP conditions are optimized to achieve this as

efficiently as possible, trying to minimize the formation of multiply-chargedand molecular ions.

These positive ions are sampled from the plasma through a sampling

interface and directed into the mass spectrometer, where they are

separated according to mass (strictly mass to charge ratio), beforereaching the instrument detector.


13/4713

13

ICP-MS: characteristic mass spectrum

Simple spectra (primarily M+ions)

- Simple interpretation

Very high signal tobackground

- Low detection limits

ICP-MS Spectrum - Vanadium (51V)ICP-AES Spectrum - Vanadium 10 mg/L

Many emission lines

High continuum background

The mass spectra of all elements contain a small number of peaks. Even

ifall elements in the Periodic Table were present in the sample there

would only be 211 peaks in the mass spectrum. Every element (with theexception of indium) has at least one isotope that is at a unique mass. The

peak position along the mass scale (x axis) identifies the isotope present

(ie provides qualitative analysis) and the peak intensity (y axis) is

proportional to the concentration. Quantitative analysis is obtained by

analyzing standards of known concentration for each of the isotopes of

interest and comparing the peak intensities of the standards and samples.

Mass spectra tend to be much less complex and far more predictable than

optical spectra. For example, the mass spectrum for vanadium comprises

two peaks - one isotope at mass 51 and a small peak at mass 50 is

extremely simple compared to the rather complex ICP optical emissionspectrum comprising many lines, and begins to illustrate why ICP-MS is

often less troubled by interferences than ICP-OES and other atomic

spectroscopy methods. You will also notice that optical spectra often have

quite high continuum backgrounds, whereas the mass spectrum is often

characterized by low backgrounds, yielding ppt detection limits for mostelements.


14/4714

14

Location of ICP-MS components

This simplified schematic of an ICP-MS will be used to explain the

positions, functions and relationship between the key components of the

system. The following slides will then deal with each area in turn, followingthe journey of the sample through the ICP-MS:

Sample introduction system (spray chamber and nebulizer)

ICP (torch and power supply)

Mass spectrometer Interface

Lens system (ion optics)

Mass analyzer

Vacuum system (Mechanical and turbomolecular pumps)Detector

Data handling and system controller


15/4715

15

Components of an ICP-MS

Sample introduction system

ICP

Interface

Lens system

Mass analyzer

Vacuum system

Detector

The first area to look at is the sample introduction components.

For the purposes of this workshop we will consider only solution samples.

However, it is possible to directly sample solid samples (for example usinglaser ablation) and gases (for example by hydride generation).


16/4716

16

Sample introduction

Most common samples are aqueoussolutions

Sample delivery by peristaltic pump

Fine (


17/4717

17

Sample introduction - nebulizer types

Pneumatic nebulizers

Concentric

Excellent sensitivity and stability

~ 1 ml/min uptake rate

Micro concentric

Low flow nebulizers

< 0.1 mL/min uptake rate for micro samples, organics or radioactive samples

PTFE, PFA or PVDF for excellent corrosion resistance

Low blank levels - ideal for semiconductor labs

Parallel path or cross flow

Heavier matrix, higher TDS samples or particulate matter

The job of the nebulizer is to generate a fine aerosol of the sample. It does

this by contacting the flow of liquid sample with a high velocity stream of

argon gas at its tip. The gas makes the liquid break up into small dropletswhich are carried into the spraychamber by the gas stream. A number ofdesigns of nebulizer are available:

Glass concentric nebulizer

Advantage: stable aerosol; easy setup; widely available

Disadvantages: will not tolerate high total dissolved solids (TDS) oraggressive acids (e.g.HF)

Other nebulizers are available for use in different applications e.g. Inertnebulizers such as the concentric Burgener and polyimide types

Inert nebulizer

Advantages: can tolerate aggressive acids and high TDS

Disadvantages: less stable, less sensitive.

Inert low flow nebulizers such as the PFA-50 are also available

Advantages: HF resistant; useful for small sample volumes; contaminationfree (ideal for semiconductor applications)

Disadvantages: will not tolerate high TDS.

Applications using high TDS samples or very fine particulates, emulsified

samples and colloidal materials can be successful, using suitablenebulizers such as cross-flow or parallel path designs.

Micro-volume samples, organic matrices, radioactive samples are all

ideally suited to the range of micro-concentric nebulizers, includingcorrosion-resistant designs.


18/4718

18

Sample introduction droplet size

Fine (


19/4719

19

Sample introduction spraychamber cooling

Spraychamber cooling to ~ 2oC 5oC

Water-jacket or Peltier device

Less solvent to plasma

Stable plasma

Lower moleculars (oxides)

Spraychamber cooling to 15oC

Removal of organic solvents

Performance can also be improved by cooling of the spraychamber.

Cooling removes most of the sample solvent (water), making processes in

the plasma even more efficient, enhancing sensitivity for most elements.Eliminating solvent also reduces some solvent-derived interferences and

reduces oxide formation, again simplifying spectra even further. Cooling

also tends to improve long-term stability as the processes occurring in the

chamber are under temperature control and are therefore not sensitive to

lab temperature changes. The chamber can be cooled using water jacketor using a Pelter device (electrothermal cooling).


20/4720

20



ICP

Interface

Lens system

Mass analyzer

Vacuum system

Detector

Once the sample aerosol is generated, it is introduced to the inductively

coupled plasma (ICP). The plasma is a very high temperature (6000-

10000 Kelvin) environment of atoms, ions and electrons. At thesetemperatures many elements make the transition from the atomic to

the ionic state since there is sufficient energy to completely remove

their outer shell electron. A very useful characteristic of the argon ICP

is that the great majority of the elements form mainly singly-charged

positive ions. This helps to generate a simple mass spectrum evenwhen lots of elements are present in the sample.

The inductively coupled plasma source consists of two primarycomponents.

1. RF generator

2. Torch assembly


21/4721

21

RF generator

Solid state generator

RF generator frequency is typically 27 MHz or 40 MHz

27 MHz provides higher sensitivity with ICP-MS

RF power is typically 0.6 - 1.5 kW

Water-cooled copper load coil

Coupling efficiency 70-75%

Actual power from generator that reaches the plasma

Most instruments use solid state RF generators that operate at either 27

or 40 MHz. The load coil carries this oscillating RF energy from the

generator to the area around the end of the torch and is often made fromcopper and is usually water-cooled. Most of the energy produced by thegenerator reaches the plasma, so coupling efficiency is quite high.


22/4722

22

Matching networks

Compensates for changes in impedance Impedance A materials resistance to the flow of an electric

current

Hence compensates for changes in sample matrix or solventvolatility

Crystal controlled Frequency locked at 27.12 MHz or 40.68 MHz

Uses servo-driven capacitors

Free running Frequency at ~40 MHz

Electronic tuning of frequency changes (impedance) due tomatrix/solvent

Most instruments use a matching network to control the impedance of the

plasma. Plasma impedance changes are the content of the plasma

changes (e.g. when the plasma contains air of a sample). As theimpedance changes, the plasma energy must continue to be balanced to

maintain a stable plasma. This is the job of the matching network. This is

normally performed by some motor driven capacitors that adjust the drive

impedance dynamically as the plasma impedance changes.

An alternative design is the so-called free-running generator which adjustsimpedance by shifting the RF frequency.


23/4723

23

The ICP-MS torch

Torch consists of three

concentric tubes

One piece or demountable

types available

Torch Is mounted horizontally

(axially)

Approx 10-20 mm from

interface

Gas flows optimized for M+ ion

formation and low molecular

(oxide) species

Most elements achieve their first ionization potential at a temperature of

approximately 7,000 K. This temperature is achieved at approximately

12-14 mm from the first turn of the load coil.The torch comprises three concentric quartz tubes, mounted axially (the

end of the torch faces the sample cone, separated by 10-20mm).

1. The inner tube (the torch injector) is used to direct the nebulizedsample into the plasma.

2. The next tube, supplying argon gas (the auxiliary gas), serves to keepthe plasma away from the torch injector to prevent it from melting.

3. The outer tube extends through the load coil and is supplied with

enough argon gas (the cool gas) to support the plasma and to prevent

the torch from melting.

The plasma is initially started without sample introduction, and then

optimized for singly charged positive ion formation and low molecular(oxide) species.


24/4724

24

The ICP

Arcoolant flow

Magnetic fields

High temperature

plasma -generatesions

Sample aerosol in Arnebulizerflow

++

++

+ +

++

Arauxiliary flow

Load coil

A very hot plasma of ionized argon

and free electrons.

Plasma sustained by a radio

frequency (RF) induction coil

Plasma constrained within a quartz

torch

The ICP is an argon plasma, constrained inside a quartz torch. It is

inductively coupled, which means it is sustained by a 2 turn induction

coil, supplied with a radio frequency (27 or 40 MHz) oscillating field. Thiscoil induces a powerful oscillating magnetic field inside the torch which

oscillates ionized argon gas. The rapidly oscillating argon ions

energetically collide with other argon atoms and cause ionization of those

atoms. This process of ionization is self-sustaining with continuously

applied RF to the load coil and continuously supplied argon into the torch.

There are 3 argon flows in 3 concentric channels. The outer annulus

carries most of the supplied argon (coolant gas). The central injector tube

carries the argon sample aerosol. The annulus in between carries the

auxiliary flow which controls the position of the plasma and prevents

damage to the injector tube by the high temperature plasma.

The sample aerosol punches a channel through the axis of the plasma

and the ionization zone is positioned above the load coil, from whereanalyte ions can be easily sampled.


25/4725

25

Processes in the plasma

+

Very fine aerosol droplets

ensure high efficiency in 4

key processes

As a sample aerosol droplet passes through the plasma, the following fourprocesses occur rapidly:

1) Solvent evaporates from the sample matrix.

2) The sample matrix is vaporized.

3) Vaporized sample is completely atomized.

4) Analyte atoms are ionized.

Introducing a very fine sample aerosol to the plasma ensures maximum

efficiency in these important plasma processes. The end goal is the

efficient production of single charged ions that can be sampled into the

mass spectrometer.


26/4726

26

Degree of ionization

H0.1

Li100

Na100

Ru96

K100

Rb100

Cs100

Fr

Sr96(4)

Ca99(1)

Mg98

Be75

Tl100

Ra

Ba91(9)

Ac

Sc100

Y98

La90(10)

Ti99

Zr99

Hf98

Cr98

Mo98

W94

Ta95

Nb98

V99

Th100

Fe96

Re93

Tc

Mn95

Ni91

Au51

Ir

Co93

Rh94

Os78

Ag93

Cu90

Pd93

Pt62

Nd99

Ar0.04

B58

Hg38

Zn75

Cd85

C5

Si85

In99

Ga98

Al96

F9e-4

N0.1

Pb97

Sn96

Ge90

O0.1

Ne6e-6

Cl0.9

S14

Bi92

As52

Sb78

P33

Ce98(2) Eu100 Tm91(9)

U100

Pa

Pr90(10) Sm97(3)

Gd93(7

)

Xe8.5

Kr0.6

AtPo

Te65

Se33

Br5

I29

Rn

Tb99

Dy100

Yb92(8)Er99Pm Ho

Np Pu Am Cm Bk Cf Es Fm Md LwNo

He

Calculated values of degree of ioni zation of M+ and M+2

(T=7500K, ne = 1e15cm

-3

) *Houk 1986

Lu

M+

M+ + Mx 100%

The nebulizer gas flow is optimized to enable a relatively long sample

residence time in the plasma to allow the aerosol solvent to be

evaporated, analytes to vaporize, atomize and finally ionize. The ICPsource is highly efficient and enables 80% of elements to be more than

75% ionized. Many of these elements are 100% ionized. The large

majority of elements yield only singly-charged ions because their second

ionization potential is too high to allow doubly-charged ions to form.

However, some elements, such as the rare earth group, can form doubly-

charged ions (typically < 3 % of the singly charged ion level). Even so, the

predominance of singly charged ions is a significant factor in both thesensitivity of the method and the simplicity of the mass spectra.

Ionization efficiency is element-specific. All elements do not ionize to the

same extent. The first ionization potential is the energy required to releasethe first electron from the outermost orbit of an atoms electron shell. The

periodic table of the elements is characterized in blocks and each block

e.g. transition element group or the actinide group have broadly similarionization potentials within their group.

Most elements are very efficiently ionized in the plasma. The most

challenging are the halogens, phosphorus, sulphur, arsenic, selenium,

mercury, which have ionization efficiencies ranging from around 50%,down to 1% or even less. In practice, F, Cl, noble gases, C, N, O are notreadily measured by ICP-MS.


27/4727

27

In an argon plasma, 80% of elements are > 75% ionized

Few oxide molecular ions

- Worst case Ce, forms around 2% [CeO]+

Most elements yield mainly singly charged ions

- Worst case is Ba, at around 3% Ba++

Plasma Ionization: Oxides, 1+ and 2+ Ions

0

20

40

60

80

100

120

5 6 7 8 9 10 11 12 13 14 15

First ionization potential (eV)

Degree

of

Ionization

in

the

ICP

(%)

Everything present in the plasma (such as argon and oxygen) also give

rise to peaks, and there are also combinations of these. The plasma

properties are normally optimised to minimise these interferences andmaximise singly charged positive ion signal. Under normal operatingconditions, approximately 80%of elements are greater than 75% ionized.

In ICP-MS, positive, singly charged analyte ions are the species measured

by the mass spectrometer. The ICP is a very efficient ionization source for

the great majority of elements. A few elements have doubly charged ionstoo, up to 3% of their ion population at worst.

A small number of elements form oxide molecular ions in the plasma.Again, plasma conditions are optimized to keep this to a minimum. The

worst element is Ce, which has an oxide contribution of 2%, relative to the

Ce+ signal.

The factors that control these contributions are:

-type of nebulizer, spraychamber, torch and sampling interface design

-sample uptake rate

-nebulizer gas flow rate-distance from the plasma to the sampling cone.

The last three can normally be optimized to produce satisfactorycharacteristics.


28/4728

28



ICP

Interface

Lens system

Mass analyzer

Vacuum system

Detector

The interface is the region where ions generated in the plasma areextracted and introduced to the mass spectrometer as an ion beam.

The interface region, where ions are extracted from the plasma, is also the

first stage of the 3-stage vacuum system. A vacuum of around 2 mbar is

maintained between the interface cones, using a mechanical roughingpump.


29/4729

29

The interface - sampling ions

ICP at 1 atmosphere

Vacuum between cones ~ 2x10-3

bar (rotary pump)

Supersonic gas and ion jet

extracted by sample cone

Ions admitted to mass

spectrometer by skimmer cone

Interface water cooled

The job of the sampling interface is to sample the plasma-generated ions

from the atmospheric pressure region of the plasma into the high vacuum

region inside the mass spectrometer.

The interface consists of a pair of metal cones with their apertures

positioned sequentially. The region between the two cones is continuously

evacuated of gas by a mechanical roughing pump. The region behind the

second cone (the skimmer) is at lower pressure, generated by a

turbomolecular vacuum pump.

A nickel sampling cone, with a 1 mm orifice, is positioned in the plasma at

the point of maximum ionization. Plasma gas and ions pass through theorifice, accelerating into the partial vacuum as a supersonic jet.

A second (skimmer) cone has a smaller orifice and an even lower vacuum

behind it. This skimmer cone samples ions (and gas) from the supersonic

jet, which contains only gas and ions derived from the plasma.

The whole configuration is designed to maximise the number of ions

reaching the mass spectrometer, whilst minimising the gas load admittedwith the ions.

Beyond the skimmer cone, ions continue as a narrow axial beam towardsthe electrostatic lenses.


30/4730

30

The sampling interface

Ions generated in the plasma are

transported into the mass

spectrometer

Sample cone extracts ions

from the plasma Skimmer cone admits ions to

the mass spectrometer

There is a region of the plasma, above the load coil, where the population

of analyte ions is at its highest. The sampling interface is simply a pair of

metal (nickel) cones, with a partial vacuum between them. The samplercone (outer) is placed in the high ionization region. The cone has a 1mm

orifice, and gas and ions pass through the orifice, accelerated towards thesecond (skimmer) cone.

There is an even higher vacuum behind the skimmer and ions are

admitted to the mass spectrometer through the skimmer cones orifice.

The interface is designed to admit as many ions as possible, of similarkinetic energy, with as little gas loading as possible.

The whole interface is water cooled.


31/4731

31


Sample introduction System ICP

Interface

Lens system

Mass analyzer

Vacuum system

Detector

Focusing: Once in the high vacuum region of the mass spectrometer, the

sampled ions are accelerated and focused into the mass filter by a set of

charged plates or ion lenses.

The ion lenses are positioned immediately after the interface, allowing the

ion beam extracted from the plasma to be focused and steered towards

the quadrupole mass analyzer. The ion lenses are housed in the second

stage of the 3-stage vacuum system, maintained by the first of 2 turbo-

molecular pumps.


32/4732

32

The lens system

Three electrostatic lenses focusand steer the ion beam

Immediately behind the interface Key design criteria

- Low background

- High transmission

- Stable signal

Various designs, e.g.:Photon-stop

Chicane

Reflecting fields

Collision/reaction cells in thisregion

DA

Skimmer Cone

Ion Lens

Extraction

Lens

Slide Valve

Increasing vacuum

Ions are charged particles and are therefore affected by electrostatic fields. The

positively charged ions of interest in ICP-MS are attracted towards a negative potential (voltage)and deflected away from a positive potential.

The ion lenses are supplied with user- and software- adjustable voltages , whichexert forces of attraction and repulsion on the transmitted ions, focusing the beam for maximum

performance by the mass analyzer (quadrupole). The sampled beam includes positive ions, neutral

species, electrons and photons from the plasma. The positive ions are selected and focused by thelenses, whilst negative species are rejected and photons and neutrals are not influenced.

The spectrometer detector is light-sensitive, so photons must be eliminated from

the ion beam. The axis of the quadrupole is not in direct line of sight of the plasma so that photons

are not transmitted to the detector and the job of the ion lenses is to ensure that the ion beam is

efficiently transmitted from the interface into the mass analyzer. There are a number of methods of

doing this. These include the use of a photon-stop, the use of chicane deflectors and the use ofreflecting fields.

A photon-stop is simply a metal plate positioned in the centre of the path of the

material entering the mass spectrometer from the interface. A lens or set of lenses allows the ionsto pass around this plate, while photons and neutrals collide with it and are lost. This method tends

not to be very efficient (lower sensitivity) and results in the need to frequently clean the lens

system. On the positive side, no strong negative fields are required to extract ions from the

sampling interface, resulting in very low spectral contamination from material removed from thecones (good backgrounds and detection limits).

A reflecting field (sometimes called an ion mirror) consists of a ring electrode

that has a repelling positive charge applied to it and another electrode opposite it that has a large

negative charge applied to it. The ions are repelled by the ring electrode and attracted by the

opposite electrode. This makes the ions take a path through a 90 degree bend, while photons and

neutrals pass straight on. This design has quite a high transmission efficiency (high sensitivity), but

normally requires that ions are extracted from the interface with a strong negative extraction field,

meaning that contaminant species from the cones can often be observed in the background

spectrum (causing poor backgrounds and detection limits).Chicane lens designs utilize a pair of deflector plates to move the ion beam axis

laterally, separating the ions from any transmitted photons and neutrals. This design is efficient

(good sensitivity) and can be operated with extraction fields ranging from large negative potentialsto low positive potentials, offering high transmission and low backgrounds and detection limits.

Additionally, on modern instruments, collision/reaction cells are positioned in theion lens part of the system for the removal of polyatomic interferences.


33/4733

33


Sample introduction system ICP

Interface

Lens system

Mass analyzer

Vacuum system

Detector

Having created an ion beam and transported it through a vacuum

chamber we now have to select ions of a particular isotope. In most

designs of ICP-MS, this is achieved by passing the ion beam through aquadrupole mass analyzer which filters out ions of a specific mass tocharge ratio, to generate the mass spectrum.

The quadrupole mass analyzer is positioned out of the line-of-sight and

behind the ion lenses. The quadrupole is in the high vacuum region,

evacuated by the second turbo-molecular pump.


34/4734

34

Quadrupole design criteria

Quadrupole design can be 4 cylindrical or hyperbolic rods

Approx 80-230 mm in length and 9-12 mm in diameter

Typically made from stainless steel or molybdenum

Sometimes ceramic rods with gold coating used

Rods matched to ceramic supports to maintain high precision alignment

May include pre and post filters

Quadrupole pre-filterpost-filter

to detector

A quadrupole essentially consists of four parallel cylindrical or hyperbolicrods.

These rods are typically constructed from stainless steel or molybdenum.Sometimes, ceramic rods may be used which are coated in gold toprovide electrical conductivity.

The rods are precisely fixed within the analyser.

The operation of the quadrupole is enhanced using a pre- or post-filter.


35/4735

35

Quadrupole mass filter

+ve DC

+ve RF

-ve DC

-ve RF

+

-

t = 1 t = 2 t = 3 t = 4

Having created an ion beam and transported it through a vacuum

chamber we now have to select ions of a particular isotope. This is

achieved by passing the ion beam through a quadrupole mass analyzerwhich filters out ions of a specific mass to charge ratio.

A quadrupole consists of four rods, placed equidistant from each other.Taking an ions view of the quadrupole, there are two rods in the

horizontal, x-plane, and two in the vertical, y-plane. The rods in the x-

plane are supplied with a voltage +Vo. The rods in the y-plane are

supplied with an equal and opposite voltage -Vo. The z-axis is the one

along the axis of the rods. Opposite pairs of poles supplied with positiveand negative DC voltages and RF voltages 180o out of phase


36/4736

36

Quadrupole mass filter - continued

Ions traverse in a complex spiral motion One mass transmitted to the detector -

all others collide with the quadrupole

rods

Scanning the voltages (along the scan

line) transmits different masses in turn

-+

+

+ DC and RF

- DC and RF +

Ions already have forward velocity an travel into the space between the

axis of the four rods and move in response to an applied electrical field.

The force which moves them is proportional to the applied voltages. Thevoltage applied changes during the passage of the ion through the

quadrupole. The oscillation of the ion in the changing conditions leads to

ions being repeatedly pulled towards one rod, then another, such that, asthe RF and DC fields are oscillated, the ion adopts a corkscrew motion

along the axis of the rods.


37/4737

37

Quadrupole stability diagram

DC

Offset

RF

MM-1

M+1

Analyzer yields mass spectrum with peak

width of 1 a.m.o.

The amplitude of the applied voltages is automatically scanned and the magnitude of this, dictates

whether the path of an ion of one fixed mass to charge ratio travels along a stable trajectory

through the quadrupole. As the ion is accelerated e.g. to the left hand rod, the voltage changes

and it is now pushed away and attracted to the top rod. As the ion moves towards the top rod, thevoltage reverses again and the ion moves to the right hand rod and so on, acquiring a spiraling

forward trajectory. A given applied voltage will accelerate a light mass ion to high speed, but a

heavy mass to a slower speed. If the ion is accelerated to a speed that is too fast or slow for the

next voltage change to steer it back onto a stable path, it will hit the rods, lose its charge and be

lost to the vacuum system. Since this acceleration process is mass dependent, only an ion of one

particular mass to charge ratio will maintain a stable path through the quadrupole for a given

applied voltage. Thus, by varying the applied voltages, a particular mass-to-charge ratio ion can beselected to transmit on a stable path through the quadrupole.

The characteristics of a quadrupole can be described by using a diagram known

as a quadrupole stability diagram. This diagram explains the correlation between the DC offset

and the RF. The diagram shows the range of voltage values at which an ion of a particular mass to

charge ratio will adopt a stable path through the mass filter. At voltages outside of this stability

region, the ion will not pass through the quadrupole.Looking at the red area we can see that there are a large number of voltage combinations that willresult in ion transmission for mass M.

Repeating the exercise with an adjacent mass at M-1 results in a similar transmission shape butwith lower voltage values at the apex of the area.

Completing the exercise with ions of mass M+1 gives the green area.

This means that there is a unique combination of voltages that will transmit mass M, whileexcluding ions of mass M-1 and M+1.

Thus for a quadrupole to be used as a mass analyzer it has to be used under conditions that willisolate just the right combinations for all masses.

The criteria used to isolate these conditions is that the ratio between U and Vp must remain

constant. Thus the only voltage combinations of interest fall on the dotted line shown in the above

diagram. This is known as the mass scan line. Changing the slope of the line and/or the DC offset

potential at zero RF changes the intersection of the line with the ion stability curves. This change isreflected as an increase or decrease in the peak width (resolution) of the masses being scanned.

With a quadrupole mass analyzer, a mass resolution of one amu or less can be easily achieved, up

to several hundred mass units. For ICP-MS, it is only necessary to be able to scan from mass 2 to

mass 250, as all naturally occurring isotopes and many currently identified non-natural isotopes liein this mass range.


38/4738

38

Quadrupole resolution

Standard Resolution

High Resolution

Changing the slope of the mass scan line (the ratio of DC offset to rf

voltage) changes the intersection of the line with the transmission curves.

This is reflected in the parameter known as Resolution.

Most instruments typically give peak widths of ~0.7 amu in standardresolution mode.

Some instruments quadrupole and DC/RF supplies have been designed

to allow resolution switching under software control during analysis.

Efficient peak separation can often be achieved at a peak width of 1 amu.

Some isotopes are best measured at higher resolution, even though

transmission of ions is reduced. To achieve maximum sensitivity for allelements, higher resolution is only used for elements which benefit from it.


39/4739

39

Quadrupole mass spectra

Mass spectrum contains one peak per isotope

Some elements (Na, As, Bi) are monoisotopic (one isotope)

Other elements have many more (e.g. Sn has 10)

Most elements have fixed isotopic composition

(exceptions include Pb and U)

The quadrupole mass spectra can be easily interpreted and elements can

be identified by their characteristic masses and isotopic fingerprints. Some

elements (Na, As, Bi) are monoisotopic (one isotope), while otherelements have many more isotopes (e.g. Sn has 10). Most elements have

fixed isotopic composition (exceptions include Pb and U), meaning that

their concentrations can be measures using any of their isotopes. Isotopic

ratio measurement can provide a good element identification method and

can provide interesting geological information as long-timescale geologicalprocesses often fractionate different isotopes of an element.

The sensitivity of an element is dictated by its ionisation efficiency, its

mass (high masses transmitted more efficiently than low) and its isotopic

splitting pattern (the lower the abundance of an isotope, the lower itssensitivity).


40/4740

40



ICP Interface

Lens system

Mass analyzer

Vacuum system

Detector

For ions to be successfully transmitted through the ion lenses and

quadrupole, they must not collide with gas particles. For this reason, the

entire mass spectrometer must be kept under high vacuum. This is donewith the vacuum system. This section describes typical vacuum systems.


41/4741

41

Vacuum systemThree-stage differential pumping

3 chamber differential vacuum system

-Accommodation of plasma gas loading

1st stage (interface between cones)

Rotary pump ~ 1.5 torr

2nd stage (ion optics)

Turbomolecular pump ~ 10-4 torr

3rd stage (mass analyzer and detector)

Turbo molecular pump ~ 10-6 torr

For optimum results, the quadrupole analyser needs to operate at

pressures below 10-6mbar. To achieve this is it necessary to utilise a 3

stage differential pumping system.

The 3 stages achieve a final vacuum below 10-6 Torr at the quadrupoleand detector.

The 3-chamber differential vacuum system accommodates the gasloading from the ICP source.

ICP-MS instruments have a 3-stage vacuum system, designed to achieve

good performance in ion transmission, abundance sensitivity, lowbackground and instrument reliability.

The expansion chamber has a pressure of ~2mbar. This is reduced in the

intermediate chamber to ~10-4 mbar. At this pressure the mean free path

of species in the plasma gas is sufficiently long for the ions to pass

through to the quadrupole without being lost by collisions with residual gasspecies in the vacuum system

The pressure in the intermediate chamber is limited by the quantity of gasintroduced through the skimmer cone. The amount of gas reaching the

aperture between the intermediate and analyzer chambers is much

reduced and therefore the analyzer pressure is reduced still further, toaround 10-6 to 10-7 mbar.


42/4742

42

X Series ICP-MS vacuum systemThree-stage differential pumping via split flow turbo pump

Advanced split flow turbo-

molecular pumping

Single rotary pump

Improved reliability and reducedcost of ownership

Upgrade to CCT without changeto pumping configuration

Some latest generation instruments, such as Thermos XSeriesII, include

traditional 3-stage pumping but using a novel pumping design. A split-flowturbo pump backed by a rotary pump pumps the analyser. The same

rotary pump also evacuates the interface region. This means that the

instrument only uses two pumps. The pumping performance of this

configuration is excellent and this single pump design results in improvedreliability and reduced cost of ownership there is less heat output into

the laboratory and this can be reduced even further since the rotary pump

can be sited remotely.


43/4743

43



ICP Interface

Lens system

Mass analyzer

Vacuum system

Detector

Ions transmitted through the quadrupole are finally detected using a dualmode detector which has linearity over nine orders of magnitude.

The detector is behind the quadrupole mass analyzer.


44/4744

44

Ion detection

Most instruments use discrete dynode

multiplier detectors. Ion strikes first dynode, releases electrons

Electrons are accelerated by voltage applied

to the multiplier and strike next dynode,

releasing more electrons

End result is a large electron cascade (pulse)

at the multiplier exit, for each incident ion

The discrete dynode detector is also known as an active film multiplier. It

is positioned off-axis from the quadrupole to minimize continuum

background. Ions emerging from the quadrupole strike the first dynode.Electrons are liberated by the impact, which are accelerated towards the

next dynode by a potential difference between dynodes. These electrons

impact the next dynode and more electrons are liberated. The process is

repeated down the detector, generating an electron pulse, which is

captured at the anode at the end of the dynode chain. The multiplier

electronics count the pulses over an integration period, leading to a countrate for the isotope being measured.


45/4745

45

The detector cross calibration

Ion counting up to ~4x106

cpsAnalogue mode up to 2x109 cps

Modes accurately cross-calibrated

Simultaneous

ppt to ppm analysis

Ba at 7ppm

Analog si gnal

PC signa l

Na: blank to 300 ppm Cr: blank to 100 ppb

In order to achieve a continuous, and wide, dynamic range, a secondmode of counting analog counting is used. This essentially measures

the ion current part way down the dynode chain on the detector. Themagnitude of the current is proportional to the concentration of the analyte

in the sample. The pulse counting and analog acquisition modes operate

simultaneously. It is important to have an accurate conversion factor

between the two modes of operation to ensure continuity over the

complete dynamic range. This is achieved by performing a pulse counting

and analog detector cross-calibration. This is a calibration record thatautomatically converts analog counts to equivalent pulse counts.

The detector cross calibration is a mass dependent numerical factor which

scales up the low sensitivity analog signal by the correct amount to match

the pulse counting mode sensitivity. In this way, a single, continuouslinear calibration can be achieved from low ppt to high ppm levels.


46/4746

46

Pulse counting & analog linearity

ICPS

Concentration

ppt ppb ppm

Pulse counting detector

Analog detector

0.1ppt 10ppt 1ppb 100ppb 10ppm 1000ppm

Concentration

0.000001

0.0001

0.01

1

100

10,000

Response (ICPS)

Most simultaneous detectors are designed to detect and count ion signals

from


47/47

47

ICP Ion optics and mass

spectrometerIon detection

ICP-MS in a nutshell

Sampling

interfaceSample intro

Most elements possible (around 80)

Elemental and isotopic information given

Concentration range ppq (pg/L) to mid-ppm (100s mg/L) Rapid analysis 2-6 minutes per sample

Good precision ~2% RSDs

To summarize, ICP-MS comprises five steps:

1) Sample introduction as an aerosol and droplet size filtering;

2) Sample droplet desolvation, vaporization, atomization and ionization ina high temperature plasma;

3) Sampling of the ions from atmospheric pressure into the high vacuumof the mass spectrometer;

4) Focusing and transmission of the ions by the lens system, which alsodecouples the ions from electrons, neutrals and photons;

5) Mass separation (element/isotope selection) in the quadrupole; and

6) Detection with an electron multiplier

This powerful technique allows sensitive detection of upto 80 elements

and provides both elemental and isotopic information. Detectable

concentrations range from ppq to hundreds of ppm (thousands on

some instruments). Modern instruments also allow multi-element

analyses in timescales of 2-6 minutes per sample with excellentprecision of typically between 1 and 5 % RSD.

An Introduction to ICP MS

Documents

Transcript of An Introduction to ICP MS