Trace element analysis of
geological materials by ICP-MS I
DSP analytical geochemistry
Markéta Holá, MU Brno
Tento učební materiál vznikl v rámci projektu Rozvoj doktorského studia chemie
č. CZ.02.2.69/0.0/0.0/16_018/0002593
C9067
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Outline
1. Mass spectrometry. General introduction and history.
2. Ion sources for mass spectrometry. Inductively coupled plasma.
3. Interface. Ion optics. Mass discrimination. Vacuum system.
4. Spectral interferences. Resolution, ion resolution calculations.
5. Mass analyzers. Elimination of spectral interferences.
6. Non-spectral interference.
7. Detectors, expression of results.
8. Introduction of samples into plasma.
9. Laser ablation for ICP-MS.
10.Excursion in the laboratory.
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The purpose of the sample introduction system is to transport the analyte to the ICP in a form that can be
converted into elemental ions for ICP-MS.
In most cases, an aerosol is generated from the sample, which is transported into the plasma. One possibility is
the nebulizing of liquid samples, where the nebulizer is connected to the plasma head or the wet aerosol passes
through the desolvation unit. In the case of dry aerosol generation, for example by laser ablation or
electrothermal vaporization, the aerosol is entrained by the carrier gas through the transport tube to the ICP.
The sample can also be introduced into the plasma in the form of vapors and gases, for example when using the
method of generating gaseous hydrides or in conjunction with ICP-MS with gas chromatography (GC-ICP-MS).
Sample introduction
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The sample is transported to the ICP discharge in the form of a wet or dry aerosol, or in the gas phase.
A key parameter affecting the properties of the ICP-MS analytical result is the introduction of the sample into
the plasma in a defined volume, form and time.
Fáze Forma při vstupu do ICP Technika
kapalná / suspenze (polo)vlhký aerosol pneumatický zmlžovač (standardní technika)
kapalná vlhký aerosol vysokotlaká tryska
kapalná polosuchý aerosol termosprej
kapalná polosuchý aerosol ultrazvukový zmlžovač (standardní technika)
kapalná směs plynů a par generace plynných hydridů
kapalná / suspenze suchý aerosol elektrotermické vypařování ETV
pevná / suspenze suchý aerosol elektrotermické vypařování ETV
pevná suchý aerosol laserová ablace
pevná suchý aerosol jiskrová ablace
pevná / suspenze páry přímé vnášení do speciální plazmové hlavice
plynná směs plynů GC-ICP-MS
Sample introduction
To obtain reliable analytical results, you need a generation technique aerosol
exhibited the following properties:
• independence of aerosol generation efficiency from sample properties,
• the same chemical composition of the aerosol and the sample,
• dominant proportion of small aerosol particles (<1 μm),
• stability of aerosol generation and transport into the plasma,
• good aerosol transport efficiency,
• minimal interference with the sample matrix.
The sample is transported to the ICP discharge in the form of a wet or dry aerosol.
Sample introduction
aerosol
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The process of aerosol generation from solutions and transport to the ICP
can be divided into three phases, which follow each other or partially
overlap.
1. Primary aerosol generation by the nebulizer – by acting against the
surface tension of the nebulized liquid, a polydisperse aerosol is
created with a primary particle size distribution dependent on the
properties and operating conditions of the nebulizer, the properties
of the solution and the energy source
2. Secondary aerosol modification – reduction of mean particle
diameter by loss or shattering on a solid barrier or interaction with
another energy source, e.g. a gas stream
3. Tertiary modification of the aerosol – during transport, particles
exceeding the limiting aerodynamic cross-section are lost depending
on the mechanism limiting the passage of particles along the
transport route: gravitational losses, centrifugal losses, impacts on
walls, turbulence. Further, the particles are reduced by natural or
controlled evaporation in the carrier gas stream.
to ICP
tertiary aerosol
Ar
primary aerosol
drain
liquid
sample
Aerosol generation
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They are the most commonly used devices for generating aerosols from
solutions.
Nebulizing is dependent on the carrier gas flow rate.
The aerosol is created by kinetic energy through the interaction of the carrier
gas with the liquid flow. The gas flow must have a sufficient speed to break
the flow of liquid, which is transported to the mouth of the nozzle either by
force (peristaltic or other pump) or by the suction effect.
− capillary nebulizers
concentric (with suction) – Meindhard
cross-flow (with/without suction) – Kniseley
− Babingtonova type nebulizers
V-groove
grid (Hildebrand)
fritted disc
Material
− glass
− polymers (resistant to HF)
vzorek
Ar
Ar
Pneumatic nebulizers
Sample introduction
nebulizers
Concentric nebulizers
Concentric nebulizers have a central
capillary with the liquid and an outer
capillary with the gas. The gas draws the
liquid into the gas stream through
induction, and the liquid is broken into a
fine mist as it moves into the gas stream.
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(a) Schematic diagram of a typical pneumatic concentric nebulizer with the tip geometry under magnification,
(b) front view of the nebulizer tip geometry, and (c) scheme of the nebulization process.
Sample introduction
nebulizers
Meinhard concentric glass nebulizer (CGN)
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Sample introduction
nebulizers
Microconcentric nebulizers
10 – 100 µl/min (concentric ~ 1 ml/min)
Microconcentric nebulizers use the principle of
concentric nebulizer, but use higher gas pressures
at low flow rates of liquid solution. This makes
them ideal for cases where only a small amount
of sample is available.
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Sample introduction
nebulizers
Cross flow nebulizers
Cross flow nebulizers have a gas capillary set
at right angles to the liquid capillary. The gas is
blown across the liquid capillary and this
produces a low pressure that draws the liquid
into the gas stream. Generally the suction is
similar to what is produced in a concentric
nebulizer. The benefit of a cross flow is that
the liquid capillary have a larger inside
diameter allowing for more particles to pass
through without plugging the nebulizer. The
disadvantage is that the mist is usually not as
fine or as consistent. 12
Sample introduction
nebulizers
V-groove nebulizers
The liquid is delivered in a capillary at right
angles to the gas capillary, but the liquid is
poured down a vertically orientated groove that
flows past a gas orifice. The gas pulls the liquid
into the gas flow and forms a fine mist.
High content of dissolved contents (up to 20 %)
Poor stability
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Sample introduction
nebulizers
Droplet size
Sample introduction efficiency
varies between 1 – 80 %
(Concentric nebulizer < 5%)
Spray Chamber
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The spray chamber controls not only what proportion of the analyte is transported to the ICP, but also the
amount of aerosol and solvent vapor that reaches the plasma. If too much aerosol or solvent vapor enters the
plasma, the temperature of the plasma in the central channel will decrease or the plasma will switch off. The
spray chamber also reduces the pulses during aerosol formation.
The spray chamber is part of the system for introducing the liquid sample into the plasma. Its task is to filter
the aerosol produced by the nebulizer (primary/secondary aerosol) so that only the smallest droplets reach the
source (tertiary aerosol). Cyclonic, cylindrical (Scott type) and conical spray chambers are most often used in
plasma spectrochemistry.
Spray chambers
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ICP
sample
waste
carrier Ar
• Removal of large droplets
• Often with waterjacket for cooling
Spray chambers
Scott type
It is the most used type of spray chamber. It consists of two concentric
tubes. The aerosol enters the spray chamber through the inner tube and
at its end is forced to change its flight path by 180˚ towards the outer
tube, at the end of which there is a supply to the plasma. Due to the
influence of gravity, impact of droplets on the walls and coagulation
between them, larger droplets on the walls of the chamber are eliminated
on the way to the exit to the plasma, and only a fine aerosol enters the
plasma. This spray chamber is most often associated with a concentric
nebulizer.
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• Higher sample introduction efficiency
Spray chambers
Cyclonic type
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The most common arrangement for nebulization the solution is a pneumatic nebulizer + spray chamber.
Efficiency:
1 – 4 % at sample flow rate > 0.5 ml min-1
The low efficiency is a prevention for unwanted cooling of the plasma and its switching off
Chamber cooling is usually necessary
60 – 100 % at sample flow rate < 10 µml min-1
Without spray chamber heating
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Efficiency of nebulization
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The limiting factor for elemental analysis can be the available sample volume.
Recently, sample introduction systems used in plasma spectrometry (ICP-MS) have evolved to expand the
applicability of these techniques to the analysis of micro- and nano-samples. Another reason for
introducing a small amount of sample is to minimize matrix or solvent sampling into the plasma.
Common nebulizers: flow rate 0.5 – 1.0 ml min-1
Micronebulizers: flow rate < 200 µl min-1
Princip:
- Miniaturizace běžně používaných zmlžovačů
- Direct Injection High Efficiency Nebulizer –
bez mlžné komory, 100% účinnost, 1-100 µl min-1
DIHEN MEINHARD® Nebulizer
Micronebulizers
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Jet impact
The aerosol is formed from a solution flowing at high speed from a capillary into the headspace of a spray
chamber or against an impactor due to inertial effects that are greater than the surface forces in the liquid.
The nebulizer requires the supply of the solution by a high-pressure pump under a pressure of 10 - 40 MPa
into a capillary with a diameter of 10 - 30 µm. The resulting aerosol is transported by the carrier gas
independently of the operation of the nebulizer.
Thermospray
The aerosol is formed by shock heating the solution above the boiling point in stainless steel or quartz
capillaries. During the expansion of superheated vapors in a capillary with an internal diameter of 150 µm with
an outlet opening of a diameter of 20-150 µm, an aerosol is created with an efficiency of up to 60%.
Utrasonic nebulizer
The aerosol is formed from a film of solution using acoustic energy generated by the
vibration of the piezoelectric plate of the transducer. The sample is taken from the
flowing carrier gas and introduced into the spray chamber. Due to their high efficiency
(~10x higher than pneumatic nebulizer), ultrasonic nebulizers are coupled to the ICP
through a desolvation stage, as it is necessary to reduce the load of the discharge
with solvent and its vapors.
Nebulizers independent of gas flow
Sample introduction
Electrothermal vaporization ETV
Nakata K., Talanta vol. 138, Pages 279-284
• Introduction of small volume – liquid, solid
• Sample subjected to multi-step t° programme
- drying step
- pyrolysis step (removal of matrix components)
- vaporization step
- high t° cleaning step
• Sample vapours transported to ICP (Ar)
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Sample introduction
Electrothermal vaporization ETV
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Sample introduction
Electrothermal vaporization ETV
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Sample introduction
Laser ablation LA
Günther, D., Hattendorf, B., 2005, Trends in Analytical Chemistry, vol. 24, no. 3, p. 255-265.
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pulsed
laser
carrier gas
sample
transport tube
ablation cell
Sample introduction
Laser ablation LA
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Sample introduction
Laser ablation LA
Laser beam
Heating and melting
Cracking (shock wave)
Solid
sample
Evaporation
Atomization
Excitation
Ionization
Molten material
Atoms, ions,
clusters, particles
Ablation process is influenced by:
Laser wavelength, pulse energy, pulse duration, repetition rate… 26
Sample introduction
Laser ablation LA
Advantages
• Solid sampling approach
- no (minimal) sample pre-treatment
(disslolution) required
- high sample throughput
• Broad application range
• Spatially resolved analysis
Disadvantages
• Quantification complicated
• Purchase price
• Small ablation craters
- higher LOD
- problems with microheterogeneity
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