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
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.
Ion sources in mass spectrometry
in general
• Electron ionization (EI)
• Chemical ionization (CI)
• Matrix assisted laser desorption ionization (MALDI)
• Electrospray ionization (ESI)
• Inductively Coupled Plasma (ICP)
• Secondary ion mass spectrometry (SIMS)
• Glow discharge (GD)
• Plasma desorption (PD)
• Laser desorption (LD)
molecules
atoms
Electron ionization (EI)
electron impact, electron bombardment
• Electrons are emitted form glowing filament (tungsten, rhenium) by thermionic emission
and accelerated by a potential of 70 V applied between the filament and anode
M + e- M·+ + 2e•
“Hard” ionization method – ionization and
fragmentation occur simultaneously
• Incompatible with liquid streams
• Widely used with gas chromatography
where M is the atom or molecule being ionized
Chemical ionization (CI)
• Lower energy process than electron ionization because it involves ion/molecule
reactions rather than electron removal.
• Similar to EI, but with reaction gas G (CH4, butane, H2, NH3, etc.)
• „soft“ ionization technique
G+ + M M+ + G
G + e- G+ + 2e- G+ production
Charge transfer
Matrix assisted laser desorption ionization
(MALDI)
• Energy of the laser is absorbed by the matrix
e (matrix) >> e (analyte), c (matrix) >> c (analyte)
Matrix MH+, M+, M*, fragments, ion fragments
Analyte, dispersed in matrix, is vaporized with the
matrix
• Matrix causes ionization of analyte (proton transfer)
MH+ + A M + AH+
• UV (IR) absorbing matrix is energized by the laser matrix molecules desorbed from the
surface carry analyte molecules into the gas phase proton transfer from matrix (acid) to
analyte in the gas phase (UV (IR) lasers (N2, Nd:YAG…))
Figure from Wikimedia Commons
Matrix assisted laser desorption ionization
(MALDI)
α-cyano 4 hydroxycinnamic acid
(CHCA)
dihydroxy benzoic acid
(DHB)
sinapic acid
• Analyte samples are co-crystalized with matrix molecules
• Matrices – usually acids due to easy H+ transfer
3-hydroxy picolinic
acid
(3-HPA)
Electrospray ionization (ESI)
• Liquid is pushed through a very small, charged and usually metal capillary. This liquid
contains the analyte, dissolved in a large amount of solvent, which is usually much more
volatile than the analyte
• As the solvent evaporates, the
analyte molecules are forced closer
together, repel each other and
break up the droplets (Coulombic
fission)
Secondary ions (SI)
• Used for Secondary Ion Mass Spectrometers (SIMS) – ion beam is focused to a selected sample
domain. A small percentage of the material sputtered from the polished surface of the sample is
ionized, and these ions are accelerated into a mass spectrometer.
• High sensitivity - the ability to count individual ions results in detection limits in the parts-per-billion
range for many elements. Isotopic analyses of very small amount of the sample (200 pg).
• Primary ion beam – Cs+ (probe size ~ 10 nm), O2
+, Ar+, Ga+ (provided the smallest probe size ~ 50 nm)
https://www.toray-research.co.jp/en/technicaldata/techniques/SIMS.html
Thermal ionization (TI)
• Used for Thermal Ionization Mass Spectrometry (TIMS) – Ions are created by passing a current
through a conducting metal filament, on which the sample is on, to temperatures often exceeding
1000°C under vacuum. Single or double heated filaments of varying materials can be used,
dependent on the element to be isotopically analysed. The ions generated are accelerated under
vacuum to a magnetic sector where the ions are separated according to their m/z ratio and a
detection system.
https://www.nu-ins.com/
Photo: Lars Eivind Augland, UiO
Gas-discharge ion sources
Plasma source or electric discharge to create ions
• Inductively-coupled plasma ICP – energy is supplied by electrical current which is
produced by electromagnetic induction (by time-varying magnetic field), 8000 K
• Microwave-induced plasma MIP - typically, a 2.45 GHz microwave generator
(magnetron) produces a wave that travels through a cable and is focused via a tuning
system where a torch sits creating a plasma. The carrier gas continues to flow through
the torch and the plasma is self-sustaining. The plasma has a high electron density,
1000 K
• Glow discharge GD - a plasma formed by the passage of electric current through a
low-pressure gas. It is created by applying a voltage between two metal electrodes in
an evacuated chamber containing gas, < 1000 K
• is one of the four fundamental states of matter,
and was first described by chemist Irving
Langmuir in the 1920s.
• plasma contains free electric charges, so it is
electrically conductive. Thanks to the electrical
conductivity, a strong magnetic field also acts on
the plasma. With increasing concentration of
charged particles, the coefficients of thermal
conductivity and dynamic viscosity of the ionized
gas change.
• Plasma is mostly quasineutral - the density of
positive and negative charges equalizes
Plasma
physical properties
Simplified diagram of the dependence of temperature and state
of a chemically pure substance on the supplied thermal energy.
(Krejčí, 1974)
• solid – the atoms are tightly bound
• liquid – loss of regular arrangement
in the grid
• gas – molecules move freely
• plasma - the molecular gas gradually
becomes an atomic gas. Atoms begin
to divide - ionize - into free electrons
and positively charged residues ions.
Free-moving electrons and ions
can carry an electric current, and the
substance changes from a gaseous
state to a plasma state.
Plasma
physical properties
Plasma
physical properties
• 99% of the mass in space is in the plasma state,
ie in the form of an electrically conductive gas
with atoms dissociated into positive particles
and electrons
• the interior and atmosphere of the stars, the
solar wind, most of the interstellar hydrogen and
gas nebulae are plasma
we live in 1% of the universe, where plasma does
not occur naturally
Plasma
occurrence on earth
Naturally: flash, solar wind, aurora
Artificially: light source - discharge lamp, laboratory use
sample m/z separation
Inductively Coupled Plasma
ion source for mass spectrometry
Is a type of plasma source in which the energy is supplied
by electric currents which are produced by electromagnetic
induction, that is, by time-varying magnetic fields.
Inductively Coupled Plasma
ion source for mass spectrometry
In ICP-MS, a plasma or gas consisting of ions,
electrons and neutral particles, is formed from
Argon gas, which is then utilized to atomize and
ionize the elements in the sample matrix. These
resulting ions are then passed through a series
of cones into a high vacuum mass analyzer
where the isotopes of the elements are
identified by their mass-to-charge ratio. The
intensity of a specific peak in the mass
spectrum is proportional to the amount of the
elemental isotope from the original sample.
Inductively Coupled Plasma
ion source for mass spectrometry
Energy of processes in plasma maintained by high-frequency electromagnetic
field, it is not burning = oxidation processes (therefore it is not possible to call
plasma torch ICP burner), it is primarily the kinetic energy of electrons and Ar
ions accelerated by hf field
hf → e- + Ar → e- + e- + Ar+
Inductively Coupled Plasma
plasma torch
carrier gas ~ 1 l/min
+
sample aerosol
quartz torch
rf coil
plasma gas
12-18 l/min
auxiliary gas
0-1 l/min
The plasma torch is designed in such a manner that the sample is then injected
directly into the heart of the plasma. The injected sample consists of a fine
aerosol, which can be derived from, but not limited to, nebulized liquids and
ablated solids. As this aerosol sample passes through the plasma, it collides with
free electrons, argon cations, and neutral argon atoms, causing any molecules
initially present in the aerosol to be quickly and completely broken down into
charged atoms. Some of these charged atoms will recombine with other species
in the plasma to create both stable and meta-stable molecular species, which will
then be transmitted into the mass analyzer along with the charged atoms. At this
point, a special set of metal cones and ion-focusing elements are used to extract
the charged atoms from the plasma into the mass analyzer.
Inductively Coupled Plasma
plasma torch
Inductively Coupled Plasma
plasma torch
• Presence of charged particles
energy supply via induction
• Annular plasma (cf. Donut shape!)
Plasma is punctured by central gas flow
• Rf frequency
27.12 MHz quartz-controlled oscillator
(Free-running generator 40.68 MHz)
Inductively Coupled Plasma
plasma torch
• rf current through coil
• time-dependent magnetic field
• electrons accelerated in circular paths
• collision with Ar atoms: ionization
• seed electrons: spark (Tesla-generator)
analyte ionization
electron impact e- + M → M+ + 2 ePenning
ionization Ar* + M → M+ + echarge
transfer Ar + + M →M+* ± ΔE
Inductively Coupled Plasma
formation of ICP
Inductively Coupled Plasma
analyte ionization
Residence time of
analyte in ICP: ~ ms
• desolvation
• dissociation
• atomization
• excitation
• ionization
Ed McCurdy and Don Potter, Spectroscopy Europe, 13/3, 2001
Inductively Coupled Plasma
analyte ionization
Energy achieved by the Ar ICP is sufficient to cause the majority of sample atoms passing through it to
exceed their first, but not second, ionization potentials. The significance of this is two-fold: (1)
elements from most of the periodic table will be ionized to a +1 state, and (2) because the ions
generated will principally differ by mass, not charge, they can be focused and separated on the basis
of their inertial masses within an electrostatic field.
Energetics of progressive ionization of 56Fe. The Ar plasma temperature is sufficient to convert the vast majority of iron
atoms into singly charged species (remove a single outer shell electron), with negligible production of doubly charged
species (remove two outer shell electrons).
Inductively Coupled Plasma
analyte ionization
Inductively Coupled Plasma
analyte ionization
Ionization energy: the energy required to remove an electron from a neutral atom.
Electronegativity: the ability of an atom in a molecule to draw bonding electrons to itself. Higher values of
electronegativity have those elements that reach the electronic configuration of the following noble gas by forming
an anion. Such elements are referred to as electronegative elements.
Atomic size: the atomic radius increases with each filled shell
of electrons. For any column in the periodic table, the size
increases down a column. The attraction between the
positively charged protons and the negatively charged
electrons causes a contraction, or a decrease in size as the
number of protons increases. In any row, increasing the
number of protons decreases the size of the atom even
though the number of protons always equals the number of
electrons.
Inductively Coupled Plasma
analyte ionization
Inductively Coupled Plasma
analyte ionization
Ionization energy (kJ/mol) SI ~ Ionization potential (eV) non-SI (It corresponds to the kinetic energy obtained by
an electron accelerated in a vacuum with a voltage of one volt)
1 eV = 1.602 176 634 x 10-19 J
Ar: 1520.6 kJ/mol vs. 15.76 eV
Inductively Coupled Plasma
analyte ionization
Degree of ionization vs. Ionization energy for singly charged ions in ICP. https://www.agilent.com/
Inductively Coupled Plasma
analyte ionization
It is important to realize that although most elements are substantially ionized in the high temperature Ar
plasma (Houk 1986; Douglas and Tanner, 1998), some elements having first ionization potentials
approaching or exceeding that of Ar (15.76 eV) can have much lower ionization efficiencies. Quantitative
ion measurements require both consistent +1 ionization and adequate sensitivity. Arsenic (9.81 eV),
selenium (9.75 eV), and phosphorous (10.49 eV), for example, have fairly low first ionization efficiencies
on the order of 52, 33 and 33%, respectively, which are still sufficient for robust quantitative
determinations. By comparison, sulfur (10.36 eV) and fluorine (17.42 eV) have respective first ionization
efficiencies of only 14% and 0.00091%, for which quantitative determinations are impractical or
impossible. With well-optimized plasma conditions, doubly charged ions for most elements occur in
negligible amounts, with worst case maxima typically on the order of 1-3% for elements having lowest
second ionization potentials (e.g., Ba2+).
Inductively Coupled Plasma
analyte ionization
Elemental first ionization efficiencies (as percents) calculated for 7500°K and electron density of 1 x 1015/cm3.
Inductively Coupled Plasma
analyte ionization
Ion population as a function of
plasma temperature and ionization
potential, calculated using Saha
equation.
Ed McCurdy and Don Potter, Spectroscopy Europe,
13/3, 2001
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Inductively Coupled Plasma
Limitations, detection limits