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.
2
3
Interface links the atmospheric pressure ICP
ion source and the high vacuum mass
spectrometer.
Interface
ICP-MS
Selection of positive ions - efficient transport &
introduction into mass analyser
4
Schematic cross-section of plasma torch and ICPMS interface (after Houk 1986).
Interface
ICP-MS
interface
5
Interface
Sampler & Skimmer cones • Cones are made of nickel (Ni) most of the time, but they
could also be made of Pt, Cu or Al.
• The metal must be characterized by high thermal
conductivity, otherwise it will melt! A high melting point is
therefore important and it should also be as hard as
possible.
• However thermal conductivity seems to be the best criteria
with regards to performance/price ratio
6
Interface
Sampler & Skimmer cones
7
matrix-induced signal drift (high TDS):
- internal standard
- external standard
Interface
Sampler & Skimmer cones
Wear of cones due to
salinisation, settling of samples.
Regular cleaning needed.
8
The sampling cone is usually damaged by erosion over time, the skimmer cone is
covered with salt deposits, which are formed by the condensation of compounds
with a high boiling point during adiabatic expansion in the interface.
Interface
supersonic gas expansion
- transition from 1 ATM to 100 Pa
- supersonic gas expansion
- rapid cooling
- formation of turbulence and Mach disc
The structure of an idealized free jet. Gas expands
into a reduced background pressure (p1) from a
source with pressure (p0), number density (n0),
and temperature (T0). The expansion through a
nozzle with diameter D0 forms a “zone of silence,”
bounded radially by a barrel shock and axially by a
Mach disk. The distance from the nozzle to the
Mach disk is zm. To achieve ideal skimming, a
skimmer cone should be placed with its tip well
inside the zone of silence, typically at about 2/3
zm. 9
https://www.milmag.cz/znaceni-rychlosti-letadel/
Ernst Mach
(18. února 1838 Chrlice – 19. února 1916 Vaterstetten)
The Mach number (Ma or M), often only Mach is a
dimensionless quantity in fluid dynamics representing
the ratio of flow velocity past a boundary to the local
speed of sound. It is named after physicist and
philosopher Ernst Mach.
Ma =
𝑣
𝑐
where v is the local flow velocity, c is the speed of sound in the given medium. Speeds Ma less
than 1 are subsonic, if Ma is greater than 1 they are supersonic.
https://www.youtube.com/watch?v=45yQ3nfu3SQ
https://en.wikipedia.org/wiki/Sonic_boom#
Interface
supersonic gas expansion
10
Sensitivity of ICP-MS
The increase in the sensitivity of the MS signal with the ion mass is common to all
constructions of modern mass spectrometers.
This means, for example, that a 1 ng/ml 238U solution provides a higher signal than
a 1 ng/ml 7Li solution.
The reason for the higher sensitivity (response) of heavier ions lies in the processes
that take place in the ICP/MS interface space and in the following ion optics:
• Space charge effect
• Collisional scattering !!! mass discrimination !!!
11
- The phenomenon of non-stoichiometric transition of ions
through the mass spectrometer depending on their mass
- Origins in ICP-MS:
- plasma nozzle effect
- space charge effects in the interface during supersonic
expansion of ions through the sampler cone
- Coulombic repulsion
- Results in ICP-MS: measured isotope ratio shows
significant bias to the true value
- Numerical Correction needed:
- mass bias drifts with time (time and matrix dependent)
- Instrumental Isotope Fractionation (IIF)
12
J. Anal. At. Spectrom., 2022, 37, 701–726
Mass discrimination
Sensitivity of ICP-MS
space charge effect
Interface and ion lens to focuse all ions
Separation of particles according to
their electric charges:
• different mobility of ions and
electrons
• differences in particle sizes
• High kinetic energy of electrons
Light ions with lower kinetic energy
are pushed out - ion optics required
13
Sensitivity of ICP-MS
space charge effect
International Journal of Mass Spectrometry 408 (2016) 33–37
More of the ion beam can be input
to the interface, which introduces
more ions into the mass
spectrometer and thus improves
the instrument’s sensitivity.
The sensitivity of inductively coupled
plasma mass spectrometers can be
significantly improved using different
sample and skimmer cones with
different orifice diameters and angles.
The regular nickel skimmer cone The nickel X
skimmer cone
14
- specific isotope amount ratio is used to calibrate the amount ratio of another pair of isotopes (same or
different element) - SSB (Sample-standard bracketing) correction
- mass bias: time-dependent, mass-dependent
Correction factor: linear correction law 𝐾𝑖/𝑗 = 1 + 𝑓(𝑡)(𝑚𝑖 − 𝑚𝑗)
Russell‘s law 𝐾𝑖/𝑗 = (
𝑚 𝑖
𝑚 𝑗
) 𝑓(𝑡)
exponential law ln 𝐾𝑖/𝑗 = 𝑓(𝑡)(𝑚𝑖
𝑛
− 𝑚𝑗
𝑛
)
- principle: calibrator with known isotope amount ratio is measured to obtain the f(t) value as a difference
between known and measured value; this f(t) value is then used to calculate the correction factor Ki/j
- Matrix effect: target element needs to be isolated – separation process
15
Mass bias correction
Inductively Coupled Plasma
sensitivity
Elemental first ionization efficiencies (as percents) calculated for 7500°K and electron
density of 1 x 1015/cm3.
16
Interface
Sampler & Skimmer cones
Spectrochimica Acta Part B 146 (2018) 1–8
17
Sensitivity of ICP-MS
beam divergence
Two-cone design: wide ion beam divergence resulting
from a single, large pressure reduction
Three-cone design: small ion beam divergence
resulting from two small pressure reductions
https://resources.perkinelmer.com/corporate/cmsresources/images/44-74849tch_icpmsthirtyminuteguide.pdf
18
Ion optics
The ion optics are positioned
between the skimmer cone and the
mass separation device, and consist
of one or more electrostatically
controlled lens components.
The function of the ion optic system:
• to take ions from the plasma at atmospheric pressure via the interface cones and steer them
into the mass analyzer, which is under high vacuum.
• to stop particulates, neutral species, and photons from getting through to the mass analyzer
and the detector.
intermediate chamber
19
Ion optics
comparison with light optics
20
Ion optics
comparison with light optics
https://engineering.purdue.edu/BioMS/Pdf/Lecture%20Notes%20for%20MS%20Short%20Course/L9-L10%20Instrumentation_Fina.pdf
21
Mass Spectrometry
basics
Moving of particles in an electric field
+
-
+The
electric (Coulomb) force Fel exerted
by the field on the positive charge is:
𝑭 𝒆𝒍 = 𝒒 𝑬 =
𝒎 𝒗 𝟐
𝒓
q charge magnitude
E electric field (N·C−1; V·m−1)
m mass (kg)
v velocity (m·s−1)
r radius of the ion path
𝒓 =
𝒎 𝒗 𝟐
𝒒 𝑬
22
Ion optics
Einzel lenses
After entering into the evacuated region, a number of lenses are used to manipulate the path of the ions
flowing from the plasma. First and foremost are the accelerator lenses. An accelerator lens consists of two to
three plates with a relatively large hole in them (typically larger than the hole in the cones). Each plate has an
increasingly negative charge placed across them that result in the attraction of the cations towards the plate
increasing their kinetic energy. The hole in the center allows most of the cations to pass directly through the
plate. The imposed kinetic energy is needed to pass the cations through the subsequent reaction cell, mass
filter, and on to the detector with sufficient energy to dislodge electrons on the surface of the detector (an
electron multiplier device).
The next type of lens used in the MS is a focusing lens that centers the cations into a small beam.
23
Ion optics
Einzel lenses
An Einzel lens is comprised of three metal cylinders/electrodes arranged coaxially in a row. The first and third
electrodes have the same voltage applied to them, while the second electrode has a voltage that is "uphill" to ions
of the polarity of interest.
24
Ion optics
Einzel lenses
25
Ion optics
If photons or neutral species reach the detector, they will elevate the background noise and
therefore degrade detection capability.
Both photons and neutrals are detected by the universal detector that can give false signals and increase the
instrumental noise if they are not filtered. Besides causing increased noise, neutrals passing through the
mass filter can become adsorbed onto metal components that can interfere with their proper function.
There are two major types of lenses that remove neutral particles and photons; a Bessel box and omega
lens.
26
Ion optics
Einzel lenses + photon stop
Bessel box, also referred to as a photon stop, is comprised of two photon stops, an Einzel lens, and a set of three lenses. The
first photon stop (located before the Einzel lense) prevents particles from flowing directly down the evacuated chamber. The
Einzel lens focuses the particles into the Bessel box and around the second photon stop. The positive voltage (+4 V) on the
outside of the Bessel box and the negative voltage on the second photon stop (-9 V) direct the cations back to the exit slit.
Neutral particles and photons are unaffected by the electrical field and are removed.
http://people.whitman.edu/~dunnivfm/FAASICPMS_Ebook/CH4/4_2_4.html
27
Ion optics
Omega lens
Omega lens, filters out the photons and neutral particles. A cross-section of an Omega lens consists of four electrodes, two
near the top and two near the bottom of the ion beam. The lens works by carefully balancing the charges of the electrodes
to deflect the beam of cations, but not the neutral species or the photons from the plasma. This deflection is accomplished
by placing a positive charge on the first top electrode and a negative charge on the first bottom electrode that acts to deflect
the beam of cations downward in the front of the lens
28
Ion optics
extraction lenses
29
Ion optics
transfer lens system – HR-ICP-MS
https://www.thermofisher.com/order/catalog/product/IQLAAEGAAMFABWMAFC 30
Vacuum system
Pressure units:
Pascal: [ Pa ] = N/m². The basic unit of SI (the International System of Units).
Standard Atmosphere: [ Atm ] it is about equal to the atmospheric
pressure. Technically, the atm is defined as 101 325 Pa, but it was originally
designated as the air pressure at sea level and 0 ºC.
torr: [ Torr ] = mmHg = 133.32 Pa. 1 Atm = 760 Torr. Outdated, unauthorized
pressure unit.
bar: [ bar ] = 100 000 Pa
1 Pa =
1𝑁
𝑚2 =
1𝑘𝑔
𝑚2
𝑠
Atmospheres are most commonly used when the precise
air pressure is unimportant. For example, I might say that I
heated my sample in dry air at 1 atm. In this case I mean
that I didn’t measure the air pressure, but I assumed it
was basically the pressure of the room, which is more or
less 1 atm.
With the prefix “hecto-” which means 100, Pa can be
easily converted to bar. 1 hectopascal (hPa) is equivalent
to 1 mbar.
1 Atm = 101 325 Pa = 760 Torr = 1013.25 mbar
31
Vacuum system
Why vacuum?
the mean free path (λ) – the distance an
atom travels between collisions with other
particles
λ(1 atm) ≈ 70 nm
λ(10-6 Pa) ≈ 1 km
Jennings, S (1988). "The mean free path in air". Journal of Aerosol
Science. 19 (2): 159.
How to reduce pressure?
- lowering the temperature of the system
- increasing the volume of the system
- reduction of the amount of the substance
in the system
32
Vacuum system
Spectrometers always have min. 2 degrees of vacuum.
1. stage – mechanical pump
2. stage – turbo pump
33
Vacuum system
HR Element2
Five-stage differential pumping system
Atmospheric pressure to a high vacuum < 10-7 mbar
34
Vacuum system
The interface (mechanical) pump is the first
pumping stage of the system. It is switched
independently and runs only when the
system is in operation (i.e. the pump is
switched OFF during standby operation).
1. stage
35
Vacuum system
1. stage is provided by rotary oil pumps – vacuum approx. 10-2 mBar – serves for operation of the
2nd stage of vacuum
https://vacaero.com/
36https://www.youtube.com/watch?v=oXgdnnkIKZ4
Vacuum system
rotary oil pumps maintenance – oil change
The entire assembly is machined and assembled with tight
tolerances so that the gap between the top of the rotor and
the stator wall (often referred to as the “Dou seal”) is
approximately 0.025 mm. This seal is filled with oil, providing
a seal between the inlet and outlet sides. The oil is circulated
from the oil reservoir into the pump interior and is exhausted
through the exhaust valve with the pumped gas.
https://vacaero.com/
37
Vacuum system
2. Stage - turbomolecular pump 10-5 – 10-8 mBar
does not work at atmospheric pressure =>
necessity to use a rotary pump
Rotation speed upto 90000 min-1
Oil-free
https://youtu.be/f1SErZyhMe4
38
Vacuum system
Do not move the device when the turbo pump is on, there is a risk of damage!!!
39