Introduction to the Varian ICP-MS
What is a mass spectrometer?
The Inductively Coupled
Plasma Mass Spectrometer
(ICP-MS) is a fast, precise and
extremely sensitive
multielement analytical
technique for the determination
of trace elements.
What is a mass spectrometer?
The system measures elemental concentrations by,
ionising the sample, then filtering each isotope by its
individual atomic mass to charge ratio.
Forming the ion…
All elements are different because they have a
unique atomic structure consisting of protons,
electrons and neutrons.
What is an isotope?
Each element has a unique number of protons and an
equal number of electrons.
The protons and neutrons give the atom a definite
mass.
Elemental Analysis by Mass Spectrometry
Atomic Number
•The number of protons in the
nucleus of the atom
•Also the number of electrons in a
ground state (electrically neutral)
atom
Isotopes
•Atoms of elements which have the
same number of electrons and
protons but different numbers of
neutrons
Mass Number
Atomic Number
11
B
5
Mass Number (common ICP-MS
terminology)
•The total number of protons and
neutrons in the nucleus of a
particular isotope
AMU
•Atomic Mass Unit; the mass of a
proton or neutron
The two isotopes of Boron shown
here will have different masses but
will otherwise be essentially
identical in their chemical properties
10
B
5
What is an isotope?
Each element can have a number of different masses, called
isotopes. For example, 204, 206, 207 and 208 are all
isotopes of Lead. The units are AMU.
The difference between the atomic mass of each isotope is
the number of neutrons in the nucleus.
What is an isotope?
Not all elements have multiple isotopes. For example,
Beryllium has a single isotope at mass 9, therefore mass 9 is
100% abundant. Lead however has multiple isotopes each
with a different percentage of abundance, Pb204=1.4%,
Pb206=24.1%, Pb207=22.1% and Pb208=52.4%.
Ionized State Atom
Ionization Occurs When the Energy is
High Enough to Cause an Atom to Lose
One or More Electrons (Electron is moved
to Infinity)
B (5n, 5p, 4e)+
e-
+
Orbitals
Neutrons
Protons
Electrons
Why use ions?
Ions all have the same positive charge, therefore they can
be controlled using electrostatic fields.
And, as each ionised element has a unique mass it can
be identified.
Basically, mass spectrometer ion optics forms the ion
beam from the plasma and the quadrupole separates the
ions by their individual mass to charge ratio.
Extent of Ionization
H
Calculated values of degree of ionization
of M+ and M+2 (T=7500K, ne = 1e15cm-3 )
*Houk 1986
0.1
Li
Be
100
75
M+
Na Mg
100
x 100%
M+ + M
98
He
B
C
N
O
F
6e-6
58
5
0.1
0.1
Al
Si
P
S
Cl
Ar
96
85
33
14
0.9
0.04
Br
Kr
33
5
0.6
Sc
Ti
V
Cr Mn Fe Co Ni Cu Zn Ga Ge
As Se
100 99(1) 100
99
99
98
52
Rb Sr
Zr
K
Ca
100 96(4)
Y
Nb Mo
95
96
93
91
90
75
Tc Ru Rh Pd Ag Cd
93
93
90
In
Sn
Sb
Te
I
Xe
99
96
78
65
29
8.5
Po
At
Rn
99
98
98
Cs Ba La
Hf
Ta
W
Re
Os Ir
Pt Au
Hg Tl
Pb
Bi
100 91(9) 90(10) 98
95
94
93
78
62
38
100
97
92
Ac
94
98
98
Fr Ra
96
85
51
Ce Pr Nd P Sm Eu Gd Tb
99 m 97(3) 100 93(7 99
100
99
Np Pu Am Cm Bk
Cf
Es F
m
98(2) 90(10)
Th
100
P
a
)
U
100
Ne
9e-4
Dy Ho Er Tm Yb Lu
91(9
)
92(8)
Md No Lw
Different elements in the periodic table are ionised with
different amounts of efficiency in the argon plasma.
Some elements are ionised to the +1 state almost
completely, some others only have about 50%
ionisation, and some even less so.
As you can see from the table, most elements are very
efficiently ionised to M+ and M++ states, but a few do
not ionise well at all, and these will have much lower
sensitivity by the ICPMS technique.
Two of the most common examples of these poor
performers are As and Se.
Some elements have almost no ionisation potential
whatsoever in an argon plasma and these are
essentially impossible to measure by ICPMS.
What is the plasma…
What is the plasma?
The plasma is ionised argon gas.
It is generated when a spark
passes through gas contained
within a strong electric field
operating at radio frequency.
Once the spark stops the plasma
will remain lit provided the RF field
is maintained and there is a
constant supply of argon gas.
Why use a plasma for MS?
The plasma is a very aggressive ion source because it
is capable of operating at high temperatures of up to
approximately 10,000 degrees Kelvin.
Virtually all molecules presented to the plasma are
ionised making it ideal for mass spectrometry.
Any disadvantages with plasma?
Analyte ions from the sample are typically in the M+
state however some M++ are also formed. They are
known as doubly charged ions.
Atomic recombinations also occur within the plasma,
resulting in the formation of stable and meta-stable
species.
ICP-MS is a destructive sampling technique therefore
all samples presented to the plasma cannot be reused.
Any disadvantages with plasma?
Many of these species are positively charged and will
therefore join the ion beam and be delivered into the
mass analyser. This is a non-ideal situation because it
misrepresents what is contained within the sample.
One measure of a mass spectrometer performance is
its ability to provide an accurate indication of the sample
whilst minimising the generation of doubly charged and
oxide interferences.
Why use argon for the plasma?
ICP’s always use argon as the gas for the plasma
because the ionization energy of argon gas is higher than
the first ionization potential for almost all elements in the
periodic table, but lower than the second ionisation
potential.
Why use argon for the plasma?
This essentially means that any sample delivered into an
argon plasma will have one electron removed from each
atom, but the plasma does not have enough energy to
remove two electrons in most cases. The result is a
stream of singly-charged M+ ions being ejected from the
plasma (and into the mass spectrometer).
Multi-isotope elements that are not subject to
radioactive decay will have a constant, fixed ratio of
their different isotopes.
This characteristic is useful to double-check the
accuracy of an elemental concentration as more than
one isotope should give the same answer and also to
quantify an unknown signal or interference.
Peaks can be checked against “fingerprints” of known
isotopic patterns to see which one they most closely
match.
Some elements that are active in geological, biological
or environmental processes may show slight
discrepancies from the ideal isotopic ratios for that
element. This data can be used to more closely
investigate these processes.
Element Isotopes in ICP-MS
Example: Nickel
58
Ni
60
Ni
61
Ni
62
Ni
64
Ni
No. of protons (p+)
28
28
28
28
28
No. of electrons (e-)
28
28
28
28
28
No. of neutrons (n)
30
32
33
34
36
Atomic mass (p+ + n)
58
60
61
62
63
Atomic number (p+)
28
28
28
28
28
Natural abundance
68.1%
26.2%
1.14%
3.63%
0.93%
Atomic weight
58.69
ICP-OES Spectrum for Ni
Many emission lines for Ni
The previous diagram shows the optical emission
spectrum for Nickel. There are many emission lines
present, some of which are fairly close together. This has
two important effects.
The optical spectrometer must be well-designed to be
able to separate the Ni peaks as well as any
interferences from any other elements – a much more
challenging problem).
Also, the energy in the plasma used to ionise the nickel
atoms is dispersed amongst all these different ionic and
atomic transitions – if there were fewer lines the signal
would be more concentrated amongst them. Compare
this with the Ni mass spectrum on the next page…
ICP-MS Spectrum for Ni
This is the mass spectrum for the five isotopes of Ni. There are far fewer lines
(peaks) in this case, which makes the spectrum much simpler to interpret, and the
signal to noise ratio is also better. Simpler spectra and greater sensitivity are the
two key advantages of ICP-MS over ICP-OES.
The ion optics…
The ion optics system.
There are 11 lenses in the Varian ICP-MS ion optics
system. Each lens operates at a DC voltage.
Ion Mirror Optics for ICP-MS
Focuses all analyte ions
into the mass analyzer,
irrespective of the
unwanted particles
energy spread
electrostatic
field
Focuses all
analyte ions
into the mass
analyzer,
irrespective of
the energy
spread
Why have ion optics?
The DC voltages on the lenses produce a series of
electric fields. They create an environment suitable for
accelerating ions of all masses into the mass analyser.
The geometry of the lenses and the resultant electric
fields also serve to reject photons, neutrals and solids
from the sample beam.
This minimises interferences and the generation of new
molecules within the system.
How are the isotopes separated?
Isotopes are separated according to their mass to charge
ratio (m/z) by the quadrupole mass analyser.
At any given moment the RF and DC voltages applied to
the quadrupole will create an environment suitable only
for ions of a specific mass to charge ratio.
How are ions detected?
Ions are detected when they exit the quadrupole by a
pulse counting ion detector.
Each ion generates electrons within the detector. The
electrons are multiplied and the resultant current pulse
is amplified and finally counted.
The detector also forms an integral part of the signal
attenuation system. This allows the instrument to
measure samples over a wide range of concentrations.
How is the spectrum formed?
The spectrometer is capable of detecting ions from
mass 4 AMU to 256 AMU.
The quadrupole scans the spectrum sequentially from
low mass to high mass.
The spectrometer is calibrated via the software by
aspirating a solution containing known isotopes.
How is the spectrum calibrated?
Initially, mass calibration is a manual process. The
operator scans two segments of the spectrum, a low
mass and a high mass.
The locations of two specific peaks are found then
corrected using a software algorithm to the theoretical
positions.
This is followed by a similar process for several
isotopes throughout the complete mass spectrum.
How is the data collected?
Data is collected in increments defined via the software
by the instrument operator.
Data is collected by setting the RF and DC voltages
applied to the quadrupole to suit an ion of a specific
mass to charge ratio then maintaining that condition for
a set time.
The amount of time spent scanning a specific mass is
called the Dwell time.
How is the data collected?
The spectrum is divided into channels by the software.
Each mass is divided into 40 channels.
The more data points that are selected the longer each
scan will take.
The user can opt to scan segments or just single peaks,
these are called segmented scans or peak hopping.
How is the data collected?
Segmented scanning methods take longer but will
provide you with a peak profile.
Peak hopping methods are faster and can provide you
with better scanning statistics because longer time can
be spent scanning each mass.
The instrument performance test uses peak hopping
mode. But to calibrate the instrument segmented
scanning must be used.
All ICPMS instruments, no matter who the manufacturer
is or what kinds of technology they use for the various
components, all operate on the same principles and use
the same general hardware (They all must have RF
generators, vacuum pumps, etc etc).
ICP-MS -Overview

Sample nebulized in spraychamber

Argon transports sample and sustains plasma

RF generator supplies energy to induction coil

Sample atomized and ionized in the plasma

Ions are transmitted through the interface, most of
the gas removed

Quadrupole filters the ions by mass

Detector counts the ions
Atomic spectroscopy techniques
Atomic Spectroscopy Techniques
•A brief comparison of three common techniques for
the analysis of metals…
– Atomic Absorption (Flame and Furnace)
– ICP-AES
– ICP-MS
Advantages of ICP-MS
•Routine trace element analysis (ppb and below)
•Fast (~simultaneous) multi element analysis
•Wide range of elements (>75)
•Analysis of a wide variety of sample types
•A wide variety of sample introduction methods
•High sensitivity with DL’s of <1ppt for some elements in
solution
•Simple spectra produced
•Isotopic analysis capability
Disadvantages of ICP-MS
•Relatively high capital cost (~2x ICPOES)
•More operator experience needed
•TDS < 0.2%
•Matrix interferences
•Isobaric interferences
•Drift
•Relatively high running costs
•Can be “too sensitive” for higher concentrations
Comparison of Techniques
ICP-MS
ICP-ES
GFAAS
FAAS
Detection Limits
Excellent
Good
Excellent
Good
Productivity
Excellent
Excellent
Low
Good
108
105
102
103
Precision
1-3%
0.3 – 2 %
1–5%
0.1 – 1 %
Spectral Interferences
Few
Common
Very Few
Almost None
Chemical Interferences
Moderate
Few
Many
Many
Minimal
Minimal
Minimal
Some
Mass Effects
High on low
mass
None
None
None
Dissolved solids
0.1 – 0.4 %
2 – 25 %
Up to 20 %
0.5 – 3 %
75
73
50
68
Sample Usage
Low
Medium
Very Low
High
Semi-Quantitative
Yes
Yes
No
No
Isotope Analysis
Yes
No
No
No
Skill required
Skill required
Skill required
Easy
High
High
Medium
Low
Very high
High
Medium
Low
Linear Dynamic Range
Ionization
# Elements
Method Development
Running Costs
Capital Costs
Detection Limits (1) g/L
ELEMENT ICP-AES
As
12
ICP-AES
w/USN
2.5
GFAAS
ICP-MS
0.25
0.1
Se
37
3.6
0.35
0.5
Pb
14
1.0
0.15
0.006
Cd
1.5
0.06
0.01
0.06
Graphite Furnace has very low limits of detection for most elements, in some
cases they may be even better than ICPMS, however the trade-off is that GFAAS
is VERY slow compared to ICPMS for analysing multiple elements. ICPMS can
measure ALL of the elements requested almost as fast as a GFAAS can
measure one element.
Example Detection Limits –2 (ug/L)
ELEMENT ICP-AES
Al
1.5
ICP-AES
w/USN
0.15
Cr
4.0
0.08
0.075
0.04
Ni
6.0
0.3
0.5
0.012
Ti
0.6
0.06
0.75
V
4.0
0.2
0.5
GFAAS
ICP-MS
0.2
0.036
0.02
Location of ICP-MS components
A schematic overview of a quadrupole ICPMS (not to scale). The sample introduction,
torch, RF system, interface, vacuum system, mass spectrometer and detector can all
be see from right to left.
Why does the MS need a vacuum?
The mass spectrometer system operates within a
vacuum because:
• low pressures reduce the chance that analyte ions will
collide with air molecules and
• vacuum allows the system to operate with high
voltages between components with small mechanical
tolerances.
Tips regarding the vacuum system…
Components within the vacuum system should be
kept clean and free from dust, oil and fingerprints.
Tips regarding the vacuum system…
A few pairs of gloves are provided with each instrument
but you will have to include a supply in your service kit.
Source lint free nylon gloves and some powder free
latex gloves. Be aware that if the gloves have been
treated with some chemicals you may see these in the
spectrum after you have handled the mass analyser
components.
Optimisation
The instrument must be optimised in order to achieve
the best results possible for analysis.
Essentially optimisation maximises the signal
sensitivity whilst minimising the background counts
and the formation of interferences and doubly charged
ions.
Optimisation
The system will require some optimisation or
‘tuning’each day.
Plasma alignment is critical. A movement of 0.1mm for
the plasma relative to the cones will produce a
significant change in ion signal throughput.
Optimisation
Optimisation is typically performed in the following
sequence:
•
•
•
•
•
•
•
Warm-up
Plasma alignment
Resolution and Trim
Mass Calibration
Ion optics and gas flows
RF power, peri pump
Method settings - scan and dwell times, scan mode
Optimisation
The system can be optimised for high and normal
sensitivity modes and for hot and cold plasma.
The difference between the two modes is in the ion
optics voltages, gas flows, RF power and peri pump
rate. The torch position and mass calibration are the
same between sensitivity modes.
Optimisation is always performed using a segmented
scanning method. These are referred to as System
Setup methods in the Expert software.
Optimisation
Cool plasma conditions
•
•
•
•
•
provide ultimate Detection Limit’s for elements
such as: Na, K, Ca, Fe
are tuned using similar ion optics conditions to
hot plasma high sensitivity mode, typically
generating moderate sensitivity
extends concentration range into 100’s ppm
maintains robust conditions for high matrix
samples
operates with low oxide interference's of <1%
Performance testing
Once the system has been optimised running a System
Test worksheet will provide an indication of the
instrument’s analytical performance.
A different worksheet is used for each sensitivity mode.
Review:
What is a System Setup worksheet used for?
What is a System Test worksheet used for?