Instrumentation & Methods:
ICP/MS, Uranium
Jeff Brenner
Minnesota Department of Health
EPA Method 200.8
Overview and Fundamentals of ICP-MS
Determination of Metals Using Inductively
Coupled Plasma Mass Spectrometry
Overview & Fundamentals of ICP-MS
What we will cover

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Overview and Fundamentals
ICP-MS Theory
Interferences
Reports
EPA 200.8
ICP-MS Definition


An analytical technique to
determine Elements using Mass
Spectrometry from Ions generated
by an Inductively Coupled Plasma.
Mass Spectroscopy

Separation and measurement of the
mass of individual atoms making up a
given material
EPA 200.8
Analytical Benefits of ICP-MS
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Rapid multi-element quantitative
analysis
Very low detection limits
Rapid semi-quantitative analysis
Wide dynamic range
Isotopic analysis
Spectral simplicity
Speciation (with HPLC)
EPA 200.8
Isotopes and Mass Spectra


Isotopes of an element differ in the
number of neutrons in the nucleus
U Atomic Number 92
 234U


has 142 neutrons
235U has 143 neutrons
238U has 146 neutrons
EPA Method 200.8
U Isotope Abundance

Isotope Half Life
Years
234U
235U
238U
246,000
700 million
4.47 billion
Natural
Abundance
Specific
Activity (pCi/ug)
0.0055 %
0.72 %
99.27 %
6208.2
2.17
0.336
EPA Method 200.8
Isotopes and Mass Spectra



The Isotopic abundance of most elements
is constant
Pb may differ slightly based on the source
of the Pb
Pb is analyzed as the sum
206 Pb
207 Pb
208 Pb
EPA Method 200.8
Ions and Mass Spectra

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Positive ions are produced by the
energy in the plasma
In order to utilize a mass
spectrometer an ion is necessary
ICP-MS analyze isotopic ions
The ions are “steered” throughout
the ion path of the spectrometer.
EPA Method 200.8
ICP-MS Spectrum
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A series of peaks that correspond to
mass to charge ratio (m/z)
Peaks could be the sum of different
isotopes of different elements
Doubly charged ions will appear ½
its mass
138Ba double charges will appear at
138/2 = 69
EPA Method 200.8
Isobaric Spectral Overlaps



Signal at given amu is the
summation of all the isotopes at
that amu
It is best to avoid potential overlaps
by monitoring a “clean” mass
Overlaps are correctable in software
EPA Method 200.8
Isobaric Spectral Overlaps

Several factors must be considered
when selection an isotope:
Concentration of analyte
 Concentration of interferences
 Abundances of isotopes at the given
mass

EPA Method 200.8
Molecular Overlaps

Polyatomic or molecular ions will
occur


Common ones are Ar, O, and H based
Be aware of molecular overlaps that
are formed:

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
Plasma (Ar)
Solvents (O, H, Cl, N)
Samples (C, Cl, S)
EPA Method 200.8
Molecular Overlaps


Elements in the ICP do not fully
break apart and recombination of
highly concentrated elements will
occur
Example
56Fe

and
40Ar+16O
Background spectral features have
been well characterized
EPA Method 200.8
Factors Affecting Ion Intensities

Isotopic Abundance Intensity


Intensity of an isotope is proportional to
its natural abundance
The sum of the signals from all isotopes
of an element are compared to the signal
from a mono-isotopic element, the
signals ideally should be equal
Example: Element
Isotope
55Mn
234U
235U
238U
Percent
Abundance
100.0
0.0055
0.7200
99.2745
Relative
Intensity
100.0
0.0055
0.7200
99.7245
EPA Method 200.8
Factors Affecting Ion Intensities

Percent Ionization
Element
Na
As
Se
F
% Ionized
100
50
34
0.001
Most elements are ionized greater than
90%.
EPA Method 200.8
ICP-MS System
Courtesy: Perkin Elmer
EPA Method 200.8
Spray Chamber and Nebulizer
EPA Method 200.8
ICP-MS Ion Source Region


Plasma creates ions from the components in the
sample.
Heat from 6,000K-10,000K dries, aerosol, then
atomize, and ionize components of the sample.
EPA Method 200.8
ICP-MS Ion Source Region (Plasma)
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
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Plasma is formed by a stream of argon gas flowing
between to quartz tubes.
Radio frequency (RF) power is applied through the
coil, and an oscillating magnetic field is formed.
An electrical discharge creates seed electrons and
ions.
EPA Method 200.8
ICP-MS Ion Source Region (Plasma)


Inside the induced magnetic field,
the charged particles are forced to
flow in a closed annular path.
As they meet resistance, heating
takes place and additional ionization
occurs.
EPA Method 200.8
Reaction Cell

Pressurized with a reactive gas

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Convert isobar to a different ion which does
not interfere
Convert analyte to polyatomic ion which is not
interfered
The specific chemistry is dependent on:


Nature and density of the reactive gas
Electrical fields within the cell
EPA Method 200.8
ICP-MS Ion Source Region (Lens)
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Before sampler cone 760 torr
Before skimmer cone 3 torr
After skimmer cone 1e-3 torr
EPA Method 200.8
ICP-MS Ion Source Region (Lens)

Material extracted from the plasma are
composed of a mixture of the following:
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Neutral atoms (Ar) Molecules (O2)
Positively charged atomic and molecular ions
(Ar+, O2+)
Reactive metastable atoms and ions
Negatively charged atomic and molecular ions
Photons
Electrons
EPA Method 200.8
ICP-MS Ion Source Region (Lens)
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The lens captures and guides the
positively charged ions to the quadrupole.
By applying a positive potential to the
lens, the ions will be focused to the center
of the lens.
Small ions are optimized at lower
voltages. As the voltage is increased,
higher mass ions are better focused.
If the voltage is to high the ions are
repelled.
EPA Method 200.8
Reaction Cell or Collision Cell

A reaction gas is introduced into the
cell. The reaction of the gas with
the interfering species is set up to
remove these interferences from
the path.
EPA Method 200.8
Quadrupole
Courtesy: Perkin Elmer
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Mass Filtering System


Separates on type of element (ion) from another with
an electromagnetic field.
Only one mass (m/z) will make it through at a time.
Many masses enter, only one makes it out.
EPA Method 200.8
Perkin Elmer Optimization
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

After initiating the plasma, allow the
instrument to warm up while
aspirating a blank solution for at
least 15 minutes.
Mass Calibration Tune
DRC II Tuning Solution

(1 ppb Mg, In, Ce,Ba,Pb, U) and check
for responses and RSDs. Generate and
evaluate a tune report.
Perkin Elmer DRC II Optimization
Suggestions
Suggested guidelines for an acceptable tune for method
200.8
 Sensitivity:

Mg > 8,000 cts/0.1 sec/10 ppb

In >40,000 cts/0.1 sec/10 ppb
U >30,000 cts/0.1 sec/10 ppb
Precision:
Mg
In
U


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< 5 % RSD
< 5 % RSD
< 5 % RSD
Oxides:
Ba++/Ba+
Background:
Mass 220
(0.1 sec integration time)
(“)
(“)
< 3.0%
< 3.0%
< 2 cps
Mass Accuracy: +/- 0.05 AMU
EPA Method 200.8
Daily Performance Check

Sensitivity
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Oxides to High:
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Reduce nebulizer flow (plasma temperature increases)
Dirt cones
Reduce peristaltic pump speed
Increase RF power
Double Charged ions too high:
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Nebulizer
Autolens
x-y adjustment
Detector Optimization
Decreased RF power
Increase nebulizer flow
Check skimmer 0-ring
Poor precision

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
Check entire sample introduction system
Check the nebulizer
Check that the correct method is used
Perform a visual check of the plasma! Is it stable?
EPA Method 200.8
Isobaric Correction
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

Counts at mass 114 = 114Cd + 114Sn
114Cd = mass 114 - 114Sn
We cannot measure the counts of Sn at
mass 114 directly since 114Cd can also be
present. However, we can measure
another isotope of Sn (118) that is free
from overlap by Cd. Therefore:
114Cd = mass 114 –
(a114Sn/a118Sn)*(118Sn)
EPA Method 200.8
Isobaric Correction

The abundance ratio (a114Sn/a118Sn) of
these two isotopes is (0.65%/24.23%)
and is reasonably constant. Therefore:

114Cd
= mass 114 –(0.65%/24.23%)*(118Sn)

Correction = -(0.0268)*(118Sn)
EPA Method 200.8
Polyatomic Correction

Interference of Chloride on Arsenic


High concentrations of chloride react with argon
in the plasma to form the following:
 40Ar35Cl interfering on 75As
 40Ar37Cl interfering on 77Se
As has only one isotope at mass 75
 40Ar35Cl


can cause isobaric overlap &
Erroneously high results
Must measure 40Ar35Cl contribution and subtract
it from the total counts at mass 75
Total counts mass 75 = counts from
plus counts from 40Ar35Cl
75As
= mass 75-
40Ar35Cl
75As
EPA Method 200.8
Polyatomic Correction


We cannot measure the ArCl contribution at mass
75, however, we can measure the ArCl contribution
from 40Ar37Cl at mass 77
The equation then becomes:
 75As

The relative intensities of 40Ar35Cl and 40Ar37Cl are
determined by the isotopic ratio of 35Cl to 37Cl.



= mass 75- (a40Ar35Cl/a40Ar37cl)*(40Ar37Cl)
75.77%/24.23%=3.127
75As = mass 75-3.217*(40Ar37Cl)
Correction = -3.127*
77Se
EPA Method 200.8
Polyatomic Correction


If Se is present in the sample, the
correction becomes more complicated. 77Se
will contribute intensity counts to mass 77.
Therefore, measure Se at mass 82 and
multiply the result by the ratio of 77Se to
82Se.
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


75As
= mass 75-3.127*(mass77-77Se)
75As = mass 75-3.127*[(mass77-(a77Se/a82Se)*82Se]
75As = mass 75-3.127*[(mass77-0.874*82Se]
Correction -3.127*77Se+2.733*
82Se
EPA Method 200.8
Types of Methods Measuring Uranium

Total concentration method 200.8
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Uranium analysis by ICP-MS
Results reported as ug/L
Not very labor intensive
Limitations

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Can not detect 234U and 235U isotope
Conversion is accurate if isotopes are present
in natural abundance
Bias radioactivity concentration low
EPA Method 200.8
Uranium Calculation

Uranium radioactivity

A
(pCi/L)
=U
(ug/L)
* 0.67
(pCi/ug)
Where: A = activity of uranium
U = uranium concentration
0.67 = conversion factor
40 CFR part 141.25 Analytical methods for radioactivity.
Footnote 12
EPA Method 200.8
Types of Methods Measuring Uranium

Total activity method 908.0
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Uranium chemically separated
Analyzed on alpha-beta proportional counter
Total activity of all three uranium isotopes
Reported as pCi/L
Limitations

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Can not distinguish isotope
Conversion is accurate if isotopes are present
in natural abundance
Bias mass concentration high
Labor intensive
EPA Method 200.8
Types of Methods Measuring Uranium

Isotopic activity method
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Uranium chemically separated
Similar to total activity
Alpha spectrometer
Able to distinguish uranium isotope
Results can be reported as pCi/L or
ug/L
Limitations

Labor intensive
EPA Method 200.8
U Isotope Abundance

Isotope




Half Life (years)
Natural Abundance
Specific Activity (pCi/ug)
Relative Intensity
234U
235U
238U
246,000
700 million
4.47 billion
0.0055 %
0.72 %
99.27 %
6,208
2.17
0.336
0.0055
0.72
99.27