Instrumentation

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Instrumentation Methods ICP/MS, Uranium

• Jeff Brenner
• Minnesota Department of Health

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EPA Method 200.8Overview and Fundamentals of
ICP-MS

Determination of Metals Using Inductively Coupled
Plasma Mass Spectrometry
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Overview Fundamentals of ICP-MS What we will
cover

• Overview and Fundamentals
• ICP-MS Theory
• Interferences
• Reports

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EPA 200.8ICP-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

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EPA 200.8Analytical Benefits of ICP-MS

• Rapid multi-element quantitative analysis
• Very low detection limits
• Rapid semi-quantitative analysis
• Wide dynamic range
• Isotopic analysis
• Spectral simplicity
• Speciation (with HPLC)

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EPA 200.8Isotopes 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

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EPA Method 200.8U Isotope Abundance

• Isotope Half Life Natural Specific
• Years Abundance Activity (pCi/ug)
• 234U 246,000 0.0055 6208.2
• 235U 700 million 0.72 2.17
• 238U 4.47 billion 99.27 0.336

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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

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EPA Method 200.8 Ions and Mass Spectra

• 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.

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EPA Method 200.8 ICP-MS Spectrum

• 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

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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

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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

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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
• Plasma (Ar)
• Solvents (O, H, Cl, N)
• Samples (C, Cl, S)

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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 40Ar16O
• Background spectral features have been well
  characterized

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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 Percent Relative
• Isotope Abundance Intensity
• 55Mn 100.0 100.0
• 234U 0.0055 0.0055
• 235U 0.7200 0.7200
• 238U 99.2745 99.7245

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EPA Method 200.8 Factors Affecting Ion
Intensities

• Percent Ionization
• Element Ionized
• Na 100
• As 50
• Se 34
• F 0.001
• Most elements are ionized greater than 90.

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EPA Method 200.8 ICP-MS System
Courtesy Perkin Elmer
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EPA Method 200.8 Spray Chamber and Nebulizer
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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.

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EPA Method 200.8 ICP-MS Ion Source Region
(Plasma)

• 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.

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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.

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EPA Method 200.8 Reaction Cell

• Pressurized with a reactive gas
• 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

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EPA Method 200.8 ICP-MS Ion Source Region (Lens)

• Before sampler cone 760 torr
• Before skimmer cone 3 torr
• After skimmer cone 1e-3 torr

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EPA Method 200.8 ICP-MS Ion Source Region (Lens)

• Material extracted from the plasma are composed
  of a mixture of the following
• 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

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EPA Method 200.8 ICP-MS Ion Source Region (Lens)

• 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.

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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.

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EPA Method 200.8 Quadrupole
Courtesy Perkin Elmer

• 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.

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EPA Method 200.8 Perkin Elmer Optimization

• 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.

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Perkin Elmer DRC II Optimization Suggestions

• Suggested guidelines for an acceptable tune for
  method 200.8
• Sensitivity
• Mg gt 8,000 cts/0.1 sec/10 ppb
• In gt40,000 cts/0.1 sec/10 ppb
• U gt30,000 cts/0.1 sec/10 ppb
• Precision
• Mg lt 5 RSD (0.1 sec integration time)
• In lt 5 RSD ()
• U lt 5 RSD ()
• Oxides lt 3.0
• Ba/Ba lt 3.0
• Background
• Mass 220 lt 2 cps
• Mass Accuracy /- 0.05 AMU

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EPA Method 200.8 Daily Performance Check

• Sensitivity
• Nebulizer
• Autolens
• x-y adjustment
• Detector Optimization
• Oxides to High
• Reduce nebulizer flow (plasma temperature
  increases)
• Dirt cones
• Reduce peristaltic pump speed
• Increase RF power
• Double Charged ions too high
• Decreased RF power
• Increase nebulizer flow
• Check skimmer 0-ring
• Poor precision
• 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?

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EPA Method 200.8 Isobaric Correction

• 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)

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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)

• 114Cd mass 114 (a114Sn/a118Sn)(118Sn)

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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 75As plus
  counts from 40Ar35Cl
• 75As mass 75- 40Ar35Cl

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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 mass 75- (a40Ar35Cl/a40Ar37cl)(40Ar37Cl)
• The relative intensities of 40Ar35Cl and 40Ar37Cl
  are determined by the isotopic ratio of 35Cl to
  37Cl.
• 75.77/24.233.127
• 75As mass 75-3.217(40Ar37Cl)
• Correction -3.127 77Se

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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.
• 75As mass 75-3.127(mass77-77Se)
• 75As mass 75-3.127(mass77-(a77Se/a82Se)82Se
  
• 75As mass 75-3.127(mass77-0.87482Se
• Correction -3.12777Se2.733 82Se

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EPA Method 200.8 Types of Methods Measuring
Uranium

• Total concentration method 200.8
• Uranium analysis by ICP-MS
• Results reported as ug/L
• Not very labor intensive
• Limitations
• Can not detect 234U and 235U isotope
• Conversion is accurate if isotopes are present in
  natural abundance
• Bias radioactivity concentration low

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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

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EPA Method 200.8 Types of Methods Measuring
Uranium

• Total activity method 908.0
• Uranium chemically separated
• Analyzed on alpha-beta proportional counter
• Total activity of all three uranium isotopes
• Reported as pCi/L
• Limitations
• Can not distinguish isotope
• Conversion is accurate if isotopes are present in
  natural abundance
• Bias mass concentration high
• Labor intensive

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EPA Method 200.8 Types of Methods Measuring
Uranium

• Isotopic activity method
• 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

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EPA Method 200.8U Isotope Abundance

• Isotope 234U 235U 238U
• Half Life (years) 246,000 700 million 4.47
  billion
• Natural Abundance 0.0055 0.72 99.27
• Specific Activity (pCi/ug) 6,208 2.17 0.336
• Relative Intensity 0.0055 0.72 99.27