Advanced Analytical Chemistry

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Advanced Analytical Chemistry CHM 6157 Y.
CAI Florida International UniversityUpdated on
9/26/2006 Chapter 3 ICPMS-3

• 2.1.5 Interference equations
• Isobaric interferences can usually corrected for
  by the use of elemental interference equations.

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Advanced Analytical Chemistry CHM 6157 Y.
CAI Florida International UniversityUpdated on
9/21/2006 Chapter 3 ICPMS

• Arsenic determination in a Cl matrix
• ArCl polyatomic ions formed, one of which has
  the same m/z as As (75).
• Cl 35 (75.77), 37 (24.23)
• As a result, quantitative analysis of arsenic can
  have an error due to ArCl.

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Advanced Analytical Chemistry CHM 6157 Y.
CAI Florida International UniversityUpdated on
9/21/2006 Chapter 3 ICPMS

• ArCl is present at m/z 75 and m/z 77 in the
  proportion to the isotope ratio of 35Cl 37Cl,
  75.77 24.23, can be used to correct for the
  interference at m/z 75.
• The ArCl counts at m/z 75 are calculated based on
  the m/z 77 ArCl count. By subtracting ArCl from
  the count at m/z 75, the correct As concentration
  can be obtained.
• As (75) M (75) (75.77/24.23) x ArCl (77)
• M (75) 3.132 x ArCl (77) 1
• Where M (75) is the count number measured at m/z
  75, As (75) is the count number contributed only
  by arsenic at m/z 75, and ArCl (77) is the count
  number contributed by polyatomic ion ArCl at m/z
  77.

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Advanced Analytical Chemistry CHM 6157 Y.
CAI Florida International UniversityUpdated on
9/21/2006 Chapter 3 ICPMS

• However, as selenium has an isotope at m/z 77,
• Se 74 (0.89), 76 (9.36),
• 77 (7.63), 78 (23.78),
• 80 (49.61), 82 (8.73).
• By measuring the Se at m/z 82, the Se count at
  m/z 77 can be estimated, and subtracted from the
  counts at m/z 77 to calculate the counts due to
  ArCl.
• ArCl (77) M (77) (7.63/8.73) x Se (82)
• M (77) 0.874 x Se (82) 2
• Where M (77) is the count number measured at m/z
  77 and Se (82) is the count contributed by
  selenium at m/z 82.

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Advanced Analytical Chemistry CHM 6157 Y.
CAI Florida International UniversityUpdated on
9/21/2006 Chapter 3 ICPMS

• Then equation 2 can be applied to equation 1
• As (75) M (75) 3.132 x M (77) 0.874 x Se
  (82)
• M (75) 3.132 x M (77) 2.736 x Se
  (82) 3
• So far, we have only considered ArCl and Se.
• What else?
• Kr interference at m/z 82!
• Kr 78 (0.35), 80 (2.25), 82 (11.6), 83
  (11.5), 84 (57.0), and 86 (17.3).

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Advanced Analytical Chemistry CHM 6157 Y.
CAI Florida International UniversityUpdated on
9/13/2006 Chapter 3 ICPMS

• In some cases, Kr is found in the Argon gas
  supply (mainly from bottle Ar), therefore the
  signal at m/z 82 should be corrected
• Se (82) M (82) (11.6/11.5) x Kr (83)
• M (82) 1.009 x Kr (83) 4
• If this equation is applied to the equation 3
• AS (75) M (75) 3.132 x M (77) 2.736 x M
  (82) 1.009 x Kr (83)
• M (75) 3.132 x M (77) 2.736 x M (82)
  2.760 x Kr (83)

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Advanced Analytical Chemistry CHM 6157 Y.
CAI Florida International UniversityUpdated on
9/13/2006 Chapter 3 ICPMS

• 2.2 Matrix effects
• 2.2.1 Observation and mechanisms
• High dissolved solids
• blockage of the entrance aperture of the sampling
  cone
• The deposition of salts leads to a decrease in
  the aperture diameter, so that the sensitivity
  worsens and the signals gradually decrease as a
  function of time.

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Advanced Analytical Chemistry CHM 6157 Y.
CAI Florida International UniversityUpdated on
9/13/2006 Chapter 3 ICPMS

• Suppression and enhancement effects
• Ionization suppression
• M M e-
• Introduction of an easily ionized element
  contributes strongly to the electron density in
  the plasma and therefore shifts the ionization
  equilibrium so that the analyte elements are
  ionized to a lesser extent.
• Space charge effects
• Lighter analyte ions can be expected to suffer
  more from this effect than heavier ones, and are
  thus preferentially lost from the transmitted ion
  beam.

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Advanced Analytical Chemistry CHM 6157 Y.
CAI Florida International UniversityUpdated on
9/13/2006 Chapter 3 ICPMS

• 2.2.2 Methods to correct for or overcome matrix
  effects
• Dilution
• Easy
• Detection limits sacrificed
• Matrix matching
• Of course, when the analyzed matrix is also
  added to the standards, correction for matrix
  effects is possible. This method can only be
  applicable for simple matrices, e.g. metals, but
  is clearly not applicable for complex matrices of
  varying composition.

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Advanced Analytical Chemistry CHM 6157 Y.
CAI Florida International UniversityUpdated on
9/13/2006 Chapter 3 ICPMS

• Use of internal standards
• Allows correction for random fluctuations of the
  signal
• Allows correction for systematic variations of
  the analytical signal in samples and standards
  due to matrix effects
• The signal for the internal standard element
  should be influenced in the same way as that for
  the analyte
• Choose the internal standard with a mass number
  as close as possible to that of the analyte

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Advanced Analytical Chemistry CHM 6157 Y.
CAI Florida International UniversityUpdated on
9/13/2006 Chapter 3 ICPMS

• Standard addition
• A safe method for samples of unknown composition
  and thus unknown matrix effect.
• Time consuming
• Chemical separation
• Allow pre-concentration of the analyte elements
• Avoidance of spectral interference.
• Isotope dilution

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Advanced Analytical Chemistry CHM 6157 Y.
CAI Florida International UniversityUpdated on
9/13/2006 Chapter 3 ICPMS

• Calibration and quantification
• External calibration
• Internal standard
• Standard addition
• Isotope dilution

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Advanced Analytical Chemistry CHM 6157 Y.
CAI Florida International UniversityUpdated on
9/13/2006 Chapter 3 ICPMS

• Isotope Dilution
• Isotope dilution is a super internal standard
  addition method on the basis of isotope ratios.
• Add a known amount (spike) of a stable enriched
  isotope of the element considered, which has at
  least two stable isotopes 1 and 2, to the sample
• Measure the isotope ratio of isotopes 1 and 2 in
  the Spike, the unspiked sample and finally the
  spiked sample.
• The concentration of the element of interest can
  then be deducted from these isotopic ratios and
  from the amount of spike added.

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Advanced Analytical Chemistry CHM 6157 Y.
CAI Florida International UniversityUpdated on
9/13/2006 Chapter 3 ICPMS

• Advantages
• Simplified chemical and physical separation
  procedures
• Elimination (reduction) of matrix effects
• Elimination of the effect of instrumental drift

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Advanced Analytical Chemistry CHM 6157 Y.
CAI Florida International UniversityUpdated on
9/13/2006 Chapter 3 ICPMS

• Theory
• In principle, any element with at least two
  isotopes that can be measured is suitable for
  determination by isotope dilution. The two
  selected are designed 1 and 2.
• Three solutions will be used
• Sample (s) Standard (t) Spiked sample (m)

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Advanced Analytical Chemistry CHM 6157 Y.
CAI Florida International UniversityUpdated on
9/13/2006 Chapter 3 ICPMS

• 1ns is the number of moles of isotope 1 in the
  sample.
• 2ns is the number of moles of isotope 2 in the
  sample.
• 1nt is the number of moles of isotope 1 in the
  standard.
• 2nt is the number of moles of isotope 2 in the
  standard.
• Rs is the ratio of isotope 1 to isotope 2 in the
  sample solution.
• Rt is the ratio of isotope 1 to isotope 2 in the
  standard.
• Rm is the ratio of isotope 1 to isotope 2 in the
  spiked sample.

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Advanced Analytical Chemistry CHM 6157 Y.
CAI Florida International UniversityUpdated on
9/13/2006 Chapter 3 ICPMS

• Assuming the molecular sensitivity 1S/2S of the
  MS for isotope 1 and 2 are the same, then
• For the sample solution
• Rs 1ns/2ns 1
• For the standard solution
• Rt 1nt/2nt 2

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Advanced Analytical Chemistry CHM 6157 Y.
CAI Florida International UniversityUpdated on
9/13/2006 Chapter 3 ICPMS

• For the spiked sample solution
• Rm (1ns 1nt)/(2ns 2nt) 3
• Substitution of equations 1 and 2 into equation
  3
• Rm (Rs2ns Rt2nt)/(2ns 2nt) 4
• Rearranged to
• 2ns 2nt (Rm-Rt)/(Rs-Rm) 5
• Convert the number of moles of isotope 2 in the
  sample to the total number of moles of the
  elements in the sample.
• ns (2nt/?2)(Rm-Rt)/(Rs-Rm) 6
• ?2 is the isotopic abundance of isotope 2 in the
  sample.

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Advanced Analytical Chemistry CHM 6157 Y.
CAI Florida International UniversityUpdated on
9/13/2006 Chapter 3 ICPMS

• The mass of the element in the sample is then
  given by
• Ms M(2nt/?2)(Rm-Rt)/(Rs-Rm) 7
• M is the molecular weight of the element.

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Advanced Analytical Chemistry CHM 6157 Y.
CAI Florida International UniversityUpdated on
9/13/2006 Chapter 3 ICPMS

• New developments in ICP/MS
• Instrumental development/improvement to
  eliminate the polyatomic spectral interferences
• Cool Plasma
• Collision Reaction Cell
• Dynamic Reaction Cell
• High resolution (Double-focusing analyzer)

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Advanced Analytical Chemistry CHM 6157 Y.
CAI Florida International UniversityUpdated on
9/13/2006 Chapter 3 ICPMS
22
Advanced Analytical Chemistry CHM 6157 Y.
CAI Florida International UniversityUpdated on
9/13/2006 Chapter 3 ICPMS

• Cool Plasma
• The first breakthrough to reduce some of the
  severe polyatomic overlaps
• Use low temperature plasma to minimize the Ar and
  matrix-based polyatomic species that form under
  normal plasma conditions (1-1.4 KW rf power)
• Cool plasma uses 500-800 KW rf power

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Advanced Analytical Chemistry CHM 6157 Y.
CAI Florida International UniversityUpdated on
9/13/2006 Chapter 3 ICPMS

• Unfortunately cool plasma
• Useful only for a small number of elements
• Element that form strong bond with O2 and F
  cannot be decomposed because of the low plasma
  energy.
• Elements with high ionization potential cannot be
  ionized.

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Advanced Analytical Chemistry CHM 6157 Y.
CAI Florida International UniversityUpdated on
9/13/2006 Chapter 3 ICPMS

• Collision Reaction Cell

Hexapoles
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Advanced Analytical Chemistry CHM 6157 Y.
CAI Florida International UniversityUpdated on
9/13/2006 Chapter 3 ICPMS
26
Advanced Analytical Chemistry CHM 6157 Y.
CAI Florida International UniversityUpdated on
9/13/2006 Chapter 3 ICPMS

• Dynamic Reaction Cell

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Advanced Analytical Chemistry CHM 6157 Y.
CAI Florida International UniversityUpdated on
9/13/2006 Chapter 3 ICPMS
28
Advanced Analytical Chemistry CHM 6157 Y.
CAI Florida International UniversityUpdated on
9/13/2006 Chapter 3 ICPMS

• High resolution (Double-focusing analyzer)
• (Moens and Jakubowski, Anal. Chem. 1998, 251A
  256A)

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Single Focusing Magnetic Sector
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Advanced Analytical Chemistry CHM 6157 Y.
CAI Florida International UniversityUpdated on
9/13/2006 Chapter 3 ICPMS
Skoog et al, 1998
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Advanced Analytical Chemistry CHM 6157 Y.
CAI Florida International UniversityUpdated on
9/13/2006 Chapter 3 ICPMS