Advanced Analytical Chemistry – CHM 6157 ® Y. CAI
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2.1.5 Interference equations
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Isobaric interferences can usually corrected for by the use of elemental interference equations.

Advanced Analytical Chemistry – CHM 6157 ® Y. CAI
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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.

Advanced Analytical Chemistry – CHM 6157 ® Y. CAI
Updated on 9/21/2006 Chapter 3
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
ArCl is present at m/z 75 and m/z 77 in the proportion to the isotope ratio of 35 Cl : 37 Cl, 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.

Advanced Analytical Chemistry – CHM 6157 ® Y. CAI
Updated on 9/21/2006 Chapter 3
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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.

Advanced Analytical Chemistry – CHM 6157 ® Y. CAI
Updated on 9/21/2006 Chapter 3
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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%).

Advanced Analytical Chemistry – CHM 6157 ® Y. CAI
Updated on 9/13/2006 Chapter 3
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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)
If this equation is applied to the equation 3:
[4]
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)

Advanced Analytical Chemistry – CHM 6157 ® Y. CAI
Updated on 9/13/2006 Chapter 3
2.2 Matrix effects
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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.

Advanced Analytical Chemistry – CHM 6157 ® Y. CAI
Updated on 9/13/2006 Chapter 3
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
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.

Advanced Analytical Chemistry – CHM 6157 ® Y. CAI
Updated on 9/13/2006 Chapter 3
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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.

Advanced Analytical Chemistry – CHM 6157 ® Y. CAI
Updated on 9/13/2006 Chapter 3

Use of internal standards
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 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

Advanced Analytical Chemistry – CHM 6157 ® Y. CAI
Updated on 9/13/2006 Chapter 3

Standard addition
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
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

Advanced Analytical Chemistry – CHM 6157 ® Y. CAI
Updated on 9/13/2006 Chapter 3
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3. Calibration and quantification

External calibration

Internal standard
 Standard addition

Isotope dilution

Advanced Analytical Chemistry – CHM 6157 ® Y. CAI
Updated on 9/13/2006 Chapter 3
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
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.

Advanced Analytical Chemistry – CHM 6157 ® Y. CAI
Updated on 9/13/2006 Chapter 3

Advantages:
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
Simplified chemical and physical separation procedures

Elimination (reduction) of matrix effects

Elimination of the effect of instrumental drift

Advanced Analytical Chemistry – CHM 6157 ® Y. CAI
Updated on 9/13/2006 Chapter 3
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
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)

Advanced Analytical Chemistry – CHM 6157 ® Y. CAI
Updated on 9/13/2006 Chapter 3
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 1 n s is the number of moles of isotope 1 in the sample.
 2 n s is the number of moles of isotope 2 in the sample.
 1 n t is the number of moles of isotope 1 in the standard.
 2 n t is the number of moles of isotope 2 in the standard.

R s is the ratio of isotope 1 to isotope 2 in the sample solution.

R t is the ratio of isotope 1 to isotope 2 in the standard.

R m is the ratio of isotope 1 to isotope 2 in the spiked sample.

Advanced Analytical Chemistry – CHM 6157 ® Y. CAI
Updated on 9/13/2006 Chapter 3
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Assuming the molecular sensitivity 1 S/ 2 S of the MS for isotope 1 and 2 are the same, then
For the sample solution:
R s
= 1 n s
/ 2 n s
For the standard solution:
R t
= 1 n t
/ 2 n t
[1]
[2]

Advanced Analytical Chemistry – CHM 6157 ® Y. CAI
Updated on 9/13/2006 Chapter 3
Florida International University
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For the spiked sample solution:
R m
= ( 1 n s
+ 1 n t
)/( 2 n s
+ 2 n t
) [3]
Substitution of equations 1 and 2 into equation 3:
R m
= (R s
2
Rearranged to: n s
+ R t
2 n t
)/( 2 n s
+ 2 n t
)
2 n s
= 2 n t
(R m
-R t
)/(R s
-R m
) [5]
[4]
Convert the number of moles of isotope 2 in the sample to the total number of moles of the elements in the sample.
θ
2 n s
=( 2 n t
/θ
2
)(R m
-R t
)/(R s
-R m
) [6] is the isotopic abundance of isotope 2 in the sample.

Advanced Analytical Chemistry – CHM 6157 ® Y. CAI
Updated on 9/13/2006 Chapter 3
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The mass of the element in the sample is then given by:
M s
= M( 2 n t
/θ
2
)(R m
-R t
)/(R s
-R m
) [7]
M is the molecular weight of the element.

Advanced Analytical Chemistry – CHM 6157 ® Y. CAI
Updated on 9/13/2006 Chapter 3
Florida International University
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4. “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)

Advanced Analytical Chemistry – CHM 6157 ® Y. CAI
Updated on 9/13/2006 Chapter 3
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Advanced Analytical Chemistry – CHM 6157 ® Y. CAI
Updated on 9/13/2006 Chapter 3

Cool Plasma
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
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

Advanced Analytical Chemistry – CHM 6157 ® Y. CAI
Updated on 9/13/2006 Chapter 3
Unfortunately cool plasma:
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 Useful only for a small number of elements

Element that form strong bond with O
2 and F cannot be decomposed because of the low plasma energy.

Elements with high ionization potential cannot be ionized.

Advanced Analytical Chemistry – CHM 6157 ® Y. CAI
Updated on 9/13/2006 Chapter 3

Collision Reaction Cell
Hexapoles
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Advanced Analytical Chemistry – CHM 6157 ® Y. CAI
Updated on 9/13/2006 Chapter 3
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Advanced Analytical Chemistry – CHM 6157 ® Y. CAI
Updated on 9/13/2006 Chapter 3

Dynamic Reaction Cell
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Advanced Analytical Chemistry – CHM 6157 ® Y. CAI
Updated on 9/13/2006 Chapter 3
Florida International University
ICPMS

Advanced Analytical Chemistry – CHM 6157 ® Y. CAI
Updated on 9/13/2006 Chapter 3
Florida International University
ICPMS
High resolution (Double-focusing analyzer)
(Moens and Jakubowski, Anal. Chem. 1998, 251A 256A )

KE
 ze V
Magnetic

1 m v
2
2
Force

F
M

Bze v
Centripeta l Force

F c
 m v
2 r
F
M

F c
Bze v
 m v
2 r v

Bzer m r m z


B
2
2 r
V
2 e radius of curvature

Advanced Analytical Chemistry – CHM 6157
® Y. CAI
University
Updated on 9/13/2006 Chapter 3
Florida International
ICPMS
Skoog et al, 1998

Advanced Analytical Chemistry – CHM 6157 ® Y. CAI
Updated on 9/13/2006 Chapter 3
Florida International University
ICPMS