Sources of Errors in Trace Element and Speciation Analysis

Sources of Errors in Trace Element and
Speciation Analysis
Zoltan Mester,
National Research Council of Canada, Institute for National Measurement Standards
Outline
Definitions
Sources of errors in the analytical process
Detection methods in trace element analysis
Trace element measurement methods and the associated errors
Summary
TAQC-WFD, Budapest, November 2-4, 2006
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Expression of an analytical result
Value 1 ± Value 2
Expression of an analytical result
Value 1 ± Value 2
Free of systematic error (bias)
Uncertainty
Free of systematic error (bias)
Uncertainty
Verification of traceability of
the results:
• CRMs
• Primary methods (or reference
methods)
Consideration of all sources of error
of the analytical process:
1. Random errors: Method precision
2. Correction of systematic errors
Verification of traceability of
the results:
• CRMs
• Primary methods (or reference
methods)
Consideration of all sources of error
of the analytical process:
1. Random errors: Method precision
2. Correction of systematic errors
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Measurand
Particular quantity subject to measurement
Measurement
Set of operations having the object of determining a value of a quantity
Result of a measurement
Value attributed to a measurand, obtained by measurement
Uncertainty (of measurement)
Parameter associated with the result of a measurement, that characterises the
dispersion of the values that could reasonably be attributed to the
Measurand
“Conventional” true value (assigned value, best estimate of the value, conventional
value or reference value)
Value attributed to a particular quantity and accepted, sometimes by convention, as
having an uncertainty appropriate for a given purpose.
Error (of measurement)
The result of a measurement minus a true value of the measurand
Systematic error
Mean that would result from an infinite number of measurements of the same
measurand carried out under repeatability conditions minus a true value of the
measurand
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Arithmetic mean
x Arithmetic mean value of a sample of n results
Sources of Errors in Trace Element and Speciation Analyses
Sample Standard Deviation
s An estimate of the population standard deviation σ from a sample of n results.
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Sources of Errors in Trace Element and Speciation Analyses
Sources of Errors in Trace Element and Speciation Analyses
Perhaps…
Perhaps it should be…
Sources of Errors and Uncertainities in Trace Element and Speciation
Analyses
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(i)
(ii)
(iii)
(iv)
It is easy to recognize that increasing the complexity of an analytical protocol
(i.e.: more steps during the analytical process) increases the chance of errors. In
an ideal situation, analytical measurements would be done:
in-situ, removing the issues related to sampling and sample integrity.
Directly in the matrix even for solids, without extensive sample preparation such
as leaching, decomposition, etc.
Using highly selective, absolute measurements, with the dynamic range of the
measurement method extending from percentage to ppt levels, thus removing
potential sources of error due to interferences, calibration etc.
without human intervention, i.e., automated, removing the human variable from
the system.
(i)
(ii)
(iii)
(iv)
It is easy to recognize that increasing the complexity of an analytical protocol
(i.e.: more steps during the analytical process) increases the chance of errors. In
an ideal situation, analytical measurements would be done:
in-situ, removing the issues related to sampling and sample integrity.
Directly in the matrix even for solids, without extensive sample preparation such
as leaching, decomposition, etc.
Using highly selective, absolute measurements, with the dynamic range of the
measurement method extending from percentage to ppt levels, thus removing
potential sources of error due to interferences, calibration etc.
without human intervention, i.e., automated, removing the human variable from
the system.
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Overview of the analytical process
Overview of the analytical process
Sampling
Sample preparation
Instrumental analysis
Data evaluation
Sampling
Sample preparation
Instrumental analysis
Data evaluation
Sampling and sample preparation are by far the largest
contributors to measurement errors and uncertainty!!!!
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Contamination, sample loss
-Sample Container Material
-Container Transpiration
-Stability of Metals at ppb Concentration Levels
-Environmental Contamination
-Contamination From Reagents
-Contamination From the Analyst and Apparatus
Overview of the analytical process
Sampling
Sample preparation
Instrumental analysis
Data evaluation
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Detection Technologies
Optical
Absorption, AA, GF-AA
Emission, ICP, Flame
Fluorescent,
Mass spectrometry
ICP
GD
MIP
Atomic Absorption spectrometry (AA)
Discovered in the late fifties
Shine light specific for an element through an atom reservoir (flame or furnace)
Light is absorbed by the ground state atoms of given element
The absorption rate is proportional with the number of analyte atoms in the atom source
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AA
FAST, (seconds per determination)
Low cost per analysis
Low capital investment and low operating cost
Single element determination
Easy to use
Well understood interferences (mature technique)
Versatile
Requires separate lamp for each element to be determined
Hollow Cathode (HCL) or Electrodeless Discharge Lamp (EDL)
Interferences
Technique Type of Interference Compensation Method
Flame AA Ionization Ionization buffer
Chemical Releasing agent or nitrous -
oxide-acetylene flame
Physical
Dilution, matrix matching or
method of additions
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Graphite Furnace Atomic Absorption (GF-AAS)
Low detection limits for most elements
parts per billion (μg/L)
Slower then flame analysis
Min per determination
Single element determination
Small sample volume (μL)
Interferences
Cost per analysis higher than Flame AA
More technical experience needed
Interferences
Technique Type of Interference Compensation Method
GFAA Molecular absorption background correction
Matrix interferences modifiers
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Special case of AA or AFS
•Dedicated system for cold vapor Hg
analysis
•Detects ppt levels of Hg
•High intensity Hg lamp
•Long path length cell
•Solid state detector
•Extremely sensitive but prone to matrix interferences
Inductively Coupled Plasma – Optical Emission Spectrometry
• ICP is an argon gas discharge (~6-9000K)
• Excites atoms in sample and Light being emitted. The emitted light is separated and
measured.
• Wavelengths are characteristic of element
• Intensity of light emitted is proportional to amount of element in the sample
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ICP-OES
Fast or very fast analysis time
Many elements in a few minutes
Low detection limits
Parts per billion in axial view mode
Wider dynamic range
Over 3 times better than flame AA
(Two viewing options)
Cost per analysis
Comparable to flame AA
Lower than GF AA
Some technical experience needed
Interferences
Technique Type of Interference Compensation Method
ICP-OES Spectral Background correction
or use of alternate
analytical wavelengths
Physical
Internal standardization
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Spectrum of a high Ca containing matrix (red line) causing a sloping
background for the Cu 219.959 or Ge 219.871 lines are used. Background
correction is difficult at best in these situations.
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Spectrum of a high concentration of Fe (red line) showing a direct spectral overlap upon the B
208.892 line and a wing overlap on a B 208.959
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Wing overlap interference of Fe (red line) upon the Ba 233.527 nm line
Overview of ICP-OES
Detection limits not as good as GFAA or ICP-MS
• Some specific applications require lower detection limits
Some elements in line-rich area of spectrum
• Thousands of wavelengths for Fe
• Rare earths (Dy, Eu, La, Ce)
Interferences for particular elements
• Transuranics (U, Am, Pu, Tc)
Sensitivity for some elements not sufficient
• Halogens
Higher initial investment than AA
More gas consumption than AA
• Requires ~16 L/min of argon gas
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Advantages of ICP-OES
Multielement technique
Speed
samples can be analyzed in seconds to minutes
Excellent linear dynamic range
up to 100-1000 mg/L
Detection limits moderate to excellent
0.4 - 2 ug/L
Interferences
spectral interferences controlled through instrument design / software
Plasma view: axial or radial
axial view has lower DLs
radial view has wider linear range and less interferences
Lower cost per (similar to AA)
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Nebulizers
ICP-MS
Same type of plasma source as in ICP-OES
Ions are generated by intense ICP ion source, separated and detected by a mass
spectrometer
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Nebulizers
The main function of the sample introduction system is to generate a fine aerosol of
the sample. It achieves this purpose with a nebulizer and a spray chamber
Nebulizers generate an aerosol from the aqueous sample and spray chambers filter
out heavy droplets and dampen noise.
In general, nebulizers designed for use with ICP-OES are not recommended for
ICP-MS 1-2% total dissolved solids (TDS) for OES and only 0.1-0.2% for MS
Nebulizers
Concentric design. In the concentric nebulizer,the solution is introduced through a
capillary tube to a low-pressure region created by a gas flowing rapidly past the end of
the capillary.
Concentric pneumatic nebulizers can provide excellent sensitivity and stability,
particularly with clean solutions. However, the small orifices can be plagued by
blockage problems, especially if large numbers of heavy matrix samples are
aspirated
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Nebulizers
Crossflow design. For samples that contain a heavier matrix or small amounts of
undissolved matter, the crossflow design is probably the best option. With this
design the argon gas is directed at right angles to the tip of a capillary tube, in
contrast to the concentric design, where the gas flow is parallel to the capillary.
Spray chambers
Crossflow nebulizers are generally not as efficient as concentric nebulizers
at creating the very small droplets needed for ICP-MS analyses.
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Schematic of a cyclonic spray chamber (shown
with concentric nebulizer).
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Spray chambers
Ion source
Schematic of a Scott double-pass spray chamber
(shown with crossflow nebulizer
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Plasma
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Plasma
Mass analyzer is the region of the ICP mass spectrometer that
separates the ions according to their mass to charge ratio (m/z)
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Mass analyzers
Quadrupole
Quadrupole
TOF (Iinerar, ortogonal)
Sector instruments
Schematic of a quadrupole mass analyzer.
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Sensitivity comparison of a quadrupole operated at 3.0, 1.0,
and 0.3 amu resolution (measured at 10% of its peak height)
Quads are low res analyzers
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Ions entering the quadrupole are slowed down by the filtering process and
produce peaks with a pronounced tail or shoulder at the low-mass end.
The abundance sensitivity for quadrupoles is always worse on the low mass side
than on the high mass side and is typically 1 x 10 -6 at M - 1 and 1 x 10 -7 at M + 1
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Sector instruments
Schematic of a double-focusing magnetic-sector ICP mass spectrometer
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• Why we need high mass resolution?
Resolution required to resolve some common polyatomic interferences
from a selected group of isotopes.
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Q
Ion transmission with a magneticsector instrument decreases as
the resolution increases.
HR
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TOF (on-axis, ortogonal design)
Schematic of an orthogonal acceleration TOF analyzer.
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m/z = 2Ut 2 /D 2
Where (U) is the accelerating voltage, mass-to-charge ratios
(m/z), time (t), length of the flight path (D)
Schematic of an on-axis acceleration TOF analyzer.
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• Mass analyzer
Resolution
Quad 200-300
TOF 2000-3000
Sector 10,000
Reflectron TOF MS design 59
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Resolution required to resolve some common polyatomic interferences
from a selected group of isotopes.
ICP-Mass Spectroscopy Overview
Very fast analysis
• Comparable to simultaneous ICP-OES
Superior detection limits
• Parts per trillion
Semi-Quant analysis
Isotope analysis
Cost per analysis
• Same as ICP-OES
Some technical experience required
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Technique Type of Interference Compensation Method
ICP-MS Mass Overlap Interelement correction,
use of alternate mass
values or higher mass
Resolution
Dynamic Reaction Cell
Physical
Internal Standardization
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ICP MS
Very fast analysis
• Comparable to simultaneous ICP-OES and standard ICP-MS
Superior detection limits
• Up to parts per quadrillion
• Removes isobaric and argon interferences
Cost per analysis
• Slightly more than standard ICP-MS
Some technical expertise required
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Flame AA
• A few elements per sample to analyze
• 5 mL/min sample consumption
• Detection levels needed are in the sub-mg/L region
GFAA
• Detection levels of μg/L to sub- μg/L levels
• 10-50 uL sample consumption
ICP-OES
• Several elements per sample (5+)
• 1-5 mL/min sample consumption
• Detection levels are in the sub-mg/L and μg/L region
ICP-MS
• Detection levels of sub- μg/L to sub-ng/L levels
• Typical 1-2 mL/min sample consumption
Can use low consumption nebulizers
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Interferences in ICP MS
Isobaric Interferences
Polyatomic (Molecular) Interferences
Doubly Charged Ion Interferences
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Isobaric Interferences:
Isobaric interference is a result of equal mass isotopes of different elements present in
the sample solution. Low-resolution instruments (quad, TOF) cannot distinguish
between the isotopes. There are many examples in the intermediate mass regions
where the second and third row transitions and the rare earths appear. There are no
elemental singly charged isotopes that overlap with monoisotopic elements (9Be,
23Na, 27Al, 45Sc, 55Mn, 75As, 89Y, 103Rh, 127I, 133Cs, 141Pr, 159Tb, 165Ho,
169Tm, 197Au, and 232Th). For elements having more than one isotope, the quickest
fix may be to use another isotope of that element.
Isobaric Interferences:
If the interference is from an isotope with roughly the same or lower peak intensity, it is
possible to perform a correction by measuring the intensity of another isotope of the
interfering element and subtracting the appropriate correction factor from the intensity
of the interfered isotope. If you are working with an unknown sample composition, a
semi-quantitative analysis is suggested with low-resolution instruments using a quick
scan of the sample and the rather sophisticated semi-quantitative programs available
on current instrumentation.
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Polyatomic (Molecular) Interferences:
Molecular interferences are due to the recombination of sample and matrix ions with Ar
and other matrix components such as O, N, H, C, Cl, S, F, etc. The light elements (Li,
Be, B) are not affected due to their small masses.
Starting with 39K, this type of interference becomes a significant issue. For example,
39K is interfered with by 38ArH and 23Na16O. Some polyatomic interferences can be
avoided by eliminating certain matrix elements such as the classic 40Ar35Cl
interference upon the monoisotopic element 75As, where the use of HCl in the sample
preparation is to be avoided. The isotopes 56Fe, 39K, and 44Ca or 40Ca are all
interfered with by combinations of the Ar, O, and N isotopes.
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Polyatomic (Molecular) Interferences:
As we go to the heavier elements the major polyatomic interferences come from
isotopes that are 16 atomic mass units lower than the analyte isotope through
molecular oxide (MO) interference. The lanthanide element isotopes are especially
prone to molecular oxide formation. The use of cool plasma techniques, reaction /
collision cells, desolvation, and chromatographic separations -- to name a few
approaches -- have resulted in reduction and, in some cases, complete elimination of
many polyatomic interferences.
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Polyatomic (Molecular) Interferences:
The severity of the MO interference can be reduced through reduction of the sample
argon gas flow rate. Mass corrections may be an option in cases where the use of an
alternate isotope is not an option. Polyatomic interferences are particularly
troublesome in the determination of first row periodic table elements (K thru Se) due to
the vast number of combinations of Ar with matrix components.
Doubly Charged Ion Interferences:
Doubly charged ion interference is due to doubly charged element isotopes with twice
the mass of the analyte isotope. For example, interference from 206Pb++ (m/e = 103)
upon 103Rh is likely at high Pb concentration levels. Reduction in the sample Ar will
minimize this interference. Fortunately, this type of interference is not as prominent in
Ar plasmas, but care should be exercised in matrices containing high levels of mid to
heavy mass element isotopes. The alkaline and rare earth elements form doubly
charged ions to a extent that is greater, relative to the other elements.
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ICP MS background mass spectrum of high-purity water.
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Common polyatomic spectral interferences in ICP-MS.
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Space Charge Effects:
These effects are thought to occur at the MS interface, the region between the
skimmer tip and ion optics and in the ion optics region. The net result is a suppression
of the signal in high concentrations of a matrix element. The kinetic energy of the ion
element matrix affects the degree of suppression with larger masses (higher kinetic
energy) causing more depression than lower masses. Due to differences between
instruments in interface and ion optic designs it is difficult to predict the conditions
under which the effect is minimal. Under 'cool plasma' conditions, this suppression
effect is more pronounced. Keep the matrix element concentration at or below the 100
µg/g!
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How to eliminate spectral interferences?
isobaric interferences from isobaric element ions cannot be eliminated easily
solution: use an other isotope if you can
Front end solutions (chromatography, hydride generation etc)
polyatomic interferences could be eliminated by
solution: cool plasma conditions
increased mass resolution
gas phase chemistry
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How to eliminate spectral interferences?
Collision/Reaction Cell Technology
isobaric interferences from isobaric element ions cannot be eliminated easily
solution: use an other isotope if you can
Front end solutions (chromatography, hydride generation etc)
polyatomic interferences could be eliminated by
solution: cool plasma conditions
increased mass resolution
gas phase chemistry
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Examples of polyatomic interferences:
• 40Ar16O on the determination of 56Fe
• 38ArH on the determination of 39K
• 40Ar on the determination of 40Ca
• 40Ar40Ar on the determination of 80Se
• 40Ar35Cl on the determination of 75As
• 40Ar12C on the determination of 52Cr
• 35Cl16O on the determination of 51V.
Elimination of the ArO interference with a dynamic reaction cell.
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Calibration
“Collision/reaction cells have
given a new lease on life to
quadrupole mass analyzers used
in ICP-MS.”
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External Calibration:
Use this approach for matrices that are known and can be matched. The use of
internal standards is helpful in accounting for drift.
Must know your sample composition! A semi-quantitative analysis using a scanning
approach for the entire mass range allows the analyst to predict interferences
and select internal standards and analyte isotopic masses.
Perform interference check analysis. Prepare for the variations in the matrix and
analyte composition and determine if corrections that have been built into the
procedure are capable of providing the required accuracy.
External Calibration:
Use this approach for matrices that are known and can be matched. The use of
internal standards is helpful in accounting for drift.
Must know your sample composition! A semi-quantitative analysis using a scanning
approach for the entire mass range allows the analyst to predict interferences
and select internal standards and analyte isotopic masses.
Perform interference check analysis. Prepare for the variations in the matrix and
analyte composition and determine if corrections that have been built into the
procedure are capable of providing the required accuracy.
Simple and cheap BUT prone to interferences
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Standard Additions:
This approach is common with ICP-OES & ICP-MS.
Could correct interferences but not for instrument drift
Isotope Dilution:
Isotope dilution in mass spectrometry is a type of internal standardization. ID is a
primary (definitive) analytical method for the determination of metals in a variety
of sample types. (other primary analytical methods are gravimetry, titrimetry,
coulometry, differential scanning calorimetry) and nuclear magnetic resonance
spectroscopy.
Only applicable for multi isotope elements. (not for: 9Be, 23Na, 27Al, 45Sc, 55Mn,
75As, 89Y, 103Rh, 127I, 133Cs, 141Pr, 159Tb, 165Ho, 169Tm, 197Au, and
232Th)
ID could correct both for systematic and random errors in the analysis.
But it is expensive and involves complicated mathematics.
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Expression of an analytical result
Value 1 ± Value 2
Free of systematic error (bias)
Verification of traceability of
the results:
• CRMs
• Primary methods (or reference
methods)
Uncertainty
Consideration of all sources of error
of the analytical process:
1. Random errors: Method precision
2. Correction of systematic errors
Thank you for your attention!
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