Trace element analysis of
geological materials by ICP-MS I
DSP analytical geochemistry
Markéta Holá, MU Brno
Tento učební materiál vznikl v rámci projektu Rozvoj doktorského studia chemie
č. CZ.02.2.69/0.0/0.0/16_018/0002593
C9067
Outline
1. Mass spectrometry. General introduction and history.
2. Ion sources for mass spectrometry. Inductively coupled plasma.
3. Interface. Ion optics. Mass discrimination. Vacuum system.
4. Spectral interferences. Resolution, ion resolution calculations.
5. Mass analyzers. Elimination of spectral interferences.
6. Non-spectral interference.
7. Detectors, expression of results.
8. Introduction of samples into plasma.
9. Laser ablation for ICP-MS.
10.Excursion in the laboratory.
• Spectral
mass overlap of the interfering particle and the measured
isotope (same m/z - indistinguishable from each other)
• Non-spectral
influencing the signal intensity of the analyte by the presence of
various substances in the sample matrix
Interferences
ICP-MS
Non-spectral interferences
matrix effect
• Matrix of sample solutions to be analyzed
- Total content of dissolved solids < 1 – 2 g/l (< 0.2 %)
- Acid concentration < 10 % (tipically 2-5 %)
• ‘Heavier’ matrices:
- Irreversible effects (Clogging torch, sampling cone aperture, …)
- Reversible effects (Very pronounced matrix effects or non-spectral interferences
=Matrix-induced signal suppression or enhancement
- Dilution required
Non-spectral interferences
matrix effect
• Physical effects - Difference in viscosity between sample & standard
- difference in nebulization efficiency
- difference in droplet size distribution
• Shift in ionization equilibrium
• Ambipolar diffusion in ICP
• Shift of zone of maximum M+ density
• Space charge effects during ion extraction
• Specific effects
Non-spectral interferences
matrix effect
Physical effects
- Matrix-induced signal suppression or enhancement
Non-spectral interferences
matrix effect
Shift in ionization equilibrium
Non-spectral interferences
matrix effect
Ambipolar diffusion in ICP
Non-spectral interferences
matrix effect
Ambipolar diffusion in ICP
Non-spectral interferences
matrix effect
Shift of zone of maximum ion density
Non-spectral interferences
matrix effect
Shift of zone of maximum ion density
- The phenomenon of non-stoichiometric transition of ions
through the mass spectrometer depending on their mass
- Origins in ICP-MS:
- plasma nozzle effect
- space charge effects in the interface during supersonic
expansion of ions through the sampler cone
- Coulombic repulsion
- Results in ICP-MS: measured isotope ratio shows
significant bias to the true value
- Numerical Correction needed:
- mass bias drifts with time (time and matrix dependent)
- Instrumental Isotope Fractionation (IIF)
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J. Anal. At. Spectrom., 2022, 37, 701–726
- specific isotope amount ratio is used to calibrate the amount ratio of another pair of isotopes (same or
different element) - SSB (Sample-standard bracketing) correction
- mass bias: time-dependent, mass-dependent
Correction factor: linear correction law 𝐾𝑖/𝑗 = 1 + 𝑓(𝑡)(𝑚𝑖 − 𝑚𝑗)
Russell‘s law 𝐾𝑖/𝑗 = (
𝑚𝑖
𝑚 𝑗
) 𝑓(𝑡)
exponential law ln 𝐾𝑖/𝑗 = 𝑓(𝑡)(𝑚𝑖
𝑛
− 𝑚𝑗
𝑛
)
- principle: calibrator with known isotope amount ratio is measured to obtain the f(t) value as a difference
between known and measured value; this f(t) value is then used to calculate the correction factor Ki/j
- Matrix effect: target element needs to be isolated – separation process
13
Non-spectral interferences
matrix effect
Space charge effects during ion extraction
Non-spectral interferences
matrix effect
Elimination of matrix effect:
• Trace / matrix separation
- (Chromatographic) separation technique
Labor-intensive & time-consuming
Increased risk of contamination & analyte losses
- Alternative sample introduction technique
e.g., ETV, hydride generation
• Dilution
- Not canalyte/cmatrix, but cmatrix is determining factor
- Deterioration of LODs
• Use of internal standard
• Appropriate calibration techniques
- Single standard addition
- Isotope dilution
Non-spectral interferences
matrix effect
Use of internal standard:
• Element (same concentration) added to sample, standard, blank
• Assumption - Undergoes same suppression / enhancement as analyte
• Selection - Most important criterion: close agreement in m/z
• Further conditions
Not present in sample
No precipitation, no volatilization
No spectral overlap
• All calculations with Ianalyte / Iinternal standard
• Additional advantage: improved precision - Correction for signal drift & instrument
instability
ICP-Q-MS Users Guide
Sample Collection and Preparation
No Particles:
Samples for solution mode ICP-Q-MS should be particulate free in order to prevent clogging of
sample tubing and the nebulizer capillaries (particularly for concentric and micro-concentric types).
Samples containing solids (even nanoparticles) will not ionize the same way as liquid samples, and
thus can negatively affect quantitative analysis. Particles may be removed by filtration or by
centrifugation and decanting.
Dilute Samples in 1-2 % Nitric Acid:
Sample solutions should ideally share a matrix similar to calibration standards. Commercially
available single and multielement standards often have matrices comprised of 1-5% nitric acid.
Acidification prevents dissolved species from readily sorbing onto tubing and container walls. Nitric
acid is preferred because it generates fewer polyatomic interferences compared to other acids.
ICP-Q-MS Users Guide
Sample Collection and Preparation
Dilute Samples to <<< 0.2 wt% (2000 ppm; preferably ≤200 ppm) Total Dissolved
Solids:
It is important to limit total dissolved sample contents in order to minimize matrix effects that can
affect the consistency of ionization in the plasma as well as focusing and collimation of the ion
beam beyond the interface region of the ICP-Q-MS.
Dilution is also important to ensure that measurement conditions remain consistent throughout
the analytical session. Analysis of high TDS samples can rapidly deposit coatings onto sampler and
skimmer cones that change orifice shapes and flow dynamics. Such deposits can also constitute a
contamination source if partially ionized during subsequent analyses.
Example: salinity of seewater 3.5 % (35 000 ppm). Dilution 175x to obtain 200 ppm. Will trace
elements be detectable?
ICP-Q-MS Users Guide
Sample Collection and Preparation
Plan Quality Control in Advance:
Quality control statistics typically derive from analyses of method blanks, matrix spikes, and
certified reference materials of known concentration. Method (or procedural) blanks, sample
processed identically to real samples except that no sample is actually added, provide a measure of
all processing-related contamination (airborne, reagents/acids, labware, personal) that could
potentially contribute to measured concentrations. Matrix spikes provide a means of evaluating
the extent to which quantitative results may suffer as a result of matrix differences between
samples and calibrations standards. Quality control standards are often analyzed periodically
throughout a run of unknowns. Accuracy and precision obtainable by the data acquisition method
can be evaluated from recoveries and standard deviations relative to certified concentrations.
Reference standards should be independently sourced from stocks used for calibration standards.
https://www.jsg.utexas.edu/icp-ms/icp-ms/
ICP-Q-MS Users Guide
Sample Collection and Preparation