Sample Analysis Design - University of Notre Dame · · 2012-09-28Sample Analysis Design ... introduction system – e.g. DSN 100) • Optimize instruments settings so that formation

Sample Analysis Design

PART II

Sample Analysis Design •  Generating high quality, validated results is the

primary goal of elemental abundance determinations

•  It is absolutely critical to plan an ICP-MS analysis carefully – from sample gathering to final analysis on the ICPMS

•  Always create and follow an analysis design that shall permit you to follow the procedure for future samples, AND know what you did when looking at old data

Sample Analysis Design •  Goal: high quality, validated, quantitative determination

of elemental concentrations/isotope ratios

•  This goal can be achieved by:

•  Specificity - only detecting isotope(s) of interest, not interferences, OR making sure that matrix effects do not play a role

•  Sensitivity - can we differentiate the isotope of interest from the background signal?

•  Accuracy - does the analysis represent the real value?

•  Precision - how repeatable is the value?


•  Sensitivity and Precision: – generally defined by how well the instrument

is tuned (assuming dilution factor is appropriate)

•  Specificity and Accuracy: – generally governed by how well you prepare

your sample and set up your analysis method/parameters


•  Spectroscopic interferences

•  Matrix effects

Sample Analysis Design •  Interference – analytical artifact which causes

the ion signal in a sample to vary from an idealized (true) signal –  Interferences cause inaccurate analyses

•  Two main types:

– 1. Spectroscopic

– 2. Nonspectroscopic

Sample Analysis Design •  Spectroscopic Interferences in ICP-MS

– 1. Isobaric overlap

– 2. Polyatomic ions

– 3. Refractory oxides

– 4. Doubly charged ions

Sample Analysis Design •  Isobaric Overlap

•  Two different elements having the same nominal mass

•  E.g., 40Ca (96.9%) and 40Ar (99.6%)

•  Monoisotopic elements:

•  B, Na, Al, Sc, Mn, As, Nb, Y, Rh, I, Cs, Pr, Tb, Ho, Tm, Au, Th

•  None have isobaric interferences

Sample Analysis Design – Isobaric Overlap

•  A few multi-isotope elements have no interference free isotopes:

–  E.g., In - 113In → 113Cd, and 115In → 115Sn

•  Usually possible to choose an interference free isotope or one that is unlikely to be interfered upon strongly (either due to very low natural abundance of interfering element (isotope) or the interfering element (isotope) is not present in sample)

Sample Analysis Design – Isobaric Overlap

•  IF an isobaric interference is unavoidable, you can correct for it by measuring the counts on a non-interfering isotope of the interfering element

•  CPSanalyte = CPStotal -CPSinterferent*(Xa/Xb)

•  Xa = abundance of interfering isotope •  Xb = abundance of non-interfering isotope

Sample Analysis Design – Polyatomic Interferences

•  More serious than isobaric interferences

•  Result from possible, short-lived combination of atomic species in the plasma or during ion transfer

•  Common recombinants are Ar, H, and O

•  Dominant elements in reagents also form polyatomic interferences - N, S, and Cl


•  Analyzing deionized water with ICP-MS instrument – we have solely H and O present (within the ‘matrix’)

•  Thus, peaks would be visible at masses:

–  41 (40Ar + 1H) → 41K

–  56 (40Ar + 16O) → 56Fe

–  80 (40Ar + 40Ar) → 80Se

–  As well as other minor peaks from the minor isotopes of Ar and O


•  If you acidify the deionized water with HNO3 or H2O2 , these behave in the same manner as deionized water

–  these acids are considered “ideal matrices” as they don’t add unnecessary polyatomic interferences

•  Acidification with HCl or H2SO4 changes the situation since Cl and S can be used to form polyatomic ions


•  HCl matrix:

– 35Cl + 16O = 51V

– 37Cl + 16O = 53Cr

– 40Ar + 35Cl = 75As

– 40Ar + 37Cl = 77Se


•  H2SO4 matrix:

– 32S + 16O = 48Ti

– 32S + 16O + 16O, or 32S + 32S = 64Zn, 64Ni

– 34S + 16O = 50Ti, 50V, 50Cr

– 33S + 16O + 16O = 65Cu

Sample Analysis Design •  These interferences will be present just from the

gas and solvent

•  Therefore, if possible, solutions should be prepared with a weak (1-5% v/v) HNO3 acid matrix

•  Why not deionized water? –  acid helps keep elements from sticking to sides of test

tubes and tubing during transport –  ionization efficiency is significantly lower relative to

HNO3


•  The most serious polyatomic interferences are formed from the most abundant isotopes of H, C, N, O, Cl, and Ar

•  So, if polyatomic interferences are present in plain deionized water with no other ions present, what happens in natural (complex) samples/matrices?


•  Need to know the most abundant elements present in sample

–  Is (are) there any one (or more) elements present that are in very high abundances?

•  If so - then it’s likely interferences could be formed from such abundant element(s)

•  E.g., rock samples - high Si


•  Typically, most polyatomic interferences are only present below mass 82 - since Ar, H, and O are by far the most abundant isotopes in the plasma, they form most interferences

•  Counteractive measures:

–  Proper machine settings (especially nebulizer flow rate and RF power) can minimize formation


•  If a polyatomic interference is unavoidable, then a correction equation like the one for isobaric interferences can be used (assuming you are certain of the ‘species’ of the interference)

Sample Analysis Design – Refractory Oxides

•  Refractory Oxides

•  Occur because of incomplete sample dissociation or from recombination

•  Always occur as an interference as an integral value of 16 mass units above the interfering element

•  MO, MO2, or MO3 where M is the interfering element


•  Elements with high oxide bond strength are most likely to form refractory oxide interferences

•  Severity is expressed as MO/M as a percentage

•  MO/M should be as low as possible, typically <5%

•  MO/M minimized by adjusting nebulizer flow rate, z-axis position, and RF power


•  Si, Ce, Zr, Ti, Sm, Mo, and P all form strong oxide bonds and may have severe oxide interferences

•  Usually monitor the CeO/Ce ratio to make sure it’s below 5%


•  Examples:

•  LREE on the HREE

•  BaO on Eu

•  MoO on Cd


•  Again, correction equations can be applied but these lead to increased error, especially if the interference is large

•  Oxide formation tends to not be stable and the corrections accumulate error quickly

•  If there is a severe oxide interference, it’s usually better to try to separate the interference from the analyte


•  Typically,

– MO > MO2 > MO3 > MO4

Sample Analysis Design – Doubly Charged Ions

•  Form when the 2nd ionization potential is less than the first ionization potential of Ar

•  Typical of alkaline earths, a few transition metals, and some REE

•  Low nebulizer flow rates increase doubly charged ion formation

•  Interference signal is always M/2 where M is the element becoming doubly charged

How to avoid spectral MS interferences?

•  Attempt to ‘dry’ solutions prior to introduction to plasma - remove O and H gets rid of many polyatomic species (i.e. use a desolvating introduction system – e.g. DSN 100)

•  Optimize instruments settings so that formation is minimized and corrections are not severe

•  Use simple matrices whenever possible

•  Process samples prior to analysis to isolate elements of interest or remove potential interfering elements; e.g. ion exchange chromatography

How to avoid spectral MS interferences?

•  Alternatively - use higher mass resolutions, MR – medium resolution or HR – high resolution

•  Most overlaps are not exact using medium and high resolution modes

•  Can completely separate the isotope of interest from interfering species

•  However, consequence is loss of sensitivity


•  PART III

•  Spectroscopic interferences

• Matrix effects

Matrix Effects

•  Physical – physical effects from dissolved or undissolved

solids present in solution

•  Chemical

•  Ionization – suppression and enhancement effects

Matrix Effects

•  Physical

–  variations in rate of atomization/ionization of samples and standards

–  samples and standards should have similar viscosities

–  matrix acid concentrations should be the same in samples and standards

Matrix Effects

•  Chemical – properties of the solution may inhibit or

suppress the formation of ions

– high TDS, refractory species

–  typically increasing RF power or lowering nebulizer flow rates can reduce this type of interference

Matrix Effects

•  Ionization

– space charge effects

Matrix Effects

•  Effects from high TDS

– Significant sample drift observed when TDS is > 2000 ug/ml

– 200 mg of sample in 100 ml of solute –  rock analysis = 50 mg in 100 ml

Matrix Effects

•  Ways to avoid high TDS effects:

– most obvious - DILUTE

– 2nd way - prime the system → most high TDS effects will cause signal drift until a lower (or sometimes higher) analyte signal is reached after that point, the change is minimal

Matrix Effects •  Ion signal Suppression and Enhancement

–  suppression more common than enhancement

–  suppression through space charge effects

–  in general, large concentrations of heavy elements will cause signal suppression through space charge effects

–  S - a classic ion signal suppressor

Matrix Effects •  Enhancement is more rare as ion signal

increases over time are more likely the result of changing machine conditions than true matrix effects

•  Overall - matrix effects caused by low mass elements are poorly understood

–  plasma dynamics? ion transfer changes? supersonic expansion?

Matrix Effects

•  Matrix effects are difficult to quantify

•  Corrections are typically quite error prone

•  Hard to determine if signal degradation is due to instrumental drift or due to suppression/enhancement

Ways to overcome matrix effects?

•  DILUTE

•  internal standarization

•  instrument optimization

•  matrix matching standards and samples

•  standard addition

•  matrix separation


Element2 - Basic Software Concepts

Element2 – Software Basic Concepts





Element2 – Software Method Editor

•  Before you acquire data, you must define a suitable measuring program – this results in a Method

•  The Method contains all the parameters:

–  Related to the elements to be measured (e.g. mass range & isotopes)

–  Related to conditions of data sampling (e.g. sample time, samples per peak, replicates- runs & passes)

–  Related to the measuring method (e.g. resolution, scan mode, detection mode)

Element2 – Software Method Editor

•  Method editor application features two different views:

– Periodic system of the elements (PSE)

– Display of the measuring parameters in spreadsheet format

Method Editor Views

•  Element2 – Software Method Editor –

•  TOOL BAR

Software definitions •  Pass:

–  Defined as one measurement cycle in the selected resolution

•  Run:

–  To improve the statistical ‘numbers’, several passes can be combined (averaged) into Runs

•  Scans: –  The # of scans is the product of the # of runs and the

# of passes –  Scans = Runs * Passes

Settling time for acquisitions using multiple mass resolutions.

Scan Modes •  Magnetic Scan (BScan):

–  the electric field is kept constant and the magnetic field is varied as a function of time

–  the BScan is suitable for scanning large, continuous mass ranges to obtain a complete spectrum from an unknown sample.

–  for repeated scanning, a fast ‘flight-back’ follows each scan –  depending on the (mass) distance for the flight-back, a

settling time for the magnet must be allowed –  the settling time takes into account the time needed for the

magnet to recover from the fast flight-back of each cycle –  this scan mode is also used for Mass Calibration in order

to account for the non-linear behavior of the magnet

Scan Modes

•  Electric Scan (EScan): –  the magnetic field is kept constant and scanning is

performed electrically by varying the accelerating voltage and the ESA high voltage

–  this scan mode is the fastest scan –  the EScan starts with setting the magnet to a mass at

the lower end of the Mass Range –  the available mass range for the electric scan

depends on the magnet mass; it ranges from -6% to +30% of the magnet mass

Scan Modes •  EScan advantages:

–  fast –  scans can be conducted over the detectable mass

range without huge energy transfer in the magnet = this results in settling times that do not have to be observed even when performing the fastest repeated scans

–  thus possible to scan the mass ranges of the isotopic pattern and to skip the ranges of no interest

–  possible to carry out replicate measurements in quick succession because of the low energy transfer

–  the advantages of EScan make it the preferred and default scan mode

•  Integration Window (%): – Expressed as a percentage of the peak width, is

used to calculate peak intensity. A value of 100% means that the intensity will be calculated from samples, measured in a window of ± ½ peak width of peak center

•  Peak Search Window (%):

– This specifies the mass window, where the software performs a search (calculation) for the peak center. For example, a value of 100% means that all samples are used in a window of ± ½ peak width of the accurate mass as specified during the elements/isotope selection.

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Sample Analysis Design - University of Notre Dame · · 2012-09-28Sample Analysis Design ... introduction system – e.g. DSN 100) • Optimize instruments settings so that formation