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AGILENT 7500

1.Fundamental aspects of ICPMS

Briefly, in this technique, singly charged analyte ions generated in an ICP are extracted into and measured with a mass analyzer.

The rapid development of ICPMS has been fueled by its unique measurement capabilities

Detection Limit: 1 to 100 ng/L in many cases, these limits are 100 to 1000 times superior to those that can be routinely achieved by ICP-AES.

Spectral interferences

Isotopic ratio measurement capability

Inductively Coupled Plasma Mass Spectrometry (ICPMS)

ICP as an ion source

When a system is in thermal equilibrium, the degree of ionization of an atom is given by the Saha equation (Note that the degree of excitation of an atom is described by Boltzmann equation:

nine/na = 2Zi/Za(2πmkT/h2)3/2exp(-Ei/kT)

whereni, ne and na are the number densities of the ions, free electrons and atoms in the plasma, respectively.Zi and Za are the ionic and atomic partition functions, respectively.m is the electron mass.k is the Boltzmann constant.T is the temperature.h is the Planck’s constant.Ei is the first ionization energy.

Degree of ionization is dependent on: electron number density temperature ionization energy

Assume:electron number density for an argon ICP: 4 x 1015 cm-3

ionization temperature: 8730 K

Then the degree of ionization as a function of first ionization energy, predicted by the Saha equation.

Over 80% ionized for elements having 1st ionization energies of less than 9 eV.The most poorly ionized elements:

He, Ne, F, O, and N (<1%); Kr and Cl (1 – 10%)C, Br, Xe and S (10 –30%), P, I, Hg, As, Au and Pt (30 –80 %).

Note that this calculation is carried out based on assumption of a local thermodynamic equilibrium (LTE) be reached.

From Jarvis et al., 1997

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Jarvis et al., 1997

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Jarvis et al., 1997

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From Houk ICP Course

YO, Y(I), Y(II) EMISSION ZONES COURTESY VARIAN

Distribution of ions in the plasma

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Jarvis et al., 1997

Cooler central

Hotter area

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Jarvis et al., 1997

The optimized location of the orifice is determined by the elements of interest and other plasma conditions.

Ionization needs time!

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Jarvis et al., 1997

The optimized location of the orifice is determined by the elements of interest and other plasma conditions.

Ionization needs time!

1.3 Ion samplingICP/AES vis ICP/MS

1.3.1 Ion sampling interface

Boundary layer and sheath

Interaction between plasma and sampling cone:

Intermediate temperature zone between plasma and cone (oxide formation) in this boundary layer (red color).

Plasma potential (sheath electrical by the interaction between the plasma and the conducting sampler)

Boundary vs. Sheath region

Supersonic jet

Barrel shock

Mach disk

1.3.3 Supersonic jet From Houk ICP Course

The barrel shock and Mach are caused by collisions between fast atoms/ions from jet and the back ground gas, which reheat the atoms/ions and induce emission. To avoid losses of ions due to collisions and scattering, position the skimmer with its open tip inside the Mach disc.

Mach diskSamplerSkimmer

The position of the onset of the Mach disc is given byXm = 0.67 D0(P0/P1)1/2

Xm = position of Mach dsic from the sampling orifice along the central axisD0 = diameter of the samplingorificeP0 = pressure of ICPP1 = back ground gas pressure in extraction chamber

A skimmer-sampler separation of 2/3 of the distance to the onset of the Mach disc usuallyProvides optimum ion transmission

Ion focusing• Serves to focus ions from the skimmer into the mass filter

(analyzer)

• Rejects neutral atoms

• Minimize the passage of any photons from ICP (Electron Multiplier is photo/neutral sensitive)

Extraction(Extract ions fromPlasma)

Einzel(Focus ion beam)

Omega (eliminate photons&neutrals)

10-2 Torr10-5 Torr

Space charge effects

Space charge effects are a consequence of the mutual repulsion between particles of like charge.

It is a consequence of Coulomb’s law, which quantifies the force (F) between two point charges (q and q’) separated by a distance r as

F = k(qq’/r2)

where k is a proportionality constant that depends upon the unit chosen for the other variables in the equation.

Adapted from Ken Busch, June 2004, 19(6) Spectroscopy.

Interferences in ICPMS

Mass spectral interferences

Skoog et al., 1999, Instrumental Analysis

Isobaric overlap

Isobaric interferences are due to two elements that have isotopes having substantially the same mass.

Quadrupole instruments: differ in mass by less one unit.

Generally Most elements in the periodic table have one (e.g. 59Co),

two (e.g. Sm, Samarium), or even three (e.g. Sn) isotopes that are free from isobaric overlap.

An isobaric interference occurs with the most abundant (sad!) and thus the most sensitive isotope, e.g. the very large peak for 40Ar+ overlaps the peak for the most abundant calcium isotope 40Ca+ (97%) making it is necessary to use the second most abundant isotope 44Ca+ (2.1%).

Isotopes with odd masses are free from overlap, while with even masses are not.

No isobaric peak interferences below 36 m/z.

Isobaric overlaps are exactly predictable!

Polyatomic

Polyatomic ion interferences result from interactions between species in the plasma and species in matrix or atmosphere.

Argon, hydrogen and oxygen are the dominant species present in the plasma and these may combine with each other or

With elements from the analyte matrix or The major elements present in the solvents or acid

used during sample preparation (e.g. N, S. and Cl)

Vandercasteele and Block1997

??3000000

Vandercasteele and Block1997

This type of interference is found largely at m/z values of below 82.

Jarvis et al., 1997

Polyatomic ion peaks in both H2O2 and HNO3 are identical to those identified in de-ionized water and these media are therefore considered ideal matrices. However, the spectra in an HCl or H2SO4 matrix are more complex.

Vandercasteele and Block, 1997

Vandercasteele and Block, 1997

Vandercasteele and Block, 1997

Corrected for using a blanks Estimate the response of the interference

relative to the analyte Reduce water entering Plasma

Refractory oxide ions

Refractory oxide ions occur either as a result of incomplete dissociation of the sample matrix or from recombination in the plasma tail.

16 (MO+), 32 (MO2+) or 48 (MO3+) mass units above the M+ peak

The relative level of oxides can be predicted from the monoxide bond strength of the element concerned. Those elements with the highest oxide bond strength usually give the greatest yield of MO+ ions.

Plasma operating conditions can dramatically influence the formation of oxide ions

Jarvis et al., 1997

Jarvis et al., 1997

Doubly charged ions

The formation of doubly charged ion in the plasma is controlled by the second ionization energy of the element and the condition of plasma equilibrium.

Only those elements with a second ionization energy lower than the first ionization energy of Ar will undergo any significant degree of 2+ formation.

The effect of 2+ ions is two-fold: Sensitivity for the singly charged species Spectrum interferences for others

Jarvis et al., 1997

04/21/23

Interference equations

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

04/21/23

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.

04/21/23

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.

04/21/23

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.

04/21/23

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%).

04/21/23

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)

04/21/23

Matrix effects

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.

04/21/23

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.

04/21/23

Methods to correct for or overcome matrix effects

Dilution» Easy

» Detection limits sacrificed

Matrix matchingOf 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.

04/21/23

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

04/21/23

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

04/21/23

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

04/21/23

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.

04/21/23

Collision Reaction Cell

Hexapoles

04/21/23

04/21/23

Dynamic Reaction Cell

04/21/23