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AGILENT 7500. 1. Inductively Coupled Plasma Mass Spectrometry (ICPMS). 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. - PowerPoint PPT Presentation

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  • 1.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 capabilityInductively 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(2mkT/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 Plancks constant.Ei is the first ionization energy.

  • Degree of ionization is dependent on:electron number densitytemperatureionization energy

    Assume:electron number density for an argon ICP: 4 x 1015 cm-3ionization 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.Jarvis et al., 1997

  • 1.Jarvis et al., 1997

  • 1.

  • From Houk ICP CourseYO, Y(I), Y(II) EMISSION ZONES COURTESY VARIANDistribution of ions in the plasma

  • 1.Jarvis et al., 1997Cooler centralHotter area

  • 1.Jarvis et al., 1997The optimized location of the orifice is determined by the elements of interest and other plasma conditions.

    Ionization needs time!

  • 1.Jarvis et al., 1997The optimized location of the orifice is determined by the elements of interest and other plasma conditions.

    Ionization needs time!

  • 1.3Ion samplingICP/AES vis ICP/MS1.3.1Ion 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 jetBarrel shockMach disk

  • 1.3.3Supersonic jetFrom Houk ICP CourseThe 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 diskSamplerSkimmerThe position of the onset of the Mach disc is given byXm = 0.67 D0(P0/P1)1/2Xm = 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 chamberA skimmer-sampler separation of 2/3 of the distance to the onset of the Mach disc usuallyProvides optimum ion transmission

  • Ion focusingServes 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 Coulombs 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 interferencesSkoog et al., 1999, Instrumental Analysis

  • Isobaric overlapIsobaric interferences are due to two elements that have isotopes having substantially the same mass.Quadrupole instruments: differ in mass by less one unit.

  • GenerallyMost 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!

  • PolyatomicPolyatomic 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 orWith elements from the analyte matrix orThe 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 blanksEstimate the response of the interference relative to the analyte Reduce water entering Plasma

  • Refractory oxide ionsRefractory 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+ peakThe 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 speciesSpectrum interferences for others

  • Jarvis et al., 1997

  • *Interference equations

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

  • *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.

  • *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.

  • *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.

  • *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%).

  • *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)

  • *Matrix effects

    Observation and mechanisms

    High dissolved solids

    blockage of the entrance aperture of the sampling coneThe 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.

  • *Suppression and enhancement effects

    Ioni

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