Docstoc

Chapter 11 Atomic Mass Spectrome

Document Sample
Chapter 11 Atomic Mass Spectrome Powered By Docstoc
					     Atomic Mass Spectrometry
   Nearly all elements in the periodic table can
    be determined by mass spectrometry
   More selective and sensitive than optical
    instruments
   Simple spectra
   Isotope ratios
   Much more expensive instrumentation
Still spectroscopy?
    What is a mass spectrometer?



Illustration of the basic components of a mass spectrometry system.



            Ionization              Mass
                                                          Detector
            Source                  Analzyer



                                               selected              Data
   Inlet                 all ions              ions
                                                                     System
        Lets talk about mass!
   Atomic mass of Carbon

   Atomic mass of Chlorine

   Atomic mass of Hydrogen
          Lets talk about mass!
   Atomic mass of Carbon
       12.000000000000000000000000000 amu
   Atomic mass of Chlorine
       35.4527 amu
   Atomic mass of Hydrogen
       1.00794 amu


         1amu = 1 dalton (Da)
          What about isotopes?
   Atomic mass of Carbon
       12.000 amu for 12C but 13.3355 for 13C
   Atomic mass of Chlorine
       34.9688 amu for 35Cl and 36.9659 for 37Cl
   Atomic mass of Hydrogen
       1.00794 amu for H and 2.0141 for D!
        Just for clarification
   Atomic mass
   amu, atomic mass units (uma??)
   “Da” or Dalton.
   kD (kiloDalton for macromolecules)
   1 amu = 1.66056*10-27 kg.
   proton, mp = 1.67265*10-27 kg,
   neutron, mn = 1.67495*10-27 kg.
    Ways to define and calculate the mass of an
              atom, molecule or ion

   Average mass: calculated using the atomic weight, which
    is the weighted average of the atomic masses of the
    different isotopes of each element in the molecule.
    Often used in stoichiometric calculations.
   Nominal mass: calculated using the mass of the
    predominant isotopes of each element rounded to the
    nearest integer value that corresponds to the mass number.
   Monoisotopic mass: calculated using the extract mass of
    the most abundance isotope for each constituent element.
    Use monoisotopic mass if possible in MS
                Differences between Masses




           C20H42                                       C100H202
Nominal:            (20 x 12) + (42 x1) = 282 u                      (100x12) + (202x1) = 1402u
Monoisotopic:       (20 x12) + (42 x 1.007825) = 282.33 (100x12) + (202x1.007825) = 1403.5807
Average:            (20 x 12.011) + (42 x 1.00794) = 282.5535        (100x12.011)+(202x1.00794) =
1404.7039
                 Exact Masses of Some Common Elements and Their Isotopes:
       Element          Symbol          Exact Mass (u)                    Rel. Abundance %
Hydrogen         1H               1.007825037                   100.0
Deuterium        2H or D          2.014101787                   0.015
Carbon 12        12C              12.00000                      100.0
Carbon 13        13C              13.003354                     1.11223
Nitrogen 14      14N              14.003074                     100.0
Nitrogen 15      15N              15.00011                      0.36734
Oxygen 16        16O              15.99491464                   100.0
Oxygen 17        17O              16.9991306                    0.03809
Oxygen 18        18O              17.99915939                   0.20048
Fluorine         19F              18.998405                     100.0
Sodium           23Na             22.9897697                    100.0
Silicon 28       28Si             27.9769284                    92.23
Silicon 29       29Si             28.9764964                    5.0634
Silicon 30       30Si             29.9737717                    3.3612
Phosphorus       31P              30.9737634                    100.0
Sulfur 32        32S              31.972074                     100.0
Sulfur 33        33S              32.9707                       0.78931
Sulfur 34        34S              33.96938                      4.43065
Sulfur 36        36S              35.96676                      0.02105
Chlorine 35      35Cl             34.968854                     100.0
Chlorine 37      37Cl             36.965896                     31.97836
    (a) only one chlorine atom   (b) only one bromine atom




              3:1                  1:1




35Cl: 75.77
37Cl: 24.23


79Br: 50.69
81Br: 49.31
                    3:4:1



          c) one chlorine and one bromine atom
Types of Atomic mass spectrometers
  Atomic mass spectrometer
                             AMS
MMS
YO, Y(I), Y(II) EMISSION ZONES
COURTESY VARIAN




     Dr. Houk Presentation, 2002
Sampler

                                            Skimmer




   Photo by A. L. Gray
              Dr. Houk Presentation, 2002
AGILENT 7500   OMEGA LENS
                 Ionization Source
                   8,000 to 10,000 oC




Plasma Torches
  Mass Analyzer (Quadrupole)


                                            Two pairs of rods:
                                                Attach + and - sides of a
                                                variable dc source
                                                Apply variable radio-
                                                frequency ac potentials to
                                                each pair of rods.

                                            Ions are accelerated into
                                            the space between the rods by
                                            a small potential (5-10V)

                                            Ions having a limited range
                                            of m/z value reach the
                                            transducer.
Skoog et al., 1999, Instrumental Analysis
Ion trajectories in a Quadrupole
   A pair of positive rods (as lying in the xz plane).
    In the absence of a dc potential:
   Positive half of the ac cycle: Converge (ion in the channel will
    tend to converge in the center of the channel during the
    positive half of the ac cycle).
   Negative half of the ac cycle: Diverge (ions will tend to
    diverge during the negative half).

                                Whether or not a positive ion strikes
                                the rod will depend upon the rate of
                                movement of ion along the z axis, its
                                m/z, and the frequency and magnitude
                                of the ac signal.




                                 Skoog et al., 1999, Instrumental Analysis
   A pair of positive rods (Cont’d)

    With dc potential:
    Heavier ions: less affected by ac (largely by dc).
    Lighter ions: deflected during negative cycle of ac.



    The pair of positive rods: a high-pass mass filter for
    positive ions traveling in the xz plane.
   The pair of negative rods
In the absence of the ac potential:
   All positive ions will tend to strike the rods.

With ac potential:
  For the lighter ions, however, this movement may be
  offset by the positive half cycle of ac potential.


    Thus, the pair of negative rods operates as a low-pass
    mass filter.

    The mass that can be analyzed can be varied by
    adjusting the ac and dc potential.
How does it work?




          -U - Vcoswt
                        .   m/z=900
                        . m/z=990
                        .
          U + Vcoswt    .
                        .
                            m/z=1000
                            m/z=1010
                            m/z=1100
Typical mass spectrum
                    How good this is?




Linear calibrations over 4 orders of magnitude   Multi-elemental analysis of a standard
ICP-MS: a handy tool!
      Spectral Interferences?
   Isobaric overlap
    Due to two elements that have isotopes having
    substantially the same mass
             40Ar+   and 40Ca+
   Polyatomic
    Due to interactions between species in the plasma
    and species in matrix or atmosphere
             56Fe and 40Ar16O
             44Ca and 12C16O16O.
Isobaric interferences?
      Spectral Interferences?
   Refractory oxide
    As a result of incomplete dissociation of the sample
    matrix or from recombination in the plasma tail
             MO+, MO2+, MO3+

   Doubly charged ions
           Matrix Effects?
   Space charge effects
Advanced Analytical Chemistry – CHM 6157   ® Y. CAI    Florida International University
Updated on 9/13/2006                       Chapter 3   ICPMS



Isotope Dilution
            Isotope dilution is a super internal standard
             addition method on the basis of isotope
             ratios.
            Add a known amount (spike) of a stable
             enriched isotope of the element considered,
             which has at least two stable isotopes 1 and
             2, to the sample
            Measure the isotope ratio of isotopes 1 and
             2 in the Spike, the unspiked sample and
             finally the spiked sample.
            The concentration of the element of interest
             can then be deducted from these isotopic
             ratios and from the amount of spike added.
Advanced Analytical Chemistry – CHM 6157   ® Y. CAI    Florida International University
Updated on 9/13/2006                       Chapter 3   ICPMS



       Advantages:

            Simplified chemical and physical separation
             procedures

            Elimination (reduction) of matrix effects

            Elimination of the effect of instrumental drift
Advanced Analytical Chemistry – CHM 6157     ® Y. CAI    Florida International University
Updated on 9/13/2006                         Chapter 3   ICPMS



       Theory
        In principle, any element with at least two
        isotopes that can be measured is suitable for
        determination by isotope dilution. The two
        selected are designed 1 and 2.


        Three solutions will be used:

        Sample (s)                    Standard (t)       Spiked
        sample (m)
Advanced Analytical Chemistry – CHM 6157   ® Y. CAI    Florida International University
Updated on 9/13/2006                       Chapter 3   ICPMS


       1n  is the number of moles of isotope 1 in the
             s
        sample.
       2n is the number of moles of isotope 2 in the
          s
        sample.
       1n is the number of moles of isotope 1 in the
          t
        standard.
       2n is the number of moles of isotope 2 in the
          t
        standard.
       Rs is the ratio of isotope 1 to isotope 2 in the
        sample solution.
       Rt is the ratio of isotope 1 to isotope 2 in the
        standard.
       Rm is the ratio of isotope 1 to isotope 2 in the
        spiked sample.
Advanced Analytical Chemistry – CHM 6157   ® Y. CAI    Florida International University
Updated on 9/13/2006                       Chapter 3   ICPMS



        Assuming the molecular sensitivity 1S/2S
        of the MS for isotope 1 and 2 are the
        same, then

For the sample solution:
   Rs = 1ns/2ns
      [1]
For the standard solution:
   Rt = 1nt/2nt
      [2]
Advanced Analytical Chemistry – CHM 6157   ® Y. CAI    Florida International University
Updated on 9/13/2006                       Chapter 3   ICPMS


For the spiked sample solution:
    Rm = (1ns + 1nt)/(2ns + 2nt)               [3]
Substitution of equations 1 and 2 into equation 3:
    Rm = (Rs2ns+ Rt2nt)/(2ns + 2nt)
       [4]
Rearranged to:
    2n = 2n (R -R )/(R -R )                    [5]
       s    t   m t     s m


        Convert the number of moles of isotope 2 in the
        sample to the total number of moles of the
        elements in the sample.

     ns =(2nt/θ2)(Rm-Rt)/(Rs-Rm)                [6]
θ2 is the isotopic abundance of isotope 2 in the
     sample.
Advanced Analytical Chemistry – CHM 6157   ® Y. CAI    Florida International University
Updated on 9/13/2006                       Chapter 3   ICPMS



        The mass of the element in the sample is
        then given by:

        Ms = M(2nt/θ2)(Rm-Rt)/(Rs-Rm)
         [7]

        M is the molecular weight of the element.

				
DOCUMENT INFO
Shared By:
Categories:
Stats:
views:15
posted:10/11/2010
language:English
pages:40