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 instrumentationAtomic spectrometry systems containing free atoms and ions of the element of interest. Figure 1-3 shows theinstrumental arrangements for four different techniques used to detect these atomsor ions.In atomic absorption spectrometry (AAS), light of a wavelength characteristic of theelement of interest is shone through this atomic vapor. Some of this light is thenabsorbed by the atoms of that element. The amount of light that is absorbed bythese atoms is then measured and used to determine the concentration of thatelement in the sample.In optical emission spectrometry (OES), the sample is subjected to temperatureshigh enough to cause not only dissociation into atoms but to cause significantamounts of collisional excitation (and ionization) of the sample atoms to take place.Once the atoms or ions are in their excited states, they can decay to lower statesthrough thermal or radiative (emission) energy transitions. In OES, the intensity ofthe light emitted at specific wavelengths is measured and used to determine theconcentrations of the elements of interest.One of the most important advantages of OES results from the excitation proper-ties of the high temperature sources used in OES. These thermal excitation sourcescan populate a large number of different energy levels for several different elementsat the same time. All of the excited atoms and ions can then emit their characteristicradiation at nearly the same time. This results in the flexibility to choose from severaldifferent emission wavelengths for an element and in the ability to measure emissionfrom several different elements concurrently. However, a disadvantage associatedwith this feature is that as the number of emission wavelengths increases, theFigure 1-3. Atomic spectrometry systems.An Overview of Elemental Analysis via Atomic Spectroscopy 1-5ICP-OES Instrument 3 ICP-OES INSTRUMENTATIONIn inductively coupled plasma-optical emission spectrometry, the sample is usuallytransported into the instrument as a stream of liquid sample. Inside the instrument,the liquid is converted into an aerosol through a process known as nebulization. Thesample aerosol is then transported to the plasma where it is desolvated, vaporized,atomized, and excited and/or ionized by the plasma (see Chapter 2). The excitedatoms and ions emit their characteristic radiation which is collected by a device thatsorts the radiation by wavelength. The radiation is detected and turned into elec-tronic signals that are converted into concentration information for the analyst. Arepresentation of the layout of a typical ICP-OES instrument is shown in Figure 3-1.Figure 3-1. Major components and layout of a typical ICP-OES instrument.ICP-Quadrupole Mass AnalyzerThe$four$main$components$of$a$Atomic$mass$spectrometer$Ionization Source / Ion Accelerator Inlet Mass Analyzer Detector All ions Selected ionsLets talk about mass! • Atomic mass of Carbon • Atomic mass of Chlorine • Atomic mass of HydrogenLets 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 (atomic mass units) = 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 • “Da” or Dalton. • kD (kiloDalton for macromolecules) • 1 amu = 1 Da = 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 (find it in periodic table). • 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 MSAverage mass!!!element nominal exact Percent average ! mass mass abundance mass ! C 12 12.00000 98.9%! 13 13.00335 1.1% 12.00115!H 1 1.007825 99.98%! 2 2.0140 0.02% 1.00794 Mass (Weights) of Atoms and MoleculesMass or Molecular Weight of molecules C20H42 Average: (20 x 12.011) + (42 x 1.00794) = 282.5535 Nominal: (20 x 12) + (42 x1) = 282 u Monoisotopic: (20 x12) + (42 x 1.007825) = 282.33 C100H202 Average: (100x12.011)+(202x1.00794) = 1404.7039 Nominal: (100x12) + (202x1) = 1402u Monoisotopic: (100x12) + (202x1.007825) = 1403.5807Exact 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 31.97836Mass%to%Charge!Ra.o!• Unitless ratio of its mass number to the number of fundamental charge • m/z • For 12C1H4+, m/z =16.0313/1 = 16.0313. •