Atomic Mass Spectrometry Yongsik Lee 2004

Introduction ► Atomic mass spectrometry  Versatile and widely used tool  All elements can be determined ► Advantage over AOS  Detection limits are 1000 times better  Simple spectra  Measure atomic isotope ratios ► Disadvantage  Cost – 100 or 1000 times expensive  Instrumental drift – 5% per hour  Interference effects

11A General Features ► Atomization = same as AOS ► Conversion to ions = same as AOS ► Separation of ions by mass-to-charge ratio ► Counting the number of ions of each type

Atomic weights in MS ► Atomic mass unit (amu)  = dalton (Da)  Relative scale where mass of C12 is 12 amu  Carbon-12 scale  1 amu = (1/12)(12g / Navo)  = x g/atom of C12 ► Exact mass  CH4, CH3D, CH2D2, CHD3, CD4 ► Nominal mass  A whole-number precision

Chemical atomic weight ► Average atomic weight  Fractional abundance x isotope mass ► m/z  Atomic or molecular mass / charges of ions  If Z=+1, m/z = mass

Atomic mass of Zr ► (51.5 x 90) + (11.2 x 91) + (17.1 x 92) + (17.4 x 94) + (2.8 x 96) = ► The average mass of these 100 atoms would be / 100 = 91.3 (to 3 significant figures). ► 91.3 is the relative atomic mass of zirconium.

MS of chlorine molecule ► Fragmentation  When chlorine is passed into the ionisation chamber, an electron is knocked off the molecule to give a molecular ion, Cl2+.  These ions won't be particularly stable, and some will fall apart to give a chlorine atom and a Cl+ ion. ► If the Cl atom formed isn't then ionised in the ionisation chamber, it simply gets lost in the machine - neither accelerated nor deflected. ► The Cl+ ions will pass through the machine and will give lines at 35 and 37, depending on the isotope. ► You will also record lines for the unfragmented Cl2+ ions. ► Both atoms could be 35Cl, both atoms could be 37Cl, or you could have one of each sort. That would give you total masses of the Cl2+ ion of:  = 70  = 72  = 74 ► That means that you would get a set of lines in the m/z = 70 region

MS of Chlorine

Types of atomic MS ► Hyphenated methods  ICP-MS  DCP-MS  MIP-MS ► SSMS (spark source) ► TIMS (thermal ionization) ► GDMS (glow discharge) ► LMMS (laser microprobe) ► SIMS (secondary ion)

11B Mass spectrometers

Ion deflection ► Atoms can be deflected by magnetic fields - provided the atom is first turned into an ion. ► Electrically charged particles are affected by a magnetic field although electrically neutral ones aren't. ► Stage 1: Ionisation  The atom is ionised by knocking one or more electrons off to give a positive ion. This is true even for things which you would normally expect to form negative ions (chlorine, for example) or never form ions at all (argon, for example). Mass spectrometers always work with positive ions. ► Stage 2: Acceleration  The ions are accelerated so that they all have the same kinetic energy. ► Stage 3: Deflection  The ions are then deflected by a magnetic field according to their masses. The lighter they are, the more they are deflected.The amount of deflection also depends on the number of positive charges on the ion - in other words, on how many electrons were knocked off in the first stage. The more the ion is charged, the more it gets deflected. ► Stage 4: Detection  The beam of ions passing through the machine is detected electrically.

Basic Principle - deflection ► subject something to a sideways force, it will move in a curve  deflected out of its original path ► The amount of deflection for a given sideways force  depends on the mass of the ball. ► If you knew  the speed of the ball  the size of the force, ► Then you could calculate the mass of the ball  The less the deflection, the heavier the ball.

Mass analyzer ► ► separate ions to measure m/z and intensity ► resolution

Electron multipliers ► General characteristics  Rugged and reliable  High current gains  Nanosecond response time  Requires enough Kinetic energy ions ► Acceleration required for quadrupoles ► types  Discrete dynode emp  Continuous dynode emp

Electron multipliers ► Discrete dynode electron multiplier (figure 11-2 a)  Similar to PMT  Cu/Be surface ► Burst of electrons ► when struck by energetic ions or electrons  20 dynodes = gain of 10 million ► Continuous dynode electron multiplier (figure 11-2 b)  Trumpet shaped  Made of glass of highly Pb doped  Electrical potential across the length of transducer  Gain of 100 thousand – 100 million

Multi Cannel Plate (MCP)

Faraday Cup ► principle  Ions strike the collector electrode  Charge of the positive ions is neutralized by a flow of electrons  Electron flow cause a potential drop over a resistor  High impedance amplifier ► advantages  Independent of the energy, mass, chemical nature of the ion  Inexpensive and simple ► disadvantages  Low response speed  Less sensitive than EMP

Photographic plate ► Photographic plate  Silver bromide emulsion are sensitive to energetic ions  Frequently used for spark source instrument  Simultaneous observation of wide m/z possible

Magnetic sector analyzer

Analyzing ions

► ► For fixed radius and charge can   use permanent magnet, vary A and B potential (V)   or variable electromagnet, fixed A and B potential (V)

Double focusing analyzer ► ► Single-focusing magnetic sector analyzers have Rmax < 2000   translational energy aberrations   angular aberrations   Addition of electrostatic analyzer simultaneously minimizes both ► ► Electrostatic analyzer focuses ions of unique m/z at entrance slit to magnetic sector

Quadrupole analyzer ► ► ions travel parallel to four rods   opposite pairs of rods have rapidly alternating potentials (AC)   ions try to follow alternating field in helical trajectories   stable path only for one m/z value for each field frequency ► ► Harder to push heavy molecule - m/zmax < 2000 ► ► Rmax ~ 500

Time of flight analyzer ► ► Generate pulse of ions (by laser, electrons) with same initial energy ► ► Ions travel down field-free tube separate according to mass   light ions arrive first, heavy ions arrive later   Unlimited mass range m/zmax > 100 kDa ► ► Poor resolution Rmax < 1000 ► ► Poor sensitivity

TOF

Summary ► ► One of most powerful analytical tools:   sensitive (10-6 to <10-13 g)   range of ion sources for different situations   elemental composition for small and large MW- biomolecules   limited structural information   qualitative and quantitative analysis of mixtures   composition of solid surfaces   isotopic information in compounds ► ► But   complex instrumentation   expensive ► ► high resolution   structure obtained indirectly   complex spectra/fragmentation for hard ionization sources   simple spectra for soft ionization sources

ICPMS

Spark source MS