instrumental chap 11

atomic mass spectroscopy

quantitative and qualitative detection of elements in parts per million and parts per billion concentrations

detection limits higher than optical methods, simple spectra, measures atomic isotopic ratios

advantages

expensive, instrumental drift, interference

disadvantages

atomization, formation of ions, separation of ions based on m/z, counting number of ions

processes involved

mass spectrometer

an instrument used to determine the relative masses of atoms by the deflection of their ions on a magnetic field

electron multipliers

ions hit metal surface or semiconductor and breaks off to make secondary electrons. secondary electrons also fragment and make more secondary electrons. 1 electron can make 1x10^6 electrons, destructive detection. good for ion trap and quadrupole

faraday cup

Relatively simple MS detector; uses an aligned or tilted collector electrode that is connected to ground through a resistor; voltage drop is amplified using high impedance amplifier; main disadvantage is the slow response of the detector due to the high impedance amplifier.

electron multipliers, faraday cup

transducers for mass spectrometry

quadrupole mass analyzer

A set of 4 conducting rods, each with an oscillating voltage.

This establishes an oscillating electric field between the rods.

Frequency of voltage oscillations is varied, to select for ion with

an m/Z that can successfully pass between the rods.

separates ions of different mass based on m/z through 4 rods

quadrupole mass analyzer

Time of flight mass analyzer

Equal kinetic energy ions enter the tube. Drift velocity and thus arrival time at the detector depend on mass

ions are accelerated into field free drift tube, lighter ions arrive first

time of flight

double focusing mass analyzer

Electrostatic focusing followed by magnetic field detection. Trajectories depend on m/z values.

ions accelerated into electrostatic field and magnetic, lightest deflected and dispersed on plane

double focusing

more compact, less expensive, more rugged, m/z needs to be less than 2000

quadrupole traits

unlimited mass range, simple rugged

TOF advantages

limited resolution, limited sensitivity, requires fast eletronics

TOF disadvantages

ICP-MS

a form of mass spectrometry used to detect inorganic materials and metals. the best and most modern technique for metal analysis

nebulizer creates microdroplets, argon carries to plasma torch, atomization takes place, taken to quadrupole

ICPMS

atomic is complex, mass is simple, clear

atomic emission spectra vs mass spectra

isobaric ions, polyatomic or adduct ions, doubly charged ions, refractory oxide ions

ICPMS spectroscopic interferences

high sensitivity, selectivity, determines most elements in periodic table

why is ICPMS widely used