Adv. Inst. Techs

 flame emission (eg flame photometer) known as low temperature emission ( K) ◦ first form of spectroscopy ◦ used in commercial analytical instrument in 1930s  atomic absorption important from 1960s-90s  this chapter will cover high temperature ( K) emission methods ◦ have been around for more than 50 years ◦ two sources of energy:  electricity (arc or spark)  plasma  both remain important

 What advantages would high temperature sources have over low temperature ones?  higher intensity => better sensitivity  better atomisation => less matrix interference  more elements (some non-metals)

 for high T sources, the number of atomic lines requires v. high resolution (beyond a slit monochromator)  for scanning instruments, the detector moves on a circular path (like XRF) Wavelength Separation Source (incl. sample) Collimator Detector

ExcitationEmission Desolvation Evaporation Atomisation Ionisation MX(aq) MX(s) MX(g) M(g) M + (g)

 sequential – one emission line can be measured at any one time; only one detector moving on a circular path to pick up the dispersed lines via a grating (no exit slit)  simultaneous – where multiple detectors are available, either as single units (eg PMTs) or a bank of semiconductor devices (eg diode array); ◦ polychromator – multiple exit slits at fixed positions for different emission wavelengths; scanning is not possible

Simultaneous multiple fixed position detectors

 older instruments could not scan nor vary the set wavelengths  new instruments have a array detector with 100,000+ pixels like a TV (a diode array on steroids)  allows instant full spectrum – no moving parts

 arc/spark instruments have dominated metals analysis  multi-element, solid sampling  as long as the matrix can be matched by standards  otherwise AAS now ICP  use high power electricity between electrodes, one being the sample  sample is vapourised, so this isn’t non-destructive  arc & spark differ in the means of applying the electrical energy

 a plasma is a contained vapour of ions and electrons  argon forms the most stable plasma  contained within a torch:three concentric quartz tubes, each carrying a stream of argon  copper coils surround the end of the torch  RF energy applied which creates the conditions to excite and contain the plasma  maximum temp. 10,000K

 the extreme temperatures produce almost perfect atomisation  mixture of ions and atoms giving more lines per element  the linear detection range is very great; typically 10,000 times  more elements emit, eg P and S  lower detection limits, typically 50 ug/L

 have to be less efficient than AAS to avoid the plasma becoming unstable due to too much sample  pneumatic – where gas pressure directly or indirectly breaks up the liquid, eg concentric, Babington, V-groove  ultrasonic – oscillations from an ultrasonic generator break up the sample into the aerosol, which is then carried by the gas; good for slurries

 three concentric quartz tubes, with the copper RF coils at the top  plasma flow – the outer tube provides the bulk of the argon for the plasma, but also achieves cooling for the quartz  auxiliary flow –buffer between the inner and outer flows, helps in plasma stability  inner flow – from the nebuliser plasma flow auxiliary flow inner flow RF coils

 radial was first  axial required better cooling  axial is more sensitive due to greater pathlength  has more “interference” by other elements for same reason (a) (b) radial axial

 not completely free of sample variations, especially flow rates & uptake through the nebuliser  samples in organic solvents or with significant organic content (eg wine) will cause the plasma to become unstable due to extra uptake compared to aqueous  require an increase in RF power, more argon flow and lower nebuliser pump rate to achieve stability  critical to use an internal standard in such situations

 typically 10 times better sensitivity  the ability to measure non-metals such as P, S, N and the halogens  low levels of chemical interferences, such as ionisation and non-atomisation  multi-component analysis (though a new design of flame AAS by Varian allows up to six wavelengths to be measured in rapid succession without resetting the instrument)  linear ranges of 1000 times rather than 10 times

ICP disadvantages:  they still cost at least four times more, but the price difference is coming down all the time  spectral interferences are greater, because there so many peaks, some are bound to overlap  relative precision is higher  very high argon use, which makes the running costs significantly higher  fast sequential AAS with multi-element lamps counter the multi-component advantage of ICP  ICP-MS will probably replace electrothermal AAS in next decade