1 / 18

4. Flame AAS

4. Flame AAS. Hollow cathode lamp. Mono-chromator. Detector & readout. Sample aspirator. Revision 1. Draw a block diagram showing the components of a typical flame atomic absorption spectrophotometer. MX (aq) solution. MX (aq) mist. MX (s). MX (g). M (g). atomisation. nebulisation.

amma
Télécharger la présentation

4. Flame AAS

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  1. 4. Flame AAS

  2. Hollow cathode lamp Mono-chromator Detector & readout Sample aspirator Revision 1 • Draw a block diagram showing the components of a typical flame atomic absorption spectrophotometer.

  3. MX (aq) solution MX (aq) mist MX (s) MX (g) M (g) atomisation nebulisation solvent loss evaporation Revision 2 • Describe the process by which a solution of an metal salt (MX) in a flask is converted into an atomic vapour in a flame-based atomic spectrophotometer.

  4. Revision 3 & 4 • What gases are generally used in flame AAS? • fuel: natural gas or acetylene • oxidant: air or nitrous oxide • What do the terms reducing and oxidising mean in the context of AA flames? • reducing: excess fuel, cool • oxidising: excess oxidant, hot

  5. Revision 5 • What sort of flame would be used for analysis of (i) sodium, (ii) iron and (iii) calcium? • sodium – coolest, easy to atomise, NG/air • iron – medium, acet/air • calcium – hard to atomise, acet/N2O

  6. Exercise 4.1 (a) 2 aspects of flame AAS (not related to the instrument components) in common with UV/VIS • wavelength range is UV/VIS • still electrons buzzing around between states (b) 2 aspects of flame AAS (not related to the instrument components) different to UV/VIS • the very different sample treatment required to generate the atomic vapour • line spectra

  7. Forms of atomic absorption spectro. • flame • cold vapour • electrothermal

  8. Radiation source • monochromator is needed to block out radiation of wavelengths not absorbed by the analyte • atomic absorption lines have bandwidth of 0.005 nm • normal monochromator systems do no better than 0.1 nm • normal UV/VIS radiation sources cannot be used • bandwidth of an atomic emission line is 0.001 nm • this would be the ideal source for atomic absorption • basis for hollow cathode lamp

  9. Ne or Ar gas fill anode radiation hollow cathode with lining of element Hollow cathode lamp • a high voltage causes ionisation of the inert gas • ions dislodge some of the metal atoms from cathode • produce cloud of excited atoms which emit the characteristic lines of the element

  10. Nebuliser & burner

  11. Burner • burner is a long thin slot because • pathlength is increased, therefore increasing sensitivity • if the analyte emits significantly at the analysis wavelength (self-emission) Exercise 4.2 (a) What effect on absorbance would self-emission have? • increase radiation going to detector => lower Abs. (b) Why does a thin burner reduce this problem? • fewer atoms “pointing” at the detector (c) Name an element likely to be particularly prone to self-emission. • easily excited ones, e.g. Na, K

  12. Monochromators • still needed even though HCL generates specific wavelength for analyte • standard types Exercise 4.3 • What would produce the other wavelengths of radiation that the monochromator needs to remove? • the element produces multiple wavelengths for the same element • the flame • matrix emission

  13. Experimental aspects (from manual) • lamp current – there is an optimum current • below: lamp emission is too low • above: the ionised gas has too much energy and will ionise the lamp element giving the spectrum of metal ion, not the neutral atom • wavelengths and working ranges • most lamps produce a number of wavelengths of radiation at varying intensities • allow measurement of different concentration solutions for a given element • interferences • vary from one metal to another • generally well-documented • provides means of preventing them

  14. Interferences

  15. MX (aq) solution MX (aq) mist MX (s) MX (g) M (g) atomisation nebulisation solvent loss evaporation Exercise 4.4 What stage of the atomisation process do the interferences occur? • Non-vaporisation • Formation of stable oxides • Ionisation • Physical interferences

  16. Instrument performance • sensitivity usually refers to how low a concentration the technique can measure • in AAS, it has a specific numerical value • the concentration giving an absorbance of 0.0044 (99%T, 1% absorbance) • sensitivity value for a given element and wavelength • quoted in the manufacturer’s literature • can be used to check how well the instrument is performing at any given time

  17. Using the sensitivity value • most useful is to calculate expected absorbance from a given std • Aexp = 0.0044 c ÷ S Exercise 4.5 • The manufacturer’s sensitivity value is 0.06 mg/L. What is the expected absorbance of a 10 mg/L solution? • Aexp = 0.0044 x 10 ÷ 0.06 = 0.73 • 4 mg/L Cu std => Abs 0.44. S is 0.04 mg/L. How well is the instrument working? • Aexp = 0.0044 x 4 ÷ 0.04 = 0.44 • working perfectly

  18. Fast Sequential FAAS • advent of ICP in the 1980s => the beginning of the end of flame AAS • superior in almost all ways, except purchase and running costs. • to increase the life of flame AAS, Varian developed a flame AAS with: • the capacity to be set up beforehand for multiple lamps, • a monochromator drive capable of very rapidly jumping between wavelengths • an optical system capable of switching between the installed lamps • parallel development of multi-element HC lamps • fast sequential FAAS becomes a multi-component instrument

More Related