4. Flame AAS

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