HGAAS, CVAAS and interferences Flashcards
Name 3 deficiencies in FAAS
nebulisation efficiency, residence time, sample size
Give 2 reasons for the inefficiency of conventional nebuliser-burner
is less than 10%, most sample never reaches flame (droplets too big to desolvate)
What is the issue with residence time?
particles only in flame for a few hundredths of a second
what is the issue with sample size?
continuous flow of sample required for a steady signal - difficult to use less than 1ml
another deficiency in Flame atomisation is form of sample, why?
only vicscous liquids or solution
another deficiency in Flame atomisation is flame background absorption, why? 2
No wavelength below 90nm can be observed, some important elements cant be analysed
another deficiency in Flame atomisation is flame background emission, why?
gives rise to increased noise levels at various wavelengths
another deficiency in Flame atomisation is flame temp, why?
hot enough to atomise most elements, but not hot enough to produce emission from many elements
what does HGAAS stand for?
hydride generation atomic absorption spectrophotometry
Elements in which groups form volatile hydrides?
IV, V, VI
what is the burner assembly in standard AA replaced with?
heated silica tube
What temp is the tube? is this sufficient for atomisation?
900oC, yes
how is metal hydride generated?
by acidifying sample with solution of HCl and then adding NaBH4
When can hydrides be separated from the sample matrix?
before being introduced to the light path
the hydride is then decomposed and atomised with what?
negligable background interference
What does this have considerable advantages for?
analysing volatile elements which escape from atom cell before a measurement can be made in techniques such as GFAAS
Why is the window made of quartz?2
transparent in UV VIS wavelengths with high thermal stability
What are 2 advantages of HGAAS over FAAS?
cheap and simple modification, detection limits for suitable elements are comparable to GFAAS
Resonance lines of elements such as As (193.7nm) and Sb (217.6nm) are in a region where flame gas absorption is strong. what does this mean?
no flame in HGAAS so no flame gas absorption
A disadvantage would be the efficiency of hydride generation depends on what?
oxidation state of elements `
A disadvantage would be the presence of what in sample matrix?
easily reducible ions
Give 3 examples, what would they suppress?
Co, Cu, Ni, can suppress hydride formation
A disadvantage would be that wet chemistry sample prep is more complex than what?
an automated furnace measurement
A disadvantage would be that it lacks generality - why is this?
only a limited number of elements form hydrides
what does CVAAS stand for?
Cold vapour atomic absorption spectrometry
what element is it used to detect?
mercury
The main absorption wavelength of Hg is at 184.9nm, meaning what?
less sensitive lines have to be used if using flame methods
Why is this?
it is too close to maximum absorption wavelength of flame gases
Hg is the only element with high vapour pressure of what?
monoatomic atoms at room temp
The cold vapour Hg cell is simialir to what technique?
HGAAS
What temp is the silica tube in CVAAS?
room temp
Why might the cell be open ended?
to prevent window absorption at most sensitive resonance line (184.9nm)
Hg vapour is generated by reducing inorganic Hg compounds with what?
NaBH4 or SnCl2
when are organic Hg compounds oxidised to inorganic Hg?
before analysis
Elemental Hg is flushed from solution - how is the vapour flushed from solution?
bubbling Ar through the solution
Light is then transmitted along what?
axis of tube from a hollow cathode mercury lamp
What are 2 advantages of CVAAS over FAAS?
simple and cheap modification, detection limits are 100x more sensitive to flame.
Why are the detection limits so much higher in CVAAS than FAAS? 2
use of most sensitive line (184.9nm v 253.7nm) and longer residence time of atoms in beam
What are 2 disadvantages of CVAAS over FAAS?
interferences in the reduction from very reducible elements, background correction may be requied
why is atom cell heated in HGAAS but not CVAAS?
no heating is applied to obtain gaseous mercury atoms because it is already in elemental form with high vapour pressure after reduction reaction
what are the 4 types of interferences?
spectral, chemical, ionisation, physical
what would a spectral interference on line spectra caused by atomic absorption look like?
interference between 2 overlapping absorption lines - occurs only occastionally
how about atomic emission line spectra?
Interference from adjacent line of another element in matrix - depends on resolution from monochromator
Why are interferences more common in AE then AA?
due to number of emission lines observed
How can AE spectral interferences be avoided?
by careful choice of wavelength considering other possible matrix components
What is broad band spectra AA due to?
molecules or particles in flame leading to higher absorbance than expected
if broad band spectra interference is due to combustion, what can be done?
instrument can be zeroed using a blank
what 3 things can be done if broad band spectra interference is due to sample matrix?
use background correction, use higher temp flame, use radiation buffer
How can emission lines which are superimposed on bands emitted by oxides and other molecular species be corrected?
correct by scanning background either side of emission line and subtracting mean of the 2
What is a chemical interference in AA and AE?
presence of compounds of low volatility which are not readily atomised
what does this chemical interference lead to?
leads to low results
How can chemical interferences be avoided? 3
higher temp flame, releasing agents, protective agents
what is a releasing agent
cations which preferentially react with interfering species
what is a protective agent?
compounds which form stable volatile species with analyte
In ionisation interferences, what happens when a hotter flame causes ionisation?
samples and standards ionised to same extent, leads to loss in sensitivity
If analyte contained an easily ionisable element, what would happen in the event of ionisation interference?
it would also be ionised
There would be increased energy in the flame, which would suppress what?
ionisation of analyte leading to high results
How can this be avoided?
by use of ionisation buffer - large excess of easily ionisable element is added to both samples and standards
Name 3 types of physical interference?
density, surface tension, viscocity
What do these factors affect? 2
rates of aspiration and nebulisation
What is another physical interference?
incomplete volatilisation
in incomplete volatilisation, matrix components in flame may form what? what do they do?
stable compounds which physically trap analyte
how can incomplete volitilisation be avoided? 2
dilution and higher t flame
In self absorption interferences, the centre of flame is hotter than exterior. what does this mean for atoms?
excited atoms emitting in centra are surrounded by cool region of unexcited atoms
In self absorption interferences, an emitted photon travelling towards detector may be absorbed by what?
unexcited atom at edge of flame
In self absorption interferences, what factor increases the chances of this occuring?
higher conc
how can self absorption interferences be avoided?
using a non-resonance line for emission measurements
