GFAAS and system optimisation Flashcards
What does GFAAS stand for?
graphite furnace atomic absorption spectrometry
What is the flame replaced with?
graphite furnace
Is it safer than FAAS?
yes
Does it require a nebuliser?
no
Can it be used to analyse solids
yes
Electrical heating is used for what?
volatilisation and atomisation of sample
In the first step of GFAAS procedure, sample is injected by micropipettee via what?
injection hole at top of tube
in second step of GFAAS procedure, why is tube heated rapidly by electric current?
to volatilise and atomise sample
the graphite material resists the passage of what?
electric current and converts electrical energy into heat
In step 3 of GFAAS procedure, an inert gas flow of Ar is used for what? 3
to flush tube and prevent formation of refractory oxides and prevent tube incineration
In the final step of the GFAAS procedure, atoms take several seconds to diffuse down tube giving rise to what?
transient absorption signal
how much better are detection limits in GFAAS than FAAS?
100-1000x
what are the detection limits of flame and furnace?
flame: 10-1000ppb, furnace: 0.01-10ppb
How long are atoms present in the atom cell?
a relatively long time
how can elements with resonance lines near 200nm be measured?
Because of absence of flame gases
Because of this, what may be more severe?
background absorption of light by other species
Can GFAAS detect more elements than FAAS?
no, fewer
GFAAS also has high background absorption, what is this caused by?
long residence time of atoms in furnace
What kind of samples in particular cause more background absorption in GFAAS?
biological and geological
How can the chances of scattering of light beam by smoke occuring be reduced?
temperature programming
The first step of temp programming is drying - what does this do and what temp for how long?
100oC for 30 seconds, removes solvent
what might a too high temp cause in drying stage?
sputtering of the sample
second step of temp programming is ashing. what does this do? and what temp?
350-550oC pyrolyses organic matter
What might happen if ashing temp is too high?
loss of analyte
in the ashing stage, organic molecules break down to small volatile moleules - what happens to them?
flushed from the furnace
third step of temp programming is atomisation, what temp and what occurs?
1500-2800oC - gives transient absorption peak
How can the minimum temp required for the sample be found out?
by working down from a high temperature until absorption peak begins to decrease
Step 4 of temp programming is tube clean - What occurs?
prevent memory effects
What stage of temp programming might a loss of analyte occur?
ashing stage
Halide salts absorption increases sharply below 220 nm, what elements does this cause a problem for?
ones that absorb below 220nm, lead arsenic and cadmium
how can the sample be stabilised?
use of matrix modifier
How can error in volume when manually pipetting be improved?
using autosampler
What does an IR light sensor do?
used to monitor radiation from furnace walls and control electrical supply
Why can the signal not be integrated to average out noise?
as furnace AA signal is transient
How is the sample throughput a limitation in furnace atomisation?
temp program takes approx 2 mins, 10x longer than flame
What 2 factors make GFAAS more expensive than FAAS?
due to autosampler and method development
Temp gradients are another limitation in GFAAS. Why is this?
gas inside furnace heats up more slowly than furnace wall
What might this cause atoms to do?
form too rapidly on wall of furnace and migrate into relatively cool gas, condensation can occur
how can this be avoided
L’vov platform inside furnace
What does a L’vov platform consist of?
raised graphite platform on which sample is placed
What does L’vov platform ensure?
gas in tube and sample are at same temperature
How is the sample and platform then heated?
indirectly by radiation and convection from furnace walls
this delays atomisation until what?
gas is heated up sufficiently to prevent condensation
What is matrix modification used for?
to get better separation of analyte elements from concomitants
How does it achieve this?
makes the matrix more volatile and/or stabilises analyte element
What is added to sample in high concentration?
a reagent ie acid or salt
according to the law of mass action, what is analyte element transformed into?
well defined compound with known properties
What does this allow for?
easily reproducible conditions for thermal pretreatment for a variety of matrices
matrix modification has 5 advtantages - one being that is converts analyte into what?
well defined compounds
Another advantage is that it allows same well defined thermal pretreatment conditions to be used for a variety of samples - what does this mean?
dont need to confirm applicability each time
Another advantage is that is helps avoid multiple peaks during what?
atomisation caused by different compounds of analyte element
Another advantage is that it allows highest thermal pretreatment temps to be applied - what does this ensure?
best separation from accompanying materials, minimum interferences present
another advantage is that there is a small temp difference between pretreatment and atomisation - why is this important?
for reaching stabilised temp conditions
What happens when the sample contains large quantities of volatile organic material?
organics removed during thermal pretreatment, allow analysis without any sample prep
Chemical modification can be done directly into the furnace to reduce what?
spectral and/or non spectral interferences
What are the 2 types of modification that can be done in the furnace?
those that reduce volatility of analyte, those that increase volatility of matrix
If the element is determined to be volatile, such as chlorides/organo-metallic compounds, sample may be modified chemically using what?
adding ammonium hydroxide directly into furnace - renders it less volatile
Why do samples containing NaCl usually produce high background signals?
due to high temp required to volatilise >1000oC
What is added to the furnace to make NaCl more volatile to aid removal?
ammonium nitrate
what does the addition of ammonium nitrate form? and what temp are these chemicals volatilsed at?
forms ammonium chloride and sodium nitrate, all 3 chemicals volatilised at less than 500oC
What are 2 main background corrections for furnace measurements?
deuterium lamp and Zeeman background correction
What does Zeeman background correction use to split atomic energy levels and thus absorption lines?
intense magnetic field
What does this allow for?
background signal to be measured very close to atomic absorption line
What is the term for most atoms having several electronic configurations that have the same energy?
degenerate
Transitions between different pairs of configurations correspond to what?
single spectral line
how does the presence of magnetic field break degeneracy?
as it interacts in different ways with electrons with different quantum numbers and slightly modifies energies - causes splitting of levels
describe the commercial experimental arrangements?2
magnet around lamp, magnet around atom cell
What happens if the magnet is placed around atom cell?
absorption of sample that is split
what happens if magnet is placed around HCl?
emission spectrum of source that is split rather than absorption spectrum of sample
What does the splitting of a singlet transition lead to?
central π line and equally spaced σ lines
The π line has an absorbance of ……. times that of each σ line?
two
The σ lines are usually separated enough from π line meaning what?
little or no atomic absorption is observed
The zeeman affect is applied to atomic absorption based upon what?
differing response of types of absorption peaks to polarised radiation
The π peak absorbs only what radiation?
radiation that is plane-polarised in a direction parrallel to external magnetic field
The σ peak absorb only what radiation?
polarised at 90 degrees to external magnetic field
The ……. line ,π, is polarised ……… to the direction of magnetic field?
central, parallel
the …. lines ,σ, are polarised ……….. to magnetic field?
wing, perpendicular
What can be used to isolate either the centre or wing transitions?
polariser
What are 2 advantages of zeeman background correction?
only 1 light source meaning no alignment problems, background measured close to atomic line
what are disadvantages of zeeman background correction?
expensive, loss of sensitivity and linear range
The smith-Hieftje background correction is based on what?
self-reversal behaviour of radiation emitted from HCls when operated under high currents
The high currents cause a large concentration of non-excited atoms which are capable of what?
absorbing radiation emitted from excited species
what else does the high current cause?
a broadening of excited species emission line
The overall effect is a line with a minimum in its centra corresponding to what?
exact wavelength of absorption peak
The lamp is run at alternating high and low current, what is measured at high/low currents?
total absorbance measured at low current, background is measured at high current
The signals are then subtracted to give what?
corrected value
