GFAAS and system optimisation

Question 1

What does GFAAS stand for?

Answer 1

graphite furnace atomic absorption spectrometry

Question 2

What is the flame replaced with?

Answer 2

graphite furnace

Question 3

Is it safer than FAAS?

Answer 3

yes

Question 4

Does it require a nebuliser?

Answer 4

no

Question 5

Can it be used to analyse solids

Answer 5

yes

Question 6

Electrical heating is used for what?

Answer 6

volatilisation and atomisation of sample

Question 7

In the first step of GFAAS procedure, sample is injected by micropipettee via what?

Answer 7

injection hole at top of tube

Question 8

in second step of GFAAS procedure, why is tube heated rapidly by electric current?

Answer 8

to volatilise and atomise sample

Question 9

the graphite material resists the passage of what?

Answer 9

electric current and converts electrical energy into heat

Question 10

In step 3 of GFAAS procedure, an inert gas flow of Ar is used for what? 3

Answer 10

to flush tube and prevent formation of refractory oxides and prevent tube incineration

Question 11

In the final step of the GFAAS procedure, atoms take several seconds to diffuse down tube giving rise to what?

Answer 11

transient absorption signal

Question 12

how much better are detection limits in GFAAS than FAAS?

Answer 12

100-1000x

Question 13

what are the detection limits of flame and furnace?

Answer 13

flame: 10-1000ppb, furnace: 0.01-10ppb

Question 14

How long are atoms present in the atom cell?

Answer 14

a relatively long time

Question 15

how can elements with resonance lines near 200nm be measured?

Answer 15

Because of absence of flame gases

Question 16

Because of this, what may be more severe?

Answer 16

background absorption of light by other species

Question 17

Can GFAAS detect more elements than FAAS?

Answer 17

no, fewer

Question 18

GFAAS also has high background absorption, what is this caused by?

Answer 18

long residence time of atoms in furnace

Question 19

What kind of samples in particular cause more background absorption in GFAAS?

Answer 19

biological and geological

Question 20

How can the chances of scattering of light beam by smoke occuring be reduced?

Answer 20

temperature programming

Question 21

The first step of temp programming is drying - what does this do and what temp for how long?

Answer 21

100oC for 30 seconds, removes solvent

Question 22

what might a too high temp cause in drying stage?

Answer 22

sputtering of the sample

Question 23

second step of temp programming is ashing. what does this do? and what temp?

Answer 23

350-550oC pyrolyses organic matter

Question 24

What might happen if ashing temp is too high?

Answer 24

loss of analyte

Question 25

in the ashing stage, organic molecules break down to small volatile moleules - what happens to them?

Answer 25

flushed from the furnace

Question 26

third step of temp programming is atomisation, what temp and what occurs?

Answer 26

1500-2800oC - gives transient absorption peak

Question 27

How can the minimum temp required for the sample be found out?

Answer 27

by working down from a high temperature until absorption peak begins to decrease

Question 28

Step 4 of temp programming is tube clean - What occurs?

Answer 28

prevent memory effects

Question 29

What stage of temp programming might a loss of analyte occur?

Answer 29

ashing stage

Question 30

Halide salts absorption increases sharply below 220 nm, what elements does this cause a problem for?

Answer 30

ones that absorb below 220nm, lead arsenic and cadmium

Question 31

how can the sample be stabilised?

Answer 31

use of matrix modifier

Question 32

How can error in volume when manually pipetting be improved?

Answer 32

using autosampler

Question 33

What does an IR light sensor do?

Answer 33

used to monitor radiation from furnace walls and control electrical supply

Question 34

Why can the signal not be integrated to average out noise?

Answer 34

as furnace AA signal is transient

Question 35

How is the sample throughput a limitation in furnace atomisation?

Answer 35

temp program takes approx 2 mins, 10x longer than flame

Question 36

What 2 factors make GFAAS more expensive than FAAS?

Answer 36

due to autosampler and method development

Question 37

Temp gradients are another limitation in GFAAS. Why is this?

Answer 37

gas inside furnace heats up more slowly than furnace wall

Question 38

What might this cause atoms to do?

Answer 38

form too rapidly on wall of furnace and migrate into relatively cool gas, condensation can occur

Question 39

how can this be avoided

Answer 39

L'vov platform inside furnace

Question 40

What does a L'vov platform consist of?

Answer 40

raised graphite platform on which sample is placed

Question 41

What does L'vov platform ensure?

Answer 41

gas in tube and sample are at same temperature

Question 42

How is the sample and platform then heated?

Answer 42

indirectly by radiation and convection from furnace walls

Question 43

this delays atomisation until what?

Answer 43

gas is heated up sufficiently to prevent condensation

Question 44

What is matrix modification used for?

Answer 44

to get better separation of analyte elements from concomitants

Question 45

How does it achieve this?

Answer 45

makes the matrix more volatile and/or stabilises analyte element

Question 46

What is added to sample in high concentration?

Answer 46

a reagent ie acid or salt

Question 47

according to the law of mass action, what is analyte element transformed into?

Answer 47

well defined compound with known properties

Question 48

What does this allow for?

Answer 48

easily reproducible conditions for thermal pretreatment for a variety of matrices

Question 49

matrix modification has 5 advtantages - one being that is converts analyte into what?

Answer 49

well defined compounds

Question 51

Another advantage is that it allows same well defined thermal pretreatment conditions to be used for a variety of samples - what does this mean?

Answer 51

dont need to confirm applicability each time

Question 52

Another advantage is that is helps avoid multiple peaks during what?

Answer 52

atomisation caused by different compounds of analyte element

Question 53

Another advantage is that it allows highest thermal pretreatment temps to be applied - what does this ensure?

Answer 53

best separation from accompanying materials, minimum interferences present

Question 54

another advantage is that there is a small temp difference between pretreatment and atomisation - why is this important?

Answer 54

for reaching stabilised temp conditions

Question 55

What happens when the sample contains large quantities of volatile organic material?

Answer 55

organics removed during thermal pretreatment, allow analysis without any sample prep

Question 56

Chemical modification can be done directly into the furnace to reduce what?

Answer 56

spectral and/or non spectral interferences

Question 57

What are the 2 types of modification that can be done in the furnace?

Answer 57

those that reduce volatility of analyte, those that increase volatility of matrix

Question 58

If the element is determined to be volatile, such as chlorides/organo-metallic compounds, sample may be modified chemically using what?

Answer 58

adding ammonium hydroxide directly into furnace - renders it less volatile

Question 59

Why do samples containing NaCl usually produce high background signals?

Answer 59

due to high temp required to volatilise >1000oC

Question 60

What is added to the furnace to make NaCl more volatile to aid removal?

Answer 60

ammonium nitrate

Question 61

what does the addition of ammonium nitrate form? and what temp are these chemicals volatilsed at?

Answer 61

forms ammonium chloride and sodium nitrate, all 3 chemicals volatilised at less than 500oC

Question 62

What are 2 main background corrections for furnace measurements?

Answer 62

deuterium lamp and Zeeman background correction

Question 63

What does Zeeman background correction use to split atomic energy levels and thus absorption lines?

Answer 63

intense magnetic field

Question 64

What does this allow for?

Answer 64

background signal to be measured very close to atomic absorption line

Question 65

What is the term for most atoms having several electronic configurations that have the same energy?

Answer 65

degenerate

Question 66

Transitions between different pairs of configurations correspond to what?

Answer 66

single spectral line

Question 67

how does the presence of magnetic field break degeneracy?

Answer 67

as it interacts in different ways with electrons with different quantum numbers and slightly modifies energies - causes splitting of levels

Question 68

describe the commercial experimental arrangements?2

Answer 68

magnet around lamp, magnet around atom cell

Question 69

What happens if the magnet is placed around atom cell?

Answer 69

absorption of sample that is split

Question 70

what happens if magnet is placed around HCl?

Answer 70

emission spectrum of source that is split rather than absorption spectrum of sample

Question 71

What does the splitting of a singlet transition lead to?

Answer 71

central π line and equally spaced σ lines

Question 72

The π line has an absorbance of ....... times that of each σ line?

Answer 72

two

Question 73

The σ lines are usually separated enough from π line meaning what?

Answer 73

little or no atomic absorption is observed

Question 74

The zeeman affect is applied to atomic absorption based upon what?

Answer 74

differing response of types of absorption peaks to polarised radiation

Question 75

The π peak absorbs only what radiation?

Answer 75

radiation that is plane-polarised in a direction parrallel to external magnetic field

Question 76

The σ peak absorb only what radiation?

Answer 76

polarised at 90 degrees to external magnetic field

Question 77

The ....... line ,π, is polarised ......... to the direction of magnetic field?

Answer 77

central, parallel

Question 78

the .... lines ,σ, are polarised ........... to magnetic field?

Answer 78

wing, perpendicular

Question 79

What can be used to isolate either the centre or wing transitions?

Answer 79

polariser

Question 80

What are 2 advantages of zeeman background correction?

Answer 80

only 1 light source meaning no alignment problems, background measured close to atomic line

Question 81

what are disadvantages of zeeman background correction?

Answer 81

expensive, loss of sensitivity and linear range

Question 82

The smith-Hieftje background correction is based on what?

Answer 82

self-reversal behaviour of radiation emitted from HCls when operated under high currents

Question 83

The high currents cause a large concentration of non-excited atoms which are capable of what?

Answer 83

absorbing radiation emitted from excited species

Question 84

what else does the high current cause?

Answer 84

a broadening of excited species emission line

Question 85

The overall effect is a line with a minimum in its centra corresponding to what?

Answer 85

exact wavelength of absorption peak

Question 86

The lamp is run at alternating high and low current, what is measured at high/low currents?

Answer 86

total absorbance measured at low current, background is measured at high current

Question 87

The signals are then subtracted to give what?

Answer 87

corrected value