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analytical chemistry > GFAAS and system optimisation > Flashcards

GFAAS and system optimisation Flashcards

1
Q

What does GFAAS stand for?

A

graphite furnace atomic absorption spectrometry

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2
Q

What is the flame replaced with?

A

graphite furnace

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3
Q

Is it safer than FAAS?

A

yes

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4
Q

Does it require a nebuliser?

A

no

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5
Q

Can it be used to analyse solids

A

yes

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6
Q

Electrical heating is used for what?

A

volatilisation and atomisation of sample

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7
Q

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

A

injection hole at top of tube

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8
Q

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

A

to volatilise and atomise sample

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9
Q

the graphite material resists the passage of what?

A

electric current and converts electrical energy into heat

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10
Q

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

A

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

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11
Q

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

A

transient absorption signal

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12
Q

how much better are detection limits in GFAAS than FAAS?

A

100-1000x

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13
Q

what are the detection limits of flame and furnace?

A

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

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14
Q

How long are atoms present in the atom cell?

A

a relatively long time

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15
Q

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

A

Because of absence of flame gases

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16
Q

Because of this, what may be more severe?

A

background absorption of light by other species

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17
Q

Can GFAAS detect more elements than FAAS?

A

no, fewer

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18
Q

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

A

long residence time of atoms in furnace

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19
Q

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

A

biological and geological

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20
Q

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

A

temperature programming

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21
Q

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

A

100oC for 30 seconds, removes solvent

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22
Q

what might a too high temp cause in drying stage?

A

sputtering of the sample

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23
Q

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

A

350-550oC pyrolyses organic matter

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24
Q

What might happen if ashing temp is too high?

A

loss of analyte

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25
Q

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

A

flushed from the furnace

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26
Q

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

A

1500-2800oC - gives transient absorption peak

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27
Q

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

A

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

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28
Q

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

A

prevent memory effects

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29
Q

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

A

ashing stage

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30
Q

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

A

ones that absorb below 220nm, lead arsenic and cadmium

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31
Q

how can the sample be stabilised?

A

use of matrix modifier

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32
Q

How can error in volume when manually pipetting be improved?

A

using autosampler

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33
Q

What does an IR light sensor do?

A

used to monitor radiation from furnace walls and control electrical supply

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34
Q

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

A

as furnace AA signal is transient

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35
Q

How is the sample throughput a limitation in furnace atomisation?

A

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

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36
Q

What 2 factors make GFAAS more expensive than FAAS?

A

due to autosampler and method development

37
Q

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

A

gas inside furnace heats up more slowly than furnace wall

38
Q

What might this cause atoms to do?

A

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

39
Q

how can this be avoided

A

L’vov platform inside furnace

40
Q

What does a L’vov platform consist of?

A

raised graphite platform on which sample is placed

41
Q

What does L’vov platform ensure?

A

gas in tube and sample are at same temperature

42
Q

How is the sample and platform then heated?

A

indirectly by radiation and convection from furnace walls

43
Q

this delays atomisation until what?

A

gas is heated up sufficiently to prevent condensation

44
Q

What is matrix modification used for?

A

to get better separation of analyte elements from concomitants

45
Q

How does it achieve this?

A

makes the matrix more volatile and/or stabilises analyte element

46
Q

What is added to sample in high concentration?

A

a reagent ie acid or salt

47
Q

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

A

well defined compound with known properties

48
Q

What does this allow for?

A

easily reproducible conditions for thermal pretreatment for a variety of matrices

49
Q

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

A

well defined compounds

50
Q
A
51
Q

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

A

dont need to confirm applicability each time

52
Q

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

A

atomisation caused by different compounds of analyte element

53
Q

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

A

best separation from accompanying materials, minimum interferences present

54
Q

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

A

for reaching stabilised temp conditions

55
Q

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

A

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

56
Q

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

A

spectral and/or non spectral interferences

57
Q

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

A

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

58
Q

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

A

adding ammonium hydroxide directly into furnace - renders it less volatile

59
Q

Why do samples containing NaCl usually produce high background signals?

A

due to high temp required to volatilise >1000oC

60
Q

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

A

ammonium nitrate

61
Q

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

A

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

62
Q

What are 2 main background corrections for furnace measurements?

A

deuterium lamp and Zeeman background correction

63
Q

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

A

intense magnetic field

64
Q

What does this allow for?

A

background signal to be measured very close to atomic absorption line

65
Q

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

A

degenerate

66
Q

Transitions between different pairs of configurations correspond to what?

A

single spectral line

67
Q

how does the presence of magnetic field break degeneracy?

A

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

68
Q

describe the commercial experimental arrangements?2

A

magnet around lamp, magnet around atom cell

69
Q

What happens if the magnet is placed around atom cell?

A

absorption of sample that is split

70
Q

what happens if magnet is placed around HCl?

A

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

71
Q

What does the splitting of a singlet transition lead to?

A

central π line and equally spaced σ lines

72
Q

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

A

two

73
Q

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

A

little or no atomic absorption is observed

74
Q

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

A

differing response of types of absorption peaks to polarised radiation

75
Q

The π peak absorbs only what radiation?

A

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

76
Q

The σ peak absorb only what radiation?

A

polarised at 90 degrees to external magnetic field

77
Q

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

A

central, parallel

78
Q

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

A

wing, perpendicular

79
Q

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

A

polariser

80
Q

What are 2 advantages of zeeman background correction?

A

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

81
Q

what are disadvantages of zeeman background correction?

A

expensive, loss of sensitivity and linear range

82
Q

The smith-Hieftje background correction is based on what?

A

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

83
Q

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

A

absorbing radiation emitted from excited species

84
Q

what else does the high current cause?

A

a broadening of excited species emission line

85
Q

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

A

exact wavelength of absorption peak

86
Q

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

A

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

87
Q

The signals are then subtracted to give what?

A

corrected value

88
Q
A

analytical chemistry (8 decks)

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