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analytical chemistry > HGAAS, CVAAS and interferences > Flashcards

HGAAS, CVAAS and interferences Flashcards

1
Q

Name 3 deficiencies in FAAS

A

nebulisation efficiency, residence time, sample size

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2
Q

Give 2 reasons for the inefficiency of conventional nebuliser-burner

A

is less than 10%, most sample never reaches flame (droplets too big to desolvate)

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3
Q

What is the issue with residence time?

A

particles only in flame for a few hundredths of a second

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4
Q

what is the issue with sample size?

A

continuous flow of sample required for a steady signal - difficult to use less than 1ml

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5
Q

another deficiency in Flame atomisation is form of sample, why?

A

only vicscous liquids or solution

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6
Q

another deficiency in Flame atomisation is flame background absorption, why? 2

A

No wavelength below 90nm can be observed, some important elements cant be analysed

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7
Q

another deficiency in Flame atomisation is flame background emission, why?

A

gives rise to increased noise levels at various wavelengths

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8
Q

another deficiency in Flame atomisation is flame temp, why?

A

hot enough to atomise most elements, but not hot enough to produce emission from many elements

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9
Q

what does HGAAS stand for?

A

hydride generation atomic absorption spectrophotometry

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10
Q

Elements in which groups form volatile hydrides?

A

IV, V, VI

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11
Q

what is the burner assembly in standard AA replaced with?

A

heated silica tube

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12
Q

What temp is the tube? is this sufficient for atomisation?

A

900oC, yes

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13
Q

how is metal hydride generated?

A

by acidifying sample with solution of HCl and then adding NaBH4

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14
Q

When can hydrides be separated from the sample matrix?

A

before being introduced to the light path

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15
Q

the hydride is then decomposed and atomised with what?

A

negligable background interference

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16
Q

What does this have considerable advantages for?

A

analysing volatile elements which escape from atom cell before a measurement can be made in techniques such as GFAAS

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17
Q

Why is the window made of quartz?2

A

transparent in UV VIS wavelengths with high thermal stability

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18
Q

What are 2 advantages of HGAAS over FAAS?

A

cheap and simple modification, detection limits for suitable elements are comparable to GFAAS

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19
Q

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?

A

no flame in HGAAS so no flame gas absorption

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20
Q

A disadvantage would be the efficiency of hydride generation depends on what?

A

oxidation state of elements `

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21
Q

A disadvantage would be the presence of what in sample matrix?

A

easily reducible ions

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22
Q

Give 3 examples, what would they suppress?

A

Co, Cu, Ni, can suppress hydride formation

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23
Q

A disadvantage would be that wet chemistry sample prep is more complex than what?

A

an automated furnace measurement

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24
Q

A disadvantage would be that it lacks generality - why is this?

A

only a limited number of elements form hydrides

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25
Q

what does CVAAS stand for?

A

Cold vapour atomic absorption spectrometry

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26
Q

what element is it used to detect?

A

mercury

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27
Q

The main absorption wavelength of Hg is at 184.9nm, meaning what?

A

less sensitive lines have to be used if using flame methods

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28
Q

Why is this?

A

it is too close to maximum absorption wavelength of flame gases

29
Q

Hg is the only element with high vapour pressure of what?

A

monoatomic atoms at room temp

30
Q

The cold vapour Hg cell is simialir to what technique?

A

HGAAS

31
Q

What temp is the silica tube in CVAAS?

A

room temp

32
Q

Why might the cell be open ended?

A

to prevent window absorption at most sensitive resonance line (184.9nm)

33
Q

Hg vapour is generated by reducing inorganic Hg compounds with what?

A

NaBH4 or SnCl2

34
Q

when are organic Hg compounds oxidised to inorganic Hg?

A

before analysis

35
Q

Elemental Hg is flushed from solution - how is the vapour flushed from solution?

A

bubbling Ar through the solution

36
Q

Light is then transmitted along what?

A

axis of tube from a hollow cathode mercury lamp

37
Q

What are 2 advantages of CVAAS over FAAS?

A

simple and cheap modification, detection limits are 100x more sensitive to flame.

38
Q

Why are the detection limits so much higher in CVAAS than FAAS? 2

A

use of most sensitive line (184.9nm v 253.7nm) and longer residence time of atoms in beam

39
Q

What are 2 disadvantages of CVAAS over FAAS?

A

interferences in the reduction from very reducible elements, background correction may be requied

40
Q

why is atom cell heated in HGAAS but not CVAAS?

A

no heating is applied to obtain gaseous mercury atoms because it is already in elemental form with high vapour pressure after reduction reaction

41
Q

what are the 4 types of interferences?

A

spectral, chemical, ionisation, physical

42
Q

what would a spectral interference on line spectra caused by atomic absorption look like?

A

interference between 2 overlapping absorption lines - occurs only occastionally

43
Q

how about atomic emission line spectra?

A

Interference from adjacent line of another element in matrix - depends on resolution from monochromator

44
Q

Why are interferences more common in AE then AA?

A

due to number of emission lines observed

45
Q

How can AE spectral interferences be avoided?

A

by careful choice of wavelength considering other possible matrix components

46
Q

What is broad band spectra AA due to?

A

molecules or particles in flame leading to higher absorbance than expected

47
Q

if broad band spectra interference is due to combustion, what can be done?

A

instrument can be zeroed using a blank

48
Q

what 3 things can be done if broad band spectra interference is due to sample matrix?

A

use background correction, use higher temp flame, use radiation buffer

49
Q

How can emission lines which are superimposed on bands emitted by oxides and other molecular species be corrected?

A

correct by scanning background either side of emission line and subtracting mean of the 2

50
Q

What is a chemical interference in AA and AE?

A

presence of compounds of low volatility which are not readily atomised

51
Q

what does this chemical interference lead to?

A

leads to low results

52
Q

How can chemical interferences be avoided? 3

A

higher temp flame, releasing agents, protective agents

53
Q

what is a releasing agent

A

cations which preferentially react with interfering species

54
Q

what is a protective agent?

A

compounds which form stable volatile species with analyte

55
Q

In ionisation interferences, what happens when a hotter flame causes ionisation?

A

samples and standards ionised to same extent, leads to loss in sensitivity

56
Q

If analyte contained an easily ionisable element, what would happen in the event of ionisation interference?

A

it would also be ionised

57
Q

There would be increased energy in the flame, which would suppress what?

A

ionisation of analyte leading to high results

58
Q

How can this be avoided?

A

by use of ionisation buffer - large excess of easily ionisable element is added to both samples and standards

59
Q

Name 3 types of physical interference?

A

density, surface tension, viscocity

60
Q

What do these factors affect? 2

A

rates of aspiration and nebulisation

61
Q

What is another physical interference?

A

incomplete volatilisation

62
Q

in incomplete volatilisation, matrix components in flame may form what? what do they do?

A

stable compounds which physically trap analyte

63
Q

how can incomplete volitilisation be avoided? 2

A

dilution and higher t flame

64
Q

In self absorption interferences, the centre of flame is hotter than exterior. what does this mean for atoms?

A

excited atoms emitting in centra are surrounded by cool region of unexcited atoms

65
Q

In self absorption interferences, an emitted photon travelling towards detector may be absorbed by what?

A

unexcited atom at edge of flame

66
Q

In self absorption interferences, what factor increases the chances of this occuring?

A

higher conc

67
Q

how can self absorption interferences be avoided?

A

using a non-resonance line for emission measurements

68
Q
A

analytical chemistry (8 decks)

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