Diffusive Gradients in Thin Films for Inorganic Arsenic Speciation and Electrothermal Atomic ...

342 Pages·2008·3.35 MB·English

Diffusive Gradients in Thin Films for Inorganic Arsenic
Speciation and Electrothermal Atomic Absorption
Spectrometry with a Coupled Microcolumn for Trace
Metal Speciation

A thesis submitted in partial fulfillment of the requirements for the degree of
Doctor of Philosophy in Chemistry

at the

University of Canterbury
Christchurch
New Zealand

Jared G. Panther
August 2007
Abstract
This thesis is directed towards the development of the diffusive gradients in thin films (DGT)
technique for the measurement of total dissolved As, and for As speciation measurements. In
addition, a preliminary investigation of a novel laboratory-based method for measuring labile
metal species was carried out; this method involved the coupling of a microcolumn of
adsorbent with a standard electrothermal atomic absorption spectrometer.

An iron-oxide adsorbent was utilized for As measurements by DGT. The diffusion
coefficients of inorganic AsV and AsIII were measured through the polyacrylamide diffusive
gel using both a diffusion cell and DGT devices. A variety of factors that may affect the
measurement of total As by DGT were investigated. These factors, which included pH,
anions, cations, fulvic acid, FeIII-fulvic acid complexes, and colloidal Fe, may affect the
adsorption of the As species to the iron-oxide, or may affect the diffusion coefficients of the
individual As species.

The DGT method was further developed to selectively accumulate the AsIII species in the
presence of AsV. This was achieved by the placement of a negatively charged Nafion
membrane at the front of the DGT device which slowed the diffusion of the negatively
charged AsV species (H AsO -) considerably, relative to the uncharged AsIII species (H AsO ).
2 4 3 3
The effect that pH, anions, and cations may have on the selective accumulation of AsIII, in the
presence of AsV, was investigated. DGT devices without a Nafion membrane and with a
Nafion membrane were deployed in natural waters to determine the total inorganic As and
AsIII concentrations, and to evaluate its performance.

A preliminary investigation of the coupling of a microcolumn of Chelex-100 resin with a
standard electrothermal atomic absorption spectrometer was undertaken to establish its value
as a laboratory-based speciation method. This involved the examination of various
microcolumn materials to accommodate the Chelex-100 resin, and finding an appropriate
buffer that could be used to buffer the Chelex-100 resin without interfering with the ETAAS
measurement. Furthermore, factors that may affect the uptake of metal by the Chelex-100
resin, such as concentration of buffer in solution, ionic strength, and conditioning of the
Chelex-100 resin, were investigated.
Acknowledgements

First, I would like to thank my supervisors, Alison Downard and Kip Powell, for their support,
encouragement, and guidance during the course of this research. I greatly appreciate Alison
and Kip’s dedication to this project, and I am very grateful for the many hours they both
spent reading this thesis. I also appreciate the time spent in this department with Dr Hao
Zhang, who showed me the finer points of the DGT technique; this was very much
appreciated.

I would like to thank the past and present members of the Downard research group for their
friendship and support during the course of this research, and who have made my PhD
experience an enjoyable one.

I would like to thank the technical staff of the chemistry department, who without their
expertise much of this research would not have been possible. A special thanks goes to Sandy
Ferguson, for keeping the very temperamental ETAAS behaving; Russell Gillard, for helping
out with designing and making various components required for this research; and both Bruce
Reid and Wayne MacKay, for always being available when issues relating to the clean room
arose.

Financial assistance in the form of a Tertiary Education Commission Top Achiever Doctoral
Scholarship and a Canterbury Scholarship, is gratefully acknowledged.

Lastly I would like to thank all my family and friends who have been very patient and
understanding during my time at University, especially to Mum and Dad, who without their
love, care, and financial assistance, it would not have been possible to complete this thesis.
i

Contents

Chapter 1: Introduction
1.1 Speciation...................................................................................................................1
1.1.1 Speciation measurements......................................................................................2
1.1.2 Sampling and storage in speciation studies............................................................2
1.1.2.1 Sources of contamination...............................................................................3
1.1.2.2 Potential changes in sample composition during storage.................................3
1.2 Diffusive gradients in thin films (DGT)....................................................................5
1.2.1 DGT theory...........................................................................................................5
1.2.2 DGT assembly......................................................................................................8
1.2.3 Diffusive boundary layer.......................................................................................9
1.2.4 Speciation measurements using DGT..................................................................11
1.2.4.1 DGT for determining the concentrations of labile inorganic and organically
bound metal.............................................................................................................12
1.2.4.2 Effect of adsorbent binding strength.............................................................14
1.2.4.3 DGT for CrIII and CrVI speciation.................................................................15
1.2.5 Advantages of DGT............................................................................................15
1.3 Arsenic speciation and measurement......................................................................17
1.3.1 Arsenic speciation and concentrations in natural waters......................................17
1.3.1.1 Inorganic As speciation................................................................................18
1.3.1.2 Concentration of As in natural waters...........................................................20
1.3.2 Toxicity of AsIII and AsV.....................................................................................20
1.3.2.1 Mechanism of AsIII toxicity..........................................................................21
1.3.2.2 Mechanism of AsV toxicity...........................................................................22
1.3.3 Analytical methods for inorganic As speciation...................................................23
1.3.3.1 Hydride generation.......................................................................................23
1.3.3.2 Anodic stripping voltammetry......................................................................24
1.3.3.3 Cathodic stripping voltammetry....................................................................25
1.3.3.4 Anion exchange resins..................................................................................26

DGT for inorganic As speciation and ETAAS with a microcolumn for trace metal speciation
Contents ii
1.3.3.5 Capillary electrophoresis..............................................................................27
1.3.3.6 Chromatography...........................................................................................29
1.3.3.7 Spectrophotometric methods........................................................................29
1.3.4 Preservation strategies and storage of As samples...............................................30
1.3.4.1 Photo-oxidation of AsIII................................................................................31
1.3.4.2 Preservation of As solutions using acid.........................................................32
1.3.4.3 Preservation of As solutions using EDTA.....................................................34
1.3.4.4 Evaluation of preservation methods..............................................................35
1.4 Thesis outline...........................................................................................................37
1.5 References................................................................................................................38

Chapter 2: Experimental
2.1 Clean room protocols...............................................................................................49
2.2 Reagents...................................................................................................................50
2.2.1 Water..................................................................................................................50
2.2.2 Chemicals...........................................................................................................50
2.2.3 Analytical standards............................................................................................50
2.2.3.1 AsV stock solutions.......................................................................................50
2.2.3.2 AsIII stock solutions......................................................................................52
2.2.3.3 Metal standard solutions...............................................................................52
2.2.4 Preparation of buffer stock solutions...................................................................52
2.2.4.1 Sodium acetate buffers.................................................................................52
2.2.4.2 Ammonium acetate buffer............................................................................53
2.2.4.3 Calcium acetate buffer..................................................................................53
2.2.4.4 MES buffer..................................................................................................53
2.2.4.5 Phthalate buffer (pH 4.01)............................................................................54
2.2.4.6 Phosphate buffer (pH 7.00)...........................................................................54
2.2.5 Fulvic acid samples.............................................................................................54
2.2.6 Gases..................................................................................................................54
2.3 DGT procedures and protocols...............................................................................54
2.3.1 Preparation of iron-oxide.....................................................................................54
2.3.2 Preparation of iron-oxide gel and diffusive gels...................................................55
2.3.2.1 Preparation of gel solution............................................................................55

DGT for inorganic As speciation and ETAAS with a microcolumn for trace metal speciation
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2.3.2.2 Preparation of diffusive gel..........................................................................55
2.3.2.3 Preparation of iron-oxide gel........................................................................56
2.3.2.4 Gel casting procedures.................................................................................57
2.3.3 Diffusion cell......................................................................................................57
2.3.3.1 Diffusion cell for measurement of As diffusion coefficients through diffusive
gel and membrane filter...........................................................................................57
2.3.3.2 Diffusion cell for measurement of As diffusion coefficients through Nafion
membrane................................................................................................................58
2.3.4 DGT devices.......................................................................................................60
2.3.4.1 DGT devices for measurement of As diffusion coefficients through diffusive
gel and membrane filter...........................................................................................60
2.3.4.2 DGT devices for measurement of As diffusion coefficients through Nafion
membrane................................................................................................................60
2.3.5 DGT experiments and procedures.......................................................................61
2.3.5.1 Assembly of DGT devices and DGT blanks.................................................61
2.3.5.2 Deployment of DGT devices........................................................................62
2.3.5.3 Retrieval of DGT devices and elution of iron-oxide gel................................62
2.3.6 Membrane filters and pre-treatment.....................................................................63
2.3.7 Nafion membrane and pre-treatment...................................................................63
2.4 ETAAS instrumentation and protocols...................................................................63
2.4.1 ETAAS and graphite furnace parameters.............................................................64
2.4.1.1 Instrumental parameters...............................................................................64
2.4.1.2 Graphite furnace heating programs...............................................................64
2.4.1.3 Graphite furnaces.........................................................................................65
2.4.2 Arsenic analysis by hydride generation atomic absorption spectrometry..............65
2.4.2.1 HG 3000 hydride generator..........................................................................65
2.4.2.2 ETAAS and graphite furnace parameters......................................................67
2.4.2.3 HG-AAS chemical and physical parameters.................................................67
2.4.2.4 Preparation of reagents for HG-AAS............................................................68
2.4.2.5 Preparation of standards and samples for HG-AAS......................................69
2.4.3 Coupling of microcolumn with ETAAS..............................................................70
2.4.3.1 Microcolumns..............................................................................................70
2.4.3.2 Adsorbents...................................................................................................70
2.4.3.3 Microcolumn-ETAAS setup.........................................................................70

DGT for inorganic As speciation and ETAAS with a microcolumn for trace metal speciation
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2.4.3.4 Microcolumn-ETAAS procedure and experimental conditions.....................72
2.5 Miscellaneous equipment.........................................................................................74
2.5.1 Glassware and cleaning protocol.........................................................................74
2.5.2 Micropipettes......................................................................................................75
2.5.3 pH measurement.................................................................................................75
2.5.4 Filtration.............................................................................................................75
2.5.5 Balance...............................................................................................................75
2.5.6 Orbital shaker......................................................................................................75
2.6 Speciation calculations.............................................................................................75
2.7 References................................................................................................................77

Chapter 3: Investigation and optimization of hydride generation
atomic absorption spectrometry for As determination
3.1 Introduction.............................................................................................................78
3.1.1 Generation of hydrides........................................................................................79
3.1.1.1 Hydride generation using NaBH .................................................................80
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3.1.1.2 Pre-reduction of analyte and selective hydride generation.............................81
3.1.2 Use of modifiers to trap and stabilize hydrides....................................................82
3.1.2.1 Permanent and non-permanent modifiers......................................................83
3.1.2.2 Pd modifiers.................................................................................................84
3.1.2.3 Pd modifier mechanism................................................................................86
3.1.3 Interferences.......................................................................................................87
3.1.3.1 Liquid-phase Interferences...........................................................................87
3.1.3.2 Matrix interferences.....................................................................................87
3.1.4 Efficiency of As hydride generation and analyte accumulation............................88
3.1.5 Chapter outline....................................................................................................89
3.2 Experimental............................................................................................................90
3.2.1 Pd modifier parameters.......................................................................................90
3.2.1.1 Pd pre-treatment temperature........................................................................90
3.2.1.2 Mass of Pd...................................................................................................90
3.2.1.3 Pd surface area.............................................................................................91
3.2.1.4 Accumulation temperature............................................................................91
3.2.2 Effect of NaBH and NaOH concentrations.........................................................91
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DGT for inorganic As speciation and ETAAS with a microcolumn for trace metal speciation
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3.2.2.1 Effect of NaBH concentration at constant NaOH concentration...................91
4
3.2.2.2 Effect of NaBH concentration at varying NaOH concentrations.................91
4
3.2.2.3 Effect of NaOH concentration at optimum NaBH concentration.................92
4
3.2.3 Hydrochloric acid concentration..........................................................................92
3.2.4 Effect of graphite furnace age and type...............................................................93
3.2.5 Effect of FeIII......................................................................................................93
3.2.6 Effect of argon flow rate.....................................................................................93
3.3 Results......................................................................................................................94
3.3.1 Pd modifier parameters.......................................................................................94
3.3.1.1 Pd pre-treatment temperature.......................................................................94
3.3.1.2 Mass of Pd...................................................................................................97
3.3.1.3 Pd surface area.............................................................................................99
3.3.1.4 Accumulation temperature………………………..………………...………..99
3.3.2 Effect of NaBH4 and NaOH concentrations......................................................100
3.3.2.1 Effect of NaBH4 concentration at constant NaOH concentration................100
3.3.2.2 Effect of NaBH4 concentration at varying NaOH concentration.................102
3.3.2.3 Effect of NaOH concentration at optimum NaBH4 concentration...............103
3.3.3 Hydrochloric acid concentration........................................................................105
3.3.3.1 Effect of HCl concentration on AsIII sensitivity (absence of L-cysteine).....105
3.3.3.2 Effect of HCl concentration on AsV sensitivity (presence of L-cysteine).....107
3.3.4 Effect of graphite furnace age and type……………………….…………………109
3.3.5 Effect of FeIII....................................................................................................110
3.3.6 Effect of argon flow rate...................................................................................113
3.4 Discussion...............................................................................................................113
3.4.1 Pd modifier parameters.....................................................................................113
3.4.1.1 Pd pre-treatment temperature......................................................................113
3.4.1.2 Mass of Pd.................................................................................................115
3.4.1.3 Pd surface area...........................................................................................116
3.4.1.4 Accumulation temperature..........................................................................117
3.4.2 Effect of NaBH and NaOH concentrations.......................................................119
4
3.4.2.1 Effect of NaBH concentration at constant NaOH concentration.................119
4
3.4.2.2 Effect of NaBH concentration at varying NaOH concentration..................121
4
3.4.2.3 Effect of NaOH concentration at optimum NaBH concentration................122
4
3.4.3 Hydrochloric acid concentration........................................................................123

DGT for inorganic As speciation and ETAAS with a microcolumn for trace metal speciation
Contents vi
3.4.3.1 Effect of HCl concentration on AsIII sensitivity (absence of L-cysteine).....123
3.4.3.2 Effect of HCl concentration on AsV sensitivity (presence of L-cysteine).....125
3.4.4 Effect of graphite furnace..................................................................................126
3.4.5 Effect of FeIII on sensitivity...............................................................................128
3.4.5.1 Effect of FeIII on AsIII sensitivity in the absence of L-Cysteine...................128
3.4.5.2 Effect of FeIII on AsV sensitivity in the presence of L-cysteine....................130
3.4.6 Effect of argon flow rate...................................................................................131
3.5 Conclusion..............................................................................................................133
3.6 References..............................................................................................................133

V III
Chapter 4: Measurement of As and As diffusion coefficients
through diffusive gels and membrane filters
4.1 Introduction...........................................................................................................141
4.1.1 DGT adsorbents................................................................................................142
4.1.2 Measuring diffusion coefficients.......................................................................142
4.1.2.1 Measuring diffusion coefficients using a diffusion cell...............................143
4.1.2.2 Measuring diffusion coefficients using DGT devices..................................143
4.1.2.3 Measuring elution efficiency......................................................................144
4.1.3 Use of membrane filters in DGT.......................................................................145
4.1.4 Chapter Outline.................................................................................................145
4.2 Experimental..........................................................................................................145
4.2.1 Elution efficiency from iron-oxide gel...............................................................145
4.2.2 Measurement of AsV and AsIII diffusion coefficients through polyacrylamide
diffusive gel and membrane filter at pH 5 using a diffusion cell.................................146
4.2.3 Measurement of AsV and AsIII diffusion coefficients through polyacrylamide
diffusive gel and membrane filter at pH 5 using DGT devices....................................148
4.2.4 Effect of membrane filter and pre-treatment of membrane on mass of As
accumulated by DGT.................................................................................................149
4.2.4.1 Effect of membrane filter on mass of AsIII accumulated by DGT devices....149
4.2.4.2 Effect of membrane type, pore size, and acid washing of membrane on mass
of AsV accumulated by DGT devices.....................................................................150
4.3 Results....................................................................................................................152
4.3.1 Elution efficiency from iron-oxide gel...............................................................152

DGT for inorganic As speciation and ETAAS with a microcolumn for trace metal speciation
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4.3.2 Measurement of AsV and AsIII diffusion coefficients through polyacrylamide
diffusive gel and membrane filter at pH 5 using a diffusion cell.................................153
4.3.2.1 AsV diffusion coefficients...........................................................................153
4.3.2.2 AsIII diffusion coefficients..........................................................................154
4.3.3 Measurement of AsV and AsIII diffusion coefficients through polyacrylamide
diffusive gel and membrane filter at pH 5 using DGT devices....................................155
4.3.3.1 AsV diffusion coefficients...........................................................................155
4.3.3.2 AsIII diffusion coefficients..........................................................................157
4.3.4 Effect of membrane filter and pre-treatment of membrane on mass of As
accumulated by DGT.................................................................................................159
4.3.4.1 Effect of membrane filter on mass of AsIII accumulated by DGT devices....159
4.3.4.2 Effect of membrane type, pore size, and acid washing of membrane on mass
of AsV accumulated by DGT devices.....................................................................159
4.4 Discussion...............................................................................................................161
4.4.1 Elution efficiency from iron-oxide gel...............................................................161
4.4.2 Measurement of AsV and AsIII diffusion coefficients through polyacrylamide
diffusive gel and membrane filter at pH 5 using a diffusion cell and DGT devices.....163
4.4.3 Effect of membrane filter and pre-treatment of membrane on mass of As
accumulated by DGT.................................................................................................168
4.4.3.1 Effect of membrane filter on mass of AsIII accumulated by DGT devices....168
4.4.3.2 Effect of membrane type, pore size and acid washing of membrane on mass of
AsV accumulated by DGT devices..........................................................................169
4.5 Conclusion..............................................................................................................169
4.6 References..............................................................................................................170

V III
Chapter 5: Method development for As and As accumulation
by DGT
5.1 Introduction...........................................................................................................174
5.1.1 Capacity of the adsorbent used in DGT.............................................................174
5.1.2 Adsorption of As by iron-oxides.......................................................................175
5.1.2.1 Effect of pH on As adsorption....................................................................176
5.1.2.2 Anion and cation adsorption on iron-oxides................................................177
5.1.3 Adsorption of fulvic acid by iron-oxides...........................................................179

DGT for inorganic As speciation and ETAAS with a microcolumn for trace metal speciation