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1
Literature Study on Speciation Analysis of Inorganic Arsenic in
Aqueous Samples Using Inductively Coupled Plasma Mass
Spectrometry
Assefa Yimer, 2011
Degree project in chemistry, 15hp
Department of chemistry
Umeå University
Sweden
Supervisor: Solomon Tesfalidet
Examiner: Erik Björn
2
Abstract
The oxidation state of arsenic plays an important role in its behavior and toxicity. As(III) and As(v) are
the most toxic forms and known to be carcinogens. This literature study covers speciation analysis of
inorganic arsenic species in water samples using high performance liquid chromatography inductively
coupled plasma mass spectrometry (HPLC-ICP-MS). Moreover, species stability, factors affecting
arsenic speciation and sample preservation techniques are also discussed.
3
Table of contents
1. Introduction ......................................................................................................................................... 4
2. Common Analytical Methods Used for Speciation Analysis of Arsenic ........................................ 5
2.1 High Performance Liquid Chromatography (HPLC) ................................................................... 5
2.1.1 Ion Exchange HPLC ...................................................................................................................... 5
2.1.2 Ion Pair HPLC ............................................................................................................................... 5
2.2 HPLC-ICP-MS .................................................................................................................................. 5
2.3 GC-ICP-MS ....................................................................................................................................... 6
3. Detection Limit .................................................................................................................................... 7
4. Species Stability ................................................................................................................................... 7
4.1 Factors Affecting Arsenic Speciation .............................................................................................. 7
4.1.1 pH ................................................................................................................................................... 7
4.1.2 Redox Potential (Eh) ...................................................................................................................... 9
4.1.3 Natural Organic Matter (NOM) ................................................................................................... 9
4.1.4 Iron (Fe2+
/Fe3+
) ............................................................................................................................. 10
4.1.5 Microbial Activities ...................................................................................................................... 10
5. Preservation of Arsenic Species ....................................................................................................... 10
5.1 Filtration .......................................................................................................................................... 10
5.2 Acidification..................................................................................................................................... 10
5.3 Storage and Type of Container Used for Storage ....................................................................... 10
5.4 Stabilization Using EDTA ............................................................................................................. 11
6. Conclusion ......................................................................................................................................... 15
7. Acknowledgement ............................................................................................................................. 16
8. References .......................................................................................................................................... 17
9. Appendix ............................................................................................................................................ 20
4
1. Introduction Natural sources and anthropogenic activities are encountered for the introduction of several metals in to
the environment. Among these, arsenic, the 20th most abundant element in the earth´s crust is widely
distributed in soil, sediments, water, air and in biological system. Most problems related to arsenic are due
to mobilization under natural conditions. There are also additional man made impacts through combustion
of fossil fuels, mining activities, the use of arsenic as an additive to livestock feed, the use of arsenical
herbicides, pesticides and crop dessicants. Though there are studies suggesting consumption of arsenical
pesticides and herbicides has fallen dramatically, recent uses on wood preservation is common [1].
Manufacturing semiconductor at industrial level is also accountable for the release of arsenic. The main
source of human exposure is due to its accumulation in food. Compounds like aryloarsenicals has been
used as additive for animal feed which can find its way easy to human diet [2]; The use of 4-hydroxy-3-
nitrophenyl arsenic acid in chicken feed for promoting growth [3].
Geological sources from surface weathering or underground deposits contribute much for the aquatic
environment. The biggest threat for human health is water since it can easily get in to human body. The
concentration of arsenic in water ranges from µg/L (ppb) to mg/L (ppm) level[4].
Arsenic exists in the environment in four oxidation states (As0, As
3-, As
3+ & As
5+) for both organic and
inorganic forms. There is great variation in toxicity between different arsenic compounds. Due to
chemical similarity of arsenic with phosphate, it might interfere to the phosphate transport system and yet
replaces phosphate in energy transfer phosphorylation reactions [5]. Some studies have shown that
enzyme inactivation may take place due to the high affinity of arsenic for thiol(-SH) groups in
proteins[5]. The toxicity, bioavailability and transport properties of arsenic depends highly in its chemical
form[12]. The most acutely toxic types are inorganic forms [6]. In reducing conditions, the inorganic
forms exist as trivalent arsenate (H3AsO3), and the pentavalent arsenite (H2AsO4-) is favored under
oxidizing condition [7]. Inorganic arsenite [As(III)], is extensively metabolized in humans. Arsenate
[As(v)] is less toxic than arsenite[As(III)] [8]. The organic forms like monomethyl arsonic acid [MMA(v),
CH3AsO(OH)2] and dimethyarsenic acid [DMA(v), (CH3)2AsO(OH)] show a toxicity factor of one in
four hundred when compared to inorganic types[9]. The inorganic forms are known to be carcinogens.
With an increase in the degree of methylation, toxicity generally decreases. The order of toxicity is :
arsenite > arsenate > monomethylarsonate > Dimethylarsinate [10].
Many countries in the world are suffering from a contaminated arsenic ground water. Among these;
Hungary, Finland, USA, Argentina, China, Taiwan, India, Bangladish, Japan and Vietenam are noticeable
ones[see appendix 1]. Contamination of water with arsenic is believed to bring diseases on long term
basis like gangrene, skin cancer, splenomegaly, hepatomegaly, nonpitting swelling, leucomelonsis,
cardiovascular diseases, conjunctivitis, hyperkeratosis and hypopigmentation [10]. According to
EPA(U.S) and WHO, 10µgl-1
is the recommended value for drinking water and 50 µgl-1
, the maximum
permissible concentration[46,11]. Due to these regulations sensitive and reliable analytical methods are
needed. Total quantification of an element is not sufficient but it is also mandatory to determine the
species within which the elements are found. Analyzing a sample to identify/or quantify one or more
chemical species is called speciation analysis [13]. The aim of this project is to make a literature study on
the speciation analysis of inorganic arsenic in water samples using chromatographic separation technique
coupled to inductively coupled plasma mass spectrometry. Moreover, sampling, storage conditions,
stability of the inorganic species and detection limits are also included.
5
2. Common Analytical Methods Used for Speciation Analysis of Arsenic
2.1 High Performance Liquid Chromatography (HPLC) Most compounds which are not sufficiently volatile for gas chromatography can be separated through the
use of high performance liquid chromatography. HPLC applies high pressure to force a solvent through
closed columns containing very fine particles that yields good resolution separations. The system in
HPLC consists of a solvent delivery route, a sample injection valve, a high pressure column, a detector
and a computer to control the system and display results. Below are types of HPLC in connection to ICP-
MS.
2.1.1 Ion Exchange HPLC It employs a mechanism of exchange equilibria between the mobile phase containing oppositely charged
ions and stationary phase of the surface ions [3]. It operates in two separation modes, cation exchange or
anion exchange. The most commonly used column in anion exchange chromatography is Hamilton
PRPx100. DionexAS7, ION120 and LC-SAX were also used. These columns use polar stationary phases.
Malonates, acetates, phosphates, carbonates and a small percentage of methanol were components of the
different mobile phases used at different pH (table 1). Retention of analytes and separation in ion
exchange HPLC can be affected by standard variables like flow rate and introduction of organic modifiers
in to the mobile phase. Furthermore, the PH of the mobile phase, the ionic strength of the solute,
concentration of the ionic strength of the buffer and temperature can influence the technique. For
separation of arsenic compounds, Isocratic or gradient ion exchange are applied.
2.1.2 Ion Pair HPLC Ion pair HPLC involves either anion pairing or cation pairing technique. In such a separation, surface of a
stationary phase is kept less polar than the mobile phase. Addition of a counter ion to the mobile phase in
reversed phase ion-pair chromatography is important and a secondary chemical equilibrium of the ion pair
is established to monitor retention and selectivity. Ion pair separates ionic species and uncharged
molecular species as well. Long chain alkyl ions can be used as ion pair reagents. The mobile phases in a
simple reversed phase HPLC are aqueous solutions which may partly contain organic modifiers. With a
significant change in the organic content of the mobile phase, a signal drift is possibly noticed in the
ICPMS detector [3]. Reversed phase columns like Capcell C18 pak, ODS-3 and Devilsol C30-UG-5 were
used. The mobile phase compositions and the optimization parameters are given in table 2.
2.2 HPLC-ICP-MS The big merit of liquid chromatography is the use of different mobile phases and stationary phases that
would allow suitable condition for speciation analysis. High sensitivity, wide linear dynamic range, multi
element capability, isotope ratio measurement capability are the advantages that an ICPMS can offer.
However, an optimal solvent composition should be attained when ICPMS is coupled to HPLC. Plasma
instability and accumulation of carbon residue on sampling cone may arise due to high concentration of
some organic solvents [15]. Signal suppression would occur in the ICP if there is much salt concentration
thereby increasing space-charge effects where the ion beam is defocused [16]. In using ICP-MS as a
detector, spectral interferences may arise due to the formation of 40
Ar35
Cl+
which has the same mass as 75
As. Other interferences can also originate from samples having a complicated matrix. However, such
6
problems can be solved by applying mathematical equations, using dynamic reaction cells or high
resolution ICP-MS [4].
Fig 1. HPLC-ICP-MS Coupling [50].
2.3 GC-ICP-MS Gas chromatography is applicable for the separation of volatile and semi-volatile compounds and
possesses a high resolving power. Properly derivatized compounds having high boiling and polarity can
also be separated i.e The arsenic species need to be derivatized prior to separation by GC. Compared to
HPLC-ICP-MS, GC-ICP-MS offers 100% introduction efficiency to the ICP and significant
chromatographic resolution [14]. Moreover, the stable and dry plasma conditions allow only fewer
spectral interferences.
7
Fig 2. Schematic Diagram of GC-ICP-MS [42].
3. Detection Limit In most of the cases, detection limits are calculated as three times the standard deviation of the
background signal or replicate analysis of deionized spiked water samples. Below are datas on limit of
detection obtained with LC-ICP-MS. The LOD results obtained from Duarte et.al[54] are 0.02µgL-1
for
As(III) and 0.10 µgL-1
for As(V). However, Ronkarrt et. al [44] achieved 0.017 µgL-1
for As(III) and
0.026 for As(V). Day et.al [55] conduced arsenic speciation methods on drinking water with LOD 0.067
µgL-1
for As(III) and 0.089 µgL-1
for As(V). Better detection limits than Day et. al was obtained by Roig-
Navaro et. al [47] with values 0.046 µgL-1
for As(III) and 0.03 for As(V). Worse detection limits were
obtained by Milstein et.al [48], 0.09 µgL-1
for As(III) and 0.3 µgL-1
As(V). Robust and sensitive method
for the determination of arsenic species in sea water was conducted by Nakazato et. al [56] using hydride
generation techniques coupled to ICP-MS where detection limit for As(III) was 0.0028 µgL-1
in pure
water, 0.0034 µgL-1
in 2% Cl-1
solution. For As(V), 0.0045 µgL-1
in pure water and 0.0042 µgL-1
in 2%
Cl- solution.(see Table 3)
4. Species Stability Maintaining the integrity of the original chemical species in the sample is very important to obtain
reliable speciation information. Among species of arsenic, As(III) and As(v) are the dominant forms in
water. Stability of arsenic species can be affected by several factors during the different steps in the
analytical chain.
Fig 1.Changes or processes affecting stability of arsenic speciation during different steps of sampling to
analysis [18].
4.1 Factors Affecting Arsenic Speciation
4.1.1 pH Among the different forms of arsenic, As(III), As(v), MMA(v) and DMA(v) exhibit acid base equilibria
that results various major and minor species along changes with pH. There are no analytical techniques
that allow quantification of these type of individual complexes illustrated in fig 2 and fig 3 at typical
natural concentration.
8
Fig 2. A graph showing distribution of As(III), As(v), MMA(v) and DMA(v) hydroxide species as a
function of pH at 25oC[17].
Dissociation of As(OH)3 in water follows the following (a) to (c) (Pierce and Moore,1982 cited in[17])
As(OH)3 As(OH)2O- + H
+ Pka1=9.2(a)
As(OH)2O-
As(OH)O22-
+ H+ Pka2=12.1(b)
As(OH)O22-
AsO33-
+ H+ Pka3=12.7(c)
From the figure above, As(OH)3 dominates over the others at pH=7
As(v) undergoes the dissociation below (d) to (F) (Goldberg and Johnston, 2001 cited in[17])
AsO(OH)3 AsO2(OH)2- + H
+ Pka1=2.3 (d)
AsO2(OH)2- AsO3(OH)
2- + H
+ Pka2=6.8(e)
AsO3(OH)2-
AsO43-
+ H+
Pka3=11.6(f)
Almost the same amounts of AsO2(OH)2- and AsO3(OH)
2- exist at neutral pH.
The monoprotic acid (MMA) and the Diprotic acid (DMA) undergo (g) to (i) (Cox and Ghosh, 1994 cited
in[17])
9
CH3AsO(OH)2 CH3AsO2(OH)- + H
+ Pka1=4.1(g)
CH3AsO2(OH)- CH3AsO3
2- + H
+ Pka2=8.7(h)
(CH3)2AsO(OH) (CH3)2AsO2- + H+ Pka1=6.2(i)
At pH=7, major species of MMA(v) is CH3AsO2(OH)-, minor species is CH3AsO3
2- but for DMA(v)
both (CH3)2AsO(OH) and (CH3)2AsO2- are present.
4.1.2 Redox Potential (Eh) Considering pH and Eh, it is possible to model arsenic speciation. The figure below illustrates the
relationship between redox potential and pH for inorganic arsenic compounds in the natural environment.
Fig 3.The Eh-pH diagram for arsenic at 250C,1
atm and total arsenic 10-5
mol/L, total sulfur 10-3
mol/L. Parenthesis is for solid species in the cross
hatched area with a solubility of less than 10-5.3
mol/L (Source: Fergusson and Gavis, 1972 cited and
reproduced in [17])
In oxidizing conditions, H3AsO4 is likely to exist at pH<2. For a pH range 2-11, both H2AsO4- and
HAsO42-
do sufficiently exist under the same environment (oxidizing). However, at low Eh values,
inorganic arsenic species [As(III)], H3AsO3 predominantly exists.
4.1.3 Natural Organic Matter (NOM) In aqueous systems, NOM is ubiquitous and has a number of functional groups like phenolic, carboxylic,
esteric, amino, quinine, sulfhydryl, hydroxyl, nitroso and other moieties[19]. NOM rich waters
dominantly contain As(III) since it has a reducing property[20]. NOM can form complexes with Fe(II)
thereby retarding the Fe(III) oxidation of As(III) [21]. A report has shown that As(v) is reduced because
of NOM in rain water and soil-pore water samples preserved by EDTA[22].
10
4.1.4 Iron (Fe2+/Fe3+) Iron exists as Fe(II) or Fe(III) depending on PH and Eh. When atmospheric oxygen reacts with a
dissolved iron (Fe(II)), precipitation of iron as HFOs (hydrated ferric oxides) is formed that offers
sorption sites for dissolved arsenic species. Preferential co-precipitation of As(v) occurs since it is anionic
species [23]. In the presence of Fe(III), light exposure and preservation by HCl, Fast oxidation of As(III)
to As(V) was noticed in synthetic solution [24]. With the addition of of Fe(II) on iron rich ground water
samples having PH<2, an excellent preservation of As(III) is obtained [25].
4.1.5 Microbial Activities Conversion of arsenic species by micro-organisms include redox, methylation and demethylation [26].
Reduction of As(V) and oxidation of As(III) was found using mixed microbial cultures [27]. Microbes
that reduce As(V) were obtained in oxic water samples [28].
5. Preservation of Arsenic Species
5.1 Filtration It is useful for separating particulate bound and dissolved fraction of an element. It inhibits changes in
speciation partly through removal of micro-organisms and partly through the removal hydrated ferric
oxides. During some field speciation method, loss of arsenic occurs as a result of the deposited HFOs on
filters [22]. For a sample containing dissolved iron with PH>8, filtration should be done fast (~<10
minutes) [29]. Filters of size 0.1 - 0.2µm improves stability since many of the bacteria have dimensions
less than 0.45 µm [30]. However, concentration of the dissolved arsenic obtained using filter 0.45 µm is
much higher than 0.2 µm filtered sample [31].
5.2 Acidification During acidification, precipitation of iron is suppressed by increasing its solubility. Furthermore, the
method reduces microbial activities. For preserving arsenic species, HCl is widely used [32]. Hydrogen
chloride alone could not preserve As(III) in Fe(III)) containing samples[25]. Using HNO3 as a
preservative enables oxidation of As(III) to As(V) due to photo-reduction of HNO2. The use of H2SO4 as a
preservative is noted, but not recommended for it is difficult to purify and may lead to BaSO4
precipitation. H3PO4 preserves by lowering PH and complexing Fe[34]. However, high acid
concentration may destroy chromatographic columns. In samples containing higher amount of iron (AMD
waters), insoluble phosphate, strengite [Fe3(PO4)2] is formed[35]. Redox potential (Eh) of a solution
might increase by the use of H2SO4 and H3PO4, especially in oxic water samples (Eh~>400mv), where
oxidation of As(III) is favored [36].
5.3 Storage and Type of Container Used for Storage Different types of plastics (polypropylene, polyethylene, polycarbonate) and glass containers can be used
for storing samples. Leaching of species from the material and adsorption losses should be controlled.
Containers made of glass that are rinsed with HNO3 exhibited to leach out arsenic [37]. Photochemical
reactions do also contribute for the loss of species. Opaque glass sample storage are preferred than plastic
containers which in most of the cases are translucent [38]. However, Community bureau of reference,
measurements and testing programme (BCR study) revealed that glass and plastic containers are equally
11
good at storing samples for arsenic speciation [18]. Refrigeration minimizes biotic and abiotic processes
that alter arsenic species. Many authors do agree that, 40C is recommendable for stabilizing arsenic
species[39]. Increasing temperature leads to an increase in biotic and abiotic oxidation rates of As (III),
thus stabilization period minimizes for samples at room temperature [42]. For iron deficient samples, -
200C is considered good but alternations in arsenic species were detected in iron rich samples [34].
5.4 Stabilization Using EDTA In acid-mine-drainage (AMD ) samples , the redox activities of the two iron species, Fe(II)and Fe(III),
should be lowered to stabilize the arsenic species. EDTA can form stable complexes with the iron species
thereby decreasing the free metal ion concentration. However, the chelating agent, EDTA, can also form
stable metal complexes with Ca and Mg. Thus, higher concentration of EDTA is needed to make arsenic
species stable [40]. But, PH increment is favored while using higher EDTA concentration (10mmoL/L)
due to formation of protonated metal-EDTA complex[41]. There are studies showing an adjustment of PH
to 3.2 when using EDTA to preserve arsenic speciation, this suppresses oxidation of Fe(II) [36].
Successful preservation could not be obtained if lower EDTA concentration (1.25mmoL/L) is used for
ground water samples having high Fe(II), Ca and Mg[41].
Table 1.Application of ion exchange(anion exchange) HPLC with ICP-MS for speciation analysis.
Species
matrix Column Mobile phase Optimization Retention
time(minutes)
Literatu
re cited
As(III)
As(V)
MMA(V)
DMA(V)
AsB
Well
water
Anion exchange
Dionex AS7
(250mmx4mm,10µm)
Gradient
elution
a.2.5 mM
ammonium
dihydrogen
phosphate
b.50mM
ammonium
dihydrogen
phosphate
PH=10
F.R=1ml/min
1.35,AsB
2.0, DMA(V)
3.0, As(III)
8.7, MMA(V)
14.8, As(V)
[44]
As(III)
As(V)
MMA(V)
DMA(V)
Natural
water
Anion exchange
Hamilton PRP-x100
(250mmx4.1mm)
Gradient
elution
38-75mM
phosphate
buffer
PH=5.7
F.R=1mL/min
C.T=330C
3.1, As(III)
3.6, DMA(V)
4.7, MMA(V)
6.1, As(V)
[45]
As(III)
As(V)
MMA(V)
DMA(V)
Surface
water
Anion exchange
Hamilton PRP-x100
(250mmx4.1mm,10
µm)
Isocratic
elution
20mmolL-
1NH4H2PO4
PH=5.6
F.R=1.5
mL/min
1.7, As(III)
2.5, DMA(V)
5.0, MMA(V)
9.7, As(V)
[46]
As(III)
As(V)
MMA(V)
DMA(V)
Surface
water
Anion exchange:
ION
120(120mmx4.6mm,1
0 µm)
Gradient
elution
4mM-0.3M
ammonium
hydrogen
carbonate/2%
PH=10.3
F.R=1ml/min
2.5, DMA(V)
3.5, As(III)
7.0, MMA(V)
8.5, As(V)
[47]
12
methanol
Anion exchange
Hamilton PRP-x100
(250mmx4.1mm,10
µm)
Gradient
elution
10mM-
100mM
Ammonium
dihydrogen
phosphate/2
% methanol
pH=6
F.R=1ml/min
2.5, As(III)
3.7, DMA(V)
4.7, MMA(V)
5.8, As(V)
Anion exchange
Hamilton PRP x100
(250mmx4.1mm,1 0
µm)
Gradient
elution
4mM-0.3M
ammonium
hydrogen
carbonate/2%
methanol
PH=8
F.R=1ml/min
2.5, As(III)
4.25, DMA(V)
5.5, MMA(V)
7.0, As(V)
As(III)
As(V)
MMA(V)
DMA(V)
roxarsone
Natural
water
Anion exchange
Dionex AS7
(4mmx250mm)
Gradient
elution
2.5-50mM
nitric acid in
0.5%
methanol
F.R=1ml/min
C.T=330C
1.7, As(III)
2.3, MMA(V)
4.4, DMA(V)
5.6, As(V)
6.7,Roxarsone
[45]
As(III)
As(V)
Anion exchange
LC-SAX
(4mmx50mm)
Isocratic
elution
12.5mM
malonate
And 17.5
mM acetate
PH=4.8
F.R=1ml/min
C.T=330C
0.9, As(III)
1.5, As(V)
As(III)
As(V)
MMA(V)
DMA(V)
Drinki
ng
water
Anion exchange
Hamilton PRP x100
(250mmx4.1mm,10
µm)
Isocratic
elution
14mM
phosphate
buffer
PH=6
F.R=1.5ml/mi
n
C.T=250C
2, As(III)
4.7, DMA(V)
6.5, MMA(V)
11.5, As(V)
[48]
Anion exchange
Hamilton PRP x100
(250mmx4.1mm,10
µm)
Gradient
elution
30ml-100mM
TRIS acetate
buffer
PH=7
F.R=1.5ml/mi
n
C.T=250C
2, As(III)
4.7, DMA(V)
6.5, MMA(V)
11.5, As(V)
Table 2.Application of ion pairing HPLC with ICPMS for arsenic speciation
species Matrix column Mobile phase optimization Retention
time(minutes)
Literatu
re cited
As(III)
As(V)
River
Water
Reversed
phase
Capcell C18
pak
Isocratic elution
5mM butane sulfonic
acid,2mM malonic
acid,0.3mM hexane
PH=2.5
F.R=1mL/mi
n
2.4, As(V)
3.0, As(III)
[51]
13
(250mmx4.6
mm)
sulfonic acid,0.5%
methanol
As(III)
As(V)
MMA(V)
DMA(V)
Well
water
Reversed
phase
ODS-
3(150mmx4.
6,3µM)
Isocratic elution , 5mM
TBAH,3mM malonic
acid,5%methanol
F.R=1.5mL/
min
C.T=500C
1.5, As(III)
2.4, DMA(V)
3.2, MMA(V)
4.7, As(V)
[52]
As(III)
As(V)
MMA(V)
DMA(V)
AsB
AsC
TMAO
TeMA
Hot
spring
water
Reversed
phase
Devilsol
C30-UG-5
(250mmx4.6
mm,5µM)
Isocratic elution,10mM
sodium butane
sulfonate,4mM
malonic acid,4mM
tetramethyl ammonium
hydroxide,0.1(v/v)%m
ethanol,20mM
ammonium
tartarate(pH=2.0)
mixed solution.
PH=2
F.R=0.75ml/
min
Column
temperature
is same as
room
temperature.
5.2, As(V)
5.5, As(III)
5.75,
MMA(V)
7.0, DMA(V)
8.5, AsB
9.7, TMAO
10.4, TeMA
10.8, AsC
[53]
C.T=column temperature MMA(V) = Mononethylarsonic acid
F.R= Flow rate DMA(V) = Dimethylarsinic acid
HPLC= High performance liquid chromatography AsB = arsenobetaine
ICP-MS=Inductively coupled plasma mass spectrometry AsC = Arsenocholine
As(III)=Arsenic(III) TMAO = Trimethylarsenic oxide
As(V)=Arsenic(V) TeMA = Tetra methylarsonium ion
Table 3 – summary for the work done on speciation of inorganic arsenic species.
Type of
sample
containers
used
Sample
storage
Sample preparation Type of
matrix
Analytica
l method
Detection limit Refe
renc
e As(III)
(µgL-1
)
As(V)
(µgL-1
)
Polyethylene
bottles
<40C Manually shaken,
centrifuged (10,000rpm
for 10 minutes) and
filtered through 0.45µm
filter
Water from
industrial
shale
processing
LC-ICP-
MS
0.02 0.1 [54]
Polyethylene
flasks
40C - Well Water LC-ICP-
MS
0.017 0.026 [44]
Opaque, clean,
high density
propylene
bottle
EDTA - Drinking
water
LC-ICP-
MS
0.067 0.089 [55]
Polyethylene
flasks
40C Filtered through
0.45µm membrane
Filter
Surface
water
LC-ICP-
MS
0.046 0.03 [47]
Polypropylene -200C Filtered through Drinking LC-ICP- 0.09 0.3 [48]
14
bottles 0.45µm filter water MS
Polypropylene
bottles
Stored
in
darkne
ss at
40C
- Sea water LC-ICP-
MS
[56]
For pure
water
0.015 0.057
For 2%
Cl-
solution
0.057 0.116
LC-ICP-
ORS-MS
For pure
water
0.025 0.020
For 2%
Cl-
solution
0.022 0.021
LC-HG-
ICP-MS
For pure
water
0.0028 0.0045
For 2%
Cl-
solution
0.0034 0.0042
LC-ICP-MS=Liquid chromatography inductively coupled plasma mass spectrometry
LC-ICP-ORS-MS=Liquid chromatography inductively coupled plasma mass spectrometry using reaction
cell
LC-HG-ICP-MS=Liquid chromatography inductively coupled plasma mass spectrometry using hydride
EDTA=Ethylene diamine tetra acetic acid
Table 4. Some studies showing stability durations of inorganic arsenic species [As(III) and As(V)]
No Observations made Reference cited
1 No change of As(III) and As(V)was observed in pure water for 24 hours both
in light and dark conditions(no additives were used)
As(III) and As(V) species up to five days were preserved by the addition of
EDTA to experimental solutions(both in light and dark conditions)
[57]
2 At 40C, 20
0C, 40
0C with PH=1.6 and 7.3, As(V) was stable for at least 4
months in waste water.
4 months stability period was attained for As(III) in treated waste water at
PH=7.3 However, at PH=1.6,complete conversion of As(III) to As(V) was
noticed within two months at 400C.
At PH=7.6,complete transformation of As(III) to As(V) was achieved in raw
waste waters within 50 days ,even at 40C
[58]
3 As(V) in surface waters did not show show alternations during 15 day storage
at 40C.
[59]
4 At room temperature, reduction of As(V) to As(III) within few days was [60]
15
observed in Otawa river and deionized water samples spiked with As(III) and
As(V)[0.5-20µg/L]
A natural water sample filtered with 0.45µm and a total concentration of 21
µg/L, stored in a filled bottle at about 50C preserved As(III) and As(V)
concentration for about 30 days.
5 Slow transformation of As(III) to As(V) was noticed during the 8 day
observation period in spiked natural water samples with ascorbic acid.
As(V) was reduced to As(III) within 3 days in rain water in the presence of
ascorbic acid.
[61]
6 Less than 1 µg/L change in As(III) in 16 days was noticed in reagent water
containing As(III),Fe(III) and EDTA.
Capability of EDTA for preserving original As(III)+ As(V) in three well
waters for ten days has been noted.However,2-3 µg/L of As(III) changed to
As(V) after 14-27 days.
[62]
7 Stability period of 1 week for As(III) with the addition of HCl (PH<2) and
excess Fe(III)) to groundwater samples.
[25]
8 At 40C, Samples containing As(III) and As(V) at concentration levels of 0.5
µg/ml or 1 µg/ attained stability for 21 days. However, some alternations were
noted after 29 days of storage.
[63]
9 Different tests on aqueous mixtures at -200C, +3
0C and +20
0C were conducted
for storing arsenic species. Excellent storage temperature was +30C but species
transformations occurred at -200C.
[64]
6. Conclusion Speciation of arsenic in water is vital to assess toxicity, transport and bioavailability. The type of
analytical method used and the knowledge of the different processes that affect inter-conversion of
arsenic species is important to reach to a representative data. Depending on the type of the sample matrix,
preservation by EDTA and acids help in As(III)/As(v) stabilization. Furthermore, filtration, optimum
storage conditions and suitable sample containers play significant role in maintaining integrity of arsenic
species. HPLC-ICP-MS is the most commonly used hyphenated system for speciation analysis of arsenic.
HPLC is a versatile separation technique since it uses different types of stationary phases (Normal,
reversed) and various mobile phases. Separation can also be enhanced by adding ion pair reagents to the
mobile phase. Changing mobile phase during analysis (gradient elution) is also another merit of liquid
chromatography. ICP-MS can serve as an element specific detector and offers better sensitivity. During
speciation analysis of arsenic using ICP-MS, polyatomic interferences like 40
Ar35
Cl+
(the same mass as 75
As) occur due to the presence of chloride in the water samples and the gaseous argon plasma. To
overcome this problem, reaction cells, mathematical equations or high resolution ICP-Ms can be applied.
16
7. Acknowledgement
I would like to thank my supervisor, Dr. Solomon Tesfalidet for providing me valuable
comments, good instruction, proper guidance and encouragement. Moreover I would like to
extend my gratitude to my examiner, Dr. Erik Björn for his constructive comments.
17
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9. Appendix