CHAPTER 12
CHEMICAL VAPOR GENERATION WITH SLURRY SAMPLING:
A REVIEW OF ATOMIC ABSORPTION APPLICATIONS
Henryk Matusiewicz
Department of Analytical Chemistry,Poznan University of Technology,
00-965 Poznań, Poland
ABSTRACT
This review summarizes and discusses the analytical methods and techniques described
in the literature for hydride generation as a mode of slurry sample introduction into
atomization cells. A brief comparison of detection limits for analytical atomic
absorption methods and practical applications to analytical samples that utilize slurry
hydride generation is discussed. The current state-of-the-art, including advantages and
limitations of this approach is discussed.
Chapter 12
1 INTRODUCTION
Chemical vapor generation combined with atomic absorption spectrometric detection in
the forms of cold vapor generation (CVAAS) for the determination of Hg and Cd and
hydride generation (HGAAS) for elements forming gaseous covalent hydrides (As, Bi,
Ge, Sb, Se, Sn, Te and even In, Pb and Tl) has become one of the most powerful
analytical tools for the determination of these elements. The generation of gaseous
analytes and their introduction into atomization cells offers several significant
advantages over conventional solution phase pneumatic nebulization of samples. These
include enhancement of analyte transport efficiency (approaching 100 %), elimination
of the need for a nebulizer/spray chamber assembly, higher selectivity due to a
significant reduction of interferences, better detection limits (at the µg l-1 level or
lower), automation of methods and possibility of speciation studies and coupling with
different techniques. The continuous interest in this technique is reflected in the number
of recent reviews [1-4], a new book [5] and a book chapters [6,7]. However, as a rule,
CVAAS and HGAAS requires complete decomposition and/or dissolution of the
samples prior to analysis, and the quantitative production of a single labile analyte
species, which increases both analysis time and the risk of sample contamination and/or
losses of the analyte. In addition, the problem with using a large amount of reagents
during pretreatment leads to increased blank values and higher detection limits.
Pretreatment, by avoiding sample digestion by wet or dry oxidation methods, of solid
samples by slurrying in liquid medium overcomes these problems and has the
advantages of rapid analysis, reduction of blank levels and risk of analyte losses.
In recent years, the slurry sampling approach coupled with AAS techniques with
electrothermal and flame atomization has been extensively employed for the analysis of
analytical samples, in order to simplify sample preparation procedures and to avoid
some inconveniences related to wet digestion and dry ashing methods. Introduction of
slurry samples combines the advantages of direct solid sampling (reduction of sample
preparation time; in sample contamination; decrease in analyte losses through
volatilization prior to analysis and/or associated with retention by insoluble residues)
and liquid sampling (sampling dispensing by using a conventional liquid sample
handling approaches; straightforward automation; flexibility in slurry preparation and
the advantage that slurries may be prepared in advance). The extensive reviews
published in the last decade [8-12], has confirmed the great applicability of this
approach. The method of CV/HG-AAS directly from slurried samples has recently been
proposed as an attractive alternative to avoid the need for intensive treatments (e.g., acid
digestion). This analytical technique is based on efficient extraction of the analyte into
the liquid phase. It is difficult to understand how the reducing agent, which is inside the
solid particles, can act on the analyte. A possible explanation for this fact could be that
analyte adsorbed onto solids can also take part in the hydride generation reaction.
However, no review has dealt with the use of slurried samples coupled with HGAAS
and CVAAS.
This review is an attempt to fill this void and will consider the historical
development of direct hydride generation of vapors from slurried samples coupled with
batch and flow injection formats, and detection via AAS. Current state-of-the-art,
including basic properties, advantages and limitations of this approach will be
discussed.
203
Chapter 12
2 CHEMICAL VAPOR GENERATION WITH SLURRY SAMPLING
Analyses involving the formation of a slurry (i.e., a solid suspension in a liquid
medium) are much simpler than direct analyses on a solid. This method has been used
for the determination of a variety of elements by flame and electrothermal AAS. Plasma
emission analyses on slurries have reached the same extent of development. Slurries
also allow the application of the hydride generation technique which offers the
advantage of simpler and faster sample pretreatment than conventional methods, plus a
detection technique (hydride generation) that is selective and sensitive enough to
determine low analyte levels in complex matrices and is easy and cheap to operate. This
combined system is a novelty as it has been used for this purpose in a several cases.
2.1 Hydride Generation Atomic Absorption Spectrometry (HGAAS)
The concept of a method of hydride generation directly from slurried samples with
subsequent atomization was first reported by Haswell et al. [13]. This approach has
been applied for the determination of arsenic in complex matrices such as solid
environmental samples. Samples were weighed directly into the glass hydride
generation vessel and 10 ml of 4 M HCl added. Arsine was generated by adding 1%
(m/v) sodium tetrahydroborate (III). The influence of particle size, homogeneity and
matrix on the reproducibility and amount of analyte released was examined. The
influence of interfering elements in the sample matrix has also been examined. The
technique gave similar results to those obtained by the hot acid extraction with aqua
regia which commonly requires lengthy handling procedure. The method developed
shows promises as a rapid screening procedure for a range of environmental sample
types (soil, sewage sludge, coal fly ash, incinerator ash) being able to achieve
acceptable accuracy together with precision at the order of 7% RSD.
Subsequent to the above work, a major series of studies based on that concept
have been carried out by Cámara and co-workers [14-18]. Madrid et al. [14] reported a
simple and rapid method for the determination of lead in foodstuffs and biological
samples that combines a slurry procedure with lead hydride generation. Powdered
samples were suspended in Triton X-100 as dispersing agent and shaken with 10 g of
blown zirconia spheres until the slurry was formed. A few drops of silicone antifoaming
agent were added before the slurry was diluted to minimize the foam formed on addition
of NaBH4 to the viscous slurry. This grinding procedure ensured that 90% of the
particles had a diameter of less than 25 µm, a particle size small enough to permit lead
determination by HGAAS. Three oxidant media, namely H2O2-HNO3, K2Cr2O7-lactic
acid and (NH4)2S2O8-HNO3, were evaluated for the generation of lead from slurry
samples and their application to the determination of lead in vegetables and fish by
HGAAS was investigated [15-17]. HNO3-H2O2 medium was unsuitable for the
generation of lead hydride from slurry samples because of decomposition of the
hydrogen peroxide by the organic matrix. Further, the low sensitivity provided by this
medium made it necessary to increase the concentration of the powdered sample in the
slurry, resulting in higher errors owing to sampling difficulties and increased matrix
effects. (NH4)2S2O8-HNO3 gave reliable results for the determination of lead in
vegetables but only semi-quantitative results with fish slurry samples. K2Cr2O7-lactic
acid provided the best results for the determination of lead in slurried vegetable and fish
samples, and also resulted in lower detection limits owing to its high sensitivity and low
blanks. When mussel was analyzed, however, this medium gave lower results than the
(NH4)2S2O8-HNO3 and wet digestion procedures, perhaps because potassium
dichromate was unable to remove the lead completely from this sample, unlike
ammonium peroxodisulfate, which is a sufficiently strong oxidant to eliminate lead
204
Chapter 12
completely. A simple and rapid flow injection-lead hydride generation atomic
absorption spectrometry (FI-HG-AAS) method was optimized for the determination of
lead in fruit [18]. The lead hydride was generated from slurries of the fresh sample. No
matrix effect was found in the determination of lead. The method enabled the direct
determination of lead in untreated samples with use of an aqueous calibration graph.
Procedure based on the use of slurries on lead hydride generation for determination of
lead in commercial iron oxide pigments has been evaluated [19]. No pretreatment was
required. The samples were suspended in water containing 0.01% hexametaphosphate,
and lead hydride was generated from a 0.7 M HNO3 acid and 14% ammonium
peroxodisulfate medium by addition of 10% tetrahydroborate solution. In this way, an
improvement in reproducibility and sensitivity as well as a saving of time and effort was
achieved. Therefore, the slurry hydride generation approach can be of use for routine
quality-control analyses. de la Calle Guntiñas et al. [20] have also utilized the slurry
hydride generation approach. A method was described for the determination of total
antimony, extractable into 4 M HCl, which combines a slurry procedure with HGAAS.
Further, when slurry is prepared with HCl instead of water, a recovery of 100% of the
total Sb is found in the supernatant of the antimony is adsorbed on the surface of the
sediment. This suggest that antimony can be removed from the sample with minimum
sample pretreatment. The lack of a matrix effect allows the analytical results to be
obtained with a simple aqueous working curve. The proposed method might be applied
to the determination of antimony in complex samples such as biological materials and
foodstuffs, which would be of interest if there were contamination problems.
Nerín et al. [21] and López García et al. [22] extended the work of Cámara et al.
[14-18]. For the determination of As, analyte introduction was accomplished with the
generation of arsenic hydride on a fly ash slurry from a thermal power plant burning
lignite [21]. The most critical variable of the method is the particle size, as it has been
shown in the article. For the method to be successful the particle size of the ash must be
below 8.5 µm. In the next article [22], the conditions for the determination of As and Hg
in coal fly ash and diatomaceous earth samples using the vapor generation-slurry
methodology are discussed. Data for the fraction of the analyte extracted into the
supernatant as a consequence of the slurry preparation are reported. Calibration is
performed using aqueous standards. The results agree well with those obtained with
procedures based on whole dissolution of the samples. Mierzwa et al. [23,24] have
provided a further examples of the generation of arsine directly from a slurry samples.
A batch procedure for the determination of As in cigarette tobacco in which a slurry of
the sample was injected into the reactor has been developed [23]. Slurries were prepared
with the aid of an ultrasonic bath and a microwave oven. Pre-treatment of samples
slurried in nitric acid by ultrasonication permitted the extraction of about 90% of the
total arsenic from tobacco samples. Further improvement in the recovery efficiency (up
to 93-94%) was accomplished by the use of an additional step of short microwaveaccelerated treatment. Ŀ-Cysteine was used as a pre-reduction agent. The results
suggested that all of the analyte was extracted into the solution phase prior to injection.
The main factors that influenced the reliability of the method were sample homogeneity,
particle size and slurry concentration. A slurry sampling procedure has been used [24]
for the analysis of sediments. The slurry was first treated by microwave heating (total
time 2 min) and then sonicated (12 min). Brindle’s reagent (Ŀ-Cysteine) was added to
reduce As(V) to As(III), followed by Triton X-100. Copper, Fe and Ni did not interfere
at concentrations up to 10 mg l-1. Recently, a flow injection procedure has been
developed for the determination of acid-extractable arsenic in soils by HGAAS [25].
Several parameters, including acid and borohydride concentrations, exposure time to
205
Chapter 12
microwave energy, and the microwave power applied, were optimized. The on-line
microwave extraction increased the recovery of the adsorbed arsenic significantly;
whereas, preparation of the slurry in 10% HCl instead of water increased the recovery
only when the microwave oven was off.
Another technique, which has been successfully used for increasing the
sensitivity of vapor generation systems is the use of an in situ preconcentration
(trapping) technique [2], which couples a hydride generation system with the graphite
furnace. The subsequent atomization procedure is the same as in conventional
electrothermal atomization systems [26-29]. The most attractive advantages of the in
situ trapping HGAAS technique are the very low detection limits and the decrease of the
kinetics interferences in HG and interferences in the atomizer. Moreda-Piñeiro et al.
[26] have used Ir-treated graphite tubes for pre-concentration and atomization of the As,
Bi and Se hydrides generated from acidified slurries of marine sediment, soil and coal
samples. A batch mode generation system was used for the hydride generation. The
variables affecting the acidified slurry preparation procedure (assisted by ultrasonic
energy) and the hydride generation/trapping/atomization processes were studied by
using a Plackett-Burman design. Subsequent work [27] reported different procedures of
tin hydride generation from aqueous and acidified slurries of marine sediment, soil, coal
fly ash and coal samples, coupled to ETAAS and were optimized by using factorial
design. A batch mode generation system and Ir-treated graphite tubes were used for the
hydride generation and atomization, respectively. Eight variables, affecting the hydride
generation and hydride transport efficiency (HCl and NaBH4 concentrations, particle
size, acid volume and argon flow), the hydride trapping efficiency (trapping temperature
and trapping time) and the atomization efficiency (atomization temperature) were
studied and optimized. In addition, acid pre-treatment procedures assisted by ultrasonic
energy were used for soil and coal matrices, to obtain acidified slurries and acid
leachates. The involved variables were hydrochloric and nitric acid concentrations,
exposure time to ultrasound, particle size and leaching solution volume. By using
acidified slurries, the hydride generation occurs from the acid liquid phase and also
from solid particles. Thus, the use of acid increases the hydride generation efficiency
from solid particles. Finally, a mean particle size lower than 50 µm is small enough to
achieve adequate tin hydride generation efficiency from aqueous slurry samples. Very
recently, Vieira et al. [28] also determined As in sediments, coal and fly ash by ETAAS
after collecting the arsine, generated directly from the sample slurries by hydride
generation, in an Ir-coated graphite tube, using the addition calibration technique.
Matusiewicz and Mikołajczak [29] described a method for the HGAAS determination
of As, Sb, Se, Sn and Hg in untreated samples of wort using a batch system and in situ
preconcentration of the analytes onto the Pd- (for As, Sb, Se, Sn) or Au-pretreated (for
Hg) interior was surfaces of a graphite furnace. Determination of the total concentration
of these elements was obtained after a previous reduction with thiourea. The accuracy of
the method was confirmed by comparing the results obtained with those found for wort
using microwave-assisted digestion and by analyzing five certified reference materials.
Calibration was achieved via the method of standard additions.
2.2 Cold Vapor Atomic Absorption Spectrometry (CVAAS)
A number of papers appear recently devoted to the determination of As, Bi, Cd, Ge, Hg
and Se by cold vapor generation techniques [30-35]. Slurry procedures avoid the
problems of time consuming digestion procedures and the accompanying risks of
contamination or analyte loss. This technique has been applied successfully to the
determination of Hg by CVAAS in iron oxide-titanium oxide pigments [30]. The
206
Chapter 12
samples were suspended in water containing 0.02% (m/v) sodium hexametaphosphate
and Hg generated from an HCl medium with NaBH4. Calibration was with aqueous
solutions of Hg and there was excellent agreement between results from slurry and acid
digestion procedures. Río-Segade and Bendicho [31] reported a method for the
determination of total mercury in solid biological and environmental samples by slurry
sampling combined with flow injection-cold vapor-atomic absorption spectrometry (FICV-AAS). Aqueous calibration was not considered adequate and calibration with the
standard addition method was needed. Analytical results for total Hg determination in
real samples of mussel tissue were in good agreement with those obtained by FI-CVAAS after microwave-assisted digestion of samples. Ultrasonic pretreatment of slurries
was proved to be necessary in order to improve the low mercury recoveries obtained.
The ultrasonic pretreatment step caused an increase in the fraction of analyte extracted
into the liquid phase of the slurry, thus facilitating the reduction to elemental mercury.
A method for the analysis of solids based on slurry formation with CV chemical
vapor generation has been developed [32] in which the detection was also by AAS.
Samples (sewage sludge, city waste incineration, Antarctic krill and human hair) were
suspended in HCl and sonicated to reduce particle size and provide a homogeneous
slurry. Potassium cyanide was added to overcome the interferences from Cu, Pb, Ni and
Zn. Complete leaching from the environmental samples was obtained, but not from the
biological materials, which were analyzed by the standard additions method. The main
drawback of this technique is the non-homogeneity of the suspensions formed.
Flores et al. [33] developed a rugged and reliable method for the determination
of mercury in coal without sample digestion, based on chemical vapor generation (cold
vapor technique) from slurried coal samples. It involves collection of the mercury vapor
in a graphite tube, treated with gold or rhodium as “permanent modifier”, and
determination by ETAAS. Mercury quantitatively leached out of the investigated coal
reference materials into 1 M HNO3 within 48 h when the coal was ground to a particle
size of < 50 µm. No detectable quantity of mercury was generated directly from the
slurry particles, but it was not necessary to filter the solution.
Iridium is both a well-recognized “permanent modifier” in ETAAS and an
efficient collector for the trapping of volatile hydrides. Moreda-Piñeiro et al. [34,35]
reported on the direct CV and hydride generation procedures for As, Bi, Ge, Hg and
Se(IV) from aqueous slurries of environmental (marine sediment, soil, coal, coal fly
ash) and biological (human hair, seafood) samples by using a batch mode generation
system. Ir-treated graphite tubes have been used as a pre-concentration and atomization
medium of the vapors generated. A Plackett-Burman experimental design has been used
as a strategy for evaluation of the effects of several parameters affecting the vapor
generation efficiency from solid particles, vapor trapping and atomization efficiency
from Ir-treated graphite tubes. Optimum values of the parameters have been selected for
the development of direct cold vapor/hydride generation methods from slurry particles.
2.3 Chemical Speciation Analysis
Speciation of an element is the determination of the individual physico-chemical forms
of the element, which together makes up its total concentration in sample. The hydride
generation procedure coupled with AAS can afford several methods for inorganic
and/or organic speciation of some hydride forming elements. In practice, the use of
hydride generation for speciation analysis is dominant, when coupled to atomic
absorption spectrometric system as element specific detectors. HGAAS is perhaps the
most widely used technique for determination of volatile hydride forming elements.
207
Chapter 12
However, a modest amount of work has been published in this field, and there is still a
low level of activity related to the determination of total analyte concentration.
Speciation of Sb(III) and Sb(V) in marine sediment slurry was carried out by
determining total antimony and Sb(III) [20] by the method which combines a slurry
procedure with HGAAS. It appears that the most of the Sb(III) is released from the
sample during hydride generation and slurry formation is a promising method for
determining Sb(III) and Sb(V) selectively. de la Calle Guntiñas et al. [36] showed it was
possible to speciate Sb(III) and Sb(V) by slurry formation in soil and sediments.
Although oxidation states are unchanged in slurry formation with water and in 4 M HCl
it was shown that a low recovery is obtained. Clearly, however, there are some aspects
of the speciation of antimony.
Rondón et al. [37] devised a method for the determination of Sb(III) and Sb(V)
in liver tissue. For the determination of Sb(III) the slurried sample was dissolved with 1
M acetic acid, whereas for Sb(V) the carrier was a mixture of sulfuric acid, potassium
iodide and ascorbic acid. It was evident that the addition of organic acids leads to a
speciation.
A novel method for speciation of phenyl-mercury (PH-HG) in the biomass slurry
at trace levels, based on the retention of analyte by a living Escherichia coli strain, has
been developed [38]. The mercury vapor was generated from the Hg-biomass slurry by
treating it with Sn(II) or NaBH4 as a reducing agent and the amount of Hg was
determined by AAS. The use of living bacterial cells coupled with the specific CVAAS
detection provides a reliable procedure for characterizing mercury species based on the
differences in their relative sorption under non-equilibrium situations.
A slurry sampling hydride generation method for As(III) and total inorganic As,
without total sample digestion, has been developed using a continuous flow mode
generation system coupled with AAS determination from environmental and biological
samples [39]. It involves trapping of the arsenic vapor in a pre-heated Ir-coated graphite
tube, and determination by ETAAS. Pretreatment of samples (slurried in HCl with
addition of ozone) by ultrasonic agitation enabled the extraction of more than 95%
confidence level of the total arsenic from reference materials investigated. For the
estimation of As(III) and As(V) concentrations in samples, the difference between the
analytical sensitivities of the absorbance signals obtained for arsenic hydride, without
and with previous treatment of samples with thiourea, can be used. The concentration of
arsenate (As(V)) was calculated by the difference between total As and As(III).
Calibration was achieved via the technique of standard additions.
Selenium determination by HG techniques requires its presence as Se(IV).
Consequently, inorganic speciation by hydride generation techniques is done by first
determining Se(IV) and then, after reduction of Se(VI) to Se(IV), the total selenium
(40). For real samples (garlic, sediment) selenium was determined from slurry (without
any pretreatment). It was demonstrated, that dimethylselenium and dimethyldiselenium
(organic selenium species) are forming other volatile species by reaction with NaBH4,
applying the same reduction conditions as for inorganic selenium. These species can be
subsequently detected by AAS. The error that their presence can cause in determination
of inorganic selenium has been evaluated.
3 SUMMARY OF INSTRUMENTATION
Table 1 summarizes the instrumentation and methodology reported for the slurry
sampling hydride generation for atomic absorption spectrometry.
208
TABLE 1
Operating parameters for slurry sampling-chemical vapor generation-AAS studies
Atomic
absorption
Chemical
Atomizer
spectrometer
Thermo
Electron 951
Air-
Hydride generation
Temperature/oC
Slurry sample preparation
modifier
Element
System
Reaction
Size/ml
AVA 440
1% NaBH4,
10
acetylene
Dispersant
Homogenization
Particle
size/µm
Magnetic stirrer
<90
Trapping
Detection
Ref.
mode
Atomization
As
HGAAS
[13]
Pb
HGAAS
[14]
4M HCl
flame
Perkin-Elmer
Laboratory
8% NaBH4,
2380
constructed
10% HNO3,
5
1% Triton
Mechanical
X-100
shaker, zirconia
209
10%
spheres
ammonium
persulphate
Perkin-Elmer
MHS-10
4% NaBH4,
5
2M HCl
300
0.02%
Ball mill
<45
Hg
CVAAS
[30]
Ultrasonic bath
<4
Sb
HGAAS
[20]
sodium
hexametap
hosphate
Perkin-Elmer
Laboratory
3% NaBH4,
2380
constructed
4M HCl,
3
1% Triton
X-100
2% KI
209
Perkin-Elmer
370A
Air-
MHS-10
acetylene
3% NaBH4,
1
Vibrating,
<8.5
As
HGAAS
[21]
Ball mill
<400 mesh
Pb
HGAAS
[19]
1% Triton
Mechanical
<25
Pb
FI-HGAAS
[18]
X-100
shaker, zirconia
Sb
HGAAS
[37]
As
HGAAS
[23]
0.4 M HCl
stirring
flame
Perkin-Elmer
300
Flame-
MHS-10
10%
heated silica
NaBH4, 0.7
tube
M HNO3,
1
14%
ammonium
peroxodisul
phate
Perkin-Elmer
210
2380
VarianTechtron AA1475
Air-
FI manifold
6% NaBH4,
acetylene
15% H2O2,
flame
40% HNO3
Air-
VGA-70
0.1%
acetylene
NaBH4, 0.5
flame
M H2SO4,
0.1
spheres
10
Ultrasonic
10% KI
GBC 902
Electrically
Laboratory
1.2%
heated quartz
constructed
T-tube
1
0.005%
Ultrasonic bath,
NaBH4, 6.5
Triton X-
vortex mixing
M HNO3
100
210
<60
Perkin-Elmer
Non-heated
2380
quartz cell
FI system
0.5%
0.02%
NaBH4,
Triton X-
15% HNO3,
100
Ultrasonic
<100
Hg
FI-CVAAS
[31]
Grinding
<212
As
HGAAS
[25]
Cd
FI-CVAAS
[32]
As,Sb,S
HGAASa
[29]
15% HCl
Perkin-Elmer
3100
Perkin-Elmer
Air-
Peristaltic
0.6%
0.2
0.005%
acetylene
pump
NaBH4,
Triton X-
flame
10% HCl
100
T-quartz cell
4% NaBH4,
0.5
Ultrasonic bath,
1 M HCl,
3100
magnetic stirrer
0.5% KCN
211
Analytik
THGA
AAS5EA
graphite tube
Pd, Au
Laboratory
1% NaBH4,
constructed
HCl,
10
50 µl
Ultrasonic
110-400
1300-2600
decanol
e,Sn,Hg
thiourea
Analytik
THGA
AAS5HydrEA
graphite tube
Perkin-Elmer
Graphite
Aanalyst 800
tube
Au, Rh
HS5
3% NaBH4,
1
Manually
MSH-10
2% NaBH4,
5
1 M HCl, 5
0.02%
Ball mill,
glycerol
zirconia balls,
M HNO3
Perkin-Elmer
Graphite
Aanalyst 800
tube
Ir
MSH-10
1% NaBH4,
6 M HCl
900-1500
1000-2200
Hg
CVAASa
[33]
<50
200-1000
2000-2500
As,Bi,Se
HGAASa
[26]
<10
75
2600
Hg
HGAASa
[35]
shaking
1 M HNO3
Ir
<50(30)
ultrasonic bath
2(0.1)
0.02%
Ball mill,
glycerol
zirconia balls,
ultrasonic bath
211
Perkin-Elmer
Graphite
Aanalyst 800
tube
Ir
MSH-10
1% NaBH4,
0.02-1
6 M HCl
0.02%
Ball mill,
glycerol
zirconia balls,
<10
100-1000
2000-2600
As,Bi,G
CVAASa
[34]
HGAASa
[39]
e,Hg,Se
ultrasonic bath
Analytik
THGA
AAS5EA
graphite tube
Ir
HS5
3% NaBH4,
2% HCl
10
0.1%
Ultrasonic
Triton X100
a
In situ trapping.
212
212
<20
300
2150
As
Chapter 12
4 ANALYTICAL FIGURES OF MERIT
The analytical performance of slurry sampling hydride generation – atomic absorption
spectrometry is characterized by figures of merit, such as detection limit and
quantification limit, linear dynamic range, and precision and accuracy of measurements.
It is usual practice to quote the detection limit pertaining to a particular technique or
method, and to draw comparisons between the detection limits obtained using similar
techniques. The detection limits for slurry sample introduction – hydride generation –
atomic absorption spectrometric technique is summarized in Table 2. This approach was
adopted because the range of reported values is a reflection of differences in
instrumental applications and a variability in the slurry sampling – hydride generation
spectrometric technique capability. Detection limits are presented in terms of both mass
and concentration or characteristic mass to simplify comparison. The compiled data
refer mainly to the determination of the elements in slurry solutions containing the
analyte in question. Because of this simplification, any application of the data to
practical trace analysis must be subject to some restrictions. The limit of detection is
only one of several figures of merit characterizing a technique and should not be used
alone as a criterion of choice. Nevertheless, the data compiled here may be useful as an
initial survey of the effectiveness of slurry sampling hydride generation AAS technique
with respect to the determination of trace levels of these analytes. In addition, because
of the wide range of slurry sampling hydride generation technique and atomizers and
the resulting differences in optimized experimental conditions, it is very difficult to
accurately compare published data with regard to analytical performance of the slurry
sampling – hydride generation – AAS. Moreover, there is much confusion over the
definition of the term “detection limit”, so users of such data as shown in Table 2 and in
the literature should always check the definition applied in the original papers. Some
relative standard deviation (RSD) values reported for the analysis of analytical samples
are summarized in Tables 3 and 4.
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Chapter 12
TABLE 2
Analytical characteristics for volatile hydride-forming element determination by slurry sampling-AAS
Element/nm
214
As
As 193.7
As 193.7
As 193.7
As 193.7
As 193.7
As 193.7
As 193.7
As 193.7
Bi 223.1
Bi 223.1
Cd 228.8
Ge 265.1
Hg 253.6
Hg 253.7
Hg 253.7
Hg 253.7
Hg 253.7
Hg 253.7
Hg 253.7
Hg 253.6
Pb 217.0
Pb 217.0
Pb 217.0
Pb 283.3
Sb 217.6
Sb 217.6
Sb 217.6
Sb(III) 217.6
Sb(III) 217.6
Sb(III) 217.6
Se 196.0
Se 196.0
Se(IV) 196.0
Sn 286.3
Sn 286.3
a
Limit of detection (LOD)a
Detection
Mode
HGAAS
HGAAS
HGAAS
HGAAS
HGAAS
HGAASb
HGAASb
CVAASb
HGAASb
HGAASb
CVAASb
FI-CVAAS
CVAASb
HGAAS
CVAAS
CVAASb
CVAASb
CVAASb
FI-CVAAS
HGAAS
HGAASb
HGAAS
HGAAS
FI-HGAAS
HGAAS
HGAAS
HGAASb
HGAAS
HGAAS
HGAAS
HGAAS
HGAASb
HGAASb
CVAASb
HGAASb
HGAASb
ng/l
200
ng/g
ng
2.8 (2σ)
2.6 (3σ)
2.75 (3σ)
7
108-1800 (3σ)
540-700 (3σ)
28 (3σ)
11.5 (3σ)
1.2 (3σ)
40-100 (3σ)
1.5
48 (3σ)
50-200
600 (3σ)
10 (3σ)
5 (2σ)
9 (3σ)
Limit of quantification (LOQ)a
Characteristic
ng/g
mass/pg
940
360-6000 (10σ)
65
93 (10σ)
40 ng/l (10σ)
4.8 (10σ)
140-340 (10σ)
160 ng/l (10σ)
125-600
2000 ng/l (10σ)
28
60
15
75
80
220
180 ng/l (10σ)
70-570 (10σ)
110
370
390
0.09
55 (3σ)
21-170 (3σ)
3-28
50 (3σ)
90 (3σ)
320 (10σ)
120-2100
40-700 (3σ)
40-100 (3σ)
1 (3σ)
200 (3σ)
8.35 (3σ)
3.2
600 (10σ)
21 (3σ)
100 (3σ)
92
440
2000
69 (10σ)
21
3
70-170 (10σ)
40 (10σ)
40 ng/l (10σ)
120-267 (10σ)
180 (10σ)
100
13
110
160
50
13.3 (3σ)
150
2.97
20-50 (3σ)
10 (3σ)
11 (3σ)
36-80 (3σ)
50 (3σ)
Detection and quantification limits were calculated according to IUPAC rules, based on a 3σblank criterion.bIn situ trapping.
214
Ref.
[21]
[23]
[24]
[25]
[26]
[28]
[29]
[34]
[39]
[26]
[34]
[32]
[34]
[22]
[30]
[33]
[34]
[35]
[31]
[38]
[29]
[15]
[16]
[18]
[19]
[20]
[29]
[37]
[20]
[37]
[36]
[26]
[29]
[34]
[27]
[29]
TABLE 3
Applications of slurry sampling hydride generation atomic absorption spectrometric techniques to the analysis of environmental and mineral
substances and technical products
Detection
Amount
Matrix
mode
of samplea/g
Soil BCR 142, sewage sludge BCR 144, fly ash BCR 38,
HGAAS
>0.025
CVAAS
1-5
Element
As
Slurry-hydride generation approach
Ref.
Cold acid soluble technique, calibration with aqueous standards, 7% RSD
[13]
Samples suspended in water containing 0.02% sodium hexametaphosphate (HMP)and
[30]
waste incineration ash BCR 176
Hg
Iron(III) oxide and titanium oxide pigments
mercury vapor generated from HCl medium by adding NaBH4, 1-7% RSD
Sb
Marine sediments, soils
HGAAS
3
Slurry samples prepared by sonication. Antimony hydride was generated from HCl and KI
[20]
medium by adding NaBH4. Calibration by standard addition method
215
As
Fly ash
HGAAS
0.01
Slurry samples prepared by stirring for about 1 min. Aliquot (1 ml) of the fly ash slurry was
[21]
placed in a reaction vessel (0.4 M HCl, 3% NaBH4) to yield the As hydride. Calibration by
standard addition method. Method validated against BCR 38 fly ash, ca. 5% RSD
Sb
Sediments, soils
HGAAS
0.5
Slurries prepared by mixing 0.5 g of sample and 5 ml of 1% Triton X-100 and drops of
[36]
silicone, diluted to 25 ml with water or 4 M HCl. Antimony hydride was generated by
adding NaBH4, ca. 5% RSD
Pb
Iron oxide pigments
HGAAS
1
Slurries prepared by suspending 1g of sample in 50 ml of water containing 0.01% HMP.
Lead hydride was generated from 0.7 M HNO3 by addition of 10% NaBH4. Calibration by
standard addition method, 3.2% RSD
219
[19]
As, Hg
Coal fly ash, diatomaceous earth
HGAAS
0.1-1.5
Slurries prepared from the unsieved ground samples, suspended in 3 M HCl (As) or 5 M
[22]
HCl (Hg). Suspensions were sonicated, NaBH4 solution was used as the reducing agent.
Calibration was performed using aqueous standards
As
Sediments
HGAAS
0.05-0.35
Slurries pretreated by ultrasonic agitation and microwave-assisted extraction (0.75 M HNO3,
[24]
0.04% L-cysteine, 0.005% Triton X-100) and vortexed prior to each measurement for about
30s. Arsenic hydride was generated by 1.25% NaBH4. Calibration technique based on the
aqueous standard solutions was applied, 8.5% RSD
Hg
Mussel tissues
FI-CVAAS
0.4
Slurries suspended in 15% HNO3 containing 0.02% Triton X-100 subjected to ultrasonic
[31]
pretreatment for 2 min. Mercury vapor was generated by adding 0.25% NaBH4. Calibration
with the standard addition method was needed. Method validated against CRMs BCR 278
mussel tissue, BCR 60 aquatic plant, BCR 320 river sediment, BCR 145R sewage sludge,
216
4-7% RSD
As
Soils
HGAAS
0.6
25 ml slurries prepared by stirring and microwave-assisted acid-extractable arsenic in
[25]
samples was determined by hydride generation (NaBH4). Calibration by using aqueous
standards, 5% RSD
Hg
Coal
CVAASb
0.02-0.1
Slurries manually shaken, the mercury vapor transported from the reaction flask to, and
[33]
collected in the pre-heated graphite tube with a permanent modifier for 60s. Method
validated against coal reference materials: NIST SRM 1630a, 1632b, SACCRM SARM 19,
20, BCR CRM 40, 180, 181.
As, Bi,
Se
Marine sediment, soil, coal
HGAASb
0.25
As, Bi and Se hydrides generated (batch mode) from acidified (HNO3, HCl) slurries. Vapors
transported to, and collected in the pre-heated graphite tube with a permanent modifier (Ir).
Standard additions method is required. Method validated against NRCC PACS-1 marine
sediment, GBW 07401 soil, NIST SRM 1632c coal, <10% RSD
219
[26]
Hg
Marine sediment, coal, human hair, seafood
CVAASb
0.25
Slurries magnetically stirred, the mercury vapor generated from aqueous slurry samples
[35]
(without acid-pretreatment) transported from the reaction flask to, and collected in the
graphite tube with a permanent modifier (Ir). Method validated against reference materials:
PACS-1 marine sediment, DORM-2 dogfish muscle, GBW-07401 soil, NIST-1632a coal,
BCR CRM-397 human hair, <15% RSD
Sn
Marine sediment, soil, coal fly ash, coal
HGAASb
0.25
Slurries magnetically stirred, the tin hydride generated (batch mode) from aqueous or
[27]
acidified slurries transported to, and collected in the pre-heated graphite tube with a
permanent modifier (Ir). Standard addition method is mandatory. Method validated against
reference materials: NRCC PACS-1 marine sediment, GBW-07401 soil, NIST-1633b coal
fly ash, NIST-1632c coal, <8% RSD
As, Bi,
Coal fly ash
CVAASb
0.25
Slurries magnetically stirred, hydrides generated (batch mode) from aqueous slurries
217
Ge, Hg,
transported to, and collected in the pre-heated graphite tube with a permanent modifier (Ir).
Se(IV)
Method validated against NIST SRM-1633a coal fly ash, <8% RSD
As
Sediments, coal, fly ash
HGAASb
0.05
Slurries mixed with aqua regia and HF by ultrasonication. Arsenic hydride was generated
by adding NaBH4 and arsine vapor was transported to, and collected in the pre-heated
graphite tube, treated with iridium. Method validated against reference materials: NRCC
MESS-2, PACS-2, HISS-1 marine sediments, NIST SRM 2704 Buffalo River Sediment,
SRM 1646a Estuarine sediment, SRM 1632b coal bituminous, SRM 1633b coal fly ash,
BCR 181 coking coal, BCR 180 gas coal, SARM 19 coal, SARM 20 coal. Calibration was
performed using aqueous standard solutions (containing the same acid concentrations as the
slurries), <3.5% RSD
a
b
Amount of sample refers to original test portion used in the analytical procedure.
In situ trapping.
219
[34]
[28]
Chapter 12
TABLE 4
Applications of slurry sampling hydride generation atomic absorption spectrometric techniques to the analysis of biological and foodstuff
materials
Element
As
Matrix
Cigarette tobaccos
Detection
Amount
mode
of samplea/g
HGAAS
0.05-0.25
Slurry-hydride generation approach
Slurries prepared by mixing of sample with 0.005% Triton X-100 and then sonicated and
Ref.
[23]
microwave treated. Arsenic determined by hydride generation (NaBH4). Method validated
against CRM CTA-OTL-1 oriental tobacco leaves. Calibration technique based on aqueous
standard solutions, <7.6% RSD
As
Wort, gel, waste water
HGAASb
10 ml
Slurry sample solutions were subjected to sonication-ozonation procedure. As hydrides
[39]
generated (continuous flow mode) from acidified slurries. Vapors transported to, and
218
collected in the pre-heated graphite tube with a permanent modifier (Ir). Method validated
against nine certified reference materials. Calibration by standard addition method, 7.8%
RSD
Cd
Sewage sludge, krill, human hair
FI-CVAAS
0.25-0.50
Ultrasonic slurry formation. Volatile Cd species formation (HCl and NaBH4) by adding 4%
[32]
NaBH4 (0.5% KCN). Method validated against reference materials: CRM BCR 176 waste
incineration ash, MURST-ISS-A2 Antarctic krill, CRM BCR 397 human hair. Application
of external calibration method, 6-12% RSD
Hg
Living bacterial cells
CVAAS
0.006
Mercury vapor generated from Hg-biomass slurry by treating it with Sn(II) or NaBH4 as a
reducing agent. Calibration was achieved by treating standards in the same way as samples,
2.2-5.3% RSD
218
[38]
Pb
Lettuce, mussel, tomato
HGAAS
0.25-1.0
Powdered samples suspended in Triton X-100. Lead hydride generation carried out in an
[14]
ammonium persulphate-nitric acid medium by adding 8% NaBH4. Method validated against
IAEA H-9 whole total diet. Calibration by standard addition method
Pb
Vegatables, fish
HGAAS
0.25-1.0
Powdered samples suspended in Triton X-100. Lead hydride generation carried out in an
potassium dichromate-lactic acid medium by adding 4% NaBH4. Method validated against
[15],[
16]
IAEA V10 hay, CRM BCR 281 ryegrass. Calibration by standard adiition method, 5.1%
RSD
Pb
Fruit
FI-HGAAS
1.0
Lead hydride generated in HNO3-H2O2 medium using NaBH4 as reducing agent from slurries
[18]
of fresh sample
Sb
Liver tissue
HGAAS
0.5
Slurry prepared by sonication and microwave-assisted acid-extractable antimony. The
[37]
stibine generated from slurry sample by adding 0.1% NaBH4. Calibration by standard
219
addition method, ca. 3% RSD
Se
Sediments, garlic
As,Sb,Se, Wort
Sn,Hg
HGAAS
HGAASb
10 ml
Selenium determined from slurry samples by hydride generation (HCl, NaBH4)
[40]
Wort slurry solutions were subjected to sonication by adding 50 µl of decanol. Hydride
[29]
forming elements determined by hydride generation (batch system) and in situ
preconcentration of the analytes onto the Pd-(for As,Sb,Se,Sn) or Au-pre-heated (for Hg) of
a graphite furnace. Method validated against reference materials: NRCC CASS-2 nearshore
seawater, NASS-2 open ocean seawater, TORT-1 lobster hepatopancreas, IAEA W4
simulated fresh water, Seronorm, urine. Calibration was achieved via the method of standard
addition, ca. 5% RSD
a
Amount of sample refers to original test portion used in the analytical procedure.
In situ trapping.
b
219
Chapter 12
Acceptable precisions in most instances, reported as percentage RSDs usually
range from 1% to slightly higher than 10%, with the most frequent value about 5%.
Thus, in general, slurry sampling hydride generating elements can be detected at
concentrations below 500 ng/g, and concentrations than are 10 or more times the
detection limit can be measured with precision less than 10% RSD.
The accuracy of the present method for the analysis of analytical samples has
been checked by different approaches. These include: recovery test and standard
addition, use of independent analytical methods of proved validity and verification of
the method by means of standards or certified reference materials (CRMs), the latter
two methods being mostly applied. In the specific case of biological and environmental
samples, a great variety of CRMs such as those issued by the NIST, BCR and IAEA are
available. In consequence, the accuracy of the present technique has been mainly
checked against these standards. From the survey of the literature it is evident that the
accuracy of the slurry sampling hydride generation technique compares favorable with
the accuracy of other techniques for these kind of materials.
Linear dynamic ranges for the slurry sampling – hydride generation – AAS vary
from two to four orders of magnitude, depending on the particular method used. Since
the figures of merit for solution nebulization are comparable to those for slurry
nebulization, and the operating procedure is simpler, slurry sample introduction
combined with hydride generation technique is the present “method of choice” for
sample introduction of the elements.
5 PRACTICAL APPLICATIONS
Illustrative applications of slurry sampling hydride generation in AAS have been
summarized in Tables 3 and 4. These applications are listed for sample type (matrix)
and elements determined, the analytical AAS instrumental mode, the methodological
approach are given. In addition, the tables include the reported standard deviation. The
RSD is only an informative value and does not differentiate between in-slurry and
between-slurry precision, because this is often not stated in the literature and because of
very variable numbers of individual measurements. The aim of this section is to
examine publications, not merely to present potential users with established methods,
but rather to point to the reasons why slurry sampling hydride generation AAS has been
used to solve particular problems and to stimulate further interest in its application. The
references cited may contain additional determinations, or trials, for a particular sample
type. A wide range of applications is clearly evident, showing that slurry-hydride
generation approach is applicable widely throughout biological, environmental and
foodstuff analysis. The variety and number of samples indicate that future studies
involving slurry sample introduction combined with hydride generation technique
would be readily applied to the analysis of more complex samples.
6 CONCLUSION AND FUTURE PROSPECTS
It is evident from this review that there is considerable research being done, in the last
decade, in the area of sampling of pretreated slurry combined with the hydride
generation AAS method. The most advantageous designs are probably those which
generation of vapor from slurried samples produces reliable analytical data and uses less
time for analysis since the full sample digestion (decomposition/dissolution) step is
avoided. Consequently, the sample pretreatment is reduced to a slurry preparation
procedure. Thus, this combined sample introduction method should complement
“conventional” hydride generation for pretreated (i.e., by hazardous acid mixtures) solid
materials. The technique, though being inexpensive and having considerable promise; is
220
Chapter 12
in the author’s opinion yet to be firmly established. The ultrasonication of the samples
was the most important pretreatment part, and the additional microwave-assisted
extraction step was useful for the further improving of analytes extraction efficiency.
Although sample homogeneity is a critical factor influencing precision, it is not a
determining factor in achieving accurate results. Reliable procedures for
homogenization such as magnetic and ultrasonic agitation, and vortex and gas mixing
are available. It would be fair to say that most of the fundamental parameters and
requirements of this slurry sampling hydride generation technique have been established
and virtually all of the work examined in this review mainly concerns applications
(reports are summarized in Tables 3 and 4).
Very few observations have been reported concerning the speciation of slurry
samples, thus making it difficult to draw any conclusion on this matter, although
encouraging preliminary results were obtained for the speciation of analytes in slurried
samples.
The present technique may be subject to a number of positive and/or negative
systematic errors, which depend on the element to be determined, the analytical AAS
instrumental technique, the matrix composition and other factors. However, as shown in
this review (Tables 3 and 4) there is tendency of using the method of standard additions
to eliminate some possible matrix effects and ensure accuracy of results. Nevertheless, it
appears from the survey of the literature that the slurry sampling hydride generation
introduction technique compares favorably with the accuracy of the other AAS methods
for the determination of trace elements in analytical samples.
The slurry analysis using hydride generation technique should encourage their
adoption and be consistently useful in AAS. Continued study and research into
improving the analytical performance such as detection limits, precision and accuracy is
required.
Further area of growing interest is speciation of elements to supplement the total
element figure. In order to obtain data relating to the speciation, in situ hydride
generation pre-concentration procedures is especially suitable for speciation work,
because this approach allows direct slurry sample analysis without sample preparation
(destroying the matrix), and thus also the original speciation of the analyte of interest.
Accordingly, it would be desirable that the slurry sampling hydride generation AAS
methodology is accepted both as a regular quality control technique and/or as a
screening approach in different processes (environmental, biological, foodstuff, etc.,
fields). In this respect, one can recall its reduced sample manipulation requirements, low
turnaround time and relative low cost of implementation. On the other hand, hydride
generation from slurried samples for analytical purposes deserves mention for, although
not yet combined with atomic emission spectrometric techniques (ICP, MIP, DCP),
implementation of this approach for hydride forming elements should be
straightforward.
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