Determination of 17 trace elements in coal and ash reference
materials by ICP-MS applied to milligram sample sizes
H. Lachas,a R. Richaud,a K. E. Jarvis,b A. A. Herod,*a D. R. Dugwella and R. Kandiyotia
a
Department of Chemical Engineering and Chemical Technology, Imperial College,
University of London, Prince Consort Road, London, UK SW7 2BY.
E-mail: a.herod@ic.ac.uk
b NERC ICP-MS Facility, T. H. Huxley School, Imperial College, University of London,
Silwood Park, Buckhurst Road, Ascot, Berkshire, UK SL5 7TE
Received 8th October 1998, Accepted 8th December 1998
This paper describes the evaluation of two digestion methods used to extract 17 elements (Be, V, Cr, Mn, Co, Ni,
Cu, Zn, Ga, As, Se, Mo, Cd, Sn, Sb, Ba and Pb) from coal and coal ash, obtained as standard reference materials.
An acid digestion method in open vessels using sulfuric, hydrofluoric, perchloric and nitric acid was compared
with a sealed microwave digestion method using nitric acid only. The microwave method cannot break down
silicates, which harbour many trace elements, but can extract As and Se quantitatively. Volatile elements such as
As and Se might be lost during the open vessel digestion; therefore, the closed vessel is the method of choice. The
effect of reducing the sample size from several hundred milligrams to amounts as small as 5 mg on the accuracy
of determinations was investigated. No significant differences were observed so long as the total dissolved solids
and the dilution factors of the final solutions remained constant.
1. Introduction
It is well known that coal contains most elements in the Periodic
Table.1 They can be classified into three groups, based on their
concentration: major elements (C, H, O and N with contents in
excess of 1% m/m or 10 000 ppm), minor elements (Al, Ca, Fe,
K, Mg, Na and Si with contents between 1 and 0.1% m/m) and
trace elements (mainly all the remaining elements with contents
below 0.1% m/m or 1000 ppm). These elements, bound in coal,
are mobilised during coal burning and may be released either
associated with particles or as vapours.2–7. The damaging
effects caused to gas turbine blades by the release of some of the
elements2,8 and the health hazard constituted by others9–11
necessitate the control of their emission during coal gasification
and combustion processes. From an environmental perspective,
trace elements have attracted more attention than the major and
minor constituents; within a context of progressively more
stringent regulation of pollutant discharges from coal fired
power plant, there is a need for information on the trace element
content of coal and coal derived residues. The US Clean Air Act
Amendments (CAAA) of 1990 identified 11 trace elements and
their compounds commonly found in coal as potentially
‘hazardous air pollutants’ (HAPs): Be, Cr, Mn, Co, Ni, As, Se,
Cd, Sb, Hg and Pb.2 The efficiency of gas cleaning devices
developed by the coal industry relies mainly on an evaluation of
their retention properties for a large number of elements.12 To
this end, seven additional elements (V, Cu, Zn, Ga, Mo, Sn and
Ba) have been added to the list of elements of prime concern.
Many analytical methods aim at measuring elements associated with coal and coal by-products at concentrations down to
the ultra-trace level. A detailed review of the methods used was
provided by Vandecasteele and Block.13 During the last decade,
analysing so many elements required a large range of analytical
techniques, including X-ray fluorescence (XRF) spectrometry,14 flame, electrothermal vaporisation and cold vapour
atomic absorption spectrometry (FAAS,15 ETVAAS and
CVAAS, respectively) and various combustion techniques
[electrothermal (graphite furnace) AAS15,16]. X-ray absorption
fine structure (XAFS)17 and neutron activation analysis
(NAA)18,19 have also been used. More recently, plasma based
instruments have been introduced with the extensive development of both inductively coupled plasma atomic emission
spectrometry (ICP-AES) and inductively coupled plasma mass
spectrometry (ICP-MS). Although ICP-AES is a rapid, multielement technique that exhibits few matrix or chemical
interferences,20 it lacks the sensitivity and often the selectivity
to determine accurately many of the less abundant but
environmentally more important elements in coal and its byproducts. In contrast, ICP-MS is extremely sensitive and the
efficient ionisation by means of a plasma coupled with the
sensitive detection of the mass spectrometer results, at its best,
in parts per trillion detection limits that are generally 2–3 orders
of magnitude lower than those in ICP-AES. Although analysis
of solid samples by ICP-MS is possible, information remains
mainly qualitative or at best semi-quantitative. The technique is
at its most sensitive when coupled with liquid sample
introduction by solution nebulisation.21
Samples are therefore usually dissolved into a liquid form.
Ashing is widely used to preconcentrate a coal sample in trace
elements before the dissolution step. Successive digestion steps
involving several mineral acids are necessary to dissolve and
destroy the minerals and carbonaceous materials and to extract
completely the whole range of elements contained in both the
organic and inorganic constituents of coal. However, concerns
have been raised regarding the loss of volatile elements when
coal or its by-products are subject to relatively high temperatures during the open vessel acid attack.22,23 Alternative
sample preparations such as microwave extraction23 or fusion
using an alkaline flux24 have therefore been suggested, but
methods based on fusion are often considered susceptible to
increased blank values due to contamination mainly from the
alkaline flux and also to losses due to volatilisation.16
Analyst, 1999, 124, 177–184
177
Consequently, the concentrations of trace elements contained in
coal and related materials have extensively been determined
using both open vessel acid digestion and closed vessel
microwave extraction.
In the various studies undertaken in our laboratories to try to
understand the partitioning of some trace elements during coal
combustion and gasification, small bench-scale reactors, described elsewhere (wire mesh reactor,25,26 hot rod27 and
fluidised bed28 rigs), are being used. Products formed during
coal processing under various conditions in these reactors
(ashes, tars and chars) are prepared in very small amounts,
sometimes as small as 1 mg. The small sample size available
required some novelty of approach as sample preparation for
ICP-MS normally involves relatively large samples, usually
between 200 and 500 mg. Following the preliminary experiments reported previously,29 this study involved the development of both extraction techniques toward the quantification of
up to 17 trace elements (Be, V, Cr, Mn, Co, Ni, Cu, Zn, Ga, As,
Se, Mo, Cd, Sn, Sb, Ba and Pb) from coal and ash standards
using sample amounts as small as 5 mg. Although the need to
control mercury emissions makes it an element of potential
interest for the analyst, it is known to be readily volatile and it
is lost from solution following any of the sample preparations;
it has therefore not been quantified by ICP-MS. An alternative
instrument based on an atomic absorption spectrometer specifically designed to determine Hg content without requiring any
sample preparation has been successfully used. Details of the
technique and results obtained have been described.30 Results
obtained by open vessel acid digestion and HNO3 leaching in a
microwave, prior to ICP-MS analysis, for two coal and two ash
standards are presented here. Comparison between our results
obtained for small sample sizes (10 mg and even 5 mg for the
ash standards) and certified values (or results obtained for 250
mg by other studies) suggests that the sample size does not have
an influence on the recovery of the elements. As the total
dissolved solids (TDS) level in the final solution is a major
concern in ICP-MS analyses, all solutions were initially
prepared according to a solid to final solution dilution factor of
5000. As the concentrations of most trace elements in coal are
very low, preparing solutions at such high dilutions proved to be
a limitation and prevented several elements from being
accurately quantified. Both trace element recovery methods
using 10 mg of coal were therefore carried out with two
different final dilutions and the results obtained are reported
below. The mineral content of coal rather than the total mass of
coal represents the total dissolved solids for carbonaceous
samples. No such test was performed for the ash standards
whereas the diluted solutions (prepared with 10 mg of solid and
a final dilution factor of 5000) proved concentrated enough to
allow an accurate analysis for all elements. Issues concerning
the dilution to be applied are also discussed. Following the
replication of the analysis and the repetition of the sample
preparation for such low masses, the long term reproducibility
for each sample preparation coupled with ICP-MS determination was evaluated and data obtained for between 9 and 12
replicates are presented.
2.
2.1.
Experimental
Instrumentation
All ICP-MS analyses were performed on a PQ 2+ STE
instrument from Fisons Instruments (Manchester, UK). It was
equipped with a de Galan V-groove nebuliser and a Gilson
(Worthington, OH, USA) Minipuls 3 peristaltic pump. A
standard PC 486 computer provided overall system control.
Wet ashing (open vessel) digestions were performed in Pt
crucibles and microwave digestions in a Milestone MLS
178
Analyst, 1999, 124, 177–184
microwave oven system provided with a rotating 10-position
carousel containing 120 ml Teflon vessels from CEM (Matthews, NC, USA) with pressure release valves resistant up to
110 bar.
2.2. Reagents
Sulfuric acid, hydrofluoric acid (40% m/m), nitric acid (69%
m/m), perchloric acid (60% m/m) were all of Aristar grade
(Poole, Dorset, UK). Distilled water (Milli-Q Micropore grade,
Millipore, Bedford, MA, USA) was used throughout for rinsing
vessels and preparing solutions. All the laboratory ware was
cleaned using Decon and dilute acids (mainly HNO3). Standard
solutions (SPEX solutions, supplied by Glen Spectra, Stanmore,
UK) at 10 mg ml21 were used to prepare multi-element aqueous
calibration standards.
Four standard reference materials, two certified reference
coals and two certified reference ashes, were analysed: SRM
1632b, a bituminous coal [certified by the National Institute of
Standards and Technology (NIST), Gaithersburg, MD, USA];
SRM 1635, a sub-bituminous coal (from NIST); SRM 1633b, a
bituminous coal fly ash (from NIST); and CTA-FFA-1, a fine
fly ash (certified by the Institute of Nuclear Chemistry and
Technology, Warsaw, Poland). All were obtained from Promochem (Welwyn Garden City, UK). Only results for coal
SRM 1632b and ash CTA-FFA-1 are presented in the
subsequent tables. Results for coal SRM 1635 and ash SRM
1633b are provided as Electronic Supplementary Information.
2.3. Sample preparation
Wet ashing (open vessel acid) digestion. Coal samples were
ashed in concentrated H2SO4 to digest the organic materials and
retain volatile metals as sulfates. The mixture of coal and
sulfuric acid was heated in a platinum crucible, first on a hotplate for 3 h and then in a muffle furnace at 520 °C. The ashed
product or an ash sample was treated with perchloric and
hydrofluoric acid and heated at 200 °C to incipient dryness.
When fume emissions ceased, the residue was dissolved in 4%
HNO3 and washed into a 25 ml calibrated flask. Clear solutions
were obtained, indicating complete dissolution of solid samples.
Volumes of acid were matched to the initial sample size as far
as possible. The initial volumes of sulfuric, perchloric and
hydrofluoric acid (1, 3 and 5 ml, respectively, for digestion of a
150 mg sample) were reduced for smaller sample sizes: for 10
mg samples, the amounts of the three acids used were 100, 300
and 500 ml, respectively. They could not be decreased any
further for the 5 mg sample since a minimum working volume
was necessary to rinse the crucibles properly.
Microwave extraction. For the 10 mg sample preparation,
the sample and 1 ml of 70% HNO3 acid were mixed in a clean
7 ml PTFE vessel with a screw top. A 10 ml volume of
concentrated HNO3 was added to the 120 ml vessel to maintain
the pressure inside the vessel. The small vessel was loosely
closed and introduced into the large vessel, which was then
placed on the carousel. The heating programme consisted of
three steps: first the power was set at 25% for 5 min, then at 40%
for 5 min and finally at 60% for 5 min. The maximum power
available was 1000 W. After 5 min rest under ventilation, the
carousel was removed and kept at 218 °C for 1 h to minimise
the loss of volatiles and gases on opening. The sample was
finally poured from the small vessel into a 25 ml calibrated flask
and diluted in 2–3% HNO3. The yellowish solution was filtered
to eliminate the remaining solids (mineral matter such as
silicates that did not dissolve in the hot nitric acid). When
solutions were prepared with larger sample sizes (approximately 150 mg), 15 ml of concentrated HNO3 were added
directly to the sample in the large 120 ml vessel. The same
procedure (heating programme and dilution) was then used to
obtain the final solution.
Dilution factor (DF). As the total dissolved solids level in
the final solution is a major concern in ICP-MS analyses,
dilution of the final solution was required. For both coal and ash
standard preparations, all trace elements were determined using
a solid to final solution dilution factor of 5000.
However, coals generally contain of the order of 10% ash.
Most of the sample is therefore formed of carbonaceous
material which is destroyed and lost as carbon dioxide during
the dissolution. The ash level defines the TDS level, which is
clearly lower than if all of the solid materials were retained in
solution, and therefore more concentrated solutions may be
analysed. In order to increase the sensitivity of the determination for the least concentrated trace elements, analyses with less
diluted solutions were therefore carried out. Since the minimum
volume of solution used to wash the crucibles or the PTFE
vessels proved to be 6 ml, solutions prepared from a 10 mg
sample were introduced into 10 ml calibrated flasks and the
lowest dilution factor was therefore 1000.
The complete dissolution of ash matrices implies that all of
the mainly inorganic mineral sample was dissolved with no
significant loss of carbonaceous material; solutions prepared
from ashes contained relatively high TDS levels. Reduction of
the dilution factor was therefore not desirable and all the
solutions prepared from ash standards were analysed with a
dilution factor of around 5000.
2.4.
ICP-MS operating conditions
Typical instrumental parameters and operating conditions are
listed in Table 1. The instrumental background was checked at
5 and 220 u. The instrument was optimised for multi-element
determination from 6Li to 210Pb. The ion lenses were optimised
using an isotope in the middle of the mass range (115In) so that
the count rate was relatively uniform across the mass range
considered. As shown in Table 1, mass regions corresponding to
elements of no interest for this study were skipped; only
selected ions were monitored.
Owing to the wide dynamic range of the technique (calibration curves in ICP-MS are linear over five or six orders of
magnitude), quantification was obtained for all the elements by
using external standard calibration curves drawn through the
origin and three points, 10, 20 and 30 mg l21, using multielement aqueous standard solutions in 2 m HNO3. Isotopes
chosen for the analyses are listed in Table 2. Each sample
determination was the mean of three separate sequential
Table 1
ICP-MS instrumental parameters and operating conditions
Forward power
Reflected power
Plasma gases
Nebuliser gas flow rate
Coolant gas flow rate
Auxiliary gas flow rate
Sample uptake flow rate
Nebuliser
Spray chamber
Detector
Dwell time
Ion lenses
Sample time
Washout time
Number of replicates
Scan region
Skipped regions
Calibration mode
Curve fit
1.3 kW
<5 W
All argon
0.92 l min21
14.0 l min21
1.5 l min21
0.5 ml min21 (pumped)
de Galan V-groove
Water cooled (5 °C)
Pulse counting
320 ms
Optimised on 115In
66 s
60 s
3
5.6–210.4 u
11.4–49.6, 55.4–56.6, 79.4–80.6, 139.4–200.6 u
External calibration
Linear through zero
determinations and a reagent blank correction was made. Signal
drift was monitored by running the 10 mg l21 standard solution
after every three samples and a linear drift correction was
applied to the raw count integrals. Each analytical run consisted
of a calibration blank and a procedural blank, calibration
solutions, a batch of three coal or ash standard samples as
unknowns, a drift solution check, a batch of three coal or ash
standard samples as unknowns, followed by drift solutions and
three samples, repeated as necessary, and finally a drift solution
check. All results are given with the relevant confidence
intervals. The precision (e) was expressed as a 95% confidence
limit and calculated using the following equation:
ε≡
tσ
n
where: t (Student’s t-factor) = 4.3 when a 95% confidence limit
is required and three replicates are available and n = number of
replicates (three here).
3. Results and discussion
3.1. Limits of quantification
Although they bring to light the high sensitivity of ICP-MS,
elemental detection limits for ICP-MS, available from many
sources,21 often provide limited information. Such figures
usually represent the technique at its best. In contrast, quoting
the limits of quantification (LOQ) allows the analyst to estimate
the lowest practical level that can be reached during the
analysis. Taking into account both instrumental sensitivity and
sample preparation, the LOQs are calculated as the concentration equal to 10 times the standard deviation of the background
signal (procedural blank), 10s, multiplied by the dilution factor
(5000 or 1000 here), and are quoted as concentration in the solid
(mg g21) for each preparation procedure. With respect to the
ongoing variability of analytical blanks and instrument performance, the figures presented in Table 2 were calculated as
the mean of the LOQ established in each analytical session.
With the exception of a few elements (Cr, Ni, Cu, Zn, As, Se and
Sn), values were low ( ≤ 1.0 mg g21) for both extraction
methods, despite the very large dilution factor of 5000. As
expected, diminishing the dilution factor to 1000 reduced the
LOQ. However, in both cases it was not surprising to find that
Table 2 Limits of quantification in the solid by ICP-MS using (A) wet
ashing acid digestion and (B) microwave extraction
LOQ (A)/mg g21
LOQ (B)/mg g21
Element Isotope
DFa = 5000 DF = 1000 DF = 5000 DF = 1000
Be
V
Cr
Mn
Co
Ni
Cu
Zn
Ga
As
Se
Mo
Cd
Sn
Sb
Ba
Pb
0.4
0.5
3.2
1.0
0.4
4.0
6.0
(15)b
0.4
4.0
16.0
1.5
1.0
2.2
0.4
2.0
1.0
a
b
9
51
52
55
59
60
65
66
71
75
82
95
111
118
121
137
208
0.1
0.2
0.4
0.1
0.1
2.4
1.6
2.4
0.1
0.5
3.0
0.2
0.6
0.3
0.1
0.7
0.3
0.4
0.4
3.0
0.6
0.4
5.0
1.8
(20)b
0.3
1.2
15.0
1.2
0.6
1.0
0.3
3.0
0.6
0.2
0.2
1.1
0.3
0.1
2.4
1.0
4.2
0.1
0.6
3.0
0.2
0.2
0.2
0.1
0.8
0.2
DF = dilution factor.
Figures in parentheses are means of widely spread values.
Analyst, 1999, 124, 177–184
179
the LOQ for the microwave extraction was usually lower than
for the acid digestion. The possibility of adventitious contamination during the open digestion was much higher using
three acids than in the closed vessel digestion using one acid.
Indeed, contamination by reagents has been reported for
elements such as Ni, Cu, Zn, As, and Sn.23 Owing to the
formation of polyatomic interferences arising from the argon
plasma and air gases ([40Ar12C]+), variable background was
thought responsible for the large LOQ in the case of Cr.31 The
poor precision of the Se determination was undoubtedly linked
with the insufficient counts recorded during the calibration of
the instrument. Usually 1500 counts s21 Se were recorded for
the 10 mg l21 multi-element standard solution in comparison
with 15 000 for Ba (lowest counts after Se) and 200 000 counts
s21 for V (largest counts).
3.2.
Accuracy
The validation of both the sample preparation and the ICP-MS
determination was carried out by using four reference materials,
two ashes and two coals. The results are presented in Tables 3
and 3A (the latter available as Electronic Supplementary
Information†) for the ash reference samples and Tables 4 and 4A
(the latter available as Electronic Supplementary Information)
for the coal reference samples. Although most of the solutions
were prepared with 10 mg of sample (and even 5 mg in a few
cases), several solutions were also obtained using larger
amounts, similar to those used in sample preparations widely
described in the literature, for comparison.
The main purpose of this work was to establish if decreasing
the amount of sample used for the analysis would reduce the
accuracy of analysis. The overall impression is that no obvious
change in the analytical accuracy can be discerned when the
sample size is reduced. Where the large sample preparation
proved capable of extracting completely one element, the
extraction was as effective for the smaller sample size.
Similarly, when the large scale procedure was found to be
unsuccessful for some elements, the same lack of success was
recorded for the smaller sample size. Inconsistent results were
occasionally obtained when the sample size was changed; for
instance, the Mn, Cu or Ba results from small samples of ash
SRM 1633b (Table 3A, available as Electronic Supplementary
Information†) by the open vessel digestion were slightly lower
† Available as Electronic Supplementary Material; see http://www.rsc.org/
supdata/an/1999/177.
Table 3
(125–127, 120–116 and 660–661 mg g21, respectively) than
those values for the solution prepared with the larger sample
(158, 131 and 777 mg g21, respectively). However, the same
inconsistency was not found using the second ash reference
material, CTA-FFA-1 (Table 3), where the results obtained for
Mn, Cu and Ba proved impressively consistent whatever
amount of sample was used. The analyses of solutions of coal
reference material prepared by microwave HNO3 extraction
(Table 4 for SRM 1632b and Table 4A available as Electronic
Supplementary Information for SRM 1635†) showed a similar
lack of dependence on initial sample size. It can therefore be
concluded that despite whether the sample preparation method
is able or unable to release the trace elements into solution, the
results are completely independent of the initial mass of sample
used. Nevertheless, major differences between sample preparation methods were observed and proved dependent on the nature
of the sample or the mode of occurrence of the trace
elements.
Open vessel acid attack and microwave extraction on ash
reference materials. The results reported for the ash reference
show that the open vessel acid digestion is far superior to the
microwave procedure. With the exception of As, Se and to some
extent V, the acid mixture procedure allows the determination
of all elements with good accuracy. The poor agreement with
certified values for V and As is a direct consequence of residual
chlorides that can cause well known polyatomic interferences.21,23,24 The use of hydrochloric acid was avoided but it
has proved impossible to replace perchloric acid as the powerful
oxidising agent used to destroy inorganic bonds and dissolve
metals. For both ash reference materials, the total removal of
chlorine as HCl gas by taking the solution to dryness several
times has proved to be effective to some extent. Using this latest
procedure only recently, much improved results for V and As
have been obtained for the 5 mg sample, as shown in Table 3.
A similar problem of interference with selenium was avoided
because the mass 82 isotope is almost free from interference.
However, selenium was not measured successfully in CTAFFA-1 mainly because the certified value was lower than the
LOQ. A loss of this element, known to be volatile,9 cannot be
completely ruled out, even if the acceptable recovery of Se in
SRM 1633b (13.3 ± 2.2 and 8.9 ± 0.6 mg g21 for the 10 and 5
mg samples, respectively against a certified value of 10.26 ±
0.17 mg g21; see Table 3A, provided as Electronic Supplementary Information†) seems to indicate a tendency to stay in
solution under such conditions.
ICP-MS analysis of ash CTA-FFA-1: results in mg g21 (dilution factor = 5000)
Open vessel acid digestion
a
Element
250 mg
10 mg
5 mg
Certified
valuea
Be
V
Cr
Mn
Co
Ni
Cu
Zn
Ga
As
Se
Mo
Cd
Sn
Sb
Ba
Pb
26.3 ± 1.8
204 ± 23
147 ± 6
1058 ± 48
39.3 ± 2.8
95.6 ± 3.0
158 ± 6
520 ± 11
58.9 ± 1.4
66.6 ± 2.7
< 16
15.5 ± 1.3
2.3 ± 0.7
14.2 ± 5.4
16.5 ± 1.8
733 ± 5
372 ± 7
28.2 ± 1.1
225 ± 13
149 ± 3
1074 ± 31
41.1 ± 0.8
97.7 ± 2.2
159 ± 1
604 ± 23
62.3 ± 2.4
66.4 ± 0.5
< 16
13.9 ± 0.9
2.4 ± 0.4
15.0 ± 0.5
18.5 ± 0.5
697 ± 18
392 ± 20
28.5 ± 1.0
251 ± 29
160 ± 17
1135 ± 60
44.2 ± 2.9
94.8 ± 12.3
157 ± 14
655 ± 59
66.0 ± 3.8
55.8 ± 2.5
< 16
15.1 ± 2.2
2.3 ± 0.4
11.1 ± 2.5
19.0 ± 1.1
712 ± 37
398 ± 21
[27]
260 ± 10
156 ± 8
1066 ± 41
39.8 ± 1.7
99.0 ± 5.8
158 ± 9
569 ± 58
[49]
53.6 ± 2.7
[4.6]
[17]
[2.8]
n.c.
17.6 ± 2.5
835 ± 56
369 ± 49
[ ] = Informational value only; n.c. = not certified. b () = Below the LOQ.
180
Analyst, 1999, 124, 177–184
Microwave
extraction
(10 mg)b
16.8 ± 2.
128 ± 13
111 ± 13
840 ± 88
22.9 ± 3.1
53.9 ± 8.8
102 ± 12
411 ± 41
34.3 ± 4.6
49.9 ± 0.1
(4.9 ± 1.3)
8.0 ± 0.7
1.8 ± 1.1
6.0 ± 0.7
2.0 ± 0.3
392 ± 47
234 ± 26
With the exception of arsenic and selenium, which were
completely recovered in both ash materials by the HNO3
leaching in the microwave digestion, the results obtained by this
method were lower than the expected values for all the other
elements (see Tables 3 and 3A, the latter supplied as Electronic
Supplementary Information†). These disappointing figures
result from the inability of the leaching process to destroy
inorganic species, particularly silicates, when only nitric acid is
used. Some trace elements remained in the minerals, especially
the silicates, and were not recovered into solution. Similar
results have already been reported by Laitinen et al.,16 who
compared three different ash sample preparations involving
HNO3 alone and two mixtures, HNO3–HCl and HNO3–HF.
They found that the suitable method had to include HF for the
successful extraction of Be, V, Cr, Mn, Co, Ni, Cu, Zn, Cd and
Pb in solution. However, in our work, the good agreement with
the certified values for As and Se shows that the use of only
nitric acid is sufficient to extract these elements from the ash
matrix, mainly formed of silicates. These results suggest that As
and Se are associated with minerals that are digested by nitric
acid rather than in silicates. Indeed, avoiding the introduction of
chlorides during the sample preparation and the avoidance of
polyatomic interferences in the ICP-MS determination, with
operation in a closed vessel, make the determination of these
two elements very easy and secure. In contrast to the situation
during the open vessel digestion, the low recovery of V cannot
be associated with poor analysis due to interference from
chloride ions and it must be assumed that a large part of the V
and many of the other elements were retained in materials not
dissolved by nitric acid alone.
Open vessel acid attack and microwave extraction on coal
reference materials. Tables 4 and 4A (the latter provided as
Electronic Supplementary Information†) give the results obtained for the coal reference materials, with a solid to final
solution dilution factor of 5000. Both sample preparations were
carried out for two different sample sizes (10 and 140–125 mg).
Be, V, Cr, Mn, Co, Ga, Ba, Pb and to some extent Ni using the
open vessel acid digestion and Be, Mn, Co, Ni, Cu, As and Ga
and to some extent Pb using the microwave leaching all
compare reasonably well with values given for SRM 1632b
either by the Bureau of Certification or in other work.
Unfortunately, the Mo and Sn contents are so low in this
material that they were below the LOQ calculated for such a
high dilution factor (5000), although still agreeing very well.
The LOQ were also much too high to allow the determination of
Cu (in the open vessel acid digestion) and Zn, Se, Cd and Sb (by
either sample preparation). Over-estimation of As in solutions
prepared by digestion in a mixture of acids is likely to be a direct
consequence of residual chlorides introduced with the
perchloric acid, as mentioned above. Contamination during the
leaching in the microwave oven has been found to be the only
reasonable explanation for the excess of Zn in the relevant
solutions. As with ash samples, the slightly low recoveries for
V, Cr and Ba suggest that a small proportion of these elements
is not released in solution when only nitric acid is used. Very
similar conclusions can be drawn for SRM 1635 (Table 4A,
provided as Electronic Supplementary Information†) as for
SRM 1632b.
In order to enhance the sensitivity of the analysis, more
concentrated solutions were prepared from a 10 mg coal sample
by using a solid to final solution dilution factor of 1000. Prior to
analysis, checks were first carried out to assess whether
suppression effects were present. The total dissolved solids
(TDS) content was kept low and no suppression occurred. As
expected, the H2SO4 wet ashing step of the open vessel acid
digestion made sure that the coal matrix, mainly organic, was
completely destroyed and escaped as a mixture of gases. In the
case of the HNO3 leaching procedure in the microwave vessel,
nitric acid was not sufficient to dissolve all of the coal matrix
completely, and the elimination of the remaining solids by
filtration of the final solution ensured that the solutions were
clear enough to be properly analysed.
This procedure led to the much reduced LOQ as shown in
Table 2, thus allowing the determination of elements previously
detected at concentrations which could not be quantified.
Results for coals SRM 1632b and SRM 1635 are given in Tables
5 and 5A, respectively (the latter available as Electronic
Supplementary Information†). Ni, Cu, Mo, Sb, and to some
extent Zn were successfully quantified in SRM 1632b after the
open vessel acid digestion. As already mentioned, the determination of As has been much improved for the latest analyses by
the effective removal of chlorides by taking the solution to
dryness several times. Unfortunately, the improvement of the
technique sensitivity has not proved sufficient to allow the
quantification of Se, Cd and Sn. However, only Mo was added
to the list of successfully extracted and determined elements by
the microwave procedure. Low recoveries for V, Cr, Ga, Sn and
Ba and contamination by Zn were confirmed; Sb, which
previously could not be quantified since the concentration was
below the LOQ for the more diluted solution (dilution factor =
5000), was also not completely recovered from the coal matrix
Table 4 ICP-MS analysis of coal SRM 1632b: results in mg g21 (dilution
factor = 5000)
Table 5 ICP-MS analysis of coal SRM 1632b: results in mg g21 (dilution
factor = 1000)
Element
Open vessel
acid digestionb Certified
(10 mg)
valuea
Microwave extractionb
140 mg
10 mg
Be
0.63 ± 0.09
0.70c
0.68 ± 0.02
0.68 ± 0.14
V
13.6 ± 0.9
[14]
9.3 ± 1.1
8.1 ± 0.3
Cr
11.2 ± 0.8
[11]
9.0 ± 0.6
9.2 ± 0.4
Mn
12.2 ± 1.1
12.4 ± 1.0
12.9 ± 0.4
10.7 ± 0.8
Co
2.09 ± 0.16
2.29 ± 0.17 2.39 ± 0.34
2.05 ± 0.08
Ni
4.87 ± 1.1
6.10 ± 0.27 6.19 ± 1.65
5.62 ± 0.90
Cu
<6
6.28 ± 0.30 7.85 ± 1.02
6.04 ± 0.78
Zn
< 15
11.89 ± 0.78 30.9 ± 6.0
32.4 ± 4.4
Ga
2.9 ± 0.3
2.8c
2.4 ± 0.3
2.2 ± 0.1
As
14.2 ± 3.6
3.72 ± 0.09 3.58 ± 1.17
3.29 ± 0.47
Se
< 16
1.29 ± 0.11
< 15
< 15
Mo
(0.7 ± 0.3)
[0.9]
0.6 ± 0.2)
(1.0 ± 0.2)
Cd
<1
0.0573 ± 0.0027 < 0.6
< 0.6
Sn
(0.4 ± 0.1)
0.6d
<1
<1
Sb
< 0.4
[0.24]
< 0.3
< 0.3
Ba
65.1 ± 4.1
67.5 ± 2.1
56.3 ± 2.1
52.0 ± 1.2
Pb
3.27 ± 0.12
3.67 ± 0.26 2.45 ± 0.46
3.04 ± 0.19
a,b See Table 3. c From Fadda et al.23 d From Ebdon and Wilkinson.32
Element
Open vessel
acid digestion
(10 mg)
Certified valuea
Microwave
extraction
(10 mg)
Be
0.74 ± 0.14
0.70b
0.73 ± 0.08
V
11.7 ± 1.2
[14]
10.0 ± 0.4
Cr
10.8 ± 1.8
[11]
8.6 ± 0.4
Mn
13.3 ± 1.0
12.4 ± 1.0
12.3 ± 0.1
Co
2.07 ± 0.17
2.29 ± 0.17
2.15 ± 0.09
Ni
6.09 ± 1.29
6.10 ± 0.27
6.66 ± 0.45
Cu
6.60 ± 0.97
6.28 ± 0.30
6.06 ± 0.52
Zn
8.64 ± 2.68
11.89 ± 0.78
23.2 ± 1.5
Ga
3.1 ± 0.3
2.8b
2.2 ± 0.1
As
4.41 ± 0.30
3.72 ± 0.09
3.76 ± 0.16
Se
<3
.29 ± 0.11
<3
Mo
0.7 ± 0.1
[0.9]
0.8 ± 0.3
Cd
< 0.6
0.0573 ± 0.0027
< 0.2
Sn
Poor precision
0.6c
< 0.2
Sb
0.26 ± 0.11
[0.24]
< 0.1
Ba
62.0 ± 2.4
67.5 ± 2.1
42.0 ± 3.0
Pb
3.48 ± 0.18
3.67 ± 0.26
3.43 ± 0.17
a See Table 3. b From Fadda et al.23 c From Ebdon and Wilkinson.32
Analyst, 1999, 124, 177–184
181
by this method. Results for coal SRM 1635 (Table 5A, provided
as Electronic Supplementary Information†) followed the same
trends for both extractions even if the low concentrations for
many elements prevented more trace elements from being
quantified.
3.3.
Reproducibility
The long-term precision was assessed by analysing nine
different digestions of both ash reference material CTA-FFA-1
and coal reference material SRM 1632b prepared and analysed
over a 2 month period. Relative standard deviations (RSD),
calculated as the standard deviation of the analysis over nine
results divided by the mean of these nine measurements and
expressed as a percentage, are presented in Tables 6 and 7,
respectively.
Table 6 Long-term reproducibility (RSD) for the determination of 17
elements in coal ash CTA-FFA-1 by ICP-MS analysis and open vessel acid
digestion or microwave extraction (mass uptake = 10 mg; dilution factor =
5000)
Open vessel acid digestion
Mean/
Element mg g21
Be
V
Cr
Mn
Co
Ni
Cu
Zn
Ga
As
Se
Mo
Cd
Sn
Sb
Ba
Pb
Microwave extraction
Long-term
reproducibility: Mean/
RSD (%)
mg g21
28.2
1.7
237
5.5
151
5.3
1093
3.2
43.0
3.6
95.2
3.4
151
6.8
630
4.1
63.8
3.2
63.4
9.1
Poor precision
14.1
6.6
2.4
7.2
13.9
11.2
18.9
2.1
716
3.0
390
2.8
Long-term
reproducibility:
RSD (%)
Low recovery
Low recovery
Low recovery
Low recovery
Low recovery
Low recovery
Low recovery
Contamination
Low recovery
48.6
3.1
4.5
9.4
Low recovery
Low recovery
Low recovery
Low recovery
Low recovery
Low recovery
Table 7 Long-term reproducibility (RSD) for the determination of 17
elements in coal SRM 1632b by ICP-MS analysis and open vessel acid
digestion or microwave extraction (mass uptake = 10 mg; dilution factor =
1000)
Open vessel acid digestion
Mean/
Element mg g21
Long-term
reproducibility: Mean/
RSD (%)
mg g21
Be
0.70
5.8
V
11.7
4.7
Cr
10.7
3.5
Mn
13.9
12.8
Co
2.0
2.1
Ni
6.2
7.1
Cu
7.4
11.0
Zn
8.4
9.2
Ga
2.9
5.1
As
4.6
8.5
Se
Below LOQ
Mo
0.76
10.4
Cd
Poor precision
Sn
Poor precision
Sb
0.23
5.7
Ba
65.3
4.2
Pb
3.72
11.3
a Accurate value but below the LOQ.
182
Microwave extraction
0.61
Low recovery
Low recovery
12.4
2.0
6.3
6.3
Contamination
Low recovery
3.6
Below LOQ
0.73
(0.04)a
Low recovery
Low recovery
Low recovery
3.39
Analyst, 1999, 124, 177–184
Long-term
reproducibility:
RSD (%)
16.1
6.8
5.1
7.1
6.1
5.5
10.9
31.6
7.4
A comparison of the results for the open vessel acid digestion
shows that, whereas the RSD was acceptable for any element, a
higher imprecision was found for coal for almost all the
elements, whereas the precisions of the V, Cr, Co and As
determinations in both types of sample were very similar. If it
was thought that the ashing process necessary to digest the
organic constituents of the coal sample might be held responsible for the higher variation, other studies have suggested that
little imprecision was added by the use of H2SO4 during that
step of the process.33 Alternatively, the trace element contents
in the solid are between 10 and 100 times lower in the coal than
in the ash and not much higher than the LOQ even when
quantified in solution with a solid to solution dilution factor of
1000 as defined in Table 2. Consequently, it is not unusual to
find that the less concentrated solutions lead to a less precise
analysis, all the more so when the analyte concentration is so
close to the limits of detection of the analysis.
Ash reference material CTA-FFA-1 (Table 6). As mentioned previously, the open vessel acid digestion proved to be
the only method capable of achieving an accurate recovery for
almost all trace elements studied here, with the exception of Se
and to some extent As. Based on the data obtained using this
method, the RSD for most elements (Be, Mn, Co, Ni, Zn, Ga,
Sb, Ba and Pb) in the ash standard is better than 5%, with
slightly increased values (below or around 7%) for five others
(V, Cr, Cu, Mo and Cd). Only Sn has an uncertainty exceeding
10% RSD.
The determination of As, for which the accuracy by open
vessel acid digestion has been described as unsatisfactory owing
to the presence of interferences caused by remaining chlorides,
is also poorer in terms of precision than most of the other
elements. However, the microwave HNO3 leaching, already
identified as more reliable to quantify As (Tables 3 and 3A, the
latter available as Electronic Supplementary Information†)
exhibits a very good long-term reproducibility (3% RSD).
Finally, the precision for the determination of Se was only
worked out for the microwave extraction since sometimes
inaccurate and often inconsistent results were obtained for the
open vessel acid digestion. With an uncertainty slightly below
10% RSD, the microwave extraction used here proved acceptably reproducible for an element rarely well determined and as
precise as reported in other studies (8.9% RSD for a similar
preparation23).
Coal reference material SRM 1632b (Table 7). The longterm reproducibility for half the elements of interest is below
7% RSD for the open vessel acid digestion. In addition to these
eight elements (Be, V, Cr, Co, Ni, Ga, Sb and Ba), the
determination of five others (Cu, Zn, As, Mo and Pb) can also
be considered reasonably precise as the RSD does not exceed
11%; some of these elements have been reported to be sensitive
to contamination from the chemical reagents used for the
preparation (Cu, Zn and Pb) or affected by possible interferences (As) or memory effects (Mo).23 Mn alone presents a high
uncertainty (12.8% RSD), which is not completely understood
as the LOQ is low. The only explanation may be that some
contamination occurred during the sample preparation since
biomass samples, Mn rich materials, were digested at the same
time as part of complementary studies. As already mentioned, it
has not been possible to determine Se or Cd in either coal
sample while the concentration in solution was far too small to
be detected (10 mg sample uptake and dilution factor of 1000).
The determination of Sn, for which no certified value was
available, exhibited such poor precision that the analysis was
not considered successful.
For the elements previously mentioned as being well
recovered by the microwave procedure (Be, Mn, Co, Ni, Cu, As,
Mo and Pb; see Table 4), the RSDs are generally below 8%,
with the exception of Be (16.1%) and Mo (10.9%). The
variability of Mo concentration may be explained by the
occurrence of a memory effect such that the contribution to the
final level determined is even greater at such low levels (0.7
mg g21 in the solid, i.e., 0.7 ng g21 in solution). The reasons for
the inconsistency of some Be analyses remain mainly unknown,
and also the LOQ values were surprisingly greater with
microwave than with open vessel acid digestion. Some Cd
determinations in SRM 1632b led to a mean value of 0.04
mg g21, which is reasonably close to the certified value (0.057
± 0.003 mg g21), suggesting that HNO3 alone may be successful
in releasing Cd from the coal matrix. However, the certified
value is so low that it is far below the LOQ. The large RSD of
31% clearly confirms that the determination of Cd at this level
is not reliable. In this context, any concentration detected,
however close it may be to the true value, can only be taken as
an indication.
3.4.
Analysis of plant samples
Based on the methods developed using the reference compounds, a series of samples from a gasification plant was
examined. Although a preliminary study has already been
published,34 final results are still in the process of publication.35
They indicate that the methods are applicable to plant derived
samples where element volatilities agree with the expected
behaviour under gasification conditions.
4.
Conclusions
The trace elements chosen for determination are of particular
interest for the coal industry. As expected, for a multi-elemental
analysis covering wide ranges of atomic masses and levels of
occurrence, a single extraction method is not adequate. The
comparison with certified values for reference materials showed
that a number of factors must be considered when choosing
which is the more suitable digestion technique of the microwave
HNO3 leaching procedure and the open vessel acid digestion:
1. the acid attack must include hydrofluoric acid to dissolve
the silicate-enriched ash matrix completely since, except for Se
and As, none of the other elements studied here is completely
extracted from the matrix by HNO3 alone;
2. the loss of volatile elements has to be expected during the
open vessel acid digestion and an alternative preparation,
involving closed vessels such as in the microwave attack, is
often preferable for these elements (As and Se);
3. the final solutions must be suitable for the ICP-MS
instrument (total dissolved solid kept low and possible interferences mainly due to the use of HCl or HClO4 avoided); the total
solid to final solution dilution factor has to be kept minimal to
allow the determination of some elements present in coal at very
low level. For the small sample size of 10 mg, a dilution factor
of 1000 has proved necessary to quantify several elements (Ni,
Cu, Mo, Sb and to some extent Zn).
The mixture of acids (HF, HClO4 and HNO3) combined
during the open vessel acid digestion has proved to be by far the
most effective sample preparation for all the elements in ash and
for most of the elements in coal. The microwave HNO3 leaching
procedure appears more convenient not only for volatile
elements such as arsenic and selenium but also for minimising
polyatomic interferences during analysis and contamination
during the sample preparation using several acids together, even
of the purest quality. However, the microwave procedure as
used here presents the problem of incomplete solubility of
mineral phases resistant to HNO3 acid alone. By combining the
results obtained for the two procedures evaluated in this work,
up to 17 elements were determined in coal and coal ash
matrices. The very good recoveries for all the elements when
only 10 mg, and even 5 mg, of sample are used to prepare the
solution led to the conclusion that the decrease in the sample
size does not affect the accuracy or the precision of the analysis,
provided that solutions sufficiently highly concentrated in trace
element are prepared for a matrix such as coal in which the trace
element contents are often very low. These preparations are now
routine extraction procedures to determine, by ICP-MS and
ICP-AES, trace element contents in by-products formed during
the combustion and gasification of coal in laboratory-scale
reactors and in large power plants.
5. Acknowledgements
Support for this work by the European Union under Contract
Nos. JOF3-CT95-0018, JOF3-CT95-0024 and ECSC 7220-ED/
069 is gratefully acknowledged. The ICP-MS facility at
Imperial College is supported by the NERC. The authors
express their gratitude to Mr. A. E. Healey of CRE Group, Stoke
Orchard, for advice, assistance and helpful discussions.
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Paper 8/07849A