American Journal of Analyt ical Chemistry, 2011, 2, 66-74
doi:10.4236/ajac.2011.21007 Published Online February 2011 (http://www.SciRP.org/journal/ajac)
Microwave Assisted Acid Digestion of Biomorphic Ceramic
Obtained from Cedar Wood Infiltrated with ZrO2
Fuensanta Sánchez Rojas, Catalina Bosch Ojeda, José Manuel Cano Pavón
Department of Analytical Chemistry, Faculty of Sciences, University of Málaga, Málaga, Spain
Received October 22, 2010; revised November 15, 2010; accepted November 17, 2010
Abstract
This work proposes the use of experimental design for optimization of microwave-assisted digestion of bio-
morphic ZrO2-ceramic. Cedar wood is used as template and it was infiltrated with ZrO2 via sol-gel process.
XPS, SEM and XRD have been used to verify the effectiveness of the synthesis. The effects of different
combination of acids in the digestion of this sample have been optimized taking as response the concentra-
tion of several metallic ions measured by ICP-MS.
Keywords: Cedar Wood, Microwave Digestion, ICP-MS, Metallic Ions, Experimental Design
1. Introduction
Sol-gel processing is a new technology that is being de-
veloped more quickly for the manufacture of ceramic
and glasses. In the ceramics, this method allows the uni-
form and fine particle formation of high purity to rela-
tively low temperatures. These dusts can be sintered next
until obtaining to a high density with their corresponding
good mechanical properties. In such techniques, the fun-
damental characteristic is the formation of an orga-
nometallic dissolution. The phase disperses “sol” be-
comes a rigid “gel”, which, as well, is reduced until its
final composition by means of diverse heat treatments. A
key advantage of the process sol-gel is that the product
the forms initially by means of this procedure of liquid
phase can be roasted to lower temperatures, compared
with those of the conventional processes for ceramics.
The aim of this work is to optimize a microwave acid
digestion procedure for the determination of metals in
biomorphic ceramic samples. Microwave radiation is
widely used in modern chemistry and technology as a
source of energy for intensification of physicochemical
processes. Microwave-assisted digestion has clear ad-
vantages over the traditional acid digestion using con-
vective heating systems in terms of recovery, precision,
short time needed (minutes) to perform decomposition of
the sample, direct heating of samples and reagents (the
vessels are only indirectly heated by the hot solution),
minimal contamination and looses of volatile elements. It
reduces the possibility of cross contamination and the
consumption of reagents. The use of small amounts of
reagents decreases the blank signal. The combination of
different acids, such as H2SO4, HNO3, HCl and HF is a
very frequent method for preparing samples before their
quantitative elemental analysis.
The association of factorial design [1] and micro-
wave-assisted [2-6] digestion can contribute to accelerate
the pre-treatment step, improving the accuracy of the
results. On the other hand, ICP-MS is an ideal technique
for determining elements, with rapid data acquisition and
low detection limits. This work proposes the use of ex-
perimental design for optimization of microwave-as-
sisted digestion of biomorphic ZrO2-ceramic [7-13].
2. Experimental
2.1. Instrumentation
A Lenton Tube furnace, model LTF 16/180, was em-
ployed for the synthesis of biomorphic ceramics.
X-ray photoelectron spectroscopy (XPS) analysis was
performed with a Physical Electronics 5700 instrument
with a Mg Kα X-ray excitation source (hν = 1253.6 eV);
binding energies (BE) were determined with respect to
the position of the C1s peak at 284.5 eV. The residual
pressure in the analysis chamber was maintained below
133 × 10−9 Pa during data acquisition.
Scanning electronic microscopy (SEM) JEOL, Model
SM-6490LV was used to obtain the micrographs shown

F. S. ROJAS ET AL.
67
in Figure 1 and semi-cuantitative analysis shown in Ta-
ble 1.
(a)
(b)
(c)
(d)
(e)
Figure 1. Micrographs of biomorphic ceramic obtained
with SEM at different magnification a) ×150, scale 1:10 µm;
b) ×500, scale 1:50 µm; c) ×900, scale 1:20 µm; d) ×1000,
scale 1: 10 µm; e) Site of interest for results showed in Ta-
ble 1.
Table 1. Semi quantitative analysis made from Figure 1(e).
Espectrum C O Ca Zr Total
Espectrum (2)73.98 3.71 22.31 100.00
Espectrum (3)67.945.58 13.03 13.45 100.00
Espectrum (4)87.264.40 8.35 100.00
Espectrum (5)88.043.80 8.16 100.00
Espectrum (6)93.81 6.19 100.00
Máx. 93.815.58 13.03 22.31
Min. 67.943.80 3.71 6.19

68 F. S. ROJAS ET AL.
Powder patterns were collected on a X’Pert Pro MPD
automated diffractometer equipped with a Ge(111) pri-
mary monochromator (strictly monochromatic CuKα1 ra-
diation) and an X’Celerator detector. The overall meas-
urement time was approximately 30 min per pattern to
have very good statistic over the 2 range of 10˚ - 70˚
with 0.017˚ step size. The patterns were identified using
the PDF (Powder Data File).
A Panasonic (National) microwave oven, model NN-
8507, and a Parr Microwave Acid Digestion Bomb,
model 4782, were used for sample digestion. The bombs
were cleaned before use with 10% (v/v) HNO3 for 1 day
followed by repeated rinsing with water.
The measurements of Zr, Cu, Fe, Mg and Ni elements
were performed on a PerkinElmer ELAN DRCe ICP-MS
quadrupole spectrometer. The samples were introduced
into the ICP-MS via a RytonTM cross-flow nebulizer
(PerkinElmer), Scott spray chamber (PerkinElmer) and
Cetac ASX-510 autosampler. The sample transport from
the autosampler to the nebulizer was established by a
peristaltic pump. The operating parameters of the spec-
trometer are summarized in Table 2.
2.2. Reagents
Analytical reagent grade chemicals were used throughout.
Sol was prepared from zirconium n-propoxide (ZNP)
Zr[O(CH2)2CH3]4 (Merck) for infiltration into the porous
biological templates. Prior to hydrolysis, ZNP was modi-
fied with acetic acid (HAc) in a molar ratio of 1:4 to re-
duce reactivity with H2O.
Standard 1000 μg·mL−1 Zr(IV), Cu(II), Ni(II), Fe(III)
and Mg(II) solutions (Fluka) were used. Standards of
working strength were made by appropriate dilution as
required, immediately prior to use. Water was deionised
with a Milli-Q system. Concentrated acid HCl, HF,
H2SO4 and HNO3 (Merck) were used for digestion of the
samples.
2.3. Synthesis
Based on previously described synthesis [14] in the lit-
erature, rectangular specimens of native cedar of dimen-
sions (3.5 × 3.5 × 1) cm3 were dried (130˚C/24 h) and
later subjected to vacuum infiltration for 2 h with
ZrO2-sol. The specimens were dried at 110˚C for 1 h in
air to form the gel in the wood cells after infiltration. A
schematic diagram of the device used for the vacuum
infiltration is shown in Figure 2.
The infiltration and drying process was repeated for
three times. Multiple infiltrations were used to increase
the content of the ZrO2 precursor in the specimen. After
the infiltration steps, the cedar samples were pyrolyzed at
Table 2. ICP-MS operating parameters used for element
determination.
RF power 1100 W
Nebulizer gas flow 0.92 L·min−1
Plasma gas flow 15 L·min−1
Auxiliary gas flow 1.2 L·min−1
Nebulizer Cross-flow
Spray chamber Scott type
Cones Nickel
Lens voltage 6.5 V
Analog stage voltage −1800 V
Pulse stage voltage 850 V
Sweep/reading 20
Reading/replicate 1
Replicate 3
Scan mode Peak hopping
Figure 2. Schematic drawing of the experimental set-up for
the vacuum infiltration process.
800˚C for 1 h in Ar atmosphere in order to decompose
cellulose, hemicellulose, and lignin into carbon. After
repeated infiltration steps, the specimens were annealed
up to 1200˚C during 3 h (temperature ramp 5 ˚C/min up
to 1200˚C).
The infiltration/drying process was repeated up to
three times to increase the ZrO2 content in the biomor-
phic samples. The infiltration/annealing processing are
described by the schematic diagram in Figure 3 [13].

F. S. ROJAS ET AL.
69
Figure 3. Flow chart for the manufacturing of biomorphic
ZrO2 cedar.
2.4. Microwave-Assisted Pressure Digestion with
Acids
In a preliminary step, ten samples of 10 mg each one
were weighed accurately and then different acid mixtures
(combination of binary and ternary mixtures of HCl, HF,
HNO3 and H2SO4) were added to the digestion vessels
and were treated for 2 min to maximum power. The dis-
solutions obtained were completely transferred into a 25
mL calibrated flasks and diluted to volume with water.
Then, the optimum mixture of acids was obtained by
means of the ICP-MS and taking as response variable the
concentrations of several ions to select the better mixture
of acids. From these data we concluded the best acid
mixture was (H2SO4 + HF).
Later, a 52 multi-level factorial design with 25 runs (in
duplicates) was developed in order to determine the in-
fluence of the factors and their interactions on the system.
Two factors were studied: volumes of acids selected
(H2SO4 and HF).
The sequence of the experiments carry out is shown in
Table 3. The experimental data were processed making
use of the STATGRAPHICS 5.1 plus program [15]. The
significance of the effects was done by analysis of vari-
ance (ANOVA) and using p-value significance levels.
This value represents the probability of the effect of a
factor being due solely to random error. Thus, if the
p-value is less than 5%, the effect of corresponding fac-
tor is significant. The effects and significance of the
variables in the microwave-assisted digestion system
were evaluated using Pareto’s charts.
3. Results and Discussion
3.1. XPS Analysis
XPS has been used for the study of the surface composi-
tion of the sample. For this study the biomorphic materi-
als were powdered and homogenized in an agate mortar.
Measurements were performed on samples mounted in a
cup (1 mm × 3.5 mm, i.d.) and pressed manually.
XPS spectrum obtained is shown in Figure 4. Conse-
quently, the performance of the synthesis of biomorphic
ceramics can be evaluated approximately from these
data.
The atomic concentration calculation is expressed as a
percentage in a tabular form based on the area under the
peak, multiplied by the sensitivity factor for each ele-
ment, and provides a ratio of a single element to the sum
of the others elements present [16] (Table 4). The error
of the method is approximately of 10%.
3.2. XRD Analysis
The identification is based on PDF data base. The con-
tent of amorphous phase in the sample is high as can be
seen from Figure 5, due to the background curvature
between 20˚ - 30˚ 2 theta and 40˚ - 50˚ 2 theta. From this
figure we can conclude the sample is especially mono-
clinic ZrO2 (PDF number 01-086-1450), although the
sample shows in less proportion tetragonal phase of the
same oxide (PDF number 01-080-0784).
3.3. Sample Dissolution
Factorial design approach is an useful tool to establish
and improve analytical procedures. Although it seems
more complex than the univariate procedure from the
operative point of view, it is advantageous because it
makes use of fewer experiments and provides important
information on interactions among the studied variables
[17,18]. A multi-level full factorial 52 with 25 runs was
carried out in order to determine the main factors of the
microwave-assisted biomorphous ZrO2-ceramic diges-

F. S. ROJAS ET AL.
70
Table 3. Design matrix and the results for Zr, Cu, Fe, Mg and Ni concentration.
Experiment Vol. H2SO4 (mL)Vol. HF (mL) [Zr] (mg/g) [Cu] (mg/g) [Fe] (mg/g) [Mg] (mg/g) [Ni] (mg/g)
1 2 4 411.8 1.9 1.8 7.6 0.7
2 5 4 672.2 3.6 2.5 9.6 1.4
3 4 4 469.7 2.1 1.6 12.3 1.2
4 5 2 417.0 3.0 2.5 9.4 1.2
5 4 5 391.4 2.7 2.7 9.4 1.3
6 3 6 626.9 2.4 1.6 13.0 1.2
7 2 5 631.0 1.6 1.7 10.0 1.3
8 3 5 555.3 1.8 2.1 8.2 1.0
9 2 3 566.8 1.5 1.0 10.1 0.7
10 6 5 531.7 4.5 2.0 7.7 2.0
11 3 3 387.8 2.0 1.6 7.7 0.8
12 3 2 518.6 2.2 1.7 34.2 1.1
13 6 4 531.6 4.0 1.4 14.0 1.7
14 5 6 766.6 3.2 2.0 14.1 1.5
15 2 6 548.2 1.4 4.0 10.3 0.8
16 5 5 692.8 3.3 1.3 13.5 1.5
17 2 2 468.0 1.2 1.5 8.9 0.6
18 6 2 685.8 4.1 1.4 12.6 1.8
19 4 3 575.9 2.2 1.4 10.0 1.0
20 4 2 536.5 2.3 1.2 8.3 1.0
21 6 3 502.7 3.5 1.5 11.8 1.6
22 3 4 619.8 1.8 1.3 10.0 0.9
23 6 6 588.4 3.0 2.2 12.6 1.5
24 5 3 558.0 2.4 1.6 12.3 1.2
25 4 6 831.4 2.1 1.8 15.4 1.1
(a) (b)
Figure 4. XPS spectrum for biomorphic ceramic from cedar wood a) survey b) Zr3d5/2 (183.8 eV).

F. S. ROJAS ET AL.
71
(a)
(b)
Figure 5. XRD spectrum for biomorphic ceramic from cedar wood (a) between 10º - 70º 2 theta (b) Amplification between 15º
35˚ 2 theta. -

F. S. ROJAS ET AL.
72
tion. The results of the ANOVA carried out on the data
given in Table 3 are shown in Table 5.
The two-order effect combined Pareto charts (P = 95%)
were obtained for the two factors considered (Figure 6).
After the analysis of the results, it was observed that the
studied variables have no significant effects for Mg
however significant effects were demonstrated for HF
volume in the case of Zr, Fe and Ni and H2SO4 volume
in the case of Cu and Ni.
3.4. Comparison of Samples
(b)
Also, we have made the comparison of the content of
ions in samples of biomorphic ceramics of cedar and oak,
and of cedar and pine. To carry out this comparison have
employed a statistical procedure designed for to compare
two sets of data, in our case concentrations of a certain
Table 4. Analysis of biomorphic ceramic by XPS.
Concentration (%)
Element Area
(cts-eV/s)
Sensitivity
factor Atomic Mass
C1s 131082 7.439 93.65 89.15
O1s 20606 18.619 5.88 7.46
Zr3d 5675 63.731 0.47 3.4
(c)
Table 5. P-value for data analysis given in Table 3 for metal
ions concentration.
Design
variable
P-Value
(Zr)
P-Value
(Cu)
P-Value
(Fe)
P-Value
(Mg)
P-Value
(Ni)
A: Vol. H2SO4 0.2754 0.000 0.7339 0.8055 0.000
B: Vol. HF 0.0341 0.7507 0.0335 0.6178 0.0451
AA 0.5789 0.1545 0.7738 0.4867 0.1106
AB 0.7541 0.5671 0.1417 0.4739 0.2814
BB 0.1950 0.4585 0.2498 0.1331 0.6803(d)
(e)
(a) Figure 6. Pareto’s charts for a) Zr; b) Fe; c) Ni; d) Mg; e) Cu.

F. S. ROJAS ET AL.
73
a) (
)
io
(b
Figure 7. Comparison of ion contents from different b.
filtrant agent
k and zirconium
-propoxide for the cedar and pine).
The above-mentioned procedure calculates statistical
data and it gives several charts for every shows. In addi-
tion executes several tests to determine if there are statis-
tically significant differences between two samples.
Below the F test is made to compare the variances of
the two samples. Additionally intervals or confidence
limits are established for every standard deviation as well
as for the reason of the variances. In addition are com-
pared also the content of ions specific to the wood as are:
Mg, Fe, Ni and Cu. The results obtained are shown in
Figure 7.
HCl, HNO3, H2SO4, HF). Of all of
them the mixture (H2SO4 + HF) was selected in order to
obtain maximum concentration of the ions.
Differences on Zr contents in two ceramics (cedar and
oak) can be due because synthesis have been made from
different templates and different reagents to obtain ZrO2
sol-gel (zirconium oxichloride and zirconium n-propo-
xide), also the experimental design for the digestion
process of these samples were different.
Similar Zr content were obtained from different tem-
plates (cedar and pine) when the infiltration agent (zirco-
nium n-propoxide) is the same for the two samples.
morphic ceramic samples a) cedar-pine b) cedar-oak
ion in two biomorphic materials synthesised in the same
conditions whose only difference is the employed wood
as precursor (cedar, oak and pine) and the in
rconium oxichloride for the oa
4. Conclusions
Different acid mixtures were tested for the samples di-
gestion processes (
(zi
n

F. S. ROJAS ET AL.
74
5. Acknowledgements
The authors thank to the Ministerio de Ciencia e Innovación
for supporting this study (Projects CTQ2009-07858) and
also the Junta de Andalucia.
6. References
[1] G. Hanrahan and K. Lu, “Application of Factorial and
Response Surface Methodology in Modern Experimental
Design and Optimization,” Critical Reviews in Analytical
Chemistry, Vol. 36, 2006, pp. 141-151.
Hdoi:10.1080/10408340600969478
[2] P. E. Smith and E. A. Arsenault, “Application of Facto-
rial and Response Surface Methodology in Modern Ex-
perimental Design and Optimization,” Talanta, Vol. 48,
1996, pp. 1207-1268. Hdoi:10.1016/0039-9140(96)01882-6
[3] K. J. Lamble and S. J. Hill, “Microwave Digestion Proce-
dures for Environmental Matrices, Critical Review,” Ana-
lyst, Vol. 123, 1998, pp. 103-133. Hdoi:10.1039/a800776d
[4] I. V. Kubrakova and E. S. Toropchenova, “Microwave
Heating for Enhancing Efficiency of Analytical
Operations (Review),” Inorganic Materials, Vol. 44,
2008, pp. 1509-1519. Hdoi:10.1134/S0020168508140069
[5] H. M. Kinston and S. J. Haswell, “Microwave-Enhanced
Chemistry, Fundamental, Sample Preparation and Appli-
cations,” American Chemical Society, Washington DC,
1997.
[6] C. Bosch Ojeda and F. Sánchez Rojas, “Sample
Dissolution for Elemental Analysis/Microwave Diges-
tion,” Encyclopedia of Analytical Chemistry, 2nd Edition,
Elsevier, 2005, pp. 153-163.
Hdoi:10.1016/B0-12-369397-7/00540-9
[7] M. Luo, J. Gao, G. Qiao and Z. Jin, “Synthesis of
Wood-Derived Ceramics from Biological Templates,”
Progress in Chemistry, Vol. 20, 2008, pp. 989-1000.
[8] H. Sieber, J. Cao, C. R. Rambo and P. Greil,
“Manufacturing of Porous Oxide Ceramics from
Bioorganic Preforms,” Ceramic Engineering and Science
Proceedings, Vol. 23, 2002, pp. 175-181.
Hdoi:10.1002/9780470294758.ch20
[9] M. Singh and B. M. Yee, “Reactive Processing of Envi-
ronmentally Conscious, Biomorphic Ceramics from
Natural Wood Precursors,” Journal of the European Ce-
ramic Society, Vol. 24, 2004, pp. 209-217.
Hdoi:10.1016/S0955-2219(03)00244-9
[10] J. Cao, C. R. Rambo and H. Sieber, “Preparation of
Porous Al2O3-Ceramics by Biotemplating of Wood,”
Journal of Porous Materials, Vol. 11, 2004, pp. 163-172.
Hdoi:10.1023/B:JOPO.0000038012.58705.c9
[11] J. Cao, O. Rusina and H. Sieber, “Processing of Porous
TiO2-Ceramics from Biological Preforms,” Ceramics
International, Vol. 30, 2004, pp. 1971-1974.
Hdoi:10.1016/j.ceramint.2003.12.180
[12] J. Cao, C. R. Rambo and H. Sieber, “Manufacturing of
Microcellular, Biomorphous Oxide Ceramics from Native
Pine Wood,” Ceramics International, Vol. 30, 2004, pp.
1967-1970. Hdoi:10.1016/j.ceramint.2003.12.181
[13] C. R. Rambo, J. Cao and H. Sieber, “Preparation and
Properties of Highly Porous, Biomorphic YSZ Ceramics,”
Materials Chemistry and Physics, Vol. 87, 2004, pp.
345-352. Hdoi:10.1016/j.matchemphys.2004.05.031
[14] C. R. Rambo, J. Cao, O. Rusina and H. Sieber,
“Manufacturing of Biomorphic (Si,Ti,Zr)-Carbide Cera-
mics by Sol-Gel Processing,” Carbon, Vol. 43, 2005, pp.
1174-1183. Hdoi:10.1016/j.carbon.2004.12.009
[15] Statgraphics Plus (version 5.1), Reference Manual,
Manugistics: Rockville, MD, 1992.
[16] Physical Electronics, Multitechnique ESCA Reference
Manual for the PCACCES Software Version 6.0; Physical
Electronics: Minneapolis, USA, 1995.
[17] M. Nechar, M. F. Molina and J. M. Bosque-Sendra,
“Application of Doehlert Optimization and Factorial
Designs in Developing and Validating a Solid-Phase
Spectrophotometric Determination of Trace Levels of
Cadmium,” Analytica Chimica Acta, Vol. 382, 1999, pp.
117-130. Hdoi:10.1016/S0003-2670(98)00768-5
[18] L. M. Costa, S. L. C. Ferreira, A. R. A. Nogueira and J. A.
Nóbrega, “Use of Factorial Design for Optimization of
Microwave-Assisted Digestion of Lubricating Oil,”
Journal of the Brazilian Chemical Society, Vol. 16, 2005,
pp. 1269-1274.