ANALYTICAL SCIENCES DECEMBER 2001, VOL.

17 1395
2001 © The Japan Society for Analytical Chemistry

Comparison of Different Digestion Methods for Total

Decomposition of Siliceous and Organic Environmental Samples
Yuh-chang SUN,*† Po-hsiang CHI,** and Mei-yun SHIUE**

*Nuclear Science and Technology Development Center, National Tsing-Hua University,

30043 Hsinchu, Taiwan
**Department of Nuclear Science, National Tsing-Hua University, 30043 Hsinchu, Taiwan

A comparison was made of different digestion methods for the total decomposition of siliceous and organically
environmental samples prior to their analysis by inductively coupled plasma optical emission spectrometry (ICP-OES).
In the present study, three different digestion methods, including microwave-assisted, hot plate heating and pressurized
digestion (pressure bomb), were employed for the determination of nine heavy metals, i.e. Ag, Cd, Cr, Cu, Fe, Mn, Ni, Pb
and Zn in sediment, soil, sludge and oil. The investigation of different combinations of acids through their analytical
performance demonstrated that HCl plays a vital role in the determination of silver. The combination of HNO3 and HCl
possesses more reactive ability in oxidizing organic matter. The recoveries of all elements of interest in sediment (NIST
2704) obtained by different digestion methods were found to be 86% to 113%, while microwave assisted digestion with
various combinations of HNO3–HCl–HF and HNO3–HClO4–HF was considered to be a viable alternative to the
conventional digestion systems because of its more intensive reaction conditions. The analytical results of four certified
reference materials with different matrices, including sediment (GBW 07305), soil (GBW 07411), sludge (BCR R-143)
and oil (NIST 1085a), by the microwave-assisted acid digestion method indicated that the recoveries of all elements of
interest were more than 85% and the throughput of applied analytical method could be elevated significantly.

(Received April 28, 2001; Accepted September 17, 2001)

precise and consistent analytical results, the variation of

Introduction digestion temperature is reported as a critical parameter which
should be properly controlled during the digestion.10–12
With a view to investigate the geochemical properties, Moreover, for the purpose of achieving the goal of total
physicochemical availability and biological availability of decomposition and the requirements of analytical results
sediment, soil and sludge, several complicated fractionation quality, the use of acid combinations for the digestion is also
methods have been developed.1 Nevertheless, considerable considered very critical.13,14
problems remain or are generated when one attempts the Due to the complexity and diversity of environmental
interpretation of the information obtained from fractionation samples, the variation of decomposition efficiency is found to
methods. Improper selection and buffer capacity of extracting be strongly dependent on the natural composition of sample and
solution and re-adsorption and precipitation which occurred acids used for the digestion.13 Several obstacles, such as the
during sequential extraction always lead to ill-defined analytical effect of HCl on the decomposition of high Al and Fe
results. If total decomposition methods are used, more precise containing sample13 and the effectiveness of decomposition of
and well-defined analytical results can be achieved, otherwise oil, always cause ill-defined analytical results. These still need
the data can not be employed directly to assess the potential to be overcome, if the goal of total decomposition is to be
effects of pollution. According to sediment management, accomplished. Furthermore, from literature reports,15,17 the
several correction methods have been established to gain the apparent recoveries of some problematic elements, such as Fe,
environmental or geochemical significance through the use of Cr, Ti, Mg and Al, were always found to be suppressed because
data obtained from total decomposition methods.2,3 of the formation of refractory chromite (FeCr2O4), corrundum
Hot plate,4,5 pressure digestion system (pressure bomb)6,7 and (Al2O3), rutile (TiO2) and Ti-Mg mineral. In order to solve such
microwave assisted8–11 heating technique are widely used problems, various acid mixtures were developed to dissolve
methods for solid waste digestion prior to instrumental solid environmental samples.15–18
detection. Based on the considerations of shortening working The analysis of heavy metals in solid samples is one of the
time and minimizing volatile loss of analytes, microwave- most dynamically developing scientific areas. The
assisted digestion technique is regarded as a more convenient decomposition of samples with siliceous matrices, such as soil
and precise sample pretreatment method. In recent years, and sediment, or with long hydrocarbon chain organically-based
microwave-assisted digestion in closed vessels has been matrices, such as grease, wax and oil, is difficult to accomplish.
employed with increasing success. In the light of the demand of In order to decompose and dissolve analytes out of sample
matrices, several conventional and microwave-assisted
† To whom correspondence should be addressed.
digestion methods were developed.9,10 For achieving more
E-mail: ycsun@mx.nthu.edu.tw consistent and controllable analytical results, various digestion
1396 ANALYTICAL SCIENCES DECEMBER 2001, VOL. 17

Table 1 Summary of tested digestion methods and established acid digestion procedures

Parameter Microwave-1 Microwave-2 Microwave-3 Hot plate Pressure bomb

Vessel PTFE PTFE PTFE PTFE PTFE

Sample/g sediment: 0.51 sediment: 0.5 sediment: 0.5 sediment: 0.5 sediment: 0.5
soil: 0.52
sludge: 0.53
oil: 0.14
Acids A B C C A
Acid volume/mL 9:3:31 5:2 2:5:2 5:5:5 5:5:5
9:3:32
9:3:43
9:2:0.54
Heating time 9.5 min 10 min 10 min 45 min 7h
Tmax/˚C 180 180 180 — 180
Pmax/psi 180 110 110 — —

A, HNO3:HCl:HF; B, aqua regia:HF; C, HNO3:HClO4:HF.

parameters, such as heating power, pressure and temperature, Equipment

were applied to monitor the digestion performance. Based on The microwave digestion procedures were performed using
the suggestion of Kingston,11,12 the digestion temperature was the microwave digestion system MDS-2000 from CEM Corp.
employed as one vital parameter to be controlled in subsequent (U.S.A.). The pressure digestion system was obtained from
studies. A consistent digestion temperature (175 ± 5˚C) was Berghof Corp. (Germany). An inductively coupled plasma
employed for both microwave-assisted and pressure bomb optical emission spectrometer (Optima 3000 DV, Perkin-Elmer
digestion procedures to eliminate any possible discrepancy Corp. U.S.A.) was used for the determination of interesting
arising from the reaction rate and attack capability of acids. elements under standard operating conditions.
Microwave-assisted, hot plate and pressure bomb digestion
methods were compared with respect to digestion efficiency, Vessel cleaning
accuracy and acids used. From the viewpoint of total For analysis of trace level of analytes, it is necessary to keep
decomposition, three different combinations of acids: aqua regia the blank value as low as possible. This is because some solid
+ HF, HNO3 + HClO4 + HF, and HNO3 + HCl + HF, were waste samples, which contained very high concentrations of
employed in the experiment. In order to figure out the total various elements, may remain in the vessels even after the
decomposition problems of various environmental samples, digestion procedure. In order to avoid the serious cross-
nine elements, i.e., Ag, Cd, Cr, Cu, Fe, Mn, Ni, Pb and Zn in contamination between different digestion assignments, it will
sediment, soil, sludge and oil were chosen for subsequent be necessary to immerse the vessels in dilute nitric acid after
comparative experiments. In addition, for the purpose of each digestion. Furthermore, if the blank value is still not
figuring out the applicability of different sample preparation satisfied, the vessel should be dipped in boiling nitric acid for
methods in total decomposition, microwave-assisted and drastic cleaning. In general, the blank values of elements of
conventional digestion methods, such as hot plate and pressure interest were not detectable by the ICP-OES system employed.
bomb heating procedures, were discussed in this study along
with the uses of different combinations of acid. To prove the Digestion procedures
compatibility of overall procedures, commonly used inductively The total decomposition of the siliceous and organic based
coupled plasma optical emission spectrometry (ICP-OES) was matrix samples was difficult because of their distinctive
applied for the measurement of analytes in samples preparing constitution. For the determination of the trace elements in
by digestion methods tested in this study. these materials, the microwave-assisted acid digestion method
and two conventional wet-chemical digestion methods, hot plate
digestion and pressure bomb digestion, were compared. These
Experimental are discussed below, based on their analytical performance.

Reagents and standards Procedure for conventional hot plate digestion method
To test the analytical performance of the digestion methods The conventional hot plate digestion was performed with an
employed, five certified reference materials were used in this acid mixture by using Teflon beakers. The samples were
study. NIST SRM 2704 river sediment and NIST SRM 1085a weighed in Teflon beakers with pre-determined amounts of acid
oil were obtained from the National Institute for Standards and mixture. The samples were heated on a hot plate up to 110˚C
Technology (U.S.A.). GBW 07305 river sediment and GBW and then maintained at this temperature for 15 min. After the
07411 soil were obtained from the National Research Center of sample temperature was cooled to less than 70˚C, 5 mL of
Certified Reference Materials (China). CRM 143 sludge was concentrated HNO3 was added and the samples were refluxed at
obtained from the European Commission Joint Research Center ~110˚C for 30 min. Subsequently, the samples were evaporated
(Belgium). The acids, including hydrofluoric acid, perchloric to ~5 mL without boiling. After cooling to room temperature,
acids, nitric acid and hydrochloric acid, were obtained from E. the samples were diluted to 100.0 mL using deionized water
Merck Company (Germany). with double distilling. The acid mixtures and experimental
parameters are summarized in Table 1.
ANALYTICAL SCIENCES DECEMBER 2001, VOL. 17 1397

Table 2 Comparison of results of sediment (NIST 2704)

obtained by different sample digestion methods

Recovery, %
Element
Hot Pressure
Microwave-1a Microwave-2b Microwave-3c plated bombe

Ag 91 — — — —
Cd 94 — 95 113 95
Cr 113 96 94 104 107
Cu 96 105 101 93 90
Fe 96 99 99 95 95
Mn 97 100 102 86 86
Ni 102 95 102 93 97
Pb 98 101 98 110 105
Zn 102 102 101 92 98

a. 9 mL HNO3 + 3 mL HCl + 3 mL HF/microwave-assisted digestion.

b. 5 mL aqua regia + 2 mL HF/microwave-assisted digestion.
c. 2 mL HNO3 + 5 mL HClO4 + 2 mL HF/microwave-assisted Fig. 1 The effect of HCl on the recovery of Ag in different
digestion. matrices.
d. 5 mL HNO3 + 5 mL HClO4 + 5 mL HF.
e. 5 mL HNO3 + 5 mL HClO4 + 5 mL HF.

with or without HF were used for the decomposition in the

present work. Generally, according to the analytical results of
Procedure for pressure bomb digestion method siliceous base samples obtained by the use of aqua regia,
The conventional high-pressure digestion with acid mixture perchloric acid + nitric acid or nitric acid, we found that the
was performed in the pressure bomb system (Berghof, recoveries of all elements of interest in soil and sediment
Enningen, Germany) using PTFE digestion vessels. The samples fluctuated seriously (10 – 20%) and thus only non-
samples were weighed in the PTFE digestion vessels and a pre- quantitative results were achieved. However, in the use of
determined amount of acid mixture was added. Due to the different combinations of acid along with HF, as shown in
thermal properties of PTFE, the maximum temperature should Table 1, the recoveries of elements of interest measured by ICP-
be below 180˚C during the digestion process in the aluminum OES became quantitative and could be controlled within 90% to
heating block. Since the temperature and pressure inside the 115% (see Table 2) when the microwave-assisted technique was
digestion vessels could not be monitored during the digestion, employed. Such a result indicates that HF may be indispensable
the longer heating period (7 h) was employed to ensure the to open the samples with siliceous matrices.
achievement of total decomposition. The optimal experimental Effect of HCl. The effect of HCl on the recovery of silver in
conditions and acid mixtures for the digestion are summarized four different matrices is shown in Fig. 1. For the samples with
in Table 1. siliceous matrix, such as sediment and soil, it was found that the
recovery of silver increased gradually with increasing amount of
Procedure for microwave-assisted digestion method HCl used in the digestion. For organically-based samples, such
The microwave-assisted acid digestion was performed in the as sludge and oil, the recovery of silver increased abruptly when
microwave digestion system (MDS-200, CEM Corp., U.S.A.) 2 mL of HCl was used for the digestion. It is interesting to note
using PFA digestion vessels (max pressure 120 psi) and that the recovery of silver shows significant dependence on the
advanced composite vessels (ACV, max pressure 200 psi) with concentration of HCl. This observation can be explained on the
pressure and temperature feedback control. In digestion vessels, basis of the relative tendency of forming soluble chloro-silver
0.5 g samples (0.1 g for oil) were weighed and pre-determined complexes with chloride ion, such as AgCl2–, AgCl32– and
amounts of acids were added (see Table 1). These vessels were AgCl43–, in the HCl medium.
capped, sealed and heated in the microwave system with Owing to the tremendous amount of gas generated and the
simultaneous temperature and pressure monitoring. Since the exothermic reaction occurring during the acid digestion,11
temperature profile was specified to permit specific reactions, organic samples, such as oil, may contribute additional stress
the digestion temperature is considered to be a vital factor (high temperature and pressure) when decomposed in a closed
which should be controlled during digestion. According to system. In order to explore the limitation of the organically-
proposed microwave heating program, the completion of based samples in the closed digestion system, different
specific reactions can be accomplished when 180˚C of digestion combinations of acids were used in this study. Figure 2 shows
temperature can be reached and maintained for 9.5 min. The the temperature and pressure profile of 0.1 g of oil sample
microwave heating programs and acid mixtures for the digestion digested with different combinations of acid in a closed Teflon
are summarized in Table 1. vessel exposed to 500 W power. It was worthy to note that the
rate of accumulated system pressure and temperature inside
closed vessels was increased with increasing amount of HCl
Results and Discussion used for the digestion. The cause may be related to the
formation of the strong oxidant NOCl19 when HCl was mixed
Effect of acid with HNO3 and to the larger amount of gaseous products
Effect of HF. Owing to the existence of the aluminosilicate present due to the much more violent oxidation reaction which
matrix in natural environmental solid samples8 and the effect of occurred between NOCl and oil. However, except for silver,
the chemical form of analytes,11 different combinations of acid more than 85% of analytes could be recovered (see Fig. 3 and
1398 ANALYTICAL SCIENCES DECEMBER 2001, VOL. 17

Fig. 3 The effect of HCl on the recovery of analytes in oil sample.

pressure bomb digestion are estimated to be around 0.5, 3 and 7

h, respectively. Therefore, based on the consideration of
working time and analytical performance, microwave assisted
method is considered to be the more efficient and effective
method among the methods tested.

Analysis of different samples

In order to assess the applicability and compatibility of the
microwave-assisted acid digestion method, four different
matrices with certified values, including sediment (GBW
07305), soil (GBW 07411), sludge (BCR-143) and oil (SRM
1085a), were used for validating the accuracy of the overall
Fig. 2 The effect of HCl on the digestion parameters of oil sample. analytical procedure. The most suitable combinations of acids
used for the digestion were applied as shown in Table 1. The
analytical results shown in Table 3, indicate that the
Table 3), irrespective of the use of HCl, if the digestion concentrations of all elements of interest in different matrices
temperature was 180˚C and the conditions persisted for 9.5 min. determined by ICP-OES were in agreement with certified
To be safe, 0.1 g of oil sample and 2 mL of HCl were added for values. The results obtained from the overall analytical
the quantitative recovery of silver and to avoid possible sudden procedures were considered accurate for the determination of
increase in the pressure. nine heavy metals in tested matrices. Additionally, in order to
In order to minimize the possible contamination introduced examine the analytical stability of the applied method, the
through the use of HCl and to retain silver ion in the solution, a within-run and between-run results were obtained by replicate
2 mL and 3 mL volume of concentrated HCl was employed for determination of these four certified materials. Since the
the subsequent digestion of organically-based samples (sludge concentrations of some elements were below the detection
and oil) and siliceous based samples (sediment and soil), limits of ICP-OES, the experiments were carried out by addition
respectively. of pre-determined amounts of desired elements before the
digestion. The results obtained are given in Table 4. In general,
Comparison of different digestion methods the precision of overall procedure was below 10% for four
In order to compare the applicability of the microwave- different matrices.
assisted acid, hot plate heating and pressure bomb digestion
procedures, sediment samples (NIST 2704) were digested
according to the methods described in Table 1. The results in Conclusion
Table 2 show that the recoveries for all elements of interest
were more consistent and quantitative when the microwave From the obtained experimental results, it can be concluded that
energy was applied for the digestion using the conditions of the proper addition of hydrochloric acid plays a very critical
minimum temperature 180˚C and minimum time of 9.5 min. role not only in the determination of silver, but also in the
The open-vessel digestion coupled with hot plate and close- avoidance of possible sudden increases in the pressure during
vessel digestion coupled with heating block (pressure bomb) the digestion of organic-rich samples. To achieve total
methods were also used for the sediment digestion. Except for decomposition of sample matrices and dissolution of elements
the recovery of Mn, which appeared somewhat lower than that of interest, the combinations of HNO3 + HCl + HF and HNO3 +
obtained from microwave method, the recoveries of other HClO4 + HF accompanying with microwave-assisted digestion
elements of interest were found to be quantitative, as shown in technique are considered as practical options for the digestion of
Table 2. The minimum working times, including heating and environmental solid wastes.
cooling time, for microwave-assisted, open-vessel hot plate and
ANALYTICAL SCIENCES DECEMBER 2001, VOL. 17 1399

Table 3 The analytical results of four certified reference materials containing different matrices by ICP-OES

Sediment Soil Sludge Oil

Matrix
(GBW 07305) (GBW 07411) (BCR R-143) (NIST 1085a)

Certified/ Observed/ Certified/ Observed/ Certified/ Observed/ Certified/ Observed/

Element
mg kg–1 mg kg–1 mg kg–1 mg kg–1 mg kg–1 mg kg–1 mg kg–1 mg kg–1

Ag (0.36 ± 0.02) — — — — — 305.7 ± 5.8 309 ± 11

Cd (0.82 ± 0.03) — 28.2 ± 1.3 29.3 ± 0.1 71.8 ± 1.2 73.3 ± 0.2 — —
Cr 70 ± 3 59.8 ± 2.0 59.6 ± 5.0 57.6 ± 1.0 426 ± 12 453 ± 13 296.3 ± 3.3 290 ± 1
Cu 137 ± 3 116 ± 3 65.4 ± 4.7 59.4 ± 0.1 130.6 ± 1.4 126 ± 12 295.1 ± 6.8 300 ± 1
Fe (48408) 41146 ± 300 55800 ± 2400 550500 ± 830 — — 296.8 ± 27 293 ± 6
Mn 1160 ± 15 1060 ± 10 9700 ± 100 9110 ± 50 904 ± 13 854 ± 10 — —
Ni 34.0 ± 1.2 31.0 ± 1.0 24.2 ± 2.1 23.2 ± 0.2 299 ± 5 273 ± 6 302.9 ± 6.8 273 ± 6
Pb 112 ± 4 116 ± 4 2700 ± 100 2650 ± 10 179.7 ± 2.1 168 ± 4 297.4 ± 9.6 277 ± 12
Zn 243 ± 7 223 ± 2 3800 ± 300 3580 ± 10 1055 ± 14 953 ± 9 — —

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