Microchim Acta (2008) 162: 127–132

DOI 10.1007/s00604-007-0838-4

Printed in The Netherlands

Original Paper

The microdetermination of thallium by ICP-AES after previouspreconcentration on modified silica. Comparison withintegrated-platform graphite atomic absorption spectrometry

Kristyna Urbankova, Lumır Sommer

Faculty of Chemistry, Institute of Environmental Chemistry and Technology, Brno University of Technology, Brno, Czech Republic

Received 23 April 2007; Accepted 9 July 2007; Published online 14 January 2008

# Springer-Verlag 2008

Abstract. The extensively used technique of ICP-

AES determination of thallium at optimal lines

190.864(1) nm and 190.864(2) nm in different orders

n1¼ 135 and n2¼ 136 shows just limited sensitivity.

Thus, TlIIICl4� was preliminarily separated and pre-

concentrated on modified silica C18 in the form of ion

associate with various cationic surfactants. The sur-

factant Zephyramine with 0.1 M HCl served as a sor-

bent surface activator during quantitative sorption of

5–20mg of thallium(III) from 15–1000 mL of aque-

ous solutions. The subsequent elution with 96% etha-

nol and the final determination with ICP were carried

out after evaporating the solvent. Limited interference

by SO42�, NO3

�, Cl�, Naþ, Kþ, Ca2þ, Mg2þ, Al3þ and

Fe3þ during the sorption was observed. The above

procedure is applicable for the determination of thalli-

um in surface and mineral waters.

The use of PdMg(NO3)4 or NH4VO3 modifiers and

Zeeman splitting for the sample background elimina-

tion with the integrated-platform graphite AAS tech-

nique at 276.8 nm was found to be suitable for the

direct determination of thallium(I) up to 30 ng in the

presence of the following species: large amounts of

chlorine, 100 fold excess of NO3�, SO4

2�, Ca2þ,

Mg2þ, Kþ, Al3þ and Fe3þ. The temperature program

starting by stepwise drying between 90 and 110 �C

followed by pyrolysis at 700 �C and the atomisation

at 1700 �C was optimized this way. This method can

be applied for the determination of thallium in urine

and spruce needles.

Keywords: Thallium; ICP-AES; preconcentration on Silica C18;

cationic surfactants; integrated-platform graphite AAS

Thallium represents an outstanding element that is

more toxic for humans than lead. Moreover, it is

retained longer than it in human body. It causes seri-

ous damage of the organism. Thallium(III) is more

toxic than thallium(I). On the other hand, it is easily

reduced in the presence of biological material [1].

Thallium in the Tl(I) speciation is characterized for

its ubiquitous presence in the environment, especially

in atmosphere, waters and soil but often in limited

concentrations. The concentrations of thallium rang-

ing from 0.06 ng �m�3 in clean air to 14 ng �m�3 in

industrial and urban areas have been reported. In areas

not contaminated by thallium its content is <0.01–

0.02 mg �L�1 in seawater and 0.01–1mg �L�1 in river

water. In wastewaters it increases up to 2400mg �L�1.

Thallium often penetrates into the soil (more than

�20 mg � kg�1 in dry residue). Increased thallium lev-

Correspondence: Lumır Sommer, Faculty of Chemistry, Institute

of Environmental Chemistry and Technology, Brno University of

Technology, 612 00 Brno, Czech Republic

e-mail: sommer@fch.vutbr.cz

els are found in plants growing on such soil [2]. At

present, advanced and instrumental analytical meth-

ods must be used for the determination of micro

amounts of thallium [3–5].

The use of ET-AAS with graphite for the determi-

nation of thallium has been frequently described but

the results are sometimes questionable [6–13]. They

are often influenced by volatility of thallium species

from various matrices, especially in the presence of

chlorides. Modifiers of analytes are necessary. For the

determination of thallium in wine a previous separa-

tion by extraction must be applied [14].

The ICP-AES determination of thallium in aque-

ous solutions was mentioned by Barnes [15] and

Floyd et al. [16]. The determination on the line

Tl(I) 276.787 nm needs background correction on

276.710 nm. The ionic line Tl(II) 190.816 nm is par-

tially overlapped by Tl 190.801 nm. The technique has

limited detection efficiency at 0.15 mg �mL�1 Tl, thus

preconcentration is required. The preconcentration of

Tl(III) on dynamically modified silica C18 in the pres-

ence of cationic surfactants was apparently used by

Otruba et al. [17] for the first time.

New aspects concerning preconcentration of thal-

lium on modified silica C18 in the presence of cat-

ionic surfactants done before the determination of

thallium by ICP-AES and its application for various

kinds of waters are described in this paper. A mod-

ified procedure for the determination of ng Tl(I)

amounts in urine and spruce needles not employing

sorption but using integrated-platform graphite AAS

and Zeeman lines splitting is also described in this

article.

Experimental

Chemicals

All chemicals and solvents used were of analytical grade quality.

Thallium standard as thallium(I) nitrate contains 1.000 �0.002 g Tl L�1. It was purchased from AnalyticaTM Prague, Czech

Republic (www.analytika.cz).

Surfactants: Benzyldimethyl tetradecylammonium chloride

(Zephyramine, M.w. 404,0) from Merck (www.merck.de) Germany,

benzyldimethyldodecylammonium bromide (Ajatin, M.w. 384,45)

from Profarma, Czech Republic (www.profarma.cz) and 1-ethoxy-

carbonylpentadecyltrimethylammonium bromide (Septonex, M.w.

422,14) from Aventa, Czech Republic. 0.1 M aqueous stock so-

lutions were prepared by dissolution of appropriate amounts

of substances in water and stored not longer than one week.

Ammonium vanadate, palladium nitrate, magnesium nitrate and

ascorbic acid were purchased from AnalyticaTM Prague, Czech

Republic. PdMg(NO3)2 was prepared from 0.1% Pd(NO3)2 to

0.05% Mg(NO3)2. Sorbent: octadecylsilica SeparonTM SGX C18

with particle size 60mm, TessekTM Prague, Czech Republic

(www.tessek.com) were placed in plastic cartridges, size 20�9 mm.

Samples

Surface water was sampled from the river Svitava (Moravia). The

samples were filtered on a dense filter paper, acidified with HCl to

0.1 M and stored in dark bottles. Mineral waters of different prove-

nience were acidified with HCl to 0.1 M. and degassed by short

boiling. The concentration of natural thallium, if present, was be-

low the detection limit. Suitable amounts of thallium were added

to samples and the solutions were equilibrated and analyzed after

24 h.

Instrumentation

A single beam absorption spectrophotometer ZEEnit 650, Analytik

Jena A. G. Germany, with a grating monochromator containing

1400 grooves per mm and photomultiplier tightly placed behind the

grating and Zeeman splitting device were used. A HLC discharge

lamp enabled to record lines with their half-width �0.002 nm at

current intensity 4 mA. The atomisation took place in graphite tube

with an integrated platform. The Zeeman splitting was used for

background elimination. The line 276.8 nm served for thallium de-

termination at an argon flow rate 300 mL min�1.

An echelle based ICP-spectrometer with a prism-predisperser

IRIS APTM, (Thermo Jarell ash) and CID detector with 512�512

pixels for the region of 195–900 nm; axial plasma discharge and

echelle grating with 54.4 lines were used. The plasma source

generator was set at 27.12 MHz and the power output at 1.15 or

1.35 kW. Argon flows through the plasma torch were chosen at

12 L, 0.5 L and 1 L �min�1. The Meinhard nebulizer was fed with

a peristaltic pump. Signal integration time was 30 sec, results were

calculated as an average of 3 measurements. A two-point back-

ground correction lowered the noise of the plasma. The increased

power output of 1.35 kW enabled decomposition of interfering

organic compounds in the plasma. It also raised the slope of cal-

ibration plots.

Retention and elution devices

Vaccum pump-operated vacuum suction device DorcusTM (Tessek,

Prague, Czech Republic). A peristaltic pump PCD 81=82.4K

(Kou�rrilTM, Kyjov, Czech Republic) was attached with 3 mm wide

silicon tubing to the cartridges containing the sorbent. Solution flow

rate was 1 mL �min�1.

Limit of detection

Two ways of calculation were used Eq. (1) for the evaluation of

detection limit from the 10 times measured blank according to

IUPAC [19]

LOD ¼ 3 s=S ð1Þ

where s represents the standard deviation of one measurement and S

is the slope of the calibration plot. The second way of calculation

follows: from the zero values of the lower and upper confidence

interval of the calibration plots XD� and XD

� are calculated ac-

cording to Graham [20]. Evaluation of two different kinds of detec-

tion limits is performed since they show considerable differences

when background data or confidence intervals of the calibration

plots are used for calculations.

128 K. Urbankova, L. Sommer

Results and discussion

Optimized heating program for the integrated-platform

graphite AAS for micro sampling of thallium

The drying must be performed in the following steps:

from 75–100, 100–150 and 150–200 �C. The sample

pyrolysis is performed between 600 and 1000 �C and

the atomisation step between 1500 and 1800 �C. A

number of heating programs were tested with regard

to the course of calibration plots between 0 and

30 ng Tl �mL�1.

5mL of 30 ng �mL�1 TlNO3 and 5mL PdMg(NO3)4

modifier in the presence of 0.65% HNO3 were sampled

into the sampling device of the spectrometer under

300 mL �min�1 argon flow. Absorbance was measured

at 276.8 nm. The argon flow-stop was used during the

atomization step. The highest slope of the linear plot

and lowest background was observed for the following

heating program. This program is more precise than

those used in literature [8, 10, 12] (Table 1).

Effect of modifiers

The effects of PdMg(NO3)4, PdNH4 (NO3)3, Pd(NO3)2,

NH4VO3 and ascorbic acid on calibration plots men-

tioned in literature [9, 11] were tested (Table 2). 5 mL

of PdMg(NO3)4 and NH4VO3 represent the optimum

for the determination of less than 30 ng �mL�1 Tl. The

calibration plot was strictly linear with a correlation

coefficient of 0.996. The regression equation for

the integrated absorbance is A¼ 0.0026xþ0.00073.

There is no difference between Tl(I) and Tl(III) in the

sample. The detection limits for the optimized proce-

dure are LOD 0.72mg �L�1, XD� 0.73mg �L�1 and XD

�

1.99mg �L�1 for the PdMg(NO3)4 and 0.79mg �L�1,

XD� 0.81 mg �L�1 and XD

� 1.85mg �L�1 for the

NH4VO3 modifier.

Interferences

Interferences of selected modifiers were tested. No

interferences with 250 fold excess of Cl� and 100 fold

excess of NO3�, SO4

2�, Ca2þ, Mg2þ, Kþ, Al3þ and

Fe3þ in the presence of 0.65% HNO3 and with mod-

ifiers PdMg(NO3)4 or NH4VO3 were observed. For

10mg Tl L�1 and 5mL of modifiers the recoveries were

more than 99%. Samples were evaluated in triplicate.

Sorption of Tl(III) on the octadecylsilica sorbent

SeparonTM SGX C18 and determination by ICP-AES

The following spectral lines (nm) of high orders were

tested: Tl 190.864, Tl 276.787, Tl 351.924, Tl

377.572 and Tl 535.046. The lines 190.864(1) and

190.864(2) nm differing in order (n1¼ 135, n2¼ 136)

were selected for thallium finally. They show the

highest intensity of the signal, the highest slope of

the calibration plots and very low values of the back-

ground intensity. Four-point linear calibration plots

were used for the evaluation of thallium concentra-

tions in the range of 0–20mg �mL�1. The slope of

calibration plots and the detection limit are influenced

by the presence of cationic surfactant.

Evaluation of recovery during the preconcentration

of thallium on modified silica

The retention and elution efficiency were tested inde-

pendently by ICP-AES. The evaluation of the input

Table 1. Optimal graphite furnace temperature program for deter-

mination of thallium

Step Temp. (�C) Ramp (�C � sec�1) Hold time (sec)

Drying 1 90 5.0 20

Drying 2 105 3.0 20

Drying 3 110 2.0 10

Pyrolysis 700 250 10

Atomisation 1700 1500 3.0

Clean-out 2700 500 4.0

Table 2. Comparison of slope of the calibration plot and back-

ground absorbance for various modifiers

Modifiera Slope of the

calibration plot

Background

absorbance

PdMg(NO3)4 0.0026 0.0007

PdNH4(NO3)3 0.0013 0.0027

Pd(NO3)2 0.0008 0.0016

NH4VO3 0.0023 0.0009

C6H8O6 0.0009 0.0005

a 5 mL of 30 ng Tl �mL�1 and 5 mL of modifier sample.

Table 3. Comparison of detection limits for selected surfactants

Surfactant Detection limits (mg �mL�1)

LOD XD� XD

�

No surfactant 0.31 0.27 1.13

Zephyramin 0.24 0.23 0.91

Ajatin 0.25 0.25 1.17

Septonex 0.22 0.26 1.43

Comparison with the integrated-platform graphite atomic absorption spectrometry 129

and output concentration of thallium was evaluated

from ICP calibration plots. The confidence interval

of the mean recovery resulted from 3 independent

sorptions and elutions using the span between the

lowest and highest values of the variation interval

[18]. The recovery is defined by the expression

R ¼ cðTlÞeluted=cðTlÞapplied on the column � f

where f is the enrichment factor.

Effect of surfactants on the sorption

TlCl4� is assumed in the solution for the preconcen-

tration on SeparonTM SGX C18 with various cationic

surfactants. Various kinds for the oxidation of 20mg Tl

were tested, but the use of 0.02 M KMnO4 in 0.1 M

HCl at 80 �C and during 20 min is most suitable. The

Tl(III) was usually sorbed from 0.1 M HCl. Another

sorption environment represents solutions of 0.1–

0.3 M HCl in various volumes which can be used after

cooling. Prior to the sorption the column of the sor-

bent was always conditioned by 10 mL of 0.1 M

HCl and 10 mL of 0.005 M surfactant, mostly by

Zephyramine. The flow rate of the solution was al-

ways 1 mL �min�1. The column was rinsed with

10 mL of distilled water prior to the elution of thalli-

um with 10 mL ethanol. The ethanol was removed by

evaporation in the presence of 0.3 M HCl and the

residual solution was filled with distilled water to

10 mL in a volumetric flask and analyzed by ICP.

The recovery of sorbed 0.5–4.0 mg Tl �mL�1 from

50 mL of solution in the presence of Zephyramine,

Ajatine and Septonex is practically 100%. However,

the most successful approach is the conditioning of the

column with 0.1 M HCl and 0.005 M Zephyramine

and without surfactant in solution.

The efficiency of sorption of Tl(I) on SeparonTM

SGX C18 activated with the Zephyramine under opti-

mal conditions is very low compared to Tl(III). The

recoveries are even 15.00� 2.01% in the presence of

0.3 M NaCl and 68.90� 1.42% in the presence of 0.3 M

NaBr. The efficiency of Tl(I) and Tl(III) of various con-

ditionings during the sorption is summarized in Fig. 1.

Concerning Fig. 1, Tl(I) in the presence of 0.1 M

HCl. The sorbent was conditioned with 0.1 M HCl and

0.005 M Zephyramine (1). Tl(I) was acidified with

0.1 M HCl and 0.3 M NaCl was added and the sorbent

was conditioned in the same way as before (2), Tl(I)

was acidified with 0.1 M HBr and the sorbent was

treated with 0.1 M HBr and 0.005 M Zephyramine

(3), Tl(I) was acidified with 0.1 M HBr in the presence

of 0.3 M NaBr, the sorbent was adjusted as before (4),

Tl(III) was acidified with 0.1 M HCl, the sorbent was

conditioned with 0.1 M HCl without Zephyramine (5),

Tl(III) was acidified with 0.1 M HCl and 0.005 M

Zephyramine was added, sorbent was treated with

0.1 M HCl without Zephyramine (6), Tl(III) was acidi-

fied with 0.1 M HCl and 0.005 M Zephyramine was

added, sorbent was conditioned with 0.1 M HCl and

0.005 M Zephyramine was added (7), Tl(III) was acid-

ified with 0.1 M HCl, the sorbent was conditioned with

0.1 M HCl and 0.005 M Zephyramine was added (8).

The effect of sample volume on the retention

of thallium

The sorption efficiency of silica SGX C18 in depen-

dence on the volume of the solution containing 0.4–

0.02 mg Tl(III) in the presence of 0.1 M HCl and

0.005 M Zephyramine on the sorbent allows the sorp-

tion with the maximum enrichment factor 100. The

recovery of Tl(III) is 99.32%.

Table 4. Recoveries for thallium(III) using various surfactantsa

Surfactants Concentration of Tl(III) (mg �mL�1)

0.5 1.0 2.0 3.0 4.0

No surfactant 6.08 6.98 6.71 6.88 6.79

Zephyramine 96.45 98.45 99.15 101.2 100.4

Ajatine 96.83 97.77 99.36 99.10 99.38

Septonex 96.58 97.30 97.88 99.58 100.3

a 50 mL of solutions, sorbent surface activated by the surfactant,

190.864(2) nm.

Fig. 1. Comparison of various conditions for the sorption Tl(I)

and Tl(III) on SeparonTM SGX C18 in 50 mL of solution with

20 mg of Tl, the elution with 10 mL of 96% ethanol, the flow rate

1.12 mL �min�1, calculated in triplicate. Short segments corre-

spond to the confidence intervals

130 K. Urbankova, L. Sommer

Interferences during the sorption

The influence of selected ions on the retention of 10mg

of Tl on Separon SGX C18 in 50 mL of solution in the

presence of 0.1 M HCl and 0.005 M Zephyramine for

the flow rate of 1.12 mL �min�1 were tested. The col-

umn was washed with 10 mL of distilled water prior

to the elution with 10 mL of 96% ethanol. Limiting

mol and mass ratios for interferents and the recoveries

are given in Table 5. Unfortunately, the sorption effi-

ciency is influenced by various cations and anions

which are present in waters.

Applications

Determination of thallium in spruce needles

and in urine with ET-AAS

0.2 g of needles were dried at 60 �C and then pulver-

ized in a porcelain mortar and mineralized with 5 mL

of conc. HNO3 and 5 mL of 30% hydrogen peroxide.

The solution was partly evaporated under the IR lamp

and spiked with 1, 5 and 10 ng of Tl �mL�1. The sol-

ution was equilibrated for 24 h. 5 mL of the sample and

5 mL of PdMg(NO3)4 modifier were placed into the

sampler of the absorption spectrometer ZEEnit 650

and analyzed under 300 mL �min�1 argon and Zeeman

splitting. The analysis was carried out in triplicate.

20 mL of morning urine were acidified with HNO3

to 0.65% and spiked with 1, 5 and 10 ng of Tl �mL�1.

The sample was equilibrated for 24 h in a cool place.

5 mL of the sample with 5 mL of PdMg(NO3)4 were

placed into the instrument sampler and analyzed in the

above way. The recoveries can be found in Table 6.

Determination of thallium in river and mineral

water with ICP-AES

100 mL of river water or degassed mineral water with

less than detectable amounts of thallium were acidi-

fied with HCl to 0.1 M concentration and spiked with

0.5, 2 and 4mg �mL�1 of Tl(I) in the form of TlNO3.

The solution was equilibrated for 24 h and Tl(I) was

oxidized with 0.02 M KMnO4. The solution was ap-

plied on the column of SeparonTM SGX C18 activated

with 10 mL of 0.01 M HCl and 10 mL of 0.005 M

Zephyramine at flow rate 1 mL min�1. The column

was then rinsed with 10 mL of distilled water and

thallium was eluted with 10 mL of ethanol and pro-

cessed as mentioned before. The ICP calibration plot

for the sampled water was constructed after further 3

spikes of 1, 3, and 5 mg of Tl �mL�1 and evaluated

Table 6. The recovery (%) of thallium(I) in various samplesa

Spikes of Tl (mg �L�1) Needles Urine

0 not detectable not detachable

1 99.20 � 0.67 98.60 � 0.58

5 98.62 � 0.31 100.7 � 0.29

10 99.47 � 0.93 99.75 � 0.97

a The analysis was carried out in triplicate.

Table 5. Recovery and limiting mol or mass ratio of selected

interferents against thalliuma on the sorption

Ion Mass ratio Recovery (%)

Cl��

105:l 99.34 � 1.12

NO3� 100:1 95.58 � 0.53

SO42� 50:1 97.31 � 0.41

Ca2þ 100:1 98.15 � 0.01

Mg2þ 10:1 99.53 � 0.01

Kþ 100:1 97.30 � 0.12

Al3þ 1:1 99.20 � 0.44

Fe3þ 1:1 85.61 � 0.16

a Evaluated for samples in triplicates at 190.864(1) nm; � mol

ratio.

Table 7. The recovery (%) of thallium in water samplesa

Spikes of Tl (mg �L�1) The recovery (%) Concentration of some accompanying ions in water sample (mg �mL�1)b

Naþ Kþ Ca2þ Mg2þ Al3þ Fe3þ

Mineral water

0 not detectable 111.1 25.16 86.51 30.86 – 0.02

0.5 98.91 � 2.10

2.0 99.62 � 2.63

4.0 99.67 � 0.42

River water

0 not detectable 19.32 4.19 45.26 5.42 0.49 0.63

0.5 99.55 � 2.51

2.0 100.3 � 2.41

4.0 99.24 � 2.40

a The analysis was carried out in triplicate, evaluated for 190.864(2) nm.b Evaluated preferently by ICP-AES.

Comparison with the integrated-platform graphite atomic absorption spectrometry 131

at 190.864(2) nm. Recoveries for the spikes are eval-

uated for the surface and mineral waters in triplicate.

The concentration of macrocomponents does not in-

terfere with the sorption of thallium on the column;

they were previously determined by ICP-AES.

Conclusion

The graphite-platform AAS is suitable for the direct

and selective determination of thallium(I) and (III)

in very low concentrations (below 30 ng) and mi-

crovolumes 5–25 mL in the presence of selected

PdMg(NO3)4 modifier using the optimal temperature

program. A number of ions did not interfere in this

case. This technique of determination of thallium is

suitable for analyses of spruce needles and urine. It

provides satisfying results.

Determination of thallium by ICP-AES at

190.864(1) nm and 190.864(2) nm in environmental

compartments like waters is not sufficiently sensitive.

The previous sorption of thallium(III) on silica SGX

C18 with particle size 60mm activated by cationic

surfactants Zephyramine is implemented and thallium

is preferably oxidized to Tl(III) with permanganate in

slightly acidic environment.

The prevailing mechanism employed during the

sorption is the interaction of TlCl4� on the sorbent

with dynamically bonded cationic surfactant, which

is accompanied by the formation of ion associate on

the sorbent-solution interface. The sorption is quan-

titative after 100 fold preconcentration of thallium

from 1 L of sample solution in the presence of the

Zephyramine-surfactant. This enables significant de-

crease of determination limit of thallium to 20 ng for

the analysis with ICP-AES.

After previous preconcentration on modified silica

SGX C18 the following detection limits were found:

0.72mg �L�1 (IUPAC) and XD� 1.99mg �L�1 (Graham)

for optimized integrated-platform graphite AAS and

0.24 mg �L�1 (IUPAC) and XD� 0.91 mg �L�1

(Graham) for ICP-AES at 190.864(2) nm.

The sorption from 0.1 M HCl and 0.005 M

Zephyramine could be used for the separation of

Tl(III) from Tl(I) and the determination of Tl(III) in

the presence of Tl(I).

Acknowledgement. Thanks are due to Associated Professor Miroslav

Fi�ssera for encouraging discussions.

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132 Comparison with the integrated-platform graphite atomic absorption spectrometry