American Journal of Anal yt ical Chemistry, 2011, 2, 902-908
doi:10.4236/ajac.2011.28104 Published Online December 2011 (
Copyright © 2011 SciRes. AJAC
Surfactant Enhanced Chemofiltration of Zinc Traces
Previous to Their Determination by Solid Surphase
Mabel Vega1, Miriam Augusto1, María C. Talío2, Liliana P. Fernández2*
1Instituto de Ciencias Básicas, FFHA, Universidad Nacional de San Juan,
San Juan, Argentina
2Área de Química Analítica, Facultad de Química, Bioquímica y Farmacia, Universidad Nacional
de San Luis, Instituto de Química de San Luis
(INQUISAL-CONICET), Chacabuco y Pedernera, San Luis, Argentina
E-mail: *
Received October 19, 2011; revised November 27, 2011; accepted December 8, 2011
Surfactant enhanced chemofiltration on Nylon membranes pre-treated with hexadecyltrimethylammonium
bromide (HTAB) and eosin dye (eo) is proposed for zinc traces quantification by solid surphase spectro-
fluorimetry (SSF, λexc = 532 nm; λem = 548 nm). Operational variables which have influence on quantitative
retention of metal complex have been studied and optimized. At optimal experimental conditions, quantita-
tive recovery was reached with a detection limit of 0.662 pg·L–1 and quantification limit of 2.20 pg·L–1. The
calibration sensitivity was of 1.22 L·pg–1 for the new methodology with a linear range of 2.20 pg·L–1 to 779
pg·L–1 Zn (II). The tolerance levels of potential interfering ions were studied with good results. Recuperation
studies were carried out by standard addition method applied to natural water samples (San Juan, Argentine)
without previous treatment. The reproducibility (between-days precision) was also evaluated over 3 days by
performing five determinations each day. CV% was 0.37. The performing obtained in sensitivity and selec-
tivity thanks to chemofiltration step, converts the proposed methodology in an adequate alternative to con-
ventional techniques for Zn (II) traces determination.
Keywords: Zinc Traces, Eosin Dye, Surfactant Enhanced Chemofiltration, Solid Surphase Fluorescence,
Natural Water Samples
1. Introduction
Water sources contamination is a frequent form of envi-
ronmental pollution. Despite of great advances in mod-
ern analytical instrumentation, preconcentration proce-
dures are still often required for the precise and accurate
determination of trace metals in natural waters. Many
techniques, e.g. solvent, cloud point and solid-phase ex-
tractions, volatilization, electrodeposition, ion-exchange
and coprecipitation, have been combined with instru-
mental analytical methods so far [1-4].
Solid phase extraction (SPE) is a rapid, simple, eco-
nomical preconcentration step, environmentally-friendler
than the traditional liquid–liquid extraction. SPE fol-
lowed by electrothermal atomic absorption spectrometry
(ET-AAS), inductively coupled plasma atomic emission
(ICP-AES) or ICP-mass spectrometry are suitable for
analysis of metal traces [5,6].
Nowadays, investigators are interested in improving
the selective preconcentration of the sorbents used in
SPE. This objective is particularly important when ana-
lyzing complex matrices. Several solid materials as filter
papers, silica gel, exchange resins, aluminium oxide,
poly(vinylalcohol), C18 membranes, cyclodextrines, be-
tween others have been successfully employed as sup-
ports for SPE. Recently, Nylon has proved to be an ade-
quate support for luminescent detection of organic com-
pounds [7-9]. Experimental results shown that this sup-
port possesses good selectivity, low background signal
and can be used without previous treatment.
Zinc is an essential trace element of great importance
in living beings. It plays an important role in several bio-
chemical processes [10], however, if it is in excess, this
metal can also produce damage in the human body, in-
cluding nausea, vomiting, fever, arrhythmias/dysrhyth-
mias and increase susceptibility to autoimmune reactions,
between others [11].
Spectrophotometry [12,13], atomic absorption spec-
trometry [14,15], neutron activation analysis [16], induc-
tively coupled plasma-atomic emission spectroscopy
[17,18], and inductively coupled plasma-mass spectro-
metry [19,20] are widely applied to the determination for
zinc at trace levels.
The use of micellar media has been investigated in
order to improve analytical parameters of instrumental
methodologies such as sensitivity and selectivity [21-25].
Particularly, micellar enhanced separation method have
been extensively studied for removal of heavy metals
and organic pollutants by using ultrafiltration membranes
[26-28]. Additionally, the feasibility of realizing chemi-
cal separation processes without polluting solvents has
turned to surfactant organized media in a very attractive
In this work, chemisorption of zinc on nylon mem-
branes previously treated with diluted solution of cati-
onic surfactant HTAB and eo dye is proposed for subse-
quent quantification by SSF in natural water samples.
The study was carried out analyzing the different factors
which influence on the chemisorption processes and
fluorescence intensity of the Zn (II)-HTAB-eo associa-
tion. Additionally, the stability of samples was also ex-
plored during a period of one month.
2. Experimental
2.1. Reagents
1 × 10–9 mol·L–1 Zn (II) stock solutions were prepared by
dilution of 100 g ·mL–1 standard solution plasma-pure
(Leeman Labs, Inc.)
1 × 10–7 mol·L–1 eosin (sodium bromofluorescein,
C20H6O5Br4Na2, H.E-Daniel Ltd., England) stock solu-
tion was weekly prepared by dissolution of the appropri-
ate amount in ultrapure water.
A 1 × 10–2 mol·L–1 HTAB purchased from Tokyo Ka-
sei Industries (Chuo-Ku, Tokyo, Japan) was prepared by
dissolution of the appropriate amount in ultrapure water.
A 1 × 102 mol·L1 sodium tetraborate (Mallinckrodt
ChemicalWorks, New York, Los Angeles, St. Louis,
USA) solution was prepared, obtaining the desired pH by
addition of dilute HClO4 (Merck) or NaOH (Mallinck-
rodt Chemical Works) solutions.
Nylon membranes (Millipore, Sao Paulo, Brazil) 0.45
μm pore size and 47 mm diameter were used in chemi-
sorption studies.
All used reagent were analytical quality.
2.2. Apparatus
Fluorescence measurements were done using a Shimadzu
RF-5301 PC spectrofluorometer equipped with a 150 W
Xenon lamp and solid sample holder with GF-UV35 filter.
Instrument excitation and emission slits were both ad-
justed to 1.5 nm.
A combined glass electrode and a pHmeter (Orion
Expandable Ion Analyzer, Orion Research, Cambridge,
MA, USA) Model EA 940 were used for pH adjust-
A centrifuge was used in water sample processing.
A Gilson Minipuls 3 peristaltic pump with PVC
pumping tubes coupled to an in-line filter holder 47 mm
(Millipore) was use for filtrating sample/standard solu-
All used glass materials were previously washed with
a 10% v/v HNO3 water solution and then with ultrapure
2.3. General Procedure
Nylon membranes were impregnated in batch by contact
with 100 µL HTAB 1 × 10–2 mol·L–1 and then in 5 mL of
1 × 10–7 mol·L–1 eo solution during 5 min. Membranes
were dried at room temperature and reserved in dried
ambient (20˚C - 25˚C) up to filtration step. Later, a dried
membrane was put in filtration holder.
Adequate volume of sample/standard Zn (II) solution
(2.20 pg·L–1 - 779 pg·L–1), 100 μL buffer borax solution 1
10–2 mol·L–1 (pH = 9.22), were placed in a 10 mL gradu-
ated centrifuge tube. The whole mixture was diluted to
10 mL with ultrapure water. Systems were filtrated
across eo-HTAB-impregned membranes, using peristal-
tic pump at 0.1 mL·min–1 and dried at room temperature.
Zinc was determined on the membranes by SSF at λem
= 548 nm and λexc = 532 nm, using a solid sample holder
(Figure 1).
2.4. Interferences Study
Different amounts of ions (1/1, 1/10, 1/100 and 1/1000
Zn (II)/interferent ratio) were added to the test solution
containing 0.49 ng·L–1 Zn (II) and general procedure was
2.5. Accuracy Study
250 μL of water samples were spiked with increasing
amounts of Zn (II) (0.33 - 0.65 ng·L–1). Zinc contents
were determined by proposed methodology.
Copyright © 2011 SciRes. AJAC
Copyright © 2011 SciRes. AJAC
Step1 Step 2 Step 3
Nylon membrane
impregned in
5 minutes
Membrane dried at
room temperature
Step 4 Step 5
Zn(II) sample/standard –
buffer pH 9.22
Nylon membrane
Filtration teflon module
Filtration teflon module h.
Solid surphase fluorescence
Figure 1. Schematic representation of general procedure of proposed methodology.
2.6. Precision Study
The repeatability (within-day precision) of the method
was tested for replicate water samples (n = 3) spiked
with 0.49 ng·L–1 Zn (II); metal contents were determined
by proposed methodology.
2.7. Recovery Procedure
250 μL water was spiked with increasing amounts of
Zn(II) (0.33 - 0.65 ng·L–1) at three levels and treated, fol-
lowing proposed methodology.
2.8. Stability Test of Water Samples
250 μL of water samples were spiked with increasing
amounts of Zn (II) 0.33 - 0.65ng L–1). Zn (II) contents
were determined by proposed methodology at different
times (1 day, 1 week, 2 weeks, 1 month after sampling)
using preservation in refrigerator at 4˚C.
3. Results and discussion
3.1. Study of Zn (II)-Eo System: Support and
Surfactant Selection
Eosin yellowish [2’, 4’, 5’, 7’-tetrabromo-3’, 6’-di-hy-
is a xanthene group dye with strong absorption in the
visible region of electromagnetic spectrum. Previous
studies have shown that this dye presents the feasibility
of forming association with metallic ions [25,29,30].
In preliminary assays, aqueous systems containing eo
and Zn (II) were prepared adjusting a pH from 6.0 to
10.0. Fluorescent signals were checked without alteration
of fluorescent emission of eo for presence of Zn (II). As
a consequence, SPE was explored for eo-Zn (II) system;
Nylon membrane as solid support was put in contact with
eo solution in order to assuring the retention of this dye.
In this step, it didn’t observe variation on eo fluorescent
signal for the presence of Zn (II).
Surfactants have been used in enhancing membrane
filtration for the removal of metal ions in aqueous solu-
tions due to their high selectivity properties [26,27].
Taking into account previous reports, the support im-
pregnation was carried out with different surfactants and
eo dye. Then, Zn (II) solution was filtrated across the
pre-treated support. In presence of Zn (II), an enhance-
ment of fluorescent emission of eo was observed for
systems with the cationic HTAB surfactant (Figure 2).
Enhanced separation by surfactant can be explained for
the adsorption of metallic ions on the polar head of the
surfactants surface.
Several solid supports were assayed for SPE: cellulose
acetate, Nylon and esters mixture. Among examined
supports, Nylon membranes showed to be the most ade-
quate for the quantitative metal retention. Likewise, Ny-
lon membranes are usable without purification, are
commercially available and it poses low cost
The use of Nylon membranes resulted satisfactory to
performing of fluorescent eo signal with low background.
Additionally, the pre-treated solid support with HTAB-
eo mix allowed Zn (II) retention put in evidence by the
increased fluorescent response. Quantitative metal reten-
tion was verified by double filtration on a new pre-
treated membrane and later fluorescent detection with a
signal similar to eo dye (blank).
Therefore, Nylon membrane support was selected to
following experiences.
3.2. Effect of Eosin and HTAB Concentration
To ensure quantitative Zn (II) retention on Nylon support,
tests were carried out by varying the concentration of eo
from 1 × 10–8 to 5 × 10–7 mol·L–1. A concentration of 1 10–7
Figure 2. Fluorescent spectra (SSF) of Zn (II)-HTAB-eo
system. (a): Membrane of nylon treated with HTAB and eo
1 × 10–7 mol–1; (b): Idem A with Zn(II) 0.33 ng·L–1; (c):
Idem A with Zn(II) 0.49 ng·L–1.
mol·L–1 was selected as optimal because it resulted
enough to warrant an adequate eo excess respect to
expected Zn (II) contents in natural water samples.
HTAB concentration was too optimized. With this
propose, systems were preparing varying HTAB con-
centration from 0 to1 × 10–3 mol·L–1, maintaining con-
stant eo concentration at 1 × 10–7 mol·L–1. The best im-
provement in fluorescent signal was achieve at HTAB
concentration of 2 × 10–4 mol·L–1 (Figure 3). At this
concentration level, monolayer on solid support are
formed for surfactant monomers characterized by
extensive surface area, facilitate Zn (II) retention.
This concentration was selected for following studies.
3.3. Membrane/HTAB-eo Contact Time
Nylon membranes were submerged in 2 × 10–4 mol·L–1
HTAB and 1 × 10–7 mol·L–1 eo solution during different
time. Quantitative eo retention was reached in five min-
utes of contact time checked by SSF eo signal. Following
assays were realized using Nylon membranes pre-treated
during five minutes.
3.4. Influence of Filtration Flow Rate
With the purpose of optimize the sampling rate, the fil-
tration speed was varied between 0.05 and 0.25
mL· mi n –1, maintaining constant other experimental con-
ditions. A filtration flow rate of 0.1 mL·min–1, was the
most adequate for the quantitative Zn (II) retention.
3.5. pH of Retention. Buffer Concentration
pH is an experimental variable of weight when exists an
association equilibrium as in the eo-Zn(II) system. In
order to obtain the optimal retention of metal and the
maximum SSF signal, aqueous systems containing con-
Figure 3. Influence of HTAB concentration on Zn (II) re-
tention. Zn (II) 0.49 ng·L–1 on Nylon membrane treated
with eo 1 × 10–7 mol·L–1.
Copyright © 2011 SciRes. AJAC
stant concentration of Zn (II) were prepared, adjusting
their pH values between 7.0 and 10.5 by addition of
buffer solution sodium tetraborate. Then, solutions were
filtrated across the pre-treated support and SSF signals
were determined.
The obtained results showed a maximum level of re-
tention of Zn(II)-eo association for pH values of 8.7 to
9.5 (Figure 4). For following experiences, a pH value of
9.2 was chosen.
Then, the buffer concentration was tested in order to
obtain the maximum fluorescent signal. The concentra-
tion of sodium tetraborate buffer was varied from 5 ×
10–3 to 0.01 mol·L–1. Buffer concentration of 5 × 10–2
mol·L–1 was chosen as optimal.
Table 1 summarizes studied experimental parameters
and their optimal values. At optimal experimental condi-
tions, Zn (II) quantitative retention was verified by a
second filtration procedure through a new pre-treated
membrane; the obtained fluorescence signal was similar
to eo signal (blank).
Figure 4. Influence of pH on Zn (II) retention. Zn (II) 0.49
ng·L–1 on Nylon membrane treated with 2 × 10–4 mol·L–1
HTAB and eo 1 × 10–7 mol·L–1.
Table 1. Studied optimal experimental conditions and ana-
lytical parameters for Zn (II) determination by SSF.
Parameters Studied range
Optimal condi-
pH 7.0 - 10.5 9.20
Buffer sodium
tetraborate 5 × 103 - 0.01 mol·L1 5 × 102 mol·L1
Eo concentration 1 × 108 – 5 × 107
mol·L1 1 × 107 mol·L1
HTAB concentration 0 – 1 × 103 mol ·L 1 2 × 10-4 mol·L1
Contact time 0 - 700 min 5 min
LOD - 0.66 pg·L1
LOQ - 2.20 ng·L1
LOL - 2.20 - 779 pg·L1
Calibration sensitivity - 1.22 L·pg1
4. Applications
Stability Studies of Water Samples
Natural water samples belonging to San Juan (Argentine)
were spiked with increasing amounts of Zn (II) and then
processed at different times by proposed methodology.
Samples were maintained in refrigerator at 4˚C. The re-
sults are presented in Table 2; it can be inferred that wa-
ter samples show optimal stability for Zn (II) determina-
tion by developed methodology, during the studied pe-
riod of one month.
5. Tolerance Studies
In order to study the effects of representative potential
interfering species on Zn (II) determination using devel-
oped methodology, assays were carried out at the con-
centration levels at which they may occur in the natural
water samples. An ion was considered interfering, when
it caused a variation in the fluorescent signal of the sam-
ple greater than ±5%. Figure 5 shows the obtained re-
Table 2. Stability test of water samples.
Time (h) Zn(II) (ng·L–1) CV
0 1.10 0.37
24 1.12 0.21
168 (seven days) 1.14 0.26
360 (fifteen days) 1.14 0.22
720 (thirty days) 1.12 0.32
Figure 5. Tolerances of cations for developed methodology
for 1/1000 Zn (II)/interferent ratio. 1: Zn (II) 0.49 ng·L–1; 2:
Zn (II) in presence of K (I); 3: Zn (II) in presence of Na (I);
4: Zn (II) in presence of Ni (II); 5: Zn (II) in presence of Fe
(III); 6: Zn (II) in presence of Ca (II); 7: Zn (II) in presence
of Mn (II); 8: Zn (II) in presence of Ba (II); 9: Zn (II) in
presence of Sr (II); 10: Zn (II) in presence of Pb (II); 11: Zn
(II) in presence of Cd (II); 12: Zn (II) in presence of Mg
Copyright © 2011 SciRes. AJAC
sults for assayed cations. Obtained results put in evi-
ence the usefulness and robustness of the new method-
ology for the quantification of Zn (II) traces in presence
of different ions at 1/1000 Zn (II)/interferent ratio.
6. Analytical Performance
Standard addition method was applied in order to evalu-
ating the accuracy of the methodology. The reproducibil-
ity of the method was estimated repeating the proposed
approach, 3 times for each sample. Recoveries of Zn (II)
in eight natural water samples based on the average of
replicate measurements are illustrated in Table 3.
For determine the repeatability (within-day precision)
of the method, replicate water samples (n = 5) were ana-
lyzed by proposed methodology. The precision was bet-
ter than 0.15 CV% for zinc contents. The reproducibility
(between-days precision) was also evaluated over 3 days
by performing six determinations each day. CV% was
7. Conclusions
Zinc traces determination has been realized by SSF using
separation/preconcentration step on Nylon membranes
containing cationic surfactant HTAB at sub-micellar
concentration and eo dye. Enhanced separation and se-
Table 3. Recovery study. Zn (II) determination in natural
water of San Juan (Argentine).
Sample Zn(II) added
Zn(II) found ±SD
(%, n = 3)
1.10 ± 0.37
1.42 ± 0.21
1.57 ± 0.14
1.46 ± 0.11
1.80 ± 0.18
1.93 ± 0.12
0.98 ± 0.06
1.31 ± 0.05
1.46 ± 0.15
1.08 ± 0.16
1.37 ± 0.06
1.57 ± 0.08
1.00 ± 0.03
1.36 ± 0.14
1.54 ± 0.03
1.02 ± 0.14
1.35 ± 0.19
1.52 ± 0.03
1.09 ± 0.13
1.47 ± 0.03
1.57 ± 0.08
0.41 ± 0.12
0.73 ± 0.04
0.92 ± 0.03
lectivity can be explained for the adsorption of metalli-
cions on the polar head of the surfactants surface. The
reached sensitivity was comparable at those arrived with
atomic spectroscopies. The good tolerance at elevated
levels of regular foreign constituents put in evidence the
high selectivity and versatility of the new methodology.
Stability of natural water samples during a month was
studied with good results. Precision and accuracy were
tested with good results. The proposed methodology
represents a contribution to zinc environmental monitor-
ing and a suitable alternative to routine metal analysis
methods, with advantages referred to simplicity, low cost
and adequate sampling rate, using a simple and inexpen-
sive instrumental. The developed methodology was suc-
cessfully applied to Zn (II) quantification to natural wa-
ter samples belonging of different sites of sampling of
San Juan (Argentine).
8. Acknowledgements
The authors wish to thanks to Instituto de Química San
Luis-Consejo Nacional de Investigaciones Científicas y
Tecnológicas (INQUISAL-CONICET) and National
University of San Luis (Project 22/Q828) for the finan-
cial support.
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