Volume 7 Number 2 21 January 2015 Pages 383–800
Analytical
Methods
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ISSN 1759-9660
PAPER
Pavel N. Nesterenko et al.
Determination of trace magnesium and strontium in calcium carbonate
and calcareous skeletons of marine planktonic organisms using high
performance chelation ion chromatography
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Methods
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Cite this: Anal. Methods, 2015, 7, 416
Determination of trace magnesium and strontium
in calcium carbonate and calcareous skeletons of
marine planktonic organisms using high
performance chelation ion chromatography†
Yan Li,a Brett Paull,a Marius N. Müller‡b and Pavel N. Nesterenko*a
A new high performance chelation ion chromatography method for the simultaneous determination of
trace magnesium and strontium in various calcium carbonate samples was developed. Separations were
performed on a monolithic silica column (Chromolith Si, 100 4.6 mm I.D.) chemically modified with
hydroxyethyliminodiacetic acid functional groups. At a flow rate of 1.0 mL min 1, an eluent containing
80 mM NaCl and 20 mM picolinic acid at pH 5.30 was found to provide complete separation of Ni2+,
Cu2+, Mg2+, Cd2+, Sr2+ and Ca2+ in 15 minutes, in matrices with 20 000 fold excess of Ca2+. Two postcolumn reagents, o-cresolphthalein complexone (o-CPC) and ZnEDTA-PAR, were compared for postcolumn reaction based photometric detection at 570 and 490 nm, respectively. This method provides
sensitive detection of Mg2+ (LOD 20 mg L
2+
Sr
(LOD 200 mg L
1
and 39 mg L
range was from 0.5 to 24 mg L
Received 11th September 2014
Accepted 11th October 2014
1
and 5 mg L
1
for o-CPC and ZnEDTA-PAR, respectively) and
for o-CPC and ZnEDTA-PAR, respectively). Using o-CPC, the linear
for Mg2+ and from 1.0 to 32 mg L
linear range was from 0.5 to 4 mg L
1
for Mg
2+
and 0.1 to 32 mg L
1
1
for Sr2+. For ZnEDTA-PAR, the
for Sr2+. The method was applied
to the analysis of a variety of calcium carbonate samples, including laboratory reagents, limestone NIST
DOI: 10.1039/c4ay02126f
certified reference material, and the calcite based shells of marine microorganisms. The accuracy of the
method was confirmed using inductively coupled plasma mass spectrometry.
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Introduction
Determination of trace metal ions in calcium carbonate
matrices (commonly calcite) is of signicant interest in a variety
of scientic elds, particularly for geological and environmental
studies. For example, limestone and dolomite diagenesis is
known to be related to salinity and the Mg/Ca ratio.1 Additionally, the trace metal chemistry of calcareous skeletons of marine
microorganisms, such as coccolithophore calcite,2,3 can be utilised to provide insight into the nature of ancient oceans and
environments. Furthermore, the rate of inorganic minor
element incorporation (e.g. Mg and Sr) into calcium enriched
matrices such as coccolithophore calcite can be of great
signicance for the evaluation of changes in the current marine
environment,4 including changes associated with global
Australian Centre for Research on Separation Sciences (ACROSS), School of Physical
Sciences, University of Tasmania, Hobart, Tasmania, Australia. E-mail: Pavel.
Nesterenko@utas.edu.au
a
Institute for Marine and Antarctic Studies (IMAS), University of Tasmania, Private Bag
129, Hobart, Tasmania, 7001, Australia
b
† Electronic Supplementary
10.1039/c4ay02126f
1
1
Information
(ESI)
available.
See
DOI:
‡ Present address: Institute of Oceanography, University of São Paulo, Praça do
Oceanográco 191, 05508-120 São Paulo, SP, Brazil
416 | Anal. Methods, 2015, 7, 416–422
warming and increasing CO2 emissions. However, in the latter
case, complicating factors associated with removal of residual
sea salts and organically bound magnesium from coccolithophore calcite samples3 means that currently there are few
practical analytical methods for the accurate determination of
trace inorganic magnesium, present as MgCO3, in such materials. In addition, the excess of C and Ca within all forms of
calcite can cause signicant isobaric interference for the precise
determination of trace Mg2+ and Sr2+ (e.g. limestone5) when
using techniques such as inductively coupled plasma mass
spectrometry (ICP-MS).6 The measurement of magnesium
isotopes (24Mg+, 25Mg+ and 26Mg+) can suffer from isobaric
interference from carbon-dimer ions (12C2+, 12C13C+ and 13C2+).
Similarly, strontium isotopes, 84Sr, 86Sr, 87Sr and 88Sr, can suffer
from isobaric interference from Ca-dimer ion signals
(40Ca44Ca+, 42Ca2+, 40Ca46Ca+, 42Ca44Ca+, 43Ca2+, 40Ca48Ca+,
42
Ca46Ca+, and 44Ca2+). To fully eliminate such interference, it is
oen necessary to apply additional chromatographic separation
of these elements prior to introduction into the ICP-MS. For
example, Chang et al. determined magnesium in CaCO3 (ref. 7)
using multiple collector ICP-MS, by including a two-step ionexchange chromatographic pre-treatment procedure. Likewise,
extraction chromatography with polyacrylate porous resin
coated by a solution of 4,40 (50 )-di-tert-butylcyclohexano-18-
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crown-6 in octanol-1 was applied in a similar study for the
determination of strontium.8,9 Obviously, such coupled
methods are rather complicated, expensive, and time
consuming. Furthermore, the low recovery of elements
following the separation procedure can also present a potential
problem.
Other methods have also been reported for the determination of traces of magnesium or strontium in calcite, such as
ame photometry,10 neutron activation analysis (NAA)11 and
atomic absorption spectroscopy (AAS).12,13 However, these
methods also have their limitations. For example, the linear
range of both atomic emission and absorption spectroscopy is
rather narrow,13,14 and due to the high ionic strength and
complexity of samples, analysis generally requires the removal
of the matrix and/or dilution.
Thus, alternative accurate and practical methods for the
determination of Mg and Sr in calcium carbonate based
matrices are in signicant demand. Ion chromatography (IC)
has obvious potential for such samples,15 and has been widely
used for the determination of Mg and Sr in various complex
sample matrices, including seawater,16 plant-tissues17 and
ceramic superconductors.18 However standard IC, using traditional cation-exchange columns, suffers considerably when
applied to samples of high ionic strength or those containing
very diverse ion ratios, such as magnesium in concentrated
calcium carbonate. To overcome these issues, high performance chelation ion chromatography (HPCIC) was developed,
and specically presented as a solution to the determination of
alkaline earth and transition metal ions in such complex
samples. The separation mechanism of HPCIC includes the
formation of kinetically labile complexes between chelating
groups and the sample metal ions, so that their retention and
separation are not affected by the ionic strength of the
samples.19,20 Generally, chelating ligands are conjugate bases,
resulting in their strong affinity to hydrogen ions. As a result of
this affinity, pH can be used to control the separation selectivity
and time.
The determination of alkaline earth metals in complex
samples is one of the most effective applications of HPCIC. As
far back as 1994, Jones et al.21 published the determination of
Ba2+ and Sr2+ in calcium-containing matrices using HPCIC.
However, this chromatographic system could not separate Mg2+
from the huge amount of Ca2+. In addition, since the technique
was in its infancy at that time (and column limitations), the
whole separation took an excessive 50 minutes. Recent studies
reported much faster (5–15 min) determination of Be2+ in
stream sediments,22 and Mg2+, Ca2+ and Sr2+ in seawater6,23,24
and in saturated brines for the chlor-alkali industry.19
The current work presents the results of determination of
trace magnesium (and strontium) in calcium carbonate based
matrices, such as certied argillaceous limestone, using HPCIC.
The HPCIC methodology developed is based on hydroxyethyliminodiacetic acid (HEIDA) bonded monolithic silica
columns and optimised post-column reaction (PCR) detection.
The applicability of the method to the analysis of calcareous
skeletons of marine microorganisms, such as coccolithophores
(in particular Emiliania huxleyi), is also demonstrated, with its
This journal is © The Royal Society of Chemistry 2015
Analytical Methods
potential as a new method for inorganic magnesium determination in coccolithic calcite discussed.
Experimental
Instrumentation
A Metrohm Model 844 Compact IC was used throughout this
study. This system comprised of a built-in high pressure pump,
a peristaltic pump, a UV-Vis detector and a post-column reactor
(Metrohm, Herisau, Switzerland). The post-column reactor
consisted of a plastic tee and a 2.5 m long PTFE capillary
reaction coil. Two polyetheretherketone (PEEK) sample loops,
10 mL and 20 mL, were used, with manual sample injection.
ICNet 2.3 SR6 soware (Metrohm, Herisau, Switzerland) was
used for IC data acquisition and processing of chromatograms.
A 100 4.6 mm I.D. Onyx silica monolithic column was
purchased from Phenomenex (Cheshire, UK) and modied with
HEIDA functional groups according to the procedure described
elsewhere.25 High-resolution (sector eld) inductively coupled
plasma mass spectrometry (HR-ICP-MS) was applied as a
conrmatory method, using an ELEMENT 2 instrument
(Thermo Fisher, Bremen, Germany).
Chemicals and reagents
Analytical or higher grade reagents and Milli-Q water (Millipore,
Bedford, MA, USA) were used for preparing all solutions. oCresolphthalein complexone (o-CPC, 90% dye content) was
sourced from Fluka (Buchs, Switzerland). 4-(2-Pyridylazo)
resorcinol (PAR) (99.5% dye content) was obtained from SigmaAldrich (Sydney, Australia). Picolinic acid (pyridine-2-carboxylic
acid, 99%), dipicolinic acid (pyridine-2,6-dicarboxylic acid,
99%) and sodium chloride (NaCl, 99.5%) were purchased
from Sigma-Aldrich (Sydney, Australia). Sodium tetraborate
(Na2B4O7$10H2O, 99.5%) and ZnEDTA were purchased from
BDH chemicals (Poole, UK). CaCO3 was purchased from BDH
chemicals (Poole, UK), Strem Chemicals (Miami, USA) and AJAX
chemicals (Sydney, Australia). Nitric acid (69%) and ammonium
hydroxide (25%) were obtained from Merck (Sydney, Australia).
Boric acid and Spectrosol atomic absorption standard solutions
of Ni2+, Fe2+/3+, Ca2+, Sr2+, Mg2+, Mn2+, Co2+, Cd2+, Zn2+, La3+ and
Cu2+, with concentrations of 1.00 g L 1, were purchased from
BDH Chemicals (Poole, UK).
A certied reference sample (1d Limestone (Argillaceous))
was purchased from the National Institute of Standards and
Technology, NIST, USA. Full details of its composition can be
found at the web-page http://www.nist.gov/srm.
Preparation of post-column reagents
Three PCR reagents for the detection of alkaline earth metal
cations and transition metal cations were used: (1) 0.4 mM oCPC, 0.25 M boric acid adjusted to pH 11 using NaOH; (2) 0.15
mM PAR, 0.4 M NH4OH, adjusted to pH 10.65 with nitric acid;
(3) 0.2 mM ZnEDTA and 0.15 mM PAR in 2 M NH4OH, adjusted
to pH 10.65.
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Sample preparation
For CaCO3 and limestone samples, nitric acid (30 mM, 5–50 mL)
was used to dissolve sample solid (50 mg) for 40 minutes under
ultra-sonication. The nal pH of the samples was adjusted to
2.0–3.0 to ensure that all the carbonates were fully dissolved. It
was found that a bigger volume of acid is required to dissolve
CaCO3 samples containing more magnesium.
The coccolithophore Emiliania huxleyi (isolated from the
Southern Ocean, strain EHSO 5.14) was cultured in 0.20 mm
ltered natural sea water, with a salinity of 35. Macro- and
micro-nutrients were added in excess to ensure the supply of all
necessary nutrients for growth. The medium was continuously
bubbled with sterile air supplying carbon dioxide. Cultures
received a constant daily illumination of 150 mmol quanta
m 2 s 1 and were harvested at a density of 3 000 000 cells per
mL. Sample pellets were produced by centrifugation and
subsequently dried and stored at 60 C till further processing.
Aer rinsing with water, drying in air and weighing the sample
(40 mg) was suspended in 9 mL of deionised water. Titration
with Cu2+ was carried out in suspension to replace the labile Mg
(see the ESI†), and then the suspended sample was dissolved by
addition of nitric acid (0.1 mol L 1) until the pH of the solution
reached 2.0 to ensure complete dissolution of the calcite matrix.
Results and discussion
The minor element content in calcareous marine microorganisms depends on seawater composition. The literature values of
Sr/Ca and Mg/Ca ratios in coccolithic calcite varied from 2.40 to
4.38 mmol mol 1 and from 0.066 to 98.6 mmol mol 1, respectively.2 So, in order to determine the traces of Mg2+ and Sr2+ in
the presence of a huge excess of Ca2+, it is essential to develop
highly selective separation of these metals and ideally have both
target cations of interest elute prior to the matrix Ca2+ peak.
Additionally, for application to coccolithophore samples, for
the determination of non-labile inorganic magnesium only, the
further resolution of an additional divalent cation (in this work
Cu2+), added during titration of the sample to release labile
organically bound magnesium (see the ESI†), must be achieved.
Therefore, herein the separation of Cu2+ from Mg2+, Sr2+ and
excess Ca2+ and other possible minor metals from coccolithophore samples was required.
Generally, the affinity to alkali metal cations is very low for
most of the iminodiacetic acid (IDA) type chelating columns.
Hence high alkali metal concentrations originating from the
marine microorganism samples should not affect the resultant
separation.19 Additionally, with non-complexing eluents, alkaline earth metals such as Mg2+, Ca2+ and Sr2+ are generally less
retained on such chelating phases than the majority of transition metal ions, including Cu2+.23,25,26 Therefore herein, particular attention to the eluent conditions was required to pre-elute
residual titrated Cu2+ ions.
Method development
The separation of metal ions using HEIDA functionalised
substrates has been investigated and reported previously.23,27–29
418 | Anal. Methods, 2015, 7, 416–422
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Upon such substrates, both ion-exchange and chelation are the
two main interactions simultaneously responsible for the
retention selectivity displayed for specic metal ions.19,20 When
using non-complexing acidic eluents, of relatively low ionic
strength, simple ion-exchange processes dominate. The
suppression of ion-exchange interactions by using eluent with
high ionic strength results in dominance of surface chelation in
the retention mechanism and in a higher separation selectivity.19,20 For the alkaline earth metals, previous studies6,24 have
shown that with such high ionic strength eluents, the observed
selectivity is as follows: Mg2+ < Ba2+ < Sr2+ < Ca2+. Generally,
nitrates, perchlorates or chlorides of alkali metals are applied to
regulate the ionic strength of eluents. Herein, with elevated
concentrations of sodium and chloride within the marine
derived samples, sodium chloride was obviously the most
appropriate eluent additive for controlling ionic strength.
Further control of selectivity can be obtained through the
inclusion of complexing ligands within the eluents, to compete
with the complexing ligand on the surface of the stationary
phase.30 Recent work6 using HPCIC for the direct determination
of Sr2+ in seawater demonstrated the separation of Mg2+, Sr2+
and Ca2+ using an eluent containing glycolic acid (complexing
agent) and sodium chloride (ionic strength regulator). However,
for the current application, greater resolution of the above
metals was required, due to a diverse concentration ratio, and
the additional resolution of the alkaline earth metal ions from
Cu2+.
In 2008,30 Jones and Nesterenko investigated the effects of
different complexing agents as eluent additives. As Cu2+
exhibits very strong retention on all IDA based columns,
including HEIDA, the use of complexing agents is generally
required. Dipicolinic acid and picolinic acid have been proven
to provide appropriate affinity to Cu2+, compared to other
complexing reagents,30,31 and so were applied in this application. The addition of picolinic acid to the eluent has little effect
on the separation selectivity of Sr2+, Ca2+ and Mg2+, as the
stability of complexes of these metals with picolinic acid is
lower than that of complexes formed with the surface bonded
HEIDA group. Alternatively, the b values of Sr2+, Ca2+ and Mg2+
with dipicolinic acid are higher than that with the surface
bonded HEIDA group, and resultant selectivity reects this.
Using a 0.15 mM dipicolinic acid eluent, with 6 mM HNO3 as
a starting point, a poor separation of Mg2+ and Ca2+ was achieved, whilst Cu2+ failed to elute (Table 1). Obviously, a higher
concentration of dipicolinic acid was required to elute Cu2+,
however, as expected from the high b of Mg2+ and Ca2+ with
dipicolinic acid, increased eluent concentrations of the acid
reduced both the retention and resolution of Mg2+ and Ca2+,
and was therefore unsuitable. A change of complexing agent to
picolinic acid and an increase in concentration (4 mM) provided
the elution of Cu2+, whilst the separation selectivity (a) between
Mg2+ and Ca2+ remained relatively unaffected (Table 1).
With the addition of sodium chloride (0.25 M) and a slight
increase of pH (5.0), Mg2+, Ca2+ and Cu2+ could be separated
with a retention order of Mg2+ < Ca2+ < Cu2+. However, the
elution of Cu2+ aer Ca2+ is not applicable to the analysis of the
samples with massive excess of Ca2+. Therefore, a signicant
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Table 1 The effect of addition of complexing agents to 6 mM HNO3 used as an eluent on the retention times (min) and separation selectivity of
Mg2+, Ca2+ and Cu2+a
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Complexing additives
Metal ions
0.15 mM dipicolinic acid
2 mM picolinic acid
4 mM picolinic acid
Mg2+
Ca2+
Cu2+
a (Mg2+ and Ca2+)
4.69
5.13
No peak
1.15
4.56
4.99
13.72
1.16
4.57
5.00
6.39
1.16
a
Note: “No peak” means strongly retained.
increase of picolinic acid concentration (20 mM) in the eluent
was used to further reduce the retention of Cu2+, whilst a
simultaneous reduction in eluent sodium chloride concentration (from 0.25 to 0.1 M) provided slightly increased retention of
both Mg2+ and Ca2+. Under these conditions the desired elution
order of Cu2+ < Mg2+ < (Cd2+, potential internal standard cation)
< Sr2+ < Ca2+ was achieved. The optimisation was completed by
attenuation of the eluent pH (5.0) and column temperature (40
C) and resulted in the separation of ve metals of interest in
less than 10 minutes (see Fig. 1).
These conditions provided not only excellent resolution for
minor Mg2+ and excessive Ca2+ peaks, but also a reasonable
separation of Sr2+ and Ca2+ (aSr2+/Ca2+ ¼ 1.15). However, for the
latter pair the quantitative determination of strontium still
could be a problem in the analysis of real samples (Fig. 2(a)).
However, according to work,6 the retention of Ca2+ on HEIDA
functionalised silica is more sensitive to the changes in eluent
pH as compared with Mg2+ and Sr2+. So, a slight increase in
eluent pH to 5.3 improved dramatically the separation selectivity of Sr2+ and Ca2+ (aSr2+/Ca2+ ¼ 1.30), while the retention of
magnesium increased insignicantly (Fig. 2(b)).
Fig. 2 HPCIC chromatogram of coccolithic calcite model solution
(Mg2+ 0.024 mg L 1, Sr2+ 1.75 mg L 1, Ca2+ 400 mg L 1). Column:
100 4.6 mm I.D., injection volume: 10 mL, flow rate: 1 mL min 1,
eluent: (a) 20 mM picolinic acid, 100 mM NaCl, pH ¼ 5, (b) 20 mM
picolinic acid, 80 mM NaCl, pH ¼ 5.3. Photometric detection at
570 nm after post-column reaction with o-CPC. PCR reagent flow
rate: 0.44 mL min 1.
Under the above nal conditions the retention of a range of
divalent and trivalent cations (Ni2+, Mn2+, Fe2+/3+, Zn2+, Co2+,
Cu2+, Mg2+, Cd2+, Sr2+, Ca2+, and La3+) was also checked to
ensure the absence of the possible interference for the determination of magnesium and strontium (see Table 2).
Detection optimisation
Fig. 1 HPCIC chromatogram of a test mixture of metal cations (Cu2+,
Mg2+, Cd2+, Sr2+, Ca2+). Column: 100 4.6 mm I.D., injection volume:
10 mL, flow rate: 1 mL min 1, eluent: 20 mM picolinic acid, 0.1 M NaCl,
pH ¼ 5. Photometric detection at 490 nm after PCR with ZnEDTAPAR. PCR reagent flow rate: 0.44 mL min 1.
This journal is © The Royal Society of Chemistry 2015
The use of PAR, ZnEDTA-PAR, calmagite, arsenazo I, 8-hydroxyquinolinol or o-CPC has been reported as reagents for the
photometric detection of alkaline earth metals aer PCR in
HPCIC.19 Recently, Nesterenko et al.6 found that ZnEDTA-PAR
and o-CPC provide the most sensitive photometric detection
aer PCR for these cations.
Herein, for quantitative analysis, the linear range test for
alkaline earth metals was checked for both ZnEDTA-PAR (at pH
10.65) and o-CPC (at pH 11.0), with photometric detection at
570 and 490 nm, respectively. The ow rate of the eluent was 1.0
mL min 1 and PCR reagent was delivered at a ow rate of 0.44
mL min 1. With o-CPC, the linearity was conrmed over the
concentration range of 1 to 29 mg L 1 for Mg2+, 1 to 32 mg L 1
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Analytical Methods
Table 2
The retention time (minutes) of metal ionsa
Metal ions
Ni2+
Mn2+
Fe2+/3+
Zn2+
Co2+
Cu2+
Mg2+
Cd2+
Sr2+
Ca2+
La3+
Time (min)
2.18
2.17
1.71/2.18
2.26
2.87
2.93
3.57
5.51
7.12
10.19
No peak
a
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Note: “No peak” means strongly retained.
for Sr2+, n ¼ 6, with R2 values ¼ 1.00 and 0.999, respectively. For
ZnEDTA-PAR, the linear range was found to be only 0.05–4 mg
L 1 for Mg2+ and 0.1 to 32 mg L 1 for Sr2+, (n ¼ 6, R2 ¼ 0.999 for
both metals).
The limits of detection (LODs) for Mg2+ and Sr2+ with each
PCR reagent were calculated using the signal to noise criteria S/
N ¼ 3. These were found to be 20 mg L 1 and 200 mg L 1 for oCPC, and 5 mg L 1 and 39 mg L 1 for ZnEDTA-PAR, for Mg2+ and
Sr2+, respectively.
In the following experiments ZnEDTA-PAR was used because
of higher sensitivity for Mg2+ and Sr2+and the possibility of
detection of various transition metal ions.
Applications
The accuracy of the proposed method was rst veried with the
analysis of NIST certied reference material (1d Limestone
(Argillaceous)) for the determination of magnesium and strontium. The concentration of Mg2+ in the limestone reference
material was found to be 1.49 0.04 mg g 1, with RSD ¼ 2.4%
at n ¼ 5. This value was 18% below the certied value 1.82
0.06 mg g 1, so a further conrmatory analysis was completed
using sector eld HR-ICP-MS. This conrmed a value of 1.49 mg
g 1 for Mg2+, matching exactly that found using the HPCIC
method. The concentration of Sr2+ determined within the
HPCIC chromatogram of a sample solution of 1 mg mL 1
limestone certified reference material dissolved in HNO3 (0.03 M).
Column: 100 4.6 mm I.D., injection volume: 10 mL, flow rate: 1 mL
min 1, eluent: 20 mM picolinic acid, 80 mM NaCl, pH ¼ 5.3. Photometric detection at 490 nm after PCR with ZnEDTA-PAR. PCR reagent
flow rate: 0.44 mL min 1. According to the passport of the reference
material the sample contains 0.018 mg mL 1 Zn2+, 1.82 mg mL 1 Mg2+,
0.26 mg mL 1 Sr2+ and 378 mg mL 1 Ca2+.
limestone certied reference material was found to be 0.254
0.005 mg g 1 (RSD ¼ 2.1%), which matched very well with the
certied value 0.256 0.008 mg g 1. The complete data of HRICP-MS analysis are presented in the ESI.† The HPCIC chromatogram obtained for the certied reference limestone materials can be seen in Fig. 3. Traces of Zn2+, Fe2+/Fe3+, and Cu2+
were also detected in the limestone sample, but were well
resolved from the Mg2+ and Sr2+ peaks of interest. Technically,
copper and zinc can be determined quantitatively in the reference material under these conditions, but it was not under
scope of this work.
To evaluate the suitability of the developed method to the
determination of Mg2+ and Sr2+ traces in the samples with
high matrix calcium, three laboratory grade CaCO3 samples
from different producers were also analysed. Fig. 4(a) and (b)
show the chromatograms resulting from the injection of a 10
Fig. 3
420 | Anal. Methods, 2015, 7, 416–422
HPCIC chromatograms of laboratory grade CaCO3 sample
solutions. (a) 10 mg mL 1 CaCO3 (AJAX), dissolved in HNO3 (0.3 M). (b)
10 mg mL 1 CaCO3 (BDH) dissolved in HNO3 (0.3 M). (c) 1 mg mL 1
CaCO3 (Strem Chem.) dissolved in HNO3 (0.03 M).
Fig. 4
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mg mL 1 solution prepared from CaCO3 obtained from AJAX
and BDH, respectively. In both cases the sample concentrations were high enough to quantitate both Mg2+ and Sr2+
without loss of resolution between the peak of Sr2+ and the
massive peak of Ca2+. However, a diluted solution (1 mg
mL 1) was prepared for the analysis of the CaCO3 sample
obtained from Strem Chemicals, Inc. because this contained
a signicant impurity of Mg2+ (see Fig. 4(c)), which otherwise
will be at concentration beyond the linearity range. In the
latter case, both the peaks for Mg2+ and Sr2+ impurities can
be clearly identied. Using a standard addition method, the
concentration of Mg2+ in this sample was found to be 5.05
0.04 mg g 1, with RSD ¼ 0.76% (n ¼ 5). This again matched
well with the results of HR-ICP-MS analysis, with which the
concentration of Mg2+ was found to be 5.10 mg g 1, which
corresponds to less than 1% difference between methods.
Finally, the developed method was applied to the determination of non-labile inorganic magnesium and the Sr/Ca
ratio in coccolithic calcite skeletons (Emiliania huxleyi). For
this purpose the sample of calcite was pre-titrated using Cu2+
to replace organically bound or physically adsorbed magnesium in coccolithic skeletons. Organic magnesium is mainly
presented as chlorophyll and can be substituted in its molecules by copper. The content of inorganic or calcite entrapped
magnesium was obtained by subtraction of the concentration
of organic magnesium measured in the ltrate of the sample
titrated with Cu2+ from total concentration of magnesium in
coccolith. The latter concentration was measured for the
sample completely dissolved in nitric acid. The chromatogram of the ltrate of the sample titrated with Cu2+ is shown
in Fig. 5. The ratio of non-labile inorganic Mg to Ca in coccolithic calcite skeletons was determined to be 5.39 0.23
mmol mol 1, with RSD ¼ 4.3% (n ¼ 5). This value is in good
agreement with the literature data,2 where the Mg/Ca ratio
varied in the range of 0.066–98.6 mmol mol 1.
HPCIC chromatogram of a sample solution of 4.4 mg mL 1
coccolithic calcite solution pH ¼ 2, adjusted by HNO3 (0.1 M),
pre-titrated with Cu2+ (60 mg mL 1). The concentrations of Mg2+,
Sr2+ and Ca2+ were 10.28 mg mL 1, 2.48 mg mL 1 and 481.45 mg
mL 1, respectively. Cu2+ was not quantitatively measured as it was
the titrant.
Fig. 5
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Analytical Methods
Using the HPCIC method, the ratio of Sr to Ca in coccolithic
calcite skeletons was found to be 2.79 0.22 mmol mol 1, with
RSD ¼ 7.8% at n ¼ 7. This is in good agreement with 2.73 0.22
mmol mol 1 reported in the literature3 for this sample, when
ICP-MS was used for the analysis.
Conclusion
A simple and sensitive HPCIC method for the determination of
magnesium and strontium in calcite based sample matrices has
been developed. The results achieved for the various samples
compared well with either certied values or values obtained
from HR-ICP-MS. The chromatographic method demonstrates
excellent analytical performance, and importantly provides an
analytical tool for further study into the important issue of
environmental impact upon coccolith shell chemistry.
Acknowledgements
The authors wish to thank Dr Ashley Townsend from the
Central Science Laboratory of the University of Tasmania for the
ICP-MS analysis. This work was supported by a grant from the
Australian Research Council (DP130101518).
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