American Journal of Anal yt ical Chemistry, 2011, 2, 675-682
doi:10.4236/ajac.2011.26077 Published Online October 2011 (
Copyright © 2011 SciRes. AJAC
Simultaneous Separation and Quantification of Iron and
Transition Species Using LC-ICP-MS
Qinhong Hu
Department of Earth and Environmental Sciences, The University of Texast, Arlington, USA
Received June 27, 2011; revised July 28, 2011; accepted August 5, 2011
Using liquid chromatography-inductively coupled plasma-mass spectrometry (LC-ICP-MS), this work inves-
tigates the simultaneous separation and quantification of seven transition metal species (Fe, Mn, Co, Ni, Cu,
Zn, and Cd), based on a separation scheme published by Dionex company that used the spectrophotometric
method for quantification. The LC-ICP-MS method overcomes the shortcomings of conventional ferrozine
approaches of measuring Fe(II) and total Fe by two separate runs and calculating Fe(III) by the difference of
two runs. The advantage is particularly evident in that organo-iron species are found to be the predominant
iron species in many natural waters, and the difference method cannot measure the concentration of Fe(III)
because ferrozine will not complex with organo-iron species. In the work reported here, the LC-ICP-MS
method is successfully applied to the separation of dissolved iron species, as well as six other divalent transi-
tion metals in tap water, deionized water, river water, hot springs, and groundwater samples.
Keywords: LC-ICP-MS, Fe(II), Fe(III), Organo-Fe, Transition Metals
1. Introduction
Concentration determination of soluble reactive species
is key to understanding biogeochemical processes in
aquatic and terrestrial environments. Iron is one of the
most reactive elements in aquatic and geological envi-
ronments, and is involved in the cycling of many major
chemicals, as well as trace elements [1]. For example,
hydrous ferric oxides (e.g., ferrihydrite) are the most
reactive soil components with respect to arsenic sorption
and can take up hundreds of mg/kg As, either as As(III)
or As(V) [2]. The reduction of Fe oxyhydroxides and
release of arsenic has been invoked as a probable
mechanism of elevated As concentration in groundwater
used for drinking and responsible for the poisoning of
millions of people [3,4] .
Iron is present in th e hydrosphere under two oxidation
states, Fe(II) and Fe(III), which are thermodynamically
stable under anoxic and oxic conditions, respectively [5 ].
Measurements of both dissolved Fe(II) and Fe(III) con-
centration are important in assessing iron’s contribution
in mediating numerous biogeochemical processes that
involves many elements [6]. Most analytical approaches
require a separate analysis of dissolved Fe(II) and total
dissolved Fe, and the calculation of Fe(III) by the differ-
ence [4]. First proposed by Stookey [7], the ferrozine
(monosodium salt hydrate of 3-(2-pyridyl)-5, 6-diphenyl-
1,2,4-triazine-p, p’-disulfonic acid) method is the most
widely-used way of determining Fe(II) and Fe(III).
Ferrozine reacts with Fe(II) to form a stable magenta
complex species, with a maximum absorbance at 562 nm,
which is measured spectrophotometrically. When Fe(III)
is also present in the aqueous samples, either as a true
dissolved complex or in colloids, a separate reduction
step with hydroxylamine (NH4OH·HCl) is performed to
measure the total iron, with the difference ascribed to
Fe(III) [5,6]. The approach lacks sufficien t sensitivity for
determining iron con centrations in natural waters at µg/L
levels, and therefore a pre-concentration is usually re-
Alternative approaches to the ferrozine method have
been proposed [6,8,9]. Yan [6] reported a method of
on-line coupling of flow injection separation and pre-
concentration with ICP-MS, with a sample volume of 2.5
mL and detection limit of 0.08 µg/L. However, the con-
centration of Fe(II) was obtained as the difference be-
tween the combin ed Fe(III) and Fe(II), and Fe(II I) alone.
This was done by controlling the sample acidity range,
and detecting the Fe(III) by the Fe(III)-pyrrolidinecar-
bodithioate (PDC) complex. In summary, relatively little
Q. H. HU
has been published on the simultaneous measurement of
both Fe(II) and Fe(I II) in a single run.
Metal ions can exist in several different forms, which
are determined by the extent of complexation and the
oxidation state. In many aqueous samples, metal ions are
present in their hydrated forms. Hydrated metal ions can
also be complexed by weak ligands such as organic acids
or amino acids. These ligands are generally displaced by
the complexing agents used in liquid chromatography
eluents, with the total of both hydrated and weakly com-
plexed metal ion s determined [10]. Hydrated and weakly
complexed transition metals can be separated as cations
on a cation exchange column. By adding a carboxylic
acid chelating agent to the eluent, the net charge on the
metal is reduced, because the carboxylic acids are ani-
onic in solutions above their pKas. The selectivity of the
separation is related to the different degrees of associa-
tion between the metals and the chelating agen ts produc-
ing different net charges on the metal complexes. If
strong enough chelating agents are used in high enough
concentration, the net ch arge of the metal complexes can
be negative. The resultant anionic metal complexes can
be separated by an anion exchange process. The Dionex
IonPac® CS-5A column has both cation and anion ex-
change capacities, allowing metals to be separated as
cations or anions on a single column. The 9 µm poly-
meric pellicular packing of the CS-5A column has an
ethylene-vinylbenzene divinylbenzene resin core, 55%
cross-linking, consisting of two layers of latex particles,
functionalized with both anion exchange alkyl quarter-
nary amine (internal layer) and cation exchange sulfonic
acid groups (outer layer), with capacities of 40 and 20
µequivalents, respectively. With pyridine-2, 6-dicarboxy-
lic acid (PDCA) used as the chelating agent in the eluent,
the transition metals are separated as anionic complexes
in the analytical column of the IonPac CS-5A [10].
The following seven transition metals can be separated
and quantified: iron, both ferric (Fe3+) and ferrous (Fe2+),
divalent cations of copper, nickel, zinc, cobalt, cadmium,
and manganese [10]. The present study was primarily
focused on the separation of iron species, as well as the
detection and prevalence of organo-Fe species in natural
waters. However, the separation of the other six transi-
tion metals will also be described in this paper, and some
examples of applications in natural waters will be given.
2. Materials and Methods
Except for Fe2+ and Fe3+, transition metal standards of
1000 mg/L were obtained from CPI International (Santa
Rosa, CA). These stock solutions were purchased in
dilute acid (1% - 2% nitric acid) solutions and diluted to
different concentrations in deionzied (DI) water for LC-
ICP-MS analyses. Ferrous ammonium sulphate (Mohr’s
salt, [NH4]2[Fe][SO4]2·6H2O) was purchased from Alfa
Aesar (Ward Hill, MA) and ferric chloride hexahydrate
(iron III) from Mallinckrodt Baker (Phillipsburg NJ).
These chemicals were used to prepare the standard
solutions for Fe (I I) and Fe(III), res pective ly . Each ind ivi-
dual transition metal was prepared to obtain the retention
time from LC separation for peak identification. All
solutions were prepared using purified deionzied (DI)
water (Milli-Q Ultrapure Water Purification System,
Millipore, Billerica, MA).
As reported in Dionex (2011), the eluent for separating
the transition metals includes the following mixture: 7.0
mM (PDCA), 66 mM potassium hydroxide, 74 mM for-
mic acid, 5.6 mM potassium sulfate. Dionex carries the
MetPac™ PDCA eluent concentrate, which is to be di-
luted 5 times with 200 mL of the co ncentrate added with
800 mL DI water to make up 1-L eluent. To adjust the
complexing ability of PDCA to optimize the separation
time, we also purchased the individual chemicals in mak-
ing th e eluent: PDCA from Alfa Aesar (Ward Hill, MA),
potassium hydroxide from EMD Chemicals (Gibbstown,
NJ), formic acid from EMD Chemicals, and potassium
sulfate from Mallinckrodt Baker (Phillip sburg NJ).
Both ferrous and ferric ions can be separated under the
above separation conditions, using PDCA as the chelat-
ing agent to form anionic complexes. Because the ferrous
ion is easily oxidized to ferr ic iron, oxygen was removed
from the eluent by degassing the eluent solution bottle
with helium for half an hour. To remove oxygen from the
analytical and guard columns, a solution of 0.1 M so-
dium sulfite (12.6 g/L Na2SO3) was pumped through the
columns for 2 hours befor e the sample analysis [10].
We used an advanced analytical method of LC-ICP-
MS for separating and quantifying iron, and the other 6
transition metal species, by modifying the quantification
method of Dionex [10], to enable sensitive and simulta-
neous analyses in aqueous samples. Briefly, the LC sys-
tem consisted of a PerkinElmer Series 200 Quaternary
Pump and a Series 200 Autosampler (PerkinElmer/
SCIEX, Sheldon, CT) with an IonPac CS5A analytical
and CG5A guard columns from Dionex (Sunnyvale, CA).
Separation conditions included the following: mobile
phase of MetPac PDCA eluent mixture at pH of 4.2 (ad-
justed with ammoniu m hydroxid e, with 20% - 22% NH 3),
flow rate 1.2 mL/min, and sample injection volume 50
In the Technical Note of Dionex [10], the metal com-
plexing agent 4-(2-pyrid ylazo) resorcinol (PAR) is added
postcolumn to form a light-absorbing complex with the
hydrated and weakly complexed metals. These transition
metals are detected by measuring the absorbance at 530
nm of the complex. We used the ICP-MS for sensitive
Copyright © 2011 SciRes. AJAC
Q. H. HU677
determination of transition metal species. The effluent
from the LC column was directly connected, via 60 cm
of trifluoroacetic acid capillary tubing (1.6 mm o.d. × 0.5
mm i.d.), to the nebulizer of a PerkinElmer/SCIEX
ELAN DRC II (Sheldon, CT) for the determination of
transition metal concentration. The sample introduction
system components of the ICP-MS consisted of a cyc-
lonic spray chamber, a Meinhard® type A nebulizer, and
platinum cones.
3. Results and Discussion
3.1. Stability of Fe(II) and Fe(III) Solutions
We used the DRC (Dynamic Reaction Cell) capability of
ELAN DRC II ICP-MS, with ammonium as the reaction
gas at a flow rate of 0.75 L/min and rejection parameter
RPq of 0.45, to minimize the polyatomic interference of
40Ar16O to 56Fe [11-14]. From the monitored signal in-
tensity results at the atomic mass unit of 56 with only the
eluent passing through the column, we observed a base-
line level at 22,000 cps (counts per second) under the
DRC condition (Figure 1(a)), compared to 240,000 cps
at the standard mode (i.e., no DRC). A factor of 10 times
reduction of baseline level is critical for sensitive meas-
urements of 56Fe.
We next focused on the standards of both Fe(III) and
Fe(II) prepared from FeCl3 and [NH4]2[Fe][SO4]2·6H2O.
Fe(III) standards from FeCl3 chemicals exhibited only an
Fe(III) peak, as shown in Figure 1(b), where the chro-
matogram for 1000 mg/L solution of Fe(III) was ac-
companied by only a very small peak at the retention time
of Fe(II). H ow e ver , Fe(II) standa rd prepared fr om amm o-
nium iron(II) sulfate ([NH4]2[Fe][SO4]2·6H2O, Mohr’s
salt) chemicals essentially only shown an Fe(III) peak
(Figure 2(a)), indicating the oxidation of Fe(II) chemicals
over time. Supposedly, Mohr’s salt is preferred over
iron(II) sulfate as it is much less affected by oxygen in
the air than iron(II) sulfate, solutions of which tend to
oxidize to iron(III). The oxidation of solutions of iron(II)
is very pH dependent, occurring much more readily at
high pH. The ammonium ions make solutions of Mohr’s
salt slightly acidic, which prevents this oxidation from
We used a reducing agent, 0.1 M NH2OH·HCl, and
boiling condition [15] to treat the solu tion prepared from
Mohr’s salt, and stored the resultant standard of 1000
mg/L in an autoclaved amber glass bottle.
Standards of Fe(II) with different concentrations were
prepared from a working standard of 100 mg/L Fe(II) for
LC-ICP-MS studies. Figure 2(b) shows the Fe(II) peak,
with a negligible Fe(III) peak which likely is caused by
the DI water used in standard preparation (to be dis-
Figure 1. (a) LC-ICP-MS 56Fe signal intensity baseline of
the eluent solution; (b) Signal intensity of Fe(III), at the
retention time around 4 min, prepared from FeCl3 chemi-
cals indicating minimal presence of Fe(II) with a retention
time of 12.5 min.
cussed later), of the 100 mg/L Fe(II) working standard.
Figure 3 shows the chromatograms of mixed stan-
dards of Fe(II) and Mn(II) at different concentrations.
Compared with 24,000 cp s back ground level f or 56Fe, the
background for 55Mn is only at about 800 cps, indicating
much lower polyatomic interference for Mn detection.
Multiple measurements of 0.5 µg/L Fe(II) exhibit some-
what different peak height (and area), indicating that the
detection limit for Fe(II) by the LC-ICP-MS method is
slightly higher than 0.5 µg/L (Figure 3(a)). However,
this is still a simple and versatile method to measure
simultaneously both Fe(II) and Fe(III) at low µg/L con-
centration levels. In addition, all chromatograms show
sharp peaks with reproducible retention times for Fe(II),
Fe(III), and Mn(II) species (Figure 3). Furthermore,
there are Fe(III) peaks reaching 120,000 cps levels from
all standards of Fe(II) and Mn(II); this level of Fe(III)
probably originates from the DI water used for standard
Copyright © 2011 SciRes. AJAC
Q. H. HU
3.2. Prevalence of Organic-Fe Species in Natural
The Technical Note of Dionex [10] describes the use of 7
mM PDCA as the complexing agent for seven transition
metals, with Fe(III) appearing first and Fe(II) last in the
chromatograms. We tested the effect of PDCA concen-
tration on the retention time of Fe, as well as that of other
transition metal species; the results are shown in Table 1.
Figure 2. (a) Fe(II) standard prepared from untreated
[NH4]2[Fe][SO4]2·6H2O chemical indicating its oxidation to
Fe(III); (b) Fe(II) standard solution obtained from reduc-
tant NH4OH·HCl under boiling condition.
Table 1. PDCA concentration and associated retention of
iron species.
PDCA Species and retention
time (min) Note
3.5 mM Fe3+ (7.0) Too long separation time
7.0 mM organo-Fe (2.8); Fe3+
(5.0); Fe2+ (11.2) Reasonable
separation time
14 mM@ organo-Fe (2.1); Fe3+
(3.8); Fe2+ (6.5) Best scheme for separating all
7 transition metals in 8 min
@Preparation of 14 mM pyridine-2, 6-dicarboxylic acid (PDCA), 66 mM
potassium hydroxide, 74 mM formic acid, and 5.6 mM potassium sulfate:
warm the solution to fully dissolve PDCA; the measured pH is 3.58; add
about 10 mL concentrated NH4OH into 4 L eluent solution to adjust pH to
4.22; otherwise, the baseline is not stable in LC-ICP-MS runs.
Figure 3. Signal responses of Fe(II) and Mn(II) standard
solutions at the concentration of (a) 0.5 µg/L (four duplicate
injections are presented); (b) 40 µg/L; and (c) 1000 µg/L.
Slightly different PDCA concentrations (Table 1) were used
to produce different retention time s.
It was observed that a lower PDCA concentration will
lead to a longer retention time in the sep aration cond ition.
We suggest the use of a concentration of 14 mM PDCA
to reduce the run time to less than eight minutes for the
separation of all 7 transition metal species; note that
Copyright © 2011 SciRes. AJAC
Q. H. HU679
some figures shown in this work were obtained at a
lower PDCA concentration; this had no effect on the
interpretation of results, although the chromatograms
show a longer run time (e.g., 12 min). An adjustment of
pH to 4.22 is necessary to produce a stable baseline in
the eluent mixture of 14 mM PDCA, since PDCA is
slightly acidic, with a measured pH of 2.38 for the14
mM PDCA solution.
Figure 4 shows example chromatograms for ground-
water and river water samples collected in Texas; the
samples were filtered through a 0.25 µm membrane filer
with no other preservation step (e.g., acidification), and
stored at 4˚C in a refrigerator. Both samples show a
small Fe(III) peak and no Fe(II), while the groundwater
shows Mn(II) pr esence. In add ition, both samples sh ow a
large 56Fe peak before the 56Fe(III) peak; this is likely
related to an organic-Fe species, such as an Fe-complex
with natural organic matter NOM. Typically, NOM is a
complex mixture of organic substances containing a va riet y
of functional groups, such as carboxyls, phenols, thiols
and amines, many of which interact strongly with Fe(II)
and/or Fe(III). It has been reported that Fe(III) forms
Figure 4. Detection of iron species and Mn(II) in (a) gr oun d-
water; and (b) river water samples in Texas.
complexes with organic acids. At low Fe concentrations
and pH 3.0 - 7.2, mononuclear Fe(III)-NOM complexes
completely dominate the speciation, and a substantial
amount of the total Fe (>50%) is in the form of organic
complexes [16].
We further measured the Fe chromatograms of several
solutions of organic materials containing 10 mg/L carbon;
these solutions were prepared from the standard refer-
ence materials including Suwannee River natural organic
matter, Suwannee River fulvic acid, Pahokee Peat humic
acid, and Summit Hill soil humic acid (all from the In-
ternational Humic Substan ces Society, Denver, CO). We
consistently observed the organic-Fe peak for these four
materials; the chromatograms of Suwannee River natural
organic matter and Summit Hill soil humic acid are
shown in Figure 5. The iron species in these solutions
are predomi nantl y organo-Fe.
In addition, we analyzed the effluent samples from
columns packed with sediment samples, where we stud-
ied the transport behavior of arsenic species in different
sediments [17]. We consistently observed the predomi-
nant organo-Fe peaks in the column effluents from both
Hanford and Datong basin sediments (Figure 6). In a
report on determination of the structures and reactivities
of Fe associated with NOM (considering that these in-
teractions influence the redox, hydrolysis and solubility
of Fe18), Rue and Bruland [18] reported that 99.97% of
the dissolved Fe(III) in central North Pacific surface wa-
ters is chelated by natural organic ligands. We are not
aware of the direct detection, or of the prevalence and
predominance, of organo-Fe species in natural water. It
is important to note that the conventional ferrozine me-
thod of an alyzing f or Fe(III ) is ba sed on the diff erence of
measured total Fe and Fe(II), and that the presence of
organo-Fe species will affect the measurement of inor-
ganic Fe species. As discussed above, iron forms com-
plexes with dissolved organic matter; where this occurs
Figure 5. Detection of iron species in (A) Suwannee River
natural organic matter; and (B) Summit Hill soil humic
acid; both at the concentration of 10 mg/L carbon.
Copyright © 2011 SciRes. AJAC
Q. H. HU
Copyright © 2011 SciRes. AJAC
species in a single run. Using individual standards, we
obtained the retention times of these species in the eluent
with 14 mM PDCA concentration (Figure 7, Table 2).
Table 2 also presents the isotopes monitored for these
seven transition metals, the baseline levels, and the in-
tensity responses for these isotopes. Other than Fe-56
and Cu-63, the isotopes for monitored transition metals
have low mass interference. After normalizing the natu-
ral abundance for these isotopes, the signal intensities for
all of these elements are fairly close (Table 2), which is
expected from their closeness in atomic mass units
Using the separation method for seven transition met-
als, we further analyzed both the tap water and the DI
water in a research lab; the DI water was produced by-
passing the tap water through the water purification sys-
tem, which consisted of reverse osmosis and a 254-nm
UV lamp for organic molecule oxidation and bacteria
destruction. We observed a peak of organo-Fe in the tap
water, and this peak was removed in the DI water, possi-
bly from the UV oxidation (Figure 8). On the other hand,
the heights of Fe(III) and Fe(II) peaks are similar for
both tap and DI waters, indicating the in efficiency of the
water purification system to remove very low (below
about 0.2 µg/L) concentrations of these Fe species. The
tap water also has higher concentrations of Cu and Zn
than the DI water, probably from the piping system
(Figure 8).
Water samples from the Gallinas River and the Mon-
tezuma Hot Springs in New Mexico also indicate the
presence of organo-Fe, as well as Fe(III) and Fe(II), spe-
cies (Figure 9). In addition, there is a detectable amount
of divalent Cu, Ni, Zn, and Mn species in these waters.
Figure 6. Detection of iron species and Mn(II) in the efflu-
ent samples collected from a column packed with (a) Han-
ford sediment in the State of Washington; and (b) Datong
Basin in Shanxi province, China.
4. Conclusions
in seawater, the color development of ferrozine chelates
is hindered. To eliminate this factor, samples were heat-
ed and exposed to UV irradiation to decompose the
organo-iron complex in the work of Kononets et al. [19]. Using advanced analytical tools of LC-ICP-MS and
modification of a separation scheme for seven transition
metals published by Dionex, we present a versatile
method of simultaneous analysis for iron species, with
micro-liter injection volumes. The method is assisted by
the use of the Dynamic Reaction Cell technique to re-
duce the detection of 56Fe, and the species check of
3.3. Simultaneous Separation of Seven
Transition Metals
The separation scheme can detect seven transition metal
Table 2. Retention time and signal intensity of 7 transition metals simultaneously separated and quantified by LC-ICP -M S.
Cu-63 Ni-60 Zn-66 Co-59 Cd-114 Mn-55 Fe(II)-56
Retention time (min) 3.9 4.3 4.7 5.2 5.8 6.6 6.9
Natural abundance (%) 69.09 26.33 27.81 100 28.86 100 91.66
Baseline level (cps) 25,000 2,600 1,100 140 110 800 25,000
LC-ICP-MS intensity of 1 ppm standard (cps ) 7,443,7372,753,4641,859,57810,385,8283,383,030 14,665,3787,338,520
Abundance normalized cps/ppb 10,773 10,457 6,687 10,386 11,722 14,665 8,006
Q. H. HU
Copyright © 2011 SciRes. AJAC
Figure 7. Retention times and signal intensities of 7 transi-
tion metals in a single run.
Figure 8. Presence of 7 transition metal species in the (a)
tap water; and (b) DI water.
Fe(III) and Fe(II) increases the confidence of iron spe-
cies determination. In particular, we found that most of
the iron species in natural waters and geological samples
could exist in complexation with natural organic matter.
Attention should be paid to the prevalence of organo-Fe
species in speciation studies of iron, and to its role in
Figure 9. Presence of 7 transition metal species in (a) Galli-
nas River; and (b) Montezuma Hot Springs water samples
in New Mexico.
modifying the biogeochemical cycling of elements in
aquatic and terrestrial environments.
5. Acknowledgments
This work was financially supported by the University of
Texas at Arlington.
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