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Accepted Manuscript
Quantification of paramagnetic ions in solution using time domain
NMR. PROS and CONS to optical emission spectrometry method
Flávio V.C. Kock, Maiara P. Machado, Geisamanda P.B. Athayde,
Luiz A. Colnago, Lúcio L. Barbosa
PII:
DOI:
Reference:
S0026-265X(17)30956-6
doi:10.1016/j.microc.2017.10.013
MICROC 2934
To appear in:
Microchemical Journal
Received date:
Revised date:
Accepted date:
9 September 2017
18 October 2017
19 October 2017
Please cite this article as: Flávio V.C. Kock, Maiara P. Machado, Geisamanda P.B.
Athayde, Luiz A. Colnago, Lúcio L. Barbosa , Quantification of paramagnetic ions in
solution using time domain NMR. PROS and CONS to optical emission spectrometry
method. The address for the corresponding author was captured as affiliation for all
authors. Please check if appropriate. Microc(2017), doi:10.1016/j.microc.2017.10.013
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ACCEPTED MANUSCRIPT
QUANTIFICATION OF PARAMAGNETIC IONS IN SOLUTION USING TIME
DOMAIN NMR. PROS AND CONS TO OPTICAL EMISSION
SPECTROMETRY METHOD
Flávio V. C. Kocka, Maiara P. Machadob, Geisamanda P. B. Athaydeb, Luiz A.
Colnagoc, Lúcio L. Barbosad,*
a
SC
RI
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Instituto de Química de São Carlos, Universidade de São Paulo, Avenida
Trabalhador São Carlense 400, 13566-590, São Carlos, São Paulo, Brasil.
b
Laboratório de Pesquisa e Desenvolvimento de Metodologia para Análises de
petróleo-LabPetro. Universidade Federal do Espírito Santo, Av. Fernando Ferrari, 514,
CEP 29075-910, Goiabeiras, Vitória – ES, Brasil.
c
Embrapa Instrumentação, Rua XV de Novembro 1452, 13560-970, São Carlos,
São Paulo, Brasil.
d
Universidade Federal de São Paulo, Rua Carvalho de Mendonça 144, 11070-010,
Santos, São Paulo, Brasil. *email: luciolbar@gmail.com
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Solutions containing paramagnetic ions (PMIs) have been widely used in several
industrial and agricultural applications. Although the concentrations of these ions in
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solutions are very important, they are rarely checked due to the high cost of the
measurements. In this paper, we demonstrate the viability of 2 MHz time-domain
nuclear magnetic resonance (TD-NMR) relaxometer to quantify PMIs in solution. The
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method was used to quantify the concentration of Co2+, Cr3+, Cu2+, Fe3+ and Mn2+ and is
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based on linear correlation between the transverse relaxation rate (T2-1) of the hydrogens
of water and PMIs concentration. The method was validated using inductively coupled
plasma optical emission spectrometry (ICP-OES), which is the standard method for
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quantitative elemental analyses. The concentration of PMIs were analyzed between 0.5
and 10 mg.L-1, with r2  0.99 between the two methods. Although the limits of detection
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(LOD) and quantification (LOQ) of TD-NMR are several orders of magnitude higher
than those of ICP-OES, they are in the range of concentration normally found in
commercial applications. Therefore, this work demonstrate that TD-NMR can be an
affordable alternative for certificate the concentration of PMIs concentration in small
laboratory in industrial or agricultural facilities.
Keywords: low field NMR, ICP-OES, Paramagnetic ions
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1. INTROCUCTION
Solutions containing paramagnetic ions (PMIs) have been widely used in several
researches and industrial applications such as electroplating [1,2], tannery [3] and
agriculture, specifically, in hydroponics [4] and foliar fertilization [5]. Although, the
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concentrations of these ions are very important in these areas, they are rarely checked
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due to the time consuming and laborious procedure of the traditional wet chemical
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methods or the high cost of emission/absorption spectroscopic standard approaches. In
some cases, only the total ionic concentrations are monitored using electrical
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conductivity [4].
In this paper, we are demonstrating that time-domain nuclear magnetic
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resonance (TD-NMR) relaxometry can be a simple, rapid, precise and non-destructive
method to quantify PMIs in solution [6]. The TD-NMR relaxometry can also be applied
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direct in the storage flash [7] reducing the possibility of contamination and does not
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need specialized operator [8,9].
The relaxometry analysis is based on linear correlation between the
1
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concentration of the PMIs and the longitudinal (1/T1) and transverse relaxation rates (T2) of the hydrogens present on the solvent. In 1946, Bloch, Hansen and Packard
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observed this phenomenon for the first time using an aqueous solution of Fe3+. In 1948,
Bloomberg, Purcell and Pound explained that the increase on the solvent relaxation
rates in presence of PMIs is a function of the hydrogen gyromagnetic ratio (γp),
effective magnetic moment (μef), number of the ions by volume unity (Nion), viscosity
(η), Boltzmann constant (κ) the and temperature (T) in Kelvins according to equation 1.
(1)
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Nothnagel and Weiss in 1970 show the first use of T1 relaxation time, measured
with inversion-recovery (IR) pulse sequence, to quantify PMIs in solution using the
equation 1. Schluter & Weiss in 1970 decade shows the application of the same method
to measure T1 in redox and complexometric titration and in the indirect determination of
PMIs species trough precipitation [10-13]. They also show that T2-1 are dependent on
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equation 1 and can be used to measure the concentration of PMIs in solution, but they
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did not use it due to instrumental limitations at that time. Nowadays, Carr-Purcell-
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Meiboom-Gill (CPMG) pulse sequence, widely in relaxometric measurements, is a
standard sequence in low field NMR spectrometers and much faster that IR method [6,
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14-16].
T2 relaxometry have been used by Nunes et al. 2012 [6], Gomes et al., in 2014
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[15] and Gomes et al, 2015 [16] to in situ quantify Cu2+ in aqueous solution during an
electrodeposition reaction. Cobra et al., 2015 use the T2 relaxometry to determine the
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concentration of PMIs in aqueous solution in function of pH to determine the solubility
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product constant (Ksp) of the hydroxides ions [14]. Kock & Colnago in 2015 studied the
T1 and T2 relaxometry properties of EDTA-paramagnetic ions chelates in solution, in
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function of pH using a rapid method that measure both relaxation times in a single shot
pulses sequence, named CWFP [17]. Gomes et al., in 2016 compared the quantification
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of PMIs determined by relaxometry with Atomic Absorption Spectroscopy (AAS) data
found in literature [18]. Kock et al, 2017 study the complexation of PMIs with chitosan
in function of pH using both T1 and T2 relaxometry measured with CWFPx-x sequence
[19]. All the above experiments were performed in 0.23 (9 MHz) or 0.47 T (19.9 MHz)
bench top spectrometers.
In this paper, we are demonstrating that TD-NMR relaxometry in very low
magnetic field 0.05 T (2 MHz for 1H) is a simple, fast, low cost and precise method to
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quantify paramagnetic ions in aqueous solution. The method was checked using
Inductively Coupled Plasma Optical Emission spectrometry ICP-OES that is standard
method for quantitative elemental analyses.
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2. EXPERIMENTAL
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The HNO3 P.A. (Vetec) used for dilution of samples was previously distilled in a
Subboiling Destillacid BSB-939-IR Berghof. The solutions were prepared in the
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concentration range from 0.5 to 10 mg.L-1 using ultrapure water with resistivity of 18
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MΩ.cm at 25°C and standard solutions of Mn2+, Cr3+, Fe3+ and Cu2+ (SpecSol®). For
Co2+ solution, a salt of CoCl2.6H2O (Vetec) was solubilized in HNO3 0.2% v/v. Finally,
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dilute nitric acid was used as "blank" solution in the experiments.
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NMR analyses
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The T2 measurements were performed in a MARAN Ultra-2 equipment, Oxford
Instruments Molecular Biotools Ltd., equipped with a magnet of 0.052T, equivalent to
H resonance frequency of 2.2 MHz and a 51 mm probe. Three samples (triplicates) of
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1
25 ml of each solution were transferred to the glass sample holder (50 mm diameter and
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180 mm height) and left in the probe for about 5 min to stabilize the temperature of 27 ±
1 °C. The T2 values were obtained employing the CPMG pulse sequence, using 90o and
180o pulses of 8.3 and 16.6 s, respectively, number of echoes equal to 32K, echo time
of 200 s and eight scans. The recycle time was 12 s that is more than five times the
longest T1 that was 1.5 s.
The T2 values were obtained by fitting the CPMG decay with exponential function,
using WinFit software (oxford).
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ICP-OES Analyses
The Inductively Coupled Plasma Optical Emission Spectrometric (ICP-OES)
analyses were performed in an Optima 7000 DV spectrometer (PerkinElmer) with dual
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view (axial/radial), equipped with optics Echelle and RF solid state generator (40 MHz).
The concentrations of all PMIs were determined in triplicates. The emission lines at
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228.616nm for Co (I); 327.393nm for Cu (I); 257.610 nm for Mn (II), 267.716 nm for
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Cr (II) and 259.939 nm for Fe (II) were used to quantify the respective PMIs in solution.
The symbols (I) and (II) indicate atomic and ionic lines respectively. The instrumental
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parameters used in the analyses were: plasma power of 1300 W, plasma gas flow rate of
15 L.min-1, auxiliary gas flow rate of 0.2 L.min-1, nebulization gas flow rate of 0.8
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mL.min-1, nitrogen purge (2.5 mL.min-1, nebulizer model Munhard and a cyclonic
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nebulization chamber.
3. RESULTS AND DISCUSSION
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Figure 1 shows the CPMG decay of the pure water and aqueous nitric acid (0.02
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%) solution (a) and the decays of the five nitric acid aqueous solutions of Co2+ from 0.5
and 10 mg.L-1 (b) at 0.05 T (2 MHz for 1H). The CPMG decays of the solutions
containing, Cr3+, Fe3+, Mn2+ and Cu2+ions are shown in the supplementary material. The
signals to noise ratio (SNR) of these signals are not very high as measured in higher
field (0.2 to 0.5 T) but it does not interfere in the monoexponential fitting use to
measure T2. If necessary the SNR can be improved by data compression [19].
Figure 1a shows that the water and nitric acid solution has similar T2,
approximately 2.77 s. Figure 1b shows the strong effect of Co2+ concentration in the
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water relaxation times. The T2 values in this figure range from 2.2 to 0.61 s, for the 0.5
and 10 mg.L-1 solutions. This T2 reduction is associated to dipolar interaction between
the magnetic moments of unpaired electrons from paramagnetic ions and the magnetic
moments present on the 1H water nuclei. The relaxation times for the others PMIs for
the 0.5 and 10 mg.L-1 solutions varies from 2.12 and 0.40 s for Cr3+, 1.67 to 0.21 s for
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Fe3+, from 1.49 to 0.12 s for Mn2+ and from 2.77 to 1.55 s for Cu2+. These differences in
H2O
0.8
-1
0.5 mg.L
-1
2 mg.L
-1
6 mg.L
-1
10 mg.L
0.8
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0.6
0.4
0.2
0.0
-0.2
(b)
1.0
Blank solution
Normalized amplitude
Normalized amplitude
1.0 (a)
[20,21].
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for
-2
0
2
4
6
8
10
12
14
0.6
0.4
0.2
0.0
-0.2
-2
0
2
4
6
8
10
12
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Time/s
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Time/s
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The
) of each ion.
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relaxivity are direct related to the difference in the magnetic moment
Figure 1. Normalized CPMG decay curves of the pure water and aqueous nitric acid
(0.02 %) solution (a) and five nitric acid aqueous solutions of Co2+ from 0.5 and 10
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mg.L-1(b)
Figure 2 show the linear correlation between transverse relaxation rate (T2-1)
with the concentration of PMIs. The variation ranges obtained for (T2-1) were 0.45 to
1.63 s-1 for Co2+, 0.47 and 2.51 s-1 for Cr3+, 0.36 to 0.64 s-1 for Cu2+, 0.59 to 4.67 s-1 for
Fe3+ and 0.67 and 8.05 s-1 for Mn2+. This range values and the difference in the slopes
(relaxivity) for each ion experimentally demonstrate the effect of the magnetic moment
on the relaxation of 1H of the solvent.
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Co2+
Cr+3
Cu2+
Fe+3
Mn2+
8
7
-1 -1
T2 /s
6
5
4
3
2
1
0
2
4
6
8
Concentration (mg.L-1)
10
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0
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Figure 2. Variation of transverse relaxation rate (T2-1) of the water versus the
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concentration of Co2+, Cr3+, Cu+2, Fe3+ and Mn2+ in the solutions.
The results of figure 2 shows that the method can be used to determine the
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concentration of these ions in solution. However, to certify the solutions concentrations,
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the same samples were analyzed by ICP-OES. The correlation curves between (T2-1)
and the optical emission intensity in counts per seconds rate (cps-1) is shown in figure 3.
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These curves show r2 > 0.99 indicating a high linear correlation between the two
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methods. As observed in figure 2, the slope of the Cu2+ in figure 3 shows the smallest
slope due to its lower relaxivity.
T2-1 (s-1)
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9
8
7
2+
6
Co
3+
Cr
2+
Cu
3+
Fe
2+
Mn
5
4
3
2
1
0
0
20
40
60
80
100
120
Intensity (105 cps-1)
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Figure 3. Correlation between transverse relaxation rates (T2-1) obtained from TDNMR relaxometry versus optical emission intensity obtained from ICP-OES: (a) Co2+,
Cr3+, Fe3+ Cu2+ and Mn2+.
The figures of merit, limit of detection (LOD) and limit of quantification (LOQ)
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of TD-NMR relaxometry and ICP-OES are shown in table 2. The NMR measurements
were obtaining by measuring ten times the relaxation of 0.5 mg.L-1 solutions and for
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ICP-OES ten measurements of the blank solution. The results of Table 2 shows, as
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expected, that TD-NMR is several orders of magnitude less sensible than ICP-OES.
However, the NMR relaxometry technique is suitable for estimating the concentration
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of PMIs in industrial or agriculture applications. Due to the low relaxivity, the LOD and
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LOQ of Cu2+ measured by TD-NMR have the higher values or less sensibility. Cu2+ has
also the lowest determination correlation with ICP-OES measurements.
Table 2. Comparison of analytical figures of merits (LOD and LOQ) obtained by TD-
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NMR and ICP-OES and the determination coefficients obtained from the validation
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curve constructed from the relaxation rate and emission intensity data.
LOD (mg.L-1)
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TD-NMR
LOQ (mg.L-1)
R2
ICP-OES TD-NMR
ICP-OES
0.000078 0.9996
0.32
0.000024
1.08
Cr3+
0.13
0.00035
0.43
0.0012
0.9999
Cu2+
1.14
0.00020
3.79
0.00067
0.9872
Fe3+
0.10
0.00044
0.34
0.0014
0.9989
Mn2+
0.11
0.00032
0.37
0.0011
0.9996
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Co2+
TD-NMR relaxometry can also be used to check the total content of PMIs or to
monitor the precipitation of PMIs as function of pH (figure 4a). This figure shows that
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in strong acidic mediums (pH lower than 2.5), the constant T2 values are lower than ≤
100 ms. This result leads to infer that all five PMIs are fully protonated and solubilized
in solution and its paramagnetic relaxation contribution is maxima. As the pH increased
there is at approximately pH 3 a small increase of T2 that can be related to the
precipitation of Fe(OH)3, with a turning point at pH 3.5 [14]. Between pH 4 to 6.5 and
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from 6.5 to 7.5 T2 increases again due the precipitation of Cr(OH)2, with a turning point
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at pH 6.5 for this hydroxides and pH 7 for precipitation of Cu(OH)2. The pH shows a
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rapid increase in the range between 7.5 and 10 that can be related to precipitation of
Co(OH)2 and Mn(OH)2 with turning points at pH 8.7 and 9, respectively. From pH 9.5
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to 12, the T2 did not varies and is close to the pH of “pure water” (in this case, the term
in quotes is used to refer the alkaline water in the supernatant), indicating that all PMIs
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were precipitates and are not interfering in the relaxation times. However, for pH >12
the relaxation times decreases. This decay has been assigned to the presence Co 2+ that
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shows similar behavior when this ion analyzed in solution in this pH range (Figure 4b).
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This decay of T2 at pH>11 indicated an alkaline hydrolysis of Co(OH)2 producing a
soluble form of Co2+ ions. From the result of figure 4 it is possible to infer that the TD-
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NMR relaxometry can also been used to monitor the different PMIs (speciation) that is
important in commercial solutions.
2.6
2.4
T2 (s)
2.3
(b)
1.8
1.6
T2 (s)
2.5
2.0
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(a)
2.2
0.3
0.2
0.1
0.0
-0.1
-0.2
1.4
1.2
1.0
0.8
2
4
6
8
pH
10
12
14
0
2
4
6
8
10
12
14
pH
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Figure 4. a) Variation of T2 of the water of a mixture of Co2+, Cr3+, Cu+2, Fe3+ and
Mn2+ versus pH. b) Variation of T2 of the water in a Co2+ solution versus pH.
4. CONCLUSIONS
Given the results, it is possible to conclude that TD-NMR relaxometry even at
very low filed (0.05 T) can be used to quantify the concentration for PMIs in solution,
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from 0.5 to 10 mg.L-1. Consequently, this spectroscopy can be a simple, rapid and non-
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destructive method for certifies the concentration of PMI in several trades where this
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simple method is necessary. Besides, a new generation of small portable Halbach and
unilateral magnets [22-24] and NMR spectrometer–on-a-chip [25] can significantly
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reduce the instrumentation costs. In addition, the development of new pulse sequences
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that leads the simultaneous estimative for T1 and T2 in a single and fast measurement,
can reduce the experimental time and improve the information about the system [2627]. Therefore, TD-NMR has the potential to be a popular method for measuring PMIs
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in solution due to its simplicity, speed and no cost with chemicals for the analyses,
similarly to high field NMR spectroscopy that already is used to quantify
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simultaneously several compounds [28].
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ACKNOWLEDGEMENTS
The authors thank CAPES and FAPESP grants 2012/23169-8 and 2017/12864-0 and
CNPq grant 303837/2013-6 for financial support.
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Highlights
 The analysis performed in a benchtop time-domain NMR spectrometer
showed be great alternative to the ICP OES.
 Detection of precipitation of the several ions in the same chemical solution.
 NMR has the potential to be a popular method for measuring paramagnetic
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ion concentration in solution due its simplicity, speed and no cost with
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chemicals for the analyses.
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