A sensitive and accurate simultaneous continuous analysis for six arsenic species including arsenobetaine (AsB), arsenocholine (AsC), monomethylarsonic acid (MMA), dimethylarsinic acid (DMA), arsenite (AsIII), and arsenate (AsV) has been developed by high performance liquid chromatography with inductively coupled plasma mass spectrometry (HPLC-ICP-MS). An anion-exchange column of Hamilton PRP-X100 (Switzerland) was applied for separation of the six arsenic species with gradient elution of 1.25 mmol/L Na2HPO4 and 11.0 mmol/L KH2PO4 as the mobile phase A and 2.5 mmol/L Na2HPO4 and 22.0 mmol/L KH2PO4 as the mobile phase B. The linearity ranges for AsB, AsC, MMA, DMA, AsIII, and AsV were between 0.5 and 50.0 μg/L, and the detection limits of the six arsenic species were all within 0.01–0.35 ng/L. The relative standard deviations (RSDs) were within 2.26–3.68% and the recovery rates of samples ranged from 95 to 103%. The proposed method was applied for the arsenic speciation analysis of sediment pore-water samples, which were taken from the supernatant after centrifugation and filtration.
National Natural Science Foundation of China21773170Yangtze Scholars and Innovative Research Team in Chinese UniversityIRT_17R811. Introduction
Arsenic as a typical toxic element [1, 2] is considered as one of the primary pollutants, and the detriment of arsenic to human body is secular and chronic. Mcneill and Edwards [3] reported that toxicity of arsenite (AsIII) is 60 times higher than that of arsenate (AsV), and the organic forms such as dimethylarsinic acid (DMA) and monomethylarsonic acid (MMA) are much less toxic [4] and the sequence of toxicity is in order of AsH3 > AsIII > AsV > MMA > DMA > AsB ≈ AsC [5]. In natural environment, arsenic in sediment, water body, and atmosphere mainly existed in the form of inorganic compounds including arsenate and arsenite.
As we well know, most of speciation analytical methods in the literature are based on separation techniques hyphenated with high-precision detectors, for instance, capillary electrophoresis (CE) [6], ion chromatography (IC) [7], or high performance liquid chromatography (HPLC) [8], coupled to atomic fluorescence spectrometry (AFS) [9], atomic absorption spectrometry (AAS) [10], or inductively coupled plasma mass spectrometry (ICP-MS) [11]. Nevertheless, simultaneous continuous measurement of six arsenic species is still crucial but challenging. Compared with other combined technologies, high performance liquid chromatography (HPLC) hyphenated with ICP-MS is an ultrasensitive method for the determination of all arsenic species in complex sediments [12–14] since ICP-MS has an advantage that it can achieve trace analysis. It is worth noting that ICP-MS has been proverbially used in all kinds of samples such as in urine [15, 16], ground water [17, 18], and food [19, 20], but the arsenic species simultaneous continuous measurement using HPLC-ICP-MS in the sediment pore-water samples has not been reported. In this work, we present a practical and sensitive method for the quantification of arsenite, arsenate, and organic arsenic species in sediment pore-water samples.
2. Experimental2.1. Apparatus
An iCAP Q ICP-MS (Thermo Scientific, USA) was used for quantitative analysis of arsenic speciation. A Thermo U3000 HPLC system was successfully used to separate arsenic species with the injection volume of 20 μL in this study. Separation of arsenic species was achieved using a Hamilton PRP-X100 column (250 mm length × 4.1 mm i.d, 10 μm particle size) and the column temperature was at room temperature. A buffer solution of sodium hydrogen phosphate and monopotassium phosphate was used through gradient elution for separation of arsenic of six species. The outlet of the chromatographic column was forthright connected to the concentric nebulizer using a 0.18 mm i.d. PEEK tubing. ICP-MS was fitted with Kinetic Energy Discrimination (KED) mode allowing target isotope ions to enter the mass analyzer while preventing polyatomic interfering ions from entering the mass analyzer. By this way, background values can be reduced and obtain good peak shape. The HPLC and ICP-MS system working conditions were summarized in Table 1. The pH value was measured by a high-precision pH meter (WTW PH-7310, Shanghai Precision Scientific Instruments Co. Ltd., China) with an uncertainty of ± 0.003.
Working conditions of the HPLC and ICP–MS system.
Parameters
Value
ICP-MS system
RF power (W)
1500
Cool flow (L min−1)
14.00
Auxiliary gas flow (L min−1)
0.80
Nebuliser gas flow (L min−1)
1.015
Collision gas flow (L min−1)
He/4.09
CCT1 flow (mL min−1)
5.198
Sampler depth (mm)
5.0
Spray chamber temperature/°C
2.70
Operation mode
KED mode
Isotopes monitored
75As
Dwell time (ms)
200
HPLC system
Column
Hamilton PRP–X100 column (250 mm × 4.1 mm i.d., 10 μm)
Mobile phase
Phase A: 1.25 mmol/L of Na2HPO4 and 11 mmol/L of KH2PO4, pH 6.116
Phase B: 2.5 mmol/L of Na2HPO4 and 22 mmol/L of KH2PO4, pH 6.057
Flow–rate (mL/min)
Gradient elution of range 0.6-1.5 mL/min
HPLC elution program
0-4.9 min:100% A, 4.9-5.3 min: 100% A to 100% B, 5.3-12 min:100% B0-4.3 min: 0.6 mL/min, 4.3-4.6 min: 0.6 to 1.0 mL/min, 4.6-4.9 min: 1.0 to 1.5 mL/min, 4.9-12 min: 1.5 mL/min.
Column temperature (°C)
Room temperature
Quantitative loop (μL)
20 μL
2.2. Reagents and Standard Solution
Ultrapure water (resistivity, 18.2 MΩ cm−1) obtained from a Milli-Q ultrapure water purification system (Millipore, Bedford, MA, USA) was used for all dilution in the experiment. Sodium hydrogen phosphate (A.R., Sinopharm Chemical Reagent Co., Ltd) and monopotassium phosphate (A.R., Sinopharm Chemical Reagent Co., Ltd) were used to prepare mobile phase, which were filtered through a 0.45 μm membrane filter and bubbles were excluded in an ultrasonic bath before use. Standard solutions of AsB, AsC, MMA, DMA, AsIII, and AsV were purchased from Chinese Academy of Metrology (Beijing). Six species of arsenic stock solution (1.00 mg/L) were prepared and stored in polytetrafluoroethylene bottle at 4°C in the refrigerator. Working standard solution of 0–50 μg/L was prepared from stock solution by gradient dilution for calibration curve prior to use and stored at 4°C.
2.3. Sample Collection and Preparation
Teflon fiber membrane balance sampler was adopted to collect samples, which was described in detail [22]. The sampler was immersed with 1% nitric acid for 3 days, then washed with distilled water, packed in plastic bags, and tightly wrapped in preservative bags in the laboratory. There was one sediment sample chosen in Tuojiang River in the west of China (104°31′19.0′′E, 30°43′44.2′′N). The core sampler was inserted into sediment slowly to sample about 20 to 25 cm in length and then carefully placed into the big plastic bag filled with nitrogen gas in situ, and then the samples were divided centimeter by centimeter from the bottom to the top with plastic knife to load into a series of numbering high-density polyethylene bottles and stored in low temperature preservation to move to laboratory quickly. For analysis, the samples were thawed firstly in the glove box (UNIlab Plus, MBraun, Germany) with nitrogen gas filled and refrigerated centrifuge (HERMLE Z326K, Germany) at speed of 18000 rpm, the pore-water sample was filtered with 0.45 μm membrane into the high-density polyethylene bottle, and then a certain amount of hydrochloric acid was added to make the pH about 2 and then stored at 4°C for analysis as quickly as possible.
2.4. Speciation Analysis Procedures for Water Samples
For the speciation analysis of sediment pore-water samples collected in the high-density polyethylene bottles, the samples were removed from the refrigerator and returned to the room temperature without any other treatment and then directly injected using manual injection needle with quantitative loop volume of 20 μL. The concentrations of six arsenic species in the pore-water sample were determined directly by HPLC-ICP-MS.
3. Results and Discussion3.1. Selection of Chromatographic Conditions
The chromatographic column and buffer solution as mobile phase were indispensable to establish a successful separation and analysis method for arsenic species using high performance liquid chromatography with inductively coupled plasma mass spectrometry (HPLC-ICP-MS). For the anion-exchange column, the retention of arsenic species can be influenced by the type of column, the ionic strength, concentration, and the flow rate of the mobile phase. In order to find the optimal mobile phase, Na2HPO4 (5 mmol/L) and KH2PO4 (44 mmol/L) were used to achieve a preferable separation of the arsenic species. AsC, MMA, DMA, and AsV were completely separated and the analysis time was abbreviated, but it was worth noting that AsB and AsIII were not fully separated from each other. Meanwhile considering that the sampler and skimmer cones of ICP-MS would be blocked if the sodium salt concentration was too high, resulting in signal suppression [23], reducing the mobile phase concentration was adopted to analyze the arsenic species. Fortunately, AsB and AsIII were fully separated; at the same time the analysis time would be applicable. Therefore, a gradient elution procedure consisting of 1.25 mmol/L of Na2HPO4 and 11 mmol/L of KH2PO4 and 2.5 mmol/L of Na2HPO4 and 22 mmol/L of KH2PO4 was adopted for the optimized chromatographic operating conditions in this experiment, and the procedure was listed in Table 1. The separation result is shown in Figure 1.
Chromatograms of arsenic speciation by gradient elution using mobile phase A: 1.25 mmol/L of Na2HPO4 and 11 mmol/L of KH2PO4; mobile phase B: 2.5 mmol/L of Na2HPO4 and 22 mmol/L of KH2PO4, 20 μg/L for each arsenic speciation.
The Hamilton PRP-X100 anion-exchange chromatographic column was based on polymer anion-exchange filler, in which the elution behavior can be explicated by means of ion exchange mechanism. In order to discuss the elution behavior as described in this paper, the ionic forms of arsenic were estimated using the dissociation constants and the pKa values of each species were shown in Table 2. Each speciation of arsenic has different anion-exchange capacity due to different pKa value; the smaller the value is, the easier the corresponding acids dissociate and the stronger the retention in the column is. AsC, a cation irrespective of the pH, implied that the elution was almost unaffected; hence, it was the first form to be separated. Since AsB existed as the zwitterionic form while AsIII existed as a weakly ionized compound [21], the interaction with stationary phase was weak, so AsB and AsIII cannot be completely separated through high concentration in the mobile phase. Owing to the fact that other three arsenic species have low pKa values in descending order of DMA, MMA, and AsV, they sequentially appeared at the end in the separation process.
pKa values and formulas of arsenic species [21].
Species
Formula
pKa
AsC
(CH3)3As+CH2CH2OH
−
AsB
(CH3)3As+CH2COOH
2.18
AsIII
H3AsO3
9.28
DMA
(CH3)2AsO(OH)2
6.3
MMA
CH3AsO(OH)2
2.6, 8.2
AsV
H3AsO4
2.3, 6.8, 11.6
3.2. Analytical Performances
Under the optimum conditions, six arsenic species were achieved with the symmetrical peaks and have a good resolution. The analytical performance using the HPLC-ICP-MS was determined by the linearity of calibration curves, detection limits, and relative standard deviation. The calibration plot was obtained by drawing peak area of signal (cps) against the concentration of the homologous target ions. In Table 3, it was shown that the linearity of six arsenic species ranged from 0.5 to 50.0 μg/L with 20 μL of standard solution injection.
Analytical figures of arsenic speciation by HPLC–ICP–MS.
Parameters
Analytical features
AsB
AsC
AsIII
DMA
MMA
AsV
Linear range (µg/L)
0.5–50.0
0.5–50.0
0.5–50.0
0.5–50.0
0.5–50.0
0.5–50.0
Coefficient (r)
1.0000
0.9999
1.0000
1.0000
1.0000
1.0000
LOD (ng/L)
0.01
0.05
0.11
0.28
0.20
0.35
R.S.D. (%, n = 5)
3.68
3.04
3.19
3.02
3.36
2.26
The calibration curve has achieved good linearity with correlation coefficients (r) values more than 0.9999. Method detection limits (3σ/k) were calculated, where σ was the standard deviation for three replicates of the lowest concentration of standard solution and k was slope of the calibration plot, ranging from 0.01 to 0.35 ng/L. The detection limit can be increased with a larger volume injection. Nevertheless, a larger volume injection can bring about column overloading and make the salt accumulation on the skimmer and sampler cones, which caused sensitivity reduction. The reproducibility was expressed by calculating the RSD of five repeated experiments using 20 μg/L of standard mixture solution of each arsenic species. The results showed that RSDs for AsB, AsC, AsIII, DMA, MMA, and AsV were 3.68, 3.04, 3.19, 3.02, 3.36, and 2.26%, respectively. The results demonstrated that satisfactory reproducibility and sensitivity were achieved for arsenic speciation using this method.
3.3. Analytical Application
According to the speciation analysis procedures in Section 2.4, the proposed method was employed to determine arsenic speciation and corresponding content in sediment pore-water samples in depths of 0, -5, and -10 cm (from the surface to the depth of bottom) in Tuojiang River.
The analytical results of arsenic species and the recoveries of different species in the sediment pore-water samples via standard adding are shown in Table 4. The analytical results indicated that the concentrations of AsB and AsC were not detected in the whole core samples. And the three replicates of samples and standard additions through biking in the samples showed that recoveries for the six arsenic species of AsB, AsC, AsIII, DMA, MMA, and AsV were between 95 and 103%, and the RSDs for parallel experiments were below 3%. Furthermore, with the increase of underground depths, the concentration of AsIII and MMA increased, the concentration of AsV decreased, and that of DMA changed indistinctively. Those results indicated that the proposed method in this work is valid for the determination of arsenic species in water samples.
Analytical results and recoveries of the sediment pore-water samples.
Depth(cm)
Arsenic species
Concentration(µg/L) ± S.D.a
Added (µg/L)
Found(µg/L)
Recovery(%)
0
AsB
N.D.b
0.500
0.503 ± 0.002
100.6
AsC
N.D.b
0.500
0.495 ± 0.005
99.0
AsIII
0.217 ± 0.003
0.500
0.708 ± 0.002
98.2
DMA
0.530 ± 0.002
0.500
1.032 ± 0.009
100.4
MMA
0.012 ± 0.002
0.100
0.115 ± 0.007
103.0
AsV
2.431 ± 0.020
2.500
4.970 ± 0.008
101.6
-5
AsB
N.D.b
0.500
0.499 ± 0.006
99.8
AsC
N.D.b
0.500
0.501 ± 0.003
100.2
AsIII
0.389 ± 0.014
0.500
0.872 ± 0.011
96.6
DMA
0.563 ± 0.005
0.500
1.061 ± 0.021
99.6
MMA
0.019 ± 0.004
0.100
0.114 ± 0.006
95.0
AsV
2.302 ± 0.018
2.500
4.862 ± 0.029
102.4
-10
AsB
N.D.b
0.500
0.489 ± 0.009
97.8
AsC
N.D.b
0.500
0.497 ± 0.005
99.4
AsIII
0.447 ± 0.006
0.500
0.952 ± 0.010
101.0
DMA
0.534 ± 0.010
0.500
1.040 ± 0.004
101.2
MMA
0.074 ± 0.009
0.100
0.169 ± 0.013
95.0
AsV
2.057 ± 0.015
2.500
4.562 ± 0.004
100.2
aThe values are presented as average ± confidence interval (n = 3).
bNot detectable.
4. Conclusions
In this work, it was clearly shown that high performance liquid chromatography with inductively coupled plasma mass spectrometry (HPLC-ICP-MS) can be perfectly applied in the determination of six arsenic species (AsB, AsC, MMA, DMA, AsIII, and AsV) by one single anion- exchange column using sodium hydrogen phosphate and monopotassium phosphate as eluent. In order to avoid clogging sampler and skimmer cones, arsenic species were separated by reducing mobile phase concentration and setting gradient elution procedure for further optimization. In addition, operating mode of ICP-MS was adapted with Kinetic Energy Discrimination (KED) to improve instrument sensitivity. Under the optimized conditions, determination of six arsenic species has been achieved with good repeatability, high precision, and low detection limits. The proposed method was sufficient to detect the arsenic species and determine the corresponding concentration of each species in sediment pore water, whose form mainly existed in MMA, DMA, AsIII, and AsV, and this method has also been validated accurately by recovery tests in sediment pore-water samples.
Data Availability
The data used to support the findings of this study are included within the article.
Conflicts of Interest
The authors declare that they have no conflicts of interest and we approve of data availability via the corresponding author.
Acknowledgments
Financial supports from National Natural Science of China (21773170) and the Yangtze Scholars and Innovative Research Team in Chinese University (IRT_17R81) are acknowledged.
CostagliolaP.BenvenutiM. M.BenvenutiM. G.Di BenedettoF.LattanziP.Quaternary sediment geochemistry as a proxy for toxic element source: a case study of arsenic in the Pecora Valley (southern Tuscany, Italy)20102701-4808910.1016/j.chemgeo.2009.11.0072-s2.0-74449085174BhattacharyaP.WelchA. H.StollenwerkK. G.McLaughlinM. J.BundschuhJ.PanaullahG.Arsenic in the environment: Biology and Chemistry20073792-31091202-s2.0-3424933874810.1016/j.scitotenv.2007.02.037McNeillL. S.EdwardsM.Soluble arsenic removal at water treatment plants199587410511310.1002/j.1551-8833.1995.tb06346.xHuangJ.-H.ScherrF.MatznerE.Demethylation of dimethylarsinic acid and arsenobetaine in different organic soils20071821-431412-s2.0-3424832867610.1007/s11270-006-9318-4JainC. K.AliI.Arsenic: occurrence, toxicity and speciation techniques200034174304431210.1016/S0043-1354(00)00182-22-s2.0-0034579068ZhangP.XuG.XiongJ.ZhengY.YangQ.WeiF.Capillary electrophoretic analysis of arsenic species with indirect laser induced fluorescence detection20022531551592-s2.0-003612320610.1002/1615-9314(20020201)25:3<155::AID-JSSC155>3.0.CO;2-KChenZ.Farzana AkterK.Rahman MahmudurM.NaiduR.Speciation of arsenic by ion chromatography inductively coupled plasma mass spectrometry using ammonium eluents20062917267126762-s2.0-3384600516310.1002/jssc.20050030417313108ChoiJ. Y.KhanN.NhoE. Y.ChoiH.ParkK. S.ChoM. J.YounH. J.KimK. S.Speciation of arsenic in rice by high-performance liquid chromatography–inductively coupled plasma mass spectrometry201649121926193710.1080/00032719.2015.11259122-s2.0-84978164188Gómez-ArizaJ. L.Sánchez-RodasD.GiráldezI.MoralesE.A comparison between ICP-MS and AFS detection for arsenic speciation in environmental samples20005122572682-s2.0-003854378510.1016/S0039-9140(99)00257-XMaitaniT.UchiyamaS.SaitoY.Hydride generation-flame atomic-absorption spectrometry as an arsenic detector for high-performance liquid chromatography1987391116116810.1016/S0021-9673(01)94313-42-s2.0-0023666175ChenS.GuoQ.LiuL.Determination of arsenic species in edible mushrooms by high-performance liquid chromatography coupled to inductively coupled plasma mass spectrometry201710374074810.1007/s12161-016-0629-92-s2.0-84982290332ThomasP.FinnieJ. K.WilliamsJ. G.Feasibility of identification and monitoring of arsenic species in soil and sediment samples by coupled high-performance liquid chromatography - inductively coupled plasma mass spectrometry199712121367137210.1039/a704149g2-s2.0-0031337957RattanachongkiatS.MillwardG. E.FoulkesM. E.Determination of arsenic species in fish, crustacean and sediment samples from Thailand using high performance liquid chromatography (HPLC) coupled with inductively coupled plasma mass spectrometry (ICP-MS)20046425426110.1039/b312956j2-s2.0-2042542070Montes-BayónM.DeNicolaK.CarusoJ. A.Liquid chromatography-inductively coupled plasma mass spectrometry200310001-245747610.1016/S0021-9673(03)00527-22-s2.0-0037512359RitsemaR.DukanL.Roig I NavarroT.Van LeeuwenW.OliveiraN.WolfsP.LebretE.Speciation of arsenic compounds in urine by LC-ICP MS1998128-959159910.1002/(SICI)1099-0739(199808/09)12:8/9<591::AID-AOC767>3.0.CO;2-E2-s2.0-0000835382MoldovanM.GómezM. M.PalaciosM. A.CámaraC.Arsenic speciation in water and human urine by HPLC/ICP/MS and HPLC/MO/HG/AAS1998591899910.1006/mchj.1997.15562-s2.0-0002193319AkterK. F.ChenZ.SmithL.DaveyD.NaiduR.Speciation of arsenic in ground water samples: a comparative study of CE-UV, HG-AAS and LC-ICP-MS200568240641510.1016/j.talanta.2005.09.0112-s2.0-27744558938PolyaD. A.LythgoeP. R.Abou-ShakraF.GaultA. G.BrydieJ. R.WebsterJ. G.BrownK. L.NimfopoulosM. K.MichailidisK. M.IC-ICP-MS and IC-ICP-HEX-MS determination of arsenic speciation in surface and groundwaters: preservation and analytical issues200367224726110.1180/00264610367200982-s2.0-0038545669MoreiraC. M.DuarteF. A.LebherzJ.PozebonD.FloresE. M. M.DresslerV. L.Arsenic speciation in white wine by LC-ICP-MS201112631406141110.1016/j.foodchem.2010.11.1202-s2.0-78751579153RaberG.StockN.HanelP.MurkoM.NavratilovaJ.FrancesconiK. A.An improved HPLC–ICPMS method for determining inorganic arsenic in food: application to rice, wheat and tuna fish2012134152453210.1016/j.foodchem.2012.02.1132-s2.0-84860374645MaL.YangZ.TangJ.WangL.Simultaneous separation and determination of six arsenic species in rice by anion-exchange chromatography with inductively coupled plasma mass spectrometry201639112105211310.1002/jssc.2016002162-s2.0-84973923020DengT.-L.ChenY.-W.BelzileN.Antimony speciation at ultra trace levels using hydride generation atomic fluorescence spectrometry and 8-hydroxyquinoline as an efficient masking agent2001432229330210.1016/S0003-2670(00)01387-82-s2.0-0035967253B'HymerC.CarusoJ. A.Evaluation of HPLC systems for the separation and quantification of arsenic compounds from apple extracts20022546396532-s2.0-0036121686