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Determination of trace cadmium in geological samples by membrane desolvation inductively coupled plasma mass spectrometry
Microchemical Journal 148 (2019) 561–567
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Determination of trace cadmium in geological samples by membrane desolvation inductively coupled plasma mass spectrometry Dong Hea, Zhenli Zhu Shenghong Hua,d
a,⁎
T
, Xin Miaoa, Hongtao Zhengb, Xiuli Lia, Nicholas S. Belshawc,
a
State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences, Wuhan, China, 430074 Faculty of Material Science and Chemistry, China University of Geosciences, Wuhan 430074, China c Department of Earth Sciences, University of Oxford, OX1 3PR, UK d Faculty of Earth Sciences, China University of Geosciences, Wuhan 430074, China b
ARTICLE INFO
ABSTRACT
Keywords: Cadmium Membrane desolvation ICP-MS
The accurate determination of Cd in geological samples by inductively coupled plasma mass spectrometry (ICPMS) suffers significant interference from polyatomic ions especially the oxides and hydroxides of Mo and Zr. In this study, we coupled membrane desolvation system with ICP-MS to reduce Mo and Zr based interference and accurately measured Cd content in geological samples. With the solvent in samples solution were removed by membrane desolvation system, Zr based interferences were reduced to blank level and Mo based interferences were reduced by about 10 times. In addition, the membrane desolvation sample introduction system also increased the Cd sensitivity 13 times compared to conventional pneumatic nebulizer with spray chamber. Under the optimal conditions, a linear working curve is obtained at least for Cd concentrations of 0.1–10 μg/L, with a correlation coefficient of 0.9999 and detection limit is calculated to be 0.004 μg/L for Cd. The proposed method was demonstrated by analysis of 25 geological standard reference samples and the results showed that membrane desolvation significantly improved analytical accuracy for Cd by ICP-MS, especially for the low Cd samples.
1. Introduction The determination of cadmium (Cd) in environmental and geological samples has attracted significant attention in recent years because of its high toxicity and unique geochemical characteristics. Cadmium is a toxic trace element, which is not only capable of causing damage to a number of organ systems leading to kidney failure, respiratory issues, gastro-intestinal problems, but also increasing the risk of neurological complications [1–4]. Cd is a nutrient-like element in marine systems and it closely follows the distribution of phosphate and zinc. In the past years, the measurement of cadmium and its isotopes have been usefully applied in pollutant and nutrient elements tracing [5,6], marine environment [7–9] and biotic activities etc. [10,11] [12]. Additionally, accurate determination of Cd concentration is important for high precision Cd isotopic analysis as the accurate sample Cd content would help to achieve the right sample-spike proportion, which have been proved with great association to the measured Cd isotope ratio by double spike technique [13]. Therefore, the accurate determination of trace and ultra-trace Cd is important.
⁎
Inductively coupled plasma mass spectrometry (ICP-MS) is the most commonly used technique to determine Cd in a variety of samples because of its high sensitivity, multi-element capability, and wide dynamic range [14–17]. However, isobaric and polyatomic interferences complicate the technique, making it a challenge to precisely measure Cd at trace and ultra-trace levels by ICP-MS. Measured by ICP-MS, all Cd isotopes suffer potential interferences from isobaric and/or polyatomic ions from K, Ca, Pd, Sn, In, Zr, Mo, Ru, Nb and Y [8,18–22]. Among these elements, the oxides and hydroxides of Mo and Zr are regarded as the primary potential interference for Cd determination by ICP-MS. [8,23–27] In typical geological samples, the contents of Mo and Zr are much higher than that of Cd, for example, the abundances of Zr, Mo, and Cd in the earth's crust are 162 μg/g, 1.2 μg/g and 0.16 μg/g, respectively [28].. This could lead to erroneous results in Cd determination by the ICP-MS if interfering species are not carefully controlled. Therefore, it is highly desirable to develop methods to reduce or eliminate Mo and Zr oxide and hydroxide interferences for accurate determination of trace Cd in geological samples by ICP-MS. Several techniques have been developed for accurate trace Cd
Corresponding author at: State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences, Wuhan 430074, China E-mail address: [email protected] (Z. Zhu).
https://doi.org/10.1016/j.microc.2019.05.042 Received 16 April 2019; Received in revised form 16 May 2019; Accepted 16 May 2019 Available online 17 May 2019 0026-265X/ © 2019 Elsevier B.V. All rights reserved.
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determination by ICP-MS. Chemical separation is one of the most effective methods to reduce both isobaric and polyatomic interference caused by matrix elements [21,24,26,29,30]. Several chemical separation strategies, including the use of anion exchange resin [26,31], chelating resins [32] and solid phase extraction have been applied to remove matrix for the measurement of Cd and enrich the trace Cd in samples to an appropriate concentration for analysis by ICP-MS. Though useful these separation methods suffer a number of disadvantages which include being slow, the requirement of a means to compensate for analyte recovery and high blank level [20,24,33–35]. Alternative separation techniques such as inverse aqua regia extraction [23], precipitation [26,36], vapor generation [29,30] and electrothermal volatilization [37,38] etc. have also been developed to separate Cd from matrix for measurement. These methods are unduly complex or employ hazard chemicals. Instrumental approaches involving collision/ reaction cell techniques might offer convenient and practical methods to eliminates several polyatomic interferences in ICP-MS. [16,39,40] This approach would use H2 or He as the cell gas to remove a number of the polyatomic interferences by the collision process between atoms/ molecular and ion, which effectively attenuates the polyatomic ion intensity, but may also reduce the analyte ion signals leading to a relatively higher detection limit and poor precision [41–43]. The technique of membrane desolvation provides a highly effective method for sample introduction which can separate solvent vapours from a sample aerosol using PTFE membrane. With less solvent entering the ICP, signal intensity can be increased about 10–20 times while the MO+/M+ signal ratio is dramatically reduced [44]. Using these advantages, membrane desolvation has been successfully used in a number of trace/ultra-trace element studies [33,45–49]. For example, Duan et al. developed a method for rapid determination of 26 elements in iron meteorites using membrane desolvation ICP-MS. [48] In their work, molecular interferences derived from solutions and gas, as well as from the sample matrix were significantly reduced, and as a result allowing wider range of trace elements determination. Pamela et al. applied membrane desolvation-ICP-MS to investigate the lanthanide distribution in human placental tissue by membrane desolvation. With a membrane desolvation system, low relative detection limits for the lanthanides (0.57–6.1 ng/L) and lower oxide formation rates were obtained when compared with other sample introduction systems [47]. However, few works have utilized membrane desolvation to improve Cd analytical performance by ICP-MS [48,50], especially on geological samples. In the present study, we describe a practical method to accurately determine Cd in geological samples. Coupling a membrane desolvation system with ICP-MS, the Zr based interferences were decreased to blank levels and Mo based interferences were reduced by about 10 times. The experimental parameters which include adding N2, sweep gas flow rate, carrier gas flow rate and ICP power, were optimized by evaluating the Cd signal intensity and production of MoO+ and ZrO+. The performance of this method was evaluated for sensitivity, MO+/M+ signal ratios and compared with a conventional nebulizer plus spray chamber sample introduction system under optimized conditions. Finally, the proposed method was successfully applied to the analysis of 25 standard reference geological materials with Cd content ranging from 0.033 μg/g to 4.0 μg/g. The results demonstrate this method provides an accurate and practical approach for measuring trace levels of Cd in geological samples.
Table 1 Instrumental operating condition. Instrument
Parameter
Value or description
Agilent 7700x
RF power Reflected power Plasma gas Auxiliary gas Carrier gas Sampling depth Dwell time Monitored isotopes Nebulizer type Sample uptake rate Spray chamber temperature Nebulizer type Sample uptake rate Sweep gas argon flow Nitrogen gas flow Membrane temperature Spray chamber temperature
1400 W 3W 15.0 L/min 0.90 L/min 1.05 L/min 7.0 mm 30 ms 111 Cd Glass 0.96 mL/min 5 °C PFA 0.17 mL/min 3.20 L/min 8 mL/min 160 °C 130 °C
Micromist APEX-Ω
Scott-type spray chamber, and dry mode with an Apex-Ω membrane desolvation system (ESI Technologies, USA) incorporating a PFA microconcentric nebulizer. In the wet mode, sample delivery to the glass nebulizer was driven by a peristaltic pump with an uptake rate of 0.96 mL/min. In dry mode the free-flow nebulizer uptake rate was 0.17 mL/min. The Apex-Ω (ESI Technologies, USA) comprises a multistage Peltier cooled desolvation system and a helical PTFE fluoropolymer membrane desolvator, which can provide a maximum ICP-MS signal while simultaneously showing the lowest levels of oxide and polyatomic ion interferences. A mix gas of N2 could be introduced at the end of membrane desolvation and mixed with the carrier gas to improve the analytical performance. Ar is used as the sweep gas and carrier gas whose flow rates have obvious influence on the efficiency of water removal. It is necessary to evaluate and optimize the flow rate of mix gas (N2), sweep gas and carrier gas before sample measurement. The initial instrumental settings were optimized to obtain maximum signal intensities for Li, Y, Ce, Ba and Pb, while keeping the CeO+/Ce+ ratios below 2%. Details of operating conditions and measurement parameters are summarized in Table 1. Ultra-pure water with a resistivity of 18.2 MΩ·cm was obtained from a Milli-Q water purification system (Millipore, Bedford, MA, USA). Commercially available HNO3 (Sinopharm Chemical Reagent Ltd., China) and HF (Sinopharm Chemical Reagent Ltd., China) were further distilled twice in sub-boiling distillation system. And the distilled HNO3 were further diluted to 30% and 2% (m/m) concentration with ultrapure water for the sample digestion. Standard solutions were diluted by 1000 μg/mL and included Ga, Ge, Zr, Mo, Ru, Rh and Cd (National Analysis Centre for Iron and Steel, China). Standard reference materials for stream sediments GBW07303a (GSD-3a), GBW07304 (GSD-4), GBW07305 (GSD-5), GBW07306 (GSD-6), GBW07310 (GSD-10), GBW07311 (GSD-11) and GBW07312 (GSD-12), soils GBW07403 (GSS3), GBW07406 (GSS-6), GBW07423 (GSS-9) and GBW07428 (GSS-14), rock GBW07107 (GSR-5), GBW07108 (GSR-6), GBW07110 (GSR-8), GBW07111 (GSR-9), GBW07112 (GSR-10), GBW07113 (GSR-11), GBW07114 (GSR-12), GBW07122 (GSR-15), GBW07725 (GSR-16), GBW07726 (GSR-17) and GBW07727 (GSR-18) were purchased from the Institute of Geophysical Geochemical Exploration (Langfang, China), and 3 international rock standards materials (AGV-2, BCR-2 and BHVO-2) from United States Geological Survey were also used to assess the performance of the proposed method.
2. Experimental details 2.1. Instrumentation, regents and standards
2.2. Sample preparation and analytical procedure
Analyses was performed using an ICP-MS Agilent 7700× ICP-MS instrument (Agilent Technologies, USA). Two types of sample introduction systems were employed in this work, conventional wet mode with a MicroMist glass nebulizer (GE, Australia) coupling to a vertical
The procedures used for sample digestion in our laboratory were as follows: about 0.1 g homogenized sample powder was weighed and placed in an in-house PTFE-lined stainless-steel bomb (consisting of a 562
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10 mL PTFE inner vessel with lid, which fitted tightly into an outer stainless-steel pressure jacket). After wetting with a few drops of ultrapure water, 1 mL concentrated HNO3 (after twice distilled) and 1 mL HF (30%, m/m) were slowly added. The sealed bomb was heated to 190 °C in an electric air oven (Jinghong DHG-9030A, Shanghai) for 48 h. After cooling, the bomb was opened and placed on a hotplate at 115 °C and evaporated to incipient dryness (but not baked). This was followed by adding 1 mL concentrated HNO3 and evaporating to dryness. Then 3 g weighted 30% HNO3 was added to each sample and the bombs were resealed again then placed in the oven for another 12 h heating at 190 °C to dissolve the residue completely. By this process, > 60 samples can be digested at a time. After cooling, samples were transferred to polyethylene bottles and 0.3 g solution were weighed to polyethylene bottles, with addition of 0.1 g of 1.0 μg/mL Rh internal standard solution and diluted to 4 g with of 2% HNO3. Calibration solutions (0.1, 1 and 10 μg/L) were prepared by gravimetric serial dilution from 1 mg/L Cd stock standard solutions. The internal standard concentration of Rh was constant at 25 μg/L in all the final sample solutions and calibration solutions. The samples were measured both in dry mode and wet mode. Inevitably, dry mode suffers from higher risk of memory effects than wet mode as the samples may adhere in the membrane and be difficult to clean thoroughly. To minimize memory interferences, a wash process (at least 3 min) was applied after each sample measurement by using 2% HNO3 until the signal decreased to the blank level. Also, the time resolved data (using TRA mode) was used to record all signals continuously with 0.026 s integral time thus allowing offline data processing by excel to reject obvious outliers and correct the isobaric interferences.
Table 3 The signal intensity of Cd, Mo, Zr, Ga and Pd in the m/z of 111. Element
Concentration (μg/L)
Wet mode (counts)
Dry mode (counts)
Blank Cd Mo Zr Ga Pd
0 1 1000 1000 1000 1000
21 5669 615 952 17 10
22 11,904 105 18 24 27
Note: A peristaltic pump was used for sample introduction in wet mode and the nebulizer uptake rate was 0.96 mL/min, while in dry mode sample was introduced by the free flow-mode nebulizer with the 0.17 mL/min flow rate.
shown in Table 2, the spectral interferences on 111Cd mainly come from polyatomic interferences originating from the plasma gas and matrix components in the sample. Although the spectral interferences on 111Cd isotope can be predicted theoretically, contributing elements in the interfering signal need to be identified. The main interferences on 111Cd come from polyatomic ions of Pd, Mo, Ga and Zr. To identify interference contributions from these elements on 111Cd, signals at m/z = 111 where measured while introducing 1000 μg/L Pd, Mo, Ga, Zr as single elements solution respectively and the results were compared with 1 μg/L Cd on the ICP-MS instrument in both wet mode and dry mode. As shown in Table 3, in wet mode the 1 μg/L Cd and 1000 μg/L interference ion solutions produced Cd, Mo and Zr signals of 5669, 615, and 952 counts at m/z = 111 while Ga and Pd produced only 17 and 10 counts. The 1000 μg/L Pd and Ga produced similar level signals as blank solutions which suggests the production of 110Pd1H+ and 71Ga40Ar+ were extremely low and their interference on 111Cd could be ignored. However, the Mo based interferences (including MoO+, MoN+ and MoOH+) and ZrOH+ causing 615 and 952 counts signals at m/z = 111, correspond to about 0.11 and 0.17 μg/L bias in Cd concentration. In the case of dry mode, the signal from 1 μg/L Cd solution was 11,904 counts while 1000 μg/L solutions Zr, Pd and Ga produced blank level signals at m/z = 111. The 1000 μg/ L Mo solution contributed about 105 counts at m/z = 111 which equals approximately 0.01 μg/L apparent Cd signal. From these results, it is apparent that the presence of Pd and Ga in solution contributes little interference to 111Cd and can be ignored in both wet and dry mode. Mo and Zr based polyatomic ions are the main interferences to 111Cd in wet mode. The membrane desolvation system effectively reduces the Zr based interference to blank level, but cannot eliminate the Mo based interferences (97Mo14N+, 95Mo16O+ and 94 Mo16O1H+) completely.
3. Results and discussion 3.1. The evaluation of potential interference Cd has 8 stable isotopes, but each potentially suffers from multiple spectral interferences during mass spectrometric analysis. Polyatomic ion interference with > 3 atoms in plasma is very complex and has too many possible combinations. Fortunately, it is very unlikely to produce the observable polyatomic ions which consisted by > 3 atoms. Therefore, in all potential interference, isobaric, diatomic and triatomic ion are most common and should be considered as primary interference. Generally, eH, eN, eO and eAr were the most present ligand for polyatomic ions in ICP source, and 1H, 14N, 16O and 40Ar comprise > 99% of all isotopes, thus interferences beyond triatomic ions or containing the low abundance isotopes of H, N, O and Ar can be ignored. The abundances and likely potential interferences on Cd isotopes are summarized in Table 2. Routinely, 111Cd and 114Cd were the most widely used isotopes in ICP-MS for Cd concentration determinations [1]. Although 114Cd (28.73%) is the most abundant isotope of Cd, it suffers from an isobaric interference with 114Sn, which makes it not the best choice for Cd determination. While 111Cd with a lower abundance (12.80%), it is the only isotope of Cd free from isobaric interference and therefore it is most suitable for Cd quantification by ICP-MS. [23] As
3.2. Optimization of membrane desolvation operating conditions for ICPMS Previous studies have shown that instrumental conditions such as the addition of N2, sweep gas flow rate, carrier gas flow rate and ICP power etc. may have a significant effect on the sensitivity and oxide/ hydroxide ion yields [44,47,48,50,51]. In order to obtain the best analytical performance, 100 μg/L Cd standard solution and MoeZr
Table 2 Abundance and typical interference of Cd isotopes. Cd isotope
Abundance
Interference
106
1.25% 0.89% 12.49% 12.8% 24.13% 12.22% 28.73% 7.49%
106
Cd Cd 110 Cd 111 Cd 112 Cd 113 Cd 114 Cd 116 Cd 108
Pd, 105Pd1H, 92Zr14N, 92Mo14N, 90Zr16O, 66Zn40Ar, 89Y16O1H Pd, 107Ag1H, 92Zr14N, 92Mo14N, 92Zr16O, 92Mo16O, 68Zn40Ar, 1Zr16O1H 110 Pd, 109Ag1H, 96Zr14N, 96Mo14N, 96Ru14N, 94Zr16O, 4Mo16O, 70Ge40Ar, 93Nb16O1H 110 Pd1H, 97Mo14N, 95Mo16O, 71Ga40Ar, 94Zr16O1H, 94Mo16O1H 112 Sn, 98Mo14N, 98Ru14N, 96Mo16O, 96Ru16O, 96Zr16O, 72Ge40Ar, 95Mo16O1H 113 In, 112Sn1H, 99Ru14N, 97Mo16O, 73Ge40Ar, 96Mo16O1H, 96Zr16O1H, 96Ru16O1H 114 Sn, 113In1H, 100Mo14N, 100Ru14N, 98Mo16O, 98Ru16O, 74Ge40Ar, 97Mo16O1H 116 Sn,115Sn1H, 115In1H, 102Ru14N, 102Pd14N, 100Mo16O,100Ru16O, 76Ge40Ar, 99Ru16O1H 108
563
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proportion of the sweep gas may cross the membrane inside the fluoropolymer tube and act to increase the carrier gas [52]. As a result, the excess carrier gas affects the sampled depth of plasma and results in a decreased signal intensity and higher MO+/M+ signal ratio. Similar behaviour may also occur when the carrier gas flow rate is in the range 0.90–1.15 L/min (Fig. 2b). As a compromise for optimum Cd intensity and MO+/M+ ratio a sweep gas flow rate of 3.20 L/min and carrier gas flow rate of 1.05 L/min were used for the remaining experiments. 3.2.3. The effect of ICP RF power ICP RF power has been shown to have a significant effect on both signal intensity and MO+/M+ signal ratio [52,53]. It is generally observed that higher RF power conditions result in lower levels of refractory oxides. As Fig. 3, the effect of changing ICP RF power in the range from 1250 to 1550 W caused Cd signal intensity to increase by > 20%, from ~30,000 to ~37,000 counts (with integral time of 0.026 s) though remaining stable when the ICP RF power was further increased to the upper limit of 1500 W. At the same time the ZrO+/Zr+ ratio remained relatively stable, in the range of 0.010–0.016% while the ZrO+/Zr+ ratio showed a sharp decreasing when the ICP RF power increased from 1450 W to 1500 W. Although 1500 W ICP power achieved a combination of best sensitivity and MO+/M+ signal ratio, to avoid the RF generator working at its upper power limit for extended periods, 1400 W was chosen for the subsequent experiments.
Fig. 1. The effect of N2 flow rate on the Cd intensity, ZrO+/Zr+ and MoO+/ Mo+.
mixed standard solution were used to assess Cd sensitivity and oxide formation. The mass to charge ratios of 106/90 and 114/98 were used to track the ratios of ZrO+/Zr+ and MoO+/Mo+, respectively. The yields of MoOH+ and ZrOH+ were not evaluated as their signals are much lower than the signal of oxide ion.
3.3. Analytical performance and measurement of standards materials
3.2.1. The effect of N2 flow rate It has been reported that the N2eAr mixed gas plasma can significantly affect the sensitivity and MO+/M+ signal ratio in the plasma as the addition of N2 can increase plasma temperature and ion density [46,50,52]. According to reported works, with different elements the minimum values of oxides most present with the N2 flow rate at about 10 mL/min while the maximum signal intensity were occurred with N2 flow rate range at 8–20 mL/min, depends on different instrument setting [46,50]. In this work, N2 (> 99.999%) was introduced though the APEX-Ω and mixed with the carrier gas with the N2 flow rate ranging from 0 to 12 mL/min. Fig. 1 presents the observed behaviour of the 111 Cd signal, MoO+/Mo+ and ZrO+/Zr+ ratios with increasing N2 flow rate. The intensity of the Cd signal increased 2.8 times with N2 flow rate increasing from 0 to 8.0 mL/min. With further increase in the N2 flow rate, the intensity of Cd then decreased. The addition of N2 did not affect the MO+/M+ signal ratio significantly in this study. In the investigated N2 flow range, the ZrO+/Zr+ and MoO+/Mo+ varied in the range 0.011–0.013% and 0.053–0.081% respectively with the MoO+/ Mo+ ratio showing an increase with increasing N2 flow rate. Though the addition of N2 does not reduce the interferences from Mo and Zr, the 2.8 times Cd signal enhancement improves the overall analytical performance. Based upon these observations a N2 flow rate of 8 mL/min was chosen for the following experiments.
The comparison between wet mode and dry mode proven the membrane desolvation could significantly improve the analytical performance of Cd. But in practical, the memory effect cannot be ignored as the large surface area of membrane in dry mode. Especially when measuring samples whose Cd content varied by several orders, the memory effect would lead to large errors for the samples with trace Cd. So that the memory effect should be taken into consideration seriously. According to our observation, in dry mode the memory signal of Cd decreased rapidly after finished sample introduction and reached the blank level (30–50 cps) in about 30 s. But different with wet mode, some strong transient signals (several thousand cps) would appear randomly (about 3–5 spike signals per minute and their appearing frequency decreased slowly with the wash time) in the next minutes. This phenomena could be caused by the residual samples which attached on the surface of membrane and blow off brokenly by the carrier gas. As the transient signal caused by residual samples cannot be eliminated in a short time, the measured results will deviate from their true values and lead to poor repeatability and accuracy, especially for the trace Cd samples. To reduce the memory effect caused by membrane desolvation, the TRA mode was applied to collect data. By this way the strong transient signal could be recognized and picked out by offline data processing in Excel. Generally, the spike value often serval times higher than the mean value. And in practice, the spike signal two times higher than the average value were regarded caused by the memory effect and should be rejected. Operating under optimized conditions, analytical performance in both wet and dry mode were evaluated. The calibration curves obtained using 0, 0.1, 1 and 10 μg/mL Cd standard stock solution gave coefficients (R2) of regression of 0.9997 (wet) and 0.9999 (dry) respectively. The limits of detection (LOD) defined as the ratio of three times the standard deviation of the blank determination to the slope of analytical curve, for wet and dry modes were 0.017 μg/L and 0.004 μg/L, respectively. In wet mode, sample was driven into the nebulizer by a peristaltic pump with an uptake rate of 0.96 mL/min, while in dry mode the sample flow rate was only 0.17 mL/min. Under optimized conditions, the signal intensity in dry mode was about 2.4 times higher than wet mode. On considering the different uptake rates, dry mode produced a signal enhancement factor of 13 over wet mode. Also, the ratio of ZrO+/Zr+ and MoO+/Mo+ decreased from 1.2% and 0.77% (wet
3.2.2. The effect of sweep gas flow rate and carrier gas flow rate The water in a sample aerosol is removed by the Ar sweep gas in a membrane desolvator inside a heated fluoropolymer tube. The sweep gas flow rate is likely to directly affect measured analyte signal intensity and MO+/M+ signal ratio. The flow rate of the sweep gas in the experimental arrangement described was investigated over the range 1.70–3.70 L/min. As can be seen in Fig. 2a, Cd signal intensity improves rapidly when the sweep gas increases from 1.50 to 2.45 L/min, then less rapidly in the range of 2.45–3.20 L/min. When the sweep gas flow is > 3.20 L/min, Cd signal intensity starts to decrease. In comparison the ZrO+/Zr+ and MoO+/Mo+ ratio remained stable when sweep gas was in the range 1.50–3.20 L/min but increased significantly above 3.20 L/min. The result suggests that increasing the sweep gas flow rate can remove more water vapor and so increase the excitation efficiency of the material being analysed observed as an increased signal intensity. However, when the flow rate exceeds a certain value (3.20 L/min), a 564
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Fig. 2. The effect of sweep gas flow rate (a) and carrier gas (b) on Cd intensity, ZrO+/Zr+ and MoO+/Mo+.
error. The bias of results might come from the memory effect of APEX-Ω and analysis error, as the measured Cd concentration of these samples solutions were only sub-ppb level after about 300 folds diluted. In general, membrane desolvation ICP-MS has been proven an effective method to eliminate the Mo and Zr based interference and improve the analytical performance for trace Cd determination in geological samples. From the comparison of wet and dry plasma mode, we can conclude that conventional pneumatic nebulizer with spray chamber ICP-MS suffers significant interference from Mo and Zr which cannot be ignored when used for trace Cd determination in geological samples. Accurate results can be obtained using membrane desolvation ICP-MS due to the elimination or reduction of the Zr and Mo based interference, making this method especially suitable for trace Cd determination in geological samples. Fig. 3. The effect of ICP power on Cd intensity, ZrO+/Zr+ and MoO+/Mo+.
4. Conclusion In this study, a method for the accurate determination of trace-level Cd in geological samples was developed, based on membrane desolvation sample introduction coupled to ICP-MS. Compared with standard pneumatic nebulizer with spray chamber operation, Cd sensitivities using membrane desolvation were enhanced 13 times, which is very helpful for determination of low abundance Cd. The LOD was 0.004 μg/L for solutions, which is equal to about 4 ng/g in solid samples when considering the 1000-fold dilution factor. In addition, with membrane desolvation, Zr based interferences were reduced to a blank level and Mo based interferences were reduced about 10 times. The proposed method was validated by the analysis of a variety of standard materials including 4 soils, 7 sediments and 14 rocks. The measured results accorded well with their reference values. All these experimental results demonstrate this method has great potential for the accurate determination of trace levels Cd in geological samples. Supplementary data to this article can be found online at https:// doi.org/10.1016/j.microc.2019.05.042.
mode) to 0.014% and 0.068% (dry mode) respectively. These results demonstrate that the membrane desolvation system provides superior performance for trace-level Cd determination of solid geological samples). The application and accuracy of the present method was demonstrated by the determination of geological standard reference materials including 4 soils (GSS-3, GSS-9, GSS-6, and GSS-14), 7 stream sediments (GSD-3a, GSD-4, GSD-5, GSD-6, GSD-10, GSD-11 and GSD-12) and 14 different kind of rocks (GSR-5, GSR-6, GSR-8, GSR-9, GSR-10, GSR-12, GSR-11, GSR-15, GSR-16, GSR-17, GSR-18, AGV-2, BHVO-2 and BCR2). The sample details and analytical data are summarized in Table S1. The reference Cd contents of these standards range from 0.033 μg/g to 4.0 μg/g while the Mo/Cd and Zr/Cd ranges 1–1689 and 43–4100, respectively. Fig. 4 present the reference values and measured values of these standard reference samples by both wet mode (a) and dry mode (b). In wet mode, the measured values were mostly above the regression line of X = Y (dotted line), which results indicated the polyatomic ions obviously interfere the Cd signal in ICP-MS. For some low Cd samples, the results are even several times beyond their reference values. When it comes to dry mode, the measured values were much approach to their reference values. These results proved that the membrane desolvation could effectively minimize the potential polyatomic ions interference to Cd determination. However, it should be noted that for some samples with Cd content lower than 0.1 μg/g, the measured results can not completely accord with their reference values within the margin of
Acknowledgement Financial support from the National Natural Science Foundation of China (Nos. 21822405, 41673014, and 41521001), National Key Research and Development Program (2017YFD0801202), and Natural Science Foundation of Hubei Province (2016CFA038), are gratefully acknowledged. 565
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Fig. 4. The reference values vs measured values in wet plasma mode(a) and dry plasma mode (b).
References
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Determination of trace cadmium in geological samples by membrane desolvation inductively coupled plasma mass spectrometry Dong Hea, Zhenli Zhu Shenghong Hua,d
a,⁎
T
, Xin Miaoa, Hongtao Zhengb, Xiuli Lia, Nicholas S. Belshawc,
a
State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences, Wuhan, China, 430074 Faculty of Material Science and Chemistry, China University of Geosciences, Wuhan 430074, China c Department of Earth Sciences, University of Oxford, OX1 3PR, UK d Faculty of Earth Sciences, China University of Geosciences, Wuhan 430074, China b
ARTICLE INFO
ABSTRACT
Keywords: Cadmium Membrane desolvation ICP-MS
The accurate determination of Cd in geological samples by inductively coupled plasma mass spectrometry (ICPMS) suffers significant interference from polyatomic ions especially the oxides and hydroxides of Mo and Zr. In this study, we coupled membrane desolvation system with ICP-MS to reduce Mo and Zr based interference and accurately measured Cd content in geological samples. With the solvent in samples solution were removed by membrane desolvation system, Zr based interferences were reduced to blank level and Mo based interferences were reduced by about 10 times. In addition, the membrane desolvation sample introduction system also increased the Cd sensitivity 13 times compared to conventional pneumatic nebulizer with spray chamber. Under the optimal conditions, a linear working curve is obtained at least for Cd concentrations of 0.1–10 μg/L, with a correlation coefficient of 0.9999 and detection limit is calculated to be 0.004 μg/L for Cd. The proposed method was demonstrated by analysis of 25 geological standard reference samples and the results showed that membrane desolvation significantly improved analytical accuracy for Cd by ICP-MS, especially for the low Cd samples.
1. Introduction The determination of cadmium (Cd) in environmental and geological samples has attracted significant attention in recent years because of its high toxicity and unique geochemical characteristics. Cadmium is a toxic trace element, which is not only capable of causing damage to a number of organ systems leading to kidney failure, respiratory issues, gastro-intestinal problems, but also increasing the risk of neurological complications [1–4]. Cd is a nutrient-like element in marine systems and it closely follows the distribution of phosphate and zinc. In the past years, the measurement of cadmium and its isotopes have been usefully applied in pollutant and nutrient elements tracing [5,6], marine environment [7–9] and biotic activities etc. [10,11] [12]. Additionally, accurate determination of Cd concentration is important for high precision Cd isotopic analysis as the accurate sample Cd content would help to achieve the right sample-spike proportion, which have been proved with great association to the measured Cd isotope ratio by double spike technique [13]. Therefore, the accurate determination of trace and ultra-trace Cd is important.
⁎
Inductively coupled plasma mass spectrometry (ICP-MS) is the most commonly used technique to determine Cd in a variety of samples because of its high sensitivity, multi-element capability, and wide dynamic range [14–17]. However, isobaric and polyatomic interferences complicate the technique, making it a challenge to precisely measure Cd at trace and ultra-trace levels by ICP-MS. Measured by ICP-MS, all Cd isotopes suffer potential interferences from isobaric and/or polyatomic ions from K, Ca, Pd, Sn, In, Zr, Mo, Ru, Nb and Y [8,18–22]. Among these elements, the oxides and hydroxides of Mo and Zr are regarded as the primary potential interference for Cd determination by ICP-MS. [8,23–27] In typical geological samples, the contents of Mo and Zr are much higher than that of Cd, for example, the abundances of Zr, Mo, and Cd in the earth's crust are 162 μg/g, 1.2 μg/g and 0.16 μg/g, respectively [28].. This could lead to erroneous results in Cd determination by the ICP-MS if interfering species are not carefully controlled. Therefore, it is highly desirable to develop methods to reduce or eliminate Mo and Zr oxide and hydroxide interferences for accurate determination of trace Cd in geological samples by ICP-MS. Several techniques have been developed for accurate trace Cd
Corresponding author at: State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences, Wuhan 430074, China E-mail address: [email protected] (Z. Zhu).
https://doi.org/10.1016/j.microc.2019.05.042 Received 16 April 2019; Received in revised form 16 May 2019; Accepted 16 May 2019 Available online 17 May 2019 0026-265X/ © 2019 Elsevier B.V. All rights reserved.
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determination by ICP-MS. Chemical separation is one of the most effective methods to reduce both isobaric and polyatomic interference caused by matrix elements [21,24,26,29,30]. Several chemical separation strategies, including the use of anion exchange resin [26,31], chelating resins [32] and solid phase extraction have been applied to remove matrix for the measurement of Cd and enrich the trace Cd in samples to an appropriate concentration for analysis by ICP-MS. Though useful these separation methods suffer a number of disadvantages which include being slow, the requirement of a means to compensate for analyte recovery and high blank level [20,24,33–35]. Alternative separation techniques such as inverse aqua regia extraction [23], precipitation [26,36], vapor generation [29,30] and electrothermal volatilization [37,38] etc. have also been developed to separate Cd from matrix for measurement. These methods are unduly complex or employ hazard chemicals. Instrumental approaches involving collision/ reaction cell techniques might offer convenient and practical methods to eliminates several polyatomic interferences in ICP-MS. [16,39,40] This approach would use H2 or He as the cell gas to remove a number of the polyatomic interferences by the collision process between atoms/ molecular and ion, which effectively attenuates the polyatomic ion intensity, but may also reduce the analyte ion signals leading to a relatively higher detection limit and poor precision [41–43]. The technique of membrane desolvation provides a highly effective method for sample introduction which can separate solvent vapours from a sample aerosol using PTFE membrane. With less solvent entering the ICP, signal intensity can be increased about 10–20 times while the MO+/M+ signal ratio is dramatically reduced [44]. Using these advantages, membrane desolvation has been successfully used in a number of trace/ultra-trace element studies [33,45–49]. For example, Duan et al. developed a method for rapid determination of 26 elements in iron meteorites using membrane desolvation ICP-MS. [48] In their work, molecular interferences derived from solutions and gas, as well as from the sample matrix were significantly reduced, and as a result allowing wider range of trace elements determination. Pamela et al. applied membrane desolvation-ICP-MS to investigate the lanthanide distribution in human placental tissue by membrane desolvation. With a membrane desolvation system, low relative detection limits for the lanthanides (0.57–6.1 ng/L) and lower oxide formation rates were obtained when compared with other sample introduction systems [47]. However, few works have utilized membrane desolvation to improve Cd analytical performance by ICP-MS [48,50], especially on geological samples. In the present study, we describe a practical method to accurately determine Cd in geological samples. Coupling a membrane desolvation system with ICP-MS, the Zr based interferences were decreased to blank levels and Mo based interferences were reduced by about 10 times. The experimental parameters which include adding N2, sweep gas flow rate, carrier gas flow rate and ICP power, were optimized by evaluating the Cd signal intensity and production of MoO+ and ZrO+. The performance of this method was evaluated for sensitivity, MO+/M+ signal ratios and compared with a conventional nebulizer plus spray chamber sample introduction system under optimized conditions. Finally, the proposed method was successfully applied to the analysis of 25 standard reference geological materials with Cd content ranging from 0.033 μg/g to 4.0 μg/g. The results demonstrate this method provides an accurate and practical approach for measuring trace levels of Cd in geological samples.
Table 1 Instrumental operating condition. Instrument
Parameter
Value or description
Agilent 7700x
RF power Reflected power Plasma gas Auxiliary gas Carrier gas Sampling depth Dwell time Monitored isotopes Nebulizer type Sample uptake rate Spray chamber temperature Nebulizer type Sample uptake rate Sweep gas argon flow Nitrogen gas flow Membrane temperature Spray chamber temperature
1400 W 3W 15.0 L/min 0.90 L/min 1.05 L/min 7.0 mm 30 ms 111 Cd Glass 0.96 mL/min 5 °C PFA 0.17 mL/min 3.20 L/min 8 mL/min 160 °C 130 °C
Micromist APEX-Ω
Scott-type spray chamber, and dry mode with an Apex-Ω membrane desolvation system (ESI Technologies, USA) incorporating a PFA microconcentric nebulizer. In the wet mode, sample delivery to the glass nebulizer was driven by a peristaltic pump with an uptake rate of 0.96 mL/min. In dry mode the free-flow nebulizer uptake rate was 0.17 mL/min. The Apex-Ω (ESI Technologies, USA) comprises a multistage Peltier cooled desolvation system and a helical PTFE fluoropolymer membrane desolvator, which can provide a maximum ICP-MS signal while simultaneously showing the lowest levels of oxide and polyatomic ion interferences. A mix gas of N2 could be introduced at the end of membrane desolvation and mixed with the carrier gas to improve the analytical performance. Ar is used as the sweep gas and carrier gas whose flow rates have obvious influence on the efficiency of water removal. It is necessary to evaluate and optimize the flow rate of mix gas (N2), sweep gas and carrier gas before sample measurement. The initial instrumental settings were optimized to obtain maximum signal intensities for Li, Y, Ce, Ba and Pb, while keeping the CeO+/Ce+ ratios below 2%. Details of operating conditions and measurement parameters are summarized in Table 1. Ultra-pure water with a resistivity of 18.2 MΩ·cm was obtained from a Milli-Q water purification system (Millipore, Bedford, MA, USA). Commercially available HNO3 (Sinopharm Chemical Reagent Ltd., China) and HF (Sinopharm Chemical Reagent Ltd., China) were further distilled twice in sub-boiling distillation system. And the distilled HNO3 were further diluted to 30% and 2% (m/m) concentration with ultrapure water for the sample digestion. Standard solutions were diluted by 1000 μg/mL and included Ga, Ge, Zr, Mo, Ru, Rh and Cd (National Analysis Centre for Iron and Steel, China). Standard reference materials for stream sediments GBW07303a (GSD-3a), GBW07304 (GSD-4), GBW07305 (GSD-5), GBW07306 (GSD-6), GBW07310 (GSD-10), GBW07311 (GSD-11) and GBW07312 (GSD-12), soils GBW07403 (GSS3), GBW07406 (GSS-6), GBW07423 (GSS-9) and GBW07428 (GSS-14), rock GBW07107 (GSR-5), GBW07108 (GSR-6), GBW07110 (GSR-8), GBW07111 (GSR-9), GBW07112 (GSR-10), GBW07113 (GSR-11), GBW07114 (GSR-12), GBW07122 (GSR-15), GBW07725 (GSR-16), GBW07726 (GSR-17) and GBW07727 (GSR-18) were purchased from the Institute of Geophysical Geochemical Exploration (Langfang, China), and 3 international rock standards materials (AGV-2, BCR-2 and BHVO-2) from United States Geological Survey were also used to assess the performance of the proposed method.
2. Experimental details 2.1. Instrumentation, regents and standards
2.2. Sample preparation and analytical procedure
Analyses was performed using an ICP-MS Agilent 7700× ICP-MS instrument (Agilent Technologies, USA). Two types of sample introduction systems were employed in this work, conventional wet mode with a MicroMist glass nebulizer (GE, Australia) coupling to a vertical
The procedures used for sample digestion in our laboratory were as follows: about 0.1 g homogenized sample powder was weighed and placed in an in-house PTFE-lined stainless-steel bomb (consisting of a 562
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10 mL PTFE inner vessel with lid, which fitted tightly into an outer stainless-steel pressure jacket). After wetting with a few drops of ultrapure water, 1 mL concentrated HNO3 (after twice distilled) and 1 mL HF (30%, m/m) were slowly added. The sealed bomb was heated to 190 °C in an electric air oven (Jinghong DHG-9030A, Shanghai) for 48 h. After cooling, the bomb was opened and placed on a hotplate at 115 °C and evaporated to incipient dryness (but not baked). This was followed by adding 1 mL concentrated HNO3 and evaporating to dryness. Then 3 g weighted 30% HNO3 was added to each sample and the bombs were resealed again then placed in the oven for another 12 h heating at 190 °C to dissolve the residue completely. By this process, > 60 samples can be digested at a time. After cooling, samples were transferred to polyethylene bottles and 0.3 g solution were weighed to polyethylene bottles, with addition of 0.1 g of 1.0 μg/mL Rh internal standard solution and diluted to 4 g with of 2% HNO3. Calibration solutions (0.1, 1 and 10 μg/L) were prepared by gravimetric serial dilution from 1 mg/L Cd stock standard solutions. The internal standard concentration of Rh was constant at 25 μg/L in all the final sample solutions and calibration solutions. The samples were measured both in dry mode and wet mode. Inevitably, dry mode suffers from higher risk of memory effects than wet mode as the samples may adhere in the membrane and be difficult to clean thoroughly. To minimize memory interferences, a wash process (at least 3 min) was applied after each sample measurement by using 2% HNO3 until the signal decreased to the blank level. Also, the time resolved data (using TRA mode) was used to record all signals continuously with 0.026 s integral time thus allowing offline data processing by excel to reject obvious outliers and correct the isobaric interferences.
Table 3 The signal intensity of Cd, Mo, Zr, Ga and Pd in the m/z of 111. Element
Concentration (μg/L)
Wet mode (counts)
Dry mode (counts)
Blank Cd Mo Zr Ga Pd
0 1 1000 1000 1000 1000
21 5669 615 952 17 10
22 11,904 105 18 24 27
Note: A peristaltic pump was used for sample introduction in wet mode and the nebulizer uptake rate was 0.96 mL/min, while in dry mode sample was introduced by the free flow-mode nebulizer with the 0.17 mL/min flow rate.
shown in Table 2, the spectral interferences on 111Cd mainly come from polyatomic interferences originating from the plasma gas and matrix components in the sample. Although the spectral interferences on 111Cd isotope can be predicted theoretically, contributing elements in the interfering signal need to be identified. The main interferences on 111Cd come from polyatomic ions of Pd, Mo, Ga and Zr. To identify interference contributions from these elements on 111Cd, signals at m/z = 111 where measured while introducing 1000 μg/L Pd, Mo, Ga, Zr as single elements solution respectively and the results were compared with 1 μg/L Cd on the ICP-MS instrument in both wet mode and dry mode. As shown in Table 3, in wet mode the 1 μg/L Cd and 1000 μg/L interference ion solutions produced Cd, Mo and Zr signals of 5669, 615, and 952 counts at m/z = 111 while Ga and Pd produced only 17 and 10 counts. The 1000 μg/L Pd and Ga produced similar level signals as blank solutions which suggests the production of 110Pd1H+ and 71Ga40Ar+ were extremely low and their interference on 111Cd could be ignored. However, the Mo based interferences (including MoO+, MoN+ and MoOH+) and ZrOH+ causing 615 and 952 counts signals at m/z = 111, correspond to about 0.11 and 0.17 μg/L bias in Cd concentration. In the case of dry mode, the signal from 1 μg/L Cd solution was 11,904 counts while 1000 μg/L solutions Zr, Pd and Ga produced blank level signals at m/z = 111. The 1000 μg/ L Mo solution contributed about 105 counts at m/z = 111 which equals approximately 0.01 μg/L apparent Cd signal. From these results, it is apparent that the presence of Pd and Ga in solution contributes little interference to 111Cd and can be ignored in both wet and dry mode. Mo and Zr based polyatomic ions are the main interferences to 111Cd in wet mode. The membrane desolvation system effectively reduces the Zr based interference to blank level, but cannot eliminate the Mo based interferences (97Mo14N+, 95Mo16O+ and 94 Mo16O1H+) completely.
3. Results and discussion 3.1. The evaluation of potential interference Cd has 8 stable isotopes, but each potentially suffers from multiple spectral interferences during mass spectrometric analysis. Polyatomic ion interference with > 3 atoms in plasma is very complex and has too many possible combinations. Fortunately, it is very unlikely to produce the observable polyatomic ions which consisted by > 3 atoms. Therefore, in all potential interference, isobaric, diatomic and triatomic ion are most common and should be considered as primary interference. Generally, eH, eN, eO and eAr were the most present ligand for polyatomic ions in ICP source, and 1H, 14N, 16O and 40Ar comprise > 99% of all isotopes, thus interferences beyond triatomic ions or containing the low abundance isotopes of H, N, O and Ar can be ignored. The abundances and likely potential interferences on Cd isotopes are summarized in Table 2. Routinely, 111Cd and 114Cd were the most widely used isotopes in ICP-MS for Cd concentration determinations [1]. Although 114Cd (28.73%) is the most abundant isotope of Cd, it suffers from an isobaric interference with 114Sn, which makes it not the best choice for Cd determination. While 111Cd with a lower abundance (12.80%), it is the only isotope of Cd free from isobaric interference and therefore it is most suitable for Cd quantification by ICP-MS. [23] As
3.2. Optimization of membrane desolvation operating conditions for ICPMS Previous studies have shown that instrumental conditions such as the addition of N2, sweep gas flow rate, carrier gas flow rate and ICP power etc. may have a significant effect on the sensitivity and oxide/ hydroxide ion yields [44,47,48,50,51]. In order to obtain the best analytical performance, 100 μg/L Cd standard solution and MoeZr
Table 2 Abundance and typical interference of Cd isotopes. Cd isotope
Abundance
Interference
106
1.25% 0.89% 12.49% 12.8% 24.13% 12.22% 28.73% 7.49%
106
Cd Cd 110 Cd 111 Cd 112 Cd 113 Cd 114 Cd 116 Cd 108
Pd, 105Pd1H, 92Zr14N, 92Mo14N, 90Zr16O, 66Zn40Ar, 89Y16O1H Pd, 107Ag1H, 92Zr14N, 92Mo14N, 92Zr16O, 92Mo16O, 68Zn40Ar, 1Zr16O1H 110 Pd, 109Ag1H, 96Zr14N, 96Mo14N, 96Ru14N, 94Zr16O, 4Mo16O, 70Ge40Ar, 93Nb16O1H 110 Pd1H, 97Mo14N, 95Mo16O, 71Ga40Ar, 94Zr16O1H, 94Mo16O1H 112 Sn, 98Mo14N, 98Ru14N, 96Mo16O, 96Ru16O, 96Zr16O, 72Ge40Ar, 95Mo16O1H 113 In, 112Sn1H, 99Ru14N, 97Mo16O, 73Ge40Ar, 96Mo16O1H, 96Zr16O1H, 96Ru16O1H 114 Sn, 113In1H, 100Mo14N, 100Ru14N, 98Mo16O, 98Ru16O, 74Ge40Ar, 97Mo16O1H 116 Sn,115Sn1H, 115In1H, 102Ru14N, 102Pd14N, 100Mo16O,100Ru16O, 76Ge40Ar, 99Ru16O1H 108
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proportion of the sweep gas may cross the membrane inside the fluoropolymer tube and act to increase the carrier gas [52]. As a result, the excess carrier gas affects the sampled depth of plasma and results in a decreased signal intensity and higher MO+/M+ signal ratio. Similar behaviour may also occur when the carrier gas flow rate is in the range 0.90–1.15 L/min (Fig. 2b). As a compromise for optimum Cd intensity and MO+/M+ ratio a sweep gas flow rate of 3.20 L/min and carrier gas flow rate of 1.05 L/min were used for the remaining experiments. 3.2.3. The effect of ICP RF power ICP RF power has been shown to have a significant effect on both signal intensity and MO+/M+ signal ratio [52,53]. It is generally observed that higher RF power conditions result in lower levels of refractory oxides. As Fig. 3, the effect of changing ICP RF power in the range from 1250 to 1550 W caused Cd signal intensity to increase by > 20%, from ~30,000 to ~37,000 counts (with integral time of 0.026 s) though remaining stable when the ICP RF power was further increased to the upper limit of 1500 W. At the same time the ZrO+/Zr+ ratio remained relatively stable, in the range of 0.010–0.016% while the ZrO+/Zr+ ratio showed a sharp decreasing when the ICP RF power increased from 1450 W to 1500 W. Although 1500 W ICP power achieved a combination of best sensitivity and MO+/M+ signal ratio, to avoid the RF generator working at its upper power limit for extended periods, 1400 W was chosen for the subsequent experiments.
Fig. 1. The effect of N2 flow rate on the Cd intensity, ZrO+/Zr+ and MoO+/ Mo+.
mixed standard solution were used to assess Cd sensitivity and oxide formation. The mass to charge ratios of 106/90 and 114/98 were used to track the ratios of ZrO+/Zr+ and MoO+/Mo+, respectively. The yields of MoOH+ and ZrOH+ were not evaluated as their signals are much lower than the signal of oxide ion.
3.3. Analytical performance and measurement of standards materials
3.2.1. The effect of N2 flow rate It has been reported that the N2eAr mixed gas plasma can significantly affect the sensitivity and MO+/M+ signal ratio in the plasma as the addition of N2 can increase plasma temperature and ion density [46,50,52]. According to reported works, with different elements the minimum values of oxides most present with the N2 flow rate at about 10 mL/min while the maximum signal intensity were occurred with N2 flow rate range at 8–20 mL/min, depends on different instrument setting [46,50]. In this work, N2 (> 99.999%) was introduced though the APEX-Ω and mixed with the carrier gas with the N2 flow rate ranging from 0 to 12 mL/min. Fig. 1 presents the observed behaviour of the 111 Cd signal, MoO+/Mo+ and ZrO+/Zr+ ratios with increasing N2 flow rate. The intensity of the Cd signal increased 2.8 times with N2 flow rate increasing from 0 to 8.0 mL/min. With further increase in the N2 flow rate, the intensity of Cd then decreased. The addition of N2 did not affect the MO+/M+ signal ratio significantly in this study. In the investigated N2 flow range, the ZrO+/Zr+ and MoO+/Mo+ varied in the range 0.011–0.013% and 0.053–0.081% respectively with the MoO+/ Mo+ ratio showing an increase with increasing N2 flow rate. Though the addition of N2 does not reduce the interferences from Mo and Zr, the 2.8 times Cd signal enhancement improves the overall analytical performance. Based upon these observations a N2 flow rate of 8 mL/min was chosen for the following experiments.
The comparison between wet mode and dry mode proven the membrane desolvation could significantly improve the analytical performance of Cd. But in practical, the memory effect cannot be ignored as the large surface area of membrane in dry mode. Especially when measuring samples whose Cd content varied by several orders, the memory effect would lead to large errors for the samples with trace Cd. So that the memory effect should be taken into consideration seriously. According to our observation, in dry mode the memory signal of Cd decreased rapidly after finished sample introduction and reached the blank level (30–50 cps) in about 30 s. But different with wet mode, some strong transient signals (several thousand cps) would appear randomly (about 3–5 spike signals per minute and their appearing frequency decreased slowly with the wash time) in the next minutes. This phenomena could be caused by the residual samples which attached on the surface of membrane and blow off brokenly by the carrier gas. As the transient signal caused by residual samples cannot be eliminated in a short time, the measured results will deviate from their true values and lead to poor repeatability and accuracy, especially for the trace Cd samples. To reduce the memory effect caused by membrane desolvation, the TRA mode was applied to collect data. By this way the strong transient signal could be recognized and picked out by offline data processing in Excel. Generally, the spike value often serval times higher than the mean value. And in practice, the spike signal two times higher than the average value were regarded caused by the memory effect and should be rejected. Operating under optimized conditions, analytical performance in both wet and dry mode were evaluated. The calibration curves obtained using 0, 0.1, 1 and 10 μg/mL Cd standard stock solution gave coefficients (R2) of regression of 0.9997 (wet) and 0.9999 (dry) respectively. The limits of detection (LOD) defined as the ratio of three times the standard deviation of the blank determination to the slope of analytical curve, for wet and dry modes were 0.017 μg/L and 0.004 μg/L, respectively. In wet mode, sample was driven into the nebulizer by a peristaltic pump with an uptake rate of 0.96 mL/min, while in dry mode the sample flow rate was only 0.17 mL/min. Under optimized conditions, the signal intensity in dry mode was about 2.4 times higher than wet mode. On considering the different uptake rates, dry mode produced a signal enhancement factor of 13 over wet mode. Also, the ratio of ZrO+/Zr+ and MoO+/Mo+ decreased from 1.2% and 0.77% (wet
3.2.2. The effect of sweep gas flow rate and carrier gas flow rate The water in a sample aerosol is removed by the Ar sweep gas in a membrane desolvator inside a heated fluoropolymer tube. The sweep gas flow rate is likely to directly affect measured analyte signal intensity and MO+/M+ signal ratio. The flow rate of the sweep gas in the experimental arrangement described was investigated over the range 1.70–3.70 L/min. As can be seen in Fig. 2a, Cd signal intensity improves rapidly when the sweep gas increases from 1.50 to 2.45 L/min, then less rapidly in the range of 2.45–3.20 L/min. When the sweep gas flow is > 3.20 L/min, Cd signal intensity starts to decrease. In comparison the ZrO+/Zr+ and MoO+/Mo+ ratio remained stable when sweep gas was in the range 1.50–3.20 L/min but increased significantly above 3.20 L/min. The result suggests that increasing the sweep gas flow rate can remove more water vapor and so increase the excitation efficiency of the material being analysed observed as an increased signal intensity. However, when the flow rate exceeds a certain value (3.20 L/min), a 564
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Fig. 2. The effect of sweep gas flow rate (a) and carrier gas (b) on Cd intensity, ZrO+/Zr+ and MoO+/Mo+.
error. The bias of results might come from the memory effect of APEX-Ω and analysis error, as the measured Cd concentration of these samples solutions were only sub-ppb level after about 300 folds diluted. In general, membrane desolvation ICP-MS has been proven an effective method to eliminate the Mo and Zr based interference and improve the analytical performance for trace Cd determination in geological samples. From the comparison of wet and dry plasma mode, we can conclude that conventional pneumatic nebulizer with spray chamber ICP-MS suffers significant interference from Mo and Zr which cannot be ignored when used for trace Cd determination in geological samples. Accurate results can be obtained using membrane desolvation ICP-MS due to the elimination or reduction of the Zr and Mo based interference, making this method especially suitable for trace Cd determination in geological samples. Fig. 3. The effect of ICP power on Cd intensity, ZrO+/Zr+ and MoO+/Mo+.
4. Conclusion In this study, a method for the accurate determination of trace-level Cd in geological samples was developed, based on membrane desolvation sample introduction coupled to ICP-MS. Compared with standard pneumatic nebulizer with spray chamber operation, Cd sensitivities using membrane desolvation were enhanced 13 times, which is very helpful for determination of low abundance Cd. The LOD was 0.004 μg/L for solutions, which is equal to about 4 ng/g in solid samples when considering the 1000-fold dilution factor. In addition, with membrane desolvation, Zr based interferences were reduced to a blank level and Mo based interferences were reduced about 10 times. The proposed method was validated by the analysis of a variety of standard materials including 4 soils, 7 sediments and 14 rocks. The measured results accorded well with their reference values. All these experimental results demonstrate this method has great potential for the accurate determination of trace levels Cd in geological samples. Supplementary data to this article can be found online at https:// doi.org/10.1016/j.microc.2019.05.042.
mode) to 0.014% and 0.068% (dry mode) respectively. These results demonstrate that the membrane desolvation system provides superior performance for trace-level Cd determination of solid geological samples). The application and accuracy of the present method was demonstrated by the determination of geological standard reference materials including 4 soils (GSS-3, GSS-9, GSS-6, and GSS-14), 7 stream sediments (GSD-3a, GSD-4, GSD-5, GSD-6, GSD-10, GSD-11 and GSD-12) and 14 different kind of rocks (GSR-5, GSR-6, GSR-8, GSR-9, GSR-10, GSR-12, GSR-11, GSR-15, GSR-16, GSR-17, GSR-18, AGV-2, BHVO-2 and BCR2). The sample details and analytical data are summarized in Table S1. The reference Cd contents of these standards range from 0.033 μg/g to 4.0 μg/g while the Mo/Cd and Zr/Cd ranges 1–1689 and 43–4100, respectively. Fig. 4 present the reference values and measured values of these standard reference samples by both wet mode (a) and dry mode (b). In wet mode, the measured values were mostly above the regression line of X = Y (dotted line), which results indicated the polyatomic ions obviously interfere the Cd signal in ICP-MS. For some low Cd samples, the results are even several times beyond their reference values. When it comes to dry mode, the measured values were much approach to their reference values. These results proved that the membrane desolvation could effectively minimize the potential polyatomic ions interference to Cd determination. However, it should be noted that for some samples with Cd content lower than 0.1 μg/g, the measured results can not completely accord with their reference values within the margin of
Acknowledgement Financial support from the National Natural Science Foundation of China (Nos. 21822405, 41673014, and 41521001), National Key Research and Development Program (2017YFD0801202), and Natural Science Foundation of Hubei Province (2016CFA038), are gratefully acknowledged. 565
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Fig. 4. The reference values vs measured values in wet plasma mode(a) and dry plasma mode (b).
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