Enhancement of the Capabilities of Inductively Coupled Plasma Mass

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Enhancement of the Capabilities of Inductively Coupled Plasma Mass Spectrometry using Mono-Segmented Flow Analysis Ram P Lamsal, Gregory Jerkiewicz, and Diane Beauchemin Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.8b03488 • Publication Date (Web): 01 Nov 2018 Downloaded from http://pubs.acs.org on November 2, 2018

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Analytical Chemistry

Enhancement of the Capabilities of Inductively Coupled Plasma Mass Spectrometry using Mono-Segmented Flow Analysis Ram P. Lamsal, Gregory Jerkiewicz and Diane Beauchemin* Queen’s University, Department of Chemistry, 90 Bader Lane, Kingston, ON K7L 3N6, Canada ABSTRACT: Flow injection (FI) in combination with inductively coupled plasma mass spectrometry (ICPMS) is advantageous for the analysis of volume-limited samples and is invaluable for the analysis of corrosive samples that would prematurely degrade ICPMS components. However, the dispersion process with 50-µL injections in FI degrades ICPMS sensitivity. Mono-segmented flow analysis (MSFA), where the sample plug is in the middle of 1 mL of air, eliminates dispersion while preserving the rinsing effect of the carrier. More reproducible as well as sharper, narrower and more symmetrical peaks result with MSFA than FI, leading to a 2fold improvement in detection limit and a 5-fold increase in sample throughput versus FI. Furthermore, by facilitating the formation of small droplets during nebulization, the air surrounding the sample even enhances sensitivity by 20-40%, depending on the element, compared to that obtained with direct sample aspiration. Coupling MSFA to ICPMS, which does not degrade analytical performance, is advantageous for the determination of Pt in 0.5 M H2SO4 electrolyte from a simulated fuel cell. It also enables the multi-element analysis of a 150-µL buffer sample containing as little as 60 µg of plant protein, thus further extending the range of applications of ICPMS.

Inductively coupled plasma (ICP) mass spectrometry (MS) is known for its multi-elemental detection at ultra-trace levels1 and its isotope ratio measurement capabilities.2 The high sensitivity of ICPMS is despite 95% of the sample typically being wasted through the spray chamber, which is necessary to remove large droplets that might overload the plasma.3 However, the physical sampling of ions through a differentially pumped interface is not a passive process like the measurement of emitted light in ICP optical emission spectrometry (OES). As a result, ICPMS cannot handle as complex and concentrated matrices as ICPOES, typically leading to greater sample dilution prior to ICPMS than ICPOES analysis. However, dilution can only be afforded if analyte concentration is not near the quantification limit. For example, the ultra-trace analysis of electrolyte for electro-dissolution of Pt catalyst used in polymer electrolyte fuel cells is required to improve them, as debate is ongoing on whether Pt dissolves anodically in competition to oxide formation, cathodically during oxide reduction, or by chemical dissolution of the formed oxide.4 The working conditions of electro-catalysts in such fuel cells can be simulated using cyclic voltammetry (CV) to gain a basic understanding of the kinetics of degradation mechanism(s). However, because CV lacks the sensitivity to accurately measure dissolved Pt after only a few CV cycles, 1-mL aliquots of 0.5 M H2SO4 CV electrolyte were removed from the CV cell and analyzed by ICPMS, after 5-fold dilution.5 Despite this dilution, direct aspiration of such solutions led to extensive corrosion of the cones and the load coil, which all had to be replaced. To minimize such rapid degradation of ICPMS components, subsequent analyses of 5-fold diluted CV electrolyte were carried out with flow injection (FI) of 100-µL aliquots.6–8 Discrete injection of sample into a continuous flow of carrier enabled the latter to wash out the sample introduction system between injections, thereby reducing exposure to corrosive

electrolyte. Another benefit of FI is that, because the sample is injected in an inert closed system, contamination is minimized.9 However, dispersion of the sample plug into the carrier stream as it is transported to the detector may lower sensitivity and degrade quantification limits. The extent of dispersion depends on the injection volume as well as the length and size of the tubing connecting the injection port to the nebulizer.10–13 As the edges of the injected sample plug are in direct contact with the carrier, dispersion at its edges is greater than in its center, and the analyte concentration gradient is maximum at the center of the sample plug.13 A number of strategies, for instance, the use of coiled, packed reactors,14 capillary tubing,15 supercritical carrier streams 16 and the stopped-flow technique 17 have been employed to minimize dispersion in FI systems. A gaseous carrier can also be used to significantly reduce dispersion of the sample plug, resulting in narrower and sharper peaks; however, it leads to significant memory effect as a carrier solution is no longer used to wash out the system.18,19 To get the best of both worlds, FI into air bubbles of an airsegmented water carrier,10–13,19 by switching between air and water carriers,18 or by injecting water and sample plugs into an air carrier,20 was performed to reduce dispersion while preserving the washing effect of water. The air surrounding the sample plug essentially eliminated the longitudinal dispersion of the sample along the flow path.21 The same approach has been used for decades in air-segmented flow analysis.22 Airsegmented FI was used to overcome the element-specific timedependent dispersion that is observed with conventional FI, which in turn enabled the detection of mass-dependent matrix effects in ICPMS and their correction.23 It was also used to study the role of the spray chamber in matrix effects in ICPOES.24 Several studies used air-segmented FI in combination with a heated torch integrated sample introduction system to mitigate non-spectroscopic interferences caused by inorganic concomitant components and to reduce plasma loading in ICPOES during the determination of metals in fuels

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and biofuels.25–28 However, reproducibly injecting into moving air bubbles can be challenging,10–13,19 and the flow rate of an air carrier delivered by a peristaltic pump can be subject to significant variations.18 Pecked sampling,14,29 where each sample slug results from dipping the sample probe into the sample solution for a given length of time (and is preceded by four air segments through repetitive insertion and withdrawal of the sample probe) makes the segmented flow system operationally cumbersome. The injected sample volume is then dependent on any fluctuation in peristaltic pump rate in contrast to FI with a sample injection valve. One way to reproducibly surround the sample slug with air was proposed by Pasquini and Oliveira,30 where the sample slug from the injection loop is simply sandwiched by two air segments of fixed volume provided by another injection loop. The Internal Union of Pure and Applied Chemistry later named this approach mono-segmented flow analysis (MSFA).31 It was further expanded by Brito and Raimundo Jr, 32 to allow multiple injections, enabling sample and reagents or diluent to be simultaneously injected. However, to the best knowledge of the authors, MSFA has yet to be coupled to ICPOES or ICPMS.

In the present work, for the first time, MSFA is used in combination with ICPMS to reproducibly surround each sample aliquot with air plugs of equal volumes, and only introduce air when a sample is injected into an aqueous carrier to maximize washout time. The performance of MSFA is compared to that of regular FI and of direct aspiration to demonstrate that significant improvements in ICPMS sensitivity, detection limit (DL), precision and sample throughput result from a simple modification of a FI manifold. The approach is then used to measure the dissolved fraction of Pt electro-catalyst in CV electrolyte without diluting it 5 fold. It is also applied to the determination of metals in minute amounts of plant proteins to demonstrate that it is applicable to any type of analysis. EXPERIMENTAL SECTION Chemicals. All chemicals used in this work were of high purity. The 0.10 M or 0.5 M H2SO4 electrolyte solution was prepared from dilution of high-purity H2SO4 (≥96%, Suprapur®, Millipore Sigma, Oakville, ON, Canada)) with doubly deionized water (DDW) having a resistivity ≥ 18.2 MΩ cm−1). A 2% HNO3 carrier solution was prepared by diluting sub-boiled HNO3 (ACS grade; Fisher Scientific, Ottawa, ON, Canada). All DDW was purified using an Arium Pro UV|DI water purification system (Sartorius Table 1. ICPMS operating conditions Parameter Ar plasma gas flow rate (L min-1) Ar auxiliary gas flow rate (L min-1) Ar sheath gas flow rate (L min-1) Nebulizer gas flow rate (mL min-1) Sample uptake rate (mL min-1) Sampling position (mm) RF power (kW) Dwell time (ms) Monitored signals

Setting 18 1.80 0.05-0.07 0.97-0.99 0.80-0.90 6.0-6.5 1.40 100 24Mg+, 52,53Cr+, 75As+, 105,106Pd+, 112,114Cd+, 115In+, 192,194-196,198Pt+, 206-208Pb+

Stedim Biotech, Göttingen, Germany). HNO3 was purified with a DST-1000 sub-boiling distillation system (Savillex, Minnetonka, MN, USA). Standard solutions ranging from 1 to 100 μg L-1 of Mg, Cr, As, Pd, Cd, In, Pt and Pb were prepared in 2% v/v HNO3 from 1000 mg L-1 mono-element standard solutions (SCP Science, Baie d’Urfé, QC, Canada). Furthermore, working Pt standard solutions in the 1−75 µg L−1 range and a blank solution were prepared in each of 0.1 M and 0.5 M H2SO4. (All acid sampling was done in a fume hood while wearing safety goggles, lab coat and appropriate gloves.) Instrumentation. Table 1 summarizes the operating conditions used throughout this study. The research was conducted on a Varian 820MS (Varian Inc., Melbourne, Australia) quadrupole-based ICPMS instrument equipped with Ni sampler and skimmer cones having 0.9 and 0.4 mm diameter orifices, respectively. The sample introduction system consisted of a MicroMist concentric nebulizer (Glass Expansion, Victoria, Australia) fitted into a Peltier-cooled Scott double-pass spray chamber (SCP Science) maintained at 0 °C to reduce solvent loading and thereby minimize corrosion of the interlaced load coils and cones. Data acquisition was carried out in time-resolved analysis mode using the instrument’s ICPMS expert software with three points per peak (0.025 amu spacing), and one scan per replicate. The instrument was tuned daily (including torch alignment) using a solution containing 5 μg L−1 of Be, Mg, Co, In, Ce, Pb, and Ba in 2% (v/v) HNO3 (prepared by dilution of a 10 mg L-1 Varian tuning solution) for a compromise between sensitivity, oxide and double-charged ion levels. CV Sample Collection. Approximately 1 mL of 0.1 M H2SO4 or 0.5 M H2SO4 electrolyte was withdrawn for analysis using a long, flexible tubing attached to a syringe and lowered into the working cell’s electrolyte without interrupting CV measurements. The same amount of blank electrolyte was added to the electrochemical cell to keep the electrolyte volume constant. Isolation of Differentially Glycosylated Purple Acid Phosphatase (PAP) glycoforms. A pair of differentially glycosylated purple acid phosphatase (PAP) glycoforms (AtPAP26-CW1 and AtPAP26-CW2) was fully purified from cell cultures of the model plant Arabidopsis thaliana.33 Each sample was desalted in 50 mM CH3CO2Na buffer (pH 5.6). For the analysis, the 150 μL of sample containing 60 μg of protein provided was diluted to 0.5 mL with 50 mM CH3CO2Na buffer prior to analysis by MSFA-ICPMS. Standard solutions prepared in the same buffer were used for external calibration. Flow Injection set-up. A sample injection valve (Rheodyne, Inc., Cotati, CA) was inserted downstream of the peristaltic pump and as close as possible to the nebulizer to maximize sensitivity and minimize washout time. Replicate 100-μL injections of 0.1 M H2SO4 electrolyte were made into 2% HNO3 carrier whereas 50-μL injections of 0.5 M H2SO4 electrolyte were made into DDW to minimize corrosion of the cones. Sample was manually sucked through the loop into a 5-mL plastic syringe to avoid contamination by its plunger. A Minipuls 3 peristaltic pump


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Analytical Chemistry the equation of the line of best fit, the slope of which corresponded to sensitivity. The DL was calculated as three times the standard deviation of the blank count rate (3σ, n = 9) divided by the corresponding sensitivity, i.e., the slope of the calibration curve established with standard solutions of 1–75 µg L-1.

RESULTS AND DISCUSSION

Figure 1. Schematic representation of the MSFA manifold, where carrier, sample solution and air segments are blue, orange and green respectively.

(Gilson Medical Electronics, Middleton, WI, USA) controlled the carrier flow rate. Mono-Segmented Flow Analysis set-up. Figure 1 illustrates the MSFA manifold. Two 500-μL loops simply connect the FI valve to a second low-pressure six-port sample injection valve (Rheodyne Inc.). They were manually filled with air using a 50-mL disposable plastic syringe while the two valves were in loading position, and the 50- or 100-μL sample loop was manually filled as described in the FI set-up section. Meanwhile, 2% HNO3 or DDW carrier was continuously pumped to the nebulizer. When both valves were switched to the injection position (starting with the sample valve and then the air-sandwich valve), the carrier pushed the air-sample-air sandwich towards the nebulizer.13 Washing with 2% HNO3 pumped through the whole sample path between injections prevented carry-over. The 500-μL slugs of air immediately preceding and succeeding the sample slug did not dry the spray chamber (with an internal volume of around 250 mL) between injections. Data Processing. The raw data points were exported to Microsoft Office Excel 2013 for data processing. Either peak height or peak area was used to construct calibration curves after blank subtraction. Linear regression analysis using the data analysis function in Microsoft Office Excel 2013 provided

Comparison of Direct Aspiration, FI and MSFA. Three figures of merit (sensitivity, DL, and precision expressed as percent relative standard deviation (%RSD)) obtained using peak height by direct aspiration, FI and MSFA (using 50 μL injections into DDW carrier with FI and MSFA) of multielemental standard solution prepared in 2% HNO3 are compared in Table 2. As expected, FI degraded sensitivity and DL compared to direct aspiration because of the dispersion that is inherent to the FI process, which inevitably dilutes the sample plug, thereby reducing peak height. Precision also deteriorated in FI mode. In contrast, the ratio columns for sensitivity and DL (which are arranged so that a ratio above 1 indicates an improvement with MSFA) show that sensitivity is systematically enhanced with MSFA compared to that observed with continuous aspiration by 20-40%, depending on the element. The average sensitivity improvement for the 8 elements is 1.3 ± 0.1 fold, which is very similar to that in DL of 1.3 ± 0.3, although the improvement in DL does not match that in sensitivity for all elements. In any case, precision, as assessed using a 100 μg L1 standard solution, is similar with MSFA as with direct aspiration. Hence, MSFA provides similar or improved performance as with direct aspiration. Furthermore, MSFA provides on average a 2.2 ± 0.3 fold improvement in sensitivity and a 1.9 ± 0.6 one in DL compared to FI while preserving all the benefits of FI. The systematic increase in sensitivity with MSFA versus direct aspiration is in contrast to previous studies with air segmented FI,10,11 where a mass-related trend was observed, only the sensitivity of light analytes being enhanced while that of heavier elements was degraded compared to those obtained by direct aspiration. The systematic improvement with MSFA in this work suggests that the air sandwich, where each 50- or 100-μL sample plug is in the middle of 1 mL of air, increases sample transport efficiency in addition to eliminating dispersion. The introduction of air in the carrier stream indeed increases liquid-gas interactions, which may facilitate the

Table 2. Sensitivity (counts s-1 ng-1 L ± standard deviation), detection limits (ng L-1) and precision (n=3-4) obtained using peak height by direct aspiration (normal) of standard solutions in 2% v/v HNO3 or their 50 µL injection into DDW carrier by FI and MSFA. Analyte Sensitivity Detection limit %RSD Normal FI MSFA MSFA MSFA_ Normal FI MSFA FI_ Normal Normal FI MSFA FI Normal MSFA MSFA 24Mg 17.0 ± 0.6 12.0 ± 0.5 23.0 ± 0.3 1.9 1.4 90 100 70 1.4 1.3 4.5 8.6 6.4 52Cr 50.0 ± 0.5 30 ± 1 70 ± 1 2.3 1.4 50 90 40 2.3 1.3 9.7 8.6 3.9 75As 9±1 5.3 ± 0.2 11 ± 1 2.1 1.2 9 10 7 1.4 1.3 3.4 5.2 3.9 106Pd 23.0 ± 0.6 15.0 ± 0.3 28.0 ± 0.1 1.9 1.2 3 6 3 2 1 5.4 6.2 4.0 114Cd 16.0 ± 0.4 11.0 ± 0.3 23.0 ± 0.1 2.1 1.4 2 3 1 3 2 1.0 6.7 1.5 115In 70.0 ± 0.7 36.0 ± 0.7 100 ± 2 2.8 1.4 8 10 7 1.4 1.1 3.2 4.4 3.1 195Pt 8.0 ± 0.1 4.50 ± 0.01 10.00 ± 0.02 2.2 1.3 2 3 2 1.5 1 3.7 4.2 3.4 208Pb 12.0 ± 0.3 7.0 ± 0.1 14.0 ± 0.2 2.0 1.2 9 20 8 2.5 1.1 2.2 3.6 2.5



Table 4. Sensitivity (in counts ng-1 L (area) or counts s-1 ng-1 L (height)) and DL (ng L-1) for Pt determination using FI and MSFA in 0.1 M (v/v) H2SO4 (with 100-µL injections) or 0.5 M (v/v) H2SO4 (with 50-µL injections). Figure of Injection FI MSFA FI_ merit volume ASFI Peak area 50 µL 125.0 ± 0.7 220.0 ± 0.5 0.57 sensitivity 100 µL 230 ± 2 600 ± 2 0.38 Peak height 50 µL 6.5 ± 0.1 11.00 ± 0.05 0.59 sensitivity 100 µL 10.0 ± 0.2 25.0 ± 0.2 0.4 Peak area 50 µL 30 10 3 DL 100 µL 20 6 3 Peak height 50 µL 50 20 2.5 DL 100 µL 40 20 2

Figure 2. Time-resolved profiles of 195Pt+ for 50-μL injections into DDW of 0.5 M H2SO4 electrolyte sample collected from a CV cell after 500 cycles and for 100-μL injections into 2% HNO3 carrier of 0.1 M H2SO4 electrolyte sample from a CV cell, with FI and MSFA.

formation of small droplets that pass through the spray chamber. Comparison of FI and MSFA for CV Electrolyte Analysis. Figure 2 shows typical transient peaks obtained by FI and MSFA upon the injection of 100 µL of 0.1 M H2SO4 electrolyte into 2% HNO3 carrier and of 50 µL of 0.5 M H2SO4 electrolyte into DDW carrier. The FI peaks are asymmetrical and broader than the MSFA peaks, which are symmetrical and sharper, confirming the elimination of dispersion. Table 3 shows that the peak width at 10% peak height and corresponding washout time (i.e. time taken by the signal to return to baseline from the maximum) with FI are systematically larger than those with MSFA, especially with 50-µL injections, as is also evident in Figure 2. In FI, the larger washout time for 50-µL than 100-µL injections stems from the inverse relationship between dispersion and injected sample volume.18,21,34 Hence, with the 50-μL loop, nearly 5 injections can be made with MSFA during a single injection with FI. This thus translates into a 5-fold increase in sample throughput. Switching from FI to MSFA also systematically improved the precision of replicate injections whether peak height or area is used (Table 3). This indicates that there is no problem of shearing of the air segments on the way to the nebulizer; Table 3. Peak width (s) at 10% peak height, washout time (s) and precision (% RSD) for 4-5 injections of 0.1 M or 0.5 M (v/v) H2SO4 sample collected after 500 CV cycles for Pt determination using FI and MSFA. Figure of Injection FI MSFA FI/MSFA merit volume (µL) Peak width 50 17.90 ± 0.90 5.30 ± 0.80 3.3 100 12.70 ± 0.80 8.30 ± 0.60 1.5 Washout 50 28.50 ± 0.90 5.90 ± 0.90 4.8 100 18.20 ± 0.80 12.60 ± 0.50 1.4 time Peak area 50 3.5 1.5 2.3 %RSD 100 3.1 1.1 2.8 Peak height 50 5.0 1.9 2.6 %RSD 100 3.9 1.5 2.6

otherwise, precision would be degraded with MSFA compared to FI. Sensitivity and DL by FI and MSFA based on peak area or height for Pt in 0.1 M or 0.5 M H2SO4 with 100-μL and 50-μL injections respectively are summarized in Table 4. Sensitivity with MSFA was 1.7-2.6 times that with FI, with a concurrent improvement in DL. Using the sum of all five Pt isotopes or the sum of the three major Pt isotopes instead of solely 195Pt+ did not offer any advantage. The results obtained for two different CV experiments, using 0.1 and 0.5 M H2SO4 electrolytes, are summarized in Table 5. In both cases, the Pt concentrations measured by MSFA and FI are not significantly different according to a Student’s t test at the 95% confidence level, which demonstrates that accuracy is not jeopardized by using MSFA instead of FI.

Determination of Metal Content in Differentially Glycosylated Purple Acid Phosphatase (PAP) glycoforms. All the analytical service laboratories contacted for the multi-elemental analysis of 60 μg of protein in 150 μL of solution declined, saying that there was an insufficient amount of sample. With MSFA-ICPMS, after diluting the sample to 500 μL to enable replicate 50-μL injections with enough extra sample to rinse the injection loop prior to loading, the results in Table 6 were obtained for three replicate injections of each sample. Evidently, the small amount of protein and the dilution did not prevent detection of each metal. Plant PAP glycoforms typically contain an Fe(III)-X(II) active center, where X is either Zn or Mn. The results confirmed that both Mn and especially Zn coordinated with their bimetallic active center. For each Table 5. Pt concentrations in μg L-1 (± standard deviation, n=3) found in two different CV electrolytes using FI and MSFA Injection Electrolyte FI MSFA volume (µL) 50 0.5 M H2SO4 3.32 ± 0.03 3.41 ± 0.01 100 0.1 M H2SO4 1.38 ± 0.03 1.42 ± 0.01

Table 6.

Metals concentrations in μg L-1 (± standard deviation, n=3) found in two differentially glycosylated purple acid phosphatase (PAP) glycoforms by MSFA-ICPMS using 50-μL injections Metal Monitored AtPAP26 isotope CW1 CW2 55Mn Mn 46 ± 9 45 ± 6 57Fe Fe 266 ± 33 225 ± 26 66Zn Zn 200 ± 20 207 ± 23


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Analytical Chemistry metal, there is no difference in concentration between the two samples according to a Student’s t test at the 95% confidence level. However, the precision of 10-20% RSD is degraded compared to that obtained with standard solutions (1.5-6.4% RSD in Table 2). This is likely due to a suppressive matrix effect from the 0.05 M Na present in the Na acetate buffer solution. In any case, this application demonstrates that MSFA can further extend the capabilities of ICPMS towards even tiny samples.

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CONCLUSIONS For the first time, this work demonstrated that MSFA facilitates the ICPMS analysis of sample with corrosive matrix, exemplified by the direct analysis of 0.5 M H2SO4 CV electrolyte. Compared to FI, sensitivity and DL improved by 2-3 fold with MSFA. In addition, the narrower peak and shorter washout time allow a significant increase in sample throughput or sampling rate, by up to 5 fold with 50-μL injections, as a result of a huge reduction of analyte dispersion into the carrier. The reproducibility of injections with MSFA is also better by 2-3 fold than that observed with FI. Moreover, the approach was invaluable for the multielemental analysis of minute samples. It will systematically be used for the analysis of fuel cell and CV electrolytes. Future work will check if MSFA has a similar beneficial effect on single particle ICPMS, where any improvement in DL is desired, as it may translate into a decrease in the size of the smallest nanoparticles that can be measured.

AUTHOR INFORMATION

(10) (11) (12) (13) (14) (15) (16) (17) (18) (19) (20) (21)

Corresponding Author *Phone: +1 613 533 2619. Fax: +1 613 533 6669. E-mail: [email protected].

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Conflict of Interest Disclosure

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The authors declare no competing financial interest.

Funding Sources This research was conducted as part of the Engineered Nickel Catalysts for Electrochemical Clean Energy project administered from Queen’s University and supported by Grant No. RGPNM 477963-2015 under the Natural Sciences and Engineering Research Council of Canada (NSERC) Discovery Frontiers Program.

The authors are grateful to NSERC for research funding. The authors also thank William Plaxton and Mina Ghahremani from the Department of Biology at Queen’s University for the challenge to determine metals in such tiny samples. RPL thanks the School of Graduate Studies of Queen’s University for a graduate award.

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