Evaluation of solution-cathode glow discharge atomic emission spectrometry for the analysis of nanoparticle containing solutions☆
Graphical abstract
Introduction
The solution-cathode glow discharge (SCGD) is a very simple, inexpensive alternative to the inductively-coupled plasma (ICP) for atomic emission analysis. In contrast to the ICP, the SCGD uses relatively low operating power (<100 W), does not require radio-frequency electronics, operates in ambient air without supporting gas flow (e.g. Ar), and requires no external nebulization equipment. Through iterative improvement, the SCGD has developed to the point where it holds analytical figures of merit on par with radially viewed ICP-AES [1], and this level of analytical performance paired with its benefits suggests that SCGD-AES is an attractive approach for mobile, field-portable, continuous onsite analysis [2,3].
A variety of applications of the SCGD have also been reported. Environment samples have been analyzed for industrially relevant metals (Te, Rh, In) with detection limits below 1 mg/L [4], and even complex samples like honey have been analyzed for Na, Ca, Cu, Fe, Li, Mg, Mn, Rb and Zn with detection limits almost entirely below 1μg/g [5]. Because the SCGD is based on a flowing liquid stream, it is very well suited for analysis of transients such as those from chromatographic separations. The plasma has been used with flow injection analysis [6], as a chromatographic detector [7], and with a novel type of ion-exchange chromatography [8]. The SCGD has even been employed as an ionization source for molecular mass spectrometry for the fragmentation of peptides [9]. Recently, trace metal contaminants in colloidal samples have been analyzed by SCGD-AES. Wang et al. quantified trace Li, Na, Mg, and K in aqueous dispersions of 22 nm dia. silica nanoparticles using the SCGD, reporting concentrations with relative accuracy within 7%–20% of the true value in slurry samples containing up to 20 mg/mL using external calibration [10]. Previously, a maximum of 10 mg/mL was achieved [11]. This result has generated significant interest, since analysis of refractory material slurries and nanoparticles are notoriously difficult without extensive sample treatment and dissolution steps.
In this paper, we report the first systematic study of the viability of using SCGD-AES for the analysis of NPs. NPs are increasingly seen as a subject of environmental concern as they are increasingly manufactured, used, and released into the environment [[12], [13], [14]]. NPs of various types have been shown to be potentially useful as anti-cancer drugs and sensing techniques [15,16], but also to pose potential biological issues, with some showing toxic effects [[17], [18], [19]], while others appear relatively benign [20]. Thus, accurately determining the identity, presence, size, and concentration of these species is of significant interest [21]. Unfortunately, current analytical strategies are relatively expensive and not amendable to continuous field-based monitoring. For example, hyphenated ICP-based techniques are currently commonly used in the analysis of NP solutions [22]. Here, a chromatographic sizing step such as size-exclusion chromatography [23] or field flow fractionation [24] is followed by quantitation by atomic spectrometry. Alternatively, single-particle ICP-MS is able to directly analyze and size sample solutions without previous separation [25]. However, ICP-MS analysis comes at a higher cost as it requires vacuum conditions, high-power electronics, complicated data processing components, and high gas flow rates of purified Ar.
In this report we examine the efficacy of the SCGD for the analysis of nanoparticle materials likely to be encountered in environmental samples with the eventual goal of using SCGD-AES for on-site, portable, and continuous analysis of effluent streams for potential contaminants. Because elements within the NP are introduced into the plasma as particles, as opposed to solvated ions as in normal conditions, analysis of various metal particulate samples by SCGD-AES also provides insights into the mechanisms by which material is removed from the liquid cathode of the SCGD and vaporization within the plasma. Lastly, analysis of NP atomic emission also provides some insight into the possible factors influencing the synthesize NPs using the SCGD plasma [26].
Section snippets
Experimental
The SCGD used here is shown in Fig. 1 and was similar to that reported previously elsewhere [27]. Briefly, sample solution was introduced into a quartz capillary (0.38 mm I.D. and 1.1 mm O·D.) at a rate of 2.5 mL/min using a peristaltic pump, where it overflowed into a waste reservoir of roughly 175 mL volume. The SCGD discharge was sustained between the sample solution as it exited the capillary (cathode) and a tungsten electrode (anode) by the bias network shown in Fig. 1. A high voltage
Relative sensitivity of nanoparticle solutions
In order to accurately quantitate nanoparticle concentrations as total metal in a sample using a simple external calibration approach, the sensitivity of a measurement for the analyte element should be identical whether the element is present as free-ion in solution or present as nanoparticles, or at worst, these sensitivities should be related by a reproducible factor. To determine if nanoparticles and free-ion standard solutions showed similar atomic emission response in the SCGD, calibration
Conclusions
The analysis of dispersions of NPs by SCGD-AES has been reported for the first time. Direct analysis of NP showed a significant decrease in observed sensitivity as compared to free-ion standard solutions, with NPs showing lower sensitivity from 14.3 ± 7.1% to 93.2 ± 1.5% as compared to the fee-ion solutions depending upon the NP composition, size, and morphology. Conventional acid digestion and matrix matching was shown to permit accurate quantitation of NP solutions, with limits of detection
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This material is based upon work supported by the National Science Foundation under Grant No. (CHE-1622531). The authors also acknowledge support from the State University of New York at Buffalo. The authors gratefully acknowledge the contribution of the UB Department of Chemistry Mass Spectrometry Facility, the UB CAS Machine Shop, and the UB Department of Chemistry Electronic Shop. The authors also thank and acknowledge the ICP Information Newsletter and Professor Ramon Barnes, and Indiana
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This paper is dedicated to Paul Farnsworth, following his retirement, in recognition of his outstanding contributions to the fields of optical emission spectroscopy and mass spectrometry.