Letter pubs.acs.org/ac
Conceptual Demonstration of Ambient Desorption-Optical Emission Spectroscopy Using a Liquid Sampling-Atmospheric Pressure Glow Discharge Microplasma Source R. Kenneth Marcus,* Htoo W. Paing, and Lynn X. Zhang Department of Chemistry, Clemson University, Clemson, South Carolina 29634, United States ABSTRACT: The concept of ambient desorption-optical emission spectroscopy (AD-OES) is demonstrated using a liquid samplingatmospheric pressure glow discharge (LS-APGD) microplasma as the desorption/excitation source. The LS-APGD has previously been employed for elemental analysis of solution samples and particulates introduced via laser ablation in both the optical emission and mass spectrometries (OES, MS) modes. In addition, the device has been shown to be effective for the analysis of elemental and molecular species operating in an ambient desorption/ionization mass spectrometry (ADI-MS) mode. Proof-of-concept is presented here in the use of the LS-APGD to volatilize three very diverse sample forms (metallic thin films, dry solution residues, and bulk materials), with the liberated material excited within the microplasma and detected via OES, i.e., ADOES. While the demonstration is principally qualitative at this point, it is believed that the basic approach may find application across a broad spectrum of analytical challenges requiring elemental analysis, including metals, soils, and volume-limited solutions, analogous to what has been seen in the development of the field of ADI-MS for molecular species determinations. ne of the most active fields within analytical chemistry over the past decade has been ambient desorption/ ionization mass spectrometry (ADI-MS).1−6 The underlying premise for the phenomenal activity lies in the capacity to “analyze things as they really are”.7 A plethora of ADI source designs have been described in the literature, with most seeking to affect the sampling (volatilization) of a surface under ambient conditions (outside of the vacuum in simple terms) with subsequent ionization in the gas phase by the same or some supplemental form of energy. Clearly, the most evolved of these approaches are desorption electrospray ionization (DESI)8 and direct analysis in real time (DART)9 sources. These approaches typically operate with the low temperature (65% HNO3, are typically needed to dissolve bulk Ni. If acid dissolution was the operable mechanism for solid sampling in the LS-APGD, one would expect that Cu would indeed generate a large analytical response and Ni not so much, with the potential for very large matrix effects. In fact, to have both species’ OES transitions yield values of the same order of magnitude seems to suggest that, indeed, Ni is quite well vaporized. Herein, we see that it is perhaps the ability to generate high densities of H3O+ in the vapor phase at elevated temperatures that is the important reactive species and not the identity of the starting acid (or more specifically the conjugate base) that allows vaporization of these diverse materials. Another potential mechanism could be the formation of volatile nickel nitrate salt. Changing the acid identity, or operating the microplasma with a salt electrolyte such as NaCl instead of an acid at all, would provide insights to the processes. Obviously, this is the point that demands a detailed evaluation, but at this stage, it is considered to be a positive attribute that the volatilization of metals is not driven (nor limited) by specific acids.
Letter
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
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REFERENCES
This work was supported by the Defense Threat Reduction Agency, Basic Research Award #HDTRA1-14-1-0010, to Clemson University. Mr. Daniel Willett of the research group of Professor George Chumanov in this department is acknowledged for the preparation of the vapor-deposited metal thin films.
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CONCLUSIONS Described and demonstrated here is ambient desorption-optical emission spectroscopy (AD-OES), affected through the use of a liquid sampling-atmospheric pressure glow discharge (LSAPGD). The AD-OES is consistent in concept with ADI-MS in that samples are analyzed in their native states without any form of sample preparation, one of the principal driving forces for the LIBS technique. While LIBS provides great opportunities for micrometer-scale sampling, AD-OES using a microplasma source would seem to offer simplicity in terms of operational overhead and general utility. Certainly, comparisons are warranted at the appropriate levels of development. Demonstrations were provided for three diverse matrices: vapor-deposited thin metal films, dry solution residues, and bulk metals. While the precise mechanisms of operation are not fully understood at this point, it is clear that the LS-APGD possesses the capabilities to volatilize surface species into the microplasma environment where analyte atoms are excited and emit characteristic radiation. The combination of the general operation space of the LS-APGD (low solution flow rates, low power, and small footprint) and the potential for using portable, array-based optical spectrometers matches well with methods that can be affected in the field. Clearly, much work remains in terms of gaining better fundamental understanding. In terms of quantification, the ability to place an internal standard in the LS-APGD electrolyte feed stream is an exciting option. Ultimately, it is believed that, just as in the case of ADIMS, there are many opportunities for AD-LS-APGD-OES to solve real-world analytical challenges. E
DOI: 10.1021/acs.analchem.6b00751 Anal. Chem. XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.analchem.6b00751 Anal. Chem. XXXX, XXX, XXX−XXX
