The
spatial distributions of elemental and molecular species are
vital pieces of information for a broad number of applications such
as material development and bio/environmental analysis. There is currently
no single analytical method that can simultaneously acquire elemental,
molecular, and spatial information from a single sample. This paper
presents the coupling of an NWR213 laser ablation (LA) system to the
liquid sampling-atmospheric pressure glow discharge (LS-APGD) microplasma
for combined atomic and molecular (CAM) analysis. The work demonstrates
a fundamental balance that must be considered between the extent of
fragmentation of molecules and ionization of atoms for CAM analysis.
Detailed studies showed that the interelectrode gap to be a critical
parameter for controlling the ionization efficiency of atomic and
molecular species. Utilizing Design-of-Experiment (DoE) procedures,
the discharge current was also found to be a significant parameter
to control. Elemental lead, caffeine, and simultaneous lead and caffeine
analysis via LA-LS-APGD-MS was made possible through improved understanding
of the influence of plasma parameters on the product mass spectra
of laser-ablated particles. Finally, a chemical map of elemental lead
and molecular caffeine, from lead nitrate and caffeine residues, was
generated, demonstrating the comprehensive mapping capabilities of
LA-LS-APGD-MS. The practical relevance of the capabilities is demonstrated
by mapping glutamic acid from a cryosectioned chicken breast with
a thallium spike deposited within the tissue. It is believed that
the LA-LS-APGD-MS could be a valuable methodology for the simultaneous
mapping of elemental and molecular species from a variety of samples.
The use of LS-APGD-MS is demonstrated for applications in cell culture media analysis and validated by ICP-OES. Use of simple polymer fiber injector loop allows determinations of the relative free vs. bound fractions of the metals.
The concept of ambient desorption-optical emission spectroscopy (AD-OES) is demonstrated using a liquid sampling-atmospheric 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., AD-OES. 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.
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