Advanced Methods Measure Skin Penetrants at the Parts-Per-Billion Level: Part II

Editor’s Note: Part I of this Dermatologic View column appeared in the January issue of C&T magazine and described advanced methods to measure skin penetration such as Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) (See Figure 1) and Atomic Absorption Spectrophotometry (AAS). Here, the authors continue this investigation of advanced methods to measure skin penetration.

Electron Spin Resonance (ESR) applies to transitions between electronic spin states in a magnetic field. The magnetic field of a probe is swept and a resonance indicates a single unpaired electron. If the unpaired electron registers proximity of magnetic nuclei with a nuclear spin ≥ 1/2, the single absorption is split into hyperfine structures identifying interacting nuclei and their number in the vicinity of the unpaired electron.

Stable organic, organometallic, and inorganic species with unpaired electrons can be detected in liquid as well as solid materials. Thus presence, identity and oxidation state (speciation) of paramagnetic transition metals causing ESR absorption can be determined, e.g., Mn, Cu, V, Al, Ti, Ni, etc. Such investigation facilitates detection of xenobiotics upon experimental or occupational exposure, for scientifi c investigation and occupational safety assesment, respectively.

The nuclear spin of transition metals and the natural abundance of the magnetic isotopes of a metal assist in this intent. Transient species such as organic free radicals must be continuously generated in the ESR probe to maintain a sufficient steady state concentration for detection. Low temperature (freezing) as well as spin trapping techniques are used to visualize transient radicals in the biological matrices.

Changes in the structure and properties of biological membranes (e.g., skin) under the infl uence of changing physical parameters (e.g., UV radiation) or under the infl uence of xenobiotic penetrants can be studied by ESR. Skin is a potential target of oxidative injury due to exposure to ultraviolet (UV) radiation and high oxygen concentration whereby lipid radicals and hydroxyl radicals are induced, potentially resulting in injury in the stratum corneum and epidermis.1 Perturbation and conformational changes in natural constituents in the skin (i.e., the lipid bilayers of human skin) due to experimental penetration enhancers could be investigated by incorporating various (ESR-detectable) stearic acids as spin labels.

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