Extruded dielectric sample tubes of complex cross section for EPR signal enhancement of aqueous samples

Abstract

This paper builds on the work of Mett and Hyde (2003) and Sidabras et al. (2005) where multiple flat aqueous sample cells placed perpendicular to electric fields in microwave cavities were used to reduce the RF losses and increase the EPR signal. In this work, we present three novel sample holders for loop-gap resonators (LGRs) and cylindrical cavity geometries. Two sample holders have been commissioned and built by polytetrafluoroethylene (PTFE) extrusion techniques: a 1mm O.D. capillary with a septum down the middle, named DoubleDee, and a 3.5mm O.D. star shaped sample holder, named AquaStar. Simulations and experimental results at X-band show that the EPR signal intensity increases by factors of 1.43 and 3.87 in the DoubleDee and AquaStar respectively, over the current TPX 0.9mm O.D. sample tube in a two-loop-one-gap LGR. Finally, combining the insight gained from the constructed sample holders and finite-element solutions, a third multi-lumen sample holder for a cylindrical TE₀₁₁ cavity is optimized, named AquaSun, where simulations show an EPR signal intensity increase by a factor of 8.2 over a standard 1mm capillary.

Keywords: Aqueous samples; Cylindrical symmetry; Electron paramagnetic resonance; Loop-gap resonator; Multi-lumen sample holders.

Figure 1

Two LGR geometries: (a) a two-loop–one-gap with a 1 mm sample loop and b) a five-loop–four-gap with a 5 mm sample loop.

Figure 2

Three aqueous sample tube cross sections: (a) the AquaStar; (b) the DoubleDee; and (c) the AquaSun. Grey represents PTFE.

Figure 3

Cylindrical electric field components highlighting the three types of losses along a 2.5mm radius circle concentric within a TE₀₁₁ cavity. A) Cross-section of a TE₀₁₁ with a 8 × 0.2 mm flat-cell sample B) Electric field, E_ϕ, showing the reduction of electric field in the sample from Type II losses, C) Electric field, E_ρ, showing the tangential electric field null down the center of the sample cell, and D) Electric field, E_ϕ, along the center of the sample showing the Type III complexity where the electric field increases towards the end of sample cell.

Figure 4

Electric field 2D plot showing the three types of losses established in Mett and Hyde [3] A) Plot of E_ϕ showing Type II and Type III loss. B) Plot of E_ρ showing the electric field tangential to the sample surface and C) a zoom of E_ρ highlighting the Type I and Type III loss.

Figure 5

Illustration of the electric vector fields in the sample loop of A) a two-loop–one-gap 1.2 mm inner diameter and B) a five-loop–four-gap 5 mm inner diameter LGR. Shown are positions of zero electric field, marked as 0. Illustration of the two sample holder geometries with dimensions: C) the DoubleDee and D) the AquaStar.

Figure 6

Electric field profiles comparing the TPX capillary (A–D) with the DoubleDee (E–H) in a 1 mm I.D. 2-loop–1-gap LGR. A) E_ϕ and B) E_ρ along a 0.25 mm radius circle concentric with the sample loop, showing an electric field peak at the LGR gap at 0°. C) E_ϕ along the line parallel to the gap (solid) and perpendicular to the gap (dashed), D) E_ρ perpendicular to the gap. E) E_ϕ showing discontinuities of the electric field at the DoubleDee septum and F) E_ρ showing tangential electric field null. Plots are for septum orientations perpendicular (solid) and parallel (dashed) to the electric field along a 0.5 mm concentric circle. G) E_ϕ perpendicular to the gap for the septum orientation parallel to the gap (solid) and E_ϕ parallel to the gap for the septum orientation perpendicular to the gap (dashed). H) E_ρ perpendicular to the gap for the septum orientation parallel to the gap.

Figure 7

A) Electric field for the AquaStar geometry in the five-loop–four-gap LGR. B) E_ϕ is plotted showing the Type II reduction in each sample cell and C) E_ρ showing tangential electric field nulls in each sample cell at 45° increments. D) E_ϕ along the center of the sample showing the Type III increase towards the end of sample cell for a cell parallel to the gap (solid) and parallel to the electric field null (dashed). Increase in electric field at 0.3 mm is formed from circulating tangential fields where the arms connect.

Figure 8

A) The proposed AquaSun sample tube designed for a cylindrical TE₀₁₁ cavity, where each of the 12 azimuthal flat cells and two crossed flat cells are aligned with the perpendicular electric field in the azimuthal direction. Cylindrical electric field components are plotted: B) E_ϕ showing the Type II reduction in each sample cell at a 2.5 mm radius; and C) E_ρ showing tangential electric field nulls in each sample cell at 22.5° increments over half the circumference along the center of the flat cells. D) E_ϕ along the center of the sample showing the Type III increase towards the end of the azimuthal sample cells (dashed) and through one of the crossed flat cells (solid). A small increase due to circulating electric fields occurs near the center.