Why does a low-conductivity electrolyte give better images in SVET and LEIS?

Key points

• For a point source the potential at the probe is φ = I/(2πκR), where κ is the solution conductivity and R the distance from the source. As conductivity increases the gradient and the signal weaken; simulations show that low conductivity also improves the localisation of features.
• If a high-conductivity electrolyte must be used, the signal can be increased by moving the probe closer to the sample, although this is not always possible.
• Conductivity is assumed constant throughout the measurements; evaporation increases it and leaves the calibration valid only at the start. A large-volume cell such as the TriCell is a simple precaution.
• In LEIS conductivity also affects the usable frequency range: low conductivity improves the resolution of the ac current, high conductivity reduces the effect of stray capacitance at high frequency.
• Conductivity can be determined from the ohmic resistance fitted from EIS instead of a conductivity meter.
• Examples: SVET in tap water (145 µS/cm) and 5 mM NaCl (590 µS/cm); LEIS at 134 µS/cm and 655 µS/cm. At low conductivity the signal is stronger and the contrast clearer.

Figures

Images: Bio-Logic Science Instruments; taken from the source article.

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