Electron probe x-ray analysis of single ferritin molecules

Area of Science:

• Materials Science
• Biophysics
• Analytical Chemistry

Background:

• Ferritin is a key protein for iron storage in cells.
• Accurate quantification of iron at the single-molecule level is challenging.
• Electron probe X-ray microanalysis (EPXMA) offers potential for elemental mapping.

Purpose of the Study:

• To assess the sensitivity of EPXMA for detecting iron in single ferritin molecules.
• To validate theoretical predictions of EPXMA sensitivity.
• To establish detection limits for iron quantification.

Main Methods:

• Transmission electron microscopy (TEM) with a liquid nitrogen cooled stage.
• Electron probe X-ray microanalysis (EPXMA) of single and small clusters of ferritin molecules.
• Measurement of Fe Kalpha peaks during 100-second counts with varying probe currents.

Main Results:

• Significant Fe Kalpha peaks were detected from single ferritin molecules.
• A linear correlation was observed between the number of ferritin molecules and count rates.
• Experimental yields closely matched theoretical calculations.

Conclusions:

• Current EPXMA technology meets predicted sensitivity levels for elemental analysis.
• The minimal detectable mass of iron is estimated at 0.9 x 10^-19 g with 95% confidence.
• EPXMA is a viable method for sensitive iron quantification in biological samples.