Apparent binding of ionized calcium by various buffers.

Area of Science:

• Analytical Chemistry
• Biochemistry
• Clinical Chemistry

Background:

• Accurate measurement of ionized calcium (Ca2+) is crucial for various physiological and clinical applications.
• Ionic strength and pH significantly influence Ca2+ measurements, necessitating careful control in reference solutions.
• Selecting appropriate buffers is essential for maintaining stable conditions and ensuring reliable Ca2+ quantification.

Purpose of the Study:

• To investigate the impact of different buffer substances and concentrations on the accuracy of ionized calcium (Ca2+) measurements.
• To identify a suitable buffer for use in aqueous reference solutions for Ca2+ determination.
• To assess the performance of buffers concerning electrode bias and linearity of response.

Main Methods:

• Six different buffer substances were tested in aqueous solutions with controlled ionic strength (160 mmol/L) and varying calcium chloride (CaCl2) concentrations.
• Ionized calcium (Ca2+) levels were measured using ion-selective electrodes.
• The effect of increasing buffer concentrations and solution pH on Ca2+ measurements and electrode bias was analyzed.
• The performance of 4-(2-Hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES) was specifically evaluated for pH stability and linearity of Ca2+ response.

Main Results:

• Measured Ca2+ levels decreased with increasing ionic strength and pH.
• Tris buffer exhibited a positive Ca2+ electrode bias, while the other five buffers showed a negative bias with increasing concentration.
• HEPES buffer at 10 mmol/L effectively maintained a stable pH (7.39) in reference solutions.
• HEPES demonstrated a linear Ca2+ response across instruments from three different suppliers.

Conclusions:

• Ionic strength and pH are critical factors affecting Ca2+ electrode measurements.
• HEPES buffer is a suitable choice for aqueous reference solutions due to its ability to maintain stable pH and provide a linear Ca2+ response.
• The findings provide guidance for selecting buffers to improve the accuracy and reliability of ionized calcium measurements.