Recent Research Developments

September 19, 2001
 

Ion Sensors for Clinical Chemistry

Ion sensors for various applications have been devloped by Phil Buhlmann and co-workers. For example, the complexing agent used in a chloride sensors developed for measurements in human blood is now commercially available. An acetate selective sensor was shown successful for measurements in food products, and a sulfate selective sensor was incorporated into a measurement unit for a Mars lander. In their most recently published work, Buhlmann and co-workers report on a newly discovered phenomenon that they serendipitously discovered when carefully testing creatinine-selective electrodes developed for use in clinical chemistry (Anal. Chem. 2001, 73, 3199).
Creatinine is one of the most important analytes in clinical chemistry. Because creatinine is produced in the human body in relatively constant concentration and passes through the kidney into urine, creatinine measurements provide an important measure of kidney health. Creatinine measurements are not only extremely relevant for the diagnosis of kidney diseases, but provide crucial information for the dosage of drugs in the treatment of cancer or heart diseases.

When Buhlmann and co-workers optimized potentiometric membrane electrodes, they found selectivities that were comparable or superior to previously reported creatininium-sensitive sensors. Applying these electrodes to measurements in urine, they discovered that certain lipids occurring naturally in the human body may lead substantial undesirable selectivity changes. Potentiometric, chromatographic, nuclear magnetic resonance spectroscopic, and mass spectrometric evidence revealed that these selectivity changes are caused by electrically neutral lipophilic compounds of low molecular weight, which are easily extracted into the sensor membranes. The ubiquitous lipids coproporphyrin, phosphatidylserine, taurocholic acid, cholic acid, phosphatidylethanolamine, and octanoic acid were identified as such lipids. Follow-up experiments confirmed that the same lipids also affect the responses of other potentiometric membrane electrodes. This discovery opens ways to improve the robustness of ion-selective potentiometric sensors for applications in clinical chemistry, which represents an application field with over a billion measurements per year.