Anal. Chsm. 1983, 55, 1807-1809
1807
Errors in Determination of Potassium in Physiological Fluids with Valinomycin IEIectrodes Sir: The accurate determination of potassium for both clinical and research purposes is of recognized importance. In the past such determinations were performed by flame photometry, but recently direct potentiometric techniques, particularly those based on valinomycin electrodes, have been utilized (1,2). Such electrodes are now routinely used for the determination of potacasium in whole blood (3), plasma or serum (4), and diluted urine (4-6). Potassium microelectrodes for investigative use have been fabricated from glass (7),tetra@-chlorophenyl) borate (8),and more recently valinomycin (!+11). Valinomycin is attractive for such uses due to its high selectivity for potassium over Na+ and H+. Prior work from this laboratory (4), using an instrument that monitors potassium in urine with a valinomycin electrode after a 1:2.5 dilution of the samplie, gave an indication of a concentration-dependent discrepancy compared to flame photometric values. We further evaluated this discrepancy by measuring urinary potassium by direct potentiometry and by flame photometry. I n this report, we piresent evidence that the commonly utilized valinomycin-poly(viny1 chloride) electrodes may be subject to interference in biological fluids. Thus the potassium data obtained from such electrodes should be interpreted with ex heme caution. EXPERIMENTAL SECTION Apparatus. Potassium was determined automatically by a direct potentiometric instrument (Nova-1 analyzer, Nova Biomedical Inc., Newton, MA; modified t o operate in the urine mode without diluting the sample) and biy flame photometry (KLiNA flame, Beckman Instruments, Inc., Fullerton, CA.). Potassium was also determined manually with a valinomycin electrode obtained from Orion Research, Cambridge, MA (Model 931900),and with a glass1 K+ electrode (Beckman Instrumenta). These measurements, refierenced to a saturated calomel reference electrode, were recorded with a Model 135 specific-ion meter (Corning Glass Works, Corning, NY). Ultrafiltration of urine used stirred cells (Amicon Corp., Danvers, MA) under refrigeration with membranes with nominal molecular weight cutoffia of 500, 1000, or 10000 (Amicon type UM05, UM2 and PM10). Gel filtration of urine was performed on glass columns (Pharmacia, Inc., Piscataway, NY) packed with Sephadex G-10 (Sigma (ChemicalCo., St. Louis, MO). Anion exchange chromatography used QAE-Sephadex(Sigma) in similar columns. Fractions from the columns were collected with the aid of an automated fraction collector (Isco Corp., Lincoln, NB). Reagents. NaCl and KC1 were purchased from Mallinckrodt (St. Louis, MO). Tris buffer was made with Trizma base (Sigma). Batch anion exchange resin experiments were performed with Dowex 2 anion exchange resin (C1 form, 100-200 mesh, Sigma). All water used was distilled and deionized. Sample Preparation. Urine samples we re collected randomly from healthy humans, or 24-h timed specimens were collected without preservative from patients in Barnes Hospital. For the red blood cell hemolysate, human red cells were washed in saline, the saline was removed, and the cells were lysed by freezing and thawing. Following centrifugation, the supernatant was assayed for Na and K. RESULTS AND DISCUSSION Urine potassium concentrations were lower in nearly all cases by the Nova-1 direct potentiometric technique than by flame photometry. For 131 urine samples from patients hospitalized at Barnes Hospital, a plot of the flame value 0,) vs. the valinomycin electrode value (x) gave least-squares statistics of slope 1.597 and y intercept -!3.23. To emphasize the strong dependence of the lower elect rode results on the
potassium concentration, a plot of the ratio of potassium values by the two methods vs. the flame photometric value for these 131 samples is shown in Figure 1. The concentration-dependent bias of the direct potentiometric values was not due to instrument response as both analytical methods were linear up to 200 mmol/L K+. Various studies were undertaken to characterize the substance(s) responsible for the lower value with the valinomycin-poly(viny1 chloride) electrode in urine and to determine whether it caused a physical-chemical association with K+ or an analytical artifact. Ultrafiltration of urine showed that the substance responsible for the lower potentiometric K+ values had a molecular mass of
