An artificial receptor has been designed to bind creatinine with a color change (chromogenic response) caused by proton transfer from one end of the receptor to the other. The receptor was synthesized and found to extract creatinine from water into chlorocarbon solvents. The color change in the organic layer is specific for creatinine relative to other organic solutes, and it is selective for creatinine relative to sodium, potassium, and ammonium ions. The chromogenic mechanism is revealed by x-ray crystal structures of creatinine, the free receptor, and the complex, showing "induced fit" binding resulting from electronic complementarity between host and guest.
Previously described ion-selective membrane electrodes useful for determination of total carbon dioxide in biological fluids in automated analyzers show interference from several sources--e.g., fatty acids, keto acids, salicylate, heparin. Judicious selection of membrane components has produced a membrane with superior performance characteristics: short conditioning time, long lifetime in storage, rapid and stable response, low drift, and significantly less susceptibility to interference than other electrodes thus far reported. Samples can be analyzed at 240 samples or more per hour and results correlate well with those by, e.g., the Technicon SMAC II method for total carbon dioxide. Mounted on the Technicon RA-1000, the carbon dioxide sensor is arranged in tandem with sodium and potassium ion-selective electrodes. When all three species are measured in the same buffered sample stream, the sampling rate thus becomes 720 tests per hour, done on 25-microL samples.
We report a colorimetric method for determining lithium in blood serum without sample pretreatment or solvent-extraction steps. The method is based on a novel chromogenic ionophore that exhibits exceptionally high selectivity (much greater than 4000:1) for lithium over sodium. The standard curve for the method is linear up to 3.5 mmol/L and exceeds the therapeutic range for lithium. The results for 57 patients' samples correlated well with results obtained with an Instrumentation Laboratory flame photometer (r = 0.97).
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