Report for Analytical Chemists
electrometric procedure based on the glass electrode. T o calibrate these electrodes accurately, the NBS several years ago certified a special ratio of the phosphate buffer ( S R M 186 Ic and 186 He) for the physiologic range of p H from 7.35 to 7.45 at 37°C. I n spite of the fact t h a t this material was not compatible with blood (phosphate causes calcium precipitation), it has been widely used as a primary standard in the clinical laboratory. T o alleviate this and other problems, the N B S has been working for some time to evaluate and certify special S R M s of t r i s ( h y droxymethyl) aminomethane and its hydrochloride salt. These new physiologic p H standards (SRM 922 and 923) are now available in solid form, and work is in progress to certify this material in isotonic saline solution for greater accuracy and convenience for the clinical chemist. Ionic Activity Standards. T h e recent development of ion-selective electrodes for such physiologically important ions as calcium, sodium, potassium, and chloride has resulted in a need for activity standards comparable to the clinical p H standards.' Although still not widely or routinely used in the clinical laboratory, perhaps because, to some extent, standards were not available, such sensors have been extremely useful to reseach physiologists and biochemists in monitoring the activities of these ions. As has been discussed in a variety of biologic studies, it is ionic activity, rather t h a n concentration, t h a t is of utmost importance in a wide variety of physiologic phenomena. I t has been suggested t h a t more accurate measurements and interpretations could be made from activities t h a n from concentrations. Ion-selective electrodes offer a very simple and convenient way of determining ionic activities. N o t only are the measurements rapid and precise, but these sensors easily lend themselves to automation using continuous flow-through analysis systems requiring submilliliter sample volumes. Using procedures similar to those developed in the certification of the p H standards, N B S has recently
from Matheson...
Figure 5. Ion-Selective Electrodes Standards—SRM 2 2 0 1 and 2 2 0 2 — r e cently issued by NBS
issued the first in a series of ionselective electrode standards (Figure 5 ) . Two standards, N a C l (SRM 2201) and KC1 ( S R M 2202), have been certified for the activities of sodium, potassium, and chloride over the range from 0.001 to 2m. Although not specifically developed for clinical applications, the certified ranges encompass the normal physiologic ranges for these ions, which are the three most a b u n d a n t electrolytes in the body. We cite just a few diseases accompanied by changes in the activities of these ions: diabetic coma, toxemia, alimentary t r a c t infections, renal and cardiac failure, cellular breakdown, and adrenal cortical insufficiency. Of all the recently developed ion-selective electrodes, the calcium electrode may prove to be the most useful for diagnosis in the clinical laboratory. Again, t h e measurement of ionic calcium, believed to be the physiologically active component of the total serum calcium, is of considerable value in diagnosing such conditions as hyperparathyroidism, tetany, bone diseases, and cardiac dysrhythmias. W o r k is in progress to develop a calcium-ion activity standard in solution form. Our work is ultimately directed toward the development of a multicomponent clinical standard in which the activities of several of the most important biologic electrolytes are certified and can be used for the calibration of clinical analyzer systems based on ion-selective electrodes.
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ANALYTICAL CHEMISTRY, VOL. 4 3 , NO. 6, MAY 1 9 7 1
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