The basic principles of potentiometric measurement of ion concentration using the ion-selective electrode chain are discussed in greater detail elsewhere. Similarly, the marvellous properties of ion-selective electrode membranes are interesting enough to merit their own chapter. Additionally, as a main reference for this topic, I refer the readers to Nallanna Lakshminarayanaiah's Membrane Electrodes (2012), as well as Martin Frant's two articles.

Structure of the sodium-sensitive electrode

One cannot speak too broadly, having experience of only one blood gas analyser. The locally available unit apparently features a ceramic pin in lieu of a PVC-coated membrane (though the latter is a valid alternative, and is used in numerous industrial applications).

Unfortunately, Radiometer aren't very specific in their description of their electrode. Given the high degree of selectivity of the electrode, and the absence of a solvent reservoir, one might surmise that this is not one of those "saturated wick" liquid membrane electrodes, where the selectivity is conferred by a hydrophobic oil-based solvent impreganted with ionophores, soaked into a porous ceramic frit. Rather, it may be a ceramic loosely based on one of the old-school glass sodium electrodes, a staple of the electrochemistry lab since the 1950s.

In view of this uncertainty, one cannot help but take the following diagram with a grain of salt. It is not the result of careful scrutiny of the modern ABG sodium electrode, but rather the composite of many ancient articles, and therefore possibly quite incorrect.

Possible sodium-sensitive ceramic pin electrode diagram

Glass seems to be an effective sodium-selective membrane material. All glasses containing more than 0.01mol Al2O2 seem to have a high cation selectivity. What's more, the selectivity for a given cation species changes predictably with changing glass composition, and the glass seems indifferent to anion species. An excellent article by George Eisenman (1962) exists to inform and delight the ion-selective electrode enthusiast. Building on this work, In 1963, Moore and Wilson were the first to evaluate such glass membranes in electrodes designed for measuring the cation concenration of huma body fluids. "Samples of urine, serum, whole blood, and plasma were obtained from normal hospital personnel" who (it seems) were glad to provide everything but CSF (which was obtained over many months "from hospitalized patients with a variety of diseases"). The specific glass used in this early work references Eisenman, and uses his NAS11-18 glass, containing 11mol of Na2O, 18mol of A12O3 and 71mol of SiO2 per every 100ml. The electrode was essentially a slightly modified glass electrode with a membrane thickness of around 0.25mm.

A diagram of a similar setup is available in Richard A. Durst's "Ion Selective Electrodes (1969)", pp. 304.

sodum selective electrode from DURST

As you can see, the tip forms a pin of sorts. On can use this fragile relationship to imagine that the modern radimerter E722 sodium electrode has a similar sort of "pin", even though the above-depicted device is only pin-like to suit the purposes of Khuri et al (1963), who shoved these things up into the proximal tubules of pet salamanders.

Anyway. The properties of the cation-selctive glass membrane are discussed in a classic article by Lee and Fozzard (1974). In brief, it would appear that the amorphous crystal lattice of glass contains numerous negatively charged regions into which cations gladly fit. The lattice has enough gaps in it (perhaps about 4 angstrom wide) that cations are able to migrate from gap to gap. Perhaps no single ion would ever migrate across the comparatively astronomical width of the entire membrane, but enough of them change position that some end up in the reference solution inside the electrode, and - more importantly - a potential difference develops across the membrane, which is proportional to the cation concentration in the sample solution.

Limitations

Generally speaking, glass sodium electrodes tend to have rather extreme measurement ranges. For instance, the linear response range for the Metrohm 6.0501.100 ISE is from 1 × 10-5 mol/L to 1mol/L.

The ABG machine's sodium electrode follows this trend; its linear response range is well outside the normally expected physiological limuts (from 10mmol/L to 250mmol/L).

Other cations can interfere with measurement. Unlike the highly selective ionophore membranes, the glass electrodes only discriminate according to size of the ion, and to a lesser extent charge. This means the sodium-selective glass electrode can be confused by high concentrations of other cations, particularly monovalent ones like lithium and potassium. The selectivity is fortunately high enough that at survivable lithium or potassium concentrations this should never be a clinically relevant source of error.

But is it any good?

The other methods of sodium measurement are flame spectrophotometry and the indirect ion-selective electrode method. There is a well-known difference in results between these methods and the direct ISE, which is probably entirely due to the protein and lipid content of the plasma. The indirect methods measure a sample diluted 1/10th, and then relate the result to the original volume. This routinely returns higher results, and the difference seems to be greatest in patients with the lowest protein levels (conversely, the presense of hyperproteinaemia leads to pseudohyponatremia). The difference is not trivial - Siggaard-Andersen reports a theoretical discrepancy of 20mmol/L in patients who are severely hyperproteinaemic (eg. where the mass concentration of water has decreased by 15% to 0.8kg/L). Additionally, the γ-value (activity coefficient) of sodium changes between theoretical equations and direct measurement, which introduces inaccuracy into both direct and indirect ISE results.

In this case, the more accurate measurement is actually the point-of-care ABG result, because it measures undiluted whole blood. Some authors have even gone on to suggest that central laboratories consider changing to direct ISE measurement techniques.

References

Device-specific information in all these ABG pages refers to the ABG machine used in my home unit.

Other machines may have different reference ranges and different symbols.

For my ABG analyser, one can examine this handy operations manual.

There is also an even more handy reference manual, but one needs to be an owner of this equipment before one can get hold of it. Its called the "989-963I ABL800 Reference Manual"

Kurzweil, Peter. "Metal oxides and ion-exchanging surfaces as pH sensors in liquids: state-of-the-art and outlook." Sensors 9.6 (2009): 4955-4985.

Breathnach, C. S. "The development of blood gas analysis." Medical history 16.01 (1972): 51-62.

Lakshminarayanaiah, Nallanna. Membrane electrodes. Elsevier, 2012.

Buck, RICHARD P., and Erno Lindner. "Recommendations for nomenclature of ionselective electrodes (IUPAC Recommendations 1994)." Pure and Applied Chemistry 66.12 (1994): 2527-2536.

Frant, Martin S. "Historical perspective. History of the early commercialization of ion-selective electrodes." Analyst 119.11 (1994): 2293-2301.

Frant, Martin S. "Where did ion selective electrodes come from? The story of their development and commercialization." Journal of chemical education 74, no. 2 (1997): 159.

Young CC."Evolution of blood chemistry analyzers based on ion selective electrodes."Journal of chemical education 74, no. 2 (1997): 177.

Yim, Hyoung-Sik, et al. "Polymer membrane-based ion-, gas-and bio-selective potentiometric sensors." Biosensors and Bioelectronics 8.1 (1993): 1-38.

Oesch, Urs, Daniel Ammann, and Wilhelm Simon. "Ion-selective membrane electrodes for clinical use." Clinical Chemistry 32.8 (1986): 1448-1459.

Sollner, Karl. "Membrane electrodes." Annals of the New York Academy of Sciences 148.1 (1968): 154-179.

Bloch, René, Adam Shatkay, and H. A. Saroff. "Fabrication and evaluation of membranes as specific electrodes for calcium ions." Biophysical journal 7.6 (1967): 865-877.

Oesch, Urs, and Wilhelm Simon. "Lifetime of neutral carrier based ion-selective liquid-membrane electrodes." Analytical Chemistry 52.4 (1980): 692-700.

Dimeski, Goce, et al. "Disagreement between ion selective electrode direct and indirect sodium measurements: estimation of the problem in a tertiary referral hospital." Journal of critical care 27.3 (2012): 326-e9.

Dimeski, Goce, Peter Mollee, and Andrew Carter. "Effects of hyperlipidemia on plasma sodium, potassium, and chloride measurements by an indirect ion-selective electrode measuring system." Clinical chemistry 52.1 (2006): 155-156.

Maas, A. H., et al. "Ion-selective electrodes for sodium and potassium: a new problem of what is measured and what should be reported." Clinical chemistry31.3 (1985): 482-485.

Eisenman, George, Donald O. Rudin, and James U. Casby. "Glass electrode for measuring sodium ion." Science 126.3278 (1957): 831-834.

Moore, Edward W., and Donald W. Wilson. "The determination of sodium in body fluids by the glass electrode." Journal of Clinical Investigation 42.3 (1963): 293.

Eisenman, George. "Cation selective glass electrodes and their mode of operation." Biophysical journal 2.2 (1962): 259-323.

Khuri, Raja N., et al. "Single proximal tubules of Necturus kidney. VIII: Na and K determinations by glass electrodes" (1963).

Lee, Chin Ok, and Harry A. Fozzard. "Electrochemical Properties of Hydrated Cation-Selective Glass Membrane: A Model of K+ and Na+ Transport."Biophysical journal 14.1 (1974): 46-68.