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 calcium-sensitive electrode

One cannot speak too broadly, having experience of only one blood gas analyser. The locally available unit uses calcium electrodes which use an ionophore-doped PVC membrane, and are similar in construction to the potassium-sensitive valinomycin ionophore electrode.

calcium-sensitive PVC membrane electrode

Probably the most relevant historical papers to quote here would be either the 1971 piece by Li and Piechocki or the earlier 1970 article by Moore. Both used a then-new commercially available liquid-membrane system, relying on a porous ceramic membrane saturated with a water-immiscible organic liquid ion-exchanger. Subsequent revisions on this theme have (like for potassium) yielded several calcium-selective ionophores which mix well with PVC, allowing the creation of a durable electrode membrane. Of these, the first were 1:3:1 proportion composites of PVC, tributyl phosphate and thenoyl trifuoroacetone (a calcium chelator). Since the late 1960s, calcium-selective ionophores have proliferated in such vast excess that it would be totally unreasonable to list them. A totally random molecule (ETH 129) was used in the diagram above; it happens to be more selective for lantanum cations than for calcium, but hey- when are your patients' bidy fluids ever going to be inundated with those? In any case, it is impossible to discern which specific ionophore Radiometer use in their E733 electrode without contacting them directly and begging for information.

The limits of measuring ionised calcium with the ABG machine

The range for this electrode is - as for all of them - totally unreasonable. The E733 model can go from 0.20mmol/L to 9.99mmol/L. It can be safely said that living human organisms will never touch either of these limits.

As one can discern from the diagram above, the generation of a potential difference across this membrane rests on the traffic of free calcium ions. Obviously, if abovementioned ions are tied up in some sort of chelation complexes or are busy encrusting albumin, they will not be measured by the analyser. Nor will they perform any useful work.

Thus, the ABG-derived ionised calcium is the only physiologically relevant calcium measurement.
All those serum calcium values you get from the automatic lab machines are poor surrogates.

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.

Li, Ting-Kai, and Joseph T. Piechocki. "Determination of serum ionic calcium with an ion-selective electrode: evaluation of methodology and normal values."Clinical chemistry 17.5 (1971): 411-416.

Moore, Edward W. "Ionized calcium in normal serum, ultrafiltrates, and whole blood determined by ion-exchange electrodes." Journal of Clinical Investigation49.2 (1970): 318.