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 chloride-sensitive electrode
One cannot speak too broadly, having experience of only one blood gas analyser. The locally available unit uses chloride electrodes which use an ionophore-doped PVC membrane, and are similar in construction to the potassium-sensitive valinomycin ionophore electrode. Like for the calcium-sensitive electrode membrane, Radiometer do not release the composition of their secret ionophore recipe, and so we are left to guess as to what it might be.
There is a vast selection of chloride-sensisive ionophores available. Traditionally, quaternary ammonium salts have been used as a chloride-selective ionophore admixture into PVC, together wih a plastiiser like o-nitrophenyl octyl ether (NPOE). However, researchers and clinicians comlained that these membranes showed poor selectivity over salicylate anions and heparin (both with a high anionic charge). Subsequently, ionopores such as mercury(n) EDTA and indium porphyrins were developed. Less rare-earth-dependent solutions today include such unpronounceable neutral carriers as tridodecylmethylammonium chloride (TDMAC). The poor selectivity of the PVC membrane for intensely lipophilic anions (such as the abomentioned salicylates and bromide) were overcome by the use of a organic-inorganic hybrid sol-gel matrices, which are 10 % TDMAC, 57 % diisodecyl adipate, and 33% PVC. The reference solution is usually a 100mmol/L potassium chloride; which means that at a patient serum Cl- of 100 mmol/L, the potential difference across the membrane will be 0 mV.
But is it any good?
The E744 Radiometer brand of chloride-sensitive electrode reports a linear response between 7 mmol/L and 350 mmol/L, which is well ouside the normal physiological reference range. Generally, ABG-derived electrolyte results are good enough for government work and perhaps better in the case of sodium, as they are not befouled by hyperproteinaemic pseudo-hyper-electrolyteamia effects (the artifactual problem of using the measured volume of a sample where much of the volume is occupied not by water but by protein).
In short, the chloride value from the ABG machine is a valid measurement, worth considering.