Can I use an ISAAC reference electrode?
Sencelll, Flexcell, VT03 and other Antec flowcells.
Antec flowcells are available with different reference electrodes. The ISAAC (in situ Ag/AgCl) reference is user friendly and easy maintenance. To use such reference electrode, the mobile phase must contain 2 mmol/L KCl. The chloride is a counter ion needed to stabilize the REF potential. The ref potential is chloride concentration dependent, hence must be constant and well-defined. To our knowledge, in most methods there are no issues known with such low concentration, no drawbacks. The flowcell manuals contain further detailed information.
Suppliers of clinical kits for catecholamines in urine and plasma usually don’t reveal the exact content of the mobile phase. The situation known to Antec -at date below- about the mobile phase for ECD applications is:
Biorad: no chloride added
RECIPE: no chloride added
Chromsystems: 2 mmol/L chloride added
In conclusion, the Chromsystems mobile phase is fully compatible with an ISAAC reference electrode. When using a Biorad or RECIPE kit icw an ISAAC, 2 mmol/L KCl must be added (149.1 mg/L). If for some reason this is not possible, a salt bridge Ag/AgCl reference electrode must be used.
Note: this chloride concentration is a counter ion for proper use of the REF. In addition, a mobile phase needs 10 – 100 mmol/L electrolyte. For example, a phosphate, citrate or acetate buffer set to a well-defined pH will do. Do not use chloride salts for this, because that will change the total chloride concentration (which should be a fixed concentration as explained above).
The reference potential is defined by the equilibrium of Ag + Cl- < = > AgCl. At lower Cl- concentration, the reference potential is higher. The actual working potential in the measurement cell is the sum of this reference electrode potential + the set potential of the potentiostat. Therefore, when using an ISAAC reference electrode with 2 mM KCl the applied working potential must be significantly lower compared to a method using a salt bridge reference electrode in saturated (3 M) KCl. The difference is 200 mV, so when the method with a salt bridge ref (vs saturated KCl) uses a working potential of +800 mV, a lower working potential of +600 mV should be used with an ISAAC vs 2 mM KCl. It is not pH dependend.