Ion sensitive electrodes (ISE) are widely used in numerous analytical laboratories which measure single anions and cations in aqueous samples. Typical ions include chloride, flouride, sodium, and proton (pH).The ISEs provide a simple, inexpensive, and easy method of measuring these ions. ISEs of various designs are offered by a number of companies marketing analytical laboratory equipment.
Most ISE measurements are carried out using a manual procedure. The procedure involves calibration of the ISE with 2 or 3 standards followed by measurement of the samples. The manual procedure is time intensive, especially if multiple samples are analyzed.
Numerous publications have appeared over the years describing automation of the ISE procedure using continuous flow methods such as FIA. However, in most cases the flow cells and often the ISEs themselves are ‘home-made”. What is lacking are commericially available flow cells suitable for use with common, robust commercially available ISEs. While flow cells are available, they generally have high volume flow channels whereas for use in FIA or SIA low volume (<50 uL) are needed.
Therefore, we have designed a low volume flow cell in which a 12 mm epoxy flat-bottomed Cole-Parmer ISE can be mounted. A photo of the flow cell is shown in Figure 1. The flow cell has an internal volume of about 20 uL and is compatible with any FIA or SIA system. We have also developed a Zone Fluidics instrument which incorporates this flow cell for automation of the measurement of ions. A schematic of the system is depicted in Figure 2. The instrument automates both the calibration and the measurement of samples. A calibration curve for chloride is shown in Figure 3 using duplicates of 3 calibrants. Precision and linearity are excellent. The instrument substantially reduces labor required for ion measurements using ISEs, especially for batches of samples.
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