September 1, 2011

The New Water Lab

By David G. Daniels, M&M Engineering Associates Inc.

New Competitor: Capillary Electrophoresis

Until recently, only IC could detect multiple anions or cations in a single run at the ppb levels needed by steam cycles. There have been specific ion electrodes for chloride, for example, but IC is alone in its ability to produce both a chloride and sulfate analysis on the same run.

Another emerging measurement technique is gel electrophoresis, used for many years to separate DNA into its various base pairs. The simple analytical premise is that when you apply a voltage, molecules will pass at different rates through a medium, based on their affinity for the charge at the far end of the plate.

A similar process can be applied to inorganic ions. Capillary electrophoresis is currently under development by Advanced Microlabs of Fort Collins, Colo. A very small sample of known volume is injected into a capillary channel. The channel is flushed with a continuous supply of an electrolyte buffer. The ions separate as they pass through the channel and past a micro-conductivity meter at the end of the channel. This meter senses the change in conductivity as the ions are eluted out of the channel. The online instrument will take a continuous sample and filter a small portion of it through a 1-micron filter before sending it to the electrophoresis module. It takes a little more than 2 minutes for the sample to traverse the capillary electrophoresis channel and approximately 5 minutes between sample cycles. This equipment is designed to be continuously (every 5 minutes) sampling, analyzing, and producing data (Figure 2).

2. The capillary electrophoresis module. The thin S-shaped line in the middle of the “circuit board” is the ion channel through which the sample passes. The module can analyze levels of chloride and sulfate in parts per billion. Courtesy: Advanced Microlabs

The manufacturers of the equipment anticipate that a module will be in the analyzer for approximately one month and then be changed out with a new one. They are able to achieve detection limits of less than 1 ppb not only for chloride and sulfate, but soon should also be able to achieve these limits for silica, thereby pushing the limits of the classic spectrophotometric analyzer. Cations that can currently be determined include sodium, calcium, and magnesium; others, such as ammonium ion, are expected the near future.

To date, Advanced Microlabs has been working with nearby Colorado utilities to develop this process specifically for a utility market. Test units are currently in place, and Advanced Microlabs anticipates commercially available units in the second quarter of 2012.

Dissolved Oxygen Probes —Using Light

For many years, the most common dissolved oxygen probes were based on the amperometric principle. Dissolved oxygen in the sample first passed through a gas-permeable membrane into a chamber filled with an electrolyte. Here the oxygen would be consumed at an inert metal cathode, while a large anode was converted to an oxide, completing the circuit. The amount of dissolved oxygen in the sample was related to the amount of current generated by the probe by the electron transfer from the cathode to the anode.

There were a number of difficulties with the measurement approach for an online environment in a power plant, usually with the membrane. Over time, the outside of the membrane would foul with iron oxide, slowing the response of the probe. Also, because the probe would use the oxygen in the sample for the determination, the probe was very dependent on sample flow rate for a correct analysis. Replacing the membranes on some probes required dexterity and a little luck. Occasionally, electrolyte also needed to be replaced and the probe had to be refurbished. Better membrane replacement systems were developed by some manufacturers. Some even offered a factory-refurbished probe replacement service.

A recently introduced dissolved oxygen probe operates on a completely different technology: A light source is used to determine the dissolved oxygen concentration. The principle of operation is that a blue light source illuminates an area coated with a dye that produces a red fluorescence. As the sample passes by this area, any dissolved oxygen in the sample causes a phase shift in the response time of the red fluorescent light that is proportional to the amount of dissolved oxygen in the sample (Figure 3).

3. Glowing report. According to Hach, “the oxygen sensor is made up of a clear, oxygen impermeable hard substrate. An oxygen sensitive luminescent dye, along with a scattering agent, is pad-printed on the substrate. A final overlay of dark pigment is added to prevent stray light from entering the measurement cell. The luminescent dye emits red light when exposed to blue light. The scattering agent distributes the emitted light throughout the sensor matrix and contributes to the opacity of the sensor. Pulses from a red LED serve as an internal reference. The duration of the luminescence is proportional to the concentration of dissolved oxygen in the sample.” Next-generation luminescent sensors will measure dissolved oxygen in parts per billion. Courtesy: Hach Co.

This technology was developed by Hach in 2006 and has been used successfully in many applications that operate at higher (parts per million) dissolved oxygen levels. However, the probe and electronics have been further improved so that now it can operate in the low ppb range required by steam cycles.

Head-to-head testing of a luminescent probe against an electrochemical Orbisphere dissolved oxygen analyzer (Orbisphere is now also owned by Hach) showed excellent correlation at levels in the single-ppb range in an actual power plant setting. Obviously, the membranes or electrolytes with a luminescent probe are not accessible, so the probe requires far less maintenance than conventional dissolved oxygen probes, and the maintenance required is very simple.

Special thanks to David Gray of Mettler-Toledo Thornton, Joachim Weiss of Dionex Corp., Vickie Olsen of Hach, and Uwe Michalak of Advanced Microlabs for their help in the preparation of this article.