Name:

Contact details

Qualifications

Ph.D., Analytical Chemsitry
University of Washington, June 2004
Advisor: Professor James B. Callis
Dissertation: A thin film oxygen sensor for the study of insect flight

M.S., Analytical Chemistry
University of Washington, June 2001

B.S., Chemsitry
University of Alaska, June 1999

Project Summary

Phosphate

Phosphorus, in the form of phosphate, is a limiting nutrient in many water systems. Excessive amounts of this element can lead to an overabundance of algae (Figure 1) and significantly impact the health of an aquatic ecosystem. This process, known as eutrophication, is the most common water quality problem in the world.

Figure 1. Algae covered pond

Monitoring phosphate levels is a key step in the protection of vulnerable areas. The Adaptive Sensors Group of the Adaptive Information Cluster has developed a portable sensor for long-term measurements of phosphate in natural waters. This is a completely autonomous device which incorporates sampling, reagent and waste storage, colorimetric detection, wireless communication, and a power supply into a complete, miniaturized system.

The sensor

The phosphate sensor was designed to be left in a lake or stream for continuous measurements of phosphate levels. The system can operate autonomously for one week on a single rechargeable, 12V battery.

During this period, aquired data can be transferred from the sensor to the lab using a wireless modem incorporated into the sensing device.

The phosphate sensor uses the yellow method for phosphate determination. This is a simple colorimetric technique which involves the formation of vanadomolybdophosphoric acid when a phosphate-containing sample is mixed with an acidic reagent containing ammonium molybdate and ammonium metavanadate. The intensity of the resulting solution depends on the amount of phosphate in the original sample. Figure 2 shows the yellow solution when the reagent is mixed with water samples containing 0 ppm to 50 ppm phosphate.

Figure 2. The resulting solution when the yellow reagent is mixed with water samples containing 0 ppm (left) to 50 ppm (right) phosphate.

The system

In the portable device, the reaction between the reagent and the water sample takes place on a microchip with a volume of only 2 uL. By building the system around a microfluidic chip, only small volumes of reagent and waste need to be stored and contained.

The microfluidic chip was integrated into a small, waterproof sensor box, designed to rest directly in the water system.  The sensor box incorporates a sampling port, miniature peristaltic pump, the microfluidic chip and holder, waste and reagent containment bags, and a control board for detection of the yellow reaction product.

This sensor box is controlled and powered by a base station, housed in a second waterproof box. The base station sits on the river bank or lake shore and contains a power supply, microprocessor and memory board, GSM modem for wireless communication and a connection to the sensor box.

Figure 3 shows the phosphate system during measurements in a creek running into the Black River near Newport, Co. Mayo. The base station can be seen sitting on the bank of the creek; the sensor box is submerged in the river.

Figure 3. The phosphate device during field tests near the Black River, Co. Mayo. The base station is visible on the bank; the sensor box is submerged in the creek. The sensor box contains a miniature peristaltic pump (A), sampling port (B), microfluidic chip and holder (C), detection system control board (D), and waste and reagent storage bags (E).

Publications

< to submission for preparation in Diamond, Dermot Stitzel, Shannon McGraw, Christina detection, phosphate system microfluidic>

Autonomous microfluidic system for phosphate detection, Christina McGraw, Shannon Stitzel, Dermot Diamond, In preparation for submission to Talanta

Dynamic surface pressure measurements on a square cylinder with pressure sensitive paint, Christina McGraw, James Bell, Gamal Khalil, James Callis, Accepted Exp. Fluids, August 2005

The phosphorescence microphone: A device for testing oxygen sensor and films, Christina McGraw, Hari Shroff, Gamal Khalil, James Callis, Rev. Sci. Instrum., 74 (2003) 5260-5266

Further Information

Contact Christina McGraw at +353 1 700 7602