Multi-Sampling Device for DR/4000 Spectrophotometer
Senior Capstone Project
Terina A. Rollins
Capstone Advisor: Professor Tom Christensen
Course Instructor: Professor Darrell Donahue
Designed for: Ken Johnson, Senator George J. Mitchell Center

Executive Summary
Mr. Ken Johnson works for the Senator George J. Mitchell Center on the University of Maine campus. The Center works closely with the Environmental Protection Agency (EPA) on acid rain monitoring. To aid in this process, Mr. Johnson had recently purchased a DR/4000 Spectrophotometer. The instrument utilized visible light to perform total and ortho-phosphorus analysis of surface water samples which come in test tubes. Currently, each sample must be loaded and discarded manually. The sample cell must be rinsed between each sample with de-ionized water. The cell is extremely fragile and costs approximately $400. Since the Center analyzes thousands of samples a year, Mr. Johnson wanted a system designed to make the process more automated and eliminate unnecessary handling of the cell.

The solution was to design an automated system to manually load and discard the sample, as well as flush the cell between each sample. This was accomplished the following way:

1. 1. A test tube holder was designed our to UHMW(ultra-high molecular weight polyethylene). A stepper motor and timing belt drive cause the holder to rotate about a stainless steel rod, slowly pouring the sample into the cell. A fiber optic sensor detects the fill level. A limit switch in the bottom of the holder determines whether a test tube is present.
2. The cell moves in and out on a stainless steel carriage. The carriage has rollers and rides on a rail. The cell is secured in place by rubber backed UHMW fasteners. A stepper motor and timing belt drive are attached to a ball screw. A nut rides on the ball screw, causing linear motion.
3. The sample is discarded by rotation of the above (number 2) system. Rotation is caused by a stepper motor and timing belt drive. Waste is caught in a small container and discarded through a tube into the sink.
4. Rinsing occurs while the cell is upside down with the usage of a peristaltic pump. The tubing runs from a provided de-ionized water container, through the pump, and is directed upward into the cell.
5. The system is automated by the usage of a PLC (programmable logic controller).

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