Student Research: Jo Ann Johnson

, , 1997
Faculty Advisor: David A Kalman

A Cryogenic Technique Using Differential Temperature for Sampling Volatile Organic Compounds in Air


Abstract

Breath analysis provides a non-invasive method for exposure assessment of volatile organic chemicals (VOC's). Low concentrations of VOC's in exhaled breath require concentration prior to analysis, typically by use of a sorbent or cryogen. Tenax, the most common sorbent, requires thermal desorption for analysis. Both thermal desorption and cyrotapping do not allow repeat analysis, and also require long analysis times. I developed a method to concentrate breath samples by cryogenic condensation followed by distillation of atmospheric gases and dissolution of the residual frozen VOC's in a solvent vehicle. This form for the final sample allowed full use of automated instrumental analysis, required half to one-third as much instrument time as does thermal desorption, and permitted repeated or multiple assays from a single breath sample.

My design approach was: 1) literature review, 2) design of a series of prototypes fabricated in glass or copper, 3) limited recovery testing, 4) development of final design configuration, 5) fabrication of final design, and 6) performance evaluation with a select group of aromatic hydrocarbons.

Analysis of breath began with collection of a sample in a Tedlar bag. The Tedlar bag was then connected to the condensing unit. The vacuum, created by cooling the condensing unit with liquid nitrogen (-196°C), pulled the sample into the unit as the air condensed. The unit was then transferred to liquid argon (-186°C) which allowed the nitrogen and oxygen from the sample to boil off. These gases were vented to the ambient air. Organics remained frozen on the inner surfaces of the unit. The unit was allowed to warm to ambient temperature before 2 ml of acetone were added to rinse the organics from the inside walls and then to collect them in the devices. The acetone solution was analyzed by CG/MS.

Prior to the development of the final design and method, several alternatives were evaluated. Initial attempts with glass collection reservoirs failed because of poor thermal conductivity of glass. Metal reservoirs, either copper or stainless steel, allowed a slow condensation of the sample. Addition of a copper helical coil (3 ½' x ¼ O.D.) greatly reduced the condensation time.

Reduced pressure distillation of the liquid oxygen and nitrogen caused erratic and uncontrollable losses of organics. Boiling away the air at liquid argon temperatures allowed successful recovery of organics after a secondary venting path was created. An early design vented the volatilized air back through the entire condensing unit and caused loss of residual organics trapped in the condensing unit. Addition of a vent tube between the collection reservoir and the condensing unit solved this problem.

This design was a prototype for four stainless steel devices which were constructed and evaluated in a series of experiments. Results of these experiments showed that the precision and accuracy of this technique were reasonable and that this technique reliably assessed the concentration of VOC's in air samples.