Persistent Organic Pollutants (POPs) in Alaska. New GC-MS Experiments and Experiences for College and Pre-College Students
 

America's Arctic University
Dept of Chemistry & Biochemistry

Organic chemistry laboratory (Chem 324) 
Mass spectral analysis of fragrance molecules using Head Space-Solid Phase Micro Extraction (HS-SPME) 
John Keller and Tom Clausen

This experiment was introduced in two sections (Keller TR, and Green WF) of Organic Chemistry Lab (Chem 324) during the Spring 2008 semester. To see Keller's student handout click here. To see Keller's grading rubric, which is handed out before the reports are due so that students have some notion about how the reports will be graded and what to make sure they incorporate in the reports, click here.

Keller's section (see course homepage here) had 10 students (the maximum size due to space limitations in the laboratory) working in five pairs. This year the students investigated samples of cheese, spice tea, orange peel vs orange juice, and others. Some students had trouble getting enough sample in terms of the number and size of GC-MS peaks. It would appear that in the future we should use longer absorption times and higher sample temperatures to get more accurate head space sampling of these materials. (If students do preliminary research in the literature, the articles should tell which HS-SPME conditions were used to obtain a given GC-MS trace.)

The HS-SPME experiments this semester were carried out using our currently installed Hewlett-Packard GC-MS and a manual SPME fiber holder. In future years, the HS-SPME sampling process can be automated using the CombiPal injection robot on the new Agilent GC-MS. To see this instrument and the various sample introduction features, go back to the project homepage and follow the link at the bottom to the CombiPal Injector.

Computational chemistry. The additional features in this experiment, which are new compared to several of already published student SPME experiments, are the suggestions that students (a) make a detailed analysis of several fragmentation reaction mechanisms, and (b) carry out quantum chemical calculations on the molecular ion radical cations. The student handout gives sample analyses of these activities for the phenethyl alcohol radical cation. One could question the recommended approach of using a restricted Hartree-Fock (RHF) method, whereas the unrestricted method (UHF) may be more appropriate for an odd electron species. However, both methods give essentially the same results at least for this ion. This issue is addressed in more detail in .pdf document here.

Assessment. The proposed assessment tool for this experiment will be how students score on a mass spectra fragmentation question that is part of the current ACS organic chemistry examination. We give this test each semester as the final exam in Chem 322, the second-semester organic chemistry lecture course. We have plenty of data from previous years that suggests our students have done relatively poorly on this question. Hopefully, the students who carry out this SPME experiment will do better on that question than students have in the past: we will see!