Thirty Years of Cometary Spectroscopy from McDonald Observatory
  We report on the results of a spectroscopic survey of 130 comets that was conducted at McDonald observatory from 1980 through 2008. Some of the comets were observed on only one night, while others were observed repeatedly. For 20 of these comets, no molecules were detected. For the remaining 110 comets, some emission from CN, OH, NH, C$_{3}$, C$_{2}$, CH, and NH$_{2}$ molecules were observed on at least one occasion. We converted the observed molecular column densities to production rates using a Haser (1957) model. We defined a restricted data set of comets that had at least 3 nights of observations. The restricted data set consists of 59 comets. We used ratios of production rates to study the trends in the data. We find two classes of comets: typical and carbon-chain depleted comets. Using a very strict definition of depleted comets, requiring C$_{2}$ \underline{and} C$_{3}$ to both be depleted, we find 9% of our restricted data set comets to be depleted. Using a more relaxed definition that requires only C$_{2}$ to be below a threshold (similar to other researchers), we find 25% of the comets are depleted. Two-thirds of the depleted comets are Jupiter Family comets, while one-third are Long Period comets. 37% of the Jupiter Family comets are depleted, while 18.5% of the Long Period comets are depleted. We compare our results with other studies and find good agreement. 
Anita L. Cochran, Edwin S. Barker and Candace L. Gray
astro-ph.EP