Cavity-ringdown spectroscopy is an alternative way of measuring small fractional absorptions down to sub-ppm levels per pass in the cavity.In this chapter, the basic principles behind cavity-ringdown spectroscopy are discussed. The areas where ultra-sensitive absorption measurements in the gas phase are important are illustrated, and existing methods of making these measurements are presented. The experimental requirements for performing cavity-ringdown spectroscopy are discussed, and an expression for the detection limit in cavity-ringdown spectroscopy is derived from statistical considerations. Finally, future prospects for the technique are discussed.Cavity-ringdown spectroscopy is an alternative way of making ultra-sensitive absorption measurements, particularly with gas-phase samples. This volume will discuss the research being conducted on cavity-ringdown spectroscopy by the leading research groups in the area. In the chapters that follow, various aspects of the cavity-ringdown experiment will be discussed and its applications described. Although the origins of the technique can be traced back to attempts to measure mirror reflectivities in the early 1980s (1-4), the spectroscopic application of the concept has only recently been reviewed (5,6) and is still not widely known to the general scientific community. Because the technique can provide the ability to measure small fractional absorptions (down to sub-ppm levels per pass), it should be of interest to broad segments of the analytical-, physical-, and engineering communities. This includes analytical chemists, spectroscopists, physical chemists doing laser spectroscopy and molecular beam work, combustion scientists, atmospheric chemists, and chemical engineers involved with process measurements-in fact, anyone involved with measuring trace components in gas-phase samples by absorption spectroscopy.This chapter will provide a basic introduction to cavity-ringdown spectroscopy for the reader who is not familiar with the technique. The intent is to show the basic principles of the technique without introducing undue mathematical complexity at the outset. However, before describing the technique, it is worthwhile to review some of the areas where sensitive gas-phase absorption measurements are important.

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A novel spectrophotometric method for the determination of capsaicinoids in habanero pepper extracts is described that does not require prior analyte separation. The method uses partial-least-squares (PLS-1) multivariate regression modeling techniques in conjunction with ordinary UV absorption spectral data obtained on alcoholic extracts of habanero peppers (Capsicum chinese). The PLS-1 regression models were developed by correlating the known total concentration of the two major capsaicinoids (capsaicin and dihydrocapsaicin) in the extracts as determined by high-performance liquid chromatography with the spectral data. The regression models were subsequently validated with laboratory-prepared test sets. The validation studies revealed that the root-mean-square error of prediction varied from 4 to 8 ppm, based on the results obtained from models prepared from nine test sets. Once a regression model has been developed and validated, analyses of the extracts can be accomplished rapidly by ordinary spectrophotometric procedures without any prior separation steps.

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