Liquid-phase analytical applications of UV-visible broadband cavity-enhanced absorption spectroscopy (BBCEAS)

Absorption spectroscopy can be considered to be one of the most useful and also most widely used analytical techniques. One of the reasons for its usefulness and wide use is its experimental simplicity; a typical measurement requires just the intensity of a light source to be recorded with and without a sample. It is also a universal technique in that all analytes absorb somewhere in the electromagnetic spectrum and, further, it is an absolute technique, as an absorption measurement can be used directly to determine a fundamental property of an analyte such as an absorption wavelength-specific molar extinction coefficient, el, through the application of the well-known Beer–Lambert law; UV-23_2-Eq1 where I0(l) is the intensity of light reaching the detector in the absence of a sample and I(l) is the intensity transmitted through the sample at a given wavelength, l, C is the concentration of the sample and l is the path length of the sample. Knowledge of el and l allow absorption spectroscopy to be used in a quantitative manner, whilst the wavelength of absorption can be used qualitatively to identify an analyte or functional groups in an analyte. One disadvantage of conventional absorption spectroscopy is its relatively poor sensitivity when compared to other spectroscopic techniques such as fluorescence, which is, typically, more than 103 times more sensitive. This is a consequence of the manner of measurement. Whilst a fluorescence measurement is, typically, made relative to a “zero background”, a small absorption requires accurate discrimination between two large signals. An analogy attributed to the eminent Stanford spectroscopist, Professor R.N. Zare, describes absorption spectroscopy as being similar to determining the weight of a ship’s captain by first measuring the weight of the ship with the captain on board and subtracting from this the weight of the ship alone. From the Beer–Lambert law, it is seen that the absorbance, and thus the sensitivity of measurement, can be improved by increasing the path length. Conventionally, this has been achieved by using either a long physical path length, as in atmospheric measurements such as differential optical absorption spectroscopy (DOAS), where path lengths of several kilometres are used, or by using multi-pass cells such as a White cell or a Herriot cell. In these, segmented gold mirrors are used to multi-pass light, typically 20 times through a cell of a volume >1 L. However, in many experimental set-ups, long physical path lengths are not possible and even the volume of a multi-pass cell is too large.