Article pubs.acs.org/ac
Absorption Measurements in Liquid Core Waveguides Using Cavity Ring-Down Spectroscopy Klaus Bescherer, Jack A. Barnes, and Hans-Peter Loock* Department of Chemistry, Queen’s University, Kingston, ON, K7L 3N6, Canada ABSTRACT: Short liquid core waveguides (LCWs) were included into a fiber-loop cavity ring-down absorption spectrometer to reduce the detection limit over, both, single pass absorption in a LCW and cavityenhanced absorption using a conventional fiber-loop cavity. LCWs of 5 and 10 cm length were interfaced with a pressure-flow system and a multimode fiber-loop cavity using concave fiber lenses with matching numerical apertures and diameters. Two red dyes, Allura Red AC and Congo Red, were detected with a 532 nm pulsed laser at a 5 nM limit of detection in a detection volume of less than 1 μL, corresponding to a minimal detectable absorbance of less than 4 × 10−4 cm−1 and a minimal detectable change in absorption cross section, σmin = Vdet × ε × CLOD, of about 14 μm2 (Allura Red AC) and 37 μm2 (Congo Red).
T
consisting of two mirrors, the transmission term, T, is usually given by reflectivities of the two mirrors, i.e. ln(T) = 2 ln(R), where R is the average reflectivity of its mirrors. With known absorption or scattering cross sections of the analytes, εi, one can then determine the concentration, Ci. Here, d is the absorption length through the sample, which may be identical to the entire length of the cavity, L. The application of cavity ring-down spectroscopy to liquids has been the subject of a recent and thorough review.2 For example, van der Sneppen et al. demonstrated a small ring-down cavity in which mirrors were only spaced about 2 mm apart. Using a laser at 532 nm (5 ns pulse width), they were able to measure ring-down times of around 65−75 ns in a 12 μL detection volume. They determined absorption losses of crystal violet at 2.5 nM concentrations under static conditions3 and a variety of azo-dyes at 15−20 nM concentration following chromatographic separation.2 Their setup may be understood as a miniaturized version of an earlier experiment by Zare and co-workers in which a much larger cavity was filled with solution.4,5 Other systems fill only part of the cavity with liquid analyte, for example, by the use of a liquid flow cell5 or liquid sheet.6 Already in 2003, Xu et al. measured the fifth overtone of a vibration of neat benzene in a 1 cm Brewster-angled cuvette in a 48 cm long cavity with a minimal detectable absorbance of αLOD = ε CLOD = 2 × 10−5 cm−1 to 5 × 10−5 cm−1.7 Similar experiments by Islam et al., but using an LED at 630 nm and a cuvette at normal incidence, demonstrated a detection limit for Brilliant Blue-R dye below 1 nM which corresponds to αLOD = 5 × 10−5 cm−1.8 Our own contributions have focused on the use of fiber-optic ring cavities, where the sample is injected
he recent development in microanalytical absorption detectors was driven to a large extent by a demand of the pharmaceutical and health industries to detect and identify analytes at concentrations of μM or less in liquid samples, which have a volume of typically less than 1 μL. In absolute numbers, one therefore requires the detection of 10−12 mol or about 1012 molecules and frequently much less. Fluorescence detection is the method of choice for most microanalytical detectors, but in many applications, it is important that the analyte remains unlabeled and fluorescence detection of weakly fluorescing analytes is then impractical. Detection of less than one picomole of analytes can been realized by a variety of techniques, of course, but optical absorption spectroscopy stands out because of its simplicity, low cost, and compatibility with existing microseparation equipment.1 Unfortunately, the sensitivity of single-pass absorption spectroscopy is not very high and requires the detection of a small intensity variation on top of a large background intensity. Our group and others have therefore developed variants of cavity-enhanced spectroscopy that measure the optical loss in a high-finesse optical cavity. The most common method, cavity ring-down (CRD) spectroscopy, does not require the long absorption paths and the stable light sources that are essential in single-pass absorption spectroscopy. In CRD spectroscopy, light from a pulsed light source is injected into a high-finesse optical cavity and the ring-down time is extracted from the exponential decay of the pulses emitted from the cavity. The ring-down time τ=
nL c0(αcavityL + ∑i εiC id − ln T )
(1)
depends on the attenuation coefficient of the cavity medium, α, and its refractive index, n, the length of the cavity, L, and the vacuum speed of light, c0. In a cavity containing a gas and © 2013 American Chemical Society
Received: October 2, 2012 Accepted: March 12, 2013 Published: March 12, 2013 4328
dx.doi.org/10.1021/ac4007073 | Anal. Chem. 2013, 85, 4328−4334
Analytical Chemistry
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between the ends of a multimode fiber loop.9 For this technique, the term ln(T) in eq 1 is the transmission through the sample path in the absence of analyte, and d is the distance between the fiber ends. Reviews of the method and its different applications in chemical, thermal, and mechanical sensing have been given earlier.2,10−12 In our previous work, the detection volumes were between 275 pL11 and 100 nL;13 i.e., they were containing liquid volumes considerably smaller compared to what could be achieved with mirror-based cavities, and they were comparable to recent fiber ring-down studies by Vallance and co-workers, who used fiber-loop gaps between 20 pL14 and 19 nL.15 The detection limits Vallance’s group achieved were αLOD = 0.001 cm−1 (210 nM potassium permanganate), which was a considerable improvement over our previous results (αLOD = 0.02 cm−1 or 900 nM tartrazine dye). The molar limits of detection tend to be somewhat higher for fiber-loop cavities compared to CRD experiments with mirrors, because the cavity roundtrip losses are higher. However, when comparing the absolute number of detected molecules, the molecular detection limits are comparable or superior to conventional cavities, since the detection volumes are smaller by several orders of magnitude. Note that chemical detectors using fiber-cavity ring-down methods with either tapers,16 field access blocks,17 etched fibers,18 or long period gratings19,20 usually require sample volumes that are at least several tens of microliters, since the analyte solution has to be spread over the entire active region and only a small fraction of the propagated modes interacts with the sample. In a related effort to miniaturize absorption detectors, very low detection limits can be realized simply by extending the length of the absorption path. A recent review by Pena-Pereira et al. provides a thorough survey of the different technologies.1 Of particular relevance for the present study are liquid core waveguides (LCW), which have been developed for a variety of applications in, e.g., environmental analysis and characterization of biological and medicinal samples.21,22 Commercial instruments incorporating LCWs are now available. Since LCWs direct the entire light beam through the sample by total internal reflection, the sample liquid has to have a higher refractive index than the walls of the surrounding material, typically, a capillary tube. While glass capillary tubes can be used with some solvents, such as dimethylsulfoxide (DMSO) and toluene, only very few materials, such as low-density Teflon AF 2400, are suitable for use with water as a solvent. Notably, Bragg fibers23 and hollow core photonic crystal fibers,24 which guide light due to a photonic bandgap, do not require that the solvent has a higher refractive index and may be used with water. In this report, we attempt to combine the advantages of fiber-cavity ring-down spectroscopy with the enhanced absorption path that liquid core waveguides can provide. We inserted a liquid core waveguide made from a silica capillary into a multimode fiber loop and thereby enhanced the interaction path from typically d = 10−100 μm to d = 50 mm. As one might expect, we found that the detection limit improves accordingly, whereas the absolute amount (in moles) of detectable analyte is comparable to that of smaller gaps. By incorporating a liquid core waveguide into a fiber-loop ringdown spectroscopy setup, we demonstrate a viable compromise between low detection limits (nanomolar concentration) and small detection volumes (
