Moving beyond Traditional UV−Visible Absorption Detection: Cavity Ring-Down Spectroscopy for HPLC

A rigid, monolithic but still scannable cavity ring-down spectroscopy cell

A novel cell for continuous wave cavity ring-down spectroscopy (cw-CRDS) is described and tested. The cell is monolithic and maintains a rigid alignment of the two cavity mirrors. Two high-resolution and high-force piezoelectric transducers are used to sweep the length of the cell by elastic deformation of the 2.86 cm outer diameter stainless steel tube that makes up the body of the cell. The cavity length is scanned more than 1/2 wavelength of the near-IR light used, which ensures that at least one TEM(00) mode of the cavity will pass through resonance with the laser. This allows the use of a frequency-locked-laser cw-CRDS technique, which increases the precision of the measurements compared to the alternative of sweeping the laser more than one free spectral range of the cavity. The performance of the cell is demonstrated by using it to detect the absorption spectrum of methane (CH(4)) at the wavenumber regions of around 6051.8-6057.7 cm(-1).

Polarized normal-incidence cavity ring-down spectroscopy: probing spiropyran photochromism in thin PMMA films and toluene

A normal-incidence geometry, polarization-resolved cavity ring-down spectroscopy technique (polarized NICRDS) is described to probe the polarized absorbance of surface-adsorbed thin films and short-path length liquid samples. The technique is demonstrated by a kinetic study of the photochromic behavior of the spiropyran dye 6,8-dibromo-1', 3'-dihydro-1', 3', 3'-trimethylspiro[2H-1-benzopyran-2, 2'-(2H)-indole]. The technique is shown potentially to have monolayer coverage sensitivity and can measure the angular orientation distribution of analyte molecules. The photochromic kinetics of 6,8-dibromoBIPS in toluene solution were qualitatively consistent with a previous study of this molecule using conventional absorption spectroscopy. The absorption polarizations and slow ring-closing kinetics measured in a thin poly(methyl methacrylate) film are consistent with a strong interaction of the spiropyran and merocyanine forms with the polymer matrix.

Absorption detection using optical waveguide cavities

Cavity ring-down spectroscopy is a spectroscopic method that uses a high quality optical cavity to amplify the optical loss due to the light absorption by a sample. In this presentation we highlight two applications of phase-shift cavity ring-down spectroscopy that are suited for absorption measurements in the condensed phase and make use of waveguide cavities. In the first application, a fiber loop is used as an optical cavity and the sample is introduced in a gap in the loop to allow absorption measurements of nanoliters of solution at the micromolar level. A second application involves silica microspheres as high finesse cavities. Information on the refractive index and absorption of a thin film of ethylene diamine on the surface of the microresonator is obtained simultaneously by the measurements of the wavelength shift of the cavity mode spectrum and the change in optical decay time, respectively.

Chemical sensing using fiber cavity ring-down spectroscopy

Waveguide-based cavity ring-down spectroscopy (CRD) can be used for quantitative measurements of chemical concentrations in small amounts of liquid, in gases or in films. The change in ring-down time can be correlated to analyte concentration when using fiber optic sensing elements that change their attenuation in dependence of either sample absorption or refractive index. Two types of fiber cavities, i.e., fiber loops and fiber strands containing reflective elements, are distinguished. Both types of cavities were coupled to a variety of chemical sensor elements, which are discussed and compared.

Liquid-phase and evanescent-wave cavity ring-down spectroscopy in analytical chemistry

Due to its simplicity, versatility, and straightforward interpretation into absolute concentrations, molecular absorbance detection is widely used in liquid-phase analytical chemistry. Because this method is inherently less sensitive than zero-background techniques such as fluorescence detection, alternative, more sensitive measurement principles are being explored. This review discusses one of these: cavity ring-down spectroscopy (CRDS). Advantages of this technique include its long measurement pathlength and its insensitivity to light-source-intensity fluctuations. CRDS is already a well-established technique in the gas phase, so we focus on two new modes: liquid-phase CRDS and evanescent-wave (EW)-CRDS. Applications of liquid-phase CRDS in analytical chemistry focus on improving the sensitivity of absorbance detection in liquid chromatography. Currently, EW-CRDS is still in early stages: It is used to study basic interactions between molecules and silica surfaces. However, in the future this method may be used to develop, for instance, biosensors with high specificity.

Evanescent-wave cavity ring-down spectroscopy for enhanced detection of surface binding under flow injection analysis conditions

The feasibility of liquid-phase evanescent-wave cavity ring-down spectroscopy (EW-CRDS) for surface-binding studies under flow-injection analysis (FIA) conditions is demonstrated. The EW-CRDS setup consists of an anti-reflection coated Dove prism inside a linear cavity (with standard or super-polishing of the total internal reflective (TIR) surface). A teflon spacer with an elliptical hole clamped on this surface acts as a 20 muL sized flow cell. The baseline noise of this system is of the order of 10(-4) absorbance units; the baseline remains stable over a prolonged time and the prism surface does not become contaminated during repeated injections of the reversibly adsorbing test dyes Crystal Violet (CV) and Direct Red 10 (DR10). At typical FIA or liquid chromatography (LC) flow rates, the system has sufficient specificity to discriminate between species with different surface affinities. For CV a much stronger decrease in ring-down time is observed than calculated based on its bulk concentration and the effective depth probed by the evanescent wave, indicating binding of this positively charged dye to the negatively charged prism surface. The amount of adsorption can be influenced by adjusting the flow rate or the eluent composition. At a flow rate of 0.5 mL/min, an enrichment factor of 60 was calculated for CV; for the poorly adsorbing dye DR10 it is 5. Super-polishing of the already polished TIR surface works counter-productively. The adsorbing dye Crystal Violet has a detection limit of 3 muM for the standard polished surface; less binding occurs on the super-polished surface and the detection limit is 5 muM. Possible applications of EW-CRDS for studying surface binding or the development of bio-assays are discussed.

Discrete sums for the rapid determination of exponential decay constants

Several computational methods are presented for the rapid extraction of decay time constants from discrete exponential data. Two methods are found to be comparably fast and highly accurate. They are corrected successive integration and a method involving the Fourier transform (FT) of the data and the application of an expression that does not assume continuous data. FT methods in the literature are found to introduce significant systematic error owing to the assumption that data are continuous. Corrected successive integration methods in the literature are correct, but we offer a more direct way of applying them which we call linear regression of the sum. We recommend the use of the latter over FT-based methods, as the FT methods are more affected by noise in the original data.

Detection of Separated Analytes in Subnanoliter Volumes Using Coaxial Thermal Lensing

A collinear-beam thermal lens detector has been constructed and its properties were characterized. Its application to the high-performance liquid chromatography (HPLC) separation of a mixture of five anthraquinone dyes dissolved in water shows a linear response over 3.5 orders of magnitude and a detection limit that is subnanomolar in the dye concentrations. These results are compared with those obtained previously using cavity ring-down spectroscopy (CRDS) in a Brewster's angle flow cell (Bechtel, K. L.; Zare, R. N.; Kachanov, A. A.; Sanders, S. S.; Paldus, B. A. Anal. Chem. 2005, 77, 1177-1182). The peak-to-peak baseline noise of the thermal lensing detection is 3.5 x 10(-8) absorbance units (AU) with a path length of 200 microm, whereas the peak-to-peak baseline noise of CRDS detection is approximately 2 x 10(-7) AU with a path length of 300 microm. Both of these figures of merit should be compared to the peak-to-peak baseline noise of one of the best commercial UV-vis HPLC detection systems, which is approximately 5 x 10(-6) AU with a path length of 10 mm (1-s integration time). Therefore, the thermal lensing technique has a demonstrated sensitivity of subnanomolar detection that is approximately 140 times better than that of the best commercial UV-vis detector and approximately 5 times better than that of CRDS.

Liquid-phase broadband cavity-enhanced absorption spectroscopy measurements in a 2 mm cuvette

A novel implementation of broadband cavity enhanced absorption spectroscopy (BBCEAS) has been used to perform sensitive visible wavelength measurements on liquid-phase solutions in a 2 mm cuvette placed at normal incidence to the cavity mirrors. The overall experimental methodology was simple, low cost, and similar to conventional ultraviolet-visible absorption spectroscopy. The cavity was formed by two concave high reflectivity mirrors. Three mirror sets with nominal reflectivities (R) of R = 0.99, 0.9945, and 0.999 were used. The light source consisted of a high intensity red, green, blue, or white LED. The detector was a compact charge-coupled device (CCD) spectrograph. Measurements were made on the representative analytes, Ho(3+), and the dyes brilliant blue-R, sudan black, and coumarin 334 in appropriate solvents. Cavity enhancement factors (CEF) of up to 104 passes for the high reflectivity mirrors were obtained. The number of passes was limited by relatively high scattering and absorption losses in the cavity, of approximately 1 x 10(-2) per pass. Measurements over a wide wavelength range (420-670 nm) were also obtained in a single experiment with the white LED and the R = 0.99 mirror set for Ho(3+) and sudan black. The sensitivity of the experimental setup could be determined by calculating the minimum detectable change in the absorption coefficient alpha(min). The values ranged from 5.1 x 10(-5) to 1.2 x 10(-3) cm(-1). The limit of detection (LOD) for the strong absorber brilliant blue-R was 620 pM. A linear dynamic range of measurements of concentration over about two orders of magnitude was demonstrated. The overall sensitivity of the experimental setup compared very favorably with previous generally more experimentally complex and expensive liquid-phase cavity studies. Possible improvements to the technique and its applicability as an analytical tool are discussed.

Cavity ring-down spectroscopy for detection in liquid chromatography at UV wavelengths using standard cuvettes in a normal incidence geometry

Liquid chromatography (LC) with cavity ring-down spectroscopy (CRDS) detection, using flow cuvettes (put under normal incidence inside the ring-down cavity), is demonstrated. Fresnel reflections are maintained within the capture range of a stable cavity of 4 cm length. This method circumvents the need for specific Brewster's angles and possible mirror degradation is avoided. The flow cuvettes are commercially available at low cost. At 355 nm (the frequency-tripled output of a Nd:YAG laser), the system surpasses the performance of conventional absorbance detectors; the baseline noise was 1.3 x 10(-5)AU and detection limits (injected concentrations) were between 40 and 80 nM for nitro-polyaromatic hydrocarbons with an extinction coefficient epsilon of 7.3-10.2 x 10(3)M(-1)cm(-1). The system was also tested at 273 nm, but in the deep UV the reflectivity of the currently best available mirrors (R>or=99.91%) is still too low to show a significant improvement as compared to conventional UV-vis detection.

Characterization of two types of silanol groups on fused-silica surfaces using evanescent-wave cavity ring-down spectroscopy

Evanescent-wave cavity ring-down spectroscopy has been applied to a planar fused-silica surface covered with crystal violet (CV+) cations to characterize the silanol groups indirectly. A radiation-polarization dependence of the adsorption isotherm of CV+ at the CH3CN/silica interface is measured and fit to a two-site Langmuir equation to determine the relative populations of two different types of isolated silanol groups. CV+ binding at type I sites yields a free energy of adsorption of -29.9 +/- 0.2 kJ/mol and a saturation surface density of (7.4 +/- 0.5) x 10(12) cm(-2), whereas the values of -17.9 +/- 0.4 kJ/mol and (3.1 +/- 0.4) x 10(13) cm(-2) are obtained for the type II sites. The CV+ cations, each with a planar area of approximately 120 A2, seem to be aligned randomly while lying over the SiO- type I sites, thereby suggesting that this type of site may be surrounded by a large empty surface area (>480 A2). In contrast, the CV+ cations on a type II sites are restricted with an average angle of approximately 40 degrees tilted off the surface normal, suggesting that the CV+ cations on these sites are grouped closely together. The average tilt angle increases with increasing concentration of crystal violet so that CV+ cations may be separated from each other to minimize the repulsion of nearby CV+ and SiOH sites.

Cavity ring-down spectroscopy for detection in liquid chromatography: extension to tunable sources and ultraviolet wavelengths

In earlier studies, it was demonstrated that the sensitivity of absorbance detection in liquid chromatography (LC) can be improved significantly by using cavity ring-down spectroscopy (CRDS). Thus far, CRDS experiments have been performed using visible laser light at fixed standard wavelengths, such as 532 nm. However, since by far most compounds of analytical interest absorb in the ultraviolet (UV), it is of utmost importance to develop UV-CRDS. In this study, as a first step towards the deep-UV region, LC separations with CRDS detection (using a previously described liquid-only cavity flow cell) at 457 and 355 nm are reported for standard mixtures of dyes and nitro-polyaromatic hydrocarbons (nitro-PAHs), respectively. For the measurements in the blue range a home-built optical parametric oscillator (OPO) system, tunable between 425 and 478 nm, was used, achieving a baseline noise of 2.7 x 10(-6) A.U. at 457 nm, improving upon the sensitivity of conventional absorbance detection (typically around 10(-4) A.U.). An enhancement of the sensitivity can be seen at 355 nm as well, but the improvement of the baseline noise (1.3 x 10(-5) A.U.) is much less pronounced. The sensitivity at 355 nm is limited by the quality of the UV-CRDS mirrors that are currently available: whereas the ring-down times as obtained at 457 nm are around 70-80 ns for the eluent, they are only 20-25 ns at 355 nm. Critical laser characteristics for LC-CRDS measurements, such as pulse length and mode structure, are given and prospects for going to shorter wavelengths are discussed.

Flowing Liquid-Sheet Jet for Cavity Ring-Down Absorption Measurements

The use of a flowing liquid-sheet jet in cavity ring-down absorption measurements is described. A mechanical gear pump was used to pump solvent at low pressure through a circular orifice. The resulting cylindrical jet of solvent was fired at a flat surface. A flat sheet of liquid was formed in a small portion of the resulting spray, which was sufficiently stable to be positioned at the Brewster angle in a linear ring-down cavity setup operated with a pulsed laser. The path length through a sheet-jet of ethylene glycol was measured to be 23.2 +/- 0.6 mum. Malachite Green dye was used as an analyte to demonstrate a linear dynamic range of 12.6 dB (73.9 nM to 1.34 microM). The limit of detection for the system was determined to be alpha LOD = 0.0162 cm(-1), or 71 nM (at epsilon = 9.975 x 10(4) M(-1) cm(-1), 628 nm, 3 sigma). The technique is shown to have promise for analytical and spectroscopic measurements, for example, in studies of gas-liquid interfaces.