ARTICLE pubs.acs.org/ac
Simultaneous and Continuous Multiple Wavelength Absorption Spectroscopy on Nanoliter Volumes Based on Frequency-Division Multiplexing Fiber-Loop Cavity Ring-Down Spectroscopy Helen Waechter,* Dorit Munzke, Angela Jang, and Hans-Peter Loock* Department of Chemistry, Queen’s University, Kingston, ON, K7L 3N6, Canada
bS Supporting Information ABSTRACT: We demonstrate a method for measuring optical loss simultaneously at multiple wavelengths with cavity ring-down spectroscopy (CRD). Phase-shift CRD spectroscopy is used to obtain the absorption of a sample from the phase lag of intensity modulated light that is entering and exiting an optical cavity. We performed dual-wavelength detection by using two different laser light sources and frequencydivision multiplexing. Each wavelength is modulated at a separate frequency, and a broadband detector records the total signal. This signal is then demodulated by lockin amplifiers at the corresponding two frequencies allowing us to obtain the phase-shift and therefore the optical loss at several wavelengths simultaneously without the use of a dispersive element. In applying this method to fiber-loop cavity ring-down spectroscopy, we achieve detection at low micromolar concentrations in a 100 nL liquid volume. Measurements at two wavelengths (405 and 810 nm) were performed simultaneously on two dyes each absorbing at mainly one of the wavelengths. The respective concentrations could be quantified independently in pure samples as well as in mixtures. No crosstalk between the two channels was observed, and a minimal detectable absorbance of 0.02 cm-1 was achieved at 405 nm.
C
avity ring-down (CRD) spectroscopy is an established method for very sensitive absorption spectroscopic measurements. While it is commonly used for gaseous samples, i.e. either for trace gas analysis or for weak absorption lines, there have been numerous reports by us and others that demonstrate liquid absorption measurements.1-3 Extremely sensitive measurements on liquid samples can be performed by filling a “conventional” cavity with liquid sample,4,5 but for many applications in analytical spectroscopy, a smaller sample volume is preferred. A cavity of microliter dimensions may be made by reducing the distance between the mirrors to millimeters.6-8 Alternatively, one can insert either a cuvette or flow cell9-13 or a liquid film14,15 into a larger two-mirror cavity. Another possibility is to use total internal reflection as a third cavity mirror and probe the sample with the evanescent wave.16-20 Even smaller volumes of much less than 1 μL can be interrogated by using a fiber optic waveguide as the cavity medium.1,21,22 We have demonstrated such measurements in several versions at various wavelengths and with different fluid delivery designs.1,21-28 To take full advantage of the very broad wavelength range that is accessible with silica waveguide cavities (about 2501700 nm), we now extend the fiber-loop CRDS technique to simultaneous and continuous dual-wavelength (or multiwavelength) detection. In phase-shift CRDS, the ring-down time is obtained from the phase-shift between intensity modulated light entering and exiting the cavity. When using multiple light sources that are modulated at different frequencies, one can distinguish the wavelengths by demultiplexing the output of the single broadband photodetector. r 2011 American Chemical Society
Related methods of broadband CRD spectroscopy include time-division multiplexing of ring-down signals29 and Fourier transform CRDS,30-33 as well as cavity enhanced absorption spectroscopy (CEA) with a wavelength dispersive element.3,34,35 Yet, this is—to the best of our knowledge—the first time that multiwavelength CRD spectroscopy is conducted with quasicontinuous light sources and without a dispersive element or interferometer.
’ THEORY OF PHASE-SHIFT CAVITY RING-DOWN AND DUAL-WAVELENGTH DETECTION Cavity Ring-Down Spectroscopy. CRD spectroscopy using fiber loops is very similar to mirror-based CRDS, i.e. the measured ring-down time describes the losses of the fiber-loop cavity containing the sample. While the loss term of interest is the absorption due to the sample, in a fiber cavity there are additional losses due to absorption in the optical fiber, Rfiber, and losses at splices and at the sample gap. The ring-down time of a fiber loop with length L, sample gap width d, and round trip time tRT is given by
τ¼
tRT nL ¼ cð-lnðTgap, splice Þ þ Rfiber L þ C~ε dÞ losses
ð1Þ
The term -ln(Tgap,splice) describes the losses per roundtrip due to the sample gap and fiber splices, C is the concentration Received: December 17, 2010 Accepted: February 3, 2011 Published: February 28, 2011 2719
dx.doi.org/10.1021/ac103210w | Anal. Chem. 2011, 83, 2719–2725
Analytical Chemistry
ARTICLE
of the sample, ~ε is the molar extinction coefficient of the sample based on the natural logarithm (related to the decadic extinction coefficient by ~ε = ε ln(10)), c is the vacuum speed of light, and n is the effective refractive index of the propagating modes. There are several ways of determining the ring-down time. For example, in pulsed or in continuous wave (cw) CRD spectroscopy, the decay of the light intensity is monitored as a function of time. In phase-shift CRD spectroscopy, the intensity of the laser is modulated sinusoidally and the cavity emits light that is phase-shifted due to the lifetime of the photons in the cavity, i.e. the ring-down time.36,37 The ring-down time can then be obtained by measuring the phase-shift Δφ between the light entering and exiting the cavity at modulation frequency ω = 2πν:37 tanðΔφÞ ¼ - ωτ
ð2Þ
Therefore, for a single-exponential decay there is a linear relationship between the ring-down time τ, the tangent of the phase-shift Δφ and the modulation frequency ω. For multiexponential decays, the relationship between tan Δφ and τ is no longer linear:36 N
tanðΔφÞ ¼ - ω
Rj τ j ∑ 2 ω τ2þ1 j¼1 2
j
N
Rj τ j 2 j ¼ 1 ω τj 2 þ 1
∑
ð3Þ
The ring-down times can nevertheless be determined by measuring the phase-shift at several modulation frequencies. In optical fibers, biexponential or triexponential decays are observed frequently, since the light is traveling not only in the core but also in the cladding and coating. Other methods of CRD spectroscopy have been described in a series of recent reviews.3 Dual-Wavelength Detection by Frequency-Division Multiplexing. To achieve simultaneous multiwavelength detection without a dispersive element, we combined phase-shift CRD spectroscopy with frequency-division multiplexing. As a demonstration, light at two different wavelengths from two lasers sources was coupled into a fiber cavity, and each is intensity modulated at a distinct frequency, ω. A single broadband detector records the wavelengths simultaneously and the output is directed to two separate lock-in amplifiers which demodulate the total signal at their respective modulation frequency to extract the phase information. The phase-shift between input and output contains the information about the ring-down time at the specific wavelength. The present method may be compared to the frequency modulation multiplexing technique for wavelength-modulation spectroscopy, suggested first by Oh et al.38 and also applied later by others.39-42 These groups have modulated a number of light sources simultaneously at different frequencies to perform wavelength-modulation spectroscopy at multiple wavelengths. A single, broadband detector recorded the total intensity of the three-or-more light sources after a single pass through the sample gas. The signal was demultiplexed by sending the detector output to three-or-more mixers, which were each referenced to one of the modulation frequencies. In these previous cases, the wavelength of each laser is modulated at a different frequency but their intensity is constant, whereas in our
Figure 1. Setup of the experiment. Each laser diode (LD) is modulated at a different modulation frequency ωi with an arbitrary function generator (AFG). The broadband detector (PMT) measures both wavelengths simultaneously. By demodulating the signal at the modulation frequencies, the corresponding phase-shift Δφi can be obtained for each wavelength. The fiber ends are mounted in the grooves of the fiber-liquid interface. The delivery fibers shine light on the loop fiber end to couple light into the loop. The liquid sample enters the interface through the hole in the bottom plate in front of the loop fiber, flows along the grooves, and exits the interface through the two holes in the top plate in front of the delivery fibers.
case the intensity is modulated but the wavelength of each laser is constant. Crosstalk in Multiwavelength Phase-Shift CRD Detection. An important point when measuring several wavelengths simultaneously is the amount of crosstalk between the different channels. For example a quick change in the modulation amplitude A(t) creates new frequencies, which might lead to crosstalk. In our case the amplitude change is so slow that these additional frequencies cannot overlap with the modulation frequency of the other laser. Also a phase change φ(t) caused, for example, by a sudden appearance of an analyte creates new frequencies. Again, the change is on the time scale of seconds and too slow for crosstalk to occur. In our case the modulation frequencies are separated by a few hundred kilohertz, whereas phase changes and amplitude changes give a frequency change in the millihertz range. The most important source for crosstalk is due to imperfect electronics. If the modulation of the laser intensity is not a pure sine wave but also contains other frequencies, a component of the signal may be observable at the modulation frequency of the other laser. We observe only very small contributions (
