Lens-coupled liquid core waveguide for ultraviolet-visible absorption spectroscopyTony Robles, David Paige, and Cort Anastasio Citation: Review of Scientific Instruments 77, 073103 (2006); doi: 10.1063/1.2219973 View online: http://dx.doi.org/10.1063/1.2219973 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/77/7?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Broadband ultraviolet-visible transient absorption spectroscopy in the nanosecond to microsecond time domainwith sub-nanosecond time resolution Rev. Sci. Instrum. 84, 073107 (2013); 10.1063/1.4812705 Ultraviolet-visible spectroscopy of graphene oxides AIP Advances 2, 032146 (2012); 10.1063/1.4747817 Measurement technique of electric field using ultraviolet/visible spectroscopy in cylindrical plasmas Rev. Sci. Instrum. 75, 4121 (2004); 10.1063/1.1789584 High-temperature and high-pressure cell for kinetic measurements of supercritical fluids reactions with the use ofultraviolet-visible spectroscopy Rev. Sci. Instrum. 74, 3073 (2003); 10.1063/1.1573747 A high pressure fiber-optic reactor with charge-coupled device array ultraviolet-visible spectrometer formonitoring chemical processes in supercritical fluids Rev. Sci. Instrum. 70, 4661 (1999); 10.1063/1.1150129

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Lens-coupled liquid core waveguide for ultraviolet-visibleabsorption spectroscopy

Tony RoblesDepartment of Land, Air, and Water Resources, University of California, Davis, California 95616

David PaigePaige Instruments, Woodland, California 95776

Cort Anastasioa�

Department of Land, Air, and Water Resources, University of California, Davis, California 95616

�Received 19 May 2005; accepted 12 June 2006; published online 18 July 2006�

We have developed and characterized a modular liquid core waveguide �LCW� assembly for use ina commercial spectrophotometer to measure very low light absorbance values for dilute aqueoussolutions in both the ultraviolet and visible wavelengths. The sample cell in this assembly iscomposed of a �1 m, pure Teflon® AF 2400 tube that is coupled to the spectrophotometer lightpath with a quartz lens and windows. Spectrophotometric absorbance measurements of test solutions�e.g., dilute sodium nitrate and hydrogen peroxide� produce clear and distinct spectra that exhibit alower limit of detection of 3 mAU between 240 and 640 nm. At absorbances above 10 mAU ourLCW system produces spectra from multiple independent injections that have a relative standarddeviation of less than 10% and average absorbances within 5% of calculated values based onpublished molar absorptivities. © 2006 American Institute of Physics. �DOI: 10.1063/1.2219973�

I. INTRODUCTION

In the 1960s research began on a spectrophotometric testcell that would use a longer path length to dramaticallyamplify absorbance signals1 while also maintaining smallsample volumes. This was achieved by using a liquid filledwaveguide, or as it has been called, a liquid core waveguide�LCW�. In the early years of this research, the materials cho-sen for the waveguide cladding �i.e., the tube� started withpolished metal or quartz. This approach provided long pathlength LCWs, but the high attenuation through the deviceseffectively counteracted any benefit of a path lengthincrease.1,2 Following the advent of the polymer Teflon® AF2400 by DuPont® in 1989, the research associated withLCWs took a significant leap forward. For the first time apolymer was made available that had a lower index of re-fraction �n=1.29� than that of water3 �n=1.33�. According toSnell’s law, total internal reflection of light requires that thecore of a waveguide �e.g., the aqueous solution or sample� beof a higher index of refraction than that of the cladding �e.g.,Teflon® AF 2400 tubing�. Teflon® AF 2400 not onlyachieved this requirement for the index of refraction, but italso possesses the ability to be molded or used as a film orcoating. The combination of these characteristics makes for acompact, long path length, low attenuation, and chemicallyresistive liquid waveguide.

Current commercial LCW absorbance spectrophotom-eters couple the illumination source and photodetector to theLCW with an optical fiber, which allows precise coupling tothe small inner diameters of the LCW tube.1–4 An alternative

scheme is to couple the LCW tube to the optical path usinglenses and windows, which has been used in a few applica-tions such as Raman spectroscopy and chromatographydetectors.5,6 Although this lens coupling has not been usedpreviously for absorbance spectrophotometers, the benefit isthat it would allow a liquid core waveguide assembly to beused with a commercial, open bay spectrophotometer thatnormally utilizes cuvettes.

The goal of our research was to develop a modular LCWassembly that �a� can measure light absorption of highly di-lute aqueous environmental samples in the UVA and UVB,�b� can be easily installed and removed from the test cham-ber of a commercial spectrophotometer, and �c� can be thor-oughly characterized. To achieve this goal we have created amodular, lens-coupled, LCW assembly that utilizes a pureTeflon® AF 2400 tube with a lens and window system thatefficiently focuses the incident light. We describe in this ar-ticle the components of this LCW system and characterize itsperformance using dilute solutions of nitrate and hydrogenperoxide, two common light absorbing chemicals in environ-mental samples.

II. SYSTEM

The custom LCW assembly that we have designed �Fig.1� allows for easy integration into the test chamber of aShimadzu UV-2501 PC ultraviolet/visible �UV/Vis� spectro-photometer, and could be easily modified to fit many otheropen bay spectrophotometers. Our spectrophotometer pos-sesses dual monochromators, a double-blazed holographicgrating, sample and reference beams, a photomultiplier de-tector, and two light sources �tungsten and deuterium�. Weset the bandwidth at the maximum value of 5 nm to allow for

a�Author to whom correspondence should be addressed; electronic mail:canastasio@ucdavis.edu

REVIEW OF SCIENTIFIC INSTRUMENTS 77, 073103 �2006�

0034-6748/2006/77�7�/073103/4/$23.00 © 2006 American Institute of Physics77, 073103-1

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the greatest light throughput. Bandwidth in our spectropho-tometer is selected via a Czerny-Turner configuration with apre- and postslit around a double-blazed holographic grating.

All components of our LCW assembly are mounted to analuminum baseplate that replaces the spectrophotometer cu-vette holder apparatus. The components of our LCW assem-bly capture the incident light from the spectrophotometersample beam by focusing the light with a UV optimized lens�Edmund Optics, 18.0 mm diameter, UV DCX lens, un-coated� into the lumen of the LCW tube. This light travelsthrough the LCW tube and is ultimately emitted from theoutlet of the LCW tube into the spectrophotometer detector.To provide balanced light throughput between the referenceand sample beams of the spectrophotometer, our LCW as-sembly utilizes an adjustable slit mounted to the baseplatealong the path of the reference beam. Prior to sample analy-sis, this slit is adjusted and permanently set to an optimalwidth.

The LCW tube within our assembly is held between twoidentical, PEEK t-adapters with Teflon® nuts and ferrules.The t-adapters �Fig. 2� possess a counterbore around the endof the tube to reduce the development and impact of en-trapped gas bubbles. At each end the LCW tube is physically,but not optically, separated from the open bay via a window

�Edmund Optics, 10.0 mm diameter, 1 mm thick, UV silicawindow, uncoated� within each t-adapter. While the inlet sideof the LCW assembly uses a lens to couple to the incidentlight, we found that a lens on the outlet is unnecessary. Foroptimal coupling of the incident light to the LCW we use anxyz micropositioner �Siskiyou Design Instruments, DT3-100�connected to the inlet t-adapter to center the LCW in the pathof the focused beam of light. Liquid samples are introducedinto the LCW tube via the t-adapter from an inlet tube. Toavoid contamination and sample incompatibilities from ma-terials such as metal, all of the parts in our LCW assemblythat are in contact with the flow path �inlet tubing, t-adapter,windows, and LCW� are made from PEEK, Teflon®, orfused silica.

The Teflon® AF 2400 LCW tubing that ourassembly uses is manufactured by Random Technologiesin San Francisco, CA, and has an outer diameter�o.d.� of 1/16 in. �1.59 mm�, a wall thickness of0.001 in. �0.03 mm�, and a length of 83.0±0.2 cm. Wefound similar performance with a slightly narrower tube�1 mm o.d., 0.1 mm wall, length of 117.0±0.2 cm� fromBiogeneral �San Diego, CA�. Care must be taken to not con-taminate the LCW tube �e.g., it should be handled only withlaboratory gloves� and to not kink the tube when coiling it ortightening the fittings. To fit the �1 m long tubing into thetest bay of our spectrophotometer, we coil the tubing intotwo loops. From Snell’s law the minimum radius of curva-ture of the loop is 1.94 in., which means that as long as thecoils extend to near the full width of our test bay �5.9 in.�,light will not be lost due to coiling.

III. OPERATION

We inject samples and record spectra using the followingfive steps: �1� flush the system with 2 ml of purified water�Milli-Q, �18 M� cm�, �2� inject 1.7 ml Milli-Q, �3� recordthe spectra of this Milli-Q as a baseline, �4� inject 1.7 ml ofsample solution, and �5� record the spectra from the sample.The Milli-Q spectrum recorded in step 3 is manually sub-tracted from the sample spectrum of step 5, making Milli-Qwater the baseline for each sample. Typically, we repeat thisprocess several times and average the resulting spectra for agiven sample. Between injections of different solutions �e.g.,between steps 3 and 4�, a small air bubble is sent through theLCW tube to remove the previous solution and prevent mix-ing and sample dilution.

IV. TEST AND PERFORMANCE

A. Detection limits

Based on standard methods7 we define the lower limit ofdetection �LLD� �often referred to as the default “detectionlimit” for a device� as three times the standard deviation �1��of the measured absorbance in a low absorbing sample.Similarly, the limit of quantitation �LOQ�, which is the pointabove which any measured absorbance is considered reliablyquantitative under “routine operating conditions,” is calcu-lated as ten times the standard deviation. The standard devia-tion ��� of absorbance from our system was determined fromtime dependent and wavelength dependent readings. While

FIG. 1. Schematic of the lens-coupled LCW adapter assembly: �1� spectro-photometer test bay, �2� LCW assembly mounting plate, �3� convex lens, �4�inlet t-adapter, �5� LCW fitting, �6� LCW tube, �7� outlet t-adapter, �8� xyzmicropositioner, and �9� reference beam shutter. All parts are to scale withrespect to the test chamber.

FIG. 2. LCW t-adapter. Components: �1� window, �2� threaded port for theinjection �inlet� or waste �outlet� fitting, �3� LCW tube, and �4� threaded portfor the LCW fitting.

073103-2 Robles, Paige, and Anastasio Rev. Sci. Instrum. 77, 073103 �2006�

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monitoring a solution of Milli-Q at 290 nm for 10 min, themeasured absorbance from our LCW assembly exhibited astandard deviation of 1 mAU. Within the range of our UVAand UVB testing, this wavelength provides the highest stan-dard deviation and is thus considered a conservative value.As shown in Fig. 3, multiple independent spectra of the samesample solution coincidentally produce a maximum standarddeviation of 1 mAU, as derived from the figure which showsa 3� of 3 mAU or less. Thus, based on these wavelength andtime dependent standard deviations our LLD is 3 mAU andour LOQ is 10 mAU.

B. Precision

We have used our LCW assembly to acquire more than500 spectra of purified water and dilute aqueous samples,and throughout this time the system has produced highlyrepeatable results. This precision of our device is illustratedby the relative standard deviations �RSDs� of the measuredabsorbance from independent injections of highly dilute so-dium nitrate �Fig. 3� and moderately concentrated hydrogenperoxide �Fig. 4�. RSD values from these tests are well be-low 10% when the measured absorbance is above our LOQof 10 mAU. Furthermore, the RSD is still reasonable �lessthan 20%� for absorbance readings as low as the LLD of theequipment. Although the RSD rapidly increases as the absor-bance approaches zero, the standard deviation remains rela-tively constant at all wavelengths, indicating that the systemis robust and highly repeatable.

C. Accuracy

A measure of accuracy for a spectrophotometric systemis whether the sample absorbance matches previouslypublished results, and whether absorbance is directlyproportional to species concentration �i.e., whether it followsBeer’s law�. To test for this with our LCW we took theabsorption spectra of low concentration sodium nitrate�25 �M–1 mM�, as shown in Fig. 5. The measured values�symbols� are compared with theoretical values �dashedlines�, which are calculated from Beer’s law using published

molar absorptivities.8 The average percent deviation �PD�between the measured absorbances �for all wavelengthswith values between 10 mAU and 3 AU� and the calculated�theoretical� values is less than 5%, where the PD= ��experimental−theoretical� / theoretical��100%. In addi-tion, our measured absorbance values are fully linear �R2

�0.99� between 10 mAU and 3 AU.Analogous measurements with moderate to high concen-

trations of hydrogen peroxide �300 �M–60 mM; data notshown� give similarly good results. Measured absorbancevalues were linearly related to concentration �R2�0.99� andagreed closely �average PD�5%� with calculated9 �theoret-ical� absorbances in the range of 10 mAU–3 AU. Finally, asa test of the accuracy of the LCW with dilute organic com-pounds in aqueous solution, we also measured the light ab-sorption of 400 �M acetone as a representative species. Theresulting spectrum from our LCW �data not shown� producesnearly an exact reproduction of the calculated absorbancein the ultraviolet, based on previously reported molar

FIG. 3. Absorbance spectrum of aqueous 25 �M sodium nitrate �solid line�,determined as the average of six independent injections, along with the 3�values �circles�. The dashed line represents the calculated �denoted as theo-retical� absorbance for 25 �M sodium nitrate using published molar absorp-tivities �Ref. 8� with Beer’s law. The �’s represent the relative standarddeviation of the measurements, with values above 50% not shown.

FIG. 4. Measured absorbance spectrum of 4.0 mM hydrogen peroxide inaqueous solution �solid line� determined as the average of six independentinjections. The dashed line represents the calculated �theoretical� absorbanceusing published molar absorptivities �Ref. 9� with Beer’s law. The �’s rep-resent the relative standard deviation of the measurements from the multipleinjections, with values above 50% not shown.

FIG. 5. Linearity in absorbance vs concentration of aqueous sodium nitratefrom 25 �M to 1 mM. The dashed lines represent calculated absorbancesusing published molar absorptivities �Ref. 8� with Beer’s law. For eachwavelength shown the experimental results fit a line with R2 greater than0.99, and deviate less than 5% from the theoretical absorbance.

073103-3 Lens-coupled liquid core waveguide Rev. Sci. Instrum. 77, 073103 �2006�

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absorptivities,10 in the range where the absorption measure-ments are above the LLD of the device.

D. Limitations of use

While the LCW system described here providesvery accurate and sensitive absorbance measurements of di-lute aqueous chromophores, it does have limitations whenused with highly volatile and/or hydrophobic organiccompounds.6 For example, we find that when we overfill theLCW with 45 �M aqueous benzene �as opposed to the in-jection procedure described in Sec. III�, the measured lightabsorbance of the solution increases due to adsorption ofbenzene to the LCW wall. Hence, this system is not suitedfor measuring hydrophobic organics using continuous flowfor sample delivery. In addition, we found that benzene out-gasses from this solution in the LCW tube. Although theinitial scans of our benzene solutions provided very accuratespectra when compared with literature values, the absorbancevalues decreased �e.g., �0.2 AU min−1 at the peak of thespectrum� when we took multiple scans on the same injec-tion. Despite these limitations, for most environmental aque-ous samples, e.g., very clean surface waters, slightly solubleand volatile species such as benzene will be present at muchlower concentrations and as such will not contribute signifi-cantly to light absorbance.

V. DISCUSSION

This new LCW system is useful for those who needexcellent UV/Vis performance for weakly absorbing solu-tions and/or a device that can be integrated into an existingopen bay UV/Vis spectrophotometer. Our assembly has anexceptionally low detection limit �3 mAU, equivalent to anoptical density of 36 �AU/cm� and provides absorbancereadings that are very precise and highly accurate. While thesystem cannot be used for all light absorption applications,6

it is very well suited for measuring light absorbance in diluteaqueous solutions such as surface waters and melted snowand ice. For those investigators who already have an openbay spectrophotometer, this new LCW assembly represents anovel means to dramatically boost the sensitivity of the de-vice while maintaining a high level of accuracy andprecision.

ACKNOWLEDGMENTS

The authors thank the following people for assistance:Dr. Ilia Koev �Biogeneral�, John Imbalzano �DuPont�, TomMorris �Ocean Optics�, Heidi Habhegger �World PrecisionInstruments, Inc.�, Dr. Alan Jackman �UC Davis�, and Dr.Amos Gottlieb �Random Technologies�. This work was madepossible by funding from the Office of Polar Programs at theNational Science Foundation �Grant No. ANT-0230288�.

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