Detectirw chlorooreanirc in soundwater U
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Fiber-optic-based sensors permit in situ analysis
By Fred I! M i b v i c h
Optical fibers, chemistry, lasers, and fluorescence spectroscopy have been d ito form remote fiber flue rimetry 0. The key to RFF is the use of fiber-optic chemical sensors (F0CSs)-optical fibers with small chemical sensors attached to their distal ends. Because each FOCS has a chemistry of its own, its sensitivity and specificity to a compound or class of com-
poundsareoptimized. The RFF system is ideal for in situ detection and quantification of very low levels of contamination. It has been used to measure volatile chlomrganics in groundwater and the vadose zone. This new technology not only ofkrs an inexpensive alternative to m v e n tional sampling technology and insmments but also provides an analytical capabiity where traditional system have not been practical or feasible. In situ RFF measurements solve the difficult Droblems associated with sam~lina and klyzing trace.quantities of volatile chlomorganicssuch as chloroform. The integrity of the resultant data permits quantitative measuremeot. The abiiity to do RFF depends on the use of low-loss optical fibers for transmitting visible light; the availability of these fibers originally gave rise to fiber-optic sensors. The first sensors, designed to collect information from remote locations, relied on alterations in a specific physical property of the medium being probed. Such alterations cause a predictable change in the optical fiber’s transmission properties. The applicationsand versatility of r e mote f i b seslsors increased greatly with the addition of POcSs and optical sptmscopy to the remoteaetection scheme. The technique is no longer restricted to measuring physical properties but has been extended to organic and inorganic chemical analysis and
other physical measurements. The key components of the RFF system are the spectrometer, an optical coupler, and the FOCS (Figure 1). Here, excitation light of the appropriate wavelength is focused into singlestrand optical fibers. The use of single fibers greatly simplifiesthe optical system and eliminates alignment problems. The optical fiber transmits this excitation to the sampling region and to the sensor. The interaction of the FOCS with the m e t molecule mults in chanaes in flu&e.wnce. A small amount Gf chis fluorescentli,@t is collected by the o p tical fiber and returned to an optical coupler that has been specifically designed to separate the excitation light fromthe returning fluorescent light. Finally, he fluorescence is directed info a spectmmeter for specVal analysis. The intensity of signal in a specific wave-
length band (or bands) is related to the identification and quantification of the target molecule. One of the first FOCSs developed was used both for early-warning and for long-term monitoring of chlomganics. The chemical basis for this FOCS is a modified Fujiwara reaction (1). The FOCS, in conjunction with a field fluorimeter, has been used to take in Situ measuremts from a chlomform-contaminated well. Preliminary data indicate good agreement between the FOCS data and indeuendent eas chromatography analysis df samples: The work of Fujiwara demonstrates that the absorbance of basic pyridine changes when it is exposed to various chlomrganics. This change in absorb ance is caused by the formation of a chromophore and is a quantitative measure of the presence of these compounds. In the chlomrganic FOCS,
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Enviran. Sci. Techd.,
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m g a n i c FOCS at 600 n m a n d p a s a through the dichroic mirror into the signal channel. Here it is purified by a narrow-band fdter and deteaed by a photodiode. Provisions are made for measuring and displayingthe individual reference and signal channel output, as well as the ratio of the two. The RFF systemis6Ocm x 36cm x 15cmand weighs ll kg without batteries. RFF systems now under development include a 2-kg, httery-opexated unit that measures 3 cm x 6 cm x 9 cm and a s i n g l e d model that wntains the RFF s-meter, which fiLq into a small computer.
In the Reld
fluorescence of the chromophore is usedtodemmlne ' how much absorbimg product is formed. Thus, fluoresceace intensities can be directly related to concentrationsof chlomrganics. There are two types of FOCSs for chlomganics: A two-phase system uses pyridine and 10-11 M potassium hydroxide, and a single-phase FOCS uses pyridine and tetra(n-propyl)ammonium hydroxide. For most monitoring the reaction chemistry is pmtected from the aqueous matrix by a membrane that allows volatile chloroorganics to pass thmugh but is imperviow to water. This memhram gives a lifetime of months to the sensor. Without the membrane the lifetime of the FOCS is a matter of hours.
minator and sent to a fiber switch, which directs the light to an optical splitter. An electronic shutter is used to control analysis time, greatly redwing or eliminating photo bleaching of the iluorophore. With the chlomorganic FOCS, the light entering the optical splitter is fdtered so that only the green light passes through. This is then divided into two beams by a dichroic mirror set at a 45O angle to the incident beam. Here the dichroic mirror acts as the optical coupler, designed to reflect the green light and to pass light at 600 nm. When the entering green light reaches the dichroic mirror, it is reflected into the FOCS. The mirror, however, is not perfect, and a small amount of the
How it works In operation, the light fromthe incandescent lamp is conditioned by an illu-
turns through the fiber from the chlo-
442 Envimn. Sci. Techml.. MI.20, NO.5,1986
The two-phase and Single-phase organic chloride FOCSs were tested in Henderson, Nev., in wells that were known to contain chloroform (2). These wells are continuonsly monitored using grab samples and gas chromatography. The k k u p analysis prw vided an independent reference for the in situ measureme3lts. Each FOCS test is charaaenzed ' by an initial time lag, and its respoose (count rate) as a function of time is liiear. Because the organic chloride FOCSs are integrating devices, the slope ofthe response curve can be directly related to sample concentrations. Abrnpt changes in Concentrationare indicated by deffitive slope shifts. To increase respntse time,the single phase FOCS measnrements were made in a well without using a membrane. The system operated properly for several hours, its response time. was nearly instantanmus, and several different FOCSs gave essentially the same response curve. The results fromseveral hour-long runs were almost identical and are shown in Figure 2. Au of the expimental poims fell on the average curve within f3%. Because of the fast response time of the FOCS, the effect of pumping a well is shown (Figure 3). l l e curves in this figure repnsmt a series of hour-long measurements started immdiately after the well was pumped and sampled. BY e g the data in Figures 2 and 3 it is apparent that more than three hours passed be fore the chloroform concentration in the well reblrned to its quiescent state.
Refereaces
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