Combining Preconcentration of Air Samples with Cavity Ring-Down

Quantitative detection of small volatile organic compounds in ambient air is demonstrated using a combination of continuous wave cavity ring-down ...
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Anal. Chem. 2004, 76, 7329-7335

Combining Preconcentration of Air Samples with Cavity Ring-Down Spectroscopy for Detection of Trace Volatile Organic Compounds in the Atmosphere Alistair M. Parkes, Ruth E. Lindley, and Andrew J. Orr-Ewing*

School of Chemistry, University of Bristol, Cantock’s Close, Bristol BS8 1TS, UK

Quantitative detection of small volatile organic compounds in ambient air is demonstrated using a combination of continuous wave cavity ring-down spectroscopy (cwCRDS) and the preconcentration of air samples with an adsorbent trap. The trap consists of a zeolite molecular sieve, selected for efficient trapping of the test compounds ethene (ethylene) and ethyne (acetylene). Upon heating of the trap, these organic compounds desorb into a smallvolume ring-down cavity, and absolute concentrations are measured by CRDS at 6150.30 cm-1 (ethene) and 6512.99 cm-1 (ethyne) without the need for calibration. The efficiency of the trapping and desorption was tested using commercial standard gas mixtures and shown to be 100% in the case of ethene, whereas some ethyne is retained under the current operating conditions. Samples of indoor and outdoor air were analyzed for ethene content, and measurements were made of mixing ratios as low as 6 ppbv. Removal of water vapor and CO2 from the air samples prior to trapping was unnecessary, and the selectivity of the trapping, desorption, and spectroscopic detection steps eliminates the need for gas chromatographic separation prior to analysis. With anticipated improvements to the design, measurements of these and other trace atmospheric constituents should be possible on time scales of a few minutes. Monitoring the low, but significant abundances of trace volatile organic compounds (VOCs) in the atmosphere, which impact on many aspects of the chemistry of the troposphere, including ozone and particulate production,1,2 usually necessitates preconcentration of air samples to bring the VOC concentrations up to the detection ranges of the instruments used.3 This preconcentration can be achieved by selective adsorption of the target species onto one of a variety of adsorbents. By accumulation over a period of time, removal of most of the volume of N2 and O2 from the air sample, and desorption into a small detector volume using a limited * To whom correspondence should be addressed. Phone: +44 117 928 7672. Fax: +44 117 925 0612. E-mail: [email protected]. (1) Atkinson, R. Atmos. Environ. 2000, 34, 2063-2101. (2) Volatile Organic Compounds in the Atmosphere, Issues in Environmental Science and Technology; Hester, R. E., Harrison, R. M., Eds.; The Royal Society of Chemistry: Letchworth, 1995; and references therein. (3) Clement, R. E.; Eiceman, G. A.; Koester, C. J. Anal. Chem. 1995, 67, R221255, and references therein. 10.1021/ac048727j CCC: $27.50 Published on Web 11/03/2004

© 2004 American Chemical Society

amount of carrier gas, the concentration of the sample can be substantially increased. The extent of preconcentration is then used to deduce the atmospheric mixing ratio. Current methods for VOC detection in the atmosphere frequently couple such preconcentrating adsorbent traps to gas chromatographic (GC) separation and either a flame ionization detector (FID),4 an electron capture detector (ECD),5 or a mass spectrometer (MS).6 These methods are well-established and provide an excellent way of measuring a large suite of organic compounds at mixing ratios in the atmosphere down to a few parts per trillion by volume (pptv). The major drawbacks of these instruments are 2-fold. First, the detectors are not generally species-selective (although MS does give speciation information), so careful separation (e.g., by GC) of the sample is required prior to detection. This is a time-consuming step and limits the temporal resolution of the instruments, sometimes to as long as 90 min.4 The second limitation is that the detection methods are not intrinsically quantitative, and thus, they require careful and frequent calibration to ensure the analytical integrity of the measurements. Spectroscopic detectors can provide quantitative, speciesdependent information, and small Fourier transform infrared (FT-IR)-probed flow cells have been successfully combined with GC separation to provide functional group information.7 Conventional FT-IR, however, has only limited sensitivity and spectral resolution. Laser-based techniques can provide greater specificity, so may circumvent the need for GC separation and decrease the overall sampling time. Moreover, laser absorption gives quantitative information on concentrations via Beer-Lambert law analysis without calibration, and recent developments in cavity enhanced spectroscopy8-11 ensure that the high sensitivities required for direct atmospheric monitoring are approached or, in favorable (4) Greenberg, J. P.; Lee, B.; Helmig, D.; Zimmerman, P. R. J. Chromatogr., A 1994, 676, 389-398. (5) Bassford, M. R.; Simmonds, P. G.; Nickless, G. Anal. Chem. 1998, 70, 958965. (6) Wevill, D. J.; Carpenter, L. J. Analyst 2004, 129, 634-638. (7) Smith, S. L.; Adams, G. E. J. Chromatogr. 1983, 279, 623-630. (8) Scherer, J. J.; Paul, J. B.; O’Keefe, A.; Saykally, R. J. Chem. Rev. 1997, 97, 25-51. (9) Wheeler, M. D.; Newman, S. M.; Orr-Ewing, A. J.; Ashfold, M. N. R. J. Chem. Soc. Faraday Trans. 1998, 94, 337-351. (10) Berden, G.; Peeters R.; Meijer, G. Int. Rev. Phys. Chem. 2000, 19, 565607. (11) Atkinson, D. B. Analyst 2003, 128, 117-125.

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cases, reached.12-14 As we and others have shown previously, nearinfrared diode lasers offer excellent prospects for atmospheric sensing of VOCs via overtones of C-H stretching vibrations or combination bands when coupled with high-finesse optical cavities, either via cavity ring-down spectroscopy (CRDS)8-15 or cavity enhanced absorption spectroscopy (CEAS).16-18 The lasers and associated technology are compact, of low cost, and have low power demands, so they are well-suited to development of remote monitoring instrumentation. We demonstrated that the sensitivity toward small hydrocarbons (HCs), such as methane, ethyne and ethene, is greater than that for larger HCs (such as butenes) because of sharp absorption features in the near-IR spectra of the smaller molecules.12,13 Quantitative measurements of CH4 at a wavenumber of 6086.79 cm-1 (via the 2ν3 band) were made in ambient laboratory air at a mixing ratio of 3.2 ppmv,12 and we showed that C2H2 could be monitored at 6578.58 cm-1 (via the R(9) line of the ν1 + ν3 combination band) at levels approaching the 1 ppbv mixing ratio representative of the lower atmosphere in a remote U.S. location,19 despite the deleterious effects of pressure broadening on the isolated rovibrational spectral features.13 Our demonstrated detection limits for ethene (∼78 ppbv for measurement at 6149.74 cm-1, since improved by a factor of 2) and larger alkenes, such as 1,3-butadiene (g1 ppmv), were, however, well above the sensitivities required for direct monitoring by CRDS (representative ethene mixing ratios for remote and urban environments are, respectively, 0.8 and 3.2 ppbv19,20). Photoacoustic spectroscopy (PAS), using a CO2 laser and a carefully designed sample cell, has been demonstrated to offer outstanding sensitivity to gases such as ethene (in the pptv range),21 but limitations of the type of laser and the need for calibration have so far prevented its development into an ultrasensitive sensor outside the laboratory. Rapid progress is being made in the use of new mid-infrared sources for trace gas sensing, such as quantum cascade lasers,22 and difference frequency generation in efficient nonlinear materials.23 Here, we explore further the potential of near-IR lasers because of their ease of use, compact size, and availability and demonstrate an alternative strategy for laser-based trace gas monitoring in which the VOC of interest is concentrated prior to measurement. (12) Fawcett, B. L.; Parkes, A. M.; Shallcross, D. E.; Orr-Ewing, A. J. Phys. Chem. Chem. Phys. 2002, 4, 5960-5965. (13) Parkes, A. M.; Fawcett, B. L.; Austin, R. E.; Nakamichi, S.; Shallcross, D. E.; Orr-Ewing, A. J. Analyst 2003, 128, 960-965. (14) Stry, S.; Hering, P.; Mu ¨ rtz, M. Appl. Phys. B: Lasers Opt. 2002, 75, 297303. (15) Awtry, A. R.; Miller, J. H. Appl. Phys. B: Lasers Opt. 2002, 75, 255-260. (16) Engeln, R.; Berden, G.; Peeters, R.; Meijer, G. Rev. Sci. Instrum. 1998, 69, 3763-3769. (17) O’Keefe, A.; Scherer, J. J.; Paul, J. B.; Chem. Phys. Lett. 1999, 307, 343349. (18) Baer, D. S.; Paul, J. B.; Gupta, M.; O’Keefe, A. Appl. Phys. B: Lasers Opt. 2002, 75, 261-265. (19) Hagerman, L.; Lonneman, W. J.; Aneja, V. P. Atmos. Environ. 1997, 31, 4017-4038. (20) Derwent, R. G.; Davies, T. J.; Delaney, M.; Dollard, G. J.; Field, R. A.; Dumitrean, P.; Nason, P. D.; Jones, B. M. R.; Pelper, S. A. Atmos. Environ. 2000, 34, 297-312. (21) Hekkert, S. T.; Staal, M. J.; Nabben, R. H. M.; Zuckermann, H.; Persijn, S.; Stal, L. J.; Voesenek, L. A. C. J.; Harren, F. J. M.; Reuss, J.; Parker, D. H. Instrum. Sci. Technol. 1998, 26, 157-175. (22) Bakhirkin, Y. A.; Kosterev, A. A.; Roller, C.; Curl, R. F.; Tittel, F. K. Appl. Opt. 2004, 43, 2257-2266. (23) Limpert, J.; Zelimer, H.; Tunnermann, A.; Lancaster, D. G.; Weidner, R.; Richter, D.; Tittel, F. K. Electron. Lett. 2000, 36, 1739-1741.

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Figure 1. Schematic diagram of the gas sampling, adsorbent trap, and diode laser cavity ring-down spectrometer. Not shown are the diode laser beam, which enters the cavity through one end mirror, and the detector, which is placed behind the opposite mirror.

In the current study, we demonstrate that the sensitivities necessary for monitoring small VOCs in the atmosphere by continuous wave diode laser CRDS (henceforth denoted as cwCRDS) or CEAS can readily be achieved by combining these absorption spectroscopy methods with preconcentration of air samples on an adsorbent trap of the type commonly employed with GC-based detectors. The efficiency of the trap for retaining particular compounds, heating-induced desorption, and spectroscopic detection provide sufficient selectivity to measure both ethyne and ethene without the need for chromatographic separation. The designs of the trap and the associated, low-volume CRD spectrometer are described, and enhancements in sensitivity are demonstrated for ethene measurements in air. CRDS determinations of concentrations are quantitative and accurate without the need for detector calibration, and detection of ethene and ethyne by this method should provide a useful alternative to GC-FID detection, for which the error is large compared to larger HCs.4,24 As will be demonstrated, the combination of preconcentration methods and diode laser detection offers excellent prospects for quantitative and rapid analysis of low-abundance trace gases. EXPERIMENTAL SECTION The new instrument is shown schematically in Figure 1. Air is sampled at a flow rate regulated by a mass flow controller (MFC) through an adsorbent trap on which the compound of interest is retained while N2 and O2 are removed. The adsorbed sample is then desorbed into a vacuum cell bounded by two highreflectivity mirrors that constitute an optical ring-down cavity (RDC). Desorption is aided by an inert carrier gas. The number density of the analyte is determined in a static sample by absorption spectroscopy (cw-CRDS) at total pressures, including the carrier gas, below 25 Torr. The amount of preconcentration is determined simply by the ratio of the volume of air sampled to the cavity volume. For optimum performance of any preconcentration stage, the volume of the RDC must be sufficiently small to avoid the time-consuming sampling of very large volumes of (24) Martin, D. Ph.D. Thesis, University of Bristol, 2002.

air. Design and characterization of the adsorbent trap and the optical cavity are described in turn. The Adsorbent Trap. When designing an adsorbent-filled trap for the preconcentration of very dilute samples, several features of the trap require consideration, such as the size and internal volume, the mass and type of adsorbent used, and the temperature of operation. These properties can be tailored for optimum performance, depending on the analyte of interest and the amount of preconcentration required to bring that compound into the detection range of the detector. A variety of different types of adsorbent are used when trapping VOCs from the atmosphere, including graphitized carbon materials such as Carbotrap;25 carbon-based molecular sieves (MSs), such as Carboxen and Carbosieve SIII (CS3);24,26 and zeolite molecular sieves (ZMSs), such as MS 4A, 5A, and 13A. Because of their advantageous properties, we focused on evaluation of CS3, MS 4A, and MS 5A. To prepare the adsorbent trap, stainless steel tubing (0.125 in. o.d., 0.085 in. i.d.) was cut into a 15-cm length. The tube was cleaned by repeated flushing with ethanol and acetone, followed by oven drying. Silanized glass wool, drawn into a fiber, was inserted ∼2 cm into the trap and was secured by packing the end of the trap with compacted glass wool. Glass beads (∼5 mg) were added to help contain the adsorbent in the trap. A 250-mg portion of adsorbent was drawn into the trap under vacuum, and glass beads and wool were added to the open end, which was again secured using compacted glass wool. Wires were connected to the tubing at either end, and a second pair of terminals was attached to a copper heating wire coiled around the trap, which could then be resistively heated to a temperature of up to 225 °C in 99%, BOC Gases) in argon (BOC Research Grade, 99.98%) was introduced into the stainless steel canister and zero air again added to a pressure of 5.4 bar. Each sample was allowed to mix for at least 1 h before use. MS 5A has the highest BTV (2076 cm3 of 3.7 ppmv ethene and 7411 cm3 of 9.8 ppmv ethyne at 25 °C) and, therefore, the (25) Brancaleoni, E.; Scovaventi, M.; Frattoni, M.; Mabilia, R.; Ciccioli, P. J. Chromatogr., A 1999, 845, 317-328. (26) O’Doherty, S. J.; Simmonds, P. G.; Nickless, G.; Betz, W. R. J. Chromatogr. 1993, 630, 265-274. (27) Bertoni, G.; Tappa, R. J. Chromatgr., A 1997, 767, 153-161. (28) Gold, A.; Dube, C. E.; Perni, R. B. Anal. Chem. 1978, 50, 1839-1841.

best retention of both analytes. The BTV is limited by the strength of adsorption to MS 5A and not by the number of active sites in the trap. There was no immediate breakthrough for either ethene or ethyne. Plots of log(BTV) versus 1/T are linear for the three adsorbents, and experiments at elevated temperatures thus indicate appropriate desorption temperatures for each adsorbent. The best-fit lines for CS3 and MS 5A intersect at around 60 °C, indicating that adsorbents will desorb more readily from MS 5A upon heating. This useful property, together with the larger BTVs at lower temperatures, guided the choice of MS 5A as adsorbent in the trap. A trap temperature of 160 °C gives a breakthrough volume of 1 mL for ethyne and 0.3 mL for ethene. The capacity of MS 5A for ethene and ethyne means that, for a trap containing 250 mg of adsorbent, at least 1 L of a dilute sample of either gas could be passed through the trap without risk of breakthrough. In similar experiments using an 11.7 ppmv sample of methane, 100% breakthrough occurred immediately, demonstrating no retention of methane on MS 5A. This is advantageous when sampling air because of the effective removal of this very abundant VOC. MS 5A will, however, adsorb H2O and CO2, together with H2S, SO2, NH3, and small nonmethane hydrocarbons.29 Provided the atmospheric abundance of these species is small, they should not significantly reduce the sieve’s ability to retain the analyte of interest, and spectroscopic selectivity in the CRDS detector makes it “blind” to these coadsorbents. The Ring-Down Cavity, Laser, and Optics. The design of the RDC is shown schematically in Figure 1. A key condition to ensure the effectiveness of the proposed combination of preconcentration and CRDS is that the RDC volume is much smaller than the initial sample size, encouraging a design based on a linear cavity. The mirrors available for this work (Layertec GmbH, 12.5-mm diameter, planoconcave, 1-m radius of curvature) had a manufacturer specified reflectivity of >99.97% over a wavelength range ∼1520 to 1650 nm. Increasing the mirror separation extends the ring-down time (RDT) and, thus, sensitivity, but it reduces the portability of the instrument and increases the volume of the cavity. The RDC design parameters, considering sensitivity, ease of alignment, cavity volume, sampling time, portability, and the cost and ease of manufacture, were chosen to include a 300-mmlong, 5-mm-i.d. tube bored from a 28.5-mm-diameter steel rod. For excitation of a cavity TEM00 mode, the laser beam diameter at the mirrors is ∼0.94 mm,30 and the 5-mm internal diameter thus ensures negligible diffraction losses. Additional 3.1-mm-diameter holes were drilled for sample in- and outlets. The surface areato-volume ratio is large for the narrow-bore cavity, and thus, loss of gases by adsorption to the walls must be carefully tested. Flanges were fixed to the cavity structure to allow mirror mounts to be held securely in place. The front faces of the ringdown mirrors made contact with O-rings mounted on the main cavity structure to ensure a good vacuum seal. Mirror and PZTstack tilts were finely adjusted for optimal cavity alignment by three micrometer screws at each end of the cavity.The 300-mm separation of the mirrors gives a cavity free spectral range of 500 MHz that is considerably greater than the ∼2-MHz bandwidth of the distributed feedback (DFB) diode laser (Fitel FOL15DCWB(29) http://www.alltechweb.com. (30) Siegman, A. E. Lasers; Oxford University Press: New York, 1986.

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M63-18440-B, λ ∼ 1625 nm) used for CRDS. One mirror was thus mounted on a piezoelectric transducer (PZT, Piezomechanik GmbH) stack to which a modulation voltage (50 Hz, 30 V peakto-peak triangular wave) was applied to sweep the cavity length, and thus the frequencies of cavity modes, periodically to tune the cavity into resonance with the laser frequency. The laser bandwidth is >2 orders of magnitude smaller than Doppler line widths of the samples, so quantitative retrieval of absorber concentrations from ring-down measurements is assured. The pressure of the sample inside the RDC was measured using a capacitance manometer (Edwards Barocel). This, along with connecting tubing on the inlet and outlet of the RDC, brought the total internal volume up to 29.5 cm3. The dead volume associated with the pressure gauge and tubing will be minimized in a refined cavity design currently under construction. For a sample volume of 1 L, small enough to avoid significant breakthrough of the MS 5A trap and loss of the target VOC, the maximum preconcentration that can be achieved with the current design is, thus, a factor of ×33.9. This preconcentration factor can, however, be increased by using a smaller volume RDC (we anticipate factors in excess of ×100), or by cooling the trap to increase the sample volume that can be analyzed. The fiber-coupled output from the DFB laser was directed to an acousto-optic modulator (AOM, Isle Optics, 80 MHz). The resultant first-order deflected beam was coupled into the RDC using two prisms and a mode-matching lens. Following excitation of a cavity mode to a preselected threshold level, the AOM extinguished the deflected laser beam. Time-dependent light intensity escaping from the RDC was monitored using a highgain photodiode (New Focus IR-DC, 150 MHz), and digitized with a digital oscilloscope (LeCroy 9361, 8-bit vertical resolution) connected to a PC by a GPIB interface. Custom software was used to fit individual ring-down decay curves to single-exponential functions to determine the decay rate. The residual standard deviation of a user-selected number of decay rates was calculated to test cavity stability, and the PC also recorded wavelength readings from a wavemeter (Burleigh WA-1000) and controlled the scanning of the laser. Typical stabilities, calculated as the standard deviation about the mean of 100 RDT measurement, were between 0.7 and 1.0% for a 10.1-µs RDT. This measure of stability ultimately determines the limiting sensitivity of the spectrometer. Changes in the ring-down rate constant, ∆k, caused by absorption of near-IR by the sample were converted to absolute concentrations using standard formulas and knowledge of wavelengthdependent molecular absorption cross sections.9 The cavity was used to make CRDS measurements over a twoweek period without the need for mirror realignment. Repeated measurements over a 1-h time period of a 10 ppmv sample of ethene in zero air contained within the RDC showed no measurable loss due to adsorption to the walls of the cavity. Combination of Trapping and CRDS Detection. First tests of the trapping and desorption of ethene into the RDC demonstrated that even with prior evacuation of the RDC to a base pressure of a few milli-Torr and rapid heating of the trap to 200 °C, a small flow of inert carrier gas was necessary to ensure the efficient desorption and transfer of the gas. The flows of sample and carrier gas were kept separate by using a 6-port, two-position valve (VICI microactuator), as shown in Figure 1. Trap loading 7332

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was achieved by flowing the air sample through the MFC to the valve, onward through the trap, and to the vent (A f E f D f F). When the valve was switched, the N2 purge flowed for g90 s at 0.50 sccm through the heated trap to the RDC (B f D f E f C) while the sample flow bypassed the trap and flowed straight to the vent (A f F). Following transfer of gas from the trap to the RDC, two types of data were recorded. Fastest data acquisition was achieved if the output of the 1625-nm DFB laser was fixed to the peak center of an ethene absorption line at 6150.30 cm-1 of known absorption cross section, and the difference in optical decay rate between the cavity containing the sample and the evacuated, or zero-air filled, cavity was used to determine the concentration of the sample. Typically, the mean of 100 shots was used to determine the decay rate, and the error quoted in concentrations is 2σ about the mean. Alternatively, as a means of ensuring absorption was due to ethene, a voltage ramp was applied to the laser driver, and the wavelength was scanned across one or more absorption features, averaging between 5 and 15 shots per point and using ∼100 wavelength steps per absorption feature. The areas under the peaks in the spectra were then used to determine the concentration of the sample in the cavity, and the 2σ error was derived from the confidence of the fit. RESULTS Measurements of Ethene Mixing Ratios. Analysis of the absorption of near-IR by target VOCs in the small-volume CRD spectrometer requires precise knowledge of the absorption cross sections of the gas at the wavelength used, and their dependence upon total pressure in the RDC. Our measurements of these physical properties are presented elsewhere.31 CRDS measurements presented herein exploit Q-branch rotational lines of the ν5 + ν9 combination band in the wavenumber range 6149.7-6150.3 cm-1. Residual N2 and O2 in the trap will be transferred to the small-volume RDC along with the VOC analytes and may broaden spectral features if the resultant pressure is sufficiently high. We previously determined a HWHM air pressure broadening coefficient31 of 0.1060 ( 0.0011 cm-1 atm-1 that, together with our measured absorption cross sections, can be used to extract ethene concentrations from absorption measurements over a wide range of total pressures. From the evidence of pressure broadening studies, it is advantageous to make CRDS measurements of ethene at total pressures in the RDC below 50 Torr to avoid extensive pressure broadening and an associated reduction in signal-to-noise ratios. A further advantage of our preconcentration approach is, thus, that spectra can be measured at pressures well below ambient. The residual air pressure in the trap was relieved before commencing heating of the trap to desorb the ethene. The RDC was evacuated by a mechanical pump, then opened to the trap to transfer residual air, and this process was typically repeated three times. The trap was not exposed directly to the pump to avoid loss of adsorbent material or partial and uncontrolled desorption of the adsorbate. Measurement of the ring-down decay rate after each opening of the cavity to the unheated trap showed no detectable change in absorbance, indicating no loss from the trap during these serial evacuations. (31) Parkes, A. M.; Lindley, R. E.; Orr-Ewing, A. J. Phys. Chem. Chem. Phys., in press.

Table 1. Ethene Integrated Peak Areas and Mixing Ratios Derived from Three Separate Experiments Using the NPL Standard Mixturea sample 1 2 3 av manufacturer specification a

Figure 2. Top: Ethene concentrations derived from 11 consecutive ring-down measurements for different samples of fixed volume of the 10 ppmv standard (see text for details). Bottom: Ethene concentrations derived from measured changes in the cavity ring-down decay rate for four different volumes of the 10 ppmv standard plotted against the expected concentration, assuming transfer of all the ethene from the sample to the RDC.

Before making measurements of ethene concentrations in ambient air samples, tests were undertaken to ensure quantitative trapping and desorption into the RDC. First tests were conducted with a relatively concentrated sample of ethene (10 ppmv), with net dilution rather than preconcentration in the course of the experiments, and were followed by tests using a 19.1 ppbv standard sample of ethene to demonstrate the effectiveness of the preconcentration. A standard containing 10 ppmv ethene in zero air (Scotty Speciality Gases) was flowed through the trap at 24 sccm for 20 s, and the trap was then isolated from the sample flow. Following removal of residual pressure of N2 and O2, the trap was resistively heated for 90 s under static conditions with no carrier gas flow. When the temperature of the trap was ∼200 °C, the carrier gas flow (0.50 sccm) was switched on, and the trap was opened up to the previously evacuated RDC. The carrier gas was flowed until the pressure in the cavity was around ∼20 Torr and not greater than 25 Torr. This procedure was repeated 11 times to obtain separate samples, with measurements made of the ethene absorption in the RDC for each, and the results were converted to concentrations using the known absorption cross section of σ ) 3.24 × 10-20 cm2 molecule-1 at a fixed wavenumber of 6150.30 cm-1. The outcomes are shown in the upper panel of Figure 2. By comparing to the number of ethene molecules known to be delivered in the standard gas sample for a selected flow rate and time, the efficiency of trapping and desorption was determined to be 100 ( 12.0% (with 2σ uncertainties). The uncertainty may

peak area s-1 cm-1

mixing ratio ppbv

240.8 ( 5.4 236.8 ( 7.2 240.3 ( 5.9

20.39 ( 0.82 20.05 ( 1.22 20.34 ( 1.02 20.26 ( 1.78 19.1

Uncertainties are 2σ from the fits.

be a consequence of possible variations in the laser wavelength and inaccuracies in the flow rate set by the MFC. Further experiments were conducted in which the flow of 10 ppm ethene in zero air through the trap was maintained for periods of 10, 20, 30, and 40 s. The lower panel in Figure 2 shows the measured concentrations plotted against the calculated concentrations expected for transfer of all the ethene in the sample to the trap and RDC. The gradient of the fit through the data set demonstrates an efficiency of 101.1 ( 2.2%. These experiments used a standard mixture of ethene in zero air, but the effects of the presence of other hydrocarbons in the sample must also be tested. Experiments with more dilute ethene samples, comparable to typical mixing ratios in air, are also necessary to test the performance of the trap. A multicomponent hydrocarbon standard mixture (NPL) containing 19.1 ppb of ethene as well as many other HC components (27 alkanes, alkenes, and aromatic compounds from C2 to C9 at mixing ratios up to a few tens of ppbv) was flowed through the trap at 200 sccm for 5 min. The trap was then evacuated and desorbed following the procedure described earlier. Three consecutive experiments were carried out, in part to test for carryover effects, in which spectra were recorded of the same volume of standard mixture, with no additional purging or cleaning of the trap between each run. Subtraction of the baseline, measured for a zero air sample, from the spectrum of the NPL standard, reveals spectral lines of ethene atop a weaker broadband absorption attributed to the artificially high concentration of high molecular weight hydrocarbons in the hydrocarbon standard. To calculate the concentration of ethene in the cavity, the ethene line centered at 6150.30 cm-1 was fitted with a Gaussian function with a width constrained to the calculated Doppler width for T ) 295 K (0.0143 cm-1 FWHM). The integrated area was determined and converted to a concentration from knowledge of the line center absorption cross section. The ethene concentration in the cavity was then used to work out the concentration in the original sample using the ratio of volumes of the gas sample analyzed and the RDC. Table 1 shows the integrated areas for the three consecutive multicomponent standard measurements and the mixing ratios deduced from these peak areas. The uncertainties in the data (2σ) are those derived from the fitting error alone. A peak area of ∼240 s-1 cm-1 corresponds to a concentration inside the RDC of ∼1.6 × 1013 molecules cm-3 (0.5 mTorr). The mixing ratios calculated for the standard from the experimental measurements after due allowance for the preconcentration factor are in excellent agreement (to within 6%) with Analytical Chemistry, Vol. 76, No. 24, December 15, 2004

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Figure 3. Changes in cavity ring-down rate, ∆k, with laser wavenumber for two ethene samples. Top: Spectrum of 2.5 mTorr of ethene in argon, together with a fit to three Gaussian functions (dashed lines). Bottom: Co-added spectrum for the indoor air sample (see text for details). The additional fitted Gaussian function centered at 6150.258 cm-1 (solid line) is attributed to OC18O.

the value specified by the manufacturer. The experiments also lend confidence that there is no significant wall loss of ethene anywhere in the apparatus on the time scale of the experiment or carryover of trapped ethene from one measurement to the next. Unique detection of ethene in a mixture of numerous saturated and unsaturated C2-C9 VOCs is clearly demonstrated without the need for a GC separation step. Having carefully characterized the apparatus, samples of indoor and outdoor air were analyzed after collection in an evacuated 5-L glass bulb. The 1-L samples were admitted to the apparatus using a small battery-powered pump to provide a positive pressure to the inlet of the MFC, and experiments were conducted as described above. No sample manipulation, such as water removal, was used, but the trap was precleaned by heating and purging with zero air. For the indoor air sample, three cycles of heating and purging the trap with carrier gas were required to desorb all the ethene (with ∼45% desorption in the first cycle), perhaps because of water and CO2 retention, but no systematic studies of the effects of relative humidity on desorption efficiency have yet been undertaken. Spectra were scanned following each purge and were coadded with the result shown in Figure 3 after background subtraction. The integrated area of the peak near 6150.26 cm-1 relative to that of the peak centered at 6150.30 cm-1 was greater than expected for an ethene spectrum, indicating that a species with an overlapping absorption feature is present in the indoor air sample. This additional absorption was only observed for the first cycle of heating and desorption from the trap. The HITRAN database32 reveals only OC18O as a plausible interference. Although methane has absorption features in this wavenumber region, BTV experiments using GC-FID showed that MS 5A does not retain any measurable amount. Moreover, if the extra absorbance at 6150.266 cm-1 was due to methane, additional peaks would be observed at 6150.24, 6150.30, and 6150.33 cm-1. A spectrum of ethene (8.2 × 1013 molecules cm-3, 2.5 mTorr) diluted in argon in the wavenumber range 6150.24(32) The HITRAN database is available from www.hitran.com.

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6150.32 cm-1 was fitted with three Gaussian curves, each with its width constrained to the calculated Doppler width. The peak centers, widths, and relative areas from this fit were used to guide the fitting of the spectrum obtained from the air sample, as shown in Figure 3. A further Gaussian function was included in the second fit with its center constrained to 6150.258 cm-1, as specified in the HITRAN database for OC18O, and its width was fixed at the Doppler limited FWHM of 0.0114 cm-1. The area of the fourth function corresponded to an OC18O concentration in the RDC of (8.4 ( 2.6) × 1014 molecules cm-3. Assuming quantitative trapping and, therefore, a preconcentration factor of ×33.9, the concentration of OC18O in the air sample would be (2.5 ( 0.6) × 1013 molecules cm-3, equivalent to a mixing ratio of 1.03 ( 0.24 ppmv in 1 atm. This is close to the expected OC18O atmospheric abundance of 1.4 ppmv, with the difference a consequence of imperfect retention of CO2 in the MS 5A trap. The area of the Gaussian curve with its center at 6150.305 cm-1 used to fit to the spectrum in Figure 3 can be converted to an ethene concentration of 1.78 × 1013 molecules cm-3 in the RDC using the previously determined absorption cross section.31 With correction for the preconcentration factor, the mixing ratio of ethene is deduced to be 22.0 ( 1.4 ppbv for this indoor air sample. The spectrum of ethene from the air sample shows negligible underlying absorption by higher hydrocarbons, unlike the NPL standard. The ethene concentrations can thus be determined by rapid, fixed wavelength (at line center) absorption measurements. Care must be taken with such an approach because of possible interferences, but even for the NPL standard, which was much enriched in many compounds, such as isoprene (52.6 ppbv) and toluene (40.2 ppbv), the selectivity of the trap and the spectral detection resulted in only a weak additional offset. Analysis of 1 L of a new sample of laboratory air at a wavenumber of 6150.30 cm-1 by this method gave a change in decay rate of ∆k ) 6484 ( 1516 s-1. Given a preconcentration of 33.9 times and taking the absorption cross section at line center as 3.24 × 10-20 cm2 molecule-1, the ethene concentration in the laboratory air at the time of measurement was determined as 1.97 ( 0.40 × 1011 molecules cm-3 or 8.1 ( 2.2 ppbv. Analysis of an outdoor air sample, collected at a location above Bristol city center in a 5-L bulb filled to ambient pressure, was conducted by flow of a 1-L sample across the adsorbent trap, followed by heating for 2 min and purging until the pressure in the RDC was 35 Torr. A fit to the peak at 6150.30 cm-1 yielded an ethene concentration for the urban air of 1.41 ( 0.08 × 1011 molecules cm-3, equivalent to 5.79 ( 0.34 ppbv. Repeating the trapping and desorption part of the experiment immediately after this measurement but using a zero-air flow showed no retention of ethene by the trap. The problem of ethene retention for the indoor air sample was thus eliminated by the additional 30 s of heating and use of additional purge gas. Measurements of Ethyne Mixing Ratios. Simply by changing the laser wavelength, in this case by use of a different DFB laser (Marconi LD 6204 DFB), ethyne can be measured via the ν1 + ν3 combination band at λ ∼ 1530 nm in the same air samples as were employed in the work on ethene. No changes in ring down mirrors, other optical components, or detector were required.

A 60-mL portion of the NPL standard (containing 42 ppbv of C2H2) was sampled for three consecutive experiments. Analysis of the spectra shows an increasing response with each successive experiment, corresponding to mixing ratios of 23.4, 53.6, and 69.5 ppbv. These observations indicate that the trap contents are not completely desorbed in each experiment. The P(17) line of the ν1 + ν3 ethyne combination band at 6512.99 cm-1 used for analysis is isolated, and there are no overlapping absorption features of other trace air constituents in this region. This spectral simplicity facilitates analysis of broadened peaks, so larger amounts of carrier gas (and, therefore, higher pressure in the RDC) can be used to purge the trap, despite the consequently greater pressure broadening. After cleaning of the trap, a 1-L sample of Bristol air was flowed through, and the trap was purged with ∼1.0 mL of N2, giving a pressure in the cavity of 60 Torr. Consecutive experiments with the local air sample, however, also showed partial retention of ethyne on the trap. The mixing ratios of ethyne in the successively analyzed air samples were estimated to be 2.2 and 3.5 ppbv. The increased affinity of ethyne toward adsorption on the molecular sieve probably accounts for this behavior. The BTV of ethyne at 200 °C is estimated to be 0.202 mL, which is close to the volume of carrier gas used and could be the cause of inefficient gas transfer. An immediate challenge is, thus, to ensure that ethyne is desorbed from the trap with an efficiency approaching unity. The removal of water might facilitate analyte gas transfer, and the use of a smaller trap would enable higher desorption temperatures via resistive heating. DISCUSSION AND CONCLUSIONS The experiments described in the preceding section show the successful combination of preconcentration via the use of an adsorbent trap and cw-CRDS. The trapping and desorption method was demonstrated to be quantitative through testing with standards containing very different ethene mixing ratios, and the use of CRDS detection removes the need for any further instrument calibration. Concentrations of ethene in ambient air samples as low as ∼6 ppbv, which would be below the detection limit of a conventional cw-CRD spectrometer, were determined without the need for GC separation. In the current method, samples are drawn through a trap for 5 min, and evacuation of residual air from the trap takes ∼1 min, with heating for 2 min followed by desorption for ∼30 s. The CRDS measurements then require times in the range of 30 s to 5 min, depending on the method chosen. Using the current procedure, the total measurement time is, therefore, between 9 and 14 min. This time could, however, be much reduced by using a smaller trap with a faster heating rate and a greater flow rate through the trap. Sample flows of 500 mL min-1 have been reported33 (33) Betz, W. R.; Maraldo, S. G.; Wachob, G. D.; Firth, M. C. Am. Ind. Hyg. Assoc. J. 1989, 50, 181-187. (34) Berden, G.; Peeters, R.; Meijer, G. Chem. Phys. Lett. 1999, 307, 131-138. (35) Paul, J. B.; Lapson, L.; Anderson, J. G. Appl. Opt. 2001, 40, 4904-4910. (36) Boudries, H.; Touppance, G.; Dutot, A. L. Atmos. Environ. 1994, 28, 10951112.

with no impact on the trapping properties of the adsorbent. Scanning a smaller region of the spectrum (one spectral line) is also feasible, enabling the overall measurement time to be reduced by 50%. The automation of the trapping and desorption procedure may also result in an increased sampling rate, as would the use of two or more adsorbent traps in parallel, connected to the same RDC. CEAS measurements34,35 are generally more rapid than CRDS, but are typically less sensitive and would require a small increase in the diameter (and, hence, the volume) of the cavity for optimum performance. An improved design RDC is currently under construction, which should give increased stability for the ring-down measurements (and, hence, better S/N ratios), together with the possibility of using CEAS. Other rapid and sensitive absorption methods, such as frequency modulation spectroscopy, could alternatively be employed. The total detection time, including sampling, can realistically be reduced to ∼1 or 2 min. This compares very favorably with the GC-FID instruments for the detection of hydrocarbons, whose time resolution is limited by the GC separation of a large number of compounds trapped. The detection limit for the current spectrometer design for measurement of ethene in air, estimated from the smallest measurable change in decay rate given above, is 4.7 × 1010 molecules cm-3 in the air sample before preconcentration, equivalent to 1.9 ppbv. This detection limit could be significantly improved by use of higher reflectivity mirrors, more stable cavity alignment, discrimination against ring-down decays that deviate from single exponential behavior, and by use of a digitizer with greater depth than the 8-bit oscilloscope. For a smallest measured change in decay rate of 0.7%, and a ring-down time of 10.1 µs, the detection limit for ethene is 2.15 × 1010 molecules cm-3, or a mixing ratio of 0.885 ppbv. Greater preconcentration prior to desorption into the RDC could further enhance this detection limit, either through the use of a cooled adsorbent trap or by reduction in the volume of the RDC, with 100-fold preconcentration ratios readily attainable. With straightforward improvements, we anticipate measurements of ethene below 200 pptv, sufficient for monitoring in many rural sites.36 By tailoring the material in the adsorption trap and the laser wavelength to the target analyte, field-deployed detectors can be designed for a range of compounds, such as oxygenated VOCs, N2O, and NH3, with sensitivities in the sub-ppbv range. ACKNOWLEDGMENT We thank Dr. D.E. Shallcross, Dr. S. O’Doherty and Dr. D. Martin (University of Bristol), S. Enami and J. Ueda (Kyoto University), and S. Nishida (Nagoya University) for their contributions to this work. Financial support from the EPSRC Portfolio Grant LASER and the EPSRC and Royal Society of Chemistry Analytical Trust Fund is gratefully acknowledged. Received for review August 26, 2004. Accepted September 20, 2004. AC048727J

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