Anal. Chem. 2006, 78, 6765-6773
Ultrafast Gas Chromatographic Separation of Organophosphor and Organosulfur Compounds Utilizing a Microcountercurrent Flame Photometric Detector Shai Kendler,*,†,‡ Shaelah M. Reidy,† Gordon R. Lambertus,† and Richard D. Sacks†
Department of Chemistry, University of Michigan, 930 North University, Ann Arbor, Michigan 48109, and Department of Physical Chemistry, Israel Institute for Biological Research, 24 Lerer Street, P.O. Box 19 Ness-Ziona, 74100 Israel
A microcountercurrent flame photometric detector (µccFPD) was adapted and optimized for ultrafast gas chromatographic (GC) separation and detection of organophosphor (OP) and organosulfur (OS) compounds on short chromatographic columns. Air and hydrogen are introduced to the µcc-FPD from opposite directions, creating a hydrogen-rich flame. In this µcc-FPD, combustion takes place between the burner tips without touching them. The separation between the tips and the flame reduces heat loss from the flame to the surrounding environment, resulting in low hydrogen consumption and a compact flame. The µcc-FPD is capable of detecting very narrow (13 ms) chromatographic peaks. An ultrafast GC separation of a group of six OP and OS compounds is achieved within less than 5 s using fast temperature programming of a 0.5-m-long microbore column. Very fast separations are also demonstrated on a 1-m-long microfabricated column consisting of 150-µm-wide, 240-µmdeep channels, etched in a 1.9-cm square silicon chip, covered with a Pyrex wafer, and statically coated with dimethyl polysiloxane. With a hydrogen flow rate of 10 mL/min, the detection limit for OP is 12 pg of P/s and 3 ng of S/s for OS compounds at a signal-to-noise ratio of 2. The coupling of a microfabricated column and a miniature FPD is an important step toward the development of a miniaturized GC-FPD capable of ultrafast detection of low levels of OP and OS compounds. Miniaturization of detectors and gas chromatography (GC) systems for real-time detection of high-priority chemicals such as explosives, toxic industrial compounds, and chemical warfare agents (CWA) is a field of increasing interest over the past few years.1-8 The motivations behind miniaturization of analytical * Corresponding author. E-mail: [email protected]
Tel: 972-8-9381457. Fax: 972-8-9381743. † University of Michigan. ‡ Israel Institute for Biological Research. (1) Lambertus, G. R.; Fix, C. S.; Reidy, S. M.; Miller, R. A.; Wheeler, D.; Nazarov, E.; Sacks, R. D. Anal. Chem. 2005, 77, 7563. (2) Overton, E. B.; Carney, K. R.; Roques, N.; Dharmasena, H. P. Field Anal. Chem. Technol. 2001, 5 (1-2), 97. (3) Whiting, J. J.; Lu, C.-J.; Zellers, E. T.; Sacks, R. D. Anal. Chem. 2001, 73, 4668. 10.1021/ac060851a CCC: $33.50 Published on Web 09/07/2006
© 2006 American Chemical Society
systems are to reduce weight and resource consumption while providing sensitive and reliable detection devices that can be used for on-site analysis. GC separations can be accelerated by using short columns, fast temperature programming, and increased carrier gas flow rates. In many cases, accelerating the GC separation degrades column resolution. In this case, the reduction in analysis time, as well as the limited sample pretreatment techniques available in the field, may result in an increase in the false alarm rate. Using selective detection is an elegant way to reduce the false alarm rate for fast GC separations.9 Mass spectrometers (MS) are considered to be the gold standard in chemical analysis thanks to their fast response time, sensitivity, and most importantly, mass selectivity. Thus, it is not surprising that a lot of effort is devoted to miniaturization of MS systems. However, the complexity of peripheral modules, especially vacuum pumps, still impedes application of MS devices for field applications.10 Flame photometric detectors are known to be highly sensitive and selective to organophosphor (OP) and organosulfur (OS) compounds,11 thus making them a natural choice for the analysis of sulfur- and phosphorus-containing CWA and pesticides.12 Detection in a flame photometric detector (FPD) is based on the combustion of the analytes in a hydrogen-rich flame. Combustion of OP compounds yields the electronically excited POH* species that has a strong optical emission around 526 nm. The limit of detection for OP compounds is in the range of 10-12 g of P/s, and the response is linear with the mass flow over more than 4 orders of magnitude.12 Detection of OS compounds is done by monitoring the optical emission of S2* at 394 nm. The limit of (4) Kendler, S.; Zifman, A.; Gratziany, N.; Zaltsman, A.; Frishman, G. Anal. Chim. Acta 2005, 548, 58. (5) Frishman, G.; Amirav, A. Field Anal. Chem. Technol. 2000, 4, 170. (6) Snyder, A. P.; Maswadeh, W. M.; Parsons, J. A.; Tripathi, A.; Meuzelaar, H. L. C.; Dworzanski, J. P.; Kim, M. G. Field Anal. Chem. Technol. 1999, 3, 315. (7) Eiceman, G. A.; Nazarov, E. G.; Tadjikov, B.; Miller, R. A. Field Anal. Chem. Technol. 2000, 4 (6), 297. (8) Deng, C.; Yang, X.; Li, N.; Huang, Y.; Zhang, X. J. Chromatogr. Sci. 2005, 43, 355. (9) Koryta´r, P.; Janssen, H.-G.; Matisova´, E.; Brinkman, U. A. Th. Trends Anal. Chem. 2002, 21, 558. (10) Badman, E. R.; Cooks, R. G. J. Mass Spectrom. 2000, 35, 659. (11) Brody, S. S.; Chaney, J. E. J. Gas Chromatogr. 1966, 4, 42. (12) Westmoreland, D. G.; Rhodes, G. R. Pure Appl. Chem. 1989, 61, 1147.
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detection for OS is in the range of 10-11 g of S/s, and the response is quadratic with the OS mass flow over 3 orders of magnitude.12 Thanks to its selectivity and sensitivity, FPD technology has been successfully adapted for field applications.13,14 Frishman and Amirav developed a prototype of a portable GCpulsed flame photometric detector capable of analyzing part-pertrillion levels of five CWA simulants in air within 30 s.5 Zimmermann and Muller developed atomic emission flame spectrometer utilizing microelectromechanical systems fabrication techniques.15 This detector is based on a premixed air-rich flame and equipped with a miniaturized electrolysis cell that supplies oxygen and hydrogen to the flame. Detection limits obtained for sodium and potassium in liquid samples were 35 times higher than those obtained with conventional atomic emission spectrometers.15 The sensitivity toward OP and OS compounds as well as the compatibility of this device to GC has not been reported. The main limitation in miniaturization of the FPD is the increased rate of heat losses as flame size goes down, which may result in flame extinction.16 Ballal and Lefebvre defined the quenching distance (dq) as the critical size the flame volume must exceed in order to propagate unaided.17 Quenching distance is affected by the type and flow rate of the fuel and oxidant and also by the surrounding temperature. The main routes for heat losses are through the burner tip and to the walls of the combustion chamber. Thurbide and Aue explored the possibility of miniaturizing the FPD by introducing the oxidant and fuel streams from opposite directions (microcountercurrent, µcc).18-21 In that way, the fuel and the oxidant are mixed inside the combustion chamber and combustion occurs between the burner tips. The exact location of the flame inside the combustion chamber is determined by the linear velocities of the fuel and the oxidant. By choosing the appropriate fuel and oxidant flow rates, the flame may be located between the burner tips without actually touching either one of thems“between burner” operating mode.18 In these conditions, the main route for heat loss is through the walls of the combustion chamber. Since very small flames are obtainable with µcc-flame, they may be suitable for the miniaturization of GC-FPD. Thurbide et al. studied the chemical performance of µcc-FPD, using air and oxygen as an oxidant. The minimum detectable limit (MDL) obtained with a µcc-FPD, occupying a volume of only 30 nL, between two fused-silica capillaries, was ∼100 higher than those obtained with a conventional FPD. The main reason for this difference is optical glow obtained from the fused-silica capillaries, which increases the noise level. It was also reported that the flame was less stable when air was used as an oxidant in comparison to oxygen.19 An improved sensitivity (comparable to that of conventional FPD) was achieved with stainless steel burners inside a thick wall chamber having internal diameter (i.d.) of 0.9 mm and (13) Sun, Y.; Ong, K. Y. Detection Technologies for Chemical Warfare Agents and Toxic Vapors; CRC Press: Boca Raton, FL, 2005; pp 38-41. (14) Kendler, S.; Zaltsman, A.; Frishman, G. Instrum. Sci. Technol. 2003, 31, 357. (15) Zimmerman, S.; Muller, J. Microsyst. Technol. 2000, 6, 241. (16) Vlachos, D. G.; Schmidt, L. D.; Aris, R. Comb. Flame 1993, 95, 313. (17) Ballal, D. R.; Lefebvre, A. H. Proc. R. Soc., London A 1977, 357, 163. (18) Thurbide, K. B.; Cooke, B. W.; Aue, W. A. J. Chromatogr., A 2004, 1029, 193. (19) Thurbide, K. B.; Anderson, C. D. Analyst 2003, 128, 616. (20) Thurbide, K. B.; Hayward, T. C. Anal. Chim. Acta 2004, 519, 121. (21) Hayward, T. C.; Thurbide, K. B. J. Chromatogr., A 2006, 1105, 66.
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using oxygen instead of air. It was noted that the optical glow from the stainless steel burners is very low, resulting in improved sensitivity.20 Similar sensitivity was obtained using air when the combustion chamber i.d. was as wide as 10 mm in a large ccFPD.18 The use of oxygen in µcc-FPD also improves the linear range and sensitivity for OS compounds.21 An important factor in field analytical instrumentation is the reduction of resource (e.g., pure gas) consumption. For FPD, the main resource consumable is hydrogen gas. The typical hydrogen flow in conventional FPD varies between 50 and 100 mL/min. Frishman and Amirav reduced hydrogen consumption to 12 mL/ min by means of flame pulsation.5 Thurbide et al. reduced hydrogen consumption to 5 mL/min in their µcc-flame.18 This paper describes the adaptation and optimization of the µcc-FPD for fast GC separation of several OP and OS compounds. Flame properties and the effects of gas flow rates (air and hydrogen) on its performance are explored. Interfacing between the µcc-FPD and a fast GC is described. Fast separations obtained by coupling the µcc-FPD to commercial capillary columns and silicon glass microfabricated columns are presented. The µcc-FPD is extremely fast and selective; thus, coupling it to a microfabricated column seems to be very attractive for real-time on-site detection and identification of low levels of CWAs. EXPERIMENTAL SECTION Materials. Dimethyl methyl phosphonate (DMMP; 97%, bp ) 181 °C, Alfa Aesar, Ward Hill, MA), diisopropylmethyl phosphonate (DIMP; 95% bp ) 209 °C, Alfa Aesar), triethyl phosphate (TEP; 99%, bp ) 216 °C, Acros Organics Morris Plains, NJ), tributyl phosphate (TBP; 98%, bp ) 289 °C, Alfa Aesar), and benzothiophene (BT; 97% bp ) 21 °C, Acros Organics) were used without further purification. These chemicals were chosen due to their similarity to common CWAs in terms of volatility and chemical structure.5 During this work, a contamination (1-2%) in the DIMP sample was found and later identified by time-offlight mass spectrometry as diisopropyl phosphite (DIP; bp ) 7172 °C at 8 mmHg). Construction of the µcc-Flame Photometric Detector and Coupling to a Fast GC. Three combustion chambers were tested, differing in wall thickness and bore diameter. Type A was made of a quartz tube (QSI, Fairport Harbor, OH) with and i.d. of 1 mm and 2.5-mm wall thickness. Type B was made from a quartz tube with 0.3-mm i.d. and 1.35-mm-thick wall (QSI). This tube was etched with concentrated HF in order to obtain a combustor having 0.8-mm i.d and 0.5-0.6-mm-thick wall. Type C was made of fused-silica tubing (Restek Corp., Bellefonte, PA) with an i.d. of 0.52 mm and wall thickness of 0.05 mm. The polyimide layer that coated the type C combustor was removed with a torch prior to use. Similar designs have been previously described in the literature18-21 and were applied for slow (few minutes) GC separations. This work is focused on the characterization and optimization of the µcc-FPD using air/hydrogen flame (airoperated µcc-FPD) as a sensor for fast (few seconds) GC separations of OS and OP compounds. A design schematic of the µcc-FPD is shown in Figure 1. In type A and B combustors (Figure 1a), air enters the combustion chamber through a stainless steal tube having an i.d. of 0.5 mm and o.d. of 0.8 mm, while hydrogen is delivered through a thinner tube (0.15 mm i.d. × 0.3 mm o.d.). The separation between the
Figure 1. A scheme of the µcc-FPD. (a) Type A and B combustors; (b) type C combustor. Air and hydrogen flows are shown with solid arrows; carrier gas (He) flow is shown with straight dashed arrow; combustion products vent flow is marked with curved dashed arrows. The µcc-FPD is housed inside a heated aluminum chamber (not shown in the figure). Key: 1, air entrance; 2, hydrogen entrance; 3, column effluent; 4, combustion chamber; 5, quartz light guide; 6, optical filter and light sensor; 7, T-shaped union; 8, vent; 9, nitrogen cooling gas line.
end of the hydrogen and air capillaries (burners tips) can be set from 1 to 15 mm, and a stable flame exists over the entire range. Experiments presented in this paper were performed with burner tips separated by 4 mm. Hydrogen and air flow were set between 1 and 100 mL/min by means of digital mass flow controllers (model EW-32907-91, Cole-Parmer, Vernon Hills, IL). Light emitted from the flame was collected with quartz light guides (40 mm long and 6 mm in diameter, QSI). The light was filtered with two interference filters (46033 and 43069 Edmund Optics, Barrington, NJ) having 10-nm band-pass and maximum transmission at 394 nm for OS compounds and 520 nm for OP compounds. In this respect, the terms “S” and “P” channels will be used in relation to the optical emission at 394 and 520 nm, respectively. Emission intensities of the S and P channels were detected with two integrated sensors (Hamamatsu, H5784-04, Bridgewater, NJ). Each integrated sensor is composed of a PMT (operated at a gain of 2 × 105) and a built-in amplifier (1 × 106 V/A) with a bandwidth of 20 kHz. Signals were digitized with a fast A/D card (PCI-6221, National Instruments, Austin, TX) and recorded with a personal computer using data acquisition software written in-house (LabView, National Instruments). Signals were acquired simultaneously from both P and S channels. Integration time for each data point was 1 ms, and acquisition rate was 1000 Hz. Simultaneous detection of both OP and OS compounds may be important in many applications where high throughput is required. However, some cross sensitivity between the two channels was noticed. This effect was more noticeable for OS compounds due to the broad emission of S2*, which partially overlaps the emission spectrum of POH*.12 Due to this overlap, introducing OS compounds to the miniature FPD gave rise to signal in the P channel that was 4-7 times lower than the signal
obtained in the S channel. On the other hand, the signal recorded in the S channel due to combustion of OP compounds was ∼40 times lower than the signal recorded in the P channel. The GC and the miniature FPD were coupled by inserting the end of the chromatographic column through the air tubing. The end of the column was aligned flush with the end of the air tube. To avoid water condensation inside the combustor, which would have extinguished the flame, the system was housed inside a heated (150 °C) aluminum chamber (no optical glow was observed at this temperature). The light sensors were connected to the aluminum chamber and cooled by blowing room air around them. The flame was ignited by heating the combustor with a torch (Alltech, 14602, Columbia, MD). Type C combustor had a smaller i.d. (0.52 mm); thus, a slightly different approach was used for coupling it to the GC. The air supply and column outlet were connected to the upper burner tip with a “T”-shaped union (Swagelok, SS-100-3, Solon, OH). The third port of the fitting was connected to the combustor (type C) and served as both the air line and the combustion chamber. The hydrogen line was inserted 6 mm into the combustion chamber. Since the type C combustor has a very thin wall and low thermal mass, an intense glowing (“background emission”) was obtained without adding any analytes to the flame. To reduce this background emission, the combustor was cooled with a stream (10600 mL/min) of nitrogen around the combustor during operation (without changing flame stoichiometry). Admittedly, such flow rates might be too high for a field-portable system and further development is needed in order to improve the combustor cooling technique. All other details are the same as described for type A and B combustors. GC separations were performed on several commercial columns coated with a film of 5% phenyl/95% methyl polysiloxane. Column length, i.d., and film thicknesses were 2 m × 0.18 mm × 0.2 µm (medium bore), 1 m × 0.1 mm × 0.2 (narrow bore), and 0.5 m × 0.05 mm × 0.05 µm (microbore), respectively. The medium- and narrow-bore columns were made of Silco-steel (Restek) while the microbore column was made of fused silica (Agilent, Palo Alto, CA). The results obtained with the commercial columns are compared to the separation performed on a 1-m column microfabricated in the Engineering Research Center for Wireless Integrated Micro Systems (WIMS) at the University of Michigan. The columns were microfabricated by etching a 150µm-wide, 240-µm-deep channel in a 1.9 × 1.9 cm square silicon chip and covered with a Pyrex wafer, as has been described in detail.22-24 Two ports (350 µm wide by 250 µm deep) were etched at opposite corners of each column to accommodate fused-silica connection lines (0.1 m long, 0.1-mm i.d., Polymicro Technologies, Phoenix, AZ). The connection lines were glued to the columns with epoxy resin (Loctite Corp., Hysol, Rocky Hill, CT) prior to column coating. The microfabricated columns were passivated prior to coating by passing a solution (20% v/v) of 1,1,1,3,3,3hexamethyldisilazane (98%, Acros Organics Organics) in toluene through the column at 100 °C overnight. Microfabricated columns were statically coated with a 0.2-0.3-µm-thick film of poly(dimethylsiloxane) (PDMS). Coating procedure and column characterizations have been previously described.22-25 Briefly, (22) Agah, M.; Potkay, J. A.; Lambertus, G. R.; Sacks, R. D.; Wise, K. D. IEEE J. Microelectromech. Systems 2005, 14, 1039.
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20 mg of PDMS (Specialty Chemical, OV-1, Marietta, OH) was dissolved in 2.0 mL of a 1:1 (v/v) mixture of n-pentane and dichloromethane. A thermally activated cross-linking agent, (dicumyl peroxide, 1 wt %, Aldrich, Milwaukee, WI) was added to the liquid polymer phase. The column was completely filled with the coating solution, and the column outlet was sealed. The solvent was then evaporated at a pressure of ∼3.25 psi and temperature of 40 °C until the column appeared empty (∼15 min). After coating, the film was thermally cross-linked by heating the column to 180 °C at 5 °C/min and holding for 4 h under nitrogen flow. Fast temperature programming was achieved by wrapping heaters around the columns. The medium- and narrow-bore Silcosteel columns were wrapped with adhesive heat tape (Clayborn, B-16-2, Truckee, CA) while the fused-silica column was inserted into a spring that was made in-house from NiCr heating wire (Omega, NI80-010, Stamford, CT). The microfabricated column was wrapped with adhesive Kapton tape (Kaptontape, Torrance, CA) and then wrapped with NiCr heating wire (Omega). Column temperatures were measured with a thin thermocouple (type J, Omega). Fast injection was achieved using two heated segments in the column, the first one (5-10-cm-long “focuser”) was used to trap the analytes at room temperature (25 °C) after the injection. The trapped analytes were then thermally desorbed from the focuser by means of ultrafast heating (∼100 °C/s for 2 s). The sample then recondensed as a narrow plug on the second segment of the column (0.5-2 m long), which served for the separation. For the microfabricated column, the inlet connection line was used as the focuser. The focuser was heated with the NiCr spring in a manner similar to the commercial columns. Separations were performed by heating the separation column from room temperature to 250 °C. Column heating rates and carrier gas flow rates were as follows: 1000 °C/min and 8 mL/ min (medium bore), 600 °C/min and 2.5 mL/min (microfabricated column), 2200 °C/min and 2.6 mL/min (narrow bore), and 2100 °C/min and 0.7 mL/min (microbore). Carrier gas for GC separations was high-purity helium (Metro Welding Supply Corp., Detroit, MI). The carrier gas supply was passed through filters for hydrocarbons, oxygen, and water vapor. Before starting the experiments, the carrier gas flow rate was measured at the column outlet by means of a digital mass flow meter (model ADM1000, J&W Scientific, Folsom, CA). Separation conditions were selected to provide narrow chromatographic peaks and short analysis times while maintaining peak capacity of at least 20, to test the µccFPD compatibility with fast GC separations. Liquid samples containing known concentrations of the analytes (0.2-0.5 µL of hexane solution) or headspace samples were injected to a standalone, heated (275 °C) split/splitless inlet (Restek, 22721) that was connected to the chromatographic column. Liquid samples were split during injection (split ratio of 10-100), while headspace samples were injected in splitless mode. (23) Lu, C-. J.; Steinecker, W. H.; Tian, W-. C. Agah, M. Potkay, J. A.; Oborny, M. C.; Nichols, J.; Chan, H.; Driscoll, J.; Sacks, R. D.; Pang, S. W.; Wise, K. D.; Zellers, E. T. Lab Chip 2005, 5, 1123-1131. (24) Lambertus, G. R.; Elstro, A.; Sensenig, K.; Potkay, J. A.; Agah, M.; Scheuering, S.; Wise, K. D.; Dorman, F.; Sacks, R. D. Anal. Chem. 2004, 76, 2629. (25) Reidy, S.; Lambertus, G. R.; Reece, J.; Sacks, R. Anal. Chem. 2006, 78, 2623.
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RESULTS AND DISCUSSION Flame Characteristics. The minimum hydrogen flow rate required to sustain a stable flame for the various combustors was explored. In most cases, the flame was located between the burner tipssthis mode of operation will be referred to as between burner mode.18 In type A, the minimum hydrogen flow rate needed to sustain the flame was 15 mL/min while the air flow rate can be set anywhere in the range of 15-70 mL/min. For type B, the minimum hydrogen flow rate was 8 mL/min (air flow rate, 7-50 mL/min) and only 5 mL/min in type C (air flow rate, 5-30 mL/ min). The main impediment in using a type C combustor in a FPD is the intense optical glow obtained due to self-heating of the combustor by the flame, making the measurement of low light level due the combustion of analyte in the FPD difficult. The selfheating phenomenon has some advantage, in that elevating the combustor temperature decreases the quenching distance17 and enables enclosure of the flame inside a smaller chamber. Comparing these findings to those previously reported for µccFPD shows that the minimum hydrogen flow rate needed to sustain a stable flame in a type C combustor is lower (5 mL/min) than that reported by Thurbide et al. (15 mL/min).18 It also shows that the type C combustor provides more stable combustion. However, in both cases, background levels are high. It was also reported that using oxygen instead of air increases flame stability while decreasing the hydrogen flow rate needed to sustain a stable combustion to 6 mL/min.18 Since this work is focused on minimizing resource consumption, the tradeoffs involved in using air were further explored. The possibility of dissipating some of the heat from the combustor by delivering nitrogen gas around it was explored, to reduce the glow without extinguishing the flame. It was found that the light intensity recorded in the P channel was more than 20 times higher than that measured in the S channel. Cooling the flame with a flow of 350 mL/min of nitrogen significantly reduced the glowing in both channels but did not eliminate it entirely. Further cooling of the combustor by increasing the nitrogen flow above 550 mL/min resulted in extinguishing the flame. Addition of BT, TEP, and SF6 to the cooled flame did not yield any optical signal. Noise levels obtained with the uncooled flame were too high to obtain sensitive detection based on optical emission. Thus, all other experiments were performed with type A and B combustors. Figure 2 shows the noise and background levels recorded with type A and B combustors as a function of the ratio between the hydrogen and air flow rate. Hydrogen flow rates were 20 mL/ min for type A and 10 mL/min for type B. The measurements were performed by varying the air flow rate and recording the emission intensity (background) and the standard deviation of the baseline fluctuations (noise) caused by the flame. Carrier gas (He) flow rate during this measurements was 0.7 mL/min; no analytes were added to the flame. In both cases, noise levels, which are ∼2 times higher than the electronic noise level without the flame, are achievable at the optimal air flow. The noise level, generated from a stable flame (i.e., no flickering), can be calculated according to,18,26 noise ) (background × eg/t)1/2; noise and background are measured in amperes, e is the electron charge (1.6 × 10-19C), g is the PMT gain (2 × 105), and t is the integration time (0.001 s). (26) Aue, W. A.; Singh, H.; Sun, X.-Y. J. Chromatogr., A 1994, 687, 283.
Figure 2. Noise and background levels as a function of the ratio between the hydrogen and air flow rate, (a) Type A combustor, hydrogen flow, 20 mL/min; (b) type B combustor, hydrogen flow, 10 mL/ min. Background emission intensities are displayed with triangles (solid for the P channel and open for the S channel), and noise level is displayed with diamonds (solid for the P channel and open for the S channel). Electronic noise level recorded without the flame is shown with a dashed line.
Noise and background currents were obtained by dividing the experimental results (in volts) by the integrated light sensor amplification factor (1 × 106 V/A). Noise levels, calculated this way are 20-30% lower than experimental values for a type A combustor and 50-60% for a type B combustor. It was reported that calculated noise levels for a large cc-FPD (combustor diameter of 10 mm) differ from the experimental value by 12%,18 indicating that miniaturizing the combustor reduces flame stability. For the type A combustor, the noise and background levels follow the same trend in both channels, which is not the case for the type B combustor. The most noticeable difference is obtained in a hydrogen-lean flame, where noise and background levels in the P channel were much higher than in the S channel. It was noticed that at these conditions the flame touched the bottom burner and increased its temperature, resulting in a strong orange glow. This orange glow is filtered away by the narrow band-pass filter used in the S channel (maximum transmission at 394 nm) but recorded in the P channel (maximum transmission at 520 nm), resulting in increased noise level. Signal-to-noise (SNR) measurements as a function of hydrogen/ air flow, were performed by injecting known amounts of BT and TEP to the GC. Figure 3 shows the SNR obtained with type A and B combustors; for the sake of simplicity, each set of results was normalized to 1. It shows that the optimal condition for operating the miniature FPD depends strongly on the combustor type. For a type A combustor, maximum SNR is achieved at hydrogen/air flow rate ratio of 0.821 and 0.995 for the P and S
Figure 3. Normalized SNR as a function of the ratio between the hydrogen and air flow rate. (a) Type A combustor, hydrogen flow, 20 mL/min; (b) type B combustor, hydrogen flow, 10 mL/min. Signals in the P and S channels are marked with solid and open diamonds, respectively.
channels, respectively. While, the maximum SNR for a type B combustor is achieved at hydrogen/air flow rate ratio of 0.559 and 0.457 for the P and S channels, respectively. Figure 3 also shows the tradeoffs involved in constructing a µcc-FPD that is capable of measuring both OP and OS compounds simultaneously. In this case, the same gas flow rates have to be set for both channels. By choosing to operate a type A combustor at the optimum flow rate for either P or S channels, the SNR in the other channel will be 20-30% lower than the SNR obtained at optimum conditions. Setting the gas flow rates to the optimum point of the S channel in a type B combustor will decrease the SNR in the P channel by 40%. Furthermore, around this working point the SNR in the P channel might vary by a factor of 2 when the hydrogen/ air flow rate ratio is changed only by 10%. Such sensitivity to the operational parameters is not desirable in analytical instrumentation. Setting the hydrogen/air flow rate ratio to the optimum point for the P channel where the SNR in the S channel is 17% lower than in optimal conditions and the effects of fluctuations in the gas flow rates are much lower, seems to be a better choice in this case. Similar to previous work describing miniature µcc-FPD,18 optimal flame conditions are achieved closer to a stoichiometric flame than in conventional FPD. In our case, optimal H2/O2 was 2.3-5 (which corresponds to H2/air of 0.46-1.0) while in a conventional FPD H2/O2 varies between 2.5 and 10 depending on the specific combustor design.27 (27) Farwell, S. O.; Barinaga, C. J. J. Chromatogr. Sci. 1986, 24, 483.
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Figure 4. µcc-FPD response to phosphorus (as TEP) and sulfur (as BT). Bottom frames show chromatograms obtained close to the limit of detection for BT (b) and TEP (c). Mass injected to the microbore column was 1 ng (BT) and 5 pg (TEP), full peak width at half-height were 17 (TEP) and 23 ms (BT), resulting in mass flow of 4 ng of S/s and 25 pg of P/s. Calibration curve and separation of TEP were obtained using type A combustor (hydrogen flow rate 20 mL/min, air flow rate 25 mL/min). Calibration curve and separation of BT were obtained using type B combustor (hydrogen flow rate 10 mL/min, air flow rate 20 mL/min).
The MDL obtained with the µcc-FPD was measured by injecting known amounts of BT and TEP dissolved in hexane. MDLs obtained with the µcc-FPD are 10-12 pg of P/s and 3-4 ng of S/s at SNR of 2 for both type A and B combustors (when operated at optimal conditions). Calibration curves obtained with the µcc-FPD are shown in Figure 4, as well as the chromatograms obtained when injecting low levels of BT and TEP. Figure 4 shows a linear response in the P channel with a concentration range spanning more than 3 orders of magnitude. The response in the S channel was quadratic over 1.5 orders of magnitude and reached a plateau at a mass flow of ∼250 ng of S/s. The MDL for OS compounds is 300 times higher, and the dynamic range is 1 order of magnitude narrower in comparison to conventional FPD.12 The µcc-FPD MDL of OP compounds is ∼1 order of magnitude higher than conventional FPD, and the response is linear with the mass flow over 3 orders of magnitude. Reducing the amount of light that reaches the sensor increases the linear range to 3.7 orders of magnitude, which is slightly lower than the linear dynamic range obtained with a conventional FPD.12 This was achieved by recording the light emitted during the combustion of high concentrations of OP compounds in the S channel. In this way, the amount of light that reaches the light sensor is 40 times lower than in the P channel and saturation of the light sensor is avoided. This study is oriented toward the detection of low levels of OP and OS compounds; thus, further attempts to extend the linear dynamic range are beyond the scope of this work. It is noted that sensitivities which are comparable to those obtained with con6770
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ventional FPD may be obtained with µcc-FPD using oxygen instead of air. The response to OP compounds is linear with the concentration over 5 orders of magnitude while the response to OS compounds shows quadratic dependence on the mass flow over 3.5 orders of magnitude.20 A linear response to OS compounds over more than 4 orders of magnitude was achieved by using counterstreams of oxygen and oxygen/hydrogen mixture in the µcc-FPD and recording the emission from SOH*. Detection limits in this mode of operation are 1 order of magnitude higher than in conventional FPD.21 Several sources for the difference between the performances obtained with air-operated µcc-FPD and those obtained with conventional FPD and oxygen-operated µcc-FPD were considered, especially in the case of OS compounds. It was reported that, in air-operated large cc-FPD, where the analytes are introduced through the bottom burner (inside the hydrogen stream), sulfur emission occurred close to the upper burner.18 Thus, a possible explanation for the increased MDL for OS compounds is that emission occurs upstream where light cannot reach the light sensor. This possibility was tested by introducing sulfur hexafluoride to the µcc-FPD (mass flow of 500-5000 ng/s for 2 s) through the chromatographic column and observing the light emitted from the flame by eye in a dark room. The emission occurred between the burners, which were only 4 mm apart. Since the diameter of the light guide is 6 mm, it is reasonable to assume that most of the light reaches the sensor. Another possibility that was considered is that the noise level is high, resulting in increased MDL. As mentioned earlier, the noise level generated from the flame was only 20-50% higher than the noise level expected from an ideal (no flickering) light source. This difference cannot account for the performance obtained with the air-operated µcc-FPD. Possible masking of the light emitted during the combustion of OS compounds by the background emission was also considered. Examining the results in Figure 2 shows that the background levels are 5 and 8 mV in the P and S channels, respectively. This difference between the two channels cannot account for the difference between the performance obtained for OP and OS compounds. Another possibility that was considered is reduced combustion efficiency in air-operated µcc-FPD. Combustion efficiency is affected by the time needed to complete the combustion reactions (t, reaction time) and the analyte residence time in the flame (R). The ratio between t and R is defined as the Damko¨kler number (Da) which is often used to quantify combustion efficiency.28 Changing the combustor dimensions or using oxygen instead of air also changes the Da number and might change also the combustion efficiency. Combustion efficiency might be evaluated experimentally by measuring the amount of analyte that passes through the flame without undergoing combustion. This can be done by sampling the gas emanating from the combustor and analyzing the content of the sample. These measurements are part of future experiments aimed to characterize the tradeoff involved in miniaturizing the air-operated µcc-FPD. Very-Fast and Ultrafast Gas Chromatographic Separations. The µcc-FPD compatibility with very-fast and ultrafast GC separations was tested by coupling it to short, narrow-bore GC columns employing fast temperature programming, high carrier (28) Mastorakos, E.; Taylor, A. M. K. P.; Whitelaw, J. H. Combust. Flame 1995, 102, 101.
gas velocities, and narrow injection plugs. The terms “very-fast” and “ultrafast” GC separation were defined previously by several groups.9,29,30 In this work, the full width at half-maximum (fwhm) was used to distinguish between very-fast GC separations (fwhm between 30 and 200 ms) and ultrafast GC separations (fwhm