Anal. Chem. 2009, 81, 6422–6428
Gas Chromatograph-Combustion System for 14 C-Accelerator Mass Spectrometry Cameron P. McIntyre,*,† Sean P. Sylva,‡ and Mark L. Roberts† National Ocean Sciences Accelerator Mass Spectrometry Facility, Department of Geology and Geophysics and Department of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543 A gas chromatograph-combustion (GC-C) system is described for the introduction of samples as CO2 gas into a 14 C accelerator mass spectrometry (AMS) system with a microwave-plasma gas ion source. Samples are injected into a gas chromatograph fitted with a megabore capillary column that uses H2 as the carrier gas. The gas stream from the outlet of the column is mixed with O2 and Ar gas and passed through a combustion furnace where the H2 carrier gas and separated components are quantitatively oxidized to CO2 and H2O. Water vapor is removed using a heated nafion dryer. The Ar carries the CO2 to the ion source. The system is able to separate and oxidize up to 10 µg of compound and transfer the products from 7.6 mL/min of H2 carrier gas into 0.2-1.0 mL/min of Ar carrier gas. Chromatographic performance and isotopic fidelity satisfy the requirements of the 14C-AMS system for natural abundance measurements. The system is a significant technical advance for GC-AMS and may be capable of providing an increase in sensitivity for other analytical systems such as an isotope-ratio-monitoring GC/MS. 14
C-accelerator mass spectrometry (AMS) is a well established technique for age dating and tracer studies in environmental sciences, archeology, and bioanalytic research. Recently, separation techniques such as gas chromatography and high performance liquid chromatography have been combined with 14 C-AMS so that routine molecular level measurements can be made.1,2 At The National Ocean Sciences AMS facility at Woods Hole Oceanographic Institution we are developing a continuousflow radiocarbon dating system that consists of a microwaveplasma gas ion source for our compact 14C-AMS system and a GC-combustion (GC-C) interface for sample introduction.3 It is expected to significantly improve our capacity for compound * Corresponding author. E-mail:
[email protected]. † National Ocean Sciences Accelerator Mass Spectrometry Facility, Department of Geology and Geophysics. ‡ Department of Marine Chemistry and Geochemistry. (1) Flarakos, J.; Liberman, R. G.; Tannenbaum, S. R.; Skipper, P. L. Anal. Chem. 2008, 80, 5079–5085. (2) Skipper, P. L.; Hughey, B. J.; Liberman, R. G.; Choi, M. H.; Wishnok, J. S.; Klinkowstein, R. E.; Shefer, R. E.; Tannenbaum, S. R. Nucl. Instrum. Methods Phys. Res., Sect. B 2004, 223-224, 740–744. (3) Roberts, M. L.; Schneider, R. J.; von Reden, K. F.; Wills, J. S. C.; Han, B. X.; Hayes, J. M.; Rosenheim, B. E.; Jenkins, W. J. Nucl. Instrum. Methods Phys. Res., Sect. B 2007, 259, 83–87.
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specific 14C measurements and afford greater sensitivity and analyte resolution over existing GC-AMS systems that use solidtarget cesium sputter sources modified to accept CO2 gas.1,4,5 Such sources suffer from memory effects and limited output, making them less than ideal for monitoring chromatographic effluents.2 It is possible to radiocarbon date gas chromatographic effluents; however, it requires the use of cryogenic trapping to preconcentrate samples, which is an intensive procedure and limits the number of compounds that can be processed at one time for a given sample.6-8 While a GC can provide quantities of carbon adequate for analysis of 14C at natural abundance, it presents a significant technical challenge to couple one to the microwave-plasma ion source. For natural abundance radiocarbon measurements, at least 2 µg of carbon is required to obtain sufficient 14C+ ions.3 The source uses an Ar/CO2 gas system as Ar readily forms a plasma and is the preferred gas for stable operation. In a previous study, a microwave-plasma gas ion source was successfully operated with an argon carrier gas flow rate of 0.24 mL/min.3 CO2 is preferred as it can be quantitatively produced by combustion of analytes, is easy to transfer, and is established in 13C isotope-ratio-monitoring GC/MS (irm-GC/ MS).9 For gas chromatography, however, the carrier gas for maximum efficiency is hydrogen or helium. Moreover, compounds containing more than 2 µg of carbon require a 0.53 mm i.d. capillary column and carrier gas flow rates of 4-9 mL/ min.10 Therefore, the GC system needs to be capable of continuously separating and oxidizing micrograms of sample and transferring the resulting CO2 into relatively low flows of argon gas. Some irm-GC/MS systems use an open split to deliver a constant flow of gas to the MS, but the resulting loss of sample in those systems is unacceptable for 14C-AMS.9 Molecular separators such as diffusion, effusion, jet, and semipermeable membrane separators have also been used to (4) Ramsey, C. B.; Ditchfield, P.; Humm, M. Radiocarbon 2004, 46, 25–32. (5) Kjeldsen, H.; Churchman, J.; Leach, P.; Ramsey, C. B. Radiocarbon 2008, 50, 267–274. (6) Eglinton, T. I.; Aluwihare, L. I.; Bauer, J. E.; Druffel, E. R. M.; McNichol, A. P. Anal. Chem. 96, 68, 904–912. (7) Rottenbach, A.; Uhl, T.; Hain, A.; Scharf, A.; Kritzler, K.; Kretschmer, W. Nucl. Instrum. Methods Phys. Res., Sect. B 2008, 266, 2238–2241. (8) Zencak, Z.; Reddy, C. M.; Teuten, E. L.; Xu, L.; McNichol, A. P.; Gustafsson, O. Anal. Chem. 2007, 79, 2042–2049. (9) Merritt, D. A.; Freeman, K. H.; Ricci, M. P.; Studley, S. A.; Hayes, J. M. Anal. Chem. 95, 67, 2461–2473. (10) Barry, E. F. Modern Practice of Gas Chromatography, 4th ed.; Grob, R. L., Barry, E. F., Eds.; Wiley-Interscience: Hoboken, NJ, 2004; p 136, p 141. 10.1021/ac900958m CCC: $40.75 2009 American Chemical Society Published on Web 07/02/2009
Figure 1. Schematic diagram of the gas chromatograph-combustion system.
concentrate samples but they have limited efficiency or compatibility with downstream combustion reactors.11 Accordingly, alternative GC-C strategies are required. We present here a novel procedure that involves combustion of the GC H2 carrier gas and sample with O2 and subsequent removal of the water vapor with a nafion dryer. A small, supplementary flow of Ar is added prior to combustion to carry the product CO2 to the ion source. In this paper we describe the system and the results of experiments that assess its chromatographic performance and isotopic fidelity. EXPERIMENTAL SECTION Instrumentation. A schematic diagram of the system in analysis mode is shown in Figure 1 with details of the individual components given below. Gas Chromatograph. We used a HP5890 Series II (HewlettPackard, Avondale, PA) gas chromatograph fitted with a programmable, cool on-column capillary inlet, a flame ionization detector (FID), and a 60 m, Restek Rxi-1 ms, 0.53 mm i.d., 1.5 µm film thickness, megabore capillary column (Restek, Bellefonte, PA). The required sample capacity (micrograms) precluded the use of smaller diameter columns. H2 and He were used as carrier gases and selected as required by means of three-way valve. The carrier gas pressure was set to give an average linear velocity of 49 cm/s at 30 °C (7.6 mL/min) for H2 and was not changed for He which was used during standby, purging, and conditioning. The temperature of the GC oven was programmed from 30 to 90 °C at 25 °C/min, from 90 to 320 °C at 10 °C/min, and held at 320 °C for 20 min. The GC was operated in constant pressure mode with the temperature of the injector set to track that of the oven, and the GC/furnace interface was held at 300 °C. To regulate heating of the combustion furnace and nafion dryer, the GC was retrofitted with two temperature controllers, solid state relays, and K-type thermocouples (Omega, Stamford, CT). (11) McFadden, W. H. Techniques of Combined Gas Chromatography/Mass Spectrometry: Applications in Organic Analysis; Wiley: New York, 1973; pp 157-221.
Combustion Furnace. The combustion reactor used a metal catalyst together with excess O2. Residual O2 in the Ar carrier gas is acceptable for the operation of the microwave plasma. The combustion furnace was a 12 in. long × 0.5 in. i.d., 120 V, 350 W, ceramic fiber heater (Watlow, St. Louis, MO), and a microvolume reactor tube. The reactor tube was a round, single-bore extrusion 99.8% alumina tube, 360 mm long, 1.57 mm o.d., 0.79 mm i.d. (McDanel, Beaver Falls, PA). The i.d. of the reactor tube was selected to be sufficient to minimize blockages and leave an annular gap similar to the i.d. of the capillary column so as to conserve the linear flow rate. The catalyst consisted of three Cu, one Ni, and one Pt, 0.1 mm diameter wires (Alpha Aesar, Ward Hill. MA). These had a minimum purity of 99.994% and were twisted together to give a final diameter of 0.3-0.34 mm and 6-8 turns per cm. A 27 cm length of twisted catalyst wire was inserted into the reactor tube and positioned 25 mm from the bottom. GC/Combustion Furnace Interface. The metal block provided to house the FID was modified to act as a heated transfer line between the GC and the combustion reactor (Figure S1 in the Supporting Information). The blank end of a tube adapter was machined to fit in place of the FID jet and to seal the reactor tube with a Valcon polyimide ferrule (Figure S2 in the Supporting Information). The tube adapter was collared with a sliding-fit brass tube 37 mm long × 6.35 mm i.d. × 10 mm o.d. to improve heat conduction from the FID block and combustion furnace. The capillary column was inserted through the axis of the adapter directly into the reactor tube and positioned 2-3 mm from the catalyst. This arrangement allowed the supplementary gases (O2/ Ar mixture) to flow through the annular space between the adapter and capillary column and merge coaxially with H2 carrier gas exactly where the column end met the catalyst. This was intended to minimize the volume of the mixing zone prior to oxidation to maintain peak shape. Also, it meant that we could avoid adding Ar before the analytical column where its viscosity would change with temperature, thus achieving a constant flow of Ar through the system. The ceramic-fiber heater was positioned over the reactor tube so that its base Analytical Chemistry, Vol. 81, No. 15, August 1, 2009
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was inline with the nut of the tube adapter and a layer of Kaowool insulation was placed around the top of the FID block between the GC and furnace. Nafion Dryer. The nafion dryer was constructed from a 325 mm length of 6.35 mm o.d. quartz tubing fitted with two tees. Nafion tubing (Permapure, Toms River, NJ) of two diameters was used: (A) A 27 cm length of nafion tubing 0.635 mm i.d. was shrink-fitted onto two pieces of 0.53 mm i.d. fused silica capillary 10 and 90 cm long. Shrink fitting was accomplished by expanding the end of nafion tubing in methanol for 30 s, sliding it over the capillary tubing and drying with compressed N2. (B) A 27 cm length of nafion tubing 0.356 mm i.d. was shrink-fitted onto a 10 cm length of 0.32 mm i.d. fused silica capillary and epoxied onto a 90 cm long of 0.25 mm i.d. fused silica capillary. Configuration A was used unless where specified otherwise. The nafion/capillary tubing was pulled through the quartz tubing assembly and held in place using Valcon polyimide ferrules. The inlet-side (bottom) half of the dryer was double wrapped with heater tape 120 V, 104 W, 0.5 in. × 4 ft (Omega, Stamford, CT), and the second thermocouple was positioned between the layers. The nafion tubing was positioned to maximize the amount (length) in the heated zone. Ar for the annular, countercurrent drying gas was connected to the side port of the top tee and regulated by a metering valve. The inlet capillary (10 cm length) was trimmed so that 30 mm extended from the back of the nut, and the dryer was connected to the reactor tube via a stainless steel union and Valcon polyimide ferrules. The union was kept at >100 °C via radiant heat from the furnace and Kaowool insulation. The outlet end-capillary (90 cm length) was attached to the six-port valve. Calculation of water vapor residence and diffusion times showed that both configurations of dryer were capable of removing >99% of the water vapor at 50 °C.12 For this study, we operated conservatively at 110 °C with a high external flow rate of drying gas (500 mL/min) which resulted in a signal for water (18 amu) that was comparable to background CO2 and N2. Water vapor is far less of a concern for AMS than for irm-GC/MS (where it produces interference at m/z 45 due to HCO2+) because molecular bonds are broken by the microwave plasma and interfering ions are removed by the AMS system. Gas Management. The system used UHP grade Ar, He, H2, and O2. A six-port switching valve (Vici Valco, Houston, TX) was used to direct the solvent peak to the vent off the six-port valve by reversing flow through the combustion furnace with backflush argon. When required, a second four-port valve was connected in series so that the gas stream could be diverted to a transfer line for offline trapping of the CO2. Control of the GC and switching of the valves was accomplished using NI Labview software and a NI USB-6009 DAQ (National Instruments, Austin, TX). The flow rates of O2, carrier Ar, and backflush Ar where controlled with 0-30 psi regulators (Porter, Hatfield, PA) and fused-silica capillaries. The O2 was connected to 15 m of 0.25 mm i.d. nondeactivated fused silica capillary located in the GC oven. It was necessary to locate the capillary in the oven so that its flow rate was maintained at a constant ratio with that of the H2 for stoichiometric combustion, i.e., H2/O2 ) 2:1. The rate of change in the viscosity of O2 with respect to temperature (12) Leckrone, K. J.; Hayes, J. M. Anal. Chem. 97, 69, 911–918.
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is greater than that for H2, and the ratio increases by approximately 5% from 30-320 °C. When the GC was at 30 °C, an excess of 50-100 µL of O2/min was set, and the metal oxide catalyst supplied additional O2 as the ratio exceeded 2:1 over the course of the GC run. The use of electronic pressure controllers would remove the necessity of locating a capillary for O2 in the oven. A three-way switching valve was used inline to release O2 pressure when required. The Ar carrier gas was connected to the six-port valve via 90 cm of 0.1 mm fused silica capillary and was set at a flow rate of 1 mL/min unless stated otherwise. The Ar carrier gas from the sixport valve and the O2 gas from the capillary were combined at a tee, the outlet of which was connected to the H2 inlet line of the FID block. This allowed O2 and Ar carrier gas to be mixed and added to the GC carrier gas precisely at the end of the analytical column immediately prior to oxidation. The Ar backflush gas was connected to the six-port valve using 1 m of 0.25 mm fused-silica capillary and was operated at a flow rate of 25 mL/min. Gas Monitoring. Gases were monitored using a Dycor LC100 residual gas analyzer (RGA) connected to an Alcatel ATH31+ hybrid turbo pump backed by an Adixen Drytel 1025 compact dry pumping system. A 1.6 m length of 0.05 mm i.d. fused silica capillary was connected to the RGA which sampled the gas stream from the system at 0.022 mL/min. For the Ar flow rate experiment, a 1.0 m length of 0.075 mm capillary was also used to sample at 0.144 mL/min. The quadrupole was tuned for maximum sensitivity using an electron energy of 100 V, a focus voltage of 100 V, and a repeller voltage of 102 V. Chromatograms were recorded using Dycor System 200 PC software in selected ion monitoring mode (trend mode) with a dwell time of 10 ms. Masses utilized were 4, 18, 28, 32, 40, and 44 amu for He, water, N2/CO, Ar, and CO2, respectively. The chromatogram for CO2 (m/z 44 amu) was exported and integrated manually. Operational Procedure. The system was initially conditioned for 1 h with He carrier gas, the combustion furnace at 650 °C, 1 mL/min Ar carrier gas, and 0.1 mL/min O2. The Ar drying gas was set to 100 mL/min, and the nafion dryer was heated to 50 °C. The O2 pressure was increased to give a flow rate of 5.5 mL/min (a large excess), and the carrier gas was switched to H2. He (4 amu) was monitored and when the signal fell to zero (signaling that the H2 had passed through the GC), the nafion dryer temperature was increased to 110 °C and the nafion tubing was retensioned. The O2 pressure was reduced to give a flow rate 4 mL/min (0.2 mL/min excess), and the system was left to stabilize overnight as the residual He was purged. Next, the Ar drying gas and backflush gas were set to 500 and 25 mL/min, respectively, and the combustion furnace was raised to 850 °C. The O2 pressure was reduced so that it was in 0.05-0.1 mL/min excess. This was determined by monitoring the partial pressure of O2 with the RGA and also by measuring the total flow rate through the system, i.e., Ar + O2 ) 1.05-1.1 mL/min. A volume of 0.1 µL of hexane was injected, and the retention time of the peak was noted for the backflush time window. The system was now ready for operation. The system could be operated stably for several months at a time, and when not in use, it was left in standby mode. The combustion furnace was reduced to 650 °C, the nafion dryer to 50 °C, and the
carrier gas switched to He. When the He had passed through the GC, the O2 flow rate was reduced to 0.1 mL/min, the Ar drying gas to 10 mL/min, and the backflush Ar was switched off. Performance Experiments. Standards. Stock solutions of compounds in hexane were prepared and diluted with hexane as required: standard A, 10 ± 1 µg/µL per component of C10-C30 even carbon number n-alkanes (Alltech, Deerfield, IL); standard B, 12 ± 1 µg/µL per component of n-decane, 2-methylnaphthalene, methyl laurate, pristane, n-nonylbenzene, and 1-octadecene; standard C, 10.21 µg/µL eicosane (n-C20). Chromatography. Standard A was analyzed with the system in GC-C mode and in conventional GC-FID mode, i.e., with the FID reassembled. In GC-C mode, both configurations of the nafion dryer were tested. Stock standard A was diluted to 1 µg/µL per component, and 1 µL samples were injected. The average peak full width at half-maximum (fwhm) and standard deviation was determined for each compound. Standard B was a mixture of compounds from different classes. Stock standard B was diluted to 1.2 µg/µL per component, and 1 µL was injected. Yield. The yield of CO2 was determined by diverting the gas stream by means of a four-port switching valve and 1/8 in. stainless steel tubing transfer line to an offline trapping apparatus.13 The valve was switched for 0.5 min before the peak eluted and was held for 2.5 min. Stock standard C (1 µL) was injected 5 times, and the quantity of the trapped CO2 was determined using a capacitance manometer and a reproducible volume and temperature. The average quantity of CO2 and the standard deviation was used to calculate yield. Linearity. The linearity of the CO2 response was investigated for a single compound and a mixture of compounds over a concentration range of 1-10 µg/µL per component. Stock standard C was diluted to give final concentrations of 1, 2.5, 5, 7.5, and 10 µg/µL. A volume of 1 µL of each solution was injected three times (n ) 6 for 10 µg/µL), and the peak area was plotted against the concentration. Stock standard A was used neat and diluted to 1, 2, and 5 µg/µL with hexane. A volume of 1 µL of each solution was injected, and the peak area and width of each compound were plotted against the mass of carbon. Linear regression was performed on both plots to determine a correlation coefficient. Ar Carrier Gas Flow Rate. The effect of lowering the Ar carrier gas flow rates was investigated with the nafion dryer in both configurations. Stock standard A was diluted to 1 µg/µL per component. A volume of 1 µL was injected with the carrier Ar carrier gas set at 1.0, 0.8, 0.6, 0.4, and 0.2 mL/min. The peak fwhm and area of each compound at each flow rate was determined and plotted against the Ar flow rate. The change in total flow rate of the system and split ratio of the open split as a peak elutes was modeled numerically using a Gaussian shaped peak with a width of 4σ divided into 100 time slices. The fractional height was used to determine the flow rate of CO2 from a peak and the total flow rate (Ar + CO2) of the system. A corrected split rate was then calculated and used to estimate the flow rate of CO2, overall yield, and mole fraction of CO2 delivered to the source.
Isotopic Fidelity. Eicosane and 5R-androstane were selected as petrogenic and nonpetrogenic 14C standards. Eicosane was fossilfuel derived and therefore contained no radiocarbon, while the 5R-androstane was known to be of recent origin. δ13C and 14C Fm values where determined by conventional methods. Four 1 mg replicates of each were combusted using an elemental analyzer, and the resulting CO2 graphitized by standard Fe-H2 reduction. δ13C and 14C Fm values were determined using conventional IRMS and AMS techniques. δ13C and 14C Fm values of the CO2 generated by the GC-C system were determined. Five 1 µL injections of 10 ± 0.5 µg/ µL solutions of eicosane and 5R-androstane were chromatographed, and the resulting CO2 was trapped offline (as for yield). Five injections were pooled to obtain sufficient quantities of carbon for graphitization and AMS analysis. A single graphite sample for eicosane and duplicate samples from 5R-androstane were prepared and analyzed. RESULTS AND DISCUSSION Chromatography. The n-alkanes in standard A were clearly resolved and had symmetrical peak shapes (Figure 2). At the lowest Ar flow used (0.25 mL/min), slight tailing occurred due to dead volumes in the valve and fittings. The width of the peaks and their precision was comparable to that of the GC-FID (Table 1). With the Ar carrier at 1 mL/min, peak widths were 21 ± 4% wider with the nafion dryer in configuration A and 13% ± 3% for configuration B. The band broadening was due extracolumn factors, that is, the combustion furnace, nafion dryer, fittings, and connecting tubing. The changes in tubing diameters and linear velocities cause spreading that has an additive effect. It was symmetrical in nature indicating that it was due to processes such as laminar flow and “new” spreading rather than dead volumes which result in tailing.14 Removal of the relatively large quantity of water in the nafion dryer is also likely to contribute as this concentrates the CO2 in the Ar stream. In each mode, the peaks for higher molecular weight alkanes (n-C26-n-C30) indicated overloading. The chromatogram for standard B demonstrates that the system can combust various compound classes that might typically be analyzed for radiocarbon including condensed aromatics.6 These results demonstrate that the chromatographic performance was satisfactory and that the combustion, drying, and transfer processes did not have a significant negative impact. Yield. Yield was determined to test if the system could quantitatively and reproducibly combust a sample. The most concentrated eicosane standard was used to minimize a contribution from the background. The quantity of eicosane per injection was 10.21 ± 0.1 µg which theoretically produced 0.723 ± 0.007 µmol of CO2. The 5 injections gave an average of 0.738 ± 0.010 µmol, which corresponded to a yield of 102.1 ± 2.4%. This demonstrated that oxidation was quantitative and repeatable. Linearity. The linearity of CO2 response was determined to confirm that combustion was quantitative over a range of sample sizes and that overloading was absent. This is of interest for quantitative enrichment studies and was also required for assessing the magnitude of band broadening from extracolumn
(13) Rosenheim, B. E.; Day, M. B.; Domack, E.; Schrum, H.; Benthien, A.; Hayes, J. M. Geochem. Geophys. Geosyst. 2008, 9, DOI: 10.1029/2007GC001816.
(14) Sternberg, J. C. In Advances in Chromatography, Vol. 2; Giddings, J. C., Keller, R. A., Eds.; Marcel Dekker: New York, 1966; pp 205-270.
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Figure 2. Chromatograms of even carbon number n-alkanes (upper) and compounds from different classes (lower) combusted to CO2 and transferred from 7.6 mL/min H2 carrier gas (30 °C) into a constant flow of Ar carrier gas as indicated in each panel. Table 1. Peak Full Width at Half Maximum (fwhm, s) for n-C10-n-C30 Even Carbon Number Alkanesa method
GC-FID
GC-C
nafion dryer nafion tubing i.d. (mm) total flow rate (mL/min) replicates 7 compound n-C10 n-C12 n-C14 n-C16 n-C18 n-C20 n-C22 n-C24 n-C26 n-C28 n-C30
2.3 2.3 2.4 2.5 2.5 2.6 2.6 3.1 3.9 5.0 6.7
± ± ± ± ± ± ± ± ± ± ±
0.1 0.1 0.0 0.0 0.0 0.0 0.0 0.1 0.1 0.1 0.2
GC-C
A
B
0.635 1.05 5
0.356 1.02 5
2.7 2.8 3.0 3.0 3.1 3.1 3.3 3.8 4.5 5.8 7.6
± ± ± ± ± ± ± ± ± ± ±
0.1 0.1 0.1 0.0 0.1 0.1 0.0 0.1 0.1 0.1 0.1
2.5 2.5 2.7 2.7 2.8 2.9 3.1 3.6 4.4 5.9 7.7
± ± ± ± ± ± ± ± ± ± ±
0.0 0.0 0.1 0.1 0.0 0.1 0.0 0.1 0.1 0.1 0.1
A
B
0.635 0.22 1
0.356 0.26 1
9.4 10.4 9.4 9.4 9.4 9.4 9.6 9.4 9.6 10.7 11.2
3.0 3.0 3.3 3.3 3.5 3.5 3.6 4.1 4.9 6.1 7.9
a H2 flow rate at 30 °C ) 7.6 mL/min, total flow rate ) Ar + O2 at 30 °C measured at the outlet on the six-port value.
factors. The concentration range of 1-10 µg/µL covered the anticipated working range of the system for a 1 µL injection and was within the linear response range of the RGA. CO2 peak areas for both eicosane and the mixture of n-alkanes increased linearly with concentration, and linear regression gave an r2 of 0.999 and a y-intercept at 0. Peak areas were highly reproducible with a % RSD of 2.2 ± 1.0% for the eicosane standards. Correspondingly, peak widths increased linearly with concentration. The results confirmed complete oxidation of the samples. Ar Carrier Gas Flow Rate. The Ar flow rate that the GC-C will use is dependent on leak flow rate of the source and is being tested in the range of 0.2-1 mL/min. An open split will be 6426
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employed initially to maintain a constant flow of gas to the ion source. The efficiency of delivery of sample to the ion source is, therefore, primarily controlled by the sample size and the split ratio of the open split. However, the split ratio does not remain constant as a peak elutes from the system. The CO2 increases the total flow rate of gas eluting from the GC-C system and, correspondingly, the split ratio and efficiency decrease. This becomes more significant at low Ar flow rates where the mole fraction of CO2 becomes higher. The length of connecting tubing after the nafion dryer was kept to a minimum to prevent restriction of the flow which would result in the development of a pressure pulse in the gas stream and slowing of the hydrogen carrier in the analytical column. The residence time of the gas in the connecting tubing postdrying was calculated to be 3-13 s, which was typically less than the peak width. Therefore, a change in the mole fraction of CO2 in the gas stream postdrying corresponded closely to a change of flow rate at the open split. Figure 3 shows what is anticipated to be a typical operating situation where sufficient carbon is delivered to the source. The split ratio dips with a Gaussian profile as the peak elutes. An initial split ratio of 0.9 results in an average split ratio of 0.85, and 81% of the carbon is delivered to the source. Peak width is also a factor to consider as narrower peaks have a higher maximum flow rate which further lowers the split ratio and yield to the source. This has a lesser effect when compared with sample size and split ratio, and for example, with the conditions presented in Figure 3, halving the peak width to 5 s lowered the calculated yield to 73%. Normal system operation therefore will require optimization of sample size, split ratio, and chromatographic parameters. We varied the Ar flow rate of the GC-C from 0.2-1 mL/min to investigate the effects on peak shape and chromatography. When the nafion dryer was in configuration A, the peak widths
Figure 3. Calculated changes in split ratio and sample delivery to the AMS as a peak elutes from the GC-C system. Sample ) 3 µg of n-C20, Gaussian peak width ) 4σ ) 10 s, GC-C/AMS flow rates ) 0.30/0.27 mL/min, yield to MS ) 81% ) 2.06 µg of C.
increased exponentially with decreasing Ar flow rate. At a total flow rate (Ar + O2) of 0.22 mL/min, the peaks were up to 350% wider than those obtained with GC-FID (Table 1). This was an unacceptable loss of resolution. Calculations showed that this was due to laminar flow spreading in the nafion dryer and connecting tubing and that by reducing the diameter to
