Anal. Chem. 1997, 69, 1248-1255
Electrolyzer-Powered Flame Ionization Detector Aviv Amirav* and Nitzan Tzanani
School of Chemistry, Sackler Faculty of Exact Sciences, Tel Aviv University, Tel Aviv 6997, Israel
We describe here a novel water electrolyzer-powered flame ionization detector (EFID), characterized by enhanced portability, reduced cost of operation and improved safety of operation and storage. A conventional FID operates with a hydrogen diffusion flame, which has a central flow of pure hydrogen and column effluents, surrounded by a much higher coaxial flow of pure air. In contrast, the EFID is based on the combustion of a premixed (unseparated), hydrogen and oxygen, stoichiometric gas mixture. This premixed gas mixture is provided by a simple water electrolyzer with low power and water consumption, without separation, compression, or pressure stabilization of the hydrogen and oxygen. The EFID is very similar to FID with two minor modifications: (a) The flame tip must have a narrow hole (∼250 µm) to prevent flame flashbacks. (b) The entire detector structure is maintained above 100 °C to prevent water condensation due to the lack of diluting air. The EFID sensitivity is similar to that of the FID, and up to twice improved detectivity is demonstrated with the EFID. The flame chemical ionization yield of the EFID linearly increases with the electrolysis current. The EFID response is linear over almost 6 orders of magnitude. The response is selective to carbon compounds where the response in aliphatic compounds is ∼30% lower than with aromatic compounds, and no observable difference for N-, S-, P-, and Clcontaining compounds. The use of splitless solvent injections with a megabore column (0.53 mm i.d.) quenches the flame. This flame extinction is eliminated by the use of a miniature air pump during the solvent elution time. Typical electrolyzer operating parameters are current of 1.5 A, 12 mL/day water consumption and 4 W electrolyzer power requirements. Thus, a relatively small size water electrolyzer can provide the total gas consumption of the EFID for up to 40 days before water replenishing is required. This EFID can also be operated either as an EFID or as an FID, simply by replacing the gas sources. The flame ionization detector (FID) is the most popular and widely used detector of gas chromatograph (GC) instruments. It is also used as a stand-alone detector, portable or stationary, for monitoring the total hydrocarbon compound concentration in air. Recently, it has also been employed as the detector of choice in supercritical fluid chromatograph (SFC) instruments. The FID was developed by McWilliam and Dewar1 and Harley et al.,2 and its operating characteristics are well reviewed by McMinn and Hill3 and by Bocek and Janak.4 Traditionally, the (1) McWilliam, I. G.; Dewar, R. A. Nature 1958, 181, 760. (2) Harley, J.; Nell, W.; Pretorius, V. Nature 1958, 181, 177-178.
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FID is based on a hydrogen diffusion flame where a hydrocarbonfree (zero grade) hydrogen gas is fed through a flame tip (jet) surrounded by a coaxial, much higher flow of purified (zero grade) air. In typical GC detection, helium gas is also added to the central hydrogen flow as a make-up gas to further enhance the performance of the FID. Typical flow rates for a conventional FID are 30 mL/min hydrogen, 30 mL/min helium, and 300-400 mL/min air. FID operation is based on the H2/O2 combustion decomposition of organic compounds, which form some CH radicals, followed by the flame chemical ionization reaction CH + O f CHO+ + e-. The resulting flame-induced current is then measured, and it is proportional to the flux of organic compounds over 5-7 orders of magnitude linear dynamic range. Typical flame chemical ionization yield is 15 mC/g of carbon. The response is selective to carbon atoms only and is generally uniform among the organic compounds, but with some secondary molecular effects. Certain heteroatoms, such as organo-oxygen or nitrogen reduce the detection of carbon and CO, CO2, and CS2 are practically undetectable. While the vast majority of FIDs are operated with air, some studies have been performed with pure oxygen instead of air in the hydrogen diffusion flame.5-7 A few investigators have also studied the effect of gas composition in premixed hydrogen air flames.8-10 The FID is not operated with premixed H2/O2 alone, probably due to the danger of flame penetration through the flame tip to the gas source in an unsafe or uncontrolled manner (flashback). The FID became the GC industry standard detector of choice due to its sensitivity, carbon selectivity, large linear dynamic range, and simple and robust operation. In spite of the above desirable features, the FID suffers from a one major disadvantage of having high gas consumption. This aspect is translated into several undesirable features. 1. High cost of gases and operation. 2. Increased cost of installation and purchase: An additional major cost of the FID is that of empty hydrogen and air gas cylinders, pressure regulators, high-pressure gas line installation, and water and hydrocarbon impurity traps. The GC includes FIDspecific penumatics, flow control valves, and gas lines. In addition, laboratory space for the cylinders and their safety construction is needed. (3) McMinn, D. G.; Hill, H. H. In Detectors for Capillary Chromatography; Hill, H. H., McMinn, D. G., Eds.; John Wiley Publishing: New York, 1992; pp 7-21. (4) Bocek, P.; Janak, J. Chromatogr. Rev. 1971, 15, 111-150. (5) Jones, K.; Green, R. Nature 1965, 205, 67-68. (6) Jones, K.; Green, R. Nature 1966, 210, 1355. (7) Simpson, C. F.; Gough, T. A. J. Chromatogr. Sci. 1981, 19, 275-282. (8) Blades, A. T. J. Chromatogr. Sci. 1973, 11, 251-255. (9) Miller, W. J. 12th International Symposium on Combustion, The Combustion Institute, Pittsburgh, PA, 1969; pp 311-320. (10) Bolton, H. C.; McWilliam, I. G. Proc. R. Soc. London 1971, A321, 361380. S0003-2700(96)00484-2 CCC: $14.00
© 1997 American Chemical Society
3. Safety: A compressed hydrogen gas cylinder is a major hazard in the laboratory, as this gas is flammable and can explode in certain large-quantity mixtures with air. 4. Weight and size: The hydrogen and air cylinders are very heavy and bulky, weighing up to 100 kg total. Thus, the FID severely restricts the portability or even transportability of GC instruments as well as of an independent FID. Recently there has been a growing use of zero grade hydrogen and air generators. Hydrogen is generated through the electrolysis of water and is then separated from the cogenerated oxygen by a polymeric or palladium membrane. After separation, the hydrogen is compressed to a preset stabilized pressure of several atmospheres and undergoes final stages of purification from water and oxygen before the delivery to the GC. For pure air generation, air from the room is compressed to the usual delivery pressure by a mechanical compressor and its hydrocarbon residual content is removed to a low acceptable level, usually through a form of catalytic combustion such as on a heated platinum wire. Again, the available instrumentation is expensive and bulky and requires extensive maintenance. THE NEW APPROACH: ELECTROLYZER-POWERED FID (EFID) In conventional water electrolysis, water is electrochemically separated into hydrogen and oxygen gases. In general use, a hydrogen generator requires the hydrogen to be separated, purified, and compressed (for serving as a carrier gas). However, the FID requires no such separation and purification if a premixed, approximately stoichiometric, H2/O2 gas mixture can be used. In addition, the FID imposes only a small gas flow resistance; thus, only a few psi pressure difference in the electrolyzer is sufficient to drive the gas mixture into the detector even through a frit flow restrictor. This novel approach (patent applied for) is based on the realization that FID can be powered by very simple water electrolysis and, in contrast to the conventionally used hydrogen and zero grade air generators, has the following characteristics: (1) The hydrogen gas generated is not thoroughly cleaned or separated from the oxygen. (2) Oxygen is provided to the flame from a water electrolyzer instead of air. (3) The FID can be powered by a premixed H2/O2 gas instead of the normal hydrogen diffusion-air flame. (4) The FID can be powered by a nearstoichiometric H2/O2 gas mixture.11 (5) Water vapor is only partially removed since as long as it does not condense, it is harmless to the flame where water vapor is also formed. (6) The electrolysis-generated gases do not have to be compressed or pressure stabilized for use as a carrier gas in a GC column. In fact, the premixed gas flow is regulated by the electrolysis current, which acts as an “electronic flow control” that creates a small (but constant) driving pressure gradient. (7) Water electrolysis provides “zero grade” gases without hydrocarbon impurities. (8) A relatively low total combustible gas flow rate can sustain a stable flame due to the use of pure oxygen. Thus, our new approach of EFID is based on the provision of the total FID gas requirements through the simplest water electrolysis, without gas separation and/or compression, resulting in a “gas bottle-free FID”. ELECTROLYZER-POWERED FID This research was performed on two gas chromatographs, a Hewlett-Packard 5890 series II and a Varian 3600. The EFID is (11) Tzanani, N.; Amirav, A. Anal. Chem. 1995, 67, 167-173.
Figure 1. Schematic diagram of the EFID constructed for a HewlettPackard GC. The water electrolyzer consists of a container (10) having water with KOH (typically 1 M concentration) (11) and two nickel mesh electrodes (12) connected to an external power supply through electrical feed-throughs (13). The H2/O2 gas mixture generated passes the membrane (Teflon) (14) for partial separation from the water mist and it is further dried using the silica gel drying material (15). It passes the frit flow restrictor element (16), and then the gas mixture flows to the EFID flame source (20). The flame source accepts a GC effluent from a column (21) sealed by a seal (22). The H2/O2 mixture is mixed with the sample flow from the flame tip (23) into the flame (24). The flame can be surrounded by auxiliary air provided from the air source (25). The flame-produced charge carriers are collected by the charge collector electrode (32), and the current is amplified by an amplifier (33) and recorded by an integrator (34). The EFID temperature gradient is reduced by an aluminum heat conductor (26) that is thermally insulated by a piece of glass tube (27) and is electrically insulated from the charge collector by the Teflon insulators (28). The hole structure is covered by a short cover (29) that is clamped (30) onto the FID base (31).
schematically described in Figure 1. The water electrolyzer is shown on the bottom left side. Two water electrolyzers were constructed and used. The smaller unit (shown in Figure 1) was made out of PVC plastic (polypropylene is also a suitable material), with a 66 mm diameter and 112 mm length, similar in its dimensions to a standard soft drink can. Its lower chamber can contain up to 120 mL of water with 7 g of KOH added (initially) to increase the water conductivity (even 1 g of KOH is enough). Two round pieces of nickel mesh are placed electrically separated at the bottom of the container, to serve as electrodes for the water electrolysis. A vertical concentric mesh electrode arrangement was also used with slightly better results in terms of flow stability. These meshes are connected through electrical feed-throughs to a standard power supply providing typically 1.5 A at 2.6 V, with total power consumption of ∼4 W. Under these conditions ∼18 mL/min stoichiometric H2/O2 gas mixture is formed. The gas mixture flows to the upper chamber through a Teflon membrane, intended to separate the water mist formed by the gas bubbles Analytical Chemistry, Vol. 69, No. 6, March 15, 1997
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during the electrolysis. The gas mixture is further passed through a silica gel water drying material (35 g). The volume of the drying material is planned to be just enough for the water treated below, and it is replaced or redried in an oven, during every cycle of water refilling. The 120 mL water reservoir enables the electrolysis consumption of ∼100 mL of water. Care was taken to use only clean triple-distilled water to minimize the flame background current. At 1.5 A, the water consumption is 12 mL/day, thereby providing the total gas consumption of the EFID for over a full week. A larger water electrolyzer with up to 0.55 L water capacity, with a separate dripping chamber and drying material tube (standard 120 mL drying tube with stainless steel connectors), and a safety check valve was also constructed. This larger device enables seven weeks of continuous operation. The unseparated H2/O2 mixture flows through a 100 mL/min frit flow restrictor element (standard element, made by Mott). This frit is essential as a flame arrester, to ensure the safety of the electrolyzer by the complete elimination of any possible danger of flame flashback into the electrolyzer. It is also used to build a low pressure (a fraction of an atmosphere) within the electrolyzer in order to stabilize the flow rate out of the electrolyzer. The generated H2/ O2 gas mixture is introduced directly into the FID through an additional low-flow resistance safety frit in the hydrogen line that was disconnected from its control valve. The FID (see Figure 1) is the standard Hewlett-Packard FID mounted on a 5890 series II GC. The detector structure was modified to increase the heat transfer from the base to the charge collector, in order to avoid water condensation, which otherwise would accumulate in the EFID due to the absence of the high flow rate of diluting air that is conventionally used in FID. The sample to be analyzed is fed from the GC column, which is positioned beneath the flame tip in the ordinary way. The sample is swept by the H2/O2 gas mixture into the flame, where it is combusted to form a flameinduced current as in a conventional FID. No additional external hydrogen, air, or helium make-up gas is required (or used) except the normal column flow of helium. The charged particles produced in the flame are collected in the normal way by the electrically biased collector, and the current intensity is recorded versus time. Note that the hole of flame tip is narrower than usual, having a diameter of 280 µm, to ensure the absence of flashbacks, since the flame cannot propagate back through such small holes. This flame tip is the standard “highperformance” narrow flame tip of HP. A modified flame tip based on a quartz capillary column with 200 µm i.d. provides better response by up to a factor of 2, but no results with this flame tip are shown here. In case hydrogen is used as a carrier gas, the flame becomes slightly hydrogen rich without any major perturbation. Note that, in addition to the aluminum heat conductor and glass insulating tube, the charge collector was also made out of aluminum, was shorter, and was installed beneath a homemade cover. We found that we had to replace all these FID items in order to increase the collector temperature to above 100 °C to avoid water condensation and reflux that induced severe electronic noise and could extinguish the flame. With the Varian 3600 or 3400 GCs, no modification of the FID was required, and the EFID could be operated simply by connecting the water electrolyzer through the hydrogen or helium make-up gas line. The narrow 250 µm flame tip had to be used with the Varian FID. 1250 Analytical Chemistry, Vol. 69, No. 6, March 15, 1997
Figure 2. Comparison of chromatograms of the indicated compounds (45-50 pg each) achieved with (A) FID, (B) EFID, and (C) FID in the modified EFID construction. A Hewlett-Packard 5890 series II GC and FID was used, and its electrometer had the same gain in all the chromatograms. The EFID electrolysis current was 1.5 A, and the FID conditions are 30 mL/min H2, 35 mL/min He make-up, and 400 mL/min air, using a 280 µm flame tip.
EFID SENSITIVITY AND CARBON RESPONSE In Figure 2, chromatograms of toluene, octane, xylene, and nonane at a concentration of 20 ppm in methanol (volume) are shown. Samples of 0.15 µL were injected with a 50:1 split ratio into a 6 m narrow-bore (0.20 mm i.d.) column, amounting to about 45-50 pg of each compound. Trace A shows the chromatogram obtained with the standard HP FID using zero grade air and hydrogen and 99.999% helium make-up gas with an in-line hydrocarbon trap. Trace B was obtained in the same manner as (A), except that the EFID was used, solely powered by the water electrolyzer shown in Figure 1. Trace C was obtained using the same EFID structure but with the standard FID gases as in trace A. Clearly traces A and C are practically identical; thus the minor modifications made in the conversion of the FID to an EFID do not change its operation as an FID in any noticeable way. The only difference between the conditions of traces C and B is that in (B) the gas lines were disconnected and the FID was operated as an EFID. Note that the peak height of the aromatic compounds toluene and xylene is similar in the EFID to that of the FID, while the response of the aliphatic octane and nonane compounds is lower by ∼30%, as known for premixed H2/air flames.8 Since the noise levels are the same, the EFID detectivity is practically the same as that of the standard FID. In Figure 3 the dependence of the relative EFID response on the electrolyzer current is plotted and also versus the emerging total gas flow rate (measured at room temperature). A linear dependence of the chemical ionization yield on the total H2/O2 gas flow rate is clearly observed. At this time we are not sure about the exact mechanism responsible
Figure 3. Relative EFID response (normalized to the FID response) dependence on the water electrolysis current and resulting H2/O2 gas flow rate. Toluene (2.5 ng) and octane (3 ng) were injected into the column.
for this observation, but this result is clear and highly reproducible. Below an electrolyzer current of 0.7 A, the flame becomes unstable and can self-quench. At 1 A the ionization yield is lower by more than a factor of 2 than at 1.5 A. Note that at 2 A the ionization yield for octane is similar to that of the FID with the high-performance nozzle (assumed to be ∼18 mC/g of C) while that of toluene is higher by ∼30%. At 2.5 A both yields are higher with the EFID than with the FID. However, a careful comparison must include the noise level. At 1.5 A the EFID produces a background current of 2 pA. This current grows to 5-6 pA at an electrolysis current of 2 A and is over 10 pA at an electrolysis current of 2.5 A. While these background currents can be equivalent or lower than our 5-6 pA FID background current, the actual noise due to background current instability is equivalent. It is believed that temporarily trapped gas bubbles or other electrolysis irregularities induce flow instabilities in conjunction with the higher dependence of the background current on the total flow, these are the source of the limiting noise. Thus, considering all factors, we concluded that the signal to noise ratio has a maximum at an electrolyzer current of 2 A that is only slightly better than that achieved with 1.5 A. The 1.5 A current was chosen as our standard electrolysis current to sustain longer independent operating time. Note also that this demonstrated performance is nearly equivalent to that of an FID operated with zero grade gases. The FID dark current of 5 pA emerges from a calculated total hydrocarbon content of 0.1 ppm. Consequently, the EFID detectivity should be better than that of an FID operated on conventional clean air, H2, and He. In Figure 4 we compare the EFID and FID response to compounds with N, S, P, and Cl heteroatoms. Aside from differences emerging from the 30% lower response to aliphatic carbon, no other heteroatom effect is practically observed with the EFID. Thus the EFID, like the FID, is a universal carbon-selective detector with an approximately uniform carbon response. This is however, with the penalty of reduced response uniformity between aromatic and aliphatic hydrocarbon.
Figure 4. Comparison of chromatograms of the indicated compounds (2 ng each) achieved with (A) FID and (B) EFID. Similar gain was used, and the EFID electrolysis current was 1.5 A.
In Figure 5, a higher EFID sensitivity is demonstrated in comparison with that of the conventional FID. Benzene, toluene, and xylene were mixed at liquid volume ratio of 1:3:10, correspondingly, and 0.1 µL of the headspace gas mixture was injected with a 100:1 split ratio. The larger size electrolyzer was used with 2 A electrolysis current to increase the EFID ionization yield. As shown, the benzene EFID signal is higher by a factor of 1.8 from that of the FID, while the EFID noise is lower by a factor of ∼1.2, demonstrating that the EFID’s detectivity is twice better than that of the FID. This enhanced EFID detectivity is slightly reduced with toluene and xylene due to 30% lower EFID ionization yield of the aliphatic methyl groups. EFID CHARACTERISTICS The linearity of the EFID response is shown on a log-log plot in Figure 6. The full circles represent the toluene peak areas as measured by the integrator. The two empty squares were obtained by calculating the normalized peak area from its peak height, since the area measurements were hampered by baseline noise. A linear response is clearly observed over a range of almost 6 orders of magnitude, with a slope of 0.993 (when the two lowamount points are neglected). We believe that the EFID is as linear as FID, and any deviation in our curve is due to systematic errors in our procedure of sample dilution. Due to the relatively low total gas flow rate, the EFID is expected to deviate from its linear response at high carbon flux before the conventional FID, but these ranges could not be explored with the narrow-bore column used. Thus, it is concluded that the EFID is characterized by a linear response over all the useful carbon flux range that is relevant for capillary GC, up to a few micrograms of C per second. The EFID was tested with relatively nonvolatile compounds, and Analytical Chemistry, Vol. 69, No. 6, March 15, 1997
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Figure 5. Comparison of chromatograms and detectivity of the indicated compounds achieved with FID (upper) and EFID (lower). A 0.1 µL headspace gas mixture above a 1:3:10 benzene, toluene, and xylene liquid mixture was injected with 100:1 split ratio. A 6 m 0.2 mm i.d. column was used at 90 °C during the injection followed by 30 °C/min temperature programming.
Figure 6. EFID toluene signal dependence on its injected amount on a log-log presentation. GC peak area was used from the integrator indicated by full circles while the empty squares represent normalized peak height.
in Figure 7, the chromatograms of a C12-C40 linear-chain hydrocarbon mixture obtained by both the EFID and FID are compared. Note that the two chromatograms are practically identical and tail free. This means that although the total gas flow rate at the flame tip is only 18 mL/min as compared with the 60-65 mL/min in FID, the sample compounds are effectively swept, including C40H82. However, compounds that tend to show flame tip-induced 1252 Analytical Chemistry, Vol. 69, No. 6, March 15, 1997
Figure 7. Comparison of A (FID) and (B) EFID in the analysis of a mixture of linear hydrocarbons with concentration of 10-3 (0.2 µL, split 50:1) of C12H26, C16H34, C20H42, C24H50, C2H58, C32H66, C36H74, and C40H82. The FID base temperature was 340 °C.
tailing need to be further studied with the EFID, especially in view of its lower sweeping gas flow rate. We believe that this problem should not exist, since one of the conjectures that can be advanced to explain the onset of EFID zero chemical ionization yield at about 5-6 mL/min H2/O2 gas mixture flow rate is catalytic flameless combustion within the flame tip. Accordingly, the H2/ O2 flame is much hotter than the helium-diluted H2/air flame of the FID, and the eluting compound begins to pyrolyze prior to its exit from the flame tip through flameless partial combustion of the O2/H2 gas mixture. Further research will be required to probe these possible phenomena. At this time, from Figure 7 we assert that the EFID can be used with relatively nonvolatile compounds. In Figure 8, chromatograms of six compounds in methanol are shown using either helium or hydrogen as the carrier gas at column flow rate of 4 mL/min. No problems are observed with the use of hydrogen as a carrier gas, and in fact, the relative yield of the aliphatic compounds octane, nonane, and decane is slightly increased. This result is in agreement with the results of Blades,8 who concluded that a uniform carbon response requires a hydrogen-rich environment during the pyrolysis stage. Also note that the increase of the column helium flow rate to 4 mL/min cooled the EFID flame and reduced its background current. EFID FLAME QUENCHING AND ITS PREVENTION WITH AN AUXILIARY SMALL AIR PUMP The EFID is based on a relatively low flow of H2/O2 mixture. Although the total flow is ∼18 mL/min, it is a pure oxygen flame with an oxygen flow rate equal to that in 30 mL/min air, which is much lower than the flow rate usually encountered in FID. This lower total gas flow rate results in a major limitation of the EFID, namely, possible flame quenching.
Figure 8. Effect of the carrier gas on the EFID. The two chromatograms of the indicated compounds at a concentration of 10-3 (0.2 µL split 50:1) were achieved with the EFID: (A) with a column flow rate of 4 mL/min helium; (B) with a column flow rate of 4 mL/min hydrogen.
In Figure 9 we show chromatograms of the indicated compounds, achieved with a megabore column (0.53 mm i.d.). We studied the high column flow rate limitation and found that the flame becomes unstable and tends to self-quench above a helium column flow rate of 8 mL/min. This observation is not fully understood since, even with the addition of 8 mL/min helium, the mixture is richer in oxygen than a stoichiometric H2/air mixture (that contains a higher partial flow rate of nitrogen). However, since we limited ourselves to the use of the existing commercial HP flame tip, we did not try to alleviate this problem through the use of different flame tip designs. This problem was solved through the use of an auxiliary enveloping air flow, supplied by an external miniature diaphragm pump (ASF Model 3003 pump, 0.5 W power consumption). This pump was connected to the GC air line through a 500 or 1000 mL/min Mott flow restrictor element (stainless steel frit). This combination provided 80-130 mL/min coaxial air flow around the flame and stabilized the EFID flame when a high column flow rate was used. In Figure 9, the lower trace was achieved with a column flow rate of 7.5 mL/min helium without coaxial air flow. The upper trace was achieved under the same chromatographic conditions except that the helium flow rate in the column was increased to 24 mL/min and external 80 mL/min coaxial air was provided by the small pump. Thus we conclude that the EFID can work with the full range of capillary GC column flow rates. However, for flow rates above 8 mL/min, additional air must be provided. We note that the external room air, although unfiltered, raised the EFID dark current only by 1 or 2 pA. This small increase is the result of the fact that, unlike FID the EFID is based on a premixed, stoichiometric H2/O2 composition that can be fully combusted indepen-
Figure 9. GC-EFID analysis of the indicated compounds with high column flow rate. A 4 m megabore column (0.53 mm i.d.) was used with a 7.5 (B) and 24 mL/min flow rate of helium (A). In order to avoid self-quenching of the flame by the high column flow rate in trace A, an auxiliary miniature air pump was used, which provided 80 mL/ min of air through the FID air line to stabilize the EFID flame. An electrolyzer current of 2 A was used.
dent of the sweeping gas environment. The coaxial air acts mostly to stabilize the flame and does not participate in the combustion and, thus, can be used without filtration directly from the environment. The more important reason for flame quenching is solvent-induced flame extinction. The low H2/O2 flow rate also restricts the maximum amount of solvent that can be tolerated by the EFID without flame extinction. In Figure 10 we show a chromatogram in the upper trace that was obtained by the splitless injection of 1 µL of toluene into a narrow-bore 0.20 mm i.d., HPcolumn. As shown, this injection could be handled by the EFID without flame quenching. However, above a column flow rate of 1 mL/min, or when more than 1 µL of toluene was injected, the EFID flame was quenched. Similarly, the use of a megabore (0.53 mm i.d.) column with splitless injection could not be explored due to flame quenching resulting from the increased hydrocarbon flux with this larger size column. This problem of flame quenching was practically solved by the use of an external air pump compressing air through a 1000 mL/min frit flow restrictor. With the provision of a coaxial air flow rate of 120 mL/min, the large flux of solvent turned the flame into solvent-air diffusion flame during the period of solvent elution. Note that the air pump could be used only during the period of solvent elution and thus did not affect the analytical performance of the EFID during the chromatography of the other compounds that subsequently eluted. In Figure 10, the use of external air-flame stabilization is shown in the lower chromatogram, obtained with 3 µL splitless injection of toluene into a megabore column having a column flow rate of 8 mL/min helium. In this case, the relay was closed for 0.4 min, Analytical Chemistry, Vol. 69, No. 6, March 15, 1997
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Figure 11. Diesel fuel analysis with the EFID. A 2 µL solution of 10 ppm diesel fuel in methanol (with 1% cyclohexane) were injected splitless (1 min). The column temperature was 50 °C for 1 min and then ramped to 300 °C at 15 °C/min. The column helium flow rate was 2.5 mL/min at the injection temperature. A 6 m capillary column with 0.20 mm i.d. was used. Figure 10. Splitless injections with the EFID. The upper chromatogram shows the EFID analysis of toluene (Analytical Reagent grade) with 1 µL splitless injection into a 6 m narrow-bore column (0.20 mm i.d.) having a flow rate of 1 mL/min helium. The lower chromatogram was achieved with 3 µL splitless injection into a 4 m megabore column (0.53 mm i.d.) having a column flow rate of 8 mL/ min helium. In order to avoid solvent-induced flame extinction, room air was provided by an external miniature air pump at a flow rate of 130 mL/min for the first 1.5 min.
but the relay times does not affect the flame quenching since the highest carbon flux appears at the solvent front. Thus, as shown in Figure 10, when supplemented by a 80-130 mL/min flow of room air provided by a miniature pump, the EFID can handle all the range of samples encountered in capillary GC. We believe that the provision of this external air is a better solution than automatic flame ignition. In Figure 11, we demonstrate the application of the EFID for the detection of a “real world” diesel fuel sample. A 2 µL solution of 10 ppm diesel fuel in methanol (with 1% cyclohexane) was injected splitless using 0.20 mm i.d. capillary column with a 2.5 mL/min helium column flow rate. It is shown that the EFID can be applied for the detection of a low-level complex mixture, and in this case, the methanol solvent did not extinguish the flame even without the use of an external small air pump. DISCUSSION AND CONCLUSIONS It can be summarized that the FID can be easily converted into an EFID through the following minor modifications in addition to the utilization of the water electrolyzer as its combustible gas source: (1) The flame tip should have a nozzle of small diameter (i.e., 250-280 µm) to prevent flame flashbacks. Both in the HP and Varian GCs, the use of the high efficiency small flame tip is thus required. (2) The EFID body should be heated to above 100 °C to avoid water condensation, which is more likely to occur 1254 Analytical Chemistry, Vol. 69, No. 6, March 15, 1997
due to the lack of the high diluting air flow rate used in FID. This was the major problem in the conversion of the HP-FID into an EFID, but was not an issue with the Varian FID. (3) No helium make-up gas (and gas line) is required, and in fact, the use of make-up gas in the EFID is undesirable. (4) The igniter should be redesigned for easy ignition of the low flow H2/O2 gas mixture. In the EFID on the HP-GC, the flame was ignited by an external lighter. No change was required with the Varian FID. (5) An external miniature air pump is essential if splitless injections are required with 0.32 or 0.53 mm i.d. capillary columns. The conversion of the Varian FID to EFID is very simple and requires only the use of the small (250 µm) flame tip and the connection of the water electrolyzer and miniature air pump. No changes were introduced to the current amplifier and electronics. The EFID is characterized by the following advantages over FID: (1) Reduced cost of installation and operation. The price of distilled water is negligible compared to that of the gases it replaces, while the cost of construction of this simple electrolyzer is much lower than that of conventional hydrogen and zero grade air generators, or empty gas cylinders and their pressure regulators, gas valves, tubes, pneumatics, and the safety requirements of hydrogen handling. (2) The EFID is safer. No compressed hydrogen is required and only a few hundred milliliters of combustible gas mixture is stored at any time. The total flow rate of combustible gases is also much smaller and thus, even if uncombusted, this mixture is easily diluted in air to a safe, unignitable level. (3) Independent total gas supply. The EFID can be operated continuously, eliminating the dependence on external gas sources and gas shipment, storage, and exchange of cylinders. (4) Comparable or better sensitivity. The EFID is inherently operated with “zero grade” gases without hydrocarbon impurities. Thus, its noise level is lower than that of FID when it is operated with conventional gases. (5) Enhanced portability. The EFID provides the ultimate in FID portability. The electrolyzer shown
in Figure 1 weighs only 450 g and consumes much less energy (and water) than all its alternatives (less than 4 W and 0.5 mL of water/h). In view of the above, it seems that the EFID is ideally suited for field portable or transportable GCs. In the laboratory, the EFID can solve the hydrogen safety issues, provide intrainstitute transportability, and reduce the cost of gases involved in the operation of FID. A crude calculation shows that when operated
can be saved by using the EFID. Finally, while we discussed the EFID operated on a GC, its implementation for monitoring total hydrocarbon content in air is straightforward and is under investigation with both continuous and pulsed flames.11 ACKNOWLEDGMENT We thank Tsvi Shahar for helping in the design of the first electrolyzer.
continuously, FID consumes ∼200 m3 total gas volume of air,
Received for review May 16, 1996. Accepted December 20, 1996.X
hydrogen, and helium per year. Depending on the quality of
AC960484A
gases, these gases can cost in the range of $1500-4000/year that
X
Abstract published in Advance ACS Abstracts, February 1, 1997.
Analytical Chemistry, Vol. 69, No. 6, March 15, 1997
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