Use of Penning Ionization Electron Spectroscopy in Plasma for

Mar 4, 2009 - A breadboard GC detector based on Penning ionization electron spectroscopy in plasma (PIES) was investigated. The PIES detector was set ...
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Anal. Chem. 2009, 81, 2626–2632

Use of Penning Ionization Electron Spectroscopy in Plasma for Measurements of Environmental Gas Constituents Vadim P. Stepaniuk,*,† Gotze H. Popov,† and Valery A. Sheverev†,‡ Lenterra Incorporated, 7 Tenney Road, West Orange, New Jersey 07052, and Polytechnic Institute of New York University, 6 Metrotech Center, Brooklyn, New York 11201 A breadboard GC detector based on Penning ionization electron spectroscopy in plasma (PIES) was investigated. The PIES detector was set up in series with a gas chromatograph and a thermal conductivity detector. Twodimensional PIES chromatograms were recorded for carbon monoxide, carbon dioxide, and methane. The analytes were identified independently of the GC retention time, and their concentrations were measured in a range between 1 and 100 ppm. PIES spectra for methane were observed for the first time and displayed two characteristic peaks with electron energies of 7.1 and 5.4 eV. Rate coefficients for Penning ionization due to collisions between 23S helium metastable atoms and analyte molecules under study were found to be k*CO ) (0.7 ( 0.2) × 10-10, k*CO2 ) (1.8 ( 0.7) × 10-10, k*7.1 CH4 ) (4.7 ( 0.6) × 10-10, and k*5.4 CH4 ) (8 ( 2) × 10-10 cm3/s. The work provides the basis for the development of a portable and robust analytical platform capable of in situ real-time monitoring of greenhouse gases, with a perspective toward laboratory-on-chip realization. Penning ionization electron spectroscopy in plasma (PIES) was recently introduced to analytical science as a simple analytical technique capable of independent identification of unknown species in a gas sample.1 PIES is based on the measurement of the concentrations and energies of electrons in the afterglow of a glow discharge. Typically these discharges are ignited in helium, although other gases such as neon can be used. Electrons of interest are generated in collisions between metastable helium atoms and analyte molecules in Penning ionization (PI) reactions: He* + X f He + X+ + e(εx)

(1)

Here He* is a metastable helium atom, X is an analyte molecule, and He and X+ are a ground-state helium atom and an analyte ion, respectively. The energy of PI electrons, εx, is specific for each analyte and equal to εx ) Em - Ex, where Em is the excitation energy of the helium metastable atom (19.8 eV for * To whom correspondence should be addressed. Fax: 973-623-0755. E-mail: [email protected]. † Lenterra Incorporated. ‡ Polytechnic Institute of New York University. (1) Sheverev, V. A.; Khromov, N. A.; Kojiro, D. R. Anal. Chem. 2002, 74 (21), 5556–5563.

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the 23S state) and Ex is the excitation energy for the analyte ion (Ex ) Ei if a ground-state ion is formed). Since the Ex values are unique characteristics of the species, by measuring the energy and the number of PI electrons it is possible to identify the analyte and by modeling the reaction 1 measure its concentration. The measurement is accomplished by simply placing an additional electrode collector in the plasma and recording its current-voltage (i-v) characteristic. The sensitivity and resolution of this method are inferior to those of other spectral analytical techniques, but the ability to independently identify analytes, the extreme simplicity of the device, its small size, and the prospect of on-chip realization could overcome these shortcomings for applications where portability, size, and weight considerations are priorities. One of such applications is monitoring greenhouse gases (GHG) and other constituents that have negative effects on the Earth’s biosphere. New cost-effective detection technologies capable of on-site quantitative measurements of GHG are needed to follow changes in the carbon cycle and global warming. Helium is a well-established carrier gas in GC columns and, incidentally, the buffer gas of choice for PIES. A PIES apparatus can be combined with a GC column which functions as a preliminary separator. No temperature stabilization for the column is required since PIES can identify analytes independently from GC retention time. Since the PIES measurement method is sufficiently fast, short GC columns can be used. For the analysis in the ppb (parts per billion) region, a microconcentrator2 can be employed. Each element of the PIES analytical platform could be miniaturized, and the total instrument could be made portable with a prospective toward realization on a single chip (“lab-on-chip” or LOC). This paper presents measurements of PIES spectra for carbon dioxide, methane, and carbon monoxide by a breadboard PIES detector set up in-line with a fast gas chromatograph and a nonselective thermal conductivity detector (TCD). PIES DETECTOR The PIES detector is a dc glow discharge cell with an additional electrode (collector) placed inside the plasma. In the experiments described here, a collector that is commonly known as a Langmuir probe (a thin wire fully insulated except a small portion on the tip) was used, see Figure 1. The electron energy spectrum is obtained by measuring the collector i-v characteristic in the (2) Mitra, S.; Yun, C. J. Chromatogr. 1993, 648 (2), 415–421. 10.1021/ac8025674 CCC: $40.75  2009 American Chemical Society Published on Web 03/04/2009

Figure 1. PIES discharge cell.

Figure 3. Fast GC coupled with breadboard PIES system.

acquired in a pure helium discharge from that of the discharge in which an analyte is present.

Figure 2. Measurement of the collector i-v characteristic in timedelayed afterglow.

afterglow phase of the discharge (after the voltage on the discharge electrodes is set to zero) and calculating the second derivative of the collector current, i, with respect to the collector voltage, v, d2i/dv2. The i-v characteristic is measured by ramping the negative electrical potential on the collector so that, at a particular time, only electrons having energies greater than the instantaneous potential of the collector impact the probe surface and contribute to the collector current. Electrons with lower energies are repelled away. The negative part of the collector i-v characteristic therefore shows a number of features that separate different groups of high-energy electrons produced according to reaction 1, (see ref 1 for details). Double differentiation of the collector i-v characteristic yields a “raw” PIES spectrum that ideally consists of a number of narrow peaks whose positions correspond to energies εx. In comparison to the previous study,1 a faster approach for obtaining the second derivative of the collector current, d2i/dv2, was employed. The i-v characteristic is measured by applying an instantaneous voltage pulse to the collector following a controlled delay time from the end of the discharge (see Figure 2) and measuring the collector current at this voltage level. The i-v plot is acquired by repeating the discharge pulse and changing the collector voltage amplitude by a predetermined increment. The process continues with every successive discharge pulse until the collector voltage reaches its maximum value (Vmax). The resulting set of data ij(vj), where j is the discharge pulse number, is differentiated numerically using a standard algorithm.3 Finally, the differential PIES spectrum (referred to below as simply “PIES spectrum”) is calculated by subtracting d2i/dv2 (3) Andrei, H.; Covlea, V.; Covlea, V. V.; Barna, E. Rom. Rep. Phys. 2003, 55 (2), 51–56.

SETUP The experimental setup included a gas chromatograph (Shimadzu GC-2010), PIES detector, a controller, and host computer (Figure 3). The GC was preinstalled with a TCD and a gas sampling valve (not shown in Figure 3). The PIES detector was connected in series with the TCD detector so that the TCD chromatogram was recorded along with the PIES spectra. The gas sampling valve was connected to the gas manifold that was used to prepare a desired gas mixture. The GC in all experiments was operating in the constant flow mode. The flow rate through the column was 10 mL/min. The temperature of the GC oven was kept constant at 60 °C, resulting in peak widths on the TCD chromatograms on the order of a few tens of seconds. A ShinCarbon ST 100/120 micropacked column, a product of the Restek Corporation, with i.d. of 1/16 in. and length 2 m, was installed in the GC. The ShinCarbon column is capable of separating permanent gases, including CO, CO2, CH4, and light hydrocarbons. The PIES detector was placed outside of the GC and connected to the TCD outlet with capillary tubing via a set of valves. The detector, a U-shaped discharge cell similar to that shown in Figure 1, was made from Pyrex glass tubing with an i.d. of 1.6 cm. The electrodes (anode and cathode) were manufactured from 0.1 mm thick nickel foil, formed into hollow cylinders 25 mm long and 5 mm in diameter, and separated from one another by 7 cm. The collector was a tungsten wire 0.1 mm in diameter, with 4 mm of its length exposed to the discharge, positioned midway between the discharge electrodes. The PIES detector was connected to a mechanical pump, and the gas pressure in the cell was kept constant at 3.5 Torr for all measurements. The PIES controller that was developed in-house was used to sustain a periodic discharge and to measured the collector current-voltage characteristic. The controller consists of two major parts: the high-voltage (HV) pulser circuit and the collector i-v characteristic measurement circuit. The HV pulser generated short rectangular pulses (discharge pulses in Figure 2) that periodically ignited a glow discharge in the PIES cell. The typical Analytical Chemistry, Vol. 81, No. 7, April 1, 2009

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Figure 4. “Raw” PIES spectra obtained in pure helium and in methane with Cmax ) 41 ppm.

parameters were a voltage amplitude of 410 V, a discharge current of 60 mA, and an HV pulse repetition rate of 2.8 kHz. The i-v characteristic measurement circuit applied voltage to the collector according to the above-described algorithm and amplified the measured collector current. Typical parameters of the measurement circuit were a measurement delay of 55 µs (after the shutoff of the discharge), a total scanning range of 20.8 V with a minimum of Vmin ) -18.9 V, and a voltage increment -0.04 V. Each i-v characteristic contained 520 individual measurements of collector current, taking on average approximately 0.21 s to acquire. Data acquisition and control software performed the following basic functions: (1) synchronization of GC and PIES detector operation, (2) control of discharge duration and repetition rate, (3) control of the collector i-v characteristic measurements, and (4) computation of the PIES spectrum. Software was developed using the National Instrument LabVIEW programming environment. A separate software package supplied with the GC was used tooperatethegaschromatographandtoobtainTCDchromatograms. PIES SPECTRA PIES spectra and PIES chromatograms were recorded for gas samples of carbon monoxide, methane, and carbon dioxide diluted in helium. Five different gas samples were prepared by releasing a known amount of analytes into a mixing cylinder that contained a controlled amount of helium. The analyte concentrations in the mixture injected into the GC, Cinj, were 11 300, 4700, 1570, 520, and 170 ppm for CO and 14 100, 5900, 1950, 650, and 215 ppm for CH4 and CO2. Before reaching the PIES detector, the gas eluted from the GC was further diluted in the TCD by its makeup gas (helium), and therefore the concentration of each analyte in the PIES detector was lower than that injected into the GC. The time variation of the analyte concentration in the discharge 2 cell, CPIES, can be approximated by CPIES(t) ) Cmax e-[(t - t0/∆t)] 2628

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where Cmax is the maximum analyte concentration in the PIES detector, t is retention time, t0 is time instant when analyte concentration is highest (center of the peak on a chromatogram), and ∆t characterizes the peak width. ∆t was found from TCD chromatograms and is equal to 8.2, 13.2, and 37.4 s for carbon monoxide, methane, and carbon dioxide, respectively. Assuming that the total number of analyte molecules both captured by the gas sampling valve and passed through the PIES and TCD detectors is the same, one can obtain the following expression for Cmax: Cmax )

Vinj

˙c Q

˙c + Q ˙ m) ˙c + Q ˙ m)√π (Q ∆t(Q

Cinj

(2)

where Vinj is the volume of the injected gas sample (0.25 mL) ˙ c and Q ˙ m are the column and TCD makeup gas flow rates, and Q respectively. The first factor in the right-hand side of eq 2 describes the reduction of analyte concentrations in the PIES cell as compared to that in the gas sample valve due to the adsorption/ desorption process in the stationary phase of the column, and the second factor is due to the dilution of the analyte in the TCD detector by the makeup gas. In the presented experiments Q˙m was varied between 20 and 50 mL/min. The resulting maximum analyte concentrations in the PIES detector (Cmax) were 1-100 ppm for methane, 2-85 ppm for carbon monoxide, and 1-35 ppm for carbon dioxide. Typical “raw” PIES spectra (d2i/dv2) obtained in pure helium and in helium with methane (Cmax ) 41 ppm) are given in Figure 4. Each spectrum is an average of 75 individual i-v characteristics, with a total acquisition time of 15 s. The presence of an analyte affects plasma parameters, in particular, leading to changes in the plasma potential which leads to a shift along the voltage axis of the i-v characteristic. The presence of a helium peak that appears in every “raw” PIES spectrum due to PI resulting from collisions of two helium metastable atoms

He(23S) + He(23S) f He+ + He + e (14.5 eV)

(3)

allows for absolute calibration of the voltage axis. The x-coordinate of the helium peak in the spectrum is set at -14.5 V, as predicted by reaction 3, εx ) 2Em - Ei ) 14.5 eV, where the electron energy, ε, is related to the collector voltage v after calibration as ε ) -ev. Here e is the elementary charge. The PIES spectrum is further obtained by subtracting a calibrated “raw” PIES spectrum obtained in pure helium from that with the analyte present. The resulting spectrum for methane along with the spectra for carbon monoxide and carbon dioxide are shown in Figure 5. The PIES spectra reported here are, to the authors’ knowledge, the first observation of Penning ionization electron spectra for methane. The methane PIES spectrum given in Figure 5 can be satisfactorily described with two overlapping peaks, with the positions of their maxima at -7.1 and -5.4 V. The -7.1 V peak is likely due to a PI reaction that results in a molecular ion in the ground state: He(23S) + CH4 f CH4+ + He + e (7.2 eV)

(4)

since the ionization energy of methane is 12.61 eV, and εx ) 19.8 - 12.61 ) 7.19 eV. The nature of the second peak is not yet clear, but it appears in every observed methane spectrum. The carbon monoxide and carbon dioxide fingerprints are also shown in Figure 5 for comparison. The CO peak with the center at -5.8 V is that due to the reaction He*(23S) + CO f CO+(X2Σ) + He + e (5.8 eV)

(5)

and the CO2 peak at -6.0 V is due to the following reaction: He(23S) + CO2 f CO2+(X2Πg) + He + e (6.0 eV)

(6)

CO and CO2 PIES spectra are consistent with those previously observed in molecular beam and glow discharge experiments.1,4 PIES CHROMATOGRAMS An example of a PIES chromatogram is shown in Figure 6. The analyte concentrations in the PIES cell, Cmax, were 10 ppm for carbon monoxide, 8 ppm for methane, and 3 ppm for carbon dioxide. A chromatogram acquired by TCD is shown on the right-hand side. Both chromatograms are compressed in time to show three segments: between 2.3 and 3.5 min (with CO peak), from 4.5 to 7 min (with CH4 peaks), and from 18 to 20 min (with CO2 peak). The total number of PIES spectra shown in Figure 6 is 33, and the interval between two consecutive spectra is 11 s. Also, since the CO and CO2 peaks were lower than the CH4 peak, spectra with the retention time between 4.5 and 7 min are reduced by the factor of 6. The positions of the peak maxima are better seen in the intensity graph given in Figure 7. CONCENTRATION ESTIMATES The Langmuir probe theory provides the following tool for calculating the analyte concentration.1 The quasi-stationary concentration of electrons formed in the discharge afterglow due to (4) Ohno, K.; Mutoh, H.; Harada, Y. J. Am. Chem. Soc. 1983, 105, 4555–4561.

reaction 1 on the axis of a cylindrical tube of radius b can be found from NmNakb2

ne(εx) )

(7)

µ12D(εx)

where Nm and Na are concentrations of helium metastable atoms and analyte molecules on the discharge cell axis, k is the reaction 1 rate coefficient, µ1 is the first root of the Bessel function J0, and D(εx) ) [1/(3σ(εx)NHe)][(2εx/me)1/2] is the diffusion coefficient for electrons in helium with energy εx, where me is the electron mass, σ(εx) is the elastic collision cross section for electrons with helium atoms (values for σ(εx) were taken from ref 5), and NHe is the helium concentration in the PIES detector. PI electron concentration can be also found from the electron energy distribution function (EEDF), f(ε), that is proportional to the second derivative of the collector i-v characteristic: f(ε) ) [me2/(2πAe3)][d2i/dv2] (where A is the collector area). Integrating the EEDF in the vicinity of the characteristic energy εx and taking into account that ε ) -ev, one can get

∫

ne(εx) )

εx+∆ε

εx-∆ε

-



-8mev 1 d2i dv e3 A dv2

(8)

Here ∆ε is half of the full peak width in the PIES spectrum (the full peak width is the width of the peak at its base as shown in Figure 5). Equations 7 and 8 allow computation of the analyte concentration, Na, provided the concentration of helium metastable atoms, Nm, is known. This value can be obtained utilizing the helium peak that is present in the “raw” PIES spectrum (see Figure 4). Letting Na ) Nm in eq 7 and combining eqs 7 and 8 one can get an expression for the concentration of helium metastable atoms participating in the reaction 3 such that Nm )

(

µ12D(14.5 eV) 2

kmb

)∫ (  14.5eV+∆ε

14.5eV-∆ε

-

)

-8mev 1 d2i dv e3 A dv2 (9)

Here km ) 1 × 109 cm3/s is the rate coefficient for reaction 3.6 Knowing Nm, Na can be found using eqs 7 and 8 such that Na )

(

µ12D(εx) 2

Nmkb

)∫ (  εx+∆ε

εx-∆ε

-

)

-8mev 1 d2i dv e3 A dv2

(10)

The PI rate coefficient for reaction 1 is known for carbon monoxide7 and is kCO ) 0.5 × 10-10 cm3/s. There are no data available for methane and carbon dioxide. To evaluate these parameters, we found the best fit for the model function (eq 10) to the analyte concentration estimated from the flow rate calculations (eq 2). CO concentrations computed from the PIES chromatogram, Cmax(PIES), using the rate coefficient from ref 7, are presented (5) Raizer, Y. P. Gas Discharge Physics; Springer: Berlin, 1991. (6) Massey, H. S.; Burhop, E. H. S. Electronic and Ionic Impact Phenomena; Clarendon Press: Oxford, U.K., 1952. (7) Ionikh, Y. Z.; Kolokolov, N. B.; Kudryavtsev, A. A.; Khromov, N. A.; Yakovitskii, S. P. Opt. Spectrosc. 1991, 71 (6), 542–544.

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Figure 5. PIES spectrum for methane (Cmax ) 41 ppm), carbon monoxide (Cmax ) 85 ppm), and carbon dioxide (Cmax ) 35 ppm).

Figure 6. PIES chromatogram obtained in a carbon monoxide (Cmax ) 10 ppm), methane (Cmax ) 8 ppm), and carbon dioxide (Cmax ) 3 ppm) mixture.

in Figure 8 as a function of the peak CO concentration in the PIES detector, Cmax(TCD), that is calculated using eq 2. One can see that the values of Cmax(PIES) are consistently higher than those estimated from eq 2 (the solid line is Cmax(PIES) ) Cmax(TCD)). The dotted line represents linear regression of 2630

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the experimental data points (excluding the highest two points corresponding to the measurements with highest carbon monoxide concentrations). The rate coefficient for reaction 5 incurred from the linear regression data is k*CO ) (0.7 ± 0.2) × 10-10 cm3/s.

Figure 7. PIES chromatogram (intensity graph) obtained in a carbon monoxide (Cmax ) 10 ppm), methane (Cmax ) 8 ppm), and carbon dioxide (Cmax ) 3 ppm) mixture. Dots indicate peak maxima.

Figure 8. Computed maximum CO concentration in the PIES detector. The dashed line is a linear fit to all but the two highest data points. The solid line represents Cmax(PIES) ) Cmax(TCD) for reference.

The nonlinearity is most probably due to the fact that at high CO concentrations the number of carbon monoxide molecules exceeded that of helium metastable atoms, a condition under which eq 7 becomes invalid. Rate coefficients for reaction 1 involving CO2 and CH4 were computed by fitting their concentrations found using PIES spectra to those evaluated using eq 2, i.e., requiring for the linear regressions that Cmax(PIES) ) Cmax(TCD) (Figures 9 and 10). Again, data for high analyte concentrations (Cmax(TCD) above 15 ppm for CO2 and 30 ppm for CH4) were ignored in fitting

Figure 9. Computed maximum CO2 concentration in the PIES detector with linear fit.

procedures. The resulting coefficients were k*CO2 ) (1.8 ± 0.7) × 10-10 cm3/s for the carbon monoxide peak, k*7.1 CH4 ) (4.7 ± 0.6) × 10-10 cm3/s for the methane peak at -7.1 V, and k*5.4 CH4 ) (8 ± 2) × 10-10 cm3/s for the methane peak at -5.4 V. DISCUSSION Providing the capability of identifying analytes selectively and independently of retention time with a device that is comparable in size and energy consumption to a nonselective GC detector is advantageous, primarily in view of the modern trend toward LOC systems. PIES technology also proposes means for quantitative analysis and the ability to measure two or three different analytes Analytical Chemistry, Vol. 81, No. 7, April 1, 2009

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16 eV. The random deviation of the characteristic peak positions, as incurred from PIES chromatograms, is currently of the order of 0.2 V, leading to a total number of channels of less than 35. Although noticeably lower compared to that for other spectral analytical technologies such as mass spectrometry or optical emission spectrometry, such resolution may prove sufficient for identification and measurement of a number of environmental gas constituents, as well as those found in other targeted analytical applications.

Figure 10. Computed maximum CH4 concentration in the PIES detector with linear fit.

coeluting from the GC column. The detector cell potentially can be made as small as the cells found in modern plasma TV screens and, if realized at atmospheric pressure, would require minimal flow and temperature control. Discharges in microchannels at atmospheric pressures have recently been successfully applied in analytical science.8 A drawback for the method is low spectral resolution. Most of the molecules have ionization energies in a range between 9 and (8) Eijkel, J. C. T.; Stoeri, H.; Manz, A. Anal. Chem. 2000, 72 (11), 2547–2552.

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CONCLUSION A breadboard GC detector based on PIES was investigated. Admixtures of carbon monoxide, carbon dioxide, and methane were identified, and their concentrations were measured independently of the GC retention time in a range between 1 and 100 ppm. ACKNOWLEDGMENT This work was supported through SBIR Grants by NASA (NNA06CA55C) and DOE (DE-FG02-07ER84894). The authors are deeply grateful to Daniel R. Kojiro of NASA-Ames for continuous and long-lasting support of the project.

Received for review December 4, 2008. Accepted January 26, 2009. AC8025674