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Detection of trace concentrations of helium and argon in gas mixtures by laser-induced breakdown spectroscopy
Spectrochimica Acta Part B 64 (2009) 1111–1118
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Spectrochimica Acta Part B j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s a b
Detection of trace concentrations of helium and argon in gas mixtures by laser-induced breakdown spectroscopy☆ E.D. McNaghten a,⁎, A.M. Parkes a,1, B.C. Griffiths a, A.I. Whitehouse b, S. Palanco b a b
AWE Aldermaston, Reading, Berkshire, RG7 4PR, United Kingdom Applied Photonics Ltd., Skipton, North Yorkshire, BD23 2DE, United Kingdom
a r t i c l e
i n f o
Article history: Received 22 December 2008 Accepted 22 July 2009 Available online 3 August 2009 Keywords: Laser-induced breakdown spectroscopy Trace gas detection Noble gases
a b s t r a c t We report what we believe to be the first demonstration of the detection of trace quantities of helium and argon in binary and ternary gas mixtures with nitrogen by laser-induced breakdown spectroscopy (LIBS). Although significant quenching of helium transitions due to collisional deactivation of excited species was observed, it was found that losses in analytical sensitivity could be minimized by increasing the laser irradiance and decreasing the pressure at which the analyses were performed. In consequence, limits of detection of parts-per-million and tens of parts-per-million and linear dynamic ranges of several orders of magnitude in analyte concentration were obtained. The results of this study suggest that LIBS may have potential applications in the detection of other noble gases at trace concentrations. British Crown copyright/MOD © 2009. Published by Elsevier B.V. All rights reserved.
1. Introduction A unique feature of laser-induced breakdown spectroscopy (LIBS) is its ability to enable remote, real-time determination of the elemental composition of a sample, whatever its nature, physical state or environment [1–3]. Although the LIBS technique has found widespread application in the elemental analysis of solids and liquids there is only limited scope for using it in trace gas analysis. This is due to the significant loss of molecular information in laser-produced plasmas during the atomization and ionization processes which occur during plasma formation. Scope for using LIBS for unambiguous identification of gases is, in the main, restricted to monatomic species such as the noble gases. Although elemental signatures from breakdown products of polyatomic species can be obtained by LIBS, the information derived is usually of limited value [4,5]. Use of LIBS for elemental analysis of diatomic gases such as hydrogen and deuterium has been reported [6–9], but there is limited scope for quantifying the molecular hydrogen/deuterium content of a sample by this method as it is impossible to discriminate between hydrogen/deuterium atoms originating from the parent diatomics (H2, D2, HD) per se and those originating from other hydrogenated/deuterated species (e.g. water vapor and methane). Detection of metals in airborne particulates and ☆ This paper was presented at the 5th International Conference on Laser-Induced Breakdown Spectroscopy (LIBS 2008), held in Berlin, Adlershof, Germany, 22–26 September 2008, and is published in the Special Issue of Spectrochimica Acta Part B, dedicated to that conference. ⁎ Corresponding author. Tel.: +44 1189825757; fax: +44 1189825024. E-mail address: [email protected] (E.D. McNaghten). 1 Present address: De La Rue Currency, Overton, Hampshire, RG25 3SE, United Kingdom.
aerosols by LIBS has also been achieved [10,11], but such applications fall outside the remit of bona-fide trace gas analysis. The demand for trace gas detection is largely driven by the environmental and industrial sectors and the analytes of interest to these communities are usually molecular (e.g. CO, CO2, CH4, NH3, NOx (x = 1,2) and SOy (y= 2,3)) and are either pollutants or greenhouse gases. Detection of these gases by optical methods is usually straightforward and can be achieved by infrared absorption and photoacoustic spectroscopy, usually with high sensitivity. In contrast, there is no significant demand for detecting noble gases in the environment as they do not have an atmospheric, chemical or biological source and are not usually regarded as pollutants. Detection of helium and argon is mainly of interest to radio- and geochemists as they are produced by radiogenic processes, decay of 40K producing 40Ar and decay of the uranium/ thorium series (235U, 238U and 232Th) producing 4He. Detection of noble gases by optical techniques presents a significant challenge to the spectroscopist. Absorption spectroscopy involving transitions originating from their ground electronic states would require excitation with radiation in the extreme ultraviolet region [12,13] and is not a practical proposition. Absorption techniques can only be applied to noble gases if the excitation processes involve transitions between electronically excited states. Population of such states can be achieved by exciting the gas in an electric discharge or microwave source. For example, the first infrared absorption spectra of noble gases were obtained using excitation in discharge tubes by Humphreys and Kostkowski in 1952 [14]. More recently, Uhl et al. demonstrated this approach by introducing a gas sample containing argon and krypton into a low pressure electrical discharge and probing excited state transitions using 811 nm diode laser radiation [15]. Detection limits in the parts-per-billion (ppb) range were obtained and use of the narrow
0584-8547/$ – see front matter. British Crown copyright/MOD © 2009. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.sab.2009.07.011
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linewidth TDL source enabled the 78Kr/86Kr ratio to be determined. Atomic emission spectroscopy techniques such as LIBS offer an alternative (and potentially simpler) means of detecting trace quantities of noble gases by optical methods. We have therefore investigated this approach and report what we believe to be the first demonstration of the detection of trace quantities of helium and argon in binary and ternary mixtures with nitrogen by LIBS. Although LIBS experiments are frequently performed under helium or argon atmospheres at reduced pressure in order to optimize the plasma conditions, the emission spectra of the gases themselves have received relatively little attention. Early work conducted by Rosen and Weyl [16] in the 1980s focused on the experimental and theoretical investigation of laser-induced breakdown in nitrogen, argon, neon and xenon using an Nd:YAG laser operating at its second and third harmonic wavelengths. In the following decade Iida [17] and Lee [18] investigated the influence of the confining atmosphere on the optical emission from laser-generated plasmas in order to assess the influence of the surrounding gas on the analyte emission lines. Around the same time Xu et al. [19] studied the time and wavelength resolved laser-induced plasma emission spectra in helium, argon, nitrogen, air and various gas mixtures over the 200– 700 nm wavelength range and found that plasmas generated in helium provided a stable spectrum over this region. Plasmas created in argon had the longest decay times, followed by helium and nitrogen. In more recent years Hanafi et al. [20] investigated the spectral emission of pure helium, argon, nitrogen and air in plasmas generated using a pulsed ruby laser and recorded the temporal behavior of the emission of each analyte for the different spectral regimes (continuum, ionic, and atomic). The influence of gas pressure and laser power on the intensity of the emission for the three gases was determined using their intense emission lines and a relationship between the intensity of the spectral lines as a function of the laser power at different pressures was observed. In 2007 Henry et al. [21] investigated the role of helium addition on the analyte signal enhancement in LIBS for analysis of pure gaseous systems and noted that the 388.9 nm and 587.6 nm helium
emission lines were found to be quenched by nitrogen. Addition of 25% nitrogen was found to essentially eliminate the observed helium emission lines. Similar behavior was observed for other helium lines. The strong quenching was attributed to the quenching of the metastable states via collisional deactivation processes (Penning ionization). 2. Experimental A schematic of the experimental arrangement is shown in Fig. 1. A 6way cube (Kurt Lesker CU6–0275) served as the sample cell for this investigation. The cube was fitted with two BK7 viewports, one of which was used to admit the laser beam and one for direct observation of the plasma. A fused-silica viewport allowed collection of the plasma emission in the UV–visible region. The remaining faces of the cube were fitted with ancillaries to allow gas admittance, venting and measurement of the cell pressure. An extension tube served as the laser beam dump. The sample cell was connected to a turbomolecular pump (Edwards EXT100/200) and a gas feed line which enabled the sample composition to be controlled by means of two mass flow controllers (MKS 1179A) with maximum flow rates of 20 and 200 sccm nitrogen. The flow controllers were regulated by a multichannel flow controller (MKS 647C). In addition, the chamber pressure was adjusted using a pulsed valve (MKS 0248A, 2000 sccm) which was placed between the chamber and the vacuum pump. The valve was controlled by a pressure controller (MKS 250E) connected to a Baratron absolute pressure transducer (MKS 627B). By combining the action of the mass flow controllers with that of the pulsed valve, a constant gas flow and pressure regime was achieved. The entire system was contained in an optical enclosure (Applied Photonics Ltd. System 600) which provided the necessary laser safety controls. In addition to the pressure controls provided by the regulators used in conjunction with the gas cylinders, the gas handling line was fitted with a 3–50 psi pressure relief valve to avoid over-pressurizing the sample cell. Two Q-switched Nd:YAG lasers (Quantel Brilliant B and Big Sky CFR400) were used in this investigation. The maximum energy available
Fig. 1. Experimental arrangement showing the layout of the different components inside the optical enclosure: 1, Laser head. 2, Sample chamber (a six-way cube). 3, Baratron transducer. 4, Automated valve. 5, Foreline trap. 6, Manual valve. MFC, mass flow controller. SV, safety valve. FM, folding mirror. BE, beam expander. FL, focusing lens. CL, collection lens. SOF, second order filter. XYZ, three-axis fiber positioner. FOC, fiber optic cable.
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from the Quantel Brilliant B was 850 mJ per pulse in pulses of 6 ns duration at 10 Hz repetition rate. The peak power provided by this laser (defined as pulse energy divided by the pulse duration) was 142 MW and the irradiance was estimated to be 8.5 × 1010 W cm− 2. The maximum energy available from the Big Sky CFR400 laser was 400 mJ per pulse in pulses of 6 ns duration at 10 Hz. The peak power provided by this laser was 67 MW and the irradiance was approximately 1.3× 109 W cm− 2. These irradiance levels are typical of those required for gas-phase LIBS studies. The Nd:YAG lasers were used separately as required and both were operated at their fundamental outputs (1064 nm). In each case the laser radiation was focused into the centre of the sample cell to generate plasmas in the gas mixtures. In order to allow tighter focusing the laser beam was expanded using a lens with a magnification factor of 4 and then focused into the sample chamber with an f/1 lens. The emission from the laser-induced plasmas was focused into a fiber optic cable by a fused silica lens of 25.4 mm focal length. The fiber was used to guide the emission to a f/3.8, 125 mm focal length Czerny–Turner spectrograph (Oriel MS125) fitted with a 300 groove/mm grating and a 1024 × 128 pixel ICCD detector (Oriel Instaspec V) working in full vertical binning mode. The low-dispersion spectrograph/grating combination afforded by the Oriel MS125 helped to maximize the sensitivity. The selected spectral window covered the 300–800 nm region and allowed monitoring of the continuum (Bremsstrahlung) emission as well as the discrete line emission of the analytes (helium and argon in nitrogen). Eight certified gases/gas mixtures supplied by Air Products were used for the investigation. These comprised cylinders of pure argon, helium and nitrogen as well as mixtures containing 1% argon in nitrogen, 1% argon in helium, 1% helium in argon, 1% helium in nitrogen and 1% argon plus 1% helium in nitrogen. The binary mixtures of the noble gases were studied in order to determine the analytical performance of the LIBS technique in the absence of nitrogen. Comparison of the data obtained using these gases with data acquired using the nitrogen-based mixtures enabled the extent of emission quenching by nitrogen to be assessed. In order to produce the calibration curves for the wide range of concentrations required, two procedures were used: (i) diluting the analyte gas in nitrogen for high concentrations and (ii) diluting the 1% concentration samples to produce the lowest concentration range. The He(I) 587.56 nm, Ar(I) 763.51 nm and N(I) 746.83 nm emission lines were selected for measurement as these had minimum spectral interference and high intensities. The latter feature ensured higher sensitivity over a large linear dynamic range of measurement. Limits of detection (LODs) were taken as three times the ratio of the standard deviation of the continuum emission (SDC) originating from the diluent gas, as measured at the analyte pixel number, to the total continuum emission (analyte plus diluent), C, at the same pixel number. 3. Results 3.1. Emission spectra Laser-induced breakdown spectra of helium, argon and nitrogen were acquired using samples of the pure gases at 1000 mbar pressure and the Big Sky CFR400 laser (400 mJ pulse energy). Spectra from the three samples are shown in Fig. 2. Argon and nitrogen exhibited strong continuum contributions whereas continuum emission was absent from the helium spectrum. The signal-to-noise ratios (SNRs) of the emission spectra were taken as the inverse of the relative standard deviation (RSD) of the net signal, i.e. the raw signal of the peak minus the continuum background signal. The contribution from the continuum emission was measured at locations close to the helium and argon emission lines in order to determine the signal-to-continuum ratios (SCRs). The helium and argon emission lines exhibited very different temporal behaviors. Argon produced a strong continuum emission which lasted for several microseconds with the 763.51 nm line intensity only becoming of comparable intensity to the continuum after 400 ns and reaching its maximum at ca. 1000 ns. In contrast, helium exhibited a
Fig. 2. From top to bottom, laser-induced breakdown spectra of Ar, N2 and He. Ar, acquisition delay, td = 1 µs, integration time, tg = 100 ns. He, N2, td = 100 ns, tg = 100 ns. Pressure was 1000 mbar for the three samples. Laser pulse energy = 400 mJ.
very low continuum contribution with line emission starting immediately after the laser pulse. Dilution of argon to 1% (by volume) with nitrogen had a significant effect on the temporal behavior of the argon line emission, reducing the delay time at which the maximum emission occurred from 1000 ns to approximately 300 ns. In contrast, similar dilution of helium had negligible effect on the line emission. The LIBS signal stability was assessed by accumulating successive laser shots and it was evident that minimal signal enhancement occurred after 50 laser pulses. RSDs below 6% were obtained in most cases by averaging 50 laser pulses.
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3.2. Influence of sample pressure The sample pressure was shown to have a strong influence on the helium and argon emission line intensities. Measurements were carried out at various pressure increments over the 13–1000 mbar range for argon and the 200–1000 mbar range for helium using the Quantel Brilliant B laser (850 mJ pulse energy). For both analytes the maximum SCRs and emission decay times decreased as the pressure was reduced; this was attributed to lower confinement of the plasmas at reduced pressures. Lower confinement of a LIBS plasma results in (i) a lower particle interaction, leading to a lower electron density and (ii) a higher particle velocity which enables particles to leave the region observed by the emission collection fiber in a shorter period of time. The influence of pressure on the delay at which the maximum SCRs occurred is presented in Fig. 3. Both signals exhibited monotonic increasing behavior but at different rates, as evidenced by the slopes of the SCR/pressure plots. It is clear that the delay at which the maximum SCR occurred for argon was strongly dependent on the sample pressure whereas, in the case of helium, the pressure dependence was much less pronounced. Fig. 3 shows that, for any given pressure, helium reaches the maximum SCR much earlier in the plasma life than argon. This effect has immediate consequences when the helium and argon concentrations are to be determined simultaneously. 3.3. Influence of laser pulse energy/irradiance The influence of the laser irradiance can be observed by comparing the SCR/sampling gate delay plots obtained using 1000 mbar of pure argon and both lasers (Fig. 4). When 400 mJ pulses provided by the Big Sky CFR400 laser were used, the SCR for argon decayed to a half of its maximum in approximately 0.5 µs. In contrast, the SCR obtained using 850 mJ laser pulses remained very high across the delay range investigated (0–20 µs). It must be noted that the irradiance at the focus is dependent on the beam divergence and beam diameter. Thus, although the pulse energy of the Quantel Brilliant B laser was more than twice that of the Big Sky CFR400, its beam diameter was larger by a factor of 1.33 and its divergence was approximately 20 times lower. The net effect of this was to increase the irradiance at the focus by over one order of magnitude for the Quantel laser. 3.4. Calibration Calibration curves were obtained for argon and helium in nitrogen (separately and in the same mixture) as well as for binary mixtures of argon and helium. Mixtures were prepared using the pure gases and two calibration curves were obtained for each element, corresponding
Fig. 3. Variation of the delay at which the maximum signal-to-continuum ratios for argon (■) and helium ( ) emission lines occurred as a function of pressure. Data were acquired using samples containing 100% argon and 100% helium and a laser pulse energy of 850 mJ.
Fig. 4. Signal-to-continuum ratio v sampling delay plots obtained for 1000 mbar pure argon at laser pulse energies of 400 mJ ( ) and 850 mJ (■).
to low and high analyte concentrations. In each case the calibration plots were obtained by plotting the line emission signal for each analyte against the mole fraction (X) of the analyte in the mixture. Although the calibration plots only exhibited linearity in three cases, a simple fitting function was found to describe the signal behavior. The reasons for this rely both on the plasma formation phenomena and on the properties of the elements under study. 3.4.1. Argon in nitrogen The LIBS spectra of argon/nitrogen mixtures provided examples of the many processes occurring in the plasma. Fig. 5 shows the calibration curves for detection of argon in nitrogen at low and high mole fractions respectively. Data for mixtures containing low mole fractions of argon (XAr) (Fig. 5a) were acquired using the Quantel Brilliant B laser (850 mJ pulse energy) whereas data for mixtures containing high mole fractions of argon (Fig. 5b) were obtained using the Big Sky CFR400 laser (400 mJ pulse energy). The differences in the signal scale axes on the plots shown in these figures are due to the differences in the laser pulse energies and signal integration times: a higher gain setting allowed higher sensitivity to be achieved at low analyte concentrations (Fig. 5a) whilst a lower gain setting helped to prevent saturation of the detector at high analyte concentrations (Fig. 5b). The calibration curves presented in Fig. 5 show three well defined regions. At very low analyte concentrations (Fig. 5a), the argon emission signal increased due to the increase in the number of species in the plasma. The emission increased in a linear manner up to an argon mole fraction of 0.002, thus covering approximately two orders of magnitude in terms of argon concentration. The calibration curve for the complete range of concentrations studied (Fig. 5b) exhibited a third order polynomial growth. Although Fig. 5b shows a linear region up to an argon mole fraction of 0.2, this cannot be taken to be a continuation of Fig. 5a since both plots were obtained under different irradiance conditions. The saturation behavior shown in Fig. 5a is well known in LIBS and is related to the use of a transition with a relatively low energy upper level. The calibration curve presented in Fig. 5b can be explained by interaction processes occurring between excited states of nitrogen and argon, leading to a higher number of emitting species. Interactions of this nature have been observed by Koslowski et al. [22] and mechanisms of single electron capture by N2+ 2 through collisions with noble gases to yield the cation of the noble gas and N+ 2 have been proposed. Koslowski et al. also observed emission from atomic nitrogen following electron impact of nitrogen gas. This is in good agreement with the increased N(I) 746.83 nm emission observed up to an analyte concentration of 50% in this work and proved that dissociation of molecular nitrogen is achieved
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Fig. 5. (a) Calibration plot for detection of argon in nitrogen by LIBS at argon mole fractions up to 0.01. XAr represents the mole fraction of argon in the gas samples. Measurement conditions: P = 1000 mbar, laser pulse energy= 850 mJ. (b) Calibration plot for detection of argon in nitrogen by LIBS at argon mole fractions up to 0.5. Measurement conditions: P = 1000 mbar, laser pulse energy= 400 mJ.
in the plasma. This suggests that ionization and subsequent dissociation of the nitrogen molecule is competitive with the collisional de-excitation of argon species with nitrogen, leading to quenching of the argon emission (Penning ionization). The results suggest that the balance between these processes is influenced by the concentration of nitrogen in the samples and that the border between the regions in which a particular process dominates occurs at a nitrogen mole fraction of 0.8. 3.4.2. Helium in nitrogen In order to obtain calibration data for the helium/nitrogen mixtures the intense continuum background caused by the presence of nitrogen had to be minimized. In addition, quenching of the helium emission by collisional de-excitation had to be minimized in order to achieve high sensitivity. The calibration curves for detection of helium in nitrogen presented in Fig. 6 indicate that helium exhibited a similar behavior to argon. Further evidence to support this hypothesis was provided by the binary calibration mixtures of argon and helium. Fig. 6b shows that there are two distinct linear regions and that these are separated by an abrupt change of slope in the vicinity of 20% helium. The slope change in the argon/nitrogen calibration plot for high argon mole fractions also occurred at 20% argon (Fig. 5b); this suggests that nitrogen is probably the common cause of this effect. The plot in Fig. 6a indicates that, at low helium mole fractions (XHe b 0.012), the helium calibration plot exhibits an excellent linear dynamic range which extends over five orders of magnitude down to the parts-per-million level. The manner in which the helium SCRs and SNRs varied with sample pressure is shown in Fig. 7. Data for this plot were acquired
Fig. 6. (a) Calibration plot for detection of helium in nitrogen by LIBS at helium mole fractions up to 0.01. Measurement conditions: P = 80 mbar, td = 500 ns, laser energy = 850 mJ. (b) Calibration plots for detection of helium in nitrogen by LIBS at helium mole fractions up to 0.5. Measurement conditions: P = 1000 mbar, laser pulse energy = 400 mJ.
Fig. 7. Variation of the signal-to-continuum (SCR, ■) and signal-to-noise (SNR, ) ratios of the He(I) 587.56 nm emission line as a function of total sample pressure. Data were acquired using the 1% helium in nitrogen mixture. Measurement conditions: laser pulse energy = 850 mJ.
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using a mixture containing 1% helium in nitrogen and the Quantel Brilliant B laser (850 mJ). The SCR of the helium emission line increased as the pressure was reduced; this behavior was consistent with the lower electron density and particle cross-section under such conditions. Operating at lower pressures ensured that the collision probability was kept as low as feasible and the deactivation of excited helium species by collisional processes (Penning ionization) was minimized. The net effect of this measure was to maximize the number of excited helium atoms in the plasma. As an additional benefit, the SNR was also found to improve at low pressures. It is evident from these plots that the optimum pressure range for analysis is approximately 80 mbar. 3.4.3. Argon and helium binary mixtures Fig. 8 shows a calibration curve of the full concentration range of the argon/helium binary mixtures at 1000 mbar. Calibration was achieved by normalizing signals from each element to the sum of the signals from both elements. The concentrations of both elements could be calibrated using a simple function and exhibited linear dynamic behavior over the full concentration range. The linear calibration plot shown in Fig. 9 for mixtures containing low helium mole fractions indicates that quenching of the He(I) 587.56 nm emission does not occur in binary mixtures with argon. The linear behavior exhibited by the argon/helium binary mixtures is in marked contrast to the behavior exhibited by the nitrogen-based binary mixtures (Figs. 5b and 6b). Fig. 10 depicts the variation of the helium SCRs and SNRs as a function of pressure. Data were acquired using the 1% helium in argon mixture and 850 mJ laser pulse energy. Although the maximum SCR occurred at approximately 13 mbar the SNR was poor at this pressure, mainly due to the lack of reproducibility of plasma formation at pressures under 40 mbar. It is expected that a higher irradiance at the laser focus would provide the scope for performing a reliable analysis at 13 mbar sample pressure. 3.4.4. Ternary mixtures: argon and helium in nitrogen A range of ternary mixtures containing argon, helium and nitrogen were prepared by diluting a mixture containing 1% argon, 1% helium and 98% nitrogen with nitrogen in order to assess how the presence of both
Fig. 8. Calibration plots for detection of argon ( ) and helium (■) in the argon/helium binary mixtures by LIBS. Measurement conditions: P = 1000 mbar, laser pulse energy = 850 mJ.
Fig. 9. Calibration plot for detection of helium in the argon/helium binary mixtures by LIBS at helium mole fractions up to 0.008. Measurement conditions: P = 837 mbar, laser pulse energy = 850 mJ.
analytes (argon and helium) in nitrogen influenced their respective calibration curves. Given the differences between the optimum analysis conditions for both elements two different experiments were performed. In the first experiment, the sampling gate delay timing and sample pressure were slightly different to those used for the binary mixtures of helium in nitrogen, yet still sufficient to record an argon emission signal of good analytical quality. The chamber pressure was set to 100 mbar during measurements and the optimum sampling gate delay and integration times were 500 ns and 5 µs respectively. Under these conditions linear calibration plots were obtained for both analytes at concentrations up to 1% (Fig. 11). It should be noted that the concentration range investigated was limited by the gas mixtures available for the study. Analyte detection limits obtained using the ternary mixtures are presented in Table 1. Whilst the limit of detection for helium in the ternary mixtures (46 ppm) was similar to that found with the helium/nitrogen binary mixtures (57 ppm) and better than that recorded for the helium/argon mixtures (160 ppm), the LOD for argon in the ternary mixtures (270 ppm) was much higher than those obtained for the argon/nitrogen binary mixtures (17 ppm) and the argon/helium binary mixtures (4.9 ppm). In order to determine whether the reduction in sensitivity observed for the detection of argon in the ternary mixtures (relative
Fig. 10. Variation of the signal-to-continuum (SCR, ■) and signal-to-noise (SNR, ) ratios of the He(I) 587.56 nm emission line as a function of pressure. Data were acquired using the 1% helium in argon mixture and a laser pulse energy of 850 mJ.
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cross-sections for the lower pressure regime. Another consequence arising from the differences in pressure is that a different region of the plasma is sampled by the fiber optic coupler during each experiment. The optimum position of the fiber optic coupler for data acquisition at low pressure (Fig. 11) corresponded to an outer region of the plasma at 1000 mbar (Fig. 5). Consequently, the argon calibration plot obtained at 1000 mbar (inset of Fig. 11) was acquired by monitoring a cooler region of the plasma where the emission line curve of growth was not saturated for the range of concentrations under study (up to 1000 ppm). The results are therefore not only strongly influenced by the experimental conditions (laser irradiation and sample pressure) but also by the collection geometry used. The ternary mixtures all exhibited linear dynamic behavior over the concentration range investigated (0–1% analyte concentration in nitrogen). The actual LDRs could be larger than those reported as the full range of concentrations was not tested. Table 1 summarizes the analytical figures of merit for all elements concerned. Fig. 11. Calibration plots for detection of argon ( ) and helium (■) in nitrogen by LIBS at analyte mole fractions up to 0.01. Measurement conditions were optimized for detection of helium (P = 100 mbar, laser pulse energy = 850 mJ). Inset: calibration plot for detection of argon in the argon/helium/nitrogen ternary mixtures by LIBS at argon mole fractions up to 0.01 (1% concentration by volume). Measurement conditions: P = 1000 mbar, laser pulse energy= 850 mJ. The experimental conditions were optimized for argon. Helium could not be detected under these conditions.
Table 1 Analytical figures of merit for detection of helium and argon by LIBS. Analyte
Diluent
P (mbar)
R2
LOD (ppm)
CDR
LDR
Argon Helium Argon Helium Argonb Argond Heliumd
Nitrogen Nitrogen Helium Argon Nitrogen Nitrogen Nitrogen
1000 80 1000 37 1000 100 100
0.9979 0.9993 0.9938 0.9973 0.9996 0.9953 0.9997
17 57 4.9 160 76 270 46
0–50% 0–50% 0–100% 0–100% 0–1%c 0–1%c 0–1%c
0–17%a 0–20% 0–0.025% 0–20% 0–1%c 0–1%c 0–1%c
P = sample chamber pressure, R2 = correlation coefficient of the calibration plot. LOD is the limit of detection, CDR is the calibrated dynamic range and LDR is the linear dynamic range. a The calibration curve was in a log/log scale. b Experimental conditions optimized for argon. Helium could not be detected under these conditions. c The actual ranges could be larger than those reported as the full range of concentrations was not tested. d Experimental conditions optimized for helium.
to the nitrogen/argon and helium/argon mixtures) was due to the unfavorable acquisition conditions or merely to the presence of helium, a second calibration curve was obtained under similar experimental conditions as used for the calibration of argon/nitrogen mixtures (Fig. 5). The position of the fiber optic coupler was adjusted to optimize conditions for this study. The calibration plot obtained under these conditions is presented in the inset of Fig. 11 and exhibited linear behavior (R2 = 0.9996). The detection limit (76 ppm) was greater than that obtained for the argon/nitrogen mixtures (17 ppm, Fig. 5b), confirming that the presence of low concentrations of helium do not significantly affect the Ar(I) 763.51 nm emission line curve of growth. Although helium was detected at 100 mbar sample pressure (Fig. 11, main plot) it was not possible to detect it when the conditions were optimized for detection of argon (1000 mbar) (see inset to Fig. 11). It should be noted that the linear relationships between the argon emission intensity and mole fraction shown in the plots in Fig. 11 are in marked contrast to that presented in Fig. 5. The change in behavior can be explained by the different laser irradiation conditions and sample pressure used in each experiment, the latter leading to much lower
4. Conclusions The detection of trace concentrations of helium and argon in gas mixtures by the LIBS technique has been demonstrated for the first time. The analytes have been detected in binary and ternary mixtures with nitrogen using a low resolution spectrometer. Limits of detection in the parts-per-million and tens of parts-per-million ranges have been achieved in most cases, despite the quenching effects associated with the gases involved in the analysis. Although significant quenching of the helium emission due to collisional deactivation of electronically excited helium atoms was observed, it has been shown that the consequent losses in analytical sensitivity could be minimized by increasing the laser irradiance and decreasing the pressure at which the analysis was performed. Linear dynamic ranges of several orders of magnitude in analyte concentration were obtained for determination of both gases by LIBS. It is evident that the optimum conditions for determination of helium and argon in nitrogen by LIBS differ markedly and simultaneous analysis of both gases involves a degree of compromise. In order to obtain high analytical sensitivities for both gases by LIBS the analyses should be performed sequentially under the optimum conditions pertaining to each analyte (i.e. sample pressure and sampling gate delay time). This will ensure that the best possible SCRs and SNRs are obtained for each analyte. The results of this study suggest that LIBS may have potential applications in the detection of other noble gases at trace concentrations. Acknowledgement The work described in this paper was fully funded by AWE Corporate Technical Outreach. References [1] D.A. Cremers, L.J. Radziemski, Handbook of Laser-Induced Breakdown Spectroscopy, John Wiley and Sons, 2006. [2] A.W. Miziolek, V. Palleschi, I. Schechter, Laser-Induced Breakdown Spectroscopy (LIBS), Fundamentals and Applications, Cambridge University Press, 2006. [3] J.P. Singh, S.N. Thakur (Eds.), Laser-Induced Breakdown Spectroscopy, Elsevier, 2007. [4] E.D. Lancaster, K.L. McNesby, R.G. Daniel, A.W. Miziolek, Spectroscopic analysis of fire suppressants and refrigerants by laser-induced breakdown spectroscopy, Appl. Opt. 38 (1999) 1476–1480. [5] V. Sturm, R. Noll, Laser-induced breakdown spectroscopy of gas mixtures of air, CO2, N2, and C3H8 for simultaneous C, H, O, and N measurement, Appl. Opt. 42 (2003) 6221–6225. [6] K.H. Kurniawan, K. Kagawa, Hydrogen and deuterium analysis using laser-induced plasma spectroscopy, Appl. Spectrosc. Rev. 41 (2006) 99–130. [7] A.J. Ball, V. Hohreiter, D.W. Hahn, Hydrogen leak detection using laser-induced breakdown spectroscopy, Appl. Spectrosc. 59 (2005) 348–353. [8] K.H. Kurniawan, T.J. Lie, M.M. Suliyanti, R. Fedwig, S.N. Abdulmadjid, M. Pardede, N. Idris, T. Kobayashim, Y. Kusumoto, K. Kagawa, M.O. Tjia, Detection of deuterium
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and hydrogen using laser-induced helium gas plasma at atmospheric pressure, J. Appl. Phys. 98 (2005) 093302–1– 093302-3. A. D'Ulivo, M. Onor, E. Pitzalis, R. Spiniello, L. Lampugnani, G. Cristoforetti, S. Legnaioli, V. Palleschi, A. Salvetti, E. Tognoni, Determination of the deuterium/ hydrogen ratio in gas reaction products by laser-induced breakdown spectroscopy, Spectrochim. Acta Part B 61 (2006) 797–802. D.W. Hahn, M.M. Lunden, Detection and analysis of aerosol particles by laserinduced breakdown spectroscopy, Aerosol Sci. Technol. 33 (2000) 30–48. J.E. Carranza, B.T. Fisher, G.D. Yoder, D.W. Hahn, On-line analysis of ambient air aerosols using laser-induced breakdown spectroscopy, Spectrochim. Acta Part B 56 (2001) 851–864. V.P. Belozerova, Absorption spectra of several atmospheric gases in the vacuum ultraviolet region, Sov. J. Opt. Technol. (USA) 34 (1967) 184–192. J.A. Coleb, Near ultraviolet–visible atomic absorption spectra of the noble gases, Anal. Chem. 38 (1966) 1059–1061. C. Humphreys, H.J. Kostkowski, Infrared spectra of noble gases (12000 to 19000 Å), J. Res. Natl. Bur. Stand. 49 (1952) 73–84. R. Uhl, J. Franzke, U. Haas, Detection of argon and krypton traces in noble gases by diode laser absorption spectrometry, Appl. Phys. B 73 (2001) 71–74.
[16] D.I. Rosen, G. Weyl, Laser-induced breakdown in nitrogen and the rare gases at 0.53 and 0.35 µm, J. Phys. D: Appl. Phys. 20 (1987) 1264–1276. [17] Y.I. Iida, Effects of atmosphere on laser vaporization and excitation processes of solid samples, Spectrochim. Acta Part B 45 (1990) 1353–1367. [18] Y.-I. Lee, K. Song, H.-K. Cha, J.-M. Lee, M-C. Park, G-H. Lee, J. Sneddon, Influence of atmosphere and irradiation wavelength on copper plasma emission induced by excimer and Q-switched Nd:YAG laser ablation, Appl. Spectrosc. 51 (1997) 959–964. [19] N. Xu, V. Majidi, Wavelength and time-resolved investigation of laser-induced plasmas as a continuum source, Appl. Spectrosc. 47 (1993) 1134–1139. [20] M. Hanafi, M.M. Omarb, Y.E.E-D. Gamal, Study of laser-induced breakdown spectroscopy of gases, Radiat. Phys. Chem. 57 (2000) 11–20. [21] C.A. Henry, P.K. Diwakar, D.W. Hahn, Investigation of helium addition for laserinduced plasma spectroscopy of pure gas phase systems: analyte interactions and signal enhancement, Spectrochim. Acta Part B 62 (2007) 1390–1398. [22] H.R. Koslowski, H. Lebius, V. Staemmler, R. Fink, K. Wiesemann, B.A. Huber, Collisions of doubly charged nitrogen molecules with rare gas atoms, J. Phys. B: Atmos. Mol. Opt. Phys. 24 (1991) 5023–5034.
Contents lists available at ScienceDirect
Spectrochimica Acta Part B j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s a b
Detection of trace concentrations of helium and argon in gas mixtures by laser-induced breakdown spectroscopy☆ E.D. McNaghten a,⁎, A.M. Parkes a,1, B.C. Griffiths a, A.I. Whitehouse b, S. Palanco b a b
AWE Aldermaston, Reading, Berkshire, RG7 4PR, United Kingdom Applied Photonics Ltd., Skipton, North Yorkshire, BD23 2DE, United Kingdom
a r t i c l e
i n f o
Article history: Received 22 December 2008 Accepted 22 July 2009 Available online 3 August 2009 Keywords: Laser-induced breakdown spectroscopy Trace gas detection Noble gases
a b s t r a c t We report what we believe to be the first demonstration of the detection of trace quantities of helium and argon in binary and ternary gas mixtures with nitrogen by laser-induced breakdown spectroscopy (LIBS). Although significant quenching of helium transitions due to collisional deactivation of excited species was observed, it was found that losses in analytical sensitivity could be minimized by increasing the laser irradiance and decreasing the pressure at which the analyses were performed. In consequence, limits of detection of parts-per-million and tens of parts-per-million and linear dynamic ranges of several orders of magnitude in analyte concentration were obtained. The results of this study suggest that LIBS may have potential applications in the detection of other noble gases at trace concentrations. British Crown copyright/MOD © 2009. Published by Elsevier B.V. All rights reserved.
1. Introduction A unique feature of laser-induced breakdown spectroscopy (LIBS) is its ability to enable remote, real-time determination of the elemental composition of a sample, whatever its nature, physical state or environment [1–3]. Although the LIBS technique has found widespread application in the elemental analysis of solids and liquids there is only limited scope for using it in trace gas analysis. This is due to the significant loss of molecular information in laser-produced plasmas during the atomization and ionization processes which occur during plasma formation. Scope for using LIBS for unambiguous identification of gases is, in the main, restricted to monatomic species such as the noble gases. Although elemental signatures from breakdown products of polyatomic species can be obtained by LIBS, the information derived is usually of limited value [4,5]. Use of LIBS for elemental analysis of diatomic gases such as hydrogen and deuterium has been reported [6–9], but there is limited scope for quantifying the molecular hydrogen/deuterium content of a sample by this method as it is impossible to discriminate between hydrogen/deuterium atoms originating from the parent diatomics (H2, D2, HD) per se and those originating from other hydrogenated/deuterated species (e.g. water vapor and methane). Detection of metals in airborne particulates and ☆ This paper was presented at the 5th International Conference on Laser-Induced Breakdown Spectroscopy (LIBS 2008), held in Berlin, Adlershof, Germany, 22–26 September 2008, and is published in the Special Issue of Spectrochimica Acta Part B, dedicated to that conference. ⁎ Corresponding author. Tel.: +44 1189825757; fax: +44 1189825024. E-mail address: [email protected] (E.D. McNaghten). 1 Present address: De La Rue Currency, Overton, Hampshire, RG25 3SE, United Kingdom.
aerosols by LIBS has also been achieved [10,11], but such applications fall outside the remit of bona-fide trace gas analysis. The demand for trace gas detection is largely driven by the environmental and industrial sectors and the analytes of interest to these communities are usually molecular (e.g. CO, CO2, CH4, NH3, NOx (x = 1,2) and SOy (y= 2,3)) and are either pollutants or greenhouse gases. Detection of these gases by optical methods is usually straightforward and can be achieved by infrared absorption and photoacoustic spectroscopy, usually with high sensitivity. In contrast, there is no significant demand for detecting noble gases in the environment as they do not have an atmospheric, chemical or biological source and are not usually regarded as pollutants. Detection of helium and argon is mainly of interest to radio- and geochemists as they are produced by radiogenic processes, decay of 40K producing 40Ar and decay of the uranium/ thorium series (235U, 238U and 232Th) producing 4He. Detection of noble gases by optical techniques presents a significant challenge to the spectroscopist. Absorption spectroscopy involving transitions originating from their ground electronic states would require excitation with radiation in the extreme ultraviolet region [12,13] and is not a practical proposition. Absorption techniques can only be applied to noble gases if the excitation processes involve transitions between electronically excited states. Population of such states can be achieved by exciting the gas in an electric discharge or microwave source. For example, the first infrared absorption spectra of noble gases were obtained using excitation in discharge tubes by Humphreys and Kostkowski in 1952 [14]. More recently, Uhl et al. demonstrated this approach by introducing a gas sample containing argon and krypton into a low pressure electrical discharge and probing excited state transitions using 811 nm diode laser radiation [15]. Detection limits in the parts-per-billion (ppb) range were obtained and use of the narrow
0584-8547/$ – see front matter. British Crown copyright/MOD © 2009. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.sab.2009.07.011
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linewidth TDL source enabled the 78Kr/86Kr ratio to be determined. Atomic emission spectroscopy techniques such as LIBS offer an alternative (and potentially simpler) means of detecting trace quantities of noble gases by optical methods. We have therefore investigated this approach and report what we believe to be the first demonstration of the detection of trace quantities of helium and argon in binary and ternary mixtures with nitrogen by LIBS. Although LIBS experiments are frequently performed under helium or argon atmospheres at reduced pressure in order to optimize the plasma conditions, the emission spectra of the gases themselves have received relatively little attention. Early work conducted by Rosen and Weyl [16] in the 1980s focused on the experimental and theoretical investigation of laser-induced breakdown in nitrogen, argon, neon and xenon using an Nd:YAG laser operating at its second and third harmonic wavelengths. In the following decade Iida [17] and Lee [18] investigated the influence of the confining atmosphere on the optical emission from laser-generated plasmas in order to assess the influence of the surrounding gas on the analyte emission lines. Around the same time Xu et al. [19] studied the time and wavelength resolved laser-induced plasma emission spectra in helium, argon, nitrogen, air and various gas mixtures over the 200– 700 nm wavelength range and found that plasmas generated in helium provided a stable spectrum over this region. Plasmas created in argon had the longest decay times, followed by helium and nitrogen. In more recent years Hanafi et al. [20] investigated the spectral emission of pure helium, argon, nitrogen and air in plasmas generated using a pulsed ruby laser and recorded the temporal behavior of the emission of each analyte for the different spectral regimes (continuum, ionic, and atomic). The influence of gas pressure and laser power on the intensity of the emission for the three gases was determined using their intense emission lines and a relationship between the intensity of the spectral lines as a function of the laser power at different pressures was observed. In 2007 Henry et al. [21] investigated the role of helium addition on the analyte signal enhancement in LIBS for analysis of pure gaseous systems and noted that the 388.9 nm and 587.6 nm helium
emission lines were found to be quenched by nitrogen. Addition of 25% nitrogen was found to essentially eliminate the observed helium emission lines. Similar behavior was observed for other helium lines. The strong quenching was attributed to the quenching of the metastable states via collisional deactivation processes (Penning ionization). 2. Experimental A schematic of the experimental arrangement is shown in Fig. 1. A 6way cube (Kurt Lesker CU6–0275) served as the sample cell for this investigation. The cube was fitted with two BK7 viewports, one of which was used to admit the laser beam and one for direct observation of the plasma. A fused-silica viewport allowed collection of the plasma emission in the UV–visible region. The remaining faces of the cube were fitted with ancillaries to allow gas admittance, venting and measurement of the cell pressure. An extension tube served as the laser beam dump. The sample cell was connected to a turbomolecular pump (Edwards EXT100/200) and a gas feed line which enabled the sample composition to be controlled by means of two mass flow controllers (MKS 1179A) with maximum flow rates of 20 and 200 sccm nitrogen. The flow controllers were regulated by a multichannel flow controller (MKS 647C). In addition, the chamber pressure was adjusted using a pulsed valve (MKS 0248A, 2000 sccm) which was placed between the chamber and the vacuum pump. The valve was controlled by a pressure controller (MKS 250E) connected to a Baratron absolute pressure transducer (MKS 627B). By combining the action of the mass flow controllers with that of the pulsed valve, a constant gas flow and pressure regime was achieved. The entire system was contained in an optical enclosure (Applied Photonics Ltd. System 600) which provided the necessary laser safety controls. In addition to the pressure controls provided by the regulators used in conjunction with the gas cylinders, the gas handling line was fitted with a 3–50 psi pressure relief valve to avoid over-pressurizing the sample cell. Two Q-switched Nd:YAG lasers (Quantel Brilliant B and Big Sky CFR400) were used in this investigation. The maximum energy available
Fig. 1. Experimental arrangement showing the layout of the different components inside the optical enclosure: 1, Laser head. 2, Sample chamber (a six-way cube). 3, Baratron transducer. 4, Automated valve. 5, Foreline trap. 6, Manual valve. MFC, mass flow controller. SV, safety valve. FM, folding mirror. BE, beam expander. FL, focusing lens. CL, collection lens. SOF, second order filter. XYZ, three-axis fiber positioner. FOC, fiber optic cable.
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from the Quantel Brilliant B was 850 mJ per pulse in pulses of 6 ns duration at 10 Hz repetition rate. The peak power provided by this laser (defined as pulse energy divided by the pulse duration) was 142 MW and the irradiance was estimated to be 8.5 × 1010 W cm− 2. The maximum energy available from the Big Sky CFR400 laser was 400 mJ per pulse in pulses of 6 ns duration at 10 Hz. The peak power provided by this laser was 67 MW and the irradiance was approximately 1.3× 109 W cm− 2. These irradiance levels are typical of those required for gas-phase LIBS studies. The Nd:YAG lasers were used separately as required and both were operated at their fundamental outputs (1064 nm). In each case the laser radiation was focused into the centre of the sample cell to generate plasmas in the gas mixtures. In order to allow tighter focusing the laser beam was expanded using a lens with a magnification factor of 4 and then focused into the sample chamber with an f/1 lens. The emission from the laser-induced plasmas was focused into a fiber optic cable by a fused silica lens of 25.4 mm focal length. The fiber was used to guide the emission to a f/3.8, 125 mm focal length Czerny–Turner spectrograph (Oriel MS125) fitted with a 300 groove/mm grating and a 1024 × 128 pixel ICCD detector (Oriel Instaspec V) working in full vertical binning mode. The low-dispersion spectrograph/grating combination afforded by the Oriel MS125 helped to maximize the sensitivity. The selected spectral window covered the 300–800 nm region and allowed monitoring of the continuum (Bremsstrahlung) emission as well as the discrete line emission of the analytes (helium and argon in nitrogen). Eight certified gases/gas mixtures supplied by Air Products were used for the investigation. These comprised cylinders of pure argon, helium and nitrogen as well as mixtures containing 1% argon in nitrogen, 1% argon in helium, 1% helium in argon, 1% helium in nitrogen and 1% argon plus 1% helium in nitrogen. The binary mixtures of the noble gases were studied in order to determine the analytical performance of the LIBS technique in the absence of nitrogen. Comparison of the data obtained using these gases with data acquired using the nitrogen-based mixtures enabled the extent of emission quenching by nitrogen to be assessed. In order to produce the calibration curves for the wide range of concentrations required, two procedures were used: (i) diluting the analyte gas in nitrogen for high concentrations and (ii) diluting the 1% concentration samples to produce the lowest concentration range. The He(I) 587.56 nm, Ar(I) 763.51 nm and N(I) 746.83 nm emission lines were selected for measurement as these had minimum spectral interference and high intensities. The latter feature ensured higher sensitivity over a large linear dynamic range of measurement. Limits of detection (LODs) were taken as three times the ratio of the standard deviation of the continuum emission (SDC) originating from the diluent gas, as measured at the analyte pixel number, to the total continuum emission (analyte plus diluent), C, at the same pixel number. 3. Results 3.1. Emission spectra Laser-induced breakdown spectra of helium, argon and nitrogen were acquired using samples of the pure gases at 1000 mbar pressure and the Big Sky CFR400 laser (400 mJ pulse energy). Spectra from the three samples are shown in Fig. 2. Argon and nitrogen exhibited strong continuum contributions whereas continuum emission was absent from the helium spectrum. The signal-to-noise ratios (SNRs) of the emission spectra were taken as the inverse of the relative standard deviation (RSD) of the net signal, i.e. the raw signal of the peak minus the continuum background signal. The contribution from the continuum emission was measured at locations close to the helium and argon emission lines in order to determine the signal-to-continuum ratios (SCRs). The helium and argon emission lines exhibited very different temporal behaviors. Argon produced a strong continuum emission which lasted for several microseconds with the 763.51 nm line intensity only becoming of comparable intensity to the continuum after 400 ns and reaching its maximum at ca. 1000 ns. In contrast, helium exhibited a
Fig. 2. From top to bottom, laser-induced breakdown spectra of Ar, N2 and He. Ar, acquisition delay, td = 1 µs, integration time, tg = 100 ns. He, N2, td = 100 ns, tg = 100 ns. Pressure was 1000 mbar for the three samples. Laser pulse energy = 400 mJ.
very low continuum contribution with line emission starting immediately after the laser pulse. Dilution of argon to 1% (by volume) with nitrogen had a significant effect on the temporal behavior of the argon line emission, reducing the delay time at which the maximum emission occurred from 1000 ns to approximately 300 ns. In contrast, similar dilution of helium had negligible effect on the line emission. The LIBS signal stability was assessed by accumulating successive laser shots and it was evident that minimal signal enhancement occurred after 50 laser pulses. RSDs below 6% were obtained in most cases by averaging 50 laser pulses.
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3.2. Influence of sample pressure The sample pressure was shown to have a strong influence on the helium and argon emission line intensities. Measurements were carried out at various pressure increments over the 13–1000 mbar range for argon and the 200–1000 mbar range for helium using the Quantel Brilliant B laser (850 mJ pulse energy). For both analytes the maximum SCRs and emission decay times decreased as the pressure was reduced; this was attributed to lower confinement of the plasmas at reduced pressures. Lower confinement of a LIBS plasma results in (i) a lower particle interaction, leading to a lower electron density and (ii) a higher particle velocity which enables particles to leave the region observed by the emission collection fiber in a shorter period of time. The influence of pressure on the delay at which the maximum SCRs occurred is presented in Fig. 3. Both signals exhibited monotonic increasing behavior but at different rates, as evidenced by the slopes of the SCR/pressure plots. It is clear that the delay at which the maximum SCR occurred for argon was strongly dependent on the sample pressure whereas, in the case of helium, the pressure dependence was much less pronounced. Fig. 3 shows that, for any given pressure, helium reaches the maximum SCR much earlier in the plasma life than argon. This effect has immediate consequences when the helium and argon concentrations are to be determined simultaneously. 3.3. Influence of laser pulse energy/irradiance The influence of the laser irradiance can be observed by comparing the SCR/sampling gate delay plots obtained using 1000 mbar of pure argon and both lasers (Fig. 4). When 400 mJ pulses provided by the Big Sky CFR400 laser were used, the SCR for argon decayed to a half of its maximum in approximately 0.5 µs. In contrast, the SCR obtained using 850 mJ laser pulses remained very high across the delay range investigated (0–20 µs). It must be noted that the irradiance at the focus is dependent on the beam divergence and beam diameter. Thus, although the pulse energy of the Quantel Brilliant B laser was more than twice that of the Big Sky CFR400, its beam diameter was larger by a factor of 1.33 and its divergence was approximately 20 times lower. The net effect of this was to increase the irradiance at the focus by over one order of magnitude for the Quantel laser. 3.4. Calibration Calibration curves were obtained for argon and helium in nitrogen (separately and in the same mixture) as well as for binary mixtures of argon and helium. Mixtures were prepared using the pure gases and two calibration curves were obtained for each element, corresponding
Fig. 3. Variation of the delay at which the maximum signal-to-continuum ratios for argon (■) and helium ( ) emission lines occurred as a function of pressure. Data were acquired using samples containing 100% argon and 100% helium and a laser pulse energy of 850 mJ.
Fig. 4. Signal-to-continuum ratio v sampling delay plots obtained for 1000 mbar pure argon at laser pulse energies of 400 mJ ( ) and 850 mJ (■).
to low and high analyte concentrations. In each case the calibration plots were obtained by plotting the line emission signal for each analyte against the mole fraction (X) of the analyte in the mixture. Although the calibration plots only exhibited linearity in three cases, a simple fitting function was found to describe the signal behavior. The reasons for this rely both on the plasma formation phenomena and on the properties of the elements under study. 3.4.1. Argon in nitrogen The LIBS spectra of argon/nitrogen mixtures provided examples of the many processes occurring in the plasma. Fig. 5 shows the calibration curves for detection of argon in nitrogen at low and high mole fractions respectively. Data for mixtures containing low mole fractions of argon (XAr) (Fig. 5a) were acquired using the Quantel Brilliant B laser (850 mJ pulse energy) whereas data for mixtures containing high mole fractions of argon (Fig. 5b) were obtained using the Big Sky CFR400 laser (400 mJ pulse energy). The differences in the signal scale axes on the plots shown in these figures are due to the differences in the laser pulse energies and signal integration times: a higher gain setting allowed higher sensitivity to be achieved at low analyte concentrations (Fig. 5a) whilst a lower gain setting helped to prevent saturation of the detector at high analyte concentrations (Fig. 5b). The calibration curves presented in Fig. 5 show three well defined regions. At very low analyte concentrations (Fig. 5a), the argon emission signal increased due to the increase in the number of species in the plasma. The emission increased in a linear manner up to an argon mole fraction of 0.002, thus covering approximately two orders of magnitude in terms of argon concentration. The calibration curve for the complete range of concentrations studied (Fig. 5b) exhibited a third order polynomial growth. Although Fig. 5b shows a linear region up to an argon mole fraction of 0.2, this cannot be taken to be a continuation of Fig. 5a since both plots were obtained under different irradiance conditions. The saturation behavior shown in Fig. 5a is well known in LIBS and is related to the use of a transition with a relatively low energy upper level. The calibration curve presented in Fig. 5b can be explained by interaction processes occurring between excited states of nitrogen and argon, leading to a higher number of emitting species. Interactions of this nature have been observed by Koslowski et al. [22] and mechanisms of single electron capture by N2+ 2 through collisions with noble gases to yield the cation of the noble gas and N+ 2 have been proposed. Koslowski et al. also observed emission from atomic nitrogen following electron impact of nitrogen gas. This is in good agreement with the increased N(I) 746.83 nm emission observed up to an analyte concentration of 50% in this work and proved that dissociation of molecular nitrogen is achieved
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Fig. 5. (a) Calibration plot for detection of argon in nitrogen by LIBS at argon mole fractions up to 0.01. XAr represents the mole fraction of argon in the gas samples. Measurement conditions: P = 1000 mbar, laser pulse energy= 850 mJ. (b) Calibration plot for detection of argon in nitrogen by LIBS at argon mole fractions up to 0.5. Measurement conditions: P = 1000 mbar, laser pulse energy= 400 mJ.
in the plasma. This suggests that ionization and subsequent dissociation of the nitrogen molecule is competitive with the collisional de-excitation of argon species with nitrogen, leading to quenching of the argon emission (Penning ionization). The results suggest that the balance between these processes is influenced by the concentration of nitrogen in the samples and that the border between the regions in which a particular process dominates occurs at a nitrogen mole fraction of 0.8. 3.4.2. Helium in nitrogen In order to obtain calibration data for the helium/nitrogen mixtures the intense continuum background caused by the presence of nitrogen had to be minimized. In addition, quenching of the helium emission by collisional de-excitation had to be minimized in order to achieve high sensitivity. The calibration curves for detection of helium in nitrogen presented in Fig. 6 indicate that helium exhibited a similar behavior to argon. Further evidence to support this hypothesis was provided by the binary calibration mixtures of argon and helium. Fig. 6b shows that there are two distinct linear regions and that these are separated by an abrupt change of slope in the vicinity of 20% helium. The slope change in the argon/nitrogen calibration plot for high argon mole fractions also occurred at 20% argon (Fig. 5b); this suggests that nitrogen is probably the common cause of this effect. The plot in Fig. 6a indicates that, at low helium mole fractions (XHe b 0.012), the helium calibration plot exhibits an excellent linear dynamic range which extends over five orders of magnitude down to the parts-per-million level. The manner in which the helium SCRs and SNRs varied with sample pressure is shown in Fig. 7. Data for this plot were acquired
Fig. 6. (a) Calibration plot for detection of helium in nitrogen by LIBS at helium mole fractions up to 0.01. Measurement conditions: P = 80 mbar, td = 500 ns, laser energy = 850 mJ. (b) Calibration plots for detection of helium in nitrogen by LIBS at helium mole fractions up to 0.5. Measurement conditions: P = 1000 mbar, laser pulse energy = 400 mJ.
Fig. 7. Variation of the signal-to-continuum (SCR, ■) and signal-to-noise (SNR, ) ratios of the He(I) 587.56 nm emission line as a function of total sample pressure. Data were acquired using the 1% helium in nitrogen mixture. Measurement conditions: laser pulse energy = 850 mJ.
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using a mixture containing 1% helium in nitrogen and the Quantel Brilliant B laser (850 mJ). The SCR of the helium emission line increased as the pressure was reduced; this behavior was consistent with the lower electron density and particle cross-section under such conditions. Operating at lower pressures ensured that the collision probability was kept as low as feasible and the deactivation of excited helium species by collisional processes (Penning ionization) was minimized. The net effect of this measure was to maximize the number of excited helium atoms in the plasma. As an additional benefit, the SNR was also found to improve at low pressures. It is evident from these plots that the optimum pressure range for analysis is approximately 80 mbar. 3.4.3. Argon and helium binary mixtures Fig. 8 shows a calibration curve of the full concentration range of the argon/helium binary mixtures at 1000 mbar. Calibration was achieved by normalizing signals from each element to the sum of the signals from both elements. The concentrations of both elements could be calibrated using a simple function and exhibited linear dynamic behavior over the full concentration range. The linear calibration plot shown in Fig. 9 for mixtures containing low helium mole fractions indicates that quenching of the He(I) 587.56 nm emission does not occur in binary mixtures with argon. The linear behavior exhibited by the argon/helium binary mixtures is in marked contrast to the behavior exhibited by the nitrogen-based binary mixtures (Figs. 5b and 6b). Fig. 10 depicts the variation of the helium SCRs and SNRs as a function of pressure. Data were acquired using the 1% helium in argon mixture and 850 mJ laser pulse energy. Although the maximum SCR occurred at approximately 13 mbar the SNR was poor at this pressure, mainly due to the lack of reproducibility of plasma formation at pressures under 40 mbar. It is expected that a higher irradiance at the laser focus would provide the scope for performing a reliable analysis at 13 mbar sample pressure. 3.4.4. Ternary mixtures: argon and helium in nitrogen A range of ternary mixtures containing argon, helium and nitrogen were prepared by diluting a mixture containing 1% argon, 1% helium and 98% nitrogen with nitrogen in order to assess how the presence of both
Fig. 8. Calibration plots for detection of argon ( ) and helium (■) in the argon/helium binary mixtures by LIBS. Measurement conditions: P = 1000 mbar, laser pulse energy = 850 mJ.
Fig. 9. Calibration plot for detection of helium in the argon/helium binary mixtures by LIBS at helium mole fractions up to 0.008. Measurement conditions: P = 837 mbar, laser pulse energy = 850 mJ.
analytes (argon and helium) in nitrogen influenced their respective calibration curves. Given the differences between the optimum analysis conditions for both elements two different experiments were performed. In the first experiment, the sampling gate delay timing and sample pressure were slightly different to those used for the binary mixtures of helium in nitrogen, yet still sufficient to record an argon emission signal of good analytical quality. The chamber pressure was set to 100 mbar during measurements and the optimum sampling gate delay and integration times were 500 ns and 5 µs respectively. Under these conditions linear calibration plots were obtained for both analytes at concentrations up to 1% (Fig. 11). It should be noted that the concentration range investigated was limited by the gas mixtures available for the study. Analyte detection limits obtained using the ternary mixtures are presented in Table 1. Whilst the limit of detection for helium in the ternary mixtures (46 ppm) was similar to that found with the helium/nitrogen binary mixtures (57 ppm) and better than that recorded for the helium/argon mixtures (160 ppm), the LOD for argon in the ternary mixtures (270 ppm) was much higher than those obtained for the argon/nitrogen binary mixtures (17 ppm) and the argon/helium binary mixtures (4.9 ppm). In order to determine whether the reduction in sensitivity observed for the detection of argon in the ternary mixtures (relative
Fig. 10. Variation of the signal-to-continuum (SCR, ■) and signal-to-noise (SNR, ) ratios of the He(I) 587.56 nm emission line as a function of pressure. Data were acquired using the 1% helium in argon mixture and a laser pulse energy of 850 mJ.
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cross-sections for the lower pressure regime. Another consequence arising from the differences in pressure is that a different region of the plasma is sampled by the fiber optic coupler during each experiment. The optimum position of the fiber optic coupler for data acquisition at low pressure (Fig. 11) corresponded to an outer region of the plasma at 1000 mbar (Fig. 5). Consequently, the argon calibration plot obtained at 1000 mbar (inset of Fig. 11) was acquired by monitoring a cooler region of the plasma where the emission line curve of growth was not saturated for the range of concentrations under study (up to 1000 ppm). The results are therefore not only strongly influenced by the experimental conditions (laser irradiation and sample pressure) but also by the collection geometry used. The ternary mixtures all exhibited linear dynamic behavior over the concentration range investigated (0–1% analyte concentration in nitrogen). The actual LDRs could be larger than those reported as the full range of concentrations was not tested. Table 1 summarizes the analytical figures of merit for all elements concerned. Fig. 11. Calibration plots for detection of argon ( ) and helium (■) in nitrogen by LIBS at analyte mole fractions up to 0.01. Measurement conditions were optimized for detection of helium (P = 100 mbar, laser pulse energy = 850 mJ). Inset: calibration plot for detection of argon in the argon/helium/nitrogen ternary mixtures by LIBS at argon mole fractions up to 0.01 (1% concentration by volume). Measurement conditions: P = 1000 mbar, laser pulse energy= 850 mJ. The experimental conditions were optimized for argon. Helium could not be detected under these conditions.
Table 1 Analytical figures of merit for detection of helium and argon by LIBS. Analyte
Diluent
P (mbar)
R2
LOD (ppm)
CDR
LDR
Argon Helium Argon Helium Argonb Argond Heliumd
Nitrogen Nitrogen Helium Argon Nitrogen Nitrogen Nitrogen
1000 80 1000 37 1000 100 100
0.9979 0.9993 0.9938 0.9973 0.9996 0.9953 0.9997
17 57 4.9 160 76 270 46
0–50% 0–50% 0–100% 0–100% 0–1%c 0–1%c 0–1%c
0–17%a 0–20% 0–0.025% 0–20% 0–1%c 0–1%c 0–1%c
P = sample chamber pressure, R2 = correlation coefficient of the calibration plot. LOD is the limit of detection, CDR is the calibrated dynamic range and LDR is the linear dynamic range. a The calibration curve was in a log/log scale. b Experimental conditions optimized for argon. Helium could not be detected under these conditions. c The actual ranges could be larger than those reported as the full range of concentrations was not tested. d Experimental conditions optimized for helium.
to the nitrogen/argon and helium/argon mixtures) was due to the unfavorable acquisition conditions or merely to the presence of helium, a second calibration curve was obtained under similar experimental conditions as used for the calibration of argon/nitrogen mixtures (Fig. 5). The position of the fiber optic coupler was adjusted to optimize conditions for this study. The calibration plot obtained under these conditions is presented in the inset of Fig. 11 and exhibited linear behavior (R2 = 0.9996). The detection limit (76 ppm) was greater than that obtained for the argon/nitrogen mixtures (17 ppm, Fig. 5b), confirming that the presence of low concentrations of helium do not significantly affect the Ar(I) 763.51 nm emission line curve of growth. Although helium was detected at 100 mbar sample pressure (Fig. 11, main plot) it was not possible to detect it when the conditions were optimized for detection of argon (1000 mbar) (see inset to Fig. 11). It should be noted that the linear relationships between the argon emission intensity and mole fraction shown in the plots in Fig. 11 are in marked contrast to that presented in Fig. 5. The change in behavior can be explained by the different laser irradiation conditions and sample pressure used in each experiment, the latter leading to much lower
4. Conclusions The detection of trace concentrations of helium and argon in gas mixtures by the LIBS technique has been demonstrated for the first time. The analytes have been detected in binary and ternary mixtures with nitrogen using a low resolution spectrometer. Limits of detection in the parts-per-million and tens of parts-per-million ranges have been achieved in most cases, despite the quenching effects associated with the gases involved in the analysis. Although significant quenching of the helium emission due to collisional deactivation of electronically excited helium atoms was observed, it has been shown that the consequent losses in analytical sensitivity could be minimized by increasing the laser irradiance and decreasing the pressure at which the analysis was performed. Linear dynamic ranges of several orders of magnitude in analyte concentration were obtained for determination of both gases by LIBS. It is evident that the optimum conditions for determination of helium and argon in nitrogen by LIBS differ markedly and simultaneous analysis of both gases involves a degree of compromise. In order to obtain high analytical sensitivities for both gases by LIBS the analyses should be performed sequentially under the optimum conditions pertaining to each analyte (i.e. sample pressure and sampling gate delay time). This will ensure that the best possible SCRs and SNRs are obtained for each analyte. The results of this study suggest that LIBS may have potential applications in the detection of other noble gases at trace concentrations. Acknowledgement The work described in this paper was fully funded by AWE Corporate Technical Outreach. References [1] D.A. Cremers, L.J. Radziemski, Handbook of Laser-Induced Breakdown Spectroscopy, John Wiley and Sons, 2006. [2] A.W. Miziolek, V. Palleschi, I. Schechter, Laser-Induced Breakdown Spectroscopy (LIBS), Fundamentals and Applications, Cambridge University Press, 2006. [3] J.P. Singh, S.N. Thakur (Eds.), Laser-Induced Breakdown Spectroscopy, Elsevier, 2007. [4] E.D. Lancaster, K.L. McNesby, R.G. Daniel, A.W. Miziolek, Spectroscopic analysis of fire suppressants and refrigerants by laser-induced breakdown spectroscopy, Appl. Opt. 38 (1999) 1476–1480. [5] V. Sturm, R. Noll, Laser-induced breakdown spectroscopy of gas mixtures of air, CO2, N2, and C3H8 for simultaneous C, H, O, and N measurement, Appl. Opt. 42 (2003) 6221–6225. [6] K.H. Kurniawan, K. Kagawa, Hydrogen and deuterium analysis using laser-induced plasma spectroscopy, Appl. Spectrosc. Rev. 41 (2006) 99–130. [7] A.J. Ball, V. Hohreiter, D.W. Hahn, Hydrogen leak detection using laser-induced breakdown spectroscopy, Appl. Spectrosc. 59 (2005) 348–353. [8] K.H. Kurniawan, T.J. Lie, M.M. Suliyanti, R. Fedwig, S.N. Abdulmadjid, M. Pardede, N. Idris, T. Kobayashim, Y. Kusumoto, K. Kagawa, M.O. Tjia, Detection of deuterium
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