Microwave-induced Plasma Boosted Microsecond-pulse Glow Discharge Optical Emission Spectrometry YONGXUAN SU, PENGYUAN YANG*, DENGYUN CHEN, ZHIGANG ZHANG, ZHEN ZHOU, XIAORU WANG AND BENLI HUANG Department of Chemistry, L aboratory of Analytical Science, Xiamen University, Xiamen 361005, China A microsecond-pulse (ms-pulse) glow discharge (GD) source When in combination with a GD source, free sample atoms produced by cathode-sputtering in the GD enter the MIP by boosted by a microwave-induced plasma (MIP) has been developed and studied for optical emission spectrometry diusion and convection.However, most previous studies on MIP boosted GDs operated under the dc GD mode. (OES). The excitation processes of the tandem GD source were investigated. The analytical characteristics of the As a new member type of GD, ms-pulse GD has recently been paid more attention in analytical areas, including OES, GD-OES source in the presence and absence of the MIP were compared, including the operating parameters, signal-to- AAS, AFS and MS techniques.6–9 Experimental results have shown that the ms-pulse GD mode has an analytical perform- background ratios (S/B) and relative standard deviation (RSD).The results show that under a relatively low discharge ance that is much better than the dc GD mode.7,8 The pulsing technique is also a useful diagnostic tool for the excitation and pressure (<180 Pa), the ms-pulse GD can couple fairly well with the MIP and emit intense analytical lines.When the GD ionization processes in a GD plasma.6,10–12 In order to evaluate the possibility of using ms-pulse GD as an ion source for solid source is operated under a pressure higher than 200 Pa, two emission peaks appear, independent in time, for a given sample analysis as well as depth profile analysis on a solid surface, experiments on ms-pulse GD time of flight mass resonance atomic line, because sample atoms are independently structurally excited, first by the ms-pulse GD spectrometry (TOFMS), with some interesting results have been carried out.8,9 One of the most significant characteristics and then by the MIP.The time interval between the two peaks for Zn I 213.8 nm is longer than that for Cu I 324.7 nm, which of ms-pulse GD is that the sputtering rate of ms-pulse GD is more than two orders of magnitude higher than in dc GD is believed to be due to the faster diusing velocity of copper atoms. When the ms-pulse GD lamp is operated under a gas during the pulse-on regime, and that the average discharge power is relatively low.9 Therefore, the number of sample pressure higher than 220 Pa, the ion population is so high that Cu II ionic line at 224.7 nm ‘becomes’ two peaks because of a atoms produced in the very short pulse time is significantly high.possible self-absorption. The results show that the supplementary use of an MIP can eliminate the self-absorption In the present work, an MIP plume is superimposed on the sputtered sample atom–ion cloud in order to obtain atomic of ionic and atomic lines.When the ms-pulse GD source is coupled with the MIP, S/Bs are improved by a factor of more and ionic lines of high intensity. Based on the work of ms-pulse GD-TOFMS, an MIP boosted ms-pulse GD source has been than one order of magnitude for several analytical lines. A short-term RSD of 0.2% is achieved for the ‘ms-pulse designed in which the MIP is sustained in a tunable Beenakker TM010 resonator.The excitation processes of the OES source GD+MIP’ configuration compared with that of 1.0% for ‘mspulse GD only’ mode. The experimental results show that the were studied. The eects of the operating parameters on ms-pulse GD in the presence and absence of the MIP were MIP boosted ms-pulse GD is a promising technique for solid sample and surface analysis. investigated. A comparison of analytical characteristics for the ms-pulse GD alone and the ms-pulse GD-MIP tandem source Keywords: Microsecond-pulse glow discharge; microwaveis presented, in terms of the signal-to-background (S/B) ratios induced plasma; optical emission spectrometry; tandem source; and relative standard deviation (RSD).excitation process Glow discharge (GD) has been widely used in optical emission EXPERIMENTAL spectrometry (OES), atomic absorption spectrometry (AAS) MIP Boosted ms-Pulse GD-OES Source and atomic fluorescence spectrometry (AFS) for analytical applications, and also has served extensively as an ion source A schematic diagram of the MIP boosted GD source is shown in Fig. 1. The structure of the GD source is similar to the one for mass spectrometry (MS).1 Various techniques have been used to obtain high intensity without losing the advantage of used as an ion source for TOFMS and has been described in detail previously.9 Both the GD source and the resonator are narrow spectral lines. Leis and Steers have reviewed articles on boosted GD sources.2 The use of an MIP to enhance the made of brass.The discharge gap in GD lamp is designed to be relatively thin, such that the distance between the microwave performance of a GD has attracted increased interest since the 1980s.3–5 The advantage of a microwave-boosted GD lamp is cavity and the sample cathode is only 7 mm. Water cooling is used directly to cool the sample cathode. The anode body and that the atomization of a solid sample is performed by the process of cathode-sputtering in the GD while highly ecient the microwave cavity are earthed.A quartz tube with an id of 8 mm, separates the inner low pressure region from the outer excitation of analytical lines can be carried out in an MIP at low pressure. atmospheric pressure. Two tuning screws are used to adjust the minimum reflected power. A GD source with microwave boosting has been presented by Leis et al.3 They combined a conventional Grimm-type GD The lamp is evacuated by a two-stage mechanical pump (4 dm3 s-1, Shanghai Vacuum Plant, Shanghai, China) and source with an MIP sustained in a tunable Beenakker TM010 resonator.It is known that an MIP shows good excitation the working carrier gas is pumped continuously through the lamp. The working gas pressure is regulated by an inlet needle properties but is not particularly suitable for atomization. Journal of Analytical Atomic Spectrometry, August 1997, Vol. 12 (817–822) 817Comparison of the analytical characteristics of the GD source in the presence and absence of theMIP An ICP-AES instrument (sequential ICP-AES, Model 2070, Baird, Bedford, MA, USA) was utilized, with a built-in 1 m Czerny–Turner monochromator with a grating of 3600 grooves mm-1 and a blaze wavelength of 400 nm. The ICP torch was removed and replaced by the GD–MIP tandem source. The slit-widths of both the entrance and the exit are 17 mm. Signals are integrated for 0.2 s, and then digitized with a OS-2 microcomputer.The signal processing part of this ICPAES instrument is designed for continuous emission rather than signal pulses, such as a microsecond wide signal obtained in the present study. Thus, the signal pulse has to be averaged against a background signal of about 500 ms for a pulse rate of 2 kHz that is used in this study. Therefore, the S/B obtained in this experiment can only be used as reference values for a Fig. 1 Structure of the MIP boosted ms-pulse GD source.The solid comparison of some of the results obtained for a ms-pulse GD sample is sputtered and atomized in the GD, and the excitation occurs in the MIP tandem ms-pulse GD discharge. in the absence and presence of the MIP. Experimental Considerations valve and monitored with a thermocouple gauge (ZDO-54, The lamp can be operated in two optical configurations: ‘ms- Chengdu Instrument Factory, Chengdu, China). pulse GD only’ and ‘ms-pulse GD+MIP’. The current was The ms-pulse GD power supply is laboratory built with regulated for ms-pulse GD and argon was used as the carrier the following adjustable parameters: discharge frequency gas.A dc voltage (-750 V, with a current of several tens of (10–5000 Hz); discharge duration (0.2–5 ms); and discharge mA) was utilized to pre-burn the sample cathode. After several current (0.03 mA–3.1 A). The voltage output of the ms-pulse days operation, the reflected microwave power could increase GD power supply is kept at -750 V.owing to the redeposition of sputtered material on the inner The microwave generator produces a maximum power of surface of the quartz tube. In the case of brass samples, the 200W at a frequency of 2450 MHz (Hai Guang Instruments, quartz tube should be cleaned frequently because of the high Beijing, China). It is connected to the resonator via a 2m sputtering rate and the thin metallic film hampers the ecient length of coaxial cable with 50 V impedance.Two meters coupling of the microwave energy. indicate the forward and reflected microwave powers. RESULTS AND DISCUSSION Study of the Excitation Processes of the GD-OES Source Experimental Set-up Superimposition of anMIP on a GD Investigation of the excitation processes in the GD source When sucient microwave power (50W forward power in the A block diagram of the experimental set-up used for investigapresent experiment) is superimposed on a running GD plasma, ting of the excitation processes is shown in Fig. 2. The radiation a microwave discharge in the quartz tube can be ignited, and source is imaged by a quartz lens onto the entrance slit of a forms a plasma extention to the positive column of the GD. 0.5 m Czerny–Turner monochromator (Acton Research, Acton, No boundary between the microwave discharge and the MA, USA), with 1800 grooves mm-1 and a blaze wavelength ms-pulse GD can be observed visually. A minimum reflected of 500 nm.Both the entrance and exit slit-widths are 20 mm. power of about 5 W can be obtained by adjusting the two Signals from the preamplifier were observed with a 40 MHz tuning screws. Because charged particles are also produced by oscilloscope (Model BS-5504, Aron, Korea), they can also be the MIP, the transient pulse voltage drop between the anode integrated with a boxcar integrator (laboratory made) and and cathode decreases significantly, and the radiation of the then recorded with a recorder (Model 9176, Varian, Palo Alto, lamp is obviously enhanced when the microwave generator is CA, USA) or a compatible PC-386 computer with a 20 MPS switched on.analogue to digital converter. Eect of Discharge Parameters on Excitation Processes Dependence of discharge pressure on the coupling of an MIP with ms-pulse GD The argon pressure can clearly aect the coupling of an MIP with ms-pulse GD. As shown in Fig. 3, the sample atoms emit a strong radiation peak when the MIP boosted ms-pulse GD lamp is operated at a discharge pressure lower than 180 Pa.However, if this GD lamp is operated at a pressure higher than 200 Pa, two emission peaks appear independently in time for a given resonance atomic line (see Fig. 3). Depending on the increase in argon pressure, the MIP plume tends to shrink away from the GD region. The sample atoms are excited first by ms-pulse GD and then by the MIP when atoms move from Fig. 2 Schematic diagram of the MIP boosted ms-pulse GD-OES the GD region to the MIP region.As shown in Fig. 3(a), the system. A commercial ICP-AES is utilized to replace the monochroma- first peaks (peak I) correspond to the emission excited by tor and the signal processing system, when the analytical characteristics of this tandem GD source are evaluated. See text for details. ms-pulse GD and the second peaks (peak II) by the MIP. The 818 Journal of Analytical Atomic Spectrometry, August 1997, Vol. 12Fig. 4 Eect of discharge pressure on the time interval between peak I and peak II for dierent resonance atomic lines: &, Zn I 213.8 nm; and %, Cu I 324.7 nm. surface of the sample cathode under the operating conditions used. Eect of discharge frequency and pulse duration on emission intensities The eect of discharge frequency on the intensities of peaks I and II for Zn I 213.8 nm and Cu I 324.7 nm are illustrated in Fig. 5. It can be seen that the peak height for peak I reduces Fig. 3 Eect of discharge pressure on the time-resolved resonance with an increase in discharge frequency. If the discharge atomic lines for a brass sample: (a) Cu I 324.7 nm; (b) Zn I 213.8 nm. frequency is operated between 800 and 1200 Hz, peak II can reach a maximum value. Above this frequency range, the sputtering rate increases in accordance with discharge pressure, higher the frequency, the lower the peak height will be. It and the high pressure may also tend to slow the loss of could be supposed that a very high discharge frequency would sputtered atoms owing to diusion, and as a result peak I eliminate the sputtered and excited processes because the increases.However, because the gap between the sample and laboratory made power supply is unable to recover for the the MIP plume lengthens, the density of the atom cloud next discharge pulse owing to its limited power capacity.9 This decreases when the atom cloud passes through the MIP excited finite capacity of the discharge power could also limit operation region.For ionic lines with high energy transition radiation, of the GD pulse to a relatively wide pulse duration. Walden only the first peak appears with the superimposition of the et al. have found such results in their experiments on ms-pulse MIP. In general, the radiation emitted by the microwave GD-OES.7 The present work also indicated that intensities of boosted lamp can be characterized by those spectral lines with both peak I and peak II increased according to the duration low and mean excitation energies.of the discharge pulse, although not obviously (the maximum The time duration for peak I at its maximum value is 15 ms pulse duration can be adjusted only to within 5 ms in the pulse after the discharge pulse starts and is almost the same for both generator). Zn I 213.8 nm and Cu I 324.7 nm. This time value hardly varies with the change in discharge pressure because the shift of the most excited region in ms-pulse GD is insignificant.Eect of discharge current on emission intensities However, the duration of peak II at its peak value changes The relationship between the transient current and the radi- according to the discharge pressure, because the MIP plume ation intensities of peak I and peak II is given in Fig. 6. Visual shrinks further away from the GD excited area with an increase observation through the quartz window showed that the glow in argon pressure.The relationship between time interval of the two emission peaks and discharge pressure is illustrated in Fig. 4. It can be seen that the higher the discharge pressure, the longer the time interval, and that the time interval between the two peaks for Zn I 213.856 nm is longer than that of Cu I 324.7 nm under identical operating parameters, which is believed to be due to the faster diusing velocity of the copper atoms. When the discharge pressure is 300 Pa, the times for peak II at the peak values are about 120 and 140 ms for Cu I 324.7 nm and Zn I 213.8 nm, respectively.The distance between the sample and the central part of the quartz tube is about 18 mm (the thickness of the quartz restrictor ring is 3 mm). Therefore, the average diusing velocities of copper and zinc atoms can be estimated to be 150 and 129 m s-1, respectively, and the most reactive region for excitation in this ms-pulse GD is 1.94–2.25 mm (estimated from the diusing velocity multiplied by the time interval between the falling edge of the discharge Fig. 5 Dependence of emission intensities of peak I and peak II on discharge frequency: &, Zn I 213.8 nm; and %, Cu I 324.7 nm. pulse and peak I at its maximum value) from the sputtering Journal of Analytical Atomic Spectrometry, August 1997, Vol. 12 819tation mechanism. Such an experiment is currently in progress in this laboratory. Eects of MIP on Sputtering Rate and Sputtered Surface The addition of an MIP has an obvious eect on sputtering rate as well as on the topography of the sputtered surface.The sputtering rate of ms-pulse GD has been studied previously9. The transient sputtering rate (9.5 mg s-1 mm-2) for ‘ms-pulse GD+MIP’ is only about half of that of ‘ms-pulse GD only’ (21 mg s-1 mm-2) under the same operating conditions for a brass sample, with a discharge frequency of 1.8 kHz, pulse duration of 3 ms, current of 2 A and pressure of 180 Pa.The crater on the surface of the erosion area also becomes finer, when observed with a scanning electron microscope (Hitachi Fig. 6 Eect of discharge current on the intensities of peak I and S-300, Tokyo, Japan). The decrease in sputtering rate and the peak II: &, Zn I 213.8 nm; and %, Cu I 324.7 nm. improvement in crater shape are believed to be due to the lower voltage drop between the anode and cathode.14 For turns white and then green with an increase in discharge more details of the results of the sputtering rate and on the current, indicating very intensive sputtering and excitation sputtered surface for ms-pulse GD, see ref. 9. processes. Peak I increases sharply correspondingly with discharge current and reaches its peak when the transient current Comparison of the Analytical Characteristics of the OES is about 0.8 A, then the emission intensity tends to decrease. Source in the Presence and Absence of the MIP In this experiment, it was observed that when the discharge current increases, the line profile of peak I widens with hardly Dependence of emission intensities on discharge parameters any increase in peak intensity.It could be that a self-absorption As described above, an MIP can couple well with ms-pulse occurs, which is a result of the high density of ‘cool’ atoms GD if the discharge pressure is maintained at less than 200 surrounding the ‘hot’ atoms in the center of the plasma.Pa. In order to evaluate the analytical characteristics of the However, peak II increases steadily and tends to saturation boosted GD source, the argon pressure in the discharge only when the current is higher than 2 A. The high current chamber was kept relatively low throughout the following can result in an increase in the sputtering rate, and more experiments. sample atoms can be excited by the MIP. The self-absorption in the MIP excited region is not severe because of a relatively Eect of discharge frequency on spectral intensity.As shown in low density of ‘cool’ atoms. Fig. 7, emission intensities of the integrated signals increase steadily in accordance with the increase in discharge frequency without an MIP, and increase sharply (especially for Cu I Eect of microwave power on emission intensities The forward microwave power was optimized at 50Wthroughout the experiment. The microwave power has a minor eect on emission peaks when the power is varied between 50 and 80 W.When the forward power is high, the reflected power will increase accordingly, and is rather dicult to tune unless the discharge gas pressure is lowered for improved coupling. However, a low discharge pressure can eliminate sputtering. The experimental results are also supported by previous reports from Li et al.5 They found that the MIP and dc GD can be optimized to an ideal coupling by adjusting the relationship between discharge voltage, microwave power and discharge pressure under dierent discharge conditions.In their experiments, the maximum S/B levels could also be achieved with a microwave power of 50 W, if the GD lamp is operated under a low discharge pressure.5 Eect of MIP on the Excitation of Ionic Lines The excitation and ionization processes are very intensive when the ms-pulse GD lamp works under a high transient current and relatively high pressure (>220 Pa). Owing to its high ion populations, the ionic line of Cu II 224.7 nm appears to become two peaks because of a possible self-absorption.Supplementing with the MIP aects the excitation of sample ions and results in the alleviation of this self-absorption of Cu II 224.7 nm. Steers and Leis have reviewed many examples of charge exchange (CE) excitation between sample atoms and argon ions in a GD source,13 which has been believed to be a very important process for the excitation of ionic lines in a Fig. 7 Relationship between discharge frequency and line intensities GD with inert gases.The change in the ion density of argon of sputtered atoms, (a) in the absence of the MIP and (b) in the can be observed by a mass analyser coupled with an MIP presence of the MIP: ×+, Cu I 324.7 nm, see right y-scale; &, Cu II 224.7 nm; +, Zn I 213.8 nm; and %, Zn II 202.5 nm. boosted ms-pulse GD for a better understanding of the exci- 820 Journal of Analytical Atomic Spectrometry, August 1997, Vol. 12324.7 nm) when the microwave generator is switched on. Owing to the relatively low discharge pressure, self-absorption of emitted radiation by a cloud of ‘cool’ atoms in the outer region of the discharge is fairly serious when the discharge frequency is increased to as high as 2 kHz. The emission intensities decrease when the discharge frequency is adjusted to higher than 2 kHz. Two possible reasons account for the results: one is the possible self-absorption of radiation owing to an increase in the surrounding cold atoms, and the other results from the limited power capacity of the laboratory made ms-pulse power supply, as discussed previously.9 Eect of discharge pressure on spectral intensity.The eect of discharge gas pressure on the line intensity is shown in Fig. 8. The working carrier gas pressure is an important factor for it has a clear eect on the GD. As shown in Fig. 8(a), with the ‘ms-pulse GD only’ mode, the intensity of Cu I 324.7 nm rises rapidly in accordance with the gas pressure.The relationship between the argon gas pressure and the line intensities for Cu II 224.7 nm and Zn II 202.5 nm are similar in the two discharge configurations. However, self-absorption of spectral line Cu I 324.7 nm occurs in the ms-pulse GD+MIP mode when the argon pressure is higher than 180 Pa. As a result, the emission intensity decreases. It is interesting to note that the intensity of the Zn I 213.8 nm line is about four orders of magnitude lower than the intensity of the Cu I 324.7 nm line when the ‘ms-pulse GD only’ mode is utilized, while it is eight times lower when the ‘ms-pulse GD+MIP’ configuration is used, because the line transition for Zn I 213.8 nm originates from Fig. 9 Relationship between discharge current and signal intensities, a higher energy state. Similar results can be seen in Figs. 7 (a) in the absence of the MIP (b) in the presence of the MIP: +×, Cu I 324.7 nm, see right y-scale; &, Cu II 224.7 nm; +, Zn I 213.8 nm and and 9.%, Zn II 202.5 nm. Eect of discharge current on spectral intensity. The dependence of emission intensities on discharge current is depicted in sputtering rate. Coupling with the MIP results in a decrease Fig. 9. As shown in Fig. 9(a), a possible self-absorption of in the sputtering rate and particularly enhances the excitation selected spectral lines occurs when the discharge current is processes of the low energy lines.As a result, the self-absorption higher than 1.5 A, which is believed to be due to the high of Cu I 324.7 nm and Zn I 213.8 nm decrease because of the smaller number of ‘cool’ atoms and the intensities of Cu II 224.7 nm and Zn II 202.5 nm become lower with the increase in discharge current. Signal-to-background Ratios and the Relative Standard Deviation Signal-to-background ratios As shown in Table 1, the S/B are enhanced for the selected lines when the microwave generator is coupled to the ms-pulse GD.As the generator is turned on, the emission intensities increase sharply but the background level rises only slightly. Therefore, detection power can be improved by using a ms-pulse GD–MIP source. Leis et al. have also shown that detection powers can be significantly increased by supplementing a dc GD with an MIP discharge.3,14 Shown in Fig. 10 is the spectral profile for Cu I 324.7 nm (brass sample, 69.4% Cu), obtained with both the ‘ms-pulse GD only mode’ and ‘ms-pulse GD+MIP’ mode.It was found that the profile is still very narrow in the presence of the MIP and the S/B is fairly good with a relatively strong line intensity. Relative standard deviation The short-term stability of the lamp operated under ‘ms-pulse GD only’ and ‘ms-pulse GD+MIP’ modes was studied by measuring the intensity of the Mn I 257.610 nm line for a low alloy steel sample containing 0.122% Mn. About 2 min after ignition, the intensity of Mn I 257.610 nm becomes constant Fig. 8 Eect of discharge pressure on signal intensities, (a) in the and then the RSD measurements were carried out. The RSDs absence of the MIP and (b) in the presence of the MIP: +×, Cu I of the ‘ms-pulse GD only’ and ‘ms-pulse GD+MIP’ configur- 324.7 nm, see right y-scale; &, Cu II 224.7 nm; +, Zn I 213.8 nm; and %, Zn II 202.5 nm. ation were 1.0 and 0.2%, respectively, obtained from line Journal of Analytical Atomic Spectrometry, August 1997, Vol. 12 821Table 1 Comparison of the S/Bs of the GD source in the absence and presence of the MIP Spectral line/nm Mode Parameter Cu I 324.7 Cu II 224.7 Zn I 213.8 Zn II 202.5 ms-pulse GD only Signal (count) 113071 20987 27908 4479 Background (count) 1910 191 185 176 S/B ratio 59 110 151 25 ms-pulse GD+MIP Signal (count) 8096826 96030 1430311 96821 Background (count) 4212 331 752 234 S/B ratio 1922 290 1902 414 operation of the lamp under ‘ms-pulse GD only’ mode, the S/B ratios of ‘ms-pulse GD+MIP’ mode have been enhanced several to several tens of times for selected lines.The RSD of the level of emission intensity for the GD lamp has also been improved from 1.0 to 0.2% in the presence of MIP boosting. Further work will focus on the determination of detection limits for the boosted source and on investigating the excitation processes using TOFMS. The application of the technique to surface analysis of solid samples is also an attractive area. The Laboratory of Analytical Science is run by the State Education Commission of China (SEDC). This work was supported by the National Nature Science Foundation of China under grant number CHEM-29235110-II, and partially by the Outstanding Youth Fellowship of SEDC.The authors thank Baird for their generous donation of the ICP-AES Fig. 10 Spectral profile of Cu I 324.7 nm: discharge frequency 1.7 kHz; instruments to the laboratory and also thank Liang Feng for pulse width of 2 ms; current 1.5 A; argon pressure 180 Pa; and microwave forward power 50 W.Signals were integrated for 0.2 s, helpful discussion in designing the microwave resonator. recorded with a 3600 groves mm-1 high-resolution monochromator. 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The eects of the discharge parameters on the emission 12 Pan, C., and King, F. L., Anal. Chem., 1993, 65, 3187. intensities of the analytical lines of interest have been studied 13 Steers, E. B. M., and Leis, F., Spectrochim. Acta, Part B, 1991, and optimized as 1.8 kHz for discharge frequency, 200 Pa for 46, 527. argon pressure and 1.5 A for discharge current. Spectral self- 14 Leis, F., and Broekaert, J. A. C., Spectrochim. Acta, Part B, 1991, 46, 243. absorption appears to exist in the GD and can be reduced when the microwave generator is switched on, especially for Paper 7/00913E low energy emission lines. Received February 10, 1997 The net intensities of the spectral radiation increase sharply Accepted April 11, 1997 when the lamp is coupled with the MIP. Compared with 822 Journal of Analytical Atomic Spectrometry, August 1997, Vol.