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Laser vaporization of solid samples into a hollow-cathode discharge for atomic emission spectrometry
0584-8547/90s300+ 00 PergamonPress plc
Spectrochumco AcfoVol 45B.No 4/S. pp 427 438,1990 PnnledI"GreatBntam
Laser vaporization of solid samples into a hollow-cathode atomic emission spectrometry YASUO Government
Industrial
Research
(Received
Institute,
Nagoya
discharge for
IIDA 1, Hirate-cho,
Kita-ku,
Nagoya
462. Japan
19 May 1989; in revised form 6 September 1989)
Abstract-A hollow cathode discharge system has been developed for atomic emission spcctrometry of solid samples. In this system, a solid sample is vaporized by a Q-switched Nd:YAG laser, and the vaporized material is introduced into a hollow cathode discharge by means of an argon gas flow. A photodlode array detector is used to acquire the spectral data and the spatial intensity profiles of atomic lines in the cathode bore. Experimental
parameters which have been optimized include the pressure and flow rate of argon gas, the forms of the electrodes, and the discharge current. The laser is operated in either single pulse mode or multiple pulse mode, 20 pulses at 10 Hz repetition rate. The latter mode offers improved precision and detection limits, and quasi-continuous emission signals. Analytical curves obtained for standard samples of aluminum alloys are linear over 3 orders of magnitude with a relative standard deviation of 1.64.6%. and the estimated detection limits for the various elements range from 3 pgg- ’ for Mg to 16 p’gg- ’ for Ni.
1. INTRODUCTION LASER vaporization offers the possibility to handle conducting and non-conducting solid samples with excellent microsampling characteristics [l, 21. Nevertheless, the direct spectrochemical analysis of laser induced plasmas is limited in its use by the lack of precision and the poor linearity of the analytical curves [3, 41. Several efforts have been made to ameliorate the situation by the adaptation of the spatial and time resolution [3, 51, by the variation of the surrounding atmosphere [6-83 and by obtaining a fundamental understanding of the laser-surface interaction [9, lo]. On the other hand, several kinds of additional excitation sources, such as spark cross-excitation, ICP and MIP, have been combined with laser vaporization to improve the analytical performance, as reviewed by DITTRICH and WENNRICH [ll]. From the point of view of the sources, we might regard these works as developments of techniques for the direct introduction and microprobe sampling of solid samples. Glow discharge (GD) plasmas are known to be not in local thermal equilibrium (LTE) [12]. Owing to the non-LTE character, GDs have several excellent features as radiation sources for atomic spectrometry, i.e. high line-to-background ratio, simple spectra, narrow line width, and low matrix effects [13-161. Another attractive feature is their fairly good precision [14]. In spite of those excellent properties, some limitations and drawbacks also exist in the GD source: lack of microprobing facility, time-consuming machining of sample into a cathode of fixed form with the risk of contamination, and special sample preparation for non-conducting materials. Most of these disadvantages can be obviated by sample introduction with the laser vaporization technique. The combination of laser vaporization with excitation by a GD, having either flat or hollow cathodes, has not yet been reported. In this paper, the design of a laser vaporization system coupled with a hollow cathode discharge (HCD) source is described, and the analytical performance of the system is reported.
2. EXPERIMENTAL 2.1. Apparatus
The experimental apparatus and the operating conditions are summarized in Table 1. The laser vaporization chamber was constructed from a Pyrex glass cylinder, 90 mm high and 32 mm in diameter. The top ended in a ground flange, which was closed by a Pyrex glass window of 3 mm thickness. The bottom edge was cemented to a brass block. The chamber was equipped with a 427
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Table 1. Apparatus and operating conditions Q-switched Nd: YAG laser YG-580A (Quantel International); energy: 250 mJ/pulse; pulse width 10 ns; repetition rate: 10 Hz. Spectrometer THR-1OOOMSL(Jobin-Yvon); Czerny-Turner; focal length: 1000 mm; grating: 2400 lines/mm, holographtc; sht width, 75 pm; sht herght: 5 mm. Photodiode
array
detector
system
OMA III system (Princeton Applied Research); detector: 1421B-1024HQ, 1024 elements (element stze. 25 pm x 2.5 mm) with microchannel plate image intensifier; controller: 1460 (68000-based); detector module: 1463 (14 bit A/D); triggered by a Q-swatch stgnal from the laser; spectral wrndow 10 nm. Photomultiplier R-955 (Hamamatsu Photonics). Digital sampling oscilloscope DS-6121 (Iwatsu Electric); 2 channel (2048 word/channel); sampling rate: 20 MHz. DC power supply for the glow discharge PAD 1 K-O.2 L (Kikusui Electronics); f&1500 V, O-200 mA; the supply was connected with the discharge tamp through a 2 ka ballast resister. Vacuum gauge PT-9 P (Daia Vacuum Engineering); Puani type; calibrated for Ar gas with mercury manometer.
lateral flange of 20 mm i.d. for sample changing, which was usually closed by a glass plate. A brass sample stage, 24 mm in diameter, and adjustable in height from 35 mm to 50 mm by a screw, was positioned in the chamber. The laser beam was directed downward with a 45” prism and focused with a single lens, focal length 80 mm, on the sampIe surface through the window. The diameter of the focused spot was about 1.5 mm. Argon entered through a 4 mm id. giass tube and carried the sample vapor to the discharge Iamp through a tapered (20 mm to 8 mm i.d.) glass tube. The glass tubes were coaxial and the axis was about 5 mm above the sample surface. The argon flowed continuously under the action of a rotary pump. The pressure and the flow rate were controtled with a needle valve in the gas inlet line and a diaphragm valve in the evacuation line. The Row-through discharge lamp designed for this work is shown in Fig. 1. The evaporation cell and the lamp were connected by silicon rubber tubing (id. 10 mm, o.d. 21 mm, length 80 mm). The tubing was curved so as to prevent radiation emitted from the laser induced plasma from entering the optics. The distance between the sample vaporization point and the cathode was about 15 cm. The cathodes were prepared in three different bore sizes (Fig. 2a-2c), and were made of aluminum of 99% purity. The anode was a tungsten rod of 2 mm diameter and was soldered with silver to the UItra Torr fitting (CAJON). A similar fitting and rod were used for the electricat contact to the cathode. A He-Ne laser was aligned coaxially with the Nd:YAG laser beam to facilitate positioning of the sample. Pure argon (more than 99.9995% purity) was used as the carrier and plasma-forming gas without further purification.
2.2. Samples The following standard Al alloy samples were used: ALCOA KA-213-B, KB-356-B, KC-356-E, KA356-E, %-138-R, SS-242-C, SS-A332-D and SS-F332-D. The specified compositions are given in anode
cathode
quartz
--w
vapor1 red
window
sama 1e
4
c0011ng
water
J(
3to rotary
cwmp
Fig 1. Schematic diagram of the flow-through discharge lamp with a hallow cathode. The three parts (l-3) of the lamp and the window are fixed together with Aptezon wax W.
429
Laser vaporization of solid samples
a
b
c
(mn)
Fig. 2. Aluminum hollow cathodes. Cathodes of three different bore sizes were used. Table 2. Standard alummum alloy samples Alcoa number
MS
cu
KA-213-B KB-356-B KC-356-E KA-356-E SS-138-R SS-242-C
0.041 0.38 0.27 0.15 0.27 1.53
7.08 0.055 0.040 0.20 10.06 3.97
SS-A332-D SS-F332-D
1.16 0.95
1.01 3.22
Nt W)
Fe
Mn
0.21 2.01
0.96 0.20 0.075 0.48 0.97 0.54
0.30 0.13 0.026 0.034 0.22 0.085
2.61 0.50
0.66 0.70
0.072 0.26
0.20 0.012
*Otherelements contained (%). Si(O.49-12.03). ~r(O.~l~.O31), Ti(O.O51~.17), PqO.O6~.10), Sn(O.O~.57~
Zn(O.O3~.99),
Table 2. The flat and smooth surfaces of the samples were obtained by polishing with a No. 800 silicon
carbide paper. The polished surfaces were washed with distilled water and acetone, and dried with hot air. 2.3. Procedure A sample was positioned on the flat stage in the evaporation cell, and the position of the laser shot was adjusted by means of the coaxial He-Ne laser beam. After evacuation, the argon gas was introduced, then the discharge was struck and allowed to be steady for several minutes. The laser was fired in a single pulse mode or a multiple pulse mode (20 pulses at 10 Hz). Since the first few shots on a fresh surface gave relatively high intensities, the corresponding data were discarded and only the subsequent data were evaluated. Spectral data were measured with a photodi~e array detector system, except for the emission time profiling, where a photomultip~ier with a digital sampling oscilioscope was used. The cathode bore was imaged on the slit of the spectrometer with unity magnification and the center of the cathode was conjugate to the center of the slit. Thus the observation region was 2.5 mm, being limited by the height of the photodiode detector. The photodiode array system offered some flexible data acquisition facilities, i.e. triggering, accumulation, and background subtraction, in a desired sequence. The trigger was fed from the Q-switch signal of the laser. The emission was accumulated for 1 sin the single pulse mode and 2 s in the multiple pulse mode. The background emission was also accumulated for the same period just before the laser shots, and used for background subtraction. Values of the emission intensity were obtained by integration of peak area with automatic baseline correction. The spectral window corresponded to l&30 photodiode elements (0.1-0.3 nm) and varied with the degree of spectral interference. The spatial distribution of the emission in the hollow cathode bore was also measured with the photodiode array detector. In this case, the diode array was directed vertically, rotated over 90” from the usual position, and a slit of 200 pm width was installed on the detector window. A complete vertical image of the cathode bore at the desired wavelength was thus obtained. 3. RESULTS 3.1.Discharge characteristics The discharge was maintained
showed the characteristic
in the constant current mode. The voltage-current curve of the abnormal GD, i.e. the voltage increased as the current
Y~suo
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;:k-_ 2
Time,
200 ms/DlV
Time,
500 ms/OlV
205
Fig. 3. Change
of the electrode voltage as a result of sample vapor introduction vaporization. (a) Single pulse mode; (b) multiple pulse mode.
by laser
increased, and the voltage decreased as the pressure increased. For cathodes of different bore sizes, the voltage was higher for the cathode of smaller bore size. Under the conditions of this experiment, the voltage was in a range between 180 and 240 V. By the introduction of sample vapor with the laser pulse, the electrode voltage changed as shown in Fig. 3. For the single pulse mode (Fig. 3a), the voltage drops immediately after the laser pulse. This drop seems to stem from a pressure change by the expansion of the lasergenerated plasma or from the electron flow from the sample [17]. The next large decrease of the electrode voltage occurs after about 200 ms from the start of the laser pulse and corresponds to the impedance change by the sample introduction. Figure 3b shows the voltage change in the multiple pulse mode. The voltage decreases by the continuous introduction of sample vapor and reaches a steady value of about 10 V below its initial value. By the end of the sample introduction, the voltage returns gradually to its initial value. These changes in the discharge conditions produce some perturbations in the background emission as well as the sample emission, so that accurate compensation of the background emission should be done by measuring the dynamic background emission with sample introduction. However, as a matter of convenience, we used the static background emission just before the sample introduction for the background subtraction. The differences of those background emissions would appear as a rough baseline in spectral data or a deviation of the y-intercept from origin in the analytical calibration curve. 3.2. EfSect of experimental parameters The effects of several experimental parameters were investigated, and the parameters were optimized for analytical performance. Figure 4 shows the effect of discharge current on the emission intensity of Mn 1403.08 nm for an Al alloy sample introduced into the discharge with laser vaporization. Cathodes of bore sizes 4,6 and 8 mm (Fig. 2a-2c) were used. An increase in the current usually leads to an increase in intensity. The discharge current had to be limited to 160 mA in view of the heat generation at the cathode and the poor thermal conductivity of the surrounding glass wall. The size of the cathode bore is one of the important factors related to the emission intensity. The relative surface area for the cathodes of bore sizes of 4,6 and 8 mm were 1.O, 1.2 and 1.4, respectively. The calculated current density was 15 mA/cm’ for the 4-mm bore cathode at 120 mA. Comparing the same current density points in Fig. 4, e.g. 100, 120 and 140 mA for the 4,6 and 8 mm bores, respectively, the emission intensity is still higher for the cathode of smaller bore size. Thus, the difference of emission intensity could not be explained only by the difference in the current density. It is well known that a HCD has complex radial
431
Laser vaporization of solid samples
8-
6-
4.
2.
0 0
SO
101)
150
200
Discharge current, mA
Fig. 4. Dependence of the emission intensity of Mn I 403.08 nm on the discharge current. Three aluminum cathodes of different bore size (4 mm ( l), 6 mm (A), 8 mm ( n)) were investigated. An aluminum alloy sample (SSF332D) was vaporized by a single laser shot and introduced into the discharge by an argon flow of 80ml/min. Pressure: 800 Pa. The error bars show the standard deviation of five measurements.
in the cathode bore [Is]. So, the decrease of the bore size may increase the opportunity of the sample vapor to pass through the effective excitation region of the discharge. Also, the bore volumes change with the bore size in the ratio 1.0: 1.5: 2.3 for the 4, 6 and 8 mm bores, respectively. The volume dictates the concentration of the sample vapor in the excitation region. These effects, as well as the current density, appear to be important for increasing the emission intensity. Figure 5 shows the effect of flow rate, which was regulated at atmospheric pressure, on the emission intensity. In view of the residence time of the sample vapor in the discharge region, one should make the flow rate as small as possible. However, in a system where the sample vapor must be carried by the gas flow, which is the case in this study, a higher flow rate is preferable to diminish capture of sample vapor at the wall, and to transport the larger particles from the vaporization cell to the discharge. The optimal flow rate is determined by the balance between these factors. Figure 6 shows the emission time profile of Mn I at 403.08 nm for the Al alloy sample, at 800 Pa, 120 mA, and various flow rates for the 4-mm structure
10 l-
8
0 Flow rate, ml/min
Fig. 5. Dependence of the emission intensity of Mn I 403.08 nm on the argon flow rate. Three aluminum cathodes of different bore size (4 mm (O), 6 mm (A), 8 mm ( n)) were investigated. An aluminum alloy sample (SSF332D) was vaporized by a single laser shot and introduced into the discharge by an argon flow. Discharge current: 120 mA. Pressure: 800 Pa. The error bars shows the standard deviation of five measurements.
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Laser pulse Time, 100 ms/DIV
Ftg. 6. Emission-time profile of Mn I 403.08 nm for the aluminum alloy sample fSSF332D). Flow rate: (a) 80 ml/min; (b) 40 ml/min; (c) 20 mljmin. Discharge current: 120 mA. Pressure: 800 Pa.
bore cathode. The delay of the emission corresponds to the linear velocity of the gas flow. The duration of the emission, which exceeds the pulse interval of 100 ms, indicates the possibility of using the discharge lamp as a quasi-continuous emission source. Figure 7 shows the effects of the pressure on the emission intensity of the various elements introduced into the discharge. All the five investigated elements showed the same dependence upon the argon pressure, and the emission intensity peaks are obtained at about 800 Pa. Figure 8 shows the effect of pressure on the emission intensity of the concomitant species in the discharge without sample introduction. The emission intensity of Ar decreases with increasing argon pressure. For Al, the intensity decreases more rapidly than for Ar and the intensity peak appears in the same pressure region as those of the sample constituents introduced by laser vaporization. The emission from OH molecules, which exist as an
J
1 0
500
1000
1500
2000
Pressure. Pa
Fig. 7. Dependence of the emission mtensrties of various elements in aluminum alloy samples on the pressure: Mg I 285.21 nm (O), Cu I 324.75 nm (0), Ni I 352.45 nm (A), Fe I 371.99 nm (A), Mn I 403.08 nm (W). The maximum peak in each plot is assigned a value of unity. The samples were vaporized by a single laser shot and introduced into the discharge by an argon flow. Discharge current: 120 mA. Flow rate 80 ml/mm.
Laser vaporization of sohd samples
I
I
0
500
433
1 1000
1500
2000
Pressure. Pa Fig. 8. Dependence of the emission intensity of Al 396.15 nm ( l), Ar 415.86 nm (A), and OH-band 309.8-309.9 nm (U) on the pressure. Discharge current: 120 mA. Flow rate: 80 ml/min. The intensity of each plot is normalized to a constant level.
impurity of the argon, gradually increases with the pressure. The pressure affects almost all processes in the system, i.e. laser vaporization, sample transportation, and excitation. If one compares the result for Al with those for the samples introduced into the discharge, one notices the coincidence of peaks at a pressure of about 800 Pa. This seems to indicate that the excitation processes are not specific for the sputtered cathode material but are similar for the elements introduced by the laser vaporization. For the latter, an increase in emission at lower pressure is counteracted by the reduced efficiency of the transportation processes. The effects of current and pressure will be discussed in the following section in relation to the excitation mechanism in the discharge. On the basis of the results stated above, the standard measurement conditions are fixed as follows: discharge current; 120 mA, argon gas pressure; 800 Pa, argon gas flow rate; 80 ml/min at atmospheric pressure, cathode bore size; 4 mm. 3.3. Analyticalperformance Table 3 shows the analytical results, the slopes and the correlation coefficients of the double logarithmic analytical curves, the relative standard deviation, and the estimated Table 3. Slopes and correlation coefficients of log-log plots of the analytical calibration curves and estimated detection limits for standard aluminum alloy samples
Element
Line (nm)
A. Single pulse mode Mg 285.21 cu 324.75 Ni 352.45 Fe 371.99 Mn 403.08 B. Multiple (20) putse mode Mg 285.21 CU 324.75 Ni 352.45 Fe 371.99 Mn 403.08
Concentration range W)
N’
Slope
0.041- 1.53 0.04(r10.06 0.20 - 2.61 0.07s- 0.97 0.026 0.30
8 8 S 8 8
0.977 1.014 0.947 1.072 1.136
0.041o.O4(r 0.0120.075 0.026
6 5 6 6 7
0.922 0.928 1.012 1.009 1.187
1.53 7.08 2.61 0.91 0.30
RSDf W)
Detectiont limit (Mgg-‘)
0.9938 0.9998 0.9980 0.9973 0.996 1
6.8 6.4 5.1 4.8 6.9
51 101 111 150 28
0.988 1 0.9993 0.9925 0.9949 0.9916
4.6 1.6 3.1 2.1 2.4
Correlation coefficient
2.7 6.5 18 11 4.3
* Number of standard samples. t Relative standard deviation, calculated as the average for each standard sample. : Calculated from 3 times the standard deviatton of the background emission without sample introduction using the same integration period (1 s for the single pulse mode and 2 s for the multtple pulse mode).
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Method
Sample
Laser-MIP [19]
2.1
Al alloy
Laser-ICP [20]
9
steel
Laser-ICP [213
20
steel
2
steel
Laser-ICP/MS Laser-AA [23] This work single pulse multiple (20) pulse
[22]
32
Al ahoy
101
Al alloy
6.5
Laser Q-switched ruby Q-switched ruby Q-switched ruby Q-switched Nd : YAG Q-switched ruby Q-switched Nd:YAG
s~tr~hemi~al
Sampling weight 019) 0.8 1 1 continuous va~rization 0.8
0.2 4
detection limits, for the standard Al alloy samples. The slopes range from 0.922 to 1.187 and the correlation coefficients are larger than 0.988 for all elements. The relative standard deviation is about 6% for the single pulse mode and 3% for the m~tiple pulse mode. The analytical performance is thus much improved in comparison with that of spectrochemical analysis using the laser induced plasmas directly [3, 43. The detection limits are almost comparable with those of the other methods using laser vaporization [19-231. The detection limits for Cu are compared in Table 4. For this GD system, the main restriction upon the detection limit stems from the background emission in the form of molecular bands. Because of the non-LTE character of the discharge, the intense continuum as found in the ICP is not observed but complex molecular bands are present in some regions of the spectrum. Analysis of the spectrum indicates that it chiefly comprises OH, NO and N, bands [24]. Application of a better vacuum system and a purer carrier gas would improve the detection limit. 3.4. Life of caboose The cathode is exposed to sputte~ng by argon and to the sample vapor stream, so that the excitation conditions will change with the time of operation. Without sample introduction or in the single pulse mode, and for a cathode with large bore of 8 mm, such a change with the operation time is so small that one would not notice it after several days of operation. However, with sample introduction in the multiple pulse mode and for a cathode with a bore size of 4 mm, the excitation conditions change perceptibly. For example, although the scale in Fig. 3b does not permit us to see it, the introduction of a sample of 20 pulses produces a rise in electrode voltage of about 0.1 V. This phenomenon may be related to the adsorption of sample vapor or oxide formation on the cathode surface. After 3000-5000 pulses, gray spots were observed near the edge where the diameter changes, and the observed intensity for the sample decreased by l&30% from its initial value. Such a degradation of the cathode can be partly avoided by using a su~~iently long time interval, e.g. 5 mins, between successive sample introductions. In this interval, the cathode surface degraded by the sample introduction might be refreshed by the sputtering. By monitoring the electrode voltage, the time for the replacement of the cathode can be recognized, e.g. from a 10 V rise from the initial voltage. In connection with the cathode life, one might also worry about the memory effects. However, even when pure copper was ablated for a few hundred pulses, the background emission from the cathode itself showed only a slight increase in the emission intensity of the copper line. This indicates that the larger portion of the sample vapor flows through the cathode bore, while the sample adsorbed on the cathode wall is well removed by sputtering.
Laser vaporization of solid samples
435
4. DISCUSSION In this section, the excitation processes in the flow through cathode lamp are discussed. Figure 9 shows log-log plots of the intensity-current dependences for Al sputtered from the cathode, Ar and OH molecules as gas components, and Mn and Cu introduced by the laser ablation. Since self-absorption has not been corrected for and the current range varies over an order of magnitude, an exact correspondence to mechanisms cannot be expected to be found. However, as a first approximation, the following features may be noted. The slope for Al is 3-4, that for Ar is 1-2, while the slope for the OH band varies from 1 to 0. The slopes for Mn and Cu are 334 in the low and high current regions. In the intermediate current region, the behavior seems to be affected by self-absorption. The resuming of the initial upward trend in the higher current region might be related to a thermal effect caused by heat generation in the cathode region. From the slope of the log-log plots of emission intensity vs discharge current one might conclude to the following excitation processes [253. Slope = 1: direct electron collision excitation, A+e-
+ A*+e-
(1)
Slope = 2: ionization and three-body recombination with an electron and a neutral carrier gas atom as third partner, A-l-e- + A+ +2e(21 A++e-+A-,A*+A
(3)
Slope = 3: ionization and three-body recombination A+e-
with an electron as third body.
+ A+ +2e-
(21
A+ +2e- + A*+e-
(41
where A denotes an argon atom or another species in the discharge.
x *
104
VI C 01 44 c
103
al > u 1 w oz
102
10'
1
1
10
I
20
Discharge
50
1
1
100
200
current,
mA
Fig. 9. Log-log plot of the dependence of the emission intensity on the discharge current. Mn I 403.08nm (0) and Cu I 324.75 nm (O), Al I 396.15nm (A), Ar I 415.86nm (Cl), OH-band 309.8-309.9 nm (m). Mn and Cu are introduced by the laser vaporization. Flow rate: 80 ml/min. Pressure: 800 Pa.
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I.. 6
1
42
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Fig. 10. Effect of pressure (expressed in Pa) and cathode bore stze (8 mm: a, c and e; 4 mm: b, d and f) on the lateral distribution of the intensity for the following spectral lines: Ar 1415.86 nm ( a and b); Al i 396.15 nm (c and d); Mn I 403.08 nm (e and f). The Al line emission originated from the cathode itself. For Mn, a sample (SSF332D) was vaporized by a single laser shot and introduced into the discharge by an argon flow. Discharge current: 120 mA. Flow rate: 80 ml/min. Emission from the anode region contributed at the left hand side. The relative intensity ofeach figure (a-f) is normalized to a constant level.
Laser vaporization
of solid samples
431
For the carrier gas and non-sputtered concomitants, argon and OH molecules, the slope does not exceed 2, whence processes (lH3) may represent the governing mechanisms. The decrease in slope in the high current region for the OH plot indicates decomposition of OH into atomic species. On the other hand, the situation is different for the species sputtered from the cathode. First, the vapor density of the sputtered species varies with the lst-3rd power of the discharge current [25-271. Then the slope of the log-log plots indicates the total effect of the current on the two processes, production of atomic vapor by sputtering and the excitation process. As a result, the slope of the plot for Al becomes 34. For Mn and Cu, which are introduced by the laser ablation, the slope clearly exceeds 2. This means that the sputtering, which may occur both at the cathode surface and on small sample particles flowing in the cathode bore, seems to be involved. Thus not only the initially atomized sample but also sample particles decomposed by the sputtering are excited in the hollow cathode discharge. The lateral distribution of emission intensity of Ar, Al, and Mn were also measured to obtain information about the excitation processes. The results are shown in Fig. 10. The lateral distribution of the emission intensity can be related to the electron energy distribution [27]. The phenomenon that the Ar emission prevails close to the cathode surface at relatively high pressure (Figs 10a and lob) is explained by the electron energy distribution with a high density of fast electrons in the dark space near the cathode surface. The asymmetry indicates a contribution from emission near the anode. For Al (Figs 1Ocand lOd), emission is maximum in the center. In view of the difference in excitation energy between the Ar (14.53 eV) and Al (3.14 eV) lines, this may be explained by effective excitation by slow electrons [27]. Further, the pressure at which the emission intensity peaks, varies with the cathode bore diameter, and is 500 and 800 Pa for 8 and 4 mm diameter, respectively. This can be explained by the mechanism of the “hollow cathode effect” [28,29]. Figures 10e and 10f show for Mn, introduced into the discharge by laser vaporization, the same emission pattern as for Al sputtered from the cathode. This means that the sample vapor is likely to be excited in the same way as the cathode material. STIRLINGand WESTWOOD[30] have indicated that, for the hollow cathode discharge, some sputtered material is ejected from the cathode as molecules or groups of atoms, which are subsequently decomposed into atoms in the cathode dark space by collisions with high energy ions bombarding the cathode. The same processes seem to be active for the vapor introduced by the laser vaporization. 5. CONCLUSIONS
A spectrochemical analysis method with good linearity and precision has been developed by coupling laser vaporization with a hollow cathode discharge. The detection power is rather low with the single pulse vaporization, but in a multiple pulse mode the detection power reaches a few micrograms per gram. The main restriction upon the detection limit stems from background emission in the form of molecular bands. Application of a tighter vacuum system and a trap to eliminate most of the gas impurities would improve the detection limit. One may regard this technique as a unique method for local analysis of solid samples, or as a method for the introduction of non-conducting samples into a discharge. An investigation aimed at the analysis of ceramic materials using this method is in progress. The excitation of the sample vapor introduced into the discharge by the laser vaporization seems to show an interesting feature: some part of the sample vapor which is not in atomic form appears to be decomposed into atoms by the sputtering and is then excited in the discharge. The hollow cathode effect plays an important role in the excitation processes and in the enhancement of the emission intensity. Acknowledgements-The author wishes to thank Dr. T. ISHIZUKA for his helpful advice and encouragement work, and Dr. K. KITAGAWA for helpful discussions.
in this
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Laser Micro-SpectrochemlcnI
Analysrs. Adam
Hilger,
London
438 [2] [3] [4] [S] [6] [7] [8] [9]
[lo] [l l] [12]
[13] [14] [15J [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27]
[28] [29] [30]
YAsuo
IIDA
K. Laqua, Analytica/ Laser Spectroscopy (Ed. N. Omenetto), Chap. 2, Wiley, New York (1973). R. H. Scott and A. Strasheim, Specrochim. Acta ZSB, 311 (1970). W. Van Deijck, J. Balke and F. J. M. J. Maessen, Spectrochim. Acta 34B, 359 (1979). W. J. Treytl, K. W. Marich and D. Click, Anal. Chem. 47, 1275 (1975). E. H. Piepmeler and D. E. Osten, Appl. Spectrosc. 25, 642 (1971). W. J. Treytl, K. W. Marich, J. B. Orenberg, P. W. Carr, D. C. Miller and D. Glick, Anal. Chem. 43,1452(1971). Y. Iida, Appl. Spectrosc. 43, 229 (1989). R. Kirchheim, U. Nagorny, K. Maier and G. T61g, Anal. Chem. 48, 1505 (1976). C. W. Huie and E. S. Yeung, Anal. Chem. 58, 1989 (1986). K. Dittrich and R. Wennrich, Prog. Anal. Atom. Spectrosc. 7, 139 (1984). B. N. Chapman, Glow Discharge Processes. Wiley, New York (1980). H. Falk, E. Hoffmann, I. Jaeckel and CH. Ludke, Spectrochim. Acta 348, 333 (1979). S. Caroli, 0. Senofonte, N. Violante, F. Petrucci and A. Alimonti, Specrrochim. Acta 39B, 1425 (1984). S. Caroli, J. Anal. Atom. Spectrom. 2, 661 (1987). R. K. Marcus and W. W. Harrison, Spectrochim. Acta 4OB, 933 (1985). J. F. Ready, E$xts ojHigh-Power Laser Radiation. Academic Press, New York (1971). S. Caroli, A. Alimonti and F. Petrucci, Improoed Hollow Cathode Lamps for Atomic Spectroscopy (Ed. S. Caroli), Chap. 1, Ellis Horwood, Chichester (1985). T. Ishizuka and Y. Uwamino, Anal. C’hem. 52, 125 (1980). T. Ishizuka and Y. Uwamino, Spectrochim. Acta 38B, 519 (1983). M. Thompson, J. E. Goulter and F. Sieper, Analyst 106, 32 (1981). P. Arrowsmith, Anal. Chem. 59, 1437 (1987). T. Ishizuka, Y. Uwamino and H. Sunahara, Anal. Chem. 49, 1339 (1977). R. W. B. Pearse and A. G. Gaydon, The Identification ofMolecular Spectra. Chapman & Hall, London (1965). H. Falk, Improved Hollow Cathode Lampsfor Atomic Spectroscopy (Ed. S. Caroli), Chap. 4, Ellis Horwood, ChIchester (1985). V. Orlinov, G. Mladenov and B. Goranchev, Int. J. Electron. 30, 233 (1971). M. E. Pillow, Spectrochim. Acta 368, 821 (1981). P. F. Little and A. Von Engel, Proc. Roy. Sot. Lond A 224. 109 (1954). B. E. Warner, K. B. Persson and G. J. Collins, J. Appl. Phys. 50, 5694 (1979). A. J. Stirling and W. D. Westwood, J. Appl. Phys. 41, 742 (1970).
Spectrochumco AcfoVol 45B.No 4/S. pp 427 438,1990 PnnledI"GreatBntam
Laser vaporization of solid samples into a hollow-cathode atomic emission spectrometry YASUO Government
Industrial
Research
(Received
Institute,
Nagoya
discharge for
IIDA 1, Hirate-cho,
Kita-ku,
Nagoya
462. Japan
19 May 1989; in revised form 6 September 1989)
Abstract-A hollow cathode discharge system has been developed for atomic emission spcctrometry of solid samples. In this system, a solid sample is vaporized by a Q-switched Nd:YAG laser, and the vaporized material is introduced into a hollow cathode discharge by means of an argon gas flow. A photodlode array detector is used to acquire the spectral data and the spatial intensity profiles of atomic lines in the cathode bore. Experimental
parameters which have been optimized include the pressure and flow rate of argon gas, the forms of the electrodes, and the discharge current. The laser is operated in either single pulse mode or multiple pulse mode, 20 pulses at 10 Hz repetition rate. The latter mode offers improved precision and detection limits, and quasi-continuous emission signals. Analytical curves obtained for standard samples of aluminum alloys are linear over 3 orders of magnitude with a relative standard deviation of 1.64.6%. and the estimated detection limits for the various elements range from 3 pgg- ’ for Mg to 16 p’gg- ’ for Ni.
1. INTRODUCTION LASER vaporization offers the possibility to handle conducting and non-conducting solid samples with excellent microsampling characteristics [l, 21. Nevertheless, the direct spectrochemical analysis of laser induced plasmas is limited in its use by the lack of precision and the poor linearity of the analytical curves [3, 41. Several efforts have been made to ameliorate the situation by the adaptation of the spatial and time resolution [3, 51, by the variation of the surrounding atmosphere [6-83 and by obtaining a fundamental understanding of the laser-surface interaction [9, lo]. On the other hand, several kinds of additional excitation sources, such as spark cross-excitation, ICP and MIP, have been combined with laser vaporization to improve the analytical performance, as reviewed by DITTRICH and WENNRICH [ll]. From the point of view of the sources, we might regard these works as developments of techniques for the direct introduction and microprobe sampling of solid samples. Glow discharge (GD) plasmas are known to be not in local thermal equilibrium (LTE) [12]. Owing to the non-LTE character, GDs have several excellent features as radiation sources for atomic spectrometry, i.e. high line-to-background ratio, simple spectra, narrow line width, and low matrix effects [13-161. Another attractive feature is their fairly good precision [14]. In spite of those excellent properties, some limitations and drawbacks also exist in the GD source: lack of microprobing facility, time-consuming machining of sample into a cathode of fixed form with the risk of contamination, and special sample preparation for non-conducting materials. Most of these disadvantages can be obviated by sample introduction with the laser vaporization technique. The combination of laser vaporization with excitation by a GD, having either flat or hollow cathodes, has not yet been reported. In this paper, the design of a laser vaporization system coupled with a hollow cathode discharge (HCD) source is described, and the analytical performance of the system is reported.
2. EXPERIMENTAL 2.1. Apparatus
The experimental apparatus and the operating conditions are summarized in Table 1. The laser vaporization chamber was constructed from a Pyrex glass cylinder, 90 mm high and 32 mm in diameter. The top ended in a ground flange, which was closed by a Pyrex glass window of 3 mm thickness. The bottom edge was cemented to a brass block. The chamber was equipped with a 427
428
YAW0
IIDA
Table 1. Apparatus and operating conditions Q-switched Nd: YAG laser YG-580A (Quantel International); energy: 250 mJ/pulse; pulse width 10 ns; repetition rate: 10 Hz. Spectrometer THR-1OOOMSL(Jobin-Yvon); Czerny-Turner; focal length: 1000 mm; grating: 2400 lines/mm, holographtc; sht width, 75 pm; sht herght: 5 mm. Photodiode
array
detector
system
OMA III system (Princeton Applied Research); detector: 1421B-1024HQ, 1024 elements (element stze. 25 pm x 2.5 mm) with microchannel plate image intensifier; controller: 1460 (68000-based); detector module: 1463 (14 bit A/D); triggered by a Q-swatch stgnal from the laser; spectral wrndow 10 nm. Photomultiplier R-955 (Hamamatsu Photonics). Digital sampling oscilloscope DS-6121 (Iwatsu Electric); 2 channel (2048 word/channel); sampling rate: 20 MHz. DC power supply for the glow discharge PAD 1 K-O.2 L (Kikusui Electronics); f&1500 V, O-200 mA; the supply was connected with the discharge tamp through a 2 ka ballast resister. Vacuum gauge PT-9 P (Daia Vacuum Engineering); Puani type; calibrated for Ar gas with mercury manometer.
lateral flange of 20 mm i.d. for sample changing, which was usually closed by a glass plate. A brass sample stage, 24 mm in diameter, and adjustable in height from 35 mm to 50 mm by a screw, was positioned in the chamber. The laser beam was directed downward with a 45” prism and focused with a single lens, focal length 80 mm, on the sampIe surface through the window. The diameter of the focused spot was about 1.5 mm. Argon entered through a 4 mm id. giass tube and carried the sample vapor to the discharge Iamp through a tapered (20 mm to 8 mm i.d.) glass tube. The glass tubes were coaxial and the axis was about 5 mm above the sample surface. The argon flowed continuously under the action of a rotary pump. The pressure and the flow rate were controtled with a needle valve in the gas inlet line and a diaphragm valve in the evacuation line. The Row-through discharge lamp designed for this work is shown in Fig. 1. The evaporation cell and the lamp were connected by silicon rubber tubing (id. 10 mm, o.d. 21 mm, length 80 mm). The tubing was curved so as to prevent radiation emitted from the laser induced plasma from entering the optics. The distance between the sample vaporization point and the cathode was about 15 cm. The cathodes were prepared in three different bore sizes (Fig. 2a-2c), and were made of aluminum of 99% purity. The anode was a tungsten rod of 2 mm diameter and was soldered with silver to the UItra Torr fitting (CAJON). A similar fitting and rod were used for the electricat contact to the cathode. A He-Ne laser was aligned coaxially with the Nd:YAG laser beam to facilitate positioning of the sample. Pure argon (more than 99.9995% purity) was used as the carrier and plasma-forming gas without further purification.
2.2. Samples The following standard Al alloy samples were used: ALCOA KA-213-B, KB-356-B, KC-356-E, KA356-E, %-138-R, SS-242-C, SS-A332-D and SS-F332-D. The specified compositions are given in anode
cathode
quartz
--w
vapor1 red
window
sama 1e
4
c0011ng
water
J(
3to rotary
cwmp
Fig 1. Schematic diagram of the flow-through discharge lamp with a hallow cathode. The three parts (l-3) of the lamp and the window are fixed together with Aptezon wax W.
429
Laser vaporization of solid samples
a
b
c
(mn)
Fig. 2. Aluminum hollow cathodes. Cathodes of three different bore sizes were used. Table 2. Standard alummum alloy samples Alcoa number
MS
cu
KA-213-B KB-356-B KC-356-E KA-356-E SS-138-R SS-242-C
0.041 0.38 0.27 0.15 0.27 1.53
7.08 0.055 0.040 0.20 10.06 3.97
SS-A332-D SS-F332-D
1.16 0.95
1.01 3.22
Nt W)
Fe
Mn
0.21 2.01
0.96 0.20 0.075 0.48 0.97 0.54
0.30 0.13 0.026 0.034 0.22 0.085
2.61 0.50
0.66 0.70
0.072 0.26
0.20 0.012
*Otherelements contained (%). Si(O.49-12.03). ~r(O.~l~.O31), Ti(O.O51~.17), PqO.O6~.10), Sn(O.O~.57~
Zn(O.O3~.99),
Table 2. The flat and smooth surfaces of the samples were obtained by polishing with a No. 800 silicon
carbide paper. The polished surfaces were washed with distilled water and acetone, and dried with hot air. 2.3. Procedure A sample was positioned on the flat stage in the evaporation cell, and the position of the laser shot was adjusted by means of the coaxial He-Ne laser beam. After evacuation, the argon gas was introduced, then the discharge was struck and allowed to be steady for several minutes. The laser was fired in a single pulse mode or a multiple pulse mode (20 pulses at 10 Hz). Since the first few shots on a fresh surface gave relatively high intensities, the corresponding data were discarded and only the subsequent data were evaluated. Spectral data were measured with a photodi~e array detector system, except for the emission time profiling, where a photomultip~ier with a digital sampling oscilioscope was used. The cathode bore was imaged on the slit of the spectrometer with unity magnification and the center of the cathode was conjugate to the center of the slit. Thus the observation region was 2.5 mm, being limited by the height of the photodiode detector. The photodiode array system offered some flexible data acquisition facilities, i.e. triggering, accumulation, and background subtraction, in a desired sequence. The trigger was fed from the Q-switch signal of the laser. The emission was accumulated for 1 sin the single pulse mode and 2 s in the multiple pulse mode. The background emission was also accumulated for the same period just before the laser shots, and used for background subtraction. Values of the emission intensity were obtained by integration of peak area with automatic baseline correction. The spectral window corresponded to l&30 photodiode elements (0.1-0.3 nm) and varied with the degree of spectral interference. The spatial distribution of the emission in the hollow cathode bore was also measured with the photodiode array detector. In this case, the diode array was directed vertically, rotated over 90” from the usual position, and a slit of 200 pm width was installed on the detector window. A complete vertical image of the cathode bore at the desired wavelength was thus obtained. 3. RESULTS 3.1.Discharge characteristics The discharge was maintained
showed the characteristic
in the constant current mode. The voltage-current curve of the abnormal GD, i.e. the voltage increased as the current
Y~suo
430
IID.~
;:k-_ 2
Time,
200 ms/DlV
Time,
500 ms/OlV
205
Fig. 3. Change
of the electrode voltage as a result of sample vapor introduction vaporization. (a) Single pulse mode; (b) multiple pulse mode.
by laser
increased, and the voltage decreased as the pressure increased. For cathodes of different bore sizes, the voltage was higher for the cathode of smaller bore size. Under the conditions of this experiment, the voltage was in a range between 180 and 240 V. By the introduction of sample vapor with the laser pulse, the electrode voltage changed as shown in Fig. 3. For the single pulse mode (Fig. 3a), the voltage drops immediately after the laser pulse. This drop seems to stem from a pressure change by the expansion of the lasergenerated plasma or from the electron flow from the sample [17]. The next large decrease of the electrode voltage occurs after about 200 ms from the start of the laser pulse and corresponds to the impedance change by the sample introduction. Figure 3b shows the voltage change in the multiple pulse mode. The voltage decreases by the continuous introduction of sample vapor and reaches a steady value of about 10 V below its initial value. By the end of the sample introduction, the voltage returns gradually to its initial value. These changes in the discharge conditions produce some perturbations in the background emission as well as the sample emission, so that accurate compensation of the background emission should be done by measuring the dynamic background emission with sample introduction. However, as a matter of convenience, we used the static background emission just before the sample introduction for the background subtraction. The differences of those background emissions would appear as a rough baseline in spectral data or a deviation of the y-intercept from origin in the analytical calibration curve. 3.2. EfSect of experimental parameters The effects of several experimental parameters were investigated, and the parameters were optimized for analytical performance. Figure 4 shows the effect of discharge current on the emission intensity of Mn 1403.08 nm for an Al alloy sample introduced into the discharge with laser vaporization. Cathodes of bore sizes 4,6 and 8 mm (Fig. 2a-2c) were used. An increase in the current usually leads to an increase in intensity. The discharge current had to be limited to 160 mA in view of the heat generation at the cathode and the poor thermal conductivity of the surrounding glass wall. The size of the cathode bore is one of the important factors related to the emission intensity. The relative surface area for the cathodes of bore sizes of 4,6 and 8 mm were 1.O, 1.2 and 1.4, respectively. The calculated current density was 15 mA/cm’ for the 4-mm bore cathode at 120 mA. Comparing the same current density points in Fig. 4, e.g. 100, 120 and 140 mA for the 4,6 and 8 mm bores, respectively, the emission intensity is still higher for the cathode of smaller bore size. Thus, the difference of emission intensity could not be explained only by the difference in the current density. It is well known that a HCD has complex radial
431
Laser vaporization of solid samples
8-
6-
4.
2.
0 0
SO
101)
150
200
Discharge current, mA
Fig. 4. Dependence of the emission intensity of Mn I 403.08 nm on the discharge current. Three aluminum cathodes of different bore size (4 mm ( l), 6 mm (A), 8 mm ( n)) were investigated. An aluminum alloy sample (SSF332D) was vaporized by a single laser shot and introduced into the discharge by an argon flow of 80ml/min. Pressure: 800 Pa. The error bars show the standard deviation of five measurements.
in the cathode bore [Is]. So, the decrease of the bore size may increase the opportunity of the sample vapor to pass through the effective excitation region of the discharge. Also, the bore volumes change with the bore size in the ratio 1.0: 1.5: 2.3 for the 4, 6 and 8 mm bores, respectively. The volume dictates the concentration of the sample vapor in the excitation region. These effects, as well as the current density, appear to be important for increasing the emission intensity. Figure 5 shows the effect of flow rate, which was regulated at atmospheric pressure, on the emission intensity. In view of the residence time of the sample vapor in the discharge region, one should make the flow rate as small as possible. However, in a system where the sample vapor must be carried by the gas flow, which is the case in this study, a higher flow rate is preferable to diminish capture of sample vapor at the wall, and to transport the larger particles from the vaporization cell to the discharge. The optimal flow rate is determined by the balance between these factors. Figure 6 shows the emission time profile of Mn I at 403.08 nm for the Al alloy sample, at 800 Pa, 120 mA, and various flow rates for the 4-mm structure
10 l-
8
0 Flow rate, ml/min
Fig. 5. Dependence of the emission intensity of Mn I 403.08 nm on the argon flow rate. Three aluminum cathodes of different bore size (4 mm (O), 6 mm (A), 8 mm ( n)) were investigated. An aluminum alloy sample (SSF332D) was vaporized by a single laser shot and introduced into the discharge by an argon flow. Discharge current: 120 mA. Pressure: 800 Pa. The error bars shows the standard deviation of five measurements.
432
YAsuo
IIDA
Laser pulse Time, 100 ms/DIV
Ftg. 6. Emission-time profile of Mn I 403.08 nm for the aluminum alloy sample fSSF332D). Flow rate: (a) 80 ml/min; (b) 40 ml/min; (c) 20 mljmin. Discharge current: 120 mA. Pressure: 800 Pa.
bore cathode. The delay of the emission corresponds to the linear velocity of the gas flow. The duration of the emission, which exceeds the pulse interval of 100 ms, indicates the possibility of using the discharge lamp as a quasi-continuous emission source. Figure 7 shows the effects of the pressure on the emission intensity of the various elements introduced into the discharge. All the five investigated elements showed the same dependence upon the argon pressure, and the emission intensity peaks are obtained at about 800 Pa. Figure 8 shows the effect of pressure on the emission intensity of the concomitant species in the discharge without sample introduction. The emission intensity of Ar decreases with increasing argon pressure. For Al, the intensity decreases more rapidly than for Ar and the intensity peak appears in the same pressure region as those of the sample constituents introduced by laser vaporization. The emission from OH molecules, which exist as an
J
1 0
500
1000
1500
2000
Pressure. Pa
Fig. 7. Dependence of the emission mtensrties of various elements in aluminum alloy samples on the pressure: Mg I 285.21 nm (O), Cu I 324.75 nm (0), Ni I 352.45 nm (A), Fe I 371.99 nm (A), Mn I 403.08 nm (W). The maximum peak in each plot is assigned a value of unity. The samples were vaporized by a single laser shot and introduced into the discharge by an argon flow. Discharge current: 120 mA. Flow rate 80 ml/mm.
Laser vaporization of sohd samples
I
I
0
500
433
1 1000
1500
2000
Pressure. Pa Fig. 8. Dependence of the emission intensity of Al 396.15 nm ( l), Ar 415.86 nm (A), and OH-band 309.8-309.9 nm (U) on the pressure. Discharge current: 120 mA. Flow rate: 80 ml/min. The intensity of each plot is normalized to a constant level.
impurity of the argon, gradually increases with the pressure. The pressure affects almost all processes in the system, i.e. laser vaporization, sample transportation, and excitation. If one compares the result for Al with those for the samples introduced into the discharge, one notices the coincidence of peaks at a pressure of about 800 Pa. This seems to indicate that the excitation processes are not specific for the sputtered cathode material but are similar for the elements introduced by the laser vaporization. For the latter, an increase in emission at lower pressure is counteracted by the reduced efficiency of the transportation processes. The effects of current and pressure will be discussed in the following section in relation to the excitation mechanism in the discharge. On the basis of the results stated above, the standard measurement conditions are fixed as follows: discharge current; 120 mA, argon gas pressure; 800 Pa, argon gas flow rate; 80 ml/min at atmospheric pressure, cathode bore size; 4 mm. 3.3. Analyticalperformance Table 3 shows the analytical results, the slopes and the correlation coefficients of the double logarithmic analytical curves, the relative standard deviation, and the estimated Table 3. Slopes and correlation coefficients of log-log plots of the analytical calibration curves and estimated detection limits for standard aluminum alloy samples
Element
Line (nm)
A. Single pulse mode Mg 285.21 cu 324.75 Ni 352.45 Fe 371.99 Mn 403.08 B. Multiple (20) putse mode Mg 285.21 CU 324.75 Ni 352.45 Fe 371.99 Mn 403.08
Concentration range W)
N’
Slope
0.041- 1.53 0.04(r10.06 0.20 - 2.61 0.07s- 0.97 0.026 0.30
8 8 S 8 8
0.977 1.014 0.947 1.072 1.136
0.041o.O4(r 0.0120.075 0.026
6 5 6 6 7
0.922 0.928 1.012 1.009 1.187
1.53 7.08 2.61 0.91 0.30
RSDf W)
Detectiont limit (Mgg-‘)
0.9938 0.9998 0.9980 0.9973 0.996 1
6.8 6.4 5.1 4.8 6.9
51 101 111 150 28
0.988 1 0.9993 0.9925 0.9949 0.9916
4.6 1.6 3.1 2.1 2.4
Correlation coefficient
2.7 6.5 18 11 4.3
* Number of standard samples. t Relative standard deviation, calculated as the average for each standard sample. : Calculated from 3 times the standard deviatton of the background emission without sample introduction using the same integration period (1 s for the single pulse mode and 2 s for the multtple pulse mode).
434
YAsuo IIDA Table 4. Comparison of the detection limit of Cu for various laser-va~ri~tion analysis method Detection limit (Wr-‘)
Method
Sample
Laser-MIP [19]
2.1
Al alloy
Laser-ICP [20]
9
steel
Laser-ICP [213
20
steel
2
steel
Laser-ICP/MS Laser-AA [23] This work single pulse multiple (20) pulse
[22]
32
Al ahoy
101
Al alloy
6.5
Laser Q-switched ruby Q-switched ruby Q-switched ruby Q-switched Nd : YAG Q-switched ruby Q-switched Nd:YAG
s~tr~hemi~al
Sampling weight 019) 0.8 1 1 continuous va~rization 0.8
0.2 4
detection limits, for the standard Al alloy samples. The slopes range from 0.922 to 1.187 and the correlation coefficients are larger than 0.988 for all elements. The relative standard deviation is about 6% for the single pulse mode and 3% for the m~tiple pulse mode. The analytical performance is thus much improved in comparison with that of spectrochemical analysis using the laser induced plasmas directly [3, 43. The detection limits are almost comparable with those of the other methods using laser vaporization [19-231. The detection limits for Cu are compared in Table 4. For this GD system, the main restriction upon the detection limit stems from the background emission in the form of molecular bands. Because of the non-LTE character of the discharge, the intense continuum as found in the ICP is not observed but complex molecular bands are present in some regions of the spectrum. Analysis of the spectrum indicates that it chiefly comprises OH, NO and N, bands [24]. Application of a better vacuum system and a purer carrier gas would improve the detection limit. 3.4. Life of caboose The cathode is exposed to sputte~ng by argon and to the sample vapor stream, so that the excitation conditions will change with the time of operation. Without sample introduction or in the single pulse mode, and for a cathode with large bore of 8 mm, such a change with the operation time is so small that one would not notice it after several days of operation. However, with sample introduction in the multiple pulse mode and for a cathode with a bore size of 4 mm, the excitation conditions change perceptibly. For example, although the scale in Fig. 3b does not permit us to see it, the introduction of a sample of 20 pulses produces a rise in electrode voltage of about 0.1 V. This phenomenon may be related to the adsorption of sample vapor or oxide formation on the cathode surface. After 3000-5000 pulses, gray spots were observed near the edge where the diameter changes, and the observed intensity for the sample decreased by l&30% from its initial value. Such a degradation of the cathode can be partly avoided by using a su~~iently long time interval, e.g. 5 mins, between successive sample introductions. In this interval, the cathode surface degraded by the sample introduction might be refreshed by the sputtering. By monitoring the electrode voltage, the time for the replacement of the cathode can be recognized, e.g. from a 10 V rise from the initial voltage. In connection with the cathode life, one might also worry about the memory effects. However, even when pure copper was ablated for a few hundred pulses, the background emission from the cathode itself showed only a slight increase in the emission intensity of the copper line. This indicates that the larger portion of the sample vapor flows through the cathode bore, while the sample adsorbed on the cathode wall is well removed by sputtering.
Laser vaporization of solid samples
435
4. DISCUSSION In this section, the excitation processes in the flow through cathode lamp are discussed. Figure 9 shows log-log plots of the intensity-current dependences for Al sputtered from the cathode, Ar and OH molecules as gas components, and Mn and Cu introduced by the laser ablation. Since self-absorption has not been corrected for and the current range varies over an order of magnitude, an exact correspondence to mechanisms cannot be expected to be found. However, as a first approximation, the following features may be noted. The slope for Al is 3-4, that for Ar is 1-2, while the slope for the OH band varies from 1 to 0. The slopes for Mn and Cu are 334 in the low and high current regions. In the intermediate current region, the behavior seems to be affected by self-absorption. The resuming of the initial upward trend in the higher current region might be related to a thermal effect caused by heat generation in the cathode region. From the slope of the log-log plots of emission intensity vs discharge current one might conclude to the following excitation processes [253. Slope = 1: direct electron collision excitation, A+e-
+ A*+e-
(1)
Slope = 2: ionization and three-body recombination with an electron and a neutral carrier gas atom as third partner, A-l-e- + A+ +2e(21 A++e-+A-,A*+A
(3)
Slope = 3: ionization and three-body recombination A+e-
with an electron as third body.
+ A+ +2e-
(21
A+ +2e- + A*+e-
(41
where A denotes an argon atom or another species in the discharge.
x *
104
VI C 01 44 c
103
al > u 1 w oz
102
10'
1
1
10
I
20
Discharge
50
1
1
100
200
current,
mA
Fig. 9. Log-log plot of the dependence of the emission intensity on the discharge current. Mn I 403.08nm (0) and Cu I 324.75 nm (O), Al I 396.15nm (A), Ar I 415.86nm (Cl), OH-band 309.8-309.9 nm (m). Mn and Cu are introduced by the laser vaporization. Flow rate: 80 ml/min. Pressure: 800 Pa.
436
YASUO
L..
6
4
.
:
.
.
2
0
2
4
IIDA
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6
4
2
IC
2
4
6
Id
I.. 6
0
mm
mm
202
4
46
64
I
:.
2
0
.
24
.
6
mm
I.. 6
1
42
i
02
mm
I
I.
46
I. 6
1
c
4
20246
j
I..
mm
Fig. 10. Effect of pressure (expressed in Pa) and cathode bore stze (8 mm: a, c and e; 4 mm: b, d and f) on the lateral distribution of the intensity for the following spectral lines: Ar 1415.86 nm ( a and b); Al i 396.15 nm (c and d); Mn I 403.08 nm (e and f). The Al line emission originated from the cathode itself. For Mn, a sample (SSF332D) was vaporized by a single laser shot and introduced into the discharge by an argon flow. Discharge current: 120 mA. Flow rate: 80 ml/min. Emission from the anode region contributed at the left hand side. The relative intensity ofeach figure (a-f) is normalized to a constant level.
Laser vaporization
of solid samples
431
For the carrier gas and non-sputtered concomitants, argon and OH molecules, the slope does not exceed 2, whence processes (lH3) may represent the governing mechanisms. The decrease in slope in the high current region for the OH plot indicates decomposition of OH into atomic species. On the other hand, the situation is different for the species sputtered from the cathode. First, the vapor density of the sputtered species varies with the lst-3rd power of the discharge current [25-271. Then the slope of the log-log plots indicates the total effect of the current on the two processes, production of atomic vapor by sputtering and the excitation process. As a result, the slope of the plot for Al becomes 34. For Mn and Cu, which are introduced by the laser ablation, the slope clearly exceeds 2. This means that the sputtering, which may occur both at the cathode surface and on small sample particles flowing in the cathode bore, seems to be involved. Thus not only the initially atomized sample but also sample particles decomposed by the sputtering are excited in the hollow cathode discharge. The lateral distribution of emission intensity of Ar, Al, and Mn were also measured to obtain information about the excitation processes. The results are shown in Fig. 10. The lateral distribution of the emission intensity can be related to the electron energy distribution [27]. The phenomenon that the Ar emission prevails close to the cathode surface at relatively high pressure (Figs 10a and lob) is explained by the electron energy distribution with a high density of fast electrons in the dark space near the cathode surface. The asymmetry indicates a contribution from emission near the anode. For Al (Figs 1Ocand lOd), emission is maximum in the center. In view of the difference in excitation energy between the Ar (14.53 eV) and Al (3.14 eV) lines, this may be explained by effective excitation by slow electrons [27]. Further, the pressure at which the emission intensity peaks, varies with the cathode bore diameter, and is 500 and 800 Pa for 8 and 4 mm diameter, respectively. This can be explained by the mechanism of the “hollow cathode effect” [28,29]. Figures 10e and 10f show for Mn, introduced into the discharge by laser vaporization, the same emission pattern as for Al sputtered from the cathode. This means that the sample vapor is likely to be excited in the same way as the cathode material. STIRLINGand WESTWOOD[30] have indicated that, for the hollow cathode discharge, some sputtered material is ejected from the cathode as molecules or groups of atoms, which are subsequently decomposed into atoms in the cathode dark space by collisions with high energy ions bombarding the cathode. The same processes seem to be active for the vapor introduced by the laser vaporization. 5. CONCLUSIONS
A spectrochemical analysis method with good linearity and precision has been developed by coupling laser vaporization with a hollow cathode discharge. The detection power is rather low with the single pulse vaporization, but in a multiple pulse mode the detection power reaches a few micrograms per gram. The main restriction upon the detection limit stems from background emission in the form of molecular bands. Application of a tighter vacuum system and a trap to eliminate most of the gas impurities would improve the detection limit. One may regard this technique as a unique method for local analysis of solid samples, or as a method for the introduction of non-conducting samples into a discharge. An investigation aimed at the analysis of ceramic materials using this method is in progress. The excitation of the sample vapor introduced into the discharge by the laser vaporization seems to show an interesting feature: some part of the sample vapor which is not in atomic form appears to be decomposed into atoms by the sputtering and is then excited in the discharge. The hollow cathode effect plays an important role in the excitation processes and in the enhancement of the emission intensity. Acknowledgements-The author wishes to thank Dr. T. ISHIZUKA for his helpful advice and encouragement work, and Dr. K. KITAGAWA for helpful discussions.
in this
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