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The microwave-coupled hollow cathode — a novel radiation source in emission spectroscopy
trends in analytical chemistry, vol. I, no. 15, 1982
368
The microwave-coupled novel radiation source
hollow in emission
cathode - a spectroscopy
The development of new excitation sources accounts for the renaissance during the 1970s of emission spectroscopy as a powerful analytical tool. Among these, the hollow cathode discharge is still expanding the scope of its applications. S. Caroli, A. Alimonti and F. Petrucci Rome, Italy Several decades have elapsed since Paschen first described the use of a low pressure emission source named the hollow cathode’. Although interest in this field was kept alive by a number of significant contributions it was not until the late 1950s that the hollow cathode gained wide popularity, when it made possible the introduction and development of atomic absorption and fluorescence techniques. It is worth noting that, rather paradoxically, the peculiar properties of the hollow cathode were generally recognized, not for their direct applicability in emission spectroscopy, but only as an auxiliary to other methods. However, the last 15 years have seen a steady increase in the number of papers which testify to a renewed interest in this type of excitation source. This can largely be ascribed to Grimm’s introduction2*3 of a combined hollow cathode and glow discharge lamp, particularly suitable for the analysis of solid, electrically conducting samples. This encouraged further research, often with remarkably successful results. The glow discharge and hollow cathode have some fundamental features in common. However, the Grimm lamp, operating in the obstructed glow is now well established and is discharge mode, currently employed in various fields, particularly metallurgical analysis. This is not the case for the hollow cathode, though this discharge lends itself to a wider variety of analytical applications for gaseous, liquid, solid and powdered samples. Despite its namely high stability and intrinsic advantages, reproducibility, sharp spectral lines which are almost unaffected by self-absorption, low background, negligible Doppler and Stark effects, as well as reduced matrix and third element effects, this excitation source has not yet received the attention it deserves. The complex operating conditions required for the lamp, involving the use of high purity noble gases at reduced pressure, can only partially account for this. Another limiting factor is that the intensity of spectra emitted by the hollow cathode is, as a rule, lower than those obtained with other sources, especially in the case of ICP torches. This makes it inconvenient for sub-trace analysis. A number of workers have devoted considerable 0 165-9936/82/CKlOOX1OWxwx)/$o1.oo
efforts to solving this problem. They have endeavoured to develop new versions of the hollow cathode lamp capable of higher efliciency. Examples of this are the magnetic field-coupled tube devised by Rudnevskii et aL4, and the transitional type proposed by Eichhoff”. We have recently6 investigated the possibility of superimposing microwave irradiation on a hollow cathode discharge. The results obtained have been highly promising. Although none of the hollow cathode properties were altered, the intensity ofspectra emitted by the cathodic material increased markedly while that of the filler gas was lower.
Processesoccurring in the lamp The essential aspects of the mechanism of the hollow cathode discharge are well known. Very briefly, the electrons emitted by the cathodic inner surface, under the applied dc voltage difference, undergo acceleration in the Hittorf dark space within the cavity (the Hittorf dark space is the region where the voltage drop is almost completely localized. It develops between the luminescent layer on the cathodic surface and the negative glow, i.e. the region where emission occurs. In a hollow cathode the dark space is annular, as shown in Fig. 1.) Further electrons are generated by collision with the carrier gas atoms until a state ofequilibrium is attained between atomic and ionic species. This is a result of the oscillating movement of the electrons from wall to wall. The positively charged ions ofthe filler gas
Cathode ,/
-,’
Luminescent cathodic layer
Hittorf dark space
Negative
Fig. 1. Cross section of a hollow cathode showing the various characteristic zones. @
1982 Ekvier
Scientific
Publishing
Company
trends m analytical chemistry, vol. 1, no. 15, 1982
369 TABLE
I. Cathodic
discharge
Cathodic
. 0
ekctron targetatom
0
target
atom ( ionized
)
0
target
atom (excited
)
0 Q 0
rare
(neutral
materials
in the MW-coupled
and solutions
subjected
hollow cathode
Material
to
lamp
Solution
Aluminium
Zinc
Copper
Iron
Graphite
Chromium (in Steel)
Molybdenum
Nickel
(in Steel)
Copper Nitrate
Arsenic Pentoxide
Zinc Nitrate
Aluminium Nitrate
Lead Nitrate
(in Steel)
)
gas atom (neutral)
rare
gasatom
rare
gas atom (metastable
(ionized) )
Fig. 2. Surface of a solid sample sputtered under ion bombardment.
are attracted by the cathode, onto which they impact, thus extracting particles from the surface. The sputtered material enters the discharge zone, is excited by collision with electrons and gas atoms to emit its characteristic radiation and finally redeposit. Since it only escapes with difficulty from the bore, the process can be repeated. There are a number of variations of this general mechanism and these depend on the type of noble gas used for the discharge. Because of its considerable atomic weight and ionization potential, argon is most commonly employed. In this case metastable atoms with rather long lifetimes (= 10-3s, i.e. lo5 times that of other excited states) play an essential role in the excitation process. Processes occurring on the cathodic surface are depicted in a simplified way in Fig. 2. Microwave (MW) excitation is a consequence of the highly energetic motion imparted to atoms in the gaseous phase. In practice, this means that the electrodeless discharge works well with elements of relatively low vapour pressure at the operating temperature of the MW field (400-500°C). It seems logical, therefore, to assume that the hollow cathode and MW excitation processes should mutually reinforce one another if performed simultaneously. Material sputtered from the cathode is available for further excitation by the MW field, or, to look at it another way, the MW field can exert its influence on a greater number of sample particles. However, in practice it can be assumed that the two phenomena are not simply superimposed, but contribute to an overall more complex process. The analytical implications of the above observations are of paramount importance and have been thoroughly investigated.
The composite discharge tube The practical realization of the MW-coupled hollow cathode lamp was achieved by modifying a commercially available Grimm’s glow discharge device as
outlined in previous paper?,‘. Basically, the modifications consist of separating the anodic and cathodic blocks of the lamp by means of a cylindrical glass chamber through which MW irradiation could be adequately transmitted into the discharge zone. The MW source used was a diathermy unit (capable of generating heat in body tissue by means of microwave radiation) produced by Bosch (FRG). This delivers a MW frequency of 2450 MHz with power adjustable between O-230 W. A simple antenna with a metallic reflector concentrates and directs the MW beam. No physical connection is therefore necessary between the lamp and the MW generator, which makes it much easier to assemble. The optimum position for the antenna is perpendicular to the hollow cathode axis; this directs the MW beam towards the middle of the glass chamber. The experimental set up is shown in Fig. 3 and Table I lists the materials investigated. Details on instrumentation and working conditions are extensively reported in previous papers6s7. The working conditions were optimized for each combination of sample and discharge gas. Preliminary experiments showed that the otput stability and reproducibility of the MW generator are optimal at 200 W (nominal value). This power was therefore adopted for all measurements. The procedure chosen E
-G
Fig. 3. Schematic diagram of the instrumentation. (A) hollow cathode; (B) water-cooled cathodic plate; (C) glass chamber; (0) water-cooled anodic block; (E) connection to vacuum spectrograph; (F) magnesium fluoride window; (G) argon supply; (H) antenna with reflector; (I) MWgenerator; (J) current-stabilized electrical unit; (L) connections to pumping systems.
trends in analytical chemistry, vol. 1, no. l& 1982
Fig. 4. Normalized blackening variations in the microwave-coupled hollow cathode discharge as afunction of the normalizedpower ratio. (a) Steel cathodes, argon 690 Pa. 0, Fe(I) 304.760 nm; 0, Fe(II) 238.204 nm; V, Cr(I) 428.972 nm; & Cr(II) 286.511 nm; x, Ni(I) 305.082 nm; 0, Ni(II) 226.446 nm; l , Ar(II) 303.352 nm; A, As(I), 338.835nm. (b) Molybdenum cathodes, argon 690 Pa.0, MO(I) 379.825 nm; a, Mo(II) 281.615nm; 0, MO 280.775nm; l , Ar(II) 303.352 nm; A, Ar(I) 338.835 nm.
TABLE
II. Oualitative
Cathodic Material
Wavelength (nm)
Aluminium
Copper
Graphite
(I) (I) (I) (I)
257.510 265.249 266.039 394.403 396.153
(II) (II) (I) (I) (I) (I)
221.026 224.261 224.699 299.736 301.084 327.982
(I) (II)
247.857 392.068
(II) (II) (I) (I)
280.775 281.615 284.823 379.825 386.411
(I) (II) (II) (I) (I) (I) (I) (I) (I)
213.856 250.200 255.796 280.087 307.590 328.233 330.259 334.502 334.572
Molybdenum
Zinc
behavior
of spectral
Blackening Variation in Argon=
lines emitted Blackening Variation in Heliuma
+ ) 0” +,-c -I- ) .- (
0 0 0 0 0
-
)
Ob
0,
-e
0,
+ + +
Ob
)
Iron (in steel)
0
+ +
+
Od
)
-e -e
0 0 0 0 + , Ob
+ Ob )
0 0 0 +
)
Cathodic Material
-,Od - ,Od
+
0,
by a hollow cathode
Ob
0 0 0 0 0 0 0
Chromium (in steel)
Nickel (in steel)
under the effect of a superimposed Wavelength (nm)
Blackening Variation in Argo@
(II) (II) (II) (II) (II) (II) (II) (I) (I) (I) (I) (I) (I)
235.191 236.483 238.204 239.562 240.488 241.052 241.331 284.398 302.064 302.403 304.760 305.909 344.388
+ + + +
(II) (II) (I) (I) (I) (I)
286.257 286.511 302.156 302.435 427.480 428.972
+ + + + + +
(II) (II) (I) (I) (I) (I) (I) (I) (I) (I)
226.446 227.877 300.363 305.082 344.626 345.289 345.847 346.165 347.254 349.295
+ + + + + + + + + +
MW-field Blackening Variation in Heliuma
0 0 0
+ + +,-c
+ + +
0 0 0 0 0
0 0
a Symbols + and - are attributed to blackening variations exceeding + 10% and - lo%, respectively. 0 is assigned for smaller variations. b Positive up to 100 mA, then practically unaffected; c positive up to 100 mA, then progressively negative; d negative up to 100 mA, then practically unaffected; e largely negative, except for current intensity values lower than 25-50 mA.
371
trend;in analytical chemistry, vol. I, no. 15, 1982
to ascertain the effect of MW irradiation on the hollow cathode discharge consists of taking exposures in duplicate for a given sample under a given set of working conditions, both with and without the MW field. This allows a direct comparison of the variations in emission intensity when passing from one excitation mode to the other. Since, in emission spectroscopy, a photographic emulsion is used as the radiation receiver, emission intensity is measured through the effect caused by the emitted radiation on the silver halide grains. After development of the emulsion spectra appear as a series of lines which are blackened to varying degrees. Blackening is easily correlated to radiation intensity by means of the so-called densitometric curve.
Comparison of the two operation modes Argon systems The superimposition of the MW field on the discharge of the materials which formed the hollow cathode itself or which were contained therein invariably produced a marked effect. This took the form, in most cases, of a large increase (as great as one order of magnitude) in the spectral lines, accompanied by a simultaneous decrease in the emission output of the carrier gas. This was generally true when current flowing through the lamp did not exceed = 100 mA. At higher values the effect of the MW irradiation progressively diminished. There are, however, exceptions to this general pattern of behaviour, consequently no simple rules can be deduced. The extent of the MW effect is set out qualitatively in Tables II, III and IV. As stated previously, the diathermy unit employed in this study enabled a reliable application only at a fured value of 200 W. It was therefore impossible to verify experimentally whether an increase in the MW output proportional to that in the electrical power supplied to
TABLE III. Qualitative behavior of spectral lines emitted by solution residues in a hollow cathode with superimposed MW-field Element
Aluminium
(as nitrate)
Arsenic
(as pentoxide)
Copper
(as nitrate)
Lead
(as nitrate)
Zinc (as nitrate)
Wavelength
(nm)
Blackening Variation in Argon=
(I) (I)
394.403 396.152
+ +
(I) (I)
228.812 234.984
+ , -b +,-b
(II
) -b , .-b
) 224.699 276.637
+ +
) 220.350 368.347 465.782
+ + +
328.233 334.502
+ +
to blackening variations a Symbols + and ‘- are attributed exceeding + 10% and .-lo%, respectively. b Positive for the first 2 min of discharge, then progressively negative.
TABLE carrier
Carrier
IV. Qualitative behavior of spectral lines emitted by gas in hollow cathode discharge with superimposed MW-field
Gas
Wavelength
(nm)
Blackening Variationa
235.760 279.545 279.665 302.675
Argon
_ .._ _ _ -
(ID302.893 303.320 (ID303.352 306.094 337.646 (I)
Helium
338.835 393.124
-
, Ob
(II) 273.332 (I) 276.380 (I) 294.510 (II) 320.314 (I) 344.759 (I) 361.364
+ + + + + +
, Ob , Ob ) Ob ) Ob , Ob ) Ob
, Ob
a Symbols
+ and .- are attributed to blackening variations exceeding + 10% and - 1096, respectively. 0 is assigned for smaller variations. b Depending on the nature of the cathodic material.
the discharge tube would produce a constant effect on the total emission. In order to overcome this practical limitation the ratio n S/S (where nS is the blackening difference between the discharge in the presence of MW irradiation and in its absence, and St is the blackening with MW superposition) was plotted as a function of the ratio of the MW power to the total lamp power input (i.e. electrical and MW irradiation). On the basis of these normalized ratios it could be established that in all instances the diminution of MW influence is attributable to a decrease in the MW power. Power ratio dependence of the normalized blackening ratios is shown for a few spectral lines in Fig. 4. Results also show that the higher the carrier gas pressure, the more pronounced is the effect on the spectral emission of the cathodic material. A similar pattern is observed, although sometimes less markedly, in all the spectral lines investigated for cathodic material. Analogous results were obtained for solutions. Blackening vs. time curves indicated clearly that the general pattern already observed for elements discharged as massive electrodes does not change substantially if they are introduced as solutions into the electrode cavity and dried therein. An example of such curves is given in Fig. 5. For samples contained within the hollow cathode the analyte signal is generally improved throughout the duration of the discharge. This is an important prerequisite for performing analytical determinations of trace elements with better detection limits, as future investigations will no doubt demonstrate.
trendsin analyticalchemistry,vol. 1, no. 13, 1982
372 AS
Helium systems In contrast to the general behavior observed in argon, in helium the emission of the cathodic material or carrier gas was scarcely affected by the presence of the MW field. There is a slight overall variation of blackening for every species studied, but this is usually almost negligible. The variation can be either positive or negative and no particular trend can be identified for emission from the hollow cathode. Blackening of helium spectral lines is, on the other hand, always slightly more positive (about 5-10%). Fig. 6 illustrates one of the cases where the increase in blackening is most manifest. In the case of solutions, preliminary results have shown that if any increase in emission does take place, it is almost undetectable. No systematic investigation in helium has therefore been carried out.
A possible interpretation A thorough evaluation of experimental data reveals some general trends which can be summarized as follows for systems involving argon as the carrier gav (i) coupling of a MW field to a hollow cathode discharge results in two distinct behaviors which are characteristic of the cathodic material and the noble gas used and have no particular dependence on wavelength or atomic state (either for neutral or for ionized species); (ii) the increase in emission intensity, measured as normalized blackening ratios, is positive for all the cathodic samples studied, using up to cu. 100 mA current intensity (roughly corresponding to l/(nPlPt = 1.2), and remains positive for higher current values for many elements, as shown in Table II. Moreover, a steady state appears to occur in most instances, so that 5 0.6 i
O.?-
0.6-
0.3 I 0.2 0.1 1 0.0
II 0123456789
t (min)
Fig. 5. Blackening v. time curvesfor zinc in steel hollow cathodes. 50 pg zinc (as nitrate), 100 mA, Argon 690 Pa. Solid line: with MWfLeld; broken line: without MWjTeld.
St 0.5
1
\
0.4 0.3
0.2 Ii
----------&,
yj 1.0
1:1
Fig. 6. Normalized blackening variations in the microwave-coupled hollow cathode discharge as a function of the normalized power ratio. Molybdenum cathodes, helium 440 Pa. 0, MO(I) 386.411 nm; l , He(I)
276.380 nm.
a further increase in current intensity causes no further diminution in the overall MW effect; (iii) this increase in emission intensity due to the MW field occurs irrespective of whether the sample sputtered is the hollow cathode itself, elements present as minor constituents in the electrode, or solution residues contained therein; (iv) a qualitatively opposite behavior is shown by argon emission lines, for which there is almost always a decrease in intensity; (v) the spectral background, generally low in the hollow cathode discharge, is further decreased by the presence of the MW field, thus allowing better detection limits to be reached. As far as helium systems are concerned, it can be stated that: (i) with few exceptions there is no appreciable variation in emission intensity of either cathodic material or the filler gas; (ii) the effect, if any, tends to disappear at currents of around 100 mA, corresponding to an electrical power of between 2040 W; (iii) as in the case of argon systems, the above observations are independent of the wavelength used and the atomic and physical state of the sample. These findings indicate that there are clearly different reaction mechanisms in the two rare gases. This is also reflected by the differences in behavior observed when a MW field is superimposed. In the case of helium systems it can be deduced that the energetic contribution of microwaves is somehow undifferentiated, and simply reinforces the overall emission, particularly of the gas atoms. This phenomenon tends to become less noticeable as the percentage of MW power decreases. When, on the other hand, argon is employed the MW field causes an increase in the emission of cathodic material and a decrease in the emission of the carrier gas. This would suggest that the
trends % a&ytical
chemistry, vol. I, no. 15, 1982
density of argon metastable atoms is increased. Consequently, there is a greater transfer ofenergy from the gas to sputtered atoms. At the moment these are preliminary hypotheses which will need to be more adequately validated by further experiment. More conclusive information on the reaction mechanism will, hopefully, be available in the near future. For the present, we can take advantage of this simple method of enhancing emission from a hollow cathode. As demonstrated above, the increase in emission using argon as the carrier gas can be as great as one order of magnitude, especially at current intensities not exceeding 100 mA (i.e. under conditions of optimal stability of discharge). This, together with the decreased spectral background, should make it much easier to determine trace and sub-trace elements in a variety of matrices, particularly those of biological origin.
References 1 2 3 4
Paschen, F. (1916) Ann. Physik (IV) 50, 901 Grimm, W. (1967) Naturwissenschaften 54, 588 Grimm, W. (1968) Spectrochim. Acta 23 B, 443 Rudnevskii, N. K., Maksimov, D. E. and Lazareva, L. P. (1974) Zh. Anal, Khim. 29, 1422 5 Eichhoff, H. J. (1964) Rozpravy Narod. Techn. Muz. Praze Fys. Chem. Rada 13, 7 6 Caroli, S., Alimonti, A. and Petrucci, F. (1982) Anal. Chim. Acta, 136, 269 7 Caroli, S., Alimonti, A. and Senofonte, 0. (1980) Spectrosc. Lett. 13, 302
373 Dr S. Caroli obtained his Ph.D. in 1968at the University of Rome. Dr Caroli has since that date carried out his activities at the Zstituto Superiore di San& (National Institute for Health), Laboratorio di Tossicologia, Viale Regina Elena 299, 00161 Rome, Italy, where he is at present Head Researcher. His research interests are in physical chemistry and emission spectroscopy using low pressure radiation sources, with particular reference to trace analysis in biological materials. Dr A. Alimonti obtained his Ph.D. in 1978 at the University of Rome and since that date has been working at the Laboratory of Toxicology of the Istituto Superiore di Sanita. His main activities are development of new analytical techniques in emission spectroscopy and atomic absorption spectrometry, and the evaluation of the characteristics of dangerous substances. Dr F. Petrucci took his degree in Biological Sciences at the University of Rome in 1980. His research interests are directed to the application of sputtering sources in emission spectroscopyfor the analysis of trace elements in biological materials.
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368
The microwave-coupled novel radiation source
hollow in emission
cathode - a spectroscopy
The development of new excitation sources accounts for the renaissance during the 1970s of emission spectroscopy as a powerful analytical tool. Among these, the hollow cathode discharge is still expanding the scope of its applications. S. Caroli, A. Alimonti and F. Petrucci Rome, Italy Several decades have elapsed since Paschen first described the use of a low pressure emission source named the hollow cathode’. Although interest in this field was kept alive by a number of significant contributions it was not until the late 1950s that the hollow cathode gained wide popularity, when it made possible the introduction and development of atomic absorption and fluorescence techniques. It is worth noting that, rather paradoxically, the peculiar properties of the hollow cathode were generally recognized, not for their direct applicability in emission spectroscopy, but only as an auxiliary to other methods. However, the last 15 years have seen a steady increase in the number of papers which testify to a renewed interest in this type of excitation source. This can largely be ascribed to Grimm’s introduction2*3 of a combined hollow cathode and glow discharge lamp, particularly suitable for the analysis of solid, electrically conducting samples. This encouraged further research, often with remarkably successful results. The glow discharge and hollow cathode have some fundamental features in common. However, the Grimm lamp, operating in the obstructed glow is now well established and is discharge mode, currently employed in various fields, particularly metallurgical analysis. This is not the case for the hollow cathode, though this discharge lends itself to a wider variety of analytical applications for gaseous, liquid, solid and powdered samples. Despite its namely high stability and intrinsic advantages, reproducibility, sharp spectral lines which are almost unaffected by self-absorption, low background, negligible Doppler and Stark effects, as well as reduced matrix and third element effects, this excitation source has not yet received the attention it deserves. The complex operating conditions required for the lamp, involving the use of high purity noble gases at reduced pressure, can only partially account for this. Another limiting factor is that the intensity of spectra emitted by the hollow cathode is, as a rule, lower than those obtained with other sources, especially in the case of ICP torches. This makes it inconvenient for sub-trace analysis. A number of workers have devoted considerable 0 165-9936/82/CKlOOX1OWxwx)/$o1.oo
efforts to solving this problem. They have endeavoured to develop new versions of the hollow cathode lamp capable of higher efliciency. Examples of this are the magnetic field-coupled tube devised by Rudnevskii et aL4, and the transitional type proposed by Eichhoff”. We have recently6 investigated the possibility of superimposing microwave irradiation on a hollow cathode discharge. The results obtained have been highly promising. Although none of the hollow cathode properties were altered, the intensity ofspectra emitted by the cathodic material increased markedly while that of the filler gas was lower.
Processesoccurring in the lamp The essential aspects of the mechanism of the hollow cathode discharge are well known. Very briefly, the electrons emitted by the cathodic inner surface, under the applied dc voltage difference, undergo acceleration in the Hittorf dark space within the cavity (the Hittorf dark space is the region where the voltage drop is almost completely localized. It develops between the luminescent layer on the cathodic surface and the negative glow, i.e. the region where emission occurs. In a hollow cathode the dark space is annular, as shown in Fig. 1.) Further electrons are generated by collision with the carrier gas atoms until a state ofequilibrium is attained between atomic and ionic species. This is a result of the oscillating movement of the electrons from wall to wall. The positively charged ions ofthe filler gas
Cathode ,/
-,’
Luminescent cathodic layer
Hittorf dark space
Negative
Fig. 1. Cross section of a hollow cathode showing the various characteristic zones. @
1982 Ekvier
Scientific
Publishing
Company
trends m analytical chemistry, vol. 1, no. 15, 1982
369 TABLE
I. Cathodic
discharge
Cathodic
. 0
ekctron targetatom
0
target
atom ( ionized
)
0
target
atom (excited
)
0 Q 0
rare
(neutral
materials
in the MW-coupled
and solutions
subjected
hollow cathode
Material
to
lamp
Solution
Aluminium
Zinc
Copper
Iron
Graphite
Chromium (in Steel)
Molybdenum
Nickel
(in Steel)
Copper Nitrate
Arsenic Pentoxide
Zinc Nitrate
Aluminium Nitrate
Lead Nitrate
(in Steel)
)
gas atom (neutral)
rare
gasatom
rare
gas atom (metastable
(ionized) )
Fig. 2. Surface of a solid sample sputtered under ion bombardment.
are attracted by the cathode, onto which they impact, thus extracting particles from the surface. The sputtered material enters the discharge zone, is excited by collision with electrons and gas atoms to emit its characteristic radiation and finally redeposit. Since it only escapes with difficulty from the bore, the process can be repeated. There are a number of variations of this general mechanism and these depend on the type of noble gas used for the discharge. Because of its considerable atomic weight and ionization potential, argon is most commonly employed. In this case metastable atoms with rather long lifetimes (= 10-3s, i.e. lo5 times that of other excited states) play an essential role in the excitation process. Processes occurring on the cathodic surface are depicted in a simplified way in Fig. 2. Microwave (MW) excitation is a consequence of the highly energetic motion imparted to atoms in the gaseous phase. In practice, this means that the electrodeless discharge works well with elements of relatively low vapour pressure at the operating temperature of the MW field (400-500°C). It seems logical, therefore, to assume that the hollow cathode and MW excitation processes should mutually reinforce one another if performed simultaneously. Material sputtered from the cathode is available for further excitation by the MW field, or, to look at it another way, the MW field can exert its influence on a greater number of sample particles. However, in practice it can be assumed that the two phenomena are not simply superimposed, but contribute to an overall more complex process. The analytical implications of the above observations are of paramount importance and have been thoroughly investigated.
The composite discharge tube The practical realization of the MW-coupled hollow cathode lamp was achieved by modifying a commercially available Grimm’s glow discharge device as
outlined in previous paper?,‘. Basically, the modifications consist of separating the anodic and cathodic blocks of the lamp by means of a cylindrical glass chamber through which MW irradiation could be adequately transmitted into the discharge zone. The MW source used was a diathermy unit (capable of generating heat in body tissue by means of microwave radiation) produced by Bosch (FRG). This delivers a MW frequency of 2450 MHz with power adjustable between O-230 W. A simple antenna with a metallic reflector concentrates and directs the MW beam. No physical connection is therefore necessary between the lamp and the MW generator, which makes it much easier to assemble. The optimum position for the antenna is perpendicular to the hollow cathode axis; this directs the MW beam towards the middle of the glass chamber. The experimental set up is shown in Fig. 3 and Table I lists the materials investigated. Details on instrumentation and working conditions are extensively reported in previous papers6s7. The working conditions were optimized for each combination of sample and discharge gas. Preliminary experiments showed that the otput stability and reproducibility of the MW generator are optimal at 200 W (nominal value). This power was therefore adopted for all measurements. The procedure chosen E
-G
Fig. 3. Schematic diagram of the instrumentation. (A) hollow cathode; (B) water-cooled cathodic plate; (C) glass chamber; (0) water-cooled anodic block; (E) connection to vacuum spectrograph; (F) magnesium fluoride window; (G) argon supply; (H) antenna with reflector; (I) MWgenerator; (J) current-stabilized electrical unit; (L) connections to pumping systems.
trends in analytical chemistry, vol. 1, no. l& 1982
Fig. 4. Normalized blackening variations in the microwave-coupled hollow cathode discharge as afunction of the normalizedpower ratio. (a) Steel cathodes, argon 690 Pa. 0, Fe(I) 304.760 nm; 0, Fe(II) 238.204 nm; V, Cr(I) 428.972 nm; & Cr(II) 286.511 nm; x, Ni(I) 305.082 nm; 0, Ni(II) 226.446 nm; l , Ar(II) 303.352 nm; A, As(I), 338.835nm. (b) Molybdenum cathodes, argon 690 Pa.0, MO(I) 379.825 nm; a, Mo(II) 281.615nm; 0, MO 280.775nm; l , Ar(II) 303.352 nm; A, Ar(I) 338.835 nm.
TABLE
II. Oualitative
Cathodic Material
Wavelength (nm)
Aluminium
Copper
Graphite
(I) (I) (I) (I)
257.510 265.249 266.039 394.403 396.153
(II) (II) (I) (I) (I) (I)
221.026 224.261 224.699 299.736 301.084 327.982
(I) (II)
247.857 392.068
(II) (II) (I) (I)
280.775 281.615 284.823 379.825 386.411
(I) (II) (II) (I) (I) (I) (I) (I) (I)
213.856 250.200 255.796 280.087 307.590 328.233 330.259 334.502 334.572
Molybdenum
Zinc
behavior
of spectral
Blackening Variation in Argon=
lines emitted Blackening Variation in Heliuma
+ ) 0” +,-c -I- ) .- (
0 0 0 0 0
-
)
Ob
0,
-e
0,
+ + +
Ob
)
Iron (in steel)
0
+ +
+
Od
)
-e -e
0 0 0 0 + , Ob
+ Ob )
0 0 0 +
)
Cathodic Material
-,Od - ,Od
+
0,
by a hollow cathode
Ob
0 0 0 0 0 0 0
Chromium (in steel)
Nickel (in steel)
under the effect of a superimposed Wavelength (nm)
Blackening Variation in Argo@
(II) (II) (II) (II) (II) (II) (II) (I) (I) (I) (I) (I) (I)
235.191 236.483 238.204 239.562 240.488 241.052 241.331 284.398 302.064 302.403 304.760 305.909 344.388
+ + + +
(II) (II) (I) (I) (I) (I)
286.257 286.511 302.156 302.435 427.480 428.972
+ + + + + +
(II) (II) (I) (I) (I) (I) (I) (I) (I) (I)
226.446 227.877 300.363 305.082 344.626 345.289 345.847 346.165 347.254 349.295
+ + + + + + + + + +
MW-field Blackening Variation in Heliuma
0 0 0
+ + +,-c
+ + +
0 0 0 0 0
0 0
a Symbols + and - are attributed to blackening variations exceeding + 10% and - lo%, respectively. 0 is assigned for smaller variations. b Positive up to 100 mA, then practically unaffected; c positive up to 100 mA, then progressively negative; d negative up to 100 mA, then practically unaffected; e largely negative, except for current intensity values lower than 25-50 mA.
371
trend;in analytical chemistry, vol. I, no. 15, 1982
to ascertain the effect of MW irradiation on the hollow cathode discharge consists of taking exposures in duplicate for a given sample under a given set of working conditions, both with and without the MW field. This allows a direct comparison of the variations in emission intensity when passing from one excitation mode to the other. Since, in emission spectroscopy, a photographic emulsion is used as the radiation receiver, emission intensity is measured through the effect caused by the emitted radiation on the silver halide grains. After development of the emulsion spectra appear as a series of lines which are blackened to varying degrees. Blackening is easily correlated to radiation intensity by means of the so-called densitometric curve.
Comparison of the two operation modes Argon systems The superimposition of the MW field on the discharge of the materials which formed the hollow cathode itself or which were contained therein invariably produced a marked effect. This took the form, in most cases, of a large increase (as great as one order of magnitude) in the spectral lines, accompanied by a simultaneous decrease in the emission output of the carrier gas. This was generally true when current flowing through the lamp did not exceed = 100 mA. At higher values the effect of the MW irradiation progressively diminished. There are, however, exceptions to this general pattern of behaviour, consequently no simple rules can be deduced. The extent of the MW effect is set out qualitatively in Tables II, III and IV. As stated previously, the diathermy unit employed in this study enabled a reliable application only at a fured value of 200 W. It was therefore impossible to verify experimentally whether an increase in the MW output proportional to that in the electrical power supplied to
TABLE III. Qualitative behavior of spectral lines emitted by solution residues in a hollow cathode with superimposed MW-field Element
Aluminium
(as nitrate)
Arsenic
(as pentoxide)
Copper
(as nitrate)
Lead
(as nitrate)
Zinc (as nitrate)
Wavelength
(nm)
Blackening Variation in Argon=
(I) (I)
394.403 396.152
+ +
(I) (I)
228.812 234.984
+ , -b +,-b
(II
) -b , .-b
) 224.699 276.637
+ +
) 220.350 368.347 465.782
+ + +
328.233 334.502
+ +
to blackening variations a Symbols + and ‘- are attributed exceeding + 10% and .-lo%, respectively. b Positive for the first 2 min of discharge, then progressively negative.
TABLE carrier
Carrier
IV. Qualitative behavior of spectral lines emitted by gas in hollow cathode discharge with superimposed MW-field
Gas
Wavelength
(nm)
Blackening Variationa
235.760 279.545 279.665 302.675
Argon
_ .._ _ _ -
(ID302.893 303.320 (ID303.352 306.094 337.646 (I)
Helium
338.835 393.124
-
, Ob
(II) 273.332 (I) 276.380 (I) 294.510 (II) 320.314 (I) 344.759 (I) 361.364
+ + + + + +
, Ob , Ob ) Ob ) Ob , Ob ) Ob
, Ob
a Symbols
+ and .- are attributed to blackening variations exceeding + 10% and - 1096, respectively. 0 is assigned for smaller variations. b Depending on the nature of the cathodic material.
the discharge tube would produce a constant effect on the total emission. In order to overcome this practical limitation the ratio n S/S (where nS is the blackening difference between the discharge in the presence of MW irradiation and in its absence, and St is the blackening with MW superposition) was plotted as a function of the ratio of the MW power to the total lamp power input (i.e. electrical and MW irradiation). On the basis of these normalized ratios it could be established that in all instances the diminution of MW influence is attributable to a decrease in the MW power. Power ratio dependence of the normalized blackening ratios is shown for a few spectral lines in Fig. 4. Results also show that the higher the carrier gas pressure, the more pronounced is the effect on the spectral emission of the cathodic material. A similar pattern is observed, although sometimes less markedly, in all the spectral lines investigated for cathodic material. Analogous results were obtained for solutions. Blackening vs. time curves indicated clearly that the general pattern already observed for elements discharged as massive electrodes does not change substantially if they are introduced as solutions into the electrode cavity and dried therein. An example of such curves is given in Fig. 5. For samples contained within the hollow cathode the analyte signal is generally improved throughout the duration of the discharge. This is an important prerequisite for performing analytical determinations of trace elements with better detection limits, as future investigations will no doubt demonstrate.
trendsin analyticalchemistry,vol. 1, no. 13, 1982
372 AS
Helium systems In contrast to the general behavior observed in argon, in helium the emission of the cathodic material or carrier gas was scarcely affected by the presence of the MW field. There is a slight overall variation of blackening for every species studied, but this is usually almost negligible. The variation can be either positive or negative and no particular trend can be identified for emission from the hollow cathode. Blackening of helium spectral lines is, on the other hand, always slightly more positive (about 5-10%). Fig. 6 illustrates one of the cases where the increase in blackening is most manifest. In the case of solutions, preliminary results have shown that if any increase in emission does take place, it is almost undetectable. No systematic investigation in helium has therefore been carried out.
A possible interpretation A thorough evaluation of experimental data reveals some general trends which can be summarized as follows for systems involving argon as the carrier gav (i) coupling of a MW field to a hollow cathode discharge results in two distinct behaviors which are characteristic of the cathodic material and the noble gas used and have no particular dependence on wavelength or atomic state (either for neutral or for ionized species); (ii) the increase in emission intensity, measured as normalized blackening ratios, is positive for all the cathodic samples studied, using up to cu. 100 mA current intensity (roughly corresponding to l/(nPlPt = 1.2), and remains positive for higher current values for many elements, as shown in Table II. Moreover, a steady state appears to occur in most instances, so that 5 0.6 i
O.?-
0.6-
0.3 I 0.2 0.1 1 0.0
II 0123456789
t (min)
Fig. 5. Blackening v. time curvesfor zinc in steel hollow cathodes. 50 pg zinc (as nitrate), 100 mA, Argon 690 Pa. Solid line: with MWfLeld; broken line: without MWjTeld.
St 0.5
1
\
0.4 0.3
0.2 Ii
----------&,
yj 1.0
1:1
Fig. 6. Normalized blackening variations in the microwave-coupled hollow cathode discharge as a function of the normalized power ratio. Molybdenum cathodes, helium 440 Pa. 0, MO(I) 386.411 nm; l , He(I)
276.380 nm.
a further increase in current intensity causes no further diminution in the overall MW effect; (iii) this increase in emission intensity due to the MW field occurs irrespective of whether the sample sputtered is the hollow cathode itself, elements present as minor constituents in the electrode, or solution residues contained therein; (iv) a qualitatively opposite behavior is shown by argon emission lines, for which there is almost always a decrease in intensity; (v) the spectral background, generally low in the hollow cathode discharge, is further decreased by the presence of the MW field, thus allowing better detection limits to be reached. As far as helium systems are concerned, it can be stated that: (i) with few exceptions there is no appreciable variation in emission intensity of either cathodic material or the filler gas; (ii) the effect, if any, tends to disappear at currents of around 100 mA, corresponding to an electrical power of between 2040 W; (iii) as in the case of argon systems, the above observations are independent of the wavelength used and the atomic and physical state of the sample. These findings indicate that there are clearly different reaction mechanisms in the two rare gases. This is also reflected by the differences in behavior observed when a MW field is superimposed. In the case of helium systems it can be deduced that the energetic contribution of microwaves is somehow undifferentiated, and simply reinforces the overall emission, particularly of the gas atoms. This phenomenon tends to become less noticeable as the percentage of MW power decreases. When, on the other hand, argon is employed the MW field causes an increase in the emission of cathodic material and a decrease in the emission of the carrier gas. This would suggest that the
trends % a&ytical
chemistry, vol. I, no. 15, 1982
density of argon metastable atoms is increased. Consequently, there is a greater transfer ofenergy from the gas to sputtered atoms. At the moment these are preliminary hypotheses which will need to be more adequately validated by further experiment. More conclusive information on the reaction mechanism will, hopefully, be available in the near future. For the present, we can take advantage of this simple method of enhancing emission from a hollow cathode. As demonstrated above, the increase in emission using argon as the carrier gas can be as great as one order of magnitude, especially at current intensities not exceeding 100 mA (i.e. under conditions of optimal stability of discharge). This, together with the decreased spectral background, should make it much easier to determine trace and sub-trace elements in a variety of matrices, particularly those of biological origin.
References 1 2 3 4
Paschen, F. (1916) Ann. Physik (IV) 50, 901 Grimm, W. (1967) Naturwissenschaften 54, 588 Grimm, W. (1968) Spectrochim. Acta 23 B, 443 Rudnevskii, N. K., Maksimov, D. E. and Lazareva, L. P. (1974) Zh. Anal, Khim. 29, 1422 5 Eichhoff, H. J. (1964) Rozpravy Narod. Techn. Muz. Praze Fys. Chem. Rada 13, 7 6 Caroli, S., Alimonti, A. and Petrucci, F. (1982) Anal. Chim. Acta, 136, 269 7 Caroli, S., Alimonti, A. and Senofonte, 0. (1980) Spectrosc. Lett. 13, 302
373 Dr S. Caroli obtained his Ph.D. in 1968at the University of Rome. Dr Caroli has since that date carried out his activities at the Zstituto Superiore di San& (National Institute for Health), Laboratorio di Tossicologia, Viale Regina Elena 299, 00161 Rome, Italy, where he is at present Head Researcher. His research interests are in physical chemistry and emission spectroscopy using low pressure radiation sources, with particular reference to trace analysis in biological materials. Dr A. Alimonti obtained his Ph.D. in 1978 at the University of Rome and since that date has been working at the Laboratory of Toxicology of the Istituto Superiore di Sanita. His main activities are development of new analytical techniques in emission spectroscopy and atomic absorption spectrometry, and the evaluation of the characteristics of dangerous substances. Dr F. Petrucci took his degree in Biological Sciences at the University of Rome in 1980. His research interests are directed to the application of sputtering sources in emission spectroscopyfor the analysis of trace elements in biological materials.
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