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Spectrochemical excitation with low-temperature arc
Spectrochfmiaa Acta,Vol. 25B.pp. 646to 658. PergamonPress1970.Printed1x1 NorthernIreland
~p~~he~~
excitation with io~-ternpe~~
arc
M. KLISKA and M. ~IXKOV~~ The Boris KidriE Institute of Nuclear &&noes, Beograd, Yugoslavia (Received 13 Jammy
1970)
Abstract_A eystematia investigation haa been made conaerning excitation featurea of the Iow-temperature arc. The following experimental variables have been studied in detail: buffer effect of several alkali eIementz at different concentrationz, orific size in stabilizing dizcz, flow rate of aerosol and the uze of organic aolventa. The obtained results ihuztrate the possibility and tlexibihty of the source and zuggezt the way of cthoozing optimum operating oondition.
low power plasma jets as excitation sources in spectroohemical suggested first by MAR~OSHESand SCRIBNER. [X] and rtlmost simultaneously by KOROLEV and VAISHTEIN[Z]. Since then, the interest for ttpplication of stabilized &rcs &s excitation sources has been growing continu&lly. A considerable number of sources of this type are described in the literature [3-61. In an earlier p&per [7] one of the present authors described & new version of & stabilized &rc of the &bove mentioned kind. The m&in new feature of this arc w&s the use of spectroohemical buffers for reducing and controIling the temperature of the arc column. For this re&son it w&s called the low-~mperature arc. This &rc h&s pro~rties which make it very suitable for use &s an excitation source for certain analytical de~rminations. The two most impo~ant ~h&r&cter~tics &re: (&) high stability of emission of spectral lines, and (b) low detection limits for & considertlble number of elements. In this paper the results of an investigation of the roles of the m&inexperiment&l variables of the new source will be presented. The aim w&s to test the anzllytical possibilities of the source and to suggest w&ya for selecting optimum working conditions. In the course of our experimental work the influence of source geometry, &ro current, kind and concentration of buffer, aerosol flow rate, &nd use of organic solvents h&s been investigated. THE
USE
analysis
of
has
been
The &rc, &heady mentioned and described elsewhere 171,was modified in such & way that it could be operated with different openings for the introduction of the aerosol in the central segment. The solutions are spr&yed into the argon stream by means of&n angle type indirect sprayer. This type of sprayer is more suitable than the concentric one because clogging troubles c&n be &voided. The argon [l] M. MARoos~sos and B. F. SCIUBNER,Spectrochim. Acta 16,138 (1969). 123V. V. KOROLEV and E. E. V~SWJZIN, 2. And Khim. 14,668(1969).
[37 L. E. OWEN,Appt. Spectry. 16,160 (1961). [4] M. RIE-, 2. Anab. C-&m. 215,407 (1966). [6f E. Ho-
and G. HOLDT, iktmtaber. Deb
Akad. W&s. Be&m 7, 693 (1965). 308 (1963). Aeta 28B, 267 (1968).
[s] E. EXZANZ, .&fomt&e~. De&. Akad. W&w. Bed& 6, [7] M. M,UINEO~I~ snd B. DIXITIWE~I~, S~e~oc~~~. s
646
flow rate was normally kept at 24.3l/min* and the spray rate was appro~ately f-4 ml/m& with an &ciency of 345%. It was found that this technique permitted the addition of buffer elements to the aerosol at mole fractions up to 6, UP* by means of spraying solutions contain~g the respective element up to 1*5 molar concentration.
Stobiliting Inner
Fig.
discs
zone
1, Thi3 appearanceof the anaIytical~frrp.Spmyed liquide: (a) pure w&a-, (bf, (of, and (d) eok~tionswith graduafly inamedl potassium oonoentrstion.
1, The use of elements which have low ionization potential as s~ctroscopic buffers is essential for the investigated are,These elements have a con~derable inffuence on both the spectral emission and the visual appearance of the arc oolumn. When pure water is sprayed, a narrow and very bright column is formed (Rg. 1(a)), accompanied by an emission of argon, hydrogen and oxygen lines and an intense continuous spectrum. On spraying a solution containing a suitable amount of an element of low ioniaation potential two distinct zones in the arc column can be observed (Fig. 1 (b)), a bright axial zone slightly narrower than the one of Fig. 1(a), whose width decreases with buffer concentration and a wide periphery zone whose brightness depends upon the kind of the buffer employed. At a certain buffer * Except in Section 5.
Spectrochemical
excitetion with low-temperature
src
547
concentration the axial zone breaks off in two parts, Fig. 1 (c). A further increase results in the shrinking of the remaining part of the axial zone, causing eventually its complete disappearance from the analytical gap, Fig. l(d). In this last case the lines of argon, hydrogen, oxygen, and the intense background (emitted in the axial zone) disappear, leaving in the spectrum only potassium lines and OH bands. Following the notation introduced in the earlier paper [7], the arcs as characterized
4 c
Argon
in
Fig. 2. Stabilizedam. 1, anode; 2,3,4,6,7, and 8, the metal segments, electrioaUy insulated from one enother and water cooled; 5, the central segment, not water cooled; 9, cathode. The am burns in the axial channel which consists of openings on eaoh segment. The analytical gap is between segments 6 and 6.
Figs. 1 (a) and 1 (b) will be referred to as the argon arc (AA) and that in Fig. 1 (d) as the low temperature arc (LTA). The minimum concentration of buffer required for obtaining the LTA is an impo~ant parameter for ana~ytica1 applications of this source. A ~o~~pon~g attention has been paid to it t~oughout this paper. The considerable differences in the spectrum of AA and LTA are demonstrated by Figs. 3 and 4 respectively. These spectra have been obtained with the glass prism monocbromator of a Hilger Uvispek spectrophotometer, in the region of 390-620 nm. The wave lengths were taken from reference [8]. in
2. Injluence of the opedngs of the stabilizing segmem% By means of the segments numbered 4 and 6, Fig. 1, the are column is stabilized. However, the sizes of these openings are not important for the characteristics of [S] A. R. STRIQMOV and N. S. SVENTWKII, T&z@ rstmm.
Atomizdat,
Bloskva (1966).
q~kt7aZ’ gh Zinii ~~tr~~yh
i s-okwvasg&
M.
KLJSKA
and M.
&fARINKOVI6
‘r
ti:391.4.
r
Call 393.37
*r
\rr
Call 396.85
t
-
I( 404..1 I( 404.72
394.75 3e4.90
n 337.00
:----
k ,cw
-CN -CI
-cw
415.24 415.01 416.78 418.10
t
I-
Fig. 3. Recorder tracing of the spectrum of the AA; pure water sprayed; arc ourrent: 6 amp.
Fig. 4. Recorder tracing of the spectrum of the LTA; potassium chloride of O-6M sprayed; arc current: 3 amp.
Spectroahemical exoit&ion with low-temperature are
MB
th% c?entraI pastof the EbrG column. On the other hand, the sixes of the holes in the centrsl segment numbered 5, Fig. 2, eonsiderabfy inffnenee the char~ter of the are discharge, especiaiiy in the wrt;vbuffer elements of low ionization potentials act. These holes have been varied from 3 to 12 mm dia. by inserting Werent graphite rings. The dependence of the minimum concentration of potasdum required for obt&ning LTA on the sizes of these openings and on the arc ourrent has been investig&ed, The results are shown in Fig. 6 in which the minimum mole
5
6
Current, amp Fig+ 5. Dqwndenoe of the miaimmn mole fraction of potassium in the swoaoXfar obtaining lJ?A on tie MOcurrentand opetigs of cent& eegment. Numerator denotea &m&ax of the upper openingand denominator denotes,diameter of the lower opening of the centrfxlsegment, in millimeters.
fraction of pottassium in the aerosool sufficient to obt&n LTA are plotted versus &POeurrent with hole d&meters as parameters, It csn be seen that larger buffer ~n#n~~tio~s correspond to sm& &meters of the openings. It is remarkable that the size of the upper opening is much more critic&. At IBgiven #~~~~~tion of potassium transition from AA to LTA occurs & partieufar v&e of the arc ourrent. This v&e is usually the same regardless of whether the transition is obtained by increasing or decreasing current. The exception is at low current and small openings of the central segment. In that o&se two values are observed one with increasing and another with decreasing arc current. Dashed lines have been obtained by increasing the arc current, Large openings in the central segment are generally preferable sinae they permit I (a) obtaining LTA with lower buffer concentrations and (b} long continuous operation of the arc before memory effe&s beoome significrant. For these reasons the experiments have been ctied out with the upper opening of 12 mm Itncithe lower opening of 10 mm.
Besides potassium, other elements which h&ve low ionization potentials could be used as spectrascopic buffers. With the exception of cesium, all alkali metals hftve therefore been examined experiment&y as spectroscopic buffers. It was found convenient to use them as chlorides. Again the minimum mole fraction for each of the examined met& sufficient to obtain LTA, for a given arc current was determined. The results for the different buffers are presented grrtphiaally in Fig. 6. It can be seen that for a complete suppression of the argon spectrumI i,e. for obtaining LTA, the required concentration of a buffer element is higher for
a
5
Current,
Fig. B, Depen&noe of the miniium
amp
mole fraction of alkali met& required fQr obtaining the LTA. on MO current.
in
elements with higher ionization potentials and it also increases with the &ro current. For ans,lytieaJ ~pp~~~tio~ it is important to know the relation between the kind and ~on~ntr~tion of a buffer element and the ~~sities of emission of speetrsl lines and that of the background. As sn example the infiuenoe of two of the most common buffer elements, e.g. potassium and sodium, on emission of aluminium and lithium has been studied, For these measurements the two W&S focused on the oollimator lens of the monechromator, Consequently, light from the whole analytical gap was conducted to the photosensitive area, of the photomultiplier. The results &re represented graphically in Figs. 7 and 8. Here, the intensities of aluminium and lithium lines, in arbitrary units, but obtained under identical conditions, are plotted versus concentrations of buffer elements. As ortn been seen the influence of the buffer element is very significant. Maximum emission occurs in the vicrinity of the ~n~ntr~tio~ at which the argon spectrum is just completely suppressed. At the same eoncentmtion the background intensity is
Spmtmchemie~ excitation with low-~~p~t~
561
are
minimum, thus giving the best me-~-b&~k~ouud ratio for a&iwing low detection limits, Owing to the high spatial inhomugenitg of emission of AA, line-to-baGwound ratio is very dependent on the method of illumination of the monochromator slit. at a
0
2
4
6
Mole fractfon of bufferin aerosd x IO-
Sharp focusiug of the arc on the monoc~om~to~ slit by means of a single condenser lens gives a considerably smaller line to background r&o, especially for the AA, as compared to the ratio obtainable if the axrcco1um.nis focused upon the collimator mirror. In the former wse steeper curves than those in Figs. 7 and 8 will be obtaiued.
662 6.
M.Xkxsga and1yf.&%.mmx5vI6
Isji?lecenceof aerosol$ow rate
To investigate the influence of the aerosol flow rate on the intensities of spectral lines, a capillary flow meter has been designed, Fig. 9. This design of the flow meter almost completely eliminates aerosol losses. It functions as follows: from the sprayer, which operates with a constant argon flow rate, the aerosol enters at
Molefractionof buffeer in aerosol x 10-a Fig. 8. Intensities of the Li 6704 line and the background at 6704 nm, plotted ~48functions of the mole fraction of buffer in the zserosol, Line intensities for 6 “np am mx.&ipliedby 1% 0,s s.bmp,potasaiuntchIoride e, 3 smp, eodium &l&de V, 6 amp, potmsium chloride V$ 6 twq, aodium chloride.
A: part of it can escape via B the rest passes through the capillary C (4 2 x 30 mm) and enters chamber D. Turbulent motion of the gas in this chamber reduces the amount of aerosol which condenses on the chamber walls. The pressure in a waterfilled U-tube E is employed for the measurement of the flow rate. Aerosol losses are less than 10%. The intensities of lithium and aluminium lines against the aerosol flow rate are plotted in Fig. 10, For the sake of clearness of ~prese~ta~o~ the &f&rent curves are arbitrary displaced along the ordinate and can therefore not be compared with each other in respect to their absolute level. It can be seen that the maxima of the curves are situated between flow rates of 0%143 l/min. The aluminium curve exhibits a particularly sharp maximum. The concentration of
Spectrochemicd excitation with low-temperature are
663
the aerosol particles does not depend upon the rate of flow. Consequently, changes in the intensity of spectral lines can be explained in terms of the changes in excitation conditions and of the changes in time of residence of the aerosol particles in the arc column, i.e. changes in the degree of their evaporation. At low flow rates the aerosol particles spend longer times in the arc column, thus their evaporation is more complete. This could explain the increase of the intensity of lines at reasonably small flow rates, especially of aluminium which, after solvent evaporation, forms refractory aerosol particles. At flow rates smaller than l-2 l/min
To ox
-3 Fig. 9. Aerosol flow-meter.
the intensity of the lithium line decreases. This may be due to the air penetrating the arc column and diluting the aerosol, i.e. at low flow rates the arc column is not completely screened from the surrounding air. Since aluminium forms refractory particles, which for complete evaporation need a longer time of residence in the arc zone the aluminium curves reach maximum emission at smaller flow rates than the curves for lithium. If KCL is present in the sprayed solution the aerosol particles become larger, therefore the time for complete evaporation is longer. As a result the maxima are shifted towards smaller flow rates. 6. Effect of organic solvents The influence of organic solvents in flame photometry is well known. It results in an essential increase in the intensity of lines. In so far as the electric arcs are concerned, however data in the literature are quite controversial. This is probably due to considerable differences in design and characteristics of the applied arcs. For this reason it was decided to investigate the effect of organic solvents on the
554
M. KLISKA end Bd. MABINKOVI~
present souroe. Several different solvents, all mixable with water, have been examined. The presence of an organic solvent is found to increase the voltage drop between the electrodes. The concentration of an organio solvent in the aerosol which enters the are zone is proportional to: (a) its evaporability, (b) its
1
I
I
FIow rote of ~otrosol
I 2
0
1
I 4
I
I 6
, Vmin
Fig. IO. Intensities of Al 396.2 and Li 670-S lines plotted tssfun&ions of aerosol flow rate. o,AA,3amp V,AA,%-p 0, LTA (1 M KCl), 3 emp Y, LTA (1 M KCl), 6 amp.
ooncentration in the solution, and (c) the nebulizer efficiency. The results of measurements at two different currents are shown in Fig. 11, It can be seen that the voltage drops are higher at lower currents (the measurements were made at 3 amp and 6 amp, without KCL and with a O-5 H KCL solution). It is noteworthy that this ~~uence is strongest with acetone, which has the highest e~aporabi~ty of all examined solvents. As a consequence of spraying organic soivents there appear in the spectra moleoular band spectra, particularIy of C?, and CN. At low am-rents and high
~~~ntr&ti~ns af ~~si~rn 8 ~~ide~~ble number of these bzmds disr4ppaar,and the resuH5ng spectra are very simi1s.zto the ones produced by the LTA, If 8. sprayed solution cont&ns potrtssium, the presence of organic solvents greatly pareses the co~centr&tio~ of pot~sium in the arc, This, owing to the accompanying reduction of temperature, may reduce the intensity of some spectrsJ lines. The experimental results are shown in Pigs. 12 and 13.
Drsuossrax
The ~ro~~i~ of the described arc, as an e~~it&tion source in ~~~~~~~~~~~ polyps, could be vmied ~~sider&bly~ thaw to, p~rn~ily~ the ~pp~~tiou of of the MO column is high which ocuxbe s~~ros~opi~ btiers. ft the tern~r~t~ a&eyed if the bufFer ~on~ntr~tio~ is below a certain oritic&l vske, lines of high excitation potentials (e-g the lines of Ar, IX, 0, C, etttc.)asd a&o the lines of the ehments which form refractory oxide (e.g. Be, La, etc.) will be excited, If on the other hand the buflFerconcentration exceeds a certain critical value, the szc column tern~r~tu~ is co~ider&bly redumd, making the properties of the azo aim&r to those of some hot chemica;l fhmes. The buffer efficiency is good if the co~st~ctiou of the arc column is small, the excitation potential of the buffer low, amd the arc current small. Therefore, (a) wide openings on the central segment (ltbX2 mm) (b) bufFerelements of iow excitation po~nti&ls~ md fof small ~bllo currents (3-B &mpjt should preferably be used, The fluent of buEers on the emission of ~~~~~1 lines is ex~ptiuu~~y strongAs the buffer concentration incremes the line intensity at first increlases very shyly and then the rate of &ream slows down and ag& increa+sesbefore the
maximum is attained. On the other hand, the baakground intensity at first decreases and after passing through a minimum starts rising, The best line-to-background ratio is obtained with different buffer concentrations for different elements, depending primarily on the excitation potential
of the line and on the thermal stability of the corresponding oxide. For the determination of elements which form highly refractory oxides or which have analytical lines of high excitation potentials the AA mode is required. In determinations of elements which have medium or low excitation potentials and which do not form highly refractory oxides the beat me-to-ba~k~ound ratio is obtained in the vicinity of the minimum buffer ~on~ntra~o~ at which the LTA mode can be obtained. IIowever, for routine analysis it is advisabIe to use somewhat higher buffer concentrations {e.g. twice the minimum ~o~~~tration~ because of a better stability of emission in this case.
Spectroohemiealexoitation with low-temperature am
657
A comparison of pot~~urn and sodium as buffer elements shows that with potassium a lower concentration of it is required for obtaining LTA, Also, with potassium better line-to-background ratios for the two studied elements (Al and Li) have been found. From the curves obtained when the influence of the aerosol flow rate was studied it can be concluded that the evaporation of the aerosol particles in the are
0
20
40
0
Concentration
of ethanol
20
40
, vol. %
Fig. 13. Intensity of the Li 670-S h8 and the line-to-backgroundratio as functions of ethauol concentration in spayed solution. o,AA.3amp VIA&6amp l , LTA (O-5111KCl), 3 amp v, LTA (O-SM KCl), 6 amp.
column, is despite a very high temperature, far from complete. This can be attributed to the relatively small dimensions of the arc column, i.e. the aerosol particles spend only short times in the high temperature zone. Although the intensity of some speotral lines is very high at small flow rates, the most suitable flow rates for analytical purposes are from 2 to 3 l/min because of the arc stability. Application of organic solvents in flame photometry eliminates the reduction of the flame temperature caused by spraying of water. However, this effect is not impo~ant for eIectric arcs. In addition, the presence of organic solvents leads
668
M[. K~XCA
and lsd.&fAlZINKOVIb
to the appearance of molecular bands in the spectra. Therefore, it can not be expected that their application with LTA will be as profitable as it is in flame photometry. High flexibility of the described arc might be considered its weakness, for it requires careful standardization of the operations and special precautionary measures for avoiding matrix effects. On the other hand, this is also its merit since it can be successfully used for solving many different analytical problems.
~p~~he~~
excitation with io~-ternpe~~
arc
M. KLISKA and M. ~IXKOV~~ The Boris KidriE Institute of Nuclear &&noes, Beograd, Yugoslavia (Received 13 Jammy
1970)
Abstract_A eystematia investigation haa been made conaerning excitation featurea of the Iow-temperature arc. The following experimental variables have been studied in detail: buffer effect of several alkali eIementz at different concentrationz, orific size in stabilizing dizcz, flow rate of aerosol and the uze of organic aolventa. The obtained results ihuztrate the possibility and tlexibihty of the source and zuggezt the way of cthoozing optimum operating oondition.
low power plasma jets as excitation sources in spectroohemical suggested first by MAR~OSHESand SCRIBNER. [X] and rtlmost simultaneously by KOROLEV and VAISHTEIN[Z]. Since then, the interest for ttpplication of stabilized &rcs &s excitation sources has been growing continu&lly. A considerable number of sources of this type are described in the literature [3-61. In an earlier p&per [7] one of the present authors described & new version of & stabilized &rc of the &bove mentioned kind. The m&in new feature of this arc w&s the use of spectroohemical buffers for reducing and controIling the temperature of the arc column. For this re&son it w&s called the low-~mperature arc. This &rc h&s pro~rties which make it very suitable for use &s an excitation source for certain analytical de~rminations. The two most impo~ant ~h&r&cter~tics &re: (&) high stability of emission of spectral lines, and (b) low detection limits for & considertlble number of elements. In this paper the results of an investigation of the roles of the m&inexperiment&l variables of the new source will be presented. The aim w&s to test the anzllytical possibilities of the source and to suggest w&ya for selecting optimum working conditions. In the course of our experimental work the influence of source geometry, &ro current, kind and concentration of buffer, aerosol flow rate, &nd use of organic solvents h&s been investigated. THE
USE
analysis
of
has
been
The &rc, &heady mentioned and described elsewhere 171,was modified in such & way that it could be operated with different openings for the introduction of the aerosol in the central segment. The solutions are spr&yed into the argon stream by means of&n angle type indirect sprayer. This type of sprayer is more suitable than the concentric one because clogging troubles c&n be &voided. The argon [l] M. MARoos~sos and B. F. SCIUBNER,Spectrochim. Acta 16,138 (1969). 123V. V. KOROLEV and E. E. V~SWJZIN, 2. And Khim. 14,668(1969).
[37 L. E. OWEN,Appt. Spectry. 16,160 (1961). [4] M. RIE-, 2. Anab. C-&m. 215,407 (1966). [6f E. Ho-
and G. HOLDT, iktmtaber. Deb
Akad. W&s. Be&m 7, 693 (1965). 308 (1963). Aeta 28B, 267 (1968).
[s] E. EXZANZ, .&fomt&e~. De&. Akad. W&w. Bed& 6, [7] M. M,UINEO~I~ snd B. DIXITIWE~I~, S~e~oc~~~. s
646
flow rate was normally kept at 24.3l/min* and the spray rate was appro~ately f-4 ml/m& with an &ciency of 345%. It was found that this technique permitted the addition of buffer elements to the aerosol at mole fractions up to 6, UP* by means of spraying solutions contain~g the respective element up to 1*5 molar concentration.
Stobiliting Inner
Fig.
discs
zone
1, Thi3 appearanceof the anaIytical~frrp.Spmyed liquide: (a) pure w&a-, (bf, (of, and (d) eok~tionswith graduafly inamedl potassium oonoentrstion.
1, The use of elements which have low ionization potential as s~ctroscopic buffers is essential for the investigated are,These elements have a con~derable inffuence on both the spectral emission and the visual appearance of the arc oolumn. When pure water is sprayed, a narrow and very bright column is formed (Rg. 1(a)), accompanied by an emission of argon, hydrogen and oxygen lines and an intense continuous spectrum. On spraying a solution containing a suitable amount of an element of low ioniaation potential two distinct zones in the arc column can be observed (Fig. 1 (b)), a bright axial zone slightly narrower than the one of Fig. 1(a), whose width decreases with buffer concentration and a wide periphery zone whose brightness depends upon the kind of the buffer employed. At a certain buffer * Except in Section 5.
Spectrochemical
excitetion with low-temperature
src
547
concentration the axial zone breaks off in two parts, Fig. 1 (c). A further increase results in the shrinking of the remaining part of the axial zone, causing eventually its complete disappearance from the analytical gap, Fig. l(d). In this last case the lines of argon, hydrogen, oxygen, and the intense background (emitted in the axial zone) disappear, leaving in the spectrum only potassium lines and OH bands. Following the notation introduced in the earlier paper [7], the arcs as characterized
4 c
Argon
in
Fig. 2. Stabilizedam. 1, anode; 2,3,4,6,7, and 8, the metal segments, electrioaUy insulated from one enother and water cooled; 5, the central segment, not water cooled; 9, cathode. The am burns in the axial channel which consists of openings on eaoh segment. The analytical gap is between segments 6 and 6.
Figs. 1 (a) and 1 (b) will be referred to as the argon arc (AA) and that in Fig. 1 (d) as the low temperature arc (LTA). The minimum concentration of buffer required for obtaining the LTA is an impo~ant parameter for ana~ytica1 applications of this source. A ~o~~pon~g attention has been paid to it t~oughout this paper. The considerable differences in the spectrum of AA and LTA are demonstrated by Figs. 3 and 4 respectively. These spectra have been obtained with the glass prism monocbromator of a Hilger Uvispek spectrophotometer, in the region of 390-620 nm. The wave lengths were taken from reference [8]. in
2. Injluence of the opedngs of the stabilizing segmem% By means of the segments numbered 4 and 6, Fig. 1, the are column is stabilized. However, the sizes of these openings are not important for the characteristics of [S] A. R. STRIQMOV and N. S. SVENTWKII, T&z@ rstmm.
Atomizdat,
Bloskva (1966).
q~kt7aZ’ gh Zinii ~~tr~~yh
i s-okwvasg&
M.
KLJSKA
and M.
&fARINKOVI6
‘r
ti:391.4.
r
Call 393.37
*r
\rr
Call 396.85
t
-
I( 404..1 I( 404.72
394.75 3e4.90
n 337.00
:----
k ,cw
-CN -CI
-cw
415.24 415.01 416.78 418.10
t
I-
Fig. 3. Recorder tracing of the spectrum of the AA; pure water sprayed; arc ourrent: 6 amp.
Fig. 4. Recorder tracing of the spectrum of the LTA; potassium chloride of O-6M sprayed; arc current: 3 amp.
Spectroahemical exoit&ion with low-temperature are
MB
th% c?entraI pastof the EbrG column. On the other hand, the sixes of the holes in the centrsl segment numbered 5, Fig. 2, eonsiderabfy inffnenee the char~ter of the are discharge, especiaiiy in the wrt;vbuffer elements of low ionization potentials act. These holes have been varied from 3 to 12 mm dia. by inserting Werent graphite rings. The dependence of the minimum concentration of potasdum required for obt&ning LTA on the sizes of these openings and on the arc ourrent has been investig&ed, The results are shown in Fig. 6 in which the minimum mole
5
6
Current, amp Fig+ 5. Dqwndenoe of the miaimmn mole fraction of potassium in the swoaoXfar obtaining lJ?A on tie MOcurrentand opetigs of cent& eegment. Numerator denotea &m&ax of the upper openingand denominator denotes,diameter of the lower opening of the centrfxlsegment, in millimeters.
fraction of pottassium in the aerosool sufficient to obt&n LTA are plotted versus &POeurrent with hole d&meters as parameters, It csn be seen that larger buffer ~n#n~~tio~s correspond to sm& &meters of the openings. It is remarkable that the size of the upper opening is much more critic&. At IBgiven #~~~~~tion of potassium transition from AA to LTA occurs & partieufar v&e of the arc ourrent. This v&e is usually the same regardless of whether the transition is obtained by increasing or decreasing current. The exception is at low current and small openings of the central segment. In that o&se two values are observed one with increasing and another with decreasing arc current. Dashed lines have been obtained by increasing the arc current, Large openings in the central segment are generally preferable sinae they permit I (a) obtaining LTA with lower buffer concentrations and (b} long continuous operation of the arc before memory effe&s beoome significrant. For these reasons the experiments have been ctied out with the upper opening of 12 mm Itncithe lower opening of 10 mm.
Besides potassium, other elements which h&ve low ionization potentials could be used as spectrascopic buffers. With the exception of cesium, all alkali metals hftve therefore been examined experiment&y as spectroscopic buffers. It was found convenient to use them as chlorides. Again the minimum mole fraction for each of the examined met& sufficient to obtain LTA, for a given arc current was determined. The results for the different buffers are presented grrtphiaally in Fig. 6. It can be seen that for a complete suppression of the argon spectrumI i,e. for obtaining LTA, the required concentration of a buffer element is higher for
a
5
Current,
Fig. B, Depen&noe of the miniium
amp
mole fraction of alkali met& required fQr obtaining the LTA. on MO current.
in
elements with higher ionization potentials and it also increases with the &ro current. For ans,lytieaJ ~pp~~~tio~ it is important to know the relation between the kind and ~on~ntr~tion of a buffer element and the ~~sities of emission of speetrsl lines and that of the background. As sn example the infiuenoe of two of the most common buffer elements, e.g. potassium and sodium, on emission of aluminium and lithium has been studied, For these measurements the two W&S focused on the oollimator lens of the monechromator, Consequently, light from the whole analytical gap was conducted to the photosensitive area, of the photomultiplier. The results &re represented graphically in Figs. 7 and 8. Here, the intensities of aluminium and lithium lines, in arbitrary units, but obtained under identical conditions, are plotted versus concentrations of buffer elements. As ortn been seen the influence of the buffer element is very significant. Maximum emission occurs in the vicrinity of the ~n~ntr~tio~ at which the argon spectrum is just completely suppressed. At the same eoncentmtion the background intensity is
Spmtmchemie~ excitation with low-~~p~t~
561
are
minimum, thus giving the best me-~-b&~k~ouud ratio for a&iwing low detection limits, Owing to the high spatial inhomugenitg of emission of AA, line-to-baGwound ratio is very dependent on the method of illumination of the monochromator slit. at a
0
2
4
6
Mole fractfon of bufferin aerosd x IO-
Sharp focusiug of the arc on the monoc~om~to~ slit by means of a single condenser lens gives a considerably smaller line to background r&o, especially for the AA, as compared to the ratio obtainable if the axrcco1um.nis focused upon the collimator mirror. In the former wse steeper curves than those in Figs. 7 and 8 will be obtaiued.
662 6.
M.Xkxsga and1yf.&%.mmx5vI6
Isji?lecenceof aerosol$ow rate
To investigate the influence of the aerosol flow rate on the intensities of spectral lines, a capillary flow meter has been designed, Fig. 9. This design of the flow meter almost completely eliminates aerosol losses. It functions as follows: from the sprayer, which operates with a constant argon flow rate, the aerosol enters at
Molefractionof buffeer in aerosol x 10-a Fig. 8. Intensities of the Li 6704 line and the background at 6704 nm, plotted ~48functions of the mole fraction of buffer in the zserosol, Line intensities for 6 “np am mx.&ipliedby 1% 0,s s.bmp,potasaiuntchIoride e, 3 smp, eodium &l&de V, 6 amp, potmsium chloride V$ 6 twq, aodium chloride.
A: part of it can escape via B the rest passes through the capillary C (4 2 x 30 mm) and enters chamber D. Turbulent motion of the gas in this chamber reduces the amount of aerosol which condenses on the chamber walls. The pressure in a waterfilled U-tube E is employed for the measurement of the flow rate. Aerosol losses are less than 10%. The intensities of lithium and aluminium lines against the aerosol flow rate are plotted in Fig. 10, For the sake of clearness of ~prese~ta~o~ the &f&rent curves are arbitrary displaced along the ordinate and can therefore not be compared with each other in respect to their absolute level. It can be seen that the maxima of the curves are situated between flow rates of 0%143 l/min. The aluminium curve exhibits a particularly sharp maximum. The concentration of
Spectrochemicd excitation with low-temperature are
663
the aerosol particles does not depend upon the rate of flow. Consequently, changes in the intensity of spectral lines can be explained in terms of the changes in excitation conditions and of the changes in time of residence of the aerosol particles in the arc column, i.e. changes in the degree of their evaporation. At low flow rates the aerosol particles spend longer times in the arc column, thus their evaporation is more complete. This could explain the increase of the intensity of lines at reasonably small flow rates, especially of aluminium which, after solvent evaporation, forms refractory aerosol particles. At flow rates smaller than l-2 l/min
To ox
-3 Fig. 9. Aerosol flow-meter.
the intensity of the lithium line decreases. This may be due to the air penetrating the arc column and diluting the aerosol, i.e. at low flow rates the arc column is not completely screened from the surrounding air. Since aluminium forms refractory particles, which for complete evaporation need a longer time of residence in the arc zone the aluminium curves reach maximum emission at smaller flow rates than the curves for lithium. If KCL is present in the sprayed solution the aerosol particles become larger, therefore the time for complete evaporation is longer. As a result the maxima are shifted towards smaller flow rates. 6. Effect of organic solvents The influence of organic solvents in flame photometry is well known. It results in an essential increase in the intensity of lines. In so far as the electric arcs are concerned, however data in the literature are quite controversial. This is probably due to considerable differences in design and characteristics of the applied arcs. For this reason it was decided to investigate the effect of organic solvents on the
554
M. KLISKA end Bd. MABINKOVI~
present souroe. Several different solvents, all mixable with water, have been examined. The presence of an organic solvent is found to increase the voltage drop between the electrodes. The concentration of an organio solvent in the aerosol which enters the are zone is proportional to: (a) its evaporability, (b) its
1
I
I
FIow rote of ~otrosol
I 2
0
1
I 4
I
I 6
, Vmin
Fig. IO. Intensities of Al 396.2 and Li 670-S lines plotted tssfun&ions of aerosol flow rate. o,AA,3amp V,AA,%-p 0, LTA (1 M KCl), 3 emp Y, LTA (1 M KCl), 6 amp.
ooncentration in the solution, and (c) the nebulizer efficiency. The results of measurements at two different currents are shown in Fig. 11, It can be seen that the voltage drops are higher at lower currents (the measurements were made at 3 amp and 6 amp, without KCL and with a O-5 H KCL solution). It is noteworthy that this ~~uence is strongest with acetone, which has the highest e~aporabi~ty of all examined solvents. As a consequence of spraying organic soivents there appear in the spectra moleoular band spectra, particularIy of C?, and CN. At low am-rents and high
~~~ntr&ti~ns af ~~si~rn 8 ~~ide~~ble number of these bzmds disr4ppaar,and the resuH5ng spectra are very simi1s.zto the ones produced by the LTA, If 8. sprayed solution cont&ns potrtssium, the presence of organic solvents greatly pareses the co~centr&tio~ of pot~sium in the arc, This, owing to the accompanying reduction of temperature, may reduce the intensity of some spectrsJ lines. The experimental results are shown in Pigs. 12 and 13.
Drsuossrax
The ~ro~~i~ of the described arc, as an e~~it&tion source in ~~~~~~~~~~~ polyps, could be vmied ~~sider&bly~ thaw to, p~rn~ily~ the ~pp~~tiou of of the MO column is high which ocuxbe s~~ros~opi~ btiers. ft the tern~r~t~ a&eyed if the bufFer ~on~ntr~tio~ is below a certain oritic&l vske, lines of high excitation potentials (e-g the lines of Ar, IX, 0, C, etttc.)asd a&o the lines of the ehments which form refractory oxide (e.g. Be, La, etc.) will be excited, If on the other hand the buflFerconcentration exceeds a certain critical value, the szc column tern~r~tu~ is co~ider&bly redumd, making the properties of the azo aim&r to those of some hot chemica;l fhmes. The buffer efficiency is good if the co~st~ctiou of the arc column is small, the excitation potential of the buffer low, amd the arc current small. Therefore, (a) wide openings on the central segment (ltbX2 mm) (b) bufFerelements of iow excitation po~nti&ls~ md fof small ~bllo currents (3-B &mpjt should preferably be used, The fluent of buEers on the emission of ~~~~~1 lines is ex~ptiuu~~y strongAs the buffer concentration incremes the line intensity at first increlases very shyly and then the rate of &ream slows down and ag& increa+sesbefore the
maximum is attained. On the other hand, the baakground intensity at first decreases and after passing through a minimum starts rising, The best line-to-background ratio is obtained with different buffer concentrations for different elements, depending primarily on the excitation potential
of the line and on the thermal stability of the corresponding oxide. For the determination of elements which form highly refractory oxides or which have analytical lines of high excitation potentials the AA mode is required. In determinations of elements which have medium or low excitation potentials and which do not form highly refractory oxides the beat me-to-ba~k~ound ratio is obtained in the vicinity of the minimum buffer ~on~ntra~o~ at which the LTA mode can be obtained. IIowever, for routine analysis it is advisabIe to use somewhat higher buffer concentrations {e.g. twice the minimum ~o~~~tration~ because of a better stability of emission in this case.
Spectroohemiealexoitation with low-temperature am
657
A comparison of pot~~urn and sodium as buffer elements shows that with potassium a lower concentration of it is required for obtaining LTA, Also, with potassium better line-to-background ratios for the two studied elements (Al and Li) have been found. From the curves obtained when the influence of the aerosol flow rate was studied it can be concluded that the evaporation of the aerosol particles in the are
0
20
40
0
Concentration
of ethanol
20
40
, vol. %
Fig. 13. Intensity of the Li 670-S h8 and the line-to-backgroundratio as functions of ethauol concentration in spayed solution. o,AA.3amp VIA&6amp l , LTA (O-5111KCl), 3 amp v, LTA (O-SM KCl), 6 amp.
column, is despite a very high temperature, far from complete. This can be attributed to the relatively small dimensions of the arc column, i.e. the aerosol particles spend only short times in the high temperature zone. Although the intensity of some speotral lines is very high at small flow rates, the most suitable flow rates for analytical purposes are from 2 to 3 l/min because of the arc stability. Application of organic solvents in flame photometry eliminates the reduction of the flame temperature caused by spraying of water. However, this effect is not impo~ant for eIectric arcs. In addition, the presence of organic solvents leads
668
M[. K~XCA
and lsd.&fAlZINKOVIb
to the appearance of molecular bands in the spectra. Therefore, it can not be expected that their application with LTA will be as profitable as it is in flame photometry. High flexibility of the described arc might be considered its weakness, for it requires careful standardization of the operations and special precautionary measures for avoiding matrix effects. On the other hand, this is also its merit since it can be successfully used for solving many different analytical problems.