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The effect of controlled atmospheres in the graphite arc method
wu-8547/H 53.a?+40 0 1985. Pcrgamon Rnr Ltd.
4OB.No. 8, pp. 1047-1057,1985. Rimedio GreatBritain.
Specmch&nica Acm, Vol.
The effect of controkd atmospheres in the graphite arc metbod N. NEDJALKOVA and N. KRASNOBAEVA Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, Sofia 1000, Bulgaria
V. VUKANOVJC* Faculty of Sciences, University of Beograd, Yugoslavia and B. PAVLOVlC Faculty of Technology and Metallurgy, University of Beograd, Yugoslavia (Received 17 April 1984; in revised~~
31 January 1985)
Abstract-The effect of different atmospheres-air, nitrogen, argon and helium--on the analytical properties of the “graphite arc” method used to analyse solutions were investigated. It was shown that the use ofan argon atmosphere enables an analyst to achieve detection limits for elements with ionization potentials greaterthan 8.5 eV thirty to one hundred times lower than those that can be achieved in air.
1. INTRODUCTION
EXPERIENCE has shown that the “graphite arc” method for the analysis of solutions gives low detection limits while using a minimum volume of sample. In this method a drop of dilute solution is evaporated to dryness on the top surface of a carbon electrode impregnated with polystyrene. The electrode is then arced either in the presence of additives containing elements with low ionization potentials [I, 21 or in an external inhomogeneous magnetic field [3]. The “graphite arc” method in these two variants gives detection limits which are low enough for many practical analytical problems involving elements with ionization potentials lower than 8.5 eV. The use of a controlled argon atmosphere makes it possible to extend the range of analytes to those with high ionization potentials [4,5]_ The present communication reports the results of an investigation into the influence of air, nitrogen, argon and helium atmospheres respectively on the analytical properties of the “graphite arc” burnt in the presence of barium nitrate, the latter additive ensuring a uniform entry of the elements into the plasma. 2. EXPERIMENTAL The apparatus used is given in Table 1. 2.1. Solutions The basic anaiyte solutions were prepared by dissolving metals or metal salts in nitric acid in concentrations lOtI mgl- ‘. The test solution was made by diluting basic anaiyte solutions in 0.4 M [ 1J
Ch. I. ZILEERSTEIN, R. GERBATSCH, 0. N. NIKITINA, P. M. SEMOV and G. ARTUS, Reinstsroflprobleme, Ed. REXER, Vol. II, p. 187. Akademie, Berlin (1966). 123 N. KRASNOBAEVA,Ju. HARIZANOVand Z. ZADGORSKA, Bull. hr. Phys. Rech. Arom.Acad. Bulg. Sci. 21,171 (1971). [33 N. KRASNOBAEVA,Ju HARUNOV and Z. ZADGORSKA, ~~fr~~~. ACM 24B, 473 (1969). [4] N. KRASNOBAEVAand N. N~DJA~OVA-DA~LOVA, lrv. Otd. Khim. Nauki, Buig. Akad. Nauk 7,385 (1974);7, 399 (1974). [S] N. NEDJALKOVA,Dissertation, Sofia (1982).
*Present address: Rochester Institute of Technology, College of Sciences, Department of Chemistry, Rochester, N.Y. 14623, U.S.A. 1047
Spectrograph Optics Power supply Arc current Electrodes
Controlled atmosphere E&z&ode temperature Exposures
PC%-2 VEB Carl Zeiss Jena Spectrograph used in the 2nd order of a grating with 650 lines/mm blazed towards 570 mm. Slit width was 20 w. The arc was focused on the slit of the spectrograph; magnification 1: 1. For radial distribution studies the image of the arc was rotated 90” using a Dove prism. A d.c. power supply with pulse ignition. 8 A when the arc was burnt in air and nitrogen, 10 A when the arc was burnt in argon and helium. Electrode No. TO manufactured by VEB Elektrokohle Lichtenbexg (Dimensions are given in Ref. [43). The electrodes were held in water cooled clamps with an arc gap of 4 mm. The arc atmosphere was controfted using a modified Stallwood Jet [4]. Gas flow rates were IO0 f h- ’ in argon and nitrogen, 300 1h- ’ in helium. Made with an optical pyrometer OPPIR-017 (USSR) 10 s when atmosphere is air or nitrogen, 40 s when atmosphere is argon or helium.
nitric acid in the following concentrations:
lOOmgl_’ 1Omg I-’
P, Hg, As, Zn, Te Cd, Bi, Fe, Co, Ni, Mn, T1
2mgI-’
Be
1 mg I- ’
Pb, In.
The test solution contained all the above efements at the above con~nt~tions= Solutions of barium nitrate, the additives were prepared by dissolving Ba(NUSjz in water with ca~~ni~~ons of 9200, 18 400,26 gW.55 200 and 73 600 mg I- I, respective& The upper surface of tower electrode was impregnated with 30 ~1 of 0.5 % polystyrene in benzene. Using a micropipette, 20 ~1 ofappropriate barium nitrate solution was placed on the upper surface of the impregnated lower electrode, allowed to dry and then 20 ~1 of the sample solution added and similarly allowed to dry before arcing. 2.2. Spectral lines Tables 2 and 3 summarise the wavelengths, excitation energies and ionization spectral lines of the analytes.
potentials of the
The ratio of the spectral line intensity to the background is an important factor in spectral trace analysis. It is known that the precision and detection limit of the spectral determination
are dependent on the value of this ratio. A large value of this ratio can be obtained either by reducing the background intensity or by enhancing the line intensity. Both can be achieved either by changing the conditions of analyte entry in the plasma or by changing the excitation parameters. The investigations were made with 0.7 mg Ba (NO,), as additive since in the previous investigation of the “graphite arc” method in air, KRASNOBAEVA [6} has found that this amount of additive was optimal. 3S:l. ~n~~ence ofthe gusems ~#~sp~re on the elm~mle r~per~rure. ~va~mtion of elements with different physico~hem~~ properties from the electrode cavity has been investigated by many authors [7-91. If an additive is used [lo], the entry ratios of different elements are approximately equal in the case of the “graphite arc” method. [6] N. KRASNOBAEVA, Dissertation, Sofia (1968). [7] R. J. DECKER,Spectrochim. Acra 26B, 137 (1971). [83 R. J. DECKERand D. J. EVE, Appl. Spectr. 22, 13 (1968). [9] P. W. J. M. BOUMANS and F. J. M. J. MAESSEN,Spectrochim Acta 24B, 585 (19693; UB, 611 (1969)” [lOI 2. ZADGORSKA,N. KXA~NO~AEVA and D. APOSTOLOV, Specrrochim. Acta 3OB, 527 (1975).
1049
253.56 253.65 278.01 328.23 334.50 332.13 238.57 326.fD 306-77 3mB6 304.40 30108 279.48 283.30 422.67 276.19 325.60 334.22
Zn Cd Mg cu Co Co Cd Be Fe Fe La CaII Ca
II II II II 11 Ii II fI If II I? II
250.20 274.86 279.08 276.97 2BI.W 285.49 284.58 JI3.04 m.33 325.90 334.46 39336 396.84
7.22 4.88 6.77 7.78 7.711 6.45 5.78 3.80 4.04 4.fl 4.07 4.M 4.44 4.40 2.93 4.40 4.10 3.94
9.39 8.99 7.64 7.72 7.88 7.88 -US8 9.32 7.%x ?%G 5.65 6.10 6.10
10.48 10.43 9.81 9.39 9.39 9.32 9.0% 8.99 8.00 7.86 7.79 7.63 7.43 7.42 6.11 6.10 5.79 5,61
He, Ar He, Ar He He Ar AC Ar He, Ar Ar AZI-k& He, AT He
When an inert atmosphere is applied, the rate of volatilization of the sample isccmsiderably reduced, because the energy input at the anode is markedly changed fl$ Theelectrode tem~rat~re~ at 1 mm below the top surfa~ of ~l~tr~~an~~ are listed in Table 4 along with the ~o~s~nd~ng burn-off times ~b~~~~ from the “graphite arc” method in different a~m~s~beres. The resufts from Table 4 show the fo~~o~~g futures, fi) Anode temperatures are the same in air and ~~~0~~~ (ii) Anode tem~rat~~~ in argon and h&urn are a~r~~jrn~t~~y the same. The burn-&T times are apFro~~~t~~~ the same, too. Therefore, we can a~urn~ that the entries ofeleme~ts in the arc plasma are not s&nificantly different. (iii) Comparing our r@&ts of anode temperature measurements in the “graphite are*’ method with the other authors’ measurements of anode temperatures, when the samples were
N. NEDJALKOVA et al.
1050
Table 4. The electrode temperatures at 1 mm below the top surface of
the electrode and burn-off time obtained from the d.c. arc in different gaseous atmospheres, in the presence of 0.7 mg Ba(NO,),
Our measurement; “graphite arc” method
Gas
Electrode ( = anode) temperature (R)
M~surements by other authors, when the samples are placed in the electrode cavity Electrode
Burn-off time (s)
Air Nitrogen Helium
1870 1870 1330
10 10 35
Argon
1220
40
(= anode) temperature (R) 1940 [12] 1480 1360 1140 965
[12] rl3] [tzj [13]
Burn-off time (s) 110 [12] > 900[12] -
placed in the electrode cavity [12, 131, we can conclude that these temperatures are approximately the same. But, the burn-off time in argon is RS800 s longer if the sample is placed in the electrode cavity, while in the “graphite arc” the burn-off time does not differ more than 30 s. 3.1.2. Influence of the gaseous atmosphere on the excitation parameters. The arc temperatures were calculated from the intensity ratios of zinc lines and the electron pressure from the calculated temperature and the intensity ratios of magnesium lines as well as the intensity ratios of calcium lines using standard equations [ 14, 151. The values obtained are accurate only if local thermal equilibrium (LTE) exists otherwise they can only be regarded as an estimate of the parameters and an indicator of changes in these parameters. The results obtained are given in Table 5. There is little difference between the values obtained when the arc burnt in air or when it burnt in nitrogen. This is probably due to the fact that nitrogen is a major component of air and in addition the functions of heat conductivity vs temperature have the same shape for air and nitrogen and approximately the same values in the interval 5~8000 K [ 16,173. The measured values of temperature and electron pressure obtained in atmospheres of argon and helium are higher than those obtained in air and nitrogen. KOLESNIKOV [18] made an experimental study of thermal equilibrium in arcs burning in argon or helium atmospheres. He has demonstrated the existence of LTE in a d.c. argon arc with a current of 10 A and with electron concentrations greater than 3 x 10’ J cm- 3, while the existence of LTE in the helium atmosphere is achieved at currents greater than 300 A. Therefore we suppose that LTE exists in argon atmosphere, while LTE does not exist in helium atmosphere, since we have applied a 10-A d.c. arc in our experiments. 3.1.3. Influence of gaseous atmosphere on the background intensities. In practice, the background may be appreciably reduced by decreasing the arc temperature [19] or by eliminating the contribution of certain molecular bands. In Table 6 are present the background intensities in various spectral regions when the arc burnt in different atmospheres and in the presence of 0.7 mg Ba(NO,), . The background has been determined during the burn-off time. The burn-off times are given in Table 4. 1121 V. A. MARZUVANOV, Izv. Acad. Sci. USSR 23, 1059 (1959). [13] R. E. THIERSan& B. L. VALLEE, Proc. 6th Call. Spectrosc. hf., Amster&m 1956, Spectr~c~im. Acta 11, 179 (1957). [14] P. W. J. M. BOUMANS, Theory of Spectrochemicai Excitation. Hilger and Watts, London (1966). [15] B. PAVLOVI& N. IKONOMOVand V. VUKANOVIC, Bull. Chem. Sot. (Beograd) 34, 313 (1969). [16] V. VUKANOVI~, N. IKONOMOVand B. PAVLOV&, Spectrochim. Acta 26B, 95 (1971). [17] N. IKONOMOV, B. PAVLOVI~and V. VUKANOVI~, Bull. Inst. Phys. Reck Atom. Acad. Bulg. Sci. 21,29 (1971). [18] V. N. KOLESNIKOV,Trans. P.N. Lebedev Physics Institute, Acad. Sci. USSR 30,66 (1964). [19] J. P. WILLIS,M. KAYE and H. AHRENS, AppI. Spectr. 18, 84 (1964).
Effect of controlled atmospheres in graphite arc
1051
Table 5. Effective values of temperature and electron pressure in air, nitrogen, argon and helium atmospheres
Temperature (K)
Gas Air Nitrogen Argon Helium
5800 5900 7300 8000.
Electron pressure (Pa) 81 91 1010 6700
*The electron temperature.
Table 6. Background intensities in different spectral regions when the arc burnt in gaseous atmospheres Spectral region
Air
250 nm 300 nm 330 nm* 385 nmt
0.30 0.40 0.70 0.90
Background intensity Argon Nitrogen 0.30 0.40 0.80 0.90
0.40 0.52 0.30 0.28
Helium 0.40 0.54 0.30 0.32
*Region of N,-molecular bands. tRegion of cyanogen emission.
From the results presented in Table 6 it is evident that in the spectral region 250-300 nm the background is not so high, and it is not significantly changed in different atmospheres. But, in those spectral regions where the increase of background is due to the molecular bands influenced by the presence of nitrogen, the background intensity will be significantly lower in argon or helium atmosphere than in air or nitrogen. 3.1.4. lnjuence of the atmosphere on the line intensities. The effect of nitrogen, argon, and helium, respectively, on the spectral line intensities in the “graphite arc” method was investigated and compared with those obtained when the arc burnt in air. We have found the following. (i) The use of nitrogen not only has an insignificant effect on the electrode temperature and excitation parameters, but also affects line intensities insignificantly. (ii) With an atmosphere of helium, the observed atom lines decreased in intensity (Table 7). The ratios lair/lHe for atom lines of other elements, which are given in Table 2, have approximately the same values (3.7-3.8). Therefore, in the helium atmosphere, the decrease of atom lines is not significantly dependent on the ionization potential of investigated elements. (iii) The magnitude of the increase of the ion line intensity is related to the total excitation energy, i.e. ionization plus excitation energy, of the lines as shown in Fig. 1. This relationship does not show a maximum since the lowest excited levels of helium lie between 19 and 20 eV and this corresponds to the upper limit of the total excitation energies of the investigated lines. Under the applied experimental conditions, resonance excitation probably occurs although this would affect only a small number of energy levels. We want to remark here that recently KARJAKINet al. [20] have tried to explain the mechanism of action of inert gases on the spectral line intensities in emission spectrochemical analysis by taking into account the collisions of the second kind. (iv) A considerable enhancement of both atom and ion lines occurs with the elements that have ionization potentials greater than 8.5 eV when an argon atmosphere is used. It was established that the enhancement of the spectral lines is related to the ionization potentials of the elements as shown in Fig. 2. This figure also shows that the shapes of the curves for the [20] A. V.
KARJAKIN, L. I. PAVLENKO, L. I. KARPENKOand L. P. SHTEPA, 2%.
Anal. Khim. 39,411 (1984).
1052
N.
NEDJALKOVA
et al.
Table 7. The change of the intensity for atom lines in an arc burning in air and helium atmosphere, respectively
Anaiyte
Spectral line (nm)
Ionization potential (V)
P I Zn I co1 In I
253.56 328.23 304.40 325.60
10.48 9.39 7.79 5.79
‘Ir
3.80 4.27 2.24 3.16
‘air llhelium
14.18 15.85 8.51 12.02
3.72 3.71 3.75 3.80
*Line intensity in helium atmosphere. +Line intensity in air.
I m
I Aor 15 cu
10 -
5
-~
0, 3
I 10
15
20
Eq=VI
1 25
+ Vq.
eV
Fig. 1. Ratio of the ion lines intensities (lhelium/lair) as a function of total excitation energy (E, = vi + V,).
atom lines and for the ion lines are similar to that of the curve for the relative degree of ionization, a, as a function of the ionization potential of the relevant elements. Having in mind the above results, we have concluded that a controlled argon atmosphere can extend the application of the “graphite arc” method for the determination of elements with ionization potentials higher than 8 eV. Therefore, from the spectroanalytical point of view, argon atmosphere is worth further investigation. 3.2. Additional investigation of argon atmosphere influence on line intensities 3.2.1. The injuence of the amount of Ba(NO& in an argon atmosphere on line intensities and on the excitation parameters. It was found that the atom line intensities of different elements displayed maxima as the amount of barium nitrate added to the electrode was increased as shown in Fig. 3. As we have already seen, changing the amount of barium nitrate results in changes in the plasma temperature and in the electron density (Fig. 4). Thus the maxima occurs at specific temperatures and electron densities. The temperatures at which the maxima occur are in close agreement with the optimum temperatures calculated using the method suggested by DE GALAN and BOUMANS [21] as shown in Table 8. For the calculation of optimum temperatures the following values of electron concentrations in the arc plasma were assumed. For elements from P to Cd: ne = 1 x 1Ol6 cm-3; the corresponding amount of additive is 0.7 mg of Ba(N03)2; for other elements: [21] L. DE GALANand P. W. J. M. BOUMANS,Z. Anal. Chem. 214, 161 (1965).
Effect of controlled atmospheres in graphite arc d’
.
5 Intensity ratio *
of the
atom lines as a fuktion of the ionization pdentials V,
Fig. 2. (I&
_
I
Zn
10 Intensity ratio e
1053
&Argon dn,r t
Xl
5
V, ,eV
V, ,eV
The dependence of the
of the
ratio d = *
lines as a function of the ionization potentials V,
ion
dAir
On the
ionization potentials Vi d q degree of ionization
/rht ) of the atom lines, (I&,, /IzJ of the ion lines, and (a.&apir as a function of ionization potentials.
) degree of ionization
109IA&Q”
7
1.5 -
I,,(Mn)-T=6300
I,., (Te)-T=6800
1 xiv
1x10-
K
K
1x10-l0gC.g
Fig. 3. Atom tine intensities of the elements with different ionization potentials as a function of barium nitrate amount.
nc = 2 x 1016 cm- 3; the amount of E3a(NO& is 1.4 mg. In Table 8 the elements are arranged in order of decreasing ionization potential. It follows therefore that by adjusting the amount of barium nitrate added, it is possible to obtain optimum conditions for the excitation of specik atom lines in argon atmosphere. 3.2.2. Spatial inhomogeneity of the argon plasma in the presence of barium nitrate. Many authors have investigated spatial inhomogeneity in the d.c. arc plasma [ 14, 15,223. We have compared only axial and radial distributions of line intensities in the d.c. arc burnt in air and argon, respectively. [22] R. J.
DECKER
and P. A.
MCFADDEN,
Spectrochim. Acta 3OB, 1 (1975).
N. NEDJALKOVAet al
1054
Fig. 4. Effective values of temperature and electron pressure measured in argon temperature as a function of the barium nitrate amount. TabIe 8. Optimum temperatures for the emission of distinct spectral lines Toptimum
Toptimum
(calculated) (K)
(experimental) (K)
Hg As Zn Be Te Cd
7300 7200 7100 7ooo 6900 6700 6700
7100 7100 7100 6800 6800 6800 6700
Bi Fe co Ni Mn
6300 6250 6200 6200 6200
6.500 6500 6500 6500 6300
Analyte P
Pb
6200
6300
TI
5300
6300
In
SO@0
6300
3.2.2.1. Axial distribution. The influence of barium nitrate as an additive and of the argon atmosphere has been investigated earlier by using the techniques of imaging the arc on the slit of the spectrograph 123,241 and of using a s&less spectrograph combined with equidensitometry [ZS]. These investigations have shown the foilowing. (i) For a given distribution of the arc parameters the particle density of the elements determines the axial distribution. The observed enhancement of the spectral lines in the vicinity of the electrode corresponds to a high concentration of the particles of a given element in these regions. (This enhancement is shown in the equidensity contour diagrams for Tl I 276.8 and for Hg I 253.7 given in Figs 5 and 6.)
[23]
N. KRASNOBAEVA, 2. ZADGORSKA and N. NEDJALKOVA, Spectrochim. Acta 33B, 655 (1978).
[24]
8, PAVLOVIC,D. PETROVI~,N. KRASNOBAEVAandN. NEDJALKOVA,Bu#. Chem. Sot. (Beogd)
[25]
K. D~TFRICH, K. NIEBERCALL,N. KRASNOBAEVAand N. NEDJALKOVA,S~ctr~h~m.
4,203 (1979). Acta 38B, 461 (1983).
Effectof controlled atmospheres in
1055
i&S tOM 2 K
Tl Argon
1
276.78 Air
b)
Fig. 5. Radial distribution of rnercurjt fine Hg I 253.65 (pi = tQ.43V) with anode (a) and cathode (b) excitation. Curves: (1) for arcing in argon, (2) for arcing in air. The carresponding equidensitograms are shown for comparison [25].
Ul
Fig. 6. Radial distribution of thalium line TI I 276.78 (Vi = 6.1 V) with anode (a) and cathode (b) excitation. Curves: (1) for arcing in argon, (2) for arcing in air. The carresponding equidensitograms are shown for comparison [25]. sA
1056
N. NEDJALKOVA et ol.
(ii) When optimum conditions are being selected for trace analysis using the “graphite arc” method, it is useful to take into account the axial distribution of the analytical signal. 3.2.2.2. Radial distribution. Experiments were performed in air and in argon by evaporating dry residues from the lower electrode to which 0.7 mg barium nitrate had been added and the radial intensity distributions measured at 1 mm from the lower electrode, in the centre of the plasma and at 1 mm in front of the upper electrode. Above experiments were performed in cathode and anode excitation, respectively. The intensity distribution obtained directly from the photographic image represents intensities integrated along the line of observation. From this intensity distribution, the radial distribution was calculated by solving the Abel inversion integral [26]. Figure 5 gives the results obtained with Hg I 253.6 in air and in argon and Fig. 6 gives the results from Tl I 276.8. These figures show the following. (i) Anode excitation in air is characterised by a classical cathode-layer enhancement of lines with low ionization potentials. In argon the enhancement near the anode is observed for all elements. (ii) Cathode excitation in both air and argon results in an enhancement of the spectral line intensity in the vicinity of the cathode. (iii) The argon plasma is characterised by a more rapid radial decrease in the spectral line intensity than when the arc burns in air. This is the consequence of smaller radius of the arc column in argon in comparison with one in air. Table 9. Detection limits of chemical elements when the samples are arced under different conditions Spectral line (nm)
Element P Hg As Zn* Be* Be Te Cd Bi Fe co cu Mg+ Ni Mn Pb TI In
I I I I I II I I I I I I I I I I I I
253.56 253.65 278.02 334.50 332.13 313.10 238.57 326.10 306.77 302.06 304.40 324.70 285.21 305.08 279.48 283.30 276.79 325.60
Air* 1200 1200 450 100 60 0.1 10000 20 7 0.4 0.3 20 6 0.1
Detection limits (ng) Nitrogen@ Argon11 1200 1200 450 100 60 0.1 10000 20 7 0.4 7 0.3 20 6 0.1
1 5
5 1
30 50 30 2 2 0.005 300 1 1 0.1 2 0.1 5 2 0.08 0.7 3 0.8
*Spectral lines are in the region of molecular bands which can be eliminated in argon atmosphere. ‘Limited only the blank. *Free-burning arc in air in presence of 0.7 mg Ba(NO&. Current 8 A. @Nitrogen-stabilized d.c. arc in presence of 0.7 mg Ba(NO,),. Current 8 A. l/Argon stabilized d.c. arc. The elements P, Hg, As, Zn, Be are determined in the presence of0.7 mg Ba(NO&; the elements Te, Cd, Bi, Fe, Co, Cu, Mg, Ni are determined in the presence of 1.4 mg Ba(NO& and Mn, Pb, Tl, In are determined in the presence of 2.1 mg Ba(NO,),.
[26] F.
KUMMEL, Tobellenfiir
die L~Gung der Abelschen Inregrolgleichung.
Akademie, Berlin (1960).
E&ct of controlled atmospheres in graphite arc
1057
Table 10. Detection limits of different elements; arc in helium in presence of 0.7 mg Ba(NO&
Element
cu Zn cd Mg Be
II II II II II
Spectral line (nm)
Total excitation energy (ev)
276.97 250.20 274.84 279.08 313.14
21.10 20.36 19.27 16.51 13.14
Detection limit (ng) 230 62 41 40 0.03
3.3. detection limits and precision
Table 9 summarizes the analytes and the detection limits obtained when the arc burnt in different gaseous atmospheres. As we have shown in Section 3.1.4, only ionic lines are enhanced in helium atmosphere. The investigated ionic lines of elements, with a total excitation energy value close to the lowest excitation level of helium, and which show significant intensity enhancement under the above experimental conditions, are characterized by a low intensity in the arc [27j. Therefore, by using these lines, it was not possible to achieve lower detection limits. Table 10 shows the detection limits for when the arc burns in helium atmosphere in the presence 0.7 mg Ba(NO&. Relative standard deviations calculated for n = 20 measurements in the ‘*graphite arc” method using 0.7 mg Ba(NO& as additive were 6 y0 in argon and helium atmosphere and 1% in air and nitrogen atmosphere, respectively. 4. SUMMARY ANDCONCLUSION This investigation of the influence of different atmospheres in the presence of barium nitrate on the plasma parameters and on the line intensities has shown the following. (i) A nitrogen atmosphere gives detection limits very similar to those obtained when the arc burns in air. In some instances the gross signal is lower than that obtained in air but the background level is also lower due to the isolation of the plasma from the environment. Thus nitrogen can only be profitably used as a stabilising gas flow. (ii) The different excitation characteristics found when the arc burns in argon or helium makes it possible to choose an atmosphere which is appropriate for a given analytical problem. (The ion lines of the elements whose total excitation energies are close to the lowest energy levels of helium have low intensities in the arc, and thus little is gained in terms of detection limits when helium is used as the atmosphere.) (iii) The use of an argon atmosphere in the presence of barium nitrate extends the possibilities offered by the “graphite arc” method because lower detection limits can be achieved for elements with high ionization potentials than in an atmosphere of air. Acknowledgntents-We are thankful to Dr R. J. DECKERfor numerous useful suggestions and remarks that have considerably improved the manuscript. We wish also to express our gratitude to Dr. P. W. J. M. BOUMANSfor commenting on this manuscript.
1271A. N. ZAIDE~ V. K. PROKOV’EV,S. (1962).
M. RAJSKIJ anil E. Ju. SCHREIDER, T&icy SpektraPnykh L&ii, Moskow
4OB.No. 8, pp. 1047-1057,1985. Rimedio GreatBritain.
Specmch&nica Acm, Vol.
The effect of controkd atmospheres in the graphite arc metbod N. NEDJALKOVA and N. KRASNOBAEVA Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, Sofia 1000, Bulgaria
V. VUKANOVJC* Faculty of Sciences, University of Beograd, Yugoslavia and B. PAVLOVlC Faculty of Technology and Metallurgy, University of Beograd, Yugoslavia (Received 17 April 1984; in revised~~
31 January 1985)
Abstract-The effect of different atmospheres-air, nitrogen, argon and helium--on the analytical properties of the “graphite arc” method used to analyse solutions were investigated. It was shown that the use ofan argon atmosphere enables an analyst to achieve detection limits for elements with ionization potentials greaterthan 8.5 eV thirty to one hundred times lower than those that can be achieved in air.
1. INTRODUCTION
EXPERIENCE has shown that the “graphite arc” method for the analysis of solutions gives low detection limits while using a minimum volume of sample. In this method a drop of dilute solution is evaporated to dryness on the top surface of a carbon electrode impregnated with polystyrene. The electrode is then arced either in the presence of additives containing elements with low ionization potentials [I, 21 or in an external inhomogeneous magnetic field [3]. The “graphite arc” method in these two variants gives detection limits which are low enough for many practical analytical problems involving elements with ionization potentials lower than 8.5 eV. The use of a controlled argon atmosphere makes it possible to extend the range of analytes to those with high ionization potentials [4,5]_ The present communication reports the results of an investigation into the influence of air, nitrogen, argon and helium atmospheres respectively on the analytical properties of the “graphite arc” burnt in the presence of barium nitrate, the latter additive ensuring a uniform entry of the elements into the plasma. 2. EXPERIMENTAL The apparatus used is given in Table 1. 2.1. Solutions The basic anaiyte solutions were prepared by dissolving metals or metal salts in nitric acid in concentrations lOtI mgl- ‘. The test solution was made by diluting basic anaiyte solutions in 0.4 M [ 1J
Ch. I. ZILEERSTEIN, R. GERBATSCH, 0. N. NIKITINA, P. M. SEMOV and G. ARTUS, Reinstsroflprobleme, Ed. REXER, Vol. II, p. 187. Akademie, Berlin (1966). 123 N. KRASNOBAEVA,Ju. HARIZANOVand Z. ZADGORSKA, Bull. hr. Phys. Rech. Arom.Acad. Bulg. Sci. 21,171 (1971). [33 N. KRASNOBAEVA,Ju HARUNOV and Z. ZADGORSKA, ~~fr~~~. ACM 24B, 473 (1969). [4] N. KRASNOBAEVAand N. N~DJA~OVA-DA~LOVA, lrv. Otd. Khim. Nauki, Buig. Akad. Nauk 7,385 (1974);7, 399 (1974). [S] N. NEDJALKOVA,Dissertation, Sofia (1982).
*Present address: Rochester Institute of Technology, College of Sciences, Department of Chemistry, Rochester, N.Y. 14623, U.S.A. 1047
Spectrograph Optics Power supply Arc current Electrodes
Controlled atmosphere E&z&ode temperature Exposures
PC%-2 VEB Carl Zeiss Jena Spectrograph used in the 2nd order of a grating with 650 lines/mm blazed towards 570 mm. Slit width was 20 w. The arc was focused on the slit of the spectrograph; magnification 1: 1. For radial distribution studies the image of the arc was rotated 90” using a Dove prism. A d.c. power supply with pulse ignition. 8 A when the arc was burnt in air and nitrogen, 10 A when the arc was burnt in argon and helium. Electrode No. TO manufactured by VEB Elektrokohle Lichtenbexg (Dimensions are given in Ref. [43). The electrodes were held in water cooled clamps with an arc gap of 4 mm. The arc atmosphere was controfted using a modified Stallwood Jet [4]. Gas flow rates were IO0 f h- ’ in argon and nitrogen, 300 1h- ’ in helium. Made with an optical pyrometer OPPIR-017 (USSR) 10 s when atmosphere is air or nitrogen, 40 s when atmosphere is argon or helium.
nitric acid in the following concentrations:
lOOmgl_’ 1Omg I-’
P, Hg, As, Zn, Te Cd, Bi, Fe, Co, Ni, Mn, T1
2mgI-’
Be
1 mg I- ’
Pb, In.
The test solution contained all the above efements at the above con~nt~tions= Solutions of barium nitrate, the additives were prepared by dissolving Ba(NUSjz in water with ca~~ni~~ons of 9200, 18 400,26 gW.55 200 and 73 600 mg I- I, respective& The upper surface of tower electrode was impregnated with 30 ~1 of 0.5 % polystyrene in benzene. Using a micropipette, 20 ~1 ofappropriate barium nitrate solution was placed on the upper surface of the impregnated lower electrode, allowed to dry and then 20 ~1 of the sample solution added and similarly allowed to dry before arcing. 2.2. Spectral lines Tables 2 and 3 summarise the wavelengths, excitation energies and ionization spectral lines of the analytes.
potentials of the
The ratio of the spectral line intensity to the background is an important factor in spectral trace analysis. It is known that the precision and detection limit of the spectral determination
are dependent on the value of this ratio. A large value of this ratio can be obtained either by reducing the background intensity or by enhancing the line intensity. Both can be achieved either by changing the conditions of analyte entry in the plasma or by changing the excitation parameters. The investigations were made with 0.7 mg Ba (NO,), as additive since in the previous investigation of the “graphite arc” method in air, KRASNOBAEVA [6} has found that this amount of additive was optimal. 3S:l. ~n~~ence ofthe gusems ~#~sp~re on the elm~mle r~per~rure. ~va~mtion of elements with different physico~hem~~ properties from the electrode cavity has been investigated by many authors [7-91. If an additive is used [lo], the entry ratios of different elements are approximately equal in the case of the “graphite arc” method. [6] N. KRASNOBAEVA, Dissertation, Sofia (1968). [7] R. J. DECKER,Spectrochim. Acra 26B, 137 (1971). [83 R. J. DECKERand D. J. EVE, Appl. Spectr. 22, 13 (1968). [9] P. W. J. M. BOUMANS and F. J. M. J. MAESSEN,Spectrochim Acta 24B, 585 (19693; UB, 611 (1969)” [lOI 2. ZADGORSKA,N. KXA~NO~AEVA and D. APOSTOLOV, Specrrochim. Acta 3OB, 527 (1975).
1049
253.56 253.65 278.01 328.23 334.50 332.13 238.57 326.fD 306-77 3mB6 304.40 30108 279.48 283.30 422.67 276.19 325.60 334.22
Zn Cd Mg cu Co Co Cd Be Fe Fe La CaII Ca
II II II II 11 Ii II fI If II I? II
250.20 274.86 279.08 276.97 2BI.W 285.49 284.58 JI3.04 m.33 325.90 334.46 39336 396.84
7.22 4.88 6.77 7.78 7.711 6.45 5.78 3.80 4.04 4.fl 4.07 4.M 4.44 4.40 2.93 4.40 4.10 3.94
9.39 8.99 7.64 7.72 7.88 7.88 -US8 9.32 7.%x ?%G 5.65 6.10 6.10
10.48 10.43 9.81 9.39 9.39 9.32 9.0% 8.99 8.00 7.86 7.79 7.63 7.43 7.42 6.11 6.10 5.79 5,61
He, Ar He, Ar He He Ar AC Ar He, Ar Ar AZI-k& He, AT He
When an inert atmosphere is applied, the rate of volatilization of the sample isccmsiderably reduced, because the energy input at the anode is markedly changed fl$ Theelectrode tem~rat~re~ at 1 mm below the top surfa~ of ~l~tr~~an~~ are listed in Table 4 along with the ~o~s~nd~ng burn-off times ~b~~~~ from the “graphite arc” method in different a~m~s~beres. The resufts from Table 4 show the fo~~o~~g futures, fi) Anode temperatures are the same in air and ~~~0~~~ (ii) Anode tem~rat~~~ in argon and h&urn are a~r~~jrn~t~~y the same. The burn-&T times are apFro~~~t~~~ the same, too. Therefore, we can a~urn~ that the entries ofeleme~ts in the arc plasma are not s&nificantly different. (iii) Comparing our r@&ts of anode temperature measurements in the “graphite are*’ method with the other authors’ measurements of anode temperatures, when the samples were
N. NEDJALKOVA et al.
1050
Table 4. The electrode temperatures at 1 mm below the top surface of
the electrode and burn-off time obtained from the d.c. arc in different gaseous atmospheres, in the presence of 0.7 mg Ba(NO,),
Our measurement; “graphite arc” method
Gas
Electrode ( = anode) temperature (R)
M~surements by other authors, when the samples are placed in the electrode cavity Electrode
Burn-off time (s)
Air Nitrogen Helium
1870 1870 1330
10 10 35
Argon
1220
40
(= anode) temperature (R) 1940 [12] 1480 1360 1140 965
[12] rl3] [tzj [13]
Burn-off time (s) 110 [12] > 900[12] -
placed in the electrode cavity [12, 131, we can conclude that these temperatures are approximately the same. But, the burn-off time in argon is RS800 s longer if the sample is placed in the electrode cavity, while in the “graphite arc” the burn-off time does not differ more than 30 s. 3.1.2. Influence of the gaseous atmosphere on the excitation parameters. The arc temperatures were calculated from the intensity ratios of zinc lines and the electron pressure from the calculated temperature and the intensity ratios of magnesium lines as well as the intensity ratios of calcium lines using standard equations [ 14, 151. The values obtained are accurate only if local thermal equilibrium (LTE) exists otherwise they can only be regarded as an estimate of the parameters and an indicator of changes in these parameters. The results obtained are given in Table 5. There is little difference between the values obtained when the arc burnt in air or when it burnt in nitrogen. This is probably due to the fact that nitrogen is a major component of air and in addition the functions of heat conductivity vs temperature have the same shape for air and nitrogen and approximately the same values in the interval 5~8000 K [ 16,173. The measured values of temperature and electron pressure obtained in atmospheres of argon and helium are higher than those obtained in air and nitrogen. KOLESNIKOV [18] made an experimental study of thermal equilibrium in arcs burning in argon or helium atmospheres. He has demonstrated the existence of LTE in a d.c. argon arc with a current of 10 A and with electron concentrations greater than 3 x 10’ J cm- 3, while the existence of LTE in the helium atmosphere is achieved at currents greater than 300 A. Therefore we suppose that LTE exists in argon atmosphere, while LTE does not exist in helium atmosphere, since we have applied a 10-A d.c. arc in our experiments. 3.1.3. Influence of gaseous atmosphere on the background intensities. In practice, the background may be appreciably reduced by decreasing the arc temperature [19] or by eliminating the contribution of certain molecular bands. In Table 6 are present the background intensities in various spectral regions when the arc burnt in different atmospheres and in the presence of 0.7 mg Ba(NO,), . The background has been determined during the burn-off time. The burn-off times are given in Table 4. 1121 V. A. MARZUVANOV, Izv. Acad. Sci. USSR 23, 1059 (1959). [13] R. E. THIERSan& B. L. VALLEE, Proc. 6th Call. Spectrosc. hf., Amster&m 1956, Spectr~c~im. Acta 11, 179 (1957). [14] P. W. J. M. BOUMANS, Theory of Spectrochemicai Excitation. Hilger and Watts, London (1966). [15] B. PAVLOVI& N. IKONOMOVand V. VUKANOVIC, Bull. Chem. Sot. (Beograd) 34, 313 (1969). [16] V. VUKANOVI~, N. IKONOMOVand B. PAVLOV&, Spectrochim. Acta 26B, 95 (1971). [17] N. IKONOMOV, B. PAVLOVI~and V. VUKANOVI~, Bull. Inst. Phys. Reck Atom. Acad. Bulg. Sci. 21,29 (1971). [18] V. N. KOLESNIKOV,Trans. P.N. Lebedev Physics Institute, Acad. Sci. USSR 30,66 (1964). [19] J. P. WILLIS,M. KAYE and H. AHRENS, AppI. Spectr. 18, 84 (1964).
Effect of controlled atmospheres in graphite arc
1051
Table 5. Effective values of temperature and electron pressure in air, nitrogen, argon and helium atmospheres
Temperature (K)
Gas Air Nitrogen Argon Helium
5800 5900 7300 8000.
Electron pressure (Pa) 81 91 1010 6700
*The electron temperature.
Table 6. Background intensities in different spectral regions when the arc burnt in gaseous atmospheres Spectral region
Air
250 nm 300 nm 330 nm* 385 nmt
0.30 0.40 0.70 0.90
Background intensity Argon Nitrogen 0.30 0.40 0.80 0.90
0.40 0.52 0.30 0.28
Helium 0.40 0.54 0.30 0.32
*Region of N,-molecular bands. tRegion of cyanogen emission.
From the results presented in Table 6 it is evident that in the spectral region 250-300 nm the background is not so high, and it is not significantly changed in different atmospheres. But, in those spectral regions where the increase of background is due to the molecular bands influenced by the presence of nitrogen, the background intensity will be significantly lower in argon or helium atmosphere than in air or nitrogen. 3.1.4. lnjuence of the atmosphere on the line intensities. The effect of nitrogen, argon, and helium, respectively, on the spectral line intensities in the “graphite arc” method was investigated and compared with those obtained when the arc burnt in air. We have found the following. (i) The use of nitrogen not only has an insignificant effect on the electrode temperature and excitation parameters, but also affects line intensities insignificantly. (ii) With an atmosphere of helium, the observed atom lines decreased in intensity (Table 7). The ratios lair/lHe for atom lines of other elements, which are given in Table 2, have approximately the same values (3.7-3.8). Therefore, in the helium atmosphere, the decrease of atom lines is not significantly dependent on the ionization potential of investigated elements. (iii) The magnitude of the increase of the ion line intensity is related to the total excitation energy, i.e. ionization plus excitation energy, of the lines as shown in Fig. 1. This relationship does not show a maximum since the lowest excited levels of helium lie between 19 and 20 eV and this corresponds to the upper limit of the total excitation energies of the investigated lines. Under the applied experimental conditions, resonance excitation probably occurs although this would affect only a small number of energy levels. We want to remark here that recently KARJAKINet al. [20] have tried to explain the mechanism of action of inert gases on the spectral line intensities in emission spectrochemical analysis by taking into account the collisions of the second kind. (iv) A considerable enhancement of both atom and ion lines occurs with the elements that have ionization potentials greater than 8.5 eV when an argon atmosphere is used. It was established that the enhancement of the spectral lines is related to the ionization potentials of the elements as shown in Fig. 2. This figure also shows that the shapes of the curves for the [20] A. V.
KARJAKIN, L. I. PAVLENKO, L. I. KARPENKOand L. P. SHTEPA, 2%.
Anal. Khim. 39,411 (1984).
1052
N.
NEDJALKOVA
et al.
Table 7. The change of the intensity for atom lines in an arc burning in air and helium atmosphere, respectively
Anaiyte
Spectral line (nm)
Ionization potential (V)
P I Zn I co1 In I
253.56 328.23 304.40 325.60
10.48 9.39 7.79 5.79
‘Ir
3.80 4.27 2.24 3.16
‘air llhelium
14.18 15.85 8.51 12.02
3.72 3.71 3.75 3.80
*Line intensity in helium atmosphere. +Line intensity in air.
I m
I Aor 15 cu
10 -
5
-~
0, 3
I 10
15
20
Eq=VI
1 25
+ Vq.
eV
Fig. 1. Ratio of the ion lines intensities (lhelium/lair) as a function of total excitation energy (E, = vi + V,).
atom lines and for the ion lines are similar to that of the curve for the relative degree of ionization, a, as a function of the ionization potential of the relevant elements. Having in mind the above results, we have concluded that a controlled argon atmosphere can extend the application of the “graphite arc” method for the determination of elements with ionization potentials higher than 8 eV. Therefore, from the spectroanalytical point of view, argon atmosphere is worth further investigation. 3.2. Additional investigation of argon atmosphere influence on line intensities 3.2.1. The injuence of the amount of Ba(NO& in an argon atmosphere on line intensities and on the excitation parameters. It was found that the atom line intensities of different elements displayed maxima as the amount of barium nitrate added to the electrode was increased as shown in Fig. 3. As we have already seen, changing the amount of barium nitrate results in changes in the plasma temperature and in the electron density (Fig. 4). Thus the maxima occurs at specific temperatures and electron densities. The temperatures at which the maxima occur are in close agreement with the optimum temperatures calculated using the method suggested by DE GALAN and BOUMANS [21] as shown in Table 8. For the calculation of optimum temperatures the following values of electron concentrations in the arc plasma were assumed. For elements from P to Cd: ne = 1 x 1Ol6 cm-3; the corresponding amount of additive is 0.7 mg of Ba(N03)2; for other elements: [21] L. DE GALANand P. W. J. M. BOUMANS,Z. Anal. Chem. 214, 161 (1965).
Effect of controlled atmospheres in graphite arc d’
.
5 Intensity ratio *
of the
atom lines as a fuktion of the ionization pdentials V,
Fig. 2. (I&
_
I
Zn
10 Intensity ratio e
1053
&Argon dn,r t
Xl
5
V, ,eV
V, ,eV
The dependence of the
of the
ratio d = *
lines as a function of the ionization potentials V,
ion
dAir
On the
ionization potentials Vi d q degree of ionization
/rht ) of the atom lines, (I&,, /IzJ of the ion lines, and (a.&apir as a function of ionization potentials.
) degree of ionization
109IA&Q”
7
1.5 -
I,,(Mn)-T=6300
I,., (Te)-T=6800
1 xiv
1x10-
K
K
1x10-l0gC.g
Fig. 3. Atom tine intensities of the elements with different ionization potentials as a function of barium nitrate amount.
nc = 2 x 1016 cm- 3; the amount of E3a(NO& is 1.4 mg. In Table 8 the elements are arranged in order of decreasing ionization potential. It follows therefore that by adjusting the amount of barium nitrate added, it is possible to obtain optimum conditions for the excitation of specik atom lines in argon atmosphere. 3.2.2. Spatial inhomogeneity of the argon plasma in the presence of barium nitrate. Many authors have investigated spatial inhomogeneity in the d.c. arc plasma [ 14, 15,223. We have compared only axial and radial distributions of line intensities in the d.c. arc burnt in air and argon, respectively. [22] R. J.
DECKER
and P. A.
MCFADDEN,
Spectrochim. Acta 3OB, 1 (1975).
N. NEDJALKOVAet al
1054
Fig. 4. Effective values of temperature and electron pressure measured in argon temperature as a function of the barium nitrate amount. TabIe 8. Optimum temperatures for the emission of distinct spectral lines Toptimum
Toptimum
(calculated) (K)
(experimental) (K)
Hg As Zn Be Te Cd
7300 7200 7100 7ooo 6900 6700 6700
7100 7100 7100 6800 6800 6800 6700
Bi Fe co Ni Mn
6300 6250 6200 6200 6200
6.500 6500 6500 6500 6300
Analyte P
Pb
6200
6300
TI
5300
6300
In
SO@0
6300
3.2.2.1. Axial distribution. The influence of barium nitrate as an additive and of the argon atmosphere has been investigated earlier by using the techniques of imaging the arc on the slit of the spectrograph 123,241 and of using a s&less spectrograph combined with equidensitometry [ZS]. These investigations have shown the foilowing. (i) For a given distribution of the arc parameters the particle density of the elements determines the axial distribution. The observed enhancement of the spectral lines in the vicinity of the electrode corresponds to a high concentration of the particles of a given element in these regions. (This enhancement is shown in the equidensity contour diagrams for Tl I 276.8 and for Hg I 253.7 given in Figs 5 and 6.)
[23]
N. KRASNOBAEVA, 2. ZADGORSKA and N. NEDJALKOVA, Spectrochim. Acta 33B, 655 (1978).
[24]
8, PAVLOVIC,D. PETROVI~,N. KRASNOBAEVAandN. NEDJALKOVA,Bu#. Chem. Sot. (Beogd)
[25]
K. D~TFRICH, K. NIEBERCALL,N. KRASNOBAEVAand N. NEDJALKOVA,S~ctr~h~m.
4,203 (1979). Acta 38B, 461 (1983).
Effectof controlled atmospheres in
1055
i&S tOM 2 K
Tl Argon
1
276.78 Air
b)
Fig. 5. Radial distribution of rnercurjt fine Hg I 253.65 (pi = tQ.43V) with anode (a) and cathode (b) excitation. Curves: (1) for arcing in argon, (2) for arcing in air. The carresponding equidensitograms are shown for comparison [25].
Ul
Fig. 6. Radial distribution of thalium line TI I 276.78 (Vi = 6.1 V) with anode (a) and cathode (b) excitation. Curves: (1) for arcing in argon, (2) for arcing in air. The carresponding equidensitograms are shown for comparison [25]. sA
1056
N. NEDJALKOVA et ol.
(ii) When optimum conditions are being selected for trace analysis using the “graphite arc” method, it is useful to take into account the axial distribution of the analytical signal. 3.2.2.2. Radial distribution. Experiments were performed in air and in argon by evaporating dry residues from the lower electrode to which 0.7 mg barium nitrate had been added and the radial intensity distributions measured at 1 mm from the lower electrode, in the centre of the plasma and at 1 mm in front of the upper electrode. Above experiments were performed in cathode and anode excitation, respectively. The intensity distribution obtained directly from the photographic image represents intensities integrated along the line of observation. From this intensity distribution, the radial distribution was calculated by solving the Abel inversion integral [26]. Figure 5 gives the results obtained with Hg I 253.6 in air and in argon and Fig. 6 gives the results from Tl I 276.8. These figures show the following. (i) Anode excitation in air is characterised by a classical cathode-layer enhancement of lines with low ionization potentials. In argon the enhancement near the anode is observed for all elements. (ii) Cathode excitation in both air and argon results in an enhancement of the spectral line intensity in the vicinity of the cathode. (iii) The argon plasma is characterised by a more rapid radial decrease in the spectral line intensity than when the arc burns in air. This is the consequence of smaller radius of the arc column in argon in comparison with one in air. Table 9. Detection limits of chemical elements when the samples are arced under different conditions Spectral line (nm)
Element P Hg As Zn* Be* Be Te Cd Bi Fe co cu Mg+ Ni Mn Pb TI In
I I I I I II I I I I I I I I I I I I
253.56 253.65 278.02 334.50 332.13 313.10 238.57 326.10 306.77 302.06 304.40 324.70 285.21 305.08 279.48 283.30 276.79 325.60
Air* 1200 1200 450 100 60 0.1 10000 20 7 0.4 0.3 20 6 0.1
Detection limits (ng) Nitrogen@ Argon11 1200 1200 450 100 60 0.1 10000 20 7 0.4 7 0.3 20 6 0.1
1 5
5 1
30 50 30 2 2 0.005 300 1 1 0.1 2 0.1 5 2 0.08 0.7 3 0.8
*Spectral lines are in the region of molecular bands which can be eliminated in argon atmosphere. ‘Limited only the blank. *Free-burning arc in air in presence of 0.7 mg Ba(NO&. Current 8 A. @Nitrogen-stabilized d.c. arc in presence of 0.7 mg Ba(NO,),. Current 8 A. l/Argon stabilized d.c. arc. The elements P, Hg, As, Zn, Be are determined in the presence of0.7 mg Ba(NO&; the elements Te, Cd, Bi, Fe, Co, Cu, Mg, Ni are determined in the presence of 1.4 mg Ba(NO& and Mn, Pb, Tl, In are determined in the presence of 2.1 mg Ba(NO,),.
[26] F.
KUMMEL, Tobellenfiir
die L~Gung der Abelschen Inregrolgleichung.
Akademie, Berlin (1960).
E&ct of controlled atmospheres in graphite arc
1057
Table 10. Detection limits of different elements; arc in helium in presence of 0.7 mg Ba(NO&
Element
cu Zn cd Mg Be
II II II II II
Spectral line (nm)
Total excitation energy (ev)
276.97 250.20 274.84 279.08 313.14
21.10 20.36 19.27 16.51 13.14
Detection limit (ng) 230 62 41 40 0.03
3.3. detection limits and precision
Table 9 summarizes the analytes and the detection limits obtained when the arc burnt in different gaseous atmospheres. As we have shown in Section 3.1.4, only ionic lines are enhanced in helium atmosphere. The investigated ionic lines of elements, with a total excitation energy value close to the lowest excitation level of helium, and which show significant intensity enhancement under the above experimental conditions, are characterized by a low intensity in the arc [27j. Therefore, by using these lines, it was not possible to achieve lower detection limits. Table 10 shows the detection limits for when the arc burns in helium atmosphere in the presence 0.7 mg Ba(NO&. Relative standard deviations calculated for n = 20 measurements in the ‘*graphite arc” method using 0.7 mg Ba(NO& as additive were 6 y0 in argon and helium atmosphere and 1% in air and nitrogen atmosphere, respectively. 4. SUMMARY ANDCONCLUSION This investigation of the influence of different atmospheres in the presence of barium nitrate on the plasma parameters and on the line intensities has shown the following. (i) A nitrogen atmosphere gives detection limits very similar to those obtained when the arc burns in air. In some instances the gross signal is lower than that obtained in air but the background level is also lower due to the isolation of the plasma from the environment. Thus nitrogen can only be profitably used as a stabilising gas flow. (ii) The different excitation characteristics found when the arc burns in argon or helium makes it possible to choose an atmosphere which is appropriate for a given analytical problem. (The ion lines of the elements whose total excitation energies are close to the lowest energy levels of helium have low intensities in the arc, and thus little is gained in terms of detection limits when helium is used as the atmosphere.) (iii) The use of an argon atmosphere in the presence of barium nitrate extends the possibilities offered by the “graphite arc” method because lower detection limits can be achieved for elements with high ionization potentials than in an atmosphere of air. Acknowledgntents-We are thankful to Dr R. J. DECKERfor numerous useful suggestions and remarks that have considerably improved the manuscript. We wish also to express our gratitude to Dr. P. W. J. M. BOUMANSfor commenting on this manuscript.
1271A. N. ZAIDE~ V. K. PROKOV’EV,S. (1962).
M. RAJSKIJ anil E. Ju. SCHREIDER, T&icy SpektraPnykh L&ii, Moskow