Axial distribution of the analytical signal in the “graphite arc” method in the presence of additives and in different atmospheres

Spectrochimica Acta,Vol.33B,pp.655to 664 Pergamon PressLtd.1978 Printed m GreatBritain

Axial distribution of the analytical signal in the “graphite arc” method in the presence of additives and in different atmospheres* N. KRASNOBAEVA, Z. ZADGORSKAand N. NEDJALKOVA Institute

of General

and Inorganic

Chemistry,

Bulgarian

Academy

of Scieqces, Sofia 1000, Bulgaria

(Received 28 April 1978)

Abstract-The influence of various potassium salts and barium nitrate on the axial distribution of AY = was investigated in the case of the line In 303.9 nm. Comparative studies of the axial log Illne - log Ibackground distribution were made for lines of the elements: Tl, Pb, Mg, Co, Cd, Zn, Hg in the presence of Ba(NO& in air and in argon. The axial distributions of the effective temperature and the electron pressure were studied in the presence of various additives in air and in argon with barium nitrate as additive. It is shown that a most favourable region of emission exists for every element for a given additive and atmosphere.

1.

INTRODUCTION

IMPROVEMENT of the signal-to-noise ratio in the development of new analytical methods or in the improvement of already known ones has become one of the major trends in spectral trace analysis after the works of KAISER [l, 21 who introduced the concepts of analytical signal (in the photographic recording of the spectra: AS = Sline- Sbackground or AY = log Iline - log Ibackground, where S is blackening and I intensity), noise level and a statistically defined detection limit for the characterization of analytical methods. Practice has shown that the dry residue method makes it possible to achieve low detection limits. In this method a drop of a dilute solution is evaporated on the top surface of a carbon electrode impregnated with polystyrene. The electrode is arced in the presence of additives containing elements with low ionization potentials [3-51 or in a controlled argon atmosphere [4,6]. On the analogy of the “copper spark” method this technique is sometimes called “graphite arc” method [IS]. We shall adopt this term here. The further improvement of the detection limits for the individual elements in the “graphite arc” method may be achieved by selecting the optimum region of arc emission which is characterized by the maximum analytical signal in the sense defined above. Tne noise level to be connected with this signal is expressed in terms of the standard deviation of the log background intensity. This noise level depends entirely on the properties of the photographic emulsion, the developing procedure, the photometered area, etc., but is independent of the source characteristics (see e.g. KAISER[9], LAQUA[lo] and BOUMANS and MAESSEN[ll, 121. This noise level will not be further considered here. As is well known, the inhomogeneity of the distribution of the arc cloud is a consequence

THE

* Dedicated [l] [2] [3] [4] [S] [6] [7] [8] [9] [lo] [ll] [12]

to the memory

of Professor

HEINRICHKAISER.

H. KAISER,Speccrochim. Acta 3,40 (1947). H. KAISER,Z. Anal. Chem. 209, 1 (1965). CH. I. ZILBERSTEIN,R. GERBATSCH,0. N. NIKITINA, M. P. SEMOV and G. ARTUS, Reinststofprobleme (Edited by E. REXER),Vol. II, p. 187. Akademie, Berlin (1966). N. KRASNOBAEVA,Dissertation, Sofia (1968). N. KRASNOBAEVA,Ju. HARIZANOV and Z. ZADGORSKA, Bulletin de I’ Institut de Physique et de Recherche Atomique, Acad. Bulg. Sci. 21, 171 (1971). N. KRASNOBAEVA,N. NEDJALKOVA-DASKALOVA, Zh. Prikl. Spektrosk. 23,768 (1975); 26,201 (1977). M. FRED, N. H. NACHTRIEBand F. S. TOMKINS,J. Opt. SOC. Am. 37, 279 (1947). J. CZAKOW, Proc. 14th Coil. Spectrosc. Int., Debrecen 1967, Vol. I, p. 219. Hilger, London (1968). H. KAISER,Optik 21, 309 (1964). K. LAQUA, Z. Anal. Chem. 221,44 (1966). P. W. J. M. BOUMANSand F. J. M. J. MAESSEN,Z. Anal. Chem. 220,241 (1966). P. W. J. M. BOUMANSand F. J. M. J. MAESSEN,Z. Anal. Chem. 225,98 (1967).

S.&(B) 33,9--H

655

656

N. KRASNOBAEVA, Z. ZADGORSKA and N. NEDJALKOVA

of the heterogeneous composition of the arc plasma [13-151. By changing the composition of the plasma it is possible to act on the distribution of the emission and, in particular, on the axial distribution from anode to cathode. Numerous references can be found in the literature concerning the analytical application of the inhomogeneity of the arc emission during sample evaporation from the cavity of the electrode. In the first place the classical work of MANNKOPFF and PETERS [16] on the use of the enhancement in the cathode region should be mentioned.

One can also

point to [17] and [18]. In the case of the “graphite

arc” method, indications are given only on the enhancement in the cathode region of lines of elements with a low ionization potential [3] and in the absence of additives while the influence of the electrode polarity on the axial distribution of the line intensity has been also studied [19]. The present work deals with the results of an investigation performed to establish the influence that the factors which act on the plasma composition (additives and controlled atmosphere) have on the axial distribution of the effective arc parameters and the analytical signal in the “graphite arc” method with evaporation of a dry residue from the top surface of the electrode. 2. EXPERIMENTAL 2.1. Optics 2-m grating spectrograph PGS-2; grating 650 rulings mm-‘, used in the second order; blaze 570 nm. Slit width : 20 pm. Entrance optics : projection of the arc gap with a si,ngle lens on the slit of the spectrograph. 2.2. Arc operution D.c. arc with pulse ignition; in argon.

current:

8 A at excita;ion

of the spectra

in air; IO A at excitation

of the spectra

2.3. Electrodes Graphite electrodes, VEB Elektrokohle Lichtenberg: Spektralkohlen Electrodes placed in water-cooled clamps. Analytical gap: 4 mm.

TO, with shape and size shown

in [20].

2.4. Solutions The analyte solutions were prepared by dissolving metals and salts in HNOs. The test solutions had the following concentrations (mg/ml): In-l x 10e3, Tl-1 x lo-‘, Pb-3 x IO-j, Mg-1 x 10-3, Co-l x lo-‘, Cd--- 1 x lo-‘, Zn-1 x lo-‘, Hg-1 x 10-l. A solution containing all elements in the above concentrations was prepared. By means of a micropipette 0.02 ml of the solution was placed on the top surface of the anode. An amount of 0.02 ml of the additive solution with the corresponding concentration was applied after drying. Aqueous solutions of the following additives were used : KCI, KBr, KI, KN03, KzC03, K2S04 and Ba(N03)2. They contained either 40 mg/ml K, 10 mg/ml Ba (Sections 3.1 and 3.2) or 18.4 mg/ml Ba (Sections 3.2 and 3.4). 2.5. Controlled Control

atmosphere

of the atmosphere

was achieved

by a modified

Stallwood-jet

type device [21]

2.6. Erposures In Sections 3.1 and 3.2, exposures were taken immediately after arc ignition for 10.5 s duration In Sections 3.3 and 3.4, the preliminary exposure was 1 s and the main exposure was 2 s. 2.7. Spectrul

lines

Table 1 summarizes

the analytes

and the wavelengths

and excitation

energies of the spectral

lines.

P. W. J. M. BOUMANS, Theory ofSpectrochemicul E.xcitution. Hilger & Watts, London (1966). V. VUKANOVI~, Proc. 20th Coil. Spectrosc. Inf. and 7th 1111. Cmf Atomic Spectrosc,, Inaited Lectures, Prrrgue 1977, Vol. I, p. 45. Statni Pedagogicke Nakladatelstvi, Prague (1977). [ 151 K. DITTRICH, K. NIEBERCALL und H. ROSSLER, Spectrochirn. Actcr 31B, 331 (1976). [16] R. MANNKOPFF und C. PETERS, 2. Physik 70,444 (1931). [17] R. AVNI, H. HARELL and A. S. LOURENCO, Preprints 16th Co/l. Spectrosc. Int., Heidelberg 1971, Vol. I, p. 1. Hilger, London (1971). [18] K. ZIMMER, S. KEREKES und E. SZABO, Preprints 16th Co/l. Spectrosc. Int., Heidelberg 1971, Vol. I, p. 7. Hilger, London (I 971). [19] CH. 1. ZILBERSTEIN, 0. N. NIKITINA, M. P. SEMOV and S. S. LEGESA, Iza. Sib. otd. Akad. Nauk USSR. Ser. Khim. 2, 87 (1967). [13] [14]

[20]

N. KRASNOBAEVAand Z. ZADGORSKA, Spectrochim.

[21]

N. KRASNOBAEVAand N. NEDJALKOVA-DASKALOVA,Izu. Otd. Khim.

Acta 26B, 301 (1971).

Nauki. Bulg. Akad. Nauk 7, 385 (1974).

Axial distribution of the analytical signal in the “graphite arc” method

657

Table 1. Data of the analytes Analyte

Spectral line (nm)

Excitation energy (ev)

In Tl Pb Mg co Cd Zn Hg

1 303.9 I 216.7 1283.3 1285.2 1304.4 I 326.1 I 328.2 1253.65

4.1 4.40 4.44 4.30 4.07 3.80 7.78 4.88

Ionization potential (V) 5.78 6.10 7.41 7.64 7.87 8.99 9.39 10.43

3. RESULTS

3.1. Injuence of the anionic component of the additive on the distribution of the analytical signal AY It was previously shown [22] that in the graphite arc method the total chemical composition of the additive affects the axial distribution of the line intensity of the element under investigation as well as the background near these lines and consequently the analytical signal BY. As an example, Fig. 1 shows the distribution of AY for the line In I 303.9 nm in the presence of potassium salts. 3.2. Injluence of the anionic component of the additive on the axial distribution of the eflective arc parameters It was previously shown [23] that the effective values of the temperature and the electron pressure in the central part of the arc, which is generally used as analytical region, depend not only on the nature of the cationic component of the additive but on the form of the anionic component as well. The results of the investigation of the effect of the anionic component of the additive on the distribution of the effective arc parameters are illustrated in Figs. 2 and 3. Data obtained in the absence of additives are given for comparison purposes. From the intensity ratio of the zinc lines Zn I 328.2 nm and Zn 1307.6 nm we derived an effective temperature and from the intensity ratio of the magnesium lines Mg 1285.2 nm and Mg II 279.5 nm an effective electron pressure, using well-known relationships and transition probabilities [13]. We see from these curves the following : (i) The introduction

of an additive brings about a drastic change in the axial distribution 6

3

1*5

2.0 5

1.0 >

1s

~~

a

1

7

0.5

L

3

1.0

0

0,5 12

3L

123L

Distance, mm Cathode

Allode

Distance,mm Anode

Cathode

Fig. 1. AI’ = lo&! Iline - log Ibackground for the line In 303.9 nm in dependence on the distance from the anode in the presence of (1) KCI, (2) KBr, (3) KI, (4) KN03, (5) K2C03, (6) KzS04, (7) Ba(NO&. [22] N. KRASNOBAEVA and Z. ZADGORSKA, [23] Z. ZADGORSKA and N. KRASNOBAEVA,

Zh. Prikl. Spectrosk. C.R.

Acud.

Bulg.

29, (1978).

Sci. 28,4 (1975).

658

N. KRASNOBAEVA,Z. ZADWRSKA and N. NEDJALKOVA

,L I

1 2 Distance,

4 n-m3

Anode

Cathode

Fig. 2. Effective plasma temperature in dependence addition (1) and in the presence of additions

on the distance from the anode of (2) KBr, (3) KN03, (4) K2S04.

without

of the temperature and electron pressure. The shape of this distribution depends on the anionic component of the additive for one and the same cation. (ii) The shape of the temperature curves is identical for different halides. This is why Fig. 2 shows the temperature curve for KBr only. The temperature minimum in the middle of the interelectrode space and a temperature increase towards the electrodes are noticeable. The temperature increase is more pronounced near the cathode. The axial distribution of the electron pressure in the presence of halides is characterized by an increase in electron pressure near the anode.

k

No additw

KNO, F

-3.00

I--

,“3.50

l-

8 --4.00

I-

- 4.50

‘f

KI I

1 2 Distance, Anode

’

rnrr? Cathode

K,%

1 1

/

4 Distake.

Anode

t-m? Cathode

Fig. 3. Effective values of the electron pressure (atm) in dependence of the distance from the anode without additive and in the presence of KCI, KBr, KI, KN03 or K2S04.

Axial distribution

of the analytical

signal in the “graphite

arc” method

659

1

1 2 Distance, mm3 Anode

Distance, Cathode

Anode

mm Cathode

Fig. 4. Logarithm of the ratio of the intensities I, and 1~ upon dry residue evaporation from the surface of the lower electrode (= anode) in dependence on the distance (d) from the anode. I, = line intensity at a point m located at d mm from the anode. 1~ = intensity at a point A located at 0.14 mm from the anode for the elements (1) TI, (2) Pb, (3) Mg, (4) Co, (5) Cd, (6) Zn, (7) Hg.

(iii) The curves that depict the temperature distribution in the presence of KN03 and K2S04 show a small maximum close to the anode. In the presence of KN03 the location of the maximum on the curve of the electron pressure coincides with that of the maximum temperature. However, in the presence of KzS04 the shape of the electron pressure curve resembles more that found in the presence of KCl. 3.3. Comparative study of the influence of the gaseous atmosphere on the axial distribution of the line intensity in the presence of Ba(NO& It has been already shown that in the “graphite arc” method the addition of Ba(NO& [4,6] is more effective than that of NaCl since it leads to lower detection limits. In [22] it was proved that in the presence of Ba(N03)2 the axial distribution of the background in an arc burning freely in air is rather homogeneous. The homogeneity in the distribution of the background and the lower levels of the latter are also typical when the arc burns in an argon atmosphere with addition of Ba(NO&. Therefore the axial distribution of the line intensity links up in both cases with the distribution of the analytical signal. The stabilizing effect of barium nitrate on the arc has been noted in [6]. In the present work, experiments were performed by evaporating dry residues from the lower electrode with addition of Ba(NO& at different electrode polarities, in air and argon. Figure 4 illustrates the distributions of the line intensities of the elements with different ionization potential in air and argon and Fig. 5 shows the same distributions for cathode excitation. The intensity of each line is normalized to the intensity of the line recorded at the point located closest to the anode (0.14 mm from the anode). Figure 5 shows the following : (i) Anode excitation in air is characterized by a classical cathode-layer enhancement for lines of elements with low ionization potentials. Qualitatively the enhancement effect can be correlated with the ionization potential with a slight deviation in the case of cobalt.

N. KRASNOBAEVA. Z. ZADGORSKA and N. NEDJALKOVA

660

4

2 1 Distance, mm3

0 Anode

Cathode

0 1 2 3 Anode Distance, mm

4 Cathode

Fig. 5. Logarithm of the ratio between the intensities I, and IA upon dry residue evaporation from the surface of the lower electrode (= cathode) in dependence on the distance from the anode. The notation is the same as in Fig. 4. (1) Tl, (2) Pb, (3) Mg, (4) Co, (5) Cd, (6) Zn, (7) Hg.

(ii) During cathode excitation in air the enhancement effect near the cathode is still more pronounced for elements with low ionization potentials (Tl, Pb and Mg). For the remaining elements the effect of line enhancement near the cathode is not so strong and does not depend on the ionization potential. (iii) In the case of anode excitation in argon (Fig. 4) enhancement near the anode is observed for all elements. The strongest enhancement was observed in the case of heavy elements (Pb, Tl and Hg). (iv) In the case of cathode excitation in an argon atmosphere (Fig. 5) line enhancement is observed near the cathode for all elements. Near the anode an additional maximum is observed for Tl and Mg and to a very small extent for Pb. There is no correlation with the ionization potential. Table 2 contains the maximum values obtained for BY for the various elements under different conditions of evaporation of the dry residues. Clearly, when the electrode polarity is reversed and the atmosphere in which the spectra are excited is changed, it is possible to select the most favourable region for recording the signal of a given trace element. Table 2. Maximum

values of BY for different conditions

of evaporation Argon

Air Analyte

Tl Pb Mg co Cd Zn Hg

Anode excitation AY 0.66 1.13 1.46 1.31 1.12 1.29 0.77

Cathode

excitation AY 1.60 1.78 1.98 1.40 1.16 1.34 0.66

Anode excitation AY 1.04 0.86 1.15 0.64 1.32 1.75 1.15

Cathode

excitation BY 1.48 1.55 1.41 I .02 1.64 1.60 1.06

Axial distribution of the analytical signal in the “graphite arc” method 80 Air

’

661

I

Distance, mm ^ . Cathode Fig. 6. Effective values of temperature and electron pressure in dependence on the distance from the anode in air and argon with anode (2) and cathode excitation (1). AK&

3.4. Influence of the gaseous atmosphere on the axial distribution of the effective plasma parameters Under the assumption that in an arc burning in argon at a current of 10 A the state is close to LTE [24], effective values of temperature and electron pressure were derived. Figure 6 represents the axial distribution of the effective temperature and electron pressure for anode and cathode excitation of a dry residue with addition of Ba(N03)2 in air and in argon. From Fig. 6 it can be concluded that the axial distribution of temperature and electron pressure depends on the gaseous atmosphere and on the polarity of the lower electrode.

4. DISCUSSION

AND

CONCLUSIONS

When discussing the results obtained in the present work the following specific features of the “graphite arc” method should be borne in mind. (i) The dry residue of the solution under investigation is deposited as a thin layer on the top surface area of the electrode and is subjected to the direct action of the arc to a larger extent than in the case of evaporation of the substance from the cavity of the electrode. (ii) Since the method is applicable to very dilute solutions, the additive constitutes the matrix df the residue and its effect on the electrode phenomena and on the plasma processes is stronger than’in the case of evaporation of a powdered substance in the presence of an additive. [24] V. N.

KOLESNIKOV,

Trms. P.N. Lebedev Physics Insritute. Acad. Sci. USSR 30, 66 (1964).

N.

662

KRASNOBAEVA,

Z. ZADG~RSKA

and N.

NEDJALKOVA

Argon

t

2 1 Distance, mm3

i

Distance, mm3 Cot hode

Anode

1

4

Cathode

Anode

0.5 +--Q

I

.-t& 0

(9

s

62 - 0,E

1 2 Anode Distance, Anode

3 mm

4 Cathode

Cathode

Fig. 7. Logarithm of the ratio between the Boltzmann factors in dependence on the distance from the anode. i;, is the effective temperature at a point m and PA is the effective temperature at a point A for the elements (1) Tl, (2) Pb, (3) Hg ; upper frames : anode excitation ; lower frames cathode excitation.

I:_,1 7 0

1 2 Distance,

4

mn? Cathode

Anode

0 1 2 3 Anode Distance, mm

i Anode

Distance, mm

Cathode

0

i

:

1 Cathode

J

Anode Distanze. mm3 Cothodz

Fig. 8. Change of the ionization factor in dependence on the distance from the anode. .vrn is the degree of ionization at point m and .Q the degree of ionization at point A for the elements (1) Tl, (2) Pb, (3) Hg; upper frames: anode excitation; lower frames: cathode excitation.

Axial distribution of the analytical signal in the “graphite arc” method

663

It should also be taken into account that the arc used in the present work is a so-called short arc [25]. It is known that the intensity of a given spectral line is determined by the temperature, the electron pressure and the particle density in the plasma. Therefore the axial distribution of the line intensity will depend on these three factors. As shown by results given in Section 3.2, the introduction of an additive that contains a potassium salt leads to different distributions of temperature and electron pressure in the interelectrode space in dependence on the anionic component of the additive. This difference is probably connected with a different rate of entry of the additive depending on the nature of the compound [23]. The rate of entry of the investigated elements in the plasma also depends on the form of the additives [26]. The high ionization potential of argon and its low thermal conductivity determine the effective values of the arc parameters. However, the distribution is affected by the rate of the entry of Ba(N03)2 in the plasma. In an argon atmosphere the electrodes are heated to lower temperatures than in air. According to our measurements performed with an optical pyrometer the wall temperature at a distance of 1 mm from the top surface of the lower electrode reaches 1870 K in air and 1220 K in argon and in the presence of barium nitrate. The rate of entry of the investigated elements is responsible for the effect of both an additive and the atmosphere on the distribution of line intensities. According to [13,14,27] the distribution of the particle density in the plasma is a consequence of the dynamic equilibrium reached between the number of particles which enter the plasma in unit time and the rate at which particles leave the plasma as a consequence of transport processes. With the data given in Sections 3.3 and 3.4 and using the well-known Boltzmann-Saha equation for the intensities of atomic lines we estimated the change in the Boltzmann factor (Fig. 7), the change in the ionization factor (Fig. 8) and the change in the relative particle density (atoms and ions) of a given element (Fig. 9). Results are given for elements I-

Argon

(a)

1

Distance, Anode

mm Cathode

Anode

1

2 Distance,

1

3 mm Cathode

[25] P. W. J. M. BOUMANS,Excitation of Spectra, Analytical Emission Spectroscopic (Edited by E. L. GROVE), Part II, p. 33. Dekker, New York (1972). [26] 2. ZADGORSKA, N. KRASNOBAEVA and D. APOSTOLOV, Spectrochim. Acta 30B, 527 (1975). [27] I. A. KRINBERGand E. V. SMIRNOVA, Zh. Prikl. Spektrosk. 10,400 (1969).

664

N.KRASNOBAEVA,Z.ZADGORSKA

and N.NEDJALKOVA

1.5

1.0 f

z”

a

I

I3 -(

(b)

-O,! 5_ 0

1 H”O(le

1

1

2

Distance,

L mrr? ^.. Lolnoae

-O-5!

u

1

1

_

DistanL. A”Od@

I

J

L

t-nrff Cathode

Fig. 9. Log N,/NA as a function of the distance from the anode. N, is the total particle concentration at point m and NA is the total particle concentration at point A for the elements (1) Tl, (2) Pb, (3) Hg; (a) anode excitation; (b) cathode excitation.

with similar atomic weight, different ionization potentials and similar excitation potentials of the lines under consideration (Tl, Pb and Hg). These results confirm that for a given distribution of the effective arc parameters the particle density of the elements determines the axial distribution of the signal. The observed enhancements of the lines near the electrodes correspond to an enrichment in particles of a given element in these regions. The general conclusion of the present work is that it is useful to take into account the axial distribution of the analytical signal when selecting optimum conditions for trace element analysis by means of the “graphite arc” method.