Halogenation with CCl4 and CF2Cl2 vapours in arc emission spectrometry

Spearochimica Acta, Vol.358,~~. 401to420 @PergamonPress Ltd., 1980. Printed inGreat Britain

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Halogenation with Ccl, and CF,CI, vapours in arc emission spectrometry” T. NOR, I%. HANAK-JUHAI and E. PUNGOR Institute for General and Analytical Chemistry, Technical University of Budapest, H-1521, Hungary (Receiued 15 January 1980) Absiract-Halocarbon vapour diluted with air or nitrogen was applied during the arc excitation of carbide forming elements in solution form, of metal samples (copper, aluminium) and powder samples (zilumina, glass, RU-powder) on graphite supporting electrodes. Means were developed for the carrier vaporization of Ccl_, and for the introduction of the halocarbon vapour into the arc discharge. The gaseous agent was supplied continuously during excitation. This possibility was also subjected to some theoretical predictions. On applying Ccl, with samples introduced in solution form, the volatilization rates of the most refractory elements (e.g. W) were found to increase at least 50 times on the basis of line intensities. Fractional distillation could be attained on constituents and matrices similar to those reported with solid agents, but the gaseous agents could be applied more easily and without contamination problems. The overall effect of halogenation on excitation processes was evaluated from line intensities integrated over the total evaporation time of a complex powder sample, with and without graphite powder dilution. A high intensification (a factor of 3-12) was obtained for the U.V.lines of the refractory constituents with halogenation, which was attributed to the increased efficiency of these elements in entering the excitation zone. Decrease in the intensities of barium atom and ion lines in the VIS range and a decrease of self-absorption were found as a consequence of halide formation in the arc fringe.

1. INTRODUCTION THE HALOGENATIONof samples heated in excitation sources is applied to increase the volatility of the determinants. The extended literature of halogenation used in arc emission spectrometry was reviewed recently by NICKEL [l]. Accordingly, in these methods solid agents are mostly used which release halogen gas at elevated temperature. These agents are inorganic (e.g. silver and copper halides) and organic (e.g. teflon, perchloro-propylene) compounds, either mixed to powder samples or deposited on to metal chips [2] and excited in a graphite cup electrode. MAFWJVANOV [3] has found chlorine atmosphere advantageous in the arc excitation of heavy volatile elements. However, such a corrosive gas might be quite difficult to handle in routine work. The decomposition products of halocarbons introduced into arcs and radio-frequency plasma torches have been the subject of several molecular spectroscopic investigations as reviewed and contributed by KANA’AN et al. [4]. The use of halocarbons or other easily handled gaseous agents has been introduced recently in various techniques of atomic spectrometry. NICKEL [S] studied the vaporization of zirconium oxide labelled with g5Zr isotope and dispersed in a graphite pellet electrode (anode) of a d.c. arc. Investigating different atmospheres he has found that in the presence of Freon-12 (CF,CI,) vapour in the discharge atmosphere zirconium was completely evaporated from a 16 mm part of a rectangular pellet electrode due to the formation of volatile zirconium fluoride and chloride. SEELY and SKOGERBOE [6] elaborated an arc emission spectrographic method for the analysis of atmospheric

*Parts of this paper have been presented Poland (1979).

at the School of Analytical Atomic Spectroscopy,

Zaborow,

[l] H. NICKEL, Preprints 20th Coil. Spectrosc. Int. and 7th Znt. Conf. Atomic Spectrosc., Prague, 1977, Volume I, pp. 27-38 (1977). [2] P. TYMCHUK, A. MYKITIUK and D. S. RUSSEL, Appl. Specfrosc. 22, 268 (1968). [3] V. L. MARZUVANOV, Zzu.Akad. Nauk SSSR, Ser. Fiz. 23, 1059 (1959). [4] A. S. KANA'AN, C. P. BEGUN and J. L. MARGRAVE, Appl. Spectrosc. 20, 18 (1966). [S] H. NICKEL, K&m. Kiizl.(Proc.Hungarian Academy of Sciences) 39, 303 (1973). [6] J. L. SEELY and R. K. SKOGERBOE, Anal. Chem. 46,415 (1974). S.&(B) 35/7-B

401

T. K~oR,

402

pollutants

filtered

vaporization

and collected

of several

I?. HANAK-JUHAI and E. PUNGOR on the internal

elements

surface

could be attained

of a graphite

cup electrode.

from the deep electrode

Fast

by introduc-

ing HCl gas. The same reagent was used later [7] for producing volatile chlorides in a heated quartz vessel. The halide vapours were injected either into a flame for atomic absorption,

or into a microwave

induced

plasma for emission

et al. [8,9] used Ccl, or CF,Cl, vapours tion of carbide forming elements, and coupled

plasma

thermal

(ICP)

purification

use of Ccl,

source.

and CF,Cl,

arc, graphite [11-i31:

vapours

furnace,

has also been extended flame

samples

here is one of the simplest liminaries

in a d.c. carbon samples.

atomization/excitation

were

KIRKBRIGHT

used

to perform

detailed The

Arc

of these

may

The in d.c.

sources

processes

spectrography

information

results

[lo].

halogenation

on the excitation arc.

to aid the

and titanium

and in the combinations

ways of attaining

of multi-component to other

vapours

from aluminium

with the effect of halocarbons

and non-conducting

halogenation

tetrachloride

tube furnace

acetylene-air

This paper is concerned metals

Carbon

of a graphite

spectrometry.

in a graphite furnace to attain fast vaporizacombined this furnace with an inductively

of

applied

on high temperature also

be useful

as pre-

sources.

2. THEORETICAL CONSIDERATIONS Halogen-gas formed by thermal decomposition constituents of sample in both the condensed

from phase

the agent may react with the and the gas phase (plasma).

Generally, the formation of volatile halides in heterogeneous reactions can be exploited to improve the analytical performance. However, the formation of halides in the gas phase in the presence of free atoms two groups 2.1

of processes

Heterogeneous

Indirect shown

measurements

signal.

may also decrease Some

general

in the following

with a combined are formed

that at this temperature reacts

calculation

of the reaction

systems

of halogen

emission

are considered

species

gas which simple

amount

the concentration

predictions

for these

sections.

reactions

that reactive

expected

of excess

and thus the atomic

with

[14].

the

the major

sample

temperature

However,

graphite above

in solid

furnace

560°C

product or

Ccl,

vapour.

of the decomposition

molten

phase.

of the heterogeneous

the real spectrochemical

and flame system [13] have

from

The

chlorination

samples

It might

be

is chlorine

thermodynamic is possible

for

are usually multicom-

ponent systems and the reactivity-volatility characteristics of a certain component in such a matrix can hardly be treated theoretically. The small amount of sample applied in spectrochemical methods evaporates mostly between the melting and boiling points of the substance concerned [15,16]. Nevertheless, the boiling/sublimation data of pure substances are characteristic to a first approximation of the relative volatilities of the components in a complex matrix as well. The boiling/sublimation temperatures of metal [7] R. K. SKOGERBOE,D. L. DICK, D. A. PAVLICAand F. E. LICHTE, Anal. Chem. 47, 568 (1975). [8] G. F. KIRKBRIGHT,Analytiktreffen -Atomspektroskopie. Finsterbergen, GDR (1978). [9] G. F. KIRKBRIGHT, D. L. MILLARDand R. D. SNOOK, Preprints 21st Co/l. Spectrosc. Int. and 8th Conf. [lo] [ll] [12] [13] [14] [15] 1161

Atomic Spectrosc., Cambridge 1979, Paper No. 346 (1979). T. KANTOR, S. A. CLYBURN and C. VEILLON, Anal. Chem. 46, 2205 (1974). T. KANTOR and E. PUNGOR, Sci. Session on Atomic Absorption Spectrometry. Dobo&kb, Hungary (1977). T. KANTOR and E. PUNGOR, Analytiktreffen-Atomspektroskopie. Finsterbergen, GDR (1978). T. KANTOR, E. GRAF-HARSANYI, L. BEZ~~R and E. PUNGOR, Preprints 21st Coil. Spectrosc. Int. and 8th Int. Conf. Atomic Spectrosc., Cambridge, 1979, Paper No. P70 (1979). W. FRECH and A. CEDERGREN, Anal. Chim. Acta 82, 83 (1976). T. KANTOR and E. PUNGOR, J. Therm. Anal. 6, 521 (1974). R. RAUTSCHKE, G. AMELUNG, N. NADA, P. W. J. M. BOUMANS and F. J. M. J. MAESSEN, Spectrochim. Acta 30B, 397 (1975).

Halogenation

Table

with CCI, and CF,Cl,

1. Temperature

T(“C)

ranges

vapours

in arc emission

of boiling/sublimation

of chlorides

spectrometry

and fluorides

Chlorides

403

[17]

Fluorides

B, Si, P, Ge, Sn, Cr, As Ti, V, Se, Al, Ga, Ta, Nb MO, Sb, Hg, Hf, Zr, W, OS Te, Bi, In, Re

B, Si, P, Se, Ge, W MO, Te, Re, U, As V, Ta

Be, U, Fe, Zn, Ru, Lu, SC, Tl, Th, Ni, Pb, Cd

Zr, T1, Bi, Sn, Be, Fe, Ni

1000-1500

Co, Mn, Cs, Sr, Li, Cu, Rb, Mg, Na, Tm, K, Ho, Dy, La

Ga, Cr, Cs, Al, Pb In, Co, Rb, Zn

1500-2000

Er, Y, Ba, Ca, Nd, Pr, Ce

K, Li, Na, Cd, Er

(500

500-1000

Ho, Lu, Tm, Y, Mg Ba, Eu, Tb, Nd, Sm, Sr, Ca

2000-2500

For each element the most volatile of increasing temperature.

compound

is considered.

Elements

in each group

are arranged

in order

chlorides and fluorides [17] are summarized in Table 1. As can be seen, the majority of the chlorides and fluorides boil or sublimate below 15OO”C, the exceptions are primarily the alkaline earth and rare earth halides with a corresponding range of 1500-2500°C. As a comparison it is noted that 14 elements in the form of metal, oxide or carbide have boiling/sublimation points above 3400°C [17], i.e. above a temperature at which the graphite supporting material also sublimates at a substantial rate. The halides of these 14 elements belong to the most volatile compounds. The solid halogenation agents release their halogen content within lo-20 s during the initial period of arcing [2] and ensure a high halogen concentration in the electrode cup for this short time. In contrast, the gaseous agents can be introduced continuously into the arc (see below), but the resulting halogen gas can enter the cup only by diffusion through its opening or through the porous graphite wall. As the halocarbon is applied in a diluted form, the concentration of halogen in the cup is probably lower than that produced by a solid agent in the initial period of arcing. However, because of the continuous supply of the gaseous agent the halogenation of the less reactive components can take place in a later period of arcing. Thus it may be expected that gaseous agents generally promote the complete evaporation of samples, and the rate of vaporization can be controlled to a certain degree by the rate of supply of the agent. 2.2 Reactions in the plasma The dissociation of monoxides and monohalides in the arc column and its flamy fringe (6000-4000 K) has been treated theoretically by BOUMANS[18]. He considered that in a carbon arc operated in air the concentration of oxygen atoms is the order of 1017 cme3, while the concentration of halogen atoms originated from the sample matrix does generally not exceed 1015 cmW3. Under these conditions the formation of certain monohalides in addition to the oxide formation may decrease the concentration of the corresponding metal species appreciably in the arc fringe only (e.g. at 4000 K). Thus it was concluded [18] that emission can be expected to decrease only for low level atom lines of easily-ionized elements emitted predominantly from this outer region of the arc. R. C. WEAST (Editor), Handbook of Chemistry and Physics, 54th Edition. Chemical Rubber Co., Cleveland, Ohio (1974). [18] P. W. J. M. BOUMANS, Theory of Spectrochemical Excifafion. Adam Hilger, London; Plenum Press, New York (1966). [17]

T. NOR,

404

Table 2. Dissociation

Ed(eV)

energy

8. HAN&+JUHAI

ranges

of monoxides,

MO

and E. PUNGOR

monofluorides

and monochlorides

MF

[17]

MC1

Ag, Pd, K, Na, Rb, Zn, Cd, Cs, Ga

Bi

Hg, Ti, Cd, Zn

In, Mg, Bi, Cu, Pt, Ca, Co, Ni, Sb, Rh, Pb

Cd, Ag, Pb, Cu, Ni,

Bi, Pb, Ag, Sn, Mg, SC, Au, Ge, Y, Ra, Cu, Fe, Sb, Mn, Cr, Ni, Tl

Sr, Te, Mn, Fe, Yb, Be, Se, Cr, Ru, MO, As

Mn, Cr, P, Sb, Tl, Mg, Sn, Na

Be, Ca, Sr, Na, Rb, K, In, Si, Ba, Cs, Ga

Al, P, Tm, Sn, Eu, Ba, Sm

Ge, Si, Rb, K, Cs, In, Ca, Eu, Sm, Pu, Sr, Nd, Be, Ti, Ga

Al, B

6-7

Dy, Er, Ho, V, SC, W, Ti, Ge, Lu

Ba, SC, Gd, Y, Al

7-8

Y, Gd, Pu, Tb, Nd, Np, Zr, U, Ta, Pr, Si,

B

8-8.4

Hf, Ce, La, Nb, Th, B

2-3

3-4

4-5

5-6

Elements

in each group

are arranged

in order

of increasing

dissociation

energy.

VUKANOVI~ et al. [19-221 have investigated the effect of introducing high amounts of NH,Cl and NH,F. These substances were vaporized separately from the samples either from an additional hole of the anode [20] or from the cathode [21,22]. The intensities of U.V. atom lines of Sn, Sb, Bi, Pb, Ge, In, Mg and Zn [20] and VIS atom lines of Ca, Al, Ga and Ag [21] were found to decrease significantly, and this effect was attributed in part to halide formation in the arc plasma. More recently the radial distribution of temperature and of the species involved as well as the transport properties of the species were investigated [22]. These studies confirmed that the intensities of calcium atom and ion lines were decreased owing to CaF formation. Under the conditions applied in our work (see below) the concentration of halogen atoms was ca. 1017 cme3 assuming a complete dissociation of halocarbons introduced, i.e. it was of the same order as the oxygen atom concentration in a free-burning carbon arc [18]. If we consider the dissociation energies of monohalides and monoxides (Table 2), an appreciable decrease in free atom population can be expected due primarily to the formation of monofluorides. The ratio of atom concentrations with and without halogenation (R) can be expressed on the basis of known relationships [IS] as

where n (cm-‘) is particle concentration, K is dissociation constant, Ki is ionization constant, suffixes M, MO, MF, MC1 and e denote metal, monoxide, monofluoride, [19] V. VUKANOVIE, Preprints 20th Coff. Spectrosc. Int. and 7th lnt. Conf. Atomic Spectrosc., Prague 1977 Volume I., pp. 45-80 (1977). [20] M. TRIPKOVII: and V. VUKANOVIC, Spectrochim. Acta 26B, 131 (1971). [21] J. B. RADIX-PER& and V. M. VUKANOVI?, Z. Anal. Chem. 274, 177 (1975). [22] J. RADIE-PERIL, V. M. VUKANOVI~, M. PERIL: and M. REKALI~, Preprints 21th Coil. Spectrosc. Int. and 8th Int. Conf. Atomic Spectrosc., Cambridge 1979 Paper No. 341.

Halogenation with CCI, and CF,CI, vapours in arc emission spectrometry

405

monochloride and electron species respectively and the prime refers to the conditions with halocarbon introduction. Equation (1) applies when oxygen atoms are the major competitors to halogen atoms in the formation of metal containing molecules, and these are predominantly diatomic species [18]. Fluorine and chlorine atoms are considered to arise from the decomposition of CF,Cl,, and halogen atoms originating from the sample matrix are neglected. As known, the dissociation and ionization constants in Equation (1) increase with temperature and decrease with the corresponding dissociation and ionization energies exponentially to a first approximation. The detailed calculation of I?, Boltzmann factor and transport function also considering the radial distribution [18,22] is beyond the scope of this paper. However, a qualitative interpretation of our experimental results with respect to the role of halide formation in the plasma will be attempted on the basis of Equation (1) and some additional assumptions. Line intensities integrated over the complete evaporation time of samples will be used to assess the influence of vaporization rate and its variation under halogenation. The time-integrated intensities of several elements which form stable fluorides (Table 2) will be related to the line intensity of gold, for which the formation of halides at high temperature is negligible. Using this internal reference method we assumed that the problems related to the variation of transport velocities in evaluating halide formation are, at least in part bypassed. 3. EXPERIMENTAL 3.1 Supply of halocarbon vapours The apparatus used for carrier vaporization of Ccl, is shown in Fig. 1. The water thermostat was operated at 26°C and the magnetic stirrer at the lowest possible rotation speed. The total gas flow rate supplied to the arc and the fraction used as carrier gas above the liquid were regulated to maintain an optimum by the needle valves shown. When no Ccl, vapour is to be introduced, its diffusion to the arc can be prevented by closing the valve near the outlet of the container, and flushing the tubing with air for an appropriate period. The height of the liquid was kept constant within a limit of 15 mm difference.

Fig. 1. Apparatus for the carrier vaporization of carbon tetrachloride.

According to chemical engineering studies [23, 241, the volume rate of the vapour of a liquid (VI) carried by a gas at a flow rate of V, at atmospheric pressure (1.013 x 10’ Pa) can be estimated from the vapour pressure (P,) of the liquid:

v, = v,

PI 1.013x105-P,‘

[23]J.H. PERRY (Editor), Chemical Engineer’s Handbook. McGraw-Hill, New York (1950). [24] J. 2. K~~RTHYand M. FARKAS, Proc. Res. Inst. for Telecommunication Technology (Hungary) 8, 57 (1968).

406

T. K~QOR, E. HAN&-JUHAI and E. P~JNGOR

The validity of equation (2) is limited at high vapour pressures close to the boiling point and in the case of imperfect contact between the carrier gas and the vaporizing medium. According to reference [23], for Ccl, at 26°C we have PI = 1.52~ lo4 Pa. Under real conditions P, in equation (2) should be replaced by the partial pressure (pr) determined experimentally. This was performed by measuring the weight loss of the liquid and the absorbance of Ccl, vapours in a flow cuvette at 220 nm using an U.V. spectrophotometer. It was found that the partial pressure approximated a constant value (pl = 8 x lo3 f lo3 Pa) when the carrier flow rate (V,) was between 0.2-2 1. mitt-‘. Below this range p1 was lower, presumably as a consequence of the condensation or adsorption of CCI, vapours in the transport tubes. The decrease of pr above a carrier flow rate of 2 1. min-’ was probably caused by insufficient thermostating of the liquid. Optimization studies showed that CCL, vapour carried by 2 1. mine1 air was sufficient for halogenation in the arc, without dilution by additional air. According to equation (2) and the experimental value of p1 = 8 x lo3 Pa, 174mlmin-1 of CCI, vapour was supplied to the arc. Freon-12 was taken from a commercially available cylinder. By means of needle valves and rate meters shown in Fig. 1, 174 ml min-’ of CF2C1, was diluted with 2 1. min-’ of air, and this mixture was transported to the arc (8 vol. % for both Ccl, and CF,Cl,). For reference measurements an air flow of 2 1. min-’ was supplied to the arc. 3.2 Spectrographic conditions The device used to introduce the diluted halocarbon vapour into the arc discharge and the electrode arrangement are shown in Fig. 2. Metallic parts close to the supporting electrode could not be used because of possible corrosion, therefore most of the parts were made from graphite. With this open system the supporting electrode can be replaced easily without contamination problems. Attention must be drawn here to the fact that the decomposition products of halocarbons are poisonous, therefore an effective laboratory hood above the arc must be used.

3.2.1 Source parameters. Available voltage, 220 V d.c., sample as anode, 3 mm arc gap, lo-12 A current (unless otherwise noted). The supporting electrode (Fig. 2) was machined by a Rank-Hilger Electrode Sharpener from high purity graphite rods supplied by Elektrokarbon Tapol’Eany (CSSR).

Fig. 2. Electrode arrangement and device for arc excitation in a flowing atmosphere containing halocarbon vapour. (1) graphite counter electrode, (2) graphite supporting electrode, (3) graphite ring, (4) water-cooled copper mantle, (5) clamp of the electrode stand (commerical), (6A) and (6B) cross section and top view of the electrode holder made of graphite.

407

Halogenation with CCI, and CF,Cl, vapours in arc emission spectrometry

3.2.2 Spectrograph. Carl Zeiss (Jena) Model Q-24, internal focusing (Zwischenabbildung), 0.01 mm slit width, total attenuating filters and step filters as appropriate. Spectra were recorded by Agfa Geveart Scientia 34B50 and 31D65 emulsions developed in Kodak D19 at 20°C for 5 min. 3.2.3 Evaluation. Transmittances of lines and background were measured by Carl Zeiss (Jena) Model G II Schnellphotometer (microphotometer) and were transformed to intensities according to ARRAK’S function [25,26]. The transformation constants were determined by the two step filter method. Line intensities corrected for background were calculated by a Hewlett-Packard HP 65 calculator.

4. RESULTS AND DISCUSSION

Samples in solution, metallic (chips) and non-metallic (powder) forms were investigated, as they all occur in the practice of arc spectrometry. Line intensity vs arcing time relationships were evaluated from racking plate spectra. These studies aimed primarily at determining the eventual increase in selectivity of vaporization of certain constituents, under the effect of halogenation. As well known, when the trace elements are distilled from a matrix, the detection power can generally be improved provided that exposure is made during the distillation period (cut-off exposure). Using gaseous agents an other important aspect is the continuity of vaporization of the major constituents during the total evaporation time of the sample. This continuity is in a close correlation with the analytical precision [27,28] because the vaporization rate of the major constituents (or of the buffer added) controls the temperature and electron pressure in the arc plasma at certain electric parameters [18]. The overall effect of halogenation on the excitation was evaluated from the line intensities integrated over the time of complete evaporation of a complex powdered sample.

4.1 Solution samples

Carbide formation taking place in metal oxide/graphite powder mixtures was studied in detail by NICKEL [5,29], and some of the elements known to form refractory carbides were studied by a solution method in this work. The supporting electrode shown in Fig. 2 was modified for solution samples to have a hole with a depth of 2.5 mm in the centre. Matrix free standard solution 10 ~1 in volume was applied for each element dissolved in a suitable acid and dried on a hot plate. When acids other than oxyacids were used for dissolution, 10 ~1 of concentrated HNO, was pipetted to the residue and dried again. The amount of each element applied was lo-20 times higher than the corresponding HARVEY sensitivity factor (k, %) of the most sensitive U.V. line [30] transformed to weight units (pg = 100 k). Table 3 shows the arcing times required to evaporate at least 95% of the loaded sample in air and air + Ccl, atmospheres. The line intensities of B, MO, Nb, Ta and W excited in air did not change significantly during 30 s arcing, thus no estimate is given on the fraction evaporated under these conditions. It is clear that volatile compounds were formed in the presence of Ccl, at a high rate from the typical carbide forming elements when these were applied separately, i.e. when no other metallic elements were present. It is very probable that the ‘carbon’ supporting material also plays an important chemical role in these fast reactions (see below). [25] A. ARRAK, Appl. Specrrosc. 11,38 (1957). [26]R. J. DECKER and D. .I. EVE, Spectrochim. AC& 25B, 479 (1970). [27] T. KANTORand E. PUNGOR,Spectrochim. Acta 29B, 139 (1974). [28] J. KUBOVAand E. PLSKO, Chem. Analit. 22,855 (1977). [29] H. NICKEL, Spectrochim. Acrd 23B, 323 (1968). [30] C. E. HARVEY, A Merhod of Semi-Quanrirariue Spectrographic Laboratories, Glendale, CA (1947).

Analysis,

Applied

Research

408

T. KANTOR, 8. HAN&-JUHAI

Table

and E. BUNGOR

3. Evaporation times of some carbide forming applied spearately in solution form

elements

Atmosphere Evaporation time (s)

Air

Air + CCI, Al, B, Cr, MO, Nb, Si, Ta, Ti, V, W, Zr

5-10

10-20

Al, Cr, Si

Be, Ce, La, Th, U

20-30

Be, Ce, La, Th, Ti, V, U, Zr

>30

B, MO, Nb, Ta, W

The addition of alkali and alkaline earth compounds to solution samples analysed by the dry residue technique of arc excitation generally improves the detection power [31-331. We investigated the cumulative effect of chlorination and barium chloride on the excitation of tungsten and boron. The results were similar for both elements; those obtained for tungsten are shown in Fig. 3. In these studies an aqueous BaCl, solution (10 IL!) was added to the residue of W (HF+HNO,) solution. As can be seen, the intensity vs time characteristics of the tungsten line change primarily under the effect of CCl,, but barium chloride has a strong influence with and without CCl, as well. The intensification caused by chlorination is about 50-fold in the first 10 s of arcing. The

200 r c 3 x

loo-

E 5

50-

-

Air

b i 2

zo-

L c -

lo-

5-

2-

l-

IO

20

10

30 Arcing

20 time,

30

s

Fig. 3. Intensity vs arcing time curves for a tungsten atom line with and without CC& vapour and BaCl, additive. Samples: 70 pg W (HF+HNO,) and 1 mg BaCI, in solution. -C- W in air, -O- W + Ba in air, -C- W in air + Ccl,, -C W + Ba in air + Ccl,.

[31] N. KRASNOBAEVA and Z. ZADGORSKA, Specrrochim. Acta 26B, 301 (1971). [32] Z. ZADGORSKA, N. KRASNOBAEVA and D. APOSTOLOV, Spectrochim. Acta 30B, [33] T. KANTOR, Talanta 10,507 (1963).

527 (1975).

409

Halogenation with Ccl, and CF,Cl, vapours in arc emission spectrometry

intensities

integrated

over 30 s were also calculated, yielding the following intensifica-

tion factors

IW+Ba) = I(W)

2

IW+Ba+CCLJ =

g.

.

’

I(W +CC1,)

I(W+Ba+CCl,)=31 I(W +Ba)

5

I( w+CCLJ=

7.

. ’

m+9

16

’

’

Accordingly, the enhancing effect of BaCI, is about twofold in the presence of CCL, and the enhancing effect of Ccl, is similarly doubled in the presence of BaCI,. To understand the stronger effect of barium in the presence of CCL, it should be noted that the vaporization of tungsten and barium takes place simultaneously only in this case. Therefore the buffer effect of barium causing an increase in the residence time of tungsten atoms [31] is more pronounced under the action of CC14. It can also be concluded from Fig. 3 that the BaCl, matrix does not adversely affect the rate of chlorination of tungsten by Ccl, thus it can be used to advantage as an additive. It is also obvious from these results that purification of graphite electrodes from contaminants can be performed effectively by pre-arcing them in chlorinating atmosphere. The use of N, + CCI, atmosphere is more advantageous for this purpose since it minimizes the consumption of the electrodes. It has also been observed that under this atmosphere a pyrolytic graphite layer develops on the high temperature surface of the electrode cup. The penetration of solution samples into the inner part of the cup is hindered by this layer. The importance of the pyrolytic graphite coating in this respect has been recognized fully in graphite furnace atomic absorption methods. A pre-arcing of electrodes in N, +CCl, atmosphere for 1.5s was applied throughout this work. Due to the presence of Ccl, weak emission bands were observed in the 275-295 nm region of pure carbon arc spectra arising from the emission of CC1 species [4]. Apart from these bands the background radiation was not increased in the U.V. range under these conditions. However, monohalide spectra [34] of several major components were detected (see below). 4.2 Metal samples The metal globule procedure of arc excitation has been one of the best suited methods for the analysis of high purity copper. Several elements can be distilled selectively from copper matrix without any additive and determined with a high sensitivity [35]. It is also important that arcing to completion can readily be performed, and this is used for the determinands which evaporate simultaneously with the main constituent. TYMCHUK er al. have applied CuCl, [36] and CuF, [2] as halogenation agents, enabling to perform fractional distillation for much more elements. Recently, an arc-vaporization and flame-atomization method has been developed for the determination of volatile trace elements in copper [37]. The results discussed here may apply as preliminaries to this combined technique as well. Chips of copper standards from Johnson Matthey (U.K.) of 15 * 0.5 mg were loaded into pre-arced graphite electrodes (Fig. 2). Flowing air atmosphere with an without halocarbons was used in which total evaporation was attained within about 2 min. Nitrogen was also studied as carrier gas of halocarbons, and similar enhancement in the volatility of certain components was found as with air carrier gas (see below). However, under these conditions the evaporation time of the globule was too long (about 6 min). [34] R. B. B. PEARSE and G. A. GAYDON, The Identification of Molecular Spectra. Chapman and Hall, London (1963). [35] W. E. PUBLICOVER, Anal. Chem. 37, 1680 (1965). [36] P. TYMCHUK, .I. A. H. DESAKJLNIERS, D. S.RUSSELLand S.S.BERMAN, Appl. Spectrosc. 21, 151 (1967). [37] T. KANTOR, P. FODOR and E. FLINGOR, Anal.Chim. Acta 102, 15 (1978).

410

T. KANTOR;6. HANAK-JUHAI and E. PUNGOR

160t AI~+CF,CI, 120 -

I-

IS0 Ni Si CU

Alr+CCI, 120 -

40

60 Evaporation

80 time

100 I” air,

120

140

s

Fig. 4. Evaporation time of copper constituents in halogenating atmospheres related to that in air. Upper part: air + CF,Cl,, lower part: air + Ccl,.

The vaporization patterns of the components with and without halocarbons are shown in Fig. 4. A straight line is drawn from the origin to the evaporation time of the main constituent (copper), and its slope gives the factor by which the vaporization rate of the major component is changed. The variation of volatility of a component relative to that of copper (change of relative volatility) is given by the vertical distance between the straight line and the position of the symbol of that particular element. It can be seen that under the effect of Ccl, the relative volatility of Be, Al, Mn, Cr, Fe and Ga increases considerably and a small enhancement for Mg is also observable. In the presence of CF.&l,, Si and Sn also become volatile. The elements enclosed in the square co-evaporate with copper. The specific effect of fluorination on the intensit, v vs time curves for Sn and Si is shown in Fig. 5 together with the corresponding curves for copper. It can be seen that copper vaporizes at a higher rate in the first 75 s of arcing with both agents although the time of the total evaporation is not shortened. Consequently, copper vaporizes more smoothly under halogenation thereby producing a steadier arc discharge. This is illustrated by the arc voltage recordings shown in Fig. 6. In the upper traces the effect of Ccl, in air carrier gas can be seen, and this can be related to the corresponding curves in Fig. 5. In the presence of CCL, the higher vaporization rate of copper during 7.5 s resulted in a lower arc voltage (higher electron concentration and lower temperature in the plasma) in accordance with theoretical predictions [18]. The high instability of the voltage in the period of 75-90 s is due to a random oscillation (creeping) of the anode spot between the globule and the graphite wall. This is followed by a very high vaporization rate of copper as a consequence of direct heating by the anode spot which stays then permanently on the globule. The arc voltage drops accordingly in this last period of evaporation and it rises sharply when evaporation is completed. The lower traces of Fig. 6 show the effect of CCL, in N, carrier gas. The arc is extremely unstable in N, atmosphere because of the creeping of the anode spot mentioned above. Upon introducing Ccl, with N,, the anode spot rotates on the rim of the electrode cup while a continuous and slow vaporization of copper takes place. After 2 min of arcing the arc was switched off without complete evaporation of the globule. Figures 5 and 6 suggest the conclusion that the increase in the volatilization rate of copper by halogenation in the first period of arcing creates more favourable excitation conditions in the plasma for the elements distilled from the globule during this time.

Halogenation with 021, and CFQ,

Si

2L3.5

vapours in arc emission spectrometry

nm

I

16

Cu

263.0,-m

12 6

Arcing

time,

3

Fig. 5. Intensity vs arcing time curves of silicon, tin and copper lines in different atmospheres. + air, -O- air+ Ccl,, -KS air + CF,Cl,.

Air CCCL,)

N, IO&)

I

0

24

46

I

I

I

72

96

120’

Arcing

’

time,

0

I’

I

I

I

I

I

24

48

i2

96

120

s

Fig. 6. Arc voltage recordings during excitation of 15 mg of copper in various atmospheres. Upper traces: air (left), air + CCl, (right). Lower traces: nitrogen (left), nitrogen + Ccl, (right), arc gap 3 mm, current 10 A.

411

412

T. K.&NTOR,g. HANAK-JUHAI and E. PUNGOR

Comparing the results obtained with CF,Cl, to those with solid CuF, reagent [2] for the elements studied with both reagents, the following differences can be observed. With CuF*, As and Sb traces could also be distilled selectively [2], which does not apply to CF,Cl*. Furthermore, with the solid agent, 10 s arcing time was reported to be sufficient for complete distillation of the elements; this required 60-70 s in the presence of the gaseous agent. Although these differences tip the balance unfavourably for the gaseous agents, their effectiveness is unambiguous from Fig. 4. The drawbacks mentioned might be overcompensated in practice by the simplicity of the use of gaseous agents without any risk of contamination since the solid agents may require tedious purification procedures [36] to become suitable for trace analyses. The effect of Ccl, on the arc excitation of a2uminium samples has also been studied but it will be discussed here only briefly. Aluminium is rather volatile as a metal, but it forms refractory oxide, nitride and carbide when arced in air using graphite supporting electrode. Owing to these reactions and products the arc discharge is extremely unstable and useless for quantitative analysis. In air +CCl, atmosphere the relative volatility of minor constituents studied (common in industrial samples of 99% Al) did not change appreciably, but the vaporization of the whole aluminium sample became highly continuous. Total evaporation of a 10 mg sample was attained in 2 min, and a quantitative method based on this excitation technique could be elaborated. With air +CF,Cl, atmosphere both the AlF 227.5 nm and AlCl 261.4 nm band systems [34] were rather intense, the former showing self-reversal. The intensity of the A10 bands in the VIS range decreased at the same time. 4.3 Powder samples The effects of “spectroscopic carriers,” including metal halides, on the excitation mechanism of powder samples were described in a series of papers by STRYZEWSKA [38] (also see references therein). Among several matrices, alumina was also studied and some of the results may be compared to our observations described below. An industrial (calcinated) alumina sample of 5 ho.5 mg was placed into a graphite electrode (Fig. 2) and this small amount of sample could be evaporated completely in air atmosphere without any additive (arc current 8 A). The vaporization characteristics of the constituents under the action of Ccl, and CF,Cl, are compared in Fig. 7 (for interpretation, see Fig. 4). As can be seen, the relative volatilities of B, Zr, Be, Cr, V, Ca, Fe and Mn were increased by both of the agents with some minor differences only. The evaporation time of aluminium decreased by a factor of 0.8 and 0.65 with CF,Cl, and CCL, respectively, which is consistent with the sublimation temperatures of aluminium fluoride and chloride (Table 1). The relative volatility of magnesium decreased in CF,CI, containing atmosphere, as could also be expected from volatility data, the behaviour of calcium, however, contradicts these results (Table 1). The relative volatility of silicon (related to aluminium) decreased in both halogenating atmospheres. However, its evaporation time remained nearly constant, i.e. silicon was not halogenated. Titanium evaporated together with the matrix in the presence of halocarbons, while in air it evaporated only after aluminium (the latter holds for zirconium as well). This may cause an important difference in the excitation of titanium in the plasma. Obviously, the buffer effect of aluminium on the plasma parameters [39] affects excitation of titanium when they are co-evaporated. It is also exceptable that in this case aluminium is a more suitable reference element to titanium. Experiments with AgCl [38] have shown a definite chlorination effect for manganese and iron in alumina, in agreement with our results. For the determination of Ba, Ca, MO and Zr a mixture of AgCl +LiF was suggested as additive [38]. [38] B. STRYZEWSKA,Spectrochim. Acta 27B, 227 (1972). [39] T. KANTOR, Emission Spectroscopy, Wilson and Wilson’s Comprehensive Analytical Chemistry (Edited by G. SVEHLA), Volume 5, Chapter 1, pp. 52-61. Elsevier, Amsterdam (1975).

Halogenation with Ccl, and CF,Cl, vapours in arc emission spectrometry

413

140 AirtCF,CL, 120 @ 100 -

60 60 -

20

40

I 60

I 80

I 100

I 120

I 140

L 160

I 180

Evaporation time in (1~. s

Fig. 7. Evaporation time of alumina constituents in halogenating atmospheres related to that in air. Upper part: air + CF,CI,, lower part: air + Ccl,.

A mixture of alumina and graphite (1 + 1) of 10 mg was investigated under the conditions described above. The results are shown in Fig. 8. In comparison with Fig. 7 it can be seen that the evaporation time of the main constituent decreases by about 20 s in all the three atmospheres and the increase in relative volatilities in the presence of halocarbons becomes less pronounced for the majority of constituents. Titanium evaporated somewhat earlier than aluminium with Ccl, and CF,C12, whereas without halocarbons it evaporated after aluminium. It is evident that chemical reduction of metal oxides is highly promoted by admixing graphite powder, which ensures a good contact between the reactants. The importance of reduction processes prior to chlorination was emphasized by PSZONICKIand MINCZEWSKI [40] who studied the chemical effect of AgCl by a separated distillation technique. According to our results summarized above, the halogenation of the majority of the components of alumina is promoted by the reduction processes and this results in similar volatilities for a large number of constituents. The well known physical effect of graphite powder dilution to decrease fractional distillation could also be important. It has been proved [41-431 that the fusion of powder samples with alkali metal borates prior to arc excitation improves the analytical precision and accuracy for several samples and constituents. Because of its general importance, some preliminary data are provided here on the effect of halocarbons on the excitation of samples in a glass state. Sodium-boron-glass sample (5 mg) was excited without any additive and the intensity vs time curves of the less volatile components obtained with and without CF,Cl, are shown in Figs. 9 and 10. As can be seen, the total evaporation of the most refractory elements (B, Ti) becomes shorter by about 60 s and the vaporization of Mg, Ba, Ca, Si, Fe, Al, B and Ti takes place almost simultaneously under the effect of halogenation. It should be added here that sodium as one of the main constituents [40] L. PSZONICKIand .I. MINCZEWSKI,Spectrochim. Acra 18, 1325 (1962). [41] H. W. RADMACHERand M. C. DE SWARDT, Spectrochim. Acta 23B, 353 (1968). [42] P. W. J. M. BOUMANSand F. J. M. .I. MAESSEN, Specrrochim. Actu 24B, 611 (1969). [43] F. J. M. J. MAESSEN,J. W. ELGERSMAand P. W. J. M. BOUMANS,Spectrochim. Acta 31B, 179 (1976).

414

T. KANTOR, fi.HANAK-JUHAI

+

and E. PUNGOR

Air CF,Ct,

100

60-

20

40

60

60

100

Evoporatnn time in or,

120

140

160

5

Fig. 8. Evaporation time of alumina constituents using graphite powder dilution (l+ 1) in halogenating atmospheres related to that in air. Upper part: air + CF,CI,, lower part: air + Ccl,.

evaporates in about 30 s irrespectively of halogenation; consequently its buffer effect on plasma parameters does not prevail over the total evaporation time even with halogenation. However, an efficient buffering is expected by the alkaline earth elements when halogenation is applied. The effect of Ccl, was similar to that seen in Figs. 9 and 10 with CF,Cl, from most aspects, but co-evaporation proceeded somewhat better with the latter agent. 4.4 Overall effect of halogenation

on time-integrated

intensities

For studying the effect of halogenation on time-integrated intensities, a complex powder sample containing 50 elements was excited. this was the so called RU-powder prepared by Johnson Matthey (U.K.) for the identification of the most sensitive lines (raies ultimes) in arc spectra. The RU-powder, a mixture of oxides with major components of zinc, magnesium and calcium [44], was arced without additive and with the addition of graphite powder (1+ 1). The arc was operated first at 5 A for 15 s to avoid the violent vaporization of zinc, and the current was kept at 9 A thereafter. Preliminary racking plate spectra had shown that 120 s arcing time was needed for the total evaporation of 8 mg of sample without graphite powder in air atmosphere. The corresponding time was shorter by 30 s when exciting 16 mg of diluted sample. These arcing and exposure times were also applied in the presence of halocarbons (8 ~01%). Line intensitives corrected for background were evaluated from duplicate spectra and normalized to the highest intensity found under the six different conditions. In Table 4 normalized intensity values are shown for the constituents known to form refractory carbides under conventional conditions [29], whereas in Table 5 data are summarized for noble metals (Au and Pt), volatile elements (Ga, Pb and Sb) and for elements of medium volatility (Ba, Cu, Mg, Mn, Ni and Sn). One significant figure is given only for [44]

The Use of R.U.

Powder

in Spectroscopic

Analysis.

Johnson,

Matthey,

London

(1951).

Halogenation with CCI, and CF,CI, vapours in arc emission spectrometry

41.5

1.0

O.@

0.6

0.4

.

0.2

Arclng

tome,

s

Fig. 9. Vaporization curves of Mg, Ba, Ca and Si in a sodium-boron-glass sample (normalized intensity vs arcing time) in various atmospheres. Upper part: air+CF,CI,, lower part: air. GMg 277.98, +Ba 307.16, -U Ca 317.93, -V- Si 243.52 (nm).

I

0.6

-

0.4

-

0.2

-

Fe

Al

8

i

I,

I

20

Ti

40

60

I

60 Arcing

100 time,

120

vs arcing time) in various -O-Fe 272.36, -O-AI

140

s

Fig. 10. Vaporization curves of Fe, Al, B and Ti in a sodium-boron-glass intensity

....

sample (normalized atmospheres. Upper part: air+CF,Cl,, lower part: air: 265.24, -Cl- Ti 323.45, -V-B 249.68 (nm).

416

T. KANTOR,

Table

I?. HAN&-JUHAI

4. Normalized line intensities of refractory with and without graphite powder

Sample

and E. F'UNGOR

elements in RU-powder excited in different dilution (total evaporation and exposure)

8mgRU

atmospheres

8mgRU+8mgC Atmosphere

(nm)

Air

+cc1,

0.04 0.2 0.2 0.07 0.1 0.08 0.06 0.09 0.05 0.04 0.06 0.05 0.08 0.05

0.2 1 0.9 0.3 0.7 0.7 0.5 0.5 0.2 0.6 0.5 0.8 0.8 0.3

Al1 B I Be I CaI Cr II MoII Nb SiI Ta Ti II VI WI Y II Zr II

308.22 249.68 234.86 300.69 283.56 281.62 309.42 250.69 268.51 328.45 318.54 294.70 320.03 343.82

Table

5. Normalized line intensities pheres with and without

Sample

+CF,Cl, 0.5 1 1 0.4 0.6 0.7 0.2 0.5 0.2 0.5 0.4 0.6 1 0.3

Air

+cc1,

0.9 0.5 0.6 0.6 0.7 0.5 0.3 0.8 0.08 0.3 0.4 0.2 0.6 0.2

0.9 0.8 0.4 1 1 0.9 0.7 0.7 0.5 0.8 1 0.7 0.9 0.6

of less refractory elements in RU-powder graphite powder dilution (total evaporation

8mgRU

+ CF,Cl, 1 0.9 0.6 0.8 0.9 1 1 1 1 1 0.9 1 1 1

excited in different and exposure)

atmos-

8mgRU+8mgC Atmosphere

(nm) Au1 Ba II CuI GaI MgI Mn II NiI PbI PtI SbI SnI

267.60 233.53 324.75 294.36 278.14 259.37 300.25 283.31 265.95 259.81 284.00

Air

+cc1,

0.6 0.4 0.6 0.6 0.8 0.5 0.4 0.6 0.4 0.7 0.6

0.7 0.6 0.7 0.4 0.6 0.8 0.6 0.5 0.5 1 0.6

+CF,Cl, 0.6 1 0.7 0.4 0.7 0.7 0.8 0.4 0.5 1 0.8

Air

+cc1,

0.5 0.4 0.8 1 1 1 0.5 1 0.5 0.6 1

0.7 0.3 1 1 1 0.9 1 1 0.8 0.6 0.9

+CF,Cl, 1 0.3 1 0.9 0.8 0.9 0.8 0.7 1 0.6 0.9

each value as an average of duplicate measurements because of the rather poor precision of the spectrographic “absolute intensities,” particularly near the detection limit. Nevertheless, these data are appropriate for an overall comparison. It is clear that a high intensification is attained by halogenation and/or graphite powder dilution for the line intensities of refractory elements (Table 4) and there is no unanimous variation in this respect in the line intensities of the less refractory elements (Table 5). For the former elements the ratio of the highest and lowest intensities of a certain line is between 5 and 25, whereas for the latter it is below 3 if one considers the six sets of conditions. For noble metals (Table 5) this ratio reaches 2 in spite of the expectation that halide formation is irrelevant with these elements either in the condensed phase or in the plasma. This suggests an effect due to a change in the excitation parameters, those not directly related to halide formation also play an important role in determining line intensities.

HalogenationwithCCI, and CF,Cl, vapoursin arc emissionspectrometry

417

Considering the refractory elements in Table 4 and the conditions without the use of graphite powder dilution, we observe that the intensification factor obtained with halogenation is in a range between 3 and 12 for all 14 elements. On the other hand, with the use of graphite powder dilution a similar intensification upon halogenation is obtained only for the lines of Ta, W, Zr, Nb and Ti, i.e. for those known to form the most refractory carbides [29]. One may, therefore, assume that the change in vaporization processes and in sample introduction under halogenation must be the dominant factors in these intensification phenomena. The efficiency of particle transport from the electrode cavity to the excitation zone and its role in the analytical performance have been studied in detail by BOUMANSand MAESSEN[45]. In the light of their results it can be supposed that an extremely high sample loss occurs with refractory elements when excited without graphite powder dilution and halogenation. The possible mechanism of sample loss with refractory elements was explained as follows [45]. A part of an element forming refractory carbide (e.g. W) “infuses into the bottom of the cavity and might be only partly liberated at the end of the arcing period.” Graphite powder dilution can be expected to decrease the possibility of refractory elements infusing into the bottom of the cup and to simultaneously increase the possibility of spattering of these elements bound to graphite particles. It was concluded [45] that the efficiency of sample introduction into the source is increased by this spattering mechanism for refractory elements. This might be one of the reasons for the higher intensities found with graphite powder dilution when the arc is operated in air (Table 4). However, for Ta, W, Zr, Nb and Ti, which form the most refractory carbides, the loss must be still very high using graphite power dilution without halogenation (Table 4). This suggests that the highly refractory elements spattered together with graphite particles are only partially evaporated in the arc plasma due probably to their fast travel through this high temperature zone. It may also happen, of course, that not all the particles escaping the electrode enter the arc plasma. It can be concluded that halocarbon introduction is very effective in reducing the vaporization losses of carbide forming elements, as it was also found with LiF additive [45]. 4.5 Eflecfs of halocarbon introduction on plasma processes The appearance of monohalide bands in the spectra of major components is an evidence that some loss of free atoms must occur in the plasma with halogenation. Halide formation is, of course, stronger in the boundary of the discharge and therefore a decrease in self-absorption of the resonance lines is expected [18]. This was found extremely striking on the Mg I 285.21 nm line when the RU-powder spectra obtained with and without CF,Cl, were compared, magnesium being one of the major components in this sample. This effect was studied in further spectra taken with a spectrograph slit of 5 pm resulting in a spectral band width of 0.0057 nm at 285 nm. From the spectra obtained with and without CF&l, those two were selected in which the peak intensities of Mg I 278.14 nm non-resonance line were equal within 5%. A recording microphotometer was used to measure the profile of the Mg I 285.21 nm resonance line in a transmittance scale and this was transformed to an intensity scale (Fig. 11). As can be seen, the self-reversal found in air atmosphere disappears upon halogenation to produce a sharp line profile. It is also mentioned that the peak intenisty of Mg II 279.55 nm (ion) line is enhanced by a factor of .1.6 in the presence of CF,C$. Thus it can be expected that halogenation improves the linearity of the analytical curves, i.e. with halogenation a larger dynamic concentration range can be obtained for sensitive atom or ion lines (exceptions are probably the noble metal lines). In the measurements connected to Tables 4 and 5 the decrease in self-absorption with halogenation was

[45] P.

W. J. M. BOUMANS and F. J. M. J. MAESSEN, Spctrochim.

s.*.(e) 35/7-c

Acta

24B, 58.5(1969).

418

T.

K.bro~,

8.

-AK-J-

Wavelength,

and E.

P~GOR

nm

Fig. 11. Intensity profiles of Mg I 285.213 nm resonance line in various atmospheres. -CL air, -C air + CF,Cl, (8 ~01%).

probably not important because resonance lines were selected only for the trace elements of the RU-powder. As mentioned in Section 2.2, intensity ratios with respect to a gold line as internal reference were also evaluated. In these studies RU-powder diluted with graphite (1 + 1) was excited in air and air +CF,Cl, atmospheres with increasing concentration of the agent (total evaporation and exposure). The U.V.atom lines of Al, B, Be, Ca, Mg and Si were studied because no strong vaporization losses were expected for these elements, even in air atmosphere provided that graphite powder dilution was used (Tables 4 and 5). Normalized intensity ratios (averages of duplicate measurements) are shown in Fig. 12 for the line pairs which vary significantly with the CF,Cl, concentration (Be, B, Si and Mg). The intensity ratio of Al 1308.22/Au 1267.60 varied only randomly (RSD of 23%, n = 12) with increasing CF,Cl, concentration, and the same was found for the Ca I 300.69/Au I 267.60 line pair (RSD of 31%, n = 12). From the variation of the intensity ratio of calcium atom/ion lines (reciprocal excitation index) given in Fig. 12 it can be concluded [lg] that the electron pressure increases and the temperature decreases in the plasma with increasing halocarbon introduction. These changes in plasma parameters may be due to a more efficient introduction of calcium as an easy ionized matrix component into the plasma under halogenation. However, it is also possible that halocarbon alone increases the electron concentration and reduces the temperature in the arc as it is reasonable to conclude from the results found with CH, introduction [46]. If we recall equation (1) and bear in mind the variation of plasma parameters discussed above, some qualitative explanation can be given of the curves in Fig. 12. B, Si and Be have fairly high ionization energies, and thus their atom lines are emitted primarily from the arc core, and the change in electron concentration does not affect strongly the R-factors of these elements. With decreasing temperature (T’ < T) however, the R-factors of these elements must decrease not only because of monofluoride formation but also because the monoxides are less dissociated. The latter may be dominant for B and Si since the relevant dissociation energies are EBF< EBO and E,,, < E,,, (Table 2). The formation of monofluoride is, however, more responsible for [46] G.

HEINRICH, H. NICKEL, M. MAZURKIEWICH

and R.

AWI,

Spectrochim. Acta 33B, 635 (1978).

Halogenation with Ccl, and CF,Cl, vapours in arc emission spectrometry

I

I

1

I

I

I

I

I

I

0

2

4

6

8

10

12

14

CF&I,

419

,Vol.%

Fig. 12. Normalized intensity ratios as a function of CF,CI, concentration. Reference line: Au I 267.60 nm. 0 Be I 234.86/Au, -0-B I 249.68/Au, -C-S I 250.69/Au, -C Mg I 27&14/Au, -xCa 1300.69/Ca II 315.89.

the decrease in R-factor and relative line intensity with beryllium because EBeF> EBeO. (Fig. 12) might be explained as follows. With increasing concentration of CF,Cl, the degree of dissociation of CF radicals probably decreases because these species have a rather high dissociation energy (5.6 eV). In addition, the concentration of oxygen atoms at high halocarbon introduction rate may be reduced considerably (n&c no) through the formation of CO thereby increasing the R-factor As mentioned above, the relative intensities of Al and Ca did not change significantly on halocarbon introduction. These elements are relatively easy to ionize and their U.V. atom lines are emitted mainly from the intermediate and outer regions of the arc [18]. The increase in electron concentration (n: > n,) by halocarbon introduction must increase their R-factors considerably. However, halide formation is also significant for these elements (EAIF> Ealo and ECaF> Ecao) which in turn decreases the R-factors. Therefore these two effects can balance each other. The ionization energy of magnesium is between those of the two groups discussed, which is represented by the small change in the relative intensity of its lines (Fig. 12). It is clear that the behaviour of the relative intensities of the U.V. atom lines with increasing halocarbon concentration could not be explained on the basis of halide formation in the plasma alone. However, the changes in plasma parameters necessary to interpret the variation of relative intensities should also be considered to understand the behaviour of “absolute intensities” under halocarbon introduction (Tables 4 and 5). The drop in plasma temperature and rise in electron concentration must increase the residence time of the atoms in the plasma, which may balance or overcompensate the loss of free atoms caused by plasma reactions. This might be the reason why absolute intensities of the U.V.lines of Be, B, Si and Mg were relatively invariant with respect to halocarbon introduction. To investigate the behaviour of visible lines of low excitation energy (E,), spectra of RU-powder (1 + 1) were recorded on a panchromatic emulsion. Air and air + CF,Cl, (8 ~01%) atmospheres were used without internal reference. The intensity of Ba I 553.55 nm line (Eq = 2.24 eV) was found to decrease by a factor of 0.3 and that of Ba II 493.41 nm line (E, = 2.51 eV) by a factor of 0.5 under halogenation. Consequently, the favourable change in plasma processes considered above was insufficient to compensate for the halide formation in the excitation of low energy atom and ion lines. The shapes of the curves of these elements

420

T. KANTOR, 8. HAN.&-JUHAI

and E. PIJNGOR

5. CONCLUSIONS The experimental results presented in previous sections show that halogenation with halocarbons increases to a smaller or larger extent both the instantaneous and timeintegrated intensities in the U.V. region as a result of a complex mechanism. The improved buffer effect found in several cases may reduce the matrix effect, but this is the subject of future studies. From the point of view of vaporization, the effect of halogenation can be exploited in two different ways. There might be a “specific effect” which accounts for the fractional and complete distillation of certain constituents during a short arcing time i.e. for an increase in relative volatility by halogenation. As it also exists with solid halogenation agents, the matrix plays an important role in the reactivity-volatility characteristics of a component, the possibility of fractional distillation should be explored experimentally for every particular case. Fractional distillation and cut-off exposure may enhance detection power, and this effect might be promoted by a buffer element vaporized during this period. However, to obtain an appropriate precision as well, an internal reference element similar in volatility to the determinants is required. Apart from the constituents which become extremely volatile by halogenation (relatively to the main constituents) or which are a priori volatile enough, the continuous supply of the agent increases both the continuity and the rate of vaporization. (To avoid possible confusion the terms “relative volatility” and “volatilization rate” must be clearly distinguished). Thus all the less volatile elements will co-evaporate in a shorter time under halogenation, which may be considered as a “general effect.” These characteristics can be exploited for elaborating quantitative methods based on the principle of complete evaporation and exposure. An internal reference element and buffer element belonging to the less volatile group is expected to be more suitable in these cases. There will be samples in practice for which the excitation procedures with both partial and total evaporation are worth performing when ultimate detection power is required. For the remaining cases the latter excitation method is preferred concerning all aspects of analytical performance [42,43]. The experimental results presented here may serve as preliminaries in elaborating quantitative methods for the analysis of the sample types studied. Acknowfedgemenr-The mental work.

authors

gratefully

acknowledge

the assistance

of Miss I. CYRANSKI in the experi-