Some observations on the mechanisms of atomization in atomic absorption spectrometry with atom-trapping and electrothermal techniques

Analytica Chimica Acfa. 117 (1980) 257-266 Q Elsevier Scientific Publishing Company, Amsterdam

-

Printed

SOME OBSERVATIONS ON THE MECHANISMS ATOMIC ABSORPTION SPECTROMETRY WITH ELECTROTHERMAL TECHNIQUES

J. KHALICHIE, ‘l%e

Macaulay

(Received

A. $1. URE

Netherlands

OF ATOMIZATION ATOM-TRAPPING

IN AND

and T. S. WEST*

lnstitu te for Soil Research.

2nd January

in The

Craigicbrcckler.

-4 berdcen

A B9 3QJ (G&. Britain)

1980)

SURIMARY

The mechanisms of collection and release of sisteen elements in atom-trapping atomic absorption spectrometry with a water-cooled silica trap in an air-acetylene flame are examined. Ag. Au. Cd. Co. Cu. Fe, Ni. Pb. Se and Zn appear to accumulate as metals whiist K, Li, Na. Cr. Rig and Mn are trapped as silicates or oxides. Al and V are also trapped as oxides, but were not studied further. No evidence could be found that the surface temperature of the trap exceeds 1700 K during the release cycle. Plots of appearance time of atoms vs. m.p. suggest that while direct evaporation can play a part in atomization, sputtering by energetic species in the scouring flame gases may explain the appearance of gaseous atoms at the relatively low temperatures involved. The atomization phenomena are related to those observed with electrothermal atomizers based on carbon and tantalum. It is suggested that sputtering processes may also be involved in such atomizers.

It has already been shown [l] that copper may be determined by the technique of atom trapping in an air-acetylene flame to yield a characteristic concentration (ppm for 1% absorption) of 0.0008 ppm, whereas the corresponding value for the conventional technique of atomic absorption with the same equipment and flame was only 0.04 ppm. The technique [ 21 involves trapping analyte species. generated in the lower parts of the flame, on a water-cooled silica tube mounted in the body of the flame. Subsequently, the -trapped species are released int,o the flame gases for measurement by atomic absorption when the coolant water is blown out of the tube to allow it to heat up to ca. 1700 K. Since our previous paper was published, a considerable amount of work has been done on other elements. Current interest. in the mechanisms of atomization from electrothermally-heated graphite surfaces [ 3. -71 prompts some observations on coliection and atomization at the surface of the silica atom trap,. \Vhilst these observations are concerned principally with the atom-trapping technique, they may have some relevance to atomization by electrothermal techniques.

EXPERIMENTAL

=ipparatus The esperiments were carried out with virtually the same equipment as described previously [ I]_ The atom trap was a silica tube (4 mm 0-d.) positioned ca. 5 mm above the tip of the primary reaction zone of an airacetylene flame huming on the adapted burner head of a Ovarian-Techtron _A_-16spectrometer. The gas, air and water flow rates were as described previously and the atomic absorption measurements were made at the approptiate resonance wavelength with the beam of the hollow-cathode lamp set at grazing incidence (25% obscuration of the beam) on the upper surface of the silica tube. To permit measurement of the absorption signal as close as possible to the surface of the silica tube where the atom population would be expected to be most dense during the release cycle, 2-mm horizontal slits were mounted at either side of the burner assembly. Apart from allowing observations to be made in more precisely clefined regions of the flame, this had the effect of improving the sensitivity of measurement by a factor of 2. -2.5 for copper, i.e. the characteristic concentration was reduced to <0.0004 ppm. RESULTS

AND

DISCUSSION

Collection processes Lau et al. [ 21 showed that when concentrated solutions of copper, silver or iron were sprayed in to an ah-acetylene flame a metal-like film was formed on the surface of the cooled silica tube. Observations with 500 ppm copper(II) solutions have confirmed the formation of this film of metallic copper. Since the efficiency of atomization of copper in an airacetylene flame is ca. 9Slo [S] , it may be concluded that the copper film originates from direct condensation of free copper atoms formed in the lower regions of the flame to form an aggregated deposit on the cooler surface of the tube (ca. 3’iO K) [2] _ At such a temperature, there is little possibility of further dissociation or reduction mechanisms operating during the collection cycle at the silica

surface for the very small proportion of unatomized copper species in the flame gases. A similar situation may be postulated for other metallic species have high atomization efficiencies in such flames. The fraction trapped as free metal will be smaller for those elements that have lower atom fractions in the flame, and species other than free metal, e-g_ oxides, will condense on the silica surface. For esample, the visible deposition on the collector tube of alumina from a concentrated aluminium salt solution nebulized into the air-acetylene flame has been noted [ 1] _ In the present investigation, x-ray examination of the yellow stain produced on the surface of a silica collector tube after nebulization of a 500 ppm ammonium metavanadate solution showed the x-ray diffraction pattern of vanadium(II1) oside and no evidence of metallic vanadium_ These observawhich

259

tions are consistent with the known lack of sensitivity of the stoichiometric air-acetylene flame for these two elements for measurements of atomic absorption. It is probable that most elements will be collected from stoichiometric air-acetylene flames as a misture of free metal and oxides, the proportions depending on the free atom fraction in the flame under the conditions of collection_ A few species may deposit partially as carbides or even combine with the silica. A tomization processes If it is assumed that the analyte

species trapped on the surface of the silica tube is mainly free metal, it is reasonable to expect the appearance temperature of the atoms to be related to the melting or boiling point of the metal and to deduce that the efficiency of atomization from the surface of the tube will be poorer for those elements that have high melting or boiling points. It is also logical to assume that more than one release peak may be observed

for

elements

that

condense

in more

than

one

chemical

form

or

physical modification_ The temperature of the surface of the silica collector tube may be assumed to be at ca. 370 K at the beginning of the release or atomization cycle when the coolant water is ejected from the tube, and to increase to a maximum equilibrium value over a period of several seconds so that the length of time, tx,, elapsing between the start of the release cycle and the attainment of the ma_.imum atomic absorption signal may be related to the boiling point of the metal. Figure 1 illustrates how the values of t&i and the less easily pin-pointed appearance time, t_.,, may be measured from the absorbance vs. time curve. Table 1 gives the relevant data for the sixteen elements which have so far been studied. Figure 2 shows a plot of tli vs.

i

Zn #KNo -/
/‘Cd -iie

300

700

t (3) --

I :30

..-

I500

I900

1300

M.p. I;<)

Fig:. 1. Temporal shape of atomic absorption signal from silica atom trap: f,%. time interval between appearance of absorption signal and start of atomization cycle; t&l, time interval between attainment of maximum signal and start of atomization cycle. Fig. 2. Relationship

between

lb, and the melting

points

of the metals.

260 TABLE

1

Data for fu, I :,, melting melting points Etemcnt

K Na Li SC Cd Ph Zil Mg

dl.

p.

and

boiling

points

(Kl

B. p.
h (~1

t.3 (~1

336 3Ti ‘15-I 190 59-1 600 693 923

1039 1163 lGO4 955 1035 20421 118% 1390

4.0

3.0 2.2 -l.S 0.9 1-S 1.3 3.2 6.5

4.0 S-0 2.0 2.S 3.0 5.0 il.5

for several

Element

elements

111.p.

(K) 1234 1339 1356 1517 172s

Ag

AU CU

Mn Ni co Fe Cr

l’i68 IS12 2176

listed

in order

of their

13. p. (K)

t&l (~1

is”

2450 3239 2S55 2314 3110 3150 3160 2913

10.5 10.5 II.0 9.0 15.0

8.5 6.5 5.5 5.5 9.5

16.0 15.0 16.0

10.0 9.5 9.0

melting point. Ten of the elements show a linear relationship, but magnesium, which forms a very stable oside, and lithium, sodium and potassium, which additionally attack silica, show a positive deviation. Rlanganese and chromium, which form easily reduced oxides, show a negative deviation. The corresponding relationship between i si and the boiling point of the metaI (Fig. 3) is less well defined although it is perhaps more obvious for the lower-boiling metals. The position of the alkali metals is more in keeping with the other metals, but magnesium and lead diw-ge considerably from any linear refaticnship. The attainment of t, (see Fig. 1) however, involves extensive as wcil as intensive factor5; lx1 values, although more relevant to quantitative measurements, arc less diagnostic for the present purpose than the appearance time,

lK

30

rC?l

.

i de L._ 7co

--__.I. I30c1

. 31.03 B.0

.--.w2003

3700

*a

,

lZn

Ica

ser .‘*Pb L--_-__-_ 303 700

. _

_^._ rtoo

_ _.-,_ 1500

M c, ;Kl

idI

Fig. 3. Relationship

between

t, and the boiling points of the met&

Fig. f. Rcintioriship

between

I,

rtntl the melting

points

of the metals.

I.._

_.” ._-_. 1900

-. 23oc

261

t,, defined according to the criteria used in studying the behaviour of electrothermal atomizers (3, 6, 9]_ A plot of t,, vs. melting point (Fig. 4) shows a smooth curve which is nearly linear at tower temperattlres, with magnesium and chromium giving a much closer fit but still leaving the alkali metals and manganese well above and below the curve, respectively_ A plot of tA vs. boiling point again sl~ows a less clearly defined relationship. The appearance time, fA, would at fist sight be expected to be related to the instantaneous surface temperature, T, of the silica tube, which may be assumed to be ca. 370 K at the start of the release cycle 12 j and to increase asymptotically to an equilibrium value several hundred degrees fess than the 2600 K of the a-acetylene flame [lo] _ If, therefore, there is a linear relationship between appearance temperature, T,, and met ting point, the experimentally measured tit vs. melting point p!ot should mirror the asymptotic temperature vs. time cuwe. Figure 4 indeed shows a typical asymptotic shape with a marked feve!ling off at ca. 1500 K, but the curvature is in the wrong direction, i.e. towards the melting point rather than the time asis. Elements that melt above 1000 K sflow appearance times, i.e. produce free atoms, weft before their melting points are reached. It is doubtful if the melting points of Co, Cr. Fe, Mn and Ni are attained, although it has as yet been impossible to obtain a reliable measure of the instantaneous effective surface temperature of the silica tube as it warms up through the ca. 10-s (t_&) heating range. Thermocouple methods tend to give erroneous results

because of poor contact with the tube surface and tfieir temporaf inertia within this time scale. The maximal readings obtained were ca. 1600 K. The under surface of the tube could not be examined by optical pyrometry and, in any it is doubtful if the technique could measure the surface rather than the bulk temperature of the tube. No change was observed in the crystalline form of a mixture of andafusite and waterglass deposited on the surface of the tube, which is said to change at about 1700 K [ 11 j _ In addition, it was noticeable that the silica tube did not melt or show an appreciably distorted surface under the conditions of use i.e. ma..imum heating times of 10-12 s during the releax cycle, so it may be deduced again that the surface temperature of the silica did not esceed the nieftino: b point of quartz (lST3 K) or silica (19S6 K). Before the probable mechanism of atomization from the metallized silica surface in the flame is discussed, it is interesting to note (Fig. 4) the relatively late appearance of Li, Na, K and klg. All these metals form very stable osides and are unlikely to condense as free metal. Strong solutions of the alkali metals produce an easily brushed-off white deposit on the cool silica tube. If, however, the white deposits are allowed to heat up during the release cycle, the tube quickly becomes etched, suggesting reaction with the silica. Although the appearance times of the alkali metals are ‘fate’ relative to the attainment of temperatures corresponding to the melting points of the free metals they are ‘early’ re!ative to the melting points of their silicates, osides or carbonates (with the esception of lithium carbonate).

event,

Manganese and chromium, phi& show early appearance times in Fig. 4 relatisc to the other non-alkali metals, seem to deposit on the silica tube as oxides rather than metals; the formation of c~~romiurn~rir) oside on a silica tube has already been noted [ 2]_ The ~~lecj~a~~i~~~ of atom production qxxt relertse for the other elements tfxxt zu-etrapped as free metals may invofve several pathways. For elements that meft at Iow temperatures, an appreciable ~o~~e~tration of vapourized atoms wiH obvioust_v be generated by thermal motion of the atoms in the liquid phase. Many metals show appreciabie vapour pressures at temperatures below their melting points, and so will produce a detectabie population of free atoms by a direct evaporative mechanism from a solid metal film on the atom trap. L’Vov [ 121 has c~~tl~~ted that species that produce saturated vapour pressures > 1W3 Prr would be detectable in a graphite furnace. The conditions obtaining in the present experiments can be evaluated easily against such a criterion by reference ta Table 2 which links the temperatures f 13. 12 f at. which the sixteetl efcments produce saturated vapour pressures of CLZ.101” Pa. Figure 5 shows a plot of t, vs_ the temperatures corresponding

to XW3 Pa. it cat1 be seen that these resufts are fair& widely scattered, showing no overall linear relationship. The order of appearance predicted from a 103 Pa vapour pressure requirement is Se, K, Cd, Xa, Zn, hlg, Li. Ph, 3111,Ag, Cr, Cu, Fe, Au, Xi, Co, This fits the observed appearance order (viz. Se, Pb, Cd, Nn, K, Zn, Li, &In, Jig, f_Ag---Au--%%), Cr. (Fe-Xi), Co) better than the order that tvaufd be observed for it melting point rel~tiotls~li~ (i.e. K, Sa, Li, Se, Cd, Pb. 2.n. Mg, Ag, Au, Cu, &In, Xi, Co, Fe, Cr), thus indicating a cfoser reiationship to dependence on vapour pressure thtut on melting pOtIlt. it cm be argued from Fig. 6 that those elements that produce saturated vapour pressures of 1W3 Pa below fUO K show a fairly well defined linear r&ationship for t, whilst those that require temperatures >1500 K show httlc tfcpcndence of I,, 011vapour pressure. The dotted curve in Fig. 5 is an

Tempcrntures elements E~lcmcn f

required f 13, I-r J

to praduce

3 saturahd

Temp. Tar 10” Pn srtt. vapaur pressure (K)

vapour

Element

pressure

111.p.

Wf -triS 924 1517 371 1728 601 490 699

of

IO’-’

Pa of

snriaus

Temp. r0r lo-’ P:tsat. vapour pressure (K) 699 565 1376 627 1952 1105 539 675

estimate of the surface temperature/time curve for the atom trap. From this it can be deduced that the heating rate at temperatures above ca. 1300 I< is too slow to permit use of L’Vov’s lo-’ Pa criterion_ Nevertheless it can be

seen from Table 2 and Fig. 5 that even if the surface

of the atom

trap did

heat up at a sufficiently rapid rate, the temperatures actually attained in these experiments would be insufficient to yield 10m3 Pa for Au, Co, Cu, Fe, Ni and possibly Ag and Cr. From these observations, it can be concluded that in addition to direct vaporization from a liquid for the lower melting elements, or from solid: metal oxide or other solid phase for the other elements, it is probable that there is a scouring action of the flame gases involving bombardment of the analyte film by energetic molecules or free radicals that leads to the ejection of free metal atoms, mo!ecules or small clusters that will rapidly atomize in the flame gases. It is postulated, therefore, that a process akin to the electrical sputtering of metals in hollow-cathode discharges occurs at the surface of the silica atom trap in the air-acetylene flame. These observations of the independence of tA and melting point for the higher melting point metals, and the early appearance of oside-trapped species relative to the melting point of the osides, suggest that the atomtrapping atomic absorption technique may be applied to a much wider range of elements than was envisaged originally. It is also interesting to relate these observations to those recently advanced for atom-formation processes with electrothermal atomizers. Sturgeon and Chakrabarti [9] concluded that there was no relationship between the appearance temperature, T.,, and the melting or boiling points when a carbon

furnace was used, and they related the appearance time of elements to their socalled energies of atomization. Campbell and Ottaway [ 3 1, also working with a carbon tube furnace, postulated that the reduction of metal osides by carbon is rapid; they correlated TA to the temperature of the surface of the graphite cell at which the reduction of the oxide becomes thermodynamically favourablc. Subsequently, llowston and Ottaway [ 151 concluded that the noble metals atomize by direct evaporation of the metal with appearance temperatures below their melting points, whilst the appearance temperatures of zinc, cadmium, lead and aluminium are above their melting points. Johnston et al. [6 ] implied, as far as the unenclosed carbon filament is concerned, that several energies associated with the vaporization processes have to be taken into account and that the heat of vaporization of the metal or the dissociation energy of the metal oxide bond, whichever is the greater, is the decisive factor. Aggett and Sprott [ 41 compared T,, for various analytes in both carbon filament and tantalum strip) electrothermal atomizers. Of the ten elements investigated, only Fe, Ni, Co and Sn showed strong evidence of the involvement of reduction by carbon. In Fig. 6 Sturgeon and Chakrabarti’s data [9] arc used to plot t,, against melting point. The relationship between the two for most of the elements corresponds fairly well wit!) the behaviour of the atom-trap device (Fig. 2)

264

and the appearance order of the elements is similar. The refractory oxideforming elements AI, Ca and V show delayed appearances relative to their melting points, as does tin which is well know to exhibit anomalous bchaviour in atomic absorption spectrometry f 161. Manganese and chromium appear earfy as in atom trapping, but the behaviour of nickel diverges slightly. Sturgeon and Chakrabarti postulated carbide formation to explain the behaviour of nickel and calcium following the penetration of these elements into the pores of the heated graphite.. A plot of taxvs, boiling point is erratic except for the oxide-forming elements Mg, Mn, AI and V. Figure 7 shows a plot of appearance temperature, T,, vs. melting point for

-il-c ._

y:

Fig. 5. RelationshipbetweenTVand temperatures required to produce 1W3 Pasaturated metal vapour pressure. Dotted Iine is estimatedchange of silica surface tcmpemture with time.

Fig, 6. Relationshipbetween I&zand the melting points of metals obtained by Sturgeon and Chakrabarti [9] with a carbon furnace technique,

265

Fig. 7. Relationship metals for (A) carbon

Fig. 6. Relationship

between appearance temperature, furnace [9 ) ; (:I) carbon filament [G

between

vnpour pressures of met&

a wider

range

T,,. and the melting

1; (.1) tantalum

strip

points [-I

of

J.

T, nnd the temperature required to produce 10 ’ Pa saturated for(~) carbon furnace;(n) carbon filament; (. ) tantalum strip.

i.e. using data from Sturgeon and et al. [6] (carbon filament) and Aggett and Sprott [ 41 (tantalum strip). An almost linear graph could be drawn for Cd, Zn. Ag, Au, Mn, Co, Xi and 310 for the carbon filament and the behaviour of the other atomizers tends to confirm a straight-line relationship for these elements and copper and iron. Again, however. there are delayed appearances for the alkali metals, alkaline earths and several other metals. Chromium tends to appear early, but manganese fits in all three cases. Figure 8 shows a similar plot of appearance temperature, 7’,, vs. the temperature required to produce a saturated vapour pressure of 10e3 Pa for the various metals. It will be seen that the correlation is no better than that with melting point. These data from electrothermal atomizers show that there is only a moderate correlation between appearance temperatures and the melting points or the 10e3 Pa saturated vapour pressure temperatures. Those elements that are reported to fan stable oxides show appearance temperatures greater Chakrabarti

of atomizers

(91

(carbon

and elements,

furnace),

Johnston

than would be predicted on the basis of the melting point or vapour pressure of the free metal but are generaiIy earlier than the corresponding properties of the osides would suggest.

Staveral chemical species condense as free metal from air-acetylene ff ames onto the silica atom trap. Others are trapped, at least in part, as osidcs, or in the case of tfie alkali metals, perhaps as silicates. Atomization of the annlytc deposited on the silica trap may be caused by evaporation directly from the solid metal (or oside etc.) or the melt. The low temperatures involwd in atomization, however+ suggest that sputtering of the analyte species by tlnergetic flame species afso contributes to Uie atomization process. There qqlears to I)e a general similarity in behaviour of the silica atom trap and electrothermal atomizers. III the latter, collisional excitation of atomic vapours witfl excited-state purge gas species has been suggested [ 1’71 as a mechaiulism for atomic emission. It seems reasonabfe, tflerefore, that sputtering by such escited species could also account for part of the atomization found in electrothermal atomizers_ The results obtained in this study show that atom-trapping atomic absoq>tion spectrometry has a much wider range of application tharl was hitherto sup&~ose
thank Dr D. C. Bain for the s-ray cfiffrwtion

results quoted.

ZE?!?HENCES I J. Khaiighic, 3. 31. Ure and T. S. West, .4x11_Chim. _-kta, IO7 ( 1979) 19 1. 3 C. Lrtu, i\. Herd and R. Stephens, Can. J. Spectrosc., 3-t (1976) 100. 3 W. C. Crtmpte!I and J. 11. Ott;lw;Ly’,Trtiantn, 31 (19i4) 63’7. 1 J. Aggett nnd -4. J. Sprott, -Anal. Chim. hctn, 7‘2 (197.1) -ID. .i C. K’. Fulicr, ,Inrtlyst. 99 (1971) ‘139. 6 0. J. Johnston, B. L. Sflarp and T. S. West, Annl. Chem., 17 (1975) 1233. 7 R. E. Sturgeon. C. L. Cbakrabwti and 0. H. Langford. Aati. Chem., -I8 (19iiB) 11793. S C. F. Kirkbright and 31. Sargent, Atomic Absorption and Fluorcscenn? Spectroscopy,

Academic Press, London, 1974, p. 225. 9 R. E. Sturgeon and C. L. Clmktmbarti. Prog. _4nrti._4tom. Spectrosc., 1 (197s) 40. 10 J. ,1. Dean and T. C. Rains, Flame Emission and Atomic Ahsorption Spectrometry, Deliker, New York, Vol. 1, 1969, p. 191. 1 1 iS. Eitel, The Physica Chemistry of the Siticnte8, University of Chiago Press, Chic~1go. 19.2. p. 697. 13 B. V. L’Vov, Atomic :\bsorption Spectrochcmicot Anniysis, Adnm Hilger, London, 1970. p. 201. 13 II. C. Weast (Ed.), Handbook of Chemistry nnd Physics, 51st edn., Chemical Rubkr Co., Cleveland, OH, 1970, p_ D.115. 1-l G. C. Cr3u and T. Schrifcr, Landok--Bornstein Tables. Springer-Vertag, Berlin, 1960. 13 \V. B. Rowston nnd J. M. Ottnwrty, Analyst, IO.? (1979) 616. 16 II. L. Kahn and J. E. Schallis. At. Absorpt. Newsl., ‘7 (1966) 5. 1’7 D. Litttejohn and J. M. Ottaway, Analyst, 101 (1979) 20&