Anal. Chem. W81, 53, 1573-1576
1573
Treated Graphite Surfaces for Determination of Tin by Graphite Furnace Atomic Absorption Spectrometry Thomas M. Vlckrey” and Gabrlelle V. Harrlson Department of Chemlstry, Bowllng Green State Universi@,Bowling Green, Ohio 43403
Gerald J. Ramelow Department of Chemistry, Texas A&M UnlverslyY, College Station, Texas 77843
The coating of the graphlte furnace surface wlth refractory metal salts Increases tho atomization efficiency of tin. The use of zirconyi acetate irnproves the atomization of tin, seienium, and arsenlc. The treated surface shows granules contalnlng zirconium. The zlrconlum concentration on the inside of the tube decreases with Increasing atomizations. The uniformlty of the zirc:onium-treatedsurface is compared to treatment with sodiuni salts of molybdate and tungstate.
Graphite furnace atomic absorption if3 a very sensitive technique for elemental analysis at the ultratrace (partsper-billion) level for many elements. The rate of generation of the atomic vapor from the graphite suirface is a complex function of the temperature, the rate of temperature increase, and the physical and chemical nature of the graphite furnace surface (1-4). This absorption signal variation and the nature of the surface, coupled with the possibility of volatilization of the element of interest in some molecular form(s) rather than an atomic cloud, are termed chemiical interferences. These interferences have led to the experimental development of matrix modifiers and of graphite surface treatments (5-11) which will reduce the extent of each chemical interference in the electrothermal atomization analysis. One method of surface modification which has been shown to improve the atomizatioin efficiency of tin 11sa treatment with zirconyl salts (6,8-10).The treatment method is simply the application of aqueous solutions of zirconium salts to the surface and subsequent drying and surface activation at ca. 2000 “C.This paper will focus on the surface of the treated graphite used for tin analysis. For comparision purposes other refractory metals have been briefly investigated and are described. The influence of these treated surfaces on atomization of several elements in adldition to tin is also described. EXPERIMENTAL SECT1ON Graphite Surface Treatment. The methods of surface treatment have been detailed elsewhere (9). The overnight soaking of the cuvettes in ca. 5-10% (w/v) aqueour, solution was the general procedure. Scanning Electron Microscopy. The SEM images were obtained on an Hitachi Model 450 LB SEM with a Kevex X-ray detector. The beam energy was 20 keV for the zirconium investigation and 30 keV for the observation of Mo and W. Atomization Profile. The atomic absorption as a function of time was obtained with either a Hitachi 180-70 or 170-70 graphite furnace atomic abriorption spectrometer. The standard microprocessor controller-data station played back the atomization transients on the strip chart recorder for the Hitachi 180-70 and a Cromemco sptem 2D with D+74 board was wed to monitor the Hitachi 170-70.
RESULTS The effect of the zirconium surface in increasing atomization efficiency is demonstrated in the atomization profile com0003-2700/81/0353-1573$01.25/0
Table I. Zirconium Concentrationa as a Function of Position on the Graphite Furnace old tube new tube Zr concn Zr concn end
Inside 4763 27 27
3455
2893
46
center end center edge
Outside 6421 6144 5199
1029
3018 2178
1750 1033c
a Determined as X-ray counts per 100 s interval in the Zr X-ray window (440 eV wide), The area of sampling The end is the “cool” portion of the was 3 X 4 mm. tube. Center is the zone where samples are applied. Sample integration area-not as for the rest of the data.
parison in Figures 1-4. In each case 1ng of element (from standard solution) is atomized from an untreated (U) graphite furnace surface and from a zirconium-treated (Zr) graphite furnace surface. The major feature of this comparison is the increase in signal height and total observed signal area for Sn, As, and Se, while Cd is not appreciably affected. Surface Distribution of Zirconium. The distribution of the zirconium along the surface of the furnace was investigated by using electron microprobe analysis. The emitted X-rays from the 3 x 4 mm sample area were integrated for 100 s. The results are shown for a newly treated tube (less than 5 atomizations) and an old tube (62 atomizations). The tubes were quartered. The electron beam sampled the inside surface near the end approximately half way between the end and what was originally the center of the tube and near the center of the furnace tube. The outside surface was sampled similarly. These results are tabulated in Table I. While direct comparison of the absolute numerical values is not appropriate, there are several trends which are obvious from these data. First, the zirconium concentration is much higher at the ends of the tube relative to the middle. That is consistent with the center of the furnace becoming hotter than the ends. The thermal loss from this section is increased after many atomizations, the reading in the center-inside being reduced to roughly 1% of the value of the ends of the tube after many atomizations. Second, the outside of the furnace shows less loss of zirconium than the inside. The reason for this could be the etching of the inside relative to the outside because of the acidic solutions which were analyzed. The fumes created by drying and charring these solutions may corrode the interior surface and cause more rapid loss than is observed on the outside. Third, total apparent zirconium decreases with age. The zirconium on the surface of the “old” tube is sig0 1981 Amerlcan Chemical Society
1574
ANALYTICAL CHEMISTRY, VOL. 53, NO. 11, SEPTEMBER 1981
As.
ZR
ING
AQ
1.0~~
SN
1
8
U
A
Flgure 3. As
in Figure 1 for A s standard. CD -
Flgure 1. Atomic absorption signal as a functlon of time for the graphite furnace atomization analysis of SnCi, standard. The total amount of Sn as the element was 1 ng. The untreated tube (U) and
the zlrconium oxide treated tube (Zr) were subjected to the same temperature program.
Se lONJ
U
1.ohiG
Zr
Zr
Flgure 4. As
In Flgure 1 for Cd standard. Cd shows no enhancement
with surface coating.
Flgure 2. As
in Figure 1 for Se standard.
nificantly lower than that observed for the “new” tube. This is especially noticeable in tlm tsnter-inside of the tube where the most severe depletion of the zirconium occurs. Finally, the edge (across the cut) of the sample was integrated to determine the diffusion of the zirconium into the graphite tube. The value (1033 counts) shows an appreciable zirconium concentration along the depth of the furnace. The area of integration was much smaller in this case, and the surface was a much different texture; therefore, except for qualitative comparison these data cannot be compared to the other “old tube” data. Electron Microscopy Investigation. In order to characterize the surface chemistry involved in the coated graphite furnace atomization, scanning electron microscopy and electron microprobe analysis of the surface were performed. While low magnification observations do not appear any different than untreated graphite furnace surfaces, at higher (10 000 X ) magnification, it can be seem that the zirconium
is concentrated in granules. Figure 5 shows these granules and the zirconium content of the granules. The data in this figure were obtained on the same “new” tube as is referenced in Table I. The aging of the tube is manifested in the migration of these particles into the graphite. At the same magnification the old tube shows the same zirconium-containing particles (Figure 6); however, the shadowy appearance of the majority of these particles results from the slight difference in depth of the particles. In fact, an X-ray density map obtained for the exposure in Figure 6 has the high count regions corresponding to the shadows in the micrograph. The nonzero background count observed in both Figures 5 and 6 can be interpreted in terms of uniform level of zirconium throughout the depth of the cuvette. The uniformly distributed zirconium may well be of a different chemical form than the particles. It has been observed previously that the major zirconium species on the surface is a zirconium oxide species (9), with a reduced (presumably carbide) zirconium species in minor surface amounts. This observation on the zirconium is consistent with other workers’ surface analysis of refractory metal coatings (11).
Other Metal Coatings. The metal carbide species is formed in some surface coating procedures. For example, the
ANALYTICAL CHEMISTRY. VOL. 53. NO. 11. SEPTEMBER I981
Double exposure showing the SEM scan of ( I ) a newly lreated zirconiumcoated graphne cuvette at I O O O O X magnification and (2) the electron mlcroprobe analysis for Zr along the horizontal line midway up the photo. Fbure 5.
1575
Fipm 7. Medun magriRcaKm (IOOOX) pichre of the cwette M a , In a tantalum-coated cwette using the method of Zatka ( 7 ) .
I
I
..
i .;
c:
:
,,
. ..
,!
P
.
rI ,.~
J Flgwa 6. Parameters
Zr-coated cuvette.
as in Figure 2. The tube examined is an "old"
tantalum coating procedure of Zatka (7)is reported to yield a surface of tantalum carbide. At moderate magnification (lOOOX), the graphite surface coated by that procedure does a o w to be a metal carbide (Fiewe 7). Althoueh not shown in Figure 7, the concentration Gofile'of tantalum acrosa the surface is essentially constant, and only small variations due to surface irregularities are observed. The use of molybdenum, tungsten, and vanadium solutions to coat the graphite surface has been shown to increase the atomization signals observed for tin compounds (6.9, IO). This is true for graphite surface freshly coated with vanadiumcontaining solutions. However, the vanadium is rapidly lost from the surface. After ca. 25 atomizations the vanadium content of the surface is negligible. Tungsten and molybdenum are mole stable on the surface. In fact, the molybde-
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m e 8. Low magniflcation (IOOX) SEM image of the sufiace ot a gapMte fur~r.8cuvette coated with NaJdoO,. The second exposue 1s the MO X-ray emission across the horizontal line midway up the photo. num-coated graphite shows particles of molybdenum similar to the zirconium described above. The problem with using molybdenum- or tungsten-coated tubes (althoueh there are manv aoolications of the molvb.. denum coating 6 6 ) )is that the coating efficiency is poorer than that observed with the zirconium. The difference may well be in the salts used in the coating procedure. A solution of zirconyl acetate is used for zirconium coating while molybdate and tungstate salts are used for the Mo and W coatings. The acetate salt seems to yield a much more uniform surface coating while the alkali metalate solutions yield what appear to he salt ridges (see Figures 8 and 9). This lack of uniform coverage apparently results in the variation of observed increases in atomization efficiency for W- and Mo-coated graphite furnaces. Although, the previous report (9)on the effect I
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1981. 53. 1576-1579
using molten metal chlorides appeara to improve the coverage; however, the structural integrity of the surface is rapidly degraded (11).
ACKNOWLEDGMENT We wish to thank NSA Hitachi Analytical Instrument Division for support of this project and Naito for technical assistance with the scanning electron microscope. LITERATURE CITED (1) Fulla. C. W. ' " E b c V o ~Ass~cbtiOn l lor At& Absuptbn S p e c U ~ l?m : chrmlcal Society: London. 1977; p 81. (2) Chakrabartl. C. L. Rcg. Anal. At. Spscnar. 1978. 1. 1. (3) Shagean. R. E.; chakrabartl. C. L.: Langlad. C . H. Anal. chan. 1976. u1. 1792. (4) ChakraGrtl. C.C.; am. H. A.: Wan. c. c.: u. w. c.: ~ertea.P. c.; Cregohe. D. C.; Lee. S. Anal. chem. 1980. 52. 187. (5) Manning. D.C.; Sbvh.W. A M I . C h m . 1978. 50. 1234. (6) Fritzche. H.; Wegsctmlder. W.: Knnpp. G.: OItner. H. rnhntu 107% 20. 219. (7) Zatka. V. J. Annl. Chem. 1970. 50. 538. (8) Vickrey. T. M.: kkn!son. 0. V.: Ramebr. 0. J. At. Spacaosc. 1900. 1
I .
npue 9.
.=
I I".
(9) Vlckrey. 1.M.: Hanlson. G. V.: Ramebw, G. J.: C a w r , J. C. Anal. Len. i980. 13(A9). 781. (10) Vlckrey, T. M.; Howen. H. E.: Hnniwn. Q. V.: R a m . G. J. Anal. Chem. 1980. 52. 1743. C o n l a a a , on A n a w l (11) Almeida. M. C.: Sdz. W. R. Chemisby and ApplM Swctroscopy: A U n m CRY. NJ. March 1980: Abstract No. 590.
As in Flgure 5 but lhs surface was coated wkh Na,WO,.
of surface treatment with W or Mo salts is, in some cases, roughly the same 88 with the Zr used here. however, the imprecision of the coating does not encourage the use of these salts as a general procedure. The increased coating efficiency
RECEIVED for review February 23, 1981. Accepted June 1, 1981.
Dissimilarity Measure of Concentration Ratios Hideo Nlshida' AppIM StatlstlCs Labwatory. NrppOn Instlhrte of
Technology, Miyashkwnachl, Saltam, Japan
Sadao Ikeda Dsparlm3nt of Economics 8 Insthie of Information Sclems, Soka Univers#y, Tangcmachl, t&chb/f, Tokyo, Japan
Masaya Mlyal Depamnwrt of S ~ s f e n sE n g l w , Nippon Insthie of Technology, M&ashko-macM,Saltam, Japan
Shlzuo Sutukl FacuW Of Phamaceullcal &/em, science UniversHy of
Tokyo.
Fumgawara-mchl SMn/uku-ku, Tokyo, Japan
The conMallon number, proposed by Anders in analyzing concentratlon-ratlo data, has been widely applied to the analysis of environmental data. especially in Japan. The present artkle points out (10meundesirable popetiler of this number a s a similarity measure. An aiternatlve measure of
d/ss/m//ar/ty Is proposed, whlch has ceriain desirable prop etiles for analyzlng concentrallon-ratio data.
Suppo~?we are given a set of n data points obtained by an independent measurement on the concentrations of p elements or substances of n individuals (say, p heavy metals in the bottom sediment of a river)
X(k)
( X , ( k ) ,X,(k), ..., X J k ) )
k = 1, 2, .-, n (I)
0003-2700/8110353-1576~01.2510
where the unit of measurement may be different from element to element. There are some situations in which the ratiostructure of the data is to he investigated rather than the absolute values of the original data (eq 1). The ratioatructure of the data is given by x;j(k) = X ; ( k ) / X j ( k ) ij = 1, 2, p (2) k = 1, 2, ..., n
...,
In such a case,we need a suitable dissimilarity (or, similarity) measure defmed between a pair of individuals, say Ixij(k)l and Ixij(l)\, in comparing the two individuals and/or in clustering the data into a certain number of groups of "similar" individuals. Anders (1) has proposed a sort of similarity measure, ru(M), called the correlation number for the purpme of comparing the concentration-ratio structure of two individuals. The 0 1981 AmerlCan Wrtmlcal society
