ANALYTICAL CHEMISTRY, VOL. 51, NO. 12, OCTOBER 1979
I
R Figure 1. Structure of basic beryllium compound
absorbance measured at 490 nm against a blank prepared in the same manner with deionized water. The value of the blank is subtracted from the absorbances of the samples. Data for the different fluoride samples are shown in Table I. The curve is linear to 15 ppm fluoride. Interferences. Common anions and cations were investigated as interferences (Table 11). Anions were tested for interference as their sodium or potassium salt for positive interference at zero fluoride concentration. Hydroxide ion, because of its close similarity in size and charge density to fluoride, interferes as expected. pH control at 6.0 is essential. Cations which weakly complex or precipitate fluoride, such as calcium and magnesium, can be adequately removed by the cation-exchange resin.
RESULTS AND CONCLUSIONS The 6:4 ratio of dye anions to beryllium cations in the basic complex was confirmed by the method of continuous variations, using M solutions of each component. Some variation in sensitivity with the type and porosity of filter
2045
paper was observed; Whatman No. 40 paper provided maximum sensitivity. The choice of ligand was severely restricted by several considerations: first, the carboxylate ligand must be highly colored and an azo dye appeared to be the most feasible choice; second, in order to admit six ligands around the {OBe4)6+ nucleus, groups no larger than substituted benzoic acid would appear necessary to avoid steric hindrance; and third, selection of an azo dye of desirable hue and sufficient molar absorptivity would suggest a naphthol group as the coupling component. Further, experience in the preparation of azo dyes suggested that a single pure compound would be obtained if @-naphthol were the coupling component rather than a-naphthol, which often produces a mixture of dyes. The method as described lacks the sensitivity necessary for such purposes as control of fluoridation of public water supplies, but the absence of interference by sulfate makes it potentially useful in other applications.
LITERATURE CITED (1) Snell, F. D.; Snell, C. T. "Colorimetric Methods of Analysis", Vol. 11, 3rd ed.; Van Nostrand Co.: New York, 1949; Chapter 54. (2) Bellack, E.; Schouboe, P. J. Anal. Chem. 1958, 3 0 , 2032. (3) Hensley, A. L.; Barney, J. E.. 11. Anal. Chem. 1960, 32, 828. (4) Belcher, R.;Leonard, M. A,; West, T. S. Talanta, 1959, 2, 92. (5) Cotton, F. A,; Wilkinson, G. "Advanced Inorganic Chemistry", 3rd ed.; John Wiley 8 Sons: New York, 1972; Chapter 7. (6) Sillen, L. G.; Martell, A. E. "Stability Constants", 1964, Special Pub. No. 17; The Chemical Society: London, 1971; Supplement No. 1, Special Pub. No. 25. (7) FierzDavid, H. E.; Bbngey, L. "Fundamental Processes of Dye Chemistry",
transl. from the 5th Austrian ed., by P. W. Vittum; Interscience: New York, 1949; p 262.
RECEIVED for review September 29, 1978. Accepted July 9, 1979. This work was supported in part by National Science Foundation Grant CHE-7403024.
Carbide Coating Process for Graphite Tubes in Electrothermal Atomic Absorption Spectrometry Elsa Norval," H. G. C. Human, and L. R. P. Butler National Physical Research Laboratory, CSIR, P.O. Box 395, Pretoria 0007, Republic of South Africa
Automated or semiautomated atomic absorption spectrometers using electrothermal atomization are capable of handling large numbers of samples without the need for continuous operator attendance. The weak link in the automated process has thus far been the graphite tube, whose physical characteristics change on aging ( I , 2). It has also been shown that the type, structure, and reactivity of the graphite influence sensitivities and limits of detection ( 3 ) . The use of tubes coated with pyrolytic graphite solves some problems and in many cases an increase in sensitivity is obtained ( 4 , 5 ) . A major disadvantage of this type of coating is that it is often progressively removed from the surface and recoating of the tubes requires recalibration. Moreover, when graphite surface interference effects are involved, the problem is not always eliminated by such a coating. Bokros (6) has shown that the wear resistance of a pyrolytic carbon-coated article is markedly improved if a thin layer, made up of pyrolytic carbon plus an amount of a carbide additive having good frictional wear characteristics, is created near the outer surface. Such a finding may point to a different type of bonding of the carbon which could also have greater 0003-2700/79/0351-2045$01.00/0
resistance to elevated temperatures and chemical attack. A number of reports have been published which describe treatment of graphite tubes with solutions of metals capable of forming interstitial carbides (7-12). These papers all describe soaking the tube in a solution or applying the solution to the inner tube surface. Both these methods preclude forming a layer of the metal as a first step. This is considered an advantage as any mass of the metal can be sputtered, whereas the concentration of solutions of interstitial carbide-forming metals is generally limited. In addition the use of such solutions results in the metal atoms being interspersed with other substances. A barrier of the protective carbide is therefore not obtained. Furthermore, nowhere is any mention made of the reproducibility of the coating or reinforcing process and information on tube lifetime is scanty. Ortner and Kantuscher (9) stated that the lifetime of tubes treated by their method was similar to that of untreated tubes, Runnels et al. (IO) reported that their coating was stable for the normal useful life of the furnace, and Zatka (12) gave a figure of 400 firings a t temperatures of 2700-2800 O C . This paper describes the application of tungsten and 1979 American Chemical Society
2046
ANALYTICAL CHEMISTRY, VOL. 51, NO. 12, OCTOBER 1979 GLASS DISCS
h
r PJRE
1
Table I. Comparison of Tube Lifetimes and Reproducibilitiesu Coated Commercial pyrolytic tube tube
from detmn no. Flgure 1. Diagram of sputtering chamber used for coating interior surfaces of graphite tubes
16
EXPERIMENTAL Instrumentation. The results of this study were obtained using a Perkin-Elmer Model 460 spectrometer equipped with a Model HGA 2200 graphite furnace atomizer, an AS1 automatic sampler, and a Perkin-Elmer Model 056 recorder (fast response). Coating Process. Both standard and pyrolytically coated commercial tubes were coated by the process described here. A sputtering chamber (Figure 1)consisting of an acrylic cylinder was fitted with water-cooled brass end plates which served as electrical contacts for a 1.5-mm diameter tungsten or tantalum rod 50 mm long. The tube was supported concentrically with the metal rod by two glass disks, while two further glass disks centered the rod, with the edges of the disks fitting into slots in the acrylic cylinder. A brass electrode passing into the cylinder through an O-ring seal made contact with the tube. The metal rod was connected as the cathode and the graphite tube as the anode. The chamber was evacuated to 6.7 X lo-' Pa and subsequently purged with purified argon a t 133 Pa, then again evacuated to 2 X lo-' Pa after which the argon supply was restored to give a pressure of approximately 170 to 267 Pa. At this pressure and with a direct current of 20 mA passed at 600 V, profuse cathodic sputtering occurred. The continuous flow of argon through the chamber was directed to enter the graphite tube at both ends and to pass out through its sample introduction hole. The sputtered metal formed an even coating on the inner surface of the tube, some metal emerging from the sample introduction hole and covering a collar-shaped area immediately surrounding the hole on the outer tube surface. The mass of metal deposited was easily controlled by periodic weighing, and on average 10-20 mg of metal were applied to each tube. After application of the metal, the tubes were taken through a series of heating stages in an argon-methane atmosphere using an induction furnace to convert the metal to the carbide and to deposit a layer of pyrolytic carbon. The argon flow was kept constant at 10 L/min and the methane flow was varied according to the temperature program. A satisfactory program was found to be: 1850 "C for 35 min, 2100 "C for 1 min, 2350 "C for 1min, and 2550 "C for 0.5 min. Methane flow was increased from 0.03 L/min a t the initial temperature to 0.14 L/min a t the final temperature, which resulted in the application to the tube of a total mass of 200 mg graphite. On completion of the firing, the inner surface of the tube and an area on the outer surface extending for about 2 mm around the sample introduction hole were provided with a coating of metal carbide followed by one of pyrolytic graphite which covered both inner and outer tube surfaces. Coated tubes were found to take about 2 s longer than commerical ones to reach a required atomizing temperature. Atomizing temperatures were measured with an optical pyrometer and those obtained with coated tubes were 100" to 150" lower than those obtained with commercial tubes. Adjustment of the temperature setting compensated for the difference in resistance which had been indicated by this finding. Resistance may also be increased by removing a fraction of the tube wall before it is coated.
7.1
from detmn no. 1
0.63
(50 values)
71 tantalum carbide coatings by means of a process which is highly reproducible. Tubes coated using this method show greatly improved resistance to oxidation and chemical attack and have greatly extended lifetimes.
RSD for Cub in HNO, and HClO,, %
RSD for c u b in HNO, RSD for and Cuc in HClO,, HNO,, %, %, 50 25 values values
126
2 (30 values)
200
2.7
6.1
400
2.8
(30 values) 1000 1100
1500
2.2 1.1 1.1
1.7 1.6
1600 2 000 2 500
2 560 2 700
1.9 1.9
2.3 2.6 a Instrumental parameters: wavelength, 324.7 nm; sample volume, 20 p L ; argon flow, 30 (instrumental setting); dry 110 'C, 50 s (ramp 40 s ) ; char 500 " C , 1 0 s (ramp 5 s ) ; atomize 2700 "C (as measured with a n optical pyrometer) 6 s for the commercial tube and 8 s for the coated one, 0.0125 ppm Cu in 1 0 % (viv) nitric acid and 2% !v!v) 0.0125 ppm Cu in 10% (vlv) nitric acid. perchloric acid. 3 000
RESULTS A N D D I S C U S S I O N Lifetime of Tungsten Carbide-Coated Tube. A test solution consisting of 0.0125 ppm copper in 10% (v/v) nitric acid and 2% (v/v) perchloric acid was used for lifetime tests. A commercial pyrolytically coated graphite tube served as reference. This tube required 65 firings t o give reproducible readings after which a short series of 30 determinations gave satisfactory reproducibility. Following this, t h e tube deteriorated rapidly and was unusable. The carbide coated tube also required about 65 firings to stabilize owing to the perchloric acid attacking loosely bound carbon. Stabilization could possibly be achieved with fewer firings if a more concentrated solution of this acid were used. In order to determine reproducibility in the absence of perchloric acid for this tube, the determinations with the test solution were interspersed with groups of 25 determinations where copper was present in 10% nitric acid only. The coated tube lasted for more than 3000 determinations with the first signs of erosion around the sample introduction hole being detected after 2700 firings. T h e absorbance obtained for copper remained the same for t h e duration of the tube's lifetime and was similar to that obtained with the commercial tube. The lifetimes of the two tubes with their respective reproducibilities are given in Table I while in Figure 2 reproductions of the recorder tracings obtained are given. Samples in a perchloric acid medium have thus far not generally been analyzed by the electrothermal atomization technique, partly because of tube erosion and partly because of the chloride matrix. Results have been reported showing the degree of suppression of the absorbance of various metals in the presence of perchloric acid (13). Our results for copper obtained with the coated tube showed less than a 10% decrease in sensitivity when perchloric acid was present and, as reproducibilities were satisfactory, it is clear that during sample preparation an evaporation step for removal of perchloric acid can be omitted when a coated tube is used.
ANALYTICAL CHEMISTRY. VOL. 5
Table 11. Determination of Lead in a Sodium Chloride Matrix, Comparison of Three Types of Tube commercial pyrolytically commercial stancoated tube dard tube detmn RSD, RSD, no.
%
21-50 61-90
1.4 11.0
detmnno.
tube unusable
1-40
151-180 345-373
%
6-35
0.8 1.7
121-150
tube powdery and unusable from 600 f i rings onwards because of adhesion of carbon particles to sampler tip
301-330 601-630
1.9 1.3 2.3 1.5
i41
tube virtually unaffected. N o interference by car-
b
I
bon particles
Determination of Lead i n a Sodium Chloride Matrix. A tungsten carbide-coated tube was tested for lead and compared with both the commercial pyrolytically coated and standard graphite tubes by using 0.33 ppm lead in 0.5% sodium chloride and 0.5% (v/v) nitric acid. Operating parameters (similar for all three tubes) were the same as those used for the previous test, except that the atomizing temperature was 2300 "C and the atomizing time 6 s. Molecular absorption, due to sodium chloride was eliminated by background correction. The results are given in Table 11. The test was terminated after lo00 determinations with the metal-coated tube as its longer lifetime had a t that stage been proved since it was virtually unaffected. Although still giving good reproducibility, the commercial standard tube was very badly eroded after 800 determinations. I t had become progressively more powdery and from about 600 firings the sampler tip was completely blackened and required continual wiping. It is interesting to note that Fuller (2)suggested that the difficulties experienced with lead determination in the presence of magnesium chloride could be overcome by using old tubes which have a soft powdery inner coating. Sensitivity a n d R e s i s t a n c e t o Oxidation w i t h a T a n t a l u m Carbide-Coated Tube. Copper was selected as the test element. Operating parameters and instrumentation were the same as those previously described except that the AA 460 spectrometer was replaced with an AA 272 with which somewhat poorer sensitivities were obtained. A tantalum carbide-coated tube (17 mg Ta) was tested using two solutions. Solution A, used for testing resistance to oxidation, consisted of 5% (v/v) perchloric acid and 10% (v/v) nitric acid. After every 25 firings with solution A, the mass loss of the tantalum carbide-coated tube was determined, after which a number of absorbance measurements were obtained using solution B (0.02 ppm Cu in 10% "OB). Two standard and two pyrolytically coated commercial tubes served as reference. For
2047
I
coated t u b e RSD, detmnno. % '
1.5
NO 12, OCTOBER 1979
30:
Figure 2. Reproducibilityof copper signals. Test solution: 0.0125 ppm Cu in 1 0 % HNOBand 2 % HC104. Number of firings at base line. (a) Commercial pyrolyticaliy coated tube. (b) Tungsten carbidecoated tube
these copper absorbance was measured (solution B) after every 20 firings with solution A. Results are given in Table 111, the mass loss values serving as a measure of oxidation. Commercial pyrolytically coated tubes had very little resistance to solution A, the coating flaking off within 30 firings, after which reproducibility was so poor that the tubes were unusable. After 100 firings with solution A, the mass loss found for commercial standard tube 1was about 8 times, and for tube 2 about 10 times greater than for the tantalum carbide-coated tube. Absorbance values obtained with the tantalum-coated tube were 2 to 3 times greater than those obtained with the commercial standard tubes, and a t the termination of the test the tantalum carbide-pyrolitic graphite layer was still intact. With these tests mass loss was not considered to be a good indicator of resistance to oxidation on the interior surfaces of the tubes, as most of the erosion appeared t o have taken place on the external surfaces. On examination, the interior of the tantalum carbide-coated tube appeared virtually unaffected. Because of their poor condition it was difficult t o judge where the standard tubes were most affected. This indicates a need for exterior as well as interior coating of tubes. CONCLUSIONS Tubes coated according to the method described have greatly extended lifetimes in comparison to standard graphite
Table 111. Comparison of Mass Loss and Absorbance Values for Three Types of Tube commercial pyro- commercial pyro-
number of firings with perchloric acidinitric acid mass loss, mg
initial and final
lytically coated tube (1)
lytically coated
27
52
pyrolytic layer
pyrolytic layer
flaked off 0.2 6-0.1 2
commercial standard tube
tube ( 2 )
(2IU 100
100
80
100
tantalum carbidecoated tubeu 100
10.6
flaked off
0.27-0.16
0.10-0.14
absorbance a
commercial standard tube
These tubes were gently wiped to remove loose carbon particles before weighing.
0.11-0.14
0.30-0.32
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ANALYTICAL CHEMISTRY, VOL. 51, NO. 12, OCTOBER 1979
tubes while giving excellent reproducibilities. This should make metal carbide-coated tubes particularly suitable for routine operation or for use on automated systems. Copper sensitivity is appreciably increased when the coating consists of tantalum carbide. This may indicate a different atomization process from the inert impermeable carbide layer. A study of sensitivities obtainable for analytes normally forming stable carbides in graphite tubes is at present being undertaken by the authors to determine whether reactivity is less in carbide layer tubes. The fact that samples may be introduced into the carbide layer tubes in a perchloric acid medium, normally considered inadvisable in graphite tubes, opens up some interesting possibilities. It may be possible to destroy organic matter directly before atomization and to prevent the deposition of carbon filaments which shield off radiation. The possibility of coating the tubes externally is also being investigated as this could further extend the lifetimes of tubes. The possible coatings are of course not limited to the carbides of tungsten and tantalum as any material which is
conductive may in principle be sputtered.
LITERATURE CITED (1) M. K. Chooi, J. K. Todd, and N. D. Boyd, Clin. Cbem. ( Winston-Salem, N.C.). 21. 632 (1975). (2) C. W: Fuller, At: Absorpt. Newsi., 18, 106 (1977). (3) G.Volhnd, G.Kolblin. P. Tsct-d~el.and G. Tob. Fresenius' 2.Anal. Chem.. 284, 1 (1977). (4) D. C. Manning, F. J. Fernandez, and G. E. Peterson, Ind. Res., 19(2), 82 (1977) I . - .. I .
(5) RT E. Sturgeon and C. L Chakrabarti, Anal. Cbem., 49, 90 (1977). (6) J. C. Bokros, U S . Patent 3969 130 (1976). (7) R . Cioni, A. Mazzucotelli, and G. Ottonello. Anal. Cbim. Acta, 82, 415 (1976). (8) I. A. Kuzovlev, Y. N. Kuznetsov, and 0. A. Sverdlina. Zavodsk. Lab.. 39, 428 (1973). (9) H. M. Ortner and E. Kantuscher, Talanta, 22, 581 (1975). (IO) J. H. Runnels, R. Merryfield, and H. B. Fisher, Anal. Cbem., 47, 1258 (1975) (11) T. Stiefel, K. Schulze, G. Tolg, and H. Zorn, Anal. Cbim. Acta, 87, 67 (1976). (12)V. J. Zatka, Anal. Cbem., 50, 538 (1978). (13) K. Julshamn, At. Absorpt. News/.. 16, 149 (1977)
RECEIVED for review April 24, 1979. Accepted June 18, 1979.
Determination of C8 and Heavier Molecular Weight Alkylbenzenes in Petroleum Naphthas by Gas Chromatography Joseph J. Pesek" Department o f Chemistry, San Jose Stafe University, San Jose, California 9 5 792
Bruce A. Blair Safety-Kleen Corporation, Elgin, Illinois 60 120
ASTM D1319 ( I ) , fluorescent indicator absorption (FIA), has been utilized as the standard procedure in the petroleum industry for the determination of aromatic content in petroleum hydrocarbon fractions having a distillation end point below 316 O C . This method consists of a special glass adsorption column packed with activated silica gel and a mixture of fluorescent dyes. The petroleum distillate sample to be treated is introduced into the column with alcohol and is separated into distinct hydrocarbon groups according to polarity. Under ultraviolet light the various hydrocarbon groups can be determined quantitatively by the length of each zone in the column. Unfortunately, the time required for the analysis of the aromatic content of various petroleum fractions ranges from 1 to 4 h and C5 and lighter hydrocarbons must be removed from the sample. Sautoni, Garber, and Davis ( 2 ) have developed a high pressure liquid chromatographic method for the determination of saturates, olefins, and aromatics in hydrocarbon fractions having a distillation range of 60-271 "C. Their technique employs a low polarity perfluorocarbon mobile phase and a small particle silica column. After resolution of the saturates and olefins, a backflush valve is operated and the aromatics are eluted as one peak. The total analysis time is about 10 min, results are reasonably accurate and precise, and the C5 and lighter hydrocarbons do not have to be removed from the sample. Several gas chromatographic methods have been developed for the determination of hydrocarbon group types in petroleum fractions. Boer and Van Arkel (3)have described an automatic analyzer system using several columns in conjunction with flow switching and hold-up fractions. This can be used for petroleum fractions with distillation end points up to 200 OC and it gives the aromatic and saturate content at each carbon number. However, the analysis time is 2 h. Stuckey ( 4 ) has 0003-2700/79/0351-2048$01 .OO/O
developed a method using an open tubular column coated with 1,2,3-tris(2-cyanoethoxy)propane.This method is not useful for a wide range of petroleum samples because benzene is eluted after n-Cll and the analysis time is 40 min. ASTM D2267-68 (5)is used for aviation gasolines with a boiling point below 177 "C and other petroleum products with a boiling point below 149 "C. Any one of five stationary phases can be chosen and the analysis time is only 10 min, but the aromatic constituents begin eluting after n-Clo. Another ASTM method, D3606-77 (6),uses N,N-bis(2-cyanoethyl)formamide (CEF) as a stationary phase. Using this column with a rotary backflush valve and a 3-m length of 0.04-mm capillary tubing results in benzene being eluted after n-CI5. In this method the saturated hydrocarbons are eluted first followed by benzene and toluene. Then the backflush valve is activated and the remainder of the aromatic constituents are eluted as one peak. The total analysis time is about 15 min, but the method requires a reproducible sample volume. This leads to relatively poor agreement between laboratories on the same sample with overall accuracy about *3%. If the latter problems could be overcome, a CEF column in conjunction with a backflush valve and a flow restrictor could be used for petroleum naphthas because they contain no benzene, very little toluene, and saturated hydrocarbons no heavier than n-C15.
EXPERIMENTAL Apparatus. The gas chromatograph used was a Perkin-Elmer Model 910 equipped with a flame ionization detector and a Sargent
Model DSRG recorder. The He flow rate was maintained at about 26 mL/min. The injection port and detector were operated at 225 "C and the oven temperature was maintained at 145 OC. The CEF column was 2.44 m X 0.31 cm i.d. stainless steel. The stainless steel flow restrictor column was 3.05 m X 0.040 cm i.d. The system was equipped with a Velco Instruments Company
C 1979 American Chemical Society
