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to an exhaust duct via the dummy column. The valve is turned only to let the peak of interest pass on to column 2; an operation noted by the dark rectangle on the time axis of Figure 2 .
RESULTS AND DISCUSSION Figure 2 shows regular chromatograms obtained from EC-1 for PCNB (pentachloronitrobenzene, a much-used fungicide) and Aroclor 1254 (a typical polychlorinated biphenyl product), followed by a mixture of the two, in which P C N B overlaps with peak 5 of the Aroclor. In each case, the valve was turned at the same time, allowing PCNB, Aroclor peak 5, and their EC-induced products to enter column 2 . Residual analyte (starred) and products were monitored by EC-2 as shown in Figure 3. As expected, the product patterns of pentachloronitrobenzene and the polychlorinated biphenyl(s) are drastically different and allow unequivocal distinction, even in mixture. I t should be noted that, even though columns 1 and 2 are packed with the same stationary phase, the analyte peaks overlap severely in Figure 2 but are almost resolved in Figure 3. This is caused by the temperature difference between the two columns and the superior chromatographic conditions for resolution on the second one. T h e effect could have been
easily enhanced by using a different column packing in column 2; and, mutatis mutandis, there are obvious analytical advantageous to such an approach. While the method appears to work well, an obvious limitation and caveat should be kept in mind. First, not all compounds which respond well in the EC detector give rise to useable product patterns. Second, the requirement for a "clean" system, so typical of EC-GC, is even more stringent in this case, where peaks in the picogram range are being manipulated. This taken into account, the described approach should prove valuable for the detection and confirmation of pesticide residues.
LITERATURE CITED (1) (2) (3) (4)
C. R. Hastings, T. R. Ryan, and W. A. Aue, Anal. Chern., 47, 1169 (1975). S. Kapiia and W. A. Aue, J . Chromatogr., 108, 13 (1975). S. Kapila, Ph.D. thesis, Dalhousie University, 1976. C . R. Hastings and W. A. Aue, J , Chromatogr., 89, 369 (1974).
RECEIVED for review August 19, 1977. Accepted October 10, 1977. This study was supported by AC grant 6099 and NRC grant 9604.
Tantalum Treated Graphite Atomizer Tubes for Atomic Absorption Spectrometry Vladimir J. Zatka J. ROY Gordon Research Laboratory, INCO Metals Company, Sheridan Park, Mississauga, Ontario, Canada, L5K 1Z9
Atomic absorption analysis in electrically heated graphite atomizers has found widespread acceptance as a routine method in many research and application laboratories. Atomization of the sample in graphite tubes heated up to 3000 "C makes the method exceptionally sensitive and capable of determining a large number of trace elements directly in diverse sample matrices. Unfortunately, practicing analysts have not always been able to take full advantage of the high temperatures. The tubes rapidly deteriorate and a frequent use of standards is then required due to a steadily changing response. At 2700 "C a useful lifetime of 30 to 50 firings is not unusual. By lowering the atomization temperature, the lifetime of a tube can be increased but sensitivity for many elements is thus sacrificed and potential problems from an incompletely volatilized sample matrix may be generated. T o alleviate the sensitivity problems, matrix modification (I, 2) or various pre-treatments of the graphite tube, sometimes of questionable value, have been tried (3-5) including the in situ coating with pyrolytic graphite (6-9). In all these approaches, only the interior of the tube is affected and the exterior surface is left unprotected towards oxidation. No substantial improvement in the lifetime of the tube is so achieved. Obviously, a much more useful approach is one where the whole graphite tube surface, both interior and exterior, is involved in the protective treatment. Tubes with a complete pyrolytic coating are available from Varian-Techtron. It is the purpose of the present paper to document the outstanding properties of tantalum carbidized graphite tubes. Developed originally for handling lanthanum matrices, the treated tubes showed such a steady response and long lifetime a t 27OC-2800 "C that they are now being used for over two years for general electrothermal atomic absorption practice. In the meantime, two papers were published dealing with specific determinations of silicon in tungsten (10) and beryllium in biological material (11) for which tubes impregnated with tungsten, 0003-2700/78/0350-0538$01 .OO/O
tantalum, or zirconium salts were used. In the present paper, a rapid soaking method is described. The procedure is simple enough to be carried out in any laboratory and is easily amenable to commercial mass production of carbidized tubes. T h e general performance of the tantalum carbidized tubes is distinctly superior to that of tubes with internal pyrolytic coating (9).
EXPERIMENTAL Apparatus. The work was done on a Perkin-Elmer Model HGA 74 graphite furnace atomizer mounted in a Model 306,4A spectrophotometer with a Model 165 recorder. Also used were a Leeds and Northrup disappearing-filament optical pyrometer, a Cameca Model MS 64 electron microprobe, and a Siemens x-ray diffraction unit. Regular graphite tubes were utilized for the treatment by tantalum.Sample solutions were injected by Eppendorf microliter pipets. The atomizer system was operated in the gas-stop mode with argon as the purge gas. Tantalum Soaking Solution (670Ta). Weigh 3 g of tantalum metal into a 100-mL PTFE beaker, add 10 mL of dilute hydrofluoric acid (1 + l), 3 g of oxalic acid dihydrate, and 0.5 mL of 30% hydrogen peroxide. Heat carefully to dissolve the metal. Add more peroxide when the reaction becomes too slow. When dissolution is complete, add 4 g of oxalic acid and approximately 30 mL of water. Dissolve the acid and dilute to 50 mL. Store in a plastic bottle. Tube Treatment. Vertically immerse the graphite atomizer tubes in the 6% tantalum soaking solution co7tained in a plastic vial. Transfer the vial into a desiccator, evacuate (water pump), and maintain under reduced pressure for 20-30 s. Release the air bubbles formed on the tube walls by tapping the exterior of the desiccator. Restore the atmospheric pressure in the desiccator, remove the tubes from the bath and dry them first in the air (30 min) and then at 105 O C (1h). Mount each tube in the atomizer unit fitted with new unused graphite rings and, while the argon purge gas flow and the water cooling are on, raise the temperature gradually (30 s) to 1000 "C and then for a few seconds to 2500 "C. Repeat the treatment once again but soak the tubes for only C 1978 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 50,
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ANALYTICAL CHEMISTRY, VOL. 50, NO. 3, MARCH 1978
Table 11. Accuracy in Copper Metal AnalysisC Bi
-
Parts per million Pb
Sb
Sn
NBS Designation Found NBS Found NBS Found NBS Found NBS 0.3 0.35 28. 26.5 3.7 4.8 65 SRM 394 (Cu I ) SRM 395 (Cu 11) 0.4 0.50 3.4 3.25 7.8 7.5 1.7 1.5 SRM 396 (Cu 111) 0.1 0.07 0.44 0.41
