ANALYTICAL CHEMISTRY, VOL. 51, NO. 2, FEBRUARY 1979
Sir: Yao and Zollinger express their concern about the derivatization methods for the determination of bis(ch1oromethyl) ether (BCME) in air. the methods in question were published in A n a / j , t i c a l Chenzistr3, (1. 2 ) .e% '! have carefully reviewed their comments, specifically with respect to the issues of concern they have raised. Because of the carcinogenicity of BCME (3, 4 ) and its impact on the environment, extensive research has been conducted in many laboratories throughout the world. The development of analbTica1 methods, the study on the stability of BCME and its possible formation from many sources has been rhoroughly researched in our laboratory over the past few years ( I , 2,5512). Yao and Zollinger have reviewed many analytical techniques that have been developed in the scientific community to monitor BCME at parts per billion levels in air for environmental control. However. most of these techniques require expensive and sophisticated instrumentation that would not be accessible to a small laboratory or convenient for on-site plant analysis. The derivatization procedure ( I , 2) developed in our laboratory has adequately met our objectives for a rapid, practical. and reliable analbtical technique. I t is well established that the reaction of BCME with sodium methoxide and the sodium salt of 2,4.6-trichlorophenol will produce three main derivatives. 2,4,6-trichlorophenoxy m et h y 1 met h ox y me t h y 1 et her ( 4 1. bi s ( 2.4,6-t r i ch lor ophenoxymethyl) ether (B) ( I . 2 ) . and bk(methoxymethy1) ether (C). All these products were confirmed by both mass spectrometry and nuclear magnetic resonance (13).
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T h e distribution of these products in this reaction depends on the relative reactivitv of BCME and concentration of phenoxide and methoxide species We also realize from the chemistry of this rraction that 100% conversion of BCME to any single product is impossible However. for a reliable analytical procedure. the reaction products need not be stoichiometric, but the percent conversion must be constant and reproducible relative t o the amount of the particular product heing determined In respect to derivative A. reference 1 clearly shows the linearitv of the procedure relative to the concentration of RCME (Table I) for derivative A Even though the absolute conversion of BCME t o this derivati\e is unknown, it is very reproducible BCME recoveries of 86- l l h % were obtained utilizing derivative A by comparing the response from standard. of BCME: in air to the response for similar amounts of BCME added to the derivatizing solution directly. It is not a measiire of the percent conversion of BCME to derivative A. The procedure was further verified and validated by analysis of prepared vapor standards by the GC derivative procedure and GC-Mass Spectrometric technique ( 5 ) (Table 111,reference 1 ) of the rinderivatized sample 0003-2700/79/0351-0301$01 O O / O
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Since the first GC procedure was designed for derivative A, there was no attempt a t this point to determine derivative B. Later when the above procedure ( I ) was used on certain samples, possible interferences were encountered. I t was realized, as previously stated, that derivative B was also present from the chemistry involved but did not elute from the GC column under the conditions employed. A different GC procedure was developed for detecting derivative B. The linearity of response was determined for derivative B ( 2 )and a recovery study was carried out as described previously for derivative A. Again it is not necessary to know the conversion efficiency of BCME to the products as long as the conversion is reproducible. It has been demonstrated that sensitivity could be increased by changing the amounts of sodium methoxide and 2,4.6-trichlorophenol, thus increasing the conversion efficiency for derivatives A and B relative t o the previous conditions ( I ) . T h e excess sodium methoxide was added to form the sodium salt of 2,4,6-trichlorophenol so that the trichlorophenol would not be extracted into the hexane. In the improved procedure, the BCME is allowed to react before addition of NaOH to eliminate any free trichlorophenol. The derivatizing reagents are used in fixed proportion and at concentrations many orders of magnitude higher than that of the BCME present in the sample, and thus a constant reagent environment is maintained for all samples, and hence the same product mixture is always produced. Yao and Zollinger have clearly misinterpreted the role of the possible interference, chloromethylal, reported in our study (2). It was pointed out ( 2 )that if chloromethylal were present, it would be expected to yield the same derivative A as BCME and thus would constitute an interference. It is common in trace analytical work to encounter interferences that may be difficult to identify and whose source is not readily identified. In fact. the use of the later procedure ( 2 ) measures the derivative B for which chloromethylal is not an interference. if present, and thus provides a more specific determination of RCME. Chloromethylal is not a main issue. Yao and Zollinger question the validity of the derivative method. Factual data that would support their comments or data that would refute that of the published methods, however. are not presented. The derivative method for the determination of BCME has been used successfully by many analytical chemists in our laboratory and others in the scientific community (14,15). This method has also been recommended (16) for use by subcommittee 5 of the APHA Intersociety Committee. Yao himself has evaluated and used the derivative method quantitatively during an extensive field survey with many tables of data presented in his final report (17). The derivatization method is specifically designed to detect and measure parts per billion BCME in environmental air, and it has been repeatedly shown to be technically and chemically sound. We are not aware of any data which do not support the analytical integrity of the published derivative methods.
LITERATURE CITED (1) R. A . Solomon and G . J. Kallos, Anal. Chem., 47, 955 (1975). (2) J. C. Tou and G. J. Kallos, Anal. Chem , 48. 958 (1976). (3) B. L. VanDuuren, E. M. Goldschrnidt, C . Katz, L. Langseth, G. Mercado, and A. Sivak, Arch. €nviron Health. 16, 472 (1968) (4) R. T.Drew, S.Laskin, M. Kuschner, and N. Nelson, Arch. Environ. Heath, 30, 61 (1075), and references therein. ( 5 ) L. A. Shadoff, G. J. Kallos, and J. S. Woods, Anal Chem., 45, 2341 ( 1973). (6) G. J. Kallos and R. A. Solomon, Am. I d . Wg. Assoc. J., 34, 469 (1973). (7) J. C . Tou, L. 8. Westover. and L. F. Sonnabend, J . Phys. Chem.. 78, 1096 (1974) (8) J. C. Tou and G. J. Kallos, Am. Ind. Hyg. Assoc. J . , 35, 419 (1974).
? 1979 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 51, NO. 2 , FEBRUARY 1979
(9) J. C. Tou, L. 8. Westover, and L. F. Sonnabend. A m . Ind. Hyg. Assoc. J., 36, 374 (1974). (10) J. C. Tou and G. J. Kallos. Anal. Chem., 46, 1866 (1974). (11) G. J. Kallos and J. C. Tou. fnviron. Sci. Techno/., 11, 1101 (1977). (12) G. J. Kallos, U.S. Patent 4042326 (1977). (13) G. J. Kallos and R. A. Solomon, unpublished data. (14) G. M. Rusch, A. R. Sellekumar. S. L. LeMendola. G. V. Katz, S. Laskin, and R. E. Albert, Am. Ind. Hyg. Assoc., Abstr. 176, May 1978, Los Angeles, Calif. (15) K. S. McCallum. Rohm and Haas Co., private communication. (16) E. Sawicki, T. Belsky, R. A . Friedel, D. L. Hyde, J. L. Monkman. R. A . Rasmussen, L. A. Ripperton. and L. D. White, "Methods of Air Sampling and Analysis", American Public Health Association, 1977, pp 874-877. (17) C. C. Yao and G. C. Miller, Final Report "Research Study on Bis(chloromethyl) Ether Formation and Detection in Selected Work
Environments", Sept. 17, 1976, Contract No. 210-75-0056, The Bendix Corporation.
G . J. Kallos* R. A. Solomon J. C. Tou Analytical Laboratories Dow Chemical U.S.A. Midland, Michigan 48640
RECEIVED for review October 16, 1978. Accepted October 17, 1978.
Thin Carbon Foils for the Elimination of Charging Effects in Proton Induced X-Ray Emission Spectrometry Sir: The advantages of ion excitation, particularly proton excitation in X-ray emission analysis ( I ) have been debated at length in the literature (2). The most important advantages of ion excitation over electron or X-ray excitation are the high cross sections for X-ray emission and the low background contribution from bremsstrahlung. If the target consists of a thin uniform sample, the continuous background radiation is low, but if the target thickness is increased to such an extent t h a t the proton beam is stopped completely, a considerable increase in the background continuum occurs, especially if the sample is a good insulator. The acceleration of electrons toward the sample targets that have acquired a positive charge from the proton beam results in an increase in the background bremsstrahlung in the 0-20 keV region. The effects of this increase in the bremsstrahlung on the X-ray emission spectra are twofold: many subtle features in the emission spectra are obscured and the accurate measurement of peak heights or peak areas for purposes of quantitation is almost impossible. Several experimental modifications for minimizing the charge buildup on the sample target and reducing the
bremsstrahlung have been proposed. The evaporation of a conductor onto the sample ( 3 , 4 ) or mixing the sample with a conducting compound ( 5 ) are unsatisfactory because the sample can be readily contaminated. An increase in the pressure of the sample chamber has also been shown to be effective ( 6 ) ,hut this also increases the probability of contaminating the sample. The most satisfactory device that has been used is a hot filament that is positioned close to the sample target. Electrons emitted from the hot filament prevent the accumulation of positive charge on the target. Commercially available tungsten filaments, however, were found to be a severe source of contamination (6, 7). In a successful modification of this approach, a commercial carbon filament was clamped between two carbon rods, to form an "electron gun" and the electrons from the carbon filament effectively neutralized the charge on the sample target. A perforated aluminum cap that was maintained at +lo0 V was placed over the carbon filament to prevent contamination of the sample with impurities in the carbon filament (6). A simple and effective alternative to the "electron gun" is
Proton Beom
Somple changer
Faraday cup current
Figure 1. Sample chamber showing the position of the carbon foil in the proton b e a m . (The figure is not drawn to scale) 0003-2700/79/035 1-0302$0 1.OO/O
@ 1979 American Chemical Society
