ANALYTICAL CHEMISTRY, VOL. 51, NO. 2, FEBRUARY 1979
Table I. Comparison of Absorptivities of Two Sets of Solutions Calculated from the Best Straight Lines and from the Best Straight Lines through the Origin" rice equation s, equation s, brown A = 2 . 3 5 ~- 0.011 0 . 0 0 3 A = 2 . 3 0 ~ 0.003 milledb A = 2 . 0 6 ~+ 0.051 0.008 A = 2 . 3 0 ~ 0 . 0 1 2 Another publication of Cell thickness 1.000 cm. protein determination in brown and milled rice b y the Kjeldahl reaction also shows a larger standard error of estimate for milled rice ( 3 ) .
299
The results calculated from the two equations are shown in Table I, the most striking being the different absorptivities using Equation 10 (2.35 and 2.06) and identical ones (2.30) using Equation 11. A slight difference in the two absorptivities might be expected, since the layer removed by milling contains a small amount of protein that might be different in nature. However, there should then also be a difference when using Equation 11, but there is not. Hence, the identity of absorptivities from best straight lines through the origin demonstrates the desirability of the proposed method.
LITERATURE CITED analyzed for total nitrogen by the Kjeldahl method (1.170 to 1.886% N and 1.051to 1.864% N for the two sets, respectively) and used to test a procedure developed by the authors for determining protein nitrogen by the biuret reaction. The absorbances of the solutions of the copper-nitrogen complex for the two sets of samples are shown plotted vs. the nitrogen concentration in Figure 1. Introducing symbols for spectrophotometry into Equations 1 and 2 gives, respectively,
A = a,
+ abc
(10)
and
A = abc
(11)
(1) "Official Methods of Analysis of the A.O.A.C.", William Horwitz, Ed., 12th ed., Association of Official Analytical Chemists, Washington, D.C., 1975, xvi-xvii. (2) "A Moderately Rapid, Accurate, Room-Temperature Method for the Spectrophotometric Determination of Protein in Rice by the Biuret Reaction", F. C. Strong 111and P. Theis-Maimone, submitted for publication. (35 L. C. Parial, L. W. Rooney, and B. C. Webb, Cer. Chem.,47, 38-43 (1970).
Frederick C. Strong I11 Faculdade de Engenharia de Alimentos e Agricola Universidade Estadual de Campinas Caixa Postal No. 1170 13100 Campinas, S.P., Brasil
RECEIVED for review May 24,1978. Accepted October 30,1978.
Exchange of Comments: Analytical Methods of Bis(chloromethy1) Ether in Air Sir: Since the carcinogenicity of bis(chloromethy1) ether (BCME) was established, its potential presence in the industrial environment became a grave concern (1-4). T o ascertain its actual existence and level in suspected industries and to protect the workers from this occupational hazard, air monitoring is essential. Thus, various quantitative analytical procedures for BCME have been reported. Among them are: (A) Direct gas chromatography (GC) (B) On-Column Concentration-GC. The BCME, together with some of the other contaminants, is first allowed to adsorb on the front section of the column a t room temperature and then analyzed a t a programmed higher temperature ( 5 ) . (C) Adsorber-GC combination. The BCME, together with some of the other contaminants, is first allowed to adsorb on the packing in B trapping tube and then thermally flashed onto a GC analytical column or a set of analytical columns (6, 7 ) . (D) Adsorber-mass spectrometry (MS) combination. The BCME, together with some of the other contaminants, is first allowed to adsorb on the packing in a trapping tube, and then is thermally eluted into the reservior of a high-resolution mass spectrometer (8). (E) Adsorber-GC-MS combination. The BCME, together with some of the other contaminants, is first allowed to adsorb on the packing in a trapping tube, then thermally flashed onto a GC analytical column, and finally the BCME fraction is gated into a mass spectrometer (9, I O ) . The preceding methods are all technically sound and straightforward in application. They differ in sensitivity, selectivity, cost of equipment, and requirement for trained personnel. In addition to the above methods, there is a unique derivatization method which first appeared in this journal (Analytical Chemistry) in 1975, followed by a modified version a year later (11, 12). 0003-2700/79/0351-0299$01.00/0
This derivatization method involves the conversion of BCME, by means of trichlorophenol and methoxide in methanol, into a derivative which is then to be assayed by GC. The stock reagent consists of 25 g of sodium methoxide and 5 g of trichlorophenol in 1 L of methanol. In other words, methoxide is more than 18-fold in molar excess. The derivative was identified, as stated in the article, as C13CsH2OCH20CH20CH,but no spectral or other evidence was given to substantiate its identity. An 86 to 115% recovery was reported according to the data in Table I1 of the original article. (Data in Table I11 of the same article show an 84 to 160% variation.) No reason was offered for an 18-fold molar excess of sodium methoxide, in spite of the fact that BCME undergoes extensive decomposition in the presence of methoxide, and that it has been used in scrubbers to destroy BCME. At the end of the original article, a statement was made to the effect that the sensitivity could be increased 6or 8-fold by using stoichiometric quantities of sodium methoxide and trichlorophenol, with recoveries varying from 82 to 100%. This realization of the basic principle of chemical stoichiometry did not, however, prompt a re-examination of the experiments, nor promote a critical evaluation of the chemistry involved. T h e derivatization apparently involves the following equilibrium:
OH
+
CH30Na
ci
._ C I CI
Both the methoxide and the trichlorophenoxide then react with BCME and can give one unsymmetric derivative and two symmetrical derivatives as follows: 'C 1979 American Chemical Society
300
ANALYTICAL CHEMISTRY, VOL. 51, NO. 2 , FEBRUARY 1979
CICH20CH2CI
+
t CH30Na
Cl
ci CIfiOCH2OCH20CH3
CICH20CH2CI
+
+
2NaCl
2Ci CI
f
CICH20CH2CI f 2 C H 3 0 N a
-
2NaCI
Chloromethylal can conceivable originate from three sources. One is the reaction of BCME and methoxide. It happens to be the product of the intermediate step in the formation of either derivative A, or derivative C, or the decomposition of BCME, all of which originate from the actual presence of BCME, and therefore it is not a false signal. Another possible source is from the reaction of formaldehyde, hydrogen chloride, methanol, and the unisolable chloromethanol. But it certainly cannot take place in the basic derivatization mixture inside an impinger. Putting it in general terms, while hemiacetal formation is catalyzed by either acids or bases, the subsequent acetal formation is catalyzed by acids only. Mechanistically speaking, bases can do nothing t o hemiacetals except attack their hydroxyl functions, which is exactly the reverse of hemiacetal formation. RCHOH OR
(B) CH30CH20CY20CH3
t 2NaCI
B Lr i 1J RCHO-
F='
+
R'O-
OR'
Only acids can consumate the acetal formation as follows.
rH
(C) T h e product distribution naturallv depends on the relative reactivities and relative concentrations of the methoxide and trichlorophenoxide species. In addition, the decomposition of BCME in the presence of methoxide is known to be extensive. T h e formation of deri\iative C from BCME and methoxide is possible but this species is also invisible to an electron capture detector. The formation of derivative B from BCME and trichlorophenoxide is expected but is unmentioned and unidentified in the derivatization mixture in spite of the fact that it is twice as visible to an electron capture detector in comparison with derivative A. Furthermore the degree of decomposition of BCME and the relative extents of formation of the derivatives, A, B, and C, may be different for different samples under the crudely controlled conditions. In view of all these uncertainties, the yield of derivative A alone can hardly be taken as a measurement of the BCME existing in the air. Nevertheless. an accuracy of 86- 115% recovery was reported. Claims of absence of BCME were positively stated when the tests by this method gave negative responses. A year later, an improved version of the method by the same author (12) appeared in this journal. This improved iersion involves no change in the dericatization procedure. just a change in chromatographic conditions. One of the possible symmetrical derivatives, Cl3C6H2OCH2OCH2OC6H2C1, (derivative B), was now identified from the same derivatization medium of a known sample in addition t o the previously identified unsymmetrical derivative (derivative A). What promoted their effort to look for the new product in the same derivatization mixture were the positive responses of BCME in three of their experiments by the original technique, a fact they could not accept. Since negative responses were found for the new symmetrical derivative in their three experiments in question, the quantification of BCME is now t o be determined in terms of this newly found symmetrical derivative which, as was reported in the article, again gives high recoveries. T h e original unsymmetrical derivative which has been established a t 86 -115% recovery from known amounts of BCME samples is now attributed to the unproved and unexplained existence of chloromethylal (D) in the following reaction (12):
RCHO
i
H
A third source of chloromethylal is its pre-existence in air. Appropriate control experiments should be run to determine its presence or absence. In addition, the detection of the new symmetric derivative (B) in a known sample has not always yielded the expected response as experienced by its originator (13).
Therefore, the reasoning for this derivatization method for determination of BCME in air is confusing and chemically unsound.
LITERATURE CITED (1) B. L VanDuuren et al., Arch. Envlron. Health, 16, 472-476 (1968). (2) J. L. Gargus, W. H. Reese, Jr., and H. A. Rutter, Toxic Appl. fharmacol.. 15, 92-96 (1969). (3) B. K . J. Leong. H. N. Macfarland, and W. H. Reese, Jr., Arch. Enwiron. Health, 22, 663-666 (1971). (4) S. Laskin et al., Arch. Environ. Health, 23, 135-136 (1971) (5) F. W. Williams and M. E. Limstead, Anal. Chem.. 40, 2232-2234 (1968). (6) D. G. Parkes et al., A m . Ind. Hyg. Assoc. J , , 37, 165-173 (1976). (7) R. L. Wilkins and L. S. Frankel. U.S. Patent 3, 807,217. (8) L. Collier, Environ. Sci. Techno/.,6, 930-932 (1972). (9) L. A. Shadoff, G. J. Kallos, and J. S. Wocds, Anal. Chem. 45, 2341-2344 (1973). (10) K . P. Evans et al.. Anal. Chem., 47, 821-824 (1975). (11) R . A . Solomon and G. J. Kallos, Anal. Chem., 47, 955-957 (1975). (12) J. C. TOU and G. J. Kallos, Anal. Chem., 48, 958-963 (1976). (13) Final Report, "Research Study on Bis(cloromethy1) Ether Formation and Detection in Selected Work Environments." Sept. 17. 1976, Sec. XVII D, Contract No. 210-75-0056, The Bendix Corporation.
C h a r l e s C. Yao* ~ ~ l Laboratories, ~ ~ p.0. B~~ 15623 orlando, ~ l ~ 32858 ~ i d
~
~
h
~
H e i n r i c h Zollinger Technisch-chemisches Laboratorium Eidgenossische Technische Hochschnle ( E T H ) 8092 Zurich. Switzerland
+
C ' ~ O C H Z O C H ~ O C HN a~C l
CI
RECEIVED for review January 26, 1978. Resubmitted June 6. 1978. Accepted July 31, 1978.
