Elimination of Interferences in Determination of DDT Residues L. B. NORTON A N D BARBARA SCHMALZRIEDT, Cornel1 Uniuersity, Zthaca, N . Y . ERTAIN crops give interfering colors in the Scherhter (3) colorimetric method for determining D D T residues. Cauliflower curds and alfalfa give colors sufficient to mask small quantities of DDT, and even to make a reading impossible on the extract from a large sample. A sulfuric acid treatment surh as that used in t h e analysis of DDT in milk (3) will remove these interferences. However, this procedure requires a change of solvents, tends toward low recoveries, and is somewhat unpleasant and tedious to use. A direct removal of the interfering material from the benzene extract by selective adsorbants was attempted, and gave satisfactory results. Preliminary tests with alumina, magnesia, and Attapulgus clay-Celite mixtures ( 1 ) as adsorbants all gave promising results. Alumina was chosen as the best of these because of more consistent D D T recovery, ease of regeneration of the adsorbant, and superior physical properties. All the data reported here were obtained with alumina.
Table I.
D D T Added.
The physical state of the alumina was such that the benzene passed through the column a t a convenient rate without pressure or suction. It was therefore possible to set up several columgs a t once to save time. The alumina was recovered by driving off the benzene with gentle heating and then igniting for several hours in a muffle furnace at dull red heat. It could be used again without further treatment. RECOVERY q F DDT
Known quantities of D D T were added to the benzene strippings from untreated alfalfa and these solutions were carried through the procedure described. The results are presented in Table I. A series of samples of cauliflower leaves had been run without any preliminary treatment. As a check on the possibility that small fragments of curd might have been included and might have contributed some interference, aliquots of some of the same strip solutions were treated with alumina and analyzed again. There was evidence of interference in only one sample, so that the rest of the samples furnished useful recovery data on residues from actual field treatments. The results are presented in Table 11. The deviations in Table I were considered within the experimental error, especially because test tubes were used as absorption cells in the photometric readings. The 64% drop of the fourth sample in Table I1 was considered due to interference in the first analysis rather than to iriromplete recovery in the second, in view of the full recovvies i n all other samples. The slight tendency toward high' values was very probably due to concentration of some strip solutions by evaporation, inasmuch as the samples were stored for several weeks between the analyses in bottles closed with corks covered with tinfoil. I t was concluded that the alumina treatment would remove the interfewncrs without causing significant loss of DDT. Similar
Y
D D T Found.
1
570 Deviation
83 81 93 91 86 93 114 117 161 159
79 90
113 157
+5
+2 t 3 +1 -4 +3 +1 +4
+2 +1
Table 11. Recovery of DDT from Aliquots of Strippings from Cauliflower Leaves with and without Alumina Treatment D D T Found in Aliquot, 7 Without With alumina alumina 108 93 108 11 32 58 93 4
PROCEDURE
A slurry of alumina (General Chemical Company aluminum oxide, ignited powder, reagent, Code 1236, mean particle diameter approximately 100 microns) in benzene was poured into a column 1 inch (2.5 cm.) in diameter and 18 inches long, containing a small plug of cotton. A bed of adsorbant about 2 inches deep was usually sufficient. As soon as t8helevel of benzene had reached the top of the adsorbant bed, a 5- to 50-ml. aliquot of the benzene strip solution was passed through, and was followed by approximately 50 ml. of benzene added in small portions, to carry through all unadsorbed material. After being passed through the column, the benzene solution was carried through the usual Schechter procedure. The interferences from both cauliflower curds and alfalfa were completely eliminated the absorption spectrum of the final solution being identical with that obtained with pure DDT.
Recovery of DDT from Alfalfa Strippings after Alumina Treatment
%
Deviation
11s 99 108 4 32 65 100 4
+ 6 + 6
0 64 0 +12 f 8 0
-
tests with D D D or T D E [l,l-dichloro-2,2-bis(p-chlorophenyl)ethane] indicated that it would also come through the alumina without significant 1.0s~. BENZENE INTERFERENCE
A reddish-brown interfering color results if the last traces of benzene are not removed before the nitration step of the Schechter colorimetric procedure. Alfalfa is particularly susceptible to this interference because of the large quantity of extracted material, which holds the last traces of benzene tenaciously on evaporation. Only part of this extracted material is removed by alumina. The addition of alcohol to the residue as suggested by Schechter et al. ( 4 ) is not satisfactory in this case because alcohol forms a gelatinous mass which is extremely slow t o dry a t room temperature. Heating was not considered because serious losses of D D T have resulted in this laboratory from heating dry extracts even on the steam bath. I t was found that chloroform would not form a gelatinous mixture, and that it would yield a film type of deposit. Although not so efficient in removing benzene under normal conditions, it was much more satisfactory with alfalfa extracts. The procedure finally adopted t o remove all interferences from alfalfa strip solutions was a treatment with alumina, evaporation of the benzene, two evaporations with chloroform, and finally exposure of the residue to the air for a t least a day. A transfer to carbon tetrachloride and passage through a column of Celite-sulfuric acid as recently described by Davidow ( 2 ) may offer a better method of treatment for such voluminous extracts, although it has not as yet been tried on alfalfa by the authors. The alumina treatment appears a t its best as a rapid and convenient method for removing direct interferences where secondary effects are not serious. LITERATURE CITED
(1) Averell, P. R.,and Norris, M. V., ANAL.CKEM., 20, 753 (1948). (2) Davidow, B.,J. Assoc. Ofic.Agr. Chemists, 33, 130 (1950). (3) Schechter, M. S.,Pogorelskin, M. A., and Haller, H. L., ANAL. CHEM., 19, 51 (1947). (4) Schechter, M.S.,Soloway. S. B., Hayes, R. A., and Haller, H. L.. I N D . ENG. C H E M . , A N A L . ED.,17,704 (1945). RECEIVED April 27, 1950
1451
