fered with morphine readings, as previously noted by Passwater ( 4 ) . Codeine does not interfere under conditions used for the analyses. However, if body fluids or tissues from a codeine fatality were tested, one would obtain readings for morphine, which is one of the biotransformation products of codeine. Thirty random blood samples which were found positive for morphine by spectrofluorometric analysis were selected and the results were compared by GLC determinations. A qualitative correlation was shown in every case because each positive GLC value for morphine was confirmed by a positive fluorescence. A correlation coefficient of 0.94 which was calculated for the two sets of data appeared to give a favorable overall agreement. Calibration. Standard curves for morphine and codeine in blood were linear over the concentration range of 0.02 t o 0.12 Mg. Linear plots were also obtained from the extraction of these compounds in liver, bile, and urine in the concentration range of 0.5 to 2.5 Mg. Table I11 shows the range of recovery of morphine and codeine from the various specimens.
Since morphine, codeine, and the internal standard (nalorphine) were added to these specimens to obtain the data, these curves afforded analytical values which included a recovery factor for each of the tissues run. T o offset any changes in peak area over a time span, recovery tests on blood and other tissue samples containing known amounts of added morphine and codeine were periodically run to obtain a nalorphine/morphine value to be used as a calculation factor. LITERATURE CITED (1) G. R. Wilkinson and E. L. Way, Biocbem. Pbarmacol, 18, 1435 (1969). (2) H. W. Elliott, K. D. Parker, J. A. Wright, and N. Nomof, Clin. Pbarmacoi. Tber., 12, 806 (1971). (3) H. Kupferberg, A . Burkhaiter, and E. L. Way, J. Pbarm. Exp. Tber., 145, 247 (1964). (4) R. A. Passwater. "Fluorescence News," American instrument Go., Silver Springs, Md., 6, 8 (1971).
RECEIVEDfor review September 9,1974. Accepted December 11,1974.
I CORRESPONDENCE Determination of Low Surface Areas by the Continuous Flow Method Using an Interrupted Flow Technique Sir: In a previous publication ( I ) , we described our improved continuous flow method. A shoulder was sometimes obtained high on the leading edge of the desorption peak. Unfortunately, we did not rigorously define the conditions under which we obtained these shoulders and this misled Lowell and Karp ( 2 ) into the assumption that they were characteristic of peaks obtained from samples of low surface area. This was not so and, in the present communication, it is hoped to resolve this and also to describe a method for the determination of low surface areas. EXPERIMENTAL The apparatus was similar to that described in reference ( I ) with a few exceptions. For rapid turnover of control samples, individual metering of the gases was replaced by premixed cylinders of helium and nitrogen. Two-way valves (Drallim Couplings Ltd. Whyteleafe, Surrey) were provided each side of the sample tube so that it could be isolated from the gas stream. The practice of cooling a tube in the reference stream a t the same time as the sample was discontinued. The single point method at a relative pressure of 0.2 was used. The sample tube had side arms of 2-mm internal diameter and a region 3.5 cm long of 13-mm internal diameter to contain the sample. The flow rate used was 15 ml/min. The adsorbent used was crystallized unmilled ammonium perchlorate obtained from three different sources and one milled sample of higher surface area.
RESULTS AND DISCUSSION Problems have arisen previously in the measurement of low surface areas (2-4). Similar problems occurred in the present case, as shown by Figure l a . However, the shoulder referred to in ( I ) was obtained only on an alumina sample. This had a surface area of 67 m2/g and the measurements were performed on a sample weighing 0.0960 g. The volume of gas adsorbed ranged from 1.24 to 1.97 ml, depending on the relative pressure, and the measuring system was attenuated by a factor of more than thirty below the level a t which anomalous peaks became clearly visible. 778
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With the type of sample tube used in the present work, which allowed the use of a reasonable sample size, anomalous peaks were of no importance in measurements on samples having surface areas greater than about 0.2 m2/g. Unmilled ammonium perchlorate has a very low surface area (
