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Anal. Chem. 1981, 53, 1144-1145 -
Table I. Effect of pH on Precipitation of Methylcellulose viscosity, CP s 1.015 1.014
1.016
PH 1.5 3.3 6.2
viscosity CP s 1.016 1.016
PH 8.0
11.8
Table 11. Recovery Study of Pharmaceutical Compounds from 0.5% Methylcellulose
generic name or MK no. aspirin hydrochlorothiazide MK-4 2 1 mevinolin cyclobenzaprine HCl diflunisal plus cyclobenzaprine HCI
mg/mL of methylcellulose recovered spiked concn concn 0.54 7.93 0.55 7.93 0.51 8.30 0.48 7.93 8.89 6.61 0.10
0.51 7.83 0.55 8.14 0.48 8.44
indicated close to 100% efficiency for the process (Table 11). Both columns performed well after the analysis and no significant column pressure change was observed after separate cleaning of the columns. The compounds either are already on the market or are undergoing clinical trials and were selected for the diversity of their functional groups. Application of this technique to compounds with other functional groups, though not yet attempted, is very likely possible. Acidic or alkaline compounds will not affect the amount of methylcellulose removed. The sensitivity is limited by the detection capability of the instrument. However, since Samples are not quantity restricted, this aspect has not been explored. The type and quantity of compounds using this cleanup procedure will be restriced by their solubility in acetonitrile.
ACKNOWLEDGMENT Sincere thanks to colleagues who have offered helpful comments and assistance and to the library staff of Merck Sharp and Dohme Research Laboratories for their literature search.
0.47
7.90 8.70 6.62 0.10
thylcellulose precipitated (Table I). T o test the applicability of the above observations, we modified the procedure to analyze suspensions as well as solutions of compounds in 0.5% aqueous methylcellulose. A number of drugs were assayed individually or in combination. Acetonitrile acted as a solvent and later as an extraction medium to separate the drug from the aqueous phase and precipitate. The recovery data using an external standard
LITERATURE CITED (1) Whistler, R. L., Smart, C. L., Ed. “Polysaccharide Chemistry”; Academlc Press: New York, 1953; p 76. (2) Isshiki, Y. Kagawa Dabaka Nogakubo Gakujutsu Hokoku 1977, 28 (60), 33-36. (3) McCollister, S. E.; Kociba, R. J.; McColiister, D. D. Fd Cosmet. ToxlCOl. 1973, 11, 943-953. (4) Lehman, A. J. Quant. Bull. Assoc. Fd. Drug Officials U . S . 1950, 74, 3, 82-96. (5) Greminger, G. K., Jr.; Savage, A. E. I n “Industrlal Gums”; Whistler, R. L., BeMlller, J. N., Eds.; Academic Press: New York, 1959; p 565. (6) Spurlin, H. M. I n “Cellulose and Cellulose Derivatlves: Hlgh Polymers Part 11”; Ott, E., Spurlin, H. M., Grafflln, M. W., Eds.; Interscience: New York, 1954; Vol. 5, p 935.
RECEIVED for review January 1, 1981. Accepted March 23, 1981.
Determination of Trace Solvent in Waxes and Lubricating Oils Bachan S. Rawat” and Guru Prasad Indian Institute of Petroleum, Dehra Dun, India
Solvent dewaxing of lubricating oil stocks with methyl ethyl ketone (MEK)-toluene mixture is well-known in the petroleum industry. Lubricating oil and wax stocks after solvent removal should not normally contain more than 50 and 150 ppm of toluene and 50 and 100 ppm of MEK, respectively. Determination of solvents in small quantites in these stocks is, therfore, necessary. The gas-liquid chromatographic (GLC) method devised for this purpose (1) uses a solvent-stripping manifold system consisting of a freeze-out trap at -50 OC to retain solvents before they are flashed to the GLC column. Although the precision reported in this technique is good, the method is cumbersome and needs special care to resolve such small quantities of toluene/MEK from these high boiling and viscous stocks. In the present study, a simple technique was used which consists of azeotroping toluene and MEK with methanol. As wax and lube oil stocks are not miscible with methanol, the solvent molecules trapped within wax or lube oil molecules cannot be easily brought in contact with methanol during 0003-2700/81/0353-1144$01.25/0
distillation. For this purpose moderately polar solvents like cyclohexanol, cyclohexanone, hexanol, and hexyl acetate were tried which dissolve both methanol and wax or lube oil. Such a solvent, however, should not form an azeotrope with methanol (e.g., carbon tetrachloride, 55.7 O C ) . Carbon tetrachloride and chloroform otherwise also do not form azeotropes with toluene. Methanol-toluene and methanol-MEK form azeotropes (2) at 63.8 and 63.5 “C, respectively. Toluene (bp 110.6 OC) and MEK (bp 79.6 “C) do not form azeotropes.
EXPERIMENTAL SECTION All the chemicals used had purities of more than 99% as checked by GLC. The wax (congealing point 58.60 O C ) and lubricating oil (370-500 O C ) were free from toluene and MEK. About 800 g of wax or lubricating oil was placed in a distillation flask along with about 400 g of cyclohexanone or cyclohexanol, 10 mL of methanol, and known quantities of toluene/MEK. The quantities of toluene and MEK were taken in such a way that wax or lube oil contained them in the range of about 30-200 ppm. The flask was connected to the distillation assembly as shown 0 1981 American Chemical Society
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Anal, Chem. 1981, 53, 1145-1146
Table I. Determinaii%of
sample lubricating oil
slack wax
Toluene and MEK in Wax and Lubricating Oil Stocks toluene amt amt wt % in added, determined, % wt % in distillate ppm PPm error distillate
1.37 0.84 1.9 1.35 0.28 1.05 0.9 0.7 0.29
150 105 196 139 40 150 120 105 38
137 94 185 128 37 138 109 97 35
Figure 1. Schematic dlragram of distillation assembly: (1) to condenser, (2) thermometer pocket, (3) cannon packed column, (4) to buret.
in Figure 1. The contmts were slowly heated and homogenized. The unit was run undler total reflux for 1 h before the collection of the distillate at about 63-90 "C. About 7-11 mL of the sample was collected, weighed, and analyzed by GLC. The quantities of toluene and MEK were calculated in the sample.
RESULTS AND DISCUSSION The synthetic samples of toluene, MEK, and wax or lube oil were distilled with methanol in the presence of cyclohexanone. The quantities of toluene and MEK added and
1.3 0.8 1.2 0.98 0.21 0.97 0.67 0.8 0.18
8.6 10.4 5.6 7.9 7.0 8.0 9.1 7.6 7.9
MEK amt amt added, determined, ppm PPm
137 99 126 99 31 140 90 125 29
129 92 118 93 29 127 82 111 26
%
error
5.8 7.0 6.3 6.0 6.0 9.2 8.9 11.2 10.3
determined by the method are presented in Table I. Although the concentration of MEK and toluene in the synthetic samples (Table I) was varied from about 30 to 200 ppm, there is no limitation in determining reasonably lower or higher concentrations of these solvents in the samples. The technique of azeotropic distillation is reported (3) to recover as low as 0.01-1.0ppm of volatile polar organic solvents like acrolein acrylonitrile and alcohols from water in a microdistillation apparatus. The data given in Table I show that the technique used is quite accurate as the errors are less than about 11% by weight. This error is quite low particularly when such small concentrations are involved. There is no other method available for this purpose in the knowledge of the authors except the GLC method ( I ) described in the beginning. This method is quite cumbersome and needs a freeze-out trap at -50 "C and reports errors up to 13.3% by weight. Determination of these solvents in wax or lube oil is important particularly to control the parameters in a solvent recovery process for complete recovery of the solvents and to get products of required specifications. The solvents presesnt in concentrations as low as 0.1% affect the properties ( 4 ) of these products, for example, flash point. The method is being used in the laboratory for determining solvents in waxes and lubricating oils obtained in a solvent dewaxing step. Other chemicals can also be tried which will dissolve azeotroping components like methanol and wax or lube oil.
LITERATURE CITED (1) (2) (3) (4)
Durrett, L. R. Anal. Chem. 1959, 37, 1824-1825. Horsly, L. H., Ed. Advan. Chem. Ser. 1952, No. 6 , Table I. Peters, T. L. Anal. Chem. 1980, 52, 211-213. Kalichevsky, V. A.; Kobe, K. A. "Petroleum Refining with Chemlcals", 1st ed.; Elsevier: London, 1956; Chapter 7.
RECEIVED for review June 12, 1980. Accepted February 6, 1981.
Multisampling of Microgram Quantities for Infrared Spectrometric Analysis P. J. Zanzucchi" and W. R. Frenchu RCA Laboratories, Princeton,
New Jersey 08540
Small amounts of contaminants, typically in microgram quantities, when found on the surface of electronic devices need to be identified to determine the origin of the unwanted material (I). Devices for supporting small quantities of sample for infrared analysis are available, e.g., the Perkin-Elmer microsampling accesriories (Perkin-Elmer Corp., Nonvalk, CT, Catalog No. 0186-03lO),but they often require use of separate supports for each sample. Preparing one sample a t a time or preparing samples on a number of separate salt disks, as 0003-2700/81/0353-1145$01.25/0
is done with commercially available devices, is a slow process. Moreover, since the sampling procedure is spread over a considerable time period, the possibility of contaminating the sample and sample transfer equipment is increased. We have found that a single device which will support a number of samples of microgram quantity is an aid to infrared analysis. For example, all sample transfer can be done in the shortest time period on the same sample holder reducing the possibility of error in the identity of the samples. 0 1981 American Chemlcal Society
