fact, the theoretical plates number for the two components is decreased from about 700 to about 350, and because the peak area of quinoline is practically unaffected, the peak area of caprolactam continuously decreases. Consequently, an average difference of 5.5 between the actual and the experimental concentrations is found, which is constant in the entire concentration range. No unexpected peak or anomalous baseline disturbance is observed; and so decomposition phenomena may be excluded which means that caprolactam begins to be partially lost in the column. The mechanism by which this loss occurs is not clear; it may be generally assumed that one of the causes of the irreversible absorption of polar substances, like caprolactam, is hydrogen bonding (#). In addition, a possible interaction between liquid phase, solid support, and analyzed substances should be taken into account. For these reasons it may be similarly supposed that the progressive decrease of caprolactam response is due to a continuous absorption on the solid support, after the solid substrate has been saturated by formic acid. Consequently, after a certain time, calibration should be checked regularly at frequent intervals. The precision of the determination, expressed as average standard deviation, is = t O . O O l ; accuracy, expressed as average relative error, is 1 2 . 6 z ; this value, by excluding the first determination which regards the lowest concentrations, becomes + l .7 (see Table 11). The gravimetric method of Wiloth, the gas chromatographic
method applied to the water extractions, and the direct gas chromatographic method applied to the formic acid solutions of polymers have been compared. The caprolactam content according to the Wiloth method has been calculated as a difference between the water extractables and the oligomers content. The caprolactam content according to the Ongemach method has been determined on an aliquot of the abovementioned water extracts. Quinoline in aqueous-alcoholic solution was used as internal standard. Five aqueous extractions based on 10-gram polymer samples for the first two methods and five determinations on about 0.2-gram polymer solutions for the direct method were carried out. In Table 111, the results obtained from the same polymer sample containing 10% of water extractables are reported. By comparing the results of the three methods, it can be seen that the precision of the direct caprolactam determination is of the same order as that supplied by Wiloth and OngemachMoody methods. The direct one which requires less handling, reduces the possibility of casual errors. The analysis time for a single determination is reduced to about 2.5 hours, because only one weighing and one dissolution step are required. The sensitivity of the method allows the determination of monomer in polymer down to 0.1 The advantages of the present method, compared with the others described in the literature, are: simplicity, high-speed performance, and sensitivity, all of which make it particularly suitable for control analyses. The method can be also applied to polymerization kinetics studies.
(4) V. Kusy, ANAL.CHEM., 37,1748 (1965).
RECEIVED for review April 18, 1969. Accepted July 2, 1969.
z
z
z.
Determination of Pentaerythritol Tetanitrate and Other Nitric Acid Esters with p-Nitroaniline and Azulene V. Hankonyi and V. Karas-Gaiparec Institute of Chemistry and Biochemistry, Faculty of Medicine, Zagreb, Salata 3, YugosIavia
DURING the hydrolysis of nitric acid esters, certain quantities of nitrite ions along with other products are produced. This effect is used for the quantitative determination of esters. In earlier publications ( I , 2), we presented a micromethod, based o n the determination of nitrite ions produced by hydrolysis for the determination of some esters. However, this method is not suitable for the determination of pentaerythritol tetranitrate because of its poor solubility in water and alcohol. Existing methods (3) were tried and found to be unsatisfactory also. This paper presents an application of Garcia's method (4) to the determination of some nitric acid esters (erythritol tetranitrate, pentaerythritol tetranitrate, and glyceryl trinitrate) and particularly of pentaerythritol tetranitrate in pharmaceutical preparations. The principle is as follows: nitrite ions produced by hydrolitic decomposition of esters under the influence of a strong base react with p-nitroaniline to form a diazonium ion, which is then coupled with azulene. The (1) V. Hankonyi and V. Karas-Gasparec, Acta Pharm. Jugoslau,
17, 41 (1967). (2) V. Hankonyi and V. Karas-Gasparec, ibid., in press. (3) T. Bihan, Food and Drug Administration, Zagreb, Yugoslavia, personal communication, 1968. (4) E. F . Garcia, ANAL.CHEM., 39, 1605 (1967).
absorbance of the azo dye produced is measured at 515 nm. The method is rapid and sensitive and permits the determination of nitric acid esters in the concentration range of 5-50 pg in 10 ml of the reaction mixture, EXPERIMENTAL
Apparatus. Absorption intensity measurements were made with a Unicam SP 600 spectrophotometer using 1-cm cells. Reagents. The p-nitroaniline was of reagent grade and the azulene was obtained from Gerhardt Schmidt, Hamburg. The nitric acid esters were obtained from Pliva, Zagreb. The tablets analyzed contained pentaerythritol tetranitrate and had been obtained on the market. ~NITROANILINE. Dissolve 0.4 gram of p-nitroaniline in 100 ml of glacial acetic acid. AZULENE.Dissolve 0.08 gram of azulene in 100 ml of glacial acetic acid. This reagent lasts one week. PERCHLORIC ACID. 7.8 M . PENTAERYTHRITOL TETRANITRATE STOCKSOLUTION.Dissolve 0.04 gram in acetone to a volume of 100 ml. T o prepare the standard solution containing 40 pg of pentaerythritol tetranitrate per milliliter, dilute the stock solution with acetone. ERYTHRITOLTETRANITRATE STOCK SOLUTION.Dissolve 0.015 gram in distilled water at 90 "C and dilute to 50 ml.
ANALYTICAL CHEMISTRY, VOL. 41, NO. 13, NOVEMBER 1969
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_ _ _ _ ~ ~
Pentaerythritol tetranitrate Taken 19.30 pg Absorbance Found," pg 0.534 18.86 0.548 19.35 0.533 18.83
Table I. Reproducibility of Method Erythritol tetranitrate Taken 26.40 pg Absorbance Found,a pg 0.445 26.79 0.423 25.50 0.434 26.15
Average 19.02 Std dev f0.37 Re1 std dev 11.96% a Calculated from Equations 1, 2, and 3, respectively.
Preparation Vasocor, lingualettoe Jugodietetika, Zagreb, Yugoslavia Retol Rutin, dragee Pliva, Zagreb, Yugoslavia
Glyceryl trinitrate Taken 28.56 pg Absorbance Found,a pg 0.542 27.95 0.533 27.47 0.560 28.89 0.550 28.37 28.17 50.65 52.32z
26.15 10.58 52.23%
Table 11. Results of the Pentaerythritol Tetranitrate Determination Pentaerythritol tetranitrate, mg Average with 95% Declaration Proposed method5 confidence limit 10 mg/ling. 9.75 9.58 9.80 9.73 10.16 9.80 10 mg/drag. 9.90 9.90 9.70 9.88 =k 0.20
Re1 std dev, 2 1.07
1.27
10.00 a
Duplicate analyses.
Preparation Vasocor Retol Rutin
Table 111. Recovery of Pentaerythntol Tetranitrate Pentaerythritol tetranitrate, pg In sample Added Total 8 5.5 13.5 8 10.9 18.9 10 8 18 10 15 25
To prepare the standard solution containing 30 pg of erythritol tetranitrate per milliliter, dilute the stock solution with water. Prepare fresh daily. GLYCERYL TRINITRATE STOCKSOLUTION. One per cent alcoholic solution. The exact concentration is determined according to the Ph. Helv. V. regulation. The standard solution, containing about 20 pg of glyceryl trinitrate per milliliter, is prepared by diluting the stock solution with 9 6 x alcohol. Procedure. Transfer a n aliquot of the solution containing between 5 and 50 pg of pentaerythritol tetranitrate to a 10-ml volumetric flask and add 1 ml 1 N NaOH. Keep the mixture in a boiling waterbath for 15 minutes then allow it to cool to room temperature. Add 1 ml 2N HC1 and immediately after it 1 ml of the p-nitroaniline reagent, 1 ml of the azulene reagent, and 4 ml of perchloric acid. Dilute to mark with water, mix well, and measure the absorbance a t 515 nm against a reagent blank. The procedure for the determination of other esters is the same with the only difference that the alkaline ester solution need not be heated but is left standing for some minutes at room temperature. PROCEDURE FOR DETERMINING PENTAERYTHRITOL TETRANITRATE IN PHARMACEUTICALS. Put the weighed quantity of the finely ground sample containing about 1 mg of pentaerythritol tetranitrate into a 25-ml volumetric flask and dilute to mark with acetone. Mix the solution well and allow it t o stand for about 2 hours or preferably overnight. Pipet 0.5 ml of the solution into a 10-ml volumetric flask and proceed as described for pure pentaerythritol tetranitrate. 1850
Found 13.3 19.5 18 24.9
Recovery, 98.5 103.2
z
100.0
99.6
RESULTS AND DISCUSSION
Adherence to Beer's Law. All systems conformed to Beer's law over the concentration range studied. The calibration curves for the three esters were prepared by following the described procedures with varying amounts of esters. Equations were calculated using the least squares method, for pentaerythritol tetranitrate, Equation 1, for erythritol tetranitrate, Equation 2, and for glyceryl trinitrate, Equation 3. A
=
0.028 C
+ 0.006
(1)
A
=
0.017 C
- 0.010
(2)
A
=
0.019 C
+ 0.011
(3)
A is the absorbance at 515 nm and C is the concentration expressed in pg/lO ml. Rate of Adding the p-Nitroaniline. The examinations of the hydrolytic decomposition of esters in alkaline and acid solutions have shown that the proposed method can be used successfully for the determination of esters only if the hydrolysis takes place in strongly alkaline solutions. On acidifying the mixture with 1N HCI, it is necessary to add the first reagent, p-nitroaniline, as quickly as possible to the mixture, otherwise much lower results are obtained. The rate of adding the other reagents does not matter for it does not influence the results.
ANALYTICAL CHEMISTRY, VOL. 41, NO. 13, NOVEMBER 1969
Reproducibility. The results of 3-4 replicate analyses of a standard nitric acid esters solutions are presented in Table 1. These determinations were conducted over a period of four consecutive days. From the absorbance obtained with pure ester solutions and the absorbance produced by definite quantities of nitrite ions, the ratio between the quantities of produced nitrite ions and the quantity of the individual ester was calculated for the given condition of hydrolysis. For every mole of ester, 1.63 mole of nitrite ion is released from pentaerythritol tetranitrate, 0.91 mole from erythritol tetranitrate, and 0.79 mole from glyceryl trinitrate. Determination of Esters in Tablets. Tablets containing either pentaerythritol tetranitrate or glyceryl trinitrate were
analyzed. The examination showed that the proposed method can be used successfully only for the determination of pentaerythritol tetranitrate. When determining glyceryl trinitrate, high results were always obtained. A method described earlier ( 2 ) is satisfactory for determining glyceryl trinitrate in tablets. Table 11 presents the results of four duplicate analyses of two kinds of pharmaceutical preparations containing pentaerythritol tetranitrate. Table 111 presents the recovery of pentaerythritol tetranitrate added to the acetone solution of the sample. The recoveries are good in both cases with a maximum error of approximately f 3 %, RECEIVED for
review May 7, 1969. Accepted July 10, 1969.
Microwave Excited Electrodeless Discharge Tubes Containing Organo-Sulfur and Phosphorus Compounds K. M. Aldous, R. M. Dagnall, S. J. Pratt, and T. S. West Imperial College, Chemistry Department, London S.W .7 , England RECENTLY much interest has been shown in the use of microwave excited plasmas as gas chromatographic detection systems (Z-7). These detectors operate by allowing the eluted compound, and the carrier gas, to flow along a silica tube within a resonant microwave cavity. When the pressure of the carrier gas (usually argon or helium) is maintained at a low value (7) (ca. 1-5 torr) a stable plasma is produced which subsequently excites emission from the eluted compound as it passes through the discharge. If a monochromator is used to select a particular emission line or band, the device becomes specific for a particular group or class of compound, or for an elemental constituent. The emission spectra obtained when organic compounds are excited in this type of plasma are complex and variable, and it is often not feasible to scan the complete emission spectrum in a flowing system. We have devised a static system in order to evaluate the areas of the spectrum which may most usefully be examined. This has been achieved by the construction of sealed electrodeless discharge tubes (hereafter called EDTs), and the method of preparing these sources is described below. Once the standard source is available the most sensitive and selective emission region can quickly be found and a dynamic -Le., chromatographic-system monitored at this wavelength in order to detect the trace quantities of the compound present in the carrier gas. Apart from this direct application, the sealed organic EDT offers a quick qualitative method of organic elemental analysis. If the emission spectrum obtained from an unknown sample is scanned, the presence of C, S, P, C1, Br, and I may be rapidly determined as well as As, Mg, Pb, Hg, etc. in organo-metallic compounds. (1) A. J. McCormack, S . C . Tong, and W. D. Cooke, Anal. C/iem., 37, 1470 (1965). (2) C. A. Bache and D. J. Lisk, ibid., 37, 1477 (1965). (3) Ibid., 38, 783 (1966). (4) Ibid., p 1757. (5) Ibid., 39, 787 (1967). (6) H. A. Moye, ibid., p 1441. (7) R. M. Dagnall, S.J. Pratt, T. S . West, and D. R. Deans, Tulurm, 16, 797 (1969).
EXPERIMENTAL
Preparation of Organic EDTs. The general procedure for the preparation of organic EDTs is similar to that for atomic spectral line-sources which have previously been reported for metallic elements (8). However, because many of the organic compounds investigated were volatile liquids, a different method was devised for introducing them into the tube. The silica envelopes were prepared from 8 mm i.d. transparent tubing and the sealed tubes were made ca. 6 cm long. After degassing the bulb on the vacuum line, at red heat, the liquid sample was introduced using a 0-10 p1 Hamilton syringe. Two microliters of sample were placed into the bulb, taking care not to allow the liquid to come into contact with the walls of the constriction, which subsequently formed the seal. The bulb was flushed several times with helium so that all of the liquid was vaporized and the pressure was finally adjusted to 3 torr before sealing. Enough sample vapor remains in the tube to yield a discharge characteristic of the compound. This procedure prevents the development of a high pressure within the bulb during operation and there is therefore no explosion hazard. The heliumto-sample vapor ratio is not critical. When prepared under these conditions, the tubes could be operated at 2450 MHz using either the 3/4 wave, 3/4 wave-foreshortened, or ' 1 4 wave cavities at powers up to 40 watts. Details of the apparatus used for excitation of the spectra have been given elsewhere (7). Because of the thermal instability of most organic compounds, the tube lifetime was limited and was found to be dependent on operating power. Below 40 W there was normally little change in output for a running period in excess of 20 hours. Stability plots at principal emission peaks-e.g., CS for sulfur compounds, P atomic lines at ea. 253 nm for phosphorus compounds-showed only 9=2% intensity fluctuations over one hour when operated in the 3/4 wave-foreshortened cavity, which is favored for gas chromatographic detection (7). The shelf-life of these tubes was found to be as good as that of the corresponding inorganic sources (8) and after several months without use they could be initiated immediately and without difficulty. (8) R. M. Dagnall, and T. S. West, Applied Optics, 7, 1287 (1968).
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