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).
ANALYTICAL CHEMISTRY, VOL. 41, NO. 13, NOVEMBER 1969
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CARBON DISULPtiIDE
D I M E T H Y L SULPHOXIDE DISCHARGE TUBE
DISCHARGE TUBE
I
B
I C
530 5 0 3
530 500 THIOGLYCOLLIC
270 ACID
250,
DISCHARGE TUBE
270
240
I THIOPHEN
DISCHARGE TUBE
H
5 3 0 500
270
250
+----c i30 KM
270
250
NANOMETERS
Figure 1. Emissions from organo-sulfur tubes on the regions 530-500 nm and 270-250 nm A and B, CS2; C and D ,DMSO; E and F, TGA; G and H,Thiophen
The compounds examined were, sulfur, carbon disulfide, thiophen, thioglycollic acid, dimethylsulfoxide, phosphorus trichloride, phosphorus pentachloride, phosphorus oxychloride, trimethylphosphate, trimethylphosphite, triethylphosphate, tributylphosphate, and phosphorus. Spectrographic Study of Organic EDTs. All of the tubes were used in the wave cavity a t a power of 40 W (reflected power ca. 5 W). The spectra were examined using a Hilger Large Quartz Spectrograph with a slit width of 0.025 mm and were recorded on Kodak B10 plates in order to obtain an overall view of the spectrum at high resolution. An emission spectrophotometer was used to study the distribution and relative sensitivities of specific molecular and atomic emissions. RESULTS AND DISCUSSION Spectrographic Examination. Cyanogen spectra were observed prominently in all of the EDTs which contained carbon compounds. This can only be due to the presence of nitrogen in the carrier gas (10-20 vpm) or traces of adsorbed air. This nitrogen impurity also accounts for the appearance of some N2 emission. The presence of oxygen (less than 100 vpm) in the carrier gas may also account for the CO and PO spectra where none would be expected. SPECTRAOF SULFURCOMPOUNDS. The significant structures in the spectra of most of the sulfur compounds consist of CS, S2, C (atomic), and C N ; all of the sulfur compounds have very similar spectra (9). 1852
0
CS Emission. These emissions are all degraded to the red and show close double-headed bands, with fairly obvious sequences at: 266.3, 262.2, 260.6, 257.6, and 247.7 nm. The head of the ( 0 , O )band at 257.6 nm is most prominent. S? Emission. These bands are degraded to the red, some having a fainter head to violet. The system consists of extensive, roughly equally-spaced bands and is apparent in our spectra chiefly at 295.5 and 282.9 nm--i.e., those which are predominant in the sulfur EDT. C (atomic) Emission. The carbon (atomic) line at 247.8 nm was observed in all organo-sulfur compounds. C N and NLEmissions. These emissions are not characteristic for organo-sulfur compounds but were observed in all the tubes for reasons given above. The N? spectra appear at 380.5, 375.5, 357.7, 337.1, and 315.9 nm and are degraded to the violet. C N band heads appeared at 421.6, 419.7, and 388.3 nm and were degraded to the violet. SPECTRAOF PHOSPHORUS COMPOUNDS. The significant structures seen in these spectra consist mainly of CO, PO, C (atomic), P (atomic), and C N ; the spectra are similar for most phosphorus compounds. CO Emission. The CO emissions appearing in the phosphorus compounds are: 439.3 nm in tri-methyl, tri-ethyl, and (9) “Identification of Molecular Spectra,” R. W. B. Pearce and A. G . Gaydon, Chapman and Hall Ltd., London (1963).
ANALYTICAL CHEMISTRY, VOL. 41, NO. 13, NOVEMBER 1969
tri-butyl phosphates, 412.4 nm in tri-methyl, tri-ethyl, and tributyl phosphates, 349.0 nm in tri-methyl, tri-ethyl, and tributyl phosphates and POC13,313.4 nm in tri-methyl, tri-ethyl, and tri-butyl phosphates and P0Cl3, 297.8 nm in all samples except P, PC13, and PClj, 283.3 nm in all samples except P, PCI3, and PCl:, 266.4 nm in all samples except P, PCb, and PC16, 257.5 nm in tri-methyl, tri-ethyl, and tri-butyl phosphates. The first two bands are part of the Angstrom system which is a progression (from 412.4 nm to higher wavelengths) of bands with strong single heads degraded to violet. The next four bands are the third positive bands, which consist of a progression of five strong bands (330.6 nm is missing), with five close subheads; the strongest members of the (0,v”) progression. A weaker progression of bands of a similar type which form the 5B group, appears in the spectra as the bands at 266.4 nm. All of these I)ands are degraded to the violet. The last band (at 257.5 nm) is the strongest and highest member of the Cameron series, which is degraded to the red. There are five close heads to each band. C (Atomic) Emission. The C (atomic) line at 247.8 nm appears in tri-methyl phosphate, tri-methyl phosphite, tributyl phosphate, and tri-ethyl phosphate. PO Emission. Four bands appear in the spectra recorded for tri-methyl phosphate and phosphite, tri-ethyl phosphate, and tri-butyl phosphate at 255.5, 254.0, 247.8, and 246.4 nm. These are the strongest bands in the y system, which are degraded to the blue. They are double double-headed, a n d the sequences are fairly well marked. P (Atomic). P (atomic) lines were observed at 255.5,255.3, 253.6, and 253.4 nm. These were completely resolved from each other and from the PO bands except the one at 255.49 nm. C N and N2 Emissions. These were similar to those observed for the sulfur containing compounds. Evaluation of the Organic EDTs Using the SP900 Spectrophotometer. Following identification of the major emission lines and bands on the spectrographic plates, a Unicam SP900 flame emission spectrophotometer was used to evaluate the characteristic emissions thought to be most useful in the possible GLC application of this type of excitation. Only the most prominent diagnostic emissions are discussed below. SULFURCOMPOUNDS.The CS emission (Figure 1) was studied by scanning the wavelength region between 270 and 250 nm. The appearance of the CS band in this region is of close double-headed bands degraded to the red; the sequence is fairly obvious and the head of the ( 0 , O )at 257.6 nm is predominant and of most use analytically. Of the major peaks expected (9), only those at 266.3, 262.2, 260.6, 257.6, and 247.7 nm were identified. This system appears quite plainly in all of the organo-sulfur compounds, but is most intense in carbon disulfide and dimethylsulfoxide. The C? emission which was not observed on the spectrographic plates because of the insensitivity of the emulsion at long wavelengths was studied by scanning the region from 500 to 530 nm (Figure 1). In all cases the CZpeak which is the strongest of the Swan series ( 0 , O ) is well defined. The carbon (atomic) line at 247.9 nm is well defined in all compounds containing carbon and may be used analytically. The characteristic sulfur atomic lines at 190.0 and 191.4 nrn appear in all of the sulfur containing compounds studied. These were not observed in the spectrographic examination because they lay outside the spectral range examined. In all cases the S (atomic) emission was approximately equal in in-
P ATOMIC EMISSION
253
256
S ATOMIC EMISSION
I89
192
NANOMETERS
Figure 2. Atomic emissions from organosulfur and phosphorus compounds A . Phosphorus lines 253-256 nm B. Sulfur lines 189-192 nm
tensity and the ratio of the 190.0 to the 191.4 nm line was always 2 to 1. S2 emission occurs strongly in the elemental sulfur tubes and consists of a very extensive, roughly equally-spaced system of bands, in the range 600 to 270 nm, all degraded to the red. SZ emissions from organic compounds were not judged to be analytically useful in relation to other emissions because of their spectral band spread. SPECTRA OF PHOSPHORUS COMPOUNDS. Only the two major peaks for PO and P are discussed here. The CO, C, Nz, and C N emissions have been discussed under spectrographic examination. The strongest bands of the PO y system appear as double, double-headed bands degraded to the violet, and constitute a fairly well defined series at 255.5, 254.0, 247.8, and 246.4 nm. The first two were not resolved from the atomic lines by the spectrophotometer. This system was found in all of the phosphorus containing compounds with the exception of phosphorus tri- and pentachlorides. The P (atomic) lines identified were at 255.5, 255.3, 253.6, and 253.4, but due to the lack of resolution of the SP900, the pairs of lines were not resolved and appeared as two lines at 255.4 and 253.5 nm, which predominated over the PO bands upon which they are superimposed, see Figure 2. These lines were present in all of the phosphorus containing tubes. Electrodeless discharge tubes containing organic compounds yield intense characteristic band and line spectra when excited by microwaves at 2450 MHz. These tubes may serve as useful calibration standards for microwave excited emissive detector systems in gas-liquid chromatography, etc., or for characterizing unknown organic compounds. Most tubes when operated at powers of 40 W, or less, have running lives of more than 20 hr and an indefinite shelf-life. ACKNOWLEDGMENT
The authors are grateful to Electromedical Supplies Ltd., for the loan of the Microtron excitation equipment. RECEIVED for review February 14, 1969. Accepted July 10, 1969. We thank the Science Research Council and the Agricultural and Heavy Organic Chemicals Divisions of I.C.I., Billingham, for their financial support of this research.
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