ACKNOWLEDGMENT The technical assistance of Mary Lee Holcomb is acknowledged. We are indebted to Paul O’Connell, Upjohn Co., for a generous supply of 1,1,3-tricyano-2-amino-l -propene used early in this study and to Carleton B. Moore, Center for Meteorite Studies, Arizona State University, for meteorite and steel samples.
Received for review June 10, 1968. Accepted August 16, 1968. Work taken in part from the thesis submitted by Kim Ritchie in partial fulfillment of the requirements for the doctoral degree, Department of Chemistry, Arizona State University, Tempe, Ariz. Work supported in part by grants from the National Institute of Mental Health (MH07692) and the Epilepsy Foundation.
Gas Chromatographic Determination of Plasticizers and
Stabilizers in Composite Modified Double-Base Propellants J. M. Trowell and M. C. Philpot Hercules Incorporated, Bacchus Works, Magna, Utah
of the various plasticizers and stabilizers The determination in both nitroxy base propellants is required for control of the finished product. However, with the development of more exotic and higher energy propellants, there has been an increasing need to know accurately the true or remaining stabilizer contents of surveillance propellants. This is particularly true for the more exotic single-, double-base, and composite modified double-base (CMDB) propellants for the prediction of the safe shelf life of the item. The classical methods used for these analyses on propellants are found in MILSTD 286B. These methods are time-consuming, lack specificity, and the determination of some components by difference results in decreased accuracy. Recently, workers in the field of propellant analysis have reported more sophisticated methods of analysis. Spectrophotometric techniques (7) are faster than “wet” methods, but these methods can be subject to interferences which generally can be eliminated only by time-consuming separations. Investigators in this laboratory have published gas chromatographic (GC) methods applicable for the analysis of plasticizers and stabilizers, 2-nitrodiphenylamine (2-NDPA) in single-base propellants (2). However, no investigators have reported any degree of success with the GC analysis of doublebase propellant stabilizers because of the uncontrolled decomposition of nitroglycerin (NG) in the GC resulting in the nitration of the stabilizers. Therefore, to obtain a workable GC method for the analysis of stabilizers and plasticizers in double-base or CMDB propellants, a scheme had to be developed to avoid the interferences caused by NG breakdown. EXPERIMENTAL
Apparatus. A commercial gas chromatograph, F&M 5754B with dual flame and thermal conductivity detectors was used in this investigation. The instrument was modified by placing an injection port inside the oven compartment to allow true on-column injection. Detailed instrument parameters are summarized in Table I. Column packings were prepared using the Applied Science Laboratories Hi-Eff Fluidizer and the method described by Kruppa and co-workers (3). The columns were packed under (1) G. F. Macke, presented at the 22nd ICRPG Working Group on Analytical Chemistry, Denver, Colo., 1966. (2) J. A. Kohlbeck and R. Dalton, 25th ICRPG Working Group on Analytical Chemistry, Dover, N. J., 1968. (3) R. F. Kruppa, R. S. Henly, and D. L. Smead, Pittsburgh Conf. on Anal. Chem. and Applied Spectroscopy, Pittsburgh, Pa., 1967.
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Table I.
Gas Chromatography Conditions
Dual Columns: 4 ft X Vs inch tubing Packed with 5% OV-17 on 60-80 mesh Gas Chrom Q Column temp 70-250 °C Column temp program rate, 15 °C/min Detector temp 330 °C Injection port temp 70 °C Helium flow, 15 cc/min Auxiliary gas flow, N2, 30 cc/min Hydrogen flow, 28 cc/min Air flow, 500 cc/min Sample size, 4 \µ
vacuum with vigorous vibration. The ends of the column were then plugged with silanized glass wool and coiled to fit the oven compartment of the GC. Before using any column for a sample separation, it was treated for 8 hours at
275 °C.
Chemicals. The silanizing reagents, 7V,0-bis-(trimethylsilyl)acetamide (BSA) and trimethylchlorosilane (TMCS) were obtained from Pierce Chemical. The column packing materials were all obtained from Applied Science Laboratories. Other chemicals used in the investigation were available through plant production samples and were found to be of suitable purity for calibrations and preparation of standards. Procedure. Samples of CMDB propellant extracts were obtained by extracting 1 to 2 grams of propellant with methylene chloride or diethyl ether. The extract volume was then reduced to approximately 2 ml and reacted with 1 ml of BSA catalyzed with 1% TMCS. Reaction of the BSA with the resorcinol was essentially instantaneous at room temperature. The reacted sample was diluted to a known volume, usually 25 ml with dichloroethane. The GC analysis of the extract solution was performed as shown in Table I. Calibrations. An internal standardization method was used for calibrations. Dimethyl phthalate (DMP) was chosen as the internal standard. Response factors were determined for each of the expected components in CMDB propellant extracts by preparing standards of known concentrations and analyzing the standards as a sample. Once the linearity of the response was established over the expected concentration range of the components, a single standard was run daily as a check on the consistency of the response factors.
Table II.
Retention Time of Various Components Possible in Double- and Single-Base Propellants Relative retention time
Component Resorcinol
0.82 0.89 1.00
TA DMP DMS 2-Nitroresorcinol 2.3- Dinitroresorcinol 2-NDPA 2.4- Dinitrosoresorcinol
1.08 1.10 1.34 1.38 1.46
Resazurin Resorufin
2,4'-DNDPA 2.4- DNDPA °
No definite peaks
were
obtained for these compounds.
packings because the extremely low bleed rate at 250 °C and the preferred order of elution of the trimethylsilyl ether of resorcinol and NG. This column packing, however, did not separate NG from triacetin (TA), a plasticizer in certain double-base propellants. The interference of NG with TA was removed by increasing the detector temperature to 330 °C to completely destroy the NG prior to the flame ionization detector. Of course, this also precluded measurement of
Figure 1. DMS
NG.
Chromatogram of propellant extract with added Attenuation:
103
The problem of the NG decomposition was solved by incorporating the technique of programmed temperature oncolumn injection of the sample. This was achieved by placing an additional injection port inside the oven compartment so that the injection port temperature was programmed simultaneously with the column temperature, thus allowing low temperature injection and avoiding decomposition of the NG on the vaporizer. Upon programming the column temperature, the NG was rapidly removed from the injection port area and thus avoided any possible nitration of the stabilizers. To use this injection port, it was necessary for the operator to raise momentarily the oven top to inject a sample. However, by having an oven start temperature of only 70 °C and a 2-minute post injection time interval, the brief opening did
X 32
RESULTS AND DISCUSSIONS
In the course of this investigation, numerous combinations of solid supports and liquid phases were evaluated as possible GC column packings capable of removing the interferences of NG as well as applicable for the separation of the stabilizers and plasticizers of interest. However, initial data demonstrated the problem of NG decomposition was not necessarily a function of GC column packing but instead vaporizer temperature. The initial data also indicated that practically any high temperature silicone gum or oil coated on an inert solid support would give the desired separation of stabilizers and plasticizers—i.e., resorcinol, triacetin (TA), dimethylsebacate (DMS), DMP, and 2-NDPA. A column packed with 5% OV-17
on
Gas Chrom Q was selected
over
other possible
Table III. Propellant
1.00
1
0.63
2
Std dev
Reliability Data for Propellant Extracts DMS, %
TA, %
Resorcinol, % GC UV
Std dev
not affect the analysis. Determination of Interferences. A brief study was undertaken to ascertain if any known potential contaminants in CMDB propellant extracts would interfere with the determination of TA, DMS, resorcinol, and 2-NDPA. As previ-
IR 5.00
1.02 0.010 0.66 0.009
2.48
Table IV.
IR
GC 5.09 0.040 2.49 0.031
Effect of NG
on
GC
TA
99 80 75
0
60
Resorcinol
1.00
1.00 0.021
0.95
1.00 0.012 0.98 0.010
TA and Stabilizer Values Percentage found by GC
2-NDPA
TA
Resorcinol
0
0 1.01 0
18
1
24 35
0
1.00 1.00 1.00
2.5
2.5
0
2-NDPA, % GC
1.00
Percentage added
NG
Vis
17.8 24.1 35.2
2.50
2-NDPA 0.98 1.02 1.01
2.48
VOL. 41, NO. 1, JANUARY 1969
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167
ously described, the NG interference with TA was minimized by thermally decomposing the NG. The retention times of several of the most likely nitrated derivatives of resorcinol and 2-NDPA were measured. The relative retention times are shown in Table II. As shown by these data, all of the components studied were separated from TA, DMS, resorcinol, and 2-NDPA or the compounds in question did not elute from the column. Precision Study. The precision study was conducted using extracts obtained from two different propellant formulations, 1 and 2. Because these propellants did not contain DMS, a known amount of this compound was added to the propellant 2 extracts. The recovery of the added DMS was then determined. A typical chromatogram for these propellant extracts is shown in Figure 1. The data obtained from the precision study are shown in Table III. Two different propellant formulations were analyzed to emphasize any effects of different ratios of the components on the results. These data showed that the concentration of any one component did not influence the results of another component. The coefficient of variance for each of the two pro-
pellants was essentially the same—i.e., TA, 0.03%; DMS, 0.02%; resorcinol, 0.01%; and 2-NDPA, 0.01%. In addition, data in Table III show the close agreement obtained between the GC and present IR-TLC (4) methods, about ±2% relative or less between all the components. Because the method would be applied to large numbers of samples, the precision calculations were based on peak height instead of the more accurate area measurements by triangulation to decrease the time required for calculation of the results. The results definitely indicated that NG was not a problem because low coefficient of variance for each component was obtained. These data were confirmed by analyzing samples with widely varying NG/TA and NG/stabilizer ratios as shown in Table IV. These results proved that the effect of NG on the stabilizers had been removed and, further, the interference of NG with TA had been eliminated.
Received for review July 15, 1968.
Accepted September 16,
1968.
(4) G. F. Macke, J. Chromatogr., (in press).
Solution of Blank Problems in 14-MeV Neutron Activation Analysis for Trace Oxygen E. Suddueth, and S. L. Birkhead S. S. Nargolwalla, E. P. Przybylowicz,1 C. 20234 National Bureau of Standards, Washington, D. 234J.
The accuracy of trace oxygen determinations by 14-MeV neutron activation analysis can be seriously affected if the oxygen contribution from the container is significant. If the count from the blank is merely subtracted from the total sample-in-container count, errors as large as 100% may be introduced. Solutions by other workers (7-4) are specific to the individual systems being used and are not of general
applicability. With a general purpose activation analysis facility, an attempt has been made to provide a generalized solution to the problem. In our previous approach (5), leak problems, associated with the encapsulation technique for loading steel samples in polyethylene containers under a nitrogen atmosphere, introduced imprecision in about 20% of the experiments. The present technique gives consideration to the sample-in-container geometry and any attenuation effects which may arise during irradiation and counting. The application of this method does not presuppose the availability of low-oxygen containers, but can be used in any analysis where the blank problem is considered to be significant. 1
Research Associate from Eastman Kodak Co., Rochester,
N. Y.
(1) O. U. Anders and D. W. Briden, Anal. Chem., 36, 287 (1964), (2) R. F. Coleman, Iron Steel Inst. (London), Spec. Rept. 68. (1960). (3) K. G. Broadhead and . H. Heady, Anal. Chem., 37, 759 (1965).
(4) H. F Priest U.S. Army Materials Research Agency, Watertown, Mass., private communication, June 1968. (5) S. S. Nargolwalla, M. R. Crambes, and J. R. DeVoe, Anal. Chem., 40, 666 (1968). (6) F. A. Lundgren and S. S. Nargolwalla, ibid., p 672.
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analytical
chemistry
This study contributes to the state of the art in the following ways: determination of the attenuation of the activity from the container by samples of different diameters; definition of a geometrical model which quantitatively expresses the observed attenuation by the samples; and design of a flow-through container for solid samples which reduces the capsule blank contribution and eliminates the need to encapsulate samples in a nitrogen atmosphere provided the propelling gas in the pneumatic system is dry nitrogen. The solution of these blank problems considerably improves the reliability of the 14-MeV neutron activation technique for the precise and accurate determination of trace oxygen. EXPERIMENTAL Equipment. The facility at NBS consists of a 2.5-mA beam current Cockcroft-Walton neutron generator, a dualsample pneumatic system with a rotating sample assembly (6), a sequence programmer, and a detector system of two 4 inch by 3 inch NaI(Tl) detectors coupled to a 400-channel pulse height analyzer. For a quantitative study it was necessary that a container be constructed of a material with macro amounts of oxygen—viz. nylon (ca. 12% oxygen) identical to the polyethylene vials normally used. The samples were steel rods containing small amounts of oxygen (
