Full Paper

A Desorption Electrospray Ionization Mass Spectrometry Study ofAging Products of Diphenylamine Stabilizer in Double-BasePropellants

Andre Venter*, Demian R. Ifa, R. Graham Cooks

Chemistry Department, Purdue University, West Lafayette, Indiana (USA)

Sara K. Poehlein, Anton Chin, Dan Ellison

Naval Surface Warfare Center, Crane, 300 Highway 361, Crane, IN 47522 (USA)DOI: 10.1002/prep.200600064

Abstract

Double-base propellants consisting of nitrocellulose, nitrogly-cerin and stabilizer undergo chemical and physical changes uponaging, leading to changes in ballistic power and presentingexplosive hazards. During aging, PTFE seals of the glass ampoulesused in the aging studies undergo a yellow discoloration. Thisreport studies the discoloration of the liners using desorptionelectrospray ionization (DESI), a gentle surface analysis tech-nique based on electrospray ionization. The color bodies in thePTFE liners were identified by DESI together with tandem massspectrometry to be the nitrated derivatives of the diphenylaminestabilizer: dinitro-, trinitro-, and tetranitrodiphenylamine. Whileincreased nitration decreases vapor pressure of the DPA species,an increase in solubility in the PTFE liners occurs. This mayaccount for these species not previously being observed duringearly aging studies as they are preferentially absorbed into theliners, which were not extracted prior to high performance liquid-chromatography analysis.

Keywords: Double-Base Propellants, Diphenylamine, DesorptionElectrospray Ionization (DESI), Polymer Analysis, Color Bodies

1 Introduction

Double-base propellants contain nitrocellulose (NC),nitroglycerin, and stabilizer. Chemical and physical changesoccur during aging, leading to ballistic performance changessuch as ignition and burning rate. Chemical processes createthe release of heat and gaseous products that accelerate theaging process. Over time propellants become unstable,presenting an explosive hazard. For this reason the agingprocess of propellants needs to be understood. The mech-anism of degradation ofNChas been studied formany yearsand it has become accepted that the instability of the nitrateester groups in the polymer is the main cause [1]. Theformation of NO2 is a radical process and in the presence of

moisture can lead to the formation of HNO2, whichcatalyzes the hydrolysis of the nitrate ester groups. Stabil-izers are added to scavenge the freeNO2 in order to increasethe shelf life of explosives and to reduce the danger of auto-ignition. Diphenylamine (DPA) was first added as astabilizer by Alfred Nobel in 1889 and is still widely usedtoday [1]. It reacts with the nitrogen dioxide through anelectrophilic aromatic substitution to initially form n-nitroso-DPA, followed by rearrangement to the mono-nitrated DPA. These reaction products also have a stabiliz-ing effect, however more highly nitrated forms of DPA aresaid to only occur during the final stages of aging when thepropellant becomes unstable [2, 3, 4]. This state is reachedwhen the DPA concentration is consumed to the pointwhere it is less than 0.2% [1]. Older studies have indicatedthat only 60% of the used stabilizer could be accounted forin aged propellants by quantitative analysis of the mono-nitrated DPA reaction products [5]. High PerformanceLiquid Chromatography (HPLC) has most often been usedto measure stabilizer content [6]. The stabilizer is extractedfrom the propellant prior to analysis. Some of the degrada-tion products such as 2 and 4-NDPA and 2,4-DNDPA arereadily available as standards, while the highly nitrateddegradation products such as trinitrodiphenylamine andtetranitrodiphenylamine are not. Coelution is also a majorproblem. Trinitrodiphenylamine and tetranitrodiphenyla-mine coelute with 2,4’-dinitrodiphenylamine, and 2,2’-diphenylamine. For real samples, characterized by thepresence of complex matrix components, the identificationof the analyte peaks are virtually impossible by HPLCwithout selective detection. Past analyses of aged propel-lants within Naval Surface Warfare Center, Crane Divisionlaboratory, could not confirm the presence of higherdegradation DPA products such as trinitrodiphenylamineand tetranitrodiphenylamine.

* Corresponding author, e-mail: aventer@purdue.edu

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Condensation of yellow discolorants inside a sampleampoule is a common phenomenon during the artificialaging of double-base propellants. The polytetrafluoroethy-lene (PTFE) seals or other surfaces on top of the sampleampoule absorb the yellow condensate and separate it fromthe main propellant composition. Several studies haveattempted to identify the unknown discolorant [1, 4, 6] inthe hope of gleaning information about the complexreaction steps in the mechanism of aging of nitrocelluloseand nitroglycerin.Kimura [7] used Electron Impact (EI) mass spectrometry

to study the yellow condensate formed during aging ofdouble-base propellants. He identified glycerol aldehydedinitrite as a yellow degradation product of nitroglycerin.High temperatures experienced by the sample while vapor-izing the sample for electron impact and the intrinsic harshnature of the ionization method may have resulted in theloss of chemical information during this mass spectrometricanalysis.In contrast to EI, electrospray ionization (ESI) is a soft

ionization technique that is used to ionize molecules fromnebulized aqueous solutions. It produces singly or multiplycharged molecular ions of a wide range of molecules withpolar functionality.Recently a new ionization techniquewasdeveloped that produces ions in a similar fashion to ESI, butdirectly from surfaces without the need for sample prepa-ration or extraction. Desorption electrospray ionization(DESI) is shown in Figure 1. During DESI analysis, anaqueous solvent, delivered by a syringe pump, is pneumati-cally nebulized through a capillary spray tip to which a highvoltage (relative to the grounded inlet to a mass spectrom-eter) is applied. The fine droplets of solvent impact on asample surface in the ambient atmosphere of the laboratory.During the interaction of the spraywith the surface, analytesare desorbed and ionized before being sampled into a massspectrometer for analysis. Recently DESI was used tocharacterize the formation of color bodies in a specializedpolymer [8]. As will be shown, DESI provides an idealionization technique for the mass spectrometric analysis ofthe trace amounts of yellow degradation products thatdefuse into the PTFE cap liners of ampoules used for agingstudies. This is the first time a desorption ionizationtechnique has been used in the identification of degradationproducts of the diphenylamine stabilizer. This could giveinsight into the mechanism of the degradation of stabilizers,which may help to determine a more accurate shelf life ofpropellants.

2 Experimental

2.1 DESI Analysis

Experiments were carried out using a commercial Ther-mo Finnigan LTQ (San Jose, CA) linear ion trap massspectrometer. Figure 1 shows the DESI source, which washome-built as previously described [9]. The optimum sourcesettings are summarized in Table 1. DESI spectra were

obtained in the negative mode using a mixture of chloro-form/methanol/water (20 :70 :10) as the desorption spray.

2.2 Aging Process and Sample Preparation

A microcalorimeter was used to monitor the rate of heatgeneration during the breakdown of the propellant underisothermal conditions. The microcalorimeter simulatesnormal aging at ambient conditions at an accelerated rate.At 90 8C, 2 hours approximates six years of natural aging.Microcalorimetry of propellants was conducted using

4 mL glass or stainless steel ampoules with PTFE seals(Figure 2). The PTFE seal was also replaced by clear orsuperFrostedK microscope slides. Approximately 3.8 g ofWC868 double-base propellant was loaded into the am-poules using a PTFE cap with a silicon o-ring. Samples weresealed at ambient room temperature and 40% relativehumidity. Using a Thermal Activity Monitor Model III(TAM) calorimeter, the samples were subjected to aging for2 hours, 3 days, and 6 days at 90 8C. The extended aging doesnot represent real service life aging.

Figure 1. The DESI source. �4kV is applied to the solventdelivered at 5 mL/min to the capillary tip by a syringe pump. The100 mm i.d. solvent capillary is surrounded by a gas capillary towhich 827 kPa nitrogen is delivered to aid nebulization of thespray solvent. The incident (a) and collection (b) angles areindicated.

Table 1. DESI source settings

Electrospray voltage �4 kVElectrospray flow rate 5 mL/minIncident angle (a) 508Collection angle (b) 108MS inlet-sample distance 5 mmSpray tip-sample distance 3 mmNebulizing gas pressure 827 kPaCapillary voltage 15 VTube lens voltage 65 VCapillary temperature 150 8C

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The experimental conditions for the microcalorimetertests were set to simulate the conditions in KimuraMs paper[7]. It was hoped that the nitrate esters thatKimura reportedwould also be observed.

3 Results and Discussion

Figure 3 shows the negative ion DESI analysis of thePTFE liner obtained from the inside of a sealed ampouleafter 6 days of aging at 90 8C. The ions observed at m/z 258,303 and 348 were stable, reproducible and not observed in asimilar analysis of a blank PTFE seal that was not subjectedto the aging step. DESI is a soft ionization technique thatproduces ions similar to those obtained by electrosprayionization. Thus the ions observed in the negative ion modeare themolecular ions at [M-1]-orM- of the analytes presentin or on the PTFE liner.With continuous spray at the same position theDESI-MS

signal quickly decreased and the sample had to bemoved toa fresh, unsprayed spot.Also, when a samplewaswipedwithacetonitrile to remove surface contamination, the ions atm/z 258, 303 and 348 were not observed. This could indicatethatDESI was only analyzing the surface and not the yellowcontamination that was deeply diffused into the PTFE liner.To show that this was not the case, a section of discoloredPTFE was sonicated for 15 minutes in chloroform/metha-nol/water (20 :70 :10) and reapplied to a blank PTFEsurface. The extract had a yellow color and produced amass spectrum very similar to the original direct DESIanalysis, showing the m/z 258, 303 and 348 ions in high

abundance.A sprayed anddepleted surface also reproduceda good signal of the threemasses after resting for a few days.Thus, the yellow contaminants were desorbed and ionizedonly from the outer layer of the PTFE by the DESI processand the spray removes analytes from the surface faster thanthey can diffuse back to it.The ions at m/z 258, 303 and 348 were subjected to

Collision-Induced Dissociation (CID) to obtain structuralinformation in order to identify the colorproducing entities.CID was performed in the ion trap using helium as collisiongas and with normalized collision energy of 25%. Thisproduced ample fragmentation while the precursor ion wasalso still observed. An isolation window of 2 mass to chargeunits was used.The ion of m/z 258 corresponds to deprotonated dinitro-

diphenylamine. This compound is a predictable reactionproduct between the stabilizer DPA and the free NO2produced from the hydrolysis of nitroester groups in nitro-glycerin and nitrocellulose. The CID mass spectrum of m/z258 (Figure 4) shows two intense ions atm/z 228 and 198 thatcorrespond to the consecutive losses of two NO· fragments.Loss of NO· is a well-established process in the EI and CIDmass spectra of nitroaromatic compounds in both thenegative and positive ionization modes [10]. Loss of HNOhas also previously been reported [11].The ion at m/z 303 corresponds to deprotonated trinitro-

diphenylamine, the next higher homologue in the nitrationof DPA. The CID spectra presented in Figure 5 shows a lossof OH (m/z 286) followed by a loss of NO·(m/z 256). Acompeting and more favorable pathway (based on therelative abundance of the fragment ions) is an initial loss ofHNO (m/z 272), followed by a further loss of NO2 (m/z 226).In a third pathway, an initial loss of NO·(m/z 273) isexperienced, again followed by the loss of NO2 (m/z 227).The ion atm/z 348 is again 45mass units higher, indicating

the substitution of yet another NO2 group to DPA. The

Figure 2. The glass ampoule into which 3.8 grams of propellantis loaded for artificial aging studies using microcalorimetry. Themicroscope slide and PTFE liner used for DESI analysis areindicated.

Figure 3. DESI analysis of the PTFE liner after aging for 6 days.Ions m/z 258, 303 and 348 were not present in the backgroundanalysis and are formed during the aging process of double-basepropellant.

474 A. Venter, D. R. Ifa, R. G. Cooks, S. K. Poehlein, A. Chin, D. Ellison

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fragmentation pattern presented in Figure 6 supports thisassumption. In an analogous fashion to the trinitro-DPACID spectrum, a loss ofOHorNH3 initially occurs (m/z 331)followed by the loss of NO·(m/z 301) and NO2 (m/z 285).The ion at m/z 301 loses NO2 to produce an ion at m/z 256.Two consecutive losses of NO·groups produce m/z 228 andm/z 198. An alternative route leads to the initial loss of NO2,producing an intermediate at m/z 303 (not observed), whichdecomposes further by loss of NO·(m/z 273) andHNO (m/z272). Two consecutive losses of NO·occur, producing a verysmall signal at m/z 242 and a strong signal at m/z 212.Other surfaces were investigated for collection of the

decomposition products during the aging process. Smoothor superFrostedK glasswas placed on top of the propellant todetermine if the yellow discolorant would also collect onnon-Teflon surfaces. None of the ions under investigationcould be observed from the smooth or superFrosted glass

surfaces with direct DESI analysis. In the case of the smoothglass surface there is almost no surface area for analytecollection. However the superFrosted glass surface showeda distinct yellow appearance after collection of products for6 days. The superFrosted glass collection platewas sonicatedin chloroform/methanol/water (20 :70 :10) for 15 minutes.The light yellow extract was applied to a clean, blank PTFEsurface and analyzed by DESI. The spectrum again con-tained the ions at m/z 258, 303 and 348 but these werepresent in a different ratio than observed directly from aPTFE liner. Dinitro-DPA (m/z 258) was the most abundant,followed by trinitro-DPA (m/z 303) and only a very smallsignal was observed for the tetranitro-DPA isomer (m/z348). This is in contrast to when a discolored PTFE surfacewas sonicated, as discussed in the previous paragraph,wherethe m/z 348 ion was the most abundant.The improvement inDESI signal fromPTFE is consistent

with previous studieswhere PTFE surfaceswere found to bebeneficial for the DESI analysis of small molecules in thenegative ionization mode while glass surfaces producedbetter signals in the positive ionization mode [12].While increasing the level of nitration ofDPA reduces the

volatility of the breakdown products, it also increasessolubility into the Teflon, thereby reducing or completelyremoving the more highly nitrated species from thepropellant in the ampoule. In the past, whenHPLC analysesfollowed extraction of the aged propellant, the presence ofthe tri- and tetranitrated DPA was overlooked as it wasabsorbed into theTeflon seal and removed from the reactionvessel.

4 Conclusions

Desorption electrospray ionization serves as an idealsurface analysis technique for the analysis of color bodies inpolymericmaterials.DESI extracts only the smallmoleculesfrom large polymeric material without destroying the

Figure 4. Collision induced dissociation mass spectrum of theion of m/z 258 ion assigned as dinitro-DPA.

Figure 5. CID mass spectrum of precursor ion at m/z 303assigned as deprotonated trinitro-DPA.

Figure 6. CID mass spectrum of precursor ion at m/z 348assigned as tetranitro-PDA.

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polymeric matrix, as is likely in the case with MALDI orSIMS analysis. For this particular investigation it wasserendipitous that PTFE is also a good surface for analysisof small molecules in the negative ion mode of DESI.The correlations seen in the CID mass spectra of the ions

at m/z 258, 303 and 348 serve as an indication that the threecompounds belong to a homologous series. Furthermore,the fragmentation patterns are consistent with the knownroutes for decomposition of nitroaromatic compoundscontaining multiple nitro groups.Therefore, we conclude that the yellow discoloration of

the Teflon liners is caused by highly nitrated reactionproducts of DPA and the free nitro groups produced duringthe aging of base propellants. These have been identified bytandem mass spectrometry to be dinitro-, trinitro- andtetranitrodiphenylamine. Since these products are removedfrom the double-base propellants by the Teflon liner, theywere previously overlooked when aged propellants wereextracted forHPLC analysis. Future quantitative extractionof the liners may indicate that the higher nitrated reactionproducts account for the deficiency in DPA recoveries.The heat flow evaluation of these sampleswill be reported

in a following paper. Preliminary evaluation of the heat flowdata suggests that the surface material surrounding thepropellant also affects the stability of the propellant.

References

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[11] J. Yinon, Mass Spectral Fragmentation Pathways in SomeDinitroaromatic Compounds studied by Collision-InducedDissociation and Tandem Mass Spectrometry, Org. MassSpectrom. 1992, 27, 689.

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Acknowledgements

This work was supported by the Office of Naval Research undergrant number BAAONR 04-024.

(Received June 7, 2006; Ms 2006/126)

476 A. Venter, D. R. Ifa, R. G. Cooks, S. K. Poehlein, A. Chin, D. Ellison

Prop., Explos., Pyrotech. 31, No. 6, 472 – 476 www.pep.wiley-vch.de B 2006 WILEY-VCH Verlag GmbH&Co. KGaA, Weinheim