low-mass ions observed in plasma desorption mass spectrometry of high explosives

JOURNAL OF MASS SPECTROMETRYJ. Mass Spectrom. 35, 337–346 (2000)

Low-mass ions observed in plasma desorptionmass spectrometry of high explosives

Kristina H akansson,1* Ramal V. Coorey,1,2 Roman A. Zubarev,3 Victor L. Talrose 4 andPer Hakansson11 Ion Physics Division,Angstrom Laboratory, Uppsala University, Box 534, SE-751 21 Uppsala, Sweden2 Department of Physics, University of Colombo, Colombo-3, Sri Lanka3 Chemistry Department, University of Southern Denmark/Odense University, Campusvej 55, DK-5230 Odense M,Denmark4 Institute of Energy Problems of Chemical Physics, Leninsky Prospekt, 38, V-334 Moscow, Russia

The low-mass ions observed in both positive and negative plasma desorption mass spectrometry (PDMS) ofthe high explosives HMX, RDX, CL-20, NC, PETN and TNT are reported. Possible identities of the mostabundant ions are suggested and their presence or absence in the different spectra is related to the propertiesof the explosives as matrices in PDMS. The detection of abundant NOY and NO2

− ions for HMX, RDX andCL-20, which are efficient matrices, indicates that explosive decomposition takes place in PDMS of these threesubstances and that a contribution from the corresponding chemical energy release is possible. The observationof abundant C2H4NY and CH2NY ions, which have high protonation properties, might also explain the higherprotein charge states observed with these matrices. Also, the observation of NO2

−, possibly formed by electronscavenging which increases the survival probability of positively charged protein molecular ions, completes thepattern. TNT does not give any of these ions and it is thereby possible to explain why it does not work as aPDMS matrix. For NC and PETN, decomposition does not seem to be as pronounced as for HMX, RDX andCL-20, and also no particularly abundant ions with high protonation properties are observed. The fact thatNC works well as a matrix might be related to other properties of this compound, such as its high adsorptionability. Copyright 2000 John Wiley & Sons, Ltd.

KEYWORDS: desorption ionization; explosives; matrix effects; protonation; multiply charged ions

INTRODUCTION

Plasma desorption1 was the first ionization techniquewhich was able to produce intact gas-phase molecular ionsof biomolecules with a molecular mass above 5 kDa.2

Owing to this breakthrough, mass spectrometry became atechnique with great potential in biochemistry and mole-cular biology since it can determine molecular mass to anaccuracy orders of magnitude higher than other methodsused in these fields. However, the performance of plasmadesorption mass spectrometry (PDMS) is limited, primar-ily in sensitivity and analysis time. In 1986, 3 years afterthe first detection of a small protein, another breakthroughcame with the introduction of the nitrocellulose (NC)matrix.3 The use of nitrocellulose resulted in a substan-tial increase in the molecular ion yield and partly solvedthe earlier problems. In the late 1980s, two techniquessuperior to PD for the soft ionization of large molecules,

* Correspondence to: K. Hakansson, Ion Physics Division,AngstromLaboratory, Uppsala University, Box 534, SE-751 21 Uppsala,Sweden.E-mail: [email protected]

Contract/grant sponsor: Swedish Natural Sciences Research Coun-cil (NFR).

Contract/grant sponsor: International Association for the Promo-tion of Cooperation with Scientists from the New Independent Statesof the Former Soviet Union (INTAS).

electrospray ionization (ESI)4 and matrix-assisted laserdesorption/ionization (MALDI),5 were introduced and theinterest in PDMS for biomolecular analysis has declinedsince then. However, PDMS has still been used, since itis a simple and robust technique and in many cases hasa sufficient performance. Also, great efforts have beenmade to elucidate the mechanisms behind the ionizationand desorption processes involved.6 In contrast, the exactmechanisms of molecular ion production in MALDI andESI remain poorly understood owing to the complexityof the processes involved. The PDMS mechanism is alsocomplicated, but has been more thoroughly investigatedduring the past two and a half decades.7 In our view,PDMS may serve as a model giving insights into themechanisms of other ion production techniques.

Our current interest in the PDMS mechanism was to alarge extent triggered by the discovery of new explosivematrices with the intriguing ability to assist the forma-tion of multiply protonated molecular ions of proteinsand peptides.8,9 The most efficient explosive matrix alsoresults in a higher molecular ion yield than NC (by a fac-tor of about two).8,9 Achieving a higher charge state isarguably a way to improve the analytical capabilities ofMALDI owing to increased sensitivity (via an enhanceddetector response) and extended capabilities for tandemmass spectrometry (MS/MS). In terms of the degree ofprotonation, explosive matrix-assisted PDMS performancelies between MALDI and ESI. The octogen (HMX) matrix

Copyright 2000 John Wiley & Sons, Ltd. Received 24 September 1999Accepted 30 November 1999

338 K. HAKANSSON ET AL.

is the most efficient yielding spectra resembling thoseobtained with ESI when an excess of explosive is usedunder a thin (<300 A) analyte layer.10

The first investigation into the mechanism of explosivematrix assistance supported the hypothesis of a contribu-tion from the chemical energy released during conversionof the explosive.8 As additional assisting mechanisms,enhanced protonation of the analyte through collisions withproducts of the explosive decay, and also electron scav-enging by other products which leads to a higher survivalprobability of positively charged protein molecular ions,have been discussed.10 Elucidating the exact mechanismhas been hampered by the absence of PD mass spectra ofpure explosives, which might give hints about the targetcomposition after the passage of the swift primary ion orfission fragment. In this work, we studied such spectra ofhigh explosives, both those which were found to work wellas matrices and those which did not have any effect.

EXPERIMENTAL

Sample preparation

The explosive substances cyclotetramethylenetetran-itramine (octogen or HMX), hexahydro-1,3,5-trinitro-5-triazine (hexogen or RDX), 2,4,6,8,10,12-hexanitrohexa-azaisowurtzitane (CL-20 or HNIW), nitrocellulose (NC),pentaerythritol tetranitrate (pentrit or PETN) and trinitro-toluene (trotyl or TNT) were dissolved in acetone at aconcentration of 1 g l�1. All substances were kindly pro-vided by J. Hansson and H.Ostmark, National DefenceResearch Establishment, Stockholm, Sweden. PDMS sam-ples were prepared by electrospraying 200µl of thesesolutions on to aluminized Mylar foils. These sampleswere rinsed with deionized water in order to remove saltcontaminants. A pure protein sample was prepared byapplying 200µl of a 1 g l�1 aqueous solution of bovineinsulin (Sigma, Stockholm, Sweden) to an aluminizedMylar foil and drying under a flow of nitrogen. Unfor-tunately, this sample could not be rinsed since the pro-tein adsorbs only weakly on the metal surface. Finally,an HMX–insulin sample was also prepared according tothe procedure described previously.10 A 200 µl volume ofHMX was electrosprayed on to an aluminized Mylar foil,resulting in an average film thickness of 1µm. On top ofthis layer, 10µl of a 3.5ð 10�5 M solution of insulin inwater containing 0.1% TFA was applied, giving an aver-age insulin thickness of 200A.

Mass spectrometry and data analysis

A BioIon 20 mass spectrometer (BioIon, Uppsala, Swe-den) with a time-of-flight (TOF) analyzer equipped withan Einzel lens and a reflectron was used in all experi-ments. Positive and negative PD mass spectra were taken atš10 kV accelerating voltage andš12 kV reflecting volt-age, respectively. The Einzel lens voltage wasC3 kV in thecase of positive ions, and�1 kV in the case of negativeions. Data acquisition was performed using a multi-stoptime-to-digital converter (CTN-M2, Institut de PhysiqueNucleaire, Orsay, France). All spectra were taken at a time

resolution of 0.5 ns and summed over 200 000 start events.The spectra were recorded on an Atari computer with suit-able software, developed by Professor W. Ens at the Uni-versity of Manitoba, Canada. Internal mass calibration wasperformed using the HC and CH3

C ions for positive spectraand the H� and CN� ions for negative spectra. Them/z val-ues of the remaining ions were determined using centroidsat the 50% level. Possible identifications of the low-massions (belowm/z 100) were obtained using a program com-paring the observed mass values with masses correspond-ing to different elemental combinations. A mass error of<20 mDa was assumed. As a second criterion, a maxi-mum of four C, N or O atoms were allowed. The NationalInstitute of Standards and Technology (NIST) ChemistryWebBook (http://webbook.nist.gov) was used as referencematerial in order to check if the identified species are likelyto exist. The NIST Chemistry WebBook was also used forobtaining the values for proton and electron affinities ofsome neutrals, which are given below. Electron ionization(EI) mass spectra of HMX, RDX, PETN and TNT werealso obtained from this site.

RESULTS AND DISCUSSION

Structures of all the explosives discussed are shown inFig. 1. In earlier studies it was found that HMX, RDX,

Figure 1. Structures of the explosives studied.

Copyright 2000 John Wiley & Sons, Ltd. J. Mass Spectrom. 35, 337–346 (2000)

LOW-MASS IONS IN PDMS OF HIGH EXPLOSIVES 339

CL-20 and NC work well as matrices with HMX being themost efficient in terms of both the degree of protonationand total molecular ion yield, as mentioned above.8,9

RDX, CL-20 and NC show mutually similar behavior inboth respects.8,9 PETN and TNT do not work as matrices.8

In the present study, it was seen that none of the explosivesproduced molecular ions or molecular cluster ions inPDMS. Therefore, attention was directed to the observedlow-mass ions where identifications are possible.

Positive ion PD mass spectra

Positive ion PD mass spectra of the explosives in theregion up tom/z 100 are shown in Fig. 2. The mainpeaks in each spectrum together with possible identitiesare listed in Table 1. Species with a deviation>20 mDabetween observed and theoretical mass values are notlisted. From the table it is seen that all substances stud-ied, except TNT, give a high-intensity NOC peak. Thision is not present in the spectrum of the correspondingpure protein sample [Fig. 3(a)]. Neutral NO is a possi-ble product from explosive decomposition. As discussedpreviously,8,10 decomposition is likely to occur under theaction of υ-electrons taking place after the252Cf fissionfragment impact. However, no detonation of the wholesample is initiated owing to the small excited area in

PDMS.8 The time-scales of the decomposition and desorp-tion processes also seem to be correlated.8,10 Conversionof the explosive is a prerequisite for the chemical energyrelease,11 and the presence of an ion which is likely tooriginate from such a process supports the idea of explo-sive matrix assistance related to this phenomenon. Theabsence of NOC in the TNT spectrum might contribute toan explanation of why the latter is not an effective matrix.There is a probability that the ion identified as NOCinstead corresponds to CH2OC (see Table 1). However,the mass deviation between the observed and theoreti-cal masses is fairly large (on average 16 mDa comparedwith 3 mDa for NOC) for this ion. On the other hand,CH2O is also known to be a product from explosivedecomposition, although an intermediate one.11 In a pos-sible reaction scheme, CH2O is one of the species whichabsorbs significant levels of vibrational excitation as a rel-atively stable intermediate preceding the formation of thefinal products.11

HMX, RDX and CL-20.For HMX and RDX, intense peaksat m/z 42.03, most probably corresponding to C2H4NC,are observed. Species with such an elemental composi-tion are protonated CH3CN, which has a proton affinityof 779.2 kJ mol�1, or protonated CH3NC (proton affin-ity 839.1 kJ mol�1). These ions might supply protons

Figure 2. Positive low-mass ion PD mass spectra of all the explosives. Ion identifications were performed by comparing observedm/z values with theoretical values corresponding to different elemental combinations. A mass accuracy of better than 20 mDa wasassumed. For a more complete list, see Table 1.

Copyright 2000JohnWiley & Sons,Ltd. J. MassSpectrom. 35, 337–346 (2000)


Table 1. Relative (peak) intensities of the 10 most abundant peaks in the positive ion PD massspectra of all the explosives studieda

Possible TheoreticalObserved m/z elemental monoisotopic Mass deviation Relative

Explosive (Da) composition(s) m/z (Da) (mDA) intensity (%)

HMX 29.993 NOC 29.997 �4.4 100CH2OC 30.010 �17

42.032 (C2H4NC 42.034 �1.8) 97C3H6

C 42.046 �1428.014 CH2NC 28.018 �4.2 28

C2H4C 28.031 �17

COC 27.994 2074.097 2543.050 C3H7

C 43.054 �4.2 2341.030 C3H5

C 41.039 �8.6 20CHN2

C 41.013 171.007 HC 1.007 0 (calibration peak) 17

57.064 C4H9C 57.070 �5.9 14

55.043 C4H7C 55.054 �11 13

75.023 (CH3N2O2C 75.019 4.1) 12

RDX 29.994 NOC 29.997 �3.4 100CH2OC 30.010 �16

42.030 (C2H4NC 42.034 �3.8) 80C3H6

C 42.046 �16C2H2OC 42.010 20

28.016 CH2NC 28.018 �2.2 31C2H4

C 28.031 �1557.043 C3H5OC 57.034 9.5 2784.051 C3H4N2OC 84.032 �19 2630.032 CH4NC 30.034 �1.8 2443.040 (CH3N2

C 43.029 11) 20C3H7

C 43.054 �1441.027 C3H5

C 41.039 �12 18CHN2

C 41.013 141.007 HC 1.007 0 (calibration peak) 12

75.014 (CH3N2O2C 75.019 �4.9) 11

CL-20 29.996 NOC 29.997 �1.4 100CH2OC 30.010 �14

28.020 CH2NC 28.018 1.8 30C2H4

C 28.031 �1182.049 (C2N3OC 82.041 7.7) 29

(C3H4N3C 82.040 9.0)

43.042 C3H7C 43.054 �12 14

(CH3N2C 43.029 13)

55.041 C4H7C 55.054 �13 12

45.998 NO2C 45.992 5.6 12

CH2O2C 46.005 �6.9

69.048 C2H3N3C 69.032 16 11

41.036 C3H5C 41.039 �2.6 11

94.047 (C4H4N3C 94.040 7.0) 10

81.041 (C4H5N2C 81.045 �3.7) 10

NC 29.993 NOC 29.997 �4.4 100CH2OC 30.010 �17

45.994 NO2C 45.992 �1.6 41

CH2O2C 46.005 �11

28.999 CHOC 29.002 �3.2 3427.020 C2H3

C 27.023 �2.9 2741.038 C3H5

C 41.039 �0.6 2619.017 H3OC 19.018 �0.8 2531.015 CH3OC 31.018 �2.8 25

HNOC 31.005 9.757.037 C3H5OC 57.034 3.5 2539.018 C3H3

C 39.023 �4.9 2243.016 C2H3OC 43.018 �1.8 21

(CH3N2C 43.029 �13)



Table 1. (continued)


Explosive (Da) composition(s) m/z (Da) (mDA) intensity (%)

PETN 29.996 NOC 29.997 �1.4 100CH2OC 30.010 �14

45.994 NO2C 45.992 �1.6 50

CH2O2C 46.005 �11

43.017 C2H3OC 43.018 �0.8 12(CH3N2

C 43.029 �1231.017 CH3OC 31.018 �0.8 11

HNOC 31.005 1255.018 C3H3OC 55.018 0.2 1019.017 H3OC 19.018 �0.8 1041.038 C3H5

C 41.039 �0.6 927.022 C2H3

C 27.023 �0.9 929.001 CHOC 29.002 �3.2 857.033 C3H5OC 57.034 �0.5 8

TNT 1.007 HC 1.007 0 (calibration peak) 10041.033 C3H5

C 41.039 �5.6 63CHN2

C 41.013 2043.052 C3H7

C 43.054 �2.2 5839.014 C3H3

C 39.023 �8.9 5155.046 C4H7

C 55.054 �8.2 4457.066 C4H9

C 57.070 �3.9 3927.017 C2H3

C 27.023 �5.9 3429.033 C2H5

C 29.039 �5.6 3451.026 C4H3

C 51.023 3.1 2453.024 C4H5

C 53.039 �15 22

a Possible elemental compositions were determined by comparing the observed m/z values withtheoretical values to an accuracy of 20 mDa. A maximum of four C, N or O atoms were allowed.Species which cannot be found in the NIST Chemistry WebBook are given in parentheses.

Figure 3. Positive low-mass ion PD mass spectrum of (a) a pureprotein sample and (b) an HMX insulin sample. Identities of themost abundant ions are given.

for positively charged quasi-molecularprotein ions. Forexample,the protonaffinity of the quadruplychargedB-chainof insulin (which is themostbasicpartof theinsulinmolecule)hasbeenmeasuredto be 852š 24 kJ mol�1.12

Sincethegas-phasebasicity,which is relatedto theprotonaffinity, of peptidesandproteinsis knownto increasewithdecreasingchargestate(partlydueto Coulombicrepulsioneffects),13,14 it is probablethat C2H4NC donatesa protonif encounteringan insulin molecularion with chargestateC4 or lower. Insulin protonatesup to charge stateC5in explosivematrix-assistedPDMS when a thin analytelayer is used.10 This is consistentwith the proton affini-ties mentionedaboveand indicatesthat small protonatedions,originatingfrom theexplosive,might be involved inthe proteinmolecularion formation.For CL-20 andNC,which alsowork well asmatrices,no C2H4NC peaksareobserved.

HMX and RDX also give fairly intensepeaksat m/z28.02,asdoesCL-20.This ion mostprobablycorrespondsto CH2NC. NeutralCH2N is a possibleintermediateprod-uct from explosive decomposition,as is CH2O whichwas discussedabove.Deprotonationof CH2NC resultsin the formation of CHN, which has a proton affinityof 712.9kJ mol�1 (hydrogencyanide)or 772.3kJ mol�1

(hydrogen isocyanide).Also theseproton affinities arelower thanfor a typical neutralor moderatelyprotonatedprotein, which meansthat CH2NC can act as a protondonor if encounteringsuchspecies.For insulin, protona-tion to charge stateshigher than C5 seemspossibleifonly proton affinity is considered.However, the degree



of protonation of insulin in PDMS is most likely limitedby the number of basic sites in the molecule, similarly toelectrospray ionization.10

In order to investigate further the credibility of pro-tein protonation through reactions with products fromthe explosive decomposition, the low-mass region of anHMX–insulin sample was also studied [Fig. 3(b)]. Thisspectrum is very similar to the pure HMX spectrum(Fig. 2) in terms of possible proton donors, indicating thatproper mixing of matrix and analyte occurs.

The remaining ions listed in the spectra of HMX, RDXand CL-20 are mostly low-abundance hydrocarbon ionsor unidentified species. The fairly abundant ion atm/z 82in the CL-20 spectrum is also observed after EI of thissubstance (see below).15

Comparison with electron ionization of HMX, RDX and CL-20.A comparison of the PD mass spectra of HMX, RDXand CL-20 to with their EI spectra shows that EI andPDMS produce noticeably different mass spectra fromthese substances . After EI, the dominating peak ism/z 46(NO2

C), which can be formed by electron loss combinedwith direct fragmentation of the molecules (Fig. 1). High-abundance NOC ions are also seen. These ions do notnecessarily originate from any decomposition since the

abundant NO2C ion can fragment further to produce NOC.This is supported by the fact that the most abundant ionin the EI spectrum of NO2 is NOC. However, for CL-20 it was noted thatm/z 30 and 28 (designated COC)increase in abundance at slightly higher temperatures,which is explained by decomposition started by the hightemperature.15

NC, PETN and TNT.Of the remaining explosives, NC andPETN produce similar PD mass spectra in terms of thekinds of ions observed, although different from HMX,RDX and CL-20. However, the relative abundances ofthese ions differ in the two cases. In both cases the basepeak is NOC and the second most intense peak is NO2

C(41% of the base peak for NC and 50% for PETN).NO2 is also a possible product from explosive decom-position, although its low abundance in the spectra ofHMX, RDX and CL-20 suggests that the decompositionpathways for NC and PETN are different to those forthese explosives. The difference in behavior of NC andPETN might be explained by the chemical environmentof the nitro groups in these two explosives. Here, NO2is linked to oxygen atoms, in contrast to the other explo-sives where N—NO2 structures are present. However, as

Figure 4. Negative low-mass ion PD mass spectra of all the explosives. Ion identifications were performed by comparing observedm/z values with theoretical values corresponding to different elemental combinations. A mass accuracy of better than 20 mDa wasassumed. For a more complete list, see Table 2.



mentioned above and as seen in the EI spectra, NO2C

can also be formed by direct fragmentation, which meansthat the detection of this ion does not necessarily indicatethat decomposition takes place. Among the lower inten-sity peaks, species such as some hydrocarbon ions andCHOC, H3OC, CH3OC, C3H5OC and C2H3OC are seen.All the latter ions should be good proton donors. Pro-ton affinities for the corresponding neutrals are 594 (CO),691 (H2O), 712.9 (CH2O), 797.0 or 834.1 (C3H4O) and825.3 kJ mol�1 (C2H2O). All of these ions are observedat lower relative abundances for PETN than NC, indicat-ing that PETN should be a poorer proton donor. This isconsistent with the fact that PETN does not work as aPDMS matrix. TNT shows its own particular behavior,giving only hydrocarbon ions at low total abundances.

Comparison with electron ionization of PETN and TNT.A com-parison with EI mass spectra was also performed forPETN and TNT. Unfortunately, no EI spectrum was avail-able for NC. For PETN, the most intense peak observedafter EI ism/z 76. This ion is most likely CH2ONO2

C,which can be formed directly from the molecule by loss ofone of the ‘arms’ (Fig. 1). The second most intense peak

corresponds to NO2C. NOC is not observed. For TNT , themost intense peak is the [M�OH]C ion atm/z 210. Also,intense peaks atm/z 89 and 63 are observed, in additionto a fairly intense NOC peak. No NO2

C is observed in thiscase. In summary, the low-mass regions of all the explo-sives are noticeably different in PDMS and EI, indicatingthat separate processes are under operation.

Negative ion PD mass spectra

Negative low-mass ion PD mass spectra of all the explo-sives are shown in Fig. 4. The main peaks in each spec-trum and possible identities are listed in Table 2. Thespectra from HMX, RDX and CL-20 have a similarappearance with high yields of the nitrogen-containingions CN�, CNO� and NO2

�. The same ions are observedat moderate peak intensities for NC and PETN. However,the most intense peak in the PD mass spectra of the lattertwo substances is the nitrate ion, NO3

�. The observationof this ion is consistent with the structure of these twoexplosives since it can be formed after cleavage of theC—O bond and uptake of an electron (Fig. 1).

Table 2. Relative (peak) intensities of the 10 most abundant peaks in the negative ion PD massspectra of all the explosives studieda


Explosive (Da) composition(s) m/z (Da) (mDa) intensity (%)

HMX 26.003 CN� 26.004 Calibration peak 100(truncation error)

46.000 NO2� 45.994 6.5 88

CH2O2� 46.006 �6.0

42.001 CNO� 41.999 2.5 32C2H2O� 42.011 �10

102.046 C4H6O3� 102.032 14 26

66.017 C3H2N2� 66.022 �5.3 10

62.000 C4N� 62.004 �3.6 7NO3

� 61.988 12129.069 6

40.009 C2H2N� 40.019 �10 6C2O� 39.996 14

25.009 C2H� 25.008 0.6 459.999 4

RDX 45.997 NO2� 45.994 3.5 100

CH2O2� 46.006 �9.0

26.003 CN� 26.004 Calibration peak 75(truncation error)

42.000 CNO� 41.999 1.5 37C2H2O� 42.011 �11

66.010 C3H2N2� 66.022 12 17

102.030 C4H6O3� 102.032 �2.2 15

61.992 NO3� 61.988 �3.6 8

C4N� 62.004 �12129.036 7

25.005 C2H� 25.008 �3.4 640.005 C2O� 39.996 9.5 6

C2H2N� 40.019 �1447.997 C4

� 48.001 �3.5 5

CL20 26.003 CN� 26.004 Calibration peak 100(truncation error)

45.997 NO2� 45.994 3.5 46

CH2O2� �9.0

66.013 C3H2N2� 66.022 �9.3 45

(continued overleaf)



Table 2. (continued)


Explosive (Da) composition(s) m/z (Da) (mDa) intensity (%)

41.999 CNO� 41.999 0.5 43C2H2O� �12

93.026 15133.015 13

61.992 NO3� 61.988 3.6 12

C4N� 62.004 �1289.007 1186.001 940.007 C2O� 39.996 12 8

C2H2N� 40.019 �12

NC 61.993 NO3� 61.988 4.6 100

C4N� 62.004 �1145.992 NO2

� 45.994 �1.5 38CH2O2

� 46.006 �1426.004 CN� 26.004 Calibration peak 1971.021 1142.003 CNO� 41.999 4.5 9

C2H2O� 42.011 �8.145.001 CHO2

� 44.998 2.8 859.016 799.022 525.008 C2H� 25.008 �0.4 572.997 C2H3NO2

� 73.017 20 4

PETN 61.990 NO3� 61.988 �1.6 100

C4N� 62.004 �1445.995 NO2

� 45.994 1.5 28CH2O2

� 46.006 �1126.004 CN� 26.004 Calibration peak 1541.999 CNO� 41.999 0.5 10

C2H2O� 42.011 �1299.007 3

124.987 325.007 C2H� 25.008 �1.4 217.002 OH� 17.003 �1.3 2

1.008 H� 1.008 Calibration peak 268.999 2

TNT 26.003 CN� 26.004 Calibration peak 100(truncation error)

25.008 C2H� 25.008 �0.4 811.008 H� 1.008 Calibration peak 68

48.996 5173.003 3247.995 C4

� 48.001 �5.5 31H2NO2

� �14 �1442.001 CNO� 41.999 2.5 29

C2H2O� 42.011 �1034.971 2159.995 1945.996 NO2

� 45.994 2.5 19CH2O2

� 46.006 �10

a Possible elemental compositions were determined by comparing the observed m/z values withtheoretical values to an accuracy of 20 mDa. A maximum of four C, N or O atoms were allowed.Only the most probable species are listed.

H� and CN� were observed in all spectra and couldtherefore be used for calibration. These ions were alsoobserved in the spectrum obtained from a pure proteinsample, in addition to the CNO� ion [Fig. 5(a)], and theirrole in the explosive assistance process therefore seemsto be of limited importance. NO2� ions, however, arenot observed in the pure protein spectrum, which might

reflect the efficiency of explosives as PDMS matrices,particularly the phenomenon of increasing the averagecharge state of protein molecular ions. It has been sug-gested earlier that the presence of electron scavengers inthe gas phase should increase the survival probability ofpositively charged protein molecular ions.10 The rate con-stant for recombination of such ions with the relatively



Figure 5. Negative low-mass ion PD mass spectrum of (a) a pureprotein sample and (b) an HMX insulin sample. Identities of themost abundant ions are given.

stable negative ions createdby electron scavengingismuchlower (two to threeordersof magnitude16) thanthecorrespondingconstantfor recombinationwith free elec-trons.NeutralNO2, asa possibleproductfrom explosivedecomposition,is a goodcandidatefor actingasan elec-tron scavengerin the mannerdescribedabovesince itselectronaffinity is high (2.3 eV). Suchan action wouldcreatethe observedNO2

� ions andis consistentwith theproposedmechanismfor the explosiveassistance.10 Theabsenceof NO2

� in the spectrumof TNT is anothercon-tribution to anexplanationof why this explosivedoesnotfunction asa PDMS matrix.

The hypothesisthat NO2, originating from the explo-sive, actsas an electronscavengerin explosive–proteinsampleswasfurtherinvestigatedby studyingthelow-massregionof an HMX –insulin samplein negativeion mode[Fig. 5(b)]. NO2

� is presentat high abundancealso inthis case.

Corr elation of the observedions with matrixproperties for NC and PETN

The results for NC and PETN, in both the positiveand negativemodes,are not as clear as for the otherexplosives.However, the fact that NOC and NO2

� arenot the dominant peakssuggeststhat decompositionisnot as pronouncedin thesetwo explosivesas for HMX,RDX and CL-20. The detectionof abundantNO2

C andNO3

� ions,which canbe formedby direct fragmentationwith no needfor a decompositionpathway,supportsthishypothesis.Also, no particularlyabundantions with highprotonationpropertiesare observedfrom NC or PETN.

This picture indicatesthat thesetwo explosivesshouldnot be efficient as matrices.However, NC works wellwhereasPETNdoesnot. Thefact thatNC givesionswithgoodprotonationpropertiesat higherrelativeabundancesmight be part of an explanationbut the solution to theparadoxcanalsobe hiddenin the macroscopicpropertiesof the substances.PETN has a crystalline structure,incontrastto NC, which is a polymer.Earlierstudiesof NCas a matrix in PDMS also indicatedthe high adsorptionpropertiesof this compoundasa majorcontributionto itsefficiency.3

The high efficiency of HMX

The datapresentedanddiscussedabovegive manycluesto themechanismswhich areinvolvedin explosivematrixassistancein PDMS. However, it is still not completelyclearwhy HMX is so muchmoreefficient thanthe othermatrices.As shown, the yield of ions with good pro-tonation properties(C2H4NC and CH2NC) is high fromthis explosivebut the sameis true for RDX, which haspoorermatrix performance.The first study of explosivesubstancesasmatricesin PDMSdiscussedtheimportanceof detonationspeedfor the matrix efficiency.8 A posi-tive correlationwas seensinceHMX hasa higher valuethan RDX (9110 comparedwith 8200m s�1). However,the experimentswith CL-20, which has an even higherdetonationspeed(10100 m s�1), diminishedthe impor-tanceof this parameter.9 Here, it is shownthat the yieldof good proton donors is much lower for CL-20 thenHMX and RDX, suggestingthat both of theseparame-tersshouldhavehigh valuesfor the bestperformance.Apossibleexplanationmaybethat theproton-donatingions(if present)are formed faster in an explosivewith highdetonationspeed.

CONCLUSIONS

Low-massions observedin both positive and negativePDMS of high explosiveshelp to explain why HMX,RDX and CL-20 are efficient matrices.The detectionofabundantNOC and NO2

� ions indicatesthat explosivedecompositiontakesplacein PDMSof thesethreeexplo-sives. Decompositionis a prerequisitefor the chemicalenergy releasewhich is thought to be involved in theexplosivematrix-assistanceprocess.8,10 The observationof abundantC2H4NC andCH2NC ions, which havehighprotonationproperties,might explain the higher chargestatesobservedwith thesematrices.Also, theobservationof NO2

�, possiblyformedby electronscavenging,whichincreasesthe survival probability of positively chargedprotein molecularions, completesthe pattern.TNT doesnot give any of the ions mentionedhereandit is therebypossible to explain why it does not work as a PDMSmatrix. The resultsfor NC andPETNarenot asclearbutit seemsas though decompositionis not as pronouncedin PDMS of thesetwo explosivesand also no particu-larly abundantions with high protonationpropertiesareobserved,indicating that NC and PETN should not beefficient matrices.The fact that NC works well mightbe attributedto other propertiesof this compound,such



as its high adsorption ability or very homogeneous filmformation.

The emerging insight into the mechanisms behindexplosive assistance in PDMS can hopefully be ofvalue in future work concerning the search for new,more efficient matrices for other, more generally usedparticle- or photon-induced desorption mass spectrometrictechniques such as fast atom bombardment (FAB)17 orMALDI. 5

Acknowledgements

This work was supported by the Swedish Natural Sciences ResearchCouncil (NFR) and the International Association for the Promotionof Cooperation with Scientists from the New Independent Statesof the Former Soviet Union (INTAS). The International ScienceProgram at Uppsala University is acknowledged for providing aresearch fellowship for R.V.C. We are also grateful to J. Hansson andH. Ostmark for providing the explosive substances and to J. Kjellbergfor technical assistance.

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low-mass ions observed in plasma desorption mass spectrometry of high explosives

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