Electron Impact Mass Spectrometry of Alkanesin Supersonic Molecular Beams

Shai Dagan and Aviv AmiravSchool of Chemistry, Sackler Faculty of Exact Sciences, Tel Aviv University, Tel Aviv, Israel

The electron impact mass spectrometry of straight chain alkanes CSH1S-C411Hs2' squalane,methylstearate, 'l-chlorohexadecane, I-bromohexadecane, and dioctylphthalate was studiedby sampling them with supersonic molecular beams. A fly-through Brink-type electronimpact ion source was used, utilizing a vacuum background ion filtration technique based ondifferences between the kinetic energy of the supersonic beam species and that of thermalmolecules. The 70-eV electron impact mass spectra of all the alkanes were characterized by apronounced or dominant molecular weight peak together with all the fragment ions nor­mally exhibited by the standard thermal 70-eV EI mass spectra. In contrast, the NIST libraryof most of these molecules did not show any molecular weight peak. By eliminating theintramolecular thermal vibrational energy we gained control over the degree of molecularion fragmentation by the electron energy. At an electron energy of 18 eV the molecular iondissociation was further reduced considerably, with only a small absolute reduction in thepeak height by less than a factor of 2. The effect of vibrational cooling increased with themolecular size and number of atoms. Pronounced differences were observed between themass spectra of the straight chain triacontane and its branched isomer squalane. Similar massspectra of octacosane (C 2sH ss) achieved with 70-eV EI in a supersonic molecular beam wereobtained with a magnetic sector mass spectrometer by using an electron energy of 14 eV andan ion source temperature of 150°C. However, this ion source temperature precluded the gaschromatography-mass spectrometry (GC-MS) of octacosane. The GC-MS of alkanes wasstudied with an ion trap gas chromatograph-mass spectrometer at an ion source temperatureof 230°C. Thermal peak tailing was observed for C 211H 42 and heavier alkanes, whereas forC 2sH s8 and heavier alkanes the severe peak tailing made quantitative GC-MS impractical. Incontrast, no peak tailing existed even with C 411H s2 for GC-MS in supersonic molecularbeams. The minimum detected amount of eicosane (C 211H 42) was shown to be 60 fg. This wasdemonstrated by using single ion monitoring with the quadrupole mass analyzer tuned tothe molecular weight peak of 282 u. The coupling of electron impact mass spectrometry insupersonic molecular beams with hyperthermal surface ionization and a fast GC-MS inlet isbriefly discussed. (J Am Soc Mass Spcctront 1995, 6, 120-13])

Electron impact (ED is the major ionization methodused in mass spectrometry and gas chromatogra­phy-mass spectrometry (GC-MS). EI is a rela­

tively Simple ionization method, yet it provides highsensitivity and a uniform semiquantitative response toall molecules and atoms. In addition, the rich fragmen­tation pattern observed, combined with the very richlibraries of 70-eV EI mass spectra, result in substantialmolecular identification and structural informationcapability.

Despite its strength, EI suffers from several limita­tions:

1. In many important cases the molecular weight peakis either unobserved or it is too small to be reliable.

Address reprint requests to Professor Aviv Arnirav, School of Chem­istry. Sackler Faculty of Exact Sciences, Tel Aviv University, RamalAviv 69978, Tel Aviv, Israel.

© 1995 American Society for Mass Spectrometry1044-0305/95/$9.50SSDl1044-0305(94){)0095-H

2. The ion source must be heated to reduce the re­sponse time, and the ion source metal surface caninitiate thermal or catalytic molecular and molecu­lar ion decomposition.

3. Relatively nonvolatile molecules undergo lengthycycles of thermal adsorption-desorption in the ionsource that result in gas chromatography (GC) peaktailing. Eventually, this peak tailing dictates theupper limit of molecular volatility in GC-MS.

We note that these three limitations are interrelated.The actual ion source temperature is set to a compro­mised value of - 150°C for direct sample insertionand 230-250 °C for GC-MS. Although increasing theion source temperature can decrease its response timeand ability to handle relatively nonvolatile molecules,it also substantially reduces the observed relativeheight of the molecular weight peak and promotescatalytic and thermal molecular decomposition.

Received June 1, 1994Revised September 7, 1994

Accepted September 7,1994

J Am Soc M.1SS Spectrorn IYY5. b. 120-131 ElMS OF ALKANES IN SUPERSONIC BEAMS 121

This compromise can be eliminated by the use ofsupersonic molecular beams [1-3] for molecular sam­pling in a slightly modified EI ion source. Supersonicmolecular beams (SMB) [4-8] are characterized byunidirectional motion with controlled hyperthermalmolecular kinetic energy (0.1-30 cv). intramolecularvibrational-rotational supercooling, mass focusing asin an ideal high load jet separator, a very high flowrate in the range of 100-500 ml.yrnin, and a conve­nient method for introducing samples at atmosphericpressure.

Supersonic molecular beams are used in mass spec­trometry predominantly in conjunction with laser­induced multiphoton ionization followed by time-of­flight mass analysis [9-14]. In these experiments thevibrational supercooling is used mostly to improve thewavelength selectivity in resonance-enhanced two ormultiphoton ionization schemes.

In a supersonic molecular beam, the vibrational­Iy supercooled molecules undergo a single passagethrough the "fly-through" EI ion source. Thus, the lowmolecular vibrational temperature is independent ofthe ion source temperature, the molecular weightpeak height is substantially increased, and molecular­thermal decomposition and adsorption-desorption onthe ion-source walls is avoided.

Amirav and Danon [2] have demonstrated the con­siderable effect of vibrational supercooling encoun­tered in 5MB on the electron impact mass spectrome­try (EI-MS) of bromopentane isomers. This effect wasalso studied in cholesterol [15] and various isomers ofoctane [3]. In these studies electrostatic backgroundion filtration [2, 3, 16, 17] was used. This method isbased on the observation that in a seeded supersonicmolecular beam, when hydrogen or helium is used asthe carrier gas, heavy organic molecules acquire hyper­thermal kinetic energy. The ions formed in the ion cagepossess a kinetic energy that is the sum of their electri­cally given ion energy and their directional hyperther­mal kinetic energy. Because the thermal molecules ofthe vacuum background have only small « O.l-eV)random kinetic energy, the use of a simple smallretarding field along the ion trajectory results in effec­tive background ion filtration. This effect was used formass spectrometry of atoms and small molecules toimprove the beam signal-to-vacuum background ratio[16,17]. We have found that this procedure could havea major impact on organic mass spectrometry for thefollowing reasons: (1) Background ion filtration alsoeliminates the self-background of sample moleculesscattered from the ion source and vacuum chamberwalls. This self-background elimination uncovers thegenuine mass spectra of the vibrationally coldmolecules. (2) Eliminating the self-background alsoresults in a very fast ion source « 1 ms) response timeregardless of the molecular volatility because any sur­face-scattered molecule loses its kinetic energy and isfiltered out after ionization. (3) Because only moleculesin the 5MB are analyzed, the electron emission fila-

ment can be close to the ion cage and errussion cur­rents over 10 mA can be routinely used. Therefore,increased ionization emission current and reducedbackground can compensate for the loss of sensitivityassociated with the fast passage and lack of multiplescattering of the molecule in the EI ion source.

Although the combination of EI and 5MB alreadyhas been demonstrated, no detailed study of its mani­festation in organic mass spectrometry is available.This paper presents our results from the systematicstudy of the homologous alkane series CRH1/l-C40H /l2and their derivatives. These molecules have the samecharacter and their size and vibrational heat capacitysystematically increase. This group of molecules waschosen because it represents molecules that are impor­tant to petrochemical mass spectrometry, whose massspectra usually do not exhibit a molecular weight peakat the 1% relative height scale. In addition, chemicalioniza tion is less effective with alkanes both in termsof extensive fragmentation and also because of re­duced sensitivity that results from their relatively smallproton affinity.

Experimental

In Figure 1, the supersonic molecular beam-massspectrometry (SMB-MS) apparatus is shown (upper),together with an expanded view of the inlet, whichalso serves as a fast gas chromatograph (lower). Thecarrier gas (H 2 or He) is introduced via needle valveVI at a flow rate of about 120-150% above the flowrequired by the column. The excess gas flows to theatmosphere and serves as a "septum flush". The sam­ples are introduced as in a direct insertion device fromthe lower portion of the vaporizing oven in a conven­tional small test tube. No air lock and bypass pumpingis required, and the upper entrance remains open tothe atmosphere. The temperature gradient in the inletallows easy control of the sample vaporization ratewith mass spectrometry feedback. The upper openingserves for fast GC injection [18] with conventionalsyringes such as the SGE 1.0-JLL syringe (SGE, Aus­tralia). Manual injection is fast and simple because noseptum has to be penetrated. Only below a certainpenetration depth of the syringe into the injector doesthe carrier gas vaporize the injected amount and sweepit into the column (above this level the sample is sweptoutside as in septum flush devices). The injector istemperature controlled up to 500 °C, and the injectorliner is made either of Vespel'" or glass. All seals aremade of graphite.

The injector is mounted on a 3/4-in. stainless steeltube that contains a 50-cm-long, 0.53-mm-i.d. mega­bore column [DB-lor DB-5MS, 1.5-JL film thickness(J & W Scientific, Folsom, CA)]. The GC column isseparately heated and its temperature is controlled. Ata relatively high temperature it merely serves as atransfer line for the vaporized molecules. The outputof the column is mixed with make up gas (helium or

122 DAGAN AND AMIRAV J Am Soc Mass Speclrom 1995. 6, 120-131

Figure 1. The sample inlet and mass spectrometer apparatus.The carrier gas flow He or H 2 , is controlled by needle valves 0)and (2). The sample is injected or introduced into the vaporizingoven (3) and flows through the temperature-controlled capillarycolumn (4). The gas mixture expands through a supersonic noz­zle (5) and is double differentially pumped. The sample moleculesare ionized in the EI ion source (6), deflected by 90° by the iondeflector (7) and mass analyzed by the quadrupole mass spec­trometer (8). The molecular beam also can be square wave­modulated by the beam chopper (9). The data are processed andpresented on a PC (10). The lower section shows an expandedview of the fast gas chromatograph and sample inlet.

diffusion pump) and enters the electron impact ionsource (6) in the high vacuum mass spectrometrychamber (63-mm diffusion pump). The nozzle can bealigned (XYZ) with respect to the skimmer from out­side the vacuum chamber to maximize the beam inten­sity as measured by the increase of pressure in themass spectrometry chamber.

The supersonic molecular beam is ionized by thefly-through Brink-type [20] EI ion source (6), which isopen both at the entrance and exit of the 5MB. Theionization cage is 25 mm long and 5 mm in diameter.The filament is made of a O.2-mm-diameter thoriatedtungsten wire purchased from Vacuumetrics. The ionsare extracted by the ion optics attached to the EI ionsource and guided by 90° deflection (7) into aquadrupole mass analyzer (8) [VG-SXP 600 (VG Ana­lytical Ltd., Manchester, UK)]. This quadrupole massanalyzer does not have a prefilter element and exhibitsa significant reduction in ion transmission with mass.We measured an exponential transmission reductionfactor of about 60 between masses 69 and 502 underconventional unit mass resolution conditions. The orig­inal commercial EI ion source was removed from themass analyzer and its entrance contains an ion opticslens system.

Between the quadrupole mass analyzer and thechanneltron ion detector there is a circular plate with a3-mm hole denoted "exit lens". This plate is externallyvoltage biased to filter out ions produced from thethermal molecules of the vacuum background, and ittransfers only ions produced in the hyperthermal 5MB,which are distinguished by their higher kinetic energy[2, 3, 16, 17]. As mentioned, in addition to eliminatingthe vacuum background, the background-free EI ionsource facilitates ultrafast response time and exposesthe mass spectrum of vibrationally cold molecules.

The 5MB also can be square wave modulated by abeam chopper (9), which can be inserted into the beamtrajectory or removed. This beam modulation allowsthe use of lock-in amplification for background sub­traction when conventional EI is used.

The 900 ion deflector can also be moved upward onan XYZR vacuum manipulator and be replaced by asuitable surface for hyperthermal surface ionization(HSI) [3, 21, 22] of the hyperthermal beam species.

Compatibility with standard GC was investigatedby using a conventional gas chromatograph [CarloErba (Milan, Italy) Vega 6000] coupled to the fast gaschromatograph via a homemade, separately heated,transfer line. The GC column (Ifi-m megabore J& WDB-I; 1.5-p. film thickness) ended inside the injectornear the fast GC column, which served as a transferline at that time by heating it to above the conven­tional GC column temperature. We used the ShraderSystem TM as the GC-MS software and for most of thedata presentation.

Some additional important dimensions are thenozzle-skimmer distance, which is 5-8 mm, theskimmer-surface (or ion deflector) distance of - 60

sc AND SYRINGEINLET

M5 in 5MB

AT".OSPHERIC PRESSURE SAMPLING

VACUUM

FAST GC

HEATERNOZZLE \

hydrogen) provided by needle valve Vz. This needlevalve controls the pressure behind the nozzle and thusaffects the flow rate through the column. When Vz isclosed the linear velocity of the carrier gas is about2000 cmys, whereas when it is open the column flowrate can be regulated to any value, including a reverseprotective flow. The supersonic beam nozzle is anall-ceramic nozzle with either a 100-p.m pinhole in awatch jewel [19] or a I-mm-long channel that has adiameter of 105 p.m drilled in alumina (Kulicke andSoffa-Israel), Both the gas mixing zone and the nozzleare separately heated. The carrier gas mixture and thesample molecules expand through the nozzle (5) (inthe upper scheme), into the first vacuum chamber,which is pumped by a 4-in. diffusion pump [VarianVHS 4 (Varian Associates, Lexington, MA) backed by a550-L/min rotary pump]. The emerging supersonicfree jet is skimmed and differentially pumped (63-mm

J Am Soc Mass Speclrom 1995. 6, 120-131 ElMS OF ALKANES IN SUPERSONIC BEAMS 123

Figure 2. EI mass spectra in 5MB of octacosane and a compari­son with the NIST library thermal 70-eV mass spectrum (trace c).Trace a was obtained in 5MB with 18-eV electron energy. Trace bwas obtained with 70-eV electron energy.

Octocosone- C28H58394

A .IBeV-SMB

B.70eV-SMB 394

57M

71

4j r k1/1111

M'2,.

43C. 70eV-L1BRARY57

71

C

a molecular weight of 394 u. Octacosane and two of itsisomers appeared to be the best three candidates. Onthe other hand, we failed to locate octacosane throughthe mass spectrum obtained from an ion trap GC-MS[Finnigan ITO model 8100 (Finnigan-MAT, San Jose,CA)] with the standard library search algorithm. Thesesearch results demonstrate that when the molecularweight peak is a prominent mass spectrometry peak,the chances to obtain high confidence level librarysearch results are increased. This observation emergesfrom the existence of all the usual fragments with onlysmall changes in their relative peak heights. This resultis rationalized by noting that the molecular weight ioncan only dissociate, whereas all other peak heights canbe increased by the dissociation of a higher mass peakand can be decreased by their own fragmentation.Therefore they are in a quasi-steady state. The molecu­lar weight peak in octacosane is increased by morethan 3 orders of magnitude compared with thermal70-eV EI mass spectrum, whereas the peak heightsfrom other fragments changed only slightly.

Because in 5MB the thermal intramolecular vibra­tional energy is removed, the electron energy is theonly parameter that governs the degree of fragmenta­tion. The effect of the electron energy on the degree offragmentation of the molecular weight ion is wellknown. However, the effect of reduced electron energyto achieve a noticeable increase in the relative molecu­lar weight peak height also is associated with a consid-

rnm, the beam diameter of - 3.5 mm in the EI ionsource, and the 900 ion deflector of 15 mm from thequadrupole mass spectrometer entrance lens.

Vibrational Cooling Effect on the ElectronImpact-Mass Spectrometry of Octacosane

Vibrational temperature effects on EI-induced ion frag­mentation are well known [23-28] including in longchain alkanes [29]. In many books on mass spectrome­try , at least one example of the pronounced effect ofthe ion source temperature on the degree of ion frag­mentation appears [30, 31]. The subject of low electronenergy, low temperature EI-MS is reviewed by Maccolland references therein [32].

Octacosane [CH3(CHz)Z6CH3] is a relatively longstraight chain alkane molecule that has 86 atoms. It is asolid at room temperature and has a boiling point of705 K. Octacosane, like all the other long chain alkanes,is known to exhibit a mass spectrum that consists oflow mass fragment ions without a molecular weightpeak. The high temperature limit of the vibrationalheat capacity of octacosane is 11.4 eV at an ion sourcetemperature of 250 °C, whereas it is estimated, basedon model calculations [33], that the actual heat capac­ity is about 3.5 eV. This intramolecular vibrationalenergy, together with the electron imparted vibrationalenergy, is sufficient to break the majority of all molecu­lar ions. In a supersonic molecular beam, however, weanticipated that this thermal contribution would bepractically eliminated. In Figure 2 the electron impactmass spectra of octacosane are shown. In trace a, the18-eV electron impact mass spectrum is plotted. Intrace b the 70-eV mass spectrum is shown. The lowertrace (c) was taken from the NIST mass spectra libraryand it represents the standard 70-eV EI mass spectrumof thermal molecules at unspecified ion source temper­ature (probably in the range 150-250 0C). In contrast totrace b, trace c does not show any molecular weightpeak nor does it show any other high mass fragmentpeaks. In fact, the molecular weight peak does notexist in the library list of ions even at the level of 0.1%.On the other hand, the 70-eV EI mass spectrum in asupersonic molecular beam is characterized by a domi­nant molecular weight peak that is the most prominentfeature in the mass spectrum. The expanded massscale near the molecular weight shows that all themolecular weight peaks exist, and their relative heightsreflect 13C natural abundance of 1.08 ± 0.02%, which isin good agreement with the literature value of 1.10 ±0.03%. Thus, one can obtain the elemental informationdirectly from the relative peak height ratios and deter­mine the empirical formula. We note that the mass 394u reflects the genuine molecular weight without anychemical ionization-related process of proton transfer.

We used a library search to identify the mass spec­trum of trace b by eliminating its molecular weightpeaks and by limiting the search to molecules that had

20 120 220 320

MASS(omu)

420 520

124 DAGAN AND AM IRAV J Am Soc Mass Spectrum 1995,6,120--131

562I

450

338

L:'H"226 -----I

220 320 420 520

MASS (o rnu)

oscillator. In Figure 3 it is observed clearly that eachion has a different normalized electron energy depen­dence. We also observe that the smaller the ion frag­ment is, the weaker is its dependence on the electronenergy. This observation suggests that fragmentationoccurs via a chain of fragmentation events, each to aclose, but lower mass. It is believed that the measure­ments outlined in Figure 3 contain structural and ener­getic information, but this subject is beyond the scopeof this article.

85

70eV (I-SMB

Effects of the Length of the Alkane Chainon its Mass Spectrum in SupersonicMolecular Beams

114

Because the vibrational heat capacity depends approxi­mately linearly on the number of atoms in the molecule,one can expect an increase of the vibrational coolingeffect with the length of the alkane chain.

We have studied the mass spectra of alkanes CHH1Hthrough C 4oH H2, and in Figure 4 the 70-eV EI massspectra in 5MB of the indicated straight chain alkanesare portrayed. All these mass spectra are characterized

Figure 4. 70-eV electron impact ionization mass spectra in su­personic molecular beams of the indicated straight chain alkanesCMH1M-C40HR2'

80

oElo-

oZ

(J)

uco-0c~

.o«-0(J)

N

erable reduction in the EI ionization cross section. Wehave measured an EI ionization cross section reductionfactor of 8 at 18 eV, in comparison with 70-eV electronenergy. In trace a we show that at 18 eV, the EI massspectrum in 5MB is dominated almost totally by themolecular weight peaks, whereas all other fragmentpeak heights are less than 5% of the normalized scale.More important is the observation that this consider­able simplification of the mass spectrum was achievedwhere the absolute height of the molecular weightpeak was reduced only by a factor of 1.5. This observa­tion results from the demonstrated degree by whichthe molecular ion dissociation is reduced, which com­pensates for the reduced ionization cross section.

We believe that the ability to achieve EI mass spec­tra that consist almost entirely of the molecular weightpeak, without losing sensitivity, is of considerable im­portance. If any molecule in a complex mixture ex­hibits a single or very few mass spectral peaks, thiscould considerably extend the ability of GC-M5 tohandle its analysis in complex mixtures, because boththe mass spectrometer and the gas chromatographcould serve as orthogonal separation tools with simpli­fied deconvolution of overlapping GC peaks.

In addition, because the electron energy alone deter­mines the degree of fragmentation of the various ions,this dependence can be explored and monitored. InFigure 3 we plot the normalized abundance of theindicated fragment and molecular weight ions versusthe electron energy. Each ion peak height was normal­ized to its maximum value in the range of 10-80 eV.The electron energy was scanned by programming theelectron energy power supply through a slow ramping

Figure 3. The dependence of the normalized octacosane frag­ment ion peak heights on the electron energy.

J Am Soc Mass Spectrom 1995. 6. 120-131 ElMS OF ALKANES IN SUPERSONIC BEAMS 125

by a dominant or strong molecular weight peak. Infact, the molecular weight peak is always the strongestpeak in the mass spectra. However, these mass spectraare distorted due to the substantial reduction in trans­mission at high masses of our quadrupole mass ana­lyzer. In addition, a very long progression of peaksevery 14 u (CH z) is observed, which extends to frag­ments heavier than those in the library mass spectra ofthese compounds. Note that the library mass spectraalso do not show a molecular weight peak for most ofthese alkanes.

It seems, therefore, that electron impact mass spec­trometry in supersonic molecular beams exhibits theinformation content usually achieved by electron im­pact together with chemical ionization (Cl) mass spec­tra in one scan. However, CI provides a degree ofselectivity against alkanes and in favor of compoundsthat have high proton affinity. Moreover, the molecularweight peaks in EI provide elemental and isotopicinformation through the relative M+, [M + 1]+, and[M + 2]+ peak height rat ios.

In Figure 5 we show the EI mass spectra in 5MB ofthe same alkanes as in Figure 4, obtained at 18-eVelectron energy. It is clear that as the length of the

18eV EI-SMB

85 114

A .C8 HI8

,iI170

B . CI2H26

IIII226

C . CI6H34

338

D. C24 H50

450

E .C32H 6 6

562

F. C40H82

20 120 220 320 420 520MASS (ornu )

Figure 5. 18-eV electron impact ionization mass spectra in su­personic molecular beams of the indicated straight chain alkanesCSHlS -C40Hs2'

alkane chain increases, the 18-eV EI mass spectrum isdominated further by the parent undissociated molec­ular weight peak. For certain noncritical applications,one can say that above an alkane chain length of C16'

the 18-eV EI mass spectra are essentially characterizedby a single molecular weight peak and only smallindication of other fragments . This claim is furthersupported by consideration of the strong transmissionreduction of our quadrupole mass analyzer with mass.

As mentioned before, this simplification of the massspectra at reduced electron energies also involves areduction in the ionization cross section by a factor of 8at 18 eV in comparison with 70 eV (these data wereobtained by total ion current measurements normal­ized to the emission current in a DuPont magneticsector mass spectrometer). However, this reduction inthe ionization cross section is compensated by thesubstantially reduced degree of fragmentation encoun­tered in EI mass spectra in 5MB. In Figure 6, thereduction factor of the molecular weight signal at 18eV in comparison with that of 70-eV EI mass spectra isplotted versus the chain length of the straight alkane.We note that the electron energy value is accurate towithin ± 1.5 V due to a voltage gradient on the elec­tron emission filament. From Figure 6 we observe that(1) the reduction of M +, and thus the reduction indetection sensitivity, is much smaller than the reducedionization cross section by a factor of 8; (2) for alkaneswith over 12 carbon atoms, this reduced sensitivity isless than a factor of 2; (3) the factor by which thesensitivity falls is reduced with the length of the alkanechain and approaches unity (no reduction) for longchain alkane molecules. We conclude therefore, that in5MB the simplification of the mass spectra that resultsfrom lowering the electron energy is achieved with asmaller penalty in sensitivity as compared with ther­mal electron impact mass spectrometry.

Comparison of Figures 4 and 5 indicates that al­though the effect of reduced electron energy on themass spectra is relatively small in octane, it graduallyincreases with the length of the alkane chain. In Figure7, we plot the ratio of the molecular weight peak to the57-u prominent fragment peak achieved at 18-eV elec­tron energy, divided by the same ratio achieved at anelectron energy of 70 eV. This normalized ratio is ameasure of the relative degree of control over thedegree of fragmentation achieved by the electron en­ergy. Note that this measure is normalized and unaf­fected by inconsistencies in the response of thequadrupole mass analyzer with the ion mass. Figure 7demonstrates a purely exponential increase of the de­gree of control (C) over the fragmentation achievedwith increased chain length of the alkane. Althoughthe mass spectrum of octane is hardly affected by theelectron energy in the ranges 18-70 eV, where C ::::; 1.2,the degree of control is increased to more than 50 forthe C 36 and C 40 alkanes. The small deviation of C 4oH82

from the straight line is attributed to incomplete cool­ing in the 5MB.

126 DAGAN AND AMlRAV

2.8

2.6 ..2.4

;;- 2.2u00......~

2::E-.;;- 1.8 ..u

0 ..t:,

1.6::E ....1.4 ..1.2 .. ..

1+----~-~-.______,_-......____,,.____r-.___.-__14 8 12 16 20 24 28 32 36 40 44

Number of Carbon Atoms

Figure 6. The ratio of the height of the molecular weight peaktaken with 70- and 18-eV electron energy, plotted against thenumber of carbon atoms of straight chain alkanes C~H 1~-C.lIH6Z'

The observation shown in Figure 7 can be rational­ized by using the following assumptions: (1) The vi­brational energy given by the electron to the moleculeis independent of the length of the alkane chain both at18- and 70-eV electron energies (localized ionization);(2) the amount of intramolecular vibrational energy isnegligible and can be ignored in comparison with thevibrational energy imparted by the electrons. By usingthese reasonable assumptions one finds that the amountof vibrational energy is molecular size independent,

100..------------,--,

........;>.,0t:,ell~..~.......::E....... 10

;>.,00-......ell

'"..~::E

1+--.-L..,----r-..-----.----r-____,-.---,-_.---14 8 12 16 20 24 28 32 36 40 44

Number of Carbon Atoms

Figure 7. The ratio of the molecular weight peak divided by the57-u fragment peak height achieved at 18- and 70-eV electronenergies, plotted against the number of carbon atoms of straightchain alkanes CgHlS-C.oHgz.

J Am Soc Mass Spectrom 1995, 6, 120-131

but the amount of vibrational energy per atom isreduced linearly with the number of atoms (chainlength).

Accordingly, as the molecular size increases, itsdissociation to small fragments such as the 57-u frag­ment is less likely, especially in view of the consecu­tive fragmentation processes implied from the resultsshown in Figure 3. If we further assume an exponentialdependence of the rate of ion fragmentation on theexcess vibrational energy and number of atoms, thedegree of fragmentation will be reduced exponentiallywith reduced intramolecular vibrational energy con­tent per atom at lower electron energy. In addition, theratio of these reduction ratios also will behave in anexponential fashion as is demonstrated in Figure 7.Although the foregoing conjectured explanation maynot be based on theoretically solid arguments, ourexperimental results clearly demonstrate increasedcontrol over the degree of molecular ion fragmentationwith electron energy for larger molecules. We notethat the extrapolation of our measurements to largeralkanes should result in the appearance of the molecu­lar ion exclusively even at 70-eV electron energy. Infact, if we correct our results for the reduced sensitiv­ity of our quadrupole mass analyzer with increasedmass, the molecular weight of C 4oH H2 at 562 u will behigher than all other fragments by a factor close to 20in the 70-eV electron energy mass spectrum.

Up to now we have addressed the question of thevibrational supercooling effect on the EI mass spectraof straight chain alkane molecules. In Figure 8 the70-eV EI mass spectrum in 5MB of the straight chaintriacontane molecule (C 30H 62 ) is compared with thatof its branched isomer squalane. Pronounced isomermass spectral differences are observed and new highmass major peaks appear through the enhanced frag­mentation at the branching points that form secondaryions. A closer examination of the NI5T library massspectra of these molecules reveals that these isomereffects exist even in the thermal mass spectra, but theyare considerably amplified in the EI-M5 of the vibra­tionally cold molecules in the 5MB. The mass spectrumof squalane contains relatively abundant ions at 182,238, 266, and 337 u as well as neighbors to thesemasses. These ions are not straightforward cleavagesof the carbon chain and are hard to explain. Again allthese ions appear in the NI5T library, whereas in 5MBtheir relative abundance is largely amplified. In Figure9 the 70-eV EI mass spectra in 5MB of the indicatedalkane derivatives are shown. Figure 9 demonstratesthat the feature of a considerably enhanced molecularweight peak is universal to all molecules. In fact, thegeneral trend we observed in these and many othermolecules [2, 3, 15, 20] is that the smaller the molecu­lar weight peak is in thermal 70-eV EI-M5, the larger isthe enhancement achieved by vibrational cooling in5MB. Methylstearate exhibits a 21% molecular weightpeak in the library (NI5T) thermal 70-eV EI massspectra. In the 5MB its relative peak height is increased

J Am Soc Mass Spectrom 1995. 6. 120-1 31 ElMS OF ALKANES IN SUPERSON IC BEAMS 127

4H

70 eV EI- SM B-MS of (30 H62 70 eV EI-SM8 298

"'.

A. TriacontoneA .Methylsteorote

7487

14 3

I I

8 .1,Chlorohexodecone260

91

262

135

C. I , 8romohexodecane238

B. Squalane

T14 9

422

D.Dioctylphtholote279

167

71

Mass (amu)

Figure 8. 70-eV electron energy EI mass spectra of the straightcha in triacontane (a) and its branched isomer squalane (b) .

by about 1 order of magnitude. 1-Chlorohexadecaneand 1-bromohexadecane do not show any molecularweight peaks in their thermal 70-eV EI mass spectraand thus an enhancement of at least 3 orders of magni­tude in peak height is obse rved. Dioctylphthalate in5MB exhibits only a 4% molecular weight peak, whichis the lowest we have measured to date. We havestud ied the 70-eV EI mass spectrum of dioctylphtha­late in a DuPont magnetic sector instrument and foundthat at an ion source temperature of 250 °C, the relati veheight of its M + peak is - 2 X 10- 6 of the 149-u peak,and that the [M + 1] peak is higher than expected ,probably due to the ionization of dimers or due toresidual chemical ion ization. Thus, the small M+ peakof dioctylphthalate in EI-SMB represents an increase ofover 4 orders of magnitude in the relative height of themolecular weight peak.

Figure 9. 70-eV electro n ene rgy El mass spectra in 5MB of theindicated alkane der ivati ves .

Tradeoff of Response Time, Sensitivity,and Relative Molecular WeightPeak Height

The effect of the vibrational temperature on the degreeof molecular ion fragmentation is a very well knownand established phenomenon [23-33]. Reduction of theion source temperature indeed improved the molecu­lar ion pea k appearance , but involved a tradeoff oflowered ion source response time and reduced cleanli­ness. The relative height of the molecular weight peakalso can be increased through the use of a better massanal yzer. A magnetic sector, in contrast to a quadrupolemass analyzer, does not exhibit a marked reduction insensitivity with increased mass and enhances the ap­parent M + peak height. Another way to increase therelati ve height of the molecular weight peak is toreduce the electron energy with the tradeoff of detec­tion sensitivity . In Figures 10 and 11 we demonstratethese three methods and discuss the tradeoffs in­volved. In Figure 10 mass spectra of octacosane(C2sH ss) measured by a magnetic sector mass spec­trometer are shown. They were taken at the indicated

170 220 270 320 370

MASS (ornu )

12070

5,7" d J

20

182

11:3

127 266

155 237

t j It I I T IDO 180 230 280 330 380 430

j

~7

30 80

128 DAGAN AND AM IRAV

C28 H S8 Thermal EI-MS

J Am Soc Mass Spectrom 1995, 6, 120-131

ALKANES GC - MS

A .GC-ITD-MS

230°C

Magnetic Sector

A. T1s=230°C

Ee=18eV

o 5 10Cl6

15 20TIME(m in}

25 30 35

394

B. TIS =ISO°C

Ee= 18eV

o

B GC-SMB-MS

2 3TIME (m in)

4 5

c.Tls=ISO°C

Ee=14eV

o

C FAST GC-SMB-MS

8 10

L.U.,LL.,........y...u,..I...L~+-'-.J+LJ..,JLJ..--.

40 120 200 280 360 440MASS(amu)

Figure 10. Electron impact ionization-magnetic sector massspectrometry of octacosane achieved with a thermal ion source atthe ind icated ion source temperatures and electron energies.

electron energies and ion source temperatures. In con­trast to the library 70-eV EI mass spectra, the magneticsector instrument does provide a 6% normalizedmolecular weight peak at 18-eV electron energy and anion source temperature of 230 °C as shown in trace a.This increased M + peak is the result of the flat massresponse of the magnetic sector mass analyzer as wellas the effect of reducing the electron energy to 18 eV.However, the tradeoffs are both in price and complex­ity through the use of a magnetic sector instrumentinstead of a quadrupole one and reduction of theionization cross section (sensitivity) at the reducedelectron energy by a factor of 8. We note that reducedelectron energy cannot be achieved with an ion trapmass spectrometer unless an external ion source isused. In trace b, the effect of the ion source tempera­ture is demonstrated by its reduced temperature from230 (trace a) to 150 °C. The molecular weight peak isnow strongly increased to about 90% of the normal-

Figure 11. GC-MS and fast GC-MS of a mixture containingC8H 18-C40H S2 alkanes achieved us ing the following instru ­ment s: (a) Commercial Finni gan model 8100 ion trap gas chr o­matograph-mass spectrometer with a 30-m narrow bore capil­lary column. (b) Superson ic molecular beam mass spectrometrycombined with a gas chromatograph with a 4-m narrow borecolumn <injection at 100 °C and ramping at 49 °Cjmin to 300 °c).(e) Fast GC-MS in a supersonic molecular beam, achieved withthe fast gas chromatography interface shown in Figure 1.

ized scale. This is ach ieved together with the addi­tional penalty that at 150 °C octacosane cannot bequantified with GC-M5 due to a severe loss of ionsource response time and associated peak tailing. Intrace c the magnetic sector mass spectrum of octa­cosane was measured at an ion source temperature of150 °C and 14-eV electron energy. At this electronenergy the ionization cross section is lower than at 70eV by a factor of 30, but now the apparent massspectrum seems similar to that of octacosane achievedwith our quadrupole mass analyzer with 70-eV EI-M5in 5MB, as shown in Figure 2. Therefore, from Figure10 it can be concluded that the use of supersonicmolecular beams for 70-eV EI-M5 can provide massspectra like those achieved by magnetic sector instru­ments at low electron energy and reduced ion sourcetemperature, without the involved penalties of re­duced sensitivity, much longer ion source response

J Am Soc Mass Spcclrom 1995,6,120-131 ElMS OF ALKANES IN SUPERSONIC BEAMS 129

time, and increased complexity of the mass analyzer.Among these three features we believe that the ionsource response time is the most significant because itconcerns the mere ability to perform GC-M5 experi­ments.

In Figure 11 we compare a total ion chromatogramobtained by GC-MS (Finnigan ITD 8100) with a con­ventional ion source and a chromatogram taken withsupersonic molecular beam mass spectrometry. A mix­ture of C

IIH 2 11 + 2 (11 = 8,12,16,20,24,28,32,36,40)was injected into the gas chromatograph. It is observedclearly that the ion trap response is characterized byextensive chromatographic peak tailing for the C 24H 50

and heavier alkanes. A closer examination revealedthat even the eicosane (C 2oH 42 ) exhibited noticeableion source-related peak tailing. As shown in the ex­panded scale ( X 16), the extensive peak tailing rendersGC-MS measurement of C 32H 66 , C 36H 74 , and C 4oH R2

molecules impractical. In addition, the peak tailingfound with C 2oH 42 , C 24H 50 , and C 2RH 4R results in areduction of detection sensitivity and hampers quanti­tative measurement by considerably poorer repro­ducibility, as the exact amount of peak tailing maychange due to long term thermal instabilities of the iontrap. Trace b shows the EI-SMB GC-MS of the samealkane mixture by using a Carlo Erba Vega 6000 gaschromatograph with a 4-m narrow bore column (250-jLi.d. J& W DB1; 0.25-jL film thickness). No peak tailingis observed although the chromatographic peak heightsabove hexadecane are reduced due to chromatographicpeak broadening and due to the reduced response ofthe quadrupole mass spectrometer at higher masses. InFigure 11c we also demonstrate the ultrafast GC-~S[18] of a similar mixture of C 24-C 40 alkanes. In thiscase, with the help of flow programming after theelution of octacosane, the GC-MS trace ended after 8 sand even the C 4oH R2 alkane molecule was character­ized by a l-s tail-free peak width. Thus, we concludethat, as expected from the use of background ionfiltration, no tailing exists and the full temperature andvolatility limit of the gas chromatograph can be ex­ploited without mass spectrometric-related limitations.In addition, we note that we have examined the ex­panded mass spectra of hexadecane and eicosane ob­tained with the ion trap gas chromatograph-massspectrometer and failed to find even a sing~e ion. attheir molecular weight masses at the conventional IOnsource temperature used (230 °C).

Detection Sensitivity with Electron ImpactMass Spectrometry in SupersonicMolecular Beams

One of the most important parameters of any massspectrometry ionization method is its associated detec­tion limits. However, detection limits are usuallyquoted for a given molecule and experimental condi­tions that do not reveal the other interrelated parame-

ters. Electron impact in supersonic molecular beams isdifferent from conventional EI in several importantaspects that affect the detection limits:

1. The supersonic molecular beam is inherently associ­ated with jet separation, which reduces the molecu­lar flux by a factor in the range of 2-20. The heavierthe molecule is, the smaller is the reduction factor.On the other hand, the very high flow (240 ml.yrnin)jet separator allows fast sampling of very largesplitless injections and thus actually improves theachievable minimum detected concentration limit.

2. The molecules pass the ion source in fast motiononly once instead of an estimated number of 10-20cycles of intraion source adsorption-desorptionrounds of thermal molecules which are 10 timesslower. This means that the molecular ionizationprobability is smaller in the 5MB by a factor of100-200 times.

3. Because pyrolysis on the electron emitter filamentand temperature gradients inside the ion source arenot a problem, an open Brink-type ion source [20]. isused with a filament close to the ion cage and WIthan electron emission current of up to 20 mAo Theconsiderably increased electron emission current andthe high efficiency of the open Brink-type ion source,which allows several electron trajectories through it,compensate for most of the sensitivity losses due. tothe single fast molecular flight through the IOnsource.

4. The ion source ion optics and quadrupole massanalyzer transmission obviously affect the detectionsensitivity. Typically, we detect less than 2% of theions that exit the ion source at the channeltron iondetector (mostly quadrupole losses). The quadrupoletransmission is further reduced exponentially athigher masses.

5. The vacuum background ion filtration process re­duces the noise. This feature does not provide betterdetection limits, but rather allows peak performancenear the detection limits to be achieved everydayregardless of the "history" of the apparatus. Novacuum system bake-out is needed to approach theminimum detection limit.

6. The supersonic molecular beam allows coupling ofa fast gas chromatograph with narrower peak widthsand thus higher peak flux, which contributes toimproved detection limits. .

7. The considerably enhanced molecular weight peakimproves the single ion monitoring of chosenmolecules through their molecular weight peak by afactor that can be over 103

.

Figure 12 demonstrates the detection sensitivityperformance of our apparatus through the fast GC-MSof eicosane (C 2oH 42 ) . The upper trace (a) demonstratesthe single ion monitoring of eicosane through itsmolecular weight peak at 282 U. A solution of 100-ppb

130 DAGAN AND AMlRAV J Am Soc Mass Speclrom 1995.6,120-131

Discussion and Overview

We have shown that the EI-M5 of alkanes in super­sonic molecular beams is characterized by a substan­tial increase in the relative height of the molecularweight peak. The electron energy provides good con­trol over the degree of molecular ion fragmentationwith only a small sacrifice in the absolute signal of themolecular weight ion. In addition, the EI-M5 in 5MBprovides tail-free GC-M5 chromatograms without massspectrometry ion source related restrictions on themolecular volatility.

As is well known and established, the enhancedmolecular weight peak is of considerable importancefor the following reasons:

1. This is a very important piece of information fordetecting unknown molecules.

2. It helps to improve library searches.3. This is the best mass spectrum peak to resolve

overlapping gas chromatography peaks.4. M + is the best peak for the single ion monitoring of

target compounds.5. The M+, [M + 1]+, and [M + 2]+ peak ratios pro­

vide elemental information.

ions per time constant of 0.3 s. The noise originatespredominantly from a mass-independent backgroundion signal, which depends on the total pressure. Webelieve this background signal originates from the in­sufficient ion filtration dynamic range of our quadru­pole mass analyzer.

The lower trace (b) shows the full scan total ionchromatogram obtained by five consecutive injectionsof 1 ng of eicosane (0.1 JLL, splitless, 10 ppm), eachevery 12 s. The extrapolated MDA in this full scanmode is 30-40 pg. The lower trace (c) represents thesame chromatogram as in trace b, but with a computerreconstructed single ion chromatogram at the molecu­lar weight peak. In this case the detection limit isimproved to below 10 pg . We note again that evenwith 100 ng of eicosane we could not find even a singleion at the eicosane molecular weight in the conven­tional GC-M5 shown in Figure 11. Thus, in this modethe 5MB-M5 is much more sensitive than an ion trapgas chromatograph-mass spectrometer. It should bementioned also that when one searches for low detec­tion limits, mass spectrometry in supersonic molecularbeams also facilitates the addition of hyperthermalsurface ionization [3, 21, 22] which is a very sensitiveionization method with a demonstrated MDA of below1 fg [18]. However, hyperthermal surface ionization isrelatively inefficient with alkanes and results in exten­sive molecular ion fragmentation. It is, however, anoptimal ion source for the detection of aromaticmolecules and amines that supplement the capabilityof the EI ion source and that is present in the sameapparatus.

48

x to~

A 10 Picogrom

SIM 2820mu

12o

I , ,6 i

0 2 4 8 10TI ME (sec)

B. 1 Ng FU LL SCAN TIC

~\ 1

c. 1 Ng FULL SCAN RSIM 2820mu

24 36TIME (sec)

Figure 12. Sensitivity evaluation of EI-M5 in 5MB. (a) O.l -J.tLsolution of lOO-ppbeicosane 00 pg ) was injected into the fast gaschromograph with the mass spectrometer tuned to the molecularweight mass of 282 u. (b) Five repetitive O.l-J.tL injections, eachafter 12 s, of a lO-ppm eicosane solution into the fast gas chro­matograph-mass spectrometer. A full scan GC mass spectrum isshown with the mass spectrometer scanning. (c) As in b, butcomputer reconstructed single ion monitoring is shown at themolecular weight of 282 u.

eicosane in cyclohexane (0.1 JLL) was injected (split­less) into the O.5-m megabore column of the fast gaschromatograph inlet [18]. The GC-MS measurementwas completed in 3 s and the observed chromatogramof 10 pg of eicosane was practically noise-free. Fromthe expanded ( X 10) noise trace we estimate the signal­to-two rms noise ratio to be 180, and the linearlyextrapolated minimum detected amount (MDA) to be60 fg (this value is 250 fg if a three peak-to-peak noisevalue is considered). We believe this result is betterthan that which can be achieved with any other massspectrometer for heavy alkanes by single ion monitor­ing at their molecular weight peak. We also havemeasured the signal by ion counting and found theoverall signal to be about one molecular weight ion atthe channeltron ion detector per femtogram of eicosane.The background noise count rate was 2000 ions persecond and the rms noise fluctuations were about 25

J Am Soc Mass Spectrum 1995, 6, 120-131 ElMS OF ALKANES IN SUPERSONIC BEAMS 131

6. The existence of a noticeable molecular weight peakcan provide isotope abundance geochemical infor­mation as well as enable deuterium exchange andisotope dilution analysis.

Obviously, all these aspects have long been attributedto EI-M5 for those molecules that exhibit a pronouncedmolecular weight peak. The contribution of supersonicmolecular beam sampling is merely in the extension ofthe range of molecules that exhibit molecular weightpeaks. In this article we have demonstrated this foralkanes.

This work on EI-M5 of alkanes in 5MB is part ofour continuous research on mass spectrometry insupersonic molecular beams. Our goal is to create anew mass spectrometric approach that will combinefast GC-M5, extended temperature limits, increasedcapability for handling thermolabile molecules, en­hanced molecular weight peak height with the above­mentioned features, possible selective ionization andincreased sensitivity with hyperthermal surface ioniza­tion, possible coupling of gas chromatographs withany column and gas load, and the ability to use verylarge amounts of splitless injections.

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