Plasma Chromatography of Normal Alkanes and Its Relationship to Chemical Ionization Mass Spectrometry Francis W. Karasek, Donald W. Denney, and Edward H. DeDecker Department of Chemistry, University of Waterloo, Waterloo, Ontario
Product ion spectra produced in chemical ionization mass spectrometry (CIMS) (ca. 1 Torr) possess similarities to the product ion mobility spectra obtained in plasma chromatography (PC) (ca. 760 Torr). Positive ion mobility spectra observed for a series of n-alkanes, nC5H12 to n-ClSH32, show molecular weight region ions and fragmentation product ions that can be compared to their CIMS spectra. PC spectra of the n-alkyl bromides provide a similar comparison, apparently forming a bundant (M - Br)+ ions whose mobilities display a correspondence to ions in the n-alkane PC spectra.
The ionization and fragmentation of molecules in chemical ionization mass spectrometry (CIMS) results from ion-molecule and charge-exchange reactions with ions which are created in a host reactant gas by electron ionization (EI). By operating the ion source of the mass spectrometer a t pressures of 1 TOR or greater with reactant gases like methane or propane a t a concentration ratio of lo3 to the analytical sample, mass spectra of the sample are produced which are more abundant in the molecular 1)+ ions, along weight region of M+, (M - 1)+ or (M with simpler fragmentation patterns than those found in E1 spectra. Following the initial work by Munson and Field ( I ) using reactant gases primarily of hydrocarbons, further work by these authors and others has established the CIMS spectra of several classes of compounds and demonstrated the analytical value of the technique (2). The type of CIMS spectra obtained depends greatly upon the reactant gas used. Studies have been conducted using reactant gases of nitrogen ( 3 ) , ammonia ( 4 ) , water and argon-water mixtures ( 5 ) , and acetone or acetaldehyde (6). Processes that occur in high pressure (ca. 1.5 Torr) charge exchange mass spectrometry using non-hydrogen containing reactant gases (He, Ne, Ar, Kr, Xe, Nz, CO, C02, 02, CFI, NO) are described by Einolf and Munson (7). The rare gases show virtually complete dissociative charge exchange, while carbon monoxide gives relatively abundant molecular ions. Nitric oxide gives mostly molecular ions and the additive (M NO)+ ions with very little fragmentation. Although there are large differences in instrumentation and technique between plasma chromatography (PC) and CIMS, there exists a similarity in the basic fundamentals employed and the phenomena being observed in each. The PC technique creates ions in a nitrogen reactant gas a t atmospheric pressure and then reacts these ions with the sample molecules. The reactant gas and analytical sample are present in a concentration ratio of 106 to 10IO.
+
+
(1) M . S. B. Munson and F. H Field, J. Amer. Chem. SOC.,88, 1621 (1966). (2) M . S. B. Munson,Ana/. Chem., 43 (13), 28A (1971). (3) T . A. Whitney, L. P. Klemann. and F . H . Field, Anal. Chem., 43, 1048 (1971). (4) D. F . H u n t , C . N McEwen, and R . A. Upham, Tetrahedron Lett., 47, 4539 (1971). ( 5 ) D . F . H u n t and J. F . Ryan I I I , A n a / . Chem., 44, 1306 (1972). (6) D F . Huntand J. F . Ryan I l l , Tetrahedron Left., 47, 4535 (1971) (7) N Einolf and B. Munson, lnt. J. Mass Specfrom. /on Phys., 9, 141 (1972).
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A N A L Y T I C A L CHEMISTRY, VOL. 46, NO. 8, J U L Y 1974
The charged species resulting from the ion-molecule reactions are observed as the mobility spectra of an ion-drift spectrometer. Both positive and negative mobility spectra can be observed independently. Using a nitrogen reactant gas, the negative reactant species are electrons of about 0.5 eV energy. These electrons undergo associative and dissociative electron attachment reactions to produce simple negative mobility spectra with compounds that undergo electron attachment, such as aromatic halides and other substituted aromatic compounds (8, 9). The positive reactant ionic species in nitrogen are of the type (HzO),H+ and (HzO),NO+ ( I O ) . Using a quadrupole mass spectrometer coupled to a plasma chromatograph, Griffin et al. observed that these ions react with aromatic compounds to give primarily (M 1)+ ions (11). These protonated molecular ions are also observed as the most abundant species formed for aromatic compounds in CIMS using a methane reactant gas (12). In both CIMS and PC as the concentration of the sample increases, the sample molecules frequently form (2M 1)+ ions (2, 13). In PC, ions of this type are particularly observed for polar compounds such as alcohols and esters (13, 14). The CIMS spectra of normal alkanes possess an abundant (M - 1)+ion, formed by hydride extraction, along with welldefined fragment ions formed by decomposition of the (M - 1)+ ions. Therefore, these compounds provide a series that can be used to compare the results obtained with the CIMS and PC techniques. The PC mobility spectra obtained for the normal alkanes from CSH12 to C10H32 reveal spectra quite similar to their corresponding CIMS spectra.
+
+
EXPERIMENTAL Instrumentation. The basic design and operating characteristics of the BETA-VI model plasma chromatograph used in these experiments have been described previously (8, 9). Ions formed in a flowing carrier gas a t atmospheric pressure by a *3Ni source are moved by an electric field through an ion-molecule reactor section toward the drift spectrometer. Separation of a pulse of ions injected into the drift spectrometer occurs because of the different mobilities of the ions as they move through a countercurrent flow of nitrogen gas. To record the millisecond mobility spectra in time spans of 1 to 10 minutes, an electronic dual grid gating technique is used. A first shutter grid injects a pulse of ions into the delay between the injection pulse and the gating pulse from 0 to 20 msec over a 2-minute interval, 20-msec mobility spectrum have traversed the drift spectrometer. By continuously varying the delay between the injection pulse and the gating pulse from 0 to 20 msec over a 2-minute interval, a 20-msec mobility spectrum
(8) F. W . Karasek and 0. S. Tatone, Anal. Chem., 44, 1758 (1972) (9) F . W . Karasek, 0. S. Tatone. and D. M . Kane, Anal. Chem., 45, 1210 (1973). (10) F. W . Karasek and D. W . Denney, Anal. Chem., 46,633 (1974). (11) G. W. Griffin, I . Dzidic. D. I . Carroll, R . N. Stillwell, and E . C. Horning, Anal. Chem., 45, 1204 (1973) (12) M . S. 8 . Munson and F H Field, J . Amer. Chem. SOC.,89, 1047 (1967). (13) F . W . Karasek and D. M . Kane, J. Chromarogr. S o . , 10, 673 (1973) (14) 0. S. Tatone, "Studies of Halogenated and Oxygenated Compounds by Plasma Chromatography," Thesis submitted to the University of
Waterloo in partial fulfillment of requirements for MS Degree in Chemistry,June 1973.
~~
~~
Table I. Ion Mobilities Observed from Mobility Spectra of the n - A l k a n e s and n-Alkyl Halidesa Compound
n-Bromobutanec n-Pentane n-Bromopentanec n-Hexane n-Bromohexane" n-Heptane n-Bromoheptanec n-Octane n-Bromooctane" n-Nonane n-Decane n-Undecane n-Dodecane n-Tridecane n-Tetradecane n-Pentadecane
C4
CS
CS
Cl
cn
CR
c
ci2
C11
10
ClS
C14
c 1 3
2.32 2.18 2.16 2.05 2.03 1.92 1.92
2.33
1.83 1.81 1.83
2.06
2.32
2.06 2.04 2.04 2.04 2.04 2.04 2.04
1.92 1.92 1.92 1.92 1.92 1.93
1.84 1.83 1.82 1.85 1.86
1.72 1.71 1.71 1.72 1.72 1.72 1.72
1.62 1.62 1.62 1.62 1.62 1.62
1.54 1.54 1.54 1.54 1.54
1.46 1.46 1.46 1.47
1.40 1.34 1.34
1.40
1.28
*
I
:=
Reduced mobility, K,6 (cm2/volt-sec) of ions (CnHn,+i)'. Ko = ( d / r E ) ( P / 7 6 0 ) ( 2 7 3 , ' T )where d = drift distance, 6 cm; E = field strength, 250 V/cm; drift time, sec; T = degrees K ; P = pressure, Torr. Ref. 16.
can be recorded in 2 minutes. Complete details of this technique have been given previously ( 1 5 ) . Because of the high sensitivity of the plasma chromatograph, sample quantit,ies of 10- to 10- l2 gram must be used. To facilitate sample entry and ensure high purity, a direct interface of the plasma chromatograph to the effluent of a gas chromatograph was constructed (16). The system uses a Carle series 2000 valve mounted in a Model 4300 valve oven (Carle Instruments, Inc., Fullerton, Calif., 92631) which directs the GC effluent to vent, except when i t is desired to inject a selected peak, or portion of a peak, into the plasma chromatograph inlet. The entire interface can be heated to 220 "C and is kept 25 "C above GC detector temperature. The electrically operated switching valve functions in less than 0.2 sec per cycle. This technique avoids a continual entry of column bleed into the plasma chromatograph, which could very quickly sat,urate the PC instrument. With this GC/PC interface, it is possible to inject gram or less of a highly purified compound. Unless indicated differently in figure and table captions, the experimental parameters used to obtain these data are: carrier gas flow rate, 100 ml/min; drift gas flow rate, 450 ml/min; electric field, 250 V/cm; ion injection pulse width, 0.2 msec; scan pulse width, 0.2 msec; recorded scan time, 2 min; pressure, atmospheric (728-735 Torr); temperature, 135 "C. Reagents. The normal alkanes used in this study were obtained from PolyScience GC Kit No. 21A (PolyScience Corporation, Evanston, Ill.). The alkyl halides were reagent grade and were obtained from Analabs (Analabs Inc., North Haven, Conn. 06473). The carrier and drift gases were nitrogen (Linde, high purity grade: 99.996%). Prior to entry into the PC instrument, both carrier and drift gases were passed through individual metal traps of 2.25-liter capacity packed with Linde Molecular Sieve 13X. This procedure removes impurities from the gases and gives a water concentration estimated to be about 10 ppm. Procedure. The preferred method of sample introduction is to use the GC/PC system and actuate the switching valve during a GC peak to direct a portion of the peak into the PC instrument. 4 n alternate method is to inject less than 0.1 fil of sample vapor directly into the carrier gas stream using a GC liquid sampling syringe. Both methods give sample concentrations sufficient to produce mobility spectra €or 5 to 20 min, depending upon the amount of sample introduced, reactivity of the compound with the reactant ions, and temperature of the inlet. Mobility spectra of relatively comparable sample concentrations can be obtained by recording and comparing spectra a t times after sample injection when reactant ion concentrations are similar. Many runs were made on each sample reported here using both injection methods. The stability of the instrument and freedom from previous sample contamination is monitored by examination of the positive reactant ion spectra. These spectra are very stable from day to day, as indicated by relative peak heights, drift times under (15) F W . Karsek and M J. Cohen. J . Chromatogr. Sci.. 9, 390 (1971) (161 F W Karasek. 0. S. Tatone, and D. W. Denney, J . Chromatogr., 87, 137 (1973)
I b
I
2 i l 2il A 5
4
4
4
4
40
Ib
112
lh
116
D R I F T TImI- M I L L I S I C O N D S
figure 1. Mobility spectrum of the positive reactant ions obtained with a nitrogen carrier gas
the same operating parameters, and reduced mobility values (KO) calculated from the Equation given in Table I. The precision of these determinations is f0.02 cmZ/V-sec. The three ion peaks shown in Figure 1 are found in the positive reactant spectra a t all temperatures from 25 to 180 "C. Relative abundances are a function of temperature and water concentration (17). Reduced mobility values of the reactant ions are a slight function of temperature, whereas product ion peaks are generally not. The ionic species indicated in Figure 1 are consistent with relevant published data (IO, 11, 15).
RESULTS AND DISCUSSION Positive mobility spectra were obtained for each normal alkane from C5HI2 to C15H32.No negative spectra were observed, as is e x p e c t e d f r o m the lack of response these c o m p o u n d s exhibit in the e l e c t r o n - c a p t u r e detector (9). By contrast to the s i m p l e molecular-type spectra o b t a i n e d for aromatic c o m p o u n d s , each alkane gives a series of product ions p e a k s , m a n y of w h i c h show coincident mobilities f r o m compound to compound. T h i s is demonstrated in Figure 2, w h i c h shows the coincident p r o d u c t ions of e a c h c o m p o u n d and the relative a b u n d a n c e s us. KO values. The KO v a l u e s of each product i o n are given in Table I. AS the molecular weight and l e n g t h of c a r b o n c h a i n increases i n t h i s homologous series of c o m p o u n d s , product ions of higher molecular weight (lower K O )and a greater number of lower molecular weight product ions are produced. These s p e c t r a l patterns and the regularity of (17) L. J
Puckett and M W. Teague, J. Chem. Phys.,
A N A L Y T I C A L C H E M I S T R Y , VOL. 46,
NO. 8,
54, 2564 (1971)
JULY 1974
971
CHEMICAL l O N l Z A l l O N (CW, REACTANT)
"-c15w32
I I ,
"-Cl,h
I
I
I
I
"'C,2*,6
I
"-c11M24
I
"'C9H20
I
"'COHII
Figure 4. Comparison of the PC mobility spectrum and the CIMS spectrum for n-C12H26.The CIMS spectrum comes from reference (78).Unidentified ion peak in PC spectrum ( K O = 1.38) is postulated to be (M + NO)+ ion
1 "-c7MIb I
"-CbRl~ 1!20
0
2100
li
1!M
1!bO
1!40
I.x)
" 1.
1.2
I
5'7
7'1
5
9
~b
>AI
I 7 145 MOLfCULAR W f l O H l I A M U I
Id9
I/,
I 7
I
AI
Figure 3. Relation of ionic mobility and molecular weight for the alkyl ions assigned in Table I coincident ions suggest that the ions are predominately alkyl fragment ions. Although important differences exist, these spectral patterns resemble those obtained for the nalkanes in CIMS. To further interpret the positive mobility spectra, an identification of the most probable structure of the ion peaks observed is desirable. Data from an earlier study of the mobility spectra of the n-alkyl halides are helpful (16). The negative mobility spectra of those compounds gave only the halogen ion by dissociative electron attachment. The positive mobility spectra gave primarily a major product ion peak for each of the n-C4H9Br to nC8H17Br compounds. The mobilities of the major ion peaks decreased in a regular manner with increasing molecular weight. There was also a coincidence of the major product ion peak observed in the spectra for all three halogen derivatives (chloro-, bromo-, and iodo-) of a given alkyl group throughout the series, As the alkyl radical was the only molecular entity that was common to all three halogenated compounds in a group, it seems reasonable to postulate that the major product ions formed represent the alkyl radicals C4H9+, CsHll+, CsH13+, CTH15+, and C8H17+. These assignments agree with results obtained in CIMS, where the most abundant ion for alkyl halides is 972
ANALYTICAL CHEMISTRY, VOL. 46, NO. 8, J U L Y 1974
usually the (M - X)+ . This is also in accordance with the known sensitivity of alkyl halides to fragmentation in electron impact ionization, where the alkyl ion appears as the most abundant ion. Application of ion assignments from the n-alkyl halide spectra (16) to the n-alkane of this work permits the tabulation of ions with coincident mobilities and the structural assignments indicated in Table I. According to Griffin et al. ( I I ) , structurally related ions show a curve of ionic mass us. reduced mobility that correlates to within &2%. A plot of the mobility data in Table I is given in Figure 3, and provides a test of the ionic assignments with a correlation that tends to confirm them, although not proving their identity. These data suggest that the positive reactant ions and alkane molecules undergo ion-molecule reactions a t the atmospheric pressure conditions of the plasma chromatograph to produce molecular-type and alkyl fragment ions similar to those obtained in CIMS (18). Figure 4 shows a comparative plot of the CIMS and PC spectrum of nC12H26. While there are significant differences in relative intensities of individual alkyl ions, a general similarity between these two spectra is observable. The ion with the lowest mobility seen in the PC spectrum is postulated to be the (M NO)+ ( I O ) . An ion of this relative mobility to the molecular-type ion is observed in the positive mobility spectra of the majority of compounds studied. Mode of Ionization. The information available is insufficient to ascertain the exact mode of ionization occurring for the normal alkanes in the plasma chromatograph under the conditions described in these studies. In addition to being unable to define definite ionization pathways, it is not possible to distinguish between the M + , (M - l ) + ,or (M 1)+ ions because of the low resolution of the drift spectrometer. However, consideration of the most plausible ionization mechanisms can be useful in attempts to relate these results to established concepts, and to provide guidance for increased understanding of the PC technique. I t is, therefore, worthwhile to briefly examine related mechanisms which have previously been reported in the literature. The PC positive reactant ions are primarily of the types ( H z O ) ~ H +and (HzO),NO+. The product ion mobility spectra observed for the n-alkanes represent the sum of the charge exchange reactions these reactant ions undergo with the sample under the experimental conditions. In
+
+
(18) F H Field M S B M u n s o n and D A S e r , 58, 167 (1966)
Decker Advan Chem
*REACTAN1
ION5
41
-PRODUCT
I O N S 4
D l l f l 1 I M l - MILLISECONDS
Comparison of the positive PC mobility spectra of nand f l ' C 1 3 t i 2 8 . Note the decrease in reactant ions (cf.,
Figure 5. C9H20
Figure 1 ) CIMS studies, these same reactant ions have been used as charge exchange reagents, but generally not in studies involving the n-alkanes. Hunt and coworkers, using both HzO and DzO as CIMS reactant gases, found that abundant reactant ions of the type (H20),H+ and (D20),D+ were formed (5, 19).These ions function as Bronsted acids in the gas phase to protonate many organic compounds, forming abundant and stable (M + 1)+ions with compounds such as 4-decanone, di-n-pentylamine, and 2,5dihydrocoticosterone. In charge exchange studies using reactant gases not containing hydrogen, Einolf and Munson noted that the presence of water in the reactant gases was the major cause of the appearance of (M 1)+ ions (7). These studies, involving large molecules such as chloresterol, testosterone, and parathion, also indicated that NO as a reactant gas produced mostly M+ and (M NO)+ ions. Evidence that reactions of the NO+ ion with alkanes proceeds somewhat differently than with aromatic classes of compounds is provided by Searles and Sieck (20). Using a high pressure photoionization mass spectrometer, they found that reaction of the NO+ ion with normal, branched, or cyclic alkanes from Ca through C7 proceeds exclusively via a hydride transfer mechanism to give (M - 1)+ions. Of the compounds studied, it was ob-
+
+
D F Hunt, C N McEwen. and R A Upharn, Ana/ Chem 1292 (1972) (20) S K Searles and L W Sieck, J Chem Phys , 53, 794 (1970) (19)
44,
served that the reactions become increasingly exothermic in the progression to higher alkane homologs, and that the 3-methylhexane reaction was sufficiently exothermic to produce the C4Hg+ fragment ion as well as the (M - 1)+ ion. Consideration of this related charge exchange work with reactant ions similar to those in PC, suggests that the (HzO),NO+ ions play a leading role in producing the PC mobility spectra observed for the n-alkanes. Further evidence for this role can be seen by comparison of the reactant ions shown in Figure 1 and those still present after reaction with alkanes, shown in Figure 5. These data indicate that the (H20)NO+ is the most reactive, since it is completely absent in Figure 5 . This observation is supported by recent experimental data in which the quantity of the (HzO)NO+ ion in the plasma chromatograph was greatly increased by addition of NO to the nitrogen reactant gas (10). When this altered group of reactant ions was reacted with n-octane, an increase in the number of fragment ions and abundance of the ion peak attributed to the (M - 1)+ ion was observed in the mobility spectrum. It appears that work done in both PC and CIMS techniques defines the role of the (HzO),H+ ions in producing molecular weight region ions for aromatic and other, high molecular weight compounds observed in PC. The role of these ions in producing the mobility spectra abundant in fragment alkyl ions observed for the alkanes is less clear. From the available evidence, the most plausible ionization pathway appears to be primarily via the (H20),NO+ ions in reactions where the exothermicity provides sufficient energy to lead to fragmentation of the alkyl chain. A complete study of the ionization modes in PC also should take into account the collisionally-induced stabilization and decomposition of the alkyl ions occurring at the atmospheric pressures involved (21). Further studies are now under way to determine more exactly the modes of ionization in the plasma chromatograph. Received for review July 9, 1973. Accepted March 7, 1974. Paper presented in part at the 21st Annual Conference of the American Society for Mass Spectrometry in San Francisco, Calif., May 1973. The research for this paper was supported by the Defence Research Board of Canada, Grant Number 9530-116, and the National Research Council of Canada, Grant Number A5433. (21) M S B Munson and F H Field J Amer Chem SOC 91, 3413 ( 1 969)
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