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Drift Tube Chemical Ionization Mass Spectrometry of Esters P. C. Price,' H. S. Swofford, Jr., and S. E. Buttrill, Jr.** Department of Chemistry, University of Minnesota, Minneapolis, Minnesota 55455
A series of carboxylic acid esters are examined by a new technique: drift tube chemical ionization mass spectrometry. Using water as the chemical ionizatlon reagent gas, there Is very little fragmentation of most esters at low values of E / P in the drift tube. At higher E / P , the extent of formation of analytically useful fragments is greater than can be obtained by increasing temperature or switching to methane reagent gas. As E / P is Increased, the onset of each fragment is correlated with the endothermicity of the reaction which forms it. The absence of M+ and (M - 1)' ions in water chemical ionlration mass spectra increases the usefulness of the technique for assessing sample purlty whlle the ability to induce extensive fragmentation with the Urift tube enables one to obtain structural information from the same sample.
Recently we described a new ion source for chemical ionization (CI) mass spectrometry (1)which incorporates the features of both a conventional CI source and a 4-cm drift tube. Ions formed by chemical ionization (2) of the sample enter the drift tube where they experience a uniform electric field. The ratio of this electric field strength to the pressure, E / P , determines the average translational energy of the ions in the drift tube (3). Inelastic ion-neutral collisions convert some of this translational energy into internal (rotational and vibrational) energy (4).The increased energy of the ions may be described in terms of an "effective temperature" defined by Mason (5) as
Teff= T ( l + M v 2 / 3 k T ) where T and M are the temperature and mass of the neutral gas and u is the ion drift velocity through the drift tube. Effective ion temperatures in excess of lo00 K can be obtained in the drift tube CI source, causing extensive fragmentation of the quasi-molecular ions (1). Since a single drift voltage setting controls the extent of fragmentation, it is possible to obtain both molecular ion spectra and extensive fragmentation on successive scans of a single sample. Carboxylic acid esters were one of the first classes of compounds examined by CI mass spectrometry (6). There are also considerable data available on the temperature dependence of ester CI spectra (7), and the mechanisms of the formation of the major fragment ions have been elucidated (8) through deuterium labeling studies. Furthermore, since most esters are at least sparingly soluble in water, they may be introduced very conveniently into the CI mass spectrometer as dilute aqueous solutions (9, I O ) , thereby allowing precise control of the pressures of both the ester and the water reagent gas. All of these considerations, plus their widespread occurrence both in nature and in widely used chemical processes and products, led us to choose esters for this initial study of the potential analytical applications and utility of the new drift tube chemical ionization source. Present address, Union Carbide 770-120, P.O. Box 8361, South Charleston, W. Va. 25303. Present address, SRI International, 333 Ravenswood Avenue, Menlo Park, Calif. 94025. 0003-2700/78/0350-1127$01 .OO/O
EXPERIMENTAL Figure 1shows a schematic drawing of the drift tube CI source. Since the entire instrument has been described in detail earlier ( I ) , only a brief description is given here. The 1300-eVelectron beam enters at about a 45" angle to minimize the electron current entering the drift tube while forming as many ions as possible near the axis of the source. While in the source region, ions experience only the weak fields due to the 5 V applied to the conical repeller (A). Thus, ions entering the drift tube (C) are formed under normal CI conditions and must travel at least 1 cm before coming under the influence of the drift potential which at present can be up to 135 V. Source pressures are measured by an MKS Baratron Model 90-1H capacitance manometer connected directly to the source via a static (no gas flow) 3-mm 0.d. tube. Temperature is measured by two chromel-constantan thermocouples mounted in plates (B) and (D). Except as noted, all spectra were obtained at 110-115 "C and 0.10 to 0.13 Torr total pressure. Esters used in this study were all commercial materials used as received. Purity was confirmed by GC and CIMS. The water used as reagent gas was purified as described previously (9). Spectra were obtained by injecting aliquots of 500-ppm solutions of the esters in water into the 120 "C batch inlet system. The value of E I P was changed by adjusting the drift voltage during the intervals between magnet scans, and several spectra at various settings could be recorded from a single sample. The water reagent ions observed in the drift tube CI source are H30+,H602+,H703',H904', and H;106'.at masses 19, 37,55, 73, and 91. The distribution of these ions is established by the set of reversible reactions, H+(H,O), + H,O H+(H,O),+ and the relative amounts of each reagent ion depend on the source pressure, temperature, and E I P value ( I ) . For this study, the use of water reagent gas has the advantage that the reagent ions do not interfere with the detection of alkyl fragment ions. These fragments contain information which would be very useful in determining the structure of an unknown compound, but they are often masked by reagent ions when methane or isobutane is used as reagent gas.
RESULTS AND DISCUSSION Relative Effects of Temperature, Reagent Gas, and E/P. Table I shows the CI spectra of ethyl acetate obtained under six different sets of experimental conditions in the drift tube source. Both methane and water were used as reagent gases. The relevant proton transfer reactions in the two reagent gases are: A H = - 72 CH,+ + EtAc CH, + EtAcH' A H = - 39 C,H,+ + EtAc C,H, + EtAcH' A H = - 29 H,O+ + EtAc + H,O + EtAcH+ -+
-+
where the enthalpy changes (11) are in kcal/mol. The two major fragmentation pathways of protonated ethyl acetate lead to the formation of CH,COt and protonated acetic acid as the ionic products (8) and have activation energies (11-13) of 33 and 26 kcal/mol, respectively. Comparison of spectra A and D in Table I shows that, as expected, the extent of fragmentation of the quasi-molecular ions is greater in methane than in water reagent gas. The two major fragments comprise 10.2% of the sample ionization in methane a t 116 O C and a low drift field while the corresponding fraction in water is 3.4%. Increaging the temperature greatly increases the extent 0 1978 American Chemlcal Society
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ANALYTICAL CHEMISTRY, VOL. 50, NO. 8, JULY 1978
Table I. Drift Tube CI Spectra of Ethyl Acetate (Percent Total Sample Ionizationa) Reagent Spectrum gas
T,"C 116 116 230
EIP (V/cm- m / e : 1 5 Torr) Ion: CH,' 34 b
A CH, 170 b B CH, 38 b C CH, ... 110 35 D H, 0 110 174 9.1 E HZ0 220 35 F H*O a Spectra do not add to 100%since minor peaks are omitted. 1 . .
33
CH,O+
...
... ... ...
4.9 .,.
43 61 CH,CO+ CH,COOH,+ 5.8 4.4 57 23 17 18 1.9 1.5 29 33 19 21
____ 3 15 cm
u
Flgure 1. Cross section of the drift tube chemical ionization source,
drawn to scale. All parts are stainless steel except those shaded, which are machinable glass ceramic of fragmentation in both reagent gases, but increasing E I P to approximately 170 V/cm-Torr is even more effective (spectra B and E). The amount of internal energy which can be deposited in an ion at high E I P is illustrated by the appearance of CH3+in spectrum E. The lowest energy process (11-13) producing this fragment from protonated ethyl acetate requires propionic acid as the neutral product and is 92 kcal/mol endothermic. More likely fragmentation pathways are even more endothermic. Thus the internal energy available a t high E I P is greater than can be obtained by raising the source temperature and comparable to that seen with the most energetic reagent gases (14). The ion at mle 33 observed in spectrum E (Table I) is most likely a complex of CH3+and HzO. With DzO reagent gas, this ion occurs a t mle 35, showing that i t contains two hydrogens from the reagent gas. The existence of this complex is consistent with the fact that methyl cation does not react with water, even though an exothermic channel exists (15). A more complete study of the effect of E I P on the water CI spectrum of ethyl acetate at 110 "C is shown in Figure 2. At low values of E I P , the two quasi-molecular ions MH+ and MH+.H20 account for over 95% of the total sample ionization. These conditions facilitate molecular weight determination and assessment of sample purity. As E I P is increased, the first change in the spectrum is the disappearance of the hydrogen-bonded ion MH+.HzO which dissociates to the MH' ion. At higher values of E I P , fragment ions appear a t mle 61 and 43. These ions have been shown by deuterium labeling studies (8) to be protonated acetic acid and the acylium ion, respectively. At about 130 V/cm-Torr the methyl cation appears, as described above. For clarity, the small amount of CH60+ ion associated with the methyl cation peak is not shown in Figure 2. Above approximately 160 V/cm-Torr, the fraction of unfragmented MH+ ions begins to increase with increasing E / P .
... ... ...
68 0.5
...
Obscured by large reagent gas peak.
80
CERAMIC
107 MH'.H,O
89 MH' 78 17 63 28 21 54
107
1lOiC
1
Figure 2. Variation of the water chemical ionization spectrum of ethyl
acetate as a function of E / P in the drift tube
While this unexpected behavior is not well understood a t present, it probably results from the fact that the collision cross section drops off rapidly with increasing energy leading to a rapid decrease in the number of ion-molecule collisions within the drift tube as the drift voltage is increased. There is therefore an increasing probability that an ion can be accelerated through the drift tube without experiencing a sufficiently strong collision to cause fragmentation. One important aspect of the data in Figure 2 is the fact that the onsets of the four dissociation processes are correlated with their endothermicities (11-13). This observation strongly suggests that the drift tube chemical ionization source may be capable of providing thermochemical data for the various fragment ions. This aspect of the new source will be examined in future studies. D r i f t T u b e Chemical Ionization Spectra of Esters. Table I1 summarizes the water chemical ionization spectra of 15 carboxylic acid esters obtained at 110 "C under both low and high drift field conditions. The nomenclature used to designate the various ions is the same as in previous studies (6, 8) with W = water molecule. At low E I P , the extent of fragmentation of the quasimolecular ions is considerably less than in either conventional methane CI (6) or in the low pressure conditions of the ICR spectrometer (8). This is expected in view of the higher proton affinity of water compared to methane. The quasi-molecular ions MH+ and MH+.W account for at least 60% of the total ionization at low E I P for all of the esters except benzyl acetate. At high EIP, in most cases, at least 50% of the ester ionization corresponds to fragments which are structurally significant. The principal fragment ions from protonated esters (RCOOR')H+ are RCO', RC02H2+,and R", but their relative intensities vary greatly. As we have noted before ( B ) , this variability is due to differences in the thermochemistry of the various fragmentation processes. The fragment ion which is formed via the lowest energy process is always the most
ANALYTICAL CHEMISTRY, VOL. 50, NO. 8 , JULY 1978
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Table 11. Chemical Ionization Spectra of Esters Obtained in the Drift Tube Source (% of Ester Ionization) Compound RCO, - RCO, MH+wd MH+ H,+W RCOOR RCO' ROH,' R' R" R'OH,' EIP H, + ... ... ... ... ,.. ... Ethyl 33 88 9.3 2.7 ... ,.. ... formate 200 6.2 48 39 1.0' 1.0' 6.2 .,. ... ... ... ... ... Butyl 14 33 50 36 ... ,.. ,.. ... ... *.. 200 formate 13 3.5 84 ... ... ... ... ... ... 33 Methyl 23 5.1 72 200 11a acetate 40 5.1 0.3 2.0 24 17 17b 11' ... ... ... .., ... 35 Ethyl 68 28 1.5 1.9 ... ... acetate 21 33 4.9 9.1 174 0.5 2.5 29 .., ... ... ... Propyl 59 15 22 3.4' 3.40 8.5 ... ... acetate 31 19a 2.0 16 154 8.8 23 1ga ... ... .*. ... Butyl 10 41 33 24 1.3 ... ... ... acetate 14 182 27 5.4 12 38 2.9 ... ,.. .,. ... ... ... Benzyl 10 25 19 55 ... ... *.. ... ... ... acetate 182 1.1 3.9 95 ... ... ... ... ... ... Methyl 53 28 46 0.7 ... ... ... ... propionate 139 79 1.3 0.4 1.7 18 ... ... ... ... ... Ethyl 10 23 75 1.8 0.5 ... propionate 182 43 14 27 8.5 4. 3' 1.5b 4.3' 1.5b ... ... ... *.. Propyl 10 26 10 58 3.4 2.7 ... ... ... ... propionate 19 182 4.1 3.3 61 13 ... ... ..* 64 21 3.8' Butyl 8.5 5.0 5.7 3.8' ,.. ... ... propionate 154 13 1.1 1.2 15 7 Oa 7 0' ... ... ... ... ... ..* ... Vinyl 44 10 56 ... ... .*. propionate 182 2.0 3.0 22 65 1.9 6.1 ... ... ... .*. ... ... 20 10 15 Allyl 65 ... ... ... ... propionate 182 50 1.5 14 32 2.4 ... ... ... ... ... 27 10 69 2.1 Ethyl 2.1 ... ... .., ... 182 43 31 1.7 butyrate 15 9.0 ... ... ... *.. *.. 14 10 79 2.0 Ethyl-3-chloro 1.6 ... ... ... ..* 182 40 18 19 propionateC 16 0.9 Intensities shown are the sum of the peaks for both chlorine isotopes. a , b Same mass as another possible fragment. The (M - Cl)' ion accounts for 2.8% and 5.7% of the ionization at 10 and 182 V/cm-Torr, respectively. W = water molecule. d , .
intense. In other words, the major fragments appear to all arise from a single protonated ester species which may decompose by any one of several competing channels. The lowest energy process is most often the formation of RCOOHz+,corresponding to the protonated carboxylic acid (8). The neutral product is an olefin formed from the R' group. This process is almost completely absent in methyl esters, because of the very large endothermicity associated with the CHz diradical. Deuterium labeling experiments have shown that the formation of RCOOH2+involves a cyclic transition state I,
1 I R-C
I
I
\;
the RCO' ion is the major fragment. In most cases, it is comparable to, or somewhat less intense than, the RCOOH2+ ion. The COOH2+-W ion corresponds to a protonated carboxylic acid molecule hydrogen-bonded to water. There are at least two possible mechanisms for the formation of this ion. The first is the simple association reaction, RCOOH,' t H,O
RCOOH,+...OH,
which is known to occur in the water chemical ionization spectra of carboxylic acids (16). The second is analogous to the formation of R-CO-OH+-C2H5 ions in the methane chemical ionization of esters (6, 8). This latter mechanism is similar to that for the formation of RCOOH2+except that the decomposing protonated ester is hydrogen bonded to a water molecule, as in 111.
/HZ
H
and therefore has a low frequency factor. A less endothermic fragmentation requiring only a simple bond cleavage, such as formation of R'+ via I1 can easily dominate the decomposition. Examples of this situation are butyl formate and benzyl acetate.
o I
R-C
/""
/"\
0'
I
p
\+
H 111
fH2
0
\ \
H
H
\
The formation of RCO' usually has a higher activation energy than formation of RCOOHz+,except when the neutral fragment for the latter process has an unusually high heat of formation, as noted above for methyl esters. This is also the situation in both vinyl and allyl propionate and, as a result,
Since the hydrogen bond strength between protonated carboxylic acids and water is significantly stronger than that between protonated esters and water, the effect of the water molecule is to favor the decomposition. In other words,
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ANALYTICAL CHEMISTRY, VOL. 50, NO. 8, JULY 1978
(155.,,,,,'"
30 0 1 0 3 Torr
%TIC 2 0
'0 40
60
83
803
20
E T H Y L BUTYRATE 1'0DC 129 Vlcm Torr
%TIC
30
2oL__.d 10
43
60 "ROPYL
80
103
120
PRCP OluA-E
Flgure 3. Water CI spectra of ethyl butyrate and propyl propionate obtained in the drift-tube CI source with water reagent gas at 110 OC and 0.103 Torr. The drift field strength was 129 Vkm-Torr, sufficient to produce substantial amounts of fragmentation characteristic of these two structural isomers
\
in propyl propionate are RCO+ ( m / e 57), RCOOH2' ( m / e 75), and RCOOHzf.0H2 ( m / e 93). The differences in the spectra are so great that these two structural isomers are readily distinguished. There is one important difference between methane (6,8) and water CI spectra of esters which should be noted, and that is the complete absence of M+ and (M - 1)+ions in the water CI spectra. The absence of these peaks is a great advantage when attempting to assess the isotopic or chemical purity of a compound from its water CI spectrum at low E / P .
preferential solvation of the product ion favors formation of RCOOH2+-OH2from (RCOOR')H+.OH, over formation of RCOOH2+ from (RCOOH')H+. Inasmuch as the origins and analytical utility of all of the various ions observed in the CI spectra of esters have been discussed extensively in previous work ( 6 , 8 , 1 4 ) ,there is no need to repeat that information here. As an example of the ease of identifying esters from their water CI spectra a t high E / P in the drift tube source, Figure 3 compares the spectra of ethyl butyrate and propyl propionate a t 129 V/cm-Torr. The MH+ ions at m / e 117 are present for both esters, but none of the other peaks are identical. The fragment ions in ethyl butyrate are R+ ( m / e 43), RCO' ( m / e 711, RCOOH2+ ( m / e 89), and RCOOH2+.0H2( m / e 107), while the fragment ions
LITERATURE CITED (1) P. Price, H. S. Swofford, Jr., and S. E. Buttrill, Jr., Anal. Chem., 49, 1497
(1977). (2) For recent reviews, see B. Munson, Anal. Chem., 49, 772A (1977)and F. H. Field in "Ion Molecule Reactions", J. L. Franklin, Ed., Plenum Press, New York, N.Y., 1972,pp 261-313. (3) E. W. McDaniel, "Collision Phenomena in Ionized Gases", John Wiley and Sons, New York, N.Y., 1964,pp 426-440. (4) W. Lindinger, M. McFarland, F. C. Fehsenfeld, D. L. Albritton, A. L. Schmeltekopf, and E. E. Ferguson, J. Chem. fhys., 63, 2175 (1975). (5) H. E. Rivercomb and E. A. Mason, Anal. Chem., 47, 970 (1975). (6) M. S.B. Munson and F. H. Field, J. Am. Chem. Soc., 68, 4337 (1966). (7) F. H. Field, J . Am. Chem. SOC., 91,2827 (1969). (8) C. V. Pesheck and S. E. Buttrill, Jr., J. Am. Chem. Soc., 96, 6027 (1974). (9) P. Price, H. S. Swofford, and S. E. Buttrill, Jr., Anal. Chem., 48, 494
(1976).
(IO) P. C. Price, D. P. Martinsen, R. P. Upham, H. S. Swofford, Jr., and S. E. Buttrill, Jr., Anal. Chem., 47, 190 (1975). (11) H. M. Rosenstock, K. Draxl, B. W. Steiner, and J. T. Herron, J. fhys. Chem. Ref. Data, 6,Suppl. 1 (1977). (12) R. Yamdagni and P. Kebarle, J . Am. Chem. SOC.,98, 1320 (1976). (13) W. A. Chupka and J. Berkowitz, J . Chem. f h y s . , 54, 4256 (1971). (14) C. W. Tsang and A. G. Harrison, J. Chem. Soc., ferkln Trans. 2 , 1718 (1975). (15) W. T. Huntress, Jr., R. F. Pinizzotto, Jr., and J. B. Laudenslager, J . Am. Chem. SOC.,95,4107 (1973). (16) Unpublished data from this laboratory.
RECEIVED for review January 27, 1978. Accepted April 10, 1978. This work was supported by NSF Grants GP-38764X, MPS-7510940, and CHE 76-20096.
Surface Chemical Ionization Mass Spectrometry Gordon Hansen and Burnaby Munson" Department of Chemistry, University of Delaware, Newark, Delaware 1971 I
The use of "in-beam" sample introduction to obtain chemical ionization (CI)mass spectra of thermally labile compounds and salts of low volatilHy is descrlbed. Samples are introduced on the surface of Teflon tubing and the effects of changing the sample position in the source are shown. The time and temperature dependence of some CI mass spectra are shown. The results are interpreted in terms of a model of sample ion production whlch involves surface ionization by ion-molecule reactlons followed by thermal desorption.
A major limitation in the use of mass spectrometry for the analysis of thermally labile compounds is the requirement that a sample must first be vaporized before it can undergo ionization. For many polar compounds, the energy required to disrupt the bonds between molecules may be greater than the energy required to break intramolecular bonds; therefore,
decomposition of the molecules may occur a t temperatures lower than those required for volatilization of the intact molecules. Field desorption mass spectrometry has been used successfully for the analysis of thermally labile compounds and of salts of low volatility. The spectra produced by this technique contain few ions for structure determination but generally give a n indication of the molecular weight of the compounds. Some complications arise from the presence of ions of masses higher than the molecular weight of the analyte ( I , 2). Recently, the mechanism of ion production in field desorption has been challenged ( 3 ) . Another recent and highly successful technique in obtaining mass spectra of thermally labile compounds is that developed by MacFarlane and Torgerson, known as Fission Fragment Desorption or Plasma Desorption ( 4 ) . This technique utilizes energetic fission fragments from 252Cfto volatilize and ionize solid samples deposited on thin Ni-foils. Ions characteristic
0003-2700/78/0350-1130$01.00/0 0 1978 American Chemical Society
