Anal. Chem. 1993, 65, 1426-1430
1426
Field Ionization Mass Spectrometric Sensitivities of One-Ring Aromatic Hydrocarbons Christine Schulz, Swapan K. Chowdhury,’ and Saul C. Blum Corporate Research Laboratories, Erron Research and Engineering Company, Annandale, New Jersey 08801
Field ionization mass spectrometric sensitivities of xylene and nine monosubstituted one-ring aromatic hydrocarbons have been investigated toward the establishment of an alternative method for quantitative hydrocarbon characterization and “typeanalysis”. The molar sensitivity, relative to toluene, is found to increase nonlinearly as the chain length of the alkyl substitution on the ring is increased. This increase of sensitivity is consistently observed under the different ion source conditions investigated. The source temperature is found to affect the sensitivity significantly.The sensitivity of compoundswith longer chain length was found to increase more, relative to those with shorter chain length, when the temperature is lowered. The standard deviations of the data also increased with the decrease of temperature. The most reproducible results were obtained at 250 “C, the highest temperature studied. These effects may be attributable to adsorption phenomenon on the emitter as pointed out earlier by others. There was no discernible change in the measured sensitivities with changing solute concentration when hexane and pentane were used as solvents.
INTRODUCTION Characterization of complex hydrocarbon mixtures derived from petroleum and coal products has been a challenging task because of the presence of a large number of different types of compounds and their homologous series. Mass spectrometry has played an important role in the characterization of complex hydrocarbon mixtures. Several ionization techniques have been used to generate hydrocarbon ions for mass spectrometric analyses, such as, high-voltage (70 eV) electron impact ionization (HVEI),1-3 low-voltage ( 10 eV) electron impact ionization (LVEI),4-7 charge exchange,a12and field ionization (FI).13-18 N
* Corresponding address: Sterling Winthrop Inc., 9 Great Valley Parkway, Malvern, PA 19355. (1)Brown, R. A. Anal. Chem. 1951,23,430. (2)Lumpkin, H. E.; Thomas, B. W.; Elliott, A. Anal. Chem. 1952,24, 1389. Lumpkin, H. E.Anal. Chem. 1956,28,1946. (3)Clerc, R. J.; Hood, A.; ONeal, M. J., Jr. Anal. Chem. 1955,27,868. (4)Field, F. H.; Hastings, S. H. Anal. Chem. 1956,28, 1248. (5)Johnson, B. H.; Aczel, T. Anal. Chem. 1967,39,682.Lumpkin, H. E.; Aczel, T. Anal. Chem. 1964,36,81. (6)Aczel, T.;Hsu, C. S. Int. J. Mass Spectrom. Ion Processes 1989, 92,1. (7)Hsu, C. S.;McLean, M. A.; Qian, K.; Aczel,T.;Blum, S. C.; Olmstead, W. N.; Kaplan,L. H.;Robbins, W. K.; Schulz, W. W. Energy Fuels 1991, 5 , 395. (8) Hatch, F.; Munson, B. Anal. Chem. 1977,49,731. (9)Subba Rao, S.C.; Fenselau, C. Anal. Chem. 1978,50,511. (10)Mercer, R. S.;Harrison, A. G. Org. Mass Spectrom. 1986,21,717. (11)Dzidic, I.; Peterson, H. A.; Wadsworth, P. A.; Hart, H. V. Anal. Chem. 1992,64,2227. (12)Qian, K.; Hsu, C. S. Anal. Chem., in press. 0003-2700/93/0365-1426$04.00/0
In high-voltageelectron impact ionization of hydrocarbons, ions undergo extensive fragmentation and sometimes no molecular ion is even observed. Although the amount or even the presence of a given molecular species cannot be deduced, those of a given type (i.e., z number of the formula CnHzn-J can be, from the characteristic fragment ion intensities using a precalibrated “inverse matrix” procedure.19~20LVEI produces less fragmentation and is highly selective for aromatic compounds, because of their low ionization potential (IP < 10 eV), by discriminating against saturated linear and cyclic hydrocarbons (IP > 10eV). The sensitivity in this technique is low because of small ionization cross sections resulting from low electron flux and energy and is highly dependent on the energy of the bombarding electrons. A small fluctuation in this energy not only changes the ion yield but also affects the degree of fragmentation. Any change in the degree of fragmentation in different channels may distort the quantitative analysis of mixtures. Ionization by the method of charge exchange depends on the ionization potential of the analyte molecule and the electron/ion recombination energy (RE) of the reagent ion. Only analyte molecules with ionization potential lower than the RE of the reagent ion will be ionized. The sensitivity in this technique depends upon the magnitude of the difference (A) between the RE of the reagent ion and the IP of the analyte molecules. The sensitivity increases with the increase of the difference (A); however, at the same time, fragmentation of analyte ions also increases. Field ionization is one of the few mass spectrometric techniques that produces molecular ions of both paraffins and aromatics with virtually no fragmentation.13-18 This technique is, therefore, well-suited for obtaining quantitative analyses of individual hydrocarbons as well as various hydrocarbon “types” and does not require the use of the complicated inverse matrix method for processing data. In order to perform hydrocarbon characterization and “type analysis”using FI mass spectrometry, the relative ionization yield or sensitivities of many analyte molecules must be determined. Several reports concerning the FI sensitivities of hydrocarbons are in the literature. Kuras andco-workers15reported the molecular weight and structure dependence of sensitivity coefficients of C&12 paraffins and C&14 cycloparaffins. Schepple et al. investigated the FI sensitivities of a large (13)Beckey, H. D. Field Ionization Mass Spectrometry; Pergamon Press: Oxford, England, 1971. (14)Mead, W.L. Anal. Chem. 1968,40, 743. (15)Kuras, M.; Ryska, M.; Mostecky, J. Anal. Chem. 1976,48,196. Ryska, M.; Kuras, M.; Mostecky, J. Int. J. Mass Spectrom. Ion Phys. 1975,16,257. (16)Scheppele, S.E.; Grizzle, P. L.; Greenwood, G. J.; Marriott, T. D.; Perreria, N. B. Anal. Chem. 1976,48,2105. (17)Scheppele,S.E.;Hsu,C.S.;Marriott,T.D.;Benson,P.A.;Detwiler, K. N.;Perreira, N. B. Int. J. Mass Spectrom. Ion Phys. 1978,28,335. (18)Gallegos, E. J. Presented at the 37th ASMS Conference on Mass Spectometry and Allied Topics, Miami Beach, FL, 1989;p 296. (19)Hastings, S. H.; Johnson, B. H.; Lumpkin, H. E. Anal. Chem. 1956,28, 1243. (20)Ashe, T. R.; Colgrove, S. G. Energy Fuels 1991,5 , 356. 0 1993 Amerlcan Chemical Society
ANALYTICAL CHEMISTRY, VOL. 65, NO. 10, MAY 15, lSS3
number of smallaromatic hydrocarbons typical of those found in coal-derived liquids16 and saturated hydrocarbons from fossil origin.” Data were presented for gasoline,13some low molecular weight hydrocarbons,’s and heavy petroleum fractions.14 However, FI sensitivity information on individual hydrocarbons of a given series (or z number) is currently unavailable. As a first step toward filling the void, we report here field ionization sensitivities for xylene and eight monosubstituted one-ring aromatic hydrocarbons (z = 6) withchain lengths ranging from C1 to C17. The substituents chosen in this study are linear alkyl groups (C,H2,+1). In addition, several parameters that may affect sensitivity measurements were investigated. The parameters include ion source temperature, concentration of the analytes in solution, solvents, and amount of material injected into the sample inlet system. The majority of the previous investigati~ns~”’~ were performed using FI emitters prepared from razor blades.21922 In the present study, we examined activated carbon microneedle emitters23-2son 10-pm tungsten wires. These emitters can be routinely prepared in the laboratory and have several advantages.29 The presence of numerous microneedles provides an anormous number of high electric field sites for ionization, thus enhancing sensitivity. In addition, these emitters are routinely used for field desorption (FD) measurements in our laboratory and the same emitters, therefore, can be used for FI studies without changing ion source conditions.
EXPERIMENTAL SECTION Field ionization mass spectra were acquired on a Kratos MS50 double-focusingmass spectrometer equipped with a combined EI/FI/FD ion source. The emitters were prepared in the laboratory by spot welding l0-lm tungsten wire on two metal posts mounted on a ceramic base. The quality of spot welding was tested by measuring the electrical resistance (typically 3.04.5 fl for acceptable welding) between the two posts. The welded emitters were then sonicated in dichloromethane for 5 min to clean the surface before putting them into the emitter activating chamber (Linden Chromaspec,Auf dem Berge 25D 2803 Leeste, Germany), which was evacuated by a small diffusion pump. The activation of the emitters was performed using indene vapor28to produce carbonaceous whiskers on the tungsten wire at high temperature and high field. The procedure is similar to those described earlier.23-28Briefly, the pressure of the chamber is initially adjusted to 50 mTorr by controlling the rate of indene leakage into the chamber. The wires are then heated for 2 min without the application of any high voltage, by passing a current (50 mA) through them to pyrolyze the indene on the wire. Pyrolyzed materials on the wire then act as nucleation sites for growing whiskers during the subsequent high-voltage operation. After 2 min the emitter current is brought to zero in order to apply high voltage to the wires. The pressure of the chamber is lowered to 25 mTorr. With the voltageon the emitters maintained at 8 kV relative to countereledrodes, the emitters are then heated until they glow brightly. After some time the emitters become faint. The emitter current is increased every hour to a point where they again glow brightly. This process is terminated after (21) Robertson, A. J. B.; Viney, B. W. J. Chem. SOC.A. 1966, 1843. (22) Derrick, P.; Robertson, A. J. B. Int. J.Mass Spectrom. Zon Phys.
1969, 3, 409. (23) Schulten,H.-R.; Beckey, H. D. Org. Mass Spectrom. 1972,6,885. (24) Beckey, H. D.; Hilt, E.; Schulten,H.-R. J.Phys. E: Sei. Instrum. 1973, 6, 1043. (25) Barofsky, D. F.; Barofsky, E. Znt. J. Mass Spectrom. Ion. Phys. 1974, 14, 3. (26) Okuyama, F.; Beckey, H. D. Int. J. Mass Spectrom. Ion Phys. 1978, 27, 391. (27) Lehrnann, W. D.; Fischer, R. Anal. Chem. 1981,53, 743. (28) Rabrenovic, M.; Ast, T. Int. J. Mass Spectrom. Ion Phys. 1981, 37, 297. (29)For a recent overview on the subject, see: Prokai, L. Field Desorption Mass Spectrometry, Practical Spectroscopy-A Series: Brame, E. G., Jr., Ed.; Marcel Dekker Inc.: New York, 1990. Vol. 9.
1427
Table I. Purity of Aromatic Hydrocarbons Analyzed by GC/FID compound purity ( % ) comwund Duritv (%) toluene 99.0 phenyloctane 97.0 ethylbenzene 97.0 phenylnonane 94.0 xylene 96.0 phenyldecane 98.0 o-xylene 17.0 phenyldodecane 96.0 m-xylene 64.0 phenyltridecane 98.0 p-xylene 15.0 heptadecylbenzene 97.3 phenylhexane 89.0 ~
~~~
1h of setting the current at 100 mA. The emitters are kept in a vacuum desiccator to prevent any oxidation or other unwanted reactions. Samples for mass spectrometric analyses were prepared by dissolvingaromatic hydrocarbons in hexane or pentane to obtain approximate desired concentrations. Different number of hydrocarbonswere mixed together in each experimentwith different known concentrations. In a majority of experiments toluene was a common component,because molar sensitivitieswere calculated relative to toluene (see later). The concentrations used ranged from -10-3 M to neat mixtures. The aromatic hydrocarbons investigatedwere toluene, ethylbenzene,xylenes,1-phenylhexane, 1-phenyloctane, 1-phenylnonane, 1-phenyldecane, l-phenyldodecane, 1-phenyltridecane, and 1-phenylheptadecane. Phenyldodecane and phenyltridecane were purchased from Aldrich Chemical Co. Phenylheptadecane was obtained from Chemical Samples Co., ethylbenzene from Pfaltz and Bauer, and toluene and xylene from J. T. Baker and Co. Xylene was a mixture of ortho, meta, and para isomers. The purity of the aromatic hydrocarbonsused was determined by gas chromatographyusing a flame ionization detector (FID). With the exception of 1-phenylhexane (89%), they were all highly pure with a purity ranging from 94% to 99 % (see Table I). The two solvents used, hexane and pentane, were purchased from EM Science and American Burdick & Jackson, respectively. The FI mass spectrometric analyses of the hydrocarbon mixtures were carried out on the day they were prepared to avoid any change in composition due to evaporation. Samples were introduced into the ion source through an allglass heated batch inlet system from which the sample vapors leaked at a constant rate into the ion source. The temperatures of the inside oven, glass reentrant, and transfer line were all held at approximately 260-270 O C . Injection volumes ranged from 1 to 6 pL, and measurements with each injection volume were repeated two or three times. Massspectra were acquired using the Kratos DS90 data system in “Raw”acquisition mode. The mass spectrometer was scanned from mass/charge (m/z) 90 to 900 at a rate of 10 s/decade at a resolution of -2OOO. The acquired data were then processed using Kratos Mach3 software on a SUN computer. In general, 20-30 scans were summed. Emitters were initially conditioned prior to acquisition every day by passing a current from 0 to 30 mA, which was increased at a rate of 2.9 mA/min in the presence of acetonevapor. Beforesubsequent data acquisition,the emitter surface was cleaned by resistively heating with up to 30-mA current. However, the emitters were not cleaned between scans because of practical difficulties. Molar sensitivities were calculated for each compoundrelative to toluene using the following equations:
molar intensity
SI = Zf,/m,
(1)
and relative molar sensitivity Sf= Zf,/m,S, X 100 (2) where Z, is the intensity obtained from the mass spectrum corresponding to the 1*Ccomponent peak of the species i, f, is the 13C isotopic correction factor, m, is the number of moles of i, and St is the molar intensity of toluene. The Srvalues can be readily converted to gram sensitivities preferred by others16J7by dividing them by the respective molecular weight.
RESULTS AND DISCUSSION Relative Molar Sensitivities. The relative molar sensitivities (Sr)of the hydrocarbons obtained in the present
1428
ANALYTICAL CHEMISTRY, VOL. 65, NO. 10, MAY 15, 1993
2,500 +
2,000
250 C
+ 200 C
3,000 -
T
-
x 4._ 5 ._ ._ v)
A
E 1,500
v)
2
P
~1,000 ._ c
-m
U
500
0
I
1
1
'
1
10,000
l
'
l
'
l
5
0
,
l
I
l
I
10
I
I
'
I
I
0
15
1
Figure 2. Average relathre molar senslthrltles (9)of nlne monosubstituted one-rlng aromatic hydrocarbons obtalned at Ion source temperatures of 100 ('), 200 (+), and 250 OC (0) from -10-l M solutions. 9 values are plottedagalnstthe substltuent carbon number. See also Table 11. 17
__
I6
A
,l
, =-
14-
o
l3 *
i 10
'
5 10 15 Chain length of the substituent, X
Chain length of the substituent, X
I
0
l
4
I
'
a
l
l
I
I
l
l
12
I
I
I
I
I
16
Chain length of the substituent, X
Flgurr 1. (a) Average relathre molar senslthrltles (9)of nlne monosubstltutedonerhgaromatic hydrocarbonsplottedagalnst number of carbon atoms, x, of the +alkyl substltutlon. The 9 values are all relative to toluene (S = 100). Data were obtalned from neat mixtures of hydrocarbons at the Ion source temperature of 250 OC (Tables I 1 and 111). (b) Plot of relathre molar sensitMtles from (a) In log scale versus substltuent chain length x.
investigation, under different experimental conditions, are given in Figures 1-3 and Tables I1 and 111. Shown in Figure 1are the Srvalues of nine one-ring aromatic hydrocarbons plotted against the substituent chain length (i.e., the number of CH2 groups designated as x ) . The data obtained from neat mixtures of hydrocarbons (Figure 1) are an average of 11 determinations with different numbers of hydrocarbons in each determination. These 11determinations are multiple measurements with different amountsinjected into the batch inlet system, all performed a t a source temperature of 250 "C. The complete results including the standard deviations are given in Tables I1 and 111. The reproducibility of the measurements, in general, is better than 10?6 . The results presented in Figure l a clearly show the sizedependent FI sensitivity of substituted aromatic hydrocarbons. The sensitivity of aromatic hydrocarbons increased with the increase of substituent chain length. The increase of molar sensitivity (SI)appears to be slow for chain lengths c1-C~. However, the increase is more rapid for x = 9 and higher. For example, Srvalue for x = 8 is a factor of 2.1 higher than toluene, whereas a value of 13.7 was obtained for x = 17, i.e., 1-phenylheptadecane. The 9values of aromatic hydrocarbons are also plotted in a log scale against their corresponding chain length in Figure lb. A more linear correlation between the log Srand the substituent chain length is observed.
0
-
no'' 'C-IMlhr X-2MlhrI -lO-lMipenl
-I
1
/ ,
I 5
7
8
13
II
15
IT
x 01 Phn"1-c"
Figure 3. Average relative molar senslthrltles of monosubstituted aromatic hydrocarbons plotted agalnst substltuent carbon number. The data shown (Ion source temperature 250 OC) are obtalned from M In (a) neat mlxtures (O), (b) -10-l M In hexane (+), (c) hexane ( 0 ), (d) lo-' M In pentane (A), and (e) lo-*M In pentane (X). See also Table 111.
-
-
The observation of substituent chain length dependent molar FI sensitivities of monosubstituted aromatic hydrocarbons in the present investigation differs from that reported by Mead,14 who reported the relative sensitivity of alkylbenzenes and other hydrocarbons to be constant over the mass range 280-700. However, the procedure adopted by Mead to obtain relative sensitivities of hydrocarbons using the results from "conventional mass analysis" may lead one to obtain erroneous sensitivities and whether the reported values are molar or gram sensitivities is not clear from the report. In addition, the sensitivity values of individual aromatic hydrocarbons were not reported in the early work of Mead.14 Scheppele et d.16investigated a number of hydrocarbon mixtures obtained from a coal-derived liquid using FI MS. The monosubstituted monoaromatic hydrocarbons studied were toluene, ethylbenzene, and propylbenzene. The measured molar sensitivities relative to ethylbenzene (SI= l) were 1.02 for toluene and 1.02 for propylbenzene. The value obtained in the present investigation after being converted relative to ethylbenzene (Sr= 1)is 0.93 for toluene; propylbenzene was not investigated. The present results are comparable to those reported earlier,I6 considering that the reproducibility observed in the present study is within 10?6 (Tables I1 and 111).
ANALYTICAL CHEMISTRY, VOL. 65,
NO. 10, MAY 15, 1993 1429
Table 11. Effect of Temwrature on the Relative Molar Sensitivity of Monoaromatic HYdrocarbonssrb ~~
~~~~
~~~
relative molar sensitivitiesc compounds
substituent C no.
toluene ethylbenzene xylene phenylhexane phenyloctane phenylnonane phenyldecane phenyldodecane phenyltridecane heptadecylbenzene
1 2 2 6 8 9 10 12 13 17
250 O C 100.0 107.4 f 4.6 135.0 f 13.7 180.4 f 16.0 228.1 f 37.9 256.6 f 42.2 358.2 f 82.5 463.3f 50.4 607.1f 71.2 1673.9 f 312.4
100 "C 100.0 121.1 f 15.5 141.0 f 15.6 251.6 f 27.9 455.3 f 37.4 606.7 f 95.3 1045.9 f 162.2 1359.6 f 359.0 1758.0 f 253.0 1105.6 f 212.5
200 O C 100.0 105.3 f 5.6 133.5 f 3.8 198.9 f 3.9 277.7 f 7.2 319.5 f 34.3 493.5 f 37.0 778.9 f 70.0 1086.1 f 148.4 3386.3 f 625.2
Relative to toluene. Data taken from -1V' M solutions. The data are averages of multiple determinations.
Table 111. Relative Molar Sensitivity of Monoaromatic Hydrocarbonsa-c relative molar sensitivities comuound
substituent C no.
-10-' M neat
(hexane)
-
lo-*M
(hexane)
-10-3
M
(hexane)
-
toluene ethylbenzene xylene phenylhexane phenyloctane phenylnonane phenyldecane phenyldodecane phenyltridecane heptadecylbenzene a
-1V2M
lo-' M
100.0 111.4 f 1.9
(uentane) _ _ _ 100.0 97.1f 5.2
170.3 f 0.4 203.8f 10.5 228.1 f 3.2 291.8f 8.0 448.5 f 22.5 537.6 f 50.5 1612.2 f 511.5
198.3 f 13.1 256.9 f 27.2 310.4 f 12.5 399.4 f 31.8 584.3 f 40.2 695.3f 89.9 1442.3f 26.0
(Dentane)
~~
~
1 2 2 6 8 9 10 12 13 17
100.0 111.0 f 4.2 132.0 f 5.2 173.6 f 7.5 210.6 f 6.6 232.9 f 19.1 193.5 f 23.3 462.2 f 41.6 613.3 f 76.6 1319.5 f 71.8
100.0 103.4 f 6.4 135.0 f 13.7 183.7 f 17.4 236.2 f 40.9 263.6 f 44.8 371.4 f 85.8 470.7 f 62.1 641.8 f 52.8 1631.7 f 253.3
100.0
100.0
108.9 f 13.4 170.7 f 42.5 240.0f 67.6 194.3 f 33.5 344.0f 157.1
265.7 f 60.0 366.5 f 225.8 424.3 f 167.9 328.0 f 113.4 472.7 f 176.8
Relative to toluene. Source temperature 250 O C . The relative molar sensitivity values are averages of multiple determinations.
Ryska et al.15 also observed chain-length dependence of FI sensitivity of C6-C12 n-paraffins using emitters prepared from razor blades. Similar molecular weight dependent molar sensitivities of paraffins were also reported by Scheppele et al.17 These reports support the substituent chain length dependent molar FI sensitivities of monoaromatic hydrocarbons obtained in the present study. The former authors attributed this dependence to be related to preferential adsorption of some molecules on the emitter surface.13J5 Addition of a large excesa of ethylbenzene to paraffins lowered the dependence of sensitivity on size, probably because the aromatic compound preferentially adsorbed on the emitter surface. We have also observed similar effects in our studies involving linear paraffinsa30 The present data involving the chain-length dependence of sensitivity of aromatic hydrocarbons may also be an effect of adsorption phenomenon on the emitter. The activated carbon microneedles on a thin wire used in the present investigation appears to affect the measured sensitivities of larger molecules more than those of smaller ones. The rapid increase in sensitivity of aromatic hydrocarbons with the increase of substituent chain lengths may be considered to be resulting from two different "sample supply sources" for ionization. The "gas-phase source" supplies gas molecules to the highest field region resulting in "field ionization" in the gas phase.'3>29 The second source is the surface-adsorbedmolecules. Field ionization from this source is known.13831332Ions may form inside the condensed phase (adsorbed layer) near the tip of the microneedles31or are field ionized after supply to the needle tip via surface migration or e v a p o r a t i ~ n .The ~ ~ ions formed in the adsorbed layer can be extracted via a field-induced desolvation33 or ion evapo(30) Chowdhury, S. K.; Shultz, C., unpublished results. (31) Giessmann,U.; Heinen, H. J.;RBllgen,F. W. Org. Mass Spectrom. 1979, 14, 111. (32) Beckey, H. D.; RBllgen, F. W. Org. Mass Spectrom. 1979,14,188.
ration me~hanism.3~It is expected that the amount of monoaromatic hydrocarbons adsorbed on the emitter surface will increase with the increase of the substituent chain length as the boiling point increases. The second sample supply source for ionization appears to contribute to the rapid, nearly logrithmic increase in sensitivity with chain length (Figure lb). From the investigation of paraffins, it was shown that when the ionization from the surface-adsorbed source is lowered by adding ethylbenzene, for example, to the analyte molecules, the chain-length dependence of molar FI sensitivities decreased substantially and became linear.15~30 It is also interesting to note that the relative molar sensitivity obtained for xylene (B = 132) is a factor of 1.19 higher than that of ethylbenzene (SI= 1111,despite the fact that they have same molecular weight (Table 111,column 3). This value is comparable to that reported by Schepple et al.,'6 who obtained an average value of 1.1for xylenes relative to ethylbenzene. The reason for the higher sensitivity for xylene compared to ethylbenzene and the effect of number of substitutions on the FI sensitivity of aromatic hydrocarbons is currently being investigated. The higher ionization potential of ethylbenzene (IP = 8.76 eV) relative to xylene (average IP = 8.5 eV) may contribute to the observed higher sensitivity for xyleneS35 Effect of Ion SourceTemperature. The FI sensitivities of aromatic hydrocarbons have also been investigated at different ion source temperatures. Figure 2 shows the averaged S r values of nine monosubstituted aromatic hydrocarbons plotted against the substitution chain length, n, measured a t ion source temperatures of 100,200, and 250 "C. Complete results are given in Table 111. The temperatures (33) RBllgen, F. W.; Giessmann, U.; Wong, S. S.; Okuyama, F. Proceedings, 32nd ASMS Conference on Mass Spectrometry and Allied Topics, Boston, MA, 1983; p 513. (34) Derrick, P. I. Fresenius' 2. Anal. Chem. 1986, 324, 486. (35) Rosenstock, H. M.; Draxl, K.; Steiner, B. W.; Herron, J. T. J . Phys. Chem. Ref. Data 1977, 6, Supplement 1.
~
~
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ANALYTICAL CHEMISTRY, VOL. 65, NO. 10, MAY 15, 1993
of the batch inlet system and the gas-transfer line were kept unchanged. The FI sensitivities are found to be ion source temperature dependent. The sensitivities of hydrocarbons with longer chain length increased relative to toluene with decreasing source temperature. This increase is more rapid for hydrocarbons with x = 9 and higher. The only exception is the observed low sensitivity of 1-phenylheptadecaneat 100 “C, which was consistently obtained in repeat experiments. The reproducibility of measurements decreased at a lower temperature, as seen from the standard deviations in Table 11. The most reproducible results were obtained at the highest source temperature, 250 “C. The reason for the observed temperature dependence of FI sensitivities of aromatic hydrocarbons is not yet elucidated. The nature of adsorption of aromatic hydrocarbons on activated carbon microneedle emitters and the effect of temperature on the adsorptionJdesorption phenomena is not clearly understood. Further investigations are underway to probe the effects of temperature on FI sensitivities of aromatic hydrocarbons (Table 11). It is also important to note that, at lower source temperatures, analyte vapors may condense on the inside walls of the ion source. Differential condensation of different compounds may also affect the data reproducibility at lower temperatures. Solvent Dependence and the Effect of Solution Concentration. In addition to neat mixtures, solutions in two different solvent systems were investigated. Figure 3 depicts the averaged molar sensitivities of nine monosubstituted aromatic hydrocarbons obtained from neat mixtures, 10-l and M solutions prepared separately in pentane and hexane plotted against substitution chain length, x . All the measurements were performed at an ion source temperature of 250 “C. No readilyvisible effect of concentration or solvent on molar sensitivity is evident from the data presented in Figure 3. Although the averaged intensities are similar, the standard deviations were higher for more dilute solutions.
-
-
Neat mixtures provided the best results. For lO-3M solutions the reproducibility was poor (Table IV). We have also investigated the effect of changingthe amount of sample injected into the inlet system of the mass spectrometer. Neat mixtures of 1-and 2-pL volumes and solutions of 2-, 4-, 5-, and 6-pL volumes were examined. No significant differences in sensitivities were observed.
CONCLUSIONS Field ionization mass spectrometry is a viable technique for the quantitative characterization of individual hydrocarbons and type analysis because of its nondestructive nature of ionization.13-1s However, preferential adsorption on the emitters of certain hydrocarbons in mixtures causes the response of hydrocarbons to be nonuniform for molecules with different alkyl chain lengths.13J5 The present investigation involving one-ring aromatic hydrocarbons with different chain lengths supports earlier observations. Determination of FI sensitivities for other classes of hydrocarbons (z number) with different chain lengths will allow the establishment of a data base that could be used to characterize hydrocarbons (both molecular and “type”) in unknown mixtures. The present study represents the first step toward achieving the goal.
ACKNOWLEDGMENT The authors thank P. Grosshans, G. Kramer, R. S. Polizzotti, and W. K. Robbins for their helpful comments and R. R. Wehman and G. D. Dupre’ for their assistance with the GC/FID measurements. RECEIVED for review October 1, 1992. Accepted January 27, 1993.
