ANALYTICAL CHEMISTRY, VOL. 51, NO. 11, SEPTEMBER 1979
(31) (32) (33) (34) (35) (36)
B. R . Culver, "Analytical Methods for Carbon Rod Atomizers", Varian Techtron F'ty. Ltd., Springdale, Victoria, Australia, 1975. R. K. Skogerboe and C. L. Grant, Spectrosc. Lett., 3, 215 (1970). A. Woodriff, Momir Marinkovic, R. A. Howakl, and I.Eliezer, Anal. Cbem., 49, 2008 (1977). C. W. Fuller, Proc. Anal. Div. Chem. Soc., 13, 273 (1976). C. W. Fuller, Ana/yst(London), 101, 798 (1976). D. D. Siemer and R. W. Stone, Appl. Spectrosc., 29, 240 (1975). G. R. Harrison (Massachusetts Institute of Technology), "Wavelength Table
1849
of 100000 Spectrum Lines", John Wiley and Sons, New York, 1939. (37) B. V. L'vov, Federation of AMlytical Chemistry arid Spectroscopy Societks, 1976 Meeting, Philadelphia, Pa., Nov. 1976, Paper 2'14. (38) B. R. Culver and T. Surles, Anal. Cbem., 47, 920 (1975). (39) P. Barren, L. J. Davidowski. Jr., K. W. Penaro, and T. R. Copeland, Anal. Cbem., 50, 1021 (1978).
RECEIVED for review April 16, 1979. Accepted June 22, 1979.
Analysis of Coal Liquids by Mass-Analyzed Ion Kinetic Energy Spectrometry D. Zakett, V. M. Shaddock, and R. G. Cooks* Department of Chemistry, Purdue University, West Lafayette, Indiana 47907
Particular nitrogen-containing compounds, in some cases individual isomers, can be identified in coal liquid samples by mass-analyzed ion kinetic energy spectrometry (MIKES). This is done directly, without prior treatment or fractionation of the sample. Isobutane chemical ionization (CI)is employed to give an approximate molecular welght profile of the coal liquid. Nitrogen-containing ions are recognized by their enhanced tendency for charge strlpplng as evident in E / 2 mass spectra or by using ammonia as the CI reagent. I n selected cases, comparison with authentic compounds has been used to verify the MIKES identifications.
This paper represents an attempt to apply the powerful new method of mass-analyzed ion kinetic energy spectrometry (MIKES) (1,2) t o coal liquefaction products. T h e complexity of these samples ( 3 ) ,their growing practical importance ( 4 ) , and the laboriousness of chromatographic analysis (5), all indicate the desirability of evaluating all available alternatives. MIKES is a nonchromatographic procedure in which individual components are mass-selected after ionization and then identified by mass analysis of their dissociation products. Qualitative identification of individual components in natural products, particularly plant tissue, has been achieved with minimal sample pretreatment (6, 7) and these successes have encouraged the present undertaking. T h e complexity of coal liquids is such that it is desirable to confine attention to certain groups of constituents. Indeed, selectivity for particular functional groups must be seen as a decided advantage in any analytical technique. Such selectivity can he achiev.td in MIKES by controlled pyrolysis (8) or by use of selective ionization techniques, as in negative C I I M I K E S (9). In the latter approach only compounds of interest are ionized. We have chosen to explore selective ionization as well as an alternative approach in which a general ionization method (isobutane chemical ionization) is employed. This is followed by mass analysis with functional group screening being done using a high energy reaction of t h e selected ion. T h e reaction we use, charge stripping, has a n enhanced cross section for nitrogen-containing compounds and these can therefore be recognized. This is most efficiently done in the course of a single magnetic scan taken with a n appropriate setting of the second analyzer. Since this electrostatic analyzer measures kinetic energy to charge ratio, doubly charged ions occur a t one half the normal setting, E. From the resultant E / 2 mass spectrum (IO),one can pinpoint 0003-2700/79/0351-1849$01,00/0
those ions likely to contain nitrogen. Each such ion can then be examined individually by means of' its MIKES spectrum. In addition t o evaluating these approaches to functional group screening, we are concerned here with how readily individual compounds are identified by MIKES and with whether or not isomers can be distinguished. 'The use of a relatively soft ionization method (isobutane CIl should also provide a qualitative picture of the molecular weight distribution in the sample, as has been done using field ionization mass spectrometry (11).
EXPERIMENTAL The MIKES instrument has been described previously (12). The coal liquids sample was placed in a glass capillary ( - 1 pL) and introduced to the mass spectrometer source using a direct insertion probe. Chemical ionization was employed using isobutane as reagent gas a t a pressure of approximat.ely 0.3 Torr. Ammonia chemical ionization was done using r m isobutane ammonia mixture (101) (13). MIKES spectra were typically taken at scan rates of 1 amu s-'. They are reproducible to within a few percent relative abundance. The source pressure was monitored using a capacitance manometer (MKS Baratron type 221). E / 2 scanning was performed by first setting t,he magnet to pass an arbitrarily chosen ion which showed a relatively strong charge stripping peak in its MIKES spectrum. The electrostatic analyzer voltage was then optimized to allow detection of the charge stripping peak maximum. The actual analyzer voltage was slightly less than 50% E due to the endothermicity of charge stripping (10). The magnet could then be scanned and the E : / 2 spectrum of the entire sample recorded. The relative charge stripping abundances (Table I) were obtained a t a constant collision gas pressure (indicated as 5 X 10-bTorr, estimated as 1-2 X Torr) in the second field free region. The reported values represent an #averageof three or more measurements. Nitrogen bases were extracted from the coal liquids by the following simple procedure: 1 g of coal liquids was diluted with 15 mL of toluene and extracted first with 15 mL of aqueous 2 N HC1, then twice with 15 mL each of 6 N HCl. The aqueous layer was retained and neutralized with concentrated ",OH to pH 10. The basic mixture was then extracted with three 15-mL volumes of toluene. The toluene fraction was dried over anhydrous MgSO, and then evaporated to minimal volume. Approximately 1 pL of the resulting basic extract was analyzed. The coal liquefaction sample was obtained from K. C. Chao, Purdue University, and is otherwise uncharacterized.
RESULTS AND DISCUSSION Figure 1 shows part of the isobutane CI mass spectrum of the coal liquid evaporated from a direct insertion probe at 90 "C. Only low abundance ions occurred at lower and higher @Z1979 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 51, NO. 11, SEPTEMBER 1979
Table I. Relative Charge Stripping Abundance for Protonated Compounds name
structure
charge stripping enhancement
naphthalene
MH2 /MH 2.0 x 10-4
quinoline
3.3 x 10-4
1.6
isoquinoline
2.3 x 10-4
1.1
2-methylnaphthalene 2-methylquinoline l12,3,4-tetrahydronaphthaiene
4.1 X 9.8 x 10-4 3.6 x 10-5 1.3 x 10-4 3.4 x
+
1,2,3,4-tetrahydroquinoline 1,2,3,4-tetrahydroisoquinoline
anthracene
+
+
2.4 3.6 9.4
1.2 x 10-4
"
acridine
2.4 x
lo-"
2.0
COAL LIQUID I34
179
I 1
I55
I
TETRAHYDROQUINOLINE
(Miti1
1
+
+IO00
H
m/z
250
200
5C
Figure 1. Chemical ionization (isobutane)mass spectrum of a coal liquid sample
masses so the spectrum shown gives an approximate representation of the molecular weight profile of the evaporating sample (minimal fragmentation and adduct formation occurs with isobutane). The prominence of odd mass ions (M + H)' corresponding to even molecular weight compounds is noteworthy. This indicates that most constituents of this particular sample do not contain nitrogen. Evaporation a t higher temperatures gave CI mass spectra which showed more components, especially those of higher molecular weight. There is also a relative increase in the even over the odd mass ions, showing increased evaporation of nitrogen compounds a t higher temperature. All the work described refers to the lower probe temperature products. A possible means of identifying the individual compounds is to obtain MIKES spectra on each ion and either interpret these spectra from first principles ( 1 4 , 1 5 ) or compare them with those of authentic compounds. This approach is illustrated in Figure 2 where the ion 134' has been selected by the first analyzer and subjected to collision-induced dissociation. Figure 2a shows the resulting MIKES spectrum. The odd molecular weight indicates a compound containing an odd number of nitrogen atoms, and tetrahydroquinoline and tetrahydroisoquinoline are obvious candidates. (The former is a known coal liquid constituent (16).) Comparison of the coal liquid spectrum with those of the authentic strongly suggests that 134' from the coal liquid consists largely (al-
TE TRAHY DROISOQUINOLINE
~
(M+H)+
flOOO I
Flgure 2. MIKES spectra obtained using nitrogen collision gas on the ion m / z 134 from (a) coal liquid, (b) tetrahydroquinoline, (c) tetrahydroisoquinoline. The abscissa is labeled both in terms of the mass of the fragment ion and as a percentage of the kinetic energy ( E ) required to transmit the 134' stable ion beam
though not entirely) of tetrahydroquinoline. Particularly telling for this characterization is the difference in intensity of the sharp peak a t 50% E between the isoquinoline and quinoline spectra. This peak is due to charge stripping (Reaction 1). m+
+N
-
m2++ N
+ e-
(1)
Differences in collision-induced dissociation are also evident. In order to screen for nitrogen-containing compounds, use can be made of the tendency of N to stabilize a charge and hence the expected greater abundance of charge stripping peaks in nitrogen-containing compounds than in their non-
ANALYTICAL CHEMISTRY, VOL. 51,
SCAN MAGNET DETECT
M
++
WITd FOR
ESP,
EACH
FIXED AT
COMPONENT
SEPTEMBER 1979
1851
E
50%
THE
OF
NO. 11,
VIXTURE
Flgure 3. Principle of E i 2 mass scan COAL LIQUID CI MASS SPECTRUM
COAL LIQUID
203
180+
PROBE TEMP 9 0 ° C 1'9
I
a)
235 217
I93
I55 I 4 4
130
--__
m h
I50
200
250
COAL LIQUID € 1 2 MASS SPECTRUM PROBE TEMP
90°C
203
2 3 4 218
194 180
I55 144
I30
b)
P,CR IDINE
N
COAL LIQUID BASE EXTRACT CI MASS SPECTRUM
PROBE TEMP.
I M+nl+
+H'
I
-300
1
li
I
90°C 194 IS0
C)
158
144
I30
1 1 ___-A
m/z
J
1!.d-!!h~*~,-!,-2>
-.,--%A
150
200
Figure 4. Comparison of the isobutane C I mass spectrum of coal liquid with the E / 2 mass spectrum taken under the same conditions. The third spectrum shows the mass spectrum of a base extract of the coal liquid
nitrogenous analogues. This expectation is based on considerable previous experience with charge stripping ( I 7, 18) but it was felt that it was desirable to substantiate it for the t-ypes of conipounds expected to be encountered in coal liquids. Moreover, most previous experience refers to molecular ions (M'.) not protonated molecules, ( M + H)'. Table I gives data for four sets of compounds, each comprising an aromatic compound and one or more heteroaromatic analogues. Without exception the intensity of the ion due to stripping is greater for the nitrogen-containing compounds. Stripping is not the only process which can give a peak a t 50'70 E , the other is symmetrical dissociation (Reaction 2).
ml+ + N
-
mlj2+ + m1/2
+N
(2)
As shown in detail elsewhere ( I O ) , these processes are readily distinguished, Reaction I giving the characteristic narrow peaks seen in Figure 2 while Reation 2 gives the broad peaks which characterize dissociation reactions a t high velocity. An efficient means of recognizing compounds which are potentially nitrogen-containing therefore is to record a mass spectrum taken with the second analyzer set to transmit only the peak a t 50% E. The principle of the measurement is
Figure 5 . MIKES spectra of 180' derived from coal, 5,6-t)enzoquinoline, and acridine
illustrated in Figure 3. Because of possible contributions from dissociations such as Reaction 2, t,hese scans can only be used as a guide to the desired class of compounds. Figure 4 compares the E / 2 and the normal CI niass spectra of the coal liquid. It is evident that the even mass ions (odd molecular weight) generally show substantial increases in abundance relative to the odd mass ions. (Compare the l79/180, 193/194, and 203/204 pairs, as examples.) Of course the odd-even mass rule refers only to the presence of an odd num7oe:r of nitrogen atoms, hence its limited usefulness and the need for such alternatives as E / 2 mass spectra. Further evidence that these spectra do indeed assist in indicating nitrogen-containing compounds was obtained from the CI mass spectrum (Figure 4c) of a base-containing extract of the coal liquefaction product. On the basis of the E j 2 spectrum (and the CI mass spectrum of the extract) the ion m / z 180 is nitrogen-containing. Its MIKES spectrum (Figure 5) is extre:mely similar to those of acridine and 5,6-benzoquinoline. In this case it is not possible to specify which isomer is present in the coal liquid although the ratio of fragment to main beam (180') ions favors 5,6-benzoquinoline over acridine. Basic compounds can be selectively ionized using the ammonium reagent ion generated from isobutane/ammonia
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ANALYTICAL CHEMISTRY, VOL. 51, NO. 11, SEPTEMBER 1979 COAL
LIQUIDS
CIMS
ionization of the base-containing extract. A further example of the manner in which MIKES can be used to characterize the constituents of coal liquids is illustrated by consideration of the ion 194'. This ion is due to a basic compound according to the criteria advanced above and it may therefore be a methylated acridine or a n isomer. It is not necessary to obtain authentic compounds to establish the conclusion, a t least to this point of rigor. The MIKES spectrum of 194' (Figure 7 ) shows an abundant ion due to loss of 15 mass units, a result which parallels the rearornatizations observed (19) in other protonated aromatic ions on dissociation. Much of the rest of the spectrum, especially below 152', is very similar to that of protonated acridine, as would be expected if the compounds shared a common ring system.
203
ISOEUTANE
211 2:-
9-
91
,
ISOBUTANE / NH,
~
