reliable quantitative determination of iduronic acid. Fklier methods such as that of Boas ( 1 ) for purification of amino sugars could be used in combination with silicic acid chromatography. Our studies indicate, however, that by refinement of the techniques presented this separation can be conveniently accomplished using adsorption chromatography of trimethylsilyl ethers.
LITERATURE CITED
(1) Boas, N. F., J. BWZ. Chem. 204, 553 (1953). (2) Dziewiatkowski, D.,Biochim. Biophys. Acta 56, 167 (1958). (3) Haahti, E.,Nikkari, T., Karkkainen, J., J. Gas Chranatog. 4, 12 (1966). (4) Karkkainen, J., Lehtonen, A., Nikkari, T.,J. Chromatog. 20, 457 (1965). (5)Lehtonen, A.,Karkkainen, J., Haahti, E., Ibid., in press.
(6) Schiller, S., Slaver, G., Dorfman, A,, J . BWl. Chem. 236, 983 (1961). (7)Sweeley, C. C., Bentley, R., Makta, M., Wells, W. W., J. Am. Chem. Soc. 85, 2497 (1963). (8) Sweeley, C. C.,Walker, B., ANAL. CHEM.36, 1461 (1964). RECEIVED for review March 28, 1966. Accepted May 26, 1966. Work supported by PHS research grant HE0681805
from the National Heart Institute, Bethesda, Md., and by the Emil Aaltonen Foundation, Helsinki, Finland.
Qualitative Analysis of Petroleum and Related Materials Using Linear Elution Adsorption Chromatography L. R. SNYDER Union Oil Company of California, Union Research Center, Brea, Calif.
b By means of linear elution adsorption chromatography (LEAC) narrow petroleum fractions can be assigned an adsorptivity range, which in turn defines the various compound types which can or cannot be present in the fraction. This can simplify the subsequent qualitative analysis of the fraction by other techniques, since the number of possible component types which must be considered is greatly reduced. Application of the principle of the method to several past literature studies suggests some erroneous conclusions regarding the identificationof certain compound types in petroleum related samples.
A
CHROMATOGRAPHY has been used frequently in the separation of petroleum and related materials prior to analysis by other techniques. Its great value in this connection is based upon the fact that different compound types exhibit sharp differences in relative adsorptivity and are hence often separated cleanly by adsorption chromatography. By the same token the relative adsorptivity of a narrow petroleum fraction defines its composition to some extent. Until the present time, however, there has been no systematic attempt to use relative adsorptivity per se for the qualitative analysis of petroleum fractions. Two major problems have discouraged efforts in this direction: variability of adsorptivity with sample composition and separation conditions, and the complexity of petroleum. I n certain simple cases, however, it has been found that the techniques of linear elution adsorption chromatography (LEAC) can overcome these limitations on qualitative analysis of petroleum via
RP
-
1010
E,
1
0.305
\ I
1.0
El& I
I
0.1
I
I
I
I
0.2 0.3
0.4
l
0.5
l
1
06
t=
Figure 1. LEAC analysis of a narrow fraction from a heavy gas oil; fraction R ” values vs. eluent t o values
VSORPTION
adsorption. Thus the presence of nonvicinal aromatic and/or polysulfide types in petroleum was first demonstrated by qualitative LEAC analysis (see Figure 2 of ref. 8). With the recent publication (16)of an extended set of LEAC adsorptivity data for possible petroleum compound types, it is now possible t o apply LEAC routinely to the qualitative analysis of petroleum. The present paper describes this technique in detail, verifies its accuracy by several examples, and illustrates its usefulness by application to some previous literature studies. EXPERIMENTAL
Reagents. Philip’s 99% n-pentane is purified by passage over activated silica gel (20 ml. per gram). Other solvents used are reagent grade. Chromatographically standardized 3.8% H,O-AltOa (Alcoa F-20, equivalent linear retention volume 1.60 ml. per gram for elution of naphthalene by
n-pentane) is prepared as reported previously (9). Preparation of Narrow Fractions. The following procedure for qualitative analysis by LEAC is restricted to narrow fractions prepared by adsorption chromatography. Other samples must first be separated by nonlinear adsorption chromatography, the individual fractions analyzed by LEAC, and the resulting data composited as described in a following section. For nonlinear separation, 25 mg. of the sample are charged to a 10gram dry column as reported previously (7) and elution is begun with the series of eluents listed in Table I under “nonlinear separation.” Fifty-milliliter portions of each eluent are applied to the column in sequence (after wetting the column with the first eluent), beginning with n-pentane, and 50-ml. fractions are collected. When it is known that the original sample contains no very weakly or very strongly adsorbing components, one or more of the initial or terminal eluents of Table I may be omitted. The various fractions collected are evaporated under nitrogen and weighed, or redissolved in pentane or CCL, and their ultraviolet absorbance is determined (assumed proportional to weight, if necessary). Only those fractions which contain a p preciable sample are retained for LEAC analysis. LEAC Oualitative Analvsis. Elution curve‘s for the abovk fractions are obtained by LEAC in the usual manner (9, 10) (ultraviolet absorbance of small fractions), using 3.8% HeOAlrOa and solvents selected from the list of Table I under “LEAC Analysis.” Two or more elution curves (different eluents) must be obtained, using solvents whose eo values are similar to those for the solvent used to elute the original fraction in the nonlinear separation. One hundred micrograms of sample per gram of adsorbent (or smaller sample) is charged. values VOL 38,
NO. 10,
SEPTEMBER 1966
1319
(retention volumes in milliliters per gram) are calculated in the usual manner (9) (50% elution volumes), along with corresponding values for 10 and 90% elution ( R o , and ~ RO.g, respectively). The resulting two or more values should fall in the range 0.5-100 ml. per gram, and at least one govalue must be 52 ml. per gram. For elution by binary solvents which contain less than 10% of the strong eluent component, charging of the sample to the column should be preceded by a column wash with the eluent in question (2 ml. per gram of adsorbent). The column wash is discarded and ignored in the calculation of 3'. The initial guess of which eluent and column size to use in LEAC analysis is fairly critical, and should be guided by the t o value of the solvent used to elute the fraction in the preceding nonlinear separation. Use of an eluent whose t o value is the same as that used during nonlinear separation will generally give a R" value close to 1 ml. per gram. Decreasing to for the eluent by 0.06 unit generally increases 3" by a factor of 2 to 5. The temperature of the column should be held within z k 2 O of 24' C. The resulting values for a fraction are plotted semilogarithmically us. the eluent to values, as in Figure 1. The fraction adsorptivity parameters RP and t1 can be calculated as shown. The ratios R O , ~ / _ R O and Ro.9/Ro are formed, using values of R o l and R0.9 in the range 1-10 ml. per gram, and Bo values for the same eluent used to measure either or RO.g. The quantities €1 (0.1) and €1 (0.9) are calculated as
Bo
Table 1. to
0.00 0.06 0.12 0.18 0.24 0.30 0.36 0.42 0.48 0.54 0.60 0.66 0.72 0.78 0.84 0.90b
e l (0.1) is the eluent strength at which the first eluted 10% of the sample fraction is just eluted by 1 ml./gram of eluent. el (0.9) is the corresponding quantity for elution of the first 90% of the sample (or retention on the column of the last 10% of sample).
0.0 3.5 9.2 20 41 70
DISCUSSION
0.6
Development of Present LEAC Procedure. The basic theory assumed
0.5
in the present analytical method is discussed in detail elsewhere (6, IS). The fundamental equation relating LEAC retention volume Bo to separation conditions and sample (solute) structure is
LogloV, and a are adsorbent parameters (equal 1.80 and 0.64, respectively, for 3.8% H20-A1203), So is the solute adsorption energy, A , is the relative area covered by the solute molecule upon adsorption, t o is an eluent strength parameter, and the term ZAaas represents contributions from so-called "anomalous" adsorption effects. Equation l, with ZA... assumed equal to zero, can be rearranged to give the various relationships of Figure 1. R, is the value of Bo for pentane as eluent ( t o equal 0.00), and el is the value of t o for Bo equal 1.0. As discussed previously (16), Equation 1 is of greatest value when the term ZAeas can be made equal to zero. This is largely a matter of selecting the right eluent system. For the eluents of Table I, only those solvents which contain an alcohol give sizeable ZA,.. values. This was confirmed for the
-
M-PI
LEAC analysiso A-Ma I-P*
3 . 2 (0.05)'
1* 5 (0.1)' 5.5 (0.2) 14 (0.2) 40 (0.2) 100 (0.4)
16(0'os) 43 (0.10)
Numbers refer to 7,by volume of indicated binaries. Volume methanol-benzene. Benzene-pentane. d Acetonitnle-benzene, Isopropanol-benzene, Methylene chloride-pentane. 0 Acetonitrile-methylene chloride. h Isopropanol-pentane. Numbers in parentheses refer to % volume added water. i This eluent series is not used if possible.
* 50%
c
1320
ANALYTICAL CHEMISTRY
I
I
I
I
I
4
/
/
C
C
0.4
0.3 0.2
0.1 ,
0.1 eo
1
0.3 0.5 0.7 of Eluting Solvent
1
1
0.9
Figure 2. Experimental LEAC parameters for narrow fractions vs. eo value of eluting solvent
0.0 2.0 5.5 11 19 32 52 100
7.0 21 43 83 (0.1)'
I
6
Eluent Series for Nonlinear Separation and LEAC Analysis
Nonlinear separation@ B-PI A-Bd I-Be
I
0 . 9 (0.05)'~i 1.7 (0.08) 3.5(0.10) 8 . 0 (0.15) 20 (0.22) 44 (0.31)
0 Heavy gas oil bases titrable in glacial acetic acid 0 V Heavy gas oil weak baser titrable In acetic anhydride (two different samples)
acetonitrile solutions by the data of Table I1 (this solvent had not previously been studied for anomalous adsorption effects). As noted in Table 11, the resulting ZAeaa values are generally small and can be ignored. The alcohol containing solvents of Table I are used in LEAC analysis only as a last resort, as eluents for quite strongly adsorbing fractions. Values of Bo obtained with these solvents should be compared with values (same sample fraction) for elution by other solvents, so that 2A,,, for the alcohol solvent may be evaluated for the fraction in question by means of Equation 1. The resulting alcohol Eo values can then be corrected to a basis of LA,, equal zero, using Equation 1. Fortunately most petroleum fractions will not require elution by the alcohol solvents in LEAC analysis. The data of Tables I1 and I11 permit the derivation of eo values for the acetonitrilemethylene chloride and methylene chloride-pentane solvents of Table I. to values for the remaining solvent systems of Table I were derived from previously reported data (IO, 1 2 ) . The experimental LEAC parameters, R,, So, A,, and el are each characteristic of sample structure, but the parameter €1 is the only generally useful quantity for qualitative analysis. A , is a complex function of both alkyl substitution and compound type for the average sample molecule, and it cannot be directly related to sample type. Values of So and R,, which are linearly related, do not increase regularly with increasing sample adsorption strength (as inferred from chromatographic sequence), so that compound types which fall in different chromatographic fractions may have identical So and R, values. This is
illustrated in Figure 2a, where LEAC So values for a series of chromatographic fractions (petroleum bases) are plotted us. the eo value of the eluent used to elute the fraction in the preceding nonlinear separation. As seen in Figure 2b, sample el values do not suffer from this limitation. There are several other reasons for preferring the use of €1 values in the LEAC qualitative analysis of petroleum. Comparison of the €1 value of a narrow petroleum fraction with values tabulated previously (16) can provide information on the compound types likely to be present in the fraction. Alternatively the experimental €1 range of the fraction [e1 (0.1) to (0.9)] permits the exclusion of compound types falling outside this range. Graphical presentation of the final LEAC analysis is useful for summarizing the analysis of broad fractions (as the sum of subfraction analyses). A Gaussian (or skewed Gaussian) distribution of component el values can be assumed in each narrow fraction, and such a curve passed through the points el (O.l), el, and el (0.9) for the fraction in question. This is illustrated in Figure 3a for a hypothetical example with el (O.l), el, and el (0.9) equal to 0.2, 0.3, and 0.4, respectively. The quantity h, is made proportional to the relative concentration of the fraction divided by the quantity [c1(0.9) c1(0.1)]. The Gaussian distribution can be conveniently approximated by the triangular distribution (dashed lines) shown in Figure 3a. In Figure 3b the LEAC analysis of a previously described (1) quinolone concentrate is shown in composited form. Two basic nitrogen concentrates (obtained by ion exchange) are similarly displayed in Figures 3c and 3d. Verification of Present LEAC Technique. The LEAC analysis of narrow fractions in terms of their el value range is quite reproducible. The major source of error in LEAC analysis lies in the decision of what compound types can or cannot correspond to the measured el range of a sample. The magnitude of this error is determined by the accuracy of previously tabulated €1 values (16) for various possible com-
z
0 Figure 3. el Distribution curves for broad fractions
+ z w
0 a Hypothetical example b Quinolone concentrate ( I ) c Heavy gas oil bases titrable in glacial ocetic acid d Heavy gas oil weok bases titrable in acetic anhydride e Estimated LEAC analysis of fractions described by Jewel1 and Hartung (4) . , Total curve for basic fractions of
. .
z
0 0 J
a
5
UJ
w
Fignre 3c and 3 d
-I
w
a 0.1 0.2 0.3 0.4 0.5 0.6
El pound classes present in petroleum. The accuracy of these latter values has been justified in terms of their derivation, but additional checks on their reliability are desirable. Table IV
Table II.
provides a number of comparisons which are useful in this connection. Several narrow petroleum fractions containing predominantly a single compound type were available from pre-
Eluent Strength Data for Acetonitrile-Methylene Related ZA,, Contributions
-
Table 111.
sa
Solute 2,6-Dimethylphenol
Mc 1.02
Perinaphthenone
0.29
N-Methyl-2-quinolone -
0.98
%Acetyl pyridine
0.80
gNitroaniline
0.49
Acetanilide
0.88
Phenanthrenequinone
0.96
Chloride Binaries and
log R ofor indicated eluenta 5% A-Md 10% A-Md 25% A-Md 50% A-Md 0.64 0.33 (0.10) (0.21) -0.34 (0 * 09) 0.42 0.28 -0.01 -0.32 (-0.10) (-0.16) 0.10 -0.34 (-0.19) (-0.16) 0.05 -0.54 (0.19) (0.16) 0.17 -0.49 (-0.11) (-0.23) 0.30
(0.03) (0.42)i 0.49 0.52 0.57 0.61 ZAeSa values in parentheses. Calculated value (IO). Methvlene chloride. Acetdnitrile-methylene chloride binaries (% by volume). e Average values. Assumed in calculation of remaining eo and ZAem values (ref. IO).
Ab 7.5 11.5 9.5 8.0 10.0
9.5 11.0
Eluent Strength Data for Methylene Chloride-Pentane Binaries
log Ro for indicated eluento Solute 0% M-P 3.5% M-P 4.0% M-P 8.5%-M-P 10% M-P 16% M-P Fluoranthene 1.42 0.78 0.23 Triphenylene 1.96 1.30 0.78 0.27 Chrysene 1.98 1.21 0.66 1 2 4,5,8,%Tribenzpyrene 2 (ex t1.P (0.000) 0.086 0.096 0.153 0.170 0.220 eo (cagd.)d 0.000 0.084 0.095 0.156 0.173 0.224 a M-P refers to meth lene chloridepentane (% by volume). Calculated value (107. * Average values. From Equation 2 or ref. IO.
20% M-P 0.18 1.42 0.247 0.249
40% M-P
0.47 0.335 0.328
VOL 38, NO. 10, SEPTEMBER 1966
A,b
11.0 12.0 12.0 17.0
1321
ceding and current studies in these laboratories. Their composition was in every case known within narrow limits, and LEAC analysis of each fraction was carried out as described in the Experimental section. The aromatic ester fraction of Table IV could consist either of benzoic acid esters or of esters of phenyl acetic or propionic acid. The el values of Table 11 suggest that the latter ester type predominates in this aromatic ester fraction. Two separate LEAC bands are observed for the quinolone concentrate of Table IV (shown in Figure 3b), and it appears that the two bands correspond, respectively, to quinolones which are N-alkyl substituted and those which are not. The nitrogen bases in petroleum gas oils are believed to consist primarily of pyridine derivatives and higher benzologs, and a broad range in el values is predicted (16) for the basic nitrogen fraction of Table IV. As seen in Table IV (see also Figure 3c), the experimental el range for this nitrogen base fraction is indeed quite broad and close to the predicted range of values. On balance the data of Table IV suggest a reliability in previously calculated el values (and of LEAC analysis) of about +0.05 unit. Application of LEAC Analysis to Some Previous Literature Studies. The principal advantage of LEAC analysis as described will probably prove to be a negative one: the ability to rule out certain compound types in individual chromatographic fractions obtained by separation over alumina. I n this connection i t is frequently unnecessary to experimentally determine the exact el value of a fraction. Thus in some cases approximate tl values can be inferred from the separation procedure (e.g., using Fig-
Table IV.
ure 2b) or the separation order of identifiable compound types in the sample. As an example of this type of applicstion some previous literature studies concerned with the identification of petroleum compound types will now be re-examined. Chromatographic separation over alumina was involved in each of these studies. Gordon et d. (2) have reported the analysis of polyaromatic hydrocarbon types present in a heavy cracked gas oil. On the basis of the ultraviolet analysis of chromatographic fractions in which tetraaromatic hydrocarbons predominate, these authors conclude that naphthacenes are absent from this m p l e . The tetraaromatic fractions examined should have el values in the range 0.2 to 0.25, on the basis of other tetraaromatic types identified. A carbazole fraction, for which el equal about 0.35 can be estimated, was not analyzed by these workers. Naphthacenes, if present in the original sample, have el values equal to 0.35 to 0.38, and would thus be concentrated in the carbazole fraction. Consequently, the absence of naphthacenes in the tetraaromatic fractions does not guarantee the absence of naphthacenes from the original sample, as claimed by Gordon et d. Naphthacenes are probably not a significant component of cracked or other petroleum samples, but the work of Gordon et al. cannot be used as evidence one way or the other. Horton et d. (3) have reported the presence of both N-alkyl and N-H substituted carbazoles in cracked gas oil, the former being eluted after the latter from alumina. The distinction between these two compound types was made on the basis of solvent shifts of the ultraviolet spectra of the carbazole fractions (an extremely unreliable
el Values for Petroleum Fractions of Known Composition: Comparisons
with Tabulated Values (16) e1
Fraction description Methyl esters of naphthenic acids isolated from 500-600OF. gas od aliphatic aromatic Carbazole, benzcarbazole concentrate from 600-1000" F. gas oilc Quinolone, benzquinolone concentrate from 650-850' F. gas oild
Fraction (LEAC)
Table (16)
0.19 0.30
0.25 0.22; 0.30'
0.36
0.34-0.41
0.45, 0.65'
0.46-0.52' 0.6l-O.68"
Basic nitrogen compounds from 6000.10.45' 0.13-0.51 lO00" F. gas oilh Alkyl methyl benzoates. * Methyl esters of 2-phenyl propionic acid derivatives (calcd.from ref. 11). Described in Table I, ref. 16. Described in ref. 1. value corrected for presence of e Two distinct bands of LEAC analysis; fiRt band carbazoles and benzcarbazoles. N-Alkyl quinolones, benzquinolones. 0 Hydroxy quinolines, benzquinolineg. * Total bases titrable in acetic acid (Isolated by ion exchange). i Cl(O.1) to s(O.9). 5
1322
a
ANAL'YTICAL CHEMISTRY
criterion for the identification of petroleum fractions, in our opinion). Since the €1 values of N-alkyl carbazoles actually fall about 0.12 unit below values for the N-H carbazoles, the assignment of Horton et d.(3) is clearly in error. Other work (14) suggests that the carbazoles in cracked samples are almost exclusively of the N-H variety. Jewell and Hartung (4) have recently reported the detailed analysis of a basic nitrogen concentrate isolated from a heavy gas oil by means of aqueous HC1 extraction. The concentrate was separated on Alcoa F-20 alumina (as received, probably 1-2% HZO-AlzO3 in our experience), with elution by approximately 10 ml. per gram of successively stronger eluents (4, 5 ) . I n addition to quinoline and hydroxy quinoline derivatives (and higher benzologs), a number of new compound types were reported as present in this basic nitrogen concentrate: indoloquinolines, carbazdoquinolines, 1,IOphenanthroline, and tetrahydrocarbazolenines. It is possible to estimate the el range of the various fractions separated on alumina by Jewell and Hartung, as shown in Figure 3e. For comparison the composited LEAC curves for the sum of strong and weak bases isolated by us from a similar gas oil sample are also shown in Figure 3e (dotted curve, sum of curves from 3c and 3). The two samples differ principally in the presence of some quite weakly adsorbing material in the Jewell and Hartung sample (fraction I). Jewell and Hartung report quinoline and benzoquinoline derivatives predominating in their fractions I1 and 111, which is reasonable on the basis of the estimated c1 values for these fractions. Fractions I1 plus I11 in Figure 3e are also seen to resemble the sample of Figure 3c fairly closely (the latter consists predominantly of the higher benzologs of pyridine). Jewell and Hartung also report that hydroxyquinolines predominate in fractions I11 and IV, and these fractions overlap the €1 region in which N-alkyl quinolones are found (compare Figure 3b). Thus this assignment also agrees with LEAC analysis. Indolo- and carbazoloquinolines are reported as concentrated in fraction I by Jewell and Hartung, but this identification is incompatible with the el values predicted for these compound types: 0.35 $ e, $0.60 for the indoloquinolines, with el values for the carbazoloquinolines slightly higher. The evidence cited by Jewell and Hartung for the presence in fraction I of these compound types is actually quite weak, and i t now appears that the even numbered mass spectral peaks originally assigned to the* diazaaromatic types are actually due to some oxygen compound or hydrocarbon contamination of the start-
ing basic nitrogen concentrate. This suspicion is supported by comparison of the total LEAC curves of the samples of Figures 3c and 3e, which show Jewell and Hartung’s fraction I apparently absent from an ion exchange concentrate of similar petroleum bases. It is concluded that the presence of indolo and carbazoloquinolines in petroleum has not yet been demonstrated. Jewell and Hartung also report the presence of 1,lo-phenanthroline in fraction 11. Since the el value of the latter compound is 0.75, and the range in values for fraction I1 is estimated at 0.1 to 0.25, it is clear that 1,lOphenanthroline could not have occurred originally in this fraction. The presence of the latter compound in petroleum must therefore be regarded as unproved at the present time. Alkyltetrahydrocarbaolenines are reported by Jewell and Hartung in fraction VI1 (e1 > 0.5). Since an
e, range for this compound class of 0.4 to 0.6 is predicted, the identificsr tion of this compound type is consistent with LEAC analysis. The evidence used in this identification appears otherwise weak, however, and this assignment should be accepted only conditionally. ACKNOWLEDGMENT
The author is grateful to E. C. Copelin and B. E. Buell of these laboratories for editing the original manuscript and for helpful discussions. The various ion exchange fractions were prepared by B. E. Buell, and the esterified naphthenic acid fraction by E. C. Copelin. LITERATURE CITED
(1) Copelin, E. c., ANAL. CHEM. 36, 2274 (1964). (2) Gordon, R. J., Moore, R. J., Muller, E. E., Ibid., 30, 1221 (1958).
(3) Horton, A. W., Burton, M. J., Tye, R., Bingham, E. L., Division of Petro-
leum Chemistry, ACS, New York,
N. Y . , Sept. 8-13, 1963. (4) Jewell, D. M., Hartung, G. J., J . Chem. Eng. Data 9 , 2 9 7 (1964). (5) Jewell, D. M., Yevich, J. P., Gulf Research & Development Co., Pittsburgh, Pa., personal communication, 1966. (6) Snyder, L. R., “Chromatography,” E. Heftmann, ed., 2nd Ed., Chap. 4, Reinhold, New York, 1966, (in press). (7) Snyder, L. R., ANAL.CHEM.33, 1535 (1961). ( 8 ) Ibid:, p. 1538. (9) Snyder, L. R., J . Chromatog. 6 , 22 iiafii) \-.,”*,.
(10) (11) (12) (13) (14)
Ibid., 8, 178 (1962). Ibid.; p. 319. Ibid., 16, 55 (1964). Ibid., 20, 463 (1965).
Snyder, L. R., Buell, B. E., ANAL. CHEM.36, 767 (1964). (15) Snyder, L. R., Buell, B. E., Anal. Chim. Acta 33, 285 (1965). (16) Snyder, L. R., Buell, B. E., J . Chem. Eng. Data, in press.
RECEIVEDfor review May 9, 1966. Accepted June 24, 1966.
Analysis by Nuclear Magnetic Resonance Spectrometry Taking Advantage of the Uncertaidy Principle ROBERT J. DAY and CHARLES N. REILLEY Department of Chemistry, University of Norfh Carolina, Chapel Hill, One manifestation of the uncertainty principle is the effect of decreasing the lifetime of a nucleus in a given environment on the nuclear magnetic resonance spectrum of the same nucleus or one to which it is coupled. These kinetic effects may have analytical significance by giving rise to spectra whose chemical-shift values or line widths may be used as a measure of concentration or to spectra which are simplified so that the resonances are more suitable for quantitative measurements. The shifting and merging of spectra by kinetic processes may also be used to shift an interfering resonance away from a resonance of interest if the interfering species can be made to undergo rapid exchange. Some examples of these applications are presented and discussed, including the use of quadrupole and paramagnetic relaxation.
T
effect of various rate processes on the spectra of absorbing molecules, a manifestation of the uncertainty principle, has applications in diverse areas of spectrometry. The magnitude of the effect can be estimated from the uncertainty relationship, M A t = h/2a. Since 4 E = h A v and At may be identified with 7, the average lifetime of the molecules in the various exchanging HE
N. C.
states, the relationship becomes 7 4 ~ slow exchange between different environments, separate resonances are = (1/2 7 ) . Thus, the magnitude of the observed for each environment; at rate constant that will give an observable effect will vary for different higher rates of exchange, the resonances types of spectrometry, depending on the are broadened and shifted toward each frequency differences involved. other; and with fast exchange, a single Table I gives the approximate order resonance is observed at a position which of magnitude of the 7 values that might is a weighted average of the positions be studied by various types of spectromand relative populations of the various etry. For example, Wicke, Eigen, environments. An example of the appearance of a spectrum at different and Ackermann (21)used the broadness of the OH stretching band in the Raman rates of exchange for a simple case involving two environments is given spectra of aqueous acids to estimate the relaxation time for the exchange of by Pople, Schneider, and Bernstein (16). protons between HaO+ and HzO; KreeThese effects can also arise from other voy and Mead (8) measured the mean relaxation phenomena such as nuclear lifetime of trifluoroacetate ion in quadrupole relaxation as well as from aqueous solutions of trifluoroacetic acid chemical exchange. from its Raman spectrum, and Pearson I n addition to their use in the study and Buch (12)used the ESR line widths of kinetic processes, the changes in the of paramagnetic cations to study rates NMR spectrum are, in themselves, of ion-pair formation. Line broadening analytically useful. For example, the also may arise from a distribution of energy states whose lifetimes are long compared to Av, especially in cases such Table 1. Approximate Lifetimes Necesas electronic or Mossbauer spectrometry sary for Observation of Kinetic Hfects where AV is very large, but because this in Spectra is not an “uncertainty” effect, this form of broadening is not discussed Av, Type of spectra sec.-l 7 , sec. here. Vibrational 10P-lO1* 10-~0-10-1~ The widest application of these Rotational 107-109 10-*-10 -10 principles has been in nuclear magnetic Electron pararesonance spectrometry, especially promagnetic 106-108 10-‘10-0 ton NMR. When a system under NMR 3-10-’ 5 X 10-”10-’ observation by NMR is undergoing VOL 38, NO. 10, SEPTEMBER 1966
0
1323
