sium chloride solution to 100 ml. of distilled water in the cell and taking simultaneous voltage and low-frequency specific conductivity readings. The titration curve a t 30 N c . (Figure 10) was obtained Tvith the higher concentrations of electrolytes permitted a t that frequency and is typical of the results obtained. Hydrochloric acid, 0.01N, was added to a solution of 10.0 ml. of 0.10S sodium hydroxide and 100.0 ml. of m t e r . The transfer plot readings were obtained by adding 0.155 potassium chloride solution to 100.0 nil. of distilled n-ater. The end point of the titration is approximately at the point of inflection on the transfer plot. A titration of a strong base by a iveak acid a t 30 X c , is demonstrated by the results shown in Figure 11. Curve 1 represents the voltage readings obtained by titrating a solution of 10.0 nil. of
0.10.N sodium hydroxide and 100.0 ml. of water with approximately 0.10N acetic acid solution. Curve 2 is a graph of the low-frequency specific conductivity for the same titration. CONCLUSIONS
One important advantage of the unbalanced circuit instrument is that the data are obtained in the form of voltmeter readings which can be plotted readily against volume without further calculation. The output also is in a form suitable for feeding into automatic reading or monitoring devices. HOTever, the operator must assume that the initial instrument settings remain fixed and that cell admittance is the only variable during a titration. This is not always so, as some drift may occur in the instruments. Drift would also
be a problem to some extent even under balanced bridge conditions. The new titration cell utilizing magnetic stirring and having the top of the magnetic stirrer as one electrode proved successful with this instrument and should be suitable for other types of high-frequency titration instruments. LITERATURE CITED
(1) Blaedel, W. J., Malmstadt, H. V., Petitjean, D. L., Anderson, K. K., ANAL.CHEX 24,1240 (1952). ( 2 ) Hall, J. L., Zbid., 24, 1236 (1952). (3) Hall, J. L., Gibson, J. A., Jr., Zbid., 23.966 (1951'). (4) Reilley, C. h., hIcCurdy, \I-. H., Jr., Zbid., 25,86 (1953). RECEIVEDfor review August 21, 1958. Accepted October 8, 1959. Portion of a diseertation presented a8 partial fulfillment of the requirement for the degree of doctor of philosophy in chemistry by Joe M. Walker at Kansas State College, 1958.
Low Voltage Mass Spectrometric Sensitivities of Aromatics G.
F.
CRABLE, G. L. KEARNS, and M.
S. NORRIS
Gulf Research & Development Co., Pittsburgh, Pa.
b A study of the mass spectral sensitivities of a number of aromatic compounds using low energy electrons has resulted in correlations useful for the prediction of low voltage sensitivities. The effects of substitution on an aromatic ring are evaluated for both hydrocarbon and nonhydrocarbon substitutions. These results permit sensitivities to b e predicted for compounds not available for direct study.
T
use of low energy electrons to decrease the complexity of mass spectral data obtained with the normal ionizing voltages of 50 to 70 volts has become an accepted analytical tool. Applications to the analysis of unsaturated hydrocarbons have been made by Field and Hastings (a), Lumpkin (15), and Kearns, Maranowski, and Crable ( I S ) ; Sharkey, Robinson, and Friedel (17) have used low voltage techniques in the analysis of oxygen-containing compounds from coal hydrogenation. For the majority of mass spectrometric analyses of low molecular weight materials, the spectra obtained using 50- t o 70-volt electrons are adequate. Individual components can usually be determined by standard matrix procedures. HoFvever, in the high molecular weight range, approximately Clz and above for hydrocarbons, standard HE
techniques yield only group-type information (3, 10). I t is in applications to mixtures of unsaturated high molecular weight compounds that low voltage techniques, with their ability to determine both compound types and the molecular weights of the compounds within a type, become a source of valuable information on compositions. In common with all high molecular weight mass spectrometric techniques, the major problem in developing low voltage procedures is that of obtaining suitable calibration data. Lumpkin (15) has related low voltage sensitivities to the inverse of the molecular weight by drawing smooth curves through a number of experimental values for aromatic compounds of different degrees of ring condensation. Kearns, Maranowski, and Crable (13)shon-ed that the sensitivities for aromatics were a function of the degree of substitution. When expressed as divisions per microliter (or divisions per unit of liquid volume), sensitivities increased rrith increasing degree of ring condensation and with an increase in the number of alkyl substitutions. Sharkey, Robinson, and Friedel (I?') have observed similar substitution effects in their study of phenols. The present work is a report of a study of possible correlations of low voltage sensitivity data with molecular properties of aromatic compounds. Its
purpose was to develop generalizations which would explain the results observed for alkyl aromatics and would predict low voltage sensitivities for additional aromatic types. Sensitivities and ionization potentials are shown to be approximately linearly related on a semilogarithmic plot. A comparison of sensitivities and electron-accepting and -donating properties of substituent groups has shown a good correlation. The electron removed in producing the parent ion for an aromatic compound is assumed to be derived from the aromatic ring. Substituent groups affect the ease of removing a ring electron in a predictable manner. These conclusions provide a reasonable explanation of the observed sensitivity data and permit sensitivity predictions to be made for compounds not available for direct study. INSTRUMENTATION
A Consolidated Electrodynamics Corp. Model 21-103 mass spectrometer modified as described (13) mas used for obtaining the experimental data. All data were obtained with a repeller voltage of 1.5 volts and a nominal ionizing voltage of 8.0 volts as determined by the voltmeter in the electron-accelerating circuit. A comparison of the spectroscopic ionization potentials of argon and krypVOL. 32, NO. 1, JANUARY 1960
13
ton with experimental values showed that the voltmeter readings of nominal electron energy are low by 2.0 volts. The actual maximum electron energy in the beam a t a nominal energy of 8.0 volts is 10 0 volts. Because the ionization potential of normal Clo paraffin is 10 19 volts ( 7 ) and the ionization potentials of higher paraffins change very little with increasing molecular weight, a nominal value of 8.0 volts will eliminate all parent ions from paraffins. Both olefins and aromatics exhibit parent peaks at this voltage. To eliminate the effect of small changes in instrument operating conditions and the problem of quantitative introduction of small liquid volumes, all calibration data were obtained from blends by comparison with a n internal standard. Although a number of compounds were used as internal standards, all were compared either directly or indirectly with ethylbenzene. The sensitivities per unit liquid volume were standardized to a value of 100 divisions per microliter for ethylbenzene. Molar sensitivity data were standardized to a value of 1.00 division per micron for ethyIbenzene. The term “molar sensitivity” is used throughout for sensitivity in divisions per micron. Molar sensitivity is an appropriate term, as divisions per micron can be interpreted as divisions per mole. I n order to compare low voltage semitivities correctly on a true mole per mole basis. the effects of the several isotopic species in a given compound must be considei ed. For example. dodecylbenzene contains the following molecular of CiiI&o, 16.3 species: 82.1 mole mole 70 of Ci3C::Hj0 and C:;H2$D, 1.6 mole % of C:3C:iH30, and negligible concentrations of heavier species. If, as is usually done, the low voltage sensitivity is calculated as the number of CiiH30 ions per unit of pressuie of dodecylbenzene introduced into the instrument, the calculated sensitivity will be Ion-er than the true molar sensitivity by 17.9%. The true molar sensitivity is obtained either by measuring and summing the parent ions of all isotopic species, or by measuring only the principal parent ion and making appropriate corrections. TTithout this correction a bias in a direction to decrease the molar sensitivities of hydrocarbons gradually with increasing number of carbon atoms would be present, All molar sensitivity data presented here have been corrected to a true molar basis as discussed above. However, the sensitivity data in divisions per unit liquid volume were not corrected, because they are used in analytical applications to determine the total concentration of a compound from a measurement of the principal parent ion only-Le., the molecular ion containing the most abundant isotope. I n the case of hydro14
*
ANALYTICAL CHEMISTRY
0
I
BENZENE
0 MONOSUBSTITUTED BENZENES A DISUBSTITUTED BENZENES V TRISUBSTITUTED BENZENES
0 TETRASUBSTITUTED BENZENES
I
0
Figure 1.
Molar sensitivities of alkylbenzenes
carbons the principal parent ion contains only CI2’s and H’s. RELATION OF LOW VOLTAGE SENSITIVITIES TO IONIZATION POTENTIALS
Lossing, Tickner, and Bryce (14) have shown that plots of the logarithm of observed ion currents as a function of electron energy are essentially linear within a range of several volts of the ionization potential. Honig (11) has derived expressions for ion current as a function of the electron-accelerating potential for a standard mass spectrometer having a spread of electron energies because of thermal effects. His results indicate that the ionization efficiency curve near onset of ionization will be very nearly exponential with respect to the electronaccelerating potential. From these results this portion of the ionization efficiency curves can he approximated by I,
&b(v-r-t)
or log I ,
=
log A
+ b(V - V,)
(1)
where
It
=
A. b
=
5;t
= =
V
ion current in arbitrary units per unit of sample pressure constants rrhich include instrumental factors ionization potential electron-accelerating voltage
b is a function of the inverse of the filament temperature and can be considered as a constant for the usual low voltage operation. A is related to the ionization cross section of the molecule and is assumed here to be constant for molecules of the same type, in this case aromatics, Such an assumption is reasonable for substituted benzenes if the electron removed in producing a molecule ion (parent ion) is a pi electron from the benzene ring. The number and location of the pi electrons will be similar for these compounds and the processes for ionization by electron impact should be similar. Equation 1 predicts that for low voltage spectrometry where the accelerating
voltage, V , is held constant, a plot of the log of sensitivity as a function of ionization potential should be a straight line. Experimental molar sensitivities plotted as a function of literature ionization potentials confirm this. However, there is so much disagreement among published ionization potentials that such a plot cannot be quantitatively utilized. Nicholsoii (16) has recently published a critical review of ionization potential measurements by various electron-inipact methods. He shows that the usual methods of measurement ordinarily give results accurate to hO.1 e.v., but can be very much in error in some cases. As the reported ionization potentials of the compounds of interest differ by only 0.1 to 0.2 volt, only a rough correlation of reported ionization potentials and sensitivities can be expected. These results indicate that literature ionization potentials should not be used for more than a first approximation to low voltage sensitivities. The present data suggest that molar sensitivities of alkylbenzenes of a given degree of substitution are nearly alike, 4 comparison of the data for benzene and the mono- and disubstituted benzenes indicates that the addition of an alkyl group reduces the ionization potential by approximately 0.3 e.v. Similar results were discussed by Field and Franklin (6) in their study of ionization potentials, in which they ohserved that the ionization potentials of a number of alkylbenzenes decreased rapidly as additional alkyl groups were substituted on a benzene nucleus. Increasing the lengths of the substituent groups had only a minor effect on ionization potentials. Although Equation 1 is written in terms of the ionization potential, it is not necessary to know the ionization potentials of the compounds of interest in order to estimate sensitivities. From sensitivity data for a set of similar compounds, sensitivity changes resulting from the addition of a particular substituent group can be estimated. This is based on the assumption that the addition of a substituent group at a par-
ticular position on an aromatic results in the same change in ionization potential regardless of the structure of the original aromatic. The magnitude of the change in sensitivity from the addition of a particular substituent group will depend on the ionization potential of the original aromatic. As a n example of this type of estimation, from a knowledge of the molar sensitivities of phenol, and 0-.m-, and pcresols from Table 11, and a value for the molar sensitivity of aniline, reasonable sensitivities for the o-, M - , and p toluidines can be calculated. Calculated sensitivities are 5.13, 6.39, and 7.21 compared n ith experimental values of 5.20, 5.79. and 6.43, respectivelj. for the 0- m-, and p-toluidines. A maximum error of 12% occurs for the para position n i t h smaller errors for the meta isomers. Of practical interest to low voltage anal>tical work is the fact that Equation 1 slions that the ratio of molar senzitivitips for any two compounds is a constant which is independent of the electronaccelerating voltage. This conclusion involves no assumptions concerning the constancy of A for different conipounds. It is, however, invalid for compounds having ionization potentials less than or equal to the actual electronaccelerating voltage; this limitation is inherent in the derivation of Equation 1 [iee Honig ( 1 1 ) ] . SENSITIVITIES OF ALKYLBENZENES
In their study of low voltage sensitivities of a number of alkylbenzenes, Kearns e2 al. (18) observed that the sensitivities were a function of the numher of substituent alkyl groups. Conversion of these results from sensitivities per unit liquid volume to molar sensitivities permits one to study structural effects on sensitivities on a per molecule basis. Figure 1 shows alkylbenzene molar sensitivity data plotted as a function of molecular weight. The monosubstituted sensitivities show an increase in sensitivity with increasing size of the substituent group a t the low molecular weight end of the curve. Beyond a propyl substitution the molar sensitivity is essentially independent of the length of the substituent group. T h e sensitivities of the more highly substituted alkylbenzenes behave in a similar manner, at least within the limits of the available experimental data. The curve for tetrasubstituted alkylbenzenes is essentially an extrapolation based on the assumption that it follows the same general form as the mono-, di-, and trisubstituted curves. The observed independence of molar sensitivities with the size of the substituent group strongly suggests that the electron removed in the ionization of aromatic molecules is derived from the
aromatic ring and not from the substituent group. Because the least strongly bound ring electrons are the pi electrons, one would expect factors which affect the electron density of the aromatic ring and thus the binding energy of the pi electrons to influence low voltage sensitivities. The effect of alkyl substitution can be explained as resulting from changes in the electron configuration of the ring b y the substituent groups. T h e results indicate that the effect of alkyl substitution is t o decrease the energy required to remove an electron in approximately an additive fashion-i.e., sensitivities increase stepwise with increasing degree of substitution. Although some increases in sensitivities are seen in increasing the size of the substituent group from methyl to propyl, the effect of substituent size is removed for larger groups through the insulating effects of the methylene groups adjacent to the ring. Effects of substituent groups on the electronic configurations of aromatic rings are commonly discussed in terms of electron-donating or -R ithdrawing properties of the substituent groups (12) and their consequent effects on electron densities of the ring. However. other effects, sucli as electrostatic polarization as discussed recently by Coggeshall (4), play an important role in treating aromatic molecules. Throughout the remainder of the discussion, the terms electron-donating and electron-withdrawing are used with the understanding that other explanations of the effect of substituent groups are not excluded. Using this terminology, alkyl groups are considered to be electron donors to the aromatic ring. As such, they increase the ring electron density, decrease the energy of electron removal, and thus result in a molar sensitivity increase when attached to an aromatie ring. GENERAL STRUCTURAL CORRELATIONS WITH AROMATIC SENSITIVITIES
The changes in ring electron densities as a function of the particular substituent group are not, in general, measured directly. A source of indirect information on the relative electron densities in aromatic rings and thus l o x voltage sensitivities is the relative substitution-directing powers of substituent groups on a benzene ring ( 1 , 9). For some time the directing properties of substituent groups have been explained in terms of a shift of charge to or from the ring and the resulting distribution of charges on the carbon atoms of the ring ( I d ) . Ortho-para-directing groups donate electrons to the ring and the additional charge appears at t h e para and the two ortho positions. A meta director removes or accepts electrons from the ring, leaving the ortho and para positions positive with respect to the two meta
Table I. Correlation of Molar Sensitivities with Directing Properties of Substituent Groups l*Iolar SubSensitivity of
stituent Directlng Substituted Property Benzene Group Strong ortho-para 3 42 -KHz Strong ortho-para 1 12 -OH 1 70 -OCHs Intermediate ortho-para -CH, Weal, ortho-para 0 SS -Br Weal, ortho-para 0 69
-H
-CHO -COOH -CS
-3 0
Weak meta Intermediate meta Intermediate meta Strong meta
0 0 0 0 0
35 13 21
07 03
carbons. A strong ortho-para-directing group increases the electron density of the ring, R hile a strong meta-directing group decreases the electron density of the ring. These ideas are supported by nuclear magnetic resonance data ( 2 , 6) of cheniical shifts of ring hydrogens as a function of the substituent group. A comparison of KO2 and Tc"2 as substituent groups, a strong electron acceptor and a strong electron donor, respectively, shon s a large negative shift for nitrobenzene and a large positive cheniical shift for aniline. These shifts indicate that the ring positions of nitrobenzene are more positive than benzene in increasing order nieta, para, and ortho. The shifts ill aniline show the ring positions of aniline to be more negative than benzene in increasing order meta, para, and ortho. Although Corio and Dailey (5) point out the iniportance of polarization in addition to electron density distribution as a factor in determining directing properties. their experimental results agree, in general, n ith the thesis that ortho-para directors donate electrons while meta directors accept electrons from the ring. Because nonalkyl substituent groups provide a wide range of electron-accepting and -donating powers, a comparison of their effects on Ion- voltage sensitivities should provide a good measure of the correlation between ring electron density and sensitivity. Table I gives a list of substituent groups in order of decreasing ortho-para-directing properties and relative molar sensitivities wherever the corresponding substituted benzenes were available. Table I1 contains the collected sensitivity data for all aromatics studied in this work. A number of qualitative correlations can be obtained from these data. For example, bromobenzene has approximately the same sensitivity as toluene, while a-bromonaphthalene (1bromonaphthalene) has the same sensitivity as a-methylnaphthalene (1methylnaphthalene). T h e low sensitivity for o-chlorophenol as compared with o-bromophenol is in agreement n ith VOL. 32, NO. 1, JANUARY 1960
0
15
Table II.
Tabulation of Sensitivity Data
Relative Molar RelaSensitive tivity Liquid (Mono- Volume isotopic SensiBasis) tivity 0.35 49 0.83 97 1.00 100 1.05 91 0.89 77 1.18 90 1.06 81 0.91 70 1.19 61 1.33 50 1.51 154 1.69 168 172 1.73
Relative Molar Sensitivity (Monoisotopic Basis) 10 2 a 1 68 2 72 1 12 1 68 2 09 2 36 2 33 1 19
Relative Liquid Volume Sensitivity 370b 179
Compound Compound Benzene Hexaethylbenzene Toluene Styrene Ethylbenzene Biphenyl 2000 n-Propylbenzene Isopropylbenzene Phenol 160 n-Butylbenzene o-Cresol 200d sec-Butylbenzene m-Cresol 246 tert-Butplbenzene p-Cresol 279 1-Phenyloctane m-Ethylphenol 238 o-Chlorophenol 1-Phenyldodecane 143. o-Bromophenol 1 48 160d o-Xylene Aniline 3 42 46gd m-Xylene o-Toluidine 5 20 608d p-Xylene m-Toluidine 5 79 66id l-llethyl-3p-Toluidine 6 43 783d ethylbenzene 1.97 172 Kitrobenzene 0 03 4f l-llethyl-4m-Nitrotoluene 0 14 15~ ethylbenzene 2.09 176 p-Xitrotoluene 0 08 9d 1,2-Diet,hylbenzene 1.90 149 Anisole 1 70 193d 1,3-Diethylbenzene 2.09 161 o-Dimethoxybenzene 3 01 288h 1,4-Diethylbenzene 2.18 167 0 07 Benzonitrile 8h 135 1,2-Diisopropylbenzene 2.19 0 13 Benzaldehyde 16h 1,3-Diisopropylbenzene 2.27 140 0 21 2Th Benzoic acid 1,2,3-Trimethylbenzene 3.43 219 Bromobenzene 0 69 83h 1,2,4-Trimethylbenzene 2.82 2 76 249 Indene 289 1,3,5-Trimet.hylbenzene 2.48 Tetralin 216 1 95 171 1,a-Dimet hyl-4Naphthalene 4 59 490 et'hglbenzene 2.95 230 a-Methylnaphthalene 465 5 46 1,3-Dimethyl-5p-Methylnaphthalene 656d 7 68 3.04 ethylbenzene 234 2,6-DimethylTriethylbenzenes naphthalene 9 67 737d (mixture of isomers) 3.23 202 1,2,3,4,6,7,8,9-0cta1,2,3,5-Tetramethylhydroanthracene 4 41 260' benzene 3.58 284 Anthracene 10 0 801i 1,2,4,5-Tet,ramethylPhenanthrene 590 9 02 benzene 4.13 326 a-Bromonaphthalene 5 28 453d Pentamethylbenzene 4.48 327 a-Fluoronaphthalene 3 88 360 Hexamethylbenzene 6.36O 490 a The difference in molar sensitivities observed for hexamethylbenzene and hexaethylbenzene appears large compared with differences in molar sensitivities of other methyl- and ethyl-substituted benzenes. Where needed, the liquid volume sensitivities were calculated using density data taken under the following conditions: b
Q
h i
12.50
16O
c
c.
250/250
c.
88" C. i 27'/4" C.
the fact that chlorine is a slightly poorer electron donor than bromine. The low sensitivity of fluoronaphthalene compared with unsubstituted naphthalene agrees with the classification of fluorine as a poor electron donor. A discussion of the effects of two substitutions on a n aromatic ring is complicated by the fact that the individual effects of each substitution are not completely independent. An examination of data for electron-donating groups from Table I1 shows that although molar sensitivities increase with additional substitution, the final results are considerably modified by the position of the substituent groups. For example, the 0-, rn-, and p-toluidines all have sensitivities greater than aniline. However, the 16
ANALYTICAL CHEMISTRY
position of the additional methyl group in the toluidines greatly affects the sensitivities of the three isomers, increasing the sensitivity in the order: ortho, meta, and para. A similar effect is seen for the cresols, whose sensitivities again increase in the order: ortho, meta, and para. T h e m- and p-nitrotoluenes show a reversal, the meta isomer having a slightly higher sensitivity than the para. The effect is small and may be a result of the electron-accepting properties of the NO2 group. Although substitution position effects similar to those noted here for electron-donating groups are discernible in the alkylbenzenes, the magnitude of the effect is smaller. Included in Table I1 are sensitivities for a number of condensed aromatics.
A large change in sensitivity is observed with the increase in the size of the aromatic nucleus in the series benzene, naphthalene, and phenanthrene. The increased conjugation with the more highly condensed ring system results in a decrease of the energy binding a pi electron to the ring. This fact plus the increase in number of pi electrons results in an increase in molar sensitivities with number of condensed rings. Biphenyl is interesting in that it has a larger number of pi electrons per molecule than does napthalene but a smaller degree of conjugation, with only a single bond connecting the twc rings. Its molar sensitivity is 2.72, or about halfway between those of benzene and naphthalene. This result indicates that the degree of conjugation is more important than the number of available pi electrons. A considerable difference in the molar sensitivities of a- and 8-methylnaphthalenes was observed, with the betasubstitution having the higher sensitivity. This same effect was seen in the results of Sharkey, Robinson, and Friedel (I?') for the CY- and p-naphthols. Because the results for the methylnaphthalenes showed an increase in sensitivity of 4@4 in going from alphato beta- substitution while the naphthol data increased by only lo'?&, no quantitative generalization can be made for this effect. Condensed ring systems having both aromatic and saturate rings can be explained b y assuming that a saturate ring has the same effect as two ethyl A consideration of substitutions. Tetralin as a benzene ring with two alkyl substitutions in the 1,2 positions indicates that the molar sensitivity should be similar to that of 1,2-diethylbenzene. The agreement between the observed sensitivities for Tetralin and 1,2diethylbenzene, 1.95 and 1.90, is very good. 1, 2, 3, 4, 6, 7, 8, 9-Octahydro anthracene is an example of a compound having two saturate rings attached to an aromatic ring. From the positions of the saturate rings, the above gcneralization would predict that its molar sensitivity should be similar to a tetraethylsubstituted alkylbenzene having the 1,2, 4,and 5 positions occupied. This compound n-as not available. However, the observed molar sensitivities of 1, 2, 4,5tetramethylbenzene and octahydroanthracene are 4.13 and 4.41, respectively, in reasonable agreement. Indene is a particularly interesting case, because i t consists of an aromatic ring and a condensed ring having one double bond. $n extension of the explanations given for Tetralin and octahydroanthracene to indene would predict a sensitivity equal to a benzene ring with one -C=Cgroup and one alkyl group. No such compound was available for study. However, a comparison of the sensitivities for styrene and ethyl-
benzene shows that the double bond in the styrene produces 0.68 division per micron increase in sensitivity. T h e addition of 0.68 to the molar sensitivity observed for Tetralin gives a n estimated sensitivity of 2.63 for indene, in good agreement with the observed value of 2.76. A further prediction can be made concerning the effects of substitution on the saturate ring compared with a substitution on the aromatic ring of a compound having condensed saturate and aromatic rings. T h e substitution on the saturate ring should have a negligible effect on the molar sensitivity, while a substitution on the aromatic ring should have a n effect similar to the effect of substitution of the group on a benzene ring. Unfortunately, no experimental data are available here to test this prediction. Although the discussions above have been concerned with the prediction of sensitivities from information on electron-accepting and -donating properties of substituent groups, one can now use the established generalizations to obtain useful structural information about relatively pure compounds. T h e type and positions of many substituents can be estimated from observed sensitivities. Because the correlations observed apply only to sensitivities on a per molecule basis, all data have been discussed in terms of molar sensitivities. The data are easily converted to sensitivities per unit liquid volume. as used in a practical analytical scheme, by multiplying by a factor containing liquid density/ molecular weight. Table I1 contains sensitivities converted to liquid volume units for all compounds studied.
CONCLUSIONS
The following set of structura1 correlations for predicting low voltage molar sensitivities has resulted from this study: T h e effect of a single substitution can be predicted from a tabulation, such as Table I, of relative electron-accepting and -donating properties. Increasing the length of a n alkyl substitution beyond two or three carbons produces no change in sensitivity. Molar sensitivities increase with increasing number of donor substituents, T h e position of donor substituents on disubstituted benzenes produces a sensitivity change increasing in the order ortho, meta, and para. Condensed saturate rings on an aromatic have the same effect on sensitivity as two alkyl substitutions. I n a compound having condensed saturate and aromatic rings, substitution on the saturate ring has little effect on the sensitivity. Sensitivities increase with an increase in the number of condensed aromatic rings per molecule. Donor substitution on the position of p-naphthalene produces a larger increase in sensitivity than a similar substitution in the alpha position. Higher condensed aromatics probably shox similar changes with position of substitution,
for obtaining the mass spectrometric experimental data. LITERATURE CITED
(1) Brewster, R. Q., “Organic Chemistry,” 2nd ed., p. 497, Prentice-Hall,
New York, 1954. ( 2 ) Chamberlain, K.F., ANAL.CHEM.31,
56 i1959). ( 3 ) Clerc, k. J., Hood, A,, O’Neal, 31. J., Ibid., 27, 868 (1955). (4) Coggeshall, N. D., J . Chem. Phys., in press. (5) Corio, P. L., Dailey, D. P., J . Am. Chem. Soc. 78, 3043 (1956). (6) Field, F. H., Franklin, J. F., “Electron Impact Phenomena,” p. 119, Academic Press, Kew York, 1957. ( 7 ) Ibid., p. 270. (8) Field, F. H., Hastings, S. H., AZIAL. CHEX.28, 1248 (1956). (9) Glasstone, S., “Textbook of Physical Chemistry,” 2nd ed., p. 591, Van Nostrsnd, Princeton, N. J., 1916. (10) Hastings, S. H., Johnson, B. H., Lumpkin, H. E., AXAL.CHEXI. 28, 1243 (1956). (11) Honig, R. E., J . Chem. Phys. 16, 105 ( I 948). ( 1 2 ) Ingold, C. K., “Structure and hlechanism in Organic Chemistry,” Chap. 6, Cornell University Press, Ithaca, S . Y., 1053. ( 1 3 ) Kearns, G. L., Ilaranowski, K. C., Crable, G. F., ANAL.CHEX. 31, 1646
(1959).
(14) Lossing, F. P., Tickner, A. W., Bryce, K. A , , J . Chem. Phys. 19, 1254
(1951).
115) Lumokin. H. E., ASAL. CHEW 30.
321 (1968). ’ (16) Sicholson, A. J. C., J . Chern. Phys. 20. 1x12 - _ - - (1Q.m). (lfj Sharkey, A. G., Jr., Robinson, C. F., Friedel, R. A., 4STM Committee E-14, Conference on Mass Spectrometry, New Orleans, La., May 1958. RECEIVED for review June 29, 1959. AcI
ACKNOWLEDGMENT
The authors thank K. D. Coggeshall and D. E. O’Reilly for a number of interesting discussions of this work and P. TT. hfazak, J. P. Klems, and D . J. Clancy
\ - - - - ,
cepted October 1, 1959. Seventh Meeting
of ASTRI Committee E-14 on Mass Spectrometry, Los Sngeles, Calif., May 1959.
Modifkation of S pectrofIuorometric Determinuti on of Aminochromes in Human Plasma A. N. PAYZA and MARGARET MAHON Psychiafric Research Unit, Deparfmenf o f Public Health, University Hospital, Saskatoon, Saskatchewan, Canada
b In connection with the assay method for adrenochrome (2,3-dihydro-3hydroxy-N-methylindole 5,6 quinone), originated in this laboratory, a modification has been developed which allows the method to b e used for whole tissues-Le., red blood cells, etc. A theoretical objection to the earlier method has been met in that the present procedure has a stable blank. A large series of indoles was also tested and found not to interfere with the basic method.
-
T
-
HE procedure for determining the reagent blank in a previous method
(4)was criticized by Feldstein ( 1 ) on the basis of interference of zinc acetateascorbic acid fluorescence, Kith the fluorescence arising from the aminochrome-zinc acetate complex. The present reaction of aminochrome with zinc acetate does not use ascorbic acid, the source of the unstable reagent blank. MATERIALS AND METHOD
T h e chemicals described in the previous method were used (4). Yew compounds tested were: 5-hydroxyindole, 5-hydroxyindole-3-acetic acid (Regis Chemical Co.), 3,4-dihydroxyphenyl-
serine (Bios Laboratories), 3-(2-aminoethyl)-5-indolol (serotonin) (Nutritional Biochemicals), and N-isopropyladrenalone sulfate, ephedrine of unknown origin. The following additional compounds were studied: N-ethylnoradrenochrome, AT-isopropylnoradrenochrome, 3-iOdOadrenochrome, 2-iodo-2-methylnoradrenochrome, 2-iodo-N-ethylnoradrenochrome, 2-iodo-N-isopropylnoradrenochrome, adrenochrome methyl ether, adrenochrome ethyl ether (3-ethylepinchrome) , leucoadrenochrorne (5>6dihydroxy-hr-methylindole), 2-iodoleucoadrenochrome, and adrenolutin (l-methyl-3,5,6-indoletriol), Reagents. 1. Trichloroacetic acid VOL. 32,
NO. 1, JANUARY 1960
17
