tives of thiophenoquinoline as shown in the table. Part or all of the 241 mass could be due to the dithiophenoquinoline structure or to chrysene and its isomers. The last two peaks of still smaller magnitude (202 and 302) can be ascribed to pyrene, benzoperylene, and/or dibenzopyrene types of hydrocarbon structures. Interestingly, there were no significant peaks due to naphthalene, phenanthrene and/or anthracene, or isomers of pentacene. It may be well a t this point to consider the methods used to identify the major fragment peaks. A mass spectrum shows the distribution by mass of the ionized products that result from electron bombardment of the sample. Unfortunately, the spectrum tells us nothing about the identity of an ion except its mass. The problem, then, is to relate mass to molecular structure. The relationships between mass and molecular empirical formula can be expressed as follows: m
=
CnH2rrti
or m = 12n
+ 2n + z = 14%+ z
where m = mass n = C N Q 25 z = 2-20-2R N 2Q 45 C, N, Q, and S = number of carbon, nitrogen, oxygen, and sulfur atoms, respectively D = number of double bonds (including aromatic) R = number of rings
+ + ++ + +
Thus an ion of a given mass can be definitely associated with certain possibilities of structural formulation within the limits set by the above equations.
These possibilities can be further limited by other considerations. For example, mass 302 cited above could be caused by CnHB (2 = -6), a hexadecylbenzene. This type of molecule can be eliminated because of the kind of sample under consideratmion. Another possibility would be CsH26 (2 = -20), which is equivalent to a molecule containing a dihydropyrene nucleus with a 7-carbon atom side chain. This would be a light oil component and also not a likely possibility in an asphalt sample. A more likely pos(2 = -34), which is sibility is dibenzopyrene and/or its isomers. However, the molecular weight distribution of this asphalt sample would indicate that the peak a t mass 302 is not caused by dibenzopyrene itself, but rather by a dibenzopyrene nucleus linked to other groups within the asphalt molecule. In this case the dibenzopyrene nucleus (mass 302) is formed by rearrangement (hydrogen saturation) of the dissociated nucleus. Such rearrangements are prevalent for highly condensed aromatics. Since little is known about the molecular composition of asphalt, Table I can be considered only as an indication of possible structures within the asphalt molecule. It appears, for example, that asphalt does not consist of very highly condensed polyaromatic structures. Rather, these mass spectrometric studies have indicated that the degree of condensation is unlikely to be very great although the number of different ring systems may be very large. Similar conclusions based on the physical properties of asphalts have been reported (2, 6). This preliminary investigation has
indicated that mass spectrometric techniques might be useful by providing information about the molecular structure of asphalts that is not obtainable a t the present time by any other technique. The full possibilities of this technique can be realized only after further study with very high molecular weight pure compounds and the simplification of asphalt samples by various separation methods. ACKNOWLEDGMENT
The authors acknowledge the help of A. Hood, C. K. Hines, C. E. Davis, and R. Y. Seaber in connection with this investigation. LITERATURE CITED
(1) Brown, R. A., Skahan, D. J., Cirillo,
V. A., Melpolder, F. W., ANAL.CHEM. 31, 1531 (1959). (2) Hillman, E. S., Barnett, B., Proc. Am. Sac. Testing Materials 37, 11, 558 (1937). (3) O’Neal, M.,, J., in “Applied Mase Spectrometry, p. 27, Institute of Petroleum, London, 1954. (4) O’Neal, M. J., Hood, A., Clerc, R. J., Andre, M. L., Hines, C. K., Fourth World Petroleum Con ress, Section V/C. Remint 3. Carlo 8olombo Pub.. Rome, 19’55. ’ (5) Traxler, R. N., Romberg, J. W., Petroleum Engr. 30, No. 11, G37 (1958). (6) Voorhies, H. G., et a$,, in “Advances in Mass Spectrometry, p. 44, J. D. Maldron, ed., Pergamon Press, London, 1959. -_._
(7) Washburn, H. W., in “Pfiysical Methods in Chemical Analysis, Vol. I, p. 630. Academic Press, New York, 1950. RECEIVEDfor review July 18, 1960. Accepted November 7, 1960. Division of Petroleum Chemistry, 138th Meeting, ACS, New York, N. Y., September 1960.
Flame Spectrophotometric Study of Silver JOHN A. DEAN and CHARLES B. STUBBLEFIELD‘ Department of Chemisfry, Universify o f Tennessee, Knoxville, Tenn.
b The flame emission characteristics of the two ultraviolet emission lines of silver at 328.0 and 338.3 mp from aqueous solution and from solutions of several organic solvents have been thoroughly studied in oxygen-acetylene and oxygen-hydrogen flames. Parameters investigated include oxygen and fuel-gas flows; optimum ratio of flows, oxygen/fuel gas; effect of different burner heights; and background emission. The spectral and radiation interferences of metals and anions that are commonly associated with silver have been examined. 382
ANALYTICAL CHEMISTRY
F
methods for silver have been reported by Rivkina (Q),and Pungor and KonkolyThege ( 7 ) have studied briefly the behavior of the 338.3-mp silver line. Rathje (8)applied the flame photometric method to zinc-cadmium phosphors and Galloway (6) determined silver in blister copper. Nevertheless, there is a distinct lack of information regarding the optimal conditions for the determination of silver, as well as information about interferences arising from anions and cations commonly encountered during silver determinations. This inLAME SPECTROPHOTOMETRIC
vestigation describes a thorough study of the flame emission characteristics of silver in oxygen-hydrogen flames, in oxygen-acetylene flames, and in flames into which an organic solvent is aspirated. REAGENTS AND APPARATUS
Reagents. Silver, standard solution, 10,000 pg. per ml. Dissolve 15.74 grams of ACS grade silver nitrate, 1 Present address, Columbia-Southern Chemical Corp., Corpus Christi, Tex.
50
5or O X Y G E N FLOW C Y I f . per h r
40 (3
z 9W 30 K
~
s
8 2
13 7. I
&//
OXYGEN FLOW
cu. I t . Der hr.
3.00 2.45 e.05 1.60 1.25 0.95
0 4.3
35
4.8
\
rnl ~ e mi". r
0 7 1 48
I
ASPIRATION RATE
rn 8 2 A 63 A 5 5
A 63 A 5.5
40 45 5 0 55 6 0 6 5 ACETYLEhE FLOW,, c d ++ per i r
70
Figure 1 . Emission intensity of silver line at 338.3 mp as function of acetylene flow at various oxygen flows Present, 200 p.p.m. silver; burner height, 10 mm.; multiplier phototube, 52 volts/dynodei ERA, % adjust, 0; mirror blocked; slit width, 0.030 mm.
dried at 110" C., in deionized water and dilute to exactly 1 liter. All other salts were reagent grade chemicals Apparatus. The Beckman Model DU flame spectrophotometer employed has been described (1). Data mere recorded on a 10-mv. Bristol recorder with a 1-second pen response by the Beckman energy recording attachment (ERA). These instrument settings were employed, unless otherwise specified: I
Selector switch, position 0.1 Sensitivity control, ERA yo adjust 0 Phototube resistor, megohms 22 Phototube (RCA 1P28), volts per dynode 60 Slit, mm. 0.030 Spectral band width, mp 0.17 Mirror (except for hydrogen flames) Blocked Recommended fuel flows, cu. It. per hr. Hydrogen 13.5 Oxygen 4.7 Acetylene 5.0 Oxygen 5.5 OPTIMUM INSTRUMENT SETTINGS
Slit Width. The choice of slit width affects the ratio of signal-toflame background and also the interference ratio of adjacent emission lines. Throughout the investigation no problem existed in obtaining the desired emission sensitivity since a multiplier phototube was used. The slit width employed was 0.030 mm., which corresponds to a band width of 0.17 mp at one half the maximum intensity. The spectral band width is narrower than that used by Whisman and Eccleston (IO), which mas 0.30 mp, and considerably narrower than those used by Rathje (8) and by Pungor and Konkoly-Thege (7), which were 1.2 and 2.9 mp, respectively. Burner Height. The atomizerburner was mounted on a rack-and-
O 0.6
0 43
i IO
I.5
25
2.0
3.0
4.0
35
R A T I O O F THE FLOW RATE; O X Y G E N / A C E T Y L E N E
Figure 2. Emission intensity of silver line at 338.3 mp as function of ratio of flow rate, oxygen/acetyIene, at various oxygen flows Conditions same as for Figure 1
pinion mechanism. By adjusting the height of the burner, one is able to study the variation in silver emission intensity from different areas of the flame. This mas done from the tip of the burner (0 mm.) to the tip of the flame. Various ratios of oxygen/ fuel were employed. I n the normal burner position (10 mm.), as fixed by the manufacturer in the burner housing, the multiplier phototube is receiving light from an area between 10 and 16 mm. above the tip of the burner. Changes in the height of the burner produced little change in the silver emission intensity from an oxygenacetylene flame. The areas observed included the ranges 0 to 6 mm. and up to 12 to 18 mm. By contrast, changes in burner height brought about a marked effect upon the silver emission intensity from an oxygen-hydrogen flame. Readings taken a t a position 20 mm. above the burner tip are twice those obtained either at 10-mm. or a t 30-mm. positions (see also Figure 4). Interestingly, the silver line a t 328.0 mp is somewhat stronger than the line a t 338.3 mp a t the normal burner position (10 mm.) whereas the opposite is true a t the other flame positions studied. Fuel and Oxygen Flows. Optimum oxygen and fuel flows are those which give the greatest emission and a t the same time provide a low flame background. The emission of the 338.3-mw silver line as a function of the acetylene flow, with the oxygen flow as a parameter, is shown in Figure 1. Only a slight gain in emission intensity is achieved when the acetylene flow exceeds 5.0 cubic feet per hour and the oxygen flov exceeds 5.5 cubic feet per hour. Whatever the oxygen and acetylene flows, the optimum ratio of oxygen/ acetylene flows is approximately 1.1, as shown in Figure 2. At the optimum flow ratio there is a slight gain in emission intensity as the total amount of oxygen is increased. The slight gain is
O X Y G E N FLOW C Y f t / hr
80-
I 0
OXYGEN FLOW
60
VI
-
,
IO
6
12
1-
16
I8
12 14 16 18 20 2 2 2 4 26 HYDROGEN FLOW, C U f t 1 hr 8
IO
Figure 3. Emission intensity of silver line at 328.0 mp as function of hydrogen flow at various oxygen flows Present, 100 p.p.m. silver; multiplier phototube, 60 volts/dynode; ERA adjust, 751 slit width, 0.15 mm. Main group, burner height of 20 mm. lower right, 10 mm.
70
probably due to the additional silver atoms which are aspirated into the flame as the oxygen flow is increased and which more than offset the cooling effect of the additional water molecules introduced into the flame gases (3, 4). Though only the 338.3-mp silver line is plotted, the 328.0-mp line was also observed and paralleled the data obtained for the 338.3-mp line. The emission intensities of the two lines are virtually identical. At the optimum flow ratio, the fuel mixture is very rich, in fact much richer than a stoichiometric ratio (equal to 2.5). The optimum flow ratio corresponds more closely to a burned gas composition H20 (rather which consists of CO HZO) and which would rethan COn quire a theoretical ratio of oxygen/ acetylene equal to 1.5. Experimental and theoretical values agree well when one considers the large amount of air entrained a t the base of the flame. Unpublished studies of other elements in this laboratory show that maximum
+
+
VOL. 33, NO. 3, MARCH 1 9 6 1
383
into the burned flame gases. The emission characteristics of the two silver lines are similar in an oxygenhydrogen flame; however, the silver line a t 338.3 mp possesses 1.2 times greater emissivity than does the line a t 328.0 mp. The optimum oxygen and hydrogen flows, and the corresponding aspiration rate of the aqueous solution, were compared with the values predicted by Fuwa et al. (4). Using their formula, the optimum ratio (moles water/moles oxygen) is calculated to be 0.445 for the silver lines. From our experimental data the closest agreement was achieved at the lowest oxygen flow employed, 4.7 cubic feet per hour, a n oxygen flow which aspirated 1.14 ml. of solution per minute. The ratio is then 0.716. Actually, no significant gain in the maximum emission intensity of either line was achieved with larger oxygen flows. Unfortunately, smaller oxygen flow did not produce proper aspiration with the aspirator-burner employed since the oxygen pressure was considerably below the value recommended (10 pounds per square inch) by the manufacturer. Background Emission. I n the vicinity of the silver lines, the background emission arises from the remnants of the OH band systems and the continuum due to carbon monoxide molecules. As a guide to the contribution of the O H band systems to the general background radiation, the intensity of the 306.4-mp band head of the OH system was measured a t various oxygen/acetylene flow ratios (Figure 5 ) . The ratio which gave maximum OH emission is considerably larger than the ratio which provided the maximum silver emission. I n an oxygen-hydrogen flame, the background readings were essentially zero from 325 to 340 mp. Apparently, most of the variation in the background reading arises from the continuum due to carbon monoxide molecules and, consequently, ryill be significant only in a
! 4 1
ok
2o
0 0
0.2 0 4
0.6 -0.8
1.2
1.0
1.4
R A T I O O F T H E FLOW RATE; OXYGEN/HYDROGEN
Figure 4. Emission intensity of silver line a t 328.0 mp as function of ratio of flow rate, oxygen/hydrogen, at various burner heights Conditions same as for Figure 1 Individual oxygen flaws for the family of curves obtained at burner height of 10 mm.
emission intensity is often achieved when the composition of the burned gases is largely CO HzO. A plot of the silver emission as a function of the flow of hydrogen, with the oxygen flow as parameter, is shown in Figure 3 for two burner positions. The silver emission a t a burner height of 20 mm. is twice the value a t the normal burner position of 10 mm. No gain in emission intensity ensues when oxygen flows larger than 4.7 cubic feet per hour are employed. The optimum ratio of oxygen/hydrogen flows varies from 0.32 to 0.37 at a burner height of 10 and 20 mm., as shown in Figure 4. A family of curves was obtained at the 10-mm. burner position; only the curve for the lowest oxygen flow lies under the maximum for the other burner positions. Perhaps this indicates that equilibrium conditions do not prevail a t this position within the flame when larger amounts of water are injected
+
Table I. Influence of Organic Solvents Present, 200 pg. of silver per ml. hlultiplier phototube, 52 volts per dynode. ERA 70 adjust, 80 Silver Emission Readings in Type of Flame
Oxygen-acetylene (Ratio: 2.5/1.0\ Enhancement, xfold Oxygen-hydrogen (Ratio: 0.4/1.0) Enhancement, xfold Aspiratipn rate, ml. per min.
384
Water
Methanol
h- M rJ l-- -e-t.h.-
2-Propanol
pentan-2-one__
328
338
328
338
328
338
328
338
mp
mp
mfi
mp
mu
mp
mp
mil
24.5
27.0
...
...
7 8
9.3
,..
... 1.76
ANALYTICAL CHEMISTRY
134
145
5 5
5 4
72
84
9.2 2.18
9 0
90
120
3 7
4 5
55
52 6 7
5 9
0.88
177
166
6 8 78
6 6
89
10.0 2.28
9.6
50r
I
0
z
2 30
W
DH 0 l N D
0 16
-014
0
1 10
I
I
15
2 0
. 5
1
R A T I O OF T H E FLOW R A T E , OXYGEN / A C E T Y L E Y E
Figure 5. Background emission vicinity of silver lines
in
Emission intensity of 306.4-mp band head of O H system and ratio, background/net line emission, for silver line at 338.3 mp, Background taken as average of readings at 3 3 7 and 3 4 0 mp. O H band readings scaled down; other conditions same as for Figure 1
hydrocarbon flame. The backgroundto-emission ratio in an oxygen-acetylene flame was determined by operating the flame spectrophotometer manually and taking readings directly from the % 5"-scale of the instrument. The data obtained are shown in Figure 5 . The background in the vicinity of the 338.3mp line was taken as the average of readings a t 337 and 340 mp. Use of Organic Solvents. T o study the influence of organic solvents upon the flame emission characteristics of silver, silver nitrate wis either dissolved in the pure organic solvent or extracted from an aqueous solution 6 M in nitric acid with a 30 volume per cent solution of triiso-octyl thiophosphate in 4-methyl-pentan-2-one (6). rl plot of the emission intensity of silver as a function of the ratio of oxygen/acetylene flow is shown in Figure 6 at four burner positions. By comparison mith the optimum flow ratio of 1.1 found for aqueous solutions, in the organic system the optimum value shifts to 1.3 and 1.4 when the oxygen flow rates are 4.8 and 6.3 cubic feet per hour, respectively. The shift in the ratio would be anticipated since the organic solvent which is aspirated would require additional oxygen for combustion. An exhaustive study of organic solvents was not performed. However, several other solvents have been investigated by Dean and Kuper ( 2 ) . Some of their results are given in Table I. The largest enhancements are observed when using 4-methyl-pentan-2-one or methanol in a n oxygen-hydrogen flame; hon ever, the largest net emission intensity is found with an oxygen-acetylene flame into which is aspirated a solution of silver in 4-methyl-pentan-2-one.
KO special significance should be attached to the choice of solvents investigated. Possible mechanisms involved in the enhancing action of organic solvents will be discussed in a forthcoming paper. WORKING CURVE
The silver lines show the onset of selfabsorption a t rather low concentrations of silver (Figure 7 ) . Consequently, the R orking curve is nonlinear over most of the concentration range. In logarithmic coordinates, and a t optimum instrument settings for an oxygen-acetylene flame, the line a t 328.0 mp has a slope of 0.95 for silver concentrations of 30 p.p.m. and less. At higher concentrations the slope (log emission plotted us. log concentration) slon ly decreases owing to self-absorption. For concentrations of silver between 120 and 2000 p.p.m., the slope again becomes constant nith a value of 0.45. Until the inception of self-absorption, the emission intensity of the 328.0-mp line is 1.2 times greater than the emission of the 338.3mp linc. Measurements in ouygen-hydrogen, while hampered a t 1017 silver concentrations due to small emission readings, shoned that the logarithmic slopes for both emiqsion lines are approximately equal to those for oxygen-acetylene a t lon concentrations of silver. HOKever. in the concentration interval 100 to 500 p.p.m. of silver, the 338.3mp line cvhihits less rapid changes in slope and, consequently, the working curves for the two emission lines cross a t about 150 p.p.m. of silver. This may explain the discrepancies reported in the literature regarding the relative sensitivities of the silver lines in oxygenhydrogen. For the instrummt and the instrument settings employed in this n ork, the 7:r
emission sensitivity is 0.4 pg. of silver per milliliter in oxygen-acetylene, but only 4.0 pg. per ml. per % T in oxygenhydrogen. These emission sensitivities are considerably less than those reported by previous workers (8, 10). Rathje (8) and Whisman and Eccleston (10)employed wider slit widths. Equipment used was different in each case, and multiplier phototubes differ among themselves in sensitivity. The emission sensitivities observed for the oxygenacetylene and the oxygen-hydrogen flames are in reasonable agreement according to the Boltzmann equation, taking the temperature of the wet acetylene flame to be 3250' K. a value obtained by calculating the adiabatic flame temperature, and assuming the temperature of the wet hydrogen flame to be about 300" lower than the dry flame temperature ( 3 ) . The reproducibility of a series of net readings, after subtraction of the background reading, varied between 2.5 and 3.0% of the net reading, expressed as Table II.
Anion Interference Effects Present, 50 p.p.m. of silver
Per Cent Anion Added5 Error Acetate 2 Carbonate -4 Chloride 1 Ntrate -2 Oxalate 12 Perchlorate 1 Phosphate -1 Sulfate 2 a As sodium salts in a concentration of 10,000 pg. of anion per ml. the standard deviation of the reading, when the latter is 30 to 50 scale divisions (yoT scale). An estimate of accuracy is not possible since standard samples were not analyzed. INTERFERENCE STUDY
The interference effect of the anions which are commonly encountered is summarized in Table 11. For those solutions which formed silver precipitates, a few drops of aqueous ammonia or of nitric acid were added to bring the precipitate into solution. Only carbonate and oxalate ions caused interference that could be considered significant. Investigation of the carbonate ion a t concentrations from 200 to 10,000 p.p.m. revealed that the depressant effect remained constant. With oxalate ion, the enhancement of silver became constant when the oxalate ion concentration exceeded 5000 p.p.m., and became insignificant a t 200 p.p.m. or less of oxalate. Cation Interference. All the cations which possessed spectral lines Influence of Anions.
10
I 2
I 4
I6
I8
20
R A T I O OF T H E F L O W R A T E , O X I G E N / A C E 7 Y L E N E
Figure 6. Emission intensity of silver as function of ratio of flow rate, oxygen/acetylene, a t various burner positions
70
Present, 100 pg. siiver/ml. in 30 volume solution triiso-octyl thiophosphate in 4-methylpentan-2-one; oxygen flow, 4.8 cu. ft./hr.
0 A9
328.0
80
[
::p 10
0
0
50
100
2OC
150
SILVER C O N C E N T R A T I O N , pg
p e r ml
Figure 7. Emission reading of silver as function of concentration Conditions, recommended instrument settings and aqueous solutions Upper curves, oxygen-acetylene flame; slit,
0.025 mm. lower curves, oxygen-hydrogen flame;
slit,
0.050 mm.
in the vicinity of the silver lines, as well as many others which might possibly offer radiation interference, were included in the study of cation interferences. The results are shown in Table I11 and discussed subsequently. The silver line a t 328.0 mp is completely resolved from the cadmium line a t 326.1 mp a t the spectral band width
Table 111.
Cation Interference Effects
Present, 50 p.p.m. of silver concen-Per Cent Error Measured at Cation tration, P.P.M. 328.0 mp 338.3 mp Tested -3 Aluminum 5,000 -1 Ammonium (or NH,)
Cadmium Calcium Cerium Chromium Cobalt Copper Iron Lead Magnesium
5,000 2,000 5,000 5,000 5,000 2,000
-8 -2 -5 -4 3 -1
1,000
-3 1 0 1 -2 -7 -5 -2
5,000 5,000 5,000 1,000 5,000 2 000 5,000 5,000 5,000 10,000 5,000 5,000 2,000 ~
Manganese Nickel
Potassium Sodium Tin Zinc 5
-2 -5 1 - 14
-4 -1
-7 -5 -2
0 5
3 -3 n
-2 3 -2 -8 -5 -6
-6 -2 ,-
16 -5
0
Direct spectral interference.
VOL. 33, NO. 3, MARCH 1961
385
employed. Cadmium appeared to depress the silver emission slightly. Cobalt possesses weak lines a t 335.4, 336.8, 338.5, 338.8, and 339.5 mp, and a strong line a t 340.5 mp. The silver line a t 338.3 mp cannot be resolved from the cobalt lines; specificity factor is approximately 130 for a spectral band width of 0.14 mp (0.025 mm. slit width). At a slit width of 0.025 mm., the silver line a t 328.0 mp is not completely resolved from the copper line a t 327.4 mp. However, the peak of the silver line is separated sufficiently from the underlapping copper emission to permit its height to be determined. The correct silver reading is obtained when the base-line method of measuring the background a t 326 and a t 330 mp is used. Kickel possesses an emission line a t 338.1 mp; the specificity factor is approximately 5.
Calcium and tin increased the background readings about 40%. In this region tin possesses a series of weak molecular band systems. Whereas tin also definitely depressed the silver emission, calcium appeared to have little effect upon the silver readings. In the presence of calcium, the background is altered in the region of the silver line a t 328.0 mp and necessitates securing a background reading only a t 327 mp. Cerium enhanced slightly the silver readings but did not affect the background readings. Ammonium, chromium, magnesium, manganese, potassium, and zinc depressed slightly the silver emission. In each case, a slight increase in the background was observed, especially in the vicinity of the 338.3-mp silver line. The spectral and radiation interferences of the other cations investigated were negligible.
LITERATURE CITED
(1) Dean, J. A., “Flame Photometry,” Chap. 6, McGraw-Hill, New York,
1960. (2) Dean, J. A., Kuper, H. S., unpublished studies to be included in Ph.l). thesis of H. S. Kuper. (3) Foster, W. H., Hume, D. X., AXAL. CHEW31,2028 (1959). (4)Fuwa, K., Thiers, R. E., Vallee, B. L., Baker, M. R., Ibid., 31,2039 (1959). (5) Galloway, N. McK., Analyst 83, 373 f lCJ.58).
(6j Handley, T. H., Dean, J. -4.,AXAL. CHEM.32,1878 (1960). ( 7 ) Pungor, E., Konkoly-Thege, I., dcta Chim. Acad. Sci. Huna. 13, 235 (1958). (8) Rathje, A. O., A ~ A L’ .CHEM. 27, 1583 (lCL5.5).
RECEIVED for review September 12, 1960. Accepted November 30, 1960. Taken in part from the M.S. thesis of C. B. Stubblefield at the University of Tennessee, June 1960.
Correlation of Mass Spectra with Structure in Aromatic Oxygenated Compounds Methyl Substituted Aromatic Acids and Aldehydes THOMAS ACZEL and H. E. LUMPKIN Manufacturing Department, Research and Development Division, Humble Oil & Refining
Co., Humble Division, Baytown, Tex.
b The correlations existing between mass spectra and chemical structure are examined in detail for a group of aromatic acids and aldehydes. Particular emphasis is given to the study of ions which can b e used for analytical applications, as qualitative and quantitative determinations and prediction of sensitivities of compounds currently unavailable for calibration. As in the case of aromatic alcohols and phenols, discussed in a precedent publication, the results obtained in the present work confirm that the fragmentation pattern is greatly influenced by the position of the substituents on the benzenic ring.
Sources of the compounds used in this work were commercial, if available; otherwise, the compounds were prepared in these laboratories. A list of the coinmercial sources is given in the tables.
T
HE CORRELATIONS that can be established between mass spectra and chemical structure and the usefulness of their applications in analytical mass spectrometry have been discussed by several authors and for a number of chemical classes. Need for extending similar studies to new types of compounds lies in the immediate applicability of the results obtained to ordinary analytical work, and, eventually, to a better understanding of the
386
ANALYTICAL CHEMISTRY
multiple factors involved in the formation of ions under electron impact. I n a precedent paper ( I ) , we discussed the mass spectra of several aromatic alcohols and phenols. The present communication deals with the spectra of aromatic acids and aldehydes. Spectra of aromatic esters, as well as the considerations that might be applied to most of the compound types hitherto examined, will be discussed in a following communication. EXPERIMENTAL
The data reported were obtained on Consolidated Electrodynamics Corp. Models 21-102 and 21-103C mass spectrometers, and recorded either on an oscillograph or with the CEC peak digitizer (Mascot). Experimental conditions were comparable in every detail to that described in the previous work (1).
The availability of a solids inlet system (3) was particularly helpful in the case of the higher molecular weight acids, which could not be otherwise introduced into the mass spectrometer. Peak heights are expressed as per cent of the total ionization, calculated as the summation of the peak intensities from m/e 73 to m/e (parent 2) (1).
+
DISCUSSION OF SPECTRA
This study confirms that the respective position of the various substituents on the benzene ring has a leading importance in the formation of fragment ions. The so-called proximity effect, by Lumpdiscussed by McLafferty (4, kin and Nicholson ( 8 ) ) and in our previous communication ( I ) , is observed in a number of ions formed from the compounds discussed. Particularly remarkable is the regularity of this, and similar trends, in more complex types of substitution. Aromatic Acids. A number of aromatic acid spectra have recently been discussed by McLafferty and Gohlke ( 4 ) . Our data, though based on a larger quantity of compounds, confirm essentially the findings contained in their publication. For convenience, the spectra studied are divided into three main classes,
