In T
[ar2(1
+ T-’) + 26, + cr2(1+ T-2 - T-1)]1/2
(‘4-3)
Equation A-3 is the same as Equation 36 except for the additional term contained in the multiplier for c r z . Thus limiting Cases I1 and 111, where cr2is negligible, are unaffected if only dark current measurement is performed. Only limiting Case I is influenced by the number of dark current measurements. If c r 2variances predominate, and one dark current measurement is made, Case Ia results and the (S/N)A is given by Equation A-4.
(A-4)
The only differences between Case Ia and Case I, where two independent dark current measurements are made, are that the optimum transmittance shifts to 38.8% and the T-’ term in the denominator of Equation A-4 tends to increase the signal-to-noise ratio. For transmittances between 1 and 0.1, the for Case l a is 5-41 % better than for Case I. Case Ia gives better precision only if dark current drift is negligible between measurements. At low transmittances, Cases Ia and I become identical because T-2begins to predominate over the T-1 term in Equation A-4.
RECEIVED for review September 27, 1971. Accepted March 30, 1972. Work partially supported by NSF Grant No. GP-181123 and an American Chemical Society, Analytical Division Fellowship sponsored by Perkin-Elmer Corporation.
Nondestructive Charged Particle Activation Analysis Using Short-Lived Nuclides Jean-Luc Debrun,’ David C. Riddle, and Emile A . SchweikerV Activation Analysis Research Laboratory and Department of Chemistry, Texas A&M UniEersity, College Station, Texas 77843 The objective of this study was to combine the features of nondestructive determination with speed of analysis. Accordingly, this study was restricted to radioisotopes with half-lives ranging from 1 second to -1 minute. Furthermore, only those species were considered which emit one or several ?-rays besides the 511 keV annihilation peak. Thirty elements were irradiated with protons or 3He particles. Activation with 3He proved to be of little interest in the cases studied. Proton activation was found suitable for the trace determination of the following elements: Se, Br, Y, Zr, La, Pr, Dy, and Nd. Data on their respective specific activities, activation curves, and pertinent ?-ray energies ( 1 0 . 3 keV) are given. These elements can be determined in 13 matrix elements which yield little or no activity under the experimental conditions described. Among these are matrices where neutron activation analysis cannot be applied easily because of high neutron absorption or activation cross sections. FEWREPORTS have appeared so far on the use of charged particle activation in conjunction with short-lived nuclides (t1’2 < 1 min). Markowitz et af.( 1 , 2 ) have reported on oxygen-18 and fluorine determination methods based on the detection of 2 0 F (t”2 = 11 sec). Several authors have considered the use of I7F( P ’ 2 = 66 sec) for detecting oxygen (3-7). Ricci et al. 1 On leave of absence from Laboratoire d’Analyse par Activation Pierre Sue, Saclay, France. 2 To whom correspondence should be addressed.
(1) J. F. Lamb, D. M. Lee, and S. S. Markowitz, Proc. 2nd Conf. on Practical Aspects of Activation Analysis with Charged
Particles, Euratom Report Eur-3896 d-f-e, 225 (1968).
(8) have measured the specific activities of several radioisotopes in the 1- to 20-min range produced by 3He activation on boron, carbon, and oxygen. In this laboratory, we have recently studied the determination of sulfur by measuring 32Cl of 300 msec half-life obtained by proton activation (9). With the exception of the investigations dealing with 20Fand 32Cl, the applications proposed so far involve the measurement of the 511 keV annihilation peak. The procedures based on the detection of this y-ray appear, however, for reasons outlined below, to be only of limited usefulness. The present study was motivated by the intrinsic features of an approach combining nondestructive determinations with the speed of analysis associated with the use of shortlived nuclides. The objective was to obtain a survey on the analytical possibilities offered by such an approach on a wide range of elements. To evaluate these possibilities from a practical standpoint, a selection was made among the large number of possible activation reactions and potentially suitable radioisotopes according to the following three requirements : (a) minimal or no interfering reactions yielding the nuclides of interest (b) half-lives ranging from -1 sec to -1 min (one exception was made as noted below) (c) y-ray spectra consisting of one or several y-rays in addition to the 51 1 keV annihilation peak. The reason for this last condition was that a majority of nuclides produced by charged particles are p+ emitters. The resulting 511-keV peaks are thus often too complex to be
(2) D. M. Lee, J. F. Lamb, and S. S. Markowitz, ANAL.Cmhf.,
43, 542 (1971). (3) P. Sue, C.R. Acad. Sci.Paris, 242,770 (1956). (4) R. R. Sippel and E. Glover, Nucl. Znsfrum. Methods, 9, 37 (1960). ( 5 ) S. n i i i , ~ ;and M. Peisach, ANAL.CHEM.,34, 1305 (1962). (6) L. Hammar and S. Forsen, J . Ztrorg. N L ~Clzem., . 28, 2111 (1966). 1386
ANALYTICAL CHEMISTRY, VOL. 44, NO. 8, JULY 1972
(7) M. J. Lacroix, M. D. Tran, and J. Tousset, Proc. 2nd Conf. on Practical Aspects of Activation Analysis with Charged Particles, Euratom Report Eur-3896 d-f-e, 351 (1968). (8) E. Ricci and R. L. Hahn, ANAL.CHEM., 39, 794 (1967). (9) J. P. Thomas and E. A. Schweikert, Nucl. Zmtrurn. Merhods, 99, 461 (1972).
resolved by decay curve analysis. Possible exceptionsnamely some rare earth characterizations based on p+ counting-will be discussed below. In examining the Q values of applicable nuclear reactions, it became apparent that only those induced by protons and 3He particles could prove useful under practical circumstances. In selecting the proper proton energies a number of (p, n) and (p, p’) reactions can occur on certain elements, while many others are not activated or give long lived nuclides only. In 3He activation the thresholds for ( aHe,n), ( aHe,p) or ( 3He,a) are uniformly low; however, advantage can be taken of the Coulomb barrier for determining low Z elements in high Z matrices (10). No discrimination of impurity cs. matrix activation is possible with deuteron induced reactions which have low thresholds for too many elements, and with a particles where all reactions have high thresholds.
EXPERIMENTAL Preparation of Targets. Thin targets were prepared by depositing approximately 1 mg/cm2 of a given compound onto thin vanadium backing foils (14.9 mg/cm2). Vanadium gave no activity in the half-life range of interest (0.5 sec 5 T”2 5 5 min) and for the particles and bombarding energies used. Pelletized powders were used for the thick target irradiations. Irradiation, The irradiations were performed at the 88-in. variable energy cyclotron of Texas A & M University. The samples toget her with a thin monitor foil were bombarded in the air, the particles emerging from the beam line through a thin beryllium window (250 /I thick). The beam on target was controlled with a movable Faraday-cup far away from the irradiation site. The beam energy was varied by rotation of a beam degrader disk containing 12 aluminum absorbers of different thicknesses. Depending upon the targets and the bombarding energies, the irradiations lasted from 1 to 5 sec, and beam intensities varied from 10 nA to several hundred nA. Nominal irradiation energies were 18 MeV for protons and 23 MeV for 3He particles. Maximum irradiation energies on the sample after energy losses due to the beryllium window and the monitor foil were 16.8 MeV and 11.3 MeV for protons and aHe, respectively. Sample Transfer. Two fast sample transfer systems were used in this study. The first system (Figure 1) was similar to one described by Markowitz et al. ( I , 2 ) . A thin monitor and a sample, each mounted on a slide, were irradiated one behind the other. The slides were held in position during bombardment by an electromagnet which was de-activated automatically at the end of irradiation with the slides falling by gravity into their respective counting positions. Sample transfer time to the lower counting position was less than one second. For successive bombardments on the same target, a remotely controlled elevator allowed the sample and monitor to be lifted back into the irradiation position. A second sample transfer system was built for counting with a Ge(Li) detector placed outside the irradiation cave to avoid any radiation damage to the detector. This setup consisted of a modification of the system described above. The monitor foil was still irradiated and counted in the same way. The sample mounted in a small rabbit was irradiated directly behind the monitor and could, after activation, be transported by pneumatic tube to the counting station (Figure 2). Transfer time for the rabbit was -1 sec for a distance of -8 m. Detection and Monitoring, The detector setup for in-cave counting consisted of a heavily shielded 3 X 3-in. NaI(T1) crystal in each of the counting positions connected to 400 channel analyzers used either in the pulse height or multiscale (10) S. S. Markowiti and J. D. Mahony, ANAL. CHEM.,34, 329
(1962).
W B E A M DEGRADER DISC MONITOR HOLDER S L I D E 7 CLIP L E A D TO CURRENT INTEGRATOR ALUMINUM BACK P L A T E
Figure 1. Experimental set-up for in-cave counting with NaI(T1) detectors CYCLOTRON BEAM PORT BEAM DEGRADER
r BEAM MONITOR ELECTROMAGNET
/-RABB’T
ALUMINUM TUBE HOLDER /-PNEUMATIC
TUBE
Figure 2. Sample rabbit in the irradiation position modes. As already mentioned, a Ge(Li) detector placed outside the irradiation cave was used for high resolution y-ray spectrometry. Characteristics of the Ge(Li) detector were as follows: Resolution: 2.0 keV FWHM for the 1.332 MeV y-ray of 60Co; Peak to Compton Ratio: 20: 1 ; Photopeak efficiency relative to 3 x 3-in. NaI(T1): 3.05%. This detector was coupled to a 4096 channel Nuclear Data analyzer. Relative monitoring of the beam was accomplished either by measuring the short lived p+ activity induced in thin aluminum foils or by integrating the beam current falling onto the target. Usually the beam was stable enough to give integrated current values, varying no more than 10% in a series of successive similar irradiations. The entire sequence, bombardment, delay (to allow for sample transfer), and counting, was carried out automatically with preset electronic timers.
RESULTS AND DISCUSSION 3He Activation. The 7-ray spectra measured had similar features in all cases: the 511 keV annihilation peaks were for each target very intense; however, the characteristic y-rays expected from the nuclides of interest were either only barely or not detectable. The 511 keV activities resulted apparently from several other short-lived radioisotopes decaying principally by p+ emission. Consequently, the 3He reactions studied (see Table I) cannot be considered suitable for trace analysis. Proton Activation. For 16 of the 24 elements studied, the induced activities measured on the y-rays listed in Tables ANALYTICAL CHEMISTRY, VOL. 44, NO. 8,JULY 1972
1387
~
~~~
~~
Table I. Helium-3 Induced Reactions Yielding Short-Lived Radioisotopes
Element Be B
S
Reaction $Be(3He,2n)l0C log(3He,T)10C 1 6 0 ( 3He,n) 18Ne 24Mg(3He,n)2%i %Si(3He,n)30S 3*S(3He,n)Ar
Ca
40Ca(3He,p) ZmSc
0
Mg Si
Q value, MeV
Principal y-ray energies (keV) and abundances 717 keV 98% 717 keV 98z 1040 keV 7% 820 keV 34% 687 keV 80% 670 keV
z
Half-life, sec
-5.5 -3.62 -3.2
19.3 19.3 1.56
-0
2.1
-0.54
1.4 0.9
-1.1
Isotopic abundance of target nuclide,
z
100 19.6 99.8 78.7 92.2 95
438) >O
3
61
97
100%
keV
Table 11. (p,n) and (p,2n) Reactions Yielding Short-Lived Radioisotopes Principal y-ray energies Reaction Q,value, MeV Half-life, sec and % abundances l0B(p,n)loC -4.4 19.3 717 keV 100% l4N(p,n)l40 -5.9 71.3 2312 keV 99% Z3Na(p,n)23Mg 12 -4.8 9% 440 keV 35Cl(p,n)35Ar 1.8 -6.8 1220 keV 5%
Element B N Na
e1 Fe
4Fe(p,n)64Co
-9
Ge Se
4Ge(p,n)74As 8OSe(p,2n)79mBr 7gBr(p,n)78mKr 8 lBr(p ,n)81mKr 89Y(p,n)SgmZr BoZr(p,n)gOmNb 127I(p,n)127mXe
-3.4 -10.5 -2.4 -1.1
Br Y Zr I
139La(p,n)13smCe la1Pr(p,n)14lmNd lelDy(p,n)16lmHo 16 3Dy(p,n)163mHo 203Tl(p,n) 20 3mPb
La Pr
DY T1 ~
Element Se Br Y Zr Ba Er Yb
Hf Au Pb
~
90
-6.9 -1.5
8 4.8 55 13 4.2 24 70
-1 -2.6 -1.6 -0.8 -1.6
56.5 64 6.1 1.1 6 -
~~
100%
65% 872 712 93z 53% 70%
ANALYTICAL CHEMISTRY, VOL. 44, NO. 8, JULY 1972
'OB: 19.6 14N: 99.6 23Na: 100 75.5
3 5 ~ 1 :
54Fe:
5.8
74Ge: 80Se: 79Br: 81Br: "Y: 90Zr: 1271:
36.5 49.8 50.5 69.5 100 51.1 100
139La: I41Pr: 161Dy: 163Dy: *03T1:
99.9 100 19 25 29.5
~~
Table 111. (p,p ') Reactions Yielding Short-Lived Radioisotopes Principal y-energies (keV) Reaction Half-life, sec and % abundances 77Se(p,p')77mSe 17.5 161 keV 50% 7BBr(p,p')7QmBr 4.8 210 keV 16 910keV 99% 89Y(p,p ')81mY BOZr(p,p ')gOmZr 0.8 2320 keV 86% 0.32 818 keV 100% 136Ba(p,p') 3emBa 1050 keV 100% 167Er(p,p')167mEr 2.3 208 keV 632 190 keV 12 290 keV 176Yb(p,p ') 176mYb 390 keV 1.1 208 keV 80% 177Hf(p,p')177mHf 48% 228 keV 97% 421 keV 43 326 keV 94% 75% 214 keV 18.6 217 keV 94% 7.5 279 keV 75% 98% 0.8 570 keV 83x 1064 keV
I1 and 111, were too low to permit determinations at trace levels. Accordingly only data for the eight remaining elements are reported, namely: Se, Br, Y , Zr, La, Pr, Nd, and Dy. Because of the possible applications dealing with the determination of rare earth impurities in a rare earth matrix, we have studied the activation reactions listed in Table IV in spite of the fact that the nuclides produced are pure or almost pure p+ emitters. In these specific applications, the counting 1388
~-
410 11,] keV 1410) 283 keV 210 keV 127 keV 190 keV 588 keV 122 keV 125 keV 175 keV 746 keV 755 keV 211 keV 305 keV 825 keV
Isotopic abundance
of target nuclide, %
Isotopic abundance of target nuclide, Te: 7.6 79Br: 50.5 89Y: 100 90Zr: 51.5 136Ba: 7 . 8
16'Er:
22.9
176Yb: 12.7 177Hf: 18.5 I7*Hf: 27. 1 179Hf: 13.7 Ig7Au: 100 207Pb: 22.6
can be performed on the 511 keV annihilation peak because rare earths are usually major impurities in rare earth materials ; furthermore, the radioisotopes obtained from Nd, Sm, and Ce have distinctly different half-lives. Activation curves (relative excitation functions) for the different nuclear reactions on Se, Br, Y , Zr, La, Pr. Nd, and Dy have been obtained to provide the information necessary on experimental thresholds and peak energies for proper
Table IV. Proton Reactions on Rare Earths Yielding Predominant p+ Emitters Element Ce Nd Sm
Q value, MeV -4.15 -5.6 -7.1
Reaction 14°Ce(p,n)1@Pr 42Nd(p,n)142Pm 144Sm(p,n)'44Eu
Half-life 3.4 min 40.5 sec 10.5 sec
4Fl m
a
a
IO
15
5'
IO
15
o
Isotopic abundance of target nuclide, % laace: 8 8 . 5 142Nd: 27.1 144Sm: 3.1
Values of Remeasured Y-Ray Energies Previously published Main y-energy, values, from ( I ] ) , keV Target Radionuclides keV,a this work Se iimBr 106.4 108 iirnSe 162.5 161 iYm& 207,3 2 10 Br 79mKr 129.3 127 SimKr 190.3 190 iymBr 207.3 210 Y 89mZr 587. 7 588 SYmy 908 6 910 Zr 90mNb 122 2 122 La 139rnCe 154.2 746 Pr 1 4 1mNd 156.3 755 16lmH0 211.1 211 DY 163niHo 297.5 305 Values accurate to
[email protected] keV. Table V.
2
5'
511 keV, 100 190 200
a
Thick Target Yields for Protons of 16.8 MeV Yield" Element Target Reaction counts1pA Se Se sOSe(p ,2n)' ylliBr 6 X 108 Br KBr 7gBr(p,n)igmKr 33 x 108 7yBr(p,p')7yniBr 2.2 x 106 16 X 106 'Br( p ,n)*lmKr Y yzos 89Y(p,p ')*9mY 0 . 8 X 106 8yY(p,n)8Yn1Zr 10 x 106 Zr Zr goZr(p,n)90111Nb 23 X 106 La La203 13yLa(p,n)139mCe 7 . 5 x 106 Pr Pr203 141Pr(p,n)l lmNd 7 x 106 20 x 106 Nd Nd2O3 2Nd(p,n) 2Pm DY Dy203 161Dy(p,n)161t1>Ho 2 . 8 x 106 l 63Dy(p,n)163mH0 0 . 8 X 106 The yields correspond to the following conditions: The samples are counted 5 cm away from the 3 X 3-in. NaI(T1) detector. Irradiation of 1 sec at 1 pA, number of counts correspond to the end of the irradiation (io). The number of counts at r,, is the number of counts in the photopeak calculated by Covell's method (12). It corresponds also to a counting time long enough to ensure complete disintegration of the radionuclide of interest. The experimental count rates from the targets were corrected (when needed) to correspond to 100% of the element of interest. Table VI.
U
IO
2m
5'
IO
15
10
15
0I
5'
IO
15
5
ENERGY ( MeV 1
Figure 3. Activation curves for the following reactions: ( A )8oSe(p,2n) 7ymBr; ( B ) iyBr(p, n) 7ymKr; (C) *lBr(p, n) *ImKr; (D)8yY(p, n) symZr; ( E ) b3Y(p, and ylZrp') 8gmY;(F)yOZr(p,n) yOmNb (p, 2n) yOmNb;( G )13yLa(p,n)13ymCe;( H ) 141Pr(p,n) 141mNd; (I) 142Nd(p,n 142Pm and 143Nd(p,2n) '"Pm; (J)161Dy)(p, n) 161mHo and l6*Dy(p,2n) I6lmHo selection of the bombarding energies in analytical applications (Figure 3). Each data point is the average result of at least three measurements. In most cases, these were in agreement Wider dispersions were found and within less than * 5 % . are indicated accordingly on the activation curves for the reactions 89Y(p,n)8Ymr,42Nd(p,n)14*Pm, and 161Dy(p,n)161mHo l'j2Dy(p,2n) I6ImHo. Furthermore, the pertinent y-ray energies have been remeasured with a high resolution Ge(Li) detector (Table V). Accurate y-ray energies are of the utmost importance in this analytical technique since they constitute, together with the half-life information, the criteria of identification for the species sought. The relative yields for thick targets as a function of bombarding energy are given in Figures 4 and 5 . Data on these
+
yields obtained with protons of 16.8 MeV are given in Table VI. These figures correspond to arbitrarily chosen irradiation conditions and matrices; no attempt was made to calculate sensitivities. Indeed the sensitivities will have to be assessed in each practical case depending upon the nature of the matrix to be analyzed and the counting conditions, particularly the type and performance of the detector used and the extent of the ambient background. Based on the irradiation conditions, it appears that the yields listed in Table VI could easily (11) C. M. Lederer, J. M. Hollander, and I. Perlman, Ed., "Table of Isotopes," J. Wiley and Sons, New York, N.Y., 1968. (12) D. F. Covell, ANAL.CHEM., 31, 1785 (1959). ANALYTICAL CHEMISTRY, VOL. 44, NO. 8, JULY 1972
1389
Table VII. Matrices in Which Nondestructive Analysis by Proton Activation Using Short-Lived Nuclides Appears Feasible 3-
a
i L
I 8 F 2 x
Element
Remarks
Li
If E 5 10 MeV, only the 53.4 d 7Beis produced. No radioisotopes are produced if E 5 17 MeV. If E 5 12.7 MeV, only the 27.8 d 51Cr and 330 d 49Vare produced. When E 5 24 MeV, 53Mn(1.9 X 106 y, no y)-j4Mn (303 d)- and jjFe(2. 6 y, no y) are the only radioisotopes produced. If E _< 10.46 MeV, the only radioisotope produced is jgNi(7.5 X 104 y. no y), If E 5 9.33 MeV, only “Ge(l1 d, no y) and 69Ge(39 h) are produced. When E _< 10.2 MeV the only radioisotope produced is i5Se(125d). E < 7 MeV only isotopes with half-lives longer than 17 d are produced. E < 6 , 8 MeV only 33Ba(38,9h) and 133Ba (10.7 y) are produced. E < 6 . 8 MeV, l 3 X s ( 6 . 5 d) is also produced. E < 5 . 7 MeV only ‘5IGd (120 d) and 153Gd (242 d) are produced. E < 6 MeV only ljgDy(144 d) is produced. E < MeV only l+jaEr(10.3 h, no y) is produced. E < 18 MeV only isotopes with half-lives longer than 2,89 y are produced.
Be 2-
a
V
wF
Mn
A
co
Ga As Sb
cs Eu Tb Ho
Bi
IO
17 12 24 11
8
10 9 10 8 8 8
18
be increased 20 to 150 times (depending upon the half-lives of the nuclide considered) by increasing the beam intensity to -10 pA and the irradiation time up to -1.5 sec. These latter irradiation conditions appear feasible for a large number of samples. The counting geometry used could be improved considerably if necessary. Table VI1 lists those elements for which no or only low activities were expected and which consequently could be considered as matrix elements. However, only Be, Mn, and Bi can be irradiated at 16.8 MeV while for the other elements lower proton energies would have to be used. This would result in a decreased sensitivity for the trace elements sought compared to that at 16.8 MeV. In some cases, notably Se, where the experimental threshold is high, the analysis might become impossible.
8-
74
a
\
Y)
c
6-
m 0
x
0 -I
10
-
9-
5 8
Maximum “Safe” proton energy, MeV
5-
w> 4-
CONCLUSIONS 3-
2-
I -
ENERGY
(MeV)
Figure 5. Relative thick target yields as a function of proton bombarding energy (irradiation and counting conditions are given in Table VI) 1390
ANALYTICAL CHEMISTRY, VOL. 44, NO. 8, JULY 1972
This study indicates that nondestructive trace analysis of Se, Br, Y, Zr, La, Pr, Dy, and N d by proton activation yielding short-lived nuclides is feasible. Thirteen other elements (Li, Be, V, Mn, Co, Ga, As, Sb, Cs, Eu, Tb, Ho, and Bi) as well as combinations of these elements ( e . g . , GaAs) give little or no activity with short irradiations. Practical applications can be envisioned dealing with the determination of one or several of the elements detectable via proton activation in samples where the major components consist of elements which are not activated. While inherently limited in its scope to a set of trace element and matrix combinations, the approach outlined here features, however, several important advantages :
The irradiations are short thus providing for rapid determinations. Maximum sensitivities can be obtained because the use of high beam intensities (10 pA or more) appears feasible for short irradiations. High beam intensities might have to be restricted, however, in cases involving the determination of very volatile elements (Se, Br). Further increases in sensitivity may be obtained by repetitive irradiation-counting cycles. The samples remain intact; even easily destroyed matrices which cannot withstand the heat dissipated in a prolonged irradiation could be considered for analysis. The cost of analysis per sample is minimized due to the short beam time required and the elimination of radiochemical processing necessary in many cases involving longer irradiations. Finally it is interesting to contrast the possibilities of short lived proton activation with reactor neutron activation.
Most of the thirteen elements which gave little or no activity have high neutron absorption and/or activation cross sections, resulting either in severe flux depression problems or in the build-up of high levels of radioactivity. In these cases proton activation appears to be much better suited for nondestructive trace characterization. ACKNOWLEDGMENT
We thank L. E. Fite and W. Kuder who developed and built the special electronics needed in this study. The assistance of the Cyclotron operations personnel is also gratefully acknowledged.
RECEIVED for review December 8, 1971. Accepted March 15, 1972. Work supported by the National Science Foundation (Grants GP-11630 and GP-l1630A#l) and by the Texas Engineering Experiment Station.
Ion-Exchange, Coordination, and Adsorption Chromatographic Separation of Heavy-End Petroleum Distillates D. M. Jewell,’ J. H. Weber, J. W. Bunger, Henry Plancher, and D. R. Latham2 Laramie Energy Research Center, Bureau of Mines, US.Department of the Interior, Laramie, W y o . 82070 Information about the composition of heavy-end petroleum distillates is needed because of the increased use of this material as an energy source. The complexity of the heavy ends requires extensive separation to give fractions amenable to compositional studies. A separation scheme is described that separates heavy-end petroleum distillates into acid, base, neutral-nitrogen, saturate, and aromatic fractions. The analytical techniques used include anion- and cation-exchange chromatography, coordination chromatography, and adsorption chromatography. The scheme has been applied to heavy-end distillates from a variety of crude oils having different compositional characteristics. Data from mass spectral analysis of the saturate and aromatic fractions are presented.
INCREASED ENERGY REQUIREMENTS together with increased environmental constraints require more efficient use of the heavy ends of petroleum. These factors will require increased processing, which to be efficiently accomplished requires a more intimate knowledge of the composition of the heavy ends of petroleum. To obtain this knowledge, a research project was established under the joint sponsorship of the Bureau of Mines and the American Petroleum Institute. One of the objectives of this project is to develop methods for the characterization of petroleum fractions boiling above 400 “C. To achieve this objective, the chemically complex, high-boiling distillate cut must be subdivided into simpler fractions that are amenable to suitable analysis. The separation scheme should meet several requirements. First, the individual separation procedures as well as the overall procedure should be analytically repeatable. Second, they must be applicable to highmolecular-weight distillates and residual fractions. Third, Present address, Gulf Research and Development Co., Pittsburgh, Pa. * To whom correspondence should be addressed.
they should be experimentally convenient and involve a minimum investment of man hours. Finally, they should allow the recovery of the compound types present with no chemical alteration. This paper describes a separation scheme that removes the “polar” nonhydrocarbon compounds as acid, base, and neutral-nitrogen fractions. The acid and base fractions are removed with anion- and cation-exchange resins, respectively, while the neutral-nitrogen fraction is obtained by coordination-complex formation with ferric chloride supported on Attapulgus clay. The remaining hydrocarbon and “nonpolar” nonhydrocarbon compounds are separated into saturate and aromatic fractions by adsorption chromatography using silica gel. The scheme has been applied to heavy-end distillates from several crude oils. Mass spectral analyses have been obtained on some of the saturate and aromatic fractions. The use of ion-exchange resins to remove acids and bases from petroleum fractions was first suggested by Munday and Eaves (]),and several workers (2-6)have used the technique for that purpose. Hartung and Jewell (7) reported the use of ferric chloride to form coordination complexes with nitrogen (1) W. A. Munday and A. Eaves, World Petrol. Cong. Proc. 5th N . Y., Sect. V , Paper 9 (1959). ( 2 ) L. R. Snyder and B. E. Buell, ANAL.CHEM., 34,689 (1962). (3) D. M. Jewell, J. P. Yevich, and R. E. Snyder, Amer. SOC.Testing Mater., Spec. Tech. Publ., 389,363 (1965). (4) P. B. Webster, J. N. Wilson, and M. C. Franks, Anal. Chim. Acta, 38,193 (1967). ( 5 ) I. Okuno, D. R. Latham, and W. E. Haines, ANAL.CHEM.,39,
1830(1967). (6) L. R. Snyder and B. E. Buell, J. Chem. Eng. Data, 11, 545 (1966). (7) G. K. Hartung and D. M. Jewell, A d . Chim. Acta, 27, 219 ( 1962). ANALYTICAL CHEMISTRY, VOL. 44, NO. 8, JULY 1972
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