Another look a t the change in aromaticity is obtained from the refractive index chromatograms (Figure 9). In the original sample, the large positive area (above baseline) indicates a large amount of condensed aromatics with a molecular weight range from about 20 000 to 250. This is within experimental agreement of the UAD data. T h e negative alkyl area in the positive profile indicates the presence of saturates plus possibly substituted noncondensed aromatics. Their molecular weight range is 610 to 320 with the peak a t 425. As the sulfur content is reduced to 0.6%, the amount of condensed aromatics decreases and the molecular weight range is decreased to approximately 9000 to 225. The amount of saturates plus alkyl-substituted monoaromatics increases and their molecular weight range extends from 275 to 900 with a peak molecular weight of 400. The information provided here supplements that determined on a class basis, Le., percentage of asphaltenes, resins, aromatics, and saturates. One of this technique’s main limitations is the qualitative nature of the information. Not all condensed aromatic systems absorb a t 313 nm and 365 nm, while some noncondensed heterocyclic compounds do. With refractive index, some noncondensed aromatics do have a refractive index greater than benzene. However, the data to date on isolated petroleum fractions indicate that these detectors (UAD and RID) respond to the condensed aromatic systems regardless of whether they are in the asphaltenes, resin, or aromatic class; and the changes they depict are those
of the whole sample. Because of the different absorptivities and refractive indices of the various compounds, absolute values cannot be placed on the aromaticities. However, comparison of one chromatogram with another clearly shows the changes in condensed aromaticity. T h e same problem exists for the alkyl-substituted monoaromatics and aliphatic compounds as shown by the RID. Because of the variation in refractive index with structure, absolute values have not been calculated. However, with a series of crude oils, a semiquantitative correlation was found between saturate content and the corresponding area used here. When the saturates were removed from these samples, the negative peak disappeared indicating a very low alkyl-substituted monoaromatic contribution.
LITERATURE CITED W. Albaugh and P. C. Taiarico, J. Chromatogr.,74, 233 (1972). (2) E. W. Albaugh, P. C. Talarico, €3. E. Davis, and R. W. Wirkkala,Prepr. Pop. Nat. Meet. Div. Petrol. Chem.,Am. Chem. Soc., 15, No. 2 A225 (1970). (3) D. M. Jewell, E. W. Albaugh, B. E. Davis, and R. G. Ruberto, lnd. Eng. Chem., Fundam., 13, 278 (1974). (4) J. H. vanDijk, J. Chromatogr. Sci., I O , 31 (1972). (5) Technical Manual, Pye LCMP Liquid ChromatographPublication No. 299-365, Pye Unicam, Cambridge, England. (6) Operating Instructions for Dohrmann Micro.Coulometric Titrating System, Model C200 Dohrmann,Mountainview, Calif. 94040. (1) E.
RECEIVEDfor review May 21, 1975. Accepted May 21, 1976.
Molecular Sieve Adsorption Method for Determination of nParaffins in C12-C32 Petroleum Distillates V. R. Sista and G. C. Srivastava”’ Indian Institute of Petroleum, Dehadrun, India
Based on the principle of preferential adsorptionof n-parafflnic hydrocarbons on molecular sieves, a method was developed for direct estlmation of n-paraffins In petroleum distillates (boiling range, 200-470 “ C ) .The effect of temperature on the desorption of n-paraffins was studied. The method is accurate to f l wt %.
The demand for n-paraffinic hydrocarbons (np) is increasing with time. For the production of single cell protein and detergents, n p present in the mi‘ddle distillates are preferred. It has been felt that the presence of hydrocarbons other than n p may deteriorate the quality of the final product especially in the case of single cell protein production. Thus, need arises to check the purity of the feed. Besides, knowledge of n p content in kerosene and gas oil fractions is essential for process development studies. Based on the principle of adsorption of n p on molecular sieves, methods developed (I, 2) have been summarized by Chen and Lucki and these authors have given two methods for the determination of np content in gas oil fractions ( 3 ) .As has been reported by them, the methods are suitable only when there is a significant quantity of cyclic and branched
Present address, IIP Proteins Project, Gujarat Refinery, Baroda, India. 1582
hydrocarbons. When these methods were applied to determine the percentage purity of n p in the normal paraffinic feed, prepared by any of the processes, inaccurate results were obtained. All the methods reported deal with the estimation of unadsorbed hydrocarbons (cyclic and branched) which, if present in low quantities, would give erroneous results. In case larger samples are used, a fairly high quantity of molecular sieves (40 g/g of np) would be required. The method described here is the direct estimation of n p using a lesser quantity of molecular sieves as well as a test sample and can be applied to determine the percentage purity of the n p sample as well as for quantitative determination of n p content in kerosene and gas oil fractions.
EXPERIMENTAL Materials. Molecular Sieve type 5A, lhs-inch pellets (Linde Co.); naphtha (boiling range: 80-100 “ C ) ; acetone (LR Grade, BDH); n hexadecane (BDH); n-dodecane (Fluka); n-docosane (Fluka); ndotriacontane (Fluka); dewaxed gas oil fractions (boiling range: 200-370 “C and 300-460 O C ) ; np prepared from gas oil fractions of Ankleshwar crude oil by the urea adduction method. Preparation of Dewaxed Gas Oil. An Ankleshwar gas oil fraction was subjected to successive urea adduction till no more adduct was formed. This dewaxed gas oil was used as a sample representing cyclic and branched hydrocarbons. Experimental Setup. The U-shaped sorption tube (herein called “tube”) employed was similar to the one used by Nelson et al. ( 4 ) .A suitable furnace having capacity up to 350 “ C was used. The disturbance caused in the system due to pressure variation was overcome
ANALYTICAL CHEMISTRY, VOL. 48, NO. 11, SEPTEMBER 1976
Table I. n-Paraffin Content in Test Samplesa Test sample
Figure 1. Experimental setup for quantitative determination of nparaffinic hydrocarbons
Quantity used, g
n-Paraffin c o n t e n t , Wt % Actual
Determined
n-Hexadecane 0.881 100 99.9 n-Hexadecane + p-Xylene 0.775 69.8 70.5 n-Hexadecane + DGO (i)* 0.493 89.8 89.2 n-Hexadecane + DGO (i)* 0.765 49.9 50.1 n-Hexadecane + DGO (i)* 2.3723 30.14 29.9 n-Dotriacontane + DGO (ii)" 0.6431 45.70 45.72 np (Single stage)d 1.036 87.75e 87.0 np (Double stage)d 0.5872 94.7e 95.0 a Experimental conditions: Activation: at 300 "C at 0.5 mm mercury pressure. Adsorption: 1 h at room temperature and 1 h a t 150 "C at reduced pressure. Removal of unadsorbed material: At 300 "C at 0.5 mm mercury pressure. DGO (i): Dewaxed Gas Oil (boiling range: 200-370 "C).C DGO (ii): Dewaxed Gas Oil (boiling range: 300-460 "C). d np hydrocarbon samples prepared by urea adduction method. e np content determined by temperature programmed gas chromatography.
*
20 I
# f
-
The difference between the weight of the tube with sample and loss in weight represents the amount of np in the sample.
I
-2
R E S U L T S AND DISCUSSION
to w
e
3 a
4 cZDO
260
300
400
by incorporating a pressure relieving device which may either be a closed mercury manometer or simply a thick glass round bottom flask. The assembly of the experimental setup is shown in Figure 1. Procedure. Preparation of Sorption Tube. About 12-15 g of Molecular Sieves pellets (MS) were charged into the cleaned and dried tube. The stopcocks were then properly lubricated and fitted in the
tube.
Dehydration of Molecular Sieves. The tube containing MS was suspended in the furnace which was gradually heated to 300 "C while maintaining a vacuum of 0.5-1.0 mm mercury in the tube as well as in the pressure relieving system. The tube under evacuated conditions was cooled and weighed to the nearest 0.1 mg. Addition of Sample. About 1 g of sample was allowed to drain through the funnel without allowing any air bubbles. The side tube and funnel were cleaned of the residual sample with naphtha followed by acetone and dried. The tube was then weighed to determine the exact quantity of sample added. Adsorption. The tube was kept at room temperature and adsorption was allowed to take place under that condition for 1h. It was then connected to the pressure relieving system and suspended in the oven which was gradually heated to 150-200 "C and maintained at that temperature for 1 h. Estimation of n-Paraffins. After the adsorption step, the tube was connected to a vacuum pump to maintain in the system a vacuum of known value between 0.1-0.5 mm mercury. The temperature of the furnace was raised to maintain in the tube a temperature corresponding to 560 "C at 760 mm mercury. After 1 h, the tube was taken out, cooled t o room temperature, and weighed under evacuated conditions. The tube was again heated in a similar way for 15-20 min, cooled, and weighed to constant weight.
It was found necessary to conduct adsorption of n p in two stages; for 1h a t room temperature and another hour a t elevated temperature. At room temperature, some of the n p fractions of low boiling range, under reduced pressure, remained in vapor stage and are adsorbed on MS. I n order to accelerate the adsorption of n p of higher boiling ranges, the tube was required to be heated to about 200 "C for 1h, and the total time of 2 h was found to be sufficient as reported earlier (3). After adsorption, when the tube was heated under vacuum (0.1-0.5 mm mercury), keeping the temperature corresponding to 560 "C a t 760 mm mercury for 1 h, all the unadsorbed fractions (cyclic and branched) were removed from the tube without affecting the n p already adsorbed. A study was made to show the effect of temperature on desorption or loss in weight of adsorbed n p of different molecular weights. From Figure 2 , it can be observed that desorption of np, irrespective of their molecular weights, takes place only when the temperature of the furnace exceeds 560 "C a t 760 mm mercury. However, the effect of temperature on desorption depends on the molecular weight of np. Since at high temperature, n p would isomerize and crack and the structure of MS would also be damaged when heated to 540 O C or above ( 5 ) ,the experiments were conducted under reduced pressure. T h e present method thus altogether avoids successive solvent extraction of unadsorbed hydrocarbons and stripping of solvents as adopted in earlier methods (2,3). T h e results of different experiments conducted with n hexadecane, synthetic blends of nCl6-p-xylene, ncls-dewaxed gas oil, nC32-dewaxed gas oil are given in Table I. The result of n p content of the two samples of n p feed prepared by the urea adduct method (6) was checked on a HewlettPackard Model 5705 G dual column temperature programmed chromatograph. T h e gas chromatographic column used was a 1.8-meter X 2-mm column packed with 10%Ucc-W.982 on Chromasorb W, AW, DMCS. From the results, it is observed t h a t this method is accurate to fl wt % irrespective of the nature of the hydrocarbons present in the test sample (Table
I).
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1583
LITERATURE CITED (1) J. G. O'Connor and M. S . Norris, Anal. Chem., 32, 701 (1960). (2) J. G. O'Connor,F. H. Burow, and M. S. Norris, Anal. Chem., 34,82 (1962).
(5) V. R. Sista et al., IIP Internal Report No. 20.008(unpublished). (6)"Modern Petroleum Technology", Institute of Petroleum, London, 1962,p 438.
Determination of Aromatic Amines by an Adsorption Technique with Flame Ionization Gas Chromatography Barry E. Bowen Polymer Intermediates Department, E. 1. du Pont de Nemours and Company, Experimental Station Laboratory, Wilmington, Del. 19898
Quantitative adsorption and desorption of aromatic amines using Tenax GC has been demonstrated at the nanogram level. Linear calibration data were obtained over two to four decades and detailed response curves are reported for the first time. The sensitivity of an adsorption-FlGC technique to detect sub-ppm levels of aromatic amines in organic or aqueous solutions or in air is shown to approach that achieved by electron capture and nitrogen specific detectors.
The toxicity of aromatic amines to man has been studied for many years ( I ) . Recently, the Occupational Safety and Health Organization (OSHA) established exposure limits for 14 human carcinogens (2, 3 ) . This list includes 4-aminodiphenyl and several other amines. Numerous other aromatic amines are cancer-suspect agents. Secondary and tertiary amines, when reacted with nitrous acid, yield nitrosamines, another cancer-suspect class of compounds. Numerous workers have recently published trapping and/or detection schemes for aromatic amines, using GC, LC, or TLC, sometimes coupled with ancillary techniques like IR and UV spectrometry (4-15). Tenax GC has been found to be a good general purpose adsorbent for efficiently adsorbing and desorbing organics. This study presents a general analytical technique for detection and quantitation of trace aromatic amines in dilute solutions, in air, and in solids. Well-defined limits of detection and linearity ranges, work not previously reported, for aromatic amines u p to b p 300 "C have been established for our system which uses only commercially available components. Tenax GC, a 2,6-diphenyl-p-phenylene oxide solid GC support stable to 375 "C, was used t o absorb submicrogram amounts of aromatic amines from dilute organic and aqueous solutions. The solvent was vented using a heated injection valve. The amines were then desorbed and backflushed onto a glass Silar 1OC GC column, then detected by flame ionization.
EXPERIMENTAL The amines listed in Table I were obtained from various chemical suppliers. They were purified by crystallization and sublimation to at least 99 mol % purity. A Model 1047 Concentrator (Chromalytics Corp., Unionville, Pa.) was used to concentrate the amines either by trapping with 15 cm X 0.5 cm 0.d. glass sampling tubes filled with 8 cm of adsorbent or by trapping with 30 cm X 3/s in. 0.d. stainless steel U-tube filled with 10 cm of adsorbent. Adsorbents tried were Tenax GC, Porapak QS, Chromosorb 103,and Silar 1OC on Chromosorb WHP obtained from Applied Science Laboratories, Inc., State College, Pa. 1584
The Model 1047 is a heated gas sampling valve that attaches to many commercial gas chromatographs. The device permits normal injection onto the GC column or concentration of a dilute gaseous or liquid sample by injection onto the U-trap. A sampling tube, used for collection of airborne amines in a remote location, can be attached to the unit. The amines are desorbed from the adsorbent by rapid heating and backflushing onto the GC column. The temperature of the concentrator oven is limited to 250 "C by the internal Teflon seals. The temperature of the adsorption tube or trap can be controlled separately and can be held at "raised ambient" levels for venting solvents. The rate of temperature rise of the sampling tube or U-trap, for rapid desorption, is obtained from a variable potentiometer and is nominally set at 1-5 O C / s . Several other sample concentrators are commerciallyavailable and adaptable to GC work besides the Model 1047 (Chromalytics Corp., Unionville, Pa. 19375)used in the work reported here. They include the Single Hollow Fiber Concentrator for liquids and the Model 105 T for gases (MDS Scientific, Inc., Park Ridge, I11 60068),the DC-50 series (Environtech.Dohrmann Division, Santa Clara, Calif. 95050), the Model LSC-1 (Tekmar Co., Cincinnati, Ohio 45222) and the Bendix Personnel Monitoring Collection Column with accessories (National Environmental, Inc., Warwick, R.I. 02888). Silar 1OC (Applied Science Laboratories, Inc., State College, Pa.), a polar liquid phase stable to 250-275 "C, was coated at 8%by weight onto Chromosorb WHP by slowlywithdrawingthe solvent chloroform using a rotary vacuum apparatus. Single 6 f t X 6 mm 0.d. X 2 mm i.d. glass columns were placed into a Perkin-Elmer 3920 gas chromatograph equipped with a flame ionization detector. The hydrogedair pressure was optimized to 16/45 psi using the methane bleed method. Chromatographicdata were collected by the Du Pont Experimental Station Real Time Computer System (16).
RESULTS Linearity of Response. Dilute xylene solutions of the aromatic amines in Table I (to be used as a glossary for abbreviations in the discussion below) were adsorbed onto the Tenax U-trap. The xylene was vented and the amines were desorbed. Our data show t h a t most of these amines can be quantitatively adsorbed and desorbed from Tenax GC. Figures 1and 2 show that response of some amines for our analytical system is linear in the 10-104 ng range. Silar 1OC on Chromosorb WHP gives sharp well-defined peaks (Figure 3), so that peak height response varies linearly with the amount injected, similar to area response. Our original work with Tenax GC as the analytical column proved t o be less satisfactory, since peaks were broad and higher mol wt amines could not be eluted within a reasonable time period. Diamines (OPD, PPD) are apparently "irreversibly" adsorbed t o some degree on the Tenax trap below the 500-ng level, since the response curves take on a new slope near t h a t point. Similar behavior was observed in this laboratory using
ANALYTICAL CHEMISTRY, VOL. 48, NO. 11, SEPTEMBER 1976
