studying the metabolism of phenolic compounds in experimental animals [see e.g. Bray and Thorpe (30)]and especially for their identification using paper chromatographic techniques [ e . g . Bray, Thorpe, and White (31);McIsaac and Williams (32); Mead, Smith, and Williams (33)]. The need for caution in the interpretation of the result of the Gibbs color reaction should now be clear; even more so in view of the additional facts that many highly substituted phenols with the para position unsubstituted d o not give a positive test [see e.g. Birkinshaw, Bracken, Morgan, and Raistrick ( 1 2 ) ; Briggs and Locker (29)], while many compounds, e.g. amines (Castle, 34) and other amino derivatives (Fearon, 35) also give purple, violet, or other characteristic colors.
Table 111. Some Para Substituted Phenols Giving a Negative Gibbs Reaction p-Halogenophenols 2,4-Dibromophenol 2,4,6-TrichlorophenoI 2,4,6-Tribromophenol 2,4,6-Triiodophenol 2-Hydroxy-5-chlorobenzaldehyde p-Hydroxybenzaldehydes P-Resorcyl-aldehyde 3-Ethoxy-4-hydroxybenzaldehyde p-Hydroxybenzyl alcohol p-Benzyl-phenol p-Hydroxybenzoic acids Protocatechuic acid P-Resorcylic acid Gallic acid
RECEIVED for review August 31, 1970. Accepted December 14, 1970.
(30) H. G. Bray and W. V. Thorpe, in “Methods of Biochemical Analysis,” D. Glick, Ed., Interscience Publishers, New York, N. Y . ,Vol. 1, p 27 (1954). (31) H. G. Bray, W. V. Thorpe, and K. White, Biochenz. J.,46, 271 ( 1950). (32) W. M. McIsaac and R. T. Williams, ibid., 66, 369 (1957). (33) J. A. R. Mead, J. N. Smith, and R. T.Williams, ibid., 68, 61 ( 1958). (34) R. Castle, Chem. I d . (Lorzdorz),313 (1950). (35) W. R. Fearon, Biochem. J., 38, 399 (1944).
ticularly those working on the elucidation of the structure of naturally occurring organic compounds [see e.g. King, King, and Manning (27); Hems and Todd (?8); Briggs and Locker (29)]. The reaction has also been applied both by those
_
~
_
(27) F. E. King, T. J. King, and L. C . Manning, J. Chem. Soc., 1957,563. (28) B. A. Hems and A. R. Todd, ibid., 1940, 1208. (29) L. H. BriggsandR. H. Locker, ibid.,1951,3131.
Rapid Hydrocarbon-Type Analysis of Gasoline by Dual Column Gas Chromatography R. E. Robinson, R . H. Coe, and M. J. O’Neal Shell Oil Company, Houston Research Laboratory, Deer Park, Texas 77536 AN OFTEN REQUIRED characterization of gasoline-range hydrocarbon streams is type composition in terms of total saturates, olefins, and aromatics. This analysis is conventionally carried out by the fluorescent indicator adsorption (FIA) method (1). The technique lacks good precision and becomes extremely difficult to apply to highly colored samples derived from pyrolysis, coking, hydrocracking, etc. Moreover, the procedure is rather lengthy. For light gasoline fractions, the analysis time is increased further because the sample must be depentanized first by distillation, with subsequent analysis of the Cs-and-lighter fraction by gas chromatography and finally analysis of the Cs- fraction by the FIA method. A more rapid and direct approach is t o employ a gas chromatographic method, wherein the entire untreated sample is injected into the chromatograph. Martin ( 2 ) successfully utilized a combined method for the separation of gasoline into aromatics, olefins, and total saturates, based on group chromatographic separation and chemical absorption. Separation of aromatics from saturates and olefins was obtained with a @,@’-thiodipropionitrile packed column; olefins were then separated from saturates by reaction with mercuric perchlorate. This technique was (1) Am. SOC.Testing Materials, “1968 Book of ASTM Standards,”
Method D-1319-66T,Part 17, p 506. ( 2 ) R. L. Martin, ANAL.CHEM., 34, 896 (1962).
later extended to include n-paraffin separation from other saturates and individual n-paraffin analysis (3, 4). More recently, a subtractive method for the rapid analysis of hydrocarbon types has been described by Soulages ( 5 ) . The aromatic and olefinic hydrocarbons are selectively retained by two chemical absorbers in parallel while the total saturates pass unaltered, and the resulting peaks are detected by a single flame ionization detector. In the present paper, a new, but related, technique for rapidly determining hydrocarbon-type content of gasoline is described. A parallel capillary column system and a n olefinabsorbing trap are employed with valving. One column is uncoated and the other is coated with the highly polar N , N bis(2-cyanoethy1)formamide (6). This coated column is in series with the trap. By proper adjustment of flow rates, each column receives the same total sample from a splitter. The empty column passes its charge directly to the detector where a measure of total sample is obtained. The other column retards the aromatics and the trap removes the olefins
(3) D. K. Albert, ANAL.CHEM.,35, 1918 (1963). (4) L. E. Green, D. K. Albert, and H. H. Barber, J. Gas Chromatogr., 4, 319 (1966). (5) N. L. Soulages and A. M. Brieva, ANAL.CHEM.,in press. (6) M. Rogozinski and I. Kaufman, J . Gas Chromatogr., 4, 413 (1966).
ANALYTICAL CHEMISTRY, VOL. 43, NO. 4, APRIL 1971
591
r--------
HELIUMCARRIER^ GAS
!
FLOW CONFIGURATION 1
SAMPLE
7
1
I/
HEATING MANTLE 1
4 G I
1
L--% I I
FLOW CONFIGURATION 7.
FLOW CONFIGURATION 3
--"fl
I 0
I
I
I
I
5
10
I
5
TIME, MINUTES
Figure 2. Chromatogram of hydrocarbontype separation in gasoline Figure 1. Flow diagram of apparatus for rapid hydrocarbon-type analysis
so only the saturates go to the detector. The aromatics are backflushed to the detector. Thus total sample, saturates, and aromatics are measured, and olefins are obtained by difference. EXPERIMENTAL
Apparatus Requirements and Procedure. The gas flow in the assembled apparatus is shown in Figure 1. This figure illustrates the order in which the two valves are positioned in each of three flow configurations. With the apparatus in flow configuration 1, an injected liquid sample (0.1-0.4 111) is vaporized and split approximately lOOjl (Varian Aerograph capillary inlet splitter) in the usual fashion for open tubular GLC. The smaller sample fraction then passes through a section of 0.01-inch i.d. stainless steel capillary tubing approximately 60 inches long. The purpose of this column is to allow sufficient time for the sample and carrier gas to become thoroughly mixed before reaching the splitter (Swagelok zero-dead-volume tee). The sample then passes through the splitter and is divided nearly equally between columns A and B. By adjusting the length and temperature of Column A, approximately equal flow rates through A and B are obtained. Column A is a n empty threefoot section of 0.003-inch i.d. stainless steel tubing. By means of a "Glas-Col" heating mantle, the empty column is maintained at about 250 "C in order to prevent adsorption on the capillary wall by heavy ends present in full-range gasoline (upper boiling point of 216 "C, n-C12). Column B is 100 feet of 0.01-inch i.d. stainless steel tubing coated with a 25 wt % solution of N,N-bis(2-cyanoethyl)formamide (CEF) in chloroform. The C E F column is thermostated at about 50 O C in a separate "Glas-Col" heating mantle. At a column temperature of 50 "C, the aromatic-saturate selectivity is adequate (C12 n-paraffin elutes before benzene) and column bleed is negligible. That portion of sample passing rapidly through Column A is detected as a single peak, the area of which is proportional to the sample size. The fraction entering column B, uia a 4-port high-temperature micro-valve V1 (Model CV-4-HT, Valco Company, Houston, Texas) is delayed by the partition592
ANALYTICAL CHEMISTRY, VOL. 43, NO. 4, APRIL 1971
ing process. The C E F column selectively separates the aromatics from the saturates and olefins. After the sample is eluted from column A but before the combined saturates and olefins emerge from column B, valve V2 (low-temperature micro-valve, Model CV-4-HP) is rotated to give flow configuration 2. With this change in flow pattern, an olefinabsorbing trap C is placed into the sample path and the saturates are measured directly. The temperature of valve V2 and absorber is maintained within the temperature range 60-75 O C by heating tapes. After the saturates are eluted, but before benzene emerges from column B, both valves V1 and V2 are rotated to give flow configuration 3. This flow pattern allows the aromatics to be backflushed through column B and measured as a single unsymmetrical peak. A typical chromatogram is shown in Figure 2. Analysis time is about 15 minutes. Typical times between sample injection and switching olefin absorber in and switching absorber out are 2.5 and 7.5 minutes, respectively. The backflush procedure is employed solely for minimizing analysis time. In the event analysis of individual aromatics is of interest, the backflush operation is eliminated (valve V1 is not rotated in flow configuration 3) and the analysis time is extended 20-30 minutes. Helium is used as the carrier gas. At an inlet pressure of 20 psig, a flow rate of 1.6 ml per minute is obtained. The air and hydrogen flows to the flame ionization detector (FID) are 41 1 ml per minute and 16.7 ml per minute, respectively. The injector splitter temperature is controlled at 310 'C and the detector temperature is 160 O C . The signals from the FID are amplified by a Keithley Model 410 Micro-Microammeter. The electrometer is monitored by an Esterline-Angus Model S601S recorder and a Computer Measurements Model 315A digital integrator. Olefin Absorber. The absorber used to remove olefins quantitatively is a mixture of 40 wt mercuric perchlorate and 15 w t z of 70% perchloric acid retained on 60-to-80-mesh Chromosorb P. The absorber was prepared according to the procedure reported by Martin ( 2 ) . For routine analyses a 1-inch section of 0.062-inch i.d. Teflon (DuPont) tubing is packed loosely with the absorber and attached directly to the 0.062-inch 0.d. stainless steel tubing of valve V2. In general, the absorber will last through 15-20 gasoline-range analyses.
z
Blend 1
2
Table I. Quantitative Hydrocarbon-Type Analysis of Synthetic Blend Replicate measurements, % Weighed Run 4 Run 2 Run 3 Run 1 Hydrocarbon type composition, % 34.3 34.6 34.5 33.8 33.6 Saturates 26.6 25.9 26.1 26.5 26.9 Olefins 39.1 39.5 39.4 39.7 39.5 Aromatics 100.0 100.0 100.0 100.0 100.0 Total 29.1 28.7 29.0 Saturates 36.2 36.8 35.6 Olefins 34.7 34.5 35.4 Aromatics 100.0 100.0 100.0 Total
Table 11. Quantitative Hydrocarbon-Type Analysis of Gasoline Hydrocarbon Undepentanized Mass Sample type FIA spectrometry Stabilized reformate Saturates 61.9 ... Olefins 0.0 ... Aromatics 38.1 ... Light catalytically Saturates 55.0 ... 29.0 , . . cracked gasoline Olefins 16.0 18.0 Aromatics Intermediate Saturates 45.6 ... 21.2 ... catalytically Olefins cracked gasoline Aromatics 33.2 32.1 a Average of three replicate measurements. * Average of four replicate measurements. c Average relative deviation from mean.
To ensure complete retention of olefins, the general practice is to replace the absorber after 15 analyses. The absorber is never left in the system overnight but is simply discarded, and a freshly packed column inserted the following morning. Calculations. The following procedure is used for calculating on a weight per cent basis, the distribution of saturates, olefins, and aromatics in gasoline. Under isothermal conditions of operation, a ratio, R, of “peak area measured from the empty column”/“peak area measured from the partitioning column” is determined for the system with a reference compound (e.g.,n-heptane). A sample of gasoline is injected. After the peak from the empty column is eluted, the olefin absorber trap is placed into the flow path of the saturates and olefins eluting from the partitioning column. After the saturates peak is eluted, the partitioning column is backflushed and the aromatics peak is eluted. The peak area from the empty column is divided by R to obtain the total area of saturates, olefins, and aromatics eluted from the partitioning column. The olefins area is then obtained by subtracting from the total area the combined peak areas of saturates and aromatics. In summary: Empty column peak area
R
- Total peak area of saturates, olefins, and aromatics
(Total peak area of saturates, olefins, and aromatics) (Peak area of saturates peak area of aromatics) = Peak area of olefins
+
The areas are normalized to 100 %. For improved accuracy, the respective peak areas should be corrected by detector sensitivity factors determined from synthetic mixtures containing typical gasoline-range hydrocarbon types. In this work the peak area for each hydrocarbon type was corrected by the following factors: Total saturates, 1 . O l Olefins, 1.02 Aromatics, 1 .04
Mean 34.3 26.3 39.4 100.0 28.9 36.5 34.6 100.0
Gas-liquid chromatography 6 1 . 9 (0.27p 0.oa 38.50 (0.43)c 5 1 . 9 (0.0f1)~ 32.8. (0.10)~ 15.7R(0.43)~ 45.3b (0.61)~ 23.lb (1.82)~ 31 .6* (0.48)~
These factors were used with the gasolines analyzed. For a gasoline of unusual composition, different factors would probably be required. Splitter Performance. The accuracy of analysis achievable with the method is dependent upon the dynamics of the system used for splitting the sample between the two columns. To be effective, the sample-splitting device must be nondiscriminatory; that is, it must divide all components of the sample in the same ratio (7,8). The performance of the tee-splitter was evaluated with various hydrocarbon types representative of those present in gasoline. Pure compounds injected singly as well as synthetic mixtures were examined. The precision of splitting was estimated by integrating the areas of the peaks from the two columns and calculating the ratio of these peak areas. Analyses of 22 consecutive measurements under typical operating conditions for isothermal operation indicated that the variation in the ratio was randomly distributed and that the average peak ratios for the various samples agreed very well with the limits of precision found for n-heptane (mean ratio of peak areas = 1.031, standard deviation, u = 0.011). Moreover, no significant change in the ratio was found when the sample size of n-heptane was varied. RESULTS AND DISCUSSION
Accuracy was determined by analyzing two synthetic mixtures, each containing various hydrocarbon types representative of those present in gasoline. The analyses are summarized in Table I. The differences between the weighed and found values average approximately 1.6 of the amount found. The largest variance in the data appears in the olefin concentration; this would be expected because olefins are determined by difference. (7) L. S.Ettre, “Open Tubular Columns in Gas Chromatography,” Plenum Press, New York, N. Y., 1965,p 114. (8) C . Merritt, Jr., and J. T. Walsh, ANAL.CHEM., 34,908 (1962). ANALYTICAL CHEMISTRY, VOL. 43, NO. 4, APRIL 1971
593
To illustrate further the accuracy of the method, a comparison of results from undepentanized FIA analyses and GLC analyses is presented in Table I1 for three gasolines of differing compositions. FIA data normally reported in per cent volume are converted to per cent weight to allow direct comparison with GLC data. The aromatics content of the gasoline samples was also determined by mass spectrometry and these results are included in Table 11. An inspection of these data shows that relatively good agreement is found among the different analytical procedures. Interestingly, the GLC method shows lower aromatics and higher olefin values than those measured by FIA for both olefinic gasolines. Since the boundary between aromatic and olefin fractions in the FIA analysis is the more difficult to determine accurately, the aromatics content measured by the GLC method is probably more reliable. In the case of the light catalytically cracked gasoline analysis, the high saturates value obtained by FIA may be a consequence of not depentanizing the sample. In general, spreading of the
saturate zone by the volatile C6-and-lighter hydrocarbons leads to erroneously high saturates content. On the basis of four replicate measurements of the intermediate catalytically cracked gasoline, the relative deviations from the mean values average about 1 %. Again, the largest variance appears in the olefin value. The results in Table I1 also show that a similar, if not better, precision is achieved for the triplicate analyses of the reformate and light catalytically cracked gasoline. ACKNOWLEDGMENT
The authors are indebted to J. B. Maynard, Wood River Research Laboratory, for his initial development work on hydrocarbon-type analysis by gas chromatography. RECEIVED for review September 28, 1970. Accepted December 14, 1970. Permission to publish granted by Shell Oil Company.
New Spectrophotometric Method for Determination of Submicrogram Quantities of Selenium Robert L. Osburn Department of Chemistry, Louisiana State University at Eunice, Eunice, La. 70535
A. D. Shendrikar and Philip W. West Coates Chemical Laboratories, Institute for the Environmental Sciences, Louisiana State University, Baton Rouge, La. 70803
SELENIUM DIOXIDE is an oxidizing agent for unsaturated hydrocarbons, aldehydes, ketones, heterocyclic nitrogen compounds, terpenes, sterols, fatty oils, and other natural products (I). Analytical methods which are based on its oxidizing properties usually lack sensitivity and hence cannot be used for the determination of selenium. However, Postowsky, Lugowkin, and Mandryk (2) investigated the oxidation of arylhydrazines by selenous acid, and they found that diazonium salts were produced which could be coupled with aromatic amines to produce intensely colored azo dyestuffs. Fiegl and Demant (3) employed this reaction for the detection of small amounts of arylhydrazines, and Kirkbright and Yoe ( 4 ) have developed a spectrophotometric method for the determination of selenium based on the spot test developed by Feigl. However, the useful analytical range of this procedure is 2 to 40 pg of selenium in a maximum sample volume of 2 ml. Feigl (5) has mentioned that hydroxylamine hydrochloride is oxidized to nitrous acid by selenous acid under strongly acidic conditions. It is commonly known that nitrites react with primary aromatic amines in acidic solution with the formation of diazonium salts which will couple with certain (1) G. R. Watkins and C. W. Clark, Chem. Rev., 36,235 (1945). (2) J. J. Postowsky, B. P. Lugowkin, and G. T. Mandryk, Ber., 69, 1913 (1936). (3) F. Feigl and V. Demant, Mikrochim. Acta, 1, 134 (1937). 35,808, (1963). (4) G. R. Kirkbright and J. H. Yoe, ANAL.CHEM., (5) F. Feigl, “Spot Tests in Organic Analysis,” 7th ed., Elsevier, New York, 1966, p 237. 594
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ANALYTICAL CHEMISTRY, VOL. 43, NO. 4, APRIL 1971
compounds to form intensely colored azo dyes. Because of the extreme sensitivity of the diazotization-coupling reactions sequence with which nitrite determinations are routinely made in the parts per billion range, this reaction offers a unique and attractive approach for the determination of submicrogram quantities of selenium. Various combinations of reagents for the diazotizationcoupling reactions have been used by different workers including sulfanilic acid and 1-aminonaphthalene (6-8) sulfanilic acid and N-N-dimethyl-1 -aminonaphthalene ( 9 ) , sulfanilic acid and N-( 1-naphthyl)-ethylenediamine hydrochloride (IO), and sulfanilamide and N-(1 -naphthyl)-ethylenediamine dihydrochloride (11,12). Because the last combination of reagents is somewhat more sensitive than others, it was chosen as the basis for the development of a new spectrophotometric method for the determination of selenium which can be represented by the following
(6) “Standard Methods of Water Analysis,” American Public Health Association, New York, N. Y., 1936, p 46. (7) “Official and Tentative Methods of Analysis,” Association of Official Agricultural Chemists, Washington, D. C., 1940, pp 222, 527. (8) W. P. Mason and A. M. Bushwell, “Examination of Water,” John Wiley and Sons, New York, N. Y., 1931, p SO. (9) F. G. Germuth, IND.ENG.CHEM., ANAL.ED.,1,28 (1929). (10) B. E. Saltzman, ANAL.CHEM., 26,1949 (1954). ANAL,ED., 12,325 (1940). (11) M. B. Shinn, IND.ENG.CHEM., (12) N. F. Kershaw and N. S. Chamberlin, ibid.,14,312 (1942).
