2620
Anal. Chem. 1984, 56,2620-2622
Figure 1 shows the relationship between flow rate and ta,wc LITERATURE C I T E D for 25-m and 50-m columns with both hydrogen and helium, (1) Leferink, J. G.; Leciercq, P. A. J. Chromafogr. 1974, 91, 385-391. enabling ready estimation of one parameter from the other (2) Vangaever, F.; Sandra, P.; Verzele, M. Chromafographia 1979, 72, 153-154. for these typical columns. Figure 2 illustrates the effect of (3) Sellier, N.; Guiochon, G. J. Chromatogr. Sci. 1870, 8, 147-150. column diameter, length, and carrier gas on the additional (4) Varadl, P. F.; Ettre, K. Anal Chem. 1963, 3 5 , 410-412. average velocity necessary to maintain the same flow rate a t (5) Giddings, J. C. Anal. Chem. 1982, 3 4 , 314-319. (6) Hatch, F. W.; Parrish, M. E. Anal. Chem. 1978, 5 0 , 1164-1168. vacuum outlet pressure as that produced by a particular av(7) Cramers, C. A.; Scherpenzeel, G. J.; Leclercq, P. A. J. Chromatogr. ta,opt,vac for each erage velocity a t atmospheric outlet (Oat,). 1981, 203, 207-216. ~ Hoiland, ~ ~ L.;. Steckeimacher, W.; Yarwood, J. "Vacuum Manual"; E. & situation can thus be found readily from knowledge of o ~ ~ ~ , (8) F. N. Spon: London, 1974; p 26. Taking- the case of a 25 m X 0.5 mm column usine" helium. if uoptah = 30 cm/s, the additional velocity required is 47 cm/s for a total of 77 cm/s. Hence ta,opt,vac= (25/0.77) = 32 s. RECEIVED for review April 4, 1984. Accepted June 18, 1984.
Microanalysis of Reaction Products in Sealed Tube Wet Air Oxidations by Capillary Gas Chromatography M a r g a r e t K. Conditt' and Robert E. Sievers*
Cooperative Institute for Research i n Environmental Sciences and Department of Chemistry, University of Colorado, Campus Box 215, and Campus Box 449, Boulder, Colorado 80309 Capillary gas chromatography can be used to analyze the reaction products of the oxidation of a compound in aqueous solution. A sealed glass capillary tube is used as the reaction vessel in which the compound and its products of oxidation are confined. The contents of the sealed tube are released upon crushing and are swept directly into the carrier gas stream of the gas chromatograph using a novel technique. As an example, the technique outlined in this paper is used to determine the combustion products of the wet air oxidation treatment of phenol; however, it may be used for any reaction in which volatile products are formed. Wet air oxidation is a promising wastewater treatment technique that is capable of detoxifying many hazardous organic compounds in aqueous waste streams. The process relies upon conditions of elevated temperature and oxygen pressure to partially oxidize the organic matter in aqueous solution and has been reported to produce low-molecular weight oxygenated compounds as the principal products (1). A thorough study of this thermal oxidation treatment method requires analytical methodology that can determine the relative rate of degradation of a specific pollutant as well as identify its products of combustion. Equipment and methods for studying the fate of specific organic compounds during wet air oxidation have been described previously by Baillod et al. (1)and Randall and Knopp (2). Traditionally, stainless steel or titanium pressure vessels have been used to establish the conditions required for dissolved oxygen to react with difficult to oxidize organic compounds (ca. 315 OC and 15 MPa). However, we wish to describe a microanalysis technique which, in our experience, offers advantages over previously reported methods. The oxidation reaction a t elevated pressure and temperature occurs in a sealed glass capillary tube and the reaction products are subsequently quantitatively introduced directly into a fused silica capillary column coated with a cross-linked polymer for gas chromatographic separation and analysis. EXPERIMENTAL S E C T I O N As a demonstration of the effectiveness of the technique for microanalysis of reaction products in aqueous solutions, the wet air oxidation of phenol was performed in a small, sealed glass capillary tube at 300 OC in the presence of oxygen. A relatively 'Present address: The Procter & Gamble Co., Food Products Division, 6071 Center Hill Rd, Cincinnati, OH 45224.
high concentration of phenol (1000 mg/L) was studied in order to see both the major and minor products of oxidation. With a modified inlet system, shown in Figure 1,the glass capsules were crushed, releasing the products of combustion into the carrier gas stream of a capillary gas chromatograph equipped with either flame ionization or mass spectral detectors. Preparation and Oxidation of Phenol. A standard containing 10000 mg/L of phenol was prepared by dissolving 1.000 g of phenol in 100 mL of distilled, deionized water. One milliliter of this stock solution was diluted 10-fold with distilled, deionized and oxygen-saturated water to make the standard solution containing 1000 ppm of phenol. Two microliters of this standard solution was added to a micro reaction capsule made from a borosilicate melting point capillary 1.7 mm 0.d. x 1.0 mm i.d.) cut to a length of 4 cm and sealed at one end. To increase the oxygen available for oxidation, the air in the capsule was purged with oxygen using a syringe needle, while making certain that the needle did not come in contact with the phenol solution. The capsules were carefully sealed with a flame, leaving a headspace volume of approximately 30 pL that contained a 6-fold excess of oxygen available for the complete oxidation of phenol to carbon dioxide and water. The capsules were then heated at 300 f 2 "C in a laboratory oven for varying periods of time (0, 20,30,60,90, 120,150, and 180 min). Subsequently, they were cooled to room temperature and the contents were analyzed on the same day as the high temperature reaction, in most cases. The pressure inside the glass capsule increased at higher temperatures as a result of the vapor pressure of water (8.6 MPa or 1246 psi at 300 "C) and of the formation of gaseous oxidation products. Since this increased pressure resulted in the explosion of a few improperly sealed glass capsules, the capsules were isolated from one another in the oven by heating a few together in a small test tube. Care was taken to ensure that the capillary did not balloon during sealing. Appropriate eye protection was used whenever handling the heated micro reaction capsules. Modified Inlet System. The carrier gas stream of the gas chromatograph was modified to allow diversion through an external apparatus designed to introduce the contents of the sealed capillary tubes into the instrument. The apparatus, shown in Figure 1,consists of a bayonet-type F&M solid sample injector, SI-4, that has a hollow stainless steel tube into which the micro reaction capsule is inserted. The SI-4 injector was originally designed for introducing solid samples but works equally well for injecting aqueous aliquots. A tightly fitted plunger (2.2 mm) was slipped into the hollow tube (3.0 mm 0.d. X 2.3 mm i.d.), forcing and breaking the capsule against a beveled stop, releasing the contents into the carrier gas stream. The heated inlet system was maintained at 220 O C . This SI-4 injector was attached vertically to a stainless steel tee with a U-tube (6.35mm 0.d. X 4.5 mm i.d.)
0003-2700/84/0356-2620$01.50/00 1984 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 56, NO. 13, NOVEMBER 1984 ,MOVABLE
2821
PLUNGER
COMPRESSION NUT WITH O-RING
HEATED TO 220 'C
Figure 1. Apparatus used for breaking sealed glass micro reaction capsules and releasing contents into carrier gas stream of a gas
chromatograph. placed at the bottom to collect the glass broken upon crushing the sealed capillaries. The helium carrier gas entered the apparatus from the opposite end of the U-tube and exited from the side port of a tee that was fitted with a stainless steel tube (1.6 mm 0.d. X 0.5 mm i.d.) and a syringe needle (Unimetrics Copr., Anaheim, CA) for entrance into the injection port of gas chromatograph. The needle assembly was preceded with a medium porosity glass frit to prevent powdered glass from plugging the syringe. Instrumental Procedure. Sealed capsules were inserted into the heated SI-4 injector and crushed, releasing the contents directly into the helium carrier gas stream of a Hewlett-Packard 5730A gas chromatograph (Avondale, PA) equipped with a flame ionization detector. The combustion products were trapped cryogenically at the entrance of a fused silica gas chromatographic column by pulsed injections of liquid carbon dioxide into the chromatographic oven to maintain a temperature of 0 "C for 2 min, after which time the carrier gas flow was diverted away from the SI-4 injector and back to the normal flow through the injection port of the gas chromatograph. The temperature of the column was programmed then from 0 "C to 200 "C at a rate of 32 OC/min. The DB-5 fused silica capillary column (J&W Scientific, Inc., Rancho Cordova, CA), 30 m X 0.32 mm id., with a 1.0 pm film thickness was used to separate the products of oxidation. The relatively inert surface of the fused silica made the irreversible adsorption of polar constituents to the walls of the capillary column unlikely. The cross-linked stationary phase (94% methyl, 5 % phenyl, 1% vinyl polysiloxane) exhibited excellent thermal stability and was able to withstand repeated injections of aqueous samples without significant degradation of column performance. The identities of compounds giving rise to all of the peaks were established by combined gas chromatography/massspectrometry. Analysis of the combustion products was performed with a Hewlett-Packard 5982A GC/MS/DS operated in the electron impact mode and scanning at 167.5 amu/s in the range of 29 to 250 amu. The temperature program of the column was the same as that described above; however, in this case, the analytical column used to resolve the individual compounds was a Hewlett-Packard Ultra No. 2 fused silica capillary, 30 m x 0.31 mm i.d. with a 0.52 pm cross-linked polymer film thickness. Identification of the combustion products was made by comparison of retention times with those of reference compounds and by comparison of mass spectra with a reference compilation [EPA/NIH Mass Spectral Data Base (NSRDS-NBS 63)]. Quantitation of phenol and the products of oxidation was achieved by comparing the area of the chromatographic peaks obtained by a flame ionization detector to the peak area of a known concentration of a standard compound using a Hewlett-
30
60
TIME
90 120 I50 ( minutes 1
180
Figure 2. Concentrations of phenol and its oxidation products after heating in the presence of oxygen-enrichedair to 300 "C for various reaction times. Two-microliter aliquots of aqueous solutions containing 1000 ppm of phenol in contact with air were sealed in borosilicate glass
capillary tubes and heated for varying periods. Packard 3390A recording integrator. The standard solutions of authentic compounds were sealed in micro reaction capsules and introduced into the gas chromatograph through the modified heated inlet system. Detector responses were compared to analytical standards that were injected directly into the gas chromatograph to assess the adsorption properties of the apparatus and to determine the precision of the capsule injection technique. RESULTS AND DISCUSSION The results of the sealed tube oxidation of phenol are presented in Figure 2. The data indicate that phenol was completely destroyed at 300 " C in 3 h. The products of oxidation identified by GC/FID and GC/MS were acetic acid, acetone, acetaldehyde, and carbon dioxide. These oxidation products are a result of multiple simultaneous oxidation reactions. The major intermediate is acetic acid, which is formed in 93 mol % yield initially, but is subsequently destroyed. The oxidation products acetone and acetaldehyde each constitute approximately 3 mol % of the phenol reacted. These two products did not oxidize appreciably further during the course of this experiment; however, there appears to be an indication that acetaldehyde is slowly being oxidized (see Figure 2). Acetic acid was completely destroyed after 3 h a t 300 "C, producing carbon dioxide. The rapid increase in C 0 2over this reaction time was observed by GC/MS as a large, early eluting peak with mle 44. Quantitation and Precision. Two microliters of aqueous standard solutions ranging from 100 to 1000 ppm of phenol, acetone, acetic acid, and acetaldehyde was introduced into the gas chromatograph by both syringe injection and capsule injection via the modified heated inlet system. A split injection (split ratio = 1/12) was used in each case to reduce the amount of water that entered the DB-5 fused silica capillary column. A comparison of responses from syringe injection vs. the capsule injection technique (Table I) reveals that the latter method has a higher relative standard deviation (RSD) than the former. The inlet system did not appreciably adsorb and retain phenol, acetaldehyde, and acetone; however, 87% of the acetic acid was irreversibly adsorbed on either the broken glass fragments from the capsule, the stainless steel walls of the apparatus, or the fritted glass filter. Silanization and
2622
ANALYTICAL CHEMISTRY, VOL. 56, NO. 13, NOVEMBER 1984
Table I. FID Responses of Phenol and Its Oxidation Products syringe injection,O capsule injection integrator integrator counts/ng RSDb counts/ng RSDb phenol acetaldehyde acetone acetic acid
140.2 f 15.5 83.7 i 4.8
11.1
5.7
129.4 i 18.5 78.1 i 8.4
18.5 10.7
98.5 f 13.8 34.9 i 5.1
14.0 14.8
99.7 i 15.5 4.4 f 2.8
15.5 64.0
Split injection was used. bRSD,relative standard deviation. Table 11. Mass Balance of Organic Carbon in Sealed Tube Oxidation of Phenol at 300 OC % C
time,
unaccounted
min
organic carbon, ppm
0 20 30 60 90 120 150 180
765 783 867 650 818 429 339 66
0 0 0 0 0 43 55 91
fora
% standard deviation averaged 10% in the first 90 min.
repeated conditioning with acetic acid covered some of the active sites and reduced the adsorption to approximately 15% of that injected. A mass balance of the organic matter in the sealed capsule was determined by summing the organic carbon concentration of the compounds identified by the flame ionization detector. These data, presented in Table 11, indicate that all of the organic products were accounted for by GC/FID up to a reaction time of 90 min. After 90 min a t 300 "C, the sum of the organic carbon did not balance with the original 765 ppm of organic carbon present. This decrease in organic carbon at the longer reaction times may be explained by the presence of oxidation products that either do not respond to the flame ionization detector (e.g., carbon dioxide) or are not amenable to analysis by gas chromatography without prior derivatization. The presence of carbon dioxide was observed by GC/MS a t the longer reaction times; however, since this product was not quantitated, the latter explanation cannot be ruled out. Comparison of Results. The wet air oxidation of phenol has been studied by Pruden and Le (3),Katzer, Ficke, and Sadama ( 4 ) ,Randall and Knopp (Z), and Baillod, Faith, and Masi ( I ) . While each of these researchers studied the kinetics of the oxidation of phenol, only two groups attempted to identify and quantitate the products of oxidation of phenol. Acetic acid was identified as a major product of oxidation by Randall and Knopp (2) and Bailled et al. (1). Acetone and acetaldehyde were identified also by Baillod et al. as stable products of oxidation (i.e., still present after 1h at 232 "C). Higher molecular weight products (succinic, maleic, and oxalic acids) reported by Baillod et al. were not identified by this method and may, indeed, not be present since our oxidation reaction was conducted for a longer time at a higher temperature. Carbon dioxide was identified in the present study by gas chromatography/mass spectrometry (GC/MS), since the flame ionization detector (FID) does not respond to carbon dioxide. The GC/MS method also was used to search for formic acid; however, this product was not detected.
Advantages and Disadvantages. A significant advantage of this sealed tube wet air oxidation method over previously reported methods (1-4) is that the micro reaction capsule retains both the aqueous and gaseous products of oxidation and makes the analysis of these products relatively simple. With the portable crushing apparatus described here, the entire contents of the sealed capsule may be analyzed by any gas chromatograph with any appropriate detector. The sealed capsule technique also prevents the loss of reactants and products through the volatilization that occurs when air is released from a vessel under pressure. Indeed, some reports of the destruction of 99.9% of specific pollutants may actually be a result of volatilization of compounds with high vapor pressures. Since it is time-consuming to perform large volume batch reactions in a pressure vessel, a set of micro reaction capsules can be prepared and heated at one time. The capsules can be removed from the oven at various reaction times to produce many duplicate data points in a short period of time. The thin glass wall and small sample size permit attainment of rapid thermal equilibrium, so that the experimental reaction times do not involve uncertainties arising from long periods of heating and cooling. Also the reaction temperature is limited only by the softening point of the borosilicate glass (475 "C). The F&M solid sample SI-4 injector is no longer commercially available; however, the design is simple to reproduce and alternative techniques for breaking capillaries in a carrier stream can be envisioned. Additional methods of breaking sealed tube reactions have been reported in the literature (5-8) and may be used in place of the SI-4 injector design. This sealed tube oxidation method suffers from some disadvantages with respect to the previous methods (1,2). For example, with this method it is less convenient to esterify the organic polyacids to facilitate the chromatographic separation. In fact, these acids, if present, may irreversibly adsorb on the surfaces of the modified inlet system before even entering the chromatographic column. Silanization of the glass components of the inlet system may alleviate this problem. In conclusion, an apparatus is described that will confine and then release the contents of a sealed reaction tube directly into the carrier gas stream of a gas chromatograph for analysis. The apparatus is simple to operate, is less hazardous than high-pressure vessels, allows rapid attainment of thermal equilibrium, and produces results rapidly with a minimal amount of sample handling. ACKNOWLEDGMENT The advice of Ralph Franklin is greatly appreciated. Registry No. Phenol, 108-95-2; acetic acid, 64-19-7; acetone, 67-64-1; acetaldehyde, 75-07-0; carbon dioxide, 124-38-9. LITERATURE CITED (1) Baillod, C. R.; Faith, B. M.; Mash 0. Envlron. Prog. 1982, 7 , 217-227. (2) Randall, T. L.; Knopp, P. V. J.-Water Pollot. ControlFed. 1980, 52,
21 17-2130. ( 3 ) Pruden, B. 6.; Le, H. Can. J. Chem. f n g . 1978, 5 4 , 319-325. (4) Katzen, J. R.; Flcke, H. H.; Sadana, A. J.-Water Pollut. ControlFed. 1978, 48, 920-933. (5) Buchanan, D. L.; Corcoran, B. J. Anal. Chem. 1959, 3 7 , 1635-1637. (6) Des Marals, D. J.; Hayes, J. M. Anal. Chem. 1978, 4 8 , 1651-1652. (7) Coleman, D. D. Anal. Chem. 1981, 53, 1962-1963. (8) Caldwell, W. E.; Odoin, J. D.;Wllllams, D. F. Anal. Chem. 1983, 5 5 ,
1175-1176.
RECEIVED for review February 21, 1984. Accepted May 18, 1984. This work was supported by the Department of Energy under Contract DOE-10298-3.
