Anal. Chem. 1988, 6 0 , 858-883
858
LITERATURE CITED Sparks, C. J., Jr. Synchrotron Radlatlon Research, Wlnlck, H., Doniach. S.,Eds.; Plenum: New York. 1980; p 459. Jones, K. W.; Gordon, 8. M.; Hanson, A. L.; Hastlngs, J. 6.; Howells, M. R.; Kraner, H. W. Nucl. Instrum. Methods Phys. Res., Sect. B 1984, 231, 225. (3) Chen, J. R.; Gordon, B. M.; Hanson, A. L.; Jones, K. W.; Kraner. H. W.; Chao, E. C. T.; Mlnkln, J. A. I n Scannlng Nectron Mlcroscopy: SEM, Inc., AMF O'Hare: Chicago, IL, 1984; Vol. 4, p 1483. Prlns, M.; Davles, S.T.: Bowen. D. K. Nucl. Instrum. Methods Phys. Res. 1984, 222, 324. Prlns, M.; Kulper, J. M.; Viegers, M. P. A. Nucl. Instrum. Methods Phys. Res. Sect. B 1884, 231, 246. Ilda, A.; Gohshl, Y. I n Advances h X-Ray Analysis; Barrett, C. S., Predeckl, P. K., Eds.; Plenum: New York, 1985; No. 28. pp 61-68. Underwood, J. H.; Thompson, A. T.; Wu, Y. Nucl. Instrum. Methods Phys. Res ., in press. Pella, P. A,; Newbury, D. E.; Steel, E. B. Anal. Chem. 1986, 58,
1133.
(9) Glauque, R. D.; Garrett, R. 6.; Qcda,
L. Y.
Anal. Chem. 1979, 5 7 ,
RECEIVED for review August 24, 1987. Accepted December 21, 1987. This work was supported by the Director's Office of Energy Research, Office of Health and Environmental Research, U.S. Department of Energy Contract No. DEAC03-76SF00098. The experiment was carried out a t the NSLS X-26C beam line which is supported by the Processes and Techniques Branch, Chemical Sciences Division, Office of Basic Energy Sciences, US.Department of Energy Contract No. DE-AC02-76CH00016; the National Institutes of Health as a Biotechnology Research Resource, Grant No. P41RR01838; and the National Science Foundation, Grant NO. EAR-8618346.
Oxygen-Selective Microwave-Induced Plasma Gas Chromatography Detector for Petroleum-Related Samples Cherlynlavaughn Bradley*
Amoco Corporation, Amoco Research Center, Naperville, Illinois 60566
Jon W. Carnahan Department of Chemistry, Northern Illinois University, DeKalb, Illinois 60115
An atmespherlc-pressure, mkrowavelnduced, hellun plasma system for oxygen-selectlve gas chromatographlc detectlon of petroleum-related samples Is presented. Extensive purlflcatlon of the helium plasma gas, exclusion of atmospheric oxygen, and use of an approprlate plasma contalnment tube were cruclal to mlnlmire oxygen spectral emlsslon produced by lmpuritles In the plasma gas, alr leaks, back dlffuslon of alr Into the plasma, and the plasma'contalnment tube ltsetf. With these precautlons, the oxygen-to-carbon selectlvlty Is IO8. Thls Is the best oxygen selectlvity reported In the Ilterature to date for a mlcrowave-lnduced plasma detector system. The system Is sensltlve down to 2 ppm, (parts per milllon by welght) oxygen for narrow-boillng range dlstlllates and simple mlxtures and 500 ppm, oxygen for wide-bolllng range, complex samples. The detector Is Hnear over 3 orders of magnitude and has a unlform response to different types of oxygenates. Examples Illustrate the feaslbllty of gas chromatography/mlcrowave-Induced plasma for detecting organic oxygenated compounds in dmple mlxtures and In petroleum and synfuel dlstHlates bolllng up to ca. 500 O F .
Selective detectors, responding primarily to species containing specific atoms or functional groups, have had a tremendous impact in chromatography (1). But, an important deficiency in the array of selective gas chromatographic (GC) detectors is a sensitive and selective detector for organic compounds containing oxygen. Such a detector is attractive for identifying and quantifying oxygenates influencing the refining and stability of petroleum and synfuels. In 1965, McCormack and co-workers ( 2 ) attempted oxygen-selective GC detection by using an elemental emission spectrometer system equipped with a microwave-induced plasma (MIP) excitation source. They investigated both a
reduced-pressure, helium plasma and an atmospheric-pressure, argon plasma for the analysis of organics and monitored the oxygen content by observing the OH molecular emission band. However, they found that the presence of a high oxygen background in their system precluded any meaningful use of the OH band for selective oxygen detection. In the 1970s, several researchers investigated the use of atomic emission lines from reduced-pressure helium plasmas for oxygen-compound detection in organic materials. McLean et al. (3) investigated the atomic oxygen emission triplet at 777.19,777.41, and 777.54 nm and obtained a 3.0 nanogram per second (ng/s) oxygen detection limit at the most intense oxygen line (777.19 nm). Van Dalen et al. (4) reported oxygen detection limits in the 2.7-12 ng/s range for 2-propanone and methanol. Brenner (5) reported an oxygen detection limit of 4.0 ng/s and an oxygen to carbon selectivity of 500 to 1. However, this selectivity is based on a change in the plasma continuum. It is clear that selectivity for oxygen is determined by on-line shifts and cannot be based on a continuum change calculation. An important chromatographic factor discussed in Brenner's paper was that when using packed columns, stationary phases such as polyglycol and polyamide caused negative responses to oxygen and nitrogen compounds but less polar, more thermally stable stationary phases showed this behavior to a lesser extent. In the 198Os, capillary columns have been increasingly used with the MIP (6-9). The high resolution achievable with these columns is essential for the complexity found in petroleum and environmentally related samples. Moreover, the inertness of fused-silica capillary columns is an important and desirable factor when analyzing reactive and polar oxygen-containing compounds. Yu et al. (IO) obtained a 0.3 ng/s oxygen detection limit in their reduced pressure helium plasma/GC capillary column system by minimizing air leakage into the system and purifying the helium carrier and plasma gases (11). Besides im-
0003-2700/88/0360-0858$01.50/00 1988 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 60, NO. 9, MAY 1, 1988
proving the oxygen detection limit, this approach also eliminated the negative oxygen responses observed by Brenner. Yu reported a 103 linear dynamic range but provided no data on oxygen to carbon selectivity. Tanabe et al. examined the potential of an atmospheric pressure helium MIP for oxygen-selective detection (12,13). He reported that the detection of oxygen (as well as nitrogen) was a problem due to air leakage into the GC-MIP system
(13). Besides the problems with minute air leaks that Tanabe and other investigators experienced, decomposition of the plasma containment tube itself presents problems in oxygen-selective detection. The MIP normally employs a fused quartz @ioz) discharge tube to contain the plasma. These tubes continually undergo changes from carbon and metallic oxide deposition that result in etching and, consequently, devitrification of the fused quartz tube wall (14-16). The continual change in the transparency of the optical emission window also creates a major reproducibility problem if the plasma is viewed laterally (side-on, through the fused quartz) or axially due to long-term changes in plasma characteristics. Devitrification of the quartz also causes oxygen background emission from the vaporized oxygen. Slatkavitz, Uden, and Barnes (17) recently reported taking extensive precautions to exclude oxygen background spectral emission produced by impurities in the helium plasma. This group used a boron nitride discharge tube. These precautions resulted in a reduced oxygen background and an improved sensitivity (0.2 ng/s oxygen). However, oxygen to carbon selectivity was only 10 to 1. Summarizing, although promising, the practical use of this detector for selective oxygen-compound detection has been an elusive pursuit for gas chromatographers. The key problem has been to minimize oxygen spectral emission produced by impurities in the plasma gas, air leaks, back diffusion of air into the plasma, and the plasma containment tube itself. In this paper we report changes made to an atmospheric pressure GC-MIP system to effectively reduce oxygen background emission and substantially improve the system as an oxygen-selective detector for organic compounds. With the described GC-MIP system, we obtained an oxygen to carbon selectivity of lo3 and an oxygen detection limit of 0.03 ng/s (2 ppm, oxygen) for narrow-boiling-range distillates and simple blends containing oxygenated compounds. For wide-boiling-range, complex samples, the system is limited to samples containing 0.05% or more total oxygen. The detector is linear over 3 orders of magnitude for oxygen. Response to different types of oxygen compounds is uniform, making quantitation simple. Repeatability of detector response is better than 10% relative. Examples of petroleumrelated samples are illustrated. EXPERIMENTAL SECTION
Apparatus. The gas chromatograph was a Hewlett-Packard Model 5890. A 15 m X 0.30 mm i.d. DB-1fused-silica capillary column with a 0.25-gum film was used. The MIP system was a Model 850 (Applied Chromatography Systems, Luton, England). This system is equipped with a 0.75 m focal length spectrometer and a 960 groves/mm grating. The entrance and exit slit widths were 75 gm. Two elemental channels (oxygen and carbon) were monitored simultaneously in most of the applications. Amplifier gain was varied to observe the analyte signal, background, and dark current signals. The GC to MIP transfer line was a copper tube (l/* in. o.d., in i.d.) layered with woven g h tape, Teflon-covered nichrome wire, and outer glass tape insulation with a platinum resistance thermometer. The transfer line was heated and monitored by the auxiliary heating controller of the gas chromatograph. The transfer line was operated at 250 "C. The capillary column was
lHEl Getterer
859
I
I
Flow control
Spectro-
Capillary
GC Mass flow
Stub tuner
z
Figure 1. GC-MIP system schematic. inserted through the transfer line and to within 5-10 mm from the plasma. The microwave power generator (Raytheon) was operated at 2450 MHz with forward power of 100 W. A resonant cavity was used with a coaxial three-stub tuner ( M a w Microwave Corp., Model HMC-1878B). Cavity dimensions were 10 cm diameter, 2.3 cm depth with a 1-cm central hole for the plasma containment tube. The cavity design was based on Beenakker's TMoloconfiguration (18). The plasma tube was a 2 in. length X 6 mm 0.d. X 2 mm i.d. of boron nitride (hot pressed, HBC grade from Union Carbide). The total oxygen content in the boron nitride was 0.2% as determined by using a method described by Oita (19). The plasma tube was held in place in the cavity with a 1/4-in.ceramic-filled Teflon ferrule (Supelco, Inc.) and a Swagelok fitting. To eliminate back diffusion of air into the plasma, the front of the boron nitride tube was enclosed in a copper cylinder face plate attached to the front of the cavity. The cylinder had a in. thick quartz window attached to the front and a port for introducing nitrogen sheathing gas. The cylinder was removable from the cavity for igniting the plasma. High-purity helium (99.9999%) from Liquid Carbonic used for the plasma and carrier gas was further purified through a titanium gettering furnace (Model 2B-20-Q, Centorr Associates, Inc.). A two-stage, Model 18 regulator (Scott Specialty Gases, PA) with a stainless steel diaphragm was used on the helium cylinder. A mass flow controller (Omega Engineering) was used to set and maintain the helium gas flow to the plasma tube as well as the flow of the hydrogen used as a scavenger gas. Nitrogen was used as a sheathing gas for the plasma tube. Oxygen filters (Chrompack) were used to purify the scavenger and sheath gases. The helium plasma gas and the scavenger gas were teed into the transfer line via a low-volume '/16-in. stainless steel Swagelok union cross tee. Figure 1 shows an overall schematic diagram of the GC-MIP system. Details of the cavity-transfer line configuration are shown in Figure 2. Strip chart recorders (Linear, Model 555 (Alltech Associates, Deerfield, IL)) were 0-10 mV full scale for the oxygen channel and 0-100 mV full scale for the carbon channel. Chart speed was 1 cm/min for both recorders. Peak area determinations for data quantitation were handled by an in-house GC data system. Materials. All gas lines were stainless steel that had been cleaned with methylene chloride and dried with N2 prior to installation and heated to 100 O C under He purge after installation. Teflon tape was used to seal fittings in the transfer line connection to the cavity. All chemicals were of reagent grade. Test solutions were prepared by either diluting the pure chemical with isooctane or toluene or blending the pure chemicals without solvent.
880
ANALYTICAL CHEMISTRY, VOL. 60, NO. 9, MAY 1, 1988 TMOlO cavity
Ill
Cu face+
plate
Heated transfer
scavenger gas
Figure 2. GC-MIP interface.
Petroleum and synfuel distillates examined were a gasoline (100-400 O F boiling range) and a coal liquid (200-450 O F ) . Operating Procedure. The flow rate of purified He gas through the plasma tube was set and maintained at 160 mL/min. The copper cylinder of the face plate attached to the front of the cavity was removed so that the plasma could be ignited. The plasma was ignited by inserting a copper wire held in a fused quartz tube into the boron nitride plasma containment tube. The copper cylinder was then reattached to the front of the cavity to enclose the front of the boron nitride tube in purified nitrogen. Nitrogen sheath gas was ca. 400 mL/min. Hydrogen scavenger gas was set at 4 mL/min. Carrier gas flow through the column was 2 mL/min with an inlet split ratio of 101and the temperature of the GC oven set at operating leveL In this study, the GC column oven was either set at 30 "C isothermal or temperature programmed from 30 (1min hold) to 250 "C at 10 deg/min unless otherwise indicated. The injection port temperature was 250 OC. After plasma ignition, the system stabilized within an hour and was checked for leaks with an electronic gas leak detector (GOW MAC Instrument Co.). Split injections (101ratio) of 0.1-pLsample volumes were used. No solvent venting was employed prior to the effluent entering the ME'. Because of the small amount of sample actually entering the MIP, we observed only a momentary change in the plasma color (reddish pink to blue) as the solvent in the sample went into the plasma. The plasma was not extinguished nor was the discharge tube contaminated with carbon deposits by these procedures. RESULTS AND DISCUSSION Oxygen Background Signal. Reduction of background oxygen emission from impurities in the helium plasma gas was crucial before the MIP system could be effectively evaluated as an oxygen-selective detector. Ideally, the oxygen background signal should be so small that it is indistinguishable from the background noise. While this "ideal" situation was not actually obtained, the steps taken in this study reduced the oxygen background signal to background noise ratio to the point that proper system evaluation could be achieved to determine applicability of the GC-MIP as an oxygen-selective detector. Purity of Plasma Gas and Plasma Tube Sheathing. The titanium gettering furnace provided a 70% reduction in the oxygen background signal coming from helium impurities. Moreover, sheathing of the front of the plasma tube with purified nitrogen to decrease back diffusion of air into the plasma afterglow and/or air absorption by the porous boron nitride further reduced oxygen background emission. These results indicate that extensive purification of even high-purity helium (99.9999%) is critical for diminishing oxygen background response. The results also indicate that a large portion of the background signal comes from excitation of atmospheric oxygen. Consequently, the plasma tube must be shielded from the atmosphere. We used nitrogen as the sheath gas because
q
Oxygen 777.2 nm
0 2 4 6 Minutes
b Oxygen 777.2 nm
0 2 4 Minutes
Figure 3. GC-MIP response for tetrahydrofuran (peak A, 110 ng of 0) in isooctane (peak B): carbon response (top), attenuation 64;
oxygen response (bottom),attenuation 16, (a) without scavenger gas and (b) with H, scavenger gas. of its abundance and low cost. Helium is another possible sheath gas. However, we found that helium used as a sheath gas filled the T N l ocavity and caused arcing in the cavity at the high flow rate employed. Effect of Scavenger Gas Concentration on Detector Response. We found in this work, as well as in the work of other investigators (17), that hydrogen scavenger gas added to the helium plasma reduces the level of oxygen I emission in the helium plasma and is important for selective oxygen detection. Figure 3 shows MIP response in the carbon and oxygen channels for a blend of 6% tetrahydrofuran (THF) in isooctane. The top chromatogram is the carbon response and the bottom two chromatograms (a & b) are of the response in the oxygen channel. In particular, parts a and b of Figure 3 show detector response without and with scavenger gas. Note that the T H F signal is positive while response for isooctane appears as a negative signal. While in our system the hydrocarbon response is totally negative (no false positive response), we desired minimum negative response. Slatkavitz, Uden, and Barnes (17) have reported that the negative response in the oxygen channel is reduced by adding hydrogen to the helium plasma. The explanation of this phenomenon is that hydrogen consumes background oxygen and forms OH. We tested the THF/isooctane blend with hydrogen scavenger gas concentrations ranging from 0.6 to 5.8 vol % for their effect on the T H F and isooctane signals. As the scavenger gas concentration increased from 0.6 to 5.8%, the negative response due to isooctane decreased. However, the T H F response also decreased. This decrease is not surprising since one would expect that doping the helium plasma gas reduces the effective excitation efficiency of the helium plasma resulting in a decrease in analyte signal (10, 17). In this case, both the positive THF response and negative isooctane response decrease. A working concentration range of 0.644% (v/v) H2scavenger gas was selected to give reasonable oxygen sensitivity and selectivity with a minimal amount of negative
ANALYTICAL CHEMISTRY, VOL. 60, NO. 9, MAY 1, 1988
Table 11. Minimum Oxygen Detectability
Table I. Measured vs Expected Peak Area Data for Various Oxygen Compounds
compound methyl ethyl ketone THF 2-propanol dioxane cyclohexanone methyl tert-butyl ether 1-butanol 2-ethyl-I-butanol
oxygen injected, meaex% in samples ng sured' pected error 1.4 1.1 1.1 4.2
894
7.0 1.4
1480 149
25 23 22 81 144 11
0.2 0.2
190 380
300
253 250
881
27 23
-7.4 0
23
-4.3
81 134 12
+7.5
15
17
-5.9
32
35
-8.9
0 -9.1
Based on THF (C,H,O)' 200 pg of 0 absolute detection limit time-based detection limit* 25 pg of 01s concentration-based detection limit 2 ppm (w/w)0 Based on Complex Sample' 40 ng of 0 absolute detection limit 5 ng of O/s time-based detection limitb 500 ppm (w/w) 0 concentration-based detection limit In isooctane. * 8-s peak width at half peak height. Spiked gasoline. Table 111. Measured Peak Area Precision
'Based on a THF calibration. hydrocarbon response. Figure 3b shows the same blend as in Figure 3a but with 1.0% hydrogen content in the plasma gas. While the T H F response is only half the level it was without the scavenger gas, very little negative response from the hydrocarbon is observed when the scavenger gas is used. These conditions were used for all further studies. System Evaluation. Signal to Background Ratio. With nitrogen sheath gas and hydrogen scavenger, the signal-tobackground ratio (S/B) was determined by measuring the detector response level while adjusting the lateral position of the primary slit so that maximum intensity of the most intense oxygen line is obtained and then changing the position of the primary slit to a region of minimum base-line intensity. The primary slit was adjusted via the profile scanning control knob located on the front of the spectrometer. These measurements were made and related to the level of the dark current (the level of response when the entrance slit of the spectrometer is blocked). We observed a signal to background ratio of approximately 10, indicating that while the oxygen background emission was not completely eliminated, it was considerably lower than the SIB = 70 observed before using nitrogen sheath gas and hydrogen scavenger. In all preliminary studies, the SIB was monitored with the goal of minimizing this parameter in the absence of analyte. As expected, we found that minimizing SIB also reduced the effect of non-oxygen-containing compounds on the oxygen base-line signal. Linearity. The linear range of response of the MIP for oxygen, measured as the mass of oxygen entering the plasma to the signal size, was 3 orders of magnitude. A log-log plot gave a slope of 0.962 and a correlation coefficient better than 0.999. Uniformity of Response. The MIP gave a uniform response to different types of oxygen compounds. All the oxygencontaining reference compounds used in this study contained one (e.g., THF) or two (e.g., dioxane) oxygen atoms. Measured vs expected peak area data obtained for various oxygen-containing compounds are shown in Table I. Expected peak areas, calculated using a THF calibration plot, were acceptable. The percent error of the relative response measurements was better than i 9 % compared to the expected values. This was also true when using a dioxane calibration plot to calculate expected peak areas. This indicates that response varies little from compound to compound. This aspect is a very useful characteristic of the detector since the need for different response factors when using internal standards is eliminated and a single calibrant can be used for the various oxygen components in a sample. Detection Limits. The minimum detectable amount of oxygen in a sample must give a response twice as great as the detector noise. On the basis of this definition, minimum
peak area average of 10 runs
mass of oxygen injected
oxygen compound THF 2-propanol dioxane cyclohexanone
253
23 22 81 143
250 894 1480
std dev fl f2
fl f3
re1 std dev, % 4.3 9.2 1.2 2.1
oxygen detectability is 2 ppm, for oxygenates in simple blends composed of one to three components. Note that this concentration corresponds to a O.l-rL injection with a 1 0 1 inlet split ratio. For complex samples composed of a host of oxygenates in a bulk hydrocarbon matrix, minimum detectability is 500 ppm, total oxygen content. For complex samples, the apparent higher oxygen detection limit is due to the difficulty of distinguishing an individual oxygen signal from the other oxygen-containing compounds in the chromatogram. Detection limit data are given in Table I1 calculated in terms of absolute detection limits, picograms of oxygen per second, and ppm (w/w) oxygen. Selectivity. Oxygen to carbon selectivity is defined as follows: selectivity =
peak height of oxygen-containing cmpd X peak height of hydrocarbon mass of hydrocarbon mass of oxygen
where the mass values are the amounts of hydrocarbon and oxygen in the sample injected and where the hydrocarbon peak height is the absolute value of the hydrocarbon response observed in the oxygen channel. A typical peak for a hydrocarbon appears as a depression of the background signal (or a negative peak). The major source of the non-oxygen response appears to be background signal depression caused by the hydrocarbon content of a sample. The oxygen to carbon selectivity in our system is lo3. This value, to our knowledge, is the best oxygen selectivity value reported to date for a MIP system. Repeatability. Results of analyzing in replicate a blend of four oxygen compounds in isooctane are given in Table 111. Shown are the averaged peak areas of 10 runs made over a 5-day period. Relative standard deviation is better than i 1 0 % (indicating MIP day-to-day consistency). Applications. Several examples are cited below to suggest possible applications of a GC-MIP system for analyzing oxygenated compounds in simple or complex mixtures. Synthetic Blends. A variety of oxygen-containing compounds were used in preparing the synthetic blends shown in Table IV. These references blends range from 0.3% to 2% total oxygen content. With 2-propanol as a standard, the percent oxygen recoveries ranged from 89% to 113% with an average recovery of 100.1 7.0% as shown in the table, in-
*
862
ANALYTICAL CHEMISTRY, VOL. 60, NO. 9, MAY 1, 1988
Table IV. Quantitation of Oxygen Compounds in Synthetic Blends and a Spiked Gasoline
blend Ab measd' ng of 0 % recovery
expd ng of 0
component methyl tert-butyl ether THF methyl ethyl ketone %propanol' dioxane cyclohexanone
900 872 890 1820
888 844 890 1938
expd ng of 0
99 97 100 107
blend measdn
expd ng % recovery of 0
ng of 0
120
113
94
120 450 728
120 410 825
100 91 113
spiked gasoline" measd" ng of 0 % recovery
506
479
95
500 470 790
472 483 781
94 103 99
"Average of duplicate runs. *Hydrocarbon matrix is pentane, isooctane, and toluene. 'Refer to Figure 4. dHydrocarbon matrix is cyclohexane, isooctane, toluene, and decane. "Refer to Figure 5. 'Used as an internal standard in all these samples. Carbon 247.9 nrn
Oxygen 777.2 nrn
0 I
I
0
5 10 Minutes
I
15 Minutes
10
5
1
15
Flgure 4. Chromatograms of a synthetic blend B: carbon response at top (attenuation 64); oxygen response at bottom (attenuation 4); tetrahydrofuran (A), 2-propanol (B), isooctane (C), dloxane (D),toluene (E), cyclohexanone (F), ndecane (0);temperature program, 30 OC (3 mln hold) to 170 OC at 4 deg/mln.
dicating reasonable and uniformly quantitatie recovery of the oxygen compounds tested. Figure 4 shows the simultaneous GC-MIP response for oxygen and carbon emission for synthetic blend B described in Table IV. Again, positive response is seen only for the oxygen-containing compounds. Gasoline. Oxygen to carbon selectivity achievable in a complex sample is shown in Figure 5. Simultaneous GC-MIP element-selective detection of oxygen and carbon is shown for a gasoline sample. Also shown is the oxygen response of the gasoline spiked with oxygen compounds (Figure 6c). Detector response for the oxygen compounds is pronounced in the spiked sample, with little response from the hydrocarbon matrix in either the spiked or original gasoline sample. Synfuels. Shale oil and coal liquid distillates contain a high concentration of oxygen and nitrogen compounds as contrasted with petroleum distillates, e.g., light catalytic cycle oils. The latter contain organic nitrogen but only trace quantities of oxygen. GC-MIP can be used to determine oxygenated compounds in such complex mixtures as synfuels
20
25
Flgure 5. Chromatograms of a gasoline: (a) carbon response of unspiked gasoline; (b) oxygen response of unspked gasdine; (c) oxygen response of spiked gasoline; methyl fert-butyl ether (A), methyl ethyl ketone (B), 2-propanol (C), dioxane (D). la)
ibi
Minutes
{Ci
Minutes
Manuter
Figure 6. Chromatograms of a light coal liquid distillate: (a) carbon emission, (b) oxygen emlssbn, (c) FID trace of phenollcs concentrate
of the same dlstlllate; phenol (A), o-chlorophenol (B), o-cresol (C), mand p-cresols (D), C,-phenols (E);temperature program, GC-MIP, 30 OC (1 mln hold) to 250 OC at 10 deg/min; GC-FID, 50 to 250 OC at 10 deglmin.
to confirm the presence of a particular type of oxygenated compound or to determine the boiling point distribution of
ANALYTICAL CHEMISTRY, VOL.
Table V. Analysis of Phenolics in a Coal Liquid" peak
idb
A B
phenol o-chlorophenol o-cresol m- p-cresols C,-phenols
C
D E
+
wt % component MIP-0' FIDd 16.3
14.9
e
e
8.11
8.05
24.3 23.7
26.6 24.7
" A light coal liquid distillate. *Based on retention times of standards, verified b y GC/MS (see footnote d). c D a t a obtained on t h e original, untreated coal liquid. dData obtained o n the phenolrich fraction o f the same coal liauid. eInternalstandard.
the different types of oxygenates in such materials. A coal liquid (450 OF distillate) was used to evaluate the capability of the system for complex samples. The major oxygen-containing components detected were phenolic compounds (phenol, cresols, and C2-phenols (dimethyl- and ethylphenols)). These components were qualitatively identified by retention time comparisons with standards compared with a GC-FID trace and verified by GC/MS. In order to verify identification of these oxygen-containing components by GC/MS, the coal liquid had to be base extracted to obtain a phenol-rich fraction (20) since the host of hydrocarbons in the untreated coal liquid interferes with the GC/MS analysis of oxygenates. Similarly, sample pretreatment is also necessary for GC-FID analysis of oxygenates in coal liquids. In this case, base extraction is followed by preparative liquid chromatography (21) to further isolate phenols as a class from interfering components. Table V shows the phenolics detected by using a MIP and FID. Quantitation of the phenolics by MIP compares well (within *lo%) with the FID data. Figure 6 shows the GCMIP chromatograms of the untreated coal liquid in the oxygen (b) and carbon (a) channels and the FID chromatogram (c) of the phenolics concentrate of the treated sample. The MIP-0 and FID chromatograms show the same oxygen components in similar quantitative amounts as given in Table V. The MIP-carbon chromatogram shows the same host of components as would an FID trace of the untreated sample. These results suggest that the MIP-0 has an advantage over the FID for light coal liquids in that no sample pretreatment to concentrate oxygenates is necessary. CONCLUSIONS Extensive precautions to minimize oxygen spectral emiasion produced by impurities in the plasma gas, air leaks, and back diffusion of air into the plasma have resulted in an improved
60,NO. 9,
MAY 1, 1988
863
GC-MIP system for organic oxygen compound detection. Our system appears best suited to light distillates having high levels of oxygenates. Possible applications are gasohols and gasohol feedstocks and low molecular weight distillates of coal liquids and shale oils. ACKNOWLEDGMENT The authors acknowledge L. J. Duffy and J. A. Meyer of Amoco Corp. who provided helpful discussions and support throughout the research and W. E. McDaniels and E. G. Lesko of Amoco Corp. and A. S. Viscomi of Northern Illinois University for their technical assistance. Thanks also goes to D. Martin of Scientific Equipment Services, England, who serviced the spectrometer and offered several helpful suggestions throughout the course of this work. Registry No. THF, 109-99-9; isooctane, 540-84-1. LITERATURE CITED (1) Ettre, L. S. J . Chromatogr. Sci. 1978, 76,396. (2) McCormack, A. J.; Tong, S. C.; Cooke, W. D. Anal. Chem. 1985, 3 7 ,
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RECEIVED for review July 23,1987. Accepted December 28, 1987. This work was presented in part at the Federation of Analytical Chemistry and Spectroscopy Societies meeting, St. Louis, MO, 1986 (poster session no. 290).
