Table 11. Cell Compensation of Background C u r r e n t s
Using Auto-Compensationa Average % error for 5 replicates Current, nA
Auto-compensation
Offset compensation
1.o 5 .O 10 .o 20 .o 50 . O
0.02 0.05 0.02 0.12 0.08
0.50 1.20 2.42 5.12 23.25
a lif = 500 ohms, Ri = 1000 ohms, R , = 10 Megohms, l i d = 1000 ohmil, c d = 1.0 pf, 7 = 2.5 seconds, step = 1.0
v.
and Ci = 0.1 pf.
Prior to the development of the auto-compensator, our standard scheme made use of an offset compensation principle. This scheme used two FET’s to implement a N.O. switch between the integrator input resistor and the summing point and a N.C. switch across the feedback capacitor. In this circuit, the two switches changed states 10.8 lsec prior to the step which ensured that the integrator was set to zero prior to the experiment. For comparison, an analogous series of experiments was performed utilizing this form of offset compensation. The results also summarized in Table I indicate the superiority of the auto-compensator. In electrochemistry, situations do exist where the value of i, in Figure 1 is non-zero at the potential imposed prior to the potential step. These currents can arise from a number of sources and frequently it would be desirable to null out their contribution automatically. For this reason, a second set of experiments was undertaken to test the performance of the auto-compensation circuit in eliminating finite i, values. Several values of i, have been simulated by injecting a known current into the transducer amplifier summing point using the circuit in Figure l.4.The
auto-compensation circuit does an excellent job of eliminating response from these currents. The offset compensation scheme is not designed to correct for this effect, and the results using this scheme are included in Table I1 solely for magnitude comparison. CONCLUSIONS The auto-compensation circuit described appears to do an excellent job of eliminating integration drifts for times of up to 5 seconds. The basic input specifications for the transducer, integrators, and compensation amplifiers can be met by almost any FET input operational amplifier. This removes many limitations from the choice of instrumentation for transient chronocoulometry. Thus, very high power and high slew rate operational amplifiers can now be used for the transducer amplifier since only the dynamic error terms contribute to the experimental error with the use of the auto-compensation circuit. This circuit also may be applied in other applications such as derivative pulse polarography. In this case, the gain set by R7 and T1 in Figure 3 should be centered a t unity and the filter time constant must be lowered to a value consistent with the time scale of the experiment. ACKNOWLEDGMENT The authors would like to thank C. Michael Elliott for extensive practical testing of this circuit and many helpful suggestions. Received for review November 12, 1973. Accepted January 7, 1974. Various aspects of this work were supported by the Air Force Office of Scientific Research (AF0SR)USAF, under Grant AF-AFOSR-69-1625 and by the Materials Research Center, UNC, under Grant Number GH33632 from the National Science Foundation.
New Sampling Device for the Recovery of Petroleum Hydrocarbons and Fatty Acids from Aqueous Surface Films Russell Miget, Howard Kator, and Carl Oppenheimer Marine Science Institute. University of Texas, Port Aransas, Texas 78373
John L. Laseter’ and Enoch J. Ledet Department of Biological Sciences. Louisiana State University in New Orleans. New Orieans, La. 70122
Replacement of coal by petroleum as the major world source of energy has resulted in increasing amounts of petroleum products being released into the environment. The need to quantify and to identify petroleum hydrocarbons in natural waters has presented methodological difficulties-especially with hydrocarbon films a t the air/ water interface. Since petroleum spills generally result in surface slicks of non-uniform thickness which can cover relatively large areas, a surface film sampler was required which would permit rapid, consistent, and efficient retrieval of surface hydrocarbons. We have developed an inexpensive sampler which fulfills these requirements, and which can be operated under moderately rough surface conditions. Field comparison of this sampler with the screen technique ( 2 ) and the sorTo whom correspondence should be addressed. ( 1 ) W . D. Garrett, Limnoi. Oceanogr.. 10,
1154
602 (1965).
ANALYTICAL CHEMISTRY, VOL. 46, NO. 8, JULY 1974
bent-in-a-can sampler (2) showed it to be easier to use and to require less sampling time. While we have used this sampler for both inshore and offshore field work for the past year under a variety of sea surface conditions, the purpose of this report is to discuss laboratory studies performed to determine the selectivity of the sampler for hydrocarbons and related compounds as a function of molecular structure, and the efficiency of recovery for petroleum films of varied thicknesses. EXPERIMENTAL Sampling Apparatus and Procedure. The surface film sampler (Figure 1) consists of a disk of 2-mm Teflon which is attached to a 4-mm marine aluminum backing by means of 24 aluminum countersunk bolts. The original prototype tested by one of us (RM) utilized a “heat cured” epoxy coated wooden backing Estes, P. G Mikolaj. R. R. Thaman, and L. W . Senger, Proceedings 1973 Conference on Prevention and Control of Oil Spills, American Petroleum Institute, Washington, D.C., 1973.
(2) J. E .
gas chromatograph equipped with a flame detector was employed. The injector was at 250 "C and the detector was a t 350 "C. No in. jedion split was used. Separation of the aromatic compounds was carried out using a 3.25-m X 2-mm i.d. glass column ?mploying 1%GE SE-30 on SO/lW mesh Chmmosorb Q and temperature programmed from 50 "C to 2W "C a t 4"/min. A 5750 HewlettPackard gas chromatograph equipped with a flame detector was employed. The injector was at 250 "C and the detector at 350 "C. A 9.5-m X 0.318-em i.d. stainless steel packed column employ.ing ethylene glycol succinate (12%) on Chromosorb P a t 180" isothermal was used to separate the fatty acids. A Hewlett-Packard 5150 gas chromatograph was employed with the injector at 250 'C. and the flame detector a t ~ 3 5 0"C. He flow was a t 18 ml/min a t 180 "C. T h e identity of each recovered compound was determined with gas chromatography-mass spectrometry. The system consisted of a Hewlett-Packard model 5750 gas chromatograph connected hy means of a jet-type separator to a model 21-491 duPont double focusing mass spectrometer as described previously (3). The transfer line and separator were maintained at temperatures of 225 to 250 "C. All spectra were taken a t 70 eV and recorded by use of an oscillographic recorder and a Digital Equipment Corporation PDP-12 LDP computer. Identifications were made by comparison with authentic laboratory standards. Pristane was ohtained from K & K Labs., Ine. (Plainview, N.Y.), whereas phenan-
Figure 1. Photograph of the sampling device
I
260
2
TEMPERA-
Figure 2. Gas chromatographic separation of the paraffinic hy drocarbons from Louisiana ciuUc;Y I I 18,ecr; and from the s a m e crude recovered from water surface by means of the sampling device (Trace 8 ) . Pristane and phytane are indicated by 'a' and 'b', respectively plate. It was later found, however, that the epoxy would contrihute small quantities of aromatic compounds in the 220- to 524molecular weight range to the samples collected. Substitution with marine aluminum eliminated the contamination problem and produced a stronger and lighter weight device. A unidirectional hinge is located in the center of the backside of the aluminum disk. For field use, a wooden pole is attached to the hinge at such an angle that the Teflon face and water surface are parallel. In order to collect a sample, the device is first touched to the water surface as lightly a8 possible, and the disk is then rested vertically an a large glass funnel. Organics adhering to the disk are then washed through the funnel into a container by B gentle stream of CClr. The sampler is slowly rotated and CCI4 applied in a fine stream from the center of the disk downward to the washed surface. A 30- to 40-ml solvent rinse is generally sufficient, although unusually thick films require more solvent. When crude oil slicks are sampled, the dark oil contrasts against the white Teflon aiding in an efficient removal. Other than for allowing the solvent to dry, no additional treatment of the Teflon surface is required before taking the next sample. Thus, several samples may be collected a t the same site and washed into the same tuhe for a pooled sample. Further treatment of the samples for qualitative and quantitative analysis will be described in the following sections. Gas Chromatography and M a s s Spectrometry. Gas chromatographic separation of hoth the paraffinic hydrocarbons from Louisiana crude oil and paraffinic standards was achieved using a stainless steel capillary (270-m X 0.05-cm i.d.) column: initial 100 m coated with 3% Apiezon L and the remainder with 3% OV-17. The column was programmed from 125 "C to 280 "C/min with an initial hold of 15 min and a final hold for 45 min. He flow rate of 5-6 m b i n was measured at 125 "C. A 7620A Hewlett-Packard
threne and diphenylmethane were obtained from Aldrich Chemical Company (Milwaukee, Wis.). All other standards were purchased from Applied Science Laboratories (College Park, Pa.). Spectrograde solvents were obtained from Matheson Coleman & Bell (Nowood, Ohio). Qualitative Recovery of Petroleum Paraffinics. The crude oil employed was obtained from the 5380-m depth Production Zone of a Louisiana offshore well operated by Exxon, located in South Timhalier Block 54 approximately 16 km southwest of Grande Isle, La.,a t latitude 28.5 N, longitude 90.3 W. The petroleum is typical of the lighter crudes produced in the Southern Louisiana area. Crude oil (0.1 m1)'was carefully pipetted onto deionized water (27 "C) contained in a stainless steel tray. The water was left undisturbed for 20 minutes prior to sampling during which time the petroleum spread to form a thin film. The Teflon surface of the sampler was then gently placed in horizontal contact with the water surface and immediately removed. The petroleum was washed from the disk using spectrograde CC4. The solvent was evaporated under reduced pressure at 40 "C and the residual material taken up in 5 ml of n-heptane for fractionation on silica gel (activated at 250 "C for 24 houh) in a 1- X 20-em column. Prior to use, the column was washed with three column volumes of n-heptane. The paraffinic hydrocarbons were eluted with 40 ml of n-heptane. The resulting fraction was taken to dryness under a stream of purified nitrogen. A duplicate 0.1-ml volume of crude oil used as a control was taken up in 100 ml of CClr and treated as above, with the exception that the sampler was not employed. In both cases, the residues were dissolved in 50 P I of n-pentane and injected directly on a capillary column for gas chromatographic analysis (Figure 2). (3) J. L. Laseter, G . ' C . Lawler. and G. W. Griffin. Anal. Lett., 6, 735 (1973).
ANALYTICAL CHEMISTRY, VOL. 46, NO. 8. JULY 1974
1155
-
/
I30
I
170
.__ I 200
TEMPERATURE “C Figure 3. Gas chromatographic separation of standard mixture of n-hexadecane, 1 -hexadecene, 2-methylhexadecane (ISOC17), hnethylhexadecane (ANTEISO-C17) and pristane (Trace A) and the same mixture following recovery from the water surface by means of the sampling device (Trace B)
130
150 170
TEMPERATURE “C Figure 4. Separation of diphenylmethane, phenanthrene, and f?-Cl8 standard mixture (Trace A) along with that same mixture following recovery from the water surface by means of the sampling device (Trace B)
Qualitative Recovery of Unsaturated a n d Branched Chain Paraffins, Aromatics, and Free F a t t y Acids. Pristane (2,6,10,14-tetramethylpentadecane),1-hexadecene, n-hexadecane, 3-methylhexadecane, and 2-methylhexadecane were dissolved in nhexane. Aliquots containing from 50 to 500 pg of each hydrocarbon were carefully pipetted onto the water surface and recovered using the sampler. Following solvent removal as described above, the residue was injected onto a capillary column (Figure 3). An aliquot of the original mixture was used as the non-sampled control. Recovery of aromatic hydrocarbons was likewise tested using diphenylmethane and phenanthrene with n-octadecane as a n internal standard. After solvent removal, the organics in the recovered residue were dissolved in benzene and injected directly onto a packed column which employed 1%GE SE-30 (Figure 4). An aliquot of the original mixture was chromatographed as the control. Approximately 250-750 pg each of free stearic, oleic, and linoleic acids along with n-eicosane were also recovered from the water surface using the sampler. The recovered residue after sol1156
ANALYTICAL CHEMISTRY, VOL. 46, NO. 8 , JULY 1974
Figure 5. Separation of a standard mixture of stearic (Cls:o), oleic (c18:1), and linoleic (c18:2) fatty acid methyl esters plus n-eicosane (Trace A) and the same mixture recovered from the water surface by means of the sampling device (Trace B)
vent removal was taken up in 3 ml of BFa-methanol and methylated ( 4 ) . The methyl esters and n-Czo standard recovered from the methylation procedure were taken up in 50 p1 of benzene and samples directly injected onto a packed gas chromatographic column employing 12% ethylew glycol succinate (Figure 5 ) . A portion of the original fatty acid-alkane mixture was methylated directly and served as the control. Quantitative Recovery of a Louisiana Crude Oil. The crude oil was artificially “weathered” by evaporation under low heat (50 “C) for 2 hours. This process essentially removes all compounds with boiling points below n-CI6. A small volume of the “weathered” petroleum (0.1 ml) was mixed with 5 ml of benzene. The oil in 50 p1 of this mixture formed a n incomplete sheen on distilled water contained in an aluminum pan (30-cm diam. X 20-cm deep) after the benzene evaporated. A 100-pl aliquot left a slightly broken sheen, whereas the oil in 200- and 300-p1 aliquots covered the water surface with a continuous film. A sampler with the same diameter as the pan was used to recover the oil so that the uneven distribution of the films would not affect the recovery efficiencies. Sets of eight replicate samples for each of four selected film thicknesses were collected. The pan was washed with soap and repeatedly rinsed with cold water between samples. For controls, identical quantities of “weathered” oil from each set were carefully added directly to the Teflon disk using a 50-p1 syringe, the solvent was allowed to evaporate (20 “C) and the residual crude washed off with cc14 using the recovery procedure described. Each cc14 washing was collected in a glass test tube, the water drawn off the top with a glass pipet, and the solvent plus oil carefully transferred to a tared aluminum weighing dish. The solvent was slowly evaporated (from the dish) on a heating tray (40 “C) in a fume hood. After the solvent evaporated, the dishes were placed in a desiccator and weighed until constant weights were obtained. Blanks using CC14 were run by evaporating 50 ml of solvent (approximate amount used for each wash) in tared dishes. In a similar experiment, three sets of relatively thicker oil films were quantitatively collected using the sampler. After evaporating the solvent under reduced pressure a t 40 “C, each residue was taken up in exactly 100 p l of n-heptane and analyzed using gas chromatography. Analyses was carried out using a Hewlett-Packard 7620A Gas Chromatograph with flame ionization detectors and +’*-in. X 6-ft stainless steel columns packed with 10% UCCW-982 on Chromosorb W. Nz carrier flow was 40 ml/min. Temperature was programmed from 100 to 300 “C a t 4 “C/min. Peak heights of n-paraffins from n-Cl7 to mC26 were measured, corrected for attenuation factor, and summed. Controls again consisted of oil Laseter. G . C. Lawler, C. H. Walkinshaw, and J . D . Weete, Phytochemistry. 12, 817 (1973)
(4) J. L.
Table I. Retrieval Efficiency of the Surface Film Sampler Using a “Weathered” Louisiana Crude Oil-Gravimetric Analysis Amount of oil added, mg, control
Calculated film thickness, nma
0.623 1.216
10.4 20.2
2.532 4.181
42.2 69.6
a
Amount of oil retrieved, mg
Appearance
Partial sheen Continuous sheen Dull sheen Continuous film
Std dev, mg
Efficiency, %
0.551 1,110
h0.035 =t0.045
88 91
2.165 3.596
=kO0l14 f0.251
85 86
Density = 0 . 8 5 g/ml.
Table 11. Retrieval Efficiency of the Surface Film Sampler Using a “Weathered” Louisiana Crude Oil-Gas Chromatographic Analysis Amount of oil added, control, mg
Calculated film thickness, rima
4
66.6
13
216.5 499.6
Appearance
Peak height control
Continuous film Continuous film Continuous
Peak height retrieved
Std dev
17.77
Efficiency, 57,
158
130
82
309
277
*13.43
89
232
205
+9.19
88
film a
Density = 0 . 8 5 g/ml.
added directly to the sampler. The efficiency of recovery was calculated using the following formula:
hydrocarbons tested. Also, the presence of a carboxyl group on a hydrocarbon chain containing one, two, or no double bonds does not appear to influence the recovery of % Efficiency = the compound. In all instances, the quantitative recovery Av. Z peak heights of recovered oil of the various classes of organics tested in the above exX 100 Av. Z peak heights of control oil periments ranged from 87 to 96%. It should be uoted that extensive secondary washing of the Teflon surface did not RESULTS AND DISCUSSION yield additional organics. The ability of the sampler to recover crude oil comTable I shows the quantitative recovery of a “weathpounds from aqueous surface films is illustrated in Tables ered” Louisiana crude oil using the sampler. The chroI and I1 and Figure 2. Not included in Table I is the informatograms in this experiment were similar to those in mation from control experiments where the weathered Figure 2 and revealed no discrimination in the recovery of crude oil was added directly to the disk surface (100 ~ 1 ) alkanes above n-CI6. with an equal volume of oil added to a weighing pan conThese laboratory studies indicate that the sampler taining approximately 50 ml of CC14. The oil was collectshould prove to be valuable for both quantitative and ed from the disk in the usual manner. Following evaporaqualitative retrieval of surface film paraffins, aromatics, tion of the solvents in both cases, the weight of residual and fatty acids in the environment. The sampler has been oil in the weighing pan and that recovered from the disk employed in actual field exercises to collect surface organwere found to be essentially the same, considering the ics from offshore Louisiana, offshore Florida, and Lake limits of error of the microliter syringe used. The chroPontchartrain. Analogously, the ability of the sampler to matograms in Figure 2 clearly demonstrate that low mocollect petroleum type organics suggests that it will also lecular weight alkanes below n-Cl6 are not recovered be useful in collecting related surface film organics such efficiently by the sampler. However, there appears to be as pesticides. no discrimination on either a qualitative or quantitative ACKNOWLEDGMENT basis with respect to alkanes above n-C16. All major compounds were identified by mass spectrometry. The authors wish to thank D. Carlisle, C. W. Schuler, and Figures 3, 4, and 5 illustrate the ability of the sampler R. Evans for technical assistance. to recover normal alkanes, aromatics, and free fatty acids, respectively. In each of these experiments, a normal alReceived for review October 31, 1973. Accepted February ,kane was included as an internal standard to determine if 15, 1974. This work was supported by funds provided by there was selective discrimination between it and the the Environmental Protection Agency (Grant No. 15080 other organics being evaluated by the Teflon surface. The EHF), and the Offshore Ecology Investigations Project addata suggest that the sampler does not discriminate with ministered through the Gulf Universities Research Conrespect to branching, saturation, or aromatic nature of the sortium.
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