Analysis of Petroleum for Trace Metals1-Determination of Trace Quantities of Cadmium in Petroleum by Atomic Absorption Spectrometry Winston K. Robbins Analytical & Information Division, Exxon Research & Engineering Company, Linden, N.J. 07036
Harry H . Walker Analytical Section, Mobil Research & Development Corporation, Paulsboro, N.J. 08066
Two atomic absorption methods are described for the determination of cadmium at or above the 10 ng/g level in petroleum and petroleum products. In both methods, the sample is digested with sulfuric acid and then ashed. The preliminary digestion with sulfuric acid avoids loss of cadmium during ashing. This treatment avoids problems that would be caused by high-boiling components if direct heat vaporization were attempted. Ashing also allows the cadmium to be concentrated to levels that can be detected by the flame method, which is not sensitive enough to permit direct analysis. On the basis of data obtained in four laboratories as part of an interlaboratory cross-check program, the standard deviation at the 30 ng Cd/g level is 7 ng/g for the heated vaporization method and 5 ng/g for the flame method.
The presence of traces of cadmium in the environment has caused concern because of its cumulative toxic effects ( I ) . Much attention has been given to the analysis of hair, tissue, blood and urine, foods, cigarette smoke, air, water, and a variety of other materials for cadmium content ( 2 - 9 ) . However, although cadmium has been reported to be present in crude oils (10, 111, few data are available because there has been no simple, validated method capable of determining cadmium in complex petroleum matrices a t the nanogram-per-gram level (12). Neutron activation analysis for cadmium, which is frequently used in trace analysis, is complicated by radionuclide interferences, requiring the use of ashing, isotope dilution, and separation techniques ( 1 3 ) . Other methods for the determination of trace quantities of cadmium, involving colorimetric, polarographic, and atomic absorption techniques were reviewed ( 1 4 ) . Atomic absorption techniques have received the greatest attention because of sensitivity, absence of interferences, and wide applicability (15-1 7 ) . The recent introduction of heated vaporization atomic absorption (HVAA) has extended the sensitivity to nanogram levels (18--20). However, cadmium has a high vapor pressure and may be prematurely lost under conditions needed in the analysis of samples which require decomposition (21, 22). This paper describes two methods capable of measuring cadmium in crude oil and petroleum products down to a concentration level of 10 ng Cd/g. The methods are based on destruction of the organic matter by wet oxidation and Contribution of Trace Metals Project Participating Laboratories: Atlantic Richfield Company, Harvey, Ill.; Chevron Research Company, Richmond, Calif.; Exxon Research and Engineering Company, Linden, N.J.; Mobil Research and Development Corporation, Paulsboro, N.J.; and Phillips Petroleum Company, Bartlesville, Okla.
subsequent measurement of the cadmium in the residue by either flame atomic absorption, using air-acetylene ( 2 4 , 2 3 ) , or by heated vaporization atomic absorption (HVAA) using a carbon rod atomizer (22, 2 4 ) . The flame method requires a large sample (20-100 g), while the heated vaporization method uses a 0.5-g sample decomposed in a crucible (Mini-ash technique). In the flame method, the cadmium signal is referred to a calibration curve; whereas in the heated vaporization method, the cadmium is neasured by standard additions.
EXPERIMENTAL S t a n d a r d s a n d Reagents. All reagents and solvents were ACS rea,gent grade, except for H2SO4 which was C'ltrex grade (J. 'r. Baker, Phillipsburg, N.J.). Distilled water was deionized before use. Organic standards were prepared either from commercially available cadmium in oil standards (5000 pg Cd/g as a sulfonate, Conostan, Ponca City, Okla.) or from cadmium cyclohexane butyrate ( U S . National Bureau of Standards, Washington, 11.C.). The cadmium in oil standard was diluted with a 1 to 1 mixture of 1methyl-2-pentanone (MIBK) and xylene to a concentration of 1000 p/ml; further dilutions were made with tetrahydrofuran (THF) immediately before use. An aqueous standard (1 pglml) f o r the standard additions in the HVAA method was prepared by diluting a commercial 1000-ppm (1 mg/ml) cadmium standard (F&.J Scientific) with 1N HpSO4 immediately before use. Calibration standards for the flame method were prepared by adding 1-, 5 - , lo-, o r 20.~1portions of the 1000-ppm aqueous cadmium standard to 100-ml polypropylene flasks and diluting to volume with 1:19 HC1 to give solutions containing from 10 to 200 ng Cd/ml. P r e p a r a t i o n of Samples f o r t h e Interlaboratory Crossc h e c k Program. Gasoline, No. 2 Heating Oil, J e t Fuel, and South Louisiana, Nigerian, and Light Arabian crude oils were spiked with approximately 30 ng Cd/g by adding a measured volume of standard (1000 pg/ml) cadmium sulfonate solution to a weighed amount of oil. Apparatus a n d Operating Parameters. Both methods were developed on a Jarrell-Ash 82-532 Atomic Absorption Spectrorneter (Jarrell-Ash Div., Fisher Scientific Co., Waltham, Mass.). The operating parameters of the spectrometer are listed in Table I. For the flame atomic absorption method, the five-pass optics of the iiistrument was removed to accommodate a Techtron partial-consumption burner (Model No. BNA-5). The operating parameters are listed in Table 11. T h e decomposition for the flame method was carried out in an 800-ml Vycor dish contained in a heated bath. The air bath consisted of a cylindrical aluminum container with a diameter about one inch larger, and a height '/a inch shorter, than the Vycor dish. Heating was by means of a hot plate and an infrared lamp as described below. The heated vaporization atomic absorption procedure was developed with a Varian-Techtron Model 63 Carbon Rod Atomizer (CRA-63, Varian-Techtron Corp. Palo Alto, Calif.) mounted in place of the burner. The components and function of the CRA-63 have heen previously described (22, 25). The spectrometer was modified by raising the monochromator and second lens turrent 1% inch to the plane of the hollow cathode lamp and workhead as described previously (25). The operating parameters are listed in Table 11. ANALYTICALCHEMISTRY, VOL. 47, NO. 8, JULY 1975
1269
~
Table I. Atomic Absorption Spectrometer Conditions Analytical wavelength
228.8 n m
Background wave length
226.7 n m
Slit wid t 11 Band p a s s Photomultiplier
100 p m 0.2 n m R-106
(Jarrell A s h L a m p , 5 mA) (Non-absorbing wavelength i n Cd lamp)
~~~
~~~~~~~~
~
Table 11. Operating Parameters For the Techtron Burner Bur ne r head : a i r a c e t y l e n e with 10-cm s l o t 10 scale d i v i s i o n s 6.2 s c f h a t 10 p s i 17.5 s c f h at 1 8 p s i 1.5 m l / m i n A b s o r b a n c e o r digital concentration
B u r n e r height: Acetylene flow r a t e : A i r flow r a t e : Nebulization r a t e : Read out:
(Hamamatsu)
For the Carbon Rod Atomizer
Procedures. Flame Method. SAMPLE DECOMPOSITION.Weigh into a 800-ml Vycor dish an amount of sample estimated to contain 100 ng of cadmium (100 g maximum). Add 5 ml of concentrated HzS04, mix a glass stirring rod, and place the dish in the air bath. At the same time, start a reagent blank with 5 ml of H2S04. Place the bath on a hot plate and suspend an infrared lamp above it so that the face of the bulb is 1to 2 inches above the top of the dish. Decompose the sample, slowly a t first with heat from the lamp, and then with low heat from the hot plate. Stir frequently with a glass rod to break up the surface crust to reduce spattering. A protective shield should be used to prevent possible injury from spattering of oil or acid. When fumes of HzSO4 are no longer evolved, transfer the dish to a muffle furnace a t 550 "C and heat until all organic matter has been burned off. Cool, wash the walls of the dish with 10 ml of 1:19 HC1, cover the beaker, and dissolve the ash by warming on the steam bath. Transfer the solution to a 10-ml volumetric flask, dilute to volume with 1:19 HC1 and mix thoroughly. ATOMIC ABSORPTION MEASUREMENT.Using Table I as a guide, optimize the instrument for maximum absorbance while nebulizing a standard cadmium solution. Calibrate the instrument immediately before measuring cadmium in samples by adjusting the base line to zero absorbance with 1:19 HC1 and nebulizing each standard solution in turn. Plot the absorbance readings obtained against concentration. Again adjust the base line to zero with 1:19 HCI, and measure the absorbance on the blank and sample solutions, interspersing an occasional standard to ensure that response is stable. Record the instrument reading and convert it to ng Cd/ml by use of the calibration curve. Calculate the concentration of cadmium by the equation:
where A = ng Cd/ml in sample solution, B = ng Cd/ml in blank solution, V = final volume of solution, ml, and W = weight of sample, g. Mini-Ash HVAA Method. SAMPLE DECOMPOSITION.Weigh into the 30-ml Vycor crucible an amount of sample (0.5 g maximum) estimated to contain not more than 30 ng of cadmium, and add 10 drops of concentrated H2S04. A t the same time, start a blank. Place the crucible on a hot plate set a t low heat. Suspend an infrared lamp 4 inches above the surface of the hot plate. Gradually increase the heat a t a rate sufficient to avoid spattering and heat until the evolution of sulfuric acid ceases. Transfer the crucible to a muffle furnace and heat a t 550 "C until carbonaceous matter is destroyed. Cool, add 1 ml of 1:l HC1 to the crucible and heat to incipient dryness. After cooling again, add 1.0 ml of 1 N HzS04 and rotate the crucible so that the walls are wetted. ATOMICABSORPTIONMEASUREMENT.Position a graphite atomization tube and two support electrodes in the workhead of the CRA-63. Using Table I as a guide, adjust the instrument to give zero absorbance. Optimize the optical alignment of the workhead so that a minimum tube blank is obtained when maximum voltage is applied in the atomization cycle. Inject a 1-111 aliquot of the sample solution into the graphite tube, atomize the sample, and record the signal. If the signal is greater than that which corresponds to 30 ng/g, dilute the solution with 1 N &So4 and work with a 1 - ~ 1 aliquot of both sample and blank. Measure the absorbance on three 1-111 aliquots of the sample solution. If each of the three signals falls within 10% of their average, use that average. Otherwise, measure the absorbance twice more and calculate the average of the five signals. Add 5 ~1 of a 1 wg/ml working standard ( 5 ng Cd total) to the sample solution. Mix well and obtain an average cadmium response. In a similar manner, obtain an average cadmium response after a total of 10 and 15 ng of cadmium have been added 1270
ANALYTICAL CHEMISTRY, VOL. 47, NO. 8, JULY 1975
Atomization tube: Support e l e c t r o d e s :
P y r o l y t i c coated g r a p h i t e FX 9 g r a p h i t e ( P o c o G r a p h i t e , D e c a t u r , Texas) Nitrogen (4 I./min.) 4 l./min. 90 s e c o n d s 1 iJ-1
Inert gas: Cooling w a t e r : Injection frequency: Sample size: C R A - 6 3 program
Dry Ash Atomize Read-out: Recorder Time constant S c a l e expansion Read-out a
Supply Setting
3.5 5 4 Speedomax W
Power (kWf
Seconds
0.004 0.07 0.30
15 15 7
(Leeds & Northrup)
0.5 sec 2.5X P e a k height
Calculated from measured VandA as V x A p 0 3 ( 2 5 ) .
to the sample solution. After the final addition, measure the background absorbance for each sample a t the 226.7-nm nonabsorbing cadmium wavelength. Calculate the apparent concentration of cadmium in the sample by the equation:
ng Cd/g = A 0 - B x (5ng)i x F W A i - A,, ~
( 2)
where A, is the average recorder signal for the sample solution, B is the signal a t the non-absorbing line, i is the number of the addition (1, 2, or 3), A; is the average signal after the ith addition of cadmium, W is the weight of sample, g, and F is dilution factor if the sample solution required dilution. Record the average of the three apparent concentrations. Measure the absorbance on the reagent blank in the same manner as the sample and calculate the concentration of cadmium in the blank by assuming a weight equivalent to that of sample. Subtract this concentration of cadmium from that found in the sample; the difference is the cadmium content of the sample. The blank is typically in the range of 1-2 ng Cd/g.
RESULTS AND DISCUSSION Flame Method. Choice of Burner and Matrix. In att e m p t s to o b t a i n m a x i m u m sensitivity for c a d m i u m , the response obtained with a force-fed total c o n s u m p t i o n b u r n e r ( m o u n t e d on the Jarrell-Ash s p e c t r o m e t e r w i t h the fivepass optics included) was c o m p a r e d to that o b t a i n e d with a partial-consumption burner. Both b u r n e r s gave linear response. The total-consumption b u r n e r gave a greater response t h a n the partial-consumption b u r n e r for a q u e o u s solutions (Figure 1).However, the total c o n s u m p t i o n b u r n er caused considerable spectral noise, resulting i n poorer precision of m e a s u r e m e n t s and yielding a detection l i m i t n o lower than that of the partial c o n s u m p t i o n burner. It also produced a disturbing a m o u n t of audible noise. The possibility of d e t e r m i n i n g c a d m i u m directly w i t h o u t decomposing the petroleum m a t r i x was also investigated. A gasoline s a m p l e was s p i k e d with varying a m o u n t s of c a d m i um cyclohexanebutyrate and the p e r c e n t absorption f r o m e a c h solution was m e a s u r e d directly. The data are also p l o t t e d i n Figure 1. In c o n t r a s t to t h e findings w i t h a q u e -
Table 111. Detection Limits a n d Sensitivities for Cadmium ScnTitiviqa I
I
.=4 0
0
”
Cd in 1:19 HC1
ng Cd!rnl,’l
Partial -consumption burner Total -consumption burner
-1
Detection l i m i t , ‘
abs.
nq Cd/:.;l
22
5
5
5
12
5
20
10
Cd in Gasoline
Partial -consumption burner Total -consumption burner 18 : d , d
Figure I . Determination of cadmium in dilute hydrochloric acid and
gasoline (A - -) Total consumption burner; aqueous soh. (0 - -) Partial consumption burner: aqueous soh. (A -) Total consumption burner: gasoline. (0 -) Par-
a Sensitivity is defined as ng Cd/ml for 1%absorption. Detection limit is defined as concentration in ng C.d/ml t h a t produces an absorption equal to twice the magnitude of the fluctuation of the blank or background noise (30).
tial consumption burner; gasoline. I
I
I
I
Table IV. Determination of Cadmium i n Petroleum by Flame Atomic Absorption C d , nri, 4
V C
Samplea
Gasoline No. 6 Fuel Oil Nigerian Crude Nigerian Crude Nigerian Crude a
FlO.
Rdfe,
1‘:re)mi”
Figure 2. Effect of flow rate of NPof the absorption of 1 ng Cdlg
ous solutions, the partial-consumption burner gave a higher response than was obtained with the total consumption burner. Detection limits and sensitivities for the determination of cadmium are given in Table 111. The detection limits are possibly low enough to permit direct flame measurements on gasoline, but they are not low enough for heavier petroleum fractions and crude oil, which would require dilution to a less viscous matrix before they could be measured directly. Spectral interference was significant in experiments on t h e direct analysis of gasoline requiring measurement of the absorption a t the analytical wavelength (228.8 nm) to be corrected by the amount of absorption found a t t h e 226.7-nm cadmium non-absorbing wavelength. However, spectral interference was insignificant with aqueous solutions. Therefore, a uniform procedure was adopted for all types of petroleum samples. In this procedure a 20 to 100-g sample was decomposed with sulfuric acid and the residue was dissolved in a small volume of 1:19 HC1 and the final measurement made with a partial-consumption burner. Recovery. Cadmium compounds dissociate readily in the air-acetylene flame, and it has been reported that there are no major interferences (26). Although losses of cadmium have been reported when organic samples are dry ashed, no such difficulties are encountered when samples are sulfated prior to muffle furnace treatment (27). No samples containing natural cadmium were found in this study. Therefore, as a test of cadmium cyclohexanebutyrate were added to gasoline, No. 6 fuel oil, and Nigerian crude oil and analyzed by the proposed procedure. Recoveries were quanti-
.Added
1.0 10 10
20 40
Found
2.2, 2.4 12 8, 12 21 39
