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Application of atomic absorption spectroscopy to trace analyses of petroleum
MICROCHEMICAL
JOURNAL
Application
10,
148-157 (1966)
of Atomic
to Trace
Absorption
Analyses
Spectroscopy
of Petroleum’
E. J. MOORE, 0. I. MILNER,
AND J. R.
GLASS
Mobil Oil Company, Inc., Research Department, Paulsboro Laboratory, Paulsboro, New Jersey
SOCO~Y
Received June 10, 1965
During the past several years we have investigated the application of atomic absorption to the determination of traces of metals in petroleumderived hydrocarbons. This technique seemed particularly suitable for certain analytical problems related to refining and storage of petroleum and its products, not only because of its inherent sensitivity for some metals of special concern, but also because the sensitivity is usually highest when the solution is nonaqueous. Although atomic absorption is finding increasing use for direct measurements on hydrocarbons (1-4, 6-8), the use of this technique for measurements at the level of 1 ppm or lower does not appear to have received the attention it merits. This paper presents several procedures that have given a high degree of precision or sensitivity or both. Because little or no preliminary processing of the sample is involved, the procedures are unusually rapid and have the further advantage of being relatively free from contamination possibilities. This should lead to improved accuracy. APPARATUS
AND
GENERAL
METHOD
OF OPERATION
The instrument used in our investigations was the Perkin-Elmer model 214. This is a double-beam spectrophotometer using a 60-cycle modulated hollow cathode source, a 600 line per millimeter grating monochromator, and two lP28 photomultipliers. The readout unit is an AC servo ratiometer which converts the ratio of the voltages from the sample and reference beam photomultipliers to a digital readout corresponding to percentage absorption. In our work, the digital readout was replaced by a strip 1 Paper presented at the International Symposium on Microchemical 1965, held at The Pennsylvania State University, University Park, U.S.A., August 22-27, 1965. 148
TechniquesPennsylvania,
TRACE
ANALYSIS
OF
PETROLEUM
149
chart recorder. This permitted the “noisy” signal, resulting from mechanical and electrical variations, to be averaged over the desired interval and not only was more convenient but more precise. The instrument was originally fitted with a partial-consumption burner constructed of brass. Because copper and zinc were among the elements of special interest, the burner was replaced by an all-aluminum burner that was constructed in two sections to facilitate cleaning. In preliminary tests, no significant differences were observed with the various fuels-hydrogen, acetylene, or liquefied petroleum gas. Mobilflame, a liquefied petroleum gas, was most convenient to use. It was further found that the position of the beam with respect to the flame height, within reasonable limits, had little effect on the sensitivity with the systems studied. A position just above the blue inner cone was selected. It was noted that in aspirating solutions from a tall container, such as a graduated cylinder, the sample flow rate gradually decreased as the level dropped, causing a slight but noticeable decrease in absorbance. To overcome this effect of the changing hydrostatic head, measurements were made at a fixed sample level as described in the procedures below. Determination of copper. Copper may be present in petroleum stocks not only because of its presence in the original crude oil, but more often as a contaminant introduced during refining or storage and from copperbearing alloys or treating processes that use copper salts. This element is undesirable, even in minute concentration, since it poisons certain common catalysts and accelerates oxidative decomposition of finished products. Stocks of interest include gasolines, domestic heating fuels, and jet fuels. Because products as diverse as those listed above differ greatly in viscosity, the rate of flow into the atomizer and hence the response varies widely. In our initial tests, gasoline gave excellent sensitivity but heavier products did not. To improve the response with heavy stocks, and at the same time to minimize effects of base stock variations, a number of organic solvents were tried as diluents. Acetone was most suitable in enhancing absorbance. At moderate dilution, the increase in sensitivity resulting from the greater rate of sample flow compensated for the loss in sensitivity due to dilution (Table 1). An acetone-to-sample volume ratio of 4 to 1 was selected for use. We found that gradual changes in combustion conditions during the course of the day made it unwise to use a fixed calibration curve. A standard addition technique was adopted. In this procedure, the flame back-
150
E.
J.
MOORE,
0.
I.
MILNER,
AND
J.
R.
GLASS
ground is established at 3247& the wavelength of the most sensitive copper line; then 4 ml of the sample-acetone blend is atomized from a graduated cylinder containing 25 ml of the blend. To the remaining 21 ml of solution, a standard addition of 50 ~1 of a solution of copper cyclohexanebutyrate in xylene (approximately 200 mg Cu per liter) is made. The cylinder is raised on a block to make the liquid level the same as at the start of the initial aspiration, and the sample plus standard is atomized for approximately the same length of time. Finally, the background is again recorded. This standard addition technique compensates not only for instrumental drift and changing combustion conditions but also for any matrix variations not eliminated by the acetone solvent. Figure 1 illustrates the measurement of the absorption record. A straight line is drawn through each series of absorption values, with the peaks TABLE COPPER IN FUEL
OIL?
RELATIONSHIP
1
BETWEEN
ABSORPTION
Volume ratio, acetone/fuel oil 0 1 3 5
AND DILUTION
RATIO
% Absorption 2.8 2.8 2.8 2.5
10
2.1
20
1.5
a Copper content of original fuel oil = 0.6 mg/liter.
being averaged by eye. Each line is marked at the point corresponding to 1 minute of aspiration. The two baseline l-minute values (A and D) are connected by a straight line; the height of the l-minute values for the sample (B) and for the sample plus standard addition (C) is measured and converted to absorbance, which is then used to calculate the copper content of the sample. It is not necessary to make a standard addition to each sample in a series, provided density and viscosity are reasonably the same, but it is advisable to repeat it several times during the analysis of the series, Table 2 shows the results obtained by this method on analyzing a home heating fuel containing known amounts of copper naphthenate. Table 3 compares results obtained on two types of fuels by the atomic absorption method and by a chemical method; the latter involved ashing a large sample and measuring the copper in the dissolved ash calorimetrically
TRACE
ANALYSIS
151
OF PETROLEUM
as the diethyldithiocarbamate in the presence of EDTA. Based on these and similar data, the standard deviation of the atomic absorption method is 0.030 mg per liter over the range of 0.0-1.4 mg per liter, the same as by the chemical method. The statistical t-test for bias showed no difference between the methods.
0
I
3
2
FIG. 1.
m.4,~
Typical TABLE
DETERMINATION
6
MIN”&
7
chart record. 2
OF COPPER IN HOME
HEATING
FUELS
CU initially present
Cu added
Total Cu present
Cu found
0.03 0.03 0.03 0.03
0.00 0.08 0.42 0.83
0.03 0.11 0.45 0.86
0.03 0.12 0.46 0.88
a Values in m&liter.
Determination of nickel. Nickel is one of the elements that is indigenous to crude oil, being present, at least in part, in the form of volatile metal porphyrins. It will thus accompany the hydrocarbons during fractionation, and even in the sub part-per-million range in feed stocks creates problems in catalytic cracking units where it may deposit on, and help poison, the catalyst. Recently, trace amounts of an oil-soluble nickel organic salt have been added to some gasolines.
152
E.
J.
MOORE,
0.
I.
MILNER,
AND
J.
R. GLASS
The method for nickel is essentially the same as for copper; the 3414 A line is used. As in the case of copper, gasoline samples are aspirated without dilution. Gas oils used as feed stocks to catalytic cracking units require dilution with a solvent to increase fluidity, and in some cases to actually dissolve waxy material that deposits at room temperature. Heptane, xylene, tetrahydrofuran, cyclohexane, and dioxane all were suitable solvents, but the best combination of fluidity, solvent power, and burning characteristics was given by dioxane. Limiting sensitivity of the method is about 0.03 mg per liter (0.1% absorption) for undiluted samples (gasolines). The sensitivity for diluted samples decreases in proportion to the dilution; in the usual case, 3- or 4-fold dilution is needed, making the sensitivity limit TABLE DETERMINATION
Fuel Gasoline
Home heating fuel
3
OF COPPER IN MOTOR GASOLINE AND HOME
HEATING
FUELS”
Atomic absorption
Chemical
0.15 0.03 0.24 0.39,0.33 1.29,1.31 0.85,0.77 0.25,0.22 0.41,0.45
0.14 0.05 0.29 0.37,0.41 1.40, 1.43 0.80, 0.80 0.20,0.24 0.44,0.46
a Values in mg/liter.
about 0.1 mg per liter. If greater sensitivity is needed, and a hollow cathode tube with a quartz window is available, a somewhat longer method based on use of the 2320 A line and a series of background corrections may be applied (8). Table 4 shows the results obtained by the atomic absorption method and compares them with calorimetric treatment of the ash from a large sample (5). Determination of zinc. Zinc is sometimes determined in aviation jet fuels, in which ash-forming constituents are undesirable. It is also of interest in connection with gasoline quality where contamination by zinc may lead to oxidation instability. It is believed that this element may be introduced into the fuel in minute amounts, i.e., less than 1 ppm, from zinc-coated storage tanks. Determining such low concentrations by chemical methods is lengthy, and eliminating all interferences is troublesome. By atomic absorption the procedure is extremely rapid.
TRACE
ANALYSIS
OF
153
PETROLEUM
The method is generally similar to that used for copper and nickel, but in this case no diluent is used, as the dilution effect was found to offset the effect of more rapid aspiration of the sample. The 2138 IL resonance line is used. As was not the case with copper and nickel, which use resonance lines at higher wavelengths, this line falls in a region of the spectrum in which there is a strong flame absorption band (about 2100-2400 A). However, no difficulties are encountered if the recorder is adjusted so that the flame background corresponds to zero absorbance. Figure 2 illustrates the response. TABLE 4 DETERMINATTON OF NICKEL IN GASOLINE AND GAS OIL+ Samde
Atomic absorDtion
Calorimetric
Gasoline type A Sample Sample Sample Sample
1.63 1.73 1.16 1.68
1.58 1.71 1.15 1.66
1 2 3 4
2.15 1.68 0.77 1.32
2.09 1.67 0.95 1.39
1 2 3 4
0.7 1.4 1.9 3.7
0.8 1.4 2.0 2.9
1 2 3 4
Gasoline type B Sample Sample Sample Sample Gas oil Sample Sample Sample Sample
a Values in mg/liter.
The signal itself is somewhat noisier than in the case of nickel and copper. The fluctuations seem to be due to the hollow cathode lamp or the electronic detection circuit, or both, since the “noise” is the same with and without a flame. However, the fluctuating signal can easily be averaged visually with good repeatability. Based on several replicate absorption measurements on samples and the scatter of points about the calibration curve (Fig. 2)) values in the range of O-1 mg per liter are precise to about 0.05 mg per liter. It will be noted from the figure that below 1 mg per liter the response is linear with concentration, but above 1 mg per liter the sensitivity gradually decreases. Thus, while higher zinc concentrations can be determined
154
E.
J.
MOORE,
0.
I.
MILNER,
AND
J.
R.
GLASS
by interspersing standards, in effect recalibrating with each series of analyses, we have preferred to dilute to a concentration where the response is linear so that the standard addition technique will apply. ‘Determimztion of lead. The precise analysis of gasoline for tetraalkyllead content is one of the most frequently requested determinations in the petroleum industry. However, the lead concentration in leaded gasolines is usually of the order of 500 mg per liter, a concentration level at which direct atomic absorption measurements cannot be made with any useful .07 0
.06 -
/
.05 Z
2 .04:: u
/ 0
$ * .03-
/
Q .02-
/
0 zn, FIG.
2.
Determination
I MC/LITER
r 2
of zinc in jet fuel.
degree of precision. Several investigators have shown that lead can be determined in gasoline after dilution to the level of 20-50 mg Pb per liter (3, 6). We too found this to be feasible, but, although precision is improved by dilution, the atomic absorption method still falls short of the repeatability given by other methods. Nevertheless, where variations of up to 2 or 3% of the amount present can be tolerated, lead can be determined quickly and conveniently. We found a more propitious application of atomic absorption to be the determination of that form of lead which results from decomposition of
TRACE
ANALYSIS
OF
155
PETROLEUM
the tetraalkyllead in the gasoline. Lead in this form (alkyllead salts) is likely to be at the low milligram per liter level, and provides valuable clues relating to the storage stability of various gasolines. The usual method of analysis involves extracting the lead present as alkyllead salts with dilute aqueous nitric acid (0.8 vol.55 concentrated HNOs). In chemical measurements, the extract is treated with bromine and evaporated to dryness with perchloric acid to destroy residual organic matter. The lead is finally measured calorimetrically as the dithizonate. The method is somewhat long and 0.3
d
O.‘y
/ of .4
o/ 0
I IO
I 20
I 30 Pb,
FIG. 3.
Determination
I 40 MC/LITER
I 50
I 60
70
of lead in aqueous solution.
requires meticulous skills, and avoiding contamination is a problem. We thought it likely that the lead in the acid extract could be determined by direct atomic absorption spectroscopy. Preliminary to evaluating the proposed atomic absorption method on actual extracts, standards containing from 2 to 60 mg Pb per liter, prepared by adding lead nitrate to 0.870 nitric acid, were aspirated and the absorbance was measured at 2833 A. The response was sensitive and linear up to above 50 mg per liter (Fig. 3). Since base stock variations are not involved in the case of these aqueous extracts there is no need to use a standard addition technique. However. gradual changes in combustion conditions and instrumental drift make it necessary to check the calibration with each series of samples and adjust the slope of the curve. One or two standards will usually suffice.
156
E. J. MOORE, 0. I. MILNER,
AND J. R. GLASS
To compare results obtained by atomic absorption with those obtained by the conventional method, an aliquot was removed from the acid extract for atomic absorption measurement while the bulk of the acid extract was analyzed chemically. Comparative values are shown in Table 5. It is apparent that the direct measurement of the lead in the extract by atomic absorption is completely satisfactory. TABLE 5 DETERMINATION OF TETRAETRYLLEAD DECOMPOSITION PRODUCTSIN GASOLINE Extractable
Pb (ppm) Chemical
Atomic absorption
44 2 7
43 2 7 53 85 97 4 15 5 25 41 92
49 81 108 3 15 5 23 39 91
SUMMARY The successful use of atomic absorption for the determination of certain metals in hydrocarbons is aided by the enhanced sensitivity in nonaqueous media. By the use of a standard addition technique, high precision can be achieved together with high accuracy.Even where the hydrocarbon solution itself is not used directly, a significant reduction in time and in the number of manipulations may be achieved, thereby minimizing the possibility of error. REFERENCES 1. 2.
BARRAS, R. C., AND HELWIG, J. D., Rapid metals analysis for plant control. Proc. Am. Petrol. Inst. Sect. IIf 43, 223-228 (1963). BURROWS, J. A., HEERDT, J. C., AND WILLIS, J. B., Determination of wear metals in used lubricating oils by atomic absorption spectrometry. Anal. Chem. 37, 579-582
3.
4.
(1965).
DAGNALL, R. M., AND WEST, T. S., Observations on the atomic absorption spectroscopy of lead in aqueous solutions, in organic extracts and in gasoline. Takta 11, 1553-1557 (1964). MEANS, E. A., AND RATCLIFF, D., Determination of wear metals in lubricating oils by atomic absorption spectroscopy. Perkin-Elmer Atomic Absorption Newsletter 4 (January), 174-179 (1965).
TRACE ANALYSIS 5.
OF PETROLEUM
157
MILSER, 0. I., GLASS, J, R., KIRCHNER, J. P., AND YURICK, A. If,, Determination of trace metals in crudes and other petroleum oils. Anal. Chem. 24, 1728-1732 (1952). 6. ROBINSON, J. W., Determination of lead by atomic absorption spectroscopy. Anal. Chim. Acta 24, 451-455 (1961). 7. SPRAGUE, S., AND SLAVIN, W., The application of atomic absorption spectroscop] to the analysis of petroleum products. Perkin-Elmer Atomic Absorption Newsletter 12 (April), 1-6 (1963). 8. TRENT, D., AND SLAVIX, W., The direct determination of trace quantities of nickel in catalytic cracking feedstocks by atomic absorption spectrophotometry. Prrkin-Elmer dtomic Absorption Newsletter 3 (November), 131-140 (1964).
JOURNAL
Application
10,
148-157 (1966)
of Atomic
to Trace
Absorption
Analyses
Spectroscopy
of Petroleum’
E. J. MOORE, 0. I. MILNER,
AND J. R.
GLASS
Mobil Oil Company, Inc., Research Department, Paulsboro Laboratory, Paulsboro, New Jersey
SOCO~Y
Received June 10, 1965
During the past several years we have investigated the application of atomic absorption to the determination of traces of metals in petroleumderived hydrocarbons. This technique seemed particularly suitable for certain analytical problems related to refining and storage of petroleum and its products, not only because of its inherent sensitivity for some metals of special concern, but also because the sensitivity is usually highest when the solution is nonaqueous. Although atomic absorption is finding increasing use for direct measurements on hydrocarbons (1-4, 6-8), the use of this technique for measurements at the level of 1 ppm or lower does not appear to have received the attention it merits. This paper presents several procedures that have given a high degree of precision or sensitivity or both. Because little or no preliminary processing of the sample is involved, the procedures are unusually rapid and have the further advantage of being relatively free from contamination possibilities. This should lead to improved accuracy. APPARATUS
AND
GENERAL
METHOD
OF OPERATION
The instrument used in our investigations was the Perkin-Elmer model 214. This is a double-beam spectrophotometer using a 60-cycle modulated hollow cathode source, a 600 line per millimeter grating monochromator, and two lP28 photomultipliers. The readout unit is an AC servo ratiometer which converts the ratio of the voltages from the sample and reference beam photomultipliers to a digital readout corresponding to percentage absorption. In our work, the digital readout was replaced by a strip 1 Paper presented at the International Symposium on Microchemical 1965, held at The Pennsylvania State University, University Park, U.S.A., August 22-27, 1965. 148
TechniquesPennsylvania,
TRACE
ANALYSIS
OF
PETROLEUM
149
chart recorder. This permitted the “noisy” signal, resulting from mechanical and electrical variations, to be averaged over the desired interval and not only was more convenient but more precise. The instrument was originally fitted with a partial-consumption burner constructed of brass. Because copper and zinc were among the elements of special interest, the burner was replaced by an all-aluminum burner that was constructed in two sections to facilitate cleaning. In preliminary tests, no significant differences were observed with the various fuels-hydrogen, acetylene, or liquefied petroleum gas. Mobilflame, a liquefied petroleum gas, was most convenient to use. It was further found that the position of the beam with respect to the flame height, within reasonable limits, had little effect on the sensitivity with the systems studied. A position just above the blue inner cone was selected. It was noted that in aspirating solutions from a tall container, such as a graduated cylinder, the sample flow rate gradually decreased as the level dropped, causing a slight but noticeable decrease in absorbance. To overcome this effect of the changing hydrostatic head, measurements were made at a fixed sample level as described in the procedures below. Determination of copper. Copper may be present in petroleum stocks not only because of its presence in the original crude oil, but more often as a contaminant introduced during refining or storage and from copperbearing alloys or treating processes that use copper salts. This element is undesirable, even in minute concentration, since it poisons certain common catalysts and accelerates oxidative decomposition of finished products. Stocks of interest include gasolines, domestic heating fuels, and jet fuels. Because products as diverse as those listed above differ greatly in viscosity, the rate of flow into the atomizer and hence the response varies widely. In our initial tests, gasoline gave excellent sensitivity but heavier products did not. To improve the response with heavy stocks, and at the same time to minimize effects of base stock variations, a number of organic solvents were tried as diluents. Acetone was most suitable in enhancing absorbance. At moderate dilution, the increase in sensitivity resulting from the greater rate of sample flow compensated for the loss in sensitivity due to dilution (Table 1). An acetone-to-sample volume ratio of 4 to 1 was selected for use. We found that gradual changes in combustion conditions during the course of the day made it unwise to use a fixed calibration curve. A standard addition technique was adopted. In this procedure, the flame back-
150
E.
J.
MOORE,
0.
I.
MILNER,
AND
J.
R.
GLASS
ground is established at 3247& the wavelength of the most sensitive copper line; then 4 ml of the sample-acetone blend is atomized from a graduated cylinder containing 25 ml of the blend. To the remaining 21 ml of solution, a standard addition of 50 ~1 of a solution of copper cyclohexanebutyrate in xylene (approximately 200 mg Cu per liter) is made. The cylinder is raised on a block to make the liquid level the same as at the start of the initial aspiration, and the sample plus standard is atomized for approximately the same length of time. Finally, the background is again recorded. This standard addition technique compensates not only for instrumental drift and changing combustion conditions but also for any matrix variations not eliminated by the acetone solvent. Figure 1 illustrates the measurement of the absorption record. A straight line is drawn through each series of absorption values, with the peaks TABLE COPPER IN FUEL
OIL?
RELATIONSHIP
1
BETWEEN
ABSORPTION
Volume ratio, acetone/fuel oil 0 1 3 5
AND DILUTION
RATIO
% Absorption 2.8 2.8 2.8 2.5
10
2.1
20
1.5
a Copper content of original fuel oil = 0.6 mg/liter.
being averaged by eye. Each line is marked at the point corresponding to 1 minute of aspiration. The two baseline l-minute values (A and D) are connected by a straight line; the height of the l-minute values for the sample (B) and for the sample plus standard addition (C) is measured and converted to absorbance, which is then used to calculate the copper content of the sample. It is not necessary to make a standard addition to each sample in a series, provided density and viscosity are reasonably the same, but it is advisable to repeat it several times during the analysis of the series, Table 2 shows the results obtained by this method on analyzing a home heating fuel containing known amounts of copper naphthenate. Table 3 compares results obtained on two types of fuels by the atomic absorption method and by a chemical method; the latter involved ashing a large sample and measuring the copper in the dissolved ash calorimetrically
TRACE
ANALYSIS
151
OF PETROLEUM
as the diethyldithiocarbamate in the presence of EDTA. Based on these and similar data, the standard deviation of the atomic absorption method is 0.030 mg per liter over the range of 0.0-1.4 mg per liter, the same as by the chemical method. The statistical t-test for bias showed no difference between the methods.
0
I
3
2
FIG. 1.
m.4,~
Typical TABLE
DETERMINATION
6
MIN”&
7
chart record. 2
OF COPPER IN HOME
HEATING
FUELS
CU initially present
Cu added
Total Cu present
Cu found
0.03 0.03 0.03 0.03
0.00 0.08 0.42 0.83
0.03 0.11 0.45 0.86
0.03 0.12 0.46 0.88
a Values in m&liter.
Determination of nickel. Nickel is one of the elements that is indigenous to crude oil, being present, at least in part, in the form of volatile metal porphyrins. It will thus accompany the hydrocarbons during fractionation, and even in the sub part-per-million range in feed stocks creates problems in catalytic cracking units where it may deposit on, and help poison, the catalyst. Recently, trace amounts of an oil-soluble nickel organic salt have been added to some gasolines.
152
E.
J.
MOORE,
0.
I.
MILNER,
AND
J.
R. GLASS
The method for nickel is essentially the same as for copper; the 3414 A line is used. As in the case of copper, gasoline samples are aspirated without dilution. Gas oils used as feed stocks to catalytic cracking units require dilution with a solvent to increase fluidity, and in some cases to actually dissolve waxy material that deposits at room temperature. Heptane, xylene, tetrahydrofuran, cyclohexane, and dioxane all were suitable solvents, but the best combination of fluidity, solvent power, and burning characteristics was given by dioxane. Limiting sensitivity of the method is about 0.03 mg per liter (0.1% absorption) for undiluted samples (gasolines). The sensitivity for diluted samples decreases in proportion to the dilution; in the usual case, 3- or 4-fold dilution is needed, making the sensitivity limit TABLE DETERMINATION
Fuel Gasoline
Home heating fuel
3
OF COPPER IN MOTOR GASOLINE AND HOME
HEATING
FUELS”
Atomic absorption
Chemical
0.15 0.03 0.24 0.39,0.33 1.29,1.31 0.85,0.77 0.25,0.22 0.41,0.45
0.14 0.05 0.29 0.37,0.41 1.40, 1.43 0.80, 0.80 0.20,0.24 0.44,0.46
a Values in mg/liter.
about 0.1 mg per liter. If greater sensitivity is needed, and a hollow cathode tube with a quartz window is available, a somewhat longer method based on use of the 2320 A line and a series of background corrections may be applied (8). Table 4 shows the results obtained by the atomic absorption method and compares them with calorimetric treatment of the ash from a large sample (5). Determination of zinc. Zinc is sometimes determined in aviation jet fuels, in which ash-forming constituents are undesirable. It is also of interest in connection with gasoline quality where contamination by zinc may lead to oxidation instability. It is believed that this element may be introduced into the fuel in minute amounts, i.e., less than 1 ppm, from zinc-coated storage tanks. Determining such low concentrations by chemical methods is lengthy, and eliminating all interferences is troublesome. By atomic absorption the procedure is extremely rapid.
TRACE
ANALYSIS
OF
153
PETROLEUM
The method is generally similar to that used for copper and nickel, but in this case no diluent is used, as the dilution effect was found to offset the effect of more rapid aspiration of the sample. The 2138 IL resonance line is used. As was not the case with copper and nickel, which use resonance lines at higher wavelengths, this line falls in a region of the spectrum in which there is a strong flame absorption band (about 2100-2400 A). However, no difficulties are encountered if the recorder is adjusted so that the flame background corresponds to zero absorbance. Figure 2 illustrates the response. TABLE 4 DETERMINATTON OF NICKEL IN GASOLINE AND GAS OIL+ Samde
Atomic absorDtion
Calorimetric
Gasoline type A Sample Sample Sample Sample
1.63 1.73 1.16 1.68
1.58 1.71 1.15 1.66
1 2 3 4
2.15 1.68 0.77 1.32
2.09 1.67 0.95 1.39
1 2 3 4
0.7 1.4 1.9 3.7
0.8 1.4 2.0 2.9
1 2 3 4
Gasoline type B Sample Sample Sample Sample Gas oil Sample Sample Sample Sample
a Values in mg/liter.
The signal itself is somewhat noisier than in the case of nickel and copper. The fluctuations seem to be due to the hollow cathode lamp or the electronic detection circuit, or both, since the “noise” is the same with and without a flame. However, the fluctuating signal can easily be averaged visually with good repeatability. Based on several replicate absorption measurements on samples and the scatter of points about the calibration curve (Fig. 2)) values in the range of O-1 mg per liter are precise to about 0.05 mg per liter. It will be noted from the figure that below 1 mg per liter the response is linear with concentration, but above 1 mg per liter the sensitivity gradually decreases. Thus, while higher zinc concentrations can be determined
154
E.
J.
MOORE,
0.
I.
MILNER,
AND
J.
R.
GLASS
by interspersing standards, in effect recalibrating with each series of analyses, we have preferred to dilute to a concentration where the response is linear so that the standard addition technique will apply. ‘Determimztion of lead. The precise analysis of gasoline for tetraalkyllead content is one of the most frequently requested determinations in the petroleum industry. However, the lead concentration in leaded gasolines is usually of the order of 500 mg per liter, a concentration level at which direct atomic absorption measurements cannot be made with any useful .07 0
.06 -
/
.05 Z
2 .04:: u
/ 0
$ * .03-
/
Q .02-
/
0 zn, FIG.
2.
Determination
I MC/LITER
r 2
of zinc in jet fuel.
degree of precision. Several investigators have shown that lead can be determined in gasoline after dilution to the level of 20-50 mg Pb per liter (3, 6). We too found this to be feasible, but, although precision is improved by dilution, the atomic absorption method still falls short of the repeatability given by other methods. Nevertheless, where variations of up to 2 or 3% of the amount present can be tolerated, lead can be determined quickly and conveniently. We found a more propitious application of atomic absorption to be the determination of that form of lead which results from decomposition of
TRACE
ANALYSIS
OF
155
PETROLEUM
the tetraalkyllead in the gasoline. Lead in this form (alkyllead salts) is likely to be at the low milligram per liter level, and provides valuable clues relating to the storage stability of various gasolines. The usual method of analysis involves extracting the lead present as alkyllead salts with dilute aqueous nitric acid (0.8 vol.55 concentrated HNOs). In chemical measurements, the extract is treated with bromine and evaporated to dryness with perchloric acid to destroy residual organic matter. The lead is finally measured calorimetrically as the dithizonate. The method is somewhat long and 0.3
d
O.‘y
/ of .4
o/ 0
I IO
I 20
I 30 Pb,
FIG. 3.
Determination
I 40 MC/LITER
I 50
I 60
70
of lead in aqueous solution.
requires meticulous skills, and avoiding contamination is a problem. We thought it likely that the lead in the acid extract could be determined by direct atomic absorption spectroscopy. Preliminary to evaluating the proposed atomic absorption method on actual extracts, standards containing from 2 to 60 mg Pb per liter, prepared by adding lead nitrate to 0.870 nitric acid, were aspirated and the absorbance was measured at 2833 A. The response was sensitive and linear up to above 50 mg per liter (Fig. 3). Since base stock variations are not involved in the case of these aqueous extracts there is no need to use a standard addition technique. However. gradual changes in combustion conditions and instrumental drift make it necessary to check the calibration with each series of samples and adjust the slope of the curve. One or two standards will usually suffice.
156
E. J. MOORE, 0. I. MILNER,
AND J. R. GLASS
To compare results obtained by atomic absorption with those obtained by the conventional method, an aliquot was removed from the acid extract for atomic absorption measurement while the bulk of the acid extract was analyzed chemically. Comparative values are shown in Table 5. It is apparent that the direct measurement of the lead in the extract by atomic absorption is completely satisfactory. TABLE 5 DETERMINATION OF TETRAETRYLLEAD DECOMPOSITION PRODUCTSIN GASOLINE Extractable
Pb (ppm) Chemical
Atomic absorption
44 2 7
43 2 7 53 85 97 4 15 5 25 41 92
49 81 108 3 15 5 23 39 91
SUMMARY The successful use of atomic absorption for the determination of certain metals in hydrocarbons is aided by the enhanced sensitivity in nonaqueous media. By the use of a standard addition technique, high precision can be achieved together with high accuracy.Even where the hydrocarbon solution itself is not used directly, a significant reduction in time and in the number of manipulations may be achieved, thereby minimizing the possibility of error. REFERENCES 1. 2.
BARRAS, R. C., AND HELWIG, J. D., Rapid metals analysis for plant control. Proc. Am. Petrol. Inst. Sect. IIf 43, 223-228 (1963). BURROWS, J. A., HEERDT, J. C., AND WILLIS, J. B., Determination of wear metals in used lubricating oils by atomic absorption spectrometry. Anal. Chem. 37, 579-582
3.
4.
(1965).
DAGNALL, R. M., AND WEST, T. S., Observations on the atomic absorption spectroscopy of lead in aqueous solutions, in organic extracts and in gasoline. Takta 11, 1553-1557 (1964). MEANS, E. A., AND RATCLIFF, D., Determination of wear metals in lubricating oils by atomic absorption spectroscopy. Perkin-Elmer Atomic Absorption Newsletter 4 (January), 174-179 (1965).
TRACE ANALYSIS 5.
OF PETROLEUM
157
MILSER, 0. I., GLASS, J, R., KIRCHNER, J. P., AND YURICK, A. If,, Determination of trace metals in crudes and other petroleum oils. Anal. Chem. 24, 1728-1732 (1952). 6. ROBINSON, J. W., Determination of lead by atomic absorption spectroscopy. Anal. Chim. Acta 24, 451-455 (1961). 7. SPRAGUE, S., AND SLAVIN, W., The application of atomic absorption spectroscop] to the analysis of petroleum products. Perkin-Elmer Atomic Absorption Newsletter 12 (April), 1-6 (1963). 8. TRENT, D., AND SLAVIX, W., The direct determination of trace quantities of nickel in catalytic cracking feedstocks by atomic absorption spectrophotometry. Prrkin-Elmer dtomic Absorption Newsletter 3 (November), 131-140 (1964).