Quantitative Determination of Nickel in Oils by X-Ray Spectrography C. W. DWIGGINS, Jr., and H. N. DUNNING Bureau o f Mines Petroleum Experiment Station,
b A rapid x-ray spectrographic method for the direct determination of nickel in oils utilizing a cobalt internal standard has been developed, As little as 3 grams of a sample containing a few parts per million nickel may b e used. The internal standard method minimizes effects of the matrix, variation of x-ray tube emission, evaporation of the samples, density changes of the sample, and iron interference. The close correlation between the results of the internal standard method and those of other methods indicates that it is accurate, precise, and suitable for a large variety of oils.
U. S.
Department of the Interior, Bartlesville, Okla.
bration. The use of an internal standard greatly reduces these requirements, thus ensuring greater accuracy with moderate additional effort. EXPERIMENTAL METHODS
Equipment. A North American Phillips x-ray spectrograph was used, which incorporated both voltage and current stabilization. T h e x-ray beam was controlled with a n open tube primary collimator and a 4-inch long parallel plate receiving collimator with a 0.010-inch plate spacing. The spectrometer n-as adjusted for mauimum intensity of t h e nickel K, line, resulting in a shift of the 20 readings from their true value. This procedure gave only a slight increase in intensity. Peaking of the spectrometer is recommended only if the spectrometer is to be used exclusively for nickel determination for several days. It is probable that many x-ray spectrometers will produce the maximum nickel K , intensity at the true 28 reading. A helium attachment was used with a helium flow of 1 liter per minute, A lithium fluoride crystal gave somewhat higher intensities than other available crystals. X-ray fluorescence was detected with a Geiger counter, although a stable scintillation counter and a pulse height analyzer probably would give lower background readings and eliminate coincidence corrections. A water-cooled aluminum sample holder which has a sample well 1 inch in diameter and 3/8 inch deep was used, and a 0.00025-inch Mylar sample cover could be attached to the top of the holder with heavy grease. The sample well could be filled to a mark, but the height of filling was not found to be critical for the internal standard method. The removable cover allowed rapid filling and cleaning with chloroform for viscous oils. A refrigerated water bath was maintained a t 10" f 2' C. and water was circulated through the sample
T
HE quantity of nickel in various petroleums is of considerable practical as well as theoretical interest. Even traces of nickel may poison cracking catalysts, and x-ray methods have been used for the determination of nickel on such catalysts ( 3 ) . At least part of the nickel occurs as nickelporphyrin complexes, which may hare considerable bearing on the origin of petroleum. Various ashing methods have been used extensively, follolved by analysis of the ash for nickel by x-ray. colorimetric, arc spectrographic, or other methods. I n many ashing methods some of the nickel is lost, probably through volatility of the nickel-porphyrin complexes. Much work has been done on ashing, including comparison of the various methods (4, 6, 9). The procedures that have appeared in the literature are tedious a t best. Therefore, a method for the direct determination of nickel in petroleum and other oils is particularly desirable. I n x-ray fluorescence methods, external standards as commonly used require longterm x-ray stability and frequent recali-
Table I. Short-Term Stability of Spectrograph with Crude Oil Containing 3 4 . 3 P.P.M. Nickel and 0.1 7 4 Weight of 6% Cobalt Internal Standard 28 Settingn Counts per Second, Time of Period, Minutes in Degrees, Identifications LiF Crystal 5 15 30 241.0 241.3 241. G 49.275 Nickel K ,
yo
355.4 355.8 53.300 355.2 Cobalt K , 101.2 99.9 52.000 102.2 Background a Actual instrumental 28 readings after peaking of spectrometer for maximum nickel Ka intensity.
1040
ANALYTICAL CHEMISTRY
holder at approximately 1 liter per minute. A water bath stabilized at approximately room temperature is satisfactory, if the cooled water bath is unavaiIable. KO corrosion of the sample cell was noted after long use. Intensities were recorded with the count register and the timer of the xray unit by the fixed count method. A Rlachlett OEG-50-S tungsten tube was operated a t 40 kv. and 30 ma., which offered the best compromise between intensity and short term stability. Instrument Stability. T h e method depends on t h e short-term stability of the instrument. Therefore, great care n-as necessary t o produce and maintain satisfactory stability. A helium f l o ~rate of 1 liter per minute was satisfactory; the helium flow may vary by 25y0 without detectable changes in intensities. The constant temperature water b a t h prevented density changes of the sample, as well as evaporation losses. The x-ray unit was allowed to stabilize for approximately 2 hours, to obtain maximum short-term stability, before use. With these precautions, the intensities recorded for a typical crude oil containing 0.174 weight yo of a cobalt standard containing 6% cobalt remained relatively constant, as shown in Table I. This short-term stability is necessary so that the K , peaks of nickel and cobalt, as me11 as the background reading, may be read accurately for a particular sample, but the long-term stability of the x-ray unit is of little importance. Selection of Internal Standard. Cobalt n-as selected as t h e internal standard for several reasons. Cobalt has been used n i t h satisfactory results in emission spectrography ( 5 ) , and an x-ray spectrographic survey of the various crude oils studied failed to show a significant quantity of cobalt compared to the amount of cobalt added. Also, the spectrum of the x-ray tube was smooth in the cobalt K , range. The cobalt K , peak is as close to the nickel K , peak as is practical. This is necessary to minimize matrix effects which may become important with wide separation of the analytical and internal standard peaks. Although an iron absorption edge and iron Kp peak occur between the nickel and cobalt K , peaks, iron would be expected to contribute little to the total mass absorption coefficients at the cobalt and nickel K , wave lengths. Thus, the presence of the iron absorption edge may be unimportant. However, in ashed samples, it usually is neces-
sary to make corrections for the presence of iron due to the increase in relative iron concentrations after ashing ( 2 ) . Preparation of Standards a n d Oils. Kational Spectrographic Laboratories metallo-organic nickel and cobalt standards containing 6% metal were used. T h e calibration standards mere prepared in a n SAE 30 lubrication oil stock. T h e stock was additive-free a n d proved t o be free of nickel, iron, a n d cobalt t o t h e limits of detection. The nickel compound was added to the oil to give the desired nickel concentration in the oil standards. Then 0.174 n-eight To of the cobalt standard was added to the oil-nickel standard mixture. It is most convenient to weigh the nickel and cobalt standards onto 1-inch aluminum disks rapidly and seal these in 6-ounce bottles. Then the required amount of oil is neighed into the bottle containing the nickel and cobalt standards. The cobalt internal standard is added to the crude oils in the same manner as to the calibration standards. Thorough mixing of the calibration standards and crude oils after addition of the internal standard is essential. The calibration standards and very viscous crude oils should be heated to 100' C. to allow thorough mixing, but the less viscous crude oils should be hrated only slightly to avoid the danger of explosion on heating in scaled bottles. The samples were mixed by vigorous shaking and agitation nit11 a n ultrasonic generator. Vigorous hand or mechanical shaking is satisfactory, if a n ultrasonic generator is unavailable. This mixing procedure Kas repeated before each sample was used to avoid possible sedimentation of the components. Intensity Determinations. The cobalt and nickel K , peaks n-ere located a n d helium was alloned t o flow for 5 minutes before intensity measurenieiits for a sample were made. T h e \.-ray unit Tvas allowed t o stabilize, t h e sample holder was precooled, a n d t h e water b a t h was a t thermal equilibrium. F o r t h e standards a n d oils, t h e time required t o accumulate 32,000 counts was recorded. Intensity measurements were made at 0.025' (20) readings on each side of the cobalt and nickel K , peaks and, if the peak positions drifted, new peak readings were made. The background intensity was taken at 52.000' (20). This was found to be the smoothest region, between the two peaks, as determined from a strip chart recording of intensity in the region of the cobalt and nickel K , peaks. The times for the accumulation of 32,000 counts n ere recorded and converted to counts per second. The intensities for each calibration Sample were determined three times on separate samples, and the intensities for the oils were determined three times on the same sample. Coincidence corrections were made using methods described by Klug and Alexander ('7) with a resolving time correction factor of K , of 2.55 X 10-d used as described by these authors. This simple correction was adequate for
this work and resulted in a linear calibration curve. The background intensity was subtracted from both the cobalt and the nickel K , intensities and the ratio
R
IC0 i
was calculated for each standIK ard or crude oil with the coincidence corrected intensities. =
RESULTS AND DISCUSSION
Ideally the internal standard method should be described by the equation: Xi (p.p.m.) =
K
This equation applies to both x-ray diffraction and spectrographic analyses (8)for ideal cases were K is a constant, provided the same concentration of internal standard is aln ays added to each sample. This assumes that the nickel and cobalt peaks are close enough together so that matrix effects are not important. Hoiverer, in practice the concentration of nickel should be evpressed by: S i (p.p.ni.)
=
f(R) ~
R
where f ( R ) is a variable, mildly dependent on the ratio R. This was found to be the case, and the departure of f ( R ) from a constant probably is due to several factors, such as a small
Table II.
15.6
57.8 36.3 51.8
(chloroform-oil)
57 8 (100 p.p.m. Fe) 57 8 (25 liv., 25 ma.)
f ( R ) = -7.175R
+ 75.81
(3)
The deviations between the experimentally determined f ( R ) values and f ( R ) values determined by Equation 3, obtained by the least squares method, also are given in Table 11. Thus, for a petroleum sample containing 0.174 neight 70 of the cobalt standard the ratio R is calculated, the function f ( R ) is determined by Equation 3, and the concentration of nickel is determined by use of Equation 2. Matrix Effects. The mass absorption coefficient in a purc hydrocarbon should be approximately 5.6 a t the wave length of the nickel K , line, depending on the carbon-hydrogen ratio of the hydrocarbon. Even an oil containing h l drocarbon n ith a mass absorption
Calculated and Observed f(R) Values
Ni, P.P.M. 00.3
nickel K , peak in the x-ray tube spectrum, possible interaction of the nickel and cobalt K , peaks at the background wave length due to the rather crude collimation necessary for sufficient intensity, and other factors. However, the concentrations of nickel determined are not dependent on matrix effects. The calibration results are shown in Table 11. The experimentally determined f ( R ) values nere fitted to a straight line by the method of least squares and obeyed the equation:
I(E
R
Obsd.
Calcd.
Dev.
0,7796 3.32% 1.156 1.750 1.28
T O . 40
+o. 19
66.82 63,85 66.3
70.21 S i . 97 87.51 63.19 66.6
1.197
69 2
67 2
-2.0
1 160
67 0
67 5
-0 5
Table 111.
j i . si
-0.15 -0.69 +0.66 +0.3
Nickel Contents of Various Oils
X-Ray Internal Standard
Spectrographic Results Essob . on Ashed Samplea X-Ray CommerGovernExternal cia1 ment Standard
Oil Location Tatums Oklahoma 57 Tatums" 4.4 Tatumsd 252 N . Belridge California 107 Wilmington 53 Rio Bravo 2.2 Bachaquero Venezuela 52 Lagunillas 34 Tia Juana Mexico 20 Coleville Canada 36 Rhodes Kansas 36 Oil shale extractC Colorado 4Tf U. S. Geological Survey, Denver, Colo. Esso Research and Engineering Co., Linden, N. J. c Propane deasphalted raffinate. Propane precipitated asphaltene. 6 Benzene extract of oil shale. f Colorimetric method.
5G
"TO So'
56 3 225 83
...
...
60
...
53 41 24 32 32
4
... 38 29 16
...
38 4ia
...
...
...
...
..
..
0
VOL. 31, NO. 6, JUNE 1959
1041
coefficient of approximately 5.6 aiid containing 10% sulfur, 5% nitrogen, 0.1% vanadium, and 0.01% copper would have a total mass absorption coefficient of less than 60. I n order to test for matrix effects, a standard containing 51.8 p.p.m. of nickel was prepared in a 50 weight yo chloroform-oil solution. Because the mass absorption coefficient of this oilchloroform mixture is calculated to be approximately 60, any matrix effects that are produced by differences in the mass absorption coefficients between the nickel standards prepared in oil and the various oils analyzed should be indicated. However, as shown in Table 11, the calculated and observed f ( R ) values are very nearly the same as the values of f ( R ) observed for the oil-chloroforni based standard and the f(R) predicted by the pure oil standards. This indicates that the method is independent of ordinary matrix effects. I n view of the usual range of iron concentrations in petroleum samples it \Todd be predicted that, although iron has an absorption edge between the nickel and cobalt K , peaks, it would produce little interference. To test this, 100 p.p.m. of iron Kere added to a nickel standard solution containing 57.8 p.p.m. of nickel and the usual amount of cobalt internal standard. As shown in Table 11,this concentration of iron would result in approximately a 3% error in f(R), and a 3% error in nickel concentration. However, the concentration of iron usually found in crude oil is less than 100 p.p.m. and the presence of iron should result in not more than about 1% error for a typical crude oil. However, if a crude oil sample contained 100 pap.m. or more of iron, the iron concentration could probably be determined by the internal
standard method using the cobalt peak as the internal standard peak, and a correction could be applied to the nickel concentration. As shown in Table 11, the change of operating conditions to 25 kv. and 25 ma. resulted in approximately the same R value for the 57.8 p.p.m. nickel standard, thus indicating that the long-term stability of x-ray emission is of little importance so long as shortterm stability is maintained. Indeed, after 1 month, the same calibration curve was found to apply. Thus, the method eliminates the frequent recalibrations required for external standard methods.
The close correIation between the internal standard method and other methods indicates that it is accurate, as well as precise and suitable for a large variety of oils. The precision for duplicate determinations was found to be within 0.6% for all determinations as compared with a theoretical precision of 0.4%. The accuracy was within 3%, with an accuracy approaching 1% for oils containing little iron. The method is rapid, and frequent recalibration is unnecessary. Precautions were taken to eliminate sources of error due to matrix effects, sample evaporation, density changes, and x-ray emission variability.
COMPARISON WITH OTHER METHODS
ACKNOWLEDGMENT
The nickel contents of various oils obtained by the internal standard method are compared to those obtained by other methods in Table 111. The results with Tatums crude oil and its fractions are in excellent agreement with spectrographic methods. With the other oils the two x-rays methods give results generally in good agreement, despite the difference in methods. The spectrographic methods in some cases give low results. This is particularly apparent in the N. Belridge oil. This oil contains the highest content of nickel-porphyrin complex of any oil studied in this laboratory (3). Nearly half of the nickel and vanadium of this oil is present as the porphyrin complexes. It appears that methods involving ashing may give low nickel contents because of the volatilization of nickel complexes during the ashing process. Such loss would be minimized by wet-ashing methods, but still may be sizable. The x-ray fluorescence methods, which do not require ashing, avoid this error.
The authors gratefully acknowledge the assistance of J. R. Lindley, of this station, who designed the special sample holder and adjustment mechanism. LITERATURE CITED
(1) Davis, E. N., Hoeck, B. C., ANAL. CHEM.27, 1880 (1955). (2) Dunning, H. N. Myers, A. T., Moore, J. W., Ink Eng. C h m . 46, 2000 (1954). (3) Dyroff, G. V., Skiba, P., ANAL. CHEW26, 1774-8 (1954). (4) Gamble, L. W., Jones, W. H., Zbid., 27, 1456 (1955).
(5) Hansen, John, Skiba, Paul Hodgkins, C. R., Zbid., 23, 1362 (1951j. (6) Horecay, J. T. Hill, B. N , Walters, A. E., Schutze, h. G., Bonner, W. H., Zbid., 27, 1899 (1955). ( 7 ) Klug, H. P., Alexander, L. E., “XRay Diffraction Procedures,” pp. 28190, John Wiley, New York, 1954. (8) Zbid., pp. 415-16. (9) Milner, 0. I., Glass, J. R., Kirchner, J. P., Yurick, A. N., ANAL. CHEV. 24, 1728 (1952). RECEIVED for review September 15, 1958. Accepted December 22, 1958. Divisioii of Analytical Chemistry, 135th Meeting, ACS, Boston, Mass., April 1959.
Spectrophotometric Determination of Cycloheximide ARLINGTON A. FORIST and SUSAN THEAL Department of Physical and Analytical Chemistry, The Upjohn Co., Kalamazoo,
b The antibiotic cycloheximide (actidione) hus been determined routinely b y a microbiological assay utilizing the inhibition of the growth of Saccharomyces pastorianos. A chemical method has been developed based on the reaction with alkaline hydroxylamine to produce a hydroxamic acid, followed by conversion to the highly colored ferric hydroxamate. Analysis of standard samples indicates a mean recovery =k standard deviation of 1042
ANALYTICAL CHEMISTRY
Mich. n
100.2 & 1.7%. Analysis of typical bulk cycloheximide preparations b y the chemical method gives results in excellent agreement with those obtained by solubility analysis and by bioassay, CH3
T
antibiotic cycloheximide (I) (5) isolated from Streptomyces griseus (3, 6) is highly active against a large number of yeasts, but has HE
I no marked antibacterial activity (10). Recently, cycloheximide has received
