Automated Determination of Elements in Organic Samples Using X-Ray Emission .. Spectrometry C. W. DWIGGINS, Jr. Bureau of Mines, Bartlesville Petroleum Research Center, U. S. Department o f the Interior, Bartlesville, Okla.
b New automated x-ray methods for determining elements in organic materials, including petroleum and petrochemicals, use bath x-ray emission and scatter methods. Methods for matrix effect and background corrections suitable fot automated analyses were investigated in detail. When possible, correction methods were based only on x-ray data, and more generalized methods capable of simplification for less complex analyses were developed. Specific analytical applications illustrate the utility of the direct and ashing methods for determining vanadium and nickel and direct methods for determining sulfur, carbon, and hydrogen. Direct-reading analyses for some of the more routine determinations were achieved. The use of the new correction methods and techniques speeds analyses and requires much less operator attention for both routine and special analyses.
H
IGHLY automated x-ray methods
for on-the-line analyses have been described; highly specialized instruments are ' ordinarily used. However, many of the analytical problems in petroleum and organic research require analyses of many different types of samples, where a single, linear calibration curve does not apply. Manually operated x-ray equipment usually is used for research and development analyses, and the operator often has little time for other duties. The necessary calculations, although usually simple, can be time-consuming. Hence automation of x-ray analytical methods for research and development analyses is highly desirable. The objective of this research was to develop correction methods suitable for automated analysis of many types of samples. GENERAL EXPERIMENTAL
Automated X-Ray Equipment. T h e instrument used for this work was a n X-Ray Autrometer (11), a sequential automated instrument. Other sequential and simultaneous instruments are available, and could be used with no or slight modifications of methods. The instrument was equipped with a crystal changer holding three crystals, : lithium fluoride, ethylenediamine Dtartrate, and ammonium dihydrogen
phosphate. A tungsten target x-ray tube and a helium atmosphere were used. A 0.010 X 4 inch primary collimator was the best compromise for a general-purpose instrument. A multichannel sequential pulse height analyzer is necessary if pulse height analyzer window and base line settings must be different for different wavelengths. The pulse height analyzer used was a five-channel unit; it is available as a n option with the Autrometer. The instrument that was used prints intensity ratios. Many types of analyses require only that a linear equation be solved to obtain concentrations of elements. Thus, the instrument was modified to allow direct print-out of concentrations when a linear equation applies. Details of this modification are available from the author. Intensity Ratio Values. I n the usual method of operation, the instrument first counts the reference material for a preselected number of counts and then counts the unknown for the same time as the reference was counted. Finally, the intensity ratio, Rx, is printed for a chosen wavelength where intensity unknown Rx = (1) intensity standard Intensity ratios can be used directly to calculate concentrations, or the usual equations based on intensities can be modified and used. I n many cases, it is best to consider an intensity ratio as just another, different form of intensity information (in this case dimensionless) and base calibration methods directly on intensity ratios rather than intensities. In contrast to earlier work (2, 6), all esperimentally determined intensity ratios result from a comparison of intensities of unknown and arbitrary reference standard at the same wavelength. Preparation and Use of Primary Metalloand Working Standards.
organic, oil-soluble standards from the National Bureau of Standards and h'ational Spectrographic Laboratories were used. Directions for miving these standards in oils are supplied with t h e standards. Scandium wa5 the internal standard for determining trace metals when ahhing was used. Scandium 2-ethylhelanoate was prepared from scandium nitrate and 2-ethylhexanoic acid by precipitation from ethanol. Stable sohtions of this compound containing up to a t least a few hundred parts per million of scandium can be prepared in
a mixture of 15 ml. of 2-ethylhexylamine and 85 ml. of xylene. Because of the inconvenience of using primary standards for all work, solid, stable secondary working standards are desirable. Standards consisting of the analytical element dispersed in polyethylene disks were used (14). Disks molded from microcrystalline wax of high melting point also were tested, but are not as stable as the plastic disks, and tend to crack after long exposure to x-radiation. USE AND PREDICTION OF BACKGROUND CORRECTIONS IN ORGANIC SOLUTION ANALYSIS
Theory and Principles. When trace elements are determined in organic solutions, peak-to-background ratios often are small, and t h e background correction becpmes very important. T h e use of intensity ratio values in the automated determination of trace elements requires a somewhat different approach to the background problem than is usual. Formally, i n t e q i t y ratios can be used in the same. why as intensities in most equations for calculations of concentrations. When an equation based on intensities is used, the concentration, c, often is given by the relation: c =
~ ( IuIbd
(2)
where q = a constant
I, Ibt,
=
total peak intensity
= background inteiisity
The corresponding equation, based on intensity ratios, is c =
m(R - Rb)
(3)
where
m = a constant R = intensity ratio for the analytical element Rb = background intensity ratio
A background taken near a peak often must be corrected to give the background a t the peak for use in the above equations. Previous work has shown that coherent and incoherent scatter peaks have extremely high intensities ( 2 ) , Thus to determine intensity ratios for two scatter peaks takes much less time VOL. 36, NO. 8, JULY 1964
1577
Table 1.
Calculation of Nickel Background Intensity Ratios from Coherent and Incoherent Scatter lntensiiy Ratios Intensity ratios" RSb*, Sample Ra * R7 * Rsb* calcd. 2,2,4-Trimethylpentane 0,997 1.038 1.021 1.016 Dibutyl phthalate 0.873 0,647 0.723 0.726 Cyclohexane 1.003 1.007 1,000 0.999 Benzene I . 086 0.957 0.884 0,948 2-Ethylhexylamine 0.936 0.932 0.950 0,936 cis-Decahydronaphthalene 0.982 0.991 0.999 0,983 n-Butyl alcohol 0.735 0.834 0.768 0.757 274 sulfur in white mineral oil 0.766 0.643 0,677 0.685 Water 0.572 0.385 0,453 0.455 Heavy water (99.5 mole %) 0.387 0.575 0.454 0.455 0.472 0.287 3.239% KCl in water 0,357 0.354 0.491 0.278 2.543% Ba(N03)*in water 0.358 0.356 0.300 0,501 4,468% NaCl in water 0.373 0.374 0.564 0.780 Polyethylene (solid) 0.643 0.652 0.447 Silicone oil 0.190 0,292 0.285 0.513 Inorganic mixtureb 0.292 0.373 0.375 0,444 Organic mixture" 0.185 0.286 0.278 Rsb* = 0.39309Re* -k 0.60691Ri* - 0.005817 R,* = R,K,:k, = l/Rnv 9b, nickel background; 6, coherent scatter; 7, incoherent scatter; n, nth value; r, calibration reference material having approximately same composition as reference standard (white mineral oil was used for these data). b Aqueous solution having percentage composition: 0.146 Na, 0.509 Br, 0.308 Ba, 0.070 Ca, 0.091 Sr, 0.322 I, 0.164 N, 0.444 C1, 0.192 Cd, 0.050 Li, 0.097 Mn, 0.099 K, 10.846 N, 86.649 0. c Contains in approximately equal weights: cyclohexane, iodoform, ethyl alcohol, butyl alcohol, xylene, ethylene bromide, 2,2,4-trimethylpentane, 6-methyl-2,4heptanedione, phenyl sulfide, quinoline, dibutyl phthalate, carbon tetrachloride, 2-ethylhexylamine, glycerol, 2-ethylhexanoic acid, acetone, paraffin oil, silicone oil, Kel F polymer oil.
background intensity ratios can be calculated with sufficient accuracy for many purposes from a knowledge of the two scatter intensity ratios. The range of elements studied covers the bulk of organic matrices of interest. However, caution should be observed if the method is extended to more exotic organic compounds, especially metalloorganic compounds. I n general, the use of both types of scatter intensity ratios resulted in better agreement than the use of only one type. However, for some applications one type of scatter may be sufficient. Intensity ratios for vanadium background were calculated using intensity ratios measured near the wavelength of the characteristic vanadium K a transition. The background intensity ratio, &a*, a t the vanadium peak was found to depend on the background intensity ratio near the peak, RI4* in a linear fashion.
5
than to determine one general background intensity ratio with the same precision. If the intensity ratios of the incoherent and coherent scatter peaks can be used to predict background intensity ratios for" several elements having wavelengths not too far removed from the scatter wavelengths in multielement analysis, even greater advantages are realized.
Experimental. Channels of the xray instrument were programmed for the tungsten L,, coherent scatter peak, the corresponding incoherent scatter
Table It. Determination of Nickel in Carbon-Sulfur-Hydrogen System Using Incoherent Scattering to Correct for Matrix Effects Nickel, p.p.m. Sulfur, Intensity ratios pres72 R7 Rg ent Detd. 0 1.282 0.885 0 0.1 80 78.2 0 1.304 2.129 0 1.287 1.514 40 40.4 20 20.9 0 1.287 1.210 0 0.7 3 0.793 0.645 40 40.0 3 0.800 1.023 80 77.3 3 0.808 1.390 5 0.642 0.566 0 0.4 0.864 40 38.9 5 0.649 Concentration of nickel, C N ~ given , in p.p.m. by: - Rs - 0.49949R7 - 0.24227 ,NI 0.01241R7 - 0.0004 ,
1578
ANALYTICAL CHEMISTRY
peak, and elements of interest such as nickel and vanadium. The initial x-ray tube voltage was set at 64 kv. a t 1-ma. x-ray tube current. At a full current load of 50 ma., the voltage is slightly less than 50 kv. The channel current controls were adjusted to produce a counting rate of less than 20,000 counts per second for the scatter peaks and to the maximum for the other peaks. The flow counter only was used because it produced a more favorable peak to background ratio. Programmed pulse height analysis was used for all channels. Channel counting precision controls were set to accumulate 400,000 counts for the reference standard for the higher atomic number elements; 40,000 counts for vanadium and elements of lower atomic number. The reference material in the standard sample holder can be a white mineral oil or one of the polyethylene disks. Plastic disks were more convenient in most cases and were used for this work. The reference material is used only as a dummy reference in this application, so exact knowledge of its composition is not necessary.
Results and Discussion. Intensity ratio d a t a were collected for a wide variety of metal-free materials. Equations were then developed t h a t allowed calculation of the background intensity ratios for the various elements from intensity ratios of the two types of scatter. A small sample of these results for nickel is given in Table I. These data give evidence that
MATRIX CORRECTION METHODS FOR DETERMINATION OF CONCENTRATION OF TRACE ELEMENTS IN ORGANIC LIQUIDS
Theory and Principles. Several x-ray methods have been used t o determine trace metals such as vanadium, nickel, iron, cobalt, and chromium in organic liquids (5-8,10,12, IS). Several of the methods are rather specialized and are designed for determining metals in only one type of sample, such as petroleum, but nearly all could be adapted for use with automated x-ray equipment. However, a different approach seemed desirable to take full advantage of the intensity ratio method and the automated x-ray equipment. Methods that were versatile and suitable for several types of samples encountered in research and development analysis were desired. Scattered radiation, either general background or essentially coherent scatter, has been used to make matrix corrections. No treatment of the sample and no information other than the x-ray intensity data are necessary in many cases. The use of incoherent scatter, or both coherent and incoherent scatter, intensity ratios was tested to make matrix corrections for rather wide variations in organic matrices. Experimental. NBS nickel standards were prepared in a white mineral oil base, and various concentrations of sulfur, as phenyl sulfide, were added. D a t a were obtained in the same manner as for the background studies. Results and Discussion. SIMPLIMETHOD FOR DETERMINING TRACEMETALS. Using experimental d a t a for many concentrations of niciel and sulfur, a n equation was developed for determining nickel b y FIED
the method of least squares. Compositions of typical samples a n d calculated concentrations of nickel are given in Table 11. This method could be used to determine the concentrations of nickel in most crude oils. Sulfur causes the important matrix effects in crude oils, and in one method sulfur concentrations only were used to calculate matrix corrections (10). I n the method illustrated only scatter intensity ratios, quantities t h a t are easily determined to high precision in 1 minute or less, are necessary for corrections. A total of 400,000 counts was taken for the reference standard for all intensity ratio determinations. A wax reference standard was in the standard sample holder to collect these d a t a ; later a polyethylene disk was found to be superior.
MOREGENERAL METHOD FOR DETERMINING TRACEMETALS. If organic samples contain appreciable quantities of major elements in addition to carbon, hydrogen, and sulfur, a more elaborate method for matrix correction will give somewhat better results. The background intensity ratios for samples varying widely in matrix composition are predicted more accurately by using both coherent and incoherent scatter intensity ratios or a background intensity ratio. Results obtained using this more exact method of correction are shown in Table 111. Constants in the equations were determined using the method of least squares. This method gives good compensation for matrix effects for a very wide variety of samples, and only scattered radiation was used for all corrections. Calculation of concentrations using this method can be time-consuming. -4 digital computer can be used to convert the intensity ratios to corrected intensity ratios and to calculate concentrations. Results obtained using a slightly different method for vanadium determination are shown in Table IV. Background intensity ratios were calculated using intensity ratios taken near the vanadium peak, as explained previously. Barium can interfere. I n such a case a correction based on the barium concentration would have to be used, or the less sensitive vanadium Kp line could be used. DETERMINATION OF MAJOR ELEMENTS IN ORGANIC MATERIALS USING SCATTERED INTENSITY RATIOS
Table 111.
Determination of Nickel Using Both Coherent and Incoherent Scattering to Correct for Matrix Effects
Nickel. Present 98.7 94.2 96.0 148.7 40.8 0.7 20.2 97.4 101,l 102.1 116.8 107.2 140.6 20.2 22,l 53 45 36
Corrected intensity ratios RE* R7 * Re*' 0.472 1.361 0,299 1.281 0.492 0.288 1,402 0.313 0,500 2.496 0.572 0.408 1.648 0.758 0.647 0.762 0,692 0.643 1.164 0,763 0.648 Dibutyl phthalate 0.865 0.668 3.095 Dibutyl phthalate-Decalin 0.912 0.804 3.726 Benzene 1.086 0.991 4.386 0,450 0.202 1,058 555 silicone oil 0.515 0,305 1.462 Inorganic mixtureb 0.194 1.118 Organic mixtureb 0.443 1.198 Crude oil-dibutyl phthalate 0.818 0.649 0.531 0.629 Crude oil-silicone oil 0.310 1.794 Bachaquero petroleum 0.729 0.605 1.613 Lake Medium tar 0.732 0.610 Rhodes oetroleum 0.694 0.564 1.300 ~.~~~ ~. . 0.472 0.287 0.357 3.239% KCl, aqueous 0.491 0.358 2.543% Ba(NOI)*,aqueous 0.278 0.501 0.300 0.374 4 . 468y0 NaCI, aqueous 0.575 0.387 0,461 Heavy water (99.5 mole %) 0,950 0.932 0.936 2-E thvlhexvlamine 0.834 0.735 0.757 n-But$ alc"oho1 0.866 0.645 0.724 Dibutyl phthalate 1.021 0.997 2,2,3-Trimethylpentane 1 ,038 1.002 Cyclohexane 1.008 1 ,004 Concentration of nickel, Cg, given in p.p.m. by:
I
Nickel-containing sample 3.239% KCl, aqueous 2.543% Ba(NOs)2,aqueous 4 , 468y0 XaC1, aqueous Water Petroleum, 2% sulfur
r -
a
b
~
C, Rjs* Rob*
=
FgRjo* RQ*- Rea* 0.39309 RE'
Fg
=
31.15611
= =
0 0 0 0 0 0 0 0 0
I
I
0.10
0.46 -0.12 -0.42 -0.09 -0.08
0.11
+ 0,60691 R,* - 0.005817
(&) - 4.24827
Rs. Kickel intensity ratio. Compositions given in Table I.
Table IV.
Determination of Vanadium Using incoherent Scattering and Vanadium Background to Correct for Matrix Effects
Corrected intensity ratios Rle*a R7* Ri,* 1.769 0.288 0.444 2.280 0,382 0.538 6.641 0.647 0.715 3.742 0.648 0.719 1.092 0.643 0.706 1.985 Petroleum-silicone oil 0.310 0.444 Petroleum-dibutyl phthalate 0.649 0.720 3.690 0,558 Heavy water ( 9 9 , 5 mole %) 0,387 0,529 0.983 2-Ethylhexylamine 0.936 0.940 0.749 0.668 0.734 Dibutyl phthalate 0.770 n-Propyl alcohol 0.693 0.761 1.023 2,2,4-Trimethylpentane 1.038 1.013 0.915 0.915 0.881 Benzene Concentration of vanadium, CN, given in p.p.m. by:
Vanadium, p.p.m. Present Found 56.9 56.0 55.6 56.9 111.4 111.5 56.8 56.7 6.2 7.1 60.6 60.5 55.1 55.4 0 0.44 0 0.45 0 0.05 0 -0.03 0 0.01 0 -0.12
Vanadium-containing sample 3,239% KC1, aqueous Water Petroleum, 2% sulfur
c 1 6
=
R/IB*=
0
D.o.m. Found 99.9 95.5 97.3 146. 42.3 0.4 20.9 99.8 99.5 102.2 114.6 105.8 130.1 21.3 22.9 54.1 45 35.2 0.35 0.22
Fl&/16*
R16b*
=
- R16b* 0.98268 Rid*
F16
=
12.4148 (j$)
e* & l'
+ 0.02451 - 0.33102
Symbols as previously. R16) vanadium intensity ratio.
Theory and Principles. DETERMIKATIONS O F Low ATOMICKUMBER ELEMENTS INCLUDIKG CARBONAND HYDROGEN.Previous work (2) has shown that coherent and incoherent scattering of x-rays can be used to determine hydrogen and carbon in hydrocarbons. If a sulfur determina-
tion is available, the method can be used for sulfur-containing materials. For samples containing small amounts of nitrogen or oxygen, the method can be used if oxygen or nitrogen analyses are
available. I t was decided to adapt this method for automated x-ray analyses and to extend the method. Suppose the carbon-hydrogen-sulfur system is considered. The scattering VOL. 36, NO.
a,
JULY 1964
1579
Table V.
Determination of Carbon in Sulfur-Hydrogen-Carbon Scattered Radiation Intensity Ratios Only
Carbon concn. detd., % 91.69 92.69 84.32 90.72
Scatter intensity ratios
R7'
86'
1,164 1.325 1.191 1.291 0.953 1.064 0.871 1.123 0.932 0.827 0 790 0 814
0,9975 1.159 1.349 1 ,202 0.737 1.139 0.840 1.001 0.758 0.598 0 711 0 617
87.61 80 13 85 99
System Using
Sample composition, % Carbon Sulfur 91.34 1 92,25 0 84.11 0 90.50
87.71 80 05 86 06
5 5 5
A = 0 000007940 B = -0 00180 R, - 0 00310 Re - 0 0003321 C = 0 05266R7 0 4423 Re 0 11038
+
0
-
Average of three determinations taking 400,000 counts for reference standard.
produced by a characteristic L-transition ( 2 ) of a tungsten target x-ray tube is used. In contrast to the previous work, no background corrections are made. The concentration of hydrogen is taken as the dependent variable, while concentrations of sulfur and carbon are, taken as independent variables. Intensity ratios, in relation to an arbitrary reference standard, are determined at both coherent and incoherent wavelengths. DETERMINATIOK OF SULFUR. Many x-ray emission spectrographic methods have been reported for determining sulfur in organic materials. If the matrix remains reasonably constant, a linear calibration equation applies. However, if the hydrogen-to-carbon ratio of the matrix varies greatly, errors of up to several per cent can result and various corrections must be applied ( 9 , 15). An internal standard method or a dilution method often can be used for the more difficult cases. However, another method of correction for matrix effects, based on x-ray scattering only, is given.
Experimental. M a n y solutions containing different concentrations of sulfur, hydrogen, and carbon were prepared by adding phenyl sulfide to sulfur-free organic solvents. Solutions were similar to those used in previous work ( 2 ) . The reference material used in the standard cell was a polyethylene disk containing approximately ITo sulfur. Intensity ratios relative to this reference disk were obtained.
coherent and incoherent wavelengths depended on carbon concentration in a linear fashion. Also, a t fixed carbon Concentration, the scatter intensity ratios depended on sulfur concentration in a linear fashion. These two experimentally determined facts can be expressed as two sets of partial differential equations:
I n the equations x, and x, are concentrations of carbon and sulfur, respectively, and R6 and R7, the coherent and incoherent intensity ratios, were defined previously. These two sets of partial differential equations can be solved easily and yield :
Rg = kl f ksxa R7 = ks
+ k f l , + k4.1'~Xa(5)
+ kGa + k7~c+ ksxcxa
(6)
Once the constants are known, the concentrations of sulfur and carbon can be determined knowing the intensity ratios, R6 and R7, only. Because the method is not as sensitive as the x-ray emission spectrographic method, the equations were not used for sulfur determination. However, the method is sensitive for carbon determination, and rearrangement of the equations for carbon determination yields: xc =
- B - (BZ - 4AC).'
(7) 2A DETERMINATION O F HYDROGEN A N D CARBON. A = ksk3 - k4k7 h theoretical prediction of the total scatter a t the coherent and incoherent B = k4R7 - ksR6 k&3 kski wavelengths would be very difficult k4k5 - kzk7 (8) t o make to the necessary accuracy; thus, a phenomenological approach C = kzR7 - k&e - kska koki was used. At various fixed concenResults of using this method to trations up to a t least 10% sulfur, determine carbon are given in Table V, the intensity ratios of scattering a t the Results and Discussion.
+
+
+
1580
e
ANALYTICAL CHEMISTRY
along with the constants used for solution of the equation. This method applies exactly only to the hydrogen-carbon-sulfur system. However, if relatively small quantities of nitrogen and oxygen are present, the concentration determined based on carbon represents the sum of the concentrations of carbon, nitrogen, and oxygen (2). To test this concept, water was used instead of a hydrocarbon. When the equation for determining carbon is used, the concentration of oxygen determined is in error by approximately 6%. Thus, the method of calculation gives sufficiently good results for most work when only small amounts of oxygen are present. DETERMINATION OF SULFUR. h typical linear calibration equation obtained for sulfur standards prepared in a white mineral oil matrix was: X # = 1.278Ris
- 0.012
(9)
where 2. represents the weight per cent of sulfur and R15is the intensity ratio for sulfur. The intercept constant varies only slightly with changes in the carbon-to-hydrogen ratio and can be assumed a constant for sulfur concentrations in the 0.01 to a few per cent concentration range. However, the slope in Equation 9 varies considerably with the carbon-to-hydrogen ratios, and such variations must be considered for accurate analyses. I n the carbonhydrogen-sulfur system, the slope m, in the sulfur calibration equation was found to be a linear function of the carbon concentration, and a typical equation was:
m,
=
0 . 0 1 8 6 ~-~ 0.326
(10)
As explained previously, the concentration of carbon can be determined using only scatter intensity ratios. Thus, the correct' slope for the sulfur calibration equation can be calculated. Then the concentration of sulfur is given by :
xa = m,Rls - 0.012
(11)
Slight variations in the window t'hickness of the sample cells can cause slight variations in calibration constants when windows are changed. Thus, recalibration, which requires little time, was done every time the window was changed. If the sulfur-containing sample contains small amount's of oxygen and nitrogen, the sum of nitrogen, oxygen, and carbon concentrations will be obtained instead of the concentration of carbon. Since oxygen and nitrogen have absorption properties similar to those of carbon, the presence of small concentrations of theTe elements will not introduce significant errors for most purposes. However, recalibration or further correction would be required t o
Table VI.
Sample White mineral oil
Sulfur Determination
Sulfur, 76 Uncor- Corrected recteda
Present
0.511 0.511 0.510 0.990 0.990 0.991 0.305 0.305 0.30 0.206 0.206 0.204 0.115 0.115 0.106 Benzene 0.929 0.999 0.997 2,2,4-Trimethylpentane 1.053 1.020 1.013 p-Xylene 0.956 1,005 1.007 a Average of five determinations.
determine sulfur in high oxygen or nitrogen content samples. Some results for sulfur analyses with and without the correction are given in Table VI. The sulfur in mineral oil was determined using the direct-reading feature previously described. The importance of the correction is demonstrated. If it is desired to determine very low concentrations, the variation of the intercept of the sulfur calibration equation with carbon concentration should be considered. DETERMINATION OF TRACE METALS USING ASHING METHODS
Theory and Principles. Several recent ashing methods and sample mounting techniques (1, 12, I S ) have been described for preparing organic samples for x-ray analysis. Of the several methods, a modification of the benzene eulfonic acid ashing method was used. Benzenesulfonic acid has been reported to prevent losses of volatile metal compounds, such as porphyrin complexes ( I S ) . Toluenesulfonic acid (12) or sulfur (1) is also satiffactory for ashing. The use of an internal standard is desirable. Cobalt can be used as an internal standard (12, I S ) , although the samples must not contain significant quantities of cobalt. However, a scandium internal standard was prepared as described previously. Theoretically, it qhould be better than cobalt for vanadium analysis. However, unlesq cobalt is present in samples, cobalt would be theoretically the best standard for iron and nickel determination. I n this work approximately a 100-p.p.m. scandium solution was used. Experimental. Ashing usually was done in 50-ml. high-form Vycor crucibles. If these crucibles were not sufficiently large, a 300-ml. quartz evaporating dish was used. T h e crucibles were cleaned after each use by boiling in aqua regia. The weight of the simple depends on the type of sample and can range from a few milligrams for microanalysis in the high parts per million range to 100
Table VII.
Determination of Vanadium and Nickel in Organic Materials Using Ashing
Weight of Concentration metal, sample, p.p.m. Metal Sample grams X-ray Present V Petroleum distillate 100,O 0.0042 0.0042 9.312 0.178 0.188 0.3168 6.6 6.2 Crude oil 0.03099 56.9 56.8 0.02356 111.4 111.4 Ni Petroleum distillate 100.0 0,0086 0.0090 10.05 0,508 0.466 99.72 0.016 0,018 Crude oil 0.1345 39.8 40.8 V: V, pg. = 0.33090Q - 0.0159 Q (Rle - R14)/(R4 - R14Ia Xi: Ni, pg. = 8.446P - 0.321 p = (R9 - R14)/(R4 - Rl4) a
Metal present, rg. 0.420 1.749 0.210 1 ,760 2.625 0.896 4.683 1.79 5.488
R4 - Sc ratio.
grams for analysis in the extremely low parts per billion concentration range. One milliliter of internal standard solution and 5 ml. of melted benzene sulfonic acid were added to the sample in the crucible. Samples were a t first heated on a hot plate until low-boiling point materials escaped and charring began. The crucibles next were transferred to a higher temperature hot plate maintained a t approximately 300' C., and heating was continued until fumes no longer escaped. Samples can be ignited when placed on the second hot plate to speed the ashing process, but in some cases evaporation without ignition resulted in slightly better reproducibility. Samples next were placed in a muffle furnace controlled a t 550' k 2' C. The furnace was automatically turned off a t a predetermined time. The minimum time necessary for firing is best determined experimentally for various types of samples. .4 higher furnace temperature will result in much more rapid afhing, but reproducibility was best when the sample was fired at a relatively low, controlled temperature for a definite time. After samples were fired and cooled, 3 drops of high-purity hydrochloric acid and 1 drop of high-purity nitric acid were added to the crucible. A few drops of doubly distilled water were added, and the sample was evaporated nearly to dryness using a hot plate. A few more drops of water were added, and the sample was evaporated until only one or two drops remained. The sample then was transferred to dialysis film mounted on a sample cell. The acid treatment and sample transfer were repeated two times, using only on? drop of hydrochloric acid and one drop of nitric acid each time. If the samples are not evaporated nearly to dryness after addition of acid, excess acid may cause the dialysis film to crack on drying. The solution should be confined to the middle portion of the dialysis film of the sample holder and not allowed to touch the stainless steel ring holding the film, or contamination can occur. Approximately the same area of the dialysis film of the sample
holder should be covered with the sample each time, but exact control of sample area is not necessary. The prepared sample holders are transferred to a vacuum desiccator and thoroughly dried using high vacuum. Preparation of the dialysis film sample holders can be troublesome unless the correct techniques are used. The thickness of the dialysis film used was approximately 1.5 mils. Thinner film (about 0.8 mil) was tried. However, because this thinner film often cracked on drying and resulted in only a slightly lower background, the thicker film was used. A piece of the wet film is placed over the metal sample holder window ring, and a polyethylene ring is pressed over the metal ring to hold the film. The sample holder ring is dried in a vacuum d,esiccator. The film shrinks on drying and is stretched very tightly over the sample holder ring. The sample preparation method described perhaps appears complicated and time-consuming. However, many samples can be prepared at one time, so the total operator time required for each sample is not great. Calibration was based on terms of total micrograms of trace metal. A piece of untreated dialysis film was used as the arbitrary reference standard to give maximum sensitivity in the low concentration range. Lead liners for sample cells were used for nickel and iron analyses; without these liners, sensitivity was lowered drastically. Results and Discussion. VANADETERMINATION. Some typical results for vanadium are given in Table V I I . These results are the average of three determinations with a total of 10,000 counts taken for the arbitrary reference standard for each peak. Interference checks showed that a t least 300 p g . of nickel or iron can be present without introducing appreciable interferences. T h u f , appreciable interference should not occur for crude oil and distillate analysis. If it is desired to use oils containing brine, the brine should be removed by DIUM
VOL. 36, NO. 8, JULY 1964
158 1
ultracentrifugation or some other means before the samples are used. Otherwise, the large amount of solid in the brine may produce interferences in ashed samples. Calibration should be checked frequently when using this method-at least two times a week. Variations in the thickness of the dialysis film and other factors could result in considerable error if this is not done. SICKEL DETERMINATION.The technique for nickel determination is the same as for vanadium, except that the nickel intensity ratio must be measured. Some typical results for determination of nickel in petroleum and petroleum distillates are given in Table VII. Interferences usually are not encountered
for distillate and crude oil analyses. For some other types of samples, correction factors for other elements will have to be determined experimentally. Otherwise errors of several per cent of the amount present can result. LITERATURE CITED
(1) Agazzi, E. J., Burtner, D. C., Critt.en-
den, D. J., Patterson, D. R., ANAL. CHEM.35,332 (1963). (2) Dwiggins, C. W.,Jr., Zbid., 33, 67 (1961). (3) Dwiggins, C. W., Jr., “Applications of X-Ray Spectrography to Analysis of Organic Materials with Emphasis on Petroleum Products,” ACS Eastern Analytical Symposium, New York, N. Y., Nov. 15,1961. (4) Dwiggins, C. W.,Jr., U. S. Bur. Mines, Rept. Invest. 6039 (1962).
( 5 ) Dwiggins, C. W., Jr., Dunning, H. N., ANAL.CHEM.31. 1040 (1959). (6)Zbid., 32, 1137’(19603. ’ (7) Dwiggins, C. W., Jr., Lindley, J. R., Eccleston, B. H., Ibid., 31, 1928 (1959). (8)Hale, C. C., King, - W. H., Jr.. Ibid.. 33, 74 (1961). (9) Jones, R. A , , Ibid., 33, 71 (1961). (IO) Kang, C. C., Keel, E. W., Solomon, E.,Ibid., 32,221 (1960). (11) Miller, D. C.,Norelco Reptr. 4, 2 I,1- 9.57) - - . ,. (12) Rowe, W. A , , Yates, K. P., ANAL. CHEM.35,368(1963). (13) Shott, J. E.,Garland, T. J., Clark, R. 0..Ibid.. 33. 506 (1961).
RECEIVED for review February 25, 1964. Accepted April 16, 1964.
Mass Spectra of N-Substituted Ethyl Carbamates CARL P. LEWIS O h Research Center, O h Mathieson Chemical Corp., New Haven, Conn.
b The spectra of 25 N-substituted ethyl carbamates have been determined and correlated with structural features. Interactions between the N substituents and the carbamate grouping give rise to a number of intense, characteristic peaks. Fragmentation paths responsible for many of these peaks have been elucidated so that mass spectrometry may b e used for the identification of compounds of this type.
B
(3) has pointed out that the mass spectrometric fragmentation of an organic molecule depends upon the mutual interaction of all of the atoms within a molecule and not upon the fragmentation of an isolated functional group. Nevertheless, fragmentations characteristic of a functional group do occur, and these must be recognized before the modifying effects of the remainder of the molecule can be appreciated. This was particularly true in the case of carbamates (urethanes). Their spectra were too complex to permit the assumption of sample analogies. Not until the numerous rearrangements and fragmentations of this functional group had been characterized was it possible to carry out a satisfactory structural examination of such compounds. The characteristic fragmentations of N-substituted ethyl carbamates are presented below. Much of the IEMANN
1 Present address, The Johns Hopkins University, School of Medicine, 725 N. Wolfe Street, Baltimore, Md.
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ANALYTICAL CHEMISTRY
corroborative evidence for the suggested cleavage paths has been previously published in a detailed report describing the behavior of ethyl N-phenylcarbamate and ethyl N-ethylcarbamate upon (4). The electron bombardment volatility of carbamates, the small amount of sample required for an analysis, and the wealth of structural information contained in a spectrum make mass spectrometry an excellent analytical approach for carbamates as well as for compounds which can be easily converted into carbamates. EXPERIMENTAL
The compounds utilized in these studies were either prepared from ethyl chloroformate and the corresponding primary or secondary amine or purchased from Aldrich Chemical Co. Solids were purified by recrystallization from hexane. Liquids, including several of the purchased ones, were purified by vacuum distillation or gas chromatography. The following compounds are considered in the present report : ethyl N-methylcarbamate, Figure 1 (purchased) ethyl N-n-propylcarbamate, Figure 2 (purchased) ethyl N-n-butylcarbamate, Figure 3 (prepared) eth 1 N allylcarbamate, Figure 4 (purcEmed ) ethyl N-cyclohexylcarbamate, Figure 5 (purchased) ethyl N-benzylcarbamate, Figure 6 (prepared) ethyl N-p-tolylcarbamate, Figure 7 (prepared) ethyl N-o-tolylcarbamate, Figure 8 (prepared)
ethyl N-m-tolylcarbamate, Figure 9 (prepared) ethvl N-2,6-dimethylphenylcarbamate, Figure 10 (prepared) ethyl N-p-chlorophenylcarbamate, Figure 11 (purchased) ethyl N-p-nitrophenylcarbamate, Figure 12 (purchased) ethvl N-a-naphthylcarbamate, Figure 13 (prepared) ethyl N-a-naphthylcarbamate, Figure 14 (prepared) ethvl N,N-diphenylcarbamate, Fimre 15 (purchased) ethvl N-methvl-N-phenylcarbamate, Figure 16 (prepared) ethyl N-ethyl-N-phenylcarbamate, Figure 17 (prepared) .hi-n-butvl-AT-phenylcarbamate, ethyl Figure 18 (purchased) ethvl N-n-allyl-N-phenylcarbamate, Figure 19 (purchased) ethyl Y,Y-dimethylcarbamate, Figure 20 (prepared) ethyl N,N-diethylcarbamate, Figure 21 (purchased) ethyl N,N-dipropylcarbamate, Figure 22 (prepared) ethyl N-piperidinocarboxylate, Figure 23 (Durchased) ethyl IN-piperaxinocarboxylate, Figure 24 (purchased) ethyl morpholinocarbamate, Figure 25 (purchased) Deuteration experimentq, similar to those described in the earlier publication concerned with the fragmentation of ethyl N-phenylcarbamate and ethyl N-ethylcarbamate ( 4 ) , were performed, as necessary, to elucidate the composition of many ions. In these experiments, deuterium was exchanged for active protons by simply saturating the inlet y y t e m with D20 before the introduction of the undeuterated sample. An 80 to 90% conversion of N-H into N-D could be achieved in this way. The spectrum of the pure deuterated
