Examples of the quantitative separation of binary mixtures of several derivatives of aniline are presented in Table I. Solutes which have a poor separation factor and low distribution ratios are best separated using a long column, illustrated by the separation of 0- and m-methoxyaniline on a 60-cm. column. The elution curve for this separation is shown in Figure 4. The flow rate of the 60-cm. column was 1.3 ml./minute. The height equivalent to a theoretical plate was calculated for the 60-cm. column using parameters obtained from the experimental shape of the two solute peaks, and was found to be 0.45 cm. I n general, the flow rates for columns packed with 70- to 80-mesh support is high. The separation of binary mixtures of amines on 1.1- x 20-cm. columns was performed a t a flow rate of 2.0 ml./minute. The separation of 1.7 mg. of aniline from 300 mg. of N,N-dimethylaniline was attempted using a large 2.1- X 45cm. preparative-scale column. The column was operated a t a maximum flow rate of 5.0 ml./minute. The aniline peak emerged from the column
after 55 ml. of eluate had been collected. The elution of the aniline was complete after the collection of an additional 100 ml. A constant background of ultraviolet-absorbing material followed the aniline peak and prevented the quantitative analysis of the aniline. Experience with high solute ratios indicates that the separation of traces of solute from large quantities of wellextracted material using large columns can be recommended as a method of purification but not high accuracy quantitative analysis. When solute peaks are only partially separated, quantitative analysis by collection of fractions is not useful because of mutual contamination of the fractions. A satisfactory correlation was found between the peak height recorded on the chart record and the mole fraction of the solute in the mixture. This correlation was successfully applied in several separations. The results of these experiments are presented in Table 11. The separation of small quantities of 4-picoline from ten times as much N-methylaniline shows that the method is useful for heterocyclic
aromatic amines as well as the homologous series of aniline derivatives. If the peaks are completely resolved, the relative error of the mole fraction of each solute is approximately 5%. I t should be noted that this system is used in conjunction with an ultraviolet flow monitor which senses a very wide wavelength range. If the wavelength range is small, as would be the case if a n ultraviolet monochromator were used, differences in the absorption spectra of different solutes would be expected to give different instrument responses. LITERATURE CITED
(1) Amin, El S., J . Chem. SOC.1959, 1619. (2) Clayton, R. A., Strong, F. AI., ANAL. CHEM.26, 579 (1954). ( 3 ) Fritz, J. S.,Hedrick, C. E., Zbid., 37, 1015 (1965). ( 4 ) Roberts, J. C., Selby, K., J . Chem. SOC.1949, 2785. C. E. HEDRICK Department of Chemistry University of Pennsylvania Philadelphia, Pa. 19104 WORKsupported by the Department of Che,mistry of the University of Pennsylvania. Presented at 149th Meeting, ACS, Detroit, Mich., April 1965.
A Study of Spectrochemical Detection Limits of Selected Elements in Tungsten SIR: Tungsten is one of the purest metals commercially available, and its physical properties are uniquely affected by trace impurities ( 5 ) . For this reason better detection and estimation of impurity levels remain a constant challenge to the analyst. The complexity of the tungsten emission spectrum with its concomitant high probability of interference is well known. It has been shown, however, that the tungsten spectrum can be suppressed and impurity elements enhanced by intimately mixing tungsten powder with graphite prior to arcing ( 1 , 2 ) . The technique of forming tungsten carbide in situ and selectively distilling out impurities has been explored ( 1 , 2, 6) and used to advantage. The purpose of this work was to broaden the potential of this approach and to obtain data comparing the technique to other methods. The behavior of 33 elements as trace impurities in tungsten was examined.
Electrodes: 11/4T X 1/4-inch coneshaped SPK graphite electrodes with craters inch deep and inch in diameter as sample-bearing electrodes, and 11/4- X l/r-inch cone-shaped counter electrodes. The electrodes were shaped to these specifications and purified after shaping by Union Carbide Corp. Preparation of Standards. Pure tungsten powder with a n average particle size of approximately 2 mi-
Equipment. Spectrograph: Bausch and Lomb Dual Grating with standard preslit optics. Source: Direct current, 240 volts. Plates: Eastman Type 33 and Eastman T y p e I S . 1046
ANALYTICAL CHEMISTRY
Table I. Preparation of Standards Elements added Form of addition Dilute hydrochloric acid solution All Ca, Cr, Cu, Fe, Ki, M g Si Aqueous potassium silicate solution Potassium meta arsenites Aqueous boric acid solution Aqueous nitrate solution Cadmium pyrophosphate" Antimony trioxide0 Dilute hydrochloric acid solution Aqueous nitrate solution Beryllium oxide. Cadmium sulfiden Dilute hydrochloric acid solution Gallium sesquioxidea Ga Germanium dioxidea Ge Zinc sulfiden Zn 4 110, Nb, Ta, Ti, Z, Zr Pure oxidesa Aqueous chloride solutions 5 Ba, K, Na, Sr Fine enough to pass a No. 400 sieve.
Group 1
EXPERIMENTAL
a
crons and synthetic standards were prepared as described by Dyck and Veleker (1). Five groups of standards were prepared in order to minimize interelement effects. The final reduction temperature for each group was regulated to preclude loss of volatile impurities. Some elements were added as very finely divided compounds. The procedure was to prepare a master standard for each group and make lower standards by dry dilution with
tion limits for those elements whose main lines are interfered with by tungsten. An example of such an element is silicon. The two most intense lines, 2881.58 A and 2516.12 A ( 7 ) , have tungsten interferences, although neither is listed in the M I T tables. Laun (3) lists tungsten spark lines a t 2881.54 A and 2516.14 A, which apparently count for the interferences. The latter of these is a weak line and can probably be eliminated by graphite dilution. With a detection limit of 1 p.p,m. and
\or.-mo\ 100 0-I5
15-30
30-43 43-60
TIME
-
60.75
Table II. Instrumental Conditions Source: D.C. Current, amp: 6-7 gap) mm.:
~ order),~ (2)270O-370Oa ~2c’~l)?&{~-2~800. (second ~ (second e ’ order),
(3) 3700-4700 (first order), (4) 57007700 (first order). Reciprocal linear dispersion, A/mm. = 2,(2) 4,(3)4,and (4)8. Slit(1)width, p : 20 Primary aperture, mm.: 1.0 a Photographed simultaneously.
N O -DIRECT
73-W
Table 111.
90-105 1)3-120
SECONDS
Figure 1. Moving plate study of Ag, Cd, and N a
Impurity
2
pure blank powder. Table I shows how the elements were grouped and in what form the impurities were added. Procedure. Each standard was diluted with graphite in the ratio of 1.0 gram of tungsten to 200 mg. of SP-2 graphite, and with high purity zinc oxide in t h e same ratio. Mixing was done in 1-inch plastic vials using a Spex Industries Wig-L-Bug. The mixtures and a n undiluted sample of each standard were tamped into the electrode craters. All samples were arced under the conditions listed in Table 11. Since the continuum was much more intense for zinc oxide diluted and undiluted samples, it was necessary to place neutral filters of various densities before the slit such that equivalent background levels were obtained. Selective filters were used to avoid overlapping orders. Residues remaining in electrode craters after arcing were removed and examined by DebyeScherrer x-ray patterns. The criterion for detection limits was arbitrarily chosen t o be a line intensity at least 10% transmittance units darker than background, which was held within the range of 90-95y0 transmittance. Moving plate studies were carried out in the conventional manner using intensities of selected arc lines a t 15second consecutive intervals. The same line of an element was used for each method of arcing. Moving plate data were recorded for representative elements from each group of the periodic table.
As B
Ba Be Bi Ca Cd co Cr cu Fe Ga Ge
In
K Mg Mo Na Ni Nb P Pb
Sb Si Sn Sr Ta Ti
V Zr Zn
RESULTS AND DISCUSSION
Table I11 lists comparative detection limits. The marked difference in detection limits is the most significant advantage gained from adding graphite before arcing. I t enables the spectrographer to achieve good detec-
Wavelength, A 3280 68 3092 71 3961 53 2860 44 2496 78 2497 73 4554 03 3131 07 2348 61 3067 72 2897 98 3933 67 4226 73 3261 06 3466 20 3453 50 2843 25 4254 35 3273 96 2599 40 2943 64 4172 06 2651 18 3256 09 4511 31 7664 91 7698 98 2795 53 3132 59 3902 96 5889 95 3002 49 4058 94 2553 28 2833 06 2614 18 2598 05 2877 92 2506 90 2863 33 4607 33 2714 67 3349 41 3088 02 3093 11 3391 98 3302 59
Comparative Detection Limits Detection limits, p.p.m.
Graphitediluted 0.03 0.09 b
30 2 0.8 0.5 0.02 0.2 0.7 6
