Mair, Glasgow, and Rossini (6) using 4930 cal./mole for the heat of fusion. These results, along &.ith the initial freezing points, give the melting Point of pure hexamethylbenzene as 165.75 f 0.02O c. The uncertainty arises mainly from discrepancies in the initial freezing points (column 2, Table I) and to some extent from the uncertainty in the total time required for complete crystallization, from which the fraction frozen at any time was calculated.
(7) Moessen, G. W., Ph.D. thesis, The
LITERATURE CITED
( 1 ) Frankosky, M.9 Aston, J. G.9 J. PhYs. Chem. 69, in press. (2) Herrington, E. F. G., Handley, R., Cook, A. J., Chem. Ind. (London) 1956, p. 292. (3) How, H. J., Brickwedde, F. G., J. Res. Natl. Bur.+Std. 28, 217 (1942). (4) ~ ~ fH, H,, f parks, ~ ~G , s,, ~ ~ , ~ ~ A. C., J.Am. Chem. Sac. 52,1547 (1930). ( 5 ) MacDougall, F. H., Smith, L. 1.) Ibid., p. 1998. (6) Mair, B. 'J., Glasgow, A. R., Jr., Rossini, F. D., J. Natl. Bur. Standards 26,591 (1941).
Pennsylvania State University, University Park, Pa., 1955. (8) Seki, S., Chihara, H., Coll. Papers Faculty Sci., Osaka Univ., Ser. C, Chem. 11, NO. 1, 1-8 (1943-9). (9) Spaght, hl. E., Thomas, S.B., Parks, G. S., J. Phys. Chem. 36, 882 (1932). J. E. OVERBERGER i ~ l ~ , J. G. ASTON Department of Chemistry The Pennsylvania State University University Park, Pa. WORKaided by a grant from the National Science Foundation.
Factors Affecting the Selection ,of a Cobalt Analysis Line for Atomic Absorption S pectro met ry SIR: Information available in the literature concerning cobalt investigations by atomic absorption spectrometry has been generally limited to the evaluation of detection limits and analytical methods. Allan (1, b), Gatehouse and Willis (6),and McPherson, et al. (6) indicate the 2407.2 A. line as the most sensitive analysis line. Fuwa and Vallee (4) established very low detection limits for cobalt using the 2424.9 A. line. Robinson (9) listed detection limits for three relatively low absorbing lines, the strongest of which was 3529.0 A. in a n oxycyanogen flame. Menzies (8) found the 3533.4 A. line best suited to his apparatus. A study of several cobalt absorption lines indicates the proper choice of analysis line to be influenced by experimental parameters. The factors controlling such selection are discussed with respect to atomic absorption variables.
slot atomizer-burner (Perkin-Elmer Corp., Norwalk, Conn.) assembly was operated at the following conditions: 2.2 liters/minute acetylene, 7.1 liters/minute .*.atomizer air, and 10.9 liters/minute .auxiliary air. The sample uptake rate was 7.0 ml./minute. A D.C. solenoid shutter system mounted on the adjustable 'slit in front of the tube allowed the hollow cathode radiation to be cut off. An H T V R106 photomultiplier tube was used as the detector in the Model 139 photomultiplier attachment. The readout device was the direct reading meter on the spectrophotometer. A Heath EUW20'4 potentiometric recorder was also used as a readout for some measurements. RESULTS A N D DISCUSSION
Cobalt exhibits a line rich spectrum in both emission and absorption. Table I shows the relative absorption of both ground state and non-ground-state transition lines of cobalt (7) for a 50-
p.p.m. solution. There are eight lines which show a n absorption greater than 25% and could prove reasonably useful in atomic absorption cobalt determinations in the range of 0-100 p.p.m. The 2424.9 A., 2521.4 A., 2411.6 A, and 2407.2 A. lines would be suitable for analyses at the low p.p.m. range. The complexity of the cobalt hollow cathode emission spectrum necessitates the use of narrow slits to avoid the inclusion of radiation other than the analysis line. The absorption values shown were determined under conditions which do not represent maximum sensitivity and could be improved if necessary for specific determinations. Working curves were prepared for several cobalt absorption lines to determine their linearity over a particular concentration range. Figure 1 shows working curves run at a hollow cathode tube current of 25 ma. Most of the lines show a pronounced curvature
EXPERIMENTAL
Reagents. T h e cobalt stock solution was prepared from reagent grade CoC12.6H20 dissolved in distilled water. Working concentrations (0100 p.p.m.) were obtained by appropriate dilutions. Apparatus. The atomic absorption unit was a D.C., single beam system assembled from a Hitachi-PerkinElmer Model 139 spectrophotometer and a n optical bench containing the atomizer-burner, hollow cathode source, and associated equipment. The Westinghouse WL 22814 cobalt hollow cathode tube was powered by a Kepco Model Al3C 42511 0.05% regulated I1.C. poner supply (Kepco, Inc., Flushing, K . Y.), operated in the constant current configuration (Kepco Instruction Manual, &lode1 ABC 425M). X small beam, constricted by adjustable slits on each side of the burner, as collimated prior to passage through the flame and then focused on the monochromator entrance slit with bi-convex quartz lenses. A 10-cm. 1 168
a
ANALYTICAL CHEMISTRY
Table I.
Wavelength
% Absorption
4234.0 4190.7 3909.9 3526.8 3474.0 3465.8 3412.6 3121.4
ND ND
A.
Absorption of Selected Cobalt Lines
% Wavelength Absorption A. A. Ground state transitions ND 2424.9 3082.6 Wavelength A.
3044.0 3013.6 2989.6 2987.2 2928.8 2521.4 2435,8
ND
9.5 4.2 7.0 10.3
ND
Absorption %
2407,2 2384.9 2365.1 2309.0 2295.2 2274,5 2174,6
17.5
ND
5.2 3.4
ND
68.6 27.3
75.2 58.1
ND
ND 4.5 4.0 2.2
ND
B. Non-ground state transitions 3.8
3453.5 3442.9 3.0 3431,6 5.2 3405,l 4.2 2536.0 3.5 2529.0 Cobalt solution = 50 p.p.m.
3575 3533 3529. 3513, 3506 3502
ND
ND
=
not detectable
10.0 3.6 4.2 6.0 16.0 26.4
2439.0 2432.2 2419.1 2415.3 2411.6
Slit width = 0.10 mm. HCT current = 18 ma.
12.6 42.2 1.5 28.8 59.6
.80.70 -
.30 -
.60 -
A
50 -
\
24022A 2424.9 A
A -
.40 -
.20-
30 -
20
\ I
/
2411 6 A
2432:2 A
0
10
20
30
40
5 0 60 [c p prn
70
80
90
100
0.
01
Figure 1.
Cobalt working curves
Hollow cathode tube current = 25 ma.
past 50 p.p.m. I n the case of the 2407.2 A. line, the severe deviation from linearity arises from the presence of a n adjacent interfering line which is too close to allow resolution with the monochromator employed in this study. David (3) also mentions interference from this line. Reduction of the slit width to 0.022 mm., which corresponds to a spectral band pass of approximately 0.5 A., fails to provide sufficient resolution to correct the nonlinearity of the working curve. The 2424.9 A. and the 2521.4 A. lines are of comparable sensitivity, but the latter shows somewhat greater linearity. The 3044.0 A. line, though of low absorption strength, produces a nearly linear working curve which might be suitable for those cobalt determinations in which high sensitivity is not a major requirement. Figure 2 shows the change in absorbance produced by varying the hollow cathode tube current from 10 to 30 ma. Four of these lines show a relatively small increase in absorbance as the tube current is decreased and the emission line width becomes more narrow. The sensitivity of the 2407.2 A. line, however, is considerably altered, particularly in the region from 10 to 20 ma. This is possibly due to a n increase in Z A L / Z I L with a decrease in current, where Z A L is the intensity of the 2407.2 A. analysis line and Z I L is the intensity of the unresolved interference line, possibly the
2407.7 A. cobalt line. Absorption measurements taken with a 500-l1.p.m. cobalt solution, a concentration sufficient to absorb essentially all of the 2407.2 A. radiation, show 550/, of the incident radiation unabsorbed a t 30 ma., while only 19% remains unabsorbed at 10 ma. The most sensitive analysis would suggest the use of the 2407.2 A. line at a very low current, narrow slit width, and a n analysis concentration of less than 50 p.p.m. However, since cobalt does not show particularly strong hollow cathode emission at such low currents and slit width, the line emission intensity from the hollow cathode tube may be so low as to make stable and reproducible measurements difficult to obtain. It may be advantageous to increase the tube current to a point where the signal-to-noise ratio is more favorable, if maximum sensitivity is not required. At these higher currents (above 15 ma.) the sensitivity of the 2407.2 A,. line is less than that of certain other lines (Figure 2). Furthermore, since the sensitivities of these lines are ‘not as dependent on tube current, the emission intensity from the hollow cathode tube may be increased with the sacrifice of only a small amount of absorption. The greater linearity of the 2521.4 A. and 3044.0 A. lines may be of considerable importance if the concentration of the cobalt analysis
20 p.p.rn. cobalt
solutions may vary over a range such as 10-100 p.p.m. The sharp curvature of the working curves of several of the analysis lines would make determinations in the range above 50 p.p.m. quite difficult. The 3044.0 A. line appears to be very suitable for analyses where high sensitivity is not required. However, this emission line is rather weak from the hollow cathode tube. There is also more flame background around this line which would require compensation in D.C. systems and which might introduce photomultiplier noise even in modulated instruments. LITERATURE CITED
(1) Allan, J. E., Nature 187, 1110 (1960). (2) Allan, J. E., Spectrochim. A c t a 18, 259 (1962). (3) David, D. J., Ibid., 20, 1185 (1964). (4) Fuwa, K., J’allee, B. L., ANAL. CHEM.35, 942 (1963).
(5) Gatehoiise, B. M., Willis, J. B., Spectrochim. A c t a 17,710 (1961). (6) X!cPherson, G. L., Price, J. W., Scaife, P. H., Nature 199, 371 (1963). (7) SIeggers, W. F., Corliss, C. H., Scribner, B. F., “Tables of Spectral
Line Intensities,” National Bureau of Standards hlonograph 32, Part I, 1961. (8) Menzies, A. C., ANAL. CHEM. 32, 898 (1960). (9) Robinson, J. W., Ibid., (1961).
33, 1067
W. W. HARRISON
Department of Chemistry University of Virginia Charlotteeville, Va.
VOL 37, NO. 9, AUGUST 1965
1169
