ANALYTICAL CHEMISTRY, VOL. 50, NO. 11, SEPTEMBER 1978
0
2
4
bDN + s T T f
Figure 2. Transition energy (AETR)equivalent of the Soret shift for ZnTPP vs. the summation of donor strength and donor polarity terms (Units are kcal/mol ) Numbered points correspond to the donors listed in Table I
be justified within the reliability of the present numerical data base in DN and A * . T h e two parameter model in Equation 3 gives a more satisfactory correlation to the Soret red shift than any single parameter function proposed to date, since the correlation coefficient for t h e line in Figure 2 is 0.986 for all 12 donors. Also, it is instructive to compare the relative importance of each of the terms for those donors which are outlying on the AH,vs. Ah,= graph (12) and in Figure 1. For such stronger Lewis bases as hexamethylphosphoramide, triethylamine, and pyridine, t h e bBd term dominates (bDN > S T * ) , in contrast to the condition for most of the remaining donors in which t h e basicity and dipolar contributions t o t h e bathochromic shift are similar in magnitude. Benzene represents the reverse extrema in which bDN
9 z
P
300
400
500
600
EMISSION WAVELENGTH, nm
Figure 1. Fluorogram of 7-hydroxybenzo[alpyrene (0.2 ng/mL), adapted from ref. 2. Curves A and B are the conventional excitation and emission spectra, respectively. Curve C is the spectrum obtained by a synchronous scan with AA = 50 nrn. Curve C is a plot of the emission intensities vs. ,A, measured along the dashed diagonal line. The contours are 10% increments of the maximum emission intensity. Curve A is the profile section of the three-dimensional surface taken at ,A, = 440 nm. Curve B is a similar profile section taken at A, = 390 nm. Curve C is the profile section taken along the dashed line and projected onto the emission wavelength axis
would be generated by a synchronous scan where AA = ,A, - ,X , = (440 - 390) = 50 nm, the wavelength difference between the maxima of the conventional excitation and emission spectra. The spectrum labeled C is the synchronous spectrum obtained by plotting the variation of emission intensity along the diagonal dashed line vs. the emission wavelength. A synchronous measurement made according to the method of Vo-Dinh ( 1 ) will give spectrum C if AA is 50 nm.
Vo-Dinh’s experimental arrangement can produce the three-dimensional contour plot of Rho and Stuart by making repeated scans where AA is changed in fixed increments. Rho and Stuart‘s method can give Vo-Dinh’s results by the technique described above. Vo-Dinh’s experiment is simpler and is considerably faster if only a single scan is necessary, but it discards much information that is retained in the three-dimensional plot. T h e main difficulty with the synchronous scan method is that the best value for AA must be known beforehand for optimum results. Also, in some multicomponent systems, several different values of AA might be necessary for complete identification. The best values for AA can be determined easily from a three-dimensional plot, such as Figure 1. A threedimensional plot also shows whether it would be possible, in a synchronous scan, to suppress some components and observe only selected components of mixture. On the other hand, the three-dimensional plotting method, by itself, will often be more complicated than necessary for specific analytical purposes and it requires interpolation between contours for quantitative results. I t appears that the simplest adaptation of these two methods would serve many analytical applications. This would be to use the synchronous scan method, but by first generating a three-dimensional plot to completely characterize the sample spectrum. We have found t h a t a crude threedimensional plot can be generated in 2 or 3 h by hand plotting, if we wish only to locate the “mountain peak” positions without quantitative details of the contour shapes. In complicated systems, these crude plots have been completely adequate for determining the most useful values of AX for subsequently more rapid and quantitative analysis. LITERATURE CITED (1) T. Vo-Dinh, Anal. Chem., 5 0 , 396 (1978). ( 2 ) J. H. Rho and J. L. Stuart, Anal. Chem., 5 0 , 620 (1978)
E u g e n e R. Weiner Department of Chemistry University of Denver Denver, Colorado 80208 RECEIVED for review May 1, 1978. Accepted J u n e 22, 1978.
Comments on Determination of Fluorine in Petroleum and Petroleum Process Catalysts with a Fluoride Electrode Sir: In 1973 we published a paper describing the determination of fluorine in petroleum and related materials which included analytical results for the fluorine contents of ten crude oils indicating levels, with one exception. substantially below 1 mg/kg ( 2 ) . Subsequently, data from a cooperative exercise presented by other workers ( 2 ) suggested that even the low levels we had measured might be erroneously high. T h e toxic effects of fluorine and its compounds can be very severe, and we considered it important to resolve this difference. In outline, our procedure consists of C-F bond cleavage under nonaqueous conditions with sodium biphenyl. followed by aqueous extraction of NaF and measurement of fluoride ion concentration with a solid state fluoride electrode. Because of the strong tendency of crude oils and residues to form stable emulsions during the extraction process, the size of sample which could be taken was limited to a maximum of 2 g. 0003-2700/78/0350-1584$01 .OO/O
However, we have found that addition of a cationic detergent such as methyl-tricaprylyl ammonium chloride. (Aliquat 336, General Mills Chemicals Inc., Des Plaines, Ill.), immediately before extraction prevents emulsion formation. This not only gives a cleaner separation but also permits the use of much larger samples (in practice 20-25 g.). The crude oils which we originally examined were Alaskan (North Slope), Arabian Light and Heavy, Iranian Light and Heavy, Iraq (Basra) and Iraq (Med), Kuwait, Libyan (Sarir), and Nigerian (Forcados). Re-examination of these materials by the modified procedure has established fluorine levels for these crude oils to be below 0.01 mg/kg, and probably below 0.005 mg/kg. These figures are in complete agreement with the results of the cooperative program (2). We have concluded that our original results were high because of interference with the electrode response by traces of entrained organic material. These new findings do not affect results reported in our 8 1978
American Chemical Society
