as the mercaptan. It will be noted that precision is within 3% of the mean value.

DISCUSSION

The present column system only partially resolves sec-butyl and isobutyl mercaptan. If both are present in the sample mixture in about the same con- centration, identification is unequivocal. However, if only one peak is obtained, identification is difficult. Firm identifi- cation would require addition of one of the isomers to the sample and a second analysis. This type of problem would be encountered more frequently with higher molecular weight mercaptans because of the larger number of isomers.

Most GLC methods require calibra- tion with known mixtures. The prepa- ration of gas mixtures of reactive sub- stances-i.e., mercaptans-in the low p.p.m. range is a time consuming and

difficult task. In coulometry, which in this case is an absolute method of determination, no calibration standards are required and the quantitative interpretation of results is greatly simplified. In addition, it also allows the method to be readily applied quantitatively to mercaptans not pre- viously determined.

The coulometer was found to be 100% efficient with a sample of nonvolatile halide. However, one possible source of error might occur in the analysis of large amounts of volatile materials which could escape from the acidic titration solvent before reacting with silver ion. Slower column flow rates or smaller samples should be used if peaks with flat tops occur in the chromato- gram. Such peaks indicate that silver ion generation is not keeping up with the rate of emergence of the volatile mer- captan.

LITERATURE CITED

(1) Amberg, C. H., Can. J. Chem. 36,590 f 1958).

(21 Carson, J. R., Weston, W. J., Ralls,

(3) Coulson, D. M., Cavanaugh, L. A.,

(4) Karchmer, J. H., Zbid., 31, 1377

J. W., Nature 186,801 (1960).

ANAL. CHEM. 32,1245 (1960).

f19.59'). (5j-~ii&, P. J., Ibid., 33,1851 (1961). (6) Liberti, A., Cartoni, G. P., Chim.

( 7 ) Ryce, S. A., Bryce, W. A., ANAL. I n d . ( M i l a n ) 39, (10) 821 (1957).

CHEM. 29.925 (1957). (8) Spencer; C. F., Baumann, F., John-

(9) Sporek, K. F., Danyi, M. D., Zbid.,

(10) Sullivan. J. H.. Walsh. J. T.. Merritt.

son, J. F., Ibid. , 30, 1473 (1958).

35,956 (1963).

~ C.,Zbid. , 31, 1826(1959).' ' (11) Sunner, S., Karrman, K. H., Sunden,

V., Mikrochim. Acta 1144 (1956). (la) Tamele, hl. W., Ryland, L. B.,

IND. ESG. CHEM., ANAL. ED. 8, 16 (1936).

RECEIVED for review August 14, 1963. Accepted October 22, 1963.

Quantitative Determination of Chromium(II1) Hexa flu o roace ty lace tona te by Gas Chroma tog ra p hy

WILLIAM D. ROSS and GUTHRIE WHEELER, Jr.

Dayton laboratory, Monsanto Research Corp., Dayton, Ohio

b A gas chromatographic method is reported for determination of Cr(lll) hexafluoroacetylacetonate [Cr(hfa),] through a range of to gram per ml. This high sensitivity is achieved through use of an electron capture detector. The method involves passage of Cr(hfa)3 in toluene through an 1 1 -foot X '/8-inch stainless steel column packed with 2070 Dow Corning Silicone Fluid 710R at 90' C. Cali- bration curves were prepared using the internal standard method.

HE extension of gas chromatog- T raphy to the analysis of inorganic and organometallic compounds is of increasingly widespread interest. I ts application to metal acetylacetonates has been reported by Biermann and Gesser ( I ) , Brandt and Heveran ( 2 ) , and Floutz (4) . More recently, it has been shown by Moshier et al. (10) and Sievers et al. (13-16) that substitution of fluorine in the ligands significantly increases the vapor pressure of metal acetylacetonates. As a result, the fluo- rinated derivatives can be chromato- graphed a t much lower column and injection port temperatures than the parent compounds. This discovery broadens the spectrum of metal chelates separable by gas chromatography.

Lovelock has described the use of

the electron capture detector on halo- genated compounds (8,9). The electron capture detector is remarkably sensitive to fluorine-containing chelates (12). In our laboratory we have used this to advantage in detecting microquantities of the fluorinated analogs of Al(II1) and Cr(II1) acetylacetonates-e.g., 3.3 X

gram for Cr(II1) hexafluoro- acetylacetonate [Cr (hfaja]. The lower limit of detectability for Cr in solutions of Cr(hfa)3 was found to be ca. 2 X

Extending our earlier qualitative work, we can now report a method for quanti- tative determination of Cr(hfa)a in con- centrations in the range of 10-8 t o gram per ml.

gram.

EXPERIMENTAL

Instrumentation. A Barber-Colman Model 20 gas chromatograph equipped with an ionization detector (diode type) cell, Model A-4150, was used. Electron capture was achieved by reducing the cell potential, using the variable autotransformer of the power supply to control the primary voltage. The cell potential was reduced to less than 200 volts. The only modification needed was one to provide a range change in the existing cell potential voltmeter. This modification permitted more accurate voltage readings in the 0- to 200-volt range.

Description of Method. The in- ternal standard method as described by Ray (11) and Dal X'ogare and Juvet (3) was used as the basis of this quanti- tative analysis. This method eliminates most of the apparatus variables by using peak height ratios referred to an internal standard.

The calibration is performed by the addition of a constant amount of the standard to a specified volume of several synthetic mixtures containing known amounts of the material under investigation. The calibration curve is obtained by plotting grams per milliliter of solute in the original synthetic mixtures against the ratio of the solute: standard peak heights. The actual analysis is performed by adding the same amount of internal standard to the same volume of known mixture as was determined during the calibration procedure. Ray (11) re- ports an analysis with results obtained within 0.1% actual composition of a three-component mixture by the inter- nal standard technique.

We have determined Cr(hfa)3 over the concentration range 5.2 X 10-€ to 1.3 x 10-3 gram per ml. of Cr(hfa)s. To cover this range requires two calibra- tion curves for two reasons: To have the internal standard: sample ratio as near unity as possible (5), and to avoid

266 ANALYTICAL CHEMISTRY

06

10-8 10-5 10-4

C o n c n , g./ml.

Figure 1 . gram per ml. of Cr(hf'a)a

Calibration curve of concentration range 5.2 X 1 0-8 to 1.3 X 1 0-5

the inconvenience and possibility of error if the signal is attenuated to keep the peaks on scale. Therefore, two curves are plotted with overlapping points. The range of cwrve 1 (Figure I) extends from 5.2 X 10-* to 1.3 X gram per ml. Curve 2 (Figure 2) covers the range 1.3 x t o 1.3 x gram per ml.

Standard Samplls Preparation. Chloroform was seltbcted as an in- ternal standard because i t fulfills the requirements specified (3) and contains relatively few electron-capturing im- purities that might interfere with the detection of Cr(hfa) I. Since chloro- form has high electroi affinity, to ob- tain calibration curve 1 (Figure 1) the chloroform internal standard was diluted to 6.4 X 10-6 ml. per ml. of toluene, t o have its eluted peak a t a desirable height. The solven: (toluene) was selected because it was eluted at a dif- ferent retention time than either the chelate or the internal standard, and i t contained relatively low concentration of electron-capturing impurities.

The standard soluticns were prepared by weighing 0.0065 gram of Cr(hfa)a into a 5.0-ml. volunietric flask and making up to volume a i t h reagent grade toluene (Baker and Adams, Lot T165) which contained relatively little elec- tron-capturing impurities. From this solution a 1-ml. aliquot was diluted 1 to 25, producing a concentration of 5.2 X gram per nil. This parent solution was then dilutcd so as to obtain solutions of 11 separate concentrations. Measured volumes of these 11 solutions were diluted in 5 ml. of toluene. The internal standard soliition was then added to these standards in a ratio of 2 to 3. The standard curves were prepared by plotting Concentration vs. the ratio of Cr(hfa)3 per peak heights of internal standard on calibration curve 1 (Figure 1).

For plotting calibration curve 2, a concentration of 3.2 X 10-5 ml. per ml. of CHCL in toluene was used as an internal standard.

A parent solution of 1.3 X 10-3 gram per ml. was made and diluted by the same technique as for the solutions used on calibration curve 1 for con- centrations of 1.3 X through 2.6 X lou4 gram per ml. For the re- maining six points, the samples were weighed and diluted independently. Instrument Operating Conditions.

The operating conditions were as follows:

Eluent

Column

Column packing

Column temp. Flash heater temp. Detector temp. Column pressure Flow rate Split flow

Scavenge flow Electrometer

setting Detector cell

voltage Sample size

Prepurified Airco ni- trogen with an in- dicated 4-p.p.m. 0 2 contaminant

11-foot X '/*-inch stainless steel

20% Dow Corning Silicone Fluid 710R by weight on Gas Chrom Z (Ap- plied Science) 60- 80 mesh

goo c. 170" C. 200" c. 40 p.s.i.g. 67 ml./min. Curve'l, 75 ml./min. Curve 2, 300

ml./min. 200 ml./min. Curve 1, 1000 gain Curve 2, 300 gain Curve 1, 12 volts Curve 2, 17 volts 2 .0 rl.

RESULTS AND DISCUSSION

Several conventional methods of quantitative analysis for chromium or chromium-containing compounds are described in the literature-e.g., 0.01 p.p.m. of Cr can be detected by the s-diphenylcarbazide test (7) which ap- pears to be the most sensitive test for chromium. Chromium in the same order of magnitude can be detected by emission spectrography: 0.01 p.p.m. of Cr was detected in heavy water samples which were concentrated on the elec- trode (17'). Koch (6) reports 1 p.p.m. of Cr detectable in aluminum and Wood- ruff (18) reports 10 p.p.m. of Cr in alloy steel by emission spectrography.

Our gas chromatogra+phic method with electron capture detector for the

Concn., q . l m l .

Figure 2. gram per ml. of Cr(hfa)a

Calibration curve of concentration range 1.3 X to 1.3 X

VOL. 36, NO. 2, FEBRUARY 1964 267

quantitative determination of chromium as Cr(hfa), covers a total concentration range of 0.004 to 100 p.p.m.

Reproducibility of Calibration Curve. Figure 3 shows two calibra- tion curves obtained on different days from the same standard samples in the range 5.2 X IO-* to 1.3 X gram per ml. Curve A was prepared from fresh solutions, curve B from solution that had aged 3 days.

The shift in the curve is insignificant considering the extremely low con- centrations involved. For greatest precision the calibration curve should be prepared close to the time of the analysis of the unknown sample.

Attempts were made to prepare the calibration curves of Figure 3 without changing the instrument conditions. During the 3-day interim, a slight de- crease in the line voltage decreased detector sensitivity for the higher con- centration standards. This explains the shift in the slope of curve B. -4 determinate error has been found in

the reaction of Cr(hfa)l with the glass vessel used in dilution and storage. During a week’s storage of the standard solutions in 2-dram vials with poly- ethylene-lined caps, the concentration of Cr(hfa)r decreased. I n contrast, the loss of Cr(hfa)3 when contained in borosilicate glass volumetric flasks for the same period of time was negligible and no difficulties were encountered.

Analysis of Unknown. Four un- known samples were prepared by an independent worker and analyzed. The da ta obtained are indicated in Table I.

0- Curve A-Run 6 / 2 1 163

A - C u r v t E - R u n 6 /24/63

Table 1. Application of Method to Unknown Samples Cr(hfa), concentration, g./ml. Actual Found

Deviation - Total, g./ml. 70

. . . . . . 4 . 0 x lo-‘ 2 . 1 x 10-6 2 . 1 x 10-6 . . . 2 . 0 x 10-7 2 . 1 x 10-7 0 . 1 x‘ io-7 5 . 0 7 . 6 x 8 . 6 X 1 . 0 x 10-8 13.2

4 . 0 X lo-’’

Extension of Range. The presence of electron-capturing impurities in the solvents is the limiting factor with respect to detection of smaller concentrations of Cr(hfa)r. We believe that removal of these impurities would permit even lower concentrations of Cr(hfa), to be detected.

At the upper end of the curve the limiting factor is the saturation of the electron capture detector. However, this limitation can be eliminated by further dilution of samples, and/or by using the sample splitter to introduce much smaller samples.

CONCLUSION

Ultimate application of this method to the determination of chromium in ores and alloys is dependent, on develop- ment of a method for the quantitative extraction of chromium as chromium (111) hexafluoroacetylacetonate. That this might be possible is indicated by the work of Heveran and Brandt (d) , who have shown that the quantitative extraction of Cr from steel is possible using the related ligand, acetylacetone

P a”

Investigations are currently under way to extend this technique to other metals and to mixtures of metals.

ACKNOWLEDGMENT

The authors acknowledge the many helpful discussions and samples con- tributed by R. E. Sievers. The techni- cal advice of J. V. Pustinger, Jr., is also appreciated.

LITERATURE CITED

( I ) Biermann, W. J., Gesser, H., ANAL. CHEW 32, 1525 (1960).

(2) Brandt, W. W., Heveran, J. E., Division of Analytical Chemistry, 142nd Meeting, ACS, Atlantic City, N. J., September 1962.

(3) Dal Nogare, S., Juvet, R. S., “Gaa- Liquid Chromatography,” Interscience, New York, 1962.

(4) Floutz, W. V., M.S. thesis, Purdue University, Lafayette, Ind., 1959.

(5) Harvey, D., Chalkley, D. E., Fuel 34, 191 (1955).

(6) Koch, 0. G., Mikrochim. Acta 1958,

(7) Kolthoff, I. M., Elving, P. J., “Treatise on Analytical Chemistry,” Part 11, Vol. 8, p. 317, Interscience, Kew York, 1963.

(8) Lovelock, J. E., ANAL. CHEM. 33, 172 (1961).

92-103.

(9) Zbid.; 35 , 474 (1963). (10) Moshier, R. W., Schwarberg, J. E.,

Morris, M., Sievers, R. E., Pittsburgh Conference ’ on Analytical. ChemisGy and Amlied SuectroscoDv. March 1963.

(11 ) Ra,; N. H.; J . AppL’ehem. (London) 4, 21 (1954).

(12) Ross, W. D., ANAL. CHEM. 35, 1596 (1963).

(13) Sievers, R. E., 16th Annual Summer Svmoosium. ACS Division of Analvtical Chehistry, ’Tucson, Ariz., June 19”63.

(14) Sievers, R. E., Moshier, R. W., Morris, M. L., Inorg. Chem. 1, 966 (1962).

(15) Sievers, R. E., Moshier, R. W., Ponder, B. W., Division of Inorganic Chemistry, 141st Meeting, ACS, Wash- ington, D. C., March 1962.

(16) Sievers, R. E., Ponder, B. W., Morris, M. L., Moshier, R. W., Znorg. Chem., in press.

(17) Webb, R. J., United. Kingdom At. Energy Research Establishment, AM8 - - (1959). - -

- - (18) Woodruff, J. F., J . Opt . SOC. Am. I I I I I I I I I I I I I I I I I - 40, 192 (1950). ot I I I I 1 1 1 1

10-8 10-7 10-6 10-5 10-4 RECEIVED for review August 6, 1963.

Concn., Q /ml. Accepted October 25, 1963. Work sup- ported by the Aerospace Research Lab- oratory, Wright-patterson &r F~~~~ Figure 3.

tiviiy Ohio. Two standardization curves superimposed, illustrating changes in sensi-

268 ANALYTICAL CHEMISTRY