Detection of Metal Chelates in Gas Liquid Chromatography by Electron Capture WILLIAM D. ROSS Dayton laboratory, Monsanfo Research Corp., Dayton, Ohio
b An electron capture detector is capable of detecting trace quantities of chromium(lll) and aluminum(lll) acetylacetonates, trifluoroacetylacetonates, and hexafluoroacetylacetonates. Instrument conditions for maximum sensitivity are defined. Limits of detection are: 3.3 X lo-" gram of Cr hexafluoroacetylacetonate and 4.8 X gram of AI hexafluoroacetylacetonate. Fluorine containing chelates have higher electron affinity than nonfluorinated compounds, the degree of affinity increasing with degree of fluorination. Chromium chelates show higher electron affinity than those of aluminum. A mixture of AI hexafluoroacetylacetonate and Cr hexafluoroacetylacetonate has been rapidly and efficiently separated by gas liquid chromatography.
-
G
in the past has been used primarily for the analysis of organic compounds. It has now been extended to inorganic and organometallic compounds. In the recent literature, the analysis of metal chelates is prominent. For example, Floutz (3) investigated I1 metal acetylacetonates by gas liquid chromatography (GLC) but successfully eluted only A,Be, and Cr acetylacetonates. Bierman and Gesser (1) established gas chromatographic conditions for resolving Be and A1 acetylacetonates. Recently Brandt and Heveran (2) have studied the analysis of chromium by gas chromatography. The major problem involved in analyzing metal chelates by gas chromatography arises from their relatively low volatility. As a result, the temperature required to vaporize and elute them causes thermal degradation of all but the most stable chelates. However, Sievers, Ponder, Morris, and Moshier (7-10) have found that chelates of the fluorine-containing ligands, trifluoroand hexafluoroacetylacetonates, have significantly higher vapor pressures, hence require lower injection port and column temperatures for elution. This discovery has increased substantially the number of metal chelates separable by gas liquid chromatography. Fluorinated metal acetylacetonates have the added advantage of high AS CHROMATOGRAPHY
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electron affinity, which permits extremely small concentrations to be detected. Lovelock (5, 6) has described the affinity of halogenated compounds for low energy electrons such as are found in the electron capture detector. This investigation was initiated to determine the sensitivity of the electron capture detector for metal chelates containing fluorinated and nonfluorinated ligands. The study comprised a comparison of the sensitivity of the electron capture detector for Cr(II1) and Al(II1) acetylacetonate, trifluoroacetylacetonate, and hexafluoroacetylacetonate and the influence of two eluents (nitrogen and argon) on sensitivity. EXPERIMENTAL
A Barber-Colman Model 20 gas chromatograph equipped with an ionization detector (diode type), cell Model A-4150 was used in the investigation. (Model A-5120 gave comparable results.) 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. Compounds Investigated. Six metal chelates were investigated. They include: Cr(II1) acetylacetonate [Cr(acac)a]; Cr(II1) trifluoroacetylacetonate [Cr(tfa)a]; Cr(II1) hexafluoroacetylacetonate [Cr(hfa)s]; A1 (111) acetylacetonate [Al(acac),]; AI(II1) trifluoroacetylacetonate [Al(tfa),]; and Al(II1) hexafluoroacetylacetonate [Al(hfa)s]. Selection of Solvents. Solvents for these compounds were selected based on the solubility of the compounds and elution time of the solvent as compared with elution time of the compound being analyzed, to assure no interference by the solvent. The electron affinity of the solvent or impurities in the solvent may determine whether the solvent will interfere with detection of the compound. It was therefore important to select a solvent with little or no electron affinity and one which does not contain electroncapturing impurities. Benzene proved suitable for all compounds investigated except Cr(hfa)3 and Al(hfa)3. The latter two were eluted so rapidly that
a solvent (toluene), which was eluted after these compounds, was used. Preparation of Samples. Samples of the six compounds were prepared by accurately weighing 10- to 15-mg. quantities in a 5.0-ml. volumetric flask, and making up to volume with solvent. From this initial solution, three consecutive 1:25 dilutions were made. The samples were injected into the column via a Hamilton 10-fil. syringe. The sample sizes varied from 0.2 to 10.0 fil. Column and Instrument Conditions.
The columns used were '/winch 0.d. stainless steel packed with 20% by weight of Dow Corning Silicone Fluid 710R on Gas Chrom Z. For the very volatile (hence fast-eluting) hexafluorinated compounds, an 11-foot column was used. The other samples were analyzed on a %foot column. Each of the six compounds required unique instrument conditions to obtain the elution times and symmetry of elution peaks that would allow the ultimate signal intensity by the particular compound. Peak sharpness was critical since the response was measured by peak heights. The respective volatilities and elution properties of the compounds determined the column flow rates, injection port temperatures, and column temperatures to be used. Table I summarizes the instrument conditions used for the evaluation of the six compounds listed above. Data in Table I1 were collected using both argon and prepurified nitrogen eluents. The gain setting of 1000 and positive polarity were used on the electrometer. The maximum recorder noise was always less than 1 mm. (24 cm. full scale, 0 to 5.0 mv.) in height. Table I1 lists the peak heights of the lowest detected concentrations of the compounds evaluated and the signal to noise ratios based on 1 mm. of noise. RESULTS A N D DISCUSSION
A1 and Cr acetylacetonates and their trifluorinated and hexafluorinated analogs were investigated to establish the instrument condit'ions for ultimate signal-to-noise ratios. Table I1 compares the detector sensitivity for these compounds under the instrument conditions indicated in Table I. To determine the optimum instrument operating conditions, column and injection port temperatures, injection technique, column conditioning, cell
B
A
-1
Minutes
0
3
5
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0
3
6
1. Chromatograms of A: 1.2 X lo-' gram Al(hfa)3 and B: 3.0 X 1 O-s gram ,4l(hfa)s Figure
1. 2.
Al(hfa)n
T(3luene
voltage, column inlet pressure, and scavenge flow rate aere considered. The cell voltage, scavenge flow, and column flow rates all were found to have critical and perhaps interdependent influences on the sensitivity of the detector for these samples. Effect of Column Temperature. The relatively high thermal stability of these particular metal acetylacetonates permitted $he use of higher column temperatures than normally used for other metal chelates. As a result, the eluted peaks were sharp, symmetrical, and fast-eluting (less than 5 minutes). The sharpness of peaks permitted measurement of signal intensity by comparing peak heights, measured from the base line t o the apexes. No evidence of extraneous peaks due t o thermal degradation was found. Column temperature was held constant after satisfactory peaks were attained. Effect of Injection Port Temperature. Again the thermal stability of these chelates prevented this instrument condition from being too critical. The temaerature was maintained high enough to permit rapid and total vaporization, to aid in the elution of sharp, symmetrical peaks. Injection port temperature was held constant during the evaluation at 165' C. Sample Injection Technique. The method of sample delivery was critical in achieving reproducible peak sizes. It was necessary to hold the syringe needle in the injection port septum for 20 seconds to obtain reproducible peak heights. If this is not done, the entire sample is not volatilized. Column Conditioning It was essential to condition the column to obtain maximum sensitivity. Suc-
cessive injections of the compound under investigation were made until maximum and reproducible peak heights were obtained. At least six 1-p1. samples containing in the range of gram per ml. of the compound being measured were necessary to condition the column adequately. Determination of Optimum Cell Voltage. With all other instrument conditions static, the cell voltage was varied by &volt increments from 5 to 50 volts for each sample, until the voltage for highest response was found, This was considered the optimum voltage. Determination of Optimum Column Inlet Pressures and Scavenge Flow Rates. Changes in inlet pressure, hence changes in column flow rates, influence detector sensitivity. The range of pressures for maximum sensitivity was 20 to 50 p.s.i.g. Scavenge flow rates influence cell sensitivity, the optimum flow being different for each compound. The optimum flows ranged from 75 to 300 ml. per minute. Table I tabulates the optimum instrument conditions found in this investigation for the detection of each compound, using argon and nitrogen as eluents. Each compound, because of different volatilities and different electron affinities, required a different set of instrument conditions. The conditions were determined to attain the sharpest and most symmetrical peak. The detection limits were determined using the best instrument conditions found to date. Figures 1 and 2 are the actual curves obtained for solutions of hl(hfa)3 and Cr(hfa),. Figures 1A and 2A illustrate the peak symmetry and response t o 1.2 X lo-' gram of iU(hfa), and 5.2 X gram of Cr(hfa)a, respectively, and Figures 1B and 2B the curves obtained for the lowest detectable concentration.
Table I.
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Figure
2.
5.2 X 8.3 X 10-lo
Minutes
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Chromatograms o f A: gram Cr(hfa)o and B: gram Cr(hfa)a 1. 2.
Cr(hfa)a
Toluene
Chroniatograms in Figure 111 and B were obtained a t 95" C. column temperature a t an inlet pressure of 50 p.s.i.g., scavenge flow of 300 ml. per minute, and cell voltage of 15 volts. The curves in Figure 2A and 2B were obtained a t 95' C. column temperature, an inlet pressure of 20 p.s.i.g., scavenge flow of 200 ml. per minute, and cell voltage of 20 volts. Argon was used as an eluent in the analyses shown in both figures. It is interesting to note that the hexafluoroacetylacetonate complexes are so volatile that they are eluted more rapidly than toluene. Effect of Fluorination. Lovelock has reported that the halogens capture electrons readily to form stable negative ions, and several electron volts of energy are released by the reaction (6)'
The data of Table I1 indicate that Cr(acac)a and Al(acac)a have relative electron affinities as indicated by the lower limit of the amount of sample detected (8.8 X 10-8 gram and 5.1 x 10-5 gram, respectively). However, by fluorinating the ligands of these compounds,
Instrument Conditions Used in Investigation
Compound
Inlet pressure (p.s.i.g.)
Cr(acac), Cr(tfa)s Cr(hfa)a Al(acac)s Al(tfn)s AI( hfa)s
50 24 40 50 24 40
Cr(acac), Cr(tfa)a Cr(hfa)a Al(acac)a Al(t fa)^ Al(hfa)8
20 22 20
Scavenge flow
(ml./min. ) Nitrogen eluent
Cell voltage
100 200 200
300
25 20 15 40
250
30
200
Column temp. ( " C.)
15
215 162 90 185 162 90
Argon eluent
20 22 50
240 75 200 100 200 300
20 20 20 16 25 15
VOL. 35, NO. 11, OCTOBER 1963
215 175 95 185 160 95
0
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Table
II.
Lower Detection Limits Determined for AI and Cr Chelates
Sample size Conipound ( ~ 1 . ) Cr(acac)J Cr(tfah Cr(hfa)3 Al(a~ac)~
Al(tfa)3 hl(hfa)a
5.0 1.0 0.4 5.0 1.0 1.0
Sample concn. (g./ml.) 1.76 8.96 8.32 1.02 2.68 4.8
X 10-6 X X X X x 10-7
Peak Signal/ height noise (mm.) ratio Nitrogen eluent 10 4 7 17 2 2
1011
4/1 7/1 17/1 2/i 2/1
Amt. sample detected (grams)
Amt. metal detected (moles)
s . 8 x 10-8 9 , O X lo-” 3 . 3 x 10-11 5.1 X 2 . 7 x 10-9 4.8 x 10-10
2.5 x IO-~O 1 . 8 x 10-18 4 . 9 x 10-14 1.6 X 5 . 9 x 10-12 7 . 4 x 10-13
1.8 x 2.2 x 8.3 X 1.1 x 1.3 X 4.8 X
5 . 0 x 10-10 4.4 x 10-13
Argon eluent Cr(acac)3 Cr(tfa);l Cr(hfa)t Al(a~ac)~ Al(tfa)d Al(hfa),
10.0 0.1 0.4 0.5 0.2 1.0
1.76 X 2.24 X 2.08 X 2.1 X 6.7 X 4.8 X
10-5 10-6 10-6 10-6
5
5
5 10
9 8
their electron affinity is increased considerably [Cr(tfa)B - 9.0 x gram, Al(tfa), - 2.7 X gram]. Also the increase in response may be due in part to improved chromatographic properties of the fluorinated compounds. However, an effort was made to find the optimum conditions for each compound. The chelates of hexafluoro ligands appear to have higher electron affinity than those of the trifluoro ligands gram, Al[Cr(hfa)l - 3.3 X gram], the only (hfa)p - 4.8 X exception being the higher affinity of Cr(tfa), than Cr(hfa)C when eluted with argon. Effect of Metallic Ion on Electron Amity. The influence of the metallic ion of the chelate on the electron affinity appears to be significant. The chelates of Cr(II1) exhibit greater electron affinity than the corresponding chelates of Al(III), as shown in Table 11. Effect of Eluent on Electron AfEnity. The eluent is an important factor in determining the highest sensitivity of the electron capture detector. Knox (4) has stated that when argon is used as the carrier gas, the average electron energy is high since the collisions of electrons with argon are in general highly eIastic. When nitrogen or hydrogen is used, the electron energy is lower. I n agreement with this postulate, our data (Table 11) indicate that higher sensitivity is achieved in nitrogen than in an argon environment for both the A1 and Cr chelates.
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CHEMISTRY
511
5/l 5/1
1011 9/1 8/1
10-7 10-10 10-lo 10-5
1 . 2 x lo-’* 3 . 2 x 10-8 2 . 8 X lo-” 7.4 X
Separation of Al(II1) and Cr(II1) Hexafluoroacetylacetonate. Figure 3 illustrates the elution peaks of Al(hfa), and Cr(hfa)r (peaks 1 and 2, respectively) which were resolved in 3 minutes. Peak 3 is a n electroncapturing impurity in the toluene solvent. The toluene has no electron afinity and does not cause any detector response in a nitrogen eluent. The elution time of toluene was 6 minutes. This was determined when this same mix was analyzed in argon and a negative peak resulted for toluene. This chromatogram was obtained a t a column temperature of 65’ C., an inlet pressure of 30 p.s.i.g., a cell voltage of 15, and a scavenge flow of 200 ml. per minute.
CONCLUSION
This investigation has shown that the Sievers-Moshier method of gas chromatography of fluorine-containing metal chelates is a highly promising approach to trace as well as conventional analysis of metals. This conclusion is based on the following: The electron capture detector is capable of detecting quantities of metals as small as 2 x 10-’2 gram. Because of the increased volatilities of the fluorinated chelates, the elution system temperatures required should be within a range where many of the chelates are thermally stable. The increase in electron affinity of the
h 0
I
1
1
d
3
6
9
12 Minutes
Figure 3. Chromatogram of mixture of Al(hfa)s and Cr(hfa)3 1. 2. 3.
Al(hfa), Cr(hfa)a Impurity in solvent
fluorinated compound< permits the ube of much mslller *ample sizes, with resultant increa.e in efficiency of the chromatographic system. ACKNOWLEDGMENT
The author acknowledges the many helpful discussions and samples contributed by R. E. Sievers. The technical advice of J. V. Pustinger, Jr., and the assistance of J. W. Johnson and G. Wheeler, Jr., is also sincerely appreciated, LITERATURE CITED
(1) Bierman, R. J., Gesser, H., ANAL. CHEM.32, 1225 (1960). (2) Brandt, W. W., Heveran, J. E., 142nd
Meeting, ACS Atlantic City, N. J., September 1962. (3) Floutz, W. V., M.S. thesis, Purdue Cniversity, Lafayette, Ind. (1959). (4) Knox, J. H., “Gas Chromatography,” p. 80, Wiley, 1962. ( 5 ) Lovelock, J. E., ANAL. CHEM.33,
172 (1961). (6) Lovelock, J. E., Ibid., 35, 474 (1963). (7) Sievers, R. E., “Gas Chromatographic Separation of Metal Chelates,” 16th
Annual Summer Symposium on Analytical Chemistry, Tucson, Ariz., June
1963. (8) Sievers, R. E., Moshier, R. W., Morris, M. L., Inorq. Chsm. 1, 966 (1962): (9) Sievere, R. E., Moshier, R. W., Ponder, B. W., 141st Meeting, ACS Washington, D. C., March 1962. (10) Sievers, R. E., Ponder, B. W.,
Morris, M. L., Moshier, R. W., Znorg. Chem. 2,693 (1963).
RECEIVEDfor review A ril 4, 1963. Accepted July 11, 1963. g o r k supported by the Office of Aerospace Research, Aero-
nautical Research Laboratory, WrightPatterson Air Force Base, Ohio.
