Rapid separation of metal chelates by column liquid-liquid

Rapid Separation of Metal Chelates by Column Liquid-Liq uid Chromatography Using Ultraviolet Detection J. F. K. Huber and J. C. Kraak Laboratory for Analytical Chemisfry, Unioersity of Amsterdam, Amsterdam, Netherlands

Hans Veening Department of Chemistry, Bucknell Unioersity, Lewisburg, Pa. I7837 The separation of metal-pdiketonates has been accomplished by column liquid-liquid chromatography. Included in the study are the acetylacetonates and the trifl uoroacetylacetonates of Be( I I), AI( I I I), Cr( I I I), Fe (Ill), Co(ll), Co(lll), Ni(ll), Cu(ll), Zn(ll), Zr(lV), and Ru(lll). A ternary liquid-liquid system composed of water, 2,2,44rimethylpentane and ethanol was employed. The water-rich phase was used as the stationary medium while the water-poor phase served as eluent. Several of these two phase systems, varying in quantitative composition, were evaluated for chromatographic selectivity. The addition of a trace of chelating ligand to the phase system suppressed undesirable hydrolysis reactions of chelates. A sixcomponent mixture of metal acetylacetonates can be separated in less than 25 minutes.

THESEPARATION OF METALS has often been accomplished by taking advantage of the distribution of the metal between an aqueous solution and an organic extracting liquid in the presence of a chelating ligand. In these procedures the pH of the aqueous solution, the organic extracting liquid, and the ligand are chosen in such a manner that a single metal or group of metals can be extracted selectively. Extraction is carried out discontinuously in one or more distribution steps. Multiple extraction can be carried out in an automated procedure as well. The extended literature on metal chelate extraction is reviewed in several monographs (1-3). Metal chelate separations have also been effected by means of gas chromatography (GC) (4-6). Prior to GC, the metals are usually converted into neutral, volatile metal P-diketonates and extracted from aqueous solution. It has become obvious that the most efficient separation method based on liquid-liquid distribution is column chromatography. Modern column liquid chromatography offers the advantages of high speed, and automated operation (7-11). Also, the choice of mobile phase can provide special selectivity (1) G. H. Morrison and H. Freiser, “Solvent Extraction in Analytical Chemistry,” John Wiley and Sons, Inc., London, 1957. (2) J. Stary, “The Solvent Extraction of Metal Chelates,” Mac-

millan, New York, N.Y., 1964. (3) Y.Marcus and A. S. Kertes, “Ion Exchange and Solvent Extraction of Metal Complexes,” Wiley-Interscience, London, 1969. (4) R. W. Moshier and R. E. Sievers, “Gas Chromatography of Metal Chelates,” Pergamon Press, Oxford, 1965. (5) H. Veening, W. E. Bachman, and D. M. Wilkinson, J. Gas Cliromatogr.,5,248 (1967). (6) H. Veening and J. F. K. Huber, ibid., 6,326 (1968). (7) J. F. K. Huber and J. A. R. J. Hulsman, Alia/. Cltim. Acta, 38, 305 (1967). (8) J. F. K. Huber, “Comprehensive Analytical Chemistry,” Vol. IIB, Elsevier, Amsterdam, 1968, pp 1-54. ( 9 ) J. F. K. Huber, J. Cliromatogr. Sci., 7,85(1969). (10) Ibid., p 172. (11) J. F. K. Huber, 5th International Symp., Column Chromatography, Lausanne 1969, published as supplement to Cliimia, (1970), p 24. 1554

effects in LC. Some work on the LC separation of metal a-complexes has already been reported (12). The present paper is intended to demonstrate the analytical potential of applying liquid-liquid partition chromatography for the separation of metal /3-diketonates. EXPERIMENTAL

Apparatus. The liquid chromatograph was assembled in part from commercial and home made parts. A block diagram is shown in Figure l. The reservoir contained the eluent stock, and was thermostated at 25.0 “C by means of a constant temperature bath (Haacke FT). A high pressure pump (Orlita DMP 1515) was used to pump the moving phase. Pump pulsations were eliminated by means of a Bourdon tube and a capillary restrictor connected in series (8,9). A thermostated pre-column (25.0 “C) served to establish equilibrium between the moving and stationary phases prior to entering the separation column. Construction and preparation of the. pre-column was similar to the separation column, but coarser solid support was used in order to reduce the pressure drop. The sampling device consisted of a precision syringe (Hamilton 701N or 705N) and an injection chamber of minimal volume, thus guaranteeing insignificant mixing (7, 8). The injection chamber was sealed by a silicone rubber septum through which samples could be injected. Separation columns were constructed from thick-walled borosilicate glass tubes, 2.7-mm i.d. The unit lengths were 10 and 25 cm; longer column lengths were achieved by appropriate combinations of several sections. The column was thermostated at 25 “C by means of a water jacket and a constant temperature bath (Haacke FT). The liquid-liquid systems were prepared from distilled water, absolute ethanol (Merck), and 2,2,4-trimethylpentane (British Drug Houses, 9575, boiling range 98-99.5 “C). The liquid phases were prepared separately according to their equilibrium compositions, thus avoiding the time-consuming procedure of phase equilibrium attainment. The solid support consisted of diatomaceous earth (Kieselguhr, Merck, 0.15 to 0.20 mm) ground and sieved to a particle size range of 5 to 10 and 10 to 20 pm. Iron and other metals were extracted with boiling concentrated hydrochloric acid for 5 hours. The acid was washed off with water, and the support was dried at 200 “C for 2 hours. The column was prepared by packing it tightly with the solid support; 1- to 2-mm sections at a time (7). The two ends of the column were next connected to the injection chamber and the detector by means of capillary tubing without significant dead volume (7, 10). The feed line from the column to the detector consisted of stainless steel capillary tubing (0.5-mm i.d. X 50-mm length). The detector consisted of a low noise spectrophotometer (Zeiss PMQ 11) equipped with a homemade 7.5-pl micro flow cell with a path length of 10 mm. This detector has been very satisfactory in previous high speed liquid chromato-

(12) H. Veening, J. M. Greenwood, W. H. Shanks, and B. R. Willeford, Cltem. Commim., 1969, 1305.

ANALYTICAL CHEMISTRY, VOL. 44, NO. 9, AUGUST 1972

eluent

- pump

reservoir

I

1 damping

pre-

sampling

--c

unit

--c

column

device

I I

I

____---_ thermostat --.

- - - - - - - - ----------- ---- ----

graphic separations (10). Measurements were made at 310 nm. Reagents. Two types of metal chelates were used; acetylacetonates (AA) and trifluoroacetylacetonates (TFA). The following compounds were synthesized according to standard literature references ( 4 ) and were characterized according to melting point, I R and UV spectra: Be(TFA)2, AI(TFA)3, Cr(TFA)3, Fe(TFA)3, CO(TFA)~,CU(TFA)~,Ru(TFA)~,and Ru(AA)3. The following compounds were obtained commercially (Baker Reagent grade) : Be(AA)2,Al(AA)3, Cr(AA)3, Fe(AA)3, Co(AA)*, Co(AA),, Ni(AAI2, Cu(AA)%,Zn(AA)2, and Zr(AA)*. Procedure. Two procedures were used to coat the stationary liquid on the solid support. The eluent flow was adjusted, and after displacement of air, successive small portions of the stationary liquid were injected, thus loading the solid support. Second, stationary liquid was first pumped through the column until all air was displaced; eluent was then pumped through, displacing the excess stationary liquid from the interspace between the particles. THEORY

Total Distribution Coefficient of a Metal. In order to achieve a distribution of a metal between an aqueous and an organic phase, the metal ion must first be converted to a neutral molecule. Such a neutral molecule can be obtained by the formation of a chelate in which the ligands neutralize the metal ion and displace the water of hydration. The equilibrium distribution of the neutral metal chelate between the aqueous and the organic phase is described by the partition coefficient (K). In addition to the distribution equilibrium of the neutral chelate, a number of other competing equilibria are involved. For this reason, all complexes which are formed in aqueous solution have to be considered, if an overall distribution coefficient (Khl) of the metal is to be defined. KM is derived from a number of equations which describe the single equilibria involved. The distribution equilibria are characterized by their respective partition coefficients; the dissociation equilibria by their formation constants. Figure 2 represents a schematic situation for a simple case of an equilibrium in which it is assumed that: no chemical reactions occur in the nonpolar (organic) phase, and intermediate and anionic complexes are not formed in the polar (aqueous) phase.

separation

Figure 1. Schematic diagram of liquid chromatograph used

column

I

1

non-polar phase

polar phase

I

n HL

It

I

The metal is present as the hydrated ion and the neutral chelate in the polar phase, and as the neutral chelate in the nonpolar phase. Therefore, the total distribution coefficient of the metal is given by:

where [XI,

=

[XInp =

M L n rn

= = = =

the concentration of the species X in the polar phase the concentration of the species X in the nonpolar phase metal chelating ligand the ionic charge the coordination number of the metal

The protonated ligand and the metal chelate will undergo distribution between the two phases. The partition coefficients for each of these equilibria are:

In addition, the protonated form of the ligand and the metal chelate also undergo dissociation in the polar phase. The formation constants for these equilibria are defined as follows:


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stant if the term which contains the equilibrium constants and the concentrations of the hydrogen ion and the chelating agent, becomes small compared to 1. A large metal chelate formation constant and a high pH are therefore favorable conditions which lead to constant KMvalues. Chromatographic Peak Shape and Resolution. The effect of the nonlinearity of distribution isotherms on the symmetry of chromatographic peaks has been described previously (11, 13). Also, it has been shown that the degree of chromatographic separation of two compounds, j and i, can be described by the resolution (Rji), which is related to other chromatographic parameters as follows ( 4 1 4 ) :

I

time -.

0

where r j l = Kj,/KI, (The ratio of distribution coefficients called the selectivity factor or retention ratio) XI = Ki,Vs/Vm (The partition ratio or capacity ratio; V, and V, refer to the volume of the stationary and mobile phase, respectively) Ni = the number of theoretical plates (which depends on the capacity ratio)

Figure 3. Asymmetric elution peaks for Ni(AA)2 obtained with varying sample sizes 2

6

Equation 6 enables one to calculate the number of theoretical plates which will be required for a given resolution of two compounds. This number will depend on the value of the selectivity factor as well as the capacity ratio. Both depend on the partition coefficient and, therefore, on the nature of the liquid-liquid system and the temperature. The value of the capacity ratio is further determined by the phase ratio (VJV,). The choice of the optimum two phase system for a given separation therefore depends on the relative as well as the absolute values of the partition coefficients. It is, however, a useful guideline to base the choice of a suitable liquid-liquid system primarily on the selectivity factor (rji).

+time

Figure 4. Successive elution profiles for AI(AA)3

RESULTS AND DISCUSSION

Curve 1 : Fresh solution of AI(AA)3 Curve 2: One-week old solution of AI(AA)3 Curve 3: Two-week old solution of AI(AA),

Equations 1,2, and 3 can be combined to yield an expression which describes the dependence of the total metal distribution coefficient (&) on the hydrogen ion concentration in the polar phase and the ligand concentration in the nonpolar phase.

{

KM = KML, 1

+7 1 (-) KHL* KXL, KHL

(-)“}

[H+], [HLInp

(4)

For a more general case, the stepwise formation of the metal chelate complex and the participation of hydroxy and other anions have to be considered as well. The following general equation can be written for the total distribution coefficient of the metal between the two phases.

KM = where j k 1

= = =

’

I;[{ M(H20),_2j_r_zLj(OH)rAz~+n-~-rIp CMLnlnp

(5)

the number of chelating ligands the number of hydroxy anions the number of anions of type A

From Equation 5 it is evident that the total distribution coefficient of the metal is not constant. For the simple case, however, described by Equation 4, KM becomes almost con1556

Choice of a Liquid-Liquid System. It was decided to investigate a ternary system for the separation of metal chelates by column liquid-liquid chromatography. In such a system, the polarity difference of the phases can be easily controlled by changing the quantitative composition of the ternary system. Water and 2,2,4-trimethylpentane (TMP) were chosen as the immiscible pair; ethanol was the third component of the ternary system. Metal-P-diketonates are not very soluble in either water or TMP. Ethanol mixes easily with both, and serves to establish the polarity difference between the two liquid phases of the system, which decreases as the ethanol fraction is increased, until finally the liquid interface between the two phases disappears and a homogeneous one-phase system results. Table I represents the composition of five liquid-liquid systems which were investigated. The equilibrium composition of the phases was determined by gas chromatography. The precision of the measurements of the mass fractions was characterized by a standard deviation of better than & 2 z , with a minimum of 0.0001 (absolute). For liquid chromatographic separations, the water-rich phase was used as the stationary phase, and the water-poor phase as the eluent. (13) J. F. K. Huber, “Gas Chromatography 1962” M. van Swaay, Ed., Butterworth, London, 1963, p 26. (14) J. F. K. Huber and A. I. M. Keulemans, Z . Anal. Chem., 205, 263 (1964).


Table I.

Composition of Each Phase for Five Liquid-Liquid Systems Composed of Water, Ethanol, and 2,2,4-Trimethylpentane (TMP) Mass fraction

System A B

C

D E

Water 0.080 0.085 0.153 0.221 0.343

More polar phase Ethanol 0.696 0.699 0.752 0.735 0.641

Water 0.0040 0.0032 0.0017 0.0008 0.0007

TMP

0.224 0.216 0,095 0.044 0.016

Initial screening experiments indicated that few chelates gave symmetrical elution peaks. This indicated that the total distribution coefficient of the metal (Kbf)was not constant and that the distribution isotherm was nonlinear. Figure 3 indicates the results obtained for Ni(AA)2 using system E in Table I. The peak is highly distorted and it can be seen that the retention time increases as the concentration decreases, thus indicating a convex, nonlinear isotherm. The inertness of the solid support was demonstrated by experiments in which the chelates were injected into a column packed with uncoated support. No significant retention or peak distortion was observed in this case. Figure 4 illustrates the anomalous results obtained when solutions of Al(AA)3 dissolved in the stationary phase, were injected after measured time intervals. Curve 1 shows a symmetrical peak obtained when a freshly prepared solution of A1(AA)3 was injected. Curves 2 and 3 represent the chromatograms obtained for 1-week and 2-week old solutions of Al(AA),, respectively. It is likely that a dissociation or hydrolysis reaction of the aluminum chelate is responsible for the production of what are apparently three different species. In the freshly prepared solution, the chelate has presumably not had sufficient time to react and a single symmetrical peak is obtained, whereas after 1-week standing, a prominent second peak and a smaller third peak have appeared. After a two-week time interval, the third peak increased in size whereas the middle peak decreased. It is possible that mixed hydroxy-acetylacetonato-aluminum species could form on standing; these species subsequently separate from one another on the column. The effects described for Ni(AA)2 and Al(AA)3 become more pronounced as the water content of the stationary phase increases. The distribution of Al(AA)3and other acetylacetonates between water and organic solvents has been studied by Hopkins and Douglas (15). They found that the aluminum chelate was very reactive toward water, and was easily hydrated. It is therefore possible that liquid chromatography could serve as a powerful technique for studying the thermodynamic and kinetic behavior of metal chelates in solution. The selectivity factors for three pairs of metal-@-diketonates giving symmetrical peaks are listed in Table 11. It is clear that the selectivity factor is determined by the metal and is nearly independent of the AA or TFA ligand. Also, the selectivity factor decreases with a decreasing polarity difference of the phases in the ternary liquid-liquid systems. When the composition of the two phases becomes equal at the critical mixing point (“plait point”), where the system becomes homogeneous, the selectivity factor approaches unity. The use of phase systems yielding very high partition coefficients is (15) P. D. Hopkins and B. E. Douglas, Inorg. Chem., 3, 357 (1964).

Less polar phase Ethanol 0.111 0.096 0.056 0.032 0.022

TMP

0.885 0.901 0.942 0.967 0.977

Mass fraction ratio of ethanol, polar : nonpolar 6.2 7.3 13.4 22.9 29.0

Table 11. Selectivity Factors for Five Ternary Liquid-Liquid Systems (Water, Ethanol, and 2,2,4-Trimethylpentane) Measured by Chromatography CO(AA)~: Co(TFA)?: Ru(TFA)3: System Cr(A.413 Cr(TFA)3 Cr(TFA)3 A 1 16 1 17 1 06 B 1 19 1 19 1 07 C 1 36 1 35 1 13 D 1 48 1 47 1 15 E 1 60 1 57 1 18 Table 111. Selectivity Factors of a Number of Metal Acetylacetonates Measured by Chromatography (System E with a Trace of Acetylacetone) A1:Fe Cr:Al Ru:Cr Co:Ru Cu:Be Fe:Cu 1.07 1.08 1.48 1.09 1.09 3.42

limited. This is so because the partition coefficient is related to the ratio of solubilities in the two phases, thus the solubility in the moving phase decreases to the point where detection becomes difficult if the partition coefficient increases to a very high value. The difficulties encountered due to hydrolysis and other chelate reactions, were rectified by adding a very small amount of the protonated ligand (HAA) to the phase system, utilizing the most selective phase system E in Table I. It can be seen from Figure 2 and Equation 4, that the addition of (HAA) would tend to suppress undesirable chelate reactions in the polar phase and to improve the linearity of the distribution isotherm. An amount of ligand was added such that a mass fraction of 0.008 in the moving phase and 0.002 in the stationary phase resulted. In most cases, the presence of a trace of ligand apparently suppressed undesirable, competitive dissociation reactions and resulted in symmetrical elution peaks with constant retention times. The choice of how much ligand is to be added to the moving phase represented a compromise between attaining a low detection limit @-diketones are strong UV absorbers) and achieving maximum suppression of undesirable reactions, thus yielding constant Khf values (Equation 4). The selectivity factors for six pairs of metal acetylacetonates using liquid-liquid system E in the presence of a small amount of ligand were determined chromatographically by measuring the capacity ratio for each single compound. The data are given in Table 111. It is seen from a comparison with Table I1 that the selectivity factors are not changed by the presence of a trace of ligand in the ternary system (rcocr = V C ~ RX ~T R ~ c ~ ) . The partition coefficients of a number of metal acetylacetonates have been determined for system E (Table I) containing a


1557

ABSOPBAHCL

Be

f4

Cr

Co

I

22 min.

0

f

Figure 8. Separation of six metal acetylacetonates

Column: 100 X 2.7 mm; particle size 10-20 pm; fluid velocity 0.29 mmsec-I

Column: 500 X 2.7 mm; particle size 5-10 pm; fluid velocity 1.8 mm sec-'; pressure drop 17 bar; theoretical plate number for Co(AA)s, 2200

absorbance

t

I

Table IV. Partition Coefficients and Selectivity Factors of Some Metal Acetylacetonates in the Liquid-Liquid System E (Table I) Measured by the Static Method

co

+ time

0

Acetylacetonate Be(I1) Cu(I1) Fe(II1) Al(II1) Cr(II1) Ru(II1)

20 min

Figure 6. Separation of Be(AAh, Fe(AA)3, Cr(AAh, and CO(AA)~ (in the presence of ligand in the mobile phase) Column: 500 X 2.7 mm; particle size 10-20 pm; fluid velocity 2.6 mm sec-l; pressure drop 26 bar; theoretical plate number for Co(AA)3, 1090

Zn(I1)

Co(II1) Zr( IV) Co(I1)

nn+1)R

4.4 16.1 18.4 20.3 22.0 23.2 29.8 33.5 60.2 loo. 0

3.66 1.14 1.10 1.08 1.06 1.29 1.12 1.80 1.66

...

Q

b r(,,+l),,=

time I

6 min

Figure 7. Rapid separation of Be(AAh, Cu(AA)*, Ru(AA)a, and Co(AAh Column: 500 X 2.7 mm; particle size 5-10 pm; fluid velocity 3.0 mm sec-l; pressure drop 28 bar; theoretical plate number for Co(AA)r, 960

trace of acetylacetone. The following procedure was used: the chelate was dissolved in a known volume of the water-poor phase and the absorbance of the solution a t the wavelength of the absorption maximum in the UV-range was measured; the solution was shaken with a known volume of the waterrich phase until equilibrium was achieved. The absorbance of the water-poor phase was measured again and the partition coefficient was calculated (16) from the absorb(16) J. F. K. Huber, C. A. M. Meijers, and J. A. R. J. Hulsman, ANAL.CHEM., 44,111 (1972). 1558

KO.

The partition coefficients, KO, are the average of four measurements, the relative standard deviation of the measurements being 42.

0

25 min

0

Figure 5. Separation of Be(AAh, Fe(AA)3, Cr(AA)3, and CO(AA)~ (no ligand in the phase system)

selectivityfactor of two successive compounds.

ance values and the volume ratio. The results are compiled in Table IV. The slight difference in the values of the selectivity factors obtained by the static and the chromatographic measurements are attributed t o the impurity of the chelates which makes the static determination based on the measurement of the UV-absorption somewhat questionable. Separations. A number of separations were carried out. Examples are shown in Figures 5 t o 8. Figure 5 shows a four-component separation of Be(AA)2, Fe(AA)3, Cr(AAh, and CO(AA)~, which was carried out using liquid-liquid system E without the ligand. It is seen that the Fe(AAh peak is rather distorted presumably due t o dissociation. A comparison with Table I1 shows that the Cr(AA)3 peak is displaced by the Fe(AA), toward Co(AA)3. Figure 6 shows the same four component separation using liquid-liquid system E with a trace of ligand in the phase system. The interesting feature is that the iron peak has become symmetrical under these conditions and the retention ratio of cobalt and chromium is in agreement with Table 11. This separation can be done in less than 20 minutes. Figure 7 shows a rapid four-component separation of Be(AA)2, CU(AA)~,RU(AA)~,and Co(AA)3 in the presence of a trace of ligand in the phase system. Figure 8 shows a six-component separation of Be(AA)2, Cu(AA)z,


Al(AA),, Cr(AA)3, Ru(AA)3, and Co(AA)s, accomplished in 25 minutes. The best method to characterize the speed of a chromatographic separation is by the use of “peak capacity” (17, 18) which is the maximum number of peaks which can be separated with a given resolution in a given time under given conditions. Peak capacity can be obtained graphically from a plot of the standard deviation of a number of peaks as a function of the retention time. The peak capacity derived from the chromatogram shown in Figure 8 was 14 peaks in 20 minutes.

(17) J. C. Giddings, ANAL.CHEM., 39, 1027 (1967). (18) J. F. K. Huber and H. C. Smit, Z . Anal. Chem.,245,84(1969).

CONCLUSIONS The results of this investigation indicate that the use of high speed liquid-liquid chromatography for the separation of neutral metal chelates can be a potentially powerful technique. The search for selective phase systems for other metal chelates containing different ligands will be continued, and application of the liquid chromatographic method to the quantitative analysis for metals will be developed.

RECEIVED for review December 27, 1971. Accepted April 18, 1972. Presented at the 21st Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Cleveland, Ohio, March 1970. Acknowledgement is made to the donors of the Petroleum Research Fund administered by the American Chemical Society (PRF 3516-B) and the National Science Foundation (GP-18755) for partial support of this work.

Determination and Prediction of Anomalous Response Factors for Halogenated Substances with the Thermal Conductivity Detector Eugene F. Barry,’ Richard S. Fischer, and Douglas M. Rosie Department of Chemistry, University of Rhode Island, Kingston, R .I. 02881 The recorded peak area arising from an eluted solute in a thermal conductivity detector is a function of the molecular properties of the solute and carrier gas. I n this study we have determined signal strengths in the form of relative molar response (RMR) factors for nearly 40 halogenated hydrocarbons. These substances exhibit substantially different responses compared to other classes of compounds. The RMR data have also been predicted through the molecular diameter approach suggested by Littlewood. Our RMR equation can adequately describe the trends in the experimental RMR data. The anomalous response of the halogenates can be explained by their molecular diameters which are comparable with analogous hyd rocarbons.

PREVIOUS STUDIES of the gas chromatographic thermal conductivity detector have shown that the use of relative molar response (RMR) factors is essential for accurate quantitative analysis (1-3). Other investigations have confirmed that the signal strength arising from the presence of an eluted solute in a typical detector depends on the nature of the solute (4-9). The RMR data published by Messner et al. (3) established that relative response is a linear function of molecular weight Present address, Department of Chemistry, Lowell Technological Institute, Lowell, Mass. 01854 (1) A. B. Littlewood, Nature, 184,1631 (1959). CHEM., 29,1263 (1957). (2) D. M. Rosie and R. L. Grob, ANAL. (3) A. E. Messner, D. M. Rosie, and P. A. Argabright, ibid.,31, 230 (1 9..5. 9.) . ~ ,_ (4)E. F. Barry and D. M. Rosie, J. Chromatogr., 59,269 (1971). ( 5 ) Zbid.,63,203 (1971). (6) E. G. Hoffmann, ANAL.CHEM., 34,1216(1962). (7) J. Novak, S . Wicar, and J. Janak, Collect. Czech. Chem. Commun., 33,3642(1968). (8) H. Luy, Z . Anal. Chem., 194,241 (1963). (9) R. Mecke and K. Zirker, J. Chronzatogr.,7,1(1962).

within an homologous series and invariant over a wide range of experimental conditions. Compounds containing heavy atoms represent a special case as their thermal response differs markedly from other classes of compounds. For example, Hoffmann (6) reported that the RMR of tetraethyl lead is approximately one third of the value expected from the empirical molecular weight proportionality rule. Carbon tetrachloride has a response equivalent to an alkane which is one half the molecular weight of CC1,. This decrease in response observed with the insertion of heavy atoms in a molecule results from an increase in electron or molecular density within the molecule. It is well known that halogenates form “dense” gases with small collision cross-sections compared to analogous hydrocarbons (10). Recently RMR factors have been accurately predicted ( 4 , 5 ) by the molecular diameter approach suggested by Littlewood ( I ) . With helium as carrier gas, RMR data for nearly 100 hydrocarbons and oxygen-containing compounds was computed by the equation

where u and M indicate molecular diameter and molecular weight, respectively. The subscripts i, 1, and C$refer to the eluted solute, helium, and benzene, respectively. The factor of 100 represents the response of benzene arbitrarily assigned 100 response units per mole. The first term in Equation 1 was proposed by Littlewood and we included the second expression to explain the overall increase in RMR with molecular weight ( 4 ) . (10) A. B. Littlewood, “Gas Chromatography,’’2nd ed., Academic Press, New York, N.Y., 1970, p 370.

ANALYTICAL CHEMISTRY, VOL. 44, NO. 9 AUGUST 1972

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Rapid separation of metal chelates by column liquid-liquid

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