the reaction mixture with three aliquots of 0.250 ml of hexane, and concentrating the combined extracts gave recoveries of 92.1 f 2.5%SEM ( a = 12). This procedure gave a value for the Orchard Leaves of 2.41 f 0.12 SEM (n = 6) which is 92.7% of the NBS value. In conclusion, the feasibility of introducing the effluent from a gas chromatographic column directly into the burner of an atomic absorption spectrometer has been successfully demonstrated. This selective detection system for the analysis of chromium has been shown to have high sensitivity, somewhat better than flame analysis by aspiration of sample solutions. This increased sensitivity is due mainly to the improved efficiency of introducing the metal into the flame as a volatile species. Although this system does not have the extremely high sensitivity of graphite furnace atomization systems, the freedom from interferences as a result of the separation of the metal from the bulk matrix in the chelation-extraction-chromatographic procedure is a very great advantage in working with complex samples. While this system has been developed for use in the analysis of the essential nutrient chromium in foods and other biological samples, a great number of other metals have previously been chromatographed as volatile chelates and should readily be detected by this system. Indeed, the entire area of metal analysis utilizing volatile chelates should find use for this simple method
of obtaining a selective, sensitive, precise detection system that can be used with the procedures already established.
LITERATURE CITED R. W. Moshier and R.E. Sievers, “GasChromatography of Metal chelates”,
Pergamon Press, New York, 1965. G. Guiochon and C. Pommier, “Gas Chromatography in lnorganlcs and Organometallcs”, Ann Arbor Science Publishers, Ann Arbor, Mlch.,
1973. (3)J. Savory, P. Mushak, F. W. Snderman, Jr.. R. H. Estes. and N. 0. Roszel, Anal. Chem., 42, 294 (1970). (4)L.C. Hansen, W. G. Scribner, T. W. Gilbert, and R. E. Sievers, Anal. Chem., 43. 349 (1971). GIH. Booth, J,.; and W. J. Darby, Anal. Chem., 43, 831 (1971). W. R. Wolf, M. L. Taylor, 6. M. Hughes, T. 0. Tierman, and R. E. Slevers, Anal. Chem., 44, 616 (1972). E. E. Cary and W. H. Allaway, J. Agric. FoodChem., 19, 1159 (1971). W. R. Wolf, W. Mertz, and R. Masironi, J. Agric. Food Chem.. 22, 1037 (1974). W. R. Wolf, Federation of American Scieties f w Experimental Biology, April, 1975,fed. froc., 34, 927 (1975). W. R. Wolf, R. E. Sievers, and G. H. Brown, lnorg. Chem., 11, 1995 (1972).
RECEIVEDfor review March 4,1976. Accepted June 30,1976. Mention of a trademark or proprietary product does not constitute a guarantee or warranty of the product by the U.S. Department of Agriculture, and does not imply its approval to the exclusion of other products that may also be suitable.
Optimization of Gas Chromatographic Analysis of Complex Mixtures of Unknown Composition R. J. Laub and J. H. Purnell* Department of Chemistry, University College of Swansea, Swansea, Wales SA2 8PP
The method Is based on the prlnciple that if unknown mixtures are chromatographed with columns of A, of S, and of their mixtures, the data points for Individual solutes must accord with Equatlon 1, i.e. values of KR must be linear In ( b ~ .Supplementary Information such as peak height, peak wldth, etc. can be used to complete unambiguous linking of peaks from chromatogram to chromatogram. Subsequently, relatlve retentlons (a)can be calculated and, vla the previously described window diagram procedure, optlmum column composition and length for complete resolution can be calculated. The method provides means to achieve resolution In Ignorance of the nature of the mixture studied and can be applied to GLC or GSC with any type of column. The analysis of an Industrial still residues sample of ten malor and 33 minor unknown components is used to Illustrate the method.
In a recent series of papers (1-4), we have established that the general solution law is obeyed for 400 systems described in the literature. Here, K R is the infinite dilution partition coefficient of any solute between a solvent mixture (A + S) and the gas phase, K’R(A)and K’R(s) are the corresponding quantities for the pure liquids, A and S, respectively, and represents a volume fraction. Self-evidently, Equation 1 must apply for a binary mixture of adsorptive solids. Subsequently, we showed (5, 6) that Equation 1 provides a quantitative basis for optimizing GC separations of known mixtures. We now describe how the
+
1720
method may be applied to optimize separations of complex mixtures of unknown components.
METHOD The technique we describe below is based on the general validity of Equation 1,and for illustration we use binary stationary phases, although ternary mixtures, a t least, can also be treated similarly (6). Briefly, the theory is that, for any given solute eluted from A, S, and AIS mixtures, the corresponding K R / + A data must lie on a straight line. First, two solvents, A and S, of differing type are chosen, and chromatograms of the unknown mixture are obtained with a column of each a t some chosen and common temperature. The suitability of the two solvents can be assessed immediately, since, a t this stage, the only matter of interest is to obtain symmetrical peaks corresponding to reasonable numbers of theoretical plates ( N ) .If a satisfactory result, e.g., not less than 400 theoretical plates per foot with 100-120 mesh support, is achieved, we then assess the suitability of the chosen temperature from the point of view of both analysis time and of overall capacity factors (k’).Having then decided on a working temperature, the chromatograms are re-run. A standard solute for which K’R(A)and K’R(s) are accurately known is included in these and all subsequent runs which allows conversion of retention times to K R values. Hypothetical chromatograms for pure A and S columns are shown in Figure 1, ( a ) and (e). In each, four peaks appear, indicating, superficially, a four-component mixture, which is satisfactorily separated. The sample is then hypothetically run on three other columns containing, respectively, 2:1,1:1, and 1:2 (vol/vol) mixtures of the two solvents, and chro-
ANALYTICAL CHEMISTRY, VOL. 48, NO. 12, OCTOBER 1976
QA
Flgure 3. Window diagram for the 5 components identified via Figure
Figure 1. Hypothetical chromatograms of what appears to be an unidentified 4-component mixture
WINDOW 8 5
WINDOW A
5
4 3 2
1
3 4
21
I
KR
-
0
@A
Time
0
Figure 4. Hypothetical chromatograms for the 5 components of Figure 2 at the windows indicated in Figure 3
1
Flgure 2. Plots of KR vs. q4A constructed from the data contained in Figure 1
matograms (b)-(d) illustrate possible results. Only three peaks appear in ( d ) ,but we cannot a t this point assume that this is the only case where overlap has occurred. (We recognize that there will be changes in peak heights and areas which will be indicative of identities, but we shall return to this matter later.) The data from Figure 1 are now represented as a plot of K R vs. +A as shown in Figure 2, where each vertical set of points refers, from left to right, to chromatograms (a)-(e). Straight lines are drawn through the points, and we see for the first time that there are, in fact, at least five components. A window diagram (5,6) may now be constructed, since K R for each component is known as a function of q 4 and ~ so, CY values for all component pairs can be calculated over the whole range 4~ = 0-1. Figure 3 shows the plot (window diagram) of CY vs. +A wherein we see three almost equivalent optimum windows at the indicated values of +A and CY.Figure 4 shows the skeletal chromatogram to be expected with columns of +A corresponding to each window peak composition, where, for convenience, we assume the column dead volume to be negligible. The order of elution is determined by reading up the window lines indicated in Figure 2. Further, each peak can unambiguously be assigned an identification number,% even though their chemical identities are unknown. For example, we can now look back at Figure 1and specify that the peaks shown correspond, in order of elution, to: ( a ) 1 2,4,3,5; ( b ) 1 , 2 , 3 4,5; ( c ) 1 , 2 , 3 , 4 5; ( d ) 1 + 3 + 5,2,4;and ( e )5 , 3 , 1 , 2 4. It will be appreciated that the foregoing example illustrates a situation of highly complex retention behavior, a feature which emphasizes the power of the technique.
+
+
+
+
in'
I
40
30
20 r,me,m,ns
IO
0
Figure 5. Chromatograms showing the major constituents of an industrial still residue mixture at 100 OC with columns of: (a)di-n-nonyl phthalate (DNNP); ( b ) squalane and DNNP (4 = 0.6276); (c)squalane and DNNP (4 = 0.3315); (d)squalane Columns: 180 cm X 0.4 cm (i.d.) glass. Support: Chromosorb G (AW-DMCS, 100-120 mesh), wt YODNNP: 4.4024; wt Y squalane: 3.7719. Inlet pressure: 30 psig. Packings (b) and (c) were made by mechanicallymixing appropriate amounts of packings (a)and (0).Perkin-Elmer Model F-1 1. All off-scale peaks were singlets at lower sensitivity settings
ANALYTICAL CHEMISTRY, VOL. 48, NO. 12, OCTOBER 1976
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I
IC
Figure 6. Partition coefficients for peaks of
Figure 5 plotted vs. ~ D N N P (a) : without connecting lines; ( b ) lines drawn in for components identlfied
by inspection; (c)all possibilities using all points
Table I. Partition Coefficients for Peaks of Figure 5 . K R at
Peak No. 1
2 3 4 5 6 7 8 9
6~ =
= 0.0749)
0.0000
0.3315
0.6276
1.0000
35.9 69.9 92.8 116 137 154 257 333
49.1 92.2 117 130 151 158 176
62.1 113 138 162 178 206 261 521 749
76.2 136 163 204 215 248 321 673 998
127
138
...
10
...
Toluene
108
212
395 554 117
...
...
Finally, Figures 3 and 4 indicate that a column of $A = 0.65 (window C), of sufficient length to provide the number of theoretical plates ( N ) required by the predicted window maximum a value, will provide marginally, the best separation. Furthermore, reference to Figure 2 shows that it provides the fastest analysis since the last emerging peak has the lowest K R in this window. It is recognized that from time to time a situation may arise where two or more solutes have virtually identical values of both K’R(A) and K’R(s) and so appear as a single peak at all times, irrespective of +A. However, this is not so difficult a problem as might seem since much ancillary information is contained in the original chromatograms. First, although it is not relevant to the above problem, unlike the situation depicted in Figure l, all peaks will not be equal in height. For instance, we can now see that had we assumed equal amounts of components 1-5 in the data of Figure 1, the areas of the multiple peaks would have been: ( a i) = 2, ( b iii) = 2, (c iv) = 2, ( d v) = 3, and ( e iv) = 2, all other peaks being of unit area. Thus, inspection of the chromatograms will be a considerable aid in making the assignments involved in Figure 2 and in constructing Figure 3. Second, peak widths can also be useful, 1722
Table 11. Partition Coefficients for Peaks of Figure 7 ( 4 ~
’
Peak No. 1
2 3 4
5 6
KR 39.2 74.8 98.0 102 125 146
Peak No. 7 8 9 10 Toluene
KR 155 168 288 384 110
particularly in the context of the problem cited above. Any peak which is suspiciously broad, that is, shows a low value of N with respect to its neighbors, may indicate lack of separation. Finally, having optimized the separation, ancillary techniques, such as mass spectrometry, can be used both for identification and as an indication of lack of separation. If, for any reason, it is still thought that the mixture may not have been completely resolved, the whole procedure can be repeated with other pairs of solvents. This is not a timeconsuming matter, since, once a laboratory has a set of “standard” pure solvent columns, and has amassed the corresponding K R data for the added standard solutes, the necessary data can be obtained quickly. Indeed, any solvent pair can be fully explored in a day or two for a mixture of almost any degree of complexity. We now consider the final choice of working temperature. The foregoing procedure will have specified a column composition and length which will achieve complete separation of the mixture at the working temperature. What, then, might be the object of searching for an alternative temperature? First, overall analysis time may be undesirably long unless low solvent/support ratios are used. This however, may introduce one or both of two significant problems: (a)unacceptable solid andlor liquid surface adsorption effects and, even if these are absent, ( b )the increasing N requirement for separation as the capacity factor (k’)is reduced. Second, it is generally,although not always, true that reduction of analysis temperature increases a values which, in principle, allows separation with
ANALYTICAL CHEMISTRY, VOL. 48, NO. 12, OCTOBER 1976
Figure 7. Chromatogram of the industrial stili residue with a column of &"p = 0.0749; other conditions as specified in Figure 5
i
ODNNP Figure 9. Window diagram for solutes constructed from data contained in Figure 8. Highest windows occur at $D"P = 0.2940 (a= 1.081), and ~ D N N P= 0.7340 (CY= 1.084)
1000
60
50
40
30 Tim.,
20
10
1
0
mlns
Figure 10. Chromatogram showing complete resolution at 100 O C of the major components of the industrial stili residue mixture with column packing of &,"P = 0.2940. Column: 360 cm X 0.15 cm (Ld.). All other condltions as in Figure 5 except use of Pye 104
@A
Figure 8. Correct KR vs. &"p plot for the 10 unambiguously identified major componentsof the industrial still residue mixture at 100 O C shorter columns. However, this corresponds to increased values of K R which, since it means longer analysis times, takes us back to, and to some extent, exacerbates, the first of the problems listed. Thus, while we might consider repeating the entire optimization procedure a t a lower temperature, the most profitable initial approach would be to carry out test runs with the already-optimized column a t lower temperatures. The optimum volume fraction of the binary stationary phase may vary slightly with temperature but unless drastic temperature changes are used, this effect will be inconsequential. Since CY generally decreases with increased temperature, it is unlikely that the optimized column will provide complete (60)separation a t a temperature higher than the test temperature, and so, only if column length is of no consequence and all materials are well below the point of thermal instability, should a move in this direction be undertaken. Although a quantitative procedure for temperature optimization can be developed along the lines we indicate, we feel it to be of little value to discuss it further here, since the strategy is self-evident. We also note that the entire procedure could easily be computerized.
EXPERIMENTAL All chromatogramswere obtained at 100 O C with silanized glass columns which were packed with 100/120-mesh Chromosorb G (AW-DMCS).Both a Perkin-Elmer Model F-11and a Pye Unicam Model 104 gas chromatograph were employed. The solvents,squalane (S) and di-n-nonylphthalate (DNNP) (A), were reagent grade from B.D.H. Ltd., and were used without further purification. Liquid loadingsfor the pure solvent columns were 3.77 w t % (S) and 4.40w t % (A) and binary stationary phases comprised mechanical mixtures of these. The mixture analyzed below is an industrial still residues sample of composition unknown to us. Other details and procedures have been published elsewhere (46). RESULTS Only very rarely does any mixture contain all components at about the same concentration level. Whatever the ultimate analytical objective, the first requisite is complete separation of the major components. Thus, the most suitable initial strategy is to work at detector attentuations which reveal only the majors and keep them "on scale" for accurate K R measurement. Figure 5 shows four chromatograms of the industrial still residues mixture, only the major constituents being shown. The DNNP column (Figure 5a) yields 9 peaks, a column of (PA = 0.6276 (Figure 5b) gives 9 peaks also while one of (PA = 0.3315 (Figure 5c) gives 10 peaks and a pure squalane column (Figure 5 4 resolves only 8. Toluene was eluted from each of
ANALYTICAL CHEMISTRY, VOL. 48, NO. 12, OCTOBER 1976
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20
24
12
0
1
1
270
I
I
I
240
210
180
I
150 Time mins
I
I
I
I
120
90
60
30
I 0
Flgure 11. Chromatogram of the industrial still residue mixture with column as specified in Figure 10 at 75 "C All conditions Identical, except sensltivlty increased by a factor of 16 and inlet pressure reduced to 15 psig. Numbering now sequential
the columns and, since its K R for each was accurately known, those for the unknowns were calculated from relative retention data. The data for all peaks and columns are given in Table I, and Figure 6a shows the K R values plotted vs. $A. The numbered data points in the figure correspond to the peak numbers in Table I and Figure 5. By inspection of the chromatograms in Figure 5 and the points in Figure 6a, we can immediately draw several connecting lines between K"R(s)and K"R(A)values. As one example, the data for the peaks numbered 8 ($A = 0),10 ( 4 =~ 0.3315), 9 ($A = 0.6276), and 9 ( 4 =~ 1) all lie on a single straight line and, furthermore, the peaks have the same area. Therefore, these peaks must be due to the same component. In contrast, we can draw a line which connects data point No. 9 ( 4 =~0.6276) with data point No. 8 ( 4 =~ l),but extrapolation of such a line predicts a peak appearing after No. 10 a t (PA = 0.3315; since no such peak was found, we eliminate this possibility. Figure 6b shows the same data points as in Figure 6a, except that now, those points which, by inspection (as outlined above) have been shown to relate to a given component, have been connected by straight lines. We see that 7 components have, in this process, been unambiguously identified. We must now turn to a consideration of the several points left over and, as yet, unconnected. Clearly, the components involved must, at some value or other of 4 ~overlap , others. To decide which ones they overlap, all possible straight lines are drawn through the various unconnected points such that each line has 4 points on it, as shown in Figure 6c. Previously-used points are used again, if necessary. We now see that there are 13 lines in total, i.e., 13 possible components. Some of the lines in Figure 6c may be fictitious, that is, it may be coincidental that a straight line can be drawn through a set of points. To test this prospect, a column composition is selected by consideration of Figure 6c, such that all (possible) components would be at least partially resolved if present. Such a column composition occurs at C$A = 0.075; Figure 7 shows the corresponding chromatogram where, still, only 10 peaks are seen. The partition coefficients are given in Table 11,and are shown plotted in Figure 8, where the three fictitious lines of Figure 6c have been eliminated. We are now in a position to optimize the separation of these 10 components; Figure 9 shows the window diagram, where the two best cy values indicated are 1.081 ( 4 =~ 0.294) and a = 1.084 ($A = 0.734). Referring back to Figure 8, we find that analysis times for the latter window will be considerablylonger than those for the first. Since the two windows are for all practical purposes identical in terms of a,thus offering no real choice on the basis of resolution, we choose the window at 4~ = 0.294. Figure 10 shows the chromatogram obtained with a column containing packing made up at the indicated 4~ value, 1724
and of appropriate length to provide the number of theoretical plates demanded by the relevant value of cy where all 10 components are resolved in 52 min. If the window at $A = 0.734 had been used, the corresponding analysis time would have been about 80 min. Finally, Figure 11 shows a chromatogram of the mixture with the column of 4~ = 0.294 operated a t lower flow rate but at a higher sensitivity setting a t 75 "C. There are now 43 clearly visible peaks. In order completely to resolve the entire mixture, the optimization procedure would have to be repeated at the higher sensitivity setting, but this is a relatively trivial task as was indicated earlier. We have now applied the above approach to a number of complex mixtures of initially unknown compositionwith equal success. It seems likely that the method will prove a powerful addition to chromatographic technique since it can be applied to GLC and GSC and for any type of column, provided only that these can be accurately packed or coated. We recognize, of course, that what we have provided is a solution specific for mixtures of two particular packings. Obviously, a better optimization might be achieved with some other pair in the sense of requiring shorter columns, faster analysis, or some other desirable feature. To some extent, the judgment will be subjective; to a further extent, it may be defined by physical limitations imposed by equipment. If, in the end, an unsatisfactory solution is achieved, even though it has provided the desired complete resolution, further studies according to our method can be carried out until an acceptable situation is found. As we have stated earlier, a binary packing can be exhaustively studied in a matter of days so no great time is involved. Finally, although we have so far (1-6)been unable to find a solvent pair which does not behave in accord with Equation 1,the possibility exists and it might be argued that this destroys the generality of our method. Mechanical mixtures of packings of pure solvents always obey Equation 1; hence, their use avoids any ambiguity arising on this account.
LITERATURE CITED (1) J. H. Purnell and J. M. Vargas de Andrade, J. Am. Chem. Soc., 97, 3585 (1975). (2) J. H. Purnell and J. M. Vargas de Andrade, J. Am. Chem. Soc.. 97. 3590 (1975). (3) R. J. Laub and J. H. Purnell, J. Am. Chem. Soc., 98, 30 (1976). (4) R. J. Laub and J. H. Purnell, J. Am. Chem. Soc., 98, 35 (1976). (5) R. J. Laub and J. H. Purnell, J. Chromafogr., 112, 71 (1975). (6) R. J. Laub and J. H. Purneii, Anal. Chem.. 48, 799 (1976).
RECEIVEDfor review May 10,1976. Accepted July 9,1976. We acknowledge the support of the Science Research Council.
ANALYTICAL CHEMISTRY, VOL. 48, NO. 12, OCTOBER 1976
High-pressure Liquid-Liquid Partition Chromatography of Metal Chelates of Tetradentate ,&Ketoamines Enrico Gaetani and Carlo F. Laureri lstituto di Chimica Farmaceutica e Tossicologica, Universita di Parma, 43 100 Parma. Italy
Alessandro Mangia' and Giovanni Parolari lstituto di Chimica Generals ed Inorganica, Universita di Parma, 43 100 Parma, Italy
High pressure liquid-liquid partition chromatography was applied to some metal chelates of the tetradentate p-ketoamines: N,N' -ethylenebls(acetylacitonelmine), (H2(en)AA), N,N'-trimethylenebls( acetylacetoneimine), (H2(tm)AA) and N,N'-ethylenebb( benzoylacetonelmlne) (H2(en)BA). The complexes Coli, Ni", Cull, Pd"(en)AA, NI", Cu"(en)BA, Cu"(tm)AA have been consldered. Nickel and copper were separated by using H2(en)AA and H2(en)BA on two different columns. Palladiumwas successfully separated from copper but not from nickel. The dependence of the response of the wv detector (254 nm) on the amount of metal In aqueous solutions is reported for Ni and Cu(en)AA. The detection limits are about 0.2 and 0.5 ng of metal injected for NI and Cu, respectlvely.
High-pressure liquid chromatography is most widely employed in organic chemistry, but some examples of its application to inorganic and organometallic compounds have been reported. Ion exchange technique has been used for separation and determination of FeI" ( I ) , SbIII, BPI, Crvl, A@, HgII, PdI1, PtIV,RuIV,TlIII, SnIV(2), PblI ( 3 ) ,ThIV,Ca, CuII, Mn", Nil1 (41, rare earths (5), and transplutonium elements using di(2-ethylhexyl)orthophosphoric acid as a stationary phase (6). With liquid-solid high-pressure chromatography, the separation of HgII, CUI' (7), SnlI (8)has been achieved. The partition liquid-liquid technique, with direct or reversed phase, has been chosen for organometallic CrlI1 (9, 10) and FelI1 (11) compounds and for the P-diketonates of several divalent and trivalent metals (12). We applied the reversed phase liquid-liquid partition chromatography to some metal complexes of the tetradentate B-ketoamines
of CoII(en)AA, NilI(en)AA, Cdl(en)AA, PdlI(en)AA, Ni'I(en)BA, CuYen)BA, Cu"(tm)AA. The dependence of tlie detector response on the amount of Ni and Cu in aqueous solutions is also reported.
EXPERIMENTAL Preparation of Ligands and Metal Chelates. Described procedures were followed in the preparation of Hz(en)AA, Hz(tm)AA, Hz(en)BA, Ni(en)AA, Cu(en)AA, Cu(tm)AA, Cu(en)BA ( 1 5 ) and Pd (en)AA( 16). Except for Hz(tm)AA,the compounds were characterized by elemental analysis, mp, and mass spectra. The mass spectra of the complexesof Hz(en)AAand Hz(tm)AAagree with those reported (17). The mass spectra of Ni(en)BA and Cu(en)BA show the expected fragmentation patterns with parent ion peaks. The cobalt(I1) complex was not isolated, but prepared by adding small excess of cobalt acetate to the methanolic solution of Hz(en)AA in nitrogen atmosphere. The yellow solution obtained was directly used for the chromatographic analysis. High-pressure Liquid Chromatography. Apparatus. A Varian Aerograph 8500,with a single wavelength uv detector (254nm) was used; the full-scale sensitivity was 0.005 absorbance unit; flow cell volume, 8 ~ 1 . Columns. The columns used were stainless steel 25 cm X 0.2cm i.d. MicroPak CH (Octadecylsilaneon silica gel 10-kmdiameter),stainless steel 50 cm X 0.2cm i.d. slurry packed (18) with silica gel (10-fimdiameter) bonded to 3-aminopropyltriethoxysilane(19) (-NH2 column). M phosphate or borate buffer As moving phase, methanol-6 X mixtures were used. The pH of the buffer solutions ranged between 7.0 and 11.0.The flow rate of the eluent was 1or 1.5 cm3min-l at a pressure of 225-300 atm. Spectroscopicgrade solvents were used. Peak areas were measured by a Varian CDS 101 integrator. Mass Spectrometry. Mass spectra of ligands and chelates were run on a Varian MAT CH5 spectrometer at the ionizing voltage of 70 eV. The solid samples were directly introduced into the source by means of a direct insertion probe. Source temperature was 220-240 "C.
Ultraviolet-Visible Spectrometry. Electronic spectra of the chelates were run on a Perkin-Elmer model 402 spectrophotometer. Methanol and tetrahydrofuran (spectroscopic grade) were used as solvents. HC C ' -OH
/
HO-C
/;""
RESULTS AND DISCUSSION N,N'-Ethylenebis(acety1acetoneimine)and its analogues,
\
R with B = (CH,), R = CH3 (H,(en)AA) B = (CH,), R =CHj (H,(tm)AA) B = (CH,), R = CeH5(Hden)BA) R
Our aim was to study the behavior of metal chelates in high pressure partition chromatography and to evaluate the possibility of using this method in the determination of metals, The separation of Ni and Cu using the ligand Hn(en)AAhas been briefly reported by us (13). Independently and at the same time, other authors published the separation of Ni(en)AA and Cu(en)AA (14);in this case liquid-solid chromatography was used, with microparticulate silica as a stationary phase. The present paper deals with the chromatographic behavior
particularly the fluorinate ones, have been successfully employed in the gas chromatographic analysis (17,20-22).Owing to the high values of the stability constants of their complexes (log K = 23 for Cu(en)AA (23)),which are soluble in medium and low polarity solvents, these ligands are also suitable for liquid-liquid partition Chromatography; at high pH, the values of the total distribution coefficient of the metals are approximately coincident with those of the partition coefficient of the complexes (12). Moreover the high values of the molar absorptivities in the uv region allow good sensitivity with photometric detectors. The low number of metals with which they form stable complexes ( 2 4 ) ,reduces the problem of interferences. On the MicroPak CH column different moving phases were used, varying the volume ratio between methanol and the
ANALYTICAL CHEMISTRY, VOL. 48, NO. 12, OCTOBER 1976
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