separation of metal chelates and organometalic

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J. Biochem. Biophys. Methods 43 (2000) 273293www.elsevier.com/locate/jbbm

Separation of metal chelates and organometalliccompounds by SFC and SFE/GC

*C.M. Wai , Shaofen Wang Department of Chemistry , University of Idaho , Moscow , ID 83844 -2343, USA

Abstract

Supercritical uid chromatography (SFC) combines the high diffusion coefcients of gaschromatography (GC) and the solubility properties of liquid chromatography (LC). SFC generallyrequires lower temperatures for chromatographic separations and thus is more suitable foranalyzing thermally labile compounds including a number of metal chelates and organometalliccompounds. SFC also allows interfacing between supercritical uid extraction (SFE) andchromatographic analysis of metal-containing compounds. A large number of metal chelates andorganometallic compounds can be separated by SFC. This article summarizes SFC separation of

various chelates of transition metals, heavy metals, lanthanides and actinides as well asorganometallic compounds of lead, mercury, and tin reported in the recent literature. This articlealso discusses SFC detection systems and the determination of solubility of organometalliccompounds by SFC. 2000 Elsevier Science B.V. All rights reserved.

Keywords : Supercritical uid chromatography; Metal chelates; Organometallic compounds; Solubility

Abbreviations : acac, acetylacetone; AFS, atomic uorescence spectrometry; CO , carbon dioxide; CCl F ,2 2 2dichlorodiuoromethane; Cu(DDC) , copper diethyldithiocarbamate; Cu(FDDC) , copper bis(tri-2 2uoroethyl)dithiocarbamate; Cr(Phacac) , tri(1-phenyl-1,3-butanedione)chromium(III); DMA, dimethylarsinic3acid; Et NHDDC, diethylammonium diethyldithiocarbamate; F B(Phacac), diuoro(1-phenyl-1,3-2 2butanedione)boron(III); FID, ame ionization detection; FOD, 2,2-dimethyl 6,6,7,7,8,8,8-heptauoro-3,5,-octanedione; FPD, ame photometric detection; FT-IR, Fourier-transform infrared spectroscopy; GC, gaschromatography; GC-AED, gas chromatography with atomic emission detection; GC-ECD, gas chromatog-raphy with electron capture detection; GC-FPD, gas chromatography with ame photometric detection;H DIB, 2,6-diacetylpyridine bis(benzoylhydrazone); HFA, hexauoroacetylacetone; ICP-AES, inductively2coupled plasma-atomic emission spectrometry; ICP-MS, inductively coupled plasma-mass spectrometricdetection; LC, liquid chromatography; LiFDDC, lithium bis(triuoroethyl)dithiocarbamate; MIP, microwave-induced plasma; MMA, monomethylarsonic acid; NAA, neutron activation analysis; NaDDC, sodiumdiethyldithiocarbamate; Phacac, 1-phenyl-1,3-butanedione; SFC, supercritical uid chromatography; SFE,supercritical uid extraction; TBP, tributyl phosphate; TBPO, tributylphosphine oxide; TFA, triuoro-acetylacetone; THD, dipivaloylmethane; TOPO, trioctylphosphine oxide; TTA, thenoyltriuoroacetone; TTA ?Py, thenoyltriuoroacetonepyridine

*Corresponding author. Tel.: 1 1-208-885-6552; fax: 1 1-208-885-6173. E -mail address : [email protected] (C.M. Wai)

0165-022X/00/$ see front matter 2000 Elsevier Science B.V. All rights reserved.PI I : S0165-022X(00)00061-0


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274 C . M . Wai , S . Wang / J . Biochem . Biophys . Methods 43 (2000) 273 293

1. Introduction

Supercritical uid extraction (SFE) of metal species and organometallic compounds

has been the subject of many reports in recent years. Carbon dioxide (CO ) is a gas of 2choice in SFE because of its moderate critical constants ( T 5 318C, P 5 73 atm,c cr 5 0.47 g/ml), inertness, and availability in pure form. Major advantages of extractingcmetal species using supercritical CO include minimization of waste solvent generation2and direct removal of analytes from solid samples. One approach of SFE of metals is byconverting charged metal ions into neutral metal chelates using organic chelating agentsdissolved in supercritical CO [14]. Important considerations for the selection of 2suitable ligands for this in situ chelationSFE of metal ions include good solubility andstability of the complexing agents and their metal complexes in supercritical CO . 2Organometallic compounds usually are extractable by supercritical CO . Derivatization2is sometimes needed to enhance the solubility of certain organometallic compounds insupercritical CO . Quantication of the extracted metal chelates and organometallic2compounds can be achieved by supercritical uid chromatography (SFC).

SFC combines the high diffusion coefcients of gas chromatography (GC) with thesolubility properties of liquid chromatography (LC). Compared with GC, SFC generallyrequires lower temperatures for chromatographic separations and thus is more suitablefor analyzing thermally labile compounds. Furthermore, using CO as the mobile phase2in SFC will enormously reduce the waste organic solvent generation. Although most of the SFC reports to date have dealt with organic compounds, the rst report regarding theapplication of SFC in 1962 was with organometallic compounds [5]. Using GC forseparation of metal chelates, difculties often arise due to their low vapor pressures. Toincrease volatility of metal chelates by raising temperatures often causes decompositionof the analytes during GC separation. In LC, problems arise from limited resolution,degradation, and irreversible adsorption of the analytes to the stationary phase. SFC,which combines the solubility property of LC and the diffusivity of GC, can oftenovercome the detection and separation problems associated with GC or LC analysis of metal-containing compounds. Since solubility is an important factor in SFC separation,we shall start our discussion with the solubilities of metal chelates and organometalliccompounds in supercritical CO .2

2. Solubility of metal chelates and organometallic compounds in supercriticalCO 2

The solubilities of metal complexes in supercritical CO reported in the literature vary2signicantly depending on the chemical nature of the complexes. In 1991, Wai et al. [6]rst reported the solubilities of a number of metal dithiocarbamates in supercritical CO 2determined by UV-VIS spectroscopy using a high-pressure view-cell with quartzwindows. A signicant nding in this study is the observation that uorinated metal

dithiocarbamates exhibit unusually high solubilities relative to their non-uorinatedanalogues, typically by 2 to 3 orders of magnitude. For example, the solubility of copper


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C . M . Wai , S . Wang / J . Biochem . Biophys . Methods 43 (2000) 273 293 275

diethyldithiocarbamate Cu(DDC) in CO at 50 8C and 100 atm was found to be around2 22 61.1 3 10 mol/ l (M). However, the solubility of copper bis(triuoroethyl)dithiocarba-mate Cu(FDDC) , a uorinated analogue of Cu(DDC) , was reported to be of the order2 22 4of 9.1 3 10 M at the same temperature and pressure [6]. The CO -philic behavior of 2uorinated metal chelates observed in this study is very important for the subsequentdevelopment of SFE techniques for metals. After this report, a number of uorinatedchelating agents were tested for SFE of metals and for SFC of metal complexes[14,710]. Metal chelates formed by uorinated b-diketones and macrocyclic com-pounds were also found to have signicantly higher solubilities in supercritical CO 2relative to their non-uorinated analogues [2,11]. Table 1 shows the solubilities of somemetal dithiocarbamate chelates in supercritical CO [1,6].2

In recent years, high-pressure ber-optic cells were developed for spectroscopicmeasurements in supercritical CO [12,13]. These high-pressure ber-optic cells are2simple and economic to build. Fiber-optic cells of different pathlengths can beassembled together allowing solubility measurements to be carried out over a wideconcentration range. Using this technique, Carrott and Wai [12] measured the solubilitiesof UO (NO ) ?2TBP (uranyl nitrate tributyl phosphate) and UO (TTA) ?TBP in2 3 2 2 2supercritical CO , where TTA is thenoyltriuoroacetone, a uorinated b-diketone2commonly used for solvent extraction of lanthanides and actinides. These uranylcomplexes show very high solubilities in supercritical CO . An interesting observation is2that if a water molecule is coordinated with uranylTTA complex as in the case of UO (TTA) ?H O, the solubility of the complex in supercritical CO is about two orders2 2 2 2of magnitude lower than its TBP coordinated complex UO (TTA) ?TBP. Apparently,2 2coordinated water molecules due to its polar nature can reduce the solubility of metalchelates in supercritical CO . If the coordinated water molecule is replaced by a suitable2organic ligand, the solubility of the metal complex in supercritical CO can be2signicantly increased. This solubility behavior was utilized by Lin et al. [14,15] for

Table 1Solubility of uorinated (FDDC) and non-uorinated diethyldithiocarbamates (DDC) in supercritical CO at 2508C [1,6]

Metal chelate Solubility (mol / l) Pressure (atm) SF phase2 4Cu(FDDC) (9.1 6 0.3) 3 10 100 CO2 22 6Cu(DDC) (1.1 6 0.2) 3 10 100 CO2 22 4Ni(FDDC) (7.2 6 1.0) 3 10 100 CO2 22 7NI(DDC) (8.5 6 1.0) 3 10 100 CO2 22 4Co(FDDC) (8.0 6 0.6) 3 10 100 CO3 22 6Co(DDC) (2.4 6 0.4) 3 10 100 CO3 22 4Bi(FDDC) (7.3 6 1.0) 3 10 150 CO3 22 6Bi(DDC) (9.0 6 0.6) 3 10 150 CO3 22 3Hg(FDDC) (5.0 6 0.4) 3 10 150 CO2 22 6Hg(DDC) (8.2 6 0.6) 3 10 150 CO2 22 2

Hg(FDDC) (1.2 6 0.4) 3 10 150 CO 1 5% MeOH2 22 5Hg(DDC) (3.0 6 0.5) 3 10 150 CO 1 5% MeOH2 2


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Fig. 1. SFC chromatograms of lanthanide FOD adducts with (A) TBPO and (B) TOPO. Peaks: A (1)chloroform, (2) TBPO, (3) Er(FOD) ?2TBPO, (4) Eu(FOD) ?2TBPO, (5) Pr(FOD) ?2TBPO, (6) La(FOD) ?3 3 3 32TBPO. B (1) chloroform, (2) TOPO, (3) Er(FOD) ?2TOPO, (4) Eu(FOD) ?2TOPO, (5) Pr(FOD) ?2TOPO,3 3 3(6) La(FOD) ?2TOPO. SFC conditions: initial 80 atm, 5 atm/min increase, oven temperature: 80 8C, FID:33908C (Ref. [14]).

SFC separation of a number of lanthanide b-diketone complexes by means of adductformation with organophosphorus reagents (Fig. 1). Some relevant solubilities of lanthanide b-diketonates in supercritical CO are shown in Table 2 [16].2

Organometallic compounds usually are soluble in supercritical CO . The solubility of 2ferrocene was studied extensively by Clifford and Bartles group at the University of Leeds [17,18]. These authors used SFC retention data combined with a small number of experimentally determined solubility measurements to model the solubility of ferrocene

in supercritical CO . It was also shown that the same equation-of-state, with the same2parameters, could be used to predict quantitatively both supercritical uid solubility and

Table 2Solubility of lanthanide b-diketonates in supercritical CO at 40 8C [16]2

Metal complex Solubility (mol / l) SF phase

UO (TTA) ?X 200 atm 250 atm CO2 2 2X5 TBP 0.008 0.01 CO 2X5 TOPO 0.0015 0.003 CO 2

X5 TBPO 0.0002 0.0003 CO 2X5 H O 0.0001 0.00017 CO2 2UO (NO ) ?2TBP 0.2 0.4 CO2 3 2 2


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the temperature dependence of retention in SFC. Experimental results also indicated theinverse relationship between solubility and retention data in SFC if the same column wasused [1719]. Wai et al. [20] demonstrated that the retention behavior of a range of

ferrocene derivatives in SFC could be predicted based on a knowledge of simpleparameters such as the solubility parameter of the solute, molar volume of the solute andthe physical characteristics of the chromatographic column. The prediction was furthertested by studying a range of ferrocene derivatives with very different functionalities,ranging from the parent compound ferrocene to N , N -dimethylaminomethyl ferrocene, asshown in Table 3 [21].

Lagalante et al. [22] have studied the solubilities of 10 copper(II) and vechromium(III) b-diketonates in supercritical uid CO . A direct correlation was2observed between the metal complex solubility and the Hildebrand solubility parameterof the free ligand. This illustrates that for a given metal system, the nature of the ligandside-chain inuences the solubility of the metal complex in accordance with the regularsolution theory [23]. Lagalante et al. [22] observed that the uorinated b-diketonatesshowed higher solubilities, with the hexauoroacetylacetone complexes having thehighest values. The lowest solubility was observed for the benzyl-substituted b -diketonecomplexes, correlating with the general observation for organic compounds wherearomatic substituted systems showed the lowest solubilities. Similar trends wereobserved for the solubilities of the Cr(III) b-diketone complexes in supercritical CO 2(Fig. 2). The observed trends indicate that the solubility is strongly dictated by thecharacter of the hydrocarbon or uorocarbon shell surrounding the central metal atom.

Akgerman et al. [24] recently developed a dynamic measurement technique todetermine solubilities of chelating agents and metal chelates in supercritical CO . 2Solubilities of cupric acetylacetonate and copper diethyldithiocarbamate were measuredin supercritical CO . The authors also measured the solubility of a number of metal-2containing complexes for thermodynamic modeling and prediction of solubility insupercritical CO at varying operational conditions. The PengRobinson equation-of-2state and van der Waals mixing rules were employed in their modeling. The solubilityof copper diethyldithiocarbamate in supercritical CO was found to agree with the2results reported earlier by Laintz et al. [6]. The predicted solubilities tted well with the

Table 3Solubility parameters of ferrocene derivatives in supercritical CO [21]2

Compounds Solubility parameter Molar volume3(mol / l) (cm /mol)

Ferrocene 10.8 122.7Ethyl ferrocene 9.81 172.3t-Butyl ferrocene 9.39 204n-Butyl ferrocene 9.62 204.5di-t-Butyl ferrocene 8.73 285.3

tri-t-Butly ferrocene 8.33 366.6tetra-t-Butyl ferrocene 8.07 447.9N,N-Dimethylaminomethyl ferrocene 9.75 196


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Fig. 2. Plot of Ln( y ) solubility of various copper(II) and chromium(III) b -diketonates in supercritical CO at2 22 1a density of 0.874 g ml versus Fendors solubility parameter of the free ligand (Ref. [22]).

experimentally measured data for Cu(II) acetylacetonate and Cu(II) diethyldithio-carbamate.

3. SFC separation of metal chelates-transition metals and heavy metals

Arsenic and antimony are two toxic elements which show different toxicological and

physiological behavior depending on their oxidation states and chemical forms. Forinstance, inorganic arsenite (As(III)) is more toxic than arsenate (As(V)) and organicarsenic compounds show less toxicity than the inorganic species. Monitoring arsenicspecies in natural waters or waste streams is important for environmental studies. Laintzand Wai [8] in 1992 demonstrated that arsenic and antimony-bis(tri-uoroethyl)dithiocarbamate complexes could be separated by SFC from a mixturecontaining Zn, Ni, Co, Fe, Hg, As, Sb and Bi with excellent resolutions using a 5-mSB-Methyl-100 100- mm i.d. column (Fig. 3). The overall analytical results agreed wellwith those determined by solvent extraction followed by neutron activation analysis(NAA) and inductively coupled plasma-atomic emission spectrometry (ICP-AES). A

number of inorganic and organic species of arsenic can also be determined using solventextraction or SFE followed by the SFC method [8,25]. The organic arsenic compounds,CH AsO(OH) (monomethylarsonic acid, MMA) and (CH ) AsO(OH) (dimethylarsinic3 2 3 2


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Fig. 3. Chromatogram produced using a Lee Scientic 5 m 3 100 mm I.D. SB-methyl-100 Superbond capillarycolumn. SFC conditions: 80-nl sample injected at 100 8C oven temperature with a hold time of 6.5 min at 100atm followed by a 4.0 atm/min ramp to 200 atm. Peaks: a, Na(FDDC); b, Zn(FDDC) ; c, Ni(FDDC) ; d,

2 2Co(FDDC) ; e, Fe(FDDC) ; f, Hg(FDDC) ; g, As(FDDC) ; h, Sb(FDDC) ; i, Bi(FDDC) . (Ref. [8]).3 2 2 3 3 3

acid, DMA), in aqueous solutions were converted to CH AsI and (CH ) AsI,3 2 3 2respectively, using a mixture of potassium iodide, sodium thiosulfate, and sulfuric acid[8]. The iodides were extracted with FDDC into chloroform as CH As(FDDC) and3 2(CH ) As(FDDC). The resulting FDDC chelates, As(FDDC) , CH As(FDDC) and3 2 3 3 2(CH ) As(FDDC), were separated by SFC and the recovery was generally . 90%. In a3 2subsequent report, simultaneous separation and quantication of As(III) and Sb(III) innatural water and urine samples were demonstrated by the same authors using SFE with

LiFDDC as an extractant followed by SFC analysis of the extracted FDDC complexes[9]. A second aliquot of sample was reduced using sodium thiosulfate and potassiumiodide for the determination of total As and Sb. Pentavalent concentrations were then


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determined by subtraction. This analytical method produces satisfactory recovery resultsfor arsenic and antimony speciation studies. Concentrations ranging from 1 to 150 ppbfor natural water and 1 to 10 ppm for urine samples for both As and Sb trivalent and

pentavalent species can be determined using this method.A number of transition metals were investigated in SFE-gas chromatography with

atomic emission detection (GC-AED) [26]. The extraction of Cu(II), Co(II), Cd(II), andZn(II) ion from solid samples was carried out with LiFDDC in 5% methanol modiedCO at 200 atm and 40 8C followed by GC-AED. The recoveries of Cu(II), Co(II),2Cd(II), and Zn(II) ions from lter paper, sand, and silica gel ranged from 70% to 100%.Metal diisobutyldithiocarbamate complexes including Zn(II), Cd(II), Cu(II), Ni(II),Pb(II), Co(III), Fe(III) and As(III) were also separated by capillary SFC using a 10m3 50 mm i.d. SB-Phenyl-50 column at 120 8C by Manninen et al. [27]. Analyses werecarried out using a Lee Scientic Series 600 SFC equipped with ame ionizationdetection (FID).

Metal b-diketonates are the most commonly studied metal chelate system using GC,LC and SFC. In the rst SFC experiments with b-diketonates, 14 different metalchelates of TTA were studied, using dichlorodiuoromethane (CCl F ) as the mobile2 2phase with a column packed with 2% Kel-F wax on Chromosorb W [28]. CCl F was2 2also used in a subsequent investigation of 43 different metal acetylacetonates [29].Utilizing a packed column, SFC was also able to separate a mixture of b-diketonatesincluding Fe(acac) (acetylacetone), Cr(acac) , Co(acac) and F B(Phacac) (diuoro(1-3 3 3 2phenyl-1,3-butanedione)boron(III)), with a phenyl column at 100 8C and inlet pressure of 4750 psi [30]. Good resolutions were obtained in the separation of Cr(HFA) (hexa-3uoroacetylacetone), Cr(acac) , and Cr(Phacac) (1-phenyl-1,3-butanedione) on a phenyl3 3packed column at 99 8C and an inlet pressure of 3800 psi [30].

Supercritical CO was used as the mobile phase in several SFC studies on b-2diketonate complexes [30,31]. In these studies, methanol was used as a modier toreduce the retention times of the metal chelates and a UV detector was employed forquantication. It was observed that increasing the concentration of the modier wouldreduce the retention times of the analytes. This probably is due to the fact that methanolincreases the solubility of the b-diketonates in CO and reduces their activity in the2stationary phases. Column adsorption and decomposition of some of the acetylacetonates

also occurred during the SFC analysis, as evidenced by broad peaks with considerabletailing. Cr(HFA) , a uorinated acetylacetonate, showed a shorter retention time than its3non-uorinated analogue Cr(acac) . The best results for the SFC analysis of metal3b -diketonates using methanol modied CO were obtained with columns packed with2polar stationary phases [30]. Separation of 23 metal acetylacetone complexes includingRu(II), Co(III), Rh(III), Ir(III), Pd(II) and Pt(II) using supercritical CCl F as the2 2mobile phase and with columns made of Epon 1001 resin on Chromosorb W at 115 8Cand 800 psi was carried out by Karayannis and Corwin [29]. Twenty-three out of these43 metal acetylacetonates studied were eluted without apparent decomposition. Laintz etal. [32] separated the geometric isomers of Cr(III) and Rh(III) chelates of triuoro-

acetylacetone (TFA) and TTA using packed column SFC. Neat CO or methanol2modied CO was used as the mobile phase, and the most favorable isomeric resolution2was obtained with a phenyl stationary phase. The chromatographic separation was better


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for chelates with TFA than for the corresponding chelates with TTA. SFC has also beeninvestigated by Caruso et al. [33] for the determination and separation of chromiumb -ketonate complexes and a thermally labile organochromium dimer. The determination

of chromium complexes was done by both FID and inductively-coupled plasma withmass spectrometric detection (ICP-MS).

Reports on the utilization of macrocyclic compounds as chelating agents in SFE andSFC are limited. Calixarenes are macrocyclic complexing agents that possess a lipophilicmoiety (upper rim) and a metal coordinating moiety (low rim). They are described asmolecular baskets since they have high ionophoric selectivity and form inclusioncomplexes with many metal ions. A number of metal ions are known to form lipophiliccomplexes with the deprotonated forms of the simplest phenolic calixarene. Macrocycliccalixarene compounds can be made quite soluble in supercritical uid CO by2uorination. Glennon et al. [34] reported SFC and SFE of metals with the hexameric andtetrameric p-tert-butylcalixarenes. They demonstrated the extraction of Fe(III) usingsupercritical uid CO containing a new uorinated calix[4]arene tetrahydroxamic acid.2In this case, the ferric ions were extracted into supercritical CO as a heptade-2cauorodecane derivatized calix[4]arene hydroxamate Fe(III) complex. SFC studieswere carried out with p-tert-butylcalix[4]arene ( C1 ) and p-tert-butylcalix[6]arene ( C2 )using a mobile phase of methanol/chloroform-modied CO at 3700 psi and 55 8C, and a2ow rate of 3 ml /min; 10% chloroform modied supercritical uid CO showed a better2separation for the calixarenes C1 and C2 compared with that using methanol as themodier.

4. SFC separation of lanthanide and actinide complexes

A new approach of determination of uranium by packed-column SFC was reported byMartin-Daguet et al. [35]. 2,6-Diacetylpyridine bis(benzoylhydrazone) (H DIB) was2used as a chelating agent for uranium. SFC properties on different stationary phases suchas Nova-Pack C18, Spherisorb ODS2, Zorbax Rx C18, Inertsil ODS2, Capcell-Pack C18, Asahipac C18 and PRP-1 and the inuence of the methanol content in the CO 2mobile phase were investigated. Capcell-Pack C18 provided the best results in terms of

retention time, methanol consumption, peak efciency, and symmetry for the study of chromatographic behavior of uranyl complex UO DIB. This stationary phase was also2chosen for a chromatographic study of the chelating agent H DIB, and the copper2complex CuDIB. The stability of metal complexes and high solubility in methanolmodied supercritical CO allowed a uranyl determination in SFC in less than 2 min. A2lesser amount of solvent, | 500 ml, is required for each analysis in SFC, compared to the10 ml used in reversed phased-liquid chromatography. The results indicate that packed-column SFC can provide quantitative, reliable and fast uranium and copper de-termination.

Wai et al. [14,15] studied the separation of lanthanide b-diketone complexes using

CO as the mobile phase in SFC. The hydrated lanthanide-FOD (2,2-dimethyl26,6,7,7,8,8,8-heptauoro-3,5-octanedione) complexes exhibit strong intermolecular inter-actions and decomposition in SFC. Formation of adducts of the lanthanide b-diketones


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with a neutral donor such as tributylphosphine oxide (TBPO) or trioctylphosphine oxide(TOPO) could greatly improve their SFC behavior. Adduct formation was shown toimprove the extraction efciencies of lanthanides from different matrices by supercritical

CO containing a uorinated b-diketone and an organophosphorus reagent [2]. The2stability sequence of lanthanideFOD organophosphorus adducts was shown to followthe order Ln(FOD) ?2TOPO . Ln(FOD) ?2TBPO . Ln(FOD) ?2TBP [15].3 3 3

The chromatographic performance of lanthanide chelates of acac, TFA, TTA, FOD,THD (dipivaloylmethane), and TTA ?Py (thenoyltriuoroacetonepyridine) adducts wasinvestigated by Laintz et al. [36]. SFC separation of Pr(acac) , Pr(FOD) , and Pr(THD)3 3 3

2complexes showed excellent resolution at 150 8C and 120 kg/ cm , and a CO ow rate2of 0.3 ml/min and ethanol modier ow rate of 0.03 ml/min. The SFC separation wasdone with an Inertsil 2.1 i.d. 3 250 mm 5 mm phenyl packed column. Lanthanidecomplexes of acac, FOD and THD displayed high solubilities in supercritical CO and2have better chromatographic behavior compared to the corresponding chelates of TFA,TTA, and TTA ?Py. Typically, Eu(TTA) showed characteristic thermal decomposition3in the mobile phase as a function of temperature. Furthermore, retention behavior wasdemonstrated to be a temperature dependent function of volatility and solubility [36].

5. SFC separation of organometallic compounds of lead, mercury, and tin

SFE of ionic compounds such as organotins (R SnX , m5 13) from aqueousm 42 msamples cannot be accomplished with neat CO due to the charge neutralization2requirement and weak solutesolvent interaction. However, when ionic compounds arebound to organic ligands or by ion-pair formation, their solubility in supercritical CO is2signicantly increased, thus enabling the extraction of these compounds from en-vironmental samples.

Organotin compounds are becoming one of the most versatile groups of organometal-lic chemicals, in terms of industrial and agricultural applications. They are widelydistributed in the environment, and are under intense scrutiny owing to their signicantenvironmental impacts. Six tetraalkyltin and seven ionic organotin compounds from

spiked topsoil samples were extracted by 5% methanol modied supercritical CO by2Liu et al. [37]. Analysis of the soil extracts was done by GC-AED. The ionic organotincompounds were derivatized to their corresponding pentylated derivatives before GC-AED analysis. Six tetraalkyltin compounds spiked to the topsoil samples were extractedby SFE with recoveries in the range of 90% to 110%. Recoveries ranging from 70% to90% were obtained for most of ionic organotin compounds when sodium diethyl-dithiocarbamate (NaDDC) ligand was used. The organotin compounds are identied bycomparison with the chromatograms obtained from a standard containing nine organotincompounds. In situ chelation-SFE and GC-AED for the determination and speciation of 13 organotin compounds in soils and sediments were also developed and evaluated by

the same authors [38]. Soil or sediment samples, mixed with a chelating agent,diethylammonium diethyldithiocarbamate (Et NHDDC), were extracted with 5% metha-2nol modied CO at 60 8C and 450 atm. The extracted analytes were treated with2


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pentylmagnesium bromide to convert the ionic organotin compounds into their neutralderivatives, which later were analyzed by GC-AED. These results indicate that SFEfollowed by GC-AED analysis is a promising technique for the determination of trace

organotin compounds in environmental soil and sediment samples.A single ame photometric detector (FPD) has been optimized for capillary SFC of

trialkyltin chlorides by Bayona et al. [39]. Detection variables including temperature,hydrogen ow rate, and air ow rate were optimized by a combination of factorialexperimental design. Dibutyltin, diphenyltin, and triphenyltin chlorides were successful-ly analyzed using a linear programming in SFC [39]. An approach for simultaneousdetermination of butyl-, phenyl-, and cyclohexyltin compounds in aqueous environmen-tal samples was subsequently investigated by the same authors [40]. Ionic organotinswere ethylated by sodium tetraethylborate rst, followed by trace enrichment of thederivatized organotins on a C18 disk and extracted with supercritical CO . The analytes2recovered were analyzed by GC-FPD. The recoveries of organotins from spikedseawater ranged from 82 to 104% at the higher spiked level and from 79 to 115% at thelower level. With a hexyl derivatization by hexylmagnesium bromide to the organotincompounds on the sediment, SFE was conducted in mild thermal conditions at 40 8C and350 atm. In situ derivatization before SFE using neat CO or modied CO , on line2 2SFE-GC and SFE/SFC were also studied by the same authors [41].

Organotin compounds in a marine paint and food matrices were determined andseparated by SFC using formic acid modied CO and polymer encapsulated silica-2based stationary phases with FID [42]. The retention time of the individual compoundswas found to depend only upon the structure of the cation. The favorable chromato-graphic properties of the ionic tin compounds are probably due to the formation of formate salts which show reduced tailing and decomposition, and less irreversibleadsorption in a separation system saturated with formic acid. Blake et al. [43] recentlyreported that organotin compounds could also be determined by capillary SFC withICP-MS detection.

Optimization of SFE of methylmercury in marine samples followed by GC analysiswith electron capture detection (GC-ECD) was also developed [44]. Factor designs havebeen used for the simultaneous determination of various analytical SFC parameters suchas CO ow rate and density, temperature, pressure, static extraction time, amount of 2

HCl and contact time between HCl and the sample prior to extraction, and so on. Arecent review summarized the SFE conditions for methyl mercury extraction fromaquatic sediments. The analytical procedures for methyl mercury determination werebased on GC-ECD [45]. Liu et al. [46] showed the application of supercritical CO 2modied with 5% of MeOH to extract individual organomercury compounds includingCH HgCl, C H HgCl, (C H ) Hg and inorganic mercury compounds (HgCl , HgO,3 6 5 6 5 2 2HgS) in soil and sediment samples, followed by capillary GC-AED determination.Knochel et al. [47] demonstrated that interfacing SFC with atomic uorescencespectrometry (AFS) is suitable for the determination of organomercury compounds.Diorganomercurials can be analyzed without any prior treatment, whereas mono-

organomercurials and inorganic mercury compounds have to be converted into com-pounds with lower polarity in order to be soluble in supercritical CO . This can be done2by complexation of monoorganomercurials with NaDDC. Even for thermal labile


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compounds, such as alkoxyethylmercurials, which decompose in GC conditions, can stillbe analyzed using this SFC-AFS system.

A procedure for the SFE of ionic alkyl-lead species from solid samples was reported

by Johansson et al. [48]. Methanol was found to be a better modier compared withwater and acetone in this separation. The ionic alkyl-leads were extracted as diethyl-dithiocarbamate complexes into hexane, propylated using a Grignard reagent, andquantied by GC-MS. Recoveries of 96%, 106%, and 80%, for trimethyl-lead, triethyl-lead, and diethyl-lead, respectively, from spiked sediment samples were reported.

6. SFC separation of other organometallic compounds

In the early work on high pressure gas chromatography above critical temperatures,Klesper et al. [5] separated nickeletioporphyrin complexes using a column packed with33% polyethyleneglycol (Carbowax 20 M) on Chromosorb W, at a maximum pressure of 2000 psi, with CCl F (112 8C), and CHClF (96 8C) as the mobile phases. The same2 2 2mobile phase was later used to separate Cu, Mg, Ni, Sn and VO etioporphyrin IIcomplexes [49]. Etioporphyrin II complexes of other transition metals were analyzed byKarayannis et al. [50] in subsequent studies. These etioporphyrin II metal chelates werestudied at 145 8C and 10001700 psi and separation of Ag, Co, Cu, Mg, Mn, Ni, Pd, Pt,TiO, VO, and Zn etioporphyrin II complexes was carried out with a column of 10%Epon 1001 resin on Chromosorb W [50]. Four nickel and vanadyl porphyrin complexeswere investigated as a function of temperature (50 to 100 8C) and pressure (3000 to 4000psi) by Taylor et al. [51]. These nickel and vanadyl porphyrin complexes weresuccessfully separated using 20% methanol modied CO as the mobile phase on a C182packed column and with UV detection. The results suggest that the C18 stationary phaseis more selective for the separation of porphyrinic derivatives compared with silica orphenyl stationary phases. The major problems with separation of porphyrin-typecomplexes seem to be their irreversible adsorption on the chromatographic column. Thismay be associated with the incomplete coordination of the metals with the porphyrinring and their subsequent ionic interactions with the chromatographic column.

In addition to metalloporphyrins, cyclopentadienyl complex and its derivatives were

studied by Jentoft et al. [52]. A synthetic mixture of ferrocene, cyclopentadienylMn(CO) , and dicyclopentadienyl TiCl was successfully separated on a 1-m by3 21/8-inch o.d. stainless steel column packed with a commercially available Carbowax400, using CO as the mobile phase [52]. In another report, a mixture of ferrocene,2acetylferrocene and 1,1 9-diacetylferrocene was separated by CO /methanol (98/ 2)2mobile phase on four different SFC columns of ODS, phenyl, PRP-1, and silica, at5080 8C and 24005000 psi [30]. Methanol modied CO was required to elute the2substituted ferrocenes from the columns in | 30 min.

Wai and co-workers [20] demonstrated the separation of a range of ferrocenederivatives using capillary SFC and discussed the prediction of SFC retention behavior

of ferrocene derivatives using the Hildebrand Equation [23]. The separation of a mixtureof the seven ferrocene derivatives shown in Fig. 4a is given in Fig. 5 [21]. Thechromatography illustrates that a wide range of ferrocene derivatives can be baseline


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Fig. 4. (a) Structures of ferrocene derivatives shown in Fig. 5: ferrocene (1), ethyl ferrocene (2), t -butylferrocene (3), n-butyl ferrocene (4), phenyl ferrocene (5), benzoyl ferrocene (6), dibenzoyl ferrocene (7). (b)Structures of ferrocene, ruthenocene and osmocene derivatives shown in Fig. 6: acetyl ferrocene (1), acetylruthenocene (2), acetyl osmocene (3).


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Fig. 5. Typical chromatogram demonstrating baseline separation of a range of ferrocene derivatives of varyingphysical properties: ferrocene (1), ethyl ferrocene (2), t -butyl ferrocene (3), n-butyl ferrocene (4), phenylferrocene (5), benzoyl ferrocene (6), dibenzoyl ferrocene (7). Solvent: methylene chloride. Conditions:T 5 1008C. Insert: pressure ramp used.

resolved using pressure-programmed capillary SFC. The compounds of ferrocene, ethylferrocene, t -butyl ferrocene and n-butyl ferrocene are all eluted within 10 min at lowpressures (80120 atm). Phenyl ferrocene, benzoyl ferrocene and dibenzoyl ferrocenewere all strongly retained on the column at low pressures but were eluted when the

pressure reached 400 atm. A pressure program was thus used which caused all sevencompounds to elute with baseline resolution being achieved under 12 min. Theseparation of cyclopentadienyl complexes involving row 2 metals from row 3 metals inthe Periodic Table is more challenging since for these groups the transition metals arevery similar in terms of size and physical properties. SFC separations of ferrocene,ruthenocene, and osmocene, a different central atom in a dicyclopentadienyl system(Fig. 4b), were conducted at 100 8C using a pressure ramp of 2.5 atm/min from 80 to400 atm. The results (Fig. 6) suggest that SFC can provide baseline resolution for thesethree compounds [21].

7. SFC determination of solubility of organometallic compounds

A number of studies concerning the behavior of organometallic compounds in


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Fig. 6. Chromatogram demonstrating baseline separation of acetyl ferrocene (1), acetyl ruthenocene (2), acetylosmocene (3). Solvent: methylene chloride. Conditions: T 5 1008C, using a 2.5 atm/min ramp from 80 to 400

atm.

supercritical CO have been carried out in terms of solubility, reactivity, and catalysis2[5355]. These studies indicate that a wide range of organometallic systems show highsolubility in supercritical CO , as evidenced by SFC investigations. The catalytically2active compound RuCl [P(CH ) ] was shown to be soluble in supercritical CO . The2 3 3 4 2solubility of Ru catalyst was measured quantitatively by passing Ru catalyst/supercriti-cal CO through a ne lter at 50 8C and 120 atm [53].2

Ferrocene and its derivatives have been extensively studied with regard to supercriti-

cal CO solubility [17,18,20,21]. The high solubility of ferrocene can be interpreted as2being due to the nature of bonding. Ferrocene is a system in which the metals 3d, p, ands orbitals are lled so that no free coordination sites are available for interaction with thesolvating CO , resulting in a system having properties resembling those of an aromatic2compound.

SFC provides a means for investigating the behavior of ferrocene and its derivatives,since it has been shown that the retention behavior of solutes is directly related to thesolubility in the supercritical uid [17,18]. Cowey et al. [18] used SFC retention datacoupled with a small number of experimentally determined solubility data to determinethe solubility of ferrocene in supercritical CO under various conditions. Wai et al. [21]2

studied the behavior of ferrocene and its derivatives and separated ferrocene derivativesusing capillary SFC. The capacity factors of the ferrocene derivatives were found to varysignicantly with the chromatographic temperature and pressure. It was shown that theSFC retention could be predicted from knowledge of simple parameters such as the


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solubility parameter of the solute, molar volume of the solute, and the physicalcharacteristics of the chromatographic column. In general the more soluble a compoundis in supercritical uid CO , the quicker it will elute from the column. This general2relationship has been used to determine solubility [19,56]. There are some assumptionsthat must be made for a specic equation to work. A discussion of these assumptionswas given by Bartle et al. [19].

As the supercritical uid temperature increases at constant pressure, the density of theuid decreases, which leads to a decrease in the solvating power of the supercritical uidand leads to an increase in the analytes retention. Thus, at relatively low temperaturesand low analyte vapor pressures, the density of the supercritical uid is the dominantfactor. However, as the supercritical uid temperature is increased, the volatility of theanalyte also increases until the analyte vapor pressure becomes more important thansupercritical uid density. At this point, the solubility of the analyte in the supercriticaluid starts to increase, resulting in a decrease in analyte retention time. Thisphenomenon has been observed for the retention of lanthanide b-diketone complexesand polyaromatic compounds analyzed by packed column SFC [36,57]. The theory wasfurther veried by a range of ferrocene derivatives with very different functionalities,ranging from the parent compound ferrocene to N , N -dimethylaminomethyl ferrocene[20].

The following equation was derived by Brauer et al. [20] to show the relationshipbetween d and ( RT / V ) Ln(k 9 / r ),i i

2 2( RT / V ) Ln(k 9 / r ) 5 2 (d 2 d ) 2 2(d 2 d )d (1)i sp sf sp sf i

1 / 2 2 3 / 2where d is the solubility parameter of the solute (cal cm ); d is the solubilityi sf 1 / 2 2 3 / 2parameter of the supercritical uid phase (cal cm ); d is the solubility parametersp

1 / 2 2 3 / 2of the stationary phase (cal cm ); k 9 is the capacity factor, the ratio of the solutebetween the two phases; r is the ratio of the volume of the stationary phase to the

3volume of SF in the column; V is the pure solute molar volume (cm ); R is the ideal gasi2 1 2 1constant (cal mol K ); and T is temperature (K).For isobaric and isocratic SFC, Eq. (1) predicts a linear relationship between d andi

( RT / V ) Ln(k 9 / r ), hence allowing a quantitative means of predicting retention behavioriof solutes in SFC.

8. Detection system for SFC of metal-containing compounds

Currently, most commonly used detectors for SFC and GC are UV absorption and FID[8,20,31,51], which are universal, sensitive, and have the potential for high selectivity.For instance, Wenclawiak [31] and Ashraf-Khorassani et al. [30] described the separationof metalacetylacetone complexes and porphyrinic derivatives by SFC with UVdetection. Wai et al. [20] utilized SFC equipped with a FID with a 5-cm methyl silicone

open tubular capillary column for determination of ferrocene. The limitations of UVdetection for SFC are related to sensitivity considerations, whereas FID is constrained touids yielding no signicant ionization during detection (such as CO ).2


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In addition to UV and FID detection, element-selective detectors seem attractive forthe detection of metal-containing compounds in SFC. Several element-selective opticaldetectors, such as the microwave-induced plasma detector (MIP) [5860], radio-

frequency plasma detector [61], and ICP [62,63], have been studied extensively for SFC.A single FPD attached to a capillary SFC system was used by Bayona et al. [39] foranalyzing trialkyltin chlorides.

Among several element-selective techniques, ICP-MS provides excellent selectivity[6466]. The use of a high-resolution MS can further enhance the capacity to resolvecomponents of the same nominal molecular weight, to aid identication, and to eliminatebackground complication. A number of authors [64,66] succeeded in coupling theICP-MS with SFC for the separation and determination of organometallic compounds.For example, Shen et al. [64] interfaced the ICP-MS with SFC to separate thetetraalkyltin compounds with detection levels in the sub-picogram range. The linearrange was over three orders of magnitude (11000 pg). This new approach suggests thepotential for simultaneous determination of mixtures of organometallic pollutants of environmental interest such as As, Pb, Sn and Hg at ultra-trace levels. The SFC-ICP-MSsystem was also evaluated by comparing the results from an SFC-FID for thedetermination of tri- and tetraorganotin compounds [66]. Better resolution was obtainedwith FID, and differences were attributed to uctuations in the temperature of thetransfer line used in the SFC-ICP-MS interface. Baseline resolution for the organotincompounds is accomplished in both FID and ICP-MS detection by using a longercolumn. For the same organotin compounds, ICP-MS detection limits are superior to theFID results by one order of magnitude. The major advantages of FID are low cost andease of operation, whereas the ICP-MS detector is more sensitive and selective. For theanalysis of a real sample, more interference will appear by FID than by ICP-MS.

Both ICP-AES [67] and microwave induced plasma-atomic emission spectrometer(MIP-AES) [58] have been successfully used as element selective detectors in the SFCseparation of ferrocene compounds. These detectors were employed in conjunction withpacked microbore columns of 1-mm i.d. and packed capillary columns of 0.53-mm i.d.The use of a micro-column in SFC allows the total efuent to be introduced into theplasma. The performance was evaluated by monitoring the Fe emission line at 259.94nm.

Signicant progress has been reported using Fourier transform infrared spectroscopy(FT-IR) as a detection method [56,68]. The off-line approach for FT-IR detection isparticularly attractive because of the enhanced detection limits and the practicalelimination of the solvent modier [56,68] that greatly limits on-line detection. Theadvantage of FT-IR is that one can obtain molecular structure information from thecompounds eluting from the column and not just retention times or area counts, as in thecase of nonspectrometric detectors such as FID. FT-IR has the ability to interpret quicklyand unambiguously the presence or absence of specic organic functional groups in asample molecule. The major disadvantages of off-line FT-IR detection include me-chanical complexity, expense, sensitivity (in some cases), and the absence of commercial

instrumentation.Jinno et al. [69] used photodiode array UV and ICP serial detectors coupled with

packed micro-column SFC for determination of metalacac complexes. These metal


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acac complexes included Al(III), Co(II), Co(III), Cr(III), Cu(II), Mn(II), Mn(III),Ni(II), and Zn(II). The UV detector can provide UV spectra of acac complexes separatedby SFC, while ICP detector can supply elemental information for complex identication.

9. Future developments

With increasing demands for speciation information of elements of biological andenvironmental concern, SFC should play a role in preparation and analysis of metal-containing compounds in complicated matrices. SFC offers the possibility of performingchromatographic analysis of metal chelates and organometallic compounds at lowertemperatures and in less time than obtained with traditional GC. With modiers, SFC

analysis of more polar compounds is feasible. The range of application and sensitivity of detectors will probably be the major factor in determining the future growth of SFC. Oneattraction of SFC with CO as the mobile phase is its compatibility with both liquid- and2gas-phase detectors and particularly with FID. The development of more selectivestationary phases for both packed and capillary column SFC is desirable. Thepreparation of columns that are stable to the solvating inuences of polar supercriticaluids at elevated temperatures remains challenging.

The extraction and separation of metal chelates and organometallic compounds bySFE/SFC depend largely on the proper selection of chelating agents. Fluorinated ligandsare effective for in situ chelation-SFE of metal ions and SFC of metal-containing

compounds. Replacing coordinated water in a metal chelate with an organic ligand hasalso been shown to increase its solubility in supercritical CO and improving SFC2separation. Adduct formation is another technique for improving SFE and SFCefciency of metal chelates. Phosphorus and silicon-containing ligands also form highlysoluble metal complexes in supercritical CO . Selective extraction and separation of 2metals using macrocyclic compounds in supercritical CO is another area of interest for2future development of SFE/SFC technology. It is expected that SFC applications willexpand into the areas new to chromatographic methods or in which shortcomings of either GC or LC are well established. The compatibility of SFC with GC-type detectorsand its usefulness for analyzing metal-containing compounds not amenable to GC or LCindicate that the future of SFC is bright.

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separation of metal chelates and organometalic