liquid extraction

Tahro

Pergamon

Vol 41, No IO. pp 1765-1769. 1994 CopyrIght (‘ 1994 Elswer Saence Lid Pnnted I” Grert Bntam All rlnhts reserved 0039-914oi9457 00 + 0 00

0039-9140(94)00165-O

FLOW INJECTION ANALYSIS WITH THE CHROMATOMEMBRANE-A NEW DEVICE FOR GASEOUS/LIQUID AND LIQUID/LIQUID EXTRACTION LEONID N

MOSKVIN’

and JURGEN SIMON’?*

‘Department of Chemistry, State Umverslty of St Petersburg. Russia *Department of Chemistry. Free Unrverslty of Berhn, Germany (Recemed 18 February 1994 Rewed

28 March 1994 Accepred 28 March 1994)

new device makes extractIon procedures work contmuously by reahzmg chromatographlc pnnclples m flow-injectton analysis The method allows Independent mass transfer between two phases within a chromatomembrane cell In spite of the small size of the cell-volume (about 3 cm’) the relevant contacting area IS extended to 2 m* A mlxmg of phases IS simply prevented, and an additional step of phase separatron IS no longer necessary A chromatomembrane IS generated from porous hydrophobic material (PTFE) with two types of pores. namely, macropores and mlcropores Whenever two phases flow wlthm the cell the aqueous one exclusively fills the large pores because of the capdlary pressure produced by polar hqulds m micropores On the other hand these micropores remam available only for the extraction agent. e g non-polar liquids or gases The mass exchange IS slgmficantly Increased compared wtth conventional techmques The wide field of practical apphcatlons can be seen from several results obtamed from trace determmatlons m hqutd and gaseous phases Summary-A

Liquid/liquid extraction is one of the most commonly used procedures m analytical chemtstry, especially if removal of interfering matrices is necessary or preconcentrations of compounds under investigation is desirable. In order to transfer this procedure to the microscale of flow inJection apphcations, three separate steps are necessary. ” First, two immiscible hquid phases must be dispersed which each other with a constant volume ratio, then the two phases have to remain m close contact with each other attaining effective mass transfer, and finally a phystcal separation of both of the phases has to be implemented. The simultaneous control of these three different processes, forces compromises and reduces effectiveness. The realization of contmuous chromatographic separation can solve this problem, allowmg the three steps of extraction procedures to be combined m one device. To develop a suitable module, a compact block of foamed hydrophobic material containmg both micropores and macropores is required. The macropores are filled with a polar liquid, whereas the micropores can be filled with a non-polar phase *Author to whom all correspondence shall be addressed Fabeckstrasse 34/36, D 14195 Berlin. Germany

Taking into account the captllary pressure of polar hqutds m the mtcropores, the determmmg factor in producing a stable system is to control the ratio of pressures used to fill the two pore types. Moskvm34 produced btporous PTFE by hand, guaranteeing uniform sizes of macropores and mtcropores. The cooperation of both research groups in St. Petersburg and Berhn aims to apply this material to FIA. EXPERIMENTAL

Chromatomembrane

cell

Biporous PTFE suitable for analytical applications requires pore sizes as uniform as possible, e.g. 0.3 pm for micropores and 200 pm for macropores (being the average diameter of each pore-size). The main item of the chromatomembrane cell, which can be seen from Fig. 1, IS a rectangular block of the biporous PTFE coated with thm microporous PTFE membranes (I.e. sheets with 0.8 mm thickness) on two opposite surfaces. This is surrounded by a shell of synthetic material being equipped with inlets and outlets for the polar and the non-polar phases. The arrangement of Fig. I clearly shows the purpose of the thin layers with micropores they prevent the polar phase penetrating mto

1765

L N MOSKVIV and J 9~0s

1766 unpolar llquld or

4

pa,

Fig

I

Chromatomembrane membrane

gas

I

cell

f$j MIcroporous

W Blporous

PTFE

PTFE

block

regions exclusively reserved for the non-polar phase. Withm the biporous block the exclusion of mutual phase mixing IS guaranteed by the difference of pressure under which polar (P,) and non-polar (P,) phases are supplied mto the system Withm the total volume the pressure occupied by the non-polar phase has to be kept lower than the polar phase pressure, which means the polar phase pressure at the outlet from the cell (P?) has to surpass the pressure at the mlet of the non-polar liquid or gas (P,):

Fig 2 Contmuous

separation wth

the chromatomembrane

cell

case of a separation procedure which works contmuously are shown m Fig. 2. Because the dtrections of the fluxes are perpendicular to each other the fundamental principles used for the description of chromatographic separation processes at stationary phases are vahd The rate of component-i-zoneshift (V,) decreases with regard to the flux rate (Or,) of the aqueous phase according to the distribution coefficient K, and the ratio V, /VZ, bemg the ratio of volumes occupied by the two phases m the micropores and the macropores. respectively

P, < Pr.

As a result the non-polar phase cannot penetrate from the micropores mto the macropores As long as the condition P,cP,+P, 1s fulfilled, micropores remam inaccesstble to the polar phase. PC denotes the capillary pressure,

that polar hquids addttionally entermg mto micropores PC= (0 = surface

2 0.

have to expend

The quotient (I/U,) IS proportional to the retention time of the component-r-zone within the cell. During this time the component I has to be extracted by the non-polar phase Whenever its flow-rate is (V,,), the retention time of any zone mside the cell IS proportional to (h/U,,) Combmmg these quahties with each other one obtains the condition for complete extraction’

~0~8

r

e = contact angle, tension, r = radius). The value of PC IS negative. Under these conditions an independent flux IS ensured for each of the liquids within the chromatomembrane cell. Let us set I for the length of the rectangular biporous PTFE-block. Moreover, 1shall also be the direction of the polar hquid flux. The character h denotes its height and also the direction of the non-polar hquid flux, and ic’is its width. Experiences show this block to have dimensions (cm) varying m the range: 1.5<1<30; 0.4 < h < 0.8; 0.5 < u’ < 1.0. The conditions guaranteeing complete extraction within the chromatomembrane cell in the

(the zone-dispersion due to re-extraction IS neglected). The use of chromatomembrane-cells IS of great advantage on account of the fact that massexchange can occur m any direction required, e.g. between polar hqutd and non-polar hquid (gas) or vice versa However, it is necessary to obey exactly the conditions required by the theory A characteristic defect of PTFE is the change of the contact angle during a long run time with aqueous solution. In practice this defect is limited. In general the large biporous PTFE does not produce any serious problems However, due to the shell surrounding the real chromatomembrane there are some difficulties

Flow InJectIon analysts

1767

W D 1

=* I

Fig 3 Arrangement for contmuous separation

especially m the case of incorrect treatment, e.g. neglectmg the working condttions of the cell required from the theoretical pomt of view. One has to take into account that all chromatomembrane cells workmg at present are handmade Consequently the experience of synthetics manufacturers and their advice are required to solve the problems in the near future. In order to adjust the cell characteristics to the own special analytical problem the dimensions of the chromatomembrane cell must be varied. In practice one needs several standard sizes to be optimized with respect to the requirements of the analysts RESULTS

Several results are presented in order to demonstrate the principles of applying a chromatomembrane cell to flow-injection analysis.

hg

The normal mode is the continuously separating system which the cell is provided for with respect to well balanced flow-rates for both of the phases. A typical arrangement is shown m Fig 3. A series of different concentrattons of aqueous solutions (sample) is pumped through the chromatomembrane cell, whilst a suitable organic phase is continuously extracting the compound under investigation which is detected by photometric or electrochemtcal devices. Figure 4 shows an example for Cu2+ determination in drinking water being extracted by CClJdithizone mixture. A photometric detector measures the absorbance of the copper-dithizone complex formed. Whenever any preconcentration 1s necessary in the case of a more extreme trace analysts the flux of the extracting phase is stopped within the chromatomembrane cell as long as the

4 Cu(ll) separation and photometric detectlon. Aqueous phase (pH = 2, CI,, _ 3 ml/mm), 0, 10, 30ppb Cu*+ Orgamc phase and reagent (CJe,,- 0 IS ml/mm) Ccl, + 5 *10m6 mol/l dlthlzone

1768

L N

M~SKVIV

and J

9~0%

a)

w

Ah Y W

v4

b)

Pumvl

.. Y

Pump2

Fig 5 Arrangement for preconcentratlon

enrichment of traces which have to be extracted from the continuously flowing phase (sample) is suffictent for their subsequent detectton. An arrangement which 1s useful in the case of a required preconcentratton can be seen in Fig. 5. The detection takes place during stopped sample flux through the chromatomembrane cell In this period the flux of the extractmg phase

a) Preconcentratlon

step, b) DetectIon step

transports the zone of enriched traces to the detector. An application of thts mode IS the determmation of SO2 in air (Fig. 6) An aqueous solution of Fe’+ and l,lO-phenanthroline IS used as reagent, in which Fe’+ 1s reduced by SO2 forming the well known coloured complex Other examples requu-mg a preconcentratlon step are

T A

X = 508 nm

Frg 6 PhotometrIc detectlon of Fe(H)-I,lO-phenanthrohne complexes bemg produced from SO1 as reducmg agent Preconcentrahon time* a) 0 5 mm, b) 1 0 mm, c) 1.5 mm A An contammg 0 5 rg SO,/1 Flow rate <20 ml/mm R Reagent aqueous solution of Fe’+ and I.lO-phenanthrohne flow rate. Q 2 ml/mm

Flow qectton Table I The chromatomembrane cell used as a devtce gtvmg donor-blood arttfictal respxatton Charactensttcs of blood P (0,)

mmHg

p (CO:) mmHg PH

Inlet 32 I36 6 63

Outlet v,, v,hl0d

I 2

I5 I

97 38 681

I15 33 6 92

the determmation of hydrogen fluoride m air using an aqueous buffer solution as extracting agent and a fluoride selecttve electrode for its detection or the use of 2,2’-diquinolyl, dtssolved in hexanol. for selective Cu(1) separation, respectively. A chromatomembrane cell with extended macropores (diameter 0.2 mm) permits blood oxygen-extractton from au. Results wtth respect to the physiology are convemently obtamed by varymg the volume-ratio V (gas): V (blood), as can be seen from Table 1. The CO,-content of blood decreases stmultaneously. But extendmg its apphcation m medicine require further mvestigations to imitate the function of a lung

1769

analysts

chromatomembrane cell permits one to perform two procedures contmuous extraction and preconcentration. A small alteration m procedure permits the two dtfferent techmques, whilst conventtonal preconcentration requtres the use of column techniques.’ The application of chromatomembrane cells IS not restricted to analyttcal laboratories The use of ‘oversized’ chromatomembranes w11l offer a different kind of significance m the field of medicine if blood oxygenation is requtred. In this respect the chromatomembrane works as a model of a rear lung. The composition of au (0,, CO,, NJ controls both the uptake of O2 and the removal of CO? from the blood as the physician considers desirable Ac/cno~ledgemenrs-We thank the German Bundesmnuster fiir Forschung und Technologte for Its support of the mutual cooperatton of the two research groups from St Petersburg (Russta) and Berhn (Germany)

REFERENCES B Karlberg and S Thelander. Anal Chrm Acra. 1978, 98, I 2 H Bergamm, J X Mederros, B F Rets and E A Cl Zagatto, Anal Chum Acra, 1978, 101, 9 3 L N Moskvm, m Proceedmgs of XV Mendeleer Congress on General and Applred Chemrsrr), Belarus, Mmsk, May l993-Navuka I tekhntka, Mmsk 1993, I

OUTLOOK

A wldespread interest 1s necessary whenever a new method 1s Introduced to compete successfully with estabhshed ones. Moreover, as commercial manufacturers recogmse the growing interest, they will be forced to optimize the equipment, which IS m its Infancy now. The

J

Chromatogr

J Chromarogr

A. 669, 81, 1991

4 L N Moskvm. regtstered for Russran Patent 5 H Bergamm, B F Rets, A 0 Jacrntho and E A G Zagatto, Anal Chum Acta, 1980, 117, 81