Anal. Chem. 1900, 52, 692-700
692
Life Time of Neutral Carrier Based Ion-Selective Liquid-Membrane Electrodes Urs Oesch and Wilhelm Simon* Department of Organic Chemistry, Swiss Federal Institute of Technology, Universitatstrasse 16, CH-8092 Zurich, Switzerland
Models for the estimation of the llfetme of neutral carrier based Ion-selective Ilquld-membrane electrodes are presented. They are based on the kinetlcs of loss of plastidrer and/or ionophore from the solvent polymerlc membrane phase Into the sample solutlon. The experimentally observed lifetime Is In good agreement with the theoretically estlmated value. For analytically relevant membrane systems, both the Ion carrier and the plastlclzer used should have partltion coefflclents between sample and membrane larger than about 10’ to ensure a contlnuous-use lifetime of at least 1 year.
Ion-selective liquid-membrane electrodes based on neutral carriers such as valinomycin are widely accepted in analytical chemistry (1,2) although serious limitations as to lifetime have t o be expected in certain applications. For K+-selective electrodes using valinomycin lifetimes of more than 3 years have been claimed ( 3 )while a replacement of membranes is recommended after about 3 months of operation ( 4 ) for some clinical flow-through analyzers ( 5 ) . Among the different matrices used for neutral carrier based membranes such as silicone rubber (6),copolymers of poly(bispheno1-A carbonate) (3), poly(methylmethacry1ate) (7), poly(urethane) (71, and poly(vinylisobuty1ether) (8);poly(viny1chloride) (PVC) (9-11) is a t present the most popular one ( I ) . In such solvent polymeric membranes, the loss of ion-selective components (ligand, ionophore, ion carrier) and membrane solvent (plasticizer, softener) will undermine the electromotive behavior of the corresponding electrode. This loss may occur through transfer either into the solutions contacting the membrane or into the membrane support. A rough estimate of the transfer of ionophore into the sample solution, based on equilibrium partition, leads to membrane lifetimes which are always considerably shorter than those actually observed (12). It has been shown previously (13)that there are kinetic limitations in the transfer of neutral species from a membrane to the adjacent solution, changing from bulk diffusion control a t low to phase boundary region control a t high lipophilicity of the species in question (13). Here we report on the application of these findings in the prediction of the lifetime of analytically relevant neutral carrier based solvent polymeric membranes.
THEORETICAL Clearly the major factors limiting the lifetime of a solvent polymeric membrane are the loss of the selectivity-inducing component and that of the membrane solvent into the contacting solution. For an assessment of membrane lifetimes, three models will be considered: an equilibrium model; a kinetic model with membrane bulk diffusion control only; and a model with rate control a t the phase boundary and/or by diffusion through the unstirred Nernst diffusion layer only. In all three models, a stirred aqueous phase of volume V1 is in contact with a membrane phase of volume V 2that has an initial ligand concentration c ~ , ~ :
with c1,o = 0
where n2,0is the initial amount of ligand in the membrane phase [mol] and c ~is ,the ~ initial concentration of ligand in the aqueous phase. (a) Equilibrium Partition. It is assumed that during each contact with the sample solution, a partition equilibrium is reached, i.e.
K = czq/c;q
where K is the partition coefficient of the neutral species ~ ~ cleq, between membrane and aqueous phase, and C Z and respectively, are the equilibrium concentrations in the membrane and aqueous phases. The remaining concentration c2m of the species in the membrane after m contacts with sample solutions (each initially being free of the species considered) is (4)
with
= Vi/V,K
CY
If equilibrium partition for the neutral species is not reached within one contact period, the transfer process will be limited to a large extent by the exposed membrane surface, f, which here enters into the following parameters:
a1 = V l / f and
a2 = V 2 / f
(7)
The parameters al and a2 are formal diffusion lengths for the two phases. (b) Kinetic Control by Transfer of Species through the
Phase Boundary Region (Unstirred Aqueous Diffusion Layer (Film Diffusion) and Phase Boundary (Interfacial Diffusion)). The contribution due t o film diffusion is determined by the diffusion coefficient D1in the aqueous phase and the thickness 6 of the aqueous diffusion layer. On the other hand the contribution by interfacial diffusion is described by the exchange reaction rate constant k2 at the phase boundary (dimension of kp: cm s-*) (13). As described earlier (13) one obtains for the overall transfer coefficient k i (dimension: cm 5-l)
1
-
= -1+ - K6
122’
k2
D1
(8)
During the first contact with a sample solution, the timedependent concentration of ligand in the membrane phase, c ~ , will ~ , be (14):
c2,o = n2,o/V2 0003-2700/80/0352-0692$0 1.OO/O
(3)
0 1980 American Chemical Society
693
ANALYTICAL CHEMISTRY, VOL. 52, NO. 4, APRIL 1980 CONCENTRATION
We obtain after m contacts of period At:
If one considers only a short contact time At, which means:
At
.a,),bis(2-ethylhexyl) sebacate (DOS. pract.). dibutyl phthalate (DBP. puriss p.a.)$bis(2-ethylhexyl) phthalate (DOP. pract.). 1 -chloronaphthalene (CN, puriss p.a.). ethyl salicylate (ES, puriil!i i iind 1-hydroxy-5-phenylpentane (HPP, purum): Fluka AG. Ruchs. Switzerland; dibutyl sebacate (DBS, grade 11): Sigrnic (?hemica1 Company, St. Louis, Mo. 63118; dipentyl phthalate IDI'P, pracr.): Eastman Organic Chemicals. Rochester. N.Y. 14650: bisi3.5,5trimethylhexyl) phthalate (DNP, for gas chromatography) arid tris(2-ethylhexyl) phosphate (TEHP, for sqnthesis 98% ): hferck AG, Darmstadt, GFR; 1-isopropyl-4-methyl-2-nitrobenzene (IMNB, techn. 90% ): EGA-Chemie, Steinheim, GFR; pdy(viny1 chloride) (PVC, SDP hochmolekular): Loma AC,. R a d , Switzerland. Double quartz distilled wat,er and chenic:ds of the highest purity available were used for all aqueous electrcd>-te wlutions. Membranes. The solvent polymeric memimines ,rinsisting of variable amounts of ligand, membrane solvent, and I'VC were prepared as described in ( 1 0 ) . The thickness of e x h circular membrane was estimated from the mass. density. m d diameter of the membrane. For membranes with 5 5 wt 7 ( 4 ligand. thc density of the membrane pM can be esiimated f(br a wir!t. rangc of PVC content (weight ratio of amuumt PYC to t.u!a?: I I sufficient accuracy by means of a linear relationship density o f PVC ppvc 11.38 g cm-j (251%2 0 "C) WK! :nrnibra:ne solvent psulv, respectively 14): PM
= /kdv -I-U
I'VC
([JPv.-8.2 -8.4
-10.3 -10.4 -10.6 -11.9 18 h for all ligands considered except ligand 8 (ttr = 24 s) if one assumes diffusion coefficients as they are suggested in the text above. Thus, for the analytically most relevant membrane systems (wpvc around 0.3 and high lipophilicity of the ligand), there is a kinetic control by the overall transfer through the phase boundary region within the supposed contact time At of 30 min; if the electrode system is used continuously with rapid sample change, the overall time of use can exceed P,and therefore bulk diffusion will participate in the control of the transfer process. For ligands with log K 5 5 this happens before the lifetime-limiting ligand concentration is achieved (assumed parameters: electrode of type A, c ~ ~ ~= 0.001 / c ~and , log ~ D , = -7.3). Thus, for ligands 4-7 the lifetimes for continuous use indicated in Table VIa, columns 9 and 11,underestimate the real lifetimes because the calculations take into consideration only control by overall transfer a t the phase boundary. Since the diffusion coefficients of different ligands in the same membrane system are roughly the same, the lifetimes indicated in Table VIa for ligand 8 are representative minimal lifetime values in the same system for any other ligand, independent of its lipophilicity. A change in the membrane system (membrane solvent and/or wpvc) will induce a change in lifetime inversely proportional to the diffusion coefficient of the ligand in the membrane in question (see Equation 14). While the magnitude of the ligand concentration on the membrane side of the phase boundary is important for any specific electromotive behavior, calculations of the average concentration as the lifetime-determining limiting concentration can lead to false statements. The largest uncertainty in the estimated lifetime is given by the standard deviations of the kinetic parameter k2' and of the partition coefficient K , if these values are not experimentally available and have to be determined via Equations 23 and 24 from log P. An uncertainty in k2' is linearily correlated with the uncertainty in the estimated lifetime (see Equation 12). Table VIa clearly shows that kinetic limitations increase the lifetime of neutral carrier membranes by a factor of about 30 relative t o estimations based on equilibrium systems (columns 7 and 8 in Table VIa). As expected, electrodes of type A have higher lifetimes than those of type B because of a more favorable ratio of the membrane volume to the membrane surface exposed to the sample. Similar lifetime estimates may be performed for plasticizers (see Table VIb). The correlation between log P and log K is so poor, however, that reliable estimates have t o be based on experimental partition coefficients. For all the calculations presented in Table VIb, a loss of plasticizer from wpvc = 0.3 to 0.7 (see Figure 4a) and control a t the phase boundary (see Equation 25) has been considered. Since the self diffusion
698
ANALYTICAL CHEMISTRY. VOL. 52,
NO. 4,
APRIL 1980
w
I
,
,
,
,
,
,
ANALYTICAL CHEMISTRY, VOL. 52 NO. 4 , AFRIL 1980
699
___
~
Table VII. Comparison between Experimentally Observed and Theoretically Estimated Lifetimes" membrane system ( w p v c = 0.3) ______ membrane log c : solvent selective for X O ~r., -
(81
ionophore 1
DOA
K'
4 6
O-NPOE 0-NPOE
Ca2
--2.81 --1.67 -4.98
+
Ba?'
experiment a 111observed lifetime, h
theoretically est i mated
>, cj 00 b
6300 177 110
>;
210 93
life time, h
" For definition
of lifetime, see Figure 7 ; for parameters for theoretical atid rxperimcxntal estimations, see footnotes in Figure 7. Selectivity factor after 600 h still the same as at t h e beginning Differences between these values and those in Table VIa are due t o the use of different parameter sets (see lext a n d corlespunding tables). -
SELECTIVITY log KPot BaM C
----+----
WITH
- 1
,?
LIFE TIME
LIGAND
-2
6
(0-NPOE , wpvc = 0 3 !
'
-3
THEORETICAL LIFE TIME
-i
_______
________
WITHOUT +-SAMPLE
CHANGE
-5 0
10
20
30 TIME [days]
Flgure 7 . Comparison of the experimentally observed with the theoretically estimated lifetime of a Ba*+-selectiveelectrode based on ligand 6 (electrode of type A: r = 3.5 mm, r' = 2 mm, d = 0.2 mm). Points: theoretical values estimated by Equation 10 using the parameter a , = 0.03 cm, At = 12 h, V , = 200 mL, log c * , = ~ -1.98 and a c,: K,,Pot-correlationfor ligand 6 as shown in Figure 4b. Circles: experimental values obtained with a stirred sample of pure water changed every 12 h ( V , = 200 mL, upper curve) and without any change of t h e sample ( V , = 10 mL, lower curve). The occasional KhMgPo'measurements were performed during brief interrupts
coefficient of plasticizers decreases with increasing upVc(see Figure 5 and Table IV), the assumed control a t the phase boundary may be lost after some time in favor of diffusion control, and therefore the lifetimes indicated in Table VIb may underestimate the actual situation. A comparison of the results in Table VIa with those of Table VIb (columns 7 to 11)indicates that acceptable membrane lifetimes can only be expected if highly lipophilic plasticizers are used. Although DBS has been recommended as plasticizer for solvent polymeric potassium- and sodium-selective membranes ( 3 4 , 4 3 ) , Table VIb shows this to be a rather poor choice in this view. According to Table VI both plasticizers and ionophores are available that lead to membranes of very high lifetime. An extreme case is the Na+-selective ionophore 11 with theoretically estimated membrane lifetimes of >IO8 years (column 9 in Table VIa). In order t o obtain electrodes of type A (Figure 2 ) with a lifetime of a t least 1 year under the experimental conditions specified (see text and Table VI), a lipophilicity log K of the ionophore of 25.5 is necessary. Similarly, for the plasticizer log K should be 2 6 which corresponds to a water-solubility of 510-5 mol L-', for a relative molar mass M , of about 300. Similar estimates of electrode lifetimes may be made for flow-through applications ( 5 ) if Equations 9 and 14 are used with (Y = a. The flow-rate of the sample, however, is of some importance since it determines the thickness of the Nernst diffusion layer which in turn is a relevant parameter in the overall transfer process at the phase boundary (see Equation 8). Assuming identical diffusion layers in flow-through and batch applications (same value of k i ) , the lifetimes indicated in columns 9 and 11 of Table VI hold for both systems (see Equations 9 and 12). Indeed electrodes based on o-NPOE
and ionophore 3 have been in use in flow-through water supply contrcjl systems for periods in excess of 8 months and were still showing adequate electromotive behavior ( 4 4 ) . A more quantit,ative comparison between theoretically estimated and measured lifetime is given in Figure 7. For this purpose a stirred sample of pure water in contact with a membrane arrangement of type A (Figure 2; in contrast to this Figure and Table I the exposed membrane surface was hemispherical instead of flat. thu.s doubling the surface f , was repiaced every 12 b and the electromotive behavior was determined ar regular intervals. This behavior is compared to the one estimated using Equation 10 as well as the correlation between ligand concentration and selectivity of the type presented in Figure 4a. Other ionophores lead to a similar agreement between experimentally observed artd theoretically estimated lifetime (see Table VII). 'The correlation in Figure 7 as we!l as in Table \'I1 shows that analytically relevant prediction.; of membrane lifetimes can be made.
LITERATURE CITED (1) "Ion-Selective Electrodes in Analytical Chemistry', Vol. 1, H. Freiser, Ed., ?lenum Press, New York. 1978. (2) I. M. Kolthoff, Anal. Chem., 51, 1R (1979). (3) 0.H. LeBlanc, A,, and W. T. Grubb. Anal. Chem., 48, 1658 (1976). (4) H. W. Holy. Technicon International Division S.A , Geneva, private communication. (5) E. C. Barton. S. Bauei, W. E. Chapman, W. Groner, I. Hicks, J. 8 . Levine, T. J. Macek. J. F. Murdock, K. L. Roth. M. H. Shamos, L. R . Snyder, and P. P. Tong, Eds.. "Advances in Automated Analysis", Voi. 1. Technicon Internaticnai Congress 1976,Tarryiown, N. Y., 1977. (6) J. Pick, K T6th. E. Pungor, M. V a z k , arid W. Simon, Anal. Chim. Acfa, 64, 477 (1973) (7) U. Fiedier and J. RitiiEka, Anal. Chim. Acta, 67, 179 (1973). (8) 0.F. Schafer, Anal. Chim. 4cta, 87. 495 (!976). (9) R. Bloch, A Shatkay, and H. A. Saroff, Biophys. J . , 7,865 (1967). 110) G. J. Moody, R. 8.Oke, and J. D. R. Thomas, Analys:(London). 95,910
(1970) (11) G. J. Moody and J. D R. Thomas, Ref. 1 , p 287. (12) N. N. L. Kirsch, Ph.D. Thesis, ETH 5842.Zurich, 1976. ~13)U. Oesch and W. Simon, Helv. Chim. Ac:a. 62, 7!54 (1979). (14) U. Oesch. Ph.D Thesis, ETH 6249,Zurich, 1979 (151 D. Ammann, R . Bissig, M. Guggi. E. Prstsch, W. Sinion, 1. J. Borowitz, and L. Weiss, Helv. C h n . Acta, 5 8 , 1535 (1975). (16) D. Ammann, E. Pretsch. and W. Simon, Helv. Chim. Acta, 56. 1780 (1973). (17) M. Guggi, E. Pretsch, and W. Simon, Anal. Chirn Acta, 91. 107 (1977). (18) N. N. L. Kirsch. R. 2 . J. Funck. E. Pretsch, and W. Simon, Helv. Chim. Acta. 60, 2326 (1977). (19) U. Oesch, D. Ammann. E Pretsch. and W. Simon. Yelv. Chim. Acta, 82, 2073 (1979) 120) M. Guggi. M Oehme, E Pretsch, and W Simon, Hehi. Chm. Acta, 5 8 ,
2417 (19761. (21) J. Senkyr, D. Amniann, P. C. Meier, W. E. Morf, E . Pretsch, and W. Simon, Anal. Chem., 51. 786 (1979). (27) J. K. Schneider. P. Hofstetter, E. Pretsch. D. Ammann, and W. Simon, Helv. Chim Acta, in press.
(23) "Organic Syntheses Collection", Vol. I11 Wiley, New York, 1955,p 140. (24) D. Erne, W. E. Morf, S. Arvanitis. Z. Cimerman, D. Ammann, and W. Simon. Helv. Chim. Acta, 62. 994 (1979).
,.r51 n
Lonza AG, Visp, Switzerland, private communication to K. Hartman.
(26)R. A. Steiner, M. Oehme, D. Ammanri, and W. Simon, Anal. Chem.. 51, 351 (19793. (27) G. G. Guilbault, R. A. Durst, M. S. Frant, H. Freiser, Ei. H. Hansen, T. S. Light, E. Pungor, G. Rechnitz. N. M. Rice, T. J. Rohm, W. Simon, and J, D.R. Thomas, Pure Appl. Shem., 48. 127 (19763). ! 2 R ) P. C. Meier. 3 . Ammann. W. E. Morf. and W. Simon, in "Medical and Biological Applications of Electrochemical Devices", J. Koryta, Ed., John Wiley & Sons. Chichester, 1979 (in press). 129)E. L. Eckfeldt and G. A. Periev. J . Necfrochem. Soc.. 98. 37 (19511. ?30! A. P. Thona. A. Viviani-Naue;. S. Arvanitis, W. E. Morf. and W.'Simon Anal. Chem.. 49. 1567 (1977).
700
Anal. Chem. 1980, 52, 700-704
(31) A. Craggs, G. J. Moody, and J. D. R. Thomas, J . Chem. fduc,51, 541 (1974). (32) T. Treasure, Intens. Care Med., 4, 83 (1978). (33) N. A. J. Platzer, "Plasticization and Plasticizer Processes", Advances in Chemistry Series, Vol. 48, American Chemical Society, Washington, D.C., 1965. (34) J. L. Hill, L. S. Gettes, M. R . Lynch, and N. C. Hebert, Am. J. Physiol. 235, H455 (1978). (35) D. Ammann, R . Bissig, 2. Cimerman, U. Fiedler, M. Guggi, W. E. Morf, M. Oehme, H. Osswald, E. Pretsch, and W. Simon, in "Proceedings of the International Workshop on Ion Selective Electrodes and on Enzyme Electrodes in Biology and Medicine", M. Kessler et al., Eds., Urban & Schwarzenberg, Munich, 1976, p 22. (36) W. Simon, D. Ammann, M. Oehme. and W. E. Morf, Ann. N . Y . Acad. Sci., 307, 52 (1978). (37) M. Guggi, Ph.D. Thesis, ETH 5866, Zurich, 1977.
(38) J. A. Riddick and W. B. Bunger, "Organic Solvents", Techniques of Chemistry, Vol. 11, Wiley-Interscience, New York, 1970. (39) W. D. Stein, "The Movement of Molecules across Cell-Membranes", Academic Press, New York, 1967. (40) A. Leo, C. Hansch. and D. Elkins, Chem. Rev., 71,525 (1971). (41) J. Petrgnek and 0. Ryba, Anal. Chim. Acta, 72, 375 (1974). (42) A. M. Y. Jaber, G. J. Moody, and J. D. R. Thomas, Analyst(London) 101, 179 (1976). (43) U. Fiedler, Anal. Chim. Acta, 89, 101 (1977). (44) J. W. Ross, Orion Research Inc., Cambridge, Mass., private communication.
RECEIVEDfor review October 30, 1979. Accepted December
26, 1979.
Reversed-Phase Liquid Chromatography of Basic Drugs and Pesticides with a Fluorigenic Ion-Pair Extraction Detector Carmen van Buuren, J. F. Lawrence,' U. A. Th. Brinkman, I. L. Honigberg,2 and R. W. Frei" Department of Analytical Chemistry, Free University, De Boelelaan 1083, 108 7 HV Amsterdam, The Netherlands
fluorescence detection of some basic drugs, pesticides, and their metabolites is described. The addition of the ion-pairing reagent dimethoxyanthracene sulfonate (DAS) prior to the column permits a drastic simplification of the detector design. The Influence of the DAS concentration, pH, buffer concentration, and organic polarity modifier (methanol) on the separation of the above compounds carried out in the reversed-
DAS
--a
QRG
=
7
-
3 -
g
e x t r a c t i o n coil
-
9-
wastu
1
f l u o r o r e s c o n c o detuctor
B -
l r o m HPLC
DAS AIR
The use of post-column reaction detectors ( 1 , Z ) to expand the usefulness of existing detectors in liquid chromatography (LC) has gained widespread acceptance. Different types of reactor designs ranging from tubular ( 3 )and bed ( 4 ) reactors with nonsegmented streams to segmented-flow principles have been proposed ( Z ) , the latter being particularly useful for longer reaction times on the order of 5 min and longer ( 4 ) . Recently it has been shown (5,6)that segmentation techniques can also be used to advantage for relatively fast reactions, mainly in cases where the signal of the excess reagent interferes with the signal of the product. In these cases, a dynamic micro-extraction procedure has been adopted to separate the excess reagent from the less polar product. The feasibility of such an approach has been tested with tertiary amines of pharmaceutical and agricultural importance,
'On a transfer o f w o r k 1978 79 f r o m the Food Directorate, H e a l t h P r o t e c t i o n B r a n c h , Ottawa, danada. 2 0 n sabbatical leave 1Y78 from t h e U n i v e r s i t y of Georgia, School of P h a r m a c y , A t h e n s , Ga.
QRG
+ I
waste
k
g g
extraction coils
Y-
Figure 1. Schematics for ion-pair extraction and fluorescence detection. A = with air segmentation. B = without air segmentation (solvent segmentation)
and dimethoxyanthracene sulfonate (DAS) proposed earlier by Westerlund and Borg (7) as a fluorescent counterion for ion-pair formation. The schematic of the first detector construction coupled to reversed-phase separation and based on a three-phase segmentation system (5,6) is shown in Figure 1A. The reagent is added to the column effluent right after the column, air segmentation being used to efficiently reduce band broadening.
