1155
Anal. Chem. 1905. 57. 1155-1157
also open many possibilities including the use of traditional liquid chromatography detectors, including electrochemical detectors, in series with a tandem mass spectrometer to yield simultaneous qualitative and quantitative results. This note has emphasized a sophisticated application of the membrane interface used with a complex mass spectrometer, but it may find most utility when coupled to a simple mass spectrometer for routine environmental analysis.
MULTIPLE REACTION MONITORING' DAUGHTERS OF 141'
-
m .... h
55 81
123
136 blank
I ". I
I ,
.It
,.
XI000
141
a
___-
ACKNOWLEDGMENT We thank L. B. Westover (Dow Chemical) for helpful discussions. Registry No. Cyclohexanone,108-94-1;methylcyclohexanone, 1331-22-2; dimethylcyclohexanone, 1333-44-4; trimethylcyclohexanone, 50874-76-5; tetramethylcyclohexanone, 95421-59-3. LITERATURE CITED
Flgure 4. Comparison of several dlfferent reactions used to monitor formation of trimethylcyclohexanone in a reacting mixture. R I C is the total ion current (MS scan), 141 is a scan for the selection ion, while 130, 123, 81, and 55 denote reactions of 141 to give these fragment ions.
reaction leading from 141' to 136+ is included as a blank to indicate background signal to noise levels since the parent 141' does not fragment to 136+after CAD. Furthermore, the ion profiles for each of the selected fragments are smoother than those for the unfragmented parent, as would be expected since reduced noise levels are often observed in MS/MS relative to MS. The RIC (reconstructed ion current) represents the total ion current in the mass spectrum, viz., the sum of all ion signals detected at any particular time. The features of this interface that underline its value and versatility include the simplicity of design, its use with small sample sizes, and its adaptability to virtually any mass spectrometer. Although its response (1-5 min) to changes in sample concentrations was rapid, problems were encountered with memory effects. This may be the result of slow diffusion of some compounds but is alleviated by the disposable nature of the very simple and inexpensive interface. Naturally many compounds do not diffuse through the membrane at all, but this selectivity is key to its operation. The membrane interface seems particularly promising when used in conjunction with a robot-controlled organic reactor or as an industrial process monitor. Rapid product monitoring with the ability for feedback control of reaction conditions to optimize for desired product should be possible. Successful coupling of the membrane to a liquid chromatograph should
Alcock, N. J.; Kuhny, W.; Games, D. E. I n t . J . Mass Spectrom. Ion Phys. 1983, 4 8 , 153-156. McFadden, W. H.; Schwartz, H. J.; Evans, S. J . Chromatogr. 1976, 122,389-396. Takeuchl, T.; Hlrata, Y.; Akumura, Y. Anal. Chem. 1978, 50, 659-660. - -.- - -.
Dark, W. A.; McFadden, W. H.; Bradford, D . J. J . Chromatogr. Sci. 1977. 15. 454-59. McFadden, W. H . J . Chromatogr. Scl. 1980, 18, 97. Liberato, D. J.; Fenselau, C. C.; Vestal, M. L.; Yergey, A. J. Anal. Chem. 1883, 55, 1741-1744. Hayes, M. J.; Tankmayer, E. P.; Vouros, P.; Karger, B. L.; McGuire, J. M. Anal. Chem., 1983, 55, 1745-1752. Weaver, J. C.; Abrams, J. H. Rev. Scl. Instrum. 1979, 50 (4). Westover, L. B.; Tou, J. C.; Mark, J. H. Anal. Chem. 1974, 46 (4), 568. Jones, P. R.; Yang, S. K. Anal. Chem., 1975, 4 7 , 1000. Kallos, G. J.; Mahle, N. H. Anal. Chem. 1983, 55,813-814. Calvo, K. C.; Weisenberger, C. R.; Anderson, L. B.; Klapper, M. H. J . Am. Chem. SOC. 1883, 105,8935-6941. Calvo, K. C.; Weisenberg, C. R.; Anderson, L. B.; Klapper, M. H . Anal. Chem. 1081, 53, 981-985. Weaver, J. C.; Mason, M. K.; Jarrell, J. A,; Peterson, J. W. Blochim. Biophys. Acta 1976, 438, 296-303. Tetler, L. W.; Watson, J. M.; Kirkbright, G. F.; Elliot, M.; Walder, R.; Scrlvens, J. H. Paper presented at the British Mass Spectrometry Society, Fourteenth Meeting, Edinburgh, Sept 16-21, 1984. Johnstone, R. A.; Rose, M. E. Tetrahedron 1979, 35,2169-2173.
Jennifer S . Brodbelt R. Graham Cooks* Department of Chemistry Purdue University West Lafayette, Indiana 47907
RECEIVED for review November 19,1984. Accepted January 22,1985. This work was supported by the National Science Foundation (CHE-8408258).
Simplified Procedure for Forming Polymer-Based Ion-Selective Electrodes Sir: There are a variety of known approaches (1)to prepare polymer-based ion-selective membranes (ISM) and incorporate these membranes into ion-selective electrodes (ISE). All of these procedures for making membranes require a mixture of the polymer, ion-selective reagent, and plasticizer(s), as needed, dissolved in a suitable volatile solvent. The ISM's are then formed by such techniques as casting into a mold and letting the solvent slowly evaporate. An ion-selective electrode can then be made by attaching a piece of the polymer-based membrane to a suitable body or by dip coating
the membrane solution onto a porous substrate (2) or onto a wire (3). We will describe a new simplified procedure for fabricating polymer type ISM's and ISE's. Our process is based on the impregnation of a preformed polymeric electrode body or part with the ion-selective reagent(s). This usually eliminates the need for preparing a separate membrane or a polymeric based membrane solution and then attaching it to an electrode body. In our process the desired ion-selective reagent is dissolved in a liquid which is a swelling agent for the polymer. A
0003-2700/85/0357-1155$01.50/00 1985 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 57, NO. 6, MAY 1985
selective. The following examples are provided for making potassium and pH sensitive impregnated ion-selective electrodes: Potassium. An impregnating solution of 10 mg of valinomycin in 2.5 mL of xylene was used. One end of a piece of SR tubing was soaked in this solution for 1-2 min, removed, and allowed to air-dry. The tip of the impregnated end was plugged with a suitable material such as Dow Corning Silastic Medical Adhesive and allowed to cure. The tube was filled with 0.1 M KCl, and a Ag/AgCl wire was inserted (Figure 1A). It was then ready for use as a potassium-selective electrode. p H . An impregnating solution of 0.5 g of TDDA and 0.15 g of THF (tetrahydrofuran) was used. One end of a piece of PVC tubing was soaked in this solution for 1h. When removed from the impregnating solution, excess solution was cleaned off with acetone and distilled HzO and the tube dried at 70 "C for 15 min. The impregnated end was plugged with a PVC glue (e.g., PVC dissolved in THF) which was allowed to cure. The tube was then filled with a pH 4.6 buffer and a Ag/AgCl wire inserted. It was then ready for use as a pH probe. In addition to the design described in the examples above (Figure 1A) we have used this technique to fabricate discrete ion-selectivemembranes which can be attached to a separate body (Figure 1B) and also to make a flow-through device (Figure IC). For the discrete membrane design (Figure 1B)a disk was cut from the wall of the tube with a cork borer, impregnated, and then glued back into the tube. The flow-through configuration (Figure 1C) involved impregnating a segment of the tubing and then fitting it with an external jacket to house the reference electrodes. The other ion-selective reagents and polymers that have been tried are summarized in Table I. Commerical reference electrodes and research grade pH/mV meters were used to make all measurements.
Table I. Electrodes Prepared Using Impregnation Process species design" polymerb
K+ K+
K+ NH4+
c1PH PH K+
A, c A, B A A A A A A
SR PVC urethane SR SR SR PVC C-Flex
ion-selective reagent
swelling reagent
valinomycin valinomycin valinomycin nonactin aliquat TDDA TDDA valinomycin
xylene DPP acetone CHzClz xylene Freon DPP/THF xylene
additives
KTPB
DPP
a See Figure 1 for designs. *See Experimental Section for material abbreviations.
preformed electrode body or substrate piece (tube, disk, etc.) is soaked in this solution and the swelling agent carries the ion-selective reagent into the matrix of the polymer. After the polymer piece is removed, the swelling agent evaporates and the polymer returns to its original size and shape. However, it now has the ion-selective material distributed throughout its matrix, and it is now an ion-selective membrane. We will present preliminary results demostrating that this process can be used to fabricate a variety of ion-selective membranes (pH, potassium, ammonium, and chloride), using several different polymers (silicone rubber, poly(viny1 chloride), urethane, and C-Flex) in a variety of configurations.
EXPERIMENTAL SECTION Materials and Reagents. The ion-selective reagents and plasticizers used were as follows: valinomycin from Aldrich Chemical Co., Milwaukee, WI; tridodecylamine (TDDA),dipentyl phthalate (DPP), and dioctyl sebacate (DOS) from Eastman Kodak, Rochester, NY; nonactin from Sigma Chemical Co., St. Louis, MO, and methyltricaprylammonium chloride (Aliquat 336) from Henkel Corp. Minneapolis, MN. The polymer tubings used were as follows: silicone rubber (SR) from Dow Corning, Midland, MI, and from SilMed Corp., Tauton, MA; poly(viny1 chloride) (PVC), Tygon formulations R3603 and 50HL with durometers of approximately 60 Shore A from Norton Chemical Co., Akron, OH, and a custom extrusion with a durometer of 70 Shore A from Excel Plastics,Shrewsbury, MA, urethane from Medtronic, Energy Technology, Brooklyn Center, MN; and C-Flex (a styreneethylene/butylene-styrene end block copolymer modified with poly(dimethylsi1oxane) and mineral oil) from Concept, Inc., Clearwater, FL. All other reagents and solutions were analytical grade. Procedure. Impregnating solutions were prepared by dissolving the desired ion-selective reagent in a solvent which is also a swelling agent for the polymer. Note that this solution is not a solvent for the polymer but rather a swelling medium. The polymeric part is soaked in this solution until swollen. It is then removed and the swelling agent allowed to evaporate. By this procedure, the ion-selective reagent remains entrapped in the polymeric matrix and the area which has been swollen is now ion
RESULTS AND DISCUSSION The results obtained from the calibration curves for each of the ion-selective electrodes described are summarized in Table 11. All of the devices with the exception of the potassium/urethane and the chloride/SR combinations gave acceptable Nernstian responses. These two, however, had sub-Nernstian responses of 44 mV/decade change in activity. At the time these studies were completed, we were interested only in demonstrating the versatility of this unique process for making polymeric-based ion-selective electrodes and no elaborate attempts were made to optimize the performance of any of these devices. However, it has been reported by Fiedler et al. ( 4 ) that urethane-based potassium-sensitive membranes without plasticizers such as DOP have subNernstian responses. We believe this is the reason these particular combinations of devices did not work well here. Incorporation of suitable plasticizers should enhance the response of our urethane system. The impregnation time depends primarily on the polymer and the swelling reagent. Silicone rubber tubes with wall thicknesses ranging from 0.006 in. (0.15 mm) to 0.025 in. (0.64 mm) would generally swell in less than 1 min in either Freon
Table 11. Summary of Electrode Testing species
polymer
K+ K+ K+ K+ K+ NH,+
SR SR
A
PVC PVC urethane
PH
SR SR SR
A A A A A A
PH
PVC
B
K+
C-Flex
B
c1-
a
See Figure 1 for designs.
design" C
range tested, M
x 104 to 1 x 1 x 10-4to 1 x 4 x 10-5to 1 x 1X to 6 X 1 x 10-3to 1 x 1 x 10-3 to 1 x 1 x 10-3to 1 x 1 x 104 to 1 x 5
(PH 6-81
10-2
io-* 10-2
lo-$
io-* 10-1
10-1 10-8
3.2X10-5 to 3.2 X (pH 4.5-8.5) 1X to 6 X
slope
r
58.8
0.9999 0.9999 0.9993 0.9999 0.9998
57.4 55.7 59.1 44.3 57.3 -44.2 54.1
1.0000 0.9992 0.9996
56.3
0.9994
59.0
0.9995
Anal. Chem. 1985, 5 7 , 1157-1160
2
ordinary electrodes since the effective membrane surface area is significantly larger (example 1above had a typical resistance of 10-20 MQ). The response times of these devices, however, is believed to be similar to other polymeric based ion-selective membrane electrodes. Furthermore, we have not observed a dependence of response time on the thickness of the membrane (i.e., the wall thickness of the impregnated tubes). Although this impregnation process can be used to make discrete ISMSfrom preformed polymeric parts (Figure lB), we believe its greatest utility comes in the fabrication of ISEs with no visually recognizable membrane area (Figure lA, C). Therefore, any problems that may be associated with forming uniform membranes of desired size, shape, and thickness as well as with attaching the membrane to the electrode bodies are eliminated. We are continuing this work and will be reporting on an extensive in vitro and in vivo evaluation of potassium sensors fabricated using these processes, which were designed for medical applications. Registry No. PVC (homopolymer), 9002-86-2; (butylene). (ethylene).(styrene) (copolymer), 57271-36-0.
3\
3\
7' 4 '
3\
1157
IJ2
LITERATURE C I T E D MPLE
1c
Figure 1. (A) Impregnated tube design. (B) Impregnated disk. (C) Flow-through design. 1, impregnated ion-selective membrane areas; 2, Ag/AgCI internal reference; 3, internal filling solution; 4, plug.
or xylene. Typical impregnation times were less than 5 min. The plasticized PVC tubing would neither swell as much nor as fast in the solutions tried and therefore required longer impregnation times (e.g., 1-2 h). Wall thicknesses for the PVC tubing used ranged from 0.011 in. (0.28 mm) to 0.062 in. (1.5 mm) . The overall impedance of an ion-selective electrode made using this approach is usually smaller than for comparable
(1) Covington, Arthur K., Ed. "Ion Seiectlve Electrode Methodology, Vol. 1"; CRC Press: Boca Raton, FL, 1979; Chapter 7. (2) Treasure, Thomas; Band, David M. J . M e d . Eng. Techno/. 1979, 1 , 271-273. (3) Freiser, Henry, Ed. "Ion Selectlve Electrodes in Analytical Chemistry, Vol. 2"; Plenum Press: New York, 1980; Chapter 2. (4) Fiedler, U.; Ruzicka, J. Anal. Chlrn. Acta 1973, 6 7 , 179-193. 'Present address: 1225 Shirley Way, Bedford, TX 76102.
Eric J. Fogt* Patrick T. Cahalan Allan J e v n e Michelle A. Schwinghammer' Medtronic, Energy Technology 6700 Shingle Creek Parkway Brooklyn Center, Minnesota 55430
RECEIVED for review November 26, 1984. Accepted January 17, 1985.
Automatic Determination of Iodine Species in Natural Waters by a New Flow-Through Electrode System Sir: Inorganic iodine species, iodide, and iodate in seawater have been determined by a variety of classical methods of analysis. Only t h e spectrophotometric procedure using the absorbance of I,- a t 353 nm is utilized for an automatic determination ( I ) . This method, however, has a few disadvantages such as interference with nitrite and insufficient accuracy for low concentration of iodide. This paper describes a new electrochemical technique for the automatic determination of iodine species in natural waters. The iodide is electrochemically oxidized to iodine and quantitatively concentrated on a carbon wool electrode in a preconcentration cell. After the interference ions were removed, the iodine was eluted with reducing agent followed by the determination at the polished &,SI electrode in the detection cell. The iodate is determined after being reduced to iodide by reducing agent. The resulting amperometric current is proportional to iodide or iodate
concentration in the original solution. The sensitivity is 0.4-0.5 pA/pg I- and the detection limit is 5 ng of I-. Iodine species can be accurately determined as iodide and total iodine (iodide + iodate) from less than 50 mL of seawater sample by this method. EXPERIMENTAL SECTION Apparatus. The electrode system for automatic analysis (Kimoto Electric Co., Ltd.) is shown in the flow chart in Figure 1. The preconcentration cell (E) is the same as that previously reported ( 2 ) . The only modification is the use of a working electrode of carbon wool (Kureha Kagaku Co., Ltd.) packed in a Vycor glass tube (Corning Co., Ltd.). The polished Ag3SI electrode in the detection cell (D) is a new surface-renewaltype solid electrode in which a magnetic stirrer bar coated with silicon carbide is rotated at 600 rpm to polish the electrode surface continuously. The detailed construction of this electrode has been
0003-2700/85/0357-1157$01.50/00 1985 American Chemlcal Society
