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 CITED MPLE
1c
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. Figure 1. (A)
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 Jevne 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-renewal type 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$01.50/00 1985 American Chemlcal Society
--
ANALYTICAL CHEMISTRY, VOL. 57, NO. 6, MAY 1985
1158
+0.2
-o.6
v
ING
SOLN.+--K , , ,
r------POT.Z
~
VOLTAGE
VS.
NSE
0.0
-0.1 v
t
I
- 0.2
Figure 1. Flow chart of the flow electrode system for automatic analysis: E, preconcentration cell: D, detection cell; Pot., potentlostat; P, pump; V, valve.
I
to.1
Filtered seawater 1
,p$add-aceti: -+ acid- + T,
+ $ 0.0 a
3 V
+ 0.2
+0.4 t i
+ 0 6 t
j
LlA
Flgure 3. Current-voltage curves obtained with polished electrode: M bromide background current (l),2.5 X IO-' M Iodide (2), and (3)in base electrolyte.
I
I
1
somle exchange
1
(
Ill
(
15 sec.
electrol~tlcconcentratlon ) ( 60 sec,
320 sec.
washing
(
40 sec.
with cleaning solution for succesive analysis. The flow rates of sample and cleaning solution, eluent, and counterelectrolyte are 7.5, 3.0, and 1.0 mL/min, respectively.
)
)
RESULTS AND DISCUSSION
)
elution and detection ( 120-180 sec. ) total
V
washing
(
(
60 s e c 1
I-
+
IO3-
),
1
Flgure 2. Flow chart of the analytical procedure for the automatic determlnatlon of Iodine species in seawater.
described previously (3). An Ag/AgCl electrode in 1M potassium chloride solution is used as a reference electrode (normal silver electrode: NSE) for both cells. Two cells are controlled with potentiostats (Pot. 1 and Pot. 2). The potentiostat for the preconcentration cell (Pot. 1) can be changed over two arbitary potentials. Three peristaltic pumps (P) are used to send sample and cleaning solution, eluent, and counterelectrolyte of the preconcentration cell, respectively. Three couples of electromagnetic valves (V) are used to change the flow of solutions. The system is controlled with interlocking five-step timers. Reagents. Reagent grade chemicals and Milli-Q water are used throughout. Stock iodide and iodate solutions are prepared from potassium salts after drying overnight at 110 OC. Cleaning solution and eluent are 0.025 M acetic acid, the latter contains 5 X M of ascorbic acid. Counterelectrolyte is 0.4 M potassium sulfate solution. Artificial seawater is prepared according to the Lyman-Fleming formula (4). Procedures. The analytical procedure is shown by the flow chart in Figure 2. Filtered seawater is acidified to 0.07 M with M of tetradecylglacial acetic acid. At the same time, 2 X dimethylbenzylammonium chloride is added. To the sample M ascorbic acid to reduce iodate to solution is added 2 X iodide. At the first stage, the residual sample solution in the system is exchanged with a new one. Then the potential applied to the preconcentration cell is changed from -0.2 to 0.75 V vs. NSE. The sample solution is passed through the cell for 320 s in the case of iodide analysis and for 60 s in the case of totd iodine analysis. After electrolysis, the cell is rinsed with cleaning solution to remove residual chloride and bromide ions. The cell potential is again reduced to -0.2 V vs. NSE to elute the iodine from the carbon wool electrode with eluent containing reducing agent. The eluent is introduced to the detection cell and the iodide is determined at the polished electrode to which the potential of 0.1 V vs. NSE is usually applied. At the last stage the system is rinsed
In general, it is difficult to electrochemically determine the iodide in a sample solution containing large amounts of other halides, especially bromide, as in seawater due to the proximate oxidation potential of both halides. In this method, a major portion of the other halides is separated at the stage of electrolytic preconcentration of iodide, and the interference onslightly remaining chloride and bromide is negligibly small with the polished Ag3SI electrode because of its selective response against iodide. Figure 3 shows current-voltage curves obtained with the polished Ag3SI electrode in the base electrolyte (0.025 M acetic acid). Curves 1,2, and 3 are background current and diffusion currents (net value) for 2.5 X lo4 M of iodide and lov4M of bromide, respectively. These results indicate that the iodide is detected at the potential range between 0.08 and 0.13 V vs. NSE with sufficient sensitivity and without interference of bromide. The sensitivity and detection limit (noise level in the background current) of this electrode are approximately 0.1 pA/pM I- and 20 nM I- (2 nA), respectively, although the values deviate with various physical factors such as the surface condition of electrode. Silver and the mixture of silver and silver iodide are also tested as the working electrode. These electrode materials are inferior to Ag3SI in regard to the rate of current response and selectivity. The electrolytic concentration of iodide with carbon wool electrode is affected by various factors. The peak current obtained a t the detection cell changes with not only the electrolytic efficiency but also the kinetics of adsorptiondesorption of iodine on the carbon wool electrode because it is a diffusion current. For example, slow adsorption gives the endwise spread of accumulation of iodine in the column and the iodide concentration in the certain volume of eluent is reduced though the total amount of iodide in eluent is constant. As a result observed peak current is reduced. The peak current is constant at the cell potentials between 0.75 and 0.80 V vs. NSE. At more negative potentials the peak current decreases due to incomplete concentration of iodide. At positive potentials of more than 0.83 V the background current increases due to oxidative concentration of bromide. The peak current is constant at the flow rates of sample solution between 1 and 10 mL/min. The peak current is independent of the pH of sample solution over the range
ANALYTICAL CHEMISTRY, VOL. 57, NO. 6, MAY 1985
1159
L
1.5
rl
CONCENTRATION
Flgure 4. Effect of various salts on the peak current: [I-] = 5 X IO-' M; sample volume, 7.5 mL; NaBr (I), CsCl (2), NaCl (3),NaNO, (4), NaCIO, (5).
-60 min. Flgure 8. Recording chart of measurement for M; sample volume, 10 mL.
iodide: [I-] = 5
X
NA
0.3 /
0.2 IW
d
a 0
0
/'
0.1
0 x
I - or
0
1
2
3
4
5x10-45M(2) x 10- M (1 )
10-'M(2)
10,
Plot of peak current vs. concentration of iodide or iodate: sample volume, 7.5 mL (l), 40 mL (2).
Flgure 7.
CONCENTRAT ION
Flgure 5. Effect of large Ions on the peak current. [I-] = 5 X M; sample volume, 7.5 mL; tetradecykllmethylbenzytammonlum chloride (l), sodium tetraphenylborate (2).
between pH 2.6 and pH 6.0. The electrolytic efficiency is confirmed to be 100% by the fact that no peak current is obtained with the sample solution once electrolysis has been conducted. The effect of foreign ions in the sample solution on the peak current is shown in Figures 4 and 5, where the values of peak current are indicated as peak height normalized with the value of 0.2 M of acetate buffer solution. The presence of chloride and bromide gives a high value of peak current. Large anions such as perchlorate and tetraphenylborate decrease the peak current remarkably whereas large cations such as cesium and tetradecyldimethylbenzylammoniumincrease it. These results suggest that the adsorbed iodine species on the carbon wool electrode is an ion pair of large anion such as Is-, 12Cl-, and I,Br-. Since a constant and sensitive peak current is obtained in the presence of (1-4) X M of tetradecyldimethyl-
benzylammonium chloride (zephiramine), 2 X M of this salt is added to the sample solution prior to electrolysis. The most suitable reducing agent to reduce iodate to iodide is ascorbic acid because reduction is achieved within 10 min by the addition of 2 X lo4 M of ascorbic acid and successive electrode reaction is not affected by the presence of ascorbic acid less than 4 X lo-* M. A typical peak current recording is shown in Figure 6 for ten replicate measurements of an artificial seawater sample containing 5 X M of iodide. The same results are obtained with natural samples (salinity, 34.27-34.85%0;total iodine, (3.9-4.6) X lo-' M), and the result of standard addition to them gives sensitivity equal to that for artifical seawater. These results indicate that the iodide in seawater sample is determined within a standard deviation of about 2%. Figure 7 shows the plot of peak current vs. iodide (or iodate) concentration. Curves 1 and 2 are the cases using 7.5 mL (60 s concentration) and 40 mL (320 s concentration) of sample solution, respectively. The curves are almost linear in the high concentration range but are somewhat bent in the low con-
1160
Anal. Chem. 1085, 57, 1160-1163
in proportion to the concentration after 0.2 M of potassium chloride is added to the sample solution. However these sample solutions often contain a large amount of dissolved organic matter: therefore sensitivity should be corrected by the standard addition method. The detailed electrode reaction mechanism in this system and the oceanographicdata of iodine species will be mentioned in forthcoming reports. Registry No. Ag3SI, 12003-00-8; I-, 20461-54-5; iodate, 15454-31-6;water, 7732-18-5.
Table I. Results of Seawater Analysis
location
depth, m
M
W[total iodine], M
24' N, 128' E
0 50 150 320 1200 1800 0 0 0 0
1.1 1.2 0.25 0.10 0.14 0.14 1.0 0.85 0.51 1.3
3.9 4.1 4.0 4.1 4.6 4.5 4.0 4.0 3.9 4.1
lo'[ iodide],
30' N, 137' E 20' N, 1 3 7 O E 10' N, 137' E 0' N, 137' E
LITERATURE CITED (1) Truesdale, V. M. Mar. Chem. 1978, 6, 253-273. (2) Nakata, R.; Okazaki, S.; Hori, T.; Fujinaga, T. Anal. Chlm. Acta 1983, 149, 67-75. (3) Fuglnaga, T.; Kimoto, T. Talanta 1984, 31, 720-722. (4) Lyman, J.; Fleming, R. H. J . Mar. Res. 1940, 3 , 134-146. (5) Truesdale, V. W. Mar. Chem. 1978, 6 , 1-13.
centration range. The sensitivity of approximately 0.4-0.5 pA/pg of I- is given by these curves. Since total concentration of iodine species in seawater is ca. 50 pg/L (4.0 X lo-' M) sufficient sensitivity is given by 7.5 mL of sample solution for total analysis (in this case the increase of sensitivity with preconcentration is 4 to 5 times). Although the concentration of iodide in seawater varies significantly with location and depth, it is around 1 pg/L (8 X M) in the lowest case according to existing data (5). The iodide is detectable with 40 mL of sample solution even in such a case (this preconcentration gives a -20 times increase of sensitivity). Table I shows results of the seawater analysis. These values indicate good agreement with existing data (5) for total iodine and are more accurate and precise for iodide because values of iodide less than M are included within the error of iodate (or total iodine) analysis in the existing data. In addition, iodine species in freshwater samples such as lake water, river water, and rainwater can be determined similarly by applying an adequate amount of sample solution
'
Present address: Kimoto Electric Co., Ltd., 3-1, Funahashl-Cho, Tennoji-Ki, Osaka 543, Japan. *Present address: Department of Chemistry, Faculty of Science, Kyoto University, Kyoto 606, Japan.
Eiichiro Nakayama* Takashi Kimoto' Satoshi Okazaki2 Research Center for Instrumental Analysis Faculty of Science Kyoto University Kyoto 606, Japan
RECEIVED for review October 26,1984. Resubmitted January 7,1985. Accepted January 23, 1985. The research was supported by a grant from the Ministry of Education, Culture and Science, Japan.
Abundances of Molecular Ion Species Desorbed by Fast Atom 2H)'. and (M 3H)' Bombardment: Observation of (M
+
Sir: Fast atom bombardment mass spectrometry (FAB) (2,2) has seen ever increasing use in the characterization of ionic, polar, and/or labile samples. In particular, many studies on compounds with molecular ion species of over mass 1000 ( 3 , 4 )and up to nearly 10 000 (5) have been reported. With the commercial availability of magnetic sector instruments capable of upper mass limits of at least 10 000 daltons at full accelerating potential, the applications of FAB as an ionization technique for the study of biomolecules surely will continue to increase. Accurate mass measurement has been an important tool in the confirmation of molecular weight and empirical formulas in the study of electron ionized compounds up to mass 1000. Full mass scans at mass resolutions of 10000 to 30 000 can yield mass values with 1-2 ppm accuracies. Accurate masses can be determined by using FAB as well, by employing either calibrated full scan methods of more commonly peak matching. However, as one goes to masses greater than 1000 amu, for example, even a resolution of loo00 will mean a peak width or Am of 0.1 amu or greater. The number of possible elemental compositions contained within a 0.1 amu window increases dramatically with an increase in mass. High mass measurement accuracy (sub part per million) will be necessary so that the candidate formulas are restricted to a tractable number. 0003-2700/65/0357-1160$01.50/0
+
Another problem associated with accurate mass measurements of higher molecular weight compounds is that peaks are no longer singlets, but rather complex combinations of isotopic species. This has been theoretically demonstrated by Yergey et al. for the m / z 5776.6 ion of porcine insulin (6). A resolution of 5000000 is required to achieve complete separation. This is well beyond present instrumental capabilities. Any accurate mass determination becomes an "average mass" determination and loses some of its significance. In situations where there is a low abundance of ions produced, either because of very small sample amounts or because of poor ionization efficiency, accurate mass measurements requiring high resolution may be impossible. One recourse for validating elemental compositions may be the use of the isotopic patterns obtainable a t low resolution (7, 8). This strategy would be particularly useful for halogen or isotopically rich metal-containing species which often have rather unique isotopic patterns. For example, aglucovancomycin, C53H52N8Ol7Cl2,mol w t 1142, has a pattern distinctly different from that of the more than 1500 peptides containing the standard amino acids and having the same nominal mass. A peptide with a large number of cystine and methionine residues will be rich in sulfur, and its isotopic cluster should be distinguishable from those of other peptides. 0 1985 American Chemical Society
