Anal. Chem. 1996, 68, 4149-4152
Anion-Selective Electrodes Based on Long-Chain Methyltrialkylammonium Salts Satoshi Ozawa,*,† Hiroyuki Miyagi,‡ Yasuhisa Shibata,‡ Naoto Oki,‡ Toyoki Kunitake,§ and Walter E. Keller⊥
Central Research Laboratory, Hitachi Ltd., Kokubunji, Tokyo, Japan, Instrument Division, Hitachi Ltd., Hitachinaka, Ibaraki, Japan, Department of Chemical Science and Technology, Kyushu University, Fukuoka, Japan, and Fluka Chemie AG, Buchs, Switzerland
A new series of methyltrialkylammonium salts with an alkyl chain length (n) longer than the conventional methyltridodecylammonium (MTDDA, n ) 12) has been developed, and these materials were examined for use as the ion-sensing component (ligand) in anion-selective electrodes (ISEs). Syntheses of the higher ammoniums with n ) 16, 18, and 20 were carried out. In combination with an alcoholic plasticizer, the ammoniums with n ) 12, 16, and 18 led to ISEs with fundamental characteristics, such as slope sensitivity, impedance, and time response, that were sufficient for practical applications. Compared with the conventional MTDDA, the ISEs based on the ligands of n ) 16 and 18 showed marked improvement in chloride selectivity over both lipophilic and hydrophilic anions, deviating from the Hofmeister regime in some cases. Taking perchlorate as an example, the magnitude of the improvement was a factor of 20 for n ) 16 and 15 for n ) 18. When the new ISEs were applied to chloride analysis in blood serum, they improved the accuracy by a factor of 2-6. Therefore, the methyltrialkylammonium salts with alkyl chain lengths of 16 and 18 offer definite advantages over the conventional alternative and are strong candidates to become the standard compounds for use in future chloride ISEs. The concentration of electrolytes such as Na+, K+, and Cl- in blood provides fundamental information in clinical chemistry, especially with regard to osmotic pressure and acid-base balance. Ion-selective electrodes (ISEs) have found widespread use in the clinical determination of electrolytes. ISEs are preferred over the conventional flame photometry and coulometric titration because they offer advantages such as high throughput, suitability for automation, and elimination of fire hazards. Since the advent of an ISE based on a neutral ionophore,1 ISEs with high selectivity have been developed for major cations such as K+ and Na+. However, despite continuing efforts,2,3 the realization of a reliable ISE with sufficient selectivity for Clremains a formidable challenge. Most of the current ISEs applied * To whom correspondence should be addressed. E-mail: ozawa@crl. hitachi.co.jp. † Central Research Laboratory, Hitachi, Ltd. ‡ Instrument Division, Hitachi Ltd. § Kyushu University. ⊥ Fluka Chemie AG. (1) Stefanac, Z.; Simon,W. Chimia 1966, 20, 436. (2) Wuthier, U.; Pham, H. V.; Zu ¨ nd, R.; Welti, D.; Funk, R. J. J.; Bezegh, A.; Ammann, D.; Pretsch, E.; Simon, W. Anal. Chem. 1984, 56, 535-538. (3) Rothmaier, M.; Simon, W. Anal. Chim. Acta 1993, 271, 135-141. S0003-2700(96)00526-4 CCC: $12.00
© 1996 American Chemical Society
to clinical Cl- determinations are based on an ion-sensing material (ligand) of an ion-exchanger type, such as methyltridodecylammonium (MTDDA) salt.4 In principle, the selectivity of an ISE based on such an ion-exchanger is ruled by the lipophilicity of the ion (i.e., the Hofmeister series) and not by the ligand.5 However, there have been some reports suggesting that selectivity can be improved. Impregnating lipophilic anions into a plasticized anion-exchange membrane enhanced the Cl- selectivity, especially over hydrophilic anions.6 An annealed polyion complex membrane prepared from a cation-exchange polymer and excessive quaternary ammonium7,8 reportedly exhibited high Cl- selectivity at temperatures below the transition temperature. Certain plasticizers also appear to induce similar effects.4,5 With a combination of higher aliphatic alcohol9-12 and ligands of a tetraalkylammonium type,5 we have observed significant improvement in the Clselectivity over lipophilic anions and organic acid ions, especially when the alkyl chain length of the ligand was longer than 14.13,14 However, the electrodes based on such long-chain tetraalkylammoniums generally show high impedance, so a high level of expertise is needed to exploit the potential of the ligands. On the other hand, because of its relatively compact and asymmetric chemical structure, the MTDDA salt has the intrinsic advantages of high solubility and strong ionic interaction. These properties are considered the basis of the favorable characteristics observed with MTDDA-based ISEs, such as low impedance, high sensitivity, quick response speed, and easy preparation of the ISE membranes. Therefore, we presumed that the advantages of both the MTDDA and the higher selectivity of the long-chain tetraalkylammoniums could be attained by preserving the basic structure of methyltrialkylammonium salt and lengthening its three alkyl chains. One such compound was recently synthesized,15 and initial attempts have been made to apply such compounds to the (4) Hartman, K.; Luterotti, S.; Osswald, H. F.; Oehme, M.; Meier, P. C.; Ammann, D.; Simon, W. Mikrochim. Acta (Wien) 1978, II, 235-246. (5) Wegmann, D.; Weiss, H.; Ammann, D.; Morf, W. E.; Pretsch, E.; Sugahara, K.; Simon, W. Mikrochim. Acta (Wien) 1984, III, 1-16. (6) Oka, S.; Sibazaki, Y.; Tahara, S. Anal. Chem. 1981, 53, 588-593. (7) Ogata, T.; Yanagi, H. High Polym. Jpn. 1989, 38, 212. (8) Kimura, K.; Matsute, M.; Yokoyama, M. Anal. Chim. Acta 1991, 252, 4146. (9) Coetzee, C. J.; Freiser, H. Anal. Chem. 1968, 40, 2071. (10) Jyo, A.; Torikai, M.; Ishibashi, N. Bull. Chem. Soc. Jpn. 1974, 47, 28622868. (11) Hara, H.; Okazaki, S.; Fujinaga, T. Bull Chem. Soc. Jpn. 1981, 54, 29042907. (12) Ishiwada, H.; Suzuki, K.; Shirai, T. Bunseki Kagaku 1982, 31, 71-76. (13) Shibata, Y.; Ozawa, S.; Oki, N.; Miyagi, H. Bunseki Kagaku 1992, 41, 5-10. (14) Shibata, Y.; Ozawa, S.; Miyagi, H. Bunseki Kagaku 1993, 42, 77-83. (15) Kunitake, T.; Kimizuka, N.; Higashi, N.; Nakashima, N. J. Am. Chem. Soc. 1984, 106, 1978-1983.
Analytical Chemistry, Vol. 68, No. 23, December 1, 1996 4149
Table 1. Nomenclature, Abbreviations, and Sources of the Examined Methyltrialkylammoniums
a
alkyl chain length (n)
IUPAC nomenclature
12 14 16 18 20
N-methyl-N,N-didodecyl-1-dodecanaminium N-methyl-N,N-ditetradecyl-1-tetradecanaminium N-methyl-N,N-dihexadecyl-1-hexadecanaminium N-methyl-N,N-dioctadecyl-1-octadecanaminium N-methyl-N,N-dieicosyl-1-eicosanaminium
abbrevation MTDDA MTTDA MTHDA MTODA MTEA
source Fluka Chemie Dojindo Laboratories synthesizeda synthesized;a Fluka Chemie (sample) synthesizeda
See text.
ligands used for ISEs.14 This paper will focus on the development of a series of such long-chain methyltrialkylammonium compounds, their characteristics as Cl- ligands, and their applicability to Cl- determination in blood serum samples. EXPERIMENTAL SECTION Reagents. Methyltrialkylammonium salts with an alkyl chain length of 12-20 were either purchased or synthesized according to the literature.15 The names of the ammoniums, abbreviations, IUPAC nomenclature, and sources are listed in Table 1. The synthetic procedure for the MTHDA was similar to that reported in the literature,15 except that the quaternization of the trihexadecylamine was carried out with dimethyl sulfate. The resulting monomethyl sulfate salt was converted to the bromide form by washing with aqueous NaBr until the IR bands of the sulfonic group at 1240 and 1040 cm-1 disappeared. Recrystallization in ethyl acetate produced a colorless needle. The final structure was confirmed by FAB-MS spectroscopy. The analytical results are summarized in Table 2. The MTODA and MTEA were prepared according to similar procedures, except that, for the MTEA, eicosylamine was synthesized from eicosyl bromide by the Gabriel reaction, followed by Ing-Manske decomposition. Electrode Systems. The above quaternary ammonium salts were incorporated into ion-selective membranes by using a higher aliphatic alcohol as a plasticizer, according to the conventional method described elsewhere.13,16 To accelerate the response time, o-nitrophenyl octyl ether was used as an additional membrane component. The membranes were adhered either onto an open end of a stick-type electrode body or onto a flow-through-type electrode body. The stick-type electrode was combined with a double-junction reference electrode, the overall electrochemical cell configuration being as follows:
Ag|AgCl|1 M KCl|1 M LiOAc|sample||membrane|| 0.01 M NaCl|AgCl|Ag The flow-through-type electrode was combined with a free-flow reference electrode, the overall cell being:
Ag|AgCl|1 M KCl|sample||membrane|| 0.1 M NaCl|AgCl|Ag The performance of the ISEs was sometimes influenced by the initial contact with the serum samples. Therefore, the ISEs were conditioned with serum for several hours prior to use. Apparatus. Slope sensitivity, selectivity, and impedance were measured using stick-type ISEs with instruments described elsewhere.13,17 Serum sample measurements were carried out with (16) Sugahara, K.; Yasuda, K.; Mori, J. U.S. Patent 4519891, 1985. (17) Oehme, M. Dissertation, ETH Zu ¨ rich, 1977; No. 5953, p 63.
4150
Analytical Chemistry, Vol. 68, No. 23, December 1, 1996
Table 2. Analytical Results for the Synthesized Compounds elemental analysis compound
mp (°C)
MTHDABr
95-97
MTODABr
91-96
MTEABr
93-96
FAB-MS m/z (relative intensity)
calcda
found
268 (70.4) 479 (20.9) 705 (100)b 296 (55.6) 534 (20.3) 789 (100)b 324 (100) 591 (27.3) 873 (96.5)b
C, 73.27 H, 13.05 N, 1.74 C, 74.78 H, 13.23 N, 1.59 C, 75.72 H, 13.33 N, 1.45
C, 73.57 H, 13.11 N, 1.78 C, 74.75 H, 13.22 N, 1.53 C, 75.70 H, 13.29 N, 1.37
a As anhydrate for MTHDA, and as monohydrate for MTODA and MTEA. b Corresponds to [(M - X-)+].
flow-through-type ISEs mounted on a Model 7250 Hitachi automatic blood chemistry analyzer. This analyzer dilutes samples approximately 30-fold with Good’s buffer solution. For reference, serum samples were also measured with coulometry by a Model 710 Hitachi automatic electrolyte analyzer. Procedures. Selectivity was evaluated with the separate solution method, using nonbuffered 0.1 mol/L solutions. To evaluate the precision and accuracy of the ISEs, various blood serum samples were measured by different methods, and correlation studies were carried out. The precision of an ISE was evaluated by comparing two sets of successive measurements carried out in a rearranged order with the same ISE. The accuracy of an ISE was evaluated by comparing the results obtained with the ISE and the reference method (coulometry). The standard error (Syx, i.e., the standard deviation from the regression line) and the standard deviation of difference (SDdif, i.e., standard deviation of the difference between the two sets of results) were used as the parameters representing the error, the magnitudes of which are inversely indicative of the precision and accuracy. RESULTS AND DISCUSSION Fundamental Characteristics. The slope sensitivity and the impedance of the ISEs based on the long-chain methyltrialkylammonium ligands are summarized in Table 3. The ISEs based on MTTDA showed low slope sensitivity and high impedance over 100 MΩ, and their measurement ability deteriorated within a few days. The ISEs based on MTEA showed a sluggish time response and slightly low slope sensitivity. The reason for the instability of the MTTDA-based ISE is not completely clear, but it appears to be related to the fact that the MTTDA-based ISE membrane was the least uniform. The sluggishness of the MTEA-based ISE can be explained on the basis of the rather low solubility of the
Table 3. Slope Sensitivity and Impedance of the ISEs Based on Long-Chain Methyltrialkylammonium Ligands ligand
alkyl chain length (n)
slope sensitivity (mV/decade)
impedance (MΩ)
MTDDA MTTDA MTHDA MTODA MTEA
12 14 16 18 20
-56 -35a -56 -58 -55b
∼5 >100 ∼3 ∼2 ∼8
a
Low reproducibility, decreases with time. b Slightly slow response.
Figure 1. Selectivities of the ISEs based on MTDDA, MTHDA, and MTODA.
ligand in the membrane, which is likely to lead to lower mobility and diminishing ion-exchange kinetics in the membrane. Apart from these two exceptions, the other ligands led to ISEs with slope sensitivity in the range of -56 to -58 mV/decade, close to the theoretical value, and feasibly low impedance of less than 10 MΩ. They also showed sufficient linear response in the activity range of 10-4-10-0.2. Therefore, we focused on MTDDA, MTHDA, and MTODA. Selectivity. Figure 1 shows the typical selectivity coefficients (KCl,X) for chloride over interfering anions obtained with the ISEs. The chain length of the methyltrialkylammonium salt showed a characteristic effect on the selectivity, which is similar to that observed for tetraalkylammonium salts.13 Note that the selectivity pattern differs somewhat from the one reported earlier,14 because in this study the ISEs were conditioned with serum prior to use. For the lipophilic anions such as thiocyanate, perchlorate, and nitrate, the selectivity coefficient decreased (hence, the selectivity improved) as the chain length (n) was increased from 12 to 16, but then it increased slightly at n ) 18. Compared with the conventional MTDDA with n ) 12, the improvement in the selectivity over the latter two interferents even led to a notable deviation from the Hofmeister regime for the ligands with n ) 16 or 18. Taking perchlorate for example, the magnitude of the improvement was a factor of 20 and 15, respectively for n ) 16 and n ) 18. Concerning the hydrophilic anions such as acetate and sulfate, the tendency of selectivity enhancement was continuous up to n ) 18. Compared with the reported selectivity for the annealed polyion complex membrane ISE,8 the present ISEs exhibit superior selectivity over oxoacid anions such as perchlorate and nitrate.
Figure 2. Precision of the ISEs based on (a) MTDDA and (b) MTODA. Sample: control serum.
In summary, selectivity was improved by lengthening the alkyl chain of the methyltrialkylammonium ligand to 16 or 18, compared with the conventional MTDDA with a shorter chain length of 12. Such an effect may be related to the molecular size of the ligand, as has been suggested for the tetraalkylammonium-type ligands.13 Precision. The significance of the improved selectivity was confirmed by applying the ISEs to blood serum sample measurements. Figure 2 shows examples of the precision evaluations with the conventional MTDDA-based and the new MTODA-based ISEs, using a set of control serums as the sample. The self-correlation between the set of results for the first measurement run and that for the second run is quite good, the Syx values being in the range of a few mmol/L (MTDDA) or sub-mmol/L (MTODA). This indicates that the precision or reproducibility of the total experimental setup is accordingly high. It is also noteworthy that, compared with the conventional MTDDA, the MTODA-based ISEs displayed higher precision by a factor of about 2.3, using Syx as the indicative parameter. The results shown in Figure 2 were chosen to represent the typical situation; in other experiments, the factor of Syx improvement ranged between 1.2 and 4.1, depending on the kind of samples measured. Similar improvement in the precision was also observed for MTHDA-based ISEs. The difference in the precision between the MTDDA- and MTODA-based ISEs can be explained as follows. The ISEs with higher selectivity are less prone to interference from the concomitants in a serum, which leads to lower retention of the interferents in the ISE membrane. This results in smaller carryover in the EMF output and hence higher reproducibility. Accuracy. Examples of the accuracy evaluations are shown in Figures 3 and 4, obtained for 20 control serums and 30 patient Analytical Chemistry, Vol. 68, No. 23, December 1, 1996
4151
Figure 3. Accuracy of the ISEs based on (a) MTDDA and (b) MTODA. Sample: control serum, 20 samples.
serums, respectively. MTODA-based ISEs consistently showed superior correlation characteristics (regression line parameters and correlation coefficient γ) as well as accuracy (Syx and SDdif), compared with MTDDA-based counterparts. Taking the Syx value as the indicative parameter in these cases, the accuracy improved by a factor of 4.1 for the control serum and 2.1 for the patient serum measurements. The results shown in Figure 3 also represent the typical case; in other experiments, the factor of Syx improvement ranged between 3.0 and 6.3. Similar improvement in the accuracy was also observed for MTHDA-based ISEs. Such enhanced accuracy can reasonably be ascribed to the improved selectivity of the new ligands, which leads to less interference by the concomitants in a serum. The more dramatic enhancement observed for the control serum measurements compared with the patient serum measurements may be due to some additional interferents anticipated to be present in the artificially fabricated control serums. The Syx values for the accuracy measurements are within the range of sub-mmol/L, and
Figure 4. Accuracy of the ISEs based on (a) MTDDA and (b) MTODA. Sample: patient serum, 30 samples.
especially the Syx value for patient serum is not large compared with the reproducibility of the current ISE measurements; the standard deviation for the repetitive measurement of the same sample is about 0.3-0.5 mmol/L. This implies that the accuracy of the new ISEs is approaching the level where their contribution to the total error is as little as that due to the reproducibility of the entire system. The submillimolar Syx values attained with MTODA and MTHDA are less than 4% of the 95% normal concentration range of blood serum chloride (95-110 mmol/L).18 Such accuracy meets the most demanding requirements in typical clinical diagnosis. ACKNOWLEDGMENT We gratefully acknowledge the guidance and encouragement of the late Prof. W. Simon of Eidgeno¨ssische Technische Hochschule Zu¨rich. Received for review May 29, 1996. Accepted August 29, 1996.X AC960526V
(18) Oesch, U.; Ammann, D.; Pham, H. V.; Wuthier, U.; Zu ¨ nd, R.; Simon, W. J. Chem. Soc., Faraday Trans. 1 1986, 82, 1179-1186.
4152 Analytical Chemistry, Vol. 68, No. 23, December 1, 1996
X
Abstract published in Advance ACS Abstracts, October 1, 1996.
