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Anal. Chem. 1982, 5 4 , 2082-2085
of individual bile acids, such as their ability to induce intestinal fluid secretion (13) or to solubilize cholesterol (161,render the determination of “monomer concentration” of a mixture of bile acids in biological samples meaningless.
LITERATURE CITED (1) Helenius, A.; Slmons, K. Eiochim. Biophys. Acta 1975, 415, 29-79. (2) Mazer, N. A.; Benedek, G. B.; Carey, M. C. Biochemistry 1980, 19, 60 1-6 15. (3) O’Maille, E. R. L. J . fhysiol. (London) 1980, 302, 107-120. (4) Kwan, K. H.; Higuchi, W. I.; Hofmann, A. F. J . fharm. Sci. 1978, 6 7 , 1 17 1- 17 14. (5) Duane, W. C. 6ioch;m. Biophys. Acta 1975, 398, 275-286. (6) Shiau, Y.; Levine, G. M. Am. J . fhysiol. 1980, 2339,G177-Gl82. (7) Gilligan, T. J.; Cussler, E. L.; Evans, D. F. Blochim. Biophvs. Acta 1877, 497, 627-630. (8) Carey, M. C.; Small, D. M. J. Coilold Interface Sci. 1969, 31, 362-396.
(9) Singleton, W. S.; Gray, M. S.; Brown, M. L.; White, J. L. J . Am. Oil. Chem. SOC. 1965, 4 2 , 53-56. Duane, w, c, Blochem, Blophys, Res, Commun, 1977, 74 223-229. ( 1 1 ) Laurent, T. C.; Killander, J. J . Chromafogr. 1984, 1 4 , 317-330. (12) Mukerjee, P.; Cardinal, J. R. J . fharm. Sci. 1976, 6 5 , 882-886. (13) Wingate, D. L.; Phillips, S . F.; Hofmann, A. F. J . Clin. Invest. 1973, 52, 1230-1236. (14) Ammon, H. V . Gastroenterology 1979, 76, 778-783. (15) Tanford, C. “The Hydrophobic Effect: Forrnatlon of Micelles and Biological Membranes”; Wiley: New York, 1973; Chapter 10. (16) Armstrong, M. J.; Carey, M. C. J . LipidRes. 1982, 23, 70-80.
RECEIVED for review September 8, 1981. Resubmitted June 17, 1982. Accepted July 1,1982. This research was supported by the Research Service Of the Veterans Administration and NIH Grant AM 17941.
Quench Correction in Liquid Scintillation Counting by a Combined Internal Standard-Samples Channels Ratio Technique Erik Dahlberg Karolinska Instltutet, Department of Medical Nutrltion, Research Center, F 69, Huddinge University-Hospital, S- 14 1 86 Huddinge, Sweden
A well-known problem in liquid sclntlllation counting (LSC) Is that radioactlvlty cannot be measured with 100% efflclency, e.g., due to ”quenching”, which thus needs be corrected for. Three methods (vlz., those of internal standard (IS),samples channels ratio (SCR), and external standard channels ratio (ESCR)) are In common use to accomplish quench correction. None of these methods is ideal. This paper shows that a comblnatlon of the IS and SCR methods (IS-SCR) ameliorates the malor dlsadvantages of both techniques and also offers some advantages over the ESCR method. Thus, the dependence on accurate pipetting in the IS technique and the disadvantage of the SCR technlque at low count rates have been eliminated In the IS-SCR method, which also has a low volume dependence compared to the IS and ESCR methods. The IS-SCR method is not affected by tlme-dependent dlffusion of solutes and solvents Into the walls of plastic counting vials, whlch Is a major drawback of the ESCR technique. Used wlth a simple llnear regression technique, the IS-SCR quench curves may be llnearized over wide ranges of efficlencles. I n view of the wide-spread appllcation of LSC, the IS-SCR technique Is therefore llkely to be useful to many investlgat ors.
rection methods, particularly that of ESCR is affected by volume and geometry of samples (15, 17). Several authors have found the SCR method superior to the ESCR method (14-18,22,23). A major problem of the latter technique is the time-dependent changes in ESCR values due to diffusion of solvents and solutes into the walls of the high-density polyethylene vials most often used (7,16,20, 24,251. Background counting rate also increases (16). By contrast, SCR values are unaffected (16). The SCR method is not applicable for samples having low count rates (12,17,18). The fact that the ESCR technique is the least accurate method is especially true for highly quenched samples (18). The method using an IS is the most reliable quench-correction technique, but it is expensive, time-consuming, and highly dependent upon accurate pipetting (18). It would seem theoretically possible to avoid the drawback of t h e SCR method at low count rates by adding an IS to low-radioactivity samples. By then using SCR values for quench correction, instead of calculating the efficiency from the contribution of the known amount of radioactivity added (”classical”IS procedure), one could perhaps at the same time eliminate the dependence of the IS technique on accurate pipetting. The data in this paper show that both assumptions are correct. The combined qethod (IS-SCR) is a useful improvement of quench correction.
LSC is commonly applied to determine the radioactive decay from a number of isotopes often used in research, such as the p emitters 3Hand 14C (1-10). A well-known problem is that radioactivity cannot then be measured with 100% efficiency, partly due to ”quenching” (11-13), which needs to be corrected for. Three methods are in common use to accomplish this (12,13), all of which have their advantages and disadvantages (14-18). Counting efficiency is volume dependent in LSC (15, 17, 19-21), and the volume of sample and scintillator must be kept constant at an optimum. Of the three common quench-cor-
EXPERIMENTAL SECTION Apparatus. The liquid-scintillationcounter used was a WalIac 1215 RackBeta (LKB, Stockholm, Sweden),equipped with an ES using 226Raas the y source. Instrument performance was checked regularly as outlined by Patterson et al. (15). Standard 20-mL high-density polyethylene counting vials (28 X 60 mm) were purchased from Packard Instrument Co. Inc. (Downers Grove, IL). Reagents. The scintillator fluids used were Insta-Gel and Scintillator 299 (both from Packard). The former is a well-known solgel scintillator, equal or superior to many other fluors in most respects (e.g.,ref 26-30). The latter is a new scintillator containing
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“a mixture of emulsifiers dissolved in xylene” with properties as the common Triton X-lOO/toluene mixtures (31). In some experiments Insta-Gel was diluted with toluene (l:l,v/v). Since 3H, because of its low-energy p spectrum, is the most difficult isotope to detect (17,26,30),it was used as a model isotope in this study. [SH]Toluene, a suitable IS for toluene- and dioxane-based solutions (5’2),was purchased from New England Nuclear (Boston,MA). All other chemicals were from Merck A.G. (Darmstadt, West Germany). Counting Conditions and Quench Curves. After equilibration of the vials to the temperature of the counter (about 10 “C), the vials were counted until 100000 counts had been accumulated. Two adjacent channels were used for SCR (low energy/high energy) and were set according to Bush (23). For ESCR, the ES was counted for 1 min, and the ESCR values were obtained by using the factory-pireset windows. Standard curves with progressively increasing quenching were freshly prepared by adding different amounts of carbon tetrachloride, which causes chemical quenching by electron capture (6),to a series of vials that all contained 10000 dpm (1.67 kBq) of [3H]toluene and 10 mL of Insta-Gel. The I8 .determined efficiencieswere calculated by the ordinary technique and assumptions (32). The quench curves were usually computed using linear regression (least squares) and a plot of In (efficiency) vs. SCIR, because the integral count rate of a labeled quenched sample depends upon an exponential function of the quencher concentration (33, 34). For optimal performance,however, SCR values must be obtained from low-energy/high-energy counts, instead of the reverse (unpublished data). Statistical Methods. Leasbsquares linear regression, analysis of variance, and analysis of covariance were used as described by others (35, 36). Volume Dependence. Twenty microliters of [3H]toluene (37 900 dpm; 632 Bq) were added to duplicate vials containing from 1 to 20 mL (I-mL increments) of Insta-Gel. The samples were counted as above. The ESCR, SCR, and IS methods were compared with regard to volume dependence. The optimum volume for the counter was also determined in this way. Effect of the Amount of IS. To see if the amount of radioactivity added as IS affects the SCR values in the IS-SCR technique, we carried out the following experiment. Different volumes of [3H]toluene((5-50pL in 5-pL increments; 9000-90000 dpm; 0.15-1.5 kBq) were added to duplicate vials containing 10 mL of Insta-Gel. Unlabeled toluene was added to give the same volume and toluene addition (0.5%)to all vials. This toluene addition did not alter the background radioactivity. The samples were counted as described above. Effects of Efficiency, Scintillator Fluid, and Type of Quenching. Carbon tetrachloride (0-50 p L in 5-pL increments) was added to duplicate vials containing 10 mL of Insta-Gel, and 20 p L of [3H]toluene(36 2100 dpm; 603 Bq) was then added to each vial. The counting conditions were as above, except that a commercial set of quenched standards was also used (The Radiochemical Centre, Amersham, U.K.). The range of carbon tetrachloride was increased subsequently (0-200 pL in 20-pL increments). Additionally, Insta-Gel was compared to Insta-Gel/ toluene (1:l;v/v) and Scintillator 299. In these experiments, another batch of [3H]toluene was used, and 20pL (added to 10 mL fluor) of the reagent contained 53 100 dpm (885 Bq). The possible effect of type of quenching (color or chemical) on the IS-SCR method was evaluated. Potassium dichromate absorbs photons over a wide range of wavelengths (410 nm, maximum about 380 nm (37), although this varies with pH (38)) and thus causes color quenching. Yellow and red substances are most efficient color queinchers (17, 22, 23). Half milliliter of potassium dichromate in water solution at different concentrations was added to duplicate viids containing 10 mL of Scintillator 299, and 20 pL of [3H]toluene 153 100 dpm; 885 Bq) was then added. The potassium dichromate solutions were imade up from a saturated solution by diluting 5-, lo-, 20-, 40-, 60-, 80-, and 1OO-pL samples to 2 mL.
RESULTS AND DISCUSSION Maximal counting efficiency for the Wallace 1215 RackBeta counter occurred over a relatively wide range of volumes (Figure l),as has been found for other counters (15). However,
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>
I
w
v L
w
604
10
10
20
e
VOLUME
20
10
20
l m l l
Figure 1. The effect of different volumes of scintillator fluid on the counting-efficiency determination by the methods (from left to right) of IS, SCR, and ESCR. The percentage of the counting efficiency remaining at the various volumes of fluor is plotted as a function of the volume of scintillation fluor (mL). Open and filled circles denote single estimations and duplicates within the diameter of a circle, respectively.
the estimated counting efficiency and its volume dependence were dependent on the quench-correction method. The peak of efficiencies close to 100% of the maximum counting efficiency was broadest for the SCR method and less wide for the IS technique and the ESCR technique did not give a plateau at all. Below the plateau, the counting efficiency fell rapidly, which confirms the data of others (17). As seen in Figure 1, the SCR method is less volume dependent than the IS and ESCR methods. The combined IS-SCR procedure has an equal volume dependence as the SCR method. Obviously, there is no advantage of operating at efficiencies lower than necessary. In many scientific reports, however, nonoptimal volumes have been used. The disadvantages are particularly evident when low-radioactivity samples are analyzed. The IS-SCR method then offers an advantage over the more volume-dependent techniques. Also, if less scintillator fluid has been added than was intended, and this has escaped attention, the IS-SCR method is “safe” in view of its comparatively low volume dependence. The broad plateau also aids in precision related to, e.g., slight differences in sample volume. Since the same samples were used in Figure 1,it is of interest that the duplicates varied least for the SCR-determined efficiencies. This shows the drawback of the “classical” IS method is being sensitive to even minor pipetting errors. To determine whether the amount of radioactivity added as IS is important in the IS-SCR technique, we carried out the following experiment. Samples that contained different volumes (5-50 p L ) of [3H]toluenewere counted to the same error (0.3%). Within this range of radioactivity (9000-90000 dpm), no differences in efficiency were detected (one-way analysis of variance). A plot of efficiency against volume of PHItoluene did not have a slope significantly different from zero (efficiency = 0.490 0.000213 X volume (pL);r = 0.1806). Thus, 97% of the variation in efficiency values could be ascribed to “randomess”, Le., not to a relationship between the parameters. Hence, using an SCR value for quench correction after adding the IS circumvents the dependence on accurate pipetting, which is the major obstacle of the “classical” IS procedure. To evaluate the IS-SCR method for quench correction, the following experiment was performed. Different volumes of the quenching agent carbon tetrachloride (0-50 pL in 5-pL increments) were added to duplicate vials containing 10 mL of Insta-Gel and 20 p L of [3H]toluene (36200 dpm). The
+
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ANALYTICAL CHEMISTRY, VOL. 54, NO. 12, OCTOBER 1982
Table I. Regression Lines for the Quench Curves in Figure 2 after Linearization a quench curve 2 3
4 5
, 5 0
, 1 0 0
C O N C E N T R A T I O N
1 5 0 ( U N I T S 1
Figure 2. Quench curves obtained by different scintillator fluids and
different quenching agents. Details are given in the text. Units are microliters of quencher In all cases except for curve 4 (microliters of saturated potassium dichromate used for dllution). All curves were constructed in dupllcate, and all dupllcates were within the diameter of a circle. Regression lines and correlation coefficients after linearization are given in Table I.
counting conditions were as above, using a commercial set of quenched standards. The percentage efficiency was determined using both the IS and the SCR techniques, and these values were plotted against the volume of quencher (pL). From below 35% to above 50% efficiency, the SCR method gave efficiency estimates that were close to the true values (estimated by IS). The product-moment correlation coefficients for the IS and SCR linear regression lines (least squares) were -0.993 and -0.996, and the corresponding slopes were -0.362 (s 0.009 68) and -0.359 (s 0.007 48), respectively. Thus, the assumption of linearity is justified over this range of quenching. However, a slightly higher (l.6-1.8%) apparent efficiency was obtained by SCR than by IS. Since the re= -0.16; p > 0.80), this gression lines had the same slope (tdO factor can easily be corrected for. The results of the two methods were closely correlated (r = 0.985), and the slope was close to 1 (0.996). Thus, only less than 3% of the variation in SCR values over the range of quenching used could be attributed to "randomness". T o see if these data hold also at lower efficiencies, we increased the range of quenching. Additionally, different scintillator fluids were compared, and color-quenched samples were compared to chemically quenched samples. As seen in Figure 2, the long quench curves were nonlinear. Furthermore, the different scintillators gave quench curves following different functions, and color quenching differed from chemical quenching, Undiluted Insta-Gel (curve 1) gave slightly higher efficiencies than Insta-Gel/toluene (1:l; v/v) (curve 2) or Scintillator 299 (curve 3), all being chemically quenched with carbon tetrachloride. It is noteworthy that color quenching by a water solution of potassium dichromate (curve 4)leads to a much more rapid decline of counting efficiency than chemical quenching. Curve 4 is not directly comparable with curve 3, since the former samples contained 0.5 mL of water. However, this water addition only reduced efficiency by a constant 5.3%. Color quenching probably accounts for most of the quenching in biological samples, and it is therefore a common problem to find suitable sets of quenched standards for use with such samples. In view of these data, a major problem is to linearize the quench curves, particularly when the samples differ greatly in efficiency. Preferably, one should also have the possibility to use a standard curve with one type of quenching for samples with a different quench type or to use one scintillator fluor for the samples and another for the quenched standards. A minimum demand, however, is that a calculated regression line should adequately estimate the true efficiencies of the
function eff. = exp(-0.441 eff. = exp(-0.452 eff. = exp(-0.505 eff. = exp(-0.321 eff. = exp(-0.469
1
- 0.641*SCR) - 0.619*SCR) - 0.605*SCR) - 0.826*SCR) -
0.521"SCR)
r value
-0.99'75 -0.9978 -0.9975 -0.9994 -0.9959
a The numbers of the quench curves correspond to those in Figure 2, with the addition of a commercial quench curve (no. 5). All quench curves were calculated (linearized) using In (efficiency) vs. SCR values. Eff. denotes efficiency, and r values are product-moment correlation coefficients.
Table 11. Regression Parameters for the Comparison between the IS-SCR and IS Methods quench curve
regression line
2
Y = 0.0125 + 0.943*X Y = 0.0137 t 0.919*X
3 4 5
Y = 0.000182 t l.OOO*X Y = 0.0232 t 0.909"X
1
Y = 0.0119 t 0.934*X
r value
0.9968 0.9948 0.9959 0.9994 0.9918
a The numbers of the quench curves correspond to those used in Table I. For the calculation of quench curves, linear regression was used, taking In (efficiency) as a function of SCR values for computation of IS-SCR. Y is the true efficiency (determined by IS) and X is the efficiency determined by the IS-SCR method, and r values are product-moment correlation coefficients,
unknown samples. Table I shows that the first problem may be circumvented by simply using In (efficiency) vs. SCR to obtain efficiency from the calibration curve. This works well provided the channels are properly set, and the SCR values are obtained from low-energy/high-energy counts. Others suggested using polynomials to calculate from quench curves (e.g., ref 14 and 22). Another method is to divide the quench curve into several linear parts and to calculate a regression line from each part (e.g., ref 39). It is simpler and statistically sounder to use a single linear regression line, which gives the same response throughout the entire range of quenching, as is the case with the calculation technique used here. Despite the differences between the curves in Figure 2, they were all reasonably well linearized by this simple calculation (Table I), although they cover efficiencies between 5 and 50%. As seen in Table 11, the IS-SCR-determined efficiencies over this range of quenching were correlated to the true efficiencies (determined by IS). All regression lines seem to follow a simple relationship and may be fitted to a common line (analysis of covariance). The intercepts are close to 0 and the slopes and correlation coefficients are close to 1. This indicates that irrespective of the type of quenching or the scintillator solution used, the IS-SCR method in conjunction with the suggested calculation technique may be used to determine the true efficiency accurately also over a very wide range of quenching. In high-radioactivity samples, the SCR method can be used without adding an IS. Often one deals with sets of samples, which have a homogeneous quenching. The advantages of the IS-SCR technique are then evident, particularly if the sets contain low-radioactivity samples. The IS-SCR technique is then simplified if just a few samples are used to determine the efficiency of the entire set. When the degree of quenching varies within the sets, the efficiency often varies only in certain parts of the series. Also then is quench correction by IS-SCR simplified
Anal. Chem. 1982, 5 4 , 2085-2089
by using an efficiency estimated by just a few samples to correct the radioactivity values in the homogeneously quenched parts of t.he set.
LITERATURE CITED (1) Bell, C. G., Jr., Hayes, F. N., Eds. "Liquld Scintillation Countlng"; Pergamon Press: New York, 1958. (2) Birks, J. B. "The Theory and Practice of Eicintillation Counting"; Pergamon Press: Oxford, 1964. (3) Horrocks, D. L., Ed. "Organic Sclntillators"; Gordon and Breach: New York. - . 1988. (4) Bransome, E. D., Jlr., Ed. "The Current Status of Liquid Scintillation Countlng"; Grune and Stratton: New,York, 1970. (5) Horrocks, D. L., Pw~g,C.-T., Eds. Organic Scintillators and Liquid Scintillation Counting": Academic Press: New York, 1971. (6) . . Hendee, W. R. "Radloactive Isotopes in Biological Research"; Wlley: New York, 1973. (7) Horrocks, D. L. "Applications of Liquid Scintillation Counting"; Academic Press: New York, 1974. (8) Fox, B. W. I n "Laboratory Techniques in Biochemistry and Molecular Biology"; Work, T. S . , Work, E., Eds.; North-Holland: Amsterdam, 1976; Vol. 5, part 1, pp 1-333. (9) Noujaim, A. A., Ediss, C., Wiebe, L. I., Eds. "Llquid ScintlllationScience and Technology"; Academlc Press: New York, 1976. (10) Soini, E. Sci. Tools 11976, 25(3), 1-11. (1 1) Neary, M. P.; Budd, A. L. I n ref 4; pp 27:3-282. (12) Peng, C. T. I n "Advances in Tracer Methodology"; Rothchild, S.,Ed.; Plenum Press: New York, 1966; Vol. 3, pp 81-94. (13) Peng, C. T. I n ref 4; pp 283-292. (14) Barrows, G. H.; Samols, E.; Becker, B. J. Nucl. Med. 1976, 17, 1017-1 018. (15) Patterson, J. F.; Sauerbrunn, B. J. L.; Battist, L. Anal. Blochem. 1978, 86,707-715. (16) Horrocks, D. L. I n t . J . Appl. Radiat. Isol. 1975, 2 6 , 243-256. (17) Neame, K. D. Anal. Blochem. 1976, 9 1 , 323-339.
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(16) Rogers, A. W.; Moran, J. F. Anal. Biochem. 1966, 16, 206-219. (19) Gogan, F.; Gogan, P. Anal. Blochem. 1974, 60, 363-371. (20) Rummerfield, P. S.;Goldman, I. H. I n t . J. Appl. Radiat. Isot. 1972, 23,353-360. (21) Malcolm, P. J.; Stanley, P. E. Int. J. Appi. Radiat. Isot. 1976, 27, 415-430. (22) Noujalm, A.; Ediss, C.; Wlebe, L. In ref 5; pp 705-717. (23) Bush, E. T. Anal. Chem. 1963, 35, 1024-1029. (24) Sakashlta, M. Radioisotopes 1979, 26, 760-762. (25) Allen, H. J. I n t . J. Appl. Radiat. Isot. 1976, 27, 662-663. (26) Carter, G. W.; Van Dyke, K. Anal. Biochem. 1973, 54, 624-627. (27) Simpson, E.; Browning, M. C. K. Clin. Chlm. Acta 1973, 45, 135-143. (28) Moore, P. A. Clln. Chem. (Winston-Salem, N.C.) 1961, 27, 609-611. (29) Gray, P.; Van Reenen, 0.: Potgieter, G. M. Clln. Chem. (Winston-Sa/em, N.C.) 1960, 26, 1886-1887. (30) Benson, R. H. Int. J. Appl. Radiat. Isot. 1976, 2 7 , 867-674. (31) Leaflet from Packard Instrument Co., Downers Grove, IL. (32) Hendee, W. R.; Ibbott, G. S.;Crusha, K. L. Int. J. Appl. Radiat. Isot. 1972, 2 3 , 90-95. (33) Peng, C. T. Anal. Chem. 1960, 32, 1292-1296. (34) Peng, C. T. Anal. Chem. 1969, 41, 16-21. (35) Bennett, C. A.; Franklin, N. L. "Statistical Analysis in Chemistry and the Chemical Industry"; Why: New York, 1954. (36) Brownlee, K. A. "Statistical Theory and Methodology In Science and Engineering", 2nd ed.; Wiley: New York, 1965. (37) Heitzmann, M. W.; Ford, L. A. Anal. Chem. 1961, 5 3 , 1721-1723. (38) Painton, C. C.; Mottola, H. A. Anal. Chem. 1981, 53, 1713-1715. (39) Hope, H. J. Anal. Biochem. 1973, 5 3 , 295-298.
RECEIVED for review April 7 , 1982. Accepted June 14, 1982. This work was carried out as a part of studies supported by Karolinska Institutet, LEO Research Foundation, and the Swedish Medical Research Council (19P-6483 and 13X-2819).
Selectivity of the Potentiometric Ammonia Gas Sensing Electrode M. E. Lopez and G. A. Rechnltz" Department of Chemistry, University of Debsware, Newark, Delaware 197 1 1
The basicity of amine! lnterferents wasi found to be a more important factor than volatllity in determlnlng the selectlvlty of the Orlon ammonlia gas senslng electrode. Theoretical selectlvlty coefficients of seven volatile amine interferents were successfully calculated from fundamental conslderations and were found to bo In excellent agreement with experimental values provided corrections are made for the nonideal response of the Orlon Inner pH-sensing element. A dependence of the pH of the thin fllm of internal electrolyte on the osmolarity of the sample solution and InV,ernalelectrolyte was observed. The generality of the theoretlcal approach was demonstrated with a imethylamlne-based sensor where experimental selectivity coefflcients of arnmonla and dimethylamine agreed well with predlcted values.
Although the interference of volatile amines on the response of potentiometric membrane electrodes for ammonia has been documented in the literature (1-5) and recognized by the manufacturer of commercial electrodes (6),little effort has yet been made to provide a quantitative explanation of observed interference patterns in terms of fundamental parameters. A very recent paper (7) considered the response of the ammonia electrode tcr several amines from an analytical perspective.
The present study was undertaken to provide a systematic evaluation of the potentiometric response to a series of amines and other nitrogen-containing compounds having a range of volatilities and basicities. A model is proposed for the steady-state response of the electrode which takes into account the effect of amine dissociation on the mass action equilibria responsible for the electrode potential in the presence of ammonia or the volatile amines. It will be seen that quantitative agreement between experimentally measured and theoretically calculated selectivity coefficients can be obtained provided correction is made for the nonideal response of the internal sensing electrode and provided the properties of the electrolyte employed as the filling solution are known. The procedures employed and the model proposed here point the way to a quantitative explanation of the interference characteristics of potentiometric gas-sensing membrane electrodes. The model is further tested and confirmed through the construction of a gas-sensing electrode which utilizes a methylamine-based filling solution; such a sensor is seen to have the predicted response to ammonia as an interferent.
EXPERIMENTAL SECTION Apparatus and Materials. Potentiometric measurements were made with a Corning Model 12 Research pH meter and a Health/Zenith Model SR-204 strip-chart recorder. The cell was maintained at 25 "C with a Haake constant temperature circulator, Model FS. The Orion ammonia electrode, Model 95-10, using
0003-2700/82/0354-2085$01.25/00 1982 American Chemical Society
