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Amperometric detection of amines and amino acids in flow injection systems with a nickel oxide electrode
Analytica
Ciiimica
Elsevier Scientific
Acta,
134
Publishing
(1982)
AMPEROMETRIC FLOW INJECTION
DETECTION SYSTEMS
BEN
HUBER*
S. HUI and C. 0.
Department of Chemistry, 19-I53201 (U.S.A.) (Received
16th
March
211-218
Company,
Unioersity
Amsterdam
-
Printed
in The Netherlands
OF AMINES AND AMINO ACIDS IN W’ITH A NICKEL OXIDE ELECTRODE
of I~‘isconsi,z-~llilu~au~ee,
Nilwaukee,
1981)
SUMMARY The use and characteristics of a nickel oxide electrode as a detector for amines in a flow injection system are described. The anodic electrode reaction mechanism involves a higher oxidation state of nickel maintained by the applied potential (+0.49 V vs. SCE). The electroanalytical parameters are investigated and the currents for a series of amines and amino acids are compared. Two electrode configurations are compared. The flow injection technique is shown to be suitable for buffered 25~1 samples of pH as low as 3. The linear range for glycine is lo-“-lo-’ M with detection limits of a few nanograms.
Fleischmann et. al. [l] reported that alcohols and amines are oxidised at a nickel anode in aqueous alkaline solution_ They showed that the rate-determining step (r-d-s.) is oxidation of the substrate by an oxide species formed anodically on the surface of the nickel Ni
20H-
Ni (OH),
-2eNiOOH radical
+ analyte M
-e; 4
(adsorbed)
OHi
NiOOH
r.d.s:
+ Hz0
Ni (OH)?
(1)
+ radical
(2)
products.
(3)
The electrode reaction has been applied to ethanol determinations with the flow injection technique [2] . The present report describes application of the nickel oside electrode with the flow injection technique to the determination of amines. In addition, the reactions of amino acids at the nickel oxide electrode are characterized and parameters that influence electrode response are examined. Previous applications of the nickel oxide electrode have used samples adjusted to the high pH at which the electrode is most active, whereas this report describes the application to samples buffered at lower pH. Advantages of the flow injection technique have been reviewed [ 31. In the application reported here, the additional advantages of maintenance and reproducible restoration of an active electrode surface are shown. 0003-2670/82/0000-OOOO/soa.75
0 1982 Elsevier
Scientific
Pubiishing
Company
313 EXPERIMENTAL
Apparatus
The flow injection apparatus was constructed by using 0.5 mm i-d. PTFE tubing and standard high-performance liquid chromatographic type fittings_ A 4-1 plastic reservoir of background electrolyte positioned 1 m above the injection valve was used to maintain the desired constant flow of background electrolyte through the cell. Flow rate was regulated from 0.5 to 3.0 ml per minute by a stopcock located 30 cm below the reservoir. A Rheodyne Model 50 injection valve with a 25+1 sample loop was used. The detector cell was a three-electrode system consisting of a nickel oxide workin$ electrode, platinum counter electrode, and a mercury/mercury(II) oxide reference electrode_ Two types of working electrode were examined. The first type, a tubular electrode, was prepared by drilling a 0.8 mm hole through 1.3 mm thick nickel sheet. The nominal surface area was 0.03 cm’. The nickel sheet was embedded in a cast epoxy block (2 X 3 X 2 cm) which was machined to accept the necessary fittings_ The other type of working electrode consisted of a randomly coiled nickel wire (12 cm long, 0.1 mm diameter)_ The coiled wire was positioned in a 2 mm i-d. tube. The nominal working electrode area was O-5 cm’_ The counter electrode was a 1 X 10 mm platinum wire positioned in the exit tube of the detector. The reference electrode consisted of a pool of mercury covered with a paste made by grinding mercury and red mercury(I1) oxide moistened with 0.1 M NaOH together in a mortar. The filling solution was 0.1 M NaOH. Unless otherwise stated, all applied voltages are referred to the mercury/mercury(II) oxide electrode with an observed potential of -0.06 V vs. the saturated calomel electrode. A potentiostat capable of measuring nanoampere currents was constructedThe potentiostat consisted of voltage controller, currentto-voltage converter, and inverter amplifier for off-set adjustment_ The three operational amplifiers were an RCA 3140, and an RC..4 3240 dual unit, respectively. The potentiostat was housed in a small aluminum box, and was powered by a 15 V d-c. power supply. A potentiometric recorder was used for read-out. Reagents
and samples
All reagent solutions were prepared with distilled water. Unless otherwise specified, all materials were reagent grade. Background electrolyte was 0.1 M NaOH to which nickel sulfate (1 X lO+ M) had been added. In this solution, the nickel is virtually all insoluble nickel hydroxide. Procedure
After the flow rate has been set, the baseline level is adjusted via the circuit offset control. The 25-~1 sample loop is filled, the sample is introduced into the flow stream, and the resulting signal is recorded_ Peak height is used as the analytical signal_
213
RESULTS
AND DISCUSSION
To investigate the nature of the electrode reaction for amino acids, chronoamperometric data were obtained_ The current after stepping the potential from 0.26 to 0.46 V was observed. The results are shown in Fig. 1. These results indicate a steady-state current when oxidation of amino acid is occurring. This suggests that the mechanism proposed by Fieischmann [I] for alcohols and amines is also applicable for amino acids. To examine the effects of the applied potential at the working electrode, the potential was changed in 20 mV increments between 0.44 V and 0.64 V. Both background and analyte currents were determined_ The background current increased with increasing applied potential (Fig. 2). The increases in background current with applied potential may be due to the increased rate of oxidation of water. The optimum potential, defined as that which produces the largest analyte signal, is about 0.55 V for both amines and amino acids. Apparently, at potentials more anodic than 0.55 V, solvent oxidation increasingly competes with analyte osidation. The effects of hydroxide concentration at an applied potential of 0.55 V are shown in Fig. 3. At the lowest concentration, the formation of the higher oxide on the surface is limited whereas at concentrations greater than 0.1 M, solvent oxidation probably becomes competitive with that of the analyte. Analyte signals were observed to increase with ionic strength (Fig. 21). The increase in signal shows a consistent trend, but is not large.
I2
!3-
‘;‘x, f
x -,--I--x--x-* I
(A;
E i i 6-
1.6 f
‘\.,
I 1.21 !,UA)
-1
-1._
/
I
-1
G:T
-E
03;
--.-.
I
./
2- I
do
0.2
.i/"\oA O
‘0,
/
j
aL.--_____, ._._ .__-__ 90 30 60 I20
Tame
I50
o lE!O
____(
400
520
560
E opp
(s)
Fig. 1. Chronovnperometry after stepping glycine; (0) background electrolyte only. Fig. 2. Signal dependence ground baseline current.
L__,_._-__,-___
the potential
upon applied potential
(VS.
from 0.20
Hg/HgO):
--
600
Cm”) to 0.40
V. (X ) 1 m&I
(A) peak height;(B)
back-
0.1 Hydroxide
Cont. (MI
Ionic
0.2
I
0.3
Strength
Fig. 3. Current dependence on hydroxide concentration at an applied potential of 0.55 vss.Hg/HgO: (A) peak current; (B) background current. Fig. 4. Effect of ionic strength on swine (1
Effects
of
crease with rates up to rate can be Limitation
x
lo4
V
M) using sodium nitrate.
rate were examined. Peak currents for 0.1 mM serine inflow rate up to 1.5 ml min-’ with no further increases for flow 3 ml min-‘_ At flow rates up to 1.5 ml min-‘, dependence on flow attributed to increasing convective mass transport at the electrode_ of the analyte signal with increasing flow rates can be attributed flow
to the rate-limiting step for electron transfer at the electrode, which is independent of mass transport (see reaction 2). Analyte currents for a series of simple amines and common amino acids were determined. All were prepared in background electrolyte and determined at the conditions indicated above. Typical signals for 25 ~1 of 1.0 X 10e3 M samples are summarized in Table 1. The factors affecting oxidation rates are steric effects on adsorption and stoichiometry, i.e., number of oxidizable sites such as amino and hydroxyl groups. The stereochemical control of the oxidation rates can be observed. Propylamine yields a larger signal than butylamine and primary amines yield much larger currents than secondary and tertiary amines. For the amino acids, increases in molecular weight correspond to decreases in peak current. As examples, serine vs. tyrosine, serine vs. lysine, alanine vs. phenylalanine, and alanine vs. valine can be noted. Stoichiometry effects are shown by comparing serine with alanine and tyrosine with phenylalanine. Both serine and lysine contain 3 hydrogens with 2 active groups, yet the more bulky lysine yields a current only 6% of that for serine. This indicates that the effect of steric hinderance exceeds that of stoichiometry. Relating the amine and amino acid structures and oxidation currents, it may be suggested that the rate of oxidation increases with the number of cr-hydrogens and the number of active sites, and with decreasing size of
215
sd
216
the substrate molecule. These observations are consistent with the reaction mechanism described_ In chromatography, as well as in other applications, samples are often in a buffered medium at pH values considerably lower than that of the carrier electrolyte used here. Thus, it was necessary to study pH effects on the detection signal from the nickel oxide electrode. Glycine at pH 5.1 in 0.1 M acetate buffer and at pH 3.5 in 0.1 M formate buffer was used with the nickel tubular electrode. A negative peak precedes the positive peak used to quantify each component (Figs. 5 and 6). The negative peak height depends on pH and on buffer concentration. Simply injecting buffered blank samples also produces negative and positive peaks and the peak heights increase with buffer concentration as shown in Table 2. The time profiles at the working electrode shown in Fig. 5 can be used to propose a mechanism for the signals obtained_ When the sample reaches the detector, a maximum in sample concentration and a minimum in hydroxide concentration occur simultaneously_ The decrease in pH and buffering capacity result in reduction of the higher, active nickel oxide, i.e., decrease in the surface coverage, 0, of the Ni(II1) oxide. The reduction of NiOOH is experimentally evidenced by the sharp negative peak obtained when alow pH buffer solution segment is injected (see Fig. 6). Pourbaix [ 43 shows that the oxidizing nickel(II1) species when held at +0.55 V vs. Hg/HgO will be reduced to a nonoxidizing form at pH values lower than 9. As the hydroxide concentration begins to increase during the tail of the passing sample zone, the active higher
1
i
(P-A) 0
TIME Fig_ 5. Profiles passing electrode: (i) analyte current; (S) sample face coverage by higher oxide; (OH-) hydroxide concentration_ Fig. 6. Analytical
signal for low pH sample.
(s)
concentration;
(0)
sur-
217 TABLE
2
Peak height
dependence
on solution
composition peak
Positive
Glycine
Buffer
Negative
WI
01)
bA)
(PAI
1 x 1o-J 0.0 0.0
0.1 0.01 0.05
0.21 0.14 0.18
1.45 0.57 0.70
0.0
0.10
0.24
0.90
peak
oxide surface is renewed. The amino acid sample is oxidized by the newly formed disordered nickel oxide surface containing relatively many active sites. In support of this mechanism, current-voltage curves for Ni(OH)? surface electrodes were experimentally observed here to show anodic NiOOH formation beginning at +0.4 V, in agreement with earlier reports [ 1, 51. The high rate of this process conforms to Fleischmann’s report [l] showing that the layer of NiOOH initially formed is only a few monolayers in thickness and follows Langmuir (i.e., equilibrium) adsorption behavior [I] . Analyte signals were observed to be enhanced as much as two-fold over those obtained for pH-matched samples. This apparent greater activity of the freshly deposited NiOOH corresponds to reports of anodization of Ni(OH), electrodes in aqueous hydroxide solutions to yield formation of active forms of nickel(II1) with subsequent “aging” properties [6]. Precision was examined for five replicate determinations at each sample pH value for pH 11, 7.5, 5.1 and 3.5. The estimated relative standard deviations were between 1 and 2% for all of the sample pH values. Such consistent precision suggests a reproducible higher oxide layer formed after its temporary removal by the lower pH of the mid-portion of the sample zone. It should be noted that these results point up the special advantage of the flow injection technique in reproducing surface exposure conditions when active electrode materials are used in voltammetry. Analyte signal characteristics for pH-matched samples are summarized in Table 3. Plots of the means for triplicate determinations of currents were used. Reduced sensitivity at higher concentrations is presumably due to saturation of available sites on the electrode surface. For glycine samples in pH 7.4 and pH 3.5 buffers, the positive signals were generally slightly greater than for pH-matched samples while the linearity was similar over the concentration range lo-‘---10” M. The effects on the signal of the coiled wire electrode can be attributed to (a) increased turbulance of the sample solution at the electrode surface, (b) increased area of the electrode, and (c) increased dispersion of the sample plug. The third effect decreases the signal whereas the first two enhance it. Thus, although the nominal area of the coiled wire electrode is fifteen times that of the tubular electrode, the net enhancement of sensitivity observed is
218
TABLE
3
Linearity data for selected compounds Compound
Electrode
Linear range (RI)
n
Propglamine Butylamine Diisopropylamine Triethylamine
Tubular Tubular Tubular
1 o-‘-l O? 1 o-‘-l O? lo~-lo-*
10 8 9
Tubular
10%-l
Glycine
Tubular
5 x lO”-lo-’
Coiled
5 fi 107-10~
Glycine
0-x
Slope (PA mM-I)
2.2 22 2.2
Intercept (PA)
Standard error of the estimate (PA)
Detecti’ limit (fig)
0.05 0.05 0.03
0.01 0.09 0.09
0.02 0.2 0.2
0.04 0.0
0.09 0.6
0.2 0.3
0.3
02
0.16
, 7 9
10
3’) --12.0
72
typically only six-fold, Limits of detection are given as absolute masses by using the sample volume injected, 25 r_tl.The limit of detection (in pg) is computed as (0.025) (3) s’ (m.w.)/nz, where s’ is the standard error of the estimate, and nz is t.he slope for the regression line. While the coiled wire electrode had a lower limit of detection, its effective volume was about 25~1, whereas the effective volume of the tubular electrode was less than 1 ,ui_ Other amines or amino acids would yield somewhat higher limits of detection
and lower sensitivity
as indicated
in Table 1.
REFERENCES 1 M_ Fleischmann, ‘K_Korinek and D. Pletcher, J_ Chem. Sot. Perkin Trans. 2, (1972) 1396. 2T. N. Morrison, K. G. Schick and C. 0. Huber, Anal. Chim. Acta, 120 (1980) 75. 3 J. RG.tiEka and E. H. Hansen, Flow Injection Analysis, Wiley, New York, 1981. 4 M. Pourbaix, Atlas of Electrochemical Equilibria, Pergamon, Brussels, 1966, pp_ 330-341. 5 G. W_ D. Briggs, E. Jones, and W_F. K_ Wynne-Jones, Trans. Faraday Sot, 51(1966) 1433. 6 R. S. Schrebler Guzman, J. R. Vilche and A. J. Arvia, J. Electrochem. SOC., 125 (1978) 1578.
Ciiimica
Elsevier Scientific
Acta,
134
Publishing
(1982)
AMPEROMETRIC FLOW INJECTION
DETECTION SYSTEMS
BEN
HUBER*
S. HUI and C. 0.
Department of Chemistry, 19-I53201 (U.S.A.) (Received
16th
March
211-218
Company,
Unioersity
Amsterdam
-
Printed
in The Netherlands
OF AMINES AND AMINO ACIDS IN W’ITH A NICKEL OXIDE ELECTRODE
of I~‘isconsi,z-~llilu~au~ee,
Nilwaukee,
1981)
SUMMARY The use and characteristics of a nickel oxide electrode as a detector for amines in a flow injection system are described. The anodic electrode reaction mechanism involves a higher oxidation state of nickel maintained by the applied potential (+0.49 V vs. SCE). The electroanalytical parameters are investigated and the currents for a series of amines and amino acids are compared. Two electrode configurations are compared. The flow injection technique is shown to be suitable for buffered 25~1 samples of pH as low as 3. The linear range for glycine is lo-“-lo-’ M with detection limits of a few nanograms.
Fleischmann et. al. [l] reported that alcohols and amines are oxidised at a nickel anode in aqueous alkaline solution_ They showed that the rate-determining step (r-d-s.) is oxidation of the substrate by an oxide species formed anodically on the surface of the nickel Ni
20H-
Ni (OH),
-2eNiOOH radical
+ analyte M
-e; 4
(adsorbed)
OHi
NiOOH
r.d.s:
+ Hz0
Ni (OH)?
(1)
+ radical
(2)
products.
(3)
The electrode reaction has been applied to ethanol determinations with the flow injection technique [2] . The present report describes application of the nickel oside electrode with the flow injection technique to the determination of amines. In addition, the reactions of amino acids at the nickel oxide electrode are characterized and parameters that influence electrode response are examined. Previous applications of the nickel oxide electrode have used samples adjusted to the high pH at which the electrode is most active, whereas this report describes the application to samples buffered at lower pH. Advantages of the flow injection technique have been reviewed [ 31. In the application reported here, the additional advantages of maintenance and reproducible restoration of an active electrode surface are shown. 0003-2670/82/0000-OOOO/soa.75
0 1982 Elsevier
Scientific
Pubiishing
Company
313 EXPERIMENTAL
Apparatus
The flow injection apparatus was constructed by using 0.5 mm i-d. PTFE tubing and standard high-performance liquid chromatographic type fittings_ A 4-1 plastic reservoir of background electrolyte positioned 1 m above the injection valve was used to maintain the desired constant flow of background electrolyte through the cell. Flow rate was regulated from 0.5 to 3.0 ml per minute by a stopcock located 30 cm below the reservoir. A Rheodyne Model 50 injection valve with a 25+1 sample loop was used. The detector cell was a three-electrode system consisting of a nickel oxide workin$ electrode, platinum counter electrode, and a mercury/mercury(II) oxide reference electrode_ Two types of working electrode were examined. The first type, a tubular electrode, was prepared by drilling a 0.8 mm hole through 1.3 mm thick nickel sheet. The nominal surface area was 0.03 cm’. The nickel sheet was embedded in a cast epoxy block (2 X 3 X 2 cm) which was machined to accept the necessary fittings_ The other type of working electrode consisted of a randomly coiled nickel wire (12 cm long, 0.1 mm diameter)_ The coiled wire was positioned in a 2 mm i-d. tube. The nominal working electrode area was O-5 cm’_ The counter electrode was a 1 X 10 mm platinum wire positioned in the exit tube of the detector. The reference electrode consisted of a pool of mercury covered with a paste made by grinding mercury and red mercury(I1) oxide moistened with 0.1 M NaOH together in a mortar. The filling solution was 0.1 M NaOH. Unless otherwise stated, all applied voltages are referred to the mercury/mercury(II) oxide electrode with an observed potential of -0.06 V vs. the saturated calomel electrode. A potentiostat capable of measuring nanoampere currents was constructedThe potentiostat consisted of voltage controller, currentto-voltage converter, and inverter amplifier for off-set adjustment_ The three operational amplifiers were an RCA 3140, and an RC..4 3240 dual unit, respectively. The potentiostat was housed in a small aluminum box, and was powered by a 15 V d-c. power supply. A potentiometric recorder was used for read-out. Reagents
and samples
All reagent solutions were prepared with distilled water. Unless otherwise specified, all materials were reagent grade. Background electrolyte was 0.1 M NaOH to which nickel sulfate (1 X lO+ M) had been added. In this solution, the nickel is virtually all insoluble nickel hydroxide. Procedure
After the flow rate has been set, the baseline level is adjusted via the circuit offset control. The 25-~1 sample loop is filled, the sample is introduced into the flow stream, and the resulting signal is recorded_ Peak height is used as the analytical signal_
213
RESULTS
AND DISCUSSION
To investigate the nature of the electrode reaction for amino acids, chronoamperometric data were obtained_ The current after stepping the potential from 0.26 to 0.46 V was observed. The results are shown in Fig. 1. These results indicate a steady-state current when oxidation of amino acid is occurring. This suggests that the mechanism proposed by Fieischmann [I] for alcohols and amines is also applicable for amino acids. To examine the effects of the applied potential at the working electrode, the potential was changed in 20 mV increments between 0.44 V and 0.64 V. Both background and analyte currents were determined_ The background current increased with increasing applied potential (Fig. 2). The increases in background current with applied potential may be due to the increased rate of oxidation of water. The optimum potential, defined as that which produces the largest analyte signal, is about 0.55 V for both amines and amino acids. Apparently, at potentials more anodic than 0.55 V, solvent oxidation increasingly competes with analyte osidation. The effects of hydroxide concentration at an applied potential of 0.55 V are shown in Fig. 3. At the lowest concentration, the formation of the higher oxide on the surface is limited whereas at concentrations greater than 0.1 M, solvent oxidation probably becomes competitive with that of the analyte. Analyte signals were observed to increase with ionic strength (Fig. 21). The increase in signal shows a consistent trend, but is not large.
I2
!3-
‘;‘x, f
x -,--I--x--x-* I
(A;
E i i 6-
1.6 f
‘\.,
I 1.21 !,UA)
-1
-1._
/
I
-1
G:T
-E
03;
--.-.
I
./
2- I
do
0.2
.i/"\oA O
‘0,
/
j
aL.--_____, ._._ .__-__ 90 30 60 I20
Tame
I50
o lE!O
____(
400
520
560
E opp
(s)
Fig. 1. Chronovnperometry after stepping glycine; (0) background electrolyte only. Fig. 2. Signal dependence ground baseline current.
L__,_._-__,-___
the potential
upon applied potential
(VS.
from 0.20
Hg/HgO):
--
600
Cm”) to 0.40
V. (X ) 1 m&I
(A) peak height;(B)
back-
0.1 Hydroxide
Cont. (MI
Ionic
0.2
I
0.3
Strength
Fig. 3. Current dependence on hydroxide concentration at an applied potential of 0.55 vss.Hg/HgO: (A) peak current; (B) background current. Fig. 4. Effect of ionic strength on swine (1
Effects
of
crease with rates up to rate can be Limitation
x
lo4
V
M) using sodium nitrate.
rate were examined. Peak currents for 0.1 mM serine inflow rate up to 1.5 ml min-’ with no further increases for flow 3 ml min-‘_ At flow rates up to 1.5 ml min-‘, dependence on flow attributed to increasing convective mass transport at the electrode_ of the analyte signal with increasing flow rates can be attributed flow
to the rate-limiting step for electron transfer at the electrode, which is independent of mass transport (see reaction 2). Analyte currents for a series of simple amines and common amino acids were determined. All were prepared in background electrolyte and determined at the conditions indicated above. Typical signals for 25 ~1 of 1.0 X 10e3 M samples are summarized in Table 1. The factors affecting oxidation rates are steric effects on adsorption and stoichiometry, i.e., number of oxidizable sites such as amino and hydroxyl groups. The stereochemical control of the oxidation rates can be observed. Propylamine yields a larger signal than butylamine and primary amines yield much larger currents than secondary and tertiary amines. For the amino acids, increases in molecular weight correspond to decreases in peak current. As examples, serine vs. tyrosine, serine vs. lysine, alanine vs. phenylalanine, and alanine vs. valine can be noted. Stoichiometry effects are shown by comparing serine with alanine and tyrosine with phenylalanine. Both serine and lysine contain 3 hydrogens with 2 active groups, yet the more bulky lysine yields a current only 6% of that for serine. This indicates that the effect of steric hinderance exceeds that of stoichiometry. Relating the amine and amino acid structures and oxidation currents, it may be suggested that the rate of oxidation increases with the number of cr-hydrogens and the number of active sites, and with decreasing size of
215
sd
216
the substrate molecule. These observations are consistent with the reaction mechanism described_ In chromatography, as well as in other applications, samples are often in a buffered medium at pH values considerably lower than that of the carrier electrolyte used here. Thus, it was necessary to study pH effects on the detection signal from the nickel oxide electrode. Glycine at pH 5.1 in 0.1 M acetate buffer and at pH 3.5 in 0.1 M formate buffer was used with the nickel tubular electrode. A negative peak precedes the positive peak used to quantify each component (Figs. 5 and 6). The negative peak height depends on pH and on buffer concentration. Simply injecting buffered blank samples also produces negative and positive peaks and the peak heights increase with buffer concentration as shown in Table 2. The time profiles at the working electrode shown in Fig. 5 can be used to propose a mechanism for the signals obtained_ When the sample reaches the detector, a maximum in sample concentration and a minimum in hydroxide concentration occur simultaneously_ The decrease in pH and buffering capacity result in reduction of the higher, active nickel oxide, i.e., decrease in the surface coverage, 0, of the Ni(II1) oxide. The reduction of NiOOH is experimentally evidenced by the sharp negative peak obtained when alow pH buffer solution segment is injected (see Fig. 6). Pourbaix [ 43 shows that the oxidizing nickel(II1) species when held at +0.55 V vs. Hg/HgO will be reduced to a nonoxidizing form at pH values lower than 9. As the hydroxide concentration begins to increase during the tail of the passing sample zone, the active higher
1
i
(P-A) 0
TIME Fig_ 5. Profiles passing electrode: (i) analyte current; (S) sample face coverage by higher oxide; (OH-) hydroxide concentration_ Fig. 6. Analytical
signal for low pH sample.
(s)
concentration;
(0)
sur-
217 TABLE
2
Peak height
dependence
on solution
composition peak
Positive
Glycine
Buffer
Negative
WI
01)
bA)
(PAI
1 x 1o-J 0.0 0.0
0.1 0.01 0.05
0.21 0.14 0.18
1.45 0.57 0.70
0.0
0.10
0.24
0.90
peak
oxide surface is renewed. The amino acid sample is oxidized by the newly formed disordered nickel oxide surface containing relatively many active sites. In support of this mechanism, current-voltage curves for Ni(OH)? surface electrodes were experimentally observed here to show anodic NiOOH formation beginning at +0.4 V, in agreement with earlier reports [ 1, 51. The high rate of this process conforms to Fleischmann’s report [l] showing that the layer of NiOOH initially formed is only a few monolayers in thickness and follows Langmuir (i.e., equilibrium) adsorption behavior [I] . Analyte signals were observed to be enhanced as much as two-fold over those obtained for pH-matched samples. This apparent greater activity of the freshly deposited NiOOH corresponds to reports of anodization of Ni(OH), electrodes in aqueous hydroxide solutions to yield formation of active forms of nickel(II1) with subsequent “aging” properties [6]. Precision was examined for five replicate determinations at each sample pH value for pH 11, 7.5, 5.1 and 3.5. The estimated relative standard deviations were between 1 and 2% for all of the sample pH values. Such consistent precision suggests a reproducible higher oxide layer formed after its temporary removal by the lower pH of the mid-portion of the sample zone. It should be noted that these results point up the special advantage of the flow injection technique in reproducing surface exposure conditions when active electrode materials are used in voltammetry. Analyte signal characteristics for pH-matched samples are summarized in Table 3. Plots of the means for triplicate determinations of currents were used. Reduced sensitivity at higher concentrations is presumably due to saturation of available sites on the electrode surface. For glycine samples in pH 7.4 and pH 3.5 buffers, the positive signals were generally slightly greater than for pH-matched samples while the linearity was similar over the concentration range lo-‘---10” M. The effects on the signal of the coiled wire electrode can be attributed to (a) increased turbulance of the sample solution at the electrode surface, (b) increased area of the electrode, and (c) increased dispersion of the sample plug. The third effect decreases the signal whereas the first two enhance it. Thus, although the nominal area of the coiled wire electrode is fifteen times that of the tubular electrode, the net enhancement of sensitivity observed is
218
TABLE
3
Linearity data for selected compounds Compound
Electrode
Linear range (RI)
n
Propglamine Butylamine Diisopropylamine Triethylamine
Tubular Tubular Tubular
1 o-‘-l O? 1 o-‘-l O? lo~-lo-*
10 8 9
Tubular
10%-l
Glycine
Tubular
5 x lO”-lo-’
Coiled
5 fi 107-10~
Glycine
0-x
Slope (PA mM-I)
2.2 22 2.2
Intercept (PA)
Standard error of the estimate (PA)
Detecti’ limit (fig)
0.05 0.05 0.03
0.01 0.09 0.09
0.02 0.2 0.2
0.04 0.0
0.09 0.6
0.2 0.3
0.3
02
0.16
, 7 9
10
3’) --12.0
72
typically only six-fold, Limits of detection are given as absolute masses by using the sample volume injected, 25 r_tl.The limit of detection (in pg) is computed as (0.025) (3) s’ (m.w.)/nz, where s’ is the standard error of the estimate, and nz is t.he slope for the regression line. While the coiled wire electrode had a lower limit of detection, its effective volume was about 25~1, whereas the effective volume of the tubular electrode was less than 1 ,ui_ Other amines or amino acids would yield somewhat higher limits of detection
and lower sensitivity
as indicated
in Table 1.
REFERENCES 1 M_ Fleischmann, ‘K_Korinek and D. Pletcher, J_ Chem. Sot. Perkin Trans. 2, (1972) 1396. 2T. N. Morrison, K. G. Schick and C. 0. Huber, Anal. Chim. Acta, 120 (1980) 75. 3 J. RG.tiEka and E. H. Hansen, Flow Injection Analysis, Wiley, New York, 1981. 4 M. Pourbaix, Atlas of Electrochemical Equilibria, Pergamon, Brussels, 1966, pp_ 330-341. 5 G. W_ D. Briggs, E. Jones, and W_F. K_ Wynne-Jones, Trans. Faraday Sot, 51(1966) 1433. 6 R. S. Schrebler Guzman, J. R. Vilche and A. J. Arvia, J. Electrochem. SOC., 125 (1978) 1578.