JPH01503484A - Method and apparatus for amperometric detection - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed description of the invention] Method and apparatus for amperometric detection The present invention relates to a method and apparatus for t-flow measurement detection. In particular, the present invention Continuous use of microelectrodes in current detection methods for jet cells and femtoamplifiers Involves novel methods of electrochemical detection at quantitative (sub-picomolar) levels. Ru.

The flow jet cell referred to here is a flow-through type electrochemical cell, characterized by having a fluid jet inlet and a working electrode; The jet is directed toward the working electrode and is substantially perpendicular to the plane containing the working surface of the electrode. .

Significant benefits derived from using microelectrodes to solve electrochemical problems has become well known. The advantages of using microelectrodes under zero conditions will become apparent over the next few years. Despite being pioneered, their use in flow-through cells has only recently been It is about. At the outlet of a liquid chromatography (LC) column with an open tube The use of inserted cylindrical carbon fiber microelectrodes has been reported, where is a bundle of carbon fiber microelectrodes arranged in a thin channel-shaped flow cell. It incorporates.

Obviously, the application of microelectrodes as sensors within a normal flow cell type device were all in thin-layer detectors. We here have a single microdis The electrode is used as a sensor under free flow (thick layer) conditions, especially in wall jet cells. I discovered that it can be used as

Using electrochemical sensing in LC is difficult due to electrical resistance problems due to the normal phase have been excluded from application. Reduced absorption and solubility issues of organic matter and the range of action potentials In terms of extension, normal phase liquid chromatography electrochemical detection (LCEC) ) Although it is clear that the advantages of operation are great, few actual applications have been reported. do not have. In cases where normal phase LCEC is used, the eluate is such that the electrolyte is soluble Or, it is limited to mixed solvents whose conductivity can be increased by post-treatment of the column. I This is the flow diagram for introducing the sample into the jet cell of the organic solvent system. A liquid system in which a microelectrode using injection contains an electrolyte of about 10-'M. We found that it can be used in

According to our research, disk microelectrodes can be used on a large scale in jet-type flow cells. It was shown that the sensor is as effective as, or even more effective than, a sensor with a simulated surface. Faith In addition to improving the signal-to-noise ratio, these tiny sensors can also be used with high electrical resistance media. Electrochemical flow as a detector in liquid chromatography works well in the body. - Overcoming pluripotency limitations in cell usage. This advantage is further enhanced by the effect of electrical resistance. This is emphasized by optimizing the design of the reference electrode and cell configuration to minimize the effects.

The main difficulty in measuring low concentrations with microelectrodes is that the femtoamplifier's t-current is This is something that must be measured in order to develop the overall analytical ability of a person. slow potential In terms of scan speed, a picoamp is used instead of a potentiostat with microelectrodes. Despite suggestions that meters could be used, many researchers Even in the two-electrode mode the conventional potentiostat continues to be used.

We use picoammeters and femtometers as detectors for microelectrode flow cells. Researched the use of ammeter. If we can omit the potentiostat, It was shown that Karika has unique advantages. What is required of the equipment is low noise and no noise. These are the offset voltage of the resistor and a sensitive ammeter. we also femtore It is used as a means to minimize noise when measuring minute currents in the ampere range. We have developed a power supply that runs on batteries.

Thus, in an aspect of the present invention, a method for electrochemical detection of electroactive species is provided. A method is provided in which a liquid sample solution containing a species is connected to a microelectrode as a working electrode. is passed through a flow jet cell equipped with a

According to one preferred aspect of the invention, the current in the cell is picoammeter or femmeter. Measured in ammeter.

Therefore, a preferred device according to the invention is a picoammeter or a femtoammeter. The cell/microelectrode combination described above is coupled to a cell/microelectrode combination as described above.

In particular, the present invention provides an apparatus for electrochemical detection of electroactive species in a flowing stream. At the location, (i) a working electrode having a surface with a nominal diameter of less than or equal to 50 microns; , (ii) an orifice with a nominal diameter of 0.3 mm or less; directing substantially all fluid flow into the working electrode in a direction substantially perpendicular to the surface of the working electrode; is applied to the working electrode surface such that the entire electrode surface is within the diameter of the fluid flow. (iii) a reference electrode; and (iii) a reference electrode. Provided is an apparatus characterized by comprising a flow jet cell having the following characteristics.

The working electrode is a single electrode with a surface of nominal diameter less than or equal to 50 microns. consists of atmospheric elements or an arrangement thereof.

The working electrode consists essentially of metal or carbon. The preferred metal is platinum, but , other suitable metals can be used.

The present invention and various specific and preferred aspects thereof are further illustrated in the following non-limiting examples. Described and explained. The following abbreviations are used: AN=N Niacetonite Lil Ferrocene Feocian ferrocyanide potash TEAP = Tetraethylammonium Verchlorate Unless otherwise stated Electrode size refers to the nominal diameter of the electrode.

Reference is made to the accompanying figures shown below.

1゛bi　1　a... Applying a constant potential for direct current measurement detection by flow injection analysis 1 is a circuit diagram of a battery-based voltage offset device for How is Figure 1a? 1 shows how the device of FIG. 1 can be used with a cell and current measuring device.

Presentation I” Periodic portammogram of 1 mmol Fc in ANo, 01 mol TEAP (Oi is zero current). 3-electrode potentiostant mode, Scanray) = 10+nV /　s.

(a) 5-curved microdisk.

(b) 50! ! m platinum micro disc.

1. The reagents and equipment used in this example are as follows: reagent grade ferrocene ( Fc), potassium ferrocyanide (Feoc), and potassium nitride, and HPLC grade Acetonitrile (AN) was used as purchased. Tetraethylammonium bar - Chlorite (TEAP) (Southwestern Analytical Chemicals, Inc.) was subjected to pentoxidation in a vacuum desiccator prior to use. Dry on phosphorus for several hours.

脣KIhi 8 cells and aaq″ Wataru Metrohm EA 1096 wall jet type flow-through detection cell The electrodes of the two microdisks were adjusted to fit the working electrode part of the. Met In the rohoo+ cell, the disk is placed in the center of the flat end surface of an insulating iron with an outer diameter of 7 mm. Two electrodes were also designed to meet this criterion.

(i)50. A platinum wire of n is inserted into a soda glass capillary tube of outer diameter 5I. 5ffl! Glass seal by frame sealing within 11 segments A platinum micro disk with a diameter of 50 was adjusted. Pure copper wire to platinum wire Connect one end of the platinum-glass segment with Torr 5eal to length. Seal it inside one end of a glass tube with an outside diameter of 7mm on one wall of 6B, and The joint was sealed and strengthened. The ends of the electrodes are cut flat with a diamond blade and polished. to form the cable connections described below.

(ii) The second microdisk detector is a Magnamite detector with a nominal diameter of 5- (Hercules Corp.). 1o circle A conical segment is cut from a thick-walled capillary tube to create a conical segment. Gently pull it so that about half of it fits inside the glass tube with an outer diameter of 7 inches. Stretched.

The end of the glass cone is fired and guided through the orifice of the capillary by micro-manipulation. A single carbon fiber was then supplied and the fibers were protruded from both ends. Ga Connect the narrow end of the lath cone to a transparent vacuum hose and apply one drop of Torr 5eal. Place the TorrSeal on the wide end of the cone, keeping the fiber in place. A gentle suction was applied until the capillary was found to have filled the capillary. Silver processing of pure copper wire The cone was bonded to the carbon fiber at the narrow end using pre-treated epoxy.

Next, the narrow end of the cone was heated using Torr 5eal to create a glass with a length of 6G and an outer diameter of 7E. sealed inside the tube. After the epoxy has cured, circle it in place of the outer tube. I cut off the end of the tube. The copper wire that contacts each microelectrode has a coaxial cable lead wire. Soldered. Secure the cable to the electrode body using heat-shrinkable plastic and tubing. connected to. Polish the electrodes one by one with fine grade sandpaper and finally with P1200. Fine grade alumina slurry (5, 1, 0,3, 0,05p Polished with Ta). Each electrode can be easily refilled with 0.05-alumina at the beginning of each day's use. Polished.

Silver chloride standard of Metrohm 5ilver designed to fit flow cell The electrode body was filled with a 3.0 M NaCl aqueous solution. between the reference electrode and the cell For experiments in non-aqueous media, the bridge contains a 0.1 molar TRAP solution in water. The medium was filled with 0.05M KNO2 daily. The auxiliary electrode is Met It is a ROHM glass carbon electrode, and during the period of two electrode parts, it is inside the auxiliary electrode part. It was also used for sealing. The cell was housed in a grounded Faraday cage.

FIG. 8 shows a preferred configuration of a flow cell specifically designed for use in accordance with the present invention. It shows.

The components of the cell are as follows: 1 Cell body 2′″ reference electrode 3 External contact for pole action 41 working electrode 50-ring seal 6.Assemble the retaining plate for supporting and fixing 73 to the main body 1, set screw 8, and injection spout. Filled injection insert 9 Discharge insert 10” microelectrode 9 These components are metals or conductive materials. Other components are 6 and It is made of insulating materials, except that 7 is metal.

In the illustrated device, the working microelectrode 10 is a silver-plated stainless steel reference voltage. It is concentric with pole 2. This arrangement minimizes the resistance and capacitance of the cell and ensures a uniform electric field and Ensures that the entire electrode surface is at equal potential.

As a result, the distribution of current density on the working electrode depends only on the hydrodynamic pattern of the cell. Therefore, it is determined.

Yakudo rich pottery Four different systems are used to apply a potential to the working electrode and detect the cell current. It was done.

(i) The first system is a Metrohm E611 detector, which has a periodic It is also equipped with a Metrohm E612 scanner for standard voltammetry.

(ii) The second system is a Metrohm E611 potentiostat. Keithley 480 pin connected as preamplifier at working electrode input Equipped with a core meter.

The features of this system have been previously described (Bixler J, W.).

Bond A, M, Lay P, A, Thormann W, Van d er Bosch P, +F1eisch-mann M, +Pon5 S, A nal, Chem, Acta, V1B? , 1986. p67) 11(ii) System 3 uses Xe1thley 6e4 electrometer (used in current mode) and miniature standard $Weston cells, both connected in series with two pole cells. Ta. The Weston cell is physically placed inside the Faraday cage of the analytical cell and is A noiseless, low resistance +1.016Ve++t was applied to the pole.

(iv) Finally, a variable voltage battery operated by the circuit shown in Figure 1. source was used in place of the eston cell in the third system.

In FIG. 1, circuit elements and their values are indicated by conventional symbols. A The D584 is powered to generate a potential of 5.0 volts for 4 bins at pin 1. This is a programmed precision voltage reference. Botsiometer formed by R1 and RVI The circuit consists of a 7611 operational amplifier configured as a voltage follower circuit with a gain of 1. Generates a variable voltage of 0 to 3.5 cm at pin 3.

The polarity of the output (DC 0 to 3 V) can be changed to positive or negative depending on the position of the reversing switch outside 2. can be done. SWI is an on/off switch. This circuit is powered by battery B. A power supply of 9 to 18V is supplied.

Figure 1a shows the voltage Bv in Figure 1, the reference and working electrodes (RE, WE) of the cell, and the current. Keithley 614 digital electrometer (E) operating / in positive conversion mode ( (represented schematically). Voltage source ■ is common potential Set the potential of the reference electrode relative to the voltage (C) and therefore the potential of the solution. Electrometer ( The feedback of the input amplifier (A) included in E) is such that the potential of the working electrode WE is It acts to maintain close to the conducting potential.

The output of the electrometer (E) appears between the terminals (T,,T,).

Flow injection peaks were recorded using a Houston 2000 recorder. recorded in the code. The recorder is the unattenuated output of the Metrohm detector. Or connected to the normal analog output of a multimeter. detector recorder The response characteristics of the stem were evaluated by generating and recording synthetic triangular peaks.

Detector-recorder systems for rise times of 1 second or more Peak truncation of 1.5% or less was shown.

Is the peak area Waters Data? Measured using Iodule.

flow injection system If the use of a single piston BAS LC-6 bomb is specifically specified. All flow injection experiments used a 20 μL sample loop with the exception of A double piston coupled to a Rheodyne 7125 injector with It was performed with a Waters 6000A pump. Cell jet nozzle 1 2 is connected to the injector using a Teflon tube with a length of 18 mm and an inner diameter of 0.3 mm. Ta.

disaster treatment five ``Kiyu'' I γ11 The nonlinear diffusion characteristics of microelectrodes are shown in the periodic portamogram in Figure 2. , which was recorded in a static solution. They are both low to moderate exhibits a sigmoidal shape at scanning speeds of It is a characteristic of It is possible to compare the effective radii of two microdisks, but the The reason is that Equation (1) describes the control @ current of the micro disk electrode under the condition of spherical diffusion. It is from.

i, =4nFDCr (1) However, ia = diffusion control limit current n = number of electrons involved in electron transfer F = Faraday constant number D = diffusion coefficient C=concentration r=electrode radius From the data in Figure 1, we know that the diffusion coefficient of ferrocene in acetonitrile is 2.3. Assuming that X10-'Cl1l/sec, carbon and plastic The effective radius of China's microdisk electrode is estimated to be 2.4 μ and 21 tna, respectively. 2.5 and 25p, compared with the nominal values provided by the manufacturer.

A sample of ferrocene is repeatedly injected into the flow system and a series of Flow injection flow by recording peak information at fixed potentials A physical mechanical portamogram is obtained. Figure 3 shows a 50μ platinum microdisk electrode. Shows peak current and peak area plotted as a function of applied potential to the poles. are doing. The plot shows a clear sigmoidal dependence on the applied potential, Allows optimal selection of applied potentials for analytical applications.

backyard Set the retainer λ to 2 A platinum disk with a diameter of 4.6 tm and a micro disk of carbon and platinum. Data showing the dependence of the flow injection peak height on the flow rate of the measurement solvent. The data are shown in Figure 4 for the oxidation of (Fe(CN)i)'- in an aqueous medium. There is. Each plotted data point is the average of five or more replicates.

The relative standard deviation for all three electrodes is typically for these experiments It is between 3% and 4%, independent of the electrode, and the reproducibility of the peak height depends on the sample input. This suggests that it is limited by the performance of the projector.

The exponential flow rate dependence on the peak height data from the fourth column is given in Table I. ing. The flow velocity dependence of the 4.6 mm platinum disk was determined by wall jet data. The expected theoretical 0.0% for a given electrochemically active sample at the 50 Liang platinum and 5 JPm carb, which are not far from the flow velocity dependence of the 75th power. The disk consists of some kind of impinging jet electrode rather than a wall jet electrode. The reason is that their electrode radius versus nozzle radius (approximately 0.3 tin) The ratio is large and therefore both electrodes experience turbulent rather than laminar flow.

From these results, it is clear that the 5-day It becomes clear that the square electrode is superior.

The flow rate dependence of the 50-platinum and 5-carbon electrodes also shows that acetic acid is added to the flow cell. Continuously pumping a ferrocene solution containing 0.01 mole TEAP in nitrile were studied under steady-state sample flow conditions rather than injection. child Under these conditions the continuous current for a 50μ platinum and 5-carbon disk is Exponential velocity dependence of 0.74 ± 0.01 and 0.188 soil 0.003, respectively. However, these are Fe in aqueous media of ferrocene in acetonitrile. (CN) significantly lower than the value obtained for the 20 μL injection sample of b. this As such, flow rate dependence needs to be carefully evaluated for specific conditions of use. death However, the 5-carbon disk electrode is suitable for both steady-state and unsteady-state flow conditions. In both cases, the desired low flow rate dependence is clearly shown.

Furthermore, the thickness of the boundary layer, that is, the current, increases at a position near the stagnation at the center of the collision. It was found that there was no difference. This can be easily tested with a 5- carbon disc. The reason for this is that microscopic tests show that the carbon fiber is slightly This is because it clearly shows that there is a deviation. the electrode with respect to the direction of its installation. Even if the injection peak height is rotated by 120°, there is no effect on the injection peak height. This proves the dependence of current on position. From this point of view, the 5-disk electrode 50I has a large dependence of peak height on flow rate! Clearly superior to m and 4.6■ electrodes ing.

disaster relief owner Qianxian standing ball inspector Ferrosensors with concentrations ranging from 1 mmolar to submicromolar under experimental conditions The concentration dependence of the microelectrode was investigated using a sample. In each case, the flow index The injection hydrodynamics portamogram selects the potential to be applied within a limited current range. 1.4ml/l1lin measurement solution for all electrodes. Medium flow rate was used. In all cases, linearity with concentration and experimental precision were good. It is better to use the peak height rather than the peak area as the analytical signal if the It was good.

Therefore, the response of the detector system is sufficient to accurately record the peak height. In fast cases, there was no advantage to using integral.

A summary of the concentration-dependent research results for microsensors is given in Table ■. 4.6 Experiments on tIm platinum discs are also included for comparison. Forecast As expected, the relative fraction FrW degree (measured actual current amplitude) increases. The variation in sensitivity for a given electrode is due to different It reflects the fluid and detector amplification characteristics as well as area changes caused by daily polishing. death However, relative sensitivity is less important than response linearity and signal-to-noise ratio. There isn't.

Both microelectrodes are the same or lower than a regular 4.6 mm platinum disk. Exhibits a linear response range. Furthermore, the 4.6-ohm disk is of a low degree for microelectrodes. It exhibits undesirable properties that are only visible or not observed at all. injection valve If the flow is momentarily interrupted when turning, millimoles of electrolyte will be lost in a 4.6-barrel disk. Figure 5-a) Emergency after injection. Baseline stability is slowly restored. This is useful for measuring small peaks. and excludes the integration of useful peaks below micromolar concentration levels. cormorant. 4.6 - Reason why electrodes are not beneficial for dilute (10-'M) electrolytes The reason is that for ferrocene concentrations greater than 4 micromolar (or current <0.1 μA) The potentiostat controller is used to maintain control of the potential within the electrically resistive medium of the lever. This is because the appliance voltage is not sufficient. Also, the potentiostat control A linear response is obtained due to uncompensated resistance even at low concentrations, which allows do not have. Therefore, normally sized disks are clearly not suitable for solutions with high electrical resistance. It's inappropriate.

The 50n platinum disc showed significantly improved response over the 4.6x disc. and a very satisfactory response even at electrolyte concentrations of 10-5 molar. give. Figure 5-b shows an injection spike in the low current range. Despite the fact that the baseline quickly returns to a flat baseline after injection, This shows that the sline noise level is very low. Figures 5-c and 5-d The figure shows a 50-meter platinum electrode connected to a picoammeter preamplifier and potentiostat. Potentiostats are used alone in comparison to when used with potentiostats. It shows the difference in signal-to-noise ratio when signal by using a preamplifier The noise-to-noise ratio is improved by a factor of approximately 6, which is greater than the frequency recorded in a stationary solution. Consistent with results reported with similar detector devices used in periodic portamograms are doing.

5-The response of a cell employing a carbon disk sensor is pico in 3-electrode mode. When using a potentiostat preamplified with an ammeter and 2 in polar mode When using an electrometer (ammeter mode) without potentiostat control in In both cases, three different supporting electrolyte concentrations were investigated. 1st The battery power supply in the figure was used in the latter mode to minimize noise. dark An example of a degree-dependent response is shown in FIG. 10-3 as reported in Table ■ and the linear responsivity in 10-s molar TEAP is 3 to 2 than in polar mode. The electrode mode extends to a somewhat lower ferrocene concentration. Furthermore, the seventh Comparing Figure a and Figure 7b, the background noise is actually It is shown that the quality is low. Bank ground noise reduction is 1o-5 molar TIE It is also observed in AP. Figure 7b shows a dilute sample of 4 x 10-” moles. Easily detect electrolytes in two-electrode mode without a potentiostat It shows that it will be done. The measured current is in the femtoamp range. Therefore, all experiments were performed in a Faraday cage due to the considerable electrical noise. It is noteworthy that In two-electrode mode without a potentiostat in the circuit It has been observed that the connecting cable is almost unaffected by mechanically induced noise. It was done.

Table 1 0.05 mole KNO! 7o-I for the oxidation of [Fe(CNa))’- in Exponential dependence of injection peak height on solvent flow rate toe international search report N s τ fugo π Kyunou APP! Buzz noi El, pcπ layer "■45@23(146)/1

Claims (12)

[Claims] 1. A liquid sample solution containing seeds is passed through a flow jet cell using an electroactive In an apparatus for electrochemical detection of species, A device characterized in that a microelectrode is used as a working electrode. 2. In a device for the electrochemical detection of electroactive species in a flowing stream , (i) a working electrode having a surface with a nominal diameter of less than or equal to 50 microns; , (ii) an orifice with a nominal diameter of 0.3 mm or less; directing substantially all fluid flow into the working electrode in a direction substantially perpendicular to the surface of the working electrode; on the working electrode surface such that the entire electrode surface is within the diameter of the fluid flow. (iii) a reference electrode; and (iii) a reference electrode. An apparatus comprising a flow jet cell having a flow jet cell. 3. The working electrodes each have a nominal diameter of less than or equal to 50 microns. 3. A device according to claim 1 or 2, wherein the device is an array of electrode elements having a surface. 4. 4. A device according to claim 1, 2 or 3, wherein the working electrode consists essentially of metal. 5. 4. A device according to claim 1, 2 or 3, wherein the working electrode consists essentially of carbon. 6. The current flowing through the cell is measured using a picoammeter or femtoammeter. 6. The device according to any one of claims 1 to 5. 7. The solution potential with respect to the working electrode is a voltage connected between a common potential and a reference electrode. 7. Apparatus according to any one of claims 1 to 6, supplied by a source. 8. The potential of the working electrode is influenced by the action of an operational amplifier configured as a current-voltage converter. 8. The device of claim 7, wherein the device is maintained closer to a common potential. 9. 9. The reference electrode according to claim 1, wherein the reference electrode is arranged coaxially with respect to the working electrode. The device according to item 1. 10. 10. The cell according to claim 1, wherein the cell is housed in a Faraday cage. equipment. 11. In a method of electrochemical detection of an electrically active species, a sample containing said species is added to the solvent stream for injection into the apparatus according to any one of claims 1 to 10. How to do it. 12. 11. The solvent is a solvent system containing an electrolyte concentration of 10-4 molar or less. Method described.