| A new type of admittance detector for high impedance ultra-fine capillary(inner diameter≤25μm)is improved from the capacitive coupled contactless conductance detector.However,the use of an excitation signal of<10 k Hz reduces the influence of susceptance on conductivity detector,and the detection of ionic compounds in ultrafine channels is preliminarily realized.In this work,the admittance detector is deeply researched,and the main study contents are as follows:(1)Various factors of admittance detection in the high-impedance ultra-fine channel are fully considered,and a discrete element equivalent circuit model of the admittance detector is established.This model takes the electrode and solution within the capillary formation of coupling capacitance,solution resistance and capacitance,leakage capacitance between the electrodes into consideration.A MATLAB software program is used to calculate the capacitance and resistance in series and parallel.The signal response of the sample zone flowing through the admittance detector is simulated dynamically.(2)An admittance detector for the ultrafine channel is built.The experimental data and simulation data are compared and verified,finding that the experimental results are approximately consistent with the simulation results.The peak height and peak area of Na Cl samples obtained by flow injection in the ultrafine channel,increase first and then tend to be stable with the increase of solution concentration;in the range of 500～5000 Hz,the peak height of excitation signal at each frequency increases with the increase of solution concentration,and the half peak width of signal peak is affected by the change of excitation signal frequency and sample concentration.(3)In order to get the best sampling method for ultrafine channel capillary,four kinds of injection systems are built.the comparative analysis is carried out from the peak height,the peak area,back pressure of system,flow direction of solution and the diffusion injection volume;which shows that the best sample injection method—gas pressure driven carrier solution combined with a ten-port valve,having a high injection pressure,strong back pressure capability and good injection performance,the RSDs of peak height and peak area measured by this method,are 1.53%and 2.22%(n=10),respectively.(4)An admittance detectors with simultaneous input of multi-frequency excitation signals is developed.At the signal input terminal,PA15A chip is used to superimpose and amplify the signal.The electrical signals passing through the detector are collected by the data acquisition card and processed by the MATLAB program to obtain the signal response under different frequencies.The detector is applied to the detection of KCl solution in 10μm and 5μm inner diameter capillaries,and the best linear range of different excitation frequencies moves to higher concentration as the frequency increasing;at the same time.It is also found that the peak shape of the sample is greatly affected by frequency and the width at half peak of the same signal peak becomes narrower with the increase of frequency.(5)A two-in-one detection cell based on multi-material 3D printing technology is studied,which realizing the simultaneous response of admittance detection and confocal laser-induced fluorescence detection methods at a single point on the capillary column.The3D printing process uses the"print-pause-print"method to embed the two metal electrodes in the detection cell.This detection cell is used in combination with electrophoresis for simultaneous detection of inorganic ions and fluorescein isothiocyanate-labeled amino acids.On the capillary with an inner diameter of 25μm,the limits of detection for Na~+,K~+,and Li~+are 2.2,2.0,and 2.6μmol/L,respectively;the detection limits of fluorescence detection for fluorescein and FITC is 7.6 and 1.7 nmol/L,respectively.The relative standard deviation(RSD)of the two detection methods for peak height is between 0.3%and 4.5%(n=3),and the correlation coefficient of the working curve is R~2≥0.9904. |
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