Sensitivity Enhancement Factor Research Articles

Application of on-line sample preconcentration by large-volume dual preconcentration by isotachophoresis and stacking (LDIS) on straight-channel microchips.

In this study, large-volume dual preconcentration by isotachophoresis and stacking (LDIS) which is an on-line sample preconcentration technique coupling large-volume sample stacking with an electroosmotic flow pump (LVSEP) with transient isotachophoresis (tITP) was applied to microchip electrophoresis (MCE) for improving both detection sensitivities and peak shapes. To realize LDIS in MCE, we investigated experimental procedures for injecting a short plug of a leading electrolyte (LE) solution into a straight microchannel without any sophisticated injector apparatus. We found that a short LE plug could be injected into a sample-filled straight-channel only by making the liquid level of the LE solution in an outlet reservoir higher than that in an inlet one. By applying a reversed-polarity voltage to the microchip, anionic analytes injected throughout the microchannel were first enriched by LVSEP, followed by tITP. Through the second preconcentration effect by tITP in LDIS, sensitivity enhancement factor (SEF) and asymmetry factor for a standard dye were improved from 878 and 0.62 to 1330 and 1.14, respectively, relative to those in conventional LVSEP. It should be noted that more viscous running buffer containing sieving polymers could be employed to the LDIS analysis, which was effective for improving the SEF and the separation efficiencies, especially for bio-polymeric compounds. Finally, LDIS was applied to the oligosaccharide and protein analyses in MCE, resulting in the SEFs of 1410 and ca. 50 for maltotriose and bovine milk casein, respectively.

EC-QCL sensing system–incorporated adaptive baseline correction algorithm for simultaneous detection of multiple gas components

In this study, we presented a multi-gas sensor developed by employing an external cavity quantum cascade laser (EC-QCL)-integrated adaptive baseline correction algorithm for the simultaneous detection of ammonia (NH3), ozone (O3), and carbon dioxide (CO2). The EC-QCL sensing system was selected due to its high wavelength resolution and wide tuning range (1033–1068 cm−1), enabling the simultaneous measurement of multi-gases. The system complexity was effectively decreased and the superimposed noise was avoided by employing an adaptive baseline correction algorithm based on differential transformation and B-Spline approximation (ADTBA), which enabled the resolution of the uneven EC-QCL baseline signals. The performance validation of the ADTBA algorithm by comparing it with the conventional polynomial fitting for detecting the spectra of NH3, O3, and CO2 showed that the former outperformed the latter. The results showed a sensitivity enhancement factor of 3.7, indicating that the newly developed algorithm can generate a high-quality gas absorption spectrum. Allan deviation analysis of the system's responsiveness revealed an averaging time of 120 s resulting in sensitivities of 1.3, 26.7, and 37.0 ppm for NH3, O3, and CO2, respectively.

Ultrasound assisted-cloud point extraction coupled with spectrophotometry for determination of low levels of formaldehyde from milk-based products

In this study, a preconcentration method is proposed for the separation, extraction and spectrophotometric determination of free formaldehyde (FA) from milk-based products. The method is based on ion-pair formation between phenoxazine 3-one, resazurin, and hydroxymethanesulfonate (HMS), which is selectively formed by the reaction of FA with sulfite at pH 5.0, followed by its extraction into the micellar phase of Triton X-114. Resazurin was adopted as ion-pairing reagent for detection of low levels of FA at 610nm. The variables affecting the extraction efficiency were evaluated and optimized. A linear relationship was obtained in the range of 2–120μgL-1 with a detection limit of 0.501μgL-1. Under optimum conditions such as pH 5.0, 8 µmol L-1 Resazurin, 0.4mgL-1 sulfide, 0.03% (v/v) of Triton X-114, after applying ultrasonic assisted-cloud point extraction (UA-CPE), spectrophotometric determination was made at 610nm to measure the concentration of FA found in trace amounts in milk-based products such as liquid, semi-solid and solid-milk products including whole milk, semi-skimmed milk, cacao milk, banana flavored milk, strawberry flavored milk, goat milk, yogurt, fruit flavored yogurt, white cheese. The precision was in the range 2.5-4.8% (n: 5, for 5, 25 and 100μgL-1). After preconcentration of a 15mL sample, a sensitivity enhancement factor of 55.6 was obtained. To represent the sample matrix, the validation was checked by intra-day/inter-day accuracy and precision analysis of a spiked quality control sample and its recoveries at three concentration levels. Finally, the method was successfully applied to the determination of free FA in samples, and recoveries in the range 90.8-97.4% with RSD lower than 5.7% after spiking were obtained.

A CNN-assisted mid-infrared high-sensitivity exhaled ammonia sensor based on cavity ring-down spectroscopy

The cavity ring-down spectroscopy (CRDS) is becoming increasingly important in sensitive gas detection for atmospheric pollutant detection, greenhouse gas monitoring, human respiratory gas analysis, and measurement of specific gases in industrial processes. However, the accuracy and sensitivity of CRDS are greatly limited by noise and ring-down time extraction algorithms. To solve this problem, a CNN-assisted high-sensitivity exhaled ammonia sensor based on cavity ring-down spectroscopy was developed in this paper, which is the first time to introduce the convolutional neural network to cavity ring-down spectroscopy. A CNN filter and a neural network-based ring-down time extraction algorithm trained by two different datasets were applied to the sensor, solving the shortcomings of the traditional CRDS. Among several filters, the CNN filter achieves the best performance and the SNR is improved by 3.4 times. Compared to traditional nonlinear fitting algorithm, a sensitivity enhancement factor of 3.48 was obtained and a sub-ppb detection limit can be achieved by using Allan deviation analysis. The experimental results prove the feasibility of deep learning for improving the performance of cavity ring-down spectroscopy gas sensor.

Microwave-photonic Vernier effect enabled high-sensitivity fiber Bragg grating sensors for point-wise and quasi-distributed sensing

This paper reports a sensitivity-improved fiber Bragg grating (FBG) sensor system based on microwave-photonic interferometry and the Vernier effect. An incoherent microwave photonics system based on a broadband light source is employed to interrogate the FBG sensor using the wavelength-to-delay mapping technique combined with interferometry. Specifically, the sensing FBG together with a reference FBG is used to construct a microwave photonics Michelson interferometer (MI). Changes in the Bragg wavelength of the sensing FBG subject to external perturbations are encoded into the spectral shifts of the microwave interferogram of the MI. A virtual interferometer is then generated from the sensing MI based on a computational Vernier effect modality. By superimposing the spectra of the sensing MI and the virtual interferometer, the Vernier effect is generated. By tracking the spectral shift of the Vernier envelope, it is shown that the measurement sensitivity of the sensing FBG is remarkably enhanced with an expected factor. Moreover, a quasi-distributed sensor system with enhanced sensitivity based on cascaded FBGs and the proposed virtual microwave-photonic Vernier effect technique is implemented, representing the first demonstration of a Vernier effect-enhanced FBG array sensor. Additionally, the possibility of employing the harmonic Vernier effect for further sensitivity enhancement is investigated, where a remarkable sensitivity enhancement factor up to 685 with a strain sensitivity of 94 MHz/µε is successfully demonstrated.

On-line sample preconcentration by LVSEP-FASI with simple voltage control on Y-channel chips.

To eliminate complicated voltage controls for highly sensitive microchip electrophoresis (MCE) analyses on the basis of combining two online sample preconcentration techniques, large-volume sample stacking with an electroosmotic flow (EOF) pump (LVSEP) and field-amplified sample injection (FASI), cross-channel microchips and a multichannel high-voltage power supply were replaced to Y-channel chips and a conventional power supply designed for capillary electrophoresis, respectively. By simple switching of the electric circuit after the LVSEP-FASI sample enrichments, the focused analytes could be separated during anodic migration in a separation channel. In the LVSEP-FASI analysis of fluorescein using the Y-channel microchip, the maximum sensitivity enhancement factor (SEF) of 7400 was achieved, resulting in a 30-fold detectability increase compared to the conventional LVSEP. The developed method was applied to the oligosaccharide analysis in MCE. As a result, the SEF for maltotriose was improved from 450 to 2300 and the baseline separation of the oligosaccharides was achieved without any complicated voltage control in LVSEP-FASI on the Y-channel chips.

Two-step pressure injection-assisted online enrichment of herbicides from foods with affinity micelle sweeping by micellar electrokinetic chromatography

A novel online two-step pressure injection-assisted stacking preconcentration method, which involves sweeping and affinity micelles in micellar electrokinetic chromatography was developed to simultaneously measure various organic anions. The micellar solution was a mixed solution that contained 0.3 mM didodecyldimethylammonium bromide and 20 mM borax. After the micellar solution was injected for 60 s, the tested analytes prepared in 20 mM borax were introduced into the capillary for 150 s. The key experimental factors that influenced the separation and sensitivity were investigated and optimized, including the concentration and injection time of the micellar solution, the concentration of borax in the sample solution, the concentration of sodium dodecyl sulfate and borax in the background electrolyte (BGE), the content of acetonitrile in the BGE and the injection time of the sample solution. Compared with typical injection methods, this method achieved sensitivity enhancement factors ranging from 85 to 97 under optimized conditions. Good linearity for matrix-matched calibration was established for all analytes with R2 values of 0.9986–0.9996. The intraday (n = 6) and interday (n = 6) precisions of the method were less than 2.85% when expressed as relative standard deviations. When the method was applied to analyze rice and dried ginger samples, analyte recoveries ranged from 85.81% to 106.59%. Through sweeping and affinity micelles, stacking preconcentration method was successfully employed to analyze trace amounts of fenoprop and 2,4-dichlorophenoxyacetic acid in rice and dried ginger samples.

Terahertz metamaterial filter with enhanced sensing sensitivity using the suspended structure design

This paper presents a method to improve refractive index sensing sensitivity using a suspended structure design with a suspended silver split ring resonator metamaterial structure on a silica substrate, which is important in improving detection sensitivity compared to a planar structure. Its basic unit has two metal cleavage rings attached back-to-back to an etched dielectric substrate. Due to the enhancement of the electric field around the gap of the surface-modularized metal structure and the expansion effect of the electric field of the etched layer, the sensitivity is enhanced from 162.5 GHz/RIU of the planar structure to 342.5 GHz/RIU with a sensing sensitivity enhancement factor of 2.1 times compared to the planar metamaterial with an unetched substrate. When the refractive index of the surrounding environment is changed from n = 1 to n = 1.4, the resonance center produces 0.137 THz redshift. Based on this, a more practical structural design is proposed to improve the refractive index sensing sensitivity while reducing the fabrication difficulty. This work provides new ideas for the research of terahertz (THz) metamaterials with high refractive index sensitivity and offers more device options for THz sensing applications.

A pH-mediated field amplification sample stacking technique based on portable microchip electrophoresis heavy metal ion detection system.

We built a portable microchip electrophoresis heavy metal ion detection system and proposed a pH-mediated field amplified sample stacking (pH-mediated FASS) online preconcentration method. The pH-mediated FASS focuses and stacks heavy metal cations by controlling electrophoretic mobilities with a pH change between the analyte and the background electrolyte (BGE) in solution to improve the detection sensitivity of the system. We optimized and adjusted sample matrix solution (SMS) ratios and pH to create concentration and pH gradients for SMS and BGE. Furthermore, we optimize the microchannel width to improve the preconcentration effect further. The system and method analyzed soil leachates polluted with heavy metals and separated Pb2+ and Cd2+ within 90s, obtaining their levels at 58.01mg/L and 4.91mg/L with sensitivity enhancement factors (SEF) of 26.40 and 43.73. Compared with inductively coupled plasma atomic emission spectrometry (ICP-AES), the detection error of the system was less than 8.80%.

Demonstration of a superluminal laser using electromagnetically induced transparency in Raman gain.

We report the realization of a superluminal laser in which the dip in the gain profile necessary for anomalous dispersion is produced via electromagnetically induced transparency caused by the optical pumping laser. This laser also creates the ground state population inversion necessary for generating Raman gain. Compared to a conventional Raman laser with similar operating parameters but without the dip in the gain profile, the spectral sensitivity of this approach is explicitly demonstrated to be enhanced by a factor of ∼12.7. Compared to an empty cavity, the peak value of the sensitivity enhancement factor under optimal operation parameters is inferred to be ∼360.

Volatile Species Generation for Trace Element and Speciation Analysis – Current State and Future Perspectives

Volatile Species Generation for Trace Element and Speciation Analysis – Current State and Future Perspectives

Adjustment of eccentricity and split angle for improved SERS and refractive index sensing of TiN nanodonuts

The unique refractory plasmonic properties and strong enhancement of the electric field within the inherent gap of titanium nitride (TiN) nanodonuts make them excellent candidates for surface-enhanced Raman scattering (SERS)- and refractive index (RI)-sensing applications. The eccentricity and split angles are critical parameters for tuning the localized surface plasmon resonance (LSPR) properties of the donuts, which were numerically investigated using the finite element method herein. We demonstrated that the proposed donuts provided efficient SERS and RI sensing substrates capable of working in regions ranging from ultraviolet (UV) to near-infrared (NIR). By adjusting the eccentricity and split angles, the corresponding optimized RI sensitivity and SERS enhancement factor reached 1,374 nm RIU−1 and 6.8 × 104, respectively. Moreover, the effects of both incident polarisation and electromagnetic (EM) field distributions on the LSPR properties were elucidated and discussed. This study provides new insights for understanding the LSPR properties of TiN nanoparticles and enables the rational design of efficient refractory plasmon-based SERS and RI-sensing substrates.

Dominated mode switching and nanoparticle detection at exceptional points

We present a theoretical investigation of dominated mode switching and nanoparticle detection at a wavelength of 2 µm that uses coupled whispering gallery mode (WGM) resonators operating at exceptional points (EPs). The simulations show that with assisted gain and loss the system can be steered to operate at EPs by adjusting the nanoparticle-introduced backscattering. The unbalanced contributions of the clockwise and counterclockwise modes can be observed when the system approaches the EPs, and the dominated mode is switchable by transiting the system from one EP to another. Owing to the square root topology of EPs, the enhanced frequency splitting can be obtained when the system is subjected to a perturbation, which results in a more than five times sensitivity enhancement compared to a traditional sensor. Furthermore, a simulation also reveals that an increased gain–loss contrast in the resonators contributes to an enhanced frequency splitting, and thus a higher sensitivity enhancement factor. Our investigations validate the feasibility of a WGM resonator operating at EPs for unidirectional laser emitting and nanoparticle sensing.

Aluminum Plasmonic Nanoclusters for Paper-Based Surface-Enhanced Raman Spectroscopy.

Although surface-enhanced Raman spectroscopy (SERS) can rapidly identify molecular fingerprints and has great potential for analysis, the need for delicate plasmonic substrates and complex laboratory instruments seriously limits its applicability for on-site detection. This paper describes the development of an inexpensive aluminum nanoparticle (AlNP)-decorated paper that functions as a facile SERS-based detection platform (Al-PSERS). Polydopamine-protected AlNPs were chemically synthesized and then simply drop-cast onto a hydrophobic cellulose paper, forming a monolayer AlNP cluster array. Because of the abundance of hot spots arising from the plasmonic clusters, the inherent quasi-three-dimensional structure of the cellulose fibers, and the concentration effect of the hydrophobic surface, the Al-PSERS provided significant enhancements to the signal of various analytes, measured using a portable 785 nm Raman spectrometer. Near-field optical simulations and experimental spectroscopic results revealed that the local electric fields and corresponding SERS signal intensities of the AlNP array exhibited clear particle-length and cluster-size dependencies. Therefore, the Al-PSERS could be optimized to provide high sensitivity (enhancement factor: 2 × 103) and excellent reproducibility (variation: 8.72%). Moreover, the optimal Al-PSERS was capable of detecting colorants and environmental pollutants; for example, the detection limits of allura red and benzo[a]pyrene reached as low as 3.5 and 0.15 ppm, respectively. Furthermore, the Al-PSERS could rapidly identify illegal (rhodamine B) and edible (allura red, erythrosine) colorants from a mixture of multiple colorants or from adulterated candies. Because it facilitates rapid detection, is of low cost, and has minimal technical requirements, Al-PSERS should be applicable to on-site detection in, for example, food inspection and environmental monitoring.

Induced sample via transient isotachophoresis mediated with sweeping in micellar electrokinetic chromatography for the dual-stacking strategy of non-steroidal anti-inflammatory drugs in environmental water samples

Realising the need to devise a simple, sensitive, and reliable detection method, this study investigated the development of a dual-stacking transient isotachophoresis (t-ITP) and sweeping in micellar electrokinetic chromatography with diode array detector (t-ITP/sweeping-MEKC-DAD) for the determination of selected non-steroidal anti-inflammatory drugs (NSAIDs); ketoprofen, diclofenac and naproxen from aqueous matrices. Prior to the system setup, various parameters were optimised to assess the potential use of the t-ITP paired with the sweeping stacking technique in micellar background electrolyte for dual preconcentration and separation of trace amounts of NSAIDs. Once the optimum conditions have been established, the method performance was validated and applied to 17 environmental water samples. Based on the results, the combined t-ITP and sweeping approach significantly improved the stacking and separation sensitivity. A large volume of samples could also be introduced and subsequently separated by MEKC with greater focusing effects due to the sweeping. Under optimised conditions, the developed method exhibited excellent linearity at a high range (0.1–500 ng/mL, r2 ≥ 0.998), low limits of detection (LODs) of 0.01–0.07 ng/mL, and a remarkable relative recovery (RR) of 99.6–101.9% with a relative standard deviation (RSD) of 1.4–8.6% (n = 9). Ultimately, the sensitivity enhancement factors improved up to 666-fold using the optimised method. Therefore, the proposed method presents a simplified yet effective and suitable for the determination of NSAIDs from aqueous matrices.

Online preconcentration and determination of anthraquinones in Cassiae Semen tea by salting-out assisted liquid‒liquid extraction coupled with dynamic pH junction-sweeping-large volume sample stacking in capillary electrophoresis

A new strategy by coupling salting-out assisted liquid‒liquid extraction with dynamic pH junction-sweeping-large volume sample stacking in capillary electrophoresis was proposed for extraction, online preconcentration, separation and determination of trace anthraquinones in food samples. Based on the complementary advantages of salting-out assisted liquid‒liquid extraction and online capillary electrophoresis stacking, the strategy allowed for rapid extraction and sensitive analysis of anthraquinones in aqueous matrix samples. Under the optimized conditions, the proposed strategy showed good linearity (100–3000 ng/mL, r2 ≥ 0.993) with a low detection limit of 30–40 ng/mL. Sensitivity enhancement factors of up to 468-fold were reached with the optimized dynamic pH junction-sweeping-large volume sample stacking using capillary electrophoresis with a UV detector. As a proof-of-concept study, this strategy was successfully applied for analysing anthraquinones in Cassiae Semen tea, which demonstrated good potential for the efficient and sensitive determination of weakly acidic analytes in complicated sample matrices.

In-line sample concentration in capillary electrophoresis by cyclodextrin to admicelle microextraction

Cyclodextrins (CDs) as a pseudophase in pseudophase-to-pseudophase microextraction (P2ME) in capillary zone electrophoresis (CZE) are proposed. In this P2ME mode called CD to admicelle ME, a long plug of dilute analyte solution prepared in cetyltrimethylammonium bromide (CTAB) at the critical micellar concentration was injected into the capillary. This formed CTAB admicelles at the interface between the solution and the negatively charged capillary surface, where the analytes were trapped. The injection of CD solution released the admicelles and the analytes from the capillary surface due to the formation of stable CD/CTAB inclusion complexes. The analytes are concentrated at the CD front during injection and voltage separation. Various neutral CDs were found to be effective for CD to admicelle ME. To implement this in-line sample concentration technique in CZE, CD concentration, sample injection time, and sample:CD solution injection ratio were optimized. The optimized conditions for five model anionic analytes, namely, 4-bromophenol, sulindac, sulfamethizole, 4-vinylbenzoic acid, and succinylsulfathiazole, were 20 mM α-CD in 20 mM sodium tetraborate (pH 9.2) solution, sample injection time of 370 s, and CD:sample injection ratio of 1:2. The sensitivity enhancement factors (SEFs) were between 112 and 168. The SEFs of sulindac and sulfamethizole in particular were similar to previously published off-line microextraction techniques, which are typically time-consuming. The calculated values of LOQ, intra-/inter-day (n = 6/n = 10, 3 days) repeatability, and linearity (R2) of CD to admicelle ME were 0.0125–0.05 µg/mL, 1.5–4.6%, 1.8–4.8%, and ≥0.999, respectively. Finally, the potential of CD to admicelle ME to the analysis of artificial urine samples was demonstrated.

Ultrasensitive analysis of mirtazapine and its metabolites enantiomers in body fluids using ultrasound-enhanced and surfactant-assisted dispersive liquid–liquid microextraction followed by polymer-mediated stacking in capillary electrophoresis

A simple, rapid, and sensitive technique for measuring mirtazapine and its metabolites enantiomers in human fluids, such as urine and serum, was developed by applying ultrasound-enhanced and surfactant-assisted dispersive liquid–liquid microextraction (USA-DLLME) integrated with poly(diallyldimethylammonium chloride) (PDDAC)-mediated stacking in capillary electrophoresis (CE). The parameters that affect extraction and stacking performance, such as the extraction volume, surfactant types, surfactant concentrations, salt additives, extraction time, solution pH, and background electrolytes, were comprehensively studied and optimized to achieve optimal detection performance. Under optimal extraction conditions (injection of 120 µL of C2H2Cl4 into 1 mL of a sample solution containing 0.05 mM Brij-35 at pH 10.0) and separation conditions (0.9% PDDAC, 10 mM phosphate, pH 3.0, and 20 mM dimethyl-β-cyclodextrin), on-line CE stacking of mirtazapine-related chiral drugs was achieved by the two strategies: (i) neutral DM-β-CD sweep low concentrations of DL-NaSSA and (ii) DL-NASSA is stacked by the difference in the viscosity between the PDDAC and sample zone. An approximately 2,800–4000-fold improvement in detection sensitivity was revealed for mirtazapine, N-demethylmirtazapine, and 8-hydroxymirtazapine enantiomers. The linear ranges for the quantification of all analyte enantiomers were 1.2–150 nM, with a coefficient of determination higher than 0.99; the relative standard deviations in the migration time and peak areas for six analytes were less than 1.8% and 5.8%, respectively. The proposed system provided the limits of detection (signal-to-noise ratio of 3) of the six analytes as 0.3–0.5 nM. The recovery of the six separated analytes spiked in urine and serum samples was revealed to be 82.7%–109.5% and 91%–112.8%, respectively. This advanced technique with high sensitivity enhancement factors was successfully employed to analyze mirtazapine and its metabolites enantiomers in urine and serum samples with reliability.

Facile fabrication and SERS performance of polymer/Ag core-shell microspheres via the reverse breath figure accompanied by in situ reduction

A new facile fabrication process of the polymer/Ag core-shell microspheres is demonstrated via the reverse breath figure method accompanied by in situ reduction. In contrast to traditional approaches, which require the prior preparation of polymer microspheres, the star-shaped polyhedral oligomeric silsesquioxane (POSS) amphiphilic block copolymers solution containing AgNO3 is cast under ethanol vapor for the formation of polymer/AgNO3 core-shell microspheres in one step. The influence of polymer molecular structures, solvent, non-solvent and metal salt on the morphology of polymer microspheres was analyzed. Then the AgNO3 can be reduced in situ to form Ag nanoparticles on the surface of polymer microspheres, which resulted in the formation of polymer/Ag core-shell microspheres. The reduced Ag nanoparticles were homogeneously deposited on the surfaces of polymer microspheres, which can exhibit good performance in the surface-enhanced Raman scattering (SERS). The polymer/Ag core-shell microspheres substrate showed a high detection sensitivity (enhancement factor 2.49 × 106) with excellent uniformity and reproducibility for Rhodamine 6G (R6G) (relative standard deviation of the intensity at 612 cm−1 was 6.19%＜20%). The proposed strategy is facile to implement and thus a promising new method for the preparation of polymer/metal core-shell microspheres.

Determination of pepsin by capillary electrophoresis using mixed polymer coated capillary with switchable properties toward protein adsorption/desorption.

In this work, a simple online preconcentration method for quantitative detection of pepsin was realized by using the binary mixed polymer brushes coated capillary with switchable properties toward protein adsorption. Firstly, the binary mixed polymer brushes were prepared by grafting poly(2-methyl-2-oxazoline) and poly(4-vinylpyridine) onto the inner wall of the capillary through a polydopamine anchor. Then the coatings were characterized by X-ray photoelectron spectrometer and electroosmotic flow measurement. The results indicated that the composition of coating could be controlled by varying the feed ratio of poly(2-methyl-2-oxazoline) to poly(4-vinylpyridine) and the inner surface charge could be tuned toward the change of pH and ionic strength. The results showed that when the poly(2-methyl-2-oxazoline)/poly(4-vinylpyridine) mass ratio was 80/20, the highest online preconcentration effect was obtained and the sensitivity enhancement factor was 6.3. Moreover, satisfactory sensitivity (limit of detection: 7.5ng/mL) and good repeatability were obtained with the online preconcentration method. The polymer-coated capillary was still stable for online preconcentration and detection of pepsin after 50 consecutive runs. Last, the proposed method was used successfully to online preconcentrate pepsin in the saliva matrix.