大體積樣品堆積結合掃掠式線上濃縮

大體積樣品堆積結合掃掠式線上濃縮法改善了CSEI-sweeping- MEKC 以電壓注入導致電性不同的分析物進入毛細管中濃度不一的情

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分析物因為處在不同的pH 值緩衝溶液中，所以帶的電荷數也不 同，造成遷移速度的改變。例如酚類化合物在高pH 值的環境中是帶負 電荷的，而在低pH 的環境則為中性物質，此時若施加正向電壓，當分 析物由低pH 緩衝溶液進入高 pH 緩衝溶液時，會因為帶有負電荷而與 出口端的負極電性相斥，使得移動速度變慢，後方分析物會追趕上前方 分析物，形成樣品堆積。

界面活性劑微胞的種類繁多，當其與分析物結合之後，會改變分析 物的電性及遷移速度。最常應用在掃掠式線上濃縮技術，藉由微胞的作 用使線上濃縮技術能擴展至中性不帶電的分析物。

利用分析物所帶的電荷與電極相斥，導致分析物遷移速度小於電滲 流，而產生樣品堆積濃縮效應。當分析物帶負電荷，在施加負向電壓時，

樣品基質會隨電滲流而流出毛細管，帶有負電荷的分析物因移動速度慢 而堆積於毛細管末端，此時再施加正向電壓，分析物便會往出口端移 動。以上四點為分析物產生速度改變的基本原理，線上濃縮技術大都利 用上述一至兩種基本原理進行設計。而每種濃縮技術適用的分析物都不 盡相同，可依據Figure 2.13 所整理的，為分析物選擇合適的線上濃縮技 術。

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Figure 2.1 Schematic diagrams of the FASS model [28].

(A) The capillary is conditioned with a BGS (a high conductivity buffer), the sample, prepared in a low-conductivity matrix, is then injected to a certain

length, and a high positive voltage is applied; (B) focusing of the analytes occurs near the boundaries between the sample zone and the BGS; (C) stacked analytes are separated by the CZE mode.

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Figure 2.2 Schematic diagrams of the LVSS model [28].

(A) The capillary is conditioned with a BGS (a high conductivity buffer), the sample, prepared in a low-conductivity matrix, is then injected to a certain length, and then a high negative voltage is applied; (B) the anionic analytes move toward the detection end (outlet) and stack at one side of the boundary, whereas the cations and neutral species move and exit the capillary at the

injection end (inlet); (C) when the electrophoresis current reaches 95-99% of its original value, and the polarity is quickly switched to positive polarity; (D) the following separated by CZE mode.

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Figure 2.3 Schematic diagrams of the pH-mediate stacking model [19].

(A)The analytes dissolved in a high-conductivity matrix are electrokinetic injected into the capillary; (B) a plug of strong acid is also injected, and a positive separation voltage is applied; (C) the strong acid titrates the sample zone to neutral, creating a high resistance zone; (D) causing the ions to migrate faster and become stacked; (E) the electrophoretic separation proceeds through the remainder of the capillary.

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Figure 2.4 Schematic diagrams of sweeping with anionic micelles model [40].

(a) Injection of sample (S) prepared in a matrix having a conductivity similar to that of the BGS; (b) application of voltage at negative polarity, micelles

emanating from the cathodic side sweeping analyte molecules; (c) the injected analyte zone is assumed completely swept.

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Figure 2.5 Schematic diagrams of sweeping with cationic micelles model [43].

(A) The capillary is initially filled with BGS; (B) A sample zone containing nonmicellar sample matrix or water is injected for a period much longer than usual. By application of voltage at negative polarity the electroosmotic flow is directed toward the anode (as cationic micelles is adsorbed on the capillary wall) and the micelles migrate toward the cathode; (C) The separation is then achieved by MEKC.

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Figure 2.6 Sweeping of a charged analyte in electrokinetic chromatography with a neutral pseudostationary phase [47].

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Figure 2.7 Evolution of micelles and neutral analyte molecules during sweeping in the presence of high EOF [21].

(A) Starting situation, injection of S prepared in a matrix having a conductivity similar to that of the BGS; (B) application of voltage at positive polarity,

micelles emanating from the cathodic side sweeping analyte molecules; (C) the injection analyte zone is assumed completely swept.

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Figure 2.8 Schematic diagrams of a dynamic pH junction model [28].

(A) The capillary is filled with a high pH-BGS and a section of low pH-sample solution; (B) a high positive voltage is applied; (C) the anionic analytes are focused on the boundary of the pH junction; (D) separation occurs by the CZE mode.

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Figure 2.9 Schematic diagrams of a reversed dynamic pH junction model [27].

(A) Capillary is conditioned with a BGS (pH 4.5), then the analyte prepared in sample matrix (pH 2.0) is injected; (B) focusing of the analyte occurs because of its mobility changes in two zones; (C) focusing analyte zone migrates

independently of the sample matrix.

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Figure 2.10 Evolution of analyte zones in CSEI-sweep-MEKC [23]:

(A) Starting situation, conditioning of the capillary with a nonmicellar background buffer, injection of a high-conductivity buffer void of organic solvent, and injection of a short water plug; (B) electrokinetic injection at positive polarity (FESI) of cationic analytes prepared in a low-conductivity matrix or water, then cationic analytes focus or stack at the interface between the water zone and high-conductivity buffer; (C) injection is stopped and the

micellar background solutions are placed at both ends of the capillary; (D)

application of voltage at negative polarity that will permit entry of micelles from the cathodic and sweep the stacked and introduced analytes to narrower bands;

(E) separation of zones based on MEKC.

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Figure 2.11 Schematic illustration of the ASEI-sweep-MEKC model [26].

(a) After filling the capillary with low-pH nonmicellar electrolyte, a water plug is injected into the capillary; (b) negative voltage is applied, and the sample is electrokinetically injected into the capillary. Due to the high electric field, the anions move rapidly toward the outlet. At the same time, the water plug is moving out of the inlet of the capillary; (c) when the sample anions enter the boundary of water and low-pH BGS, they are neutralized and cease moving. A focused sample zone is formed (shaded area A); (d) injection is halted and both vials at inlet and outlet are changed to low-pH micellar BGE; (e) negative potential is applied and sweep the focused sample zone as a narrow band; (f) subsequent separation is achieved under MEKC mode.

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Figure 2.12 Schematic diagrams of dynamic pH junction-sweeping model [28].

(A) The micellar (such as SDS) BGS and the sample solution (a nonmicellar buffer) are injected into the capillary, respectively; (B) a positive polarity is applied to power the CE separation; (C) the neutral analytes are converted to anions and are swept by the SDS micelles; (D) separation occurs by the MEKC mode.

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Figure 2.13 The choice of on-line concentration techniques [55].

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