On-Line 1D and 2D Porous Layer Open Tubular/LC-ESI-MS Using 10-μm-i.d. Poly(styrene−divinylbenzene) Columns for Ultrasensitive Proteomic Analysis

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Abstract
Following on our recent work, on-line one-dimensional (1D) and two-dimensional (2D) porous layer open tubular/liquid chromatography-electrospray ionization-mass spectrometry (PLOT/LC-ESI-MS) platforms using 3.2 m × 10 μm i.d. poly(styrene−divinylbenzene) (PS−DVB) PLOT columns have been developed to provide robust, high-performance, and ultrasensitive proteomic analysis. With the use of a PicoClear tee, the dead volume connection between a 50 μm i.d. PS−DVB monolithic micro-SPE column and the PLOT column was minimized. The micro-SPE/PLOT column assembly provided a separation performance similar to that obtained with direct injection onto the PLOT column at a mobile phase flow rate of 20 nL/min. The trace analysis potential of the platform was evaluated using an in-gel tryptic digest sample of a gel fraction (15−40 kDa) of a cervical cancer (SiHa) cell line. As an example of the sensitivity of the system, ∼2.5 ng of protein in 2 μL of solution, an amount corresponding to 20 SiHa cells, was subjected to on-line micro-SPE-PLOT/LC-ESI-MS/MS analysis using a linear ion trap MS. A total of 237 peptides associated with 163 unique proteins were identified from a single analysis when using stringent criteria associated with a false positive rate of less than 1%. The number of identified peptides and proteins increased to 638 and 343, respectively, as the injection amount was raised to ∼45 ng of protein, an amount corresponding to 350 SiHa cells. In comparison, only 338 peptides and 231 unique proteins were identified (false positive rate again less than 1%) from 750 ng of protein from the identical gel fraction, an amount corresponding to 6000 SiHa cells, using a typical 15 cm × 75 μm i.d. packed capillary column. The greater sensitivity, higher recovery, and higher resolving power of the PLOT column resulted in the increased number of identifications from only ∼5% of the injected sample amount. The resolving power of the micro-SPE/PLOT assembly was further extended by 2D chromatography via combination of the high-efficiency reversed-phase PLOT column with strong cation-exchange chromatography (SCX). As an example, 1071 peptides associated with 536 unique proteins were identified from 75 ng of protein from the same gel fraction, an amount corresponding to 600 cells, using five ion-exchange fractions in on-line 2D SCX-PLOT/LC-MS. The 2D system, implemented in an automated format, led to simple and robust operation for proteomic analysis. These promising results demonstrate the potential of the PLOT column for ultratrace analysis.

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