Peak focusing based on stationary phase thickness gradient.

This paper reports the development of a stationary phase thickness gradient gas chromatography (GC) column that enables analyte peak focusing and improves separation resolution. Theoretical analysis and simulation demonstrate focusing via a positive thickness gradient, i.e., the stationary phase thickness increases along the column. This effect was experimentally verified by coating a 5 m long capillary column with a film thickness varying from 34 nm at the column inlet to 241 nm at the column outlet. The column was analyzed in forward (thin to thick) and backward (thick to thin) modes and compared to a uniform thickness column with a thickness of 131 nm, using alkanes ranging from C5 to C16 and aromatics. Comparison of resolutions between forward mode and the uniform thickness column demonstrated an overall focusing rate (i.e., improvement in peak capacity) of 11.7% on alkanes and 28.2% on aromatics. The focusing effect was also demonstrated for isothermal room temperature separation of highly volatile compounds and temperature programmed separation with different ramping rates. In all cases, peak capacities from forward mode separations are higher than those from other modes, indicating the ability of a positive thickness gradient to focus analyte peaks. This thickness gradient technique can therefore be broadly applied to various stationary phases and column types as a general method for improving GC separation performance.

Microfabricated porous layer open tubular (PLOT) column.

Development of micro gas chromatography (µGC) is aimed at rapid and in situ analysis of volatile organic compounds (VOCs) for environmental protection, industrial monitoring, and toxicology. However, due to the lack of appropriate microcolumns and associated stationary phases, current µGC is unable to separate highly volatile chemicals such as methane, methanol, and formaldehyde, which are of great interest for their high toxicity and carcinogenicity. This inability has significantly limited µGC field applicability. To address this deficiency, this paper reports the development and characterization of a microfabricated porous layer open tubular (µPLOT) column with a divinylbenzene-based stationary phase. The separation capabilities of the µPLOT column are demonstrated by three distinct analyses of light alkanes, formaldehyde solution, and organic solvents, exhibiting its general utility for a wide range of highly volatile compounds. Further characterization shows the robust performance of the µPLOT column in the presence of high moisture and at high temperatures (up to 300 °C). The small footprint and the ability to separate highly volatile chemicals make the µPLOT column highly suitable for integration into µGC systems, thus significantly broadening µGC's applicability to rapid, field analysis of VOCs.

Analysis of trace dicyandiamide in stream water using solid phase extraction and liquid chromatography UV spectrometry.

An improved method for trace level quantification of dicyandiamide in stream water has been developed. This method includes sample pretreatment using solid phase extraction. The extraction procedure (including loading, washing, and eluting) used a flow rate of 1.0mL/min, and dicyandiamide was eluted with 20mL of a methanol/acetonitrile mixture (V/V=2:3), followed by pre-concentration using nitrogen evaporation and analysis with high performance liquid chromatography-ultraviolet spectroscopy (HPLC-UV). Sample extraction was carried out using a Waters Sep-Pak AC-2 Cartridge (with activated carbon). Separation was achieved on a ZIC(®)-Hydrophilic Interaction Liquid Chromatography (ZIC-HILIC) (50mm×2.1mm, 3.5µm) chromatography column and quantification was accomplished based on UV absorbance. A reliable linear relationship was obtained for the calibration curve using standard solutions (R(2)>0.999). Recoveries for dicyandiamide ranged from 84.6% to 96.8%, and the relative standard deviations (RSDs, n=3) were below 6.1% with a detection limit of 5.0ng/mL for stream water samples.