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Static headspace (HS) sampling has been commonly used to test for volatile organic chemicals, usually referred to as residual solvents (RS) in pharmaceuticals. If the sample is not soluble in water, organic solvents are used. However, these seriously reduce the sensitivity in the determination of some RS. Here, mixed aqueous dilution media (a mixture of water and an organic solvent like dimethyl formamide, dimethyl sulfoxide or dimethyl acetamide) were studied as alternative media for static HS-gas chromatographic analysis. Although it has been known that mixed aqueous dilution media can often improve sensitivity for many RS, this study used a systematic approach to investigate phase volumes and the organic content in the HS sampling media. Reference solutions using 18 different class 1, 2 and 3 RS were evaluated. The effect of salt addition was also studied in this work. A significant increase in the peak area was observed for all RS using mixed aqueous dilution media, when compared with organic solvents alone. Matrix effects related to the mixed aqueous dilution media were also investigated and reported. Repeatability and linearity obtained with mixed aqueous dilution media were found to be similar to those observed with pure organic solvents.

Quadrupole mass spectrometry with electron ionization (EI-QMS) is a very popular detection technique in combination with gas chromatography. It is deployed for the analysis of volatile and semivolatile analytes in many industry domains. Although a very important factor for quantitative analysis, little is known about the stability of ion source performance. Only a few papers and patents report possible signal instabilities due to sample adsorption, degradation, or insulating deposits on the hot stainless steel surface of the ion source. In this study, a conventional stainless steel ion source was used to investigate possible signal drifts. It was observed that the EI-QMS instrument indeed suffered from continuous signal instability. It was found that the key parts which are responsible for the signal instabilities are those that regulate the ion beam toward the mass analyzer: the repeller, exit plate, and focusing lenses. The voltage of the repeller was found to have a major influence on the signal stability. The surface of the repeller, exit plate, and focusing lenses was modified by applying a gold coating. It was demonstrated that the signal stability of the MS dramatically improved when using the gold-coated parts. The contribution of each part to the stability improvement was quantitatively determined and compared with the standard stainless steel source performance. It was assumed that the signal drift observed with the stainless steel EI source originated from charge buildup on the surfaces. This hypothesis was supported by software simulations.

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