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How may I improve the recovery of organic volatiles during a GC headspace analysis of drinking water?
Headspace sampling from drinking water is typically performed by first equilibrating the vial at a temperature of 80 °C, as a higher temperature would generate excessive pressure within the vial with respect to the atmospheric boiling point of water. The first consideration is the duration of the equilibration to assure that analytes volatilize consistently, and these equilibration times are often around 30 – 45 min. Be mindful that low-boiling polar analytes, such as methanol, will still exhibit hydrogen-bonding with water, and will need that long equilibration to assure of volatilization. You may also add a kosmotropic salt, such as magnesium sulfate (MgSO4) to disrupt the ability of water to solvate your targeted volatile organic analytes, facilitating the volatility of these anlaytes.
Why do my retention times for PFAS analytes begin to shift earlier after many consecutive injections during an HPLC sequence?
PFAS compounds have properties that can both streamline and complicate an LC analysis. On the one hand, a PFAS with a carboxylate or sulfonate head will essentially always be deprotonated when working within typical LC reversed phase conditions. The electron-withdrawing capacity of the fluorines along the hydrophobic tail will significantly lower the pKa of a carboxylatis acid head to the point of essentially acting as a strong acid. The challenge is that the structure of a PFAS is comparable to an ion-pair reagent, consisting of an anionic head and a long perfluorinated carbon tail that is exceedingly hydrophobic. Consecutive injections of PFAS samples on a C18 column run the risk of residual amounts of analyte remaining in the stationary phase, such that the hydrophobic tail of the PFAS is burrowed into the hydrophobic C18 phase, while the ionic head faces the mobile phase. The accumulation of charge (negative in this instance) within the mobile phase will lower the hydrophobicity of the C18 phase, particularly as the anionic heads of PFAS compounds repel one another. This may be addressed by implementing a routing cleaning method using pure solvents intermittently during a prolonged sample sequence.
Zebron™ ZB-PAH-EU and ZB-PAH-CT GC Columns Guide