Why you must cautious while using water as a diluent in GC-HS Technique

 Water is widely used as a diluent during GC analysis due to its compatibility with packed as well as capillary columns. In addition, almost all residual solvents give highest response when analyzed using GC-HS with water as a diluent. If you have also noticed the same- please share your experience. The reason behind high response for residual solvents with water as diluent is the difference in polarity of water and organic solvents. Most of the organic solvents are non-polar –where as water is highly polar. Due to difference in polarity, high amount of organic solvent leaves water and become available in the headspace of the GC vial. Further the high amount of organic solvent’s vapor gets injected into GC column and result into higher response.

It has been also seen that sodium sulphate is added during standard and sample preparation when water is used as a diluent. The addition of sodium sulphate further saturates water and helps in taking out organic solvent from solution and eventually results into higher response.

The only challenge I found if water alone is used as a diluent - and it is high variability. I often found higher %RSD for standards when water alone is used as a diluent. However, such variability can be minimized by using hydro-organic solvent mixtures. For example, Water: DMF or Water: DMSO. The addition of organic solvent as a part of diluent helps in reducing variability with marginal reduction in response. Overall, the hydro organic mixture is the good bet. The usage of hydro-organic solvent mixture further helps in solubilization of highly non-polar organic solvents (such as Methylene Dichloride, Chloroform and Carbon tetrachloride) during standard solution preparation.

Hence, instead of water, it is prudent to use hydro-organic solvent mixtures as a diluent in case of GC-HS Technique. I look forward to see your comments…

I also urge you to join me on my upcoming webinar for career related guidance and information.

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