Linearity of Toluene

[dpogre]

Hi, I ran into a situation with Toluene while running method development for linearity. I found that it is slightly soluble in water. (Solubility in water 0.47 g/L), so i prepared my sample using a stock 30 mg into 100 mL. My reference standards are within the acceptance criteria of 85 % - 115% each time i do a check back however, linearity doesnt seem to work out. I'm getting r = 0.973 but i need r = 0.990. 80%, 100% and 120% are the concentrations that dont fall on the linear line. I'm using an Agilent Durabond DB-WAX 30m x 0.530mm, 1 ufilm.
What diluent should i use then? How can i improve the linearity.

Thank
Dina

[chromatographer1]

Your problem is sample preparation (I think).

Try using hexane or pentane instead of water. Fill your std vial-flask with a minimum of headspace.

Toluene partitions from water at room temperature VERY QUICKLY, and opening the vial-flask changes the concentration in the water also VERY QUICKLY.

Toluene gave me a linear response from 1ng per microliter to 10,000ng per microliter, that is R = 0.99998 in validated methods.

So it is all in the preparation.

Good luck,

Rod

[dpogre]

Thank you for getting back to me. I did it once in purified water and i got r = 0.991 but that was toluene on its own in purified water. This time i was trying to do acetone and toluene together. I'm using 20 mL headspace vials and fill 2 mL. I've been very careful with the preparation, but i was also wondering if vortexing the solution can change the concentration that is extracted with the 2 mL volumetric pipette. Should i not vortex and just keep the solution as is when extracting? I'm really hoping to make it work in purified water or i'm going to have to start over again. the range i'm looking for is 18 ppm - 72 ppm.

Thanks
Dina

[chromatographer1]

Pipettes? Never, never, use them.

Syringes yes.

I spiked my pure water with toluene by adding a very tiny consistent amount of DMAc which contained various amounts of toluene.

ANALYTICAL CHEMISTRY jUNE 1995 p.2221

Using less sample works better than 2mL ! ! !

I could see EASILY, 1ng of toluene in 1mL of waer, that is 1 ppb wt/wt with an R squared of 0.9995 or better.

You need to see 1.8 to 7.2 micrograms of toluene in 100 microliters of water. Piece of cake. Add 10 microliter samples of DMAc which contain 1 to 10 micrograms of toluene. (0.100 mg per mL - 1.000 mg/mL) Equilbrate for 12 minutes at 85C with shaking.

Don't use butyl rubber septa or prepare your samples more than 1 hour before use. The DMAc stock can be used a week of more with careful storage and sealing.

best wishes,

Rod