For accuracy in GC analysis, calibration involving an internal standard is recommended. What's the common practice in preparing calibration mixtures with internal standards?

To elaborate some more: do i prepare calibration mixtures at different conc. levels, but with equal amounts of calibrant and internal standard in each level (i.e internal std varied). Or, prepare calibration mixtures with calibrants at diffent conc. with conc. of internal std fixed regardless of the calibrant conc. (i.e internal std kept constant).

Joe,
The analysis C1 to C4 Alcohols and MTBE in gasoline utilizes the internal standard EGME(ethylene glycol dimethyl ether). Here's how I prep the standard curve. First off, the C1-C4 alcohols are prepared by weighing each alcohol into a tared flask of toluene as a stock solution.
I prepare standards in this manner. Weigh a volumetric flask with 3/4 toluene(tare), add 3 mL of EGME, weigh(wt IS)for each standard to be prepared. Add a volume of MTBE, weigh. Add a volume of alcohol stock and weigh.
Weighing vs. volumetric depends on the samples. Since hydrocarbon volumes vary according to temperature, weighing is more accurate.

The standards are

Joe,

The easiest way to ensure your internal standard is constant for all calibration standards and samples is to put it into the diluent/extraction solution or whatever it is you use to prepare all final dilutions with.

It is better to have the concentration of internal standard the same in all the standards. The constant concentration of internal standard then allows the determination of response ratio vs. concentration ratio. Most GC data systems are set up to use this as the default for internal standard quantitation, with the assumption made that IS concentration is constant. Constant IS concentration also allows monitoring of system performance, i.e. large changes in response indicate system problems.