the preparation of gas standards require a lot of accuracy. As an example you can read the following link. Maybe contact your gas suppliers for more information.
For injecting you can use a gastight syringe. (e.g. Headspace syringe with valve, www.sge.com ,article number 008775). http://www.sge.com/products/syringes/special-purpose/biotechnology-syringes/headspace/soil-gas-syringes