Plastic vials differ widely in their compositions, without even considering the closures and bottom finishes. The word “plastic” is a generic term used to describe various resins. While many plastic resins are visually similar, their chemical composition and properties are quite different. Plastics are organic polymers formed mostly from natural gases, petroleum or coal. The small units that make up these polymers are monomers build on the carbon atoms. For some plastics, such as polypropylene, the repeat unit is just one carbon atom and three hydrogen atoms. Other plastics may include other atoms such as oxygen and/or nitrogen.
To add complexity, after considering resin types additives and their effects must be considered. Different resin manufacturers include different additives to the formulations to alter and improve basic mechanical, physical or chemical properties. Additives may protect the polymer from the degrading effects of light, heat, or bacteria. Some additives will change the polymer’s properties such as color and other special characteristics such as improved surface appearance, reduced friction and flame retardancy. Additives may increase the risk of chemical interactions between the vial and its contents. It is important to know what additives and resins a vial manufacturer uses to prevent these interactions, as they can contaminate samples over time. 
The key message is do your research when shopping for plastic vials. Ask the manufacturer’s sales representative or product specialist about the resin used in their vials. Ask if it’s low in trace metals, phthalates or other additives. Ask if they have a consistent resin that they use, or if the use whatever resin that is available. It is important to know what you’re putting your sample into and how that may skew your results.
- Know your vial manufacturer
- Ask about additives, trace metals and/or contaminates
Below is a link to the most common types of plastic available.
Plastic Packaging Resins
 American Chemistry Council, Plastics Industry Producer Statistics Group, 2005