Shelf Life vs. Expiration Date of a Chemical Standard
The integrity of an aqueous trace metals standard is dependent upon:
- The chemical stability of the standard.
- Transpiration losses of the standard.
- The "human factor" while using the standard.
Shelf LifeThe shelf life of aqueous trace metals standards is dependent upon numbers 1 and 2 above. Shelf Life is the amount of time that a properly packaged and stored standard will last without undergoing chemical or physical changes, remaining within the specified uncertainty. A change greater than that uncertainty (±0.5% relative for our standards) means the standard has gone over (passed) its shelf life.
Inorganic Ventures manufactures single-element standards to be chemically stable indefinitely. Our chemists have been checking and testing standards for almost 20 years. Inorganic Ventures can state with certainty that there are no chemical stability problems that have not been solved. Number 1 above has been eliminated in our facility.
All standards have a limited shelf lifeA standard's finite shelf life is caused by transpiration (number 2 above). The entire chemical standard industry suffers from transpiration loss. Inorganic Ventures's scientists have studied these losses over a period of several years. Figure 1 below provides a brief presentation of our transpiration data.
| 125 mL Bottles|
500 mL Bottles
Our studies, performed on our 500 mL and 125 mL LDPE bottles, showed the following:
- Closed but untaped 500 mL bottles have a shelf life of 4 years.
- Closed but untaped 125mL bottles have a shelf life of 21 months.
- Transpiration loss occurs mainly around the cap circumference and not through the container walls.
- There is no difference between the transpiration loss of water versus hydrochloric or nitric acid aqueous solutions.
- The shelf life can be accurately predicted from the ratio of the cap circumference to the surface area of the solution exposed just below the head space.
- Transpiration loss is linear with time.
Inorganic Ventures purchases NIST SRMs that come packaged in 60 mL HDPE bottles. The cap circumference to volume ratio predicts a shelf life of up to one year. NIST has reinforced this fact, stating, "The limit on the validation period is due to transpiration of the solution... A one year shelf life can only be justified." — source
Expiration DatesA standard's expiration date is dependent upon numbers 1, 2, and 3 above. Inorganic Ventures has eliminated number 1 and greatly reduced number 2. This leaves the "human factor" (number 3). Unfortunately, this is the one element that simply can't be controlled.
Expiration dates should never exceed a yearThe expiration date of a standard is defined as the amount of time that it should remain in use after opening. Eventually, human error will contaminate and/or greatly devalue a standard. Most federal and state regulatory agencies recommend expiration dates no longer than one (1) year. Stricter agencies require expiration dates of half that time.
When you use a standard for longer than a year, you are gambling that absolutely nothing has inadvertently affected the chemical components.
Why is the "human factor" so dangerous?Once a bottle is opened, the "human factor" can cause:
- Contamination of standards from pipet tips, volumetric glassware, and/or switched bottle caps.
- Loosely screwed caps that allow head space to vent.
- Contamination of contents by dust and/or vapors.
- Solution to be poured back into the wrong bottle.
- Contamination by the "wrong" packaging container.
- Solution to be accidentally spilled.
Misleading the ChemistA manufacturer ran an advertisement making an eighteen (18) month accuracy and stability guarantee for their standards. Is this company referring to expiration date or shelf life? If shelf life, it is possible, albeit misleading. Their product may be able to remain accurate and stable while sitting on a shelf for eighteen months. However, because of the "human factor", an error can occur after it leaves their facility. Guaranteeing an eighteen month expiration date can damage your laboratory's reputation.
Another manufacturer has used the elusive term "18 Month Stability Dating". What is stability dating? Are they talking about expiration date or shelf life? These issues should be made clear so that consumers don't have to decipher the meaning behind every claim.