Restek’s Certificates of Analysis, or CofAs, offer comprehensive information to help you understand the composition, proper storage conditions, and method of shipment for your reference standard. Some customers also require “uncertainty” values for their analyses, so we include “certified values” that include “expanded uncertainty.” In this article, we’ll explain these terms, discuss when you might need to apply uncertainty values, and clarify why we provide them. We’ll also detail the differences between storage conditions and shipping conditions along with what else you need to keep in mind to ensure the long-term integrity of your standard.
In reference standards, “uncertainty” is a quantitative measurement that determines the quality of a standard. It’s used to provide a margin of error that could potentially apply to the product.
It’s important to note, however, that unless your analysis results require the presentation of uncertainties, it may not be necessary for you to use the uncertainty values provided on your CofA.
To explain certified values, we’ll examine this multiresidue pesticide reference standard (cat.# 31976) certificate of analysis and focus on its first eluting compound, thiabendazole.
|Elution Order||Compound||Grav. Conc.
(95% C.L; K=2)
|1||Thiabendazole||100.0 µg/mL||+/- 0.7091 µg/mL Gravimetric|
|CAS # 14879-8||(Lot 8881400)||+/- 4.6114 µg/mL Unstressed|
|Purity 99%||+/- 4.7285 µg/mL Stressed|
On the CofA, there is a “Certified Values” table. These values are labeled “certified” because the standard has been manufactured with its uncertainties calculated and presented in adherence to our ISO accreditations.
Certified values include the standard’s gravimetric concentration as well as its expanded uncertainty.
This value represents the actual concentration of the compound in the ampul.
For our above compound, the gravimetric concentration is 100.0 µg/mL.
Most of our customers use only gravimetric concentration for their analyses. If the results of your analysis do not require the use or presentation of uncertainties, you can skip the Expanded Uncertainty sections below and continue on to the Homogeneity, Storage, and Shipping sections.
Expanded Uncertainty takes a closer look at the gravimetric concentration and provides a detailed breakdown of several uncertainties that your ampul may be subject to. These uncertainty values will be needed if the results of your analysis require their presentation.
There are three expanded uncertainty components that we provide on our CofAs:
Gravimetric Expanded Uncertainty
This value contains all uncertainty from the physical making of the standard. It includes balance uncertainty, volumetric and pipette uncertainty, and purity uncertainty.
For our above compound, the gravimetric uncertainty is +/- 0.7091 µg/mL.
Unstressed Expanded Uncertainty
This value represents combined uncertainties present within your reference standard, including transit from Restek to your lab under standard shipping conditions (explained below).
unstressed expanded uncertainty = gravimetric uncertainty + homogeneity uncertainty + storage uncertainty
For our above compound, the unstressed expanded uncertainty is +/- 4.6114 µg/mL.
Stressed Expanded Uncertainty
This value (also referred to as “combined stressed uncertainty”) represents the uncertainties present within your reference standard, including transit from Restek to your lab under nonstandard shipping conditions (explained below).
stressed expanded uncertainty = gravimetric uncertainty + homogeneity uncertainty + storage uncertainty + shipping uncertainty
For our above compound, the stressed uncertainty is +/- 4.7285 µg/mL.
To further explain, we also display a formula on the CofA that shows the calculation we used to determine the stressed expanded uncertainty:
Where “k” is a coverage factor of 2, which gives a level of confidence of approximately 95%.
Note: You do not need to apply or use this formula itself—we already provide these uncertainties values on our CofAs. We only display the formula to show how the combined stressed value was calculated.
Homogeneity, Storage, and Shipping
Beyond the factors contributing to gravimetric expanded uncertainty, unstressed and stressed expanded uncertainties also include these additional factors:
Homogeneity – are all ampuls from the batch the same? For example, if we place 1 mL of solution into 50 ampuls, would each ampul have the same concentration of every compound?
Storage – is your standard concentration the same from the first day it was made up to the expiration date on the label?
Shipping (applies only to stressed uncertainty) – this is an assessment of the worst-case scenario the standard experienced during shipping. Essentially, we’re asking the question: what is the hottest temperature the ampul experienced and how long did this last? For this, we are evaluating whether the integrity of the standard is the same before and after shipment.
The figure below shows the relationships between gravimetric concentration as well as the expanded uncertainty values for gravimetric uncertainty, unstressed uncertainty, and stressed uncertainty.
Storage Conditions vs. Shipping Conditions
To determine the uncertainty of your standard, it’s helpful to understand the difference between storage conditions and shipping conditions.
If we continue with our multiresidue pesticide reference standard (cat.# 31976) as an example, its CofA (shown in the above image) has fields for both the storage conditions and the shipping conditions.
The storage field refers to the temperature that the product should be stored at once you receive it to guarantee its uncertainty, concentration, and expiration. This does not mean the ampul must remain at or below this temperature during shipment from Restek to you.
The shipping field refers to the packaging conditions that the product can be shipped in and still maintain stability. Most—approximately 98%—of all our reference standards will remain stable when shipped under “ambient” conditions. The remaining 2% of our reference standards require “on ice” packaging. These are the only two options for this field.
Our reference standards have been tested under sustained temperature extremes for specific time intervals—extremes that most ampuls will never experience during transit. In the table below, included on every CofA, we show the label conditions, standard shipping conditions, and nonstandard shipping conditions.
|Label Conditions||Standard Conditions||Nonstandard Conditions|
|25 °C Nominal (Room Temperature)||< 60 °C||≥ 60 °C up to 7 days|
|10 °C or colder (Refrigerate)||< 40 °C||≥ 40 °C up to 7 days|
0 °C or colder (Freezer)
-20 °C or colder (Deep Freezer)
|< 25 °C||≥ 25 °C up to 7 days|
Standard Shipping Conditions
If you receive your standard under standard shipping conditions, you should apply the Unstressed Uncertainty Value to your standard.
Our example reference standard has a label condition of 10 °C or colder. Under standard temperature conditions, the standard may experience up to 40 °C temperatures with no adverse effects expected.
Nonstandard Shipping Conditions
If you receive your standard under nonstandard shipping conditions, you should apply the Stressed Uncertainty Value to your standard.
Our example reference standard has a label condition of 10 °C or colder. Under nonstandard temperature conditions, the standard may experience temperatures equal to or greater than 40 °C temperatures for up to 7 days with no adverse effects expected.
Standard vs. Nonstandard Shipping Conditions
In our experience, it’s highly unlikely for a standard to experience extreme temperatures that are sustained throughout transit. Therefore, it’s highly unlikely that you will need to apply the Nonstandard Shipping Conditions when calculating the uncertainty of your standard.
If your shipment was in transit for more than seven days and you have reason to believe it exceeded the nonstandard temperature conditions, Contact Restek Technical Service at www.restek.com/contact-us for recommendations.
We hope the information contained in this article has helped you to better understand uncertainty, storage, and shipping conditions for your Restek certified reference standards. If you have any questions, we recommend reviewing the Reference Standard FAQ or contacting Restek’s Technical Service at www.restek.com/contact-us