Whether you’re characterizing a one-of-a-kind sample or performing routine analysis on established products, the accuracy of your multi-detector GPC/SEC data relies on your dn/dc value. Fortunately, the OMNISEC system and its OMNISEC software offer different ways to determine and implement a dn/dc value.

The available analysis type options dealing with dn/dc are:

  • Calculate dn/dc from sample concentration
  • Calculate concentration from sample dn/dc
  • Calculate concentration from method (fixed) dn/dc
  • Calculate dn/dc using the RI response and concentration from UV response (a known dA/dc is required)

As you can see from the list, the software presents multiple options so you can choose what works best for your samples. All these options are based on knowing two of the following three parameters and using the relationship presented below to solve for the third.

The following sections offer a brief description of the analysis type options and when they would be applicable.

Calculate dn/dc from sample concentration

This analysis type assumes that a 100%, quantitative recovery of the sample to determine the dn/dc value. The software accepts that the total amount of sample injected, based on the input concentration and injection volume, is responsible for the entire refractive index (RI) response designated by the limits and baseline. From there, the dn/dc value is determined and used in combination with other detector responses to calculate molecular weight, intrinsic viscosity and other molecular parameters. This method is ideal if you don’t have an established dn/dc value for your sample and you are confident in the sample’s concentration.

Embedded below is a demonstration of how to use this method to calculate the dn/dc value for your sample.

Calculate concentration from sample dn/dc / from method (fixed) dn/dc

These analysis types are grouped together because they both use a pre-programmed dn/dc value along with the observed RI response to determine the concentration of the sample. The difference between the two methods is where the dn/dc value is set.

The sample dn/dc is set in the Sample type database and then assigned to the sample in the sequence. This method works well when running a frequently analyzed sample type in which the dn/dc value is known and has been programmed. An advantage of this option is that multiple sample types, each with different dn/dc values, can be analyzed using the same method.

The method (fixed) dn/dc value is set in the method and the same value is applied to all samples analyzed with that method. This analysis type is useful when analyzing a sample using different dn/dc values, as it allows easy creation of method iterations with different dn/dc values.

In both of these analysis types, that feature an established dn/dc value, the OMNISEC software will compare the calculated sample concentration to the input concentration to provide a % recovery.

These three analysis types are the most common and are described in the video below.

Calculate dn/dc using the RI response and concentration from UV response

This analysis type is relevant for users working with samples in which the concentration is unknown, but the dA/dc value is known. Frequently used with protein samples, this analysis type uses the known dA/dc value and the observed UV response to determine the sample concentration. The combination of this concentration with the observed RI response allows the software to solve for the dn/dc value of the sample.

Regardless of what type of sample you’re working with, the OMNISEC software has the tools to ensure you’re obtaining the most accurate data possible. Please contact us with questions and to find out more!

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