> For the complete documentation index, see [llms.txt](https://knowledge.illumina.com/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://knowledge.illumina.com/library-preparation/dna-library-prep/library-preparation-dna-library-prep-faq-list/000001258.md).

# Library QC for the Illumina DNA Prep (M) Tagmentation Kit

This FAQ addresses frequently asked questions regarding Illumina DNA Prep library evaluation. See the [Reference Guide](https://support.illumina.com/downloads/illumina-dna-prep-reference-guide-1000000025416.html) for more information. **Which method is recommended to quantify final libraries prior to sequencing?**\* When using DNA inputs of 100-500 ng, the normalization features of this library protocol mean that quantification and normalization of individual libraries, generated within a single experiment (batch), are not required.

* Quantify the pool with Qubit or PicoGreen.
* For libraries prepared with less than 100 ng, quantify individual libraries with Qubit or PicoGreen.

**How to check the quality of libraries?**\* Use a Fragment Analyzer (Advanced Analytics) or Bioanalyzer 2100 (Agilent Technologies) to check the quality and intended size distribution of a tagmented sample. For examples of Bioanalyzer traces and library size distributions, see the library prep reference guide.\
**What can cause libraries to look different than the example shown in the Reference Guide?**\* The profile of the amplified library product can look different from the example depending on the type and quality of input DNA, and the size selection used.

* Inaccurate pipetting during the double sided SPRI will cause fragment size variation. Some variation is expected in the profiles of different libraries.

**Is a bump/shoulder/peak at 1000-3000 bp in traces expected?**\* It is not atypical to see a small bump/shoulder/peak at 1000-3000 bp. The appearance of the peak can vary between DNA samples and does not affect downstream sequencing.

**What is the typical size range and average size of final library?**\* 600 bp average fragment length on Bioanalyzer (150-1500 bp size range).

* 350 bp median insert size from sequencing.

**Which equation should be used to calculate the final library concentrations (nM)?** See [Converting ng/µl to nM when calculating dsDNA library concentration](https://github.com/illumina-swi/illumina-knowledge/tree/master/articles/Knowledge/LP-Converting-ng-µl-to-nM-when-calculating-dsDNA-library-concentration/README.md).

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| *For any feedback or questions regarding this article (Illumina Knowledge Article #1258), contact Illumina Technical Support* [*techsupport@illumina.com*](mailto:techsupport@illumina.com?subject=Question%2FFeedback%20Regarding%20Illumina%20Knowledge%20Article%20#%20-%20Library%20Preparation%20\&body=Dear%20Illumina%20Technical%20Support,%0D%0A%0D%0A)*.* |


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