# y-axis for chromatogram display

<p> Hello, I am wondering how the units on the y-axis are determined for Empower FR5 when looking at the Review Main Window.</p><p>If I have a chromatogram displayed the y-axis shows AU (absorbance units).  If I zoom in on one of the peaks the top of the peak</p><p>would be at approximately 0.012 AU.  In the table below the chromatogram this same peak shows a Height value of 16486 uV</p><p>and an Area value of 174261.01269 uv*sec.  So neither of these correspond to the units being displayed on the y-axis.  Can anyone</p><p>tell me what AU is and how this relates to height and/or area?  Is there any way to have the y-axis display Area instead of AU?  I have inserted</p><p>an image below to show a visual of what I am talking about.</p><p></p><p><img __jive_ID="1105" alt="Response.jpg" class="jive-image-thumbnail jive-image" height="426" onclick="" src="Response.jpg" width="899"/></p><p></p><p>Thank you!</p>

• AU stands for absorbance units and is related to transmittance via:

AU = 2 - log10(%T)

AU and height (uV) are directly correlated from the initial zeroing of the absorbance (Ie 200,000 Height = 0.2 AU).  However, as your baseline drifts you'll notice these values will not correlate as height is calculated from the baseline that you draw and the y-axis obviously does not change.  For this reason, your area will also not correlate with your axises directly most of the time.  Area uses a calculus function called integration to calculate the area under the curve in units of AUTime.  If the baseline surrounding your peak is identical to the starting AU value, I believe you should find a correlation between uV*sec and AU*min.

I don't think you can change the y-axis so that it correlates to area or height for the above reasons.  You can rename your axis but they are only labels and will be ignored in calculations.

• So we can still convert out Height (uV) to Height (AU) by dividing the uV value by 1,000,000.  It just won't line up exactly with what is seen on the chromatogram display

because of the baseline offset.  Is that correct?

Thanks!

• I believe so yes.  When you draw a baseline that has a different AU value from your initial baseline's AU (probably 0, as it's usually set to zero before injection), the correlation between height and AU is lost. You can keep the correlation if you incorporate the difference between your current baseline and your initial baseline.