Haemoglobin literature spectra does not match

I am studying haemoglobin absorption spectra in its oxygenated and deoxygenated state and I compiled values from different sources including:


* A PhD thesis by van Assendelft [1], using extinction coefficients in (mmol/L)-1/cm
* A book compiling almost one century of research on the topic by the same laboratory that helped standardizing the haemoglobin cyanide standard [2]. Extinction coefficients in the same unit as [1], numerical values are very close.
* Coefficients compiled by Scott Prahl[3]. The unit is the same but there is a factor 3.7 up to 4.0 between his data and that of Assendelft and Zijlstra.
* Coefficients measured by Kolyva[4].


All the published coefficients are defined as base 10 extinction coefficient. That is if *I*
is the light intensity, they are expressed in Beer-Lambert law as:


$I = I\_0 \cdot 10^{-c\cdot \varepsilon \cdot l}$


Where $l$ is the light path length (cm), $c$ the concentration in absorbing compound (mol/L) and $\varepsilon$ the molar extinction coefficient (L/mol/cm).


**How comes there is a 3.7-4.0 factor between Prahl and Kolyva data on the one hand and Assendelft and Zijlstra on the other hand?**


At first I thought of some $log(10)\approx2.3$ difference, as if some of the author forgot to precise that they used base $e$ extinction coefficient at some point but it does not fit. If anyone has an idea on the topic, the help would be most appreciated.


![Valid XHTML](https://i.stack.imgur.com/qjxs3.png)




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**Further information**


The cyanmethaemoglobin method is often used for the determination of the haemoglobin concentration of a solution. In particular, its absorbance at **540nm** is known to be 11.00L/mmol/cm[5] which is in agreement with Assendelft and Zijlstra published data.


### References


1. O.W. Assendelft, *Spectrophotometry of Haemoglobin Derivatives*, **1970**. ISBN: 902320560X.
2. W.G. Zijlstra, A. Buursma and O.W. van Assendelft, *Visible and Near Infrared Absorption Spectra of Human and Animal Haemoglobin*, **2000**. ISBN: 9067643173.
3. Scott Prahl, *Optical Absorption of Hemoglobin*, **1999**, <https://omlc.org/spectra/hemoglobin/>
4. C. Kolyva, *Systematic investigation of changes in oxidized cerebral cytochrome c oxidase concentration during frontal lobe activation in healthy adults*, **2012**. DOI: [10.1364/BOE.3.002550](https://dx.doi.org/10.1364%2FBOE.3.002550). Coefficients are in [**MEDIA 1**](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3469997/bin/boe-3-10-2550-s001.PDF).
5. F. Meng, *Determination of extinction coefficients of human hemoglobin in various redox states*, **2017**. DOI: [10.1016/j.ab.2017.01.002](https://dx.doi.org/10.1016%2Fj.ab.2017.01.002). Table 1.




As stated by Scott Prahl :



> 
> One of the most confusing things about looking at hemoglobin (Hb)
>  spectra is that the values are typically tabulated in equivalents. The
>  term equivalent is used to indicate the amount of hemoglobin which
>  contains 1 gm atom of Fe and combines with 1 gm molecule of O2 or CO.
>  One equivalent of hemoglobin is assumed to be 64,500/4 or 16,125 gm. A
>  concentration of 10$^{-6}$ equivalent is 16.125mg of hemoglobin per cc.
> 
> 
> 


It seems that Assendelft and Zijlstra used the **equivalent** quantification of haemoglobin, meaning that all their extinction coefficients should be multiplied by 4. This leads to the following figure:


[![Literature-extracted haemoglobin absorption spectra](https://i.stack.imgur.com/P7Ywm.png)](https://i.stack.imgur.com/P7Ywm.png)


*Sorry for not having read again Scott Prahl's description before posting. I read it several months ago, as I was gathering data from different source and forgotten it since...*


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