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 Leaf - an Example for a Biological Material

 Surface Spectroscopy on Biological Material

The usage of fertilizers in different forms are a quite common way to improve the resistivity of fruit trees. In this study we have a look on the trees that are growing next to fruit trees and to study how they react on these fertilizers.

X-ray photoelectron spectroscopy (XPS) is an extremely surface sensitive technique where the sample is illuminated with X-ray radiation of a specific energy. The emitted photoelectrons originate from the uppermost surface layers. Their energies is specific for the elements in atoms or molecules of which they escaped. When X-ray photons hit atoms in deeper layers of the surface the released electrons will be absorbed by the upperlaying layers and cannot contribute to the measurement signal. This information depth is only up to 10nm. Classical XPS systems are operated under vacuum conditions at the sample, not being compatible with most biological materials.

Three different positions where chosen on three different regions on the autumn leaf. The first was located on a green region, where we expected to find intact cells. For the second one a yellow region and for the third one a brown region on the leaf has been selected.

 
     
In the following section raw data is presented taken on different positions on a leaf collected underneeth a maple tree close to a group of fruit trees. The leaf was not specifically treated or cleaned before positioning on the sample plate by carbon tape. The SampleEnvironment was pumped down to 1 mbar and the sample was placed in front of the analyzers nozzle and the X-ray window.

 
Leaf
  Three different positions where chosen on three different regions on the autumn leaf. The first was located on a green region, where we expected to find intact cells. For the second one a yellow region and for the third one a brown region on the leaf has been selected.

1. Sample Position 1 (green surface region)

Implying that the green surface regions consist mainly of intact cells the following spectra show the XPS signals from the surface of biological plant cells.
     

Leaf underneath EnviroESCA nozzle

  The picture on the left shows the view through the main sample positioning camera of the EnviroESCA. This camera is being used to navigate on the sample surface to select regions of interest where the XPS measurements will be performed. A pilot laser passing through the lens system of the electron energy analyzer  indicates the measurement position and helps the user to address the desired region on the sample.
     
 XPS survey spectrum of a leaf
The above diagram shows an XPS survey spectrum taken on the leaf. Only peaks that results from electrons belonging to Oxygen, Nitrogen and Carbon are visible.
     
 XPS survey of a leaf
Looking at the carbon region in the spectrum in more detail no other peaks beside  the Carbon 1s is visible. 

2. Sample Position 2 (yellow surface region)

Leaf underneath EnviroESCA nozzle

  The XPS spectra taken on the yellow surface areas of the leaf look different when compared to the green ones. 
     
 XPS survey spectrum of a leaf
Besides the Oxygen, Nitrogen and Carbon peaks also signals from Calcium and Chlorine are clearly observable.
     
 XPS survey spectrum of a leaf
A closer look on the lower binding energy region shows signals resulting from the interaction of the impinging X-ray photons with molecules of calcium chloride in the uppermost surface layers of the leaf. Calcium chloride is known to be widely used as a fertilizer. It is mainly applied by spraying solutions onto the fruits and leaves of fruit trees.

 

3. Sample Position 3 (brown/dissolved surface region)

 Leaf underneath EnviroESCA nozzle   The brown or dissolved regions of the leaf allows the interaction of the X-ray photons with the remains of the plant cells. The measured spectra show signals of the chemicals that remain even when cell membranes and the leaf structure is gone.
     
 XPS survey spectrum of a leaf
The survey spectrum of this region on the leaf shows even more pronounced signals from the calcium chloride.
     
 XPS survey spectrum of a leaf
Also on the detail spectrum of the lower binding energy region the peaks of calcium and chlorine are more pronounced when compared to the spectrum from the yellow region. As the peak area can be related to the amount of material on the surface, the spectra shows that on the brown regions of the leaf more calcium chloride can be detected in the uppermost surface lasyer than on the green and the yellow regions.
     

Conclusion

EnviroESCA has shown its ability to analyze the surface of a leaf with XPS. It was also shown that Near Ambient Pressure XPS (NAP-XPS) can be easily applied to biological material. The surface analysis of the three different positions on the leaf was able to reveal traces of calcium chloride, a fertilizer used on fruit trees. It was shown that the amount of calcium chloride in the uppermost surface layers is higher in the brown/dissolved regions of the leaf compared to the yellow regions. The green regions does not show any trace of calcium chloride at all which could be a sign for the incorporation of the fertilizer into cells that are burried deeper than approx. 10nm underneath the surface.   
     



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 Application Notes

#000385
XPS surface analysis of a leaf with EnviroESCA



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