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Manuscript - Environmental Technology Laboratory

Structural analysis and identification of benzene via Raman Spectroscopy

Alexander Königseder, Student ID: 1330446, alexander.koenigseder@edu.uni-graz.at

1. Abstract

Constant technological developments and almost exploding birthrates especially in

developing countries together with an increasing middle class which strives for western living

standards are one of the major political and scientifical challenges which have to be

addressed in the 21th century. All those factors strongly affect the way how we manage our

resources and alter our environment in a way which is not sustainable, over a longer period

of time. During the SS 2014 our group participated in five different environmental analysis

laboratory exercises which were combined under the topic of sustainability. Most of the

laboratory work introduced us to analytical methods used to monitor and identify substances

in industrial products and the environment. A Life Cycle Assessment for the agricultural use

of biogas, biodiesel and petro-diesel was carried out and the impact of these different

approaches was evaluated. This manuscript manly deals with the lab work on Raman

spectroscopy since this was the main topic for my poster and presentation. During this

exercise we go to know the principles of spectroscopy analysis and used them to measure a

sample of benzene. The obtained chromatogram was verified with literature data and

compared to a FT-IR chromatogram of the same sample. Both methods were suited to

identify the sample and showed us the difference of those to methods. In addition to this our

different lab exercises were discussed regarding the topic sustainability wherever a suitable

and conclusive context could be found.

2. Problematics

The goal of the lab experiment was to get to know the basic principles of Raman

spectroscopy and other spectroscopy analytical methods in general. For this purpose we

took a Raman spectrum of benzene and plotted it over its wavenumber. We identified the

significant peaks for the structure of benzene and furthermore we verified the Raman spectra

with the FT-IR spectra of benzene.

3. State of the Art

Raman Spectroscopy gains more and more attention in very diverse fields of application.

Even though it was developed almost 90 years ago there has been a huge increase in

potential application fields in the last couple of years. Especially with the development of

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cheap lasers for industrial use, the Raman technology experienced a revival [1]. Nowadays it

is an invaluable technique for investigations of pharmaceutically and medically relevant

molecules. Chemical compounds and functional groups can be identified and the

conformation of complex biomolecules, such as proteins and DNA can be determined. It is a

standard quantitative and qualitative analysis technique in material sciences. It is used for

quality control of diamond like carbon (DLC) coatings, characterizing the structure and

chirality of carbon nanotubes, or quantitatively measuring the thickness and oxidation state of

graphene and related materials. An ideal characterization tool has to be fast and non-

destructive, offer high resolution, give structural and electronic information, and be applicable

to both laboratory and mass-production scales. Raman spectroscopy as well as Fourier

Transformation Infrared Spectroscopy are fulfilling all of these requirements and therefore

both technologies are heavily used for individual analysis purposes nowadays. While FT-IR

is a relatively inexpensive technology, it mostly requires sample preparation and cannot be

uses for aqueous solutions. Raman spectroscopy might be more expensive but it can be

used for all sample types, needs no sample preparation and can be used for molecules in

almost all states. Furthermore Raman spectroscopy is used for forensic application since it´s

the only analysis technology capable to distinguish between all body fluids [2].

When one has to find a context between Raman- and FT-IR Spectroscopy methods and

sustainability it might be hard to find an obvious one right away. Nevertheless both methods

are used to identify, distinguish and differentiate materials of all kind. For example IR-

spectroscopy is nowadays used to separate different kinds of plastics in recycling facilities.

This allows a fast and precise way to give those materials a “second life” instead of deposit

them in a landfill.

When it comes to biodiesel the context to sustainability is obvious. It is very desirable to

produce energy and fuel out of renewable resources. These resources (e.g.: biomass, plant

oil, animal fat,..) take up carbon dioxide from the atmosphere and release it again during their

combustion. Therefore a closed carbon cycle could be established instead of using fossil

fuels which contribute as source to the global carbon dioxide emissions. However it is very

hard to establish “real” CO2-neutral energy sources since all those biological materials have

to be planted, harvested and undergo several refining processes before they can be used as

easily transportable and storable energy source. All those steps are energy intense and are

still one of the biggest limiting factors for these technologies. Since it is still cheaper to refine

crude oil, the whole production process for alternative fuels has to become cheaper and

more efficient. Especially low priced, residues could be converted into high value products.

Also politics could play a regulating role by putting more pressure on car manufacturers,

energy companies or by reducing taxes on energy derived from sustainable resources.

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The topic biodiesel also played a role in the conducted Life Cycle Assessment (LCA). Four

different operational modes (petro-diesel, biodiesel, plant-oil and biogas) for an agriculturally

used tractor were compared. The ecological pressure, expressed as area needed to

sustainably support those processes, was evaluated and can be used as a decision tool for

politics and industry. It could be shown that, biodiesel and plant-oil are able to act as a

sustainable substitute for fossil petro-diesel. But it is crucial to take all steps, involved in the

process from the plant until the final fuel product, into account. Energy derived from fossil

resources was involved in many of the sub-processes and therefore none of the fuels was

completely CO2-neutral. Our resource system is still oriented on fossil resources and

therefore the use of renewables will not make a technology completely renewable. Regarding

the in-house production of biofuels for agricultural purposes, next to the positive effect on

global warming, an increase in economic value for the region and positive cyclic effects could

be confirmed. Nevertheless we experienced that it is crucial to double check on every single

sub-process and to critically evaluate the data used for a LCA. Even the smallest sub-

process which hasn´t been taken into consideration can have a huge impact on the outcome

of the analysis.

4. Materials and Methods

While most analytical methods are based on the Lambert Beers law of absorption, Raman

spectroscopy works with the scattering of light. If a sample is illuminated with monochromatic

light, most of the light gets absorbed or is transmitted through the sample. A proportion of the

light is scattered by the probe and changes its frequency due to vibrational, rotational and

other low frequency mode interactions of the light with the sample molecules. These shifted

light frequencies are characteristic for the molecules in the sample. The inelastic scattering

can shift the energy of the laser photons up (Anti-Stokes-lines) or down (Stokes-lines)

because the photons gain or lose energy when interacting with the molecules [3].

Figure 1: Different Types of light scattering and their energy level, [4]

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Typically a sample is illuminated with a laser beam. The electromagnetic radiation from the

illuminated spot is collected through a lens and sent through a monochromator. The elastic

scattered radiation (Rayleigh-scattering) is filtered out while the rest of the scattered light is

dispersed onto a detector by a notch filter or a band pass filter. The measured intensities of

the light are then plotted over the corresponding wavenumber, which gives the Raman

spectra.

For educational purposes we compared the measured Raman intensities of the benzene with

our FT-IR measurements of the same probe. FT-IR spectroscopy yields mostly similar, but

complementary information.

Figure 2: Raman and the FT-IR spectra of the same benzene sample, source: own measurement;

The Raman spectrum is plotted as Raman intensity against its wavenumber. The FT-IR

spectrum is plotted as Transmission against wavenumber. The Raman peaks therefore go

upwards while the FT-IR peaks have the highest intensity when they go down, on the graph.

Table 1 summarizes the most intense Raman and FT-IR peaks which were used to identify

the sample.

Raman: The peaks in the area of 1400 -1600 cm-1 can be used to determine aromatic

substances. The Raman spectra shows a split peak at 1600 cm-1 which derives from the

conjugated Carbon double bonds. The C-H stretch vibrations are also a sign for aromatic,

ring structures.

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Table 1: Summary of the characteristic peaks of benzene [5]

Wavenumber [cm-1] Raman spectroscopy FT-IR

1000 a1g sym. Ring stretch -

1530 - C-C stretch vibration

1600-2000 - ring structure “Overtones”

1600 e2g ring vibration -

3050 e2g stretching vibration -

3000-3100 - C-H stretch vibration

FT-IR: The absorption band at 1530 cm-1 stands for a C-C vibration swing. The small peaks

between 1600 – 2000 cm-1 are so called „overtones“ and display the ring structure of the

sample. The aromatic C-H swing is displayed as two peaks between 3000 -3100 cm-1.

5. Results and Outlooks

Benzene (C6H6) is a symmetrical molecule with 6 carbon atoms connected via conjugated

double bonds in a ring structure and each carbon atom is connected to a hydrogen atom.

Because of its symmetrical center, the molecule movements of benzene are either Raman or

IR active. This circumstance is called alternative prohibition. Overall Raman- and IR-spectra

are complementary to each other and both methods can be used to clearly identify

substances and can used simultaneously to collect more detailed information. The Raman

spectra as well as the FT-IR spectra of our measurements fit very well to existing literature

data and allowed us to identify our sample as benzene. Both methods are fully developed

and can be used for a broad field of applications. If one has to decide which method should

be used then the state of the sample, the sample type, the existing infrastructure and other

potential applications should be taken into consideration.

6. References

[1] KRENN (2013): Umweltanalytik-Skriptum. Univ. Prof., Dr. Heinz Krenn. Institut für Physik, Universität Graz. Graz, 2013. [2] http://www.renishaw.com/en/ramanspectroscopy/applications, accessed on the 28.7.2014 [3] FREEMAN (1974): Application of Laser Raman Spectroscopy. Stanley K. Freeman, Wiley Interscience publication. USA, 1974. ISBN: 0-471-27788-6. [4] http://www.doitpoms.ac.uk/tlplib/raman/raman_scattering.php, accessed on the 26.7.2014 [5] HOLLENSTEIN et al (1990): H.HOLLENSTEIN, S. PICCIRILLO, M. QUACK, M. SNELS, MOL. PHYS. 71, 759, 1990.