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Using X-Ray Diffraction To Correlate Physical Appearance and Chemical Structure
Priya Duvvuri [email protected] Grace Ko [email protected] Kristina Sanchez [email protected] Noel Krommenhoek [email protected]

Abstract
Using x-ray diffraction, we analyzed the chemical components of a variety of samples, including various brands of chocolate, peanut butter, different types of cloths, and two medications. Our objective was to compare substances with similar physical appearances and determine differences in their chemical compositions. Some of the materials were too amorphous, or irregular, to accurately assess their chemical make-up. Therefore, we compared the substances to those of similar compounds to identify differences, using the Xpert Data Viewer computer program. With the other materials, those Figure 1: A simplified x-ray machine, highlighting the source (x-ray tube) where x-rays are generwith crystalline structures, we used the Xpert Highated, the sample which is being tested, and score computer program to determine the materithe detector which detects the x-rays and als’ components. We found that samples which we converts it into a graph. thought were similar, due to physical appearances, were actually different in chemical composition.

1

Introduction

X-ray diffraction (XRD) is used to determine the chemical structure of nonamorphous materials. Nonamorphous materials are crystalline, or have atoms or molecules with a repeating structure. That structure is determined by creating xrays, shooting them at a sample, and measuring the intensity of the diffracted rays, as seen in Figure 1. Graphs are generated which compare the intensity to the incidence angle of the x-ray. These two values, measured over a period of time, can be used to calculate the d-spacings, or distance between planes of atoms in the material. Every material in the world has a characteristic set of d-spacings, analogous to a person’s unique set of fingerprints. The test sample’s graph is compared with the graphs of known materials to determine the sample’s composition. X-ray diffraction has applications in many fields because of the flexibility in its requirements for 1

a sample. While other tests require a large sample, XRD samples can be very small. This is partially because XRD is nondestructive to the sample, meaning that after being tested, that sample is unchanged and can be used again for other tests. Other tests require a larger amount of the sample, because part of that amount will be used in tests and altered by those tests. XRD has countless applications in the pharmaceutical field, and in production of food products. It can be used to detect differences in the basic make up of two products, leading to the ability to differentiate between slightly different formulas. It can also be used to determine all components of a sample. This is useful in the creation of food products, because manufacturers need to check that no impurities or contaminants have leaked into their products [3]. In this paper, we discuss the basic theory of xray diffraction and explain its process. We show how our research illuminates the usefulness of xray diffraction in the pharmaceutical field and in

the production of food products, by determining the composition of materials.

2

Science Behind XRD

In order to understand XRD, it is necessary to understand why x-rays are used, the requirements for the sample, how x-rays are generated, how they are diffracted, and how they are detected and analyzed. It is also necessary to understand how Bragg’s Law works. This law is a mathematical equation which explains the relationship between the x-rays and the sample being tested.

2.1

Using X-rays

X-rays are used because their wavelengths are similar in size to atoms and their bonds. This characteristic makes x-rays an excellent tool for investigating the arrangements of atoms and molecules inside samples [2]. The size of x-rays makes it possible to come to understand the chemical structure of samples in a simple manner.

comes into contact with the electrons, so that part is coated with pure Cu or Mo. The constant bombardment by electrons creates a substantial amount of heat, so the inside of the target is constantly water cooled. When the electrons hit the target material, characteristic x-ray radiation is generated. That generation occurs when one electron, excited by the heating process described above, hits an electron from the inner shell of the target atom and knocks it out, leaving an empty space for electrons in higher energy shells to occupy. This process releases electromagnetic radiation in the form of x-rays [4]. The x-rays that are used in diffraction are monochromated, or of only one wavelength, though natural x-rays have a range of wavelengths. This phenomenon is produced through diffracting the x-rays off a large graphite crystal. By doing so, all the xrays which are not a specific wavelength are either diverted or absorbed and do not interfere with the results.

2.4

Diffraction of X-rays

2.2

Requirements of the Sample

X-ray Diffraction will only yield meaningful data for finding components if the material being scanned is nonamorphous, which means that it has regular, repeating planes of atoms that form a crystal lattice. Specifically for powder X-ray diffraction, the particles of the sample should be of a size of no less than 10 microns [1].

2.3

Generation of X-rays

The Phillips X’Pert machine creates x-rays inside a sealed tube under a vacuum. Inside the tube, a current is applied across an anode and a cathode. The anode needs to be made of an appropriate material, because the wavelengths of the x-rays created depend upon that source. The most common elements used for the anode are Molybdenum (Mo) or Copper (Cu). Mo is used for materials with highly absorbing elements and for materials with small repeating distances, such as Aluminum or Silicon. Cu is used for poorly diffracting hydrocarbon materials with very large repeating distances. As more current is applied, the number of electrons emitted from the cathode and moving towards the anode increases [1]. A high voltage is applied (usually between 15 and 60 kV) [1], causing the electrons to accelerate and then hit the target (the anode) which is usually a thimble shaped block. The outside of the thimble is the only part of the target that 2

Once x-rays are created, they do not have a set direction in which to travel. They are collimated through the use of slits in the vacuum tube and in front of the detector. The slits in the vacuum tube direct the x-rays onto the sample. X-rays can react with the sample in various ways, but only diffraction leads to useful results. The requirement of a crystalline sample comes from this reaction between x-rays and the sample molecules. A noncrystalline sample will have molecules randomly organized inside the sample, which means that when the x-rays are diffracted, they go in random directions, depending on the angle with which they interact with the sample. X-rays diffracted in different directions can interfere with each other, changing the intensity measured, and invalidating the results. The symmetry of crystalline materials inhibits such interference. Diffracted x-rays can be diffracted in two ways, but only elastic scattering leads to useful results. Elastic scattering, also known as Thompson scattering, is the type of x-ray measured during x-ray diffraction because those are the type that can carry information about the sample. Elastic scattering occurs when the wavelength of the x-rays does not change [2].

2.5

Detecting and Analyzing X-rays

A detector is used to identify the diffracted x-rays. This detection can occur using either reflection or transmission geometry. Transmission geometry places a detector on the opposite side of the sam-

2.6

Application of Bragg’s Law

In order to analyze XRD results, Bragg’s Law, the equation seen below, is required. n ∗ λ = 2 ∗ d ∗ sin(θ) (1)

Figure 2: Examples of both types of diffraction geometry. ’S’ represents the x-ray source, and ’D’ represents the detector for reflection geometry, the type of geometry used in this experiment.

ple from the x-ray source and measures the x-rays that pass through the sample. Reflection geometry places a detector where it will detect and measure the x-rays that reflect off the sample [2]. Both are legitimate types of geometry, and both are employed in the process around the world. Also, both are pictured in Figure 2. When the detector identifies x-rays, it sends a signal, which is processed, and converted into a count rate. The number of counts is displayed on the computer in the form of a graph. The count rate is plotted against the angle of incidence of the xray, and the graph is then used to determine the composition of the sample. There are databases filled with standard reference patterns and measurements of the count rate and angle of incidence for various substances that have already been tested. One such database is X-Pert Highscore, which can be used to identify substances based on the comparison of peaks in the graph. These patterns and measurements are found by crossing the count rate with the angle (theta) on a graph. Using a computer program, it is possible to compare the pattern of the sample with patterns in that database, and extrapolate the most likely component(s) in the entire sample. Possible components are listed, and can be ranked according to how closely they correspond with the sample’s pattern. Crystalline substances create graphs with distinctive peaks. These peaks make it easy to extrapolate the most likely components the sample contains. Noncrystalline substances usually create broad maxima, which can be compared to other noncrystalline substances to rule out what it is not, but are hard to use to determine any components contained. 3

This equation depends upon the wavelength of the x-ray (λ), the order of reflection (n), and the angle of incidence of the x-ray into the sample (θ). At this point in the x-ray diffraction process, that wavelength and order of reflection are known, and the angle of incidence can be measured. These three values, can be used to calculate ’d.’ The variable ’d’ stands for the d-spacing, or the distance between planes of atoms in the crystalline sample. Each material has a unique set of dspacings. The d-spacings depend on which atoms are present and how those atoms are arranged. This further illuminates why noncrystalline samples cannot be easily tested. Since noncrystalline samples would not have a pattern among their molecules and thus lack d-spacings, crystalline materials are required for distinct patterns when using XRD.

3

Process of X-ray Diffraction

XRD requires three main steps to collect data to analyze. These include choosing test samples, preparing the chosen sample, and choosing settings on the XRD machine. Once data has been collected, it is grouped to aid in analysis.

3.1

Choosing Test Samples

We chose samples with similar physical appearances to test with XRD to determine if similar physical appearances correlate to similar chemical compositions. The samples we compared were multiple brands of peanut butters, brands of chocolates, types of clothing, and types of medications. To the eye they would appear to be composed of the same components; however they are very different at the atomic level.

3.2

Sample Preparation

The process used in this experiment was X-ray Powder Diffraction. As such, all samples tested had to be in powder form, if possible. A mortar and pestle were used to create that powder. The sample was then put into a sample holder, and pressurized to get rid of excess air pockets. However, some samples could not be crushed into powder, like the cloths. These were placed whole into a different type of sample holder, one which was created to

house whole samples. Once the sample holder was And our last group was two medications: Tylenol prepared, it was placed in the x-ray machine. The and Excedrin. Xpert machine could hold and analyze up to fifteen samples at a time.

4

Results

3.3

X-ray Diffraction Machine and We noted that the types of peanut butter had difSettings ferent patterns. As can be seen in Figure 3, Reese’s
peanut butter had a pattern with many peaks, the highest of which were found at 19, 20, and 25o . Skippy peanuts had a maximum, which was highest at around 20o , and a few peaks, found at about 32 and 46o . Skippy peanut butter had a broad maximum at around 20o , and no strong peaks. Skippy peanut butter’s pattern was very amorphous, which can be explained by the fact that it is organic. The graph of the Skippy peanuts looked very much like the graph of the Skippy peanut butter, showing that both are not very crystalline. But Reese’s peanut butter looked very different. We found that Reese’s peanut butter strongly resembled sucrose, as seen in Figure 4. In that figure, many peaks in the graph of the Reese’s peanut butter are at the locations of peaks in the sucrose pattern, including peaks at 19, 20, 25, and 41o . This indicates that Reese’s peanut butter is mainly composed of sucrose. This observation implies that it may possible for a person with a peanut allergy to potentially consume Reese’s peanut butter cups safely. We then found that the Reese’s peanut butter and the Reese’s chocolate had very similar peaks, through examining Figure 5. The major peaks at 19, 20, 25, and 41o were all shared in both the peanut butter and chocolate graphs. As we had already determined that the Reese’s peanut butter was mostly sucrose, it followed that sucrose was a major component in the Reese’s chocolate. All three chocolate patterns were very similar. They all shared the major peaks at 20, 25, and 40o . The Hershey’s and Reese’s chocolate were almost identical because they both strongly resembled the sucrose pattern. The chocolate from the Pepperidge Farm cookie also looked similar to the other two graphs, but there was a broader maximum between the angles 15o and 35o . However, the patterns were not unique. Slight differences in intensity at certain peaks, like those at 44o , indicate different amounts of certain components in each sample. These observations can be observed in Figure 6. The pattern of a broad maximum between 15o and 35o is common in amorphous materials, such as oil. It is likely that the separation of the chocolate chips from the cookie was incomplete, leading to oil in the sample. When we compared the cloths in Figure 7, we 4

The Philips X’Pert machine uses reflection geometry, meaning that the detector is placed so that it will detect x-rays refracted off the surface of the sample. Silicon is periodically tested to ensure that the machine is properly calibrated. This element is used as a calibration tool because its pattern is well known and rather constant. The pattern of silicon received can be compared to the one in the database. Inconsistencies between the two patterns means that the machine needs to be recalibrated, which is done. Once calibration is complete, settings need to be chosen for the machine. For our experiment, we tested our samples with an angle of incidence of between 5 and 60o . This testing was done in increments of 0.05o , while changing position every 0.5 seconds. The scan was continuous. The divergence slit was 0.957 millimeters wide, while the receiving slit was 0.3 millimeters wide. The machine was kept at 25o Celsius. The anode was made of copper.

3.4

Grouping Data

As our objective was to determine if similar physical characteristics led to similar chemical structure, it made sense in our case to group our samples and compare such like samples. Therefore, we put our results in the form of graphs with patterns from multiple samples. We analyzed some data by comparing those similar substances, and recognizing that the slight differences allowed us to determine that they were made of different components. For other graphs, which included crystalline samples, we used the Xpert HighScore computer program and found what the components of those samples were. Our first group was the three different peanut products: peanut butter from Reese’s peanut butter cups (hereafter referred to as Reese’s peanut butter), Skippy peanut butter, and Skippy peanuts. We then compared Reese’s peanut butter to sucrose. Third we compared the two parts of a Reese’s peanut butter cup: the Reese’s peanut butter and the Reese’s chocolate. Then we compared three types of chocolate: Hershey’s, Reese’s, and Pepperidge Farm’s. The chocolate from Pepperidge Farm came from chocolate chip cookies. Next we tested three types of cloth: lace, velvet, and lining.

Figure 3: Graph comparing results for Skippy peanut butter, Skippy peanuts, and Reese’s peanut butter.

Figure 4: Comparing Reese’s peanut butter to sucrose.

5

Figure 5: Graph comparing results for Reese’s peanut butter and chocolate.

Figure 6: Comparing three types of chocolate: Hershey’s, Reese’s, and from chocolate chip cookie.

6

knew that the two peaks at 37 and 43o could not have come from a fabric, as the cloths were not crystalline enough to have created such a pattern. Through further research, we determined that the x-rays were penetrating the fabric and diffracting off the aluminum backing of the sample container. The three patterns had no peaks, only broad maxima. This characteristic did not keep us from finding any use in the graph. Although individual components could not be found, graphs of multiple amorphous samples can be used to determine that the samples are different materials. Our graphs did not give us individual components, but we could easily conclude that the three materials were not the same type of sample. When we analyzed the two drugs in Figure 8, we found that the medications had very similar components. Peaks were found at 15, 20, 23, 24, and 26o . The peak at 15o was another example of different intensities, which indicates that the two medications had different amounts of that substance. They were both largely composed of Acetaminophen, which created those shared peaks, but the Excedrin also contained other substances. When we compared its unique peaks, like those at 8 and 23o , to other known patterns in the database, we identified acetylsalicylic acid as the other component of Excedrin. In all of the graphs, we found that even though we were comparing substances that physically appeared similar, their chemical structure differed. For example, Skippy peanut butter and Reese’s peanut butter have radically different patterns due to the fact that Reese’s peanut butter is predominantly made of sucrose.

XRD could be useful in similar circumstances relating to allergies. People with allergies may be able to eat substances which, when examining food by the naked eye, one would assume would induce an allergic reaction. If that allergen is actually a synthetic substitute, the allergic reaction may not occur, and that person may be able to consume that food.

6

Acknowledgments

Thanks to: Dr. Tom Emge Daniel Cobar Blase E. Ur, GSET Program Coordinator Ilene Rosen, GSET Program Director Kristin Frank, Head RTA Jameslevi Schmidt, Research RTA Governor’s School Board of Overseers (Marguerite Beardseley, Chair; Laura Overdeck, Vice Chair) Rutgers University Rutgers University School of Engineering Morgan Stanley State of New Jersey Lockheed Martin PSEG Tomasetta family Provident Bank NJ Foundation Silver Line Building Products families of Governor’s School alumni

References
[1] Marta J.K. Flohr. X-ray powder diffraction. U.S. Geological Survey, 1997. [2] Materials Research Laboratory. Introduction to x-ray diffraction. University of California, Santa Barbara.

5

Conclusions

In our experiment, we compared similar materials using XRD. Our samples included peanut butters, [3] PANalytical. X-ray analysis segments. 2010. chocolates, cloths, and medicines. Our results indicate that similarities in physical appearance do [4] The State University of New Jersey Rutgers. not necessarily correlate to similarities in chemical Rehs x-ray safety and awareness training. 2010. structure. Our results, although preliminary, appear to indicate that it may possible for a person with a peanut allergy to consume Reese’s peanut butter cups because they are predominately composed of sugar. The chocolates proved to be similar in composition to sucrose, although the chocolate from the chocolate chip cookie proved to have a slightly more amorphous pattern. The analysis of the clothing indicated that they were not comprised of crystalline materials. However each graphs contained different maxima, which allowed us to conclude that their chemical structures were unique. 7

Figure 7: Graph comparing results for lace, velvet, and lining.

Figure 8: Graph comparing results for Tylenol and Excedrin.

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