Identification of the Unknown Compound using Mass Spectrometry

Introduction

Mass spectrometry is a versatile technique with wide-ranging applications in various scientific fields, including chemistry, physics, geology, astronomy, biochemistry, medicine, and ecology². A fundamental tool in mass spectrometry is the mass spectrometer, available in several types. The two common variants are low-resolution and high-resolution spectrometers, each offering a level of accuracy suitable for determining the atomic composition of compounds by separating ions.

The primary objective of this experiment is to identify an unknown compound, referred to as #9. Given that this compound is a simple organic molecule, it is reasonable to assume that it consists of elements commonly found in organic compounds, including carbon, hydrogen, nitrogen, oxygen, sulfur, chlorine, bromine, or fluorine.

Identification of unknown #9 will rely on the analysis of mass spectrometry data and the deduction of the presence of specific atoms within the compound.

Experimental Method

Following the procedure outlined on page 39 of the course manual¹, we anticipate that the unknown substance is a straightforward organic compound composed of elements such as carbon, hydrogen, oxygen, nitrogen, fluorine, sulfur, chlorine, or bromine.

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To discern the presence of chlorine, bromine, and sulfur, we will look for an M+2 peak, as these elements have stable isotopes approximately two mass units heavier than their more abundant isotopes (M+ peak). Additionally, to identify the remaining elements, we will examine the ion fragments and employ the atomic weights of the elements to calculate the formula mass of the unknown compound.

Results

From the provided data, we observe that the molecular ion peak (M+ peak) appears at 58 atomic mass units (amu).

This M+ peak signifies that the atomic weight of the unknown compound is 58 and implies the presence of an even number of nitrogen ions. Furthermore, the absence of an M+2 peak indicates the absence of chlorine, bromine, and sulfur.

We can calculate the number of carbon atoms in the unknown compound using the equation: [(M+1) / (M+)] × (100% / 1.1%). Assuming that the M+ peak represents 100% and dividing by 1.1% (accounting for the 1.1% natural abundance of 13C), we determine that there are 4 carbon atoms in the compound, resulting in a total mass contribution of 4 × 12 = 48 atomic mass units. Subtracting this from the total mass of 58 (58 - 48 = 10) allows us to deduce that the remaining elements consist of 10 hydrogen atoms, resulting in a total mass of 58 when combined with the number of carbon atoms present (10 + 48 = 58).

Furthermore, the presence of nitrogen and oxygen can be excluded for two reasons. Firstly, an even number of nitrogen ions can indicate zero nitrogen atoms. Secondly, upon examination of the ion fragments, the peak at 15 amu (M-43) may be attributed to a CO2 impurity, suggesting the absence of oxygen atoms. Based on these observations and calculations, we can confidently identify the unknown compound as C₄H₁₀, which is butane.

Discussion

Upon analyzing the provided mass spectrum data, we successfully identified unknown #9 as C₄H₁₀, commonly known as butane. Butane is a linear hydrocarbon comprising four carbon atoms and is a colorless gas with various applications, including use as a food propellant and a refrigerant².

While this experiment involved the relatively straightforward process of mass spectrometry data analysis for compound identification, several factors could impact the accuracy of the results. Key considerations include potential misidentification of the molecular ion peak and interpretation of different fragments, both of which could lead to erroneous conclusions regarding the elements present in the compound. It is essential to acknowledge the possibility of diverse outcomes when analyzing unknown substances, as data misinterpretation can significantly affect the results.

Limitations of Mass Spectrometry

Mass spectrometry, although a powerful analytical technique, has inherent limitations. One notable constraint is the inability to distinguish between isomers of a compound that share the same charge-to-mass ratio. This limitation can complicate the identification of molecules with structural isomers. Additionally, mass spectrometry may encounter challenges in accurately quantifying substances in complex mixtures, making it less suitable for certain analytical tasks.

The Role of Modern Mass Spectrometry

Modern mass spectrometry is frequently integrated with other analytical methods to enhance its capabilities. One such example is Gas Chromatography-Mass Spectrometry (GC-MS). GC-MS combines a gas chromatograph with a mass spectrometer, enabling a two-step analysis process.

In GC-MS, the sample initially undergoes gas chromatography, where it is separated into its individual components using an inert carrier gas such as argon, helium, or nitrogen. Subsequently, the separated components are ionized by the mass spectrometer and further separated based on their distinct mass-to-charge ratios. GC-MS is particularly well-suited for the analysis of smaller and simpler molecules, including but not limited to benzenes, alcohols, steroids, and fatty acids.

Overall, mass spectrometry, in its various forms, proves invaluable for measuring mass-to-charge ratios, separating complex mixtures, detecting trace levels of organic contaminants, covering a broad range of mass-to-charge ratios, and collecting data for specific masses of interest. The integration of mass spectrometry with complementary techniques, such as GC-MS, greatly facilitates molecule identification, providing researchers with powerful tools for chemical analysis.

Summary

Through the utilization of information regarding the isotope abundances of 12C and 13C, coupled with the analysis and calculations based on the provided mass spectrometry data, we successfully identified the unknown compound as C₄H₁₀, or butane.

Learning Objectives

The primary learning objectives of this experiment encompassed the identification of an unknown compound using available data and the development of a deeper understanding of mass spectrometry. Throughout the course of this experiment, several key lessons were acquired:

• An appreciation for the principles of mass spectrometry
• Familiarity with the distinctions between low and high-resolution mass spectrometers and their respective impacts
• Comprehension of mass spectrometer operation
• Acquisition of analytical techniques for identifying unknown compounds through mass spectrometry