Mass Spectrometry Study Notes

Mass spectrometry is an analytical technique used to measure the molecular weight of compounds.
This technique allows the determination of molecular weight from very small samples.
Notably, mass spectrometry does not involve the absorption or emission of light.
A beam of high-energy electrons is used to break the molecule apart, generating ions that can be analyzed.
The masses of the resulting fragments and their relative abundance provide insight into the structure of the original molecule.

Molecular Ion (m+): The ion formed by the loss of one electron from the molecule.
Base Peak: The most intense peak in the mass spectrum, assigned a relative intensity of 100%.
Radical Cation: A positively charged species that has an odd number of electrons.
Fragment Ions: Lighter cations and radical cations generated from the decomposition of the molecular ion; these typically correspond to stable carbocations.
m/z: The mass-to-charge ratio of ions analyzed in the mass spectrometer.

In mass spectrometry, masses are represented in a graphical or tabulated form based on their relative abundance.
Example: A mass spectrum for 2,4-dimethylpentane displays a molecular ion (m+) peak corresponding to the molecule.

Ionization Steps:
- Persuade the target molecule to enter the vapor phase (this can be challenging).
- Produce ions from molecules in the gas phase.
- Separate ions based on their mass-to-charge ratios (m/z).
- Measure and record these ions.
Only cations experience deflection in a magnetic field.
The degree of deflection is dependent on the m/z ratio of the ion.
The signal generated on the detector correlates with the number of ions impacting it.
By varying the magnetic field strength, ions across all masses can be collected and assessed.

Mechanism:
- Gas phase molecules are bombarded with energetic electrons, causing electron ejection and creating radical cations.
- The equation representing the ionization step:
  $M + e^- \rightarrow M^+ + 2e^-$
- This process is highly energetic, leading to fragmentation immediately following ionization.

The fragmentation can make the intact molecular ion hard to detect.
Sample molecules must exist in the gas phase.
Existing databases for EI are limited, with an example being NIST'08, which contains only 190,000 unique compounds.
Interpreting EI spectra requires skill and experience.

Process:
- A dilute solution of the analyte (often <1 mg/L) is fed through a fine needle under a high electric field, creating highly charged droplets.
- The solvent evaporates, and the droplets split or eject ions to adjust charge/area ratios.
- A warm nebulizing gas helps to accelerate the evaporation process, while the ions are directed into a vacuum chamber.
- Ion source voltages typically range from ±2500 V to ±4500 V.
Advantages of ESI:
- Gentle ionization allows for observation of the molecular ion.
- Very labile analytes can be ionized without the need for volatility.
- Analyzes proteins and peptides conveniently.
- Easily coupled with High-Performance Liquid Chromatography (HPLC) and can generate both positive and negative ions.
Disadvantages of ESI:
- The analyte must possess an acidic or basic site.
- Hydrocarbons and steroids are not easily ionized.
- Solubility in a polar, volatile solvent is essential.
- ESI is less efficient than other ionization methods as most ions may not penetrate into the vacuum system.

Process:
- The analyte is mixed with a UV-absorbing matrix in a ratio of ~10,000:1.
- The analyte does not need to absorb the laser light.
- The mixture is dried on a target, and the analyte becomes embedded in matrix crystals.
- The matrix is activated by a laser pulse, which sublimates the analyte, generating ions.
- Ionization occurs due to charge exchange between the matrix and the analyte in the ion plume.
Advantages of MALDI:
- Gentle ionization technique.
- Ability to ionize high molecular weight species.
- Provides sub-picomole sensitivity and easy spectra interpretation.
- Can generate both positive and negative ions from a single location.
Disadvantages of MALDI:
- Cluster ions from the matrix can obscure low m/z range signals.
- Analytes must have a very low vapor pressure.
- The pulsed nature limits compatibility with some mass analyzers, and coupling with chromatographic techniques can be challenging.

The molecular weight of compounds is calculated by summing the atomic masses of all atoms in the molecular formula.
Example: For morphine (C17H19NO3):
$\text{Molecular Weight} = 12.011(17) + 1.008(19) + 14.007 + 15.999(3) = 285.34\text{ Da}$
Observed mass by EI-MS is 285.136, with differences attributed to various isotopic distributions.

Understanding isotopic abundances is crucial for accurate mass determination.
Elements such as Carbon have isotopes with varying relative abundances, which can affect the mass spectrum.

Resolution is defined as the ability to distinguish ions of nearly equal mass.
- Example:
- C6H5Cl (112.00798 amu) vs. C6H5OF (112.03244 amu).
- Resolution Calculation:
- $Resolution = \frac{\Delta m}{m}$
- Resolving Power is given by $\frac{m}{\Delta m}$ .

Presence of isotopes leads to unique peaks in mass spectra, observable for compounds with Cl and Br.
The presence of heteroatoms can also influence isotopic peaks in the spectra, impacting the interpretation of mass balance and molecular formula.