Collision Cross Section

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In Ion Mobility Mass Spectrometry (IM-MS), the collision cross section (CCS) provides valuable information about the shape, size, and structure of a molecule as it travels through a buffer gas under the influence of an electric field. For Per- and Polyfluoroalkyl Substances (PFAS), a class of synthetic compounds with distinct fluorinated carbon chains, the CCS helps distinguish different PFAS compounds and their conformers based on their molecular structure.

1. Overview of PFAS Molecules

PFAS molecules are characterized by their fluorinated alkyl chains, which provide unique properties such as chemical stability and resistance to heat and degradation. Common examples of PFAS include:

• Perfluorooctanoic acid (PFOA): $\text{C}_8 \text{F}_{15} \text{O}_2 \text{H}$

• Perfluorooctanesulfonic acid (PFOS): $\text{C}_8 \text{F}_{17} \text{SO}_3 \text{H}$

The fully fluorinated carbon chains result in a rigid, linear structure, which affects their collision cross section in the gas phase.

2. Ion Mobility Mass Spectrometry (IM-MS) and CCS

IM-MS combines ion mobility spectrometry (IMS) with mass spectrometry (MS), allowing for the separation of ions based on both their size, shape (via CCS), and mass-to-charge ratio (m/z). In this technique, ions are injected into a drift tube filled with a neutral gas (such as helium or nitrogen), and an electric field is applied. The ions move through the gas and are separated based on their collision cross section.

The CCS represents how the ion interacts with the buffer gas. Ions with larger CCS values experience more collisions with the gas, slowing them down, while ions with smaller CCS values move more quickly.

Factors affecting CCS in IM-MS for PFAS molecules:

• Size and Shape: PFAS molecules typically have long fluorinated carbon chains, which result in relatively larger CCS values, especially when compared to more compact molecules.

• Charge State: In IM-MS, PFAS molecules are ionized, often gaining a negative charge due to their acidic nature (e.g., PFOA and PFOS often form negatively charged ions). The CCS depends on how the molecule's structure responds to the charge state and interactions with the buffer gas.

• Conformation: PFAS molecules can adopt different conformations (e.g., folded or extended) depending on the environment and charge, influencing their CCS.

3. Measuring CCS in IM-MS for PFAS

The CCS of PFAS molecules in IM-MS is determined by measuring the drift time, or how long it takes the ionized molecule to traverse the drift tube filled with gas. The drift time depends on the CCS, the buffer gas used, and the electric field strength.

Drift Time and CCS Relationship:

The drift time $t_d$ is related to the CCS $\Omega$ through the following equation (simplified form):

$t_d = \frac{L}{\mu E}$

Where:

• $L$ is the length of the drift tube,

• $\mu$ is the ion's mobility, related to its CCS,

• $E$ is the applied electric field.

From this drift time, the CCS can be calculated using:

$\Omega = \frac{3 e}{16 N} \sqrt{\frac{2 \pi}{\mu}} \cdot \frac{t_d}{L}$

Where $N$ is the number density of the buffer gas, $e$ is the elementary charge, and $\mu$ is the reduced mass of the ion-gas system.

4. Collision Cross Section for PFAS Molecules

For PFAS molecules, CCS values tend to be larger than for many other small organic molecules due to their:

• Elongated shape: PFAS molecules have long, rigid fluorocarbon chains.

• Heavy fluorine atoms: Fluorine contributes significantly to the molecular weight, though CCS is more sensitive to spatial arrangement than molecular mass.

In practice, the CCS values for PFAS molecules can help:

• Distinguish between different PFAS molecules, even if they have similar mass-to-charge (m/z) ratios. For example, PFOA and PFOS, both having similar molecular backbones but different functional groups, will have different CCS values due to the size and nature of their sulfonate or carboxylate functional groups.

• Identify conformational isomers: PFAS can adopt different 3D structures depending on their interaction with the gas and the charge state. These conformers will have distinct CCS values.

Example CCS Values:

While exact CCS values depend on the specific PFAS compound and experimental conditions, studies have shown that PFAS molecules typically have CCS values in the range of 150–250 Ų in nitrogen or helium drift gases.

5. Applications of CCS for PFAS

Understanding the CCS of PFAS molecules through IM-MS has several important applications:

• Environmental Monitoring: PFAS are persistent environmental pollutants, and CCS can help accurately identify and differentiate between various PFAS compounds in complex environmental samples.

• Toxicology and Regulatory Analysis: IM-MS can be used in regulatory contexts to monitor PFAS levels in water, soil, and biological samples, with CCS aiding in the separation and identification of PFAS isomers.

• Structural Analysis: CCS data can provide insights into the conformational flexibility and structural characteristics of PFAS molecules, which can be relevant for understanding their behavior in biological systems or environmental media.

Summary:

• The collision cross section (CCS) for a PFAS molecule in Ion Mobility Mass Spectrometry (IM-MS) provides a measure of its size and shape in the gas phase.

• PFAS molecules, due to their long fluorinated chains and rigidity, tend to have larger CCS values compared to more compact molecules.

• CCS is used in IM-MS to distinguish between different PFAS compounds, separate conformers, and gain insights into their structural properties.

• CCS values for PFAS typically range between 150–250 Ų in nitrogen or helium drift gases, depending on the molecule and its conformation.

Method

The CCS value is calculated from the GFN2-xTB optimized structure in the following sequence of steps:

1. Deprotonation with CREST to form $N$, number of adducts.

2. Conformer generation with CREST with GFN-FF/GFN2-xTB levels of theory for the metadynamics and geometry optimization steps, respectively.

3. Conformer optimization with NWChem at B3LYP/6-31G level of theory.

4. The CCS for each conformer is calculated with the trajectory method in mobcal-shm

5. The CCS values are averaged with a Boltzmann weighting to yield the final CCS value.

Find

The CCS value can be found in the Global property table.