Mass Spectra Prediction

About

Mass spectrometry (MS) is a powerful analytical technique used to identify, quantify, and characterize the molecular composition of Per- and Polyfluoroalkyl Substances (PFAS). PFAS are a group of synthetic chemicals widely used for their water, oil, and heat-resistant properties. Due to their persistence in the environment and potential health risks, accurate detection and analysis of PFAS are crucial, and mass spectrometry plays a central role in this effort.

1. Basics of Mass Spectrometry

Mass spectrometry measures the mass-to-charge ratio (m/z) of ions to identify the molecular composition of a sample. The general process involves:

1. Ionization: The sample molecules are ionized to produce charged particles (ions).

2. Mass Analysis: The ions are separated based on their m/z ratios in a mass analyzer.

3. Detection: The separated ions are detected, and a mass spectrum is generated, showing the relative abundance of each ion as a function of m/z.

2. Application of Mass Spectrometry in PFAS Analysis

Mass spectrometry is widely used to detect and analyze PFAS in various environmental and biological samples. The technique is particularly effective because PFAS often contain multiple fluorine atoms, which can be easily ionized and detected with high sensitivity.

3. Ionization Techniques for PFAS

• Electrospray Ionization (ESI): ESI is the most commonly used ionization technique for PFAS analysis. It gently ionizes PFAS molecules in solution, typically producing [M-H]⁻ (negative ion) or [M+H]⁺ (positive ion) species. ESI is highly compatible with liquid chromatography, allowing for the analysis of complex mixtures.

• Atmospheric Pressure Chemical Ionization (APCI): APCI is another technique used for PFAS, particularly for less polar compounds. It ionizes the molecules under atmospheric pressure conditions, often resulting in protonated or deprotonated species.

• Negative Ion Mode: Due to the high electronegativity of fluorine atoms, PFAS are often analyzed in negative ion mode, where they form stable anions ([M-H]⁻). This mode enhances the sensitivity for detecting PFAS.

4. Mass Analyzers for PFAS

• Quadrupole Mass Analyzer: Widely used in PFAS analysis, especially in tandem with liquid chromatography (LC-MS/MS). Quadrupoles filter ions by their m/z ratios and are highly effective for targeted analysis of specific PFAS compounds.

• Time-of-Flight (TOF) Mass Analyzer: TOF analyzers measure the time it takes for ions to travel a fixed distance. They are capable of high-resolution measurements, making them suitable for identifying PFAS isomers and unknown PFAS compounds.

• Orbitrap and Fourier Transform Ion Cyclotron Resonance (FT-ICR): These high-resolution mass analyzers offer precise mass measurements, which are critical for distinguishing between PFAS compounds with very similar masses.

5. Sample Preparation and Chromatography

• Sample Preparation: PFAS analysis often requires careful sample preparation to remove contaminants and concentrate the analytes. Techniques like solid-phase extraction (SPE) are commonly used to isolate PFAS from environmental and biological matrices.

• Chromatography: Liquid chromatography (LC) is frequently coupled with MS (LC-MS) to separate PFAS compounds before mass analysis. LC helps in resolving different PFAS species, including isomers, based on their interaction with the chromatographic column.

6. Mass Spectrometric Analysis of PFAS

• Quantification: MS provides quantitative information about PFAS concentrations in a sample. By comparing the ion intensities of PFAS with those of known standards, accurate quantification can be achieved.

• Identification: MS can identify PFAS compounds based on their unique m/z ratios and fragmentation patterns. Tandem mass spectrometry (MS/MS) is particularly useful, where selected ions are further fragmented to provide structural information.

• Fragmentation Patterns: PFAS often produce characteristic fragment ions upon collision-induced dissociation (CID), which can be used to confirm the identity of specific PFAS. For example, the loss of CF₂ units is a common fragmentation pathway for PFAS, providing insight into the length of the fluorinated chain.

7. Challenges in PFAS Analysis by MS

• Isomer Differentiation: PFAS isomers (compounds with the same molecular formula but different structures) can be challenging to distinguish by MS alone. High-resolution mass spectrometry and advanced chromatography are often needed.

• Matrix Effects: Environmental samples can contain complex mixtures that may interfere with PFAS detection. Careful sample preparation and method optimization are necessary to minimize matrix effects.

• Detection of Emerging PFAS: New PFAS compounds with varying structures continue to be discovered. MS methods must be continually updated to detect and quantify these emerging contaminants.

8. Advances in PFAS Mass Spectrometry

• High-Resolution Mass Spectrometry (HRMS): HRMS provides accurate mass measurements and the ability to resolve closely related PFAS compounds. This is particularly important for identifying novel PFAS and understanding their environmental behavior.

• Non-Targeted Analysis: Non-targeted mass spectrometry approaches are being developed to screen for a wide range of known and unknown PFAS compounds in samples, expanding the scope of PFAS detection.

• Ion Mobility Spectrometry (IMS): IMS coupled with MS can separate ions based on their shape and size, in addition to their mass. This technique is useful for distinguishing PFAS isomers and understanding the conformations of PFAS molecules.

9. Regulatory and Environmental Implications

Mass spectrometry is critical for monitoring PFAS contamination in the environment and ensuring compliance with regulatory limits. Accurate PFAS analysis helps in assessing the extent of contamination, understanding the environmental fate of PFAS, and guiding remediation efforts.

Summary:

Mass spectrometry is an essential tool for the analysis of PFAS, offering high sensitivity, specificity, and the ability to both identify and quantify these persistent environmental pollutants. Techniques such as LC-MS/MS, coupled with advanced ionization methods and high-resolution mass analyzers, allow for the detailed investigation of PFAS in a wide range of matrices, making MS a cornerstone of modern PFAS research and monitoring.

Method

The [M-H]- spectra is predicted with CFMID 4.0

Wang F, Liigand J, Tian S, Arndt D, Greiner R, Wishart DS. CFM-ID 4.0: more accurate ESI-MS/MS spectral prediction and compound identification. Analytical chemistry. 2021 Aug 17;93(34):11692-700.

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The mass spectra prediction will run when the Mass Spectra Prediction option is selected in the Spectra category. The collision energy is divided into low, medium, and high levels. Select the appropriate option for the spectra you are interested in.