PFAS

Restek – PFAS HPLC Column Anatomy:
Which Phase, Dimensions, and Particle Types are Best?

As interest grows, the need for fast, accurate, and precise testing is expanding with it. This demand is driving the development of better methods, and LC column selection is the foundation for building an improved approach. This becomes particularly important as the list of monitored PFAS grows to include shorter and shorter alkyl chain compounds. Here, we’ll examine the properties that are important to consider when choosing an LC column for PFAS analysis.

 

Silcotek – Non-PFAS Coating Alternatives

 

Restek – How do I know if my LC system has PFAS contamination?

The easiest way to identify PFAS contamination in your LC is to pump mobile phase through the column for 30 minutes to give system-related PFAS a chance to build up at the head of the column. Then, inject a solvent blank, and when the gradient runs through the column, any PFAS that built up on the analytical column will elute and appear at the retention time matching where PFAS in your sample would elute.

Next, inject another solvent blank immediately after the solvent blank injection with the long equilibration time and look at the retention times for PFAS compounds. The purpose of this second injection is to not give enough time for the system-related PFAS to build up in your analytical LC column.

Compare the results of the immediate injection to those of the long equilibration time injection. If there are PFAS peaks in the long equilibration time injection, but not in the injection that immediately followed it, your LC contains system-generated PFAS contaminants that could interfere with trace analysis. If there are not any peaks at the retention times of the PFAS you are looking for in both injections, your instrument may not have a significant amount of system-related PFAS interference. This can occur if you are using an old LC unit with a new mass spectrometry system because most of the leachable, system-related PFAS have already leached out. However, this is not a permanent state; if you replace plastic components in any part of the instrument before the injector, it is highly likely that system-related PFAS interference will leach out unless the new parts are PFAS-free.

 

Restek – Why are system-related PFAS important to isolate from my samples?

If you are analyzing PFAS at very low levels, like parts per trillion (ppt), for accurate identification and quantitation, you need to make sure that the PFAS detected belong only to your sample. If background PFAS contamination from your LC coelutes with sample PFAS, they will interfere and cause inaccurate results.

 

Restek – How does the PFAS delay column help?

A PFAS delay column is installed before the injector, so it traps system-related PFAS and delays their elution. This prevents them from coeluting and interfering with the PFAS in the sample. Functionally, as the gradient starts, the trapped PFAS are eluted from the delay column and then travel to the analytical column. They arrive at the analytical column after the PFAS from the injected samples, so they do not coelute with the sample PFAS.

PFAS in the sample will elute as a normal-shaped, symmetrical peak but the delayed system-related PFAS have been continuously moving throughout the system with the gradient. They were never focused on the analytical column, so when they elute, the “peak” they generate is just an elevated baseline. You know it belongs to one or more PFAS because the signal matches the MS/MS transition of the compounds you are monitoring. However, because it is after the retention time window of the same PFAS in your sample, it’s not detected and quantitated as part of the sample.

 

Restek – What’s different about PFAS analysis compared to other LC-MS/MS analyses?

LC-MS/MS analysis is a very sensitive, selective technique that is used in many different applications and is ideally suited for multi-analyte analysis. Due to their chemical/physical properties and long-lasting inertness, plastic fluoropolymers, such as polytetrafluoroethylene (PTFE), are used to make many LC-MS/MS components. You can find PTFE in LC pump seals, mobile phase transfer lines, the plastic linings used in degassers, etc. Unfortunately, these plastic parts can leach PFAS into the mobile phase, which creates system-related background PFAS contamination that interferes with trace-level PFAS analysis. Because PFAS detection levels in drinking water are in ppt range, even slight interference can bias quantitative analysis.

 

PFAS interference can also come from the solvents (even new bottles) used to make mobile phase due to the ubiquitous use of PFAS compounds. PFAS are found in the air as well, so you can assume PFAS contaminants are literally everywhere at low concentration. Basically, system-related PFAS come from components in all stages of the LC-MS/MS workflow, including the sample collection bottle; mobile phase cap/lining (made of PTFE); solvent inlet tubing (often FEP or PFA); degasser; LC pump parts; and even the autosampler vial septum (many are PTFE-lined silicone).

 

The degree of interference that is observed will vary across LC instrument manufacturers and also depends on method parameters (e.g., equilibration time, solvent choice, target analytes, etc.) Note that the longer equilibration times produce more interference because PFAS have more time to leach out of the plastic parts.

 

Restek – What are typical PFAS analysis levels?

PFAS detection levels vary by region and across matrices such as drinking water, wastewater, seawater, soil, etc. Drinking water is the matrix of most concern in relation to public health and many environmental agencies are trying to set safe consumption levels. Most are in the form of health-based recommended levels with no regulations in place as of Feb 2019. Currently, in most countries, designated levels of concern are in the low ppt (ng/L) range for most PFAS compounds in drinking water. Some examples are given below.

•  United States: health advisory level of 70 ppt in drinking water for PFOS and PFOA combined

•  Sweden: action level of 90 ppt in drinking water for 11 PFAS compounds combined

•  Italy (Veneto): threshold values in drinking water, PFOS ≤ 0.03 ppt, PFOA ≤ 0.5 ppt, other PFAS ≤ 0.5 ppt

•  Germany: health-based precautionary value of 100 ppt in drinking water for PFOA, PFOS, and many other PFAS

•  Australia: 70 ppt for the sum of PFOS and PFHxS and 560 ppt for PFOA in drinking water with different levels for other types of waters

 

LCGC – A Review of the Latest Separation Science Research in PFAS Analysis

 

 
LCGC – Characterizing a Mixed Mode Fluorocarbon/Weak Anion Exchange Sorbent for PFAS Separation

 

Credit:

(14) Abdelraheem, E.; Wise, J.; Murphy, C. et al. Triple-Stage Quadrupole Mass Spectrometer to Determine Ubiquitously Present Per- and Polyfluorinated Alkyl Substances in Drinking Water at Part Per Trillion Levels Using Solid Phase Extraction Approach. Bull. Environ. Contam. Toxicol. 2023, 110, 32. DOI: 10.1007/s00128-022-03686-1

Cheryl Murphy is now working in Michigan, but her undergrad at Dal and masters at UofA.