Environmental Laboratory

The analysis of drinking water for volatile organic compounds by GC-MS

Jul 30 2019 Read 581 Times

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The United States Environmental Protection Agency Method (US EPA) 524.3 describes the analytical method for ‘measurement of purgeable organic compounds in water by capillary column gas chromatography/mass spectrometry’. EPA 524.3 is a method widely used with a purge and trap system to determine over 70 volatile organic compounds (VOCs) in drinking water to ensure water supplies are safe for consumption.

Demand for lower detection limits of VOCs in drinking water requires the use of a mixed mode GC-MS system. The ability for simultaneous full scan and selection ion monitoring (SIM) is vital for low level quantitation. The Scion Single Quad (SQ) mass spectrometer has a unique feature, Compound Based Scanning (CBS) for easy automated setup and optimisation of complex mixed mode methods. CBS makes use of libraries that store all the essential information about a compound such as retention time, time window, qualifier and quantifier ions. Compounds are loaded directly into a method; scan times are optimised with data acquisition and processing tables synchronised. Managing large number of SIM acquisitions is made easy in mixed mode.
The SCION 456-GC coupled with the SCION SQ MS and Tekmar Atomx XYZ Purge and Trap sample concentrator was used to achieve a highly automated and robust solution for the analysis of VOCs in drinking water, to EPA 524.3 specification.  Table 1 details the analytical conditions of the Atomx Purge and Trap and SCION GC-MS.
The SCION SQ was tuned to meet the requirements for the spectral resolution of Bromofluorobenzene (BFB) using target ion ratio tuning (built directly into the mass spectrometer software; mass spec work station MSWS). All acceptance criteria were exceeded so passed specifications, requirements of the method. Using CBS, a mixed-mode method was created by importing all target compounds from a library containing all of the associated SIM ions.  Table 2 details the tune report for BFB including acceptance criteria and obtained values.

Calibration standards were prepared at 0.1, 0.5, 1, 2, 5, 10, 20 and 40ppb. Standards at lower concentrations were required for the SIM acquisition; typically, in a range of 5-100ppt. A 5mL sample size was used for the analysis as specified in EPA 524.3. The purge and trap conditions for EPA 524.3 come factory installed on the Atomx XYZ system.  Figure 1 shows the chromatogram of the 5ppb calibration standard in full scan mode.
Post calibration standard analysis, calibration curves were generated in the MSWS software. The calibration correlation co-efficient for EPA 524.3 analytes were 0.9995. t-butyl alcohol (TBA) is usually poor compound to analyse on a purge and trap system. However, Figure 2 shows the calibration curve for TBA with an excellent R2 of 0.999.

The robustness of the analytical system was tested through replicate analysis of VOCs in water with a minimum of seven replicates. Linearity, repeatability, MDL and the recovery of each target compound can be found in Table 3.  All replicates analysed met the method requirements; RSD % must be below 20% with recovery of analytes being between 70% and 130%.  MDL was measured in full scan mode at 0.1ppb.
The Tekmar Atomx XYZ purge and trap sample concentrator coupled with the SCION SQ Mass Spectrometer is a total solution for EPA VOC methods. The method is easily setup for both full scan and SIM methods using the unique Mass Spec Work Station software. Excellent repeatability, recovery and linearity of the VOCs demonstrates the excellent performance of the SCION system, will all requirements of EPA Method 524.3 being exceeded.

 

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