EXPERIMENTAL
Reagents and materials
Ethylene glycol (EG), Diethylene glycol (DEG), Triethylene glycol (TEG), Tetraethylene glycol (TTEG), Propylene glycol (PG), Dipropylene glycol (DPG, mixture of isomers) and Tripropylene glycol (TPG, mixture of isomers) were purchased from CPA Chem. (Bulgaria). Ultrapure water was prepared in the laboratory using a Millipore Simplicity UV system.
Instrumentation
Gas chromatograph-mass spectrometry was carried out using a Perkin Elmer GC-MS (GC-Clarus 680, MS-Clarus SQ8T) equipped with split/splitless injector. The column used was a 30m x 0.25mm x 0.5 µm Elite WAX ETR (Perkin Elmer, USA) and Helium was selected as carrier gas at 1.5 mL/min constant flow. The temperature program used was as follows: initial temperature of 100°C (hold time 1min), then increased to 240°C, with a ramp of 10°C/min (hold time 10 min). The run time was 25.0 minutes. Injection of 1 µL solution was made in splitless mode, with a 20:1 split ratio. Injector, transfer line and source temperature were 250, 240 and 240°C.
The MS was operated in dual mode, a full-scan (m/z 30-240 amu) chromatogram was acquired, together also with one m/z ion per analyte in Single Ion Reaction (SIR). The m/z for each analyte monitored is presented in Table 1. For Dipropylene glycol we monitored and quantified both isomers present in the reference material and for Tripropylene glycol we also chosen the 2 isomers from the mixture (namely 1 and 2) for quantification. Using a larger acquisition window for Tripropylene glycol can facilitate the quantification also for other minor isomers, if they are present in a real sample analyzed.
Preparation of standards and samples
Standard stock solution of each target analyte (1.0 mg/mL) were prepared in ultrapure water using individual certified reference materials for each glycol. Work solutions were then prepared for the calibration solutions.
A number of 10 different types of commercially available anti-freeze liquids were purchased from Romanian market. To establish the glycol profile for each type a sample solution was prepared as follows: 0.5 g of liquid was diluted first at 100 mL with ultrapure water. A 100x dilution was applied to the first solution to obtain a sample solution of anti-freeze of 50 µg/mL that was analyzed using the validated GC-MS method.
Method validation
The method validation was performed by evaluating specificity, linearity and calibration range, limit of detection (LOD) and limit of quantification (LOQ), accuracy, precision as specified in the ICH guidelines for analytical method validation and ASTM E202-18 Standard test methods for analysis of Ethylene glycols and propylene glycols7.