Volatile compounds are captured using Twister™ technology and detected by GC-TOF-MS with a time-invariant FAME-based retention-index grid. (A) Sample collection. Volatile compounds are trapped on 1 cm long PDMS-coated Twisters™. (Figure provided by GERSTEL, Inc.). (B) Schema of data acquisition instrumentation (not to scale). Exposed Twisters™ are transferred into glass transport tubes together with an external solution of retention index markers (C4-C26 fatty acid methyl esters, FAMEs) in 0.5 ml glass capillaries. Volatiles are released from the Twisters™ in a thermal desorption unit (TDU). Desorbed volatiles and FAMEs are refocused in the liquid nitrogen-cooled CIS4 inlet, then re-volatilized for temperature-ramped separation on a gas chromatography (GC) column for electron ionization time of flight mass spectrometry detection (TOF MS) and primary data processing on ChromaTOF software. (C) Retention time shifts. Over long periods of operation, absolute retention times (RT) of compounds drift due to column use. Shown here is the RT shift for methyl eicosanoate (C20 FAME) from six separate samples during a two-year study covering 1,500 samples. Shifts of 3 seconds occurred during one month of operation, while a 6 second shift was observed after a column change. (D) Conversion to retention index. Adding FAME retention index markers (m/z 74, 5-fold magnified, blue trace) to every volatile profile (total ion chromatogram, red trace) establishes a stable grid of FAMEs to convert variable 'time' into invariable 'index' units. No further chromatogram alignments are needed.