Anomalous temperature dependence of gas chromatographic retention indices of polar compounds on non-polar stationary phases

• Some polar analytes on non-polar phases indicate anomalous temperature dependence of gas chromatographic retention indices, RI(T).
• These anomalies implies changing of the sign of RI(T) dependence and appearing the minima.
• The character of dependencies RI(T) is influenced by amounts of analytes injected in the column.
• The general reason of RI(T) anomalies is dynamics modification of stationary phase by analytes.

1Increasing the reliability of both GC and GC–MS identification requires appropriate interlaboratory reproducibility of gas chromatographic retention indices (I). Known temperature dependence, I(T), is the main source of non-reproducibility of these parameters. It can be approximated with a simple linear function I(T). However, since mid-1990s—beginning of 2000s some examples of anomalous temperature dependence, I(T), preferably for polar analytes on non-polar stationary phases were revealed independently by different authors. The effect implies the variations in the sign of the temperature coefficients β = dI/dT for selected compounds and, hence, the appearance of the I-extrema (usually, minima).The current work provides evidence that the character of the anomalous I(Т) dependences (ascending, descending, or with extrema) is strongly influenced by the amounts of analytes injected into the chromatographic column, but these anomalies appeared not to be connected directly with the mass overloading of separation systems. The physicochemical model is proposed to describe the observed anomalies of I(T) dependence. This model is based on three previously known principles of chromatography, namely:i)the dependence of partition coefficients of polar analytes between stationary and gas phases (Kp) can be approximated with the well-known equation (sometimes called as two-parameter Antoine relationship), ln(Kp) = a/T + b;ii)the polarity of the stationary phase of chromatographic column is varied as a result of its dynamic modification with polar analytes;iii)the variations in the retention indices of polar analytes are proportional to the quantity of these polar analyte dissolved in the stationary phase.The superposition of these objectives allows understanding both the unusual temperature dependence of retention indices, and the influence of the amounts of polar analytes injected into GC column on the parameters of this dependence.