Easy and accurate calculation of programmed temperature gas chromatographic retention times by back-calculation of temperature and hold-up time profiles

Abstract

Linear retention indices are commonly used to identify compounds in programmed-temperature gas chromatography (GC), but they are unreliable unless the original experimental conditions used to measure them are stringently reproduced. However, differences in many experimental conditions may be properly taken into account by calculating programmed-temperature retention times of compounds from their measured isothermal retention vs. temperature relationships. We call this approach “retention projection”. Until now, retention projection has been impractical because it required very precise, meticulous measurement of the temperature vs. time and hold-up time vs. temperature profiles actually produced by a specific GC instrument to be accurate. Here we present a new, easy-to-use methodology to precisely measure those profiles: We spike a sample with 25 n-alkanes and use their measured, programmed-temperature retention times to precisely back-calculate what the instrument profiles must have been. Then, when we use those back-calculated profiles to project retention times of 63 chemically diverse compounds, we found that the projections are extremely accurate (e.g. to ±0.9 s in a 40 min ramp). They remained accurate with different temperature programs, GC instruments, inlet pressures, flow rates, and with columns taken from different batches of stationary phase while the accuracy of retention indices became worse the more the experimental conditions were changed from the original ones used to measure them. We also developed new, open-source software (http://www.retentionprediction.org/gc) to demonstrate the system.

What experimental factors influence the accuracy of retention projections in gas chromatography–mass spectrometry?

“…Based on these and previous experiments [11,19], we developed a protocol to efficiently restore a system to a state where it passes the system suitability check. After each of the following steps, one should re-run the system suitability check to test whether the problem has been fixed:…”

“…The isothermal retention factors and a detailed description of their measurement are available elsewhere [19]. Briefly, we measured isothermal retention factors for each of the 37 compounds in the test mixture at 20 °C intervals from 60 °C to 320 °C, using N 2 as the hold-up time marker.…”

“…Alternatively, a new approach was recently reported to easily measure and account for such imperfections in the temperature and hold-up time profiles [19]. First, one spikes their sample with a set of 25 n -alkane samples and subjects it to temperature-programmed elution.…”

A practical methodology to measure unbiased gas chromatographic retention factor vs. temperature relationships

“…A detailed description of the isothermal measurements is available elsewhere [22] along with the measurements themselves. In short, we measured isothermal retention factors for each of the 37 compounds in the test mixture at 20 °C intervals from 60 °C to 320 °C, using N 2 as the t M marker.…”

“…However, we recently described a new approach that makes it relatively easy to measure the unintentional errors in the temperature program and t M vs. T relationship [22]. Briefly, a sample is spiked with a series of n -alkane standards and run in the desired temperature program for the analysis (currently, the DB5MS UI phase and He carrier gas must be used).…”