Journal of Chromatography A, 1200 (2008) 17–27 Contents lists available at ScienceDirect Journal of Chromatography A journal homepage: www.elsevier.com/locate/chroma Modulation-induced error in comprehensive two-dimensional gas chromatographic separations J.J. Harynuk a,∗ , A.H. Kwong a , P.J. Marriott b a Department of Chemistry, University of Alberta, Edmonton, Alberta T6G2G2, Canada b Australian Centre for Research in Separation Science, RMIT University, Melbourne, Victoria 3001, Australia article info Article history: Available online 7 March 2008 Keywords: Comprehensive two-dimensional gas chromatography Modulation ratio Error analysis Quantitation Modulation phase Integration GRAM abstract There is a fundamental difference between data collected in comprehensive two-dimensional gas chro- matographic (GC × GC) separations and data collected by one-dimensional GC techniques (or heart-cut GC techniques). This difference can be ascribed to the fact that GC × GC generates multiple sub-peaks for each analyte, as opposed to other GC techniques that generate only a single chromatographic peak for each analyte. In order to calculate the total signal for the analyte, the most commonly used approach is to consider the cumulative area that results from the integration of each sub-peak. Alternately, the data may be considered using higher order techniques such as the generalized rank annihilation method (GRAM). Regardless of the approach, the potential errors are expected to be greater for trace analytes where the sub-peaks are close to the limit of detection (LOD). This error is also expected to be compounded with phase-induced error, a phenomenon foreign to the measurement of single peaks. Here these sources of error are investigated for the first time using both the traditional integration-based approach and GRAM analysis. The use of simulated data permits the sources of error to be controlled and independently eval- uated in a manner not possible with real data. The results of this study show that the error introduced by the modulation process is at worst 1% for analyte signals with a base peak height of 10 × LOD and either approach to quantitation is used. Errors due to phase shifting are shown to be of greater concern, espe- cially for trace analytes with only one or two visible sub-peaks. In this case, the error could be as great as 6.4% for symmetrical peaks when a conventional integration approach is used. This is contrasted by GRAM which provides a much more precise result, at worst 1.8% and 0.6% when the modulation ratio (M R ) is 1.5 or 3.0, respectively for symmetrical peaks. The data show that for analyses demanding high precision, a M R of 3 should be targeted as a minimum, especially if multivariate techniques are to be used so as to maintain data density in the primary dimension. For rapid screening techniques where precision is not as critical lower M R values can be tolerated. When integration is used, if there are 4–5 visible sub-peaks included for a symmetrical peak at M R = 3.0, the data will be reasonably free from phase-shift-induced errors or a negative bias. At M R = 1.5, at least 3 sub-peaks must be included for a symmetrical peak. The proposed guidelines should be equally relevant to LC × LC and other similar techniques. © 2008 Elsevier B.V. All rights reserved. 1. Introduction Comprehensive two-dimensional gas chromatography (GC × GC) is a field that is gaining in popularity. The tech- nique relies on a pair of columns with different stationary phase selectivities coupled serially by one of several modulator types (interfaces). The modulator periodically introduces material exit- ing the first-dimension ( 1 D) column to the second-dimension ( 2 D) column, operating in a manner that allows multiple samples to be taken across a peak as it elutes from the 1 D column. The ∗ Corresponding author. Tel.: +1 780 492 8303; fax: +1 780 492 8231. E-mail address: james.harynuk@ualberta.ca (J.J. Harynuk). modulation process accurately reflects the distribution of the input chromatographic peak distribution at the modulator, according to considerations of modulation phase and frequency [1]. Several reviews of the technique are available [2–7] which can provide users with a background to the fundamentals of the technique. Gas chromatographers are increasingly turning to GC × GC in a search for solutions to the complex analytical challenges facing them in a diversity of fields including petrochemical exploration and exploitation [8,9], food and aroma profiling [10], and cosmet- ics [11] to name but a few. The impetus to shift towards GC × GC as an analytical tool is due to its significantly improved peak capacity and well documented ability to perform complex sepa- rations that were impossible just a few years ago using classical one-dimensional (1D) GC approaches. The ability to adopt this 0021-9673/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.chroma.2008.03.008