A novel injection technique for high-speed gas chromatography is demonstrated. Synchronized dual-valve injection is shown to provide peak widths as low as 1.5 ms (width at half-height) for an unretained analyte. This was achieved using a 0.5-m DB-5 column with an internal diameter of 100 μm and a film thickness of 0.4 μm operated at a temperature of 150 °C with a column absolute head pressure of 85 psi, resulting in a dead time of only to = 26 ms (∼1900 cm/s, 26 mL/min). Using the DB-5 column in a 1-m length under the same instrumental parameters, with a resulting linear flow velocity of 935 cm/s (12.7 mL/min, to = 117 ms), a minimum peak width of 3.3 ms was obtained. During an isothermal separation, 10 analytes were separated in a time window of 400 ms. A rigorous comparison of experimental and theoretical band-broadening data based on the Golay equation showed that band broadening is limited almost entirely by the chromatographic band broadening terms expressed by the Golay equation and not by extra column band broadening due to the injection process. Synchronized dual-valve injection offers a rugged and inexpensive design, providing extremely reproducible injections with peak height precision of 2.4% (RSD) and low run-to-run variation in retention times, with an average standard deviation less than 0.1 ms. Herein, synchronized dual-valve injection is demonstrated as a proof of principle using high-speed diaphragm valves. It is foreseen that the injection technique could be readily implemented using a combination of thermal modulation and high-speed valve hardware, thus optimizing the mass transfer and not significantly sacrificing the limit of detection performance for high-speed GC. Further implications are that, if properly implemented, high-speed temperature programming coupled with this new technology should lead to very large peak capacities for ∼1-s separations.
Chemical analysis using second-order data collected on hyphenated instruments has proven advantages over first-order or zero-order techniques due to what is known as the second-order advantage. The primary second-order advantage is the ability to perform analysis in the presence of unknown interferences. This work demonstrates another key advantage of second-order chemical analysis, that is, the ability to standardize data sets of a second-order chromatographic analyzer under conditions which result in retention time variations along the chromatographic axis. An objective technique to standardize second-order chromatographic−spectral data is both theoretically and experimentally developed and tested. This method corrects for retention time shifts that occur between the analysis of the calibration sample and "unknown" samples. When this technique is combined with bilinear data analysis techniques like generalized rank annihilation method (GRAM), standardization and quantitation can be performed in the presence of unknown interferences with a single calibration sample. Most signal inconsistencies in second-order chromatographic data are confined to shifts of the time axis in the chromatographic profile. This retention time shift correction method is objective because it relies upon spectral signal shape and an understanding of the instrumentation. Retention time correction of this type would not be objective for first-order chromatographic analysis because retention time is the only qualitative information present. In one example of experimental evaluation, quantitation of a single analyte in a sample of five chemical components is performed using liquid chromatography with absorbance detection (LC/UV−vis). Both the chromatographic and spectral signals of these five chemical components are highly overlapped. In this example, a retention time shift between the calibration and "unknown" data sets of 0.2 s resulted in a 20% quantitation error prior to standardization. After alignment of the data sets using second-order chromatographic standardization, quantitative error was reduced to nearly 1%. Theoretical simulations which evaluate the performance of this technique as a second-order chromatographic retention time correction method were performed for a wide range of resolution and signal-to-noise values. In simulations where chromatographic resolution was 0.3 or below, quantitative precision improvements resulting from second-order chromatographic standardization ranged from 3-fold to 10-fold. The standardization method presented should be generally applicable to chromatography hyphenated with all forms of spectroscopic detection, such as gas chromatography/mass spectrometry (GC/MS).
Comprehensive, two-dimensional gas chromatography (GC x GC) is used in conjunction with trilinear partial least squares (Tri-PLS) to quantify the percent weight of naphthalenes (two-ring aromatic compounds) in jet fuel samples. The increased peak capacity and selectivity of GC x GC makes the technique attractive for the rapid, and possibly less tedious analysis of jet fuel. The analysis of complex mixtures by GC x GC is further enhanced through the use of chemometric techniques, including those designed for use on 2-D data such as Tri-PLS. Unfortunately, retention time variation, unless corrected, can be an impediment to chemometric analysis. Previous work has demonstrated that the effects of retention time variation can be mitigated in sub-regions of GC x GC chromatograms through the application of an objective retention time alignment algorithm based on rank minimization. Building upon this previous work, it is demonstrated here that the effects of retention time variation can be mitigated throughout an entire GC x GC chromatogram with an objective retention time alignment algorithm based on windowed rank minimization alignment. A significant decrease in calibration error is observed when the algorithm is applied to chromatograms prior to construction of Tri-PLS models. Fourteen jet fuel samples with known weight percentages of naphthalenes (ASTM D1840) were obtained. Each sample was subjected to five replicate five-minute GC x GC separations over a period of two days. A subset of nine samples spanning the range of weight percentages of naphthalenes was chosen as a calibration set and Tri-PLS calibration models were subsequently developed in order to predict the naphthalene content of the samples from the GC x GC chromatograms of the remaining five samples. Calibration models constructed from GC x GC chromatograms that were retention time corrected are shown to exhibit a root mean square error of prediction of roughly half that of calibration models constructed from uncorrected chromatograms. The error of prediction is lowered further to a value that nearly matches the uncertainty in the standard percent weight values (ca. 1% of the median percent volume value) when the aligned chromatograms are truncated to include only regions of the chromatogram populated by naphthalenes and compounds of similar polarity and boiling point.
