John B. Phillips

Comprehensive two-dimensional gas chromatography: a hyphenated method with strong coupling between the two dimensions ☆

Comprehensive two-dimensional gas chromatography (GC x GC) provides a true orthogonal separation system. It is explained and demonstrated that it generates a peak capacity that is approximately equal to the product of the peak capacities of the two individual separation systems. The resulting peaks are ordered in a two-dimensional plane in bands of compounds with the same characteristics. Quantitation of the separated (groups of) components is fundamentally not different from one-dimensional gas chromatography, but the sensitivity is far better and true baseline is always available. The two co-ordinates of each peak in the plane make the identification more reliable. Instrumental considerations of GC x GC are discussed. The three designs of contemporary GC x GC systems are presented and compared. Although the technique is still very young, a number of applications on complex samples as petroleum and environmental samples have already been reported. Finally, the future perspectives of GC x GC are discussed.

Comprehensive multi-dimensional gas chromatography

In comprehensive two-dimensional gas chromatography, the entire chromatogram eluting from the primary column is submitted to the secondary column for a second independent separation. The resulting two-dimensional chromatogram has peaks scattered about a plane rather than along a line. Peak capacity can be very large allowing much more complete separation of complex mixtures such as petroleum products. Moderately complex samples can be separated much more quickly than is possible with high-resolution one-dimensional gas chromatography. The method is a true hyphenated instrument analogous to gas chromatography-mass spectrometry.

Separation orthogonality in temperature-programmed comprehensive two-dimensional gas chromatography.

In a comprehensive two-dimensional gas chromatograph, a thermal modulator serially couples two columns containing dissimilar stationary phases. The secondary column generates a series of high-speed secondary chromatograms from the sample stream formed by the chromatogram eluting from the primary column. This series of secondary chromatograms forms a two-dimensional gas chromatogram with peaks dispersed over a retention plane rather than along a line. The method is comprehensive because the entire primary column chromatogram is transmitted through the secondary column with fidelity. One might expect that a two-dimensional separation in which both dimensions are basically the same technique, gas chromatography, would be inefficient because the two dimensions would behave similarly, generating peaks whose retentions correlate across dimensions. Applying a temperature program to the two columns, however, can tune the separation to eliminate this inefficiency. The temperature program reduces the retentive power of the secondary column as a function of progress of the primary chromatogram such that the retention mechanism of the primary column is eliminated from the second dimension. Retention of a substance in the second dimension is then determined by the difference in its interaction with the two stationary phases. Retention times in the second dimension then fall within a fixed range, and the whole retention plane is accessible. In a properly tuned comprehensive two-dimensional chromatogram, retention times in the two dimensions are independent of each other, and the two-dimensional chromatogram is orthogonal. Orthogonality is important for two reasons. First, an orthogonal separation efficiently uses the separation space and so has either greater speed or peak capacity than nonorthogonal separations. Second, retention in the two dimensions of an orthogonal chromatogram is determined by two different and independent mechanisms and so provides two independent measures of molecular properties.

Thermal modulation: A chemical instrumentation component of potential value in improving portability

A thermal modulator is a chemical instrument component that controls the temperature of a short length of gas chromatographic column as a function of time and position. Simple rotation of a heater element over a capillary gas chromatography column accumulates sample substances, focuses them into a sharp concentration pulse, and accelerates the concentration pulse onto a following column or to a detector. The series of concentration pulses formed by continuous operation of a thermal modulator is a modulation signal that is potentially useful in the design of high-speed, portable instrumentation.

Applications of multiplex gas chromatography to the determination of organics in solid samples

Abstract A multiplex gas chromatographic technique for determining nicotine in cigarettes is described and multiplex gas chromatograms of volatiles from other solid samples are also presented. Direct headspace sampling, which is carried out in a sampler modified from an injector, provides a continuous gas sample stream for multiplex gas chromatography. Volatiles evaporating from the sample are picked up and carried by a carrier gas to a thermal desorption modulator and column. Concentrations to be determined are modulated by the modulator. The thermal desorption modulator is a short section at the head of the capillary column. The modulator section is coated externally with a thin electrically conductive film. An electrical current pulse applied to the thin film heats the modulator section and the stationary phase within it. When the stationary phase is heated and cooled, it releases and adsorbs substances into and from the flowing carrier gas. The signal form resembles a derivative of a chromatographic injection. Large-volume, continuously flowing or headspace samples can be accepted directly. The modulator is simple and effective and is used as both a trap and desorption device. Multiple modulation pulse signals applied during an extended sample introduction period result in a multiplex detector output signal, from which the chromatograms are recovered and computed by applying some fairly simple computational techniques such as cross-correlation. It is possible to determine volatiles in solid samples where the solid material is neither volatile nor soluble in a solvent. No sample pretreatment, preconcentration, extraction or distillation are required. Both the accuracy and precision achieved are fairly good.

Use of thermal desorption modulators in gas chromatograph/mass spectrometer

A thermal desorption modulator is comprised of a short length of tubing with some adsorbent or chromatographic stationary phase inside. A flowing stream containing a sample can be introduced into an instrument such as a gas chromatograph through a modulator. The concentration of samples in the flowing stream can be effectively modulated by modulating the temperature of the modulator. The purpose of this study is to demonstrate the application of a thermal desorption modulator in GC/MS.

Automated On-Line Analysis Using Thermal Desorption Modulators

ABSTRACT A thermal desorption modulator is a short length of the analytical column (or a piece of tubing containing an adsorbent) placed at the head of the column in a GC. A carrier gas containing sample may be continuously introduced into a chromatographic column through a modulator. Analytes transfer from the carrier gas onto a stationary phase within the modulator and can be released from the modulator by rapid heating. A concentrated pulse of analytes generated in this fashion serves the same purpose as an injection. Thus, chromatography using modulators does not need any injection device because the equivalent of an injection is generated internally within the flow stream. In addition to serving as an alternative to an injection valve, a modulator also preconcentrates the sample. The purpose of this paper is to demonstrate the use of a thermal desorption modulator in automated on-line analysis. This had been done by continuously determining the concentration of H2S in the effluent stream from a react...

Determination of activity coefficients of binary liquids by capillary gas chromatography with thermal desorption modulation for direct headspace sampling

Abstract Activity coefficients of the binary liquid mixtures benzene-toluene and acetonechloroform were determined using thermal desorption modulation for direct headspace sampling into a capillary gas chromatograph. A thermal desorption modulator is a short, heated section at the head of the column. An electrical current pulse applied to a thin conductive film heats the modulator section and the stationary phase within it, releasing any retained substances as a concentration pulse which flows into the column. The modulator acts like an automatic and highly reproducible small-volume injector for a continuously flowing sample stream. Short-term relative standard deviations obtained using this technique are approximately 2%.

Large volume sampling without preconcentration for continuous gas chromatography

A milligram quantity of adsorbent material was placed at the head of a gas chromatographic column. The initial part of the column itself served as this adsorbent in some experiments. A continuously flowing gaseous sample was passed through a length of narrow bore metal tubing, the precolumn adsorbent, and the gas chromatographic column. A computer generated signal heated the metal tubing which heated the sample gas stream and the precolumn adsorbent. The changing adsorbent temperature generated a changing concentration of adsorbable substances in the gas stream. Thus, the computer generated a chemical concentration signal usable for chromatographic analysis. This technique is related to chromatographic analysis using preconcentration but has several important advantages.