A Robust Thermal Modulator for Comprehensive Two-Dimensional Gas Chromatography

In comprehensive two-dimensional gas chromatography (GC×GC), two capillary columns are connected in series through an interface known as a “thermal modulator”. This device transforms effluent from the first capillary column into a series of sharp injection-like chemical pulses suitable for high-speed chromatography on the second column. Dramatic increases in the resolving power, sensitivity, and speed of the gas chromatograph result. This paper describes the development of a robust and reliable thermal modulator for GC×GC.     

Retention time locking procedure for comprehensive two-dimensional gas chromatography

In gas chromatography (GC) reproducible retention times are in many cases highly favorable or in some cases even required. In one-dimensional GC, retention time shifts can be eliminated or minimized using a procedure called retention time locking (RTL). This procedure is based on adjusting the (constant) column head pressure. Unfortunately, this RTL procedure cannot be used in comprehensive two-dimensional gas chromatography (GC × GC) given the fact that peaks will shift in both dimensions. Adjusting the column head pressure in GC × GC will only minimize or eliminate the primary retention time shifts. In this paper, a fast and easy to perform, two-step retention time locking procedure for two-dimensional gas chromatography (2D-RTL) is proposed and its feasibility is demonstrated. This 2D-RTL procedure involves adjustment of the column head pressure or constant column flow, followed by the adjustment of the so-called effective secondary column length. The secondary column length is increased or decreased, simply by moving it stepwise through the modulator. It is demonstrated that retention time shifts in both the primary- and secondary-dimension, which may occur after e.g. replacing the column set, can be minimized to less than half peak base width. The proposed 2D-RTL procedure is used successfully for approximately 1 year in our laboratory.     

A systematic analysis of a flow-switching modulator for comprehensive two-dimensional gas chromatography

A simple flow-switching device has been designed for use as a comprehensive two-dimensional gas chromatography modulator. The device is constructed from fused silica tubing, t-unions, and a solenoid valve. A series of experiments were conducted to determine the influence of primary flow, secondary flow, modulation time, and device dimensions on the performance of the modulator. The flow-switching device was found to produce pulses with widths near the theoretical minimum. High-performance was maintained over a wide range of modulation times. The flow-switching device did not introduce extra broadening along the primary retention axis. However, the modulator performance was optimal only over a narrow range of primary to secondary flow ratios. The ideal flow ratio is determined by the dimensions of the tubes that connect the t-unions. A simple flow resistance model has been developed that can predict the dimensions that will produce optimal results for a specified primary to secondary flow ratio. Thus, it is possible to construct a device that operates near the theoretical limit without numerous alterations. Under optimal conditions, the flow switching modulator generates peaks that are narrower than those produced by a diaphragm valve.     

Using electronic pressure programming to reduce the decomposition of labile compounds during splitless injection

A split/splitless capillary injection port has been developed for electronic pressure programming (EPP) in gas chromatography. The inlet may be operated in several modes: constant pressure, constant flow, vacuum compensation (for gas chromatography–mass spectrometry (GC-MS)), pressure-programmed, or a combination mode enabling a pressure program to be followed by constant flow. A pressure-programming technique has been tried which uses high pressure (high column flow rate) at the time of injection followed by reduction in inlet pressure to a value required for normal chromatography. Sample is swept rapidly from the inlet and into the column, reducing contact with the hot, active inlet surfaces which cause sample decomposition. The decomposition of endrin and 4,4′-DDT, two labile pesticides, can be substantially reduced using this technique and modest improvements were also observed with the carbamate pesticide carbaryl.     

Flow model for coupled-column gas chromatography systems

One of the challenges in performing comprehensive two-dimensional gas chromatographic separations is being able to predict the average linear velocities of the carrier gas in the two columns, especially when they have different diameters. The problem is compounded for loop-type modulators, where two thermal trapping zones that switch from hot to cold and back simultaneously are separated by a delay loop. If the linear velocity in the loop is not tuned to the length of the loop and the modulation period, the dual-stage modulation may cease to work properly. A model has been developed that calculates the flow rates in the columns and predicts appropriate delay loop dimensions for a given set-up. Additionally, the model determines the pressure ramp that needs to be used in order to maintain constant average linear velocity within the modulator loop throughout the course of the separation.     

Use of a recently developed thermal modulator within the context of comprehensive two‐dimensional gas chromatography combined with time‐of‐flight mass spectrometry: Gas flow optimization aspects

The present research is based on the use of a recently developed comprehensive two‐dimensional gas chromatography thermal modulator, which is defined as solid‐state modulator. The transfer device was installed on top of a single gas chromatography oven, while benchtop low‐resolution time‐of‐flight mass spectrometry was used to monitor the compounds exiting the second analytical column. The solid‐state modulator was first described by Luong et al. in 2016, and it is a moving modulator that does not require heating and cooling gases to generate comprehensive two‐dimensional gas chromatography data. The accumulation and remobilization steps occur on a trapping capillary, this being subjected to thermoelectric cooling and micathermic heating. In this study, the effects of the gas linear velocity on the modulation performance were evaluated by using two different uncoated trapping capillaries, viz., 0.8 m × 0.25 mm id and 0.8 m × 0.20 mm id. Solid‐state modulator applications were carried out on a standard solution containing n‐alkanes (C_9, C_10, C₁₂), and on a sample of diesel fuel. The results indicated that the type of trapping capillary and gas velocity have a profound effect on modulation efficiency.     

Flow-switching device for comprehensive two-dimensional gas chromatography

A simple flow-switching device has been developed as a differential flow modulator for comprehensive two-dimensional gas chromatography (GC x GC). The device is assembled from tubing, four tee unions, and a solenoid valve. The solenoid valve is located outside the oven of the gas chromatograph and is not in the sample path. The modulation technique has no inherent temperature restrictions and passes 100% of the primary column effluent to the secondary column(s). Secondary peaks are produced with widths at half maximum less than 100 ms when operating in GC x 2GC mode with a 2.0 s modulation period. The efficacy of this approach is demonstrated through the analysis of a standard mixture of volatile organic compounds (VOCs) and diesel fuel.     

Trace analysis of organics in aqueous samples by concentration in plastic tubing and multiplex gas chromatography

Summary The inside wall of an uncoated polyethylene capillary traps organic substances from a water sample pumped through it by a nitrogen gas stream. Heating the capillary in a chromatographic oven slowly releases the trapped organic substances from the wall. Nitrogen carrier gas transports sample substances released through a thermal desorption modulator and onto a chromatographic column. Pulsing the temperature of the modulator modulates the concentrations of sample components as they enter the column. Computing the cross correlation of the detector output signal against the applied modulation signal generates the chromatogram. Detection limits below 1 ppb are possible using a flame ionization detector. No sample pretreatment or cold trap is required.     

Effects of temperature and flow regulated carbon dioxide cooling in longitudinally modulated cryogenic systems for comprehensive two-dimensional gas chromatography

Two different modes of temperature regulation in longitudinally modulated cryogenic systems (LMCSs) for comprehensive two-dimensional gas chromatography (GC x GC) were compared. Carbon dioxide was used as coolant. In the first mode of operation, the temperature of the trap was regulated to pre-set temperature using a digital temperature controller ("the constant temperature mode"). In the second, the temperature was regulated to a fixed negative offset to the oven temperature by using a constant flow of CO2 ("the constant flow mode"). A number of problems were occasionally observed using the constant temperature mode: (1) severe band broadening of high boiling analytes in the second dimension; (2) non-Gaussian reconstructed first-dimension peak profiles; (3) high background due to modulation of first-dimension column bleed. It was concluded that these problems were associated with inefficient solute remobilization at low LMCS trap temperatures (1 and 2) or large trap temperature fluctuations (3). These problems could be avoided or significantly reduced by using the constant flow mode. Best results were obtained as the trap temperature was kept about 70 degrees C below the oven temperature.     

Dependence of the efficiency of a capillary column in gas chromatography on the relative pressure of the carrier gas

The effect of relative pressure on the efficiency of an open capillary column in gas chromatography was studied. It was shown that the relative pressure was not the only parameter determining the column efficiency. The pressure drop in the column is an additional parameter. At high values of relative pressure, the pressure drop in the column becomes determining for the column efficiency. The smallest value of a height equivalent to a theoretical plate (HETP) is achieved at the minimum values of the pressure drop and the relative pressure, which is accompanied by a decrease in the optimal flow rate of the carrier gas and an increase in the time of determination. The maximum improvement in the column efficiency is determined by the column properties and can exceed 12.5%, that is, the value predicted by Cramers for open capillary columns.     

Jet‐Cooled Thermal Modulator for Comprehensive Multidimensional Gas Chromatography

A jet of cool gas is used to locally cool a section of modulator tube in the presence of the stirred oven bath of a GC×GC instrument. Local cooling decouples the temperature of the modulator tube from that of the first dimension column, which was 100 meters long. Overall resolution of the GC×GC experiment was improved as a result. Another consequence of the jet‐cooled thermal modulation structure is the elimination of moving parts in the GC oven. By pulsing cold and hot jets of gas onto a modulator tube with solenoid valves, two stage thermal modulation can be obtained without the complexity of moving parts in the vicinity of the capillary columns.     

Factors affecting peak shape in comprehensive two-dimensional gas chromatography with non-focusing modulation

In the case of a non-focusing modulator for comprehensive two-dimensional gas chromatography (GC × GC), the systematic distortions introduced when the modulator loads the second-dimension column give rise to a characteristic peak shape. Depending on the operating conditions this systematic distortion can be the dominant component of the second-dimension elution profiles in the GC × GC peak. The present investigation involved a systematic investigation of peak shape in pulsed-flow modulation (PFM)–GC × GC. It is shown that low flow ratio can lead to significant peak skewing and increasing the flow ratio reduces the magnitude of peak skewing. Validation of the peak shape model is made by comparison with experimental data. The residuals from the fitting process (normalised to the maximum detector response) vary between –1.5% and +2.6% for an isothermal model and between –1.0% and +3.0% for a temperature-programmed model.     

High-speed gas chromatography: an overview of various concepts.

An overview is given of existing methods to minimise the analysis time in gas chromatography (GC) being the subject of many publications in the scientific literature. Packed and (multi-) capillary columns are compared with respect to their deployment in fast GC. It is assumed that the contribution of the stationary phase to peak broadening can be neglected (low liquid phase loading and thin film columns, respectively). The treatment is based on the minimisation of the analysis time required on both column types for the resolution of a critical pair of solutes (resolution normalised conditions). Theoretical relationships are given, describing analysis time and the related pressure drop. The equations are expressed in reduced parameters, making a comparison of column types considerably simpler than with the conventional equations. Reduction of the characteristic diameter, being the inside column diameter for open tubular columns and the particle size for packed columns, is the best approach to increase the separation speed in gas chromatography. Extremely fast analysis is only possible when the required number of plates to separate a critical pair of solutes is relatively low. Reducing the analysis time by reduction of the characteristic diameter is accompanied by a proportionally higher required inlet pressure. Due to the high resistance of flow of packed columns this seriously limits the use of packed columns for fast GC. For fast GC hydrogen has to be used as carrier gas and in some situations vacuum-outlet operation of capillary columns allows a further minimisation of the analysis time. For fast GC the columns should be operated near the conditions for minimum plate height. Linear temperature programmed fast GC requires high column temperature programming rates. Reduction of the characteristic diameter affects the sample capacity of the "fast columns". This effect is very pronounced for narrow-bore columns and in principle non-existing in packed columns. Multi-capillary columns (a parallel configuration of some 900 narrow-bore capillaries) take an intermediate position.     

Modified semi-rotating cryogenic modulator for comprehensive two-dimensional gas chromatography

A previously constructed semi-rotating cryogenic modulator was modified for comprehensive two-dimensional gas chromatography (GC×GC). The retention time repeatability was improved by replacing the modulator control program unit with a new system. Peak widths obtained with the modified modulator were comparable with those obtained with the previous modulator and other modulator types. The modulator was easy to construct and it can be installed in any commercial GC system. The constructed GC×GC–FID system and data obtained by gas chromatography–mass spectrometry (GC–MS) were used for identification of unknowns in forest aerosol samples. Figure A semi-rotating cryogenic modulator in which modulation is based on two-step cryogenic trapping with continuously flowing carbon dioxide has been developed for comprehensive two-dimensional gas chromatography     

Advances in Capillary Chromatography

  Abstract：Capillary columns are used in both capillary liquid chromatography and capillary electrochromatography. The design for capillary liquid chromatography is discussed in comparison with capillary gas chromatography. The difference of diffusion coefficient in gas and liquid phase is a key role. The study for obtaining a high performance capillary liquid chromatography is discussed. Capillary electrochromatography is recently interesting for its instinct ability to realize a high performance chromatography. Capillary electrochromatography with and without pressurized flow is reviewed briefly. Instrumentation for capillary electrochromatography with pressurized flow is discussed. The port of splitting, and gradient elution of both solution and potential are described. The new findings of both the variation of column resistance and capacity factor according to the value of applied electric voltage are also discussed.     

A flexible loop-type flow modulator for comprehensive two-dimensional gas chromatography

The present investigation is focused on a simple flow modulator (FM), for comprehensive two-dimensional gas chromatography (GC×GC). The interface is stable at high temperatures, and consists of a metallic disc (located inside the GC oven) with seven ports, which are connected to an auxiliary pressure source via two branches, to the first and second dimension, to a waste branch (linked to a needle valve) and to an exchangeable modulation loop (2 ports). The ports are connected via micro-channels, etched on one of the inner surfaces of the disc. Modulation is achieved using a two-way electrovalve, connected on one side to the additional pressure source, and to the two metal branches, on the other. An FM enantio-GC×polar-GC method (using a flame ionization detector) was optimized (a 40-μL loop was employed), for the analysis of essential oils. As an example, an application on spearmint oil is shown; the method herein proposed was subjected to validation. Finally, an FM GC×GC diesel experiment was carried out, using an apolar-polar column combination, to demonstrate the effectiveness of the modulator in the analysis of a totally different sample-type.     

Improving splitless injection with electronic pressure programming

An experimental injection port has been designed for split or splitless sample introduction in capillary gas chromatography; the inlet uses electronic pressure control, in order that the column head pressure may be set from the GC keyboard, and the inlet may be used in the constant flow or constant pressure modes. Alternatively, the column head pressure may be programmed up or down during a GC run in a manner analogous to even temperature programming. Using electronic pressure control, a method was developed which used high column head pressures (high column flow rates) at the time of injection, followed by rapid reduction of the pressure to that required for optimum GC separation. In this way, high flow rates could be used at the time of splitless injection to reduce sample discrimination, while lower flow rates could be used for the separation. Using this method, up to 5 μl of a test sample could be injected in the splitless mode with no discrimination; in another experiment, 2.3 times as much sample was introduced into the column by using electronic pressure programming. Some GC peak broadening was observed in the first experiment.     

Combining monolithic and hollow capillary columns under conditions of two-dimensional gas chromatography

A monolithic column with the optimum structure of its monolithic layer is considered as a second dimension column under the conditions of two-dimensional gas chromatography. It is shown that despite the considerable difference between the working pressures of different columns, sample transfer from a hollow capillary column to a monolithic column is possible when a looped switching valve is used. It is found that the additional broadening of the chromatographic zone observed during sample transfer can be effectively suppressed by using an additional flow splitter. At the same time, reducing the volume of the switching valve loop is inefficient and does not allow effective separation by the monolithic column.     

Comprehensive two-dimensional liquid chromatography: Ion chromatography × reversed-phase liquid chromatography for separation of low-molar-mass organic acids

In the work presented here a novel approach to comprehensive two-dimensional liquid chromatography is evaluated. Ion chromatography is chosen for the first-dimension separation and reversed-phase liquid chromatography is chosen for the second-dimension separation mode. The coupling of these modes is made possible by neutralising the first-dimension effluent, containing KOH, prior to transfer to the second-dimension reversed-phase column. A test mixture of 24 low-molar-mass organic acids is used for optimisation of the system. Three food and beverage samples were analysed in order to evaluate the developed methodology, the resulting two-dimensional separation is near-orthogonal, the set-up is simple and all instrumental components are available commercially. The method proved to be robust and suitable for the analysis of wine, orange juice and yogurt.     

Gas chromatographic – mass spectrometric analysis of compounds generated upon thermal degradation of some stationary phases in gas chromatography – Part II

The applicability of capillary columns for gas chromatography is often limited by stationary phase degradation at elevated temperatures. In order to achieve a better understanding of column thermostability, column bleed products have been analyzed qualitatively by gas chromatography-mass spectrometry (GC-MS). Three silicone stationary phases were studied: SE-52, SE-54, and OV-1701. Each of these was immobilized in the columns. The proportion of cyclics containing diphenyl in column bleed from SE-52 reflected to a large extent the composition of the polymer, while for SE-54 the proportion of such cyclics was unexpectedly high. The polar moiety of OV-1701, the cyanopropyl(phenyl)siloxy unit, was found to exert a highly destabilizing effect on the polymer, and the thermal degradation products consisted mainly of cyclosiloxanes containing this unit.