Temperature-programmed Separations Research Articles

Belt-Mounted Micro-Gas-Chromatograph Prototype for Determining Personal Exposures to Volatile-Organic-Compound Mixture Components.

We describe a belt-mountable prototype instrument containing a gas chromatographic microsystem (μGC) and demonstrate its capability for near-real-time recognition and quantification of volatile organic compounds (VOCs) in moderately complex mixtures at concentrations encountered in industrial workplace environments. The μGC comprises three discrete, Si/Pyrex microfabricated chips: a dual-adsorbent micropreconcentrator-focuser for VOC capture and injection; a wall-coated microcolumn with thin-metal heaters and temperature sensors for temperature-programmed separations; and an array of four microchemiresistors with thiolate-monolayer-protected-Au-nanoparticle interface films for detection and recognition-discrimination. The battery-powered μGC prototype (20 × 15 × 9 cm, ∼2.1 kg sans battery) has on-board microcontrollers and can autonomously analyze the components of a given VOC mixture several times per hour. Calibration curves bracketing the Threshold Limit Value (TLV) of each VOC yielded detection limits of 16-600 parts-per-billion for air samples of 5-10 mL, well below respective TLVs. A 2:1 injection split improved the resolution of early eluting compounds by up to 63%. Responses and response patterns were stable for 5 days. Use of retention-time windows facilitated the chemometric recognition and discrimination of the components of a 21-VOC mixture sampled and analyzed in 3.5 min. Results from a "mock" field test, in which personal exposures to time-varying concentrations of a mixture of five VOCs were measured autonomously, agreed closely with those from a reference GC. Thus, reliable, near-real-time determinations of worker exposures to multiple VOCs with this wearable μGC prototype appear feasible.

Compact prototype microfabricated gas chromatographic analyzer for autonomous determinations of VOC mixtures at typical workplace concentrations

This report concerns a benchtop prototype instrument containing a gas chromatographic microanalytical system (μGC) designed for the selective determination of multiple airborne volatile organic compounds (VOCs) at concentrations in the vicinity of recommended occupational exposure limits. The core microsystem consists of a set of discrete Si-microfabricated devices: a dual-cavity, adsorbent-packed micro-preconcentrator-focuser (μPCF) chip that quantitatively captures and thermally desorbs/injects VOCs with vapor pressures between ~0.03 and 13 kPa; tandem micro-column (μcolumn) chips with cross-linked PDMS wall-coated stationary phases capable of temperature-programmed separations; and an integrated array of five μchemiresistors (μCR) coated with different thiolate-monolayer protected gold nanoparticle (MPN) interface films that quantifies and further differentiates among the analytes by virtue of the response patterns generated. Other key components include a pre-trap for low-volatility interferences, a split-flow injection valve, and an onboard He carrier–gas canister. The assembled unit measures 19×30×14 cm, weighs ~3.5 kg, operates on AC power, and is laptop/LabVIEW controlled. Component- and system-level tests of performance demonstrated injection bandwidths <1 s, a μcolumn capacity of ≥8 μg injected mass, linear calibration curves, no humidity effects, excellent medium-term (that is, 1 week) reproducibility, autonomous operation for 8 h, detection limits below Threshold Limit Values (TLV) for 10 mL air samples collected in 1 min, and response patterns that enhanced vapor recognition. The determination of a 17-VOC mixture in the presence of seven interferences was performed in 4 min. Results augur well for adapting the microsystem to an all-MEMS wearable μGC currently under parallel development.

Subcritical Water Chromatography with Electrochemical Detection

Reverse phase liquid chromatography (RPLC) is a commonly used separation and analysis technique. RPLC typically employs mixtures of organic solvents and water or aqueous buffers as the mobile phase. With RPLC being used on a global scale, enormous quantities of organic solvents are consumed every day. In addition to the purchasing cost of the hazardous solvents, the issue of waste disposal is another concern. At ambient temperature, water is too polar to dissolve many organic substances. Therefore, although water is nontoxic it cannot be used to replace the mobile phase in RPLC since organic analytes will not be eluted. Subcritical water chromatography may be an alternative. The characteristics of water, such as polarity, surface tension, and viscosity, can be altered by manipulating water’s temperature, thus making it behave like an organic solvent. The aim of this study was to evaluate the feasibility of separation using water mobile phase and detection by an electrochemical (EC) detector. The classes of analytes studied were neurotransmitters/metabolites, nucleic acids/heterocyclic bases, and capsaicinoids. Both isothermal and temperature-programmed separations were carried out. The separation temperature ranged from 25 to 100 °C. For separations of all three classes of solutes, the retention time was decreased with increasing temperature, thus shortening the analysis time. The peaks also became narrower as temperature increased. The limit of detection of neurotransmitters/metabolites ranges from 0.112 to 0.224 ppm.

Rapid determination of thermodynamic parameters from one-dimensional programmed-temperature gas chromatography for use in retention time prediction in comprehensive multidimensional chromatography

A new method for estimating the thermodynamic parameters of ΔH(T0), ΔS(T0), and ΔCP for use in thermodynamic modeling of GC×GC separations has been developed. The method is an alternative to the traditional isothermal separations required to fit a three-parameter thermodynamic model to retention data. Herein, a non-linear optimization technique is used to estimate the parameters from a series of temperature-programmed separations using the Nelder–Mead simplex algorithm. With this method, the time required to obtain estimates of thermodynamic parameters a series of analytes is significantly reduced. This new method allows for precise predictions of retention time with the average error being only 0.2s for 1D separations. Predictions for GC×GC separations were also in agreement with experimental measurements; having an average relative error of 0.37% for 1tr and 2.1% for 2tr.

Rapid Temperature-Programmed Separation and Retention Prediction on a Novel Packed-Capillary Column in Gas Chromatography

Novel packed-capillary columns for gas chromatography were developed with a thin-wall stainless-steel capillary of 1.0 mm i.d.; and rapid temperature-programmed separations have been carried out after a basic evaluation concerning the compatibility of these columns to the temperature program. With a numerical integration method, the retention of several test analytes during temperature-programmed elution was successfully estimated. In order to confirm the suitability of the packed-capillary columns to relatively fast temperature programming up to 40 degrees C/min, theoretically predicted retention data were compared with that actually measured. The results suggested a good separation performance of the newly developed packed-capillary columns as a particle-packed column conventionally used. Also, the compatibility to a rapid temperature-programmed operation was quite satisfactory for almost all of the separations currently done in typical analytical laboratories.

The fabrication of all-silicon micro gas chromatography columns using gold diffusion eutectic bonding

Temperature programming of gas chromatography (GC) separation columns accelerates the elution rate of chemical species through the column, increasing the speed of analysis, and hence making it a favorable technique to speedup separations in microfabricated GCs (micro-GC). Temperature-programmed separations would be preferred in an all-silicon micro-column compared to a silicon-Pyrex® micro-column given that the thermal conductivity and diffusivity of silicon is 2 orders of magnitude higher than Pyrex®. This paper demonstrates how to fabricate all-silicon micro-columns that can withstand the temperature cycling required for temperature-programmed separations. The columns were sealed using a novel bonding process where they were first bonded using a gold eutectic bond, then annealed at 1100 °C to allow gold diffusion into silicon and form what we call a gold diffusion eutectic bond. The gold diffusion eutectic-bonded micro-columns when examined using scanning electron microscopy (SEM), scanning acoustic microscopy (SAM) and blade insertion techniques showed bonding strength comparable to the previously reported anodic-bonded columns. Gas chromatography-based methane injections were also used as a novel way to investigate proper sealing between channels. A unique methane elution peak at various carrier gas inlet pressures demonstrated the suitability of gold diffusion eutectic-bonded channels as micro-GC columns. The application of gold diffusion eutectic-bonded all-silicon micro-columns to temperature-programmed separations (120 °C min−1) was demonstrated with the near-baseline separation of n-C6 to n-C12 alkanes in 35 s.

Model for predicting comprehensive two-dimensional gas chromatography retention times

A model for approximating the relative retention of solutes in comprehensive two-dimensional gas chromatography (GC × GC) is presented. The model uses retention data from standard single-column temperature-programmed separations. The one-dimensional retention times are first converted into retention indices and then these indices are combined in a simple manner to generate a retention diagram. A retention diagram is an approximation of the two-dimensional chromatogram that has retention order and spacing in both dimensions similar to that found in the experimental chromatogram. If required, the retention diagram can be scaled to more closely resemble the two-dimensional chromatogram. The model has been tested by using retention time data from single-column gas chromatography–mass spectrometry and valve-based GC × GC. A total of 139 volatile organic compounds (VOCs) were examined. Approximately half of the VOCs had a single functional group and a linear alkyl chain (i.e., compounds with the structure Z (CH 2) n H). The retention diagrams had primary retention orders that were in excellent agreement with the GC × GC chromatograms. The relative secondary retention order for compounds with similar structures was also accurately predicted by the retention diagram. However, the relative secondary retention for compounds with dissimilar structures, such as acyclic alcohols and multi-substituted alkylbenzenes, were less accurately modeled. This study demonstrates how readily available single-column retention time data can be used to provide an a priori estimate of the relative retention of solutes in a GC × GC chromatogram. Such a capability is useful for screening possible combinations of stationary phases.

First-generation hybrid MEMS gas chromatograph

The fabrication, assembly, and initial testing of a hybrid microfabricated gas chromatograph (microGC) is described. The microGC incorporates capabilities for on-board calibration, sample preconcentration and focused thermal desorption, temperature-programmed separations, and "spectral" detection with an integrated array of microsensors, and is designed for rapid determinations of complex mixtures of environmental contaminants at trace concentrations. Ambient air is used as the carrier gas to avoid the need for on-board gas supplies. The microsystem is plumbed through an etched-Si/glass microfluidic interconnection substrate with fused silica capillaries and employs a miniature commercial pump and valve subsystem for directing sample flow. The latest performance data on each system component are presented followed by first analytical results from the working microsystem. Tradeoffs in system performance as a function of volumetric flow rate are explored. The determination of an 11-vapor mixture of typical indoor air contaminants in less than 90 s is demonstrated with projected detection limits in the low part-per-billion concentration range for a preconcentrated air-sample volume of 0.25 L.

Studies on the long-term thermal stability of particulate and monolithic poly(styrene-divinylbenzene) capillaries in reversed-phase liquid chromatography

In the present study, the long-term high-temperature (>80 degrees C) and temperature programming stability of fused silica capillaries packed with 5 microm PLRP-S 300 A and monolithic PS-DVB capillaries (both 180 microm id x 6 cm) under reversed-phase conditions has been examined. In isothermal mode, the columns were defined as temperature-stable when a less than 10% change in apparent retention factors (k) and a less than 20% change in "retention time/peak width"-factors (n) of the probe solutes (proteins) were observed after passing 7,500 void volumes of effluent through the columns (about 100 h operation). According to these criteria, the PLRP-S and monolithic capillaries were defined temperature-stable at 100 and 130 degrees C, respectively. Furthermore, when continuously running temperature programs between 50 degrees C and the upper temperature limit determined in isothermal mode, virtually no change in k or n were observed on neither of the columns after running more than 35,000 void volumes or 1,600 temperature programs. Additionally, temperature-programmed reversed-phase separations of proteins on both types of capillaries are demonstrated and discussed.

Aspects and applications of non-aqueous high temperature packed capillary liquid chromatography

The effect of using high isothermal temperatures and temperature programming above ambient in liquid chromatography has been investigated by the use of relatively long packed reversed phase capillary columns and non-aqueous mobile phases. Efficiency measurements have been performed in the temperature interval 50– 175 °C, indicating that the best efficiency was achieved at 100 °C, with a corresponding optimum linear velocity of approximately 0.07 cm s–1. The efficiency measurements revealed that the linear velocity had to be slightly increased in order to operate at the optimum reduced plate height at elevated temperatures. The contribution from extra-column volumes increased the plate height at temperatures above 100 °C where the solute had low retention. This effect was closely examined by introducing larger pre-column dead volumes, demonstrating the need to minimize dead volumes. Temperature programming has successfully been used for partial characterization and for purity testing of different polymer additives. The within and between day precision of retention times for the temperature-programmed separations gave a relative standard deviation of 0.3–2.9%.

Temperature-programmed reversed-phase liquid chromatography with packed fused-silica columns

Temperature-programmed separations using fused-silica columns with reversed-phase packings are described. The columns were specially designed for temperature programming. This allows their operation at elevated temperatures under high pressure and the use of an UV detector at room temperature without back pressure. Temperature programming has successfully been applied to the separation of p-nitrobenzyl esters of fatty acids, which are eluted over a wide range of capacity factors under isothermal conditions. It is also shown that fused-silica columns are superior to glass capillary columns, and 1-m fused-silica columns with 10- and 5-μm packings generate 60,000 and 90,000 theoretical plates, respectively.

Gas chromatographic study of acetyl derivates of chlorinated guaiacols

The gas chromatography of acetyl derivatives of guaiacol and all chlorinated guaiacols has been studied on SE-30, OV-351 and Carbowax 20M capillary columns. The retention order and relative retention times under isotermal and temperature-programmed conditions are discussed. Different retention times for all mono- and dichloro compounds have been observed on the SE-30 column. However, all trichloroguaiacol isomers were separated only on polar columns. The best overall temperature-programmed separations have been achieved on an SE-30 capillary column.

Inorganic gas chromatography at high temperatures

The potential of gas chromatography as a separation method for inorganic compounds (organometallic complexes are excluded) is demonstrated. At temperatures up to 1200 K it is possible to separate a large number of elements as their halides, oxides or hydroxides if an inorganic compound of high melting point and low volatility is used as stationary phase. Examples of isothermal and temperature-programmed separations are given. The influences of temperature, column-length, stationary phase and reactive gas or reactive gas mixture on the separation are discussed.