Fast Temperature Programming Research Articles

Fast temperature programming in routine analysis of multiple pesticide residues in food matrices

Flash gas chromatographic (GC) analysis of 15 organophosphorus pesticides commonly occurring in food crops was performed using the Thermedics Detection EZ Flash upgrade kit installed in the oven of a HP 5890 Series II Plus gas chromatograph. The temperature program and splitless time period were the main parameters to be optimized. In the first set of experiments wheat matrix-matched standards were analyzed both by: (i) the flash GC technique (resistive heating of a 5 m capillary column), and (ii) the conventional GC technique (moderate oven temperature programming of a 30 m capillary column). Using the flash GC technique, the analysis time was reduced by a factor of more than 10 compared to the conventional GC technique. Dramatically improved detectability of analytes was achieved due to much narrower peak widths. The flash GC technique was compared with another approach to faster GC analysis employing a 5 m column and fast temperature programming with a conventional GC oven. In comparison with this alternative, in the case of flash GC significantly better retention time repeatability was observed. The other superiority of resistive heating is very rapid cooling down (i.e., equilibration to the initial conditions) which contributes to the increased sample throughput.

Programmable control of column selectivity for temperature-programmed GC

A computer-driven pressure controller connected to the junction point of a series-coupled ensemble of two capillary GC columns having different stationary-phase selectivity is used to obtain on-the-fly (programmable) changes in ensemble selectivity. Changes in the junction-point pressure result in differential changes in the local carrier gas velocity in the two columns, and this results in changes in the pattern of peaks eluting from the ensemble. When used with relatively fast temperature programming (30 degrees C/min), the pattern of eluting peaks can be very sensitive to the time at which a selectivity (junction-point pressure) change is implemented. These elution pattern changes are described for a set of six PCB congeners that elute with a small range of retention times. The components are considered as a group, and changes in their elution pattern are described for a single junction-point pressure change, which is implemented at various times after sample injection. If the pressure change is implemented after the components have migrated across the junction point, the final pressure has relatively little impact on the ensemble retention pattern. Pressure changes implemented prior to the components reaching the junction can have a large effect and usually result in a pattern of peaks similar to the pattern obtained when the final pressure is used for the entire separation. For pressure changes made when the group of components is near the junction point, the observed peak pattern may be very sensitive to the time of the pressure change. The time at which the junction-point pressure change occurs is varied in 1.0-s intervals. Artifacts such as peak doubling and peak focusing or broadening are observed if a migrating band is crossing the column junction point at the time of the programmed pressure change.

Fast gas chromatography: The effect of fast temperature programming

Fast gas chromatography (GC) has been around ever since the introduction of capillary GC columns. M. Golay's patent application for capillary GC (Patent 2,920,478, January 12, 1960) showed five peaks in 15 s in 1957. Desty et al. (Desty, D. H.; Goldup, A.; Swanton, W. T. In Gas Chromatography; Brenner, N.; Caller, J. E.; Weiss, M. D., Eds.; Academic: New York, 1962; p. 105), LeClercq et al. (LeClercq, P. A.; Scherpenzeel, G. J.; Vermeer, E. A. A.; Cramers, C. A. J Chromatogr 1982, 241, 61–71), and Cramers et al. (Schutjes, C.; Vermeer, E.; Rijks, J.; Cramers, C. J Chromatogr 1982, 253, 1–16), among others, followed up this early work on fast GC. The work in this paper is a continuation of fast GC, primarily using fast oven temperature programming. This work is restricted to normal pressures, routinely used on Hewlett Packard 5890 and 6890 model gas chromatographs. The first step in doing fast GC is to reduce the column length, since the retention time is directly proportional to the length, only true for isothermal runs. Columns of 1 to 6 m are used and since this step sometimes sacrifices resolution, 100 μm inner diameter columns with thin films, 0.1 to 0.2 μm, are used with small sample sizes (split ratios of 500:1 to 2000:1). © 2000 John Wiley & Sons, Inc. J Micro Sep 12: 351–355, 2000

Peak capacity, peak-capacity production rate, and boiling point resolution for temperature-programmed GC with very high programming rates

Recent advances in column heating technology have made possible very fast linear temperature programming for high-speed gas chromatography. A fused-silica capillary column is contained in a tubular metal jacket, which is resistively heated by a precision power supply. With very rapid column heating, the rate of peak-capacity production is significantly enhanced, but the total peak capacity and the boiling-point resolution (minimum boiling-point difference required for the separation of two nonpolar compounds on a nonpolar column) are reduced relative to more conventional heating rates used with convection-oven instruments. As temperature-programming rates increase, elution temperatures also increase with the result that retention may become insignificant prior to elution. This results in inefficient utilization of the down-stream end of the column and causes a loss in the rate of peak-capacity production. The rate of peak-capacity production is increased by the use of shorter columns and higher carrier gas velocities. With high programming rates (100-600 degrees C/min), column lengths of 6-12 m and average linear carrier gas velocities in the 100-150 cm/s range are satisfactory. In this study, the rate of peak-capacity production, the total peak capacity, and the boiling point resolution are determined for C10-C28 n-alkanes using 6-18 m long columns, 50-200 cm/s average carrier gas velocities, and 60-600 degrees C/min programming rates. It was found that with a 6-meter-long, 0.25-mm i.d. column programmed at a rate of 600 degrees C/min, a maximum peak-capacity production rate of 6.1 peaks/s was obtained. A total peak capacity of about 75 peaks was produced in a 37-s long separation spanning a boiling-point range from n-C10 (174 degrees C) to n-C28 (432 degrees C).

Possibilities and Limitations of Fast Temperature Programming as a Route towards Fast GC

One possible way to speed up a gas chromatographic analysis is the application of fast temperature programming by using resistive heating techniques. With this heating technique programming rates up to 20° per second can be reached. A relative standard deviation of retention times better than 0.2% is obtained. Using fast temperature programming the analysis-times of a mineral oil sample, an industrial oligomer sample, and toxic compounds in diesel fuel have been reduced 5 to 20 times, compared to a standard temperature programmed analysis. In most cases resistive heating cannot be applied to reduce the analysis time of a complex sample. The use of fast temperature programming is preferable to the use of short columns and columns operated at above-optimum carrier gas velocities.

Fast Gas Chromatography of Various Sample Types Using Fast Oven Temperature Programming

Applications of fast gas chromatography (GC) using rapid column oven temperature programming are described for a variety of samples including standard solutions of hydrocarbons and polycyclic aromatic hydrocarbons and more complex samples including perfumed mineral oil and food products. Results for the standard solutions using fast temperature programming GC are compared with those using conventional GC and fast GC on a conventional instrument. The more complex samples are analyzed using fast oven temperature programming, demonstrating precision in the chromatographic data as well as the feasibility of quantitation when doing a fast GC analysis. Many applications can be run much faster than with published conventional GC methods without significant loss in resolution.

Fast Temperature Programming in Gas Chromatography using Resistive Heating

Fast Temperature Programming in Gas Chromatography using Resistive Heating

Fast Temperature Programming in Gas Chromatography using Resistive Heating

The features of a resistive-heated capillary column for fast temperature-programmed gas chromatography (GC) have been evaluated. Experiments were carried out using a commercial available EZ Flash GC, an assembly which can be used to upgrade existing gas chromatographs. The capillary column is placed inside a metal tube which can be heated, and cooled, much more rapidly than any conventional GC oven. The EZ Flash assembly can generate temperature ramps up to 1200 °/min and can be cooled down from 300 to 50 °C in 30 s. Samples were injected via a conventional split/splitless injector and transferred to the GC column. The combination of a short column (5 m x 0.25 mm i.d.), a high gas flow rate (up to 10 mL/min), and fast temperature programmes typically decreased analysis times from 30 min to about 2.5 min. Both the split and splitless injection mode could be used. With n-alkanes as test analytes, the standard deviations of the retention times with respect to the peak width were less than 15% (n = 7). First results on RSDs of peak areas of less than 3% for all but one n-alkane indicate that the technique can also be used for quantification. The combined use of a short GC column and fast temperature gradients does cause some loss of separation efficiency, but the approach is ideally suited for fast screening as illustrated for polycyclic aromatic hydrocarbons, organophosphorus pesticides, and triazine herbicides as test compounds. Total analysis times - which included injection, separation, and equilibration to initial conditions - were typically less than 3 min.

Temperature Programming for High-Speed GC

Fast temperature programming (20-50 °C/min) is used with relatively short separation columns to achieve high-speed separations of mixtures covering a wide boiling point range. A cryofocusing inlet is used to obtain narrow injection plugs. High-speed temperature-programmed chromatograms are evaluated by considering local peak capacity as a function of carbon number and boiling point for the normal alkanes in the range C(8)-C(19). The peak capacity generation rate (peaks per second) as a function of carbon number and the total cumulative peak capacity as a function of time are also considered for various column lengths and carrier gas flow rates. Column lengths in the range 3.6-25.4 m and average carrier gas velocity values in the range 50-200 cm/s are considered. For a 6.8-m-long, 0.25-mm-i.d. column operated at an average carrier gas velocity of about 100 cm/s and using a nominal programming rate of 50 °C/min, C(19) elutes in 178 s with a total peak capacity of 168 peaks. If the programming rate is reduced to 20 °C/min, the C(19) elution time more than doubles but the total peak capacity increases by only 20%. For a 25.4-m-long column using a nominal 50 °C/min programming rate, the C(19) retention time is 262 s with a peak capacity of 279 peaks. The use of average carrier gas flow rates greater than about 100 cm/s, which is common in isothermal high-speed GC, results in a considerable loss in total peak capacity with remarkably little reduction in analysis time.

Column Selectivity Programming and Fast Temperature Programming for High-Speed GC Analysis of Purgeable Organic Compounds

High-speed gas chromatograms are obtained by the use of relatively short lengths of capillary column operated at relatively large carrier gas flow rates. This approach is difficult for more complex mixtures because of the reduced peak capacity available with shorter columns. A solution to this problem is the use of tunable column ensembles consisting of the series (tandem) combination of a polar and a nonpolar column. By adjusting the pressure at the junction point between the columns, the selectivity of the ensemble can be adjusted within the limits imposed by the individual columns. For mixtures representing a relatively large boiling point range and containing more than ∼20 components, high-speed, isothermal separations are less effective. These limitations are significantly reduced by combining fast temperature programming with selectivity programming. Selectivity programming is obtained by changing the pressure at the column junction point one or more times during the course of an analysis. In the work described here, the column ensemble temperature and the junction pressure are initially set to give a high-quality separation of the earliest eluting components. After these components have eluted, a linear temperature ramp of ∼35 °C/min is initiated. As the temperature increases, the pressure is adjusted to change the selectivity and thus facilitate the separation of groups of components as they migrate through the column ensemble. Using this approach, a mixture of 30 purgeable organic compounds is separated in less than 2.5 min.

Microwave digestion and matrix separation for the determination of cadmium in urine samples by electrothermal atomization atomic absorption spectrometry using a fast temperature program

A method is proposed which involves sample pretreatment followed by electrothermal atomic absorption spectrometry (ETAAS) for determination of cadmium in human urine. A microwave digestion system was devised to accommodate double-closed vessels for simultaneous digestion of batches of up to 24 urine samples in about 20 min. After digestion, matrix substances which might interfere were removed using silica-immobilized 8-hydroxyquinoline (I-8HOQ) columns. The analyte adsorbed on the column was then eluted with dilute nitric acid solution and determined by ETAAS using a fast temperature program. Neither ashing steps in the furnace heating program nor use of matrix modifiers was necessary. The accuracy, precision, limit of detection, and sample throughput of the method were evaluated. With meticulous control of systematic errors which may be introduced in the pretreatment procedures, the present method can serve as a reference technique for the analysis of Cd in urine samples.

Novel Column Heater for Fast Capillary Gas Chromatography

In this paper, we present two designs for the implementation of extremely fast temperature programming of gas chromatographic columns. Both designs incorporate heating and temperaturesensing elements placed along the column. Resistive heating controls the column temperature via a feedback circuit and a software-selected temperature program. Linear temperature increases up to 10°C/s are obtained. The performance of each design is evaluated for applications to volatile and semi-volatile hydrocarbons. Retention times deviate only slightly in comparison with a standard commercial instrument, and peak widths increase for compounds beyond C 13 . The proposed column heater designs have satisfactory retention time reproducibility. They are lightweight and consume little power, making them promising alternatives for fast portable gas chromatography.

Enhanced selectivity in the analysis of chlorobiphenyls on a carborane phenylmethylsiloxane copolymer gas chromatography phase (HT-8)

The congener specific analysis of polychlorinated biphenyls (PCBs) on a new chemically bonded stationary GC phase, HT-8 (1,7-dicarba-closo-dodecarborane phenylmethyl siloxane) is investigated. The incorporation of carborane in the polysiloxane backbone enhances selectivity and temperature stability of the phase. On a 60 m × 0.25 mm I.D. fused-silica column coated with 0.25 μm HT-8, a total of 106 congeners in Aroclor mixtures elute as separated peaks. By the use of mass spectrometric detection 138 congeners can be analysed without interference from coeluting PCBs. In literature, no other GC phase has been reported with such a high PCB selectivity. An interesting feature of HT-8 is its near-baseline separation of the critical pair CB138/CB163. Separations for these congeners on other GC phases are reviewed. The performance of a thin film (0.15 μm) version of the HT-8 column with fast temperature programming is tested. It is concluded that congener-specific, rapid screening of seven indicator PCBs (CB28, CB52, CB101, CB118, CB138, CB153 and CB180) with a total GC run-time of 13 min is feasible with an accuracy sufficient for most purposes.

Fast Temperature Programming on Fused-Silica Open-Tubular Capillary Columns by Direct Resistive Heating

A method for extremely fast temperature-programmed gas chromatography is described. The column is heated by passing an electric current through a thin conductive film applied to the outer surface of a fused-silica open-tubular column. The low thermal mass of the column allows rapid heating and cooling. A computer generated voltage ramp may be empirically adjusted to produce a linear separation of the members of a homologous series. Sample introduction is by on-column thermal modulation. High-speed detection is by a micro thermal conductivity detector. Repetitive chromatograms can be generated as often as 1/s with peaks as sharp as 40 ms at the baseline

Improved congener‐specific GC analysis of chlorobiphenyls on coupled CPSil‐8 and HT‐5 columns

AbstractThe congener‐specific analysis of polychlorinated biphenyls (PCB) by high resolution gas chromatography on a 50 m × 0.25 mm fused silica column coated with a 0.26 μm film of 5% diphenyl polydimethylsiloxane (CPSil‐8) has been significantly improved by series coupling with a 25 m × 0.22 mm column coated with a 0.10 μm film of 1,2‐dicarba‐closo‐dodecarborane polydimethylsiloxane (HT‐5). Using helium as carrier gas, a total of 64 congeners in technical PCB mixtures could be analyzed as resolved peaks by ECD (86 by MS) with the CPSil‐8 column, and 84 by ECD (108 by MS) with the combination. The high upper temperature limit for these stationary phases (>300°C) enabled fast temperature programming and rapid analysis (60 min).

Separation of Toxic Congeners from PCB Mixtures on Two Series Coupled Narrow-Bore Columns (50 m SIL-8 AND 25 m HT-5)

Abstract The complete separation of the toxic non-ortho-substituted polychlorinated biphenyls (PCBs) PCB-77, PCB-126 and PCB-169 from the technical PCB mixtures Askarel and Aroclors (A1232, A1242, A1248, A1254, A1260 and A1262) was obtained on two series coupled narrow-bore columns: 50 m × 0.25 mm, 0.26 μm 5% diphenyldimethylsiloxane (CP-SIL-8) and 25 m × 0.2 mm, 0.1 μm 5% 1,2-dicarba-closo-dodecarborane dimethylsiloxane (HT-5). The high upper temperature limit for these GC-phases (>300°C) allowed for fast temperature programming and short analysis time (60 min). In addition to the non-ortho-substituted PCBs most of the toxic mono-ortho-substituted PCBs were completely separated from other PCBs (PCB-74, PCB-81, PCB-105, PCB-118, PCB-123, PCB-157 and PCB-189). Only four toxic PCBs were eluted with interference: PCB-60/PCB-56, PCB-114/PCB-134, PCB-167/PCB-128 and PCB-156/PCB-202. These could be analyzed satisfactorily on a CP-SIL-19 column i.e. 50 m × 0.25 mm, 0.20 μm 14% cyanopropylphenyl 1% vinyldimethyls...

Gasadsorptionschromatographie höherer Kohlenwasserstoffe. Normalparaffinbestimmung durch temperaturprogrammierte Segregations‐Gaschromatographie an 5A Molekularsieb

Gas Adsorption Chromatography of Higher Hydrocarbons. Determination of Normal Paraffins by Temperature‐Programmed Segregation Gas Chromatography on 5A Molecular SieveA fast temperature program makes it possible to determine the normal paraffins up to C 20 in the concentration range between 0.1 and 99.9 percent (molecule type analysis). From the group peak of the normal paraffins we can obtain on line the distribution of the carbon numbers.As the normal paraffin selectivity of the molecular sieve S ≈ 1 can be assumed. Isoparaffin memory could not be detected.The new method makes it possible to perform analysis with great accuracy within three minutes or less. Connected to a computer it is excellently suitable for large series of samples.Under the standard conditions of this method only a little paraffin cracking has been found. Type and quantity of the cracked products proved to be independent of the chain length of the individual normal paraffin.