Deactivation of small-diameter fused-silica capillary columns for gas and supercritical fluid chromatography

Summary In order to prevent plugging during deactivation of small diameter (50 m i.d.) capillary columns for gas and supercritical fluid chromatography, various high temperature deactivation methods were employed. Pure hexamethyldisilazane and hexamethyldisiloxane (a substitute for D₄) were dynamically coated on the column, while a film (0.05 m) of OV-101 was statically coated, before high temperature (450°C) treatment. Excellent deactivated columns were obtained, and no significant difference in column activity was observed using any of these three methods.Dedicated to Professor S. R. Lipsky on the occasion of his 60th birthday.     

The temperature dependence of retention indices in gas chromatography

Summary A linear dependence of (T–T₁)/[1(T)–1(T₁)] on temperature (considering the retention index 1(T₁) at temperature T₁ as a standard value) is derived. Both ther retention index at an assigned temperature and the temperature dependence of the retention index can be calculated from retention data measured at two temperature-programing rates.     

The achievement of reproducible temperature prodgrammed retention indices in gas chromatography when using different columns and detectors

Summary It has been found that the ratio , where r is the heating rate, t₀ is the gas hold-up time of the column and is the phase ratio of the column used, is a most explicit and convenient parameter in linear temperatur-programmed gas chromatography for reproducing temperature-programmed retention indices, I_TP. For two columns of different sizes (length, inner diamter), working under different heating rates with the same or different carrier gases at different gas flow-rates, as long as the initial oven temperature, T₀, phase ratio, , and their r·t₀/ ratios are kept unchanged, the I_TP value of a solute can be reproduced within 1–2 i.u. on either OV-101 or PEG-20M columns. When a combined gas chromatography-mass spectrometry technique is used applying vacuum at the column outlet reduces t₀. Nevertheless, r·t₀/ can still be kept unchanged by a proper choice of the oven heating rate, and thus a total ion chromatogram (TIC), quite similar to the corresponding gas chromatogram in shape, can be obtained.     

Revisiting the problem of correcting retention parameters for pressure and temperature in gas chromatography

Summary It has been shown [1, 2] that the compressibility correction factor equals the ratio of gas pressure at the column outlet to the average pressure in the column,j=p _o /p _c , and, therefore, by multiplying by this factor, all experimentally measured retention volumes and flow rates are converted from ambient pressure to the average pressure in the column. This makes retention volumes corrected in this way independent of pressure. In contrast, correcting retention times for gas compressibility has no physical meaning and terms such as “corrected retention time” and “net retention time” should not be used. Similarly, recalculating corrected retention volumes to a standard temperature of 273 K appears to provide a thermodynamically sound basis for comparison of data obtained at different temperatures. In reality, it distorts actual relationships and should not be used. Presented at: Balaton Symposium on High-Performance Separation Methods, Siófok, Hungary, September 3–5, 1997.     

Practical aspects of the dependence of polarity on temperature

Summary It is not commonly appreciated that retention indices are temperature dependent. It is even less common to express this fact in more practical terms by saying that polarity is temperature dependent. Although the meaning of both statements is identical, we believe the second to be particularly relevant, since the majority of practical gas chromatographers tends to handle polarity as an invariable characteristic of a stationary phase.The variability of polarity with temperature is the major source of inadequate reproducibility of exact finger-printing, this particularly when gc/ms traces have to be compared to those obtained by pure gc. On the other hand, the temperature dependence provides a practical means to optimize the polarity of a given column for a given analysis. Film thickness is an essential parameter in this context because of its influence on column temperature and, therefore, on column polarity.     

High temperature gas chromatography of trimethylsilyl derivatives of alcohol ethoxylates and ethoxysulfates

Conversion of ethoxylated alcohols to trimethylsilyl derivatives and subsequent high temperature gas chromatography allowed for the determination of alkyl chain distribution, mole average degree of ethoxylation and weight percent distribution of several commercial mixtures. Repetitive derivatizations and analyses of a mixture with a vendor specified mole average degree of ethoxylation of 3.0 (Neodol 23-3), yielded an experimental result of 2.93 ± 0.05 (n = 6). Mixtures with different alkyl chain distributions and higher degrees of ethoxylation provided similar accuracy. Extension of the method to the analysis of ethoxylated alcohol sulfate raw materials and ethoxylated alcohol sulfates in dishwashing liquids is discussed.     

Optimization and identification in any kind of multi-step temperature programmed gas chromatography

The theory which predicts the retention time, retention temperature, and peak width for any kind of multi-step TPGC and the principle of optimization has been described. Software for optimization and identification in TPGC has been developed on the basis of this theory. It has also been proven that the relationship between peak width in TPGC and the derived or “invented” retention time is similar to that between peak width and retention time in isothermal processes. The validity of the software has been proved by using it to predict retention temperature, retention time, and peak width for any kind of temperature programming and to predict the optimum temperature program for separation of a multihomolog mixture of industrial alcohols and 15 enantiomeric pairs of amino acids.     

Chemically modified teflon capillary columns for gas chromatography II. Solid surface effects in direct coated columns

Summary The preparation of teflon capillary columns employing direct coating of the stationary phase to the chemically modified teflon surface and the role of the chemically modified teflon in the separation mechanism are described. Two types of contributions from the modified teflon have been observed: nonspecific adsorption of the carbon surface and specific interactions of polar groups in the carbon skeleton. The use of polar liquid phases can eliminate adsorption due to the presence of polar groups in the modified teflon.     

Preparative gas chromatography with glass capillary columns

The present paper describes an automated system for preparative gas chromatography with glass capillary columns, controlled by a microprocessor. The effluent from the capillary column is divided by a pneumatically controlled splitter and any desired split ratio between traps and detector can be obtained. Moreover, a second pneumatic control allows instantaneous change-over to a different split ratio, thus minimizing loss of material during collection. The effluent containing the compounds of interest is passed through a multiple manifold and collected in coiled glass capillary traps. To ensure maximum trapping efficiency even for very small amounts of material, the inner walls of the capillary traps are wetted with a suitable solvent, which gives a quantitative recovery of micro- and nanogram amounts of material. After repetitive sampling, sufficient amounts of material can be obtained for NMR spectroscopy and possibilities exist to enrich trace components with the aid of a double column system. Two examples of such applications are given, employing mixtures of both synthetic and natural origin.     

A study of some deactivation methods for fused silica capillary columns by CP-MAS NMR and capillary gas chromatography

The effect of deactivating a fused silica surface by silylation with 1,1,3,3-tetraphenyl-1,3-dimethylilazane (TPDMDS), triphenylsilylamine (TPSA), and octamethylcyclotetrasiloxane (D4) and by polydimethylsiloxane degradation (PSD) is studied. Rehydrated, dried, and deactivated Cab-O-Sil M5 samples are used as model materials for ²⁹Si CP-MAS NMR analysis. At about 350 °C, TPDMDS yelds mainly diphenylmethylsiloxysilane, dimethyldisiloxysilane, and triphenylsiloxysilane groups. TPSA yields phenyltrisiloxysilane, diphenyldisiloxysilane, and triphenylsiloxysilane groups. At 400°C, the products formed initially are eventually replaced by methyltrisiloxysilane or phenyltrisiloxysilane groups, while a substantial number of silanol groups still remains. The possible consequences for wettability are discussed. D4 reacts with Cab-O-Sil even at 200°C, but a large number of silanol groups remains. This number decreases gradually at higher temperatures and becomes negligible above 400°C. The formation of methyltrisiloxysilane groups, which starts at 425°C, is predominant at 490°C.     

Measurement of the composition of gas desorbed from coal by gas chromatography

Summary Direct methods for the determination of the gas content of a coal seam are based upon extracting a coal sample, enclosing it in a sealed container and measuring the volume of the gas evolved. In our work we measured evolved gas as a function of time, type and quantity of low rank coal (lignite) at normal (atmospheric pressure and temperature) conditions. The evolved gas was, collected in gas sampling bags and analyzed by GC for its main components (carbon dioxide, methane and dimethyl sulfide). From the parameters obtained (gas volume, CH₄:CO₂ ratio, rate of gas desorption, time concentration profile and different lithotypes of lignite) a more detailed identification of hazardous areas in underground coal extraction could be ensured. Presented at: Balaton Symposium on High-Performance Separation Methods, Siófok, Hungary, September 3–5, 1997.     

Stainless steel capillary columns for high temperature gas chromatography

A layer of elemental silicon has been deposited on the surface of stainless steel tubing by means of chemical vapor deposition (CVD). Two kinds of capillary column were prepared from the deactivated tubing: cross-linked, silanol-terminated polydime-thylsiloxane wall coated open tubular (WCOT) columns and molecular sieve 13X porous layer open tubular (PLOT) columns. Unlike fused silica capillary columns, stainless steel WCOT and PLOT columns can be operated at temperatures in excess of 400°C. High temperature simulated distillation has been performed successfully with a macro bore WCOT column and rapid PNA (paraffin, naphthene, and aromatic) analysis with a multidimensional gas solid chromatographic (GSC) system using PLOT columns.     

High temperature gas chromatography on narrow bore capillary columns

The use of high-temperature-stable, medium polarity glass capillary columns coated with immobilized PS-090 (a 20 % diphenyl-substituted, CH₃O-terminated polydimethylsiloxane) has made it possible to analyze routinely, and with good separation efficiency, high molecular weight compounds such as triglycerides and free base porphyrins. Cold on-column injection was used throughout this work to avoid discrimination against involatile compounds, and disposable (fused silica) retention gaps were used to protect the column against contamination with involatile material. On-column injection into narrow bore glass columns was achieved by using glass-to-silica connections to attach wider bore (0.2 mm i.d.) deactivated fused silica tubing to the columns.     

Elution order inversions observed on using different carrier gas velocities in temperature programmed gas chromatography

Observations of chromatographic elution order inversions are reported which have occurred as a result of using different initial carrier gas velocities with oven temperature programming in GC and GC/MS systems.     

Prediction of the separation number of capillary columns in programmed temperature gas chromatographic analysis

The efficiency of capillary columns in programmed temperature analysis can be evaluated by calculation of the separation number (“Trennzahl”). A procedure for the prediction of this parameter at various initial temperatures, carrier gas pressures and heating rates, by using as the starting data the retention times and the peak widths obtained in some isobaric and isothermal runs is described. An equation is proposed that permits to obtain the values of the peak width at half height in any isothermal and linearly programmed temperature gas chromatographic run and therefore to calculate the separation number value. The effect on this parameter of the column polarity was investigated by using polar and non-polar compounds (n-alkanes and 1-alcohols).     

Fused silica capillary micro-packed columns in gas chromatography

Summary Flexibility, strength and adsorption inertness of fused-silica capillaries permits their extensive application for the preparation of micro-packed columns in gas chromatography. Decreasing the column diameter (from 0.5 to 0.15 mm) and the diameter of the sorbent particles (from 100 to 5 μm) results in a marked reduction of the height equivalent to a theoretical plate (HETP), as well as in diminishing the dependence of the HETP on the carrier gas flow rate. The chromatographic characteristics of fused-silica capillary micro-packed columns and open-tubular columns are compared. The fused-silica capillary micro-packed column can be used to advantage for performing rapid and trace analyses and have been shown to be adapted for application in gas-solid chromatography. Separation of organic and inorganic compounds on fused-silica capillary micro-packed columns is illustrated by practical examples. Presented at the 15th International Symposium on Chromatography, Nürnberg, October 1984     

Enzyme activity determination by radio gas chromatography on capillary columns

Androgen 5α-reductase from the foreskin of 48 boys in prepubertal age was analyzed by evaluation of the apparent K_m-and V_max- values after tissue incubation. Reaction rates were calculated after determination of specific radioactivity of distinct metabolites, by comparison to the radioactivity of the precursor. Reaction products were separated by radio gas chromatography on capillary columns. The chromatography system was based on multicolumn equipment with column switching facilities and a variable splitter at the outlet of the separation column. A radioactivity monitor (gas proportional counter) was used for measurement of radioactivity in the column effluents. An electron capture detector was used for mass detection of steroid acyl derivatives.     

The effect of the temperature dependence of mobile phase viscosity on the retention time in series-coupled gas chromatographic columns

A previously established model of a system of series-coupled columns, incorporating the effects of temperature and mobile phase compressibility, is generalized to take the temperature dependence of the mobile phase viscosity coefficient into account. Expressions are reported for the linear flow velocity, the effective partition coefficient, and the retention time for both the case of constant mass flow rate and that of constant overall pressure drop. Numerical results indicate that the temperature dependence of the viscosity coefficient affects the retention time and pressure drop while the effective mass distribution coefficient remains essentially unchanged.     

毛细管柱气相色谱法测定土壤中的有机氯农药

研究了使用毛细管柱气相色谱法测定土壤中有机氯农药的分离条件、萃取方法、净化方法以及用替代品进行全程质量控制的方法。有机氯农药检出限在 1 .1～ 4.0 μg L之间 ,各化合物保留时间的RSD <0 .1 % ,所测定化合物的质量浓度在 0 .0 0 6～ 0 .2 0mg L范围内 ,各物质校正曲线 (浓度与峰面积 )的相关系数在 0 .992 6～ 0 .9971之间。用正己烷 丙酮 ( 1 1 ) 1 0 0mL超声波萃取 ,有机氯农药的萃取回收率在 96.3%～ 1 1 4% ,RSD为 0～ 1 1 %。用国产硅镁吸附剂净化 ,有机氯农药的回收率在 1 0 0 %～ 1 40 %之间 ,弗罗里硅土的回收率在 1 0 4 %～ 1 33%之间 ,硅镁吸附剂可以替代弗罗里硅土用于样品的净化。