The effect of column characteristics on the minimum analyte concentration and the minimum detectable amount in capillary gas chromatography

The need for faster and more efficient separations of complex mixtures of organic compounds by gas chromatography has led to the development of small inner diameter open tubular columns. Owing to their decreased plate height, extremely narrow peaks are obtained. When differently sized columns with equal plate numbers are compared, injection of a fixed amount of a solute will give the highest detector signals for the smallest bore columns. When P is defined as the ratio of the column inlet and outlet pressures, it can be seen from theory that under normalized chromatographic conditions the minimum detectable amount (Q_º) for a mass flow sensitive detector increases proportionally to the square of the column diameter for P = 1. In the situation of greater interest in the practice of open tubular gas chromatography where P is large, a linear relationship is derived between Q_º and the column diameter. It is a widespread misunderstanding, however, that narrow bore capillary columns should be used for this reason in trace analysis. If a fixed relative contribution of the injection band width to the overall peak variance is allowed, a decreased plate height drastically restricts the maximum sample volume to be injected. It is shown that the minimum analyte concentration in the injected sample (C_º) is inversely proportional to the column inner diameter when a mass flow sensitive detector is used. For actual concentrations less than C_º, sample preconcentration is required. The effect of peak resolution and selectivity of the stationary phase in relation to C_º and Q_º will be discussed as well. The validity of the given theory is experimentally investigated. Minimum analyte concentrations and minimum detectable amounts are compared using columns with different inner diameter.     

Simple capillary flow porometer for characterization of capillary columns containing packed and monolithic beds

A simple capillary flow porometer (CFP) was assembled for through-pore structure characterization of monolithic capillary liquid chromatography columns in their original chromatographic forms. Determination of differential pressures and flow rates through dry and wet short capillary segments provided necessary information to determine the mean diameters and size distributions of the through-pores. The mean through-pore diameters of three capillary columns packed with 3, 5, and 7 μm spherical silica particles were determined to be 0.5, 1.0 and 1.4 μm, with distributions ranging from 0.1 to 0.7, 0.3 to 1.1 and 0.4 to 2.6 μm, respectively. Similarly, the mean through-pore diameters and size distributions of silica monoliths fabricated via phase separation by polymerization of tetramethoxysilane (TMOS) in the presence of poly(ethylene glycol) (PEG) verified that a greater number of through-pores with small diameters were prepared in columns with higher PEG content in the prepolymer mixture. The CFP system was also used to study the effects of column inner diameter and length on through-pore properties of polymeric monolithic columns. Typical monoliths based on butyl methacrylate (BMA) and poly(ethylene glycol) diacrylate (PEGDA) in capillary columns with different inner diameters (i.e., 50–250 μm) and lengths (i.e., 1.5–3.0 cm) were characterized. The results indicate that varying the inner diameter and/or the length of the column had little effect on the through-pore properties. Therefore, the through-pores are highly interconnected and their determination by CFP is independent of capillary length.     

窄内径多孔层开管柱的制备及在液相色谱中的应用

近年来,微纳分离技术由于其内在的优势而受到越来越多的关注.多孔层开管柱是一种重要的微分离柱形式,与粗内径的多孔层开管柱(>25μm)相比,窄内径的多孔层开管柱具有更高的分离效率和更低的试剂消耗量.本文综述了内径≤25μm的窄内径多孔层开管毛细管柱的制备方法、与质谱检测联用技术以及在液相色谱中的应用研究进展,对其发展前景进行了展望.     

Separation of mixtures of solvents with wide bore capillary columns connected in series

The gas chromatographic separation of the components of a complex mixture of industrial solvents, not possible on a single stationary phase owing to coelution of some of the compounds, has been achieved on two serially connected wide bore capillary columns of different polarity. The analysis of a mixture of twenty five compounds was optimized using the serial gas chromatography theory of Purnell and Williams. The capacity factors of sample components measured with the two columns coupled in series were found to agree with those predicted by theory, regardless of the order in which they were connected.     

PoraPLOT Q: A porous layer open tubular column coated with styrene-divinylbenzene copolymer

A porous polymer is deposited on the inner wall of fused silica capillary columns. The retention characteristics of this porous polymer were evaluated and found to be comparable with Porapak Q. The porous polymer has a high retention volume which enables the separation of permanent gases at ambient temperatures or higher. The hydrophobic character of the porous polymer allows the injection of water containing samples without changing retention due to adsorption of water. The inertness of the porous polymer allows the elution of a range of apolar and polar compounds. The maximum temperature of the porous polymer was estimated to be 250°C. With this new type of capillary column, high resolution separations are obtained in combination with short analysis times.     

Comprehensive two‐dimensional monolithic liquid chromatography of polar compounds

Two‐dimensional liquid chromatography largely increases the number of separated compounds in a single run, theoretically up to the product of the peaks separated in each dimension on the columns with different selectivities. On‐line coupling of a reversed‐phase column with an aqueous normal‐phase (hydrophilic interaction liquid chromatography) column yields orthogonal systems with high peak capacities. Fast on‐line two‐dimensional liquid chromatography needs a capillary or micro‐bore column providing low‐volume effluent fractions transferred to a short efficient second‐dimension column for separation at a high mobile phase flow rate. We prepared polymethacrylate zwitterionic monolithic micro‐columns in fused silica capillaries with structurally different dimethacrylate cross‐linkers. The columns provide dual retention mechanism (hydrophilic interaction and reversed‐phase). Setting the mobile phase composition allows adjusting the separation selectivity for various polar substance classes. Coupling on‐line an organic polymer monolithic capillary column in the first dimension with a short silica‐based monolithic column in the second dimension provides two‐dimensional liquid chromatography systems with high peak capacities. The silica monolithic C₁₈ columns provide higher separation efficiency than the particle‐packed columns at the flow rates as high as 5 mL/min used in the second dimension. Decreasing the diameter of the silica monolithic columns allows using a higher flow rate at the maximum operation pressure and lower fraction volumes transferred from the first, hydrophilic interaction dimension, into the second, reversed‐phase mode, avoiding the mobile phase compatibility issues, improving the resolution, increasing the peak capacity, and the peak production rate.     

Deactivation of fused silica capillary columns with phenylhydrosiloxanes

In this work, an investigation of new organosilicon hydride reagents with phenyl functional groups for deactivation and surface modification of fused silica capillary columns is described. Different reagents were tested for their ability to deactivate the fused silica surface, and actual critical surface tension measurements were made using the capillary rise method. The deactivation procedure required lower optimum temperatures than conventional methods. Deactivated capillaries and coated capillary columns were prepared and tested for reproducibility, efficiency, and surface inertness towards basic and acidic compounds at the low nanogram level.     

Evaluation of capillary gas chromatography columns for the determination of free volatile amines after solid-phase microextraction

Summary Gas-liquid chromatographic capillary columns coated with 14% diphenyldimethylpolysiloxane, base-deactivated 5% diphenyldimethylpolysiloxane, or poly(ethylene glycol), and deactivated porous polymer for capillary gas-solid chromatography were evaluated for analysis of low-molecular-weight (C₁−C₉) amines. Solid-phase microextraction with a polydimethylsiloxane fiber was used for headspace sampling and for introduction of the sample into the gas chromatograph. As expected, basic of aliphatic gaseous or volatile aliphatic amines (carbon number: C₁−C₄). A thick (e. g. 3 μm) film of 5% diphenyldimethylpolysiloxane enabled determination of analytes in a wider molecular-weight range (C₃−C₉) with acceptable efficiency and resolution.     

Deactivation of fused silica capillary columns with cyanopropylhydrosiloxanes

A method is described for surface deactivation and modification of fused silica capillary columns with a cyanopropyl-containing reagent. The deactivation procedure involved a dehydrocondensation reaction between a bis(cyanopropyl)methylhydropolysiloxane reagent and surface silanol groups at an optimum temperature of only 250°C. Actual critical surface tension measurements were made using the capillary rise method. Excellent deactivation for acidic and basic compounds at the low ng level, and wettability for nonpolar and polar polysiloxane stationary phases were obtained. A procedure was developed to remove acidic impurities that are present in polar stationary phases.     

Characterization of polymer monolithic stationary phases for capillary HPLC

Monolithic capillary columns have been prepared in fused‐silica capillaries by radical co‐polymerization of ethylene dimethacrylate and butyl methacrylate in the presence of porogen solvent mixtures containing various concentration ratios of 1‐propanol, 1,4‐butanediol, and water with azobisisobutyronitrile as the initiator of the polymerization reaction. The through pores in organic polymer monolithic columns can be characterized by “equivalent permeability particle size”, and the mesopores with stagnant mobile phase by “equivalent dispersion particle size”. Increasing the concentration of propanol in the polymerization mixture diminishes the pore volume and size in the monolithic media and improves the column efficiency, at a cost of decreasing permeability. Organic polymer monolithic capillary columns show similar retention behaviour to packed alkyl silica columns for compounds with different polarities characterized by interaction indices, I_x, but have different methylene selectivities. Higher concentrations of propanol in the polymerization mixture increase the lipophilic character of the monolithic stationary phases. Best efficiencies and separation selectivities were found for monolithic columns prepared using 62–64% propanol in the porogen solvent mixture. To allow accurate characterization of the properties of capillary monolithic columns, the experimental data should be corrected for extra‐column contributions.     

Packing capilllary electrochromatography columns using vacuum--a preliminary study

This paper introducesa novel method for packing Capillary Electrochromatography Columns (CEC). Using vacuum packing methodology, silica particles as small as 1 microm were successfully packed into the capillary columns with 75 microm inner diameter. The columns are verystable and show no noticeable loss in efficiency after 200 sample injections. The performance of these vacuum packed capillary columns was evaluated with a mixture of aromatic and non-aromatic compounds. A 24 cm long capillary column can produce peak efficiencies of around 45,000 plates for benzene.     

Analysis of volatile organic compounds in polymers by dynamic headspace and gas chromatography/mass spectrometry

An experimental method for the analysis of volatile organic compounds in polymers is described. The technique involves dynamic headspace sampling, collection, and concentration of the volatiles in a cold trap, followed by capillary column gas chromatography/mass spectometry. Flow switching is carried out by the Deans switching technique. Four technical polymers used as pharmaceutical packaging materials have been analyzed in order to demonstrate the method.     

Polymethacrylate monolithic and hybrid particle-monolithic columns for reversed-phase and hydrophilic interaction capillary liquid chromatography

We prepared hybrid particle-monolithic polymethacrylate columns for micro-HPLC by in situ polymerization in fused silica capillaries pre-packed with 3–5 μm C₁₈ and aminopropyl silica bonded particles, using polymerization mixtures based on laurylmethacrylate–ethylene dimethacrylate (co)polymers for the reversed-phase (RP) mode and [2-(methacryloyloxy)ethyl]-dimethyl-(3-sulfopropyl) zwitterionic (co)polymers for the hydrophilic interaction (HILIC) mode. The hybrid particle-monolithic columns showed reduced porosity and hold-up volumes, approximately 2–2.5 times lower in comparison to the pure monolithic columns prepared in the whole volume of empty capillaries. The elution volumes of sample compounds are also generally lower in comparison to packed or pure monolithic columns. The efficiency and permeability of the hybrid columns are intermediate in between the properties of the reference pure monolithic and particle-packed columns. The chemistries of the embedded solid particles and of the interparticle monolithic moiety in the hybrid capillary columns contribute to the retention to various degrees, affecting the selectivity of separation. Some hybrid columns provided improved separations of proteins in comparison to the reference particle-packed columns in the reversed-phase mode. Zwitterionic hybrid particle-monolithic columns show dual mode retention HILIC/RP behaviour depending on the composition of the mobile phase and allow separations of polar compounds such as phenolic acids in the HILIC mode at lower concentrations of acetonitrile and, often in shorter analysis time in comparison to particle-packed and full-volume monolithic columns.     

Use of small diameter WCOT columns for ultra-high resolution GC/MS

For clinical and environmental analyses utilizing capillary gas chromatography/mass spectrometry (GC/MS), increased sensitivity and speed of analysis are highly desirable. These performance advantages are realized using a WCOT column of 100 μm i.d. as compared to the more conventional 200 μm i.d. capillary columns. The improved sensitivity of capillary direct GC/MS with the 100 μm i.d. column for the confirmation of drugs of abuse will be demonstrated. For environmental analysis, the superior efficiency and resolution of the 100 μm i.d. column can be employed for the separation of priority pollutants. This approach is more amenable to capillary direct GC/MS providing a more effective interface to the mass spectrometer. As a result improved sensitivity and a considerable decrease in analysis time is achieved over that obtained with the larger diameter environmental specialty phase columns.     

气相色谱仪双毛细管柱双检测器技术在毒物检测中的应用

利用双毛细管柱双检测器技术建立常见毒物的检验方法, 并对其进行定性分析. 样品进样后通过VSOS装置同时进入两根不同极性的毛细管柱, 经分离后到达两个不同选择性的检测器. 利用VSOS装置可对毒物进行较好的分离、定性. 该方法可对常见毒物进行准确定性.     

基于窄内径多孔层毛细管开管柱的纳流高效液相色谱用于N-衍生化氨基酸对映体的分离

窄内径多孔层毛细管开管柱（NPLOT柱）在生命科学领域,特别是单细胞分析领域具有较好的应用前景。本研究采用原位热引发聚合法来制备窄内径奎尼丁类手性固定相多孔层开管柱,在6 μ m i.d.的毛细管中制备有机聚合物多孔层,考察了不同热聚合时间（3、6和9 h）对NPLOT柱形貌的影响,热聚3 h和6 h制备的NPLOT柱形貌均一,多孔层厚度分别为103±51 nm和210±51 nm。将热聚合3 h制备的NPLOT柱用于纳流高效液相色谱分离N-衍生化氨基酸对映体,在2 min内即可实现基本分离,消耗的样品量仅为皮升级别。该研究将为单细胞分析提供研究手段。     

High temperature gas chromatography: The development of new aluminum clad flexible fused silica glass capillary columns coated with thermostable nonpolar phases: Part 1

With the simultaneous development of blank aluminum clad flexible fused silica glass capillary tubing capable of withstanding temperatures up to 500°C, coincident with a series of special high temperature methyl polysiloxane polymers, it was possible to produce for the first time, long lived fused silica capillary columns containing thin films of thermostable stationary phases which could be maintained isothermally at 400425°C and temperature programmed to 425–440°C. The “bleed rate” here for a well conditioned column was 5 picoamperes or less. Under these circumstances, alkanes with carbon numbers in the C-90 to C-100 area were rapidly and efficiently eluted from these columns. By extrapolation here, one can easily detect certain compounds with boiling points in the 750°C range. Since this type of capillary column was found to possess certain favorable properties, it was thought that it will soon replace the packed column and will probably be more popular than the borosilicate capillary column for many high temperature applications. Moreover, evidence has now accumulated which leads us to further believe that the majority of analyses of “high molecular weight” compounds performed by Supercritical Fluid Chromatography (SFC), utilizing very narrow bore fused silica capillary columns at several hundred atmospheres, can be much more simply, much more rapidly, much more economically, and much more efficiently accomplished by gas chromatography utilizing this new generation of high temperature capillary columns.     

Free release static coating of glass capillary columns; rapid coating at lower temperatures and elevated pressures

The coating speed upon static coating of glass capillary columans was evaluated in terms of inner diameter and length of the column, viscosity and pressure of solvent vapor, etc. From the equation obtained it can be shown that a smaller diameter of a microbore column restricts solvent vapor transfer to the orifice of the column drastically. To compensate for this restriction, a higher pressure at the meniscus is needed. As an alternative to using a higher coating temperature, application of more volatile solvents such as n-butane and isobutane is proposed. Several glass capillary columns (130 μm i.d.) were coated with SE-54 dissolved inpentane-acetone mixed with n-butane or isobutane and the column performances were evaluated. Selection of these solvents permitted free release static coating of ca. 100 μm columns at lower or even ambient temperatures and they were equally suitable as commonly applied solvents (e.g. pentane) to coat highly efficient columns.     

The Analysis of volatile compounds by purge and trap with whole column cryotrapping (WCC) on a fused silica capillary column

The purge and trap (P&T) method of analysis has been interfaced with fused silica capillary column gas chromatography. This interfacing has been accomplished without splitting the P&T trap desorption carrier gas. Thus, 100% of the purged compounds are transferred to the column. The analytes are cryofocussed on the column using whole column cryotrapping (WCC) at ?80°C. The resulting P&T/WCC procedure is extremely well-suited to the analysis of trace purgeable aqueous organic compounds. Samples and standards containing a variety of aromatic standard compounds were analyzed. The standards included benzene, toluene, ethylbenzene, xylenes, C₃-C₄-benzenes, and naphthalene, as well as three P&T internal standard compounds. Chromatographic peak widths were uniformly less than 6 s at the base and excellent precision was obtained in the relative retention time data for all compounds. The chromatogram of a groundwater sample contaminated with aromatic gasoline compounds is also presented. Since P&T/WCC works well with fused silica capillary columns, the full sensitivity and chromatographic efficiency of capillary gas chromatography is made available to P&T analyses.     

Underivatized steroids on persilanized capillary columns with non-vaporizing on-column injection

Recent developments in the preparation of capillary columns by persilanization of the glass surfaces have resulted in highly inactive, heat-stable columns. With this type of column certain steroid hormone classes can be analyzed without derivatization; these include the 17-keto steroids, 11-oxo compounds, estrogens, and progesterone metabolites. It has been found that good quantitative analysis calls for non-vaporizing, on-column injection, as normal vaporizing injection results in breakdown of thermolabile compounds such as estriol and 17-OH progesterone metabolites. The gas chromatographic method described was used to obtain urinary steroid profiles.