Application of the “cooled needle” technique to split and splitless sampling onto capillary columns

Cooled needle sampling using syringes was applied to splitless injection and to simulated distillation analyses. Slight changes of the construction of the previous device are also described. The changes concern the temperature profile within the injector and especially the vaporization insert. Even with the low carrier gas flow through the injector during splitless injection, discrimination by component volatility can be avoided. Precision and accuracy of simulated distillation data obtained with split sampling also can be improved by the cooled needle technique.     

Temperature dependence of the resolution of diastereomeric isoprenoids in capillary GC. Experiments with 50 μm, 100 μm,and 250 μm i.d. Columns

The resolution of the diastereoisomers of norpristane, pristane, and phytane was studied as a function of the column internal diameter and/or the residence time of the compounds in the column. Increasing the residence time in the column by operating the column at a lower temperature program rate enhances the resolution more than reducing the internal diameter of the column. Practical experience with ultra narrow bore columns is also presented.     

Micro-HPLC 1: Linear “breakthrough-gradients” using the discontinuous interface between eluents in a packed gradient-generator column

A new approach is described for micro-HPLC for generating nearly linear gradients in the 40 to 550 μl volume range. These “linear” gradients have a straight line middle portion with a gentle curved onset of the gradient andd rapid attainment of the final 100% composition. Such “breakthrough-gradients” are produced by the interface between weak and strong eluent when a small (3 × 0.46 cm) packed “gradient-generator” column is switched abruptly from weak to strong eluent. The model system described here, without an analytical column, shows gradients from weak eluent (water) to strong eluent (modele with water plus acetone) using a 12-port high pressure valve to 1) vent the “flush” eluent, and simultaneously 2) begin the gradient and 3) make the sample injection. The volume of the gradient can be changed in two ways. If the breakthrough-generator is packed with very large (300 to 1600 μm nonporous particles, the gradient volume is nearly independent of the flush flow rate. The flush flow is used to quickly move the breakthrough-interface to the head of the micro-HPLC column. However, the gradient volume can be increased by using larger solid particles in the gradient-generator column because of increased eddy diffusion. If the gradient-generator is packed with porous particles (75–150 μm), the gradient volume can be changed over a broad range by simply changing the flush flow rate. This simple control over the gradient volume is due to the increased mass transfer of weak eluent out of the porous particles into the strong eluent at higher flush flows. The breakthrough-interface gradient-generator column approach provides the following advantages for micro-HPLC:

• – Gradient volumes can be readily varied over the micro-HPLC range 40–550 μl.
• – Gradient are linear with smooth and rapid onset of intial and final concentrations.
• – Gradient are produced inexpensively with a single high-pressure precision pump.
• – Gradient can be automated with a timer and single 12-port valve that provides both column regeneration and sample injection.

     

Construction of an all-glass/fused silica,zero dead volume multidimensional gas chromatographic system using press-fit connectors: Preliminary results

A multidimensional gas chromatographic system has been constructed using commercially available press-fit connectors, glass splitters, and fused silica lines. In this new design all the lines are continuously purged with carrier gas, ensuring no dead volumes in the chromatographic path. Some preliminary results and practical considerations are presented.     

Study of peak profiles in nonlinear gas chromatography: Application to the peak distortions observed with overloaded capillary columns

The elution profiles observed with overloaded capillary columns are well described by a four-parameter theoretical model derived from the basic mass balance equations which relies on an expansion of the partition isotherm to the second term and takes the sorption effect into account. The reliability of the model is demonstrated by the fact that the peak leaning coefficient obtained from a least squares fit of the model to the experimental peak is in good agreement with that obtained from the variation of peak height with retention time at peak maximum. The coefficeint of apparent axial dispersion is independent of sample size and can be used to characterize the plate height at zero concentration of an overloaded capillary column.     

High-speed gas chromatographic separations with diaphragm valve-based injection and chemometric analysis as a gas chromatographic “sensor”

A high-speed gas chromatography system, the gas chromatographic sensor (GCS), is developed and evaluated. The GCS combines fast separations and chemometric analysis to produce an instrument capable of high-speed, high-throughput screening and quantitative analysis of complex chemical mixtures on a similar time scale as typical chemical sensors. The GCS was evaluated with 28 test mixtures consisting of 15 compounds from four chemical classes: alkanes, ketones, alkyl benzenes, and alcohols. The chromatograms are on the order of one second in duration, which is considerably faster than the traditional use of gas chromatography. While complete chromatographic separation of each analyte peak is not aimed for, chemical information is readily extracted through chemometric data analysis and quantification of the samples is achieved in considerably less time than conventional gas chromatography.

Calibration models to predict percent volume content of either alkanes or ketones were constructed using partial least squares (PLS) regression on calibration sets consisting of the five replicate GCS runs of six different samples. The percent volume content of the alkane and ketone chemical classes were predicted on five replicate runs of the 22 remaining samples ranging from 0 to 50 or 60% depending on the class. Root mean square errors of prediction were 2–3% relative to the mean percent volume values for either alkane or ketone prediction models, depending on the samples chosen for the calibration set of that model. The alkyl benzenes and alcohols present in the calibration sets or samples were treated as variable background interference. It is anticipated that the GCS will eventually be used to rapidly sample and directly analyze industrial processes or for the high throughput analysis of batches of samples.     

Synthesis of a “molecular rope curtain”: Preparation and characterization of a sliding graft copolymer with grafted poly(ethylene glycol) side chains by the “grafting onto” strategy

A sliding graft copolymer (SGC) with poly(ethylene glycol) (PEG) side chains was prepared by ester formation between terminal carboxyl groups of oxidized PEG methyl ether with molecular weight of 2000 (mPEG2000‐COOH) and hydroxyl groups of a polyrotaxane consisting of PEG and cyclodextrins (CDs). Formation of the SGC structure was confirmed by ¹H NMR, attenuated total reflectance Fourier‐transformed infrared, and gel permeation chromatography. The SGC was soluble in good solvents of PEG and insoluble in poor solvents of PEG. Estimation of the number of grafted mPEG chains suggested a “rope‐curtain” like structure, in which an mPEG chain is connected to each CD ring. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Preparation of inert glass capillary columns coated with UCON 50 HB-5100. An attempt to rediscover industrially produced α, ω-polyethylene/polypropyleneglycol ethers as stationary phases in capillary gas chromatography

Using UCON 50 HB-5100, one of the most successful stationary phases in capillary gas chromatography during the past decade, as an example, we wish to redirect attention to the excellent properties of the polyethylene/polypropyleneglycol ether copolymer coatings. Based on experience gained in work with OH-terminated polysiloxanes, a method is described for preparing inert and relatively temperature stable chemically bonded and crosslinked coatings in a single working step. Methyltrimethoxysilane was used as coupling agent, and trifluoroacetic acid was introduced as an acidic catalyst. The reaction mechanisms involved as well as the influence of additional crosslinking on the column behavior are discussed, particularly with respect of the inertness of the coatings.     

A new strategy for synthesis of “umbrella‐like” poly(ethylene glycol) with monofunctional end group for bioconjugation

A novel strategy was used to synthesize poly(ethylene glycol) (PEG) with “umbrella‐like” structure containing a single reactive group at the “handle” of the “umbrella”. 1‐(Bis(2‐hydroxyethyl)amino)‐3‐(1‐ethoxyethoxy)propan‐2‐ol was used to initiate the ring‐opening polymerization (ROP) of ethylene oxide (EO) in the presence of diphenylmethylpotassium (DPMK) to obtain three‐arm PEG (PEG3), then terminated by benzyl bromide or ethyl bromide. The resultant PEG3 was hydrolyzed to generate hydroxyl group at the conjunction point, and the second step ROP of EO was carried out using PEG3‐OH as macroinitiator in the presence of DPMK. The obtained four‐arm PEG (PEG4) contained a functional hydroxyl group at the end of the fourth arm, which could be easily modified to bioactive groups such as carboxyl, active ester, amino, etc. The well‐defined structure of “umbrella‐like” PEG was characterized by GPC, ¹H NMR, and MALDI‐TOF MS in detail. Propionic acid succinimidyl ester of PEG4 (10 kDa) was utilized for protein conjugation with interferon α‐2b. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2010     

Selective enrichment with “click oligo (ethylene glycol)” column and TOF–MS characterization of simple phenylpropanoids in the fruits of Forsythia suspensa

To separate samples of complex natural products, highly efficient and selective separation methods should be developed. Herein, a selective enrichment method was developed to separate Forsythia suspensa components with “click oligo (ethylene glycol)” (OEG) column in reversed phase (RP) mode. In this method, F. suspensa aqueous extract was successfully separated. And three fractions with structure‐related compounds were obtained. Fraction I mainly consisted of C₆–C₂ natural alcohols and glycosides, fraction II mostly consisted of lignans, and fraction III mainly consisted of simple phenylpropanoids (SP). Then, the three fractions were separated with ultra‐performance liquid chromatography (UPLC). Four more lignans were observed in fraction II, and eight more SP were observed in fraction III than that without OEG column. Fraction III was successively characterized by TOF–MS, 2 acids (caffeic acid and chlorogenetic acid), 26 SP with caffeoyl, and 2 SP with coumaroyl were characterized. The results prove that a valid method has been developed to selectively enrich SP and lignans. And the method combined with UPLC can efficiently separate SP and lignans. Furthermore, the TOF–MS method is effective to confirm the substituents of SP.