Dual column analyses

Alternative dual-column analyses of the cytosine and uracil arabinosides (using reversed-phase then cation-exchange media), and of riboxamide (on silica then reversed-phase media both coated with hexadecyltrimethylammoniiim bromide), have been developed to enable the desired separation of these components from other constituents in biological samples. 

Dual column analysis involves the simultaneous analysis of a sample using two columns in parallel having stationary phases with widely differing polarities such as a dimethylsilicone and Carbowax (Figure 5.1(c)). The log-log plot of retention time on the two columns for the components to be identified are constructed, thus enabling complex mixtures to be analysed when there are problems of achieving good separation. Experiment 20, Chapter 9 illustrates this application. 

Figure 13 (A) Dual detection analysis where the effluent from the single column is split into two different detectors. (B) Dual column analysis provides for different columns to be used to attempt to maximize resolution of components in complex sample analysis.

Recently, multidimensional GC has been employed in enantioselective analysis by placing a chiral stationary phase such as a cyclodextrin in the second column. Typically, switching valves are used to heart-cut the appropriate portion of the separation from a non-chiral column into a chiral column. Heil et al. used a dual column system consisting of a non-chiral pre-column (30 m X 0.25 mm X 0.38 p.m, PS-268) and a chiral (30 m X 0.32 mm X 0.64 p.m, heptakis(2,3-di-(9-methyl-6-(9-tert-butyldimethylsilyl)-(3-cyclodextrin) (TBDM-CD) analytical column to separate derivatized urinary organic acids that are indicative of metabolic diseases such as short bowel syndrome, phenylketonuria, tyrosinaemia, and others. They used a FID following the pre-column and an ion trap mass-selective detector following the... 

Fang et al. [661] have described a flow injection system with online ion exchange preconcentration on dual columns for the determination of trace amounts of heavy metal at pg/1 and sub-pg/1 levels by flame atomic absorption spectrometry (Fig. 5.17). The degree of preconcentration ranges from a factor of 50 to 105 for different elements, at a sampling frequency of 60 samples per hour. The detection limits for copper, zinc, lead, and cadmium are 0.07, 0.03, 0.5, and 0.05 pg/1, respectively. Relative standard deviations are 1.2-3.2% at pg/1 levels. The behaviour of the various chelating exchangers used was studied with respect to their preconcentration characteristics, with special emphasis on interferences encountered in the analysis of seawater. 

As pointed out above, IC is a well-established method for the analysis of inorganic anions and has become the method of choice in many application areas. Many techniques are available using singlecolumn [46] or dual-column systems with various detection modes. IC can be used both for analytical and preparative purposes. Large sample volumes, up to 1300 pul, can be injected to determine trace anions and cations and to attain detection limits of 10-400 ng/1. For determinations at a pig/1 to mg/1 level, a sample size of 10-50 xl is sufficient. Preconcentration is necessary for lower concentrations (an additional column, a sample pump, an extra valve and an extra time are the disadvantages of this approach [47]). With an IEC column and isocratic... 

FPD measures the phosphorus or sulfur-containing substances. Because most organophosphorus pesiticides contain sulfur atoms, die FPD may be more sensitive and selective. Both these detectors may be used in a dual column dial detector system for analysis. 

Fused silica capillary columns are required. The columns shall demonstrate the required separation of all 2378-specific isomers whether a dual column or a single column analysis is chosen. Column operating conditions shall be evaluated at the beginning and end of each 12-hour period during which samples or concentration calibration solutions are analyzed (see Section 7.4). 

Analysis on a single column is accepcable if the required separation of all the 2378-specific isomers is demonstrated and the minimum acceptance criteria outlined in Sections 7.1, 7.2 and 7.3 are met. See Section 11 for the specifications for the analysis of the 2378-specific isomers using both dual columns and single columns. 

The gas composition was determined with a dual column Hewlett Packard gas chromatograph equipped with a thermal conductivity detector. Chromasorb 102 and Molecular Sieve 5A columns with a helium carrier gas were employed for gas analysis. 

Duebelbeis DO, Pieczonka G, Kapila S, et al. 1989. Application of a dual column reaction chromatography system for confirmatory analysis of polychlorinated biphenyl congeners. Chemosphere 19 143-148. 

A method verified by the Intersociety Committee in a manual on Methods of Air Sampling and Analysis has been used to separate and determine O2, N2, CO, CO2, and CH4 in gas samples by GC. A dual column/dual thermal conductivity detector system is employed. The first column contains a very polar stationary liquid phase and retains CO2 only, while the second column is packed with molecular sieve 13X and separates the rest of the components. A tube filled with 10/20 mesh Indicating Drierite is installed between the sample introduction system and the first column to retain water in the sample. The analysis is operated slightly above ambient temperature to obtain the best precision results. The detection limits for CO2 and O2 are 250 and 300 ppm, respectively. The separation can be completed within 8.5 min. 

Liquid anhydrous ammonia is used extensively as a coolant in heat exchange systems because of its chemical stability, low corrosiveness, and high latent heat of vaporization. However, anhydrous ammonia is readily contaminated during handling and storage. The gas chromatographic analysis of trace contaminants (O2, N2, CO, CH4, CO2, and water) in liquid ammonia was described by Mindmp and Taylor. An F M Model 5750 equipped with a Carle Microcavity Thermistor detector was used with dual columns for analysis. Table 9.3 lists the experimental parameters for both columns, which were conditioned for a minimum of 12 h at a temperature of 180°C and a... 

SP-2401" and 3% SP-2250. ° Detectors used by EPA standards procedures, include photoionization (PID)," electron capture (ECD)," Eourier transform infrared spectrometry (PTIR), " and mass spectrometry detectors (MSD)." ° Method 8061 employs an ECD, so identification of the phthalate esters should be supported by al least one additional qualitative technique. This method also describes the use of an additional column (14% cyanopropyl phenyl polysiloxane) and dual ECD analysis, which fulfills the above mentioned requirement. Among MSDs, most of the procedures employ electron impact (El) ionization, but chemical ionization (CI) ° is also employed. In all MSD methods, except 1625, quantitative analysis is performed using internal standard techniques with a single characteristic m/z- Method 1625 is an isotope dilution procedure. The use of a FTIR detector (method 8410) allows the identification of specific isomers that are not differentiated using GC-MSD. 

Gas Analysis. Product gas volumes were measured by a calibrated wet test meter. Gas compositions were determined with a Beckman model GC-5 dual column, dual thermal conductivity detector (TCD) chromatograph. One detector used a helium carrier with a Porapak Q column, and the other used an argon carrier with a molecular sieve column. Data reduction was aided by an Auto Lab System IV digital integrator equipped with a calculation module. 

Catalytic experiments were carried out at atmospheric pressure in a gas flow microreactor widi He as a diluent. The gas concentrations (NO, CO, O2, hydrocarbons) have been chosen to be representative of the exhaust gas mixtures of spark ignition engines. Analysis were performed by gas chromatography with a dual column (porapak and molecular sieve) and a TCD detector for O2, N2, CO, CO2, N2O, and a flame ionization detector for hydrocarbons. NO and NO2 were analyzed on-line by IR spectrometry (Rosemount analyzers). 

A method to select the appropriate stationary phase for analysis of a sample mixture is to consider the polar characteristics of the analytes and select a stationary phase of similar polarity. An analyte with similar polar character to the stationary phase will be well retained, the principle of like attracts like applies, and useful retention is then likely to occur leading to adequate selectivity and separation of the analytes primarily on the basis of volatility. Conversely, if the solute is immiscible with the stationary phase then little or no retention difference will be obtained. Further useful indication of retention characteristics may be obtained by analysing a sample on a non-polar and polar column, for example, a dual column GC fitted with Apiezon/OVlOl and Carbowax 20M columns temperature programmed 50-220°C at 10°Cmin with a final hold of 10 min. The chromatogram will indicate the polarity of stationary phase required for the components and the analysis can be repeated with columns of differing polarity, e.g. OV17, OV1701. Tables of RIs for various classes of compounds have been published, mainly for squalane, the reference non-polar stationary phase and Apiezon L, with RI values for Carbowax 20M as a reference polar stationary phase [10]. 

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