Fused-silica packed columns

Packed capiUary columns are prepared in 50-100 pm ID x 20-50 cm capillaries. They are packed with silica-based particles with the same or smaller particle diameter as in LC, but without metal frits. The frits cause disturbances by inhomogeneity of the field and formation of bubbles. Smaller particles than that in LC can be used because no pressure drop is present with potential-driven flow. The fused silica packed columns can be prepared by first packing the column against a frit. The column is subsequentiy sintered in the middle, and then flushed to remove the particles in the second part of the column. The sintering is performed hy local heating of the silica-paddng material. The next step is to sinter the inlet. Before use, the polyimide coating at the detection window is removed and the column is flushed with mobile phase by a mechanical pump. 

Figure 12.18 LC-SFC analysis of mono- and di-laurates of poly (ethylene glycol) ( = 10) in a surfactant sample (a) normal phase HPLC trace (b) chromatogram obtained without prior fractionation (c) chromatogram of fraction 1 (FI) (d) chromatogram of fraction 2 (F2). LC conditions column (20 cm X 0.25 cm i.d.) packed with Shimpak diol mobile phase, w-hexane/methylene chloride/ethanol (75/25/1) flow rate, 4 p.L/min UV detection at 220 nm. SFC conditions fused-silica capillary column (15 m X 0.1 mm i.d.) with OV-17 (0.25 p.m film thickness) Pressure-programmed at a rate of 10 atm/min from 80 atm to 150 atm, and then at arate of 5 atm/min FID detection. Reprinted with permission from Ref. (23).

Eichelberger JW, Kerns EH, Olynyk P, et al. 1983. Precision and accuracy in the determination of organics in water by fused silica capillary column gas chromatography/mass spectrometry and packed column gas chromatography/mass spectrometry. Anal Chem 55 1471-1479. 

Early work relied on the use of packed columns, but all modern GC analyses are accomplished using capillary columns with their higher theoretical plate counts and resolution and improved sensitivity. Although a variety of analytical columns have been employed for the GC of triazine compounds, the columns most often used are fused-silica capillary columns coated with 5% phenyl-95% methylpolysiloxane. These nonpolar columns in conjunction with the appropriate temperature and pressure programming and pressure pulse spiking techniques provide excellent separation and sensitivity for the triazine compounds. Typically, columns of 30 m x 0.25-mm i.d. and 0.25-qm film thickness are used of which numerous versions are commercially available (e.g., DB-5, HP-5, SP-5, CP-Sil 8 CB, etc.). Of course, the column selected must be considered in conjunction with the overall design and goals of the particular study. 

Under these chromatographic conditions, the CS2 retention time is about 3 min on a fused-silica capillary column and about 2 min on a Teflon Chromosil 330 packed column. 

Figure 1.17 Separation of large ring polycyclic aroaatic hydrocarbons extracted from carbon black on a 1.8 x 0.2 n I.D. fused silica capillary column packed with 3 micrometer spherical octadecylsllanized silica gel eluted with a stepwise solvent gradient at a flow rate of 1.1 mlcroliters/min with an inlet pressure of about 360 atmospheres. Under isocratic conditions this column yielded ca. 225,000 theoretical plates. (Reproduced with permission from ref. 238. Copyright Friedr. Vieweg t Sohn).

Extracolumn dispersion is a major problem for the packed fused silica capillary columns with internal diameters less than 0.35 mm. Peak standeunl deviations will be in the submicroliter range and extensive equipment modification is required for operation under optimum conditions. A reasonable compromise is to esploy injection voluMs of a few hundred nanoliters or less with detector volumes of a similar or preferably smaller size. This demands considerable ingenuity on behalf of the analyst since, as... 

Miniaturised SEC uses small fused-silica packed-capillary columns (0.32-1 mm i.d., 30-200cm) instead of relatively large metal columns. Miniaturisation puts stringent requirements on the quality of SEC columns. Advantages of ptSEC are (i) much smaller amounts of (toxic, expensive) solvents (ii) smaller samples (iii) better and easier temperature control (iv) increased detector compatibility (e.g. MS) and (v) greatly reduced... 

HPLC on a Cosmosil 5 Cis column, using a perchloric acid-acetonitrile eluent (pH 7.6), followed by CLD in the presence of hydrogen peroxide and bis(2,4,6-trichlorophenyl) oxalate (42), was applied to the determination of 1-aminopyrene (43a) and various diaminopyrenes (43b-d). Ascorbic acid was added to avoid oxidative degradation of the aminopyrenes in the presence of metals LOD in the sub-fmol range (SNR 3)147. A fast (less than 10 min) HPLC-ELCD method was proposed for determination of dopamine (19b) and its metabolites in microdialysates, using packed fused silica capillary columns LOD 0.05 Xg/L of dopamine in a 2 XL sample, RSD 3% (n = 10)148. 

FIGURE 4.2 The van Deemter curves for HPLC and SFC. Conditions (a) LC, 55 cm x 250 xm ID fused-silica capillary columns packed with 5 pm porous particles, nitromethane test solute, 25°C, UV detector (214 nm) (b) SFC, 75 cm x 250 pm ID fused-silica capillary columns packed with 5-pm porous particles, 45°C, 230 atm, carbon dioxide mobile phase, methane test solute, FID. (Adapted from Wu, N. et al. Anal. Chem. 71 5084-5092. With permission.)... 

Many nonvolatile and thermally labile allelochemicals can be well separated by liquid chromatography (LC). Identification of the separated components on-line by mass spectrometry (MS) is of great value. Fused-silica LC columns of 0.22 mm ID packed with small-particle material are used in the described LC/MS system. The shape of the column end allows direct connection to a electron impact ion source of a magnetic sector mass spectrometer. Separations by LC are reported and LC/MS mass spectra are shown for monoterpenes, diterpene acids, phenolic acids and cardiac glycosides. The LC/MS system provides identification capability and high-efficiency chromatography with a universal detector. 

SFC chromatographs represent hybrids between GC and HPLC instruments (Fig. 6.4). In order to deliver the supercritical fluid, syringe pumps or reciprocal pumps are used and maintained above the critical temperature using a cryostat regulated at around 0 "C. In instances where an organic modifier is used, a tandem pump is employed which has two chambers, one for the critical fluid and one for the modifier. The liquid then passes through a coil maintained above the critical temperature so that it is converted into a supercritical fluid. Stainless steel packed columns like those used in HPLC (1 to 4 mm in diameter) or fused silica capillary columns like those used in capillary GC (2 to 20 m in length, internal diameters as low as 50 pm and stationary phase film thickness of at least 1 pm) are used in SFC. 

Heat the sample for 30 min at 30°C while purging helium at a rate of 25 ml/min. Collect volatile compounds on the trap (packed with Tenax or equivalent) and thermally desorb at 180°C onto a 30-m x 0.32-pm i.d. x 1-pm film thickness fused-silica capillary column. After desorption is complete, hold the initial temperature for 1 min at -20°C and then program the temperature to ramp to 220°C at 6°C/min. Set the injector and the detector temperatures at 260°C and 280°C, respectively. Use helium as carrier gas at a flow rate of 3.0 ml/min and a split ratio of 20 1. 

Fused silica capillary columns give better separation than packed columns. Columns having inside diameters of 0.25, 0.32, and 0.53 mm and film thickness between 0.25 and 1 pm have found use in herbicides analysis. The stationary phase is generally made out of phenyl silicone, methyl silicone, and cyanopropyl phenyl silicone in varying compositions. Some common columns are DB-5, DB-1701, DB-608, SPB-5, SPB-608, SPB-1701, Rtx-5, AT-1701, HP-608, BP-608, or equivalent. Use helium as carrier gas flow rate 30 cm/s on narrowbore columns with 0.25 or 0.32 mm ID and 7 mL/min for megabore 0.53 ID columns. 

The pesticides listed above can be analyzed by GC-FPD (in P-mode) or by GC/MS following extractions. An open, tubular fused silica capillary column (35 m and 0.53 ID) gives better resolution and sensitivity than a packed column. Some compounds may coelute, which should be analyzed on an alternative column. 

GC column-packed Porapak-QS (80/100 mesh), Chromosorb 101 (60/80 mesh), or equivalent capillary VOCOL, DB-624, Rtx-502.2, DB-5, SPB-5, or equivalent fused silica capillary column. 

GC column. Packed -3% SP-2250 on Supelcoport or equivalent fused silica capillary column such as DB-5, PTE-5, or equivalent. 

GC column a nonpolar fused silica capillary column, such as DB-5, VOCOL DB-624, or equivalent packed column 1% SP-1000 on Carbopack B (60/80mesh) or equivalent. 

Column-packed 1.95%QF-l/1.5%OV-17onGas-ChromQ(80/100mesh) or 3% SP-2100 on Supelcoport (100/120 mesh) or equivalent capillary fused silica capillary column, such as PTE-5, SPB-5, DB-5, Rtx-5, or equivalent. 

GC column a fused silica capillary column DB-1, 30 m x 0.25 mm x 1 mm or equivalent packed column 10% FFAP on Supelcoport (120/100 mesh) or equivalent. 

GC column stainless steel column packed with Porapak Q (50/80 mesh). A fused silica capillary column such as DBWAX or DB-624 may also be used. 

Estes el al. [722] described a method for the measurement of triethyl- and trimethyllead chloride in potable water, using fused silica capillary column gas chromatography with microwave excited helium plasma lead specific detection. Element specific detection verified the elution of lead species, a definite advantage to the packed column method. The method involved the initial extraction of trialkyllead ions from water into benzene, which was then vacuum reduced to further concentrate the compounds. Direct injection of the vacuum concentrated solutions into the gas chromatography-microwave excited helium plasma system gave delectability of triethyllead chloride at the 30mg L 1 level... 

Fig. 10.1. Separation of polycyclic aromatic hydrocarbons (PAHs) on columns packed with Spherisorb ODS particles. Conditions (A) 35(43) cm x 50 pm i.d. fused silica capillary column packed with 3 pm Spherisorb ODS-1 particles (B) 41(53) cm x 75 pm i.d. fused-silica capillary column packed with 5 pm Spherisorb ODS-1 particles 30 kV applied voltage 5 kV, 5 s electrokinetic injection acetonitrile-50 mM Tris buffer, pH 8.1 (80 20 v/v). Peak identifications 1, benzene 2, naphthalene 3, acenaphthylene 4, fluorene 5, acenaphthene 6, phenanthrene 7, anthracene 8, fluoranthene 9, pyrene 10, benz[n]anthracene 11, chrysene 12, benzo[6]fluoranthene 13, benzo[fc]fluoranthene 14, benzo[a]pyrene 15, dibenz[n,/i]anthracene 16, indeno[7,2,3-af]pyrene 17,...

Although the reported sensitivity of TEA when it is used in combination with HPLC or packed column GC to analyse explosives is only in the low nanogram range, Douse [21] described a method that uses fused silica capillary column GC in conjunction with TEA detection for trace analysis of explosives in the low picogram range. He reported that silica capillary column GC can be used to overcome the problems of very polar explosives adsorbing on the packed columns used for GC analysis and in the transfer lines of the TEA. In this way, the detection limits were lowered. He also concluded that the minimum detectable levels (15,10 and <200pg for NG, TNT and RDX, respectively) of the compounds studied were similar to those obtained when ECD was used. 

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