Column film thickness

Increasing the speed of analysis has always been an important goal for GC separations. All other parameters being equal, the time of GC separations can be decreased in a number of ways (1) shorten the column (2) increase the carrier gas flow rate (3) reduce the column film thickness (4) reduce the carrier gas viscosity (5) increase the column diameter and/or (6) heat the column more quickly. The trade-off for increased speed, however, is reduced sample capacity, higher detection limits, and/or worse separation efficiency. 

Splitless injection is required for very dilute solutions. It offers high resolution but is poor for quantitative analysis because less volatile compounds can be lost during injection. It is better than split injection for compounds of moderate thermal stability because the injection temperature is lower. Splitless injection introduces sample onto the column slowly, so solvent trapping or cold trapping is required. Therefore, splitless injection cannot be used for isothermal chromatography. Samples containing less than 100 ppm of each analyte can be analyzed with a column fdm thickness < 1 p.m with splitless injection. Samples containing 100-1 000 ppm of each analyte require a column film thickness 1 p.m. 

The most critical properties of a capillary column are resolution, support inertness, retention reproducibility, thermal stability, and column bleed. To provide fast, reliable, and accurate analysis, it is important that the stationary phase, internal diameter (ID) of the column, film thickness, and length of the column be chosen with a view to the particular application. CWC-related chemicals differ greatly from each other in their chemical and physical properties and thus the selection of the stationary phase is in most cases a compromise between resolution and analysis time. The most suitable stationary phases for the separation of chemicals related to the CWC are listed in Table 1, along with their structures and polarities (24). 

Gas chromatographic analysis were run on a Hewlett Packard 5830A gas chromatograph (SPB-1 30 m column, film thickness 0.1pm, carrier gas 2 mL/min). 

Table 1. Retention (Rj) and Kovats Retention Indices (KRIs) of permethylated gibberellin-O-glucosides (HP 5890/5970B/GC-MS configuration, 25 mx0.31 mm i.d. crosslinked methyl silicone fused silica column, film thickness 0.17 jam. He 2.5 ml/min, temperature program from 60°C (1 min) to 260°C (25°C/min)...

Reference values were derived from 30 premature infants (<36 weeks of gestation), 34 term newborns (>36 weeks of gestation), 13 children younger than 5 years, 20 children older than 5 years, and 9 control adults. All control subjects were healthy with no evidence of severe systemic or especially metabolic disease. The compounds are listed in the order of their chromatographic appearance, i.e. in the order of their MUs (DBS 30 mxO.25 mm I.D. fused-silica capillary column, film thickness 1 pm, J W, Rancho Cordova, CA, USA). For a detailed description of the methodology see ref. (18). Abbreviations employed were Min.=minimal, and Max.=maximal values, n.d. = not detectable (<1 mmol/mol of creatinine), - = undetermined. 

To use Equation 4.13, we will need to relate the retention factor A to the distribution coefficient. Chromatographers can measure retention factors directly from the chromatogram if the unretained peak time is known. The distribution coefficient K in Equation 4.13, however, is not directly evident, but it can be computed from the measured retention factor, if the column film thickness d and inner diameter are known. These two column measurements determine the phase ratio P, which is the ratio of the gas to stationary-phase volumes in the column ... 

In contrast to clinical samples, bulk formulations provide more than an adequate amount of sample for analysis. Derivatization procedures are really unnecessary for unequivocal identification if adequate resolution can be achieved. Figure 16.7 shows the analysis of a mixture of 19 anabolic steroids on a 30-m X 0.25-mm-i.d. Rtx-5 (0.1-p.m film thickness) fused-silica capillary column. Table 16.5 lists the corresponding compounds in this mixture. An analysis time of less than 18 min can be achieved using this column. Film thicknesses greater than 0.1 p.m cause longer retention times and a corresponding deterioration in peak shape (55). 

Figure 4 GC/MS chromatograms of pickle volatiles obtained by three different sample preparation techniques (A) dynamic times headspace on Tenax with a 30-m X 0.25-inm-i.d. DB-5 column, film thickness = 1 [tm (B) sohd-phase extraction (with cartridges) with a 30-m X 0.25-mm-i.d. FFAP column, film thickness = 0.25 im and (C) headspace SPME (75- xm Carboxen/PDMS fiber) using same analytical column as in the dynamic headspace method. Peak identities appear in Table 4.

The long-range van der Waals interaction provides a cohesive pressure for a thin film that is equal to the mutual attractive force per square centimeter of two slabs of the same material as the film and separated by a thickness equal to that of the film. Consider a long column of the material of unit cross section. Let it be cut in the middle and the two halves separated by d, the film thickness. Then, from one outside end of one of each half, slice off a layer of thickness d insert one of these into the gap. The system now differs from the starting point by the presence of an isolated thin layer. Show by suitable analysis of this sequence that the opening statement is correct. Note About the only assumptions needed are that interactions are superimposable and that they are finite in range. 

Volumes in mm (liquid) radii and film thicknesses in A area in m g . The remainder of the standard columns ([1] to [9]) may be built up from Tabic 3.2A an alternative table, based on regular intervals of r, may be built up from Table 3.2B. 

Another important characteristic of a gas chromatographic column is the thickness of the stationary phase. As shown in equation 12.25, separation efficiency improves with thinner films. The most common film thickness is 0.25 pm. Thicker films are used for highly volatile solutes, such as gases, because they have a greater capacity for retaining such solutes. Thinner films are used when separating solutes of low volatility, such as steroids. 

Besides growth temperature, the column includes film thickness when known. Given as a value at a L or bracketed over the iadicated X range. 

In addition, the column diameter is usually about 360 (r=180 microns) and the film thickness of the stationary phase will be about 0.25 micron. 

Duranglas glass capillary column (30 m X 0.23 mm i.d.) coated with OV-215 (0.23 p,m film thickness)... 

Figure 10.1 Analysis of racemic 2,5-dimethyl-4-hydroxy-3[2H]-furanone (1) obtained from a strawbeny tea, flavoured with the synthetic racemate of 1 (natural component), using an MDGC procedure (a) dichloromethane extract of the flavoured strawbeny tea, analysed on a Carbowax 20M pre-column (60 m, 0.32 mm i.d., 0.25 p.m film thickness earner gas H2, 1.95 bar 170 °C isothermal) (b) chirospecific analysis of (1) from the sti awbeny tea exti act, ti ansfened foi stereoanalysis by using a pemiethylated /3-cyclodextrin column (47 m X 0.23 mm i.d. canier gas H2, 1.70 bar 110 °C isothemial). Reprinted from Journal of High Resolution Chromatography, 13, A. Mosandl et al., Stereoisomeric flavor compounds. XLIV enantioselective analysis of some important flavor molecules , pp. 660-662, 1990, with permission from Wiley-VCH.

Figure 10.2 MDGC-MS differentiation between the enantiomers of theaspiranes in an aglycone fraction from puiple passion fruit DB5 pre-column (25 m X 0.25 mm i.d., 0.25 p.m film thickness canier gas He, 0.66 ml/min oven temperature, 60-300 °C at 10 °C/min with a final hold of 25 min) permethylated /3-cyclodextrin column (25 m X 0.25 mm i.d., 0.25 p.m film thickness canier gas He, 1.96 ml/min 80 °C isothermal for 20 min and then programmed to 220 °C at 2 °C/min). Reprinted from Journal of High Resolution Chromatography, 16, G. Full et al., MDGC- MS a powerful tool for enantioselective flavor analysis , pp. 642-644, 1993, with permission from Wiley-VCH.

Figure 12.7 Cliromatograms of a polycarbonate sample (a) microcolumn SEC ti ace (b) capillary GC ti ace of inti oduced fractions. SEC conditions fused-silica (30 cm X 250 mm i.d.) packed with PL-GEL (50 A pore size, 5 mm particle diameter) eluent, THE at aElow rate of 2.0ml/min injection size, 200 NL UV detection at 254 nm x represents the polymer additive fraction ti ansfeired to EC system (ca. 6 p-L). GC conditions DB-1 column (15m X 0.25 mm i.d., 0.25 pm film thickness) deactivated fused-silica uncoated inlet (5 m X 0.32 mm i.d.) temperature program, 100 °C for 8 min, rising to 350 °C at a rate of 12°C/min flame ionization detection. Peak identification is as follows 1, 2,4-rert-butylphenol 2, nonylphenol isomers 3, di(4-tert-butylphenyl) carbonate 4, Tinuvin 329 5, solvent impurity 6, Ii gaphos 168 (oxidized). Reprinted with permission from Ref. (14).

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).

The preseparation utilized a 5 pim cyano column (250 cm X 4.6 mm i.d.) and a 5 p.m silica column (250 cm X 4.6 mm i.d.) in series, followed by GC analysis on an SE-54 column (25 m X 0.2 mm i.d., 0.33 p.m film thickness). The SFC system separated the aviation sample into two peaks, including saturates and single-ring aromatics as the first peak, and two-ring aromatic fractions as the second peak. These fractions were selectively cut and then transferred to the GC unit for further analysis. (Figure 12.20). 

I Most of the GC conditions given in this book are for 0.25-mm ID columns, but 0.32- or 0.53-mm ID columns also can be used. The wide bore fused silica columns are found to be more inert, probably because of the greater film thicknesses. A splitter arrangement with a jet separator is used with 0.53-mm ID columns. This arrangement shown in Figure 11.1 has the advantage of simultaneous flame ionization quantitation. 

Every column (including chemically bonded columns) will have some column bleed. The amount of column bleed will increase with increasing column temperature, film thickness, column diameter, and column length. The base line starts to rise approximately 25-50° below the upper temperature limit of the stationary phase. After a column is installed in a GC/MS system, a background spectrum should be obtained for future reference. 

GC assay of the organic layer showed no EIN(TMS)2 remaining after 15 min of stirring (GC conditions Restek RTX-1 column (30 m x 0.53 mm, 1 m film thickness), 2.53 mL/min, initial temperature 50°C, final temperature 300°C, rate 20 deg/min, injection temperature 200°C, detector temperature 350°C, injection volume 1 pL, inject sample neat retention times fert-butyl alcohol 1.4 min, THF 1.7 min, heptane 2.1 min, HN(TMS)2 2.6 min, ethylbenzene (present in commercial LHS) 3.1 min, te/ t-butyl acetate 4.0 min). Volume percents were determined based on standard solution counts. 

Now assuming that (k ) is a constant (which, for a given solute, will be true in SEC) and a given column is considered, then the particle diameter and the film thickness are also constants. Furthermore, as the mobile phase and stationary phases are the same liquids (the stationary phase consists of the mobile phase held in the pores), then Dm = Ds. 

Since the units of D/2 are the same as velocity we can think of this ratio as the velocity of two imaginary pistons one moving up through the water pushing ahead of it a column of gas with the concentration of the gas in surface water (Ci) and one moving down into the sea carrying a column of gas with the concentration of the gas in the upper few molecular layers (Cg). Por a hypothetical example with a film thickness of 17/im and a diffusion coefficient of 1 x 10 cm /s the piston velocity is 5m/day. Thus in each day a column of seawater 5 m thick will exchange its gas with the atmosphere. 

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