Harris: Quantitative Chemical Analysis, Eight Edition CHAPTER 23: GAS CHROMATOGRAPHY

Harris: Quantitative Chemical Analysis, Eight Edition CHAPTER 23: GAS CHROMATOGRAPHY

Chapter 23. Gas Chromatography What did they eat in the year 1,000? GC of Cholesterol and other lipids extracted from British bones (AD 500~1,800)

Chapter 23. Gas Chromatography

23-1 The Separation Process in Gas Chromatography Mobile phase : Carrier gas (He, H 2, N 2...) Stationary phase : i) nonvolatile liquid on a solid support (gas-liquid partition chromatography) ii) solid particles (gas-solid adsorption chromatography) Analyte : gas or volatile liquid

23-1 The Separation Process in Gas Chromatography - The column must be hot enough to provide sufficient vapor pressure for analytes to be eluted in a reasonable time. - The detector is maintained at a higher temperature than the column so that all analytes will be gaseous.

23-1 Open Tubular Columns Fig 23-2a Typical dimensions of open tubular gas chromatography column. Fig 23-2b Fused silica column with a cage diameter of 0.2 m and column Length of 15-100 m.

23-1 Open Tubular Columns Fig 23-2c Cross sectional view of open tubular column.

23-1 Open Tubular Columns

-Narrow columns provide higher resolution than wider columns, but require higher operating pressure and have less sample capacity

Resolution is proportional to then square root of plate number or to the column length

- Increasing thickness (0.25 to 1.0 um) increases retention time and thus increases resolution of early eluting peaks.

The choice of liquid stationary column like dissoves like

Common Stationary Phases

Choice of liquid stationary phase like dissolves like rule - Nonpolar columns are best for the nonpolar solutes. - Intermediate columns are best for the intermediate solutes. - Polar columns are best for the polar solutes.

23-1 The Separation Process in Gas Chromatography : Packed Columns Packed columns : - Stationary phase : i) Fine particles of solid support coated with nonvolatile liquid ii) or Fine particle itself - Comparison of packed column with open tubular column 1) Weak points of packed column : i)broader peaks, ii) longer retention times, iii) lower resolution 2) If uniform particle size smaller multiple path term If small particle size smaller equilibration time term but, needs higher pressure 3) Strong point : larger capacity due to a great deal of stationary phase useful for preparative separations which require a great deal of stationary phase or to separate gases that are poorly retained.

Fig. 23-8 Chromatogram of alcohol mixture using Packed Column poor resolution

23-1 The Retention Index * The relative retention times (t r ) of polar and nonpolar solutes change as the polarity of the stationary phase changes : 1) Non-polar stationary phase : the order of volatility of the solutes (Fig.23-9a) 2) polar stationary phase strongly retain the polar solutes Alcohols (3, 6, 9) > Ketones (1, 4, 7) > Alkanes (2, 5, 8, 10) (Fig. 23-9b)

Fig 23-9. Separation of 10 compounds on a) Nonpolar and b) Strongly polar 1 um thick stationary phase in open tubular column at 70 0 C.

23-1 Temperature and Pressure Programming Temperature programming : the temperature of a column is raised during separation to increase solute vapor pressure. 1) It decreases retention time of late- eluting components. 2) It sharpens peaks. *Avoid raising the temp. so high that analytes and stationary phase decompose. Pressure programming : The inlet pressure increases to increase the flow of mobile phase. 1) It decreases retention time of late- eluting components. 2) The pressure can be rapidly to the initial value for the next run, whereas in temperature programming, time is wasted waiting for a hot column to cool before the next injection. 3) Programmed pressure is useful for analytes that cannot tolerate high temperature.

Figure. Comparison of a) Isothermal (constant) and b) programmed temperature chromatography of linear alkanes.

Figure. Comparison of a) Isothermal (constant) and b) programmed temperature chromatography of linear alkanes. - At a constant temperature of 150 0 C, the more volatile compounds emerge close together, and less volatile compounds may not even be eluted from the column.

Figure. Comparison of a) Isothermal (constant) and b) programmed temperature chromatography of linear alkanes. - If the temperature increases from 50 0 C to 250 0 C at a rate of 8 0 C /min, all compounds are eluted and the separation of peaks is fairly unifrom.

23-1 Carrier Gas H A + B u x + Cu x [22-33] mass transfer = Cux = ( Cs + H C ) u m x [22-35] C s = 3( k 2k + 1) 2 d D 2 s (22 35a) C m = 1+ 6k 24( k + 11k + 1) 2 2 r D 2 m (22 35b) If the stationary phase is thin enough ( <0.5 um), mass transfer is dominated by slow diffusion through the mobile phase rather than through the stationary phase. In other words, C s << C m in eqs. 22-35a & b. -For a column of a given radius, r, and a solute of a given retention factor k, the only variable affecting the rate of mass transfer in the mobile phase is the diffusion coefficient of solute through the mobile phase. (Diffusion coefficients of solute: H 2 > He > N 2 ) decrease in C m (eq. 22-35b)

23-1 Carrier Gas (Diffusion coefficients of solute: H 2 > He > N 2 ) decrease in C m (eq. 22-35b) - H 2, He and N 2 give the same optimal plate height (0.3 mm) at different flow rates. - H 2, He give better resolution (smaller H) than N 2 at high flow rate because solute diffuse more rapidly through H 2, He than through N 2. - Faster separation can be achieved with H 2 as a carrier gas, and H 2 can be run much faster than its optimal velocity with little penalty in resolution. Fig.24-11 Van Deemter curves for GC of n-c 17 H 36 at 175 0 C

Fig 23-12. Separation of two polyaromatic hydrocarbons on a open tubular column with different carrier gases. - As the carrier gas changed from N 2 to He to H 2, resolution increased and analysis time decreased. - H 2 : speed of analysis (advantage), good column efficiency : dangerous (explosive with air) and reactive (hydrogenation of C=C bonds) at elevated temp. He : good alternative for H 2

23-3 Detectors

23-3 Detectors Thermal Conductivity Detector (TCD) Filament; hot tungsten (w)-rhenium (Re) filament Stream; Analyte of different thermal conductivity + carrier gas (He)

23-3 Detectors The thermal conductivity of the mixed stream decrease because He has the second highest thermal conductivity (next slide). The filament gets hotter. Its electrical resistance increases. The voltage drop through the filament changes. The detector (TCD) measures the change in voltage

Thermal Conductivity Detector (TCD) Remark : i) The thermal conductivity (TC) of H 2 and He = 6 ~ 10 times greater than those of most organic compounds. Consequently, the detector undergoes a relatively large decrease in TC even at the presence of small amount of organic materials. ii) H 2 and He give the lowest detection limit.

Thermal Conductivity Detector (TCD) Remark : iii) Sensitivity increases with increasing filament current decreasing flow rate decreasing detector surrounding block temp. iv) In the past, TCD was the most common in GC because they are simple and universal. v) It s not sensitive enough to detect minute quantities of analytes.

Flame Ionization Detector (FID) - Most organic compounds, when pyrolyzed at the temp. of H 2 /air flame, produce CH radicals, which are thought to produce CHO + ions in the flame. CH radicals + O CHO + + e - - The charged species (CHO + ) are attracted to and captured by a collector. - The ion current that results is then amplified and recorded.

Flame Ionization Detector (FID) - In the absence of organic solutes, the current is almost zero. (The ionization in a flame is a poorly understood process) - Only about 1 in 10 5 carbon atoms produces an ion, but ion production is strictly proportional to the number of susceptible carbon atoms entering the flame.

Flame Ionization Detector (FID) Remark : i) FID is insensitive to noncombustible gases (N 2, O 2, H 2 O, CO 2, SO 2, H 2 S, No x, etc) These properties make FID a useful general detector for most organic samples, including those contaminated with water and the oxides of nitrogen and sulfur. ii) Advantages high sensitivity ( 10-13 g/ml), The detection limit is ~100 times smaller than TCD. The detection limit is further reduced by 50 % when N 2 is used instead of He (N 2 gives best detection limit ). large linear response ( 10 7 ), low noise, ruggedness convenience iii) Disadvantages destroy the sample

Electron Capture Detector (ECD) * Most detectors other than FID and TCD respond to much more limited classes of analytes. Electron Capture Detector (ECD) - Carrier gas : N 2 or 5% CH 4 in Ar. (Moisture decrease sensitivity) - A gas (or effluent from the column) is passed over a beta emitter (Ni 63 or H 3 ). An electron from the emitter causes ionization of the carrier gas and production of a burst of electrons.

Electron Capture Detector (ECD) - Electrons thus formed are attracted to an anode, producing a small steady current, - However, in the presence of analyte molecules with a high e - affinity which capture some of the electrons, the current decreases. - The ECD responds by varying the frequency of voltage pulses between the anode and cathode to maintain a constant current.

Electron Capture Detector (ECD) - Remark : i) Highly sensitive to electronegative functional groups (halogens, conjugated C=O, -C N, -NO 2, -O-O-) See detection limit in Table 23-4. ii) Insensitive to compounds (amine, alcohols, hydrocarbons) iii) Application : detection of chlorinated pesticides iv) Advantage : Not consuming the sample to any significant extent

Fig.. Partial gas chromatogram using an ECD to measure halogenated compounds (green house gases) in the air at an altitude of 800 m.

Flame Photometric Detector (FPD) - Eluate from the column passes through a H 2 -Air flame - Excited atoms (S, P, Pb, Sn. etc.) emit characteristic light - P (536 nm), S (394 nm) emission is isolated by a narrow-band interference filter and detected with a photo multiplier tube - Application: For example, detection of pesticides containing P & S

Sulfur Chemiluminescence Detector - It takes the exhaust from FID detector where S compound H 2 -O 2 flame FID SO + Products - Mixing it with ozone to form an excited state of SO 2. SO + O 3 SO 2 * + O 2 _- Excited state of SO 2 emits blue light and UV. SO * 2 SO 2 + hν (emission of blue light and U.V.) Emission intensity mass of S in sample - A Nitrogen Chemiluminescence Detector works in an analogous manner. - 10 7 times greater sensitivity to S or N than to hydrocarbons.

Fig. 23-20 Gas chromatograms showing sulfur compounds in natural gas. a) FID, b) sulfur chemiluminescence detector.

23-3 Detectors