Gas Chromatography.pdf

8/14/2019 Gas Chromatography.pdf

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Principles of InstrumentalPrinciples of Instrumental

AnalysisAnalysis

Chapter 27Chapter 27

Gas ChromatographyGas Chromatography

Gas Chromatography (GC): vaporized analytes (solutes) are partitionedbetween a mobile gaseous phase and a liquid or a solid stationary phase

held in a column. The mobile phase does not interact with molecules of

the analytes (solutes).

Gas-Liquid Chromatography (GLC): partition between mobile phase and

liquid phase immobilized on the surface of an inert solid packing or on thewalls of a capillary tubing.

Gas-Solid Chromatography (GSC): physical adsorption

27A Princip les of GLC

Due to compressibility of gaseous mobile phases, the mathematical

relationships obtained in Chapter 26 need minor modification.

Retention volume (Vg) instead of retention time (tR):

Vg = (K/S) x (273/Tc) Tc : column temperatureS : density of liquid stationary phase


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VR = tRF, VM = tMF F: average volumetric flow rate

F = Fm x (Tc/T) x (P - PH2O)/P Tc: column temperature (ambient)P: gas pressure at column end (ambient)

PH2O: vapor pressure of water

Corrected retention volume:

V 0 = t F V 0 = t F : com ressibilit factor 27-4, ..

Specific retention volume:

Vg = (VR0 - VM

0)/mS x 273/Tc = jF(tR tM)/mS x 273/Tc

mS: mass of the stationary phase

Vg = jFtMk/mS x 273/Tc retention factor k = (tR - tM)/tM

= VM0k/mS x 273/Tc

= =

c M

S = mS/VS

Vg = K/S x 273/Tc (from eq. 27-4)

At a given temperature, Vg depends only on the distribution constant (K) and

density (S) of stationary phase (liquid).

van Deemter plot

FIGURE 26-8 Effect of mobile-phase flow rate on plate height for (a) LC and (b) GC.Note ver different flow rate and late hei ht scales.

P.772Ch26An Introduction to Chromatographic Separations

1) The minimum for LC usually occurs at flow rates well below those for GC.

2) Plates heights (H) for LC columns are an order of magnitude or more

smaller than those encountered with GC columns.


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27B Instruments for GLC

FIGURE 27FIGURE 27--11 Block diagram of a typical gas chromatograph.

P.790Ch27Gas Chromatography

FIGURE 27FIGURE 27--22

A soap-bubble flow meter.

FIGURE 27FIGURE 27--33 A set of micro-syringes for sample injection.



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FIGURE 27FIGURE 27--44 Cross-sectional view of a microflash vaporizerdirect injector.


Gas sampling valve for quantitative work.

FIGURE 27FIGURE 27--55 A rotary sample valve: valve position (a) is for filling the sample

loop ACB; position (b) if for introduction of sample into column.



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Column configuration: constructed of fused silica or stainless steel; formed ascoils (diameter: 10~30 cm); housed in a thermostatted oven.

Packed column: 1~5 m.Open tubular (capillary) column: ~100 m.

Column temperature is an important

variable: equal or slightly above the

average boiling point of a sample

reasonable elution time (2-30 min).

In general, optimal resolution is associatedwith minimal temperature; the cost of

lowered temperature is an increase in

elution time [Figure 27-7(a)-(b)].

FIGURE 27FIGURE 27--66 Fused-silica capillary columns.


FIGURE 27FIGURE 27--77 Effect of temperatureon gas chromatograms:

(a) isothermal at 45;

(b) isothermal at 145;

(c) programmed at 30 to 180.



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Characteristics of the ideal detectors:

1) Adequate sensitivity.

2) Good stability and reproducibility.

3) A linear responses to solutes (several orders of magnitude).

4) A temperature range from r.t. to at least 400oC.5) A short response time independent of flow rate.

6 Hi h reliabilit and ease of use.

TABLE 27TABLE 27--11 Typical Gas Chromatographic Detectors

7) Similarity in response toward all solutes.

8) The detector should be non-destructive.


FID responds to number of carbon

atom: A mass-sensitive rather thanconcentration-sensitive.

Suitable for organic samples:

Functional groups (carbonyl, alcohol,

halogen, and amine) yield fewer ions

or none at all in a flame. Insensitiveto non-combustible gases (H2O, CO2,

SO2

, CO, noble gases, and NOx

)

Sensitivity: ~10-13 g/s.

Linear response range: ~107

Low noise.


A typical flame ionization detector (FID).


Disadvantages: destructive


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Thermal Conductivity Detector (TCD):

Temperature at constant electrical

power depends on the thermalconductivity of the surrounding gas.

Suitable to both organic and inorganic

species.

Simplicity & non-destructive.

Low sensitivity: ~10-8 g/s.

Linear response range: ~105

FIGURE 27FIGURE 27--99 Schematic of (a) a TCD cell, and (b) an arrangement of twosample detector cells and two reference detector cells.


FIGURE 27FIGURE 27--1010 Schematic diagram of an Electron-Capture Detector (ECD).

Electronegative functional groups tend to capture electronsdecrease

Nickel-63


u . v y - u :

detection of environmental samples: pesticides, polychlorinated biphenyls.

Selective in response: high sensitivity to halogens, peroxides, quinones, and

nitro groups; insensitive to amines, alcohols, and hydrocarbons.


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FIGURE 27FIGURE 27--1111 Diagram of a Hall

electrolytic conductivity detector.


FIGURE 27FIGURE 27--12.12. An Atomic Emission Detector (AED) for GC.


AED is an element-selective detector.


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FIGURE 27FIGURE 27--13(a).13(a). Chromatogram for a gasoline sample containing a smallamount of MTBE and several aliphatic alcohols: monitoring a carbon

emission line.


FIGURE 27FIGURE 27--13(b)13(b) Chromatogram for a gasoline sample containing a small

amount of MTBE and several aliphatic alcohols: monitoring an oxygen

emission line.



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Mass Spectrometry Detectors

FIGURE 27FIGURE 27--1414 Schematic of a typical capillary GC/MS system. The effluent from

the GC is passed into the inlet of the mass spectrometer, where the molecules

in the gas are fragmented, ionized, analyzed, and detected.


FIGURE 27FIGURE 27--15 a .15 a . T ical out uts for a GC/MS s stem. In a the total ion

Time, min

(current) chromatogram were 1, N-nitrosodimethylamine, 2, bis(2-chloroethyl)ether, 3, bis(2-chloroisopropyl)ether, 4, N-nitrosodi-n-propylamine,and 5, bis(2-chloroethoxy)methane.



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FIGURE 27FIGURE 27--15 b15 b T ical out uts for a GC/MS s stem. In b the mass

m/z74

. ,

chromatogram at m/z = 74 is shown. The peak is due to the parent ion ofn-nitrosodimenthylamine (C2H6N2O).


-- -

m/z93

.. .chromatogram m/z = 93 is shown in (c). Peaks 2 and 5 give a response at

this m/z value due to fragmentation products.



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TABLE 27TABLE 27--22 Properties and Characteristics of Typical GC Columns

27C GC Columns and Stationary Phases


FIGURE 27FIGURE 27--1616 A photomicrograph of a diatomaceous earth ().

Magnification 5000X.



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Polar silanol

Adsorption on Column Packings or Capillary Walls:

silanization


Acid washing before silanization removes metal oxide impurities.

TABLE 27TABLE 27--3.3. Some Common Liquid Stationary Phases for GLC

Stationary phase (immobilized liquid) requires: (1) low volatility, (2) thermal

stability, (3) chemical inertness, and (4) solvent characteristics (k and ).



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Hydrocarbon

Polysiloxane

R= methyl: Polydimethyl siloxane

Liquid Stationary Phase fro GLC

Polarity

R= methyl, phenyl: x% Phenyl-polydimethyl siloxane

R= methyl, trifluoropropyl: x% Trifluoropropyl-polydimethyl siloxane

R= methyl, cyanopropyl: x% Cyanopropyl-polydimethyl siloxane

Polyethylene gl ycol: H-(OCH2CH2)n-OH

Polyester: H-(O-RO-CO-R-CO)n-OH

Analytes:


Polar: alcohols, acids, amines. Medium polar: ethers, ketones, aldehydes.

Non-polar: saturated hydrocarbons.

The polarity of the stationary phase should match that of the analytes. When

the match is good, the elution order is determined by the boiling point of the

analytes

FIGURE 27FIGURE 27--17(a)17(a)--(c)(c) Typical chromatograms from open tubular columns coated

with (a) polydimethyl siloxane; (b) 5% (phenyl methyldimethyl) siloxane; (c)

50% (phenyl methyldimethyl) siloxane.



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FIGURE 27FIGURE 27--17(d)17(d)--(f)(f) (d) 50% poly (trifluoropropyl-dimethyl) siloxane; (e)

polyethylene glycol; (f) 50% poly (cyanopropyl-dimethyl) siloxane.


Thalidomide was sold in a number of countries across the world from 1957 until

1961 when it was withdrawn from the market after being found to be a cause of

birth defects in what has been called "one of the biggest medical tragedies of

modern times".[4It is not known exactly how many worldwide victims of the drug

Chiral Stationary Phases (CSP): chiral liquid as stationary phase for

separation of enantiomers.

, , , .

(R)-thalidomide(S)-thalidomide


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27D Appl ications of GC

1) Perform separations

2) Analysis:

a Qualitative anal sis: retention times or volumes .

* Selectivity factors:

* The Retention Index (I): The retention index scale is

based on normal alkanes: 100 x no. carbon.

b) Quantitative analysis: peak heights or areas.


Graphical illustration of the

method for determining

retention indexes for three

compounds. Stationary phase:squalane. Temperature: 60.

Retention indexes for

normal alkane standardsnonane and hexane are

Indicated.



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27E Advances in GC1) High-speed GC

2) Miniaturized GC Systems

FIGURE 27FIGURE 27--1919 High-speed chromatogram obtained with isothermal operation (30)for 27 s followed by a 35/min temperature ramp to 90.


FIGURE 27FIGURE 27--2020 Microfabricated columns (a) and chromatogram (b). The columns in

(a) were 0.9-m-long spiral and serpentine channels.



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FIGURE 27FIGURE 27--20(b)20(b) The mixture (b) was 1, acetone; 2, 2-butanone; 3, benzene; 4,

trichloroethylene; 5, 2,5-dimethyl-furan; and 6, toluene. Air was used as the

carrier gas with an outlet pressure of 0.5 atm.


27F Gas-Solid Chromatography (GSC): physical adsorptionpacked or (porous-layer) open tubular columns (PLOT)1) Molecular Sieves: aluminum silicates2) Porous polymers: cross-linked polystyrenes

FIGURE 27FIGURE 27--2121 Typical gas-solid chromatographic separations: (a) a 5 ft. 1/8 in.

molecular sieve column; (b) a 30 m 0.53 mm PLOT column. Cn =

hydrocarbon with n carbons.


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Gas Chromatography.pdf