(6.) chromatography - lecture notes

7/23/2019 (6.) Chromatography - Lecture Notes

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GAS CHROMATOGRAPHY

INSTRUMENTATION

The G.C. instrument consists of several major parts.

1. Mobile phase (carrier gas) supply- the carrier gases: are contained in thick-walled cylindrical metal gas tanks that have two stages of

pressure regulation, flow rates through the column are controlled accurately by this regulation and a

pressure/flow controller at the column head in the instrument, must be pure and chemically inert, and are passed through traps to remove traces of

gaseous impurities, moisture, and oxygen, commonly used are: argon, helium, nitrogen, and hydrogen

2. Sample Injection Port- the injection port: is where the sample is introduced, vaporized and swept onto the chromatographic

column, consists of a metal body with an injection port that contains a high temperature, self-

sealing rubber or silicone septum (through which the sample is injected with a syringe),and it is connected to both the inflowing carrier gas and the column head,

is kept at a temperature of at least 50 above the boiling point of the highest boilingo

component of the sample in order to vaporize the sample instantaneously, to ensurethat the sample goes on to the column as a plug of narrow width,

samples must be volatile and may be introduced into the instrument as a gas, liquid, orsolid. The samples must enter the column as a narrow plugs to ensure narrow (sharp)peaks and to reduce band broadening and poor resolution.

Insert diagram here.


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3. Columns two types of columns - packed and open tubular, they are usually bent into coils of diameter 8 to 10 inches, so that they may fit into the

G.C. oven,

1. Packed column - is the earliest type of column developed and it is still widely used.- materials: glass, metals (e.g., stainless steel, nickel, aluminium, copper), teflon or

teflon-lined metal tubes, other exotic alloys for specific applications (e.g.,where corrosion and/or adsorption on walls may be problems).

- dimensions: length - 2 to 10 feetinternal diameter (i.d.): 1/8 or 1/4 inch

- stn. phase: may be liquid coated or bonded on to an inert support (GLC), or a solidadsorbent (GSC).Stationary phases and supports are discussed in detail, later.

2. Open tubular column - is one in which the stationary phase is not in the form of a packingmaterial, but is coated on the inner walls of a narrow tube. There are three main types ofopen tubular columns:

i. fused silica open tubular (FSOT)ii wall-coated open tubular (WCOT)iii porous-layer open tubular (PLOT) or support-coated open tubular (SCOT)

- materials: glass, metals (e.g., stainless steel, aluminium, copper), plastic, and fusedsilica coated with polyimide (flexible, 1979).

- dimensions: length - 30 to 300 feetinternal diameter

- WCOT ~ 0.25 - 0.75 mm- SCOT (PLOT) ~ 0.5 mm

- FSOT ~ 0.1 - 0.3 mm- megabore ~ 530um, 0.53mm, can use sample sizes similar to those

used with packed columns.- stn. phase: the stationary phases used are the same as those used for packed

columns. The stn. ph. is coated on to the inside of the column and is ~ 0.1to 1.0 um in thickness.PLOT, SCOT columns have a fine diatomaceous earth film (~30 um) thick,which can hold more stationary phase.


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4. Detectors- there are several requirements for a detector, as follows: high sensitivity, good stability and reproducibility, linear response - over several orders of magnitude, temperature range - can be used from room temperature to ~400 C,o

short response time that is independent of flow rate, high reliability and ease of use, similar response to all solutes or highly predictable and selective response to

one or more solutes, non-destruction of sample.

No one detector provides all of the above and detectors are often selecteddepending on the particular solute and level of concentration in the sample.There are several detectors that have gained wide-spread acceptance in GC: thermal conductivity detector (TCD), flame ionization detector (FID),

thermionic detector (TID) or nitrogen-phosphorus detector (NPD), electron capture detector (ECD), mass spectrometer (MS), photoionization detector (PID), flame photometric detector, atomic emission detector - microwave induced plasma (MIP).

We will discuss the first two (2) detectors very briefly.

i). Thermal conductivity detector (TCD) or Katharometer

based on changes in the thermal conductivity of the carrier gas due to thepresence of analyte species,

has a wide response to both organic and inorganic species

Advantages simple to use, large linear dynamic working range (~ 10 ),5

responds to both organic and inorganic species, non-destructive - the solute can be collected.

Disadvantage low sensitivity ( ~ 10 g solute/mL carrier gas )-8

ii) Flame Ionization Detector (FID)

based on the combustion of organic materials to produce ionic species thatare captured by a collector producing an ion current


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Response responds to organic species functional groups, e.g., C = O, - OH, - NH, - Cl, etc yield few, if any, ions

2 2 2 x does not respond to H O, SO , CO , NO , etc. (non-combustible gases) response found to be roughly proportional to the number of reduced carbon

atoms in the plasma, i.e., ~ proportional to the number of carbon atoms in thespecies

Advantage responds to carbon compounds, high sensitivity (~ 10 g/mL),13

large linear response range (~10 ),7

low background noise, rugged and easy to use.

Disadvantage

destroys the sample


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STATIONARY PHASES (ADDENDUM)

In GC stationary phases may be liquid (GLC) or solid (GSC).

Gas Liquid Chromatography

In GLC, the liquid stationary phases have to be supported on an inert solid substrate.They may be coated (physical adsorption) or bonded (chemically) to the surface of thesubstrate.

Support material

The support material for liquid stationary phases: holds the stationary phase in place is comprised of small spherical particles

has a large a surface area as possible for interaction with the solute species (aspecific surface area of ~ 1 m /g)2

is uniform in size is inert to interactions with the solute, stationary phase liquid, and the mobile phase must have good mechanical strength (so that it does not degrade and/or deform

easily) be uniformly wetted by the stationary phase usable at elevated temperatures (up to ~ 400 Co

The efficiency of the chromatographic column increases rapidly with decreasing particlediameter. Small particle sizes provide a large surface area for holding the liquid

stationary phase, however, there is a practical limit to the size of the particle that can beused.If the particle diameter is too small, then the pressure differential across the columnrequired to maintain a given mobile phase flow rate may become excessive:

pressure difference 1/(diameter)2

Pressure drops greater than ~ 30 psi are not advisable.

Several support sizes are commonly used. It is difficult to prepare particles of veryprecise diameters, thus the support material is usually sieved to contain a narrow

range (~ 20 mesh range) of particle sizes:

60 - 80 mesh ~ 250 - 170 m

80 - 100 mesh ~ 170 - 150 m

100 - 120 mesh ~ 150 - 120 m

Some newer support materials are being made with a mesh range of ~ 10 mesh.


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Several types of support materials are widely used for GLC columns:1. diatomaceous earth (Chromosorb P, W, and G)2. teflon particles (40 - 60 mesh)3. etched glass beads (60 - 80 mesh)4. porous polymer beads

The most widely used support materials, however, are those made from diatomaceousearth materials.There are two main types of supports made from the above:

1. Chromosorb P - this is the pink support and is made by crushing, blendingand briquetting the diatomaceous earth, and then heating atover 900 C. The briquettes are then ground and sievedo

into various mesh sizes

- the material has pore sizes of ~ 9 m, and a specificsurface area of ~ 4 m /g2

2. Chromosorb W or G

- this is the white support and is made by mixing thediatomaceous earth with sodium carbonate flux and heatingat about 900 C. The product is more rugged thano

Chromosorb P and does not retain solutes to the sameextent

- the material has pore sizes of ~ 2 m, and a specificsurface area of ~ 1 m /g2


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There are problems associated with the use of most support materials. Two things haveto be considered:

the effect of surface silanol groups on the support material the presence of mineral impurities in the support material

The above cause solutes to be physically adsorbed on to the support material. Suchadsorption leads to distorted peaks, i.e., peaks that are broadened and in peak tailing:

the surface of diatomaceous earths have silanol (-SiOH) groups that tend toadsorb polar and polarizable species, particularly when the support is lightlyloaded with the liquid stationary phase or when non-polar liquid stationary phasesare used.

the support material can be deactivated by reacting it with dimethylchlorosilanewhich replaces the H of the -OH group

the support is washed with alcohol and a second chloride is replaced with amethoxy group

silanized supports may still show some adsorption due to the presence of mineral

impurities in the diatomaceous earth- this problem is removed by washing the silanized support with acids prior tosilanization

nomenclature - e.g., Chromosorb P-AW-DMCS- AW = acid washed- DMCS = dimethylchlorosilane

another silanization reagent that is used is hexamethyldisilane (HMDS)

3 3 3 3(CH ) Si NH Si (CH )


3 3 2 2((CH ) ) Cl Si


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Liquid Stationary phases

requirements: low volatility (~100 C higher than the maximum operating temperature of theo

column) thermal stability chemical inertness

solvent characteristics, e.g., k, , for solutes, should fall within a suitable range

choice: important to the success of the separation guidelines exist for making this choice

- usually based on polarity parameters, particularly the polarity of the stationary

phase relative to those of the sample constituents although some information is obtained from these guidelines, trial and error

usually provides information for the final choice

examples: squalene (cycloparafin) methyl silicone gum (e.g., OV-1, SE-30) methyl silicone fluid (e.g., DC-200, OV-101) diisodecyl phthalate diethylene glycolsuccinate phenylmethyl silicone oil

polyethylene glycol (Carbowax-20M)

theoretical plates packed columns >> 250 - 1000/ft (easily) capillary columns >> 6000 - 10,000/m


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preparation: coating (several techniques are used), e.g.:

- a known mass of support is mixed with 3 - 4 times its volume of a volatile

solvent containing a known mass of stationary phase (~1 - 10 % of the massof the support materials)

- the solvent is then evaporated

chemical bonding (stationary phase is chemically bonded to the support material):- the silanol groups on the silica articles (support) are reacted with, e.g., silicon

4tetrachloride (Si Cl )

- the product is reacted with a polyol (MW ~3000 or more)

- the -OH group(s) are reacted with various reagents to provide surfaces ofdifferent polarities

- advantages: - uniform layer of stationary phase- has reproducible properties- monolayer leads to rapid equilibrium of solute between

phases, i.e., efficiencies are improved- thermally very stable up to ~ 250 350 Co

- disadvantage: - sensitive to degradation by oxygen and water



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Gas Solid Chromatography

Gas solid chromatography is based on the adsorption of gases on solid surfaces(adsorbents).

The distribution coefficients for the gases in GSC are much larger than those for GLC thus GSC is used for the separation of species that are not retained by gas liquid

2 2 2columns, e.g., air, H S, CS , N-oxides, CO, CO , rare gases, hydrocarbons (athigh temperatures)

Stationary Phases

There are two main types of solid adsorbents (stationary phases) that are used in GSC:

i) molecular sieves - these are aluminium silicate ion exchangers

- pore sizes depend on the nature of the cation on theexchanger

- have pore sizes of 4, 5, 10, 13 Ao

- come in 40/60 t0 100/120 mesh sizes, usually

- Mechanism of separation:- large molecules cannot penetrate into the pores and are

thus restricted to the outer surface of the stationaryphase particles

- molecules of dimension less than the pore size penetrateto the inside of the stationary phase particles where

adsorption takes place- the surface area is very large for such molecules

(compared to that for larger molecules)- thus large molecules are separated from small molecules

ii) porous polymers - these are beads of uniform size which are made fromstyrene cross-linked divinylbenzene- the pore sizes are controlled by the extent of crosslinking

and is uniform- can have several average pore sizes for use- often used for the separation of gaseous polar species

2 2 2such as: H S, H O, N-oxides, CO , methanol, and vinylchloride

(6.) chromatography - lecture notes

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