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11/22/2005 Chem 253 - Chapter 24 1

Gas Chromatography – Harris Chapter 24

Gaseous Mobile Phase, Solid or Liquid Stationary Phases

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Schematic & Major Components (fig 24-1)

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SampleIntroduction

Injection Port

Carrier Gas Reservoir

Gas FlowMake-up Gas(optional)

Injection Port Oven Temperature T1

Separation column

Column Oven Temperature T2

Detector Oven Temperature T3

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Summary of GC components.

Injection port – Syringe containing sample is introduced through a septum, injection port oven temperature heated to temperaturesthat ensures fast volatilization of sample, i.e. above the b.p. of all sample components, usually 275 0C. Keep in mind these terms: split and splitess injection

Carrier Gas – Sweeps analyte to separation column. Usually, He, or N2, sometimes H2.

Column – Either packed (old) or capillary (newer) columns. Each type requires its own set of plumbing components (threads, etc.)Interestingly all GC and HPLC plumbing components in the English system of measurements.

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Summary of GC components.

Column T – May be below b.p. of sample components, but high enough to keep a significant quantity in the gas phase. Temperature may be ramped up to get separation based on b.p. differences of components along with chromatographic separation.

Makeup Gas – Sometimes required since detector volume is too large for carrier gas flow. Remember that makeup gas is usually required for capillary columns.

Detector – Ideally, want universal detector, sensitive to anything that passes by. Wide dynamic range, low D.L. the usual wants. We want to keep all sample components volatilized, detector oven temperature usually 300 0C.

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Effects of the Column and Mobile Phase Flow in GC.

Why are modern GC based on a capillary column?

Back to the van Deemter Eqn. H = A + B/u + Cu

Remember, u = flow rate, A = multiple pathsB/u = longitudinal diffusion effectsCu = MT effects

We want to minimize H as much as possible.

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“Older” packed columns

usually 1/8” (3.2 mm OD, 2.2 mm I.D.) diameter, 1 – 2 m lengthdesign impedes gas flowmax flow rate about 1 mL/min or 8 cm/min.

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“Newer’ capillary columns

0.25 mm inner diameter.major point, less restricted flow path.flow rate about 20 ml/min or 20 cm/min

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We want to minimize H as much as possible.

H = A + B/u + Cu

Which of the above affects H the most in GC?• B/u effects!

Diffusion coefficients are large in the gas phase.

Simply increasing the flow rate partially addresses the B/u effects in GC

uLD2=σ

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Other major factors that influence the performance of capillary columns relative to packed ones are evident in the table below.

s

fs D

dkfC

2' )(α

•Higher sep. efficiency•Faster sep.•Better for complex mixtures

•Lower cost•Easy to make•Larger samples

Advantages

180,0004,000NHN = L10-60 m1-2 mL

MT for s.p. part of Cu0.25 µm5 µmdf

No packing material less restricted flow

0.25 mm0.1-0.53 mm

2.2 mmI.D.

CommentsTypical CapillaryTypical 1/8 “packed

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Other advantages of tubular over packed GC columns:

No “A” term, straight path.

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Example of the separation efficiency of capillary vs. packed column.

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GC stationary phases.

Older packed columns – uniform silica particles (150-250 µm) required to ensure uniform path lengths (the “A” term in the van Deemter eqn.) Surfaces are chemically modified (see below). The columns themselves were either glass or stainless steel.

Capillary columns – fused silica which like the particles in the packed column require chemical modification (below).Thestationary phase surface (silica) is a hydroylated surface. This caused problems with nonpolar stationary phases.

When polar or mildly polar species partition into the s.p. they stand a good chance of being trapped, causing a excessive band-broadening beyond what is expected from van Deemterconsiderations.

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OH OH OH OH OH OH OH OH

Silica Support

Stationary Phase

Trapped polar soluteMobile Phase

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Surface modification with trimethylchlorosilanereduces surface polarity.

O SiO

O SiO

O SiO

O SiO

OOOHOHOH

SiCH3

CH3CH3

SiCl

CH3

CH3CH3

+

+ HCl

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Effects can be dramatic:

A = EtOHB = methylethylketoneC = benzeneD = cyclohexane

Chromatogram 1 = unmodified

Chromatogram 2 = silinazied

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Liquid stationary phases

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Important Notes:

• All columns have a Tmax, never exceed this! Thermal degradation, or s.p. simply boils off.

• Most s.p. are air sensitive and easily oxidize at high temperatures.

• Capillary columns cost $150-600 (2002 dollars)

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Effect of Stationary Phase Polarity on Retention Times.

#1 n-heptane#2 tetrahydrofuran#3 2-butanone#4 n-propanol

We can reason this order by considering the polarity of the analytes and of the s.p.

In GC the s.p. characteristics have the largest effect on the order of elution.

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Column Temperature and Temperature Programming.

The oven T must be controlled to within ±0.5 0C from RT to 400 0C. A fan circulates air throughout the oven ensuring uniform heating.

Constant temperature isothermal separations are good for simple mixtures.

Typically T programming is required. Slowly ramping T throughout the separation provides a basis for the separation of sample components based on BP.

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Isothermal vs. Temp Programming

The more volatile cpdselute very close together under isothermal conditions (upper).

Ramping T from 50 to 250 oC at 8 oC/min separates out those volatile cpds (lower).

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How does T programming work?

• T programming is an example of the control of the capacity

factor: m

mrA t

ttk −=' which in turn controls the resolution term:

⎟⎟⎠

⎞⎜⎜⎝

⎛+

⎟⎠⎞

⎜⎝⎛ −

= '

'

11

4 B

Bs k

kNRα

α

• Note that T programming does not change the order of

elution.

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Carrier Gases• O2 is usually avoided since it will oxidize the s.p.

• 3 most common gases N2, H2, He.

• Each have very different Hmin characteristics.

• The lighter gases He and H2 require faster analysis flow rates 20-50 cm/min.

• Diffusion in He and H2 causes more band-broadening in m.p.• Greater MT effects decrease band-broadening at faster flow

rates• H = A + B/u + Cu

• N2 Hmin occurs at about 10 cm/min.

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Sample Injection

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Injection Port for Modern GC’s

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GC Injection Syringe

It is important to rapidly vaporize the sample.

Slow vaporization increases band broadening, by increasing the sample “plug”.

Injection port temperature is usually held 50 0C higher than the BP of the least volatile cpd.

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GC injection and band broadening and anomalies.

Manual injections – takes practice and patience. Extremely slow injections will cause band-broadening, wide sample “plug”. Jerky injections may cause double peaks for the same analyte species.

Auto-injectors – robotized carousels of sample vials. No practice needed. Can be set to make measurements all night long. Rare in academe, frequent in industry.

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Split vs. Splitless Injection

Sample injection is done by a syringe – 1 to 5 µL or ng’s of analyte for the average capillary column.

Capillary columns usually require split in injections, a sample reduction method.

Depending on the spilt ratio (adjustable) only 0.2 to 2 % of the sample injection makes its way to the column. The rest is discarded.

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Effect of split versus splitlessinjections on separation quality

The chromatogram (upper left) demonstrates ideal separation characteristics. Upper right illustrates what happens when too much analyte solute is injected onto a capillary column.

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Solid-Phase Microextraction

Analyte pre-concentration method, remember stripping voltammetry.

Typical derivatizingagents:

C18COO-Si-(O)3-

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SPME of Nerve Agents

Fig 24-21.

Advantages of SPME

“Green” method compared to solvent extraction

Fibers are reusable 20-30x

102-103 LOD enhancement

Disadvantage – lot’s of technique and method development.

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Purge and Trap Major Goal: Remove all analyte from sample.

Volatile components are removed from sample by gentle heating and a stream of purge gas, e.g. He.

Adsorbents trap targeted analytes, e.g. molecular sieves.

After complete purge of sample, contents of trap tube are injected into GC.

A purge and trap module is on the Cassiniprobe to Saturn and moons looking for hydrocarbons, H2, He, NH3.

http://saturn.jpl.nasa.gov/home/index.cfm

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GC Detectors

Ideal detector characteristics, for flowing systems (e.g. GC)

large dynamic/linear rangeresponse independent of flow rate, i.e. fast response times.Universal detection, responds to all species.

Keep this in mind when we discuss HPLC and CE.

Additional requirements for GC

operation from RT to 400 oCdetector response independent of detector oven T

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Thermal Conductivity Detector-TCDMeasures heat loss from a hot filament – nearly universal

filament heated to const T

when only carrier gas flows heat loss to metal block is constant, filament T remains constant

when an analyte species flows past the filament generally thermal conductivity goes down, T of filament will rise. (resistance of the filament will rise).

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Flame Ionization Detector (FID).

Sensitive towards organics

Analyte is burned in H2/air, which produces CH and CHO+, radicals, remember our discussion regarding the blue cone in AA.

CHO+ radicals are reduced at a cathode which produces a current proportional to the radical quantity. About 10-12 A

Specific for organic carbon, insensitive to inorganics, CO2, SO2 etc.

Generally DL 100x less than TCD about pg/s (flow rate dependent)

Dynamic range of 107

Response to specific organic depends on the number of organic carbons.

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Electron Capture Detector (ECD)

Sensitive to electron withdrawing groups especially towards organics conatining –F, -Cl, -Br, -I also, -CN, NO2

Nickel-63 source emits energetic electrons collides with N2(introduced as make-up gas or can be used as carrier gas) producing more electrons:

Ni-63 => e-e- + N2 => 2e- + N2

+

The result is a constant current that is detected by the electron collector (anode).

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As an analyte flows through past the Ni-63 source electron capture is possible by electron-withdrawing species:

A + e- => A-

Current decreases as a result of e- capture by analyte. This is one of the few instances in which a signal is produced by a decrease in detectable phenomenon.

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ECD Characrtersitics (the good)

VERY low DL for detected species 10-15g/s for many halogenated substances (PCB, DDT etc).

OK dynamic range of 104.

(The bad)

Radioactive Ni-63 source

EASILY contaminated with O2, H2O, sample overloading. High maintenance device.

Highly variable response to halogenated substances, see table below:

Can be a real headache when method developing a specific analysis, e.g. CH2Cl2 in the presence of CCl4.

Sometimes complementary information from FID helps.

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Other GC detectors

Nitrogen-Phosphorous Detector (NPD)

Also know as the thermionic detector (TID) or alkali flame detector. It is an FID tweaked for N-P cpds, and organics.

Flame Photometric Detector (FPD)

FID tweaked for S containing cpds.

Photoionization Detector (PID)

UV ionization of organic analyte, coupled with high voltage cathode and analode results in current proportional to ionized organics.

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FT-IR we’ve discussed before. (because of generally low ε in A =εbc, most analytes have a modest DL with FT-IR)

Mass Spec

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Qualitative Analysis in GC.

GC-MS offers structural determinations (remember organic?)

With other detectors identification is possible with retention times of analyteand standard, however it’s best if another method is used as a confirmation.

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Quantitative Analysis in GC.

Calibration curve

Standard Addition – generally matrix effects are not as pronounced as in GC as I have personally noticed in LC.

Internal Standard – best used with CC and SA methods. Almost always required since sample injection is not always a reproducible phenomenon, even with auto-samplers.

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Method of Internal Standards

Internal standards are often used in chromatographic methods, but useful in other techniques where sample sizes may vary from rune to run

Known quantities of both non-interfering species and analyte species are subjected to the same analytical procedure and the signal is measured:

[X] = 0.0837 mM[S] = 0.0666 mM

Independent variable

Signal

IS, S

Analyte, X

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F-ratio

The signal of the IS is 347, and that of the analyte is 423. We now can define a response factor F, defined as

0666.0347

0837.0423

][][F

SAF

XA SX ===

Solving for F = 0.9700 We should realize that F will not vary even if sample sizes may do so.

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Example - Now consider an unknown, if 10.0 ml of 0.146 M IS is added to 10.0 ml of unknown then diluted to 25.0 ml, what is the unknown analyte concentration is As = 553 and Ax is 582?

[S] = 0.146 M (10.0 ml/25.0 ml) = 0.0584 M

0584.05829700.0

][553

=X

[X] = 0.05721 M [X]sample = 0.05721 M (25.0 ml/10.0 ml) = 0.143 M