CEE 772: Instrumental Methods in Environmental Analysis Lecture #24 - - PDF document

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CEE 772 Lecture #23 12/10/2014 Updated: 10 December 2014 Print version CEE 772: Instrumental Methods in Environmental Analysis Lecture #24 Special Applications: Chromatographic Retention Time and Environmental Properties (Skoog, nothing)

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CEE 772 Lecture #23 12/10/2014 1

CEE 772: Instrumental Methods in Environmental Analysis

Lecture #24

Special Applications: Chromatographic Retention Time and Environmental Properties

(Skoog, nothing)

CEE 772 #24 1

Updated: 10 December 2014

(Harris, Nothing) ()

Print version

Stationary Phases:

Stationary phase in GC is the main factor determining the selectivity and retention

  • f solutes.

There are three types of stationary phases used in GC: Solid adsorbents Liquids coated on solid supports Bonded-phase supports 1.) Gas-solid chromatography (GSC)

  • same material is used as both the stationary phase and support material
  • common adsorbents include:

alumina

molecular sieves

(crystalline aluminosilicates [zeolites]

and clay)  silica  active carbon Magnified Pores in activated carbon

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2.) Gas-liquid chromatography (GLC)

  • stationary phase is some liquid coated on a solid support
  • over 400 liquid stationary phases available for GLC

 many stationary phases are very similar in terms of their retention properties

  • material range from polymers (polysiloxanes, polyesters, polyethylene glycols) to

fluorocarbons, molten salts and liquid crystals Based on polarity, of the 400 phases available only 6-12 are needed for most separations. The routinely recommended phases are listed below:

Name Chemical nature of polysiloxane Max. temp. McReynolds’ constants x’ y’ z’ ’ s’ SE-30 Dimethyl 350 14 53 44 64 41 Dexsil300 Carborane-dimethyl 450 43 64 111 151 101 OV-17 50% Phenyl methyl 375 119 158 162 243 202 OV-210 50% Trifluoropropyl 270 146 238 358 468 310 OV-225 25% Cyanopropyl- 25% phenyl 250 238 369 338 492 386 Silar-SCP 50% Cyanopropyl- 50% phenyl 275 319 495 446 637 531 SP-2340 75% Cyanopropyl 275 520 757 659 942 804 OV-275 Dicyanoallyl 250 629 872 763 1106 849 McReynolds’ constants based on retention of 5 standard “probe” analytes – Benzene, n-butanol, 2-pentanone, nitropropanone, pyridine

Higher the number the higher the absorption.

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Preparing a stationary phase for GLC:

  • slurry of the desired liquid phase and solvent is made with a solid support

solid support is usually diatomaceous earth (fossilized shells of

ancient aquatic algae (diatoms), silica-based material)

  • solvent is evaporated off, coating the liquid stationary phase on the support
  • the resulting material is then packed into the column

disadvantage:

  • liquid may slowly bleed off with time

 especially if high temperatures are used  contribute to background

 change characteristics of the column with time

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3.) Bonded-Phase Gas chromatography

  • covalently attach stationary phase to the solid support material
  • avoids column bleeding in GLC
  • bonded phases are prepared by reacting the desired phase with the surface of a silica-

based support reactions form an Si-O-Si bond between the stationary phase and support

  • r

reactions form an Si-C-C-Si bond between the stationary phase and support

  • many bonded phases exist, but most separations can be formed with the following

commonly recommended bonded-phases:

 Dimethylpolysiloxane  Methyl(phenyl)polysiloxane  Polyethylene glycol (Carbowax 20M)  Trifluoropropylpolysiloxane  Cyanopropylpolysiloxane

advantages:

  • more stable than coated liquid phases
  • can be placed on support with thinner and more uniform thickness than

liquid phases

Si

CH3 CH3 O

n Si

CH3 CH3 O

n Si

C6H5 C6H5 O

m

C C HO O H H H H H

n CEE 772 #24 5

  • B. retention and capacity factor: tR = tM(1+k)
  • 1. Modern methods: solute effects (Kamlet, Taft, and Abraham)

System constants (c, m, r, s, a, b, and l): depended on chromatographic system conditions: mobile phase, stationary phase, and temperature. Solute descriptors (R2, π2, Σα2, Σβ2, logL, and Vx): depended on solute properties

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Kamlet-Taft parameters

  • 2. Kovat’s Retention Index

I = 100z +100*[logtR’(x)-logtR’(z)]/[logtR’(z+1)-logtR’(z)]

Where tR’ is the adjusted retention time, z the carbon number of the n-alkane eluting immediately before the substance of interest denoted by x, and z+1 the retention number of the n-alkane eluting immediately after substance x. log k = c + rR2 + sπ2 + aΣα2 + b Σ β2 + llogL

H H H 16

(Gas chromatography)

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Retention Index (Kovats)

Based on n-alkanes

         

 Nn n N Nn NX

t t t t n I ' log ' log ' log ' log 100

) 1 (

where: t’N = Net retention time = tr – t0 and the analyte elutes between Cn and Cn+1

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Kovat’s approach is using retention of n-alkanes as standards to Index the retention of substance of interest on a certain chromatographic system. I = 100z +100*[logtR’(x)-logtR’(z)]/[logtR’(z+1)-logtR’(z)]

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356 ) 2 . 1 2 . 16 log( ) 2 . 1 . 25 log( ) 2 . 1 2 . 16 log( ) 2 . 1 6 . 20 log( 3 100               

unk unk

I I

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Retention Index (Kovats)

Based on the log-linear relationship between number

  • f carbons (n) in an n-alkane

and retention time.

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  • 3. McReynolds’ phase constants

∆I = I stationary phase x – I squalene Squalene (C30H62) ∆I = aX’ +bY’ + cZ’ + dU’ +eS’ McReynold’s phase constants

Phase constant: X’: Benzene; Y’: 1-butanol; Z’: 2-pentanone; U’: 1- nitropropane; S’: Pyridine a, b, c, d, e, constants for the solute of interest.

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Retention Index (McReynolds Constant)

Reports ΔI for a specific stationary phase (squalane), and 5 different reference compounds: benzene, n-butanol, 2- pentanone, nitropropane, pyridine ΔI = Isp – Isqualane. From a table of stationary phase ΔI values, one may choose the biggest ΔI value for the reference compound most like the solute of interest.

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∆I = aX’ +bY’ + cZ’ + dU’ +eS’ McReynold’s phase constants

Phase constant: X’: Benzene; Y’: 1-butanol; Z’: 2-pentanone; U’: 1- nitropropane; S’: Pyridine a, b, c, d, e, constants for the solute of interest. (Gas chromatography) System constants (c, m, r, s, a, b, and l): depended on chromatographic system conditions: mobile phase, stationary phase, and temperature. Solute descriptors (R2, π2, Σα2, Σβ2, logL, and Vx): depended on solute properties

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Kamlet-Taft parameters

Method by Kamlet, Taft, and Abraham Method by McReynolds

log k = c + rR2 + sπ2 + aΣα2 + b Σ β2 + llogL

H H H 16 CEE 772 #24 14

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Comparison to the method by Kamlet, Taft, and Abraham

Idea is same: use constants from systems and solute to describe retention Difference: Kamlet et al use solvatochromic parameters to index the constant of solute of interest. McReynolds uses properties of specific molecules to index constant of solute of interest.

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CEE 772 Lecture #23 12/10/2014 9

  • x

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Standard model

  • Log SP = c + eE + sS + aA + bB + lL

– Where

  • SP = solute property
  • L = gas‐hexadecane partition coefficient

– Cavity formation and solute‐solvent dispersion interactions

  • E = excess molar refraction descriptor
  • A, B = hydrogen bonding acidity and basicity descriptors

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  • x

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  • x

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  • x

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  • x

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CEE 772 Lecture #23 12/10/2014 12

  • Last lecture

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Retention Index (Rohrschnieder Constant)

Reports ΔI for different test solutes ΔI = Isp – Inon-polar s.p. From a table of ΔI values, one may choose the best stationary phase (s.p.) for a given class of solutes

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CEE 772 Lecture #23 12/10/2014 13

Rules for Retention Index

  • 1. R.I. increases 100 points for every CH2 group in a

molecule

  • 2. ∆I for 2 isomers can be calculated from boiling

points: ∆I ≈ 5 ∆bp

  • 3. R.I. for non-polar compounds is constant for any

stationary phase.

  • 4. R.I. for ANY compound is constant for ALL non-polar

stationary phases.

  • 5. ∆I for a solute between a polar and a non-polar

stationary phase is a characteristic of the solute and can be predicted.

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DB-5 slightly more polar than DB-1 C thickness > A E thickness > D

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