05 Expiremental Approach___Affecting Retention Time

8/13/2019 05 Expiremental Approach___Affecting Retention Time

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An Experimental Approach forAn Experimental Approach for

Students to Learn about FactorsStudents to Learn about FactorsAffecting Retention Time in GLCAffecting Retention Time in GLC

Scott DonnellyScott Donnelly

Arizona Western CollegeArizona Western CollegeYuma, AZYuma, AZ

Colorado River


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Presentation TopicsPresentation Topics

Introduction to AWCIntroduction to AWC

Educational Benefits of GC LabsEducational Benefits of GC Labs

Prelab Exercise/QuestionsPrelab Exercise/Questions

Postlab Assessment QuestionsPostlab Assessment Questions

Student CommentsStudent Comments


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AWC

TucsonMEXICOMEXICO

Grand CanyonGrand Canyon


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Public TwoPublic Two--YearYear

CollegeCollege

6,000 FTSE (Full6,000 FTSE (Full--

time Studenttime Student

Equivalents)Equivalents)

Partnerships withPartnerships with

NAU & UANAU & UA

New 40,000ftNew 40,000ft22

AgricultureAgriculture--ScienceScience

bldg., Spring 2007bldg., Spring 2007

Arizona Western CollegeArizona Western College


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Driving Forces for GC LabsDriving Forces for GC Labs

Earlier work with UVEarlier work with UV--Visible SpectroscopyVisible Spectroscopy

Chemistry of ColorChemistry of Color labslabs

Students could mathematically manipulate A =Students could mathematically manipulate A = --log Tlog Tbut had little understanding why red food coloring isbut had little understanding why red food coloring is

redred

Consistent exposure to instrumentation used inConsistent exposure to instrumentation used inmodern scientific inquirymodern scientific inquiry

Why wait until organic to use GC?Why wait until organic to use GC?


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Classes: Organic Chemistry, General Chemistry 2, &Environmental Science

GC Prelab:

Conceptual questions

5-10 minutes in length

Four consecutive weeks

GC Lab: runs simultaneously

with scheduled lab

GC Postlab Assessment Questions:

Embedded in quizzes and exams Take home lab exercise(s)

Chromatography/Visible Spectroscopy Exam


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Educational BenefitsEducational Benefits

Demystify instrumentsDemystify instruments-- notnot magicalmagical

black boxesblack boxes.based on known.based on knownchemical principleschemical principles

Increase breadth of labsIncrease breadth of labs Expand lecture topic coverageExpand lecture topic coverage

Illustrate complementaryIllustrate complementaryrelationship between theory andrelationship between theory and

applicationapplication


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GC Labs:GC Labs:PotentialPotentialFactorsFactors

Affecting Retention Time, RAffecting Retention Time, Rtt

Molecular WeightMolecular Weight

Boiling PointBoiling Point

Oven TemperatureOven Temperature

Injector TemperatureInjector Temperature

Carrier Gas Flow RateCarrier Gas Flow Rate

Sample Volume InjectedSample Volume Injected

Clarus 500 GCClarus 500 GC


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Chromatogram of Paint ThinnerChromatogram of Paint Thinner

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Every peak, regardless ofEvery peak, regardless of

its height, throughout theits height, throughout the

chromatogram representschromatogram represents

at least one compound.at least one compound.

xx--axis:axis: Time (minutes)Time (minutes)

yy--axis:axis: Detector ResponseDetector Response(mV)(mV)


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Clarus 500 Gas ChromatographClarus 500 Gas Chromatograph

Students shown GC and the majorStudents shown GC and the major

hardware componentshardware components-- injector,injector,

oven, column, and detectoroven, column, and detector-- areare

described briefly.described briefly.


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GasGas--liquid chromatography, or GC for short, physically separatesliquid chromatography, or GC for short, physically separates

liquids in a mixture according to a number of factors. Suppose yliquids in a mixture according to a number of factors. Suppose yououcarry out a GC analysis of an unlabeled bottle. Using a smallcarry out a GC analysis of an unlabeled bottle. Using a small

volume syringe you withdraw some sample from the bottle andvolume syringe you withdraw some sample from the bottle and

inject it into the GC. The chromatogram produced is shown below.inject it into the GC. The chromatogram produced is shown below.

Retention timeRetention time (R(Rtt) is the difference in time between when a) is the difference in time between when asubstance was injected and when it exits the GC into the detectosubstance was injected and when it exits the GC into the detector.r.

Answer the questions below.Answer the questions below.

start Time (arbitrary units)

A) Which peak represents the compound that travels through theA) Which peak represents the compound that travels through the

GC instrument in the least amount of time?GC instrument in the least amount of time?

B) Which peak has the shortest RB) Which peak has the shortest Rtt? the longest?? the longest?

Prelab 1: Retention Time, RPrelab 1: Retention Time, Rtt

A B C DA B C D EE


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Prelab 1: Molecular WeightPrelab 1: Molecular Weight

GasGas--liquid chromatography, or GC for short, physicallyliquid chromatography, or GC for short, physicallyseparates liquids in a mixture according to a number ofseparates liquids in a mixture according to a number of

factors, one of which is molecular weight (MW). Suppose afactors, one of which is molecular weight (MW). Suppose a

mixture of three liquid hydrocarbonsmixture of three liquid hydrocarbons-- AA,, BB, and, and CC-- waswasinjected into a GC with constant oven temperature, Tinjected into a GC with constant oven temperature, Tovenoven..

The relative MWs of the three hydrocarbons areThe relative MWs of the three hydrocarbons are BB >> AA >> CC..

Based on this information arrange the three compounds inBased on this information arrange the three compounds in

the order that they elute or exit from the column. Thenthe order that they elute or exit from the column. Then

explain your reasoning.explain your reasoning.


= A = B = C


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Postlab 1 Assessment Questions:Postlab 1 Assessment Questions:

RRttand Molecular Weightand Molecular Weight

1) Suppose there are four colorless alcohols labeled A, B, C & D1) Suppose there are four colorless alcohols labeled A, B, C & D, all of, all ofwhich are liquids at room temperature. With respect to theirwhich are liquids at room temperature. With respect to their

molecular weights B > D > C > A. GC analysis was done at Tmolecular weights B > D > C > A. GC analysis was done at Tovenoven==

115115OOC. Arrange these four alcohols with respect to their retentionC. Arrange these four alcohols with respect to their retention

times.times.2) A mixture of four hydrocarbons was injected into a GC at T2) A mixture of four hydrocarbons was injected into a GC at Tovenoven ==

8585OOC, giving the chromatogram below. Which compound spends theC, giving the chromatogram below. Which compound spends the

most time in the liquid phase?most time in the liquid phase?

3) Of the choices below, which is predicted to have the longest3) Of the choices below, which is predicted to have the longest RRtt??

a. CHa. CH33CHCH33 b. CHb. CH33CHCH22ClCl c. CHc. CH33CHCH22CHCH22CHCH22CHCH22OHOH

d. CHd. CH33CHCH22CHCH22CHCH33 e. CHe. CH33CHOCHO


AA B CB C DD


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Postlab Assessment Question:Postlab Assessment Question:

Molecular WeightMolecular Weight

A mixture of three liquid saturated hydrocarbonsA mixture of three liquid saturated hydrocarbons-- AA,, BB, and, and CC--was injected into a GC under isothermal heating conditions atwas injected into a GC under isothermal heating conditions at

8585OOC. The GC was outfitted with a 30m, 0.250mm i.d. capillaryC. The GC was outfitted with a 30m, 0.250mm i.d. capillary

column. The resulting chromatogram is shown below.column. The resulting chromatogram is shown below.

Suppose the three hydrocarbons wereSuppose the three hydrocarbons were22--methylhexanemethylhexane,,pentanepentane,,

andand2,2,32,2,3--trimethylhexanetrimethylhexane. Of the three which is represented by. Of the three which is represented by

B in the chromatogram above?B in the chromatogram above? Offer an explanation to supportOffer an explanation to support

your choice.your choice.

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AA BB CC


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Prelab 2: Boiling PointPrelab 2: Boiling Point

The chromatogram below is for a two component liquidThe chromatogram below is for a two component liquid

mixture, containing the alcohols 1mixture, containing the alcohols 1--propanol and 2propanol and 2--

propanol. Both alcohols have the molecular formula Cpropanol. Both alcohols have the molecular formula C33HH88OO

and hence the same molecular weight (MW). Yet, as shownand hence the same molecular weight (MW). Yet, as shown

the two alcohols have different retention times (Rthe two alcohols have different retention times (Rtt) under) underidentical GC conditions. Since their MWs are the same,identical GC conditions. Since their MWs are the same,

their differences in Rtheir differences in Rtt cannot be explained on account of acannot be explained on account of a

difference in MW. So what then is a plausible explanationdifference in MW. So what then is a plausible explanationfor why they have different retention times?for why they have different retention times?

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2.15 2.20 2.25 2.30 2.35 2.40 2.45 2.50 2.55 2.60 2.65 2.70 2.75 2.80 2.85 2.90 2.95 3.00


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Prelab 2: RPrelab 2: Rtt & Molecular Weight& Molecular Weight

As you learned last week gasAs you learned last week gas--liquid chromatography physically separatesliquid chromatography physically separates

liquids in a mixture according to a number of factors, one of whliquids in a mixture according to a number of factors, one of which isich ismolecular weight. Suppose you carry out a GC analysis of a bottlmolecular weight. Suppose you carry out a GC analysis of a bottle that ise that is

not labeled. Using a small volume syringe you withdraw some sampnot labeled. Using a small volume syringe you withdraw some samplele

from the bottle and inject it into the GC. Below is a hypotheticfrom the bottle and inject it into the GC. Below is a hypotheticalal

chromatogram of the analysis. Answer the questions below.chromatogram of the analysis. Answer the questions below.

Detector

Response

1)1) Which peak represents the compound that travels through the GCWhich peak represents the compound that travels through the GC

instrument in the least amount of time?instrument in the least amount of time?

2)2) Arrange the five peaks with respect to their relative molecularArrange the five peaks with respect to their relative molecular

weights, from heaviest to lightest.weights, from heaviest to lightest.

A B C DA B C D EE

___ > ___ > ___ > ___ > ______ > ___ > ___ > ___ > ___


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Postlab 2 Assessment Question:Postlab 2 Assessment Question:

Boiling Point & Molecular WeightBoiling Point & Molecular Weight

A mixture of three liquid hydrocarbons was injected into a GCA mixture of three liquid hydrocarbons was injected into a GC

at Tat Tovenoven = 95= 95OOC (isothermal). The resulting chromatogram isC (isothermal). The resulting chromatogram is

below.below.

1) Which compound is the most volatile?1) Which compound is the most volatile?

2) Which compound most likely has the smallest molecular2) Which compound most likely has the smallest molecular

weight?weight?

3) Which compound most likely has the highest boiling point?3) Which compound most likely has the highest boiling point?

4) Which compound spends more time in the liquid phase relative4) Which compound spends more time in the liquid phase relative

to the other two?to the other two?

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AA BB CC


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Postlab 2 Assessment Question: Boiling PointPostlab 2 Assessment Question: Boiling Point

GC physically separates liquids based on their differences, amonGC physically separates liquids based on their differences, among otherg other

factors, in boiling point (BP) temperature. Suppose there are thfactors, in boiling point (BP) temperature. Suppose there are threereeliquidsliquids-- A, B, and CA, B, and C-- with different boiling points: B > A > C. The threewith different boiling points: B > A > C. The three

component mixture is injected into a GC instrument whose ovencomponent mixture is injected into a GC instrument whose oven

temperature is held constant,temperature is held constant, i.e.i.e. isothermal. Which chromatogramisothermal. Which chromatogram

below (choices 1, 2, 3 or 4) best represents the order in whichbelow (choices 1, 2, 3 or 4) best represents the order in which the threethe threeliquids exit from the GC?liquids exit from the GC?

1)

2)

= A= A = B= B = C= C

3)

4)

start Time (arbritary units)


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Prelab 3: Oven Temperature, TPrelab 3: Oven Temperature, Tovenoven

Three separate 0.2Three separate 0.2L injections of an unknown colorless liquidL injections of an unknown colorless liquidwere done at three different oven temperatures: 125were done at three different oven temperatures: 125OOC,C,

100100OOC, and 75C, and 75OOC. The three separate chromatograms areC. The three separate chromatograms are

shown below. Match each chromatogram with one of the threeshown below. Match each chromatogram with one of the threeoven temperatures.oven temperatures.


TTovenoven = _______= _______OOCC

Toven = _______O

C

TTovenoven = _______= _______OOCC

l b 3 A Q i


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Postlab 3 Assessment Question:Postlab 3 Assessment Question:

TTovenoven & Molecular Weight& Molecular Weight

Into a test tube are placed four colorless, liquid compounds labInto a test tube are placed four colorless, liquid compounds labeled Aeled A

through D. Their relative molecular weights are:through D. Their relative molecular weights are: BB >> DD >> CC >> AA. On. On

the chromatograms below arrange the four compounds with respectthe chromatograms below arrange the four compounds with respect

to how they elute from the column at Tto how they elute from the column at Tovenoven == 115115

OO

CC andand 150150

OO

CC. All. Allother GC instrument conditions are the same for each analysis.other GC instrument conditions are the same for each analysis.


= A = B = C = D


TTovenoven = 115= 115OOCC

TTovenoven == 150150OOCC

l b


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Postlab 3 Assessment Question: TPostlab 3 Assessment Question: Tovenoven

As a new Environmental Science hire youAs a new Environmental Science hire youre given anre given anassignment involving GC. Your boss doesnassignment involving GC. Your boss doesnt have a cluet have a clue

about GC so gives you the two chromatograms. Bothabout GC so gives you the two chromatograms. Both

represent the same sample. The chromatograms wererepresent the same sample. The chromatograms weresubmitted to the Arizona Department of Environmentalsubmitted to the Arizona Department of Environmental

Quality (ADEQ) but returned because of the absence ofQuality (ADEQ) but returned because of the absence of

the GC operating conditions, namely Tthe GC operating conditions, namely T

ovenoven. Rather than. Rather than

doing the analysis again, you use your newly acquireddoing the analysis again, you use your newly acquired

knowledge of GC to argue that the two chromatogramsknowledge of GC to argue that the two chromatograms

were done at different Twere done at different Tovenoven. Which chromatogram. Which chromatogram-- A orA or

BB-- represents the analysis carried out at the higher ovenrepresents the analysis carried out at the higher oven

temperature? On the next page provide two reasons totemperature? On the next page provide two reasons to

ADEQ that support your conclusion.ADEQ that support your conclusion.

140


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Postlab 3 Assessment Question: TPostlab 3 Assessment Question: Tovenoven

As a new hire at a prestigious environmental consultingAs a new hire at a prestigious environmental consultingfirm, you are asked to conduct a GC analysis of a bottlefirm, you are asked to conduct a GC analysis of a bottle--

containing a colorless, homogeneous liquidcontaining a colorless, homogeneous liquid-- from one offrom one of

your firmyour firms clients. The bottles clients. The bottles label reads onlys label reads onlyAlcoholsAlcohols. Undaunted from such lack of information,. Undaunted from such lack of information,

you inject a small volume of the bottleyou inject a small volume of the bottles liquid contents liquid content

into a GC at Tinto a GC at Tovenoven

= 120= 120OOC (isothermal). The resultingC (isothermal). The resulting

chromatogram shows three peaks. Your boss ischromatogram shows three peaks. Your boss is

incompetent when it comes to GC so asks how you areincompetent when it comes to GC so asks how you are

certain that the bottle does not contain instead four orcertain that the bottle does not contain instead four or

five compounds, presumably all alcohols. What do you dofive compounds, presumably all alcohols. What do you do

to answer your bossto answer your boss question?question? Offer a brief but thoroughOffer a brief but thorough

explanation.explanation.


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Prelab 4: Injector Temperature, TPrelab 4: Injector Temperature, Tinjinj

22--PentanolPentanols boiling temperature is 120s boiling temperature is 120OOC. The redC. The redtriangle below represents 2triangle below represents 2--pentanol when Tpentanol when Tinjinj = 140= 140

OOC.C.

Suppose the same volume of 2Suppose the same volume of 2--pentanol was injected intopentanol was injected into

the GC but with Tthe GC but with Tinjinj = 100= 100OO

C. Using the bottom blank axis,C. Using the bottom blank axis,draw the chromatogram for 2draw the chromatogram for 2--pentanol when Tpentanol when Tinjinj = 100= 100

OOC.C.

Use a black triangle to represent 2Use a black triangle to represent 2--pentanol.pentanol.



TTinjinj = 140= 140OOCC

TTinjinj = 100= 100OOCC

P tl b 4 A t Q ti TP tl b 4 A t Q ti T


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Postlab 4 Assessment Question: TPostlab 4 Assessment Question: Tinjinj

Two separate 0.2L injections of para-xylene (bp = 135-137OC)

were done under identical isothermal oven temperature, carrier gasflow rate, and split ratio conditions but at different injector

temperatures, Tinj: 100OC and 170OC. Three chromatograms are

shown below but only two are realistic. Match a chromatogram

with either Tinj. Which chromatogram represents Tinj 100OC and

which Tinj 170OC? Give a reason for not choosing the remaining

chromatogram.

A.


Tinj = ________OC

B.

C.

Tinj = ________OC

Tinj = ________O

C

P tl b 4 A t Q ti TPostlab 4 Assessment Question T


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Postlab 4 Assessment Question: TPostlab 4 Assessment Question: Tinjinj

Below are a series of chromatograms of 2Below are a series of chromatograms of 2--octanol taken inoctanol taken in

order from top to bottom. Offer an explanation for whyorder from top to bottom. Offer an explanation for whythe chromatograms differ.the chromatograms differ.

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Student CommentsStudent Comments

Most of the time all we learn in school is very theoretical.Most of the time all we learn in school is very theoretical.We donWe dont apply it or use equipment. It apply it or use equipment. Ive always wanted tove always wanted toat least handle some lab equipment in my life before itat least handle some lab equipment in my life before itbecomes a necessity.becomes a necessity.

Being able to use instrumentation definitely demystifiesBeing able to use instrumentation definitely demystifiescertain elements of science. Itcertain elements of science. Its easier for me to assimilates easier for me to assimilate

knowledge through doing things Iknowledge through doing things I

m learning about.m learning about.

Wanted to let you know that I have seriously consideredWanted to let you know that I have seriously consideredgoing into research. A big part of this was mygoing into research. A big part of this was myintroduction to the use of instrumentation in your class.introduction to the use of instrumentation in your class.

I enjoyed using the expensive machinery [in order] toI enjoyed using the expensive machinery [in order] tounderstand what we were learning in class.understand what we were learning in class.


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AcknowledgementsAcknowledgements

1) National Science Foundation (NSF)1) National Science Foundation (NSF)

Course, Curriculum, &Course, Curriculum, &Laboratory ImprovementLaboratory Improvement

(CCLI) grant (DUE(CCLI) grant (DUE-- 0310264)0310264)

2) All my students2) All my students

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05 Expiremental Approach___Affecting Retention Time

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