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Chapter 24: Q20p (page 663)

Retention time depends on temperature, T, according to the equation log t鈥�r =(a/T) + b, where a and b are constants for a specific compound on a specific column. A compound is eluted from a gas chromatography column at an adjusted retention time t鈥�r =15.0 min when the column temperature is 373K. At 363 K, t9r 5 20.0 min. Find the parameters a and b and predict t鈥�r at 353K

Short Answer

Expert verified

The relationship between temperature T and retention time(t'r)is shown below:

logtr'=aT+b

where: a and b are constants for a specific compound and specific column

List the given:

-tr'=15.0minatT=373Kt'r=20.0minatT=363K

Step by step solution

01

constant a and b

Find:

- a and b

- t'rat353K

First, to find the parameters a and b, we can set-up two equations from the given t'rand T.

Setting up Equation 1

log(15)=a373+bmultiplyingbothsidesby373:438.6820=a+373b(Equation1)SettingupEquation2:log(20)=a363+bMultiplyingbothsidesby363:472.2739=a+363b(Equation2)

Solve for parameters a and b using Equations 1 and 2:

a=1691.658b=-3.3592

02

Step 2:b) t'r at 353 K 

log(tr')=aT+blog(t'r)=1691.658353+(-3.3592)log(tr')=1.4330t'r=27.10min

03

final answer

t'r=27.10min

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Most popular questions from this chapter

(a) What types of solutes are typically separated with a poly(dimethylsiloxane)-coated open tubular column?

(b) What types of solutes are typically separated with a poly(ethylene glycol)-coated open tubular column?

(c) What types of solutes are typically separated with a porous-layer open tubular column?

The graph shows van Deemter curves for n-nonane at . in the 3.0-m-long microfabricated column in Box 24-2 with a -thick stationary phase.

van Deemter curves. [Data from G. Lambertus, A. Elstro, K. Sensenig, J. Potkay, M. Agah, S. Scheuening, K. Wise, F. Dorman, and R. Sacks, "Design, Fabrication, and Evaluation of Microfabricated Columns for Gas Chromatography," Anal. Chem. 2004, 76, 2629.]

(a) Why would air be chosen as the carrier gas? What is the danger of using

air as carrier gas?

(b) Measure the optimum velocity and plate height for air and for carrier

gases.

(c) How many plates are there in the 3 -m-long column for each carrier gas at

optimum flow rate?

(d) How long does unretained gas take to travel through the column at

optimum velocity for each carrier gas?

(e) If stationary phase is sufficiently thin with respect to column diameter, which of the two mass transfer terms (23-40 or 23-41) becomes negligible?

Why?

(f) Why is the loss of column efficiency at high flow rates less severe for

than for air carrier gas?

(a) When a solution containing234mg of pentanol (FM 88.15) and237mg of 2,3 -dimethyl-2-butanol (FM 102.17) in10.0ml was separated, relative peak areas were pentanol: 2,3 -dimethyl-2-butanol = 0.913 : 1.00. Considering pentanol to be the internal standard, find the response factor for 2,3 -dimethyl-2-butanol.

(b) Use Equation 24-8 to find the areas for pentanol and 2,3 -dimethyl-2-butanol in Figure 24-8.

(c) The concentration of pentanol internal standard in the unknown solution was93.7mM . What was the concentration of 2,3 -dimethyl2-butanol?

Here is a student procedure to measure nicotine in urine. A 1.00-mL sample of biological fluid was placed in a 12-mL vial containing 0.7gNa2CO3powder. After 5.00渭驳of the internal standard 5 -aminoquinoline were injected, the vial was capped with a Teflon coated silicone rubber septum. The vial was heated to 80掳cfor 20 min and then a solid-phase microextraction needle was passed through the septum and left in the headspace for 5.00min. The fiber was retracted and inserted into a gas chromatograph. Volatile substances were desorbed from the fiber at 250掳cfor 9.5min in the injection port while the column was at 60掳c. The column temperature was then raised to 250掳cat 25掳c/minand eluate was monitored by electron ionization mass spectrometry with selected ion monitoring at m/z 84 for nicotine and m / z 144 for internal standard. Calibration data from replicate standard mixtures taken through the same procedure are given in the table.

(a) Why was the vial heated to 80掳cbefore and during extraction?

(b) Why was the chromatography column kept at 60掳Cduring thermal desorption of the extraction fiber?

(c) Suggest a structure for m / z 84 from nicotine. What is the m / z, 144 ion from the internal standard, 5 -aminoquinoline?

(d) Urine from an adult female nonsmoker had an area ratio m / z .84 / 144=0.51 and 0.53 in replicate determinations. Urine from a nonsmoking girl whose parents are heavy smokers had an area ratio 1.18 and 1.32.

Find the nicotine concentration (渭驳/L)and its standard uncertainty in the urine of each person.

(a) Use Trouton's rule, 鈭�Hvap掳鈮�(88Jmol-1K-1)Tbp, to estimate the enthalpy of vaporization of octane (b.p. 126掳).

(b) Use the form of the Clausius-Clapeyron equation below to estimate the vapor pressure of octane at the column temperature in Figure 24-9(70掳C)

In(P1P2)=-(鈭�HvapR)(1T1-1T2)

(c) Calculate the vapor pressure for hexane (b.p. 69掳C) at70掳C

(d) What is the relationship between solute vapor pressure and retention?

(e) Why is the technique called "gas chromatography鈥� if retained analytes are only partially vaporized?
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