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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

Oxalate is a naturally occurring substance found in plant foods such as fruits and vegetables. Within the body it can combine with calcium to form kidney or urinary stones. Determination of oxalate in food is important because of the potential for harmful effects on health. Search the literature for a headspace gas chromatography method for the determination of oxalate in food and answer the following questions.

(a) Give the citation (authors, title, journal name, year, volume, pages) for a research paper describing this analysis.

(b) What type of gas chromatography detector is used? Why is this detector appropriate?

(c) What are the precision, limit of quantification, and linear range of the method?

(d) How was the gas chromatographic determination validated?

(e) What alternative methods could be used for determining oxalate in food?

(f) Challenge question: What type of column was used?

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?

3. (a) What are the advantages and disadvantages of using a narrower open tubular column?

(b) What are the advantages and disadvantages of using a longer open tubular column?

(c) What are the advantages and disadvantages of using a thicker film of stationary phase?

(a) If retention times are 1.0 min for CH4, 12.0 min for octane, 13.0 min for unknown, and 15.0 min for nonane, find the Kovats retention index for the unknown. (b) What would be the Kovats index for the unknown if the phase ratio of the column were doubled? (c) What would be the Kovats index for the unknown if the length of the column were halved

This problem reviews concepts from Chapter 23 using

Figure 24-15.

(a) Calculate the retention factor for peak 11 given tm 5 6.7 min.

(b) Calculate the number of theoretical plates (N in Equation 23-31)

and the plate height (H) for peak 11.

(c) Find the resolution (Equation 23-23) between peaks 16 and 17.
See all solutions

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