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Chapter 24: Q17P (page 663)

Use Table 24-2 to predict the elution order of the following compounds from columns containing (a) poly (dimethyl siloxane), (b) (diphenyl)0.35(dimethyl)0.65polysiloxane, and (c) poly (ethylene glycol): hexane, heptane, octane, benzene, butanol, 2-pentanone.

Short Answer

Expert verified

a)hexane, benzene=butanol, 2-pentanone, heptane, octane

b)hexane, heptane, butanol =2-pentanone, benzene, octane

c) hexane, heptane, octane, benzene, 2-pentanone, butanol

Step by step solution

01

 a) poly (dimethyl siloxane)

Retention indexes for straight chain alkanes are fixed and do not depend on the stationary phase. From Table 24-2, the values of the retention index of the given compounds are:

- hexane =600

- heptane =700

- octane =800

The retention index determines the elution order of the compounds; the order is identified based from the lowest to highest value.

Determine the retention indexes of the other given compounds for this stationary phase.

- benzene =651

- butanol = 651

- 2-pentanone =667

Thus, the elution order is:

hexane, benzene=butanol, 2-pentanone, heptane, octane

02

Step 2:b) ( diphenyl)0.35(dimethyl)0.65 polysiloxane

Determine the retention indexes of the other given compounds for this stationary phase.

- benzene =746

- butanol =733

- 2-pentanone =733

Thus, the elution order is:

hexane, heptane, butanol =2-pentanone, benzene, octane

03

  Step 3: c) poly (ethylene glycol)

Determine the retention indexes of the other given compounds for this stationary phase.

- benzene =963

- butanol = 1158

- 2 -pentanone =998

Thus, the elution order is:

hexane, heptane, octane, benzene, 2-pentanone, butanol

04

final answer

a)hexane, benzene=butanol, 2-pentanone, heptane, octane

b)hexane, heptane, butanol =2-pentanone, benzene, octane

c) hexane, heptane, octane, benzene, 2-pentanone, butanol

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Most popular questions from this chapter

(a) What are the characteristics of an ideal carrier gas?

(b) Why do H2 and He allow more rapid linear velocities in gas chromatography thanN2 does, without loss of column efficiency (Figure 24-11)?

(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?

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.

Astandard solution containing 6.3脳10-8Miodoacetone and 2.0脳10-7Mp-dichlorobenzene (an internal standard) gave peak areas of 395 and 787, respectively, in a gas chromatogram. A 3.00-mlunknown solution of iodoacetone was treated with 0.100mLof 1.6脳10-5Mp-dichlorobenzene and the mixture was diluted to. Gas chromatography gave peak areas of 633 and 520 for iodoacetone and p-dichlorobenzene, respectively. Find the concentration of iodoacetone in the 3.00mLof original unknown.

(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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