Skoog Análise Instrumental Resolução Chapter 31 solutions

Prévia do material em texto

Fundamentals of Analytical Chemistry: 8
th
 ed. Chapter 31 
Chapter 31 
31-1 In gas-liquid chromatography, the stationary phase is a liquid that is immobilized on a 
solid. Retention of sample constituents involves equilibria between a gaseous and a 
liquid phase. In gas-solid chromatography, the stationary phase is a solid surface that 
retains analytes by physical adsorption. Here separation involves adsorption equilibria. 
31-2 Gas-solid chromatography is used primarily for separating low molecular weight gaseous 
species, such as carbon dioxide, carbon monoxide and oxides of nitrogen. 
31-3 Gas-solid chromatography has limited application because active or polar compounds are 
retained more or less permanently on the packings. In addition severe tailing is often 
observed owing to the nonlinear character of the physical adsorption process. 
31-4 In a soap bubble meter a soap film is formed in a gas buret through which the effluent 
from a gas-chromatographic column is flowing. The flow rate is then determined from 
the time required for the film to travel between two of the graduations in the buret. 
31-5 A chromatogram is a plot of detector response, which is proportional to analyte 
concentration or mass, as a function of time. 
31-6 Temperature programming involves increasing the temperature of a gas-chromatographic 
column as a function of time. This technique is particularly useful for samples that 
contain constituents whose boiling points differ significantly. Low boiling point 
constituents are separated initially at temperatures that provide good resolution. As the 
separation proceeds the column temperature is increased so that the higher boiling 
constituents come off the column with good resolution and at reasonable lengths of time. 
Fundamentals of Analytical Chemistry: 8
th
 ed. Chapter 31 
31-7 In open tubular columns, the stationary phase is held on the inner surface of a capillary, 
whereas in packed columns, the stationary phase is supported on particles that are 
contained in a glass or metal tube. Open tubular columns contain an enormous number of 
plates that permit rapid separations of closely related species. They suffer from small 
sample capacities. 
31-8 Sample injection volume, flow rate and column condition are the parameters which must 
be controlled for highest precision quantitative GC. The use of an internal standard can 
minimize the impact of variations in these parameters. 
31-9 The typical column packing is made of diatomaceous earth particles having diameters 
from 250 to 170 m or 170 to 149 m. 
31-10 (a) The thermal conductivity detector is based upon the decrease in thermal conductivity 
of the helium or hydrogen carrier gas brought about by the presence of analyte molecules. 
(b) The flame ionization detector is based on measuring the current that results from ions 
and electrons produced when organic compounds are combusted in a small air/hydrogen 
flame. 
(c) The electron capture detector is based upon the affinity of halogen-containing organic 
compounds for electrons emitted by nickel-63 leading to a reduction in the monitored 
current. 
(d) The thermionic detector is based upon the ion currents produced when the mobile 
phase is combusted in a hydrogen flame and then passed over a heated rubidium silicate 
bead. It is used primarily for detecting analytes that contain phosphorus or nitrogen. 
Fundamentals of Analytical Chemistry: 8
th
 ed. Chapter 31 
(e) The photoionization detector is based upon the ion currents that develop when analyte 
molecules are irradiated with an intense beam of far-ultraviolet radiation. 
31-11 (a) Advantages of thermal conductivity: general applicability, large linear range, 
simplicity, nondestructive. 
Disadvantage: low sensitivity. 
(b) Advantages of flame ionization: high sensitivity, large linear range, low noise, 
ruggedness, ease of use, and response that is largely independent of flow rate. 
Disadvantage: destructive. 
(c) Advantages of electron capture: high sensitivity selectivity towards halogen-
containing compounds and several others, nondestructive. 
Disadvantage: small linear range. 
(d) Advantages of thermionic detector: high sensitivity for compounds containing 
nitrogen and phosphorus, good linear range. 
Disadvantages: destructive, not applicable for many analytes. 
(e) Advantages of photoionization: versatility, nondestructive, large linear range. 
Disadvantages: not widely available, expensive. 
31-12 A hyphenated gas chromatographic method is a method in which the analytes exiting 
from a column are identified by one of the selective techniques such as mass 
spectrometry, absorption or emission spectroscopy or voltammetry. 
31-13 Megabore columns are open tubular columns that have a greater inside diameter (530 
m) than typical open tubular columns (150 to 320 m). Megabore columns can tolerate 
sample sizes similar to those for packed columns, but with significantly improved 
Fundamentals of Analytical Chemistry: 8
th
 ed. Chapter 31 
performance characteristics. Thus, megabore columns can be used for preparative scale 
GC purification of mixtures where the compound of interest is to be collected and further 
analyzed using other analytical techniques. 
31-14 (a) A PLOT column is a porous layer open tubular column, which is also called a support 
coated open-tubular (SCOT) column. The inner surface of a PLOT column is lined with 
a thin film of a support material, such as a diatomaceous earth. This type of column 
holds several times as much stationary phase as does a wall-coated column. 
(b) A WCOT column is simply a capillary tubing fashioned from fused silica, stainless 
steel, aluminum, copper, plastic or glass. Its inner walls are coated with a thin layer of 
the mobile phase. 
(c) The SCOT column is described in the answer to part (a) of this question. 
31-15 The stationary phase liquid should have low volatility, good thermal stability, chemical 
inertness and solvent characteristics that provide suitable retention factor and selectivity 
for the separation. 
31-16 Fused silica columns have greater physical strength and flexibility than glass open tubular 
columns and are less reactive toward analytes than either glass or metal columns. 
31-17 Film thickness influences the rate at which analytes are carried through the column, with 
the rate increasing as the thickness is decreased. Less band broadening is encountered 
with thin films. 
31-18 Currently, liquid stationary phases are generally bonded and/or cross-linked in order to 
provide thermal stability and a more permanent stationary phase that will not leach off 
the column. Bonding involves attaching a monomolecular layer of the stationary phase to 
Fundamentals of Analytical Chemistry: 8
th
 ed. Chapter 31 
the packing surface by means of chemical bonds. Cross linking involves treating the 
stationary phase while it is in the column with a chemical reagent that creates cross links 
between the molecules making up the stationary phase. 
31-19 (a) Band broadening arises from very high or very low flow rates, large particles making 
up packing, thick layers of stationary phase, low temperature, and slow injection rates. 
(b) Band separation is enhanced by maintaining conditions so that k lies in the range of 1 
to 10, using small particles for packing, limiting the amount of stationary phase so that 
particle coatings are thin, and injecting the sample rapidly 
31-20 
 A B C D E 
1 Compound Relative Area Correction Factor Corrected Area Percentage 
2 A 16.4 0.6 27.333333 22.85% 
3 B 45.2 0.78 57.948718 48.45% 
4 C 30.2 0.88 34.318182 28.69% 
5 
6 TotalArea 119.6002331 
7 
8 Spreadsheet Documentation 
9 D2 = B2/C2 
10 D6 = SUM(D2:D4) 
11 E2 = D2/$D$6*100 
 
Fundamentals of Analytical Chemistry: 8
th
 ed. Chapter 31 
31-21 
 A B C D E 
1 Compound Relative Area Correction Factor Corrected Area Percentage 
2 A 32.5 0.70 46.428571 21.09% 
3 B 20.7 0.72 28.750000 13.06% 
4 C 60.1 0.75 80.133333 36.40% 
5 D 30.2 0.73 41.369863 18.79% 
6 E 18.3 0.78 23.461538 10.66% 
7 
8 Total Area 220.1433062 
9 
10 Spreadsheet Documentation 
11 D2 = B2/C2 
12 D8 = SUM(D2:D6) 
13 E2 = D2/$D$8*100 
 
Fundamentals of Analytical Chemistry: 8
th
 ed. Chapter 31 
31-22 
 A B C D E F G 
1 
Percent 
Analyte 
Peak Height, 
Analyte 
 
2 0.05 18.80 
3 0.10 48.10 
4 0.15 63.40 
5 0.20 63.20 
6 0.25 93.60 
7 unknown 58.90 
8 
9 
Regression 
equation 
10 Slope 329.40 
11 Intercept 8.E+00 
12 c unknown 0.1545 
13 
14 Error Analysis 
15 sr 9.017 
16 N 5 
17 Sxx 0.03 
18 sm 57.03 
19 y bar 57.420 
20 M 1 
21 
22 SD of c 0.030 
23 RSD of c 0.194 
24 
25 Spreadsheet Documentation 
26 B10 = SLOPE(B2:B6,A2:A6) 
27 B11=INTERCEPT(B2:B6,A2:A6) 
28 B12 = (B7-B11)/B10 
29 B15 = STEYX(B2:B6,A2:A6) 
30 B16=COUNT(B2:B6) 
31 B17=B16*VARP(A2:A6) 
32 B18=SQRT(B15^2/B17) 
33 B19 =AVERAGE(B2:B6) 
34 B22 =B15/B10*SQRT(1/B20+1/B16+((B7-B19)^2)/((B10^2)*B17)) 
35 B23=B22/B12 
The percentage of the analyte in the unknown determined this way is 0.15±0.03, substantially 
less precise than the 0.163±0.008 value determined when using internal standards. The use of 
internal standards is more precise because the uncertainties introduced by sample injection, flow 
rate and variations in column conditions are minimized