Gas Chromatography

Name:

NAUMAN MITHANI

Student no.:

301016320; group C

Course:

CHEM 316

Object:

Expt. 3: GAS CHROMATOGRAPHY lab report

Due date:

13-3-2008

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ABSTRACT: The first part of this series of "Gas Chromatography" experiments explored the retention times of individual substances, in a BTEX mixture, based on their boiling points; it was deemed that retention time is proportional to boiling point. Secondly, the effect of flow rate of the carrier gas on isothermal separation of the BTEX mixture was observed; 40 cm/s (24 m/min.) was deemed to be the ideal carrier gas flow rate. By the variation of temperatures, it was determined that higher temperatures increased resolution but not significantly, and that it must be kept constant. With the aid of internal standards, the concentrations of the constituents of BTEX were calculated to be benzene: 536.8 ppm, toluene: 682.4 ppm (inaccurate), ethylene + m,p-xylenes: 566.8 ppm (b.p.’s are to close) and o-xylene: 48.1 ppm. The concentrations of these components in the gasoline sample were: 9274 ppm of benzene, 6880 ppm of toluene, 3820 ppm of ethylbenzene, 3420 ppm of m/p-xylene, 2186 ppm of o-xylene.

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INTRODUCTION: This series of experiments is one of the practical realisations of the concepts of chromatography, an analytical technique of separation (and potential identification) of substances or compounds in a mixture. It involves running the analyte in a solvent through a passageway coated with an immobile substance, known as the stationary phase. Each constituent of the mixture has a characteristic affinity for the immobile adsorbing substance and the solvent, and so adsorbs/de-adsorbs at its characteristic rate, thus is carried out by the solvent system and/or carrier gas at a characteristic rate. The analyte present in the solvent or eluent is known as the mobile phase and the opposite as stationary phase. These rates are dependant on physical factors, which this series of experiments seeks to explain. In this particular “Gas chromatography”, a carrier gas (He) is used to ‘push’ or carry through the analyte, dissolved in hexane and or bromobenzene, through the column (passageway). The different constituents of the analyte separate and are detected by an attached flame ionization detector. The first section of the experiment measures the effect of the speed of the carrier gas’ flow rate on the retention times (rate of ad/desorption) of the analyte’s constituents. The second section measures the effects of the column’s temperature on the retention times, the selectivity (affinity for the stationary to the mobile phase of the constituents in comparison with one another) of the analyte’s constituents and resolution. The third seeks to quantify the concentrations of the analyte’s constituents. The analytes were a BTEX (mixture of benzene, toluene, ethylbenzene, xylenes) standard and gasoline, dissolved in hexanes with 1,000 ppm bromobenzene.

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EXPERIMENTAL: The solvent is hexanes, each standard/sample contains 1000 ppm bromobenzene as internal standard. The was experiment was commenced by running a sample of hexanes and collecting the chromatogram data. The oven temperature was set at 55 oC and the carrier gas (He) flow rate at 30 cm/s (18 m/min.). Subsequently, 1000 ppm standards of benzene, toluene, ethylbenzene, xylenes (ortho, meta and para) and bromobenzene were run singly through the column. Upon collecting the chromatogram data of these standards, a 1000 ppm standard of BTEX (mixture of benzene, toluene, ethylbenzene, xylenes) was run. Next, the carrier gas flow rate was varied to values of 15 cm/s (9 m/min.), 20 cm/s (12 m/min.), 25 cm/s (15 m/min.), 40 cm/s (24 m/min.) and 50 cm/s (30 m/min.) and the respective chromatogram data(s) measured. The analyte was, once again, BTEX. The second section of the experiment was commenced with resetting the carrier gas flow rate to 30 cm/s. The 1000 ppm BTEX standard was run at varying temperatures of 35 oC, 55 oC and 75 oC. Next, the BTEX standard was run under a linear temperature ramp of 35 oC to 75 oC at 20 oC/min. with the temperature then constant at 75 oC for 3 minutes. This total run took 5 minutes. Under identical temperature ramp settings, BTEX standards of 100, 500, 1000 and 2000 ppm concentrations were run through the column. The experiment was concluded once a sample of gasoline (with 1000 ppm bromobenzene internal standard) was run under the same conditions.

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DATA and RESULTS: ---------------- Section 1 ---------------boiling point (oC) hexanes benzene toluene ethylbenzene p-xylene m-xylene o-xylene bromobenzene

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69 80 110 136 138 138 143 155

graph depicting retention times (min.) for the compounds analysed under typical conditions

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&! sample chromatogram of toluene standard under intial conditions: oven temperature of 55 oC, carrier gas flow rate of 30 cm/s (18 m/min.)

The peak at ~2 minutes is of the solvent hexane.

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'! sample chromatogram of BTEX standard under intial conditions: oven temperature of 55 oC, carrier gas flow rate of 30 cm/s (18 m/min.)

The peak at ~2 minutes is of the solvent hexane.

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compound chosen: TOLUENE flow rate (cm/s)

flow rate (m/min.) 15 20 25 30 40 50

N (toluene) 9 12 15 18 24 30

68672 60666 212450 57875 149077 88685

H (=L/N) 0.0004369 0.0004945 0.0001412 0.0005184 0.0002012 0.0003383

0.261 0.259 0.258 0.255 0.256 0.252

541.882 544.349 510.601 540.185 538.355

! 0.003004926 !/avg. (%) 1.170092396

13.858 2.590

! t $2 where N = # R & "W % H=

L ; N

k'=

tr ' tm tm

where L = 30 m

all quantities based containing time are evaluated in min.

concentration of toluene is calculated as follows:

area of toluene !1000 ppm area of bromobenzene where 1000 ppm is [bromobenzene]

conc. (ppm)

k'

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GC 1: Q4 peak no.

ret. Time (min.)

peak width

area %

velocity (m/min)

BTEX 15 (9 m/min.)

1 2 3 4 5

3.717 3.95 4.058 4.164 4.979

0.058 0.0058 0.038 87.1926 0.059 12.506 0.058 0.1482 0.076 0.1474

8.07102502 7.594936709 7.392804337 7.204610951 6.025306286

BTEX 20 (12 m/min.)

1 2 3 4 5 6 7 8 9 10

0.101 0.868 2.698 2.885 2.961 3.633 4.836 5.008 5.46 6.282

0.074 0.0041 0.095 0.0873 0.054 0.0022 0.033 88.5829 0.042 10.632 0.059 0.1319 0.042 0.1665 0.05 0.138 0.07 0.0117 0.088 0.2434

297.029703 34.56221198 11.11934766 10.39861352 10.13171226 8.257638315 6.203473945 5.990415335 5.494505495 4.775549188

BTEX 25 (15 m/min.)

1 2 3 4 5 6 7

2.221 2.382 2.996 3.992 4.135 4.506 5.193

0.057 0.0021 0.038 99.3624 0.026 0.1217 0.032 0.1527 0.075 0.1266 0.066 0.0108 0.049 0.2236

13.50742909 12.59445844 10.01335113 7.51503006 7.255139057 6.657789614 5.77700751

BTEX 30 (18 m/min.)

1 2 3 4 5 6 7

2.013 2.526 3.362 3.481 3.793 4.364 5.554

0.086 99.6203 0.042 0.0709 0.025 0.0892 0.063 0.0739 0.062 0.0065 0.038 0.1389 0.075 0.0003

14.90312966 11.87648456 8.923259964 8.618213157 7.909306617 6.874427131 5.401512423

1 2 3 4 5 6

1.46 1.834 2.467 2.645 2.779 3.24

0.066 99.5079 0.019 0.0935 0.04 0.2164 0.026 0.0001 0.021 0.0084 0.047 0.1731

20.54794521 16.35768811 12.16051885 11.34215501 10.79525009 9.259259259

BTEX 40 (24 m/min.)

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BTEX 50 (30 m/min.)

Toluene

;

7 8 9

3.904 4.086 4.645

0.05 0.042 0.036

0.0002 0.0002 0.0001

7.68442623 7.342143906 6.458557589

1 2 3 4 5 6 7 8 9 10 hexanes

1.189 1.489 1.989 2.242 2.591 3.135 3.28 3.725 5.098 5.215

0.081 99.4619 0.02 0.1018 0.034 0.2361 0.021 0.0094 0.032 0.1891 0.09 0.0007 0.033 0.0002 0.044 0.0002 0.043 0.0003 0.055 0.0002

25.2312868 20.14775017 15.08295626 13.3809099 11.5785411 9.56937799 9.146341463 8.053691275 5.884660651 5.752636625

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**! He flow rate (m/min.)

u (m/min.) 9 12 15 18 24 30

7.594 10.390 12.590 14.903 20.547 25.231

solvent system: hexanes

H (=L/N) 0.0004369 0.0004945 0.0001412 0.0005184 0.0002012 0.0003383

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*"! As can be seen from the graph and the table previously, the optimum solvent value for H occurs at u of 20.547 m/min., corresponding to carrier gas flow rate of 24 m/min. (40 cm/s). The first three values cannot be considered since the general trend of Van Deemter plots is not followed.

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---------------- Section 2 ----------------

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GC 2: Q1, Q3 peak no. 1 2 3 4 5 6 7 8

BTEX 35 at 35 oC

BTEX 55 at 55 oC

1 2 3 4 5 6 7 1 2 3 4 5 6 7 8 9

BTEX 75 at 75 oC

BTEX 35 to 75 at 35 to 75 oC (20 oC/min. then constant at 75 oC for 3 min.)

1 2 3 4 5 6 7 8 9 10

ret. Time (min.) 0.396 1.9 2.208 2.295 2.643 2.845 3.338 5.08 2.013 2.526 3.362 3.481 3.793 4.364 5.554 0.474 1.62 1.847 2.159 2.314 2.526 2.823 3.87 5.751 1.865 2.264 2.778 2.994 3.285 3.433 3.65 4.035 4.518 4.854

peak width 0.036 0.075 0.078 0.025 0.023 0.089 0.056 0.101 0.086 0.042 0.025 0.063 0.062 0.038 0.075 0.029 0.019 0.018 0.045 0.02 0.022 0.033 0.055 0.068 0.065 0.023 0.067 0.02 0.049 0.149 0.071 0.031 0.032 0.058

area (%)

k'

0.002 0.0022 88.3848 11.4625 0.0004 0.0022 0.1421 0.0038 99.6203 0.0709 0.0892 0.0739 0.0065 0.1389 0.0003 0.0004 99.402 0.1132 0.2622 0.0101 0.21 0.0004 0.0007 0.0011 99.4107 0.1119 0.2584 0.0104 0.2066 0.0007 0.0006 0.0002 0.0002 0.0005

-0.821 -0.139 0.039 0.197 0.288 0.512 1.301 0.255 0.670 0.729 0.884 1.168 1.759

0.140 0.333 0.428 0.559 0.743 1.389 2.550 0.214 0.490 0.605 0.761 0.841 0.957 1.164 1.423 1.603

avg. ret. times

2.588

3.585

2.598

2.065

3.693

Rs 27.099 4.026 1.689 14.500 3.607 6.800 22.191 8.016 24.955 2.705 4.992 11.420 21.062

12.270 9.905 4.769 10.095 10.800 23.795 30.585 9.068 11.422 4.966 8.435 1.495 1.973 7.549 15.333 7.467

When a sample’s constituents have wide ranging boiling points, it can be difficult to separate them; hence the rise in resolution as the temperature is raised to that of the boiling point of hexanes and close to that of benzene, two components in

avg. Rs

11.416

12.192

14.603

9.068

7.330

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the BTEX mixture (though hexane is the solvent). Temperature programming allows for better separation of components of a mixture, as lower boiling point components are more accurately separated at lower temperatures and vice versa. However, this trend is not seen clearly in the data. As the temperature rises and the lower b.p. components are more accurately separated, the resolution is high but when it is kept constant at 75 oC, it is reduced (this temperature is far from the b.p.’s of the heavier components of the mixture).

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---------------- Section 3 ---------------!

GC 3: Q2 peak no.

99.8777 0.0161 0.0025 0.027 0.0715 0.0052 0.0517 99.6747 0.0533 0.1197 0.0051 0.0945 0.001

1 2 3 4 5 6 7 8 9

1.845 2.125 2.257 2.771 2.835 2.991 3.274 3.649 4.035

0.063 0.06 0.033 0.024 0.023 0.035 0.048 0.109 0.03

99.355 0.0003 0.1217 0.1547 0.1285 0.0109 0.2267 0.0006 0.0002

10

4.855

0.083 0.072 0.066 0.021 0.02 0.023 0.115 0.074 0.076 0.028 0.027 0.052

0.0014 0.5554 98.6733 0.0019 0.2003 0.0018 0.4041 0.1614 0.0004 0.0003 0.0002 0.001

1 2 3 4 5 6 7

BTEX 1000

BTEX 2000

peak width 0.049 0.067 0.179 0.162 0.02 0.177 0.037 0.058 0.043 0.034 0.035 0.081 0.034

1 2 3 4 5 6

BTEX 100

BTEX 500

ret. time (min.) 1.832 2.248 2.563 2.763 3.257 3.702 1.691 1.842 2.256 2.767 2.991 3.263 3.707

1 2 3 4 5 6 7 8 9 10 11

1.632 1.854 2.125 2.262 2.676 2.778 3.274 3.65 3.736 4.036 4.855

area (%)

conc. (ppm)

536.8 682.4 566.8 48.1 1,000.0

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The 1000 ppm sample of BTEX was deemed to provide the best resolution and was therefore used in calculating the concentrations of the constituents of BTEX (based on the known concentration of the bromobezene internal standard of 1000 ppm, as shown previously). "

Benzene:

536.8 ppm

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Toluene:

682.4 ppm (27 % higher than calculated earlier)

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Ethylbenzene + m/p xylenes: 566.8 ppm (due to their approximate b.p.’s, they could not be resolved)

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o-xylene:

48.1 ppm

GC 3: Q3 graph of abundance vs. ret. time of “gasoline” sample containing BTEX

solvent hexane; internal standard: 1000 ppm bromobenzene

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*'! abundance

component

conc. (ppm)

0.294

benzene

9,274

0.2181

toluene

6,880

0.1211

ethylbenzene

3,820

0.1084

m,p-xylene

3,420

0.0693

o-xylene

2,186

0.0317

bromobenzene

1,000

Concentrations: component area ÷ bromobenzene area ! [bromobenzene]

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DISCUSSION: ---------------- Section 1 ---------------!

GC 1: Q2 Generally, the greater the molecular weight of the molecule, the higher its

boiling point (b.p.) and the longer its retention time, since the molecule is not so much in the more mobile gaseous phase.

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GC 1: Q5 o-xylene was relatively easier to separate from the the meta and para forms;

but the meta and para forms of xylene were more difficult to separate from ethylbenzene since there is a lesser difference amongst the boiling points of these than with o-xylene.

---------------- Section 3 ---------------!

GC 3: Q1 An internal standard is used as a reference; it is a substance, the relevant

characteristics, properties of which are known quantifiably. Signals and readings of unknowns are measured against those of the standard, the concentration of which e.g. is known, and so the concentration of the unknown may calculated by ratios.

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CONCLUSION: It was observed that the ret. times were proportional to the boiling points of the compounds tested. The variation of carrier gas flow rate saw 40 cm/s to be the most viable rate, providing the lowest H-value. By varying the temperatures, it was determined that resolution was raised slightly as lower boiling fractions e.g. hexanes and bezene were prevented from contributing to the chromatogram. Lastly, the concentrations of the constituents of BTEX were, against the 1000 ppm bromobenzene internal standard, measure to be benzene: 536.8 ppm, toluene: 682.4 ppm (inaccurate), ethylene + m,p-xylenes: 566.8 ppm (b.p.’s are to close to be resolved) and o-xylene: 48.1 ppm. The concentrations of these components in the gasoline sample were 9274 ppm of benzene, 6880 ppm of toluene, 3820 ppm of ethylbenzene, 3420 ppm of m/p-xylene, 2186 ppm of o-xylene.