Question

All Questions are in regards to Gas Chromatography

1. Explain what injection splitting is and why it is necessary to use in this and all of our experiments.

2. What are adjusted and corrected retention data? (See text book and other references if needed)

3. Do you expect peak shapes to be the same for all the standards on column A? Assume column A is non polar such as polymethylsiloxane. Explain

4. What is and what is the advantage of temperature programming the oven (ramping) in solvent analysis by GC?

5. How will you identify the unknown solvent(s) if present? Is this or can this be a positive identification? How?

6. Is there a better instrumental method for this type of analysis? What is it? What are its advantages?

Answer 1

1. Explain what injection splitting is and why it is necessary to use in this and all of our experiments.

The split injector is the most common and widely used injector in combination with capillary columns. It is derived from the direct injector and hence is also based on rapid evaporation. Injectors and Inlets are used to introduce the sample to the Gas Chromatography column. There are different classes of Injectors and Inlets available and out of which Vaporising Injector (including Split / Splitless) is used widely.

Split Injection Mechanism

1. Sample syringe pierces septum which seals around needle

2. Sample rapidly introduced into heated inlet

3. Liquid sample volatilises and the gaseous ‘plasma’ is contained within a quartz glass liner

4. The sample gas is swept by the carrier gas through the liner and EITHER into the GC Column OR between the liner and inlet body and down the Split Line

5. Percentage of sample reaching the column depends upon the relative flow rates in the column and split flow line

The Advantages of split injector are narrow injection band, minimal column pollution and uniform flow profile whereas the disadvantages are, not suitable for trace analysis (< 50 ppm), critical design and discrimination of high boiling components (> C30)

Split injection with capillary gas chromatography (GC) has typically been used when analyzing neat or concentrated samples. Examples include petroleum mixtures, essential oils, flavours, and fragrances. Split injection involves introducing only a small portion of the sample to the GC column, mainly to avoid overloaded peaks that corrupt the separation efficiency of the column. Other benefits of split injection, which in some applications are at least as important as not exceeding the sample loadability of the GC column, are:

1. Introduction of a very narrow band of sample to maintain GC column separation efficiency, especially for more volatile peaks.

2. Encouragement of highly efficient transfer of lower volatility compounds to the GC column.

3. Avoidance of sensitive compound degradation through increased inlet liner flow, or maybe better put, a decrease in residence time of compound in an inlet liner that contains glass wool or other potentially reactive surfaces such as liner wall and/or inlet seal.

4. Reduction of adsorption of active components (for example, some pesticides, phenols, or amines) to glass wool or liner wall.

5. Mitigation of late-eluting compound loss (for example, some polycyclic aromatic compounds) to liner wool during analysis of samples containing non-volatile “dirt”.

6. Introduction of less non-volatile “dirt” to the GC column to avoid column performance issues, including compound degradation and peak tailing.

7. Higher GC oven start temperatures to decrease oven cool-down times and increase throughput.

8. Circumvention of sample solvent and GC stationary phase mismatches that lead to bad peak shapes, especially with polar solvents.

2. Adjusted Retention Data:

An analyte's retention time (tR) minus the elution time of an unretained peak (tm). tR'= tR-tm Adjusted retention time is also equivalent to the time the analyte spends in the stationary phase. Adjusted retention time is the retention time adjusted for the hold-up time.

The GC technique has two negative aspects regarding qualitative analysis. The technique alone cannot confirm the presence of a single analyte molecule. Under a given set of conditions, any compound analysed by GC has a characteristic retention time; however, this retention time is not unique-other compounds could have the same retention time. The simplest qualititave tool is simply the comparison of adjusted retention data from known and unknown samples. Adjusted retention time is the difference between retention time of the sample and an inert component or some non retained component.

3. Do you expect peak shapes to be the same for all the standards on column A? Assume column A is non polar such as polymethylsiloxane. Explain

The peak shapes won't be same for all the standards as each sample may have different density, boiling point because the elution of sample depends mainly on boiling point.

4. What is and what is the advantage of temperature programming the oven (ramping) in solvent analysis by GC?

Temperature programming is one of the temperature control method in GC for the good seperation of volatile components. The temperature increases with respect to time. The advantages are such as

a) High resolution of lighter compounds

b) Sharp peaks for heavier compounds

c) Reduced run times

d) Lesser carryover

e) Higher sample throughput

f) Extended application range from a single column

5. How will you identify the unknown solvent(s) if present? Is this or can this be a positive identification? How?

Target compounds needs to be selected for analysis. If the standards are available for those compounds make different concentration in mixture of all compounds for calibration. The next step will be to identify the most suitable temperature program by injecting available standards (initial, final temperature, etc.) which will be helpful for better peak response. After completing steps above, you need to at least four point calibration experiment and calculate response factor (RF) of each compound. Now you are ready for real sample analysis.

6. Is there a better instrumental method for this type of analysis? What is it? What are its advantages?

Yes, for much better separation and peak intensity we can use GC-MS technique. This is an advanced high-finated technique used in chromatography.

Answer 2

1. Explain what injection splitting is and why it is necessary to use in this and all of our experiments.

The split injector is the most common and widely used injector in combination with capillary columns. It is derived from the direct injector and hence is also based on rapid evaporation. Injectors and Inlets are used to introduce the sample to the Gas Chromatography column. There are different classes of Injectors and Inlets available and out of which Vaporising Injector (including Split / Splitless) is used widely.

Split Injection Mechanism

1. Sample syringe pierces septum which seals around needle

2. Sample rapidly introduced into heated inlet

3. Liquid sample volatilises and the gaseous ‘plasma’ is contained within a quartz glass liner

4. The sample gas is swept by the carrier gas through the liner and EITHER into the GC Column OR between the liner and inlet body and down the Split Line

5. Percentage of sample reaching the column depends upon the relative flow rates in the column and split flow line

The Advantages of split injector are narrow injection band, minimal column pollution and uniform flow profile whereas the disadvantages are, not suitable for trace analysis (< 50 ppm), critical design and discrimination of high boiling components (> C30)

Split injection with capillary gas chromatography (GC) has typically been used when analyzing neat or concentrated samples. Examples include petroleum mixtures, essential oils, flavours, and fragrances. Split injection involves introducing only a small portion of the sample to the GC column, mainly to avoid overloaded peaks that corrupt the separation efficiency of the column. Other benefits of split injection, which in some applications are at least as important as not exceeding the sample loadability of the GC column, are:

1. Introduction of a very narrow band of sample to maintain GC column separation efficiency, especially for more volatile peaks.

2. Encouragement of highly efficient transfer of lower volatility compounds to the GC column.

3. Avoidance of sensitive compound degradation through increased inlet liner flow, or maybe better put, a decrease in residence time of compound in an inlet liner that contains glass wool or other potentially reactive surfaces such as liner wall and/or inlet seal.

4. Reduction of adsorption of active components (for example, some pesticides, phenols, or amines) to glass wool or liner wall.

5. Mitigation of late-eluting compound loss (for example, some polycyclic aromatic compounds) to liner wool during analysis of samples containing non-volatile “dirt”.

6. Introduction of less non-volatile “dirt” to the GC column to avoid column performance issues, including compound degradation and peak tailing.

7. Higher GC oven start temperatures to decrease oven cool-down times and increase throughput.

8. Circumvention of sample solvent and GC stationary phase mismatches that lead to bad peak shapes, especially with polar solvents.

2. Adjusted Retention Data:

An analyte's retention time (tR) minus the elution time of an unretained peak (tm). tR'= tR-tm Adjusted retention time is also equivalent to the time the analyte spends in the stationary phase. Adjusted retention time is the retention time adjusted for the hold-up time.

The GC technique has two negative aspects regarding qualitative analysis. The technique alone cannot confirm the presence of a single analyte molecule. Under a given set of conditions, any compound analysed by GC has a characteristic retention time; however, this retention time is not unique-other compounds could have the same retention time. The simplest qualititave tool is simply the comparison of adjusted retention data from known and unknown samples. Adjusted retention time is the difference between retention time of the sample and an inert component or some non retained component.

3. Do you expect peak shapes to be the same for all the standards on column A? Assume column A is non polar such as polymethylsiloxane. Explain

The peak shapes won't be same for all the standards as each sample may have different density, boiling point because the elution of sample depends mainly on boiling point.

4. What is and what is the advantage of temperature programming the oven (ramping) in solvent analysis by GC?

Temperature programming is one of the temperature control method in GC for the good seperation of volatile components. The temperature increases with respect to time. The advantages are such as

a) High resolution of lighter compounds

b) Sharp peaks for heavier compounds

c) Reduced run times

d) Lesser carryover

e) Higher sample throughput

f) Extended application range from a single column

5. How will you identify the unknown solvent(s) if present? Is this or can this be a positive identification? How?

Target compounds needs to be selected for analysis. If the standards are available for those compounds make different concentration in mixture of all compounds for calibration. The next step will be to identify the most suitable temperature program by injecting available standards (initial, final temperature, etc.) which will be helpful for better peak response. After completing steps above, you need to at least four point calibration experiment and calculate response factor (RF) of each compound. Now you are ready for real sample analysis.

6. Is there a better instrumental method for this type of analysis? What is it? What are its advantages?

Yes, for much better separation and peak intensity we can use GC-MS technique. This is an advanced high-finated technique used in chromatography.