Gas Chromatography

What is the McReynolds constant?

A way of quantifying the polarity of the column - it's really useful when you need to select a suitable stationary phase for whatever analysis you'll be doing

What happens in silylation?

An acidic hydrogen on the analyte is replaced with an alkyl silyl group.

What is the general classifications of McReynold's constants?

Anything with a McReynolds constant from zero to 100 is considered non polar from 100 to 400 is moderately polar - anything above 400 is considered polar

How is the separation between analytes estimated?

By calculating the RESOLUTION (RS)

How is the McReynold's constant for a given stationary phase determined?

By the retention of: benzene, n-butanol, pentan-2-one, nitropropane and pyridine (reference compounds) on a particular phase.

What differs with capillary GC columns compared to packed?

Capillary columns have a very much smaller internal diameter than packed columns, but are very much longer.

Give an example of a very polar stationary phase

Carbowax. The stationary phase is a polyethylene glycol containing hydroxyl groups, which gives it the high polarity - its corresponding McReynolds constant is greater than 2000.

What does the FID being a destructive detector mean?

Each sample injected can't be recovered and analysed by a different technique. It's not really that big of an issue as you're injecting a very small volume of sample. So there's plenty left over for other analysis. It also doesn't produce a signal for water.

What detector has the broadest range for detection?

FID

What are capillary columns made of?

FUSED SILICA WITH POLYAMIDE (plastic) COATING ON OUTSIDE TO GIVE FLEXIBILITY (to ensure they don't break)

Describe typical packed GC columns

Have an external surface of STAINLESS STEEL, GLASS or COPPER Diameters are in the range of 1.6 to 9.5 mm - where the internal packing or stationary phase is contained Length is usually less than 3 metres Sample loading capacity: approx. 1 - 2 µg per component - This is the absolute amount of the analyte that you can inject onto the column.

What would be an acceptable resolution value?

If ≥ 1.5

What is the relationship between McReynold's constant and polarity?

The higher the McReynold's constant the more polar the stationary phase.

What does the OV number after silicone show?

The higher the number after the OV in these phases means, the greater the polarity of the stationary phase

Why is the potential resolving power in capillary GC much higher than packed GC?

The increased length significantly increases the number of theoretical plates (N) in capillary columns compared to packed columns

Describe what SCOT columns are

The inner surface of the capillary is lined with a thin film of a support material, such as diatomaceous earth. This type of column holds several times as much stationary phase as does a wall-coated column and thus has a greater sample capacity - Increased surface area

Why is it easy to overload the column?

The internal diameter of the capillary columns is very small

What does temperature programming enable?

Very useful way to achieve separation of complex samples. Temperature programming enables the separation of a sample with both high and low volatility components through selected increases in temperature at specific points in the analysis.

Describe detection in GC

A detector at the end of the column monitors the effluent from the column and as each component emerges a peak is produced. The output from the detector is generally displayed by a computer The resulting trace of detector response against time is the chromatogram.

How does a split injection work?

In a split injection, 0.1-2.0 µL of sample , is introduced through a septum into a vaporisation region. A glass or quartz liner protects the sample from the hot, catalytically active metal surface of the chamber and a plug of glass wool helps to ensure complete volatilisation. Once it has evaporated, the sample mixes with the carrier gas stream - due to being in gaseous phase Most of the sample is then vented out through the split exit; typically only 0.1-10% of the sample is carried on to the column. A needle valve upstream of the splitter and a backpressure regulator at the split exit control the split ratio. A buffer volume prevents condensation within the split line and a portion of the carrier gas is used to purge septum contaminants.

What is the benefit of FID having a wide linear range?

In the analysis of unknown samples where the concentration of the analyte is not known.

Where is the best place to start looking for a suitable derivatizing agent?

In the literature to find suitable derivatising agents for your specific analytes that you want to analyse.

How is resolution calculated?

RS = difference in retention / mean of peak base widths

What are the three classifications of derivatisations that can be carried out?

Silylation Acylation Alkylation

What can columns be packed with?

Solid particles (gas solid chromatography, GSC) - stationary phase or A solid support coated with a liquid either by adsorption or chemically bonded to it(gas liquid chromatography ,GLC)

Give an example of a very non-polar stationary phase

Squalane, a stationary phase that is a hydrocarbon characterised by methyl groups or CH3 groups which give it its non polar nature. As this is the most non polar stationary phase available, all other phases are measured relative to it - that's why it has a McReynolds constant of zero.

What is the main disadvantage of FID?

That it requires three different gases to operate, including hydrogen. And it's not sensitive to inorganic compounds. So to analyse these, you need a different system.

What does chemical derivatization mean?

The analyte is changed chemically resulting in different chemical properties more suited to GC analysis - results in different or reduced polarity and increased volatility of the analyte

What are the disadvantages of chemical derivatization?

The derivatizing agent may be difficult to remove and interfere with the analysis It may cause unintended chemical changes in an analyte It will increase the analysis time It might cause extra expense

What affects how quickly the analyte travels through the column?

The different components, depending upon the extent of their interaction with the SP, migrate at different rates through the column.

How is the sample introduced in GC?

The rubber septum is there to avoid any losses of the carrier gas during the injection process. the heating chamber is set at a temperature roughly 50 degrees Celsius higher than the boiling point of the least volatile analyte in your sample. This causes the sample to vaporise, and it makes sure that all of your analytes are in the gaseous phase. However, the top of the column or the start of the column is typically set at a lower temperature than the heating chamber, causing the analytes to condense onto the column. Then return to the gaseous phase as the column is heated and carry through the column using the carrier gas. Therefore, this enables interactions with the stationary phase and separation can take place.

How do FID work?

The sample stream is burned in a jet of hydrogen and air. Ions are produced in the 2000C flame. These ions are sent to a collector electrode through the application of a potential from the jet to the electrode. The ions produce a current ranging from 10-12 to 10-5amps, which is then amplified and recorded as the analyte signal.

What does overloading cause?

The separation of two closely eluting picks so they can overlap, which results in errors and integration and a shift in the retention time

What happens in temperature programming?

The temperature of the oven increases during the course of the analysis in a series of ramps. Then decreases at the end of the analysis to the starting conditions. So it starts off at a lower temperature, which is suitable for the low volatility analytes and increases to be suitable for the separation of those less volatile analytes.

Describe GC instrumentation

Then a flow or pressure controller to control the gas. This thing goes on to a sample injection port. In the past, you had to inject samples individually and manually but most systems nowadays have an auto sampler. So the injection process is all automated now and the injection port leads to the column oven, where the column is housed. The column is what contains the stationary phase, which enables the separation and the oven very accurately controls the temperature that the column is maintained at. At the end of the column, there is a detector to pick up and measure the analytes as they leave the column, followed by an amplifier and a computer to process the data that you have.

Why are flame ionisation detectors (FID) popular?

They have a high sensitivity and can detect low changes in analyte concentration and they are universal for all organic compounds.

Describe the use of MS as a detector

This is essentially used to measure the mass or weight of analytes. It can also fragment these ions and measure the fragments to provide structural information. As each analyte will fragment differently, it can be used to unequivocally identify an analyte. It is also widely used quantitatively.

Describe why having a fan in the oven is essential

This is to make sure that all the air within the oven is at the same temperature. Controlling temperature is essential for GC in terms of getting good separation and reproducible separation. The fan is also important to reduce the waiting time between analysis - The temperature at the end of a GC run could be a few hundred degrees Celsius, higher than the start of the run. Therefore, before you can analyse the next sample, the temperature needs reduced - And you want to do this as quick and as efficiently as possible.

What is peak fronting?

This is when the first half of the peak is non symmetrical.

Why do samples need to be split?

To avoid overloading the capillary column

Why is chemical derivatization required for GC?

To increase volatility and decrease polarity of analytes To decrease thermal degradation of analytes To increase detector response To improve separation and decrease band broadening To improve extraction efficiency

What is the most sensitive detector?

mass spectrometry - can be used to measure the lowest analyte conc - most useful in lab for performing both qualitative and quantitative determinations

What does the split valve look like?

septum at the top, which is punctured using the injection needle and the sample is injected quartz liner containing glass wool and the sample is mixed with the carrier gas then we have where splitting occurs and how some of the sample goes onto the column and the rest is split off to waste - On the waste line, there is a buffer volume to avoid any condensation - This ensures the sample for waste remains in the gaseous phase until it is vented from the system.

What factors affect resolution?

•selectivity of column •efficiency of the column

Describe basically what WCOT columns are

simply capillary tubes coated with a thin layer of the stationary phase.

What are the three types of capillary columns?

wall-coated open tubular (WCOT) support-coated open tubular(SCOT). Porous-layer open tubular (PLOT)