Gas Chromatography

Almost all new GCs uses a personal computer or laboratory-wide data systems to collect and analyze data, because a good data system must measure signal with rapid sampling rates. Computers have greater flexibility in acquiring data, controlling the GC, data reduction, display and transfer the data to other devices. The detector output of most GCs is analog, and an analog to digital converter (A/D converter) is used to convert the data to a digital format for storage, analysis and display by the computer. All data systems can perform basic calculations for chromatography, including the start, apex, end and area of each peak, area percent, height percent, external standard and normalization calculation.

The detector senses the effluents from the column and provides a signal which is proportionate to the quantity of each solute or analyte, allowing to perform quantitative analysis. The record of this signals is used to generate the chromatogram. Flame ionization detector (FID) is the most common detector. Its most outstanding features are its high sensitivity, wide linear range and low detection limits. It is also relatively simple and inexpensive. Other popular detectors are the thermal conductivity detector (TCD) and electron capture detector (ECD).

The detector and its connections from the column exit must be hot enough to avoid sample condensation but cool enough to not degrade the sample or the stationary liquid phase of the column. Peak broadening and loss of peaks due to condensation are a possibility at low detector temperatures. Temperature control is dependent on the type of detector used. For instance, a thermal conductivity detector (TCD) needs to be controlled at ±0.1ºC or better to obtain good baseline stability and maximum detectivity. Flame ionization detectors (FID) do not need a strict temperature control. A temperature should just be high enough to avoid condensation of sample or water. A reasonable minimum temperature for an FID is 200ºC.

Isothermal analysis is performed at a constant column temperature. On the other hand, in temperature programmed, a linear increase of column temperature with time is used. This technique is very useful for wide boiling sample mixtures.

High enough temperatures for the sample to pass through the column at a reasonable speed should be used. Nevertheless, it is not necessary the temperature to be higher than the boiling point of the sample. It might seem illogical, but it must be remembered that the column operates at temperatures where the sample is in vapor state and not need to be in the gas state. Thus, in GC, the column temperature must be higher than the “dew point” of the sample, but not higher its boiling point. A good rule of thumb for temperature is that the initial temperature should be low enough that the first peak of interest elutes with a k value of at least 1.0. High temperatures provoke the retention times to decrease, reducing the analysis time. However, better resolutions are achieved at low temperatures.

The inlet must be hot enough to produce a fast vaporization of the sample without loss of efficiency from the injection technique. At the same time, very high inlet temperatures should be avoided, because thermal decomposition or chemical rearrangement can be produced. A general rule for flash vaporization is to have an inlet temperature 50ºC above the boiling point of the sample. Tests of the inlet temperature can be performed. If increasing the temperature produce better peak shapes or improves efficiency, the temperature was too low. If drastic changes are observed in retention times, peak areas or peak shapes, the temperature was too high, probably leading to thermal decomposition or rearrangements. On the other hand, in on-column injections, the inlet temperature can be lower.

The control of temperature is one of the easiest and most effective ways to influence separation. The control of temperature is necessary to achieve a good separation in a reasonable amount of time.

Capillary columns are columns that are not filled with packing material. Instead, they are internally covered by a thin film of a liquid phase. They are formally called “wall-coated open tubular” or open tubular (WCOT or OT columns). Their resistance to flow is very low, and long lengths are possible, up to 100m, which also permits efficient separation of very complex sample mixtures. Fused silica columns are the more inert.

The self-sealing septum provides a way to seal the injector and at the same time, allow the users to inject the samples into the GC. They are usually made of polymeric silicone. To select the correct septum, several properties should be considered: high-temperature stability, decomposition (also called “bleeding”), size, lifetime and cost. The self-seal is conserved during 50 injections or more for most septa. It should be replaced on a regular basis.

When using an autosampler, it should be programmed to inject the sample as fast as possible. For manual injections, some guidelines are useful: Exclude all air initially: can be achieved by repeatedly drawing and expelling out liquid sample into the syringe. Special care must be taken with very viscous liquid samples. A better alternative is to dilute them with an appropriate solvent. When handling the syringe manually, fill it with more liquid than the desired volume for the injection. After that, hold the syringe pointing up and the air in the syringe will go to the top of the barrel. Soft taps can be used to aid in this process. After that, press the plunger up to the desired injection volume. Wipe off the needle with a tissue and now add some air to the syringe. This serve two purposes: first it will often give a peak in chromatograms, which can be used to measure tM. Second, it prevents losing sample if the plunger is accidentally pushed. Injection technique require the us...

 Autosamplers are mechanical devices placed on top of the GC that automatize the process of injection. The main advantages of their use is that they provide rapid and highly reproducible injections, and of course, it does not need a person to be making every injection. The autosampler takes care of all the process, taking the sample from sealed vials, injection, flushing the syringe with a solvent after injection. Trays with large amounts of samples, standards and solvents are used and thus, they can run unattended overnight. The precision of an autosampler is greater than the manual injection with a typically 0.2% relative standard deviation (RSD).

 The barrel is usually made of glass, and the needle and plunger of stainless steel. The needle is epoxied into the barrel. Other models have removable needles that are screwed onto the end of the barrel. Some models also have a small wire inside the syringe needle, extending to its tip, so there is no dead volume after the injection. It is better to use a syringe whose total sample volume is at least two times the larger than the volume to be injected.

hey must be dissolved in an appropriate solvent, and the solution injected with a micro syringe.

These methods require the entire sample to be in the gas phase before the analysis. Mixtures of gases and liquids are problematic because in that case the analyst should convert all the sample to a gas, by heating the sample or to a liquid by increasing pressure. But this is not always possible. There are two main methods two introduce gas samples. The first one is the use of gas­-tight syringes. This method is cheaper, more flexible and the most frequent method used. The other method is the use of gas-sampling valves, which is used frequently with packed columns; give better repeatability, requires less skill from the analyst and can be more easily automated.

The most common way to introduce a sample is using a micro syringe and the sample is a liquid. The sample system should permit the sample to be introduced rapidly and quantitatively onto the column. Three types of inlets are the most common for capillary columns: split, split less and on-column. In practice is impossible to introduce the whole sample instantaneously, but it is desirable to introduce it as a sharp symmetrical band. The difficulty of achieving this can be seen by analyzing an example. A 1.0 µ L liquid sample of benzene vaporizes to 600 µ L after heating in the injector. If the flow rate is 1 mL/min, then 36 seconds would be needed to carry the whole sample onto the column. Hence, sampling and the size of the samples are critical aspects of chromatography. The smallest possible sample size should be used to obtain the maximum resolution and peak shape. On the other hand, the more components present in the sample, the larger the sample size may need to be. For trace work...

There are two ways of measuring the flows independently. The first one is an inexpensive soap-bubble flowmeter which consists of a calibrated tube (usually a modified pipet or buret) through which the carrier gas flows. With a rubber bulb we can create a bubble, which is raised into the path of the gas. After that, the ascension of a particular bubble to a defined volume is measured with a stopwatch. The carrier gas in mL/min is easily obtained from this measurement. There are also available electronic soap film flowmeters at a cost around $50. The second alternative is the use of a sophisticated device, composed of a solid-state sensor and a microprocessor to accurately flow measurements without using soap bubbles. Silicone-on-ceramic sensor can be used to measure flow rates of 0.1-500 mL/min for air, nitrogen, oxygen, helium, hydrogen and 5% argon in methane. The cost for this device is around $700. Very small flow rates, like the ones found in tubular columns, cannot be measured rel...

Two-stage regulator is connected to the cylinder as a first flow control system, which reduces the tank pressure of up to 2500 psig (psi gauge, or above the atmospheric pressure), down to a level useable level of 20-100 psig. The system should include a filter to prevent particulate matter from entering the regulator and a safety valve. The first valve indicates the pressure in the gas cylinder. With the second valve, one can increase the pressure delivered to the GC. The second stage regulator works better at pressures at least 20 psi higher than the maximum inlet pressure on the GC. Constant pressure is sufficient to provide a constant flow rate for isothermal operations if the pressure drop in the column is constant. In temperature-programmed operations, the flow rate decreases at constant temperature as a consequence of an increased viscosity of the gas at higher temperatures. Differential flow controllers are used to ensure a constant mass flow rate. Modern research-grad capil...

 The measurement and control of carrier gas flow is very important for GC. Column efficiency depends on the linear gas velocity. As a guideline, a typical column of 0.25-mm inner diameter (i.d.) open tubular (OT) column has an optimum of 0.75 mL/min. Nevertheless, the optimum for a given column should be determined experimentally. For qualitative analysis, it is crucial to have a constant and reproducible column flow rate. If this is accomplished, retention times can be reproduced, and the comparison of retention times is the quickest and easiest technique for compound identification. Two solutes can have the same retention time. However, a solute cannot have 2 different retention times in the same column. Thus, the retention times are characteristics of the solutes, but not unique.

The purity of the gas is very important in GC. Impurities such as oxygen and water can chemically attack the stationary phase of the columns and destroy it. Polyester, polyglycol and polyamide phases are particularly susceptible. Water impurities can lead to desorption of other column contaminants, producing high detector background or “ghost peaks”. Trace amounts of hydrocarbons can cause high background or noise signal in most ionization detectors and thus worsen their detection limits. Several ways to obtain the gas with the desired purities are possible. One of them is to purchase ultrahigh purity gas cylinders, which is very expensive. Another possibility is to use gas generators, especially for hydrogen and air. This possibility is economically feasible, but require and initial high investment and maintenance. The more common practice is to purchase high-purity gases and further purify them. Water and trace hydrocarbons can be removed by using a 5 Armstrong molecular sieve filt...

 Its main objective is to carry the sample through the column. It is mobile and it is inert, therefore it does not interact chemically with the sample or the stationary phase. It is also an appropriate matrix for the detectors to analyze the samples components.