Chemical Analysis. Francis Rouessac
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GC‐2010 model from Shimadzu).
The instrument represented is equipped with a sample holder (carousel), an injector, an automatic sampler, and a mass spectrometry detection system (GC‐MS). c) Portable model for analyses made in the field (volatile organic compounds, toxic industrial materials)
(Source: Hapsite ER, information courtesy of Inficon).
2.2 CARRIER GAS AND FLOW REGULATION
The mobile phase is a gas (helium, hydrogen or nitrogen), either drawn from a commercially available gas cylinder or obtained, in the case of hydrogen or nitrogen, from an on‐site generator (water electrolysis for H2 and air separation for N2), dedicated to the installation. To eliminate all harmful traces of water vapour and oxygen from polar stationary phases and detectors, a double filter, for drying and reducing, is placed before the injector.
The nature of the carrier gas has no significant influence upon the values of the partition coefficients K of the solutes between the stationary and mobile phases, owing to an almost total absence of interaction between the gas and the solutes. Temperature is the only significant modification factor. By contrast, the viscosity and speed of the carrier gas, related to its flow rate or pressure at the head of the column, have an effect on the analytes’ dispersion in the stationary phase and on their diffusion in the mobile phase (see the Van Deemter equation, Chapter 1, Section 1.10). These two factors have a direct impact on column efficiency N (Figure 2.2).
Figure 2.2 Efficiency as a function of the nature and linear velocity of the carrier gas. The Van Deemter curves show the relationship between HETP and linear velocity of the carrier gas for a given compound. Comparison of the viscosities of these three gases. Note the increase in the viscosity of these gases with temperature.
The pressure at the head of the column (several tens to hundreds of kPa – a few tenths to a few bar) is stabilized by electronic pressure control (EPC), so that the flow rate remains constant at its optimal value. This device is valuable because if the analysis is performed with ascending temperature programming (temperature gradient), the viscosity of the stationary phase and, by consequence, the pressure drop in the column, increase with time. By controlling the pressure, we conserve a constant and optimal speed of the carrier gas. The result is a faster analysis with the same efficiency.
The comparison of the three Van Deemter curves shows that the minimum of each is obtained for various linear velocities of the carrier gas: low for nitrogen, higher for helium and even higher for hydrogen. This means that the latter gas reduces the analysis time. For hydrogen also, the growth of the curve after the peak is less quick than for the other two gases, which gives more latitude in choosing the carrier gas speed without impacting the column’s efficiency (Figure 2.2). The three curves plotting the viscosity of these gases versus temperature T again show that hydrogen has a lower viscosity than the other two carrier gases. Lower viscosity means a lower pressure drop and an increased column life.
2.3 INJECTION CHAMBER
2.3.1 Sample Introduction
The sample to be analysed is never introduced into the chromatogram as is, whether it is a liquid or a solid, but rather in a highly diluted solution. We use either a microsyringe (or loop injector) or a device such as a headspace sampler for volatile compounds, which both concentrates the sample and introduces it into the chromatograph.
Microsyringe and septum
The most common injection method is where a microsyringe is used (Figure 2.3) to inject a very small quantity of sample in solution (1 μl or less) through a rubber septum sealing the injection chamber. Some very elaborate septa have been developed (Figure 2.4) for microsyringes, which are an integral part of the automatic injectors found in most current instruments (Figure 2.1).
Figure 2.3 Microsyringe for GC and principle of an injection loop installed in a continuous process. The model chosen for this illustration has a cone shape, adapted to several septa or automatic injectors. In this model, the piston enters the needle to deliver the entire sample and prevent any dead volume. At the bottom, there is an example of an injection loop for gas or liquids (also see Figure 3.5).
Figure 2.4 Direct vaporization injector used for packed columns. The typical septum is an elastomer disc but there are also more sophisticated versions, including the Merlin Microseal, which can be used thousands of times.
(Source: Courtesy of Agilent Technologies.)
Automatic injectors with a sample holder carousel are called autosamplers. With some of them, we can select the injection method (liquid, headspace, SPME (see Chapter 21)) and, if necessary, conduct a short preparation of each sample beforehand (dilution, addition of an internal standard, derivatization). These devices complete the chromatogram, improve results, and shorten analysis times. They have become essential for multi‐analyte methods in environmental analysis (search for PAHs, mycotoxins, pesticides).
When there is too little analyte to be detected (detectability limit) or to be quantified (quantification limit), we must make a preconcentration. To do so, we have such techniques as SPME (solid phase micro‐extraction) or SBSE (stir bar sorptive extraction). These techniques use the possibility of adsorbing an analyte on a solid phase, and inversely to desorb in the injector when subjected to heat (see Chapter 21).
Injection loops
For some applications, such as process control, there are injectors for gases or for liquids (Figure 2.3), which include a loop, i.e. a small volume waiting and ready to be injected by rotation of a valve. We find this same principle in liquid chromatography (see Chapter 3, Section 3.3).
2.3.2 Injectors
The injector, which is the sample’s entrance to the chromatograph, has different functions: to vaporize and entrain the sample mixed with the carrier gas at the head of the column. Depending on the way injection is conducted and on its rapidity, it can impact the quality of analysis.