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Chromatografy Methods for Environmental - Ando D.J.

Ando D.J. Chromatografy Methods for Environmental - Wiley publishing , 2003. - 265 p.
Download (direct link): chromatography2003.pdf
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5% 95%
Figure 11.2 The stationary phase of a DB-5 GC column, consisting of 5% diphenyl- and 95% dimethylpolysiloxanes.
Instrumental Techniques for Trace Analysis
187
The thickness of the stationary phase is of the order of 0.25 pm. The column, through which the carrier gas passes, is placed in a temperature-controlled oven. The carrier gas (e.g. nitrogen) is supplied from a cylinder.
DQ 11.3
As the column is located within an oven, can you suggest two methods of operating the gas chromatograph?
Answer
The gas chromatograph can be operated in either an isothermal or a temperature-programmed mode. In isothermal operation, the oven temperature is constant, e.g. 100°C, throughout the chromatographic run.
With temperature programming, the temperature increases from the start to the finish. The temperature-programmed mode is preferred for separation of complex mixtures.
Temperature programming allows a low initial temperature to be maintained to allow the separation of high-boiling-point analytes; this is then followed by a stepwise or linear temperature increase to separate analytes with lower boiling points. Typical column temperature changes can range from 50 to 250oC, at a ramp rate of 7°Cmin-1. Introduction of the sample requires an injector, of which there are several types. The aim of using an injector is to introduce a small but representative portion of the sample onto the column without overloading. Sample is introduced into the injector by means of a hyperdermic syringe. Two common approaches are applied. The first of these allows a larger sample volume (pl) to be introduced into the injection port and then “splits” or divides the sample - the split/splitless injector (Figure 11.3). In this case, a large volume of sample is introduced into the heated injection port where it is instantly vaporized, with only a small proportion being introduced into the column, and the rest being vented to waste. The ratio of the split flow to the column flow is called the split ratio and can be of the order of 50:1 or 100:1. The other type of injector introduces a smaller volume (nl) of sample directly onto the column - the cold on-column injector (Figure 11.4).
DQ 11.4
What do you think is the major difference in the ways that a split/splitless injector and an on-column injector introduce samples into a gas chromatograph?
Answer
In the case of an on-column injector, a syringe fitted with a very long thin needle is used to introduce all of the sample directly onto the column.
188
Methods for Environmental Trace Analysis
Figure 11.3 Schematic diagram of a split/splitless injector used in gas chromatography. Reproduced by permission of Mr E. Ludkin, Northumbria University, Newcastle, UK.
PTFE block
Ball-valve
Column end
Carrier-gas inlet
Compressed air for cooling
Syringe needle
Figure 11.4 Schematic diagram of an on-column injector used in gas chromatography. Reproduced by permission of Mr E. Ludkin, Northumbria University, Newcastle, UK.
Instrumental Techniques for Trace Analysis
189
The variety of detectors available range from the universal (flame ionization detector) to the specific (electron-capture, thermionic, flame photometric and atomic emission detectors). For example, the electron-capture detector is specific for halogen-containing compounds, e.g. organochlorine pesticides.
DQ 11.5
Can you name an organochlorine pesticide?
Answer
How about one of the following - dieldrin, aldrin or lindane.
In addition, gas chromatography has been coupled to mass spectrometry to provide a highly sensitive detector which also provides information on the molecular structure of the analyte. For a more detailed discussion of both the theoretical aspects and practical details of this approach, the reader is referred to Section 12.2 (Selected Resources) later in this text.
11.2.1.2 High Performance Liquid Chromatography
High performance liquid chromatography can also be classified into two broad categories, i.e. normal phase and reversed phase. For the purpose of this discussion, the most popular of these categories, namely reversed-phase HPLC, will only be discussed. In reversed-phase HPLC, the stationary phase is non-polar, while the mobile phase is polar.
DQ 11.6
What do you think the polarities of the stationary and mobile phases are
in normal-phase HPLC?
Answer
In normal-phase HPLC, the stationary phase is polar and the mobile
phase is non-polar.
A high performance liquid chromatograph (Figure 11.5) basically consists of a column, typically 25 cm long with an internal diameter of 4.6 mm, packed with a suitable stationary phase (octadecylsilyl (ODS), which consists of a C18 hydrocarbon chain bonded to silica particles of 5-10 ^m diameter) through which is passed a mobile phase. The latter is a water-organic solvent system, with typical solvents being methanol or acetonitrile, which is pumped by using a reciprocating or piston pump at a flow rate of 1 mlmin-1. The column is normally located in an oven which is maintained at ca. 30oC.
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