Chapter 1 Introduction; Flame Ionization Detection (Fid) - Thermo Scientific TVA2020 Instruction Manual

Introduction

Flame Ionization Detection (FID)

Flame Ionization
Detection (FID)
1-2 TVA2020 Instruction Manual
A Flame Ionization Detector (FID) measures organic compounds by
utilizing a flame produced by the combustion of hydrogen and air (see
Figure 1–1). When hydrocarbons in the sample are introduced to the
detection zone, ions are produced by the following reaction:

RH + O
where:
R = carbon compound
A collector electrode with a polarizing voltage is also located within the
detector chamber, and the ions produced by this reaction are attracted to it.
As the ions migrate towards the collector, a current is produced which is
directly proportional to the concentration of hydrocarbons introduced to
the flame. This current is then amplified and sent to a microprocessor.
The FID has a wide dynamic range. The effective dynamic range can be
further expanded by use of a dilutor kit which reduces very high volatile
organic compound (VOC) concentrations to within the dynamic range (or
even linear range) of the analyzer. The dilutor kit can also be used to enrich
oxygen deficient samples by adding ambient air that is rich in oxygen
(20.9% usually). Low oxygen can affect the characteristics of the hydrogen
flame, causing readings to be artificially elevated and possibly extinguishing
the flame. As a general rule of thumb, greater than 16% oxygen is required
to support the flame. If underground gases or samples in gas bags are to be
measured by an FID, it is advised that a dilutor be used to combat the
problem.

RHO + + e – H O + CO
2 2
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