Contaminants found
in analytical gases are Oxygen, Moisture, Hydrocarbons and those effecting
trace analysis applications include Carbon Dioxide, Hydrogen, Carbon
Monoxide and Nitrogen. They can contribute to capillary column degradation,
detector noise, interfere with the chromatographic separation and damage
to the instrument detectors. In the field of Mass spectrometry, a higher
grade of gas reduces the ionization of the contaminants giving a greater
signal/background ratio resulting in improved detection limits.
Typical effects/causes of contaminants:
Oxygen |
|
Effects: |
Column stationary
phase degradation.
Decomposition of labile analytes. |
|
Causes: |
| Most common
contaminant. |
|
Poor selection of gas |
(e.g.,
UHP grade 99.999% - 4 ppm of oxygen)
(Research grade 99.9999%- 1 ppm of oxygen)
(Purifier grade >99.99999% - 1 ppb of oxygen) |
|
Selection
of incorrect tubing. (i.e., permeable)
Fitting leaks.
Poor cylinder change out.
Out of spec gas supply.
|
Moisture |
|
Effects: |
Column stationary
phase degradation.
Damage to instrument/detector. |
|
Causes: |
|
Poor selection of gas. |
(e.g.,
UHP grade 99.999% - 3.5 ppm of H2O)
(Research grade 99.9999%- 0.2 ppm of H2O)
(Purifier grade >99.99999% - 1 ppb of H2O) |
|
Improper
handling and installation of plumbing.
Poor cylinder change out.
Out of spec gas supply.
|
Hydrocarbons |
|
Effects: |
Baseline
drift.
Decrease detector sensitivity by increasing detector background
noise. |
|
Causes: |
|
Poor selection of gas. |
(e.g.,
HP grade 99.995% - 0.5 ppm of THC)
(Research grade 99.9999%- 0.1 ppm of THC)
(Purifier grade >99.99999% - 1 ppb of THC) |
|
| Out of spec
gas supply. |
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