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Flame Ionisation Detector (FID): Typical detection limits 10-100 pg for individual hydrocarbons.
Electron Capture Detector (ECD): Highly selective detector. Used for halogenated solvents. Detection
limits well below 1 pg for compounds such as carbon tetrachloride and some heavily halogenated
pesticides.
Thermal Conductivity Detector (TCD): General purpose, low sensitivity detector. Typical detection limits in
the order of 10 ng.
2.4.10 Calculating analyte masses in the sample tube
In order to determine the required split it is necessary to have an approximate idea of the mass of analyte
which is expected to be retained/collected in the sample tube.
For direct desorption of volatiles from materials, the mass of analyte is most easily determined
experimentally from control or real-life samples. It can also be calculated from relevant material
specifications where appropriate e.g. if the specification for residual chloroform in cough medicine is 1%
w/w a 20 mg sample will contain 200 µg or less of chloroform.
For air monitoring applications the calculations are a little more complex as they depend on variables such
as diffusive uptake rate, pumped volume and molecular weight (See Markes TDTS 25).
2.4.11 Calculating splits/split modes
Once the expected mass of analyte, the detection limits of the system and the analytical capacity of the
column are all known, then an overall ideal split can be calculated. For example if the expected mass of
analyte on the sample tube is 50 µg and the analytical column capacity is 100 ng then a total split of at
least 1/500 is required to prevent column overload - i.e. only 0.2% of the sample must be transferred to
the column. One of three different split modes can be utilised:
Zero split or
splitless
Unusually among thermal desorbers,
UNITY 2 can operate in splitless mode
in conjunction with narrow bore (0.32
mm ID) columns and MS detectors as
the minimum flow required through
the cold trap during trap heat is 2
ml/min.
During tube desorption the desorb flow must
be set to at least 10 ml/min to provide
enough flow through the sample tube for
efficient thermal desorption.
During tube desorption the desorb flow should
not far exceed 50 ml/min or there may be a
risk of breakthrough from the cold trap.
Single split During single split operation the split
may either be open during tube or trap
desorption. The advantages of having
the split open on the way into the trap
(inlet split) are:
1. That a relatively fast flow can be
used through the tube to
facilitate desorption while, at the
same time, the flow passing
through the cold trap is kept low
to aid retention and focusing of
analytes.
During tube desorption the desorb flow must
be set to at least 10 ml/min to provide
enough flow through the sample tube for
efficient thermal desorption.
During tube desorption the trap flow should
not exceed 50 ml/min or there may be a risk
of breakthrough from the cold trap.
During tube desorption the trap flow should
not be less than 2 ml/min to ensure efficient
sweeping of analytes onto the trap sorbent.
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