Page 17 - 5_TOC in daily practice

SCA-130-508
■ Instrument blank value
Due to the ubiquity of certain compounds,
potential minute leaks in the instrument and
possibly persistent deposits or biofilms, can
lead to area values that originate from the
instrument itself and not from the measured
sample. To determine this value, the ‘blank
check’ procedure can be performed:
For this purpose, the system (TOC-LCPH)
carries out an automatic analysis of
circulating ultrapure water. The resulting
condensation is collected in a suitable
container inside the instrument. As soon as
enough condensed water is available, the
water is circulated, i.e. it is injected again.
This procedure is carried out 50 times and it
can, therefore, be assumed that the final
determined area value corresponds to the
actual instrument blank value.
Note: The determination of the instrument
blank value is also suitable for intensive
cleaning of the flow lines and for the ‘breaking
in’ a newly built-in catalyst. As this is a very
time consuming method, it should
preferentially be carried out overnight.
■ Reagents – blank value
Reagents, such as hydrochloric acid, often
stand unobserved and unintentionally next to
the instrument for months and absorb vast
amounts of organic carbon from the
environment (‘from a blank value
perspective’). For the determination of such
reagent blank values, the standard addition
method is suitable.
■ Contaminations from the environment
At last, the blank value is discussed that can
arise from everywhere in the environment
and can enter the analysis from various
sources. People and their industrial
landscapes are the source of a large variety
of organic carbon compounds. Humans
themselves consist of 18.2 % organic carbon
and lose, for example, 1 – 2 g skin particles
per day. These generally settle in the form of
house dust.
In addition to carbon originating from our
own bodies, carbon sources present in
cosmetics or toiletries such as soaps,
deodorants, perfumes, after-shaves skin
creams, ointments, plays an important role.
Also the laboratory harbors large sources of
organic carbon compounds.
Room air contains numerous pollutants that
can easily bind to dust particles and can
therefore also be present in house dust.
Finally, it should not be overlooked that room
air contains approximately 0.4 vol% / 0.06
mass% (400 / 600 ppm) CO 2.