Full Text Article - Millipore

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Additionally, technical hints are given as to how to
ensure reproducible high quality ultrapure water,
including after each consumable change.
One of the most widely trusted parameters used to
track ultrapure water quality is resistivity monitoring.
Previous studies: traditional HPLC profiles
The quality of purified water is often questioned,
when poor baselines are obtained (especially at
214 nm, a wavelength which most organic
contaminants absorb).
To illustrate this, a blank gradient (no sample
injected) was run by pumping for 4 minutes at initial
conditions (100% water), then ramping up to final
conditions (100% acetonitrile) over a period of
20 minutes. Next, suspecting water as the contributor
of the contaminant peaks, the same procedure was
run except that initial conditions were kept for
36 minutes, before running the gradient. Since the
size of the peaks increased, one can be certain they
came from the water and not from the acetonitrile
(Figure 1). If any of the peaks of interest were eluted
at the same time as these contaminating peaks, there
would be quantitation errors. There is also a
possibility that the contaminating peaks could be
confused with desired compounds.
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Figure 1: Blank analysis profiles at 214 nm.
Time (minutes)
36 minute delay
4 minute delay
Typical chromatograms obtained with different
purified water types are given below.
The first one is a gradient analysis of 50 ml of lab
water that is not ultrapure, concentrated on the head
of a C18 column. A gradient was then run, up to
100% acetonitrile, and the output was monitored at
two different wavelengths at a moderately sensitive
setting. Clearly, as shown in Figure 2 this water
source is not acceptable for HPLC, be it the isocratic
or the gradient method. Peaks due to the water occur
throughout the chromatogram. Two peaks come from
the acetonitrile.
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contaminants
from water
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Figure 2: Poor baseline profiles at 214 and 254 nm.
contaminants
from acetonitrile
Time
(minutes)
The R&D Notebook: From tap to ultrapure water: tracking organic contaminants
This value, however, does not measure the possible
organic contaminants that may be present in water.
Significant organic contamination will still give an
18.2MΩ.cm resistivity reading 7 .
The second one is a gradient analysis of 50 ml of
triple distilled water, concentrated on the head of a
C18 column. A gradient was then run, up to 100%
acetonitrile, and the output monitored at two different
wavelengths at a moderately sensitive setting.
Clearly, this water source is not acceptable for HPLC.
The early peaks are due to the water, the last two
peaks come from the acetonitrile (see Figure 3).
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For reverse osmosis water, gradient analysis of 50 ml
concentrated on the head of a C18 column is shown
in Figure 4. A gradient was run, up to 100%
acetonitrile, and the output monitored at two different
wavelengths at a moderately sensitive setting .
Clearly, this water source is not acceptable for HPLC,
neither isocratic nor gradient. The early peaks are
due to the water, two of the last peaks come from the
acetonitrile. Moreover, with this type of purified
water, the residual background is very high.
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contaminants
from water
Figure 3: Triple distilled water.
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contaminants
from water
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contaminants
from acetonitrile
contaminants
from acetonitrile
Time
(minutes)
1.0 214 nm
Figure 4: RO water.
254 nm
Time
(minutes)
In order to get the ultrapure water quality required to
perform organic chromatography analysis, a water
purification chain combining various technologies is
therefore necessary.
In order to highlight the efficiency in removing
organics from tap water, organic studies are
performed on the water delivered after the main
purification steps.