Waters Sep-Pak XPoSure Aldehyde Sampler - Manual (Page 8)

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Example: Measuring Acetonitrile Purity
1. HPLC analysis of a fresh sampler shows the sample contains:
Derivative
Contribution
from
Acetonitrile
Divided by
Background
Value
Times
100
Equals
Percent
Relative to
Background
Formaldehyde-
DNPH
0.01 ÷ 0.08 x 100 = 12%
Acetaldehyde-
DNPH
0.02 ÷ 0.12 x 100 = 17%
Acetone-DNPH 1.60 ÷ 0.40 x 100 = 400%
2. Analysis of the concentrations of hydrazones after reacting
with acid yields:
Derivative Concentration (µg/mL)
Formaldehyde-DNPH 0.08
Acetaldehyde-DNPH 0.12
Acetone-DNPH 0.40
All other hydrazones <0.05
3. The difference between the concentrations of hydrazone from
steps 3 and 5 represents the amount of hydrazone
contributed by the acetonitrile:
Derivative Concentration (µg/mL)
Formaldehyde-DNPH 0.09
Acetaldehyde-DNPH 0.14
Acetone-DNPH 2.00
All other hydrazones <0.05
4. The percent of the hydrazones contributed by the acetonitrile is:
Derivative
Concentration
after Reaction
with Acid
Minus
Concentration
in Blank
Equals
Contribution
form Acetonitrile
Formaldehyde-
DNPH
0.09 µg/mL - 0.08 µg/mL = 0.01 µg/mL
Acetaldehyde-
DNPH
0.14 µg/mL - 0.12 µg/mL = 0.02 µg/mL
Acetone-DNPH 2.00 µg/mL - 0.40 µg/mL = 1.60 µg/mL
Since the percent for formaldehyde and acetaldehyde arising
from the acetonitrile is less than 25% of the background level
in the sampler, the acetonitrile is considered clean for these
compounds. If the analysis considers only these compounds, the
acetonitrile is acceptable.
However, the amount of acetone arising for the acetonitrile is
4 times the amount found in the the background level. Therefore,
it is suggested that this lot of acetonitrile may be unacceptable
for use in the analysis of acetone.
b. Appendix B: Synthesizing the DNPH-Derivative Standards
High purity (99%) DNPH derivatives are commercially available
or can be synthesized from DNPH supplied by Aldrich Chemical
Company (70% DNPH and 30% water). To synthesize 98-99%
pure hydrozones:
1. Prepare one liter of 2 M HCl solution: Add 172 mL concentrated
reagent-grade hydrochloric acid (HCl) to a 1 L volumetric
flask. Fill the flask to the mark with distilled deionized water.
2. Saturate the 2 M HCl solution with DNPH: Add 8 g DNPH and
stir for one hour at 20 to 25 °C. Filter through a 0.45 µm
hydrophilic membrane (HVLP) filter (Waters Part number:
WAT200530).
3. Form the hydrazone derivative by adding a two-fold molar
excess of reagent-grade aldehyde or ketone to the filtered
2 M HCl DNPH solution. Stir for 30 minutes to one hour at
20 to 25 °C.
4. Filter the hydrazone slurry. Wash the hydrazone with 50 mL
2 M HCl 3 times. Wash with 50 mL water 3 times. Dry the
filter cake in an oven at 50 to 60 °C overnight.
5. Prepare a standard stock solution of the DNPH-derivatives
by dissolving an accurately weighed amount in acetonitrile.
Prepare a set of calibration standards using the stock
solution. The concentration of the standards should be in the
same range as the expected concentration of the samples.
The solutions are stable for at least one month when stored in
tightly-capped glass vials at 4 °C ± 2 °C.
8
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Sep-Pak XPoSure Aldehyde Sampler