INTERNATIONAL ISO

STANDARD 15033

Second edition
2007-03-15
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Plastics — Determination of caprolactam

and its cyclic and linear oligomers by
HPLC
Plastiques — Détermination du caprolactame et de ses oligomères
cycliques et linéaires par CLHP

Reference number
ISO 15033:2007(E)

© ISO 2007
 ISO 15033:2007(E)

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ii © ISO 2007 – All rights reserved

 ISO 15033:2007(E)

Contents Page

Foreword............................................................................................................................................................ iv
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Introduction ........................................................................................................................................................ v
1 Scope ..................................................................................................................................................... 1
2 Normative references ........................................................................................................................... 1
3 Terms and definitions........................................................................................................................... 1
4 Principle ................................................................................................................................................. 1
5 Reagents ................................................................................................................................................ 1
6 Apparatus .............................................................................................................................................. 3
7 Test sample ........................................................................................................................................... 4
8 Procedure .............................................................................................................................................. 4
8.1 Calibration ............................................................................................................................................. 4
8.2 Determination........................................................................................................................................ 4
9 Calculations........................................................................................................................................... 6
9.1 Calculation of calibration factors........................................................................................................ 6
9.2 Test sample ........................................................................................................................................... 7
10 Precision ................................................................................................................................................ 7
11 Test report ............................................................................................................................................. 8
Annex A (normative) HPLC parameters and injector programme ................................................................ 9
Annex B (informative) Schematic diagram of HPLC apparatus................................................................... 10
Bibliography ..................................................................................................................................................... 11

© ISO 2007 – All rights reserved iii

 ISO 15033:2007(E)

Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards bodies
(ISO member bodies). The work of preparing International Standards is normally carried out through ISO
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technical committees. Each member body interested in a subject for which a technical committee has been
established has the right to be represented on that committee. International organizations, governmental and
non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely with the
International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization.

International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2.

The main task of technical committees is to prepare International Standards. Draft International Standards
adopted by the technical committees are circulated to the member bodies for voting. Publication as an
International Standard requires approval by at least 75 % of the member bodies casting a vote.

Attention is drawn to the possibility that some of the elements of this document may be the subject of patent
rights. ISO shall not be held responsible for identifying any or all such patent rights.

ISO 15033 was prepared by Technical Committee ISO/TC 61, Plastics, Subcommittee SC 5,
Physical-chemical properties.

This second edition cancels and replaces the first edition (ISO 15033:2000), which has been technically
revised.

iv © ISO 2007 – All rights reserved

 ISO 15033:2007(E)

Introduction
The basic method specified in this International Standard can be used for HPLC determination of the cyclic
oligomers of caprolactam up to and including the hexamer (n = 6), using UV detection. If desired, after post-
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column reaction of the primary amine with 1,2-phthalic dicarboxaldehyde, on-line determination of the linear
oligomers up to and including the hexamer can also be carried out.

The determination is not quantitative for oligomers higher than the hexamer (n > 6). In the determination of
cyclic oligomers the sensitivity for the tetramer and higher oligomers is constant, which means that calibration
should take place up to and including the tetramer (n = 4).

The linear oligomers are determined by the fluorescence of the iso-indole group, which is a product of the
reaction between the primary amino group, 1,2-phthalic dicarboxaldehyde and 3-mercaptopropionic acid. The
calibration with the linear oligomers should be carried out up to and including the hexamer (n = 6).

© ISO 2007 – All rights reserved v

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 INTERNATIONAL STANDARD ISO 15033:2007(E)

Plastics — Determination of caprolactam and its cyclic and

linear oligomers by HPLC
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SAFETY STATEMENT — Persons using this document should be familiar with normal laboratory
practice, if applicable. This document does not purport to address all of the safety concerns, if any,
associated with its use. It is the responsibility of the user to establish appropriate safety and health
practices and to ensure compliance with any regulatory requirements.

1 Scope
This International Standard describes an HPLC (high-performance liquid chromatography) method for
determining the concentrations of cyclic oligomers of caprolactam, from 0,01 % by mass upwards, and linear
oligomers of caprolactam, from 5 mg/kg upwards, both up to and including the hexamer of caprolactam (n = 6),
in samples of polyamide 6, caprolactam and mixtures of rearrangement products in water.

2 Normative references
The following referenced documents are indispensable for the application of this document. For dated
references, only the edition cited applies. For undated references, the latest edition of the referenced
document (including any amendments) applies.

ISO 472, Plastics — Vocabulary

3 Terms and definitions

For the purposes of this document, the terms and definitions given in ISO 472 apply.

4 Principle
A test sample is dissolved in, or diluted with, formic acid and the oligomers separated in the presence of a
low-pH mobile phase using a column filled with reversed-phase packing material. The cyclic oligomers are
detected by UV absorption at 200 nm. If desired, the linear oligomers can be detected by fluorescence after
post-column reaction of the primary amino group with 1,2-phthalic dicarboxaldehyde and 3-mercaptopropionic
acid. The concentrations are calculated by comparison of the measured values with those of calibration
solutions.

5 Reagents
During the analysis, unless otherwise stated, use only reagents of recognized analytical grade.

5.1 Water, ultrapure or double-distilled.

5.2 Phosphoric acid, 85 % by mass.

© ISO 2007 – All rights reserved 1

 ISO 15033:2007(E)

5.3 Phosphoric acid, 1 mol/l.

Introduce 68 ml of phosphoric acid (5.2) into a 1-litre volumetric flask, make up to the mark with water (5.1)
and mix well.

5.4 Acetonitrile.

5.5 Formic acid, concentrated.

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5.6 Caprolactam.

5.7 Cyclic dimer of caprolactam, isolated by HPLC (see Note).

5.8 Cyclic trimer of caprolactam, isolated by HPLC (see Note).

5.9 Mixture of cyclic oligomers of caprolactam, isolated by HPLC (see Note).

5.10 ε-Aminocaproic acid.

5.11 Linear dimer of ε-aminocaproic acid.

5.12 Linear trimer of ε-aminocaproic acid.

5.13 Linear tetramer of ε-aminocaproic acid.

5.14 Linear pentamer of ε-aminocaproic acid.

5.15 Linear hexamer of ε-aminocaproic acid.

5.16 Helium.

5.17 Eluent.

Add 10 ml of acetonitrile (5.4) and 10 ml of phosphoric acid (5.3) to 900 ml of water (5.1). Raise the pH of the
solution to 2,6 using sodium hydroxide flakes (5.19). Make up to 1 litre and saturate with helium (5.16).

5.18 Sodium tetraborate decahydrate.

5.19 Sodium hydroxide flakes.

5.20 1,2-Phthalic dicarboxaldehyde.

5.21 Methanol, 96 % by volume.

5.22 3-Mercaptopropionic acid.

5.23 Post-column derivatization reagent.

Dissolve 76 g of sodium tetraborate decahydrate (5.18) and 6 g of sodium hydroxide (5.19) in 2 litres of water
(5.1). Dissolve 1,6 g of 1,2-phthalic dicarboxaldehyde (5.20) in 40 ml of methanol (5.21) and add this solution
to the sodium tetraborate decahydrate reagent. Add 1,5 ml of 3-mercaptopropionic acid (5.22) and mix well.

The stability of the post-column derivatization reagent is limited. Do not keep for longer than 3 days.

NOTE The cyclic dimer, the cyclic trimer and the mixture of cyclic oligomers of caprolactam can be isolated from a
methanol extract of PA6 by preparative HPLC, using the HPLC method described here. The purity of the dimer and the
possible presence of other oligomers can be checked using the method described in this International Standard.

2 © ISO 2007 – All rights reserved

 ISO 15033:2007(E)

6 Apparatus
6.1 HPLC equipment, having the following specifications:

⎯ Eluent pump, including mixer, damper and manometric module, giving an eluent flow rate of 0,51 ml/min
and a pressure drop of approximately 100 bar.

⎯ Injector, e.g. an auto-sampler capable of 1 µl to 250 µl injections, equipped to carry out a variable
injector programme (see Annex A). The injector shall be capable of accommodating at least three
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components in the sample loop, i.e. the injector programme shall be capable of controlling the “sandwich”
injection of up to three components into the sample loop plus a solvent injection in one HPLC run.

⎯ Column:

⎯ stainless steel

⎯ inside diameter: 3 mm

⎯ length: 250 mm

⎯ temperature: 40 °C

⎯ packing: reversed-phase C18 silica or equivalent

⎯ particle size: 0,005 mm

The resolution of the column shall be such that baseline separation of the components of interest is
obtained.

The lifetime of the reversed-phase C18 column is very strongly influenced by the C18-silica bonding of
the packing material. Therefore, columns equipped with a packing material containing monofunctional
silanes with diisobutyl side-chain groups are preferred. These side groups sterically protect the key
silanes from hydrolytic attack at low pH, making the stationary phase stable at such pH (pH 1).

⎯ UV detector: wavelength 200 nm and 220 nm.

And additionally for determination of the linear oligomers:

⎯ Reagent pump, including manometric module and pulse damper, giving a reagent flow rate of
0,25 ml/min and a reagent pressure drop of approximately 20 bar.

⎯ Reaction coil:

⎯ stainless steel or PEEK

⎯ length: 3 m

⎯ inside diameter: 0,25 mm

⎯ temperature: 25 °C

⎯ Fluorescence detector:

⎯ excitation: 330 nm

⎯ emission: 420 nm

A schematic diagram of an HPLC apparatus is given in Annex B.

© ISO 2007 – All rights reserved 3

 ISO 15033:2007(E)

6.2 Microbalance, accurate to 0,1 mg.

6.3 Ultrasonic vibration bath.

6.4 Volumetric flask, capacity 25 ml.

7 Test sample
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For polyamide 6 and caprolactam, the maximum test sample size shall be 0,5 g. For mixtures of
rearrangement products in water, a maximum test sample size of 1,25 g shall be used. The stability of the
dissolved samples is limited, since caprolactam is hydrolysed to ε-aminocapronic acid. Do not keep for longer
than 6 days.

8 Procedure

8.1 Calibration

To calibrate the column for the cyclic oligomers, prepare a series of three calibration solutions with
concentrations increasing from 100 mg/l to 1 500 mg/l by dissolution in formic acid (5.5) for caprolactam (5.6),
the cyclic dimer (5.7), the cyclic trimer (5.8) and the oligomer mixture (5.9). To calibrate the column for the
linear oligomers, prepare a series of three calibration solutions with concentrations increasing from 2 mg/l to
15 mg/l by dissolving in formic acid (5.5) for ε-aminocaproic acid (5.10) and each of the linear oligomers from
the dimer (5.11) to the hexamer (5.15). Pump eluent (5.17) through the column at a rate of 0,51 ml/min.
Starting with the lowest concentration of a calibration series, inject 2 µl of the calibration solution into the
column in accordance with the injector programme given in Annex A. Elute in accordance with the gradient
timetable in Annex A. Record the UV chromatogram. If applicable, immediately after UV detection, add the
post-column reagent (5.23) at a rate of 0,25 ml/min, mixing the eluent and the reagent in the reaction coil.
Record the fluorescence chromatogram. Measure the peak area of the component(s).

Repeat the calibration successively for the other calibration solutions of the same component and for the other
calibration series.

NOTE Calibration can also be carried out using a commercially available PA6 polymer with a known concentration of
cyclic and linear oligomers (see Reference [3] in the Bibliography).

8.2 Determination

Introduce a test sample (see Clause 7) into a 25 ml volumetric flask. Add 20 ml of formic acid (5.5), close the
flask and dissolve the sample, optionally by using ultrasonic vibration. Make up to the mark with formic
acid (5.5). Pump eluent (5.17) through the column at a flow rate of 0,51 ml/min. Inject 2 µl of the sample
solution into the column in accordance with the injector programme given in Annex A. Elute in accordance
with the gradient timetable in Annex A. Record the UV chromatogram (see Figure 1). If applicable,
immediately after UV detection, add the post-column reagent (5.23) at a flow rate of 0,25 ml/min, mixing the
eluent and the reagent in the reaction coil. Record the fluorescence chromatogram (see Figure 2). Measure
the peak areas of the cyclic oligomers (UV detection) and linear oligomers (fluorescence detection) up to and
including the hexamer.

NOTE If necessary, e.g. if lower sensitivity is necessary due to high(er) concentrations, caprolactam can be detected
by UV absorption at 220 nm instead of 200 nm.

4 © ISO 2007 – All rights reserved

 ISO 15033:2007(E)
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Key
X retention time (min)
Y response (mV)
1 formic acid sandwich injection
2 250 µl formic acid injection
C1 to C6 cyclic oligomers
L2 to L6 linear oligomers

Figure 1 — Determination of the cyclic oligomers of caprolactam

Key
X retention time (min)
Y response (mV)
L1 to L6 linear oligomers

Figure 2 — Determination of the linear oligomers of caprolactam

© ISO 2007 – All rights reserved 5

 ISO 15033:2007(E)

9 Calculations

9.1 Calculation of calibration factors

9.1.1 Calibration factors for cyclic oligomers

Calculate the calibration factor for the relevant component for each solution using the following equation (see
Note 1):
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Ac
= fc (1)
Cc

where

Ac is the peak area of the relevant component;

Cc is the concentration of the relevant component in the calibration solution, in mg/l;

fc is the calibration factor for the relevant component.

Calculate the calibration factors for n = 1 up to and including n = 3 by taking the mean value of the three
calculated calibration factors in the series of the relevant component.

The calibration factors for the tetramer (n = 4) up to and including the hexamer (n = 6) are assumed to be
constant. The calibration factor for the oligomers from n = 4 up to and including n = 6 is calculated from the
chromatogram of the oligomer mixture, the calibration factors for n = 1 up to and including n = 3 being known.
Calculate the calibration factor for n = 4 to n = 6 by taking the mean value of the three calculated calibration
factors in the series.

The calibration shall be repeated if the relative difference between the calibration factors for two solutions is
more than 5 %.

NOTE 1 The calibration factor for e.g. n = 3 can also be calculated by using for instance a calibration solution also
containing caprolactam and/or the cyclic dimer if the latter are known.

NOTE 2 Usually, the linear oligomers are present in polyamide 6 at a concentration lower than that of the cyclic
oligomers by approximately two orders of magnitude, and do not therefore interfere with the determination.

9.1.2 Calibration factors for linear oligomers

Calculate the calibration factor for the linear oligomers for each solution using the following equation:

Ac1
= f c1 (2)
C c1

where

Ac1 is the peak area of the relevant component;

Cc1 is the concentration of the relevant component in the calibration solution, in mg/l;

fc1 is the calibration factor for the relevant component.

Calculate the calibration factor for the linear components by taking the mean value of the three calculated
calibration factors in the series. The calibration shall be repeated if the relative difference between the
calibration factors for two solutions is more than 5 %.

6 © ISO 2007 – All rights reserved

 ISO 15033:2007(E)

9.2 Test sample

9.2.1 Calculation of cyclic oligomer concentrations

Calculate the concentration, in % by mass, of each of the cyclic oligomers from n = 1 up to and including n = 6
using the following equation and rounding the result to 0,1 % for concentrations greater than or equal to 10 %
and to 0,01 % for concentrations below 10 %:

Acs × 2,5
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= C cs (3)
f c × mg

where

Acs is the peak area of the relevant component;

fc is the calibration factor for the relevant component, calculated in accordance with 9.1.1;

mg is the sample size, in mg/25 ml;

Ccs is the concentration of the relevant component in the sample, in % by mass.

9.2.2 Calculation of linear oligomer concentrations

Calculate the concentration, in mg/kg, of the linear oligomers from n = 1 up to and including n = 6 using the
following equation, rounding the result to 1 mg/kg in each case:

Acs × 25 000
= C cs (4)
f c × mg

where

Acs is the peak area of the relevant component;

fc is the calibration factor for the relevant component, calculated in accordance with 9.1.2;

mg is the sample size, in mg/25 ml;

Ccs is the concentration of the relevant component in the sample, in mg/kg.

10 Precision
The reproducibility of this analytical method is not known because interlaboratory data are not available.

However, precision data within one laboratory have been determined by analysis of two reference
polyamide 6 samples over an extended period of time. The precision of this method is not expected to deviate
significantly from the repeatability thus determined, the data for which is summarized in Tables 1 to 4.

© ISO 2007 – All rights reserved 7

 ISO 15033:2007(E)

Table 1 — Repeatability data for determination of cyclic oligomers in a PA6 sample

with a low cyclic oligomer content

n=1 n=2 n=3 n=4 n=5 n=6

Mean result, x (% by mass) 0,11 0,05 0,18 0,25 0,30 0,34
Standard deviation, s 0,007 0,008 0,008 0,01 0,01 0,02
Coefficient of variation (%) 6,9 15,6 4,5 3,9 4,4 6,9
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Table 2 — Repeatability data for determination of cyclic oligomers in a PA6 sample

with a high cyclic oligomer content

n=1 n=2 n=3 n=4 n=5 n=6

Mean result, x (% by mass) 8,37 0,33 0,50 0,47 0,40 0,34
Standard deviation, s 0,28 0,03 0,02 0,03 0,03 0,03
Coefficient of variation (%) 3,4 8,4 4,8 6,3 5,3 5,3

Table 3 — Repeatability data for determination of linear oligomers in a PA6 sample

with a low linear oligomer content

n=1 n=2 n=3 n=4 n=5 n=6

Mean result, x (mg/kg) <4 23 50 116 206 292
Standard deviation, s — 5 9 19 25 42
Coefficient of variation (%) — 23 18 17 12 14

Table 4 — Repeatability data for determination of linear oligomers in a PA6 sample

with a high linear oligomer content

n=1 n=2 n=3 n=4 n=5 n=6

Mean result, x (mg/kg) 141 59 82 148 203 252
Standard deviation, s — 11 13 24 33 37
Coefficient of variation (%) — 18 15 16 16 15

11 Test report
The test report shall include the following particulars:

⎯ a reference to this International Standard;

⎯ all details necessary for complete identification of the sample, including type, manufacturer's code
number, source, trade name, etc.;

⎯ any deviation from the specifications for the HPLC equipment or from the procedure given in this
International Standard;

⎯ any treatment of the sample prior to the analysis;

⎯ the mass of test sample used;

⎯ the contents of the cyclic and linear oligomers of caprolactam, in % by mass and mg/kg, respectively;

⎯ the relevant chromatograms.

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 ISO 15033:2007(E)

Annex A
(normative)

HPLC parameters and injector programme

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a) Flow rate: 0,51 ml/min

b) Solvent A: Eluent (5.17)

c) Solvent B: Acetonitrile (5.4)

d) Oven temperature: 40 °C

e) Maximum pressure: 400 bar

f) Minimum pressure: Off

g) Stop time: 47 min

Table A.1 — Eluent programme

Time Solvent A Solvent B

minutes % %
0 99 1
36 68 32
36,1 99 1
47 99 1

Table A.2 — Injector programme

Line Function Volume Time Solvent

µl minutes
1 Draw 3 Formic acid (5.5)
2 Draw 2 Sample solution
3 Draw 3 Formic acid (5.5)
4 Inject
5 Wait 31
6 Bypass
7 Draw 250 Formic acid (5.5)
8 Inject
NOTE Adjust slowdown of drawing, mixing and injection as appropriate and to suit the equipment.

© ISO 2007 – All rights reserved 9

 ISO 15033:2007(E)

Annex B
(informative)

Schematic diagram of HPLC apparatus

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Key
1 acetonitrile 5 column 9 fluorescence detector (exitn. 330 nm, emissn. 420 nm)
2 eluent 6 UV detector (200 nm) 10 to waste
3 eluent pump 7 post-column reagent
4 injector 8 reaction coil

Figure B.1

10 © ISO 2007 – All rights reserved

 ISO 15033:2007(E)

Bibliography

[1] MENGERINK, Y., PETERS, R., KERKHOFF, M., HELLENBRAND, J., OMLOO, H., ANDRIEN, J., VESTJENS, M.,
VAN DER W AL, S.: Analysis of linear and cyclic oligomers in polyamide-6 without sample
preparation by
liquid chromatography using the sandwich injection method — I: Injection procedure and column
stability, Journal of Chromatography A, 876 (2000), pp. 37-50
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[2] MENGERINK, Y., PETERS, R., KERKHOFF, M., HELLENBRAND, J., OMLOO, H., ANDRIEN, J., VESTJENS, M.,
VAN DER W AL, S.: Analysis of linear and cyclic oligomers in polyamide-6 without sample
preparation by
liquid chromatography using the sandwich injection method — II: Methods of detection and
quantification and overall long-term performance, Journal of Chromatography A, 878 (2000), pp. 45-55

[3] MENGERINK, Y., PETERS, R., VAN DER W AL, S., CLAESSENS, H.A., CRAMERS C.A.: Analysis of linear and
cyclic oligomers in polyamide-6 without sample preparation by liquid chromatography using the
sandwich injection method — III: Separation mechanism and gradient optimization, Journal of
Chromatography A, 949 (2002), pp. 307-326

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ICS 83.080.01
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