Journal of Analytical and Applied Pyrolysis, 17 (1990) 251-260 251

Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

TRACE DETERMINATION OF HIGH MOLECULAR

WEIGHT POLYVINYLPYRROLIDONE
BY PYROLYSIS-GAS CHROMATOGRAPHY

INGER ERICSSON * and LENNARD LJUNGGREN

Department of Analytical Chemistry, Chemical Center, University of Lund, Box 124,

S-221 00 Lund (Sweden)

(Received November 20 1989; accepted January 26 1990)

ABSTRACT

The possibilities of using pyrolysis-gas chromatography as a technique for selective

determination of polyvinylpyrrolidone (PVP) in trace quantities and the use of an appropriate
detector such as a thermionic specific detector (TSD) have been investigated. However, the
determination down to 0.2 ppm in the presence of polyhydric alcohols and high hydrophilic
polymers such as polyethylene oxide was obtained with a flame ionization detector.

Gas chromatography; nitrogen selective detector; polyvinylpyrrolidone; pyrolysis; trace

determination

INTRODUCTION

Polyvinylpyrrolidone (PVP) was first synthesized during the 1930s and

since then the polymer has gained widespread medical applications [l]. The
best-recognized application was as a plasma extender, the predecessor of the
present dextrane solutions. Within the field of medical devices, the unique
properties of PVP are used to alter surface characteristics of polymers to
improve performance and biocompatibility [2,3], especially in membranes
used for blood purification [4]. Adverse effects can arise in connection with
unwanted, uncontrolled administration of high molecular weight PVP due to
the fact that PVP is metabolically inert and is not excreted [l]. This results
in a potential risk of accumulation and persistent storage. In this context, it
is necessary to perform selective analysis of this water-soluble polymer in
various types of solutions as well as in biological fluids. Most of the
analytical methods described in the literature, related to medical and physio-
logical applications and thereby connected to the elimination by the body,
are focused on PVP with K-values between 17 and 30 (M, 9500-49000

016%2370/90/$03.50 0 1990 - Elsevier Science Publishers B.V.

252

dalton) and based on photometric techniques utilizing PVP’s excellent

ability to form a complex with iodine [5,6] or the binding of different dyes
[7,8] to PVP in aqueous solutions. Precipitation with trichloroacetic acid [9]
and size exclusion chromatography [lo] have been used for its determina-
tion. However, indirect methods have limited use in cases where competitive
complex formation could occur with the analyte [11,12].

EXPERIMENTAL PROCEDURE

Chemicals

The following chemicals were used: PVP Kollidon 17, 25, 30 and 90
(BASF, Ludwigshafen, F.R.G.), glycerol (Merck), polyethylene oxide (PEO)
Pluronic F68 (BASF, Wyandotte, MI, U.S.A.), 2-pyrrolidone, (Janssen
Chimica, Bel~um) ~-vinyl-2-pyrrolidone, (Janssen Chimica, Belgium) l-
methyl-2-pyrrolidone (Sigma, U.S.A.).

Sample and sample preparation

Membranes of polyamide were handcasted from a physical mixture of the

hydrophilic additives PVP and PEO into the membrane polymer prior to
membrane precipitation followed by extensive washing with water. They
were then stabilized with glycerol and dried. Pieces of membrane from
different batches were extracted under heating conditions of 60 o C for 20 h
in the ratio 40 g membrane: 250 ml distilled water. From these extracts, 10
ml aliquots were concentrated by freeze drying and were then administered
to the pyrolyzer. The remaining extract was used in parallel tests utilizing
the iodine complex method 151.

~p~arat~ and conditions

A filament pulse pyrolyzer was used. The sample was placed in the middle
of a platinum filament (15 mm long, 2.6 mm wide and 0.012 mm thick) and
the solvent was evaporated by a heating lamp. The pyrolyzer was connected
to the injector of a gas chromatograph. The prototype of the pyrolyzer has
been described earlier 1131. Table 1 shows the apparatus and conditions
used.

RESULTS AND DISCUSSION

Optimal conditions

The major component formed upon pyrolysis of PVP is the monomer

~nylpyrrolidone (VP (86%) as illustrated in Fig. 1. Identification of the
 253

TABLE 1
Apparatus and conditions employed for pyrolysis-gas chromato~aphy

Apparatus Model Conditions Used in

figure
Pyrolyzer Pyrola, 7xT: 8 ms 1-7
Pyrolab, T,: 150°c
Sweden tp: 2s

Gas chromato- Vista 6000, Detector: FID l-5

graph Varian, Detector
U.S.A. temperature: 3oo”c
Injection
temperature: 250°C
Carrier gas: He, 20 ml/mm
Split: 1O:l
Column: DB-1701(J&W),
1 pm, 1,2,4, 5
30 m, 0.32 mm id.
Temperature: SO-2’75OC
(10 o C/n-tin)
Column: DB-1 (J&W),
0.25 fkrn 3
30 m, 0.32 mm id.
Temperature: 50-275 o C
(10 o C/mm)

Channel 1 and 2
Micromat Detector: FID+TSD 6, 7
HRG 420, Detector
Nordion, temperature: 290 ’ C
Finland Injection
temperature: 250 OC
Column: NB-54 + NB-54
0.25 pm, 25 m,
0.32 mm id
Temperature: 50-280 o C
(10 o C/mm)

Mass spec- Finnigan 1

trometer 4021,
U.S.A.

Integrator SP 4270 l-5

Spectra
Physics,
U.S.A.

pyrolysisproducts was performed by gas chromatography/mass spectrome-

try and measurement of retention times. In order to find the optimal
temperature for quantitative analysis, PVP was pyrolyzed at different tem-
254

4,3%

4,4%

p=0
N
A

c,.,,

lk min

Fig. 1. Pyrogram illustrating the major thermal degradation products formed upon pyrolysis
of 7.5 pg PVP at 845 o C.

peratures. It is evident, as shown in Fig. 2 that a variation in temperature

around 800 o C does not affect the amount of monomer (VP) formed.

Influence of molecular weight

The molecular weight (M,) of PVP has a great influence on the analytical
results presented by others [5,7]. To study this effect with pyrolysis-gas

22 l tot. peak area l

0 monomer l
l

b
8
036
600 800 1000
pyrolysis temp. (Xi)

Fig. 2. The influence of temperature on the amount of monomer formed and the total amount
of pyrolysis products.
 255

48 530 60

log M,
Fig. 3. Relationship between molecular weight and monomer formation at a pyrolysis
temperature of 890 o C for 2 s.

chromatography (Py-GC), PVP samples (7.5 pg) with different M, were

pyrolyzed and the amount of monomer was recorded. The results presented
in Fig. 3 indicate good reproducibility from each sample. A slightly in-
creased amount of monomer formed with increasing M, may be seen. This
is explained by the fact that samples of lower M, have a larger number of
molecules in the same amount of sample. When the molecules are thermally
degraded by unzipping at the ends of the molecule there remains a small
portion (n = 6) which will thus be evaporated instead of being pyrolyzed to
the monomer [14,15]. A systematic error is made if the M,s of the standards
differ from the unknown. However, this 20% deviation in monomer content
over an M, range of two decades is not as pronounced as in the calorimetric
assays [7].

Interaction of polyols and other water-soluble polymers

Polyhydric alcohols such as glycerol or diethyleneglycol are often used in

large quantities as plasticizers and humectants to preserve structure in
membranes. Therefore, determinations of PVP in a 10% solution of glycerol
in water and in pure glycerol were performed. The calibration curve for PVP
in 10% glycerol (Fig. 4) is not linear over the concentration interval investi-
gated. This is due to the fact that the platinum filament can exhibit catalytic
properties which decrease the amount of monomer formed.
To enable the detection of low concentrations of PVP, the glycerol was
evaporated by heating the pyrolysis probe over a heating device. In this
calibration curve (Fig. 5) the height of the monomer peak was measured, as
256

I I

2,0 2,4 2,8

log amount (ng)

Fig. 4. Calibration curve for PVP in 10% glycerol.

the baseline in this experiment was not stable and because the integrator
showed unsatisfactory reproducibility.
Substances such as polyethylene glycol and polyvinyl alcohol form similar
complexes with iodine, as previously reported [12]. We have seen that PEO
also complexes with iodine and thereby interferes with the determination of
PVP in aqueous solutions. To study the interference of other high 44,
hydrophilic polymers with our technique, Py-GC, samples were prepared

amount (ng)

24C
80o-

0-
0 2 4 6 8
volume (pl)

Fig. 5. Calibration curve for PVP in glycerol.

 257

which contained a constant amount of PVP to which PEO (of the same
concentration, and of lo-fold and lOO-fold concentration) was added. In this
test the flame ionization detector (FID) was used. The result was promising
and enables quantitative determination of PVP. However, further experi-
ments were carried out with a nitrogen selective detector to increase the
selectivity.

Selectivity

Some studies have been performed with a selective detector to increase

selectivity. These were carried out with a gas chromatograph after having
doubled the column- and detector systems (see Table 1). One of the
detectors was a nitrogen selective detector, a thermionic specific detector
(TSD) and the other was a conventional FID. The pyrograms shown in Fig.
6 illustrate that preparatory tests to evaporate glycerol totally from the
platinum filament by heating are not needed when using a TSD. This
indicates that the use of a selective detector in connection with Py-GC
simplifies sample handling and increases selectivity for the analysis of PVP
in aqueous solution with polyhydric alcohols of low M,. Figure V also shows
that the peak area can be used as the baseline is more stable when compared
with the conditions in Fig. 5.

Channel 2 Rt =327,2 Rt =327,0

Detector: TSD Area =3339 Area ~3224

h ,
Channel 1
Detector: F I D

Fig. 6. Simultaneous pyrograms (FID + TSD) from PVP with two different detectors in the
presence of and after vaporisation of glycerol.
25%

g
a 4,0
r
X

3 2,0
3
P

i
t
20 40 60

amount (ng)

Fig. 7. Calibration graph for PVP in glycerol with a nitrogen sensitive detector (TSD).
Different volumes of a 10 ppm solution were applied. o, glycerol not totally evaporated; 0,
glycerol totally evaporated (see Fig. 6).

Sample analysis

The results obtained from the extracted membranes are presented in

Table 2, and indicate that Py-GC is successful in trace analysis of PVP. The
iodine complex failed in this respect, owing to the interaction with PEO in
the samples, which explains the great discrepancy between the results
obtained. The significance of the PVP results has been established by
analysis of similar samples without PEO. Concentrations down to 0.2 ppm
are possible to analyze when 10 ~1 of the sample is pyrolyzed.

TABLE 2
Results obtained by different analytical techniques

Sample PVP t ag/mI)

(unknowns)
Iodine complex Py-GC
(ref. 5)
day 1 day 2
A 80 0.9 0.9
B 102 1.5 1.5
C 92 2.0 1.9
D 80 1.1 1.2
E 106 0.9 0.9
F 70 0.2 0.4
G 86 1.0 1.0
H 126 1.0 1.2
I 106 1.1 0.9
 259

CONCLUSION

Py-GC with a nitrogen selective detector is a very sensitive and selective

technique for trace determinations of PVP in complex mixtures. The ques-
tion which remains to be answered is whether this technique is possible to
use for trace analysis in biological fluids.
It has previously been reported that iodine complex formation and
precipitation with perchloric acid are useful for this kind of analysis.
However, when using the iodine complex on rat urine, the blank values
differ between the different urines, probably because of the fact that PVP
itself interacts with coloured substances in urine which in turn affects the
optical density in the wavelength region under analysis. This is contradictory
to the bleaching effect reported earlier [5].
In the technique which we have been using, some of these possible
interactions are avoided by the fact that we can discriminate by using a
selective detector. No reagents are present which might affect the samples in
the same way as described in earlier publications. We are, however, limited
by the sample size that could be administered onto the pyrolysis probe. If
quantification lower than 0.2 ppm is needed, sample volumes larger than 10
~1 can be used. Concentration of the sample is made possible by repeated
administration onto the filament and continuous evaporation of low M,
substances.
Since M, plays a role in monomer formation during pyrolysis, it must be
ascertained that PVP has not undergone any bacterial degradation and that
the extraction from a polymer matrix into a solution will increase the
amount of low M,,, polymers in the solution, otherwise this could give a false
M, when analyzed per se.

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