Separation and characterization of acrylic acid oligomers by nuclear magnetic resonance spectroscopy and thermospray ion-exchange liquid chromatography-mass spectrometry

Journal of Chromatography,

ElseviefScience

408 (1987) 349-355

Publishers B.V., Amsterdam -

Printed in The Netherlands

CHROM. 19 831

Note

Separation and characterization of acrylic acid oligomers by nuclear magnetic resonance spectrbscopy and thermospray ion-exchange liquid chromatography-mass spectrometry S. S. CUT&*, G. J. KALLOS and P. B. SMITH The Dow Chemical Company. Analytical Laboratories, 574 Building, Midland, MI 48667 (U.S.A.)

(Received June 16th, 1987)

Acrylic acid can be produced by one of several processes, two of the more important being the air oxidation of propylene to acrolein, followed by further oxidation to acrylic acid, and the modified Reppe process by addition of carbon monoxide, alcohol and water to acetylene in the presence of a nickel halide salt. Acrylic acid is used extensively in the chemical industry to make water-soluble polymers, coatings, plastics and resins, adhesives and sealants, etc. To assure the quality of the monomer during storage and handling, it is inhibited against free radical polymerization by the addition of the monomethyl ether of hydroquinone (MEHQ). However, MEHQ does not inhibit the Michael addition reaction in this system which yields primarily diacrylic acid (dimer). The rate of formation of the dimer is a function of temperature, time and water content. The Michael addition dimer formation is represented as: 2CH2 = CHCOOH

-+ CH2 = CHCOOCHz-CHz-COOH

An analytical technique, developed to determine dimer, trimer and tetramer in the monomer, is presented. It uses ion-exchange liquid chromatography with either an ultraviolet detector or a mass spectrometer connected through a thermospray (TSP) interface. Gas chromatography (GC)’ and liquid chromatography (LC)*s3 have been used in the past for the analysis of acrylic acid, thin-layer chromatography has been used to separate acrylic acid monomer from acrylic acid dimer4. LC is preferred to other methods because of the reactive, polar nature of acrylic acid. Also, the dimer is thermally labile and could decompose under GC conditions. EXPERIMENTAL

Equipment The LC set-up consisted of a Kratos variable-wavelength UV-VIS detector Model 773 (ABI Analytical, Ramsey, NJ, U.S.A.) and a Waters M-45 pump (Waters Assoc., Milford, MA, U.S.A.) at 0.6 ml/min with 0.004 M sulfuric acid as eluent, and post-column make-up of 0.3 ml/min of 0.5 M ammonium formate. Samples of 10 ~1 were injected with a Rheodyne 7125 injector and separated on a 10 cm x 7.8

351

NOTES

spectroscopy

NMR

The structure of the dimer was confirmed using 75.5 MHz 13C NMR spectroscopy. The standard dimer sample was dissolved in acetic acid and was analyzed using a GE NT-300 NMR spectrometer. The data acquisition parameters utilized are shown in Table I. The level of dimer in acrylic acid monomer was determined using 300 MHz ‘H NMR spectroscopy. The samples were analyzed neat using an IBM AF-300 NMR spectrometer, The data acquisition parameters utilized are shown in Table I. The spin lattice relaxation times of the protons were determined using the inversion recovery methods. These values are given in Table II. The samples were analyzed under quantitative conditions. Preparation of samples

Solutions of 0.1% acrylic acid in water were used throughout

the investigation.

Reagents

Acrylic acid oligomers standard was obtained from Polysciences, Warrington,

(a)

II!IIL A

B

c

D

I 0

I

(min)

AB

(b)

I

I

1

I

I

I

I

1

I

I

20

1 40

c

D

I

I

0

I

I

(min)

I

20

1

40

Fig. 1. LC traces of a dimer standard on Bio-Rad columns. (a) Organic acid analysis column (300 x 7.8 mm): (b) fast organic analysis column (100 x 7.8 mm). Peaks: A = acrylic acid monomer; B = acrylic acid dimer; C = acrylic acid trimer; D = acrylic acid tetramer.

NOTES

3.52 0.9 -

(a)

m/z90 90 -

100.0 r 162 -

m/z 162

234 -

m/z 234

26.2 Im/z 306 306

-

125.7 LClMS Total Ion Chromatogram RIG -

I

I 100 3:20

0.6 ,-

I

I 400 13:20

300 lo:oo

200 6:40

SCAN TIME

(b) m/z 117

117 -

m/z 189

169 -

100.0 m/z 261

261 F

I

1

I

I

33.0 r m/z 333

333 -

I 563.1

r

LCIMS

Total Ion Chromatogram

IdO 3:20

2iO 6:40

Fig. 2. LC-MS single and total ion chromatograms

3;o lo:oo

4io l3:20

SCAN TIME

for (a) positive ion run and (b) negative ion run.

353

NOTES

PA, U.S.A. Sulfuric acid A.C.S. reagent was obtained from Fisher Scientific, Midland, MI, U.S.A. Characterization of oligomers standards The oligomers standard was characterized

by NMR, and LC-MS.

RESULTS Fig.

the separation

la shows

fO)

100.0 -

obtained for the oligomers with conditions de-

,2 Dimer+NH,

50.0

-

145 95

M/Z

115

I 1 100

’

-

134

8

,

I

202

150

”

215

I’ 200

160

’

245

231

8

260

,

277

t

’

I

291

u

7

250

I

3f6

r

300

189 100s

Dimer HCOO-

50s I -

136

235 287

I 95 159

[I M/Z

150

176

207

223

,

8 ZOO

250

I

I

,

, 300

I

I

I

I

,

I

,

,

399 -,

350

Fig. 3. (a) Thermospray positive ion mass spectrum of acrylic acid dimer. (b) Thermospray negative ion mass spectrum of acrylic acid dimer.

NOTES

354

AceticAcid AceticAcid

-CHa

H& CQH @LO

CL0

-

F

i 0

200

PPM

Fig. 4. 13C NMR spectrum of the dimer standard in acetic acid.

scribed above using a 300-mm column. Fig. lb shows the separation using the same conditions but with a lOO-mm column. The analysis time was shortened by a factor of 3, without sacrificing the resolution significantly. The chromatographic peaks were identified using LGMS with the thermospray interface. Both positive and negative ion spectra of the LC separated components (Fig. 1) supported the structures of acrylic acid monomer, dimer, trimer and H&E

CH-

Acetic Acid -CHa

I

I

I

I

I

I

I

I

8

I

I

10.0

9.0

8.0

7.0

6.0

5.0

4.0

3.0

2.0

1.0

0

PPM

Fig. 5. lH NMR spectrum of acrylic acid with 3.2% dimer added.

355

NOTES TABLE III THE COMPARATIVE SampEe

Blank 0.8% dimer added 1.6% dimer added 3.0% dimer added 3.2% dimer added 5 1.4% standard

ANALYSIS OF DIMER LEVEL IN ACRYLIC ACID

Dimer level

NMR*

LC

0.1 1.4 2.3 3.7 3.8 51.2

0.74 1.6 2.4 4.0 4.2

* An estimate of the standard deviation at the 95% confidence level is 5% relative.

tetramer. Single ion and total ion chromatograms for both positive and negative ion runs are shown in Fig. 2. The positive ion mass spectra exhibit intense ammonium adduct ions as shown in Fig. 3. In the negative ion mode the formate anion adducts were obtained for the acrylic acid monomer, dimer, trimer, and tetramer. Several other peaks found in the negative ion mass spectra were probably cluster ions resulting from the sulfuric acid that was present in the mobile phase. The dimer standard was analyzed by 13C NMR spectroscopy for purity. The spectrum was consistent with the proposed structure. No organic impurities were observed in this sample at an estimated detection limit of 3 wt.% (Fig. 4). A series of standard solutions were prepared by spiking known amounts of this dimer standard into acrylic acid. Prior to spiking, the standard was diluted to 51.4% in acetic acid. The samples were analyzed by LC and ‘H NMR methods. A typical ‘H NMR spectrum of the acrylic acid samples is given in Fig. 5. The level of dimer determined by the two methods is given in Table III. The agreement between the two methods is excellent.

REFERENCES I 2 3 4 5

A. Vairavamurthy, M. 0. Andreae and J. M. Brooks, Anal. Chem., 58 (1986) 2684. L. Brown, Analyst (London) 104 (1979) 1165. M. Richards, J. Chrumatogr., 115 (1975) 259. K. Walczyk and L. Burczyk, Zesz. Nauk., Inst. Ciezkiej Syn. Org. Blachownia Poi., 4 (1972) 17. T. C. Farrar and E. D. Becker, Pulse and Fourier Tramform NMR, Academic Press, New York, 1971, Ch. 2.