TGA/APCI/MS/MS: a new technique for the study of pyrolysis and combustion products

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TGA/APCI/MS/MS: a new technique for the study of pyrolysis and

combustion products

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Str

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N21d

National Research

Consell national

no.

1376

Council Canada

de

recherches Canada

c .

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Institute for

_{Research in}

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Construction

construction

TGA/APCI/MS/MS, A New

Technique for the Study of

Pyrolysis and Combustion

Products

by Y. Tsuchiya

ANALYZED

Reprinted from

Fire Safety Science

-

Proceedings of the First

International Symposium

Gaithersburg, MD, 9

-

11 October 1985

p. 411 -420

(IRC Paper No. 1376)

Price $2.00

NRCC 25923

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La Section d16tude

du feu, Division des rechercnes en oacimenc,

Conseil national de recherches du Canada,

a

fait l'acquisition

d'un

SM/SM ICPA (ionisation chimique sous pression

atmosph6rique) SCIEX TAGA 6000, appareil d'analyse d'une

sensibilit6 et d'une rapidit6 de traitement exceptionnelles.

Celui-ci est capable de dktecter simultankment un grand nombre

de gaz produits lors de la pyrolyse/combustion.

Coup16

3 un

analyseur thermogravim6trique (ATG) Dupont

951, il a servi

a

6tudier les produits de pyrolyse du polyacrylonitrile (PAN)

a

diff6rents stades de ce processus.

Un

6chantillon de

1 mg a 6t6 pyrolysk dans 1'ATG sous courant

d'azote ou d'air et les produits ont 8t6 introduits dans le

SM/SM ICPA

a

l'aide d'un court capillaire en verre.

Les

mol6cules du produit ont kt6 ionis6es sous pression

atmosph6rique, et elles ont 6tE analvs6es

en

tamps reel au

moyen de

'

ce.

Les

principa.

tque et

une skr

n

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TGAIAPCIIMSIMS, A New Technique

6

for the Study of Pyrolysis

and Combustion Products

YOSHIO TSUCHIYA

Division of Building Research National Research Council of Canada + Ottawa, Ontario, K I A OR6

ABSTRACT

The Fire Research Section, Division of Building Research, National

Research Council of Canada has acquired a SCIEX TAGA 6000 APCI (atmospheric

pressure chemical ionization)/MS/MS, an analytical instrument unique for its

high sensitivity and high speed in analysis. The instrument is capable of

monitoring simultaneously many types of gases generated in

pyrolysis/combustion. Coupled with a Dupont 951 thermogravimetric analyser

(TGA), it has been used for studying the pyrolysis products of

polyacrylonitrile

(PAN) at different stages of the pyrolysis process.

A 1

mg

specimen was pyrolyzed in the TGA in a stream of nitrogen or air

and the products were introduced to the APCIIMSIMS through a short glass

capillary. The molecules of the product were ionized under atmospheric

pressure, and analyzed in real time with three serial quadrupole mass filters.

The main products were HCN, acetic acid, and a series of nitriles. The

generation of each product is discussed in the light of the thermogravimetric

analysis.

KEYWORDS

APCIIMSIMS, TGAIMS, polyacrylonitrile, combustion products,

nitriles, HCN.

INTRODUCTION

A combination of thermogravimetric analysis (TGA) and mass spectrometry

(MS) is a powerful tool for studying thermal degradation of materials. With a

TGA, a specimen of the material can be thermally degraded at accurately

controlled temperatures in a desired atmosphere. The effluent degradation

products from the TGA can be analyzed by a highly sensitive MS.

It is not

surprising that many applications of TGAIMS, using various types of

instrumentation, have been reported in the past 15 years, as shown in Table 1

(1-151, although the table is not intended to present a complete list.

The conventional MS, used in most of the earlier studies, had some

-

limitations. The electron impact (EI) ionization extensively fragmented the

molecules of the degradation products at the ion source, making the mass

spectra complicated and restricting the use of the technique to the analysis

of relatively simple degradation products. Chemical ionization (CI) has

simplified the spectra, but the information on molecular weights supplied with

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TABLE 1 TGA/MS S t u d i e s i n L i t e r a t u r e

I

Author Year TGA MS E I / C I mL/min OC/min M a t e r i a l

Zitomer 1968 Dupont 950 Bendix T-0-F E I 100 15 P o l y e t h y l s u l f i d e

Chang 1971 Dupont 900/PE 881 GC Dupont CEC 21-llOB 60-80 10-15 Ethyl v i n y l a c e t a t e

Gibson 1972 M e t t l e r F i n n i g a n 1015 E I 2 , 4 , 6 Geochemical samples

Mol 1974 M e t t l e r I UTI l O O C EI Vacuum 2,4,6

PVC,

ABS, PU, P o l y e s t e r

Tsur 1974 PE TGSl F i n n i g a n 1015 E I A , A i r 18 16 Polybenzimidazole

K l e i n b e r g 1974 Cahn RH Dupont 21-491 DF EI Oxid. 5-25 PVC, M , PU

2

N Baumgartner 1977 M e t t l e r TAC F i n n i g a n 3200 EI-CI 60-80 4 Ca o x a l a t e

Muller 1977 M e t t l e r B a l z e r s Quad MS EI 10 Ca oxalate

M o r i s a k i 1978 IR r a y thermobalance Quad MS Analyzer EI He, Air 150 PTFE

Shimizu 1979 D i r . i n s e r t . probe Dupont 21-llOB DF C I PVC, PMM

Chiu 1980 Dupont 990 Dupont 21-104 E I He 60 5-10 Ca a c e t . P o l y a c e t a l

Yuen 1982 M e t t l e r TA1 HP 5992 EI H e , 0 2 / H e 1 5 Ca oxalate, SB e t c .

I

Chan 1982 Temp. prog. f r a c t i o n . H i t a c h i RMS-4 EI 50 PS, peanut o i l

Dyszel 1983 PE TGS 2 SCIEX TAGA 3000 APCI N p 80 Guar gums

Whiting 1984 Dupont 951/HP 5710 GC LKB 9000 EI He 100 10 Coal

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I

t h e u s e of CI i s o f t e n n o t s u f f i c i e n t t o i d e n t i f y t h e d e g r a d a t i o n p r o d u c t s . I n s t u d y i n g o x i d a t i v e d e g r a d a t i o n , which i s of p a r t i c u l a r i n t e r e s t t o f i r e

I r e s e a r c h e r s , t h e r e i s a l s o a t e c h n i c a l d i f f i c u l t y : an o x i d a t i v e atmosphere

c a u s e s d r a s t i c r e d u c t i o n i n t h e l i f e of t h e i o n s o u r c e .

I n t h e p r e s e n t s t u d y an APCI/MS/MS (tandem) was u s e d f o r t h e a n a l y s i s of e f f l u e n t d e g r a d a t i o n p r o d u c t s of p o l y a c r y l o n i t r i l e (PAN) from t h e TGA. T h i s MS i s c o m p a t i b l e w i t h TGA, s i n c e t h e p r o d u c t s c a n b e a n a l y z e d i n a i r u n d e r

a t m o s p h e r i c p r e s s u r e . EXPERIMENTAL

-

M a t e r i a l s

An Orlon c l o t h t h a t c o n t a i n e d more t h a n 85% p o l y a c r y l o n i t r i l e was u s e d w i t h o u t f u r t h e r t r e a t m e n t . For comparison a 100% p u r e s o l i d amorphous p o l y a c r y l o n i t r i l e (Cellomer Assoc. I n c . C a t # 134c) was a l s o used. I n s t r u m e n t a t i o n

A Dupont 951 TGA and t h e SCIEX TAGA 6000 APCI/MS/MS were t h e two main i n s t r u m e n t s u s e d i n t h i s s t u d y . The SCIEX TAGA 6000 APCI/MS/MS h a s been d e s c r i b e d e l s e w h e r e ( 1 6 ) . U n l i k e o t h e r MS i n s t r u m e n t s , i t employs a l a r g e volume of sample g a s c o n t i n u o u s l y i n t r o d u c e d i n t o t h e i n s t r u m e n t ( t y p i c a l l y 2 L/min). Components of t h e g a s a r e i o n i z e d by a c o r o n a d i s c h a r g e under a t m o s p h e r i c p r e s s u r e u s i n g oxygen o r w a t e r i n t h e sample g a s a s t h e chemical r e a g e n t . A n i t r o g e n f l o w forms a g a s e o u s membrane between APCI s o u r c e and t h e h i g h vacuum a n a l y z e r s e c t i o n t o p r e v e n t un-ionized m o l e c u l e s from g o i n g i n t o t h e a n a l y z e r . The i o n i z e d m o l e c u l e s ( p a r e n t i o n s ) p e n e t r a t e t h e membrane and a r e s e p a r a t e d a c c o r d i n g t o t h e i r m a s s l c h a r g e r a t i o (M/Z) a t t h e f i r s t q u a d r u p o l e mass f i l t e r . Argon g a s f l o w s p e r p e n d i c u l a r t o t h e p a t h of p a r e n t i o n s a t t h e second mass f i l t e r . P a r e n t i o n s c o l l i d i n g a g a i n s t argon atoms f r a g m e n t w i t h a p a t t e r n c h a r a c t e r i s t i c of t h e i o n . The p a t t e r n i s a n a l y z e d by t h e t h i r d mass f i l t e r .

The MS/MS and t h e TGA were coupled w i t h a s h o r t g l a s s c a p i l l a r y (0.5 mm

d i a . , 20 mm l o n g ) . The f l o w of e f f l u e n t from t h e TGA t o t h e MS/MS t h r o u g h t h e c a p i l l a r y was measured from t h e p r e s s u r e d i f f e r e n c e a c r o s s t h e c a p i l l a r y and c o n t r o l l e d by a 'dump' v a l v e f i t t e d w i t h a micrometer. The whole i n t e r f a c e assembly was h e a t e d t o p r e v e n t t h e c o n d e n s a t i o n of d e g r a d a t i o n p r o d u c t s . To t h e e f f l u e n t f l o w , a make-up f l o w of 2 L/min of z e r o a i r ( p u r i f i e d a i r w i t h p r a c t i c a l l y n o o r g a n i c g a s e s ) was added. The s c h e m a t i c d i a g r a m of t h e

i n t e r f a c e i s shown i n Fig. 1. A H e w l e t t Packard 5996 GC/MS/Data s y s t e m w i t h a CDS 100 p y r o l y s i s u n i t was u s e d s e p a r a t e l y t o a n a l y z e t h e p y r o l y s i s p r o d u c t s and t h e r e s u l t s were compared w i t h t h o s e from t h e APCI/MS/MS.

RESULTS AND DISCUSSION

The p y r o l y s i s p r o d u c t s of PAN a n a l y z e d by t h e p y r o l y s i s GC/MS a r e shown i n Fig. 2. The p y r o l y s i s o c c u r r e d i n 3.6 atms of helium. The main p r o d u c t s were HCN, a s e r i e s of n i t r i l e s and a c e t i c a c i d . The l a s t was n o t g e n e r a t e d when t h e e x p e r i m e n t was r e p e a t e d w i t h 100% p u r e s o l i d p o l y a c r y l o n i t r i l e . I n Fig. 3, t h e r e s u l t s of TGA of PAN i n a t m o s p h e r i c n i t r o g e n a r e shown; t h e y may b e compared w i t h t h e t o t a l i o n v s t i m e from MS/MS a n a l y s i s of t h e e f f l u e n t ( F i g . 4 ) . The t o t a l c u r r e n t v a r i e d i n a manner s i m i l a r t o t h e f i r s t

d e r i v a t i v e of TGA curve. The t o t a l i o n i s t h e sum of e a c h of t h e p a r e n t i o n s ;

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TAGA 6000 APCl l M S l M S

EFFLUENT FROM TGA 300 mLlmin

--+

FIGURE 1. TGA/MS/MS i n t e r f a c e .

loco 0

m

0.4 0.8 1.2 1.6 2.0 2.4 2.8 3,2 3.6 4.0 4.4 4.8 5.2 5 . 6 6.0 6.4 6.8 7.2 TIME. m i n

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TIME, rnin

FIGURE

3.

Thermogravimetric analysis of PAN in nitrogen.

T I M E . rnin

FIGURE 4. Pyrolysis of PAN in nitrogen, total ion in M S b S analysis, positive mode.

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38 50 60 70 80 90 100 110 120 130 140 150 NORMALIZED VS MAX INT MASS. 60 (100 = 160663. COUNTS). MI2

FIGURE 5. P y r o l y s i s of PAN i n n i t r o g e n , MShS a n a l y s i s , p a r e n t s c a n , p o s i t i v e mode.

T I M E , min

FIGURE 6. P y r o l y s i s of PAN i n n i t r o g e n , MShS a n a l y s i s , s i n g l e i o n monitoring.

From both t h e GChS and t h e MS/MS experiments, i t was c l e a r t h a t a s e r i e s of n i t r i l e s were formed. M/Z = 60 was acetamide, which was very e a s i l y i o n i z e d and was dominating i n t h e p a r e n t s c a n , but i t s a c t u a l c o n c e n t r a t i o n

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i s shown a g a i n s t time. Some n i t r i l e s were g e n e r a t e d i n two s t e p s ; t h e f i r s t

!

s t e p o c c u r s a t t h e same t e m p e r a t u r e (290°C) f o r a l l t h e n i t r i l e s ; t h e second

*

_{s t e p a t i n c r e a s i n g l y h i g h e r t e m p e r a t u r e s f o r h i g h e r n i t r i l e s .} T h i s phenomenon i s n o t s i m p l y e x p l a i n e d by t h e d i f f e r e n c e i n t h e i r b o i l i n g p o i n t s . For example, b u t a n e n i t r i l e (shown a s C3-CN i n Fig. 6 ) and pentane n i t r i l e (C4-CN) have b o i l i n g p o i n t s of 117.6 and 140.7"C, r e s p e c t i v e l y , w h i l e t h e r e c o r d e d t e m p e r a t u r e s i n t h e TGA a t t h e peak of g e n e r a t i o n were 372 and 410°C, r e s p e c t i v e l y . The e l u c i d a t i o n of t h e mechanism i s l e f t f o r f u t u r e s t u d i e s .

The MS/MS can d e t e c t e i t h e r p o s i t i v e i o n s o r n e g a t i v e i o n s . I n t h e p r e s e n t s t u d y , t h e p o s i t i v e i o n mode was used f o r n i t r i l e s and t h e n e g a t i v e i o n mode f o r a c i d s . Fig. 7 shows

MSIMS

a n a l y s i s i n t h e n e g a t i v e mode. A p a r e n t s c a n a t t h e peak i s shown i n Fig. 8; major components were HCN and a c e t i c a c i d .

z

=

0 0 10 20 30 40 50 60 70 80 90 100 110 120 130 T I M E , m l n FIGURE 7. P y r o l y s i s of PAN i n n i t r o g e n , t o t a l i o n i n MS/MS a n a l y s i s , n e g a t i v e mode.

NORMALIZED V S MAX INT MASS. 59 (100 = 485186. COUNTS), MIZ

FIGURE 8. P y r o l y s i s of PAN i n n i t r o g e n , MS/MS a n a l y s i s , p a r e n t s c a n . n e g a t i v e mode.

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The TGA of PAN i n a i r i s shown i n Fig. 9. PAN degraded i n two s t e p s ,

b o t h i n a i r and i n n i t r o g e n ; t h e f i r s t s t e p i s a t 290°C and t h e second a t

400-500°C. I n a i r , t h e w e i g h t l o s s i n t h e f i r s t s t e p was s m a l l e r and t h e

second s t e p was l a r g e r t h a n t h e c o r r e s p o n d i n g s t e p s i n n i t r o g e n . G e n e r a t i o n

of HCN, a s determined by t h e MS/MS, i s shown i n Fig. 10. The c u r v e was

s i m i l a r t o t h e f i r s t d e r i v a t i v e of t h e TGA curve. The HCN was a l s o g e n e r a t e d

i n two s t e p s .

FIGURE 9. Thermogravimetric a n a l y s i s of PAN i n a i r .

T I M E , rnln

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1 The f i r s t d e r i v a t i v e of t h e TGA c u r v e determined f o r t h e s o l i d specimen,

;

and t h e t o t a l i o n c u r r e n t determined from t h e p y r o l y s i s p r o d u c t s i n t h e g a s

.

phase, were n e a r l y i d e n t i c a l i n s h a p e i n n i t r o g e n and i n a i r . Although t h i s

i s e x p e c t e d , s i n c e t h e weight l o s s i s a r e s u l t of g a s i f i c a t i o n ( g e n e r a t i o n of gaseous p r o d u c t s ) of t h e s o l i d specimen, t h e f i n d i n g u n d e r l i n e s t h e v a l i d i t y of t h e t e c h n i q u e . The two-step g e n e r a t i o n of HCN may be e x p l a i n e d by t h e u n z i p p i n g of molecules i n t h e f i r s t s t e p and t h e decomposition of N-containing c h a r i n t h e second s t e p . HCN i s t h e major t o x i c component i n t h e t h e r m a l decomposition p r o d u c t s of PAN (17) and o t h e r N-containing o r g a n i c

m a t e r i a l s ( 1 8 ) . CONCLUSION

The p y r o l y s i s of PAN i n n i t r o g e n and i n a i r was s t u d i e d by

TGA/APCI/MS/MS. HCN, a s e r i e s of n i t r i l e s , and a c e t i c a c i d were i d e n t i f i e d . The PAN degraded i n two s t e p s . N i t r i l e s and HCN were g e n e r a t e d i n b o t h s t e p s i n n i t r o g e n and i n a i r .

The TGA/APCI/MS/MS was found t o be an e f f e c t i v e t o o l f o r s t u d y i n g t h e p y r o l y s i s of polymers. P y r o l y s i s p r o d u c t s can be i d e n t i f i e d and t h e i r g e n e r a t i o n a t d i f f e r e n t s t a g e s of p y r o l y s i s can be c l o s e l y observed. F u r t h e r s t u d i e s a r e planned.

ACKNOWLEDGEMENT

The a u t h o r t h a n k s J.B. S t e w a r t f o r h i s a s s i s t a n c e i n r u n n i n g e x p e r i m e n t s and p r o c e s s i n g d a t a . T h i s paper i s a c o n t r i b u t i o n from t h e D i v i s i o n of B u i l d i n g Research, N a t i o n a l Research Council of Canada.

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