Forensic Science International, 36 (1988) 31-39 Elsevier Scientific Publishers Ireland Ltd.

31

IDENTIFICATION OF NATURAL AND SYNTHETIC FIBRES BY PHOTOACOUSTIC SPECTROPHOTOMETRY IN THE NEAR-INFRARED REGION

RAJINDER SINGH, S. R. SINGH and V. N. SEHGAL

Central Forensic Science Laboratory, Central Bureau of Investigation, Block 4, CGO Complex, Lodhi Road, New Delhi-110003 (India)

Summary

Qualitative analysis and composition among cotton and polyester (terene) fibres and their forensic characterisation have been carried out using near-infrared photoacoustic spectroscopic technique. The different ratio of their mixtures and effect of dye additive have also been evaluated in the present work. The technique is fast and little sample preparation is required and can be considered a non-destructive technique.

Key words: Fibres analysis; Polyester (terene); Cotton; Near-infrared photoacoustic spectroscopy

Introduction

Polyester, cotton and polyester-cotton fibres and their fabrics are always encountered as evidentiary material in many type of crimes due to their wide application for the manufacturer of clothings etc. Therefore, these types of fibres are often examined in forensic science laboratories as crime exhibits. The various techniques have been reported in literature for analysis of syn- thetic and cotton fibres. The most common methods which are often used in forensic characterisation of such samples include infrared spectroscopy, pyrol- ysis infrared spectrophotometry (PyIR), bifrigence properties using a polariz- ing microscope, comparison microscope, measurements of density, melting points and refractive indices etc. [l-5]. The application of photoacoustic spec- troscopy (PAS) has been described by Caroline et al. [6] as most comprehensive technique for quantitative examination of polyester-cotton fibres. The PAS technique has been successfully applied by Singh et al. [7,8] in forensic com- parison and identification of multilayered paint coatings and ink examination on documents. The analysis of solid and liquid materials by this method have been widely described by Adams et al. [9,10]. In this present paper an attempt has been made to identify and compare the number of cotton and polyester (terene) fibres and fabrics manufactured by number of Indian entrepreneurs. The effect of some additive such as mixture of cotton and terene and dye have also been found in the present work.

0379-0738/88/$03.50 ~:i;, 1988 Elsevier Scientific Publishers Ireland Ltd. Printed and Published in Ireland

32

The PAS technique offers advantages over techniques for analysis of fibres through its ability to give reproducible results without any specicfic prepara- tion of sample. This technique can be generally described as non-destructive technique.

Materials and methods

Apparatus The block diagram of a double beam photoacoustic spectrophotometer de-

ployed for this study has been designed, developed and fabricated at National Physical Laboratory [ll], New Dehli is shown in Fig. 1. The principle and detailed theoretical treatment of this instrument has been described elsewhere [9,10,12]. The system employs a 600-W tungsten halogen lamp as light source and approx. 2000 pm slitwidth (entrance and exit) of a grating monochromator. The recorder sensitivity of 200 UV, time constant of 10 s and scan speed of 50 nm/min, respectively were selected in the present work. All the samples were scanned in region of 0.3-2.8 pm under similar conditions at room temper- ature of 1520°C.

Samples preparation All the samples approx. 5 mg employed for this study were cut into small

pieces in order to get them in the form of powder. The control mixtures of cotton and terene in varying proportion i.e. 20% cotton, 80% terene; 30% cotton, 70% terene; 70% cotton, 30% terene; 50% cotton, 50% terene; were also prepared. Two samples of cotton fibres were dyed with Blue (1.5%) and Green vat (2%) dyes to find the impact of the dyes on the PAS signal.

RAOIATKXI MONO CHROMATOR ’

SOURCE

1 PAS CELL PAS CELL 1

IMICR~PHONEI ~~ICR~PH~NEI

i 1 REFERENCE PREAMPLIFIER PREAMPLIFIER

i LOCK IN AMPLIFIER

LOCK IN AMPLIFIER 1

Fig. 1. Double-beam photoacoustic spectrophotometer.

33

Results and discussion

Terene fibres Figure 2 shows the near-infrared photoacoustic spectra of four polyster

(terene) fibres of different manufacturers, with a list of their absorption bands position given in Table 1. Strict comparison of PAS spectra of these fibres show several important results, which involves interpretation of the differences among these fibres were noted in region between 0.8 and 1.3 pm. The character- istic absorption bands position in the terene fibres were noted nearly at 1.15 pm, 1.41 pm, 1.7 pm, 2.3 ,um and 2.5 pm due to -CH,, >CH, functional groups. The absorption at 2.05 pm may be due to -OH (alcoholic) group present in terene fibre. The differences among terene fibres arises from different con- centrations of functional groups which produce changes in absorbances peaks and different chemical reagents added during manufacturing process by differ- ent manufacturers.

Cotton fibres Figures 3 and 4 represent the PAS spectra of five different cotton fibres and

their positions of absorption bands are also given in Table 1. Comparison of these spectra suggested specific differences among these fibres. The character- istic absorption bands position in cotton fibres occur at 1.4 pm due to the first overtone for bonded hydroxyl groups and the combination band is observed around at 2.2 pm. Thus the near-infrared absorption studies may be employed

1. CISW I\raqpur Terene

2. Binny Terene 3. CAFI Terene 4. AOCI Terene

I

IIt n ‘1

0.8 1.0 1.2 1.8 1.8 2.0 2.2 24 2.6

Fig. 2. PAS absorption spectra of four different terene fibres.

34

TABLE 1

PAS ABSORPTION BANDS DATA OF TERENE AND COTTON FIBRES

vs. very strong; s, strong; ms, medium strong; m, medium; mw, medium weak; w, weak,

Sample Details and nature Position of PAS absorption bands in pm with their no. of sample related intensity

1 CISW Nagpur terene 2 Binny terene

3 CAFI terene

4 AOCI terene

5

6

7 8

9 Egypt cotton fibre

Pure cotton fibre (treated)

Pure hybrid sankar cotton fibre

Raw cotton fibre American cotton fibre

1.15(ms), 1.41(s), 1.7(s), 1.75(w), 2.05(mw), 2.3(w), 2.5(s), 2.6(s) 0.99(w), l.O3(mw), 1.18(w), 1.4(s), 1.61(vw), 1.7(s), 2.07(ms),

2.05(mw), 2,3(w), 2.48(s), 2.6(s) 0.97(w), l.l8(mw), 1.4(s), 1.6(vw), 1.72(s), 1.79(vw), 2.07(ms),

2.33(w), 2.54(s), 2.6(s). 0.88(w), l.O2(mw), 1.22(w), 1.42(s), 1.72(s), 2.05(ms), 2.54(s),

2.62(s). 0.97(w), 0.98(w), 1.17(w), 1.2(w), 1.42(vs), 1.5(s), 1.68(s), 1.72(s),

1.75(s), 2.1(s), 2.3(s). 0.96(w), l.OB(mw), 1.17 (mw). 1.4(s), 1.5(s), 1.5(ms), 1.72(s),

1.77(s), 2.18(s), 2.4(s). 1.4(s), 1.52(s), 2.18(s), 1.4(s). 1.2(mw). 1.39(s), 1.42(s), 1.5(s), 1.53(s), 1.57(s), 1.69(s),

1.93(mw), 2.08(w), 1.22(w), 1.45(s), 1.52(ms), 1.57(ms), 1.62(ms), 1.69(s), 1.75(ms),

1.92(ms), 2.1(w), 2.27(w), 2.3(w).

\ G’

\

\ J h \; 1 Furc Cot tonF,bre

(treated 2 Pure Hybrid Sankor

CottonFibre

3 RAwC 0 ,,o%;;,t,ed

L-_L_L_ 1.6 1.6 1.8 2.0 22 2;

LENGTH (urn 1

.)

1.

WAVE 7.6

Fig. 3. PAS absorption spectra of three different cotton fibres.

35

1 American Cotton Fib itrcntedl

Z.EgYpt CottonFibre jtRatld1

10 1.2 I.4 1.6 1.8 1.8 2.0 2.2 2.4 WAYE LENGfH Cm)

2.6

Fig. 4. PAS absorption spectra of two different cotton fibres.

to characterise the cotton fibre. The additional peaks besides the fingerprint absorption peak in cotton fibres may be assigned due to chemical impurities present in these fibres, which leads to differentiation between each fibres.

Cotton-terene fibres Figures 5 and 6 show near-infrared PAS spectra of a particular pure cotton,

terene and different composition of their mixture and bands absorption positions of cotton-terene mixtures represented by Table 2. Quantitative measurements

TABLE 2

PAS ABSORPTION BANDS DATA OF COTTON-TERENE FIBRES

Abbreviations as Table 1.

Sample Details and nature of no. sample

Position of PAS absorption bands in pm

20% cotton, 80% terene

30% cotton, 70% terene

50% cotton, 50% terene

70% cotton, 30% terene

0.08(w), 1.09(w), 1.14(w), 1.37(s), 1.45(ms), 1.52(s), 1.76(w), 1.85(vw), 2.05(mw).

0.88(w), 0.97(vw), l.l(vw), l.l5(vw), 1.21(vw), 1.33(ms), 1.41(w), 1.47(w), 1.52(ms), 1.62(s), 1.7(vw), 1.83(w), 2.12(w), 2.22(w).

0.97(vw), l.l(vw), 1.5(vw), 1.2(vw), 1.32(s), 1.42(s), 1.6(mw), 1.7(w), 1.83(vw), 1.9(w), 2.l(mw).

0.9(w), 1.03(w), l.l(vw), 1.23(vw), 1.5(s), 1.53(ms), 1.6(s), 1.67(ms), 1.82(w), 1.88(w), 2.04(mw), 2.26(mw).

‘LO,

.“‘-“-\ ‘...\ .,.l...., ,’ . .

,“ t ’

: , “// “, I’

! ,a:

#

\ ,‘. : \I :’

,,. . . ,,’ ,.‘.

‘I,. . .._.‘

l.-- CottOnF&e -J___ Terene Fibre

titon,BO% Terene

4. JO)! Cotton, 70% T&me

I 1.0 1.2 1.4 16 WAVE LEN&(um)

uJ-szz3 .

Fig. 5. PAS spectra of cotton, terene H and their mixtures.

I.-- Pure Cotton Fibre 3-4 Pure Terene Fibre

2. 50 X Cotton, 50% TS~UW

Fig. 6. PAS spectra of cotton, terene and their mixtures.

37

TABLE 3

DETERMINATION OF TERENE PERCENTAGE IN COTTON-TERENE MIXTURE

Sample IlO.

1 2 3 4

Self prepared mixture

20% cotton, 80% terene 30% cotton, 70% terene 50% cotton, 50% terene 70% cotton, 30% terene

Photoacoustic method (calculated value on the basis of C-H combination band)

88.6% 80% 64% 41%

were performed for determination of percentage of terene fibre in the mixture by taking the peak height at 1.62 pm arising from methyle group in terene and the height from that of cotton-terene mixture and results were compared with known percentage of terene in the mixture. The numerical results are sum- marised in Table 3. These show that the values obtained are in consonance with the known composition.

Dyed cotton fibres The PAS spectra of dyes and dyed cotton fibres are shown in Figs. 7 and 8

and their absorption data presented in Table 4. The additional absorption peaks in these fibres are contributed by the dyes as confirmed by the PAS spectra of respective dye.

Fig. 7. PAS spectra of dyed cotton fibre (blue) and blue reactive dye H,G.

38

Fig. 8. PAS spectra of dyed cotton fibre (green) and green vat dye XBN.

TABLE 4

PAS ABSORPTION BANDS DATA ON THE DYE AND DYED COTTON FIBRES

Abbreviations as Table 1.

Sample Details and nature Position of PAS absorption bands in pm with their no. of sample related intensity

Reactive dye H,G (Blue)

Dyed cotton fibre (Blue 1.5%)

Green vat dye XBN

Dyed cotton fibre (Green 2%)

1.05(s), 123(vw), 1.55(mw), 1.38(mw), 1.42(mw), 1.46(mw), 1.55(w), 1.63(vw), 1.68(ms), 1.69(ms), 1.77(mw) 1.97(ms), 202(ms), 2.2(ms), 2.42(vw).

l.O(vw), l.O5(vw), 1.22(vw), 1.32(s), 1.42(s), 1.55(w), 1.63(vw), 1.05(vw), 1.22(vw), 1.32(s), 1.42(s), 155(w), 1.63(vw), 1.68(mw), 1.77(ms), 2.0(ms), 2.6(ms), 2.9(ms), 2.4(vw).

0.85(s), 1.33(mw), 1.37(mw), 1.42(mw), 1.5(s), 1.54(w), 1.61(w), 1.68(w), 1.75(w), 1.98(mw).

0.85(s), 0.99(w), 1.1(w), 1.17(w), 1.23(vw), 1.33(ms), 1.41(ms), 1.48(s), 1.54(mw), 1.59)(w), 1.65(mw), 1.79(w), 1.87(w), 1.93(w), 2.1(w).

39

Conclusion

The double beam photoacoustic spectrophotometer described enables the study of absorption spectra in the near-infrared region and may be employed for the examination and forensic characterization of different terene, cotton and additive present in the samples because of low energy electronic transi- tions, overtone bands and combination bands. Quantitative analysis is possi- ble for determining the percentage of terene and cotton and their mixture by this technique. The method given above uses powdered samples; measurements can be made, however, on whole (unground) samples by putting them directly in the sample cell.

Acknowledgment

The authors wishes to thank Dr. R. S. Ram, Scientist ‘C’, NPL, Delhi (India) for assistance rendered during experimentation and Mr. M. R. Kumar, Physics Division, CFSL, CBI, New Dehli for valuable assistance in compiling this paper.

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