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Analytical pyrolysis techniques in forensic science
RE\-IEW
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ASALSTICAL
PYROLl-SIS
TECHXIQUES
IS FORESSIC
SCIESCE
B.B. WHEAIS
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1. ISTRODUCT20S
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ably fair to say that in no other type of laboratory is there a need to a?alyse such a diversity of poIymeric materials with such limited amounts of sample and so littIe background information; pyrolysis methods have been found to be particularly convenient to use and there are now few forensic laboratoties that do not make use of pyrolysis-gas chromatography (Py--GC). Reviews on forensic apphcations have appeared [ 1,2] and in order not to cover sizniIar material in this article I haye attempted to supplement the pub1isIw.l work with unpublished results produced in the Metropolitan Police Laboratory. At the outset of this report it is worth pointing out that a distinction esists between pyrolysis and thermal degradation methods ]3] and although in the past virtually all the methods used in forensic polymer analysis are covered by the first term, in the future a wider range of methods may be UStd
Before es&mining the progress of pyrolysis techniques in forensic an&-s% it is important to appreciate the task that has to be performed_ In general the problem involves the e_xaminationof polymeric materials (or complex hi@ boiling mistures) that occur as contact traces on exhibits submitted to the forensic laboratory by the police -the analyte may occur in the form of flakes, fragments, smears, fibres or particles and is often very small in size_ The forensic scientist has to e-\-aminesuch samples, repoti the fmdings, a?d may also be called upon to interpret the results in an unbiased menner to aid the processes of justice, There are two sets of circumstances that typify the submissions to the laboratory. In the fust the police are seeking information to clarify their criminal investigation; they already have a suspect and need to know Z that suspect can or cannot be linked to a particular crime by scientific evidence. In the second type of situation they are seeking guidance and have no suspect, In the latter instance objects found at the scene of a crime or incident, and thought to be associated in some way with the criminal, are submitted to see if scientific e_xamin ation may provide clues that might ultimateIy aid in the identification of the criminal or at. least provide an insight into the way in which the crime was committed. In the fast type of case exhibits thought to be associated with the scene of the crime and the criminal (e-g- a vehicle believed to have been used in criminal actirizy, or a suspect’s clothing), are sent in for searching and relevant contact traces are compared with control samples from the scene of the crime and hence of known provenance. Thus one type of esamination is essentially qualitative in character whereas the other requires a direct comparison to be performed in addition to a qualitative identification. The compleuify of these forensic exaxninations need hardly be esaggerated for by using the microscope and a few instrumental techniques the scientist is being asked to give an opinion on virkally any natural or synthetic material currently available, In the case of those analytes likely to be subjected to pyrolysis techniques this covers the whole field of surface coatings, fibres, elastomers, adhesives and plastics,
3_ TZCHSIQIXS
280
[c) Highly discriminatory - capable of detecting small differences in copolymers. (d) Rapid - not more than 30 min per analysis, and faster if possible. [e) Applicable to a wide variety of different sample types. If) Inexpensive, (g) Capable of producing date that can be readily stored in a computer for subsequent manipulation and retrieval, Sot surprisingly no such technique e-tits, but it is enlightening to consider the wy in which current methods fail to meet the requirements,
Py-GC using packed columns is a very versatile and powerful tool for forensic analysis but it has its limitations [4,S]_ Thus although it can be rated high in areas (a), (e) and (f), it has drawbacks in areas (b), (c) and (d) and is particularly weak in area (g). .Almost all these disadvantagesstem from the chromatographic stage of the analysis, for not only does this separation impose a timeconsuming step in the analysis but it is in the column that much of the irreproducibility arises that makes computerisation of the analytical data exceedingly difficult. l’ariation in the column due to stationary phase bleed and deterioration, and the build up of high boiling products makes it virtually impossible to accurately reproduce retention times of pyrolysis products over a long period of time - say 2 to 3 months, In addition to this some pyrolysis products of comparatively low mo!ecular weight, high6 polar, are irreversibly bound to the column packing and this rnax seriously weaken the discriminator\- power of the method for some r_vpes of polymer3-3. &-roI~stiapi!Ia~
gas chromatogmph_v
Provided the analyst can master the complexities of coupling a pyrolyser to a capillary GC column [6,7] there appear to be some adwntages to carrying out PJ--GC on a capillary column rather than on a packed column, The poterxial improvements derive from a higher chromatographic resolution and the ability LOelute certain polar pyrolysis products that bind to the support in conventional packed columns, If these predictions are in fact substantiated, and work is currently in progress at a number of forensic laboratories including my own, then for certain special&d areas of application the use of pyrolysis--capillary GC may provide improved discrimination_ However, there seems to be no reason to anticipate that it will be capable of displaying the long term reproducibility that would make the results compatible with a computer, nor would more rapid analyses be available. 3-4. Muitiple stage pymZ~s~‘c--gas chmmatogmph_v Many polymeric samples are plastic&d or contain other relatively volatile components that in normal Fy+X are introduced into the GC column at the same time as the pyrolysis products derived from the polymer- The pres-
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282
complex that it is not always possible to deduce the type of pyrolysis products formed during the thermal degradation step_ Anyone with experience of Py
3_6_ Tnermal gmr:imetv-mess
spectromehy
The coupling of a thermal gravimeter and a mass spectrometer was first described in 1968 [ l’i], but few applications have subsequently appeared_ In 1978 a report was published describing the application of a modified version of this equipment to a limited number of paint and fibre samples of forensic interest [ 181. Analyses were conducted by thermally ramping the samples from ambient to 7OO’C at 50’C.knin in a flow of helium. The products given off were passed continuously into a mass speckometer via a special interface and were either scanned repeatedly over a mass range of 45o--J5 8.m.u. for larger samples (10~SO0 pg) or used in a single ion monitoring mode for smaller samples (ca, 10 M)_ For the smaller samples typical of the weights found in case-work the analytical data are presented in the form of ion current versus temperature profiles. These are found to display characteristic features that enable very similar samples to be discriminated- So data on reproducibility are so far ar-ailable but it would be interesting to compare
This pynlysis iechaique, t4wz1edLUIX-.- a---al~sis,isoh+ fozskj $5: fkom a laser down a microscope on to the azz$tc sbkce [ 19]_ T&e e-?emderived from the lasw pulse pyrolyses the sample in the e-r of focus and produces a microplasma containing ior-s and neutral mokcule~ derked 5ozn both the inorganic and organic constituents of the mp:t_ Tke ~hsza is draw-n MO a time of flight mass spscuomeru and a compkte ma 5pxtrzn (either for poti%-e or negative lo%) cm be prdxec$ for es5 kez pdse_ The procedure has the advantage of yielding inXormat’;onon ail corzti:zer-rs of a sample rasher than just the organic potiioa as 4 the M wi:h EXX other thermal degrzdatios methods. zzd dtsipI-~2 zic ccsz r3Cs cccC5 hs-1 many attractions for forensic a.uAys%- However. ?I ksx5 we kiss-z~~5 wxductecl on painr, glass and synthetic fibre szsgks. t%e rs-_Szs weze ?a~ CET~L producible ta be of value. It nay be possibh? to ib~ra-+ r;pcx r?zz 5~ szxpling the analyte in sewral different axes and a~exgkg tk?ew~;pzr. bx unks the analytical precision can be brought to e~~~p*zble IN& :Er r+%nique fill not be a serious contender_
The txriety of samples met with in forer&c work 1hzsSrrdy xx+3 rz*ztioned and contact traces derived fro= clcrlhkc SC. z.recfwr, I=G:Jna? idsi form for analysis. For comparative work h pa&xlar it k ssa;izJ t.kx: z.he suspect and control samples be in a form that is visuzll~ simCzr b&xv any analysis is attempted and for non-homogeneous ma?*i.als t:hiscan lx~= difficulties. Paint flak=, for-example, often have a rnulr~~~e33;er sx3xzre zzd for comparative sal@s it is important to e==+v eitker a single Jkyer, or if
that is not possible to compare flakes of about the same size and relative layer thickness. Where single layers cannot be separated by hand it is possible to use microtoming procedures [ 21]_ 32,
Paint anal@s
- gene&
considenrtions
The characterisation and comparison of small flakes of paint is an area in which pyrolysis techniques are of considerable L-aluein fore-&c science and to esploit the methods fully it is essential to have some understanding of the chemical differences found in paint [ 221, For convenience it is best to consider paints in two separate sections -decorative gloss paints and vehicle paints -not only is their chemistry quite different but the type of cases in which they arise are also frequently very different. Despite the variations in such surface coatings the aim in forensic Py-GC has been to use chromaiographic conditions that are appropriate to both classes of material as well as other polymeric materials. The choice of packing material and statiomphase is of importance and Porapak Q [ 231, Carbowax 2011 [ 241 and hydrogenated graphitised carbon black coated with OV-101 [25] have found proponents in British forensic laboratories_ At the present time Carbow 20X seems to be the stationary phase most. widely used in the L’.K_ forensic service for Py-GC and the same material has ken used in collaborath-e studies in the I;‘_S_A_ [ 26,271, mainly because of its ability to provide separations of a wide range of polar and non-polar pyrolysis products_ Despite the utility of this material a major limitation is that compounds such as phthalic anhydride and benzoic acid, which are important p_yrolysis products of a.EFdbased decorative gloss paints, are not eluted from columns under normal operating conditions. .A useful feature of p~~lysis-capillarj;pillary GC is that compounds of this type can be eluted but it is still too early to decide on the best conditions to achieve thii, although it is an active area of research in several laboratories. The establiihment of standard&d conditions for Py-GC is highly desirable, but past esperience is discouraging and the range of packing materials and stationary phases is a complicating feature. Perhaps a step in the right direction could be taken by establishing a paint and polymer reference collection for circulation amongst forensic laboratories in several countries; with luck this might lead to sufficient interchange of information to prompt a voluntary move towards standardisation by shov5ng up deficiencies in current systems, although any laboratory that has built up reference data over several years is understandably reluctant ro abandon this1.3. Decmutir-e gloss paints Decorative gloss paints are usually produced from alhTd resins - complex polyesters prepared by reacting a phthalic acid (or anhydride) with natural drying oils - and as a class this type of paint probably poses one of the most difficult areas of p&t analysis, Py-GC of such materials suffers from the drawback that the major pox&-sis products such as phthalic anhydride (in the case of o-phthalic alkyds) and benzoic acid (in the case of isophthalic ak~ds) do not elute under the normal packed column GC conditions which
were established to observe lower mokul2r weight 2nd less poJkx cozipounds_ Curie-point and filament pyrobsers czn & tz& to dacompnse pakt flakes based on alkyd resins 1151 and a Carbowas 2OM column dlo~~ ciiscr%~ination on the basis of two major pyroXysk products - rcrolti azxi zrethacrolein [ 2]_ Becatuz acroiek is a pyrolysis product of &:;cero1 coz&k2_5le care is required to make use of acrolehjmeti~acro~eia ratios for c!iscrk~~tion; contamination of the paint surface with glycericies 5om the s’r--ir: dlurkg sample handling can give rise to misleading results_ Xodification of a.5 alkyd with polymers based on styrene or vin~lroluene are readily dexxxed by PyGC but packed column Py-GC cannot detect diffeences due to ~att-ing ratios of phihalic acid to iso~hthalic acid or fo polyta&xa.xe nxdficziors [28]_ Despite the limitations af h---GC fcr this q-pe of ezza$_&sit wzs found t,o be the most powerful discrimina%or in men tek.niqttes [ 291. a.4 pzforms well with b!ack paints [27]_ _l\swas mentioned ~ZYZZE.g>-ro!>_siscapGC promises to widen the scope of a%yd ptiS ar&ysZs. k? an alternative approach that has already been shown to ‘be *jscfuZB Py-515, z?d thermal gravimetry-mass spectrome-L3_may also be of value [IS]. Using Q-AI5 the resin type prenr in dried paint fLw czz ‘se ~d;:y classif;,ed [SJS], and al&d modifications dsue to the prence of p&yurethanes, vinyltoiuene, styrene ad different rat& of o-~iddic 7-o k~phthalic acids haw been found to produce charact~tc ior3 in the =ZIZW pyrograms 130 J, making Py-MS a most powerful tool for p&t ~x+_5k Kt have also found that Py-MS can make a signi!Eca.n~contribution to rhe d%crimination of alh~_dsthat do not produce major p_vrolysis pz-okrls deiexable by Py-GC and it is probably worth spending a &tic rime d&cuss&g this aspect of Py-MS analysis_ The key to discrimination of simii sam>ks is to produce mass pyrogmm5 that are sezsirive to minor composi:iorA cknges and are highly reproducible_ In practice once ecfxtive pyrolysis is being achieved and an optimum inMace ufilised thee is ve_~ little r%t the analyst can do about the fti area, but instrumental repmdxxibiky is EI aspecr that rewards careful study-. Detailed @-!US reprodudbZi?y stubi~ have been reported [ 15,161 and in the case of alkyd IB&A it has *a found that. ce,rtain ions of the mass p>rogram are formed -more reproducibIy A?L? others (as are dtfferenr p>oly:sis products in e--GCj_ T&us ti luskg pyregrams for discrimination it k imporivlt not to give equal weightic-gto se&c ion, for vatiation in the intensities of irrIxoduciblc ions has far ks sii+Zficance than variation in the inte_nsititsof more reproducible ions_ It is also important to appreciate that the level of pre&ion 2&e\-& ~~~5~ 3 give2 day is usuallp better than that obtained wheu comparing auaIyses made over Iouger periods of elaIxed time, WC had ako asxmei! that kip_m-‘ed rep_ris ducibility would be forthcoming if more mns wsre us& fo caspile t_h composite 111;~ss pyogram [S], and cn this basis quacirrpole rzs s_xt-rameters might be expected to offer ad\_antagesover mag?,etic zstor instr+ merits because of the farker scan time3. Hoxvex-er,in a comperisozi zade of a quadrupole with a magnetic sector instrument in 0-z l~rarwr\- 1301 the latter was found to give more reproducible resulti ihax the former, dihough it has to be appreciated Chat n-e inirohce the p~-rolysate oxz a f&l>- 10~4 period br band~preading in an empty GC cohmn f13]_ -At the present time
286
a magnetic sector instrument is used for Ry-MS and a quadrupole mass spectrometer for GC-MS at this laboratory. During 1979 further work on alkyd paint analysis was carried out in this laboratory and at the Home Office ~Ckntzal Research Establishment Ill]_ The findings were somewhat different for although the latter group only achieved moderate discrimination for a group of ten white household gloss paints, the former found high levels of discrimination. In our experiments nine different white aled paints were examined and ten different fragments (5-10 pg) of each sample were analyzed sequentially, The complete mass pyrogram could be esamined visually and major modifications such as those due to the presence of vinyltoluene and toluene di-isocyanate (polyurethane) uniquely character&d two of the samples, Typical mass pyrograms are shown in Fig. 1, In the absence of major modifications, however, a visual examination of the mass pyrograms becomes a difficult way of characterising alkyds and the way we then approached the problem was to reduce the complesity of the mass pyrogmm by discarding all but the most significant ions. To achieve this we treated the data to determine: (1) The relative standard deviation of ion intensities (for ex-cry ion in each set of ten replicate mass pyrograms). (2) The range of mean ion intensity (for every ion in each sample), (3) The degree of correlation between the data for every possible pair of ions (the mean values for three randomly selected samples wze used for this escercise). The results obtained were then inspected to pick out the 20 ions which displayed the largest differences between samples. The 20 selected ions were then further inspected to eliminate those that were too variable (i-e, relative standard deviation greater than 10%) and to retain only one ion from highly correlated pairs, The final product of this sifting process shovwd that alkyd mass pyrograms could be reduced to give highly discriminatory ions; these together with their most likely source in the pyrolysate were: mA
derived ircm
174 118 105 103
Tolcene di-iwcyanatc Vinyltoluene Benzoic acid
ii
Phthlic
hydride
Cnknown
It will be noticed that the selected ions do not include any arising from the mass spectra of acrolein or methacrolein. The Inference of this is that Py+C and Py-51s are discriminating alhyds on the basis of different pyrolysis products and hence are complementary rather than alternative methods for tackling the task; we hope to test this by conducting a much larger survey of paints by both techniques to directly compare their discriminating power_ The validity of using only five ions for d’wximination was tested by con-
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duding a blind trial in which data for eight p&ms (Le. tie seven @xs Irfz after elimination of the two obviously unique sampk, with ox re$ica~) were selecwd at random from +-hetest results and compaxci ti:h r_h_e maz ion intensities of the seven control szmpks, If dre ion izte_si* of the “X3-
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known” fell within -~2 standard deviations of the mean value of the control it was “‘scored” as a positive, if outside it was ‘kcored” as a negative, A ‘kcore” of five positives was taken to be an indication of identity and by using this approach all eight paint samples were correctly matched with the corresponding control. Thus in the short term at Ieast Fy-MS appears to
In contrast ‘a decoratire gloss paints the r&n q-srenx *us& %r forzxlating vehicle paints ax-2very xxried and rhe cdozr zxi dyer E~XZ-S~ zze 53 often distinctive that. vehicle paint analys5 is a rekiv2Iy sLxp:e zrza SZ foxetEic 2xamination. In a mc2nt study of XXI @x Czkes of 5% q-gte !31] it. was found possible to discriminate 94% of th2 ~~pks by micrczco$c nsdy of 50.3 _w$G> =j-k_g 9 e_xamination and solv2nt. Wsts alone, and a s .1tieiy of techniques enabled an ewn higher pro_xrrtion to .b2chz.ra~zSxi [ 32]_ For complete characterii~ion 2 is *s&y r;sce.sssy ~3 ixkdi- II~c::C;ods of inorganic and organic anelysk in the compkt2 s&em2 [31]_ IX qeco,? =%$-_ troscopy or FySC can readily charac~~k2 ~.Sclc pL,-_c -w< acrylatq polymethacrylat2s and their copol_vmers [26,33] a& I: 51 r.leo ina_KBrcEsc5y IR zpecz~s,u~y been found possible to ~2aczrpzi.n~ and then to dir2ctly pyrolyse the same samp!e to pro\%e &dirlorA Sormztion [ 34]_ ProAi2d good reference colkxions zr2 es-zzbE&ed r?d 2ffs=:%2 retieval methods dseloped 135,361 then the @nt pyrogram aogeth2r wit:? colour and layer structure information can ‘be a guId2 to tb2 t:>e of r21%&2 from which a paint flak2 originat2d f33]_ Top coat com:Stions can -b2325 titularly useful in this reqxct, although car-2m& ‘32 t&2= xo 2;“s~~ zk25 contamination with undercoats thar give r%e ro 2p~reciQ_h;S2k&s of P.\‘To!G &-q-s &2 :it,v =o rLcr”&#::l -L-s ysis products must be avoided -_samp&g analysis. Refinishing paints based on ni’rroce!klose r&ins zre kss zzenzble to Py-GC [2] mainly because they degrade to oxides of tir_mgm, -xa:tr znd
carbon dioxide on pyrolysis and the remain undet2ct&_
either bind to the sck2d
column or
290
W-MS of vehicle paints promises to be a most fruitful area of research, for subtle differences in formulation are detectable and the reproducibi2ity with which the most significant ions are produced is very good [ 151. Ma-&r monomers such as styrene. individual acrylates and methacrylates often have distinctive ions and these are apparent even amongst the complexities of a complete mass pyrogram. Sitrocellulose finishes givea characteristic ion of m/z 30 (SO) and nitrocellulose’alkyd formulations are readily detectable by W-MS whereas this is not the case with Fy-GC. Saferstein and BIZIWZI 1121 obtained useful mass pyrograms on car paints by using chemicaI ionisation to reduce monomer fragmentation of the pyrolysate. 4.5. Aihesites
‘There are many polymer systems used in preparing adhesives, and the analysis of this class of material is often requested in forensic work. Py+C and IR spectroscopy were compared for this type of e.xamination and -were found to be complementary techniques [ 37. Subsequent studies with the same collection of samp!es indicates that rapid adhesive identification is possible using Py-MS [30]. For some types of adhesives the mass p~mgrarn gives more distinctive qualitative information than the corresponding p,\logram or IR spectrum, but in general it would seem that each method has areas where it would be the method of choice. Insufficient work has been done on adhesives to assess the discriminating power of Py-MS against other methods of organic anaIysis_ One class of adhesives that was not esamined in 1974 were the so-called ‘-Super Glues”, materials based on cyanoacrylates. For commercial use the cyanoacrylic monomerpolymer is often mixed with a diluent such as polymethyl methacrylate, and the mass pyrogram of the resulting mated is often dominated b_v contributions from the latter. However, it has been found that in the pre.p>mlrsis equilibration period mentioned in Sectiosr 3.5. cyanoacrylic monomers are drawn off into the mass spectrometer and provide unequivocal confvmation of this type of adhesive. 1.6. Spz thetic fibres Although in the early 19’iO’s it looked as if Py--GC would have a considerable contribution to make to forensic fibre analysis [38,39] the subsequent sparsity of reports suggests that this ear& promise was not fulfiied. Certainly at the Metropolitan Police Laboratory it is IR spectroscopy that. is chiefly used for obtaining qualitative information about fibres, and for di+ crimination much reliance is placed upon the estraction and subsequent thin-layer chromatographic separation of associated dyestuffs. Studies carried out some years ago indicated that the pyrolysis products of many fibres were too polar to pass t’hrough the packed columns in routine use for other polymers [ 21, and this suggests that pyrolysis=pi.llary GC would lx a fruitful area of research if a chromatographic technique is required to pro. vide improved fibre methods. Py-MS has been shown to be very useful for fibre analysis [ll-131 but it
is still too earfy to assess esac%Iy how important it viii! ‘become in this 6&d. In retrospect the work published on the discriiination of spzheric Ebres using Py-MS and the Tit-factor” approach [ 131 was probably to simpkic 0-l . the . -b&s of and it seems quite probable that iif the data we m the scheme described for aILtd r&k then improwd perforzaxs wo~&d -be obtahable. acrylic fibres that were diffhlr X:Q chracreri.x? b_v either Py--GC or b-MS [lS]_ Further work 1=eeSsto !x done in t:his area to cIa5fy the practicaIity of using rhernogra-izerry-z~ss spazrro3etry 22 rhe muiiiple ion detection mode for examining singe fibres. v&ich ofxn kz\.e masses below 10 pg. dscmmnate
foregoing sections ha\-e covered the major areas 22 ~555 p\;rol?;s% techniques are used ;~r forask science, ht 15s me:ko& srs 50 FI~&$- r&icable ad the materials encountered so div+rse that the c?zzzc:&&ticn of toxic herbicides [do], chewing gums 1231, s_moWess powder r&dues ! 41] and bitumens [5] can aii be considered to f&i x2hin the xc?_- of 5% article. Other topics such as the discrizziration of so3 organics, ekxm~: and pIastics in general, the characteriition of &tgs i.z ‘Soi- fi~iZs E.%!:S mdy of variations in biopoiymers are ai! of undo.-ibxd izpxanre 2”-d hz\-;_ been admirably reviewed by Irwin [ l,lO] . The
5. coscLcsIoss
The future of pyroIrsi& technique in forensic s&x+ aP2zz-s to he uz~ promising and W-MS and @--capZIaty GC in particukr seem to offer f,Ecinating opportunities to the anaiysi. Proxrossshg zhe,rr,5y rhe improved and the anaiyt,~ end of the 19SO.s the methods now in use will appear estz~?mei- cr-xit.
6. .icIcsos-LEDcE31EsTs
Much practical work in the area of Py-MS &s I>Nn under%ke.=l at zht Metropolitan Police Laboratory, by Ian Jane and Chzk C-u!!!!-, and in PycapUary GC by Peter GandoIfi. In preparing this pqxr rkse co~es~~es have allow& me to draw freely on their worIi and I would Eke ‘I3 expr- my gratitude for this.
REFERESCES
Viicr,Amsterdam, 1975, p_ 89. T-H. Risby and AL Yergey, Anai. Chem.. 50 (1918) 326X J.S. Crighton, Proc. Ana!, Dir_ Chem. Sot. 14 (1975) 211_ J.C. Hughes, BB. ViheaIs and XJ. Whitehouse. Forensic Sci.. 10 (19X) 215, J.W. de Lecuw, W-L. Maters. D_ FM de Ueent and J_J. Boon. Anal_ Chcm., 49 (1977) 1881. Y-. Sugimura and S. Tsuge, Anal. Chem., 50 (19X) 1968. i J-C_ Hu, Anal, Chem., 49 (19i7) 535, 9 W. Worthy, Chem. Eng. Sews, 58 (19SO) 26. 10 K.J. Iru-ii, J. Anal, Appl. q-rol.. 1 (1979) 3. 11 R.E Ardrey. TX. Batchelor and IC_W_ Smaiidon, Home Offce CenrraI Research Establishment, Aidermaston, Great Britain, Report so. 311, 12 R, Saferstein and J-J_ -Manura, J_ Forensic Sci., 22 (19ii) il& 13 J.C_ Hughes, B_B. WheaLs and XJ. Whitehouse. Analyst (London), 103 (19iS) 482l-2 H.L_C_ Meuzelaar, >I_& Posthumus. P-G. Kistemaker and J. Ristemaker, AnaI. Cherz, -I5 (1973) 1546. 15 DA_ Hickman and I_ Jane, Analyst (Lor.don), 101(1979) 334, 16 H.L.C. Meuzelaar, Proc. ASMS Conf., 26 (19iS) 13. 11 F_ Zitomer. Anal_ Chem.. 40 (1968) 1091, Research Es+&IS RX_ Ardrey, L-R, Mxilings and R_lX_ Smalldon. Home Office Ceatd Iishmeat, Aidermaston. Great Britain, Report Xo_ 274, 19 R, Eaufmann, F. Hilienkamp and R. We&sung. Eur. Speetrasc, Seas, 20 (1976) man, Can, Sot, Forensic Sci, J., 9 (1956) 1, (London), 9s (1973) 23 R-W_ Map, E-F_ Pearson, J, Porter and 3LD_ Scothem. Aadyst 364. B_B. Wheals and Vi. Sob!e. Chromatographfa. 5 [ 19iP) 533. J-B-F. Lloyd. EL &d!ey and B-R-G. RoSe~rts,J_ Chromztopr.. 101 (1974) 417, W-D. Sizewart. J. x65_Offic_ Anal_ Chem.. 69 (1976) 35. B-W. Schurz and T-P. Perrm, J, Ass. Offk Ana!. Chem,, 58 (19X) 1150. CD. Wall, Some Anz!!yticA Studies of Paiit Sarnp!es Encountered in Foretic Science. PhD. Thesis. London University. London. 19Fi_ R-Vi_ May a-?dJ. Porter, J. Forensic Sci. Sot, 13 (1975) 137. z Metropolitan Police Laboratory;, unpublished xrork. 31 S-G_ Ryiand and R-J_ Kojti, J. Forensic Sci, 24 (1979) 143. 32 JA Gorhard, J_ Farensic Sci,, 21(X976) 636, 33 B.B. Wheals and W. Sable. J. Forensic Sri_ !8oc.. 14 (19i4) 23, 34 R-J, Audette and R-FE_ Percy, J. F~remi;c Sci, 23 (197s) 6i2, 35 R-J. Asdette and R.F.E. Percy, J. Foretic Sci.. 24 (1979) 790_ 36 R-J. Audette and R-F_& Percy, J. Foretic Sci, 25 (1980) 189, 37 W_ Sole, BB_ WheaIs and MJ. Whiteho-use, J_ Forensic Sci Sot, 14 (1974) 163. 35 JP. Bortnhk, SE. Brown urd E-H, SiId, J. Fozezsic Sci., 16 (19il) 350. 39 R-C_ Shaler and W-W. Prircharc!, J. Xs. Ofiic, Anal, Chera, 63 (1972) 632, 40 A. Cxuzrd arrd 7X-J.Griddle, _Analysz(London). 100 (1975) 848_ 41 SA SewIon and JL Booker, J. Forensic Sci., 24 (19i9) 85, 3 4 5 6
- _-
ASALSTICAL
PYROLl-SIS
TECHXIQUES
IS FORESSIC
SCIESCE
B.B. WHEAIS
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.............................................. 4. A&irriors ~.l.sr~?:ilg ............................................. 4.2. Pass az?zysis - geze.xi corzsiPc_~-.Iozs ......................... •L_3.~-cr=I~-~g~~3~-~ .................................... ......................................... 4.4. \-&sc!c pti_S ............................................ 4.5. xcix3ive5 ........................................ 4.6. Sy~rbe:Ie Ebrts ............................... 1.~.~~e’~co~~~~!I~~~o=?z.. .............................................. 5_Con~.~olrr ......................................... 6. Ack?.oskigc.~-rz-s RcTtr+zezs ................................................
1. ISTRODUCT20S
..
ably fair to say that in no other type of laboratory is there a need to a?alyse such a diversity of poIymeric materials with such limited amounts of sample and so littIe background information; pyrolysis methods have been found to be particularly convenient to use and there are now few forensic laboratoties that do not make use of pyrolysis-gas chromatography (Py--GC). Reviews on forensic apphcations have appeared [ 1,2] and in order not to cover sizniIar material in this article I haye attempted to supplement the pub1isIw.l work with unpublished results produced in the Metropolitan Police Laboratory. At the outset of this report it is worth pointing out that a distinction esists between pyrolysis and thermal degradation methods ]3] and although in the past virtually all the methods used in forensic polymer analysis are covered by the first term, in the future a wider range of methods may be UStd
Before es&mining the progress of pyrolysis techniques in forensic an&-s% it is important to appreciate the task that has to be performed_ In general the problem involves the e_xaminationof polymeric materials (or complex hi@ boiling mistures) that occur as contact traces on exhibits submitted to the forensic laboratory by the police -the analyte may occur in the form of flakes, fragments, smears, fibres or particles and is often very small in size_ The forensic scientist has to e-\-aminesuch samples, repoti the fmdings, a?d may also be called upon to interpret the results in an unbiased menner to aid the processes of justice, There are two sets of circumstances that typify the submissions to the laboratory. In the fust the police are seeking information to clarify their criminal investigation; they already have a suspect and need to know Z that suspect can or cannot be linked to a particular crime by scientific evidence. In the second type of situation they are seeking guidance and have no suspect, In the latter instance objects found at the scene of a crime or incident, and thought to be associated in some way with the criminal, are submitted to see if scientific e_xamin ation may provide clues that might ultimateIy aid in the identification of the criminal or at. least provide an insight into the way in which the crime was committed. In the fast type of case exhibits thought to be associated with the scene of the crime and the criminal (e-g- a vehicle believed to have been used in criminal actirizy, or a suspect’s clothing), are sent in for searching and relevant contact traces are compared with control samples from the scene of the crime and hence of known provenance. Thus one type of esamination is essentially qualitative in character whereas the other requires a direct comparison to be performed in addition to a qualitative identification. The compleuify of these forensic exaxninations need hardly be esaggerated for by using the microscope and a few instrumental techniques the scientist is being asked to give an opinion on virkally any natural or synthetic material currently available, In the case of those analytes likely to be subjected to pyrolysis techniques this covers the whole field of surface coatings, fibres, elastomers, adhesives and plastics,
3_ TZCHSIQIXS
280
[c) Highly discriminatory - capable of detecting small differences in copolymers. (d) Rapid - not more than 30 min per analysis, and faster if possible. [e) Applicable to a wide variety of different sample types. If) Inexpensive, (g) Capable of producing date that can be readily stored in a computer for subsequent manipulation and retrieval, Sot surprisingly no such technique e-tits, but it is enlightening to consider the wy in which current methods fail to meet the requirements,
Py-GC using packed columns is a very versatile and powerful tool for forensic analysis but it has its limitations [4,S]_ Thus although it can be rated high in areas (a), (e) and (f), it has drawbacks in areas (b), (c) and (d) and is particularly weak in area (g). .Almost all these disadvantagesstem from the chromatographic stage of the analysis, for not only does this separation impose a timeconsuming step in the analysis but it is in the column that much of the irreproducibility arises that makes computerisation of the analytical data exceedingly difficult. l’ariation in the column due to stationary phase bleed and deterioration, and the build up of high boiling products makes it virtually impossible to accurately reproduce retention times of pyrolysis products over a long period of time - say 2 to 3 months, In addition to this some pyrolysis products of comparatively low mo!ecular weight, high6 polar, are irreversibly bound to the column packing and this rnax seriously weaken the discriminator\- power of the method for some r_vpes of polymer3-3. &-roI~stiapi!Ia~
gas chromatogmph_v
Provided the analyst can master the complexities of coupling a pyrolyser to a capillary GC column [6,7] there appear to be some adwntages to carrying out PJ--GC on a capillary column rather than on a packed column, The poterxial improvements derive from a higher chromatographic resolution and the ability LOelute certain polar pyrolysis products that bind to the support in conventional packed columns, If these predictions are in fact substantiated, and work is currently in progress at a number of forensic laboratories including my own, then for certain special&d areas of application the use of pyrolysis--capillary GC may provide improved discrimination_ However, there seems to be no reason to anticipate that it will be capable of displaying the long term reproducibility that would make the results compatible with a computer, nor would more rapid analyses be available. 3-4. Muitiple stage pymZ~s~‘c--gas chmmatogmph_v Many polymeric samples are plastic&d or contain other relatively volatile components that in normal Fy+X are introduced into the GC column at the same time as the pyrolysis products derived from the polymer- The pres-
n;’ -_-
282
complex that it is not always possible to deduce the type of pyrolysis products formed during the thermal degradation step_ Anyone with experience of Py
3_6_ Tnermal gmr:imetv-mess
spectromehy
The coupling of a thermal gravimeter and a mass spectrometer was first described in 1968 [ l’i], but few applications have subsequently appeared_ In 1978 a report was published describing the application of a modified version of this equipment to a limited number of paint and fibre samples of forensic interest [ 181. Analyses were conducted by thermally ramping the samples from ambient to 7OO’C at 50’C.knin in a flow of helium. The products given off were passed continuously into a mass speckometer via a special interface and were either scanned repeatedly over a mass range of 45o--J5 8.m.u. for larger samples (10~SO0 pg) or used in a single ion monitoring mode for smaller samples (ca, 10 M)_ For the smaller samples typical of the weights found in case-work the analytical data are presented in the form of ion current versus temperature profiles. These are found to display characteristic features that enable very similar samples to be discriminated- So data on reproducibility are so far ar-ailable but it would be interesting to compare
This pynlysis iechaique, t4wz1edLUIX-.- a---al~sis,isoh+ fozskj $5: fkom a laser down a microscope on to the azz$tc sbkce [ 19]_ T&e e-?emderived from the lasw pulse pyrolyses the sample in the e-r of focus and produces a microplasma containing ior-s and neutral mokcule~ derked 5ozn both the inorganic and organic constituents of the mp:t_ Tke ~hsza is draw-n MO a time of flight mass spscuomeru and a compkte ma 5pxtrzn (either for poti%-e or negative lo%) cm be prdxec$ for es5 kez pdse_ The procedure has the advantage of yielding inXormat’;onon ail corzti:zer-rs of a sample rasher than just the organic potiioa as 4 the M wi:h EXX other thermal degrzdatios methods. zzd dtsipI-~2 zic ccsz r3Cs cccC5 hs-1 many attractions for forensic a.uAys%- However. ?I ksx5 we kiss-z~~5 wxductecl on painr, glass and synthetic fibre szsgks. t%e rs-_Szs weze ?a~ CET~L producible ta be of value. It nay be possibh? to ib~ra-+ r;pcx r?zz 5~ szxpling the analyte in sewral different axes and a~exgkg tk?ew~;pzr. bx unks the analytical precision can be brought to e~~~p*zble IN& :Er r+%nique fill not be a serious contender_
The txriety of samples met with in forer&c work 1hzsSrrdy xx+3 rz*ztioned and contact traces derived fro= clcrlhkc SC. z.recfwr, I=G:Jna? idsi form for analysis. For comparative work h pa&xlar it k ssa;izJ t.kx: z.he suspect and control samples be in a form that is visuzll~ simCzr b&xv any analysis is attempted and for non-homogeneous ma?*i.als t:hiscan lx~= difficulties. Paint flak=, for-example, often have a rnulr~~~e33;er sx3xzre zzd for comparative sal@s it is important to e==+v eitker a single Jkyer, or if
that is not possible to compare flakes of about the same size and relative layer thickness. Where single layers cannot be separated by hand it is possible to use microtoming procedures [ 21]_ 32,
Paint anal@s
- gene&
considenrtions
The characterisation and comparison of small flakes of paint is an area in which pyrolysis techniques are of considerable L-aluein fore-&c science and to esploit the methods fully it is essential to have some understanding of the chemical differences found in paint [ 221, For convenience it is best to consider paints in two separate sections -decorative gloss paints and vehicle paints -not only is their chemistry quite different but the type of cases in which they arise are also frequently very different. Despite the variations in such surface coatings the aim in forensic Py-GC has been to use chromaiographic conditions that are appropriate to both classes of material as well as other polymeric materials. The choice of packing material and statiomphase is of importance and Porapak Q [ 231, Carbowax 2011 [ 241 and hydrogenated graphitised carbon black coated with OV-101 [25] have found proponents in British forensic laboratories_ At the present time Carbow 20X seems to be the stationary phase most. widely used in the L’.K_ forensic service for Py-GC and the same material has ken used in collaborath-e studies in the I;‘_S_A_ [ 26,271, mainly because of its ability to provide separations of a wide range of polar and non-polar pyrolysis products_ Despite the utility of this material a major limitation is that compounds such as phthalic anhydride and benzoic acid, which are important p_yrolysis products of a.EFdbased decorative gloss paints, are not eluted from columns under normal operating conditions. .A useful feature of p~~lysis-capillarj;pillary GC is that compounds of this type can be eluted but it is still too early to decide on the best conditions to achieve thii, although it is an active area of research in several laboratories. The establiihment of standard&d conditions for Py-GC is highly desirable, but past esperience is discouraging and the range of packing materials and stationary phases is a complicating feature. Perhaps a step in the right direction could be taken by establishing a paint and polymer reference collection for circulation amongst forensic laboratories in several countries; with luck this might lead to sufficient interchange of information to prompt a voluntary move towards standardisation by shov5ng up deficiencies in current systems, although any laboratory that has built up reference data over several years is understandably reluctant ro abandon this1.3. Decmutir-e gloss paints Decorative gloss paints are usually produced from alhTd resins - complex polyesters prepared by reacting a phthalic acid (or anhydride) with natural drying oils - and as a class this type of paint probably poses one of the most difficult areas of p&t analysis, Py-GC of such materials suffers from the drawback that the major pox&-sis products such as phthalic anhydride (in the case of o-phthalic alkyds) and benzoic acid (in the case of isophthalic ak~ds) do not elute under the normal packed column GC conditions which
were established to observe lower mokul2r weight 2nd less poJkx cozipounds_ Curie-point and filament pyrobsers czn & tz& to dacompnse pakt flakes based on alkyd resins 1151 and a Carbowas 2OM column dlo~~ ciiscr%~ination on the basis of two major pyroXysk products - rcrolti azxi zrethacrolein [ 2]_ Becatuz acroiek is a pyrolysis product of &:;cero1 coz&k2_5le care is required to make use of acrolehjmeti~acro~eia ratios for c!iscrk~~tion; contamination of the paint surface with glycericies 5om the s’r--ir: dlurkg sample handling can give rise to misleading results_ Xodification of a.5 alkyd with polymers based on styrene or vin~lroluene are readily dexxxed by PyGC but packed column Py-GC cannot detect diffeences due to ~att-ing ratios of phihalic acid to iso~hthalic acid or fo polyta&xa.xe nxdficziors [28]_ Despite the limitations af h---GC fcr this q-pe of ezza$_&sit wzs found t,o be the most powerful discrimina%or in men tek.niqttes [ 291. a.4 pzforms well with b!ack paints [27]_ _l\swas mentioned ~ZYZZE.g>-ro!>_siscapGC promises to widen the scope of a%yd ptiS ar&ysZs. k? an alternative approach that has already been shown to ‘be *jscfuZB Py-515, z?d thermal gravimetry-mass spectrome-L3_may also be of value [IS]. Using Q-AI5 the resin type prenr in dried paint fLw czz ‘se ~d;:y classif;,ed [SJS], and al&d modifications dsue to the prence of p&yurethanes, vinyltoiuene, styrene ad different rat& of o-~iddic 7-o k~phthalic acids haw been found to produce charact~tc ior3 in the =ZIZW pyrograms 130 J, making Py-MS a most powerful tool for p&t ~x+_5k Kt have also found that Py-MS can make a signi!Eca.n~contribution to rhe d%crimination of alh~_dsthat do not produce major p_vrolysis pz-okrls deiexable by Py-GC and it is probably worth spending a &tic rime d&cuss&g this aspect of Py-MS analysis_ The key to discrimination of simii sam>ks is to produce mass pyrogmm5 that are sezsirive to minor composi:iorA cknges and are highly reproducible_ In practice once ecfxtive pyrolysis is being achieved and an optimum inMace ufilised thee is ve_~ little r%t the analyst can do about the fti area, but instrumental repmdxxibiky is EI aspecr that rewards careful study-. Detailed @-!US reprodudbZi?y stubi~ have been reported [ 15,161 and in the case of alkyd IB&A it has *a found that. ce,rtain ions of the mass p>rogram are formed -more reproducibIy A?L? others (as are dtfferenr p>oly:sis products in e--GCj_ T&us ti luskg pyregrams for discrimination it k imporivlt not to give equal weightic-gto se&c ion, for vatiation in the intensities of irrIxoduciblc ions has far ks sii+Zficance than variation in the inte_nsititsof more reproducible ions_ It is also important to appreciate that the level of pre&ion 2&e\-& ~~~5~ 3 give2 day is usuallp better than that obtained wheu comparing auaIyses made over Iouger periods of elaIxed time, WC had ako asxmei! that kip_m-‘ed rep_ris ducibility would be forthcoming if more mns wsre us& fo caspile t_h composite 111;~ss pyogram [S], and cn this basis quacirrpole rzs s_xt-rameters might be expected to offer ad\_antagesover mag?,etic zstor instr+ merits because of the farker scan time3. Hoxvex-er,in a comperisozi zade of a quadrupole with a magnetic sector instrument in 0-z l~rarwr\- 1301 the latter was found to give more reproducible resulti ihax the former, dihough it has to be appreciated Chat n-e inirohce the p~-rolysate oxz a f&l>- 10~4 period br band~preading in an empty GC cohmn f13]_ -At the present time
286
a magnetic sector instrument is used for Ry-MS and a quadrupole mass spectrometer for GC-MS at this laboratory. During 1979 further work on alkyd paint analysis was carried out in this laboratory and at the Home Office ~Ckntzal Research Establishment Ill]_ The findings were somewhat different for although the latter group only achieved moderate discrimination for a group of ten white household gloss paints, the former found high levels of discrimination. In our experiments nine different white aled paints were examined and ten different fragments (5-10 pg) of each sample were analyzed sequentially, The complete mass pyrogram could be esamined visually and major modifications such as those due to the presence of vinyltoluene and toluene di-isocyanate (polyurethane) uniquely character&d two of the samples, Typical mass pyrograms are shown in Fig. 1, In the absence of major modifications, however, a visual examination of the mass pyrograms becomes a difficult way of characterising alkyds and the way we then approached the problem was to reduce the complesity of the mass pyrogmm by discarding all but the most significant ions. To achieve this we treated the data to determine: (1) The relative standard deviation of ion intensities (for ex-cry ion in each set of ten replicate mass pyrograms). (2) The range of mean ion intensity (for every ion in each sample), (3) The degree of correlation between the data for every possible pair of ions (the mean values for three randomly selected samples wze used for this escercise). The results obtained were then inspected to pick out the 20 ions which displayed the largest differences between samples. The 20 selected ions were then further inspected to eliminate those that were too variable (i-e, relative standard deviation greater than 10%) and to retain only one ion from highly correlated pairs, The final product of this sifting process shovwd that alkyd mass pyrograms could be reduced to give highly discriminatory ions; these together with their most likely source in the pyrolysate were: mA
derived ircm
174 118 105 103
Tolcene di-iwcyanatc Vinyltoluene Benzoic acid
ii
Phthlic
hydride
Cnknown
It will be noticed that the selected ions do not include any arising from the mass spectra of acrolein or methacrolein. The Inference of this is that Py+C and Py-51s are discriminating alhyds on the basis of different pyrolysis products and hence are complementary rather than alternative methods for tackling the task; we hope to test this by conducting a much larger survey of paints by both techniques to directly compare their discriminating power_ The validity of using only five ions for d’wximination was tested by con-
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duding a blind trial in which data for eight p&ms (Le. tie seven @xs Irfz after elimination of the two obviously unique sampk, with ox re$ica~) were selecwd at random from +-hetest results and compaxci ti:h r_h_e maz ion intensities of the seven control szmpks, If dre ion izte_si* of the “X3-
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4.5
and 6 are alI o-phthak
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known” fell within -~2 standard deviations of the mean value of the control it was “‘scored” as a positive, if outside it was ‘kcored” as a negative, A ‘kcore” of five positives was taken to be an indication of identity and by using this approach all eight paint samples were correctly matched with the corresponding control. Thus in the short term at Ieast Fy-MS appears to
In contrast ‘a decoratire gloss paints the r&n q-srenx *us& %r forzxlating vehicle paints ax-2very xxried and rhe cdozr zxi dyer E~XZ-S~ zze 53 often distinctive that. vehicle paint analys5 is a rekiv2Iy sLxp:e zrza SZ foxetEic 2xamination. In a mc2nt study of XXI @x Czkes of 5% q-gte !31] it. was found possible to discriminate 94% of th2 ~~pks by micrczco$c nsdy of 50.3 _w$G> =j-k_g 9 e_xamination and solv2nt. Wsts alone, and a s .1tieiy of techniques enabled an ewn higher pro_xrrtion to .b2chz.ra~zSxi [ 32]_ For complete characterii~ion 2 is *s&y r;sce.sssy ~3 ixkdi- II~c::C;ods of inorganic and organic anelysk in the compkt2 s&em2 [31]_ IX qeco,? =%$-_ troscopy or FySC can readily charac~~k2 ~.Sclc pL,-_c -w< acrylatq polymethacrylat2s and their copol_vmers [26,33] a& I: 51 r.leo ina_KBrcEsc5y IR zpecz~s,u~y been found possible to ~2aczrpzi.n~ and then to dir2ctly pyrolyse the same samp!e to pro\%e &dirlorA Sormztion [ 34]_ ProAi2d good reference colkxions zr2 es-zzbE&ed r?d 2ffs=:%2 retieval methods dseloped 135,361 then the @nt pyrogram aogeth2r wit:? colour and layer structure information can ‘be a guId2 to tb2 t:>e of r21%&2 from which a paint flak2 originat2d f33]_ Top coat com:Stions can -b2325 titularly useful in this reqxct, although car-2m& ‘32 t&2= xo 2;“s~~ zk25 contamination with undercoats thar give r%e ro 2p~reciQ_h;S2k&s of P.\‘To!G &-q-s &2 :it,v =o rLcr”&#::l -L-s ysis products must be avoided -_samp&g analysis. Refinishing paints based on ni’rroce!klose r&ins zre kss zzenzble to Py-GC [2] mainly because they degrade to oxides of tir_mgm, -xa:tr znd
carbon dioxide on pyrolysis and the remain undet2ct&_
either bind to the sck2d
column or
290
W-MS of vehicle paints promises to be a most fruitful area of research, for subtle differences in formulation are detectable and the reproducibi2ity with which the most significant ions are produced is very good [ 151. Ma-&r monomers such as styrene. individual acrylates and methacrylates often have distinctive ions and these are apparent even amongst the complexities of a complete mass pyrogram. Sitrocellulose finishes givea characteristic ion of m/z 30 (SO) and nitrocellulose’alkyd formulations are readily detectable by W-MS whereas this is not the case with Fy-GC. Saferstein and BIZIWZI 1121 obtained useful mass pyrograms on car paints by using chemicaI ionisation to reduce monomer fragmentation of the pyrolysate. 4.5. Aihesites
‘There are many polymer systems used in preparing adhesives, and the analysis of this class of material is often requested in forensic work. Py+C and IR spectroscopy were compared for this type of e.xamination and -were found to be complementary techniques [ 37. Subsequent studies with the same collection of samp!es indicates that rapid adhesive identification is possible using Py-MS [30]. For some types of adhesives the mass p~mgrarn gives more distinctive qualitative information than the corresponding p,\logram or IR spectrum, but in general it would seem that each method has areas where it would be the method of choice. Insufficient work has been done on adhesives to assess the discriminating power of Py-MS against other methods of organic anaIysis_ One class of adhesives that was not esamined in 1974 were the so-called ‘-Super Glues”, materials based on cyanoacrylates. For commercial use the cyanoacrylic monomerpolymer is often mixed with a diluent such as polymethyl methacrylate, and the mass pyrogram of the resulting mated is often dominated b_v contributions from the latter. However, it has been found that in the pre.p>mlrsis equilibration period mentioned in Sectiosr 3.5. cyanoacrylic monomers are drawn off into the mass spectrometer and provide unequivocal confvmation of this type of adhesive. 1.6. Spz thetic fibres Although in the early 19’iO’s it looked as if Py--GC would have a considerable contribution to make to forensic fibre analysis [38,39] the subsequent sparsity of reports suggests that this ear& promise was not fulfiied. Certainly at the Metropolitan Police Laboratory it is IR spectroscopy that. is chiefly used for obtaining qualitative information about fibres, and for di+ crimination much reliance is placed upon the estraction and subsequent thin-layer chromatographic separation of associated dyestuffs. Studies carried out some years ago indicated that the pyrolysis products of many fibres were too polar to pass t’hrough the packed columns in routine use for other polymers [ 21, and this suggests that pyrolysis=pi.llary GC would lx a fruitful area of research if a chromatographic technique is required to pro. vide improved fibre methods. Py-MS has been shown to be very useful for fibre analysis [ll-131 but it
is still too earfy to assess esac%Iy how important it viii! ‘become in this 6&d. In retrospect the work published on the discriiination of spzheric Ebres using Py-MS and the Tit-factor” approach [ 131 was probably to simpkic 0-l . the . -b&s of and it seems quite probable that iif the data we m the scheme described for aILtd r&k then improwd perforzaxs wo~&d -be obtahable. acrylic fibres that were diffhlr X:Q chracreri.x? b_v either Py--GC or b-MS [lS]_ Further work 1=eeSsto !x done in t:his area to cIa5fy the practicaIity of using rhernogra-izerry-z~ss spazrro3etry 22 rhe muiiiple ion detection mode for examining singe fibres. v&ich ofxn kz\.e masses below 10 pg. dscmmnate
foregoing sections ha\-e covered the major areas 22 ~555 p\;rol?;s% techniques are used ;~r forask science, ht 15s me:ko& srs 50 FI~&$- r&icable ad the materials encountered so div+rse that the c?zzzc:&&ticn of toxic herbicides [do], chewing gums 1231, s_moWess powder r&dues ! 41] and bitumens [5] can aii be considered to f&i x2hin the xc?_- of 5% article. Other topics such as the discrizziration of so3 organics, ekxm~: and pIastics in general, the characteriition of &tgs i.z ‘Soi- fi~iZs E.%!:S mdy of variations in biopoiymers are ai! of undo.-ibxd izpxanre 2”-d hz\-;_ been admirably reviewed by Irwin [ l,lO] . The
5. coscLcsIoss
The future of pyroIrsi& technique in forensic s&x+ aP2zz-s to he uz~ promising and W-MS and @--capZIaty GC in particukr seem to offer f,Ecinating opportunities to the anaiysi. Prox
6. .icIcsos-LEDcE31EsTs
Much practical work in the area of Py-MS &s I>Nn under%ke.=l at zht Metropolitan Police Laboratory, by Ian Jane and Chzk C-u!!!!-, and in PycapUary GC by Peter GandoIfi. In preparing this pqxr rkse co~es~~es have allow& me to draw freely on their worIi and I would Eke ‘I3 expr- my gratitude for this.
REFERESCES
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