Pyrolysis—gas chromatography—mass spectrometry of a low organic matter calcareous soil

PYROLYSIS-GAS OF A LOW

CHRO~l4TOGRAPHl--,l4SS

ORGANC

51. G.L\SSIOT-MATAS

~14Tl.ER

CALCAREOUS

SE’ECTROMETRY SOIL

l

ISTRODCCTIOS

The compkxiry of the soil organic matter t0.M.~. due 10 the xvi& diversit- of components and their p&-mcric nature. greatly limits the dire:t application of analytical. chromstogaphic 2nd sptciros:qk techniques for identification. There are LWDmain swategies fcr the spproxh 10 rhe srud- of hum& j1.21: (a) Extraction of part of the 0.11. and separation into ..homogtnsxs.. fractions. to which convcnrional an;llyrical techniques are spplitd. lb) Controlled degradation (physical or chcmicsl~ of lhe originA 0.X. and analysis of the fra_ements by xzrious techniques. Both these methods have obvious limirstions. due to tic altcrztians rhe\-

c’aust in the original O.Sl.: havswr. they have permitted the rstabiishmrnt of the h&c ~haraacristics wmmon to humic subst;lnces of different ori@ns.

Arn~ng methods of an;l&tial nxayxph>(Py-GCr is one of xtcrizaion of m;lcr~~mJrs-uks. praiucs in ;1n inert 3:maphcre rcpr;tteJ b> CC. The misture

thermal fragmentation. pyrolysis-pas chrothe most frequently emplq-cd in the charRapid thermal dwompaition of p&_\-rncIx partly tdstilt frqments which can bt volatile fr+mcnts is reflected in ths

of

pyogmm

which nzs! be u?;sd 3s 3 chxactcristic fingxprint to differenriats ths m;ltcrirtIs. I&ntifisatiL~n of the frapments by gas chr~matognphy-mass spxtrLbrnctr?_ [GC-1\1S1 pra-ides evidence r>f the composition of the oripinx’l mxr0m0kuIt. The diverse biwhsmical slppiicati~ns oi Py-GC hsve bwn reviewed by Irwin and Slwk [3]. S3g3r [a] ~-3s one oi the first to rlppI~- I?_-GC for the investigation of the po.ssiblc origin of humus from Iignin or from 3 microbia! origin. N.ershaw and Bohner [_‘I used this method tc) stud_\- fuhic acids (F-4) ami humk acids tH.41: the_\- idsntifisd several c~mp+xwnts and easbIished the probable carbohydrae origin of the furan ring timber and Sesrk [6j anai_crcd the infknse of v3rious f3ctors on the prcxhxrion of benzene 2nd to!uene. 3nd bkxtin [7-F] cx3mined the low-boiling-point compounds froa the py+-sis of several humis substances. The dircxt p?--GC ~1f soi1 s3mpIcs petits differentiation bstxetn m3.x gn:upr L>fhumus I muI1. rnorf 3nd _etn&z h...rirL~n~. on thr b3& oi est3bIiA in_r py+-tic relatianj bttw-een differential peaks [9- 121. \vhich an be LYE gret use in 41 sun-e?. with mass 0 nc of the relstcd tcchr.iques is pyr+-sis in direct combination spcctromstet i?_- MS,. in which the nxws sprstrum is used in 3 simiI3r u-3>t\l diffexntirite bttwen humus rind nzzttrizls frL>m different origins [I>- 15:. This t~zhniqt. quisksr than Py-GC. pwvides 3n initi31 ids3 of the 0.M. type. but it hs 3 dr;t~vbs~k in the diffkulty &Ifintcrpreta:ion oi rhs wmpkx m;?ss -\ptitr3 obt3insJ. 3IthLwgh it is pessibk. in some cases. to establish the kgin c>f wme c)f the ions [ 16 19i. .A pat 3dv3nt;lgx ttrf P?_-GC is th3t it is not nr’ce_ss3ry tr) m3kc 3 previas estrr?ai\?n Lb,fsoil cvgsnk m3tter. thus side-steppins ths problems such 3n sstrxtic~n would imply. Ths 3im Llf this pstper ;‘a~ been to dsvci~p 3 Py-GC-MS method for the study oi ;f soil low in O.,\f. and with 3 h&h erbonrttc content. for \vhich the wpr~xkxibiiity ~4 the technique usu311~ presents some probkms.

showing ccn3in fersi3Iytic chsr3ctcrisanril>zed. They were t3ktn 3t three

depths {upper level G-3 cm. mean level IO-12 cm. lou-est level 30-22 cm). corresponding to an A1 new horizon (G-3 cm’, and 3 II .A? !‘3-3G cm). Samples H-ere taken monthly over almost three years. and fourteen of these u-tre used. corresponding to major variations in the content of o.xidizable orgnic matter. The>- have a high carbonate content i Z! .2-X.85 L pH ( H -0 ) S-0: 0.U. caxten~ of X7- 1.1!S and C/S ratios btt\vetn 9.7 and 12.6. Texture; clay loam to sand_\- clay losm. X < Z-mm wil fraction. graunJ tl s powder until it would pass through a 5%-urn -mesh. wzs sn;ll_\-ze3 in tzxh csise. The H.4. FA and humin fractions were also analyz& so as to cornpzwe their pxro_m with those of the original soil. For the cxtrzction a mixture of 0.1 .\I Sa,P,O,-G-1 .Y XaOH was used [Xj: soil-extrzctant ratio l_:‘ZG. for 23 h at room temperature. The 0.M. fraction. once extracted. was separated bv centrifugxtion at 15.GG0 g for 10 min. The residue wzs recovered. w&hed and allowed to dr\-. thtn pyrolyzed as with the soil samples.

.A soli& pyrol>xer. C.D.S. Pyoprobt 190. with platinum-coIi pr&t 2nd x 3 mm quartz -ample hol&x~ ~-3s used. The pr&s ~\-a5 s\?upkd dirc<:ly to the injector of a Pcrkin-Elmer 990 ,ea chr~~rn~w~rzph with ~?sme iclniation detsotor. Some XXI cm x 2 mm ~t;linl&+ateeI &umns x-;ere u?;cil. packed with SCt Carbowzx 2.011 on Gas-Chrom Q I !W-129 mesh!. GC conditions: carrier g=. helium 31 13 cm3 min- I: pragrrm.msJ tsrnpsr~tux 3 min at 6G’C then raised at S’C min-’ :O IX’C. Tim! rims 3 min. 3

Poor reproducibilit_\- of p_\-rolysis is the mAin drawbxk of this tsshnique. ;fs it depends on 3 number of factors [Zl]. The repradudbility of p_\T~gxn?; depends to 3 large extent on the soil sxmpk disposition insi& the .Dvro!ysis . tubes because it is quite impossible to put consecxi\-e empks in an identical position inside the quartz tube. Aforso\-sr. the plzcemsnt of tht sample must be made with re_gsrd to the str\mttry and &$gn of the p\-rol>zer. To ensure ;z uniform distribution of the At sample inside the pyrolysis probe. ;1~ a film sdherins to the inner ~-rl!Is of the gurrr-2 tube. tht following procedure u-35 cmployd.

:44

i Be: .".'e :;-

.

i

F-#

R.,,:aro.-xdB'E:'-Z~ • .

_

. .".~----..

A s u s p e n s i o n o f soil in '``,-ater ~I0"~. w.. v ) ,,,,'as p r e p a r e d by m e a n s o f a ,.-,., ,at,. rx e m u l s i f i e r ~Tria,:re,,ler. Ystral: × 1020~. stirring for 60 Imm e d i a t e l y afterwards, a ! i c u o t p o r t i o . ~ o f 1 0 - 3 0 .ul o f s u s p e n s i o n '`,.-ere t a k e n with a F i n p i p e t t e F P - I I. whi,:h is readiiv e m p t i e d i n t o the q u a r t z tube. A n e w s y s t e m for r o t a t o r y drs.'ing `,`,'as d e v i s e d to m a k e the s a m p l e a d h e r e to the walls o f the t u b e t see F i ~ 1 ). This a p p a r a t u s , l o a d e d w i t h t h e pyrolysis tubes, is p l a c e d i n s i d e an o v e n at 90=C. ,,'`'here it r e m a i n s for 3 h. so as to e v a p o r a t e ~he v,'ater. N o d e c o m p o s i t i o n '`,,'as o b s e r v e d w i t h r e s p ~ t to t h e direct i n t r o d u c t i o n o f soil samples. T h e t u b e s are t h e n w e i g h e d , to d e t e r m i n e the a m o u n t o f s a m p ! e i n t r o d u c e d . T h e c o n c e n t r a t i o n o f the s u s p e n s i o n a n d the v o l u m e to be t a k e n '.,.-ill b e f u n c t i o n o f the p e r c e n t a g e o f o x i d i z a b l e O..',I. in each soil. l--~

"v'--

•

"

-

.

•

Pvr~d"s.s co,::liriot:s

A f t e r a pre'`'ious s t a n d a r d i z a t i o n w e e m p l o y e d the f o l l o w i n g pyrolysis c o n d i t i o n s , b a s e d o n theoretical i n s t r u m e n t a l p a r a m e t e r s as it is ,drtua!ly i m p o s s i b l e to knov," the exact c o n d i t i o n s : pyrolysis t e m p e r a t u r e 700"-C. t i m e !0 ,, h e a t i n g r-ate t 1 0 : C s - : ~. s a m p l e a m o u n t s e q u i v a l e n t to 150 .~g o f O.M. Several tests ,,,,'ere carried o u t with s o m e i n o r g a n i c c o m p o u n d s , espeeialiy C a C O s. a n d soil s a m p l e matrices t O..M. r e m o v e d ) . In e'`erv case a small .-ignal was o b s e r v e d , the origin o f '`'`hich was difficult to p i n p o i n t . H o w e v e r . its .-ize was m : _ h m b , e ,,'`ith respect to t h e signal f r o m the p y r o g r a m s o f soil samp!~.

L L

E t

!

4.2

a

1

,;

:,

,j_iv

a_

/

ic

Y.-

1

_ I-, -

.\

e

L t

!

_

I

.

‘---c_-~

i

1

1

A_.-’

. --.

.

_

.

_----_.-_

:I

._ _

-1

-_

.__

._ -_

_ ..--_

-

._.--_-

-I

] I

-.

.

.

The Iqc number of fagmats obt;?ixd b_\- p!raiysIs of ti?t ON. $v-ss rise to p_\-rognms c0nzstinir.g many overlqpin_e peaks 2nd ~0 their qxx&Zcation presents a number of problems. \\‘e ha-c mainly considered peaks cxreqoxiing to tk zone of I)T~G’JSS with medium and high retention times. since these coztzin the largest frqncnts which are most significant in the soi! O.M. The zone of the first 2- 3 min of the p>-rogrLm contains the srnr?lIsst 2nd mat volatile frs_gments. which ;?re difficult to inrcyet. both from the ir rr.czniqo ;p -rogrdm ci Fig. 2. RE!X-LTS

ASD

DISCCSSIOS

pyrol!sis. to direct

using the prxsdurs dsssribed. ir.crsr?s;ss soil deposition inside the quartz tube.

.

. .

•, ~

.

.

,.e".

-~~ '

~

.

.

°

~

.

.

',~.

.

,u,".

~-".

.

~

~

•"~.. ~. '-~'.. ~".. ,C..-.~. ~. ~ :

~.

~.

~.

m

~

~'~ ~ " . ~

_

-.;

.-~

•

--

-

P ~

m

:

.

-'--:

.

~

.~-~

.

1

°

~.

.

.

•

:

°

~-

.

.

~-

;

--~.

'~"

.

w

m

m

,

w

~

p

v

~e J

,~

.~-

Table 1 shou-s comparative results for the areas of four peaks \S. L. K and J in Fig 2): the ccxfficient of variation. defined 25 i a,. ‘s :I - 100. retlests errors from the u-hole process. including errors in soil preparation. chromztopnphic variations and pe3k measurements. The reduction of the cwffkient of variation to 11.X is to 3 lx_ee extent due to the uniform distribution of the soil sample inside the quartz tube. with rtipect to the thermal graiisnt from the platinum coil. through the quartz tube to the &I ample_ Different for each error levels for each pe3k su_qest 2 different pyrolysis me&mism substance,

The application of fi--GC to soil samples and diverse organic material?; makes it possible to detect significant differences among them through their which 3re considered 3s “fingerprints”. However. bsforc propyrograms. cetdin_e to 3 soil ~ypificaticn by means of this technique_ it is necesz? to ascertain the extent of pyrogam variation within horizons. within profiles and with _xtmpling time. particularly with soils of 10x 0.11. content and youn_e soils in which fluctuations of the 031, content are much gerrter thsln in clinxicic or mature soils. The aspect of sampling time variation w-ill be dkus?;cd in a future paper. Here we discuss the p>rol>sis products_ campare wi! p!rogrzrns Cth rh\wz of their e.xtr;Zcts and examine the depth vrsriation. The identification of p_\-rolysis pr~tius~s. In Fig. 2. u-:s ,~rrisd out by GC-MS and confirmed by GC u-hen a;lndxdb w-err’ riv~.il&ls_ 11~~ L:f the identified fra_ements coincide with :hose cited in the bibliography 3~ iIr’gadstion pr~oducts of 0.M. b_\-this or simikr methods !5_Y.9.lU.l6!. wnfirming that O.!+I. from different soils h--1G cornman genera! c’hlir;?i’ttristiS3_ On the other hand. the overlappin_e of pe3ks makes it diffiah XY&x;in slertr mass spectra of some p>-rol?-sis fragnenw sxh ;1~ perrks X. E. H_ lvhish correspond to two or more substances. These rnzy LC 3 large estsnt be resolved bx p\--capiIl=ir?; GC-!US f IO]_ Besides the major fra_ements. there are quite ;t number of sma!lcr frsgmats indicative of chemical diversity and r?f differenrisrl 0.M. chsr-xrcri+ tics which may be ;15 important zu the !;lr_ee ps&s fcv differenrkting soil humus types_ Under the experiments1 conditions employed the more thsmxAl_\- stable cyclic fragments are well sparated while the tlliphatic side chain?; of the uomatic nuclei are not well detected. because the alkyl chains ;?re easily broken by pyrolysis into small fra_ements \vhich appear during the first few minutes of the pyro_gam. Polycarbos_vliL - acids can. how-ever. 31~1) gi\-e rie IO aromatic compounds by pyrolytic elimintition and Lyclization [ZS].

? ~ y tF • "F i y * C:onpari.,'on at.d q.t_htt.u.t, e relations between tke p.rr,~:,~rams o.f .¢oils. 1-1.4. 1:'.4 at;~,: rt'$id;te

From the L.x.nmparison of the pyrogram.~ corresponding to a superficial sample of soil tfrom 0 - 3 cm} taken on March 1979 tFig. 3J considerab'.'e differences in the dimensions of several peaks between the sampIes of the original soil and its extracts can be seen. In order to faciIitate the comparison between pyrograms, some histograms have been elaborated, which are repr~entative of the mean pyrograms of ea:,:h fraction and depth tFigs. 4 - 7 , . The relative area of each peak is expre.,sed as a percentage of the total for the tv,'elve peaks considered, and ;_, •,he mean from fourteen .-amples taken over almost 3 years. Complete quantification and interpretation of the inf,:rmation refit-ted in t~e pyrograms is not easy', and requires computer systems. However. pyro!y-;- ratios are .-, simpIe and useful method for comparing pyrograms. Kimber and Searle [6] defined a benzene.., toluene ratio. BracewelI and Robertson [9] proposed a pyrolysis ratio based on the height of four characteristic peaks of the central part of the pyrogram ~furfural-5-methylfurfural/pyrroIe-~cyclopentenoneL 5-.Meth'dfurfural and ,.~.'c!opentenone are not directly identified but very. probably correspond to peaks .M and H: peak H contains two

r

.

•

°

!

"

I

-

t

| Q

] 4

.... ---

i

;it

- -._

.%

:

",.-=.

•

i i

." | o.

::.2

J

|

" • .% m ~w ~

i

t

|

.

~"

_-

|

.

t

J

:

,

.

m

...

~

~."

,,_,,,..

•

F , # 3. l~roa,--~..'x'...,. . o f ,, ~.~,~.~_.reo-'-"-_ssoil znd t'r.~r r e s p ~ t i v e fr--,:t-o.-.s t l N U SUP. XL~R i 9 7 ~ : ~-,, wY.,-.".e so-l: (b~ hu,'r.~:- - ~ f r ~ c t i o n : (0 f , a l ~ : .~c;.d f r a c t i o n : ( d ) rc~d:c.

or more subsrsnces. This ratio permitted the authors to discriminate bct~veen mor and mull humus types. In spite of slight& different fi-GC conditions. and the restrictions noted above. the p_\-rolysis ratios of Bracave and Robertson [9] c&x&wd for this soil arz .-in range..: 0.3 for the upper level. and O-2. for the medium and lower levels. Thex results zre lower than the vsriation inten-al cited by this author in mull humus. po.ssibl_\- becaus= theare for a vex-y different soil and humus type (mull calcicl and different environmental conditions. Further research over more soil t_\pe~ and mare accurately “defined” raios is needed. A similar. but simpler. ratio is py-rol c ,SffurfurrzJ_ which compzrs two frynents of different origins. It seelms well cstsblishtd that furfurzl ongmares from soil carbohydrates f9.22]: pyrrole from amino acids such ;1~ proline and hydroxyproline. from other compounds with protoporphyinic structure [9] such as chlorophyll and cytochromes and from indole. Pyroie is a ver?_ stable frqement which may be used as r? rzferencc peak. The pyrrolq. ‘furfural ratio ()i_.:‘J) could be employed 33 an index of matwit>smd,‘or humification of soil 0.M. .A young 0.M.. rich in carbohydrates will ha\-e a low pyrole,S.‘furfural ratio_ Other rat& such a~ wicene_:p~rrc4e rC_.-‘.KJ 2nd phenol,.-‘pyrrzlc IL,. ‘K, L’JUI~ br c~nsidsred ixl the ?;rimt w-ay. bttiuse the\- compare ;ircmatic structures of different @ins. which xc abundant and es>- to identify in soil pyograms.

An examination of the histogmx brins Out SOIX clear difieren~a bstween the p_\-rognms oi the soils and those of their extraits. In the ~4 samples. tolusnc iCr is the Iarstit fragment. after which thtx fo!lo\v in importance phtnol + ticresol \C 1. pyrra!e 4 IL). H. bsnztx I. BJ 2nd iurfursl (JL together with 3 whole group of mixzor perzks. ths r&k-e 3re3 of \shich doss not excesd 5% of the total.

In this cake. the largest ps3k is that of phenol + cr-rograms oi humic acid extracts are richer in p&r substzmces rphenolic t_\pei and compxativcl_v p&xxer in benzene znd toluene than those of the whole soil. which is consistent with the nature of the extrzxtsnts. This is also in agreement with prexious p_\Tr’lysL stud& of HA fractions which are richer in protein-related S compounds (e._e. pyrrole. indolc the last peak in Fig. Zb1 and phenolic compounds. 2nd plwrer in carbohydrate components such a~ furfural than the whole soil [161_

The pyrograms of F-4 are vcq- different from those of the other fractions of 0.11. and chsnse considcrab& with time. Noticeable differences are also obstrvcd bttwen the upper and tower It\-&. which makes it difficult to generalize on which should be the Iarg=t peak. Colmparin_e the F-4 pyograms vrith those of the soil. a considerable diminution of the benzene (B). tolutnz (0. cylopentanonc ( F). p>moIc \ K) rind mzthylpyro!t t L, peaks and a marked increase of furfural (J). phenol I Cl and M and D is obser\-ed: thtse last two peaks are minor in a11 tht other fractions. The pyrolq.~‘furfuraJ t K..‘J) ratio is markedly less than that of the soil and of the HA. which implies important structure and composition chan_ees in this fmction. This is due to an abundance of pol_\-s3ccharide-relawd compounds in the F-4 fraction. in reiation to protein and qtochrome compc-,undS. which accompany. to 3 large extent. the humic acids during the extraction prwsdure [ 16.21~. Morexer. the abundance of 11 peaks in the F-4 frxrion su_we;ts that the\- mav. be r&ted to carbohydrate. probabia mcthylfurfural by campari?;an \vith the F-4 chromatogram of N.trshaw and Bcrhncr [S] and Bractwell et al. [Z]. Phenols are also abundant in F-4 frxtions fX] in co.rllb inarion with po!_\-saccharides. Ths amounts of phenol in our F-4 are much yestcr than those found b! Martin [23] under different chr~mstogsphic conditions. The F-4 fraction (sodium ~311 fo_nn) contains products related to carb3 h?dratti together with some comparatively unpolymerized compounds [ZZ] which exhibit 3 I3~e and unexplained degree of variabiiity: the HA. on the contraq-. is 3 fraction with a morz stable structure and composition as far as can be deduced from the pFrogr_.

compared uith thost of tht &E nrs Tht pyrog;?ms of the rtiiduti. somewhat similar: the dimensions of the benzene 4Bj. E. H and pyrrok (KI peaks 3re ,;lightl_\- insrestj. while those of II+ and p-cry1 (St. phenol t U I and tolcenc 40 are smaller. The r&dues mainly contain O.M. bound to mineral colloids with 3 higher degee of polymerization. and so remain insduble in extractants. The similarity with whole soil is explicable by the lo\\- extraction recovery icst. 30% of o.xidizable carbon) and the similar humus composition.

The variations are much larger betwzen the upper k-e1 (O-3 cm) and the m.ean level (IO-11 cm). than between this and the Iower (20-22 cm) Ieve

"1 "1 j

u-

~'o

£0

I .... ° t r o. . m.-.

I !

i'l

C

|

i

1 )

l

I

B

L-

t

,'o

2O

t r .:.--.~"

3O

C ii

3D,

10"

E-

F

~

H-

10

2o

30

~IP " .... -+"

(b) mho!e soil m ~

love! ( 1 0 - 1 2 cm): (c) m'h~!e s~+~ I+m~'+ !~'.+I t~O-'--~ ~'n:.

?._~2

ill

¢-

' J"

•

|

tC

20

i

"" ~

q

'

-

•

tr ~=-_.

3

b

c"

10

20

tr ~.:=c~:

30

u

,n ~.

.~ ,I

m

I/

r

•

10

ZO

tr :n:z ?

F - ~ "~ Co~p.~_,'ative hL~.:o~-,~,ns o f h',.~'~c a ~ d - n o . . ' : ~ .-~.e-,~. I ~ - e h ic~, l o - ~ ~ z l e v e ~-

30

pyro~-am~: ta} u F P ~ I~'el: (bj

b

(Fig. 4). X considerable increase of tolusnc ~C,I iz &smrd. which goes from 23% to 43% benzene (B) also increases as well as pfzak E alrbou_eh the lstrer onty very slightly. Furfural. H. acetophencrne (0). phenol (L.I ad crtsol (Xl dh-r-e considerably with depth and the rest_ espe&IIy p:-rr&. remain co%t3nt. That is to say. fn_ements of the benzene and roiuene tges are rht most abundant at Iowtr depths, which su_wests tit the 0.K in rho= fcvels is richer in aromatic structures. in accordance with a higher degree of conversion into humus [!9]. This has been pointed out before b>- Brzxewell rend Robertson [Q.!Oj. although a number of the frqnents ma>- origkte from highiy substituted aliphatic structures. thrw_eh pyoi>-tic elimination 2nd cylization reactions [25]- The pyro_m show a Sesstning. at lower depths.

of div-trzificstion in the py+-sk ~tab!c qciic stnxxra tsubstitutcd

fragments. snd an increse of the mwe be-nzene rings and hcter~q-ck of S,L

variation than the O.M. of the total soil- which is to be is 3 more homogeneous fraction selectively extrxted

dispersion

H-4. the F-A show-s a Isrge cwiarion with depth ;rr.din of the relative wea vrtlue~ (Fig. 6s and bi- It can be sscn that the

?CC -_

upper level is rich in fra_nments ha\ing high retcndon times and poor in the less polar ones. while the lower level shows a different pattern. There is a notable diminution of the phenol (L’). 11 and methyipy-role (L). whilst tit benzene (B). toluene (C). D and E peaks are considerably increased. This may be due to various factors: the low F_$‘HX ratio in this calcarcous soil (0.7 in the upper level: 0.5 in the mean and lower Ieve&-). the high mobilit>- of substance of the F-4 fraction and more prominent temporsl vxirstions.

A considerable increase in the benzene peak. up to dottble its size. can be obsen-ed as u-e11 as an increase of D and E. ~-hilt the H. furfural r,J j. methyIp>-rrole (Lj. phenol (U) and creso’s [-Xi dtsrease (Fig_ T’r. Summarizing. the 0.11. pyrol_\-satefrom the deepest levels is less diversified. ahho@ relatkely richer in stable aromatic fragments. This indirectly reflects the changes in composition and strxturt of the 0.X which x&e place during the process of conversion into humus: that is to ~a\-, as the depth increases. the 0.51. has more mature characteristics. The greates: differences are obstn-ed between the upper ieve1cd :ht others. The grtater similarit_v between the middle 2nd lower I--e& 5 s=rp~or.ed b>- tic kc1 rhs both of rhcm belong to one and the same gtcsti~ hx?zon i II _A? :I dSc_ m the conwaq-. the upper level mq- be considered 2s r?n incipient 1_a- 1 horizon_ The stud_\-through P\--GC of the varied and complc?r ,mixtart of substances constituting the soil 0.X is 8 way to simplify the pr&lz+m FiStXlttd b_vits analysis. thereby reducin_e the great diversity of chemical forms to the frqgnents most stable after pyrolysis. Subsquent identification of thst fqments makes it possible to acquire an idea of the 3trr?ctures which composed the soil 0.11. It must. howev-tr. bt bornt IIImind thzrt there nre a number of limitations: the p_\-rol>-sis producti some fragments N-hich did not necessaril_v exist as such in the soil: one and rht same molecule ma>- _eivc rise to \-arious fra_ements_while different molwults. with re!ritcd st~ctures. >-ield common fra_gntnts. Only the volatile compadnds of joI1 p_vrolysat= mq- be detected by GC_ When analil\zing the p\-rograms nn abundance of substituted btnzenc frqgnents is observed. as well as tolucne. phenol. cresol. acttophtnont. cpuaiacol. etc.. the or&in of w\-,hichhs to be sought_ among others. in molecules related to li_tin and its degradation ~)roduc*~ I5.9.11?] or in microbial ori_& [lS]. Soil polysaccharides are another important constituent of humus 1161 which Jieid furan dtrivstix-es- Aliphatic polycarboxylic acids. present in humus. yield cyclic ketones and also benzene_ tolutne and other compounds [3]- On the other hand, the ma-or propsrtion cf benzene and tolucnc in whole soil may be due to catalytic reactions by the soil inorganic matrix: never:heles based on the general characteristics of humus. a considerable amount of the tolutnc and benzene ma>- ark from organic structures with benzcnc nuclei.

-
methods employed diminish the probability a- s~~~~nda~ rerrc‘tia-s in the injector be~‘;fust the soil sample is Iike 3 fii!m ;rJhering tc-.thc inner qurrrtz tube ~-all. within the hottest axe of the tube. ani3 the pyre+-sis prakts are swept out. bcin_e rapid&- diluted in the arritr g;e which is nt a h-sr tsmpsrxure. Ths resdts xc’ in asswirtncs with the struc:ural models propased b> Sch~itztr [2.X] an3 FI5p r’t aI. [I]. zmanp cxhtrs. for the substances in the humus. on the basis r~f bsnzsne grazps \vith OH 2nd COOH rdicclls IInked t,yehcr by Aphxic chains and other ewups. These pyol_\-sis results are 31~0. to 2 great estcnt. in xwrdancc Lvith tk rst;trch of BracewAl and Robertson [9.10]. msinl>- on the compsition c? si! pvral>-sates snd depth variations. alrhou_eh quantitative compx-isoz is kiiifkult. Despite the lirnitstiens inherent in the technique. \ve can rezsstrt that Pv-GC is a suitable tcshnique for the t>.pificati~n of soil 0.M. It has been p~~ssibfc to vcrif~ that the pl;tstnxnc. size 2nd dispaition of the sltmpk wirhin the pyr+zcr is fundsment;?l SLYthe p_vrolxsis rspr&xibilit~. .+;tIn.

the sample dspesition

REFERESCFS