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7/23/2019 chuong1.pdf phn tch d lng thuc tr su
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CHNG 1. TNG QUAN
1.1 Gii thiu v thuc trsu
1.1.1 nh nghaThuc bo v thc vt (gi tt l thuc tr su) l cc cht hay hn hp cht t
nhin hoc tng hp c tc dng kim sot, y li cc loi su v cn trng gy tc
hi n thc phm, pht tn bnh tt. Thuc tr su bao gm ch yu 4 nhm chnh:
nhm clo hu c (organochlorines), nhm ln hu c (organophosphorus), nhm
carbamate v nhm cc (pyrethroid) [17]
1.1.2 c im
* Nhm clo hu c(organochlorine pesticides OC): gm nhng hp cht hahc nh DDT, methoxychlor, heptachlor, aldrin, lindane, mirex, rt bn vng trong
mi trng t nhin v c thi gian bn phn hy di. Nhm thuc tr su ny c
ha tan vo trong cc m mv c tch ly trong mng mbao quanh t bo thn
kinh lm ngn cn s chuyn dch ca cc rung ng thn kinh dc theo nhng axon
(ni lin cc t bo thn kinh). Kt qu l ph hy h thng thn kinh trung ng v
git cht sinh vt cn dit tr.[7]
c trng cho nhm ny l 3 loi hp cht DDT, hexachlorobenzen v cc hpcht cyclodiene. DDT l mt trong nhng cht bn v khng ha tan c trong nc
v p sut hi thp. BHC v cyclodiene s dng rng ri do kh nng tiu dit nhiu
cn trng, nhng do kh phn hy v c tnh cao ca thuc tr su h clo nn hu ht
b cm hoc b hn ch s dng. [7]
* Nhm ln hu c (organophosphorus pesticides-OP) gm nhng hp cht
ester ca acid phosphoric hay acid thiophosphoric vi cc gc ca hp cht hu c
khc. V d nh malathion, parathion, diazinon, fenitrothion Nhm ny c thi gian
bn phn hy nhanh hn nhm clo hu c, c c cao hn i vi ngi v ng
vt. Nhm ln hu ctc ng vo thn kinh ca cn trng bng cch ngn cn s to
thnh cholinesteraza, lm cho thn kinh km hot ng, yu c, gy chong vng v
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cht. y l nhm vn cn c s dng rng ri hin nay ngoi nhm pyrethroid
(cc) v carbamate. [7]
* Nhm carbamate: gm nhng hp cht ester ca acid carbamic v N-methyl
hay N,N-dimethyl. Mt s cht in hnh l: aldicarb, carbofuran, carbaryl,
methomyl,y l nhng hp cht t bn vng trong mi trng t nhin nhng c
c tnh rt cao i vi ngi v ng vt. Chng tc ng trc tip vo men
cholinesteraza ca h thn kinh. [7]
* Nhm cc: thuc tr su nhm Cc tng hp dn xut t ngun gc thc vt
ca cy h Cc, trong cng thc c cha cht Pyrethrin gy c cho cn trng. Thuc
ny t gy c cp tnh, phn hy nhanh trong mi trng, d chu tc ng ca nhsng v nhit . Tuy nhin, nu p dng lu v lin tc trn ng rung d gy tnh
khng ca cn trng. Nhm ny d bay hi v tng i mau phn hy trong mi
trng v cth ngi. Pyrethrin l loi ho cht c thn kinh, chng tn cng ln h
thng thn kinh ca cn trng. Khi nng khng tiu dit cn trng, chng vn
gy nn hiu lc xua ui i vi cn trng. Pyrethrin v cc cht cc tng hp l
nhng cht gy c ln knh mui (sodium channel) ca mng thn kinh. Cc
pyrethroid c i lc rt cao i vi cc knh mui, to ra nhng thay i nh chcnng ca knh. Cc pyrethroid thc cht l nhng cht gy c chc nng, hu qu xu
ca thuc mang tnh th cp, l hu qu ca s kch thch qu h thn kinh. iu
ny th hin r ch khng tm thy cc du hiu bnh l trong h thn kinh trung
ng, ngay c khi gy c nng nhiu ln cng nh s ch to thnh cc m hoi t
khng c trng v c th phc hi trn cc thn kinh ngoi vi ca ng vt b co git
v th hin cc triu chng ri lon vn ng nghim trng. Sau khi b pyrethroids lm
cho bin i knh mui vn tip tc hot ng bnh thng, vn duy tr c chcnng chn la cc ion mui v ni vi in th mng.
Cc ho cht nhm pyrethroid gy hi cho c, nhng rt t nh hng n chim v
ng vt c v hn nhiu loi ho cht dit cn trng tng hp khc. Chng khng bn
vng, d b phn hu khi tip xc vi nh sng v oxy. Chng thng b phn hu di
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nh sng mt tri v trong khng kh trong vng mt vi ngy, v khng nh hng
ng kn cht lng nc ngm. Chng c xem nh l nhng loi ho cht dit
cn trng an ton nht i vi ngi v mi trng.[19]
1.1.3 Tc hi ca thuc trsu
Cc loi thuc tr su tc ng ln h thn kinh, c ch men
acetylcholinesteraza v tc ng ln khong khng gian gia cc t bo thn kinh
(synapsis) ca h thn kinh ca cc c th trng c. Nu quan st, chng ta s thy
nhng hng phn thn kinh i km vi cc hot ng lon nhp, mt thng bng hay
bt nga, cc chi v u co git lin tc, yu dn v cht.
Hp cht OC rt bn vng trong cthng thc vt v c tch ly lu trongm m, lipit, lipoprotein, sa. Hp cht OP v carbamate tng i t hay khng tch
ly lu trong m m, lipit, lipoprotein nhng li ha tan v tn ti rt lu trong dn
xut ester ca acid hu cvng thm.
V bn cht ha hc, thuc tr su c thi gian bn phn hy trong t ko di
tng i lu. Trong mi trng t nhin, di tc dng ca nh sng, nhit , m
v vi sinh vt, cc ha cht ny bt u b phn hy. Sn phm chuyn ha ca OC t
c hn hp cht ban u. Nhng i vi OP, s chuyn ha din ra nhanh v phctp, ng thi xut hin nhiu hp cht trung gian c hn rt nhiu ln dng thuc ban
u vi cn trng v ng vt mu nng.
Thuc tr su c th gy ngc cp tnh cho sinh vt khi tip xc trc tip,
nut nhm hoc ht th. Chng cng c th gy ng c mn tnh cho sinh vt m
khng c triu chng ban u c th no.
D lng thuc tr su cn li trong rau qu, thc phm l nguyn nhn gy ra
nhng bnh nh ung th phi, tiu ng, bt thng v van tim, tiu ha thn kinh.Chng c kh nng lm suy yu h min dch khin cth khng chng li c cc t
bo ung th v cc loi vi sinh vt gy bnh. Ngoi ra, chng cn c th gy mt s
nh hng khc n cth con ngi.
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1.1.4 Cng thc cu to ca cc hp cht thuc trsu nghin cu trong ti
DDT, DDD, DDE
Methacriphos Tetradifon
Tribufos
Dichlorodiphenyldichloroethan
Dichlorodiphenyltrichloroethan
Dichlorodiphenyldichloroethylen
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Hexaclorobenzen Ethoprophos
Chlorpyriphos methyl Chlorfenvinphos
Tolclofos methyl Pirimifos-methyl
Isofenphos Fenpropimorph
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Tefluthrin
Bifenthrin Fenpropathrin
Permethrin Esfenvalerate
Hnh 1.1 Cng thc cu to ca cc hp cht thuc trsu nghin cu trong ti
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1.1.5 Yu cu v dlng ca thuc trsu
Do tnh c hi ca thuc tr su nh hng n sc khe con ngi v gy
nhim mi trng nn hin nay cc nc thit lp nhng gii hn cho php ti a
ca cc hp cht thuc tr su trn cc i tng thc phm khc nhau. V gii hn
ny khc nhau ty theo tng hp cht v tng loi thc phm.
Bng 1.2 Gii hn cho php ca mt vi hp cht theo quy nh chu u
(Regulation EC No 396/2005)
STT Thuc trsuTri cy
ti (ppb)Tht c(ppb)
Rau(ppb)
1 Methacrifos 50 10 50
2 Ethoprophos 20 10 203 Hexaclorobenzen 10 200 104 Tefluthrin 50 50 505 Tolclofos methyl 50 50 50-1006 Chlorpyrifos methyl 50-500 50 507 Pirimiphos methyl 50 50 508 Fenpropimorph 50 10-300 509 Chlorfenvinphos 20 10 2010 Bifenthrin 50-300 50-100 50
11 Fenpropathrin 10 - 1012 Tetradifon 20 50 2013 Permethrin 50 50 5014 Esfenvalerate 20-50 200 20
Phn ln cc hp cht c gii hn ti a khong 10ppb n 50ppb, iu i
hi pht trin phng php phn tch lm sao c nhy tng i tt c th kim
sot tt d lng thuc tr su trong thc phm.
1.2 Gii thiu mt s phng php chit thuc trsu
Mc ch chnh ca cc quy trnh chit l c th tch c cc hp cht thuc
tr su t nn mu bng quy trnh chit thch hp. C nhiu phng php chit thuc
tr su nh chit rn lng SLE (solid liquid extraction) , chit lng lng LLE (liquid-
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liquid extraction) , chit pha rn SPE (solid phase extraction) v chit vi lng pha rn
SPME (solid phase micro extraction). Vic chit thuc tr su t mu lng (v d mu
nc) th k thut chit thch hp l chit pha rn SPE hoc chit lng lng LLE. i
vi mu mi trng hoc mu thc phm dng rn th vic dng dung mi v h tr
bng k thut lc khuy trn hoc vi sng l thch hp. Trong cc quy trnh th u cn
lm sch bng chit pha rn hoc chit lng lng loi bt tp cht bn t nn mu.
1.2.1 Chit rn lng SLE (solid liquid extraction)
Phng php chit ny l phng php s dng rng ri phn tch thuc tr
su t nn mu rn. u tin dung mi chit len li su vo gia cc thnh phn mu
tip xc vi cht phn tch, sau cht phn tch khuch tn i vo dung mi chit.Yu t quan trng nht ca k thut chit rn lng ny l la chn dung mi thch hp.
Tuy nhin nhng yu t khc nh p sut, nhit cng c nh hng quan trng ln
hiu qu chit. Khi tng p sut s lm tng kh nng phn tn ca dung mi i su vo
nn mu rn, tng nhit s lm tng kh nng ha tan ca cc cht phn tch vo
dung mi. Cc k thut h tr trong chit rn lng l lc, chit soxhlet, nh siu
m, microwave. Mi k thut u c nhng u nhc im ring.
Lc : l cch n gin v thun tin thng s dng chit thuc tr su t tricy v rau qu. Ch cn cho dung mi thch hp ri lc trong mt khong thi gian xc
nh. Dung mi thng s dng l acetone v acetonitrile v kh nng trn vi nc
ko cht phn tch t nn mu ra dung dch lng, ngoi ra dichloromethane hay hexan
cng c dng cho nhng hp cht thuc tr su km phn cc.
K thut lc vi dung mi n gin v thun tin nhng khng hiu qu trong
nhng trng hp cht phn tch c lin kt cht ch vi nn mu. Trong trng hp
ny nh siu m l cch h tr tt, gip cho s khuch tn ca dung mi vo mu,tng thm s tip xc qu trnh chit c hiu qu hn, gim c lng dung mi
v thi gian chit.
Chit soxhlet : khi qu trnh lc khng tt chit, lc cn h trbng
cch gia nhit. Chit soxhlet l mt k thut chit nhit cao, k thut ny ra i t
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nm 1879, n nay vn cn c s dng. Dung mi chit c chng ct, sau
ngng t v mu c chit nhiu ln vi dung mi sch .
u im ca chit soxhlet l hiu qu chit tt nhng cng c nhc im v
thi gian chit lu, s dng mt lng ln dung mi.
Chit bng microwave : k thut ny c pht trin trong vi nm qua. Mu
v dung mi chit c gia nhit bng vi sng, c s kim sot v p sut, nhit v
nng lng. K thut ny cho hiu qu chit tng t nh chit soxhlet tuy nhin gim
c thi gian chit (thi gian chit khong 15 pht) v gim c dung mi chit so
vi k thut chit soxhlet (dung mi s dng 25-50ml) v c p dng trong nhiu
phng th nghim.1.2.2 Chit lng lng LLE (liquid-liquid extraction)
c s dng rng ri chit thuc tr su t mu lng. Chit lng lng da
trn s phn b ca cht phn tch gia hai pha lng. Hiu qu ca vic chit lng lng
ph thuc i lc ca cht phn tch vi dung mi chit, t l th tch mi pha. Chit
lng lng thng c s dng chit thuc tr su t mu nc trong mi trng,
hexane v ciclohexane l dung mi hu cthng dng chit cc cht khng phn
cc nh thuc tr su clo hu c, phosphor hu c. Dichloromethane , chloroform ldung mi chit cc cht phn cc va.
S kt hp ca dung mi v pH lm tng kh nng chit nhiu thuc tr su t
nn mu, l u im ca phng php chit lng lng. Tuy nhin phng php ny
chi ph cao tn nhiu thi gian v khng tng ha c.
1.2.3 Chit pha rn SPE (solid phase extraction)
Ging nh chit lng lng, chit pha rn da trn i lc khc nhau ca cht phn
tch gia hai pha. Trong chit pha rn, mu c cho qua cht hp ph rn (c nhiuloi cht hp ph rn phn cc, khng phn cc, trao i ion) nhi trong ct. Nhng
cht phn tch c i lc ln vi pha rn hp ph sc gi li trn ct, trong khi
nhng cht khc si qua ct. Tp cht c ra vi dung mi loi b nhng cht
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phn tch vn gi li, sau cht phn tch b gi trn ct c ra gii bng mt
lng nh dung mi thch hp chn lc vi cht phn tch.
Vi k thut chit SPE, mu c th lm giu nhiu ln tng nhy cho phn
tch cc hp cht. Vn quan trng trong chit SPE l la chn pha rn hp ph thch
hp, hiu c mi tng quan gia cht phn tch v cht hp ph l vn quan
trng.
1.2.4 Chit vi lng pha rn SPME (solid phase micro extraction)
Arthur v Pawliszyn gii thiu phng php chit vi lng pha rn SPME
vo nm 1989. Phng php chit ny n gin, khng dng dung mi hu cc hi,
v thc hin nhanh vi nhiu loi si chit c lm sn vi nhiu loi cht hp thukhc nhau ph ln si chit. Si chit silica c ph ln mt lp pha tnh tng t
nh ct sc k, si chit c t bn trong mt cy kim bo v. Tng t nh chit
pha rn, chit SPME cng da trn s phn b ca cht phn tch gia nn mu v pha
tnh. Chit vi lng pha rn gm hai bc chnh l chit v gii hp, ngoi ra cn c
th thc hin thm bc ra nu cn.
Chit : qu trnh chit c th thc hin bng cch chit phn khong khng
(i vi cc hp cht d bay hi t nn mu lng hoc mu rn), hoc a trc tip sichit tip xc vi mu. u tin mu c b vo l kn, cm cy kim vo l ri y
si chit ra hp thu cht phn tch ln pha tnh ph bn ngoi si chit. Sau si
chit c ko vo bn trong kim bo v v rt ra khi mu.
Vic la chn si chit vi thnh phn pha tnh hp l l yu t chnh ca k
thut ny. B dy pha tnh ph bn ngoi si chit v xp ca pha tnh cng l yu
tnh hng n hiu qu chit, bn cnh mt s yu t nh nhit , thi gian
chit, pH cng cn ti u t c hiu qu chit tt nht.Gii hp : cc cht phn tch b hp thu vo pha tnh ca si chit c gii
hp ra phn tch. C th gii hp trc tip bng cch gia nhit bung tim mu
sc k kh hoc ra gii bng dung mi thch hp. Gn y c h thng kt ni trc
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Mc d c th s dng dung mi khng cha clo nh acetone hay ethylacetate
nhng trong phng php QuEChERS la chn dung mi acetonitrile v khi thm hn
hp mui dung mi v nc d dng tch khi nhau hn l acetone. Ethylacetate c th
ha trn mt phn vi nc nhng li chit lun c bo v sp t nn mu, cho thu
hi thp i vi thuc tr su dng acid- baz. Dung mi acetonitrile chit khng tt
vi cc cht d tan trong m, nhng vi nhng mu c hm lng ng cao th
acetonitrile v nc d dng hnh thnh hai pha chit thuc tr su ln pha hu c.
So vi acetone th acetonitrile d loi nc hn khi thm MgSO4. Dung mi
acetonitrile thch hp cho c sc k lng v sc k kh. Do vy trong phng php
QuEChERS la chn dung mi acetonitrile chit cc hp cht thuc tr su. Tuynhin dung mi ny c mt nhc im l kh bay hi hn nn thi gian c quay lm
giu mu s lu hn.
Mc ch ca vic thm mui bc 3 l tch nc v dung mi hu c
thnh hai pha. Thm mt lng mui MgSO4 bo ha gim lng nc tng s
phn b ca thuc tr su vo pha hu c. Mui NaCl kim sot phn cc ca
dung mi chit, tuy nhin nhiu mui ny qu lm gim s phn b ca thuc tr su
khng phn cc ln pha hu c.Trc y bc lm sch thc hin da trn vic nhi cht hp thu ln ct.
Nhng trong chit pha rn phn tn, phn dung dch chit c cho vo ng nh c
cha mt lng nh PSA (amine bc 1 bc 2), lc u sau ly tm tch phn dung
dch v cht rn hp thu. Cht hp thu ch gi li thnh phn nn mu, khng gi cc
hp cht thuc tr su. Trong trng hp mu cha nhiu bo th thm bt C18 vo
cht hp thu loi bo, mu cha nhiu cht mu trong thc vt th thm GCB
(graphite carbon black). Cc lm sch ny so vi chit SPE th d thc hin hn, khngphi hot ha ct, v dng t dung mi.
Thm acid acetic v cht bo v mc ch cc hp cht thuc tr su c
bn hn, hn ch phn hy khi vo h thng sc k. Thng cht bo v s dng l
sorbitol, gulonolactone and ethylglycerol.
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1.3 Gii thiu phng php sc k kh xc nh thuc trsu
1.3.1 Slc v sc k kh [4]
Sc k kh l phng php tch da trn hai qu trnh hp ph v gii hp
ph xy ra lin tc gia hai pha: pha tnh rn hoc lng v pha ng l kh. Sc k kh
khi ni vi cc u d cho php nh tnh v nh lng cc cht da vo thi gian lu
v din tch mi sc k.
Hnh 1.3. S khi ca my sc k kh
H thng bao gm cc b phn cbn nh sau:- Ngun cung cp kh mang thng l bnh kh hoc my sinh kh
- H thng iu khin p sut hoc tc dng kh mang
- Bung bm mu
- L ct iu khin nhit ct tch
- Ct tch l ni xy ra qu trnh tch cht. Cc loi ct gm ct nhi, ct mao
qun
- u d c nhiu loi khc nhau ty theo mc ch phn tch, dng pht hincht v nh lng: FID, ECD, MS, NPD, TCD
- H thng ghi nhn v x l tn hiu: thu thp v tnh ton kt qu.
Trong sc k kh th quan trng nht l ct sc k v kh mang. Kh mang thng phi
trkhng phn ng vi cc hp cht phn tch v pha tnh, kh khng cha oxi, kh
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yu t hiu nng tch v dung lng ct. Hiu nng tch ca ct mao qun th hin
sa l thuyt (N), sa l thuyt tng khi ng knh ct gim. Dung lng
ct tng khi tng ng knh ct, khi phn tch cc cht nng cao hoc nhiu
nng tri rng th nn chn ct c ng knh ln. phn cc ca pha tnh
cng nh hng n dung lng ct, pha tnh khng phn cc cho dung lng cao
hn i vi hp cht phn cc v ngc li. Thng th chn ng knh ct
0.25mm ph hp cho nhiu trng hp phn tch, cho sa l thuyt ca ct v
dung lng trong khong chp nhn c.
- B dy lp pha tnh: chn b dy lp pha tnh nh th mi sc k s hp li, v
gim chy mu ct, nhit s dng ti a ca ct cng tng ln. S dng ct clp pha tnh mng gim c thi gian lu ca cc hp cht ngay ci vi trng
hp phn tch nhit thp. Nhng li c khuyt im l lm cho dung lng ct
gim khi chn b dy pha tnh nh. Ct c lp pha tnh mng (0.1m n 0.25 m)
thng dng phn tch cc hp cht c nhit si cao (nh cc hp cht thuc
tr su, phthalate esters, polychlorinated biphenyls).
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1.3.2 Sc k kh vi u d ECD (Electron capture detector) [5]
u d ECD l loi u d chn lc, n ch nhy vi mt s hp cht c
cha cc nguyn t c m in ln nh: O, N, F, Cl, Py l loi u d ph bin
nht xc nh thuc tr su v cc hp cht halogen trong mu mi trng, mu
nng thy sn, mu thc phm
u d ECD bao gm mt bung ion ha cha ngun phng x (Ti3H,
Sc3H, hay a chung l 63Ni) pht ra tia . Trong qu trnh phn tch, mt dng kh
Nithay Argon trn vi 5-10% metan i vo trong t bo ni khc ion ha bi
tia , v gii phng cc in t nhit t do. in cc dng thu nhn cc in t, dovy hu nh lun c mt dng in nh cnh c xem nh dng chun. Nu hp
cht cha nguyn t c m in ln nh F, Cl, O,i vo trong t bo, n s nhn
cc in t nhit t do, v vy dng chun s gim xung. Tn hiu u ra c ghi
nhn bi s khuch i v o ngc ca dng chun.
Tc dng kh vo trong u d ty thuc vo cu trc hnh hc ca t
bo, nhng thng khong 10ml/pht. Do , i vi ct mao qun, cn c kh mang
l Nithoc Argon/metan h trcho s ion ha.u d ECD c th phn tch hm lng thuc tr su nh khong 0.01ppb,
nhng ch chn lc cho cc thuc tr su h clo, km nhy phn tch cc thuc tr
su h phospho, carbamate.
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Hnh 1.4 S ca u d ECD
1.3.3 Sc k kh vi u d NPD (Nitrogen-phosphorus detector) [5]
u d NPD l loi u d chn lc, nhy vi hp cht cha nit, phosphor.
Cu trc cng tng tu d FID, khc bit l c thm mt vin mui kim loi kim
(mui Rubidi hay Cesi) ni vo cc m t gia ngn la thiu t ha cht v b thu
ion. Khi bt nng, vin tinh th ny (gi s l mui Rubidi) phng thch Rb*. Cc
Rb* phn ng vi cc gc t do CN t hp cht c ni C-N hay cc gc PO2 t hpcht c P ln lt cho ra CN- v PO2
- v tr thnh Rb+ b ht tr li vin tinh th.
Cc ion CN- tip tc phn ng vi gc t do H hay OH c trong ngn la cho HCN
hay HCNO v phng thch in t to ra dng in. Tng t PO2- phn ng vi gc
t do OH cho HPO2 v phng thch in t to dng in. Nu khng c hp cht c
C-N hay P th Rb* cng phn ng vi gc t do H,OH trthnh Rb+ v phng thch
in t to dng in nh. S khc bit gia dng in nn v dng in trong trng
hp c hp cht N hay P c khuch i v biu din di dng mi sc k. Hin
tng c tip tc xy ra nh th. Trn nguyn tc, vin mui Rubidi khng b hao ht
nhng thc cht sau mt thi gian hai ba nm th phi thay, nht l trong iu kin
kh hu m t.
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Hnh 1.5 S ca u d NPD
1.3.4 Sc k kh ghp u d khi ph ba ln tcc MS/MS
Sc k kh thch hp phn tch cc hp cht t phn cc, d bay hi v bn
nhit. Phng php sc k kh kt hp u d MS rt hu hiu trong vic nh danh v
nh lng, phn tch ng thi rt nhiu hp cht thuc tr su trong cc nn mu
phc tp. Phng php ny c ng dng rt rng ri do u im phn tch ng thi
nhiu hp cht, cc cht c thi gian lu ging nhau khng phn tch c bng sc k
thng thng, nhy v chn lc cao. Tuy nhin mt vi hp cht kh bay hi
nh carbamate th phng php ny khng phi l th mnh, trong trng hp ny sc
k lng c dng nhiu.
Cu to ca h khi ph ba ln t cc gm c: ngun to ion, b phn phn
tch khi lng , bung bn ph ion , u d ghi nhn tn hiu.
Nguyn tc hot ng ca u d khi ph ba ln t cc
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- Cht phn tch sau khi ra khi b phn GC c ion ha bung ion, sau i qua
b phn MS1. y ion mc chn sau b va p phn mnh bung bn ph
(collision cell) vi kh va p l argon, to ra cc mnh ion con. Cc ion con ny tip
tc i qua b phn MS2, ti y cc mnh c lc ra v i n u d ghi nhn tn
hiu.
- Ngun to ion: trong ti ny thc hin ion ha theo ch ion ha in t EI
(Electron Impact Ionization)
Ion ha in t: xy ra trong mi trng chn khng p th 70V, dng electron
c nng lng cao khong 70eV pht ra t si dy tc (filament), di chuyn gn hi
phn t ca cht cn phn tch M, v lm bin dng m my in t ca phn t. Dong nng electron cao nn khi bao quanh m my electron ng thi truyn ng
nng cho electron ca m my in t, khi ng nng ln chng tch ra hnh thnh
ion phn t M.+ v ng thi qu trnh tip xc va p ca electron vi hi phn t ca
cht phn tch cng to thnh ion con A+, B+ . Cc ion ny c nh hng v i vo
b lc khi (quadrupole). Do trong ion ha in t, cc electron c nng lng rt ln
nn sau khi va p cn li t ion phn t, v vy khng thu c nhiu thng tin v khi
lng phn t, tuy nhin t cc mnh ion nh c th thu thp c nhiu thng tin vcu trc phn ti vi nhng hp cht mi.
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Hnh 1.6 S ion ha EI
B phn tch t cc (quadrupole): c cu to gm bn thanh in cc song song
b tr cch u mt trung tm. Thin t c tn s tnh hay bin i c p vo 2
cp thanh song song i din to ra in trng. Cc ion m hay dng t ngun
ion si vo dc theo mt trc trung tm ca t cc. Do in trng c cng v
tn s c th nn ch cho nhng ion c khi lng thch hp i qua c b t cc
n b phn pht hin (detector). Thng thng ngi ta gi tn s khng i v thay
i th p vo gip cho vic la chn ion.
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Hnh 1.7 S b tcc
Hnh 1.8 S khi ph3 ln tcc (MS/MS)
BphnMS1
BphnMS2
B phn bnph ion sau khiqua MS1(collision cell)
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Nhng ch s dng ca u d khi ph ba ln t cc: phn tch mt cht
cn c hai cp mnh ion c cng thi gian lu, hai cp c th l mt mnh m v
hai mnh con hoc hai cp mnh khc nhau khng cng mnh m. tm c cc cp
phn mnh ion cn c cc ch qut khc nhau tm v xc nhn thng tin chnh
xc ca cc cp phn mnh ion ca mt cht
- Ch qut mnh ion con (product ion mode): mt ion mc chn MS1
sau qua bung bn ph ion, ti y ion m b phn mnh thnh nhiu ion con
khc nhau, vi ch qut mnh ion con s cho tt c cc mnh ion con i
qua c t cc n b phn pht hin (detector). Sau khi c c tng
mnh con vi cng khc nhau, chn hai mnh con c nhy tt nht v n
nh nht chy ch MRM (multiple reaction mornitoring) nh lng
cc hp cht ny.
- Ch qut mnh m (precursor ion mode): nhiu mnh m ca cc hp cht
khc nhau c th b bn ph v cho ra cng mt mnh con, khi b phn pht
hin o tn hiu cng ca ion con v tnh ra hm lng ca cht phn
tch s b sai s dng, l yu t cn nhiu ca u d khi ph. Do xc nhn li mnh con c phi l ca hp cht cn phn tch hay khng th s
dng ch qut mnh m xc nhn li thng tin.
- Ch MRM: S dng nh lng cc hp cht. ch ny, b phn t
cc MS1 ch cho mt ion mi qua c v n bung bn ph phn mnh
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Hnh 1.9 Cc ch phn tch ca u d khi phba ln tcc
- Ngoi ra vi h thng khi ph ba ln t cc cng c th s dng ring b phn t
cc MS1 hoc MS2 thc hin ch SCAN qut nhiu ion hay chn lc tng ion
SIM (single ion mornitoring) nh trong sc k kh ghp khi ph GC-MS mt ln.
ch SCAN, b t cc c p th thay i trong mt khong xc nh, v ghi nhn
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li tt c cc ion tng ng trong khong p th. Lc bung bn ph khng hot
ng. Tuy nhin mt phn ion s mt mt trong qu trnh di chuyn qua ba b phn
MS1, bung bn ph v MS2 nn lng ion n c b phn pht hin ion s gim i
so vi h khi ph MS mt ln, nn nhy s khng bng.
u im ca u d khi ph MS/MS:
Phng php c chn lc cao, kh nng nhn danh cao, t bnh hng ca
nhiu nn. Phng php x l mu cng n gin hn. Khng mt nhiu thi gian
tch mi sc k ca hn hp nhiu hp cht, c th phn tch cng lc vi trm hp
cht thuc tr su, trong khi phng php phn tch thuc tr su khc nh sc k khu d ECD, NPD phn tch c t hn nhiu.
Nhc im:
nhy so vi cc phng php chuyn dng thng thng khng bng.
Thng xuyn phi lm sch b phn MS v nh hng nhiu n nhy ca
phng php.
Gi thnh t.
1.4 Gii thiu phng php sc k lng xc nh thuc trsu
Rt nhiu hp cht thuc tr su t phn cc, d bay hi c xc nh bng
phng php sc k kh rt hiu qu. Kt hp vi cc hu d khc nhau nh ECD,
FID hay u d MS, sc k kh phn tch c nhiu thuc tr su trong thc phm
v trong mi trng. Tuy nhin cc hp cht khng bn nhit, km bay hi th li l th
mnh ca sc k lng.
1.4.1 Phng php sc k lng u d hunh quang
Phng php ny c bit chn lc cho vic phn tch cc hp cht thuc tr
su h carbamate sau khi to dn xut.[6]
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Nguyn tc ca phng php HPLC to dn xut sau ct l da vo c tnh
khng bn ca nhm carbamate trong mi trng kim. Sau khi tng loi carbamate
tch trn ct sc k pha o, c thy phn trong mi trng baz sinh ra methyl
amin, sau methyl amin phn ng vi thuc th O-phthalaldehyde (OPA) v 2-
mercaptoethanol to ra dn xut hunh quang 1-hydroxytylthio-2-methylisoimdol.
Phn ng phn hy ca carbamate trong mi trng baz (phn ng xy ra
nhanh hn di tc dng ca nhit )
Phn ng ca methyl amin vi thuc th sinh ra hp cht hunh quang 1-
hydroxytylthio-2-methylisoimdol. Cht ny c cc i hp thu khong 340nm v cc
i pht x 440-460nm.
Mu c ng nht v chit vi dung mi MeOH, lm sch bng ct C18, sau
mu c ra gii bng CH2Cl2, thi kh v ha tan li bng MeOH
Phng php ny rt nhy v chn lc cho phn tch thuc tr su carbamate
trong mu t, nc, thc phm. Gii hn pht hin 0.51-5.0ppb, gii hn nh lng
1.69-9.09ppb. Tuy nhin ch chn lc cho phn tch thuc tr su h carbamate, khng
thch hp phn tch thuc tr su thuc h khc.
1.4.2 Phng php sc k lng ghp u d khi ph (LC-MS/MS)
Phng php sc k lng kt hp vi u d khi ph MS/MS rt chn lc v
hiu qu xc nh ng thi nhiu thuc tr su nh carbamate, phenylureas, thuc
tr su phospho hu c, triazines m khng cn cc k thut to dn xut phc tp.
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u d khi ph cn cho nhiu thng tin v cu trc phn t gip don c tn
hp cht trong vic nh danh cc cht.
Nguyn tc hot ng ca h sc k lng ghp khi ph: mu sau khi tim c
pha ng li ko i v cc hp cht c tch ra trong ct sc k, sau mu c ha
hi v chuyn v dng ion bung ion ha. B phn MS1 s chn ion m tip tc
bn ph thnh ion con v MS2 s lc ra cc mnh con . Da vo cc cp phn mnh
nh tnh v nh lng cc cht.
u im ca phng php sc k lng u d khi ph l nhy v chn
lc cao, kh nng nh tnh v nh lng tt. Khi tim vi th tch ln c th h thp
gii hn pht hin ca cc cht phn tch. Thi gian phn tch nhanh hn so vi sc kkh. Qu trnh ti u ha cc thng s nh nng lng bn ph mnh ion m, mnh ion
con thc hin nhanh chng hn phng php sc k kh, c th nhn thy c mnh
ion con c nhy v bn ngay trong lc tim chun ti u m khng cn chy
th, do vy tit kim c thi gian tm phng php chy my thch hp cho mt
hp cht mi.
Tuy nhin vi phng php sc k lng cng c nhc im l cn thi gian
cn bng dung mi pha ng trc khi chy khng bnh hng thi gian lu cacc hp cht.