giáo trình phân tích phổ nguyên tử

8/14/2019 gio trnh phn tch ph nguyn t

1/295

PHM LUN

PHNG PHP PHN TCH

PH NGUYN T(In Ln th2 c sa cha v bsung)

NH XUT BN I HC QUC GIA H NI


2/295

2

LI MU

Php o ph pht x v hp th nguyn t l nhng kthut phn tch ha l, v ang c pht trin v ng dng rng ri trong nhiu ngnh khoa hc k thut,trong sn xut cng nghip, nng nghip, y dc, a cht, ha hc. c bit cc

nc pht trin, phng php phn tch ph pht x nguyn t trthnh mt trongcc phng php dng phn tch lng vt cc kim loi trong nhiu i tng khcnhau nht, nc, khng kh, thc phm, v.v... Hin nay trong cng tc nghin cu

bo v mi trng, phng php phn tch ny l mt cng cc lc xc nh cckim loi nng c hi.

nc ta, kthut phn tch theo ph pht x v hp th nguyn t cng cpht trin v ng dng trong khong hn hai chc nm nay. Mt s phng th nghim c trang b my o ph pht x v hp th nguyn t, hoc do Nh nc ta ut, hoc do s vin trca cc t chc nc ngoi theo cc chng trnh khc nhau.Mt s cn b khoa hc ca ta c c ra nc ngoi hc tp, nghin cu v oto. Song i a s khng c iu kin , nhng li cn s dng k thut phn tchny cho cng vic phn tch ca h. Hu ht cc ti liu hay sch khoa hc v k thutny li bng ting Anh. Mt khc, chng ta li cha c mt cun sch hoc cc ti liucsl thuyt bng ting Vit v kthut phn tch ny.

Do thc th, ng gp cho cng tc o to sinh vin trn lnh vc kthutphn tch ny ca Khoa Ha hc, Trng i hc Khoa hc T nhin - i hc Qucgia H Ni, cng nh to iu kin cho mt s cn b khoa hc, cng nhn vin phn

tch ca cc phng th nghim khc c ti liu tham kho, chng ti mnh dn binson cun sch ny. N c coi nh l gio trnh c s l thuyt vPhng phpphn tch phnguyn t. y cng l ni dung nhm phc vo to cao hc v kthut phn tch ny.

y l gio trnh ting Vit u tin v kthut phn tch hin i ny nn khngth trnh khi nhng hn ch v thiu st nht nh. V th rt mong sng gp thm kin ca cc bn b ng nghip v cc c gi quan tnm, tc gi c iu kin bsung cho hon chnh hn.

Nhn dp ny chng ti xin chn thnh cm n GS. TS. J.F.M. Maessen, GS. TS.

J. Kragton, GS. TS. Ban, TS. J.C. Kraak, cc ks H. Balker v J.W. Elgersma (khoaHa, trng i hc tng h p Amsterdam, H Lan), GS. TSKH. Trnh Xun Gin(Vin Ha), TS. Nguyn Hong (HQGHN), PGS. TS. Phm Gia Hu (H Dc) vcc ng nghip B mn Ha Phn tch khoa Ha hc c nhiu kin ng gpcho ni dung ca gio trnh ny.

Phm Lun


3/295

3

PPHHNN IIChng 1I CNG V PHNG PHP AES

1.1 Sphn loi ph

Phn tch quang ph l tn gi chung cho mt h cc phng php phn tchquang hc da trn c sng dng nhng tnh cht quang hc ca nguyn t , Ion,

phn tv nhm phn t. V d, tnh cht pht x hay hp th quang ca nguyn t,tinh cht hp th quang ca phn t, v.v... V vy ty theo quan nim, da theo nhngiu kin kch thch ph, phng tin thu ghi v quan st ph, cng nh bn cht caqu trnh sinh ra ph m ngi ta c mt s cch phn chia thnh nhng php o khcnhau, nh php o ph pht x nguyn t, h p th nguyn t, php o ph hng

ngoi,... Tuy th, nhng c hai cch phn chia sau y l ph hp hn:

1.1.1 Sphn chia theo c trng ca phTheo cch ny ngi ta c nhng phng php phn tch quang hc sau:

Phng php phn tch phnguyn t, gm c:a) Ph php x nguyn t

b) Ph hp th nguyn t

c) Ph hunh quang nguyn t

y l ph do s chuyn mc nng lng ca cc in t ha tr ca nguyn ttrng thi kh (hi) t do, khi b kch thch m sinh ra.

Phng php phn tch phphn t, gm c:a) Ph hp th phn t trong vng UV-VIS

b) Ph hng ngoi (IR v NIR)

c) Ph tn x Raman

Ph ny c quyt nh bi cc in t ha tr ca nguyn t trong phn t,

l nhng in t ha tr nm trong lin kt hay mt c p cn t do, chuyn mcnng lng khi b kch thch.

Ph Rn-ghen (tia X), l ph ca in t ni ca nguyn t, gm c:

+ Ph pht x tia X

+ Ph hunh quang tia X

+ Ph nhiu x tia X


4/295

4

Ph cng hng t, gm:

a) Cng hng tin t (ERMS).

b) Cng hng t proton (ht nhn: NRMS))

Phng php phn tch khi ph: Ph ny c quyt nh bi khi lng ca

cc Ion phn t hay cc mnh Ion ca cht phn tch b ct ra (t s m/z).

y l cch phn chia c s dng rng ri v c coi nh l hp l nht vtng ng vi ngay tng php o c th..

1.1.2 Phn chia theo di sngNh chng ta bit, bc xin t c mi bc sng, t sng di hng ngn

mt n sng ngn vi micromet hay nanomet. Do ph ca bc xin ty phi cha ht tt c cc vng sng . Nhng trong thc t khng c mt dng cquang hc no c th c kh nng thu nhn, phn li hay pht hin c ton b vng

ph nh th. V th ngi ta chia phin t thnh nhiu min (vng ph) khc nhau. l nguyn tc ca cch chia th hai ny (bng 1.1).

Bng 1.1Sphn chia ph theo di sng

STT Tn vng ph di sng

1

2

34

5

6

7

8

9

10

Tia gama ()

Tia X

T ngoi

Kh kin

Hng ngoi

Sng ngn

Sng Raa

Sng cc ngn

Tivi - FM

Sng raio

< 0,1 nm

0,1+ 5 nm

80 + 400 nm400 + 800 nm

1+ 400 m

400 + 1000 m

0,1+ 1 cm

0,1 - 50 cm

1 +10 m

10 + 1500 m

Trong bng 1.1, vng ph t 3 n 5 l vng ph quang hc. Cc ph ny xuthin do s chuyn mc nng lng ca cc in t ha tr ca nguyn t v phn t,khi b kch thch. Vic phn chia theo cch ny cng c nhng im cha r rng, nhtrong vng t ngoi v kh kin th c c ph ca nguyn t v phn t, m bn chtca hai loi ph v s kch thch ca hai loi li rt khc nhau.


5/295

5

Trn y l hai cch phn chia chnh cn hay c dng. Tt nhin cn c mts cch phn chia khc. Nhng c nhiu nhc im v t c s dng, nn khng cp n y.

S phn chia ny c tnh cht gii thiu chung ton b vng ph. Song, tronggio trnh ny chng ti ch cp n ph pht x nguyn t, v phng php phn

tch ng dng tnh cht pht x ca nguyn t trng thi hi c gi l K thutphn tch phpht x nguyn t.

1.2 Sxut hin ph pht x nguyn t1.2.1 Tm tt v cu to nguyn t

Hng ngy chng ta thng quen vi nhng tn gi, st (Fe), ng (Cu) ch(Pb), bc (Ao), vng (Au), nhm (Aj), km (Zn),... l nhng nguyn t ha hc.n nay ngi ta bit trn 110 nguyn t ha hc khc nhau. Nhng v mt hahc v theo thuyt ca alton, nguyn t ha hc bao gm nhng nguyn t ca cngmt loi v nguyn t l phn t nh nht cn gic tnh cht ha hc ca nguynt. Nguyn t ca mi nguyn t ha hc c cu to khc nhau nn chng c tnh chtkhc nhau. Quyt nh tnh cht vt l v ha hc ca chng l cu to ca lp velectron trong nguyn t, c bit l cc in t ha tr.

Nguyn t ca mi nguyn t ha hc u c xy dng t mt ht nhnnguyn t v cc electron (in t). Trong nguyn t, ht nhn gia, cc in tchuyn ng xung quanh ht nhn theo nhng quo (orbital) tng i. Ht nhnchim th tch rt nh trong khng gian ca nguyn t (khong 1/10.000 th tchnguyn t), nhng li chim hu nh ton b khi lng ca nguyn t. Nu coi

ng knh nguyn t l 10-8cm, th ng knh ht nhn ch chim khong 10-12

cm.Nh vy, lp v ca nguyn t ngoi ht nhn l rt rng, n chnh l khng gianchuyn ng ca in t. S chuyn ng ca in t trong khng gian ny rt phct p. N va tun theo quy lut ca chuyn ng sng, li va tun theo quy lutchuyn ng ca cc ht vi m. Song trong mt iu kin nht nh v mt cch tngi, ngi ta vn tha nhn cc in t chuyn ng trong khng gian ca nguyn ttheo cc quo. Nhng theo quan im hin i ca c lng t th l cc mmy electron.

Trong lp v nguyn t, in t phn b thnh tng lp ng vi s lng t

chnh ca nguyn t (n). Trong tng lp li c nhiu quo ng vi s lng t ph1 ca nguyn t. l cc phn lp. Nhng theo nguyn l vng bn th in t baogicng chim v lm y nhng quo c mc nng lng thp trc. Sau min nhng quo c mc nng lng cao hn. Th t sp xp l: ls, 2s, 2p, 3s,3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d, 7p, v.v... (hnh 1.1a).


6/295

6

Hnh 1.1aS phn b nng lng trong nguyn t(1)-cu to nguyn t: (2)-phn b nng lng trong lp v nguyn t.


7/295

7

Qu trnh pht x v hp th ca mt nguyn t.E0: Nng lng trng thi cbn; cm: Nng lng trng thi kch thch; + hv

v + E: Nng lng kch thch; - hv: Tia pht x.

Hnh 1.1 bS phn b nng lng trong nguyn tv sinh ph1.2.2 Sxut hin ph pht x

Trong iu kin bnh thng, cc in t chuyn ng trn cc quo ng vimc nng lng thp nht. Khi nguyn ttrng thi bn vng, trng thi cbn. trng thi ny nguyn t khng thu v cng khng pht nng lng. Nhng nucungcp nng lng cho nguyn t th trng thi khng tn ti na. Theo quanim ca thuyt lng t, khi trng thi kh, in t chuyn ng trong khng gianca nguyn t, c bit l cc in t ha tr, nu chng nhn c nng lng bn


8/295

8

ngoi (in nng, nhit nng, ha nng,...) th in t s chuyn ln mc nng lngcao hn. Khi nguyn t b kch thch. N tn ti trng thi kch thch. Nhngtrng thi ny khng bn vng. Nguyn t ch lu li trng thi ny nhiu nht l 108 giy. Sau n lun lun c xu hng trv trng thi cbn ban u bn vng.

Ngha l gii phng nng lng m chng hp thc trong qu trnh trn didng ca cc bc x quang hc. Bc x ny chnh l ph pht x ca nguyn t, n c

tn sc tnh theo cng thc:E = (En E0) = hv (1.1)

hay:

hcE=

trong :

En v E0 l nng lng ca nguyn ttrng thi cbn v trng thi kch thch n;

h l hng s Plank (6,626.10-7 erk.s) hay h = 4,1.10-15 eV.s;

c l tc nh sng (3.108 m/gy) - 2,99793.108 m/gy;

vl tn s ca bc x;

l bc sng ca bc x.

Trong biu thc trn, nu gi trE l nm ta c qu trnh hp th v khi gi trE dng ta c qu trnh pht x ca nguyn t.

Nhng trong nguyn t, s chuyn mc ca in t t mc nng lng En khngphi ch v mc E0, m c rt nhiu s chuyn mc t En v cc mc khc E01, E02,E03 cng vi mc E0. Ngha l c rt nhiu s chuyn mc ca in t c lngt ha, v ng vi mi bc chuyn mc ta c 1 tia bc x, tc l mt vch ph.Chnh v th m mt nguyn t khi b kch thch thng c th pht ra rt nhiu vch

ph pht x. Nguyn t no c nhiu in t v c cu to phc tp ca cc lp in tha tr th cng c nhiu vch ph pht x. Mt cch tng trng ta c th m t qutrnh kch thch v pht x ca nguyn t theo m hnh trong hnh 1.2.

Nu dng my quang ph thu chm ra pht x, phn li v ghi li cc chmtia pht x do nguyn t pht ra ta sc mt di ph t sng ngn n sng di. l ph pht x ca nguyn t ca cc nguyn t v n l ph vch.Nhvy, phpht

x nguyn tl sn phm sinh ra do stng tc vt cht, m y l cc nguyn tt do trng thi kh vi mt ngun nng lng nhit, in... nhtnh ph hp.

Nhng trong ngun sng, khng phi ch c nguyn t t do b kch thch, m c cIon, phn t, nhm phn t. Cc phn t ny cng b kch thch v pht ra ph pht x

ca n. Tt nhin l trong mc khc nhau ty thuc vo kh nng kch thch cangun nng lng. V vy, ph pht x ca vt mu lun bao gm ba thnh phn:

1. Nhm ph vch. l ph ca nguyn t v con. Nhm ph vch ny cacc nguyn t ha hc hu nh thng nm trong vng ph t 190-1000nm (vng UV-VIS). Ch c mt vi nguyn t kim hay kim loi kim mi c mt s vch ph nmngoi vng ny.

2. Nhm phm. l ph pht x ca cc phn t v nhm phn t. V d


9/295

9

ph ca phn t MeO, CO v nhm phn t CN. Cc m ph ny xut hin thngc mt u m v mt u nht. u m pha sng di v nht pha sng ngn.Trong vng t ngoi th ph ny xut hin rt yu v nhiu khi khng thy. Nhngtrong vng kh kin th xut hin rt m, v lm kh khn cho php phn tch quang

ph v nhiu vch phn tch ca cc nguyn t khc b cc m ph ny che lp.

3. Ph nn lin tc. y l ph ca vt rn bt nng pht ra, ph ca nh sngtrng v ph do s bc x ring ca in t. Ph ny to thnh mt nn m lin tctrn ton di ph ca mu. Nhng nht sng ngn v m dn v pha sng di. Phny nu qu m th cng s cn trphp phn tch.

Ba loi ph trn xut hin ng thi khi kch thch mu phn tch v trong phntch quang ph pht x nguyn t ngi ta phi tm cch loi bt phm v ph nn. l hai yu t nhiu.

1.3 Nguyn tc ca php o ph pht x (AES)T vic nghin cu nguyn nhn xut hin ph pht x, chng ta c th khi qut

phng php phn tch da trn cso ph pht x ca nguyn t phi bao gm ccbc nh sau:

1. Trc ht mu phn tch cn c chuyn thnh hi (kh) ca nguyn t hayIon t do trong mi trng kch thch. l qu trnh ha hi v nguyn t ha mu.Sau dng ngun nng lng ph hp kch thch m hi chng pht x.y l qu trnh kch thch ph ca mu.

2. Thu, phn 1i v ghi ton b ph pht x ca vt mu nh my quang ph.Trc y, ph c ghi ln knh nh hay phim nh. Chnh my quang ph s lmnhim v ny. Nhng nhng trang b hin i ngy nay c th thu v ghi trc tip cctn hiu cng pht x ca mt vch ph di dng cc lc trn bng giy hay chra cc sng cng vch ph trn my in (printer), ghi li vo a t ca my tnh.

3.nh gi ph ghi v mt nh tnh v nh lng theo nhng yu cu tra. y l cng vic cui cng ca php o.

Chnh v vy, ng vi cc nhim v v nguyn tc ny, thc hin mt phpphn tch da theo ph pht x ca nguyn t ngi ta phi cn mt h thng trang bcng gm ba phn tng ng nh th.

A. Trang b cbn (ti thiu phi c):Phn 1: Ngun nng lng ha hi, nguyn t ha mu v kch thch ph

ca mu phn tch, c ph ca nguyn t phn tch.

Phn 2: My quang ph thu, phn li v ghi li ph pht x ca mu phn tchtheo vng ph ta mong mun.

Phn 3: H thng trang bnh gi nh tnh, nh lng v ch th hay biuth cc kt qu.

Mt cch tng qut, ta c th m t nguyn tc ny theo shnh 1.3.


10/295

10


11/295

11

B. Trang b hon chnh:

C thm cc b phn sau:

1. B tng bm hay a mu vo o.


12/295

12

2. H my tnh v phn mm ca n.

Hnh 1.3

S nguyn l ca qu trnh phn tch AES


13/295

13


14/295

14

1.4 i tng ca phng php phn tch ph pht xBn cnh mc ch nghin cu vt l quang ph nguyn t, php o ph pht x

nguyn t l mt phng php phn tch vt l da trn tnh cht pht x ca nguyn ttrng thi hi xc nh thnh phn ha hc ca cc nguyn t, cc cht trong mu

phn tch. V vy n c tnphn tch quang phha hc. Phng php ny c sdng phn tch nh tnh v nh lng cc nguyn t ha hc, ch yu l cc kimloi trong mi i tng mu khc nhau, nha cht, ha hc, luyn kim, ha du,nng nghip, thc phm, y dc, mi trng... thuc cc loi mu rn, mu dung dch,mu bt, mu qung, mu kh. Tuy phn tch nhiu i tng, nhng thc cht lxcnh cc kim loi l chnh, ngha l cc nguyn t c ph pht x nhy, khi c kchthch bng mt ngun nng lng thch hp; sau l mt vi kim nh Si, P, C. Vvy, i tng chnh ca phng php phn tch da theo php o phpht x canguyn tl cc kim loi nng nh trong cc loi mu khc nhau. Vi i tng

kim th phng php ny c nhiu nhc im v hn ch v nhy, cng nhnhng trang b thu, ghi ph ca chng, v ph ca hu ht cc kim li nm ngoivng t ngoi v kh kin, ngha l phi c thm nhng trang b phc tp mi c th

phn tch c cc kim.

1.5 Cc u im v nhc imPhng php phn tch quang ph pht x nguyn t s dc pht trin rt

nhanh v c s dng trong nhiu lnh vc ca khoa hc, kthut cng nghip, nngnghip v i sng v n c nhng u im rt cbn:

a) Phng php ny c nhy rt cao. Bng phng php ny nhiu nguyn t

cthc xc nh t n nhy t n.103n n.104 %. Nhng vi nhng trang bhin i v vi nhng ngun kch thch ph mi (ICP) ngi ta c tht n nhyt n.10-5n n.10-6 % i. Vi nhiu nguyn t m khng cn phi lm giu mu phntch (bng 1). V th n l phng php kim tra, nh gi tinh khit ca nhiuha cht v nguyn liu tinh khit cao, phn tch lng vt cc kim loi nng c hitrong i tng thc phm, nc gii kht, mi trng. Trong khi vi nhng itng ny th phng php ha hc khng th no t c.

b) Phng php ny gip chng ta c th tin hnh phn tch ng thi nhiunguyn t trong mt mu, m khng cn tch ring chng ra khi nhau. Mt khc, likhng tn nhiu thi gian, c bit l phn tch nh tnh v bn nh lng.

c) Vi nhng tin b ca kthut hin nay v vi nhng trang b hin nay tc, th phng php phn tch theo ph pht x nguyn t l mt php o chnh xctng i cao. Trong nhiu trng hp, vi nng nh (cppm) m sai s ca phpo l di 10%. Tt nhin vi nhng trang b v my mc cin th sai s c th lnhn. Song thc t n b nhng trang b hin i y li.


15/295

15

d) Phng php phn tch theo ph pht x l mt phng php phn tch tiu tnt mu, ch cn t 1 n vi chc miligam mu l . c bit l kthut ph pht x.

e) Phng php phn tch ny c th kim tra c ng nht v thnh phnca vt mu nhng v tr (ch) khc nhau. V th cng c ng dng kim tra ng nht ca b mt vt liu.

f) Trong nhiu trng h p, ph ca mu nghin cu thng c ghi li trnphim nh, knh nh hay trn bng giy. N l nhng ti liu lu tr v khi cn thit cthnh gi hay xem xt li m khng cn phi c mu phn tch.

Bn cnh nhng u im nu, phng php ny cng c mt s nhc imv hn ch nht nh. Trc ht phi ni n l: Phng php ny ch cho chng ta

bit c thnh phn nguyn t ca mu phn tch, m khng ch ra c trng thilin kt ca n trong mu. chnh xc ca php phn tch ph thuc vo nng chnh xc ca thnh phn ca dy mu u v cc kt qunh lng u phi datheo cc ng chun ca cc dy mu u c ch to sn trc.

Mc du c mt s nhc im v hn ch, nhng phng php phn tch quangph pht x nguyn t ngy cng c pht trin v ng dng rng ri trong nhiu lnhvc khc nhau xc nh lng vt cc nguyn t trong cc i tng mu khcnhau. l mt phng php phn tch nhanh, c chnh xc bo m v nhykh cao, c bit thch hp cho phn tch t him, cc loi nc, khng kh, v.v.

1.6 Kh nng v phm vi ng dngPhng php phn tch quang ph pht x nguyn t ngy nay gi vai tr quan

trng trong ha hc phn tch. Cng vi s pht trin mnh m ca cc ngnh khoa

hc k thut, c bit l vt l v ha hc, s pht trin ca kthut o v ghi tn hiu, lm tng kh nng ng dng to ln ca n. Bng phng php ny ngi ta c thxc nh nh tnh, bn nh lng v nh lng c hn nm chc kim loi v gnmt chc nguyn t kim trong cc i tng mu khc nhau (v c v hu c).Phng php phn tch ny trthnh cng c phn tch nguyn tc lc ch nhiulnh vc, nht l sau khi c ngun kch thch ICP.

a)Phn tch quang phpht x trong ngnh ha v cng nghip ha hc.N lcng c cc nh ha hc xc nh thnh phn nh tnh v nh lng ca nhiucht, kim tra tinh khit ca cc ha phm, nguyn liu v nh gi cht lng ca

chng. N cng l mt phng php xc nh cc ng v phng x v nghin cucu trc nguyn t.

b) Phn tch quang phpht x nguyn t tronga cht. Ngay t khi mi rai, phng php ny c cc nh a cht s dng phn tch cc mu qung phcv cho cng vic thm d a cht v tm ti nguyn khong sn. V th ngnh a chtca tt c cc nc u c phng phn tch quang ph pht x rt hin i v honchnh.


16/295

16

c)Phn tch quang phpht x trong luyn kim. Luyn kim cng l mt ngnhs dng phng php phn tch quang ph pht xu tin vo mc ch ca mnhtrc c ngnh ha. Chnh tnh cht nhanh chng v nhy ca phng php ny lmt iu rt cn thit i vi ngnh luyn kim. N c th l cng c gip cc nhluyn kim xc nh ngay c thnh phn ca cc cht ang nng chy trong l luynkim; qua m h c thiu chnh nguyn liu a vo ch to c nhng hpkim c thnh phn mong mun, kim tra thnh phn, kim tra nguyn liu.

d) Phn tch quang ph pht x trong tiu ch un hc. Trc y con ngitng rng kh c th hiu bit c thnh phn ha hc ca cc hnh tinh xungquanh tri t chng ta. Nhng phng php phn tch quang ph pht x ra i mrng tm vi cho con ngi. Bng phng php ph pht x v kt hp vi mt sknh thin vn, cc nh thin vn c th quan st c thnh phn ca cc nguyn tha hc ca cc hnh tinh khc nh mt trng, cc v sao. Chnh nhng kt qu phntch thnh phn ca cc mu t do v tinh ly t mt trng v ni ln ngha ca

php o ph pht x trong lnh vc nghin cu thin vn. V nhng kt qu phn tchthc t cc mu l rt ph hp vi nhng s liu thu c trc y qua phn tch tiasng t mt trng bng h thng my quang ph v knh thin vn.

e)Phn tch quang phpht x trong nng nghip, y v sinh hc. y l nhngngnh khoa hc s dng phng php ny cm li nhiu kt qu rc r, c bit ltrong vic nghin cu th nhng, nghin cu cc nguyn t vi lng trong t trng,trong cy trng, trong phn bn ca nng nghip, hay nghin cu thnh phn thc n

phc v chn nui, phn tch nguyn t vi lng trong mu, serum, nc tiu, phc vcha bnh.

f) Phn tch quang ph trong cc lnh vc khc. Ngoi nhng lnh vc nu,phng php phn tch quang ph pht x nguyn t cn c mt s ngnh khc sdng nh l mt cng c phn tch. V d nh trong ngnh dc kim tra mt skim loi c hi trong cc dc phm (Pb, Cu, Zn, Hg,...). Ngnh nghin cu c s,cng an, to n, ngoi thng, cng l nhng ni s dng phng php phn tchquang ph trong cc lnh vc nghin cu v nng php o ph nguyn t ln ngangtm ca thi i.

Hnh 1.5Sphn chia cc loi ph theo di sng


17/295

17

Chng 2SKCH THCH PH PHT X NGUYN T

2.1 Yu cu v nhim v ca ngun kch thchTrong phn tch quang ph pht x nguyn t, ngun sng c gi l ngun

kch thch ph v c mt vai tr ht sc quan trng. V nh ngun nng lng kchthch ngi ta c th chuyn vt liu mu phn tch thnh trng thi hi ca ccnguyn t v kch thch m hi pht sng (pht x), ngha l ngun nng lng nhhng trc tip n kt qu ca php phn tch. V vy ngun nng lng mun dngc vo mc ch phn tch quang ph pht x nguyn t cn phi thc hin c hainhim v sau y:

Trc ht ngun nng lngphi ha hi, nguyn tha v chuyn c honton cc nguyn liu mu phn tch vo vng phng in (plasma). C nh th thnh

phn ca m hi trong plasma mi ng nht vi thnh phn ca vt mu. ng thingun nng lng phi c nng lng ln (nhit ) c th kch thch c ttnht cc nguyn t ca nguyn t cn phn tch i n pht x ra ph ca n.

Song nh th vn cha hon ton . V mt phng php phn tch ch c gi trkhi n c nhy v lp li cao. Chnh v th, ngoi hai nhim v trn mt ngunsng mun dng c lm ngun kch thch ph pht x n cn cn phi tha mn mts yu cu nht nh sau y:

1) Trc ht ngun sng phi m bo cho php phn tch c nhy cao vcng ca vch ph phi nhy vi s bin thin nng ca nguyn t phn tch;nhng li khng nhy vi s dao ng ca iu kin lm vic.

2) Ngun nng lng phi n nh v bn vng theo thi gian, m bo chophng php phn tch c lp li v n nh cao. Ngha l cc thng s cangun sng chn nht thit phi duy tr v lp li c.

3) Ngun nng lng phi khng a thm ph ph vo lm ln vi ph ca munghin cu. Nu khng s lm kh khn thm cng vic nh gi nh tnh v nh

lng v c khi lm sai lc c kt qu phn tch.

4) Ngun kch thch phi c s cu to khng qu phc tp; nhng li c khnng thay i c nhiu thng s, c th chn c nhng iu kin ph hp theotng i tng phn tch hay tng nguyn t.

5) Yu cu cui cng l ngun nng lng kch thch phi lm tiu hao t muphn tch v trong mt s trng hp phi khng lm h hi mu phn tch, nh trong


18/295

18

kim tra thnh phm.

Trn y l nhng yu cu chung, nu ngun sng tha mn c y cc yucu l mt iu rt l tng. Song trong thc t khng c c iu kin l tngnh th. V vy, mi ngi lm phn tch quang ph pht x cn phi hiu bit tngtn v ty theo tng trng hp c th m xem yu cu no cn c ch nghim

ngt trc ht v yu cu no c th chnm chc c.

2.2 Cc loi ngun kch thch ph pht xDo nhng yu cu nu trn, nn ch c mt vi loi ngun nng lng c

dng lm ngun kch thch cho phng php phn tch quang ph pht x. l:Ngn la n kh; H quang in dng xoay chiu v mt chiu; Tia la in; Tialc; Plasma cao tn cm ng (ICP); tia X.

Trong cc loi ngun nng lng ny, ngn la n kh, h quang v tia la in c dng t lu, nhng c nhy khng cao (ngun sng cin). Tia lc vICP l nhng ngun nng lng cho nhy cao. Nhng tia lc t c dng. V c c tia lc ph hp cho mc ch phn tch phi c nhng trang b phc tp, tnkm. Ring ICP l ngun nng lng hin ang c ng dng ph bin v c nhiuu vit. N l ngun sng mi c pht trin trong vng hn chc nm li y. Likhng t nh ngun lc. V nhy ca cc ngun nng lng ny chng ta c thxem trong hnh 1.4 v bng ph lc A.

2.2.1 Ngn la n kh2.2.1.1 Cu to v c im ngn la n kh

Ngn la n kh l ngun nng lng u tin c dng trong phn tch quangph pht x nguyn t, ngay t khi phng php ny ra i. Bunsen v Kirschoff lnhng ngi u tin dng ngun sng ny phn tch cc kim loi kim v kimth. Nhng do sn gin, n nh, nhy tng i v r tin, nn ngy nay n vnc s dng ph bin.

1. c im. Ngn la n kh c nhit khng cao (1700 32000C), c cuto n gin, nhng n nh v d lp li c cc iu kin lm vic. Do c nhit thp, nn ngn la n kh ch kch thch c cc kim loi kim v kim th. V ng

vi loi ngun sng ny ngi ta c mt phng php phn tch ring. l phngphp phn tch quang phngn la (Flame Spectrophotometry). Song v bn cht nvn l ph pht x ca nguyn t trong ngn la. Cc cht kh t to ra ngn laca n kh thng l mt hn hp ca hai kh (1 kh oxy ha v 1 kh nhin liu)c trn vi nhau theo mt t l nht nh.Bn cht v thnh phn ca hn hp khquytnh nhit ca ngn la v hnh dng cu to ca ngn la. Di y l mtvi hn hp kh c s dng to ra ngn la trong php o ph pht x:


19/295

19

Bng 2.1Nhit ca mt s loi n kh

STT Loi hn hp kh t Nhit (oC)

1 Axetylen v oxy 2400-3100

2 Butan v oxy 2000-2550

3 Kh t v oxy 2200-2500

4 Hydro v oxy 2100-2300

5 Axetylen v khng kh 2000-2450

2. Cu to. V hnh dng cu to v s phn b nhit ca ngn la, n kh gmba phn nh sau (hnh 2.3 v 2.4):

Phn mt (a). Phn trong cng st ming n l phn ti (a). Trong phn nycht t c trn v nung nng chun bt chy phn hai (b). Nhit trong

phn ny thp (di 700-12000C).

Phn hai (b) l li ca ngn la.Trong phn ny xy ra cc phn ng t chycht kh. N khng mu hay c mu xanhnht v nhit l cao nht. Chnh nh b lch c nhit cao nht ca ngn la. Nut bng hn h p kh axtylen v oxy haykhng kh nn th nhit ca ngn la ccung cp nhphn ng:

2C2H2+ 2O2 = 4CO + 2H2+ Q

Phn th ba (c) l v ca ngn la.Trong phn ny thng xy ra cc phn ngth cp, c mu vng v nhit thp. N to

thnh ui v v ca ngn la.

Do s phn b nhit ca ngn la nh vy, nn kh phn tch phi a muvo vng tnm c nhit cao nht. l vng b ca ngn la v trong vng ny ccqu trnh sn nh, v hiu sut kch thch ph s cao nht.


20/295

20

2.2.1.2n nguyn tho mu (burner head)L b phn t chy hn h p kh

to ra ngn la. Tuy c nhiu kiu n khcnhau, nhng u c nguyn tc cu toging nhau (hnh 2.4 v 2.5). l h thng

nguyn t ha mu. Song bt k mt loin no mun dng c cho php phntch quang ph, ngoi nhng yu cu nu(mc 1) n cn phi m bo trn u hnhp kh trc khi t v ngn la phi chyngoi ming n.

Hnh 2.4.n v h thng NTH mu ca hngPhilips

(1) n nguyn t ha mu; (2) Mng bo him; (3) ng thi phn mu tha;(4) ng dn cht oxy ha; (S) ng dn mu vo bung aerosol ha; (6) ngdn cht chy C2H2; (7) Vin bi to bi aersol.

H nguyn t ha mu bao gm:

+ Bung ht mu v to th si kh (1)

+ n nguyn t ha mu (2)

Hnh 2.5H thng nbiulai theo kthut pneumatic

K: Kh mang (oxy ha); S: ng dn mu; F: Kh chy; Q: Cnh qut quay u;G: Mng bo him; A: ng dn th aetosol ln n nguyn t ha.

2.2.1.3 Qu trnh kch thch phtrong ngn la

Trong phn tch quang ph pht x, nu dng ngn la lm ngun kch thch thmu phn tch phi chun b dng dung dch. Sau nh mt h thng phun


21/295

21

(Nebulizer system) a dung dch ny vo ngn la di dng th sng m (thsi kh - aersol) cng vi hn hp kh t. Khi vo ngn la, di tc dng ca nhit, trc ht dung mi bay hi, li cc ht bt mu mn ca cc cht phn tch, rin c t nng v chuyn thnh hi ca nguyn t v phn t. Trong ngn la cc

phn t hi ny chuyn ng, va chm vo nhau, trao i nng lng cho nhau,... Ktqu ca cc qu trnh lm cc phn t b phn li thnh nguyn t, b Ion ha v bkch thch. Nh vy trong ngn la tn ti c nguyn t t do, phn t, Ion, nhm phnt v cc in t. Trong tp hp ch yu chcc nguyn t tdob kch thch v

pht x. V th ph pht x ngn la l ph ca nguyn t trung ha. Nguyn nhn gyra s kch thch phy l s va chm ca cc nguyn t vi cc in t c ngnng ln trong ngn la. V th nhit ngn la cng cao th cc in t c ngnng cng ln. V th n l s kch thch nhit. Do cch ca qu trnh kch thchtrong ngn la l:

1. Trc ht l dung mi bay hi, li cc ht mu dng bt. Tip theo l

bt mu b nung nng, nng chy. Sau l cc qu trnh nhit ha ca cht mu xyra theo tnh cht ca n, v gm c.

2. Hai qu trnh chnh:Nu Eh < Ent th mu s ha hi, nguyn t ha to ra cc nguyn t t do,

chng b kch thch v pht x cphpht x (AES).

Men Xm(r).Th. Men Xm(k) mMe(k) + nX(k)

Me(k) + E Me(k)* Me O+ n(Hv)

Nng lng K Chm tia pht x

Cch ny (cch 1) cho nhy v n nh cao ca s pht x. Cc hpcht mui halogen (tr F), axetat, mt s nitrat, sunphat... ca kim loi thng theo cch ny.

Nu Eh > Ent th mu s b nguyn t ha trc, sau ha hi thnh nguyn

t, ri b kch thch ph sinh ra ph pht x (AES).

MenXm(r) Mn(r) + Xm(r) mMe(k) + nX(k)

Me(k) + E Me(k)* Me O+ n(Hv)

Nng lng K.Th. Chm tia pht x

C ch ny (c ch 2) cho nhy v n nh km so vi c ch 1 ca spht x.

Cc hp cht mui SIO2-3, PO3-

4, F-, mt s nitrat, sunphat,.. ca kim loi thng

theo cch ny.

Do c im ca 2 cch chnh , nn trong phn tch ngi ta thng to iu


22/295

22

kin s kch thch ph xy ra theo cch 1 c li. iu gii thch ti sao ngita hay dng cc mui halogen, v axetat ca kim loi kim lm nn, hay cht ph giacho mu phn tch.

3. Qu trnh phTrong ngn la thng c cc qu trnh ph km theo hai qu trnh chnh, cc

qu trnh ny ty iu kin ca ngn la m xut hin trong mc khc nhau, v nu khng c li, nn cn c loi tr trong phn tch. Cc qu trnh l:

- To ra cc hp cht bn nhit, ch yu l cc monoxit (dng MeO).

- S Ion ha to ra cc Ion, ch yu l s Ion ha bc 1.

- S hp th bc x ca nguyn t (qu trnh to).

- S pht x ph nn lin tc ca ht rn v electron b nung nng.

Ni chung, cc qu trnh ny u khng c li. Ngi ta phi tm bin php loi

tr. V d:

+ hn ch s hnh thnh hp cht bn nhit kiu MeO, ngi ta thm vo mumui Clorua ca kim loi kim (nh KCI, CsCL) lm nn, kch thch ph theo cch1, hoc kch thch ph trong mi trng kh trargon.

+ Hay loi tr s Ion ha ca nguyn t phn tch, ngi ta thm vo mucht ph gia l mui halogen kim loi kim c th Ion ha thp hn th Ion ha canguyn t phn tch. Nh th nguyn t phn tch s khng b Ion ha na.

So vi cc loi ngun kch thch khc, ngn la l ngun kch thch n gin,

nhy trung bnh (0,1 - 10g) v tng i n nh. Nhng cng ca vch phcng bnh hng bi mt s yu t nht nh, nh:

+ Trc ht l dng lin kt ca cc nguyn t trong cht mu.

+ Th hai l nh hng ca cc gc axit. Ni chung, cc axit d bay hi th nhhng ny l nh.

+ Th ba l cht nn hay thnh phn ca dung dch mu, v vy dung dch phntch v dung dch mu chun u phi c cng thnh phn vt l v ha hc, nht lcht nn phi cng loi.

+ nh hng th t l cc phn ng ha hc xy ra trong ngn la. c bit lnh hng ca cc phn ng to ra cc hp cht bn nhit. Cc hp cht ny thnglm gim cng ca vch ph, ngha l lm gim nhy ca phng php phntch. l khi qut chung, tt nhin cc yu t trn khng phi bao gi cng xuthin ng thi v tc dng nh nhau trong mi trng hp, m ty tng trng hpyu t no l ni bt v cn c ch . Song mt iu cn nhl nhit ngn la lyu t quan trng nht, n quyt nh cc qu trnh xy ra trong khi kch thch ph.


23/295

23

2.2.2 H quang in2.2.2.1c im v tnh cht1. H quang l ngun kch thch c nng lng trung bnh v cng l ngun kch

thch vn nng. N c kh nng kch thch c c mu dn in v khng dn in.Ty thuc vo cc thng s (A, C, R) ca my pht h quang v loi in cc ta chn

m h quang c nhit t 3500 60000C. Vi nhit ny nhiu nguyn t t ccnguyn liu mu khc nhau c thc ha hi, nguyn t ha v kch thch ph phtx. Nhit ca h quang ph thuc rt nhiu vo bn cht ca nguyn liu lm incc. V th trong mt iu kin nh nhau, h quang in cc than (graphite) c nhit cao nht (bng 2.2). Cng dng in trong mch h quang l yu t quyt nhnhit ca h quang.

2. H quang l ngun kch thch cho nhy tng i cao, v trong h quangmu phn tch c ha hi tng i d dng hn trong cc loi ngun nng lngkhc, c bit l h quang dng mt chiu. Nhng cng do tnh cht ny m php phntch dng h quang c n nh v lp li km ngn la v tia in. y hquang dng xoay chiu cho kt qun nh hn h quang dng mt chiu, v th hquang dng xoay chiu c s dng nhiu hn.

Bng 2.2Th Ion ha v nhit h quang ca mt s kim loi

Nguyn t lmin cc

Th Ion ha canguyn t (eV)

Dng in hquang (A)

Nhit ca hquang (oC)

C

Zn

Fe

Cr

Al

11,309,40

7,80

6,90

5,67

10

10

10

10

10

60005200

4800

4400

4000

3. H quang l ngun kch thch c s mch tng i n gin, nhng li cnhiu thng s c thiu chnh c chn c mt nhit tng i ph hp

cho php phn tch mi cht.

4. H quang l s phng in gia hai in cc c th thp (di 260V) v dngcao (t 8 - 20A). Th v dng ca mch h quang c lin h vi nhau theo cng thc:

21 RR

UIv

I

BAU m

m +=+=

Trong A v B l cc hng s, R1 l in trplasma h quang. Cng thc nygii thch ti sao h quang km bn vng. V th phi a vo mch h quang mt


24/295

24

bin trph R2 tht ln (R2 >> R1) hn ch s thay i cng dng in trongmch v gi cho h quang n nh.

2.2.2.2 Cu to v hotng ca h quangTuy c nhiu loi my pht h quang khc nhau, nhng v nguyn tc cu to, tt

c cc my pht h quang u gm hai phn chnh.

- Phn 1 l mch pht h quang(mch chnh), n c nhim v cung cp nnglng ha hi, nguyn t ha mu v kch thch m hi i n pht x. Nu hquang mt chiu th l mch ca dng in mt chiu (hnh 2.6). Nu l h quangxoay chiu, th l dng in xoay chiu.

- Phn 2 l mch chhuy (mch iu khin), c nhim v ch huy phn mchchnh hot ng theo nhng thng s c chn cho mt mc ch phn tch nhtnh. Theo shnh 2.6, nu cng tc Kc ng v tr AA, ta c h quangdng xoay chiu. Cn khi v tr BB' ta c h quang dng mt chiu. S hot ng

ca h quang din ra nh s(s hnh 2.6):

Khi ng cng tc K', ngha l a in vo my, qua h thng bin tr Ri tachn c th lm vic ph hp a vo bin th Tr1 tng th. Khi thhai ucun th cp ca bin th ny t n hng ngn vn (2000-7000V). Hai u cun nyc ni vi cun cm Ll, tin C1 v khong n ph E. Lc t C1c tch inv khi t n th Vf chn th c mt tia in c nh qua khong E, nh th lptc trn mch dao ng L1c1 xut hin mt dao ng cao tn. Do mch dao ng L2C2cng ghp vi mch dao ng LlC1, nn trn mch L2C2 cng xut hin mt dao ngcao tn cm ng nh th. Nhng s vng ca L2 l ln hn nhiu L1, nn th trn hai

ct ca t C2 cng s ln hn C1. Th ny li xut hin tc khc khi c tia in nh E v iu khin s phng in qua khong F ca hai in cc h quang. Nh th mtchu k phng in th nht c thc hin v kt thc, ri li ti p din n chu k

phng in th hai, th ba,... v c th din ra cho n khi no ta ngt cng tc K', tcl ngng s phng in ca h quang.


25/295


26/295

26

2

1R

URI += (2.3)

trong :

R1: in trct kh ca plasma gia hai in cc.

R2: in trni ca my pht h quang.

U: Th ca ngun in ca my pht t ln hai u in cc F.

2. Yu t th hai quyt nh nhit ca h quang l bn cht ca nguyn liulm in cc.Ni chung, nguyn liu no kh bay hi v cu to t nhng nguyn tha hc c th Ion ha cng cao, th h quang dng in cc nguyn t s cho nhit cng cao. V th h quang in cc than (graphite) s c nhit cao nht so vicc h quang in nguyn liu khc trong cng mt iu kin dng in th nghim(bng 2.2).

3. Yu t th ba l thIon ha ca cc cht c trong mu phn tch, ch yu lcht nn v nhng cht c nng ln. Ni chung, nguyn t no (nht l cht nn camu) c th Ion ha cng nh th cng lm gim nhiu nhit ca h quang, ngha lcc kim loi kim c tc dng mnh nht, sau n cc kim loi kim th. Chnh vth th ngi ta thng thm cc mui halogen ca kim loi kim vo mu gp

phn iu chnh nhit ca plasma ca h quang cho s kch thch ph ca ccnguyn t phn tch. Bn cnh th Ion ha, th nng ca cc nguyn t cng c nhhng n nhit ca h quang, c bit l cc nguyn t c th Ion ha thp s lmgim nhit ca h quang cng nhiu, khi nng ca n trong plasma cng ln.

4. Trong plasma ca hquang thng lun lun xy racc phn ng ha hc v vt l. l qu trnh phn li, Ion ha,t hp phn t v s kch thch

ph. Nhng qu trnh nythng l cc qu trnh nhitng hc v m hi trong

plasma cng c mt nhit

nht nh ng vi nhng thngs xc nh ca my pht hquang c chn. Nhit m bo cho plasma c mtp lc in t xc nh duytr s kch thch ph.


27/295

27

2.2.2.4 S kch thch ph trong plasma h quang in

Trong plasma ca h quang, s kch thch ph pht x c th xy ra theo cc qutrnh sau:

1. Nu l mu dung dch, th trc ht l dung mi bay hi, li cc ht btmu. Ri cc cht (c mu bt v b ca mu dung dch cn li), bt mu b nung

nng, nng chy. Sau l cc qu trnh nhit ha ca cht mu xy ra theo tnh chtca n. C th l cc ht mu s bay hi hay chuyn khi vo plasma gia 2 in cc.Ti y s xy ra cc qu trnh chnh v ph khc nhau, ty thuc vo thnh phn v

bn cht ca cht mu.

2. Nu mu l dng bt c nhi vo trong l in cc mang th c cc qutrnh trong in cc mang mu l:

+ S nung nng v nng chy,

+ S chuyn ng nhit khuch tn v i lu ca cc ht trong lin cc,

+ Cc phn ng ha hc phn hy ca cc cht mu km bn nhit,

+ S bay hi ca cc phn t cht mu vo plasma,

+ S vn chuyn ca cc ht mu cha ha hi vo plasm.

Bn cht v thnh phn ca mu trong lin cc v nhit ca in cc l yut quyt nh s din bin ca cc qu trnh trn.

Sau l cc qu trnh trong plasma. Gm c 2 qu trnh chnh sinh ra ph phtx nguyn t (AES) v mt s qu trnh ph nh sau:

A- Hai qu trnh chnh: Nu Eh < Ent th mu s ha hi, nguyn t ha to ra cc nguyn t t do,

chng b kch thch v pht x cphpht x (AES)

Men Xm(r) Men Xm(k) mMe(k) + nX(k)

Me(k) + E Me(k)* MeO+ n(hv)


Cch 2ny cho nhy v n nh cao ca s pht x. Cc hp cht muihalogen (tr F), axetat, mt s nitrat, sunphat,... ca kim loi thng theo cch ny.

Nu Eh > Ent th phn t ca cht mu s b nguyn t ha trc, sau hahi thnh cc nguyn t t do, ri mi b kch thch ph sinh raphpht x (AES).

Men Xm(r) Men (r) + Xm(r) mMe(k) + nX(k)



Cch 2 ny cho nhy v n nh km so vi cch 1 ca s pht x. Cc


28/295

28

hp cht mui SiO3, PO4, F, mt s nitrat, sunphat... kim loi thng theo cch ny.

Do c im ca 2 cch chnh , nn trong phn tch ngi ta thng to diukin s kch thch ph xy ra theo cch 1 c li. iu gii thch ti sao ngita hay dng cc mui halogen, v xetat ca kim loi kim lm nn, hay cht ph gia

cho mu phn tch v mu chun, bin i nn mu gc sang nn nhn to tt hnv li cn loi trc cc yu tnh hng, ng thi c khi cn tng c cnhy ln hng chc ln.

B- Cc qu trnh ph:Trong plasma ca h quang thng c:

- S hnh thnh hp cht bn nhit, ch yu l cc monoxit (dng MeO).



- S pht x ph nn lin tc.

Bn cht ca cht mu, thnh phn ca mu, cht ph gia trong lin cc vnhit ca in cc l yu t quyt nh din bin ca cc qu trnh trn.

Ni chung cc qu trnh ny u khng c li, ngi ta tm bin php loi tr.

V d:

+ hn ch s hnh thnh hp cht bn nhit kiu MeO, ngi ta thm vo mumui Clorua ca kim loi kim (nh KCI, CsCl,) lm nn, kch thch ph theo cch1, hoc kch thch ph trong mi trng kh trargon.

+ Hay loi tr s Ion ha ca nguyn t phn tch, ngi ta thm vo mucht ph gia mui halogen kim loi kim c th Ion ha thp hn th Ion ha canguyn t phn tch. Nh th nguyn t phn tch s khng b Ion ha na.

2.2.2.5 Cc loi my pht h quangMy pht h quang c rt nhiu, c nhiu hng cung cp, song xt v cch

mch in ca mch h quang chnh, th c th chia thnh hai loi:

+ My pht h quang dng mt chiu.

+ My pht h quang dng xoay chiu.

Tuy l hai loi, nhng u c nguyn tc cu to nh nhau (hnh 2.6). Trong mi

loi h quang li c nhiu ch khc nhau. Thng thng c mt ch lin tc vnhiu ch gin on. Chnh v th m chng ta c th chn c nhng gi tr nnglng khc nhau cho tng mc ch phn tch c th.

2.2.3 Tia la in2.2.3.1c im v tnh cht1. Tia la in l ngun kch thch ph c nng lng tng i cao. Ty theo

cc thng s ca my pht tia la in (C, L, R, T) chn, ta c tht c nhit


29/295

29

tnm plasma tia la in t 4000 6000OC. V th tia la in c gi l ngun kchthch cng (giu nng lng). Nn ph pht x ca tia la in ch yu l ph ca ccIon bc 1 ca cc kim loi.

2. Tia la in l ngun kch thch tng i n nh v c lp li cao. Nhngv nhy li km h quang in. Do thi gian ghi ph cn phi di hn h quang.

3. V bn cht ca s phng in, tia in l s phng in gia hai in cc cth hiu rt cao (10.000 - 20.000kV) v dng in rt thp (


30/295

30

V vy, mun m bo choqu trnh phng in n nh, th

phi gi cc iu kin lm vic thtn nh. Nhng cng khng nnchn cc thng s mt dngt gi tr gii hn j0 v mt dng in ny s phng in km

bn vng v khng n nh, tc llm cho nhit ca tia in daong nhiu. Mt cch gn ng,trong plasma tia la in ta c tns phng in:

LCf /12

1

=

Cng dng in: LCVI i /=

V mt dng:S

LCVhay a

S

Ij

f /==

2.2.3.3 Skch thch phtrong plasma ca tia la inTrong plasma ca tia la in s kch thch ph pht x xy ra cng tng t nh

trong h quang in. Nhng vi mu bt th s ha hi din ra rt km v khng nnh. Nhng do nhit ca plasma cao nn s hnh thnh cc hp cht bn nhit, tgp hn trong h quang. Mt khc, tia la in dng ch yu phn tch mu thp,

hp kim (mu rn) v mu dung dch. Nn cc qu trnh xy ra ch yu ch trn b mtin cc mang. C th l:

1. Nu l mu dung dch, th trc ht dung mi bay hi, li cc ht bt mu.

Ri cc cht (c mu bt v b ca mu dung dch cn li), bt mu b nungnng, nng chy. Sau l cc qu trnh nhit ha ca cht mu xy ra theo tnh chtca n. C th l cc ht mu s bay hi hay chuyn khi vo plasma gia 2 in cc.Ti s xy ra cc qu trnh chnh v ph khc nhau, ty thuc vo thnh phn v

bn cht ca cht mu.

2. Nu mu l dng rn, mu l chnh in cc mang, th c cc qu trnh trongin cc mang mu l:

+ S nung nng b mt v b bn ph,

+ S bay hi ca cc phn t cht mu vo plasma,

+ S vn chuyn ca cc ht mu cha ha hi vo plasma,

Bn cht ca mu, thnh phn ca mu, th phng in, v nhit b mt ca


31/295

31

in cc l cc yu t quyt nh din bin ca cc qu trnh trn.

Sau l cc qu trnh trong plasma. Gm c 2 qu trnh chnh sinh ra ph phtx nguyn t (AES) v mt s qu trnh ph nh sau:

A- Hai qu trnh chnhA1- Khi mu l dung dch nh ln in ccNu Eh < Ent th mu s ha hi, nguyn t ha to ra cc nguyn t t do,





Cch 1 ny cho nhy v n nh cao ca s pht x. Cc hp cht muihalogen (tr F), axetat, mt s nitrat, sunphat,.. ca kim loi thng theo cch ny.

Nu Eh > Ent th phn t ca cht mu s b nguyn t ha trc, sau ha hi thnh

cc nguyn t t do, ri mi b kch thch phsinh ra phpht x (AES).

Men Xm(r) Men (r) + Xm (r) mMe(k) + nX(k)



Cch 2 ny cho nhy v n nh km so vi cch 1 ca s pht x. Cchp cht mui SiO3, PO4, F, mt s nitrat, sunphat,... kim loi thng theo cch ny.

Do c im ca 2 cch chnh , nn trong phn tch ngi ta thng to iukin s kch thch ph xy ra theo cch 1 c li. iu gii thch ti sao ngita hay dng cc mui halogen v axetat ca kim loi kim lm nn, hay cht ph giacho mu phn tch v mu chun, bin i nn mu gc sang nn nhn to tt hnv li cn loi trc cc yu tnh hng ng thi c khi cn tng c cnhy ln hng chc ln.

A2- Khi mu l thanh kim loi lm in mang+ Lp cc nguyn t trn b mt in cc, di tc dng ca lc in trng

in mnh (v th phng in rt cao gia 2 in cc, 15- 20 kV), nn cc nguyn t bbt ra khi mt in cc v bay vo plasma, hoc l dng hi, hay l dng ht rn. Sau c cc qu trnh sau:

- Ha hi ca ht rn: Men) + E1 Me(k)

- Nguyn t b kch thch: Me(k) + E2 Me(k)*

- Nguyn t b Ion ha: Me(k) + E3 Me(k)+

- Ion b kch thch: Me(k)++ E4 Me(k)+*


32/295

32

- Pht x ca nguyn t: Me(k)* Me(k) + n(hv)

- Pht x ca Ion: Me(k)+* Me(k) + n(hv)

Trong : n(hv) l chm tia pht x ca nguyn t hay Ion b kch thch.

Cn El, E2,.. l nng lng nhit do plasma tia in cung cp.

B- Cc qu trnh phTrong plasma gm c:

S hnh thnh hp cht bn nhit, ch yu l cc monoixtit (dng MeO).



- S pht x ph nn lin tc.

Bn cht ca cht mu, thnh phn ca mu, cht ph gia trong lin cc v

nhit ca in cc l yu t quyt nh s din bin ca cc qu trnh trn.

Song qu trnh ph trong plasma tia in khng mnh nh trong ngn la vtrong h quang, ch c s Ion ha l ng k. V th cc kim loi kim phi o theo

cc vch Ion bc 1.

Ni chung, cc qu trnh ny u khng c li. Ngi ta phi tm bin php loi

tr. V d vi mu dung dch:

+ hn ch s hnh thnh hp cht bn nhit kiu MeO, ngi ta thm vo mu

mui Clorua ca kim loi kim (nh KCI, CsCL,) lm nn, kch thch ph theo cch

1, hoc kch thch ph trong mi trng kh trargon.

Vy, loi tr s Ion ha ca nguyn t phn tch, ngi ta thm vo mu cht

ph gia mui halogen kim loi kim c th Ion ha thp hn th Ion ha ca nguyn t

phn tch. Nh th, nguyn t phn tch s khng b Ion ha na.

2.2.3.4 Cc loi my pht tia in

V nguyn tc cu to, my pht tia la in cng c hai loi: my pht tia la

in c ch huy v my pht tia la in khng ch huy. Song trong mc ch phn tch

quang ph pht x, ngi ta ch dng my pht tia in c ch huy. Trong loi my

pht ny, vic ch huy (iu khin) cng theo hai nguyn tc v ng vi hai cch iu

khin ny ngi ta ch to hai loi my pht tia in khc nhau:

+ My pht tia in dng hai tia in, tia in chnh, tia in ph (hnh 2.9),

tng t nh my h quang.

+ My pht tia in dng ngt in ng b (hnh 2.10), ngha l iu khin bng

m tng b.

My pht tia in dng hai tia din (hnh 2.9)


33/295

33

Trong my ny c mt tia in chnh v mt tia in ch huy. Theo s hnh

2.9, khi ng mch in K, bin th Trc np in v tng th ln hng ngn vn

(V). Lc h tin C c tch in v thhai ct ca t C l th ca khong n

gia hai qu cu E. Lc ny trn mch in LC khng c dng in, t C c tch

in cho n khi t c th Vf (th phng in gia hai qu cu E) th tia in s

nh qua hai qu cu E, t C phng in qua E lm cho trn mch dao ng LC xuthin tc khc mt dao ng cao tn, dao ng ny khng qua c L2 (hay R1); do

bt buc phi nh qua khong n F (khong cch gia hai in cc chnh). Nh vy,

h c mt dao ng v mt tia in nh E, th cng c mt tia in phng qua F,

ngha l tia in E iu khin tia in F. Sau qu trnh c th tip din cho

n khi ta ngt cng tc K.

Mun cho qu trnh phng inc n nh th hai qu cu E phi ckch thc xc nh, khong cch

khng i v b mt phi nhn. Cnh thin th Vf mi c nhng gitr xc nh ng vi tng cp thng sca my pht tia in c chn,m bo cho qu trnh phng inF n nh V mt nng lng, v chai tia in xy ra ng thi, nn nnglng phi chia i hai khong n Ev F. V th mun c nng lng ln

(nhit cao) th th Vfphi rt ln.

Hnh 2.9

My pht tia in dng hia tia in

My pht dng ngt in ng b (hnh 2.10).My pht loi ny ch to

theo s ca Fressner. y sphng in c ch huy nh mtm tng b c gn b ngt inmc ngay trn mch ca my phttia in. My pht gm bin th Tr

tng th, mch dao ng LC,khong phng in chnh F, haicun cm L1, L2 ngn khng chodao ng cao tn trv bin th Trv m t ng b M ng vmmch in ca qu trnh phngin gia 2 in cc F.

Hnh 2.10

My pht tia in dng ngt in ng b


34/295

34

Khi cho in vo my, bin th Tr tng in, t C c tch in dn, m tMquay u c mang theo hai tip im A v A, cng quay, khi n v tr B v B1 mchin trong my c ng, lc tht C cng t cc i, khin cho tia in phi

phng qua F, sau tia in li tt. Nhng v m tM quay u lin tip, nn tia incng nh u n lin tip qua F theo s quay ca mtM tun hon n v tr B vB1 ln lt theo thi gian chu k c chn, cho n khi no ta ngt cng tc K.

Nh vy, chnh m tng b M iu khin s phng in theo cc thng sc chn.

2.2.4 Plasma cao tn cm ng ICP2.2.4.1c im v tnh chty l ngun nng lng mi c s dng trong php phn tch quang ph pht

x nguyn t khong 10 nm li y; song li c s dng rng ri v c hiu qucao v n c nhng c im sau y:

1. Ngun nng lng ny c nhit cao. Thng thng tnm ca plasma c tht c nhit t 5000 10000OC nn ha hi v nguyn t ha c ht mi trngthi ca vt liu mu vi hiu sut cao. Vi plasma ny, mi nguyn t kim loi u bkch thch to ra ph pht x ca n. Cc hp cht bn nhit cng b ha hi v

phn li thnh nguyn t t do, nhng trong ngun nng lng ny ph pht x ca Ionl ch yu.

2. ICP (Inductivity Coupled Plasma) l ngun nng lng kch thch ph pht xm bo cho php phn tch c nhy rt cao. Ni chung c tht t n.10-4: n.106%(0,l - 5) ngng i vi hu ht cc nguyn t. Tt nhin nhy ca php phn tch

y ph thuc nhiu vo cng sut ca my pht cao tn HF to ra plasma ICP (hnh2.11).

3. ICP l ngun kch thch ph khng nhng c nhy cao, m cn c n nhcao. N hn hn h quang v tia in. V th php phn tch dng ngun nng lng nycho sai s rt nh; ni chung l nh hn 10% trong vng nng t n.10-3- n.10-5%.


35/295

35

4. Tuy c nhit cao nhng s kch thch ph pht x trong ngun nng lngny li rt m du. N tng t nh s kch thch ph trong ngn la n kh. Chnh v

th m phng php phn tch t n nh cao v sai s nh.

5. Vi ngun nng lng ny, ngi ta c thnh lng c ng thi nhiunguyn t mt lc, nn tc phn tch rt cao (t 40 - 120 mu/gi). Mt khc, khidng ngun nng lng ICP, vng tuyn tnh ca phng php nh lng l rt rng(t 1 - 10.000 ln).

6.c im cui cng l rt t xut hin nh hng ca cht nn (matrix effect).

l im hn hn cc ngun nng lng cin nu.

Chnh do nhng c im v u vit ca ngun nng lng ICP nh th, nn n nhanh chng c ng dng rng ri v thay th cho ngun nng lng cin: tiain v h quang. Tt nhin, trang b to ra ngun nng lng ny t hn. Nhngtrong iu kin ngy nay v do tnh cht u vit ca n nn cc phng phn tch quang

ph pht xu c trang b loi ngun nng lng ny.

2.2.4.2 Cu to ca h thng 1CPV nguyn tc, h thng thit b to ra ngun nng lng ICP gm hai phn

chnh: my pht cao tn HF v h thng to th si kh v n nguyn t ha kchthch ph ca mu phn tch.

1. My pht cao tn HFCc my ny lm vic tn s rt cao nhng ph bin l hai tn s 27,12 MHZ

v 450 MHZ. Trong my trung tn s 27,12 MHZ c s dng nhiu hn. ltn s sng mang rai. My pht cao tn HF c nhim v pht tn s cao cung cpnng lng cho cun cm cao tn u ming n nguyn t ha mu, to ra plasmanhit cao cho s ha hi mu, nguyn t ha v kch thch ph (hnh 12a).


36/295

36

2. Phn th hai ca h thng ICP l b phn nguyn t ha mu, bao gm bphn to soi kh mu v n nguyn t ha mu (nebulizer system v khch burner).B phn burner c ch to bng thch anh chu nhit. N gm ba ng lng vonhau, gn chc vo nhau phn y, v mi ng u c ng dn kh vo (hnh2.12a). Trong , ng trong cng l mt ng mao dn dn mu vo plasma nhkhmang; ng th hai l ng to ra kh plasma. Cn ng th ba l ng to kh lm lnhcho n nguyn t ha. Pha ngoi ng th ba v ngay u c cun cm cao tn bngng. Cun cm ny c ni vi my pht cao tn HF v lun c lm lnh bngdng nc chy qua khi lm vic. ng thi chnh cng sut lm vic ca cun cmny s quyt nh nhit ca plasma kch thch ph. Cn h thng nebulizer, vnguyn tc, hon ton tng t nh trong ngn la n kh nu trn.

Kh dng trong plasma ca ICP l cc kh tr nh argon, hli, nit. Trong argon l u vit nht. y ngi ta c th dng kh mang v kh plasma l argon,cn dng nit lm lnh. Hoc c th dng ch mt kh Ar cng c. Trong tng

lng kh cho c ba dng l t 13 - 18 lt/pht v c phn chia nh nhau:

- Kh mang: 0,7 - 1,0 l/pht.

- Kh plasma: 6 - 8 l/pht.

- Kh lm lnh: 5 - 9 l/pht.

Nhng ngy nay, ngi ta ci tin v dng loi mini - ICP nn ch cn n 1/3lng kh so vi loi thng thng nu.

Hnh 2.12aH thng ngun ca ICP-AES

2.2.4.3 Nhit ca plasma ICPICP l ngun nng lng kch thch ph c nhit cao, nhng li n nh hn

so vi cc loi ngun kch thch khc, nh h quang, tia la in. Nhit ca plasmaca ngun ny c quyt nh bi dng in cm ng trong cun cm A u can nguyn t ha, tc l nng lng cao tn ca my pht HF. V vy, ng vi nhngthng s nht nh ca my pht HF th plasma ny c mt nhit xc nh. Trongcc thng s ca my pht HF, cng sut v tn s cm ng l hai thng s quan trng


37/295

37

nht. Song mi my pht thng ch dng mt tn s cm ng nht nh 27,12 MHZhay 450 MHZ, nn cng sut ca my pht ng vai tr chnh. V th cng cavch ph ph thuc vo cng sut ca my pht HF (hnh 2.11).

Tt nhin, bn cnh cn c nhng yu t khc nh hng n nhit caplasma ICP. Trc ht l tnh cht ca kh plasma, kh mang v tc chy ca chng

vo plasma. Sau l thnh phn ca hn hp mu c dn vo plasma kch thchph. Ni chung dng lin kt, th Ion ha ca cc nguyn t cng c nh hng nnhit ca plasma. Nhng cc yu t ny khng mnh nh trong h quang v tiain. Yu t matrix effect cng c tc dng, nhng cng khng ln nh trong ngn lan kh v trong h quang in. Cc yu t trn nh hng n nhit ca plasma,tc l nh hng n cng ca vch ph. Nhng khi chn c cng sut ln phhp, th nh hng ny s rt nh, hay khng cn na.

2.2.4.4 Skch thch phpht trong plasma ICPKhi ta ng mch in ca my pht cao tn HF th cun cm A u n s

xut hin tc khc dng in cao tn cm ng c nng lng ln. Nng lng nyc quyt nh bi cc thng s ca my pht HF c chn. ng thi tia la K cng c bt, hn hp kh v mu c t chy, plasma nhit cao xut hin,ngha l ngn la hnh thnh. Trong ngn la nhit cao ny, trc ht dung mi bayhi (phn trong, phn ti ca ngn la), li cht mu dng ht bt mn. Ri cc

bt mu ny b nung nng, nng chy, c ha hi, phn li, b nguyn t ha v Ionha to ra trong plasma c phn t, nguyn t, Ion, in t trng thi kh.Trong ch yu l cc nguyn t, Ion v in t. Sau cc phn t kh ny c trong

plasma s b kch thch v sinh ra ph pht x. Bn cht s kch thch phy l skch thch nhit, n ging nh s kch thch ph pht x trong ngn la n kh.

Nhng v plasma c nhit cao (nng lng ln) nn hiu sut nguyn t ha v kchthch ph cao v ph ca ngun nng lng ny vi cc kim loi kim v kim th chyu l ph ca cc Ion bc 1. C ch ca qu trnh trong plasma ICP c th tm tttheo s sau:


38/295

38

Hnh 2.12b

Hai loi my o ph pht x ICP (ICP-AES)

HF: My pht cao tn; Pm: H thng nguyn t ha mu; L: Thu knh; T: Khesng; G: Cch t; PM: H nhn quang in; >>: Hin t; Mr: My tnh; Sig: B chth kt quo; AB: Cung mt phng tiu; M1, M2: cc gng chun trc v hi t.

1. Trc ht l dung mi bay hi, li cc ht bt mu. Tip theo l bt mu bnung nng, nng chy. Sau l cc qu trnh nhit ha ca cht mu xy ra theo tnhcht ca n.

2. Ba qu trnh chnh: Cc qu trnh chnh y cng tng t nh trong ngnla n kh. Nhng c khc ngn la n kh l hiu qu ha hi, nguyn t ha vkch thch ph cao hn nhiu.

- Nu Eh < Ent, th mu s ha hi, nguyn t ha to ra cc nguyn t t do,




39/295

39



Cch 1 ny cho nhy v n nh cao ca s pht x. Cc hp cht muihalogen (tr F), axetat, mt s nitrat, sunphat,.. ca kim loi thng theo cch ny.

- Nu Eh

> Ent

, th phn t ca cht mu s b nguyn t ha trc, sau ha

hi thnh cc nguyn t t do, ri mi b kch thch ph sinh raphpht x (AES).

Men Xm(r) Men(k) + Xm(k) mMe(k) + nX(k)



Cch 2 ny cho nhy v n nh km so vi cch 1 ca s pht x. Cchp cht mui SiO3

-2, PO4-3, F, mt s nitrat, sunphat... ca kim loi thng theo c

ch ny.

- S Ion ha sinh Ion, sau cc Ion b kch thch v sinh ra ph ca n.

C th l:


Me(k) + Ei Me(k)+ + e

Me(k) + E Me(k)+* MeO+ n(hv)


V trong ngun ICP c nhit cao, nn cc kim loi c th Ion ha thp s dinra theo cch ny, v d cc kim loi kim, kim th.

3. Qu trnh ph gm c:

+ To ra cc h p cht bn nhit, ch yu l cc monoxit (dng MeO). Nhngtrong plasma ca ngun ICP th cc qu trnh ny xy ra khng ng k. chnh lu vit ca ICP.

+ S hp th bc x ca nguyn t (qu trnh to).

+ S pht x nn (nhng cng nh).

+ S Ion ha bc hai ca nguyn t kim loi kim v cc nguyn t c th Ionha bc hai thp.


40/295


41/295

41

Ngc li, vi cc nguyn t V, Zn, W,... hp cht bn nhit, li phi chn ngun kchthch c nng lng cao, h quang in dng ln hay ngun ICP.

3. Khi chn ngun kch thch phi m bo cho php phn tch c nhy cao,n nh, c th d dng phn tch c cc nguyn t c nng nh.

4. Chn ngun kch thch phi m bo cho php phn tch tiu tn t mu v khi

cn phi khng ph hy mu. Yu t ny trong mt s trng hp khng yu cu, btbuc nh ba yu t trn.

2.4 Cng vch ph pht x nguyn t2.4.1 Cng vch ph :

Trong ngun pht sng hay trong plasma, nguyn t (Ao) ca mt nguyn tc kch thch t trng thi nng lng thp Eo, ln trng thi nng lng cao. Cmc biu din theo phng trnh:

Ao+ XE = A* (a)

Vi mu phn tch c nng xc nh ca Ao th qu trnh ny lc u tngnhanh, sau chm dn v ti mt thi im nht nh th khng tng na. Ngha l snguyn t Ao b kch thch l khng i, ng vi mt nhit xc nh ca plasma.ng thi vi qu trnh trn l qu trnh ngc li, tc l nguyn t b kch thch A*gii phng nng lng m n nhn c di dng cc tia pht x n(hvi) trvtrng thi nng lng thp bn vng ban u. Chnh qu trnh ny l qu trnh pht xca nguyn t v sinh ra ph pht x ca nguyn t, c th c biu din theo

phng trnh:

A*

= hv + Ao

(b)Qu trnh ny lc u chm, sau tng dn theo s nguyn t A* b kch

thch v n mt thi im nht nh th cng khng tng na ng vi mt nhit nht nh ca plasma. Ngha l sau mt thi gian nht nh ca s kch thch ph, thhai qu trnh trn (a) v (b) st n trng thi cn bng. l hai cn bng nghc thun nghch. Cn bng ny ch ph thuc ch yu vo nhit ca plasma, nghal s nguyn t A* b kch thch v pht x l khng i, ng vi mt nhit xc nhca plasma.

Nu gi Nm l s nguyn t ca nguyn t Ao b kch thch n trng thi nng

lng cao cm, th theo quy lut Bolzmans ta c:

KtEeg

gNN m

mam /)(

0

=

go v gm l trng lng thng k ca nguyn t A trng thi ban u, nnglng Eo v trng thi kch thch nng lng cm;

Na l s nguyn t ca nguyn t Ao c trong plasma (trng thi hi);


42/295

42

Em l nng lng kch thch nguyn t o t trng thi Eo ln trng thi cm;

T l nhit ca plasma (oK);

k l hng s Bolzmans.

Nu gi Ia l cng ca vch ph do qu trnh kch thch ph ni trnsinh ra, th trong mt gii hn nht nh ca nhit plasma v nng ca nguyn tAo trong plasma, ngi ta thy Ia ph thuc vo:

- S nguyn t Ao b kch thch ln trng thi A*, (Nm).

- Thi gian tn ti ca nguyn t A*trng thi kch thch, (tm).

- Nng lng kch thch nguyn t A t trng thi c bn n trng thi kchthch, (Em).

- Xc sut chuyn mc ca nguyn t A t trng thi kch thch nng lng Cmv trng thi ban u nng lng Eo, (Amo).

V mi quan h ny c th biu din theo cng thc sau:

kTE

o

mamo

m

a ehvg

gAA

t

lfI /)(...... =

Nhng i vi mt loi nguyn t v trong mt iu kin th nghim nht nhca plasma (ch yu nhit plasma) th cc yu t tri Amo, go, gm, cm, hv l nhnghng s. Cho nn cng vch ph Ia ch cn ph thuc vo s nguyn t b kchthch Na m thi. Nh vy ta c:

aa NKI 1= (2.6)

Vi kTE

o

mamo

m

a ehvg

gAA

t

lfk /)(...... =

l cng vch ph ca nguyn t.

Song cng thc (2.6) mi ch cho ta bit mi quan h gia cng vch ph Iav nng ca nguyn ttrng thi hi trong plasma, m cha ch cho ta c miquan h gia cng vch ph v nng ca nguyn t trong mu phn tch. Munth ta phi xt mi quan h gia nng C trong mu v s nguyn t Na trong

plasma. Ni chung, mi quan h ny l phc tp, n ph thuc vo nhiu yu t, nh:

- Kh nng ha hi, bn cht ca cht mu, cht nn;

- Thnh phn ca mu, cht ph gia thm vo;

- Trng thi lin kt, tn ti ca cht mu;

- Cc iu kin ha hi, nguyn t ha mu, kch thch ph;

- Mi trng kch thch ph, v.v...

Tuy nhin ngi ta thy t thc nghim, trong mt phm vi nht nh ca nng


43/295


44/295

44

Cng thc (2.10) l phng trnh cbn ca phng php phn tch nh lngtheo ph pht x nguyn t. Phng trnh ny do Lmakins (Nga) v Saiber (c) tmra, nn gi l phng trnh Lmakins-saiber.

2.4.2 Cng vch ph v nhit plasmaNh trn chng ta bit, cng vch ph pht x ca nguyn tc tnh theo biu

thc sau: Ia = Pa.N(l-x)e-E(m)/kT

V vch Ion: Ii = Pi.N.x.e-Em/kT (2.12)

Trong , vi: hvg

gA

t

lP

o

mam

am

a ..=

iu ny cho thy cng ca vch ph ph thuc vo nhit ca plasma.

Nhng, tt nhin, s ph thuc ny i vi vch nguyn t v vch Ion l khcnhau v khng n iu, m theo hm s m (e-Em/kT). Chnh hai cng thc trn ddng cho ta thy rng, nhit no ca plasma to ra c s nguyn t N(1 x)

trong cng thc tnh cng vch nguyn t v s phn t (N.x) trong cng thcvch ph ln l ln nht, th lc s c c cng ca vch ph ln nht. lnhit ti u ca s kch thch ph.


45/295

45

Vi mt vch ph ca mt nguyn t, thc t ch c mt nhit to ra ccng I cc i. Nhit ny c gi l nhit ti hn ca vch ph v ck hiu l To. Tt nhin i vi mi vch th nhit To ny khc nhau (hnh2.15a,b,c). Ngay i vi vch ph ca mt nguyn t, nu n tn ti trong cc nn mukhc nhau (matric khc nhau) th nhit To ny cng khc nhau (hnh 2.15c). iu c ngha l, ngoi bn cht ca cht phn tch, nhit To cn ph thuc vo cthnh phn v nn ca mu phn tch.

Tnh cht ny c ngha thc t rt ln khi xc nh lng nh cc nguyn ttrong mu phn tch. V trong nhng trng hp ny ta cn phi chn nhng iu kinlm vic v iu kin kch thch ph sao cho thu c vch ph c cng ln nht,tc l chn cc iu kin kch thch ph c c nhit ca plasma gn vi nhit To ca vch ph cn o, hoc bng ng c To l l tng. Tt nhin bng cl trng hp rt him, ta ch c th lm cho nhit ca plasma cng gn vi nhit To th cng tt.

2.5 Hin tng thp th (to) trng thi hi trong plasma, hin tng mt nguyn t c kh nng hp th tia

bc x do chnh mt nguyn t khc cng loi ca chng pht ra c gi l hintng t hp th (hay to). Hin tng ny xy ra theo nh lut Kirschhoff. Nghal mt nguyn t ch hp th nhng tia bc x m chnh n c th pht ra c trongqu trnh pht x. Hin tng ny thng xy ra mnh v ch yu i vi cc vchcng hng, vch cui cng ca ph pht x ca mt nguyn t, v nng cngln, th hin tng ny xy ra cng mnh. Chnh do hin tng ny, m a n ktqu lm cho cng vch ph pht x I khng ph thuc tuyn tnh vo C, khi nng

C tng gi tr Cx > Co, Gi tr Coc gi l nng gii hn, cn c quan hgia I v C l tuyn tnh. Vi mi vch ph, Co l khc nhau. Vch cng nhy th Cocng nh. ng thi, trong qu trnh pht x, cng sut pht x ca mt vch phctnh theo cng thc:

W = f.Wo (2.13)

vi f l h s hot ng pht x ca vch ph v n nhn gi tr: 0 < f < 1.

Khi khng c hin tng to, ngha l nng Cx nh hn Co th f = 1, tcl W = Wo. Cn mi trng hp khc, khi c hin tng to th lun lun c W 99,99%),cn c nhng trang b ph tr. V th, ngy nay trong php do ph pht x hin iICP-AES, ph pht x ca mu lun c kch thch trong mi trng kh trargon.


113/295

113

6.1.8 Cht m v cht ph gia trong phn tch ph pht x

Trong phn tch quang ph pht x, mu th nghim d dng rn, dung dch haybt cng u c lm bay hi trong mt ngun sng, nh ngn la n kh, h quangin, tia in, ICP,... ngha l mu phn tch c chuyn thnh trng thi hi di tcdng ca nng lng nhit. Trong cc nguyn t, Ion, v c phn ttrng thi hi

cng c thc kch thch n pht sng v cng ca vch ph pht x do ccnguyn t pht ra c dng xc nh nng ca n trong mu phn tch, vcng vch pht x ca mt nguyn t ph thuc vo nng ca n trong mu

phn tch.

Nhng trong nhiu trng hp cng vch ph ca mt nguyn t trong muphn tch khng nhng ch ph thuc vo nng ca n, m cn bnh hng binng ca cc hp phn khc c trong mu phn tch. Nht l khi trong h c chanhiu nguyn t v thnh phn phc tp m nguyn t cn xc nh li l nguyn tvt. l yu tnh hng tng h gia cc hp phn trong mu phn tch. c bit

l khi thm vo mu phn tch mt s hp cht ph no th ngi ta c th lm thayi cng vch ph ca cc nguyn t. Cc cht thm vo c gi l cht ph

gia.Nhng cht khi thm vo mu lm thay i thnh phn ca mu, v qua lm nh hng n thnh phn ca ph v cng ca vch ph. Cc cht ph gia cth lm gim hay cng c th lm tng cng vch ph ca mt nguyn t. Sdnh vy l v s thay i thnh phn ha hc ca mu thng nh hng n mi qutrnh trong khi kch thch ph, nh:

- S thay i nhit plasma,

- Thay i nng in t (Pe) trong plasma,

- S khuch tn v chuyn vn ca cc phn t,

- Cc phn ng ha hc in cc v plasma,

- Qu trnh ha hi ca cht mu,

- S bay hi v nguyn t ha cht mu,

- Thi gian tn ti ca cc phn t trong plasma,

- S Ion ha, s phn li ca cc cht,

- S kch thch ph ca cc phn t trong plasma.

Trong tt c cc thay i ths thay i p lc in t (Pe) v nhit caplasma l yu t chnh, c tnh cht quytnh. Cn cc thay i khc ch l hu quca hai thay i ny.

Cht ph gia c dng trong phn tch quang ph c rt nhiu loi khc nhau,nhng c chia thnh mt s loi (thnh nhm) theo c trng ring v tnh chtnhit ng ha hc ca chng trong plasma kch thch ph, nh:


114/295

114

Nhm 1: Cc mui halogen ca kim loi kim v kim th: Lia, LiF, NaCl, NaF,CsF, CsCL, AgCl, CaF2, CaCL2, MgCl2, v mt vi kim loi nng Ti, Sb, Sn, Ga,...

Nhm 2: Mt s oxit kim loi, nh oxit ca Ga, Sb, Sn, Ti, Mg, Al, Fe,...

Nhm 3: Mt s mui cacbonat, borat ca kim loi kim v kim th, nhNa2CO3, Li2CO3, LiBO2, BaCO3,

Nhm 4: Mt s halogen ca hp cht hu c, nh CF4,

Nhm 5: Cc n cht v hp cht ca lu hunh: S. CaSO4, SrSO4,

Nhm 6: Cc hp cht ca Si: SiO2, Na2Sio3,

Cc cht ph gia ny c th lm tng hoc lm gim cng ca vch ph camt nguyn t. Song mt cht ph gia c th c mt hay nhiu chc nng khc nhaukhi chng c thm vo mu phn tch. N c th l cht m, cht mang mu, chtchy, cht pha long, cht tr lc cho s ha hi v bay hi ca mu,... ngha l tytng trng hp c th m nhng vai tr no l ch yu.

1. Cht m. L nhng cht khi thm vo mu c tc dng n nh mt iukin nay mt qu trnh no trong s kch thch ph. V dm cho nhit plasma,m cho p lc in t trong plasma. in hnh cho loi cht m l bt than quang

ph. Bt than quang ph khng nhng c tc dng m nhit cho plasma, m cnlm cho s bay hi ca mu c d dng v n nh. Mt khc n cn gi cho plasmac nhit cao thch hp i vi s kch thch ph ca nhiu nguyn t. V th n rtcn v quan trng i vi php phn tch cc mu bt v mu khng dn in. V d:Khi phn tch cc mu qung, t , oxit,... ngi ta thng nghin chng thnh btv sau trn vi bt than quang ph theo mt t l thch hp. Ngoi bt than, mt s

mui halogen ca cc kim loi kim cng l cc cht m quang ph, nh CsCl, CsF,LiF, LiCl,... Nhng cht ny c tc dng m cho p lc in t (P) trong plasma tthay i trong qu trnh kch thch ph. Tt nhin ngoi tc dng m, cc halogen cacc kim loi kim v kim th cn c tc dng duy tr v n nh dng in qua ctkh ca plasma.

2. Cht mang. Trong nhiu trng hp khi thm cht ph gia vo mu th s hahi v nguyn t ha ca cc hp phn trong mu li d dng hn, nhanh hn, hoc nnh hn. Nhng cht ph gia c tc dng nh thc gi l nhng cht mang. Nthng l nhng cht d ha kh, d bay hi. V th khi vo trong mu, n bay hi c

th ko theo c cc hp phn mu bay hi, hoc tc dng vi cc hp phn ca muto thnh nhng hp cht mi d ha hi. Chnh v th m mu bay hi tt hn. Tiubiu cho cc loi cht ny l cc mui halogen v cacbonat ca cc kim loi kim vmt vi kim loi kim th nh CsF, CsCl, LiF, LiCl, NaF, NaCl, Li2Coa, Na2CO3,CaCO3, SrCO3, BaCO3. ng thi c bt than quang ph cng c tc dng ny rt rrt khi phn tch cc mu khng dn in. Ngoi ra, mt s oxit d bay hi ca mt snguyn t nh Ga, Sb, cng l nhng cht mang. Cc cht mang ny trong nhiutrng hp cn c tc dng lm tng cng vch ph.


115/295

115

3. Cht trlc cho st chy, ha hi mu. Thng thng khi phn tch ccmu bt, c bit l cc mu khng dn in, nh cc mu qung, t , khong st,thng c hin tng kh t chy mu, kh ha hi v t bay hi thng khng nnh. Khi cng vic phn tch gp kh khn v kt qu thu c khng lp li ttLc ny ngi ta thng thm vo mu nhng cht lm cho s chy, s ha hi vnguyn t ha ca mu c d dng hn. Nhng cht nh thc gi l cht trlccho s bay hi ca mu. Cht in hnh cho loi ny li l bt than quang ph hay btgraphit quang ph. Sau l bt lu hunh, bt oxit nhm. Nhng bt lu hunh v

bt xit nhm khng th dng ring mt mnh, m phi lun lun dng cng vi btthan. V th ngi ta gi bt than quang ph l mt cht ph gia vn nng.

4. Cht dn in. Khi phn tch cc mu khng dn in, vn n nh dngin qua ct kh ca plasma h quang hay tia la in l mt yu t rt quan trng. Vnu khng, s phng in khng n nh v iu ny sa n nhng kt qu phntch khng tt. Nguyn nhn chnh l do trong nhng trng hp trong plasma

khng c nng ln v n nh ca cc phn t mang in tch dng v nm ( Iondng v in t). V th ngi ta thng thm vo mu phn tch nhng hp cht dbay hi ca cc kim loi d b Ion ha (c th Ion ha thp). Nhng cht c gil nhng cht dn in. i din cho nhm cht ny l cc mui halogen ca cc kimloi kim nh CsF, CsCl, LiF,... Nhc nhng cht ny dng in qua ct kh c nnh, v qua cng lm cho s phng in, s kch thch ph trong plasma cngc n nh. Sdnh vy l v nhng cht ny cung cp cho plasma mt nng ln ca Ion dng v in t, m yu t khc (s b sung khc) vin tch nm vdng l khng ng k, ngha l duy tr dng in n nh qua ct kh ca plasma.

5. Cht pha long. L nhng cht khi thm vo mu phn tch c tc dng lmlong nng ca cc hp phn trong mu, nhng li khng nh hng n cng vch ph ca nguyn t phn tch, ngc li c khi cn lm tng cng vch ph

phn tch. Thc t chng minh rng, trong hu ht cc trng hp phn tch cc muqung, khi pha long mu bng bt than quang ph, mui halogen kim loi kim, thcng vch ph ca cc nguyn t cn xc nh ch c tng hoc khng i. Ngoi

bt than quang ph th mt s mui cacbonat, halogen ca kim loi kim v kim th,mt s mui sunfat kim loi kim th, mt vi oxit, v d nh Li2COa, Na2CO3, LiF,LiCl, BaCO3, CaCO3, CaSO4, SrSO4, SiO2, cng thng c dng cng vi bt

than quang ph theo nhng t l thch hp. Tc dng ca cht pha long l lm cho phca mu n gin hn v d quan st. Trong mt s trng hp cht ny c tc dngkh bt ph ca nguyn t cs, lm tng cng vch ph ca nguyn t phn tch(bng 6).

6. Cht chy. L nhng cht d nng chy, c tc dng lm cho mu d nngchy v d ha hi. V d nh LiBO2, cc mui hydropyrosunphat ca kim loi K v

Na hay Li. i vi mt s mu c thnh phn phc tp, nh cc mu qung a kim


116/295

116

loi v cha nhng nguyn tdng hp cht bn nhit, kh bay hi ngi ta phithm vo mu phn tch cc cht chy theo nhng t l ph hp vi mc ch h thpnhit nng chy ca mu v lm mu d bay hi. Ngoi tc dng y, nhng chtchy c nhiu trng hp cn lm tng cng vch ph ca nguyn t cn nghincu. V d nh LiBO2 trong bng 6.6.

Bng 6.6Vai tr ca cht pha long mu

STT Mu + cht pha long t l: (gng) Cng vch ph

Mu Bt than LiBO2 Fe-248,30 nm

1 1 0 0 0,140

2 1 1 1 0,230

3 1 2 1 0 280

4 1 3 1 0,260

7. Cht khnn. Trong mt s trng hp phn tch quang ph pht x, nh ccmu qung, th ph thu c thng c nn ph lin tc rt en, nht l khi kch thch

ph bng h quang. C khi nn ny che lp mt c nhng vch phn tch c cng nh. Trong nhng trng hp nh vy ngi ta phi thm vo mu nhng cht c tcdng lm gim bt nn ca ph lin tc. Nhng cht c tc dng nh thc gi lcht kh nn. Cht thuc loi ny thng l mt s mui sunphat ca kim loi kimth v mt vi oxit: CaSO4, BaSO4, SrSO4, SiO2, Al2O3. Nhng cht ny c th dng

ring bit hay ng hn hp ca hai cht vi nhau, nhng chng cng phi c ngcng vi bt than quang ph. V d: Khi xc nh V v Ni trong du m, ngi tathng dng SiO2 hay SrSO4 thm vo mu khi tro ha mu du. Nhng cht nyva c tc dng l cht gp, cht bo v hay l n nh qu trnh tro ha trnh mtmt, mt khc li chnh l cht kh bt nn khi ghi ph ca mu tro du .

Trn y l mt s tc dng quan trng ca cht ph gia, nhng s trnh by v phn loi nh th ch l tng i, v trong thc t mt cht ph gia c th biu lng thi nhiu chc nng v tc dng cng mt lc. Trong c chc nng chnh, cchc nng ph. V d nh bt than quang ph va l cht m, va l cht mang.

Nhng tc dng m ca n l chnh. V th ngi ta thng gi bt than quang ph lcht m quang ph.

Cht ph gia c nhiu tc dng khc nhau, nn khi s dng n ngi ta thngxut pht t nhng mc ch khc nhau. Nhng c th khi qut theo hai hng chnh:

- Hng I. S dng cht ph gia nhm mc ch gy ra s bay hi chn lc camt s hp phn trong mu m bo cho php phn tch thc hin d dng v tkt qu tt. V d: Vic thm cc cht ph gia l cc mui halogen ca cc kim loi


117/295

117

kim thng gy ra s bay hi chn lc ca nhng nguyn t vt trong mu c khnng to vi cc halogen nhng h p cht d bay hi. Trong nhng trng hp nythng ko theo s tng cng ca vch ph. l iu kin thun li trong php

phn tch nguyn t vi lng. cng l u im ca cht ph gia.

- Hng II. Ngi ta s dng cht ph gia nhm mc ch lm thay i cc qu

trnh chnh trong s phng in v kch thch ph ca mu. V d: S ha hi, s kchthch ph ca mt h p phn trong plasma vi mc ch c th lm tng hay gimmnh cng vch ph ca mt nguyn t. Nhng kt qu nghin cu n nay chng minh rng, nhng yu t gy ra cc thay i trn phi l cc phn ng ha hctrong in cc v trong s bay hi ca mu khi c mt ca cht ph gia.

Trong cc cht ph gia th cc halogen ca kim loi kim v mt s kim loikim th c ngha ln i vi mc ch ny, bi v s c mt mt lng ln v

ph hp ca cht ph gia nh th trong mu phn tch lm thay i tt c cc qutrnh trong in cc v trong s kch thch ph ca mu.

Trong nhit v p lc in ttrong plasma l cc yu tchnh, quyt nhcc yu t khc. Nhng thay i cui cng dn n s thay i cng ca vch

ph.

Tc dng ny c th l dng v cng c th l nm, ty thuc vo tng trnghp c th. Tt nhin trong thc t phn tch ngi ta lun lun chn tc dng no cli cho cng vic phn tch, nh lm tng cng vch ph ca nguyn t phn tchv lm gim cng vch ph ca nguyn t nn.

V mt ha hc, khi thm cht ph gia vo mu phn tch tt nhin c nhiu phn

ng ha hc khc nhau xy ra, c phn ng chnh, c phn ng ph. Nhng c th tmchia thnh my loi phn ng sau:

- Phn ng oxy ha;

- Phn ng sunfua ha;

- Phn ng cacbua ha;

- Phn ng halogen ha;

- Phn ng phn li v phn hy ca cc phn t,...

Trong cc loi phn ng th phn ng halogen ha l quan trng nht v c ngha thc t nht, v nhphn ng ny cc hp phn ca mu d ha hi, c bit lcc hp phn lng vt, qua lm tng cng vch ph ca nguyn t phn tchv lm gim cng vch ph ca nguyn t csv nn.

Cht ph gia to ra phn ng ny l cc halogen ca kim loi kim v mt s kimloi kim th.

Ngc li vi phn ng halogen ha, phn ng sunfua ha v carbua ha thng


118/295


119/295


120/295

120

no ca cc mu u.

Nu en S, hay cng ca vch ph phn tch, ta quan st nm gia enca vch nng C2 v C3 th ta ni nng Cx ln hn C2 nhng li nh hnC3, ngha l ta c th vit:

C2 < Cx < C3

l ni dung ca phng php so snh.

Phng php ny n gin, d thc hin. Nhng mi khi phn tch mt mu liphi ghi ph ca mt dy mu u, nh th cng tn nhiu mu chun, in cc vmt th gi. Phng php ny thng c ng dng phn tch cc mu dung dch. Vd, xc nh kim loi trong nc khong, nc t nhin (hnh 6.5).

6.2.2 Phng php hin vchNguyn tc ca phng php ny da trn css vch ph ca mt nguyn t

xut hin cng nhiu khi nng ca nguyn t trong mu cng ln, ngha l ngvi mi mt nng nht nh th c mt s vch ph xut hin trong iu kin thnghim c chn ph hp i vi mt nguyn t trong mt i tng mu nhtnh.

V d: nng 0,001% ca Pb mu, v nu kch thch ph trong iu kin hquang dng 10A th ch c ba vch ca Pb: 2614,20; 2802,00; 2833,06 Ao xut hin.

V vy mun phn tch mtnguyn t no , trc ht ngi tacng phi chun b mt dy mu u

cha nguyn t X vi nng , vd: C1 C2, Ca, C4, C5, C6, C7,VCc mu phn tch Cx1, Cx2, trongcng iu kin. Sau ghi ph cacc mu trong iu kin chn.Quan st ph thu c ca tt c ccnng xem mi nng c baonhiu vch ph ca nguyn t phntch xut hin v lp mt bng hin

vch ca chng ng vi tng nng .Bng ny c dng phn tchnguyn t sau ny. Bng 6.8 l mtv d v xc nh Pb trong qung theo

phng php ny.


121/295

121

Bng 6.8

V d v xc nh Pb theo phng php hin vch

Nn Ph % S v ch xut hi n tn n Ao C1 0,001

C2 0,005

C3 0,01

C4 0,05

C5 0,1

C6 0,5

C7 1,0

C8 1,5

..

2614,20 ; 2802,00 ; 2933,06

Ba vch trn r, c thm vch 2663,17 (m)

Bn vch trn r, c thm vch 2873,30

Nm vch trn r nt.

Nm vch trn r nt v thm vch 2474,38

Su vch trn r, thm 2393,61 (cn m)

Ba vch u to, nhe, thm 2401,95; 2443,86

Ba vch u nhe, cc vch sau m.

Cx Xut hin: 2614,20; 2802,00; 2933,06;2633,17.

Phng php ny n gin, d thc hin v c ng dng rt rng ri, c bit

l trong ngnh a cht, v ta ch cn lp bng ph cc vch ph xut hin ln u tin,

sau c th s dng bng ny phn tch nguyn trong cc ln sau, m khng

phi ghi ph li ca mu u. Trong hai phng php bn nh lng trnh by, th

phng php hin vch c s dng rng ri, nht l trong nghnh a cht, v

phng php d thc hin v c nhiu u im.

6.3 Phn tch quang ph pht x dinh lng6.3.1 Phng php ng chun

Phng php ng chun da theo phng trnh cbn ca php o ph pht

x nh ch ra trong chng II l:

I = a.Cb (a)

hay S = . b.log C + k (b)

Trong iu kin b: 1, th y l phng trnh ca ng thng c dng: y= a.x v

y: a.x +b.

Phng trnh (a) l i vi nhng my o c trc tip cng pht x IM


122/295

122

ca vch ph, cn phng trnh (b) l trng hp o en S ca vch ph trn knh

nh.

Nh vy, mun xc nh nng Cx ca mt nguyn t X trong mt mu no ,

trc ht chng ta phi chun b mt dy mu u c cha nguyn t X vi nng

C1, C2, Ca, C4,V Cx trong cng mt iu kin nh cc mu phn tch. Song dng

1 ng chun, trong thc nghim, t nht cn ba mu u.

V th phng php ny gi l phng php ba mu u, tc l s mu u cn

thit dng mt ng chun. Nhng trong thc t, ngi ta hay dng 5 mu u,

cng v th phng php ny c gi mt tn chung lphng php ng chun.

Sau khi chun b xong cc mu u v mu phn tch, tin hnh ha hi, nguyn

t ha, kch thch ph, v ghi ph ca cc mu theo nhng iu kin ph hp

c chn (theo mt quy trnh phn tch).

Tip chn mt cp vch phn tch o en S hay chn vch phn tch

o cng I theo mt trong hai phng trnh c bn trn ty thuc vo h thng

trang b ca phng th nghim. Tnh cc gi tr trung bnh ca cng vch ph I

hay en S tng ng vi tng nng . Nh th ta c:

Bng 6.9. Dy chun ca phng php ng chunCht Dy chun

0 1 2 3 4 5

Cht phn tch X Co C1 C2 C3 C4 C5

Cc cht khc (nn, mi trng) Nh nhau trong tt c cc mu

o I Io I1 I2 I3 I4 I5

o S So S1 S2 S3 S4 S5


123/295

123

Ri t cc cp gi tr (s/log C)

hay (I/C) ta dng ng chun theoh ta S - log C (nu ph ghi trnknh nh cc my quang ph th h

1 v 2) hay I - C. ng ny chnh l

cc ng chun (hay ng cong lymu) xc nh nng Cx cha

bit ca nguyn t phn tch X. Mun

th ta ch cn cm cc gi trSx hayIx c t ln trc tung v t kng song song vi trc honh, nct ng chun ti im A, timA ny h ng vung gc vi trchonh, th ta c c Cx (nu o I)

hay l logCx (nu o S) ri suy ra Cxcn tm (hnh 6.6).

Tng chun trn, ngoi vicxc nh Cx chng ta cng c th xcnh c h s a trong phng trnh(a) v h s k cng nh y; h s btrong phng trnh (b). C th l:

Theo hnh 6.6a ta c:

a = tg (6.11)

V theo hnh 6.6b ta c:

.b = tgv k l on OD (6.12)

Phng php ny rt thun li khi phn tch hng lot mu ca cng mt itng, v ch cn mt ng chun l c th xc nh c rt nhiu mu c nng Cx cha bit.

Nhng mu phn tch v mu u phi ghi ph trong cng mt iu kin v trncng mt knh nh. V th nu phn tch nhiu i tng m mi i tng ch mthoc hai mu th tn mu u, knh nh, thi gian,... chnh l im cha u vit ca

phng php ny. V th xut hin phng php th khng i. Song trong nhngiu kin nht nh th n cng vn l mt phng php c ng dng ph bin v cnhiu u vit.


124/295

124

6.3.2 Phng php th chun khng iNguyn tc ca phng php ny l mun xc nh mt nguyn t no , trc

ht ngi ta cng phi dng mt ng chun nh trong phng php ba mu u.

ng chun ny c gi l ng chun cnh (ng chun khng i).Nhng c khc l ph ca cc mu u dng ng chun c ghi trn mt knh

nh ring, gi l knh nh cs; cn ph ca cc mu phn tch li c ghi trn mtknh nh khc, knh nh th hai - knh nh phn tch. Nh vy, mun xc nh c

nng Cx cha bit, ta phi chuyn cc gi trSxl tng ng v cc gi trSxoca knh nh csth mi chiu ln th chun xc nh Cx hay loa Cxc. gii quyt vn ny ngi ta c hai cch khc nhau:

- Cch thnht: Xc nh h s chuyn k theo cng thc:

Sxo = k.Sxl (6.13)

y kc gi l h s chuyn ca gi tren ca vch ph gia hai knh

nh (knh nh csv knh nh phn tch) trong cng mt iu kin th nghim.

Mun th trn knh nh phn tch ta ghi li ph ca mt mu u, v d nng

Ca. Nh th trn knh nh csta c Sxo-3, v trn knh nh phn tch ta cng c

Sxl-3 v h s chuyn k sc tnh theo cng thc:

kS

S

xl

xo =

3

3 (6.14)

Sau khi c h s k ta em n nhn vi cc gi tren Sxl trn knh nh phntch ta s thu c cc gi tren tng ng trn knh nh cs dng ngchun. By gich chiu cc gi tr vo ng chun cnh l tm c loa Cx'Ri t suy ra Cx.

- Cch thhai: T thc t phn tchkhi nghin cu cc ng chun ngi tathy rng, trong cng mt iu kin thnghim, i vi mt nguyn t khi o trnmt cp vch phn tch, th ng chundng c t dy ph ca cc mu u ghi

trn cc ln khc nhau ca mt loi knhnh, u l nhng ng thng song songvi nhau hoc trng nhau (hnh 6.7), nghal cc ng c cng h s gc.

Phng php th khng i rt phhp i vi php phn tch hng lot mu tngy ny qua ngy khc, v trong mi ngy


125/295

125

phn tch chng ta khng phi ghi ph lica ton b dy mu u, nn tit kim cthi gian, in cc, mu chun v knh nh,ngha l c tnh kinh t hn phng phpng chun. Tuy th, nhng phng phpng chun (Ba mu u) hay phng

php ng chun cnh u ging nhau chu phi chun b mt dy mu u c iu kin ca mu u trong php phntch quang ph nh lng. iu ny nhiukhi khng thc hin c. c bit i vinhng trng hp mu phn tch c thnh

phn phc t p m chng ta cha th bitc chnh xc.

V th cng khng th chun bc mt dy mu u c thnh phn ha hc vvt l ng nht vi mu phn tch. l thc t v trong nhng trng hp th nhhng nguyn t csv nguyn t th ba n kt qu phn tch l rt r. iu nyth hin r khi xc nh cc nguyn t vi lng. Lc ny chng ta phi hoc l thay inn ca mu (modify matrix) nu cch ny c hiu qu, hay l dng phng phpthm tiu chun.

6.3.3 Phng php thm tiu chunNguyn tc ca phng php thm l mun xc nh mt nguyn t phn tch X

no trong mu phn tch, ngi ta dng ngay mu phn tch lm nn chun bmt dy mu u bng cch ly mt lng chnh xc mu phn tch v thm vo nhng lng ph hp v theo tng bc ca nguyn t X. Nu gi mu phn tch cnng l Cx th dy mu chun s l (bng 6.10):

Bng 6.10Dy chun ca phng php thm

Cht Co C1 C2 C3 C4 C5

Cht PT X Cx Cx Cx Cx Cx Cx

X thm vo 0 C1 C1 C1 C1 C1

Cht khc Nh nhau tt c

o c I Io I1 I2 I3 I4 I5

Hay o S So S1 S2 S3 S4 S5


126/295


127/295

127

d: C1,C2 (C2 > C1).

Nh vy ta c: S2 = .b log C2+ k (a)

S1 = .b.log C1+ k (b)

Ly (a) tr (b) ta c:

(S2 S1) = .b(log C2 - log C1)

Do ta suy ra:)/log(

)(

12

12

CC

SS =

v b = 1

6.3.4 Phng php theo 1 mu chuna) Khi c mu chunTrong nhng trng hp n gin, chng ta khng cn pha mt dy chun

dng th chun, m c th tnh ngay gi tr Cx nhmt mu chun Cl ca cht phntch, ngha l chng ta c:

Vi mu phn tch: Ix = a.Cx (a)

Vi mu u: I1 = a.C1 (b)

Do em (a) chia cho (b) chng ta c:

Cx = (Ix/Io).C1 (6.17)

Nh vy khi o c gi tr Ix v I1 ta c t s ca chng, v ch vic nhn n vigi tr Cl l chng ta c gi tr nng Cx phi tm theo biu thc (6.17).

Nhng cn phi nhrng, nu o S trc ht phi tnh chuyn cc gi trS sangcc gi tr cng I tng ng ri mi ng dng c phng php ny, gi tr nng chun Cl v c nng cht phn tch phi nm trong vng tuyn tnh ca php oxc nh cht m chng ta bit r trc.

b) Khi khng c mu chun

Trong trng hp ny, chng ta cng khng cn pha mt dy chun dng th chun, m dng ngay 1 mu phn tch lm nn chun b mt mu phn tch v 1mu thm chun theo phng php thm v tnh ngay gi tr Cx nhmt lng chun

C1 ca cht phn tch c thm vo. Ngha l chng ta c:

Vi mu phn tch khng thm chun: Ix = a.Cx (a)

Vi mu u phn tch c thm chun: Itch = a(Cx+C1) (b)

Do em (a) chia cho (b) chng ta c:

Cng = [Ix/(Itch Ix)]C1 (6.18)


128/295


129/295

129

Cc phn ng kt ta nh lng trong ha hc th c rt nhiu, v d nh phnng to kt ta: AgCl, AgBr, AgI, PbSO4, BaSO4,v.v...Theo cch ny, ngi ta lymt lng nht nh cht mu (theo Vml hay a mg) cho tc dng vi mt thuc ththch hp v d, trong iu kin nht nh phn ng kt ta xy ra hon ton.Sau li tnm hay lc tch kt ta khi dung dch, ri thc hin xc nh kim loi theomt trong hai cch sau:

+ Ho tan kt ta v xc nh kim loi trong kt ta, ri suy ra hm lng chtphn tch X.

+ Xc nh lng kim loi thuc th d sau khi kt ta ht cht phn tch cn li.

T suy ra lng kim loi phn ng vi cht phn tch X, v tnh hm lngcht phn tch X.

Cch ny thng c s dng xc nh cc mui: Halgen (F, Cl, Br, I),sunphat (SO4

2-), photphat (PO43-), oxalat (C2O4

2-), v.v... Sau y l cc phn ng .

C + Ag+ AgCl xc nh Anion Cl

Br + Ag+ AgBr xc nh Anion Br

I + Ag+ AgI xc nh Anion r

C2O42- + Ca2+ CaC2O4 xc nh Anion C2O4

2-

PO4 + Ca2+ Ca3(PO4)2 xc nh Anion PO4

SO42- + Ba2+ BaSO4 xc nh Anion SO4

2-

SO4

2- + Pb2+ PbSO4 xc nh Anion SO

4

2-

PO43- + Mg2+ + NH4+ MgNH4PO4 xc nh Anion PO4

V d: xc nh hm lng Anion Cl trong nc, ngi ta ly 100 ml munc, thm 5 ml HNO3 65%, thm 2 ml dung dch AgNO3 5 mg/mL Ag (lng Agthm vo l 10 mg), lc u, sinh ra kt ta AgCl theo phn ng:

C + Ag+AgCl

Lm mui kt ta, sau lc hay li tnm ly kt ta AgCl, ra sch kt ta bng10ml HNO3 1%, thu nc lc v nc ra nh mc thnh 100 ml bng dung dch

HNO3 1%, v xc nh lng Ag d trong dung dch ny bng php o ph pht xca nguyn t Ag. Nh th ta tnh c lng Ag, v suy ra lng Ag phn ng vimun Cl, sau tnh hm lng mun Cr trong mu nc, v theo phn ng trn, 1Ion Cl s tng tc vi 1 Ion Ag+.

- To kt ta l sunphua khng tanMt s cht, v d cc hp cht hu c, dc phm thuc h thioo (c cha Ion

S2-, trong nhng iu kin thch hp c th tc dng vi mt s Ion kim loi to ra hp


130/295

130

cht sunphua khng tan, v cng c s dng xc nh cc cht hu c.

V d: xc nh vitamin B1 (trong 1 phn t B1c 1 Ion S2-), ngi ta ly a mg

mu c B1 cho tc dng vi b mg NaPbO2 trong mi trng KOH nng chy trong ngnghim, phn ng sinh ra PbS rt nh lng l c 1 phn t B1, th bao gicng tora 1 phn t PbS.

1 phn t vitamin Bl+NaPbO2 PbS

Sau ho tan hn hp phn ng trong 20ml dung dch NH4C11%, lc v ra ktta PbS, ho tan kt ta PbS trong 15 ml HNO3 1/1, nh mc thnh 25 mg v xc nhPb trong dung dch ny. Sau suy ra hm lng vitamin Bi trong mu a mg. theo

phn ng trn.

Phn ng ny trc y c dng xc nh vitamin B1, cng nh hmlng penicillin trong thuc, hay trong cc sn phm thc phm, bng phng phpkhi lng, nn ch xc nh c hm lng ln, v tin hnh rt lu cc thao tc kt

ta, sy kh, cn kt qu. Hin nay nhphp o ph AES hay AAS, xc nh c chm lng nh, m li nhanh.

2. Theo phn ng khmui kim loiy chnh l phn ng trng gng, phn ng kh Ion Ag v Ag+ kim loi, dng

thuc th Tulen, l phn ng xc nh cc cht andehit. V th phn ng ny chyu c dng xc nh hm lng cc cht c chc andehit trong phn t. Trong

phn ng ny, c 1 nhm chc andehit - CHO trong phn t cht, th lun gii phngra 1 nguyn t Ag.

R (CHO)n+ n(Ag+) nAg

o

V th cch ny hin nay c s dng xc nh cc hp cht hu cc chaandehit dng R-(CHO)n, ch yu l theo phn ng trng gng.

V d: xc nh hm lng andehit fomic, ta ly V ml mu cho tc dng viV ml thuc th Tulen, khi un nng nh, qu trnh kh Ion Ag+ xy ra theo phn ng:

HCHO + Ag+A go

Nh vy, kim loi Ag c gii phng v bm vo thnh ng nghim. Sau loi b dung dch, ho tan kt ta Ag kim loi bng 10 ml axit HNO3 1/1 nng, un

nh cho tan ht, nh mc thnh 25 ml, v xc nh Ag trong dung dch ny. Theo ktqu nng Ag, ta suy ra hm lng andehit trong V ml mu theo phn ng trn.

3. Theo phn ng ho tan mt kim loiMt s cht, m ch yu y l cc axit hu cc kh nng ho tan cc kim

loi hot ng nh Cd, hay Zn rt nh lng, nn cng c s dng xc nh n,theo phn ng to ra mui tan nh sau:

2R-(COOH) + Zn (R-COO)2Zn


131/295

131

Sau li tnm ly dung dch mui ny v xc nh nng Zn bng php o phpht x ca Zn, ri suy ra nng (hm lng) axit R-COOH tc dng vi Zn kimloi sinh ra dung dch mui Zn(R-COO)2.

4. Theo phn ng to hp cht phc bn chit cCch ny dng xc nh mt s Anion, hay cht hu c, ngha l cc Anion

no c th tc dng c vi mt Ion kim loi to ra c mt h p cht phc bntrong mt iu kin nht nh v chit ra c khi dung dch mu, th u c th sdng xc nh cc Anion .

V d: Xc nh Anion CNS-1 qua phn ng ca Anion ny vi dung dch muiFeCla trong mi trng pH=1, theo phn ng:

Fe3++ CNS Fe(CNS)

Sau chit phc Fe(CNS) vo dung mi etyl e v xc nh Fe trong dch chitny (c th gii chit phc vo dung dch HCI 2M, v xc nh Fe trong dung dch gii

chit ny). Sau ta cng tnh c nng ln CNS trong mu phn tch.

5. Theo phn ng to hp cht daCch ny thng c dng xc nh P, Si, qua thuc th molipdic trong mi

trng axit nc c 6M, sinh ra hp cht da ca P vi Mo, m chng ta dngtrong phn tch ha hc xc nh P trong t trng bng phng php trung ha.Cch lm cng hon ton tng t nh th, nhng y sau khi c c hp cht ktta d a ca P, ngi ta ho tan n trong kim 0,5 M v xc nh hm lng canguyn t molipden, bng ph pht x ca Mo, sau suy ra hm lng P trong mu

phn tch, theo phn ng sau.

PO4+NH4++ MoO4

2-+ HNO3 (NH4)3[P(Mo3O10)4]

Hp cht da ca P

6. Phn ng khhp cht c nhm Nitro (-NO2)Mt s hp cht Nitro trong mi trng pH thch hp c th b kh v amin (R-

NO2 R-NH2) bng kim loi hot ng, v d nh Cd, Zn, rt nh lng, v phnng ny c s dng xc nh mt s hp cht hu cc nhm nitro, v d nhxc nh Chloramphenicol, qua chun lng Ion Cd(II) sinh ra trong phn ng hay

o ph pht x nguyn t (AES hay ICP-OES) ca dung dch Ion kim loi Cd(II). Vd trong mi trng axit long (HCl 0,05M), bt kim loi xp Cd kh nhm -NO2 vNH2 hon ton nh lng theo phn ng sau.

3Cd + R-NO2+ 6HCl 3CdCl2+ H2O + R-NH2

Chloramphenicol

Nh vy sau khi phn ng kt thc, ly tnm ly dung dch Anion CdCl2 v xcnh Cd bng php o AES v suy ra hm lng Chloramphenicol theo phn ng trn.


132/295

132

Vi cch ny c th xc nh c Chloramphenicol n nhy 0,005 ppm.

Hon ton tng t, nhiu hp cht hu c nitro u c th xc nh gin tiptheo php o ph hp th nguyn t.

7. Theo phn ng to hp cht hydrua bay hiy l phn ng ca mt s Anion ca mt s nguyn t vi nguyn t hydro

mi sinh to ra hp cht kh hydrua dng MeHn (vi n = 3,4,5,v 6). V d: HgH2,AsHa, ASH5, SbH3, SbH5, SeH4, SeH8, TeH4, TeH3, v.v... Phn ng ny rt nhlng, trong nhng iu kin ph hp cho mi cht. V th n cng c s dng xc nh cc nguyn t Hg, As, Se, Te.. trong cc i tng khc nhau, nh trongqung, trong nc, trong thc phm,.. bng php o ph pht x v hp th nguyn t.Song trc ht l mu phn tch cn c x l a cc nguyn t ny v dngmui ca Ion tan trong dung dch nc mi trng axit, v d HgCl2, SbCl5, SnCl4,

Na3AsO4, Na2SeO0, Na2TeO4. Sau thc hin theo cc phn ng sau:

+ Vi Hg: HgCl2+Na2BH4+ HCl HgH2

+ Vi As: ASO43-+Na2BH4+ HCl AsH3

ASO43-+Na2BH4+ HCl ASH5

+ Vi Se SeO43-+Na2BH4+ HCl SeH4

SeO43-+NA2BH4+ HCl SeH6

+ Vi Te: TeO43-+Na2BH4+ HCl TeH4

TeO43-+Na2BH4+ HCl TeHs

Cc kh hydrua sinh ra trong cc phn ng trn c kh argon dn vo h thngnguyn t ha mu ca my v o ph ca cc nguyn t

giáo trình phân tích phổ nguyên tử

Documents