Why it is Called Monosegmented Flow Analysis
Celio Pasquini
Instituto de Química, Universidade Estadual de Campinas, C.P. 6154,
13083-970 Campinas - SP, Brazil
Sirs: A re cently pu blis hed pa per1 re vi ews a flow tech ni que that was ini ti ally des cri bed in 1985, ba sed on the in -tro duc ti on of the sam ple bet we en two air or inert gas bub bles in the flow analy sis ma ni fold2. This pro ce du re ge ne ra tes a re pro du ci ble sam ple mo no seg ment ha ving res -tric ted con vec ti on mi xing with the car ri er so lu ti on re sul ting in low dis per si on of the sam ple du ring its trans -port to the de tec tor, even with long re si den ce ti mes. Fi gu re 1 com pa res flow pat terns ge ne ra ted by three well known flow analy sis systems. From the che mi cal po int of view the mo no seg men ted tech ni que per mits per for ming de ter mi na -ti ons un der non-steady sta te con di -ti ons, en su red by the high ti ming re pro du ci bi lity found in the mo no seg men ted system due the pre sen ce of but two gas bub bles. This mo no seg men ted system con trasts with the pre vi ous mul ti seg -men ted systems (see Fig. 1B) ori gi nally des cri bed by Skeggs3. Ho we ver, the main ad van ta ge of the mo no seg -men ted system is that the sig nal is ob ta i ned af ter al lo wing most of the re ac ti on to take pla ce, un der the con di ti ons of res tric ted dis per si on of the sam ple (i.e. near the equi li bri um sta te). From the physi cal po int of view the ste ady sta te flow is more clo sely ap pro xi ma ted by a mo no seg men ted system than by the flow in jec ti on system.
The first pa per in tro du cing the new flow prin ci ple na -med it as Mo no seg men ted Con ti nu ous Flow Analy sis (MCFA)2. This name re ma i ned in use un til the re com men -da ti ons of IUPAC on the clas si fi ca ti on and de fi ni ti on of
analy ti cal met hods ba sed on flo wing me dia were pu blis -hed4,5. Sin ce then, many aut hors have re cog ni zed that the word “con ti nu ous” does not con tri bu te sig ni fi cantly to the des crip ti on of the flow pro cess, and have ac cep ted the name re com men ded by IUPAC: Mo no seg men ted Flow Analy sis (MSFA)4,5. A sur vey of the Insti tu te for Sci en ti fic Infor ma ti on (ISI) data base re ve als that, so far, 29 pa pers have em plo yed the “mo no seg men ted” word to des cri be this flow prin ci ple2,6-33 whi le only 3 works ap pe a ring in the ISI in de xed pe ri o di cals have em plo yed the term Seg men tal Flow Injec ti on1,34,35 re fer ring to the same flow ap pro ach. Unfor tu na tely, the aut hor of the re vi ew1 ig no res the IUPAC re com men da ti ons4,5, pre fer ring to use the name that he and his co-authors sug ges ted five ye ars af ter34 the first pa per on the mo no seg men ted tech ni que was pu blis -hed2. Note that in the pa per in which the name SFIA was first co i ned34, no men ti on the of ear li er pa pers2,6 was made. In the re vi ew, the aut hor sug gests that “MSFA is not ef fec ti ve in emp ha si zing some es sen ti al fe a tu res of the tech -ni que1”. He ar gues that, sin ce the sam ple is in jec ted, the word “in jec ti on” should be used in the name, as pro po sed by him. In fact, in most ca ses the sam ple is in jec ted into the ma ni fold by using a spe ci al in jec ti on de vi ce ca pa ble of ge -ne ra ting the mo no seg ment. Ho we ver, the mo no seg ment can also be pro du ced by suc ces si vely as pi ra ting air and sam ple and flus hing the mo no seg ment with a car ri er li quid in a flow system23,29,36 or by using a time ba sed sam pling system17. No in jec ti on per se is em plo yed in the se ca ses whi le the cha rac te ris tics of the mo no seg men ted system are ma in ta i ned. Thus the name need not/should not con ta in the term “in jec ti on”, as was no ted in the IUPAC re com men da -ti ons4,5. On the ot her hand, it is pos si ble to find at le ast one pa per in the li te ra tu re des cri bing in jec ti on of the sam ple and sub se quent mul ti seg men ta ti on37. The name Seg men tal Flow Injec ti on Analy sis does not dis tin guish bet we en mono and mul ti seg ments, alt hough they pre sent com ple -tely dif fe rent pro per ti es.
The re vi ew1 emp ha si zes early systems whe re the ne ces sity of bub ble re mo val was man da tory and re sul ted in sig ni -fi cant dis per si on, re du cing the sen si ti vity or cal ling for lar ger sam ple vo lu mes. An early pa per was pu blis hed ai -ming only at re sol ving this pro blem6. Actu ally, with the mo -dern de tec ti on systems ba sed on the mi cro com pu ter and di gi ti zed sig nals, the bub bles can pass through the de tec tion Let ter
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cell, even when spec trop ho to me tric de tec ti on is em plo -yed19,20,21,27. Fi gu re 2 shows a typi cal sig nal ob ta i ned using a com pu ter ba sed system that pre clu des bub ble re mo val. Obvi ously, it lo oks more like a ste ady sta te sig nal than a flow in jec ti on sig nal be ca u se the con cen tra ti on along the mo no seg ment is es sen ti ally cons tant and the mo no seg ment is ne arly ho mo ge ne ous21,27. This re sults in the sig nal ba sed on a mo no seg ment be ing in de pen dent of dif fe ren ces of re -frac ti ve in dex among sam ples, over co ming pro blems due to the Schli e ren ef fect27.
Not men ti o ned in the re vi ew1 is the un sur pas sed ad van -ta ge of the mo no seg men ted ap pro ach: the abi lity of pre ci se sam ple lo ca ti on which is pos si ble be ca u se the sam ple is unam bi guously pre sent bet we en two air bub bles as a sin gle seg ment. This fe a tu re al lows at ta i ning a sim ple and com ple te ma na ge ment of the flow analy zer. Re a gents can be se -lec ti vely de li ve red only into the mo no seg ment, whe re they are ho mo ge ne ously dis tri bu ted, al lo wing for a low con -sump ti on of ex pen si ve che mi cals and for the use of the same ma ni fold for a se quen ti al de ter mi na ti on of va ri ous analy tes21. Sim ple op ti cal swit ches and con di ti o ning elec tro nic cir cu its ca pa ble of dis tin guis hing bet we en the re frac -ti ve in de xes of air and so lu -ti on are used to pro du ce elec tro nic lo gi cal le vel tran si ti ons (TTL) to flag the con trol ling com pu ter about the pas sa ge of the gas/li quid boun -da ri es that de fi ne the mo no seg ment38. In con trast with ot her flow systems, in a MSFA system it is al ways pos si ble to know pre ci sely whe re the sam ple is, al lo wing the ap pro
pri a te ac ti on to be im ple men ted with the ob jec ti ve of con di ti o ning, re ac ti on or me a su re ment. For exam ple, the de tec ti -on of the mo no se ge ment is made -only when it is in si de the flow cell20. The ope ra ti on can be con trol led by lo o king at the po si ti on of the first bub ble im me di a tely at the ou tlet of the flow cell.
No body can deny the enor mous con tri bu ti on of the Flow Injec ti on con cept39 to Analy ti cal Sci en ces in ge ne ral. The prin ci ple of dis per si on con trol led by con vec ti on/ dif fu si on is at the he art of this flow tech ni que and it is achi e -ved only by the non-segmented flow ap pro ach. On the ot her hand, the mo no seg men ted ap pro ach uses a dif fe rent prin ci ple and the use of a si mi lar ter mi no logy would be con fu sing, as cor rectly in ter pre ted by IUPAC3,4. In fact, the MSFA works in a dis tinct way, res tric ting the dis per si on by com part men ta li za ti on. In do ing so it pro vi des many ad van -ta ges as well as some di sad van -ta ges.
Thus, even wit hout con si de ring the re com men da ti ons of IUPAC4,5 it should be cle ar that Mo no seg men ted Flow Analy sis (MSFA) is the ap pro pri a te name for a flow ap pro ach which pro po ses the in tro duc ti on of the sam ple in bet -we en two air bub bles, de fi ning a mo no seg ment, es pe ci ally in view of the fact that this ap pro ach per mits new fe a tu res not achi e ved in pre vi ous flow ap pro a ches. The name MSFA com mu ni ca tes the main fe a tu re of the flow tech ni -que (the exis ten ce of the mo no seg ment) and should re ma in the name for this ra pidly gro wing flow analy sis pro ce du re.
The aut hor is gra te ful to Dr. Ca rol H. Col lins for help ful sug ges ti ons and ma nus cript re vi si on.
References
1. Zhi, Z.L. Trends Anal. Chem. 1998, 17, 411. 2. Pas qui ni, C.; de Oli ve i ra, W.A. Anal. Chem. 1985, 57,
2575.
3. Skegg Jr., L.T. J. Clin. Pat hol. 1957, 28, 311. 4. Van der Lin den, W.E. Pure Appl. Chem. 1994, 66,
2494.
5. Za gat to, E.A.G.; Oli ve i ra, C.C.; Col lins, C.H. Quim. Nova 1999, 22, 143.
6. Pas qui ni, C. Anal. Chem. 1986, 58, 2346.
7. de Andra de, J.C.; Lo bos chi, E.M. Analyst 1988, 113, 295.
8. de Andra de, J.C.; Fer re i ra, M.; Bac can, N.; Ba ta glia, O.C. Analyst 1988, 113, 289.
9. Reis, B.F.; Arru da, M.A.Z.; Za gat to, E.A.G.; Fer re i ra, J.R. Anal. Chim. Acta 1988, 206, 253.
10. de Andra de, J.C.; Ei ras, S.P.; Bruns, R.E. Anal. Chim. Acta 1991, 255, 149.
11. de Fa ria, L.C.; Pas qui ni, C. J. Au tom. Chem. 1992, 14, 213.
12. de Andra de, J.C.; Bruns, R.E.; Ei ras, S.D. Analyst
1993, 118, 213.
Fi gu re 2. Typi cal sig nal ob ta i ned with a spec trop ho to me tric de tec ti on in a mo no seg men ted flow system. a and b: lar ge ab sor ban ce chan ge pro vi ded by the pas sa ge of the air bub bles through the flow cell; c: ste ady sig nal ob -ta i ned du ring the pas sa ge of the mo no seg ment through the flow cell.
13. No gue i ra, A.R.A.; Bri en za, S.M.B.; Za gat to, E.A.G.; Lima, JLFC.; Ara ú jo, A.N. Anal. Chim. Acta1993,
176, 121.
14. dos Reis, B.F.; Za gat to, E.A.G.; Mar tel li, P.B.; Bri en za, S.M.B. Analyst1993, 118, 719.
15. Fac chin, I.; Mar tins, J.W.; Za mo ra, P.G.P.; Pas qui ni, C. Anal. Chim. Acta1994, 285, 287.
16. Fac chin, I.; Pas qui ni, C. Anal. Chim. Acta1995, 308, 231.
17. de Andra de, J.C.; Pop pi, R.J.; Cos ci o ne, A.R. J. Au tom. Chem. 1996, 18, 199.
18. Bel la to, C.R.; Pas qui ni, C. Analyst1996, 121, 1923. 19. Fac chin, I.; Roh wed der, J.J.R.; Pas qui ni, C. J. Au tom.
Chem.1997, 19, 33.
20. da Sil va, M.C.H. and Pas qui ni, C., Anal Chim. Acta 1997, 349, 377.
21. Ra i mun do Jr., I.M.; Pas qui ni, C. Analyst1997, 122, 1039.
22. Ara ú jo, A.N.; Ca ti ta, J.A.M.; Lima, J.L.F.C.; Za gat to, E.A.G. Fre se ni us J. Anal. Chem.1998, 360, 100. 23. Vi e i ra, J.A.; Ra i mun do Jr., I.M.; Reis, B.F.; Za gat to,
E.A.G.; Lima, J.L.F.C. Anal. Chim. Acta1998, 366, 257.
24. da Sil va, M.C.H.; Roh wed der, J.J.R.; Pas qui ni, C.
Anal. Chim. Acta1998, 366, 223.
25. Ni el sen, S.; Han sen, E.H. Anal. Chim. Acta1998, 366, 163.
26. Fac chin, I.; Pas qui ni, C. Quim. Nova1998, 21, 60.
27. Bri to, V.O.; Ra i mun do Jr., I.M. Anal. Chim. Acta 1998, 371, 317.
28. So to ma yor, P.T.; Ra i mun do, I.M.; Neto, G.O.; de Oli ve i ra, W.A. Sen sor Actu at. B-Chem.1998, 51, 382. 29. Gan za rol li, E.M.; Lehm kuhl, A.; de Qu e i roz, R.R.R.;
Sou za, I.G. Quim. Nova1999, 22, 53.
30. Smi der le, M.; Reis, B.F.; Ro cha, F.R.P. Anal. Chim. Acta1999, 386, 129.
31. Ho no ra to, R.S.; Sil va, E.C.; de Ara ú jo, M.C.U.; Ve ras, G.; San tos, S.R.B.; Lima, J.L.F.C.; Lapa, R.A.S. Lab. Ro bo tics Au to mat.1999, 11, 97. 32. Ara ú jo, A.N.; Ca ti ta, J.A.M.; Lima, J.L.F.C. Far ma co
1999, 54, 51.
33. Mar tel li, P.B.; Reis, B.F.; Korn, M.; Lima, J.L.F.C.
Anal. Chim. Acta1999, 387, 165.
34. Tian, L.C.; Sun, X.P.; Xu, Y.Y.; Zhi, Z.L. Anal. Chim. Acta1990, 303, 183.
35. Bor zitsky, J.A.; Dvi nin, A.V.; Pe truk hin, O.M.; Uru sov, Y.I. Anal. Chim. Acta, 1993, 274, 125. 36. Pe ter son, K.L.; Lo gan B.K.; Chris ti an G.D.; Ru zic ka,
J. Anal. Chim. Acta, 1997, 337, 99.
37. Gard ner, W.S.; Malczyk, J.M. Anal. Chem.1983, 55, 1645.
38. Ra i mun do Jr., I.M.; Pas qui ni, C. Lab. Mi cro.1994, 13, 55.
39. Ru zic ka, J.; Han sen, E.H. Anal. Chim. Acta1975, 78, 145.
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