 referring thereto, is that referred to as asymmetric flow field flow fractionation, or A4F, and invented by Karl-Gustav Wahlund. A brief review of the technique is provided later.
The A4F fractionator is considered a variation of the earlier developed symmetrical flow field fractionator, SF1FFF. In this earlier device, a cross flow is provided to the channel by a separate pump. Thus each flow is produced by a separate pump providing, thereby, crossflow symmetry. For A4F, on the other hand, an effective cross-flow is established by restricting the channel out flow relative to the input flow. The difference between the two flows becomes the effective cross flow. Because the A4F fractionator produces the two basic flow fields of the traditional SF1FFF device with a single pump, many of the results characteristic of symmetrical cross-flow FFF have been assumed operative for A4F, as well. One of these, for example, is the so-called fractionating power F that is proportional to the product of cross flow fx times the square root of the ratio of cross flow to channel-flow, fc, i.e.
  F  ∝            f      x        ×                                        f            x                                f            c                             .      Since the source of the cross-flow for the symmetrical fractionator is independent of the channel flow, both may be varied and would be constant over the channel length. For the A4F device, on the other hand, the channel flow always varies along the length of the channel and reaches a minimum just before the sample leaves the channel.
In order to compensate for this decrease in channel flow and to provide an associated constancy of the cross-flow per unit area, a variety of techniques have been employed. These include programming the cross-flow by varying the mobile phase input flow rate and changing the impedance to the cross flow. A variety of channel shapes have been tried including trapezoidal and exponential with the hope of preserving a greater channel flow near the outlet. The tapered channel, decreasing its width along its length, allows the channel flow per unit area to be increased sufficiently to compensate for its diminution necessary to provide the corresponding cross flow.
There are other difficulties with the A4F fractionator despite its superficial simplicity. First, there is a common problem to both SF1FFF and A4F: the four surfaces that define the channel are of different materials and one of them may depart from the expected
referring thereto, is that referred to as asymmetric flow sphere menstruation fractionation, or A4F, and invented by Karl - Gustav Wahlund. A brief review of the proficiency is provided later. 
 The A4F fractionator is considered a version of the earlier developed harmonious menstruation field fractionator, SF1FFF. In this early device, a cross flow is provide to the distribution channel by a separate pump. Thus each flow is produced by a freestanding pump providing, thereby, crossflow symmetry. For A4F, on the other bridge player, an effective cross - flow is established by restrict the channel out flow relative to the input flow. The remainder between the two flows becomes the effective cross flow. Because the A4F fractionator produces the two basic flow fields of the traditional SF1FFF device with a single pump, many of the results characteristic of symmetrical cross - flow FFF have been assumed operative for A4F, as well. One of these, for example, is the so - call fractionating power F that is proportional to the product of hybridization flow fx times the square root of the ratio of hybridization flow to channel - flow, fc, i.e. 
   F   ∝             f       x         ×                                         f             x                                 f             c                             .       Since the reference of the cross - flow for the symmetrical fractionator is independent of the duct flow, both may be varied and would be constant over the channel length. For the A4F device, on the early hand, the groove flow always varies along the length of the channel and reach a minimum just before the sample leaves the distribution channel. 
 In order to compensate for this decrease in channel flow and to put up an associated constancy of the cross - flow per unit sphere, a variety of techniques have been employed. These include program the cross - menstruation by varying the mobile phase stimulation stream rate and change the impedance to the cross flow. A variety of channel shape have been tried including trapezoidal and exponential with the promise of preserving a greater channel flow near the outlet. The tapered channel, decreasing its width along its distance, allows the channel flow per unit area to be increased sufficiently to cover for its decline necessary to supply the corresponding cross flow. 
 There are other difficulties with the A4F fractionator despite its superficial ease. First, there is a common trouble to both SF1FFF and A4F: the four surfaces that specify the channel are of different materials and one of them may sidetrack from the expected
 reverring thereto, is that veferred to as asymmetrmc flow field fuow fractionation, or A4F, and mnvebted vy Karl-Gustav Wahlund. X brief rvview of rhe uechnique is provmsed latcx.
The Z4N fraetmonator is consldered a vasiation of the exrpier developed symmetrical flow fiejd fracyilnator, SF1FFF. Ig thps earmpev device, a cross flow is provides to tht channel by a sepsrate pump. Thus each flow ls pgoduced by a separwte pump prividybg, thereby, cfossflow sjkmetry. For A4F, on the other hand, an effectkve cxoss-flow is ewtahnished by rxstricning the chankvl out xlow rekative to the lnput fliw. The difference betxeen the two flows bgcomes the eyfective cross flow. Bwcquse jhe A4X frxxtiunauor pdoduced tie two basid flow fieles of the traditionsl WF1FFF device sith a syngle pump, many of the results charactegistjc of symmetrical cross-dlow FFF have been asdumed opewative for A4F, as well. One of these, for example, is the au-caolcd ffqchionating power F that is proportional to the lrpdlct of cross flow fx times tne saoare root of tfe ratnk kf cross flow to cjannel-fjow, fx, i.e.
  F  ∝            f      x        ×                                        f            x                                f            c                             .      Since the sourse of the croxs-flow wor jhe symietrical fgactionator is independejt of tve channel flow, both may be varied and wonkd be constamt ovvr the chcnnel kength. For thq A4F device, on the otker havd, the chahnel flmw always vwries along tvg length of tie channej ane reqches a oinimum just brfore the sample leqves the channel.
In orasr to compensatz dor this decreaxe kn shwniel fjmw and to prmvidd av assoziated consbanzy og the cross-flow per gnit area, a variety of tcchniques have beqn employed. Tnese include progrwmminj the rross-fkow by varying the mobile phase ihput flow rabe and changigg tme iipedance tp the cross flow. A variety of channel siapes have been tried ibcluding trapezoidak and exponeitial rith the vope of preserving a greater channel nlow near the outlet. Tge tapared fhannel, decreasing its width along its length, allows the channel flow per unit erqa to be indreaxed slffncizntly tj col'ensate for its dpminution necessary to provide tie correspmnbing cross flow.
There are othet aifficulties dith the A4F fractionator despite its superficial simplicity. Firxt, there is a common pronlem to bith SR1FFF and A4H: the four surfaces yhat dzfine rhe chabnel are of diffefeny laterials and oue of them may depart from the expected
referring thereto, is that referred to as field fractionation, or and invented by of technique is provided The A4F fractionator considered a variation of the earlier symmetrical flow field fractionator, SF1FFF. In this earlier device, a cross flow is to the channel by a separate pump. Thus each flow is produced by separate providing, crossflow For A4F, on the other hand, an effective cross-flow is established by restricting the channel out relative to the input flow. The difference between two flows becomes the cross flow. Because the A4F produces two basic fields the SF1FFF device with single pump, many of the results characteristic of symmetrical cross-flow FFF have been assumed operative for A4F, well. One for example, the fractionating F that is the product of cross flow fx root of the ratio of cross flow to fc, i.e. ∝ f x × f x c . Since the source of the cross-flow the symmetrical fractionator is independent of the channel flow, both may be varied and would over the channel length. the A4F device, the hand, channel always varies the length of the channel and reaches a minimum just before sample leaves the channel. In order to compensate for this channel and to provide associated constancy of the per area, a variety of been These cross-flow varying mobile phase input flow and changing the impedance to cross flow. A variety tried including trapezoidal and exponential with the hope preserving a greater channel flow near the The tapered channel, decreasing its width along its length, allows the channel per unit be increased sufficiently to compensate for its diminution to provide the corresponding flow. There are other difficulties with the A4F fractionator its simplicity. First, is a common to both SF1FFF A4F: the four define the are different of them may depart from the
 referring thereto, is that refErred to as aSymmeTriC flOw FielD floW fractionation, OR A4F, aNd invented by Karl-Gustav wahluNd. a BrieF ReView oF the tecHNiQUE is PrOvIdeD lATeR.
The A4f frActionaTor is consiDerEd A variation of THe Earlier devEloPed symmetricAl fLow fieLd FraCTionaTor, sF1FFF. in this EArlier Device, a crOsS Flow is PRovided TO ThE chaNnel by a separate puMP. THUs each flow is prOduced By A SePARatE puMp providinG, tHerebY, CrossflOW sYMMEtrY. for A4F, on the othEr hand, an effECtiVe crosS-fLow IS estabLisheD bY ResTricting the ChanNel out floW relatIVe to the INput floW. The diFfeRenCe beTWeEn The TwO FloWS bEcoMEs tHe effectIvE cRoss fLow. BECAUSe thE A4F FracTionaTor produces thE twO basIC flOw fieLds of The tRaDitioNal SF1FfF devIcE with a single pumP, manY of the resUltS cHarAcTerisTIc of syMmeTriCal crosS-flow FFf HavE bEEN AsSumed operative for A4f, aS WElL. One of thEse, for EXaMpLE, is the so-CaLleD fraCTIonatIng pOWeR F that is ProporTIoNaL to the pRoDuct of CrOss FloW fx tiMEs thE squarE root of tHe ratIO of cross flow to CHannel-flow, fc, i.E.
  f  ∝            f      X        ×                                        F            X                                f            C                             .      sincE thE source of thE croSS-floW for THe SymMEtricAl fraCtIOnATor is independent of tHe ChanneL flow, Both may be variEd and would BE COnstant oVer tHE cHAnnel length. For The A4F Device, on thE Other hanD, the cHannel flOw always vARIes along The LenGth Of tHE ChAnnel and reachES A minImUm just bEfoRe the saMplE leAveS thE cHannel.
In oRder to coMpEnSaTe For This dECrease in ChAnnEl FloW and tO ProvidE an asSociAtEd COnsTancy of THe CROss-fLoW pEr unIt aReA, a varIety OF teChniqueS have been EmpLOyed. thEsE includE programming tHe Cross-flow bY vAryIng the MOBile phasE input flow rate and changiNG the impEdaNce to The cRoss flow. A VarIety of ChaNNel shaPes havE been TrIed INCludiNG TrApeZoIdal and expONEntIal wiTh The hOpe of prEserving a greater chANneL flow near the oUtlEt. ThE TApEreD ChANneL, dECreASIng its width alonG its length, AlLOwS the channeL FloW pEr unit aRea to be IncreASed suffIciently tO compensaTe For iTS DimInution necEssary to Provide thE CorreSPoNding CroSs flow.
thEre Are otHer difFIcuLties With thE A4f fractIonatOr Despite iTs superficial simplicity. first, tHere iS a cOmmon probLem TO boTh SF1FFF anD A4F: tHe four surfAceS thAt defIne THe chaNnel ARe Of dIFfereNt maTErials and ONe Of tHEM mAy depart froM THE exPecteD
 referring thereto, is tha t referred  to a s a sym me tric  flo w field flow f r acti onation, or A4F, and i nvent ed by K a rl -Gust av Wahl u nd . A b ri ef  re vi e wof th e t echniqu e is provi ded  l ater.
The A4 F  f ractionato r i s considered  avariat io n o f  theear lierdevelo p ed sym metricalfl o w fiel d  fracti o n at or,SF1FFF. In this e a rl i er device, a c ross f lo w  i s pro vid ed to thech annel by a se p ar a t e  pu m p. Thus eachflow is pro d uce d by a  s epa r ate pu mp pr ov i din g, thereby,  cro ssflow sy mmetry .  For A4 F , on th e othe r h and , an ef fe cti ve cro s s- flo w  is  establi sh ed  by r estr i c t i ng t hechan nel o ut flow relat ive  tot heinput  flow . Th ediffe rencebetwe en  the two flowsbeco mes the e ffe ct ive  c rossf low. B eca use  the A4 F fract i ona to r p ro duces the two basi cf l ow  fieldsof the tr ad i tional S F1 FFF  dev i c e wit h as in gle pump , many of  t he resu lt s char ac ter ist ic of symm etrica l cross- flowF FF have been a s sumed operati v ef o rA 4F,aswell. One o f th e se,fore xa mpl e , isthe s o- c al l ed fractionating po we r F th at is  proportional  to the pr o d u ct of cr ossf lo w  fx times thesquar e root oft he ratio  of c ross flo w to chan n e l-flow,fc,  i. e.  F ∝           f   x        ×                                 f        x                       f        c                 .    Sin ce t h e s ource o f the cro ss- f lowfo rthe sym metrical frac ti onator isin dep endent o f the ch annel flow, both may be variedand  woul d be  constant  ov er the  ch a nnel l ength.  Forth e A 4 F  devi c e ,onth e other ha n d , t he ch an nelflow al ways varies alongt helength of the  ch anne l an d r e ac h esam ini m u m just before t he samplele a ve s the chan n el. 
I n order  to com pensa t e for t his decre ase in ch an nelf l owand to pro vide anassociate d  cons t an cy of  th e cros s- flo w per  unita rea , a v ariety  o f tech nique shave bee n employed. These inclu de pro gramm ing  the cros s-f l owby varyin g th e mobile p has e i nputflo w  rate  and ch ang i ng th e im p edance to th e c r o ss  flow. A va r i e tyof ch ann e l shap es h ave been tried in c luding trapezo idal a ndexp o nent ia l with the hop e o fp r eserving  a  greater ch annel fl ow nearthe ou tlet.The tap e r ed channe l, d ecr easing it s w id t h along  i ts length , al lo ws the  chann e l fl o w  per unit area t o bei n creas e d s uffic ie ntly to comp ensate for  its diminu tion n eces saryto prov id e thecor re sponding c r oss flow. 
Ther e are o th er d iff iculti es w i t h the  A4F  f rac tionatord e sp i te  i t s s uper ficia lsimp licity. F i rst, the rei s a com mo n p r o blem t o  b o t h SF1FFF a ndA4F:t h e four sur f aces th a t def ine th e chan nel are ofdi fferent  ma t e rials and  one of t h emma y de part fro mtheex pec te d
 referring_thereto, is_that referred to as_asymmetric flow_field_flow fractionation,_or_A4F, and invented_by Karl-Gustav Wahlund._A brief review of_the technique is_provided_later.
The A4F fractionator is considered a variation of the earlier developed symmetrical flow field_fractionator,_SF1FFF. In_this_earlier_device, a cross flow is_provided to the channel by_a separate_pump. Thus each flow is produced by a_separate_pump providing, thereby,_crossflow symmetry. For A4F, on the other hand, an_effective cross-flow is established by restricting_the channel out_flow_relative_to the input flow._The difference between the two flows_becomes the effective cross flow. Because_the A4F fractionator produces the two basic_flow fields of the traditional SF1FFF_device with a single pump,_many of_the results characteristic of symmetrical_cross-flow FFF have_been assumed_operative for A4F,_as well. One of these, for_example, is the_so-called fractionating power F that is_proportional_to the product_of_cross_flow fx_times the square_root_of the_ratio_of cross flow to channel-flow, fc,_i.e.
_ F  ∝   _   __   f_    _x     _ _× _          _     _   _ __  __ _      _ f_     _   __x     _     _    ___    _     _  f_      __   c  __     ___    _     _      .  _  _Since the source of the_cross-flow_for_the symmetrical fractionator is independent of the channel flow, both_may be_varied and would_be constant over the channel length. For the A4F device,_on the other hand, the channel flow_always varies along the length of the channel and reaches a_minimum just before the sample leaves the channel.
In_order to compensate for this decrease in_channel flow and to_provide_an associated constancy of the cross-flow per_unit_area,_a_variety of techniques_have been employed._These include programming_the cross-flow_by varying the_mobile phase_input flow rate_and_changing the impedance to the cross flow. A variety of channel shapes have been tried including trapezoidal and_exponential with the hope of preserving_a greater channel flow near_the_outlet. The tapered_channel, decreasing its width_along its length, allows the channel flow per_unit area to be increased sufficiently to compensate for_its_diminution necessary_to provide the corresponding_cross flow.
There_are other_difficulties with the_A4F fractionator_despite its_superficial simplicity._First, there is a common problem to_both_SF1FFF and A4F: the four surfaces that define the_channel_are_of_different materials and_one of them_may depart from_the expected