Peroxide Immobilization of a Silicone Stationary Phase on a Support

A imobilização de uma fase estacionária de sil i cone (SE-30) sobre Chromosorb W-AW para uso em cromatografia gasosa foi iniciada por peróxido de dicumila (DCUP). Foram estudadas concentrações apropriadas de peróxido e o procedimento utilizado para imobilização é bem descrito. Foram realizados tes tes de extrações com solventes, avaliação cromatográfica, bem como estabilidade térmica e lavagem com solventes em coluna. Estes tes tes mostraram que a preparação de SE-30 imobilizada sobre Chromosorb W-AW, usando DCUP, é simples e produz fases estacionárias eficientes e resistentes a solventes.


In tro duc tion
Liq uid sta tion ary phases used in gas chro ma tog ra phy pres ent lim i ta tions with re spect to vol a til ity and ther mal sta bil ity which im pose up per tem per a ture lim its on their use.These prob lems are ame lio rated by the im mo bi li za tion of the sta tion ary phase through bond ing be tween the polymer chains with pos si ble bond ing to the sup port.This immo bi li za tion pro cess is ini ti ated by free rad i cals which are pro duced by heat [1][2][3][4][5] , chem i cal ini ti a tors such as azocompounds [6][7][8] , per ox ides [9][10][11][12][13] or ozone 14,15 , and ion iz ing ra di a tion such as elec trons 16 , γ ra di a tion 17,18 , and low-temperature plas mas 19 .All of these meth ods have been suc cess fully ap plied to pro duce im mo bi lized nonpolar sil i cone phases within cap il lary col umns.However, very lit tle in for ma tion ex ists about im mo bi li za tion of sup ported sta tion ary phases used in packed-column gas chro ma tog ra phy.This pro ject was ini ti ated to study the per ox ide im mobi li za tion of SE-30 (sil i cone phase) on sup ports.Compared with other meth ods, per ox ide-initiated cross-linking is easier to per form, and in ad di tion, the nec es sary chem i cals are readily avail able.Ap pro pri ate per ox ide con cen tra tions were stud ied and the de tails of the im mo bi li za tion pro cedure are de scribed here.

Ex per i men tal
Ma te rials 15% SE-30 (Merck) on Chromosorb W/AW (80-100 mesh, Merck) was pre pared by evap o rat ing chlo ro form.This ma te rial was sus pended in chlo ro form con tain ing dicumyl per ox ide (DCUP, Merck) at con cen tra tions varying from 1 to 10% w/w.Af ter evap o ra tion of the sol vent, known quan ti ties of the coated solid were placed in ampoules which were sealed un der air and heated to dif fer ent tem per a tures (150, 210 and 250 °C ) for 10 h.

Equip ment
A Varian 3300 Gas Chromatograph with a flame ioniza tion de tec tor, cou pled to a Varian 4400 in te gra tor, was used with 2 m x 2 mm (i.d.) silanized glass col umns placed in such a way as to per mit on-column in jec tion.
In fra red spec tra were taken on a Bruker model IFS 28 spectrophotometer, while vis i ble spec tra were ob tained on a Micronal model B 260 spectrophotometer.

Testing pro ce dures
Sol vent ex trac tions of treated and non-treated pack ing ma te ri als were car ried out at re flux tem per a ture for six hours with each of three sol vents (meth a nol, ben zene and chlo ro form) us ing a mod i fi ca tion of the method from Sanchez et al. 20 .
Meth y lene blue ad sorp tion on the ac tive sur face, used to de ter mine the rel a tive dif fer ence be tween well-coated and non-coated par ti cles, was car ried out by a mod i fi ca tion of the method from Shapiro and Kolthoff 21 .Known quan tities of the pack ing ma te ri als were stirred in con tact with an ethanolic so lu tion con tain ing 10 µg mL -1 of meth y lene blue.Af ter a fixed time, the sus pen sion was fil tered and the absorbance of the meth y lene blue re main ing in so lu tion was mea sured at 610 nm.The ac tiv ity of the sur face was de ter mined from the dif fer ence in ab sorp tion be tween pure meth y lene blue so lu tion and the fil tered meth y lene blue solu tion af ter con tact.
For chro mato graphic test ing, the pre pared phases were packed into iden ti cal glass col umns with the aid of a vi brator.The pack ing den sity was con trolled by the weight of the pack ing used.
Two dif fer ent mix tures (A: dec ane, undecane, and dodecane, and B: methyl hexanoate and methyl octanoate) were used to eval u ate the chro mato graphic pa ram e ters of the phases at 160 °C (A) and 140 °C (B), with a flow rate of 30 mL min -1 (N 2).The t M was de ter mined with bu tane at three dif fer ent tem per a tures (170, 180, and 200 °C).
The col umns were sub mit ted to high tem per a tures (~350 °C), and the back ground cur rent was reg is tered to ver ify the ther mal sta bil ity.
The extractability of the sta tion ary phase film was checked in the fol low ing way.The packed col umn was succes sively washed with 2.5 mL ac e tone, 1.8 mL meth y lene chlo ride: isooctane (1:1), 1.8 mL meth y lene chlo ride: isooctane (1:4), and fi nally, 2.5 mL isooctane.Each column was tested prior to rins ing and af ter rins ing.
Ta ble 1.The per cent age of the liq uid sta tion ary phase (SE-30) extracted by sol vents as a func tion of the dicumyl per ox ide con cen tration used for im mo bi li za tion*.

Re sults
Sol vent ex trac tion of the sta tion ary phase is shown in Ta ble 1.The in fra red spec tra are pre sented in Ta ble 2. Table 3 com pares the chro mato graphic pa ram e ters mea sured for a non-immobilized phase with a phase pre pared with 8% DCUP at 250 °C.The ef fect of im mo bi li za tion on thermal sta bil ity is shown by de ter min ing the back ground current as pre sented in Fig. 1.The re sults of rins ing with sol vents are shown in Fig. 2.

Dis cus sion
Sol vent ex trac tion of the sta tion ary phase re moves the non-immobilized polymethylsilicone from the sup port, and the fi nal and ini tial weight dif fer ences pro vide the to tal per cent ex tracted and con se quently the to tal per cent im mobi lized.As shown in Ta ble 1, 8 and 10% of DCUP im mo bi -lize a sig nif i cant quan tity of the phase (up to 80% at ei ther  210 or 250 °C).Lower per cent ages of DCUP do not re sult in sig nif i cant lev els of im mo bi li za tion.The in fra red spec tra (Ta ble 2) show that the free silanol band (3500 cm -1 ) is ab sent in the phases pre pared with 8 and 10% of DCUP, in di cat ing the re moval or screen ing of sur face silanols.This is im por tant be cause the sur face silanols, when not coated, can re act with the sam ple dur ing chro mato graphic sep a ra tion and pro duce assymetric peaks.
The im mo bi lized col umn has a smaller plate height and higher res o lu tion than the non-immobilized col umn, while the re ten tion fac tor (k) de creased for the ma jor ity of compounds af ter im mo bi li za tion (Ta ble 3).Thus, sep a ra tions on the im mo bi lized col umn are faster and more ef fi cient.These re sults are sim i lar to those ob tained by Ghaoui et al. 22 with SE-54 on Chromosorb W/AW, which was 75 to 98% im mo bi lized us ing 0.30% DCUP at 180 °C for 3 h.The lower per ox ide con cen tra tion used by these au thors is prob a bly due to the pres ence of a vi nyl substituent in SE-54 that fa cil i tates the im mo bi li za tion pro cess.
The tem per a ture needed to bring about a 40% in crease in the back ground cur rent is 156 °C higher for im mo bi lized SE-30 than for the non-immobilized phase, in di cat ing a sig nif i cant in crease in sta bil ity with tem per a ture pro gramming (Fig 1).
Af ter the rins ing se quence, the col umns were eval uated.The plate heights of the col umns were de ter mined.
Af ter suc ces sive rins ing steps the im mo bi lized SE-30 showed a smaller plate height vari a tion than did non-immobilized SE-30 (Fig 2).

Con clu sion
The prep a ra tion of an SE-30 phase im mo bi lized on Chromosorb W/AW us ing DCUP is a sim ple method that pro duces ef fi cient and non-extractable col umns, in di cat ing a lon ger use ful life and the pos si bil ity of the re cu per a tion of a con tam i nated col umn.

Ac knowl edg ments
The au thors ex press their ap pre ci a tion to CNPq, FAPERGS, FIPE-UFSM and UNISC for fi nan cial sup port for sev eral as pects of this pro ject, and to Carol Col lins for help ful con ver sa tions .RCSS also wishes to thank CNPq for a fel low ship.

Fig ure 1 .*
Fig ure1.The ef fect of SE-30 im mo bi li za tion on the back ground current.
Ta ble 3. Plate heights cal cu lated for al kanes and methyl es ters.