Sergei Klimanski

Liquid phase RTX-1701 from Restek is one of the most suitable for application in GC analyses of organic solvents.  This study is about GC separation of mixtures of organic solvents on 0.25mm ID capillary column installed into Varian 3400 GC, as a detector was used  Finnigan MAT ion trap mass-spectrometer ITS40.  Liquid phase film thickness is 1 um, column length 30m.  Owen of the gas chromatograph programmed from 50 deg. C  ( 1 minute ),  8 deg/min to 180 deg.C.

As a standard solution used C5 – C14 mixture of n-Alkanes with each component content about  200 ppm ( wt )  in HPLC grade Acetonitrile, or 156 mg per ml, or  156 ng per 1 ul.  Injector of GC was  adjusted to split ratio 1:50, so as a result each alkane’s peak  load was about 3 ng if 1 ul of the standard solution is injected.

To determine RI index of solvents,  special sample solution was prepared  by mixing 0.2 ul of each solvent standart ( min 99 % purity ), same amount of n-Octane was added  as  internal standard.  No solvent was used in this case and for injecting used headspace sample – by adding of 0.5ul of test mixture to 12ml glass bottle ( with laboratory air inside ) and 50 ul of this headspace sample was injected into CG.

At the same time, if peak square of each solvent is measured , we can determine relative intensity indexes K  for MS detector to each sample solution component.  Values of K we can use later for quantitative analyses of a mixture with  unknown composition using same HS method of sample preparation and using n-Octane as internal standard.

Most popular solvents for paits were used to prepare test sample, here is list of solvents for determination, totally 27 components:

• n-Octane ( internal standard )
• Methanol
• Ethanol
• n-Propanol
• IPA ( iso-Propyl Alcohol )
• n-Butanol
• iso-Butanol
• 2-Butanol
• 1-Methoxy-2-Propanol
• 2-Ethoxy-1-Ethanol
• Acetonitrile
• Acetone
• 2-Butanone ( MEK )
• Cyclohexane
• Methylene Chloride
• iso-Octane
• 1,4-Dioxane
• n-Butyl-Acetate
• iso-ButylAcetate
• EthylAcetate
• MethylAcetate
• MTBE
• Diisopropyl Ether
• 1-Methoxy-2-Propyl Acetate
• Toluene
• p-Xylene
• Cumene

50 ul of  this mixture’s headspace contains about 120 ng of each component, what after  1/50 split in injector is acceptable load for 1 um liquid phase film.

Now about the tuning of mass-spectrometer. It was made using Varian Saturn 5.2 software:

• manifold temperature – 220 deg C
• transfer line temperature – 230 deg C
• filament emission current – 20 uA
• multiplier voltage – 1550 Volts
• AGC target TIC – 20 000 counts
• one microscan time – 100 us
• scans per second – 3

Varian  3400 CG injector temperature 250 deg.C, injector 1075 type with split valve ( open ), inlet helium pressure 12 PSI. For injection is used Hamilton 1ul ( for  liquids ) and 100 ul  ( for headpace ) syringe with 70mm needle.

Table of the components content on test mixture of 27 compoments

Resulting figures are listed below.

First of all it is chromatogramm of standard solution C5 – C14, on Y axis is TIC ( Total Ion Current ):

and table of retention times:

Values for first and last rows are obtained by extrapolation.

Chromatogram of the mixture of 27 compoments:

As seen,  three components –  IPA,  Methyl Acetate and MTBE ( mentioned in sequence as RI grows ) are not separated on this column and comes out as one peak. In AMDIS using SIM plots and data from mass-spectra of three above mentioned substances ( m/z=45 is main peak for IPA – red curve, m/z=43 is  peak for Methyl Acetate – yellow curve, and m/z=73 is main peak for MTBE – blue curve ) we still can get both three components separated and to calculate their RT and RI:

and finally – table of RI ( Retention Indexes ) of all solvents and calculated relative to n-Octane index K, which shows, how much times intensity of the substance ( TIC )  is higher ( if K < 1 ) or lower ( if K > 1 ), then intensity of the same amount (  by wt ) of n-Octane. As a result for future analyses we have  possibility  of quantification for  chromatograms with n-octane as the internal standard.

K indexes for IPA ( m/z 45), Methyl Acetate ( m/z=75 ) and MTBE ( m/z=73 ) have been calculated for SIM mode plots.

Thanks to Dmitrijs Dmitrijevs, who have helped me to find some pure solvents for making standard mixtures.

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