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2006.01
the Restek Advantage
IN THIS ISSUE
Professor WalterJennings
The "Repla cement" Column 2
Environmental
New Rxi'MFused Silica Columns 3
Impr oved SPECartridges for
Massachu setts EPH Analysis 4
New Reference Mixof Canadian
Drinking Water Volat iles 6
Chemical/Petrochemical
Analyze Hydrocar bons on
OPN/Res-SiI""C Bonde d GC Pac king 7
Clinical/Forensics
Sen sitive GClMSAnalysis
for Drug s of Abuse 8
Pharmaceutical
RP-HPLCAnalysis of Selective
Serotonin Reuptake Inhibitors 10
Assaying Tetrac yclines by HPLC 12
Analyzing Residual Solvent s
in Water-Soluble Articles ; 14
Foods, Flavors & Fragrances
trans Fat: Resolving cis and trans
FAME Isomers by GC 16
HPLC Accessories
Genu ine Restek Repl acemen t
Parts for Shimadzu HPLC Syste ms 17
GC Accessories
CoolTools 18
Head space Vials; Hand -Held,
Rechargeable, Crimpers & Decappers .. . 19
Peak Performers:
Avoid Septum Prob lems 20
Click-On Inline Super-Clean?' Traps 22
Erratum
Thetransfer line used inthe methyl tert-butyl
ether / tert-butyl alcohol analysisreported in
Advantage 200Sv4 (Figure 1, page 4) was the
factory-installed Eclipsetransfer line.
WethankLaura Chambersat 0.1. Analytical,
CollegeStation,Texas, for reviewing the analysis
with us, and we are verygrateful to 0.1.
Analyticalfor their generous loan of the 0.1.
4660 Eclipse purgeand trap system.
RestekTrademarks
Allure, Crossbond, IceBlue, Precision,Res-Sil,Rtx, Rxi, Silcoport,
Silcosteel, Siltek,Thermolite, Turning Visions intoReality, Restek
logo
OtherTrademarks
Auto SYS(PerkinElmer),BTO,Center6uide (Chromatography
Research Supplies, Inc.),Carbowax (Union Carbide Corp.),
Microseal (MerlinInstrument Co.), PEEK(Victrexpic),Porasil
(WatersAssociates,Inc.),Super·Clean (S6T Middleburg BV),
Teflon,Tefzel (E.I.duPont deNemours&Co., Inc. ),Tenax(Enka
Research InstituteArnhem),TRACE (Thermo Electron Corp.),
Versapak(Black & DeckerCorp.)
By ProfessorWalterJennings CWal~1
Professor Emeritus,University ofCalifornia, Davis; Co-Founder,
J&WScientific, Inc.; Co-Founder, AirToxics,Ltd.;
For the past fewyears, the agingprocess has beencatchingupwiththeJennings family.While I still
enjoy participatingin seminars and lectures,I nowfind seventotenevents on a two weektripis
more tiringthan it was just a fewyears ago. Inaddition, mywifehas beensuccessfullybattling
Parkinson's diseasefor almost twentyyears, but we realize that itisnow inexorablyadvancing.
Hence, she needs more ofmytime, and I must limitmyself toshorter absences. Because ofthese
developments,I permittedmyAgilent contracttoexpirewhenit ran outonJune30, 2005. Agilent
was sympatheticandunderstanding,our partingwas amicable, andI still valuemycontactswith
them. But after twomonths inmy home office, I sometimes feela needfor the challenge of discus­
sionandargument,andwhenfriendsat RestekaskedifI wouldbe interestedinwritinga short paper
that was purelyeducational and pushing no particular product line, it soundedappealing.Hereit is.
The"Replacement" Column, A Recurring Problem in Gas Chromatography
One of the problems that gas chromatographers frequently ask concerns the behavior of a "replacement" col­
umn , Evenskilled practitioners have been known to protest when they install a replacement column, use the
same operational parameters, and find that not only have solute retention times shifted, but peak 15 now
elutes prior to peak most such cases,they blame the column manufacturer, There are programs avail­
able to correct this problem, but some of those solutions have been so simplified that the user still has no
comprehension of the causative factors, a state of blissful ignorance which should be corrected.
Columns are produced, bought, and sold using nominal measurements, e,g., "30 meters x 0.25mm, film
thickness 0.25flm".As a specification, this is not equivalent to "30.0 meters x 250flm". Depending on the man­
ufacturer 's specifications, the actual column dimensions may be "30 +1- I meter x 250 +1- 6flm': Some man­
ufacturers now give better attention to the length tolerance, but the diametric variation will continue to be a
problem. Fused silica draw towers are often computer controlled, with the feed rate of the silica blank, the
draw rate of the capillary tube, and the temperature of the softening oven controlled by a computer whose
input comes from a laser micrometer that monitor s the tubing diameter durin g the drawing process, In our
hands, a blank could be drawn to approximately 14 kilometers of 0.25mm capillary tubing, The two ends of
that tube may show a significant variation in diameter, but those changes occur so slowly that over lengths of
a few hundred meters the diameter is reasonably constant. It is rare to find a column where the diameters at
the two ends are significantly different, but it is not unusual to find that columns from the two ends of that
draw, or from different draws, do exhibit significant differences in diameter, e.g., 244flm vs, 256flm'.
An analyst whose original 29.9m x 256flm column is replaced by one measuring 30,lm x 244flm will likely
experience difficulties if he or she uses the same operational parameters, i.e., same temperature program ,
same carrier gas, same inlet and outlet pressures. Because of the geometric differences, the columns possess
different pressure drops and under the same operational parameters, the carrier gas velocities would be dif­
ferent in the two columns. This will affect solute retention times, and this introduces the major problem.
Gas chromatography is a volatility phenomenon, and solutes elute in a sequence mandated by what I prefer
to call their "net vapor pressures". The net vapor pressure is a function of the intrinsic vapor pressure of that
solute, increased by the temperature at that point in the program, and furth er decreased by the sum ofall
interactions between that solute and that stationary phase.';The strengths of these various interactive forces
usually vary inversely with temperatur e in a non-linear manner, and for a given increase in temperature both
the rate of change and the degree of change are unique functions of that solute in that stationary phase under
these particular conditions.Asa result, the molecules of a chromatographing solute experience a specific tem­
perature-sensitive "selectivity profile" in their passage through the column , These interactions are rendered
moot while those molecules are in the mobile phase, and endure only while they are in contact with the sta­
tionary phase, Hence we are interested in keeping retention times, and particularly t'R (time in stationary
phase) constant from column to column and run to run. From the two relationships of K, =Bk and fl =LltM
we can establish that t's = cslc« x dr/dox Llu. The three terms of course are the distribut ion constant, the recip­
rocal of the phase ratio, G, and column length divided by the average linear gas velocity. K, is a function of the
solute, the stationary phase, and the temperature. While, by definition, the temperatu re changes in program
mode, the rate of change is constant, run to run and column to column, under the same program parame­
ters, and one can usually ignore this term if the two stationary phases are indeed identical", The second term
can also be ignored, provided the ratio of dr/deis constant. Column diameter, d, and column length are both
nominal values and usuallydiffer from column to can compensate for either or both of theseby
varying the gas velocity, u. This is most easily accomplished in constant pressure mode. In constant flowmode
it is more complicated and beyond the scope of this paper.
In constant pressure mode, the solution is quite simple, assuming that the replacement column has the same
stationary phase and the same phase ratio as the original column. I) Using the original operational parame­
ters (initial temperature and program parameters, column inlet and outlet pressures, same carrier) install the
new column and inject the same mixture. 2) Determine the retention time of an easily identifiable peak, and
compare this to the retention time of that peak on the original column, 3) Adjust the column inlet pressure
to make the retention time of the target peak the same as it was on the original column, Retention times on
the replacement column should now agree closely with the values observed on the original column, each
solute will now experience its original temperature-sensitive"selectivity profile",and chromatograms gener­
ated on the replacement column should essentially duplicate those from the original column.
i Fortunately, the columnphase ratio(B)isusually unaffectedby thesechanges indiameterbecausealmost all manufacturers currently
employ static coating methods. Provided the concentrationofthe stationary phaseinthe coating solutionremains constant,the ratio of the
filmthickness (d,)tocolumn diameter(d) wi ll remainconstant.
ii These interactions include (but arenot limitedto) dispersive interactions, hydrogen bondingand other protonforms ofproton sharing,
dipole interactions,and in some cases, molecularsize and shape.
iii Insome cases,surface preparationand deactivation treatments canalsoaffect retentions.These treatments aregenerallyproprietaryand
vary frommanufacturer tomanufacturer.Withcomplex mixtures, the separationsachieved oncolumns coated with the samestationary
phase butfromdifferentsuppliersmay yieldslightly different results.
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