Detectability enhancement of spectrophotometric detectors by the use of multidimensional gas chromatography

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Medina, Griffith, Houston, Winniford 1317

Juan CarlosMedina,JamesGriffith,K. Ray Houston,Bill Winniford

The Dow Chemical Company,2301 N. Brazosport Blvd.,Freeport, TX 77541, USA

Detectability enhancement of spectrophotometricdetectors by the use of multidimensional gaschromatography

Multidimensional gas chromatography (2D GC) is demonstrated as a way to improvelimits of detectability of spectrophotometric detectors. UV and IR detectors are gene-rally less sensitive than mass spectrometers or other GC detectors. This has placedsome limitations on the useful capabilities provided by spectrophotometric detectors,such as the ability to provide structure-related information for a particular analyte. Inthis paper, we report results from interfacing a 2D GC instrument to a UV detector.Symmetry factor and the ratio of retention time divided by peak width did not showdeterioration of the quality of chromatography when a megabore column was usedwith this detector. Furthermore, an increase in the limits of detectability over thatattainable in a single-column system was realized by using the 2D GC system. How-ever, the low flow (1 mL/min) imposed by the use of a microbore column (250 lm ID)caused significant tailing when the UV detector was used.

Key Words: Gas chromatography; Multidimensional chromatography; GC detectors;Spectrophotometric detectors; Cryotrapping; Chromatographic selectivity; GC-UV;1-Octene; Chromatographic selectivity; Chromatographic resolution;

Received: February 17, 2002; revised: August 16, 2002; accepted: August 30, 2002

1 Introduction

Chromatographic separations involving multiple dimen-sions are being used more frequently to improve the anal-ysis of complex mixtures. This is because coupling ofchromatographic columns significantly increases thepeak capacity of such systems compared to single-col-umn systems. Multidimensional chromatography may beperformed either in the heartcut mode, or in the compre-hensive mode [1–2]. In the first case, only a fraction of thecomponents, partially separated in the first column, aretransferred into the second column for further separation.In the second case, each peak is transferred to the secondcolumn. A powerful analytical tool is created when multidi-mensional systems are interfaced to detectors able to pro-vide structural information [3–5]. GC/MS is perhaps themost used hyphenated technique. The determination ofthe exact structure of a compound from a mass spectrumcan be accomplished in several ways. Typically, the initialapproach is to compare the spectrum with existing library

spectra. An alternative, more time-consuming approach,is the interpretation of the spectrum coupled with spectralcharacterization of the most likely candidates. In thispaper, we employ another approach, which is the use of asecond detector able to provide structure-related informa-tion, the UV detector. Since spectrophotometric detectorsare non-destructive, they may be placed in front of a massspectrometer to provide an additional dimension to theidentification process [6–8].

Spectrophotometric detectors used in GC are less sensi-tive than mass spectrometers or other detectors [8–9].This has placed some limitations on the useful capabilitiesthat they may provide. For example, isomers present inolefin mixtures may differ only in their geometry/positionof double bonds. These types of isomers are difficult to dif-ferentiate with a mass selective detector (MSD). How-ever, since it is expected that these differences will gener-ate different vibrational modes, an IR detector may addidentification capabilities [9–11]. Spectrophotometricdetectors, whether UV or IR, add extra column deadvolume to a chromatographic system. For example, theUV cell used in this study has a volume of 85 lL, while thelight pipe of a Nicolet Magna 560 with IR interface has avolume of 113 lL. The dimensions of the flow cell mayhave an impact on the quality of chromatography. Separa-tions performed using relatively low volumetric flow ratesmay be more adversely affected by cell dimensions. Workreported thus far does not provide detailed information onthe effect of column flow on chromatographic param-

Correspondence: Juan Carlos Medina, The Dow ChemicalCompany, 2301 N. Brazosport Blvd., B-1218, Freeport, TX77541, USA.E-mail: [email protected]: +1 979 238 0715Abbreviations: GC, gas chromatography; 2D GC, two-di-mension gas chromatography; UV/VIS, ultra violet/visible;IR, infra red; MSD, mass selective detector; FID, flame ioni-zation detector; FAMEs, fatty acid methyl esters.

J. Sep. Sci. 2002, 25, 1317–1324

i 2002WILEY-VCH Verlag GmbH&Co. KGaA,Weinheim 1615-9306/2002/1812–1317$17.50+.50/0

1318 Medina, Griffith, Houston, Winniford J. Sep. Sci. 2002, 25, 1317–1324

eters [12]. The major objective of this study was to evalu-ate the potential use of a UV detector for multidimensionalgas chromatography.

In the past, several approaches have been used toincrease the sensitivity of spectrophotometric detectors.The most frequently used is large volume injection, whichcan be implemented with the use of either programmedtemperature vaporizer (PTV) inlet, or retention gaps [13–14]. However, the PTV approach is not useful when com-pounds of interest have boiling points similar to that of thesolvent, such as in the case of catalyst poisons present in1-octene streams. Multidimensional GC as a way toimprove sensitivity of spectrophotometric detectors hasbeen scarcely reported [4, 15]. In the present study, weexplore the capabilities of a commercial multi-columnswitching systemwith cryo-trap enrichment interfaced to aUV/VIS detector. The usefulness of the new system isevaluated in the analysis of a commercial mixture of fourfatty acid methyl esters (FAMEs), namely, methyl palmi-tate (C16:0), methyl stearate (C18:0), methyl oleate(C18:1), and methyl linoleate (C18:2). Finally, the power-ful capabilities obtained by combining the 2D GC systemwith a UV detector are illustrated by the ability to separateand detect catalyst poisons present in 1-octene streams.

2 Experimental

2.1 Multidimensional system

Figure 1 shows an schematic diagram of the system usedfor multidimensional GC. It is a commercial system pur-

chased from Gerstel (Baltimore, MD, USA) and containedin a single Agilent 6890 GC oven (Agilent Technologies,Wilmington, DE, USA). It consists of several componentsincluding an external pneumatic control package, a crossflow tee piece, liquid nitrogen-cooled cryotrap, and anexternal control module for the cryotrap unit.

The heart of the system is a midpoint cross piece wherethe column flow from the first capillary column (V) can bedirected either to a vent (II), or into a second capillary col-umn (VI). Column effluent switching is accomplished bystopping the cross flow, which then directs the flow fromthe first column into the cryotrap (XI). The trappedmaterialis held until successive cuts are made, the run is started,or the oven temperature is lowered to the desired initialtemperature to be used for separation in the seconddimension.

The midpoint cross has five arms; two for column connec-tions (4 and 5), two for cross carrier flow (1 and 2), andone (3) for connection to a monitor detector. The monitordetector (X), most often an FID, is connected to the mid-point crosspiece by a short piece of 100 lm ID capillarytubing (III). This gives an effective split ratio of approxi-mately 100:1, with the lower flow going to the detector.The bulk of the flow either goes out the midpoint vent, oronto the second column, depending on midpoint crossflow conditions. In this configuration, the monitor detectoralways shows compounds eluting from the first column,even when performing cuts. Experimental conditions forchromatographic separations are provided with each fig-ure.

Figure 1. Graphic illustration of the single oven MCS system with CTE option.

J. Sep. Sci. 2002, 25, 1317–1324 Detectability enhancement of spectrophotomeric detectors 1319

2.2 UV/VIS detector

The UV/VIS unit is especially designed to operate as adetector for gas chromatography. A detailed descriptionof this instrument has been published [12]. Briefly, thedevice is designed around the optical bench of a liquidchromatographic detector, coupled through fiber optics toan external gas flow cell. It is used in the flow-throughmode and placed in front of either a mass spectrometerfor identification purposes, or of an FID for accurate quan-titation. A key feature that makes this cell design applic-able to GC detection is the cooled housing. Compoundsanalyzed by gas chromatography are present in the vaporphase due to the application of heat. Therefore, the trans-fer lines and detection cell need to be heated in order toavoid analyte condensation. This heat present in the cellcauses thermal electronic noise. However, the coolingbath keeps all of the detection electronics at low tempera-ture, and effectively reduces noise.

2.3 Reagents andmaterials

A FAMEs mixture was purchased from Alltech (Deerfield,IL, USA), 1-octene samples were available from ourlaboratory, dichloromethane was from EM Science(Gibbstown, NJ, USA), toluene and isooctane were fromBurdick & Jackson (Muskegon, MI, USA). Capillary col-umns were purchased from Agilent Technologies andfrom Varian (Walnut Creek, CA, USA).

3 Results and Discussion

3.1 Effect of volumetric flow rate on extra columnband broadening caused by the UV cell

Flow rates used in chromatography are proportional tocolumn diameters. It was expected that the volume of theUV cell would have a more drastic impact on the quality ofchromatography performed using small diameter col-umns. To evaluate this effect, toluene was used as targetcompound. Retention times divided by peak width at halfheight (tR/w 1/2) were calculated for the UV cell and an FIDplaced in series. These experiments were performed witha Megabore and later with a small-diameter column bothin the single-dimension mode. These runs were per-formed under temperature programming conditions.Since the equation for column efficiency holds only underisothermal conditions [16], the use of tR/w1/2 seemed moreappropriate, while still being proportional to column effi-ciency.

Table 1 lists tR/w 1/2 values for the two columns. The effectof extra column dead volumes is usually manifested aspeak tailing. Since the effect of tailing may pass unnoticedin the calculation of column efficiency, due to the fact that

this measurement is performed at half height, symmetryfactors (A s) were also calculated, and included in thesame Table. A s values were determined using Equation(1):

A s=w 10%/(w 10% – Dw 10%) (1)

where w10% is the actual peak width at 10% of the peakheight, and Dw10% is the difference between the front andthe back part of a chromatographic peak [17]. An As valueof 1 stands for a perfectly symmetric peak, where thelength of the front is equal to that of the back part of thepeak.

Some key points are learned from the data in Table 1.First, since the columns were operated at the same tem-perature and linear velocity, the difference in column effi-ciencies (tR/w 1/2 values of 110 versus 160 for the 530 and250 lm ID columns, respectively) is due exclusively to col-umn diameters, which is in good agreement with theoreti-cal predictions. Secondly, the UV cell does not show alarge impact on the column efficiency, as tR/w 1/2 values of110 and 160 are reasonable for column diameters of 530and 250 lm, respectively, when helium is used as carriergas. Thirdly, the UV cell has a marked effect on peak sym-

Table 1. tR/w 1/2 and A s values for toluene peaks using UVand FID detection.

Column ID (lm) tR (min) tR/w 1/2 A s

530 UV FID UV FID UV FIDRun 1 7.57 7.61 107 112 1.04 1.07Run 2 7.59 7.62 103 112 1.06 1.07Run 3 7.58 7.62 110 112 1.06 1.08

250Run 1 8.86 8.90 159 156 1.50 1.53Run 2 8.87 8.91 156 160 1.51 1.57Run 3 8.87 8.91 160 156 1.65 1.57

Conditions:Instrument: HP 6890 Plus GCColumns: 24 m6530 lm62 lm CP-Wax 30

m6250 lm61 lm stabilwaxColumn flows: Megabore: 4 mL/min, constant flow

mode (linear velocity 31 cm/s)microbore: 1 mL/min, constant flowmode (linear velocity 26 cm/s)

Oven: 408C (5 min hold), 208C/min, 2008C(2 min hold)

Injection port: 2258C, split ratio 1:1 for column 1; 1:20for column 2

Detector: UV absorbance @ 210 nm and 2308C inseries with FID at 2508C

Analyte: toluene at a concentration of 50 ppm inCH2Cl2.


metry. As expected, this is more drastic on the small col-umn because a lower flow rate is used. Peaks obtainedusing the megabore column are almost symmetric(A s = 1.05), while those from the small diameter columnshow significant tailing (A s = 1.5). In practical separations,this would affect resolution of close-eluting peaks, andmake accurate quantitation more difficult. Finally, tR/w 1/2

and A s values reveal that the impact of the cell on peaktailing is seen not only on the detectors downstream fromthe UV detector, but also on the UV detector itself.

3.2 Detectability enhancement by usingmultidimensional GC/UV

Advantages of using a 2D GC system include increasedresolving power, ability to separate a desired compoundfrom the matrix, and the capability of injecting a largervolume into the system. Also, by focusing and reinjectinga compound into a second, high-efficiency column, peaksbecome sharper which in turn generates a larger responsefrom the detector. When we used a single column with thisUV instrument, we found the limit of detectability fortoluene to be 5.5 ng (S/N = 3). To fairly compare results,the same column was used for experiments in the seconddimension of this study. Figure 2 shows a chromatogramof toluene at a concentration of 500 ppb. Injection volumewas 4 lL and split ratio was 1 :1. Signal-to-noise ratio forthe peak is 3.5. Comparison of these two results shows

that an improvement in detection limit by a factor of 7 hasbeen obtained, by the use of the multidimensional system.

Comparison of limits of detection between the 2D GC/UVand a single column systems for a mixture of FAMEs wasalso carried out. 4 lL of a mixture of FAMEs at a concen-tration of 25 ppm and 1:1 split ratio were injected in allcases. Initially, single dimension separations were per-formed using a Megabore CP-Wax column, and differentwavelengths of 190, 200, and 240 nm. Of the four com-pounds present in the FAMEs mixture, only methyl oleateandmethyl linoleate exhibited absorption in the UV region.It has been reported that application of heat to keep ana-lytes in the vapor phase causes a slight blue shift of theabsorption maxima relative to condensed-phase mea-surements [12]. Similar results were observed in thisstudy. When a UV spectrum of these compounds dis-solved in isooctane was taken, maximum absorption wasobserved at 200 nm, which is in agreement with literaturedata [18]. Maximum absorption for these compounds inthe gas phase was observed at 190 nm. Therefore, thisdetection wavelength was used for further detection stu-dies.

Multidimensional GC experiments were performed usingthe following array of columns: the Megabore columnused for single-dimension separations was placed as thefirst column, while a 320 lm ID column was used as sec-ond column. These columns had the same stationaryphase. The decision to use a 320 lm ID column in the sec-

Figure 2. 2D GC chromatogram of toluene at 500 ppb (UV signal at 192 nm).Conditions:Instrument: HP 6890 Plus GC.Columns: 24 m6530 lm62 lm CP-Wax (first dimension) 30 m6320 lm60.5 lm

HP-Innowax (second dimension).Oven: 408C (5 min), 208/min, 2008C (2 min)Column flows: First dimension: 7.4 mL/min (linear velocity 51 cm/s)Second dimension: 2.4 mL/min (linear velocity 37 cm/s)Injection Port: 2258C, split ratio 1 :1Detectors: UV absorbance @ 192 nm and 2308C in series with FID at 2508CPressures: 21.1 psi for megabore

115 and 110 kPa for cut 1 and cut 2, respectively.


ond dimension was based on the data obtained fromexperiments with toluene. We concluded that this columnwould generate sufficient chromatographic plates for thisseparation, while minimizing tailing caused by the UV cellextra column dead volume.

Figure 3 demonstrates that an increase in the signal-to-noise ratio is achieved by using a multidimensional sys-tem compared to a single-column system. Injection size,split ratio, and sample concentration were the same forboth systems. The peak in the 2D GC chromatogram cor-responds to C18:1 which has been cut, cryotrapped, andtransferred into the second column. Using a single columnsystem, this peak does not meet the criterion of signal-to-noise ratio of 3. However, the peak obtained using a two-dimension system is clearly above this value. Finally, theuse of a relatively high flow rate minimized the effect oftailing caused by the UV cell. Symmetry factors for thepeak corresponding to C18:1 were 1.14 and 1.16 mea-sured at the UV and the FID detectors, respectively.

3.3 Resolution enhancement by the use of amultidimensional system

The purpose of any chromatographic method is to sepa-rate, to a certain resolution value, the components presentin a mixture. Chromatographic resolution (Rs) can beexpressed in terms of Eq. (2):

Rs ¼ 1=4ðffiffiffiffiN

pÞ N a� 1

a

� �N

k1þ k

� �ð2Þ

where N is the total column efficiency, a is the separationfactor (a measurement of selectivity), and k is the reten-tion factor [19]. Equation (2) shows that chromatographicresolution is proportional just to the square root of the col-

umn efficiency, while it is linearly proportional to selectiv-ity. To double a given resolution value, N would berequired to increase by a factor of 4, all other terms keptconstant. In a practical gas chromatographic separation,this means that the column length would need to beincreased by the same factor, which would cause a corre-sponding increase in analysis time.

On the other hand, Eq. (2) also shows that a change inselectivity has a greater impact on resolution of closelyeluting compounds. However, changing selectivity is chal-lenging because mobile phases in GC do not have solvat-ing power. The premier way of altering selectivity in GC isby changing the column stationary phase, and to a lesserextent, changing temperature. Because of this, the use ofmultidimensional gas chromatography has great poten-tial; namely, the capability to separate closely eluting com-pounds by using two capillary columns that differ widely inthe chemistry (i.e. selectivity) of their stationaryphases [20]. For example, a separation based on polaritymay be achieved in the first dimension, while differencesin boiling point and/or shape may constitute the basis forseparation in a second dimension.

The cryotrap plays a very important role in a 2D GC sys-tem. It is not only a means to retain the different com-pounds before being transferred to the second column,but perhaps its focusing function is even more important.After being injected into the GC, the components of a mix-ture start traveling and partially separate in the first col-umn. At the same time, they also diffuse, which causesband broadening. When a particular zone reaches thecryotrap, the material in the front part of the peak changesfrom the vapor to either the liquid or the solid phase,depending on trapping temperature and boiling/meltingpoint of the compound. As more material builds up in thefront of the trap, the concentration of that analyte in the

Figure 3. Single dimension and 2D GC chromatograms of FAMEs (UV signal at190 nm). Conditions: same as for Figure 2, except oven temperature that is isothermalat 2208C.


zone increases, while the width of the band is reduced,and therefore, refocused.

Figure 4 shows FID signals for single dimension, and sec-ond dimension separations of compounds C18:0 andC18:1 which have been cut and transferred into the sec-ond column. This figure shows that when the cryotrap isnot used, no gain in sensitivity is realized. Instead, whiletraveling through the second column, the peaks becomeeven broader without the benefit of focusing. Widths at thebase of the peak corresponding to C18:0 are 0.3, 0.2, and

0.5 min for single column, 2D GC with cryotrapping, and2D GC without cryotrapping, respectively. On the otherhand, when multiple peaks are trapped, the cryotrapmixes the peaks again and resolution gained in the firstdimension is lost at the cryotrap. Resolution values for thepair C18:0 and C18:1 are 1.8 and 3.1 with and withoutcryotrapping, respectively.

The following example is an illustration that a UV/VISdetector may add an extra level of selectivity to a particularseparation. In our laboratory, we have observed that

Figure 4. Single dimension and 2D GC chromatograms of FAMEs (FID signal). Conditions:same as for Figure 2, except oven temperature that is constant at 2208C.

Figure 5. Single dimension chromatograms of a sample of untreated 1-octene (UV at 220 nm). Boxrepresents cutting time.Conditions:Instrument: HP 6890 Plus GCFirst dimension: 24 m6530 lm62 lm CP-WaxSecond dimension: 29 m6250 lm60.25 lm CYCLOSIL-BOven: 358C (5 min), 38/min, 458C (5 min), 208/min to 2008C (4 min)Column flow*: First dimension: 6.3 mL/min (linear velocity 45 cm/s)Injection Port: Split, 2258C, split ratio 1:5Detectors: UV absorbance @ 200 and 220 nm (2308C), in series with FID at 2508C

* Pressure values were 240, 141, and 136 kPa at the injector port, CUT 1, and CUT 2, respectively.


methylcyclopentadienes and other byproducts generatedduring production of 1-octene have a negative impact onthe activity of catalysts used in industrial processes. Iden-tification and quantitation of these contaminants in 1-octene streams is key to finding solutions leading to elimi-nation or minimization of the impact of these catalyst poi-sons. Initial separations of 1-octene samples wereaccomplished using a single column (Megabore CP-Wax)and dual detection; UV at 220 nm and FID. 1 lL of anuntreated 1-octene stream was injected at a split ratio of1 :1. Figure 5 presents both the FID and UV signal chro-matograms.

As can be seen, the main peak (1-octene) is transparentto UV light at the selected wavelength. By using this setup, it was possible to see some compounds coeluting withthe solvent. Obviously, in the FID these compounds weremasked by the large signal generated by 1-octene. In asingle column system, detection and identification ofthese compounds would be challenging even with the useof an MSD. For protection purposes, the ion source of anMSD is turned off during the solvent delay. As Figure 5shows, this is time during which contaminants present in1-octene elute. Therefore, they would be undetected byan MSD. 2D GC with UV detection is a powerful tool tosolve the problem just presented. UV detection originallyallowed us to determine the retention time of contami-nants eluting under 1-octene. Then, a selective cut of theregion depicted by the dotted box in Figure 5 was per-formed. Separation in the second dimension was with acyclodextrin column, which separated compounds basedon geometric differences. Figure 6 shows that whatseemed to be 2 peaks is actually composed of at least fivecompounds.

Additional advantages of the system developed in thisstudy may be realized by interfacing it to a mass spectro-meter. Several isomers present in 1-octene samples differonly in their geometry/position of double bonds. Thesetypes of isomers are difficult to differentiate with a MSdetector alone, but they may be identified by combinationof MS with IR/UV detectors. However, flows higher than2 mL/min are required in the second dimension to mini-mize tailing from the UV detector. Since these flows arehigher than typical flows handled by a vacuum system inthe MSD, a system may be set up with a tee placed afterthe UV cell, where the flow may be split to help the properflow reach the MSD. The remaining flow may be trans-ferred to an FID detector. An additional advantage of thesystem assembled in this study is the potential to increasethe detection capability of spectrophotometric detectorsby doing multi-trapping and collection of several firstdimension runs. Besides the focusing benefit, detectionwould also be increased by the build up of materialtrapped from successive runs.

4 Conclusion

A chromatographic system which encompasses multidi-mensional GC and UV/VIS detection has beenassembled. When toluene was used as analyte, this multi-dimensional arrangement showed a 7-times improvementin limits of detection, compared to results obtained using asingle column system placed in front of the same detector.In addition, the system has been highly useful for detec-tion and identification of contaminants present in 1-octenestreams. By using the detector at a wavelength where 1-

Figure 6. Second dimension chromatogram of cut specified in Figure 5. UV detection at220 nm. Conditions are the same as in Figure 5.


octene is transparent, contaminants coeluting with 1-octene, were selectively detected by this system.

Acknowledgements

Especial thanks are due to Dr. Steve Erskine for manyhelpful discussions and assistance in assembling the 2DGCwith UV detection.

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Detectability enhancement of spectrophotometric detectors by the use of multidimensional gas chromatography