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Journal of Chromatography A, 1261 (2012) 37– 45

Contents lists available at SciVerse ScienceDirect

Journal of Chromatography A

jou rn al h om epage: www.elsev ier .com/ locat e/chroma

eview

erson-portable gas chromatography: Rapid temperature program operationhrough resistive heating of columns with inherently low thermal massroperties

hilip A. Smith ∗

.S. Department of Labor – OSHA, Health Response Team, Salt Lake Technical Center, 8660 S. Sandy Parkway, Sandy, UT 84070 USA

r t i c l e i n f o

rticle history:vailable online 21 June 2012

eywords:erson-portableesistive column heatingas chromatographyigh speed temperature programeview

a b s t r a c t

As open tubular gas chromatography was becoming widely adopted, the potential to rapidly heat andcool the low thermal mass of an open tubular fused silica column was recognized. Numerous resistivecolumn heating approaches were subsequently described and demonstrated, often with a common objec-tive to focus heating efforts on the column alone, rather than on a large convection oven. Low thermalmass column bundles have been commercially available for about ten years, where insulated wires inclose proximity to a coiled open tubular capillary column provide resistive heating. Before this, person-portable gas chromatographs either operated isothermally at relatively low temperatures or at ambienttemperature to lessen power demands, but several person-portable gas chromatography–mass spec-trometry (GC–MS) instruments capable of temperature program operation have become available in thepast ten years based on this heating method. When low thermal mass heated zones are used, and witha direct GC–MS interface, analysis times of less than 5 min are possible for target compounds having awide range of volatilities. Previous capabilities in transportable and person-portable gas chromatographyinstrumentation are reviewed to demonstrate the scale of advancement made possible by the adoption of

open tubular columns and low power heating techniques now becoming routinely available. Microcoma-chined columns which are usually etched in a silicon wafer represent a radical break from the traditionalfused silica open tubular column design, and increasing efforts to use this column construction approachare also examined. The developments discussed have introduced the potential to rapidly analyze com-pounds with a wide volatility range in the field to protect deployed military forces, the health of workers,and the health of the general public.

Published by Elsevier B.V.

ontents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 382. Development of a strong, high performance low thermal mass GC column capable of high temperature operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 383. Column heating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

3.1. Ambient temperature instruments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 393.2. Isothermal analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 393.3. Temperature programmed analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

3.3.1. Temperature programmed heating of MEMS columns and MEMS components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

3.3.2. Temperature programmable person-portable GC–MS em

4. Conclusion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

∗ Tel.: +1 801 233 5066; fax: +1 801 233 5000.E-mail address: smith.philip.a@dol.gov

021-9673/$ – see front matter. Published by Elsevier B.V.ttp://dx.doi.org/10.1016/j.chroma.2012.06.051

ploying resistive column heating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

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. Introduction

Speed of separation with acceptable resolution is importantor a person-portable gas chromatograph if it is to be effectivelysed in the field. Analysis times should be very fast, and chro-atographic resolution for analytes having a wide volatility range

hould not be appreciably sacrificed relative to a modern tem-erature programmed laboratory instrument. A large difficulty inroviding performance that approaches the ideal is that a trulyerson-portable system must be small in both size and weight, andust operate under battery power.For analysis of organic compounds of wide interest the labo-

atory performance of a modern gas chromatograph is possibleue to three key developments: the adoption of open tubularolumns, the initial use of fused silica as a column material, and thevailability of immobilized liquid film stationary phase coatings.he same developments have been important for today’s state-of-he art person-portable gas chromatographs. However, without aourth more recent development the performance of a portableas chromatograph would be far inferior to that provided by aarge, power-hungry laboratory instrument. The development ofow power column heating without a bulky convection oven allowsigh speed, high resolution separation of analytes with a volatil-

ty range comparable to that for which laboratory instruments arepplicable. This is possible due to the low thermal mass of both theodern open tubular column, and experimental microelectrome-

hanical system (MEMS) columns.As a result of development efforts spanning many years, person-

ortable gas chromatographs are now available with a number ofetector options, including thermal conductivity, photoionization,lectron capture, flame ionization, and mass spectrometric types.ample introduction methods include direct air injection, sampleoop/valve injection, solid phase microextraction and needle trapampling with desorption in a heated injector, air sampling onto amall on-board sorbent trap for subsequent thermal desorption,nd thermal desorption of relatively large sorbent tubes onto amall sorbent trap for focusing prior to injection.

This review will briefly discuss the important developmentshat have led to the current capabilities in person-portable gashromatography instruments. Historical examples of transportablend person-portable instruments and currently available person-ortable instruments will be discussed. A major focus will be onurrently available person-portable chromatographs that employow power column heating techniques with the potential for fastemperature programs and high temperature operation. While notet widely available for routine use, compact micromachined chip-ased column modules which have low thermal mass, and that maye heated using relatively little power will also be examined.

. Development of a strong, high performance low thermalass GC column capable of high temperature operation

The theoretical and experimental parameters affecting thepeed and efficiency of gas-liquid chromatographic separationsuickly took on importance to the earliest researchers who devel-ped and described the method. Only a few years following theioneering gas-liquid chromatography work of James and Martin1] the theory of open tubular columns was described by Golay2]. Later, Golay described the relatively poor performance of theacked columns first used, and in his own words [3] he “. . .foundn enormous discrepancy between the actual and ideal values: the

acked columns were horrifyingly poor.” Advantages of open tubu-

ar gas chromatography were quickly theorized and demonstrated,ncluding superior chromatographic resolution and shorter analy-is times [4].

1261 (2012) 37– 45

The need for inertness soon led to the use of open tubularcolumns constructed of glass. Desty described a device in 1960that could produce a uniform glass capillary [5], and in 1979 Dan-deneau and Zerenner reported a method to produce GC columnsfrom fused silica [6]. The migration towards fused silica was impor-tant for field-portable gas chromatography as a glass capillary canbe easily broken, whereas a capillary constructed from high purityfused silica is both more inert and may also survive rough handlingwith less risk of breakage. Just a few years following the paper byDandeneau and Zerenner, Bertsch and Pretorius deposited a layerof silicon about 20 times thicker than needed to deactivate a glasssurface on the surface of stainless steel capillary columns. Thesewere subsequently coated with either octamethyltetrasiloxane ormethylpolysiloxane followed by a SE-30 film. Residual activitytowards free acids and bases was noted even with the deep layerof silicon used [7]. While not as easy to cut or install as a fused sil-ica open tubular column, deactivation methods for metal surfaceshave improved to the point where the chromatographic perfor-mance of a metal capillary now nearly matches that of a fused silicacolumn.

The use of column coatings that were not immobilized andbonded to the column walls limited the upper range of columntemperatures, and also limited useful column life. Cross-linking ofa pre-polymer mass into an immobilized coating covalently bondedto the inner surface of a capillary column was first reported byMadani et al. in 1976 [8]. Additional work in this area by numer-ous laboratory research groups [9–11] and commercial enterpriseshas resulted in open tubular capillary columns that may be usedat high temperatures, expanding the range of possible gas chro-matography analytes to include larger, less volatile compounds.Comparing the modern open tubular capillary column which couldbe routinely heated to temperatures greater than 300 ◦C in 1992,Ettre described the significance of these advances [4]: “. . .even in1975, one could rarely find a capillary chromatogram in which thecolumn was heated above 200 ◦C”.

Overall, the advances described above resulted in a strong,compact and lightweight capillary column with high perfor-mance separation capabilities. For analysis of many volatile andsemi-volatile compounds found in areas such as the outdoor envi-ronment or workplaces, the modern capillary column comparesfavorably to packed columns in nearly every respect except capac-ity. By the mid-1980s the open tubular design was replacing thepacked column for most laboratory applications. At this pointthe principal limiting factor for fast gas chromatographic analy-sis through rapid column heating became the thermal mass of thetypical air bath column oven. As early as 1984 Lee, Yang, and Bar-tle recognized the potential of the newly developed open tubularfused silica columns for rapid low power heating [12]:

“The low thermal mass of the flexible fused-silica columnsmay allow in the future the utilization of direct electric resis-tance heating or thermoelectric heating and cooling for columntemperature control. The direct electric resistance heating ofa metallic coated thin-wall flexible fused silica column couldoffer significant advantages, such as ultrarapid heating of thecolumn, no thermal lag problem, and minimum power con-sumption. Rapid cool-down could also be expected because ofits low thermal mass. It could also facilitate thermal focusing,multidimensional GC, and the miniaturization of open tubularcolumn gas chromatographs”.

A micromachined column described in 1979 represents a rad-ical departure from both the dominant packed and open tubular

column construction approaches of the era. In the same yearthat Dandeneau and Zerenner published their paper on the useof fused silica for construction of open tubular columns, Terryet al. described the fabrication of a miniature column (1.5 m

togr. A 1261 (2012) 37– 45 39

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P.A. Smith / J. Chroma

ength × 200 �m width × 30 �m depth, rectangular crosssection)ithin a silicon wafer of <20 cm2 area and 200 �m thickness [13].onstruction of this small, low thermal mass column proceeded. . .in a series of oxidation, photolithography, and etching stepsimilar to regular integrated circuit device processing.” In the inter-ening years since this work was completed interest in the use ofilicon and glass materials to create micromachined columns hasncreased, and it is hoped that efforts in this area will lead to thereation of smaller and more sophisticated detection devices thatill consume very little power. The use of micromachined columns

nd detectors may drastically reduce the cost of gas chromatogra-hy components through the use of batch manufacturing processes,nd ongoing research in the field of MEMS gas chromatographyolumns should improve capabilities to solve important practicalroblems using person-portable instruments.

. Column heating

The first gas-liquid chromatograph operated isothermally [1],ut the use of temperature programming was mentioned by Grif-ths et al. shortly thereafter [14]. Dal Nogare and Bennett noted

n 1958 that with a linear temperature program and constant col-mn flow “. . .each solute is eluted at a characteristic volume andemperature. By varying the column temperature over wide limits,

ixtures with wide boiling point ranges can be separated rapidly”15].

Convection column heating has been typical since the earliestommercial instruments. With convection heating the temperaturerogramming rate is limited by heater size and the power available.he large oven wall surfaces are subject to continual heat loss, and

fan is needed to lessen thermal gradients. The problem of heatoss rate becomes more pronounced at higher oven temperatures,ut even at lower temperatures the heating rate of a typical con-ection oven is limited [16]. Relatively long cool-down times arelso required for a convection oven.

.1. Ambient temperature instruments

Due to power constraints early person-portable gas chro-atographs were operated without column heating. This severely

imited the usefulness of a person-portable instrument to analy-is of light compounds. In the early 1980s Bond and Dumas used

Photovac 10A10 instrument without column heating to detecthe fumigant phosphine (PH3) at a grain elevator [17]. This instru-

ent employed a photoionization detector (PID), and operated forp to 8 h with power from a rechargeable battery and using com-ressed air mobile phase. They described a retention time for PH3f 1.61 min at an ambient temperature of 25 ◦C, and 2.1 min at1 ◦C (Fig. 1). Laboratory work revealed that “analysis of the higheroiling fumigants ethylene dibromide, carbon tetrachloride, and

,l,l-trichloroethane proved to be more difficult. . . The retentionime of these materials on the Carbopak BHT column was morehan l h, indicating the need for selection of more suitable columnackings for workable retention times”. Das reported the use of aimilar person-portable instrument with the packed column main-ained “only a few degrees above ambient” to measure naturallyccurring airborne terpenes drawn from a remote forest area into

sample loop for subsequent injection and analysis [18].The Century OVA employed a short packed column without tem-

erature control. This chromatograph was designed to be carriedrom one location to another while operating, and could be used

n survey mode as a stand-alone flame ionization detector (FID)nly, or in gas chromatograph mode with H2 supplied for the FID asobile phase. The OVA instrument was compared to a stand-alone

andheld PID in 1987, and its quantitative response was shown to

signal attenuation ×1, and PH3 retention time = 1.61 min.

Reprinted from [17] with permission. Copyright (1982) American Chemical Society.

be much less affected by humidity [19]. The person-portable OVAchromatograph remains in limited use, and at least one commercialenterprise still offers repairs for this instrument.

3.2. Isothermal analysis

As early as the mid-1970s, Analytical Instrument DevelopmentInc. (AID) offered a portable instrument with isothermal columnheating. External power was required, and the instrument weighed17 kg. A detailed description of the AID 511 gas chromatograph andits use for measurement of airborne 2,4,6-trinitrotoluene (TNT) andcyclotrimethylene trinitramine (RDX) was provided by Saltzmanet al. in 1975 [20]. A glass column was required due to the activity ofavailable packed metal columns towards both analytes. The relativestrengths of the open tubular capillary columns now available forportable gas chromatography are made clear in the report producedby these researchers after using the convection oven instrument atseveral U.S. Army ammunition plants for field analysis.

“The portable gas chromatograph was self-contained and couldbe moved from one location to another, although it was heavyand awkward to handle. The instrument performed satisfacto-rily except for breakage of the glass column. The column did notslip from the fittings, but broke from shock during handling. . ..The instrument required approximately 3.5 hours to heat fromroom temperature to operating temperature (180 ◦C). It isessential that this column temperature be attained if RDX isanalyzed to avoid peak spreading”.

Wesolowski and Alwan described a tiered approach to analy-sis in the field for environmental remediation in 1992 [21]. Initialscreening using a handheld PID without chromatographic separa-tion ability can quickly identify contaminated areas. These authorsdescribe the use of an isothermal Photovac 10S10 gas chromatogra-phy instrument with PID to complete field analysis of air samplesfrom contaminated areas thus identified. Methods for field anal-ysis of benzene and perchloroethylene using portable isothermalgas chromatography have been promulgated by the U.S. National

Institute for Occupational Safety and Health [22,23]. Isothermaltemperatures (30 and 40 ◦C for benzene and perchloroethylenerespectively) are listed in these methods.

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Conditions similar to the NIOSH method for perchloroethyleneere used by Sweet et al. with a Photovac 10S50 instrument toeasure this analyte in the breath of exposed workers [24]. The

hromatograph reportedly weighed approximately 11 kg, and itmployed a 9 m capillary column (0.53 mm I.D., 2.0 �m film thick-ess, 5% phenyl methyl silicone stationary phase) kept at 40 ◦C.orkers exhaled into tedlar bags, and a small volume of air wasithdrawn from a bag using a gas-tight syringe for injection into theortable chromatograph. These researchers wished to avoid delayetween sample collection and analysis and they noted that such aelay reduces the capability to “. . .identify activities that increasexposure. . . Thus, real-time analysis of perchlorethylene concen-rations in breath would be beneficial to worker education andxposure intervention.” Good agreement was obtained betweenhe field analysis method, and charcoal tube sampling followedy analysis with an established laboratory gas chromatographyethod (R2 = 0.97).In 2005, Suna et al. reported the use of a Photovac Voyager

nstrument for detection of benzene in the urine of workers [25]. person-portable chromatograph was selected due to the volatil-

ty of benzene and the potential for loss of this compound from arine sample during shipping. The polyethylene glycol stationaryhase column used (20 m × 0.53 mm I.D., 1.0 �m film thickness)as kept at 50 ◦C. By manually injecting a 1 ml volume of air in

quilibrium with a urine sample, higher concentrations of benzeneere detected in the urine of smokers compared to non-smokers.

Although in many examples noted above open tubular columnsere used, person-portable instruments are still manufactured

oday with packed columns kept at a constant temperature in amall convection oven, as this design is acceptable for the uncom-licated separation of light gases and vapors. For example, the GC12 manufactured by PID Analyzers is often supplied with a packedetal column for separation of light vapors and permanent gases toonitor process streams and for industrial hygiene studies. At the

resent time most other commercially available person-portablenstruments use an open tubular liquid film column that targetsnalysis of solvents and semivolatile compounds.

A person-portable gas chromatography-mass spectrometryGC–MS) instrument became available beginning in the late 1990s.he Inficon HapsiteTM maintained a constant column tempera-ure in a small convection oven. A quadrupole mass spectrometricetector was operated within a sealed enclosure kept under vac-um by a nonevaporative getter (NEG) and an ion sputter pump. Inrder to enter the high vacuum region, analytes diffused through aolymeric membrane after eluting from the column. Around 2001,his instrument was updated to include a temperature programapability as will be described below.

Temperature control for experimental micromachined columnsas often been accomplished using an available convection ovenithout temperature programming [26–28]. This is understand-

ble as researchers typically desire to study the fundamentalerformance of a newly constructed column without completingdditional work to integrate heaters and temperature sensors on

small device of this type. This approach for testing also allowshe use of a standard injector and detector, with the experimem-al column being evaluated connected to these through fusedilica transfer lines. For example, Radadia et al. [26] and Sunt al. [27] used this approach with isothermal column heatingf micromachined columns, while Vial et al. [28] mounted theiricromachined column in a convection oven kept either at ambient

emperature or 30 ◦C during testing.

.3. Temperature programmed analysis

Until the first decade of the twenty first century, column heat-ng of person-portable gas chromatographs was accomplished by

1261 (2012) 37– 45

convection heating–typically with a small oven similar (except interms of size) to those encountered in laboratory instruments.Although not person-portable, the transportable Viking GC–MSinstrument manufactured in the 1990s exemplified this approach,as it combined a Hewlett Packard quadrupole mass spectrometerwith a full length capillary column wound on a small cage in asmall convection oven. Temperature programmed operation waspossible with this instrument, but external power was required.Following developments by several research groups in the late1990s [29,30] and continuing to the near-present [31] a numberof designs are now available to use resistive column heating for thereasons enumerated by Lee et al. in 1984 [12], especially to mini-mize power consumption. A thorough review on the use of resistiveheating for gas chromatography has been completed very recentlyby Wang et al. [32].

The use of resistive column heating dates at least back to thework of Dal Nogare and Bennett, who described heating of a stain-less steel packed column with a linear temperature program in1958 [15]. Hail and Yost reported the use of direct resistive heat-ing of a capillary column with low thermal mass in 1989 [33]. Analuminum-clad column was resistively heated and the measuredresistance of the cladding material also provided the tempera-ture of the short (3 m) sections of 0.53 um column used (1.0 �mfilm thickness, polydimethylsiloxane stationary phase). Althoughthis work was completed with a laboratory mass spectrometricdetector, these authors noted that “since no large oven is needed,direct resistive heating is extremely attractive for use in portableGC instrumentation.” Six years later Jain and Phillips reported theuse of electrically conductive paint to coat short sections (<1 m)of 0.10 and 0.25 mm ID open tubular capillary columns with 5%phenyl methylsilicone and SE-30 stationary phases respectively.This allowed resistive heating by applying voltage to the coat-ing material [34]. A voltage ramp did not provide direct columntemperature linearity due to heat loss from the non-insulated col-umn, and no temperature measurement method was described.An empirically determined voltage program allowed a tempera-ture ramp to produce retention times for the n-alkanes hexane todecane that were linear with Kovats retention index values.

Ehrmann et al. used a separate temperature sensing and heat-ing element [29]. The heating element was either a nickel/iron(nickel alloy 120) wire placed along the length of the column, ora small silver-plated stainless steel tube that housed the column,and in both cases the temperature was measured by determiningthe resistance of a separate nickel alloy wire. Chromatograms for n-alkanes (pentane through eicosane) and a sample of crude oil wereobtained using a 3 m length of 0.1 mm ID fused silica column coatedwith 5% phenyl methyl silicone (0.4 �m film thickness). Some devi-ations from expected performance included slight retention timevariability and some peak broadening of heptadecane and largeralkanes relative to convection heating in a standard oven. Theseeffects were attributed to uneven column heating and one or morespots that were not thoroughly heated, due to the experimentalnature of their apparatus. The heating wire variant described bythese authors has since been commercialized as the microFastTM

system, available from Analytical Specialists Inc. that heats twocolumns simultaneously within a single bundle that contains acollinear temperature sensing and insulated heating wire. Columnsare typically 2 m long with 0.1 mm I.D. (although other diametersmay also be used), and temperature programming at rates up to25 ◦C/min are possible with dual FID detection. Battery operationof the approximately 5 kg instrument is possible, and compressedcarier/FID fuel may be supplied from a separate module that also

includes a pump to provide clean FID combustion air when thesystem is used in the field [35].

In the first peer reviewed publication describing design and per-formance details for the resistive heating method patented earlier

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y Mustacich, Sloan et al. described an approach similar to thatf Ehrmann et al. [29], although the resistive heating wires usedere wrapped around an open tubular capillary column [30]. The

esistive heating wire used in that approach was Chromel (Ni/Cr)n a ceramic fiber sheath, and a thin Pt wire was used to measurehe temperature of the low thermal mass column assembly throughhanges to resistance. The rationale for the work described by Sloant al. was to develop a “more efficient and powerful instrument thatay prove useful for on-site defensible forensic analyses”.Another resistive column heating approach, termed “flash GC”

as described by Macdonald and Wheeler in the late 1990s [36].his method involved a thin-wall metal tube surrounding a cap-llary column. Column length is typically 5 to about 10 m due tohe length of readily available steel tubing. Resistive heating ofhe metal tube and a 96 V power supply allows extremely fastemperature program rates (up to 20 ◦C/s), with temperature deter-

ined by measuring resistance over a 3 ms period between 10 mseating cycles. This heating method was used by van Deursent al. who described separation of C10–C40 alkanes in < 1.5 min [37].hese authors used a 0.32 mm I.D. capillary column with 5% phenylethyl silicone stationary phase and 0.25 �m film thickness. Using

flash GC instrument and full scan mass spectrometric detection,mith and MacDonald described a simple inlet that heated Bacil-us spores in the presence of oxygen to liberate pyridine, a volatile

arker compound known to result when spores are heated. Near-aseline resolution for the pyridine and gasoline range aromaticsas completed in <2 min [38]. A 12 m section of 0.25 mm I.D. capil-

ary with 5% phenyl, methyl silicone stationary phase and 0.25 �mlm thickness was employed with electrothermal cold spot focus-

ng followed by rapid resistive heating of the cold spot to begin annalysis cycle.

The power requirements of the flash GC system describedbove are too great to allow this approach to be used for person-ortable instruments. However, the stated aim in many papershat described the development and use of other resistive columneating approaches was to provide low power, high performanceolumn heating for use in field-portable instruments. Almost athe same time that the paper by Sloan et al. was published [30],he column heating approach they described became commer-ially available, and was adopted around 2001 for temperaturerogramming when used in the person-portable HapsiteTM GC–MS

nstrument. As noted above, prior to adoption of resistive heatingn 2001 the initial version of this instrument had operated isother-

ally with a convection oven since its introduction. The adoption ofesistive heating allowed temperature ramping without sacrificingattery operability.

Between 2004 and 2010 the resistive heating design ofustacich, first described by Sloan et al. [30] was added as

n aftermarket accessory for a transportable GC–MS instrument39], or used without a large convection oven in a custom-builtransportable GC–MS instrument that employed a standard Agi-ent 5973 mass spectrometric detector [40]. Neither of thesenstruments was person-portable, but the capability of rapid col-mn heating for field detection was demonstrated. In both casesited, fast resistive heating of an open tubular fused silica col-mn was combined with high H2 carrier gas linear velocity toapidly separate mixtures of chemical warfare agent compounds.he Griffin 450 (FLIR Technologies) and the Agilent Technologies975 T transportable instruments were both designed to employhe resistive column heating approach described by Sloan et al.30] to reduce instrument size, weight, and power consump-ion compared to instruments using convection oven heating.

he custom-built GC–MS instrument described in 2005 [40] hadootprint, weight, and analytical capabilities roughly compara-le to the more refined Agilent 5975 T that is now commerciallyvailable.

1261 (2012) 37– 45 41

In 2008 Tienpont et al. [41] described a low mass resistiveheating jacket to enclose a 5 m section of thick film open tubularcolumn (250 �m I.D., 1.0 �m polydimethylsiloxane film thickness).The jacket consisted of 0.5 mm I.D. polyimide tubing on which athin metal wire was densely braided. The wire was covered withanother polyimide layer for electrical isolation, The authors state anobvious benefit to this approach, which is that the capillary columnmay be removed and replaced without the need to carefully unwrapheating and temperature sensing wires from within a more perma-nent type of column bundle. Temperature programming rates upto 100 ◦C/min were described, with an upper temperature limit of200 ◦C.

Stearns et al. [31] recently reported a resistive heating methodemploying open tubular fused silica columns coated with 5%phenyl, methyl silicone stationary phase. A length of insulatednickel wire was wrapped around both a 15 m capillary column with0.25 mm I.D. and a 10 m column with 0.10 mm I.D. A 15 m capillarycolumn with 0.25 mm I.D. was also electroplated with a nickel coat-ing. The high temperature resistivity coefficient for Ni allowed theuse of this material to both heat the column and to provide tem-perature information by measuring resistance, avoiding the use ofan extra wire per the methods described previously by Ehrmannet al. [29] and Sloan et al. [30]. Using a 48 V power supply, heatingrates as fast as 800 ◦C/min (13.3 ◦C/s) were reported. For rampinga 15 m column heated from 100 to 300 ◦C at 800 ◦C/min, a powerrequirement of 60–70 W was described.

Lastly, a field-portable gas chromatograph that employs anotherresistive heating approach for low thermal mass open tubular cap-illary columns is commercially available from Falcon Analytical.The CalidusTM instrument is designed to be a crossover solutionfor lab, process, and field use. It is primarily used in petrochemicalprocessing, and oil/gas exploration and production measurements.It employs a column module (or modules) that house stainlesssteel capillary columns (up to 4 m length) with various liquid filmor porous layer stationary phase materials available. Heating isaccomplished by passing current directly through the metal col-umn itself, while a separate insulated wire provides temperaturefeedback based on resistance. Fast temperature programming ispossible (up to 10 ◦C/s) and modular FID, thermal conductivity, anddielectric barrier discharge detectors are available. One or two col-umn, and one or two detector modules may be attached to a singlesample processing module that houses a heated injector and cap-illary transfer line connections. The system normally uses stable110 or 220 V AC power which is converted to 24 V DC for inter-nal operation, but with power consumption <300 W (simultaneousoperation of all modules) operation with an external battery pack ora vehicle power source is also possible (unpublished information,J. Crandall, Falcon Analytical).

3.3.1. Temperature programmed heating of MEMS columns andMEMS components

Several instrument designs combine a MEMS component suchas an inlet or detector with a traditional open tubular fused silicacolumn. An example is the commercially available Thermo Scien-tific C2V system. Another is the experimental system described byTienpont et al. [41] which used a chip-based microplasma opti-cal emission detector, described previously by Eijkel et al. [42],along with the resistively heated jacket that accommodates shortsections of open tubular column as already described.

In work that shows the flexibility of MEMS processes for design-ing and building different types of chromatography columns forevaluation, Ali et al. [43] mounted both open and semi-packed

micromachined columns in a standard convection oven instrumentwith typical injector and detector (FID) to complete tempera-ture programmed analyses. The semi-packed columns were builtin a silicon wafer with a rectangular column cross-section (150

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idth × 180 �m depth), 1 m length, and included regularly spacedows of full depth 20 × 20 �m posts within the open columnhannel. As is typical in the production of micromachined gas chro-atography columns, a Pyrex glass piece was anodically bonded

o the surface of the silicon component after etching the col-mn channel to provide the finished rectangular cross-section. Atatic coating process deposited polydimethylsiloxane in the col-mn channel and on the posts, and was followed by crosslinking.elative to an open channel column of the same dimensions, the

nclusion of the square-faced posts increased the sample capac-ty without resorting to a thicker stationary phase. The authorsound that “. . .due to the uniform spacing and distribution of theosts, these columns have lower pressure drops and eddy dif-usion as compared to conventional packed columns.” While thenclusion of the posts roughly doubled the surface area within theolumn relative to a rectangular cross section column of the sameimensions, the authors described a drawback to the use of angu-

ar column geometry which is commonly expressed with regardso MEMS chromatography systems. “It is very challenging to coat

EMS-based columns with a uniform stationary phase using tra-itional static and dynamic techniques given that these methodsere developed for capillary tubing”.

Reidy et al. reported in 2007 on the design of a MEMS columnith integrated heaters and temperature sensors, recognizing that

or the “. . .separation of mixtures with a wide range of boilingoints, temperature programming is essential” [44]. Their experi-ental chip-based system was mounted in a traditional convection

ven instrument to use the available non-MEMS injector and FIDetection, while heating of the micromachined column was accom-lished using the integrated heaters at rates as fast as 1000 ◦C/min.

series of alkanes (n-pentane to n-pentadecane) were eluted in15 s when a 0.25 m column was heated at this rate. Also in 2007otkay et al. described a pressure- and temperature-programmableEMS system that included integrated heaters and that had a

emi-circular column cross-section (90 �m diameter) “to minimizetationary phase pooling” [45]. As typical for experimental MEMShromatographs, these authors also mounted their device in a con-ection oven instrument to provide an injector and FID detection.

In 2010 Lewis et al. created a fully circular MEMS column byonding together two glass plates (surface area < 1000 mm2, with.0 mm overall thickness). The glass plates had previously beentched to produce matching half-circular troughs, and by carefullyonding the two plates together with the channels aligned, a nearlyircular cross-sectional profile with 320 �m I.D. was obtained [46].irect heating of the glass wafer assembly was provided by eitheretal elements or thin film resistive elements and Peltier devices.

emperature sensing was accomplished using a polyimide encap-ulated sensor placed between the heater and glass wafer assembly.y reversing polarity, the Peltier device could be operated to eitherssist with heating, or to cool the wafer assembly, providing a lowolumn temperature of 3 ◦C at 22 ◦C ambient temperature. Theaximum column temperature was 200 ◦C and ramp rates up to

0 ◦C/min were obtained, with mean power consumption of 25 W.fter describing these heating and temperature sensing methods,hromatograms were produced by mounting the column wafer in aonvection oven for use of the traditional injector, convection ovenemperature programming, and FID.

Zampolli et al. applied a MEMS-based portable chromatographo quantitatively detect volatile aromatic compounds in urban airver a period of several days with samples collected repeatedly for5 min intervals, with each followed by heating of a MEMS-basedreconcentrator and an analysis cycle [47]. The preconcentrator

odule was packed with a selective sorbent material and an inte-

rated heater was used for sample desorption at 100 ◦C. The 50 cmong column micromachined in silicon (0.8 mm2 cross sectionalrea) was encapsulated with a Pyrex glass cover and packed with

1261 (2012) 37– 45

80–100 mesh Carbograph 2 + 0.2% Carbowax TM. The column chipwas constructed with an integrated heater and temperature sensor,and detection was completed by an array of mixed oxide sensorsthat were also constructed using a silicon micromachining process.Purified air was used as carrier gas, and the complete instrumentpackage included automated fans to rapidly cool the microcolumnafter a high temperature cycle.

3.3.2. Temperature programmable person-portable GC–MSemploying resistive column heating

On-scene identification of unknown chemicals requires a morecomplicated GC detector, such as a mass spectrometer. Sustainedefforts since Burroughs and Tabor reviewed commercially avail-able portable gas chromatographs in 1999 [48] have produced twoperson-portable GC–MS instruments, both of which use resistiveheating of a low thermal mass open tubular column. The availableinstruments listed by Burroughs and Tabor reportedly weighed upto 7 kg, and none of the instruments mentioned in their review useda mass spectrometric detector. The two person-portable GC–MSinstruments now commercially available include an instrumentavailable at the time (isothermal version) that they did not includein their review, and another design that has only been availablewithin the past five years.

With a GC–MS instrument in the field, not only may unknownand unexpected analytes be identified within minutes or hours, butsampling and health protection strategies may be quickly changedbased on the information provided from qualitative GC–MS screen-ing. The usefulness of portable GC–MS was demonstrated byEckenrode in 2001. He used a transportable instrument with aconvection oven to rapidly assess air quality degradation fromforest fires [49]. Near real-time GC–MS data informed samplingdecisions to improve the quality of information obtained from sub-sequent laboratory-analyzed air samples. Smith et al. used solidphase microextraction sampling and a van-mounted GC–MS instru-ment with convection oven heating to quickly identify volatilecompounds produced from the irradiation of mail [50]. Complaintsof irritating vapors had been made by U.S. congressional staff mem-bers who handled mail that had been irradiated to protect againstbioterrorism, and in a single afternoon the identities of many ofthe light aldehydes and other volatiles produced from the irradi-ation process were determined. Analyses occurred in the parkingarea of the irradiation facility, and the information obtained con-firmed the reports of irritation and allowed a strategy to be quicklyimplemented to lessen exposures.

An important defensive need exists for military organizationsto detect and identify dangerous chemical exposures in near real-time. This has provided an impetus to the development of the twocurrently available person-portable GC–MS instruments, and alsoprovides a relatively large single customer for this type of person-portable gas chromatograph.

3.3.2.1. First generation person-portable GC–MS with temperatureprogram operation. The HapsiteTM instrument (Inficon, SyracuseNY) has evolved additional capabilities since its introduction. Inthe earliest version the instrument was only able to complete gaschromatographic analysis by sampling directly from the air to filla sample loop. Heating a direct air inlet probe (Fig. 2) of over 1 mlength, and an internal valve required substantial power, and thuslimited the upper temperature range available. The sample probewas typically heated to around 40 ◦C and the open tubular fused sil-ica column was kept at a constant and relatively low temperature

(<100 ◦C). Electron ionization (70 eV) was used along with a NEGhigh vacuum pump. In order to maintain a useful life for the con-sumable NEG the vacuum region was isolated from the switchingvalve outlet by a polymeric membrane. The valve could be aligned

P.A. Smith / J. Chromatogr. A 1261 (2012) 37– 45 43

Fig. 2. Person-portable Inficon instrument deployed in the field. (1) Sample-collection probe; (2) control keypad on instrument faceplate; and (3) interface forattaching the instrument to a removable service module containing a turbomolec-ular pump and a membrane roughing pump (service module used only when ACp

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Fig. 3. Hapsite sampling/analysis GC–MS chromatogram: 5.0 mg/m3 air concentra-tion for each of four volatile chemical warfare agents sampled. Sample time was1.0 min, nominal sample rate was 250 ml/min with TenaxTM concentrator mod-ule used. Initial column temperature was 70 ◦C, ramped to 180 ◦C at 30 ◦C/min. (1)Air, methylene chloride, (2) Sarin, (3) N,N-dimethylacetamide (artifact present inclean Tedlar bags), (4) Phenol (artifact present in clean Tedlar bags), (5) Soman(two diastereomers not resolved here), (6) Sulfur mustard, and (7) Cyclohexyl-

ower available).

eprinted from [39], Copyright (2004), with permission of Elsevier.

or flow to either bypass the chromatograph to introduce contam-nated air directly to the mass spectrometer membrane inlet, oro first pass through the chromatograph. The use of a NEG vac-um system ruled out helium as a carrier gas, and thus N2 wassed–supplied from an on-board compressed gas cartridge.

Around 2001, the instrument was upgraded to include pro-ramming to column temperatures >200 ◦C with resistive columneating per Sloan et al. [30]. Rapid temperature ramping was stillot possible due to the limited power available, but with the

ntroduction of low-power resistive heating, restrained tempera-ure programming became a practical option. An in-line sorbented and heated 10-port diaphragm valve were also added to the

mproved instrument. The addition of the sorbent concentratorxtended the concentration range possible for detection by sam-ling through the air sample probe to sub-ppm values. In 2004mith et al. described the use of a HapsiteTM instrument withhis updated configuration to sample a mixture containing fourhemical warfare agent vapors at nominal airborne concentra-ions of 5 mg/m3 [39], and Fig. 3 provides a chromatogram fromhis paper. At that time this was the only commercially availableerson-portable gas chromatograph capable of temperature pro-ramming, but the tailing peaks that likely resulted from the GC–MSembrane inlet demonstrate limitations imposed by the instru-ent design. Similar chromatographic performance was obtained

y Sekiguchi et al. who described analysis of similar compoundsith the same type of instrument in 2006 [51]. In both cases the

nstrument was equipped with a column of 30 m length, 0.32 mm.D. and 1.0 �m polydimethylsiloxane film thickness. The terminalolumn temperature was 180 ◦C and ramping rates up to 30 ◦C/minere used.

Around 2010, a new model of this instrument was introduced,esignated as the Hapsite ERTM (“extended range”). The same gen-ral configuration is used as with the previous upgrade, althoughhe air sample probe may now be replaced with an inlet capa-le of either desorption of a solid phase mixcroextraction fiber, orn externally collected thermal desorption tube (89 mm × 6.4 mm.D.). The standard open tubular fused silica column included with

he Hapsite ERTM has a length of 15 m with 0.25 mm I.D. and 1.0 �molydimethylsiloxane film thickness. High temperature desorption

◦

e.g. from a solid phase microextraction fiber at 250 C) is now pos-ible and when operated in this mode the internal heated lines andalve oven, and the membrane mass spectrometer inlet may alsoe operated at high temperatures (>100 ◦C). Operation at the higher

methylphosphonofluoridate.

Reprinted from [39], Copyright (2004), with permission of Elsevier.

temperatures without the relatively cool air sample probe allowsthe detection of less volatile target analytes, but to attain these tem-peratures the instrument must be warmed up while connected toexternal power. The weight of this instrument is 19.0 kg.

3.3.2.2. Second generation person-portable GC–MS with tempera-ture program operation. In 2008 Contreras et al. [52] described aperson-portable GC–MS instrument weighing 12.7 kg. This instru-ment, the Guardion 7TM (Torion Technologies, American Fork UT),employed a commercially available 5 m open tubular metal columnwith 0.10 mm I.D. and 0.4 mm film thickness (5% phenyl methyl-silicone stationary phase). The column assembly was resistivelyheated per the methods described by Sloan et al. [30], and a lowthermal mass, low volume injector inlet was included for sampleintroduction from a solid phase microextraction fiber. Both the col-umn and the injector could be heated to temperatures > 300 ◦C, andcolumn ramping rates > 100 ◦C/min were possible. The unheatedmass spectrometric detector for this instrument was a toroidal iontrap design [53] using electron ionization and a frequency scan forion ejection. A direct interface to the chromatographic column wasemployed, and two-stage mechanical pumping provided vacuum.Splitless, and split injections were possible.

The use of the narrow diameter open tubular column in thisinstrument addressed several potential problems. As column flowwas limited to < 1 ml/min, this lessened vacuum system pump-ing requirements, allowing direct interface of the column to thedetector. Also, the ion trap detector was able to operate at rela-tively high pressure, and both of these factors kept the size andpower requirements of the mechanical vacuum system withinmanageable parameters. Mechanical pumping allowed the use ofhelium carrier gas, and a small on-board cartridge supplied suffi-cient helium for >100 analyses. The relatively low capacity of thenarrow column limited the amount of analyte that could reachthe detector, lessening the potential for space charge and associ-ated problems. This second concern was further managed through

feedback-controlled ionization time to keep the ion density withinthe ion trap below specific target values.

Within the past year, the GuardionTM instrument has beenupgraded through collaboration between Torion Technologies and

44 P.A. Smith / J. Chromatogr. A 1261 (2012) 37– 45

Fpc

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Fig. 5. Chromatogram resulting from passive solid phase microextraction sam-pling and analysis of volatile standard compounds using the Guardion 8TM

GC–MS instrument initially operated in splitless mode for 2 s, followed by50:1 injector split, 50 ◦C initial column temperature (held for 10 s), ramped to270 ◦C at 120 ◦C/min; peak identities: (1) acetone, (2) n-hexane, 3 benzene, 4methylcyclohexane, (5) methyl isobutylketone, (6) cis-1,3-dimethylcyclohexane,(7) trans-1,2-dimethylcyclohexane, (8) trans-1,3-dimethylcyclohexane, (9) cis-1,2-dimethylcyclohexane.

From [55], Copyright, reprinted by permission of Taylor & Francis,http://www.tandfonline.com.

Fig. 6. Chromatogram resulting from passive solid phase microextraction samplingof an 18 mg PCB sample contained within a sealed vial kept at 60 ◦C during sam-pling. Analysis conditions are identical to those used to obtain the chromatogramdisplayed in Fig. 5 except a relatively long splitless injection time of 30 s was usedfor the heavier PCB analytes to allow for slower mass transfer during desorption inthe heated injector of the person-portable GC–MS system used for analysis. Whilebiphenyl was identified, the specific PCB congeners were not identified. The respec-tive molecular mass values for the analytes that produced GC peaks correspondingto the various PCB isomers are noted.

From [55], Copyright, reprinted by permission of Taylor & Francis,

ig. 4. Guardion 8 GC–MS instrument operating in the field. (1) disposable highressure helium bottle and (2) heater block to control the temperature of samplesollected using solid phase microextraction.

miths Detection. Constant flow chromatography has been addedhrough pressure programming, the toroidal ion trap detector iseated to lengthen the time between required cleaning of trap elec-rodes, and a voltage scan is now used for ion ejection. The latterwo improvements improved mass resolution, especially at higher/z values compared to the original instrument design. The weight

f the current version of the instrument has increased to 14.5 kg,ut the physical dimensions of the instrument remain small enoughhat it may be brought aboard commercial aircraft as carry-on lug-age (after first removing the high-pressure carrier gas cartridge).he current version of the instrument (Guardion 8TM) is shown inig. 4. Start-up and operation for several hours are possible usingnly internal battery power.

In 2011 Smith et al. described the use of the Guardion 7TM

nstrument to study self-chemical ionization (self-CI) of basicnalytes related to the chemical warfare agent O-ethyl S-(2-iisopropylaminoethyl) methylphosphonothiolate (VX) [54]. By

ncreasing ion residence times in the toroidal ion trap detector,M+H]+ ions produced from self-CI were observed for VX andeveral degradation products, offering the potential to identifyompounds that produce mass spectra which lack characteristicigh m/z ions when detected using an ion beam mass spectrometernd 70 eV electron ionization. Large, semivolatile analytes such asX (molecular weight 267 u) and bis(diisopropylaminoethy) disul-de (molecular weight 320 u) were successfully chromatographedwing to the direct GC–MS interface and the ability to rapidly ramphe open tubular column used in this instrument to a temperaturef 300 ◦C.

Using the Guardion 8TM instrument, Smith et al. described rapidampling and analysis for analytes with a wide volatility range [55].ig. 5 shows a chromatogram for analysis of light solvents, andig. 6 shows the results for analysis of a sample of polychlorinatediphenyl (PCB) compounds from this paper. Solid phase microex-raction sampling of the solvents involved very brief exposure timest room temperature, while PCB sampling involved elevated tem-erature (60 ◦C) and 10 min sample duration.

To expand the range of field sampling and analysis approachesith this instrument beyond solid phase microextraction, the use

f a needle trap device provides the ability for exhaustive samplingnd quantitation without the use of solvents [56]. A chromatogram

ecently produced by the author in a laboratory setting from nee-le trap sampling of airborne solvent vapors for analysis with theuardion 8TM instrument (Fig. 7), shows how rapid temperaturerogramming can be used to estimate concentrations of volatile

http://www.tandfonline.com.

airborne contaminants with analysis times <1 min. The low vol-ume injector liner of the instrument includes a receptacle for thetip of the needle trap just above the injector/column interface, andpressure within the injector forces heated carrier gas into a side-hole above the sorbent packing, through the heated sorbent, andout through a hole in the needle tip. This desorbs trapped analyteswithout the need for external gas connections to the needle trap

device during desorption.

P.A. Smith / J. Chromatogr. A

Fig. 7. Extracted ion traces produced from a needle trap sample (1.5 mg carboxenTM

1000 behind 2.0 mg Tenax TATM in a 19 gauge needle), 25 ppm nominal analyte con-centrations. The respective relevant m/z values for extracted ion traces is indicatedfor target BTEX analytes and internal standards: (1) benzene (m/z 78), (2) toluene-d8 -internal standard (m/z 98), (3) toluene (m/z 91), (4) bromopentafluorobenzene-internal standard (m/z 117), (5) ethylbenzene (m/z 91), (6) p-xylene (m/z 91), (7)o-xylene (m/z 91). Air sample collected for target analytes by pulling a twenty mlvsc

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olume of air through the needle trap device with a gas-tight syringe, followed byampling a known volume of the internal standard vapors. The resistively heatedolumn was held at 30 ◦C for 10 s, followed by ramping to 270 ◦C at 120 ◦C/min.

. Conclusion

The need to operate early person-portable and transportableas chromatography instruments at ambient temperature orith relatively low isothermal heating did not allow high

peed, high resolution chromatography. The recent availability oferson-portable gas chromatography instrumentation capable ofemperature program operation is due to the development of the

odern open tubular gas chromatography column, and methodso heat the column that use relatively little power. With the usef low thermal mass heated zones and a resistively heated openubular capillary column, fast temperature ramping and high tem-erature operation under battery power has been demonstrated

n commercially available GC–MS instrumentation and also inxperimental MEMS-based systems. Challenges remain to furthermprove MEMS-based column construction and to fully realize theenefits of batch manufacturing that MEMS construction will even-ually allow.

Method development and validation are needed to expand these of small, high performance gas chromatography systems thatse little power into areas where useful information may be pro-ided from near real-time field analysis. These instruments nowxist, and it is clear that among all of the potential roles for person-ortable gas chromatography, it already has an important role inrotecting the health of deployed military forces, workers, and theeneral public.

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