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IN CHROMATOGRAPHY
The Saga of theElectron-Capture Detector
Peter J.T. Morris andLeslie S. Bttre
This month's installmentof "Milestones inChromatography"discusses the work ofJames E. Lovelock, whicheventually led to theinvention of the electron-capture detector.
Leslie S. EttreMilestones in ChromatographyEditor
I n addition to the universal detectorsused in gas chromatography (GC),selective detectors also have playedan important role in the rapid spreadingof the utilization of the technique. Prob-ably the most important selective GCdetector is the electron-capture detector,with a very high sensitivity to organiccompounds containing chlorine and flu-orine atoms in their molecules. The elec-tron-capture detector had a vital role inenvironmental protection and control its use helped to prove the ubiquitouspresence of chlorinated pesticides innature and halocatbons in our atmos-phere, and made us aware of the globalextent of pollution. It was the electron-capture detector that made concentrationranges of parts-per-billion (ppb: 1:10'')or even parts-per-trillion (ppt: 1:10'-)detectable. Today, these terms are usedroutinely without realizing how formida-ble such a sensitivity really is: 1 ppbmeans that a spaceship (or a UFO,depending upon one's inclination) couldpick up a particular family of six fromche whole living population oi the Earth,and 1 ppt means that it could even findone piece of chewing gum in the pocketof one of the children. Lovelock theinventor of die electron-capture detector illustrated its superior sensitivity bythe following metaphor (1): If one wouldpour about one liter of a perfluorocarbonliquid onto a blanket in Japan, and left itout to dry in the air by itself, a few weekslater one could detect on the west coastof England the vapor that had evapo-rated into the air in Japan from thatblanket and carried by the jet streamaround the worid.
The electron-capture detector is anionization detector and its response isbased upon the ability of molecules withcertain functional groups to capture elec-trons generated by the radioactive source.The detector chamber contains two elec-trodes and a radioactive foil as the radia-tion source. Using an inert carrier gaswith no afFmity for electrons, die ionsformed by the ionizing radiation can becollected, creating a steady standing cur-rent in the detector. ^X1len molecules ofcertain electron-absorbing solutes enterthe detector chamber, they will captureelectrons, resulting in a decrease of thestanding current, giving a negative peak.In practice, the recorded peaks are madepositive by reversing the polarity of therecorder.
Since its invention, the design of theelectron-capture detector has undergonea number of changes, but its principleshave remained the same, as shown inFig;ure 1. Also, different radioactivesources have been used: in Lovelock'soriginal design, the foil contained ''"Sr,but soon this was changed to tritiumoccluded in titanium foil. Today, italmost universally contains ''-^Ni. How-ever, questions regarding the deteaor'sconstruction are not our subject.
James E. LA)velock (born in 1919)graduated in 1941 as a chemist fromManchester University (Manchester, UK)and then, in 1948, obtained the Ph.D.degree in Medicine from the LondonSchool of Hygiene {Lotidon, UK). After1941. he was associated with the BritishMedical Research Council for almost 20years. From 1958 to 1959, he was a visit-ing scientist at Yale University Medical
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School (New Haven, Connecticut) andthen, from 1960 to 1964, he was associ-ated with Baylor College of Medicine inHouston, Texas, and the University ofHouston, Houston, Texas, as a professor.Since 1964, he has been a freelance sci-entist serving as a consultant to variouscompanies and institutions; among oth-ers, he also cooperated in NASAs spaceprograms. In the early 1970s, Lovelockproposed his theory of Gaia, the livingEarth, Rinctioning as a siiperorganism inwhich the physical environment and thelifeforms inhabiting the planet interact,maintaining a more-or-less steady state.His fundamental contribLitions to ourunderstanding of the impact of environ-mental pollution were recognized bythree major awards: the Heineken Prizefor the Environment of the Royal DutchAcademy of Sciences (1991), theVOLVO Prize (1996). and the BluePlanet Prize (1997), the latter generallyconsidered as che environmental equiva-lent of the Nobel Prizes.
Lovelock has written a number ofautobiographical treatments (1,3,4) in Figure 1: Schematic of a typical early electron-capture detector (2).
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which he dealt in details with hisinvolvement in GC and the inventionand development of the electron-capturedetector. Our disctission mainly is basedupon these.
InventionsThe electron-capture detector acttiallywas invented by Lovelock in four stages:around 1948, 1954-1955, 1956-1957,and finally in 1959. In the first twocases, the aim of his work was somethingelse; the electron-capturing effect
obtained was unexpected and at first notunderstood. In the third stage, the devel-opment of the argon-ionization detector,the crucial phenomenon was not an elec-tron-capturing effect, and only thefourth stage was aiming directly at thedevelopment of the electron-capturedetector. These represent a fascinatingstory, illustrating how a scientist gradu-ally perfects his work.
First stage: an anemometer: At theBritish Medical Research Council, Love-lock was seconded to its Common Cold
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phase, in other words, gas-Uquld parti-tion chromatography. Tbey illustrated itspossibility through the separation ofvolatile fatty acids (7). Lovelock asked fortheir help and soon a close collaborationwas established between them.
Originally, James and Martin usedtitration to measure the eluted fatty acidsand then Martin developed a gas densitybalance to serve as a general purpose GCdetector. However, he soon found outthat it was not sensitive enough and wastoo complicated to be operated by anaverage chemist. (A description of the
detector [8] was published only a fewyears after its actual development).Therefore, Martin suggested to Lovelockthat he invent a more sen.sitive detector.While he was working on this problem,he heard about a new GC detectorinvented by associates of two Shellresearch laboratories: Otvos and cowork-ers at the laboratories in Emeryville, Cal-ifornia (9,10), and Boer at the Konin-klijke/Shell lah in Amsterdam (11). Thisdetector was based upon the ionizationof molecules hy p-ray using Sr as theradiation source. Lovelock immediately
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tried to huild one; but he had a problemwith the carrier gas. Both Orvos andBoer used the hght gases bydrogen andhelium, but these two could not be usedat the NIMR: helium was prohibitivedue to its high price, and hydrogen wasunacceptable for safety reasons. 1 hus.Lovelock tried to work with nitrogen,but his first results were di.scouraging: thesensitivity of the device did noi meettheir requirements. At that time. Love-lock remembered his experience with theanemometer, the sensitivity of which wa.svery dependent upon the applied poten-tial and, thus, tried to use difFerentranges with the new detector to find ouiwhether the results can be improved.Indeed, when using low potentials, hesuddenly obtained dozens of large peakswhen analyzing allegedly pure sub-stances, hut their retention times differedfrom the values the compound's peakshould have had. In fact, what he nowhad was an elearon-capture detector andthe large peaks were due to trace quanti-ties of electron-absorbing impurities inthe sample.
In his recollections (4), Lovelockdescribes an interesting and most annoy-ing observation during his investigations.One of the samples he tried out on thenew detector was dissolved in carbontetrachloride, which was considered to bean unrcactive solvent. However, uponinjection, the ion current fell to zero andno further peak could be observed. Infact, it remained there for weeks and allattempts to restore the detectors opera-tion failed- Only much later did he real-ize that CCI4 is one of the most intenselyelectron-absorbing substances. Part of thevapors that exited the column becameadsorbed on the siliai rubber sealbetween the column and the detector inhis homemade GC. In fact, this amountof CCI4 slowly desorhing into the col-umn effluent became an almost perma-nent source of its vapor, making the useof the detector useless for weeks. As hesaid, they had no use for such a tempera-mental detector: at that time it seemed tobe useless for their purpose and, thus, itwas shelved and no results were pub-lished.
It is difficult to set exactly the dates ofthese developments. Martin certainlydeveloped the gas-density balance detec-tor by 1954 and, thus. Lovelock had to
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I ' * i t i t t t ^ i i t i i i i i i i i i i i t i (
Figure 2: The first chromatogram obtained at the University of Houston on November 18,1958, using a capillary column with the small-volume argon-ionization detector of Lovelock.The last three peaks are n-hexane, benzene, and toluene. Identified signatures are those ofA, Zlatkis, J.E. Lovelock, M.C. Simmons (5hell), R.E. Johnson, and H. Lilly (both with Barber-Col-man) (20).
start looking for a more sensitive detectorsoon after this time. Most likely, he triedto use the modified version of the Shelldetector around 1955 and had the argonincident, mentioned in the following, in1956, leading lo the argon-ionizationdetector.
Third state: the argon-ionizationdetector: The development of this detec-tor is parr of the series of events leadingto the electron-capture detector, andtherefore, should be summarized brieflyhere. While Lovelock was continuing hisinvestigations ;iiming the construction ofa universal, sensitive detector, utilizingthe principles of ionization, an unex-pected event occurred. The Institutesstore was temporarily out of nitrogen,hut a flask of argon was available: notwanting to wait until a new shipment ofnitrogen arrived. Lovelock tried to aseargon as the carrier gas. Applying a i^rlyhigh potential suitable to ionize thesolute molecules, he indeed obtained thedesired results: large peaks for the samplecomponents and a good, noise-free base-
line. ^Tien the new nitrogen shipmentarrived, he tried the same conditionswith that carrier gas, but the results werepoor, showing only unsatisfactory lowsensitivity.
Further srudies revealed that the rea-son for rhe unexpected results was the so-called Penning effecc. In 1934, F.M. Pen-ning of Philips (Eindhoven) discoveredtbat when rare gas (argon) atoms areexposed to radiation, the formedmetastable atoms have a relatively longlife and their concentration approachesthat of tbe ions during steady irradiation.If traces of the vapors of some othergases are present, the metastable argonatoms transfer their energy on collisionwith these molecules, as long as the ion-ization potential of the other molecule isless than the energy level of themetastable argon atoms (12). The ions soformed yield an increased cell currentrelated to the concentration of the othervapor in the detector chamber. This briefsummary explains how the ai^n-ioniza-tion detector was invented due to a lucky
Figure 3: J.E. Lovelock in Houston, in 1961(3).
coincidence.The argon-ionization deteaor was first
discussed publicly by Lovelock in 1957,at an informal meeting of the newlyformed Gas Chromatogrjphy DiscussionGroup, in Oxford. UK. and thendescribed in two publications (13,14). Itwas commercialized almost immediatelyby the British instrument company W.G.Pye & Co., which introduced the so-called argon chromatograph at the Sec-ond International GC Symposium heldin May 1958, in Amsterdam, theNetherlands.
In the spring of 1958, Lovelock wasinvited to present a paper on the argon-ionization detector at the symposiumAnalysis of Mixtures of Volatile Sub-stances organi/.ed April 10-11 in NewYork City by the New York Academy ofSciences (15). There he met S.R. Lipsky,professor at Yale University MedicalSchool. Lipsky already built an ioniza-tion detector based upon the Shelldesign, but had problems with it and,aftet Lovelock's lecture, asked his advice.At the conclusion of their discussion.Lipsky invited Lovelock to stay for sev-eral months as a visiting professor at Yale.
Lovelock arrived at Yale soon afi:er theSecond Internationai GC Symposiumheld in May 1958. in Am.sterdam, theNetherlands, where M.J.E. Golay pre-sented his fundamental paper on open-tubular (capillary) columns (16) and Lip-
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positive. As pointed out in the paper, thepotcnriiil ai which this transition occurscan be used for rhe characterizarion ofrhe major classes of organic compounds.
Commercial Realization of theElectron-Capture DetectorMajor instrument companies starred tointroduce electron-capture detectors fortheir GC systems around late 1961through the early part of 1962, andwhile in Houston. Lovelock also part-nered with Al Zlatkis, forming the smallcompany Ionics Research, producingsuch detectors for commercial use (Fig-ure 3). For a few years, they supplied thedetectors for the instruments of theI'crkin-Elmer Corporation. In general,the early detectors were fairly delicate touse and I remember that we often had tocall them in Houston to straighten out.some problems. A general complicationfor customers was that the detectore hadradioactive material, requiring speciallicense to use them. Also, at the verybeginning, the field of application of thedetector was not clear: it became onlygradually the most important de\'ice inthe environmental movement, and suchapplications first had to be demon--strated. In this respect, Keene Dimick,the founder of Wilkens Instrument andResearch Co. (the present-day Chro-matography Division of Varian) and theeditor of the company's quarterly publi-cation called Aerograph Research IVores,w;is particularly successfiil. The title pageof its Summer 1962 issue was particu-larly striking: it showed a single peak of apesticide corresponding ro 10-12 g, withthe headline "Have you ever seen apicogram?" printed in large, bold-facedcharacters. Figure 4 shows a similar pub-licity chromatogram from Perkin-Elmer,demonstrating the analysis of the extractof an earthworm, indicating the presenceot five pesticides (23).
Electron-Capture Detection andthe Environmental MovementIn general, the start of the environmentalmovement is identified with the publica-tion of the book Silent Spring by RachelCarson, in 1962 (26), documenting thedetrimental influence of pesticides on theenvironment. Its title refers to the resultot their indisctitninate tise: these chemi-cals pass from one organism through the
links of the food chain (see Figure 4),eventually poisoning the wild birds andsilencing the forests and meadows.
In the literature, one can often findimplications that Carson's book wasmade possible by the use of the electron-capture detector: even a relatively recentarticle in The New York Times expressedrhis opinion, saying that I^ovelock'sinvention was "providing a foundationfor the work of Rachel Carson" (27).This is, however, not true: Silenr Springwas based upon investigations in the1950s using other analytical techniques,and this is clear if one checks the morethan 500 literature references given inthe book. However, ic is true that the useof the electron-capture detector providedthe infallible proof of the correctness ofCarson's conclusions. Lovelock cites thefirst two papers reporting on the use ofthe electron-capture detector for thedetermination of pesticide residues: thefirst was a presentation by British scien-tists from Shell at the 18th InternationalCongress on Pure and Applied Chem-istry, in August 1961, in Montreal,Canada (28), and the other by associatesiii the U.S. Food & Drug Administra-tion (FDA) presented at the 75th AnnualMeeting of the Association of OfilcialAgricultural Chemists, held in October1961 in Washington, D.C. (29).
The Chlorofluorocarbon ProblemThere is one major field where Lovelock'spersonal activities have served as thefoundation of our awareness of the detri-mental effea of air pollution and eventu-ally led to banning the use of halocar-hons. We should finish our discussionwich a brief summary of these activities.
In 1966, Lovelock and his familyspent the summer on the westernmostcoast of Ireland and was surprised tooften see haze over the bay, dependingupon the wind. The next summer, whenreturning to Ireland, he brought withhim a portable GC system and regularlymeasured the chlorofiuorocarbon FreonF11 (trichlorofluoromethane) themost widely used halocarbon concen-tration during and after the outbreak ofhazy air. His measurements clearly estab-lished on clear days the presence of asmall steady background concentrationof about 50 ppt, which increased bythreefold in the case of a haze. This
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increase easily could be attributed to pol-lution brought hy winds from the Euro-pean conrinent; however, there was noexplanation for the steady backgroundconcentration on clear days when thewind.s came from the Atlantic Ocean: didthis mean that the air was polluted there?To investigate this, in November 1971,Lovelock joined the Research ShipShacklecon oi the British Natural Envi-ronment Research Council for a six-month voyage traveling from England toAntarctica, and he carried out regularmeasurements of atmospheric halocarhonconcentration over the Atlantic Ocean.The results of these measurements (30)dearly indicated the accumulation ofFreon 11 (and other halocarbons) usedin aerosol cans and as refrigerant in theEarths atmosphere, serving as the sourceof the steady background concentrationhe observed on the Irish coast. Lovelock'sdata collected durmg this voyage haveserved as the hasis on which F.S. Rolandand H. Molina were able to develop theirtheory on the decomposition of the halo-carbons in the stratosphere, releasingchlorine that in turn, is catalyzing thedepletion of stratospheric ozone (31). Fortheir work, Roland and Molina (togetherwith P. Crutzen of The Netherlands)received the 1995 Chemistrj' NobdPrize.
For a long time, the electron-capturedetector was the most sensitive GCdeteCTor, with its unique selectivity.Recendy, improvements in GC-massspectrometry systems have rivaled it.However, GC-elecrron-capture detectionsystems still remain the workhorseinstruments for routine pesticide deter-minations in water and soil, PCBs in thetransformer oils, and halocarbons in air.
References(1) J.E. Lovelock, Homage to Gaia (Oxford Uni-
vcrsit>- Prcss, Oxfoixl, 2000), pp, 181-190.(2) LS. Ecirc./ Chromatogr. Sci. 16, 396-417
(1978).(3) }.E. Lovelock, LS. Ettrc and A. Zlatkis, Eds.,
75 Yean of Chramatography A HiiwricalDialogue (Elscvier, Amsterdam), pp,277-284.
(4) J.E. Lovelock, in EUcfron Capture Theory andPractice in Chromatography, A. Zlatkis andCE Poolc, Eds. (Eiscvicr, Amsterdam, 1981),pp. 1-n.
(5) J.E. Lovelock and E.M. Wa.-iilewska, / Sci.
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fmirum. 26. 367-370 (1949).(6) A.J.P. Martin and R.L.M. Synge, Biochem. J.
35, 1358-1368(1941).(7) A.T. James and A.J.P. Martin, Biochem.J. 50.
67^-690 (1952).m A.j.P Manin and A.T., James. Biochem. J. 63,
138-142(1956).(9) J.W. Otvos and D.R Stevenson,/ Am. Chem.
5:. 78. 546-551 (1956).(10) C.H. Deal, J.W. Otvos. W.N. Smith, and RS.
Zucco,Anai Chem. 28, 1958-1964 (1956).(]1)H. Boer, in Vapour Phase Chromatography
(1956 Lx>ndon Symposium), D.H, Desry. Ed.(Butterworths, London, 1957), pp. 169-184.
(12) F.M. Penning, Physica (Amsterdam) 1,1028-1044(1934).
(13) J.E. Lovelock, Nature (London) 181,1460-1462(1958).
(14) J.E. lovelock, / Chromatogr. 1, 35-46(1958).
(15)J.E. Lovelock. A.T. James, and E.A. Piper,Ann. N. Y. Acad. Sci. 71, 720-730 (!959).
(16) M.J.E. Golay, in Gas Chromatography 1958(Amsterdam Symposium). D.H. Desiy, Ed.(Butterworths, London. 1958). pp. 36-55).
(17)J.E. Lovelock, Nature (London) 182,1663-1664(1958).
(18) S.R. Lipiky. R.A. I^ndowne. and J.E. Love-
lock, And. Chem. 31, 852-856 (1959).(19) S.R. Lipsky, J.E. Uvdock, and R.A.
Landowne. / Am. Chem. Soc. 81, 1010(1959).
(20)L.S. Ettre, ^naZ Chem. 57, 14I9A-14.38A(1985).
(21) A. Zlatkis and J.E. Lovelock, ^ na/ Chem. 31,620-621 (1959).
(22) I.G. McWilliam and R.A. Dewar, Nanire(London) 181,760(1958).
(23) I.G. McWiiliam and R.A. Dewar, in GWChromatography 195S (Amsterdam Sympo-sium), D.H. Desty, Ed. (Bunerworrks, lx)n-don,1958), pp. 142-152.
(24) J.R. Lovelock and S.R. Upsky, / Am. Chem.5oc. 82, 43M33(1960).
(25) E.W. Cieplinski, Instrument Nem 15(2), 7-8(1964).
(26) Rachel Carson, 6V/fn(6iDw;g(Houghton Mif-flin Co., New York. 1962), 7rh printing:1994.
(27) A.C. Rcvkin. The New York Times ScienceSection, Septemher 12, 2006.
(28) E.S. Goodwin, R. Golden, and J.G.Reynolds, Analyst 86, 697-709 (1961); 87,169(1962).
(29) J.O. Watu and A.K. Klein. / Assoc. Off. Agr.Chem. 4% 102-198(1962).
(30)J.E. Lovelock, R.J. M a ^ , and R.J. Waade,NatuK (London) 241, 194-196 (1973).
(31) ES. Roland and M.J. Molina. Fev. Geophys.s. 13> 1-36(1975).
Dr. Peter J.I Morris is Manager of Researchand Residencies at the Science Museum, inLondon SW7 2DD, England. He is a gradu-ate of Oxford University and has publishedon many aspects of modern chemistry,including chromatography.
Leslie S. EttreFrom 1988 to 2004,"Milestones in Chro-matography" editorLeslie 5. Ettre wasassociated with theChemical Engineer-ing Department ofYale University(New Haven, Con-necticut}, first as anadjunct professorand then as a research fellow. Previously, hehad been with the Perkin-Elmer Corpora-tion for 30 years. He is currently a memberof LCGC's editorial advisory board. Directcorrespondence about this column to "Mile-stones in Chromatography," LCGC, Wood-bridge Corporate Plaza, 485 Route 1 South,Building F, First Floor, Iselin, NJ 08830, e-mail [email protected].
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