~OP~C in..S
.
Edited by GALEN W. EWING, Seton Hall University, So. Orange, N. J. 07079
T k s e artielea are intended to serve the readers of T H JOURNAL ~ by calling attention lo new dewlopmenla i n the themy, deb, or amilabilily oj chemical laboratory instrumenlatian, w by presenting useful insights and ezplanatians of topics that are oj practical imporlance lo those who use, w teach the use of, modern instrumentation and instrumental techniques. The editor invites correspondence from prospective canlribulors.
1111. Liquid Chromatography ~etectoriHANS VEENING, Deportment of Chemistry, Bucknell University, Lewisburg, Po. 17837 During the last few years, there has been a marked increase of interest in column liquid chromatography (LC). One reason that this technique, whose discovery preceded gas chromatography (GC) by many years, has not been used extensively until relatively recently, has been due to the inherent short,comings of suitable detection devices to monitor the flow of effluent liquids, and the lengthy analysis times involved. Promising improvements in detector design during the last few years, however, have made it possible to use a number of different modes of detection with high speed, high efficiency liquid ehromatographic columns. High resolution column LC is a technique which is experimentally analogous t o GC, in that one makes use of small sample sizes (microliter quantities), long, narrow bore columns, fast flowing liquids, and continuous and highly sensitive detection devices. The term "liquid chromatography" includes several distinct types of interaction, i.e., (1) liquid-liquid, in which the components are separated by partitioning between a mobile and a stationary liquid; (2) liqxid-solid, in which the components me selectively adsorbed on an active surface; (3) ion ezchange, in which ionic components of the sample are sep* rated by selective exchange with replaceable ions of the support; (4) permeation, in which separations occur on a permeable gel by a sieving action based on molecular siae. High speed LC is presently in its early stage of development, comparable to the early period of GC. 'The use of specifio names of manufacturers and s description of their instrumentation does not imply endorsement, nor does the omission of commercially available products imply an unsatisfactory appraisal. Descriptions of instruments are generally listed alphabetically according to the name of the manufacturer.
The advantage of liquid chromatography is that thermally unstable, nonvolsi tile compounds which cannot he eluted by GC, can often he separsted by LC, since columns are oper&d a t or near room temperature. Applications therefore seem feasible for such high molecular weight compounds as proteins and polymers. Too, the interchange of solvents can provide special selectivity effects in LC, since the relative retention of two solutes is strongly influenced by the nature of the eluent used. Althongh LC is not likely to replace GC as an analytical technique, the two methods should complement one another. The current interest in column LC is evidenced by numerous articles which are now appearing in the literature. Column liquid chromatography has been successfully employed by several workers in the analysis of steroids (I), herbicides (S), insecticides (S),metal organic compounds (4, 6 ) and biologically active materials (6-8). Recently, reports have appeared, describing improvement in performance and efficiency of LC columns by the development of controlled surface porosity supports (2, 9 ) and by the use of high speeds and high prmsures (10-IS), enabling the technique to become competitive with GC. Huher (10) bas shown that high speeds can yield efficient end fast separations comparable to GC hy the use of finely divided, homogeneous packings, thus necessitating high pressures. Pressure, however, ha3 practically no effect on the arti it ion coefficient or the fluid velocity. The retention time (tn) of a component (A) for a given resolution is given by:
where K A and K B are the partition coefficients of two components A and B; c, and q are the fractions of column
Dr. Veening w h horn in 1931 in yelp. The Netherlands. Ire lived in Sunnnrn far eleven y e w s , and enme to the United States in 1944. He ohtained the B.A. n Chemistry in 1953 from Hope College, Holland. Mich.. and the M.S. and P1r.D. kgrees in Analytical Chemistry from Purdue University in 1955 snd 1959. He wrote his doctoral dissertation on umineseenee phenomena of metal oh* ates. I n 1958 he joined the faculty of Buokd l University, where he is presently tssooiate professor. He hashed summer ndustrial erperienoe a t American Cyananid Company and Esso Research and Engineering Company. I n 1 9 6 6 6 7 he was awarded a National Science Foundation Soience Faoulty Fellowship for a rear of study and research in ehronatography at the University of Amsterism, Holland. where he was sssoeiated with Professor J. F. K. Huher. His research interests include gas ehronatography and high speed liquid ohronatography as well as properties of netal-organic oompounds. He is pres ?ntly serving as Chairman of the S U ~ xuehanna Valley Section of the American Zhemieal Sooiety.
volume occupied by the moving phase ar and the stationary phase P ; r, is the fraction of the column volume through which flow occurs; HA is the height equivalent per theoretical plate for A; (u) is the average fluid velocity; and RBA is the resolution which is defined as:
Re*
tns
-tn~
= -
o*
where tne > tnA and s~ is the standard deviation of the elution peak for A . A typical van Deemter plot in liquid chromatography has been shown to have a minimum a t very low fluid velocities, a situation unlike the results obtained in gas chromatography. Huber (10) has also shown that this is so because the H (Continued on page A660)
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itself primarily with a discussio~lof detectors. The emphasis will he placed on detectors useful in high resolution LC nsing small bore columns. term (HETP) is an additive result of four An LC detector is a device which meacontributive processes, snres a physical property aorh as light absorption or refmctive index of the colH = Hard f Hnh HE^ HE. umn effluent,, and converts changes in Hlln and H>rcresult from mixing in t,he these properties into nsxble signals by the production of a cnrrent. or a potent,ial moving fluid by diffusion and coiwectian, which can be amplified and fed into a rerespectively. H m m d HE* result from mass transfer in the moving and stationa~y corder. Chromatographic detectors generally phases, respectively. In LC, the unioperate differentially, responding to either formity and smnllness of pwticle size is the concentration or the mass flow rate. therefore a more rigid requirement than Different,ial detectors responding to the in GC since a higher degree of convective concentration (photometric, rehnctomising for liquids will be enhanced by met&, conductanre) yield a signal (Y) irregulerly packed coltunns, thus lending which is proportional to the conrentrstion to higher H values. (C) which t.rnverses the detector. Locke and Martire (18) have developed a thermodynamic theory of solute retenY = K,C tion in LC, and Locke (14) hw reported experimental confirn~ationof this theory. an elution If Y is plotted against time (0, Excellent reviews on the subject of LC ill peal; results. I t can be shown that for and LC detectors in particular roch detectors, the ares. (A) under the have been written by Hnber (15, 18) and peak is proportional to the total mass (IIL) by Conlon (17). of componeut a.nd inversely proportional A typical liquid chromatogmph is shown to the flow rate (F) in Figure 1. I t consists essent,inlly of a m reservoir (A), for storing the moving A=K,F phase; a high pressure pump supplied with a damping device to eliminate UIII t is therefore important that flow of the desirable pnmp pulsations, ( R ) ; a premobile phase be kept constant for such colnnm to assure attainment of eqoilibdetectors. rium between the moving and st.ationary In differential detectom which respond liquids, (C); a sample injection device, to the mass flow rate, d m / d t (e.g., flame (D); a thermostatted analytical column, ionization) it cen be shown that the area. (E); a detector, (F); and a recorder, is directly pmportional to the total mass (G). Several manufactovers now make complete nnits, how.ever, it should be borne in mind that these are first generation units and as w r h will oiten need to be modified for specific s;pplications. Many and there is no dependency on flow rate. workers still prefer to assemble their own In geueral, LC detectors can be classiapparatus from componellt parts. fied according to the type of physical The detector used in LC is one of the property measured. These properties inmost critical instmmental aspects upon clude: which the success of the technique de(a) Light absorption pends. A large number of different types (b) Refractive index of detectors have recently appeared on (e) Gas ionization (preceded by pyrolythe market and no doubt will continue to sis or volatilization) do so. This report will therefore concern (d) Thermal changes (e) Fluorescence (f) Conductance (g) Radioadivity ( h ) Polarography
detector operates linearly with respect to concent,ration. The "linear dyuamic range" is defined as the ratio of the upper and lower linessit,y limits. Detectors which a=e presently commercially available, make use of some of the physicd properties listed (i.e., light
Of the methods listed, gas ionization and polarography are partially or wholly destructive to the sample. The choice of a suitable detector for a given chromat,ographic separation may therefore depend on this feature, especially if quantitative collection of sample components is desired. I t will also be based on the type of chmmatography (i.e., liquid-liquid, ion exchange, etc.) employed, and on the types of components eluted from the column. Requirements for a suitable high resolution LC detector are high sensitivity, low noise, a r i d e linear dynamic range and a small dead volume of the sample cell. The latter feature is very critical due to the great amount of convective mixing which can occur in the cell itself and thus lead to undesirable peak broadening and subsequentloss of resolution, The "linear dynamic rmge" of a detector represents the concentration range within which the
The Chromatronix model 200 UV photometer is suitable for high resolution microcolumn chromatography as well as prepavative separations. I t is a double beam instrument which uses a low pressure mercury lamp followed by an interferencefilter. The light beam is split by a vibrating mirror which alternately passes the beam through sample and reference flaw cells. The two beams are combined at the detector which is immediately fallowed by a low noise variable gain amplifier. Temperature drift due to differing temperature coefficientsin the two optical detecting circuits is thus eliminated. The unit utilizes the 253.7 nm mercury line and the noise level and drift are less than 0.0004 absorbance units. Thelinear dynamic range is 0.0001 to 3.0 absorbance unit,s. Range attenuat,ion can be varied from 0.01 absorbance unit to 5.12 full (Continued on page A662)
+
+
-
Figure
''
'lock diagram
Of
a liquid
chromatograph. A eluent reservoir, 8 high prePre,,ure and D injection E the,mo. $totted seoaration column. F detector, and G recorder.
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Journal of Chemical Educofion
high spekd LC based on radioactive emission and polnrogrephy are still in the experimental design stage, but are now beginning to be reported in the literatwe. There is presently no truly "mliversnl" LC detector which in seuitivity and utilit,y compares to the GC thermal wnduct,ivity cell.
PHOTOMETRIC DETECTORS Photometric LC flow detectors have a very wide .range of spplications. These detectors are used mainly in the ultraviolet region of the spectrum, since nnmerous systems of chemical aud biological interest are stroug absorbers. In photometric detectors, the elnted stream passes through a flow cell across which monochromatic energy is transmitted. The increase in absorbance or decrease in ttn~unlittauccof the stream is recurded. .\n rrrc~.prsblcphoromcr~.ir LC detectol. S ;holtld uoiicii weful i t ) hich i.rsolurion I certbin feai.ures including & 8 m d cell volume (a few microliters); readout in absorbance units enabling direct Beer's Law calibrations of peak area; and a high intensity of primary energy providing for high sensitivity. One interesting degree of selectivity possessed by photomet1.i~ detectors of variable wavelength, such as commercially available recording spectrophotometers, is that two ~ o o r l yresolved components eluting from the column could potentially be determined separately by repetitive runs at two differentwavelengths. Also, one is able to discriminate against compounds which are of no interest., such as the solvent. Most UV detectors which are presently marketed, however, are single wavelength detectors and utilize a low pressure mercury lamp. A description of some available units follows.
Chrornalronix UV Photometer
Chemical Instrumentation
Figure 2. Chromatronix U V photometer. tograph courtesy of Chromotronix, Inc.1
The reference cell can a volume of 8 be operated in flawing or static mode. The flow cell in this det,edor is of 8. unique design (18) as shown in Figure 3. The liquid stream is split a3 it enters the cell and the two substreams are rejoined in the detector exit line. This design has been shown to eliminate or diminish sensitivity to flow changes, since equal lengths of the liquid stream with apposing flows srein theoptical path. Thesplit stream design is superior to the "double-L," "U," or "Z" design built into other detectors (18). The photometer operates at 254 nm, however, adaptation for other wavelengths is possible. The sample cell volume is less
than 10 MI and readout is in absovhance. Full scale sensitivity is 0.005 ahsorhance unit; long term drift using camer flow is 0.001 absorbance unit per horn. This photometer is also s. component part in the DuPont 820 liquid chromxtogaph.
(Pho-
scale. The output signal can be selected either in absorbance or transmittance units. Two Teflon and Kel-F plug-in flow cells are ava.ilahle: ( a ) 1 mm bore X 10 mm path length, 8 #I volume; ( b ) 2 mm bore X 10 mm path length, 32 PI volume. Cell ( a ) has Teflon inlet and outlet lines of 3 and 5 #I internal volume. Cell ( b ) has lines of 75 and 125 #I internal volume, respectively. Cells can withstand 500 psi operating pressure. The detector is shown in Figure 2.
DuPont Precision Photometer The DuPont precision photometer is a double beam, double cell precision phot,ometer with a cell pathlength of 1 cm and
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Journol o f Chemical Education
Figure 30. ISCO UA-2 UV monitor with channel alternator and dual beam optical unit. (Photogroph courtesy lnrtrumentation Specialties, Inc.1
instrumentation Specialties Company, Inc. (ISCO)
,..ld"lO
IN
Figure 3. Split stream Row cell used in DuPmt photometer. IRedrawn from Felton 118) with permission of The Journol of Chromatographic Science.)
ISCO manufactures two basic UV monitors, either of which can be used with five different optical systems operating at wavelengths from 254 to 950 nm. The model UA-2 is a double beam monitor capable of linear absorbance recording and has three ahsorbance ranges of 0.1, 0.25 and 1.0 AU on the built in recorder. Using an external recorder, an additionnl (Continued on page A6641
rmge of 0.05 is obtained. Typicd minimum detect,hle signal over hackgro~ind noise is 0.0005 AU and t.ypicnl dvift over a 12-hr period is 0.005 AU. Flow cells have voh~mesof 10 ta 50 pl for opt,ienl paths ranging from 2 to 10 mm. The wnit, is also provided with an aut,omalic nctuato~ which operates an associated fmct,ioa colleet,or. The ISCO model TJA-2 UV-monitor is shown in Figure 38.
phototubes which memure t,he mmple and reference beam, respectively. The optical pnth is shown in Figure 4. Wavelength selection i~ accomplished hy use of inkrference filtera, and t,lu~sx much grcnt,er spertrnl seler:tivit,y is provided sincc continuous sources me used. The flow t.hrongh cell is made of quarts and ha? a 2 mm path length and a vohme of 80 wl. These det,ectors are sold with the J J C 2A liquid chromatograph.
Laboratory Data Control (LDC) UV Monitor The L1)C UV monitor is n high performance continuous flow det,cctor uniquely desiguod for high resolut.ion LC. The optical diagram of t,he detector is shown in Figure 5 and s. photogrnph of t,ho optical and conta.ol unil. in Figure 0. Light, heamn from a eommtm low premnrc merewy sonrce, S, are m,llirnntod by lens I, and passed through tho flow eoll c h a m bers C, and CZ,s. plane window W and a
"" Figure 4. Opticd path used in the Jeolco visible and U V detectors. ICourtesy of Jeolco, U.S.A., Inc.1
Jeolco Visible and UV Detectors These two detectors are essent,irtlly identical except for the light sources which consist of tungsten and hydrogen lamps. The optical system in each case employs n double beam arrangement, and two
A554 / Journal of Chemical Education
I
1
I
Figure 5. Opticd path in LDC UV monitor. 5 mercury source; L collimating lens and cell window; CIand CZAow cell chambers; W plane window; F filter; DL ond D2 detectors, (COWtesy of Laboretory Dato Control.)
Figure 6. Optical and control units, LDC UV monitor. (Photograph courtesy of Laboratory D d o Contml.1
filter F. The heams firdly impinge on d u d ttar~aducer detectors D, and 1 ) ~ . The f,wobeamsoriginate from one common wee of the mercury lamp and therefore me not subject to fluetuationa caused hy dif(erences in lamp hrightneas or condens* tiorr of liquid mercury dropletr. High sensitivii,y and high resolution are achieved by utilizing a cell path length of 10 mm and a cell volume of 8 wl. The minimum detcctahle ahsorhance is listed as 0.0002 absorbance units, and douhle heam linear absorbance readout at 224 nm is in six binary full seek ranges (0.02 to 0.04 ahsorhance unitii). This deteeler ia a component part of the Waters ALGlOO liquid chromatograph. (Contirued an page A6681
filter and finally to the detector. Adjustxble slits enable hmeline corrections. The optical path is shown in Figure 7. The optical module and the control onit are shown in Figure 8. The qnnsts flow cells rome in several sizes ranging from 0.05 to 1.2 ml in vohlme and f ~ o m0.5 to 10 mm in path length. The detector ootpot is linear in pevcent trnnsmibtnnce, thrts its npplirntions far qum~titativennnlysir otilizing high speed liquid chromatography me limited. This detector should find useful npplicntjona in the loention of interesting frnrtians during chromatographic or elect,rophoretic
LKB Uvicord I and II Both detectors utilize a mercnry light source. The Uvicord I can monitor only at 254 nm, wherens the Uvirord I1 can monitor a t 254 nm as well as 260 nm. The second wavelength ir achieved by rtllowh~gthe 254 n m mdintian from the lamp t o esrite fluorescenre in n specially activated quarts rod which floorexes at. 280 nm. The secondary emission is then passed through an interference filter, mt apertnre, the sample cell, s black glnss
~
~.- ~
.
-
(Continued on page A561)
~
Llphl Conrenrr
Fill..
r.rh.n...l.inlrf.rm..
"D.,'"..
P,..A,mP,,,,~,
Black Chrr Flllrr Mear".,np
Cell
,4e1ts1
8
.
!1
Record*. Ovlpvl
Figure 7. Optical path used in LKB Uvicord det-tor.
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Journal of Chemical Education
(Courtesy of LKB Instruments, 1nc.l
Chemical Instrumentation
Figure 8. LKB Uvicord. Control unit on the left, optical unit ot right. (Photograph courtesy of LKB Inrtruments, 1nc.I
sepemtiotions of preparative samples from wide bore columns.
Nerter/Faust UV Absorption Detector This detector is one of several options available in the Nester/Faust model 1200 and 1240 liquid chromatographs. The
I tector. (Photograph courtesy of Nerter/Faurt Manufaduring Corporotion.1 Figure 9. Optical arrangement of h e Nester/ Fourt UV absorption detector. (Courtesy of Nerter/Faust Mmufocturing C0r~orotion.1
instrument consists of a single low pressure meremy vapor lamp, a wavelength
(Catinued a page A584)
Volume 47, Number
9, September 1970
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A561
Chemical Instmentation filter and t,wo qua1.11: cells (sample and reference). The unit is provided with separate channels for dillerential monit,ori n . The reference cell may he left empty (if the solvent does not a b s o ~ h )i,t may be filled with solvent t o serve a5 a static refevenee, or if gradient elution is used, solvent may be passed through it. The phot,ocells w e solid state, UV sensitive, v s m n m photorliodes. Readant is linear in absorbance (0.01 unit full scale). The optical arrangement and a of t,he detector nro shown in Figures 9 and 10. The cell volome is 12 pl with n 1 em p t , h length. The detector e m be used a t two wavelengths, 254 and 366 nm by use of apprraprin fillers.
Varian Aerogroph U V Detector A donhle heam UV deiector made by Vxrian Aerograph is now available as 8. member of the Varinn Aerogmph 4000 and 4100 detectors. I t features high sensit,ivity and low dead volume, and is designed far high resolution LC. A fiinglewavelength (254 nm) is available and the output is lineas in absorbance units. i\laximum sensitivity is 0.003 absorbance unit,s full scale. Drift stability is less than 0.0002 onits/hr with n rninimnm detectable noise of 0.0002. The dual cell system is 1 mm in d i m e t e r , 8 pl in volume, and has a 10 mm path length. The linear dynamic rnnge is 3.2 X lo3. T h e flow-
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Journal of Chemical Education
through cell and amplifier module we packaged in separate units to provide maximum utility and eanvet~ionce. These modules are shown in Figure 11.
Cuvette volumes for this detector rnnge from 0.05 t o 1.0 ml. T h e Technicon colorimeter, shown in Figure 12, is %visibleflow-through detector in which the incident hesm from s tungsten lamp is passed through a n interference filter before traversing t h e flow cell. A second heam is collimated and allowed to shrike a reference photocell. T h e flow cell is tubular and has a volume of 0.05 to 0.1 ml and a. path length of 15 mm. This unit is useful for ehromatazraohic senarations. and has been used far anion exchange chromatography of carboxylie acids (10) in conjunction wlth R Technicon AutoAnalyzer. "
A
Figure 11. Varion Aeragroph UV detector. IPhotogroph, courtesy of Vorion Aerogroph.)
Other Photometric Detectors A number of other phot,omet,ricdetectors suitable for preparative applications are available. T h e Phoenix miero-flow calorimeter is used primarily in ion exchange work and is applied in the Phoenix amino acid analyzer. The instrrrment measures only in t h e visible spectrum a t 440 and 520 nm, and has x 50 p1 flow cell with a 10 mm path length. Schoeffel manufactures a single or double beam "Spectrofla" nseful in t,he UV and visible. lleadant is in percent T units, and the cell volome is not listed. T h e Canslco UV flow monitor (254 nm) is useful only for preparative L C work.
Figure 12. Technicon colorimeter. (Photograph, courtesy of Technicon Corporation.l
(Continued on page A666)
Chemical Instrumentation Specfrophotometers
A number of spectrophotometers have been used successfully as LC detectors. The obvious advantage is selectivity in wavelength. A disadvantage is that an expensive laboratory resenrch instrument is committed for use a? a. LC flow monitor and thus its availability for other experimentation becomes severely limited. The Beckman model UB spectrophotometer hay been nsed as an LC detector (4). Beckman flow cells, however, have a volume of 0.3 ml and are prone to trap bubbles permanently. No special wodensing syslem is avnilnble far focusing the energy on the sample. The Beckman DB as well ss s. Coleman Instruments model 124 spect,rophotomoter have been used successfully by Scott, et al., in the analysis of UV-absorbing constituents in body fluids (8). These instruments were modified by coupling a servo motor with the wnvelength mechanism, so that they wnld operate alternately a t fom different wavelengths in the ultraviolet region (250,260,280, and 290 nm). Special flaw cells manufactured by Pyrocell Menufaeturing Company were employed. The Carl Zeiss PMQ I1 single beam spectrophotometer is well suited for LC work and hns been used with success for several LC separations (5, 1 6 ) . It utilizes a deuterium lamp monochromator, sample changer, and photoelectric detec-
Figure 13. Zeirr PMQ I1 rpectrophotometer. ( P h ~ t o ~ r . ~courtery h of Corl zeirr, Inc.)
tor. The spectral rnnge exlcntls from 185 to 2500 nm. The monochramator slit. can be set continuausly between 0 and 2 mm, either by hand or with nn optical automatic slit unit, however, for LC t,his accessory is redly not necessary. Any recorder which is directly connected to the instrument records transmittance linearly.
Figure 14. Zeirr ultra-micro cantinuour Row cells for the PMQ 1 I rpectrophotometer when used for liquid chromotogrophy. (Photograph courtesy of Corl Zeirr, Inc.)
(Conlinued on page A588)
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Journal of Chemicol Education
($1 K ~ n a ~ ~ wJ. nJ.. ,
For LC, however, it is desirable to record absorbance, and it is therefore necessary to obtain a T-E converter as an additional accessory. Three absorbznce readings (0-1, 0-0.5, and 0-0.2) can be selected on the T-E converter. For continuous flow monitoring of the effluent stream, one needs the miwoeell accessory package. The microcell equipment consists of a. special sample changer with an adjustable sliding carrier on which the vmious microcells with their holders are placed; in addition i t has s n sdjustahle lens. This lens assures that t,he major part of the radiant flux coming from the monochromator traverses the cell at dl path lengths. This means that the spectral hand width is only about 1.7 times as large as for measurements taken with an 5-type cell at the same amplification. The Zeiss ultramicro fiow-through cells consist of a cylindrical, stainless steel body with two connecting nipples for the flowing stream. The cells are 15 and 20 d in volume and 0.5 and 1.0 cm in prtth length respectively. Huber has reported a modification of this cell whereby the variance of the sample peak was decreased to about one-third (16). The spectrophotometer and its associated ultramicrocells are shown in Figures 13 and 14, and the modified cell in Figure 15. A Cary 10-11 ultraviolet spectrophotometer was used successfully as an LC detector by Jentoft and Gouw ($0)in the analysis of high molecular weight hydro-
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Journal of Chemical Education
Figure 15. Cross section of the modified Row cell used with m e l e i s PMQ I1 spectrophotameter. (Redrawn from Huber I161 with permission of The Journd of Chromomwoohic Science.)
carbons using supercritical fluid chromatography. The column effluent was fed t,hrough a specially constructed 20 J flow-through cell with s. path length of 1 cm. A quartz lens was employed to condense sufficient energy on the sample.
REFERENCES (1) H m s m w . J. A. R. J.. Dootorai ~ i a s e r t a tion. University of Amsterdam (1969).
. I . Chromoto~.a d . . 7, 7 (1969). (3) WATBILB, J. L.. LITTGB,J. N., A N D H 0 ~ 0 . 4 ~ . D . F., J. Chromatag. Set.. 7,293 (1968). (4) Y E E N I N ~H.. . GREENWOOD, J. M.. SHANKS. W. H.. AND WILLEPORD. B . R . , Chem. Comm., 1969, 1305. (6) I i n e ~ .J.~ .F. K., K n u w , J. C.. A N D YEEN,NO, H., Paper No. 158. Pitteburgh Conference on Analytical Chemistry and A ~ p l i e d Speotrosoopy, Cleveland, Ohio. Mhrnh, 1970. (6) SCOTT,C. D.. Clinical Chem., 1% 521 (1968). (7) B u m s . C. A., AND WAREEN,K. S., Ciinicd Chem.. 14,280 (1868). ( 8 ) Scam, C. D., JOLLEY. R. L., PI-, W. W.. *ND J o ~ n s o r ,W. F., in press, Am. J . Clin. Patholog% 1970. (Q) K r ~ a ~ * x nJ.. J . , J . Chromatoo. Sci.. 7 , 361 (1869). (lo) H r m ~ n ,J. F. K . , J . Chromelog. Sei.. 7, 85 (1968). (II) Honv*~x.C.. m n Lmsnr, S. R.. J . Chvomotor. Sci., 7, 109 (1968). (18) 81s. S. T.,A N D V A N DEN HOED, N.. J . Chromolos. Sci.. 7, 257 (1969). D. E . , Anol. (2s) LOCPE,D. C., AND MARTIAE, Chem. 39,921 (1967). (14) LOCK=,D. C., J. Gar Chromatog., 5 , 202
~."-.,.
(TOR?>
(26) H v m n , J. F. K . , "Comprehensive Analytical Chemistry," Vol. I I B , pp. 1-54. Elsevier Publbhina Co.. Amsterdsm, 1968. (181 H n e e ~ J. , F. K., J . Chromatoe. Sci., 7, 172 (19691. (17) CONLON,R. D., Anol. Chem., 41. 107A (1969). (28) FELTOX,H., J. Chromaloo. Sci., 7, 13
~."*",. ,lDRO\
(19) ZERFLNC,R. C., A N D VEENINQ, H., Anol. Cham.. 38,1312 (1966). (20) JENTOPT, R. E., A N D DOUW.T. H., J . Chromoloo. Sci., 8, 138 (1970).
Pm/easor Vceninp's oeiclc w'll be continzed in the October issue. A list of manufacturers with tlicir oddiesaes mill be included in the find portion o/ his discvssion of " L i p i d Chiomatography Dctcctors."
