Anal. Chem. 1883, 65, 3487-3500
34@7
High-Precision Liquid Chromatography-Combustion Isotope Ratio Mass Spectrometry Richard J. Caimi and J. Thomas Brenna' Division of Nutritional Sciences, Cornel1 University, Savage Hall, Ithaca, New York 14853
Online liquid chromatography-combustion highprecision carbon isotope ratio mass spectrometry (LCC-IRMS) is demonstrated for the first time with a direct interface to the liquid source. The interface is based on a continuously coatedmoving wire which facilitates reproducible solvent removal and analyte combustion,followed by drying of theCOzand admissionto themass spectrometer. Routine precision and accuracy for compounds analyzed with the interface in the flow injection =B 0.5% over the range -28 < mode is about ~ W P D 6 l W p < ~ ~83. Precision in the liquid chromatography mode is routinely -1% and is limited by sample size. This system expands the range of online compound-specific isotope ratio analysis (CSIA) to thermally labile and nonvolatile compounds. High-precision gas isotope ratio mass spectrometry (GIRMS) based on the classic dual-inlet design is a routine tool for determination of isotope ratios for C isotope ratio analysis.' Modern instruments are capable of determining 13C/W ratios to a precision of -10 ppm relative standard deviation (RSD, coefficient of variation) with a minimum sample of -1 pg of carbon and -Vi0 this sample size for instruments equipped with a cryogenic concentration device. On-line, high-precision compound-specificisotope analysis (CSIA) was f i s t demonstrated in 1978 by Matthews and Hayes,2 using a single-collector high-precision isotope ratio instrument. Their design, based on the original work of Sano et al.," permitted for the first time high-precision determination of carbon isotope ratios for individual chemical compounds separated by gas chromatography. Briefly, separated compounds eluting from a GC column in a stream of He carrier gas are directed to a combustion furnace loaded with CuO as a source of oxygen. Organic compounds are quantitatively combusted, dried, and admitted to the ion source of a high-precision GIRMS instrument.2ia Precisions of RSD = -100 ppm are routinely obtained for samples of >10 ng of carbon. These systems have been applied in an increasingly wide variety of applications, principly, but not ~
~~~
(1)Ehleringer, J. R.; Rundel, P. W. In Stable Isotopes in EcoZogicaZ Research; Rundel, P. W . , Ehleringer, J. R., Nagy, K. A., Eds.; Springer-Verlag: New York, 1988;pp 1-15. (2)Matthewe, D. E.; Hayes, J. M. Anal. Chem. 1978,50,1465-1473. (3)Sano,M.; Yotsui, Y.; Abe, H.; Sasaki, S. Biomed. Mass Spectrom. 1976,3, 1-3. (4)Barrie, A.; Bricout, J.; Koziet, J. Biomed. Mass.Spectrom. 1984, 11,583-588. (5)Barrie, A.; Bricout, J.; Koziet, J. Spectrosc. Znt. J. 1984,3,259-260. (6)Rautemhlein, M.; Habfaat, K.; Brand, W. In Stable Isotopes in Pediatric Nutritional and Metabolic Research; Chapman, T. E., Berger, R., Fbijngoud, D. J., Okken, A., E&.; Intercept: Andover, MA, 1989;pp
133-148. (7)Silfer, J. A.; Engel, M. H.; Macko, S. A.; Jumeau, E. J. Anal. Chem. 1991,63,370-374. (8)Goodman, K. J.; Brenna, J. T. Anal. Chem. 1992,64,108&1095. 0003-2700/93/0365-3497$04.00/0
exclusively, in the areas of organic geochemist+" and metabolism.18-21 The recent increase in applications has been fueled in part by the introduction of commercial systems by at least three manufacturers worldwide. Most applications have focused on determination of variation in isotope ratio due to natural processes or on samples which are not far removed from the naturalabundance range. We recently described and evaluated the coupling of this analytical approach with the use of highly enriched precursor compounds for high-sensitivity biotracer studies in general and fatty acids in particular.8 These studies demonstrated detectionlimits superior to those available with radiotracers or survey studies by organic mass spectrometry and sensitivity comparable to high-sensitivity negative chemical ionization mass spectrometry.22 The principle drawbacks to GC analysis are the requirements for volatility and thermal stability imposed by a gasphase technique. Thermally labile and nonvolatile molecules can be chemically derivatized prior to GC analysis. However, most derivitizing agents add extraneouscarbon to the anal@ molecule, which must be characterizedand taken into account to yield true isotope ratios. Characterization of reagent isotope ratios is not satisfactory since isotope effects at reaction centers give rise to fractionation,23 and therefore isotope effects must be determined for each molecular species and each lot of reagent. Further, derivitization introduces additional steps which increases the possibility of contamination and reduces yields. Since most biological molecules are nonvolatile, we have developed an interface for liquid sources which extends this technique to thermally labile and nonvolatile molecules. (9)Des Maraia, D.J.; Donchin, J. H.; Nehring, N. L.; Truesdell, A. H. Nature 1981,292,826-828.
(lO)Yuen,G.;Blair,N.;DesMarais,D. J.;Chang,S.Nature1984,307, 252-254. (11)Hayes, J. M.; Freeman, K. H.; Popp, B. N.; Hoham, C. H. Org. Geochem. 1990,16,1115-1128. (12)Bernreuther, A.; Koziet, J.; Brunerie, P.; Krammer, G.; Christoph, N.; Schreier, P. 2.Lebensm.-Unters.-Forsch. 1990,191,2-301. (13)Engel, M. H.; Macko, S. A.; Silfer, J. A. Nature 1990,348,47-49. (14)Freeman, K. H.; Hayes, J. M.; Trendel, J. M.; Albrecht,P. Nature 1990,343,254-256. (15)Wakeham, S . G.;Freeman, K. H.; Pease, T. K.; Hayes, J. M. Geochim. Cosmochim. Acta 1993,57(l),159-165. (16)Kennicutt, M. C.; Brooks, J. M. Org. Geochem. 1990,15,193-197. (17)Pillinger, C. T.Int. J. Mass Spectrom. Zon Processes 1992,118/ 119,477-501. (18)Chapman,T.E.;Mulder,I.E.;Reingoud,D.J.;Berger,R.;Baarsma, R.; Van Asselt, W.; Okken, A.; Smith, G. P. A.; Nagel, G. T.; Muskiet, F. A. J.; Lafeber, H. N.; Sulkers, E. J.; Sauer, P.; Potaschick, R.; Moees, S. W.; Guilluy, R.; Paciudi, C.; Normand, S.; Riou, J. P.; Fernandez, J. In Synthesis and Applications ojlsotopically Labeled Compounds; Baillier, A., Jones, J. R., Eds.; Elsevier: Amsterdam, 1988;pp 163-169. (19)Tissot, 5:; Normand, S.; Guilluy, R.; Pacluaudi, C.; Beylot, M.; Laville, M.; Cohen, R.; Mornex, R.; Riou, J. P. Diabetologia 1990,33,
__
449-4.58. .._ -.
(20)Jones, D.M.; Carter, J. F.; Eglinton, G.; Jumeau, E. J.; Fenwick, C. S.BioZ. Mass Spectrom. 1991,20,641-646. (21)Koziet, J.; Gross, P.; Debry, G.; Royer, M. J. Biol. Mass Spectrosc. 1991,20,777-782. (22)Harrison, A. G.;Cotter, R. J. InMethodsoflonization;McCloskey, J. A., Ed.; Academic Press, Inc.: New York, 1990; pp 3-36. (23)Carpenter, B. K.Determinationof0rganicReactionMechniems; Wiley-Interscience: New York, 1984.
Q 1993 Amerlcan Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 85, NO. 23, DECEMBER 1. 1993
With concentration of analyte often below the part-permillion (ppm) range inLC eluent,relatively minutequantities of solvent would be expected to degrade the precision and accuracy of these measurements. For this reason, the key consideration for interface of a liquid chromatograph with a combustion IRMS system is reliable, reproducible solvent removal prior to introduction into the combustion furnace. Currently, the predominant approaches to the interface of liquid sources to organic mass spectrometers are direct liquid injection methods, themospray, electrospray, flow fast atom bombardment, and particle beam systems.24 All versions of the former four techniques admit large quantities of solvent into the mass spectrometer and therefore are not suitable for interface with a furnace. Particle beam does remove solvent prior to introduction to a furnace and has been interfaced to a microwavereaction chamber in a low-precisionapplication.= However, the completeness and reproducibility of solvent removal for that application is not as critical as in highprecision applications. Recently, the same laboratory reportedamovingheltdeviceforcouplingof LCtoamicrowave reactionchamher.26 Again, thedatafrom theselower precision determinations do not apply to high-precision work, and therefore these data do not imply that particle beam is an appropriate approach for the present application. The moving wire interface is based on a classical approach to interfacing liquid sources with flame ionization detectors (FID), which was introduced in 1964n and subsequently developed by a series of investigators.8,-1 Liquid solution containinganalyteisdepositedonamovingwire, whichpasses sequentially through a drying oven to remove solvent and a furnace to volatilize the remaining analyte. Early versions of the interface relied on simple pyrolysis in an oxygen-free atmwphere tovolatilize analyte. Thesesystemswerereplaced with combustion-reduction-based furnaces which expanded the dynamic range and improved reproducibility and quantitative analysis and had substantial commercial success in the 1970s. Improvement in competing selective detection schemes havereduceduse, but amoving transportFIDsystem is currently commercially available, and sensitive universal detection for LC has not been satisfactorily achieved by any other means. In this report we describe a successful implementation of a moving transport-combustion system for highprecision applications.
EXPERIMENTAL SECTION The mass spectrometer used in this work is a Finnigan MAT GIRMS system equipped with multiple faraday cup detectors, amplifiers, and digitizers for simultaneous monitoring of m/z 44, 45, and 46. The instrument is operated at its full accelerating potential of 10 kV and with a source chamber pressure of -2 X 106 Torr. A Pye-Unicam (Cambridge, England) LCM-2 interface was modifiedfor this work. Adiagramof the final mcdifiedinterface is shown in Figure 1. A stainless steel wire from a feed spool passes through a cleaner furnace held at 900 OC, with counterflowing He (4 mL/m). The wire speed was adjusted to 12.5 cm/s, which produced the highest signal. The wire loops around a (24JYergey,A.L.:Edmonds,C.G.;Lewis,I.A.S.;Vcstal.M.L.Liquid
Chramacogroph,lMoss S p e n m m e t ~Plenum ; Prm: New York. 1990. (251MeLean. M. A,: Vestal. M. L.: Abramson. F. P. 39th ASMS
ConfGence M&s Spe&ometrJl'and U i e d Topics, i99l; pp 1392-1393. (26)Moini, M.; Abramaon, F. P. B i d . Mass Spectrom. 1991,ZO. 308-
312. (21)James, A. T.: Ravenhill, J. R.; Scott, R.P.W.Chern. Znd. 1964, 18,146-756. (28) Maggs. R.J. Sci. Znf. Monuf. 1968, I, 43-48. (29) Scott, R. P.W.:Lamenee, J. G. J. Chmmotogr.Sei. 1970,8, B f r 71. (30) van Dijk,J. H. J. Chmmotogr. Sci. 1972,10,31-34. (31) Scott, R. P.W.Liquid ChmmotographyDetectors;Elsevier: New York, 1986.
TO KMS
spool Feed
Flgure 1. Liqum ratio MS.
8.'
spool Collect
chromatography-combustion interface for isotope
bearingandpasses throughacoatingblock. Liquidpassesacroes the wire at the coating block with most of the eluent collected in a waste container. The coated wire passes through a 5-cm drying oven, where solvent is evaporated at 150 "C with 8 mL/ min counterflowing He, and then into the combustion furnace. The combustion furnace (15 em X 5 mm) is loaded with two 8-cm lengths of Cu wire (diameter 0.003 in.) and one 8-em length of Pt wire (diameter 0.003 in.), held at 650 OC, and flushed with 0%gas for 15 h to oxidize the Cu to CuO. When in operation, the furnace temperature is maintained at 850 OC (*2% ), where the following chemical equilibrium exists ZCuO(s) =2CU(S) + O,(g) The O2 reacts with organic material passing through the furnace, giving quantitative combustion to COZ and H20. Consumption of Oz causes the equilibrium to shift to the right and thus serves as a gentle source of 02. He carrier gas enters the stream before the heated zone and sweep the COZout of the combustion region while preserving chromatographic separation. This stream is directed to a 5-cm length of Nafion tubing, which is permeable to the water of combustion but not to COz. The stream emerging from this tube now consists of bands of CO2 in He carrier gas which is then directly admitted to the GIRMS via a fused-silicacapillary of diameter 150 pm and length 5 m. Although an open split between the water trap and maas spectrometer is required for GCC systems: no such device is required for online liquid chromatography (LCC). This is the case since (a) there is no solvent entering the combustion area, and so no back-flush is required to prevent the solvent peak from entering the combustion furnace and prematurely exhausting the oxygen supply, and (b) the combustion furnace is nominally open to atmospheric pressure at the moving wire's point of entry, obviating Conpressure bursts which can occur at the ion source in GCC systems when large amounts of material enter the cornbustion furnace. In the present configuration,thecapillaryfurnace junction serves as a split because of the high gas load in the furnace. The capillary then samples the carrier stream at the appropriate rate for the mass spectrometer. Excess He and COz exit the system along with the wire. Calibrated COz gas from a separate, variable-volume inlet can be admitted directly totheionsourceviaadedicated capillary,bypaeaingtheinterface. An SSI (State College, PA) injector with 6-pL loop and highperformance liquid chromatography (HPLC) pump was used as the liquid source. The sugar separation was carried out on a Whatman Chemical Separations (Clifton,NJ) Partisphere 5 Pac column (4.6 mm X 12.5 cm) with an isocratic solvent system (80%CH2CN-Hz0)at 1.2 mL/min. For fattyacid analysis, the carrier solvent was Optima hexane (Fisher, Pittaburg, PA) and the injector was equipped with a 10-pL injection loop. Two modes of operation,referredtoasflowinjection and liquid Chromatographymodes, have been characterized for the interface. Inflow injectionmode, asingle solvent is continuouslydeposited on the moving wire with a short connecting tube intervening between the injector and the coating block. Pure sample is injected into the stream, deposited in a pulse on the wire, and analyzed with no separation. In LC mode, an LC column is installed and separation occurs prior to analysis. Analytically, thetwomodesaredistinguishedbythebreadthofpeakaobserved in each In general, LC peaks are broader because of diffusion
ANALYTICAL CHEMISTRY, VOL. 65, NO. 23, DECEMBER 1, lQQ3 3499
Table I. Flow Injection: Linoleic Acid Standards. standards
mean
SD
dual inletb difference %
'SC
SD a
A
B
C
D
-27.84 -28.26 -28.77 -27.39 -28.53 -28.31 -29.05 -28.35 -28.82 -28.49 -28.381 0.485 -28.142 -0.340 1.091 80 o.Oo0 55
-24.26 -25.38 -24.53 -24.98 -25.06 -24.60 -25.12 -24.63 -25.60 -25.47 -24.963 0.447 -24.651 -0.312 1.095 57
-20.35 -21.27 -21.30 -21.55 -22.02 -21.40 -21.40 -21.48 -21.24 -21.75 -21.376 0.433 -21.483 0.107 1.099 70 o.Oo0 49
-14.66 -14.77 -14.28 -15.05 -14.02 -14.01 -14.65 -13.81 -15.11 -15.17 -14.553 0.496 -14.056 -0.497 1.107 37 O.Oo0 56
o.Oo0 50
E -0.63 -0.37 -0.28 -0.50
-1.02 -0.26 0.23 -1.47 -0.71 -0.01 -0.502 0.491 -0.160 -0.342 1.123 16 o.Oo0 55
F
G
27.83 28.12 27.83 26.86 27.81 26.60 27.50 28.20 26.52 27.19 27.446 0.618 28.768 -1.322 1.154 56 O.Oo0 69
84.20 83.86 82.97 83.39 83.71 82.37 83.07 83.18 83.45 83.85 83.405 0.533 83.816 -0.411 1.217 44 o.Oo0 60
All data are reported in 6 1 8 c p ~(WO), ~ except where noted otherwise. b Means of four determinations.
during the period of the separation. The nonlinearity of the mass spectrometer ion source is of greater importance at low signal levels, and therefore broad peaks are generally more difficult to calibrate for comparable total sample sizes. Pure compounds were calibrated for W / W ratio by loading 1mg into Vicor tubes containing 200 mg of CuO and 25 mg of Ag flake, baking at 850 "C, and analyzing by conventional dualinlet GIRMS, as described previously.8 All calibrations are traceable to the National Institute of Standards and Technology (NIST)supplied referencematerialRM8541 (USGS24,graphite) with defined W / W . Signal traces for the three measurement channels were processed by vendor-supplied software for peak definition and isotope ratio calculation. In flow injection mode, precision and accuracy were assessed by multiple injections of a series of standards of graded 1sC/12C. Standards were calibrated by dual-inlet analysis of the COz of combustion prior to analysis. Injection quantity was -50 pg. LC mode was evaluated using three sugars, fructose, sucrose, and lactose (Sigma Chemical Co., St. Louis, MOA calibrated by dual-inlet analysis, mixed in equal proportion, and analyzed by LCC-IRMS. Injected quantity for each compound was 300 pg. Isotopically defied standards for linearity determinationwere prepared by spiking natural-abundance linoleic acid (LA; 18 carbons, 2 double bonds) with highly enriched perlabeled linoleic acid extracted from algae grown with WOz as a carbon source, as describedpreviously?* Labeled algaloils were purchased from Martek Corp. (Columbia, MD), and LA was isolated by solidphase extraction and HPLC to a chemical purity of >99%. The in every carbon extracts are typically labeled with >90% position. Highly enriched LA was added to natural-abundance LA, and the resulting spiked material was cut several times with natural-abundance LA to produce a series of exponentially decreasingisotope standards, These standards were analyzed in flow injection mode only and were calibrated against one another and against calibrated COz gas. The baseline signal drift is less than 1mV (10V full-scale)for all three detectors over a period of several hours, indicating that solvent was reliably removed in the drying oven.
RESULTS AND DISCUSSION The 6 standard notation for expressing high-precision carbon isotope ratios is defined as the relative difference in isotope ratio between the sample and an international (32)Cox, J.; Chen, H.; Kabacoff, C.; Singer,J.; Hoeksema, S.; Kyle, D. In Stable Isotopes in Pediatric Nutritional and Metabolic Research; Chapman, T.E., Berger,R., Fbijngoud,D. J., Okken, A., Me.; Intercept: Andover, MA, 1989; pp 165-178.
350
COz (Calibrated)
1 5 0 ~ ~ " ' ~ ' " ' ~ " " " ' ~ " " " ' 0 100 200 300 400 500
"
"
60
Time (s) Figure 2. LCC-IRMS chromatograph. standard, calculated as
(R ~ P -LRPDB)
los RPDB where R, refers to the '3C/W of the sample or international standard. In the case of carbon, PeeDee belemnite (PDB) with isotope ratio l3C/W = 0.011237 2 is used as the standad. Known isotope ratio vs measured isotope ratio over the range -28 < 6l3CpDB < 83 for data calibrated interndy was constructed. Data from 10 determinations along with summary statistics are presented in Table I. The correlated coefficient of a least squares fit to these data is r2 = 0,9999, with a slope of 0.996and an intercept of -0.431%. The average standard deviation associated with these LCC-IRMS determinations is 0.52%. For convenience, the percent l3C corresponding to the 6lscpDB determined by flow injection is presented. These data demonstrate that this system is capable of producing carbon isotope determinations from a liquid source at precision and accuracy comparable to GCCIRMS over a wide range of isotope ratios. An LCC-IRMS chromatograph of three sugars is presented in Figure 2. Mass 44 signal is plotted vs retention time. The two initial peaks are due to COZcalibrant admitted before analyte elutes. Carbon isotope ratios for the three sugars derived from the last three peaks are presented in Table 11. Precision is 6 p 4 ~% for ~ all three compounds. With absolute isotope ratio calibrated against the known value for fructose, the sucrose and lactose SPDB are within 0.4% and 1.9% of their known values. 6'3C(%) =
3500
ANALYTICAL CHEMISTRY, VOL. 65, NO. 23, DECEMBER 1, 1993
Table 11. LCC-IRMS Carbohydrate Analysis. fructose
sucrose
lactose
(-13.27)b
-11.59f -13.92 -13.41 -11.77 -13.60 -13.46
-21.58 -21.69 -21.27 -22.14 -22.64 -21.79
-12.958 1.008
-21.852 0.479
-13.222
-19.963
mean
SD dual inletc difference
-13.269
0.264
All data are reported in b ' 9 C p ~(960). ~ of four determinations.
-1.89
b Internal standard.
Means
The collection efficiency for moving wire interfaces can be estimated as only -0.1 7% of andyte presented at the coating block.28 From a consideration of the signals, we can estimate from previous measurements that the total COZadmitted to the mass spectrometer from these samples is -1 ng. The total error (precision + accuracy) expected for this quantity of COzis GWPDB1-3960, depending on operating conditions.8 The 1-2960 variation observed for LC mode is within these values, and we can therefore conclude that the factor limiting precision and accuracy is the total quantity of COZentering the ion source rather than any inherent limitations of the interface. We have recently completed a separate series of experiments designed to systematically characterize the response of the GCC-IRMS to sample sizes as low as 1fmol of carbon and enrichments up to 25% W.33 At sample sizes below 50 pg, the apparent 13C/12Cratio calibrated against external C02 rises in inverse relation to sample size. Because of the relatively low collection efficiency of the moving wire and the split at the furnace-capillary junction, the total C02 entering the IRMS is in this range. Reanalysis of the LC data with calibration against external C02 revealed on offset consistent with our GCC-IRMS observations. For the present work, the phenomenonappears to be manifested a t somewhathigher total sample sizes because of the much broader peaks observed for liquid introduction, -30 s (fwhh) for LC mode, compared to GC. This is consistent with our observation that the broader peaks observed in LC mode show an offset (-9%) while sharper peaks for flow injection show no offset over the entire enrichment range studied. The offsets observed in the LC mode data may also be explained in part by isotopic fractionation occurring in the liquid chromatography column. Such fractionation is wellknown for HPLC analysis of compounds artificially enriched in D,N as well as for carbon isotopomers analyzed by GCCIRMS. Schoell et al.36 have studied quantitatively the fractionation of carbon isotopomers of steroids and fatty acid
-
-
(33) Goodman, K. J.; Brenna, J. T. 41st ASMS Conference Mass Spectrometry and Allied Topics, 1993; pp 738a-738b. (34) Colaon, C.E.;Lowenatein, J.M. InMethodp inEnzymology,Lipidp; Lowenstein, J. M., Ed.; Academic Press: New York,1981; Part D, Vol. 72, pp 53-109. (35) Schoell,M.;Carleon,R. M.;Moldowan, J. M.;Fago,F. J.;Freeman, K. H.; Hayes, J. M.; Caimi, R. J.;Brenna, J. T., manuscript in preparation.
methyl esters (FAME) eluting from an LC by collecting cuts of single peaks with subsequent carbon isotope analysis. The apparent isotope ratio of dinosteranes analyzed by reversedphase HPLC changes continuously with time over the elution period, starting at a level high as +9% above the average isotope ratio and dropping to-9%. Low-pressure LC analyses of FAME vary systematically over 5460, with 13C isotopomers eluting before all-'% isotopomers. These obmrvationssuggest that isotope ratios would be sensitive to peak detection algorithms,which often fail to include the end of tailing peaks. The net effect is to increase the apparent 13C/12Cratio relative to externally admitted C02, which is precisely the phenomenon observed here. In any case, our data demonstrate that a calibrated standard internal to the analysis mixture can be used successfullyto accuratelydetermine relative andabsolute isotope ratios. The calibration data can be used to estimate that the split ratio at the furnace-capillary junction is about 5 0 1 (2%). This is due in large measure to the excess furnace volume in this particular design, which must be fiied with excess He carrier gas. This effectively decreases the amount of COZ available to the mass spectrometer. Taken together with a coating efficiency of 0.1 % ,the transport efficiency of sample carbon to the ion source is slightly above 106 compared to that achieved in GCC-IRMS. Future designs of the furnace interface should be able to improve upon this figure by orders of magnitude by, for instance, reduction of the ratio of transport volume to furnace volume, thereby minimizing He carrier. In conclusion, the system and data presented here demonstrate for the first time the online interface of a liquid source with a combustion IRMS for high-precision CSIA. Detection limits for online analysis af nonvolatile compounds in the flow injection mode are superior to those for conventional combustion in sealed tubes followed by dual-inlet GIRMS analysis. Internal standards appear critical for accurate assignment of isotopic composition for liquid chromatography mode. The limiting factor for precision of the present system is the quantity of analyte collected on the wire. This observation indicates that the overall approach is sound and can be improved by established approaches for moving transport interfaces. A second-generation system should incorporate provisions for improved collection, which include use of a belt rather than a wire, and investigation of spray methods for analyte deposition which are known to dramatically improve collection efficiency.
ACKNOWLEDGMENT The authors are grateful to Collin Simpson for providing the LCM-2 unit. This work was supported by NIH Grant GM49209 and the USDA-CSRS. R.J.C. acknowledges support from NIH Training Grant DK07158. RECEIVED for review June 30, 1993. Accepted September 10, 1993." ~~~~
0
~
Abstract published in Aduance ACS Abstracts, Odober 15, 1993.
