Putting Opposites Guidelinesfor Successfil SFC/MS
l
n the early days of on-line LC/MS, Arpino likened the union of these two instruments to the unlikely marriage of a bird and a fish: “Many believe this coupling is even more difficult to achieve than the lovematch.. . between two creatures that are at ease in their own environments but are not at home in both” ( 1 ) .At first consideration, this combination of high-pressure and vacuum techniques does seem preposterous. Removing a molecule from its high-pressure solvent, transporting it preferentially over vaporized solvent to a partial vacuum, and imparting a charge on the analyte does seem a challenging feat. However, great progress was made and LC/MS applications have become routine. But what if the analyte were dissolved in a fluid that, at high pressure, solvated like a traditional liquid but transformed to an easily removed gas when the pressure dropped, leaving the analyte free to “flylike a bird? This was the promise offered by supercritical fluid chromatography (SFC)/MS when research in the area began in the late 1960s (2-5). Admittedly, the analogy should not be camed too far. Nevertheless, the commonly used mobile phases in SFC/MS, such as CO,, are much more easily pumped from a high to moderate vacuum system than are
J. David Pinkston Thomas L. Chester The Procter & Gamble Company 650 A
The proper
chroma tographic and mass spectrometric choices can make the diference between success and failure common LC mobile phases. Although modern instrumentation provides nearly routine LC/MS, SFC/MS still offers distinct advantages, some chromatographic, some mass spectrometric. For example, the supercritical fluid mobile phase provides liquid-like interactions with solutes so that species with volatilities too low for GC can be eluted in SFC (6).In addition, because diffusion coefficients are generally higher in supercritical fluids than in liquids, separations of relatively nonpolar species can be performed more quickly in packedcolumn SFC than in LC (7). Similarly, because viscosities of supercritical fluids are lower than those of common solvents, the pressure drop required to produce mobilephase flow is lower, and therefore longer packed columns can be used in SFC than in LC (with correspondingly higher total plate counts [SI).In the MS realm, the effluent from open-tubular SFC columns can be introduced directly into electron ioniza-
Analylical Chemistry, November 1 , 1995
tion (EI) or chemical ionization (CI) ion sources with a very simple interface. E1 and CI have been studied for years and offer great versatility in characterizing unknown mixtures. Despite these advantages, there are relatively few practitioners of SFC/MS, although the range of potential applications warrants greater interest, particularly in industries such as consumer products, fossil fuels, food, and pharmaceuticals. The proper chromatographic and mass spectrometric choices made by the analyst can make the difference between snccess and failure. SFC guidelines
Factors that must be considered for a successful SFC/MS marriage include the types of analytes, injection method, and hardware. Most of our experience has involved using open-tubular SFC combined with the direct fluid introduction (DFI) interface on a triple quadrupole mass spectrometer with an m/z range of 4000 Da per unit charge. Packed-column SFC/MS is possible with this interface and instrument but requires additional pump ing or flow splitting to accommodate the larger mobilephase mass flow rate, especially with a traditional 4.6mm-i.d. packed column. Analytes. Open-tubular SFC with CO, mobile phase works best for low-tomedium-polarity solutes. Analytes that are more polar can be eluted using CO, modified with a polar solvent, or they can often 0003-270095036’-G50A$09000 ’ 1995 Amer can Cnem ca Soc.cl,
be derivatized and then separated using pure CO,. The disadvantage of this a p proacb is not the derivatization but the mass of the derivatives-those derivatives that are double the mass of the original solute effectively halve the solute up per mass limit kom the MS perspective. Nonetheless, this approach is very effective, and the added moieties may also aid in detection and/or structure elucidation. Because SFC can be performed at low temperatures, solutes too labile for GC can be separated.Although the nominal temperatures at the tip of the interface and within the ion source are usually higher than in the chromatographic oven, the actual temperatures experienced by the analytes are much lower because of JouleThompson cooling. In addition, the analytes experience these temperatures for only a brief moment Thus, any observed
degradation usually occurs in the chrotime must be allowed for transport of the matographiccolumn rather than in the in- solute through this extra volume. terface or mass spectrometer. Because the effects of sample inhomoInjection. Injectionsof up to 10 pL geneity are great& exaggerated by sub are usually not difficult with packedmicroliterinjection volumes, the solvent column SFC. Direct injection in a style es- must completely dissolve the sample, and the transport behavior of the solvent in sentially identical to that used in LC usually works well, even on microbore packed the mobile phase must be understood. columns, as long as the analyst rememThis cannot be neglected or underestimated in open-tubular SFC. Solventsthat bers that the injection solvent is usually stronger than the mobile phase. Injection are miscible in all proportions with liquid conditions must be mild enough that sol- CO, are often chosen, and it is erroneutes will be initially retained on the staously assumed that they stay mixed on tionary phase in the presence of injection- transport to the oven. This is not necessarily the case, depending on the chosen solvent modifier. In some cases, the addition of well-swept volume between the temperature and pressure. Figure 1is a pressure-temperature injector and the column may improve phase diagram that shows how binary mixthe peak shapes by providing a means of diluting the injection solvent with mobile tures can exist in a single phase in the inphase, weakening the binary mixture, and jector (at room temperature) and subseimproving the solute focusing. However, quently split into two phases upon trans-
-
Analyiical Chemistry, November 1, 1995 651 A
port to the oven. It is not desirable to deliver large volumes of liquid or high vapor-phase concentmtions of sample solvent to the analytical column, because these fluids are usually much stronger than the pure mobile phase and may d e posit solutes over a large band before d i s sipating. Open-tubular SFC has historically used flow- or timesplitting injection to avoid these pitfalls, and solvent-venting injection and other solvent elimination techniques have been used with vatying degrees of success. However, these techniques add more and often expensive hardware to the system and more steps to the analysis. We prefer direct injection onto a retention gap. Solutes are distributed in broad bands on the retention gap, then focused by the solvent effect or by phase-ratio focusing before migration begins on the analytical column (9).We have injected sample volumes up to 1pL onto Wpmi.d. columns with this approach. Its real beauty, aside from negligible cost, is that it is easy. We have already mapped the phase behavior of 13 C0,-solvent mixtures and can specify appropriate injection conditions (IO).The actual injection technique requires no additional decisions and no special operator skills; the details of the rather complicated mass transfer process take place automatically. Relative standard deviations of absolute peak areas are 1%for most solutes, which allows external standardization. E(uylul(ve.SFC/MS of solutes with molecular masses up to 4ooo Da is straightforward if the SFC instrument can elute the solutes. The pumps available on commercial open-tubular SFC systems are limited to 42.0 MPa (415 atm). A 68.9MPa (680atm, 10,Mx)psi) pump can greatly increase the analysis range. This pump can be added to a commercial SFC system with appropriate safety precantions, but needs a separate controller. Because of relatively low flow rates and small volumes, making proper connections is also critical to success in opentubular SFC. (Packed-column SFC is more forgiving.) An ideal union would be compatible with the full range of operating temperatures and pressures, easy to install, free of any dead volume, reusable, and inexpensive. Avariety of low- and zero dead-volume unions are available that
-
+o **. ,'Reaion wherb.
Figun 1. Prossun-tempentun phase diagram for COrtoiuUIo mixtuns. All COrtoluene mixtlrres. regardless of proportions. exst as a single iquid phase in
the room-temperature inlector i. However. when a plug 01 toluene IS transpnea to tne SFC oven 0. the necessary IlqJid 110 vapor v phase Separation ocxLrs as the fluid is heatea. This phase separation is necessary tor direct injection onto a retention gap. have their advantages and drawbacks. We have recently begun to explore a unionless retention gap-column-restrictor s y s tem that greatly simplifies plumbing the chromatograph.These systems consist of an uncoated but deactivated retention gap and a coated column made from a single piece of fused-silica tubing with an integral flow restrictor fashioned on the end of the column. These work very well as long as the phase is stabilized. In most SFC/MS separationsusing an unmodified supercritical fluid, the mobile phase pressure is programmed to increase the strength of the mobile phase. Because tixed flow restrictors are most often used, the increased pressure means increased flow of mobile phase into the ion source. The rate of increase of mobile phase flow depends significantlyon the type of restrictor used the frit restrictor ( l l ) the , short-tapered or integral restric tor (la,or the tapered capillary restrictor (13).Only the frit and integral restrictors are available commercially. The rate of mobilephase flow increase is less with restrictors that have more turbulent flow characteristics (e.g., integral, uimped metal capillary, and pinhole restrictors) than it is with restrictors that have more l a m i flow characteris tics (e.g., linear, thin-walled tapered, and multipath frit restrictors). The multipath frit restrictor is rngged but is not suitable for many solutes over 2000 Da (14.Integral restrictors are rugged and
652 A Ana/ytica/ Chemisiry, November 1, 1995
-
can be easily cleared by applying heat and pressure. To date, we have found no performance advantages of the tapered capillary over the integral restrictor, though some speculate that the tapered capillaty may perform better with high molecular weight solutes. We recommend the integral restrictor for most SFC/MS applications. The choice of restrictor flow rate involves a compromise. Fast-flowing restrictors (3-10 an/s on a W w open-tubular column) plug less frequently (and are ea+ ier to unplug when they do) than slowflowing restrictorswhen used to analyze relatively nonvolatile analytes. On the other hand, the high mobilephase v e locity means less chromatographic efficiency and a hgher gas load introduced into the mass spectrometer. For practical, day-to-day analyses, we opt for fastflowing restrictors. A traditional, differentially pumped mass spectrometer can maintain a reasonable analyzer pressure when operated with a DFI interface, a 5C-pm open-tubular column, and a fastflowing, integral-stylerestrictor (15). MS guidelines Mass spectrometricchoices that must be made for successful SFC/MS include the type of mass spectrometer, the configuration of the vacuum system, and the form of ionization. But perhaps the most obvious and visible choice involves the type of interface used to connect the chromatograph and the mass spectrometer. Interfoees. Typically, mobilephase flow rates in open-tubularSFC are such that the entire effluent may be directly introduced into the ion source of a modern mass spectrometer designed for GC/ MS, resulting in a DFI interface (4, 16, 17).This simple interface consists of a stem that houses the chromatographic column or a hansfer line that is held at the same temperature a s the chromatographic oven. The tip of the interface houses the flow restrictor and is typically heated at 150-450 "C to counteract the Joule-Thompson cooling of the expansion and to provide some volatility to the eluting analytes. The tip is usually positioned so that the effluent is introduced directly into the ionization region. Given its simplicity and flexibility, the DFI interface is used most often.
Because the mobilephase flow rates of conventional packed, microbore, and packed-capillary SFC columns are typically too high for direct introduction to the ion source, a variety of mobilephase elimination and flow-splitting interfaces (based on similar LC/MS interfaces) have been devised for packed-column SFC/MS. The two primary mobile-phase elimination interfaces are the moving-belt (18) and particlebeam (19) interfaces. The advantage of these interfaces is that they can “divorce” the chromatograph from the mass spectrometer. Chromatographic separation,analyte ionization, and mass analysis can each he performed under optimized conditions. Once transported into the ion source, the analytes are volatilized by heating the moving belt or the particle, so thermally labile analytes may suffer some degradation. Most attempts to use the particlebeam interface for SFC/MS have used interfaces designed for LC/MS with few moddcations (19).These attempts have generally resulted in markedly poor limits of detection, which have been attributed to the differencesbetween solvent evaporation and particle formation in LC and in SFC. A recent particle-beam interface designed specifically for SFC/MS performs much better (20). The simplest splitting interface is the pre-expansion splitting interface, in which a portion of the chromatographic effluent from the packed column is directed to the mass spectrometer using a DFI interface (21). The balance of the effluent goes to other detectors or is discarded. Other packedcolumn interfaces may be characterized as postexpansion splitting interfaces, in which the entire expanded effluent is directed to an ionization region. Much of the effluent, as well as many of the ions formed,is pumped from the ionization region and never reaches the massanalysis region of the mass spectrometer. Prominent members of this particular class of interfaces are the thermospray (22,23), the heated nebulizer for atmospheric pressure CI (24,25), the electre spray (269,and the “high-flow-rate”interfaces (27,28). These interfaces have an advantage over the pre-expansion interfaces in that focusingfields in the ion-sampling region may enhance the total number of ions di-
weight greater than a few thousand. Interfacing SFC to the ion source of sector mass spectrometers, which generally o p erate at voltages of 3-10 kV, has been accomplished with carefully designed probes (1629). The vacuum systems of most modem sector instruments can e m ily handle the gas load of open-tubular SFC. Traditionally. the primary disadvantage of sector instrnments has been their high cost relative to that of quadrupole instruments. This differential is shrinking as lower cost sector instruments reach the market. The FT-MS instrument offers ultrahigh resolution, simultaneous detection of all ions, and a wide mass range. Yet, the performance of most SFC/FT-MS combinations described in the literature suffers from the high SFC gas load and a longerthan-usualinterface line (30,31). A differentially pumped extemal ion source would remove these obstacles. The cost of an FT-MS instrument has traditionally been higher than that of many other mass spectrometers. The Paul ion trap can provide high sensitivity and a reasonable mass range, d e spite its small size and relatively low cost. However, when used for SFC/MS, it has problems dealing with the high SFC gas load (32,331. Ion traps usually operate with arelatively high pressure of helium as damping gas within the trap. Most SFC mobile phases are not good damping T instrument, a differHowever, MS/MS is a sequential-in-space gases. As with the F entially pumped external ion source couexperiment with a quadrupolehased instrument and requires multiple quadru- pled to a Paul ion trap should provide good performance. poles, which raises the cost of quadruAlthough timeof-flight (TOF) mass pole-based MS/MS instruments significantly. This stands in sharp contrast to the spectrometers designed for the detection of chromatographic effluents are not sequential-in-time MS/MS available at widely available today, advances in highrelatively little additional cost on the Fourier transform (FT)and Paul ion trap in- speed electronics and technology may soon give the nod to TOFMS for sensistruments. The low-energy (generally up tivity, cost, and versatility (34,35). Howto ZO@ev)collision-induced dissociation ever, even when the choice of mass spec(CID) available on quadrupole MS/MS instruments is sufficient for most analytes trometer has been made, other choices r e main that are at least as, if not more, with m/z below a few thousand. critical to success. The two most imporModern sector mass spectrometers tant are the vacuum system and the m/z provide high sensitivity, high resolution, and higher m/z range (up to 800~10,000 range of the instrument. Vacuum system Difterential pump for research-grade instruments) than typiing became popular in the early GUMS cal Paul ion trap or quadrupole mass instruments. In this approach, the ion spectrometers. They also provide highsource and mass analyzer vacuum reenergy CID for MS/MS, which can be gions are isolated from each other, with critical for molecules with molecular
rected to the mass analysis region. Of the postexpansion interfaces, the thermospray and heated nebulizer, used for atmospheric pressure ionization (API), are used most often. They are both reasonably simple and have good performance characteristics. Because the heated nehulizer operates at atmospheric pressure, the chromatograph and mass spectrome ter can operate more independently, and making alterations or adjustments to the interface is easier. Instrumentation. The quadrupole mass spectrometer (4,17) still holds the winner’s hand of positive attributes for SFC/MS. High sensitivity, reasonable m / z range (up to 4000 for research-grade instruments), moderate cost, and straightforward interfacing are among the reasons most practitioners have chosen quadrupole instruments for their laboratories.
The most obvious and visible choice is the type of MS inteqace.
Analytical Chemistry, November 1, 1995 653 A
the exception of a small slit or hole to allow passage of the ions. The analyzer can op erate at much lower pressure than the ion source and thus provide good performance despite a relatively high gas load entering the source. Recent design improvements have made instruments more tolerant of high gas loads because economic pressures have pushed instrument companies to manufacture singly-pumped systems. Commercial quadrupole mass spectrome ters are now available with a variety of vacuum systems, from low-end benchtop instrument systems with a single highvacuum pump to high-end researchgrade instruments that are differentially pumped. Most sector mass spectrometers are differentially pumped. Although singly-pumped instruments perform satisfactorily under some conditions in SFC/MS, the analyzer pressure rises to unacceptable levels and the performance drops at higher SFC mobilephase flow rates. For this reason, we strongly advise using a differentially pumped mass spectrometer for SFC/MS to provide satisfactory petformance over a wide range of conditions. Under certain circumstances, supplemental pumping may be required for a differentially pumped system (1.5. Range of m/z. Quadrupole mass spectrometerswith upper m/z range limits between 650 and 4000 have been used for SFC/MS. What is the "best" m/z range for a mass spectrometer for SFC/MS? The answer depends on the application for which the instrument is intended. If the anticipated analytes are low molecular weight, thermally labile compounds, then an upper mass range limit below m/z loo0 may be satisfactory. However, if the analytes will be higher molecular weight compounds of relatively low volatility, such as non-ionic surfactants or oligomeric species, a higher upper m/z range limit may be appropriate. Ultimately, the choice in upper m/z limit often pits cost against anticipated applications. The higher-cost research-grade mass spectrometers will be applicable to a wider range of analytes. Zonization mode. Open-tubular SFC using a DFI interface and a traditional E1 or CI source gives the analyst a good deal of flexibility.E1 is advantageousbecause
it is the most widely accepted ionization method for structure elucidation. E1 fragmentation mechanisms have been studied for years, many are well understood, and large libraries of E1 spectra have been compiled that may be used for automated searching and matching. CI provides a great deal of flexibility in the amount of internal energy deposited in the analyte upon ionization. A spectrum can be produced with a little or a lot of fragmentation,depending on the proton affinities of the analyte and reagent ions produced in the CI plasma of the reagent gas. Thus, reagent ions with proton affinities near those of the analytes can be used to produce spectra with little fragmentation for mixture analysis or to provide precursor ions for tandem MS experiments. Lower proton-affinity reagent ions can be used to produce a spectrum with more kagmentation for singlestage MS structure elucidation. There has been a good deal of discus sion on the influence of the SFC mobile phase on ionization when the DFI interface is used (36,371.With a fixed flow r e strictor, the partial pressure of the mobile phase in the ion source increases over the course of a pressure-programmed separation. In addition, fast-flowing - restrictors (higher linear velocities) mean higher pressures of mobile phase in the ion source. Despite these variables, certain general conclusions can he drawn. In E1 SFC/MS, mobilephasemediated chargeexchange ionization occurs at high mohile-phase flow rates (i.e.. at high mobile-phase pressure or when using a fast-flowingre strictor). This is generally not a great d i s advantage. T h e ions generated in the chargeexchange plasma of the CO, mobile phase have recombination energies that allow ionization and fragmentation of virtually all organic compounds. The spectra produced when chargeexchange conditions prevail in E1 SFC/MS resemble E1 spectra that can be searched in E1 libraries. However, the chargeexchange spectra usually exhibit less fragmentation because of differences in the amount of internal energy deposited and because of collisional stabilization. Recently we have shown that a more open E1 source, combined with a relatively fast-flowing integral restrictor, pro-
654 A Analytical Chemistry, November I , 1995
1396.8 [M+18]*
304.2
I m/z Figure 2. SFCIMS reparation of a functionalizedpolydimethyl. siloxane.
(a) Reconstructed ion chromatogram 01 0.06 pL of a solution 01 a modified polysiloxane injected directly onto a unionless retention
gap-column-restrictor system. The mobile phase was CO,. The mass spectrometer was scanned from ndz 100 to 2000 every 1.9 s. (b) NH, CI spectrum 01 the oligomer containing 16 dimethylsiloxane units from the chromatogram in (a).The spectrum shows an ammonium adduct ion at nVz 1396.8,a small protonated molecule, and an ion corresponding to loss 01 water from the protonated molecule at ndz 1361.8.
vides true E1 conditions up to the 56.7MPa limit of our SFC pump. We obtained these results using probe analytes that bracket the recombination energies of the reagent ions that exist in the CO, chargeexchange plasma. CI spectra are
inherently more variable because of the number of parameters that iniluence these spectra (reagent gas and its pressure, ion source configuration, and temperature). Therefore, there is less agreement in the literature on the influence of the SFC mobile phase on CI spectra Collisional stabilization and charge-exchange ionization likely occur at high SFC flow rates. Recently Sadoun described ESI for SFC/MS (2s)using an interface that accommodated flow rates typical of opentubular and packed-capillary SFC. The nebulizing effect of the expanding mobile phase allowed significantly higher flows of a polar organic modifier (methanol) than are possible in traditional ESI for LC/MS. However, memory effects were observed from analytes deposited on the electrosprayneedle and the authors suggested using a sheath flow of polar organic solvent to eliminate this problem. We have since designed and tested a sheathflow i n t e h e for ESI SFC/MS (38).This interface allows the use of unmodified CO, for the mobile phase, is compatible with open-tubular and packed-column flow rates, and can be used for a variety of polar and nonpolar analytes. Postexpansion splitting interfaces are most o k n used in packedcolumn SFC/ MS. With these interfaces, the mode of ionization is often dictated by the interface and mobile phase. The high mobile phase flow rate associated with these interfaces typically allows only high-pressure ionization mechanisms such as CI. When a polar organic modifier is added to the mobile phase, reagent ions from the modifier often dominate the CI plasma, but this is generally not a disadvantage. When an interface incorporating AF'I is used, the CI mechanisms typical of AF'I are usually o b served. In many cases this consists of water CI, if traces of water are present in the ionization region. Applications Nonpolar polysiloxanes are important active components in many industrial and consumer products. They may be present at relatively low concentrations and found with many other components. The distribution of the polysiloxane, as well as the nature of the terminal groups or of a functionalized moiety, may reveal important information about the process used to syn-
thesize a larger, siloxanecontaining polymer. Siloxanes with molecular weights of up to 20,000 are readily amenable to characterization by SFC, provided the proper type of flow restrictor is used (14). F i r e 2a shows the reconstructed ion chromatogram of a functionalized polydimethylsiloxaneinjected dirrctly onto a unionless retention gawolumn-restrictor system. Figure 2b shows the NH, CI spectrum of one of the peaks from F i r e 2a, t h e oligomer containing 16 dimethylsiloxane groups. We believe that this oligomer is capped with a phenyl group on one end and an alkyl chain bearing a hydroxyl group on the other. The analysis of more polar ethoxylated surfactants has been a traditional strength of SFC (39).For example, ethoxylatedalcohols are complex mixtures of considerable industrial importance. Characterization of the chain lengths and branching patterns of these alcohols and the dishibution of the ethoxylate chain are important to ensure not only proper perfor mance but also environmentalcompatibility. Ethoxylatechains that are longer than 10-15 units are not s&icientlyvolatile to be amenable to traditional GC sep aration, so a combination of GC and LC is used. The alcohol distribution is characterized by GC after the ethoxylate chain is cleaved, and the ethoxylatedistribution is obtained by LC after a chromophore is added by derivatization. In contrast, a sin-
-
-
gle SFC separation provides both alcohol and ethoxylate distribution data without derivatization because ethoxylated alcohols can be eluted with pure CO,, which is compatible with flameionization detection. SFC/MS is used to confirm peak identities in new or unusual samples and is especially useful in studying byproducts or other species that are present at low levels. Mebeverine. an antispasmodicagent marketed in Europe, is dif6cnlt to determine at trace and ultratrace levels because it irreversibly binds to GC columns and suffers thermal degradation. The LC method is satisfactory with a detection limit of 10 ng/mL of plasma, but analysts seeking a lower detection limit came to us to see whether SFC/MS could do the job. Ammonia CI and selected ion monitoring of mebeverine and of Dsmebeverine, the stableisotopelabeled internal standard, provided the improved detection limit (40). As in GC and LC, proper deactivation of the SFC column was necessary to achieve these d e tection limits. Organic ions are usually too polar to be eluted with pure CO,. However, they can often be made soluble in CO, by chemical derivatization.We faced a problem involving low molecular weight (< 4500) poly(acrylic acid) (F'AA). Project team members believed that certain terminal groups on the polymer chain might adversely affect the performance of the product into
-
-
400.0
1144.5
344.0
I
Figure 3. NH, CI mass spectrum of then I6 oligomer ot derivatized poly(acrylicacid). (Adapted with permissionfrom Reference 41.)
Ana/ytica/ Chemistry, November 1, 1995 655 A
R e p or t /
Characterization 01 Polymers: Hyphenated and Multidimensional Techniques his important new volume presents an overview of some of the significant deveiopmentl in the use of hyphenated multidimensional separation methods for polymer characterization. Divided into three sections, the book covers: I general considerations I light scattering and viscometry I analysis of compositional heterogeneity and blends. Among the chromatographic separatioi techniques discussed are size-exclusioi chromatography, liquid chromatograph! and field-flow fraction methods used in conjunction with information-rich detectors such as molecular size- or compositional-sensitive detectors and that are coupled in cross-fradion modes. Valuable reading for both academic ana industrial scientists developing chromatographic methods for polymers or conducting polymer research. Theodore Provdor, The Glidden Company, Editor Howard G. Barth, DuPont, Editor Marek W. Urban, North Dakota State University, Editor Advances in Chemistry Series NO. 241 314 pages (1995) Clothbound ISBN 0-8412-31Z-X $124.95
w -
American Chemical Society 1155 Sixteenth Street, NW Washington, DC 20036 Or CALL TOLL FREE
which the PAA was incorporated, but the P M supplier would not (or could not) reveal the nature of the terminal groups. We performed an SFC separation of the PAA after formation of the te*t-butyldimethylsilyl OBDMS) derivative (41). F i e 3 shows the NH, CI SFCIMS spectrum of one of the oligomers. Notice the successive losses of 186 Da, corresponding to TBDMSderivatized acrylic acid, the oligomeric unit (The most abundant isotope of the ammonium-adduct ion cluster is shifted by one mass unit b e cause of the silicon isotopes.) Using data from the E1 and CI SFC/MS separations, we postulated that the terminal groups were sulfonate and hydrogen, which was subsequently confirmed by the supplier. Certain choices favor a more successful marriage between SFC and MS. SFCIMS has some distinct advantages, especially in applications where GCIMS and LCIMS are difficult. Complex mixtures, such as surfactants, emulsiiiers, low molecular weight polymers, fats, oils, and waxes, that have relatively low volatility can benefit from the versatility offered by SFCIMS.
~~~~
(in Washington, DC 872-4363)and USI
your credit card! F A X 202-872-6067.
ACS
K E , Lee.M L A n d Chem 1988.60,
__
7715-14 _. __.
Pinkston,J. D. et al.Ana1. Chetn. 1988, 60,96M. Berry, A J.; Games, D. E.;Perkins, J. RI. Chrnmafw.1986,363,14748. Edlund. P. 0.: Henion. J. D. I. Chromafop.Sci 1989.27,nk.. Jedrzejewslri, P.T.; Taylor, L T.I. Ckrcm a b p . A 1995,703,489-501. Holzer, G.; Deluca, S.; Voorhees, K J. HRCCC 1985,a,528-31. Balsevich,J. et d.I. Naf. Prod 1988.5Z. 11%77. . . Sunders. C. W.:Tavlor.L.T.: Wilkes. 1.: Vestal, M; Am. h..I990.,'%i 653,'(241 Huang. E.C.. WachsT.; Conboy. J. J.: H e nion, J. D. Anal. Chem. 1990.62.713A724 A (25) Tyrefors, L.N.; Moulder, R X; Markides. K E. Anal. Chem. 1993,65,283540. (26) Sdoun, F.;Virehier, H.; Arpino, P. J.I. Chwmafom.1993.647.351-59. (27) Smith, R b.;Udseth, H.'RAwl. Chem. '
19 2-77 59 1%22 . . .,. ., . . (28) Cousin, I.;Arpino, P. J.I. Chromntogr.
1987,398,125441. (29) Kalinoski. H.T.; Udseth, H. R, Chess, E.K; Smith. R D.I. Chwmafopr. . 1987, 394,3-14. (30)Lee,E.D.; Henion,J. D.; Cody, R B.; Kin* inger, J. A And. Chem. 1987,59,130912. (31) Baumeister, E.R, West, C. D.: ljames, C. F.:Wilkins, C. LAnal. Ckem. 1991, 63,25165. (32)Todd, J.FJ. et al. Rapid Commun. Mass Spectwm. 1988,2,5M. (33)Pinkston.J. D.; Delaney,T. E.; Morand, K L:Cooks, R G.Anol. Cketn. 1992.64, References 1571-77. (1) Wino, P. J. TIAC 1982,I , 154-58. (34) Sin.C.;Pang,H.;Luhman,D.M.:Zorn,J. (2) Milne, T. A Int. I. Mass Specfrom. Ion Anal Chem 1986,58,487-90. fip. 1969,3,153-55. (35) Schultz,G.A. et d.I.Uiwmafop.1992, (3)Giddings,1.C.; Myers, M. N.;Wahrhaftig, 5so,329-39. A. L. Int. J. Mass Specfrom. Ion Phys. (36) Houben, R J.: Leclercq, P. A, Cramers, 1970,4,9-20. C.A.I. Chromatog. 1991,554,35168. (4)Smith, R D.; Felix, W.D.; Fjeldsted,J. C.; (37) Kalinoski. H.T.; Hargiss, L. 0.J. Ckro Lee. M. L. Anal. Chem 1982,54,1883mafop. 1990,505,199-213. 85. (3s) F'inkston. I.D.; Baker, T. R Rapid Com(5) Smith, R D.; Kalinoski, H. T.; Udseth, H. mun. Mass Spechm., in press. R Mass S p e d w n REV. 1987,6,44>96. (39)Pinkston,J. D.; Bowling, D. J.; Delaney, (6) Chester, T. L.; Pinkston, I. D.; Owens, T. E.I.C b m a f o m . 1989,474,97-111. G. D. Cadohydr. Res. 1989,194,273-79. (40)Pinkston,J. D. et aLJ, Chwmafop. 1993, 0 Schleimer,M.; Schurig,V. In Analysis with 622,209-14. Subwrnticd Fluidx Ertmctiox and C h m (41) Pinkston,J. D.; Delaney,T. E.; Bowling, m&ru#hy. Wenclawiak. B., Ed.; Spring. D. 1.J Mioocol. Sep. 1990,2,181-87. er-Verlag:Berlin, 1992; pp. 134-50. (8) Berger.T. A;Wilson. W. H. And. Chem. 1993,61451-55. J David Pinkston, of Procter & Gamble's (9)Chester, T. L.;Innis, D. P. Anal. Chem. Corporate Research Diuision, focuses on 1995,67,3057-63. (10) Ziegler, J. W.; Dorsey,J. G.; Chester, T. L: SFC and coupling microcolumn separaInnis, D. P. Anal. Chem. 1995,67,4% tions to MS.Thomas L. Chester, head of fil. Procter & Gamble's Separations and Opti-
1-800-227-5558 ~
(16) Huang, E C ,Jackson, B J , Marludes.
Publications Catalog now avaiiabii on Internet: gopher acsinfo.acs.org or URL http://pubs.acs.org
656 A Analytical Chemistry, NovemLmr 1, 1995
~~
~
~~~~~~~
~
~~
~~~
~
Cones, H.J.; HeiUer. C. D.; Richter. B. E.; Stevens. T. S U S Patent 4 793 920,1988. Gurhrie. E. J ; SchwKtz. H. E./. Ckrcmabp'. Sei. 1986,24,23641. Chester, T. L; Innis. D. P.; Owens, G. D. Anal Chem. 1985,57,2243-47. Pinkston,I. D.; Henkhel, R T.J. High Resolut. Chromatom. 1993,16,26%74. pinkston,J. D.; Bowling, D. J. Anal. Chem. 1993,65,3534-39.
cal SpechoscopySection,focuseshis work on analytical uses ofsupercriticalpuids, highresolution chromatography, and separations theov. Address correspondence about this arlicle to Pinkston at Procter & Gumble, Corpomte Research Division, Miami Valley laboratories, PO.Box 538707,Cincinnati, OH 452534707,
