Mass Spectrometry - American Chemical

Chapter 14

Downloaded by UNIV OF MASSACHUSETTS AMHERST on May 29, 2018 | https://pubs.acs.org Publication Date: February 22, 1990 | doi: 10.1021/bk-1990-0420.ch014

Application of Combination Ion Source To Detect Environmentally Important Compounds M. L. Vestal, D. H. Winn, C. H. Vestal, and J. G. Wilkes Vestec Corporation, 9299 Kirby Drive, Houston, TX 77054

A new u n i v e r s a l i n t e r f a c e and combination ion source is d e s c r i b e d which allows choice of i o n i z a t i o n modes among e l e c t r o n impact (EI), chemical i o n i z a t i o n (CI), and Thermospray. R e s u l t s obtained with t h i s system on a Vestec Model 201 LC-MS a r e presented f o r some t e s t compounds and some environmentally important compounds on the Appendix VIII list. The r e l a t i v e advantages of t h e d i f f e r e n t i o n i z a t i o n modes f o r compound i d e n t i f i c a t i o n and q u a n t i t a t i o n are d i s c u s s e d and data are presented on t h e performance o f the system. Thermospray i s w e l l - e s t a b l i s h e d as a p r a c t i c a l technique f o r LC-MS i n t e r f a c i n g and i s now being used r e g u l a r l y i n more than 200 l a b o r a t o r i e s f o r a v a r i e t y o f a p p l i c a t i o n s . (1) Despite t h i s obvious success, i t s a p p l i c a t i o n t o environmental a p p l i c a t i o n s has been l i m i t e d by the f a c t t h a t fragmentation i s o f t e n i n s u f f i c i e n t t o allow unambiguous compound i d e n t i f i c a t i o n . The development o f the "MAGIC" i n t e r f a c e by Browner and coworkers (2) has demonstrated the f e a s i b i l i t y o f o b t a i n i n g EI s p e c t r a o n - l i n e with LC s e p a r a t i o n , but the present v e r s i o n s o f t h i s technique have d i f f i c u l t y handling the higher flow r a t e s o f aqueous media encountered with reversed phase chromatography using standard 4.6 mm columns. The combined Thermospray/EI system d e s c r i b e d below was developed i n an attempt t o overcome the l i m i t a t i o n s o f these e a r l i e r i n t e r f a c e s . 0097-6156/90/0420-0215$06.00/0 © 1990 American Chemical Society Brown; Liquid Chromatography/Mass Spectrometry ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

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Thermospray U n i v e r s a l I n t e r f a c e The new Thermospray " U n i v e r s a l I n t e r f a c e " was been developed t o allow HPLC t o be p r o p e r l y coupled t o conventional EI and CI mass spectrometry. A block diagram of the new i n t e r f a c e i s shown i n F i g u r e 1. The LC e f f l u e n t i s d i r e c t l y coupled t o a Thermospray v a p o r i z e r i n which most, but not a l l , of the s o l v e n t i s v a p o r i z e d and the remaining unvaporized m a t e r i a l i s c a r r i e d along as an a e r o s o l i n the high v e l o c i t y vapor j e t which i s produced. The operation and c o n t r o l of the thermospray device has been d e s c r i b e d i n d e t a i l elsewhere. (1) The Thermospray j e t i s introduced i n t o a spray chamber which i s heated s u f f i c i e n t l y t o complete the v a p o r i z a t i o n process. Helium i s added through a gas i n l e t i n s u f f i c i e n t q u a n t i t y t o maintain the d e s i r e d pressure and flow r a t e . The f r a c t i o n of the s o l v e n t v a p o r i z e d i n the thermospray v a p o r i z e r and the temperature of the d e s o l v a t i o n chamber i s adjusted so t h a t e s s e n t i a l l y a l l of the s o l v e n t i s v a p o r i z e d w i t h i n the d e s o l v a t i o n r e g i o n . The Thermospray system allows very p r e c i s e c o n t r o l of the v a p o r i z a t i o n so t h a t a l l of the s o l v e n t can be vaporized while most of even s l i g h t l y l e s s v o l a t i l e m a t e r i a l s w i l l be r e t a i n e d i n the unvaporized p a r t i c l e s . A f t e r e x i t i n g the spray chamber, the a e r o s o l c o n s i s t i n g of unvaporized sample p a r t i c l e s , s o l v e n t vapor, and i n e r t c a r r i e r gas, passes through a condenser and a countercurrent membrane separator where most of the s o l v e n t vapor i s removed. The r e s u l t i n g dry a e r o s o l i s then t r a n s m i t t e d t o the mass spectrometer using a momentum separator t o increase the c o n c e n t r a t i o n of p a r t i c l e s r e l a t i v e t o t h a t of the s o l v e n t vapor and c a r r i e r gas. The c o u p l i n g between the gas d i f f u s i o n c e l l and the momentum separator employs a l e n g t h of t e f l o n tubing, t y p i c a l l y 4 mm ID and as much as s e v e r a l meters long. The length of t h i s connection i s n o n c r i t i c a l s i n c e the dry a e r o s o l i s t r a n s m i t t e d with no d e t e c t a b l e l o s s i n sample and a n e g l i g i b l e l o s s i n chromatographic f i d e l i t y . The apparatus i n i t s simplest form i s shown s c h e m a t i c a l l y i n Figure 2. The Thermospray v a p o r i z e r i s i n s t a l l e d i n the heated d e s o l v a t i o n chamber through a gas-tight f i t t i n g . The helium i s introduced through a second f i t t i n g and flows around the Thermospray v a p o r i z e r and e n t r a i n s the d r o p l e t s produced i n the Thermospray j e t . In general the c a r r i e r gas flow r e q u i r e d i s a t l e a s t equal t o the vapor flow produced by complete v a p o r i z a t i o n of the l i q u i d input. The heated zone w i t h i n the spray chamber should be s u f f i c i e n t l y

Brown; Liquid Chromatography/Mass Spectrometry ACS Symposium Series; American Chemical Society: Washington, DC, 1990.


14. VESTAL ETAL.

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long t o allow the p a r t i c l e s t o approach thermal e q u i l i b r i u m with the vapor phase. The minimum temperature f o r complete v a p o r i z a t i o n o f the s o l v e n t i s t h a t a t which the vapor pressure o f the s o l v e n t i s j u s t g r e a t e r than the p a r t i a l pressure o f the completely v a p o r i z e d s o l v e n t a t the p a r t i c u l a r flow o f l i q u i d employed. I d e a l l y , the e f f l u e n t from the d e s o l v a t i o n r e g i o n c o n s i s t s o f d r y p a r t i c l e s o f unvaporized sample, s o l v e n t vapor a t a p a r t i a l pressure somewhat l e s s than one-half o f the t o t a l pressure, and the balance i s the c a r r i e r gas. Flow v e l o c i t i e s through the system are not c r i t i c a l , but must be high enough t o e f f i c i e n t l y c a r r y the a e r o s o l , but not so high as t o cause extensive t u r b u l e n c e . Under the c o r r e c t flow c o n d i t i o n s i n which e s s e n t i a l l y laminar flow i s maintained the a e r o s o l i s c a r r i e d p r e f e r e n t i a l l y by the higher v e l o c i t y gas stream near the c e n t e r o f the tube, and the a e r o s o l p a r t i c l e s can be t r a n s p o r t e d f o r l a r g e d i s t a n c e s with n e g l i g i b l e l o s s e s . Because o f the very l a r g e mass o f these p a r t i c l e s r e l a t i v e t o the gas molecules, the d i f f u s i o n c o e f f i c i e n t s f o r the p a r t i c l e s i s s u f f i c i e n t l y small t h a t d i f f u s i o n o f a e r o s o l t o the w a l l s i s very slow, and the p a r a b o l i c v e l o c i t y p r o f i l e provides an aerodynamic r e s t o r i n g f o r c e which c o n t i n u a l l y pushes p a r t i c l e s toward the center of the tube where the gas v e l o c i t y i s h i g h e s t . On the other hand, d i f f u s i o n o f s o l v e n t molecules i n the c a r r i e r gas i s r e l a t i v e l y r a p i d . As e f f l u e n t passes from the heated zone o f the spray chamber t o the unheated zone o f the condenser the vapor may become supersaturated and begin t o condense on the w a l l s . The condenser and t r a n s i t i o n r e g i o n are arranged as shown i n F i g u r e 2 so t h a t the l i q u i d condensate flows under the e f f e c t o f g r a v i t y t o the d r a i n where i t i s pumped away t o waste. A small p o s i t i v e displacement pump, such as a p e r i s t a l t i c t u b i n g pump, i s used t o pump away the l i q u i d without a l l o w i n g a s i g n i f i c a n t amount o f gas or vapor t o escape. Membrane Separator The unique p a r t o f the U n i v e r s a l I n t e r f a c e i s the membrane separator or gas d i f f u s i o n c e l l which allows the s o l v e n t vapor t o be e f f i c i e n t l y removed with e s s e n t i a l l y no l o s s o f sample contained i n the a e r o s o l particles. In t h i s device the a e r o s o l i s t r a n s p o r t e d through a c e n t r a l channel bounded on the s i d e s by a gas d i f f u s i o n membrane o r f i l t e r medium which i s i n contact with a countercurrent flow of a sweep gas. For EI mass spectrometry helium appears t o be most u s e f u l f o r both the c a r r i e r and sweep gas. The p r o p e r t i e s o f the



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membrane appear not t o be very c r i t i c a l , but i t must be s u f f i c i e n t l y permeable t o allow f r e e d i f f u s i o n of c a r r i e r gas and s o l v e n t vapor while d i v i d i n g the macroscopic c a r r i e r flow from the o p p o s i t e l y d i r e c t e d sweep flow. In t h i s c e l l the c o n c e n t r a t i o n of vapor i n the c e n t r a l channel f a l l s o f f e x p o n e n t i a l l y , with the v a l u e of the exponent depending on the geometry of the c e l l , the e f f e c t i v e d i f f u s i o n v e l o c i t y and the r e l a t i v e v e l o c i t i e s of the c a r r i e r and sweep flows. A consequence of t h i s exponential dependence i s t h a t the s o l v e n t vapor t r a n s m i s s i o n can be reduced as low as r e q u i r e d f o r any a p p l i c a t i o n merely by i n c r e a s i n g the length of the c e l l or by i n c r e a s i n g the sweep flow. I f a c e r t a i n s o l v e n t vapor removal i s achieved under one set of c o n d i t i o n s , the square of t h i s value can be achieved by e i t h e r doubling the l e n g t h of the c e l l , or by doubling the sweep gas flow. Momentum Separator The two stage momentum separator used i n t h i s i n t e r f a c e i s shown s c h e m a t i c a l l y i n Figure 3 coupled t o the combination Thermospray/EI source. T h i s d e v i c e i s c o n c e p t u a l l y s i m i l a r t o those used i n other MAGIC (2) or p a r t i c l e beam (3,4) i n t e r f a c e s . However, s i n c e most of the s o l v e n t vapor i s removed i n the gas d i f f u s i o n c e l l , t h i s separator i s r e q u i r e d p r i m a r i l y t o remove s u f f i c i e n t helium t o allow the standard MS pumping system t o achieve the good vacuum r e q u i r e d f o r EI o p e r a t i o n . The performance of t h i s device can be optimized much more r e a d i l y f o r s e p a r a t i n g helium from macroscopic p a r t i c l e s than when copious q u a n t i t i e s of condensible vapors are present as i n the more conventional p a r t i c l e beam systems. Combination Thermosprav/CI/EI Ion Source The beam of sample p a r t i c l e s t r a n s m i t t e d by the momentum separator t r a v e l s d i r e c t l y i n t o the EI i o n source as shown i n Figure 3. The p a r t i c l e s impact the heated w a l l s of the i o n source and are v a p o r i z e d and i o n i z e d by a 70 eV e l e c t r o n beam. The EI source i s s i m i l a r i n design t o those used i n conventional GC-MS, but the system i l l u s t r a t e d i n F i g u r e 3 a l s o i n c l u d e s a high pressure Thermospray source i n tandem with the EI source. With t h i s system the Thermospray source can be used t o provide r e l i a b l e molecular weight i n f o r m a t i o n while the EI source provides the fragmentation needed f o r s t r u c t u r a l e l u c i d a t i o n and unambiguous identification. Choice of EI or Thermospray o p e r a t i o n i s s e l e c t e d by a s i n g l e switch. A l t e r n a t i v e l y the Thermospray source may be used as a more conventional CI



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source. In t h i s mode o f operation the a e r o s o l i s coupled t o the i o n source using a s i n g l e stage momentum separator i n s t a l l e d i n p l a c e o f the thermospray v a p o r i z e r , and the e x i t l i n e from the u n i v e r s a l i n t e r f a c e i s moved from the momentum s e p a r a t o r connected t o the EI source t o t h i s s i n g l e stage s e p a r a t o r employed f o r CI. Reagent gas, such as methane i s added d i r e c t l y i n t o the h i g h pressure i o n source, and the e f f i c i e n t s o l v e n t removal accomplished i n t h i s arrangement allows f r e e c h o i c e o f CI reagent without s i g n i f i c a n t i n t e r f e r e n c e from the more p o l a r s o l v e n t vapors. Results Development of a commercial i n t e r f a c e between EI-MS and HPLC using t h i s new approach has been completed r e c e n t l y , and i n t e r f a c i n g with a v a r i e t y o f other gas phase d e t e c t o r s i s p r e s e n t l y being s t u d i e d . Some recent r e s u l t s i l l u s t r a t i n g the performance are shown i n F i g u r e s 4 through 11. Sample Transmission E f f i c i e n c y A key parameter i n determining the o v e r a l l performance of an LC i n t e r f a c e i s the e f f i c i e n c y with which sample e l u t e d from the LC i s t r a n s f e r r e d t o the i o n source o f the mass spectrometer. Other f a c t o r s such as i o n source e f f i c i e n c y and chemical n o i s e may determine the u l t i m a t e d e t e c t i o n l i m i t s , but improvements i n these f a c t o r s g e n e r a l l y cannot overcome the e f f e c t o f sample l o s s on s e n s i t i v i t y . R e s u l t s o f an experiment designed t o estimate sample t r a n s f e r e f f i c i e n c y are shown i n F i g u r e 4. In t h i s experiment peak areas o f m/z 228 molecular i o n from chrysene were determined f o r a s e r i e s o f flow i n j e c t i o n inputs of chrysene i n the range between 50 an 500 ng. These measurements were then repeated by l o a d i n g small volumes of the same s o l u t i o n s on a s o l i d s probe. The probe was i n s e r t e d through a 3 mm h o l e d r i l l e d i n the s i d e of the EI i o n source o p p o s i t e the i n l e t f o r the p a r t i c l e beam, and the bare probe was i n p l a c e i n the i o n source during the flow i n j e c t i o n measurements, so that sample v a p o r i z a t i o n i n both experiments o r i g i n a t e d from the same r e g i o n o f the i o n source. Sample s o l u t i o n s were loaded onto the probe and allowed t o evaporate t o dryness a t room temperature and atmospheric pressure before i n s e r t i n g the sample i n t o the source, t o avoid sample l o s s from the probe. With data a c q u i s i t i o n i n progress and the LC i n t e r f a c e o p e r a t i n g i n the normal manner with the same mobile phase used i n the FIA measurements, the probe c a r r y i n g the sample was then i n s e r t e d i n t o the heated source (ca. 300 C) and the m/z 228 response was i n t e g r a t e d as before. Peak widths i n both experiments were s i m i l a r . From the r e s u l t s shown i n Figure 4, we conclude t h a t



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SEPARATOR

SOURCE

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F i g u r e 3. Schematic diagram o f combination Thermospray/EI i o n source with two stage momentum separator f o r EI input.

0

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SAMPLE (ng)

F i g u r e 4. Response f o r chrysene from flow i n j e c t i o n i n t o t h e LC-MS i n t e r f a c e compared with d i r e c t probe.


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under the c o n d i t i o n s employed f o r these measurements about 70% o f the sample i s t r a n s f e r r e d from the LC t o the i o n source of the EI mass spectrometer. Dependence on L i q u i d Flow Rate R e s u l t s f o r a s e r i e s o f i n j e c t i o n s o f 2 micrograms o f c a f f e i n e i n 50:50 ACN:H 0 a t d i f f e r e n t flow r a t e s i n the range from 0.4 t o 1.5 mL/min are summarized i n Figure 5. The observed peak height i s p r o p o r t i o n a l t o flow r a t e , the peak width (FWHM) i s i n v e r s e l y p r o p o r t i o n a l t o flow, and t o w i t h i n the experimental u n c e r t a i n t y o f these measurements the peak areas are independent o f flow r a t e . These r e s u l t s a l s o provide some information on the sources o f the observed sample d i s p e r s i o n . When the peak width i s p l o t t e d as a f u n c t i o n o f the volume i n j e c t e d (20 m i c r o l i t e r s i n t h i s case) d i v i d e d by the flow ( m i c r o l i t e r s / s e c ) a l i n e a r r e l a t i o n s h i p as shown i n F i g u r e 6 i s observed. The slope o f t h i s l i n e i s a measure o f d i s p e r s i o n i n the l i q u i d flow system as compared t o i d e a l plug flow, and the i n t e r c e p t provides a measure o f the flow independent d i s p e r s i o n i n the gas phase system and the mass spectrometer. As can be seen from the Figure the d i s p e r s i o n i n the l i q u i d i s 3.3 times the i d e a l and c o n t r i b u t i o n from the gas phase p o r t i o n i s only 1.6 seconds. These r e s u l t s were obtained with a v a r i a b l e wavelength UV d e t e c t o r o n - l i n e which c o n t r i b u t e s s i g n i f i c a n t l y t o the downstream band broadening. Without the UV d e t e c t o r s i m i l a r measurements gave a slope of 1.6 and an i n t e r c e p t o f 1.2 sec. C l e a r l y the UV d e t e c t o r and i t s a s s o c i a t e d connections c o n t r i b u t e s somewhat more t o l o s s o f chromatographic e f f i c i e n c y than does the U n i v e r s a l Interface.


2

Dependence o f Solvent Composition For g r a d i e n t e l u t i o n the e f f i c i e n c y o f the i d e a l i n t e r f a c e should be independent o f s o l v e n t composition. Some r e s u l t s obtained f o r i n j e c t i o n s o f 1 microgram o f c a f f e i n e i n t o mixtures of water and a c e t o n i t r i l e are summarized i n Figure 7. These r e s u l t s which show about a 30% v a r i a t i o n i n response depending on mobile phase composition were obtained without a d j u s t i n g temperatures of the Thermospray v a p o r i z e r o r d e s o l v a t i o n chamber. At higher water content some sample i s apparently l o s t i n the d e s o l v a t i o n chamber due t o condensation o f some d r o p l e t s c o n t a i n i n g sample. T h i s l o s s can apparently be minimized by o p t i m i z i n g the temperatures a t each composition.



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F i g u r e 5. Summary o f measurements on the dependence on l i q u i d flow r a t e .

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V/F (SEC.) F i g u r e 6. Measured peak width p l o t t e d as a f u n c t i o n o f volume i n j e c t e d d i v i d e d by flow r a t e .



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Dependence on s o l v e n t composition.


226



Solvent Removal F i g u r e 8 shows mass chromatograms f o r the molecular i o n of a c e t o n i t r i l e (m/z 41) and t h a t of chrysene (m/z 228) f o r a s e r i e s of i n j e c t i o n s of 100 ng of chrysene i n t o a flow of 1 mL/min of 75:25 a c e t o n i t r i l e : w a t e r as the helium sweep gas flow i s reduced i n steps from 10 L/min t o 4 L/min. At the higher flow the t r a n s m i s s i o n e f f i c i e n c y f o r chrysene i s about 2 m i l l i o n times t h a t f o r the a c e t o n i t r i l e s o l v e n t . The corresponding spectrum i s shown i n Figure 9 where the mass 41 i s seen t o be small compared t o the background of argon (m/z 40) and carbon d i o x i d e (m/z 44) due t o a small a i r leak i n t o the mass spectrometer which corresponds t o an a n a l y z e r pressure of 2x10-7 t o r r . Applications T h i s new LC-MS system i s j u s t beginning t o be a p p l i e d t o r e a l environmental a p p l i c a t i o n s . One example of a p o t e n t i a l a p p l i c a t i o n i s the hormones on the Appendix VIII l i s t . A t o t a l i o n chromatogram f o r a reversed phase s e p a r a t i o n of a mixture of hormone standards i s shown i n F i g u r e 10. These compounds y i e l d l i b r a r y matchable s p e c t r a at the 100 ng l e v e l on column. For example, the p a r t i a l l y r e s o l v e d doublet (peaks 2 and 3) are e a s i l y i d e n t i f i e d as t e s t o s t e r o n e and e t h y n y l e s t r a d i o l from t h e i r EI s p e c t r a shown i n Figure 11. On the other hand, Thermospray g i v e s only protonated molecular ions i n the p o s i t i v e i o n mode. As a r e s u l t of the very simple spectrum Thermospray g i v e s much lower d e t e c t i o n l i m i t s f o r these compounds (ca. 100 pg on column) while EI allows unambiguous compound i d e n t i f i c a t i o n , but g e n e r a l l y r e q u i r e s somewhat more sample. Methane CI i s intermediate between these extremes i n t h a t i t g i v e s a s i m p l i f i e d fragmentation p a t t e r n at intermediate t o high s e n s i t i v i t y . For higher molecular weight, more complex compounds Thermospray o f t e n g i v e s r e l i a b l e molecular weight and fragmentation p a t t e r n s t h a t are u s e f u l f o r compound i d e n t i f i c a t i o n even though i t i s n e i t h e r as extensive or as r e p r o d u c i b l e as t h a t obtained by EI on s m a l l e r molecules. In many of these cases EI f a i l s t o provide any u s e f u l information because the compound i s e i t h e r i n s u f f i c i e n t l y v o l a t i l e or too thermally l a b i l e t o t o l e r a t e being vaporized without undergoing extensive pyrolysis. Conclusion T h i s new LC-MS i n t e r f a c e appears t o o f f e r s u b s t a n t i a l promise f o r a n a l y s i s of samples not amenable t o GC-MS. The a v a i l a b i l i t y of three complementary i o n i z a t i o n


Brown; Liquid Chromatography/Mass Spectrometry ACS Symposium Series; American Chemical Society: Washington, DC, 1990. 10

10

11

8

12

14

15

Time (MIN)

13

6.5

F i g u r e 8. Mass chromatograms f o r (a) s o l v e n t (m/z 41 from a c e t o n i t r i l e ) and (b) 100 ng o f chrysene as f u n c t i o n s o f sweep gas flow r a t e .

TIME (MINUTES)

0

500.

1000.

1500

2000.

2500.

3000.

3500.

4000.

4500

5000J

5500.

6000.

6500.

7000

m/z 228 7903,


16

17

18

19

M+ CHRYSENE 100 NG k

*4

fcs>



228

SPECTROMETRY

228

Scan at 10.044 7326.

Carrier: 2 L/MIN Sweep: 10 L/MIN 75:25 A C N : H2O Solvent: 1 mL/MIN CHRYSENE: 100 ng 41

fl| i f l , , ! ^ 50

75

f

[inljll I J l| I .|M| 100

125

150

175

200

11^ )

250

MASS F i g u r e 9. Spectrum corresponding t o almost complete s o l v e n t removal a t a sweep gas flow o f 10 L/min.



14.

VESTAL ETAL.


229

El Total Ion Chromatogram 1. 2. 3. 4. 5.

! 10

, 11

B-Estradiol Testosterone Ethynylestradiol Estrone Progesterone

,IM,JMM,,.,,|, ,, ,.,,|M..,....|MM .... .... ,...|.,,.. ,,.,|,,,, ,,,.|...,,..l.|..l.,..l.|..l.j..T7|.........j 12 13 14 IS 16 17 18 19 20 21 22 23 |

|

(

1

|

1

Retention Time (Min) F i g u r e 10. Chromatogram o f hormone mixture on the Appendix V I I I l i s t .



100%

14,7759

124

6 7

180

51 107 77

124

150

133

168

MASS

200

El Mass Spectrum of Ethynylestradiol

213

F i g u r e 11. EI spectra f o r peaks 2, t e s t o s t e r o n e ( a ) and peak 3, e t h y n y l e s t r a d i o l from the chromatogram shown i n Figure 10.

El Mass Spectrum of Testosterone

100%

15.9919


250

300

296

3

E o

14.

VESTAL ETAL.


231

techniques on a s i n g l e instrument should g r e a t l y extend the range o f samples amenable t o a n a l y s i s and s u b s t a n t i a l l y improve both d e t e c t i o n l i m i t s f o r t a r g e t e d compounds and the r e l i a b i l i t y of unknown i d e n t i f i c a t i o n . References


1.

2. 3. 4.

C. R. B l a k l e y and M. L. V e s t a l , A n a l . Chem. 55, 75 (1983); M. L. V e s t a l and G. J . Fergusson, A n a l . Chem. 57, 2372 (1985); M. L. V e s t a l , Science 221, 275 (1984). R. C. Willoughby and R. F. Browner, A n a l . Chem. 56, 2626; P. C. Winler, D. B. Perkins, W. K. W i l l i a m s , and R. F. Browner, A n a l . Chem. 60, 489 (1988). A. A p f e l , L. C. F r a z i e r , and M. Brown, t h i s volume. (paper 49) R. C. Willoughby, E. S. Sheehan, P. E. Sanders, and M. D i l t s , t h i s volume (paper 52)

RECEIVED November 13, 1989


Mass Spectrometry - American Chemical

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