Electrospray Ion Mobility Spectrometry - ACS Publications - American

Chapter 16

Electrospray Ion Mobility Spectrometry Its Potential as a Liquid-Stream Process Sensor 1

C. B. Shumate and H. H. Hill

Downloaded by GEORGETOWN UNIV on August 23, 2015 | http://pubs.acs.org Publication Date: October 6, 1992 | doi: 10.1021/bk-1992-0508.ch016

Department of Chemistry, Washington State University, Pullman, WA 99164-4630 Ion mobility spectrometry i s an a n a l y t i c a l method i n which ions are produced at atmospheric pressure and allowed to drift and separate i n an e l e c t r i c f i e l d at i n t r i n s i c v e l o c i t i e s . By monitoring the a r r i v a l time spectra of these atmospheric pressure ions, both q u a l i t a t i v e and quantitative information can be obtained from an a n a l y t i c a l sample. In the past, IMS has been used e x c l u s i v e l y f o r gas phase a n a l y s i s . By employing electrospray i o n i z a t i o n techniques, t h i s study evaluates IMS for use with l i q u i d samples. Using amines as t e s t analytes, ion mobility r e p r o d u c i b i l i t y was studied as a function of time, solvents, electrospray voltage, sample matrix and drift gas temperature. In addition, electrospray ion mobility spectrometry was demonstrated as a detection method f o r flow i n j e c t i o n analysis, chromatography, and continuous sample stream monitoring. Detection l i m i t s as low as 5 X 10 mol/s were determined. -15

The a b i l i t y t o o b t a i n o n - l i n e , r e a l - t i m e chemical information from l i q u i d waste streams f o r p o l l u t i o n prevention and process c o n t r o l i s a challenge f o r a n a l y t i c a l chemistry. E s s e n t i a l l y , only two a n a l y t i c a l approaches are c u r r e n t l y a v a i l a b l e : spectrophotometry and e l e c t r o c h e m i s t r y . Spectrophotometric methods take advantage o f unique e l e c t r o n i c s t r u c t u r e s i n atoms and molecules by monitoring emission, absorption or f l u o r e s c e n c e i n an emerging stream. E l e c t r o c h e m i c a l methods r e l y on unique o x i d a t i v e or r e d u c t i v e p r o p e r t i e s of p o l l u t a n t s t o detect t h e i r presence. In general, compounds which are not s p e c t r o p h o t o m e t r i c a l l y or

1

Current address: Hamilton Company, 4970 Energy Way, Reno, NV 89520 0097-6156/92/0508-0192$06.00/0 © 1992 American Chemical Society

In Pollution Prevention in Industrial Processes; Breen, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.


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Electrospray Ion Mobility Spectrometry

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e l e c t r o c h e m i c a l l y a c t i v e must be converted t o s p e c i e s which are, or they go undetected. Recently, i n order to detect a wider v a r i e t y of compounds, much more s o p h i s t i c a t e d a n a l y t i c a l instruments such as MS, ICP, NMR, and XRF have been i n v e s t i g a t e d (1). U n f o r t u n a t e l y these instruments introduce a l a y e r of added complexity which, although i n many cases may be necessary, i s u n d e s i r a b l e f o r simple r o u t i n e process a n a l y s i s . The o b j e c t i v e of t h i s research was t o i n v e s t i g a t e an a n a l y t i c a l methodology c a l l e d i o n m o b i l i t y spectrometry (IMS) which does not r e l y on e i t h e r the presence of chromophoric or e l e c t r o a c t i v e f u n c t i o n a l groups f o r i t s a n a l y t i c a l response. Instead, response i n i o n m o b i l i t y spectrometry r e l i e s on gas phase e l e c t r o n and proton affinities. In the past, these instruments have t r a d i t i o n a l l y been used f o r gas phase a n a l y s i s although the d e t e c t i o n of organic compounds d i s s o l v e d i n s u p e r c r i t i c a l f l u i d s has been r e p o r t e d . Comprehensive reviews of IMS f o r gas and s u p e r c r i t i c a l f l u i d a n a l y s i s can be found i n the l i t e r a t u r e (2,3). I n i t i a l attempts to i n t e r f a c e IMS with l i q u i d streams were r e p o r t e d by Dole and h i s group (4-6). The i n t e r f a c e was accomplished by e l e c t r o s p r a y i n g the l i q u i d d i r e c t l y i n t o the i o n m o b i l i t y spectrometer. Since the e l e c t r o s p r a y process not only served t o n e b u l i z e the l i q u i d but a l s o t o i o n i z e the sprayed d r o p l e t s , no a u x i l i a r y i o n i z a t i o n source was r e q u i r e d . U n f o r t u n a t e l y , i n t h i s work, they used a low temperature instrument with a b i - d i r e c t i o n a l flow design and were unable to s u c c e s s f u l l y s t a b i l i z e solvent c l u s t e r e d ions before e n t e r i n g the d r i f t r e g i o n . Based on Dole's work, we have r e c e n t l y demonstrated e l e c t r o s p r a y n e b u l i z a t i o n and i o n i z a t i o n of l i q u i d samples f o r i o n m o b i l i t y spectrometry u s i n g a high temperature spectrometer and a u n i - d i r e c t i o n a l flow design (7). These p r e l i m i n a r y i n v e s t i g a t i o n s showed e l e c t r o s p r a y i o n m o b i l i t y spectrometery (ES-IMS) response t o a v a r i e t y of organic compounds d i s s o l v e d i n a v a r i e t y of s o l v e n t systems. Subsequently, t h i s ES-IMS was i n t e r f a c e d t o a l i q u i d chromatograph and evaluated as a chromatographic d e t e c t o r (8). E l e c t r o s p r a y i o n i z a t i o n i s an emerging technology which i s c u r r e n t l y f i n d i n g acceptance i n the f i e l d of mass spectrometry (9-13). In these i n v e s t i g a t i o n s we have focused on e v a l u a t i o n of the ES-IMS as a continuous process analyzer choosing t e s t compounds which cannot be detected by t r a d i t i o n a l methods and p r o v i d i n g more d e t a i l e d i n f o r m a t i o n of the a n a l y t i c a l f i g u r e s of merit of the instrument. Experimental The Ion M o b i l i t y Spectrometer. F i g u r e 1 represents a schematic c r o s s - s e c t i o n of the spectrometer used i n t h i s work. The geometrical design of the spectrometer c o n s i s t s



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of a stack of metal r i n g s separated by i n s u l a t o r s . The r i n g s are h e l d at s u c c e s s i v e l y lower v o l t a g e s t o p r o v i d e a smooth e l e c t r i c f i e l d down the center of the stack. Generally, f i r s t d r i f t r i n g of the spectrometer was maintained at 4 t o 5 kV. D r i f t tube v o l t a g e was doped l i n e a r l y through a r e s i s t o r chain which connected the d r i f t rings. The temperature of the d r i f t tube was kept at 150 C with a d r i f t gas flow of 600 mL/min of a i r . Gate v o l t a g e s were set at 60 V and the l i q u i d flow r a t e was 5 uL/min of 50/50 methanol/water except where noted. I n d i v i d u a l c o n d i t i o n s f o r each experiment are noted i n the f i g u r e c a p t i o n s . For d e t a i l e d i n f o r m a t i o n on the c o n s t r u c t i o n and o p e r a t i o n of t h i s spectrometer the reader i s r e f e r r e d to Shumate's Ph.D Thesis (14). Ions were c r e a t e d by an e l e c t r o s p r a y i o n i z a t i o n source. This source c o n s i s t e d of a metal c a p i l l a r y h e l d at a high v o l t a g e (6-10 kV) through which a l i q u i d sample was passed. A small p u l s e of ions i s then "gated" i n t o the d r i f t r e g i o n through a switchable orthogonal f i e l d . A f a s t e l e c t r o m e t e r records the a r r i v a l time of the ions at the c o l l e c t o r . This method of o p e r a t i o n i s r e f e r r e d t o as s i g n a l averaging as many of these a r r i v a l time s p e c t r a can be averaged to enhance the s i g n a l t o noise r a t i o . By p l a c i n g a second gate near the c o l l e c t o r , d r i f t time s e l e c t i v e d e t e c t i o n can be achieved. T h i s i s accomplished by p u l s i n g the f i r s t gate open and imposing a f i n i t e delay on the opening of the second gate. The second gate i s then h e l d open f o r a s e l e c t e d p e r i o d to monitor ions d r i f t i n g d u r i n g a s p e c i f i c d r i f t time window. This method of o p e r a t i o n i s u s e f u l when u s i n g the spectrometer as a tunable d e t e c t o r . The disadvantage of t h i s method i s the i n a b i l i t y to c o n c u r r e n t l y r e c o r d s p e c t r a . Concurrent d e t e c t i o n and s p e c t r a l a c q u i s i t i o n can be accomplished through software u s i n g the s i g n a l averaging mode. An a l g o r i t h m e x t r a c t s a d r i f t time s p e c i f i c i o n c u r r e n t from each spectrum and p l o t s the current as a f u n c t i o n of time; t h i s i s r e f e r r e d to as software s e l e c t i v e d e t e c t i o n . Test Compounds. Alkylamines were chosen as the t e s t compounds. The i n a b i l i t y of c o n v e n t i o n a l l i q u i d d e t e c t i o n methods to respond to these compounds made them i d e a l f o r demonstrating the unique a b i l i t y of ES-IMS as a d e t e c t o r for non-chromophoric compounds. C u r r e n t l y the d e t e c t i o n of a l i p h a t i c amines r e l i e s on the chemiluminescence d e t e c t i o n of a d e r i v a t i v e (15). Compounds used i n these s t u d i e s were the f o l l o w i n g : B u t y l amine, p e n t y l amine, hexyl amine, h e p t y l amine, o c t y l amine, d e c y l amine, dodecyl amine, t e t r a d e c y l amine, iso-butylamine, secbutylamine, t e r t - b u t y l a m i n e , di-n-butylamine, t r i - n butylamine. Operating C o n d i t i o n s . Flow i n j e c t i o n , chromatographic i n j e c t i o n and continuous i n t r o d u c t i o n methods were used during the course of these i n v e s t i g a t i o n s . In a l l



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methods, a dual p i s t o n s y r i n g e pump (MPLC micropump, Brownlee Labs Inc., Santa C l a r a , CA) served t o d e l i v e r s t a b l e l i q u i d flows down t o r a t e s as low as 1 uL/min. In the flow i n j e c t i o n mode, the c o n f i g u r a t i o n was pump-injector-detector. The i n j e c t o r used f o r the flow i n j e c t i o n experiments was a four p o r t , 60 nL i n t e r n a l volume i n j e c t o r (Valco C14W, Valco Ind., Houston, TX). A l l i o n m o b i l i t y s p e c t r a presented i n t h i s study were obtained from samples introduced by the flow i n j e c t i o n method. C a l i b r a t i o n graphs were generated by monitoring the m o b i l i t y o f the product i o n and making three i n j e c t i o n s at each c o n c e n t r a t i o n l e v e l . In the chromatographic mode, the c o n f i g u r a t i o n was pump-injector-column-detector. An i s o c r a t i c s e p a r a t i o n o f four alkylamines was c a r r i e d out i n 60:40 0.1M ammonium acetate : a c e t o n i t r i l e on a 2.1 mm i . d . , 5 urn p a r t i c l e c y a n o s i l i c a column ( A l l t e c h A s s o c i a t e s , D e e r f i e l d , I L . ) . A flow r a t e o f 200 uL/min was used which was s p l i t approximately 50:1 a f t e r the column through the use o f a tee with r e s t r i c t i n g v a l v e on the high flow s i d e o f the split. I n j e c t i o n s were made with a four p o r t , 200 nL i n t e r n a l volume i n j e c t o r (C14W, Valco, houston, TX). A s o l u t i o n o f the four t r i a l k y l a m i n e s ( t r i e t h y l , t r i - n p r o p y l , t r i - n - b u t y l , and tri-n-hexylamine) was made at 10 p a r t s per thousand corresponding t o 2 ug o f each compound on column and lOOng i n t o the IMS a f t e r the s p l i t . In the continuous monitor mode, the c o n f i g u r a t i o n was simply pump-detector. In t h i s experiment the c o n c e n t r a t i o n o f dibenzylamine was programmed from 0 t o 25 ppm i n methanol over 20 minutes and the c u r r e n t at the m o b i l i t y o f the product i o n was monitored. This program was achieved by g r a d i e n t programing the dual p i s t o n s y r i n g e pump. Syringe A was f i l l e d with pure methanol while Β contained 25 ppm dibenzylamine. The g r a d i e n t was programed from 0% Β t o 100% Β over 20 minutes and then h e l d i s o c r a t i c f o r 20 minutes. R e s u l t s and D i s c u s s i o n In ES-IMS, the i o n i z a t i o n mechanism appears t o be d i f f e r e n t from that which i s observed f o r ES-MS. Rather than t r u e ion-evaporation, experiments r e p o r t e d i n these s t u d i e s more c l o s e l y resembled t h a t o f chemical i o n i z a t i o n . A s p e c t r a l peak e x i s t e d even when no a n a l y t e was present and i s r e f e r r e d t o as the r e a c t a n t i o n peak. T h i s r e a c t a n t i o n peak was the s o l v e n t i o n background and was diminished when i o n peaks from the a n a l y t e appeared. For example, we know from past experience with r a d i o a c t i v e ion sources when water i s present, the predominant r e a c t a n t ions are hydronium i o n c l u s t e r s and t h a t the i o n i z a t i o n o f an amine occurs by proton t r a n s f e r t o the amine. (H 0) H 2

n

+

+ RNH

2

n H 0 2

+

RNH3


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Data which we have generated so f a r with ES-IMS i s c o n s i s t e n t with t h i s model although mass i d e n t i f i c a t i o n s of r e a c t a n t and product ions must be conducted before c o n c l u s i v e evidence f o r the i o n i z a t i o n mechanism can be e s t a b l i s h e d . Given t h a t a v a r i e t y o f s o l v e n t s were used throughout these i n v e s t i g a t i o n s a more general equation for response can be w r i t t e n as S H

+

+ RNH

n

2

nS + RNH

+ 3

where S represents the solvent molecule and, as before, RNH represent the a n a l y t e . Thus, one might expect t h a t response f a c t o r i n ES-IMS w i l l vary as a f u n c t i o n o f both s o l v e n t and analyte i d e n t i t y . In a d d i t i o n , l a r g e q u a n t i t i e s o f s o l v e n t ( s e v e r a l uL/min o f l i q u i d ) are introduced i n t o the i o n i z a t i o n r e g i o n o f the IMS. I f these solvent molecules are not e f f i c i e n t l y evaporated and swept from the d r i f t tube, the s o l v e n t can form c l u s t e r ions with the product i o n .


2

RNH

+ 3

+ mS < = >

RNH S 3

+ m

These c l u s t e r ions would have d i f f e r e n t m o b i l i t i e s causing the i o n m o b i l i t y s p e c t r a t o vary. S e v e r a l experiments were conducted t o i n v e s t i g a t e p o t e n t i a l i n t e r f e r e n c e s due to s o l v e n t c l u s t e r i n g . M o b i l i t y Experiments. M o b i l i t y data were r e p o r t e d i n terms o f reduce m o b i l i t y constants (K ) c a l c u l a t e d from the equation Q

K

0

= (d/tE)(Ρ/760)(273/T)

where d was the d r i f t length i n cm, t was the i o n d r i f t time i n seconds, Ε was the e l e c t r i c f i e l d i n the d r i f t r e g i o n o f the spectrometer i n V/cm, Ρ was the gas pressure i n the d r i f t r e g i o n i n Torr and Τ was the temperature o f the d r i f t gas i n the d r i f t r e g i o n i n K. In theory, reduced m o b i l i t y values are independent o f i n s t r u m e n t a l conditions. In p r a c t i c e , they o f t e n do change with i n s t r u m e n t a l c o n d i t i o n s i f these c o n d i t i o n s are v a r i e d over wide ranges. For normal a n a l y t i c a l i o n m o b i l i t y d e t e c t i o n , the optimal instrumental c o n d i t i o n s do not vary g r e a t l y and the reduced m o b i l i t y values should be constant from day t o day. R e p r o d u c i b i l i t y . Reduced m o b i l i t y constants o f three compounds (tri-n-propylamine, t r i - n - b u t y l a m i n e , and t r i - n hexylamine) were determined at three d i f f e r e n t times d u r i n g the day f o r f i v e days. For t r i - n - b u t y l a m i n e the average K Q value and standard d e v i a t i o n were determined t o be 1.64 -.01. F o r t r i - n - b u t y l a m i n e and tri-n-hexylamine the values were 1.44+-.01 and 1.17+-.01, r e s p e c t i v e l y . Over t h i s f i v e day p e r i o d the ES-IMS instrument performance was extremely r e p r o d u c i b l e . +


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Solvent. In a separate experiment, the e f f e c t of s e v e r a l s o l v e n t s on m o b i l i t y values were i n v e s t i g a t e d . The e f f e c t t h a t s o l v e n t s have on product i o n m o b i l i t y was i n v e s t i g a t e d by i n t r o d u c i n g t r i - n - b u t y l a m i n e i n t o the EIMS dissolved i n five different solvents: acetone, isopropanol, a c e t o n i t r i l e , methanol, and water. The average K value was 1.44, the same as i n the previous experiments, but the standard d e v i a t i o n of 0.02 was s l i g h t l y higher than that obtained during the f i v e day study. Nevertheless, solvent e f f e c t on m o b i l i t y appeared to be minimal.


Q

E l e c t r o s p r a y Voltage. A t h i r d m o b i l i t y experiment, i n which the e l e c t r o s p r a y v o l t a g e was v a r i e d from 8000 V to 10,000 V i n 500 V increments, produced an average product ion m o b i l i t y value f o r t r i - n - b u t y l a m i n e of 1.46 with a standard d e v i a t i o n of 0.02. Upon c l o s e r i n v e s t i g a t i o n of the data, i t was found t h a t the m o b i l i t y i n c r e a s e d from 1.44 at 8000 V to 1.50 at 10,000 V. This i n c r e a s e i n m o b i l i t y with e l e c t r o s p r a y v o l t a g e i n d i c a t e d that the e l e c t r o s p r a y v o l t a g e was c o u p l i n g t o the d r i f t f i e l d v o l t a g e and i n c r e a s i n g the e l e c t r i c f i e l d through which the ions d r i f t e d . In f u t u r e designs of the ES-IMS the d r i f t f i e l d v o l t a g e should be s h i e l d e d from the electrospray voltage. Temperature. In general, m o b i l i t i e s were r e p r o d u c i b l e from day to day and from i n j e c t i o n to injection. On occasion, however, e s p e c i a l l y a f t e r the instrument had been taken apart and reassembled, m o b i l i t i e s c o u l d be d i f f e r e n t than p r e v i o u s l y observed. For example, the m o b i l i t y of the product i o n of t r i - n butylamine was measured at 50, 100, 150, and 200 C, found to be 1.36 with a standard d e v i a t i o n of 0.02. No d e f i n i t i v e e x p l a n a t i o n can be given f o r why m o b i l i t y i s so much lower i n t h i s set of experiments compared to the others conducted with t r i - n - b u t y l a m i n e . One p o s s i b i l i t y was t h a t the spectrometer had become contaminated. In such cases, the a d d i t i o n of mass i d e n t i f i e d i o n m o b i l i t y data would a i d i n understanding the response and behavior of EIMS. Nevertheless, changing the temperature from 50 to 200 C was found to have l i t t l e e f f e c t on the m o b i l i t y . These r e s u l t s , coupled with those presented above i n d i c a t e that the solvent was e f f i c i e n t l y d e c l u s t e r e d from the product i o n . Matrix. F i n a l l y , matrix e f f e c t s on m o b i l i t y were i n v e s t i g a t e d u s i n g a mixture of e i g h t alkylamines. Figure 2 shows the complex ion m o b i l i t y spectrum of the unseparated amine mixture. E i g h t product i o n peaks could be i d e n t i f i e d . D r i f t times and m o b i l i t y values of these peaks matched those obtained from i o n m o b i l i t y s p e c t r a of the i n d i v i d u a l amines. Thus, i t appears t h a t under c o n d i t i o n s used i n t h i s study, the product ions produced



198


ΤΤΤί#ΤΤΈ Figure 1. Schematic cross-section of electrospray ion mobility spectrometer.

(ο c Φ

0

5

10

Drift

15

Time

(ms)

Figure 2. ES-IMS spectra of mixture of eight alkylamines. Operating conditions: need voltage, 8000 V; drift voltage, 4000 V; temperature, 150 °C; pressure, 698 Τοπ; liquid fl 5 pL/min of 50/50 methanol/water; drift gasflow,600 mL/min pre-pure nitrogen; 60 nL injection of 0.1 mg/mL (each) solution.




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from the t e s t amines were s t a b l e even i n the presence o f other p o t e n t i a l l y i n t e r f e r i n g amines. Flow I n j e c t i o n Mode. Response data were generated f o r the amines u s i n g the s e l e c t i v e product i o n monitoring mode. C a l i b r a t i o n curves were obtained by d i s c r e t e i n j e c t i o n s of t e s t compounds s i m i l a r t o flow i n j e c t i o n a n a l y s i s . Figure 3 i s a flow i n j e c t i o n a n a l y s i s t r a c e of a s e r i e s o f t r i - n butylamine standards at d i f f e r e n t a m p l i f i e r g a i n s . I t represents a t y p i c a l c a l i b r a t i o n study produced i n t h i s way. F i g u r e 4 shows the c a l i b r a t i o n curve generated from peak heights with a c a l c u l a t e d (S/N = 3) d e t e c t i o n l i m i t of 4.5 X 1 0 ~ mol/s with a dynamic range o f 1 0 . Downloaded by GEORGETOWN UNIV on August 23, 2015 | http://pubs.acs.org Publication Date: October 6, 1992 | doi: 10.1021/bk-1992-0508.ch016

1 5

Chromatographic Mode. F i g u r e 5 shows the s e p a r a t i o n o f four t r i a l k y l a m i n e s ( t r i e t h y l , t r i - n - p r o p y l , t r i - n - b u t y l , and tri-n-hexylamine) with d e t e c t i o n by ES-IMS. The top chromatogram was produced by monitoring a l a r g e d r i f t time window while the other chromatograms show s e l e c t i v e product i o n monitoring o f the three l a r g e s t compounds i n the same s e p a r a t i o n . The t r i e t h y l a m i n e produced a product i o n t h a t d r i f t e d b e f o r e the r e a c t a n t i o n and thus appeared as a negative d e f l e c t i o n i n the non s e l e c t i v e chromatogram. Since the monitoring window i n c l u d e d p a r t o f the r e a c t a n t i o n peak, any product i o n which before t h i s window decreased the t o t a l c u r r e n t detected w i t h i n t h i s window. The s e l e c t i v e i o n chromatograms demonstrate the t r u e s t r e n g t h of ES-IMS as a l i q u i d chromatographic d e t e c t o r . By t u n i n g t o the product i o n of i n t e r e s t , only compounds of i n t e r e s t can be detected with some degree o f c e r t a i n t y . Continuous Monitor Mode. I f the m o b i l i t i e s o f amines are not changed as a f u n c t i o n of the matrix and i f the ES-IMS can be tuned t o monitor s p e c i f i c i o n d r i f t times, then i t should be p o s s i b l e t o c o n t i n u o u s l y monitor waste and process streams f o r s p e c i f i c amines. F i g u r e 6 p r o v i d e s and example o f c o n t i n u o u s l y monitoring a sample i n which the c o n c e n t r a t i o n of the t e s t amine was c o n t i n u o u s l y v a r i e d from 0 t o 25 ppm. Tuned t o the product i o n o f t h i s t e s t amine, the ES-IMS t r a c e d the composition change i n r e a l time. By simply s h i f t i n g the d r i f t time monitored, no response would have been seen f o r these changes. Long term s t u d i e s o f the instrument response were conducted only f o r s e v e r a l hours at a time. Due t o the prototype nature of the instrument and the high v o l t a g e requirements, the instrument c o u l d not be l e f t unattended. For w e l l d e f i n e d parameter c o n d i t i o n s , s t a b i l i t y o f response appeared t o be good. Once the system was operating, i t continued t o operate i n a s t a b l e manner f o r s e v e r a l hours at a time. Day t o day r e p r o d u c i b i l i t y a l s o appeared good as r e p o r t e d above. Plugging o f the e l e c t r o s p r a y o r i f i c e d i d occur on occasion, but t h i s c o u l d u s u a l l y be t r a c e d t o u n f i l t e r e d s o l v e n t s or i n j e c t i o n s of e x c e s s i v e amount of n o n v o l a t i l e compounds. In gas phase


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Figure 4. Calibration curve for tri-n-butylamine. Conditions: 8—10 ms monitorin electrospray voltage, 6880 V; drift voltage, 4000 V; temperature, 152 °C; pressure, 6 liquid flow rate, 5 ^/min of methanol; drift gas flow, 600 mL/min pre-pure nitrogen; 60 injections.


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Figure 5. Non-selective and selective monitoring of a mixture of trialkylamines separate liquid chromatography. Conditions: needle voltage, 10,000 V; drift voltage, 5000 V; flow, 600 mL/min of air; temperature, 70 °C; liquidflowrate, 200 μL/min split 50:1; mob phase, 60/40 0.1 M ammonium acetatelacetonitrile isocratic; 200 nL injection of 100 mg (each) solution.



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Figure 6. Constant monitoring of di-benzylamine. Conditions: 9-9.8 ms monitoring; needle voltage, 6800 V; drift voltage, 4000 V; temperature, 162 °C; pressure, 703 Ton; drift gas flow 600 mL/min pre-pure nitrogen; liquidflowrate, 5 pUmin of A: methanol, B: methanol with DBA (0.0254 mglSOOmL), gradient from 100% A to 100% Β in 20 minutes.

and s u p e r c r i t i c a l f l u i d a p p l i c a t i o n s , long term s t a b i l i t y of i o n m o b i l i t y spectrometry i s t r u l y e x c e l l e n t . I f guarded from contamination, IMS instruments operate f o r months without down time or the need f o r maintenance. More experience i s needed, however, before such a statement can be made with respect t o l i q u i d sample introduction. C e r t a i n l y , more i n v e s t i g a t i o n s should be undertaken with s p e c i f i c analyte and matrix systems i n mind but coupled with the fundamental data c o l l e c t e d d u r i n g these s t u d i e s these i n i t i a l i n v e s t i g a t i o n s i n t o continuous monitor f o r p o l l u t i o n c o n t r o l appears promising. Ion Abundance Experiments. In general, the response of the d e t e c t o r i n c r e a s e d with i n c r e a s i n g v o l t a g e . However, at v o l t a g e s above 10 kV d e t e c t o r o p e r a t i o n became u n s t a b l e . For these s t u d i e s , the most s t a b l e o p e r a t i n g c o n d i t i o n s were determined to be a v o l t a g e of 9 kV on the e l e c t r o s p r a y needle and 4.5 kV on the f i r s t d r i f t r i n g of the spectrometer. Under these c o n d i t i o n s , t o t a l i o n c u r r e n t , n o i s e and compound response were determined to be as f o l l o w s : T o t a l Ion Current. F i r s t , the o v e r a l l i o n c u r r e n t produced by ES-IMS was on the order of 1 nA. This i s s e v e r a l orders of magnitude lower than that r e p o r t e d by e l e c t r o s p r a y sources used with mass spectrometry. Perhaps, c u r r e n t i s l o s t to the w a l l s of the d r i f t tube through the s p r a y i n g process and c o u l d be regained by i o n f o c u s i n g and geometric c o n s i d e r a t i o n s of the d r i f t tube.


204


Noise. With both i o n gates open and the t o t a l i o n current c o l l e c t e d at the c o l l e c t o r , noise was commonly on the order of β Χ 1 0 " amperes. When only the reactant ion current was monitored noise was reduced to 2 X 10 amperes. However, when the e n t i r e product i o n r e g i o n was monitored, but no product ions were present, the noise f e l l to β X 1 0 " amperes and with s e l e c t i v e product i o n monitoring the noise was reduced even f u r t h e r to 2 X 10 amperes. 11

13


1 3

D e t e c t i o n L i m i t . From noise c o n s i d e r a t i o n s alone, i t would appear that the d e t e c t i o n l i m i t f o r the ES-IMS would appear that the reactant ion monitoring mode would have the highest d e t e c t i o n l i m i t , the n o n - s e l e c t i v e product ion monitoring mode the next highest and the s e l e c t i v e product i o n monitoring mode the lowest. This was i n f a c t the order we observed f o r the t e s t compound t r i - n - b u t y l a m i n e . The d e t e c t i o n l i m i t i n the reactant i o n monitoring mode was found to be 1 X 10"-^ l/s. In the n o n - s e l e c t i v e product i o n monitoring mode i t was 1 X 1 0 " mol/s. And, i n the s e l e c t i v e product i o n monitoring mode i t was as low as 5 X 1 0 " mol/s. m o

13

15

Summary and

Conclusion

Although t h i s i n v e s t i g a t i o n was l i m i t e d to amines as t e s t compounds and f u r t h e r i n v e s t i g a t i o n s of other c l a s s e s of analytes w i l l be necessary, the data obtained i n these s t u d i e s were promising. The m o b i l i t y s t u d i e s i n d i c a t e d that the d e t e c t o r can be r e l i a b l e and r e p r o d u c i b l e over time and at v a r i o u s o p e r a t i n g c o n d i t i o n s . They a l s o suggested t h a t t h i s prototype design may not be the optimal c o n f i g u r a t i o n . I n v e s t i g a t i o n s i n t o the e f f e c t s of i o n f o c u s i n g , s h i e l d i n g of the e l e c t r o s p a y v o l t a g e from the d r i f t f i e l d v o l t a g e , and mass i d e n t i f i c a t i o n of r e a c t a n t and product ions are needed before the method can be developed to i t maximum p o t e n t i a l . Nevertheless, demonstration a p p l i c a t i o n s f o r flow i n j e c t i o n a n a l y s i s , chromatography, and continuous monitoring of l i q u i d streams i n d i c a t e d that ES-IMS shows c o n s i d e r a b l e p o t e n t i a l as a d e t e c t i o n method to compliment spectrophotometric and e l e c t r o c h e m i c a l methods of a n a l y s i s .

Acknowledgment s This research was sponsored i n p a r t by grants from the P u b l i c Health S e r v i c e , P r e c i s i o n A n a l i t i c s , Inc., and M i l l i p o r e Corporation.




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