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THE ROLE OF ORDERED SURFACE AGGREGATES IN ION DESORPTION BY 252-Cf-PDMS
R. Macfarlane, C. Mcneal, D. Jacobs
To cite this version:
R. Macfarlane, C. Mcneal, D. Jacobs. THE ROLE OF ORDERED SURFACE AGGREGATES IN
ION DESORPTION BY 252-Cf-PDMS. Journal de Physique Colloques, 1989, 50 (C2), pp.C2-21-C2-
25. �10.1051/jphyscol:1989204�. �jpa-00229400�
THE ROLE OF ORDERED SURFACE AGGREGATES IN ION DESORPTION BY 252-Cf-PDMS
R.D. MACFARLANE, C.J. McNEAL and D.L. JACOBS
Texas A and M University, Department of Chemistry, College Station.
TX 77843, U.S.A.
Resume - Le r6le des clusters lors de la d6sorption et de l'ionisation des ions mol6culaires utilisant 252-Cf-PDMS a 6te' gtudiek en faisant varier les forces intermolkculaires des mole/cules adsorbeks en surface et les sgquences de fragmentation.
Des ef fets signif icatif s ont 6tg observgs.
Abstract - The role of clusters in the desorption and ionization of molecular ions using 252-Cf-PDMS has been studied by varying the intermolecular forces between molecules adsorbed on the surface and studying the influence on fragmentation patterns.
Significant effects were observed.
1 INTRODUCTION
The technique of 252-Cf-plasma desorption mass spectrometry is based on the interaction of high energy nuclear fission fragments passing through condensed matter. Gas phase molecular ions are ejected with high probability even when the molecule is large and thermally unstable. Since the discovery of the method in 1974 /I/, there has been considerable interest in understanding how complex, fragile molecular ions can survive the high energy density generated by the fission fragment. Another unsolved problem has been the dependence of molecular ion yields on matrix composition, a complex interaction that often leads to total quenching of molecular ion formation even when the molecule is present in the matrix at the highest concentration. Molecular ion quenching by the substrate and has been a major yoblem in establishing quantitative analyses based on 252-Cf PDMS.
There has been growing indirect evidence that the presence of molecular clusters on the surface of the substrate may play a role in the desorption/ionization process. For example, in the formation of small protein molecular ions, it has been found that the addition of
~lutathione to the protein solution, forming a glutathione/protein cluster, increases 1-olecular ion yields of the protein /2/. While the specific function of the glutathione was qot clearly elucidated, it was clear that the presence of the mixed cluster did have a
?ositive influence on molecular ion yields.
.!ore definitive evidence for the role of clusters in the desorption/ionization process has cone from the work of Jungclas
et
where correlations in molecular and fragmentation ion yields were clearly identified /3/. Based on these measurements, a cluster model was proposed by these authors in which molecular ion formation is the result o f 3 . e dissociation of a desorbed cluster of molecules loosely couple by van der Waals interactions.Subsequently, they used this model to design a matrix isolation procedure for sample preparation that essentially eliminated the uncontrolled influence of matrix variability on
~olecular ion yields and resulted in the development of a protocol for quantitative analysis .of the anti-tumor drug, etoposide, for clinical applications / 4 / .
The central premise that is being tested in our study is that intermolecular interactions can play a role in stabilizing desorbed molecular ions so that they do not fragment as they are desorbed from the surface. Polycrystalline samples of this molecule were studied under Cifferent conditions where the intermolecular interactions within the matrix were perturbed using two different methods. The effects of this perturbation on the 252-Cf-PD mass spectra are the subject of this paper.
2 - EXPERIMENTAL METHODS
The molecule used in this study was a deoxydinucleotide (ApC) with the labile hydrogens replaced by dinethoxytrityl groups in the 5' positions and benzoyl groups in the 3' positions on the sugars; a p-chlorophenyl group is on the phosphate. Samples of this molecule were
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1989204
C2-22 JOURNAL
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PHYSIQUEdissolved in acetone with concentrations ranging from 0.1 to 1 mM. A small quantity of 14-C labelled valine was added to the solution for assay purposes. A series of targets were prepared by the electrospray method /5/ using a combination of solution concentrations and volumes such that the total amount of material deposited on the film was the same for all targets. Uniformity of total sample amount was monitored by measurement of the 14-C activity of the electrosprayed samples. A custom designed 252-Cf-PD mass spectrometer was used for the mass analysis / 6 / .
Ultra-violet irradiations were carried out using a low pressure Hg discharge UV- quartz capillary tube light source (Model 11 SCA-I, Ultra-Violet Products, Inc., San Gabriel, CA 91778). Irradiations were carried out in vacuum using a
W
grade sapphire window to transmit the radiation from the light source to the sample in vacuum. Samples were mounted on a target stick whose position in vacuum could be adjusted remotely through a rack and pinion assembly.The procedure was to first position the sample in front of the 252-Cf source and obtain tho 252-Cf-PD mass spectrum. The target was then moved to the sapphire window for the irradiation measurements. After the irradiation was completed, the sample was then returned to the 252Cf-source position for mass analysis of the irradiated sample.
3 - RESULTS AND DISCUSSION
A. 252-Cf-PDMS of the rotected deox dinucleotide - Figure 1 shows the structure of the test molecule. The dominantpions in the miss spectrum are the molecular ion (M
+
)a'~ at m/r 901 and two fragment ions at m/z 105 and 303 corresponding to the benzoyl and dimethoxytrityl ion. These groups are bloc.l:ed out in the structure given below.H H
A
C-C
,
H? \Ic-c=o
F j ? . 1 Kolecular ctruct:lre of the fully Frotected deoxy$inucleotide used in these studies.
B.
- Influence of Solution Concentration on Mass Spectrum - The rationale behind this set of experiments was to be able to change the microcrystallinity of the sample by varying the concentration of the electrospray solution used to prepare the .deposit. All other variables such as total sample thickness remained the same. By electrospraying with a very dilute solution, the fraction of microcrystalline molecular aggregates could reasonably be expected to be smaller than the fraction formed from a more concentrated solution. The question was whether this subtle difference between the samples would have an effect on the mass spectrum of the test molecule. Four ions in the 252-Cf-PD spectra were monitored in this study: the<+ . .. lon which served as a monitor of the integrated fission fragment flux, the two fragment
ions from the blocking groups, and the molecular ion (Pi
+
~a'). Four different solution concentrations were used: 0.1, 0.25, 0.5, and 1.0 ~$1.i n t e n s i t y was somewhat weaker ( 0 . 0 7 5 i o n s / F F ) . The y i e l d was e s s e n t i a l l y t h e same f o r a l l s a m p l e s c o n f i r m i n g t h a t t h e t o t a l i n t e g r a t e d f i s s i o n f r a g m e n t f l u x was t h e same f o r a l l s a m p l e s . T h e s p e c t r u m f o r t h e 0 . 5 mM s a m p l e was s i m i l a r b u t a t 0 . 2 5 a n d 0 . 1 mM t h e m o l e c u l a r i o n y i e l d was s i g n i f i c a n t l y d i m i n i s h e d w i t h d e c r e a s i n g c o n c e n t r a t i o n . A set o f b a r g r a p h r e p r e s e n t a t i o n s o f t h e d a t a i s shown i n F i g . 2 a l o n g w i t h a s c h e m a t i c d e p i c t i n g t h e i n f l u e n c e of s o l u t i o n c o n c e n t r a t i o n i n t h e e l e c t r o s p r a y o n t h e s t a t e o f a g g r e g a t i o n o f t h e s a m p l e d e p o s i t .
A q u i t e d i f f e r e n t p a t t e r n i s o b s e r v e d f o r t h e two f r a g m e n t i o n s . T h e r e i s e s s e n t i a l l y no d e p e n d e n c e o f t h e s e i n t e n s i t i e s on s o l u t i o n c o n c . e n t r a t i o n .
0 25
m/z 105' Benzoyl Group m/z 901 (M + Na')
g 0.20
-
-
$ 0 1 5
0
; 0 1 0
2 0 0 s
0 1 0 5 1.0 1 0 0.1 0 5 1 0 0 1 0 5 1.0 0.1 0 5 1 0
F i g . 2 The b a r g r a p h s show t h e a b s o l u t e y i e l d s o f 4 d i f f e r e n t i o n s from t h e p r o t e c t e d d e o x y d i n u c l e o t i d e a s a f u n c t i o n o f s o l u t i o n c o n c e n t r a t i o n u s e d i n t h e e l e c t r o s p r a y . The s c h e m a t i c d e p i c t s t h e i n f l u e n c e o f s o l u t i o n c o n c e n t r a t i o n on t h e a g g r e g a t i o n s t a t e o f t h e s a m p l e . L o s s o f i n t e n s i t y o f t h e m o l e c u l a r i o n a t low c o n c e n t r a t i o n is a t t r i b u t e d t o t h e low c o n c e n t r a t i o n o f m i c . r o c r y s t a l l i n e domains i n t h e m a t r i x .
?!e i n t e r p r e t t h e s e r e s u l t s a s e v i d e n c e f o r t h e i n f l u e n c e o f t h e mass s p e c t r u m on t h e n i c r o c r y s t a l l i n i t y o f t h e s a m p l e . It was n o t p o s s i b l e t o o b t a i n a n i n d e p e n d e n t c o n f i r m a t i o n of t h e c r y s t a l l i n i t y o f t h e s a m p l e by x-ray d i f f r a c t i o n b e c a u s e t h e s a m p l e s w e r e t o o t h i n .
< h u s , we m u s t 7 i n f e r t h a t t h e r e a r e d i f f e r e n c e s i n c r y s t a l l i n i t y o f t h e s a m p l e s a t t h e
m i c r o s c o p i c l e v e l .
We p r o p o s e t h e f o l l o w i n g model t o e x p l a i n t h e r e s u l t s . The p r i m a r y e x c i t a t i o n o f t h e p r o b e m o l e c u l e by t h e f i s s i o n f r a g m e n t i s t o d i s s o c i a t i v e s t a t e s t h a t f r a g m e n t by l o s s of t h e l o o s e l y bound p r o t e c t i n g g r o u p s . B u t t h e r e a r e d o m a l n s i n t h e m a t r i x w h e r e t h e m o l e c u l e s a r e p e r f e c t l y o r i e n t e d w i t h t h e n e i g h b o r i n g m o l e c u l e s s u c h t h a t t h e y form a h i g h l y o r d e r e d a g g r e g a t e . T h e s e a g g r e g a t e s b e h a v e a s a s u p r a m o l e c u l e i n t h a t t h e y c a n be c o l l e c t i v e l y e x c i t e d a s i f t h e y were a s i n g l e s p e c i e s . E n e r g y i s e f f i c i e n t l y d i s t r i b u t e d b e t w e e n members o f t h e a g g r e g a t e s o t h a t a s i n g l e m o l e c u l e i s n o t e x c i t e d i n t o d i s s o c i a t i v e s t a t e s . When t h l s a g g r e g a t e i s d e s o r b e d , i t d i s s o c i a t e s i n t o i t s c o m p o n e n t s b u t t h e r e i s a h i g h p r o b a b i l i t y t h a t a m o l e c u l e c a n s u r v i v e i n t a c t . With i n c r e a s i n g c o n c e n t r a t i o n o f t h e s o l u t i o n , t h e c o n c e n t r a t i o n o f t h e s e o r d e r e d a g g r e g a t e s i n t h e s o l u t i o n i n c r e a s e s a n d , d u r i n g s a m p l e p r e p a r a t i o n , t h e s e are t r a n s f e r r e d i n t a c t t o t h e s u r f a c e o f t h e f i l m d u r i n g t h e e l e c t r o s p r a y p r o c e s s .
C2-24 JOURNAL
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PHYSIQUEC.- Influence of W-irradiation on the Mass spectrum - If the matrix of sample molecules containing a proportion of ordered aggregates could be heated to a point where some of the ordered aggregates would lose their crystallinity, it might be possible to provide additional evidence for the influence of these aggregates on the probability for the desorption of intact molecular ions. This was the rationale behind the UV-irradiation studies. Most organic molecules absorb photons at wave lengths below 200 nm so this is an efficient way to transfer energy to the sample to induce "melting" of the ordered aggregates.
For this study, irradiations were carried out using the same fully protected deoxydinucleotide sample from the previous study. The sample prepared from the 1 mM solution was used where the molecular ion intensity was the greatest. The same four ions were monitored in this study. Measurements were carried out using this sample and 5 different irradiation times ranging from 30 s to 30 min.
In the 252-Cf-PD analysis of the irradiated film, it was found that the H+ intensity was not influenced by the irradiation nor was the benzoyl ion. The dimethoxytrityl ion intensity was diminished to some extent by the irradiation dropping to 50% of the initial intensity at 5 min and 25% at 30 min. But the largest effect was for the molecular ion. For a 2.5 min exposure the intensity was reduced to 25% of the original value and at 30 min, the ion was barely detectible. These results are sumrlarized in fig. 3.
Irradiation T ~ m e (rnin.)
r---
Revers~ble Process
Absorption In
M8crocryslal o l Ordered Slructure
Fig. 3 - Bar graph representation of the influence of uv-irradiation on the 252-Cf-PD mass spec-trum of the fully protected deoxydinucleotide whose structure is shown in Fie. 1.
To determine whether the loss in intensity might be due to dec.omposition of the sample by photochemical processes, the sample was washed off the foil after the 30 min irradiation and redeposited by electrospraying. The 252-Cf-PD analysis of this sample showed that the original spectrum was restored. Thus significant sample decomposition could be ruled out as the reason for the photo-induced molecular ion quenching. We propose that the explanation for this quenching is loss of the population of ordered cluster states induced by thermal excitation to a level where localized melting occ.urred with a permanent loss of the ordered structure on cooling. Thus, the influence of the radiation-induced melting had an effect similar to reducing the aggregate size by concentration chanses.
From these studies, we conclude that the presence of ordered clusters in the sample matrix for the test molecule used in this study, has a marked influence on the probability for desorption of an intact molecular ion of this species. We suggest that the reason for this is that the initial excitation by the fission fragment is distributed amongst the components of the ordered aggregdte resulting in dissociation of the aggregate into individual molecules but with excitation energies below the level of dissociative states.
ACKNOWLEDGEMENTS
This research was supported by the U.S. National Institutes of Health (GM-26096), National Science Foundation (CHE-8604609), and the Welch Foundation (A-258).
REFERENCES
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/2/ Alai, M., Demirev, P., Fenselau,C.,
and Cotter, R.J., Anal. Chem.3,
(1986), 1303./3/ Schmidt,
L.,
and Jungclas, H., in "PDMS and Clusters", E.R. Hilf, F. Kammer, and K. I*:ien, (eds.), Springer Lecture Notes in Physics269,
(1987), 234./4/ Jungclas, H., Pfluger, K.-H., Schmidt, L., Hahn, M., (these proceedings).
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