On cluster ions, ion transmission, and linear dynamic range limitations in electrospray (ionspray) mass spectrometry

Mass

ELSEVIER

International

Journal

of Mass

Spectrometry

and

Ion Processes

I62 ( 1997)

129-

Spectrometry

I47

On cluster ions, ion transmission, and linear dynamic range limitations in electrospray (ionspray) mass spectrometry D.R. Zook’, A.P. Bruins* Unirer.sit\-

Crntrr

jiu

Phartnrrc~. Received

A. Deusingluon 8 July

1996:

I. 9713 AV Groningm.

accepted

19 August

The Nethrrlunds

1996

Abstract The ion transmission in Electrospray (lonspray) Mass Spectrometry (ESMS) was studied in order to examine the instrumental factors potentially contributing to observed ESMS linear dynamic range (LDR) limitations. A variety of means used for the investigation of ion transmission demonstrated that a suspected loss in tetraalkylammonium ion signal in favour of formation of analyte ion-analyte ion-pair clusters is negligible. The ion/ion-pair cluster abundance continues to rise after the core analyte ion reaches the lO-5 M concentration LDR plateau The relative cluster ion abundance changes observed with increasing concentration appear to reflect solution phase ion/molecule clustering reactions at the surface of charged droplets produced in the electrospray. The simultaneous measurement of the ES capillary spray current and ion currents at the first three stages of ion sampling revealed that the nozzle orifice which separates the atmospheric pressure ion source from the first vacuum stage receives a current that rises continuously with sample concentration. The skimmer current rises to a plateau then falls off at higher analyte concentrations, closely matching the final mass spectrometric response. Complete coverage of the charged droplet surface can explain the plateau, but a simple model that explains tetraalkylammonium ion signal suppression cannot be given. Dynamic range limitation was less pronounced with higher atmospheric pressure chemical ionization total ion currents, which showed linearity over greater total MS ion current ranges. MS signal suppression observed at high ( > 0.2 mM) analyte concentrations may thus be attributed to unique ES ion formation/instrumental/space charge effects. 0 1997 Elsevier Science B.V. Kepwords:

Cluster ions; Ion transmission; Linear dynamic range limitations

1. Introduction Electrospray Mass Spectrometry (ESMS) is a rapidly expanding analytical technique with a diverse range of both small and very large target molecule applications. The basic requirement is that the target bears a net charge in solution. To date, ESMS has profoundly impacted the * Corresponding author. ’ Present address: Swiss Federal for Mineralogy and Petrography, Ziirich, Switzerland.

Institute of Technology, Sonneggstrasse 5.

Institute CH-8092

biosciences, including pharmacology and the study of large biomolecules [ l-31. Broad interest in ESMS continues within academia and industry, driven by new research possibilities and commercial opportunities. The theory of ion production from charged droplets originates in the study of electrostatic spraying. Kebarle and Tang recently considered four primary topics by which the fundamental mechanisms of ESMS may be partitioned and studied [3]. These are: (i) charged droplet production; (ii) charged droplet shrinkage; (iii)

0168-I 176/97/$17.00 Q 1997 Elsevier Science B.V. All rights reserved PII

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production of free gas-phase ions (from charged droplets); and (iv) modification of free gas-phase ions inside the atmospheric pressure source and during sampling into the mass spectrometer. The last topic (iv) concerns a number of important but poorly understood phenomena that actually alter the ion identity, including ion/molecule reactions at atmospheric pressure and collision-induced dissociation (CID) processes which occur during the transfer of ions into vacuum. Mass spectrometric sampling from atmospheric pressure into vacuum also presents the opportunity for ion modification through losses which may be due to non-uniform ion transmission [4], uncontrolled ion/molecule clustering reactions [5], and sensitivity-limiting electric fields [6] due to space charge effects in the atmospheric pressure source region [7,8]. It is understood that ESMS offers very high ionization efficiencies for target molecules that exist as ions in solution. The ultimate limits of detection, however, are hindered by low ion transmission efficiencies between the initial site of ion formation and final arrival at the MS detector. In this vein, Mann et al. examined the question: “Where do all the charges go in Electrospray Ionization ?“[9]. For the particular system studied (a 300 PM solution of tetrabutylammonium bromide) they concluded that charged species of low gas phase ion mobility are excluded from ion sampling, and are thus not available for mass analysis and detection. Mann and coworkers estimated that roughly 1% of target neutrals may be ionized, and that over half of the charged species entering their ion source end up as analyte ions. Of these only l/lo3 or l/lo4 finally undergo mass analysis [9]. Other workers likewise have reported that ESMS allows low (approximately l/10’) transfer efficiency of sample ions over the entire path from solution to the detector of the mass spectrometer [2,3]. The recent development of a micro(or nano-) ES technique is reported to consume less analyte and thus significantly improve ion transmission efficiency and limits of detection

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in comparison to conventional ES (Ionspray) techniques in use today [lo]. ESMS is further understood to provide a convenient means for the generation and observation of ion-molecule complexes (clusters) which may not be easily formed, if at all, under the conditions of other MS experiments [3,11,12]. Although a number of workers interested in cluster ions [3,13,14] have studied tetraalkylammonium halide surfactants (TAAX) at high concentrations by ESMS, to our surprise, no reports have yet mentioned their ion/ion-pair cluster ions. Low intensity cluster ions, TBA+(TBABr), have been observed during a dynamic range and sensitivity study of tetrabutylammonium bromide in a number of different solvents [ 15,161. This observation prompted the present examination of cluster ion formation in connection to the 10m5 M upper limit in the linear dynamic range (LDR) of ESMS. In this report, the concentration dependences of the TAA+ and the TAA+(TAAX), ion intensities are compared with spray current and currents collected on elements in the ion path into the vacuum, with primary interest in obtaining insight into instrumental factors and transmission limitations which might influence ESMS response and dynamic range.

2. Experimental

section

2.1. Chemicals

and solvents

All quaternary ammonium salts (tetramethylthrough tetrahexyl-, and dodecylethyl-dimethylammonium bromides, and tetrabutylammoniumchloride, iodide, and tetraphenylborate; each 99% purity) were obtained from Fluka (Buchs, Switzerland). Samples were prepared from stock solutions in 955 (v/v) acetonitrile/methanol (both gradient grade, Merck, Darmstadt, Germany) over a concentration range from 5 x lo-’ to 5 x 1O-3 M. Likewise, several amines (methyl-, ethyl-, butyl-, hexyl-, dimethyl-, diethyl-,

D.R.

Zook.

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Bruinshiemational

Journal

of Mass

dibutyl-, trimethyl-, triethyl-, tripropyl-, tributyl-, tripentyl-, and trihexyl-) were prepared from 5 x lo-’ to 5 x 10-l M from stock solutions in metbano1 (Merck, gradient grade). Dioctadecylamine (DODA) from Fluka (99% purity) was dissolved in methylene chloride at 5 x lO-2 M. All glassware used for stock solutions was cleaned by soaking in dilute nitric acid, followed by rinsing with milli-Q water, sonicating in methanol, and finally rinsing with gradient grade solvents. Sample vials were found to be usable without pretreatment. All samples were stored closed at 5°C until use. 2.2. Sample introduction

by Jlow injection

Liquid samples were individually introduced to ESMS by flow injection using a Jasco (Tokyo, Japan) Familic 1OON syringe pump equipped with a Rheodyne (Cotati, CA, USA) 7010 injection valve, associated loop port and 20 ~1 sample loop. A l-m length of 50 pm i.d. fused silica coupled the injection valve to the nebulizer (described below). Gradient grade 95:5 (v/v) acetonitrile/methanol was used for all TAAX studies reported here. Gradient grade acetonitrile has a very low electrolyte background relative to methanol, a desirable feature for monitoring the currents described in this study; the addition of 5% methanol reduces flow peak tailing without a substantial increase in the flow system background electrolyte concentration [ 161. A flow rate of 5 ~1 min-’ was used to obtain 4-min-wide sample flow peaks from which measurements were obtained. The entire injection valve was disassembled, cleaned by sonication with gradient grade methanol, and reassembled between TAAX studies. The fused silica transfer line was also changed between studies to eliminate cross contamination from high (to 5 mM) concentration injections from the previously studied TAAX. Experiments were conducted only after system cleanliness could be demonstrated by observation of a zero spray current increment response from solvent

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131

blanks. For each injection, a five-fold excess ( 100 ~1) of sample was passed through the sample loop using a clean Hamilton gas-tight syringe. All TAAX samples were run in duplicate from 5 x lo-’ to 5 x 10m3M concentration. Flow injection was used in an analogous manner for APCI experiments with amines (described below), using neat gradient grade methanol. The single exception was in the case of DODA which was dissolved in methylene chloride and introduced via a Brownlee Micro Gradient System syringe pump (Santa Clara, CA, USA) in a flow of methylene chloride in parallel to the Jasco pump flowing methanol for generation of methanol reagent ions. 2.3. ionization

by ES (ionspray)

and APCI

The pneumatically assisted nebulization version of ES (Ionspray) [ 171 was used in a diagonal, off-axis configuration. Nitrogen (99.8%, Air Liquide, Eindhoven, The Netherlands) was used as the nebulization gas. All experiments were carried out under ambient conditions. An electropolished stainless steel ES capillary (150 pm i.d., 300 pm o.d.) was connected to the fused silica transfer line from the flow injector, positioned diagonally, approximately 60” from the centre axis of the source, and 3 cm from the ion sampling orifice. The ES capillary was biased at +3 kV. Some of the experiments described in Section 3.4.2. were carried out with the nebulizer held parallel to the centre axis of the source. Atmospheric Pressure Chemical Ionization (APCI) experiments were carried out using a series of amines in methanol in order to determine mass-dependent ion transmission efficiencies. A high amine concentration (5 x 10e2 M) was experimentally chosen to afford saturation of MH+ response corresponding to complete conversion of all methanol reagent ions to a single protonated amine. In this way the massdependent ion transmission for a given focusing

132

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of MUSS Specwomrt~

condition was determined over a mass range of 18 (NH:) to 522 amu (DODA + 1). This general strategy has previously been employed for the determination of the relative ion transfer efficiency for positive [3,14,18] and negative ions [5] from atmospheric pressure into vacuum. A constant corona discharge current of 1.5 PA (approx. 5.5 kV) was maintained by a custombuilt constant current power supply. The corona discharge needle was housed in a glass “corona tube” of 28 mm i.d. which was wrapped in heating tape and maintained with a surface temperature of 150°C. The corona needle tip was positioned slightly off-axis, and 18 mm away from the ion sampling orifice. The fused silica flow injection transfer capillary (see above) was inserted through a septum into a block heated to 150°C through which a gentle stream of nitrogen was passed to effect the transfer of the evaporated mixture of amine and solvent past the corona discharge needle tip and directly on-axis to the mass spectrometer sampling orifice. The nitrogen flow also served to exclude room air from the ionization region. APCI concentration studies of amines from 5 x lo-’ to 5 x 10-l M were further carried out to examine LDR characteristics of the total ion sampling and detection system at ion currents including and exceeding those encountered during the ES concentration studies.

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internal diameters of 3.0,0.3 and 1.4 mm, respectively. A nitrogen gas curtain flows continuously (110 1 h-‘) between the atmospheric lens and nozzle plate (separated by 1.O mm) to prevent neutral material from entering and thus contaminating the mass spectrometer vacuum chamber. A flat-plate nozzle geometry (0.3-mm orifice in a 1.O-mm thick stainless steel disk) was chosen in order to minimize collection of ions which might only partially penetrate the curtain gas, an effect observed with a conical nozzle geometry. The atmospheric lens, nozzle and skimmer were normally biased at +500, +70, and +22 V, respectively. These focusing conditions allowed for maximum ion current signals at the MS detector with minimal “up-front” collision-induced dissociation in the free jet expansion region before the skimmer. For ion collection studies, these potentials were +500, ground, and -30 V, respectively. For studies of ion retarding in the region between nozzle and skimmer, the potentials on atmospheric lens, nozzle and skimmer were +500, ground, and variable, respectively. For studies of ion retarding behind the skimmer, a removable ion collection plate could be installed as a cap on the front end of the cylindrical lens behind the skimmer. Fig. 1 shows the ionization and ion sampling regions. 2.5. Data acquisition

2.4. Mass spectrometer All studies were performed with a modified Nermag (Argenteuil, France) R3010 triple quadrupole mass spectrometer with an ml: range of 1o-2000. The modification involved substitution of the original EYCI ion source with a multiplestage atmospheric pressure ion sampling system, and has been described previously [ 161. Charged droplets leaving the ES (Ionspray) capillary (see above) travel 3 cm before sampling into the mass spectrometer. The atmosphere-to-vacuum sampling region is composed of three electrodes: atmospheric lens, nozzle, and skimmer, with

A Digital Equipment Corporation PDPl l/73 computer with the Nermag Sidar 3.1 software comprised the MS data system. The first (Q,) and second (Q?) quadrupoles of the original instrument were operated in the RF-only mode, as was an additional transfer quadrupole (Qo) associated with the ion source modifications described above. The final quadrupole (Q3) before an off-axis detector was used for mass resolution of the ion beam. Profile data were collected (16 data points per mass unit) in order to obtain the signal for each isotopomer ion formed. Each TAAX acquisition table

D.R.

Zook,

A.P.

Bruinshtermtionul

Journal

atmospheric

atmospheric ion source

of Mass

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129-147

skimmer

pressure region to RF only analyzer

and

mass

quadrupoles

diffusion pump

sample Fig.

I. Schematic

for simultaneous

diagram current

solution

of the atmospheric

pressure

ionization

source

controlling Q3 had a slow total scan time (approx. 9 s). The multiplier high voltage was adjusted to allow for long integration times without saturation of the ion signal integrator in the Nermag IDS156 interface in order to optimize S/N and thus improve the measurements of low abundance ions. All isotopomer signals from a given cluster ion were summed and worked up as the response increment above baseline for the concentration introduced. The final results are the average of duplicate trials. It is noted that reproducibility was good, approximately 10% or better. 2.6. Simultaneous

with

pneumatically

assisted

electrospray

interface

and

electrometers

measurements.

ion current measurements

The established electrophoretic nature of droplet charging occurring in ES allows for observation of a current corresponding to neutralization of the counter ion on the walls within the ES capillary during nebulization [ 191. Part of the charge on droplets may be due to electrochemical dissolution of the stainless steel spray capillary. The nebulizer spray current was of interest in the present study, as were measurable currents on all electrodes comprising the atmosphere to vacuum ion sampling system (Fig. 1). In order to lower

unwanted current leakages, delrin insulators were replaced by Kel-F and polypropylene insulators. Leakage currents were thus decreased to negligible levels (below 0.1 pA for the skimmer current, as measured with a Keithly 602 (Keithley, Cleveland, OH, USA) battery-powered electrometer) during application of normal operating potentials. Measurement of nebulizer current is complicated by current flowing from the nebulizer tip through the liquid column inside the fused silica transfer line back into the grounded syringe pump. This leakage current can be neglected up to 50 PM sample concentration. The actual value of the leakage current through the liquid has been measured in a separate series of flow injections, while the nebulizer gas was turned off and the tip of the spray capillary was held at 3 kV, but shielded with a metal cup also held at 3 kV in order to prevent the development of an electrospray between the spray capillary and nearby grounded metal pieces. An Apple Macintoshbased MacLab 4e four-channel data acquisition system with associated Chart software (AD Instruments, Castle Hill, NSW, Australia) was used to monitor and record nebulizer current and ion sampling electrode currents simultaneously during normal operation of the mass

D.R. Zook, A.P. Bruins/intema:ional

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162 11997) 129-147

current. Tetraalkylammonium ions are not subject to pH effects in solution and, after transfer from solution to the gas-phase, cannot be lost by proton transfer to basic neutrals. In addition, ionic surfactants have very high response characteristics due to their high surface activities. As stated in Section 1, the cluster ion TBA+(TBABr), with n = 1 was observed to occur at the 10e5 M linear dynamic range (LDR) upper limit during previous ES (ionspray) concentration studies of tetrabutylammonium bromide [ 15,161. Higher clusters with IZ 1 2 were not investigated. A number of workers [3,13,14] have studied TAAX ionic surfactants at high concentrations by ESMS, but no reports have mentioned their ion/ion-pair cluster ions. Fig. 2 shows a high concentration (5 mM) full-scan mass spectrum of TBABr. The inset is an enlargement of TBA+(TBABr) , . The identity of all cluster ions was verified by m/z values and

spectrometer as shown in Fig. 1. Electrometers of appropriate sensitivity were floated at the potential of their associated electrode and connected with triaxial cables to the atmospheric lens, nozzle and skimmer respectively. The output signal of the electrometers was connected to the MacLab data acquisition system via isolation amplifiers (Burr-Brown, Tuczon, AZ, USA).

3. Results and discussion 3.1. Ion/ion-pair

and Ion Processes

cluster ions

3. I. 1. TAAX concentration studies Tetraalkylammonium halides (TAAX) are good model compounds for the examination of various fundamental aspects of ESMS. They are pre-formed ions in solution and thereby allow for a clear definition of the ES nebulizer spray Relative abundance 100s

242.47 563.84

565.84 1

TBA+(TBABr),

50

562

564

566

568

565.84

887.03 I..“...‘.

400

,...‘,‘...,....

-

600

800

1531.41 ,..‘.,.“‘,....,....,....

.

1000

1200

.

.

.

.

1400

.

.

.

.

1600 m/z

Fig. 2. Full isotope

scan electrospray

distribution

mass spectrum

of the TBA’(TBABr),

of a 5 x IO-’ cluster

ion

M solution

is identical

of tetrabutylammonium to the theoretically

bromide predicted

in acetonitrile-methanol.

distribution.

The measured

D.R.

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of Mass

isotopic patterns. Due to the non-uniform nature of ion sampling from atmosphere to vacuum, “lost or missing” cluster ions (m/z > 2000) were suspected as potential factors in the ESMS LDR upper limit, and correction of such undesired ion sampling conditions, it was supposed, might help to trace “missing ions” at concentrations above 10m5 M. Fig. 3a and b show the raw mass spectrometric and ion sampling electrode currents, respectively, obtained simultaneously during the flow injection concentration study of tetrabutylammonium bromide over a range from zero (blank) to 5 x 10-j M. The mass spectral data (Fig. 3a) show the time-dependent abundance profiles of the core TBAf ion, as well as the clusters TBA+(TBABr), where )2 = 1-2, as TBABr flow injection peaks of increasing concentration effuse through the capillary tip. Examination of the MS and electrode current data lead to the identification of three regions: A, B, and C. In Fig. 4a, the log-log plotted TBABr data, one observes in Region A the usual ESMS LDR approaching the frequently reported ESMS response plateau near 10e5 M [3,14-16,19-261. Region A also shows increases in all electrode currents (Fig. 4b). Region B corresponds to the onset of the approximately 10m5 M upper limit plateau in the ESMS LDR (Fig. 4a) also shown by the skimmer and nozzle electrodes, while the nebulizer and atmospheric lens currents continue to rise, but at slightly diminished rates. In Region B the TBA+(TBABr)i cluster ion appears, showing a linear increase in abundance with concentration In Region C, TBA+(TBABr)? has appeared, as TBA’(TBABr), reaches a plateau. Interestingly, at higher concentrations (Regions B and C) a gradual suppression of the mass spectral ion signal (in Fig. 3a) is seen to occur beyond the plateau, becoming increasingly pronounced up to the highest (5 mM) concentration. The first cluster ion (observed with much lower abundance at the detector than the core ion) exhibits a response plateau at yet higher concentrations (Region C). It is noted that each

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cluster ion response plateau occurs near the onset of the next higher (n + 1) cluster ion. Additionally, as seen in Fig. 3a, the cluster ions, TBA’(TBABr), and TBA+(TBABr)2, are not influenced by the suppression effects observed for the core ion, TBA+. It is emphasized that in the present experiments the ES nebulizer is directed diagonally and off-axis to the ion sampling aperture. The spray plume is thus not pointed at the ion sampling aperture, but is instead aimed at a position approximately lo-20 mm beyond it. This spraying configuration was chosen to prevent the nebulizing gas flow from blowing droplets through the curtain gas into the nozzle orifice. Thus in our experiments, ions are taken from the perimeter of the spray and thereby more gently drift against the curtain gas flow towards the nozzle orifice via the opening in the atmospheric lens plate. On-axis positioning of the sprayer, aimed straight at the ion sampling orifice, results in a quite different concentration dependence of cluster ions taken from the centre of the spray, see below in Section 3.4.2. 3.1.2. Solution phase routes to ion/ion-pair clusters The rise in the abundance of the TBA+ (TBABr), signals is analogous to results presented by Fenn and coworkers concerning multi-solute-multi-arginine ion clusters, which were considered to originate in the liquid phase [ 111. Multi-solute-multi-arginine cluster ions were further thought to arise via a desorption process from charged droplets according to the Ion Evaporation Theory [ 111. In studies by Kebarle and coworkers, the total absence of ion/ion-pair clusters from sodium chloride examined at high concentrations was taken as support of the Ion Evaporation Theory [3,14]. However, investigations of numerous electrolytes carried out in this study have shown that while certain electrolytes (including sodium chloride and ammonium acetate) do not yield ion/ion-pair cluster ions up to high (5 mM) concentrations,

136

D.R. Zook, A. P. Bruins/lntrrntinnnl

Ion signal (millions,

Journal

of‘ Mass Sprctromrtrx

and Ion Prowesses 162 (I 997) 12% 147

arbritary units)

12 10 -

Concentration,

x 1O-6 M

20

50

200

500

/ 2000

,100o

; 5000

a-

42I

o-

I

2

0 ,

TBA+(TBABr)z

A0 300

200

I

// I

II

600

Scan number - nebulizer (nA)

20-oconcentration

x 10-6 M:

100 -

nozzle (n A) 4-

100 1 skimmer

@A)

time -3

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of Mass

Spertromet~

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162 (1997)

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a higher or lower surface activity than the core TAA+ ion, information that might be useful in further assessing the route to their formation. From the results presented here concerning ion/ ion-pair clusters from a range of simple electrolytes, it is thus not surprising that ion/ion-pair clusters are formed from ionic surfactants at higher concentrations due to their very high surface activities.

1 ..,,,,(

.,,,, :,

1 o-6

.

,,,,,,,

1o-5

,‘,,,,,,(

1 o-4

...A 1 o-3

1 o‘2

3

‘B I

:A 10.’ Nebulizer ati

10-a

@

; *:PO

zg

‘C &+:0=@ A-=@

b+@’ Atm. Lens I I

-V/V

1o-g

?? 5 0 lo-‘0

&VNV

Nozzle

I

,v---

,v-v-~-v

I

7 TV .A-&&-+..

10-l’

=: .’

V’

.’

I

lo-‘*

‘v

1

‘v I

.v.J 1o-6 TBABr Etzentriil

(U)

loe3

lo-*

Fig. 4. Increments of ion signals and electrometer currents obtamed during repeated flow injections of TBABr solutions from 5 x IO-’ to 5 x IO-’ M.

3. I .3. Potential gas-phase routes to ion/ion-pair dusters Ikonomou et al. have recently alluded to the possibility of observing gas-phase ion/molecule clustering equilibria occurring in an ESMS interface chamber [ 191. Thus ion/ion-pair cluster ions observed here might also be the result of gasphase ion chemistry. Kebarle previously specified the criteria for attainment of gas-phase ionic equilibria [27]. These are: (i) reactants and products in thermal equilibrium with their surroundings; (ii) the forward and reverse reactions of interest must be faster than all other processes influencing ion concentrations; and (iii) sufficient reaction times must be possible to allow attainment of a steady state equilibrium. Reaction 1 depicts a three-body collision in which a core analyte ion, TAA+, collides with a neutral TAAX and a nitrogen (buffer gas) molecule TAA+ +TAAX+N,

-

TAA+(TAAX),

+N; (1)

other salts (including caesium iodide, sodium acetate, ammonium chloride, and all studied TAAX) do. In light of these results the production of ion/ion-pair clusters, though seemingly unpredictable, might offer evidence for the Single Ion in a Droplet Theory as outlined by Kebarle and Tang [3,14]. It is not known if TAA+(TAAX), clusters have

Fig. 3. (a) Mass spectral response obtained left). (b) Currents measured simultaneously

The energy of association is carried away as vibrationally excited nitrogen which is rapidly lost to the buffer gas. This process continues as shown in the equation TAA + (TAAX) , + TAAX + N? - TAA + (TAAX),

+ N;

by flow injections of TBABr solutions from 5 x IO-‘to 5 x IO-’ M (solvent to the mass spectral data recorded and shown in (a).

(2)

blank injection

shown at

138

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and may be expressed generally clustering reactions as TAA + (TAAX),

Journal

of Mass

for successive

+ TAAX + N,

- TAA + (TAAX),

+ , + N;

(3)

until the above conditions for equilibrium are satisfied. The reaction time allowed between the ES capillary tip prior to sampling into the mass spectrometer, estimated at l-2 ms [ 141, should allow attainment of a steady state equilibrium. It is not certain, however, if a sufficient number of free TAAX ion pairs are present in the gas phase as a result of evaporation of ionpair-containing droplets. Maybe the ion pairs exist as (TAAX), aggregates in our ion source operated at room temperature. 3.1.4. Potential perturbations to ion/ion-pair cluster intensities The relative intensities of cluster ions can be severely perturbed by ion-sampling errors encountered during the transfer of ions and gas from high pressure into vacuum. During ion sampling, declustering reactions shown by the reaction TAA + (TAAX), + mTAAX

+ N2 - TAA + (TAAX), + NZ

_m (4)

are possible where N2 provides collisional energy sufficient for dissociation of one or more TAAX ion pairs (m) during sampling through the nitrogen curtain gas in which ionic species spend approximately 0.2 s in transit to the nozzle [7]. Partial or complete CID reactions may result in complete declustering to a single core ion, TAA+. The extent of TAA+(TAAX), declustering within the curtain gas region is dependent on the unknown cluster stabilities. In our instrument we have no evidence of CID in the curtain gas region in front of the orifice. More important for declustering by CID in reaction Eq. (4) is the combination of electric field and gas pressure in the free jet expansion between the nozzle and the

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skimmer, and in the space between the skimmer and the RF-only quadrupole ion guide Qo. Formation of clusters by reactions Eqs. (l)-(3) in the free jet expansion inside the vacuum can be ruled out since neutrals, including neutral TAAX ion pairs, are excluded from the nozzle orifice and thus do not enter the free jet expansion. It follows that the production of the TAA+(TAAX), cluster ions reported here occurs prior to entrance into the nozzle orifice. In spite of the lack of known thermodynamic parameters for reactions Eqs. (l)-(3), and also the lack of knowledge about the gas-phase concentration of TAAX in the ES chamber, we assume that cluster formation at the surface of droplets generated by electrospray is far more important than formation in the gas phase at room temperature. 3.1.5. Ion/ion pair preferred stabilities Anomalies in the abundance distribution of cluster ions reflect stability differences. Riillgen and co-workers previously investigated the relative stability of tetramethyl ammonium chloride (TMACl) ion/ion pair clusters by Fast Atom Bombardment [28]. A slight preferential stability for TMA+(TMACl), was reported for n = 4. Very similar data were reported by Tolun and Todd [29] for FAB of concentrated solutions of TAAX salts (0.1 M to 1 M) in glycerol. In the present study it was found that the ES nebulizer position influenced the extent of ion/ion-pair clustering. Positions which directed the perimeter of the pneumatically assisted ES plume (rather than the plume’s axial centre) on an on-line trajectory toward the ion sampling orifice resulted in enhanced clustering. It is assumed that the on-axis nebulizer position results in enhanced clustering due to a mild pneumatically assisted penetration of weakly charged material through the gas curtain. With such a configuration, TBA+(TBABr), with n = l-4 could be easily observed. A slight preferential stability, analogous to the FAB studies [28,29], was invariably observed for n = 4.

D.R.

.&ok.

3.2. Ion transmission

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Bruins/InternutionaI

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of Mass

studies

3.2. I. Ion transfer efJiciency Table 1 shows the total ion current transmission efficiency from the ES nebulizer, through the atmosphere to vacuum pressure stages, and beyond the skimmer, for TBABr at 50 PM. An estimation of ion transfer efficiency through the mass spectrometer is also given. Theoretically, 50 PM TBABr flowing at 5 ~1 min-’ could cause a nebulizer current of 0.4 PA if complete removal of bromide ions occurred. The 70 nA increment obtained indicates that for these experimental conditions 18% of the TBABr in solution is leaving the nebulizer as free TBA’ analyte ions together with 82% TBABr ion pairs inside charged droplets. An ion pair is defined in this discussion as a TBA+ ion plus a Br- ion in solution; it may be a tight pair, or two individually solvated ions. About 90% of the spray current is collected on the atmospheric lens 3 cm away. It can be seen that approximately 1% of the charge monitored on the atmospheric lens is transfered through the nitrogen curtain gas and hits the nozzle plate, and approximately another order of magnitude is lost during transit through the nozzle to the skimmer. Approximately one out of four charged species exiting the nozzle pass through the skimmer orifice and can thus be analysed by the mass spectrometer. The losses during transfer through the three RFonly quadrupoles (Qo, Qi, and Q2) and the final Table 1 Ion transmission efficiency TBABr at 50 x lo* M Electrode

Nebulizer Atmospheric Nozzle Skimmer” Post-skimmer MS detecto?

from

nebulizer

plate

‘Skimmer received s Estimated.

52,200 45,200 486 47 I2

0.06% of a 3 pA ambient

to MS

detector

for

% of nebulizer current

Ion

discharge.

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3.2.2. Ion collection and retarding studies As stated above, cluster ions TAA+(TAAX),, which begin to occur at 10e5 M were suspected as potential factors contributing to the ESMS

A 120

40 1o-6

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mass-resolving quadrupole (Q3) cause a combined decrease in the initial MS ion current by at least another order of magnitude. In terms of analyte in solution, about one out of two hundred thousand sample ions appear to reach the MS detector.

z 2

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Fig. 5. (a) Electrometer current mcrements during TBABr Ion Collection experiment with skimmer biased to -30 V: (b) normalized skimmer current and MS data recorded during operation with the skimmer at +22 V.

140

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LDR upper limit. The discovery of signals which are converted to charged species (e.g. cluster ions), which experience very poor or no transfer to the MS detector, would reveal a need for improved ion transfer that might allow for extension of the LDR. Ion collection experiments were carried out on the skimmer biased at -30 V (see Section 2) while repeating the TBABr concentration study again from 5 x lo-’ to 5 x 10m3M. It is seen from the skimmer current-concentration curve in Fig. 5a that its shape is nearly identical to the shapes of the normalized skimmer and normalized MS detector results for TBABr concentration studies carried out under typical operating conditions as shown in Fig. 5b. In other words, there is no indication of charged material that is lost under normal conditions and neither observed in mass spectra nor collected on the skimmer biased at +22 V, but instead collected on a skimmer biased at -30 V. Ion retarding energy analysis refers to the repulsion of an ion moving against a potential field. The magnitude of the potential required to inhibit collection of (i.e. retard) the ion can be used as an approximate measure of m/z if ions of different mass all move at the same velocity. Identical velocity of ions and neutrals is achieved in the free-jet expansion in the first vacuum stage of our ion source. Ion retarding experiments have been shown as a simple means of testing for the presence of high mass ions with mlz: beyond the mass range of a mass spectrometer [9]. Fig. 6 shows the experimental configuration (Fig. 6a) and the resulting current-voltage curves for 5, 50 and 500 PM TBABr (Fig. 6b) obtained by application of potentials from -30 to +22 V to the skimmer while holding the nozzle at 0 V. Mann and co-workers estimated that a retarding potential of approximately +6 V is sufficient to inhibit collection of species with m/z > 2000 in their apparatus, and that TBA+ (m/z 242) is retarded just above 0 V [9]. Assuming similar velocities within the present free-jet expansion, it is seen at a retarding potential of +6 V that only 5% and 10% of the total ion current

and Ion Processes

162 (1997)

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emerging from the nozzle for 5 and 500 PM TBABr, respectively, is comprised of charged species with m/z > 2000. Fig. 6b clearly shows that at higher concentrations a small but increasing fraction of the TBABr ion current is due to charged species with m/z > 2000. The suppression effects (Fig. 3 and Fig. 4) observed on the skimmer (and at the MS detector) might thus be

supposed to be related only in part to the occurrence of these high mass ions. A study of the retarding of ions that have passed through the skimmer was done with a cap on the cylindrical lens as a retarding plate (Fig. 1). Inspection of the retarding curve for a 100 PM TBABr sample revealed that the fraction of ions that cannot be retarded is almost equal to the fraction of nonretarded ions when the skimmer is used as given in Fig. 6. Apparently there is no appreciable fraction of charged material that can be measured by a retarding plate behind the skimmer, but cannot be measured in retarding and collection experiments using the skimmer as the retarding or collection electrode. As stated in Section 1, Mann and co-workers reported considerable losses in ion sampling for an ES interface in combination with a glass capillary between the ion source and the first vacuum stage [9]. In that study, low mobility species were found to be excluded by a counter current nitrogen flow used for desolvation prior to the capillary. Their arrangement and our combination of curtain gas and electric field between atmospheric lens and nozzle both discriminate against low mobility ions. More recently, Guevremont et al. showed that at high TAAX concentrations some fraction of unidentified charged material is separated from both TAA’ and related clusters in the ion mobility stage of a combined IMS-MS instrument [30]. This finding corroborates our observation of high-mass charged materials in the ion-retarding experiment presented in Fig. 6. 3.2.3. Mass discrimination stud? As stated above, ion transfer from atmospheric pressure into vacuum is not a simple process. Space-charge phenomena, ion/molecule clustering, collision-induced dissociation (CID) reactions, mass discrimination in ion sampling, and ion/molecule scattering in ion optics stages contribute to complications in the sampling of ambient pressure ion sources. As an atmospheric pressure technique, ESMS is prone to the above

100

0

100

200

300

400

500

m/z of (M +l)+

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IB

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Fig. 7. (a) Mass-dependent ion transmission efficiency obtained from injections of solutions of amines in methanol under APCI conditions; (b) sampling-bias-corrected mass spectral response of ions

generated

from

TEABr.

ion sampling and transmission problems. The non-uniform ion sampling bias in atmospheric pressure ionization typically favours higher mass species [5]. Thus, as an additional test, mass-dependent ion sampling and transmission effects were examined as a potential cause of under-estimation of higher mass (m/z < 2000) cluster ions which were seen to appear at the upper limit of the LDR. Relative (mass-dependent) ion transmission characteristics were determined by saturation

142

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of Mass

experiments using APCI under identical ion source tuning conditions as used during ES (see Section 2). The results are shown in Fig. 7a. One observes very non-uniform ion sampling efficiencies for lower mass ions, rapidly increasing to a plateau of maximum sampling efficiency between m/z 150 and 300, followed by a gradual decrease in sampling efficiency out toward m/z 550. Fig. 7b shows ion transmission corrected results from a concentration study of tetraethyl ammonium bromide. Since the largest cluster observed has an m/z of 548, it was possible to apply correction factors obtained from Fig. 7a to the all of the raw data (not shown). Due to the favourable ion transmission characteristics of our instrument (Fig. 7a), the corrections do not significantly alter the relative abundance of the core and cluster ions. 3.3. ESMS linear dynamic limitations

range (LDR)

As stated above, several groups have reported near- 10m5 M upper limits in the ESMS LDR [3,14- 16,19-261, though other groups have reported much higher (up to lo-’ M) LDR upper limits [ 13,3 1,321. Fig. 4b, Fig. 8a, and Fig. 8b show that droplet charging at the nebulizer does not appear to be limited up to the maximum concentrations studied for various TAAX. The spray current shown has been corrected for current leakage through the liquid which at higher concentrations ( > 50 PM) becomes considerable and at the highest concentrations exceeds the current due to droplet charging. The measured atmospheric lens current however is completely free from liquid leakage currents and yet also reveals that droplet charging increases well beyond 10m5 M. Thus the LDR does not seem to be limited by droplet charging at the nebulizer and the passage of charged droplets through the atmospheric pressure ion source toward the atmospheric lens plate. Perhaps most interesting, in Region C an inverse trend was observed between the nozzle

Spectrometm

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and skimmer currents at higher concentrations. While nozzle and skimmer currents are nearly independent of sample concentration in region B, the skimmer current drops in region C, as does the sum of mass-analysed ion signals, while, in contrast, the nozzle current rises again. This effect was general as seen by other examples, tetraethyl- and tetrahexyl-ammonium bromide, shown in Fig. 8. The observed rise in the nozzle current might reflect the changing physical nature of the charged droplets leaving the nebulizer at increasing analyte concentrations (>0.2 mM). Such changes would not be discerned by either the nebulizer or atmospheric lens current measurements. As stated in Section 3.2.2, a counter current flow of nitrogen used by Mann et al. and the curtain gas in our ion sampling system apparently can serve to sweep away high mass solute-ion cluster species having low mobility [9]. It is thus very difficult to explain now why ions and maybe also charged droplets or other charged aggregates that have passed through the curtain gas preferentially hit the nozzle plate (or hit the wall of the 0.3-mm i.d. and 1.O-mm long nozzle orifice) but apparently cannot pass together with nitrogen through the nozzle toward the skimmer and subsequently on to the quadrupoles at high sample concentration in Region C of Fig. 8. The droplets formed at such high concentrations may result in high charge densities in front of and inside the nozzle orifice and thereby create a positive space charge that blocks the transmission of additional ion current. The fall in the skimmer current (see Fig. 3a, Fig. 4a, Fig. 8a and Fig. 8b) is particularly noteworthy since this effect corresponds exactly with signal losses observed in the MS response (Fig. 4b, Fig. 8c, and Fig. 8d). This behaviour has been observed during the concentration study of all investigated TAAX salts including tetramethyl-, tetrapropyl-, and tetrapentyl-ammonium bromide; tetrabutyl ammonium-chloride, -iodide, and -tetraphenyl borate; and dodecylethyldimethyl ammonium bromide. Thus neither

D.R. Zook. A.P. Bruins/International

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162 (1997)

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143

129-147

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Fig. 8. Comparison of MS signals and electrometer currents for a low mass (tetraethyl) and a high mass (tetrahexyl) bromide; (a) THxABr MS response, (b) TEABr MS response, (c) THxABr currents, and (d) TEABr currents.

counterion identity this general effect.

nor cation

shape influence

3.3.1. Reported causes for the ESMS LDR Presently unidentified experimental factors must explain the varying reports concerning ESMS LDR upper limits. These might include instrumental factors affecting droplet size distributions, temperature-dependent kinetic effects related to droplet fission, solvent evaporation,

quatemary

ammonium

or ion desorption rates. Chemical-physical factors of importance may include the influence of solvent [ 151, background electrolytes [ 14,15,33], or droplet surface blocking effects [ 151. Thomson and Iribarne initially proposed the Ion Evaporation Theory as a mechanism by which free gasphase ions are produced from charged droplets during electrostatic spraying [20]. They proposed that at sufficiently higher analyte concentrations (10m5 M) droplet size reduction is limited by the solubility of analyte which becomes increasingly

144

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of Mass

concentrated, with subsequent inhibition of ion release, termed the solid residue limit. Kebarle and Tang have attributed the ESMS LDR upper limit to background electrolytes: analyte responses may only rise up to the concentration of background electrolytes present in an estimated 10-5M concentration, thereafter reaching a plateau attributable to sample ions competing with other electrolytes for a share of a limited amount of droplet charge [3,14]. Concentration studies of metal cations show LDR well above 10m5 M, presumably due to their similarities to electrolytes, e.g. NaCl and NHQ, present as solvent background in connection to competition effects [3,14]. However, the background electrolyte explanation for the ESMS LDR is difficult to reconcile with the results of the present study and our earlier publications [ 15,16,33]. We have consistently worked with at least ten-fold lower electrolyte concentrations by using gradient grade methanol and acetonitrile, and we have observed nearly zero background electrolyte concentration in chloroform and dichloromethane [ 151. In spite of our low background electrolyte concentration, sample ion saturation still sets in at the same 10m5 M sample concentration. Thus the causes of the LDR involve other physical-chemical factors which have not yet been clearly identified. As an alternative to the solid residue limit explanation, it is thought that the appearance of TAA+(TAAX), at the onset of the upper limit of the LDR might suggest that the droplet surface may be progressively filled by TAA’ ions together with neutral TAAX at higher concentrations. Further arguments for a surfaceblocking effect which might limit the ESMS linear dynamic range, in connection with work carried out in our lab, are in preparation [ 15,341. 3.4. Space-charge

investigations

Sigmond previously reported that the efficiency of the sampling of ions formed in a low-energy corona through thin foils is highly

Spectrometn;

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dependent on the identity of the ion to be sampled, the choice of foil material, and the diameter of the orifice through which ions should pass [6]. These factors were reported to determine surface-potential characteristics, or spacecharge effects, that can hinder ion transmission. Sunner and co-workers reported theoretical results showing that such space charge is determined primarily by the electric fields, ion density, and ion drift time in the vicinity of an ion sampling orifice, and are thus the major factors determining the limits of detection achievable in APCI and ES techniques [7]. The suppression effects observed at the skimmer (see TBABr raw data in Fig. 3b and plotted skimmer currents for TBABr in Fig. 4b, for THxABr in Fig. 8c, and for TEABr in Fig. 8d) can be attributed to effects caused by space charge occurring at the nozzle. The experiments described below were designed to investigate whether the nozzle exhibits similar space charge effects under higher APCI ion currents, and if the position of the nebulizer might influence ion transmission. 3.4.1. APCI LDR Concentration studies were carried out for several amines from 5 x lo-’ M to 5 x 10-l M. The LDR of corona discharge APCI was found repeatedly to be linear for all the amines studied, beyond the window of ion current amplitudes in which ESMS was studied. The APCI response plateau was determined by the maximum number of reagent ions available for proton transfer (Fig. 9). The data shown in Fig. 9 were collected at lower multiplier voltages so the mass spectral ion currents transmitted under APCI conditions are actually higher than indicated. Fig. 9 thus shows that the total ion sampling system can clearly transmit higher total ion currents than those transmitted in the presented ES studies while maintaining response linearity. Thus, ESMS ion signal suppression as observed in the present study seems to be due to specific factors related to droplet generation and release of ions

D.R.

109

Zook.

A.P.

Bruinshterncrtioncl

Journal

9

Fig. 9. Abundance of the MH’ ion of a series of amines ttipropyl-, tributy-, and trihexyl-) during flow injection nolic solutions of increasing concentration and ionization discharge APCI. (after correction observed during quatemary depletion

of Mass

The shaded for a different concentration

ammonium of reactant

salts. ions.

(dibutyl-. of methaby corona

region is the signal abundance level electron multiplier voltage setting) studies of electrospray ionization of The

APCI

plateau

corresponds

with

from droplets. The observed degree of ES signal suppression is thus not due to a general space charge inhibition resulting from a sufficiently high ion current. Factors unique to ESMS, in particular the formation of weakly charged droplets or aggregates, might explain the apparent ion transfer inhibition observed in this study at high concentrations not observed at higher APCI total ion currents, which show good linearity over higher total ion current ranges. It is noted, again, that the higher mass cluster ions showed no signal suppression. In addition, the masses of TAA+ ions produced by electrospray were similar to those of the APCI-generated amine MH+ ions, so that a sample ion mass discrimination effect can be ruled out. 3.4.2. Nebulizer

position

studies

The influence of space-charge on the LDR was previously investigated during concentration studies with the same pneumatically-assisted

Spectrometry

and

Ion Processes

162 (1997)

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I45

ES nebulizer positioned much further away than normal, in an attempt to lower the charge density in the vicinity of the ion sampling orifice [33]. The result of increasing the nebulizer-to-orifice separation was an expected decrease in the MS response but with no influence on the ESMS 10T5 M LDR upper limit. Nebulizer position variations were carried out in the present study to examine the effect of changing nebulizer-to-sampling orifice separation and orientation on the electrode currents, and corresponding MS responses, at moderate (20 PM) and high (5 mM) TAAX concentrations. The four positions tested were: (i) 5 cm away, diagonal; (ii) 2.5 cm away, diagonal; (iii) 5 cm away, parallel; and (iv) 2.5 cm away, parallel. In all positions, the core of the spray plume was pointed to a spot on the atmospheric lens plate l-2 cm to the side of the orifice. Results showed that the nebulizer and atmospheric lens currents varied only negligibly between these four positions in every case. To our surprise it was found that position changes which caused a decrease in the nozzle current generally resulted in a corresponding (but lower magnitude) increase in the skimmer current. Conversely, increases in the nozzle also coincided with decreases in the skimmer current. In all the concentration studies discussed above, the nebulizer was positioned diagonally, 3 cm away from the orifice. The routine configuration invariably revealed opposite current trends between the nozzle and skimmer at high analyte concentrations. These observations can be taken as support for the existence of sensitivity-limiting electric fields occurring within the nozzle. With a 20 PM sample concentration, the closest axial nebulizer position (iv) relative to the other tested positions surprisingly caused an increase in the skimmer current while showing a corresponding signal decrease at the MS detector. Charged microdroplets are possibly blown through the curtain gas, pass through the nozzle, and hit the skimmer, but do not release

146

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&ok,

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Bruinshtemational

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sample ions that can be mass-analysed by the quadrupoles. Under normal experimental conditions the relative changes in the skimmer current are always reflected in the MS response, thus the decreasing MS response was unexpected. With a 5 mM sample concentration, the closest axial nebulizer position (iv) showed an unexpected increase in the skimmer current relative to the other three positions, and with a corresponding increase in the MS response. But as at low concentration, the high concentration (5 mM) results showed that nebulizer and atmospheric lens currents were fairly insensitive to nebulizer position changes, and again the nozzle and skimmer currents tended to move in opposite (though unequal) directions. The opposite trends between the nozzle and skimmer were also observed when tested without the nitrogen gas curtain at both concentrations. Although the studies in the routine configuration (Fig. 1) showed that the skimmer invariably reflected the MS response, these studies show that, depending on the nebulizer position, the skimmer current does not always reflect the MS response. It appears that the relative nozzle and skimmer currents are dependent on both sample concentration and nebulizer position, for reasons which are presently unknown.

4. Conclusions The MS signals observed in ES are the net result of ion production from charged droplets, and the modification of ions by mass spectrometric sampling. In this study, ion transmission characteristics in the common curtain-gas/nozzle/skimmer ES interface design were examined during the study of ionic surfactants and their ion/ion-pair cluster ions. MS ion signal suppression effects were found to occur at high analyte concentrations above 0.2 mM. The site of MS signal suppression was found to be the nozzle, and might be due to a

Spectrometr?,

und

Ion Processes

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(1997)

129-147

positive space charge which, based on APCI concentration studies, appears specific for the transfer of ES-generated ions, though this is unclear. The current observed on the skimmer is representative of the total ionization emerging from the nozzle, and under normal experimental conditions reflects the ion current transmitted through the mass spectrometer to the detector. The high concentration (>0.2 mM) response characteristics of ESMS are attributed to ion transmission limiting (instrumental factors), in addition to chemical-physical, factors. Cluster ion formation may be an interesting topic for further study in connection with the mechanism of ESMS. The factors which determine ion/ion-pair cluster formation for a given salt are also not known. Sodium chloride and ammonium acetate do not form ion/ion-pair clusters at high concentrations, whereas caesium iodide, sodium acetate, ammonium chloride, and all studied TAAX, do. Arguments can be found in the literature for either the Ion Evaporation or Single Ion in a Droplet mechanism for production of ion/ion-pair cluster ions. TAA+(TAAX), are increasingly formed at higher concentrations, but their occurrence does not account for the upper limit in the LDR. Numerical correction of raw data using mass-dependent ion transmission efficiency factors further verified that ion/ ion-pair cluster ions are not of obvious importance with respect to the ESMS LDR upper limit. Ion collection studies on the skimmer demonstrated that observed upper limits in the LDR are not due to CID, scattering, or other focusing losses of “missing” high m/z ionization entering the mass spectrometer. Interestingly, the mass spectral responses of ion/ion-pair cluster ions (m/z > 2000) did not exhibit suppression effects, suggesting that they might arise by condensation in the free-jet expansion, beyond the nozzle. However, the use of a curtain gas that prevents neutrals other than nitrogen from entering the nozzle refutes this possibility. It seems possible that the absence of suppression effects for ion/ion-pair cluster ions might be related to

D.R.

&ok,

A.P.

Bruinshternational

Journal

of Mass

their lower abundance relative to the core TAA’ ion. Ion retarding potential experiments showed that a limited, concentration-dependent amount of unidentified high mass clusters (m/z > 2000) is transported into the vacuum, and may at least be partly related to signal suppression effects at high analyte concentration.

Acknowledgements We thank Professor F.W. Rollgen of the University of Bonn, Germany, for helpful discussions. The Technical Services Staff at the University of Groningen Centre for Pharmacy are credited for their expertise in equipment construction.

References [I] J.B. Fenn, M. Mann, C.K. Meng, S.F. Wong andC.M. Whitehouse, Mass Spectrom. Rev., 9 (1990) 37. [2] R.D. Smith, J.A. Loo, C.G. Edmonds. C.J. Barinaga and H.R. Udseth, Anal. Chem., 92 (1990) 882. [3] P. Kebarle and L. Tang, Anal. Chem., 65 (1993) 972A. [4] D.R. Zook and E.P. Grimstud, J. Phys. Chem.. 92 (1988) 6374. [5] D.R. Zook and E.P. Grimstud, J. Am. Sot. Mass Spectrom., 2 (1991) 232. [6] R.S. Sigmond, in L.G. Christophorou (Ed.), Gaseous Dielectrics III, Proc. from the Third Int. Symp. on Gaseous Dielectrics. Knoxville, TN. USA, Pergamon Press, New York, 1982, pp. 92-96. [7] M. Busman. J. Sunner and K. Vogel. J. Am. Sot. Mass Spectram., 2 (1991) 1. [8] B. Lin and J. Sunner, J. Am. Sot. Mass Spectrom., 5 (1994) 873. [9] M. Mann, S.F. Wong and J.B. Fenn. in A. Hedin, B. Sundqvist and A. Benninghoven (Eds.), Ion Formation from Organic Solids, IFOS V, John Wiley and Sons, New York. 1990, pp. 139-144. [IO] M.S. Wilm and M. Mann, lnt. J. Mass Spectrom. Ion Processes, 136 (1994) 167.

Spectromet?

[III

and

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141

C.K. Meng and J.B. Fenn, Org. Mass Spectrom., 26 (1991) 542. [I21 M. Vincenti, J. Mass Spectrom., 30 (1995) 925. 4 (1993) 524. [I31 J.B. Fenn, J. Am. Sot. Mass Spectrom., [I41 L. Tang and P. Kebarle, Anal. Chem., 65 (1993) 3654. and A.P. Bruins. Rapid Commun. Mass SpecII51 R. Kosttainen trom., 10 (1996) 1393. [I61 A. Raffaelli and A.P. Bruins, Rapid Comm. Mass Spectrom.. 5 (1991) 269. ]I71 A.P. Bmins. T.R. Covey and J.D. Henion. Anal. Chem., 59 (1987) 2642. [I81 1. Sunner. G. Nicol and P. Kebarle, Anal. Chem., 60 (1988) 1300. A.T. Blades and P. Kebarle, Anal. Chem., 62 [I91 M.G. Ikonomou, (1990) 957. ml B.A. Thomson and J.V. Iribame. J. Chem. Phys., 71 (1979) 4451. PII J.V. lribame, P.J. Dziedztc and B.A. Thomson, Int. J. Mass Spectrom. Ion Phys., 50 (1983) 331. WI K. Hiroaka and 1. Kudaka, Rapid Commun. Mass Spectrom., 4 (1990) 519. ]231 L. Tang and P. Kebarle, Anal. Chem., 63 (1991) 2709. [I31 R.D. Smith. J.H. Wahl. D.R. Goodlett and J.A. Hofstadler, Anal. Chem., 65 (1993) 574A. Th. Dtilcks and U. Giessman, in v51 F.W. Rollgen. U. Liittgens. Proceedings from the 41st Conference of the American Society for Mass Spectrometry. San Fransisco, CA, May 30-June 4. 1993, pp. la-b. WI X. Xu, S.P. Nolan and R.B. Cole, Anal. Chem., 66 (1994) 119. ]271 P. Kebarle, Ann. Rev. Phys. Chem., 28 (I 977) 445. WI K.P. Wirth, E. Junker and F.W. Rollgen, in E.R. Hilf, F. Kammer and K. Wien (Eds.), PDMS and Clusters, Proceedmgs of the 1st lnt. Workshop on the Physics of Small Systems, Wangerooge. Germany, Springer-Verlag. Berlin. 1986, pp. 65-71. [29] E. Tolun and J.F.J. Todd, Org. Mass Spectrom., 23 (1988) 98. [30] R. Guevremont. M.K.W. Siu and L. Ding, Paper ThOA I I:30 presented at the 44th ASMS Conference on Mass Spectrometry and Allied Topics, Portland. OR, U.S.A., May 12-16, 1996. [31] M. Mann, Org. Mass Spectrom., 25 (1990) 575. [32] M.V. Buchanan. M. Shanhghoh and K.D. Cook, in Proceedings of the 4lst ASMS Conference on Mass Spectrometry and Allied Topics. San Francisco, CA. May 30-June 4, 1993, pp. 7@a-b. [33] R. Kostiainen and A.P. Bruins. Rapid Comm. Mass Spectrom., 8 (1994) 549. (34) D.R. Zook and A.P. Bruins, manuscript in preparation.