New instrumentation in mass spectrometry

51

International Journal of Mass Spectrometry and Ion Physics, 45 (1982) 51-86 Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

New Instrumentation Curt Finnigan

MAT

GmbH,

in Mass Spectrometry Brunnee

BarkhausenstraBe

2, 2800

Bremen,

W. Germany

Abstract An overview will be given about the progress in mass spectrometer instrumentation throughout the last years using the five functional elements as a guideline: sample introduction, ion production, mass separation, ion current detection and instrument control/data handling. The relations between the functional elements will be discussed together with the physics behind these elements. Historical remarks and commentary related to instrumental development will also be presented. Introduction: Unique features of mass spectrometry. cal spectrometry, Planck’s and Einstein’s laws.

Reasons

for their

existence.

Relation

to opti-

1. Sample Introduction: 1.1 Nonvolatiles: rapid heating I fast energy transfer, direct chemical ionization, particle bombardment (SIMS, FAB, laser, 252Cf), pyrolysis of biological samples. 1.2 OnLine: GCIMS- and LCIMS-interfacing, introduction and ionization of solids by inductively coupled plasma, on-line 13C/12C analyses of GC fractions. 1.3 Automation: automated direct probes, automated batch inlet systems. 1.4 Small samples: 13C/12C analyses, gas ion probe, automated introduction of thermionic samples. 2. Ion Production:

Why

do mass

spectrometers

require

ion sources?

Gravity

mass

spectrometer.

3. Mass Separation: 3.1 Transmission: basic ion optical laws, stigmatic and wide angle focusing, image error correction. 3.2 Speed: laminated magnets, fast and high resolution selected ion monitoring. 3.3 Specificity: various systems of MS/MS (sectors, quads), linked scan technique, multidimensional separation, selected ion flow tube, isotope separators, parabola spectrograph, secondary ion probe. 3.4 Mass range: high mass range sector instruments and quads, Wien-filter, time of flight instruments, Liouville’s theorem. 3.5 Resolution: ion cyclotron I Fourier transform mass spectrometry, ultra-high resolution, ion trap, ultra-high precision mass determination, 4. Ion Current Detection: Electra-optical ion detection, lector systems, multichannel averaging, positive/negative coincidence photoion-photoelectron spectrometer.

energetic ion mass spectrometer, ion detection, phase sensitive

5. Instrument Control and Data Handling: Off-line processing I central puter systems, fast data acquisition, instrument control through front dependent experiments, lab data management, future aspects.

computer, dedicated end microprocessors,

multicoldetection,

comdata-

Introduction It seems to me somewhat difficult to give a complete review about all the mentation in just one hour. What I rather will try to do in my presentation tions between the various instrumental developments using the listed five the mass spectrometer itself and the physics behind these elements as a . . . . .

Sample Introduction Ion Production Mass Separation Ion Current Detection Instrument Control I Data

OOZO-7381/82/0000-0000/$02.75

aspects of new instruis discuss some relafunctional elements of guideline:

Handling 0 1982

Elsevier

Scientific

Publishing

Company

52 Thus, this review is centered more around hardware than around application. of this talk, including new applications, will be covered in more detail by the this conference. It is an exciting mass spectrometry . . . . . . . . . . .

areas of science where A few examples are:

Analytical chemistry Biochemistry Physical Chemistry Environmental Science Medicine Pharmacy Geochemistry Mineralogy Geochronology Cosmochemistry Nuclear Science

Why is this some uniqe . . . . .

experience to see at this conference the many different has proven to be a very useful analytical tool [l-3].

Many of the topics lectures given during

so? The features

answer from in itself:

the instrumental

point

of view

is simple.

Mass

spectrometry

has

It is applicable to all elements Molecules up to a molecular weight of more than 10 000 daltons can be analyzed Single atoms or molecules can be detected Response is linear over more than 12 orders of magnitude Instrument output is directly proportional to the concentration of the compound

The reason for the existence of these features is that mass is one of the two fundamental qualities in physics that characterize matter in general. The other one is frequency. Frequency is also widely used for analytical purposes and has led to a number of important instrumental techniques: . . . . . .

Ultraviolet Spectroscopy, Infrared Spectroscopy, Raman Spectroscopy, Microwave Spectroscopy, X-Ray Spectroscopy, and NMR Spectroscopy.

Thus, tified.

by using

either

frequency,

mass,

or a combination

of both,

atoms

or molecules

can

be iden-

I might remind you that, by very simple and well-known formulas, frequency and mass are related to another entity which is the most basic one in physics, the energy E. The first one is Max Planck’s well-known formula published in 1900, which says that the energy E of any electromagnetic oscillator having the frequeny v is radiated in quantums h.v : E = h.v (h = Planck’s

I will mass

come back is equivalent

to this formula to energy:

later.

The

other

(1) constant)

one

is Einstein’s

famous

formula

which

The wide

application

of high

resolution

mass

spectrometry

that

(2)

E = m.c* (m = mass, c = velocity

says

of light)

is finally

based

on this

formula.

Since 1910, when J.J.Thomson presented his parabola mass spectrograph (Fig. l), enormous technical progress has been achieved related to the five basic functional elements of a mass spectrometer mentioned before.

Fig.

1

Sir Joseph

Let us start 1. Sample

with

John sample

Thomson

(1856-1940),

mass

spectrograph

introduction:

Introduction

In Fig. 2 I have tried to illustrate some trends in the evolution of mass spectrometer inlet systems. Every few years new systems have been derived from the main streams, leading more diversification and to new applications. Four points marked with R, 0, A and S seem be characteristic of the development throughout the last years, they are: . . . .

and his parabola

to to

Rapid Heating (analysis of nonvolatiles) On-line Analyses Automation Analyses of Small Samples.

I will discuss these points in more detail transfer to the molecule to be analyzed.

Fig. 2

now.

First:

Rapid

Trends systems Heating,

Organic compounds of low volatility represent a large fraction of Thus, throughout the last years, a great amount of research work propriate methods of introducing and ionizing these compounds pointed out very early that rapid energy transfer to the molecule us plots of the rate constants of decomposition and vaporization the vaporization is favored (Fig. 3).

in the evolution

which

means

of inlet

fast

energy

the biological material on earth. has gone in to finding the ap [5-131. Friedman and coworkers is the key point 1141. The Arrhenishow that at high temperatures

54

Inr

Fig. 3

Relation between rate temperature T [5]

A simple method placed on a wire stable compounds ionized with low and ionization of

constants

r and

Fig. 4

High

mass

end of DC/ spectrum

[15]

based on this principle is direct chemical ionization DCI, where the sample is that can be put right into the source [16]. By direct heating of the wire, such unas, for example, the high molecular weight platinum complex in Fig. 4 can be fragmentation. This technique has proven to be very effective for introduction nonvolatile organic compounds.

I have just talked about temperature T is raised. given by the well-known

the energy E a molecule The translational energy formula:

will pick up when the of the molecule is

E = 312.kT

(3)

It contains the constant k named after the famous physicist Ludwig Boltzmann who was born in Vienna and lectured there. Another fundamental formula found by Boltzmann says that the most probable status of a thermodynamic system is the one having maximum entropy, that is, disorder:

The future of mankind graved on Boltzmann’s

S = k.log

W

(S=entropy,

W=probability)

is closely tombstone

(4)

related to this formula in Vienna (Fig. 5).

[4]. It is en

Rapid energy transfer is also applied when mass spectrometry is combined with those chromatographic methods that preferably deal with compounds of low volatility; liquid chromatography, for example [17, 181. Fig. 6 shows the principle of two different LClMS interfaces: moving belt system [19] and thermospray or jet system [20]. Despite the fact that the hardware looks different, the underlying principles are Fig. 5 Boltzmann‘s tombstone quite similar: line of sight transport of solvent plus sample, desolvation during transport, rapid energy transfer to the sample within the source. In the first system the sample, together with the solvent, is collected on a belt which continuously moves through several vacuum stages into the source. In the second system, developed by Vestal and coworkers, the solvent is nebulized in a heated nozzle. The droplets pass through a differential pumping stage where 90 % of the solvent is pumped away. The droplets finally hit a heated plate where a rapid vaporization takes place. The sensitivity of both techniques is in the ng range. Fig. 7 shows a typical result obtained with the moving belt device.

55 In further experiments with his jet interface, Vestal found that ionization can take place during the nebulizing procedure in the heated nozzle [22]. The ionization efficiency is moderate. However, the background is also low, resulting in a favorable signal I noise ratio. Very little fragmentation is observed, as illustrated by the spectrum of Fig. 6.

Fig. 6

Schematic

of LCIMS

interfaces

[M+K]+ (3’361

Fig.

7

Spectrum of deoxyadenosine-5-monophosphate, moving be/t LCIMS-interface, chemical ionization 1271

MC Lafferty 1231, Henion jet system, where a few hole, Fig. 9.

Fig. 8

Spectrum of adenosine-5monophosphate, thermospray interface [22]

1241, Arpino 1251 and others have successfully percent of the eluate is directly injected into

LCIMS-

applied a relatively simple the Cl source through a tiny

effluent inlet 1

cocGiing

b-

water

1oy--m/

inlet

5 ,um pinhole

I I

Fig. 9

Direct

in/et

LUMS

interface

1241

I

Fig.

10 GUMS

open

split

coupling

Development will continue, and, presumably in the years to come, on-line LClMS systems will be applied on a routine basis as is the case with GUMS systems now 126-291. I might point out that it took about 20 years until GUMS interfaces reached the technical maturity of today as exempli-

56 fied by the open split interface (Fig. 10). It is well accepted performance: the end of the column is under atmospheric tions are drawn into the source through a capillary.

because pressure

of its simplicity and good and the chromatographic frac-

LC and GC are not the only chromatographic techniques to be combined with mass spectrometry. Future instrumental development probably will lead to automated systems combining electrophoresis, thin layer, paper chromatography and others with MS. Fig. 11 from a paper by Cooks and coworkers [61 shows the direct analysis of a neuro-transmitter compound from a paper strip using SIMS (bombardment with argon ions) as a soft ionization technique. A few words about these new soft ionization techniques, although ionization is not the topic of this overview: These techniques also use rapid energy transfer. Ionization is realized by bombardment with particles. By the way, the principle behind all these new techniques is a very old one, known for a few hundred years, Fig. 12. The l

l

.

.

Fig.

11 Analysis of choline from a paper strip

Fig.

12 Principle

of particle

and acetylcholine [S]

bombardment

primary

particles

can

be:

Fast fission fragments from 2%f (Macfarlane’s method of Plasma Desorption PD 1301) Ions (Secondary Ion Mass Spectrometry SIMS, pioneered by Benninghoven 1311, Cooks 1321, Devienne 133) and others) Atoms (Fast Atom Bombardment by Barber 1341)

FAB,

introduced

Photons (Laser Desorption LD, worked out by Kistemaker 1351, Cotter 136, 371, Hunt [38], Huber 1391, Vestal 140) and others)

Fig. 13 shows the similarity of two spectra, obtained by laser desorption and by bombardment with fast fission fragments of 252Cf. This result is quite typical. The spectra from these and the other soft ionization techniques usually do not differ too much, suggesting that the basic desorption and ionization process on the surface is similar in all cases j41, 421. Often hydrogen or alkali ions are attached to the molecule (protonation, cationization). Bombardment by fast atoms has been widely used for labile organic samples throughout the last two years. The FAB technique is simple to handle and provides relatively high and stable ion currents. The sample is normally kept in a solvent matrix of glycerol, thus providing some kind of a selfhealing surface for the impinging particles (“Liquid SIMS”). Compounds with a molecular weight of several thousand have been analyzed successfully, and there is no doubt that FAB, besides SIMS, laser and field desorption [431, will continue to be a standard technique for the analysis of nonvolatiles. Fig. 14 shows a typical spectrum.

57

LASER

desorption

Fig. 252

Cf ionization

14 High mass end from spectrum of melittin (NH,-Gln-Gln-Arg-Lys-ArgLys-//e-Trp-Ser-//e-Leu-A/a-pro-Leu-G/yThr-Thr-Leu-Val-Lys-Leu-Val-A/a-G/y-//eG/y-H), FAB ionization 1341

As already mentioned, FAB or SIMS ionization might be useful in combination with other chromatographic techniques, e.g. LClMS [44-481. It has been’used recently together with a moving belt LClMS interface [45-481. A typical result is shown in Fig. 15. Three of our inlet system key words were: nonvolatiles, fast energy transfer and on-line. In desFig. 16 another example referring to these points is shown. In this arrangement, investigated recently by Svec and coworkers, an inductively coupled plasma (ICP) was used to vaporize and ionize solids in solution 149, 501. The ions are drawn into a quadrupole for mass analysis through a fine hole. In addition to ICP, other types of discharge ion sources have been recently developed by several authors, mainly in conjunction with the analysis of solid inorganic materials 1511. Fig.

13 Spectra iodide, orption

FRACTION

Fig.

of dimethyldibutylammonium252Cf ionization and laser [41]

RAFFINOSE

15 Moving belt za tion 1481

MW501.

LCIMS-interface,

I

FAB

ioni-

Fig.

16 Sample inductive

introduction coupled

and ionization plasma [49, 501

by

I come to the next inlet system key word: Automation. Fig. 17 shows a system that can be used for off-line analysis of samples that might be preseparated by chromatographic techniques like LC, TLC or others. The fractions are placed on probe tips which are put into a turret sample magazine. A rod then inserts one probe tip after the other into the source of the MS. Ionization can be performed by El/Cl, DCI or FAB. A similar system has been designed by Meuzelaar and coworkers for the introduction of nonvolatile organic samples to be pyrolized (Fig. 18). It has been demonstrated that mass spectra of pyrolized bacilli can be successfully used for fingerprinting purposes.

Fig.

17 Automatic troduction in a turret

direct insertion probe for in. of up to 46 samples, stored magazine [52]

Fig.

18 Schematic of pyrolysis with MS/MS quadrupole spectrometer (automatic not shown) [53, 541

Automation of sample introduction is not limited to mass spectrometers stry. For isotope instruments, automatic devices have been developed, tem for unattended automated isotope analyses of gases, for example The valves are under computer control and operated electropneumatically. nanomole range can be handled with such systems.

system mass in/et

used in organic chemitoo. A batch type inlet sysCO,, is shown in Fig. 18. Gas samples in the

We have come to our last key point: small sample size. Fig. 20 shows the schematic of a system designed by Matthews and Hayes for on-line carbon or nitrogen isotope analysis of organic fractions separated by a GC [56]. The amount of CO, or N, gas that is generated in a catalytic furnace is very low. Application: studies of metabolism by isotopic labelling of compounds.

Fig.

19 Automatic analyses

batch of gas

Fig. 20 On-line isotope rated by a gas

type in/et sytem samples [55]

ratio monitoring chromatograph

for isotope

of fractions

ratio

sepa-

A similar system which also is able to handle very small amounts of gas has been used by Stahl for the carbon isotope analysis of methane contained in sediments [57]. This technique has been extremely useful during oil prospection since the 13C112C isotope ratio is related to the socalled maturity of the sediment. Maturity values (which correlate with isotope ratios) that are within an “oil window“ suggest the presence of oil in deeper regions (Fig. 21). Another system, recently designed for the analysis of small amounts of gas in solids, is shown in Fig. 22. It is the gas ion probe. The gas is freed from the metal by ion bombardment and then leaks into an electron impact source for analysis 1581.

59

I(%o) -50

I “C/“C

/

IN METHANE

2 Schematic

of gas ion probe

I will end the part on inlet systems with another example from the isotope field. Key points in this case are: small sample size and automation. Fig. 23 shows a thermionic ion source which is equipped with a turret magazine containing the solid sample (ng to pg) to be analyzed [59]. The rotation of the turret is under computer control. Thus, one sample after the other can be analyzed automatically. I have arrived at ion sources in my talk. It was number two on our list of the functional elements.

60 2. Ion Production However, I will not talk about ion sources since that is the topic of another lecture at this meeting. I will only raise the question: Do we need an ion source at all? Can a mass spectrometer be built that works with neutral particles and uses the earth’s gravitation as deflecting force? Fig. 24 is my view of such a spectrometer: a tube of 1 km in length that would not fit into a laboratory. Suppose we could accelerate neutral molecules to an energy of 3 kV and send them through the tube in form of a narrow beam. The path then is slightly curved because the ions “fall“ to the earth during their flight, Fig. 25.

Fig. 24 Gravity

mass

spectrometer

Fig. 25 Dispersion

of gravity

mass

spectrometer

The lateral dispersion D of the ions is very low. Example: Mass 100 is separated laterally from mass 200 by 0.084 cm only. The intensity would be many orders of magnitude less compared to an electromagnetic mass spectrometer since we could not focus the beam at the injection hole but would have to use apertures to form a narrow beam. Thus: such an instrument would not work in practice. This means: we need an ion source for efficient mass separation. The reason why gravity spectrometers for neutral molecules would work so poorly is that the coupling constant of the gravitational force is extremely small: 1g4n in an absolute scale. The age of the universe, given in units of the time which light needs to pass the radius of a proton, is also related to the power of 40. It is 1040. A third number beyond human ima ination is the number of f particles in the universe: lOso, which in fact is the ratio of 1040 over 1g4 It was the conjecture of nobel prize winner Paul Adrien Maurice Dirac that there might be a basic reason behind the simple numerical relationship of these three dimensionless units. We will come

3. Mass

. l

. l

to earth

and

discuss

the third

functional

element:

Separation

Performance

l

down

is characterized

by:

Transmission Speed Specificity Mass Range Resolution.

I will start with transmission and will report about single sector type instruments first. Transmission is proportional to the entrance slit width S and the aperture angle a of the ion beam the analyzer will accept. Thus the “quality“ Cl of sector instruments can be described by the product of resolution times entrance slit width times aperture angle, Fig. 26. It has been demonstrated that this product Cl equals F/r in all sector type instruments, where F is the area the ion beam sweeps out in the sector and r is the deflection radius [60-631. Therefore, one can estimate the performance of an analyzer just by looking at the size of this area. The greater this area F, the greater the Q-value, provided the radius r is kept constant.

61

\

enihnce

slit widlh

Q = (resolution).(slit

width)-(aperture

Q= ~

art!aF

radius

Fig. 26 Q-value,

angle a)

P

principle

An example (see Fig. 27): a) is the schematic of a conventional 90° magnetic sector instrument, top and side view. b) is an instrument with rotated pole faces. Radius and aperture angle a are equal in both cases. However, in case b) the area F, and hence the “quality“ value Q, is greater by a factor of two. Another advantage of system b): the ions are focused Fig. 27 Q-value: a) schematic of normal sector type instrument, top and side view; b) in two directions, which results in an additioinstrument with rotated pole face nal increase in transmission. Instruments having such ion optics have been developed in the last few years mainly for isotope ratio studies where transmission is of utmost importance 155, 591. Large areas greater the throughout rection have ments have pole faces; tical elements

F can only be obtained with large aperture angles a. The greater this angle, the image errors and therefore the smaller the maximum resolution (Fig. 28). Thus, the last few years, mass spectrometers with more or less complete image error corbeen successfully designed in a number of laboratories. More refined deflection elebeen used for this purpose (Fig. 29): magnetic sectors having rotated, curved or tilted electric sectors having toroidal instead of cylindrical cross section. Additional ion oplike hexapoles or quadrupoles have also been incorporated 163, 641.

MAGNETIC

SECTORS

a Fig. 28 Image sector

error

of magnetic

Fig.

29 Magnetic

ELECTRIC

SECTORS

@Q@

HEXAPOLE

and electric

ion optical

elements

62 As in the case of light optics, extensive computer calculations are required to find the optimum combination for the deflection and focusing fields. The schematic of an advanced double focusing system calculated by Matsuda is shown in Fig. 30. It has second order image correction. Advanced sector type and other ion optical systems of high transmission have also been developed throughout the last years for the microanalysis of surfaces by SIMS [66-691. Fig. 31 shows the schematics of an ion microscope: The secondary ions ejected are mass analyzed and produce an enlarged image of the surface under investigation.

Fig. 30 Schematic of double focusing mass spectrometer with second order image error corrections [65] Fig. 31 Schematic

of ion microanalyzer

[66] )

Before ending this section about transmission, a short remark related to the area swept out by particles in a deflection field. I have discussed, above, that equal areas lead to equal performance of the system. You might remember another law in physics which also refers to the equality of areas. This law is also related to the motion of particles in a deflecting field, the particles being somewhat larger than ions. It is Kepler’s second law about the motion of the planets which one finds in his famous “Harmonices Mundi”, published 1619 in Linz, 170 km from Vienna, where he lived at that time (Fig. 32). It says that a line (radius vector ) that is extended from the sun to the planet sweeps out equal areas of the ellipse in equal times (Fig. 33) because planets move faster at the aphelion and slower at the perihelion.

w

Imml, KL)+rl

HARMONICES MVNDI

Fig. 32

Title page Mundi“

of “Harmonices

Fig.

33 Kepler’s

second

law

63 At the beginning of this talk I have referred to the relation between mass, energy and frequency. I might mention that Kepler related the motion of the planets to frequencies, i.e. to musical harmonies. He found out that the ratios of the angular speed of the various planets in the aphelion and the perihelion are nearly equal to the frequency ratios that belong to the basic tone intervals of our musical system. The earth’s movement, for example, is given by the second, as the notes show, taken from the “Harmonices Mundi” (Fig. 34). A rationalist of today might say that is accidental; however, Kepler was convinced that this relation is another proof of God’s intention to harmonize the universe. Fig. 34 Musical harmoWe just came to speed, which was the second point on our list: connies of the plasiderable progress has been obtained with respect to the scanning nets according speed of sector type instruments by using laminated magnets. A to Kepler scanning speed of the order of 0.1 seconds per mass decade at a resolution of 1000 is feasible now with modern instruments. This feature will probably be of importance in the future in connection with the use of faster GC capillary columns. Laminated magnets can also rapidly be switched from one mass to the other in about 50 milliseconds, thus enabling the monitoring of a number of characteristic masses during a GC run (multiple ion detection MID or selected ion monitoring, Fig. 35). Years ago switching of the accelerating voltage was the only means to reach the required speed. The MID technique found wide application in the last years because of its high sensitivity (the ion current is integrated longer) and its inherent simplicity

r

-L----

360ms

CVCLE TIME

7

--^

Fig. 35 Fast peak switching magnet [70]

Fig. 36 Selected ion monitoring low and high resolution

with

-

laminated

of mass

355, b

In MID trace analysis, it often happens that the peaks one is looking for are hidden by background, as in this case of Fig. 36, GUMS analysis of plasma catecholamines [71/. Left side, low resolution fragmentogram mass 355. By tuning the mass spectrometer to high resolution, the characteristic peaks can be separated from the interfering background. There is little doubt that high resolution MID will become one of the standard techniques for trace analyses and routine screen. ing analyses in organic chemistry. Modern instrumentation has simplified the procedure. After the exact masses to be monitored have been typed in, the instrument, with its precise digital control of the electric and magnetic field, will then find these masses automatically. The extremely high specificity of this method results from the fact that two very powerful separating techniques, high resolution capillary GC and high resolution MS, having selectivity in two different dimensions (chemical adsorption and precise mass), are combined in one analytical procedure.

64 IONS OF MIXTURE

SELECTION OF PARENT ION

PARENT ION

MAGNETIC FIELD B

M,

SELECTION DAUGHTER -

t : c b SOURCE

)

Ml

DAUGHTER ION

*

Mz ) Mz c

ELECTRIC FIELD E

COLLISION CELL DAUGHTER IONS E, - M,v,2 E> - Mzv: Ez - M&

0, - Move

OF IONS

SCAN

MI

n

OF ELECTRIC

FIELD:

ENERGY ANALYSIS (MASS OF DAUGHTER IONS Fig. 37

Principle

+

ANALYSIS)

of MSIMS

The same basic principle, i.e. two-dimensional separation, is behind the MS/MS method, where the mass spectrometer itself isolates the targeted compound in a first step and identifies the compound in a second step [72-781. I will explain the principle using a double focusing mass spectrometer of reversed geometry, the magnetic field being the first, the electric field being the second stage (Fig. 37). The mixture is ionized by Cl or another soft ionization technique. The parent ion is selected by the magnetic field. It is then sent through a collision cell filled with a gas, where collisional induced decomposition (CID) into daughter ions takes place 1791. The daughter ions are characteristic of the targeted compound. The daughter ions all have the same velocity, but different masses. That means, they have different energies and thus can be separated in the electric field. This is the well-known MIKES or DADI technique.

IONS

OF

MIXTURE PLUS DAUGHTER -

SOURCE

L

=% ZB

IONS

DAUGHTER

MAGNETIC FIELD

DAUGHTER

ION M,

+

ELECTRIC FIELD E

COLLISION CELL LINKED

SCAN

E/B - v, CONSTANT:

DAUGHTER IONS OF SAME VELOCITY v, WILL PASS MAGNETIC/ELECTRIC FIELD Fig.

38 Linked

scan

ION M,

w +-

65 Similar results are obtained with the linked scan technique, where the collision chamber is placed near the entrance slit (Fig. 38). To obtain a spectrum of the daughter ions having all the same velocity v,, both fields have to be scanned simultaneously in such a way that only ions of this velocity v,, can pass through the instrument. Since B is proportional to the momentum mv and E is proportional to the energy 112 mv*, the condition is met when the ratio E/B, which is equal to v, is held constant during the scan. I will refer to the formula v= E/B later, when talking about the Wien filter. Other linked scan functions 178, 80-851, which I will not discuss here, are required to find all parent ions that belong to a certain daughter ion, or to find all those ions that have lost a certain fragment during collision. This neutral loss scan is important for functional group analyses. In modern instruments all functions are generated digitally and the fields can be controlled very precisely by D/A converters. The merits of the MS/MS technique for the detection of trace compounds in complex matrices, routine screening analyses, and for basic studies about metastaole transitions, are as follows: . . . .

Direct introduction of the sample without pretreatment Very fast response as compared to chromatographic High sensitivity and specificity Full information about all metastable transitions.

for

methods

There is no doubt that this powerful technique will find more and more applications in the years to come. However, the proven chromatograhic techniques like GUMS, LClMS will not be replaced in general. These techniques are unsurpassed in all those cases where little information exists about the mixture and a multicomponent analysis has to be performed. Thus MS/MS should be considered a complementary technique rather than a competitive one. I might mention that multidimensional separation has been used with GC alone, too. technique of column switching. The first column isolates a cut of the chromatogram, then sent on-line into a second high resolution column for further separation 1861.

EBE

It is the which is

More refined instrumentation has been developed for MS/MS in the last years (Fig. 39). System 1 has been discussed. Systems 2 and 3 consist of a first double focusing stage, which is followed by an electric sector or a magnetic sector respectively. Advantage: the primary beam can be selected by high resolution. System 4 consists of two complete double focusing analyzers in a tandem arrangement. McLafferty’s group has built such a system to overcome the limitations of systems 2 and 3 with respect to resolution of the second stage 1871.

EEIEEI

Fig.

l Fig.

40

39

Triple

MS/MS

quad

[89]

systems

66

A quadrupole can serve as a very efficient CID cell if it is operated in the RF-only mode 188, 891. The RF transports all ions through the collision cell without mass separation. In the second quadrupole, the daughter ions are analyzed. The primary beam can be selected by a single magnet field (system 5), by a double focusing system (system 6) or by a quadrupole (system 7). This last configuration is relatively simple from the instrumental point of view. It is fast and can be controlled very well by an integrated data system. As an example, Fig. 41 shows the mode of operation on the screen of the data system terminal: selection of the primary ion in the first quad, formation of the daughter ion in the second quad, scanning of the daughter ions in the third quad.

Fig. 41

Triple

quad,

mode

of operation

By slow scanning of the primary can create a complete overview

quad and simultaneous rapid scanning of the third quad, of all metastable transitions (Fig. 42, see also Fig. 18).

CYCLOHEXANE

DAUGHTER

Fig. 42 Metastable

mapping

[89]

IONS

one

67 Fig. 43 and 44 show an analytical application of the MS/MS technique 1901: screening of an ivy leaf for the insecticide parathion, having a parent ion at mass 291 and characteristic daughter ions at masses 154 and 169. Because of the high background, low level detection is impossible in the normal spectrum of Fig. 43. If the instrument is tuned to mass 291 and the daughter ions are scanned the “chemical noise“ of the matrix disappears and the contamination by parathion can be detected in the pg range. This example demonstrates the efficiency of background suppression by the two dimensional mass separation.

Fig.

43 Spectrum

The instruments has published which passes

of an ivy leaf,

negative

Fig. 44 Spectrum of daughter ions thion parent ion, collisional dissociation

CI [90]

discussed so far use gas collision to fragment the ions. Beynon [91, 92[ recently results using an instrument where the fragments are generated by a laser beam through the flight tube together with the ions (Fig. 45).

Chopper Argon-ion [

Beam locator Modified

Fig. 45 Photo

from parainduced

dissociation

mass

magnet

spectrometer

[92]

laser

68 An arrangement somewhat similar to the triple quad systems selected ion flow tube or afterglow apparatus 193, 941. Mass chamber through which a reaction gas flows under relatively of the ion I molecule reactions occurring in that chamber are end. The apparatus has been designed for the study of ion I stry).

MASS SELECTION

Fig. 46 Selected

ion flow

tube

1 REACTION

just described is the so-called selected ions are injected into a high pressure (Fig. 46). The products analyzed by the quadrupole at the molecule reactions (gas phase chemi-

CHAMBER

1 MASS DETECTION

[93]

The principle of multidimensional mass separation also found application in the radiocarbon dating method. 14C is produced continuously by cosmic rays. It decays with a half life of 5730 to decay without being replaced by fresh atmoyears. When a living object dies the 14C continues spheric 14C. Therefore, by measuring the amount of 14C in a sample containing carbon, one can determine the date of the death of that object. Because of the extremely low concentration of 14C and the presence of other abundant ion peaks having the same mass number, multistage mass spectrometers have been used in combination with additional absorption, fragmentation or strip ping chambers to suppress the background. Voltages in the megavolt range are applied in these large systems which have more resemblance to an accelerator than to an ordinary mass spectrometer (Fig. 47).

Fig. 47 Instrument for radiocarbon dating analyses [95]

Fig. 48 LOHENGHlN

system

The same is true for other big mass spectrometers which are installed in nuclear research labs. Some are named after Wagner’s operas. For example, the ISOLDE system in Geneva (Isotope Se parator On Line Danish Engineering). It was followed by a TRISTAN systems. Fig. 48 shows the

69 LOHENGRIN system [96], that has been installed line analysis of fission fragments. The total beam tric sector is +400,000 volts, The parabola principle son’s first instrument from 1910. Ions of different which the energy varies. LOHENGRIN perhaps is

Fig. 49 /on optics spectrograph

of parabola

Fig. 50 Miniature (model)

mass

spectrometer b

Fig. 50 shows one of the smallest on a shelf in my office. However, I have

discussed

at the high flux reactor in Grenoble for the onlength is 23 meters and the voltage at the elec(Fig. 49) is used in this machine as in Thomm/z are focused on different parabolas, along the largest mass spectrometer ever built.

transmission,

mass spectrometers. it has some limited speed,

specificity

It is mounted in a ring binder electronic functions only.

and will now

come

to mass

and stands

range:

Soft ionization techniques permit the ionization of biological molecules with a molecular weight above 5000. For sector-type instruments the relation between mass M (daltons), radius r (cm), magnetic induction B (tesla), and accelerating voltage U (volts), is given by the well-known formula:

For obtaining a higher mass range, it is more efficient than to lower the accelerating voltage U, since r and celerating voltage might reduce the sensitivity.

Fig. 51 Mattauch-Herzog

mass

spectrometer

for SIMS

to increase the radius r and/or induction B B are squared. In addition, lowering the ac-

analyses

70 A number of instruments have been designed in recent years having relatively large radii, up to about one meter. Some of field magnets for extending the mass range 1981. The maximum is given by saturation of the iron. It is around 2 Tesla. For 80 4 kV accelerating voltage, the mass range goes up to 18 000

(e.g. by Derrick and coworkers 1971) these instruments, also, use high value for the magnetic induction cm radius, 2 Tesla induction and daltons.

Reduction Of the accelerating voltage is another means to increase the mass range. A SIMS instrument for high mass analysis, that can operate with an accelerating voltage of only a few hundred volts has been built recently by Campana et al. (Fig. 51). It has a mass range of about 18 ggg daltons. As can be seen, the ion optics is of the Mattauch-Herzog type, I might remind you that J. Mattauch and R. Herzog designed the first instrument of this type in Vienna in 1934. Fig. 52 shows the abstract of their paper [loo]:

uber einen neuen Massenspektrographen Von J.Mattauch Mit 4 Abbildungen.

und

RHerzog

(Eingegangen

in Wien. am 23. Mai

1934.)

Es wird ein Massenspektrograph vorgeschlagen, der sowohl Geschwindigkeitswie Richtungfokussierung liefert. Er besteht aus einer Kombination zweier elektronenoptischer Zylinderlinsen (elektrisches Radialfeld und homogenes Magnetfeld), die wie ein achromatisches Objektiv den die Strahlen begrenzenden Schlitz such fur verschiedene Strahlgeschwindigkeiten auf einer Stelle der photographischen Platte scharf abbildet. Im Gegensatz hierzu ware der Astonsche Massenspektrograph ein achromatischer Prismensatz, der keinerlei Abbildung im optischen Sinne liefert. AuOer der Erhohung der Scharfe und lntensitat der Bilder weist die hier vorgeschlagene Anordnung noch eine Reihe weiterer Vorteile auf.

Fig. 52 Abstract

of paper

by J. Mattauch

and

R. Herzog

Besides increasing the radius r and/or the induction, for the extension of the mass range. One can combine single linear unit. I am talking about the Wien filter. of this town. During the preparation of this overview town in the world which has given its name to such well as schools in science and arts: Wiener Schnitzel, Mandeln, Wiener Kalk, Wiener Rad, Wiener Klassik, Secession and others.

PHILOSOPHY

PSYCHOLOGY

Positivism: Moritz Schlick Rudolf Carnap Hans Reichenbach

Sigmund Freud Alfred Adler Viktor E. Frank1

Analytic

I Logic

Ludwig Wittgenstein Karl R. Popper Paul Feyerabend Kurt Godel Fig. 53

Vienna chology

schools

of philosophy

and psy-

or lowering the voltage, another way exists the electric and magnetic fields into a This name has nothing to do with the name I realized that apparently there is no other a variety of things, historic events, ideas as Wiener WOrstchen, Wiener Roast, Wiener Wiener Kongress, Wiener Kreis, Wiener

Let me draw your attention for a moment to two of these schools that became well-known in this century (Fig. 53). The authors of the Vienna school of Positivism have contributed unique and revolutionary ideas to modern philosophy. The other famous authors listed below either were born in this town or worked here. I will refer to Gddel and Wittgenstein later in my talk. The famous Vienna school of psychoanalysis needs no comment. Back to mass spectrometry. The Wien filter was invented by the physicist Wilhelm Wien, Nobel prize winner 1911 [loll. The electric field is located within the magnetic field (Fig. 54). Only those ions, for which the electric force is compensated by the Lorentz force, can pass

71 the filter without being deflected. For those ions the velocity is equal to E/B. I might remind you that a constant E/B ratio was also characteristic of the constant velocity linked scans (MS/MS) discussed earlier. Since ions of different mass, accelerated by the same voltage, have different velocities, the Wien filter can be used as a mass spectrometer [102]. Advantages: straight ion path, practically unlimited mass range, rapid electric scan. It has a number of drawbacks, too, which I will not discuss here. In the last years Aberth has built such a Wien-instrument in combination with a volcano field ion source. It has been used for fingerprinting spectra (mass profiles) of biological material [103j.

e.v.8 -e .E

Fig. 54 Schematic

of Wien-filter

Practically unlimited mass range is also characteristic of the time of flight mass spectrometer. The drawback of this principle is the limited resolution. It can be increased somewhat by increasing the flight path. An elegant method to do this is to reflect the ions at the end of the tube. This reflection in addition has the advantage of bunching ions of different initial energy 11041. Thus a kind of double focusing is achieved (Fig. 55). Throughout the last years, instruments of this type have been successfully used for the spot analysis of biological material (Fig. 56). To sum up, TOF is the instrument for low resolution mass analysis of ions generated by a short pulse. Fig. 55

Principle flection

of TOF instrument

with

ion re-

06

Vg. 56

Laser shots in frog element distribution

Rb+

muscle (left), mass spectra showing of corresponding spots (right) [105]

Cs+

alkali

Progress with respect to the mass range has also eters. Their mass range is given by the formula:

been

M = O.14v, (M = mass in daltons,

V, = RF amplitude

f2

obtained

with

quadrupole

mass

spectrom-

rg

in volts, f = frequency

in MHz, r. = field radius m cm)

Thus, lowering the frequency f will extend the mass range. Beuhler and Friedman recently reported results obtained with a quadrupole having a mass range of 80 000 daltons 1106, 1071. It was operated with a RF frequency of 300 kHz only. A tradeoff in transmission and/or resolution cannot be avoided with very low frequencies. No tradeoff will occur with increase of the RF amplitude V, at the rods. However, since the RF power rises with the square of the amplitude, and also arcing problems arise, the practical limit for the RF amplitude is around 10 kV, which leads to a mass range of a few thousand for instruments of reasonable size. Ion optical calculations, with the aim of improving the performance of quadrupoles, have been performed the last years, as in the case of sector type instruments 1108, 1091. Fringing fields at the entrance and exit of the rods and their effects on transmission have been studied both theoretically and practically. Hyperbolic rod systems have been built to obtain a better approximation to the exact quadrupole field. It has been shown that a maximum resolution of about 8000 can be obtained with relatively small systems of this type (Fig. 57).

m/z

1166

Fig. 57 Resolution trometer

of quadrupole [110]

Fig. 58 Miniature

quadrupole

mass

structure

spec-

b

In addition to GUMS, quads have found wide application in many other fields of science like physical chemistry, surfaceand vacuum technology. Reason: quads are of small size and they can be very well adapted to special experimental setups. As an example Fig. 58 shows a miniature quad structure made of one ceramic block. It has been developed for residual gas analysis [ill ]. Fig. 59 demonstrates the application sis of implanted Boron 11 in Silicon several orders of magnitude. Quads

of very

high

sensitivity

mounted

of a quadrupole secondary ion probe [112]. The concentration of Boron can

in a van

have

been

used

for depth profile analybe measured over

for air pollution

control

(Fig.

60).

73

Fig. 59 Depth probe

profile

analysis

by secondary

ion

Fig.

60 Measurement of chlorine fion at various distances tured tank car [I131

concentrafrom a rup-

Throughout the last years much theoretical ion optical work has been put into another quad device, the three-dimensional quad, or ion trap 1108, 1091, where ions of a certain mass can be stored in stable paths for many seconds (Fig. 61). It has been used for ion physics studies. Because of the storage capability of the ion trap, it is possible to make the motion of the partic les visible /1141. Charged aluminium microparticles can be put into the trap instead of ions. An extended exposure photograph then shows the complicated Lissajous figures of the motion (Fig. 62). They are named after the French physicist J. Lissajous who lived in the 19th century.

Fig. 61 Diagram

of quadrupole

Fig. 62 Lissajous figure in a quadrupole In ion optical publications scientist, the mathematician a very basic one, not only

ion trap

of microparticles ion trap

b

on quadrupoles, reference is often made to another famous french Joseph Liouville, who died 100 years ago, 1882. Since his theorem for ion optics, I will discuss it briefly (Fig. 63):

is

74 FOCUSING WITHOUT ACCELERATION (U, CONSTANT)

m

ION BEAM

FOCUSING !flTJ VOLTAGE U,,

63 Liouville’s

I

DECREASED

DECREASED

U1)

I APERTURE DECREASED mi

SO

WIDTH OF ION BEAN

Fig.

J.@XEASED

VOLTAGE U1> U, I

<

ACCELERATION (U,, +

KCREASED

1

1

WIDTH EtREASED

1

theorem

The ion beam that has passed the first draw out plate of a source has a certain width xc. In the case shown in the upper part of Fig. 63, the ions are focused by an electric lens to the small width x, at the entrance slit without acceleration, i.e. the voltage U, is not changed. Liouville’s theorem then says that, necessarily, the aperture angle a will go up, since the product of the beam width x, the aperture angle a and the square root of the voltage U, is a constant 1631. To obtain a narrow focus x,, together with a small aperture angle a, one has to increase the voltage from U, to the greater value U,, i.e. one has to accelerate the ions (see lower part of Fig. 63). Thus, it is mainly because of.the existence of Liouville’s theorem that we accelerate the ions in our mass spectrometers. To sum up, it is impossible to improve the performance of a mass spectrometer beyond the limits given by Liouville’s theorem. I think it is always exciting when, in science, theorems come up stating that something is impossible. In Vienna, 1931, one of the most revolutionary theorems in modern logics was found by the mathematician Kurt Gejdel. In simple words it says, in axiomatic systems true statements exist which can be formulated in the language of those systems, but basically cannot be proven within those systems. These true statements can only be proven in overlaid metasystems. In these metasystems one again will find other true statements that cannot be proven. And this goes on and on. Godel was able to express this idea in an abstract mathematical formula 11151. This formula states by itself that it cannot be proven:

x B, (17

Gen

r)

-+

Bew,[

%I( r$TL))]

(7)

Thoughts of this kind were in the minds of philosophers and poets much earlier. However, they could not express them in terms of mathematics. As an example, I might cite the famous poet and philosopher Ralph Waldo Emerson who died 1882, 100 years ago. One of his words can just be read as a comment on Gbdel: “The field cannot well be seen from within the field.“ Gddel’s paper referred to the axiomatic system of Russell and Whitehead, published 1910 in their famous “Principia Mathematics”. The foundations of this fundamental book were shaken by Gbdel’s results. The consequences of Godel’s revolutionary theorem, not only for mathematical, but also for philosophical systems, are obvious: to approach truth seems to be a never-ending process of human thinking.

75 How do we find our way back to mass spectrometrv? BY the “Principia Mathematics”. the other fundamental one, that appeared about 250 years earlier (Fig. 64). Its author: Sir Isaac Newton. Besides many other important studies in mathematics he was the one who could explain Kepler’s law shown in Fig. 33. He also found that the centrifugal force of a body with mass m moving on a circle is given by m.v2/r. For an ion moving in a magnetic field this force is counterbalanced by the Lorentz force e.v.B (Fig. 65). I might remind you of the Wien filter (Fig. 54) where this Lorentz force was counterbalanced by an electric force. Putting the two formulas together one finds that the circular frequency o of the ions, the so-called cyclotron frequency, is only dependent on mass m, charge e of the ion, and magnetic induction B:

/ PHIEOSOPHIAZ NATURALIS

Fig.

64 Title page Mathematics“

of Newton’s

“Principia

Fig. 66 Diagram radius r

= 3

tilne t

= y

Cyclotmn frequency

of /CR mass

spectrometer

Pm = +

w = y=

f 4 Fig. 65 Ion cyclotron

frequency

This formula is the basis of the ion cyclotron resonance mass spectrometer ICR, which has gained much interest in recent years 1116.1191. The ICR cell of small dimension is immersed in a magnetic field. The ions generated by El or laser are excited to rotations by applying a mixture of frequencies to opposite plates of the cell (Fig. 66). After the RF pulse is switched off, the ions continue to rotate with frequencies according to their masses. Their oscillations influence small alternating output signals at the receiver plates of the cell. The frequency mixture of this output signal represents the mass spectrum (Fig. 67). To extract the individual frequencies, i.e. masses, from the composite signal, one has to apply a mathematical operation, the Fourier transformation, well-known from nuclear magnetic resonance spectrometry. Fourier transformation of this

76 signal in the time (Fig. 68).

domain

leads

Fig. 67 /CR output

signal

[1211

Fig. 68

to the corresponding

mass

spectrum

in the frequency

domain

) _ _,.-- ,i., *-.- ,,. m/, G3 A 25

Fourier transformation of the output signal from Fig. 67 results in the correF sponding mass spectrum

ICR is an excellent method for gas phase chemistry studies, since the circular flight path of the ions is long and ion molecule reactions already occur at moderate pressure. Throughout the last years a number of groups have studied the function of this technique in great detail (Wilkins et al. 11171, Mclver et al. [118], Comisarow et al. [122], Wanczek et al. [119]). New applications in chemical analysis are under investigation right now [117, 120, 122-1281. By using high field superconducting magnet technology it seems to be possible to build high resolution instruments for 10 000 daltons mass range. The . . . . .

inherent

merits

of the technique

are obvious:

Simple mechanical structure High transmission Simultaneous detection of all masses Multi-mode operation (high/low resolution, MS/MS, MID) Extremely high resolution (ultrahigh vacuum required).

by changing

electronic

parameters

only

Some limitations also exist: resolution is limited by collisions. Therefore high resolution can only be obtained at ultra high vacuum. Resolution drops with increasing mass. Dynamic range and quantitative results are only moderate. Since the digitizing rate of the data acquisition is very high, powerful array processors are required for real-time data processing. The last key point of our analyzer characteristics surpassed. With optimum operating conditions, tion, a resolution of 3.106 can be obtained (Fig.

was resolution. very low pressure, 69).

Here the extended

ICR instrument time for data

is unacquisi-

We have discussed the ion trap with its three-dimensional quadrupole structure before (Fig. 61). Combining the ion trap with an ICR instrument, that means by putting the ion trap in a magnetic field, one has an instrument with which one can measure the cyclotron frequency of particles over a very long time: minutes for protons, hours for electrons. Since frequencies can be measured extremely accurately, recently it was possible with such an instrument to determine the ratio of proton to electron mass M,/M, to an extremely high degree of accuracy 11301: M,/M,

= 1836.1502.

(9)

I would like to end this section on mass separation with a few remarks about Joseph Fourier and his equations. The mathematics required for the discussed transformation had been developed by Fourier to solve problems of heat propagation. However, the leading mathematicians of that time, in particular the famous Lagrange, did not trust his equations and rejected the manuscript. It took more than 10 years until finally, 1822, one of the most brilliant books in mathematics, Fourier’s “Theorie analytique de la chaleur” could appear. Since that time Fourier’s equations have been used in many fields of science and technology.

77

+ N2 RESOLUTION -=:jhI A nf

(

. IO6

Hz

--0.4

f’ ; s c

TWEiVE

SEPARATE

C FREQUENCY

Fig. 4 Fig.

70 Fourier 69

analysis

WTES

DOMAIN)

of tone

Ultra high resolution trometer [119, 1291

frequencies

of /CR

mass

spec-

Fig. 70 shows an example of FT which I have prepared just for fun and demonstration. Above the amplitudes of a mixture of twelve tone frequencies are shown. As in the case of ICR, where all frequencies (say all masses) are contained in the composite RF output signal, here the twelve notes are contained in the composite acoustic signal. By applying Fourier transformation one can extract the twelve separate notes shown below. It is the twelve-tone row from the fourth string quartet composed by Arnold Schonberg, who was born in Vienna in 1874. Schbnberg brings us back to the Vienna schools, mentioned before. As is well-known, he was the founder of the famous Vienna school of twelve-tone music (Fig. 71). From music to painting: throughout the last 30 years we became acquainted with a number of great painters representing the Vienna school of phantastic realism. Ending this little excursion you might have a look at an impressive self-portrait of Rudolf Hausner, which demonstrates the relation between his art and psychoanalysis (Fig. 72).

MUSIC

PAINTING

Twelve.Tona:

Phantastic

Arnold Schdnberg Alban Berg Anton Webern

Vienna music

Realism:

Erich Brauer Ernst Fuchs Rudolf Hausner F. Hundertwasser Wolfgang Hutter Anton Lehmden

Fig.

71

schools of twelve-tone and painting

Fig.

72 Selfportrait of Rudolf (* 1914 in Vienna)

Hausner b

78

I come

to the next

4. Ion Current

section

of this

overview:

Detection

One of the key features of ICR is simultaneous detection of all ions. This can be realized with sector type machines, too, by collecting the ions on a channelplate electron multiplier, coupled with a photodiode array (Fig. 73). Such systems, which have been developed throughout the last years by Giffin, Boettger et al. 1131-1331 and Ryhage et al. 1134, 1351, for example, allow the study of fast transient processes that cannot be investigated with scanning spectrometers. Very recently a mass spectrometer for space research has been developed using a channel plate detector in addition to an electrostatic deflector (Fig. 74). The system can analyze the incoming ions in space with respect to mass/charge, speed, elevation and azimuth angle.

ELECTROSTATlC

Fig.

73 Simultaneous ion current with channelplate

Fig.

74 Mass P361

spectrometer

detection

for space

research b

I

ACCELERATOR GRIDS

Simultaneous detection of ions by a multi Faraday cup arrangement is used today in thermionic mass spectrometers for high precision isotope ratio work. Fig. 75 shows a modern five collector system, set for simultaneous collection of the isotopes of strontium or neodymium respectively. I might mention that the element neodymium was detected by the chemist Carl Auer von Welsbach, who lived in Vienna (Fig. 76). He also found the metal alloy used as flints in our lighters as well as the incandescent mantle serving as a light source in the 19th century gas lamps.

Fig.

76 Monument Carl Freiherr Auer von Welsbath (1858-1929), Vienna, Boltzmanngasse; Inscription at back: Aus seltenen Erden und Metal/en schuf sein forschender Geisf das Gasglilhlicht, die elektrische Osmiomlampe, das funkensprUhende Cereisen

Advantages of isotope multicollector systems: since all ions are collected simultaneously, the negative effect of ion current fluctuations on the results is eliminated, also the sample amount can be kept small. With todays multicollector systems isotope ratios can be determined with very high precision. Fig. 77 shows a typical Sr result: 1O-5 precision.

79

1

Fig.

4Fig.

87 STRONTIUM 86 STRONTIUM

77 Results of strontium measurement [137]

75 Multicollector

system

= 0 710232

isotope

t 0.000013

ratio

[137]

Another means of increasing precision or improving signal/noise ratio is multichannel averaging 1138, 1391. This technique has been used throughout the last years by many authors for the detection of very low level signals. The spectrum is scanned repeatedly, 100 times for the quadruply charged ion of the example given in Fig. 78. The signals are integrated in their corresponding channels. The noise is averaged out and the signal is enhanced. A problem arises when the ions to be detected tron emission rate is low, due to the low velocity crease this rate by post acceleration [106, 1071. tection of negative ions (Fig. 79). These ions are ions are formed out of the negative ones, which plier 1140-1421.

are of very high mass. Since their secondary elecat which they hit the first dynode, one has to inThis post acceleration is also required for the dedirected at a conversion plate, where positive then generate secondary electrons at the multi-

In this talk I have referred to the importance of frequency in various respects. A central point of detection systems is frequency. The lower the bandwidth of the detection system, the lower the noise that can pass the system, and therefore the better the detection limit. A very small bandwidth is realized by using one fixed detection frequency. This means that one has to modulate the ion production with the same frequency the detector is tuned to. The background signal is not modulated and therefore very much reduced. This technique, phase-sensitive detection [143], for example, is applied in high temperature studies with a Knudsen cell, where the vapor beam coming out of the heated cell is modulated by a rotating wheel which is located between cell and source (Fig. 80). A similar principle is applied in the apparatus of Fig. 81. It is a coincidence photoion-photoelectron spectrometer [I451471. Photons of known energy produce the ions. Their mass is determined by a small TOF spectrometer. The energy of the electrons, released during ionization, is measured with an electrostatic analyzer. The instrument is designed in such a way that only coincident impulses of electronic signals are recorded. Since the energy of the ionizing photon is given by Planck’s formula (1) and the energy E, of the released electron is also known, because it is measured with the electrostatic analyzer, the energy E, transferred to the molecule can be very accurately determined: E, = h’v-E,. Instruments of this type have been successfully used throughout the last years for the study of energy states of molecules.

80

C”z m = 11'76.78

t+ OR VOLTAGE)

-= Z$

OUADRUPOLE ASSEMBLY

FIELD

294 ELECTRON

Fig.

78 Signal

DESORPTION

295 IONIZATION (1 SCAN)

averaging

[1391

Fig.

79 Posfacceleration and detection of positive/negative ions by means of a conversion dynode

/’ ,’

Fig, 80

Knudsen cell spectrometer dies [144]

and quadrupole mass for high temperature stu.

Fig. 81 Photoion-photoelectron spectrometer [146]

-

_

coincidence

81 I come to the last which coordinates 5. Instrument

section of this overview and will the other ones (Fig. 82):

Control

and

Data

briefly

discuss

the fifth

functional

element,

Handling

After about 50 years of MS development, in which all control and data handling was done manually, the computerization historically started at block 4 and 5 and, by proceeding left, opposite to the direction of the sample flow, took over one functional element after the other 1148, 1491.

I

Fig. 82 The five functional mass spectrometer

elements

of the

Fig. 83 Off-line puter

processing

and

dedicated

com-

Off-line processing of the digitized ion currents by a central computer was the first step, followed by on-line dedicated computer processing (Fig. 83). Later on the computer was used to control the functional element number three, mass separation. I have talked about the importance of digitally controlled MID and MS/MS in this context. The next step was to control the ion source. After that, the computer control of the inlet systems, GC or direct probe for example, was incorporated, thus enabling an unattended operation of the whole instrument. Today, quite a number of fully automated mass spectrometers for various applications are on the market. By the introduction of fast high resolution scans, FAB, MS/MS and instrument control, the data rate and the amount of data have increased continuously. Therefore, the technical concept of distributed microprocessors at the front end had been introduced recently and will be further pursued in the years to come (Fig. 84). Here, again, the introduction started at the right side and has now reached the functional element of sample introduction to the left, The host computer is released from the front end jobs and instrument control, fast data acquisition and data evaluation are decoupled. This modular hardware concept will evolve together with appropriate software concepts.

Fig. 84 Instrument processors

control

by front-end

micro-

Fig.

85 Lab data

management

system

a2 Some . . .

new

directions

data

handling

and

instrument

control

will take

in the future

are listed

here:

Multiexperimental analysis Real-time experiment definition Lab data management

Multiexperimental analysis means that the instrument can be preprogrammed for a set of different analyses, which will be performed automatically one after the other. Real-time experiment definition means that the instrument then will select the appropriate parameters and during the analysis will decide, without the operator’s interference, which way to go. For example, which masses should be selected for an MS/MS analysis. Lab data management is a third direction. It means that the data of various analytical instruments can be correlated by a central computer (Fig. 85). In such systems all analytical intelligence is concentrated in one integrated network. Obviously this will be the technique of the future. There is still a long way to go.

LUDWIG WITTGENSTEIN >k1889 VIENNA t 1951 CAMBRIDGE

Fig.

86 Ludwig

Wittgenstein.

Sentence

6.52 from

the “Tractatus

Logic0

Philosophicus“

I will not end this talk about instrumentation with such a view of a perfect laboratory where all problems can be solved and where science seems to be almightly. I would rather ask you to let me quote a word about basic limitations of science. It is from Ludwig Wittgenstein, one of the greatest modern philosophers, who was born in Vienna and lived there many years. After studies in engineering and mathematics in Berlin, he went to Cambridge where he began his philosophical work. Wittgenstein’s revolutionary book entitled “Tractatus Logic0 Philosophicus” appeared 1922 in a German/English edition. Bertrand Russell, his friend, wrote the introduction. At the very end of that book one finds the sentence which is shown in Wittgenstein’s handwriting on Fig. 86. The English version reads: “We fee/ that even when a// possible scientific questions have been answered, the problems of life remain completely untouched. Of course are then no questions left, and this itself is the answer.“

there

83 References 1

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86 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149

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